Genome-wide association studies (GWAS) identify novel genomic loci that are causally associated with important clinical traits but for which the molecular mechanisms are unknown. Currently, the pace of identifying new loci by GWAS is much greater than the pace of identifying the causal genes and understanding their biology relevant to the trait of interest. Plasma concentrations of lipids (LDL cholesterol, HDL cholesterol and triglycerides) are important risk factors for atherosclerotic cardiovascular disease and have proven fertile territory for GWAS. The Global Lipids Genetics Consortium, under the leadership of Dr Kathiresan, includes a consortium of lipid GWAS totalling approximately 100,000 subjects. In aggregating these data, we have identified a total of 97 loci that are genome-wide significantly associated with plasma lipid traits. This list includes most of the genes that are known causes of Mendelian lipid disorders as well as the molecular targets for lipid-altering drugs. Importantly, the majority are not previously associated with lipoprotein metabolism and represent an opportunity to understand in much greater detail the physiological regulation of lipoprotein metabolism and to identify novel therapeutic targets for reducing LDL-C and TG and raising HDL-C. Lipid traits are an ideal phenotype for high-throughput investigation of GWAS hits in model systems because of the relative ease in targeting the liver (the key organ regulating lipoprotein metabolism) via somatic approaches and because perturbations in plasma lipids and lipoproteins are easily assessed after liver-targeted somatic manipulation. We propose to use high-throughput approaches in mice and cells coupled with molecular network analysis to interrogate 38 of the most compelling novel genes identified by the Global Lipids Genetics Consortium. We have increased the probability of this approach bearing fruit by selecting for initial analysis those genes for which the index SNP at the locus is significantly associated with an eQTL for mRNA abundance of that gene in human liver. We will systematically interrogate all 38 genes in mice by utilizing second generation AAV8-based vectors to overexpress murine cDNAs in liver and, in collaboration with Alnylam, liposomal-siRNAs to silence the endogenous genes in liver. For the subset of genes that generate the most compelling changes in plasma lipids, we will perform lipoprotein kinetics studies in mice, lipoprotein metabolism studies in cultured hepatocytes, and molecular profiling and network analysis of livers in which the genes have been overexpressed or silenced. The deliverables of this proposal after 2 years are: 1) a detailed inventory of the metabolic and molecular effects on lipoproteins of 38 of the most compelling novel genes identified by the Global Lipids Genetics Consortium;2) validation of several novel lipid therapeutic targets;3) proof of a high- throughput approach for physiologic interrogation of novel genomic loci identified on GWAS.
Coronary heart disease is the leading cause of mortality in both men and women worldwide and incidence is highly correlated with levels of cholesterol and triglycerides (fat) in the blood. Plasma concentrations of LDL and HDL cholesterol, as well as triglycerides have a strong inherited basis and recent genetic association studies have linked previously unsuspected genes to these complex traits. The work proposed here will validate novel genes as targets for new therapies for lipid disorders and heart disease and will establish an early proof of concept of how genetic association studies can yield important new insights into biology that have the potential to impact tangibly on human health and disease.
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