Abstract 441: Zebrafish Larvae as a Model System for High-throughput, Image-based Genetic Screens in Dyslipidemia and Atherosclerosis

Background: Genome-wide association (GWAS) have identified hundreds of loci that are robustly associated with the risk of cardiometabolic risk factors and diseases. With few exceptions, the causal genes through which these loci influence disease risk remain uncharacterized. Since murine model systems are not suitable for systematic characterization of candidate genes in hundreds of loci, novel in vivo model systems are desirable.

Methods: My group has developed and validated zebrafish model systems that make optimal use of: 1) the zebrafish’ well-annotated genome, with orthologues of at least 71.4% of human genes; 2) recent developments in multiplex CRISPR-Cas9-based mutagenesis; 3) advances in automated positioning of non-embedded zebrafish larvae; 4) fluorescent transgenes and dyes; and 5) custom-written image-quantification pipelines.

Results: Five days of overfeeding, dietary cholesterol supplementation and/or exposure to glucose induce atherogenic and insulin resistant phenotypes (higher/more whole-body LDL cholesterol (LDLc) and triglyceride levels; vascular foam cell formation and inflammation; beta-cell number and volume; subcutaneous and hepatic accumulation of fat) that can largely be prevented by concomitant treatment with lipid-lowering or diabetes medication (N>4000). In line with recent results in humans, treatment with atorvastatin and ezetimibe results in higher whole-body glucose levels (N~835). Each additional mutated allele in zebrafish orthologues of APOE results in higher LDLc levels, while mutations in LDLR affect vascular infiltration by lipids and macrophages (N~330). Ongoing characterization of 37 prioritized genes in GWAS-identified loci for triglyceride levels identified genes that influence triglycerides, LDLc, and/or vascular inflammatory and atherogenic traits, while characterizing prioritized genes in loci associated with heart rate variability yielded genes that influence cardiac rate and rhythm.

Conclusions: Systematic characterization of candidate genes for cardiometabolic traits in zebrafish model systems will help increase our understanding of human disease and identify novel targets that can be translated into efficient therapeutics.

Abstract 472: Identifying and Characterizing Causal Genes in GWAS-identified Loci for Triglyceride Levels Using High-throughput, Image-based Screens in Zebrafish Larvae

Background: High triglyceride levels are an established risk factor for coronary artery disease (CAD). Unpublished results from our proof-of-principle studies show that triglyceride-levels are also associated with atherosclerosis in zebrafish larvae. Identifying and characterizing causal genes for triglyceride levels may yield novel drug targets. In 2013, a meta-analysis of genome-wide association studies identified 37 previously unanticipated loci that are associated with triglyceride levels. We aim to characterize positional candidate genes in these loci using a zebrafish model system.

Methods: We prioritized 37 candidate genes for functional-follow up. These genes together have 41 zebrafish orthologues, which were targeted in five lines of ~8 genes each using a multiplex CRISPR-Cas9 approach. Founder mutants have been raised and 384 offspring for two of the five lines have so far been screened for body size as well as fluorescently labeled lipids, macrophages and neutrophils in the vessel wall, using a high-throughput imaging set-up. Image quantification was performed using automated, custom-written pipelines in CellProfiler and ImageJ. Additionally, whole body lipid fractions and glucose levels were measured in each larva using enzymatic assays. CRISPR-induced mutations were quantified using paired-end sequencing and data were analyzed using hierarchical mixed models and (zero-inflated) negative binomial regression.

Results: Each additional disrupted allele in pepd resulted in higher triglyceride levels (beta±SE 0.21±0.09 SD). Mutants for met have less vascular infiltration by macrophages (-0.31±0.13), and less co-localization of macrophages with lipids (-0.56±0.21) and neutrophils (-0.49±0.18). Mutants for lpar2 have lower LDLc (-0.54±0.27) and HDLc levels (-0.57±0.27), while gmip mutants have less vascular lipid deposition (-1.05±0.43), and less co-localizing lipids and neutrophils (-1.26±0.56).

Conclusion: By characterizing candidate genes for triglyceride levels using a high-throughput, largely image-based approach in zebrafish larvae, we identified previously unanticipated genes that influence a range of cardiometabolic risk factors. This approach is anticipated to identify novel drug targets.

Genetic loci associated with heart rate variability and their effects on cardiac disease risk

Reduced cardiac vagal control reflected in low heart rate variability (HRV) is associated with greater risks for cardiac morbidity and mortality. In two-stage meta-analyses of genome-wide association studies for three HRV traits in up to 53,174 individuals of European ancestry, we detect 17 genome-wide significant SNPs in eight loci. HRV SNPs tag non-synonymous SNPs (in NDUFA11 and KIAA1755), expression quantitative trait loci (eQTLs) (influencing GNG11, RGS6 and NEO1), or are located in genes preferentially expressed in the sinoatrial node (GNG11, RGS6 and HCN4). Genetic risk scores account for 0.9 to 2.6% of the HRV variance. Significant genetic correlation is found for HRV with heart rate (-0.74<r_g<-0.55) and blood pressure (-0.35<r_g<-0.20). These findings provide clinically relevant biological insight into heritable variation in vagal heart rhythm regulation, with a key role for genetic variants (GNG11, RGS6) that influence G-protein heterotrimer action in GIRK-channel induced pacemaker membrane hyperpolarization.