Mitochondrial and nuclear DNA base excision repair are affected differently by caloric restriction

Caloric restriction and genomic stability

Caloric restriction (CR) reduces the incidence and progression of spontaneous and induced tumors in laboratory rodents while increasing mean and maximum life spans. It has been suggested that CR extends longevity and reduces age-related pathologies by reducing the levels of DNA damage and mutations that accumulate with age. This hypothesis is attractive because the integrity of the genome is essential to a cell/organism and because it is supported by observations that both cancer and immunological defects, which increase significantly with age and are delayed by CR, are associated with changes in DNA damage and/or DNA repair. Over the last three decades, numerous laboratories have examined the effects of CR on the integrity of the genome and the ability of cells to repair DNA. The majority of studies performed indicate that the age-related increase in oxidative damage to DNA is significantly reduced by CR. Early studies suggest that CR reduces DNA damage by enhancing DNA repair. With the advent of genomic technology and our increased understanding of specific repair pathways, CR has been shown to have a significant effect on major DNA repair pathways, such as NER, BER and double-strand break repair.

Highly resistant macromolecular components and low rate of generation of endogenous damage: two key traits of longevity

Key characteristics relating oxidative damage to aging and longevity are reviewed. Available information indicates that the specific composition of tissue macromolecules (proteins, lipids and mitochondrial DNA) in long-lived animal species gives them an intrinsically high resistance to modification that likely contributes to the superior longevity of these species. This is obtained in the case of lipids by decreasing fatty acid unsaturation, and in the proteins by lowering their methionine content. Long-lived animals also show low rates of reactive oxygen species (ROS) generation and oxidative damage at their mitochondria. On the other hand, dietary restriction decreases mitochondrial ROS production and oxidative damage to mitochondrial DNA and proteins. These changes are due to the decreased intake of dietary proteins (not of lipids or carbohydrates) of the dietary restricted animals. In turn, these effects of protein restriction seem to be specifically due to the lowered methionine intake of the protein and dietary restricted animals. It is emphasized that both a low rate of generation of endogenous damage and an intrinsically high resistance to modification of tissue macromolecules are key traits of animal longevity.

Dietary and genetic modulation of DNA repair in healthy human adults

The DNA in all cells of the human body is subject to damage continuously from exogenous agents, internal cellular processes and spontaneous decomposition. Failure to repair such damage is fundamental to the development of many diseases and to ageing. Fortunately, the vast majority of DNA damage is detected and repaired by one of five complementary DNA repair systems. However, recent studies have shown that even in healthy individuals there is a wide inter-individual variation in DNA repair capacity. Part of this variation can be accounted for by polymorphisms in the genes encoding DNA repair proteins. However, it is probable that environmental factors, including dietary exposure as well as diet-gene interactions, are also responsible for much of the difference in repair capacity between individuals. Whilst there is some evidence from human studies that generalised malnutrition or low intakes of specific nutrients may affect DNA repair, as yet there is limited understanding of the molecular mechanisms through which nutrients can modulate this key cellular process.

DNA repair, mitochondria, and neurodegeneration

Accumulation of nuclear and mitochondrial DNA damage is thought to be particularly deleterious in post-mitotic cells, which cannot be replaced through cell division. Recent experimental evidence demonstrates the importance of DNA damage responses for neuronal survival. Here, we summarize current literature on DNA damage responses in the mammalian CNS in aging and neurodegeneration. Base excision repair (BER) is the main pathway for the removal of small DNA base modifications, such as alkylation, deamination and oxidation, which are generated as by-products of normal metabolism and accumulate with age in various experimental models. Using neuronal cell cultures, human brain tissue and animal models, we and others have shown an active BER pathway functioning in the brain, both in the mitochondrial and nuclear compartments. Mitochondrial DNA repair may play a more essential role in neuronal cells because these cells depend largely on intact mitochondrial function for energy metabolism. We have characterized several BER enzymes in mammalian mitochondria and have shown that BER activities change with age in mitochondria from different brain regions. Together, the results reviewed here advocate that mitochondrial DNA damage response plays an important role in aging and in the pathogenesis of neurodegenerative diseases.

Calorie restriction in nonhuman primates: assessing effects on brain and behavioral aging

Dietary caloric restriction (CR) is the only intervention repeatedly demonstrated to retard the onset and incidence of age-related diseases, maintain function, and extend both lifespan and health span in mammals, including brain and behavioral function. In 70 years of study, such beneficial effects have been demonstrated in rodents and lower animals. Recent results emerging from ongoing studies of CR in humans and nonhuman primates suggest that many of the same anti-disease and anti-aging benefits observed in rodent studies may be applicable to long-lived species. Results of studies in rhesus monkeys indicate that CR animals (30% less than controls) are healthier than fully-fed counterparts based on reduced incidence of various diseases, exhibit significantly better indices of predisposition to disease and may be aging at a slower rate based on analysis of selected indices of aging. The current review discusses approaches taken in studies of rhesus monkeys to analyze age-related changes in brain and behavioral function and the impact of CR on these changes. Approaches include analyses of gross and fine locomotor performance as well as brain imaging. In a related study it was observed that short-term CR (6 months) in adult rhesus monkeys can provide protection against a neurotoxic insult. Increasing interest in the CR paradigm will expand its role in demonstrating how nutrition can modulate the rate of aging and the mechanisms responsible for this modulation.

8-Oxoguanosine and uracil repair of nuclear and mitochondrial DNA in red and white skeletal muscle of exercise-trained old rats

Oxoguanine DNA glycosylase (OGG1) and uracil DNA glycosylase (UDG) are two of the most important repair enzymes that are involved in the base excision repair processes to eliminate oxidative damage from mammalian DNA, which accumulates with aging. Red and white skeletal muscle fibers have very different antioxidant enzyme activities and resistance to oxidative stress. In this paper, we demonstrate that the activity of OGG1 is significantly higher in the red type of skeletal muscle compared with white fibers from old rats. Exercise training resulted in increased OGG1 activity in the nuclei of red fibers and decreased activity in nuclei of white fibers and in the mitochondria of both red and white fibers. The activities of UDG were similar in both red and white muscle fibers. Exercise training appears to increase the activity of UDG in the nuclei and mitochondria. However, exercise training affects the activity of OGG1 in nuclei and mitochondria differently, suggesting different regulation of the enzymes. In contrast, UDG showed similar activities in nuclei and mitochondrial extracts of exercise-trained animals. These data provide evidence for differential regulation of UDG and OGG1 in maintaining fidelity of DNA in oxidatively stressed cells.

Mitochondrial DNA repair: a critical player in the response of cells of the CNS to genotoxic insults

Cells of the CNS are constantly exposed to agents which damage DNA. Although much attention has been paid to the effects of this damage on nuclear DNA, the nucleus is not the only organelle containing DNA. Within each cell, there are hundreds to thousands of mitochondria. Within each mitochondrion are multiple copies of the mitochondrial genome. These genomes are extremely vulnerable to insult and mutations in mitochondrial DNA (mtDNA) have been linked to several neurodegenerative diseases, as well as the normal process of aging. The principal mechanism utilized by cells to avoid DNA mutations is DNA repair. Multiple pathways of DNA repair have been elucidated for nuclear DNA. However, it appears that only base excision repair is functioning in mitochondria. This repair pathway is responsible for the removal of most endogenous damage including alkylation damage, depurination reactions and oxidative damage. Within the rat CNS, there are cell-specific differences mtDNA repair. Astrocytes exhibit efficient repair, whereas, other glial cell types and neuronal cells exhibit a reduced ability to remove lesions from mtDNA. Additionally, a correlation was observed between those cells with reduced mtDNA repair and an increase in the induction of apoptosis. To demonstrate a causative relationship, a strategy of targeting DNA repair proteins to mitochondria to enhance mtDNA repair capacity was employed. Enhancement of mtDNA repair in oligodendrocytes provided protection from reactive oxygen species- and cytokine-induced apoptosis. These experiments provide a novel strategy for protecting sensitive CNS cells from genotoxic insults and thus provide new treatment options for neurodegenerative diseases.

Haploinsufficiency in DNA polymerase beta increases cancer risk with age and alters mortality rate

This study uses a base excision repair (BER)-deficient model, the DNA polymerase beta heterozygous mouse, to investigate the effect of BER deficiency on tumorigenicity and aging. Aged beta-pol(+/-) mice express 50% less beta-pol transcripts and protein (P < 0.05) than aged beta-pol(+/+) mice, showing maintenance of the heterozygous state over the life span of the mouse. This reduction in beta-pol expression was not associated with an increase in mutation rate but was associated with a 100% increase in the onset of hypoploidy. Aged beta-pol(+/-) mice exhibited a 6.7-fold increase in developing lymphoma (P < 0.01). Accordingly, 38% of beta-pol(+/-) mice exhibited lymphoid hyperplasia, whereas none of the beta-pol(+/+) exhibited this phenotype. beta-pol(+/-) mice were also more likely to develop adenocarcinoma (2.7-fold increase; P < 0.05) and more likely to develop multiple tumors, as 20% of the beta-pol(+/-) animals died bearing multiple tumors compared with only 5% of the beta-pol(+/+) animals (P < 0.05). In spite of accelerated tumor development, no gross effect of beta-pol heterozygosity was seen with respect to life span. However, the survival curves for the beta-pol(+/+) and beta-pol(+/-) mice are not identical. A maximum likelihood estimation analysis showed a modest but significant (P < 0.05) acceleration of the age-dependent mortality rate in beta-pol(+/-) mice. Thus, the beta-pol(+/-) mouse represents a model in which mortality rate and tumor development are accelerated and provides evidence supporting the role of genomic maintenance in both aging and carcinogenesis.

No decline in skeletal muscle oxidative capacity with aging in long-term calorically restricted rats: effects are independent of mitochondrial DNA integrity

We investigated if calorie restriction (CR) preserved skeletal muscle oxidative capacity with aging after accounting for life span extension by CR, and determined if mitochondrial content, mitochondrial DNA integrity, and peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) were involved. Ad libitum-fed (AL) and CR animals representing young adult, late middle age, and senescence were studied. Whereas citrate synthase and complex IV activities were lower in plantaris and gastrocnemius muscle of young adult CR animals, in contrast to the 15%-40% decline in senescent AL animals, there was no decline with aging in CR animals. There was no decline in citrate synthase protein in gastrocnemius with aging in either group, suggesting that CR preserves oxidative capacity with aging by protecting mitochondrial function rather than content. This protection was independent of mitochondrial DNA damage between groups. However, there was a slower decline in PGC-1alpha gene expression with aging in CR versus AL animals, suggesting a better maintenance of mitochondrial biogenesis with aging in CR animals.

Longevity in mice: is stress resistance a common factor?

A positive relationship between stress resistance and longevity has been reported in a multitude of studies in organisms ranging from yeast to mice. Several mouse lines have been discovered or developed that exhibit extended longevities when compared with normal, wild-type mice of the same genetic background. These long-living lines include the Ames dwarf, Snell dwarf, growth hormone receptor knockout (Laron dwarf), IGF-1 receptor heterozygote, Little, alpha-MUPA knockout, p66(shc) knockout, FIRKO, mClk-1 heterozygote, thioredoxin transgenic, and most recently the Klotho transgenic mouse. These mice are described in terms of the reported extended lifespans and studies involving resistance to stress. In addition, caloric restriction (CR) and stress resistance are briefly addressed for comparison with genetically altered mice. Although many of the long-living mice have GH/IGF-1/insulin signaling-related alterations and enhanced stress resistance, there are some that exhibit life extension without an obvious link to this hormone pathway. Resistance to oxidative stress is by far the most common system studied in long-living mice, but there is evidence of enhancement of resistance in other systems as well. The differences in stress resistance between long-living mutant and normal mice result from complex interrelationships among pathways that appear to coordinate signals of growth and metabolism, and subsequently result in differences in lifespan.

Bioenergetics of aging and calorie restriction

Aging is a physiological process that involves a multi-factorial set of deleterious changes. These alterations are caused by an exponential increase in damage to macromolecules. This process is likely due to the cumulative effects of oxidative stress over time. One area of ongoing research in gerontology has focused on determining why there is an age-dependent decrease in cellular bioenergetics. The aim of this review is to summarize the recent findings on the effects of aging and calorie restriction on energy metabolism. The effect of calorie restriction on age-associated changes in bioenergetic parameters will be examined.

Is the mitochondrial free radical theory of aging intact?

The present state of the mitochondrial free radical theory of aging is reviewed. Available studies do not support the hypothesis that antioxidants control the rate of aging because: (a) they correlate inversely with maximum longevity in vertebrates, and (b) increasing their concentration by different methods does not increase maximum lifespan. On the other hand, comparative studies consistently show that long-lived mammals and birds have low rates of mitochondrial reactive oxygen species (ROS) production and low levels of oxidative damage in their mitochondrial DNA. Furthermore, caloric restriction, which extends longevity, also decreases mitochondrial ROS production at complex I and lowers mtDNA oxidative damage. Recent data show that these changes can also be obtained with protein restriction without strong caloric restriction. Another trait of long-lived mammals and birds is the possession of low degrees of unsaturation in their cellular membranes. This is mainly due to minimizing the presence of highly unsaturated fatty acids such as 22:6n-3 and emphasizing the presence of less unsaturated fatty acids such as 18:2n-6 in long-lived animals, without changing the total amount of polyunsaturated fatty acids. This leads to lower levels of lipid peroxidation and lipoxidation-derived protein modification in long-lived species. Taken together, available information is consistent with the predictions of the mitochondrial free radical theory of aging, although definitive proof and many mechanistic details are still lacking.

Energy, quiescence and the cellular basis of animal life spans

Animals are routinely faced with harsh environmental conditions in which insufficient energy is available to grow and reproduce. Many animals adapt to this challenge by entering a dormant, or quiescent state. In some animals, such as the nematode Caenorhabditis elegans, quiescence is coincident with increased stress resistance and longevity. Here we review evidence that the rules of life span extension established in C. elegans may be generally true of most animals. That is, that the rate of animal aging correlates inversely with cellular resistance to physiological stress, particularly oxidative stress, and that stress resistance is co-regulated with the quiescence adaptation (where the latter occurs). We discuss evidence for highly conserved intracellular signalling pathways involved in energy sensing that are sensitive to aspects of mitochondrial energy transduction and can be modulated in response to energetic flux. We provide a broad overview of the current knowledge of the relationships between energy, metabolism and life span.

Repair of mitochondrial DNA in aging and carcinogenesis

Mitochondria are responsible for the generation of energy in the form of adenosine triphosphate. These organelles contain their own genetic material, mitochondrial (mt) DNA. This mtDNA has been hypothesized to play a role in the processes of aging and carcinogenesis. Initial reports have shown that there is no repair of cyclobutylpyrimidine dimers (CPD). More recent reports indicate however, that the mitochondrion contains several defence mechanisms against endogenous or exogenous damaging agents such as ultraviolet radiation or oxidative damage. The role of these defence mechanisms in the removal of mitochondrial DNA damage and the link to aging and carcinogenesis-associated processes are discussed in this review.

Effects of Aging and Anti-Aging Caloric Restrictions on Carbonyl and Heat Shock Protein Levels and Expression

Heat shock proteins (Hsps) are induced by stressful stimuli and have been shown to protect cells and organs from such stresses both in vitro and in vivo, and play a positive role in lifespan determination. An attenuated response to stress is characteristic of senescence and no Hsp induction is observed upon exposure to stress and no protective effect of a mild stress is observed in cells from aged individuals. The artificial over-expression of Hsps, can produce a protective effect against a variety of damaging stimuli in cells from aged rats or aged humans, in whom cardiovascular disease is a major cause of morbidity in older age. Here, we show that aging significantly decreases the levels of Hsp27, Hsp60, Hsp72 and Hsc70 in right atrium and left ventricle of the rat heart, both at level of protein and of mRNA. Two different caloric restriction regimens have been found to counteract in part the decrease in the levels of Hsp expression in the aged heart tissue as well as the tendency to an increase of the levels of carbonyl in cardiac proteins. Our data suggest that cardiac Hsp levels may be a determinant of longevity in rodents, and that generation of new regimens of caloric restriction may eventually show how to improve modulation of cardiac aging.

Toward a unified theory of caloric restriction and longevity regulation

The diet known as calorie restriction (CR) is the most reproducible way to extend the lifespan of mammals. Many of the early hypotheses to explain this effect were based on it being a passive alteration in metabolism. Yet, recent data from yeast, worms, flies, and mammals support the idea that CR is not simply a passive effect but an active, highly conserved stress response that evolved early in life's history to increase an organism's chance of surviving adversity. This perspective updates the evidence for and against the various hypotheses of CR, and concludes that many of them can be synthesized into a single, unifying hypothesis. This has important implications for how we might develop novel medicines that can harness these newly discovered innate mechanisms of disease resistance and survival.

Dietary restriction at old age lowers mitochondrial oxygen radical production and leak at complex I and oxidative DNA damage in rat brain

Previous studies in mammalian models indicate that the rate of mitochondrial reactive oxygen species ROS production and the ensuing modification of mitochondrial DNA (mtDNA) link oxidative stress to aging rate. However, there is scarce information concerning this in relation to caloric restriction (CR) in the brain, an organ of maximum relevance for ageing. Furthermore, it has never been studied if CR started late in life can improve those oxidative stress-related parameters. In this investigation, rats were subjected during 1 year to 40% CR starting at 24 months of age. This protocol of CR significantly decreased the rate of mitochondrial H(2)O(2) production (by 24%) and oxidative damage to mtDNA (by 23%) in the brain below the level of both old and young ad libitum-fed animals. In agreement with the progressive character of aging, the rate of H(2)O(2) production of brain mitochondria stayed constant with age. Oxidative damage to nuclear DNA increased with age and this increase was fully reversed by CR to the level of the young controls. The decrease in ROS production induced by CR was localized at Complex I and occurred without changes in oxygen consumption. Instead, the efficiency of brain mitochondria to avoid electron leak to oxygen at Complex I was increased by CR. The mechanism involved in that increase in efficiency was related to the degree of electronic reduction of the Complex I generator. The results agree with the idea that CR decreases aging rate in part by lowering the rate of free radical generation of mitochondria in the brain.

Creatine supplementation normalizes mutagenesis of mitochondrial DNA as well as functional consequences

Mutations of mitochondrial (mt) DNA play a role in neurodegeneration, normal aging, premature aging of the skin (photoaging), and tumors. We and others could demonstrate that mtDNA mutations can be induced in skin cells in vitro and in normal human skin in vivo by repetitive, sublethal ultraviolet (UV)-A-irradiation. These mutations are mediated by singlet oxygen and persist in human skin as long-term biomarkers of UV exposure. Although mtDNA exclusively encodes for the respiratory chain, involvement of the energy metabolism in mtDNA mutagenesis and a protective role of the energy precursor creatine have thus far not been shown. We assessed the amount of a marker mutation of mtDNA, the so-called common deletion, by real-time PCR. Induction of the common deletion was paralleled by a measurable decrease of oxygen consumption, mitochondrial membrane potential, and ATP content, as well as an increase of matrix metalloproteinase-1. Mitochondrial mutagenesis as well as functional consequences could be normalized by increasing intracellular creatine levels. These data indicate that increase of the energy precursor creatine protects from functionally relevant, aging-associated mutations of mitochondrial DNA.

Minireview: the role of oxidative stress in relation to caloric restriction and longevity

Reduction of caloric intake without malnutrition is one of the most consistent experimental interventions that increases mean and maximum life spans in different species. For over 70 yr, caloric restriction has been studied, and during the last years the number of investigations on such nutritional intervention and aging has dramatically increased. Because caloric restriction decreases the aging rate, it constitutes an excellent approach to better understand the mechanisms underlying the aging process. Various investigations have reported reductions in steady-state oxidative damage to proteins, lipids, and DNA in animals subjected to restricted caloric intake. Most interestingly, several investigations have reported that these decreases in oxidative damage are related to a lowering of mitochondrial free radical generation rate in various tissues of the restricted animals. Thus, similar to what has been described for long-lived animals in comparative studies, a decrease in mitochondrial free radical generation has been suggested to be one of the main determinants of the extended life span observed in restricted animals. In this study we review recent reports of caloric restriction and longevity, focusing on mitochondrial oxidative stress and the proposed mechanisms leading to an extended longevity in calorie-restricted animals.

Mitochondrial and nuclear DNA-repair capacity of various brain regions in mouse is altered in an age-dependent manner

Aging is associated with increased susceptibility to neuronal loss and disruption of cerebral function either as a component of senescence, or as a consequence of neurodegenerative disease or stroke. Here we report differential changes in the repair of oxidative DNA damage in various brain regions during aging. We evaluated mitochondrial and nuclear incision activities of oxoguanine DNA glycosylase (OGG1), uracil DNA glycosylase (UDG) and the endonuclease III homologue (NTH1) in the caudate nucleus (CN), frontal cortex (FC), hippocampus (Hip), cerebellum (CE) and brain stem (BS) of 6- and 18-month-old male C57Bl/6 mice. We observed a significant age-dependent decrease in incision activities of all three glycosylases in the mitochondria of all brain regions, whereas variable patterns of changes were seen in nuclei. No age- or region-specific changes were observed in the mitochondrial repair synthesis incorporation of uracil-initiated base-excision repair (BER). We did not observe any age or region dependent differences in levels of BER proteins among the five brain regions. In summary, our data suggest that a decreased efficiency of mitochondrial BER-glycosylases and increased oxidative damage to mitochondrial DNA might contribute to the normal aging process. These data provide a novel characterization of oxidative DNA damage processing in different brain regions implicated in various neurodegenerative disorders, and suggest that this process is regulated in an age-dependent manner. Manipulation of DNA repair mechanisms may provide a strategy to prevent neuronal loss during age-dependent neurodegenerative disorders.

Ageing, oxidative stress, and mitochondrial uncoupling

Mitochondria are a cell's single greatest source of reactive oxygen species. Reactive oxygen species are important for many life sustaining processes of cells and tissues, but they can also induce cell damage and death. If their production and levels within cells is not effectively controlled, then the detrimental effects of oxidative stress can accumulate. Oxidative stress is widely thought to underpin many ageing processes, and the oxidative stress theory of ageing is one of the most widely acknowledged theories of ageing. As well as being the major source of reactive oxygen species, mitochondria are also a major site of oxidative damage. The purpose of this review is a concise and current review of the effects of oxidative stress and ageing on mitochondrial function. Emphasis is placed upon the roles of mitochondrial proton leak, the uncoupling proteins, and the anti-ageing effects of caloric restriction.

Imbalanced Base Excision Repair in Response to Folate Deficiency Is Accelerated by Polymerase β Haploinsufficiency

The mechanism by which folate deficiency influences carcinogenesis is not well established, but a phenotype of DNA strand breaks, mutations, and chromosomal instability suggests an inability to repair DNA damage. To elucidate the mechanism by which folate deficiency influences carcinogenicity, we have analyzed the effect of folate deficiency on base excision repair (BER), the pathway responsible for repairing uracil in DNA. We observe an up-regulation in initiation of BER in liver of the folate-deficient mice, as evidenced by an increase in uracil DNA glycosylase protein (30%, p < 0.01) and activity (31%, p < 0.05). However, no up-regulation in either BER or its rate-determining enzyme, DNA polymerase beta (beta-pol) is observed in response to folate deficiency. Accordingly, an accumulation of repair intermediates in the form of DNA single strand breaks (37% increase, p < 0.03) is observed. These data indicate that folate deficiency alters the balance and coordination of BER by stimulating initiation without subsequently stimulating the completion of repair, resulting in a functional BER deficiency. In directly establishing that the inability to induce beta-pol and mount a BER response when folate is deficient is causative in the accumulation of toxic repair intermediates, beta-pol-haploinsufficient mice subjected to folate deficiency displayed additional increases in DNA single strand breaks (52% increase, p < 0.05) as well as accumulation in aldehydic DNA lesions (38% increase, p < 0.01). Since young beta-polhaploinsufficient mice do not spontaneously exhibit increased levels of these repair intermediates, these data demonstrate that folate deficiency and beta-pol haploinsufficiency interact to increase the accumulation of DNA damage. In addition to establishing a direct role for beta-pol in the phenotype expressed by folate deficiency, these data are also consistent with the concept that repair of uracil and abasic sites is more efficient than repair of oxidized bases.

p53 functions in the incorporation step in DNA base excision repair in mouse liver mitochondria

The tumor suppressor p53 protein stimulates nuclear base excision repair (BER) in vitro. In response to certain cellular stresses, p53 translocates to mitochondria, where it can trigger an apoptotic response. However, a potential role for p53 in modulating mitochondrial DNA repair has not yet been examined. In this study, we show that p53 also modulates mitochondrial BER. Uracil-initiated BER incorporation, which measures flux through the entire BER pathway, was lower in mitochondrial extracts from nonstressed p53 knockout mice than in wild type. The addition of recombinant p53 complemented the BER incorporation in p53 knockout extracts and stimulated BER in wt extracts. The activities of three major mitochondrial DNA glycosylases were similar in extracts from wild-type and knockout animals. Likewise, AP endonuclease activity was unaffected by the absence of p53. Gel shift experiments with recombinant p53 demonstrated that p53 did not bind to the uracil-containing substrate used in the repair assay. Polymerase gamma gap-filing activity was less efficient in p53 knockout extracts, but it was complemented with the addition of recombinant p53. Thus, we conclude that p53 may participate in mtBER by stimulating the repair synthesis incorporation step.