2 NOVEL 1-ALPHA-HYDROXYLASE MUTATIONS IN FRENCH-CANADIANS WITH VITAMIN-D DEPENDENCY RICKETS TYPE-I

Background Vitamin D dependency rickets type I (VDDR-I) is an autosoma l recessive disorder in which 25-hydroxyvitamin D 1 alpha-hydroxylase (1 alpha-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 rec ently cloned the human 1 alpha-hydroxylase cDNA and gene, making it po ssible to screen for mutations. The first VDDR-I mutations were report ed in one American and four Japanese patients. In this study, we scree ned for 1 alpha-hydroxylase mutations in French-Canadian patients with VDDR-I. Methods. The nine exons of the Icr-hydroxylase gene were ampl ified by polymerase chain reaction (PCR) from genomic DNA of four unre lated French-Canadian patients with VDDR-I and their parents, and sequ enced. Results. Three of the patients were homozygous for a single bas e-pair deletion (G) at position 262 in the cDNA that lies in exon 2, a nd causes a premature termination codon upstream from the putative fer redoxin- and heme-binding domains. The fourth patient was homozygous f or a 7-bp insertion (CCCCCCA) at position 1323 of the cDNA that lies i n exon 8, and causes a premature termination upstream from the putativ e 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 elect rophoresis of the PCR products, were not found in 25 normal French-Can adians. Conclusion. We describe two novel 1 alpha-hydroxylase mutation s that are consistent with loss of function in four French-Canadian pa tients with VDDR-I and suggest that the la-hydroxylase mutations arise from more than one founder in this population.