Calcium and vitamin D: what is known about the effects on growing bone

Dilemmas in establishing preterm enteral feeding: where do we start and how fast do we go?

Beginning and achieving full enteral nutrition is a key step in the care of preterm infants, particularly very low birth weight (VLBW) infants. As is true for many organ system-specific complications of prematurity, the gastrointestinal tract must complete in utero development ex utero while concurrently serving a physiologic role reserved for after completion of full term development. The preterm gut must assume the placental function of the interface between a source of energy, precursors for anabolism, and micronutrients, and the developing infant-through digestion and absorption of milk, instead of directly from the mother via the uteroplacental interface. The benefits of enteral nourishment in preterm infants are counterbalanced by gastrointestinal complications of prematurity: dysmotility leading to difficulty establishing and advancing feeds, and the risk of necrotizing enterocolitis (NEC). Concern for these complications can prolong the need for parenteral nutrition with an associated increase in risk for central line-associated bloodstream infection (CLABSI) and parenteral nutrition (PN)-associated cholestasis or liver disease (PNALD). Thus, a daily issue facing neonatologists caring for preterm infants is how to optimally begin, advance, and reach full enteral nutrition sufficient to satisfy the nutrient, energy, and fluid requirements of VLBW infants while minimizing risk. In this perspective, we provide an overview of the approaches and supporting data for starting and advancing enteral feeds in preterm infants, particularly very low birth weight infants, and we discuss the significant gaps in knowledge that accompany current approaches. This framework recognizes the dilemmas of preterm feeding initiation and advancement and identifies areas of opportunity for further investigation.

Burosumab for Pediatric X-Linked Hypophosphatemia

PURPOSE OF REVIEW

X-Linked hypophosphatemia (XLH) is the most common genetic cause of rickets. This review describes advances in the management of XLH using burosumab which was FDA approved for treating children with XLH in 2018.

RECENT FINDINGS

Elevated FGF23 in XLH leads to systemic hypophosphatemia and several musculoskeletal manifestations, including rachitic bone deformities, impaired growth, dental abscesses, insufficiency fractures, osteoarthritis, and enthesopathy, with lifelong consequences for physical function and quality of life. Burosumab treatment has demonstrated clinical improvement of rickets and growth in children, including during a randomized controlled trial compared with conventional therapy. Burosumab also improved pseudofracture healing in adults. Burosumab led to greater improvement in rickets and growth than conventional therapy. However, many questions remain regarding the impact of burosumab on several outcomes, including final height, nephrocalcinosis, dental disease, enthesopathy, and surgical interventions.

Congenital Conditions of Hypophosphatemia in Children

Great strides over the past few decades have increased our understanding of the pathophysiology of hypophosphatemic disorders. Phosphate is critically important to a variety of physiologic processes, including skeletal growth, development and mineralization, as well as DNA, RNA, phospholipids, and signaling pathways. Consequently, hypophosphatemic disorders have effects on multiple systems, and may cause a variety of nonspecific signs and symptoms. The acute effects of hypophosphatemia include neuromuscular symptoms and compromise. However, the dominant effects of chronic hypophosphatemia are the effects on musculoskeletal function including rickets, osteomalacia and impaired growth during childhood. While the most common causes of chronic hypophosphatemia in children are congenital, some acquired conditions also result in hypophosphatemia during childhood through a variety of mechanisms. Improved understanding of the pathophysiology of these congenital conditions has led to novel therapeutic approaches. This article will review the pathophysiologic causes of congenital hypophosphatemia, their clinical consequences and medical therapy.

Roles of Phosphate in Skeleton

Phosphate is essential for skeletal mineralization, and its chronic deficiency leads to rickets and osteomalacia. Skeletal mineralization starts in matrix vesicles (MVs) derived from the plasma membrane of osteoblasts and chondrocytes. MVs contain high activity of tissue non-specific alkaline phosphatase (TNSALP), which hydrolyzes phosphoric esters such as pyrophosphates (PPi) to produce inorganic orthophosphates (Pi). Extracellular Pi in the skeleton is taken up by MVs through type III sodium/phosphate (Na⁺/Pi) cotransporters and forms hydroxyapatite. In addition to its roles in MV-mediated skeletal mineralization, accumulating evidence has revealed that extracellular Pi evokes signal transduction and regulates cellular function. Pi induces apoptosis of hypertrophic chondrocytes, which is a critical step for endochondral ossification. Extracellular Pi also regulates the expression of various genes including those related to proliferation, differentiation, and mineralization. In vitro cell studies have demonstrated that an elevation in extracellular Pi level leads to the activation of fibroblast growth factor receptor (FGFR), Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway, where the type III Na⁺/Pi cotransporter PiT-1 may be involved. Responsiveness of skeletal cells to extracellular Pi suggests their ability to sense and adapt to an alteration in Pi availability in their environment. Involvement of FGFR in the Pi-evoked signal transduction is interesting because enhanced FGFR signaling in osteoblasts/osteocytes might be responsible for the overproduction of FGF23, a key molecule in phosphate homeostasis, in a mouse model for human X-linked hypophosphatemic rickets (XLH). Impaired Pi sensing may be a pathogenesis of XLH, which needs to be clarified in future.

New diagnostic modalities and emerging treatments for neonatal bone disease

Bone disease in the neonatal period has often been regarded as an issue affecting premature infants, or a collection of rare and ultra-rare disorders that most neonatologists will see only once or twice each year, or possibly each decade. The emergence of targeted therapies for some of these rare disorders means that neonatologists may be faced with diagnostic dilemmas that need a rapid solution in order to access management options that did not previously exist. The diagnostic modalities available to the neonatologist have not changed a great deal in recent years; blood tests and radiographs still form the mainstays with other techniques usually reserved for research studies, but rapid access to genomic testing is emergent. This paper provides an update around diagnosis and management of bone problems likely to present to the neonatologist.

Phosphate wasting disorders in adults

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.

Vitamin D deficiency among newborns in Amman, Jordan

OBJECTIVE

Vitamin D deficiency is well recognized in selected Middle Eastern countries, but neonatal vitamin D status is not well studied in Jordan and other nearby countries. The aim of this study is to determine the prevalence of vitamin D deficiency in Jordanian newborns and risk factors associated with low levels.

METHODS

This is a prospective cohort study of newborn infants who were delivered at the Al Bashir Government Hospital in Amman, Jordan, from January 31, 2010, to January 27, 2011. Heel stick blood samples for 25-hydroxyvitamin D [25(OH)D] levels were obtained within 96 hours of birth. Maternal dress pattern, vitamin supplementation, smoke exposure during pregnancy, mode of delivery, gestational age, and birth weight were documented.

RESULTS

Samples were obtained from 3,731 newborns. Median gestational age was 39 weeks, median birth weight was 3.1 kilograms, median maternal age was 27 years, and median newborn 25(OH)D level was 8.6nmol/L. A total of 3,512 newborns (94.1%) in this study were vitamin D deficient (< 50 nmol/L). Lower gestational age, maternal smoke exposure, and birth during winter months were associated with lower infant vitamin D levels, while vitamin D supplementation and time spent outside during pregnancy were associated with higher vitamin D levels.

CONCLUSIONS

The prevalence of severely low vitamin D levels in newborn infants in Amman, Jordan, is substantial, even in newborns born during the spring and summer months. Vitamin D supplementation is needed in this population.

The pleiomorphic actions of vitamin D and its importance for children

Knowledge regarding the physiological role and dietary requirements of vitamin D has dramatically expanded over the past several decades. The "new" vitamin D is not only a mediator of calcium homeostasis, it also has important immunomodulatory, antimicrobial, and antiproliferative actions. In spite of the interest in vitamin D as a mediator in many chronic diseases of adulthood such as cancer and type II diabetes, less attention has been given to the implications of the new understanding of vitamin D for child and adolescent health. Recently, rickets caused by vitamin D deficiency is resurging in developed countries. Therefore, pharmacokinetic studies and epidemiologic research that incorporates clinical and functional outcomes are needed to clarify the role of vitamin D in growth and development in Korean children and adolescents and to determine vitamin D dietary requirements.

Assessing bone health in children and adolescents

During normal childhood and adolescence, the skeleton undergoes tremendous change. Utilizing the processes of modeling and remodeling, the skeleton acquires its adult configuration and ultimately achieves peak bone mass. Optimization of peak bone mass requires the proper interaction of environmental, dietary, hormonal, and genetic influences. A variety of acute and chronic conditions, as well as genetic polymorphisms, are associated with reduced bone density, which can lead to an increased risk of fracture both in childhood and later during adulthood. Bone densitometry has an established role in the evaluation of adults with bone disorders, and the development of suitable reference ranges for children now permits the application of this technology to younger individuals. We present a brief overview of the factors that determine bone density and the emerging role of bone densitometry in the assessment of bone mass in growing children and adolescents.

No relationship between vitamin D status and insulin resistance in a group of high school students

OBJECTIVE

To investigate the effects of vitamin D deficiency on both insulin resistance and risk of metabolic syndrome in children.

METHODS

The study group consisted of 301 children and adolescents with a mean age of 14.2 ± 1.8 years. Serum 25-hydroxyvitamin D [25(OH)D] levels and insulin resistance indices were evaluated. According to serum 25(OH)D levels, the subjects were classified in 3 groups. Those with levels ≤ 10 ng/mL were labeled as the vitamin D deficient group (group A), those with levels of 10-20 ng/mL as the vitamin D insufficient group (group B) and those with ≥ 20 ng/mL as having normal vitamin D levels (group C). Metabolic syndrome was defined according to the International Diabetes Federation consensus. The participants with and without metabolic syndrome were compared in terms of 25(OH)D levels.

RESULTS

Mean 25(OH)D level of the total group was 18.2 ± 9.3 (2.8-72.0) ng/mL. Distribution of individuals according to their vitamin D levels showed that 11.6% were in group A, 53.5% in group B, and 34.9% in group C. The proportions of boys and girls in these categories were 22.9% and 77.1% in group A, 36.6% and 63.4% in group B, 54.3% and 45.7% in group C, respectively. There were no significant differences in 25(OH)D levels in the individuals with and without impaired fasting glucose or impaired glucose tolerance. No relationship was observed between insulin resistance/sensitivity indices and vitamin D status (p > 0.05). Metabolic syndrome was diagnosed in 12.3% (n = 37) of the children. There was also no difference in mean 25(OH)D levels between individuals who had and those who did not have the metabolic syndrome.

CONCLUSION

In our study, no correlations were found between insulin measurements during oral glucose tolerance test and vitamin D deficiency. Nonetheless, more extended studies including vitamin D supplementation and evaluating insulin sensitivity via clamp technique are needed to further elucidate this relationship.

Vitamin D deficiency among healthy infants and toddlers: a prospective study from Irbid, Jordan

BACKGROUND

The aim of this study was to estimate the prevalence of vitamin D deficiency among healthy infants and toddlers, as well as its associated factors, in Irbid, Jordan.

METHODS

A total of 275 subjects (136 infants and 139 toddlers) aged 6-36 months participated in this study. Information concerning sociodemographic characteristics and early feeding patterns was collected using a self-guided questionnaire. Plasma vitamin D, calcium, phosphorous, and alkaline phosphatase activity were measured.

RESULTS

The prevalence of vitamin D deficiency was 28% (16.7% for severe vitamin D deficiency and 11.3% for vitamin D deficiency) and vitamin D insufficiency was 28.4%. Plasma calcium and alkaline phosphatase levels showed no correlation with the vitamin D status of the study population. For both age groups, a significant association was found between vitamin D status and sun exposure (P < 0.001). A significant association between infant feeding practices and vitamin D status was found (P < 0.001). Infants who were exclusively breast-fed had higher risk for vitamin D deficiency and vitamin D insufficiency than those who were bottle-fed. Multivariate logistic regression analyses results showed that female sex, low sun exposure and exclusive breast-feeding were the main determinants of vitamin D levels.

CONCLUSION

The prevalence of vitamin D deficiency is considered to be high among northern Jordanian infants and toddlers. Sun exposure of less than 30 min daily and exclusively breast-feeding are the main factors for developing vitamin D deficiency.

Forum on aging and skeletal health: summary of the proceedings of an ASBMR workshop

With the aging of the population, the scope of the problem of age-related bone loss and osteoporosis will continue to increase. As such, it is critical to obtain a better understanding of the factors determining the acquisition and loss of bone mass from childhood to senescence. While there have been significant advances in recent years in our understanding of both the basic biology of aging and a clinical definition of age-related frailty, few of these concepts in aging research have been evaluated adequately for their relevance and application to skeletal aging or fracture prevention. The March 2011 Forum on Aging and Skeletal Health, sponsored by the NIH and ASBMR, sought to bring together leaders in aging and bone research to enhance communications among diverse fields of study so as to accelerate the pace of scientific advances needed to reduce the burden of osteoporotic fractures. This report summarizes the major concepts presented at that meeting and in each area identifies key questions to help set the agenda for future research in skeletal aging.

The role of vitamin D in paediatric bone health

Recently there has been renewed interest in the role of vitamin D in paediatric bone health. Its role in the development of rickets and hypocalcaemia in infants and young children, in particular, in many part of the world is well known, and the importance of the prevention of vitamin D deficiency during pregnancy and lactation has been highlighted. Less clear are the possible effects that maintaining maternal vitamin D sufficiency might have on foetal and early infant growth and bone development. There is little evidence to suggest that maintaining childhood vitamin D status well above that necessary to prevent rickets has an effect on intestinal calcium absorption or on peak bone mass. Further studies are needed in these areas prior to definitive conclusions are drawn about the optimal vitamin D requirements and circulating 25(OH)D concentrations for foetal, infant and childhood bone health.

Vitamin D status in abused and nonabused children younger than 2 years old with fractures

OBJECTIVE

To examine vitamin D levels in children with (1) suspected abusive and accidental fractures, (2) single and multiple fractures, and (3) fracture types highly associated with inflicted trauma.

DESIGN AND METHODS

A study of children younger than 2 years of age with fractures admitted to a large children's hospital was performed. Bivariate analysis and test for trend were performed to test for the association of vitamin D status and biochemical markers of bone health with the primary outcomes of fracture etiology, number, and type.

RESULTS

Of 118 subjects in the study, 8% had deficient vitamin D levels (<20 ng/mL; <50 nmol/L), 31% were insufficient (≥20 < 30 ng/mL; ≥50 < 78 nmol/L), and 61% were sufficient (≥30 ng/mL; ≥78 nmol/L). Lower vitamin D levels were associated with higher incidences of hypocalcemia (P = .002) and elevated alkaline phosphatase (P = .05) but not hypophosphatemia (P = .30). The majority of children sustained accidental fractures (60%); 31% were nonaccidental and 9% were indeterminate. There was no association between vitamin D levels and any of the following outcomes: child abuse diagnosis (P = .32), multiple fractures (P = .24), rib fractures (P = .16), or metaphyseal fractures (P = .49).

CONCLUSIONS

Vitamin D insufficiency was common in young children with fractures but was not more common than in previously studied healthy children. Vitamin D insufficiency was not associated with multiple fractures or diagnosis of child abuse. Nonaccidental trauma remains the most common cause of multiple fractures in young children.

VDR deficiency affects alveolar bone and cementum apposition in mice

OBJECTIVE

To compare the mineralisation density (MD), morphology and histology of alveolar bone and cementum amongst VDR +/+, VDR -/-, and VDR -/- groups supplemented with a diet TD 96348, containing 20% lactose, 2.0% calcium and 1.25% phosphorous.

METHODS

Four groups of mice (6 mice/group) were identified by genotyping: VDR +/+ mice (VDR wild type), VDR -/- mice (VDR deficient), VDR -/- offsprings derived from VDR -/- parents receiving a supplemental diet (early rescued), and VDR -/- mice fed with a supplemental diet beginning at age one month (late rescued). All mice were sacrificed at age 70.5 days. Micro-CT was used to compare MD and morphology of alveolar bone and cementum. H-E and Toluidine blue staining was used to examine the ultrastructure of the alveolar bone and cementum at matched locations.

RESULTS

In VDR -/- group, alveolar bone and cementum failed to mineralise normally. Early rescue increased MD of alveolar bone in VDR -/- mice with excessive alveolar bone formation, but which not observed in late rescue group. MD and morphology of cementum-dentine complex in both early and late rescue groups were comparable with VDR +/+ group when feeding with high-calcium rescue diet.

CONCLUSIONS

VDR affects alveolar bone mineralisation and formation systemically and locally. However, cementum apposition and mineralisation is mainly regulated by calcium concentrations in serum.

Regulation of calcium homeostasis and bone metabolism in the fetus and neonate

PURPOSE OF REVIEW

Regulation of calcium and phosphorus levels in the fetus and neonate is critical for proper bone development and mineralization.

RECENT FINDINGS

Parathyroid hormone-related peptide plays an important role in transferring calcium across the placenta into the fetal circulation. In contrast, the factors controlling placental phosphate transport have not yet been explored, and numerous studies have indicated that maternal and childhood vitamin D deficiency continues to be a significant clinical concern.

SUMMARY

The molecular basis for mineral ion homeostasis in the fetus and child remains incompletely understood. More attention must be paid to identifying and treating maternal, neonatal, and childhood vitamin D deficiency.

Enteral nutrient supply for preterm infants: commentary from the European Society of Paediatric Gastroenterology, Hepatology and Nutrition Committee on Nutrition

The number of surviving children born prematurely has increased substantially during the last 2 decades. The major goal of enteral nutrient supply to these infants is to achieve growth similar to foetal growth coupled with satisfactory functional development. The accumulation of knowledge since the previous guideline on nutrition of preterm infants from the Committee on Nutrition of the European Society of Paediatric Gastroenterology and Nutrition in 1987 has made a new guideline necessary. Thus, an ad hoc expert panel was convened by the Committee on Nutrition of the European Society of Paediatric Gastroenterology, Hepatology, and Nutrition in 2007 to make appropriate recommendations. The present guideline, of which the major recommendations are summarised here (for the full report, see http://links.lww.com/A1480), is consistent with, but not identical to, recent guidelines from the Life Sciences Research Office of the American Society for Nutritional Sciences published in 2002 and recommendations from the handbook Nutrition of the Preterm Infant. Scientific Basis and Practical Guidelines, 2nd ed, edited by Tsang et al, and published in 2005. The preferred food for premature infants is fortified human milk from the infant's own mother, or, alternatively, formula designed for premature infants. This guideline aims to provide proposed advisable ranges for nutrient intakes for stable-growing preterm infants up to a weight of approximately 1800 g, because most data are available for these infants. These recommendations are based on a considered review of available scientific reports on the subject, and on expert consensus for which the available scientific data are considered inadequate.

Normalisation of calcium status reverses the phenotype in dentin, but not in enamel of VDR-deficient mice

OBJECTIVE

To determine the effects of vitamin D receptor (VDR) deficiency on mouse dentin and enamel mineralisation, and how normalisation of serum calcium level affects dentin and enamel phenotypes in VDR knockout mice.

MATERIALS AND METHODS

Groups of VDR wild-type (VDR+/+), VDR deficient (VDR-/-) and VDR-/- rescued mice were sacrificed at 70.5 days of life. The rescued group was established by a high-calcium diet feeding the VDR-/- mice from postnatal 19 days. Micro-CT was used to compare enamel and dentin mineralisation density (MD) at different levels of mandibular incisors among the groups. The scanning electron microscope (SEM) was used to examine the ultrastructure of the enamel and dentin in the corresponding levels and of surface enamel after acidic treatment.

RESULTS

Micro-CT showed that in VDR-/- rescued group, dentin phenotype was reversed and dentin MD was reversed to normal; however, enamel mineralisation was not reversible, and remained as hypermineralisation in molar region and apical region of the incisors. SEM also revealed enamel hypermineralisation in the VDR-/- rescued group. This early enamel hypermineralisation was more susceptible to acidic erosion.

CONCLUSION

Vitamin D affects dentin mineralisation systemically, and it regulates enamel mineralisation locally.

Different enamel and dentin mineralization observed in VDR deficient mouse model

Vitamin D plays an important role in the bone mineralization process. Enamel and dentin are two mineralized tissues of different origins that combine to form teeth, but the mechanism by which vitamin D regulates these tissues remains unclear. We hypothesized that vitamin D affects enamel and dentin mineralization through different mechanisms.

OBJECTIVE

To examine enamel and dentin mineralization in a vitamin D receptor (VDR) deficient mouse model by micro-computerized tomography (micro-CT) and scanning electronic microscopy (SEM).

METHODS

VDR wild type mice (VDR+/+) and VDR deficient (VDR-/-) littermates were sacrificed at 70.5 days old, and their mandibles were dissected. Micro-CT was used to compare mineral density (MD) of enamel and dentin of the two groups at different levels along the axis of mandibular incisors. SEM was employed to examine the ultrastructure of incisors at the levels corresponding to the levels used for the micro-CT studies. Furthermore, an accelerated eruption procedure was performed to exclude the effect of delayed eruption on enamel and dentin mineralization.

RESULTS

Different distribution patterns of enamel and dentin MD were observed between VDR+/+ and VDR-/- groups. Early enamel maturation, mineralization, and hypomineralization in dentin were observed in the VDR deficient mice.

CONCLUSION

Vitamin D may affect the mineralization of dentin systemically, and enamel mineralization may be regulated locally.

Kidney and phosphate metabolism

The serum phosphorus level is maintained through a complex interplay between intestinal absorption, exchange intracellular and bone storage pools, and renal tubular reabsorption. The kidney plays a major role in regulation of phosphorus homeostasis by renal tubular reabsorption. Type IIa and type IIc Na⁺/P_i transporters are important renal Na⁺-dependent inorganic phosphate (P_i) transporters, which are expressed in the brush border membrane of proximal tubular cells. Both are regulated by dietary P_i intake, vitamin D, fibroblast growth factor 23 (FGF23) and parathyroid hormone. The expression of type IIa Na⁺/P_i transporter result from hypophosphatemia quickly. However, type IIc appears to act more slowly. Physiological and pathophysiological alteration in renal P_i reabsorption are related to altered brush border membrane expression/content of the type II Na⁺/P_i cotransporter. Many studies of genetic and acquired renal phosphate wasting disorders have led to the identification of novel genes. Two novel P_i regulating genes, PHEX and FGF23, play a role in the pathophysiology of genetic and acquired renal phosphate wasting disorders and studies are underway to define their mechanism on renal P_i regulation. In recent studies, sodium-hydrogen exchanger regulatory factor 1 (NHERF1) is reported as another new regulator for P_i reabsorption mechanism.

Hypovitaminosis D in obese and overweight schoolchildren

AIM

To determine the prevalence of vitamin D hypovitaminosis among obese and overweight schoolchildren.

DESIGN

A cross-sectional population based sample.

METHODS

In a cross-sectional study, 301 students (177 girls and 124 boys) aged 11-19 years were selected by multistage stratified sampling design. Subjects were classified according to their body mass index as obese, overweight and normal. Serum 25-hydroxyvitamin D (25-OHD), intact parathyroid hormone (iPTH) and alkaline phosphatase (ALP) were measured in late winter months. Vitamin D deficiency was defined as a 25-OHD 20 ng/ml.

RESULTS

The prevalence of hypovitaminosis D was found as 65% in all students. Vitamin D deficiency was found in 12% and insufficiency in 53% of all students. Vitamin D deficiency in female students was about two times more common than in males. In obese and overweight schoolchildren with hypovitaminosis D, serum 25-OHD levels decreased as BMI increased. There were no correlations between serum 25-OHD and ALP and iPTH levels.

CONCLUSION

Vitamin D deficiency and insufficiency are common in obese and overweight schoolchildren, especially in girls. Obesity could be a risk factor in terms of hypovitaminosis D in adolescents. Vitamin D supplementation should be administered particularly to adolescent girls.

Inherited hypophosphatemic disorders in children and the evolving mechanisms of phosphate regulation

Phosphorous is essential for multiple cellular functions and constitutes an important mineral in bone. Hypophosphatemia in children leads to rickets resulting in abnormal growth and often skeletal deformities. Among various causes of low serum phosphorous are inherited disorders associated with increased urinary excretion of phosphate, including autosomal dominant hypophosphatemic rickets (ADHR), X-linked hypophosphatemia (XLH), autosomal recessive hypophosphatemia (ARHP), and hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Recent genetic analyses and subsequent biochemical and animal studies have revealed several novel molecules that appear to play key roles in the regulation of renal phosphate handling. These include a protein with abundant expression in bone, fibroblast growth factor 23 (FGF23), which has proven to be a circulating hormone that inhibits tubular reabsorption of phosphate in the kidney. Two other bone-specific proteins, PHEX and dentin matrix protein 1 (DMP1), appear to be necessary for limiting the expression of fibroblast growth factor 23, thereby allowing sufficient renal conservation of phosphate. This review focuses on the clinical, biochemical, and genetic features of inherited hypophosphatemic disorders, and presents the current understanding of hormonal and molecular mechanisms that govern phosphorous homeostasis.

Inherited disorders of calcium homeostasis

In mammals a complicated homeostatic mechanism has evolved to maintain near consistency of extracellular calcium ion levels. The homeostatic mechanism involves several hormones, which comprise among others, parathyroid hormone and vitamin D. The recent resurge in vitamin D deficiency, as a global health issue, has increased interest in the hormone. In addition to vitamin D deficiency, other causes of rickets are calcium deficiency and inherited disorders of vitamin D and phosphorus metabolism. Vitamin D-resistant syndromes are caused by hereditary defects in metabolic activation of the hormone or by mutations in the vitamin D receptor, which binds the hormone with high affinity and regulates the expression of genes through zinc finger mediated DNA binding and protein-protein interaction. Current interest is to correlate the type/position of mutations that result in disorders of vitamin D metabolism or in vitamin D receptor function with the variable phenotypic features and clinical presentation. The calcium sensing receptor plays a key role in calcium homeostasis. Loss of function mutations in the calcium sensing receptor can cause familial benign hypocalciuric hypercalcemia in heterozygotes and neonatal severe hyperparathyroidism when homozygous mutations occur in the calcium sensing receptor. Gain of function mutation can cause the opposite effect causing autosomal dominant hypocalcemia. Mouse models using targeted gene disruption strategies have been valuable tools to study the effect of mutations on the calcium sensing receptor or in the vitamin D activation pathway. Dysfunctional calcium sensing receptors with function altering mutations may be responsive to treatment with allosteric modulators of the calcium sensing receptor. Vitamin D analogs which induce unusual structural conformations on the vitamin D receptor may have a variety of therapeutic indications. This review summarises recent advances in knowledge of the molecular pathology of inherited disorders of calcium homeostasis.