ABSTRACT Left ventricular hypertrophy (LVH) is a prevalent condition that conveys increased risk for cardiovascular events and all-cause mortality. Studies from our lab and others have genetic causes for Mendelian forms of LVH. Pathogenic variants in sarcomere protein genes account for ~50% of familial hypertrophic cardiomyopathy (HCM) and ~30% sporadic HCM. LVH also occurs from pathogenic variants in non-sarcomere genes including PRKAG2, GLA and LAMP2. Despite this progress, detailed genetic analyses of all known LVH genes fail to identify a pathogenic or likely pathogenic variant in >50% of individuals with isolated LVH that is unexplained (denoted as iLVH). To address this gap in knowledge we have harnessed whole genome sequencing (WGS) to comprehensively explore genetic cause of iLVH. Under the aegis of NIH TOPMed program (X01HL143310, PI, J.G. Seidman) we will obtain WGS data from 650 iLVH subjects and comprehensive transcriptional profiling (RNAseq) of associated iLVH tissues from these subjects. From WGS data we will identify rare and common variants in coding and noncoding sequences that may cause or contribute to iLVH. We will also consider the involvement of mitochondrial variants, somatic mosaicism, and/or pathogens in iLVH. We will characterize the functional impact of iLVH variants using induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) and new strategies to assess sarcomere performance and cardiomyocyte biology. From our analyses of isogenic iPSC-CMs with pathogenic variants in established LVH genes we defined precise abnormalities in sarcomere contraction and relaxation and cellular energetics. We will expand these analyses to studies of other pathogenic variants in established LVH genes so as to develop a reference dataset to which we will compare functional studies of novel variants identified in iLVH subjects. Parallel analyses of variants of unknown significance (VUS) in established LVH genes aim to improve the clinical interpretation of these abundant and enigmatic variants. By combining functional data with RNAseq analyses from cardiac tissues (obtained from LVH and iLVH subjects) and RNAseq of PSC-CMs we will begin to discern transcriptional responses to LVH and iLVH variants. We expect these studies will expand our understanding of the genetic architecture of iLVH and hypertrophic mechanisms and thereby promote the development of rationale therapeutics for LVH patients. These data will also enrich our insights of sarcomere physiology that enables life-long cardiomyocyte function or that incites disease. More broadly these studies will contribute to information about regulatory elements that modulate cardiac gene expression. Specifically we will: 1. Analyze whole genome sequences and RNAseq for variants associated with iLVH 2. Identify mechanisms induced by pathogenic variants in LVH tissues and iPSC-CMs. 3. Define the pathogenicity of coding and non-coding VUS in LVH and iLVH genes.