Endocrine Abstracts

Background: The CYP11B1 (11β-hydroxylase) and CYP11B2 (aldosterone synthase) genes are found in close proximity on human chromosome 8 and are highly polymorphic, enabling the definition of two common haplotypes. Haplotype1 includes the -344T and the Intron2 conversion polymorphisms in CYP11B2 and also the -1889T, -1859G polymorphisms in the regulatory region of CYP11B1. We and others have shown that these associate with increased aldosterone production, increased risk of developing hypertension and reduced 11β-hydroxylation efficiency. We investigated the haplotype-specific transcription of CYP11B1 and CYP11B2 in order to test whether either of these genes are expressed at significantly different levels depending on haplotype background.

Method: Using normal adrenal tissue taken from individuals heterozygous for these haplotypes, we measured the relative levels of Haplotype1- and Haplotype2-derived CYP11B1 (n=3) and CYP11B2 (n=4) mRNA in each sample. Our rigorously verified allele-specific method involved cloning of RT-PCR product, transformation of competent cells and quantitation of the resulting Haplotype1- and Haplotype2-containing colonies in order to establish the ratio of Haplotype1- to Haplotype2-derived mRNA.

Results: Haplotype1-derived CYP11B1 mRNAs were present at significantly lower levels than their Haplotype2-derived counterparts (P=0.01) in all the tissues examined, accounting for approximately 40% of total CYP11B1 mRNAs. Conversely, Haplotype1-derived CYP11B2 mRNAs were significantly more abundant than the Haplotype2 form (P<0.05), accounting for approximately 59% of total CYP11B2 mRNAs.

Conclusions: Haplotype1-derived CYP11B1 mRNA is less abundant in heterozygous adrenocortical tissue than the Haplotype2 form, suggesting transcription of the latter is more efficient. This provides an explanation for the lower 11β-hydroxylation efficiency that associates with Haplotype1. Similarly, the more efficient CYP11B2 transcription of Haplotype1 provides a plausible explanation for its association with increased aldosterone production and greater risk of developing hypertension with a raised aldosterone to renin ratio. These data are therefore consistent with a digenic effect leading to increased aldosterone secretion and decreased 11β-hydroxylation.