Genotoxicity of 3-methylcholanthrene in liver of transgenic big Blue mice

Parental exposure 3-methylcholanthrene disturbed the enterohepatic circulation in F1 generation of mice

3-methylcholanthrene (3 MC) is an environmental compound belonging to the PAHs and is reportedly thought to be a risk factor for the prevalence of hepatic function disorder. Here, a dose of 0.5 mg/kg of 3 MC was given to 4-week-old male and female mice (F0) in their diet for 6 weeks. After exposure, then the mice were mated between different groups. The first filial (F1) generation offspring of exposed or unexposed parental mice were sacrificed at the age of 5 weeks (F1-5 W), and the potential effects on the F0 and F1 offspring were evaluated. The results showed that the total bile acids (TBAs) in the serum and feces in F0 females and female F1-5 W individuals born from female mice exposed to 3 MC decreased, while the TBAs in the liver increased. The transcriptional levels of major genes participating in synthesis, regulation, transportation and apical uptake was also altered correspondingly. In addition, the transcription of some genes related to inflammation was enhanced in these mice. Further investigation revealed that in addition to distinct changes in the mucus secretion, tight junction proteins and ion transport were induced, and antimicrobial peptides were also disrupted in the intestine of F0 mice and F1-5 W female offspring of maternal mice exposed to 3 MC. Our results suggested that exposure to 3 MC, but not male exposure, had the potential to interfere with BAs metabolism, affecting gut barrier function. Females were more seriously affected than males.

Single-strand specific nuclease enhances accuracy of error-corrected sequencing and improves rare mutation-detection sensitivity

Error-corrected sequences (ECSs) that utilize double-stranded DNA sequences are useful in detecting mutagen-induced mutations. However, relatively higher frequencies of G:C > T:A (1 × 10⁻⁷ bp) and G:C > C:G (2 × 10⁻⁷ bp) errors decrease the accuracy of detection of rare G:C mutations (approximately 10⁻⁷ bp). Oxidized guanines in single-strand (SS) overhangs generated after shearing could serve as the source of these errors. To remove these errors, we first computationally discarded up to 20 read bases corresponding to the ends of the DNA fragments. Error frequencies decreased proportionately with trimming length; however, the results indicated that they were not sufficiently removed. To efficiently remove SS overhangs, we evaluated three mechanistically distinct SS-specific nucleases (S1 Nuclease, mung bean nuclease, and RecJf exonuclease) and found that they were more efficient than computational trimming. Consequently, we established Jade-Seq™, an ECS protocol with S1 Nuclease treatment, which reduced G:C > T:A and G:C > C:G errors to 0.50 × 10⁻⁷ bp and 0.12 × 10⁻⁷ bp, respectively. This was probably because S1 Nuclease removed SS regions, such as gaps and nicks, depending on its wide substrate specificity. Subsequently, we evaluated the mutation-detection sensitivity of Jade-Seq™ using DNA samples from TA100 cells exposed to 3-methylcholanthrene and 7,12-dimethylbenz[a]anthracene, which contained the rare G:C > T:A mutation (i.e., 2 × 10⁻⁷ bp). Fold changes of G:C > T:A compared to the vehicle control were 1.2- and 1.3-times higher than those of samples without S1 Nuclease treatment, respectively. These findings indicate the potential of Jade-Seq™ for detecting rare mutations and determining the mutagenicity of environmental mutagens.

Hawk-Seq™ differentiates between various mutations in Salmonella typhimurium TA100 strain caused by exposure to Ames test-positive mutagens

A precise understanding of differences in genomic mutations according to the mutagenic mechanisms detected in mutagenicity data is required to evaluate the carcinogenicity of environmental mutagens. Recently, we developed a highly accurate genome sequencing method, ‘Hawk-Seq™’, that enables the detection of mutagen-induced genome-wide mutations. However, its applicability to detect various mutagens and identify differences in mutational profiles is not well understood. Thus, we evaluated DNA samples from Salmonella typhimurium TA100 exposed to 11 mutagens, including alkylating agents, aldehydes, an aromatic nitro compound, epoxides, aromatic amines and polycyclic aromatic hydrocarbons (PAHs). We extensively analysed mutagen-induced mutational profiles and studied their association with the mechanisms of mutagens. Hawk-Seq™ sensitively detected mutations induced by all 11 mutagens, including one that increased the number of revertants by approximately 2-fold in the Ames test. Although the sensitivity for less water-soluble mutagens was relatively low, we increased the sensitivity to obtain high-resolution spectra by modifying the exposure protocol. Moreover, two epoxides indicated similar 6- or 96-dimensional mutational patterns; likewise, three S_N1-type alkylating agents indicated similar mutational patterns, suggesting that the mutational patterns are compound category specific. Meanwhile, an S_N2 type alkylating agent exhibited unique mutational patterns compared to those of the S_N1 type alkylating agents. Although the mutational patterns induced by aldehydes, the aromatic nitro compound, aromatic amines and PAHs did not differ substantially from each other, the maximum total base substitution frequencies (MTSFs) were similar among mutagens in the same structural groups. Furthermore, the MTSF was found to be associated with the carcinogenic potency of some direct-acting mutagens. These results indicate that our method can generate high-resolution mutational profiles to identify characteristic features of each mutagen. The detailed mutational data obtained by Hawk-Seq™ can provide useful information regarding mutagenic mechanisms and help identify its association with the carcinogenicity of mutagens without requiring carcinogenicity data.

3-Methylcholanthrene (3-MC) and 4-chlorobiphenyl (PCB3) genotoxicity is gender-related in Fischer 344 transgenic rats

Polychlorinated biphenyls (PCBs) are a class of persistent organic pollutants with myriad biological effects, including carcinogenicity. We present data showing gender-specific genotoxicity in Fischer 344 transgenic BigBlue rodents exposed to 4-chlorobiphenyl (PCB3), a hydroxylated metabolite, and the positive control 3-methylcholanthrene (3-MC) where female rats are more resistant to the genotoxic effects of the test compounds compared to their male counterparts. This difference is further highlighted through our examination of gene expression, organ-specific weight changes, and tissue morphology. The purpose of the present study was to explore the complex and multifaceted issues of lower molecular weight PCBs as initiators of carcinogenesis, by examining the mutagenicity of PCB3, a hydroxylated metabolite (4'-OH-PCB3), and 3-methylcholanthrene (3-MC, positive control) in a transgenic rodent model. Previous findings indicated that PCB3 is mutagenic in the liver of male BigBlue transgenic rats under identical exposure conditions. We expected that female rats would be equally, if not more sensitive than male rats, since a 2-year carcinogenesis bioassay with Sprague-Dawley rats and commercial PCB mixtures reported much higher liver cancer rates in female than in male rats. The current study, however, revealed a similar trend in the mutation frequencies across all four treatment groups in females as reported previously in males, but increased variability among animals within each group and a lower overall effect, led to non significant differences in mutation frequencies. A closer analysis of the possible reasons for this negative result using microarray, organ weight and histology data comparisons shows that female Fischer 344 rats 1) had a higher baseline mutation frequency in the corn oil control group and greater variability than male rats; 2) responded with robust gene expression changes, which may also play a role in our observation of 3) highly increased liver, spleen, and lung weight in 3-MC and PCB3-treated female rats and thus changed distribution and kinetics of the test compounds. Our analysis indicates that female transgenic BigBlue Fischer 344 rats are more resistant to PCB3 and 3-MC genotoxicity compared to their male counterparts.

Mutagenicity of 3-methylcholanthrene, pcb3, and 4-oh-pcb3 in the lung of transgenic bigblue rats

Recent findings of high levels of predominantly lower chlorinated biphenyls in indoor and outdoor air open the question of possible health consequences. Lower chlorinated biphenyls are more readily metabolized to reactive and potentially harmful intermediates, acting as mutagens and cancer initiators. The goal of this study was to assess the mutagenicity of PCB3 in the lungs of rats. Male BigBlue® 334 Fisher transgenic rats, which carry the bacterial lacI gene as a target of mutagenicity, were given intraperitoneal injections of corn oil, 3-methylcholanthrene (3-MC, positive control), 4-monochlorobiphenyl (PCB3) or its metabolite 4-hydroxy-PCB3 (4-OH-PCB3) weekly for 4 weeks. Lungs tissue was harvested to determine mutant frequencies, mutation spectra, and pathological changes. 3-MC caused a 15-fold increase in mutant frequency and an increase in transversion type mutations; a very early occurrence of this type of mutation in lung tissue was previously identified in Ki-ras oncogenes of lung tumors from 3-MC exposed mice. The 2-fold increase in the mutant frequency after treatment with PCB3 and 4-OH-PCB3 was not statistically significant, but a shift in the mutation spectra, especially with PCB3, and an increase in mutations outside of the hotspot region for spontaneous mutations (bp 1-400), suggest that PCB3 and possibly 4-OH-PCB3 are mutagenic in the rat lung.

4-monochlorobiphenyl (PCB3) induces mutations in the livers of transgenic Fisher 344 rats

4-monochlorobiphenyl (PCB3) is found in small amounts in commercial PCB mixtures, indoor and outdoor air, and in food. In contrast to highly chlorinated congeners that are more resistant to metabolic attack, PCB3 is more readily converted by xenobiotic-metabolizing enzymes to monohydroxy-PCBs and further to dihydroxy-metabolites, which can be oxidized to quinones. Our recent studies demonstrated the initiating action of PCB3 in the livers of male rats. Therefore we hypothesized that PCB3 and/or its metabolite(s) are mutagenic in rat livers in vivo. To investigate the mutagenicity and the types of mutations generated by PCB3, male Fischer 344 BigBlue rats, transgenic for the lacI gene, were injected intraperitoneally with PCB3 (600 micromol/kg), 4-hydroxy-PCB3 (4-HO-PCB3, 400 micromol/kg), 3-methylcholanthrene (3-MC, 300 micromol/kg, positive control) and corn oil (negative control) once per week, for 4 weeks. Animals were killed 17 days after the last injection and the mutant frequency of the liver lacI gene determined. 3-MC induced a 4-fold increase of the mutant frequency of the lacI gene in the liver. The mutant frequency in PCB3-treated animals was also significantly elevated. In contrast, 4-HO-PCB3 induced a non-significant doubling of the mutant frequency. The mutation spectrum of solvent control mutants was characterized by transitions, whereas in 3-MC-animals, transversion and frameshift mutations predominated. The PCB3-induced mutation spectrum was similar to that of the 3-MC-induced mutants. In contrast, the mutation spectrum of the 4-HO-PCB3 group hardly differed from that of the control animals. This study demonstrates for the first time the mutagenicity of a PCB in vivo.

Genotoxic effects of bitumen fumes in Big Blue transgenic rat lung

Road paving workers are exposed to bitumen fumes (CAS No. 8052-42-4), a complex mixture of volatile compounds and particles containing carcinogenic and non-carcinogenic polycyclic aromatic hydrocarbons. However, epidemiological and experimental animal studies failed to draw unambiguous conclusions concerning their toxicity. In order to gain better insights on their genotoxic potential, we used an experimental design able to generate bitumen fumes at road paving temperature (temperature: 170 degrees C, total particulate matter: 100mg/m3) and perform a nose-only exposure of Big Blue transgenic rodents 6h/day for five consecutive days. The mutagenic properties of bitumen fumes were determined by analyzing the mutation frequency and spectrum of the neutral reporter gene cII inserted into the rodent genome. We previously observed in mouse lung, that bitumen fumes did not induce an increase of cII mutants, a modification of the mutation spectrum, nor the formation of DNA adducts. Since DNA adducts were found in the lungs of rats exposed to asphalt fumes in similar conditions, we decided to carry out an analogous experiment with Big Blue rats. A DNA adduct was detected 3 and 30 days after the end of treatment suggesting that these genetic alterations were quite steady. Thirty days after exposure, the cII mutant frequency was similar in control and exposed rats. In addition, a slight but not significant modification of the mutation spectrum associated with an increase of G:C to T:A and A:T to C:G transversions was noticeable in the treated animals. Then, these data failed to demonstrate a pulmonary mutagenic potential for bitumen fumes generated at road paving temperature in our experimental conditions despite the presence of a DNA adduct. These results may provide information concerning the pulmonary mechanism of action of this aerosol and may contribute to the occupational health hazard assessment.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

Effects of paving asphalt fume exposure on genotoxic and mutagenic activities in the rat lung

Asphalt fumes are complex mixtures of aerosols and vapors containing various organic compounds, including polycyclic aromatic hydrocarbons (PAHs). Previously, we have demonstrated that inhalation exposure of rats to asphalt fumes resulted in dose-dependent induction of CYP1A1 with concomitant down-regulation of CYP2B1 and increased phase II enzyme quinone reductase activity in the rat lung. In the present study, the potential genotoxic effects of asphalt fume exposure due to altered lung microsomal enzymes were studied. Rats were exposed to air or asphalt fume generated under road paving conditions at various concentrations and sacrificed the next day. Alveolar macrophages (AM) were obtained by bronchoalveolar lavage and examined for DNA damage using the comet assay. To evaluate the systemic genotoxic effect of asphalt fume, micronuclei formation in bone marrow polychromatic erythrocytes (PCEs) was monitored. Lung S9 from various exposure groups was isolated from tissue homogenates and characterized for metabolic activity in activating 2-aminoanthracene (2-AA) and benzo[a]pyrene (BaP) mutagenicity using the Ames test with Salmonella typhimurium YG1024 and YG1029. This study showed that the paving asphalt fumes significantly induced DNA damage in AM, as revealed by DNA migration in the comet assay, in a dose-dependent manner, whereas the micronuclei formation in bone marrow PCEs was not detected even at a very high exposure level (1733 mg h/m3). The conversion of 2-AA to mutagens in the Ames test required lung S9-mediated metabolic activation in a dose-dependent manner. In comparison to the controls, lung S9 from rats exposed to asphalt fume at a total exposure level of 479+/-33 mg h/m3 did not significantly enhance 2-AA mutagenicity with either S. typhimurium YG1024 or YG1029. At a higher total asphalt fume exposure level (1150+/-63 mg h/m3), S9 significantly increased the mutagenicity of 2-AA as compared to the control. However, S9 from asphalt fume-exposed rats did not significantly activate the mutagenicity of BaP in the Ames test. These results show that asphalt fume exposure, which significantly altered both phases I and II metabolic enzymes in lung microsomes, is genotoxic to AM and enhances the metabolic activation of certain mutagens through altered S9 content.

Lack of genotoxicity of bitumen fumes in transgenic mouse lung

During hot application of bitumen containing materials, e.g. in hot paving or roofing, fumes are emitted that contain polycyclic aromatic compounds. Previous studies with rodents exposed to bitumen and coal-tar fume condensates showed formation of DNA adducts. In order to clarify the genotoxicity of bitumen fumes, we designed a study by using mice carrying a reporter gene for mutagenesis analysis and exposed by nose-only to a constant and reproducible aerosol of bitumen fumes. We analyzed the genotoxic activity of inhaled bitumen fumes generated under those controlled conditions through the induction of mutation and DNA adducts in Big Blue mice. Mice were exposed to bitumen fumes (100 mg/m(3) total particulate matter) 6 h per day during 5 days by nose-only in an inhalation chamber designed in our laboratory. Following a 30-day fixation period, the experiment was terminated and lung DNA was extracted for mutant frequency and adduct determinations. The mutant frequency was determined using the cII and the lacI mutant analysis systems. In, addition, 61 and 54 mutants were sequenced in control and exposed groups, respectively. The study did not show any mutation or adduct induction in the exposed group compared to the control group: cII mutant frequencies were 11.0+/-4.5x10(-5) and 11.0+/-4.8x10(-5) in control and exposed lungs, respectively. Identically, using the lacI mutation detection system, the mutant frequencies were 6.4+/-3.1x10(-5) and 5.8+/-2.0x10(-5). The mutation spectra of both series were quite similar with regard to transition and transversion frequencies. The absence of genotoxicity in the group exposed to 100 mg/m(3) bitumen is discussed with regard to dosage of inhaled polycyclic aromatic compounds and species.