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Clinical Science (2004) 107, 355364 (Printed in Great Britain) 355

R E V I E W

Mitochondrial DNA and aging

Mikhail F. ALEXEYEV, Susan P. LEDOUX and Glenn L. WILSONDepartment 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 O2 (superoxide anion radicals). O2

, 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 cellularmacromolecules 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 thephysiological integrityof the cell

must critically depend upon the integrity of its genome,

whichis maintainedby DNArepairmachinery. However,

although the organization, synthesisand 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; O2, 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 [email protected]).

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

(cytochromecoxidase), two are subunits of complex V

(ATP synthase) and cytochromeb(a subunit of complex

III) [4]. The inheritance of mtDNA is almost exclusively

maternal, although some important exceptions have been

reported [57]. mtDNA is present in one to severalthousand copies per cell [8] and is encapsulated into

mitochondria at 111 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, butare 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


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356 M. F. Alexeyev, S. P. LeDoux and G. L. Wilson

Figure 1 Map of human mtDNA

OH and OL , origins of heavy and light strand replication respectively; ND1

ND6, NADH dehydrogenase (ETC complex I) subunits 16; Cox1Cox3, cytochrome

oxidase subunits 13 (ETC complex IV); ATP6 and ATP8, subunits 6 and 8 of

mitochondrial ATPase (complex V); Cyt b, cytochrome b (complex III).

the heterodimeric DNA polymerase [13]. Replication

of mtDNA continues throughout the lifespan of an or-

ganism in both proliferating and post-mitotic cells. It oc-

curs bidirectionally, initiated at two spatially and tempo-

rally distinct origins of replication, OH and OL, for the

heavy and light strand origins of replication respectively(for review, see [14]). The mitochondrial proteome is

dynamic, i.e. both the precise polypeptide composition

and the relative abundance of a given polypeptide may

vary in mitochondrial proteomes of different tissues as

well as in the same tissue over time [15]. With the dis-

covery of mitochondrial diseases caused by mutations of

mtDNA, it has been found that wild-type (normal) and

mutated mtDNA may coexist in thesame cell, a condition

called heteroplasmy [16].

mtDNA DAMAGE AND THE MITOCHONDRIALTHEORY OF AGING

Despite the fact that in animal cells mtDNA comprises

only 13 % of genetic material, several lines of evidence

suggest that its contribution to cellular physiology could

be much greater than would be expected from its size

alone. For instance, (i) it mutates at higher rates than

nuclear DNA, which may be a consequence of its close

proximity to the ETC (electron transfer chain); (ii) it

encodes either polypeptides of ETC or components

required for their synthesis and, therefore, any coding

mutations in mtDNA will affect the ETC as a whole;

this could affect both the assembly and function of the

products of numerous nuclear genes in ETC complexes;

(iii) defects in the ETC can have pleiotropic effects

because they affect cellular energetics as a whole.

Several lines of evidence indirectly implicate mtDNA

in longevity. The Framingham Longevity Study of Coro-

nary Heart Disease has indicated that longevity is morestrongly associated with age of maternal death than that

of paternal death, suggesting that mtDNA inheritance

might be involved [17]. On the other hand, longevity

was shown to be associated with certain mtDNA poly-

morphisms. ThusItalian male centenarianshave increased

incidence of mtDNA haplogroup J [18], whereas French

and Japanese centenarians have increased incidences of

GA transition at mt9055 and CA transversion at

mt5178 respectively [19,20]. However, a study of an Irish

population failed to link longevity to any particular

mitochondrial haplotype, suggesting that factors other

than mtDNA polymorphismalso mayplay a role in aging[21].

Mitochondria have been shown to accumulate high

levels of lipophilic carcinogens such as polycyclic aro-

matic hydrocarbons [22,23]. When cells are exposed to

some of these compounds, mtDNA is damaged pre-

ferentially [24]. Other mutagenic chemicals also have

been shown to preferentially target mtDNA [23,2529].

Therefore it is conceivable that life-long exposure to

certain environmental toxins could result in a preferential

accumulation of mtDNA damage and accelerate aging.

However, perhaps the most relevant kind of insult to

which mtDNA is exposed is oxidative damage. The lack

of protective histones and close proximity to the ETC,

whose complexes I and III are believed to be the pre-

dominant sites of ROS (reactive oxygen species) pro-

duction inside the cell, make mtDNA extremely

vulnerable to oxidative stress. Indeed, the free radical

theory of aging first put forward by Harman [3033]

statesthatit is themitochondrial production ofROS, such

as superoxide and H2O2, and the resulting accumulation

of damage to macromolecules that causes aging. Cumul-

ative damage to biological macromolecules was proposed

to overwhelm the capacity of biological systems to repair

themselves, resulting in an inevitable functional decline.

The mitochondrial theory of aging can be considered asan extension and refinement of the free radical theory.

Its major premise is that mtDNA mutations accumulate

progressively during life and are directly responsible for

a measurable deficiency in cellular oxidative phosphor-

ylation activity, leading to an enhanced ROS production.

In turn, increased ROS production results in an increased

rate of mtDNA damage and mutagenesis, thus causing

a vicious cycle of exponentially increasing oxidative

damage and dysfunction, which ultimately culminates in

death (Figure 2). Since Miquel et al. [34] first suggested

that mtDNA might be damaged in aging, numerous

studies over the past two decades have supplied a wealth



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Mitochondrial DNA and aging 357

Figure 2 Vicious cycle of mtDNA damage

ROS produced at ETC lead to the inhibition of mtDNA transcription and ETC

activity, resulting in even higher levels of ROS production.

of information consistent with the predictions of the free

radical/mitochondrial theory of aging.

(i) Oxidative damage in cells and, in

particular, in mtDNA, is ubiquitous,

substantial and, like mortality rates,

increases exponentially with age [35]It has been shown that exhalation of ethane and n-pen-

thane, indicators of ROS-mediated lipid peroxidation

increases with age [36]. mtDNA was shown to accu-

mulate oxidative damage in an age-dependent manner in

skeletal muscle [37,38], the diaphragm [39,40], cardiac

muscle [4143] and the brain [44]. Additionally, an age-

related increase in oxidative damage to mtDNA appears

to be greater than oxidative damage to nuclear DNA

in houseflies [45,46]. In rodents, an age-related increase in

8-oxodG (7,8-dihydro-8-oxoguanine), a mutagenic

DNA base lesion caused by oxidative stress, was obser-

ved in mtDNA isolated from the livers of both rats and

mice [47]. Dietary restriction, which is known to retard

aging and increase the lifespan in rodents, has been found

to significantly reduce the age-related accumulation of

8-oxodG levels in nuclear DNA in all tissues of male

B6D23F1 mice and in most tissues of male F344 rats. This

study [47] also showed that dietary restriction prevented

the age-related increase in 8-oxodG levels in mtDNA

isolated from the livers of both rats and mice [47].Another study [48] has found that the activities of DNA

repair enzymes OGG1 (8-oxoguanine-DNA glycosyl-

ase), hypoxanthine- and uracil-DNA glycosylase in-

crease in liver extracts of Wistar and OXYS rats with age.

In both strains, OGG1 activities were approx. 10 times

greater in nuclear extracts than in mitochondrial ex-

tracts [48]. However, while OGG1 activity in nuclear

extracts remained relatively constant during the study,

this activity increased with age in mitochondrial extracts.

Importantly, in OXYS rats, which are characterized by

the overproduction of ROS, high levels of lipid per-

oxidation, protein oxidation and decreased life span, the

levels of mitochondrial OGG1 activity were greater than

in normal Wistar rats, and an increase in this activity

began earlier [48]. The increase in 8-oxodG levels in

mtDNA with aging appears to be a general phenomenon

and has been reported by de Souza-Pinto et al. [49] and

Hudson et al. [50] The steady-state concentration of

8-oxodG in mitochondrial DNA was shown to be in-versely correlated with MLSP (maximum lifespan) in the

heart and brain of mammals, i.e., slowly aging mammals

show lower 8-oxodG levels in mtDNA than rapidly

aging ones [51]. Furthermore, these authors show that

this inverse relationship is restricted to mtDNA, because

8-oxodG levels in nuclear DNA were not significantly

correlated with MLSP. The correlationbetween 8-oxodG

and MLSP was better in the heart and the brain,

possibly, in part, because these organs are composed

predominantly of postmitotic cells [51].

(ii) Longer life expectancy in organismsbelonging to the same cohort group

is associated with relatively higher

levels of antioxidants and lower

concentrations of the products of

oxygen free radical reactionsAll houseflies lose the ability to fly prior to death.

Therefore, in an aging population, shorter-lived flies can

be identified as flightless crawlers in contrast with their

longer-lived cohorts, the fliers. The average lifespan of

crawlers is about one-third shorter than the fliers. Levels

of antioxidant defences [SOD (superoxide dismutase),

CAT (catalase) and glutathione] and products of oxygen

free radical reactions [inorganic peroxides and TBA(thio-

barbituric acid) reactants] were compared between craw-

lers and fliers. The fliers showed greater SOD and CAT

activities and glutathione concentrations than crawlers,

whereas the amount of inorganic peroxides (H2O2) and

TBA reactants was higher in the crawlers than in fliers

[52].

(iii) An experimentally induced decrease

in oxidative stress retards age-associated

deterioration at the organelle andorganismic levels and extends

chronological and metabolic life spanSimultaneous overexpression of Cu/ZnSOD and CAT in

Drosophilawas shown [53] to increase the maximum and

average lifespan by one-third, to retard the age-related

accumulation of oxidative damage to DNA and protein,

to increase resistance to the oxidative effects of X-ray ex-

posure, to attenuate the age-related increase in the rate of

mitochondrial H2O2 generation, to increase the speed

of walking and to increase the metabolic potential defined

as the total amount of oxygen consumed per unit body

weight during the adult life. Moreover, others have been



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role in the aging process [47,50,90,91]. The important role

played by DNA-repair enzymes in the accumulation of

this lesion is underlined by the fact that liver mtDNA

from knockout mice for OGG1 (the glycosylase that

recognizes this lesion) accumulates 30 times as much

8-oxodG as mtDNA of wild-type control mice [91].

Interestingly, several studies have indicated that OGG1activity in mitochondrial extracts from old rats is higher

compared with extracts from young animals, which is

apparently at odds with the observed accumulation of

8-oxodG in mtDNA from older animals [49,50,91,92].

However, this controversy was recently resolved by

Szczesny et al. [93], who have shown that, in both hepa-

tocytes from oldmice and in senescenthuman fibroblasts,

mitochondrial import of OGG1 is impaired and that a

significant fraction of this enzyme remains localized in

the outer membrane and intermembrane space in the pre-

cursor form. Because uracil-DNA glycosylase, another

BER enzyme, has a similar age-dependent impairment inmitochondrial import, these authors [93] conclude that

there appears to be a general deficiency in the import of

BER enzymes into mitochondria from senescent animals

and cells. Therefore, although age-related impairment in

mitochondrial protein import does not necessarily ex-

tend beyond some DNA repair enzymes, it can explain

the simultaneous accumulationof 8-oxodG and increased

in vitroOGG1 activity in mitochondria from senescent

animals in some settings. It should be noted, however,

that other studies point out that the ability of mitochon-

dria to import proteins is a dynamic function that can be

modulated by various stimuli, such as thyroid hormone

[94], which may provide an alternative explanation for

the observed phenomena. Moreover, the rate of mito-

chondrial import of matrix chaperonins GRP75(glucose-

regulated protein 75) and HSP60 (heat-shock protein

60), which are essential for the import of precursor pro-

teins, in fact increases with age [95]. Also, there is an

apparent lack of consensus in the literature regarding the

increase in mitochondrial OGG1 activity with aging in

both senescent cells and animals. Some groups have re-

ported a decline in both activity and expression of several

key mitochondrial repairenzymes, including OGG1[96

98]. Therefore, to date, there is no consensus on the ques-

tion of whether DNA repair declines with age [99]. Oneof the possible explanations for such discrepancies is in

theintrinsic variability between cell types andtissues with

respect of their reliance on antioxidant defences com-

pared with DNA repair for the maintenance of struc-

tural integrity of mtDNA. Results from our laboratory

[100,101] suggest that the differential susceptibility of

glial cell types to oxidative damage and apoptosis does

not appear to be related to cellular antioxidant capacity,

but rather to differences in their ability to repair oxida-

tive mtDNA damage [100], and that different glial cell

types differ in their capacity to repair oxidative damage

[101].

CHALLENGES TO THE MITOCHONDRIAL

THEORY OF AGING

Recently, the rate of ROS generation by mitochondria

under physiological conditions, which is a cornerstone

of the mitochondrial theory of aging, has been critically

re-examined by several groups. Hansford et al. [102]have found that active H2O2 production (an indirect

measure of O2 generation) requires both a high frac-

tional reduction of complex I (indexed by NADH/

NAD+:NADH ratio) and a high membrane potential,

. These conditions are only achieved with supra-

physiological concentrations of succinate. With physio-

logical concentrations of NAD-linked substrates, rates of

H2O2 formation are much lower (less than 0.1 % of re-

spiratory chain electron flux). This H2O2 production

may be stimulated by the complex III inhibitor antimy-

cin A, but not by myxothiazol [102]. Staniek and Nohl

[103,104] reported further that mitochondria respiringon complex I and complex II substrates generate detec-

table H2O2 only in the presence of the complex III

inhibitor antimycin. They also suggested that the rates

of mitochondrial H2O2 production reported by others

are artificially high due to flaws in experimental design

[103,104]. Martin Brands group [105] capitalized on

these findings and used an improved experimental design

to show that mitochondria do not release measurable

amounts of superoxide or H2O2 when respiring on

complex I or complex II substrates, but release significant

amounts of superoxide from complex I when respiring

on palmitoyl carnitine. However, even at saturating con-

centrations of palmitoyl carnitine, in their estimation,

only0.15 % oftheelectron flow gives riseto H2O2under

resting conditions with a respiration rate of 200 nmol

electrons min1 mg1 of mitochondrial protein. Under

physiological conditions, this rate should be even lower

due to (i) lower partial oxygen pressure, (ii) lower

concentration of palmitoyl carnitine and (iii) lower mito-

chondrial membrane potential. Therefore, under physio-

logical conditions in cells with uncompromised antioxi-

dant defences, ROS are produced by ETC in quantities

that can be efficiently scavenged by mitochondrial anti-

oxidant systems. As a consequence, no significant oxi-

dative damage should be inflicted on mitochondrialcomponents, including mtDNA due to electron leak

from ETC, provided that cells have normal levels of

antioxidants. This conclusion is in agreement with the

observations of Orr et al. [106], who have recently re-

examined their earlier findings and those of others on the

effectof overexpressionof antioxidantenzymes on exten-

sion of Drosophila lifespan. They have found that

significant increases in the activities of both Cu/ZnSOD

and CAT had no beneficial effect on survivorship in

relatively long-livedy w mutant flies, and were associated

with slightly decreased life spans in wild-type flies of

the Oregon-R strain. The introduction of additional



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transgenes encoding MnSOD or thioredoxin reductase in

the same genetic background also failed to cause life span

extension. These authors [106] conclude that increasing

the activities of major antioxidative enzymes above wild-

type levels does not decrease the rate of aging in long-

lived strains ofDrosophila, although there may be some

effect in relatively short-lived strains [106]. In line withthis conclusion, Van Remmen et al. [107] in their study

of mice heterozygous for the MnSOD gene knockout

have found that, although life-long reduction of MnSOD

activity leads to increased levels of oxidative damage to

mitochondrial and nuclear DNA and increased cancer

incidence, it does not appear to affect aging.

A report on the requirement of cytosolic CAT for

phenotypic lifespan extension inCaenorhabditis elegans

daf-C and clk-1 mutants [108] was recently retracted

[109], dealing another blow to the notion that lifespan can

be routinely enhanced by increasingantioxidantdefences.

Rasmussen et al. [110,111] assayed 13 different en-zyme activities using optimized preparation techniques

and found that the central bioenergetic systems, includ-

ing pyruvate dehydrogenase, tricarboxylic acid cycle,

respiratory chain and ATP synthesis, appeared unaltered

with age. Maklashina and Ackrell [112] have recently

critically examined the literature regarding the role of

ETC dysfunction in aging. They conclude that the

available evidence for age-related inactivation of the res-

piratory chain can be challenged because of the prepar-

ation purity and the use of inadequate assay procedures,

and that recent experimental evidence does not support

the mitochondrial theory of aging.

In contrast, Jacobs [113] does not so much challenge

experimental evidence supporting the mitochondrial

theory of aging as to point out that all the evidence

available to date is indirect in its nature. He argues that

studiesperformedsofardonotaddressthecriticalissueof

cause andeffect, i.e. does thesomatic mutation of mtDNA

result in oxidative phosphorylation dysfunction and

increased oxidative stress? Does increased oxidative stress

promote mtDNA mutagenesis? Finally, he points out that

the results from his own laboratory [114] suggest that,

at least in a tissue culture model, progressive accumul-

ation of mtDNA mutations due to expression of proof-

reading-deficient DNA polymerase does not lead tosignificant phenotypic changes, despite the accumulation

of mtDNA mutations to the level three times higher than

that found in aged tissues. However, very recently [115],

the concerted effort of several groups (including that of

Howard Jacobs) led by Nils-Goran Larsson resulted in

the generationof homozygousknock-inmice thatexpress

a proofreading-deficient catalytic subunit of mitochon-

drial DNA polymerase . These mice develop an

mtDNA mutator phenotype with a 35-fold increase

in the levels of point mutations, as well as increased

amounts of deleted mtDNA. This increase in somatic

mtDNA mutations is associated with reduced lifespan

and the premature onset of age-related phenotypes such

as weight loss, reduced subcutaneous fat, alopecia (hair

loss), kyphosis (curvature of the spine), osteoporosis,

anaemia, reduced fertility and heart enlargement [115].

Thus results of this study provide the best evidence so

far for a causative link between mtDNA mutations and

aging phenotypes in mammals. However, as these authorsconcede, the detailed kinetics of the accumulation of

somatic mtDNA mutations remain to be elucidated. The

mutation load in the brain of mutator mice at 2 months

of age is already 23-fold greater than in 6-month-

old wild-type littermates. This, and the rather uniform

mutation loads between tissues, suggests that much of the

accumulation of mutations may occur during embryonic

and/or fetal development. Also, the onset of premature

aging in this model is not accompanied, temporally, by a

large de novo accumulation of mtDNA mutations

around 6 months. Therefore it appears plausible that the

premature onset of aging in this model is the resultof the cumulative physiological damage caused by the

high mutation load present during adult life and/or to

segregation or clonal expansion of specific mutations, as

supported by the observed mosaicism for the respiratory

chain deficiency found in the heart. However, since the

effects of high mutational burden in mtDNA during

embryonic and fetal development are poorly understood,

one has to consider the possibility that premature aging

in this model is predetermined at the prenatal, rather

than postnatal, stage. If this holds true, then the issue of

causative relationship between mtDNA mutations and

normal aging remains open, since, in normal aging, accu-

mulation of mtDNA mutations occurs postnatally. The

observation that knock-in mice show some features (e.g.

alopecia) that are more characteristic of human, rather

than mouse aging lends some credibility to the notion

that a high mutational load in mtDNA at the prenatal

stage might be an important limitation of this model.

Clearly, a model with inducible mtDNA mutator pheno-

type should help to resolve many of these outstanding

issues. Finally, mtDNA mutations in this study [115] are

generated by a mutator DNA polymerase rather than

by oxidative DNAdamage. Therefore this study does not

address one of the central premises of the mitochondrial

theory of aging, namely that oxidative mtDNA damageis the driving force behind the accumulation of mtDNA

mutations.

In the preceding paragraphs we have discussed support

provided for the mitochondrial theory of aging by the

discovery of mitochondrial disease. One might then re-

asonably expect, based on the mitochondrial theory of

aging, that mitochondrial ROS would be causative in

a significant fraction of pathogenic mtDNA mutations.

However, an analysis of 188 pathogenic mtDNA point

mutations [65] revealed that the mutagenic effect of

8-oxodG, widely regarded as a prime lesion resulting

from an oxidative insult to DNA, can be implicated in



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the aetiology of only a few mutations. Indeed, unrepaired

8-oxodG in mtDNA can pair with both C and A with

almost equal efficiency resulting in GT (and CA

on complementary strand) transversions, which account

for only 5.9 % of pathogenic mtDNA mutations. Even

when the potentially mutagenic pool of 8-oxodGTP

(8-oxodeoxyGTP), the product of cytoplasmic/matrixdGTP pool oxidation) is taken in consideration (TG

and AC transversions), the cumulative impact of both

types of mutation is still only 8.5 %. For comparison,

82 % (almost 10 times as many) of the pathogenic point

mutations in mtDNA can be attributed to deamination of

adenine and cytosine. 8-oxodG is not a prime oxidative

mutagenic lesion, because it is efficiently repaired by

BER pathways. Thus the exact factors required for the

accumulation of point mutations have yet to be fully

defined. Oxidative DNA damage can produce a variety

of base lesions whose mutagenic potential has not been

fully elucidated [116]. Therefore it is possible that other,less prominent and, at present, unidentified lesions are

responsible for the bulk of ROS-mediated mutagenesis.

Alternatively, it can be postulated that ROS do not play

a major role in mtDNA mutagenesis.

CONCLUSIONS

Aging is a complex multifactorial process which we

have only begun to understand. Although the available

evidence strongly suggests that mitochondria play a role

in this process, there appears to be a wide range of opi-

nions as to the exact nature of the involvement ofmitochondria in aging. In this review, we have made an

attempt to present in a balanced fashion theexisting views

on the involvement of mtDNA in aging. Clearly, more

research must be done to fully elucidate how damage to

mtDNA contributes to the aging process. It is possible

that the seemingly contradictory results of different

studies will be reconciled or explained through the use of

integrative approaches and unified model systems. The

next few years of aging research should prove exciting

as our knowledge expands dramatically as the results

of studies from several laboratories, now experimentally

testing the mitochondrial theory of aging, are published.

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Received 26 May 2004/22 July 2004; accepted 28 July 2004

Published as Immediate Publication 28 July 2004, DOI 10.1042/CS20040148


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