11β-HYDROXYLASE AND HYPERTENSION

Abstract

In humans, the CYP11B1 and CYP11B2 genes on chromosome 8q22 encode steroid 11β-hydroxylase isozymes that are 93% identical in amino acid sequence. CYP11B1 is expressed at high levels in the adrenal cortex, is up-regulated by ACTH and encodes an enzyme with 11β-hydroxylase activity. CYP11B2 is expressed at low levels in the normal adrenal cortex but at higher levels in aldosterone secreting tumors, is up-regulated by angiotensin II and encodes an enzyme with 11β- and 18-hydroxylase as well 18-oxidase activities. Thus, CYP11B1 is required for cortisol synthesis whereas CYPUB2 is required for aldosterone synthesis. Steroid 11β-hydroxylase deficiency (failure to convert 11-deoxycortisol to cortisol) causes a hypertensive form of congenital adrenal hyperplasia. This autosomal recessive form of genetic hypertension presumably results from accumulation of deoxycorticosterone and related metabolites with mineralocorticoid activity. We have now characterized a total of nine mutations in CYP11B1 causing this disorder. Eight are point (three nonsense and five missense) mutations and one is a single base pair deletion causing a frameshift (other investigators have reported one additional frameshift mutation). We have used an in vitro transfection assay to show that all five missense mutations causing 11β-hydroxylase deficiency abolish enzymatic activity. In principle, deletions of CYP11B1 could be generated by unequal crossing over between CYP11B1 and the adjacent CYP11B2 gene but no such deletions were found among the deficiency alleles in this study. Seven of the ten known mutations are clustered in exons 6 to 8, a non-random distribution within the gene (P<0.04). This may reflect the location of functionally important amino acid residues within the enzyme or an increased tendency to develop mutations within this region of the gene. Glucocorticoid-suppressible hyperaldosteronism is an autosomal dominant form of hypertension in which aldosterone synthesis is ACTH regulated and there are high levels of 18-hydroxy- and 18-oxocortisol, the latter a 17α-hydroxylated analog of aldosterone. In all 16 families examined thus far by us and by others, an unequal crossover has occurred generating a chromosome with a third CYP11B gene. This third gene is a chimera with a 5′ end (including regulatory sequences) corresponding to CYP11B1 and a 3′ end corresponding to CYP11B2. The breakpoint of the crossover is always somewhere between introns 2 and 4. In vitro expression of the corresponding chimeric cDNAs demonstrate that the chimeric enzyme retains the ability to synthesize aldosterone only if the last five or more (out of nine) exons correspond to CYP11B2. Thus, glucocorticoid-suppressible hyperaldosteronism is caused by abnormal expression and regulation of an enzyme with the ability to synthesize aldosterone from deoxycorticosterone. The observed distribution of breakpoints apparently reflects functional constraints on enzymatic activity. It is not yet known if abnormal regulation of CYP11B2 is responsible for any cases of essential hypertension.