Correcting a Pathogenic Mitochondrial DNA Mutation by Base Editing in Mice

Primary mitochondrial disorders are most often caused by deleterious mutations in the mtDNA. Here, I harnessed a mitochondrial base editor, DdCBE, to introduce a compensatory edit (m.5081G>A) in a mouse model that carries the pathological m.5024C>T mutation in the mitochondrial tRNA^Ala gene. For this, the DdCBE gene construct was packaged in recombinant AAV9 and systemically injected into mice. I found that total mt-tRNA^Ala levels, which are drastically reduced by the mutation, are restored by the m.5081G>A edit in a dose-dependent manner. However, an excessive expression of DdCBE also induces extensive mtDNA off-target editing, counteracting this positive outcome. To address this, I optimized the dosage to maximize the amount of compensatory edit generated with minimal off-target editing. These results show that mitochondrial base editors are promising candidates for gene therapy for mitochondrial disorders, but their expression needs to be carefully controlled.