Functional Consequences of Seven Novel Mutations in the CYP11B1 Gene: Four Mutations Associated with Nonclassic and Three Mutations Causing Classic 11β-Hydroxylase Deficiency

Abstract

Context: Steroid 11β-hydroxylase (CYP11B1) deficiency (11OHD) is the second most common form of congenital adrenal hyperplasia (CAH). Cases of nonclassic 11OHD are rare compared with the incidence of nonclassic 21-hydroxylase deficiency. Objective: The aim of the study was to analyze the functional consequences of seven novel CYP11B1 mutations (p.M88I, p.W116G, p.P159L, p.A165D, p.K254_A259del, p.R366C, p.T401A) found in three patients with classic 11OHD, two patients with nonclassic 11OHD, and three heterozygous carriers for CYP11B1 mutations. Methods: We conducted functional studies employing a COS7 cell in vitro expression system comparing wild-type (WT) and mutant CYP11B1 activity. Mutants were examined in a computational three-dimensional model of the CYP11B1 protein. Results: All mutations (p.W116G, p.A165D, p.K254_A259del) found in patients with classic 11OHD have absent or very little 11β-hydroxylase activity relative to WT. The mutations detected in patients with nonclassic 11OHD showed partial functional impairment, with one patient being homozygous (p.P159L; 25% of WT) and the other patient compound heterozygous for a novel mild p.M88I (40% of WT) and the known severe p.R383Q mutation. The two mutations detected in heterozygous carriers (p.R366C, p.T401A) also reduced CYP11B1 activity by 23 to 37%, respectively. Conclusion: Functional analysis results allow for the classification of novel CYP11B1 mutations as causative for classic and nonclassic 11OHD, respectively. Four partially inactivating mutations are predicted to result in nonclassic 11OHD. These findings double the number of mild CYP11B1 mutations previously described as associated with mild 11OHD. Our data are important to predict phenotypic expression and provide important information for clinical and genetic counseling in 11OHD.

Genetics of Congenital Adrenal Hyperplasia

“…69 Over 50 CYP11B1-inactivatig mutations that are distributed over the entire coding region exist, with the majority being missense and nonsense, but splice-site mutations, small deletions, small insertions, and complex rearrangements exist and result in absent or very little 11β-hydroxylase activity. 70 These mutations (Table 2) cause the classic form and tend to cluster in exons 2, 6, 7, and 8. 71 Rarely, unequal crossing-over between the CYP11B2 and the CYP11B1 genes produces a chimera that is under the control of angiotensin II and potassium, rather than ACTH, 72 resulting in glucocorticoid-remediable aldosteronism (familial aldosteronism type 1) which is not considered a type of CAH.…”

“…73 A variety of milder mutations that result in nonclassic 11-OHD exist (Table 2). 70,74 Novel mutations for both subtypes are being discovered routinely. 70,75,76 3.…”

“…70,74 Novel mutations for both subtypes are being discovered routinely. 70,75,76 3. 17α-hydroxylase deficiency 17α-hydroxylase deficiency (17-OHD) (OMIM 202110) results from a defective CYP17A1 and accounts for approximately 1% of all CAH cases.…”

Cytochromes P450: Roles in Diseases

“…Examples of mutations in CYP11B1 are W116G (affects substrate recognition), A165D (alters protein surface polarity), and R366C and R453Q (reduce the binding of the electron donor adrenodoxin). These effects are predicted based on a CYP11B1 structure model (26). Likewise, those in CYP11B2 include S315R and R374W (alter P450 stability), S308P (affects active site), and L415H (affects heme binding) as suggested by a CYP11B2 structure model (27).…”

The combination of a novel 2 bp deletion mutation and p.D63H in <i>CYP11B1</i> cause congenital adrenal hyperplasia due to steroid 11&beta;-hydroxylase deficiency

“…6c). Mutations around C450 in CYP11B1, such as p.R448H [20,21], p.R453Q [22], were reported to cause almost complete loss of 11β-hydroxylase activ- ity, and the p.L451F [23] mutation in aldosterone synthase abolished CYP11B2 enzyme activity. The truncated protein with p.L380V…R420X mutation was lack of C450 and residues around it and could not correctly interact with the heme prosthetic group (Fig.…”