The genetics of carotid atherosclerosis: Associations with clinical outcome and histological plaque characteristics

Abstract

Throughout this Thesis we explored the potential mechanisms of common cardiovascular disease susceptibility genetic variants (loci). We studied their impact, individually or in aggregate on histological plaque characteristics, gene expression, DNA methylation, and circulating biomarkers in deeply phenotyped biobank studies. We show that the coronary artery disease (CAD) susceptibility locus on chromosome 7q22 associates with less fat in atherosclerotic plaques and reduced LDL levels in the circulation. Through wet-lab circularized chromosome conformation capture sequencing and in silico experiments we prioritized the regional genes. Overall, the analyses confirm that cardiovascular susceptibility loci play a role in atherosclerotic plaque progression. We also describe the presumed role of genes (ALOX5, ALOX5AP and LTA4h) in the arachidonic acid pathway on stroke risk. Through a genetic analysis of all the variants in and around these genes, we found no evidence for a causal role of the arachidonic acid signaling pathway in atherosclerotic plaque progression leading to disease. In another chapter we show that the atherosclerotic stroke risk conferring allele of rs2107595 (A>G) near HDAC9 increases expression of HDAC9 in circulating cells, and apoE-/- mice lacking a functional Hdac9 gene show marked reduction in atherosclerotic lesion size. The same risk-allele did not associate with histological plaque characteristics. Furthermore, we systematically searched literature for putative atherosclerotic genes identified in experimental animal models. First, we mapped these genes to their human orthologue. Then we used summary statistics from large genome-wide association studies (GWAS) on CAD and large artery ischemic stroke (LAS) to calculate per-gene empirical p-values. Very few of the putative atherosclerotic genes were associated with human cardiovascular disease or a plaque characteristic. Epidemiological evidence shows that carotid IMT is a biomarker for cardiovascular disease in general. GWAS have discovered 6 loci associated with cIMT and carotid plaque presence. We tested these loci for association with cIMT and assessed their impact on other vascular beds in a cohort of secondary prevention. We replicated the association with cIMT, and associated a variant near EDNRA with CAD. This analysis added to the evidence for a role of the EDNRA locus in cardiovascular disease, as earlier work showed it also associates with blood pressure. In the last chapter we test the putative causal role of cystatin C on cardiovascular disease through a meta-analysis of data from over 250,000 individuals, including more than 63,000 cases with cardiovascular disease. We replicated the strong correlation of a genetic locus near the CST3 gene (which encodes for cystatin C) with circulating levels of cystatin C and estimated glomerular filtration rate based on cystatin C. We also replicated earlier observational reports showing increasing risk for CVD with increasing cystatin C levels. Utilizing the Mendelian randomization method, we could not found evidence for a causal role of cystatin C in cardiovascular disease.