Structural Characterization of Normal and Mutant Human Steroid 17α- Hydroxylase Genes: Molecular Basis of One Example of Combined 17α- Hydroxylase/17,20 Lyase Deficiency

Abstract

Steroid 17.alpha.-hydroxylase (cytochrome p-45017.alpha.) catalyzes both 17.alpha.-hydroxylation of pregnenolone and progesterone and 17,20-lysis of 17.alpha.-hydroxypregnenolone and 17.alpha.-hydroxyprogesterone. In the course of undertaking detailed investigation of the structure-function relationships which exist within this enzyme we have begun to elucidate the molecular basis of human deficiencies in either or both of these activities. Consequently we have determined the exonic structure of the human P-45017.alpha. gene as well as the sequences at the exon/intron boundaries and at the site of initiation of transcription. A single gene in the human genome encodes this protein, being the sole member of a unique gene family (P450XVII) within the P-450 supergene family. A protocol for exonic sequencing of the P-45017.alpha. gene has been established which permits structural analysis of the gene from patients having 17.alpha.-hydroxylase and/or 17,20-lyase deficiency. This procedure has been applied to the mutant gene from one individual having combined 17.alpha.-hydroxylase/17,20-lyase deficiencies. A four-base duplication is found in exon 8 producing a protein with an altered C-terminal amino acid sequence which results in loss of both enzymatic activities.