Clinical Science (2004) 107, 355–364 (Printed in Great Britain) 355 R E V I E W Mitochondrial DNA and aging Mikhail F. ALEXEYEV, Susan P. LEDOUX and Glenn L. WILSON Department of Cell Biology and Neuroscience, University of South Alabama, 307 University Blvd, Mobile, AL 36688, U.S.A. A B S T R A C T Among the numerous theories that explain the process of aging, the mitochondrial theory of aging has received the most attention. This theory states that electrons leaking from the ETC (electron transfer chain) reduce molecular oxygen to form O 2 •− (superoxide anion radicals). O 2 •− , through both enzymic and non-enzymic reactions, can cause the generation of other ROS (reactive oxygen species). The ensuing state of oxidative stress results in damage to ETC components and mtDNA (mitochondrial DNA), thus increasing further the production of ROS. Ultimately, this ‘vicious cycle’ leads to a physiological decline in function, or aging. This review focuses on recent developments in aging research related to the role played by mtDNA. Both supportive and contradictory evidence is discussed. INVOLVEMENT OF mtDNA (MITOCHONDRIAL DNA) IN AGING Aging can be defined as a multifactorial phenomenon characterized by a time-dependent decline in physio- logical function [1]. This physiological decline is believed to be associated with an accumulation of defects in the metabolic pathways. RNA, proteins and other cellular macromolecules are rapidly turned over and, conse- quently, are poor candidates for progressively accumulat- ing damage over a lifetime. Therefore even early studies on mechanisms of aging focused on DNA (for example, see [2,3]). In mammalian cells, mitochondria and the nucleus are the only organelles that possess DNA. It appears obvious that the physiological integrity of the cell must critically depend upon the integrity of its genome, which is maintained by DNA repair machinery. However, although the organization, synthesis and repair of nuclear DNA have been the focus of intense studies, mtDNA has received much less attention until recently. BASIC mtDNA BIOLOGY Human mtDNA is a circular double-stranded molecule that is 16 569 bp long (other sequenced mammalian mitochondrial genomes have similar lengths; Figure 1). Key words: aging, DNA damage, mitochondrial DNA, oxidative stress, reactive oxygen species, repair. Abbreviations: BER, base excision repair; CAT, catalase; ETC, electron transfer chain; O 2 •− , superoxide anion radical; OH • , hydroxyl radical; mtDNA, mitochondrial DNA; MLSP, maximum lifespan; 8-oxodG, 7,8-dihydro-8-oxoguanine; OGG1, 8-oxoguanine-DNA glycosylase; ROS, reactive oxygen species; SOD, superoxide dismutase; TBA, thiobarbituric acid. Correspondence: Dr Mikhail F. Alexeyev (email malexeye@jaguar1.usouthal.edu). It encodes two rRNAs, 22 tRNAs and 13 polypeptides, of which seven are components of complex I (NADH dehydrogenase), three are components of complex IV (cytochrome c oxidase), two are subunits of complex V (ATP synthase) and cytochrome b (a subunit of complex III) [4]. The inheritance of mtDNA is almost exclusively maternal, although some important exceptions have been reported [5–7]. mtDNA is present in one to several thousand copies per cell [8] and is ‘encapsulated’ into mitochondria at 1–11 copies per mitochondrion with the mean being two genomes per organelle [9]. The two mtDNA strands can be separated by denaturing caesium chloride gradient centrifugation [10]. Most of the infor- mation is encoded in the heavy (purine-rich) strand (two rRNAs, 14 tRNAs and 12 polypeptides). The light (pyrimidine-rich) strand contains genetic information for only one polypeptide and eight tRNAs. Mitochondrial genes have no introns and intergenic sequences are absent or limited to a few bases. Some genes overlap and, in some instances, termination codons are not encoded, but are generated post-transcriptionally by polyadenylation [11]. mtDNA is totally dependent upon nuclear-encoded proteins for its maintenance and transcription. In fact, the mitochondrial proteome consists of an estimated 1500 polypeptides [12] of which only 13 are encoded by its own DNA. mtDNA replication is conducted by C  2004 The Biochemical Society