Genetic Variation at the Locus Encompassing 11-β Hydroxylase and Aldosterone Synthase Accounts for Heritability in Cortisol Precursor (11-Deoxycortisol) Urinary Metabolite Excretion

Abstract

Genetic variation in the gene encoding aldosterone synthase (CYP11B2) has previously been shown to be associated with hypertension and left ventricular hypertrophy. The intermediate phenotype most consistently associated with variation at this locus is that of elevated plasma 11-deoxycortisol (S). However, in normal subjects, aldosterone synthase does not metabolize S, which is converted to cortisol (F) by the enzyme 11β hydroxylase, encoded by the gene CYP11B1, which lies adjacent to CYP11B2 on chromosome 8. It is possible that the quantitative trait locus for the phenotype is within CYP11B1 and that linkage disequilibrium across the extended locus could account for these observations. However, variation across the whole CYP11B1/B2 locus had not been extensively characterized with respect to these phenotypes. We genotyped six polymorphisms in the CYP11B2 gene and three polymorphisms in the CYP11B1 gene in 248 Caucasian nuclear families comprising 1428 individuals. We measured plasma levels of S and F in 460 individuals from 86 families and urinary excretion rates of tetrahydrodeoxycortisol (THS) and tetrahydrodeoxycorticosterone in 573 individuals from 105 families. We examined heritability of the phenotypes and their association with genotypes and haplotypes at this locus. All steroid phenotypes except urinary tetrahydrodeoxycorticosterone were highly heritable (P < 0.00001). There was strong linkage disequilibrium across the CYP11B1/B2 locus. There was modest evidence for association between polymorphisms of CYP11B2 and plasma levels of S (P = 0.02 for T4986C polymorphism) and the plasma S to F ratio, reflecting the activity of 11-β hydroxylase (P = 0.01 for T4986C polymorphism). There was strong evidence for association between polymorphisms of both CYP11B1 and CYP11B2 and urinary THS, which was strongest for the CYP11B1 exon 1 polymorphism (P = 0.00002). Addition of other marker data to CYP11B1 exon 1 did not improve the fit of a log-linear model. Genotype at CYP11B1 explained approximately 5% of the variance in urinary THS excretion in the population. Thus, it is likely that linkage disequilibrium between causative CYP11B1 variants and CYP11B2 polymorphisms account for the previous observations. Further fine-mapping studies across the CYP11B1 locus are required to localize the causative variant(s) for the biochemical phenotype; this may also identify susceptibility alleles for hypertension and left ventricular hypertrophy.