Investigating the role of DNA methylation in abdominal aortic aneurysms

Abdominal aortic aneurysm (AAA) is a complex disease characterised by the irreversible dilation of the abdominal aorta. Little is known about its epigenetic basis, and DNA methylation is the most extensively studied epigenetic mechanism. Global methylation was assessed in peripheral blood mononuclear cell (PBMC) DNA from 185 people using enzyme-linked immunosorbent assays (ELISAs), identifying global hypermethylation in those with a large AAA, and a linear association with increasing AAA diameter. The regulatory regions of genes proximal to AAA genomic risk loci were then bisulphite sequenced using next-generation sequencing (NGS) in PBMC DNA from 96 people and vascular smooth muscle cell (VSMC) DNA from 44 people. In PBMCs, hypermethylation in individuals with AAA was seen in LDLR, SORT1 and IL6R. In VSMCs, the same region in IL6R was hypermethylated and differential methylation was also observed in ERG, SERPINB9, and SMYD2. ELISAs were conducted on plasma from the same PBMC samples to corroborate the methylation patterns seen in LDLR, IL6R and SORT1, where there was a reduction in circulating IL6R. Gene expression analysis was also performed on mRNA from the VSMCs. SERPINB9 was downregulated in AAA but independently of DNA methylation, and a relationship between SMYD2 promoter hypo-methylation and decreased SMYD2 gene expression was shown. Downregulation of SMYD2 in AAA was further corroborated in 6 whole aortic tissue samples using immunohistochemistry. This PhD has illustrated a significant original contribution to knowledge at each stage. Global and gene specific DNA methylation changes are associated with AAA and could be involved in disease pathobiology. This is the first work to assess DNA methylation in AAA using NGS, and the first to assess methylation in VSMCs. In particular, methylation status of the SMYD2 promoter could be a causal factor of decreased SMYD2 expression, which has previously been implicated in adverse cardiovascular physiology and increased inflammation.