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

Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia

Introduction

Hereditary Vitamin D-dependent rickets type II (HVDDR-type II) is a rare autosomal recessive disorder caused by molecular variation in the gene encoding the vitamin D receptor (VDR). This study aims to evaluate phenotype and genotype characteristics and long-term follow-up of the largest group of patients with (HVDDR-type II) in Saudi Arabia.

Methodology

We conducted a retrospective chart review to collect the clinical, biochemical, and genetic data for all HVDDR-type II patients currently receiving treatment at King Faisal Specialist Hospital & Research Centre, Riyadh, Saudi Arabia.

Results

A total of 42 patients, 57.1% female, and 42.9% male were included in the study. Seven patients were treated with high doses of oral calcium, while 35 patients were treated with IV calcium infusion. The median age at presentation was 15.5 months. Alopecia was found in 97.6%, 21.4% presented with bowing legs, 14.3% with delayed walking, 9.5% with seizure, and 2.4% presented with respiratory failure, while a family history of the disease was positive in 71.4% of total patients. Molecular genetic testing of the VDR gene in our cohort identified six different gene variants c.885 C>A (p.Tyr295Ter), c.88 C>T (p.Arg30Ter), c.1036G>A (p.Val346Met), c.820C>T (p.Arg274Cys), c.803 T>C (p.Ile268Thr), and c.2T>G (p.Met1?).

Conclusion

We are describing the largest cohort of patients with HVDDR-type II, their clinical biochemical findings, and the most prevalent genetic variants in our population.

Hereditary Vitamin D-Resistant Rickets (HVDRR) associated SNP variants of vitamin D receptor exhibit malfunctioning at multiple levels

Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily. It is a primary regulator of calcium and phosphate homeostasis required for skeleton and bone mineralization. Vitamin D in active form 1α,25 dihydroxyvitamin-D3 mediates its cellular functions by binding to VDR. Active VDR forms heterodimers with partner RXR (retinoid X receptor) to execute its physiological actions. HVDRR (Hereditary Vitamin D-Resistant Rickets) is a rare genetic disorder that occurs because of generalized resistance to the 1α,25(OH)₂D3. HVDRR is caused by the polymorphic variations in VDR gene leading to defective intestinal calcium absorption and mineralization of newly forming bones. Using point and deletion SNPs of VDR we have studied several HVDRR-associated SNP variants for their subcellular dynamics, transcriptional functions, 'genome bookmarking', heterodimeric interactions with RXR, and receptor stability. We previously reported that VDR is a 'mitotic bookmarking factor' that remains constitutively associated with the mitotic chromatin to inherit 'transcriptional memory', however the mechanistic details remained unclear. We document that 'genome bookmarking' property by VDR is critically impaired by naturally occurring HVDRR-associated point and deletion variants found in patients. Furthermore, these HVDRR-associated SNP variants of VDR were found to be compromised in transcriptional function, nuclear translocation, protein stability and intermolecular interactions with its heterodimeric partner RXR. Intriguingly, majority of these disease-allied functional defects failed to be rescued by RXR. Our findings suggest that the HVDRR-associated SNP variations influence the normal functioning of the receptor, and this derived understanding may help in the management of disease with precisely designed small molecule modulators.

Vitamin D Analogs Bearing C-20 Modifications Stabilize the Agonistic Conformation of Non-Responsive Vitamin D Receptor Variants

The Vitamin D receptor (VDR) plays a key role in calcium homeostasis, as well as in cell proliferation and differentiation. Among the large number of VDR ligands that have been developed, we have previously shown that BXL-62 and Gemini-72, two C-20-modified vitamin D analogs are highly potent VDR agonists. In this study, we show that both VDR ligands restore the transcriptional activities of VDR variants unresponsive to the natural ligand and identified in patients with rickets. The elucidated mechanisms of action underlying the activities of these C-20-modified analogs emphasize the mutual adaptation of the ligand and the VDR ligand-binding pocket.

Rickets

Rickets is a bone disease associated with abnormal serum calcium and phosphate levels. The clinical presentation is heterogeneous and depends on the age of onset and pathogenesis but includes bowing deformities of the legs, short stature and widening of joints. The disorder can be caused by nutritional deficiencies or genetic defects. Mutations in genes encoding proteins involved in vitamin D metabolism or action, fibroblast growth factor 23 (FGF23) production or degradation, renal phosphate handling or bone mineralization have been identified. The prevalence of nutritional rickets has substantially declined compared with the prevalence 200 years ago, but the condition has been re-emerging even in some well-resourced countries; prematurely born infants or breastfed infants who have dark skin types are particularly at risk. Diagnosis is usually established by medical history, physical examination, biochemical tests and radiography. Prevention is possible only for nutritional rickets and includes supplementation or food fortification with calcium and vitamin D either alone or in combination with sunlight exposure. Treatment of typical nutritional rickets includes calcium and/or vitamin D supplementation, although instances infrequently occur in which phosphate repletion may be necessary. Management of heritable types of rickets associated with defects in vitamin D metabolism or activation involves the administration of vitamin D metabolites. Oral phosphate supplementation is usually indicated for FGF23-independent phosphopenic rickets, whereas the conventional treatment of FGF23-dependent types of rickets includes a combination of phosphate and activated vitamin D; an anti-FGF23 antibody has shown promising results and is under further study.

Structural aspects of Vitamin D endocrinology

1α,25-Dihydroxvitamin D₃ (1,25(OH)₂D₃) is the hormonally active form of vitamin D₃. Its synthesis and its metabolites, their transport and elimination as well as action on transcriptional regulation involves the harmonic cooperation of diverse proteins with vitamin D binding capacities such as vitamin D binding protein (DBP), cytochrome P450 enzymes or the nuclear vitamin receptor (VDR). The genomic mechanism of 1,25(OH)₂D₃ action involves its binding to VDR that functionally acts as a heterodimer with retinoid X receptor. The crystal structures of the most important proteins for vitamin D₃, VDR, DBP, CYP2R1 and CYP24A1, have provided identification of mechanisms of actions of these proteins and those mediating VDR-regulated transcription. This review will present the structural information on recognition of the vitamin D₃ and metabolites by CYP proteins and DBP as well as the structural basis of VDR activation by 1,25(OH)₂D₃ and metabolites. Additionally, we will describe, the implications of the VDR mutants associated with hereditary vitamin D-resistant rickets (HVDRR) that display impaired function.

Transcriptional activation of the wild-type and mutant vitamin D receptors by vitamin D3 analogs

The active form of vitamin D3 (1α,25(OH)2D3, also known as calcitriol) controls the expression of target genes via the vitamin D receptor (VDR). Vitamin D-dependent rickets type II (VDDRII) is a congenital disease caused by inactivating mutations in the VDR The condition is treated with high doses of calcitriol, but the therapeutic effects of other synthetic VD3 analogs have not yet been investigated. In the present study, we analyzed the transcriptional activity of seven different VD3 analogs with VDRs carrying ligand-binding domain mutations identified in VDDRII patients. Wild-type VDR (WT-VDR) and seven mutant VDRs were expressed in TSA201 human embryonic kidney cells, HepG2 human liver cancer cells, and MC3T3-E1 mouse calvaria cells, and their transcriptional activation with VD3 analogs were analyzed by performing transient expression assays, western blotting, and quantitative real-time PCR. The results demonstrated that falecalcitriol stimulated significantly higher transcriptional activation of the WT-VDR and some mutant VDRs than did calcitriol. Calcitriol showed almost no transcriptional activation of the VDR with the I268T mutation identified in a severe case of VDDRII, whereas falecalcitriol caused a dose-dependent increase in the activation of this mutant VDR. Our findings demonstrate that falecalcitriol has a VDR activation profile distinct from that of calcitriol and may exhibit therapeutic effects even on difficult-to-treat VDDRII cases resistant to calcitriol. It is also possible that VDDRII patients responding to high doses of calcitriol could be appropriately treated with low doses of falecalcitriol.

Structural Studies of Vitamin D Nuclear Receptor Ligand-Binding Properties

The vitamin D nuclear receptor (VDR) and its natural ligand, 1α,25-dihydroxyvitamin D3 hormone (1,25(OH)2D3, or calcitriol), classically regulate mineral homeostasis and metabolism but also much broader range of biological functions, such as cell growth, differentiation, antiproliferation, apoptosis, adaptive/innate immune responses. Being widely expressed in various tissues, VDR represents an important therapeutic target in the treatment of diverse disorders. Since ligand binding is a key step in VDR-mediated signaling, numerous 1,25(OH)2D3 analogs have been synthesized in order to selectively modulate the receptor activity. Most of the synthetic analogs have been developed by modification of a parental compound and some of them mimic 1,25(OH)2D3 scaffold without being structurally related to it. The ability of ligands that have different size and conformation to bind to VDR and to demonstrate biological effects is intriguing, and therefore, ligand-binding properties of the receptor have been extensively investigated using a variety of biochemical, biophysical, and computational methods. In this chapter, we describe different aspects of the structure-function relationship of VDR in complex with natural and synthetic ligands coming from structural analysis. With the emphasis on the binding modes of the most promising compounds, such as secosteroidal agonists and 1,25(OH)2D3 mimics, we also highlight the action of VDR antagonists and the evidence for the existence of an alternative ligand-binding site within the receptor. Additionally, we describe the crystal structures of VDR mutants associated with hereditary vitamin D-resistant rickets that display impaired ligand-binding function.

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.

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance

Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) is caused by mutations in the VDR gene, and its inheritance is autosomal recessive. In this report, we aimed to confirm whether HVDRR is occasionally inherited as a dominant trait. An 18-month-old Japanese boy was evaluated for short stature and bowlegs. His father had been treated for rickets during childhood, and his paternal grandfather had bowlegs. We diagnosed him with HVDRR based on laboratory data and radiographic evidence of rickets. Sequence analyses of VDR were performed, and the functional consequences of the detected mutations were analyzed for transcriptional activity, ligand binding, and interaction with the retinoid X receptor, cofactors, and the vitamin D response element (VDRE). A novel mutation (Q400LfsX7) and a reported variant (R370H) were identified in the patient. Heterozygous Q400LfsX7 was detected in his father, and heterozygous R370H was detected in his healthy mother. Functional studies revealed that the transcriptional activity of Q400LfsX7-VDR was markedly disturbed. The mutant had a dominant-negative effect on wild-type-VDR, and the ligand binding affinity of Q400LfsX7-VDR was completely impaired. Interestingly, Q400LfsX7-VDR had a strong interaction with corepressor NCoR and could interact with VDRE without the ligand. R370H-VDR was functionally similar to wild-type-VDR. In conclusion, we found a dominant-negative mutant of VDR causing dominantly inherited HVDRR through a constitutive corepressor interaction, a mechanism similar to that in dominantly inherited thyroid hormone receptor mutations. Our report together with a reported pedigree suggested a distinct inheritance of HVDRR and enriched our understanding of VDR abnormalities.

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We report the pedigree of dominantly inherited hereditary vitamin D resistant rickets.

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A novel VDR mutation (Q400LfsX7) was identified in the pedigree.

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The mutant had a dominant-negative effect on the wild type VDR and interacted strongly with corepressor NCoR.

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The mechanism for dominantly inheritance was similar to that in dominantly inherited thyroid hormone receptor mutations.

Regulation of gene expression by 1,25-dihydroxyvitamin D3 in bone cells: exploiting new approaches and defining new mechanisms

The biological actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) serve both to orchestrate calcium and phosphorus homeostasis in higher vertebrates and to regulate a diverse set of cellular functions unrelated to control of mineral metabolism. With regard to bone, mesenchymal lineage cells, including both early and late osteoblasts as well as osteocytes represent classic targets of the vitamin D hormone. Accordingly, much of the early information regarding our current understanding of the mechanism of action of 1,25(OH)2D3, of which gene regulation is central, derives from a broad array of studies in these cell types. Indeed, a gene that provided both the earliest and perhaps the most extensive information regarding this and additional mechanisms was that of osteoblast-specific osteocalcin. Subsequent work has provided much additional detail as to how 1,25(OH)2D3, through the vitamin D receptor (VDR), mediates the modulation of many bone cell genes. In recent years, however, a series of technical advances involving the coupling of chromatin immunoprecipitation (ChIP) to unbiased methodologies that involve next-generation DNA sequencing techniques (ChIP-seq) have opened new avenues in the study of gene regulation. In this review, we summarize early work and then focus on more recent studies that have used ChIP-seq analysis and other approaches to provide insight into not only the regulation of specific genes such as the VDR, TNFSF11 (RANKL), LRP5, CBS and CYP24a1, but overarching genome-wide principles of gene regulation as well. The results of these studies highlight the value of these new approaches and the increased insight that can be gained.

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.

Enteral calcium infusion used successfully as treatment for a patient with hereditary vitamin D resistant rickets (HVDRR) without alopecia: a novel mutation

BACKGROUND

We report a novel mutation in a case of hereditary vitamin D resistant rickets (HVDRR) without alopecia and successful management of this condition with the intravenous formulation of calcium chloride delivered via gastric tube.

CLINICAL CASE

A 22 month old male (length -3.4 SDS; weight -2.1 SDS) presented with failure to thrive, short stature, severe hypocalcemia and gross motor delay. He did not have alopecia. Initial blood tests and history were thought possibly to suggest vitamin D deficiency rickets: calcium 5.1mg/dL, (8.8-10.8); phosphorus 4.1mg/dL, (4.5-5.5); alkaline phosphatase 1481 U/L (80-220); intact PTH 537.1 pg/mL (10-71). Subsequently, vitamin D studies returned that were consistent with HVDRR: 25-hydroxyvitamin D 34 ng/mL (20-100); 1,25-dihydroxyvitamin D 507 pg/mL. This diagnosis was confirmed by DNA sequencing. His subsequent clinical course was complicated by the fact that IV calcium was not a viable option for this patient, and his calcium levels could not be well controlled on oral calcium citrate or calcium glubionate therapy. Eventually, we were able to maintain calcium levels above 8 mg/dL using the intravenous preparation of calcium chloride administered via gastric tube.

GENETIC STUDIES

A unique homozygous T to C base substitution was found in exon 6 in the vitamin D receptor (VDR) gene. This mutation causes leucine to be converted to proline at position 227 in helix 3 in the VDR ligand binding domain (LBD). The mutation rendered the VDR non-functional, leading to HVDRR, with absence of alopecia.

CONCLUSION

HVDRR should be considered in a patient with profound hypocalcemia which is refractory to conventional therapy of vitamin D deficiency rickets even without evidence of alopecia. We report the first case of HVDRR with a novel mutation in the LBD that was successfully treated with enteral treatment using a calcium chloride infusion.

Two siblings with a novel nonsense mutation, p.R50X, in the vitamin D receptor gene

Hereditary vitamin D-resistant rickets (HVDRR), an autosomal recessive disorder caused by inactivating mutations in the vitamin D receptor (VDR) gene. We identified two affected children from the same family, one at the age of 10 years and 9 months and the other at 9 months old. Mutation analysis by PCR-sequencing the entire coding region of the VDR gene revealed a homozygous C to T transition in exon 2 of the VDR gene (c.148C>T) resulting in a stop codon at amino acid position 50 (p.R50X) in the proband and his younger sister. The p.R50X has never been previously described. Both asymptomatic parents were heterozygous for the mutation. In addition to most of the clinical features of HVDRR including total alopecia, symptoms of hypocalcemia at a later onset and normophosphatemia, rarely found in HVDRR were present in the proband. This study also emphasizes an important role of genetic testing for early diagnosis and genetic counseling.

Novel compound heterozygous mutations in the vitamin D receptor gene in a Korean girl with hereditary vitamin D resistant rickets

Hereditary vitamin D resistant rickets (HVDRR) is a rare genetic disorder caused by a mutation of vitamin D receptor (VDR) gene. A number of cases had been reported in many countries but not in Korea. We examined a three-year old Korean girl who had the typical clinical features of HVDRR including rickets, hypocalcemia, hypophosphatemia, elevated serum calcitriol level and secondary hyperparathyroidism. The girl and her father were both heterozygous for the 719C-to-T(I146T)---> c.437C > T(p.T1461) [corrected] mutation in exon 4, whereas she and her mother were both heterozygous for 754C-to-T (R154C)---> c.472 > T(p.R158C) [corrected] mutation in exon 5 of the VDR gene. In this familial study, we concluded that the girl had compound heterozygous mutations in her VDR gene which caused HVDRR. This is the first report of a unique mutation in the VDR gene in Korea.

A novel nonsense mutation in the DMP1 gene identified by a genome-wide association study is responsible for inherited rickets in Corriedale sheep

Inherited rickets of Corriedale sheep is characterized by decreased growth rate, thoracic lordosis and angular limb deformities. Previous outcross and backcross studies implicate inheritance as a simple autosomal recessive disorder. A genome wide association study was conducted using the Illumina OvineSNP50 BeadChip on 20 related sheep comprising 17 affected and 3 carriers. A homozygous region of 125 consecutive single-nucleotide polymorphism (SNP) loci was identified in all affected sheep, covering a region of 6 Mb on ovine chromosome 6. Among 35 candidate genes in this region, the dentin matrix protein 1 gene (DMP1) was sequenced to reveal a nonsense mutation 250C/T on exon 6. This mutation introduced a stop codon (R145X) and could truncate C-terminal amino acids. Genotyping by PCR-RFLP for this mutation showed all 17 affected sheep were "T T" genotypes; the 3 carriers were "C T"; 24 phenotypically normal related sheep were either "C T" or "C C"; and 46 unrelated normal control sheep from other breeds were all "C C". The other SNPs in DMP1 were not concordant with the disease and can all be ruled out as candidates. Previous research has shown that mutations in the DMP1 gene are responsible for autosomal recessive hypophosphatemic rickets in humans. Dmp1_knockout mice exhibit rickets phenotypes. We believe the R145X mutation to be responsible for the inherited rickets found in Corriedale sheep. A simple diagnostic test can be designed to identify carriers with the defective "T" allele. Affected sheep could be used as animal models for this form of human rickets, and for further investigation of the role of DMP1 in phosphate homeostasis.

Genetic epidemiology of age-related osteoporosis and its clinical applications

Osteoporosis is an important and complex disorder that is highly prevalent worldwide. This disease poses a major challenge to modern medicine and its treatment is associated with high costs. Numerous studies have endeavored to decipher the pathogenesis of this disease. The clinical assessment of patients often incorporates information about a family history of osteoporotic fractures. Indeed, the observation of an increased risk of fracture in an individual with a positive parental history of hip fracture provides strong evidence for the heritability of osteoporosis. The onset and progression of osteoporosis are generally controlled by multiple genetic and environmental factors, as well as interactions between them, with rare cases determined by a single gene. In an attempt to identify the genetic markers of complex diseases such as osteoporosis, there has been a move away from traditional linkage mapping studies and candidate gene association studies to higher-density genome-wide association studies. The advent of high-throughput technology enables genotyping of millions of DNA markers in the human genome, and consequently the identification and characterization of causal variants and loci that underlie osteoporosis. This Review presents an overview of the major findings since 2007 and clinical applications of these genome-wide linkage and association studies.

Crystal structure of hereditary vitamin D-resistant rickets--associated mutant H305Q of vitamin D nuclear receptor bound to its natural ligand

In the nuclear receptor of vitamin D (VDR) histidine 305 participates to the anchoring of the ligand. The VDR H305Q mutation was identified in a patient who exhibited the hereditary vitamin D-resistant rickets (HVDRR). We report the crystal structure of human VDR H305Q-ligand binding domain bound to 1alpha,25(OH)2D3 solved at 1.8A resolution. The protein adopts the active conformation of the wild-type liganded VDR. A local conformational flexibility at the mutation site weakens the hydrogen bond between the 25-OH with Gln305, thus explaining the lower affinity of the mutant proteins for calcitriol. The structure provides the basis for a rational approach to the design of more potent ligands for the treatment of HVDRR.

Analysis of hairless corepressor mutants to characterize molecular cooperation with the vitamin D receptor in promoting the mammalian hair cycle

The mammalian hair cycle requires both the vitamin D receptor (VDR) and the hairless (Hr) corepressor, each of which is expressed in the hair follicle. Hr interacts directly with VDR to repress VDR-targeted transcription. Herein, we further map the VDR-interaction domain to regions in the C-terminal half of Hr that contain two LXXLL-like pairs of motifs known to mediate contact of Hr with the RAR-related orphan receptor alpha and with the thyroid hormone receptor, respectively. Site-directed mutagenesis indicates that all four hydrophobic motifs are required for VDR transrepression by Hr. Point mutation of rat Hr at conserved residues corresponding to natural mutants causing alopecia in mice (G985W and a C-terminal deletion DeltaAK) and in humans (P95S, C422Y, E611G, R640Q, C642G, N988S, D1030N, A1040T, V1074M, and V1154D), as well as alteration of residues in the C-terminal Jumonji C domain implicated in histone demethylation activity (C1025G/E1027G and H1143G) revealed that all Hr mutants retained VDR association, and that transrepressor activity was selectively abrogated in C642G, G985W, N988S, D1030N, V1074M, H1143G, and V1154D. Four of these latter Hr mutants (C642G, N988S, D1030N, and V1154D) were found to associate normally with histone deacetylase-3. Finally, we identified three regions of human VDR necessary for association with Hr, namely residues 109-111, 134-201, and 202-303. It is concluded that Hr and VDR interact via multiple protein-protein interfaces, with Hr recruiting histone deacetylases and possibly itself catalyzing histone demethylation to effect chromatin remodeling and repress the transcription of VDR target genes that control the hair cycle.

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.

Mutations in the vitamin D receptor gene in four patients with hereditary 1,25-dihydroxyvitamin D-resistant rickets

Mutations in the vitamin D receptor (VDR) are associated to the hereditary 1,25-dihydroxivitamin D-resistant rickets. The objectives of this work are: search for mutations in the VDR and analyze their functional consequences in four Brazilian children presented with rickets and alopecia. The coding region of the VDR was amplified by PCR e direct sequenced. We identified three mutations: two patients had the same mutation in exon 7 at aminoacid position 259 (p.Q259E); one patient had a mutation in exon 8 at codon 319 (p.G319V) and another one had a mutation in exon 3 leading to a truncated protein at position 73 (p.R73X). Functional studies of the mutant receptors of fibroblast primary culture, from patients' skin biopsy treated with increasing doses of 1,25(OH)2 vitamin D showed that VDR mutants were unable to be properly activated and presented a reduction in 24-hydroxylase expression level.

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.

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.

Bullous congenital ichthyosiform erythroderma associated with hypocalcemic vitamin D-resistant rickets

There are several reports of an association between ichthyosis and rickets based mainly on a causative relationship through the impaired ability of ichthyotic skin to synthesize vitamin D. Hypocalcemic vitamin D-resistant rickets represents a specific type of rickets that is attributed to vitamin D receptor defect rather than to vitamin D deficiency. Nevertheless, an extremely rare association between bullous congenital ichthyosiform erythroderma and hypocalcemic vitamin D-resistant rickets is presented.

Genetic ablation of vitamin D activation pathway reverses biochemical and skeletal anomalies in Fgf-23-null animals

Fibroblast growth factor-23 (FGF-23) is one of the circulating phosphaturic factors associated with renal phosphate wasting. Fgf-23-/- animals show extremely high serum levels of phosphate and 1,25-dihydroxyvitamin D3, along with abnormal bone mineralization and soft tissue calcifications. To determine the role of vitamin D in mediating altered phosphate homeostasis and skeletogenesis in the Fgf-23-/- mice, we generated mice lacking both the Fgf-23 and 1alpha-hydroxylase genes (Fgf-23-/-/1alpha(OH)ase-/-). In the current study, we have identified the cellular source of Fgf-23 in adult mice. In addition, loss of vitamin D activities from Fgf-23-/- mice reverses the severe hyperphosphatemia to hypophosphatemia, attributable to increased urinary phosphate wasting in Fgf-23-/-/1alpha(OH)ase-/- mice, possibly as a consequence of decreased expression of NaPi2a. Ablation of vitamin D from Fgf-23-/- mice resulted in further reduction of total bone mineral content and bone mineral density and reversed ectopic calcification of skeleton and soft tissues, suggesting that abnormal mineral ion homeostasis and impaired skeletogenesis in Fgf-23-/- mice are mediated through enhanced vitamin D activities. In conclusion, using genetic manipulation studies, we have provided evidence for an in vivo inverse correlation between Fgf-23 and vitamin D activities and for the severe skeletal and soft tissue abnormalities of Fgf-23-/- mice being mediated through vitamin D.

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.

Vitamin D-resistant rickets and type 1 diabetes in a child with compound heterozygous mutations of the vitamin D receptor (L263R and R391S): dissociated responses of the CYP-24 and rel-B promoters to 1,25-dihydroxyvitamin D3

We report here the first association between vitamin D-resistant rickets, alopecia, and type 1 diabetes in a child with compound heterozygous mutations in the VDR gene. Transfection studies suggest dissociated effects of VDR gene mutations on the regulation of genes involved in vitamin D metabolism and dendritic cell maturation.

INTRODUCTION

Whereas vitamin D may play a role in the immune tolerance process, no patient has been reported to associate hereditary vitamin D-resistant rickets (HVDRR) and an autoimmune disease, and no attempt has been made to delineate the outcome of mutations of the vitamin D receptor (VDR) on the transcription of genes controlling immune tolerance.

MATERIALS AND METHODS

The VDR gene was analyzed in a child with vitamin D-resistant rickets, total alopecia, and early childhood-onset type 1 diabetes. Patient's fibroblasts and COS-7 cells transfected with wildtype or mutant VDRs were studied for ligand-binding capacity, transactivation activity using two gene promoters [CYP-24, a classical 1,25(OH)2D3-responsive gene, and relB, a critical NF-kappaB component for regulation of dendritic cell differentiation], VDR-RXR heterodimers association to CYP 24 VDREs by gel mobility shift assays, and co-activator binding by Glutathione-S-transferase pull-down assays.

RESULTS

Two novel compound heterozygous mutations (L263R and R391S) were identified in the VDR ligand-binding domain in this child. Both mutations significantly impaired VDR ligand-binding capacity but had dissociated effects on CYP-24 and RelB promoter responses to vitamin D. CYP 24 response binding to SRC-1 and RXR-heterodimer binding to CYP24 VDREs were abolished in L263R mutants but normal or partially altered in R391S mutants. In the opposite, RelB responses to vitamin D were close to normal in L263R mutants but abolished in R391S mutants.

CONCLUSIONS

We report the first clinical association between HVDRR, total alopecia, and early childhood-onset type 1 diabetes. Mutations in the VDR ligand-binding domain may hamper the 1,25(OH)2D3-mediated relB responses, an effect that depends on the site of the VDR mutation and cannot be anticipated from VDR ligand-binding ability or CYP-24 response. Based on these results, we propose to survey the immune function in patients with HVDRR, including those with moderate features of rickets.

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.

Vitamin D-dependent rickets type 2 with characteristic radiographic changes in a 4-month-old kitten

This report describes the presenting signs, biochemical abnormalities, and radiographic changes in a 4-month-old kitten with vitamin D-dependent rickets type 2. Details of therapy are described and possible reasons for treatment failure are discussed.

Enhanced coactivator binding and transcriptional activation of mutant vitamin D receptors from patients with hereditary 1,25-dihydroxyvitamin D-resistant rickets by phosphorylation and vitamin D analogs

In this study, we report that the function of certain mutant VDRs from patients with hereditary HVDRR can at least be partially restored by phosphorylation and hexafluoro 1,25(OH)2D3 analogs. Our study provides new insights into mechanisms involved in enhancement of mutant VDR function.

INTRODUCTION

1,25-Dihydroxyvitamin D-resistant rickets (HVDRR) is a rare genetic disorder caused by inactivating mutations in the vitamin D receptor (VDR). In this study, we examined VDR from patients with HVDRR having mutations in the ligand-binding domain (F251C, I268T, H305Q, E420K). We examined methods of restoring transcriptional activity of these mutants and the mechanisms involved.

MATERIALS AND METHODS

Reporter gene transcriptional assays were used to examine the activation of mutant VDRs. Western-blot analysis, glutathione S-transferase (GST) pull-down assays, and chromatin immunoprecipitation (ChIP) assays were also used in this study.

RESULTS

Using mutant VDRs, H305Q, F251C, I268T, and 10(-8) M 1,25(OH)2D3, only 10-30% of the activity of wildtype (WT) VDR in activating 24(OH)ase transcription was observed. The transcriptional response of mutant VDR mutants was significantly enhanced 2- to 3-fold by co-treatment of VDR mutant transfected COS-7 cells with 1,25(OH)2D3 and okadaic acid (OA; inhibitor of phosphatase; 50 nM). The H305Q mutant was the most responsive (90% of the response exhibited by WT VDR was restored). The E420K mutant was unresponsive to 1,25(OH)2D3 in the presence or absence of OA. The increased transcriptional response correlated with an increase in the interaction between DRIP205 and the mutant VDR. We further provide evidence that OA induces the phosphorylation of CREB-binding protein (CBP), indicating for the first time a correlation between phosphorylation of CBP and enhanced VDR function. Hexafluoro 1,25(OH)2D3 analogs (RO-26-2198 and RO-4383561) also resulted in at least a partial restoration of the transcriptional responsiveness of mutant VDRs I268T, F251C, and H305Q. Our data indicate that the enhanced potency of the hexafluoro analogs may be caused by increased DRIP205 and glucocorticoid receptor interacting protein 1 (GRIP-1) binding to VDRs and enhanced association of VDRs with DNA, as suggested by results of ChIP assays.

CONCLUSION

Our study provides new insights into the mechanisms involved in the enhancement of VDR function by both phosphorylation and hexafluoro analogs and forms a basis for future study of vitamin D analogs or specifically designed kinase activity mediators as potential therapy for the treatment of selected patients with HVDRR.