18-Hydroxylase deficiency
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Synonyms and keywords: CMO I deficiency; Aldosterone deficiency I; familial hyperreninemic hypoaldosteronism 1; FHHA1A; corticosterone methyloxidase type I deficiency.
Overview
CMO type I deficiency is an autosomal recessive disorder caused by a defect in the penultimate biochemical step of aldosterone biosynthesis, the 18-hydroxylation of corticosterone (B) to 18-hydroxycorticosterone (18-OHB). This enzymatic defect results in decreased aldosterone and salt-wasting. In CMO I deficiency, aldosterone is undetectable, whereas its immediate precursor, 18-OHB, is low or normal. These patients have an increased ratio of corticosterone to 18-OHB.
The CYP11B2 gene product also catalyzes the final step in aldosterone biosynthesis: the 18-oxidation of 18-OHB to aldosterone. A defect in that enzymatic step results in CMO type II deficiency, an allelic disorder with an overlapping phenotype but distinct biochemical features. In CMO II deficiency, aldosterone can be low or normal, but at the expense of increased secretion of 18-OHB. These patients have a low ratio of corticosterone to 18-OHB.[1]
Aldosterone synthase (or 18-hydroxylase) is a steroid hydroxylase cytochrome P450 oxidase enzyme involved in the generation of aldosterone. The cytochrome P450 proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids, and other lipids. This protein is primarily expressed in the zona glomerulosa of the adrenal cortex. Within the cell, it localizes to the inner membrane of the mitochondria. The enzyme has steroid 18-hydroxylase activity to synthesize aldosterone and 18-oxocortisol as well as steroid 11 beta-hydroxylase activity. Mutations in this gene cause corticosterone methyl oxidase deficiency.[2] It converts 11-deoxycorticosterone to corticosterone, to 18-hydroxycorticosterone, and finally to aldosterone