Steroid 11-beta-hydroxylase deficiency caused by compound heterozygosity for a novel mutation, p.G314R, in one CYP11B1 allele, and a chimeric CYP11B2/CYP11B1 in the other allele

Aims: Steroid 11β-hydroxylase deficiency (11β-OHD) is the second most common (5-8%) cause of congenital adrenal hyperplasia (CAH), and results from homozygous or compound heterozygous mutations or deletions of the responsible gene CYP11B1. In order to better understand the molecular basis causing 11β-OHD, we performed detailed studies of CYP11B1 in a newly described patient diagnosed with the classical signs of 11β-OHD. Methods: CYP11B1 of the patient was investigated by polymerase chain reaction (PCR), sequencing, restriction fragment length polymorphism (RFLP) analysis. Southern blotting, and transient cell expression. Results: We identified two new mutated alleles in CYP11B1. In one allele CYP11B1 has a g.940G→C (p.G314R) missense mutation. On the other allele we found a chimeric gene that consists of part of the aldosterone synthase gene (CYP11B2) at exons 1-3 and part of the 11β-hydroxylase gene (CYP11B1) at exons 4-9. In in vitro studies, the g.940G→C (p.G314R) mutation abolished all hydroxylase activity in comparison with the wild-type 11β-hydroxylase. The chimeric CYP11B2/CYP11B1 protein retained 11β-hydroxylase enzymatic activity in vitro. Conclusion: This case is caused by compound heterozygosity for a nonfunctional missense mutation and a chimeric CYP11B2/CYP11B1 gene with hydroxylase activity that is controlled by the CYP11B2 promoter. The most likely explanation is that the CYP11B2 promoter does not function in the zona fasciculata/reticularis where cortisol is exclusively synthesized.