Presentation and outcome in carriers of pathogenic variants in SLC34A1 and SLC34A3 encoding sodium-phosphate transporter NPT 2a and 2c.

Pathogenic variants in SLC34A1 and SLC34A3 encoding sodium-phosphate transporter 2a and 2c are rare causes of phosphate wasting. Since data on presentation and outcomes are scarce, we collected clinical, biochemical and genetic data via an online questionnaire and the support of European professional organizations. One hundred thirteen patients (86% children) from 90 families and 17 countries with pathogenic or likely pathogenic variants in SLC34A1 or SLC34A3 and a median follow-up of three years were analyzed. Biallelic SLC34A1 variant carriers showed polyuria, failure to thrive, vomiting, constipation, hypercalcemia and nephrocalcinosis in infancy, while biallelic SLC34A3 carriers presented in childhood or even adulthood with rickets/osteomalacia and/or osteopenia/osteoporosis, hypophosphatemia and, less frequently, nephrocalcinosis, while the prevalences of kidney stones were comparable. Adult biallelic SLC34A3 carriers had a six-fold increase chronic kidney disease (CKD) prevalence compared to the general population. All biallelic variant carriers shared a common biochemical pattern including elevated 1,25(OH)2D and alkaline phosphatase levels, suppressed parathyroid hormone (PTH), and hypercalciuria. Heterozygous carriers showed similar but less pronounced phenotypes. In biallelic SLC34A1 carriers, an attenuation of clinical features was observed after infancy, independent of treatment. Phosphate treatment was given in 55% of patients, median duration two years, and resulted in significant reduction, although not normalization, of alkaline phosphatase and of hypercalciuria but an increase in PTH levels, while 1,25(OH)2D levels remained elevated. Thus, our study indicates that biallelic SLC34A1 and SLC34A3 carriers show distinct, albeit overlapping phenotypes, with the latter having an increased risk of CKD in adulthood. Phosphate treatment may promote kidney phosphate loss and enhance 1,25(OH)2D synthesis via increased PTH production.

COPB2 loss of function causes a coatopathy with osteoporosis and developmental delay.

Coatomer complexes function in the sorting and trafficking of proteins between subcellular organelles. Pathogenic variants in coatomer subunits or associated factors have been reported in multi-systemic disorders, i.e., coatopathies, that can affect the skeletal and central nervous systems. We have identified loss-of-function variants in COPB2, a component of the coatomer complex I (COPI), in individuals presenting with osteoporosis, fractures, and developmental delay of variable severity. Electron microscopy of COPB2-deficient subjects' fibroblasts showed dilated endoplasmic reticulum (ER) with granular material, prominent rough ER, and vacuoles, consistent with an intracellular trafficking defect. We studied the effect of COPB2 deficiency on collagen trafficking because of the critical role of collagen secretion in bone biology. COPB2 siRNA-treated fibroblasts showed delayed collagen secretion with retention of type I collagen in the ER and Golgi and altered distribution of Golgi markers. copb2-null zebrafish embryos showed retention of type II collagen, disorganization of the ER and Golgi, and early larval lethality. Copb2+/- mice exhibited low bone mass, and consistent with the findings in human cells and zebrafish, studies in Copb2+/- mouse fibroblasts suggest ER stress and a Golgi defect. Interestingly, ascorbic acid treatment partially rescued the zebrafish developmental phenotype and the cellular phenotype in Copb2+/- mouse fibroblasts. This work identifies a form of coatopathy due to COPB2 haploinsufficiency, explores a potential therapeutic approach for this disorder, and highlights the role of the COPI complex as a regulator of skeletal homeostasis.

Use of insulin pump therapy in children and adolescents with type 1 diabetes and its impact on metabolic control: comparison of results from three large, transatlantic paediatric registries.

AIMS/HYPOTHESIS: While the use of insulin pumps in paediatrics has expanded dramatically, there is still considerable variability among countries in the use of pump technology. The present study sought to describe differences in metabolic control and pump use in young people with type 1 diabetes using data collected in three multicentre registries. METHODS: Data for the years 2011 and 2012 from 54,410 children and adolescents were collected from the Prospective Diabetes Follow-up Registry (DPV; n = 26,198), T1D Exchange (T1DX; n = 13,755) and the National Paediatric Diabetes Audit (NPDA; n = 14,457). The modality of insulin delivery, based on age, sex and ethnic minority status, and the impact of pump use on HbA1c levels were compared. RESULTS: The overall mean HbA1c level was higher in the NPDA (8.9 ± 1.6% [74 ± 17.5 mmol/mol]) than in the DPV (8.0 ± 1.6% [64 ± 17.0 mmol/mol], p < 0.001) and T1DX (8.3 ± 1.4% [68 ± 15.4 mmol/mol], p < 0.001). Conversely, pump use was much lower in the NPDA (14%) than in the DPV (41%, p < 0.001) and T1DX (47%, p < 0.001). In a pooled analysis, pump use was associated with a lower mean HbA1c (pump: 8.0 ± 1.2% [64 ± 13.3 mmol/mol] vs injection: 8.5 ± 1.7% [69 ± 18.7 mmol/mol], p < 0.001). In all three registries, those with an ethnic minority status were less likely to be treated with a pump (p < 0.001) and boys were treated with a pump less often compared with girls (p < 0.001). CONCLUSIONS/INTERPRETATION: Despite similar clinical characteristics and proportion of minority participants, substantial differences in metabolic control exist across the three large transatlantic registries of paediatric patients with type 1 diabetes, which appears to be due in part to the frequency of insulin pump therapy.

Towards a safety net for management of 22q11.2 deletion syndrome: guidelines for our times.

UNLABELLED: The commonest autosomal deletion, 22q11.2 deletion syndrome (22q11DS) is a multisystem disorder varying greatly in severity and age of identification between affected individuals. Holistic care is best served by a multidisciplinary team, with an anticipatory approach. Priorities tend to change with age, from feeding difficulties, infections and surgery of congenital abnormalities particularly of the heart and velopharynx in infancy and early childhood to longer-term communication, learning, behavioural and mental health difficulties best served by evaluation at intervals to consider and initiate management. Regular monitoring of growth, endocrine status, haematological and immune function to enable early intervention helps in maintaining health. CONCLUSION: Guidelines to best practice management of 22q11DS based on a literature review and consensus have been developed by a national group of professionals with consideration of the limitations of available medical and educational resources.

Eiken syndrome with parathyroid hormone resistance due to a novel parathyroid hormone receptor type 1 mutation: clinical features and functional analysis.

We report on 2 patients of East African ancestry with the same novel homozygous variant in the parathyroid hormone receptor type 1 (PTH1R). Both patients shared skeletal features, including brachydactyly, extensive metacarpal pseudo-epiphyses, elongated cone-shaped epiphyses, ischiopubic hypoplasia, and deficient sacral ossification, suggestive of Eiken syndrome. Strikingly, both patients exhibited clinically manifest parathyroid hormone (PTH) resistance with hypocalcemia and elevated serum phosphate levels. These laboratory and clinical abnormalities initially suggested pseudohypoparathyroidism, which is typically associated with GNAS abnormalities. In both patients, however, a homozygous novel PTH1R variant was identified (c.710 T > A; p.IIe237Asn, p.I237N) that is located in the second transmembrane helical domain. Previously, others have reported a patient with a nearby PTH1R mutation (D241E) who presented with similar clinical features (eg, delayed bone mineralization as well as clinical PTH resistance). Functional analysis of the effects of both novel PTH1R variants (I237N- and D241E-PTH1R) in HEK293 reporter cells transfected with plasmid DNA encoding the wild-type or mutant PTH1Rs demonstrated increased basal cAMP signaling for both variants, with relative blunting of responses to both PTH and PTH-related peptide (PTHrP) ligands. The clinical presentation of PTH resistance and delayed bone mineralization combined with the functional properties of the mutant PTH1Rs suggest that this form of Eiken syndrome results from alterations in PTH1R-mediated signaling in response to both canonical ligands, PTH and PTHrP.

Eiken syndrome is rare genetic disorder of skeletal development, 7 due to alterations in the gene for parathyroid hormone receptor type 1 (PTH1R). This receptor can bind 2 hormones: parathyroid hormone (PTH), the body's main regulator of the level of calcium in blood, and PTH-related peptide (PTHrP), that regulates bone development. We report 2 new cases of Eiken syndrome sharing the same not previously reported change in PTH1R. The patients had typical findings in the skeleton reported in previous cases, but with some variation in the features. Unlike previously reported cases, the 2 patients we describe had low blood calcium levels causing symptoms. We wanted to explore how this new mutation affects the function of the receptor, particularly how it might affect the signals generated when the receptor binds to its 2 t hormones, PTH and PTHrP. We genetically reprogrammed a cell line with the new mutation, and tested those cells' responses to stimulation by the hormones. We showed that the altered receptor appears unable to bind both hormones in a stable fashion, explaining why the patients showed changes both in the skeleton (due to altered PTHrP signaling) and in the blood level of calcium (due to altered PTH signaling).

GNAQ/GNA11 Mosaicism Is Associated with Abnormal Serum Calcium Indices and Microvascular Neurocalcification.

Mosaic mutations in genes GNAQ or GNA11 lead to a spectrum of diseases including Sturge-Weber syndrome and phakomatosis pigmentovascularis with dermal melanocytosis. The pathognomonic finding of localized "tramlining" on plain skull radiography, representing medium-sized neurovascular calcification and associated with postnatal neurological deterioration, led us to study calcium metabolism in a cohort of 42 children. In this study, we find that 74% of patients had at least one abnormal measurement of calcium metabolism, the commonest being moderately low serum ionized calcium (41%) or high parathyroid hormone (17%). Lower levels of ionized calcium even within the normal range were significantly associated with seizures, and with specific antiepileptics despite normal vitamin D levels. Successive measurements documented substantial intrapersonal fluctuation in indices over time, and DEXA scans were normal in patients with hypocalcemia. Neurohistology from epilepsy surgery in five patients revealed not only intravascular, but perivascular and intraparenchymal mineral deposition and intraparenchymal microvascular disease in addition to previously reported findings. Neuroradiology review clearly demonstrated progressive calcium deposition in individuals over time. These findings and those of the adjoining paper suggest that calcium deposition in the brain of patients with GNAQ/GNA11 mosaicism may not be a nonspecific sign of damage as was previously thought, but may instead reflect the central postnatal pathological process in this disease spectrum.

Burosumab in management of X-linked hypophosphataemia: a retrospective cohort study of growth and serum phosphate levels.

BACKGROUND: Burosumab, an antifibroblast growth factor 23 monoclonal antibody, improves rickets severity, symptoms and growth in children with X-linked hypophosphataemia (XLH) followed up to 64 weeks in clinical trials. International dosing guidance recommends targeting normal serum phosphate concentration; however, some children may not achieve this despite maximal dosing. This study compares clinical outcomes in children with XLH on long-term burosumab treatment who achieved normal phosphate versus those who did not. METHODS: Single-centre retrospective review of a large paediatric cohort with XLH treated with burosumab. We evaluated growth and biochemical markers of bone health in those who did compared with those who did not achieve normal plasma phosphate concentration. RESULTS: Fifty-five children with XLH with median age of 11.7 (IQR 6.8-15.5) years were included. 27 (49%) had low plasma phosphate concentration, and 27 (49%) had normal phosphate after a median burosumab treatment duration of 3.3 (IQR 2.6-3.7) years. 1 (2%) did not have a recent phosphate level recorded. No difference in growth was found between normal and abnormal phosphate groups (p=0.9). CONCLUSIONS: Young children with XLH experience sustained growth on long-term burosumab treatment, although without normal plasma phosphate concentration in many. Consideration should be made to changing burosumab dosing recommendations to target normalisation of alkaline phosphatase, as opposed to plasma phosphate concentration.

Identification and characterization of novel parathyroid-specific transcription factor Glial Cells Missing Homolog B (GCMB) mutations in eight families with autosomal recessive hypoparathyroidism.

GCMB is a member of the small transcription factor family GCM (glial cells missing), which are important regulators of development, present in vertebrates and some invertebrates. In man, GCMB encodes a 506 amino acid parathyroid gland-specific protein, mutations of which have been reported to cause both autosomal dominant and autosomal recessive hypoparathyroidism. We ascertained 18 affected individuals from 12 families with autosomal recessive hypoparathyroidism and have investigated them for GCMB abnormalities. Four different homozygous germline mutations were identified in eight families that originate from the Indian Subcontinent. These consisted of a novel nonsense mutation R39X; a missense mutation, R47L in two families; a novel missense mutation, R110W; and a novel frameshifting deletion, I298fsX307 in four families. Haplotype analysis, using polymorphic microsatellites from chromosome 6p23-24, revealed that R47L and I298fsX307 mutations arose either as ancient founders, or recurrent de novo mutations. Functional studies including: subcellular localization studies, EMSAs and luciferase-reporter assays, were undertaken and these demonstrated that: the R39X mutant failed to localize to the nucleus; the R47L and R110W mutants both lost DNA-binding ability; and the I298fsX307 mutant had reduced transactivational ability. In order to gain further insights, we undertook 3D-modeling of the GCMB DNA-binding domain, which revealed that the R110 residue is likely important for the structural integrity of helix 2, which forms part of the GCMB/DNA binding interface. Thus, our results, which expand the spectrum of hypoparathyroidism-associated GCMB mutations, help elucidate the molecular mechanisms underlying DNA-binding and transactivation that are required for this parathyroid-specific transcription factor.

Good practice recommendations on paediatric training programmes for health care professionals in the EU.

Part of the SWEET Project: EU (European Union), Better Control in Paediatric and Adolescent Diabetes: Working to Create Centres of Reference, was specifically to examine the training of health care professionals (HCPs) across the EU. Several types of information were collected during 2009, and these included a literature search, workshops of the SWEET members, examination of the data collected by the Hvidøre Study Group and the Diabetes Attitudes, Wishes, and Needs (DAWN) Youth initiative, and a questionnaire distributed to SWEET members and professional colleagues who cared for children and young people (CYP) with diabetes. It was clear from the information collected that there was no European or global consensus either on a curriculum for the training of the paediatric diabetes multidisciplinary team (MDT) or individual professions in paediatric diabetes. A minority of countries had well-established training but, for the majority, there was little standardisation or accreditation. Moreover, most countries did not have available courses for training the diabetes MDT and training was not mandatory. Of the courses that were available more were accredited for doctors and nurses but fewer for the other professions. As a consequence, the majority of HCP posts in paediatric diabetes do not demand prior experience in the specialty. Standardised accredited training and continuous professional development (CPD) opportunities are severely limited. The SWEET Project supports a standardised, accredited approach to training and CPD of the MDT and for individual professions. As a consequence, a curriculum for the training of the MDT was developed, and this is now ready for implementation.

Non-collagen pathogenic variants resulting in the osteogenesis imperfecta phenotype in children: a single-country observational cohort study.

BACKGROUND/OBJECTIVES: In England, children (0-18 years) with severe, complex and atypical osteogenesis imperfecta (OI) are managed by four centres (Birmingham, Bristol, London, Sheffield) in a 'Highly Specialised Service' (HSS OI); affected children with a genetic origin for their disease that is not in COL1A1 or COL1A2 form the majority of the 'atypical' group, which has set criteria for entry into the service. We have used the data from the service to assess the range and frequency of non-collagen pathogenic variants resulting in OI in a single country. METHODS: Children with atypical OI were identified through the HSS OI service database. All genetic testing for children with OI in the service were undertaken at the Sheffield Diagnostic Genetics Service. Variant data were extracted and matched to individual patients. This study was done as part of a service evaluation project registered with the Sheffield Children's Hospital Clinical Governance Department. RESULTS: One hundred of 337 children in the HSS met the 'atypical' criteria. Eighty have had genetic testing undertaken; 72 had genetic changes detected, 67 in 13 genes known to be causative for OI. The most frequently affected genes were IFITM5 (22), P3H1 (12), SERPINF1 (8) and BMP1 (6). CONCLUSION: Among children with more severe forms of OI (approximately one-third of all children with OI), around 20% have pathogenic variants in non-collagen genes. IFITM5 was the most commonly affected gene, followed by genes within the P3H1 complex. These data provide additional information regarding the likelihood of different genetic origins of the disease in children with OI, which may influence clinical care.

Endocrine Diseases of Newborn: Epidemiology, Pathogenesis, Therapeutic Options, and Outcome "Current Insights Into Disorders of Calcium and Phosphate in the Newborn".

The physiology and regulation of bone minerals in the fetus and the newborn is significantly different from children and adults. The bone minerals calcium, phosphate and magnesium are all maintained at higher concentrations in utero to achieve adequate bone accretion. This is an integral component of normal fetal development which facilitates safe neonatal transition to post-natal life. When deciphering the cause of bone mineral disorders in newborns, the potential differential diagnosis list is broad and complex, including several extremely rare conditions. Also, significant discoveries including new embryological molecular genetic transcription factors, the role of active placental mineral transport, and hormone regulation factors have changed the understanding of calcium and phosphate homeostasis in the fetus and the newborn. This article will guide clinicians through an updated review of calcium and phosphate physiology, then review specific conditions pertinent to successful neonatal care. Furthermore, with the advancement of increasingly rapid molecular genetic testing, genomics will continue to play a greater role in this area of fetal diagnostics and prognostication.

Genetic Analysis of Pediatric Primary Adrenal Insufficiency of Unknown Etiology: 25 Years' Experience in the UK.

CONTEXT: Although primary adrenal insufficiency (PAI) in children and young people is often due to congenital adrenal hyperplasia (CAH) or autoimmunity, other genetic causes occur. The relative prevalence of these conditions is poorly understood. OBJECTIVE: We investigated genetic causes of PAI in children and young people over a 25 year period. DESIGN SETTING AND PARTICIPANTS: Unpublished and published data were reviewed for 155 young people in the United Kingdom who underwent genetic analysis for PAI of unknown etiology in three major research centers between 1993 and 2018. We pre-excluded those with CAH, autoimmune, or metabolic causes. We obtained additional data from NR0B1 (DAX-1) clinical testing centers. INTERVENTION AND OUTCOME MEASUREMENTS: Genetic analysis involved a candidate gene approach (1993 onward) or next generation sequencing (NGS; targeted panels, exomes) (2013-2018). RESULTS: A genetic diagnosis was reached in 103/155 (66.5%) individuals. In 5 children the adrenal insufficiency resolved and no genetic cause was found. Pathogenic variants occurred in 11 genes: MC2R (adrenocorticotropin receptor; 30/155, 19.4%), NR0B1 (DAX-1; 7.7%), CYP11A1 (7.7%), AAAS (7.1%), NNT (6.5%), MRAP (4.5%), TXNRD2 (4.5%), STAR (3.9%), SAMD9 (3.2%), CDKN1C (1.3%), and NR5A1/steroidogenic factor-1 (SF-1; 0.6%). Additionally, 51 boys had NR0B1 variants identified through clinical testing. Although age at presentation, treatment, ancestral background, and birthweight can provide diagnostic clues, genetic testing was often needed to define the cause. CONCLUSIONS: PAI in children and young people often has a genetic basis. Establishing the specific etiology can influence management of this lifelong condition. NGS approaches improve the diagnostic yield when many potential candidate genes are involved.

Adolescents on basal-bolus insulin can fast during Ramadan.

Fasting during Ramadan is a major tenet of the Muslim religion. All adults after the age of puberty are required to do so if health permits. However, there are exemptions to this requirement and having a chronic condition such as diabetes is one. Nevertheless, many adults and adolescents feel obliged to fast during Ramadan even though there is no absolute need to do so. This obligation must be respected. There are few data to support this practice in those whose condition, such as diabetes, potentially makes them vulnerable to developing problems during prolonged fasting. This study was designed to examine the ability and safety of young people with diabetes to be able to fast if they so desire. Two groups of patients were studied, those on a multiple injection, so-called basal-bolus, regimen and those on a 'conventional' twice daily pre-mixed insulin regimen. All patients showed a tendency to high blood glucose at the time of commencing their fast. Those on twice daily insulin continued to have hyperglycaemia during the day whilst those on basal-bolus insulin showed a steady fall in blood glucose towards normal by the time of breaking their fast. Although there was a greater tendency to hypoglycaemia in the basal-bolus group, this could be successfully prevented by reducing the dose of basal insulin by 10-20%. We recommend that it is safe for adolescents with diabetes to fast during Ramadan as long as they reduce their basal insulin by this amount and continue to monitor their blood glucose regularly.

Juvenile Paget's disease with compound heterozygous mutations in TNFRSF11B presenting with recurrent clavicular fractures and a mild skeletal phenotype.

Juvenile Paget's disease (JPD) is a rare recessively-inherited bone dysplasia. The great majority of cases described to date have had homozygous mutations in TNFRSF11B, the gene encoding osteoprotegerin. We describe a boy who presented with recurrent clavicular fractures following minor trauma (8 fractures from age 2 to 11). He was of normal height and despite mild lateral bowing of the thighs and anterior bowing of the shins he remained physically active. Abnormal modelling was noted in ribs and humeri on clavicular radiographs, and a skeletal survey at the age of 7 showed generalised diaphyseal expansion of the long bones with thickening of the periosteal and endosteal surfaces of the cortices. On biochemical evaluation, serum alkaline phosphatase was noted to be persistently elevated. The diagnosis of JPD was confirmed by the finding of compound heterozygous mutations in TNFRSF11B: a maternally-inherited A>G missense mutation at position 1 of the first amino acid codon (previously reported) and a paternally-inherited splice acceptor site mutation in intron 3 at a highly conserved position (not previously reported). Bioinformatics analysis suggested both mutations were disease-causing. Compound heterozygote mutations in TNFRSF11B causing JPD have been previously reported only once - in a boy who also had a relatively mild skeletal phenotype. The milder features may lead to delay in diagnosis and diagnostic confusion with other entities, but the extraskeletal features of JPD may nonetheless develop.

Case report: a 10-year-old girl with primary hypoparathyroidism and systemic lupus erythematosus.

Objectives Hypoparathyroidism is a rare disease in children that occurs as a result of autoimmune destruction of the parathyroid glands, a defect in parathyroid gland development or secondary to physical parathyroid gland disturbance. Typical symptoms of hypoparathyroidism present as hypocalcaemia and hyperphosphatemia due to decreased parathyroid hormone secretion and may lead to nerve and muscles disturbances resulting in clinical manifestation of tetany, arrhythmias and epilepsy. Currently, there is no conventional hormone replacement treatment for hypoparathyroidism and therapeutic approaches include normalising mineral levels using an oral calcium supplement and active forms of vitamin D. Case presentation We present the case of a 10-year-old girl with primary hypoparathyroidism who had no prior history of autoimmune disorders, but who subsequently developed systemic lupus erythematosus.

A New Multisystem Disorder Caused by the Gαs Mutation p.F376V.

CONTEXT: The α subunit of the stimulatory G protein (Gαs) links numerous receptors to adenylyl cyclase. Gαs, encoded by GNAS, is expressed predominantly from the maternal allele in certain tissues. Thus, maternal heterozygous loss-of-function mutations cause hormonal resistance, as in pseudohypoparathyroidism type Ia, whereas somatic gain-of-function mutations cause hormone-independent endocrine stimulation, as in McCune-Albright syndrome. OBJECTIVE: We report two unrelated boys presenting with a new combination of clinical findings that suggest both gain and loss of Gαs function. DESIGN AND SETTING: Clinical features were studied and sequencing of GNAS was performed. Signaling capacities of wild-type and mutant Gαs were determined in the presence of different G protein-coupled receptors (GPCRs) under basal and agonist-stimulated conditions. RESULTS: Both unrelated patients presented with unexplained hyponatremia in infancy, followed by severe early onset gonadotrophin-independent precocious puberty and skeletal abnormalities. An identical heterozygous de novo variant (c.1136T>G; p.F376V) was found on the maternal GNAS allele in both patients; this resulted in a clinical phenotype that differed from known Gαs-related diseases and suggested gain of function at the vasopressin 2 receptor (V2R) and lutropin/choriogonadotropin receptor (LHCGR), yet increased serum PTH concentrations indicative of impaired proximal tubular PTH1 receptor (PTH1R) function. In vitro studies demonstrated that Gαs-F376V enhanced ligand-independent signaling at the PTH1R, LHCGR, and V2R and, at the same time, blunted ligand-dependent responses. Structural homology modeling suggested mutation-induced modifications at the C-terminal α5 helix of Gαs that are relevant for interaction with GPCRs and signal transduction. CONCLUSIONS: The Gαs p.F376V mutation causes a previously unrecognized multisystem disorder.

Voyages of Discovery.

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.

Physiology of Calcium, Phosphate, Magnesium and Vitamin D.

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 calcium-sensing 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.