Exeter, HJ; (2012) The genetic architecture of Secretory PLA2 (sPLA2)genes and their impact on sPLA2 activity/mass and association with CHD risk. Doctoral thesis , UCL (University College London).

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Abstract

Secretory phospholipase A2 group (sPLA2) enzymes hydrolyze the sn-2 ester bond of phospholipids, producing lysophospholipids and free fatty acids. SPLA2s have been identified as biomarkers of atherosclerosis in observational and animal studies. The aim of this study was to identify functional variants in PLA2G2A and, to a lesser extent, PLA2G5 and PLA2G10 (sPLA2 encoding genes). These variants could then serve as tools to examine their contribution to sPLA2-activity, since this is a composite measure of sPLA2-IIA,-V and –X, and associations with CHD risk/CHD traits. Two PLA2G2A SNPs were identified as being associated with sPLA2-IIA mass/sPLA2 activity. Luciferase assays, EMSA and RNA expression were used to examine their allelic differences. Rs3767221G showed ∼55% lower luciferase activity compared to rs3767221T (p = 1.22×10-35), and stronger EMSA binding of a nuclear protein compared to the T-allele. For rs11573156 C >G there were no luciferase or EMSA allelic differences. For rs11573156 there was no differential allelic lymphocyte cDNA expression for exons 5-6, but G-allele carriers (n = 7) showed a trend to lower exon 1-2 expression versus CC individuals. In the ASAP study (n = 223), a SNP acting as a proxy for rs11573156 (r2 = 0.91) on the expression array showed allelic difference of ∼25% in liver expression of total PLA2G2A (1.67×10-17). However, exon 2 specific expression was greatly reduced (4.5×10-5) compared to exons 3-6 (10-10 to 10-20), implying allele-specific exon 2 skipping as the functional basis of rs11573156 association. Rs11573156 was used as a genetic tool in a large Mendelian randomisation (MR) collaboration to establish the causality of sPLA2-IIA for CHD. While the sPLA2-IIA mass was strongly associated with CHD events, rs11573156 was not associated with outcome. Similar approaches were taken for studying PLA2G5 and PLA2G10, using eQTL data and smaller meta-analyses. Both PLA2G2A SNPs are functional. Using rs11573156 as a genetic instrument, MR suggested that sPLA2-IIA is not causally associated with CHD. Based on initial meta-analyses, neither PLA2G5 nor PLA2G10 appeared to be associated significantly with sPLA2 activity in cardiovascular related tissues. This conclusion is consistent with the outcome of the recent Phase III trial for the sPLA2 inhibitor varespladib, which was terminated due to lack of efficacy since the drug was shown to be having no significant effect on the reduction of secondary cardiovascular events.

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