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Calcium and bone disorders in children and adolescents [2nd, revised edition]
 978-3318054668

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Citation preview

Calcium and Bone Disorders in Children and Adolescents 2nd, revised edition

Endocrine Development Vol. 28

Series Editor

P.-E. Mullis

Bern

Calcium and Bone Disorders in Children and Adolescents 2nd, revised edition

Volume Editors

Jeremy Allgrove London Nick J. Shaw Birmingham 152 figures, 22 in colour, and 43 tables, 2015

Basel · Freiburg · Paris · London · New York · Chennai · New Delhi · Bangkok · Beijing · Shanghai · Tokyo · Kuala Lumpur · Singapore · Sydney

Endocrine Development Founded 1999 by Martin O. Savage, London

Jeremy Allgrove

Nick J. Shaw

Department of Paediatric Endocrinology Royal London Hospital London, UK

Clinical Lead for Endocrinology Birmingham Children’s Hosptial Birmingham, UK

Library of Congress Cataloging-in-Publication Data Calcium and bone disorders in children and adolescents / volume editors, Jeremy Allgrove, Nick J. Shaw. -- 2nd, revised edition. p. ; cm. -- (Endocrine development, ISSN 1421-7082 ; vol. 28) Includes bibliographical references and indexes. ISBN 978-3-318-05466-8 (hard cover : alk. paper) -- ISBN 978-3-318-05467-5 (electronic version) I. Allgrove, Jeremy, editor. II. Shaw, Nick, editor. III. Series: Endocrine development ; v. 28. 1421-7082 [DNLM: 1. Calcium Metabolism Disorders--physiopathology. 2. Adolescent. 3. Bone Diseases--physiopathology. 4. Child. W1 EN3635 v.28 2015 / WD 200.5.C2] RJ482.B65 618.92’71--dc23 2015014316

1st edition published in 2009 as Calcium and Bone Disorders in Children and Adolescents by S. Karger Publishers, Basel

Bibliographic Indices. This publication is listed in bibliographic services, including Current Contents® and Index Medicus. Disclaimer. The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publisher and the editor(s). The appearance of advertisements in the book is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements. Drug Dosage. The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any change in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. © Copyright 2015 by S. Karger AG, P.O. Box, CH-4009 Basel (Switzerland) www.karger.com Printed in Germany on acid-free and non-aging paper (ISO 9706) by Kraft Druck GmbH, Ettlingen ISSN 1421–7082 e-ISSN 1662–2979 ISBN 978–3–318–05466–8 e-ISBN 978–3–318–05467–5

Contents

VII VIII

1 7 33 56 72 84 101 119 134 162 176 196 210

Foreword Whyte, M.P. (St. Louis, MO) Preface Allgrove, J. (London); Shaw, N.J. (Birmingham) Voyages of Discovery Allgrove, J. (London) Physiology of Calcium, Phosphate, Magnesium and Vitamin D Allgrove, J. (London) Physiology of Bone Grabowski, P. (Sheffield) Radiology of Osteogenesis Imperfecta, Rickets and Other Bony Fragility States Calder, A.D. (London) Bone Densitometry: Current Status and Future Perspective Crabtree, N. (Birmingham); Ward, K. (Cambridge) A Practical Approach to Hypocalcaemia in Children Shaw, N.J. (Birmingham) Approach to the Child with Hypercalcaemia Davies, J.H. (Southampton) A Practical Approach to Vitamin D Deficiency and Rickets Allgrove, J. (London); Shaw, N.J. (Birmingham) A Practical Clinical Approach to Paediatric Phosphate Disorders Imel, E.A. (Indianapolis, IN); Carpenter, T.O. (New Haven, CT) Primary Osteoporosis Arundel, P.; Bishop, N. (Sheffield) Osteoporosis in Children with Chronic Disease Högler, W. (Birmingham); Ward, L. (Ottawa) Genetics of Osteoporosis in Children van Dijk, F.S. (Amsterdam) A Practical Approach to Children with Recurrent Fractures Korula, S.; Titmuss, A.T.; Biggin, A.; Munns, C.F. (Westmead, NSW)

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226 247 259 277 291 319 414 421

422 423

VI

Miscellaneous Bone Disorders Mughal, M.Z.; Padidela, R. (Manchester) Skeletal Aspects of Non-Accidental Injury Johnson, K.; Bradshaw, K. (Birmingham) Skeletal Dysplasias: An Overview Offiah, A.C. (Sheffield, S. Yorkshire) Drugs Used in Paediatric Bone and Calcium Disorders Cheung, M.S. (London) Classification of Disorders of Bone and Calcium Metabolism Allgrove, J. (London) Case Histories Katugampola, H.; Saraff, V.; Kumaran, A.; Allgrove, J. (London); Shaw, N.J. (Birmingham) Appendix 1: Explanation of Abbreviations Appearing in Text Appendix 2: Conversion Factors between SI and ‘Conventional’ Units Author Index Subject Index

Contents

Foreword

After the first edition of ‘Calcium and Bone Disorders in Children and Adolescents,’ edited by Jeremy Allgrove and Nick Shaw, was published in 2009, this field continued to accelerate, with new discoveries concerning mineral and hard tissue physiology and pathophysiology, new means to evaluate the skeleton, and new ways to diagnose and to treat the associated conditions. Increasingly, endocrinologists and others who become involved with these disorders are called upon to translate this progress into better patient care. Skeletal dysplasias, once the purview of medical geneticists and managed by orthopaedists, are now largely understood at the gene level, and those that are still unexplained are yielding rapidly to whole exome and genome sequencing. Expectations of medical treatments for these ‘orphan’ diseases are intensifying because the disrupted molecular pathways are being revealed. Consequently, an increasing number of maladies are the charge of physicians who specialise in bone and mineral metabolism. Despite the grow-

ing use of mutation analysis to uncover and to identify gene-based disorders, there is undiminished importance for all heritable and acquired disturbances of mineral and bone metabolism in taking medical histories, performing physical examinations, and interpreting routine biochemical and conventional radiographic studies. In 2015, greater than ever clinical skill is necessary. Accordingly, this second edition is welcome, as it provides a solid overview of the pathophysiology, diagnostic tools, and treatments for the many interesting and challenging clinical problems encountered in this field. Clinicians and researchers will have an up-to-date resource available to understand bone and mineral physiology and to diagnose, treat, and study these patients. The book is a good addition to this especially dynamic specialty of paediatric medicine. Michael P. Whyte, MD Washington University School of Medicine, St. Louis, MO, USA

VII

Preface

We were pleased with the response to the first edition of this book, which was published in 2009, with many positive comments as to its usefulness in clinical practice and, in particular, its inclusion of case histories describing the disorders. We were encouraged that Karger was willing to commit to the production of a second edition. There have been significant advances in our understanding of various paediatric calcium and metabolic bone disorders in a relatively short time period, e.g. the evolving information regarding phosphate metabolism and the role of FGF23 in this process. In addition, advances in genetic technology due to the introduction of microarrays and whole-exome sequencing have led to the identification of the genetic basis of many more conditions. We have decided to expand the number of chapters in this second edition, with additions related to Skeletal Dysplasias, Genetics of Osteoporosis, Imaging of Bone and the approach to a child with recurrent fractures. The success of the International Conferences on Children’s Bone Health, which were estab-

VIII

lished in 1999 and will meet for the seventh time in 2015, has provided a forum for clinicians, basic scientists and researchers to meet and exchange information about paediatric metabolic bone disease. We have therefore expanded our authorship for this edition to include international colleagues in addition to our colleagues from the UK. When we started to develop an interest in paediatric calcium and bone metabolism 30–35 years ago, neither of us would have predicted how much the field would have progressed and expanded to the extent it has. We are grateful to all of the authors for sending us their manuscripts in a timely manner, and we would like to thank all of the clinicians who have allowed us to include their cases. Finally, we would again like to thank our wives, Vicki and Natalie, for their forbearance when we spent many hours during evenings and weekends in the production of this second edition. Jeremy Allgrove, London, UK Nick J. Shaw, Birmingham, UK

Chapter 1 Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

Voyages of Discovery Jeremy Allgrove Department of Paediatric Endocrinology, Royal London Hospital, London, UK

The metabolism of calcium and bone is controlled by five principal hormones: parathyroid hormone, 1,25-dihydroxyvitamin D, calcitonin, parathyroid hormone-related peptide and fibroblast growth factor 23, some of which have been known for several decades and some of which have only more recently been identified. The stories of the discovery of these hormones have constituted a series of complex journeys that have been undertaken over the past century or so, none of which has yet been completed. The complexities of bone and calcium metabolism have been and remain, to many people, somewhat mysterious and a daunting task to understand. This book is designed to try to unravel those mysteries and present them in an interesting and comprehensible manner. © 2015 S. Karger AG, Basel

Introduction

The study of the diseases that affect bone and calcium has made huge strides over the past few decades. The initial realisation that rickets, which was rife in industrial cities, particularly in the UK, could be cured in children by exposing them to sunlight or by supplementing them with foods such as cod liver oil was a major step in improving

the health of those children in the early part of the 20th century. Subsequently, the discovery of other hormones that are involved in mineral metabolism, both calcium and phosphate, has enabled a much wider understanding of the mechanisms of disease that can be gained. The underlying genetic mechanisms that determine calcium and bone metabolism are continually being unravelled and, with modern methods of detecting mutations, this process is accelerating. This has led to the introduction of logical treatments based on this scientific understanding. Five major hormones, i.e. vitamin D and its metabolites, parathyroid hormone (PTH), calcitonin (CT), parathyroid hormone-related peptide (PTHrP) and fibroblast growth factor 23 (FGF23), are directly involved in the control of mineral metabolism in man. In addition, several other hormones, such as oestrogens and androgens, cortisol, growth hormone and thyroxine, have modifying effects. The story of the unravelling of the identity of these hormones is a long and complicated one that has gradually revealed itself over the past century or so. For each, there has been a long voyage of discovery, some lasting longer than others, but each is still a journey in progress. Downloaded by: Chulalongkorn University 161.200.145.19 - 7/31/2019 7:39:14 AM

Abstract

Rickets is an ancient disease. It was probably known in the ancient world but has been recorded in the UK since the 17th century [1]. Rickets became widespread with the increase of industrialisation during the 19th and 20th centuries. The first breakthrough in treatment came with the realisation, shortly after the end of the First World War, that most rickets could be cured either by exposure to sunlight or with supplements of cod liver oil [2], and vitamin D was discovered to be the agent that effected the cure. As a consequence, rickets virtually disappeared from the UK until the first major wave of immigration, mainly from the old commonwealth countries. Most of this immigration was from either south Asia or the Caribbean and brought with it a greater predisposition to rickets than was present within the white population because of the need for greater sunlight exposure by those with more darkly pigmented skin in order to synthesise sufficient vitamin D [3]. This resulted in a second wave of rickets that again occurred mainly in industrialised cities. Following a pilot study, it was demonstrated that the incidence of rickets in Glasgow could be effectively reduced by a campaign of supplementation [4]. Since then, the incentives to persist in such a campaign appear to have been lost, and a third-wave resurgence of rickets has been seen in many countries of the world [5]. During the 1960s, it was discovered that vitamin D had to be metabolised in order to become effective, which elevated it from the status of ‘vitamin’ to one of ‘hormone’. As a consequence, some forms of rickets that had previously been thought to be caused by vitamin D deficiency were now understood to result from inborn errors of metabolism, which explained why some children had not previously responded to vitamin D treatment. Since then, the metabolism of vitamin D has been well worked out and has provided a logical basis for treatment.

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The third stage of investigation of vitamin D involved the demonstration that vitamin D deficiency may play an important part in contributing to the aetiology of a number of common diseases that previously had not been associated with vitamin D deficiency. These diseases include certain cancers, especially of the breast and colon, both type 1 and type 2 diabetes, and coronary heart disease. These relationships remain to be worked out, although increasing evidence is accumulating that suggests that vitamin D plays a part in the prevention of many of these diseases [5]. Whilst they are generally diseases of adulthood, it is conceivable that the origins of these diseases lie in childhood. Vitamin D remains the preferred treatment of vitamin D deficiency, but it seems extraordinary that, in modern societies, the ability to eliminate a fully preventable disease eludes us. It is arguable that effective vitamin D supplementation is the single most cost-effective treatment that could be given, at least to ‘at risk’ populations. Little has changed since the first edition of this book was published 6 years ago.

Parathyroid Hormone

During the 1920s, the role of the parathyroid glands in secreting a calcium-raising hormone was clarified [6]. The first description of hypoparathyroidism was made in 1929 [7] and was followed by the description of parathyroid hormone resistance in pseudohypoparathyroidism in 1942 [8]. Fuller Albright was an early pioneer of parathyroid physiology and pathology who laid the basis of much of what we know about basic parathyroid actions. He was correct in describing pseudohypoparathyroidism as a hormone resistance syndrome. However, as it turns out, he was incorrect in referring to it as ‘an example of Seabright Bantam Syndrome’. ‘Seabright bantam’, named after Sir John Sebright (sic) but was misspelled in the original paper, is characterised

Allgrove Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

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Vitamin D and Rickets

then, the post receptor mechanisms that control PTH function have been elucidated, and this has led to clarification of the mechanisms of several clinical conditions.

Calcitonin

This hormone was first described in 1963 [15], and its structure was elucidated in 1968 [16]. CT is a thirty-two amino acid protein and, unlike PTH, has a disulphide bond between the cysteine residues at positions one and seven. It has an action that is largely opposite that of parathyroid hormone, i.e. it has a calcium-lowering effect. There are considerable interspecies differences in the structure of CT [17], and interestingly, the salmon hormone has considerably greater activity in humans than its human counterpart. For this reason, salmon hormone has been used as a therapeutic agent to lower calcium levels in certain hypercalcaemic conditions, although its use in this respect has been largely superseded by the introduction of bisphosphonates. CT is now known to be a product of the alpha CT/CT gene-related peptide gene, which, as a result of alternative splicing, gives rise to at least two products, α-CT and CT gene-related peptide (CGRP) [18]. Each is mainly produced by different tissues: CT by the C-cells of the thyroid and CGRP by the hypothalamus. CT probably contributes to bone formation, and CGRP is mainly known as a neuropeptide that plays a part in vascular tone and may have a role in the pathogenesis of migraine. Nevertheless, it is also thought that all four proteins may play some part in bone formation [19, 20], possibly via a network of neurones that exists in bone. However, pathological states in man in which CT is produced in excess, such as Medullary Carcinoma of the Thyroid, do not result in hypocalcaemia, and the principal significance of CT is both as a marker of Medullary Carcinoma of the Thyroid and as a therapeutic agent.

Voyages of Discovery Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

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by a ‘hen-feathering’ appearance in cock birds that led to the misapprehension that they were resistant to testosterone. In fact, these birds have excessive aromatase P450 activity in extra-gonadal tissues, which converts testosterone to oestrogen [9] and causes the characteristic feathering pattern. He was correct in most other aspects, and he made a huge contribution to our understanding of bone disease. In fact, several conditions bear his name eponymously. The first immunoassays for PTH were described in 1963 by Berson and Yalow [10]. These had been developed in the wake of other immunoassays, such as that for insulin (for which Rosalind Yalow received a Nobel Prize; Berson having already deceased) and growth hormone. However, it rapidly became apparent that these assays were not straightforward, since PTH, a large molecule containing eighty-four amino acids, circulates as a number of fragments [11]. These are particularly problematic in the presence of renal failure, when the inactive fragments tend to circulate in higher quantities than normal. Since the original assays were developed, further refinements have been made that now allow for the measurement of physiological levels of intact hormone. The structure of PTH was difficult to establish, and different structures were initially proposed. Once these were resolved, it became apparent that, although PTH contains eighty-four amino acids, only the first thirty-four are required for its full biological activity [12]. The function of the remainder of the molecule remains unclear, although some studies have suggested that there may be differential effects of the N-terminal and C-terminal fragments on bone cells [13] and renal tubules [14]. Subsequent work revealed the mechanism of action of PTH via the Gsα second messenger, which is common to a number of polypeptide hormones, providing an explanation for the hormone resistance state known as pseudohypoparathyroidism that was originally described by Fuller Albright. Since

The first indication that there was a substance that had PTH-like activity but was not PTH came with a publication in 1985 demonstrating that human umbilical cord blood contained a compound that had PTH-like bioactivity and yet could not be identified as PTH by immunoassay [21]. The calcium concentration in foetal cord blood is unusual in that calcium is one of the few substances that are present at higher levels in the foetus than in the mother, i.e. there is a positive gradient across the placenta. Whilst it had previously been suggested that foetal PTH levels are suppressed because of these relatively high levels of calcium, the question had never been asked as to what maintains the gradient. It seemed that PTHrP was responsible. Subsequently, a humoral factor was identified and purified from malignant tissue and was found to be responsible for some instances of humoral hypercalcaemia of malignancy [22]. This factor also shared some properties with PTH, including its ability to stimulate cyclic-AMP, but was sufficiently different from PTH, as it was undetectable on standard PTH immunoassays. This factor was a considerably larger molecule than PTH and had some limited homology with PTH such that it bound to the PTH1 receptor. The role of PTHrP in man seems to principally be in maintaining the calcium gradient across the placenta and to have a paracrine function in promoting cartilage development in the foetus. In postnatal life, PTHrP seems to not have a classical endocrine role but is important as a mediator of humoral hypercalcaemia of malignancy [23].

Fibroblast Growth Factor 23

The factors controlling phosphate metabolism have, until relatively recently, not been well understood. The discovery of FGF23 in 2000 [24] led to an explosion of discoveries related to phosphate. The relationship of FGF23 to the BMP1,

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DMP1, ASARM and RGD motifs, to PHEX and GALNT3 in bone and to FGFR1, Klotho, and the sodium/phosphate co-transporter in renal tubular cells has considerably widened our understanding and led to a much greater knowledge of the pathological processes that explain the mechanisms of disorders of phosphate metabolism. Further details of these hormones are given in the relevant chapters. The discovery of the structure of DNA in the early 1950s and its role as a genetic blueprint has allowed the identification of a whole host of diseases that are genetically based. The diseases related to bone and calcium are no exception, and if one excludes vitamin D deficiency and secondary osteoporosis, the vast majority of all other causes of bone and calcium diseases have a genetic origin. Indeed, these diseases encompass the full gamut of genetic conditions, including autosomal and X-linked dominant and recessive, mitochondrial and imprinting disorders. More recently, the techniques of microarray and whole exome sequencing have extended this knowledge and will continue to do so for the foreseeable future. It is therefore necessary to have at least a modicum of understanding of genetics in order to be able fully to understand the mechanism of these diseases. Fortunately, modern technology allows for the rapid advances in genetics to be recorded electronically without having to ‘go back to the books’ all of the time, and it ensures that updates to discoveries can be made available to a wide audience more rapidly than previously. The most useful tool is the creation of the Online Mendelian Inheritance in Man (OMIM) website, which was the brainchild of the late Victor McKusick [25] when his original paper version became too unwieldy and difficult to update. The website is accessible at http://www.ncbi.nlm.nih. gov/sites/entrez?db=omim and gives details of all disorders that are or are thought to be genetically based, together with the genes involved. Because of the diversity of the disorders of bone and calcium metabolism and of the greater understand-

Allgrove Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

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Parathyroid Hormone-Related Peptide

ing of their underlying genetic basis, we have introduced an additional chapter that describes the classification of these disorders, both genetic and otherwise. Each chapter refers to this classification, which includes, amongst other things, the OMIM reference numbers for these disorders and their genes. Hopefully this will prove useful to readers. An explanation of any abbreviations appearing in the text that are not defined at the time can be found in the Appendix, together with relevant conversion factors for readers who are not familiar with either SI or ‘conventional’ units of measurement. The final chapter in this book is a series of case histories that has been considerably expanded since the first edition. These case histories are intended to illustrate some of the problems that are discussed in the previous chapters. When the text describes a case that is included in the Case History section, the number of that case is shown in the text. References to these cases may appear in more than one chapter. The editors are extremely grateful to all of the clinicians, some of whom are

overseas colleagues, who have kindly provided us with cases under their care and without whom we would not have been able to include such a diversity of conditions. All of these are acknowledged in the chapter. A suitable reference to the clinical description is shown where we have not included cases describing a condition. ‘Dr. Donne’s verses are like the peace of God; they pass all understanding.’ With these words, King James I of England and VI of Scotland is said to have replied to Archdeacon Plume when asked to comment on the poetry of John Donne [26]. Although the situation is beginning to change, there are many, even in the world of paediatric endocrinology, for whom the same is true of the study of bone and calcium disorders in children and for whom these disorders remain a mystery. This was recognised by the late Graham Chapman, of Monty Python fame, comedian, bon viveur and erstwhile medical student, who, in a book of collected sketches, letters and essays, wrote a brief essay entitled ‘Calcium Made Interesting’ [27]. This book is designed not only to enable mineral metabolism to be understood, but to ‘make calcium interesting’.

References 7 Albright F, Ellsworth R: Studies on the physiology of the parathyroid glands: I. calcium and phosphorus studies on a case of idiopathic hypoparathyroidism. J Clin Invest 1929;7:183–201. 8 Albright FBC, Smith PH, Parson W: Pseudohypoparathyroidism – an example of the ‘Seabright-Bantam syndrome’. Endocrinology 1942;30:922–932. 9 Matsumine H, Wilson JD, McPhaul MJ: Sebright and Campine chickens express aromatase P-450 messenger RNA inappropriately in extraglandular tissues and in skin fibroblasts. Mol Endocrinol 1990;4:905–911. 10 Berson SA, Yalow RS, Aurbach GD, Potts JT: Immunoassay of bovine and human parathyroid hormone. Proc Natl Acad Sci U S A 1963;49:613–617.

11 Berson SA, Yalow RS: Immunochemical heterogeneity of parathyroid hormone in plasma. J Clin Endocrinol Metab 1968;28:1037–1047. 12 Rosenblatt M, Segre GV, Tregear GW, Shepard GL, Tyler GA, Potts JT Jr: Human parathyroid hormone: synthesis and chemical, biological, and immunological evaluation of the carboxyl-terminal region. Endocrinology 1978;103: 978–984. 13 Kaji H, Sugimoto T, Kanatani M, Miyauchi A, Kimura T, Sakakibara S, Fukase M, Chihara K: Carboxyl-terminal parathyroid hormone fragments stimulate osteoclast-like cell formation and osteoclastic activity. Endocrinology 1994;134: 1897–1904.

Voyages of Discovery Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

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1 O’Riordan JL: Rickets in the 17th century. J Bone Miner Res 2006;21:1506– 1510. 2 DeLuca HF: The vitamin D story: a collaborative effort of basic science and clinical medicine. FASEB J 1988;2:224– 236. 3 Parra EJ: Human pigmentation variation: evolution, genetic basis, and implications for public health. Am J Phys Anthropol 2007;suppl 45:85–105. 4 Chesney RW: Rickets: the third wave. Clin Pediatr (Phila) 2002;41:137–139. 5 Bouillon R, Eelen G, Verlinden L, Mathieu C, Carmeliet G, Verstuyf A: Vitamin D and cancer. J Steroid Biochem Mol Biol 2006;102:156–162. 6 Collip JB: The internal secretion of the parathyroid glands. Proc Natl Acad Sci U S A 1925;11:484–485.

14 Garcia JC, McConkey CL, Martin KJ: Separate binding sites for intact PTH 1–84 and synthetic PTH 1–34 in canine kidney. Calcif Tissue Int 1989;44:214– 219. 15 Copp DH: Calcitonin – a new hormone from the parathyroid which lowers blood calcium. Oral Surg Oral Med Oral Pathol 1963;16:872–877. 16 Neher R, Riniker B, Rittel W, Zuber H: [Human calcitonin. Structure of calcitonin M and D]. Helv Chim Acta 1968;51: 1900–1905. 17 Niall HD, Keutmann HT, Copp DH, Potts JT Jr: Amino acid sequence of salmon ultimobranchial calcitonin. Proc Natl Acad Sci U S A 1969;64:771–778. 18 Rosenfeld MG, Lin CR, Amara SG, Stolarsky L, Roos BA, Ong ES, Evans RM: Calcitonin mRNA polymorphism: peptide switching associated with alternative RNA splicing events. Proc Natl Acad Sci U S A 1982;79:1717–1721.

19 Huebner AK, Keller J, Catala-Lehnen P, Perkovic S, Streichert T, Emeson RB, Amling M, Schinke T: The role of calcitonin and alpha-calcitonin gene-related peptide in bone formation. Arch Biochem Biophys 2008;473:210–217. 20 Naot D, Cornish J: The role of peptides and receptors of the calcitonin family in the regulation of bone metabolism. Bone 2008;43:813–818. 21 Allgrove J, Adami S, Manning RM, O’Riordan JL: Cytochemical bioassay of parathyroid hormone in maternal and cord blood. Arch Dis Child 1985;60: 110–115. 22 Burtis WJ, Wu T, Bunch C, Wysolmerski JJ, Insogna KL, Weir EC, Broadus AE, Stewart AF: Identification of a novel 17,000-dalton parathyroid hormone-like adenylate cyclase-stimulating protein from a tumor associated with humoral hypercalcemia of malignancy. J Biol Chem 1987;262:7151–7156.

23 Kaiser SM, Goltzman D: Parathyroid hormone-related peptide. Clin Invest Med 1993;16:395–406. 24 Yamashita T, Yoshioka M, Itoh N: Identification of a novel fibroblast growth factor, FGF-23, preferentially expressed in the ventrolateral thalamic nucleus of the brain. Biochem Biophys Res Commun 2000;277:494–498. 25 Obituary: professor Victor McKusick: advocate of the Human Genome Project. The Times 1999. 26 The Oxford Dictionary of Quotations, ed 2. London, Oxford University Press, 1953. 27 Chapman G: Calcium made interesting; in Yoakum J (ed): Calcium Made Interesting: Sketches, Letters, Essays and Gondolas. London, Basingstoke, Oxford, Sidgwick and Jackson, 2005, pp 88–89.

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Allgrove Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 1–6 (DOI: 10.1159/000380988)

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Jeremy Allgrove, MA, MD, FRCP, FRCPCH Department of Paediatric Endocrinology 8th Floor, North Tower, Royal London Hospital Whitechapel, London E1 1BB (UK) E-Mail [email protected]

Chapter 2 Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 7–32 (DOI: 10.1159/000380990)

Physiology of Calcium, Phosphate, Magnesium and Vitamin D Jeremy Allgrove Royal London Hospital, Honorary Consultant Paediatric Endocrinologist, Great Ormond Street Hospital, London, UK

The physiology of calcium and the other minerals involved in its metabolism is complex and intimately linked to the physiology of bone. Five principal humoral factors are involved in maintaining plasma concentrations of calcium, magnesium and phosphate and in coordinating the balance between their content in bone. The transmembrane transport of these elements is dependent on a series of complex mechanisms that are partly controlled by these hormones. The plasma concentration of calcium is initially sensed by a calciumsensing receptor, which then sets up a cascade of events that initially determines parathyroid hormone secretion and eventually results in a specific action within the target organs, mainly bone and kidney. This chapter describes the physiology of these humoral factors and relates them to the pathological processes that give rise to disorders of calcium, phosphate and magnesium metabolism as well as of bone metabolism. This chapter also details the stages in the calcium cascade, describes the effects of calcium on the various target organs, gives

details of the processes by which phosphate and magnesium are controlled and summarises the metabolism of vitamin D. The pathology of disorders of bone and calcium metabolism is described in detail in the relevant chapters. © 2015 S. Karger AG, Basel

Introduction

The metabolisms of calcium, phosphate and magnesium are intimately bound to each other; therefore, it is necessary to discuss all three together. Furthermore, this metabolism is, in many ways, different from that of most other substances by virtue of the fact that the majority of each is contained within bone, which acts as a structural material as well as a reservoir, whilst also acting as an important physiological regulator. Thus, it is required that the concentration of calcium be kept within narrow limits within plasma in order to maintain optimum neuromuscular function. Downloaded by: Chulalongkorn University 161.200.145.19 - 7/31/2019 7:39:55 AM

Abstract

Calcium Physiology

A full-grown adult contains approximately 1,200 grams of calcium. In foetal and neonatal life, the total calcium content (Ca) is related to body weight (BWt), and a very close relationship exists between the two under normal circumstances. This relationship is expressed by the following formula: Ca = 0.00075*BWt1.3093

where Ca and BWt are both expressed in grams [1]. This relationship has been observed during the foetal and neonatal periods and probably largely holds true throughout the period during which bone accretion is occurring. About 99% of calcium is normally contained within bone; the remainder is present either as an intracellular cation or circulating in plasma. There are three main fractions of calcium within plasma: ionised, protein-bound and complexed (mainly to citrate or sulphate). The ionised fraction constitutes approximately 50% of the total, and most blood gas machines found within critical or intensive care units can directly measure ionised calcium. Of the remainder of the calcium in the body, most circulates bound to albumin, and plasma albumin levels affect the total concentration of calcium. Various formulae are used to ‘correct’ total calcium to allow for this, and many laboratories automatically provide a value for ‘corrected’ calcium (see Chapter 6).

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The concentration of ionised calcium is normally kept within very narrow limits (1.1–1.3 mmol/l), a level that is necessary to maintain normal neuromuscular activity. Complex mechanisms, such as altering calcium absorption in the gut, changing excretion within renal tubules and balancing the rate of deposition into or removal from bone, are involved in maintaining this concentration. If calcium levels vary significantly from this, either upwards or downwards, symptoms may develop. These issues are discussed in more detail in the relevant chapters.

Control of Plasma Calcium

Five principal humoral factors are involved in the maintenance of normal concentrations of calcium and phosphate in plasma. Plasma calcium is  mainly influenced by PTH and the active form  of vitamin D, 1α,25-dihydroxyvitamin D (1,25(OH)2D). In addition, calcitonin (CT) and parathyroid hormone-related peptide (PTHrP) play a more minor role, at least during postnatal life, but attain greater significance in a number of pathological situations. Plasma phosphate is also influenced by PTH and 1,25(OH)2D, but another factor, Fibroblast Growth Factor 23 (FGF23), also plays an important part in its metabolism. Magnesium is influenced, though to a lesser degree, by the same factors that control calcium, and it indirectly influences calcium by altering PTH secretion in response to hypocalcaemia.

Transmembrane Calcium Transport

The calcium balance is principally controlled by transport across membranes in the gastrointestinal tract and in renal tubules. The mechanisms for both are similar but exhibit differences in their emphasis, depending on which organ is involved.

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Furthermore, phosphate is involved in virtually all metabolic processes, whilst magnesium is required to ensure optimum parathyroid hormone (PTH) secretion. The mechanisms required to maintain these levels are complex and dependent on a number of factors. It is the purpose of this chapter to describe these factors and to indicate how disorders of function give rise to clinical problems.

Calcium TRPV5

TRPV6 Claudin 16 Claudin 19

CB28k CB9k

Mainly GI Paracellular

Mainly renal

Na+

Ca++ PMCA1b

NCX1 Ca++

Fig. 1. Schematic representation of the mechanisms of calcium transport in the gut and renal tubules. Similar mechanisms are present in both tissues, although the importance of each differs between them. The principal mechanisms in the gut are shown on the right-hand side, and those that are more important in the renal tubules are shown on the left-hand side. Abbreviations are explained in the Appendix.

Transcellular Calcium Transport

The most important mechanism for calcium absorption in the gut is via active transport, and three steps are involved in this process [2]. There is initial absorption of calcium from the lumen that is followed by transcellular transport and lastly by extrusion of calcium across the basolateral membrane. A similar process involving related proteins is present in renal tubules (see below).

Two proteins, transient receptor potential V5 (TRPV5 (*606679)) and TRPV6 (*606680), which are members of the TRP channel protein family, are thought to play an important role in promoting active calcium transport [2, 3]. TRPV6 is the most important of these in the gut, whilst TRPV5 plays a larger role in renal tubules. These proteins inwardly rectify calcium channels whose affinity is greater for calcium than for magnesium. Once calcium reaches the intracellular compartment, cytosolic diffusion across the cell membrane is facilitated by two additional proteins, calbindin9K (*302020) in the gut and calbindin28K (*114050) in the kidney. These proteins bind calcium and transport it across the cytoplasm. At the basolat-

Calcium Physiology Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 7–32 (DOI: 10.1159/000380990)

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Transport occurs by both transcellular and paracellular mechanisms, which are summarised in figure 1.

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protect against the effects of vitamin D deficiency [6], presumably by increasing non-vitamin D-dependent absorption. Calcium reabsorption in renal tubules is largely passive and is influenced by a number of dietary factors, including a high sodium, protein or acid load, all of which increase calcium excretion. About 70% of the filtered load is reabsorbed passively in the proximal tubule in conjunction with sodium. A further 20% of the calcium in renal tubules is reabsorbed in the thick ascending loop of Henle by paracellular processes. The remaining 5–10% is reabsorbed in the distal tubule. Similar mechanisms to those in the gut are present, although TRPV5 is thought to be the major influence. Transcellular transport is facilitated by calbindins, particularly calbindin28k. At the basolateral surface, NCX1 is responsible for the more important mechanism, which is under hormonal influence, mainly by PTH. In the presence of hypoparathyroidism, treatment with active vitamin D analogues must be monitored carefully to prevent hypercalciuria.

Magnesium Metabolism

Magnesium is, like calcium, a divalent cation that is important for bone and calcium metabolism. Magnesium is normally present in plasma at a concentration of between 0.7 and 1.2 mmol/l, and adequate plasma magnesium is required for the normal secretion of PTH. For a more detailed description of this mechanism, see the section on the calcium-sensing receptor (CaSR). Magnesium absorption occurs in the small intestine by mechanisms that are very similar to those of calcium, although these mechanisms are not well understood [3]. They are summarised in figure 2. Two proteins, TRPM6 and TRPM7, which are related to the corresponding proteins involved in calcium absorption, facilitate transcellular magnesium transport. TRPM6 is present mainly in renal tubules and intestinal cells, whilst

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eral surface, extrusion of calcium is facilitated by  both an ATP-dependent Ca+-transporting ATPase (PMCA1b) (*108731) and by a Na+/Ca+ exchanger (NCX1) (*182305). In addition, calcium may be transported across the cell by passive diffusion or by extrusion of vesicles that are formed from calcium-calbindin complexes. TRPV6-mediated calcium absorption mainly occurs in the duodenum under conditions of depolarisation and during the non-fed state. There are vitamin D receptors in the duodenal cells, and both PMCA1b and TRPV6 are stimulated by 1,25(OH)2D. If these receptors are defective, as  in  Hereditary 1α,25(OH)2D-resistant Rickets (HVDRR) (#277440), calcium cannot be absorbed properly, and rickets results (see Chapter 8 and Case 19–38 for further details). Recently, an alternative, complementary transcellular mechanism has been proposed that occurs in the lower parts of the small intestine, particularly in the jejunum and ileum. This mechanism is dependent on another protein, the voltage-gated L-type calcium channel, Cav1.3, which operates under depolarised conditions. Cav1.3 is activated by the depolarising effects of the sodium/glucose co-transporter, SGLT1, and by other depolarising agents, such as amino acids and oligopeptides that are present in the fed state. Unlike TRPV6-mediated transport, Cav1.3-mediated transport is not saturable and is not dependent on 1,25(OH)2D but is determined by the concentration of luminal calcium [4]. It is therefore not surprising that no human conditions relating to abnormalities of calcium metabolism have so far been described in relation to mutations of TRPV6. Calcium absorption is also influenced by a number of other factors. In particular, absorption can be reduced in the presence of large quantities of calcium-binding agents such as phytate or oxalate [5]. Bisphosphonates also bind to calcium in the gut and, if used orally for therapeutic purposes, should be taken as far away from meals as possible. Alternatively, a high calcium intake helps to

Magnesium TRPM6

TRPM7

Claudin 16 Claudin 19

Paracellular

+

Na+ Pro-EGF EGFR

?

?

Mg++

į DŽ

Dž

Na-K

EGF

ATPase Mg++

Fig. 2. Schematic representation of the mechanisms of magnesium transport in the gut and renal tubules. Similar mechanisms are present in both tissues, although the importance of each differs between them. Abbreviations are explained in Appendix 1.

which is present on the basolateral membrane of the renal cells, where it is processed from ProEGF [7]. Following cleavage of Pro-EGF, EGF interacts with its receptor, the EGFR (*131550), which, amongst its other actions, stimulates magnesium absorption via TRPM6 on the luminal surface. It has recently been shown that mutations in the EGF gene disrupt the basolateral sorting of Pro-EGF, resulting in under-stimulation of TRPM6 and impaired magnesium reabsorption [8]. The resulting condition is known as Normocalciuric Renal Hypomagnesaemia (#611718) (see Chapter 6). TRPM6 activity is determined by the membrane potential, which is maintained by the

Calcium Physiology Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 7–32 (DOI: 10.1159/000380990)

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TRPM7 is more widely distributed. Mutations in TRPM6 cause Hypomagnesaemia with Secondary Hypocalciuria (#602014) as a result of impaired magnesium absorption in the gut (see Chapter 6 and Case 19–15). Transcellular transport of magnesium is less well understood than that of calcium. Renal tubular reabsorption of magnesium mostly occurs along with calcium by passive reabsorption in the ascending loop of Henle, mainly via tight junctions (see below). Further along the renal tubule, active reabsorption takes place in the distal convoluted tubule, where TRPM6 is situated. TRPM6 is under the influence of Epidermal Growth Factor (EGF),

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urinary tract obstruction and the diuretic phase of acute renal failure. Chronic use of proton pump inhibitors may also cause hypomagnesaemia by inhibiting both active and passive gastrointestinal absorption of magnesium [10]; however, this phenomenon has not been described in children.

Paracellular Mechanisms of Cation Transport

Paracellular proteins are present within the tight junctions of epithelial membranes and act as barriers between cells. In some tissues, e.g. skin, these barriers are complete, whilst in others, e.g. the gastrointestinal tract and renal tubules, they are incomplete in that they prevent the transport of noxious agents, such as bacteria, whilst allowing passage of electrolytes etc., such as calcium and magnesium. Paracellular transport through these tight junctions is facilitated by a number of proteins, including, amongst others, the claudins. The most important of these, with reference to cations, are claudin 16 (*603959) (also known as paracellin 1) and claudin 19 (*610036). These two proteins coexist, mainly in the thick ascending loop of Henle, where they form heteromeric complexes. It was originally thought that these proteins acted in a specific cation transport mechanism, but it is now known that the transport occurs in response to a high sodium/chloride gradient [11]. It seems that the function of claudin 16 is to facilitate transport of sodium, whilst that of claudin 19 is to inhibit the transport of chloride, thus maintaining this high gradient. Claudin 16 is coded for by a gene on chromosome 3q28, and it mainly acts in renal tubules, where it also facilitates passive transport of magnesium and calcium. Mutations in this gene cause Hypomagnesaemia, Hypercalciuria and Nephrocalcinosis Syndrome (#248250). Nephrocalcinosis is also present because both magnesium and calcium are poorly reabsorbed (see Chapter 6).

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voltage-gated potassium channel Kv1.1 encoded by the KCNA1 gene, which is located on chromosome 12p13.32 (*176260). Kv1.1 co-localises with TRPM6 and consists of four subunits that normally form homotetramers. Mutations in the gene result in both homotetrameric and heterotetrameric channels, which have a dominant-negative effect on the channels and result in one of the forms of Episodic Ataxia and/or Myokymia (#160120). However, it has recently been shown that this may also be associated with Autosomal Dominant Hypomagnesaemia [9]. Transcellular transport of magnesium is probably affected by proteins similar to the calbindins involved in calcium transport, but these mechanisms are not well understood. At the basolateral membrane, magnesium is transported partly by a mechanism that involves Na+/K+ ATPase, which consists of three subunits, α, β and γ, the latter of which is coded for by the FXYD2 (*601814) gene. Mutations in this gene result in the defective magnesium reabsorption found in the Autosomal Dominant Renal Hypomagnesaemia associated with Hypocalciuria Syndrome (#154020) (see Chapter 6). The thiazide-sensitive sodium chloride cotransporter is also involved in magnesium transport, and mutations in the coding gene, SLC12A3 (*600968), cause Gitelman’s Syndrome (#263800), in which hypermagnesuria is a feature. Raised urinary magnesium excretion is also present in some cases of Bartter’s Syndrome, which is caused by a variety of mutations that affect chloride and sodium reabsorption in the loop of Henle. In the last part of the renal tubules, the collecting ducts, both calcium and magnesium are again reabsorbed passively via tight junction proteins (see below). Renal tubular transport of magnesium can also be increased by several non-genetic causes, including the use of diuretics, gentamicin, mercury-containing laxatives or cisplatin, as well as diabetic ketoacidosis, kidney transplantation,

Phosphate Metabolism

A full-grown adult contains approximately 700 g phosphate. As with calcium, the total body content (PO4) of phosphate is closely related to BWt and is expressed by the formula: PO4 = 0.00037*BWt1.2409 [1]

Approximately 80% of phosphate is contained in bone. Of the remainder, 45% (9% of the total) is present in skeletal muscle, 54.5% is present in the viscera, and only 0.5% is present in extracellular fluid. Most phosphate is present in inorganic form but still plays a crucial part in many intracellular processes. In plasma, phosphate circulates in the form of phospholipids, phosphate esters, and free inorganic phosphate (Pi). Plasma Pi concentrations are not as tightly controlled as those of calcium and reflect the fluxes of phosphate entering and leaving the extracellular pool. In contrast to calcium, phosphate concentrations in plasma vary considerably during life, being highest during phases of rapid growth. Thus, the phosphate concentrations in premature infants are normally above 2.0 mmol/l (6.4 mg/dl), falling to 1.3–2.0 mmol/l (4.2–6.4 mg/dl) during infancy and childhood and to 0.7–1.3 mmol/l (2.2–4.3 mg/dl) in young adults.

It is not known precisely how phosphate is sensed in multicellular organisms, but more is known about how it is sensed in unicellular eukaryotes and prokaryotes. In bacteria, cellular phosphate uptake occurs via a phosphate transporter (Pst). The phosphate then activates a two component signalling system, which consists of a sensory histidine kinase (PhoR) that is coupled to a transcription factor (PhoB), which has downstream effects. This system is inhibited by phosphate and is stimulated by a lack of phosphate [13]. Sodium-phosphate co-transporters may also be involved in phosphate transport; however, whether such a system or a different system operates in higher eukaryotes is not yet known. Although phosphate concentrations do vary at different ages, there is tight control of phosphate that must involve a sophisticated sensing system. Phosphate transport across membranes is controlled by a series of sodium-dependent active transport mechanisms (Na/Pi co-transporters), of which three classes are known to exist. Type 1 is present at renal tubular brush borders but is not thought to have a major role in renal tubular reabsorption of phosphate. Type 2, which has three subtypes, 2a, 2b and 2c, is probably the most important for regulating phosphate absorption and reabsorption. Type 3 is present in many tissues but is thought to have more of a ‘gatekeeping’ role. Phosphate is readily absorbed throughout the small bowel by both passive and active mechanisms. Approximately 70% of phosphate in the body is absorbed via the type 2b Na/Pi co-transporter; the remainder is absorbed by passive absorption. This active transport is stimulated directly by 1,25(OH)2D and therefore indirectly by hypocalcaemia and PTH [14]. Since hypophosphataemia is a powerful stimulant of 25-hydroxyvitamin D-1-alphahydroxylase (1α-hydroxylase), phosphate deficiency itself stimulates increased absorption. However, the total amount of phosphate that is absorbed is dependent on the dietary phosphate

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The other component of the tight junction protein, claudin 19 (*610036), is coded for by a gene on 1p34.2. However, unlike claudin 16, claudin 19 is also present in other extra-renal tissues, particularly the cornea and retina. As a consequence, mutations in this gene not only cause hypomagnesaemia with nephrocalcinosis and progression to renal failure, which is often more rapid than that associated with claudin 16 mutations, but also eye abnormalities in more than 80% of patients. This is known as Renal Hypomagnesaemia with Ocular Involvement (#248190) [12] (see Chapter 6).

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age without affecting its intrinsic activity. As a consequence, circulating FGF23 levels remain high, resulting in the excessive renal phosphate loss in ADHR (#193100) (see Chapter 9 for further details). In contrast, mutations in the FGF23 molecule itself that render it inactive cause Hyperphosphataemic Familial Tumoral Calcinosis Type 2 (HFTC2) (#211900). Protection against inactivation results from O-glycosylation at position 178, which occurs under the influence of UDP-N-acetyl-alpha-Dgalactosamine:polypeptide N-acetylgalactosaminyl-transferase 3 (GALNT3) (*601756). GALNT3 is a 633-amino-acid protein that is coded for by a gene on chromosome 2q24-q31 [18], which has ten exons. GALNT3 itself has a single transmembrane spanning region and catalyses the O-glycosylation of serine and threonine residues on the native protein. The crucial role of GALNT3 in phosphate metabolism is demonstrated by the fact that inactivating mutations in this gene, resulting in FGF23 being cleaved more readily than normal and thus being unable to inhibit the reabsorption of phosphate by NaPi in renal tubules, results in either HFTC Type 1 (HFTC1) (#211900) [19] or Hyperostosis-Hyperphosphataemia Syndrome, which are allelic. The resulting hyperphosphataemia causes soft tissue calcinosis, similar to that of HFTC2 (see Chapter 9). The principal target organ of FGF23 is the renal tubule. When active, it acts on a receptor on the surface of the tubules. This receptor is part of the Fibroblast Growth Factor receptor family, FGFR1(IIIc) (*136350), and is coded for by a gene on chromosome 8p11.2-p11.1. The FGF receptors, of which four classes have been described, are involved in a wide variety of functions. Activating mutations cause Osteoglophonic Dysplasia (#166250), which may be associated with hypophosphataemia due to phosphate wasting [20] and is probably caused by secretion of FGF23 from the bone lesions that are part of the syndrome. Meanwhile, inactivating mutations in FGFR1 give rise to several conditions including

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load and may be inhibited by phosphate-binding agents such as calcium acetate (Phosex®), calcium carbonate (Tetralac®) or sevelamer (Renagel®). These are of value in hyperphosphataemic states such as chronic renal failure, when phosphate absorption needs to be limited. The metabolism of phosphate has, until recently, been relatively poorly understood. However, in 1999, a new member of the Fibroblast Growth Factor family, FGF23, was discovered [15]. This protein was subsequently shown to be mutated in cases of Autosomal Dominant Hypophosphataemic Rickets (ADHR) (#193100) [16], and it is now known to play a key role in phosphate metabolism. FGF23 is derived from bone cells, particularly osteocytes, circulates in plasma and is subject to a variety of feedback mechanisms. As a result, it is now considered a classic hormone. The synthesis and secretion of FGF23 are modified by several factors, especially phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), dentin matrix protein 1 (DMP1) and bone morphogenic protein 1 (BMP1) (see below), and its inactivation results from cleavage of the molecule between the arginine and serine residues at positions 179/180, which is protected against by O-glycosylation of the threonine residue at position 178. When active, the principal target organ of FGF23 is the renal tubule, where it stimulates renal phosphate excretion and inhibits 1α-hydroxylase activity to reduce levels of 1,25(OH)2D. Hypophosphataemia also inhibits FGF23 secretion. These actions are summarised in figure 3. FGF23 (*605380), encoded by a gene on chromosome 12p13.3, is a 251-amino-acid protein that includes a 24-amino-acid signal sequence. FGF23 has a crucial cleavage site between residues arginine179 and serine180, where it is cleaved by a subtilisin/furin-like enzyme, rendering it inactive [17]. A second arginine residue is present at position 176, and mutations in either of these residues renders FGF23 resistant to cleav-

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Fig. 3. Schematic representation of the control of phosphate metabolism. Fibroblast Growth Factor 23 sits at the centre. Its secretion is influenced by several other factors, and it has to undergo modification before becoming active. The receptor of Fibroblast Growth Factor 23 on renal tubules enables it to promote phosphate excretion. Solid lines represent stimulatory effects, and interrupted lines represent inhibitory actions. The modes of inheritance are summarised below, and the different conditions caused by mutations in the different genes involved in the pathway are shown below. White number in black circle: Autosomal recessive inhibitory mutations. Black number in white circle: Autosomal dominant activating mutations. Black number in grey circle: Somatic mutations. White number in grey circle: X-linked dominant mutations. (1) Osteogenesis Imperfecta; (2) Autosomal Recessive Form of Hypophosphataemic Rickets 1; (3) Jeune Syndrome; (4) X-linked Dominant Hypophosphataemic Rickets; (5) Autosomal Dominant Hypophosphataemic Rickets; (6) Autosomal Recessive Form of Hypophosphataemic Rickets 2; (7) TIO; (8) McCune-Albright; (9) Hyperphosphataemic Familial Tumoral Calcinosis Type 2; (10) HHRH; 11) Hyperphosphataemic Familial Tumoral Calcinosis Type 1; (12) HHS; (13) Hyperphosphataemic Familial Tumoral Calcinosis Type 3; (14) Osteoglophonic Dysplasia; (15) KS/Hartsfield Syndrome. Abbreviations are explained in the Appendix.

the degenerative processes, such as atherosclerosis, osteoporosis and skin ageing, seen in CKD [23]. KL is not capable of acting as an FGF23 receptor on its own, but requires FGFR1. Similarly, FGFR1 is not active as an FGF23 receptor if KL is inhibited or mutated [22]. Rare patients who have inactivating mutations of KL, which cause HFTC Type 3 (HFTC3) (#211900), have been described. In contrast to HFTC1 and HFTC2, the concentrations of active circulating FGF23 in HFTC3 patients are high. This suppresses vita-

Calcium Physiology Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 7–32 (DOI: 10.1159/000380990)

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one of the twenty-three forms of Kallmann Syndrome and a number of skeletal abnormalities [21]. FGFR1 is therefore not specific for FGF23. However, in 2006, another factor, α-Klotho (KL) (+604824), was found to act as a cofactor that confers specificity to FGFR1(IIIc) for FGF23 [22]. Klotho (named after the Greek Fate who spins the thread of life) is coded for by a gene on chromosome 13q12. Patients with Chronic Kidney Disease (CKD) have low renal expression of KL, and it has been suggested that this may accelerate

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five proteins, DMP1 (*241520), bone sialoprotein (*166490), osteopontin (*166490), dentin sialophosphoprotein (DSPP) (*125485) and matrix extracellular phosphoglycoprotein (MEPE) (*605912), that are all coded for by adjacent genes on chromosome 4p22. The SIBLING proteins, particularly DMP1 and DSPP, are secreted into the extracellular matrix and contribute to mineralisation through their acidic calcium binding domains. DMP1 is cleaved into two fragments, 37 kD and 57 kD in size, by BMP1 (see below), and these fragments have two particular features that are of importance to phosphate metabolism in common. The first has an acidic serine-aspartate-rich MEPE-associated (ASARM) peptide motif that is 23 residues in length, is the only known ligand for PHEX, and is an inhibitor of mineralisation. Furthermore, free ASARM can bind competitively with PHEX. The second fragment has an arginine-glycine-aspartate motif that binds to integrins on the cell surface. These interactions initiate downstream effects via the mitogen-activated protein kinase pathway [24] and inhibit FGF23 activity by facilitating its cleavage. Homozygous mutations in the DMP1 gene result in failure of this cleavage and leads to Autosomal Recessive Form of Hypophosphataemic Rickets 1 (#241520) that is clinically very similar to X-linked Dominant Hypophosphataemic Rickets and ADHR. The SIBLING proteins have a large number of phosphorylation sites that are phosphorylated by another enzyme, FAM20C (*611061), which regulates DMP1 activity. Homozygous mutations in the gene for this protein usually result in a rapid fatal bone sclerotic condition, Raine Syndrome (#259775), but there are occasional reports of survival into childhood [25]. DMP1 may act as a mechanostat that responds to changes in stresses within bone that are transmitted via the fluid-filled canaliculi within bone, where osteocytes lie. Other SIBLING proteins include MEPE (*605912), which also contains the ASARM and

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min D 1α-hydroxylation and causes relative hypocalcaemia. As a consequence, PTH secretion is stimulated and may lead to hyperparathyroidism, which may require parathyroidectomy (see Chapter 9). Once the FGFR1-KL complex has been activated by FGF23, it increases renal tubular phosphate excretion by means of the NaPi-IIc/ SLC34A3 (*609826) exchanger at the luminal surface of the cells. Mutations in the NaPi-IIc exchanger result in Hereditary Hypophosphataemic Rickets with Hypercalciuria (HHRH) (#241530). However, unlike those conditions, which are associated with high FGF23 levels, low concentrations of active FGF23 are present; 1α-hydroxylase activity is therefore not inhibited. The resulting raised levels of 1,25(OH)2D not only stimulate calcium and phosphate absorption in the gut but also increase calcium excretion in  renal tubules, resulting in the presence of hypercalciuria and renal stones. Unlike other forms of hypophosphataemic rickets, this condition should NOT be treated with active vitamin D metabolites, as it worsens the hypercalciuria. The secretion and initial processing of FGF23 is under the influence of several other factors that set up a cascade of events that eventually control FGF23 activity. The most important of these is PHEX (*300550). Several studies in the hyp-mouse, an animal model of X-linked Dominant Hypophosphataemic Rickets (#307800), have demonstrated that PHEX is somehow involved in the regulation of FGF23, despite the fact that it is not present in renal tubules. The precise mechanisms by which this occurs are not fully understood but may involve modification of the activity of the subtilisin/ furin enzyme activity that cleaves FGF23. PHEX is itself activated by DMP1 (see below). Whatever the precise mechanism, mutations in PHEX result in failure of FGF23 cleavage, which causes hyperphosphaturia and hypophosphataemia. The Small Integrin-Binding Ligand, N-linked Glycoproteins (SIBLING) proteins are a group of

tonucleotide Pyrophosphatase/Phosphodiesterase 1 deficiency leads to high FGF23 concentrations is not fully understood. Phosphate metabolism is also altered in CKD in childhood and may occasionally lead to arterial calcification and calciphylaxis. The cause of this altered phosphate metabolism is multifactorial but may respond to treatment with sodium thiosulphate [27]. Hypophosphataemia and rickets are also seen in several primary renal tubular abnormalities, such as Fanconi Syndrome (whatever the cause), in which a generalised proximal renal tubular defect, which results in bicarbonaturia, glycosuria and amino aciduria as well as a phosphate leak, is present. The most common inherited cause of Fanconi Syndrome is Cystinosis (#219800), and rickets may be the presenting feature of this condition, although it may disappear once renal failure supervenes and phosphate is retained. Hyperparathyroidism also causes a mild form of Fanconi Syndrome, and patients with parathyroid tumours may have a mild metabolic acidosis and aminoaciduria in addition to hypercalcaemia.

The Calcium Cascade

The concentration of calcium in plasma is normally maintained within very narrow limits. The initial stage of this process is binding of calcium to a specific CaSR. This then initiates a cascade of events that terminates in the action of PTH on its target organs (fig. 4).

The Calcium-Sensing Receptor Complex

Calcium is sensed in plasma via a specific CaSR (+601199), which consists of three components, CaSR itself, a G-protein-coupled second messenger (GNA11) (*139313) and an adaptor protein (AP2) (*602242) that internalises the G-proteincoupled receptor.

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arginine-glycine-aspartate motifs. In some tumours, MEPE is produced in excess, and the ASARM motif may become free in the circulation to bind competitively with PHEX, thus inhibiting its cleavage and leading to Tumour-Induced Osteomalacia (TIO). These tumours can be very small and extremely difficult to identify, but if they can be found, tumour excision usually results in a cure. Some individuals with McCuneAlbright Polyostotic Fibrous Dysplasia (#174800), which is caused by somatic mutations in the α-subunit of the stimulatory G-protein (Gsα), have an associated excess phosphate excretion that is secondary to increased FGF23 by an, as yet, ill-understood mechanism. BMP1 is one of a group of metalloproteinases that contribute to the formation of extracellular matrix by cleaving the propeptides of several collagens to yield mature fibres. As a consequence, homozygous mutations in BMP1 have been shown to cause a recently described form of Osteogenesis Imperfecta (#614856) by downregulating matrix formation. However, BMP1 is also thought to contribute to phosphate regulation by cleaving DMP1 (and DSPP) precursors into active molecules [26] at a specific, highly conserved cleavage site. Another enzyme involved in phosphate metabolism is ectonucleotide pyrophosphatase/ phosphodiesterase 1 (ENPP1) (*173335), which catalyses the conversion of Pi to pyrophosphate and, as such, has an effect opposite that of tissue non-specific alkaline phosphatase (TNSALP) (*171760). Pyrophosphate is a natural inhibitor of mineralisation, and homozygous mutations of the gene cause Generalised Arterial Calcification of Infancy (#208000) in new-borns. Generalised Arterial Calcification of Infancy has an 85% mortality rate, but many of those who survive go on to develop a form of Autosomal Recessive Hypophosphataemic Rickets 2 (#613312) that is associated with raised FGF23 and is sometimes associated with calcification of the lateral spinal ligaments. The precise mechanism by which Ec-

Ca++ CaSR Parathyroid glands PTH PTH1R

The Calcium-Sensing Receptor

The CaSR (+601199) is a large molecule consisting of 1,078 amino acids that is coded for by a gene on chromosome 3q13-q21. It has a large extracellular calcium-binding domain consisting of approximately the first 610 residues, followed by a seven-transmembrane domain consisting of the next 250 residues and a further 210 residues making up the intracellular component. The receptor is present in many tissues, especially the parathyroid (PT) glands and renal tubules, but also in bone, cartilage and other tissues [28] such as the temporal, frontal and parietal lobes of the brain and the cerebellum and hippocampus [29]. When calcium binds to the extracellular domain of CaSR, it alters PTH secretion via both phospholipase Cb and G-protein second messengers. As a consequence, PTH secretion changes in a sigmoidal fashion in response to acute changes in plasma calcium levels (fig. 5), and there is a continuous tonic secretion of PTH, which maintains ionised calcium in the plasma at whatever level is ‘set’ by the CaSR [30]. Magnesium also binds to the CaSR and influences PTH secretion in a similar, but less potent, manner to that of calcium. However, severe magnesium deficiency inhibits PTH

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Gsį, DŽDž Target organs – Kidney Bone (Gut)

secretion (see under GNA11, below, for further details [31]). Mutations within the CaSR gene result in either inactivation or activation of the receptor, which result in hyper- and hypocalcaemia, respectively. Inactivating mutations cause insensitivity to calcium, which shifts the curve of PTH secretion in response to plasma calcium to the right (fig. 5). As a consequence, PTH secretion is switched off at a higher concentration than normal, and hypercalcaemia results [28]. The receptors are also present in the renal tubule, and renal calcium excretion is thereby reduced. If these mutations are heterozygous, the resulting condition is known as Familial Benign Hypercalcaemia (FBH) or Familial Hypocalciuric Hypercalcaemia (FHH) (#145980) (see Chapter 7). If, however, the mutations are homozygous, a more serious condition, Neonatal Severe Hyperparathyroidism (#329200) may occur. CaSR inactivation can also occasionally occur when a heterozygous mutation is present and the mother is unaffected (see Chapter 7 for details), in which case, the condition is referred to as Neonatal Hyperparathyroidism, which is often self-limiting. Heterozygous mutations are also described as causing both Familial Isolated Hyperparathyroidism

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Fig. 4. The calcium cascade. Plasma calcium concentrations are controlled by a series of events that begin with the effect of calcium on the calcium-sensing receptor and end with the response of the target organs.

80 70

Inactivating mutations

60 50 40

Intact PTH

30 20

Activating mutations

10 0

Ca ++ 0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

1.45

1.50

Fig. 5. Schematic representation of the relationship between plasma ionised calcium and parathyroid hormone secretion, as determined by the calcium-sensing receptor. Inactivating mutations generally shift the curve to the right, whilst activating mutations do so to the left.

Many of the mutations found in FBH are clustered around the aspartate- and glutamaterich regions of the extracellular domain of the molecule, and it has been postulated that this region contains low-affinity binding sites for calcium. Many FBH homologs have been found to have unique mutations. Furthermore, mutations have also been detected within the transmembrane domain but only rarely within the intracellular domain. Mutations within this latter domain may have a greater effect on the CaSR in the parathyroid glands than in the renal tubules, and patients in whom inactivating mutations are associated with hypercalciuria and PT gland hyperplasia, necessitating parathyroidectomy, have been described [36]. Similarly, most activating mutations that cause Autosomal Dominant Hypocalcaemia Type 1 (ADH1) are present within the extracellular calcium-binding domain. So far, nearly three hundred mutations, a

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[32] and Tropical Chronic (Calcific) Pancreatitis (#608189) [33]; however, in the latter, some patients also have a mutation in another gene, SPINK1 (*167790), which is a pancreatic trypsin inhibitor. In contrast, activating mutations of the receptor shift the PTH secretion curve to the left (fig.  5),  causing chronic hypocalcaemia and hypercalciuria, a condition known as ADHR Type 1 (ADHR1) (#193100) (see Case 19–2). One particular mutation causes constitutive activation of the receptor independent of the calcium concentration so that, rather than shifting the curve to the left, PTH secretion remains permanently switched off [34] (see Chapter 6). A rare form of idiopathic epilepsy (#612899) with activating mutations in the CaSR has also been described [29]. In addition, there is one description of an activating mutation associated with normocalcaemic hypercalciuria and renal stones [35].

The G-Protein Second Messenger

A second locus, located on chromosome 19p13, was identified by family linkage studies to cause FHH in patients who were found not to have mutations in the CaSR gene. It is now known that this locus codes for the alpha subunit of a member of the G-protein family that is coupled to the CaSR. This G-protein family has downstream effects that are mediated via phosphatidyl inositol and cAMP. Once calcium binds to the receptor, Gq11 activity is inhibited, and vice versa. Moreover, heterozygous mutations in GNA11 result in stimulatory activity, which causes ADH Type 2 (ADH2) (#615361), whilst inactivating mutations cause FHH Type 2 (FHH2) (#145981). Both of these conditions are clinically similar to their type-1 counterparts. Both calcium and magnesium have an inhibitory effect when in high concentrations, although the effect of magnesium is two- to threefold less than that of calcium. Low concentrations of calcium stimulate PTH secretion, while moderately low magnesium concentrations (0.4–0.6 mmol/l) stimulate PTH secretion to a certain extent. This secretion is sometimes accompanied by hypocalcaemia because the moderately elevated PTH induces resistance to itself [31], and it can be shown that cAMP responses to PTH infusion under these circumstances are impaired. At very low magnesium concentrations, PTH secretion is inhibited altogether, and it seems that this paradoxical inhibition is caused by constitutive activation of the Gq11 subunit by enhancing inhibitory signalling whilst having little effect on the stimulatory signalling [37, 38].

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The Adaptor Protein

Family linkage studies of FHH also identified a third locus on chromosome 19q that is now known to be the location of a gene, AP2S1 (*602242), that codes for an adaptor protein (AP2σ1) whose function is to internalise G-protein-coupled receptors such as CaSR. AP2σ1 contains a highly conserved arginine residue at position 15 that is crucial to its function of binding to the CaSR at a dileucine motif at its C terminus. Mutations in the AP2S1 gene have been shown to cause a third form of autosomal dominant FHH (FHH3 – Oklahoma variant). Interestingly, no instances of an activating mutation of AP2S1 have been identified as a cause of putative ADH Type 3 (ADH3) [39].

The Parathyroid Glands

The PT glands, usually four in number but sometimes as many as seven, are derived embryologically from the third (lower glands) and fourth (upper glands) branchial arches. Several transcription factors are involved in their development [40]. Some, such as Hoxa3 (thyroid and thymus, chromosome 7p15-p14.2) (*142954), GATA3 (hearing sensation and kidney, chromosome 10p13–14) (*131320), TBX1 (thymus, cardiac outflow tract and the face, chromosome 22q11) (*602054) and UDF1L, are involved in the development of other structures. The latter two genes are located on the long arm of chromosome 22. Mutations within or deletion of the genes responsible for these factors result in the congenital hypoparathyroidism that is associated with other conditions such as Hypoparathyroidism, Deafness, Renal Anomalies Syndrome (#146255) (Case 19–10) and the 22q Deletion Complex, a part of DiGeorge Syndrome (#188400) (see Cases 19–3; 19–4). The homologue of Drosophila Glial Cells Missing 2 (GCM2) (*603716) is a highly conserved gene that is necessary for PT gland development and has no other known function in man. Mutations in this

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few of which are polymorphisms, have been described as causing one or another of the conditions described above, and an online database has been established to keep track of them (http://www.casrdb.mcgill.ca).

gene cause Familial Isolated Hypoparathyroidism (#146200), which, in most cases, is autosomal recessive but can sometimes be autosomal dominant [41] (see Cases 19–7; 19–8; 19–9). It is also thought that the SRY-related HMG-box gene 3 (*313430), located on the X-chromosome, is involved in PT gland development and that mutations in this gene may be responsible for X-linked recessive Familial Isolated Hypoparathyroidism (%307700). Apart from these autosomal and X-linked syndromes, several mitochondrial genes are involved in PT gland development, and mutations in these genes give rise to a variety of syndromes in which hypoparathyroidism is a feature. Because the genes are mitochondrial, these syndromes are maternally inherited. For full details of these conditions, see Chapter 6 and Case 19–11. In addition to these genetic causes, destruction of the PT glands may occur as a result of surgery (e.g. following thyroidectomy), infiltration (e.g. with iron in β-thalassaemia) or antibody use. These causes may either be isolated or associated with autoantibodies to other organs, as in Polyendocrinopathy Type 1 Syndrome, also known as Autoimmune Polyendocrinopathy Syndrome (#240300) (see Chapter 6 for further details).

thesised. Mutations in the PTH gene involving the pre-pro- sequence, which result in both autosomal dominant and autosomal recessive hypoparathyroidism, have been described (#146200) (see Chapter 6). Only the first 34 N-terminal amino acids of PTH are required for full activity, and the function of the remainder of the molecule is not understood, although it has been suggested that there may be differential binding sites for the N- and Cterminal fragments in bone cells [45] and renal tubules [46]. The half-life of PTH in the circulation is 1–2 minutes [44]. The molecule is cleaved at various sites, which results in a number of fragments that can be identified in the circulation. The best modern assays of PTH measure ‘intact’ PTH, are able to measure physiological concentrations of PTH, correlate well with bioactivity and ignore the inactive fragments. This is particularly important in conditions such as chronic renal failure, where inactive fragments are cleared less rapidly than normal. The normal levels of PTH in the circulation are about 1–6 pmol/l (10–60 pg/ml) but vary depending on the assay used.

The Parathyroid Hormone Receptors

PTH acts via two receptors. The first and principal receptor is PTH1R (also called PTH/PTHrP) (*168468), which has equal affinity for both PTH and PTHrP. PTH1R consists of 593 amino acids and is encoded by a gene on the long arm of chromosome 3 [47]. It has an extracellular binding domain of 190 residues, a seven-transmembrane domain, and a cytosolic component of 134 residues. Both inactivating and activating mutations of the PTH1R have been described and result in the very rare conditions of Blomstrand Lethal Chondrodysplasia (#215045) and Jansen Disease (#156400), respectively (See Case 19–25). A second PTH2 receptor (PTH2R) is present in the central nervous system; however, PTHrP is not a ligand for it.

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Parathyroid Hormone PTH (*168450) is a single-chain polypeptide hormone containing 84 amino acids that is encoded by a gene on chromosome 11. It is synthesised by the PT glands from prepro-PTH, which has an additional 31 N-terminal amino acids. Synthesis of PTH occurs in the ribosomes, where the initial 25-amino-acid ‘pre’ sequence acts as a signal peptide to aid transport through the rough endoplasmic reticulum [42, 43]. The ‘pre’ sequence is cleaved, and pro-PTH then travels to the Golgi apparatus, where the 6-amino-acid ‘pro’ sequence is cleaved to yield the mature hormone, which is stored in secretory vesicles that fuse with the plasma membrane prior to secretion of the hormone [44]. Very little PTH is stored within the glands, and most of the secreted hormone is newly syn-

Intracellular signalling principally occurs by coupling of the cytosolic component of the PTH1R to the G-protein second messengers Gs and Gq [48], which are heterotrimeric, consisting of α, β, and γ subunits. In the resting state, Gs and Gq are associated, and the Gsα subunit is bound to GDP (fig. 6a). Binding of the ligand to the receptor results in GDP being exchanged for GTP and dissociation of the Gsα subunit from the β,γ complex. Gsα is then free to stimulate adenylate cyclase, which results in an increase in intracellular cAMP and activation of the various actions of PTH via specific protein kinases (fig. 6b). The intrinsic GTPase activity associated with the Gsα subunit hydrolyses GTP to GDP, which causes reassociation of the components of the G-protein, and the cell reverts to its resting state. At the same time, phosphodiesterases, particularly PDE4D, inactivate cAMP to AMP, which switches off protein kinase activity (fig.  6c). This mechanism is common to several hormones, including thyroidstimulating hormone, gonadotrophins, and growth hormone-releasing hormone [48]. The Gsα subunit is encoded by a gene, GNAS1, (+139320), which is located on chromosome 20q13.3. This complex gene contains 13 exons that code for the Gsα subunit itself plus several other exons, known as A/B, XL, NESPAS (*610540) (which is an antisense transcript) and NESP55, which is only expressed in renal tubules. Alternative promoter use and splicing results in several different mRNA transcripts. In most tissues, these transcripts show biallelic expression, but those arising from the A/B, XL and NESPAS exons are paternally derived, whilst those arising from the NESP55 exon are maternally expressed. This differential expression results from methylation of these uniparental alleles that switches the activity of those alleles on or off in an epigenetic manner (fig. 7). In addition, another gene, Syntaxin (STX16) (*603666), which acts upstream of the GNAS complex, appears to influence the methylation of the A/B exon.

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Mutations within the biallelic coding region (exons 2–13) of the gene result in resistance to the action of PTH, which clinically causes Pseudohypoparathyroidism Type Ia (#103580) (see Case 19–12) if they are associated with the maternally derived transcripts but cause Pseudopseudohypoparathyroidism (#612463) and/or Progressive Osseous Heteroplasia or Osteoma Cutis (#166350) if derived from paternal sources [49] (see Case 19–55). These patients frequently have resistance to other hormones whose actions are mediated via the Gsα second messenger mechanism and many display features of Albright’s Hereditary Osteodystrophy (AHO). Activating somatic mutations in the GNAS complex are responsible for McCune-Albright Syndrome (#174800). Alterations in the methylation patterns of the monoallelic exons, particularly A/B, cause Pseudohypoparathyroidism Type Ib (#603233) when they are on maternally derived alleles (see Case 19–13). Under these circumstances, there are no mutations found in the coding regions of the GNAS gene, and the patients do not usually have evidence of AHO. Mutations in STX16 have also been associated with some forms of Pseudohypoparathyroidism Type Ib, probably by influencing the methylation of the A/B exon (see Chapter 6). The cAMP generated by adenylate cyclase in response to hormone binding binds to protein kinase A (PKA), which is a tetramer formed of two regulatory (R) and two catalytic (C) subunits. Four different R subunits, R1α, R1β, R2α and R2β, are described, of which R1α is the most widely expressed. Three different C subunits, Cα, Cβ and Cγ, are also described, giving rise to a number of PKA variants. Under the influence of cAMP, the R subunits dissociate from the C subunits, resulting in their downstream effects. R1α is coded for by a gene, PRKAR1A (*188830), on chromosome 17q. Inactivating mutations in this gene may give rise to increased catalytic activity, which causes the Carney complex, or decreased activity, resulting in the Acrodysostosis Type 1 (ACRDYS1

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Intracellular Signalling

PTH1R Adenylate cyclase

į GTPase

DŽ

ATP

Ǔ

GDP GTP PKAC1

PKAC2

PKAR1

PKAR2

cAMP Phosphodiesterase

a

1 3

2

PTH

4

PTH1R

5

$GHQ\ODWHF\FODVH

į

GTPase

DŽ

ATP

Ǔ

GTP GDP 8

PKAR1

PKAR2

PKAC1

PKAC2

7 cAMP 6 3KRVSKRGLHVWHUDVH

b

Phosphaturia

AMP

įK\GUR[\ODVH

PTH1R į GTPase

$GHQ\ODWHF\FODVH DŽ

ATP

Ǔ

GTP GDP

PKAC1

PKAC2

PKAR1

PKAR2

cAMP 3KRVSKRGLHVWHUDVH

c

Phosphaturia

įK\GUR[\ODVH

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AMP

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Fig. 6. a Resting state. The α and βγ subunits are associated, and the physiological processes are quiescent. b Stimulation of the receptor by parathyroid hormone causes dissociation of the α subunit from the β,γ subunit, activating adenylate cyclase to convert ATP to cAMP, which in turn activates protein kinase to initiate its downstream effectors. c Phosphodiesterase 4D inactivates cAMP to AMP, and GTPase converts GTP to GDP; the cell then returns to its inactive state. (1) Autosomal Dominant Hyperparathyroidism; (2) Autosomal Recessive Hyperparathyroidism; (3) Jansen Metaphyseal Chondrodysplasia; (4) Blomstrand; (5) PsHP1a; (6) Acrodysostosis Type 2; (7) Carney Complex; (8) Acrodysostosis Type 1. (Black numbered circles indicate AR and white numbered circles indicate AD conditions).

STX16 M P

Nesp55 – +

Nespas + –

XL + –

A/B 1

2

3 3N 4 5 6 7

13

Allele- specific methylation bi-allelic (most tissues)

exons 2–13 exon 1

*Vį exons 2–13

A/B Paternal

exons 2–13 XL exons 2–13 Nespas

Maternal

exons 2–13 Nesp55

Fig. 7. Diagrammatic representation of the GNAS gene showing the different products that result from alternative splicing. Native Gsα is expressed biallelically. The A/B, XL and Nespas transcripts are principally expressed from the paternal allele, whilst the Nesp55 transcript is mainly expressed from the maternal allele. Since the latter is present in renal tubules and the former are only present in other tissues, mutations in exons 2–13 result in AHO but are associated with pseudohypoparathyroidism if they are derived from the maternal allele. If the paternal allele is the origin, pseudopseudohypoparathyroidism is the result. Alterations to the methylation pattern of the various alternative splicing products lacking mutations in exons 2–13 result in pseudohypoparathyroidism Type 1b if they are of maternal origin (adapted and reprinted from Bastepe et al. [67] with permission from Elsevier).

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YS Type 2 (ACRDYS2 (ADOP4)) (#614613); however, these mutations are not usually associated with hormone resistance [50].

The Target Organs

The principal target organs of PTH are bone and kidney. In bone, PTH has two main effects. Under physiological conditions, it promotes bone formation via receptors on osteoblasts, while under cir-

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(ADOHR)) (#101800) that is often associated with hormone resistance, similar to that of Pseudohypoparathyroidism, although the skeletal phenotype is more severe (see Case 19–14). The amount of cAMP that is available to bind to PKA is regulated by PDE4D (*600129), which converts cAMP to inactive AMP and is coded for by a gene on chromosome 5q. Inactivating mutations in this gene may result in increased enzyme activity, which reduces the availability of cAMP to dissociate the PKA complex and results in ACRD-

[51]. These studies showed that neonatal cord blood contained high PTH-like activity; however, N-terminal immunoreactivity was absent. The bioactivity was related to the positive gradient of calcium across the placenta, and the authors suggested that it was this that maintained the gradient. It had also been recognised for some time that some patients with malignancy developed hypercalcaemia with undetectable levels of PTH. Subsequently, a protein was purified from lung cancer cells that had similar biological properties as PTH, but was clearly different from PTH itself [52]. This protein was subsequently identified as parathyroid hormone-related peptide (PTHrP) (+168470). PTHrP is a 141-amino-acid polypeptide that is coded for by a gene on chromosome 12p12.1p11.2. It has some homology with PTH at its Nterminal end, but this homology diverges from PTH after residue 13. PTHrP cannot normally be measured in the circulation and has no significant classical hormone action in postnatal life but does have an important paracrine role in chondrocyte proliferation and maturation. PTHrP has equal activity as PTH on the PTH1R, and some of the changes seen in Jansen’s Metaphyseal Chondrodysplasia (#156400) (see Case 19– 25) are thought to be related to the overactivity of these receptors. PTHrP is not a ligand for the PTH2R, which is mainly present in brain. However, PTHrP is secreted by the lactating breast, and women with hypoparathyroidism who are breastfeeding may need to reduce their dose of vitamin D analogues. The principal pathological importance of PTHrP in postnatal life is as a cause of hypercalcaemia of malignancy (see Chapter 6).

Parathyroid Hormone-Related Peptide

Calcitonin

The presence of a PTH-like substance with similar biological activity but different immunological properties was originally suggested in 1985

Calcitonin (*114130) is a 31-amino-acid protein that is synthesised by the C cells of the thyroid gland and is encoded by a gene on chromosome

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cumstances of hypocalcaemia, PTH stimulates bone resorption in order to retrieve calcium from the large reservoir within bone so that normocalcaemia can be restored. There are very few receptors for PTH in osteoclasts, and bone resorption occurs as a result of changes within the relationship between osteoblasts and osteoclasts. Both RANKL and osteoprotegerin are produced by osteoblasts. RANKL is coded for by a gene on chromosome 13q (TNFSF11 – *602642) and stimulates osteoclast differentiation, whilst osteoprotegerin, which is coded for on a gene on chromosome 8q (TNFRSF11B – *602634), acts as a dummy ligand for RANKL and inhibits its action. PTH alters the balance between the two in such a way as temporarily to change the balance in favour of bone resorption (see Chapter 3 for further details). In the absence of PTH for long periods, such as in unrecognised hypoparathyroidism, bone becomes undermineralised (see Case 19–4). Furthermore, homozygous mutations in TNFSF11 cause a benign form of osteoclast-poor Osteopetrosis (#259710), whilst homozygous mutations in TNFRSF11B result in Juvenile Paget Disease (#239000). In the nephron, PTH has three main actions. In the convoluted and straight parts of the proximal tubule, it stimulates the conversion of 25-hydroxyvitamin D (25OHD) to 1,25(OH)2D. Meanwhile, in the distal tubules, it promotes the reabsorption of both calcium and magnesium. It also promotes the excretion of phosphate, which allows the excess phosphate that is resorbed from bone by PTH to be excreted. There is also an effect of PTH on bicarbonate and amino acid reabsorption in the proximal tubule that results in the mild form of Fanconi Syndrome in hyperparathyroidism. This resolves when the hyperparathyroidism is reversed.

11p15.2-p15.1, which, by alternative splicing, also results in another protein, calcitonin gene-related peptide (CGRP). CT is mainly active in the thyroid gland, whilst CGRP plays more of a role in the hypothalamus. CT is secreted in response to hypercalcaemia and acts via a specific receptor that is coded for by a gene on chromosome 7q21.3 (*114131). The principal action of CT is to lower plasma calcium in a manner opposite to that of PTH. CT may also have a role in promoting skeletal mineralisation in the foetus but has a small physiological role in postnatal life. CT is sometimes used therapeutically to reduce plasma calcium in symptomatic hypercalcaemia, although bisphosphonates are now used more frequently for this purpose, but its principal value is as a marker of malignancy in Familial Medullary Thyroid Carcinoma (#155240).

or infantile forms of hypophosphatasia (#241500) (see Case 19–50), whilst heterozygous HN/HC or HN/HI cause the adult form (#146300). The intermediate childhood form (#241510) results from the HC/HC or HC/HI combination [54] (see Case 19–51). For a clinical description of these conditions, see Chapter 14. bTNAP is secreted by osteoblasts and promotes bone mineralisation. Circulating TNSAP is largely derived from liver and bone, and its levels in plasma during childhood reflect the growth rate [55] and are raised in the presence of rickets (see Chapter 8), in Juvenile Paget’s Disease (#239000) and in fibrous dysplasia (see Chapter 14). Low TNSAP levels are seen in hypophosphatasia, which results from mutations in the TNAP gene. A database that keeps track of these mutations (currently 194) has been established and can be accessed at http://www.sesep.uvsq.fr/Database. html.

Alkaline Phosphatase Vitamin D Metabolism

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Although referred to as a vitamin, vitamin D is mainly available, not from dietary sources, but as a result of the action of sunlight on 7-dehydrocholesterol. UV light of wavelength 270–300 nm breaks the B-ring of the steroid molecule, creating a secosteroid. Further rearrangement of the molecule occurs by the action of body heat to create cholecalciferol (vitamin D3). Vitamin D is also available from plant sources as ergocalciferol (vitamin D2), which is synthesised from ergosterol and differs structurally from cholecalciferol only in the presence of an additional double bond in the side chain. Both compounds are then metabolised in a similar manner and are thought to be equipotent. Collectively, they are referred to as vitamin D or calciferol. Under normal circumstances, approximately 80% of vitamin D requirements are obtained from this action of sunlight, but synthesis is dependent on the amount of sunlight exposure, the

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This enzyme is present in several tissues and exists as three main isoforms, intestinal (*171740), placental (*171810), and liver (tissue non-specific) (TNSAP) (*171760). A gene on chromosome 2q34–37 codes for the first two, and a gene on chromosome 1p36.1-p34 codes for the last [53]. Different post-translational modifications of the TNSAP enzyme result in the three tissue-specific forms found in bone, liver, and kidney that can be distinguished by their different isoelectric points and heat lability; the bone-specific form (bTNAP) being the least stable. It has been suggested that there are three codominant alleles (HN, HC and HI) of this enzyme and that the presence or absence of hypophosphatasia and its severity depends on which alleles are present. The HN allele is by far the most common and is homozygous in most individuals. The HI allele results in the most serious reduction in activity, whilst the HC allele is intermediate. Homozygous HI alleles result in the perinatal lethal

enzymes are distinguishable by their different affinities and capacities and by their intracellular localisation. The first to be cloned, a lowaffinity, high-capacity enzyme (CYP27A1) (*606530), is located in mitochondria. There are no reports of rickets resulting from mutations in this gene, but they do cause Cerebrotendinous Xanthomatosis (#213700). A second high-affinity, low-capacity enzyme (CYP2R1) (*608713), which is probably of greater physiological significance, is located within hepatic microsomes. CYP2R1 contains 501 amino acids and is coded for by a gene on chromosome 11p15.2 [60]. Rare cases are described of rickets associated with mutations in this gene (#600081) [61]. Two other enzymes, CYP3A4 (*124010) and CYP2J2 (*601258), probably also have some effect on 25-hydroxylase but are mainly involved in drug metabolism. The resulting product, 25OHD, circulates in plasma bound to the DBP in nanomolar concentrations. Assays of this compound give a measure of the vitamin D status because its level varies depending on the supply of vitamin D and shows a considerable annual variation, with a peak about 6 weeks after maximal exposure to sunlight. It is now generally agreed that vitamin D sufficiency is defined by a plasma concentration above 50 nmol/l [62]. 25OHD has some weak activity, which is not normally of clinical significance but may become significant in the presence of excess vitamin D (>300 nmol/l). Vitamin D 25 hydroxylase also catalyses the conversion of the synthetic vitamin D analogues 1α-hydroxy-cholecalciferol (alfacalcidol, One-Alpha®) and 1α-hydroxyergocalciferol (doxercalciferol, Hectorol®) to 1,25(OH)2D3 and 1,25(OH)2D2, respectively. 25OHD is metabolised to its active hormone  1,25(OH)2D by 25-hydroxyvitamin D 1α-hydroxylase, which is active only against metabolites that are already hydroxylated at position 25 [63]. A single enzyme that performs this metabolism has been identified and is located in convoluted and straight portions of the proxi-

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strength of the UV light in that sunlight and skin colour. Cultural practices that necessitate substantial covering of the skin limit sunlight exposure. In addition, in temperate climates, there is insufficient UV light available in sunlight during winter months, even if skin exposure is possible. Melanin absorbs UV light of the appropriate wavelength and, since the melanophores that determine skin colour are situated in the skin above the keratinocytes that synthesise vitamin D, darker skinned individuals require a greater degree of sunlight exposure to achieve the same effect as light-skinned people [56]. There may be as much as a six-fold difference in this requirement to overcome this barrier, and if this is achieved, darker skinned individuals are equally capable of synthesising vitamin D. Sunscreens, which are widely used, also limit UV light availability. There is generally little vitamin D in food, although some oily fish have a relatively high content, and it is a common misconception that because a child is taking a ‘healthy diet’, they are not at risk of vitamin D deficiency. If sunlight exposure is halved, vitamin D intake must be trebled to compensate for this, and the only realistic way of achieving this is by giving adequate dietary supplementation. Vitamin D is stored in liver and adipose tissue. Obese subjects have lower circulating levels of vitamin D than non-obese subjects, possibly because they sequester more vitamin D in their fat stores [57]. Following synthesis, vitamin D is bound to a specific vitamin D-binding protein and passes to the liver. Native vitamin D has little biological activity and requires metabolism via two hydroxylation steps, first at the 25- and subsequently at the 1- position in order to become fully active [58]. All of the steps in vitamin D metabolism are catalysed by cytochrome P450 enzymes (fig. 8). The first step is catalysed by vitamin D 25 hydroxylases, and at least four different enzymes influence 25-hydroxylase activity [59]. These

7-Dehydrocholesterol

Diet

Sunlight Previtamin D Body heat

įK\GUR[\ cholecalciferol (alfacalcidol)

Cholecalciferol/Ergocalciferol (Vitamin D) Calcitroic acid

Vitamin D 25-hydroxylase 25-OH vitamin D K\GUR[\YLWDPLQ'įK\GUR[\ODVH

Vitamin D 25-hydroxylase

1,25(OH)2 vitamin D Vitamin D receptor Peripheral action

Vitamin D 24-hydroxylase GLK\GUR[\YLWDPLQ' WULK\GUR[\YLWDPLQ'

Fig. 8. Diagrammatic representation of vitamin D metabolism.

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mia also stimulates 1α-hydroxylase activity; this is not a direct effect but is mediated via PTH. Plasma phosphate has a direct effect on 1α-hydroxylase activity; however, there is some evidence to suggest that this may be modulated by growth hormone or CT. The activity of 1α-hydroxylase is inhibited by FGF23. 1,25(OH)2D is a highly potent compound that circulates in picomolar concentrations. However, measurement of 1,25(OH)2D in plasma gives no measure of vitamin D status. 1,25(OH)2D synthesis is tightly controlled by the  plasma calcium concentration. In order to enable changes in 1,25(OH)2D to occur rapidly,  a  second enzyme, 25OHD 24-hydroxylase (25OHD 24-OHase) (CYP24A1) (*126065), exists. This is yet another cytochrome P450 enzyme that can use both 25OHD and 1,25(OH)2D as substrates to form 24,25-dihydroxyvitamin D (24,25(OH)2D) and 1α,24,25-trihydroxyvitamin

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mal renal tubule; furthermore, its activity is present in osteoblasts, keratinocytes, and lymphohaematopoietic cells, where 1,25(OH)2D may have an autocrine or paracrine role. During foetal life, 1α-hydroxylase activity is found in the placenta, while in pathological states, it is present in the macrophages of sarcoid tissue and in subcutaneous fat necrosis (see Chapter 7 and Case 19–29). 1α-hydroxylase is a mitochondrial enzyme (CYP27B1) (*609506) consisting of 508 amino acids with considerable homology to other P450 enzymes and is encoded by a single gene on chromosome 12q13.1-q13.3. Mutations in this gene are responsible for the condition known variously as Pseudo-Vitamin D Deficiency Rickets, Vitamin D-Dependent Rickets Type I, Prader Rickets or 1α-Hydroxylase Deficiency (#264700). 1α-hydroxylase activity is stimulated by PTH via its cAMP/protein kinase actions. Hypocalcae-

tion of osteoclasts via receptors on osteoblasts. In addition, there are receptors present in many tissues, such as skin, breast, prostate, colon, etc., that are not directly related to calcium homeostasis, and it has been postulated that 1,25(OH)2D may play a part in preventing cancers of these tissues [62]. Mutations in the vitamin D receptor occur throughout the molecule but particularly in either the ligand-binding (ligand-binding-negative) or DNA-binding (ligand-binding-positive) domains [66]. These mutations cause severe rickets, and many individuals, especially those with defects in DNA binding, also have alopecia. Originally referred to as Vitamin D-Dependent Rickets Type II (VDRR2), it is now more properly called HVDRR (#277440) (see Case 19–38). In another form of HVDRR, no mutations of the receptor have been identified, but it is thought to be caused by overexpression of a nuclear ribonucleoprotein that binds the hormone receptor complex to attenuate its action (%600785) (see Chapter 8 for details).

Summary and Conclusions

The mechanisms that are involved in maintaining normal calcium, magnesium and phosphate levels are complex and involve several different hormonal mechanisms that influence calcium, magnesium and phosphate in an independent but linked manner. Normal calcium and phosphate physiology demands that these mechanisms all function satisfactorily in order to maintain good bone health and demands a suitable milieu in which muscle and nerve function can be optimised. Disruptions in these mechanisms may be either environmental, principally due to vitamin D deficiency, or, in many instances, genetic. A thorough understanding of the physiology of these processes is required before a correct diagnosis can be made.

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D (1,24,25(OH)3D), respectively. The role of this enzyme is largely to divert the metabolism of 25OHD away from 1,25(OH)2D synthesis when this is not needed and to participate in the degradation of existing 1,25(OH)2D. This degradation process involves at least five steps that ultimately result in the formation of calcitroic acid, which is an inactive, water-soluble waste product. All of these steps are catalysed by CYP24A1 [59], which is inhibited by PTH and stimulated by 1,25(OH)2D and FGF23. Homozygous mutations in CYP24A1 have recently been shown to be responsible for some cases of Infantile Hypercalcaemia (#143880) [64] (see Case 19–28). 1,24,25(OH)3D has limited potency (about 10% of 1,25(OH)2D) and is probably an intermediate degradation metabolite of 1,25(OH)2D. The role, if any, of 24,25(OH)2D is uncertain. Some authors have argued that it has no role to play, whereas others have suggested that it may influence bone mineralisation. In addition, people of South Asian origin possess higher 25OHD 24-OHase activity than those of European origin [65], and this seems to contribute to their susceptibility to Vitamin D Deficiency Rickets. 1,25(OH)2D acts via a specific vitamin D receptor [66] (*601769) and is a member of the steroid-thyroid-retinoid superfamily of nuclear receptors, which, in many respects, is typical of this group. 1,25(OH)2D is located on the nuclear membrane and contains ligand binding, DNA binding, dimerisation, and transcriptional activation domains. Ligand binding by 1,25(OH)2D induces pseudodimerisation with the retinoid X receptor, which is encoded by a gene on chromosome 12 near the 1α-hydroxylase gene. The receptors are widely distributed in gut, parathyroid glands, chondrocytes, osteoblasts, and osteoclast precursors. 1,25(OH)2D plays a critical role in promoting calcium absorption in the small intestine, suppresses PTH secretion from the PT glands, influences growth plate mineralisation, and stimulates differentia-

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31 Allgrove J, Adami S, Fraher L, Reuben A, O’Riordan JL: Hypomagnesaemia: studies of parathyroid hormone secretion and function. Clin Endocrinol (Oxf) 1984;21:435–449. 32 Warner J, Epstein M, Sweet A, Singh D, Burgess J, Stranks S, Hill P, Perry-Keene D, Learoyd D, Robinson B, Birdsey P, Mackenzie E, Teh BT, Prins JB, Cardinal J: Genetic testing in familial isolated hyperparathyroidism: unexpected results and their implications. J Med Genet 2004;41:155–160. 33 Murugaian EE, Premkumar RM, Radhakrishnan L, Vallath B: Novel mutations in the calcium sensing receptor gene in tropical chronic pancreatitis in India. Scand J Gastroenterol 2008;43: 117–121. 34 Zhao XM, Hauache O, Goldsmith PK, Collins R, Spiegel AM: A missense mutation in the seventh transmembrane domain constitutively activates the human Ca2+ receptor. FEBS Lett 1999;448: 180–184. 35 Christie PT, Curley AJ, Harding B, Bowl MR, Turner JJO, Cappucco FP, Langman CB, Saggar AK, Taylor A, Thakker RV: An activating calcium sensing receptor mutation associated with normocalcemic (idiopathic) hypercalciuric nephrolithiasis. J Bone Miner Res 2002; 17s1:S127, Abstract 1008. 36 Carling T, Szabo E, Bai M, Ridefelt P, Westin G, Gustavsson P, Trivedi S, Hellman P, Brown EM, Dahl N, Rastad J: Familial hypercalcemia and hypercalciuria caused by a novel mutation in the cytoplasmic tail of the calcium receptor. J Clin Endocrinol Metab 2000;85:2042–2047. 37 Quitterer U, Hoffmann M, Freichel M, Lohse MJ: Paradoxical block of parathormone secretion is mediated by increased activity of G alpha subunits. J Biol Chem 2001;276:6763–6769. 38 Vetter T, Lohse MJ: Magnesium and the parathyroid. Curr Opin Nephrol Hypertens 2002;11:403–410. 39 Rogers A, Nesbit MA, Hannan FM, Howles SA, Gorvin CM, Cranston T, Allgrove J, Bevan JS, Bano G, Brain C, Datta V, Grossman AB, Hodgson SV, Izatt L, Millar-Jones L, Pearce SH, Robertson L, Selby PL, Shine B, Snape K, Warner J, Thakker RV: Mutational analysis of the adaptor protein 2 sigma subunit (AP2S1) gene: search for autosomal dominant hypocalcemia type 3 (ADH3). J Clin Endocrinol Metab 2014; 99:E1300–E1305.

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Allgrove Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 7–32 (DOI: 10.1159/000380990)

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Jeremy Allgrove, MA, MD, FRCP, FRCPCH Department of Paediatric Endocrinology 8th Floor, North Tower, Royal London Hospital Whitechapel, London E1 1BB (UK) E-Mail [email protected]

Chapter 3 Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 33–55 (DOI: 10.1159/000380991)

Physiology of Bone Peter Grabowski Faculty of Medicine, Dentistry and Health, Academic Unit of Human Nutrition, University of Sheffield, Sheffield, UK

Bone serves three main physiological functions: its mechanical nature provides support for locomotion and offers protection to vulnerable internal organs, it forms a reservoir for the storage of calcium and phosphate in the body, and it provides an environment for bone marrow production and haematopoietic cell development. The traditional view of bone as a passive tissue that responds to hormonal and dietary influences has changed over the past half century to one of bone as a dynamic adaptive tissue that responds to mechanical demands. This chapter gathers together some recent advances in bone physiology and molecular cell biology and discusses the potential application of the functional adaptation of bone to loading to enhance bone strength during childhood and adolescence. © 2015 S. Karger AG, Basel

Introduction

Bone is a dynamic mineralised connective tissue with multiple physiological functions. At the organ level, bone provides mechanical support for load bearing and locomotion, offers physical pro-

tection to vulnerable internal organs, forms a mobilisable reservoir of calcium and phosphate ions and provides an environmental niche for bone marrow production and haematopoietic cell development. At the tissue level, the coordinated activities of bone formation and resorption provide mechanisms for bone modelling – i.e. the formation of new bone during growth and development  – and remodelling – i.e. the coordinated process by which old bone is initially removed and then replaced during skeletal maintenance – and for rapidly responding to the body’s immediate calcium, phosphate and acid-base homeostatic requirements. At the cellular level, bone matrix formation and mineralisation are mediated by osteoblasts, and bone resorption is mediated by osteoclasts. At the molecular level, a range of systemic and local factors regulate cellular and tissue level processes in bone. Bones are highly dependent upon other organs for their growth and development, and particularly the intestine and the kidneys, through which mineral and nutritional factors are absorbed, reabsorbed and excreted, as well as the hypothalamus, the pituitary, the gonads, the parathyroid Downloaded by: Chulalongkorn University 161.200.145.19 - 7/31/2019 7:41:01 AM

Abstract

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molecular genetics and the ability to generate targeted transgenic mice has revolutionised the study of individual gene product functions in bone.

Skeletal Development

The evolutionary landmarks comprising the vertebrate skeleton are reflected by the developmental biology of bone tissue. Axial skeletal patterning, segmentation, growth and condensation are regulated by homeobox genes (reviewed in [5]), bone morphogenic proteins (BMPs) and other members of the transforming growth factor-β superfamily (reviewed in [6]), fibroblast growth factors (FGFs) [7] and the hedgehog [8] and Wnt proteins [9]. Many of these factors act not only in skeletal patterning and development but also in the recruitment and differentiation of osteoblasts during bone modelling and remodelling throughout life. The axial skeleton is initially laid down as a cartilaginous matrix model by chondrocytic cells of mesenchymal origin (fig.  1). Chondrocytes mineralise the matrix and, through a process of hypertrophy, eventually die by apoptosis. Metalloproteinases released by the chondrocytes dissolve some of the matrix and generate angiogenic signals to promote vascularisation and the influx of osteoclasts, which begin to resorb the mineralised cartilage. Along with osteoclasts, osteoblast precursors enter the primitive bone and begin to form true bone behind the advancing osteoclasts, giving rise to the primary spongiosa under the growth plate. In the growth plate, gradients of Indian hedgehog and parathyroid hormone (PTH)-related peptide regulate the directional proliferation and differentiation of chondrocytes into a hypertrophic phase characterised by mineralisation and metalloproteinase secretion, leading to longitudinal bone growth. In contrast, bones of the cranial vault form through a process of intramembranous ossification in membranes of mesenchymal condensations,

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glands, the liver and the skin, which produce hormonal factors regulating bone growth and mineral homeostasis. Whereas in adults, bone physiology is related to skeletal maintenance, in children, the context is one in which bones are growing in size, mass and mineral density and are at the same time being modelled into their final adult shape and form. Much of what we know about basic skeletal biology and physiology derives from the study of adults or animals, but the study of bone in children is an active and expanding area of research. Macroscopically, bone tissue is classified as either cortical or trabecular. Cortical bone is found most commonly in the shafts of long bones and consists of a dense compact tissue penetrated by blood vessels and canaliculi that surround osteocytes and their connecting cellular processes. Trabecular or cancellous bone is found at the ends of long bones, in vertebrae and near joint surfaces and consists of a network of thin plates and connecting struts surrounded by bone marrow. Cortical and trabecular bone are very similar in their cellular and molecular composition but significantly differ in their functional and mechanical properties. For much of the 20th century, bone physiology largely centred on understanding the hormonal regulation of osteoblasts and osteoclasts in skeletal maintenance and, to a lesser extent, in bone growth. Since the mid-1960s, the focus of bone physiology has changed, largely due to the efforts of Frost, Jee and others (reviewed in [1–3]), who established a view of bone as a dynamic tissue that responds at the tissue level to the mechanical demands placed upon it, developed the concept of the mechanostat [2] and encouraged increased interest in the role of cells within the bone matrix and marrow as sensors of local mechanical stimuli and regulators of local bone turnover [4]. Advances in tissue and cell culture techniques have contributed to our understanding of the development, regulation and function of osteoblasts and osteoclasts. More recently, the development of

GP

GP

a

b

c

Fig. 1. Early stages of long bone development by endochondral ossification. a The condensation of mesenchymal cells leads to the formation of a cartilage model of the bone. b Chondrocytes beneath the perichondrium (black) differentiate, become hypertrophic and eventually undergo apoptosis, resulting in cartilage mineralisation (shaded) and release of metalloproteinases. c Angiogenic factors released by chondrocytes encourage vascular invasion, recruiting osteoclast and osteoblast precursors that differentiate and convert calcified cartilage into true bone. Growth plates are established from chondrocytes in the epiphyseal regions.

Osteoblast Differentiation and Function

Osteoblasts, bone-lining cells and osteocytes arise from a multipotent precursor of mesenchymal origin, most commonly called a mesenchymal stem cell, that also gives rise to chondrocytes, adipocytes, myocytes and fibroblasts (fig.  2) [11].

The early differentiation process that leads to osteochondroprogenitor cell production involves Sox9, the key transcriptional regulator of chondrogenesis. Runx2 [12, 13] (*600211) and Osterix (SP7) [14] (*606633) have been identified as the two critical transcription factors that determine osteoblast lineage differentiation. Runx2 (also called CBFA1), a member of the Runx transcription factor family, is characterised by a DNAbinding domain that is homologous to the Drosophila gene runt. Runx2 was identified as a causative gene for cleidocranial dysplasia (#119600) (table 1) [12, 13]. This gene is expressed in mouse embryonic tissues in cells destined to become osteoblasts or chondrocytes in the developing embryo and in all osteoblasts regardless of their differentiation stage [15]. Runx2–/– mice are unable to produce either endochondral or intramembranous bone [13, 16] but can produce adipocytes and chondrocytes [17]. Osterix is a zinc fingercontaining transcription factor of the SP transcription factor family. Osterix–/– mice are deficient in osteoblasts and do not form intramembranous bone [14]. Osterix–/– mice do express

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which progress to bone formation without chondrocyte involvement. Bone formed through both mechanisms undergoes remodelling; this process is initiated by activation of osteoclasts, followed by a period of resorption, a reversal phase in which osteoclasts die and osteoblasts are activated, a period of matrix formation, a mineralisation phase and a return to the resting state. Communication between osteoblasts and osteoclasts coordinates this series of events, known as the bone remodelling cycle, within a basic multicellular unit. For a more detailed description of skeletal development, see Karaplis [10]. Hormonal influences on bone development are described elsewhere in this book.

Mesenchymal stem cell

Osteochondroprogenitor

Osteoblasts

Osteocytes Lining cells Sox9

Runx2

Osterix

Fig. 2. Osteoblast differentiation. Osteoblasts arise from a multipotent precursor cell of mesenchymal origin (mesenchymal stem cell). An osteochondrogenitor cell capable of forming both chondrocytes and osteoblasts arises under the control of the transcription factor Sox9. Runx2 is the key regulator of osteoblast differentiation and is constitutively expressed in osteoblasts at all stages of differentiation. A second transcription factor, Osterix, acts downstream of Runx2 in osteoblast differentiation. Osteoblasts can further differentiate into osteocytes that become embedded in the bone matrix or into lining cells on bone surfaces.

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blasts primarily exerts its effects on bone by regulating osteoclast formation through modulating the production of osteoprotegerin (OPG), the soluble inhibitor of the receptor activator of nuclear factor κ-B (RANK) signalling pathway [19] (see below). Mutations in Wnt1 (*164820) have recently been shown to cause OI type XV (#615220). Gain-of-function mutations in LRP5 (*603506) result in the high bone mass disorders endosteal hyperostosis and osteosclerosis (#144750), autosomal dominant osteopetrosis type 1 (#607634) and van Buchem disease type 2 (#607636) (table 1), while loss-of-function mutations in LRP5 result in the low bone mass disorders osteoporosis-pseudoglioma syndrome (#259770) and one form of osteoporosis (#166710) (table 1).

Bone and Cartilage Matrix Collagens and Their Modifying Enzymes

The major function of osteoblasts is to create a mineralised bone matrix that, until mineralised, is called osteoid. Type 1 collagen accounts for about 90% of osteoid content, and the remainder is largely composed of glycoproteins and proteo-

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Runx2, but Runx2–/– mice do not express Osterix, indicating that Osterix acts downstream of Runx2. In humans, mutations in Osterix result in osteogenesis imperfecta (OI) type XII (#613849). The Wnt signalling pathway in osteoblasts contains a number of molecules that are now viewed as amongst the most important regulators of bone formation during growth and development due to their mediation of some of the regulatory dialogue between osteoblasts and osteoclasts [18]. Wnts are glycoproteins in osteoblasts that act on receptors composed of a Frizzled (G protein-coupled receptor-like) protein and either of two low-density lipoprotein receptor-related proteins (LRPs): LRP5 or LRP6. The activation of the Wnt receptor results in the dephosphorylation and accumulation of intracellular β-catenin and its translocation into the nucleus, where it interacts with transcription factors to control osteoblast gene expression (fig. 3). The dickkopf family of proteins acts as a negative regulator of Wnt signalling by binding to LRP5/6 and another cell surface co-receptor, Kremen, causing the internalisation and destruction of the resulting complex and reducing the density of Wnt receptors at the cell surface (fig. 3). Wnt signalling in osteo-

Physiology of Bone

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OMIM *155760

*171640 *102576 *604539

*608990

*612277 *603051 *171760

*300647 *605145

*608662

HGNC gene symbol/protein or gene product name

ACAN aggrecan

ACP5 acid phosphatase 5, tartrate resistant

ACVR1 activin A receptor, type I

ADAMTS2 ADAM metallopeptidase with thrombospondin type 1 motif, 2

ADAMTS10 ADAM metallopeptidase with thrombospondin type 1 motif, 10

ADAMTSL2 ADAMTS-like 2

AGPS alkylglycerone phosphate synthase

ALPL (TNSALP)* alkaline phosphatase, liver/bone/kidney

AMER1 APC membrane recruitment protein 1

ANKH ANKH inorganic pyrophosphate transport regulator

ANO5 anoctamin 5

Gnathodiaphyseal dysplasia

Chondrocalcinosis 2 Craniometaphyseal dysplasia, autosomal dominant

Osteopathia striata with cranial sclerosis

Hypophosphatasia, infantile Hypophosphatasia, childhood Hypophosphatasia, adult

Rhizomelic chondrodysplasia punctata, type 3

Geleophysic dysplasia 1

Weill-Marchesani syndrome 1

Ehlers-Danlos syndrome, Type VIIC

Fibrodysplasia ossificans progressiva

Spondyloenchondrodysplasia with immune dysregulation

Osteochondritis dissecans, short stature, and early-onset osteoarthritis Spondyloepimetaphyseal dysplasia, aggrecan type Spondyloepiphyseal dysplasia, Kimberley type

Disorder

Table 1. Regulatory, structural and processing genes in bone and cartilage associated with known skeletal disorders in children Inheritance

#166260 AD

#118600 AD #123000 AD

#300373 XLD

#241500 AR #241510 AD #146300 AD

#600121 AR

#231050 AR

#277600 AR

#225410 AR

#135100 AD/SP

#607944 AR/AD

#612813 AR #608361 AD

#165800 AD

OMIM

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OMIM *300180 *603248 *615291

*606374 *604327

*611492 +601199

*603799 *608429

*601083 *300008 *602727

HGNC gene symbol/protein or gene product name

ARSE arylsulphatase E

BMPR1B bone morphogenetic protein receptor, type IB

B3GALT6 beta 1,3-galactosyl-transferase, polypeptide 6

B3GAT3 beta 1,3 glucuronyl transferase 3

B4GALT7 xylosylprotein beta 1,4-galactosyl-transferase, polypeptide 7

CA2* carbonic anhydrase II

CASR* Calcium sensing receptor

CHST3 carbohydrate (chondroitin 6) sulphotransferase 3

CHST14 carbohydrate (N-acetylgalactosamine 4-0) sulphotransferase 14

CHSY1 chondroitin sulphate synthase 1

CLCN5 chloride channel, voltage-sensitive 5

CLCN7* chloride channel, voltage-sensitive 7

Table 1. Continued

Osteopetrosis, autosomal recessive 4 Osteopetrosis, autosomal dominant 2

Hypophosphataemic rickets

Temtamy preaxial brachydactyly

Ehlers-Danlos syndrome, musculocontractural type 1

Spondyloepiphyseal dysplasia with congenital joint dislocations

Hyperparathyroidism, neonatal Hypocalciuric hypercalcaemia, type I Hypocalcaemia, autosomal dominant

Osteopetrosis, autosomal recessive 3, with renal tubular acidosis

Ehlers-Danlos syndrome, progeroid type 1

Larsen syndrome

Ehlers-Danlos syndrome, progeroid type 2 Spondyloepimetaphyseal dysplasia with joint laxity, type 1, with or without fractures

Brachydactyly, type A2 Chondrodysplasia, acromesomelic, with genital anomalies

Chondrodysplasia punctate 1, X-linked recessive

Disorder

Inheritance

#611490 AR #166600 AD

#300554 XLR

#605282 AR

#601776 AR

#143095 AR

#239200 AR #145980 AD #601198 AD

#259730 AR

#130070 AR

#245600 AR

#615349 AR #271640 AR

#112600 AD #609441 AR

#302950 XLR

OMIM

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OMIM +120150

*120160

+120140

*120215 *120190 +120210 *120260 *120270

HGNC gene symbol/protein or gene product name

COL1A1* collagen, type I, alpha 1

COL1A2* collagen, type I, alpha 2

COL2A1 collagen, type II, alpha 1

COL5A1 collagen, type V, alpha 1

COL5A2 collagen, type V, alpha 2

COL9A1 collagen, type IX, alpha 1

COL9A2 collagen, type IX, alpha 2

COL9A3 collagen, type IX, alpha 3

Table 1. Continued

Epiphyseal dysplasia, multiple, 3

Epiphyseal dysplasia, multiple, 2 Stickler syndrome, type V

Epiphyseal dysplasia, multiple, 6 Stickler syndrome, type IV

Ehlers-Danlos syndrome, type I Ehlers-Danlos syndrome, type II

Ehlers-Danlos syndrome, type I Ehlers-Danlos syndrome, type II

Achondrogenesis, type II; hypochondrogenesis Kniest dysplasia Legg-Calve-Perthes disease Otospondylomegaepiphyseal dysplasia Platyspondylic skeletal dysplasia (Torrance) Spondyloepiphyseal dysplasia, congenital Spondyloepimetaphyseal dysplasia (Strudwick) Spondyloperipheral dysplasia Stickler syndrome type I

Ehlers-Danlos syndrome, cardiac valvular form Ehlers-Danlos syndrome, type VIIB Osteogenesis imperfecta, type II Osteogenesis imperfecta, type III Osteogenesis imperfecta, type IV

Caffey disease Ehlers-Danlos syndrome, type I Ehlers-Danlos syndrome, type VIIA Osteogenesis imperfecta, type I Osteogenesis imperfecta, type II Osteogenesis imperfecta, type III Osteogenesis imperfecta, type IV

Disorder

AD AD AD AD AD AD AD AD AD

AR AD AD AD AD

AD AD AD AD AD AD AD

Inheritance

#600969 AD

#600204 AD #614284 AR

#614135 AD #614134 AR

#130000 AD #130010 AD

#130000 AD #130010 AD

#200610 #156550 #150600 #215150 #151210 #183900 #184250 #271700 #108300

#225320 #130060 #166210 #259420 #166220

#114000 #130000 #130060 #166200 #166210 #259420 #166220

OMIM

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*120280

*120290

*600310 *605497 *601105 *191311 *600980 *607461 *300205 *173335

*608177

COL11A1 collagen, type XI, alpha 1

COL11A2 collagen, type XI, alpha 2

COMP cartilage oligomeric matrix protein

CRTAP* cartilage associated protein

CTSK* cathepsin K

DDR2 discoidin domain receptor tyrosine kinase 2

DMP1 dentin matrix acidic phosphoprotein 1

DYM dymeclin

EBP emopamil binding protein

ENPP1 ectonucleotide pyrophosphatase/ phosphodiesterase 1

EXT1 exostosin glycosyltransferase 1

*608210

*120110

COL10A1 collagen, type X, alpha 1

EXT2 exostosin glycosyltransferase 2

OMIM

HGNC gene symbol/protein or gene product name

Table 1. Continued

Exostoses, multiple, type 2

Exostoses, multiple, type 1

Hypophosphataemic rickets, autosomal recessive, 2 Arterial calcification, generalized, of infancy, 1

Chondrodysplasia punctata, X-linked dominant

Smith-McCort dysplasia

Hypophosphataemic rickets, AR

Spondylometaepiphyseal dysplasia, short limb-hand type

Pycnodysostosis

Osteogenesis imperfecta, type VII

Epiphyseal dysplasia, multiple 1 Pseudoachondroplasia

Fibrochondrogenesis 2 Otospondylomegaepiphyseal dysplasia Stickler syndrome, type III

Fibrochondrogenesis 1 Marshall syndrome Stickler syndrome, type II

Metaphyseal chondrodysplasia (Schmid)

Disorder

Inheritance

#133701 AD

#133700 AD

#613312 AR #208000 AR

#302960 XLD

#607326 AR

#241520 AR

#271665 AR

#265800 AR

#610682 AR

#132400 AD #177170 AD

#614524 AD/AR #215150 AD/AR #184840 AD

#228520 AR #154780 AD #604841 AD

#156500 AD

OMIM

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Apert syndrome Bent bone dysplasia syndrome Craniofacial-skeletal-dermatologic dysplasia Crouzon syndrome Jackson-Weiss syndrome Pfeiffer syndrome Achondroplasia Thanatophoric dysplasia type I Thanatophoric dysplasia type II Hypochondroplasia

*134797

*607901 *605380 *136350

*176943

*134934

*607063 *300017

FBN1 fibrillin 1

FERMT3 fermitin family member 3

FGF23* fibroblast growth factor 23

FGFR1 fibroblast growth factor receptor 1

FGFR2 fibroblast growth factor receptor 2

FGFR3 Fibroblast growth factor receptor 3

FKBP10 FK506 binding protein 10, 65 kDa

FLNA filamin A, alpha

Frontometaphyseal dysplasia

Osteogenesis imperfecta, type XI

Hartsfield syndrome Jackson-Weiss syndrome Osteoglophonic dysplasia Pfeiffer syndrome Trigonocephaly 1

Hypophosphataemic rickets, autosomal dominant Tumoural calcinosis, hyperphosphataemic, familial

Leukocyte adhesion deficiency, type III (osteopetrosis)

Acromicric dysplasia Geleophysic dysplasia 2 Marfan syndrome Weill-Marchesani syndrome 2, dominant

Raine syndrome

*611061

FAMOC family with sequence similarity 20, member C

Disorder

OMIM

HGNC gene symbol/protein or gene product name

Table 1. Continued Inheritance

AD AD AD AD

AD AD AD AD

AD AD AD AD AD AD

AD AD AD AD AD

#305620 XLR

#610968 AR

#100800 #187600 #187601 #146000

#101200 #614592 #101600 #123500 #123150 #101600

#615465 #123150 #166250 #101600 #190440

#193100 AD #211900 AD

#612840 AR

#102370 #614185 #154700 #608328

#259775 AR

OMIM

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OMIM *603381

*601146 *121014 *139320

*602744 *300037 *606045 *300248

*600024 *607844 *151443

HGNC gene symbol/protein or gene product name

FLNB filamin B, beta

GDF5 growth differentiation factor 5

GJA1 gap junction protein, alpha 1, 43 kDa

GNAS (GNAS1) GNAS complex locus (guanine nucleotide binding protein alpha stimulating activity polypeptide 1)

GNPAT glyceronephosphate O-acyltransferase

GPC3 glypican 3

IFT122 intraflagellar transport 122

IKBKG inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase gamma

LBR lamin B receptor

LEMD3 LEM domain containing 3

LIFR leukaemia inhibitory factor receptor alpha

Table 1. Continued

Stuve-Wiedemann syndrome/Schwartz-Jampel syndrome, type 2

Buschke-Ollendorff syndrome; osteopoikilosis Melorheostosis with osteopoikilosis

Greenberg skeletal dysplasia

Ectodermal dysplasia, anhidrotic, with immunodeficiency, osteopetrosis, and lymphoedema

Cranioectodermal dysplasia 1

Simpson-Golabi-Behmel syndrome, type 1

Chondrodysplasia punctata, rhizomelic, type 2

Acromegaly, somatic McCune-Albright syndrome, somatic, mosaic Osseous heteroplasia, progressive Pseudohypoparathyroidism Ia Pseudohypoparathyroidism Ib Pseudohypoparathyroidism Ic Pseudopseudohypoparathyroidism

Craniometaphyseal dysplasia, autosomal recessive

Acromesomelic dysplasia, Hunter-Thompson type Chondrodysplasia, Grebe type

Atelosteogenesis, type I Atelosteogenesis, type III Boomerang dysplasia Larsen syndrome Spondylocarpotarsal synostosis syndrome

Disorder ? AD AD AD AR

Inheritance

SM SM AD AD AD AD AD

#601559 AR

#166700 AD/? #155950 ?

#215140 AR

#300301 ?

#218330 AR

#312870 XLR

#222765 AR

#102200 #174800 #166350 #103580 #603233 #612462 #612463

#218400 AR

#201250 AR #200700 AR

#108720 #108721 #112310 #150250 #272460

OMIM

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OMIM *603506

*602109 *120360 *120361 *600108 *164005 *602183 *108961 *607649 *601757 *300550

*610339

HGNC gene symbol/protein or gene product name

LRP5* low-density lipoprotein receptor-related protein 5

MATN3 matrillin 3

MMP2 matrix metalloproteinase 2

MMP9 matrix metallopeptidase 9

MMP13 matrix metalloproteinase 13

NFIX nuclear factor I/X

NKX3-2 NK3 homeobox 2

NPR2 natriuretic peptide receptor 2

OSTM1* osteopetrosis associated transmembrane protein 1

PEX7 peroxisomal biogenesis factor 7

PHEX* phosphate-regulating endopeptidase homolog, X-linked

P3H1 (LEPRE1)* prolyl 3-hydroxylase 1 (leprecan)

Table 1. Continued

Osteogenesis imperfecta, type VIII

Hypophosphataemic rickets, X-linked dominant

Rhizomelic chondrodysplasia punctata, type 1

Osteopetrosis, autosomal recessive 5

Acromesomelic dysplasia, Maroteaux type Epiphyseal chondrodysplasia, Miura type

Spondylo-megaepiphyseal-metaphyseal dysplasia

Marshall-Smith syndrome

Spondyloepimetaphyseal dysplasia type II; Metaphyseal anadysplasia 1

Metaphyseal anadysplasia 2

Multicentric osteolysis, nodulosis and arthropathy

Epiphyseal dysplasia, multiple, 5 Spondyloepimetaphyseal dysplasia

Hyperostosis, endosteal; osteosclerosis Osteopetrosis, autosomal dominant I Osteoporosis-pseudoglioma syndrome van Buchem disease, type 2 Primary osteoporosis

Disorder AD AD AR AD AD

Inheritance

#610915 AR

#307800 XLD

#215100 AR

#259720 AR

#602875 AR #615923 AD

#613330 AR

#602535 AD

#602111 AD

#613073 AR

#259600 AR

#607078 AD #608728 AR

#144750 #607634 #259770 #607636 #166710

OMIM

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OMIM *142461 *611466

*153454 *601865 *603066 *123841 *176876 *168468

*157660

*602337 *600211

*607444

HGNC gene symbol/protein or gene product name

HSPG2 (PLC) heparan sulphate proteoglycan 2

PLEKHM1 pleckstrin homology domain-containing, family M (with RUN domain) member 1

PLOD1 procollagen-lysine, 2-oxoglutarate 5-dioxygenase 1

PLOD2* procollagen-lysine, 2-oxoglutarate 5-dioxygenase 2

PLOD3 procollagen-lysine, 2-oxoglutarate 5-dioxygenase 3

PPIB (CYPB) peptidylprolyl isomerase B (cyclophilin B)

PTPN11 protein tyrosine phosphatase, non-receptor type 11

PTH1R (PTHR1)* parathyroid hormone 1 receptor (parathyroid hormone receptor 1)

RMRP RNA component of mitochondrial RNA-processing endoribonuclease

ROR2 receptor tyrosine kinase-like orphan receptor 2

RUNX2* Runt-related transcription factor 2

SBDS Shwachman-Bodian-Diamond syndrome

Table 1. Continued

Shwachman-Bodian-Diamond syndrome

Cleidocranial dysplasia Metaphyseal dysplasia with maxillary hypoplasia with or without brachydactyly

Brachydactyly, type B1 Robinow syndrome, autosomal recessive

Anauxetic dysplasia Cartilage-hair hypoplasia Metaphyseal dysplasia without hypotrichosis

Chondrodysplasia, Blomstrand type Eiken syndrome Metaphyseal chondrodysplasia, Jansen type

Metachondromatosis Noonan syndrome 1

Osteogenesis imperfecta, type IX

Bone fragility with contractures, arterial rupture and deafness

Bruck syndrome 2

Ehlers-Danlos syndrome type VI

Osteopetrosis, autosomal recessive 6

Dyssegmental dysplasia, Silverman-Handmaker type Schwartz-Jampel syndrome, type 1

Disorder

Inheritance

#260400 AR

#119600 AD #156510 AD

#113000 AD #268310 AR

#607095 AR #250250 AR #250460 AR

#215045 AR #600002 AR #156400 AD

#156250 AD #163950 AD

#259440 AR

#612394 AR

#609220 AR

#225400 ?AR

#611497 AR

#224410 AR #255800 AR

OMIM

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*610804

*608735

*614780 *605740

*608160 *606633

SLC35D1 solute carrier family 35 (UDP-GlcA/UDP-GalNAc transporter), member D1

SLC39A13 solute carrier family 39 (zinc transporter), member 13

SNX10 sorting nexin 10

SOST* sclerostin

SOX9 SRY (sex determining region Y)-box 9

SP7 transcription factor Sp7

Osteogenesis imperfecta, type XII

Campomelic dysplasia

Craniodiaphyseal dysplasia, autosomal dominant Sclerosteosis 1 Hyperostosis corticalis generalisata (Van Buchem disease type 1)

Osteopetrosis, autosomal recessive 8

Spondylocheirodysplasia, Ehlers-Danlos syndrome-like

Schneckenbecken dysplasia

Hypophosphataemic rickets with hypercalciuria

*609826

Langer mesomelic dysplasia Leri-Weill dyschondrosteosis Short stature, idiopathic familial

SLC34A3 solute carrier family 34 (type II sodium/ phosphate cotransporter), member 3

*312865

SHOX short stature homeobox

Osteogenesis imperfecta, type X

Achondrogenesis Ib Atelosteogenesis II; De la Chapelle dysplasia Diastrophic dysplasia Epiphyseal dysplasia, multiple, 4

*600943

SERPINH1 serpin peptidase inhibitor, clade H member 1

Disorder

SLC26A2 *606718 solute carrier family 26 (anion exchanger), member 2

OMIM

HGNC gene symbol/protein or gene product name

Table 1. Continued Inheritance

AR AR AR AR

#613849 AR

#114290 XLR

#122860 AD #269500 AR #239100 AR

#615085 AR

#612350 AR

#269250 AR

#241530 AR

#600972 #256050 #222600 #226900

#249700 AR #127300 AD #300582 AD

#613848 AR

OMIM

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Grabowski

*604592

*190180 *603499

*602643

*602642 *313400 *604505 *613602 *608124

TCIRG1 T cell immune regulator 1 (vacuolar proton pump α-subunit 3)

TGFB1* transforming growth factor, beta 1

TNFRSF11A* tumour necrosis factor receptor superfamily, member 11a, NFKB activator (receptor activator of NF-κB/RANK)

TNFRSF11B* tumour necrosis factor receptor superfamily, member 11b (osteoprotegerin/OPG)

TNFSF11* (receptor activator of NF-κB ligand/RANKL/OPGL)

TRAPPC2 (SEDL) trafficking protein particle complex 2 (sedlin)

TRIP11 thyroid hormone receptor interactor 11

WDR35 WD repeat domain 35

XYLT1 xylosyltransferase 1

Desbuquois dysplasia type 2

Cranioectodermal dysplasia 2 Short-rib thoracic dysplasia 7 with or without polydactyly

Achondrogenesis, type IA

Spondyloepiphyseal dysplasia tarda

Osteopetrosis, autosomal recessive 2

Paget disease, juvenile

Familial expansile osteolysis Osteopetrosis, autosomal recessive 7 Paget disease of bone

Camurati-Engelmann disease

Osteopetrosis, autosomal recessive 1

Disorder

Inheritance

#615777 AR

#613610 AR #614091 AR

#200600 AR

#313400 XLR

#259710 AR

#239000 AR

#174810 AD #612301 AR #602080 AR

#131300 AD

#259700 AR

OMIM

Genes are listed in alphabetical order using the symbols and names approved by the Human Genome Organisation Nomenclature Committee [57]. Alternative names are included where these may be more familiar. Genes and conditions arising from mutations therein are shown with their appropriate entry numbers in OMIM [58]. Genes associated with conditions that are described in more detail in the book are indicated by (*). AD = autosomal dominant. AR = autosomal recessive. SM = somatic mosaicism. XLD = X-linked dominant. XLR = X-linked recessive. ? = mode of inheritance not determined. See also [59] for a more comprehensive classification of genetic skeletal disorders.

OMIM

HGNC gene symbol/protein or gene product name

Table 1. Continued

Sost LRP5

Wnt

Wnt Frizzled

LRP5 Dkk

Frizzled

Kremen

Wnt

LRP5

Frizzled Kremen

Dishevelled DŽ-catenin accumulation

a and nuclear translocation

Kremen

b

Dishevelled

DŽ-catenin destruction

Dishevelled

c

DŽ-catenin destruction

Fig. 3. Wnt signalling in osteoblasts. a Osteoblasts express canonical Wnt pathway coreceptor molecules, including low-density lipoprotein receptor-related proteins (LRPs) and Frizzled family members. Wnts interact with LRP5/6 and Frizzled proteins to form a complex that leads to the inhibition of β-catenin destruction, mediated by Dishevelled. As a result, β-catenin accumulates and is translocated to the nucleus, where it interacts with transcription factors to modulate gene expression. b Dickkopf inhibits Wnt signalling by binding LRPs to Kremen, enhancing LRP internalisation and destruction. c Sclerostin competes with the binding of Wnt to LRPs, preventing the interaction of LRPs with Frizzled proteins.

gates towards the amino-terminus. During assembly, many proline and lysine residues in fibrillar collagens become hydroxylated, and some of the hydroxylysine residues are further modified by glycosylation. Intra- and inter-chain disulphide bonds are also formed during synthesis. Carboxy- and amino-terminal propeptides are proteolytically cleaved extracellularly after secretion, and the released monomers assemble into highly orientated, quarter-staggered fibrils that are held together through covalent cross-links promoted by the activity of lysyl oxidase. The collagen triple helix is highly resistant to proteolytic cleavage by pepsin, trypsin and papain, and the degradation of collagens is mediated by matrix metalloproteinases, cysteine proteinases (especially cathepsins B, K and L) and serine proteinases [20]. In bone, type I collagen is the most abundant fibrillar collagen. It is normally heterotrimeric, consisting of two α1(I) chains and one α2(I) chain. In the absence of pro-α2(I) chains, type I collagen α1(I) homotrimers can form. In bone, type I collagen forms heterotypic fibrils with type

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glycans. Other proteins that are important for mineralisation, including alkaline phosphatase, osteocalcin and osteopontin, are also secreted by osteoblasts into the newly forming matrix. The process of bone mineralisation is poorly understood. Collagens are a diverse family of structural proteins found in extracellular matrices. They are the most abundant proteins in the body, and there are at least twenty-eight different collagens found in vertebrates. The characteristic feature of all collagens is a triple helical structure consisting of three interwoven α-chain polypeptides. The triple helical region of collagen α-chains consists of a repetitive series of amino-acid triplets – [GlyX-Y]n, where Gly is glycine and X and Y are commonly proline or hydroxyproline. Collagens can be homo- or hetero-trimeric proteins, i.e. composed of three identical α-chains or three α-chains encoded by either two or three unique genes, respectively. During synthesis, carboxy-terminal propeptide regions of three pro-α-chains associate within the endoplasmic reticulum to initiate the formation of the triple helix, which propa-

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(table 1). For a more comprehensive review of the clinical genetics of collagen disorders, see Chapter 12 and work by Byers [21].

Glycoproteins and Proteoglycans in Bone

Glycoproteins consist of a core polypeptide that has been modified by the covalent attachment of one or more oligosaccharides. Oligosaccharides in glycoproteins are attached at either the amide nitrogen on the side chain of an asparagine residue (N-glycosylation) or the hydroxyl oxygen on the side chain of a hydroxylysine, hydroxyproline, serine or threonine residue (O-glycosylation). The post-translational glycosylation of bone matrix proteins, including collagens (see above), alkaline phosphatase and members of the  secretory calcium-binding phosphoprotein (SCPP) family that are important for mineralisation (see below), confers structural, lubricating, protective or recognition functions to the core proteins; this modification may be required for protein conformational integrity or stability and may determine the localisation of the modified protein within the cell or in the extracellular matrix. Proteoglycans consist of a core polypeptide that has been modified by the covalent attachment of one or more glycosaminoglycans (GAGs) – heparan, chondroitin or dermatan or their sulphated products. These GAGs consist of repeating disaccharide units that are composed of an Nacetylated or N-sulphated hexosamine and either an uronic acid (glucuronic acid or iduronic acid) or galactose. Proteoglycans play important roles in regulating cell-cell and cell-matrix interactions and perform important biophysical and biochemical functions in bone and cartilaginous matrices. Syndecans are membrane proteoglycans that interact with growth factors and matrix molecules by transmitting extracellular signals to their intracellular domains to modify cell function or cytoskeletal organisation. Aggrecan, the most abun-

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V collagen, a low-abundance fibrillar protein with three distinct α-chains – α1(V), α2(V) and α3(V). While the COL1A2 gene (*120160) is not essential for survival, homozygous COL1A1 (*120150) null mutations are not seen clinically. Skeletal phenotypes arising from mutations in type I collagen give rise to OI types I-IV (#166200; #166210; #259420; #166220) and various forms of  Ehlers-Danlos syndrome (EDS) (#130000; #130060) and Caffey disease (#114000) (table 1). Additionally, mutations in type V collagen (*120215) give rise to EDS types I and II (#130000; #130010) (table 1). In cartilage, type II collagen (*120140), which consists of α1(II) homotrimers, is the most abundant fibrillar collagen. Additionally, in cartilage, the pro-α1(II) chain is incorporated into heterotrimeric type XI collagen along with a pro-α1(XI) chain and a pro-α2(XI) chain. Type XI collagen is a low-abundance fibrillar collagen that forms heterotypic fibrils with type II collagen in cartilage. Yet another low abundance collagen – type IX, which is composed of three distinct α-chains – α1(IX), α2(IX) and α3(IX) – also forms heterotypic fibrils with type II collagen in cartilage. Type IX collagen has a triple helix that is interrupted by short non-helical regions, which give the molecule some flexibility. Type IX collagen is classed as a fibril-associated collagen with interrupted triple helices. Mutations in type II collagen give rise to a variety of chondrodysplasias (table  1). Type IX collagen mutations give rise to various multiple epiphyseal dysplasias and Stickler syndrome, while mutations in type XI collagen give rise to Stickler and Marshall syndromes (table 1). Type X collagen is a homotrimeric protein found in the hypertrophic cartilage of the growth plate. Mutations in type X collagen result in Schmidt metaphyseal chondrodysplasia (table  1). Mutations in some collagen-modifying and collagenprocessing enzyme genes, including ADAMTS2, ADAMTS10, CRTAP, CTSK, LEPRE1, MMP2, MMP13, PLOD1, PLOD2 and PLOD3, have also been identified to give rise to skeletal phenotypes

CHSY1 (*608183), CHST3 (*603799) and CHST14 (*608429), which encode enzymes that catalyse the formation of chondroitin and dermatan sulphates, are associated with Temtamy preaxial brachydactyly syndrome (#605282), spondyloepiphyseal dysplasia with congenital joint dislocations (#143095) and EDS musculocontractural type 1 (#601776) (table 1 and see [22, 23]).

Bone Mineralisation

Mineral accounts for about 50–70% of bone mass. The principal minerals in bone are calcium and phosphorus, along with some carbonate and magnesium. Much of the phosphorous in bone is present as a form of hydroxyapatite (Ca10(PO4)6OH2), a crystalline hydrated calcium phosphate. Osteoblasts orchestrate the complex coordination of the deposition of protein components of the extracellular bone matrix and the gradual formation and maturation of mineral crystals over time. The mineralisation process is regulated by systemic hormones, including vitamin D and PTH, and by proteins of the SCPP family that are secreted by osteoblasts into the newly forming matrix. SCPP family proteins have a high content of acidic amino acids, which enables them to bind calcium ions. A key member of the SCPP family is osteonectin, which can bind to both hydroxyapatite and type 1 collagen fibrils. Also included within the SCPP family are a number of phosphorylated proteins of the small integrin-binding ligand N-linked glycoprotein subgroup, of which secreted phosphoprotein 1 (osteopontin) and integrin-binding sialoprotein (bone sialoprotein) are important regulators of bone mineralisation that are thought to act as nucleators of mineral crystal formation. Alkaline phosphatases regulate the balance between the extracellular levels of inorganic phosphate and pyrophosphate (P2O74−) – a natural inhibitor of hydroxyapatite formation. Alkaline phosphatases may also modify the phosphorylation state of

Physiology of Bone Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 33–55 (DOI: 10.1159/000380991)

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dant proteoglycan in cartilage, forms extensive aggregates in which chondroitin and keratan sulphate GAG chains bind and organise water molecules to maintain cartilage hydration and to provide resistance to deformation. Biglycan and decorin are small proteoglycans that help to form, organise and modify collagen fibres. Glypican 3, encoded by GPC3 (table 1), can bind to and modify the bioactivity of a wide range of growth factors, cytokines, chemokines, BMPs and ligands, such as those in the hedgehog and Wnt signalling pathways. Mutations in GPC3 (*300037) have been associated with Simpson-Golabi-Behmel syndrome (#312870), an X-linked condition characterised by pre- and postnatal overgrowth, coarse facies and other congenital abnormalities. Proteoglycans can also act as receptors for proteases and protease inhibitors in the extracellular matrix, regulating their distribution and activity. In proteoglycans, GAG side chain synthesis occurs on a universal tetrasaccharide linker which is initiated by the attachment of xylose (Xyl) to a serine (Ser) residue in the protein, followed by two galactose (Gal) molecules and an uronic acid – e.g. Ser-Xyl-Gal-Gal-glucuronic acid-heparan sulphate. Mutations in enzymes that catalyse the formation of the tetrasaccharide linker, including xylosyltransferase 1 (*608124), galactosyltransferase 1 (*604327), galactosyltransferase II  (*615291), and glucuronosyltransferase 1 (*606374) have been associated with a spectrum of skeletal and other developmental disorders collectively called linkeropathies. These disorders include Desbuquois dysplasia 2 (#615777), progeroid EDS types 1 and 2 (#130070; #615349) (see Case 19–84) and multiple joint dislocations, short stature, craniofacial dysmorphism and congenital heart defects (previously known as AR Larsen syndrome) (#245600) (table 1). Mutations in EXT1 (*608177) and EXT2 (*608210), which encode enzymes that catalyse the formation of heparan sulphate, are associated with chondrosarcoma (#215300) and hereditary multiple exostoses type 2 (#133701), while mutations in

Osteocytes and Bone Lining Cells

As bone matrix forms, some osteoblasts undergo a terminal differentiation event (fig.  2) and, instead of continuing to produce matrix, they become osteocytes encapsulated in concentric layers within lacunae in the osteoid. The signals that initiate and control terminal osteocytic differentiation are not known. Numerous dendritic cellular processes connect osteocytes to each other through canaliculi both laterally and between cell layers within bone. When osteoblasts stop creating bone, they turn into bone lining cells and remain on the bone surface. The transition from osteoblasts into bone lining cells is poorly understood. Osteocytes and bone lining cells account for the largest proportion of cells in mineralised bone but are probably the least characterised and understood cells of bone. An important development in osteocyte biology has been the identification of sclerostin (*605740). Mutations in this gene cause sclerosteosis (#269500), van Buchem disease type 1 (#239100) and craniodiphyseal dysplasia (#122860) (table 1), which are characterised by progressive bone thickening [24, 25]. Sclerostin binds to a number of BMP growth factors and inhibits BMP-mediated osteoblast differentiation [26]. It also binds directly to LRP5/6, preventing the activation of the Wnt signalling pathway (fig. 3) [18].

Osteoclasts

Osteoclasts are large multinucleate cells found in close apposition to bone surfaces undergoing resorption. Osteoclast precursors share the same haematopoietic lineage as macrophages. One of

50

the most significant breakthroughs in bone biology of the last 20 years has been the identification and characterisation of the molecular pathway controlling osteoclastogenesis [27] (fig. 4). Osteoclast precursors express the receptor for macrophage colony-stimulating factor, which, when stimulated, promotes the expression of a TNF superfamily molecule, RANK (*603499). Osteoblasts control the differentiation of osteoclast precursors through the production of RANK ligand (RANKL) (*602642), a cell surface molecule that is the primary effector of the RANK receptor, and OPG (*602643), a soluble decoy receptor for RANKL. The balance between the RANKL and OPG concentrations regulates RANK activation. The RANK receptor can activate a network of intracellular pathways [27].  Macrophage colony-stimulating factor-/ RANKL-stimulated osteoclast precursors form polykaryons through a process of cell membrane fusion that is poorly understood. RANK activation is also necessary for mature osteoclast activity. When settled onto bone, osteoclasts form a region of tight contact between the cell and the bone surface known as the sealing zone, creating a tightly enclosed area underneath the osteoclast where bone resorption takes place. The cellular membranes within the sealing zone develop into a ruffled border, a structure of deeply folded cellular membranes adjacent to the bone surface, through which acid and proteolytic enzymes are secreted to mediate bone resorption. Defects in genes encoding the molecular pathway effectors controlling acid production (CA2) (*611492) and acid secretion through ion channels (TCIRG1, CLCN7, and OSTM1) (*604592; *602727; *607649) and in genes controlling vesicular trafficking (PLEKHM1 and SNX10) (*611466; *614780) have been identified as frequent causes of osteoclastrich infantile onset osteopetrosis (table 1; fig. 1; chapter 14) (reviewed in [28]). More recently, defects in RANKL (*602642) [29] and RANK (*603499) [30] have been identified in cases of

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small integrin-binding ligand N-linked glycoprotein phosphoproteins to regulate their function. The solubility of calcium phosphate is also highly sensitive to local pH.

c-fms

MCSF OPG RANKL

RANK Osteoclast precursor

Perfusion osteoclasts

Immature osteoclast Mature resorbing osteoclast

Osteoblasts

Bone

Fig. 4. Osteoclast differentiation. Osteoclasts arise from circulating haematopoietic cells of the monocyte/macrophage lineage (osteoclast precursors) that express c-fms, the receptor for macrophage colony-stimulating factor. Macrophage colony-stimulating factor, which is produced by osteoblasts, stimulates the expression of receptor activator of nuclear factor κ-B (RANK) on osteoclast precursors. The key regulator of osteoclastogenesis, RANK ligand (RANKL), is expressed by osteoblasts. RANKL binds to RANK and stimulates the fusion of osteoclast precursors to form multinucleated immature osteoclasts. Mature osteoclasts form a tight seal and generate a ruffled membrane border against the bone surface through which they secrete acid and proteolytic enzymes to form a resorption lacuna. RANKL acts on osteoclasts at all stages of differentiation. Osteoclastogenesis is regulated by osteoblasts through the balance of the production of RANKL and osteoprotegerin, a decoy receptor produced by osteoblasts that inhibits the interaction between RANKL and its receptor RANK.

Hormones and Mineral Homeostasis

PTH, PTH-related peptide and vitamin D are the key hormonal regulators of mineral homeostasis. PTH acts in bone to stimulate the release of calcium and phosphate and simultaneously acts in the kidney to enhance calcium reabsorption and to inhibit phosphate reabsorption. PTH also enhances the renal production of 1,25(OH)2D,

which acts in the intestine to enhance calcium absorption. The molecular mechanisms regulating the intestinal absorption and the renal reabsorption of calcium have recently been identified. Two calcium-selective ion channels, TRPV5 and TRPV6, mediate epithelial transcellular transport of calcium from the intestinal or renal tubular lumen to the extracellular fluid by way of intracellular calcium binding proteins and cell surface ion pumps (reviewed in Nijenhuis et al. [31]). FGF23 was identified as the gene responsible for autosomal dominant hypophosphataemic rickets (table 1) [32]. FGF23–/– mice have hyperphosphataemia, high renal reabsorption of phosphate and high circulating levels of 1,25(OH)2D [33, 34]. The receptor for FGF23 is a heterocomplex of the canonical FGF receptor and klotho [35]. Interestingly, klotho has β-glucuronidase enzymatic ac-

Physiology of Bone Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 33–55 (DOI: 10.1159/000380991)

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infantile onset osteopetrosis in which osteoclasts are completely absent (table  1; fig.  1; Chapter 14). Once bone has been demineralised, the removal of the matrix is mediated by cathepsin K (*601105). Mutations in this gene cause pyknodysostosis (#265800). The ultimate fate of the osteoclast is apoptosis.

The Nervous System and Bone

Although nerves are found throughout the periosteum and near metabolically active parts of bone, until very recently, little thought was given to the potential role of the nervous system in bone. Evidence for nerve fibres that signal through the neuropeptides calcitonin gene-related peptide, vasoactive intestinal peptide and substance P began to emerge in the 1980s (reviewed in Jones et al. [37]). The discovery that GLAST, a glutamate/aspartate transporter molecule previously associated with glutamatergic neuronal signalling [38], is expressed in bone motivated the search for glutamate signalling mechanisms that act on bone formation and resorption. Osteoblasts, osteocytes and osteoclasts have been shown to express ionotropic and metabotropic glutamate receptors, including NMDA receptors, and ion channel-controlled electrical currents consistent with the function of these receptors have been measured in osteoclasts [39]. The inhibitory action of leptin, a hormone involved in controlling body mass, on bone formation has been shown to be mediated by adrenergic signalling resulting from the activity of leptin in the hypothalamus [40, 41].

Functional Adaptation of Bone to Load Bearing

Bones need to be stiff enough to bear the loads to which they are commonly subjected without deforming or breaking under load [42]. Bone

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strength is a function of its stiffness and is dependent on several factors including size, shape and material composition/spatial distribution. The concept of the functional adaptation of the skeleton, which was consolidated into Wolff’s law [43], was developed into the mechanostat hypothesis [2] based on observations that loadbearing vertebrate bones undergo relatively few spontaneous fractures compared to traumatic fractures, indicating that bones adapt their strength to be able to endure typical peak mechanical loads without fracturing while having a sufficiently large safety margin to endure occasional supra-normal loads. In the mechanostat hypothesis, Frost [2] describes a series of rules by which mechanical competence may be achieved and maintained in load-bearing bones (which includes many non-weight bearing bones, e.g. mandibles), providing tissue-level mechanisms for the functional adaptation of the skeleton to the loads placed upon it and presenting predictable hypotheses concerning the mechanical implications of bone disease. The key element is the biological machinery that senses the level of loading and that responds by increasing or decreasing mechanical competence. To date, the nature of the mechanostat in bone is still unknown. Evidence indicates that such machinery is local to the bone under load, since bones respond to local or asymmetrical loads. This phenomenon is typically seen in athletes such as tennis players, in whom bones of the playing arm are preferentially strengthened [44]. A mechanostat model also implies upper and lower thresholds within which bone is sensed to be under normal loading, with loads above or below normal triggering a response to model or remodel the bone. In the 1970s, Clark et al. [45] developed the concept that bone cells respond to the magnitude of the strain experienced by a bone under load (the ratio of change in bone length to its original length). More recently, work in experimental animals and humans has shown that other factors related to strain are also

Grabowski Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 33–55 (DOI: 10.1159/000380991)

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tivity, which is responsible for hydrolysing the external sugar residues of TRPV5, resulting in the trapping and activation of this calcium-selective ion channel at the cell surface [36]. For a more detailed description of these mechanisms, see Chapter 2.

Healthy Bones for Life: Nutrition and Exercise in Bone Physiology

In the last 20–30 years, the impact of bone loss in the ageing population on quality of life and on health service provision has led to the consideration of public policy strategies to maximise bone mass accrual during childhood and adolescence by manipulating the diet or through exercise. Almost half of our adult bone mineral mass is accrued by the skeleton in the 3–4 years following the onset of puberty [49, 50], making adolescence one of the most critical periods of skeletal development. Peak bone mass, achieved during the third decade, is a powerful predictor of postmenopausal osteoporosis [51, 52], and it is widely assumed that optimising peak bone mass accrual during childhood and adolescence will produce bones that are better equipped to handle the inevitable loss of bone in later life. The two simplest ways to influence bone mass

accrual are nutritional intervention and exercise. A recent systematic review of twenty-two controlled trials concluded that weight-bearing exercise in children and adolescents leads to modest increases in bone parameters over 6 months [53]. However, it is difficult to assess which exercise activities are the most appropriate or the time frame within which they should be undertaken. Growth occurs heterogeneously in the skeleton throughout childhood. For example, the longitudinal growth velocity of the legs in infancy is about twice that of the spine until puberty [54]. The benefits of exercise are likely to be site-specific, dependent on the type of exercise (weight bearing, high impact) and influenced by dietary and hormonal factors [55] and even by conditions experienced in utero [54]. The mechanostat theory predicts that bone strength will decrease in response to disuse, but evidence is emerging from studies in former athletes and in animals that skeletal benefits may persist despite the lack of exercise [56]. Well-designed long-term studies are needed to  see if such benefits may be achievable in children.

Conclusions

Advances in understanding the molecular mechanisms and pathways regulating bone cell function provide opportunities for the development of novel, rational approaches to treat disorders of bone cell dysfunction in children. The knowledge gained from the systematic study of bone disorders in children provides insights into the physiology of healthy bones. Improving our understanding of the physiology of bone growth and development during childhood will lead to better prospects for finding early pharmacological, physiotherapeutic and nutritional strategies to optimise and maintain bone health from childhood into adulthood, which, in turn, may help to reduce the burden of bone loss and its related health disadvantages in old age.

Physiology of Bone Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 33–55 (DOI: 10.1159/000380991)

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important, including the strain direction, the rate of strain change, the duration of loading, the number of loading cycles, frequency, repetition and rest within and between cycles [46]. Whether bone cells sense mechanical loads through direct cellular deformation or through shear strains induced by interstitial fluid flow resulting from bone deformation is unclear, but molecular mechanisms capable of strain detection have been described in osteoblasts, osteocytes, osteoclasts and vascular endothelial cells, and ion channels, integrins and associated proteins, connexins, cell surface structures, the cytoskeleton and nitric oxide serve as potential molecular mediators [47, 48]. The mechanostat hypothesis is consistent with the observations that the maternal environment and intrauterine muscular activity influence load-bearing bones up to the time of birth, and this hypothesis accounts for the influence of non-mechanical factors on bone growth, development and maintenance.

References

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24 Balemans W, Ebeling M, Patel N, Van Hul E, Olson P, Dioszegi M, Lacza C, Wuyts W, Van Den Ende J, Willems P, Paes-Alves AF, Hill S, Bueno M, Ramos FJ, Tacconi P, Dikkers FG, Stratakis C, Lindpaintner K, Vickery B, Foernzler D, Van Hul W: Increased bone density in sclerosteosis is due to the deficiency of a novel secreted protein (SOST). Hum Mol Genet 2001;10:537–543. 25 Brunkow ME, Gardner JC, Van Ness J, Paeper BW, Kovacevich BR, Proll S, Skonier JE, Zhao L, Sabo PJ, Fu Y, Alisch RS, Gillett L, Colbert T, Tacconi P, Galas D, Hamersma H, Beighton P, Mulligan J: Bone dysplasia sclerosteosis results from loss of the SOST gene product, a novel cystine knot-containing protein. Am J Hum Genet 2001;68:577–589. 26 Kusu N, Laurikkala J, Imanishi M, Usui H, Konishi M, Miyake A, Thesleff I, Itoh N: Sclerostin is a novel secreted osteoclast-derived bone morphogenetic protein antagonist with unique ligand specificity. J Biol Chem 2003;278: 24113–24117. 27 Boyle WJ, Simonet WS, Lacey DL: Osteoclast differentiation and activation. Nature 2003;423:337–342. 28 Del Fattore A, Cappariello A, Teti A: Genetics, pathogenesis and complications of osteopetrosis. Bone 2008;42: 19–29. 29 Sobacchi C, Frattini A, Guerrini MM, Abinun M, Pangrazio A, Susani L, Bredius R, Mancini G, Cant A, Bishop N, Grabowski P, Del Fattore A, Messina C, Errigo G, Coxon FP, Scott DI, Teti A, Rogers MJ, Vezzoni P, Villa A, Helfrich MH: Osteoclast-poor human osteopetrosis due to mutations in the gene encoding RANKL. Nat Genet 2007;39:960– 962. 30 Guerrini MM, Sobacchi C, Cassani B, Abinun M, Kilic SS, Pangrazio A, Moratto D, Mazzolari E, Clayton-Smith J, Orchard P, Coxon FP, Helfrich MH, Crockett JC, Mellis D, Vellodi A, Tezcan I, Notarangelo LD, Rogers MJ, Vezzoni P, Villa A, Frattini A: Human osteoclastpoor osteopetrosis with hypogammaglobulinemia due to TNFRSF11A (RANK) mutations. Am J Hum Genet 2008;83:64–76. 31 Nijenhuis T, Hoenderop JG, Bindels RJ: TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate. Pflugers Arch 2005;451:181–192.

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1 Frost HM: From Wolff’s law to the Utah paradigm: insights about bone physiology and its clinical applications. Anat Rec 2001;262:398–419. 2 Frost HM: Bone’s mechanostat: a 2003 update. Anat Rec A Discov Mol Cell Evol Biol 2003;275:1081–1101. 3 Jee WS: Principles in bone physiology. J Musculoskelet Neuronal Interact 2000; 1:11–13. 4 Duncan RL, Turner CH: Mechanotransduction and the functional response of bone to mechanical strain. Calcif Tissue Int 1995;57:344–358. 5 Ducy P, Karsenty G: Genetic control of cell differentiation in the skeleton. Curr Opin Cell Biol 1998;10:614–619. 6 Chen D, Zhao M, Mundy GR: Bone morphogenetic proteins. Growth Factors 2004;22:233–241. 7 Xu X, Weinstein M, Li C, Deng C: Fibroblast growth factor receptors (FGFRs) and their roles in limb development. Cell Tissue Res 1999;296:33–43. 8 Iwamoto M, Enomoto-Iwamoto M, Kurisu K: Actions of hedgehog proteins on skeletal cells. Crit Rev Oral Biol Med 1999;10:477–486. 9 Church VL, Francis-West P: Wnt signalling during limb development. Int J Dev Biol 2002;46:927–936. 10 Karaplis AC: Embryonic development of bone and the molecular regulation of intramembranous and endochondral bone formation; in Bilezikian JP, Raisz LG, Rodan GA (eds): Principles of Bone Biology. San Diego, CA, Academic Press, 2002, pp 33–58. 11 Oreffo RO, Cooper C, Mason C, Clements M: Mesenchymal stem cells: lineage, plasticity, and skeletal therapeutic potential. Stem Cell Rev 2005;1:169– 178. 12 Mundlos S, Otto F, Mundlos C, Mulliken JB, Aylsworth AS, Albright S, Lindhout D, Cole WG, Henn W, Knoll JH, Owen MJ, Mertelsmann R, Zabel BU, Olsen BR: Mutations involving the transcription factor CBFA1 cause cleidocranial dysplasia. Cell 1997;89:773–779. 13 Otto F, Kanegane H, Mundlos S: Mutations in the RUNX2 gene in patients with cleidocranial dysplasia. Hum Mutat 2002;19:209–216.

32 Consortium A: Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23. Nat Genet 2000;26:345–348. 33 Shimada T, Kakitani M, Yamazaki Y, Hasegawa H, Takeuchi Y, Fujita T, Fukumoto S, Tomizuka K, Yamashita T: Targeted ablation of Fgf23 demonstrates an essential physiological role of FGF23 in phosphate and vitamin D metabolism. J Clin Invest 2004;113:561–568. 34 Sitara D, Razzaque MS, Hesse M, Yoganathan S, Taguchi T, Erben RG, Juppner H, Lanske B: Homozygous ablation of fibroblast growth factor-23 results in hyperphosphatemia and impaired skeletogenesis, and reverses hypophosphatemia in Phex-deficient mice. Matrix Biol 2004;23:421–432. 35 Urakawa I, Yamazaki Y, Shimada T, Iijima K, Hasegawa H, Okawa K, Fujita T, Fukumoto S, Yamashita T: Klotho converts canonical FGF receptor into a specific receptor for FGF23. Nature 2006;444:770–774. 36 Chang Q, Hoefs S, van der Kemp AW, Topala CN, Bindels RJ, Hoenderop JG: The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel. Science 2005;310:490–493. 37 Jones KB, Mollano AV, Morcuende JA, Cooper RR, Saltzman CL: Bone and brain: a review of neural, hormonal, and musculoskeletal connections. Iowa Orthop J 2004;24:123–132. 38 Mason DJ, Suva LJ, Genever PG, Patton AJ, Steuckle S, Hillam RA, Skerry TM: Mechanically regulated expression of a neural glutamate transporter in bone: a role for excitatory amino acids as osteotropic agents? Bone 1997;20:199–205. 39 Laketic-Ljubojevic I, Suva LJ, Maathuis FJ, Sanders D, Skerry TM: Functional characterization of N-methyl-D-aspartic acid-gated channels in bone cells. Bone 1999;25:631–637.

40 Ducy P, Amling M, Takeda S, Priemel M, Schilling AF, Beil FT, Shen J, Vinson C, Rueger JM, Karsenty G: Leptin inhibits bone formation through a hypothalamic relay: a central control of bone mass. Cell 2000;100:197–207. 41 Takeda S, Elefteriou F, Levasseur R, Liu X, Zhao L, Parker KL, Armstrong D, Ducy P, Karsenty G: Leptin regulates bone formation via the sympathetic nervous system. Cell 2002;111:305–317. 42 Currey JD: Bones: Structure and Mechanics. Princeton, NJ, Princeton University Press, 2002. 43 Wolff J: Das Gesetz der Transformation der Knochen. Berlin, Hirschwald, 1892. 44 Haapasalo H, Kontulainen S, Sievanen H, Kannus P, Jarvinen M, Vuori I: Exercise-induced bone gain is due to enlargement in bone size without a change in volumetric bone density: a peripheral quantitative computed tomography study of the upper arms of male tennis players. Bone 2000;27:351–357. 45 Clark EA, Goodship AE, Lanyon LE: Locomotor bone strain as the stimulus for bone’s mechanical adaptability. J Physiol 1975;245:57P. 46 Skerry TM: One mechanostat or many? Modifications of the site-specific response of bone to mechanical loading by nature and nurture. J Musculoskelet Neuronal Interact 2006;6:122–127. 47 Malone AM, Anderson CT, Tummala P, Kwon RY, Johnston TR, Stearns T, Jacobs CR: Primary cilia mediate mechanosensing in bone cells by a calciumindependent mechanism. Proc Natl Acad Sci U S A 2007;104:13325–13330. 48 Rubin J, Rubin C, Jacobs CR: Molecular pathways mediating mechanical signaling in bone. Gene 2006;367:1–16. 49 Bailey DA: The Saskatchewan Pediatric Bone Mineral Accrual Study: bone mineral acquisition during the growing years. Int J Sports Med 1997;18(suppl 3): S191–S194.

50 Bailey DA, Martin AD, McKay HA, Whiting S, Mirwald R: Calcium accretion in girls and boys during puberty: a longitudinal analysis. J Bone Miner Res 2000;15:2245–2250. 51 Hui SL, Slemenda CW, Johnston CC Jr: The contribution of bone loss to postmenopausal osteoporosis. Osteoporos Int 1990;1:30–34. 52 Seeman E: Reduced bone density in women with fractures: contribution of low peak bone density and rapid bone loss. Osteoporos Int 1994;4(suppl 1): 15–25. 53 Hind K, Burrows M: Weight-bearing exercise and bone mineral accrual in children and adolescents: a review of controlled trials. Bone 2007;40:14–27. 54 Cooper C, Westlake S, Harvey N, Javaid K, Dennison E, Hanson M: Review: developmental origins of osteoporotic fracture. Osteoporos Int 2006;17:337– 347. 55 Loud KJ, Gordon CM: Adolescent bone health. Arch Pediatr Adolesc Med 2006; 160:1026–1032. 56 Ducher G, Bass SL: Exercise during growth: compelling evidence for the primary prevention of osteoporosis? BoneKEy – Osteovision 2007;4:171–180. 57 Gray KA, Yates B, Seal RL, Wright MW, Bruford EA: Genenames.org: the HGNC resources in 2015. Nucleic Acids Res 2015;43:D1079–D1085. 58 Online Mendelian inheritance in man. https://www.ncbi.nlm.nih.gov/omim (accessed January 10, 2015). 59 Warman ML, Cormier-Daire V, Hall C, Krakow D, Lachman R, LeMerrer M, Mortier G, Mundlos S, Nishimura G, Rimoin DL, Robertson S, Savarirayan R, Sillence D, Spranger J, Unger S, Zabel B, Superti-Furga A: Nosology and classification of genetic skeletal disorders: 2010 revision. Am J Med Genet A 2011;155a: 943–968.

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Peter Grabowski Faculty of Medicine, Dentistry and Health Academic Unit of Human Nutrition, University of Sheffield Sheffield S10 2RX (UK) E-Mail [email protected]

Chapter 4 Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 56–71 (DOI: 10.1159/000380992)

Radiology of Osteogenesis Imperfecta, Rickets and Other Bony Fragility States Alistair D. Calder Radiology Department, Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK

This section gives an overview of radiological findings in bony fragility states, with a special focus on osteogenesis imperfecta (OI) and rickets. Conventional radiological assessment of bone density is inaccurate and imprecise and only reliably detects severe osteopaenia. However, other aspects of bone structure and morphology can be assessed, and it is possible to distinguish between osteopaenic and osteomalacic states. OI is a heterogeneous group of disorders of type 1 collagen formation and processing that are characterised by varying degrees of bony fragility, with presentations varying from perinatal lethality to asymptomatic. Radiological diagnosis of severe forms is usually straightforward, but that of milder disease may be challenging because specific features are often absent. However, a multidisciplinary approach is usually successful. Features of OI, including Wormian bones, skull base deformities, vertebral involvement and long bone fractures and deformities, are reviewed in this section. Rickets is best defined as a disorder of the growth plate characterised by the impaired apoptosis of hypertrophied chondrocytes. Vitamin D deficiency is a com-

mon cause of rickets. The patho-anatomical basis of radiological findings in rickets is reviewed and illustrated. Rickets is frequently accompanied by hyperparathyroidism and osteomalacia. Rickets used to be classified as calciopaenic or phosphopaenic but is now referred to as parathyroid hormone or fibroblast growth factor 23 mediated, respectively [1]. The radiological features of the two forms are reviewed. © 2015 S. Karger AG, Basel

Introduction

Recognising that a child presenting with fractures or skeletal deformities has bony fragility is of critical importance, both in terms of directing correct medical management and avoiding incorrect and damaging allegations of child abuse. Once a diagnosis of a bony fragility state, such as osteogenesis imperfecta (OI), has been made, radiological follow-up plays a central role in longterm management. This section describes the radiological assessment of bone density and quality Downloaded by: Chulalongkorn University 161.200.145.19 - 7/31/2019 7:41:41 AM

Abstract

Radiological Assessment of Bone Density and Quality

Assessing Bone Density on Plain Radiographs Assessment of bone density in children using radiographs is unreliable. Quantitative measurements of bone density from planar images are not possible due to the superposition of varying softtissue thicknesses and differing sizes of bones; thus, assessment is based on the subjective impressions of a radiologist. Studies of adults have indicated that radiologists show poor accuracy and reproducibility in bone density assessments compared with objective standards (such as dual-energy X-ray absorptiometry) for all but severe cases of osteopaenia/ osteoporosis [2]. A study of the detection of osteopaenia in children by radiologists has reported sensitivity and specificity as low as 50 and 76%, respectively, with poor inter-observer agreement [3]. Whilst image quality has improved with the introduction of digital imaging systems, this technology does not improve the detection of reduced bone mineral density and may make it more difficult because automatic post-processing may mask differences in bone density [4]. Thus, a radiological report of reduced bone density carries strong merit as evidence of bony fragility only in severe cases, and conversely, the absence of clear demineralisation does not exclude such a state. For example, in mild cases of OI, although dual-energy X-ray absorptiometry values are usually below the mean for age, they are often well within two standard deviations, and the radiological detection of such levels of osteopaenia is unlikely to be reliable. Some aspects of bone quality observed on radiographic projections are better suited for a quantitative, more reproducible approach, e.g. cortical

thickness, trabecular structure and other geometric analyses [5]. Although there is some evidence of the usefulness of these parameters in adults, they are yet to be usefully applied to children. The Royal College of Radiologists and the Royal College of Paediatrics and Child Health recommend that radiologists should explicitly comment on bone density for all children presenting with recurrent, multiple or suspicious fractures [6]. However, these assessments have severe limitations. Assessment of Bone ‘Quality’ There is more to the radiological assessment of bone than the attempted evaluation of bone density. Bone shape, size, proportions and other factors are also considered. In the context of bony fragility, a distinction can be made between osteopaenic (and osteoporotic) states, characterised by a reduction in the quantity of bone tissue (the thickness of the cortex and the number and thickness of the trabeculae) and osteomalacic states, characterised by the reduced mineralisation of bone tissue (with preservation of quantity), usually related to reduced serum mineral ion concentrations [7]. OI exemplifies osteopaenia because cortices and trabeculae may appear thin, but the radiographic densities of these structures are preserved. Further, hypophosphataemic rickets exemplifies osteomalacia because the quantity of trabecular bone is preserved, but it has an abnormal coarse appearance. Finally, it is also important to identify evidence of hyperparathyroidism manifested by increased osteoclastic bone resorption, typically in subperiosteal, endosteal and intracortical locations.

Radiological Findings in Osteogenesis Imperfecta

OI is a heterogeneous group of disorders of type 1 collagen formation and processing characterised by osteopaenia and bony fragility of varying severity.

Radiology of Bony Fragility States Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 56–71 (DOI: 10.1159/000380992)

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(including its severe limitations) and radiological findings in OI, rickets and other bony fragility states.

Fig. 1. AP and lateral whole body radiographs of 20 week gestation fetus terminated for suspected lethal skeletal dysplasia with osteogenesis imperfecta type 2a. There is crumpling and angulation of the long bones, extensive fracturing of the ribs producing a beaded appearance and a very poorly ossified skull.

Lethal Osteogenesis Imperfecta (OI Type II) The most common pattern of lethal OI, OI type IIa, is characterised by severe skeletal deformity with contiguous fractures of the ribs and long bones, resulting in the thickening and beading of the ribs, the thickening and shortening of the long bones and bowing deformities. The skull is poorly ossified and deforms easily (fig. 1). OI IIb has a milder but still lethal phenotype, characterised by bowing deformities and discrete rather than continuous fractures, and it overlaps phenotypically with severe (type III) OI. OI type IIc patients exhibit slender, paradoxically dense bones with multiple fractures, and the skull vault appears completely unossified and collapses easily. Severe Osteogenesis Imperfecta (Type III) Severe OI is usually diagnosed at birth and is often suspected antenatally. The bones are usually unequivocally osteopaenic in appearance. Long bone fractures and/or bowing deformities are

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typically present at birth. Multiple rib fractures may be present from birth, particularly following vaginal delivery. The skull is poorly ossified and often demonstrates an abnormal Wormian bone (WB) pattern. The spine is often normal initially, but multiple compression fractures may be seen during infancy, even before upright posture is achieved. Mild to Moderate Osteogenesis Imperfecta (Types I and IV) Mild and moderate forms of OI may present a diagnostic challenge based on the radiological features alone; fortunately, suggestive non-radiological findings are usually present, including blue sclerae, skin laxity, facial dysmorphism, positive family history and short stature. Consequently, the diagnosis of mild or moderate OI should be considered a multi-disciplinary exercise, and the involvement of experienced clinical personnel is important. A radiologist alone cannot exclude mild or moderate OI. These patients typically present with low-trauma fractures, usu-

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Overview of Features [8]

Fig. 2. AP skull radiograph in 9 month old female with severe osteogenesis imperfecta, with a confirmed mutation of COL1A2. Large fontanelle with expanded lambdoid sutures, filled with a mosaic of numerous Wormian bones.

Specific Features of Osteogenesis Imperfecta Wormian Bones

WBs are ossicles within the cranial sutures. They are a feature of OI as well as several other disor-

ders. A few WB may be found in the skulls of normal individuals, particularly in the lambdoid sutures [9]. It is therefore important to establish whether WBs are present in an abnormal pattern. Cremin et al. have defined an abnormal WB pattern as that consisting of ten or more WBs of greater than 6 × 4 mm in maximal orthogonal measurements arranged in a general mosaic pattern [10] (fig. 2). Although the number ten was chosen arbitrarily for this study, experimental data exist in support of this cut-off; for example, in a study of over 600 cranial CTs of skeletally normal individuals, none had more than eight WBs [9]. Cremin et al. have suggested that an abnormal WB pattern is nearly universal in OI. A more recent study has suggested that this not the case, particularly in mild disease. Only 35% of patients with type 1 OI exhibit an abnormal WB pattern, compared with 78% of those with type IV and 96% of those with type III disease [11].

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ally after the child starts to mobilise independently, and the presentation of multiple fractures in non-mobile infants is rare. Other than index fractures, skeletal radiography is sometimes normal, particularly for younger children. Unequivocal osteopaenia is often not present. Soft signs may be present, such as slender long bone diaphyses, slight bowing of the long bones, thin cortices or a mildly poorly ossified skull; however, in isolation, these signs are insufficient to definitively determine the diagnosis. The presence of an abnormal WB pattern in the context of low-trauma fractures is highly suggestive of OI, but more often than not, this sign is not present.

Anterior cranial base angle Lines

Skull Base Deformities Deformity of the skull base is an important, potentially lethal and difficult to treat complication of OI. Three distinct skull base disorders are defined as follows (fig. 3): Platybasia This disorder is characterised by the flattening of the anterior skull base. The floor of the anterior cranial fossa and the clivus form an angle that is more obtuse than normal. The presence of this disorder is determined according to the anterior cranial base angle, between a line connecting the nasion (the base of the nasal bone) and the centre of the pituitary fossa and a line from the centre of the pituitary fossa to the basion (the basal tip of the clivus and the anterior margin of the foramen magnum). An angle of greater than 140 degrees is considered abnormal. Basilar Impression This disorder reflects the settling of the skull base around the foramen magnum, such that the most caudal border of the skull falls below the level of the tip of the odontoid process. Its presence can

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Chamberlain’s MacGregor’s

McRae’s

be determined by measuring Macgregor’s line, drawn from the posterior nasal spine (the back of the hard palate) to the most caudal aspect of the skull base. Extension of the tip of odontoid process above this line by more than 5 mm indicates basilar impression. This measurement varies with age. Age-specific cut-offs are available, as well as age-independent measurements [12]. Basilar Invagination This disorder is characterised by the migration of the odontoid process into the foramen magnum, and it may result in cervico-medullary neural compression. It is defined by extension of the tip of the odontoid process above a line drawn between the anterior and posterior borders of the foramen magnum (McRae’s line) (fig. 4). Platybasia is present in 15–20% of individuals with OI, and basilar impression and basilar invagination affect approximately 5%. Skull base deformities are more prevalent in patients with more severe disease and are predicted by short stature, abnormal WBs and dentinogenesis imperfecta [12, 13]. However, these deformities may

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Fig. 3. Midline sagittal image of T1 weighted cranial MRI examination of 13 year old male with osteogenesis imperfecta type 4, illustrating positioning of lines for measurement of cranial base deformities.

Fig. 4. Lateral skull radiograph and midline sagittal T1W cranial MRI in 13 year old male with osteogenesis imperfecta type 4, showing platybasia and basilar invagination. There is cervicomedullary kinking.

Spinal Disease in Osteogenesis Imperfecta

Fractures and malalignment of the spine are major contributors to morbidity in OI. Spinal Fractures The majority of patients with even the mildest forms of OI demonstrate at least one vertebral fracture [14]. In mild disease, fractures occur only with upright posture, but in severe disease, they may be present in infancy or even from birth. Fractures most commonly demonstrate the anterior or biconcave wedging of the vertebral body. The most common fracture sites are the 7th and 8th thoracic vertebral bodies [14]. A degree of anterior wedging is normal, particularly in the mid-thoracic spine. The differentiation of normal wedging from mild fracturing at these sites can be difficult. By convention, a reduction in the height of the anterior vertebral body by >20% of the posterior height is considered indicative of fracture (fig. 5). Because vertebral body fractures are common, often oligosymptomatic and difficult to localise clinically, radiographic screening of the spine is a part of the regular assessment of children with OI.

Spondylolisthesis This disorder is common in children with OI and may result from fractures of the pars interarticularis/spondylolysis (fig.  5), particularly at L5, or from abnormal elongation of the pedicles, typically in the context of lumbosacral hyperlordosis [15]. Its prevalence is around 20% of all patients with OI [16]. Patients may be asymptomatic, but when symptomatic or when progressive slip is demonstrated, surgical fixation may be considered. Scoliosis and Other Alignment Disorders Scoliosis affects approximately 20% of patients with mild OI but more than 70% of those with the severe form of the disease [17]. It is indicated by a Cobb angle of 10 degrees or greater on frontal radiography. Kyphosis, typically affecting the thoracic spine, is seen in 10–20% of children with severe disease [17] and is defined by a Cobb angle on lateral projection of greater than 40 degrees. Although kyphoscoliosis may be caused by vertebral body deformities resulting from fractures, these alignment disorders also result from ligamentous laxity, and scoliosis may be seen in the absence of compression fractures (fig. 6). A delay in motor development resulting from ligamentous laxity is a predictor of the development of scoliosis [17].

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affect even those with mild disease; thus, screening via imaging (radiography, cephalometry or MRI) is required.

Fig. 6. AP radiograph of thoracic and lumbar spine in 23 month old male with severe OI and missense mutation in COL1A1. There is severe left convexity thoraco-lumbar scoliosis, with no evidence of vertebral body deformity or fracture. There is also a right hemidiaphragmatic eventration (arrow).

Long Bone Involvement

spiral fracture of the tibia/fibula [14]. Although fractures in mild OI most commonly begin after mobilisation, isolated long bone fractures may occur with normal handling in infancy, and OI should always be considered in such cases. In patients with more severe disease, fractures may occur spontaneously and involve a greater variety of sites. Apophyseal avulsion fractures are more common in children with OI and occur at unusual sites. Avulsion fractures of the olecranon, often occurring bilaterally, represent a rare presentation that strongly suggests this disease [18] (fig. 7).

Fractures Long bone fractures are common in OI. Even children with the mildest forms of OI experience a 100-fold greater frequency of fracturing than the healthy population, with a rate of 0.62 long bone fractures per patient year [14]. Fractures in patients with mild disease are similar to those seen in normal children but occur following lower levels of trauma or at a greater frequency. The most common long bone fracture in mild OI is a

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Fig. 5. Lateral and AP radiographs of the thoracic and lumbar spine in 4 year old male with severe osteogenesis imperfecta, with mutation confirmed in COL1A2. There is extensive vertebral body height loss with anterior and biconcave wedge compression fractures. There is spondylolysis of L5 (arrow), with mild L5/S1 spondylolisthesis.

Deformities Long bone deformity is a typical feature of children with severe OI, in whom multiple deformities of this type are typically present at birth. Deformity may result from the malunion of a single fracture, resulting in angular deformity. However, bowing also occurs. OI is an osteopaenic rather than an osteomalacic disorder. The bones are brittle not soft, and bowing reflects multiple microfractures. Bowing affects the lower limbs more frequently than the upper limbs; however, bowing deformities of the upper limbs can result in severe functional impairment and should be promptly evaluated because severe upper limb bowing may be impossible to correct.

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Fig. 7. Lateral radiograph of the right elbow in a 19 month old female, demonstrating an avulsion fracture of the olecranon. This was the presenting feature. A COL1A1 mutation was later confirmed.

Distinguishing Osteogenesis Imperfecta from Non-Accidental Injury A non-mobile infant presenting with fractures rightly prompts concerns regarding child protection. However, such a presentation may also indicate the presence of a disorder involving bony fragility, such as OI. Children with OI and their families are occasionally the subject of a child protection investigation, which may proceed as far as family and criminal courts before a diagnosis is established; such experiences can be damaging to families and their relationships with medical professionals. Alternatively, undiagnosed OI may be offered as a defence against allegations of non-accidental injury. In cases of possible non-accidental injury involving fractures, paediatricians and radiologists should explicitly consider the possibility of a bony fragility state. In the great majority of cases, severe OI should not be difficult to distinguish from abusive trauma because in the former, fractures and skeletal deformities are likely to be present from birth, the bones usually appear unequivocally osteopaenic and the skull is poorly ossified with an abnormal WB pattern. Cases involving milder disease may be more challenging, at least for the radiologist, because osteopaenia may be absent, and when present, it is hard to detect radiographically. Further, an abnormal WB pattern is usually not present. However, particular clinical features, such as a positive family history and scleral discolouration in particular, will be present in most cases, and such features must be actively sought. There is some evidence that mild OI rarely presents with fracture patterns typically described in abusive skeletal trauma; for example, rib fractures, which occur in severe OI, are thought to be highly unusual in non-mobile infants with milder disease, only appearing once the children are old enough to sustain falls [19]. Classical metaphyseal lesions also represent an unusual presenting feature of OI. Presentation of mild OI with simultaneous multiple fractures prior to weight bearing is rare [19]. However, the evidence base has not been sufficiently developed to allow for

increase in bone mass. Their cyclical use causes the deposition of bands of dense bone within the metaphyses, resulting in a transverse striped appearance, with regularly spaced stripes according to the treatment regimen. These metaphyseal bands, which are sometimes referred to as zebra lines, resemble growth arrest lines but are distinct because they are denser and traverse the full width of the metaphysis (growth arrest lines are usually partial). As inhibitors of osteoclasis, bisphosphonates may also result in undermodelling of the metaphyses [20] (fig. 8).

categorical statements on these matters, and bony fragility states should still be considered, even in apparently classical cases of abuse. Finally, children with bony fragility states may also be victims of physical abuse. These rare cases are particularly challenging, for example, for estimating the level of violence that caused the injuries observed. Treatment Effects Many children with OI receive long-term intravenous therapy with bisphosphonates. Bisphosphonates inhibit osteoclast function, resulting in an

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Rickets and Related Disorders

Rickets is typically defined as a bone disease of children resulting from vitamin D deficiency. However, vitamin D deficiency is neither necessary (as shown by genetic forms of rickets and those occurring due to dietary calcium deficiency)

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Fig. 8. DP Radiograph of left hand in 15 year old female with osteogenesis imperfecta type IV, on intravenous bisphosphonate therapy for many years. There are regularly spaced dense transverse metaphyseal lines (‘Zebra lines’) and undermodelling of the distal radius.

Radiology and Classification of Osteogenesis Imperfecta The classification of OI into 4 types by Sillence et al. remains the core classification system for this disorder. However, advances in the understanding of its molecular basis have led to refinements in this system, with the addition of a several types corresponding with distinct phenotypes and genotypes [8, 21] (see Chapter 7 for a more detailed discussion). Radiological findings are sometimes helpful in establishing the likely presence of some of these rare forms. OI type 5 in particular has very characteristic features on imaging, for example, fracture healing, typically accompanied by hyperplastic callus formation, in addition to ossification of the forearm interosseous membrane, with a flange-like projection extending into the membrane from the radius and/ or ulna (with or without radial head dislocation) [22] (see case 19–63). Table 1 outlines key radiological findings within the current classification of OI.

Table 1. Expanded classification of osteogenesis imperfecta, with clinical and radiological phenotypic features Inheritance

Sillence type

Gene

Clinical phenotype

Radiological features

Autosomal dominant

I

COL1A1

Mild. Blue sclerae. DI rare.

Low-trauma fractures. Osteopaenia: may not be apparent in infancy. Most do not have AWBP.

II

COL1A1/A2

Lethal.

Extensive fracturing results in shortened, thickened bones. Very poor skull ossification.

III

COL1A1/A2

Severe. DI in most.

Fractures and deformities usually present from birth. Usually have AWBP.

IV

COL1A1/A2

Moderate. Grey or white sclerae. With DI (IV A) or without DI (IV B).

Low-trauma fractures. Osteopaenia may not be apparent in infancy. Usually have AWBP.

V

IFITM5

Moderate to severe. No DI.

Hyperplastic callus formation. Interosseous membrane calcification. AWBP.

VI

SERPINF1

Moderate to severe. No DI. Grey or white sclerae. Short stature.

Fractures in infancy. ‘Popcorn’ epiphyseal and metaphyseal ossifications and bulbous metaphyses. No AWBP.

VII

CRTAP

Severe to lethal. No DI or laxity. Slightly blue sclerae.

Rhizomelic brachymelia and short stature. AWBP. Fractures present from birth. ‘Popcorn’ epiphyseal and metaphyseal ossifications.

VIII

LEPRE1

Severe to lethal. White sclerae. No DI.

Severe osteopaenia. Fractures present at birth. Bulbous metaphyses. AWBP.

IX

PPIB

Moderate to lethal. No DI.

Shortened, bowed long bones. Thin, beaded ribs. AWBP.

X

SERPINH1

Severe. Triangular face with micrognathia. Blue sclerae. DI.

Short, bowed long bones. Fractures at birth. No AWBP.

XI

FKB10

Moderate to severe. Grey to white sclerae. DI.

Severe vertebral involvement. Severe acetabular protrusion in later life. Contractures in some (‘Bruck 1’). No AWBP.

XII

SP7

Moderate. Mid-face hypoplasia and micrognathia. No DI. White sclerae.

Fractures from infancy. AWBP.

XIII

BMP1

Severe. Characteristic facial features. No DI. Umbilical hernias.

Fractures at birth. AWBP.

XIV

TMEM38B

Mild to severe. No DI. White sclerae

Fractures at birth or during early childhood.

XV

WNT1

Mild to severe. Short stature. Blue sclerae. No DI. Developmental delay.

Fractures and bowing of extremities. Brain malformations.

Bruck 2

PLOD2

Moderate to severe.

Contractures. Similar to OI XI. AWBP.

Autosomal recessive

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DI = Dentinogenesis imperfecta; AWBP = abnormal Wormian bone pattern.

Normal growth plate

In presence of low phosphate: rickets

Trabeculae Metaphyseal spur (bone ‘bark’) Primary spongiosa Phosphate-mediated apoptosis

Accumulation of hypertrophic chondrocytes causes splaying, fraying and cuping of metaphysis, with irregular widening of the physis

Hypertrophic layer Proliferative layer Germinal layer

Epiphysis

Fig. 9. Schematic representation of the patho-anatomical basis of rickets, alongside a radiograph of the left ankle in a 2 year old with severe vitamin D deficiency rickets.

Basis for Radiological Findings in Vitamin D-Deficient Rickets

The most important effect of vitamin D in terms of bone health is on calcium transport in the gut [23]. In the presence of low vitamin D and inadequate compensation by dietary calcium intake, serum calcium falls. This decrease drives hyperparathyroidism which has direct effects on bone, as well as down-regulating the expression of renal phosphate transporters, resulting in hypophosphataemia. The drivers of bone disease in vitamin D deficiency are thus hypophosphataemia, hyperparathyroidism and reduced mineral

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ion concentrations, which are responsible for radiological changes of rickets, hyperparathyroidism and osteomalacia, respectively. Although the metabolic pathways of the activities of vitamin D are complex, the final common pathway for all forms of rickets is reduced phosphate levels within the growth plate [1]. Reduced phosphate causes a delay in apoptosis of hypertrophied chondrocytes; consequently, non-apoptosed hypertrophied chondrocytes accumulate, with the delayed formation and mineralisation of primary spongiosa. Chondrocytes accumulate both longitudinally, causing the widening of the growth plate, and axially, resulting in the splaying of the growth plate and metaphysis. The haphazard accumulation causes irregular defects in ossification of the metaphysis, resulting in fraying. A peripheral rim of bone in the metaphysis forms by membranous ossification of the perichondrium, which is sometimes termed the bone bark. Bone bark ossification is less affected in rickets than endochondrally formed bone, resulting in exaggerated metaphyseal spurring and contrib-

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nor sufficient (as shown by many children with vitamin D deficiency without rickets) for rickets to develop. Rickets can alternatively be defined as a disorder of the growth plate characterised by impaired apoptosis of hypertrophic chondrocytes. It is frequently accompanied by (or may be preceded by) hyperparathyroidism and osteomalacia.

Table 2. Components of hypovitaminosis D osteopathy and radiological features Component

Mechanisms

Radiological features

Hypophosphataemia

Inhibition of apoptosis of hypertrophic chondrocytes.

Splaying, fraying and widening of the growth plate.

Hyperparathyroidism

Increased osteoclast activity and bone turnover, causing hypophosphatemia.

Resorption at subperiosteal, endosteal, intracortical and submetaphyseal sites.

Reduced mineral ion concentrations

Reduced availability of calcium and phosphate for mineralisation of osteoid.

Coarsening of trabecular pattern, poorly defined cortices, bone softening resulting in bowing, and rarely, the presence of Looser zones.

Direct vitamin D effects

Impaired osteoblast number and activity. Direct effects on growth plate.

May contribute to osteopaenia and growth plate widening.

Hyperparathyroidism and Osteomalacia In vitamin D deficiency, the hypophosphataemia required for rickets to develop is dependent on hyperparathyroidism. Hyperparathyroidism has direct effects on bone that may precede or predominate over the appearance of rickets, particularly in rare cases presenting perinatally. When hyperparathyroidism predominates, periosteal new bone formation may be seen; more typically, this is only observed after the treatment of vitamin D deficiency.

Osteomalacia is sometimes considered the adult form of rickets. This is a misleading oversimplification because it occurs equally in children, with only the absence of growth plate changes differentiating adult manifestations of vitamin D deficiency. Osteomalacia refers to the impaired mineralisation of osteoid during bone formation and remodelling [7]. It reflects the reduced availability of mineral ions for the formation of hydroxyapatite crystals. Radiologically, there is some overlap with features of hyperparathyroidism (and they frequently co-exist), with generalised demineralisation and intracortical tunnelling. Poorly mineralised osteoid adjacent to trabeculae results in a coarse appearance of the trabecular pattern. Sites of high bone turnover exhibit greater degrees of demineralisation because these sites produce more osteoid and are typically sites of stress, such as the femoral neck. Focal lucencies in these regions in osteomalacia are termed ‘Looser zones’ or ‘pseudofractures’. Although the findings of studies using animal models have suggested that only the effects of vitamin D deficiency on the gut are important for the development of rickets, it may have some direct effects on bone, including growth plate wid-

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uting to cupping. A schematic representation of the patho-anatomical basis of rickets is shown in figure 9. The radiological changes of rickets are dependent on time and growth. If growth is impaired, rachitic features may not be present. Rachitic features are most pronounced at sites of the most rapid growth, such as the distal radius and ulna and the growth plates around the knee. The distal ulna is particularly rapid-growing (because its proximal physis contributes little to longitudinal growth); thus, it is a sensitive site for rickets detection but may show a degree of cupping as a normal finding.

Rickets and Bony Fragility In severe cases, the combination of these features result in fragile bones, with susceptibility to insufficiency fractures and bowing. Insufficiency fractures commonly occur in the metaphysis at locations more proximal than those of the classic metaphyseal lesions seen in abusive skeletal injury [25]. Bowing deformities may reflect diaphyseal bowing of soft bone or angulation through the expanded growth plates themselves. Bowing is typically varus after weight bearing. Prior to weight bearing, the tibia may exhibit anteriorly convex bowing, reflecting the asymmetric pull of the Achilles tendon. Whether milder cases of rickets and vitamin D deficiency without rickets also represent bony fragility states, particularly in nonmobile infants, is a matter of some controversy [26]. Signs of Rickets in the Skull Some authors suggest that the softening of the skull (craniotabes) is an early indicator of vitamin D deficiency [27]; however, others maintain that this is a non-specific sign present in normal infants [28]. It has also been proposed that radiographic signs in the skull are sensitive early markers for vitamin D-related bone disease, such as the loss of the lamina dura of the teeth and demineralisation adjacent to sutural margins [29]. These signs have not been adequately validated in systematic studies. With chronic vitamin D deficiency and osteomalacia, skull base deformities, including platybasia, basilar impression and basilar invagination, may rarely occur. Craniosynostosis may complicate rickets, more typically in X-linked hypophosphataemic rickets but occasionally in nutritional rickets.

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Other Types of Rickets

Rickets used to be classified as either calciopaenic or phosphopaenic, according to whether the primary disorder was related to a lack of calcium or phosphate. More recently, this condition has been reclassified as either parathyroid hormone (PTH) or fibroblast growth factor 23 (FGF23)-dependent, respectively [1], although all rickets is ultimately dependent on hypophosphataemia, as we have seen. Parathyroid Hormone-Dependent (Calciopaenic) Rickets In some areas of the world (including Bangladesh, Nigeria and Lesotho), rickets due to dietary calcium deficiency is more common than that due to vitamin D deficiency, and its radiological appearance is similar to that of vitamin D deficiency rickets [30]. PTH-dependent rickets may also occur in the presence of hepatobiliary disease and malabsorption due to defective vitamin D absorption. There are two major autosomal recessive forms of PTH-dependent rickets, which are confusingly termed vitamin D-dependent rickets type 1 (defects in 1 alpha-hydroxylase) and vitamin D-dependent rickets type 2 (defects in vitamin D receptor). Both of these forms manifest severe rickets, which develops during the first few months after birth, but rickets is not present at birth [31]. Fibroblast Growth Factor 23-Dependent (Phosphopaenic) Rickets The most common form of phosphopaenic rickets is X-linked dominant hypophosphataemia (XLH), which is a disorder of PHEX (phosphate-regulating gene with homologies to endopeptidase on the X chromosome), in which, through an unknown mechanism, there are excessive levels of FGF23 [32]. FGF23 is the principal regulator of phosphate. Increased FGF23 activity down-regulates renal phosphate trans-

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ening and impaired osteoblast functioning, which may contribute to osteopaenia [24]. The skeletal consequences of vitamin D deficiency may be collectively termed hypovitaminosis D osteopathy and are summarised in table 2.

malacia. This increase occurs with a variety of often small, mesenchymal tumours that secrete FGF23 or an analogue. Fibrous dysplastic lesions and some cutaneous disorders (such as linear sebaceous nevus syndrome) may also cause this presentation. The rachitic changes and osteomalacia can be very severe in these disorders. Renal tubular disorders, such as Fanconi syndrome, may also result in hyperphosphaturia and rickets.

porters, with consequent hyperphosphaturia and hypophosphataemia. A second effect is the reduced renal expression of 1 alpha-hydroxylase, potentially resulting in reduced vitamin D function, and most patients with XLH require treatment with activated vitamin D metabolites. Low phosphate in these patients results in rachitic changes, which tend to be mild, and also in osteomalacia, typically resulting in varus bowing deformities. If varus is present, the rachitic changes are more apparent on the medial aspects of the physes, reflecting increased load bearing at these sites. Because calcium levels are typically normal, hyperparathyroidism is usually absent. The diaphyses in XLH are distinct in appearance compared with those in PTH-dependent rickets as a result, and osteomalacia, but not osteopaenia, are observed, such that the number of trabeculae is preserved (fig.  10). Looser zones may be present in older individuals. Rare autosomal dominant and autosomal recessive forms of hypophosphataemic rickets also occur and tend to be milder with a later onset [32]. Excess FGF23 causes most forms of paraneoplastic rickets or tumour-induced rickets/osteo-

Radiology Findings in Other Bony Fragility States

Bony fragility is a feature of a number of inherited and acquired conditions in childhood. The pathophysiological basis and radiological findings of some of these are summarized in table 3.

Conclusions

Although radiological evaluation of bone density is limited, morphological evaluation of the skeleton via radiography plays a key role in the assessment of children with a known or suspected bony fragility state. Diagnosis of severe OI is usually possible by radiological evaluation alone. However, the diagnosis of milder disease and its distinction from non-accidental injury may be more challenging. Radiological screening for complications in the skull and spine is part of the regular assessment of children with established OI. The radiological features of rickets can be easily understood in the light of the modern understanding of pathophysiological mechanisms. It is useful to distinguish the growth plate changes of rickets from frequently (but not always) accompanying features of hyperparathyroidism and osteomalacia. These three features vary according to the aetiology of rickets.

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Fig. 10. AP radiograph of the knees in a 10 year old male with X-linked hypophosphataemic rickets. There is only mild widening of the growth plate and mildly irregular metaphyseal ossification. The trabecular pattern is abnormally coarsened.

Table 3. Pathophysiology and radiological features of selected other bony fragility states Pathophysiology

Radiological findings

Metabolic bone disease of prematurity [33]

Affects preterm (3.5 mmol/l. There is, however, considerable variability between children with similarly high 25OHD levels in terms of the manifestation of symptoms of hypercalcaemia. Genetic predisposition to sensitivity to vitamin D due to mutations in CYP24A1 and other genes that regulate the synthesis and regulation of vitamin D may account for some of this variability. 25OHD has a strong affinity for vitamin Dbinding protein, which extends the half-life of 25OHD by 2–3 weeks. 25OHD is also lipophilic

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and can be stored in adipose tissue for months, so vitamin D intoxication may take weeks to resolve and may require prolonged treatment. By contrast, hypercalcaemia secondary to ingestion of shorter-acting vitamin D analogues (alfacalcidol and calcitriol) usually lasts 1–2 days because the shorter half-life, and thus, discontinuing calcitriol treatment and hydration with IV saline will often suffice. Treatment of vitamin D intoxication is with IV hydration with normal saline combined with a loop diuretic; glucocorticoids, such as prednisolone, have also been used. An alternative strategy is to use bisphosphonates (IV pamidronate or oral alendronate), and repeated doses may be required. In refractory or life-threatening cases, haemodialysis may be used to lower serum calcium levels acutely [47]. Other Drugs Vitamin A toxicity may also cause hypercalcaemia [48, 49]. Vitamin A analogues such as cisretinoic acid are used as a treatment for acne, and other retinoic acids are used in the treatment of certain malignancies. The mechanism of hypercalcaemia is unclear, but vitamin A has direct effects on stimulating bone resorption. Thiazide diuretics may cause hypercalcaemia, particularly in those with PHPT, by increasing renal tubular calcium reabsorption.

Granulomatous Disorders

Subcutaneous Fat Necrosis Newborns who develop subcutaneous fat necrosis (SCFN) usually are healthy at birth but may have predisposing factors, including obstetric trauma, meconium aspiration, asphyxia, hypothermia, hypoxia or prior use of therapeutic cooling. Within the first few weeks of life, hard, indurated, violaceous nodules and plaques with ill-defined overlying erythema develop on the trunk, arms, buttocks, thighs, or cheeks, which may be

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ty. Serum PTH and 1,25(OH)2D levels are suppressed, and hypercalcaemia and hypercalciuria may resolve with the onset of weight bearing. Bisphosphonates and calcitonin have been used to treat immobilisation-associated hypercalcaemia [44] (see Case 19–30).

Other Granulomatous Disorders Sarcoidosis and other granulomatous diseases such as tuberculosis can occasionally give rise to hypercalcaemia. The mechanism is thought to be similar to that of SCFN. Gestational Maternal Hypocalcaemia If infants present with persistent hypercalcaemia, one of the differential diagnoses to be considered is maternal hypocalcaemia. Investigation of neonatal hypercalcaemia should include a measurement of plasma calcium in both parents. Chronic maternal hypocalcaemia (from undiagnosed or under-treated hypoparathyroidism or pseudohypoparathyroidism) may cause secondary foetal hyperparathyroidism from reduced maternofoetal calcium transfer in order to maintain adequate plasma calcium levels in utero. Transient neonatal hypercalcaemia may occur in the infant, which may take several weeks to settle and does not require surgical intervention.

Treatment of Hypercalcaemia

Treatment of hypercalcaemia is aimed at both lowering the serum calcium concentration and correcting the underlying disease. General measures include minimising the calcium concentration in enteral and parenteral feeds and discontinuing oral calcium supplements and drugs known to cause hypercalcaemia. Weight-bearing activity should be increased, and, when relevant, sedatives should be withdrawn to promote mobility.

Increased Urinary Calcium Excretion Filtered calcium is principally reabsorbed in the proximal tubule and the ascending loop of Henle. This process is mainly passive, whereas active resorption of calcium occurs in the distal loop under the influence of PTH and, to a lesser degree, 1,25(OH)2D. During hypercalcaemia, urinary calcium excretion can be increased by inhibiting proximal and loop sodium reabsorption, thus reducing passive reabsorption of calcium. Proximal reabsorption is inhibited by volume expansion via IV saline infusion, which increases the delivery of sodium, calcium and water to the loop of Henle, and administration of a loop diuretic such as furosemide then blocks transport at this site. The majority of children with severe hypercalcaemia have volume contraction due to both decreased fluid intake and the natriuretic effects of hypercalcaemia. Loop diuretics must be used with caution in the long term, as they will increase the chances of nephrocalcinosis development. Reduced PTH Secretion In cases where excess PTH secretion is a cause of the hypercalcaemia, it may be possible to reduce secretion of the hormone. In neonatal hyperparathyroidism, the calcimimetic agent cinacalcet reduces PTH secretion sufficiently to allow the hy-

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painful. Histological findings show a panniculitis. The pathophysiology is likely secondary to excessive endogenous synthesis of 1,25(OH)2D, which is supported by the finding that skin biopsies taken from affected areas showed copious expression of 1-α hydroxylase in the inflammatory infiltrate [50]. Presentation is often with symptoms of hypercalcaemia, and especially hypotonia due to myopathy, and fever and eosinophilia may also be initial features [51]. The calcium levels are often very high. There is a high incidence of persistent nephrocalcinosis (∼80%) that may be present for years, but without evidence of renal insufficiency [51]. Initial treatment is with rehydration and glucocorticoids. Pamidronate may be considered if the hypercalcaemia is refractory [52, 53]. Lowcalcium formulae may be required for weeks following the initial treatment to normalise the calcium level and prevent rebound hypercalcaemia. Hypercalcaemia may persist for as long as 1–2 years.

perparathyroidism to settle without recourse to parathyroidectomy, particularly if the CaSR mutation is heterozygous. Similarly, in chronic renal failure, cinacalcet may help to reduce PTH secretion and reverse the hyperparathyroidism. At the same time, it is important to ensure that there is no vitamin D deficiency; treatment of this may also help to reverse the high PTH level. Decreased Intestinal Calcium Absorption If hypercalcaemia is caused by raised levels of 1,25(OH)2D, some effect can be achieved by administration of glucocorticoids, which reduce the conversion of 25OHD to its active metabolite, 1,25(OH)2D. Glucocorticoids have been used to treat hypercalcaemia in granulomatous conditions such as tuberculosis, sarcoidosis and SCFN. Inhibition of Bone Resorption Bisphosphonates Bisphosphonates are extremely effective in children with moderate to severe hypercalcaemia [40]. Pamidronate is the drug of choice in children, even in those with renal failure. It is given at a dose of 0.5–1.0 mg/kg as an infusion over 4–6 hours. A reduction in calcium is observed 12–24 hours after administration and may last for 2–4 weeks. A sustained period of hypocalcaemia may follow the initial administration of pamidronate, and additional calcium supplementation may be required. Calcitonin In the acute situation, calcitonin injections can be effective in reducing plasma calcium. The effect is rapid but wears off after a while (see Chapter 17 for further details).

Dialysis

Parathyroidectomy

Surgery is required for PHPT and should be undertaken by an experienced paediatric endocrine surgeon. NSHPT requires total parathyroidectomy, which renders the child hypoparathyroid. The decision to operate for milder forms depends on the degree of hypercalcaemia, the symptoms and the potential for renal damage. Pamidronate may be required preoperatively in those with severe or symptomatic hypercalcaemia. Rapid perioperative PTH measurements can be undertaken to determine whether or not parathyroidectomy has been effective. Following parathyroidectomy, maintenance calcium and vitamin D replacement should be commenced. Immediately following surgery, hypocalcaemia secondary to a ‘hungry bone’ syndrome may develop, despite maintenance treatment. Serial calcium measurements should therefore be taken following surgery, and additional calcium supplementation should be given if appropriate. If severe hypocalcaemia develops, IV calcium may be required.

Summary and Conclusions

Hypercalcaemia is a relatively rare problem in childhood when compared with hypocalcaemia. Nevertheless, it remains an important problem that requires proper assessment and diagnosis before correct treatment can be given. The spectrum of disease is much wider than that found in adults, less specifically emphasising hyperparathyroidism and malignancy and encompassing a wide range of conditions, many of them genetic in origin. Once a correct diagnosis has been made, it is usually possible to offer effective treatment.

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In resistant, life-threatening hypercalcaemia, haemodialysis against a low-calcium dialysate is more effective in lowering calcium levels than peritoneal dialysis is.

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30 Pronicka E, Rowinska E, Kulczycka H, Lukaszkiewicz J, Lorenc R, Janas R: Persistent hypercalciuria and elevated 25-hydroxyvitamin D3 in children with infantile hypercalcaemia. Pediatr Nephrol 1997;11:2–6. 31 Huang J, Coman D, McTaggart SJ, Burke JR: Long-term follow-up of patients with idiopathic infantile hypercalcaemia. Pediatr Nephrol 2006;21:1676–1680. 32 Udwin O, Yule W, Martin ND: Age at diagnosis and abilities in idiopathic hypercalcaemia. Arch Dis Child 1986;61: 1164–1167. 33 Schlingmann KP, Kaufmann M, Weber S, Irwin A, Goos C, John U, Misselwitz J, Klaus G, Kuwertz-Broking E, Fehrenbach H, Wingen AM, Guran T, Hoenderop JG, Bindels RJ, Prosser DE, Jones G, Konrad M: Mutations in CYP24A1 and idiopathic infantile hypercalcemia. N Engl J Med 2011;365:410–421. 34 Dauber A, Nguyen TT, Sochett E, Cole DE, Horst R, Abrams SA, Carpenter TO, Hirschhorn JN: Genetic defect in CYP24A1, the vitamin D 24-hydroxylase gene, in a patient with severe infantile hypercalcemia. J Clin Endocrinol Metab 2012;97:E268–E274. 35 Masuda S, Byford V, Arabian A, Sakai Y, Demay MB, St-Arnaud R, Jones G: Altered pharmacokinetics of 1alpha,25dihydroxyvitamin D3 and 25-hydroxyvitamin D3 in the blood and tissues of the 25-hydroxyvitamin D-24-hydroxylase (Cyp24a1) null mouse. Endocrinology 2005;146:825–834. 36 Jacobs TP, Bilezikian JP: Clinical review: rare causes of hypercalcemia. J Clin Endocrinol Metab 2005;90:6316–6322. 37 Tebben PJ, Milliner DS, Horst RL, Harris PC, Singh RJ, Wu Y, Foreman JW, Chelminski PR, Kumar R: Hypercalcemia, hypercalciuria, and elevated calcitriol concentrations with autosomal dominant transmission due to CYP24A1 mutations: effects of ketoconazole therapy. J Clin Endocrinol Metab 2012; 97:E423–E427.

38 Fencl F, Blahova K, Schlingmann KP, Konrad M, Seeman T: Severe hypercalcemic crisis in an infant with idiopathic infantile hypercalcemia caused by mutation in CYP24A1 gene. Eur J Pediatr 2013;172:45–49. 39 Stewart AF: Clinical practice. Hypercalcemia associated with cancer. N Engl J Med 2005;352:373–379. 40 Niizuma H, Fujii K, Sato A, Fujiwara I, Takeyama J, Imaizumi M: PTHrP-independent hypercalcemia with increased proinflammatory cytokines and bone resorption in two children with CD19negative precursor B acute lymphoblastic leukemia. Pediatr Blood Cancer 2007; 49:990–993. 41 Shimonodan H, Nagayama J, Nagatoshi Y, Hatanaka M, Takada A, Iguchi H, Oda Y, Okamura J: Acute lymphocytic leukemia in adolescence with multiple osteolytic lesions and hypercalcemia mediated by lymphoblast-producing parathyroid hormone-related peptide: a case report and review of the literature. Pediatr Blood Cancer 2005;45:333–339. 42 Dharmaraj P, Ball S, Johnstone H, Bailey S, Clifford SC, Hale J, Cheetham TD: Hypercalcaemia in relapsed medulloblastoma 8 years post-diagnosis; evidence to support PTHrP production by medulloblastoma cells. Horm Res 2006; 66:268–272. 43 Lakhdir F, Lawson D, Schatz DA: Fatal parathyroid hormone-related proteininduced humoral hypercalcemia of malignancy in a 3-month-old infant. Eur J Pediatr 1994;153:718–720. 44 Go T: Low-dose oral etidronate therapy for immobilization hypercalcaemia associated with Guillain-Barre syndrome. Acta Paediatr 2001;90:1202–1204.

45 Kara C, Gunindi F, Ustyol A, Aydin M: Vitamin D intoxication due to an erroneously manufactured dietary supplement in seven children. Pediatrics 2014; 133:e240–e244. 46 Vanstone MB, Oberfield SE, Shader L, Ardeshirpour L, Carpenter TO: Hypercalcemia in children receiving pharmacologic doses of vitamin D. Pediatrics 2012;129:e1060–e1063. 47 Vogiatzi MG, Jacobson-Dickman E, DeBoer MD: Vitamin D supplementation and risk of toxicity in pediatrics: a review of current literature. J Clin Endocrinol Metab 2014;99:1132–1141. 48 Kimmoun A, Leheup B, Feillet F, Dubois F, Morali A: [Hypercalcemia revealing iatrogenic hypervitaminosis A in a child with autistic troubles]. Arch Pediatr 2008;15:29–32. 49 Nagasawa M, Okawa H: All-trans retinoic acid induced hypercalcemia in a patient with acute promyelocytic leukemia: its relation to increased PTH-rP. Int J Hematol 1994;59:143–144. 50 Farooque A, Moss C, Zehnder D, Hewison M, Shaw NJ: Expression of 25-hydroxyvitamin D(3)-1alpha-hydroxylase in subcutaneous fat necrosis. Br J Dermatol 2009;160:423–425. 51 Shumer DE, Thaker V, Taylor GA, Wassner AJ: Severe hypercalcaemia due to subcutaneous fat necrosis: presentation, management and complications. Arch Dis Child Fetal Neonatal Ed 2014; 99:F419–F421. 52 Alos N, Eugene D, Fillion M, Powell J, Kokta V, Chabot G: Pamidronate: treatment for severe hypercalcemia in neonatal subcutaneous fat necrosis. Horm Res 2006;65:289–294. 53 Lombardi G, Cabano R, Bollani L, Del Forno C, Stronati M: Effectiveness of pamidronate in severe neonatal hypercalcemia caused by subcutaneous fat necrosis: a case report. Eur J Pediatr 2008;168:625–627.

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Dr. Justin H. Davies, MB BCh, MRCP, FRCPCH, MD Department of Endocrinology, Southampton Children’s Hospital Tremona Road Southampton SO16 6YD (UK) E-Mail [email protected]

Chapter 8 Allgrove J, Shaw NJ (eds): Calcium and Bone Disorders in Children and Adolescents. 2nd, revised edition. Endocr Dev. Basel, Karger, 2015, vol 28, pp 119–133 (DOI: 10.1159/000381000)

A Practical Approach to Vitamin D Deficiency and Rickets Jeremy Allgrove a · Nick J. Shaw b b Department

of Paediatric Endocrinology, Royal London Hospital, Whitechapel, London, and of Endocrinology and Diabetes, Birmingham Children’s Hospital, Birmingham, UK

Abstract Rickets is a condition in which there is failure of the normal mineralisation (osteomalacia) of growing bone. Whilst osteomalacia may be present in adults, rickets cannot occur. It is generally caused by a lack of mineral supply, which can either occur as a result of the deficiency of calcium (calciopaenic rickets, now known as parathyroid hormone-dependent rickets) or of phosphate (phosphopaenic rickets, now called FGF23-dependent rickets). Renal disorders may also interfere with the process of mineralisation and cause rickets. Only parathyroid hormone-dependent rickets and distal renal tubular disorders will be discussed in this chapter. The most common cause of rickets is still vitamin D deficiency, which is also responsible for other problems. Disorders of vitamin D metabolism or responsiveness may also cause similar issues. Distal renal tubular acidosis may also be caused by a variety of metabolic errors similar to those of osteoclasts. One form of distal renal tubular acidosis also causes a type of osteopetrosis. This chapter describes these conditions in detail and sets out a logical approach for treatment. © 2015 S. Karger AG, Basel

Introduction

Osteomalacia is a condition in which mineralisation of osteoid tissue fails to occur normally, usually because of a deficiency in the supply of mineral, calcium or phosphate for some reason. Although osteomalacia can occur in any individual, rickets can only occur in the presence of unfused epiphyses because it manifests in the growth plate. Therefore, adults cannot suffer from rickets but may develop osteomalacia. There are two principal causes of rickets, which have traditionally been referred to as calciopaenic and phosphopaenic. In addition, a third group of conditions, related to renal tubular disorders, also cause this disorder. Recently, a new classification of rickets has been proposed [1], in which calciopaenic rickets is now regarded as parathyroid hormone (PTH)-dependent. Phosphopaenic rickets will not be considered in this chapter because it is discussed elsewhere (See Chapter 9). This section will deal only with the various forms of calciopaenic and renal rickets. Downloaded by: Chulalongkorn University 161.200.145.19 - 7/31/2019 7:43:17 AM

a Department

For a detailed discussion of the physiology of vitamin D, see Chapter 2. Briefly, vitamin D is a secosteroid that is derived principally in the form of cholecalciferol by the action of ultraviolet light on dehydrocholesterol. It then undergoes two metabolic steps, involving a first hydroxylation at the 25-position to form 25OHD and a second hydroxylation at the 1α-position of the steroid molecule to form the active metabolite, 1,25(OH)2D. Following this activation, 1,25(OH)2D acts on classical steroid receptors by participating in ligand binding, followed by transfer to the nucleus and subsequent DNA binding. Defects in any of these processes can give rise to rickets, and a thorough understanding of the various processes is required to be able to make a correct diagnosis so that appropriate treatment can be instigated.

Vitamin D Deficiency

Definition of Vitamin D Deficiency There have been several recent consensus statements or guidelines that have included definitions of vitamin D deficiency. It is generally agreed that the serum concentration of 25OHD is the best marker of an individual’s vitamin D status because it is the major circulating form and reflects the combination of dietary intake and cutaneous skin synthesis. However, different thresholds for the level of 25OHD that is considered to reflect deficiency are used. For example, the Institute of Medicine’s report on the dietary reference intake for vitamin D published in 2010 [2] defined a level of 50 nmol/l as meeting the needs of 97.5% of the population, whereas the Endocrine Society Clinical Practice Guideline published in 2011 defined vitamin D deficiency as a level