Mitochondrial DNA mutations and oxidative damage in aging and diseases: an emerging paradigm of gerontology and medicine.

Human mitochondrial DNA (mtDNA) is a multi-copy extra-chromosomal genetic element, which is exposed to a high steady-state level of reactive oxygen species and free radicals generated by the respiratory chain in mitochondria. Thus, it is much more vulnerable to oxidative damage and mutation than is nuclear DNA. In the past decade, more than two-dozen mutations of mtDNA have been observed in the somatic tissues of aged individuals. Among them, the 4,977 bp and 7,436 bp deletions and the A3243G and A8344G point mutations frequently occur and accumulate exponentially with age in muscle and other human tissues. These mtDNA mutations occur alone or co-exist in old human tissues at relatively low levels (< 5%). Aside from mutation, oxidative damage to mtDNA also increases in an age-dependent manner in human tissues. On the other hand, more than a hundred mtDNA mutations have been detected in patients with mitochondrial myopathy and encephalomyopathy. The mutant mtDNA often coexists with the wild-type mtDNA in affected tissues (a condition termed heteroplasmy). Usually the clinical severity of the disease is correlated with the proportion of the mutate mtDNA in the target tissues (usually > 80%). The threshold of the mutant mtDNA which is required to elicit clinical symptoms varies with different mutations. At the same level, large-scale deletions usually cause much more severe pathologies than do point mutations. The pattern of distribution of the mutant mtDNA and the energy demand of the target tissues are important factors in determining the pathological outcome of the mutation. The mutant mtDNA is usually widely distributed in the body tissues of the patient, thereby leading to multi-system disorders, which are frequently seen in mitochondrial diseases. Although a majority of the pathogenic point mutations are maternally transmitted, large-scale deletions of mtDNA are mostly sporadic. In addition, tandem duplication and depletion of mtDNA have also been found in the muscle and other affected tissues of elderly subjects and some patients with mitochondrial myopathy. Moreover, recent work in our laboratory has shown that oxidative damage to DNA in affected tissues is significantly higher than that in normal tissues. It is now established that mutation and oxidative damage of mtDNA are contributory factors to aging and that at high levels, they cause a fall of ATP supply below the threshold of energy needed by affected tissues in patients with mitochondrial diseases. These advances have laid the foundation for the development of biomedical gerontology and mitochondrial medicine.