Restrictive Cardiomyopathy (RCM) is an autosomal dominant form of cardiomyopathy characterized by profound diastolic dysfunction yet normal or near-normal ventricular dimensions, wall thickness, and systolic function. RCM patients have fewer treatment options and notably poorer outcomes than those with other forms of cardiomyopathy. This is especially true in pediatric-onset RCM, for which the only definitive therapy is heart transplantation, often in childhood. Mutations that cause RCM predominate in the sarcomere, which is the contractile unit of cardiac muscle cells. Since the dysfunction is intrinsic to cardiomyocytes, human in vitro induced pluripotent stem cell (hiPSC)-derived cardiomyocytes are well-suited to modeling the RCM and evaluating therapeutics strategies. To date, however, there are no reported investigation of hiPSC-derived cardiomyocyte models of RCM. This proposal, therefore, seeks to use hiPSC-based models of familial, pediatric RCM to elucidate pathological features, determine whether certain mutations cause distinct pathogenetic mechanisms, and evaluate the therapeutic potential of two newly approved drugs that have shown promise for treating diastolic dysfunction in other forms of heart disease. Preliminary studies generated a patient-derived, hiPSC-based model of RCM caused by mutations in cardiac Troponin-T (TNNT2). We found that heightened Ca2+ sensitivity of force generation and increased fibrosis might underlie disease pathogenesis. Besides TNNT2, mutations in other sarcomeric mutations also cause severe pediatric RCM, and some are hypothesized to induce disease by distinct pathophysiological mechanisms. Therefore, AIM 1 of this proposal is to develop hiPSC models of RCM caused by diverse gene variants, and identify distinct and common mechanisms of contractile dysfunction. Our hypothesis is that RCM is a heterogeneous disease and distinct gene variant-specific mechanisms converge to elicit hallmark clinical features of RCM. Independently, AIM 2 is to evaluate mavacamten mecarbil and sodium-glucose cotransporter-2 inhibitors (SGLT2i) for efficacy in treating contractile dysfunction in RCM using the hiPSC models. Mavacamten and SGLT2i are newly approved for other forms of heart disease. Mavacamten, by decreasing actin-myosin cross- bridging, might be therapeutically effective for RCM independently of genetic etiology. In contrast, SGLT2 inhibitors (SGLT2i), which operate by inhibiting multiple proteins and decrease intracellular [Ca2+] in cardiomyocytes, might show selectivity for gene mutation depending on pathogenic mechanism. Characterizing the basic disease mechanisms of RCM and evaluating the efficacy of candidate therapeutics is a critical step towards improving management of this challenging disease.