COMPLETE STRUCTURE OF THE HUMAN GENE FOR THE VITAMIN-D 1-ALPHA-HYDROXYLASE, P450C1-ALPHA

The rate-limiting, hormonally regulated step in the biosynthesis of th e biologically active form of vitamin D, 1,25(OH)(2)D, is its 1 alpha- hydroxylation in the kidney by the mitochondrial P450 enzyme, P450c1 a lpha. We have recently cloned the human P450c1 alpha cDNA and shown th at 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 P450c1 alpha, we have used PCR-based tactics to clone the gene, Southern blot ting 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-c oding region can be PCR-amplified as a single 4-kb fragment, The trans criptional start site, located by primer extension and S-1 nuclease pr otection, lies 62-bp upstream from the ATG transitional start codon, A nalysis of rodent/human somatic cell hybrid DNAs show that this gene t ies on chromosome 12, Although the gene is substantially smaller than the human genes for other mitochondrial enzymes, its intron/exon organ ization is very similar, especially to that of P450sec, This indicates that although the mitochondrial P450 enzymes retain only 30%-40% amin o acid sequence identity, they all belong to a single evolutionary lin eage.