The human cytochrome P-450 21-hydroxylase genes
Deficiency of the cytochrome P-450 steroid 21-hydroxylase (21-OHase) which causes Congenital Adrenal Hyperplasia (CAH) is a monogenic autosomal recessive disorder which is linked to HLA. There are two 21-OHase genes in man, A and B, and they are mapped to the HLA class III region ~ 3 kb 3' to the complement genes C4A and C4B, respectively. Two genes encoding 21-OHase were isolated, characterized and sequenced. Both 21-OHase genes are ~ 3.3 kb in length and are split into 10 exons by nine introns. Comparison of the two genes showed that although they are highly conserved, there are three deleterious mutations in the 21-OHase A gene which cause frameshifts and introduce in phase premature termination codons. Thus the 21-OHase A gene is a pseudogene. Comparison of the 21-OHase B gene to the other cytochrome P-450 sequences revealed that although the cysteine-429 was conserved in 21-OHase, there is very little homology with other cytochrome P-450, indicating it belongs to a separate family of genes within the superfamily. Clear evidence of polymorphism in 21-OHase is apparent on comparison with other 21-OHase B sequences. There is a size polymorphism of 494 and 495 amino acids. The differing severities of 21-OHase deficiency in CAH may be due to allelic variants of the 21-OHase B gene, since in most cases the defect is not due to gene deletion (Rumsby et al., 1986). A 21-OHase B gene from a patient with CAH was characterized and sequenced. There were 13 nucleotide alterations in his single 21-OHase B gene, one of which at codon 269 caused a serine to change to a threonine residue. The G → C transversion in the 21-OHase B gene from the patient at codon 269 introduced a new NcoI restriction site into the gene. This restriction fragment length polymorphism (RFLP) was used to study other patients with CAH and normal individuals. The NcoI RFLP was found not to be confined to the 21-OHase B gene but was also present in some 21-OHase A genes. It is likely therefore that the mutation occurred in the pseudogene first and then transferred to some 21-OHase B genes.