In vivo and in vitro mechanistic characterization of a clinically relevant PolγA mutation

Mutations in POLG, encoding POL{gamma}A, the catalytic subunit of the mitochondrial DNA polymerase, cause a spectrum of disorders characterized by mtDNA instability. However, the molecular pathogenesis of POLG-related diseases is poorly understood and efficient treatments are missing. Here, we generated a POLGA449T/A449T mouse model, which reproduces the most common human recessive mutation of POLG, encoding the A467T change, and dissected the mechanisms underlying pathogenicity. We show that the A449T mutation impairs DNA binding and mtDNA synthesis activities of POL{gamma} in vivo and in vitro. Interestingly, the A467T mutation also strongly impairs interactions with POL{gamma}B, the homodimeric accessory subunit of holo-POL{gamma}. This allows the free POL{gamma}A to become a substrate for LONP1 protease degradation, leading to dramatically reduced levels of POL{gamma}A, which in turn exacerbates the molecular phenotypes of PolgA449T/A449T mice. Importantly, we validated this mechanism for other mutations affecting the interaction between the two POL{gamma} subunits. We suggest that LONP1 dependent degradation of POL{gamma}A can be exploited as a target for the development of future therapies.