Mouse bone marrow micronucleus test results do not predict the germ cell mutagenicity of N-hydroxymethylacrylamide in the mouse dominant lethal assay

A Return to the Origin of the EMGS: Rejuvenating the Quest for Human Germ Cell Mutagens and Determining the Risk to Future Generations

Fifty years ago, the Environmental Mutagen Society (now Environmental Mutagenesis and Genomics Society) was founded with a laser-focus on germ cell mutagenesis and the protection of "our most vital assets"-the sperm and egg genomes. Yet, five decades on, despite the fact that many agents have been demonstrated to induce inherited changes in the offspring of exposed laboratory rodents, there is no consensus on whether human germ cell mutagens exist. We argue that it is time to reevaluate the available data and conclude that we already have evidence for the existence of environmental exposures that impact human germ cells. What is missing are definite data to demonstrate a significant increase in de novo mutations in the offspring of exposed parents. We believe that with over two decades of research advancing knowledge and technologies in genomics, we are at the cusp of generating data to conclusively show that environmental exposures cause heritable de novo changes in the human offspring. We call on the research community to harness our technologies, synergize our efforts, and return to our Founders' original focus. The next 50 years must involve collaborative work between clinicians, epidemiologists, genetic toxicologists, genomics experts and bioinformaticians to precisely define how environmental exposures impact germ cell genomes. It is time for the research and regulatory communities to prepare to interpret the coming outpouring of data and develop a framework for managing, communicating and mitigating the risk of exposure to human germ cell mutagens. Environ. Mol. Mutagen. 61:42-54, 2020. © 2019 Her Majesty the Queen in Right of Canada.

Dr. Daniel Acosta and In Vitro toxicology at the U.S. Food and Drug Administration's National Center for Toxicological Research

For the past five years, Dr. Daniel Acosta has served as the Deputy Director of Research at the National Center for Toxicological Research (NCTR), a principle research laboratory of the U.S. Food and Drug Administration (FDA). Over his career at NCTR, Dr. Acosta has had a major impact on developing and promoting the use of in vitro assays in regulatory toxicity and product safety assessments. As Dr. Acosta nears his retirement we have dedicated this paper to his many accomplishments at the NCTR. Described within this paper are some of the in vitro studies that have been conducted under Dr. Acosta's leadership. These studies include toxicological assessments involving developmental effects, and the development and application of in vitro reproductive, heart, liver, neurological and airway cell and tissue models.

Models of germ cell development and their application for toxicity studies

Germ cells are unique in their ability to transfer genetic information and traits from generation to generation. As such, the proper development of germ cells and the integrity of their genome are paramount to the health of organisms and the survival of species. Germ cells are also exquisitely sensitive to environmental influences although the testing of germ cell toxicity, especially in females, has proven particularly challenging. In this review, we first describe the remarkable odyssey of germ cells in mammals, with an emphasis on the female germline, from their initial specification early during embryogenesis to the generation of mature gametes in adults. We also describe the current methods used in germ cell toxicity testing and their limitations in examining the complex features of mammalian germ cell development. To bypass these challenges, we propose the use of alternative model systems such as Saccharomyces cerevisiae, Drosophila melanogaster, Caenorhabditis elegans, and in vitro germ cell methods that have distinct advantages over traditional toxicity models. We discuss the benefits and limitations of each approach, their application to germ cell toxicity studies, and the need for computational approaches to maximize the usefulness of these models. Together, the inclusion of these alternative germ cell toxicity models will be invaluable for the examination of stages not easily accessible in mammals as well as the large scale, high-throughput investigation of germ cell toxicity.

Approaches for identifying germ cell mutagens: Report of the 2013 IWGT workshop on germ cell assays(☆)

This workshop reviewed the current science to inform and recommend the best evidence-based approaches on the use of germ cell genotoxicity tests. The workshop questions and key outcomes were as follows. (1) Do genotoxicity and mutagenicity assays in somatic cells predict germ cell effects? Limited data suggest that somatic cell tests detect most germ cell mutagens, but there are strong concerns that dictate caution in drawing conclusions. (2) Should germ cell tests be done, and when? If there is evidence that a chemical or its metabolite(s) will not reach target germ cells or gonadal tissue, it is not necessary to conduct germ cell tests, notwithstanding somatic outcomes. However, it was recommended that negative somatic cell mutagens with clear evidence for gonadal exposure and evidence of toxicity in germ cells could be considered for germ cell mutagenicity testing. For somatic mutagens that are known to reach the gonadal compartments and expose germ cells, the chemical could be assumed to be a germ cell mutagen without further testing. Nevertheless, germ cell mutagenicity testing would be needed for quantitative risk assessment. (3) What new assays should be implemented and how? There is an immediate need for research on the application of whole genome sequencing in heritable mutation analysis in humans and animals, and integration of germ cell assays with somatic cell genotoxicity tests. Focus should be on environmental exposures that can cause de novo mutations, particularly newly recognized types of genomic changes. Mutational events, which may occur by exposure of germ cells during embryonic development, should also be investigated. Finally, where there are indications of germ cell toxicity in repeat dose or reproductive toxicology tests, consideration should be given to leveraging those studies to inform of possible germ cell genotoxicity.

Transgenic rodent assay for quantifying male germ cell mutant frequency

De novo mutations arise mostly in the male germline and may contribute to adverse health outcomes in subsequent generations. Traditional methods for assessing the induction of germ cell mutations require the use of large numbers of animals, making them impractical. As such, germ cell mutagenicity is rarely assessed during chemical testing and risk assessment. Herein, we describe an in vivo male germ cell mutation assay using a transgenic rodent model that is based on a recently approved Organisation for Economic Co-operation and Development (OECD) test guideline. This method uses an in vitro positive selection assay to measure in vivo mutations induced in a transgenic λgt10 vector bearing a reporter gene directly in the germ cells of exposed males. We further describe how the detection of mutations in the transgene recovered from germ cells can be used to characterize the stage-specific sensitivity of the various spermatogenic cell types to mutagen exposure by controlling three experimental parameters: the duration of exposure (administration time), the time between exposure and sample collection (sampling time), and the cell population collected for analysis. Because a large number of germ cells can be assayed from a single male, this method has superior sensitivity compared with traditional methods, requires fewer animals and therefore much less time and resources.

Current Status of Short-Term Tests for Evaluation of Genotoxicity, Mutagenicity, and Carcinogenicity of Environmental Chemicals and NCEs

The advent of the industrial revolution has seen a significant increase in the number of new chemical entities (NCEs) released in the environment. It becomes imperative to check the toxic potential of NCEs to nontarget species before they are released for commercial purposes because some of these may exert genotoxicity, mutagenicity, or carcinogenicity. Exposure to such compounds produces chemical changes in DNA, which are generally repaired by the DNA repair enzymes. However, DNA damage and its fixation may occur in the form of gene mutations, chromosomal damage, and numerical chromosomal changes and recombination. This may affect the incidence of heritable mutations in man and may be transferred to the progeny or lead to the development of cancer. Hence, adequate tests on NCEs have to be undertaken for the risk assessment and hazard prediction. Compounds that are positive in tests that detect such damages have the potential to be human mutagens/carcinogens. Only long-term animal bioassays, involving lifetime studies on animals, were used earlier to classify substances as mutagens/carcinogens. These tests were cumbersome and time consuming and required a lot of facilities and personnel. Short-term tests, therefore, were brought into practice. A "battery" of three to four of these short-term tests has been proposed now by a number of regulatory authorities for the classification of compounds as mutagenic or carcinogenic. This review deals with the current status of these short-term tests.

The measurement of induced genetic change in mammalian germ cells

In vivo methods are described to detect clastogenic and aneugenic effects of chemical agents in male and female germ cells in vivo. The knowledge of stages of germ cell development and their duration for a given test animal is essential for these experiments. Commonly, mice or rats are employed. Structural chromosome aberrations can be analyzed microscopically in mitotic cell divisions of differentiating spermatogonia, zygotes, or early embryos as well as in first meiotic cell divisions of spermatocytes and oocytes. Numerical chromosome aberrations are scorable during second meiotic divisions of spermatocytes and oocytes. The micronucleus test is applicable to early round spermatids and to first cleavage embryos, and as in somatic cells, it assesses structural as well as numerical chromosome aberrations. In contrast to the somatic micronucleus assay, the timing of cell sampling determines whether the micronuclei scored in round spermatids were formed from structural or numerical aberrations, i.e. with short treatment-sampling intervals the micronuclei are formed by exposed meiotic divisions and represent induced non-disjunction. On the -contrary, after longer intervals of 12-14 days micronuclei are formed from induced unstable structural aberrations in differentiating spermatogonia or during the last round of DNA-synthesis in early spermatocytes. Furthermore, labelling with fluorescent DNA-probes can be used to confirm these theoretical expectations. The mouse sperm-FISH assay is totally based on scoring colour spots from individual chromosomes (e.g. X, Y, and 8) hybridized with specific DNA-probes. The most animal demanding assay described here is the dominant lethal test. It is commonly performed with treated male laboratory rodents and allows the determination of the most sensitive developmental stage of spermatogenesis to a particular chemical under test. Theoretically, unstable structural chromosome aberrations in sperm will lead to foetal deaths after fertilization at around the time of implantation in the uterus wall. These can be scored as deciduomata or early dead foetuses in the uterus wall of the females at mid-pregnancy. None of the tests described in this chapter provide data for a quantitative estimate of the genetic risk to progeny from exposed germ cells. The only tests on which such calculations can be based, the heritable translocation assay and the specific locus test, are so animal and time-consuming that they can no more be performed anywhere in the world and thus are not described here.

Sidestream tobacco smoke is a male germ cell mutagen

Active cigarette smoking increases oxidative damage, DNA adducts, DNA strand breaks, chromosomal aberrations, and heritable mutations in sperm. However, little is known regarding the effects of second-hand smoke on the male germ line. We show here that short-term exposure to mainstream tobacco smoke or sidestream tobacco smoke (STS), the main component of second-hand smoke, induces mutations at an expanded simple tandem repeat locus (Ms6-hm) in mouse sperm. We further show that the response to STS is not linear and that, for both mainstream tobacco smoke and STS, doses that induced significant increases in expanded simple tandem repeat mutations in sperm did not increase the frequencies of micronucleated reticulocytes and erythrocytes in the bone marrow and blood of exposed mice. These data show that passive exposure to cigarette smoke can cause tandem repeat mutations in sperm under conditions that may not induce genetic damage in somatic cells. Although the relationship between noncoding tandem repeat instability and mutations in functional regions of the genome is unclear, our data suggest that paternal exposure to second-hand smoke may have reproductive consequences that go beyond the passive smoker.

Germ cell mutagens: risk assessment challenges in the 21st century

Heritable mutations may result in a wide variety of detrimental outcomes, from embryonic lethality to genetic disease in the offspring. Despite this, today's commonly used test batteries do not include assays for germ cell mutation. Current challenges include a lack of practical assays and concrete evidence for human germline mutagens, and large data gaps that often impede risk assessment. Moreover, most regulatory assessments are based on the assumption that somatic cell mutation assays also protect the germline by default, which has not been adequately confirmed. The field is also faced with new challenges aimed at dramatically reducing animal testing, and attempts to rapidly classify thousands of chemicals using high throughput in vitro assays. These approaches may not adequately capture effects that may be particular to gametes, since many aspects of the germline are unique. In light of these challenges, an urgent need exists to develop new approaches to evaluate the potential of toxicants to cause germline mutation. The application of new technologies will greatly enhance our understanding of mutation in humans exposed to environmental mutagens. However, we must be poised to collect and interpret these data, and facilitate risk translation to regulators and the public. Genetic toxicologists must also become actively involved in the development of high-throughput tools to study germline mutation. Appropriate attention to these areas will result in the development of policies that prioritize the protection of the germline and future generations from DNA sequence mutations.

Gender differences in the induction of chromosomal aberrations and gene mutations in rodent germ cells

Germ cell mutagenicity testing provides experimental data to quantify genetic risk for exposed human populations. The majority of tests are performed with exposure of males, and female data are relatively rare. The reason for this paucity lies in the differences between male and female germ cell biology. Male germ cells are produced throughout reproductive life and all developmental stages can be ascertained by appropriate breeding schemes. In contrast, the female germ cell pool is limited, meiosis begins during embryogenesis and oocytes are arrested over long periods of time until maturation processes start for small numbers of oocytes during the oestrus cycle in mature females. The literature data are reviewed to point out possible gender differences of germ cells to exogenous agents such as chemicals or ionizing radiation. From the limited information, it can be concluded that male germ cells are more sensitive than female germ cells to the induction of chromosomal aberrations and gene mutations. However, exceptions are described which shed doubt on the extrapolation of experimental data from male rodents to the genetic risk of the human population. Furthermore, the female genome may be more sensitive to mutation induction during peri-conceptional stages compared to the male genome of the zygote. With few exceptions, germ cell experiments have been carried out under high acute exposure to optimize the effects and to compensate for the limited sample size in animal experiments. Human exposure to environmental agents, on the other hand, is usually chronic and involves low doses. Under these conditions, gender differences may become apparent that have not been studied so far. Additionally, data are reviewed that suggest a false impression of safety when responses are negative under high acute exposure of male rodents while a mutational response is induced by low chronic exposure. The classical (morphological) germ cell mutation tests are not performed anymore because they are animal and time consuming. Nevertheless, information is needed to place genetic risk extrapolations on more solid grounds and thereby to prevent an increased genetic burden to future generations. It is pointed out that modern molecular methodologies are available now to experimentally address the open questions.

Gender differences in germ-cell mutagenesis and genetic risk

Current international classification systems for chemical mutagens are hazard-based rather than aimed at assessing risks quantitatively. In the past, germ-cell tests have been mainly performed with a limited number of somatic cell mutagens, and rarely under conditions aimed at comparing gender-specific differences in susceptibility to mutagen exposures. There are profound differences in the genetic constitution, and in hormonal, structural, and functional aspects of differentiation and control of gametogenesis between the sexes. A critical review of the literature suggests that these differences may have a profound impact on the relative susceptibility, stage of highest sensitivity and the relative risk for the genesis of gene mutation, as well as structural and numerical chromosomal aberrations in male and female germ cells. Transmission of germ-cell mutations to the offspring may also encounter gender-specific influences. Gender differences in susceptibility to chemically derived alterations in imprinting patterns may pose a threat for the health of the offspring and may also be transmitted to future generations. Recent reports on different genetic effects from high acute and from chronic low-dose exposures challenge the validity of conclusions drawn from standard methods of mutagenicity testing. In conclusion, research is urgently needed to identify genetic hazards for a larger range of chemical compounds, including those suspected to disturb proper chromosome segregation. Alterations in epigenetic programming and their health consequences will have to be investigated. More attention should be paid to gender-specific genetic effects. Finally, the database for germ-cell mutagens should be enlarged using molecular methodologies, and genetic epidemiology studies should be performed with these techniques to verify human genetic risk.

Assessing human germ-cell mutagenesis in the Postgenome Era: a celebration of the legacy of William Lawson (Bill) Russell

Birth defects, de novo genetic diseases, and chromosomal abnormality syndromes occur in approximately 5% of all live births, and affected children suffer from a broad range of lifelong health consequences. Despite the social and medical impact of these defects, and the 8 decades of research in animal systems that have identified numerous germ-cell mutagens, no human germ-cell mutagen has been confirmed to date. There is now a growing consensus that the inability to detect human germ-cell mutagens is due to technological limitations in the detection of random mutations rather than biological differences between animal and human susceptibility. A multidisciplinary workshop responding to this challenge convened at The Jackson Laboratory in Bar Harbor, Maine. The purpose of the workshop was to assess the applicability of an emerging repertoire of genomic technologies to studies of human germ-cell mutagenesis. Workshop participants recommended large-scale human germ-cell mutation studies be conducted using samples from donors with high-dose exposures, such as cancer survivors. Within this high-risk cohort, parents and children could be evaluated for heritable changes in (a) DNA sequence and chromosomal structure, (b) repeat sequences and minisatellites, and (c) global gene expression profiles and pathways. Participants also advocated the establishment of a bio-bank of human tissue samples from donors with well-characterized exposure, including medical and reproductive histories. This mutational resource could support large-scale, multiple-endpoint studies. Additional studies could involve the examination of transgenerational effects associated with changes in imprinting and methylation patterns, nucleotide repeats, and mitochondrial DNA mutations. The further development of animal models and the integration of these with human studies are necessary to provide molecular insights into the mechanisms of germ-cell mutations and to identify prevention strategies. Furthermore, scientific specialty groups should be convened to review and prioritize the evidence for germ-cell mutagenicity from common environmental, occupational, medical, and lifestyle exposures. Workshop attendees agreed on the need for a full-scale assault to address key fundamental questions in human germ-cell environmental mutagenesis. These include, but are not limited to, the following: Do human germ-cell mutagens exist? What are the risks to future generations? Are some parents at higher risk than others for acquiring and transmitting germ-cell mutations? Obtaining answers to these, and other critical questions, will require strong support from relevant funding agencies, in addition to the engagement of scientists outside the fields of genomics and germ-cell mutagenesis.

Detection of induced male germline mutation: correlations and comparisons between traditional germline mutation assays, transgenic rodent assays and expanded simple tandem repeat instability assays

Several rodent assays are capable of monitoring germline mutation. These include traditional assays, such as the dominant lethal (DL) assay, the morphological specific locus (SL) test and the heritable translocation (HT) assay, and two assays that have been developed more recently--the expanded simple tandem repeat (ESTR) and transgenic rodent (TGR) mutation assays. In this paper, we have compiled the limited amount of experimental data that are currently available to make conclusions regarding the comparative ability of the more recently developed assays to detect germline mutations induced by chemical and radiological agents. The data suggest that ESTR and TGR assays are generally comparable with SL in detecting germline mutagenicity induced by alkylating agents and radiation, though TGR offered less sensitivity than ESTR in some cases. The DL and HT assays detect clastogenic events and are most susceptible to mutations arising in post-spermatogonial cells, and they may not provide the best comparisons with TGR and ESTR instability. The measurement of induced ESTR instability represents a relatively sensitive method of identifying agents causing germline mutation in rodents, and may also be useful for bio-monitoring exposed individuals in the human population. Any future use of the TGR and ESTR germline mutation assays in a regulatory testing context will entail more robust and extensive characterization of assay performance. This will require substantially more data, including experiments measuring multiple endpoints, a greatly expanded database of chemical agents and a focus on characterizing stage-specific activity of mutagens in these assays, preferably by sampling epididymal sperm exposed at defined pre-meiotic, meiotic and post-meiotic stages of development.

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.

Heritable translocations induced by dermal exposure of male mice to acrylamide

Acrylamide (AA) is an important industrial chemical used mainly in the production of polymers. It can be absorbed through the skin. AA was shown to be a germ cell clastogen that entails a genetic risk for exposed workers. The genetic risk calculation was based on mouse heritable translocation test data obtained after acute intraperitoneal (ip) exposure (Adler et al., 1994). To obtain a correction factor between ip and dermal exposure, dominant lethal and heritable translocation tests were carried out with dermal exposure of male mice to AA. In the dominant lethal test, male (102/El x C3H/El)F1 mice were exposed by dermal application to the shaved backs of 50 mg/kg AA per day on five consecutive days or to five daily ip injections of 50 mg/kg AA. One day after the end of exposure, the males were mated to untreated females of the same hybrid stock for four days and females were changed every four days for a total of five matings. Dominant lethal effects were found during matings 1-3. For ip exposure, these values were 81.7, 85.7 and 45.4%, respectively; for dermal exposure the corresponding values were 22.1, 30.6 and 16.5%, respectively. In the heritable translocation assay, male C3H/El mice were treated with five dermal exposures of 50 mg/kg AA and mated 1.5-8.5 days after the end of exposure to untreated female 102/El mice. Pregnant females were allowed to come to term and all offspring were raised to maturity. Translocation carriers among the F1 progeny were selected by a sequential fertility testing and cytogenetic analysis including G-band karyotyping and M-FISH. A total of 475 offspring were screened and 41 translocation carriers were identified. The observed translocation frequency after dermal exposure was 8.6% as compared to 21.9% after similar ip exposure (Adler, 1990). The calculated ratio of ip vs. dermal exposure of 0.39 can be applied to obtain a more realistic calculation of genetic risk for dermally exposed workers.