Cellular Responses to Different Types of Mitochondrial DNA Damage

Mitochondrial DNA (mtDNA) is a circular DNA molecule existing in multiple copies in mitochondria. mtDNA contains a higher level of chemical-induced DNA lesions than nuclear DNA due to the accumulation of lipophilic and charged chemicals in the mitochondria and the lack of certain DNA repair pathways. mtDNA lesions can alter mitochondrial function, such as the reduction of mitochondrial respiration, mitochondrial membrane potential and an elevation of mitochondrial reactive oxygen species (ROS). In response to mtDNA damage, DNA repair, mtDNA degradation and mitochondrial dynamics are the major pathways to eliminate mtDNA damage. The relative contributions of mtDNA repair, degradation, and mitochondrial dynamics in response to different types of mtDNA damage remains an outstanding question. To address this question, characterizing these pathways in mtDNA damage cell models was conducted in this study. First, inducible mitochondrial targeting uracil DNA glycosylase 1 variant (UNG1-Y147A) transduced HeLa and HEK293 cells with APE1 siRNA transfection were used as mitochondrial abasic site (AP) lesion models. The mtDNA copy numbers were significantly reduced in the UNG1-Y147A-overexpressed HeLa and HEK293 cells. However, PCR-blocking lesions on mtDNA were not increased in the UNG1-Y147A- overexpressed or APE1-knockdown cells. DNA repair and mitochondria dynamics-related genes were not significantly altered. mtDNA degradation was the main response in the mtDNA AP lesion models. Second, mitochondrial targeting chemical mt-Ox was used to generate mtDNA oxidative damage in HeLa and HEK293 cells. Different from UNG1-Y147A, mt-Ox increased PCR-blocking lesions on mtDNA without reducing mtDNA copy number. mt-Ox also induced the expressions of DNA repair and mitochondria dynamics-related genes in HeLa cells. DNA repair and mitochondria dynamics were more significant in response to mtDNA oxidative damage. The difference in response might be due to the types and amounts of lesions. The mechanisms of damage response activation and decision need further investigation. In conclusion, this study provides solid evidence that cells had different damage responses in the mtDNA AP lesions and oxidative damage models. The evidence provides insights into different cellular responses to different types of mtDNA lesions and guides future research for illustrating the mtDNA damage induced adverse outcome pathways