11beta-Hydroxysteroid dehydrogenase (11beta-HSD) catalyses the interconversion of hormonally active cortisol (F) and inactive cortisone (E). 11beta-HSD1 is an NADP(H) dependent enzyme expressed in human liver and adipose that is targeted to the ER membrane with a lumenal catalytic domain. In vivo the enzyme acts predominantly as a reductase, generating F from E. The inherited condition, Apparent Cortisone Reductase Deficiency (ACRD) is a form of PCOS characterised by ACTH mediated hyperandrogenism as a result of defective conversion of E to F, suggesting inhibition of the 11-oxo reductase activity of 11beta-HSD1. Analogous loss of reductase activity is observed in cell lysates and in recombinant enzyme. Reductase activity can be restored with a NADPH regeneration system, indicating that cofactor levels are critical for determining set point of 11beta-HSD1. The enzyme hexose-6-phosphate dehydrogenase (H6PDH) generates NADPH within the ER lumen and is co-localised with 11beta-HSD1 in human tissues.
DNA was obtained from 3 affected kindreds with ACRD together with a fat biopsy from affected case #1 and a normal sibling. All cases had characteristically low urinary THF+allo-THF/THE ratios of <0.05 (ref range 0.7-1.2). All affected cases were either homozygous (case #1) or heterozygous (cases #2,3)) for two mutations within intron 3 of HSD11B1, that were in linkage dysequilibrium (inserted A (83577) and T-G substitution (83597)). (222 matched controls; 1% homozygous; 24% heterozygous). There was a 28-fold reduction in 11beta-HSD1 mRNA from cultured preadipocytes in homozygous case #1 compared to the normal sibling.
Additional mutations were identified in exon 5 of the H6PDH gene. Case #1 was heterozygous for a 29bp insertion between aa 620/621 resulting in a premature stop codon. Expression of this mutant H6PDH in vitro totally abolishes enzyme activity. Cases 2and 3 were homozygous for a R453Q mutation. In 222 normal controls, the 29bp 620/621 mutation was absent and the R453Q mutation was observed in 2%.
ACRD appears to be a 'digenic' disease comprising mutations in both HSD11B1 and H6PDH causing a reduction in 11beta-HSD1 expression combined with altered ER redox potential.H6PDH within the ER lumen acts as an NADPH donor for 11beta-HSD1 thereby conferring reductase activity upon an inherent dehydrogenase. ACRD defines a new intracellular redox potential of importance in human obesity and PCOS.