Mitochondrial diseases are a heterogeneous group of pathologies, presenting mitochondrial oxidative phosphorylation (OXPHOS) deficiency and subsequent deterioration of affected tissues, usually the ones with high energy demand. Nuclear and mitochondrial DNA (mtDNA) mutations have been associated with mitochondrial disorders, and only palliative therapies are available, with no approach available to correct the underlining genetic problem. Our and other groups, taking advantage of mtDNA heteroplasmy, were able to develop gene therapy strategies to target the mtDNA, inducing the increase of the wild-type haplotype. The gene editing tools used by our group and others such as specific endonucleases targeted to the mitochondria, and more recently mitoTALENs and mitochondrial Zinc-Finger Nucleases were shown to effectively shift mtDNA heteroplasmy. This change towards an increase in wild-type mtDNA molecules occurred when the nuclease was specifically designed to cleave DNA regions harboring point mutations or the common deletion. In this chapter we will focus on strategies to target specific mtDNA haplotypes to shift heteroplasmy in vitro and in vivo. A brief overview on heteroplasmic animal models and viral delivery will be introduced, as these will be critical for future translation of this approach into human gene therapy.
