p.H62L, a rare mutation of the CYP21 gene identified in two forms of 21-hydroxylase deficiency

Context: Steroid 21-hydroxylase deficiency is the most common enzymatic defect causing congenital adrenal hyperplasia with good genotype/phenotype relationships for common mutations. To determine the severity of rare mutations is essential for genetic counseling and better understanding of the structure-function of the cytochrome P450c21. Objective: The p. H62L mutation was the most frequent of 60 new mutations detected in 2900 steroid 21-hydroxylase deficiency patients, either isolated or associated on the same allele with a mild mutation (p. P453S, p. P30L, or partial promoter). Because phenotypes seemed to differ between patients with isolated or associated p. H62L, a detailed phenotype description and functional studies were performed. Results: Regarding phenotype, patients with isolated p. H62L had a nonclassical form, whereas patients with the association p. H62L + mild mutation had a simple virilizing form. Functional studies showed that p. H62L reduced the conversion of the two substrates, progesterone and 17-hydroxyprogesterone, in the same way as the mild p. P453S; the association p. H62L + p. P453S decreased enzymatic activity more strongly while conserving residual activity at a level intermediate between p. P453S and p. I172N. This suggested that p. H62L was a mild mutation, whereas a synergistic effect occurred when it was associated. Analysis of p. H62L in a three-dimensional model structure of the CYP21 protein explained the observed in vitro effects, the H62 being located in a domain implied in membrane anchoring. Conclusion: According to phenotype and functional studies, p. H62L is a mild mutation, responsible for a more severe phenotype when associated with another mild mutation. These data are important for patient management and genetic counseling.