Construction of mutants of Salmonella typhimurium deficient in 8-hydroxyguanine DNA glycosylase and their sensitivities to oxidative mutagens and nitro compounds

Inflammation response, oxidative stress and DNA damage caused by urban air pollution exposure increase in the lack of DNA repair XPC protein

Air pollution represents a considerable threat to health worldwide. The São Paulo Metropolitan area, in Brazil, has a unique composition of atmospheric pollutants with a population of nearly 20 million people and 9 million passenger cars. It is long known that exposure to particulate matter less than 2.5 µm (PM_2.5) can cause various health effects such as DNA damage. One of the most versatile defense mechanisms against the accumulation of DNA damage is the nucleotide excision repair (NER), which includes XPC protein. However, the mechanisms by which NER protects against adverse health effects related to air pollution are largely unknown. We hypothesized that reduction of XPC activity may contribute to inflammation response, oxidative stress and DNA damage after PM_2.5 exposure. To address these important questions, XPC knockout and wild type mice were exposed to PM_2.5 using the Harvard Ambient Particle concentrator. Results from one-single exposure have shown a significant increase in the levels of anti-ICAM, IL-1β, and TNF-α in the polluted group when compared to the filtered air group. Continued chronic PM_2.5 exposure increased levels of carbonylated proteins, especially in the lung of XPC mice, probably as a consequence of oxidative stress. As a response to DNA damage, XPC mice lungs exhibit increased γ-H2AX, followed by severe atypical hyperplasia. Emissions from vehicles are composed of hazardous substances, with polycyclic aromatic hydrocarbons (PAHs) and metals being most frequently cited as the major contributors to negative health impacts. This analysis showed that benzo[b]fluoranthene, 2-nitrofluorene and 9,10-anthraquinone were the most abundant PAHs and derivatives. Taken together, these findings demonstrate the participation of XPC protein, and NER pathway, in the protection of mice against the carcinogenic potential of air pollution. This implicates that DNA is damaged directly (forming adducts) or indirectly (Reactive Oxygen Species) by the various compounds detected in urban PM_2.5.

Inhibitors of DNA Glycosylases as Prospective Drugs

DNA glycosylases are enzymes that initiate the base excision repair pathway, a major biochemical process that protects the genomes of all living organisms from intrinsically and environmentally inflicted damage. Recently, base excision repair inhibition proved to be a viable strategy for the therapy of tumors that have lost alternative repair pathways, such as BRCA-deficient cancers sensitive to poly(ADP-ribose)polymerase inhibition. However, drugs targeting DNA glycosylases are still in development and so far have not advanced to clinical trials. In this review, we cover the attempts to validate DNA glycosylases as suitable targets for inhibition in the pharmacological treatment of cancer, neurodegenerative diseases, chronic inflammation, bacterial and viral infections. We discuss the glycosylase inhibitors described so far and survey the advances in the assays for DNA glycosylase reactions that may be used to screen pharmacological libraries for new active compounds.

My career development with Ames test: A personal recollection

I first became acquainted with the Ames test at the very beginning of my career in 1978, when my task at the National Institute of Health Sciences (Tokyo) was to screen for mutagenicity of food additives used in Japan, using the Ames test. I also used this test to research the metabolic activation mechanisms of chemical carcinogens, in particular, the analgesic drug, phenacetin. This chemical was not mutagenic in Salmonella typhimurium TA100 with standard 9000 × g supernatant of liver homogenates (S9) from rat but was mutagenic with hamster S9. It was revealed that hamster S9 had much higher deacetylation activities than rat S9, which accounts for the species difference. Then, my work was focused on molecular biology. We cloned the genes encoding nitroreductase and acetyltransferase in Salmonella typhimurium TA1538. Plasmids carrying these genes made strain TA98 more sensitive to mutagenic nitroarenes and aromatic amines. Because of their high sensitivity, the resulting strains such as YG1021 and YG1024 are widely used to monitor mutagenic nitroarenes and aromatic amines in complex mixtures. Later, we disrupted the genes encoding DNA polymerases in TA1538 and classified chemical mutagens into four classes depending on their use of different DNA polymerases. I was also involved in the generation of gpt delta transgenic rodent gene mutation assays, which examine the results of the Ames test in vivo. I have unintentionally developed my career under the influence of Dr. Ames and I would like to acknowledge his remarkable achievements in the field of environmental mutagenesis and carcinogenesis.

Thresholds of Genotoxic and Non-Genotoxic Carcinogens

Exposure to chemical agents is an inevitable consequence of modern society; some of these agents are hazardous to human health. The effects of chemical carcinogens are of great concern in many countries, and international organizations, such as the World Health Organization, have established guidelines for the regulation of these chemicals. Carcinogens are currently categorized into two classes, genotoxic and non-genotoxic carcinogens, which are subject to different regulatory policies. Genotoxic carcinogens are chemicals that exert carcinogenicity via the induction of mutations. Owing to their DNA interaction properties, there is thought to be no safe exposure threshold or dose. Genotoxic carcinogens are regulated under the assumption that they pose a cancer risk for humans, even at very low doses. In contrast, non-genotoxic carcinogens, which induce cancer through mechanisms other than mutations, such as hormonal effects, cytotoxicity, cell proliferation, or epigenetic changes, are thought to have a safe exposure threshold or dose; thus, their use in society is permitted unless the exposure or intake level would exceed the threshold. Genotoxicity assays are an important method to distinguish the two classes of carcinogens. However, some carcinogens have negative results in in vitro bacterial mutation assays, but yield positive results in the in vivo transgenic rodent gene mutation assay. Non-DNA damage, such as spindle poison or topoisomerase inhibition, often leads to positive results in cytogenetic genotoxicity assays such as the chromosome aberration assay or the micronucleus assay. Therefore, mechanistic considerations of tumor induction, based on the results of the genotoxicity assays, are necessary to distinguish genotoxic and non-genotoxic carcinogens. In this review, the concept of threshold of toxicological concern is introduced and the potential risk from multiple exposures to low doses of genotoxic carcinogens is also discussed.

Mutagenicity comparison of nine bioselenocompounds in three Salmonella typhimurium strains

Selenium (Se) is an essential element in animals but becomes severely toxic when the amount ingested exceeds the adequate intake level. It is known that the toxicological effects of Se are highly dependent on its chemical form. In this study, we evaluated the mutagenicity of nine naturally occurring Se compounds or the so-called bioselenocompounds, including selenite, selenate, selenocyanate, selenomethionine, selenocystine, Se-methylselenocysteine, selenohomolanthionine, N-acetylgalactosamine-type selenosugar, and trimethylselenonium ion, by using the Ames test. Salmonella typhimurium TA98, TA100, and TA1535 were used for the mutagenicity evaluation in the presence or absence of S9 mix, a metabolic activator. Only selenate showed weak mutagenicity even in the absence of S9 mix. None of the bioselenocompounds except selenate exhibited mutagenicity in all the strains tested in the presence or absence of S9 mix. Selenomethionine and selenocystine reduced the number of colonies in all the strains although no other selenoamino acids exerted the same effect. These results indicate that selenate directly or indirectly injures genome. Among the bioselenocompounds tested, selenomethionine and selenocystine show antibacterial activity, but the mechanism is unclear.

2-Nitroanisole-induced oxidative DNA damage in Salmonella typhimurium and in rat urinary bladder cells

2-Nitroanisole (2-NA) is used in the manufacturing of azo dyes and causes cancer, mainly in the urinary bladder. Previous in vivo genotoxic data seems to be insufficient to explain the mechanism through which 2-NA induces carcinogenesis, and several bladder carcinogens were reported to induce oxidative DNA damage. Thus, we examined the potential induction of oxidative DNA damage by 2-NA using bacterial strain YG3008, a mutM_ST-deficient derivative of strain TA100. Consequently, strain YG3008, when compared with strain TA100, was found to be more sensitive to 2-NA, indicating oxidative DNA damage in bacterial cells. For further investigation, we performed the comet assay using the urinary bladder and liver of rats, with and without human 8-oxoguanine DNA-glycosylase 1 (hOGG1), to confirm the potential of 2-NA for inducing oxidative DNA damage. Simultaneously, we conducted a micronucleus test using bone marrow from rats to assess the genotoxicity of 2-NA in vivo. 2-NA was administered orally to male Fischer 344 rats for 3 consecutive days. The rats were divided into 6 treatment groups: 3 groups treated with 2-NA at doses of 125, 250, and 500mg/kg; a group treated with the combination of 2-NA and glutathione-SH (GSH); a negative control group; and a positive control group. The comet assay without hOGG1 detected no DNA damage in the liver or urinary bladder, and the micronucleus test did not show clastogenic effects in bone marrow cells. However, the comet assay with hOGG1 was positive in the urinary bladder samples, indicating the induction of oxidative DNA damage in the urinary bladder for the group treated with 2-NA at 500mg/kg. Moreover, an antioxidant of GSH significantly reduced oxidative DNA damage caused by 2-NA. These results indicate that oxidative DNA damage is a possible mode of action for carcinogenesis in the urinary bladder of rats treated with 2-NA.

The mutagenic mechanism of oxygenated alkylhydrazones occurs through alkyl radicals and alkyldiazonium ions

Hydrazone hydroperoxides were formed by autoxidation and their mutagenicity was derived from the alkyldiazonium ion and the radical species.

Nitroaromatic compounds: Environmental toxicity, carcinogenicity, mutagenicity, therapy and mechanism

Vehicle pollution is an increasing problem in the industrial world. Aromatic nitro compounds comprise a significant portion of the threat. In this review, the class includes nitro derivatives of benzene, biphenyls, naphthalenes, benzanthrone and polycyclic aromatic hydrocarbons, plus nitroheteroaromatic compounds. The numerous toxic manifestations are discussed. An appreciable number of drugs incorporate the nitroaromatic structure. The mechanistic aspects of both toxicity and therapy are addressed in the context of a unifying mechanism involving electron transfer, reactive oxygen species, oxidative stress and antioxidants.

Quantitative assessment of the dose-response of alkylating agents in DNA repair proficient and deficient ames tester strains

Mutagenic and clastogenic effects of some DNA damaging agents such as methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) have been demonstrated to exhibit a nonlinear or even "thresholded" dose-response in vitro and in vivo. DNA repair seems to be mainly responsible for these thresholds. To this end, we assessed several mutagenic alkylators in the Ames test with four different strains of Salmonella typhimurium: the alkyl transferases proficient strain TA1535 (Ogt+/Ada+), as well as the alkyl transferases deficient strains YG7100 (Ogt+/Ada-), YG7104 (Ogt-/Ada+) and YG7108 (Ogt-/Ada-). The known genotoxins EMS, MMS, temozolomide (TMZ), ethylnitrosourea (ENU) and methylnitrosourea (MNU) were tested in as many as 22 concentration levels. Dose-response curves were statistically fitted by the PROAST benchmark dose model and the Lutz-Lutz "hockeystick" model. These dose-response curves suggest efficient DNA-repair for lesions inflicted by all agents in strain TA1535. In the absence of Ogt, Ada is predominantly repairing methylations but not ethylations. It is concluded that the capacity of alkyl-transferases to successfully repair DNA lesions up to certain dose levels contributes to genotoxicity thresholds.

Hydrogen peroxide and ecdysone in the cryoprotective dehydration strategy of Megaphorura arctica (Onychiuridae: Collembola)

The Arctic springtail, Megaphorura arctica, survives sub-zero temperatures in a dehydrated state via trehalose-dependent cryoprotective dehydration. Regulation of trehalose biosynthesis is complex; based in part on studies in yeast and fungi, its connection with oxidative stress caused by exposure of cells to oxidants, such as hydrogen peroxide (H₂O₂), or dehydration, is well documented. In this respect, we measured the amount of H₂O₂ and antioxidant enzyme activities (superoxide dismutases: copper, zinc--CuZnSOD and manganese containing--MnSOD, and catalase--CAT), as the regulatory components determining H₂O₂ concentrations, in Arctic springtails incubated at 5 °C (control) versus -2 °C (threshold temperature for trehalose biosynthesis). Because ecdysone also stimulates trehalose production in insects and regulates the expression of genes involved in redox homeostasis and antioxidant protection in Drosophila, we measured the levels of the active physiological form of ecdysone--20-hydroxyecdysone (20-HE). Significantly elevated H₂O₂ and 20-HE levels were observed in M. arctica incubated at -2 °C, supporting a link between ecdysone, H₂O₂, and trehalose levels during cryoprotective dehydration. CAT activity was found to be significantly lower in M. arctica incubated at -2 °C versus 5 °C, suggesting reduced H₂O₂ breakdown. Furthermore, measurement of the free radical composition in Arctic springtails incubated at 5 °C (controls) versus -2 °C by Electron Paramagnetic Resonance spectroscopy revealed melanin-derived free radicals at -2 °C, perhaps an additional source of H₂O₂. Our results suggest that H₂O₂ and ecdysone play important roles in the cryoprotective dehydration process in M. arctica, linked with the regulation of trehalose biosynthesis.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Light-dependent mutagenesis by benzo[a]pyrene is mediated via oxidative DNA damage

Benzo[a]pyrene (B[a]P) is an environmental carcinogenic polycyclic aromatic hydrocarbon (PAH). Mammalian enzymes such as cytochrome P-450s and epoxide hydrase convert B[a]P to reactive metabolites that can covalently bind to DNA. However, some carcinogenic compounds that normally require metabolic activation can also be directly photoactivated to mutagens. To examine whether B[a]P is directly mutagenic in the presence of light, we exposed Salmonella typhimurium strains with different DNA repair capacities to B[a]P and white fluorescent light at wavelengths of 370-750 nm. B[a]P plus light significantly enhanced the number of His+ revertants. Mutagenesis was completely light-dependent and required no exogenous metabolic activation. The order of mutability of strains with different DNA repair capacities was strain YG3001 (uvrB, mutMST) >> strain TA1535 (uvrB) > strain YG3002 (mutMST) > strain TA1975. The uvrB gene product is involved in the excision repair of bulky DNA adducts, and the mutMST gene encodes 8-oxoguanine (8-oxoG) DNA glycosylase, which removes 8-oxoG from DNA. Introduction of a plasmid carrying the mOgg1 gene that is the mouse counterpart of mutMST substantially reduced the light-mediated mutagenicity of B[a]P in strain YG3001. B[a]P plus light induced predominantly G:C --> T:A and G:C --> C:G transversions. We propose that B[a]P can directly induce bulky DNA adducts if light is present, and that the DNA adducts induce oxidative DNA damage, such as 8-oxoG, when exposed to light. These findings have implications for the photocarcinogenicity of PAHs.

Polymorphic variation in hOGG1 and risk of cancer: a review of the functional and epidemiologic literature

The gene encoding human 8-oxoguanine glycosylase 1 (hOGG1) is involved in DNA base excision repair. The encoded DNA glycosylase excises 7,8-dihydro-8-oxoguanine (8-OHdG), a highly mutagenic base produced in DNA as a result of exposure to reactive oxygen species (ROS). Polymorphisms in this gene may alter glycosylase function and an individual's ability to repair damaged DNA, possibly resulting in genetic instability that can foster carcinogenesis. In order to elucidate the possible impact of polymorphisms in hOGG1, we performed a literature review of both functional and epidemiologic studies that assessed the effects of these polymorphisms on repair function, levels of oxidative DNA damage, or associations with cancer risk. Fourteen functional studies and 19 epidemiologic studies of breast, colon, esophageal, head and neck, lung, nasopharyngeal, orolaryngeal, prostate, squamous cell carcinoma of the head and neck (SCCHN), and stomach cancers were identified. Although the larger functional studies suggest reduced repair function with variant alleles in hOGG1, the evidence is generally inconclusive. There is some epidemiologic evidence that risk for esophageal, lung, nasopharyngeal, orolaryngeal, and prostate is related to hOGG1 genotype, whereas risk of breast cancer does not appear related. In studies that explored potential interactions with environmental factors, cancer risk for hOGG1 genotypes differed depending on exposure, especially for colon cancer. In summary, there is limited evidence that polymorphisms in hOGG1 affect repair function and carcinogenesis. Larger, well-designed functional and epidemiologic studies are needed to clarify these relationships, especially with respect to interactions with other DNA repair enzymes and interactions with environmental factors that increase carcinogenic load.

Suppression of chemically induced and spontaneously occurring oxidative mutagenesis by three alleles of human OGG1 gene encoding 8-hydroxyguanine DNA glycosylase

8-Hydroxyguanine (8-OH-G) is an oxidatively damaged guanine base that causes G:C to T:A transversion mutations. To counteract the mutagenicity of 8-OH-G in DNA, humans possess the hOGG1 gene, which encodes 8-OH-G DNA glycosylase. Interestingly, genetic polymorphisms at codon 326 (hOGG1-Ser326 versus hOGG1-Cys326) and at codon 46 (hOGG1-Arg46 versus hOGG1-Gln46) exist in human populations. hOGG1-Ser326 and -Cys326 have Arg at codon 46, and hOGG1-Gln46 has Ser at codon 326. In this study, we examined the abilities of three forms of GST-hOGG1 (hOGG1-Ser326, -Cys326 and -Gln46) to suppress chemically induced oxidative mutagenesis using Salmonella typhimurium strains YG3001 and YG3002. These strains are the mutMST derivatives of Ames tester strains TA1535 (uvrB-) and TA1975 (uvrB+), respectively. The mutMST gene encodes a functional counterpart of the OGG1 gene. Mutations induced by 4-nitroquinoline 1-oxide were by more than 95% suppressed by the expression of any of three forms of GST-hOGG1 in strain YG3002. Expression of GST-hOGG1 also reduced by 40 and 60%, respectively, the numbers of His+ revertants induced by methylene blue plus visible light and benzo[a]pyrene plus visible light in strain YG3001. hOGG1-Gln46 displayed a slightly weaker ability to suppress the mutations induced by methylene blue plus visible light than did other two forms although the differences were not statistically significant. About 85 and 95% of spontaneous mutagenesis in strain YG3021 and YG3022, the mutMST mutYST double mutants of strain TA1535 and TA1975, respectively, were suppressed by the expression of any of hOGG1 alleles. hOGG1-Gln46 displayed a weaker suppression than did other two forms in strain YG3022 and the difference was statistically significant. These results suggest that three alleles of the hOGG1 gene efficiently suppress chemically induced and spontaneously occurring oxidative mutagenesis, and that hOGG1-Gln46 may have a weaker ability to suppress the mutations.

O(6)-alkylguanine-DNA alkyltransferase DNA repair in mycobacteria: pathogenic and non-pathogenic species differ

SETTING

DNA repair genes assist the organism in maintaining DNA integrity in the face of environmental (mutagenic) stress. The genome sequences of M. tuberculosis and M. bovis demonstrate sequences suggestive of an O(6)-alkylguanine-DNA alkyltransferase DNA repair activity similar to that seen in almost all other bacterial and eukaryotic organisms. The near ubiquitousness of this gene implies an important function.

OBJECTIVE

Our aim was to ascertain whether mycobacteria exert an alkyltransferase response to mutagen (streptozotocin) stimulation and whether alkyltransferase activity is essential for mycobacterial survival.

DESIGN

Alkyltransferase activity in slow- and fast-growing mycobacterial species was determined in the presence and absence of sublethal concentrations of an alkylating agent streptozotocin. The intracellular survival and response to anti-tuberculosis drugs of an alkyltransferase knockout strain of M. bovis BCG was also determined.

RESULTS

We demonstrate the presence of O(6)-alkylguanine alkyltransferase (cellular methyltransferase activity) in mycobacterial species and that there is an inducible and constitutive form in fast-growing mycobacteria (M. smegmatis), whereas only the constitutive form exists in the pathogenic or slow-growing species (M. bovis BCG) under the conditions tested. The overall activity of the constitutive form is high. We also show that intracellular growth of M. bovis BCG in macrophages is reduced when the alkyltransferase gene is absent. The presence of alkyltransferase activity appears to assist the organism in reducing the effects of isoniazid, since interruption of the gene confers sensitivity to the drug.

CONCLUSIONS

We conclude that for the slow-growing mycobacteria, an inducible response is not essential as their ecological niche is stable and protected, but that the presence of the alkyltransferase activity confers a growth advantage in macrophages and offers some protection against antibiotics.

Photomutagenicity of thiabendazole, a postharvest fungicide, in bacterial assays

We investigated the photomutagenicity of thiabendazole (TBZ), a postharvest fungicide commonly used on imported citrus fruits. Using UVA light (320-400 nm), we irradiated bacterial cultures with or without TBZ in a 24-well multiplate. UVA-irradiation without TBZ was not mutagenic to the tester strains, nor was unirradiated TBZ. TBZ was strongly photomutagenic in Escherichia coli WP2uvrA and WP2uvrA/pKM101 strains, weakly photomutagenic in Salmonella typhimurium TA100 and TA98, and not photomutagenic in S. typhimurium TA1535 and TA1538. The photomutagenicity of TBZ was more evident in WP2uvrA/pKM101, which carries the trpE65 ochre mutation (TAA), than in TA100, which carries the hisG46 missense mutation (CCC). In E. coli WP3101-WP3106 and the corresponding pKM101-containing strains, photoactivated TBZ induced predominantly G:C-->A:T transitions and A:T-->T:A transversions. In the plasmid-containing strains only, TBZ induced a moderate number of A:T-->G:C transitions and a few A:T-->C:G and G:C-->T:A transversions. The observation that UVA-irradiated TBZ mutated both G:C and A:T basepairs may explain why WP2uvrA/pKM101 was more sensitive to its mutagenicity than TA100. TBZ that was irradiated before it was added to the WP2uvrA/pKM101 cells was not photomutagenic, which suggests that the photomutagenic products of TBZ were unstable or rapidly reacted with other molecules before being incorporated into cells.

Oxidative mutagenesis of doxorubicin-Fe(III) complex

Doxorubicin has a high affinity for inorganic iron, Fe(III), and has potential to form doxorubicin-Fe(III) complexes in biological systems. Indirect involvement of iron has been substantiated in the oxidative mutagenicity of doxorubicin. In this study, however, direct involvement of Fe(III) was evaluated in mutagenicity studies with the doxorubicin-Fe(III) complex. The Salmonella mutagenicity assay with strain TA102 was used with a pre-incubation step. The highest mutagenicity of doxorubicin-Fe(III) complex was observed at the dose of 2.5nmol/plate of the complex. The S9-mix decreased this highest mutagenicity but increased the number of revertants at a higher dose of 10nmol/plate of the complex. On the other hand, the mutagenicity of the doxorubicin-Fe(III) complex at the doses of 0.25, 0.5, 1 and 2nmol/plate was enhanced about twice by the addition of glutathione plus H(2)O(2). This enhanced mutagenicity as well as of the complex itself, the complex plus glutathione, and the complex plus H(2)O(2) were reduced by the addition of ADR-529, an Fe(III) chelator, and potassium iodide, a hydroxyl radical scavenger. These results indicate that doxorubicin-Fe(III) complex exert the mutagenicity through oxidative DNA damage and that Fe(III) is a required element in the mutagenesis of doxorubicin.

Crystal structure of a repair enzyme of oxidatively damaged DNA, MutM (Fpg), from an extreme thermophile, Thermus thermophilus HB8

The MutM [formamidopyrimidine DNA glycosylase (Fpg)] protein is a trifunctional DNA base excision repair enzyme that removes a wide range of oxidatively damaged bases (N-glycosylase activity) and cleaves both the 3'- and 5'-phosphodiester bonds of the resulting apurinic/apyrimidinic site (AP lyase activity). The crystal structure of MutM from an extreme thermophile, Thermus thermophilus HB8, was determined at 1.9 A resolution with multiwavelength anomalous diffraction phasing using the intrinsic Zn(2+) ion of the zinc finger. MutM is composed of two distinct and novel domains connected by a flexible hinge. There is a large, electrostatically positive cleft lined by highly conserved residues between the domains. On the basis of the three-dimensional structure and taking account of previous biochemical experiments, we propose a DNA-binding mode and reaction mechanism for MutM. The locations of the putative catalytic residues and the two DNA-binding motifs (the zinc finger and the helix-two-turns-helix motifs) suggest that the oxidized base is flipped out from double-stranded DNA in the binding mode and excised by a catalytic mechanism similar to that of bifunctional base excision repair enzymes.

8-Hydroxyguanosine formed in human lung tissues and the association with diesel exhaust particles

Diesel exhaust particles consist of various organic chemicals, heavy metals, and carbon particles. Knowledge of the fate of organic chemicals and carbon particles in the lungs is important to determine the mechanisms responsible for lung tumors. In the present study, diesel particle extracts were found to show mutagenicity for YG3003, a sensitive strain to some oxidative mutagens, as well as other mutant strains, and those of lung tissues obtained from lung cancer patients exhibited potent mutagenicity. Formation of 8-hydroxyguanosine (8-OHdG) as a biomarker of oxidative damage was analyzed with in vitro and in vivo assay systems. The 8-OHdG was detected in all 22 cases of lung tissues with carcinomas tested and their levels increased with the increasing age of the patients, suggesting a correlation between age and the presence of carbon particles in lung tissues. Therefore, the formation of 8-OHdG due to diesel exhaust particles was investigated via intratracheal injections into mice. 8-OHdG formation was elevated when carboneceous particles, after removal of organic chemicals with various solvents, were administered to mice, but it was not elevated when polyaromatic compounds such as benzo[a]pyrene, 1,8-dinitropyrene, and 1-nitropyrene were used in the same procedure in mice. The carboneceous particles were formed from a giant particle that was aggregated by micro-particles with diameters of 1.47 +/- 1.34 to 1.05 +/- 0.83 microm. These results suggest that carboneceous particles, but not mutagens and carcinogens, promote the formation of 8-OHdG, and that as a mechanism, alveolar macrophages may be involved in oxidative damage. The oxidative damage may be due to the fact that the mutation is involved with the generation of a hydroxyl radical during phagocytosis, and the hydroxyl radical leads to hydroxylation at the C-8 position of the deoxyguanosine residue in the DNA.

Mutation spectra of chemical mutagens determined by Lac+ reversion assay with Escherichia coli WP3101P-WP3106P tester strains

We previously reported the development of mutation-specific Escherichia coli B tester strains WP3101 to WP3106 from strain WP2uvrA. In this study we constructed their pKM101-containing derivatives WP3101P to WP3106P, and further isolated their rfa derivatives WP4101-WP4106 and WP4101P-WP4106P. The six kinds of F' plasmids (lacI-, lacZ-, proAB+), each of which carries a different lacZ allele, contained in the above strains were originally derived from E. coli K-12 strains CC101-CC106. All the tester strains show Lac- and Trp- phenotype. Assays for transitions and transversions are based upon Lac+ reversion of a specific mutation located within the lacZ gene on an F' plasmid. The trpE65(ochre) allele in the same strains enables them to be used for Trp+ reversion assays as well. In the present paper, we evaluated the sensitivity, specificity, and usefulness of the newly developed tester strains. Strains WP3101P-WP3106P were highly sensitive to determine mutational profile of heterocyclic amines with S9 mix-mediated metabolic activation and most of the oxidative mutagens and free radical generators tested. Every type of base-pair substitutions induced by 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) or 5-diazouracil were detected in strains WP3101P-WP3106P, while A:T-->C:G and G:C-->A:T mutations induced by MeIQ, and A:T-->C:G, G:C-->A:T, and G:C-->C:G by 5-diazouracil were not detected in pKM101-free tester strains. In pKM101-carrying strains, cumene hydroperoxide induced all types of base substitutions, while formaldehyde preferentially induced G:C-->T:A transversions. Phenazine methosulfate induced predominantly G:C-->A:T transitions and G:C-->T:A transversions, while H2O2 induced predominantly G:C-->T:A and A:T-->T:A transversions. Introduction of the rfa mutation considerably enhanced sensitivity to bulky mutagens such as polycyclic aromatic compounds. All six possible base substitutions induced by 9, 10-dimethyl-1,2-benzanthracene (DMBA) were detected in tester strains WP4101P-WP4106P. In conclusion, our tester strains WP3101P-WP3106P and WP4101P-WP4106P permitted rapid and simple detection of specific mutations induced by variety of mutagens.

Thermostable repair enzyme for oxidative DNA damage from extremely thermophilic bacterium, Thermus thermophilus HB8

The mutM (fpg) gene, which encodes a DNA glycosylase that excises an oxidatively damaged form of guanine, was cloned from an extremely thermophilic bacterium, Thermus thermophilus HB8. Its nucleotide sequence encoded a 266 amino acid protein with a molecular mass of approximately 30 kDa. Its predicted amino acid sequence showed 42% identity with the Escherichia coli protein. The amino acid residues Cys, Asn, Gln and Met, known to be chemically unstable at high temperatures, were decreased in number in T.thermophilus MutM protein compared to those of the E.coli one, whereas the number of Pro residues, considered to increase protein stability, was increased. The T.thermophilus mutM gene complemented the mutability of the E.coli mutM mutY double mutant, suggesting that T. thermophilus MutM protein was active in E.coli. The T.thermophilus MutM protein was overproduced in E.coli and then purified to homogeneity. Size-exclusion chromatography indicated that T. thermophilus MutM protein exists as a more compact monomer than the E.coli MutM protein in solution. Circular dichroism measurements indicated that the alpha-helical content of the protein was approximately 30%. Thermus thermophilus MutM protein was stable up to 75 degrees C at neutral pH, and between pH 5 and 11 and in the presence of up to 4 M urea at 25 degrees C. Denaturation analysis of T.thermophilus MutM protein in the presence of urea suggested that the protein had at least two domains, with estimated stabilities of 8.6 and 16.2 kcal/mol-1, respectively. Thermus thermophilus MutM protein showed 8-oxoguanine DNA glycosylase activity in vitro at both low and high temperatures.

Genomic structure and chromosomal localization of the mouse Ogg1 gene that is involved in the repair of 8-hydroxyguanine in DNA damage

8-Hydroxyguanine (7,8-dihydro-8-oxoguanine: oh8Gua) is a damaged form of guanine induced by oxygen-free radicals and causes GC to TA transversions. Previously we isolated the hOGG1 gene, a human homolog of the yeast OGG1 gene, which encodes a DNA glycosylase and lyase to excise oh8Gua in DNA. In this study, we isolated a mouse homolog (Ogg1) of the OGG1 gene, characterized oh8Gua-specific DNA glycosylase/AP lyase activities of its product, and determined chromosomal localization and exon-intron organization of this gene. A predicted protein possessed five domains homologous to human and yeast OGG1 proteins. Helix-hairpin-helix and C2H2 zinc finger-like DNA-binding motifs found in human and yeast OGG1 proteins were also retained in mouse Ogg1 protein. The properties of a GST fusion protein were identical to human and yeast OGG1 proteins in glycosylase/lyase activities, their substrate specificities, and suppressive activities against the spontaneous mutagenesis of an Escherichia coli mutM mutY double mutant. The mouse Ogg1 gene was mapped to Chromosome (Chr) 6, and consisted of 7 exons approximately 6 kb long. Two DNA-binding motifs were encoded in exons 4 through 5. These data will facilitate the investigation of the OGG1 gene to elucidate the relationship between oxidative DNA damage and carcinogenesis.