Background. Vitamin D dependency rickets type I (VDDR-I) is an autosomal recessive disorder in which 25-hydroxyvitamin D 1α-hydroxylase (1α- 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 1α-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 1α-hydroxylase mutations in French- Canadian patients with VDDR-I. Methods. The nine exons of the 1α-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 lα-hydroxylase mutations that are consistent with loss of function in four French-Canadian patients with VDDR-I and suggest that the 1α-hydroxylase mutations arise from more than one founder in this population.
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