PROJECT SUMMARY The mitochondrial m.3243A>G variant is a common cause of genetic mitochondrial disease that causes dysfunction of the retina and other tissues. Due to the high copy number of the mitochondrial genome per cell, this variant exists in cells across a broad range of proportions. How the proportion of mutant to wildtype m.3243 allele in any given cell, tissue, or individual relates to the cellular and clinical phenotype is not well understood. In the proposed research program, we will identify how this variant causes metabolic dysfunction in ocular cells and in which cell types the variant is preferentially distributed in the retina and underlying metabolic support tissue during development. Clinical observation has implicated the retinal pigment epithelium (RPE) as the driver of retinal dystrophy in m.3243A>G disease. Using induced pluripotent stem cells (iPSCs) derived from patients carrying m.3243A>G, we will generate RPE containing various controlled proportions of the variant. We will then subject these cells to a high throughput assay to determine derangement of the normal metabolic program of RPE. This approach will allow us to determine how increasing levels of m.3243A>G proportion in RPE impacts mitochondrial and metabolic function. We and others have previously observed that the m.3243A>G variant segregates non-randomly during the development of the retina, choroid, and other tissues. Specifically, we have observed in primary ocular tissue that the choroidal endothelial cells (CEC) that supply nutrients to the RPE and outer retina select against m.3243A>G. In the proposed research program, we will generate neural retina and CECs from patient iPSCs and measure the proportion of m.3243A>G to determine if and how these cells are able to select against the pathogenic variant during in vitro development. When successful, these studies will advance knowledge of the pathogenicity of the m.3243A>G variant in the retina. Further understanding the cell type-specific segregation of this mitochondrial variant and the specific metabolic dysfunctions it causes may lead to novel therapeutic targets for the treatment of this and other retinal diseases caused by mutations in mitochondrial genes.