Project Summary/Abstract This proposal aims to provide crucial training for the applicant’s long-term career plan to develop gene therapy approaches for patients with neurological mitochondrial diseases, which currently have no approved treatment. Adeno-associated virus (AAV)-based gene therapy is a promising therapeutic option for inherited diseases evident by numerous recent clinical trials. The applicant’s previous study developed an AAV-based gene replacement therapy for SURF1-related Leigh syndrome, an early onset neurodegenerative disease, and demonstrated its efficacy and safety in vivo. Mutations in the mitochondrial DNA-encoded ATPase 6 (MTATP6) gene represents another group of common causes for neurological mitochondrial diseases. Unlike SURF1, a nuclear DNA-encoded gene, gene replacement for MTATP6 poses more challenges. Firstly, no suitable vectors deliver gene directly into mitochondria. A potential approach is the allotopic expression, in which a wildtype copy of MTATP6 is expressed in the nucleus and the ATP6 protein is relocated to mitochondria using a mitochondrial targeting sequence. Another hurdle is that there is no MTATP6 mouse model available, and the allotopic expression of MTATP6 has only been tested in the cell cultures. To address these issues, the applicant seeks to develop the first MTATP6 mouse model using a novel mitochondria-targeting base editing technology, which creates a truncating mutation in the mouse MTATP6 gene. She will then use this mouse model to evaluate the in vivo efficacy and safety of the allotopic expression of MTATP6 via AAV delivery. In the K99 phase, the applicant will develop and optimize the base editor for mouse MTATP6 gene, and generate a conditional truncated MTATP6 mouse model. She will also develop an AAV vector to deliver the allotopic MTATP6 and test the vectors in patient-derived cell lines with both MTATP6 truncating mutations and m.8993T>G missense mutation, the most common disease variant in MTATP6. In the R00 phase, the applicant will characterize the truncated MTATP6 mouse model, and evaluate the safety and efficacy of the AAV gene therapy in vivo. This study will set a clear path for a translational study for MTATP6 patients with truncating mutations, and provide a foundation for the future development for other MTATP6 variants. The applicant will acquire crucial knowledge and laboratory skills in mitochondrial biology, base editing, and transgenic mouse modeling during her K99 phase to complement her previous training on AAV gene therapy and neuropharmacology. Additionally, the applicant’s career development will be enhanced by the expertise of an exceptional advisory/mentoring committee, as well as the unparalleled resources and ample educational and training opportunities at UT Southwestern, a world class research institution. The outstanding mentoring, unmatched resources, and strong commitment from her department will strengthen the applicant’s candidacy for, and transition to, an independent tenure-track faculty position.

Project Narrative Mutations of the mitochondrial DNA-encoded ATPase 6 (MTATP6) gene lead to impaired mitochondrial ATP synthesis, which results in a variety of neurological mitochondrial diseases. Allotopic expression of MTATP6 is a plausible gene therapy approach for restoring mitochondrial dysfunction caused by MTATP6 mutations, while to date it has only been tested in vitro due to the lack of an animal model. The proposed study aims to generate the first MTATP6 targeting mouse model using a novel base editing technology, and develop an adeno- associated virus (AAV)-based gene therapy strategy for MTATP6 patients.