Abstract 339: Biomechanics and Calcium Handling of Thin Filament Hypertrophic Cardiomyopathy Variants

Abstract

The mechanism by which genetic variants of the thin filament cause hypertrophic cardiomyopathy (HCM) has not been fully elucidated. Induced pluripotent stem cell (iPSC)-derived cardiomyocytes can provide a rapidly generatable disease modeling tool to study HCM in these variants. While variants in the thick and thin filament both cause HCM, cardiomyocytes that harbor pathogenic variants in the thin filament genes troponin I ( TNNI3 ) and troponin T ( TNNT2 ) display a distinct molecular phenotype, which we hypothesize differs from the thick filament HCM phenotype. Both thin and thick filament mutant cardiomyocytes display increased measured oxygen consumption rate assessed by metabolic assays, however we hypothesize that the mechanism that drives metabolic change is not common between thick and thin filament variants. Mutations in the thick filament typically shift myosin conformations during relaxion towards a higher energy utilizing a disordered relaxed state (DRX) that enables ATP hydrolysis with more free myosin heads, and this likely accounts for their increased oxygen consumption. However thin filament mutations cause myosin heads to preferentially adopt a conformation in which the myosin heads are sequestered and unable to bind actin. This super relaxed state (SRX) is associated with energy conservation, which would predict reduced contractility. Yet contractility data from thin filament mutants replicate the classic HCM phenotype of hypercontractility and disturbed sarcomere relaxation. To further probe the mechanism by which thin filament variants drive HCM pathophysiology we have employed methodologies to assess calcium transients in iPSC-derived cardiomyocytes harboring thin filament variants. TNNT2 variants recapitulate this disrupted calcium handling.