Complete structure of the human gene for the vitamin D 1alpha-hydroxylase, P450c1alpha

The rate-limiting, hormonally regulated step in the biosynthesis of the biologically active form of vitamin D, 1,25(OH)2D, is its 1alpha-hydroxylation in the kidney by the mitochondrial P450 enzyme, P450c1alpha. We have recently cloned the human P450c1alpha cDNA and shown that this enzyme is the factor disrupted in vitamin D-dependent rickets, type 1 (VDDR-1). To facilitate the analysis of further patients with VDDR-1 and to permit studies of the regulation of the gene for P450c1alpha, we have used PCR-based tactics to clone the gene. Southern blotting studies indicate that there is only one copy of this gene in the human genome. The complete sequence of all exons and introns show that the gene consists of 9 exons spanning only 5 kb; the entire protein-coding region can be PCR-amplified as a single 4-kb fragment. The transcriptional start site, located by primer extension and S1 nuclease protection, lies 62-bp upstream from the ATG transitional start codon. Analysis of rodent/human somatic cell hybrid DNAs show that this gene lies on chromosome 12. Although the gene is substantially smaller than the human genes for other mitochondrial enzymes, its intron/exon organization is very similar, especially to that of P450scc. This indicates that although the mitochondrial P450 enzymes retain only 30%-40% amino acid sequence identity, they all belong to a single evolutionary lineage.