HUMAN HYPERTENSION CAUSED BY MUTATIONS IN THE 11-BETA-HYDROXYSTEROID DEHYDROGENASE GENE - A MOLECULAR ANALYSIS OF APPARENT MINERALOCORTICOID EXCESS

Conversion of cortisol to cortisone 11 beta-Hydroxysteroid dehydrogena se (11 beta-HSD) is a microsomal enzyme complex which, in humans, cata lyses the interconversion between biologically active cortisol and ina ctive cortisone. This prereceptor signalling mechanism is essential fo r maintaining the aldosterone selectivity of the intrinsically non-spe cific mineralocorticoid receptor and for modulating glucocorticoid acc ess to the glucocorticoid receptor. Apparent mineralocorticoid excess (AME) is a syndrome of severe low-renin mineralocorticoid hypertension associated with marked hypokalaemia which arises from a congenital de ficiency of 11 beta-HSD. In AME patients, therefore, it is cortisol an d not aldosterone which behaves as a potent mineralocorticoid. Isoform s of 11 beta-HSD Two isoforms of human 11 beta-HSD have now been chara cterized and cloned. The type 1 isoform (11 beta-HSD1) is a low-affini ty reduced nicotinamide adenine dinucleotide phosphate (NADP) dependen t dehydrogenase-oxoreductase which is expressed in predominantly gluco corticoid target tissues and the encoding sequence of which is normal in patients with AME. In contrast, the type 2 isoform (11 beta-HSDP) i s a high-affinity NADP-dependent unidirectional dehydrogenase which is expressed in placenta and mineralocorticoid target tissues such as re nal collecting ducts and distal colonic epithelia. Exon- and intron-sp ecific polymerase chain reaction amplification of the 11 beta-HSDP gen e from genomic DNA from members of a consanguinous kindred with AME co nsistently revealed a single missense mutation (C1228T) in two affecte d sibs and twin stillbirths. This mutation in codon 374 of exon 5 of t he 11 beta-HSD2 gene creates an inframe premature stop (TGA) and, as s uch, results in a truncated 11 beta-HSD2 protein lacking the carboxyl- terminal proline-rich 32 amino acids. In keeping with an autosomal rec essive mode of inheritance, both parents were phenotypically and bioch emically normal but were heterozygous for this mutation. Unique to thi s kindred were expression analyses of the native mutant 11 beta-HSD2 e nzyme in the stillbirth-affected placenta, which was almost completely devoid of NADP-dependent 11 beta-dehydrogenase activity. Immunohistoc hemical and Western blot analyses revealed the absence of 11 beta-HSD2 protein using antisera raised against synthetic peptide sequences cor responding either to the carboxyl terminus or other domains of the enz yme. Missense mutation In this kindred with AME, congenital deficiency of 11 beta-HSD activity is due to a single missense mutation in exon 5 of the 11 beta-HSD2 gene. Simultaneous studies by two other groups h ave similarly revealed no gross deletions or rearrangements of the 11 beta-HSD2 gene, but have described a number of single point mutations and oligonucleotide deletions in exons 3, 4 and 5, and adjacent to a s plice site in intron 3. Recombinant expression analysis of site-direct ed mutant 11 beta-HSD2 complementary DNA constructs suggests a correla tion between the predicted severity of these mutations and the biochem ical and clinical phenotype. AME as a cause of hypertension The mutati ons in the 11 beta-HSD2 gene, together with those currently being soug ht by us for other kindreds with AME, establishes AME as a monogenic c ause of human hypertension and will provide insight into the structure -function relationships of this important enzyme.