AAA (abdominal aortic aneurysm) is a potentially life-threatening late-onset degenerative condition. miRNAs (microRNAs), the small non-coding RNA molecules that regulate gene expression, have been shown previously to be associated with a broad range of human pathologies, including cardiovascular diseases. The aim of the present study was to identify AAA-associated miRNAs potentially contributing to AAA pathology. We analysed the expression of 124 miRNAs within AAA biopsies and serum of ten patients undergoing AAA repair, and serum from ten age- and sex-matched subjects without AAA, using the FlexmiR™ MicroRNA Assay. RNA extracted from the site of main AAA dilatation (AAA body) was compared with that extracted from the macroscopically non-dilated neck of the AAA (AAA neck). Similarly, RNA extracted from the serum of AAA patients (AAA serum) was compared with that extracted from age- and sex-matched controls (control serum). qPCR (quantitative real-time PCR), Western blot analysis and histology were performed using an independent set of six paired AAA body and neck biopsies to examine the validity of findings. Seven miRNAs were up-regulated [>2-fold difference, FDR (false discovery rate) <0.5] within AAA biopsies, of which miR-155 was the most differentially expressed (11.32-fold, FDR=0.414). This finding was confirmed by qPCR with the median relative expression of miR-155 being 3.26 and 0.63 within AAA body and AAA neck biopsies respectively (P=0.031). Circulating miR-155 was also increased in AAA patients compared with controls, with a 2.67-fold up-regulation at borderline significance (FDR=0.554). Two immunologically important miR-155 target genes, CTLA4 (cytotoxic T-lymphocyte-associated protein) and SMAD2, were assessed and found to be significantly down-regulated within AAA bodies compared with AAA necks (P=0.032 and P=0.026) as determined by qPCR and Western blotting respectively. Histology demonstrated dense accumulation of T-lymphocytes within the adventitial and outer medial layers of AAA body, but not neck tissue. The results of the present study suggest that miR-155 is overexpressed in AAA with potential implications in the pathogenesis of the condition.

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