DNA damage, mutation and fine structure DNA repair in aging

Does measurement of oxidative damage to DNA have clinical significance?

Oxidative damage to DNA is the seemingly inevitable consequence of cellular metabolism. Furthermore, despite protective mechanisms, cellular levels of damage may increase under conditions of oxidative stress, arising from exposure to a variety of physical or chemical insults. Elevated levels of oxidatively damaged DNA have been measured in numerous diseases, and as a result, it has been hypothesised that such damage plays an integral role in the aetiology of that disease. This review examines the validity of this hypothesis, exploring the mechanisms by which oxidative DNA damage may lead to disease. We conclude that further validation of biomarkers of oxidative DNA damage, along with further elucidation of the role of damage in disease, may allow these biomarkers to become potentially useful clinical tools.

The interdependence of skin aging, skin cancer, and DNA repair capacity: a novel perspective with therapeutic implications

The human body is constantly exposed to exogenous and endogenous insults that threaten its genomic integrity and that lead to changes at the molecular, biochemical, and cellular levels. As a major interface between the environment and the internal milieu, our skin is especially subject to such events. Common insults include but are not limited to infectious agents, environmental pollutions and toxins, carcinogens, and ultraviolet (UV) irradiation. It is estimated that there are thousands of DNA alterations in each cell daily. Therefore, if not efficiently repaired, our genome would rapidly be destroyed. This review focuses predominantly on UV-induced DNA damage in human skin, protective molecular responses to UV damage, and the consequences of these opposing forces for aging and photocarcinogenesis.

Cellular aging of mitochondrial DNA-depleted cells

We have reported that mitochondrial DNA-depleted rho(0) cells are resistant to cell death. Because aged cells have frequent mitochondrial DNA mutations, the resistance of rho(0) cells against cell death might be related to the apoptosis resistance of aged cells and frequent development of cancers in aged individuals. We studied if rho(0) cells have features simulating aged cells. SK-Hep1 hepatoma rho(0) cells showed typical morphology associated with aging such as increased size and elongated appearance. They had increased senescence-associated beta-Gal activity, lipofuscin pigment, and plasminogen activator inhibitor-1 expression. Consistent with their decreased proliferation, the expression of mitotic cyclins was decreased and that of cdk inhibitors was increased. Rb hypophosphorylation and decreased telomerase activity were also noted. Features simulating aged cells were also observed in MDA-MB-435 rho(0) cells. These results support the mitochondrial theory of aging, and suggest that rho(0) cells could serve as an in vitro model for aged cells.

DNA repair in human fibroblasts, as reflected by host-cell reactivation of a transfected UV-irradiated luciferase gene, is not related to donor age

The effect of donor age on the ability of mammalian cells to repair ultraviolet (UV)-induced DNA damage has been studied using several approaches, most recently via assays that measure the host-cell reactivation (HCR) of UV-irradiated reporter gene-containing plasmid vectors following their transfection into cells. Plasmid HCR assays indirectly quantify a cell line's ability to perform nucleotide excision repair (NER) by measuring the enzyme activity of the repaired reporter gene, e.g., chloramphenical acetyltransferase (cat) or luciferase (luc), and are useful in studies investigating whether increasing age may be a risk factor for the deficient repair of potentially cancer-causing, sunlight-induced, DNA lesions in skin cells. In our study, we quantified the DNA repair ability of cultured, nontransformed, human skin fibroblast lines through their HCR of a transfected UV-C-irradiated plasmid containing luc. HCR was measured at various times after transfection in five lines from normal donors of ages 21-96 years, and from one donor who had xeroderma pigmentosum (XP). The normal lines displayed increasing HCR at successive post-transfection time points and showed no significant correlation between HCR and donor age. The XP-A line, known to be markedly deficient in NER of UV-induced DNA damage, showed minimal evidence of HCR compared to the normal lines. To further assess potential variation in HCR with donor age, fibroblast lines from five old donors, ages 84-94 years, were compared with lines from five young donors, ages 17-26 years. While significant differences in HCR were found between some lines, no significant difference was found between the young and old age groups (P = 0.44). Our study provides no indication that the higher incidence of skin cancer observed with increasing age is due to an age-related decrease in the ability to repair UV-induced DNA damage.

The role of oxidative damage and stress in aging

The Free Radical/Oxidative Stress Theory of Aging, which was first proposed in 1956, is currently one of the most popular explanations for how aging occurs at the biochemical/molecular level. However, most of the evidence in support of this theory is correlative, e.g., oxidative damage to various biomolecules increases with age, and caloric restriction, which increases life span and retards aging, reduces the age-related increase in oxidative damage to biomolecules. The most direct test of the Free Radical/Oxidative Stress Theory of Aging is to specifically alter the age-related increase in oxidative damage and determine how this alteration affects life span. For the first time, investigators can use genetically altered animals to test directly the role of oxidative damage in aging. In this manuscript, we critically review the past research in this area and discuss potential future research directions in testing the Free Radical/Oxidative Theory of Aging.

Oxidative DNA damage and disease: induction, repair and significance

The generation of reactive oxygen species may be both beneficial to cells, performing a function in inter- and intracellular signalling, and detrimental, modifying cellular biomolecules, accumulation of which has been associated with numerous diseases. Of the molecules subject to oxidative modification, DNA has received the greatest attention, with biomarkers of exposure and effect closest to validation. Despite nearly a quarter of a century of study, and a large number of base- and sugar-derived DNA lesions having been identified, the majority of studies have focussed upon the guanine modification, 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OH-dG). For the most part, the biological significance of other lesions has not, as yet, been investigated. In contrast, the description and characterisation of enzyme systems responsible for repairing oxidative DNA base damage is growing rapidly, being the subject of intense study. However, there remain notable gaps in our knowledge of which repair proteins remove which lesions, plus, as more lesions identified, new processes/substrates need to be determined. There are many reports describing elevated levels of oxidatively modified DNA lesions, in various biological matrices, in a plethora of diseases; however, for the majority of these the association could merely be coincidental, and more detailed studies are required. Nevertheless, even based simply upon reports of studies investigating the potential role of 8-OH-dG in disease, the weight of evidence strongly suggests a link between such damage and the pathogenesis of disease. However, exact roles remain to be elucidated.

Dideoxynucleoside triphosphate-sensitive DNA polymerase from rice is involved in base excision repair and immunologically similar to mammalian DNA pol beta

A single polypeptide with ddNTP-sensitive DNA polymerase activity was purified to near homogeneity from the shoot tips of rice seedlings and analysis of the preparations by SDS-PAGE followed by silver staining showed a polypeptide of 67 kDa size. The DNA polymerase activity was found to be inhibitory by ddNTP in both in vitro DNA polymerase activity assay and activity gel analysis. Aphidicolin, an inhibitor of other types of DNA polymerases, had no effect on plant enzyme. The 67 kDa rice DNA polymerase was found to be recognized by the polyclonal antibody (purified IgG) made against rat DNA polymerase beta (pol beta) both in solution and also on Western blot. The recognition was found to be very specific as the activity of Klenow enzyme was unaffected by the antibody. The ability of rice nuclear extract to correct G:U mismatch of oligo-duplex was observed when oligo-duplex with 32P-labeled lower strand containing U (at 22nd position) was used as substrate. Differential appearance of bands at 21-mer, 22-mer, and 51-mer position in presence of dCTP was visible only with G:U mismatch oligo-duplex, but not with G:C oligo-duplex. While ddCTP or polyclonal antibody against rat-DNA pol beta inhibits base excision repair (BER), aphidicolin had no effect. These results for the first time clearly demonstrate the ability of rice nuclear extract to run BER and the involvement of ddNTP-sensitive pol beta type DNA polymerase. Immunological similarity of the ddNTP-sensitive DNA polymerase beta of rice and rat and its involvement in BER revealed the conservation of structure and function of ddNTP-sensitive DNA pol beta in plant and animal.

Resistance of rho(0) cells against apoptosis

Mitochondrion is one of the master players in both apoptosis and necrosis. However, most previous articles report that mitochondrial DNA-depleted cells without oxidative phosphorylation underwent apoptosis by several apoptotic effectors as efficiently as their parental cells, suggesting that intact mitochondrial function is dispensable for the progression of apoptosis. We studied the role of mitochondrial function in several apoptosis models. TRAIL, a recently identified member of the TNF family with cytotoxicity on a wide variety of transformed cells, killed SK-Hep1 cells with characteristic features of apoptosis such as DNA fragmentation, sub-G1 ploidy peak, and cytochrome c translocation. In contrast with parental cells, mitochondrial DNA-deficient SK-Hep1 rho(0) cells were resistant to TRAIL-induced apoptosis. Dissipation of mitochondrial potential or cytochrome c translocation did not occur in rho(0) cells after TRAIL treatment. Bax translocation also was absent in rho(0) cells, accounting for the failure of cytochrome c release in rho(0) cells. SK-Hep1 rho(0) cells were resistant to other death effectors such as staurosporine. Our results indicate that apoptosis of SK-Hep1 hepatoma cells is dependent on intact mitochondrial function. Because aged cells or tumor cells have frequent mutations or deletions of mitochondrial DNA, they might acquire the ability to evade apoptosis or tumor surveillance imposed by TRAIL or other death effectors in vivo, accounting for the selection advantage of cancer cells and frequent development of cancer in aged individuals.

Age-dependent deficiency in import of mitochondrial DNA glycosylases required for repair of oxidatively damaged bases

The mitochondria are the major source of chronic oxidative stress, which has been implicated in the aging process. Along with other cellular changes, aged cells accumulate mutations in both their nuclear and mitochondrial genomes, and they contain increased amounts of oxidatively damaged mutagenic bases such as 7,8-dihydro-8-oxoguanine, suggesting age-dependent inhibition of its repair. Surprisingly, the level and activity of 8-oxoguanine-DNA glycosylase (OGG1), responsible for repair of 7,8-dihydro-8-oxoguanine, was found to be higher in the liver mitochondrial extract from old rodents than in that from young ones. We addressed this paradox by analyzing OGG1 in the mitochondria of young vs. old mouse livers, as well as in replicating vs. presenescent human fibroblasts. We show here that although the total OGG1 activity is higher in old mitochondria, a large fraction of the enzyme is stuck to the membrane in the precursor form, which could not be translocated to and processed in the mitochondrial matrix. A nearly identical phenomenon was observed with the mitochondrial uracil-DNA glycosylase responsible for repair of mutagenic uracil. These results indicate an age-dependent decline in the mitochondrial import of proteins needed for DNA repair and possibly for other functions.

Compromised incision of oxidized pyrimidines in liver mitochondria of mice deficient in NTH1 and OGG1 glycosylases

Mitochondrial DNA is constantly exposed to high levels of endogenously produced reactive oxygen species, resulting in elevated levels of oxidative damaged DNA bases. A large spectrum of DNA base alterations can be detected after oxidative stress, and many of these are highly mutagenic. Thus, an efficient repair of these is necessary for survival. Some of the DNA repair pathways involved have been characterized, but others are not yet determined. A DNA repair activity for thymine glycol and other oxidized pyrimidines has been described in mammalian mitochondria, but the nature of the glycosylases involved in this pathway remains unclear. The generation of mouse strains lacking murine thymine glycol-DNA glycosylase (mNTH1) and/or murine 8-oxoguanine-DNA glycosylase (mOGG1), the two major DNA N-glycosylase/apurinic/apyrimidinic (AP) lyases involved in the repair of oxidative base damage in the nucleus, has provided very useful biological model systems for the study of the function of these and other glycosylases in mitochondrial DNA repair. In this study, mouse liver mitochondrial extracts were generated from mNTH1-, mOGG1-, and [mNTH1, mOGG1]-deficient mice to ascertain the role of each of these glycosylases in the repair of oxidized pyrimidine base damage. We also characterized for the first time the incision of various modified bases in mitochondrial extracts from a double-knock-out [mNTH1, mOGG1]-deficient mouse. We show that mNTH1 is responsible for the repair of thymine glycols in mitochondrial DNA, whereas other glycosylase/AP lyases also participate in removing other oxidized pyrimidines, such as 5-hydroxycytosine and 5-hydroxyuracil. We did not detect a backup glycosylase or glycosylase/AP lyase activity for thymine glycol in the mitochondrial mouse extracts.

Oxidative DNA damage: mechanisms, mutation, and disease

Oxidative DNA damage is an inevitable consequence of cellular metabolism, with a propensity for increased levels following toxic insult. Although more than 20 base lesions have been identified, only a fraction of these have received appreciable study, most notably 8-oxo-2'deoxyguanosine. This lesion has been the focus of intense research interest and been ascribed much importance, largely to the detriment of other lesions. The present work reviews the basis for the biological significance of oxidative DNA damage, drawing attention to the multiplicity of proteins with repair activities along with a number of poorly considered effects of damage. Given the plethora of (often contradictory) reports describing pathological conditions in which levels of oxidative DNA damage have been measured, this review critically addresses the extent to which the in vitro significance of such damage has relevance for the pathogenesis of disease. It is suggested that some shortcomings associated with biomarkers, along with gaps in our knowledge, may be responsible for the failure to produce consistent and definitive results when applied to understanding the role of DNA damage in disease, highlighting the need for further studies.

Age-related effects on induction of DNA strand breaks by intermittent exposure to electromagnetic fields

Several studies indicating a decline of DNA repair efficiency with age raise the question, if senescence per se leads to a higher susceptibility to DNA damage upon environmental exposures. Cultured fibroblasts of six healthy donors of different age exposed to intermittent ELF-EMF (50 Hz sinus, 1 mT) for 1-24 h exhibited different basal DNA strand break levels correlating with age. The cells revealed a maximum response at 15-19 h of exposure. This response was clearly more pronounced in cells from older donors, which could point to an age-related decrease of DNA repair efficiency of ELF-EMF induced DNA strand breaks.

Decline of nuclear and mitochondrial oxidative base excision repair activity in late passage human diploid fibroblasts

There are numerous studies documenting the increase of oxidative DNA damage in the nuclei and mitochondria of senescing cells as well as in tissues of aging animals. Here, we show that in IMR 90 human diploid fibroblasts, DNA repair activity is robust in both nuclear and mitochondrial extracts, however, the levels of activity differed against the three substrates tested. In extracts, cleavage of the 8-oxoguanine substrate, and to a lesser extent the dihydrouracil-containing substrate, occurred in a concerted reaction between the DNA glycosylases and the second enzyme in the reaction, hAPE. Cleavage of both the furan and the dihydrouracil-containing substrates was unchanged when nuclear extracts from early and late passage cells were compared. However, cleavage of the 8-oxoguanine substrate was substantially reduced in the nuclear extracts from late passage cells and significantly reduced transcription from the hOGG1 gene was observed. When mitochondrial extracts were examined, activity on all three substrates was significantly reduced, with the reduction in hAPE activity being the most marked. The reduction in cleavage of the furan substrate was not simply due to inactive mitochondrial AP endonuclease but a substantially reduced amount of hAPE protein; transcription from the hAPE gene was also reduced. Confocal microscopic analysis confirmed that hAPE was present in the mitochondria of early passage cells but greatly reduced in the mitochondria of late passage cells. Cytoplasmic extracts from late passage fibroblasts also showed reduced activity with all three substrates suggesting that the residual hAPE, and activities that recognized dihydrouracil, were preferentially targeted to the nuclei. Taken together the data support the concept that the increase in oxidative damage in the mitochondrial DNA of senescing cells and tissues from aging animals is due to reduced base excision repair activity.

Base excision repair capacity in mitochondria and nuclei: tissue-specific variations

Base excision repair is the main pathway for repair of oxidative base lesions in DNA. Mammalian cells must maintain genomic stability in their nuclear and mitochondrial genomes, which have different degrees of vulnerability to DNA damage. This study quantifies DNA glycosylase activity in mitochondria and nucleus from C57/BL 6 mouse tissues including brain, liver, heart, muscle, kidney, and testis. The activities of oxoguanine DNA glycosylase (OGG1), uracil DNA glycosylase, and endonuclease III homologue 1 (NTH1) were measured using oligonucleotide substrates with DNA lesions specific for each glycosylase. Mitochondrial content was normalized to citrate synthase activity and mitochondrial function was assessed by measuring cytochrome c oxidase (COX) activity. In nuclear and mitochondrial extracts, the highest DNA glycosylase activities were in testis. Brain and heart, tissues with the highest oxidative load, did not have higher levels of OGG1 or NTH1 activity than muscle or kidney, which are more glycolytic tissues. In general, mitochondrial extracts have lower DNA glycosylase activity than nuclear extracts. There was no correlation between glycosylase activities in the mitochondrial extracts and COX activity, suggesting that DNA repair enzymes may be regulated by a mechanism different from this mitochondrial enzyme.

Mitochondrial repair of 8-oxoguanine and changes with aging

Reactive oxygen species (ROS) are formed in all living organisms as a by-product of normal metabolism (endogenous sources) and as a consequence of exposure to environmental compounds (exogenous sources). Endogenous ROS are largely formed during oxidative phosphorylation in the mitochondria and, therefore, mitochondrial DNA (mtDNA) is at particularly high risk of ROS-induced damage. Mitochondria are essential for cell viability, and oxidative damage to mtDNA has been implicated as a causative factor in a wide variety of degenerative diseases, and in cancer and aging. One of the most common oxidative DNA lesions is 7,8-dihydro-8-oxoguanine (8-oxoG), which can introduce G/C to T/A transversions after DNA replication. Oxidative DNA base lesions, including 8-oxoG, are repaired primarily by the base excision repair (BER) pathway. While we know much about how this pathway functions in processing the nuclear DNA lesions, little is yet known about BER in mitochondria. We have used a number of different approaches to explore the mechanisms of DNA damage processing in the mtDNA. We have been able to demonstrate that mammalian mitochondria efficiently remove 8-oxoG from their genome, and that the efficiency of 8-oxoG incision increases with age in rats and mice. Yet 8-oxoG accumulates in mtDNA during aging. Changes in mitochondrial function with age have been observed in several organisms and accumulation of DNA lesions in mtDNA with age may be an underlying cause for numerous age-associated diseases including cancer.

Repair of oxidative DNA damage in nuclear and mitochondrial DNA, and some changes with aging in mammalian cells

Exposure to exogenous and endogenous sources cause oxidative damage to cellular macromolecules, including DNA. This results in gradual accumulation of oxidative DNA base lesions, and in order to maintain genomic stability we must have effective systems to repair this kind of damage. The accumulation of lesions is most dramatic in the mitochondrial DNA, and this may cause dysfunction and loss of cellular energy production. Base excision DNA repair (BER) is the major pathway that removes oxidative DNA base lesions, and while we know much about its mechanism in the nuclear DNA, little is yet known about this pathway in mitochondria. While nuclear BER decreases with age, the mitochondrial DNA repair may increase with age. This increase is not enough to prevent the gradual accumulation of lesions in the mitochondrial DNA with age. Accumulation of DNA lesions with age may be the underlying cause for age-associated diseases including cancer.

DNA damage and its processing. relation to human disease

We are constantly exposed to sources of agents that directly damage the genetic material. This exposure comes from environmental sources but also from within our own organisms. DNA damage occurs at a high frequency due to metabolic processes and environmental factors such as various exposures and the intake of food and drugs. The stability and correct function of the DNA is necessary for normal cellular functions and there is good evidence that damage to the DNA can lead to cellular dysfunction, cancer and other diseases, or cell death. To avoid or minimize the damage to DNA we have evolved an elaborate set of DNA repair pathways that survey the DNA and fix the errors. There are several human diseases that are known to be defective in these repair pathways, and the accumulation of DNA damage with time in their genome may then be the cause of the associated high incidence of cancer or of an expedited ageing process. The prevention and/or repair of DNA damage thus represent major concerns in biology and medicine.

Involvement of DNA polymerase beta in protection against the cytotoxicity of oxidative DNA damage

We had shown previously that DNA polymerase beta (beta-pol) null mouse fibroblasts, deficient in base excision repair (BER), are hypersensitive to monofunctional methylating agents but not to hydrogen peroxide (H2O2). This is surprising because beta-pol is thought to be involved in BER of oxidative as well as methylated DNA damage. We confirm these findings here in early-passage cells. However, with time in culture, beta-pol null cells become hypersensitive to H2O2 and other reactive oxygen species-generating agents. Analysis of in vitro BER reveals a strong deficiency in single-nucleotide BER of 8-oxoguanine (8-oxoG) by both early- and late-passage beta-pol null cell extracts. Therefore, in early-passage wild-type and beta-pol null cells, the capacity for single-nucleotide BER of 8-oxoG does not correlate with cellular sensitivity to H2O2. Expression of beta-pol protein in the late-passage null cells almost completely reverses the H2O2-hypersensitivity phenotype. Methoxyamine (MX) treatment sensitizes late-passage wild-type cells to H2O2 as expected for beta-pol-mediated single-nucleotide BER; however in beta-pol null cells, MX has no effect. The data indicate a role(s) of beta-pol-dependent repair in protection against the cytotoxicity of oxidative DNA damage in wild-type cells.

Selected contribution: Differential expression of stress-related genes with aging and hyperthermia

Aging is associated with a reduced capacity to cope with physiological stress. To study the molecular mechanisms associated with the decline in stress tolerance that accompanies aging, differences in gene expression between young and old Fischer 344 rats under euthermic control conditions or in response to hyperthermic challenge were evaluated using a cDNA array containing 207 stress-related genes. In the nonstressed control condition, aging resulted in selective upregulation of stress protein genes and transcripts involved in cell growth, death, and signaling, along with a downregulation of genes involved in antioxidant defenses and drug metabolism. Heat stress resulted in a broad induction of genes in the antioxidant and drug metabolism categories and transcripts involved in DNA, RNA, and protein synthesis for both age groups. Old animals had a robust upregulation of genes involved in cell growth, death, and signaling after heat challenge, along with a blunted expression of stress-response genes. In contrast, young animals had a strong induction of stress-response genes after hyperthermic challenge. Changes in expression of selected genes were confirmed by RT-PCR analysis. These findings suggest that aging results in altered gene expression in response to heat stress that is indicative of decreased stress protein transcription and increased expression of oxidative stress-related genes. Thus our findings support the postulate that transcriptional changes in response to a physiological challenge such as hyperthermia contribute to the loss of stress tolerance in older organisms.

Genes, telomeres and mammalian ageing

Although there appear to be several influences, which contribute to the ageing of mammals, the role of DNA appears to be pivotal. There is increasing evidence that oxidative damage is an important factor in producing mutations in genes, shortening telomeres and damaging mitochondrial DNA. Accumulation of mutations in genomic DNA could result in the gradual decline in cellular function, which is exhibited in a variety of tissues. The random nature of these mutations, could also offer an explanation for differences in the degree and time of onset of age-related changes, exhibited by different individuals. Shortening of telomeres, caused by oxidative damage or the end-replication problem, could result in the accumulation of post-mitotic cells in-vivo during ageing. This might impair certain aspects of physiology, such as wound healing. Mutation of mitochondrial DNA may also be important in causing loss of cells in post-mitotic tissues such as muscle or brain. In addition changes in the redox state during the life of an animal may alter transcription factor activities, leading to consistent changes in the gene expression profiles of mammalian tissues. The latter could explain consistent age-related changes that have been observed in cell structure and physiology. Although all of these mechanisms may make a contribution to ageing, it is likely that it is the interplay between them that produces the most prominent effects.

Attenuation of DNA polymerase beta-dependent base excision repair and increased DMS-induced mutagenicity in aged mice

The biological mechanisms responsible for aging remain poorly understood. We propose that increases in DNA damage and mutations that occur with age result from a reduced ability to repair DNA damage. To test this hypothesis, we have measured the ability to repair DNA damage in vitro by the base excision repair (BER) pathway in tissues of young (4-month-old) and old (24-month-old) C57BL/6 mice. We find in all tissues tested (brain, liver, spleen and testes), the ability to repair damage is significantly reduced (50-75%; P<0.01) with age, and that the reduction in repair capacity seen with age correlates with decreased levels of DNA polymerase beta (beta-pol) enzymatic activity, protein and mRNA. To determine the biological relevance of this age-related decline in BER, we measured spontaneous and chemically induced lacI mutation frequency in young and old animals. In line with previous findings, we observed a three-fold increase in spontaneous mutation frequency in aged animals. Interestingly, lacI mutation frequency in response to dimethyl sulfate (DMS) does not significantly increase in young animals whereas identical exposure in aged animals results in a five-fold increase in mutation frequency. Because DMS induces DNA damage processed by the BER pathway, it is suggested that the increased mutagenicity of DMS with age is related to the decline in BER capacity that occurs with age. The inability of the BER pathway to repair damages that accumulate with age may provide a mechanistic explanation for the well-established phenotype of DNA damage accumulation with age.

Mitochondrial DNA repair of oxidative damage in mammalian cells

Nuclear and mitochondrial DNA are constantly being exposed to damaging agents, from endogenous and exogenous sources. In particular, reactive oxygen species (ROS) are formed at high levels as by-products of the normal metabolism. Upon oxidative attack of DNA many DNA lesions are formed and oxidized bases are generated with high frequency. Mitochondrial DNA has been shown to accumulate high levels of 8-hydroxy-2'-deoxyguanosine, the product of hydroxylation of guanine at carbon 8, which is a mutagenic lesion. Most of these small base modifications are repaired by the base excision repair (BER) pathway. Despite the initial concept that mitochondria lack DNA repair, experimental evidences now show that mitochondria are very proficient in BER of oxidative DNA damage, and proteins necessary for this pathway have been isolated from mammalian mitochondria. Here, we examine the BER pathway with an emphasis on mtDNA repair. The molecular mechanisms involved in the formation and removal of oxidative damage from mitochondria are discussed. The pivotal role of the OGG1 glycosylase in removal of oxidized guanines from mtDNA will also be examined. Lastly, changes in mtDNA repair during the aging process and possible biological implications are discussed.

Building bridges in cancer: mismatch repair and microsatellite instability

Mismatch repair (MMR) defects and microsatellite instability (MSI) are two genetic alterations that have been documented in a wide variety of human cancers, including some that involve the skin. Available evidence indicates that these two features are sometimes directly related, although their connection seems to be indirect or nonexistent in other instances. The purposes of this review are to summarize the variable relations between MMR and MSI as deduced from analysis of a diverse array of human neoplasms and to give brief insights as to the other molecular mechanisms potentially involved in the maintenance of genomic stability.

Placental release/retention in cows and its relation to peroxidative damage of macromolecules

The disturbances in metabolic pathways reflected in clinical symptoms of illnesses may be connected, among others, with the imbalance between production and neutralization of reactive oxygen species. One of such illnesses may be the retention of fetal membranes in cows. The levels of reactive oxygen species can be measured directly or estimated indirectly by the determination of enzymatic and non-enzymatic antioxidative defence systems. The determination of parameters indicating the intensity of peroxidative processes of lipids, proteins and nucleic acids caused by reactive oxygen species is also useful. This review examined the available literature regarding peroxidative processes of lipids, proteins and nucleic acids caused by reactive oxygen species as well as parameters indicating its intensity. All information relates the importance of proper and improper placental release in cows.

Base excision repair in nuclear and mitochondrial DNA

Base excision repair mechanisms have been analyzed in nuclear and mitochondrial DNA. We measured the size and position of the newly incorporated DNA repair patch in various DNA substrates containing single oxidative lesions. Repair of 8-oxoguanine and of thymine glycol is almost exclusively via the base excision repair (BER) pathway with little or no involvement of nucleotide excision repair (NER). The repair mode is generally via the single-nucleotide replacement pathway with little incorporation into longer patches. Extension of these studies suggests that DNA polymerase beta plays a critical role not only in the short-patch repair process but also in the long-patch, PCNA-dependent pathway. Mitochondria are targets for a heavy load of oxidative DNA damage. They have efficient BER repair capacity, but cannot repair most bulky lesions normally repaired by NER. In vitro experiments performed using rat and human mitochondrial extracts suggest that the repair incorporation during the removal of uracil in DNA occurs via the short-patch repair BER pathway. Oxidative DNA damage accumulates with age in mitochondrial DNA, but this cannot be explained by an attenuation of DNA repair. In contrast, we observe that mitochondrial incision of 8-oxoG increases with age in rodents.

The impact energy metabolism and genome maintenance have on longevity and senescence: lessons from yeast to mammals

The phenomenon that caloric restriction increases life span in a variety of species from yeast to mice has been the focus of much interest. Recent observations suggest that a protein important for heterochromatin formation, Sir2, is central for caloric restriction-induced longevity in lower organisms. Interestingly, Sir2 is also capable of repairing DNA double-strand breaks by nonhomologous end joining which may be important, along with proteins that repair breaks by recombinational repair, for minimizing the age-related deleterious effects of DNA damage induced by oxygen by-products of metabolism. I propose that competition between these two distinct functions could influence longevity and the onset of senescence. In addition, sequence and functional similarities between Sir2 and other chromatin metabolism proteins present the possibility that genetic components for longevity and senescence are conserved from yeast to mammals.

Garbage catastrophe theory of aging: imperfect removal of oxidative damage?

Increasing evidence suggests an important role of oxidant-induced damage in the progress of senescent changes, providing support for the free radical theory of aging proposed by Harman in 1956. However, considering that biological organisms continuously renew their structures, it is not clear why oxidative damage should accumulate with age. No strong evidence has been provided in favor of the concept of aging as an accumulation of synthetic errors (e.g. Orgel's 'error-catastrophe' theory and the somatic mutation theory). Rather, we believe that the process of aging may derive from imperfect clearance of oxidatively damaged, relatively indigestible material, the accumulation of which further hinders cellular catabolic and anabolic functions. From this perspective, it might be predicted that: (i) suppression of oxidative damage would enhance longevity; (ii) accumulation of incompletely digested material (e.g. lipofuscin pigment) would interfere with cellular functions and increase probability of death; (iii) rejuvenation during reproduction is mainly provided by dilution of undigested material associated with intensive growth of the developing organism; and (iv) age-related damage starts to accumulate substantially when development is complete, and mainly affects postmitotic, cells and extracellular matrix, not proliferating cells. There is abundant support for all these predictions.

Levels of 8-hydroxy-2'-deoxyguanosine in cellular DNA from 12 tissues of young and old Sprague-Dawley rats

The age dependent increase of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) reported in DNA of organs of old rats appears to vary with the strain, age and sex of the animals used for the investigation. Here we report on 8-OH-dG concentrations in the cellular DNA of 12 tissues of male Sprague-Dawley rats aged 5 or 30 months and kept under standard conditions throughout their lives until being killed. DNA from frozen organs was isolated using a Qiagen DNA purification kit. Following digestion (nuclease P1, alkaline nuclease) hydrolysed DNA was applied onto a HPLC column; native nucleosides were monitored at 254 nm and 8-OH-dG by electrochemical detection. 8-OH-dG levels in organs of young rats ranged between 10 and 90 micromol/mol deoxyguanosine (dG). Highest levels (micromol 8-OH-dG /mol dG) were detected in the oesophagus (90), muscle (61), brain (65), liver (59), spleen (57), and testicles (63). 8-OH-dG in DNA from kidney, lung, heart, small and large intestine ranged between 28 and 38 micromol/mol dG. Lowest amounts were found in the glandular stomach (10). DNA of old rats generally contained higher 8-OH-dG levels with significant increases in liver (186%) and kidney (372%); other organs showed no significant decreases (spleen, brain, testicles) or increases up to 69% (heart). These findings are discussed in the context with previously published data on 8-OH-dG levels in organs from young and old rats.

Lung cancer treated surgically in patients <50 years of age

STUDY OBJECTIVES

Some investigators have suggested that lung cancer in young patients has a more aggressive course and a poorer prognosis than lung cancer in older patients. The aim of this study was to determine if the basal characteristics and survival in younger patients with lung cancer undergoing surgical resection differ from those of older patients.

DESIGN

Retrospective clinical study.

PATIENTS

Of 1,208 consecutive patients who underwent surgery for primary lung cancer between June 1984 and March 2000, we reviewed the medical records of 110 younger patients who were < 50 years of age at the time of surgery and compared them with 1,098 older patients (> or = 50 years of age). All deaths were included.

RESULTS

In the younger patient group, asymptomatic disease and adenocarcinoma was significantly more frequent, the rate of smoking was significantly higher, and the amount of smoking (Brinkman index) was significantly larger. For the 94 younger patients with complete resection, the 5-year survival rate was 61.0%, which was not significantly higher than that for the 923 older patients (57.7%). However, the 53 younger patients with stage I disease (5-year survival of 84.3%) had significantly better survival than older patients with the same condition (71.6%). Survival of patients in stage II or stage III disease was not significantly different.

CONCLUSION

The younger patients had significantly better prognoses, and a statistical difference was shown especially in the early stage, while in the advanced stage the malignancy of the lung cancer itself surpassed the difference in survival.

A unifying hypothesis of Alzheimer's disease. IV. Causation and sequence of events

Contrary to common concepts, the brain in Alzheimer's disease (AD) does not follow a suicide but a rescue program. Widely shared features of metabolism in starvation, hibernation and various conditions of energy deprivation, e.g. ischemia, allow the definition of a deprivation syndrome which is a phylogenetically conserved adaptive response to energetic stress. It is characterized by hypometabolism, oxidative stress and adjustments of the glucose-fatty acid cycle. Cumulative evidence suggests that the brain in aging and AD actively adapts to the progressive fuel deprivation. The counterregulatory mechanisms aim to preserve glucose for anabolic needs and promote the oxidative utilization of ketone bodies. The agent mediating the metabolic switch is soluble Abeta which inhibits glucose utilization and stimulates ketone body utilization at various levels. These processes, which are initiated during normal aging, include inhibition of pro-glycolytic neurohormones, cholinergic transmission, and pyruvate dehydrogenase, the key transmitter and effector systems regulating glucose metabolism. Hormonal and effector systems which promote ketone body utilization, such as glucocorticosteroid and galanin activity, GABAergic transmission, nitric oxide, lipid transport, Ca2+ elevation, and ketone body metabolizing enzymes, are enhanced. A multitude of risk factors feed into this pathophysiological cascade at a variety of levels. Taking into account its pleiotropic regulatory actions in the deprivation response, a new name for Abeta is suggested: deprivin. On the other hand, cumulative evidence, taken together compelling, suggests that senile plaques are the dump rather than the driving force of AD. Moreover, the neurotoxic action of fibrillar Abeta is a likely in vitro artifact but does not contribute significantly to the in vivo pathophysiological events. This archaic program, conserved from bacteria to man, aims to ensure the survival of a deprived organism and controls such divergent processes as sporulation, hibernation, aging and aging-related diseases. In contrast to the immature brain, ketone body utilization of the aged brain is no longer sufficient to meet the energetic demands and is later supplemented by lactate, thus recapitulating in reverse order the sequential fuel utilization of the immature brain. The transduction pathways which operate to switch metabolism also convey the programming and balancing of the de-/redifferentiation/apoptosis cell cycle decisions. This encompasses the reiteration of developmental processes such as transcription factor activation, tau hyperphosphorylation, and establishment of growth factor independence by means of Ca2+ set point shift. Thus, the increasing energetic insufficiency results in the progressive centralization of metabolic activity to the neuronal soma, leading to pruning of the axonal/dendritic trees, loss of neuronal polarity, downregulation of neuronal plasticity and, eventually, depending on the Ca2+ -energy-redox homeostasis, degeneration of vulnerable neurons. Finally, it is outlined that genetic (e.g. Down's syndrome, APP and presenilin mutations and apoE4) and environmental risk factors represent progeroid factors which accelerate the aging process and precipitate the manifestation of AD as a progeroid systemic disease. Aging and AD are related to each other by threshold phenomena, corresponding to stage 2, the stage of resistance, and stage 3, exhaustion, of a metabolic stress response.

Current issues concerning the role of oxidative stress in aging: a perspective

The main tenet of the oxidative stress hypothesis of aging is that accrual of molecular oxidative damage is the principal causal factor in the senescence-related loss of ability to maintain homeostasis. This hypothesis has garnered a considerable amount of supportive correlational evidence, which is now being extended experimentally in transgenic Drosophila over-expressing antioxidative defense enzymes. Some of these studies have reported extensions of life span, while others have not. Interpretation of life spans in poikilotherms is complicated by a number of factors, including the interrelationship between metabolic rate and longevity. The life spans of poikilotherms can be extended multi-fold by reducing the metabolic rate but without affecting the metabolic potential, i.e., the total amount of energy expended during life. A hypometabolic state in poikilotherms also enhances stress resistance and activities of antioxidative enzymes. It is emphasized that extension of life span without simultaneously increasing metabolic potential is of questionable biological significance.

Effect of DL-alpha-lipoic acid on tissue nucleic acid contents in aged rats

We have attempted to evaluate the effect of DL-alpha-lipoic acid on nucleic acid and protein contents in young and aged rats. An age-associated decrease in the deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and protein contents were observed in aged rats. DL-alpha-lipoic acid was administered intraperitoneally (100 mg/kg body weight/day) to young and aged rats from 7 and 14 days. Lipoate administration demonstrated a substantial increase in nucleic acid and protein contents in aged rats. Hence it can be justified that lipoate is functionally efficient in helping the cell to recover from oxidative damage.

Somatic cell mutations: can they provide a link between aging and cancer?

Cancers increase during aging in mammals, and an accumulating body of evidence suggests that mutational events too do likewise. Mutational events are intimately involved in the malignant process. One current view is that mutator phenotypes are required in malignant cells for a sufficient number of critical target genes to be affected. These mutator phenotypes are believed to result from underlying deficiencies in genes necessary to maintain genomic stability. This review will provide a framework for a discussion of cancer and aging by detailing with a pair of wise approach studies that address the relations between aging, cancer, and mutations. Results from these studies will be used to suggest that a mutator phenotype develops in the cells of older individuals in the absence of an underlying genetic deficiency. Instead, it is proposed that a mixture of chromosomal aberrations, DNA damage, and chronic exposure to genotoxic forces, including oxidative stress, provide the basis for this age-accelerated mutator phenotype.

The catalytic subunit of telomerase protects neurons against amyloid beta-peptide-induced apoptosis

The catalytic subunit of telomerase (TERT) is a specialized reverse transcriptase that has been associated with cell immortalization and cancer. It was reported recently that TERT is expressed in neurons throughout the brain in embryonic and early postnatal development, but is absent from neurons in the adult brain. We now report that suppression of TERT levels and function in embryonic mouse hippocampal neurons in culture using antisense technology and the telomerase inhibitor 3' -azido-2' 3' -dideoxythymidine significantly increases their vulnerability to cell death induced by amyloid beta-peptide, a neurotoxic protein believed to promote neuronal degeneration in Alzheimer's disease. Neurons in which TERT levels were reduced exhibited increased levels of oxidative stress and mitochondrial dysfunction following exposure to amyloid beta-peptide. Overexpression of TERT in pheochromocytoma cells resulted in decreased vulnerability to amyloid beta-peptide-induced apoptosis. Our findings demonstrate a neuroprotective function of TERT in an experimental model relevant to Alzheimer's disease, and suggest the possibility that restoration of TERT expression in neurons in the adult brain may protect against age-related neurodegeneration.

Gene-specific DNA repair of pyrimidine dimers does not decline during cellular aging in vitro

A large number of studies have demonstrated that various kinds of DNA damage accumulate during aging and one of the causes for this could be a decrease in DNA repair capacity. However, the level of total genomic repair has not been strongly correlated with aging. DNA repair of certain kinds of damage is known to be closely connected to the transcription process; thus, we chose to investigate the level of gene-specific repair of UV-induced damage using in vitro aging of human diploid skin fibroblasts and trabecular osteoblasts as model systems for aging. We find that the total genomic repair is not significantly affected during cellular aging of cultures of both human skin fibroblasts and trabecular osteoblasts. Gene-specific repair was analyzed during cellular aging in the dihydrofolate reductase housekeeping gene, the p53 tumor suppressor gene, and the inactive region X(754). There was no clear difference in the capacity of young and old cells to repair UV-induced pyrimidine dimers in any of the analyzed genes. Thus, in vitro senescent cells can sustain the ability to repair externally induced damage.

Overexpression of Mn-containing superoxide dismutase in transgenic Drosophila melanogaster

The general objective of this study was to examine the role of mitochondria in the aging process. Two alternative hypotheses were tested: (i) that overexpression of Mn superoxide dismutase (Mn SOD) in the mitochondria of Drosophila melanogaster would slow the accrual of oxidative damage and prolong survival or (ii) that there is an evolved optimum level of superoxide anion radical, such that overexpression of Mn SOD would have deleterious or neutral effects. Microinjection and mobilization of a transgene, which contained a 9-kb genomic sequence encoding Mn SOD, produced 15 experimental lines overexpressing Mn SOD by 5-116% relative to the parental y w strain. Comparisons between these lines and control lines containing inserted vector sequences alone indicated that the mean longevity of the experimental lines was decreased by 4-5% relative to controls. There were no compensatory changes in the metabolic rate, level of physical activity, or the levels of other antioxidants, namely Cu-Zn SOD, catalase, and glutathione. There were no differences between groups in rates of mitochondrial hydrogen peroxide release, protein oxidative damage, or resistance to 100% oxygen or starvation conditions. The experimental lines had a marginally increased resistance to moderate heat stress. These results are consistent with the existence of an optimum level of Mn SOD activity which minimizes oxidative stress. The naturally evolved level of Mn SOD activity in Drosophila appears to be near the optimum required under normal conditions, although the optimum may be shifted to a higher level under more stressful conditions.

Molecular and cellular alterations in tobacco smoke-associated larynx cancer

Tumours of head and neck belong to the most frequent types of cancer world-wide. In Poland, mortality from larynx cancer among males has been continuously increasing during the last decades up to 8.4 deaths per 100,000 men in 1993, which exceeds epidemiological records from other countries. The aetiology of laryngeal cancer is strongly associated with exposure to carcinogens present in tobacco smoke. The review describes a sequence of molecular and cellular events from carcinogenic exposure, DNA adduct formation, detection of mutations in the p53 gene, loss of heterozygosity (LOH) in chromosomal loci encoding the p53 and p16 genes, and loss of control of the cell cycle. The section concerning DNA adducts includes a discussion of the role of such confounders as exogenous exposure, the age and sex of the subject, and disease progression. The significance of genetic factors as individual risk determinants is discussed in relation to bleomycin-induced chromosome instability and in connection with the occurrence of defects in genes encoding detoxifying enzymes. The question concerning the substantial difference between men and women in larynx cancer morbidity and mortality remains open, even when the significantly higher adduct formation in male DNA compared with female material was taken into account. Preliminary experiments suggest a role of the frequently observed loss of the Y-chromosome.

Influence of sex, smoking and age on human hprt mutation frequencies and spectra

Examination of the literature for hprt mutant frequencies from peripheral T cells yielded data from 1194 human subjects. Relationships between mutant frequency, age, sex, and smoking were examined, and the kinetics were described. Mutant frequency increases rapidly with age until about age 15. Afterward, the rate of increase falls such that after age 53, the hprt mutant frequency is largely stabilized. Sex had no effect on mutant frequency. Cigarette smoking increased mean mutant frequency compared to nonsmokers, but did not alter age vs. mutant frequency relationships. An hprt in vivo mutant database containing 795 human hprt mutants from 342 individuals was prepared. No difference in mutational spectra was observed comparing smokers to nonsmokers, confirming previous reports. Sex affected the frequency of deletions (>1 bp) that are recovered more than twice as frequently in females (P = 0. 008) compared to males. There is no indication of a significant shift in mutational spectra with age for individuals older than 19 yr, with the exception of A:T --> C:G transversions. These events are recovered more frequently in older individuals.

The C-terminal domain of p21 inhibits nucleotide excision repair In vitro and In vivo

The protein p21(Cip1, Waf1, Sdi1) is a potent inhibitor of cyclin-dependent kinases (CDKs). p21 can also block DNA replication through its interaction with the proliferating cell nuclear antigen (PCNA), which is an auxiliary factor for polymerase delta. PCNA is also implicated in the repair resynthesis step of nucleotide excision repair (NER). Previous studies have yielded contradictory results on whether p21 regulates NER through its interaction with PCNA. Resolution of this controversy is of interest because it would help understand how DNA repair and replication are regulated. Hence, we have investigated the effect of p21 on NER both in vitro and in vivo using purified fragments of p21 containing either the CDK-binding domain (N terminus) or the PCNA binding domain (C terminus) of the protein. In the in vitro studies, DNA repair synthesis was measured in extracts from normal human fibroblasts using plasmids damaged by UV irradiation. In the in vivo studies, we used intact and permeabilized cells. The results show that the C terminus of the p21 protein inhibits NER both in vitro and in vivo. These are the first in vivo studies in which this question has been examined, and we demonstrate that inhibition of NER by p21 is not merely an artificial in vitro effect. A 50% inhibition of in vitro NER occurred at a 50:1 molar ratio of p21 C-terminus fragment to PCNA monomer. p21 differentially regulates DNA repair and replication, with repair being much less sensitive to inhibition than replication. Our in vivo results suggest that the inhibition occurs at the resynthesis step of the repair process. It also appears that preassembly of PCNA at repair sites mitigates the inhibitory effect of p21. We further demonstrate that the inhibition of DNA repair is mediated via binding of p21 to PCNA. The N terminus of p21 had no effect on DNA repair, and the inhibition of DNA repair by the C terminus of p21 was relieved by the addition of purified PCNA protein.

PCR assay of DNA damage and repair at the gene level in brain and spleen of gamma-irradiated young and old rats

The PCR amplification of fragments of transcribed (beta-actin, p53) and nontranscribed (IgE, heavy chain) genes in brain and spleen DNA from gamma-irradiated and unirradiated 2- and 28-month-old rats was studied. The amplification levels of fragments of these genes in DNA from old rats were substantially lower than those from young rats, which suggested that these gene fragments in old-rat DNA contained lesions blocking thermostable polymerase in PCR. The beta-actin and IgE gene fragments of spleen DNA from old rats exhibited a significantly higher level of lesions inhibiting Tth polymerase compared to analogous fragments of brain DNA from the same animals. DNA from the tissues of gamma-irradiated rats showed the amount of damage inhibiting amplification to be dependent on animal age and the postirradiation time before DNA isolation. As judged from the changes in the amplification level of gene fragments, there was no preferential fast repair of lesions in the actively transcribed gene beta-actin compared to the nontranscribed gene IgE (heavy chain) in the brain and spleen of gamma-irradiated young and old rats. The amplification results suggest that equal amounts of DNA lesions were repaired in the brain of both old and young rats during the first 0.5 h of the postirradiation time (fast-repair phase), whereas in the subsequent postirradiation period over 5 h (slow-repair phase), the efficiency of damage elimination in the brain DNA of old rats was markedly lower. As for the spleen tissue, the elimination of lesions blocking Tth polymerase was much lower in old gamma-irradiated animals for both of the repair phases.

Understanding changing risk factor associations with increasing age in adults

With an increasingly older population, there is considerable interest in understanding the potential for risk factor interventions in order to prevent, postpone, or slow down the common diseases seen in older persons. However, it is often reported that the strength of association between risk factors and common disease outcomes decreases with increasing age. Actually, many different age-related patterns are observed. Understanding these patterns requires knowledge of issues related to the pathophysiology of aging, including age-related physiologic and metabolic alterations, detection and diagnosis of disease in the elderly, measurement of risk factors, sample selection, comorbidity, competing risks, selective survival, ceiling effects, and methods of analysis in aging populations.

Age-associated increase in 8-oxo-deoxyguanosine glycosylase/AP lyase activity in rat mitochondria

The mitochondrial theory of aging postulates that organisms age due to the accumulation of DNA damage and mutations in the multiple mitochondrial genomes, leading to mitochondrial dysfunction. Among the wide variety of DNA damage, 8-oxo-deoxyguanosine (8-oxo-dG) has received the most attention due to its mutagenicity and because of the possible correlation between its accumulation and pathological processes like cancer, degenerative diseases and aging. Although still controversial, many studies show that 8-oxo-dG accumulates with age in the mitochondrial (mt) DNA. However, little is known about the processing of this lesion and no study has yet examined whether mtDNA repair changes with age. Here, we report the first study on age-related changes in mtDNA repair, accomplished by assessing the cleavage activity of mitochondrial extracts towards an 8-oxo-dG-containing substrate. In this study, mitochondria obtained from rat heart and liver were used. We find that this enzymatic activity is higher in 12 and 23 month-old rats than in 6 month-old rats, in both liver and heart extracts. These mitochondrial extracts also cleave oligonucleotides containing a U:A mismatch, at the uracil position, reflecting the combined action of mitochondrial uracil DNA glycosylase (mtUDG) and mitochondrial apurinic/apyrimidinic (AP) endonucleases. The mtUDG activity did not change with age in liver mitochondria, but there was a small increase in activity from 6 to 23 months in rat heart extracts, after normalization to citrate synthase activity. Endonuclease G activity, measured by a plasmid relaxation assay, did not show any age-associated change in liver, but there was a significant decrease from 6 to 23 months in heart mitochondria. Our results suggest that the mitochondrial capacity to repair 8-oxo-dG, the main oxidative base damage suggested to accumulate with age in mtDNA, does not decrease, but rather increases with age. The specific increase in 8-oxo-dG endonuclease activity, rather than a general up-regulation of DNA repair in mitochondria, suggests an induction of the 8-oxo-dG-specific repair pathway with age.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.

Oxidative DNA damage processing in nuclear and mitochondrial DNA

Living organisms are constantly exposed to oxidative stress from environmental agents and from endogenous metabolic processes. The resulting oxidative modifications occur in proteins, lipids and DNA. Since proteins and lipids are readily degraded and resynthesized, the most significant consequence of the oxidative stress is thought to be the DNA modifications, which can become permanent via the formation of mutations and other types of genomic instability. Many different DNA base changes have been seen following some form of oxidative stress, and these lesions are widely considered as instigators for the development of cancer and are also implicated in the process of aging. Several studies have documented that oxidative DNA lesions accumulate with aging, and it appears that the major site of this accumulation is mitochondrial DNA rather than nuclear DNA. The DNA repair mechanisms involved in the removal of oxidative DNA lesions are much more complex than previously considered. They involve base excision repair (BER) pathways and nucleotide excision repair (NER) pathways, and there is currently a great deal of interest in clarification of the pathways and their interactions. We have used a number of different approaches to explore the mechanism of the repair processes, to examine the repair of different types of oxidative lesions and to measure different steps of the repair processes. Furthermore, we can measure the DNA damage processing in the nuclear DNA and separately, in the mitochondrial DNA. Contrary to widely held notions, mitochondria have efficient DNA repair of oxidative DNA damage.