Cell sorting experiments link persistent mitochondrial DNA damage with loss of mitochondrial membrane potential and apoptotic cell death

In order to understand the molecular events following oxidative stress, which lead to persistence of lesions in the mtDNA, experiments were performed on normal human fibroblast (NHF) expressing human telomerase reverse transcriptase (hTERT). The formation and repair of H2O2-induced DNA lesions were examined using quantitative PCR. It was found that NHF hTERTs show extensive mtDNA damage (similar to4 lesions/10 kb) after exposure to 200 muM H2O2, which is partially repaired during a recovery period of 6 h. At the same time, the nDNA seemed to be completely resistant to damage. Cell sorting experiments revealed persistent mtDNA damage at 24 h only in the fraction of cells with low mitochondrial membrane potential (DeltaPsim). Further analysis also showed increased production of H2O2 by these cells, which subsequently undergo apoptosis. This work supports a hypothesis for a feed-forward cascade of reactive oxygen species generation and mtDNA damage and also suggested a possible mechanism for persistence of lesions in the mtDNA involving a drop in DeltaPsim, compromised protein import, secondary reactive oxygen species generation, and loss of repair capacity.