Impaired stimulation of 25-hydroxyvitamin D-24-hydroxylase in fibroblasts from a patient with vitamin D-dependent rickets, type II. A form of receptor-positive resistance to 1,25-dihydroxyvitamin D3

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.

Genetic evidence that the human CYP2R1 enzyme is a key vitamin D 25-hydroxylase

The synthesis of bioactive vitamin D requires hydroxylation at the 1 alpha and 25 positions by cytochrome P450 enzymes in the kidney and liver, respectively. The mitochondrial enzyme CYP27B1 catalyzes 1 alpha-hydroxylation in the kidney but the identity of the hepatic 25-hydroxylase has remained unclear for >30 years. We previously identified the microsomal CYP2R1 protein as a potential candidate for the liver vitamin D 25-hydroxylase based on the enzyme's biochemical properties, conservation, and expression pattern. Here, we report a molecular analysis of a patient with low circulating levels of 25-hydroxyvitamin D and classic symptoms of vitamin D deficiency. This individual was found to be homozygous for a transition mutation in exon 2 of the CYP2R1 gene on chromosome 11p15.2. The inherited mutation caused the substitution of a proline for an evolutionarily conserved leucine at amino acid 99 in the CYP2R1 protein and eliminated vitamin D 25-hydroxylase enzyme activity. These data identify CYP2R1 as a biologically relevant vitamin D 25-hydroxylase and reveal the molecular basis of a human genetic disease, selective 25-hydroxyvitamin D deficiency.

Vitamin D-dependent rickets Type II with alopecia: two case reports and review of the literature

Vitamin D-dependent rickets Type II is a rare autosomal recessive disorder. It usually presents with rachitic changes not responsive to Vitamin D treatment with elevated circulating levels of 1,25-Dihydroxyvitamin D3, thus differentiating it from Vitamin D-dependent rickets Type I. Alopecia of the scalp or the body is seen in some families with Vitamin D-dependent rickets Type II. This is usually associated with more severe resistance to Vitamin D. We report two Saudi brothers with this disease, and review the salient features of this disease with emphasis on the associated alopecia.

Tryptophan missense mutation in the ligand-binding domain of the vitamin D receptor causes severe resistance to 1,25-dihydroxyvitamin D

In this study, two related young children, brother and sister, exhibited severe vitamin D-resistant rickets without alopecia. Sequence analysis of the total vitamin D receptor (VDR) cDNA from skin fibroblasts revealed a substitution of the unique tryptophan of the VDR by arginine at amino acid 286 (W286R). Cultured skin fibroblasts of the two patients expressed normal-size VDR protein (immunocytochemistry and Western blotting) and normal length VDR mRNA (Northern blotting). But, these fibroblasts, as well as COS-7 cells transfected with the W286R mutant, failed to bind 3H 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The tryptophan substitution did not affect VDR trafficking toward the nucleus but abolished the 24-hydroxylase gene response to 1,25(OH)2D3, even at 10(-6) M concentrations. In conclusion, this case report of a new family with hereditary vitamin D-resistant rickets (HVDRR) emphasizes the crucial role of the VDR tryptophan for ligand binding and for transactivation of 1,25(OH)2D3 target genes. It clearly shows the clinical significance of this VDR amino acid for calcium homeostasis and bone mineralization. This observation suggests further that the presence of a stable VDR-bound ligand may not be obligatory for normal hair follicle development.

Hereditary vitamin D resistant rickets caused by a novel mutation in the vitamin D receptor that results in decreased affinity for hormone and cellular hyporesponsiveness

Mutations in the vitamin D receptor (VDR) result in target organ resistance to 1alpha,25-dihydroxyvitamin D [1,25(OH)2D3], the active form of vitamin D, and cause hereditary 1,25-dihydroxyvitamin D resistant rickets (HVDRR). We analyzed the VDR of a patient who exhibited three genetic diseases: HVDRR, congenital total lipodystrophy, and persistent mullerian duct syndrome. The patient was treated with extremely high dose calcitriol (12.5 microg/d) which normalized serum calcium and improved his rickets. Analysis of [3H]1,25(OH)2D3 binding in the patient's cultured fibroblasts showed normal abundance of VDR with only a slight decrease in binding affinity compared to normal fibroblasts when measured at 0 degrees C. The patient's fibroblasts demonstrated 1,25(OH)2D3-induction of 24-hydroxylase mRNA, but the effective dose was approximately fivefold higher than in control cells. Sequence analysis of the patient's VDR gene uncovered a single point mutation, H305Q. The recreated mutant VDR was transfected into COS-7 cells where it was 5 to 10-fold less responsive to 1,25(OH)2D3 in gene transactivation. The mutant VDR had an eightfold lower affinity for [3H]1,25(OH)2D3 than the normal VDR when measured at 24 degrees C. RFLP demonstrated that the patient was homozygous for the mutation while the parents were heterozygous. In conclusion, we describe a new ligand binding domain mutation in the VDR that causes HVDRR due to decreased affinity for 1,25(OH)2D3 which can be effectively treated with extremely high doses of hormone.

Vitamin D dependent rickets type II and normal vitamin D receptor cDNA sequence. A cluster in a rural area of Cauca, Colombia, with more than 200 affected children

Vitamin D dependent rickets type II is an autosomal recessive disease caused by the vitamin D defective receptor. More than 200 patients with different types of lower limb deformities were detected in a rural area of the Cauca department in the southwest part of Colombia. Patients were well nourished and in good physical condition in spite of their deformities. None of them presented alopecia, myopathy, seizures or aminoaciduria. Serum analysis showed significantly lower serum calcium as compared to normal relatives, though in the normal low range, normal phosphorus, high alkaline phosphatase, normal 25-hydroxyvitamin D3 and high 1,25-dihydroxyvitamin D3, indicating target organ resistance. The cDNA analysis showed normal nucleotide sequence. We suggest that our patients represent a distinct form of receptor-positive resistance to vitamin D. This report is the first extensive study on this class of patients.

Hormone-nuclear receptor interactions in health and disease. Vitamin D resistance

Tissue resistance to vitamin D, or vitamin D-dependent rickets (VDDR), can be classified as two separate conditions--VDDR type I and VDDR type II--both of which present with the classical clinical, radiological and biochemical features of rickets despite adequate vitamin D intake. VDDR II can also be associated with alopecia, for reasons that are not clear. The two syndromes result from distinct disorders of vitamin D metabolism or action. Both are inherited in an autosomal recessive fashion. VDDR I is caused by decreased production of the active form of vitamin D, 1,25-dihydroxycholecalciferol, with the proposed defect being in the gene encoding the enzyme 1 alpha-hydroxylase. VDDR II results from mutations in the gene for the intracellular receptor for 1,25-dihydroxycholecalciferol (vitamin D receptor), resulting in changes in hormone or DNA binding, depending on the mutation. These mutations are analogous to those affecting receptors for other steroid-thyroid hormones, which have also been shown to cause resistance to hormone action.

Characteristics of the 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3-24-hydroxylase(s) from HL-60 cells

1,25-Dihydroxyvitamin D3 induces the human promyelocyte leukemia cell line, HL-60, to differentiate into macrophages/monocytes via a steroid-receptor mechanism. This system is a relevant one for an investigation of the molecular mechanism of 1,25-dihydroxyvitamin D3. We have now examined the effect of 1,25-dihydroxyvitamin D3 on the induction of 1,25-dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities in HL-60 cells. The hydroxylase activities were measured by a periodate-based assay, which was validated by comparison with well-established HPLC analysis. HPLC analysis also suggested that 1,25-dihydroxyvitamin D3 induces a 23-hydroxylase in addition to the 24-hydroxylase. 1,25-Dihydroxyvitamin D3- and 25-hydroxyvitamin D3-24-hydroxylase activities were stimulated as early as 4 h after the addition of 10(-7) M 1,25-dihydroxyvitamin D3 and became maximal by 24 h. 1,25-Dihydroxyvitamin D3 stimulated both activities in a dose-dependent manner up to 10(-6) M. The Km of 24-hydroxylase for 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 were 2 x 10(-8) M and 5.2 x 10(-7) M, respectively. Cycloheximide (5 microM) inhibited 1,25-dihydroxyvitamin D3-mediated stimulation of 24-hydroxylase activity. Other differentiation inducers, such as retinoic acid and phorbol ester, did not induce either activity. 1,25-Dihydroxyvitamin D3-24-hydroxylase in HL-60 mitochondria was solubilized with 0.6% cholate and reconstituted with NADPH, beef adrenal ferredoxin, and beef adrenal ferredoxin reductase, each component being essential for 24-hydroxylase activity. These results strongly suggest that the 24-hydroxylase in HL-60 cells is a three-component cytochrome P450-dependent mixed-function oxidase.

Clinical and biochemical findings in parents of children with vitamin D-dependent rickets Type II

Vitamin D-dependent rickets type II is a rare disease caused by a disorder of the receptor for 1, 25-dihydroxyvitamin D (1, 25(OH)2D). Several parameters of this receptor-effector system were investigated to obtain biochemical information on the presumed heterozygotes of vitamin D-dependent rickets type II in parents of five patients and in their age-matched controls. It was found that the serum concentrations of 1, 25-(OH)2D and 24,25-dihydroxy-vitamin D (24,25(OH)2D), and the ratio of 1,25-(OH)2D/24,25-(OH)2D differed significantly in the parents from those of the patients and the respective control groups. In the parents' cultured skin fibroblasts, the activity of 25-hydroxyvitamin D-24-hydroxylase induced by 10(-8) mol/L 1, 25-(OH)2D3 ranged from 50 to 82% of that of their controls (versus 1-13% of controls for the patients). The binding capacity of the parents' [3H]1, 25-(OH)2D3 to the nucleus was 38-54% of that of their control subjects (versus 7-27% of controls for the patients). The parents' values were thus in a range between those of the patients and the control groups. These findings suggest that, in the parents, a partial impairment of the receptor system for 1, 25-(OH)2D led to an imbalance of vitamin D metabolism, thus confirming that vitamin D-dependent rickets type II is an autosomal recessive inherited disease. Serum concentrations of 1, 25-(OH)2D and 24, 25-(OH)2D may provide useful parameters for detecting heterozygotes of this disease.

Genetic defects of the 1,25-dihydroxyvitamin D3 receptor

Target organ resistance to steroid hormone action is known to produce clinical disorders ranging from testicular feminization in the case of androgen resistance to hypocalcemic vitamin D-resistant ricets (HVDRR) in the case of 1,25-dihydroxyvitamin D3. The etiologic basis of these disorders is thought to be genetic mutations in the gene encoding receptors for these hormones. We investigated this possibility by analyzing the vitamin D receptor (VDR) protein, mRNA, and DNA from patients with HVDRR. This autosomal recessive disease of children is characterized by early onset rickets, hypocalcemia, hyperparathyroidism, and elevated levels of 1,25-(OH)2D3. Cells from patients fall into three general classes of molecular defects: (i) decreased or absent hormone binding; (ii) decreased affinity of VDR for DNA, or; (iii) defective nuclear translocation or retention. Analysis of the DNA and/or mRNA from these cells has identified missense mutations in the DNA binding (zinc finger) domain and a nonsense mutation in the steroid binding domain of VDR. The mutations were individually recreated in wild type VDR and the expressed mutant protein behaved biochemically identically to the patient receptor. Further studies have shown that the receptor is unable to interact with the specific hormone response element (HRE) of the osteocalcin gene and activate appropriate transcription. Rapid diagnostic genotyping of these mutations is possible with either restriction digestion or allele-specific oligonucleotide hybridization. Analysis of these naturally occurring, disease producing mutations of a gene regulatory protein should provide insight into the key amino acid residues of the protein and the mechanism by which steroids modulate gene transcription.

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

Hereditary 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] resistant rickets (HVDRR) is an autosomal recessive disease caused by target organ resistance to the action of 1,25(OH)2D3, the active form of the hormone. The defect in target cells is heterogenous and commonly appears to be a mutation in the gene encoding the vitamin D receptor (VDR). We have studied cultured skin fibroblasts and Epstein-Barr virus transformed lymphoblasts of seven family branches of an extended kindred having eight children affected with HVDRR. We have previously shown that cells from three affected children in this group contain an "ochre" nonsense mutation coding for a premature stop codon in exon 7 within the steroid-binding domain of the VDR gene. In the current studies, we found that cells from affected children failed to bind [3H]1,25(OH)2D3 and had undetectable levels of VDR as determined by immunoblots using an anti-VDR monoclonal antibody. Measurement of VDR mRNA by hybridization to a human VDR cDNA probe showed undetectable or decreased abundance of steady-state VDR mRNA. Parents, expected to be obligate heterozygotes, showed approximately half the normal levels of [3H]1,25(OH)2D3 binding, VDR protein, and mRNA. The mutation at nucleotide 970 (counting from the mRNA CAP site) results in the conversion of GTAC to GTAA, which eliminates an Rsa I restriction enzyme site and facilitates identification of the mutation. We found that polymerase chain reaction (PCR) amplification of exons 7 and 8 from family members and subsequent Rsa I digestion allows detection of the specific genotype of the individuals. When Rsa I digests of PCR-amplified DNA are subjected to polyacrylamide gel electrophoresis, children with HVDRR exhibit a homozygous banding pattern with loss of an Rsa I site. Parents exhibit a heterozygotic DNA pattern with detection of both normal and mutant alleles. In summary, our data show that the genetic abnormality is a point mutation within the steroid-binding domain of the VDR in all seven related families with HVDRR. Analysis of restriction fragment length polymorphism at the 970 locus of PCR-amplified DNA fragments can be used to diagnose this mutation in both affected children and parents carrying the disease.

Lymphocyte cell lines from vitamin D-dependent rickets type II show functional defects in the 1 alpha,25-dihydroxyvitamin D3 receptor

Lymphocyte cell lines were established from five patients with vitamin D-dependent rickets, type II (VDDR-II). These lines were established by infection with human T-lymphotrophic virus type I (HTLV-I). Binding of [3H]1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) to its receptor in these cell lines was compared to binding studies using a T-lymphocyte cell line (S-LB1) from a normal individual. The 1,25(OH)2D3 receptor of S-LB1 was comparable to the well-characterized chick intestinal 1,25(OH)2D3 receptor in terms of its ligand binding affinity and capacity, its mobility on 5-20% sucrose gradients, and its adsorption to and elution properties from DNA-cellulose. Three cell lines established from patients with VDDR-II (Rh-VDR, Sh-VDR, and Ab-VDR) showed no specific binding of 1,25(OH)2D3 to a receptor and treatment of the cultured cells with 1,25(OH)2D3 did not stimulate production of 24,25-dihydroxy-vitamin D3 (24,25(OH)2D3), a response which is diagnostic of the presence of a functional 1,25(OH)2D3 receptor. In a fourth cell line, A1-VDR, the receptor for 1,25(OH)2D3 had a low binding capacity and 25(OH)D3-24-hydroxylase activity was not detectable. Induction of 24,25-(OH)2D3 synthesis by 1,25(OH)2D3 was observed in the fifth cell line, designated Ro-VDR, although the sensitivity to hormone treatment was lower than in the control cell line from a normal donor. The capacity of the receptor for 1,25(OH)2D3 was low in Ro-VDR. In all cell lines where 1,25(OH)2D3 binding to a receptor was detectable, the receptor had the typical sedimentation coefficient of 3.7 S on sucrose density gradient analysis. Binding and elution properties to DNA-cellulose, however, differed from normal in both Ro-VDR and A1-VDR cells where elution from DNA-cellulose occurred at a lower salt concentration than is typical of the 1,25(OH)2D3 receptor. While Ro-VDR cells showed typical nuclear localization of the unoccupied 1,25(OH)2D3 receptor, neither the unoccupied nor the occupied receptor from A1-VDR cells was completely localized in the nucleus. In a series of functional studies we found that modulation of the level of the mRNAs coding for both the c-myc oncogene and the growth factor known as granulocyte-monocyte colony stimulating activity by 1,25(OH)2D3 correlated with the 1,25(OH)2D3 receptor status of these cells. Use of these cell lines will facilitate further study of the molecular defect(s) in the receptor for 1,25(OH)2D3 in vitamin D-dependent rickets type II and will allow a correlation with impairment of cellular functions.

Cultured osteoblasts from normal and hypophosphatemic mice: calcitriol receptors and biological response to the hormone

The content and affinity of calcitriol receptors were analyzed in cultured osteoblasts from normal and hypophosphatemic mice. Hypertonic cell extracts were prepared by sonication followed by centrifugation at 200,000 g x 30 min. Analysis, at saturating levels of labeled 1,25(OH)2D3, revealed that binding of the hormone was dependent on the density of the cells plated and on the length of time in culture. It reached a maximum at 5 days of culture when 1.0 x 10(6) cells were plated. Under those conditions the binding capacity of Hyp osteoblasts was 6306 +/- 1267 sites/ng protein (mean +/- SEM) not different from N cells (7594 +/- 1713). The dissociation constant (Kd) was 18.3 +/- 5.4 and 20.0 +/- 5.7 pM for mutant and normal mouse osteoblasts respectively (NS). In both genotypes, a single peak for specific binding, migrating at approximately 3.0-3.5 S was observed by sucrose gradient centrifugation. 25-hydroxycholecalciferol-24-hydroxylase (24-OHase) was induced at 1 and 10 nM 1,25(OH)2D3 in a dose-dependent fashion. However, the induction was higher in mutant than in normal cells when the medium contained 1 mM and 2 mM phosphate salts. The difference vanished when cells were incubated in the presence of 3 and 4 mM phosphate salts. The effect of calcitriol on cultured osteoblasts was also analyzed in terms of collagen synthesis and alkaline phosphatase activity. In the range of 10(-10) M to 10(-7) M, 1,25(OH)2D3 was found to inhibit collagen synthesis in a dose-dependent fashion. At physiological levels, 1,25(OH)2D3 (10(-11)M-10(-10)M), stimulated alkaline phosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium metabolism

In normal individuals, 1,25-dihydroxyvitamin D (1,25-D) levels regulate calcium (Ca) absorption according to Ca intake; its synthesis is stimulated by low Ca intake, probably via increased parathyroid hormone (PTH) secretion, to increase Ca absorption, and suppressed during high intake to reduce Ca absorption. The body also adapts Ca absorption in response to renal Ca excretion, and phosphate absorption in response to phosphate intake. These adaptations may fail or be impaired in certain diseases. In disorders of overadaptation, the intestinal tract absorbs excessive amounts of Ca due to overproduction of 1,25-D, as in absorptive hypercalciuria, sarcoidosis, primary hyperparathyroidism, and tumoral calcinosis. Intestinal hyperabsorption and hypercalciuria may occur on both low- and high-Ca diets. Primary hyperparathyroidism and hypoparathyroidism are bihormonal, related to over- and underproduction, respectively, of both 1,25-D and PTH. Underadaptation disorders are typically related to low 1,25-D synthesis or resistance to this metabolite; examples include postmenopausal osteoporosis, chronic renal failure, and osteomalacia. Many of these adaptational disorders can be relieved or improved by manipulating Ca, phosphate, sodium, or protein intake or by administering exogenous 1,25-D. Overabsorption of Ca and other substances, such as oxalate, may be responsible for Ca nephrolithiasis. Hypocitraturia (which may be a complication of certain diseases or the result of unbalanced diet or excessive exercise), diets high in readily metabolizable sugars and purine-rich proteins (meat, poultry, and fish), and low fluid intake can all contribute to stone formation. Various regimens may reduce the risk of Ca nephrolithiasis.

An ochre mutation in the vitamin D receptor gene causes hereditary 1,25-dihydroxyvitamin D3-resistant rickets in three families

Hereditary 1,25-dihydroxyvitamin D3-resistant rickets is a rare autosomal-recessive disease resulting from target-organ resistance to the action of the active hormonal form of vitamin D. Four affected children from three related families with the classical syndrome of hereditary 1,25-dihydroxyvitamin D3-resistant rickets and the absence of detectable binding to the vitamin D receptor (VDR) in cultured fibroblasts or lymphoblasts were examined for genetic abnormalities in the VDR gene. Genomic DNA from Epstein-Barr virus-transformed lymphoblasts of eight family members was isolated and amplified by polymerase chain reaction techniques. Amplified fragments containing the eight structural exons encoding the VDR protein were sequenced. The DNA from all affected children exhibited a single C----A base substitution within exon 7 at nucleotide 970 that resulted in the conversion of the normal codon for tyrosine (TAC) into a premature termination codon (TAA) at amino acid 292. This mutation causes a truncation of the VDR protein thereby deleting a large portion of the steroid hormone binding domain (amino acids 292-424). Although the affected children were all homozygotic for the mutation, the four parents tested all exhibited both wild-type and mutant alleles, indicating a heterozygous state. The functional consequences of this mutation were confirmed after expression of the recreated mutant VDR cDNA in mammalian cells. Recreated mutant receptor exhibited no specific 1,25-[3H]dihydroxyvitamin D3 binding and failed to activate a cotransfected VDR promoter-reporter gene construct. Thus these findings identify an ochre mutation in a human steroid hormone receptor in patients with hereditary 1,25-dihydroxyvitamin D3-resistant rickets.

Two siblings with vitamin-D-dependent rickets type II: no recurrence of rickets for 14 years after cessation of therapy

Rickets in a 3-year-old boy and his 1-year-old sister, both with alopecia, was cured by treatment with 50,000 IU of vitamin D2 daily for 2 years and did not recur within 14 years after cessation of therapy. A diagnosis of vitamin-D-dependent rickets type II was made in these patients at the ages of 20 and 18 years based on the findings that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] did not inhibit DNA biosynthesis in phytohaemagglutinin-stimulated lymphocytes and that cultured skin fibroblasts showed impaired nuclear uptake and normal cytosol binding of [3H]1,25(OH)2D3. Surprisingly, the serum 1,25(OH)2D levels of these patients were high and their serum 24,25-dihydroxyvitamin D levels were low, although neither patient showed any symptoms except alopecia. The presence of vitamin D metabolite imbalances in the absence of rickets in these patients might be explained by differences in sensitivity to 1,25(OH)2D3 of bone formation and vitamin D metabolism. In addition, changes of sensitivity to treatment with vitamin D derivatives might be a consequence of differentiation of target cells. From the present findings, it is suggested that in this disease treatment with a sufficient dose of vitamin D derivatives should be initiated in the active phase of rickets.

Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets

Hypocalcemic vitamin D-resistant rickets is a human genetic disease resulting from target organ resistance to the action of 1,25-dihydroxyvitamin D3. Two families with affected children homozygous for this autosomal recessive disorder were studied for abnormalities in the intracellular vitamin D receptor (VDR) and its gene. Although the receptor displays normal binding of 1,25-dihydroxyvitamin D3 hormone, VDR from affected family members has a decreased affinity for DNA. Genomic DNA isolated from these families was subjected to oligonucleotide-primed DNA amplification, and each of the nine exons encoding the receptor protein was sequenced for a genetic mutation. In each family, a different single nucleotide mutation was found in the DNA binding domain of the protein; one family near the tip of the first zinc finger (Gly----Asp) and one at the tip of the second zinc finger (Arg----Gly). The mutant residues were created in vitro by oligonucleotide directed point mutagenesis of wild-type VDR complementary DNA and this cDNA was transfected into COS-1 cells. The produced protein is biochemically indistinguishable from the receptor isolated from patients.

RU486 inhibits induction of aromatase by dexamethasone via glucocorticoid receptor in cultured human skin fibroblasts

Effects of RU486 on the induction of aromatase by dexamethasone via glucocorticoid receptor were determined using cultured human skin fibroblasts. Competition of [3H]dexamethasone binding to the cytosol receptor was 7 times stronger with RU486 than with dexamethasone. The order of the strength of competition was RU486 greater than dexamethasone greater than betamethasone greater than prednisolone greater than hydrocortisone. RU486 abolished a specific 8.6 S [3H]dexamethasone binding peak in the cytosol, determined using a sucrose density gradient analysis. Dexamethasone markedly induced aromatase and this event was strongly suppressed by RU486, in a dose-dependent manner, in the cultured skin fibroblasts. A linear correlation between the strength of competition and the induction of aromatase of various glucocorticoids was observed. RU486 non-competitively inhibited aromatase induction by dexamethasone determined from a double reciprocal plot of aromatase activity, with respect to [3H]androstenedione concentration in the presence of RU486. These results show that RU486 is a peripheral noncompetitive antiglucocorticoid on aromatase induction by glucocorticoid in human skin fibroblasts and that aromatase induction is a good marker for the biological function of glucocorticoid receptor in human skin fibroblasts.

Endocrine control and disturbances of calcium and phosphate metabolism in children

Most disorders of extracellular calcium and phosphate metabolism in childhood can be attributed to primary increased or decreased secretion/action of 1,25-dihydroxyvitamin D3 and parathyroid hormone or primary increased or decreased urinary excretion of phosphate and calcium. Based on this pathogenetic classification the most important diseases related to calcium and phosphate metabolism will be discussed.

25-Hydroxyvitamin D3 metabolism in a human leukemia cell line

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is a potent inducer of monocytic differentiation of the human promyelocytic leukemia cell line, HL-60. We have noted that 25-hydroxyvitamin D3 (25(OH)D3) in high doses is also capable of promoting monocytic differentiation of this cell line. To test the possibility that the latter activity is due to conversion of 25OHD3 to 1,25(OH)2D3 by HL-60, we exposed HL-60 cells to 25OHD3 and analyzed the products by HPLC and radioreceptor assay. When chromatographed in the traditional solvent system (isopropanol-hexane), a new peak appears which migrates with authentic 1,25(OH)2D3. However, in a solvent system containing dichloromethane, 90% of the peak migrates with another metabolite, 19-Nor-10-Keto-25OHD3 (19-Nor-25OHD3). Production of this metabolite is enhanced by living cells and is synthesized by both virgin HL-60 and those which have undergone differentiation. We next determined if authentic 19-Nor-25OHD3 also promotes differentiation of this cell. As assessed by appearance of the monocyte-specific surface antigen (63D3) and macrophage-specific esterase activity, we find that this metabolite does, in fact, induce monocytic differentiation of HL-60 with a potency of approximately 1/200 that of 1,25(OH)2D3 and similar to that of 25OHD3. In agreement with the effect upon cell maturation, 19-Nor-25OHD3 displaces 3H-1,25(OH)2D3 from its HL-60 receptor with an efficiency comparable to 25OHD3. Hence, HL-60 cells convert 25OHD3 to 19-Nor-25OHD3, and 19-Nor-25OHD3 induces monocytic differentiation of HL-60 with comparable efficiency to its precursor, 25OHD3.

Analysis of the relation between alopecia and resistance to 1,25-dihydroxyvitamin D

Alopecia is a frequent feature in hereditary resistance to (1,25(OH)2D). We sought insight into this feature by analysing data from affected members of 30 kindreds. We assessed indices of mineral metabolism in one group with normal hair compared with a group with alopecia. Hereditary resistance to 1,25(OH)2D was diagnosed at an earlier age in alopecic patients (0.9 vs 3.3 years, P less than 0.05); this reflected late presentation of metabolic bone disease in some cases with normal hair and could not be attributed to early diagnosis resulting from the striking feature of alopecia. For untreated subjects, serum concentrations of calcium and 1,25(OH)2D were similar in both groups of patients. During calciferol therapy, however, the cases with alopecia showed lower serum calcium (1.9 vs 2.4 mmol/l, P less than 0.005), but higher serum 1,25(OH)2D (2900 v 340 pg/ml, P less than 0.005). Hair status did not predict the type of defect identified with cultured skin fibroblasts but did correlate with responsiveness of 25(OH)D 24-hydroxylase to 1,25(OH)2D3 in those cells. Cells from seven of eight kindreds with alopecia showed no 24-hydroxylase response to high doses of 1,25(OH)2D3 while cells from five of six kindreds with normal hair showed a 24-hydroxylase response to high doses of 1,25(OH)2D3. We conclude that in cases with hereditary resistance to 1,25(OH)2D alopecia reflects the more severe grades of this resistance based upon earlier age at time of diagnosis, lower potential for calcaemic response to calciferols, and lower potential for 24-hydroxylase response to 1,25(OH)2D3 by cultured skin fibroblasts.

Vitamin D-dependent rickets type II: extreme end organ resistance to 1,25-dihydroxy vitamin D3 in a patient without alopecia

Vitamin D-dependent rickets type II (VDDR II) is a rare syndrome resulting in severe rickets and is resistant to treatment with vitamin D and its derivatives. Patient with this disease, who are frequently the children of consanguinous marriages, present with elevated circulating concentrations of 1,25-dihydroxy vitamin D, the active metabolite of vitamin D, and in vitro studies have indicated a failure of intracellular binding of the hormone. Alopecia has been noted in many of these patients and it has been suggested that this feature may indicate a more marked resistance to treatment. However we describe a 3-year-old boy with this disease who, although having normal hair growth, displayed extreme resistance to treatment with active vitamin D metabolites. In vitro studies of skin fibroblasts disclosed not only an absence of hormone binding or 1,25(OH)2D3-induced 24-hydroxylase activity but reduced metabolism of 1,25(OH)2D3 itself. In this child, treatment with exogenous 1,25-dihydroxy vitamin D3 at doses of up to 24 micrograms/day, which increased the circulating concentration of the metabolite to greater than 100 times the normal adult mean, failed to alleviate his condition and he died at the age of 39 months. This would therefore suggest that absence of alopecia, in this condition, cannot be regarded as a constant predictive sign of a lesser resistance and of responsiveness to Vitamin D treatment.

Rapid diagnosis of vitamin D-dependent rickets type II by use of phytohemagglutinin-stimulated lymphocytes

The interactions of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] with phytohemagglutinin (PHA)-stimulated lymphocytes from normal subjects and three patients with vitamin D-dependent rickets (DDR) type II were investigated. Impaired nuclear uptake and normal cytosol binding of [3H]1,25-(OH)2D3 were observed with PHA-stimulated lymphocytes of these patients as with their cultured skin fibroblasts. Furthermore, the incorporation of [14C]thymidine into PHA-stimulated lymphocytes of the patients was not reduced by 1,25-(OH)2D3, which is known to inhibit proliferation of various cells. These findings suggest that 1,25-(OH)2D3 receptors are reduced or absent in patients with DDR type II. Thus, the capacities of cytosol binding and nuclear uptake of 1,25-(OH)2D3 in PHA-stimulated lymphocytes seem to reflect those of endo-organs such as the intestine and bone. These findings show that a test of the effect of 1,25-(OH)2D3 on thymidine incorporation into PHA-stimulated lymphocytes is useful for rapid diagnosis of DDR type II.

Cultured psoriatic fibroblasts from involved and uninvolved sites have a partial but not absolute resistance to the proliferation-inhibition activity of 1,25-dihydroxyvitamin D3

We examined the responsiveness of cultured dermal fibroblasts from biopsies of uninvolved and involved areas of skin from six patients with psoriasis to the cell-proliferation-inhibition activity of 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3). Cultured fibroblasts from age-matched controls responded to 1,25-(OH)2-D3 (at 0.01, 1, 10, and 100 microM) in a dose-dependent fashion, whereas cultured psoriatic fibroblasts from involved or uninvolved skin showed no inhibition of proliferation when exposed to 0.01 or 1 microM of 1,25-(OH)2-D3. However, 1,25-(OH)2-D3 did inhibit proliferation of cultured psoriatic fibroblasts when the concentrations were increased to 10 and 100 microM. An analysis of the 1,25-(OH)2-D3 receptors in cultured psoriatic fibroblasts from uninvolved skin revealed that the Kd, nmax, and sedimentation coefficient were identical to the receptors found in the fibroblasts from age-matched controls. Therefore, cultured psoriatic fibroblasts from involved and uninvolved skin have a partial resistance to 1,25-(OH)2-D3, suggesting that there may be a biochemical defect that is inherent in the dermal fibroblasts of psoriatic patients. Recognition of this defect may provide a new approach for the evaluation of the cause and treatment of this disfiguring skin disorder.

Vitamin D-dependent rickets type II. Defective induction of 25-hydroxyvitamin D3-24-hydroxylase by 1,25-dihydroxyvitamin D3 in cultured skin fibroblasts

1,25(OH)2D3 induces 25(OH)D3-24-hydroxylase (24-OHase) in cultured skin fibroblasts from normal subjects. We evaluated 24-OHase induction by 1,25(OH)2D3 in skin fibroblasts from 10 normal subjects and from four unrelated patients with hereditary resistance to 1,25(OH)2D or vitamin D-dependent rickets type II (DD II). Fibroblasts were preincubated with varying concentrations of 1,25(OH)2D3 for 15 h and were then incubated with 0.5 microM [3H]25(OH)D3 at 37 degrees C for 30 min; lipid extracts of the cells were analyzed for [3H]24,25(OH)2D3 by high performance liquid chromatography and periodate oxidation. Apparent maximal [3H]24,25(OH)2D3 production in normal cell lines was 9 pmol/10(6) cells per 30 min and occurred after induction with 10(-8) M 1,25(OH)2D3. 24-OHase induction was detectable in normal fibroblasts at approximately 3 X 10(-10) M 1,25(OH)2D3. [3H]24,25(OH)2D3 formation after exposure to 1,25(OH)2D3 was abnormal in fibroblasts from all four patients with DD II. In fibroblasts from two patients with DD II, [3H]24,25(OH)2D3 formation was unmeasurable (below 0.2 pmol/10(6) cells per 30 min) at 1,25(OH)2D3 concentrations up to 10(-6) M. Fibroblasts from the other two patients with DD II required far higher than normal concentrations of 1,25(OH)2D3 for detectable [3H]24,25(OH)2D3 induction. In one, [3H]24,25(OH)2D3 production reached 2.9 pmol/10(6) cells per 30 min at 10(-6) M 1,25(OH)2D3 (30% normal maximum at 10(-6) M 1,25(OH)2D3). In the other, [3H]24,25(OH)2D3 production achieved normal levels, 7.3 pmol/10(6) cells per 30 min after 10(-6) M 1,25(OH)2D3. The two patients whose cells had a detectable 24-OHase induction by 1,25(OH)2D3 showed a calcemic response to high doses of calciferols in vivo. Our current observations correlate with these two patients' responsiveness to calciferols in vivo and suggest that their target organ defects can be partially or completely overcome with extremely high concentrations of 1,25(OH)2D3. The two patients whose cells showed no detectable 24-OHase induction in vitro failed to show a calcemic response to high doses of calciferols in vivo.

IN CONCLUSION

(a) the measurement of 24-OHase induction by 1,25(OH)2D3 in cultured skin fibroblasts is a sensitive in vitro test for defective genes in the 1,25(OH)2D effector pathway. (b) This assay provides a useful tool for characterizing the target tissue defects in DD II and predicting response to calciferol therapy.

The influence of albumin on vitamin D metabolism in fetal chick osteoblast-like cells

Incubation of osteoblast-like cells with [3H]25-(OH)D3 and varying bovine serum albumin (BSA) concentrations resulted in a dramatic change in the accumulation of 1,25-(OH)2D3 and 24,25-(OH)2D3 in the medium. At 0.1% BSA 1,25-(OH)2D3 formation was transient and 24,25-(OH)2D3 was the main product after 3 h. At 2% BSA accumulation of 1,25-(OH)2D3 was sustained whereas 24,25-(OH)2D3 formation was suppressed. At low BSA levels added [3H]1,25-(OH)2D3 was rapidly metabolized to 1,24,25-(OH)3D3 and more polar metabolites. The effect of increasing BSA concentrations on 25-(OH)D3 metabolism was mimicked by addition of cycloheximide. This indicates that high BSA levels prevent the induction of 24-hydroxylase activity in this system, probably by lowering of the free 25-(OH)D3 concentration. The accumulation of 1,25-(OH)2D3 from 25-(OH)D3 not only depends on the 1 alpha-hydroxylase activity, but also on the further metabolism of 1,25-(OH)2D3 by 24-hydroxylation.

1,25-Dihydroxyvitamin D resistance, rickets, and alopecia

Two unrelated kindreds with four affected children having 1,25-dihydroxyvitamin D resistance, rickets, and alopecia are described. The children exhibited early onset of severe rickets with hypocalcemia, hypophosphatemia, elevated serum alkaline phosphatase levels, and secondary hyperparathyroidism. Radiography showed diffuse demineralization and classic changes of rickets. All affected children had total-body alopecia. Serum levels of 1,25-dihydroxyvitamin D3 were elevated and rose to extremely high values during treatment, with no apparent change in the mineral disorder. However, secondary hyperparathyroidism and hypophosphatemia did remit during treatment despite persistently low calcium levels. Skin biopsy was performed in the parents and affected children in one kindred. Analysis of 1,25-dihydroxyvitamin D3 receptors in cultured fibroblasts indicated apparent normal receptors in the parents and undetectable receptors in both affected children. After long periods of treatment with vitamin D metabolites and mineral replacement, healing took place in the older child in each kindred. These data suggest that the healing occurred spontaneously as the children reached seven to nine years of age rather than as a result of the treatment. The biochemical lesion in these children appeared to be a genetically transmitted defect in the 1,25-dihydroxyvitamin D3 receptor. The mechanisms by which healing was initiated and maintained remain to be elucidated.

Vitamin D3--resistant fibroblasts have immunoassayable 1,25-dihydroxyvitamin D3 receptors

Cultured fibroblasts obtained from patients with tissue resistance to 1,25-dihydroxyvitamin D3 (vitamin D3--dependent rickets, type II) contain normal, low, or undetectable concentrations of this hormone's receptor protein as measured by a ligand-binding assay. Extracts from these cells were evaluated for receptors by immunoassay with a recently developed monoclonal antibody to the chick receptor. The results show that a protein sedimenting at 3.7S and recognizable by the antibody exists in comparable concentrations in cells from both normal and resistant patients, irrespective of the hormone-binding abnormalities of the cells. This implies that deficiencies in hormone binding associated with inherited tissue resistance to 1,25-dihydroxyvitamin D3 probably arise from structural variations in the receptor molecule and not from defective receptor synthesis.