Crossing boundaries:
mtDNA damage response and repair

The bacterial DNA damage response dynamics made us wonder how eukaryotic cells responded to damage. In eukaryotic systems, this involves preservation of both nuclear as well as mitochondrial genomes. Indeed, close proximity of mtDNA to the electron transport chain makes them particularly susceptible to such damage. While the cellular response in case of nuclear DNA damage is well-studied, how cells sense and respond to mitochondrial DNA damage remains an outstanding and exciting question. A major challenge in studying mtDNA damage is the ability to generate mtDNA-specific damage, avoiding any confounding effects on the nuclear genome. Using our expertise in bacteria DNA damage and repair, we developed a novel tool to induce mitochondria-specific DNA damage in yeast, using a mitochondrial-targeted base modifying bacterial toxin, DarT. We uncovered the following key features about the cellular response to mtDNA damage: a). mtDNA damage triggers dynamic reorganization of the mitochondrial network, resulting in network fragmentation. b). network remodelling is associated with loss in mtDNA. c). Changes to mitochondrial organization and mtDNA levels are not driven by mitophagy. Overall, our data suggest that, under damage, the segregation of defective mtDNA and organelle can be de-coupled, with an emphasis on mitochondrial maintenance even in the absence of its DNA. More recently, we discovered that cells likely have a regulated mtDNA damage response, that is independent of nuclear DNA damage. We are now deciphering: a). The mechanisms underlying checkpoint activation, and b). How the mtDNA replicative polymerase switches action from polymerase to exonuclease function upon encountering a DNA lesion.