Low dose genotoxicity of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in gpt delta transgenic mice

Mutagenicity of carcinogenic heterocyclic amines in Salmonella typhimurium YG strains and transgenic rodents including gpt delta

Chemical carcinogens to humans have been usually identified by epidemiological studies on the relationships between occupational or environmental exposure to the agents and specific cancer induction. In contrast, carcinogenic heterocyclic amines were identified under the principle that mutagens in bacterial in the Ames test are possible human carcinogens. In the 1970s to 1990s, more than 10 heterocyclic amines were isolated from pyrolysates of amino acids, proteins, meat or fish as mutagens in the Ames test, and they were demonstrated as carcinogens in rodents. In the 1980s and 1990s, we have developed derivatives of the Ames tester strains that overexpressed acetyltransferase of Salmonella typhimurium. These strains such as Salmonella typhimurium YG1024 exhibited a high sensitivity to the mutagenicity of the carcinogenic heterocyclic amines. Because of the high sensitivity, YG1024 and other YG strains were used for various purposes, e.g., identification of novel heterocyclic amines, mechanisms of metabolic activation, comparison of mutagenic potencies of various heterocyclic amines, and the co-mutagenic effects. In the 1990s and 2000s, we developed transgenic mice and rats for the detection of mutagenicity of chemicals in vivo. The transgenics were generated by the introduction of reporter genes for mutations into fertilized eggs of mice and rats. We named the transgenics as gpt delta because the gpt gene of Escherichia coli was used for detection of point mutations such as base substitutions and frameshifts and the red/gam genes of λ phage were employed to detect deletion mutations. The transgenic rodents gpt delta and other transgenics with lacI or lacZ as reporter genes have been utilized for characterization of mutagenicity of heterocyclic amines in vivo. In this review, we summarized the in vitro mutagenicity of heterocyclic amines in Salmonella typhimurium YG strains and the in vivo mutagenicity in transgenic rodents. We discussed the relationships between in vitro and in vivo mutagenicity of the heterocyclic amines and their relations to the carcinogenicity.

Effects of DNA polymerase kappa and mismatch repair on dose-responses of chromosome aberrations induced by three oxidative genotoxins in human cells

Genotoxic carcinogens are regulated under the policy that there is no threshold or safe dose. It has been pointed out, however, that self-defense mechanisms, such as detoxification, DNA repair, and error-free translesion synthesis, may protect chromosome DNA from genotoxic insults, thereby constituting practical threshold. In this study, we examined dose responses of chromosome aberrations induced by three oxidative genotoxins, that is, hydrogen peroxide (H₂ O₂ ), menadione and paraquat, with or without DNA polymerase kappa (Polκ) activities and mismatch repair capacities in human cells. Polκ is involved in translesion synthesis across DNA damage and mismatch repair is responsible for correction of base-base mismatch in DNA. Polκ activity of the cells was inactivated either by point mutations in the catalytically essential amino acids (catalytically dead or CD) or by deletion of the POLK gene (knockout or KO). In the absence of mismatch repair, frequencies of chromosome aberrations induced by H₂ O₂ and menadione were not significantly different among CD, KO, and the wild type (WT) cells. In the presence of mismatch repair, however, cytotoxicity and clastogenicity were enhanced and Polκ modulated the sensitivity of the cells. No-observed-genotoxic-effect-levels (NOGELs) for H₂ O₂ and menadione were CD = KO < WT cells. In contrast, the sensitivities of the cells to paraquat were not significantly affected by the status of mismatch repair or Polκ activity. The results suggest that mismatch repair and Polκ coordinately modulate NOGELs for the clastogenicity of H₂ O₂ and menadione and also that DNA lesion(s) responsible for paraquat-induced chromosome aberrations are different from those induced by H₂ O₂ and menadione. Environ. Mol. Mutagen. 61:193-199, 2020. © 2019 Wiley Periodicals, Inc.

Impact of DNA polymerase ζ mutations on genotoxic thresholds of oxidative mutagens

In regulatory genetic toxicology, it is an axiom that there is no threshold for genotoxicity of chemicals, such that genotoxic chemicals may impose carcinogenic risk on humans even at very low doses. This paradigm is counterintuitive, however, because humans possess a number of self-defense mechanisms that may suppress the genotoxicity at these low doses and therefore manifest a practical threshold. DNA polymerase zeta (Pol ζ) is a specialized Pol that plays an important role in DNA synthesis across DNA damage, thereby modulating cell survival and genotoxicity. In this study, we compared the sensitivity of three types of human cells: D2781N, L2618M, and their wild-type (WT) cells, to the low dose effects of genotoxicity of the oxidizing agents, potassium bromate (KBrO₃) and sodium dichromate (Na₂Cr₂O₇). D2781N cells express a variant form of Pol ζ, whose activity is weaker than that of the WT enzyme. L2618M cells express another variant form of Pol ζ, whose fidelity of DNA replication is lower than that of the WT enzyme. D2781N exhibited the highest sensitivity for TK gene mutation and micronucleus (MN) formation and displayed the lowest practical threshold for MN induction by KBrO₃. In contrast, L2618M exhibited the lowest practical threshold for sister-chromatid exchange (SCE) induction by both chemicals. These results suggest that Pol ζ mutations have significant impacts on practical thresholds of genotoxicity; the factors affecting the practical threshold can differ depending on the endpoint of genotoxicity. Roles of the variant forms of Pol ζ in genotoxicity by the oxidizing agents are discussed.

Possible Application of Human c-Ha-ras Proto-Oncogene Transgenic Rats in a Medium-Term Bioassay Model for Carcinogens

With the aim of developing a medium-term assay for screening of environmental carcinogens, we exposed mammary carcinogen sensitive human c-Ha-ras proto-oncogene transgenic (Hras128) rats to various carcinogens, including compounds that do not normally induce mammary tumors. Seven-week-old Hras128 rats and wild-type littermates received administrations of 3-methylcholanthrene (3-MC), benzo[a]pyrene (B[a]P), anthracene, pyrene, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), dimethylarsinic acid (DMA), diethylnitrosamine (DEN) or azoxymethane (AOM) and were sacrificed at week 12 (females) (at week 10 for the 3-MC group) or week 20 (males). Female Hras128 rats receiving NNK, DEN, or DMA showed a significant increase in mammary tumor incidence and/or multiplicity compared to the respective values with olive oil or deionized distilled water (DDW) vehicles. In male Hras128 rats, a significant increase in mammary tumors was also observed in groups administered 3-MC, B[a]P, anthracene, IQ, and NNK. Mutations of transgenes were observed in codons 12 and/or 61 in the induced tumors by PCR-RFLP except in the DEN group in female and in the MeIQx group in male Hras128 rats. Thus various carcinogens, not necessarily limited to those normally targeting the breast, were found to induce mammary carcinomas in Hras128 rats, especially in females, pointing to potential use for medium-term screening.

Past, Present and Future Directions of gpt delta Rodent Gene Mutation Assays

Genotoxicity is a critical endpoint of toxicity to regulate environmental chemicals. Genotoxic chemicals are believed to have no thresholds for the action and impose genotoxic risk to humans even at very low doses. Therefore, genotoxic carcinogens, which induce tumors via genotoxic mechanisms, are regulated more strictly than non-genotoxic carcinogens, which induce tumors through non-genotoxic mechanisms such as hormonal effects, cell proliferation and cell toxicity. Although Ames bacterial mutagenicity assay is the gold standard to identify genotoxicity of chemicals, the genotoxicity should be further examined in rodents because Ames positive chemicals are not necessarily genotoxic in vivo. To better evaluate the genotoxicity of chemicals in a whole body system, gene mutation assays with gpt delta transgenic mice and rats have been developed. A feature of the assays is to detect point mutations and deletions by two distinct selection methods, ie, gpt and Spi- assays, respectively. The Spi- assay is unique in that it allows analyses of deletions and complex DNA rearrangements induced by double-strand breaks in DNA. Here, I describe the concept of gpt delta gene mutation assays and the application in food safety research, and discuss future perspectives of genotoxicity assays in vivo.

The application of hepatic P450 reductase null gpt delta mice in studying the role of hepatic P450 in genotoxic carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced mutagenesis

The cytochrome P450 (P450 or CYP) is involved in both detoxification and metabolic activation of many carcinogens. In order to identify the role of hepatic P450 in the mutagenesis of genotoxic carcinogens, we generated a novel hepatic P450 reductase null (HRN) gpt delta mouse model, which lacks functional hepatic P450 on a gpt delta mouse background. In this study, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) was used to treat HRN gpt delta mice and control littermates. Gene mutations in the liver and lungs were detected, and mutation spectra were analyzed. Pharmacokinetic analyses were performed, and tissue levels of NNK and metabolite were determined. NNK-induced mutant frequencies (MFs) were equivalent to spontaneous MFs in the liver, but increased more than 3 times in the lungs of HRN gpt delta mice compared to control mice. NNK-induced mutation spectra showed no difference between HRN gpt delta mice and control littermates. Toxicokinetic studies revealed reduced clearance of NNK with elevated tissue concentrations in HRN gpt delta mice. To our knowledge, these are the first data demonstrating that NNK cannot induce mutagenesis in the liver without P450 metabolic activation, but can induce mutagenesis in lungs by a hepatic P450-independent mechanism. Moreover, our data show that hepatic P450 plays a major role in the systemic clearance of NNK, thereby protecting the lungs against NNK-induced mutagenesis. Our model will be useful in establishing the role of hepatic versus extrahepatic P450-mediated mutagenesis, and the relative contributions of P450 compared to other biotransformation enzymes in the genotoxic carcinogens' activation.

Gene-TEQ--a standardized comparative assessment of effects in the comet assay using genotoxicity equivalents

Existing methods for the comparison of genotoxic effects in the comet assay bear considerable disadvantages such as the problem to link information about concentration dependence and severity of effects. Moreover, given the lack of standardized protocols and the use of various standards, it may be extremely difficult to compare different studies. In order to provide a method for standardized comparative assessment of genotoxic effects, the concept of genotoxicity equivalents (Gene-TEQ) was developed. As potential reference compounds for genotoxic effects, three directly acting (N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl-methanesulfonate, and N-methyl-N-nitrosourea) and three indirectly acting (cyclophosphamide, dimethylnitrosamine, and 4-nitroquinoline-oxide) genotoxic substances were compared with respect to their cytotoxic (neutral red) and genotoxic (comet assay) concentration-response profiles in the permanent fish cell line RTL-W1. For further comparison, two sediment extracts from the upper Danube River were investigated as environmental samples. Based on the results of cytotoxicity and genotoxicity testing, MNNG was selected as the reference compound. At several exposure levels and durations, genotoxic effects of both the other pure substances and the environmental samples were calculated as percentages of the maximum MNNG effect and related to the absolute MNNG effect (EC values). Thus, genotoxicity equivalent factors (Gene-TEQs) relative to MNNG could be calculated. Gene-TEQs can easily be applied to pure substances, mixtures and field samples to provide information about their toxicity relative to the reference compound. Furthermore, the Gene-TEQ concept allows a direct comparison of environmental samples from different laboratories.

Oxidative DNA damage and reporter gene mutation in the livers of gpt delta rats given non-genotoxic hepatocarcinogens with cytochrome P450-inducible potency

Previous reports have proposed that reactive oxygen species resulting from induction of cytochrome P450 (CYP) isozymes might be involved in the modes of action of hepatocarcinogens with CYP-inducible potency. In the present study, we investigated 8-hydroxydeoxyguanosine (8-OHdG) levels, in vivo mutagenicity and glutathione S-transferase placental form (GST-P)-positive foci in the livers of gpt delta rats treated with piperonyl butoxide (PBO) or phenobarbital (PhB) for 4 and 13 weeks. Significant elevations in Cyp 1A1 and Cyp 1A2 mRNA levels after PBO treatment, and in Cyp 2B1 mRNA levels after PBO or PhB treatment, appeared together with remarkable hepatomegaly through the experimental period. Time-dependent and statistically significant increases in 8-OHdG levels were observed in the PBO treatment group along with significant increases in proliferating cell nuclear antigen (PCNA)-positive hepatocytes at 4 weeks, while no increase in 8-OHdG levels was found in PhB-treated rats. No changes in mutant frequencies of gpt and red/gam (Spi(-)) genes in liver DNA from PBO- or PhB-treated rats were observed at 4 or 13 weeks. A 13-week exposure to either PBO or PhB did not affect the number and area of GST-P-positive hepatocytes. CYP 1A1 and 1A2 induction may be responsible for elevated levels of 8-OHdG in PBO-treated rats. However, neither GC:TA transversions nor deletion mutations, typically regarded as 8-OHdG-related mutations, were observed in any of the treated rats. We conclude that reactive oxygen species, possibly produced through CYP catalytic pathways, likely induced genomic DNA damage but did not give rise to permanent gene mutation.

Mutagenic potency of Helicobacter pylori in the gastric mucosa of mice is determined by sex and duration of infection

Helicobacter pylori is a human carcinogen, but the mechanisms evoked in carcinogenesis during this chronic inflammatory disease remain incompletely characterized. We determined whether chronic H. pylori infection induced mutations in the gastric mucosa of male and female gpt delta C57BL/6 mice infected for 6 or 12 mo. Point mutations were increased in females infected for 12 mo. The mutation frequency in this group was 1.6-fold higher than in uninfected mice of both sexes (P < 0.05). A:T-to-G:C transitions and G:C-to-T:A transversions were 3.8 and 2.0 times, respectively, more frequent in this group than in controls. Both mutations are consistent with DNA damage induced by oxidative stress. No increase in the frequency of deletions was observed. Females had more severe gastric lesions than males at 6 mo postinfection (MPI; P < 0.05), but this difference was absent at 12 MPI. In all mice, infection significantly increased expression of IFNgamma, IL-17, TNFalpha, and iNOS at 6 and 12 mo, as well as H. pylori-specific IgG1 levels at 12 MPI (P < 0.05) and IgG2c levels at 6 and 12 MPI (P < 0.01 and P < 0.001). At 12 MPI, IgG2c levels in infected females were higher than at 6 MPI (P < 0.05) and also than those in infected males at 12 MPI (P < 0.05). Intensity of responses was mediated by sex and duration of infection. Lower H. pylori colonization indicated a more robust host response in females than in males. Earlier onset of severe gastric lesions and proinflammatory, Th1-biased responses in female C57BL/6 mice may have promoted mutagenesis by exposing the stomach to prolonged oxidative stress.

Biological significance of DNA adducts: comparison of increments over background for various biomarkers of genotoxicity in L5178Y tk(+/-) mouse lymphoma cells treated with hydrogen peroxide and cumene hydroperoxide

DNA is affected by background damage of the order of one lesion per one hundred thousand nucleotides, with depurination and oxidative damage accounting for a major part. This damage contributes to spontaneous mutation and cancer. DNA adducts can be measured with high sensitivity, with limits of detection lower than one adduct per one billion nucleotides. Minute exposures to an exogenous DNA-reactive agent may therefore result in measurable adduct formation, although, as an increment over total DNA damage, a small increment in mutation cannot be measured and would be considered negligible. Here, we investigated whether this discrepancy also holds for adducts that are present as background induced by oxidative stress. L5178Y tk(+/-) mouse lymphoma cells were incubated for 4h with hydrogen peroxide (0, 0.8, 4, 20, 100, 500muM) or cumene hydroperoxide (0, 0.37, 1.1, 3.3, 10muM). Five endpoints of genotoxicity were measured in parallel from aliquots of three replicates of large batches of cells: Two DNA adducts, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) and 1,N(6)-etheno-2'-deoxyadenosine (varepsilondAdo) measured by LC-MS/MS, as well as strand breaks assessed with the comet assay and in vitro micronucleus test, and gene mutation as assessed using the thymidine kinase gene mutation assay. Background measures of 8-oxodGuo and varepsilondAdo were 500-1000 and 50-90 adducts per 10(9) nucleotides. Upon treatment, neither hydrogen peroxide nor cumene hydroperoxide significantly increased the DNA adduct levels above control. In contrast, dose-related increases above background were observed with both oxidants in the comet assay, the micronucleus test and the gene mutation assay. Differences in sensitivity of the assays were quantified by estimating the concentration of oxidant that resulted in a doubling of the background measure. We conclude that the increase in DNA breakage and mutation induced by hydrogen peroxide and cumene hydroperoxide observed in our in vitro experimental set-up was no direct consequence of the measured DNA adducts. In comparison with data obtained with the methylating agent methyl methanesulfonate we further conclude that the assumption of DNA adducts being oversensitive biomarkers is adduct-specific.

Mutations in the lungs of gpt delta transgenic mice following inhalation of diesel exhaust

Diesel exhaust (DE) is a major airborne pollutant of urban areas. It contains various polycyclic aromatic hydrocarbons (PAH) and nitrated PAHs. In this study, gpt delta mice were treated with inhalation of 1 or 3 mg m(-3) DE, or a single intratracheal instillation of diesel exhaust particles (DEP) or DEP extract. In the lungs of mice treated with inhalation of 3 mg m(-3) DE for 12 weeks, the mutant frequency (MF) was 3.2-fold higher than that of the control group (1.90 x 10(-5) and 0.59 x 10(-5), respectively). An instillation of DEP and DEP extract resulted in a significant dose-dependent linear increase in MF. In mice treated with 0.5 mg DEP and 0.2 mg DEP extract, the MFs were 3.0- and 2.7-fold higher than that of the control group, respectively. The mutagenic potency (MF mg(-1)) of DEP extract (5.6 x 10(-5)) was double that of DEP (2.7 x 10(-5)), suggesting that the mutagenicity of the latter is derived primarily from compounds in the extract, which itself is responsible for ca. 50% of the weight of DEP. G:C-->A:T transitions were the predominant gpt mutation induced by all three treatments and G:C-->T:A transversions were induced by DEP and DEP extract. Guanine bases centered in nucleotide sequences such as GGA, TGA, CGG, and CGT were the major mutation targets of all three treatments. Thus, our results suggest that the mutagens contained in DEP such as PAH and nitrated PAHs induce mutations and may be responsible for carcinogenesis caused by inhalation of DE.

Deletion and single nucleotide substitution at G:C in the kidney of gpt delta transgenic mice after ferric nitrilotriacetate treatment

An iron chelate, ferric nitrilotriacetate (Fe-NTA), induces oxidative renal proximal tubular damage that subsequently leads to a high incidence of renal cell carcinoma in rodents, presenting an intriguing model of free radical-induced carcinogenesis. In the present study, we used gpt delta transgenic mice, which allow efficient detection of point mutations and deletions in vivo, to evaluate the mutation spectra, in association with the formation of 8-oxoguanine and acrolein-modified adenine during the first 3 weeks of carcinogenesis. Immunohistochemical analysis revealed the highest levels of 8-oxoguanine and acrolein-modifed adenine in the renal proximal tubules after 1 week of repeated administration. DNA immunoprecipitation and quantitative polymerase chain reaction analysis showed that the relative abundance of 8-oxoguanine and acrolein-modified adenine at the gpt reporter gene were increased at the first week in the kidney. Similarly, in both 6-thioguanine and Spi(-) selections performed on the renal specimens after Fe-NTA administration, the mutant frequencies were increased in the Fe-NTA-treated mice at the first week. Further analyzes of 79 mutant clones and 93 positive plaques showed a high frequency of G:C pairs as preferred targets for point mutation, notably G:C to C:G transversion-type mutation followed by deletion, and of large-size (>1 kilobase) deletions with short homologous sequences in proximity to repeated sequences at the junctions. The results demonstrate that the iron-based Fenton reaction is mutagenic in vivo in the renal tubular cells and induces characteristic mutations.

A newly established GDL1 cell line from gpt delta mice well reflects the in vivo mutation spectra induced by mitomycin C

In order to create a novel in vitro test system for detection of large deletions and point mutations, we developed an immortalized cell line. A SV40 large T antigen expression unit was introduced into fibroblasts derived from gpt delta mouse lung tissue and a selected clone was established as the gpt delta L1 (GDL1) cell line. The novel GDL1 cells were examined for mutant frequencies (MFs) and for molecular characterization of mutations induced by mitomycin C (MMC). The GDL1 cells were treated with MMC at doses of 0.025, 0.05, and 0.1 microg/mL for 24h and mutations were detected by Spi- and 6-thioguanine (6-TG) selections. The MFs of the MMC-treated cells increased up to 3.4-fold with Spi- selection and 3.5-fold with 6-TG selection compared to MFs of untreated cells. In the Spi- mutants, the number of large (up to 76 kilo base pair (kbp)) deletion mutations increased. A majority of the large deletion mutations had 1-4 base pairs (bp) of microhomology in the deletion junctions. A number of the rearranged deletion mutations were accompanied with deletions and insertions of up to 1.1 kbp. In the gpt mutants obtained from 6-TG selection, single base substitutions of G:C to T:A, tandem base substitutions occurring at the 5'-GG-3' or 5'-CG-3' sequence, and deletion mutations larger than 2 bp were increased. We compared the spectrum of MMC-induced mutations observed in vitro to that of in vivo using gpt delta mice, which we reported previously. Although a slight difference was observed in MMC-induced mutation spectra between in vitro and in vivo, the mutations detected in vitro included all of the types of mutations observed in vivo. The present study demonstrates that the newly established GDL1 cell line is a useful tool to detect and analyze various mutations including large deletions in mammalian cells.

Lack of in vivo mutagenicity and oxidative DNA damage by flumequine in the livers of gpt delta mice

Flumequine (FLU), an anti-bacterial quinolone agent, has been recognized as a non-genotoxic carcinogen for the mouse liver, but recent reports have suggested that some genotoxic mechanism involving oxidative DNA damage may be responsible for its hepatocarcinogenesis. In the present study, we investigated this possibility in the mouse liver using male and female B6C3F1 gpt delta mice fed diet containing 0.4% FLU, a carcinogenic dose, for 13 weeks. Measurements of 8-hydroxydeoxyguanosine levels in liver DNA, and gpt point and deletion mutations revealed no significant increases in any of these parameters in either sex. Histopathologically, centrilobular swelling of hepatocytes with vacuolation was apparent, however, together with significant increase in bromodeoxyuridine-labeling indices in the treated males and females. These results suggest that genotoxicity, including oxidative DNA damage, is not involved in mouse hepatocarcinogenesis by FLU, which might rather solely exert tumor-promoting effects in the liver.

In vivo mutagenesis induced by benzo[a]pyrene instilled into the lung of gpt delta transgenic mice

Benzo[a]pyrene (B[a]P) is a ubiquitous airborne pollutant whose mutagenicity has been evaluated previously by oral and intraperitoneal administration to experimental animals. In this study, mutagenesis in the lungs, the target organ of air pollutants, was examined after a single intratracheal instillation of 0-2 mg B[a]P into gpt delta transgenic mice. Intratracheal injection of B[a]P resulted in a statistically significant and dose-dependent increase in gpt mutant frequency as measured by 6-thioguanine selection. The mutant frequencies at B[a]P doses of 0.5, 1, and 2 mg were 2.8, 4.2, and 6.8 times higher than the frequency seen in nontreated mice (0.60 +/- 0.13 x 10(-5)). The most frequent mutations induced by B[a]P treatment were G:C-->T:A transversions, which are characteristic of B[a]P mutagenesis in other models, and single-base deletions of G:C base pairs. To characterize the hotspots of B[a]P-induced mutations in the gpt gene, we analyzed sequences adjacent to the mutated G:C base pairs. Guanine bases centered in the nucleotide sequences CGT, CGA, and CGG were the most frequent targets of B[a]P. Our results indicate that intratracheal instillation of B[a]P into gpt delta mice causes a dose-dependent increase in gpt mutant frequency in the lung, and that the predominant mutation induced is G:C-->T:A transversion.

Lack of potential of low dose N-nitrosodimethylamine to induce preneoplastic lesions, glutathione S-transferase placental form-positive foci, in rat liver

Induction of liver lesions in male F344 rats by the genotoxic and carcinogenic N-nitrosodimethylamine (NDMA) was studied at a wide range of dose levels, i.e. from 0.001 to 10 ppm, in drinking water for 16 weeks. Dose related and statistically significant increase of glutathione S-transferase placental form-positive foci, endpoint markers for hepatocarcinogenesis in rats, at 1 and 10 ppm dose groups was obtained, but no increment in foci could be detected with the lower doses (0.001, 0.01, and 0.1 ppm). This finding of a no-observed effect level supports our hypothesis that a threshold, at least in practical terms, exists in carcinogenesis proposed on the basis of extensive wide range dose-dependence studies of other genotoxic carcinogens.

Molecular nature of intrachromosomal deletions and base substitutions induced by environmental mutagens

Cellular DNA is exposed to a variety of exogenous and endogenous mutagens. A complete understanding of the importance of different types of DNA damage requires knowledge of the specific molecular alterations induced by different types of agents in specific target tissues in vivo. The gpt delta transgenic mouse model provides the opportunity to characterize tissue-specific DNA alterations because small and large deletions as well as base substitutions can be analyzed. Here, we summarize the characteristics of intrachromosomal deletions and base substitutions induced by ionizing radiation in liver and spleen, ultraviolet B (UVB) radiation in epidermis, mitomycin C (MMC) in bone marrow, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in colon, and aminophenylnorharman (APNH) in liver of gpt delta mice. Carbon-ion radiation, UVB, and MMC induced large deletions of more than 1 kb. About half of the large deletions occurred between short direct-repeat sequences and the remainder had flush ends, suggesting the involvement of nonhomologous end joining of double-stranded breaks (DSBs) in DNA. UV photoproducts and interstrand crosslinks by MMC may block DNA replication, thereby inducing DSBs. In contrast, PhIP and APNH mainly generated 1 bp deletions in runs of guanine bases. As for base substitutions, UVB and MMC induced G:C-->A:T transitions at dipyrimidine sites and tandem base substitutions at GG sites, respectively. PhIP and APNH induced G:C-->T:A transversions. Translesion DNA synthesis across the lesions, i.e., UV photoproducts, intrastrand crosslinks by MMC, and guanine adducts by the heterocyclic amines, may be involved in the induction of base substitutions. These results indicate the importance of sequence information to elucidate the mechanisms underlying deletions and base substitutions induced in vivo by environmental mutagens.