Paternally inherited chromosomal structural aberrations detected in mouse first-cleavage zygote metaphases by multicolour fluorescence in situ hybridization painting

Chemically induced aneuploidy in germ cells. Part II of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

As part of the 7th International Workshops on Genotoxicity Testing held in Tokyo, Japan in November 2017, a workgroup of experts reviewed and assessed the risk of aneugens for human health. The present manuscript is one of three manuscripts from the workgroup and reports on the unanimous consensus reached on the evidence for aneugens affecting germ cells, their mechanisms of action and role in hereditary diseases. There are 24 chemicals with strong or sufficient evidence for germ cell aneugenicity providing robust support for the ability of chemicals to induce germ cell aneuploidy. Interference with microtubule dynamics or inhibition of topoisomerase II function are clear characteristics of germ cell aneugens. Although there are mechanisms of chromosome segregation that are unique to germ cells, there is currently no evidence for germ cell-specific aneugens. However, the available data are heavily skewed toward chemicals that are aneugenic in somatic cells. Development of high-throughput screening assays in suitable animal models for exploring additional targets for aneuploidy induction, such as meiosis-specific proteins, and to prioritize chemicals for the potential to be germ cell aneugens is encouraged. Evidence in animal models support that: oocytes are more sensitive than spermatocytes and somatic cells to aneugens; exposure to aneugens leads to aneuploid conceptuses; and, the frequencies of aneuploidy are similar in germ cells and zygotes. Although aneuploidy in germ cells is a significant cause of infertility and pregnancy loss in humans, there is currently limited evidence that aneugens induce hereditary diseases in human populations because the great majority of aneuploid conceptuses die in utero. Overall, the present work underscores the importance of protecting the human population from exposure to chemicals that can induce aneuploidy in germ cells that, in contrast to carcinogenicity, is directly linked to an adverse outcome.

Chromosomal mosaicism in mouse two-cell embryos after paternal exposure to acrylamide

Chromosomal mosaicism in human preimplantation embryos is a common cause of spontaneous abortions, however, our knowledge of its etiology is limited. We used multicolor fluorescence in situ hybridization painting to investigate whether paternally transmitted chromosomal aberrations result in mosaicism in mouse two-cell embryos. Paternal exposure to acrylamide, an important industrial chemical also found in tobacco smoke and generated during the cooking process of starchy foods, produced significant increases in chromosomally defective two-cell embryos, however, the effects were transient primarily affecting the postmeiotic stages of spermatogenesis. Comparisons with our previous study of zygotes demonstrated similar frequencies of chromosomally abnormal zygotes and two-cell embryos suggesting that there was no apparent selection against numerical or structural chromosomal aberrations. However, the majority of affected two-cell embryos were mosaics showing different chromosomal abnormalities in the two blastomeric metaphases. Analyses of chromosomal aberrations in zygotes and two-cell embryos showed a tendency for loss of acentric fragments during the first mitotic division of embryogenesis, whereas both dicentrics and translocations apparently underwent proper segregation. These results suggest that embryonic development can proceed up to the end of the second cell cycle of development in the presence of abnormal paternal chromosomes and that even dicentrics can persist through cell division. The high incidence of chromosomally mosaic two-cell embryos suggests that the first mitotic division of embryogenesis is prone to missegregation errors and that paternally transmitted chromosomal abnormalities increase the risk of missegregation leading to embryonic mosaicism.

DNA repair decline during mouse spermiogenesis results in the accumulation of heritable DNA damage

The postmeiotic phase of mouse spermatogenesis (spermiogenesis) is very sensitive to the genomic effects of environmental mutagens because as male germ cells form mature sperm they progressively lose the ability to repair DNA damage. We hypothesized that repeated exposures to mutagens during this repair-deficient phase result in the accumulation of heritable genomic damage in mouse sperm that leads to chromosomal aberrations in zygotes after fertilization. We used a combination of single or fractionated exposures to diepoxybutane (DEB), a component of tobacco smoke, to investigate how differential DNA repair efficiencies during the 3 weeks of spermiogenesis affected the accumulation of DEB-induced heritable damage in early spermatids (21-15 days before fertilization (dbf)), late spermatids (14-8dbf) and sperm (7-1dbf). Analysis of chromosomal aberrations in zygotic metaphases using PAINT/DAPI showed that late spermatids and sperm are unable to repair DEB-induced DNA damage as demonstrated by significant increases (P<0.001) in the frequencies of zygotes with chromosomal aberrations. Comparisons between single and fractionated exposures suggested that the DNA repair-deficient window during late spermiogenesis may be less than 2 weeks in the mouse and that during this repair-deficient window there is accumulation of DNA damage in sperm. Finally, the dose-response study in sperm indicated a linear response for both single and repeated exposures. These findings show that the differential DNA repair capacity of postmeiotic male germ cells has a major impact on the risk of paternally transmitted heritable damage and suggest that chronic exposures that may occur in the weeks prior to fertilization because of occupational or lifestyle factors (i.e., smoking) can lead to an accumulation of genetic damage in sperm and result in heritable chromosomal aberrations of paternal origin.

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.

Fluorescent in situ hybridization for sex chromosome determination before and after fertilization in mice

In mice, the relative numbers of male and female pups per litter not only can vary but can probably change over the course of pregnancy in response to numerous environmental and physiological factors. As such, a technique is required to determine gender at several developmental stages. Here we describe a robust and accurate fluorescent in situ hybridization (FISH) procedure for determining chromosomal sex that can be applied with minimal modification to sperm, pre-and post-implantation conceptuses and recovered dead post-natal pups. Sperm was prepared for FISH analysis y using a modified microwave decondensation-denaturation technique. Preimplantation conceptuses (0.5dpc) were cultured to the morula stage before sexing. They were then acid-treated to remove the zona pellucida. Tissue homogenates from postimplantational conceptuses (8.5dpc) and stillborn pups were fixed to pre-etched slides. Specimens were hybridized with identical, commercially available DNA probes for the X (FITC) and Y (Cy3) chromosomes. Sperm ratios met the expected value of 0.5 when determined by using XY FISH. Preimplantation conceptuses pre-treated with pepsin yielded distinct fluorescence of X and Y chromosomes in morulae, whereas microwave decondensation resulted in loss of conceptuses from the slide. Both 4.0 and 8.5dpc conceptuses displayed mean sex ratios of 0.5. Post-natal FISH analysis allowed gender identification of pups that could not be sexed due to developmental abnormalities or partial cannibalism. FISH analysis of sperm and of multiple conceptuses or post-natal tissue provided a cost-effective, accurate alternative to PCR-based sex determination.

Mechanisms and consequences of paternally-transmitted chromosomal abnormalities

Paternally-transmitted chromosomal damage has been associated with pregnancy loss, developmental and morphological defects, infant mortality, infertility, and genetic diseases in the offspring, including cancer. There is epidemiological evidence linking paternal exposure to occupational or environmental agents with an increased risk of abnormal reproductive outcomes. There is also a large body of literature on germ cell mutagenesis in rodents showing that treatment of male germ cells with mutagens has dramatic consequences on reproduction, producing effects such as those observed in human epidemiological studies. However, we know very little about the etiology, transmission, and early embryonic consequences of paternally-derived chromosomal abnormalities. The available evidence suggests that: 1) there are distinct patterns of germ cell-stage differences in the sensitivity of induction of transmissible genetic damage, with male postmeiotic cells being the most sensitive; 2) cytogenetic abnormalities at first metaphase after fertilization are critical intermediates between paternal exposure and abnormal reproductive outcomes; and 3) there are maternal susceptibility factors that may have profound effects on the amount of sperm DNA damage that is converted into chromosomal aberrations in the zygote and that directly affect the risk for abnormal reproductive outcomes.

Paternally Transmitted Chromosomal Aberrations in Mouse Zygotes Determine Their Embryonic Fate1

The developmental consequences of chromosomal aberrations in embryos include spontaneous abortions, morphological defects, inborn abnormalities, and genetic/chromosomal diseases. Six germ-cell mutagens with different modes of action and spermatogenic stage sensitivities were used to investigate the relationship between the types of cytogenetic damage in zygotes with their subsequent risk of postimplantation death and of birth as a translocation carrier. Independent of the mutagen used, over 98% of paternally transmitted aberrations were chromosome type, rather than chromatid type, indicating that they were formed during the period between exposure of male germ cells and initiation of the first S phase after fertilization. There were consistent one-to-one agreements between the proportions of a) zygotes with unstable aberrations and the frequencies of dead embryos after implantation (slope = 0.87, confidence interval [CI]: 0.74, 1.16) and b) zygotes with reciprocal translocations and the frequency of translocation carriers at birth (slope = 0.74, CI: 0.48, 2.11). These findings suggest that chromosomal aberrations in zygotes are highly predictive of subsequent abnormal embryonic development and that development appears to proceed to implantation regardless of the presence of chromosomal abnormalities. Our findings support the hypothesis that, for paternally transmitted chromosomal aberrations, the fate of the embryo is already set by the end of G1 of the first cell cycle of development.

Etoposide induces heritable chromosomal aberrations and aneuploidy during male meiosis in the mouse

Etoposide, a topoisomerase II inhibitor widely used in cancer therapy, is suspected of inducing secondary tumors and affecting the genetic constitution of germ cells. A better understanding of the potential heritable risk of etoposide is needed to provide sound genetic counseling to cancer patients treated with this drug in their reproductive years. We used a mouse model to investigate the effects of clinical doses of etoposide on the induction of chromosomal abnormalities in spermatocytes and their transmission to zygotes by using a combination of chromosome painting and 4',6-diamidino-2-phenylindole staining. High frequencies of chromosomal aberrations were detected in spermatocytes within 64 h after treatment when over 30% of the metaphases analyzed had structural aberrations (P < 0.01). Significant increases in the percentages of zygotic metaphases with structural aberrations were found only for matings that sampled treated pachytene (28-fold, P < 0.0001) and preleptotene spermatocytes (13-fold, P < 0.001). Etoposide induced mostly acentric fragments and deletions, types of aberrations expected to result in embryonic lethality, because they represent loss of genetic material. Chromosomal exchanges were rare. Etoposide treatment of pachytene cells induced aneuploidy in both spermatocytes (18-fold, P < 0.01) and zygotes (8-fold, P < 0.05). We know of no other report of an agent for which paternal exposure leads to an increased incidence of aneuploidy in the offspring. Thus, we found that therapeutic doses of etoposide affect primarily meiotic germ cells, producing unstable structural aberrations and aneuploidy, effects that are transmitted to the progeny. This finding suggests that individuals who undergo chemotherapy with etoposide may be at a higher risk for abnormal reproductive outcomes especially within the 2 months after chemotherapy.

In vivo cytogenetics: mammalian germ cells

This chapter summarizes the most relevant methodologies available for evaluation of cytogenetic damage induced in vivo in mammalian germ cells. Protocols are provided for the following endpoints: numerical and structural chromosome aberrations in secondary oocytes or first-cleavage zygotes, reciprocal translocations in primary spermatocytes, chromosome counting in secondary spermatocytes, numerical and structural chromosome aberrations, and sister chromatid exchanges (SCE) in spermatogonia, micronuclei in early spermatids, aneuploidy in mature sperm. The significance of each methodology is discussed. The contribution of novel molecular cytogenetic approaches to the detection of chromosome damage in rodent germ cells is also considered.

Absence of selection against aneuploid mouse sperm at fertilization

Is there selection against aneuploid sperm during spermatogenesis and fertilization? To address this question, we used male mice doubly heterozygous for the Robertsonian (Rb) translocations Rb(6. 16)24Lub and Rb(16.17)7Bnr, which produce high levels of sperm aneuploid for chromosome 16, the mouse counterpart of human chromosome 21. The frequencies of aneuploid male gametes before and after fertilization were compared by analyzing approximately 500 meiosis II spermatocytes and approximately 500 first-cleavage zygotes using fluorescence in situ hybridization with a DNA painting probe mixture containing three biotin-labeled probes specific for chromosomes 8, 16, and 17 plus a digoxigenin-labeled probe specific for chromosome Y. Hyperhaploidy for chromosome 16 occurred in 20.0% of spermatocytes and in 21.8% of zygotes. Hypohaploidy for chromosome 16 occurred in 17.0% and 16.7% of spermatocytes and zygotes, respectively. In addition, there was no preferential association between chromosome 16 aneuploidy and either of the sex chromosomes, nor was there an elevation in aneuploidy for chromosomes not involved in the Rb translocations. These findings provide direct evidence that there is no selection against aneuploid sperm during spermiogenesis, fertilization, and the first cell cycle of zygotic development.

Genetic toxicology data in the evaluation of potential human environmental carcinogens

In 1969, the International Agency for Research on Cancer (IARC) initiated the Monographs Programme to evaluate the carcinogenic risk of chemicals to humans. Results from short-term mutagenicity tests were first included in the IARC Monographs in the mid-1970s based on the observation that most carcinogens are also mutagens, although not all mutagens are carcinogens. Experimental evidence at that time showed a strong correlation between mutagenicity and carcinogenicity and indicated that short-term mutagenicity tests are useful for predicting carcinogenicity. Although the strength of these correlations has diminished over the past 20 years with the identification of putative nongenotoxic carcinogens, such tests provide vital information for identifying potential human carcinogens and understanding mechanisms of carcinogenesis. The short-term test results for agents compiled in the EPA/IARC Genetic Activity Profile (GAP) database over nearly 15 years are summarized and reviewed here with regard to their IARC carcinogenicity classifications. The evidence of mutagenicity or nonmutagenicity based on a 'defining set' of test results from three genetic endpoints (gene mutation, chromosomal aberrations, and aneuploidy) is examined. Recommendations are made for assessing chemicals based on the strength of evidence from short-term tests, and the implications of this approach in identifying mutational mechanisms of carcinogenesis are discussed. The role of short-term test data in influencing the overall classification of specific compounds in recent Monograph volumes is discussed, particularly with reference to studies in human populations. Ethylene oxide is cited as an example.

Acrylamide causes preimplantation abnormalities in embryos and induces chromatin-adducts in male germ cells of mice

Acrylamide, a known male postmeiotic germ cell mutagen, caused a dose-dependent increase in the frequency of morphologic abnormalities in preimplantation embryos. Single-cell eggs, growth retardation, and blastomere lysis were detected after paternal treatment with acrylamide (10 to 50 mg/kg, 5 d). The major effects were seen at weeks 1 to 3 after male treatment, with the highest level of abnormalities at the first week (> 90% vs. 5% in control). The frequency of abnormal four-day embryos was similar to preimplantation loss assessed at 15 to 16 d p.c. A > 100-fold elevation of chromatin adducts in sperm was observed during 1st and 2nd week after treatment, after which adduct levels decreased to baseline level. However, morphologic defects in embryos are not fully explained by the spermatid adduct curve. These findings demonstrate the effects of paternal exposure to acrylamide on preimplantation development and indicate a potential risk to the offspring of men exposed to acrylamide.

Inhibition of genotoxic effects of mammalian germ cell mutagens

Germ cell mutagens are among the most important chemicals for which chemopreventive agents should be sought and mechanistically defined. These mutagens may include environmental chemicals as well as drugs. In this investigation, the literature was reviewed for substances antimutagenic (or anticlastogenic) to compounds identified as mutagens in at least two germ cell studies. A complete matrix of test results was prepared to identify commonly tested pairs of germ cell mutagens and antimutagens. The categories of antimutagens most tested included vitamins, fatty acids, thiols, tannins and other phenolics. The most frequently studied mutagens were benzo[a]pyrene, cyclophosphamide, mitomycin C, and bleomycin. Based on the availability of the most relevant data, the analysis presented here focused on in vivo tests, specifically on bone marrow cytogenetics. The results indicated that antimutagens commonly found in the diet or endogenously in the body effectively antagonized the cytogenetic damage induced in the bone marrow by most of the germ cell mutagens studied to date. Bone marrow micronucleus and chromosomal aberration assays, which detect systemically active mutagens, may be predictive of similar mitigating effects in germ cells. Test results from antimutagenicity studies in germ cells, though limited, were comparable to the results from studies in the mouse bone marrow micronucleus test.

Reproductive toxicity of 1,3-butadiene in the mouse: cytogenetic analysis of chromosome aberrations in first-cleavage embryos and flow cytometric evaluation of spermatogonial cell killing

Reproductive effects of 1,3 butadiene inhalation have been evaluated in male mice by reduction of post-meiotic germ cells, alteration of sperm chromatin structure and transmission of chromosome aberrations to one-cell embryos. Animals were exposed for 5 consecutive days for 6 h per day to butadiene concentrations of 130, 500 or 1300 ppm. The testicular fraction of post-meiotic germ cells was measured by flow cytometric analysis on the basis of their DNA content. Round spermatids were discriminated from mature, elongated spermatids by their different degree of chromatin condensation. Butadiene-induced cytotoxic effects on differentiating spermatogonia were shown by a concentration-dependent decrease of round spermatids occurring 21 days after chemical exposure, confirmed by a similar decrease of elongated spermatids measured in testes sampled 7 days later. Statistically significant effects were seen already at 130 ppm. An incomplete repopulation of the elongated spermatid compartment observed 35 days after exposure to 1300 ppm suggested that, at the highest concentration tested, butadiene toxicity extended to stem cells. Alterations of sperm chromatin were revealed by its increased sensitivity to acidic denaturation in situ. The percentage of abnormal sperm was significantly increased after butadiene exposure of differentiating spermatogonia and spermatocytes. This suggested the induction of persistent effects interfering with chromatin remodelling during spermiogenesis. Chromosome-type structural aberrations were significantly elevated in first-cleavage embryos conceived by males mated during the first and second week after the end of exposure. The lowest effective tested concentration was 500 ppm, the same reported for dominant lethal induction under identical exposure conditions. As in the dominant lethal assay, the effect of this dose was confined to exposed sperm, while both sperm and late spermatids were affected by the inhalation of 1300 ppm. A quantitative comparison between the effects induced by intraperitoneal injections of diepoxybutane or butadiene inhalations suggested that other reactive intermediates, in addition to diepoxybutane, might contribute to mediate butadiene-induced reproductive toxicity.

Micronuclei and developmental abnormalities in 4-day mouse embryos after paternal treatment with acrylamide

The developmental consequences of paternal exposure to acrylamide (50 mg/kg i.p. for 5 days) were assessed in preimplantation embryos. There was a significant increase in the proportion of morphologically abnormal embryos after postmeiotic treatment during spermatogenesis (88.7% vs. 14.8% in control). Abnormal embryos had an average of 1.8 +/- 3.5 cells and > 80% had at least one fragmented nucleus. In addition, morphologically normal embryos were significantly delayed (34.3 +/- 12.8 cells per embryo vs. 57.6 +/- 15.7 in control, P < 0.001). Acrylamide caused 10- and 20-fold increases in frequencies of cells with micronuclei (MN) in morphologically normal and abnormal embryos, respectively (41 and 93 MN per 1,000 cells). Both centromere-negative (MN-) and centromere-positive (MN+) were induced. Nuclei of abnormal embryos were significantly larger (900 microm2 vs. 250 microm2) than controls. In addition, MN of abnormal embryos were larger than those of normal embryos (21.2 microm2 vs. 6.5 microm2, P < 0.01). Among control embryos, MN+ were significantly larger than MN- (P < 0.05). These findings suggest that the preimplantation embryo is a sensitive indicator of paternally transmitted effects on early development. Multiple mechanisms appear to be involved, including cytogenetic damage, proliferation arrest/delay, and fertilization failure. Future studies are needed to establish how induced cytological defects in preimplantation embryos contribute to birth defects and other postimplantation abnormalities.

Induction of chromosomal aberrations in mouse zygotes by acrylamide treatment of male germ cells and their correlation with dominant lethality and heritable translocations

The objectives of this research were: 1) to investigate the time course of the cytogenetic defects induced by acrylamide (AA) treatment (5 x 50 mg/kg) of male germ cells in first-cleavage zygote metaphases using PAINT/DAPI analysis, and 2) to characterize the correlation between chromosomal aberrations at first cleavage, dominant lethality, and heritable translocations. PAINT/DAPI analysis employs multicolor fluorescence in situ hybridization painting plus DAPI staining to detect both stable and unstable chromosomal aberrations at first-cleavage metaphase of the zygote. High levels of chromosomally defective zygotes were detected after mating at all postmeiotic stages (20-190-fold, P < 0.001). Early spermatozoa (6.5 d post-treatment) were the most sensitive, with 76% of the zygotes carrying cytogenetic defects. A significant 10-fold increase was also detected 27.5 d post-treatment, indicating that AA had a cytogenetic effect on meiotic stages. PAINT/DAPI analysis revealed that: 1) AA-induced chromosomal breaks occurred at random, and 2) the frequencies of symmetrical and asymmetrical exchanges were similar at all mating days, except 9.5 d after AA treatment, where significantly (P < 0.02) more asymmetrical aberrations were found. Furthermore, the proportions of zygotes carrying unstable and stable chromosomal aberrations followed a similar post-treatment time course as the proportions of dominant lethality among embryos and heritable translocations among offspring. These findings indicate that PAINT/DAPI analysis of zygotic metaphases is a promising method for detecting male germ cell mutagens capable of inducing chromosomal aberrations and for evaluating the associated risks for embryonic loss and balanced translocations at birth.