Increased left ventricular mass or hypertrophy (LVH) is an independent risk factor for cardiovascular events. Genetic variation, most commonly in genes encoding the protein constituents of the sarcomere, is an important primary cause of LVH that results in hypertrophic cardiomyopathy (HCM). Genetic studies of HCM patients identify pathogenic mutations in 50-60% of adult-onset familial HCM, ~30% of adult-onset sporadic HCM and ~40% of pediatric-onset HCM. Causes of LVH that clinically presents as HCM in ~50% of adults and children, particularly in those without familial disease, remains unknown. To address this gap in knowledge, we will perform integrated analyses of genomic DNA and cardiac tissue RNA in hypertrophic subjects without a recognized genetic etiology. We will search for undetected germline and somatic variants in the nuclear and mitochondrial genomes through sequenced-based strategies and will consider pathogenic etiologies (Aim 1). To assess the pathogenicity of novel sequence variants and genes we will build upon existing RNAseq datasets to determine if candidate genes participate in hypertrophic signaling pathways and we use high-throughput strategies to modulate candidate gene expression in mice and perform detailed cardiac analyses (Aim 2). Approximately 10% of the general population carries rare non- synonymous variants in sarcomere protein genes without developing overt HCM. To understand discordance in genotype and phenotype, we will map rare non-synonymous, definitive familial HCM mutations and population-identified sarcomere variants on 3-D structures of myofilament proteins, and assess their function in myocytes derived from differentiated isogenic iPS cells (Aim 3). Collectively these studies will expand the repertoire of genetic and acquired causes of HCM and LVH and improve the accurate prediction of phenotypes that arise from rare genetic variation. Our aims will: 1) Assess genetic variation in cardiac tissue and genomes of patients with LVH using high-throughput DNA sequencing methods. 2) Demonstrate that novel molecules cause or modify LVH in vivo. 3) Define the pathogenicity of rare human sarcomere protein gene variants found in the general population.