The mitochondrial theory of aging and its relationship to reactive oxygen species damage and somatic mtDNA mutations.

M itochondria are cellular energy factories that generate ATP via the reaction of hydrocarbons with oxygen. Every human cell contains hundreds of mitochondria, and each mitochondrion contains multiple copies of mitochondrial DNA (mtDNA). The ancestry of the mitochondrial genome can be traced to early eubacteria, and it is therefore unexpected that this organelle may have a major role in governing the pace of human aging. Three recent papers (1–3) plus a work published in a recent issue of PNAS (4) have demonstrated that accelerating the mtDNA mutation rate can result in some features suggestive of premature aging, consistent with the view that loss of mitochondrial function is a major causal factor in aging. mtDNA polymerase (Pol) is the only DNA polymerase that is known to be targeted to and that resides in mitochondria (5). In the absence of other known mtDNA polymerases, it is assumed that Polis responsible for all aspects of mtDNA synthesis, including replication of the mitochondrial genome and repair of DNA damage. As is the case for many other DNA polymerases, the high fidelity of Polderives from both selection for the correct incoming nucleotide and excision of misincorporated nucleotides by a 3 3 5 exonucleolytic proofreading activity (6). Elimination of proofreading by replacement of critical aspartic acid residues in the exonuclease domain with alanine increases misincorporation; in vitro, this misincorporation is manifested predominantly as single base substitutions (7). Both the Larsson (1) and Prolla (2) groups replaced in embryonic stem cells POLG, the gene encoding wild-type POLG, with a mutant that encodes an error-prone active Poland generated mice that accumulate mitochondrial mutations with increasing age. The mutations were predominantly single-base substitutions and deletions, the latter presumably retained by in vivo selection. Although apparently normal at birth, the mice exhibited at an early age many of the phenotypes characteristic of human aging (but perhaps not characteristic of murine aging). The results clearly indicate that Polis responsible for the attenuated lifespan and progeroid features in these mice. Conversely, Schriner et al. (3) targeted catalase to mitochondria and observed an extension of both the median and maximum lifespan in mice. The extension of lifespan was in association with reduced damage to mtDNA and increased mitochondrial resistance to exogenous reactive oxygen species (ROS) damage. Trifunovic et al. (4) now address the mechanism for accelerated aging in POLG mutator mice. Do the mitochondrial mutations introduced by Polresult in mutant mitochondrial proteins that are defective in coupling of oxygen metabolism with ATP causing increased ROS production, DNA damage, and mutations? This scenario would be consistent with the venerable ‘‘free radical theory of aging’’ (8). However, they find little or no evidence for the key intermediates in this cycle, ROS, although the production of ROS in very old animals was not examined. Despite an accumulation of mitochondrial mutations in POLG mice and the presence of ‘‘respiratory-chain dysfunction,’’ as measured by a 95% reduction in O2 consumption, they observed no increase in damaged proteins in the heart and liver in the mutator mice. Moreover, the mice did not exhibit a reduction in mitochondrial aconitase activity, a classic marker of oxidative damage to protein. In addition, mRNA levels of enzymes that scavenge ROS were unaffected, suggesting the lack of an ROS-induced stress response. Embryonic fibroblasts derived from the POLG mice and pre-