MutS recognition of exocyclic DNA adducts that are endogenous products of lipid oxidation

Nanoscale battery cathode materials induce DNA damage in bacteria

The increasing use of nanoscale lithium nickel manganese cobalt oxide (Li x Ni y Mn z Co1-y-z O2, NMC) as a cathode material in lithium-ion batteries poses risk to the environment. Learning toxicity mechanisms on molecular levels is critical to promote proactive risk assessment of these complex nanomaterials and inform their sustainable development. We focused on DNA damage as a toxicity mechanism and profiled in depth chemical and biological changes linked to DNA damage in two environmentally relevant bacteria upon nano-NMC exposure. DNA damage occurred in both bacteria, characterized by double-strand breakage and increased levels of many putative chemical modifications on bacterial DNA bases related to direct oxidative stress and lipid peroxidation, measured by cutting-edge DNA adductomic techniques. Chemical probes indicated elevated intracellular reactive oxygen species and transition metal ions, in agreement with DNA adductomics and gene expression analysis. By integrating multi-dimensional datasets from chemical and biological measurements, we present rich mechanistic insights on nano-NMC-induced DNA damage in bacteria, providing targets for biomarkers in the risk assessment of reactive materials that may be extrapolated to other nano-bio interactions.

Obesity and cancer: A mechanistic overview of metabolic changes in obesity that impact genetic instability

Obesity, defined as a state of positive energy balance with a body mass index exceeding 30 kg/m2 in adults and 95th percentile in children, is an increasing global concern. Approximately one-third of the world's population is overweight or obese, and in the United States alone, obesity affects one in six children. Meta-analysis studies suggest that obesity increases the likelihood of developing several types of cancer, and with poorer outcomes, especially in children. The contribution of obesity to cancer risk requires a better understanding of the association between obesity-induced metabolic changes and its impact on genomic instability, which is a major driving force of tumorigenesis. In this review, we discuss how molecular changes during adipose tissue dysregulation can result in oxidative stress and subsequent DNA damage. This represents one of the many critical steps connecting obesity and cancer since oxidative DNA lesions can result in cancer-associated genetic instability. In addition, the by-products of the oxidative degradation of lipids (e.g., malondialdehyde, 4-hydroxynonenal, and acrolein), and gut microbiota-mediated secondary bile acid metabolites (e.g., deoxycholic acid and lithocholic acid), can function as genotoxic agents and tumor promoters. We also discuss how obesity can impact DNA repair efficiency, potentially contributing to cancer initiation and progression. Finally, we outline obesity-related epigenetic changes and identify the gaps in knowledge to be addressed for the development of better therapeutic strategies for the prevention and treatment of obesity-related cancers.

Counteraction of Oxidative Stress by Vitamin E Affects Epigenetic Regulation by Increasing Global Methylation and Gene Expression of MLH1 and DNMT1 Dose Dependently in Caco-2 Cells

Obesity- or diabetes-induced oxidative stress is discussed as a major risk factor for DNA damage. Vitamin E and many polyphenols exhibit antioxidative activities with consequences on epigenetic regulation of inflammation and DNA repair. The present study investigated the counteraction of oxidative stress by vitamin E in the colorectal cancer cell line Caco-2 under normal (1 g/l) and high (4.5 g/l) glucose cell culture condition. Malondialdehyde (MDA) as a surrogate marker of lipid peroxidation and reactive oxygen species (ROS) was analyzed. Gene expression and promoter methylation of the DNA repair gene MutL homolog 1 (MLH1) and the DNA methyltransferase 1 (DNMT1) as well as global methylation by LINE-1 were investigated. Results revealed a dose-dependent counteracting effect of vitamin E on H₂O₂-induced oxidative stress. Thereby, 10 μM vitamin E proved to be more efficient than did 50 μM in reducing MDA. Further, an induction of MLH1 and DNMT1 gene expression was noticed, accompanied by an increase in global methylation. Whether LINE-1 hypomethylation is a cause or effect of oxidative stress is still unclear. In conclusion, supplementation of exogenous antioxidants like vitamin E in vitro exhibits beneficial effects concerning oxidative stress as well as epigenetic regulation involved in DNA repair.

Human DNA glycosylase enzyme TDG repairs thymine mispaired with exocyclic etheno-DNA adducts

Lipid peroxidation directly reacts with DNA and produces various exocyclic etheno-base DNA adducts, some of which are considered to contribute to carcinogenesis. However, the system for repairing them in humans is largely unknown. We hypothesized that etheno-DNA adducts are repaired by base excision repair initiated by DNA glycosylase. To test this hypothesis, we examined the activities of the DNA glycosylase proteins OGG1, SMUG1, TDG, NEIL1, MUTYH, NTH1, MPG, and UNG2 against double-stranded oligonucleotides containing 1,N(6)-ethenoadenine (εA), 3,N(4)-ethenocytosine (εC), butanone-ethenocytosine (BεC), butanone-ethenoguanine (BεG), heptanone-ethenocytosine (HεC), or heptanone-ethenoguanine (HεG) using a DNA cleavage assay. We found that TDG is capable of removing thymine that has mispaired with εC, BεC, BεG, HεC, or HεG in vitro. We next examined the effect of TDG against etheno-DNA adducts in human cells. TDG-knockdown cells exhibited the following characteristics: (a) higher resistance to cell death caused by the induction of etheno-DNA adducts; (b) lower repair activity for εC; and (c) a modest acceleration of mutations caused by εC, compared with the rate in control cells. All these characteristics suggest that TDG exerts a repair activity against etheno-DNA adducts in human cells. These results suggest that TDG has novel repair activities toward etheno-DNA adducts.

The effect of Msh2 knockdown on toxicity induced by tert-butyl-hydroperoxide, potassium bromate, and hydrogen peroxide in base excision repair proficient and deficient cells

The DNA mismatch repair (MMR) and base excision repair (BER) systems are important determinants of cellular toxicity following exposure to agents that cause oxidative DNA damage. To examine the interactions between these different repair systems, we examined whether toxicity, induced by t-BOOH and KBrO₃, differs in BER proficient (Mpg^+/+, Nth1^+/+) and deficient (Mpg^−/−, Nth1^−/−) mouse embryonic fibroblasts (MEFs) following Msh2 knockdown of between 79 and 88% using an shRNA expression vector. Msh2 knockdown in Nth1^+/+ cells had no effect on t-BOOH and KBrO₃ induced toxicity as assessed by an MTT assay; knockdown in Nth1^−/− cells resulted in increased resistance to t-BOOH and KBrO₃, a result consistent with Nth1 removing oxidised pyrimidines. Msh2 knockdown in Mpg^+/+ cells had no effect on t-BOOH toxicity but increased resistance to KBrO₃; in Mpg^−/− cells, Msh2 knockdown increased cellular sensitivity to KBrO₃ but increased resistance to t-BOOH, suggesting a role for Mpg in removing DNA damage induced by these agents. MSH2 dependent and independent pathways then determine cellular toxicity induced by oxidising agents. A complex interaction between MMR and BER repair systems, that is, exposure dependent, also exists to determine cellular toxicity.

Formation and repair of tobacco carcinogen-derived bulky DNA adducts

DNA adducts play a central role in chemical carcinogenesis. The analysis of formation and repair of smoking-related DNA adducts remains particularly challenging as both smokers and nonsmokers exposed to smoke are repetitively under attack from complex mixtures of carcinogens such as polycyclic aromatic hydrocarbons and N-nitrosamines. The bulky DNA adducts, which usually have complex structure, are particularly important because of their biological relevance. Several known cellular DNA repair pathways have been known to operate in human cells on specific types of bulky DNA adducts, for example, nucleotide excision repair, base excision repair, and direct reversal involving O⁶-alkylguanine DNA alkyltransferase or AlkB homologs. Understanding the mechanisms of adduct formation and repair processes is critical for the assessment of cancer risk resulting from exposure to cigarette smoke, and ultimately for developing strategies of cancer prevention. This paper highlights the recent progress made in the areas concerning formation and repair of bulky DNA adducts in the context of tobacco carcinogen-associated genotoxic and carcinogenic effects.

Modifications of nucleosides by endogenous mutagens-DNA adducts arising from cellular processes

DNA damage plays a significant role in mutagenesis, carcinogenesis and ageing. Chemical transformations leading to DNA damage include reactions of the base units with agents of endogenous and exogenous origin. The vast majority of damage arising from cellular processes such as metabolism and lipid peroxidation are identical or very similar to those induced by exposure to environmental agents. A detailed knowledge of the types and prevalence of endogenous DNA damage provides insight into the chemical nature of species involved in these modifications and may be of help in understanding their influence on the induction of cancer or other diseases. This knowledge may also be essential to the development of rational chemopreventive strategies directed against the initiation of oxidative stress- and lipid peroxidation-associated pathology. The present work reviews findings regarding the interaction between DNA bases and various reactive species arising from lipid peroxidation and other cellular processes, drawing attention to the mechanism responsible for the formation of the resulted modifications. The biological consequences of these interactions are also briefly discussed.

The importance of 3,4-epoxy-1,2-butanediol and hydroxymethylvinyl ketone in 3-butene-1,2-diol associated mutagenicity

1,2:3,4-Diepoxybutane is hypothesized to be the main intermediate involved in mutagenicity following exposure to low levels of 1,3-butadiene (BD) in mice, while metabolites of 3-butene-1,2-diol (BD-diol) are thought to become involved in both rats and mice at higher exposures. BD-diol is biotransformed to hydroxymethylvinyl ketone (HMVK), a potentially mutagenic metabolite, and 3,4-epoxy-1,2-butanediol (EB-diol), a known mutagen. To determine the relative importance of HMVK and EB-diol in BD-diol associated mutagenesis, we have examined the dosimetry of a HMVK derived DNA adduct, as well as EB-diol derived DNA and hemoglobin adducts, in rodents exposed to BD-diol. We previously demonstrated similarities in the shapes of the dose-response curves for EB-diol derived DNA adducts, hemoglobin adducts, and Hprt mutant frequencies in BD-diol exposed rodents, indicating that EB-diol was involved in the mutagenic response associated with BD-diol exposure. To examine the role of HMVK in BD-diol mutagenicity, a method to quantify the alpha-regioisomer of HMVK derived 1,N(2)-propanodeoxyguanosine (alpha-HMVK-dGuo) was developed. The method involved enzymatic hydrolysis of DNA, HPLC purification, and adduct measurement by liquid chromatography - tandem mass spectrometry. Intra- and inter-experimental variabilities were determined to be 2.3-18.2 and 4.1%, respectively. The limit of detection was approximately 5 fmol of analyte standard injected onto the column or 5 fmol/200 microg DNA. The method was used to analyze liver DNA from control female F344 rats and female F344 rats exposed to 36 ppm BD-diol. In addition, liver samples from female Sprague-Dawley rats exposed to 1000 ppm BD were analyzed. alpha-HMVK-dGuo was not detected in any of the samples analyzed. Several possible explanations exist for the negative results including the possibility that alpha-HMVK-dGuo may be a minor adduct or may be efficiently repaired. Alternatively, HMVK itself may be readily detoxified by glutathione (GSH) conjugation. While experiments must be conducted to understand the exact mechanism(s), these results, in addition to published EB-diol derived adduct dosimetry and existing HMVK derived mercapturic acid data, suggest that EB-diol is primarily responsible for BD-diol induced mutagenicity in rodents.

Lipids and DNA oxidation in Staphylococcus aureus as a consequence of oxidative stress generated by ciprofloxacin

Ciprofloxacin induced an increment of reactive oxygen species in sensitive strains of Staphylococcus aureus leading to oxidative stress detected by chemiluminescence while resistant strains did not suffer such stress. Oxidation of lipids was performed by employing thiobarbituric acid reaction to detect the formation of the amplified intermediate between reactive species oxygen and cytoplasmic macromolecules, namely malondialdehyde (MDA). The sensitive strain presented higher peroxidation of lipids than the resistant strain. The oxidative consequence for DNA was investigated by means of bacteria incubation with ciprofloxacin and posterior extraction of DNA, which was studied by high performance liquid chromatography (HPLC). Sensitive S. aureus ATCC 29213 showed an increase of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) respect controls without antibiotic; there was evident increase of the ratio between 8-oxodG and deoxyguanosine (dG) as a consequence of oxidation of dG to 8-oxodG considered the major DNA marker of oxidative stress. The resistant strain showed low oxidation of DNA and the analysis of 8-oxodG/dG ratio indicated lesser formation of 8-oxodG than S. aureus ATCC 29213.

Folate deficiency, mismatch repair-dependent apoptosis, and human disease

The vitamin that is most commonly deficient in the American diet is folate. Severe folate deficiency in humans is known to cause megaloblastic anemia and developmental defects, and is associated with an increased incidence of several forms of human cancer. Although the exact mechanisms by which this vitamin deficiency may cause these diseases are not known at the present time, recent work has shown that folate deficiency also causes genomic instability and programmed cell death (or apoptosis). Additionally, it is known that the DNA mismatch repair pathway mediates folate deficiency-induced apoptosis. This review will first describe work suggesting that folate deficiency causes genomic instability and apoptosis, then discuss possible mechanisms by which the mismatch repair pathway could trigger folate deficiency-induced apoptosis, which has either protective or destructive effects on tissue.

Malondialdehyde adducts in DNA arrest transcription by T7 RNA polymerase and mammalian RNA polymerase II

Malondialdehyde, a genotoxic byproduct of lipid peroxidation, reacts with guanine in DNA to form pyrimido[1,2-alpha]purin-10(3H)one (M(1)dG), the first endogenous DNA lesion found to be a target of nucleotide excision repair enzymes. A subpathway of nucleotide excision repair, transcription-coupled repair, is thought to occur when RNA polymerase (RNAP) is arrested at damage in transcribed DNA strands and might function for efficient removal of M(1)dG in active genes. Results presented here show that M(1)dG and its stable, exocyclic analog 1,N(2)-propanodeoxyguanine (PdG), arrest translocation of T7 RNAP and mammalian RNAPII when located in the transcribed strand of a DNA template. M(1)dG paired with thymine is exocyclic and poses a stronger block to transcription than the acyclic N(2)-(3-oxo-1-propenyl)-dG, formed upon cytosine-catalyzed opening of M(1)dG in duplex DNA. PdG is a complete block to RNAPII regardless of base pairing. The elongation factor TFIIS (SII) induces reversal and RNA transcript cleavage by RNAPII arrested at PdG. Thus, arrested RNAPII complexes may be stable at M(1)dG in cells and may resume transcription once the offending adduct is removed. The conclusion from this work is that malondialdehyde adducts in the transcribed strand of expressed genes are strong blocks to RNAPs and are targets for cellular transcription-coupled repair. If so, then M(1)dG, already known to be highly mutagenic in human cells, also may contribute to apoptosis in the developing tissues of individuals with Cockayne's syndrome, a hereditary disorder characterized by transcription-coupled repair deficiency.

Repair of exocyclic DNA adducts: rings of complexity

Exocyclic DNA adducts are mutagenic lesions that can be formed by both exogenous and endogenous mutagens/carcinogens. These adducts are structurally analogs but can differ in certain features such as ring size, conjugation, planarity and substitution. Although the information on the biological role of the repair activities for these adducts is largely unknown, considerable progress has been made on their reaction mechanisms, substrate specificities and kinetic properties that are affected by adduct structures. At least four different mechanisms appear to have evolved for the removal of specific exocyclic adducts. These include base excision repair, nucleotide excision repair, mismatch repair, and AP endonuclease-mediated repair. This overview highlights the recent progress in such areas with emphasis on structure-activity relationships. It is also apparent that more information is needed for a better understanding of the biological and structural implications of exocyclic adducts and their repair.

Induction of frameshift and base pair substitution mutations by the major DNA adduct of the endogenous carcinogen malondialdehyde

Instability of repetitive sequences is a hallmark of human cancer, and its enhancement has been linked to oxidative stress. Malondialdehyde is an endogenous product of oxidative stress that reacts with guanine to form the exocyclic adduct, pyrimido[1,2- alpha]purin-10(3H)-one (M1G). We used site-specifically modified single- and double-stranded vectors to investigate the mutagenic potential of M1G in bacteria and mammalian cells. M1G induced frameshift mutations (-1 and -2) when positioned in a reiterated (CpG)4 sequence but not when positioned in a nonreiterated sequence in Escherichia coli and in COS-7 cells. The frequency of frameshift mutations was highest when M1G was placed at the third G in the sequence. M1G induced base pair substitutions at comparable frequencies in both sequence contexts in COS-7 cells. These studies indicate that M1G, an endogenously generated product of oxidative stress, induces sequence-dependent frameshift mutations and base pair substitutions in bacteria and in mammalian cells. This finding suggests a potential role for the M1G lesion in the induction of mutations commonly associated with human diseases.

Malondialdehyde, a product of lipid peroxidation, is mutagenic in human cells

Malondialdehyde (MDA) is an endogenous genotoxic product of enzymatic and oxygen radical-induced lipid peroxidation whose adducts are known to exist in DNA isolated from healthy human beings. To evaluate the mutagenic potential of MDA in human cells, we reacted MDA with pSP189 shuttle vector DNA and then transfected them into human fibroblasts for replication. MDA induced up to a 15-fold increase in mutation frequency in the supF reporter gene compared with untreated DNA. Sequence analysis revealed that the majority of MDA-induced mutations occurred at GC base pairs. The most frequent mutations were large insertions and deletions, but base pair substitutions were also detected. MDA-induced mutations were completely abolished when the adducted shuttle vector was replicated in cells lacking nucleotide excision repair. MDA induction of large deletions and the apparent requirement for nucleotide excision repair suggested the possible involvement of a DNA interstrand cross-link as a premutagenic lesion. Indeed, MDA formed interstrand cross-links in duplex plasmids and oligonucleotides. Substrates containing the sequence 5'-d(CG) were preferentially cross-linked, consistent with the observation of base pair substitutions in 5'-d(CG) sites in the MDA-induced mutation spectrum. These experiments provide biological and biochemical evidence for the existence of MDA-induced DNA interstrand cross-links that could result from endogenous oxidative stress and likely have potent biological effects.

Binding discrimination of MutS to a set of lesions and compound lesions (base damage and mismatch) reveals its potential role as a cisplatin-damaged DNA sensing protein

The DNA mismatch repair (MMR) system plays a critical role in sensitizing both prokaryotic and eukaryotic cells to the clinically potent anticancer drug cisplatin. It is thought to mediate cytotoxicity through recognition of cisplatin DNA lesions. This drug generates a range of lesions that may also give rise to compound lesions resulting from the misincorporation of a base during translesion synthesis. Using gel mobility shift competition assays and surface plasmon resonance, we have analyzed the interaction of Escherichia coli MutS protein with site-specifically modified DNA oligonucleotides containing each of the four cisplatin cross-links or a set of compound lesions. The major 1,2-d(GpG) cisplatin intrastrand cross-link was recognized with only a 1.5-fold specificity, whereas a 47-fold specificity was found with a natural G/T containing DNA substrate. The rate of association, kon, for binding to the 1,2-d(GpG) adduct was 3.1 x 104 m-1 s-1 and the specificity of binding was essentially dependent on koff. DNA duplexes containing a single 1,2-d(ApG), 1,3-d(GpCpG) adduct, and an interstrand cross-link of cisplatin were not preferentially recognized. Among 12 DNA substrates, each containing a different cisplatin compound lesion derived from replicative misincorporation of one base opposite either of the 1,2-intrastrand adducts, 10 were specifically recognized including those that are more likely formed in vivo based on cisplatin mutation spectra. Moreover, among these lesions, two compound lesions formed when an adenine was misincorporated opposite a 1,2-d(GpG) adduct were not substrates for the MutY-dependent mismatch repair pathway. The ability of MutS to sense differentially various platinated DNA substrates suggests that cisplatin compound lesions formed during misincorporation of a base opposite either adducted base of both 1,2-intrastrand cross-links are more plausible critical lesions for MMR-mediated cisplatin cytotoxicity.

Who's on first in the cellular response to DNA damage?

Cellular DNA-repair pathways involve proteins that have roles in other DNA-metabolic processes, as well as those that are dedicated to damage removal. Several proteins, which have diverse functions and are not known to have roles in DNA repair, also associate with damaged DNA. These newly discovered interactions could either facilitate or hinder the recognition of DNA damage, and so they could have important effects on DNA repair and genetic integrity. The outcome for the cell, and ultimately for the organism, might depend on which proteins arrive first at sites of DNA damage.

Construction and purification of his6-Thermus thermophilus MutS protein

The mutS gene from the thermophilic bacterium Thermus thermophilus was PCR amplified, cloned, and expressed in Escherichia coli. The recombinant MutS protein containing an oligohistidine domain at the N-terminus was purified in a single step by Ni(2+) affinity chromatography to apparent homogeneity. The mismatch recognition properties of the his(6)-tagged MutS protein were confirmed by DNA protection against exonuclease digestion and retardation assays. The results of analytical gel filtration indicate that the predominant form of T. thermophilus MutS at micromolar concentrations is a tetramer.

The use of non-tumor data in cancer risk assessment: reflections on butadiene, vinyl chloride, and benzene

The estimation and characterization of a cancer risk is grounded in the observation of tumors in humans and/or experimental animals. Increasingly, however, other kinds of data (non-tumor data) are finding application in cancer risk assessment. Metabolism and kinetics, adduct formation, genetic damage, mode of action, and biomarkers of exposure, susceptibility, and effects are examples. While these and other parameters have been studied for many important chemicals over the past 30-40 years, their use in risk assessments is more recent, and new insights and opportunities are continuing to unfold. To provide some perspective on this field, the ILSI Risk Science Institute asked a select working group to characterize the pertinent non-tumor data available for 1,3-butadiene, benzene, and vinyl chloride and to comment on the utility of these data in characterizing cancer risks. This paper presents the findings of that working group and concludes with 15 simple principles for the use of non-tumor data in cancer risk assessment.

Interim report on genomics of Escherichia coli

We present a summary of recent progress in understanding Escherichia coli K-12 gene and protein functions. New information has come both from classical biological experimentation and from using the analytical tools of functional genomics. The content of the E. coli genome can clearly be seen to contain elements acquired by horizontal transfer. Nevertheless, there is probably a large, stable core of >3500 genes that are shared among all E. coli strains. The gene-enzyme relationship is examined, and, in many cases, it exhibits complexity beyond a simple one-to-one relationship. Also, the E. coli genome can now be seen to contain many multiple enzymes that carry out the same or closely similar reactions. Some are similar in sequence and may share common ancestry; some are not. We discuss the concept of a minimal genome as being variable among organisms and obligatorily linked to their life styles and defined environmental conditions. We also address classification of functions of gene products and avenues of insight into the history of protein evolution.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

One tube mutation detection using sensitive fluorescent dyeing of MutS protected DNA

A novel, universal method for mutation detection utilising the ability of MutS protein to recognise DNA incomplementarities is proposed. The examined and reference DNA fragments are PCR amplified. The PCR products are purified, mixed, heated and cooled to form heteroduplexes. In the case of mutation the heteroduplex DNA containing mismatch is protected against exonuclease digestion by MutS, while the DNA without mismatches is degraded. The protection effect is visualised by the direct addition of a highly sensitive fluorescent dye (SYBR-Gold) selectively binding DNA. The Thermus thermophilus recombined His-tagged MutS and 3'-5' exonuclease activity of T4 DNA polymerase were used in the assay.

Overexpression of enzymes that repair endogenous damage to DNA

A significant contribution to human mutagenesis and carcinogenesis may come from DNA damage of endogenous, rather than exogenous, origin. Efficient repair mechanisms have evolved to cope with this. The main repair pathway involved in repair of endogenous damage is DNA base excision repair. In addition, an important contribution is given by O6-alkylguanine DNA alkyltranferase, that repairs specifically the miscoding base O6-alkylguanine. In recent years, several attempts have been carried out to enhance the efficiency of repair of endogenous damage by overexpressing in mammalian cells single enzymatic activities. In some cases (e.g. O6-alkylguanine DNA alkyltransferase or yeast AP endonuclease) this approach has been successful in improving cellular protection from endogenous and exogenous mutagens, while overexpression of other enzymatic activities (e.g. alkyl N-purine glycosylase or DNA polymerase beta) were detrimental and even produced a genome instability phenotype. The reasons for these different outcomes are analyzed and alternative enzymatic activities whose overexpression may improve the efficiency of repair of endogenous damage in human cells are proposed.

Oxyradicals and DNA damage

A major development of carcinogenesis research in the past 20 years has been the discovery of significant levels of DNA damage arising from endogenous cellular sources. Dramatic improvements in analytical chemistry have provided sensitive and specific methodology for identification and quantitation of DNA adducts. Application of these techniques to the analysis of nuclear DNA from human tissues has debunked the notion that the human genome is pristine in the absence of exposure to environmental carcinogens. Much endogenous DNA damage arises from intermediates of oxygen reduction that either attack the bases or the deoxyribosyl backbone of DNA. Alternatively, oxygen radicals can attack other cellular components such as lipids to generate reactive intermediates that couple to DNA bases. Endogenous DNA lesions are genotoxic and induce mutations that are commonly observed in mutated oncogenes and tumor suppressor genes. Their mutagenicity is mitigated by repair via base excision and nucleotide excision pathways. The levels of oxidative DNA damage reported in many human tissues or in animal models of carcinogenesis exceed the levels of lesions induced by exposure to exogenous carcinogenic compounds. Thus, it seems likely that oxidative DNA damage is important in the etiology of many human cancers. This review highlights some of the major accomplishments in the study of oxidative DNA damage and its role in carcinogenesis. It also identifies controversies that need to be resolved. Unraveling the contributions to tumorigenesis of DNA damage from endogenous and exogenous sources represents a major challenge for the future.