mitoTev-TALE: a monomeric DNA editing enzyme to reduce mutant mitochondrial DNA levels

Pathogenic mitochondrial DNA (mtDNA) mutations often co-exist with wild-type molecules (mtDNA heteroplasmy). Phenotypes manifest when the percentage of mutant mtDNA is high (70-90%). Previously, our laboratory showed that mitochondria-targeted transcription activator-like effector nucleases (mitoTALENs) can eliminate mutant mtDNA from heteroplasmic cells. However, mitoTALENs are dimeric and relatively large, making it difficult to package their coding genes into viral vectors, limiting their clinical application. The smaller monomeric GIY-YIG homing nuclease from T4 phage (I-TevI) provides a potential alternative. We tested whether molecular hybrids (mitoTev-TALEs) could specifically bind and cleave mtDNA of patient-derived cybrids harboring different levels of the m.8344A>G mtDNA point mutation, associated with myoclonic epilepsy with ragged-red fibers (MERRF). We tested two mitoTev-TALE designs, one of which robustly shifted the mtDNA ratio toward the wild type. When this mitoTev-TALE was tested in a clone with high levels of the MERRF mutation (91% mutant), the shift in heteroplasmy resulted in an improvement of oxidative phosphorylation function. mitoTev-TALE provides an effective architecture for mtDNA editing that could facilitate therapeutic delivery of mtDNA editing enzymes to affected tissues.