Micronucleus test in rodent tissues other than liver or erythrocytes: Report of the IWGT working group

Micronucleus test using formalin-fixed rat glandular stomach and colon

BACKGROUND

Genotoxicity in tissues other than hematopoietic tissues, such as the liver and gastrointestinal (GI) tract, is an important focus in the risk assessment of chemicals in humans. We previously developed a rat micronucleus test for the GI tract, which is the first contact tissue where chemicals are introduced into the body through oral exposure. Target cells were obtained from fresh tissue samples by ethylenediaminetetraacetic acid disodium salt (EDTA) treatment. As an improvement to this method, we have used formalin-fixed tissues instead of fresh tissues; this approach can be used for tissues that are sampled from other toxicological tests and that are archived for several years. This new method can be used for examining micronucleus induction retrospectively when needed. In the present study, we compared the performance of the EDTA method and the new method with formalin-fixed tissues (formalin-fixation method).

RESULTS

Histological examination showed that both the EDTA and formalin-fixation methods could be used for collecting cells located in or above the proliferative zone of the GI tract tissues of rats. In addition, the collected cells were similar in shape. We conducted micronucleus tests with rat GI tract tissues by the two methods using model chemicals, which were used as positive control chemicals (a combination of diethylnitrosamine, 1,2-dimethylhydrazine dihydrochloride, and potassium bromate). The two methods showed similar results. We additionally evaluated the aging effect of tissues stored in formalin fixative. The results showed that 1 year of storage did not affect the frequency of micronucleated cells.

CONCLUSION

The equivalence of the EDTA and formalin-fixation methods was confirmed, and micronucleus analysis was possible up to at least 1 year after formalin fixation of the GI tract, indicating that the formalin-fixation method is valuable for the rat GI tract micronucleus test.

The effect of aging on the repeated-dose liver micronucleus assay using diethylnitrosamine

Background

The repeated-dose liver micronucleus (RDLMN) assay has been well-developed and applied because of its simplicity and the ease of integration into general toxicity studies which is the preferred method from the 3R’s point of view. In this assay, we observed micronucleated hepatocytes which accumulated during a rather long-term dosing period. When considering integration into general toxicity studies, the effects of age of the animals used in the micronucleus assay becomes a major issue. The effect of age on the micronucleus induction rate has been reported in bone marrow micronucleus assays, and it is considered that the decrease in cell proliferation rate due to aging is the cause of the decrease in sensitivity. A decrease in sensitivity due to aging was also reported in a liver micronucleus assay using clofibrate and the cause is considered to be a decrease in hepatocyte proliferation activity due to aging. However, no actual decrease in hepatocyte proliferation rate due to aging has been reported. In addition, there are no reports, so far, on whether similar effects of aging appear when other substances were administered. To investigate the effects of aging in the RDLMN assay, this study focused on the effects of 14-day repeated administration of DEN, a well-known genotoxic hepatocarcinogen with the hepatocyte toxicity which should cause an elevation of cell proliferation rate as a reflective regeneration.

Results

The liver micronuclei induced by DEN were equivalent between the two age groups (i.e., six and eight weeks of age at the start of dosing). In the histopathological examination for the liver, single cell necrosis, karyomegaly, and increased mitosis were observed in the hepatocytes, and the frequency and severity were increased dose-dependently. Ki-67 immunohistochemical analysis which can detect all cells in the cell cycle other than those in the G0 phase revealed dose-dependent increase of cell proliferation activity, and the difference between ages was not observed.

Conclusion

The effect of aging on the RDLMN assay could not be recognized when DEN was administered for 14 days in rats. Meanwhile, it was supported by the histopathological examination and Ki-67 immunohistochemical analysis that such an effect of aging was masked by the compensatory hepatocyte proliferation which was induced by the hepatocyte toxicity of DEN.

Evaluation of a 4-day repeated-dose micronucleus test in rat glandular stomach and colon using aneugens and non-genotoxic non-carcinogens

Background

We previously developed a rodent gastrointestinal (GI) tract micronucleus (MN) test using the glandular stomach and/or colon, and evaluated this test method using several genotoxic carcinogens (clastogens) and genotoxic non-carcinogens; we demonstrated that this test method could detect genotoxic stomach and/or colon carcinogens with target organ specificity. In the present study, we further evaluated the sensitivity and specificity of the MN test for the rat glandular stomach and colon using three aneugens (colchicine, vinblastine sulfate, and docetaxel hydrate) and two non-genotoxic non-carcinogens (sodium chloride and sucrose).

Results

Male Crl:CD (SD) rats were administered test compounds through clinical administration route (orally or intravenously) for four consecutive days and then examined for the micronucleated cell frequencies in the glandular stomach and colon. We observed that all three aneugens significantly and dose-dependently increased the micronucleated cell frequencies in the stomach and colon. In contrast, neither of the two non-genotoxic non-carcinogens increased the micronucleated cell frequency in these tissues. Notably, an increase in cell proliferation was observed in the glandular stomach of rats administered a stomach toxicant, sodium chloride, but this increase did not affect the induction of micronuclei in the gastric cells.

Conclusions

In the present study, it was demonstrated that the glandular stomach and colon MN tests could detect aneugens as positive and could adequately evaluate non-genotoxic non-carcinogens as negative, including a chemical that enhances cell proliferation. These results provide important evidence supporting good performance of the rat glandular stomach and colon MN tests with a 4-day treatment regimen.

The effect of aging on the repeated-dose liver micronucleus assay

BACKGROUND

The liver micronucleus (MN) assay is an effective and important in vivo test for detecting genotoxic compounds. In particular, the repeated-dose liver MN (RDLMN) assay which greatly facilitates incorporation of the liver MN assay into the general toxicity study has been developed. Usefulness of the RDLMN assay was appraised highly in the 7th International Workshops on Genotoxicity Testing (2017 in Tokyo) in that sufficient numbers and types of chemicals were studied and easy integration into the general toxicity study is preferred from the 3R's point of view. However, it was pointed out that it is necessary to evaluate the effect of age at the start of 4-week repeated administration, since there are limited data, where only those of rats of 6 week of age at the start of administration are available. In this study, we conducted the 4-week RDLMN assay using rats of 6 and 8 weeks of age (at the start of administration) to investigate the effect of age on the liver MN inducibility. Clofibrate, a weak inducer of liver MN, was used in this study to detect the slight difference in the liver MN induction.

RESULTS

The liver MN induced by clofibrate was detected in both rats of 6 and 8 weeks of age at the start of administration. However, the liver MN induction was lower in rats of 8 weeks of age compared to rats of 6 weeks of age at the start of administration.

CONCLUSION

These results suggest that the liver MN inducibility decreases with age. Therefore, we recommend the use of rats of 6 weeks of age at start of administration to reliably detect the liver MN induction in the RDLMN assay.

Guidance on aneugenicity assessment

The EFSA Scientific Committee was asked to provide guidance on the most appropriate in vivo tests to follow up on positive in vitro results for aneugenicity, and on the approach to risk assessment for substances that are aneugenic but not clastogenic nor causing gene mutations. The Scientific Committee confirmed that the preferred approach is to perform an in vivo mammalian erythrocyte micronucleus test with a relevant route of administration. If this is positive, it demonstrates that the substance is aneugenic in vivo. A negative result with evidence that the bone marrow is exposed to the test substance supports a conclusion that aneugenic activity is not expressed in vivo. If there is no evidence of exposure to the bone marrow, a negative result is viewed as inconclusive and further studies are required. The liver micronucleus assay, even though not yet fully validated, can provide supporting information for substances that are aneugenic following metabolic activation. The gastrointestinal micronucleus test, conversely, to be further developed, may help to assess aneugenic potential at the initial site of contact for substances that are aneugenic in vitro without metabolic activation. Based on the evidence in relation to mechanisms of aneugenicity, the Scientific Committee concluded that, in principle, health-based guidance values can be established for substances that are aneugenic but not clastogenic nor causing gene mutations, provided that a comprehensive toxicological database is available. For situations in which the toxicological database is not sufficient to establish health-based guidance values, some approaches to risk assessment are proposed. The Scientific Committee recommends further development of the gastrointestinal micronucleus test, and research to improve the understanding of aneugenicity to support risk assessment.

Chromosome aberrations induced by the non-mutagenic carcinogen acetamide involve in rat hepatocarcinogenesis through micronucleus formation in hepatocytes

Chromosome aberrations (CAs), i.e. changes in chromosome number or structure, are known to cause chromosome rearrangements and subsequently tumorigenesis. However, the involvement of CAs in chemical-induced carcinogenesis is unclear. In the current study, we aimed to clarify the possible involvement of CAs in chemical carcinogenesis using a rat model with the non-mutagenic hepatocarcinogen acetamide. In an in vivo micronucleus (MN) test, acetamide was revealed to induce CAs specifically in rat liver at carcinogenic doses. Acetamide also induced centromere-positive large MN (LMN) in hepatocytes. Immunohistochemical and electron microscopic analyses of the LMN, which can be histopathologically detected as basophilic cytoplasmic inclusion, revealed abnormal expression of nuclear envelope proteins, increased heterochromatinization, and massive DNA damage. These molecular pathological features in LMN progressed with acetamide exposure in a time-dependent manner, implying that LMN formation can lead to chromosome rearrangements. Overall, these data suggested that CAs induced by acetamide play a pivotal role in acetamide-induced hepatocarcinogenesis in rats and that CAs can cause chemical carcinogenesis in animals via MN formation.

Target-specific micronucleus induction by colon carcinogens: 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 1,2-dimethylhydrazine

Genotoxicity occurring at the target organs of carcinogenesis is important for understanding the mechanisms of chemical carcinogenicity and also for setting of threshold estimation. In vivo gene mutations have been evaluated by transgenic animal models in which any organ can be targeted; however, the methodologies that have been applied to assess chromosomal aberrations including micronucleus induction, are organ restricted, (often to bone marrow hematopoietic cells, as a common example). For food and food-related chemicals, the digestive tract is the important target organ as it is the organ of first contact. In the present study, we used 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 1,2-dimethylhydrazine (DMH) as model chemicals of carcinogens primarily targeting the colon. We evaluated the applicability of colon cells and hepatocytes, together with bone marrow cells, in the micronucleus assay. Both model chemicals induced micronuclei in the colon, which is the target organ of these carcinogens, after short- and long-term treatment(s). The results demonstrate the target specificity of micronucleus induction and the assay using organs other than bone marrow will play an important role in understanding the mechanism of carcinogenicity and predicting new carcinogenic agents.

In vivo genotoxicity testing strategies: Report from the 7th International workshop on genotoxicity testing (IWGT)

The working group reached complete or majority agreement on many issues. Results from TGR and in vivo comet assays for 91 chemicals showed they have similar ability to detect in vivo genotoxicity per se with bacterial mutagens and Ames-positive carcinogens. TGR and comet assay results were not significantly different when compared with IARC Group 1, 2 A, and unclassified carcinogens. There were significantly more comet assay positive responses for Group 2B chemicals, and for IARC classified and unclassified carcinogens combined, which may be expected since mutation is a sub-set of genotoxicity. A liver comet assay combined with the bone marrow/blood micronucleus (MNviv) test would detect in vivo genotoxins that do not exhibit tissue-specific or site-of-contact effects, and is appropriate for routine in vivo genotoxicity testing. Generally for orally administered substances, a comet assay at only one site-of-contact GI tract tissue (stomach or duodenum/jejunum) is required. In MNviv tests, evidence of target tissue exposure can be obtained in a number of different ways, as recommended by ICH S2(R1) and EFSA (Hardy et al., 2017). Except for special cases the i.p. route is inappropriate for in vivo testing; for risk evaluations more weight should be given to data from a physiologically relevant administration route. The liver MN test is sufficiently validated for the development of an OECD guideline. However, the impact of dosing animals >6 weeks of age needs to be evaluated. The GI tract MN test shows promise but needs more validation for an OECD guideline. The Pig-a assay detects systemically available mutagens and is a valuable follow-up to in vitro positive results. A new freeze-thaw protocol provides more flexibility. Mutant reticulocyte and erythrocyte frequencies should both be determined. Preliminary data are available for the Pig-a assay in male rat germ cells which require validation including germ cell DNA mutation origin.

Integration of micronucleus tests with a gene mutation assay in F344 gpt delta transgenic rats using benzo[a]pyrene

Reduction of the number of animals used in in vivo genotoxicity tests is encouraged. For this purpose, we conducted integrated toxicity tests combining gene mutation assays with multiple-organ micronucleus (MN) tests (peripheral blood, bone marrow, liver, and colon) in F344 gpt delta transgenic (Tg) rats. Seven-week-old male F344 gpt delta rats were orally administered 62.5 or 125 mg/kg/day benzo[a]pyrene (B[a]P) for 28 days. One day after the final day of treatment (day 29) and three days after the final treatment (day 31), bone marrow, liver, and colon samples were collected, and mutation assays and MN tests were performed. The gpt mutant frequency (MF) significantly increased in bone marrow, liver and colon but MN induction was only significant in bone marrow but not in liver and colon. Similarly MN induction was only observed in bone marrow in non-Tg F344 rats. In peripheral blood obtained on day 4, 15, 29, 31, a time-dependent increase was observed in reticulocyte MN frequency during the treatment. Thus, our integrated method successfully detected both gene mutations and MN induction caused by B[a]P. In addition, no significant differences were observed between sampling times (day 29 versus 31), suggesting that sampling on day 29 is also valid to evaluate gene mutations. On the other hand, MN results in bone marrow and peripheral blood were different depending on the sampling day. An appropriate sampling day should be designated according to which assays are integrated. We confirmed that integration of the MN test with a gene mutation assay using F344 gpt delta Tg rats is useful to evaluate different endpoints related to genotoxicity using the same animals and to reduce animal use.

Clarification of some aspects related to genotoxicity assessment

The European Commission requested EFSA to provide advice on the following: (1) the suitability of the unscheduled DNA synthesis (UDS) in vivo assay to follow‐up positive results in in vitro gene mutation tests; (2) the adequacy to demonstrate target tissue exposure in in vivo studies, particularly in the mammalian erythrocyte micronucleus test; (3) the use of data in a weight‐of‐evidence approach to conclude on the genotoxic potential of substances and the consequent setting of health‐based guidance values. The Scientific Committee concluded that the first question should be addressed in both a retrospective and a prospective way: for future assessments, it is recommended no longer performing the UDS test. For re‐assessments, if the outcome of the UDS is negative, the reliability and significance of results should be carefully evaluated in a weight‐of‐evidence approach, before deciding whether more sensitive tests such as transgenic assay or in vivo comet assay would be needed to complete the assessment. Regarding the second question, the Scientific Committee concluded that it should be addressed in lines of evidence of bone marrow exposure: toxicity to the bone marrow in itself provides sufficient evidence to allow concluding on the validity of a negative outcome of a study. All other lines of evidence of target tissue exposure should be assessed within a weight‐of‐evidence approach. Regarding the third question, the Scientific Committee concluded that any available data that may assist in reducing the uncertainty in the assessment of the genotoxic potential of a substance should be taken into consideration. If the overall evaluation leaves no concerns for genotoxicity, health‐based guidance values may be established. However, if concerns for genotoxicity remain, establishing health‐based guidance values is not considered appropriate.

The micronucleus test for the oral mucosa: global trends and new questions

This study reviews global trends in the publication of papers on the micronucleus test of the exfoliated cells of the oral mucosa in mammals as an approach for environmental biomonitoring. This test has been widely used due to its precision for the detection of chromosome damage. Our temporal analysis showed a significant increase (p < 0.05) in the publication of papers on the oral mucosa over the past 33 years. Brazil was the country that published most papers (24% of the total), followed by India (16%), the USA (10%), Mexico (7%), and Turkey (6%). A further 30 countries contributed the other 37% of the papers. Overall, 99% of the micronucleus studies involved human subjects, and only 1% involved other mammals. As many wild mammals are subject to the same environmental pressures as humans, in particular chemical pollution, it seems likely that many species are equally susceptible to genotoxicogical damage. We emphasize the importance of applying this technique to the analysis of the oral mucosa of wild mammals, as well as the evaluation of its effectiveness, as observed in humans, and the expansion of the available approaches to the monitoring of environmental quality.

Absence of genotoxic effects of the chalcone (E)-1-(2-hydroxyphenyl)-3-(4-methylphenyl)-prop-2-en-1-one) and its potential chemoprevention against DNA damage using in vitro and in vivo assays

The chalcone (E)-1-(2-hydroxyphenyl)-3-(4-methylphenyl)-prop-2-en-1-one), or 2HMC, displays antileishmanial, antimalarial, and antioxidant activities. The aim of this study was to investigate the cytotoxic, genotoxic, mutagenic, and protective effects of 2HMC using the Ames mutagenicity test, the mouse bone marrow micronucleus test, and the comet assay in mice. In the assessment using the Ames test, 2HMC did not increase the number of His⁺ revertants in Salmonella typhimurium strains, demonstrating lack of mutagenicity. 2HMC showed no significant increase in micronucleated polychromatic erythrocyte frequency (MNPCE) in the micronucleus test, or in DNA strand breaks using the comet assay, evidencing absence of genotoxicity. Regarding cytotoxicity, 2HMC exhibited moderate cytotoxicity in mouse bone marrow cells by micronucleus test. 2HMC showed antimutagenic action in co-administration with the positive controls, sodium azide (SA) and 4-nitroquinoline-1-oxide (4NQO), in the Ames test. Co-administered and mainly pre-administered with cyclophosphamide (CPA), 2HMC caused a decrease in the frequency of MNPCE using the micronucleus test and in DNA strand breaks using the comet assay. Thus, 2HMC exhibited antimutagenic and antigenotoxic effects, displaying a DNA-protective effect against CPA, SA, and 4NQO carcinogens. In conclusion, 2HMC presented antimutagenic, antigenotoxic and moderate cytotoxic effects; therefore it is a promising molecule for cancer prevention.

An integrative test strategy for cancer hazard identification

Assessment of genotoxic and carcinogenic potential is considered one of the basic requirements when evaluating possible human health risks associated with exposure to chemicals. Test strategies currently in place focus primarily on identifying genotoxic potential due to the strong association between the accumulation of genetic damage and cancer. Using genotoxicity assays to predict carcinogenic potential has the significant drawback that risks from non-genotoxic carcinogens remain largely undetected unless carcinogenicity studies are performed. Furthermore, test systems already developed to reduce animal use are not easily accepted and implemented by either industries or regulators. This manuscript reviews the test methods for cancer hazard identification that have been adopted by the regulatory authorities, and discusses the most promising alternative methods that have been developed to date. Based on these findings, a generally applicable tiered test strategy is proposed that can be considered capable of detecting both genotoxic as well as non-genotoxic carcinogens and will improve understanding of the underlying mode of action. Finally, strengths and weaknesses of this new integrative test strategy for cancer hazard identification are presented.