UCLA

Obesity is a well-established risk factor for multiple common disorders, such as type 2 diabetes (T2D), hypertriglyceridemia, non-alcoholic fatty liver disease (NAFLD), coronary artery disease (CAD), and certain cancers. The rates of obesity-related deaths have risen sharply globally over the last 20 years, with over 70% of adults in the United States now classified as overweight or obese according to the Centers for Disease Control (CDC). Currently, as the world faces one of the worst infectious-disease outbreaks in a century, new data are also emerging showing that obesity is a key risk factor for severe forms of COVID-19. However, the complex underlying mechanisms of obesity, especially the susceptibility genes and their regulatory mechanisms, remain elusive. To address this scientific knowledge gap, we have employed integrative multi-omics approaches on human subcutaneous adipose RNA-sequencing (RNA-seq) data from multiple cohorts; epigenomic data from chromosomal interactions and open chromatin in relevant adipose cell-types; large scale obesity genome-wide association studies (GWAS) for body mass index (BMI) and waist-hip-ratio adjusted for BMI (WHRadjBMI); and one of the largest population cohort to date, the UK Biobank (UKB).

In Chapter 2, we fine-mapped BMI GWAS loci using cis-expression quantitative trait loci (eQTLs) from the METabolic Syndrome In Men (METSIM) cohort and chromosomal interactions from adipocyte promoter Capture Hi-C (pCHi-C). We discovered that the pCHi-C interactions are enriched for central adipogenesis transcription factors (TFs), Peroxisome Proliferator Activated Receptor Gamma (PPARG) and CCAAT Enhancer Binding Protein Beta (CEBPB), and identified four key GWAS gene examples as well as 38 additional candidate genes with cis-eQTLs in chromosomal interactions whose gene expression are strongly associated with obesity-related traits, such as BMI, blood metabolites, and lipids.

In Chapter 3, I discuss my contribution to a study about context-specific changes in open chromatin and pCHi-C interactions in human primary adipocytes and the variants in those open chromatin regions that respond to lipid intake. Using these context-specific molecular data, we provide candidate gene-environment interaction (GxE) variants that significantly alter TF motifs in open chromatin regions, which are evolutionarily conserved and have a key role in adipogenesis and the responses to lipid intake. These candidate GxE variants with molecular priors were then tested for interactions with saturated fat intake on obesity in the UKB, resulting in the discovery of novel GxE variants for obesity.

In Chapter 4, we move beyond cis-eQTLs, to trans-eQTLs and master trans regulatory TFs that control adipose co-expression networks important for obesity. To advance the discovery of unknown genetic and molecular mechanisms regulating abdominal adiposity and the sex-specific distribution of body fat, we searched for genetic master trans regulators of WHRadjBMI by employing integrative genomics approaches on human adipose RNA-seq data and WHRadjBMI GWAS. We provide novel genomic evidence, verified by our functional knockdown studies in human primary preadipocytes, for the causal role of the TF, T-Box Transcription Factor 15 (TBX15), in controlling accumulation of abdominal fat and adiposity.

All in all, we have combined these omics and phenotype data using computational and functional techniques to identify genes and their regulatory mechanisms affecting obesity. Our studies suggest that by integrating the multi-omics data and elucidating the mechanisms underlying obesity, we can further the understanding of the risks associated with obesity and its comorbidities to move personalized medicine forward.