PROJECT SUMMARY. Despite recent exponential growth of the field of cardiovascular genetics leading to iden- tification of hundreds of mutations responsible for hypertrophic cardiomyopathy (HCM), the mechanism linking sarcomeric mutations to the clinical phenotype remains unknown. Clinical features of HCM are severe left ven- tricular hypertrophy, myocardial fibrosis, diastolic dysfunction, and an increased risk of arrhythmias and heart failure. As there is no disease-modifying therapy available, HCM remains the most common cause of sudden cardiac death in the young. This proposal considers worsened myocardial energetics as the unknown shared pathway that, when triggered by a sarcomeric mutation, leads to the development of the clinical phenotype. Mutations responsible for HCM increase power of contraction in extremely inefficient way, resulting in several- fold increased ATP demand. Mitochondria initially meet the increased demand and maintain normal ATP con- centration, albeit at a cost of accumulation of ADP, a product of ATP hydrolysis. Elevated ADP limits free energy of ATP hydrolysis (∆G~ATP), which is the amount of chemical energy in ATP that ATPases can use to perform work. Decreased ∆G~ATP inhibits ion pumps SERCA and Na+/K+ ATPase leading to increased diastolic Ca++ and intracellular [Na+]i, associated with diastolic dysfunction and arrhythmias. Moreover, elevated [Na+]i further impairs mitochondrial ATP synthesis and increases reactive oxygen species (ROS) production. Excessive ROS, in turn, oxidatively inhibit mitochondrial proteins and ATP synthesis. Thus, even though the primary defect is in the inefficient sarcomere, mitochondrial damage ensues, establishing a vicious cycle of energy shortage. The central hypothesis of this proposal is that interventions that improve energy balance, result in improved function, hypertrophy and fibrosis in HCM. The central hypothesis will be tested by pursuing two mechanisms to improve energetics in HCM: 1) decreasing ATP demand and 2) improving mitochondrial ATP synthesis. Sodium and phosphorus magnetic resonance spectroscopy and imaging will determine the interplay between myocardial [Na+]i, ROS, contractile function, energetics, hypertrophy and fibrosis in a murine models of HCM bearing two of the most lethal mutations, R403Q in myosin and R92L in troponin T. Specific Aim 1 will test the hypothesis that treatment with a myosin ATPase inhibitor, MYK-461, decreases excessive ATP consumption to improve ∆G~ATP, [Na+]i, oxidative stress and cardiac function in HCM mice. Specific Aim 2 will test the hypothesis that interventions that increase mitochondrial ATP synthesis improve ∆G~ATP, diastolic function and contractile re- serve in HCM. ATP synthesis in HCM mice will be increased by a) saturating mitochondria with an accessible substrate, butyrate, b) supressing excessive mitochondrial ROS by overexpressing mitochondrial catalase, and c) decreasing intracellular [Na+]i with empagliflozin, a Na+/glucose...