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The International Workshop on Genotoxicity Testing (IWGT) meets every four years to obtain consensus on unresolved issues associated with genotoxicity testing. At the 2017 IWGT meeting in Tokyo, four sub-groups addressed issues associated with the Organization for Economic Cooperation and Development (OECD) Test Guideline TG471, which describes the use of bacterial reverse-mutation tests. The strains sub-group analyzed test data from >10,000 chemicals, tested additional chemicals, and concluded that some strains listed in TG471 are unnecessary because they detected fewer mutagens than other strains that the guideline describes as equivalent. Thus, they concluded that a smaller panel of strains would suffice to detect most mutagens. The laboratory proficiency sub-group recommended (a) establishing strain cell banks, (b) developing bacterial growth protocols that optimize assay sensitivity, and (c) testing "proficiency compounds" to gain assay experience and establish historical positive and control databases. The sub-group on criteria for assay evaluation recommended that laboratories (a) track positive and negative control data; (b) develop acceptability criteria for positive and negative controls; (c) optimize dose-spacing and the number of analyzable doses when there is evidence of toxicity; (d) use a combination of three criteria to evaluate results: a dose-related increase in revertants, a clear increase in revertants in at least one dose relative to the concurrent negative control, and at least one dose that produced an increase in revertants above control limits established by the laboratory from historical negative controls; and (e) establish experimental designs to resolve unclear results. The in silico sub-group summarized in silico utility as a tool in genotoxicity assessment but made no specific recommendations for TG471. Thus, the workgroup identified issues that could be addressed if TG471 is revised. The companion papers (a) provide evidence-based approaches, (b) recommend priorities, and (c) give examples of clearly defined terms to support revision of TG471., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflicts of interest., (Copyright © 2020. Published by Elsevier B.V.)

The bacterial reverse mutation test is a mainstay for evaluation of mutagenicity predicting the carcinogenic potential of a test substance and is recommended by regulatory agencies across the globe. The popularity of the test is due, in part, to the relatively low cost, rapid results and small amount of test material required compared to most other toxicological tests as well as the near universal acceptance of the toxicological significance of a clear positive or negative result. Most laboratories follow the Organization for Economic Cooperation and Development Test Guideline 471 (TG471) or national guidelines based on TG471. Regulatory agencies in most countries are obligated to consider results from tests which meet the recommendations laid out in TG471. Nonetheless, laboratories unfamiliar with the test sometimes have trouble generating reliable, reproducible results. TG471 is a test guideline, not a detailed test protocol. A group of experts from regulatory agencies and laboratories which use the assay has assembled here a set of recommendations which if followed, will allow an inexperienced laboratory to acquire proficiency in assay conduct. These include recommendations for how to create a cell bank for the 5 Salmonella typhimurium/Escherichia coli strains and develop a laboratory protocol to reliably culture each strain to ensure each culture has the characteristics which allow adequate sensitivity for detection of mutagens using the test as described in TG471. By testing compounds on the provided lists of positive and negative test substances, the laboratory will have surmounted many of the problems commonly encountered during routine testing of unknown chemicals and will have gained the experience necessary to prepare the detailed protocol needed for performing the test under Good Laboratory Procedures and the laboratory will have generated the historical positive and negative control databases which are needed for test reports which adhere to TG471., (Published by Elsevier B.V.)

A database of 91 chemicals with published data from both transgenic rodent mutation (TGR) and rodent comet assays has been compiled. The objective was to compare the sensitivity of the two assays for detecting genotoxicity. Critical aspects of study design and results were tabulated for each dataset. There were fewer datasets from rats than mice, particularly for the TGR assay, and therefore, results from both species were combined for further analysis. TGR and comet responses were compared in liver and bone marrow (the most commonly studied tissues), and in stomach and colon evaluated either separately or in combination with other GI tract segments. Overall positive, negative, or equivocal test results were assessed for each chemical across the tissues examined in the TGR and comet assays using two approaches: 1) overall calls based on weight of evidence (WoE) and expert judgement, and 2) curation of the data based on a priori acceptability criteria prior to deriving final tissue specific calls. Since the database contains a high prevalence of positive results, overall agreement between the assays was determined using statistics adjusted for prevalence (using AC1 and PABAK). These coefficients showed fair or moderate to good agreement for liver and the GI tract (predominantly stomach and colon data) using WoE, reduced agreement for stomach and colon evaluated separately using data curation, and poor or no agreement for bone marrow using both the WoE and data curation approaches. Confidence in these results is higher for liver than for the other tissues, for which there were less data. Our analysis finds that comet and TGR generally identify the same compounds (mainly potent mutagens) as genotoxic in liver, stomach and colon, but not in bone marrow. However, the current database content precluded drawing assay concordance conclusions for weak mutagens and non-DNA reactive chemicals., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)

The present review describes available evidence about the fate of micronuclei and micronucleated cells. Micronuclei are small, extranuclear chromatin bodies surrounded by a nuclear envelope. The mechanisms underlying the formation of micronuclei are well understood but not much is known about the potential fate of micronuclei and micronucleated cells. Many studies with different experimental approaches addressed the various aspects of the post-mitotic fate of micronuclei and micronucleated cells. These studies are reviewed here considering four basic possibilities for potential fates of micronuclei: degradation of the micronucleus or the micronucleated cell, reincorporation into the main nucleus, extrusion from the cell, and persistence in the cytoplasm. Two additional fates need to be considered: premature chromosome condensation/chromothripsis and the elimination of micronucleated cells by apoptosis, yielding six potential fates for micronuclei and/or micronucleated cells. The available data is still limited, but it can be concluded that degradation and extrusion of micronuclei might occur in rare cases under specific conditions, reincorporation during the next mitosis occurs more frequently, and the majority of the micronuclei persist without alteration at least until the next mitosis, possibly much longer. Overall, the consequences of micronucleus formation on the cellular level are still far from clear, but they should be investigated further because micronucleus formation may contribute to the initial and later steps of malignant cell transformation, by causing gain or loss of genetic material in the daughter cells and by the possibility of massive chromosome rearrangement in chromosomes entrapped within a micronucleus by the mechanisms of chromothripsis and chromoanagenesis., (Copyright © 2017 Elsevier B.V. All rights reserved.)