Pathogenesis of hereditary vitamin-D-dependent rickets. An inborn error of vitamin D metabolism involving defective conversion of 25-hydroxyvitamin D to 1 alpha,25-dihydroxyvitamin D

No enzyme activity of 25-hydroxyvitamin D3 1alpha-hydroxylase gene product in pseudovitamin D deficiency rickets, including that with mild clinical manifestation

Pseudovitamin D deficiency rickets (PDDR) is an autosomal recessive disorder caused by defect in the activation of vitamin D. We recently isolated 25-hydroxyvitamin D3 1alpha-hydroxylase gene and identified four homozygous inactivating missense mutations in this gene by analysis of four typical cases of PDDR. This disease shows some phenotypic variation, and it has been suspected that patients with mild phenotypes have mutations that do not totally abolish the enzyme activity. To investigate the molecular defects associated with the phenotypic variation, we analyzed six additional unrelated PDDR patients: one with mild and five with typical clinical manifestation. By sequence analysis, all six patients were proven to have mutations in both alleles. The mutations varied, and we identified four novel missense mutations, a nonsense mutation, and a splicing mutation for the first time. The patient with mild clinical symptoms was compound heterozygous for T321R and a splicing mutation. The splice site mutation caused intron retention. Enzyme activity of the T321R mutant was analyzed by overexpressing the mutant 1alpha-hydroxylase in Escherichia coli cells to detect the subtle residual enzyme activity. No residual enzyme activity was detected in T321R mutant or in the other mutants. These results indicate that all of the patients, including those of mild phenotype, are caused by 1alpha-hydroxylase gene mutations that totally abolish the enzyme activity.

Cloning of porcine 25-hydroxyvitamin D3 1alpha-hydroxylase and its regulation by cAMP in LLC-PK1 cells

The 25-hydroxyvitamin D3 1alpha-hydroxylase, also referred to as CYP27B1, is a mitochondrial cytochrome P450 enzyme that catalyzes the biosynthesis of 1alpha, 25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) from 25-hydroxyvitamin D3 in renal proximal tubular cells. Recently, human, mouse, and rat CYP27B1 cDNA have been cloned, however the gene regulation has not been fully elucidated. In the present study, porcine CYP27B cDNA was cloned, and the effects of cAMP and vitamin D3 on the regulation of CYP27B1 mRNA expression in LLC-PK1 cells were examined. PCR cloning revealed that porcine CYP27B1 cDNA consisted of 2316 bp, encoding a protein of 504 amino acids. The deduced amino acid sequence showed over 80% identity to the human, mouse, and rat enzyme. LLC-PK1 cells were incubated with humoral factors, and expression of CYP27B1 mRNA was measured by a quantitative reverse transcription-PCR. At the completion of 3-, 6-, 12-, and 24-h incubations, 500 micromol/L 8-bromo-cAMP had significantly increased CYP27B1 mRNA expression (260 to 340%). The adenylate cyclase activator forskolin at 50 micromol/L also had a stimulatory effect at 6 h (190%). Moreover, the protein kinase A inhibitor H-89 reduced the cAMP effect. On the other hand, 1alpha,25(OH)2D3 had no effect on CYP27B1 mRNA expression at 10 and 100 nmol/L, whereas expression of 25-hydroxyvitamin D3 24-hydroxylase (CYP24) mRNA was markedly increased by 1alpha,25(OH)2D3. These findings suggest that LLC-PK1 cells express CYP27B1 mRNA, and that cAMP is an upregulating factor of the CYP27B1 gene in vitro.

Genetics of vitamin D 1alpha-hydroxylase deficiency in 17 families

Vitamin D-dependent rickets type I (VDDR-I), also known as pseudo-vitamin D-deficiency rickets, appears to result from deficiency of renal vitamin D 1alpha-hydroxylase activity. Prior work has shown that the affected gene lies on 12q13.3. We recently cloned the cDNA and gene for this enzyme, mitochondrial P450c1alpha, and we and others have found mutations in its gene in a few patients. To determine whether all patients with VDDR-I have mutations in P450c1alpha, we have analyzed the P450c1alpha gene in 19 individuals from 17 families representing various ethnic groups. The whole gene was PCR amplified and subjected to direct sequencing; candidate mutations were confirmed by repeat PCR of the relevant exon from genomic DNA from the patients and their parents. Microsatellite haplotyping with the markers D12S90, D12S305, and D12S104 was also done in all families. All patients had P450c1alpha mutations on both alleles. In the French Canadian population, among whom VDDR-I is common, 9 of 10 alleles bore the haplotype 4-7-1 and carried the mutation 958DeltaG. This haplotype and mutation were also seen in two other families and are easily identified because the mutation ablates a TaiI/MaeII site. Six families of widely divergent ethnic backgrounds carried a 7-bp duplication in association with four different microsatellite haplotypes, indicating a mutational hot spot. We found 14 different mutations, including 7 amino acid replacement mutations. When these missense mutations were analyzed by expressing the mutant enzyme in mouse Leydig MA-10 cells and assaying 1alpha-hydroxylase activity, none retained detectable 1alpha-hydroxylase activity. These studies show that most if not all patients with VDDR-I have severe mutations in P450c1alpha, and hence the disease should be referred to as "1alpha-hydroxylase deficiency."

Two novel 1alpha-hydroxylase mutations in French-Canadians with vitamin D dependency rickets type I1

BACKGROUND

Vitamin D dependency rickets type I (VDDR-I) is an autosomal recessive disorder in which 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha-hydroxylase) activity in renal proximal tubules is deficient. VDDR-I is recognized throughout the world, but occurs more frequently in a subset of the French-Canadian population. We and others have recently cloned the human 1alpha-hydroxylase cDNA and gene, making it possible to screen for mutations. The first VDDR-I mutations were reported in one American and four Japanese patients. In this study, we screened for 1alpha-hydroxylase mutations in French-Canadian patients with VDDR-I.

METHODS

The nine exons of the 1alpha-hydroxylase gene were amplified by polymerase chain reaction (PCR) from genomic DNA of four unrelated French-Canadian patients with VDDR-I and their parents, and sequenced.

RESULTS

Three of the patients were homozygous for a single base-pair deletion (G) at position 262 in the cDNA that lies in exon 2, and causes a premature termination codon upstream from the putative ferredoxin- and heme-binding domains. The fourth patient was homozygous for a 7-bp insertion (CCCCCCA) at position 1323 of the cDNA that lies in exon 8, and causes a premature termination upstream from the putative heme-binding domain. In each family, obligate carriers have one copy of the mutant allele. These mutations, which could be detected by PCR-restriction fragment length polymorphism and polyacrylamide gel electrophoresis of the PCR products, were not found in 25 normal French-Canadians.

CONCLUSION

We describe two novel 1alpha-hydroxylase mutations that are consistent with loss of function in four French-Canadian patients with VDDR-I and suggest that the 1alpha-hydroxylase mutations arise from more than one founder in this population.

Severe deficiency of 1,25-dihydroxyvitamin D3 in human immunodeficiency virus infection: association with immunological hyperactivity and only minor changes in calcium homeostasis

The serum level of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D], the biologically most potent metabolite of vitamin D, is tightly regulated within narrow limits in human healthy adults. 1,25-(OH)2D deficiency is rare and is associated with disturbances in calcium and bone metabolism. We have previously reported a marked decrease in serum levels of 1,25-(OH)2D in human immunodeficiency virus (HIV)-infected patients. The present study was designed to further examine the causes and consequences of severe 1,25-(OH)2D deficiency in these patients. The design was a prospective cohort study. Fifty-four HIV-infected patients clinically classified according to the revised criteria from Centers for Disease Control and Prevention and healthy controls were studied. Parameters related to vitamin D and calcium metabolism as well as immunological and nutritional status were determined. Twenty-nine of the patients (54%) had serum levels of 1,25-(OH)2D below the lower reference limit, and 18 of these had undetectable levels. In contrast, HIV-infected patients had normal serum levels of 25-hydroxyvitamin D and vitamin D-binding protein. HIV-infected patients as a group had modestly depressed serum calcium and PTH levels. There were, however, no correlations between these parameters and serum levels of 1,25-(OH)2D. There were no differences in serum calcium or PTH levels or nutritional status when patients with severe 1,25-(OH)2D deficiency were compared to other patients, but patients with undetectable 1,25-(OH)2D had significantly elevated serum phosphate levels. Furthermore, patients with undetectable 1,25-(OH)2D levels were characterized by advanced clinical HIV infection, low CD4+ lymphocyte counts, and high serum levels of tumor necrosis factor-alpha (TNFalpha). We conclude that inadequate 1alpha-hydroxylation of 25-hydroxyvitamin D seems to be the most likely cause of 1,25-(OH)2D deficiency in HIV-infected patients, possibly induced by an inhibitory effect of TNFalpha. The low 1,25-(OH)2D and high TNFalpha levels observed may impair the immune response in HIV-infected patients both independently and in combination and may represent an important feature of the pathogenesis of HIV-related immunodeficiency. Markedly depressed 1,25-(OH)2D serum levels are also present in certain other disorders characterized by immunological hyperactivity. Thus, the findings in the present study may not only represent a previously unrecognized immune-mediated mechanism for induction of 1,25-(OH)2D deficiency in human disease, but may also reflect the importance of adequate serum levels of 1,25-(OH)2D for satisfactory performance of the immune system in man.

Current understanding of the molecular actions of vitamin D

The important reactions that occur to the vitamin D molecule and the important reactions involved in the expression of the final active form of vitamin D are reviewed in a critical manner. After an overview of the metabolism of vitamin D to its active form and to its metabolic degradation products, the molecular understanding of the 1alpha-hydroxylation reaction and the 24-hydroxylation reaction of the vitamin D hormone is presented. Furthermore, the role of vitamin D in maintenance of serum calcium is reviewed at the physiological level and at the molecular level whenever possible. Of particular importance is the regulation of the parathyroid gland by the vitamin D hormone. A third section describes the known molecular events involved in the action of 1alpha,25-dihydroxyvitamin D3 on its target cells. This includes reviewing what is now known concerning the overall mechanism of transcriptional regulation by vitamin D. It describes the vitamin D receptors that have been cloned and identified and describes the coactivators and retinoid X receptors required for the function of vitamin D in its genomic actions. The presence of receptor in previously uncharted target organs of vitamin D action has led to a study of the possible function of vitamin D in these organs. A good example of a new function described for 1alpha,25-dihydroxyvitamin D3 is that found in the parathyroid gland. This is also true for the role of vitamin D hormone in skin, the immune system, a possible role in the pancreas, i.e., in the islet cells, and a possible role in female reproduction. This review also raises the intriguing question of whether vitamin D plays an important role in embryonic development, since vitamin D deficiency does not prohibit development, nor does vitamin D receptor knockout. The final section reviews some interesting analogs of the vitamin D hormone and their possible uses. The review ends with possible ideas with regard to future directions of vitamin D drug design.

The importance of 25-hydroxyvitamin D3 1 alpha-hydroxylase gene in vitamin D-dependent rickets

Vitamin D plays a role in a wide variety of biological events such as calcium homeostasis, bone formation and cellular differentiation. An active form of vitamin D acting as a ligand specific vitamin D receptor (VDR), 1 alpha,25(OH)2D3, is biosynthesized from cholesterol, and during this biosynthesis a renal 25-hydroxylation at the final stage by 25-hydroxyvitamin D3 1 alpha-hydroxylase is critical. Recent studies isolated the cDNA encoding 1 alpha-hydroxylase from several species, and revealed that this enzyme belongs to a member of the cytochrome p450 enzyme superfamily, with highest homologies to the p450 hydroxylases for vitamin D derivatives. One of three kinds of hereditary rickets (vitamin D-dependent rickets type I) displays an autosomal recessive trait and clinical features consistent with a defect of 1 alpha-hydroxylase activity, and the genetic analysis of the type I patients identified missense mutations of the 1 alpha(OH)ase gene that results in a loss of this enzymatic activity.

Inactivating mutations in the 25-hydroxyvitamin D3 1alpha-hydroxylase gene in patients with pseudovitamin D-deficiency rickets

BACKGROUND

Pseudovitamin D-deficiency rickets is characterized by the early onset of rickets with hypocalcemia and is thought to be caused by a deficit in renal 25-hydroxyvitamin D3 1alpha-hydroxylase, the key enzyme for the synthesis of 1alpha,25-dihydroxyvitamin D3.

METHODS

We cloned human 25-hydroxyvitamin D3 1alpha-hydroxylase complementary DNA (cDNA) using a mouse 1alpha-hydroxylase cDNA fragment as a probe. Its genomic structure was determined, and its chromosomal location was mapped by fluorescence in situ hybridization. We then identified mutations in the 1alpha-hydroxylase gene in four unrelated patients with pseudovitamin D-deficiency rickets by DNA-sequence analysis. Both the normal and the mutant 1alpha-hydroxylase proteins were expressed in COS-1 cells and were assayed for 1alpha-hydroxylase activity.

RESULTS

The gene for 25-hydroxyvitamin D3 1alpha-hydroxylase was mapped to chromosome 12q13.3, which had previously been reported to be the locus for pseudovitamin D-deficiency rickets by linkage analysis. Four different homozygous missense mutations were detected in this gene in the four patients with pseudovitamin D-deficiency rickets. The unaffected parents and one sibling tested were heterozygous for the mutations. Functional analysis of the mutant 1alpha-hydroxylase protein revealed that all four mutations abolished 1alpha-hydroxylase activity.

CONCLUSIONS

Inactivating mutations in the 25-hydroxyvitamin D3 1alpha-hydroxylase gene are a cause of pseudovitamin D-deficiency rickets.

Parathyroid hormone activation of the 25-hydroxyvitamin D3-1alpha-hydroxylase gene promoter

The DNA flanking the 5' sequence of the mouse 1alpha-hydroxylase gene has been cloned and sequenced. A TATA box has been located at -30 bp and aCCAAT box has been located at -79 bp. The gene's promoter activity has been demonstrated by using a luciferase reporter gene construct transfected into a modified pig kidney cell line, AOK-B50. Parathyroid hormone stimulates this promoter-directed synthesis of luciferase by 17-fold, whereas forskolin stimulates it by 3-fold. The action of parathyroid hormone is concentration-dependent. 1,25-Dihydroxyvitamin D3 does not suppress basal promoter activity and marginally suppresses parathyroid hormone-driven luciferase reporter activity. The promoter has three potential cAMP-responsive element sites, and two perfect and one imperfect AP-1 sites, while no DR-3 was detected. These results indicate that parathyroid hormone stimulates 25-hydroxyvitamin D3-1alpha-hydroxylase by acting on the promoter of the 1alpha-hydroxylase gene.

Mechanisms and functions of vitamin D

The vitamin D hormone, 1,25-dihydroxyvitamin D3, functions by way of a nuclear receptor (vitamin D receptor [VDR]) in a manner analogous to the other members of the steroid-thyroid hormone superfamily. Although the vitamin D receptor has been cloned, its three-dimensional structure remains unknown. The VDR binds to the direct repeat response elements called DR-3 in the promoter region of target genes to stimulate or suppress transcription of those genes encoding for proteins that carry out a wide variety of functions. The binding of the VDR to a DR-3 requires the presence of its ligand and a companion protein, namely the RXR group of retinoid receptors. The RXR binds to the 5' arm of the response element while the VDR binds to the 3' arm. In addition, the transcription factor TFIIB has been shown to bind VDR but there is currently no evidence that a co-repressor or co-activator of VDR is also involved. Phosphorylation of VDR in the transcription complex occurs as does bending of the DNA prior to the initiation or suppression of transcription. As VDR has been detected in cells not previously thought to be target organs, scientists continue to discover new functions of vitamin D. Among these new functions are those noted in the immune system. Experiments in mice have illustrated that the autoimmune diseases of multiple sclerosis and rheumatoid arthritis can be successfully treated with the vitamin D hormone and its analogs. New experiments illustrating the use of the vitamin D hormone and its analogs in suppressing transplant rejection indicate that these compounds may be superior to cyclosporin and may not have the side effects attributed to the cyclosporin immunosuppression therapies.

Molecular cloning of cDNA and genomic DNA for human 25-hydroxyvitamin D3 1 alpha-hydroxylase

The 25-hydroxyvitamin D3 1 alpha-hydroxylase (1 alpha-hydroxylase) is a cytochrome P450 enzyme that catalyzes the conversion of 25-hydroxyvitamin D3 to 1 alpha,25-dihydroxyvitamin D3. This enzyme plays an important role in calcium homeostasis. Here we report the molecular cloning of cDNA and gene for human 1 alpha-hydroxylase. The cDNA clone was obtained from a human kidney cDNA library by cross-hybridization with a previously cloned rat cDNA probe. The cDNA consists of 2469 bp and encodes a protein of 508 amino acids that shows 82.5% sequence identity with the rat enzyme. A computer-aided homology search revealed that 1 alpha-hydroxylase shares a relatively high homology with vitamin D3 25-hydroxylase (about 40% amino acid identity). Northern blot analysis showed that the 2.5-kb mRNA is most abundant in kidney. The gene for human 1 alpha-hydroxylase spans approximately 6 kb, is composed of nine exons, and is present as a single copy. This molecular cloning makes it possible to investigate the genetic mechanism of diseases related to calcium metabolism, including vitamin D-dependency rickets type I.

25-Hydroxyvitamin D3 1alpha-hydroxylase and vitamin D synthesis

Renal 25-hydroxyvitamin D3 1alpha-hydroxylase [1alpha(OH)ase] catalyzes metabolic activation of 25-hydroxyvitamin D3 into 1alpha, 25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], an active form of vitamin D, and is inhibited by 1alpha,25(OH)2D3. 1alpha(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR-/- mice), is a member of the P450 family of enzymes (P450VD1alpha). Expression of 1alpha(OH)ase was suppressed by 1alpha, 25(OH)2D3 in VDR+/+ and VDR+/- mice but not in VDR-/- mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1alpha(OH)ase through liganded VDR.

Cloning of the human 1 alpha,25-dihydroxyvitamin D-3 24-hydroxylase gene promoter and identification of two vitamin D-responsive elements

A genomic DNA clone for 1 alpha,25-dihydroxyvitamin D-3 (1,25-(OH)2D3) 24-hydroxylase was isolated from a human chromosome 20 library. It spans 2.42 kb, containing the first two exons, the first and part of the second introns, and a 1.26 kb 5'-flanking region. Putative transcription cis-elements were revealed throughout the 5'-flanking region, including TATA box, CAAT box, GC boxes, vitamin D-responsive elements (VDRE), AP1, and AP2 sites. In a CAT reporter gene expression assay, the 24-hydroxylase promoter with its 1.2 kb 5'-flanking sequence elicits a 1,25-(OH)2D3-induced transactivation activity. Gel mobility shift assays of those putative DREs have identified that two different elements can form specific complexes with porcine intestinal nuclear extract (PINE). The specificity of VDRE-PINE complexes was verified by supershift assay with VDR-specific monoclonal antibody VXIE10B6. The proximal element VDREp (-172/-143) consists of three direct repeat half-sites, GAGTCAgcgAGGTGAgcgAGGGCG, in anti-sense orientation. The distal element VDREd (-293/-273) consists of two direct repeat half-sites, GCGTTCaccGGGTGT, also in anti-sense orientation. Both VDREs can direct a reporter gene expression using a heterologous herpes simplex virus thymidine kinase (TK) promoter in a 1,25-(OH)2D3-dependent fashion. Further characterization of these VDREs in various constructs with either a native or TK promoter suggests that both VDREs are required for the optimal induction of 24-hydroxylase expression by 1,25-(OH)2D3.

Oral calcium treatment in vitamin D-dependent rickets type II

Vitamin D-dependent rickets type II is a rare hereditary disease that results from target organ resistance to the action of 1,25-dihydroxyvitamin D3. There is a great heterogeneity in the clinical presentation of this condition. The affected patients usually present early in childhood with clinical and biochemical evidence of rickets. Physiological replacement dosage of 1,25-dihydroxyvitamin D3 has no therapeutic effect. Responses to pharmacological doses of vitamin D metabolites or long-term calcium infusion have been variable. A case is reported here of an 8 year old girl, of consanguineous parents with vitamin D-dependent rickets, type II, in whom treatment with high dose oral calcium resulted in marked biochemical and radiological improvement. It is concluded that high dose oral calcium treatment is an effective treatment option for patients with vitamin D-dependent rickets type II.

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.

Tissue resistance to 1,25-dihydroxyvitamin D without a mutation of the vitamin D receptor gene

OBJECTIVE

Hereditary vitamin D resistant rickets (HVDRR) is characterized by severe rickets and is often accompanied by alopecia. Mutations in the gene encoding the vitamin D receptor have been found in this condition. In a patient with the characteristic phenotype we have investigated the functional defect and sequenced the gene to seek a mutation.

DESIGN

Patient and control cell lines prepared from skin fibroblasts and peripheral blood lymphocytes were used to measure binding of 1,25(OH)2D3 and to isolate vitamin D receptor mRNA. VDR cDNA was sequenced and transfected into receptor defective cells.

PATIENT

A child with alopecia diagnosed as having rickets due to resistance to 1,25(OH)2D3.

MEASUREMENTS

Cytosolic binding and nuclear association of 1,25(OH)2D3 were determined in patient and control cells, and functional response to 1,25(OH)2D3 assessed by measurement of 24-hydroxylase activity. VDR mRNA was prepared, reverse transcribed, and cDNA sequenced. VDR cDNA was also transfected into VDR-deficient CV-1 cells and functional response to 1,25(OH)2D3 assessed by co-transfection with a chloramphenicol acetyltransferase (CAT) reporter plasmid.

RESULTS

VDR from the patient were able to bind 1,25(OH)2D3 but showed no nuclear localization resulting in an absence of functional response to 1,25(OH)2D3. Sequencing revealed that the VDR coding region was normal. Expression studies of the patient's VDR showed functionally normal VDR as evidenced by normal transactivation in the presence of 1,25(OH)2D3.

CONCLUSION

These data indicate a new cause of tissue resistance to 1,25(OH)2D3 which occurs in the absence of mutations in the coding region of VDR gene and which is characterized by defective nuclear localization of this receptor.

A new, highly sensitive assay for 1,25-dihydroxyvitamin D not requiring high-performance liquid chromatography: application of monoclonal antibody against vitamin D receptor to radioreceptor assay

A new, highly sensitive radioreceptor assay, which does not require high-performance liquid chromatography, has been developed for the determination of 1,25-dihydroxyvitamin D3 (1,25-(OH2)D3) in serum. The assay involves rapid extraction of serum, Sep Pak silica purification, and addition of 1,25-dihydroxyvitamin D3 receptor, radiolabeled 1,25-dihydroxyvitamin D3, bovine serum albumin, and monoclonal antibody to specifically precipitate the receptor. This method is sensitive to 0.3-0.6 pg/tube, with B50 occurring at 5.8 pg/tube. This sensitivity combined with overall recovery of 1,25-dihydroxyvitamin D3 (81.5 +/- 5.2%, n = 50, mean +/- SD) allows the measurement of serum 1,25-(OH)2D3 in duplicates with only 0.5 ml of serum. Intra- and interassay coefficient of variation were 9.5 and 14.6%, respectively. Dilution analysis, analytical recovery of added 1,25-dihydroxyvitamin D3, and comparison with a standard method using HPLC have been used to validate the assay. Serum 1,25-dihydroxyvitamin D3 level was for normal adults, 36.6 +/- 10.5 pg/ml (n = 14); in primary hyperparathyroidism, 98.9 +/- 19.9 pg/ml (n = 16); in chronic renal failure, 17.8 +/- 5.1 pg/ml (n = 12). This method allows large numbers of samples to be processed at once. Further, the method is rapid and provides an accurate assay using small amounts of serum.

Molecular genetics of mineral metabolic disorders

The recent advances in molecular biology and cytogenetics have made it possible to localize, clone and characterize some of the genetic abnormalities which result in disorders of phosphate and calcium homeostasis. Thus, the genes causing X-linked hypophosphataemic rickets, Lowe syndrome, X-linked recessive hypoparathyroidism, Di George syndrome, multiple endocrine neoplasia type 1, multiple endocrine neoplasia type 2 and vitamin D-dependent rickets type I have been mapped. In addition the genes involved in the pathogenesis of the humoral hypercalcaemia of malignancy, vitamin D-dependent rickets type II, pseudohypoparathyroidism, and some of the autosomal forms of hypoparathyroidism have been cloned and the mutations characterized. The molecular and genetic studies which have mapped and identified these disease genes are described and the implications of these developments in clinical practice and in further elucidation of the mineral metabolic defects are discussed.

Population genetics of vitamin D-dependent rickets in northeastern Quebec

Vitamin D-dependent rickets (VDD1) is an autosomal recessive disorder that was recognized in Saguenay-Lac-St-Jean (SLSJ) in 1970. The great majority of the VDD1 cases reported in the French Canadian population of Quebec originated from SLSJ, Charlevoix, and the Haute Côte Nord, all regions located in northeastern Quebec. The prevalence at birth in SLSJ was estimated at 1/2916 live borns, and the carrier rate was estimated at 1/27 inhabitants in the SLSJ region. The mean coefficient of inbreeding was not elevated in the VDD1 group of SLSJ compared with three matched control groups. The mean coefficient of kinship was 2.5 times higher in the VDD1 group than in the control groups. In the SLSJ region, the places of origin of the VDD1 children and their children did not show a clustered non-uniform distribution. Endogamy was not found to be higher in the VDD1 group than in control groups. The genealogical reconstruction showed all the obligate carriers of the VDD1 gene, but one, to be related to a small set of founders who settled in New France in the 17th century. All these results, as well as a strong linkage disequilibrium between RFLPs located on the long arm of chromosome 12 and the VDD1 locus, support the hypothesis of a founder effect for VDD1. They also suggest that a unique mutation accounts for most, if not all, of the cases known in northeastern Quebec.

Development of a double antibody radioimmunoassay for quantitation of 1 alpha,25-dihydroxyvitamin D

A sensitive radioimmunoassay (RIA) for 1 alpha,25-dihydroxyvitamin D [1 alpha,25(OH)2-D] with a double antibody (DAB) separation technique to separate free from bound antigen has been developed. The hormone was extracted from 1 ml serum or plasma by Extrelut columns and normal phase high performance liquid chromatography and quantitated in the DAB-RIA. The detection limit of the assay was 3.75 ng/l. The intraassay variation coefficients were 15.9% and 10.5% for samples with 1 alpha,25(OH)2D3 concentrations of 54 ng/l and 130 ng/l, respectively. The interassay variation coefficients were 18.0% and 16.7% for these two concentrations. Mean (and SD) values for 1,25(OH)2D in serum of 40 healthy subjects and 38 patients with chronic renal failure who did not receive 1,25(OH)2D3 were 62.8 ng/ml (22.2) and 12.4 ng/ml (9.8), respectively. The mean value for 7 patients with primary hyperparathyroidism was 66.5 ng/ml (35.8) before surgery. These results compared well with those of an established charcoal-based RIA. Compared to charcoal-based RIAs, the DAB-RIA is faster and requires less laborious assay procedures.

Osteoporosis and the metabolites of vitamin D

Vitamin D is metabolized by sequential steps in the liver and kidney to its active form, a process that is strongly feedback-regulated. In old age, the activity of the enzyme, 25-hydroxyvitamin D 1 hydroxylase, which produces the vitamin D hormone, is diminished. The activities of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) extend beyond increasing intestinal absorption of calcium. The vitamin D hormone (or its analogs) is useful in the treatment of osteoporosis because it not only stimulates intestinal calcium absorption, but also is required for the stimulation of osteoblasts and many other cells in the body. Recent work demonstrates that it is possible to chemically synthesize analogs selective for specific actions of the vitamin D hormone, especially in inducing differentiation of promyelocytes and keratinocytes.

Familial renal hypophosphatemia, minor facial anomalies, intracerebral calcifications, and non-rachitic bone changes: apparently new syndrome?

We report on two brothers with renal hypophosphatemia, intracerebral calcifications, minor facial anomalies, and short distal phalanges. The children presented with recurrent dental abscesses; one had premature closure of the anterior fontanelle. Biochemical findings included hypophosphatemia and elevated serum alkaline phosphatase with normocalcemia. Blood levels of parathyroid hormone, 1,25(OH)2 and 25(OH) vitamin D levels were normal; TRP (the fractional tubular reabsorption of PO4) and TmP/GFR (the tubular maximum rate of PO4 reabsorption in relation to GFR) were low. Both parents had a normal serum phosphate and brain CT scan without evidence of calcifications. This apparently new syndrome of renal hypophosphatemia associated with intracerebral calcifications appears to be inherited as either an autosomal recessive or an X-linked trait.

Evaluating the child with short stature

Short stature is a common pediatric problem that requires us to decide whether a child's small size represents only normal variation or indicates the presence of an underlying disease. In a population of children two standard deviations (SD) below the mean for height (below the third percentile), about 20 per cent may be expected to have pathologic short stature with the remaining 80 per cent about equally divided between familial short stature and constitutional growth delay. In contrast, most children three SD below the mean for height have pathologic short stature. Set forth in this article is an orderly approach to identify normal variants of short stature and to investigate the causes of pathologic short stature.

Bone changes in rats with adjuvant arthritis: treatment with 1 alpha-hydroxycholecalciferol

Adjuvant arthritis was induced in female Lewis rats by the injection of Mycobacterium butyricum in mineral oil in a hindpaw. Bone changes due to polyarthritis were evaluated in the tibia metaphyseal region of the noninjected hind leg. A decrease in bone mass and mineral content was observed in the affected bone. accompanied by a marked increase in bone resorption. Non-arthritic and arthritic rats were treated with the synthetic vitamin D analogue, 1 alpha-hydroxycholecalciferol (0.01 to 1.0 micrograms/kg/day orally) for 28 days. Treatment resulted in a small increase in bone weight and mineral content in the non-arthritic rats. Arthritic rats exhibited a larger increase in bone weight, hydroxyproline and calcium content. These beneficial effects were correlated with the ability of 1 alpha-hydroxycholecalciferol to increase extracellular calcium and phosphorus levels, as measured by the increased urinary excretion of calcium and inorganic phosphate and by the increase in serum calcium. Bone resorption and new bone formation were not affected by the treatment.

Muscle weakness in infants with rickets: distribution, course, and recovery

We describe the distribution, progression, and resolution of muscle weakness, wasting, and hypotonia in three infants with rickets due to different causes. Progressive muscle weakness affecting preferentially the proximal muscles of the legs and failure to gain weight were the presenting symptoms. The skeletal signs appeared later in the course of the illness and the time for resolution of the neuromuscular findings varied with the etiology of the disorder.

Absence of renal 25-hydroxycholecalciferol-1-hydroxylase activity in a pig strain with vitamin D-dependent rickets

Cholecalciferol-1-hydroxylase activities were estimated in renal cortex homogenates and mitochondrial preparations from three groups of 6- to 12-week-old pigs. Five animals suffering from an inherited form of vitamin D-deficiency rickets showed symptoms of florid rickets when used for this study. Six pigs were normocalcemic heterozygous litter mates of the rachitic strain and three pigs were normal controls (German land-race and wild pigs). The renal cortex homogenates and mitochondrial preparations were incubated for 5-30 min at 37 degrees C with 25-(26-27-methyl-3H) OHD3 as substrate. 1,25(OH)2D3 was subsequently identified in normal phase (Zorbax-Sil) and reversed phase (Zorbax-ODS) HPLC eluates. 1-hydroxylase activities were demonstrated in both normal controls and heterozygote offspring and were ten times above minimum detectability of the assay. The km was 255 +/- 74 (SD) and 278 +/- 85 nmol X 1(-1) in controls and heterozygote offspring, respectively. The Vmax in the two groups was between 0.11 and 0.794 and decreased with age of the animals. Km and Vmax did not differ between the two groups. In homozygous, hypocalcemic rachitic animals no 1-hydroxylase activity was detectable in either homogenates or mitochondrial preparations. Addition of kidney homogenate from a rachitic animal to a homogenate from a normal pig did not specifically depress 1-hydroxylase activity in the mixture. Treatment of rachitic pigs with 1.0 microgram/day of 1,25(OH)2D3 for 4 weeks also had no effect on 1-hydroxylase activity. It is concluded that the rachitic pigs suffer from an inborn error of renal 1,25(OH)2D3 production.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabolism and functions of vitamin D

Vitamin D functions by stimulating intestinal calcium and phosphorus absorption, by stimulating bone calcium mobilization, and by increasing renal reabsorption of calcium in the distal tubule. These functions on bone and possibly kidney, but not intestine, require the parathyroid hormone. As a result of these functions, serum calcium and phosphorus concentrations are elevated to supersaturating levels required for the mineralization of bone to prevent rickets, osteomalacia, and hypocalcemic tetany. Recent experiments demonstrate that maintaining serum calcium and phosphorus levels in vitamin D-deficient rats in the normal range results in normal bone growth and mineralization. However, increased calcification results because bone resorption by osteoclasts is a vitamin D-dependent process. Thus, bone resorption, modeling and remodeling must be considered vitamin D-dependent processes. Vitamin D must be metabolized to 25-hydroxyvitamin D3 by the liver and subsequently by the kidney to 1,25-dihydroxyvitamin D3 before function. 1,25-Dihydroxyvitamin D3 is metabolized to a C-23 carboxylic acid (calcitroic acid) but the pathway is unknown. Although 25-hydroxyvitamin D3 is metabolized to 24R,25-dihydroxyvitamin D3, 25,26-dihydroxyvitamin D3 and 25-hydroxyvitamin D3-26,23-lactone, these pathways play no role in the function of vitamin D as shown by appropriate fluoro analogs of 25-hydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 binds to a specific receptor in the intestinal nuclei to elicit a stimulation of calcium transport. 1,25-Dihydroxyvitamin D3 plus the receptor causes transcription of specific genes that code for calcium and phosphorus transport proteins. Only one protein, the calcium binding protein, has been identified as being vitamin D dependent. Two others have been described, but no clear description of the molecular mechanism of action of 1,25-dihydroxyvitamin D3 is yet available.

The physiology and pathophysiology of vitamin D

The vitamin D endocrine system plays an important role in the maintenance of normal calcium homeostasis. Abnormalities of this system occur in many conditions, such as rickets, osteomalacia, hypoparathyroidism, and hyperparathyroidism. The diagnosis and treatment of these disorders will be facilitated if the clinician understands the general mechanisms by which defects in vitamin D metabolism and action occur. We review this information and discuss the use and limitations of vitamin D metabolite assays for diagnosis of clinical disorders of mineral metabolism.

The mechanism of end-organ resistance to 1 alpha,25-dihydroxycholecalciferol in the common marmoset

The common marmoset, a New World monkey, requires a large amount of cholecalciferol (110 i.u./day per 100g body wt.) to maintain its normal growth. In a previous report, we demonstrated that the circulating levels of 1 alpha, 25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] in the marmosets are much higher than those in rhesus monkeys and humans, but the marmosets are not hypercalcaemic [Shinki, Shiina, Takahashi, Tanioka, Koizumi & Suda (1983) Biochem. Biophys. Res. Commun. 14, 452-457]. To compare the effect of the daily intake of cholecalciferol, two rhesus monkeys were given a large amount of cholecalciferol (900 i.u./day per 100g body wt). Their serum levels of calcium, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol were markedly elevated, but the serum 1 alpha,25(OH)2D3 levels remained within a range similar to those in the rhesus monkeys fed the normal diet (intake of cholecalciferol 5 i.u./day per 100g body wt). Intestinal cytosols prepared from both monkeys contained similar 3.5 S macromolecules to which 1 alpha,25(OH)2D3 was bound specifically. However, the cytosols from the marmosets contained only one-sixth as many 1 alpha,25(OH)2D3 receptors as those from the rhesus monkeys. Furthermore, the activity of the 1 alpha,25(OH)2D3-receptor complex in binding to DNA-cellulose was very low in the marmosets. These results suggest that the marmoset possesses an end-organ resistance to 1 alpha,25(OH)2D3 and is a useful animal model for studying the mechanism of vitamin D-dependent rickets, type II.

Respiratory failure and multiple fractures in vitamin D-dependent rickets

A 20-month-old boy presented with multiple fractures and respiratory failure. He was found to have vitamin D-dependent rickets. The rickets did not improve with phosphate, calcium and calciferol. The rickets healed, however, with large doses of 1-alpha-hydroxycholecalciferol. The presentation is unusual and the response to therapy suggests metabolic heterogeneity in vitamin D-dependent rickets.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

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.

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.