Synergistic effect of partially inactivating mutations in steroid 21-hydroxylase deficiency

Lesions in the gene encoding steroid 21-hydroxylase result in congenital adrenal hyperplasia, with impaired secretion of cortisol and aldosterone from the adrenal cortex and overproduction of androgens. Mild forms of the disease cause late-onset symptoms of hyperandrogenism and are thought to be largely underdiagnosed. A limited number of mutations account for the majority of mutated alleles, but additional rare mutations are responsible for the symptoms in some patients. We previously reported a rare allele in two siblings with late-onset disease. This allele contained three sequence alterations, a C to T transition 4 bases upstream of translation initiation, a CCG to CTG change at codon 105 (P105L), and a CCC to TCC transition at codon 453 (P453S). The latter mutation has been found in other ethnic groups, whereas P105L seems to be unique to this family. We have now analyzed the functional consequences of the -4, P105L, and P453S sequence alterations by in vitro translation and after expression of mutant enzyme in cultured cells. As expected, the base substitution at position-4 had no measurable effect on gene expression. The P105L mutation reduced enzyme activity to 62% for 17-hydroxyprogesterone and 64% for progesterone, and the P453S mutation reduced activity to 68% and 46%, respectively. When present in combination, the two mutations caused a reduction of enzyme activity to 10% for 17-hydroxyprogesterone and 7% for progesterone. These results indicate that P105L and P453S can be expected to result in a very subtle disease manifestation when not found in combination, motivating their inclusion when genotyping to ascertain undiagnosed patients with the mildest forms of 21-hydroxylase deficiency.