Mitochondrial DNA Repair Processes In Oxidative Stress And Aging

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The base excision repair pathway is initiated by the action of a class of enzymes known as DNA glycosylases, which recognize and release the damaged base, and thus give specificity to the repair process. Mammalian cells carry two major DNA glycosylases for the repair of oxidized bases, oxoguanine DNA glycosylase (OGG1) and Endonuclease III homologue (NTH1). We found that OGG1 plays a crucial role in the repair of oxidized lesions in mitochondria and is probably the only DNA glycosylase for 8-oxoG removal in these organelles. In human cells two distinct OGG1 isoforms are expressed, alpha and beta. Because of the high abundance of the b-OGG1 protein in human mitochondria we are now investigating whether it has any biological function. All BER enzymes are encoded in the nucleus and transported to mitochondria; however there is very limited information on the regulation of mitochondrial BER. In mammalian mitochondria the mtDNA is found in a large protein-DNA complex known as the nucleoid. One of the most abundant protein components of mammalian nucleoids is the transcription factor TFAM, which has been postulated to have a structural function in compacting the mtDNA into the nucleoid structure. Using recombinant human TFAM we are now investigating whether TFAM modulates mtDNA repair. We find that that TFAM may modulate BER proteins through an as yet undetermined mechanism. To explore whether it is a function of TFAMs high affinity for DNA we have created TFAM DNA binding mutants and are re-evaluating the activity of the BER enzyme activities in the presence of this mutant. Additionally, we are exploring whether TFAM physically interacts with any mitochondrial BER proteins. We are now investigating whether mammalian mitochondria have any of the other repair pathways that operate in the nucleus, such as mismatch repair (MMR). Our results show that human mitochondria can catalyze mismatch repair in vitro and contain a mismatch binding activity. Using affinity purification with a mismatch-containing DNA substrate, and mass spectrometry-peptide analyses we identified 3 proteins in the mismatch-bound complex, the transcription factor YB-1, the Cytochrome oxidase-assembly factor LRP130 and an UV-resistance associated gene of unknown activity. We showed mitochondrial localization of YB-1 using both the endogenous as well as ectopic expressed protein. Interestingly, abrogation of YB1 levels by RNA interference significantly decreased mitochondrial-catalysed mismatch repair activity in an in vitro assay, indicating that this protein is involved in mitochondiral MMR. These observations, along with results from others clearly establish that mammalian mitochondria have a functional mismatch repair pathway. Another important set of proteins involved in mitochondrial DNA metabolism are the helicases SUV3 and PIF1. We have investigated the biochemical functions of SUV3, and it appears to interact with some mitochondrial and telomere proteins, making it possible that it functions both in telomeres and in mitochondria. This is under further investigation. While Oxidative damage processing is very efficient in mitochondria, little is known about the recombination DNA repair pathways in these organelles. Interestingly, we detect direct functional interactions between the OGG1 protein, present in the nucleus and in mitochondria, and the recombination protein RAD52. It is not known whether RAD52 is present in mitochondria, and this is currently under investigation.