Dinitropyrenes induce gene mutations in multiple organs of the lambda/lacZ transgenic mouse (Muta Mouse)

Quantitative relationships between lacZ mutant frequency and DNA adduct frequency in Muta™Mouse tissues and cultured cells exposed to 3-nitrobenzanthrone

The frequency of stable DNA adducts in a target tissue can be used to assess biologically effective dose; however, the utility of the metric in a risk assessment context depends on the likelihood that the DNA damage will be manifested as mutation. Previously, we employed the Muta™Mouse system to examine the induction of lacZ mutants and DNA adducts following exposure to the well-studied mutagenic carcinogen 3-nitrobenzanthrone (3-NBA). In this follow-up work, we examined the empirical relationships between total adduct frequency and mutant frequency (MF) in tissues and cultured cells following acute 3-NBA exposure. The results show a significant induction of DNA damage and lacZ mutants in liver, colon and bone marrow, as well as FE1 pulmonary epithelial cells. In contrast, lung and small intestine samples had low, but significantly elevated adduct levels, with no significant increases in lacZ MF. Additional analyses showed a significant relationship between the mutagenic efficiency of total adducts, measured as the slope of the relationships between MF and total adduct frequency, and tissue-specific mitotic index (MI). The lack of mutation response in lung, in contrast to the high in vitro MF in FE-1 lung cells, is likely related to the 100-fold difference in MI. The lack of small intestine mutagenic response may be related to limited metabolic capacity, differences in DNA repair, and /or chemically induced apoptosis that has been observed for other potent mutagens. The results indicate that interpretation of adduct frequency values in a risk assessment context can be improved by considering the MI of the target tissue; however, more generalised interpretation is hampered by tissue-specific variations in metabolic capacity and damage processing. The work provides a proof of principle regarding the use of the Muta™Mouse system to critically examine the health risks associated with tissue-specific adduct loads.

Physical-chemical and microbiological characterization, and mutagenic activity of airborne PM sampled in a biomass-fueled electrical production facility

Biomass combustion is used in heating and electric power generation in many areas of the world. Airborne particulate matter (PM) is released when biomass is brought to a facility, stored, and combusted. Occupational exposure to airborne PM within biomass-fueled facilities may lead to health problems. In March and August of 2006, airborne PM was collected from a biomass-fueled facility located in Denmark. In addition, source-specific PM was generated from straw and wood pellets using a rotating drum. The PM was analyzed for polycyclic aromatic hydrocarbons (PAHs), metals, microbial components, mutagenic activity, and ability to generate highly reactive oxygen species (hROS) in cell-free aqueous suspensions. PM collected from the boiler room and the biomass storage hall had higher levels of mutagenic activity, PAHs and metals, and a higher hROS generating potential than the source specific PM. The mutagenic activity was generally more potent without S9 activation, and on the metabolically enhanced strain YG1041, relative to TA98. Significant correlations were found between mutagenicity on YG1041 (without S9) and PAH concentration and mutagenicity on YG1041 (with S9) and hROS generating ability. PM collected in March was more toxic than PM collected in August. Overall, airborne PM collected from the facility, especially that from the boiler room, were more toxic than PM generated from straw and wood chips. The results suggest that exposure to combustion PM in a biomass-fueled facility, which likely includes PM from biomass combustion as well as internal combustion vehicles, may contribute to an elevated risk of adverse health effects.

Tissue-specific metabolic activation and mutagenicity of 3-nitrobenzanthrone in MutaMouse

3-Nitrobenzanthrone (3-NBA) is a mutagen and suspected human carcinogen detected in diesel exhaust, airborne particulate matter, and urban soil. We investigated the tissue specific mutagenicity of 3-NBA at the lacZ locus of transgenic MutaMouse following acute single dose or 28-day repeated-dose oral administration. In the acute high dose (50 mg/kg) exposure, increased lacZ mutant frequency was observed in bone marrow and colonic epithelium, but not in liver and bladder. In the repeated-dose study, a dose-dependent increase in lacZ mutant frequency was observed in bone marrow and liver (2- and 4-fold increase above control), but not in lung or intestinal epithelium. In addition, a concentration-dependent increase in mutant frequency (8.5-fold above control) was observed for MutaMouse FE1 lung epithelial cells exposed in vitro. 1-Nitropyrene reductase, 3-NBA reductase, and acetyltransferase activities were measured in a variety of MutaMouse specimens in an effort to link metabolic activation and mutagenicity. High 3-NBA nitroreductase activities were observed in lung, liver, colon and bladder, and detectable N-acetyltransferase activities were found in all tissues except bone marrow. The relatively high 3-NBA nitroreductase activity in MutaMouse tissues, as compared with those in Salmonella TA98 and TA100, suggests that 3-NBA is readily reduced and activated in vivo. High 3-NBA nitroreductase levels in liver and colon are consistent with the elevated lacZ mutant frequency values, and previously noted inductions of hepatic DNA adducts. Despite an absence of induced lacZ mutations, the highest 3-NBA reductase activity was detected in lung. Further studies are warranted, especially following inhalation or intratracheal exposures.

Genotoxicity of 3-nitrobenzanthrone and 3-aminobenzanthrone in MutaMouse and lung epithelial cells derived from MutaMouse

FE1 lung epithelial cells derived from MutaMouse are a new model system to provide in vitro mutagenicity data with the potential to predict the outcome of an in vivo MutaMouse test. 3-Nitrobenzanthrone (3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust and urban air pollution. We investigated the mutagenicity and DNA binding of 3-NBA and its main metabolite 3-aminobenzanthrone (3-ABA) in vitro and in vivo in the MutaMouse assay. Mice were treated with 3-NBA or 3-ABA (0, 2 or 5 mg/kg body weight/day) by gavage for 28 days and 28 days later lacZ mutant frequency (MF) was determined in liver, lung and bone marrow. For both compounds, dose-related increases in MF were seen in liver and bone marrow, but not in lung; mutagenic activity was approximately 2-fold lower for 3-ABA than for 3-NBA. With 3-NBA, highest DNA adduct levels (measured by (32)P-post-labelling) were found in liver (approximately 230 adducts per 10(8) nucleotides) with levels 20- to 40-fold lower in bone marrow and lung. With 3-ABA, DNA adduct levels were again highest in the liver, but approximately 4-fold lower than for 3-NBA. FE1 cells were exposed to up to 10 microg/ml 3-NBA or 3-ABA for 6 h with or without exogenous activation (S9) and harvested after 3 days. For 3-NBA, there was a dose-related increase in MF both with and without S9 mix, which was >10 times higher than observed in vivo. At the highest concentration of 3-ABA (10 microg/ml), we found only around a 2-fold increase in MF relative to controls. DNA adduct formation in FE1 cells was dose-dependent for both compounds, but 10- to 20-fold higher for 3-NBA compared to 3-ABA. Collectively, our data indicate that MutaMouse FE1 cells are well suited for cost-effective testing of suspected mutagens with different metabolic activation pathways as a guide for subsequent in vivo MutaMouse testing.

In vivo mutagenesis in the lungs of gpt-delta transgenic mice treated intratracheally with 1,6-dinitropyrene

1,6-Dinitropyrene (1,6-DNP) is a ubiquitous airborne pollutant found in diesel exhaust. In this study, mutagenesis was examined in the lungs of gpt-delta transgenic mice after intratracheal instillation of 0-0.1 mg 1,6-DNP. In addition, the 1,6-DNP-induced gpt mutation spectrum was compared with that of control mice. A single intratracheal injection of 0-0.05 mg 1,6-DNP resulted in significant dose-dependent increases in mutant frequency; the induced mutant frequency declined at the 0.1 mg dose. The average lung mutant frequencies at doses of 0.025, 0.05, and 0.1 mg 1,6-DNP were 2.9-, 4.1-, and 1.9-times higher than for control mice ((0.50+/-0.16)x10(-5)). The major mutations induced by 1,6-DNP included G:C-->A:T transitions, G:C-->T:A transversions, and 1-base deletions. Among the G:C-->A:T transitions isolated from 1,6-DNP-treated mice, five (at nucleotide positions 64, 110, 115, 116, and 418) were observed in four or more animals. These positions therefore are potential hotspots for 1,6-DNP mutation. The predominant frameshift mutations following 1,6-DNP treatment included single base pair deletions at G:C (9/13=69%). The results of this study indicate that 1,6-DNP is mutagenic for the lungs of mice.

iMARS--mutation analysis reporting software: an analysis of spontaneous cII mutation spectra

The sensitivity of any mutational assay is determined by the level at which spontaneous mutations occur in the corresponding untreated controls. Establishing the type and frequency at which mutations occur naturally within a test system is essential if one is to draw scientifically sound conclusions regarding chemically induced mutations. Currently, mutation-spectra analysis is laborious and time-consuming. Thus, we have developed iMARS, a comprehensive mutation-spectrum analysis package that utilises routinely used methodologies and visualisation tools. To demonstrate the use and capabilities of iMARS, we have analysed the distribution, types and sequence context of spontaneous base substitutions derived from the cII gene mutation assay in transgenic animals. Analysis of spontaneous mutation spectra revealed variation both within and between the transgenic rodent test systems Big Blue Mouse, MutaMouse and Big Blue Rat. The most common spontaneous base substitutions were G:C-->A:T transitions and G:C-->T:A transversions. All Big Blue Mouse spectra were significantly different from each other by distribution and nearly all by mutation type, whereas the converse was true for the other test systems. Twenty-eight mutation hotspots were observed across all spectra generally occurring in CG, GA/TC, GG and GC dinucleotides. A mutation hotspot at nucleotide 212 occurred at a higher frequency in MutaMouse and Big Blue Rat. In addition, CG dinucleotides were the most mutable in all spectra except two Big Blue Mouse spectra. Thus, spontaneous base-substitution spectra showed more variation in distribution, type and sequence context in Big Blue Mouse relative to spectra derived from MutaMouse and Big Blue Rat. The results of our analysis provide a baseline reference for mutation studies utilising the cII gene in transgenic rodent models. The potential differences in spontaneous base-substitution spectra should be considered when making comparisons between these test systems. The ease at which iMARS has allowed us to carry out an exhaustive investigation to assess mutation distribution, mutation type, strand bias, target sequences and motifs, as well as predict mutation hotspots provides us with a valuable tool in helping to distinguish true chemically induced hotspots from background mutations and gives a true reflection of mutation frequency.

3-Nitrobenzanthrone, a potential human cancer hazard in diesel exhaust and urban air pollution: a review of the evidence

Epidemiological studies have shown that exposure to diesel exhaust and urban air pollution is associated with an increased risk of lung cancer. 3-Nitrobenzanthrone [3-nitro-7H-benz[de]anthracen-7-one (3-NBA)] is an extremely potent mutagen and suspected human carcinogen identified in diesel exhaust and ambient air particulate matter. The main metabolite of 3-NBA, 3-aminobenzanthrone (3-ABA), was found in the urine of salt mine workers occupationally exposed to diesel emissions, indicating that human exposure to 3-NBA due to diesel emissions can be significant and is detectable. There is clear evidence that 3-NBA is a genotoxic mutagen forming DNA adducts after metabolic activation through simple reduction of the nitro group. Several human enzymes have been shown to activate 3-NBA and its metabolites in vitro and in cells to form electrophilic arylnitrenium and rearranged carbenium ions, leading to the formation of purine adducts at the C8 and N2 position of guanine and at the C8 and N6 position of adenine. The predominant DNA adducts in vivo, 2-(2'-deoxyguanosin-N2-yl)-3-aminobenzanthrone and N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone are also the most persistent adducts in target tissue in rodents, and are most probably responsible for the induction of GC-->TA transversion mutations observed in vivo. It is concluded that these adducts not only represent premutagenic lesions in DNA but are of primary importance for the initiation of the carcinogenic process and subsequent tumour formation in target tissue. Indeed, 3-NBA is carcinogenic in rats after intratracheal instillation, inducing mainly squamous cell carcinoma in lung. The intention of this article is to provide a critical review on the potential genotoxic effects of 3-NBA on human health. However, in general, there is a need for more mechanistic studies that relate 3-NBA to all processes that are considered to orchestrate tumour development and of studies on the ability of particles to promote 3-NBA genotoxicity. Because of its widespread environmental presence, 3-NBA may represent not only an occupational health hazard but also a hazard for larger sections of the general population. For an accurate risk assessment more epidemiological studies on 3-NBA-exposed individuals and a broader monitoring of environmental levels of 3-NBA are required.

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.

DNA adducts and mutagenic specificity of the ubiquitous environmental pollutant 3-nitrobenzanthrone in Muta Mouse

3-nitrobenzanthrone (3-NBA) is an extremely potent mutagen in the Salmonella reversion assay and a suspected human carcinogen identified in diesel exhaust and in ambient airborne particulate matter. To evaluate the in vivo mutagenicity of 3-NBA, we analyzed the mutant frequency (MF) in the cII gene of various organs (lung, liver, kidney, bladder, colon, spleen, and testis) in lambda/lacZ transgenic mice (Muta Mouse) after intraperitoneal treatment with 3-NBA (25 mg/kg body weight injected once a week for 4 weeks). Increases in MF were found in colon, liver, and bladder, with 7.0-, 4.8-, and 4.1-fold increases above the control value, respectively, whereas no increase in MF was found in lung, kidney, spleen, and testis. Simultaneously, induction of micronuclei in peripheral blood reticulocytes was observed. The sequence alterations in the cII gene recovered from 41 liver mutants from 3-NBA-treated mice were compared with 32 spontaneous mutants from untreated mice. Base substitution mutations predominated for both the 3-NBA-treated (80%) and the untreated (81%) groups. However, the proportion of G:C-->T:A transversions in the mutants from 3-NBA-treated mice was higher (49% vs. 6%) and the proportion of G:C-->A:T transitions was lower than those from untreated mice (10% vs. 66%). The increase in MF in the liver was associated with strong DNA binding by 3-NBA, whereas in lung, in which there was no increase in MF, a low level of DNA binding was observed (268.0-282.7 vs. 8.8-15.9 adducts per 10(8) nucleotides). DNA adduct patterns with multiple adduct spots, qualitatively similar to those formed in vitro after activation of 3-NBA with nitroreductases and in vivo in rats, were observed in all tissues examined. Using high-pressure liquid cochromatographic analysis, we confirmed that all major 3-NBA-DNA adducts produced in vivo in mice are derived from reductive metabolites bound to purine bases (70-80% with deoxyguanosine and 20-30% with deoxyadenosine in liver). These results suggest that G:C-->T:A transversions induced by 3-NBA are caused by misreplication of adducted guanine residues through incorporation of adenine opposite the adduct (A-rule).

In vivo transgenic mutation assays

Transgenic rodent gene-mutation models provide relatively quick and statistically reliable assays for gene mutations in the DNA from any tissue. This report summarizes those issues that have been agreed upon at a previous IWGT meeting [Environ. Mol. Mutagen. 35 (2000) 253], and discusses in depth those issues for which no consensus was reached before. It was previously agreed that for regulatory applications, assays should be based upon neutral genes, be generally available in several laboratories, and be readily transferable. For phage-based assays, five to ten animals per group should be analyzed, assuming a spontaneous mutant frequency (MF) of approximately 3x10(-5) mutants/locus and 125,000-300,000 plaque or colony forming units (pfu or cfu) per tissue per animal. A full set of data should be generated for a vehicle control and two dose groups. Concurrent positive control animals are only necessary during validation, but positive control DNA must be included in each plating. Tissues should be processed and analyzed in a blocked design, where samples from negative control, positive control and each treatment group are processed together. The total number of pfus or cfus and the MF for each tissue and animal are reported. Statistical tests should consider the animal as the experimental unit. Nonparametric statistical tests are recommended. A positive result is a statistically significant dose-response and/or statistically significant increase in any dose group compared to concurrent negative controls using an appropriate statistical model. A negative result is a statistically non-significant change, with all mean MFs within two standard deviations of the control. During the current workshop, a general protocol was agreed in which animals are treated daily for 28 consecutive days and tissues sampled 3 days after the final treatment. This recommendation could be modified by reducing or increasing the number of treatments or the length of the treatment period, when scientifically justified. Normally male animals alone are sufficient and normally at least one rapidly proliferating and one slowly proliferating tissue should be sampled. Although, as agreed previously, sequencing data are not normally required, they might provide useful additional information in specific circumstances, mainly to identify and correct for clonal expansion and in some cases to determine a mechanism associated with a positive response.

Regional mutagenicity of heterocyclic amines in the intestine: mutation analysis of the cII gene in lambda/lacZ transgenic mice

Transgenic mouse assays have revealed that the mouse intestine, despite its resistance to carcinogenesis, is sensitive to the mutagenicity of some heterocyclic amines (HCAs). Little is known, however, about the level and localization of that sensitivity. We assessed the mutagenicity of four orally administered (20 mg/kg per day for 5 days) HCAs-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) hydrochloride, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) acetate-in the intestine of male MutaMice. Two weeks after the last administration, we isolated epithelium from the small intestine, cecum, and colon and analyzed lacZ and cII transgene mutations. PhIP increased the lacZ mutant frequency (MF) in all the samples, and in the small intestine, cII and lacZ MFs were comparable. In the cII gene, G:C to T:A and G:C to C:G transversions were characteristic PhIP-induced mutations (which has also been reported for the rat colon, where PhIP is carcinogenic). In the small intestine, PhIP increased the cII MF to four-fold that of the control, but IQ, MeIQ, and Trp-P-2 did not have a significant mutagenic effect. In the cecum, cII MFs induced by IQ and MeIQ were 1.9 and 2.7 times those in the control, respectively. The MF induced by MeIQ in the colon was 3.1 times the control value. Mutagenic potency was in the order PhIP>MeIQ>IQ; Trp-P-2 did not significantly increase the MF in any tissue. The cecum was the most susceptible organ to HCA mutagenicity.