SYNERGISTIC EFFECT OF PARTIALLY INACTIVATING MUTATIONS IN STEROID 21-HYDROXYLASE DEFICIENCY

Lesions in the gene encoding steroid 21-hydroxylase result in congenit al adrenal hyperplasia, with impaired secretion of cortisol and aldost erone from the adrenal cortex and overproduction of androgens. Mild fo rms of the disease cause late-onset symptoms of hyperandrogenism and a re thought to be largely underdiagnosed. A limited number of mutations account for the majority of mutated alleles, but additional rare muta tions are responsible for the symptoms in some patients. fire previous ly reported a rare allele in two siblings with late-onset disease. Thi s allele contained three sequence alterations, a C to T transition 4 b ases upstream of translation initiation, a CCG to CTG change at codon 105 (P105L), and a CCC to TCC transition at codon 453 (P453S). The lat ter mutation has been found in other ethnic groups, whereas P105L seem s to be unique to this family. We have now analyzed the functional con sequences 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 ef fect on gene expression. The P105L mutation reduced enzyme activity to 62% for 17-hydroxyprogesterone and 64% for progesterone, and the P453 S mutation reduced activity to 68% and 46%, respectively. When present in combination, the two mutations caused a reduction of enzyme activi ty to 10% for 17-hydroxyprogesterone and 7% for progesterone. These re sults indicate that P105L and P453S can be expected to result in a ver y subtle disease manifestation when not found in combination, motivati ng their inclusion when genotyping to ascertain undiagnosed patients w ith the mildest forms of 21-hydroxylase deficiency.