Enhanced ROS production and antioxidant defenses in cybrids harbouring mutations in mtDNA

It has been suggested that mutations in mitochondrial DNA (mtDNA) can produce an increase in reactive oxygen species (ROS) and that this can play a major role in the pathogenic mechanisms of mitochondrial encephalomyopathies. Many studies exist using electron transport chain (ETC) inhibitors, however there are only a few studies that examine ROS production associated with mutations in the mtDNA. To investigate this issue, we have studied ROS production, antioxidant defences and oxidative damage to lipids and proteins in transmitochondrial cybrids carrying different mtDNA mutations. Here, we report that two different mutant cell lines carrying mutations in their mitochondrial tRNA genes (A3243G in tRNA LeuUUR and A8344G in tRNA Lys) showed an increased ROS production with a parallel increase in the antioxidant enzyme activities, which may protect cells from oxidative damage in our experimental conditions (no overt oxidative damage to lipids and proteins has been observed). In contrast, cytochrome c oxidase (COX) mutant cybrids (carrying the stop-codon mutation G6930A in the COXI gene) showed neither an increase in ROS production nor elevation of antioxidant enzyme activities or oxidative damage. These results suggest that the specific location of mutations in mtDNA has a strong influence on the phenotype of the antioxidant response. Therefore, this issue should be carefully considered when antioxidant therapies are investigated in patients with mitochondrial disorders.