Project Summary/Abstract Disruption of mitochondrial oxidative phosphorylation (OXPHOS) is associated with the development of biochemical alterations that typically affect tissues with a high energy demand, particularly skeletal and cardiac muscle. An inherited autosomal recessive skeletal myopathy and hypertrophic cardiomyopathy has been linked to loss of function of a nuclear DNA-encoded mitochondrial protein, due to a frameshift mutation in solute carrier family 25, member 4 (SLC25A4; c.523delC, p.Q175RfxX38). SLC25A4 encodes the heart-muscle isoform of the adenine nucleotide translocator-1 (ANT1, SLC25A4), which in the wild-type state is a critical component of mitochondrial metabolism. Patients with SLC25A4 deficiency display lactic acidosis, persistent adrenergic activation, and exertional intolerance secondary to both a general skeletal muscle myopathy as well as a hypertrophic cardiomyopathy. Ultimately, myocardial thickening and cardiac dysfunction progress to end- stage heart failure necessitating cardiac transplantation. There are not currently any disease-modifying therapies available for this patient cohort. However, adeno- associated viral (AAV) mediated gene replacement therapies have emerged as a powerful strategy for disease modification of inherited monogenic disorders. The long-term goal of our research is to develop a therapeutic gene replacement strategy to treat SLC25A4 deficiency. The objective of this proposal is to further characterize the disease phenotype as well as to synthesize and evaluate the efficacy of a recombinant AAV (rAAV) vector in an in vitro model of patient-derived cell lines and organoid models. The central hypothesis of this proposal is that AAV-mediated gene replacement can ameliorate the biochemical and functional effects of SLC25A4 deficiency and can more decisively prevent disease progression. The specific aims of this proposal are: 1. Characterize the SLC25A4 deficiency phenotype in patient-derived cell lines. 2. Synthesize a recombinant AAV vector for delivery of codon-optimized SLC25A4 cDNA to skeletal and cardiac myocytes. 3. Evaluate the efficacy of AAV-SLC25A4 viral transduction in patient-derived cell lines. These experiments will improve our understanding of the molecular mechanisms underlying SLC25A4 deficiency as well as allow us to evaluate the efficacy of an AAV platform in a relevant preclinical model. Moreover, the skills I will acquire during this fellowship will help to establish me as an independent investigator and a surgeon-scientist focused on the development of translational gene replacement therapies.