Endocrine Abstracts

Hypertension is a common risk factor for cardiovascular disease and up to 15% of hypertensive patients are now known to have primary aldosteronism (PA), where the adrenal glands secrete inappropriately high levels of the steroid hormone aldosterone. Of these cases, almost half are due to the presence of unilateral aldosterone-producing adenoma (APA). Aldosterone is synthesised in the zona glomerulosa of the adrenal cortex, with the final stages of production catalysed by aldosterone synthase, an enzyme encoded by the CYP11B2 gene. Previous studies showed that CYP11B2 expression is partly subject to regulation by microRNAs (miRNAs). These are small, single-stranded RNAs 20–25 nucleotides in size that target particular mRNAs, modulating gene expression at the post-transcriptional level. We have now extended our studies to investigate whether microRNA has a role in the aetiology and pathophysiology of APA.

We analysed the microRNA content of four human APA and four normal human adrenal (NA) tissue samples by microarray and then conducted bioinformatic analysis of the resulting data, using software resources including Ingenuity Pathway Analysis (IPA) and DIANA micro-T databases. Of the 78 miRNAs detected above the 500 arbitrary unit (AU) threshold level in one or both tissues, 31 were differentially expressed between NA and APA samples (P<0.05), 18 being more highly expressed in NA samples, and 13 more abundant in APA. IPA analysis confirmed endocrine system disorders as being one of the top three disease states associated with these 31 miRNAs, alongside cancer and cardiovascular. IPA was also used to identify plausible mRNA targets for these 31 miRNAs. A predicted target of particular interest is 3-hydroxy-3-methylglutaryl-CoA-reductase (HMGCR), the mRNA of which is predicted to bind four miRNAs that are each significantly downregulated in APA relative to NA. This gene represents a rate-limiting step in cholesterol production and so may be an important determinant of steroid biosynthesis. Five miRNAs are also predicted to target CYP11B2 mRNA and may therefore have a direct regulatory influence on aldosterone production.

In summary, miRNA data generated by microarray analysis was analysed bioinformatically in order to identify putative mRNA targets related to functions that may be disrupted in cases of APA, including cell proliferation, apoptosis, and steroidogenesis. These findings will direct future in vitro analyses designed to increase our understanding of the aetiology of APA and identify prospective biomarkers for its diagnosis and treatment.