Molecular cloning and functional expression of a human cDNA encoding the antimutator enzyme 8-hydroxyguanine-DNA glycosylase

The single-strand DNA binding activity of human PC4 prevents mutagenesis and killing by oxidative DNA damage

Human positive cofactor 4 (PC4) is a transcriptional coactivator with a highly conserved single-strand DNA (ssDNA) binding domain of unknown function. We identified PC4 as a suppressor of the oxidative mutator phenotype of the Escherichia coli fpg mutY mutant and demonstrate that this suppression requires its ssDNA binding activity. Saccharomyces cerevisiae mutants lacking their PC4 ortholog Sub1 are sensitive to hydrogen peroxide and exhibit spontaneous and peroxide-induced hypermutability. PC4 expression suppresses the peroxide sensitivity of the yeast sub1Delta mutant, suggesting that the human protein has a similar function. A role for yeast and human proteins in DNA repair is suggested by the demonstration that Sub1 acts in a peroxide resistance pathway involving Rad2 and by the physical interaction of PC4 with the human Rad2 homolog XPG. We show that XPG recruits PC4 to a bubble-containing DNA substrate with a resulting displacement of XPG and formation of a PC4-DNA complex. We discuss the possible requirement for PC4 in either global or transcription-coupled repair of oxidative DNA damage to mediate the release of XPG bound to its substrate.

The multiple colorectal adenoma phenotype and MYH, a base excision repair gene

We summarize the genetic and clinical features of the colorectal adenomas and cancers that occur in MYH-associated polyposis (MAP). MAP results from biallelic germline mutations in the base excision repair gene, mutY homologue (MYH). MAP has a phenotype that is often indistinguishable from classical or attenuated familial adenomatous polyposis (FAP), but the former is inherited as a recessive condition, whereas the latter is a dominantly inherited disease caused by germline mutations of the APC gene. MYH mutations seem to act by increasing the frequency of somatic APC mutations. MAP tumors may then progress to cancer along a distinct genetic pathway. MAP occurs in several different ethnic groups, the mutation spectrum appearing to differ among groups. It remains unknown, however, as to why carriers of MYH mutations specifically develop tumors of the gastrointestinal tract. In general, carriers of biallelic MYH mutations should be treated and followed up as for FAP patients with a similar phenotype. Relatives of MAP patients should be counseled as for any other recessive condition, although it remains possible that carriers of single mutations are at a modestly increased risk of colorectal cancer.

Metallothionein-III prevents gamma-ray-induced 8-oxoguanine accumulation in normal and hOGG1-depleted cells

Metallothioneins (MT) play an important biological role in preventing oxidative damage to cells. We have previously demonstrated that the efficiency of the protective effect of MT-III against the DNA degradation from oxidative damage was much higher than that of MT-I/II. As an extension of the latter investigation, this study aimed to assess the ability of MT-III to suppress 8-oxoguanine (8-oxoG), which is one of the major base lesions formed after an oxidative attack to DNA and the mutant frequency of the HPRT gene in human fibroblast GM00637 cells upon exposure to gamma-rays. We found that human MT-III expression decreased the level of 8-oxoG and mutation frequency in the gamma-irradiated cells. Using an 8-oxoguanine DNA glycosylase (OGG1)-specific siRNAs, we also found that MT-III expression resulted in the suppression of the gamma-radiation-induced 8-oxoG accumulation and mutation in the OGG1-depleted cells. Moreover, the down-regulation of MT in human neuroblastoma SKNSH cells induced by MT-specific siRNA led to a significant increase in the 8-oxoG level, after exposure to gamma-irradiation. These results suggest that under the conditions of gamma-ray oxidative stress, MT-III prevents the gamma-radiation-induced 8-oxoG accumulation and mutation in normal and hOGG1-depleted cells, and this suppression might, at least in part, contribute to the anticarcinogenic and neuroprotective role of MT-III.

Comprehensive analysis of the contribution of germline MYH variation to early-onset colorectal cancer

Mutations in the base excision repair gene MYH have recently been shown to confer recessive susceptibility to colorectal adenomas and carcinomas. To evaluate the contribution of germline MYH mutations to early-onset colorectal cancer, we screened a series of 358 unselected early-onset cases for germline changes in the coding sequence of the gene. Two cases harbored biallelic germline mutations (0.6%; 95% CI = 0.06-2.0) and 8 single MYH mutations (2.2%; 95% CI = 0.9-4.4). Both cases harboring biallelic MYH mutations had multiple polyps but not profuse polyposis. All cases had distally sited tumors. No biallelic mutations were detected among 354 controls. These results confirm that biallelic MYH mutations confer susceptibility to colorectal cancer but are unlikely to account for more than 3% of early-onset colorectal cancer.

Transcription Factors NF-YA Regulate the Induction of Human OGG1 Following DNA-alkylating Agent Methylmethane Sulfonate (MMS) Treatment

A human 8-oxoguanine-DNA glycosylase (hOGG1) is the main enzyme that repairs 8-oxoG, which is a critical mutagenic lesion. There is a great deal of interest in the up- or down-regulation of OGG1 expression after DNA damage. In this study, we investigated the effect of a DNA-alkylating agent, methylmethane sulfonate (MMS), on hOGG1 expression level and found that MMS treatment resulted in an increase in the functional hOGG1 expression in HCT116 cells. A region between -121 and -61 of the hOGG1 promoter was found to be crucial for this induction by MMS. Site-directed mutations of the two inverted CCAAT motifs substantially abrogated the induction of the hOGG1 promoter as a result of MMS treatment. In addition, the NF-YA protein (binding to the inverted CCAAT box) was induced after exposing cells to MMS. Moreover, gel shift and supershift analyses with the nuclear extracts prepared from HCT116 cells identified NF-YA as the transcription factor interacting with the inverted CCAAT box. Mutations of the inverted CCAAT box either prevented the binding of this factor or abolished its activation as a result of MMS treatment. Finally, this study showed that hOGG1-expressing HCT116 cells exhibited increased hOGG1 repair activity and resistance to MMS. Overall, these results demonstrate that MMS can up-regulate hOGG1 expression through the induction of the transcription factor, NF-YA, and increased transcription level of the hOGG1 gene correlates with an increase in enzyme activity providing functional protection from MMS.

Catalytic and DNA-binding properties of the human Ogg1 DNA N-glycosylase/AP lyase: biochemical exploration of H270, Q315 and F319, three amino acids of the 8-oxoguanine-binding pocket

The human Ogg1 protein (hOgg1) is an antimutator DNA glycosylase/AP lyase that catalyzes the excision of 8-oxo-7,8-dihydroguanine (8-oxoG) and the incision of apurinic and apyrimidinic (AP) sites in DNA. In this study, we have investigated the functional role of H270, Q315 and F319, three amino acids that are located in the 8-oxoG-binding pocket of hOgg1. Wild-type and mutant hOgg1 proteins (H270A, H270R, H270L, Q315A and F319A) were purified to apparent homogeneity. The catalytic activities and the DNA-binding properties of the various hOgg1 mutants were compared to those of the wild-type. The results show that hOgg1 mutated at H270 (H270A and H270L) or F319 (F319A) exhibits greatly reduced (50- to 1000-fold) DNA glycosylase activity, whereas the AP lyase activity is only moderately affected (<4-fold). The affinity of the hOgg1 mutants (H270A, H270L and F319A) for 8-oxoG.C-containing DNA is also greatly reduced (>30-fold), whereas their affinity for THF.C-containing DNA is only moderately reduced (<7-fold). The results also show that hOgg1 mutated at Q315 (Q315A) exhibits catalytic and DNA-binding properties similar to those of the wild-type. Therefore, H270 and F319 are essential to form the functional 8-oxoG-binding pocket, whereas Q315 is less crucial. In contrast, H270, Q315 and F319 are not required for efficient binding of THF.C and cleavage of AP sites. Finally, hOgg1 mutant proteins with a substitution of H270A or F319A are members of a new type of hOgg1 that is deficient in DNA glycosylase but proficient in AP lyase.

Probing the Requirements for Recognition and Catalysis in Fpg and MutY with Nonpolar Adenine Isosteres

The Escherichia coli DNA repair enzymes Fpg and MutY are involved in the prevention of mutations resulting from 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG) in DNA. The nonpolar isosteres of 2'-deoxyadenosine, 4-methylbenzimidazole beta-deoxynucleoside (B), and 9-methyl-1H-imidazo[4,5-b]pyridine beta-deoxynucleoside (Q), were used to examine the importance of hydrogen bonding within the context of DNA repair. Specifically, the rate of base removal under single-turnover conditions by the MutY and Fpg glycosylases from duplexes containing OG:B and OG:Q mismatches, relative to OG:A mismatches, was evalulated. The reaction of Fpg revealed a 5- and 10-fold increase in rate of removal of OG from duplexes containing OG:B and OG:Q base pairs, respectively, relative to an OG:A mispair. These results suggest that the lack of the ability to hydrogen bond to the opposite base facilitates removal of OG. In contrast, adenine removal catalyzed by MutY was much more efficient from an OG:A mispair-containing duplex (k2 = 12 +/- 2 min(-1)) compared to the removal of B from an OG:B duplex (k(obs) < 0.002 min(-1)). Surprisingly, MutY was able to catalyze base removal from the OG:Q-containing substrate (k2 = 1.2 +/- 0.2 min(-1)). Importantly, the B and Q analogues are not deleterious to high-affinity DNA binding by MutY. In addition, the B and Q analogues are more susceptible to acid-catalyzed depurination illustrating that the enzyme-catalyzed mechanism is distinct from the nonenzymatic mechanism. Taken together, these results point to the importance of both N7 and N3 in the mechanism of adenine excision catalyzed by MutY.

Mammalian DNA base excision repair proteins: their interactions and role in repair of oxidative DNA damage

The DNA base excision repair (BER) is a ubiquitous mechanism for removing damage from the genome induced by spontaneous chemical reaction, reactive oxygen species (ROS) and also DNA damage induced by a variety of environmental genotoxicants. DNA repair is essential for maintaining genomic integrity. As we learn more about BER, a more complex mechanism emerges which supersedes the classical, simple pathway requiring only four enzymatic reactions. The key to understand the complete BER process is to elucidate how multiple proteins interact with one another in a coordinated process under specific physiological conditions.

Substrate discrimination by formamidopyrimidine-DNA glycosylase: a mutational analysis

Formamidopyrimidine-DNA glycosylase (Fpg) is a primary participant in the repair of 8-oxoguanine, an abundant oxidative DNA lesion. Although the structure of Fpg has been established, amino acid residues that define damage recognition have not been identified. We have combined molecular dynamics and bioinformatics approaches to address this issue. Site-specific mutagenesis coupled with enzyme kinetics was used to test our predictions. On the basis of molecular dynamics simulations, Lys-217 was predicted to interact with the O8 of extrahelical 8-oxoguanine accommodated in the binding pocket. Consistent with our computational studies, mutation of Lys-217 selectively reduced the ability of Fpg to excise 8-oxoguanine from DNA. Dihydrouracil, also a substrate for Fpg, served as a nonspecific control. Other residues involved in damage recognition (His-89, Arg-108, and Arg-109) were identified by combined conservation/structure analysis. Arg-108, which forms two hydrogen bonds with cytosine in Fpg-DNA, is a major determinant of opposite-base specificity. Mutation of this residue reduced excision of 8-oxoguanine from thermally unstable mispairs with guanine or thymine, while excision from the stable cytosine and adenine base pairs was less affected. Mutation of His-89 selectively diminished the rate of excision of 8-oxoguanine, whereas mutation of Arg-109 nearly abolished binding of Fpg to damaged DNA. Taken together, these results suggest that His-89 and Arg-109 form part of a reading head, a structural feature used by the enzyme to scan DNA for damage. His-89 and Lys-217 help determine the specificity of Fpg in recognizing the oxidatively damaged base, while Arg-108 provides specificity for bases positioned opposite the lesion.

Inducible repair of oxidative DNA lesions in the rat brain after transient focal ischemia and reperfusion

To determine the role of oxidative DNA damage and repair in brain injury after focal ischemia and reperfusion, the authors investigated DNA base damage and DNA base excision repair (BER) capacity, the predominant repair mechanism for oxidative DNA lesions, in the rat model of temporary middle cerebral artery occlusion. Contents of 8-hydroxyl-2'-deoxyguanosine (8-oxodG) and apurinic/apyrimidinic abasic site (AP site), hallmarks of oxidative DNA damage, were quantitatively measured in nuclear DNA extracts from brains 0.25 to 72 hours after 1 hour of middle cerebral artery occlusion. In parallel to the detection of DNA lesions, the capacity for 8-oxodG- or AP site-dependent DNA repair synthesis was measured in nuclear protein extracts using specific in vitro DNA repair assays. After postischemic reperfusion, the levels of 8-oxodG and AP sites were markedly increased in ischemic tissues. In frontal/parietal cortex, regions that survived ischemia, 8-oxodG and AP sites were efficiently repaired during reperfusion. However, in the caudate, a region that was destined to infarct, the DNA lesions were poorly repaired. In consistent with the patterns of endogenous lesion repair, a markedly induced and long-lasting (at least 72 hours) BER activity was detected in the cortex but not in the caudate after ischemia. The induced BER activity in ischemic cortex was attributed to the upregulation of gene expression and activation of selective BER enzymes, particularly DNA polymerase-beta and OGG1. These results strongly suggest that inducible DNA BER constitutes an important endogenous mechanism that protects brain against ischemia-induced oxidative neuronal injury.

Oxidative DNA damage and repair in skeletal muscle of humans exposed to high-altitude hypoxia

Recent research suggests that high-altitude hypoxia may serve as a model for prolonged oxidative stress in healthy humans. In this study, we investigated the consequences of prolonged high-altitude hypoxia on the basal level of oxidative damage to nuclear DNA in muscle cells, a major oxygen-consuming tissue. Muscle biopsies from seven healthy humans were obtained at sea level and after 2 and 8 weeks of hypoxia at 4100 m.a.s.l. We found increased levels of strand breaks and endonuclease III-sensitive sites after 2 weeks of hypoxia, whereas oxidative DNA damage detected by formamidopyrimidine DNA glycosylase (FPG) protein was unaltered. The expression of 8-oxoguanine DNA glycosylase 1 (OGG1), determined by quantitative RT-PCR of mRNA levels did not significantly change during high-altitude hypoxia, although the data could not exclude a minor upregulation. The expression of heme oxygenase-1 (HO-1) was unaltered by prolonged hypoxia, in accordance with the notion that HO-1 is an acute stress response protein. In conclusion, our data indicate high-altitude hypoxia may serve as a good model for oxidative stress and that antioxidant genes are not upregulated in muscle tissue by prolonged hypoxia despite increased generation of oxidative DNA damage.

Exposing the MYtH about base excision repair and human inherited disease

Base excision repair (BER) protects against damage to DNA from reactive oxygen species, methylation, deamination, hydroxylation and other by-products of cellular metabolism. Until last year, inherited deficiencies in the BER pathway had not been causally linked with any human genetic disorder. An apparent explanation was functional redundancy between proteins in this and other pathways. However, it was recently discovered that biallelic mutations in the BER DNA glycosylase MYH lead to an autosomal recessive syndrome of adenomatous colorectal polyposis and very high colorectal cancer risk. We review the molecular mechanism of tumourigenesis in MYH polyposis, the preliminary delineation of the MYH polyposis phenotype and the functional overlap of MYH with other repair proteins.

Proportion and phenotype of MYH-associated colorectal neoplasia in a population-based series of Finnish colorectal cancer patients

Recessively inherited mutations in the base excision repair gene MYH have recently been associated with predisposition to colorectal adenomas and cancer in materials selected for occurrence of multiple adenomas. In particular, variants Y165C and G382D have been shown to play a role in Caucasian patients. To evaluate the contribution of MYH mutations to colorectal cancer burden on the population level, and to examine the MYH-associated phenotype in an unselected series of colorectal cancer patients, we determined the frequencies of Y165C and G382D MYH mutations in a population-based series of 1042 Finnish colorectal cancer patients. Four (0.4%) patients had both MYH alleles mutated. Although all these patients had multiple adenomatous polyps, the phenotypes tended to be less extreme than in previous studies on selected cases. The lowest number of colorectal adenomas at the time of cancer diagnosis was five. Cases with one mutant MYH allele were subjected to sequencing of all exons to detect possible Finnish founder mutations, but no additional changes were detected. The Y165C and G382D variants were not present in 424 Finnish cancer-free controls showing that MYH mutations are not enriched in the population. As evaluated against national Finnish Polyposis Registry data MYH-associated colorectal cancer appears to be as common as colorectal cancer associated with familial adenomatous polyposis.

Normal somatic hypermutation of Ig genes in the absence of 8-hydroxyguanine-DNA glycosylase

The hypermutation cascade in Ig V genes can be initiated by deamination of cytosine in DNA to uracil by activation-induced cytosine deaminase and its removal by uracil-DNA glycosylase. To determine whether damage to guanine also contributes to hypermutation, we examined the glycosylase that removes oxidized guanine from DNA, 8-hydroxyguanine-DNA glycosylase (OGG1). OGG1 has been reported to be overexpressed in human B cells from germinal centers, where mutation occurs, and could be involved in initiating Ab diversity by removing modified guanines. In this study, mice deficient in Ogg1 were immunized, and V genes from the H and kappa L chain loci were sequenced. Both the frequency of mutation and the spectra of nucleotide substitutions were similar in ogg1(-/-) and Ogg1(+/+) clones. More importantly, there was no significant increase in G:C to T:A transversions in the ogg1(-/-) clones, which would be expected if 8-hydroxyguanine remained in the DNA. Furthermore, Ogg1 was not up-regulated in murine B cells from germinal centers. These findings show that hypermutation is unaffected in the absence of Ogg1 activity and indicate that 8-hydroxyguanine lesions most likely do not cause V gene mutations.

Poly(ADP-ribose) polymerase-1 (PARP-1) is required in murine cell lines for base excision repair of oxidative DNA damage in the absence of DNA polymerase beta

Oxidative DNA base damage is mainly corrected by the base excision repair (BER) pathway, which can be divided into two subpathways depending on the length of the resynthetized patch, either one nucleotide for short patch BER or several nucleotides for long patch BER. The role of proteins in the course of BER processes has been investigated in vitro using purified enzymes and cell-free extracts. In this study, we have investigated the repair of 8-oxo-7,8-dihydroguanine (8-oxoG) in vivo using wild-type, polymerase beta(-/-) (Polbeta(-/-)), poly(ADP-ribose) polymerase-1(-/-) (PARP-1(-/-)), and Polbeta(-/-)PARP-1(-/-) 3T3 cell lines. We used non replicating plasmids containing a 8-oxoG:C base pair to study the repair of the lesion located in a transcribed sequence (TS) or in a non-transcribed sequence (NTS). The results show that 8-oxoG repair in TS is not significantly impaired in cells deficient in Polbeta or PARP-1 or both. Whereas 8-oxoG repair in NTS is normal in Polbeta-null cells, it is delayed in PARP-1-null cells and greatly impaired in cells deficient in both Polbeta and PARP-1. The removal of 8-oxoG and presumably the cleavage at the resulting apurinic/apyrimidinic site are not affected in the PARP-1(-/-)Polbeta(-/-) cell lines. However, 8-oxoG repair is incomplete, yielding plasmid molecules with a nick at the site of the lesion. Therefore, PARP-1(-/-)Polbeta(-/-) cell lines cannot perform 5'-dRP removal and/or DNA repair synthesis. Furthermore, the poly(ADP-ribosyl)ation activity of PARP-1 is essential for 8-oxoG repair in a Polbeta(-/-) context, because expression of the catalytically inactive PARP-1 (E988K) mutant does not restore 8-oxoG repair, whereas an wild type PARP-1 does.

Environmental exposure and lung cancer among nonsmokers: an example of Taiwanese female lung cancer

Lung cancer is the leading cause of cancer death worldwide and in Taiwan. Cigarette smoking is considered to be the most important risk factor, since about 90% of lung cancer can be related to cigarette smoking. Despite the recent decrease of cigarette smoking, lung cancer is still the leading cause of cancer death in the United States. In Taiwan, only around 50% of lung cancer incidence could be associated with cigarette smoking, particularly less than 10% of Taiwanese women are smokers. Thus, the aetiology of lung cancer for nonsmokers remains unknown. DNA damages including bulky and oxidative damage may be related with mutation of tumor suppressor genes, such as p53 gene. The high DNA adduct levels in female may be associated with frequent exposure to indoor cooking oil fumes (COF) and outdoor heavy air pollution. Oxidative stress induced by COF was also discussed. Different p53 mutation spectra and mutation frequency between genders reflected that different environmental factors may be involved in nonsmoking male and female lung cancer development. Most importantly, our recent report has demonstrated that human papillomavirus (HPV) infection was associated with nonsmoking female lung cancer. Based on our studies with Taiwanese nonsmoking lung cancer as the model, the possible aetiological factors of lung cancer incidence in Taiwanese nonsmokers were elucidated.

Product-assisted catalysis in base-excision DNA repair

Most spontaneous damage to bases in DNA is corrected through the action of the base-excision DNA repair pathway. Base excision repair is initiated by DNA glycosylases, lesion-specific enzymes that intercept aberrant bases in DNA and catalyze their excision. How such proteins accomplish the feat of catalyzing no fewer than five sequential reaction steps using a single active site has been unknown. To help answer this, we report the structure of a trapped catalytic intermediate in DNA repair by human 8-oxoguanine DNA glycosylase. This structure and supporting biochemical results reveal that the enzyme sequesters the excised lesion base and exploits it as a cofactor to participate in catalysis. To our knowledge, the present example represents the first documented case of product-assisted catalysis in an enzyme-catalyzed reaction.

Age-associated decrease of oxidative repair enzymes, human 8-oxoguanine DNA glycosylases (hOgg1), in human aging

8-Oxoguanine has been shown to be a dominant cause of oxidative DNA damage by oxygen free radicals in eukaryotic cells. The 8-oxoguanine repair-specific enzyme 8-oxoguanine-DNA glycosylase (hOgg1) was recently cloned and was observed to conduct mainly short-patch base-excision repair. It has also been suggested that reactive oxygen species play an important role in the cellular aging process. We explored the association between the hOgg1 enzyme activity in somatic cells of human subjects of various ages and the role of hOgg1(326) genetic polymorphism. An 8-oxoguanine-containing 28 mer oligonucleotide was end-labeled with gamma-32P ATP and incubated with protein extracts from peripheral blood lymphocytes (PBL) from 78 healthy individuals ranging in age from newborn to 91 years old. The hOgg1 repair activity toward the radiolabelled 8-oxoguanine-containing DNA was determined, and the results indicated a significant age-dependent decrease in the hOgg1 activity in their lymphocytes. Significantly reduced activity was also shown in those with Cysteine/Cysteine genotypes. The genders of the subjects were not shown to be associated. These results provide an important observation regarding the cellular hOgg1 activity in somatic cells during the normal human aging processes.

Multiple colorectal adenomas, classic adenomatous polyposis, and germ-line mutations in MYH

BACKGROUND

Germ-line mutations in the base-excision-repair gene MYH have been associated with recessive inheritance of multiple colorectal adenomas. Tumors from affected persons displayed excess somatic transversions of a guanine-cytosine pair to a thymine-adenine pair (G:C-->T:A) in the APC gene.

METHODS

We screened for germ-line MYH mutations in 152 patients with multiple (3 to 100) colorectal adenomas and 107 APC-mutation-negative probands with classic familial adenomatous polyposis (>100 adenomas). Subgroups were analyzed for changes in the related genes MTH1 and OGG1. Adenomas were tested for somatic APC mutations.

RESULTS

Six patients with multiple adenomas and eight patients with polyposis had biallelic germline MYH variants. Missense and protein-truncating mutations were found, and the spectrums of mutations were very similar in the two groups of patients. In the tumors of carriers of biallelic mutations, all somatic APC mutations were G:C-->T:A transversions. In the group with multiple adenomas, about one third of patients with more than 15 adenomas had biallelic MYH mutations. In the polyposis group, no patient with biallelic MYH mutations had severe disease (>1000 adenomas), but three had extracolonic disease. No clearly pathogenic MTH1 or OGG1 mutations were identified.

CONCLUSIONS

Germ-line MYH mutations predispose persons to a recessive phenotype, multiple adenomas, or polyposis coli. For patients with about 15 or more colorectal adenomas--especially if no germ-line APC mutation has been identified and the family history is compatible with recessive inheritance--genetic testing of MYH is indicated for diagnosis and calculation of the level of risk in relatives. Clinical care of patients with biallelic MYH mutations should be similar to that of patients with classic or attenuated familial adenomatous polyposis.

Limited repair of 8-hydroxy-7,8-dihydroguanine residues in human testicular cells

Oxidative damage in testicular DNA is associated with poor semen quality, reduced fertility and increased risk of stillbirths and birth defects. These DNA lesions are predominantly removed by base excision repair. Cellular extracts from human and rat testicular cells and three enriched populations of rat male germ cells (primary spermatocytes, round spermatids and elongating/elongated spermatids) all showed proficient excision/incision of 5-hydroxycytosine, thymine glycol and 2,6-diamino-4-hydroxy-5-formamidopyrimidine. DNA containing 8-oxo-7,8-dihydroguanine was excised poorly by human testicular cell extracts, although 8-oxoguanine-DNA glycosylase-1 (hOGG1) was present in human testicular cells, at levels that varied markedly between 13 individuals. This excision was as low as with human mononuclear blood cell extracts. The level of endonuclease III homologue-1 (NTH1), which excises oxidised pyrimidines, was higher in testicular than in somatic cells of both species. Cellular repair studies of lesions recognised by formamidopyrimidine-DNA glycosylase (Fpg) or endonuclease III (Nth) were assayed with alkaline elution and the Comet assay. Consistent with the enzymatic activities, human testicular cells showed poor removal of Fpg-sensitive lesions but efficient repair of Nth-sensitive lesions. Rat testicular cells efficiently repaired both Fpg- and Nth-sensitive lesions. In conclusion, human testicular cells have limited capacity to repair important oxidative DNA lesions, which could lead to impaired reproduction and de novo mutations.

Progress in the analysis of urinary oxidative DNA damage

Oxidative DNA damage has been implicated to be important in the pathogenesis of many diseases, including cancer and heart disease. The assessment of damage in various biological matrices, such as DNA, serum, and urine, is vital to understanding this role and subsequently devising intervention strategies. Despite the numerous techniques to measure oxidative DNA damage products in urine, it remains unclear what these measurements truly represent. Sources of urinary lesions may include the diet, cell death, and, of most interest, DNA repair. Were it possible to exclude the two former contributions, a noninvasive assay for DNA repair would be invaluable in the study of DNA damage and disease. This review highlights that, although progress has been made, significant work remains. Diet, cell death, and repair need continued examination to further elucidate the kinetics of lesion formation and clearance in vivo. Studies from our laboratory and others are making appreciable progress towards the interpretation of urinary lesion measurements along with the development of urinary assays to evaluate DNA repair. Upon establishment of these details, urinary oxidative DNA damage measurements may become more than a reflection of generalized oxidative stress.

Abasic sites in DNA of HeLa cells induced by lucanthone

Abasic sites in HeLa cell DNA were increased in frequency by exposing the cells to lucanthone. Cell growth in the presence of lucanthone caused progressive accumulation of abasic sites and loss of cellular DNA. After 2 hr in 8 microM lucanthone, the abundance of abasic sites was 2.4 fold greater than the background of 9.9 +/- 2.0 SE abasic sites/10(6) nucleotides; 80 microM lucanthone in the growth medium increased the level 12.6 +/- 2.5 SE fold and decreased the DNA content in HeLa cells to one-half of the value obtained in untreated cells. The frequency of abasic sites in cellular DNA was determined by the aldehyde reactive probe method, with reference to abasic sites created in plasmid pBR322. The ability of lucanthone to inhibit the normal repair of abasic sites might reflect inhibition of apurinic/apyrimidinic endonuclease (HAP1) by the drug, thereby preventing an early step in the base excision repair pathway. Unrepaired abasic sites prevalent after ionizing radiation are cytotoxic lesions that promote DNA strand breaks. These results suggest a rationale for the joint lethal effects of lucanthone and ionizing radiation in cells and the accelerated tumor regression observed in cancer patients who received the combined therapy.

Mitochondrial repair of 8-oxoguanine is deficient in Cockayne syndrome group B

Reactive oxygen species, which are prevalent in mitochondria, cause oxidative DNA damage including the mutagenic DNA lesion 7,8-dihydroxyguanine (8-oxoG). Oxidative damage to mitochondrial DNA has been implicated as a causative factor in a wide variety of degenerative diseases, and in cancer and aging. 8-oxoG is repaired efficiently in mammalian mitochondrial DNA by enzymes in the base excision repair pathway, including the 8-oxoguanine glycosylase (OGG1), which incizes the lesion in the first step of repair. Cockayne syndrome (CS) is a segmental premature aging syndrome in humans that has two complementation groups, CSA and CSB. Previous studies showed that CSB-deficient cells have reduced capacity to repair 8-oxoG. This study examines the role of the CSB gene in regulating repair of 8-oxoG in mitochondrial DNA in human and mouse cells. 8-oxoG repair was measured in liver cells from CSB deficient mice and in human CS-B cells carrying expression vectors for wild type or mutant forms of the human CSB gene. For the first time we report that CSB stimulates repair of 8-oxoG in mammalian mitochondrial DNA. Furthermore, evidence is presented to support the hypothesis that wild type CSB regulates expression of OGG1.

Conditional targeting of the DNA repair enzyme hOGG1 into mitochondria

Oxidative damage to mitochondrial DNA (mtDNA) has been suggested to be a key factor in the etiologies of many diseases and in the normal process of aging. Although the presence of a repair system to remove this damage has been demonstrated, the mechanisms involved in this repair have not been well defined. In an effort to better understand the physiological role of recombinant 8-oxoguanine DNA glycosylase/apurinic lyase (OGG1) in mtDNA repair, we constructed an expression vector containing the gene for OGG1 downstream of the mitochondrial localization sequence from manganese-superoxide dismutase. This gene construct was placed under the control of a tetracycline-regulated promoter. Transfected cells that conditionally expressed OGG1 in the absence of the tetracycline analogue doxycycline and targeted this recombinant protein to mitochondria were generated. Western blots of mitochondrial extracts from vector- and OGG1-transfected clones with and without doxycycline revealed that removal of doxycycline for 4 days caused an approximate 8-fold increase in the amount of OGG1 protein in mitochondria. Enzyme activity assays and DNA repair studies showed that the doxycycline-dependent recombinant OGG1 is functional. Functional studies revealed that cells containing recombinant OGG1 were more proficient at repairing oxidative damage in their mtDNA, and this increased repair led to increased cellular survival following oxidative stress.

Oxidative DNA damage: biological significance and methods of analysis

All forms of aerobic life are subjected constantly to oxidant pressure from molecular oxygen and also reactive oxygen species (ROS), produced during the biochemical utilization of O2 and prooxidant stimulation of O2 metabolism. ROS are thought to influence the development of human cancer and more than 50 other human diseases. To prevent oxidative DNA damage (protection) or to reverse damage, thereby preventing mutagenesis and cancer (repair), the aerobic cell possesses antioxidant defense systems and DNA repair mechanisms. During the last 20 years, many analytical techniques have been developed to monitor oxidative DNA base damage. High-performance liquid chromatography-electrochemical detection and gas chromatography-mass spectrometry are the two pioneering contributions to the field. Currently, the arsenal of methods available include the promising high-performance liquid chromatography-tandem mass spectrometry technique, capillary electrophoresis, 32P-postlabeling, fluorescence postlabeling, 3H-postlabeling, antibody-base immunoassays, and assays involving the use of DNA repair glycosylases such as the comet assay, the alkaline elution assay, and the alkaline unwinding method. Recently, the use of liquid chromatography-mass spectrometry has been introduced for the measurement of a number of modified nucleosides in oxidatively damaged DNA. The bulk of available chromatographic methods aimed at measuring individual DNA base lesions require either chemical hydrolysis or enzymatic digestion of oxidized DNA, following extraction from cells or tissues. The effect of experimental conditions (DNA isolation, hydrolysis, and/or derivatization) on the levels of oxidatively modified bases in DNA is enormous and has been studied intensively in the last 10 years.

1,N(2)-ethenoguanine, a mutagenic DNA adduct, is a primary substrate of Escherichia coli mismatch-specific uracil-DNA glycosylase and human alkylpurine-DNA-N-glycosylase

The promutagenic and genotoxic exocyclic DNA adduct 1,N(2)-ethenoguanine (1,N(2)-epsilonG) is a major product formed in DNA exposed to lipid peroxidation-derived aldehydes in vitro. Here, we report that two structurally unrelated proteins, the Escherichia coli mismatch-specific uracil-DNA glycosylase (MUG) and the human alkylpurine-DNA-N-glycosylase (ANPG), can release 1,N(2)-epsilonG from defined oligonucleotides containing a single modified base. A comparison of the kinetic constants of the reaction indicates that the MUG protein removes the 1,N(2)-epsilonG lesion more efficiently (k(cat)/K(m) = 0.95 x 10(-3) min(-1) nm(-1)) than the ANPG protein (k(cat)/K(m) = 0.1 x 10(-3) min(-1) nm(-1)). Additionally, while the nonconserved, N-terminal 73 amino acids of the ANPG protein are not required for activity on 1,N(6)-ethenoadenine, hypoxanthine, or N-methylpurines, we show that they are essential for 1,N(2)-epsilonG-DNA glycosylase activity. Both the MUG and ANPG proteins preferentially excise 1,N(2)-epsilonG when it is opposite dC; however, unlike MUG, ANPG is unable to excise 1,N(2)-epsilonG when it is opposite dG. Using cell-free extracts from genetically modified E. coli and murine embryonic fibroblasts lacking MUG and mANPG activity, respectively, we show that the incision of the 1,N(2)-epsilonG-containing duplex oligonucleotide has an absolute requirement for MUG or ANPG. Taken together these observations suggest a possible role for these proteins in counteracting the genotoxic effects of 1,N(2)-epsilonG residues in vivo.

Biological consequences of free radical-damaged DNA bases

The principal oxidized cytosine bases, uracil glycol, 5-hydroxycytosine, and 5-hydroxyuracil, are readily bypassed, miscode, and are thus important premutagenic lesions. Similarly the principal oxidation product of guanine, 8-oxoguanine, miscodes with A and is a premutagenic lesion. Most of the thymine and adenine products that retain their ring structure primarily pair with their cognate bases and are not potent premutagenic lesions. Although thymine glycol pairs with its cognate base and is not mutagenic it significantly distorts the DNA molecule and is a lethal lesion. Ring fragmentation, ring contraction, and ring open products of both pyrimidines and purines block DNA polymerases and are potentially lethal lesions. Although these breakdown products have the potential to mispair during translesion synthesis, the mutational spectra of prokaryotic mutants defective in the pyrimidine-specific and/or purine-specific DNA glycosylases do not reflect that expected of the breakdown products. Taken together, the data suggest that the principal biological consequences of endogenously produced and unrepaired free radical-damaged DNA bases are mutations.

Escherichia coli apurinic-apyrimidinic endonucleases enhance the turnover of the adenine glycosylase MutY with G:A substrates

The DNA repair enzyme MutY plays an important role in the prevention of DNA mutations resulting from the presence of the oxidatively damaged lesion 7,8-dihydro-8-oxo-2'-deoxyguanosine (OG). MutY is a base excision repair (BER) glycosylase that removes misincorporated adenine residues from OG:A mispairs, as well as G:A and C:A mispairs. We have previously shown that, under conditions of low MutY concentrations relative to an OG:A or G:A substrate, the time course of the adenine glycosylase reaction exhibits biphasic kinetic behavior due to slow release of the DNA product by MutY. The dissociation of MutY from its product may require the recruitment of other proteins from the BER pathway, such as an apurinic-apyrimidinic (AP) endonuclease, as turnover-enhancing cofactors. The effect of the AP endonucleases endonuclease IV (Endo IV), exonuclease III (Exo III), and Ape1 on the reaction kinetics of MutY with G:A- and OG:A-containing substrates was investigated. The effect of the glycosylases UDG and MutM and the DNA polymerase pol I was also characterized. Endo IV and Exo III, unlike Ape1, UDG, and pol I, greatly enhance the rate of product release with a G:A substrate, whereas the rate constant for the adenine removal step remains unchanged. Furthermore, the turnover rate with a truncated form of MutY, Stop 225, which lacks 125 amino acids of the C terminus, is unaffected by the presence of Endo IV or Exo III. These results constitute the first evidence of an interaction between the MutY-product DNA complex and Endo IV or Exo III. Furthermore, they suggest a role for the C-terminal domain of MutY in mediating this interaction.

Can DNA repair cause enhanced cell killing following treatment with ionizing radiation?

Production of DNA damage is the basis of cancer treatments, such as chemotherapy and radiotherapy. The limitation of the treatment dose tends to be how well the normal cells within the body can tolerate the therapy. Although it is possible, to some extent, to localize the treatment area during radiotherapy by targeting the beam of ionizing radiation, chemotherapy usually involves a whole body treatment. In order to improve the effectiveness of treatments, it is important to understand how cells repair the DNA damage. This review will attempt to explain how DNA repair, which would be expected to always enhance cell survival, actually may result in increased cell killing following certain types of cancer treatments, such as ionizing radiation and bleomycin sulfate. Work is underway in many laboratories to unravel how the repair systems handle specific types of DNA damage. Such information will pave the way in designing adjuvant therapies that alter a tumor cell's DNA repair capacity and increase tumor cell killing.

Global genome repair of 8-oxoG in hamster cells requires a functional CSB gene product

Cockayne syndrome (CS) is an autosomal recessive human disease characterized by UV-sensitivity as well as neurological and developmental abnormalities. Two complementation groups have been established, designated CS-A and CS-B. Traditionally, CSA and CSB have been ascribed a function in the transcription-coupled repair (TCR) pathway of nucleotide excision repair (NER) that efficiently removes bulky lesions from the transcribed strand of RNA polymerase II transcribed genes. To assess the role of the CSB protein in the repair of the highly mutagenic base lesion 7,8-dihydro-8-oxoguanine (8-oxoG), we have investigated the removal of this lesion using an in vitro incision approach with cell extracts as well as an in vivo approach with a modified protocol of the gene-specific repair assay, which allows the measurement of base lesion repair in intragenomic sequences. Our results demonstrate that the integrity of the CSB protein is pivotal for processes leading to incision at the site of 8-oxoG and that the global genome repair (GGR) of this lesion requires a functional CSB gene product in vivo.

Involvement of mammalian OGG1(MMH) in excision of the 8-hydroxyguanine residue in DNA

8-Hydroxyguanine (7,8-dihydro-8-oxoguanine, abbreviated as 8-OH-G or 8-oxoG) is the site of a frequent mutagenic DNA lesion produced by oxidative damage. MutM of E. coli and OGG1 of Saccharomyces cervisiae are known to possess 8-OH-G glycosylase and apurinic (AP) site lyase activity. cDNA clones of four isoforms (types 1a, 1b, 1c, and 2) of human OGG1 homologs (hMMH) were isolated. In order to examine whether expression of hMMH (hOGG1) protein actually occurs in human cells, we prepared type 1a specific antibody, and by using this antibody, we showed that type 1a protein isolated from HeLaS3 has 8-OH-G glycosylase/lyase activity. Furthermore, we showed that type 1a protein is a major enzyme for repair of the 8-OH-G lesion in human cells. In our second study, we generated a mouse line carrying an inactivated mutant Mmh allele by targeted gene disruption. Liver extracts of Mmh homozygous mutant mice were found to have loss of the nicking activity for the 8-OH-G site. In addition, the amount of endogenous 8-OH-G in liver DNA of the homozygous mice increased linearly with age, reaching 7-fold increase in 14 week old mice, over that of wild-type or heterozygous mice. Furthermore, when homozygous mice were fed the oxygen radical-forming agent KBrO3, to provide oxidative stress, the level of 8-OH-G in kidney DNA was tremendously increased: more than 200-fold as that of control mice without oxidative stress after 12 weeks of age. These results indicate that Ogg1/Mmh plays an essential role in the repair of the 8-OH-G residue in DNA produced by oxidative stress.

Detailed low-dose study of 1,1-bis(p-chlorophenyl)-2,2,2- trichloroethane carcinogenesis suggests the possibility of a hormetic effect

To obtain information on the effects of nongenotoxic carcinogens at low doses for human cancer risk assessment, the carcinogenic potential of the organochlorine insecticide, 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT), in the liver was assessed in F344 rats. In experiment 1, 240 male animals, 21 days old, were administered 0, 0.5, 1.0, 2.0, 5.0, 20, 100 and 500 ppm DDT in the diet for 16 weeks. Experiment 2 was conducted to elucidate the carcinogenic potential of DDT at lower levels using 180 rats given doses of 0, 0.005, 0.01, 0.1, 0.2 and 0.5 ppm. The livers of all animals were immunohistochemically examined for expression of glutathione S-transferase placental form (GST-P), putative preneoplastic lesions. Quantitative values for GST-P-positive foci in the liver were increased dose-dependently in rats given 20 ppm DDT and above with statistical significance as compared with the concurrent control value. In contrast, doses of 0.005 and 0.01 ppm were associated with a tendency for decrease below the control value, although not significantly. Western blotting analysis show that cytochrome P-450 3A2 (CYP3A2) protein expression tended to decrease at 0.005 and 0.01 ppm, a good correlation being observed with the change in the number of GST-P-positive foci. These findings suggest that a DDT hepatocarcinogenicity may show nonlinear response, that is, hormetic response at low doses. Furthermore, since CYP3A2 protein expression appears to be important for the effects of phenobarbital and the alpha-isomer of benzene hexachloride, mRNAs for IL-1 receptor type 1 (IL-1R1) and TNF-alpha receptor type 1 (TNFR1) whose ligands have roles not only in downregulating CYP3A2 expression but also in inducing antiproliferative effect or apoptosis in hepatocyte were examined. Increase was observed at low doses of DDT. Oxidative stress in liver DNA, assessed in terms of 8-hydroxydeoxyguanosine as a marker, was also decreased. These findings suggest that the possible hormetic effect that was observed in our detailed low-dose study of DDT carcinogenesis, although not statistically significant, may be linked to levels of oxidative stress and proinflammatory cytokines.

Repair of 8-oxoG is slower in endogenous nuclear genes than in mitochondrial DNA and is without strand bias

DNA is vulnerable to the attack of certain oxygen radicals and one of the major DNA lesions formed is 7,8-dihydro-8-oxoguanine (8-oxoG), a highly mutagenic lesion that can mispair with adenine. The repair of 8-oxoG was studied by measuring the gene specific removal of 8-oxoG after treatment of Chinese hamster ovary (CHO) fibroblasts with the photosensitizer Ro19-8022. This compound introduces 8-oxoG lesions, which can then be detected with the Escherichia coli formamidopyrimidine DNA glycosylase (FPG). In this report we present gene specific repair analysis of endogenous genes situated in different important cellular regions and also the first analysis of strand specific DNA repair of 8-oxoG in an endogenous gene. We were not able to detect any preferential repair of transcribed genes compared to non-transcribed regions and we did not detect any strand-bias in the repair of the housekeeping gene, dihydrofolate reductase (DHFR). In vivo, mitochondrial DNA is highly exposed to reactive oxygen species (ROS), and we find that the repair of 8-oxoG is more efficient in the mitochondrial DNA than in the nuclear DNA.

Functional interaction of MutY homolog with proliferating cell nuclear antigen in fission yeast, Schizosaccharomyces pombe

The MutY homolog (MYH) is responsible for removing adenines misincorporated on a template DNA strand containing G or 7,8-dihydro-8-oxoguanine (8-oxoG) and thus preventing G:C to T:A mutations. Human MYH has been shown to interact physically with human proliferating cell nuclear antigen (hPCNA). Here, we report that a similar interaction between SpMYH and SpPCNA occurs in the fission yeast Schizosaccharomyces pombe. Binding of SpMYH to SpPCNA was not observed when phenylalanine 444 in the PCNA binding motif of SpMYH was replaced with alanine. The F444A mutant of SpMYH expressed in yeast cells had normal adenine glycosylase and DNA binding activities. However, expression of this mutant form of SpMYH in a SpMYHDelta cell could not reduce the mutation frequency of the cell to the normal level. Moreover, SpMYH interacted with hPCNA, and SpPCNA interacted with hMYH but not with F518A/F519A mutant hMYH containing mutations in its PCNA binding motif. Although the SpMYHDelta cells expressing hMYH had partially reduced mutation frequency, the F518A/F519A mutant hMYH could not reduce the mutation frequency of SpMYHDelta cells. Thus, the interaction between SpMYH and SpPCNA is important for SpMYH biological function in mutation avoidance.

Human MutY homolog, a DNA glycosylase involved in base excision repair, physically and functionally interacts with mismatch repair proteins human MutS homolog 2/human MutS homolog 6

Adenines mismatched with guanines or 7,8-dihydro-8-oxo-deoxyguanines that arise through DNA replication errors can be repaired by either base excision repair or mismatch repair. The human MutY homolog (hMYH), a DNA glycosylase, removes adenines from these mismatches. Human MutS homologs, hMSH2/hMSH6 (hMutSalpha), bind to the mismatches and initiate the repair on the daughter DNA strands. Human MYH is physically associated with hMSH2/hMSH6 via the hMSH6 subunit. The interaction of hMutSalpha and hMYH is not observed in several mismatch repair-defective cell lines. The hMutSalpha binding site is mapped to amino acid residues 232-254 of hMYH, a region conserved in the MutY family. Moreover, the binding and glycosylase activities of hMYH with an A/7,8-dihydro-8-oxo-deoxyguanine mismatch are enhanced by hMutSalpha. These results suggest that protein-protein interactions may be a means by which hMYH repair and mismatch repair cooperate in reducing replicative errors caused by oxidized bases.

Development and application of a novel immunoassay for measuring oxidative DNA damage in the environment

We developed a facile, cost-effective competitive binding assay for the analysis of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) in DNA, using a polyclonal rabbit antiserum raised against an 8-oxodGuo hapten coupled to bovine serum albumin and radiolabeled synthetic ligand containing multiple 8-oxodGuo residues. This radioimmunoassay (RIA) displays a high affinity for 8-oxodGuo in DNA, with a detection limit of approximately 1 adduct in 10(5) bases of DNA. 8-oxodGuo standards for RIA were quantified by high-performance liquid chromatography and electrochemical detection in DNA diluted in methylene blue and exposed to visible light. As an initial application we quantified 8-oxodGuo in dosimeters deployed at increasing depths in the Southern Ocean during the austral spring of the 1998 field season or at the surface at Palmer Station, Antarctica, throughout the 1999 field season. Cyclobutane pyrimidine dimers (CPD) were quantified using an established RIA. We found that the frequency of both photoproducts decreased with depth. However, CPD induction was attenuated at a faster rate than 8-oxodGuo, correlating with the differential attenuation of solar ultraviolet wavelengths in the water column. CPD induction was closely related with ultraviolet-B radiation (UVB) attenuation, whereas the lower attenuation of 8-oxodGuo suggests that oxidative damage is more closely related to ultraviolet-A radiation (UVA) irradiance. The ratio of 8-oxodGuo: CPD was also found to covary with changes in stratospheric ozone concentrations at Palmer Station. These data demonstrate the usefulness of these assays for environmental photobiology and the potential for their use in studying the relative impacts of UVB versus UVA, including ozone depletion events.

Inherited variants of MYH associated with somatic G:C-->T:A mutations in colorectal tumors

Inherited defects of base excision repair have not been associated with any human genetic disorder, although mutations of the genes mutM and mutY, which function in Escherichia coli base excision repair, lead to increased transversions of G:C to T:A. We have studied family N, which is affected with multiple colorectal adenomas and carcinoma but lacks an inherited mutation of the adenomatous polyposis coli gene (APC) that is associated with familial adenomatous polyposis. Here we show that 11 tumors from 3 affected siblings contain 18 somatic inactivating mutations of APC and that 15 of these mutations are G:C-->A transversions--a significantly greater proportion than is found in sporadic tumors or in tumors associated with familial adenomatous polyposis. Analysis of the human homolog of mutY, MYH, showed that the siblings were compound heterozygotes for the nonconservative missense variants Tyr165Cys and Gly382Asp. These mutations affect residues that are conserved in mutY of E. coli (Tyr82 and Gly253). Tyrosine 82 is located in the pseudo-helix-hairpin-helix (HhH) motif and is predicted to function in mismatch specificity. Assays of adenine glycosylase activity of the Tyr82Cys and Gly253Asp mutant proteins with 8-oxoG:A and G:A substrates show that their activity is reduced significantly. Our findings link the inherited variants in MYH to the pattern of somatic APC mutation in family N and implicate defective base excision repair in predisposition to tumors in humans.

Human 8-oxoguanine DNA glycosylase 1 mRNA expression as an oxidative stress exposure biomarker of cooking oil fumes

Epidemiological studies have indicated that the exposure to carcinogenic components formed during the cooking of food might be associated with lung cancer risk of Chinese women. Previous studies have confirmed that cooking oil fumes from frying fish (COF) contained relatively high amount of benzo[a]pyrene, 2-methyl-3,8-dimethylimidazo[4,5-f] qunoxaline, benzene, and 1,3-butadiene, reported in fumes from heated soybean oil. Thus, we consider that oxidative stress induced by COF may play a role in lung cancer development among Chinese women. To verify whether the oxidative DNA damage was induced by COF, high-performance liquid chromatography (HPLC) analysis data showed that the levels of 8-hydroxydeoxyguanine (8-OH dG) were increased in a dose-dependent manner when calf thymus DNA reacted with various concentrations of COF. Since human 8-oxoguanine DNA glycosylase 1 (hOGG1) was a repair enzyme for removing 8- OH dG from damaged DNA, we hypothesized that hOGG1 mRNA may be used to assess the risk of oxidative damage induced by the exposure of COF. The results from reverse-transcription polymerase chain reaction showed that the hOGG1 mRNA expression was induced by hydrogen peroxide (H2O2) and COF in human lung adenocarcinoma CL-3 cells. To elucidate whether hOGG1 mRNA expression was an exposure biomarker of COF, a cross-sectional study of 238 subjects including 94 professional cooks, 43 housewives, and 101 COF-nonexposed control subjects was conducted. The hOGG1 mRNA expression frequencies of COF-exposed cooks (27 of 94, 28.7%) and housewives (6 of 43, 14%) were significantly higher than those of control subjects (4 of 101, 4%). After adjusting for age, sex, and smoking and drinking status, the odds risks (ORs) of housewives versus control and cooks versus control were 3.94 (95% confidence interval [CI] = 0.95-16.62) and 10.12 (95% CI = 2.83-36.15), respectively. These results indicated that hOGG1 may be adequate to act as an exposure biomarker to assess the oxidative DNA damage induced by COF. This also suggests that oxidative stress induced by COF may play a role in lung cancer development among Chinese women.

Escherichia coli FPG and human OGG1 reduce DNA damage and cytotoxicity by BCNU in human lung cells

The pulmonary complications of 1,3-N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) are among the most important dose-limiting factors of BCNU-containing cancer chemotherapeutic regimens. BCNU damages DNA of both cancer cells and normal cells. To increase the resistance of lung cells to BCNU, we employed gene transfer of Escherichia coli formamidopyrimidine-DNA glycosylase (FPG) and human 8-oxoguanine-DNA glycosylase (hOGG1) to A549 cells, a lung epithelial cell line, using a bicistronic retroviral vector, pSF91-RE, that encoded both FPG/hOGG1 and an enhanced green fluorescent protein. The transduced epithelial cells were sorted by flow cytometry, and expression of FPG/hOGG1 protein was determined by the level of FPG/hOGG1 RNA and enzyme activity. The single-cell gel electrophoresis (comet assay) measured DNA damage induced by BCNU. FPG/hOGG1-expressing A549 cells incubated with 40-500 microg/ml BCNU exhibited significantly less DNA damage than vector-transduced cells. In addition, FPG- and/or hOGG1-expressing cells incubated with 10-40 microg/ml BCNU showed at least a 25% increase in cell survival. Gene transfer of FPG/hOGG1 reduced BCNU-induced DNA damage and cytotoxicity of cultured lung cells and may suggest a new mechanism to reduce BCNU pulmonary toxicity.

DNA polymerase beta is the major dRP lyase involved in repair of oxidative base lesions in DNA by mammalian cell extracts

The repair of oxidative base lesions in DNA is a coordinated chain of reactions that includes removal of the damaged base, incision of the phosphodiester backbone at the abasic sugar residue, incorporation of an undamaged nucleotide and sealing of the DNA strand break. Although removal of a damaged base in mammalian cells is initiated primarily by a damage-specific DNA glycosylase, several lyases and DNA polymerases may contribute to the later stages of repair. DNA polymerase beta (Pol beta) was implicated recently as the major polymerase involved in repair of oxidative base lesions; however, the identity of the lyase participating in the repair of oxidative lesions is unclear. We studied the mechanism by which mammalian cell extracts process DNA substrates containing a single 8-oxoguanine or 5,6-dihydrouracil at a defined position. We find that, when repair synthesis proceeds through a Pol beta-dependent single nucleotide replacement mechanism, the 5'-deoxyribosephosphate lyase activity of Pol beta is essential for repair of both lesions.

An OGG1 orthologue encoding a functional 8-oxoguanine DNA glycosylase/lyase in Arabidopsis thaliana

Repair of the ubiquitous mutagenic lesion 7,8-dihydro-8-oxoguanine (8-oxoG) is initiated in eukaryotes by DNA glycosylases/lyases, such as yeast Ogg1, that do not share significant sequence identity with their prokaryotic counterparts, typified by Escherichia coli MutM (Fpg) protein. The unexpected presence of a functional mutM orthologue in the model plant Arabidopsis thaliana has brought into question the existence of functional OGG1 orthologues in plants. We report here the cDNA cloning, expression and functional characterization of AtOGG1, an Arabidopsis thaliana gene widely expressed in different plant tissues which encodes a 40.3 kDa protein with significant sequence identity to yeast and human Ogg1 proteins. Purified AtOgg1 enzyme specifically cleaves duplex DNA containing an 8-OxoG:C mispair, and the repair reaction proceeds through an imine intermediate

hOGG1 exon7 polymorphism and gastric cancer in case-control studies of Japanese Brazilians and non-Japanese Brazilians

Polymorphism of hOGG1 may be capable of serving as a genetic marker for individual susceptibility to various cancers because of its role in the repair of oxyradical DNA damage. We examined the distribution of the hOGG1 Ser326Cys polymorphism and its presumed correlation with gastric cancer risk in two case-control studies of different ethnic groups in São Paulo, Brazil. Potentially eligible Japanese (JB) and non-Japanese Brazilian (NJB) case subjects were defined as patients with newly diagnosed malignant neoplasms of the stomach in 13 hospitals in São Paulo. Ninety-six JBs and 236 NJBs were adopted as subjects. Two controls were matched for each JB case, and one control for each NJB case. The subjects were interviewed using a questionnaire and their blood samples were collected. A significant difference in the distribution of this polymorphism between the two ethnic groups was observed (chi(2)=58.3, P<0.01). The mutant type (Ser/Cys or Cys/Cys) was predominant (approximately 65%) in the JBs, but was only present in approximately 40% of the NJBs. Logistic regression analysis showed no significant increased risk for either the Ser/Cys or Cys/Cys type in either group. The odds ratios of the Cys allele for gastric cancer were 1.01 (95% confidence interval (CI): 0.52-1.93) in the JBs and 0.85 (95% CI: 0.57-1.26) in the NJBs. In the NJBs, a significant increased risk of smoking was shown only in the Ser/Ser type, and no increased risk was shown in the genotypes with the Cys allele. However, no statistically significant interactions were observed with smoking or other possible confounding factors. No statistically significant difference in the distribution of the polymorphism was observed between the intestinal type and diffuse type of gastric cancer in either the JBs or the NJBs. The ethnic difference in hOGG1 Ser326Cys polymorphism was much greater than the case-control difference, and this polymorphism is unlikely to be associated with gastric cancer.

Ascorbic acid in the 21st century - more than a simple antioxidant

Ascorbic acid (AA) is an essential micronutrient for man, with many biological roles. It is a powerful antioxidant both directly via scavenging of reactive oxygen species and indirectly through regeneration of other antioxidant systems. Paradoxically, under certain conditions (low concentration in vitro, presence of metal ions) it can exert a pro-oxidant effect, increasing oxidative damage to lipids, DNA and protein. Herein, the effects of vitamin C both in vitro and in vivo are addressed in terms of modulation of oxidative DNA damage, gene expression and protein oxidation. The view of AA as a simple scavenger is outdated, where the arrival of new bioinformatic techniques, heralds a new dawning in our understanding of ascorbate as a potential direct or indirect modulator of gene expression.

Nitric oxide in gastrointestinal epithelial cell carcinogenesis: linking inflammation to oncogenesis

Chronic inflammation of gastrointestinal tissues is a well-recognized risk factor for the development of epithelial cell-derived malignancies. Although the inflammatory mediators linking chronic inflammation to carcinogenesis are numerous, current information suggests that nitric oxide (NO) contributes to carcinogenesis during chronic inflammation. Inducible nitric oxide synthase (iNOS), expressed by both macrophages and epithelial cells during inflammation, generates the bioreactive molecule NO. In addition to causing DNA lesions, NO can directly interact with proteins by nitrosylation and nitosation reactions. The consequences of protein damage by NO appear to be procarcinogenic. For example, NO inhibits DNA repair enzymes such as human 8-oxodeoxyguanosine DNA glycosylase 1 and blocks apoptosis via nitrosylation of caspases. These cellular events permit DNA damage to accumulate, which is required for the numerous mutations necessary for development of invasive cancer. NO also promotes cancer progression by functioning as an angiogenesis factor. Strategies to inhibit NO generation during chronic inflammation or to scavenge reactive nitrogen species may prove useful in decreasing the risk of cancer development in chronic inflammatory gastrointestinal diseases.

DNA repair activity of 8-oxoguanine DNA glycosylase 1 (OGG1) in human lymphocytes is not dependent on genetic polymorphism Ser326/Cys326

8-oxoguanine DNA glycosylase 1 (OGG1) is a DNA repair enzyme that excises 7,8-dihydro-8-oxoguanine (8oxoG) from DNA. Since 8oxoG is a highly mispairing lesion, decreased OGG1 expression level could lead to a higher background mutation frequency and could possibly increase the cancer risk of an individual under oxidative stress. In order to analyse the natural variation of OGG1, we measured the DNA repair activity in human lymphocytes of healthy individuals by means of an 8oxoG-containing oligonucleotide assay. The data obtained revealed a two fold interindividual variation of OGG1 activity in lymphocytes. There was no difference in OGG1 activity due to gender and smoking behaviour. Transcriptional analyses of OGG1 showed the expression of two isoforms, 1a and b, in lymphocytes. Structural analysis of the human OGG1 (hOGG1) gene revealed a Ser326/Cys326 polymorphism in the Caucasian population with allele frequencies of 75% for Ser326 and 25% for Cys326. This polymorphism was not associated with altered OGG1 activity. The described routine test system for measuring OGG1 activity in cryopreserved lymphocytes provided highly reproducible results and is a useful tool for risk assessment associated with alterations in the repair of oxidative DNA damage.

The OGG1 gene encodes a repair enzyme for oxidatively damaged DNA and is involved in human carcinogenesis

8-Hydroxyguanine (oh8G) is a major base lesion produced by reactive oxygen species. oh8G in DNA causes G:C to T:A transversions and, thus, could be responsible for mutations that lead to carcinogenesis. A human DNA glycosylase/AP lyase encoded by the OGG1 gene has an activity to remove directly oh8G from DNA, and suppresses the mutagenic effect of oh8G. OGG1 protein has a helix-hairpin-helix-GPD motif as a domain for both DNA binding and catalysis, a nuclear localization signal, and a mitochondria targeting signal. Among multiple OGG1 isoforms, OGG1-type la is expressed predominantly in human cells and repairs chromosomal DNA in the nucleus. Inactivation of the OGG1 gene in yeast and mice leads to elevated spontaneous mutation frequency in the cells. The human OGG1 gene maps to chromosome 3p26.2, and allelic deletions of this region occur frequently in a variety of human cancers. Moreover, the OGG1 gene is somatically mutated in some cancer cells and is highly polymorphic among human populations. Repair activities of some mutated and polymorphic OGG1 proteins are lower than those of wild-type OGG1-type la-Ser326 protein and, thus, could be involved in human carcinogenesis.