Stargardt disease is the most common cause of juvenile macular degeneration leading to legal blindness. It is caused by loss-of-function mutation in the ABCA4 gene, a protein product of which is responsible for preventing the accumulation of toxic lipids in the retina. It leads to atrophy of the fovea, the central part of the retina; which impairs patient’s ability to read, drive or recognize faces. There is no accepted treatment for this seriously debilitating disorder. The most common mutations that lead to genetic diseases, including Stargardt disease, are single nucleotide variations. Precise therapeutic targeting of single nucleotide changes traditionally was difficult.Objectives: This proposal aims at developing a novel treatment modality, RNA base editing, to correct the most common single nucleotide variation (c.5882G>A, p.G1961E) responsible for Stargardt disease. Furthermore, we use innovative models to translate this approach to a clinical application. Specific aims: This proposal has four specific aims. In Aim 1, we will test different adeno-associated viral vectors to deliver the RNA base editor components to the target cells, the foveal cone photoreceptors and retinal pigment epithelial cells. In Aim 2, we will generate retinal organoid models carrying the mutation and demonstrate proof-of-concept for RNA base editing. In Aim 3, we will generate disease specific reporters and surrogate assays and directly test RNA base editing on human retina explants. In the short Aim 4, I will characterize patients carrying the c.5882G>A mutation with multi-modal imaging and functional tests to prepare for a future clinical trial.Expected results: There are two main expected outcomes from this study. First, we will generate novel models of Stargardt disease in the form of mutation carrying human retinal organoids. These organoids can not only be used to test RNA base editing but also to test other therapeutic modalities in the future. Second, we expect to deliver proof-of-concept for RNA base editing for the G4961E mutation, which will pave the way towards future clinical development in Stargardt disease.Impact: This is a high-risk proposal with an enormous impact. First if organoid models are proven valuable, they can lead to a new way of drug development for retinal disease. Second, the idea that there is a potential to correct the underlying genetic problem is unprecedented in biomedicine and the fact that this strategy can reasonably reach the clinic within 5-6 years provides an exceptional opportunity.