Context: Apparent mineralocorticoid excess (AME) is a rare autosomal recessive disease resulting from mutations within the hydroxysteroid (11β-dehydrogenase2 [HSD11B2]) gene causing a prominent mineralocorticoid receptor activation by cortisol and hypokalemic low renin hypertension as the main clinical feature.
Objective: The objective of the study was to characterize AME for possible novel HSD11B2 mutations and to define the role of HSD11B2 promoter methylation in the phenotypic expression of the disease.
Subjects: Two proband brothers and 10 relatives participated in the study.
Methods: Peripheral blood mononuclear cell DNA was used for HSD11B2 exon sequencing, and a new predicted structure of 11β-hydroxysteroid dehydrogenase type 2 was generated by an in silico three-dimensional modeling. Promoter methylation was determined by bisulfite pyrosequencing. Urinary tetrahydrocortisol plus allotetrahydrocortisol to tetrahydrocortisone ratio, a surrogate marker of 11β-hydroxysteroid dehydrogenase type 2 activity, was measured by gas chromatography-mass spectrometry.
Results: A novel homozygous variant at HSD11B2 exon 3 site (c.C662G) resulting in an alanine-to-glycine change at position 221 was discovered by sequencing the DNA of the probands. A monoallelic mutation was found in the DNA of the parents and other four relatives. In silico three-dimensional modeling showed that the Ala221Gly substitution could perturb a hydrophobic interaction by reducing the enzymatic affinity for the substrate. The HSD11B2 promoter methylation of normotensive heterozygous relatives was similar to that of wild types, whereas the hypertensive heterozygous subjects showed higher methylation than wild types, consistently with a transcriptional repressive effect of promoter hypermethylation.
Conclusions: A novel HSD11B2 functional mutation accounting for an Ala221Gly substitution causes AME. The hypertension phenotype is also epigenetically modulated by HSD11B2 methylation in subjects heterozygous for the mutation.