The molecular basis of hereditary 1,25-dihydroxyvitamin D3 resistant rickets in seven related families

Alopecia with Vitamin D-Dependent Rickets Type 2 A: A Case Report

Vitamin D-dependent rickets type II (VDDR2) is a rare inherited autosomal recessive disorder wherein the genetic mutation results in a defect in the vitamin D receptor (VDR), thus leading to target resistance to 1.25-dihydroxy vitamin D. This results in hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and rickets. We report a case of a four-year-old female child with diffuse alopecia starting 2 weeks after birth, along with frontal bossing, hypoplastic teeth, and an unusual presentation of multiple skin-colored papules over the back. Genetic testing confirmed vitamin D-dependent rickets type 2A.

Vitamin D: An Overview of Gene Regulation, Ranging from Metabolism to Genomic Effects

Vitamin D is a pro-hormone characterized by an intricate metabolism and regulation. It is well known for its role in calcium and phosphate metabolism, and in bone health. However, several studies have assessed a huge number of extra-skeletal functions, ranging from cell proliferation in some oncogenic pathways to antioxidant and immunomodulatory functions. Vitamin D exerts its role by binding to VDRs (vitamin D receptors), which are located in many different tissues. Moreover, VDRs are able to bind hundreds of genomic loci, modulating the expression of various primary target genes. Interestingly, plenty of gene polymorphisms regarding VDRs are described, each one carrying a potential influence against gene expression, with relapses in several chronic diseases and metabolic complications. In this review, we provide an overview of the genetic aspects of vitamin D and VDR, emphasizing the gene regulation of vitamin D, and the genetic modulation of VDR target genes. In addition, we briefly summarize the rare genetic disease linked to vitamin D metabolism.

Role of Neural Stem Cells and Vitamin D Receptor (VDR)-Mediated Cellular Signaling in the Mitigation of Neurological Diseases

Specific stem cell-based therapies for treating Alzheimer's disease, Parkinson's disease, and schizophrenia are gaining importance in recent years. Accumulating data is providing further support by demonstrating the efficacy of neural stem cells in enhancing the neurogenesis in the aging brain. In addition to stem cells, recent studies have shown the efficacy of supplementing vitamin D in promoting neurogenesis and neuronal survival. Studies have also demonstrated the presence of mutational variants and single-nucleotide polymorphisms of the vitamin D receptor (VDR) in neurological disorders; however, implications of these mutations in the pathophysiology and response to drug treatment are yet to be explored. Hence, in this article, we have reviewed recent reports pertaining to the role of neural stem cells and VDR-mediated cellular signaling cascades that are involved in enhancing the neurogenesis through Wnt/β-catenin and Sonic Hedgehog pathways. This review benefits neurobiologists and pharmaceutical industry experts to develop stem cell-based and vitamin D-based therapies to better treat the patients suffering from neurological diseases.

Lessons Learned from Hereditary 1,25-Dihydroxyvitamin D-Resistant Rickets Patients on Vitamin D Functions

We summarize here lessons learned from studies on skeletal and extra-skeletal functions of vitamin D in hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) patients with a mutant, nonfunctioning vitamin D receptor (VDR). During childhood, HVDRR patients are dependent on intestinal VDR, demonstrate low intestinal fraction calcium absorption, and have a bone calcium accretion rate that leads to hypocalcemia and rickets. After puberty, there is recovery in intestinal calcium absorption and in bone calcium accretion and structure. HVDRR monocytes and lymphocytes show impairment in the expression of antimicrobial proteins and demonstrate a proinflammatory cytokine profile. However, HVDRR patients do not exhibit increased rates of infections or inflammatory diseases. Vitamin D deficiency is associated with asthmatic exacerbations. Surprisingly, HVDRR patients do not usually develop asthma. They have normal allergic tests and lung functions and are protected against provoked bronchial hyperactivity. HVDRR patients have decreased IL-5 levels in their exhaled breath condensate. Given that IL-5 is a key cytokine in the development of airway inflammation and hyperactivity and that VDR is important for IL-5 generation, it is plausible that low lung IL-5 protects HVDRR patients from asthma. Vitamin D metabolites have suppressive effects on the renin angiotensin system. However, no HVDRR patient showed hypertension or echocardiographic pathology, and their renin angiotensin metabolites were normal. The VDR is expressed throughout the reproductive system, suggesting a role in reproduction. However, the reproductive history of HVDRR patients is normal despite the lack of a normal VDR. HVDRR patients provide a unique opportunity to study the role of the VDR and the role of vitamin D in various human systems.

Cholesterol homeostasis: Links to hair follicle biology and hair disorders

Lipids and lipid metabolism are critical factors in hair follicle (HF) biology, and cholesterol has long been suspected of influencing hair growth. Altered cholesterol homeostasis is involved in the pathogenesis of primary cicatricial alopecia, mutations in a cholesterol transporter are associated with congenital hypertrichosis, and dyslipidaemia has been linked to androgenic alopecia. The underlying molecular mechanisms by which cholesterol influences pathways involved in proliferation and differentiation within HF cell populations remain largely unknown. As such, expanding our knowledge of the role for cholesterol in regulating these processes is likely to provide new leads in the development of treatments for disorders of hair growth and cycling. This review describes the current state of knowledge with respect to cholesterol homeostasis in the HF along with known and putative links to hair pathologies.

Hypocalcemic disorders

Calcium is vital for life, and extracellular calcium concentrations must constantly be maintained within a precise concentration range. Low serum calcium (hypocalcemia) occurs in conjunction with multiple disorders and can be life-threatening if severe. Symptoms of acute hypocalcemia include neuromuscular irritability, tetany, and seizures, which are rapidly resolved with intravenous administration of calcium gluconate. However, disorders that lead to chronic hypocalcemia often have more subtle manifestations. Hypoparathyroidism, characterized by impaired secretion of parathyroid hormone (PTH), a key regulatory hormone for maintaining calcium homeostasis, is a classic cause of chronic hypocalcemia. Disorders that disrupt the metabolism of vitamin D can also lead to chronic hypocalcemia, as vitamin D is responsible for increasing the gut absorption of dietary calcium. Treatment and management options for chronic hypocalcemia vary depending on the underlying disorder. For example, in patients with hypoparathyroidism, calcium and vitamin D supplementation must be carefully titrated to avoid symptoms of hypocalcemia while keeping serum calcium in the low-normal range to minimize hypercalciuria, which can lead to renal dysfunction. Management of chronic hypocalcemia requires knowledge of the factors that influence the complex regulatory axes of calcium homeostasis in a given disorder. This chapter discusses common and rare disorders of hypocalcemia, symptoms and workup, and management options including replacement of PTH in hypoparathyroidism.

Phosphaturic mesenchymal tumors: what an endocrinologist should know

Tumor-induced osteomalacia (TIO), also known as "oncogenic osteomalacia", is a rare cause of osteomalacia. TIO often has an insidious onset characterized clinically by progressive muscle weakness and bone pain with fractures. The hallmark biochemical finding is a persistent low serum phosphorus concentration due to renal phosphate wasting. The vast majority of cases of TIO result from production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) by a histologically distinctive mesenchymal tumor, termed "phosphaturic mesenchymal tumor" (PMT). Circulating FGF23 induces internalization of renal sodium/phosphate co-transporters resulting in reduced proximal tubular phosphate reabsorption. FGF23 also inhibits production of 1α,25-dihydroxyvitamin D which is inappropriately low or normal in the context of hypophosphatemia. Diagnosis is often delayed owing to the rarity of the condition and an underappreciation for the role of phosphorus as a cause for the constellation of symptoms. Primary treatment for TIO is identification of the offending tumor and surgical removal. However, these tumors are notoriously difficult to find, precluding the opportunity for a curative surgery in many. In such cases, phosphate and calcitriol therapy is used to improve symptoms and heal the osteomalacia. Recently, molecular genetic studies have shown recurrent genetic events in PMT, including the novel fusions FN1-FGFR1 and less commonly FN1-FGF1. These fusion events are hypothesized to result in autocrine/paracrine signaling loops within the tumor, spurring tumorigenesis. This review will cover the clinical features, imaging characteristics, pathologic features, molecular genetic aspects, and therapy of PMT, with a brief discussion of other neoplasms that may cause TIO.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR): clinical heterogeneity and long-term efficacious management of eight patients from four unrelated Arab families with a loss of function VDR mutation

BACKGROUND

Vitamin D regulates the concentrations of calcium and phosphate in blood and promotes the growth and remodeling of bones. The circulating active form of vitamin D, 1,25-dihydroxyvitamin D, binds to the vitamin D receptor (VDR), which heterodimerizes with the retinoid X receptor to regulate the expression of target genes. Inactivating mutations in the VDR gene cause hereditary vitamin D-resistant rickets (HVDRR), a rare disorder characterized by an early onset of rickets, growth retardation, skeletal deformities, hypocalcemia, hypophosphatemia and secondary hyperparathyroidism, and in some cases alopecia.

METHODS

We describe eight new HVDRR patients from four unrelated consanguineous families. The VDR gene was sequenced to identify mutations. The management of patients over a period of up to 11 years following the initial diagnosis is assessed.

RESULTS

Although all patients exhibit main features of HVDRR and carry the same c.885C>A (p.Y295*) loss of function mutation in the VDR gene, there was heterogeneity of the manifestations of HVDRR-associated phenotypes and developmental milestones. These eight patients were successfully treated over a period of 11 years. All clinical symptoms were improved except alopecia.

CONCLUSIONS

The study concludes that VDR sequencing and laboratory tests are essential to confirm HVDRR and to assess the effectiveness of the treatment.

Biology and Mechanisms of Action of the Vitamin D Hormone

The central role of hormonal 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] is to regulate calcium and phosphorus homeostasis via actions in intestine, kidney, and bone. These and other actions in many cell types not involved in mineral metabolism are mediated by the vitamin D receptor. Recent studies using genome-wide scale techniques have extended fundamental ideas regarding vitamin D-mediated control of gene expression while simultaneously revealing a series of new concepts. This article summarizes the current view of the biological actions of the vitamin D hormone and focuses on new concepts that drive the understanding of the mechanisms through which vitamin D operates.

Functional Analysis of VDR Gene Mutation R343H in A Child with Vitamin D-Resistant Rickets with Alopecia

The functional study of different mutations on vitamin D receptor (VDR) gene causing hereditary vitamin D-resistant rickets (HVDRR) remains limited. This study was to determine the VDR mutation and the mechanisms of this mutation-causing phenotype in a family with HVDRR and alopecia. Phenotype was analyzed, and in vitro functional studies were performed. The proband and his affected sister exhibited typical HVDRR with alopecia, and their biochemical and radiographic abnormalities but not alopecia responded to supraphysiological doses of active vitamin D₃. A novel homozygous missense R343H mutation in the exon 9 of VDR residing in the retinoid X receptor (RXR)-binding domain was identified. The expression level and C-terminal conformation of R343H mutant are not different from the wild-type VDR. This mutant had no effect on the nuclear localization of VDR, VDR-RXR heterodimerization, but it impaired CYP24A1 promoter activity in the presence of 1,25 (OH)₂ vitamin D₃, at least in part, mediated through specific nuclear receptor coactivator. Simulation models revealed the vanished interaction between guanidinium group of R343 and carboxyl group of E269. Without affecting the expression, conformation, nuclear location of VDR or heteridimerization with RXR, VDR-R343H impairs the transactivation activity of VDR on downstream transcription, accounting for HVDRR features with alopecia.

Changes in bone metabolic parameters following oral calcium supplementation in an adult patient with vitamin D-dependent rickets type 2A

Vitamin D-dependent rickets type 2A (VDDR2A) is a rare inherited disorder with decreased tissue responsiveness to 1,25-dihydroxyvitamin D [1,25(OH)₂D], caused by loss of function mutations in the vitamin D receptor (VDR) gene. Approximately 50 types of mutations have been identified so far that change amino acids in either the N-terminal DNA binding domain (DBD) or the C-terminal ligand binding domain (LBD) of the VDR protein. The degree of responsiveness to 1,25(OH)₂D varies between patients with VDDR2A, which may depend on their residual VDR function. In this report, we describe a female patient with VDDR2A caused by an early stop codon (R30X) in the VDR gene that resulted in a severely truncated VDR protein. She developed alopecia and bowed legs within a year after birth and was diagnosed with rickets at the age of 2. She had been treated with active vitamin D and oral calcium supplementation until 22 years of age, when she developed secondary hyperparathyroidism and high bone turnover. The genetic diagnosis of VDDR2A promoted the discontinuation of active vitamin D treatment in favor of monotherapy with oral calcium supplementation. We observed amelioration of the secondary hyperparathyroidism and normalization of bone metabolic parameters within 6 years.

Sex-related differences in the skeletal phenotype of aged vitamin D receptor global knockout mice

The role of the vitamin D receptor (VDR) in maintaining skeletal health appears to be complex and dependent on the physiological context. Global Vdr deletion in a mouse model (Vdr^-/-) results in hypocalcemia, secondary hyperparathyroidism and bone features typical of vitamin D-dependent rickets type II. When weanling Vdr^-/- mice are fed a diet containing high levels of calcium, phosphorus and lactose, termed the rescue diet, normalisation of serum calcium, phosphate and parathyroid hormone levels results in prevention of rickets at 10 weeks of age. However, 17 week old male Vdr^-/- mice, fed the rescue diet, have been reported as osteopenic due to a decrease in bone formation when compared to wild type mice. We now report confirmation of this finding with further data on the effect of the rescue diet on appendicular and axial skeletal structures in male and female Vdr^-/- mice at 26 weeks of age compared to Vdr^+/- controls. All Vdr^-/- mice were normocalcemic with no evidence of any mineralization defect. However, male Vdr^-/- mice exhibited significantly reduced mineral in femoral and vertebral bones when compared to control littermate Vdr^+/- mice, consistent with the previously reported data. In contrast, 26-week-old female Vdr^-/- mice demonstrated significantly increased femoral trabecular bone volume although there was decreased vertebral trabecular bone volume, similar to males, and femoral cortical bone volume was unchanged. Thus, the Vdr^-/- mouse model displays sex- and site-specific differences in skeletal structures with long-term feeding of a rescue diet. Although the global Vdr^-/- ablation does not permit the determination of skeletal mechanisms producing these differences, these data confirm skeletal changes even when fed the rescue diet.

Successful intermittent intravenous calcium treatment via the peripheral route in a patient with hereditary vitamin D-resistant rickets and alopecia

BACKGROUND

Hereditary vitamin D-resistant rickets (HVDRR) is a rare genetic disorder caused by mutations in the vitamin D receptor (VDR) gene, which result in end-organ resistance to 1,25-(OH)2D3. PATIENTs with HVDRR are mostly treated using i.v. calcium therapy with a central catheter. However, central catheter-related complications have been reported.

PATIENT

The patient was a 3-year-old boy presenting with waddling gait and alopecia. He had hypocalcemia [8 mg/dl (2 mmol/l)], hyperparathyroidism (1,232 ng/l), and elevated 1,25-(OH)2D3 levels (>250 pmol/l). DNA sequence analyses of the VDR gene showed a homozygous C-T transition at codon 152, resulting in a non-sense mutation in exon 5.

INTERVENTIONS AND OUTCOMES

The patient was initially treated with calcitriol (80 ng/kg/day) and high-dose oral calcium (150 mg/kg/day) for one year. At the end of the first year, intermittent (5 days per month) i.v. calcium therapy without a central catheter was initiated because of insufficient clinical and radiological improvement. After 2 years of intermittent i.v. calcium therapy, there was a clear improvement based on clinical progress and on X-ray and biochemical findings. No peripheral complications were reported either.

CONCLUSION

HVDRR with a non-sense mutation in the ligand-binding domain and alopecia was successfully treated with intermittent i.v. calcium without a central catheter.

Identification of a novel nonsense mutation in the ligand-binding domain of the vitamin d receptor gene and clinical description of two greek patients with hereditary vitamin d-resistant rickets and alopecia

BACKGROUND/AIMS

We analyzed the vitamin D receptor (VDR) gene in 2 Greek patients who exhibited the classical features of hereditary vitamin D-resistant rickets (HVDRR) type II, including severe bone deformities and alopecia. We also describe the clinical phenotypes and the response to treatment of our patients.

METHODS

Genomic DNA was extracted from peripheral blood samples of both patients. Coding region and flanking introns of VDR gDNA was amplified and direct sequenced.

RESULTS

A unique cytosine to thymine (C>T) transition was identified at nucleotide position 1066 (c.1066C>T) in the ligand-binding domain of the VDR gene of both patients, predicting the substitution of a glutamine to a terminal codon at position 356 (Gln356stop).

CONCLUSIONS

The novel nonsense mutation c.1066C>T (Gln356stop) is expected to result in a VDR protein 71 amino acids shorter and thus to affect the normal VDR function. In particular, the missing protein part alters the VDR heterodimerization with the retinoid X receptor which has been correlated with the presence of alopecia. Both patients were introduced to treatment with supraphysiological doses of 1α-calcidiol which improved their clinical phenotypes except for alopecia.

Insights into genetic and epigenetic determinants with impact on vitamin d signaling and cancer association studies: the case of thyroid cancer

Vitamin D is a key regulator of calcium metabolism and has been implicated as a cancer preventive agent. However, clinical studies have revealed conflicting results on its cancer preventive properties, attributed in part to multiple metabolic and regulatory factors susceptible to affect individual responses to exogenous vitamin D. Vitamin D is obtained from dietary sources and sun exposure, which depends on numerous parameters such as skin type, latitude, and lifestyle factors. Focusing on thyroid cancer (TC), we document that genetic and epigenetic determinants can greatly impact individual response to vitamin D and may outweigh the classical clinical correlative studies that focus on sun exposure/dietary intake factors. In particular, genetic determinants innate to host intrinsic metabolic pathways such as highly polymorphic cytochromes P450s responsible for the metabolic activation of vitamin D are expressed in many organs, including the thyroid gland and can impact vitamin D interaction with its nuclear receptor (VDR) in thyroid tissue. Moreover, downstream regulatory pathways in vitamin D signaling as well as VDR are also subject to wide genetic variability among human populations as shown by genome-wide studies. These genetic variations in multiple components of vitamin D pathways are critical determinants for the revaluation of the potential preventive and anticancer properties of vitamin D in TC.

A mutated vitamin D receptor in hereditary vitamin D-resistant rickets prevents induction of bronchial hyperreactivity and inflammation

CONTEXT

Previous studies have reported an association between vitamin D deficiency and asthma. Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) patients provide a natural model to assess the role of the vitamin D receptor (VDR) in regulating human lung immune responses and airway hyperreactivity.

OBJECTIVES

The aim of the study was to determine the role of the VDR on lung functions, airways, and systemic markers of inflammation and allergy in HVDRR patients.

DESIGN AND METHODS

Thirteen HVDRR patients (aged 6-37 y) and 17 normal controls (aged 6-38 y) underwent spirometry, a methacholine challenge test (MCT), blood tests, allergy skin tests, determination of fractional exhaled nitric oxide, and measurement of serum and exhaled breath condensate cytokines, including IL-4, IL-5, IL-10, IL-17, and interferon-γ levels.

RESULTS

All HVDRR patients had negative MCT results, whereas six controls (35.3%) had positive MCT results (P < .014). Serum IgE levels, eosinophil counts, and fractional exhaled nitric oxide and allergy skin test results were similar for the HVDRR patients and controls, as were the serum cytokine concentrations. The HVDRR patients had different cytokine levels in their exhaled breath condensate (increased IL-4 and IL-17 and decreased IL-5, IL-10, and interferon-γ levels) compared to the controls (P < .005).

CONCLUSIONS

HVDRR patients show diverse exhaled cytokine profiles but seem to be protected against provoked bronchial hyperreactivity and clinical asthma. These findings suggest that an intact VDR has an important role in asthma pathophysiology.

Vitamin D and its receptor during late development

Expression of the vitamin D receptor (VDR) is widespread but may vary depending on the developmental stage of the animal, and therefore may differentially influence phenotypic function. Thus, the major role of the 1,25-dihydroxyvitamin D [1,25(OH)2D]/VDR system is to regulate mineral and skeletal homeostasis, although mainly after birth. Post-natally, under conditions of low dietary calcium, the 1,25(OH)2D/VDR system enhances intestinal transcellular transport of calcium and possibly paracellular calcium entry by regulating genes that are critical for these functions. This process, by providing adequate calcium, is essential for normal development of the skeletal growth plate and mineralization of bone. Furthermore, blood calcium and phosphorus homeostasis is maintained by an interplay between feedback loops of the 1,25(OH)2D/VDR system with parathyroid hormone and with fibroblast-growth factor (FGF) 23 respectively. The 1,25(OH)2D/VDR system can also modulate the expression of genes involved in both bone formation and resorption post-natally. Ligand independent activity of the VDR normally influences mammalian hair cycling after birth by potentiating Wnt and bone morphogenetic protein (BMP) signaling. Nevertheless ligand bound VDR may also modulate epidermal cell proliferation/differentiation by regulating the balance in function of c-MYC and its antagonist the transcriptional repressor MAD1/MXD1 in skin epithelia. The 1,25(OH)2D/VDR system can also modulate innate immune cells and promote a more tolerogenic immunological status and may therefore influence inflammation and the development of autoimmunity; whether this impacts the fetus is uncertain. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Mutations in the vitamin D receptor and hereditary vitamin D-resistant rickets

Heterogeneous loss of function mutations in the vitamin D receptor (VDR) interfere with vitamin D signaling and cause hereditary vitamin D-resistant rickets (HVDRR). HVDRR is characterized by hypocalcemia, secondary hyperparathyroidism and severe early-onset rickets in infancy and is often associated with consanguinity. Affected children may also exhibit alopecia of the scalp and total body. The children usually fail to respond to treatment with calcitriol; in fact, their endogenous levels are often very elevated. Successful treatment requires reversal of hypocalcemia and secondary hyperparathyroidism and is usually accomplished by administration of high doses of calcium given either intravenously or sometimes orally to bypass the intestinal defect in VDR signaling.

The rachitic tooth

Teeth are mineralized organs composed of three unique hard tissues, enamel, dentin, and cementum, and supported by the surrounding alveolar bone. Although odontogenesis differs from osteogenesis in several respects, tooth mineralization is susceptible to similar developmental failures as bone. Here we discuss conditions fitting under the umbrella of rickets, which traditionally referred to skeletal disease associated with vitamin D deficiency but has been more recently expanded to include newly identified factors involved in endocrine regulation of vitamin D, phosphate, and calcium, including phosphate-regulating endopeptidase homolog, X-linked, fibroblast growth factor 23, and dentin matrix protein 1. Systemic mineral metabolism intersects with local regulation of mineralization, and factors including tissue nonspecific alkaline phosphatase are necessary for proper mineralization, where rickets can result from loss of activity of tissue nonspecific alkaline phosphatase. Individuals suffering from rickets often bear the additional burden of a defective dentition, and transgenic mouse models have aided in understanding the nature and mechanisms involved in tooth defects, which may or may not parallel rachitic bone defects. This report reviews dental effects of the range of rachitic disorders, including discussion of etiologies of hereditary forms of rickets, a survey of resulting bone and tooth mineralization disorders, and a discussion of mechanisms, known and hypothesized, involved in the observed dental pathologies. Descriptions of human pathology are augmented by analysis of transgenic mouse models, and new interpretations are brought to bear on questions of how teeth are affected under conditions of rickets. In short, the rachitic tooth will be revealed.

Vitamin D receptor mutations in patients with hereditary 1,25-dihydroxyvitamin D-resistant rickets

CONTEXT

Hereditary vitamin D resistant rickets (HVDRR), also known as vitamin D-dependent rickets type II, is an autosomal recessive disorder characterized by the early onset of rickets with hypocalcemia, secondary hyperparathyroidism and hypophosphatemia and is caused by mutations in the vitamin D receptor (VDR) gene. The human gene encoding the VDR is located on chromosome 12 and comprises eight coding exons and seven introns.

OBJECTIVES, PATIENTS, AND METHODS

We analyzed the VDR gene of 5 previously unreported patients, two from Singapore and one each from Macedonia (former Yugoslav Republic), Saudi Arabia and Turkey. Each patient had clinical and radiographic features of rickets, hypocalcemia, and the 4 cases that had the measurement showed elevated serum concentrations of 1,25-dihydroxyvitamin D (1,25(OH)(2)D). Mutations were re-created in the WT VDR cDNA and examined for 1,25(OH)(2)D(3)-mediated transactivation in COS-7 monkey kidney cells.

RESULTS

Direct sequencing identified four novel mutations and two previously described mutations in the VDR gene. The novel mutations included a missense mutation in exon 3 causing the amino acid change C60W; a missense mutation in exon 4 causing the amino acid change D144N; a missense mutation in exon 7 causing the amino acid change N276Y; and a 2bp deletion in exon 3 5'-splice site (IVS3∆+4-5) leading to a premature stop.

CONCLUSIONS

These 4 unique mutations add to the previous 45 mutations identified in the VDR gene in patients with HVDRR.

Cell-autonomous regulation of brown fat identity gene UCP1 by unliganded vitamin D receptor

White adipose tissue stores energy in the form of lipids, and brown adipose tissue expends energy via uncoupled fatty acid oxidation, which leads to the generation of heat. Obesity reflects an imbalance between energy storage and energy expenditure and is strongly associated with metabolic and cardiovascular disease. Therefore, there are important medical and biological implications for elucidating the mechanisms that promote energy expenditure in humans. Animal models with altered vitamin D receptor (VDR) expression have changes in energy expenditure. However, the specific mechanism for this effect has not been elucidated and the relevance for humans is unclear. Here we show, using human patient samples from individuals with hereditary vitamin D resistant rickets, that the VDR directly inhibits the expression of uncoupling protein-1 (UCP1), the critical protein for uncoupling fatty acid oxidation in brown fat and burning energy. The inhibition is enforced by VDR occupancy of a negative response element in the promoter proximal region of the UCP1 gene. Deletion of VDR increases UCP1 expression and results in a "browning" of adipocytes. Importantly, we found that this process occurs cell autonomously and is independent of the physiologic VDR hormone ligand, 1,25-dihydroxyvitamin D. These results identify a mechanism for modulating energy balance in humans.

The role of vitamin D receptor in innate and adaptive immunity: a study in hereditary vitamin D-resistant rickets patients

CONTEXT

Vitamin D has regulatory effects on innate and adaptive immunity. Curiously, hereditary vitamin D-resistant rickets (HVDRR) patients show no increased incidence of infectious or autoimmune diseases.

OBJECTIVES

The aim of the study was to investigate the role of vitamin D and the vitamin D receptor (VDR) in innate and adaptive immune responses in monocytes and lymphocytes from HVDRR patients.

DESIGN AND METHODS

Fifteen HVDRR patients and 17 controls participated in the investigation. Activated monocytes (lipopolysaccharides) and lymphocytes (anti-CD3, CD28, and α-GalCer) were incubated with and without 25(OH)D3 (100 nM). The mRNA expressions of CYP27B1 and VDR; vitamin D response (TLR2); vitamin D response elements binding protein (hnRNP); antimicrobial peptides cathelicidin and β-defensin; the transcription factor enhancer binding proteins C/EBPα, C/EBPβ, and C/EBPε and enzymes involved in NO generation, Nos2, and Arginase1 were analyzed by RT-PCR. TNF-α, interferon-γ, IL-4, IL-10, and IL-17 concentrations in lymphocyte cultures media were measured by ELISA.

RESULTS

Cathelicidin expression was lower in HVDRR monocytes than in control monocytes. 25(OH)D3 increased significantly the expression of cathelicidin in control monocytes (2.3-fold) but only slightly in HVDRR monocytes. 25(OH)D3 increased the expression of VDR (2-fold), C/EBPε (2-fold), C/EBPβ (1.7-fold), and hnRNP and suppressed TLR2 only in control monocytes. Unexpectedly, 25(OH)D3 increased the expression of CYP27b1, C/EBPα, Nos2, and Arginase1 in HVDRR monocytes. TNFα and IL-17 concentrations were significantly higher in HVDRR lymphocyte cultures than in controls. 25(OH)D3 suppressed IL-17 only in control lymphocyte. 25(OH)D3 increased IL-4, IL-10, and interferon-γ concentrations in control lymphocyte media but not in HVDRR.

CONCLUSIONS

Our results demonstrate impairments in various components of innate immunity in HVDTRR patients' monocytes and a proinflammatory cytokine profile in their lymphocytes. The underlying VDR-independent compensatory mechanisms that protect HVDRR patients from infections and autoimmune diseases remain undetermined.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Genetic disorders and defects in vitamin D action

The biochemical and genetic analysis of the VDR in patients with HVDRR has yielded important insights into the structure and function of the receptor in mediating 1,25(OH)2D3 action. Similarly, study of children affected by HVDRR continues to provide a more complete understanding of the biologic role of 1,25(OH)2D3 in vivo. A concerted investigative approach to HVDRR at the clinical, cellular, and molecular levels has proved valuable in gaining knowledge of the functions of the domains of the VDR and elucidating the detailed mechanism of action of 1,25(OH)2D3. These studies have been essential to promote the well-being of the families with HVDRR and in improving the diagnostic and clinical management of this rare genetic disease.

Hereditary vitamin D-resistant rickets (HVDRR) owing to a heterozygous mutation in the vitamin D receptor

Hereditary vitamin D-resistant rickets (HVDRR) is a rare autosomal recessive disease caused by mutations in the vitamin D receptor (VDR). Patients exhibit severe rickets and hypocalcemia. Heterozygous parents and siblings appear normal and exhibit no symptoms of the disease. We analyzed the VDR gene of a young girl who exhibited the clinical features of HVDRR without alopecia. The patient had clinical and radiographic features of rickets, hypocalcemia, and elevated serum concentrations of 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. A single heterozygous missense mutation was found in the VDR gene that substituted glutamic acid with alanine at amino acid 420 (E420A). Sequencing of the girl's VDR cDNAs showed that the f/M1 allele contained the E420A mutation, whereas the F/M4 allele was completely normal. The girl's father, who was also heterozygous for the E420A mutation on the f/M1 allele, exhibited minor symptoms of vitamin D resistance. In contrast, the mother had no signs of the disease and had no mutations in her VDR gene. Both the girl and the father's skin fibroblasts showed resistance to 1,25(OH)(2)D(3) by their severely reduced induction of CYP24A1 gene expression. In transactivation assays, the E420A mutant VDR showed dominant-negative activity towards the wild-type VDR. This is the first report that we are aware of describing a patient with HVDRR caused by a single heterozygous missense mutation in the VDR gene. The E420A mutant appears to act in a dominant-negative fashion, silencing the wild-type VDR and resulting in an attenuated response to 1,25(OH)(2)D(3).

The renin-angiotensin system, blood pressure, and heart structure in patients with hereditary vitamin D-resistance rickets (HVDRR)

Vitamin D deficiency has been linked to hypertension and an increased prevalence of cardiovascular risk factors and disease. Studies in vitamin D receptor knockout (VDR KO) mice revealed an overstimulated renin-angiotensin system (RAS) and consequent high blood pressure and cardiac hypertrophy. VDR KO mice correspond phenotypically and metabolically to humans with hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR). There are no data on the cardiovascular system in human HVDRR. To better understand the effects of vitamin D on the human cardiovascular system, the RAS, blood pressure levels, and cardiac structures were examined in HVDRR patients. Seventeen patients (9 males, 8 females, aged 6 to 36 years) with hereditary HVDRR were enrolled. The control group included age- and gender-matched healthy subjects. Serum calcium, phosphorous, creatinine, 25-hydroxyvitamin D [25(OH)D],1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ], parathyroid hormone (PTH), plasma rennin activity (PRA), aldosterone, angiotensin II (AT-II), and angiotensin-converting enzyme (ACE) levels were determined. Ambulatory 24-hour blood pressure measurements and echocardiographic examinations were performed. Serum calcium, phosphorus, and alkaline phosphatase values were normal. Serum 1,25(OH)(2) D(3) and PTH but not PRA and ACE levels were elevated in the HVDRR patients. AT-II levels were higher than normal in the HVDRR patients but not significantly different from those of the controls. Aldosterone levels were normal in all HVDRR patients. No HVDRR patient had hypertension or echocardiographic pathology. These findings reveal that 6- to 36-year-old humans with HVDRR have normal renin and ACE activity, mild but nonsignificant elevation of AT-II, normal aldosterone levels, and no hypertension or gross heart abnormalities.

Hormone resistance at the clinical level

This presentation introduces clinical aspects of hormone resistance through a number of case studies that illustrate how molecular defects at various steps in hormone production, signaling, or responsiveness can produce disease in humans. The presentation also shows how careful phenotyping of such patients has triggered decades of translational research.

Two new unrelated cases of hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia resulting from the same novel nonsense mutation in the vitamin D receptor gene

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) an important regulator of bone homeostasis, mediates its actions by binding to the vitamin D receptor (VDR), a nuclear transcription factor. Mutations in the VDR cause the rare genetic disease hereditary vitamin D resistant rickets (HVDRR). In this study, we examined two unrelated young female patients who exhibited severe early onset rickets, hypocalcemia, and hypophosphatemia. Both patients had partial alopecia but with different unusual patterns of scant hair. Sequencing of the VDR gene showed that both patients harbored the same unique nonsense mutation that resulted in a premature stop codon (R50X). Skin fibroblasts from patient #1 were devoid of VDR protein and 1,25(OH)2D3 treatment of these cells failed to induce CYP24A1 gene expression, a marker of 1,25(OH)2D3 action. In conclusion, we identified a novel nonsense mutation in the VDR gene in two patients with HVDRR and alopecia. The mutation truncates the VDR protein and causes 1,25(OH)2D3 resistance.

Genetic disorders and defects in vitamin d action

Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D, the circulating hormone precursor, is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase). When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets. When the VDR is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops. Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. In this article, these 2 genetic childhood diseases, which present similarly with hypocalcemia and rickets in infancy, are discussed and compared.

Down-regulation of NF-kappaB signals is involved in loss of 1alpha,25-dihydroxyvitamin D3 responsiveness

Vitamin D anti-tumor effect is often found reduced in the late stages of cancer. To uncover vitamin D resistance mechanism, we established a vitamin D-resistant human prostate cancer LNCaP cell line, LNCaP-R, by chronic exposure of cells to 1alpha,25-dihydroxyvitamin D(3) (1,25-VD). The vitamin D receptor (VDR)-mediated transcriptional activity was reduced in LNCaP-R, whereas VDR expression level and DNA-binding capacity were similar compared to parental cells (LNCaP-P). The expressions of the key factors involved in VDR transactivity, including CYP24A1 and VDR-associated proteins are all increased in LNCaP-R cells, and yet treatment with ketoconazole, P450 enzymes inhibitor, as well as trichostatin A (TSA), a histone deacetylase inhibitor, did not sensitize LNCaP-R cells response to vitamin D, suggesting that neither a local 1,25-VD availability, nor VDR-associated proteins are responsible for the vitamin D resistance. Interestingly, nuclear factor-kappaB (NF-kappaB) signaling, which is critical for 1,25-VD/VDR activity was found reduced in LNCaP-R cells, thereby treatment with NF-kappaB activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), can sensitize LNCaP-R vitamin D response. Together, we conclude that NF-kappaB signaling is critical for vitamin D sensitivity, and dysregulation of this pathway would result in vitamin D resistance and disease progression.

Presence of a deletion mutation (c.716delA) in the ligand binding domain of the vitamin D receptor in an Indian patient with vitamin D-dependent rickets type II

Vitamin D-dependent rickets type II (VDDR-type II) is a rare disorder caused by mutations in the vitamin D receptor (VDR) gene. Here, we describe a patient with VDDR-type II with severe alopecia and rickets. She had hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and elevated serum alkaline phosphatase and 1,25-dihydroxyvitamin D(3). Sequence analysis of the lymphocyte VDR cDNA revealed deletion mutation c.716delA. Sequence analysis of her genomic DNA fragment amplified from exon 6 of the VDR gene incorporating this mutation confirmed the presence of the mutation in homozygous form. This frameshift mutation in the ligand binding domain (LBD) resulted in premature termination (p.Lys240Argfs) of the VDR protein. The mutant protein contained 246 amino acids, with 239 normal amino acids at the N terminus, followed by seven changed amino acids resulting in complete loss of its LBD. The mutant VDR protein showed evidence of 50% reduced binding with VDR response elements on electrophoretic mobility assay in comparison to the wild-type VDR protein. She was treated with high-dose calcium infusion and oral phosphate. After 18 months of treatment, she gained 6 cm of height, serum calcium and phosphorus improved, alkaline phosphatase levels decreased, and intact PTH normalized. Radiologically, there were signs of healing of rickets. Her parents and one of her siblings had the same c.716delA mutation in heterozygous form. Despite the complete absence of LBD, the rickets showed signs of healing with intravenous calcium.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia resulting from a novel missense mutation in the DNA-binding domain of the vitamin D receptor

The rare genetic recessive disease, hereditary vitamin D resistant rickets (HVDRR), is caused by mutations in the vitamin D receptor (VDR) that result in resistance to the active hormone 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3) or calcitriol). In this study, we examined the VDR from a young boy with clinical features of HVDRR including severe rickets, hypocalcemia, hypophosphatemia and partial alopecia. The pattern of alopecia was very unusual with areas of total baldness, adjacent to normal hair and regions of scant hair. The child failed to improve on oral calcium and vitamin D therapy but his abnormal chemistries and his bone X-rays normalized with intravenous calcium therapy. We found that the child was homozygous for a unique missense mutation in the VDR gene that converted valine to methionine at amino acid 26 (V26M) in the VDR DNA-binding domain (DBD). The mutant VDR was studied in the patient's cultured skin fibroblasts and found to exhibit normal [(3)H]1,25(OH)(2)D(3) binding and protein expression. However, the fibroblasts were unresponsive to treatment with high concentrations of 1,25(OH)(2)D(3) as demonstrated by their failure to induce CYP24A1 gene expression, a marker of 1,25(OH)(2)D(3) responsiveness. We recreated the V26M mutation in the WT VDR and showed that in transfected COS-7 cells the mutation abolished 1,25(OH)(2)D(3)-mediated transactivation. The mutant VDR exhibited normal ligand-induced binding to RXRalpha and to the coactivator DRIP205. However, the V26M mutation inhibited VDR binding to a consensus vitamin D response element (VDRE). In summary, we have identified a novel V26M mutation in the VDR DBD as the molecular defect in a patient with HVDRR and an unusual pattern of alopecia.

Compound heterozygous mutations in the vitamin D receptor in a patient with hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia

Hereditary vitamin D-resistant rickets (HVDRR) is a rare recessive genetic disorder caused by mutations in the vitamin D receptor (VDR). In this study, we examined the VDR in a young girl with clinical features of HVDRR including rickets, hypophosphatemia, and elevated serum 1,25(OH)(2)D. The girl also had total alopecia. Two mutations were found in the VDR gene: a nonsense mutation (R30X) in the DNA-binding domain and a unique 3-bp in-frame deletion in exon 6 that deleted the codon for lysine at amino acid 246 (DeltaK246). The child and her mother were both heterozygous for the 3-bp deletion, whereas the child and her father were both heterozygous for the R30X mutation. Fibroblasts from the patient were unresponsive to 1,25(OH)(2)D(3) as shown by their failure to induce CYP24A1 gene expression, a marker of 1,25(OH)(2)D(3) responsiveness. [(3)H]1,25(OH)(2)D(3) binding and immunoblot analysis showed that the patient's cells expressed the VDRDeltaK246 mutant protein; however, the amount of VDRDeltaK246 mutant protein was significantly reduced compared with wildtype controls. In transactivation assays, the recreated VDRDeltaK246 mutant was unresponsive to 1,25(OH)(2)D(3). The DeltaK246 mutation abolished heterodimerization of the mutant VDR with RXRalpha and binding to the coactivators DRIP205 and SRC-1. However, the DeltaK246 mutation did not affect the interaction of the mutant VDR with the corepressor Hairless (HR). In summary, we describe a patient with compound heterozygous mutations in the VDR that results in HVDRR with alopecia. The R30X mutation truncates the VDR, whereas the DeltaK246 mutation prevents heterodimerization with RXR and disrupts coactivator interactions.

TGF-beta is specifically expressed in human dermal papilla cells and modulates hair folliculogenesis

Dermal papilla cells (DPCs) in the mammalian hair follicle have been shown to develop hair follicles through epithelial-mesenchymal interactions. A cell therapy to regenerate human hair is theoretically possible by expanding autologous human DPCs (hDPCs) and transplanting them into bald skin, though much remains to be overcome before clinical success. In this study, we compared gene signatures of hDPCs at different passages and human dermal fibroblasts, and found transforming growth factor (TGF)-beta(2) to be highly expressed in cultured hDPCs. Keratinocyte conditioned medium, which is known to help preserve the hair-inducing capacity of hDPCs, up-regulated TGF-beta(2) expression of hDPCs and also enhanced their alkaline phosphatase (ALP) activity, a known index for hair-inductive capacity. Through screening of components secreted from keratinocytes, the vitamin D(3) analogue was found to promote TGF-beta(2) expression and ALP activity of hDPCs. In animal hair folliculogenesis models using rat epidermis and expanded hDPCs, inhibition of TGF-beta(2) signalling at the ligand or receptor level significantly impaired hair folliculogenesis and maturation. These results suggest an important role for TGF-beta(2) in hair follicle morphogenesis and provide insights into the establishment of future cell therapies for hair regrowth by transplanting expanded DPCs.

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.

Increased Expression of Corepressors in Aggressive Androgen-Independent Prostate Cancer Cells Results in Loss of 1α,25-Dihydroxyvitamin D3 Responsiveness

Vitamin D has antiproliferative activity in prostate cancer; however, resistance to vitamin D-mediated growth inhibition occurs. To investigate the mechanisms of vitamin D resistance, we screened two prostate cancer sublines of CWR22rv1, CWR22R-1, and CWR22R-2, with differential sensitivity to vitamin D. CWR22R-2 showed less response to the antiproliferative effect of vitamin D than CWR22R-1. The vitamin D receptor (VDR)-mediated transcriptional activity was also decreased in CWR22R-2. We further showed that the DNA-binding ability of VDR was decreased and the amount of NCoR in VDR response element was increased in CWR22R-2. Analysis of VDR-associated protein profiles found higher expression of the corepressors, NCoR1 and SMRT, in CWR22R-2 cells. Treatment with the histone deacetylase inhibitor, trichostatin A, increased vitamin D/VDR transcriptional activity and promoted the antiproliferative effect of vitamin D in CWR22R-2 cells. Targeted down-regulation of NCoR1 and SMRT by small interference RNA was able to restore CWR22R-2 response to vitamin D. Together, we showed that increased NCoR1 and SMRT expression in CWR22R-2 cells resulted in reduced VDR-mediated transcriptional activity and attenuated antiproliferative response to vitamin D. Our data suggest that the integrity of the vitamin D/VDR-mediated signaling pathway is crucial in predicting vitamin D responsiveness and thus provide a rational design to improve vitamin D-based treatment efficacy based on molecular profiles of patients.

Role of the vitamin D receptor in hair follicle biology

The vitamin D receptor (VDR) is expressed in numerous cells and tissues, including the skin. The critical requirement for cutaneous expression of the VDR has been proven by investigations in mice and humans lacking functional receptors. These studies demonstrate that absence of the VDR leads to the development of alopecia. The hair follicle is formed by reciprocal interactions between an epidermal placode, which gives rise to the hair follicle keratinocytes and the underlying mesoderm which gives rise to the dermal papilla. Hair follicle morphogenesis ends the second week of life in mice. Studies in VDR null mice have failed to demonstrate a cutaneous abnormality during this period of hair follicle morphogenesis. However, VDR null mice are unable to initiate a new hair cycle after the period of morphogenesis is complete, therefore, do not grow new hair. Investigations in transgenic mice have demonstrated that restricted expression of the VDR to keratinocytes is capable of preventing alopecia in the VDR null mice, thus demonstrating that the epidermal component of the hair follicle requires VDR expression to maintain normal hair follicle homeostasis. Studies were then performed to determine which regions of the VDR were required for these actions. Investigations in mice lacking the first zinc finger of the VDR have demonstrated that they express a truncated receptor containing an intact ligand binding and AF2 domain. These mice are a phenocopy of mice lacking the VDR, thus demonstrate the critical requirement of the DNA binding domain for hair follicle homeostasis. Transgenic mice expressing VDRs with mutations in either the ligand-binding domain or the AF2 domain were generated. These investigations demonstrated that mutant VDRs incapable of ligand-dependent transactivation were able to prevent alopecia. Investigations are currently underway to define the mechanism by which the unliganded VDR maintains hair follicle homeostasis.

Vitamin D Receptor Polymorphisms in Hypocalcemic Vitamin D-Resistant Rickets Carriers

BACKGROUND/AIMS

Hypocalcemic vitamin D-resistant rickets (HVDRR) is a rare autosomal recessive disorder characterized by severe rickets, hypocalcemia, secondary hyperparathyroidism, elevated levels of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], and occasionally, alopecia. In most cases, the disease is associated with mutations in the gene of the vitamin D receptor (VDR), the mediator of 1,25(OH)(2)D(3) action. The apparently healthy HVDRR heterozygotes express both normal and mutant VDR alleles, and they present higher levels of 1,25(OH)(2)D(3) than their respective controls. Because VDR function, except for the disease-causative mutations, might be influenced by the presence of certain polymorphisms, we investigated the distribution of four common VDR polymorphisms--BsmI, ApaI, TaqI and FokI--in HVDRR carriers compared with their respective controls.

METHODS

Sixty-seven relatives of 2 HVDRR patients, all members of an extended Greek kindred, were included in the study. VDR allelic polymorphisms were assessed by restriction fragment length polymorphisms after specific polymerase chain reaction amplification.

RESULTS

The distribution of genotypic and allelic frequencies differed between HVDRR carriers and their respective controls regarding BsmI and TaqI polymorphisms. The bb genotype and the T allele (presence of BsmI and absence of TaqI polymorphisms) were less frequent in the HVDRR carrier group than in the control group in a statistically significant manner (p = 0.029 and p = 0.025, respectively).

CONCLUSIONS

Our findings showed that the apparently healthy HVDRR carriers present a different distribution of BsmI and TaqI VDR polymorphisms than their controls, suggesting that further investigation of the HVDRR carrier population may elucidate the implication of VDR alleles in VDR function and the vitamin D endocrine system.

A unique insertion/duplication in the VDR gene that truncates the VDR causing hereditary 1,25-dihydroxyvitamin D-resistant rickets without alopecia

Hereditary vitamin D resistant rickets (HVDRR) is caused by mutations in the vitamin D receptor (VDR). Here we describe a patient with HVDRR who also exhibited some hypotrichosis of the scalp but otherwise had normal hair and skin. A 102 bp insertion/duplication was found in the VDR gene that introduced a premature stop (Y401X). The patient's fibroblasts expressed the truncated VDR, but were resistant to 1,25(OH)2D3. The truncated VDR weakly bound [3H]-1,25(OH)2D3 but was able to heterodimerize with RXR, bind to DNA and interact with the corepressor hairless (HR). However, the truncated VDR failed to bind coactivators and was transactivation defective. Since the patient did not have alopecia or papular lesions of the skin generally found in patients with premature stop mutations this suggests that this distally truncated VDR can still regulate the hair cycle and epidermal differentiation possibly through its interactions with RXR and HR to suppress gene transactivation.

A girl with a novel splice site mutation in VDR supports the role of a ligand-independent VDR function on hair cycling

Mutations in vitamin D receptor (VDR) cause hereditary vitamin D resistant rickets (HVDRR). We reported a Thai girl with HVDRR, presenting with an early onset of rickets and partial alopecia. She was a product of a consanguineous couple. Mutation analysis showed that she was homozygous for a novel splice site mutation of the VDR gene, 462 + 1 G --> C, resulting in incorporation of the whole 254 bp of the intron 4 into its mRNA. The mutated protein is expected to contain no ligand-binding domain. The fact that she did not develop total alopecia despite of no VDR ligand-binding domain supports that VDR function on hair cycling is ligand independent.

Calcium and vitamin D metabolism in hypocalcemic vitamin D-resistant rickets carriers

BACKGROUND/AIMS

Hypocalcemic vitamin D-resistant rickets (HVDRR) is a rare monogenic autosomal recessive disorder associated with mutations in the gene of the vitamin D receptor (VDR), the mediator of 1,25(OH)2D3 action. Although many investigations have discussed the clinical manifestations and molecular etiology of this disease, only a few have investigated the biochemical and hormonal status of heterozygous HVDRR. The aim of the current work was to investigate the profile of selected biochemical and hormonal parameters related to the vitamin D endocrine system in a large number of HVDRR heterozygotes.

METHODS

67 relatives of 2 HVDRR patients, all members of an extended Greek kindred of five generations with a common ancestor, were included in the study. Direct sequencing was used to identify VDR gene mutations. Serum Ca, P, 25(OH)D, iPTH, and 1,25(OH)2D levels were determined in all members of the kindred.

RESULTS

DNA analysis of the participants led to the design of two study groups: the HVDRR carriers (24) and the control subjects (43). Our results showed elevated circulating serum levels of 1,25(OH)2D3 and lower levels of PTH than their age- and sex-matched controls. No hypocalcemia or hypophosphatemia were detected in HVDRR carriers.

CONCLUSIONS

Our findings suggest that HVDRR carriers may have compensatory elevated serum levels of 1,25(OH)2D3 through which they restrain PTH secretion. The study of HVDRR carriers could be a useful tool for the investigation of the vitamin D endocrine system.

Hereditary 1,25-dihydroxyvitamin D resistant rickets due to a mutation causing multiple defects in vitamin D receptor function

Hereditary vitamin D-resistant rickets (HVDRR) is an autosomal recessive disease caused by mutations in the vitamin D receptor (VDR). We studied a young Saudi Arabian girl who exhibited the typical clinical features of HVDRR, but without alopecia. Analysis of her VDR gene revealed a homozygous T to C mutation in exon 7 that changed isoleucine to threonine at amino acid 268 (I268T). From crystallographic studies of the VDR ligand-binding domain, I268 directly interacts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and is involved in the hydrophobic stabilization of helix H12. We recreated the I268T mutation and analyzed its effects on VDR function. In ligand binding assays, the I268T mutant VDR exhibited an approximately 5- to 10-fold lower affinity for [(3)H]1,25(OH)(2)D(3) compared with the wild-type (WT) VDR. The I268T mutant required approximately a 65-fold higher concentration of 1,25(OH)(2)D(3) to be equipotent in gene transactivation. Both retinoid X receptor heterodimerization and coactivator binding were reduced in the I268T mutant. Analogs of 1,25(OH)(2)D(3) have been proposed as potential therapeutics for patients with HVDRR. Interestingly, in protease sensitivity assays, treatment with the potent vitamin D analog, 20-epi-1,25(OH)(2)D(3), stabilized I268T mutant proteolytic fragments better than 1,25(OH)(2)D(3). Moreover, 20-epi-1,25(OH)(2)D(3) restored transactivation of the I268T mutant to levels exhibited by WT VDR treated with 1,25(OH)(2)D(3). In conclusion, we describe a novel mutation, I268T, in the VDR ligand-binding domain that alters ligand binding, retinoid X receptor heterodimerization, and coactivator binding. These combined defects in VDR function cause resistance to 1,25(OH)(2)D(3) action and result in the syndrome of HVDRR.

Molecular mechanisms of end-organ resistance

Hormone resistance is caused by mutations in hormone receptors or by functional desensitization of hormone signaling pathways. Mutations that cause hormone resistance have been described for most classes of hormones, including peptides like growth-hormone releasing hormone (GHRH); large proteins such as insulin and the glycoprotein hormones; steroid hormones; and even ions such as calcium. In the case of proteins, many of these receptors include seven transmembrane, G-protein-coupled receptors such as those for thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), adrenocorticotrophic hormone (ACTH), GHRH and vasopressin. However, other classes of membrane receptors can also be affected. For example, the insulin receptor is a tyrosine kinase receptor, the growth hormone (GH) and leptin receptors belong to the cytokine family of receptors, and the Müllerian inhibiting substance (MIS, also known as anti-Müllerian hormone [AMH]) receptor is related to the transforming growth factor beta (TGFbeta) serine kinase group of receptors. G-protein mutations cause Albright's hereditary osteodystrophy, and, as might be expected, there is resistance to many different hormones that act through G-protein-coupled receptors. A variety of resistance syndromes involve members of the nuclear receptor superfamily, including resistance to androgens, vitamin D, thyroid hormone, glucocorticoids and estrogen. Mutations in orphan nuclear receptors such as steroidogenic factor-1 (SF-1) or DAX-1 result in defective glandular development, thereby indirectly causing hormone resistance. Another form of resistance, sometimes referred to as "post-receptor" resistance, is typified by acquired forms of insulin or leptin resistance, and the pathophysiology remains elusive. Studies of naturally occurring mutations have provided important insights into the structure and function of receptors. By definition, the mutations that cause disease identify important functional domains in the proteins. Although therapy is straightforward in some disorders, it is challenging or incompletely studied in others.

A unique insertion/substitution in helix H1 of the vitamin D receptor ligand binding domain in a patient with hereditary 1,25-dihydroxyvitamin D-resistant rickets

INTRODUCTION

Hereditary vitamin D--resistant rickets (HVDRR) is a genetic disorder caused by mutations in the vitamin D receptor (VDR). In this study, we examined the VDR in a young boy who exhibited the typical clinical features of HVDRR but without alopecia.

MATERIALS AND METHODS

The patient's VDR was studied using cultured dermal fibroblasts, and the recreated mutant VDR was analyzed in transfected cells.

RESULTS

The patient's fibroblasts were resistant to 1,25-dihydroxyvitamin D [1,25(OH)2D3], exhibiting only a slight induction of 24-hydroxylase gene expression when treated with 1 microM 1,25(OH)2D3 x [3H]1,25(OH)2D3 binding was absent in cell extracts from the patient's fibroblasts. Sequence analysis of the VDR gene uncovered a unique 5-bp deletion/8-bp insertion in exon 4. The mutation in helix HI of the ligand-binding domain deletes two amino acids (H141 and T142) and inserts three amino acids (L141, W142, and A143). In transactivation assays, the recreated mutant VDR was 1000-fold less active than the wildtype (WT) VDR. In glutathione S-transferase (GST) pull-down assays, the mutant VDR bound GST-retinoid X receptor (RXR) weakly in the absence of 1,25(OH)2D3; however, the binding did not increase with increasing concentrations of ligand. The mutant VDR did not bind to GST-vitamin D receptor interacting protein (DRIP) 205 at concentrations up to 1 microM 1,25(OH)2D3. We also examined effects of the three individual mutations on VDR transactivation. Only the insertion of A143 into the WT VDR disrupted VDR transactivation to the same extent observed with the natural mutation.

CONCLUSION

We describe a novel insertion/substitution mutation in helix Hl of the VDR ligand-binding domain (LBD) that abolishes ligand binding and result in the syndrome of HVDRR. This is the first time an insertion/substitution has been found as the defect-causing HVDRR.