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 2,900 21OHD patients, either isolated or associated on the same allele with a mild mutation (p.P453S, p.P30L or partial promoter). As phenotype seemed to differ between patients with isolated or associated p.H62L, a detailed phenotype description and functional studies were performed.

Results: Phenotype: Patients with isolated p.H62L had a non-classical form while patients with the association p.H62L + mild mutation had a simple virilizing form. Functional studies: p.H62L reduced the conversion of the 2 substrates, Progesterone and 17OHProgesterone, 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 while 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 patients management and genetic counseling.