Effect of ethylnitrosourea and methyl methanesulfonate on mutation frequency in Muta Mouse germ cells (seminiferous tubule cells and epididymis spermatozoa)

Transgene mutagenesis in the testicular cells of Muta™Mouse treated transplacentally with N-ethyl-N-nitrosourea at the primordial germ cell stages: Comparisons with the specific-locus test and the intragenic gene-recombination assay

N-Ethyl-N-nitrosourea (ENU) induces recessive mutations (RM) at a high frequency in male mouse primordial germ cells (PGCs)　in a dose-dependent and stage-specific manner when administered during embryonic development as confirmed by a specific locus test (SLT) (Shibuya et al., 1993, 1996 [1,2]). ENU also induces intragenic recombination (IGR) in the p^un allele at E10.5 in PGCs of male mice (Shibuya et al., 2022 [3]). In this study, the induced mutant frequencies (MF) in testicular cells of male Muta™Mousetreated at the same developmental stages of PGCs were determined with a positive selection system (MM/PS). Although the mutant frequencies　of MM/PS were consistently lower than for the SLT/RM, they showed similar stage-specificity and dose-dependency. Expressed as a linear equation, the correlation coefficient on the MF from SLT and MM/PS was extremely high (r² = 0.920).

Comparative transcriptomic analyses to scrutinize the assumption that genotoxic PAHs exert effects via a common mode of action

In this study, the accuracy of the assumption that genotoxic, carcinogenic polycyclic aromatic hydrocarbons (PAHs) act via similar mechanisms of action as benzo(a)pyrene (BaP), the reference PAH used in the human health risk assessment of PAH-containing complex mixtures, was investigated. Adult male Muta™Mouse were gavaged for 28 days with seven individual, genotoxic PAHs. Global gene expression profiles in forestomach, liver, and lung (target tissues of exposure) were determined at 3 days post-exposure. The results are compared with our previously published results from mice exposed to BaP via the same exposure regimen. Although all PAHs showed enhanced ethoxyresorufin-O-deethylase activity, DNA adduct formation, and lacZ mutant frequency in the lungs, the unsupervised cluster analysis of differentially expressed genes revealed that the transcriptional changes are both PAH- and tissue-specific, with lung showing the most response. Further bioinformatics-/pathway-based analysis revealed that all PAHs induce expression of genes associated with carcinogenic processes, including DNA damage response, immune/inflammatory response, or cell signaling processes; however, the type of pathways and the magnitude of change varied for each PAH and were not the same as those observed for BaP. Benchmark dose modeling showed transcriptomic data closely reflected the known tumor incidence for the individual PAHs in each tissue. Collectively, the results suggest that the underlying mechanisms of PAH-induced toxicity leading to tumorigenesis are tissue-specific and not the same for all PAHs; based on the tissue type considered, use of BaP as a reference chemical may overestimate or underestimate the carcinogenic potential of PAHs.

Development of the adverse outcome pathway "alkylation of DNA in male premeiotic germ cells leading to heritable mutations" using the OECD's users' handbook supplement

The Organisation for Economic Cooperation and Development's (OECD) Adverse Outcome Pathway (AOP) programme aims to develop a knowledgebase of all known pathways of toxicity that lead to adverse effects in humans and ecosystems. A Users' Handbook was recently released to provide supplementary guidance on AOP development. This article describes one AOP-alkylation of DNA in male premeiotic germ cells leading to heritable mutations. This outcome is an important regulatory endpoint. The AOP describes the biological plausibility and empirical evidence supporting that compounds capable of alkylating DNA cause germ cell mutations and subsequent mutations in the offspring of exposed males. Alkyl adducts are subject to DNA repair; however, at high doses the repair machinery becomes saturated. Lack of repair leads to replication of alkylated DNA and ensuing mutations in male premeiotic germ cells. Mutations that do not impair spermatogenesis persist and eventually are present in mature sperm. Thus, the mutations are transmitted to the offspring. Although there are some gaps in empirical support and evidence for essentiality of the key events for certain aspects of this AOP, the overall AOP is generally accepted as dogma and applies broadly to any species that produces sperm. The AOP was developed and used in an iterative process to test and refine the Users' Handbook, and is one of the first publicly available AOPs. It is our hope that this AOP will be leveraged to develop other AOPs in this field to advance method development, computational models to predict germ cell effects, and integrated testing strategies.

Subchronic oral exposure to benzo(a)pyrene leads to distinct transcriptomic changes in the lungs that are related to carcinogenesis

We have previously shown that acute oral exposure to the environmental carcinogen benzo(a)pyrene (BaP) elicits comparable levels of DNA adducts, but distinct transcriptomic changes, in mouse lungs and livers, the two main BaP bioactivating organs. Oral BaP exposure is predominantly associated with lung cancer and not hepatic cancer in some animal models, suggesting that gene expression differences may provide insight into the drivers of tissue-specific carcinogenesis. In the present study, we examine pulmonary DNA adduct formation, lacZ mutant frequency, and mRNA profiles in adult male MutaMouse following subchronic (28 day) oral exposure to BaP (0, 25, 50, and 75 mg/kg/day) and sacrificed 3 days postexposure. The results are compared with those obtained from livers of the same mice (previously published). Although there was a 1.8- to 3.3-fold increase in the levels of DNA adducts in lung compared with liver, the lacZ transgene mutant frequency was similar in both tissues. At the transcriptomic level, a transition from activation of the DNA damage response p53 pathway at the low dose to the induction of genes involved in angiogenesis, evasion of apoptosis and growth signals at the high doses was evident only in the lungs. These results suggest that tissue DNA adducts and mutant frequency are sensitive markers of target tissue exposure and mode of action, whereas early changes in gene expression may provide a better indication of the likelihood of carcinogenic transformation in selected tissues. Moreover, the study provides new information on the underlying mechanisms that contribute to tissue-specific responses to BaP.

Single-molecule PCR analysis of germ line mutation induction by anticancer drugs in mice

Understanding and estimating the genetic hazards of exposure to chemical mutagens and anticancer drugs in humans requires the development of efficient systems for monitoring germ line mutation. The suitability of a single-molecule PCR-based approach for monitoring mutation induction at the mouse expanded simple tandem repeat (ESTR) locus Ms6-hm by chemical mutagens and anticancer drugs has been validated. The frequency of ESTR mutation was evaluated in the germ line of male mice exposed to the well-characterized alkylating agent and mutagen, ethylnitrosourea, and four widely used anticancer drugs, bleomycin, cyclophosphamide, mitomycin C, and procarbazine. The dose-response of ethylnitrosourea-induced mutation was found to be very close to that previously established using a pedigree-based approach for ESTR mutation detection. Paternal exposure to the clinically relevant doses of bleomycin (15-30 mg/kg), cyclophosphamide (40-80 mg/kg), and mitomycin C (2.5-5 mg/kg) led to statistically significant, dose-dependent increases in ESTR mutation frequencies in the germ line of treated male mice. Exposure to procarbazine led to a maximal increase in mutation frequency at 50 mg/kg, with a plateau at the higher concentrations. The results of this study show that the single-molecule PCR technique provides a new and efficient experimental system for monitoring the genetic effects of anticancer drugs, capable of detecting increases in mutation rates at clinically relevant doses of exposure. In addition, this approach dramatically reduces the number of mice needed for the measurement of germ line mutation induction.

Mutagenicity testing with transgenic mice. Part II: Comparison with the mouse spot test

The mouse spot test, an in vivo mutation assay, has been used to assess a number of chemicals. It is at present the only in vivo mammalian test system capable of detecting somatic gene mutations according to OECD guidelines (OECD guideline 484). It is however rather insensitive, animal consuming and expensive type of test. More recently several assays using transgenic animals have been developed. From data in the literature, the present study compares the results of in vivo testing of over twenty chemicals using the mouse spot test and compares them with results from the two transgenic mouse models with the best data base available, the lacI model (commercially available as the Big Blue(R) mouse), and the lacZ model (commercially available as the Mutatrade mark Mouse). There was agreement in the results from the majority of substances. No differences were found in the predictability of the transgenic animal assays and the mouse spot test for carcinogenicity. However, from the limited data available, it seems that the transgenic mouse assay has several advantages over the mouse spot test and may be a suitable test system replacing the mouse spot test for detection of gene but not chromosome mutations in vivo.

Mutagenicity testing with transgenic mice. Part I: Comparison with the mouse bone marrow micronucleus test

As part of a larger literature study on transgenic animals in mutagenicity testing, test results from the transgenic mutagenicity assays (lacI model; commercially available as the Big Blue(R) mouse, and the lacZ model; commercially available as the Mutatrade markMouse), were compared with the results on the same substances in the more traditional mouse bone marrow micronucleus test. 39 substances were found which had been tested in the micronucleus assay and in the above transgenic mouse systems. Although, the transgenic animal mutation assay is not directly comparable with the micronucleus test, because different genetic endpoints are examined: chromosome aberration versus gene mutation, the results for the majority of substances were in agreement. Both test systems, the transgenic mouse assay and the mouse bone marrow micronucleus test, have advantages and they complement each other. However, the transgenic animal assay has some distinct advantages over the micronucleus test: it is not restricted to one target organ and detects systemic as well as local mutagenic effects.

Germline mutation induction at mouse repeat DNA loci by chemical mutagens

Mutation rates at two expanded simple tandem repeat (ESTR) loci were studied in the germline of male mice exposed to two monofunctional alkylating agents, ethylnitrosourea (ENU) and isopropyl methanesulfonate (iPMS), and a topoisomerase II inhibitor, etoposide. Pre-meiotic exposure to the alkylating agents resulted in a highly significant increase in ESTR mutation rate, but did not alter post-meiotically exposed cells. Pre-meiotic mutation induction by ENU and iPMS was linear within the interval of doses from 12.5 to 25mg/kg and reached a plateau at higher concentrations. Paternal exposure to etoposide resulted in ESTR mutation induction at meiotic stages but did not affect post- or pre-meiotic cells. The pattern of ESTR mutation induction after pre-meiotic and meiotic exposure to chemical mutagens was similar to that previously obtained by various traditional approaches for monitoring germline mutation in mice. The results of this study show that ESTR loci provide a new efficient experimental system for monitoring the genetic effects of chemical mutagens, capable of detecting increases in mutation rates at low doses of exposure.

Kinetics of induction of DNA adducts, cell proliferation and gene mutations in the liver of MutaMice treated with 5,9-dimethyldibenzo[c,g]carbazole

5,9-Dimethyldibenzo[c,g]carbazole (DMDBC) is a synthetic derivative of the environmental pollutant 7H-dibenzo[c,g]carbazole. DMDBC is a potent genotoxic carcinogen specific for mouse liver. Using the MutaMouse lacZ transgenic mouse model and a positive selection assay, we measured lacZ mutant frequency (MF) in the liver 28 days after a single s.c. administration of DMDBC at 3, 10, 30, 90 or 180 mg/kg. MF remained low at 3 and 10 mg/kg, but increased markedly from 30 mg/kg onwards. To investigate the reason for this non-linear response, we examined mechanisms potentially involved in mutation induction in the liver. Genotoxic effects such as DNA adduct formation were detected in 32P-post-labelling studies. Liver sections were examined for microscopic changes and cell proliferation. These parameters, and MF, were studied 2, 4, 7, 14, 21 and 28 days after a single s.c. administration of 10 or 90 mg/kg DMDBC. At 10 mg/kg, a dose found to double the MF on day 28, DNA adducts reached a level of 200-600 adducts per 10(8) nucleotides from day 4 to day 28. No changes in histology or cell proliferation were detected at this low dose. At 90 mg/kg, MF increased gradually from day 7 to day 28 (maximum 44-fold). The DNA adduct level ranged from 400 to 4500 adducts per 10(8) nucleotides on day 2, then stabilized at approximately 400 adducts per 10(8) nucleotides on day 4. An early cytotoxic effect was detected microscopically in centrilobular hepatocytes, and was followed by liver cell proliferation. These data suggest that the marked increase in MF in MutaMouse liver after treatment in vivo with DMDBC at 90 mg/kg may be explained by the induction of replicative DNA synthesis due to a cytotoxic effect, allowing the fixation of persistent DNA adducts into mutations.

Comparative mutagenicity of 7H-dibenzo[c,g]carbazole and two derivatives in MutaMouse liver and skin

7H-Dibenzo[c,g]carbazole (DBC) is an environmental pollutant that produces DNA adducts and tumors in mouse liver and skin following subcutaneous injection and topical application. The two synthetic derivatives 5,9-dimethyl-DBC (DMDBC) and N7-methyl-DBC (NMDBC) induce tissue-specific lesions. DNA adducts and tumors are observed only in liver following exposure to DMDBC and only in skin following exposure to NMDBC. We used the positive selection MutaMouse model to measure the induction of mutations in the two target organs, 28 days after a single subcutaneous injection or topical application of DBC, DMDBC and NMDBC. In liver, DBC and DMDBC induced 30- to 50-fold increases in mutant frequency (MF), while NMDBC had only a weak effect, regardless of the route of administration. After topical application, DBC and NMDBC produced 3.4- to 7.9-fold increases in MF in skin, while DMDBC had a weak effect. After subcutaneous injection, the three compounds had no or weak effect in skin. This study shows gene mutations arise in the respective target organs in which primary DNA damage and tumors are observed. These results illustrate the relevance of the MutaMouse model for testing organ-specific mutagens.