Chromosome malsegregation and embryonic lethality induced by treatment of normally ovulated mouse oocytes with nocodazole

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.

The adverse outcome pathway (AOP) for chemical binding to tubulin in oocytes leading to aneuploid offspring

The Organisation for Economic Co-operation and Development (OECD) has launched the Adverse Outcome Pathway (AOP) Programme to advance knowledge of pathways of toxicity and improve the use of mechanistic information in risk assessment. An AOP links a molecular initiating event (MIE) to an adverse outcome (AO) through intermediate key events (KE). Here, we present the scientific evidence in support of an AOP whereby chemicals that bind to tubulin cause microtubule depolymerization resulting in spindle disorganization followed by altered chromosome alignment and segregation and the generation of aneuploidy in female germ cells, ultimately leading to aneuploidy in the offspring. Aneuploidy, an abnormal number of chromosomes that is not an exact multiple of the haploid number, is a well-known cause of human disease and represents a major cause of infertility, pregnancy failure, and serious genetic disorders in the offspring. Among chemicals that induce aneuploidy in female germ cells, a large majority impairs microtubule dynamics and spindle function. Colchicine, a prototypical chemical that binds to tubulin and causes microtubule depolymerization, is used here to illustrate the AOP. This AOP is specific to female germ cells exposed during the periovulation period. Although the majority of the data come from rodent studies, the available evidence suggests that the MIE and KEs are conserved across species and would occur in human oocytes. The development of AOPs related to mutagenicity in germ cells is expected to aid the identification of potential hazards to germ cell genomic integrity and support regulatory efforts to protect population health.

The protective effect of dietary Arthrospira (Spirulina) maxima against mutagenicity induced by benzo[alpha]pyrene in mice

Benzo[alpha]pyrene (B[α]P) was used to test the possible antimutagenic effects of Arthrospira (Spirulina) maxima (SP) on male and female mice. SP was orally administered at 0, 200, 400, or 800 mg/kg of body weight to animals of both sexes for 2 weeks before starting the B[α]P (intraperitoneal injection) at 125 mg/kg of body weight for 5 consecutive days. For the male dominant lethal test, each male was caged with two untreated females per week for 3 weeks. For the female dominant lethal test, each female was caged for 1 week with one untreated male. All the females were evaluated 13-15 days after mating for incidence of pregnancy, total corpora lutea, total implants and pre- and postimplant losses. SP protected from B[α]P-induced pre- and postimplant losses in the male dominant lethal test, and from B[α]P-induced postimplantation losses in treated females. Moreover, SP treatment significantly reduced the detrimental effect of B[α]P on the quality of mouse semen. Our results illustrate the protective effects of SP in relation to B[α]P-induced genetic damage to germ cells. We conclude that SP, owing mainly to the presence of phycocyanin, could be of potential clinical interest in cancer treatment or prevention of relapse.

Cell cycle synchronization by nutrient modulation

Living cells respond to changing environments by regulating their genes and activities. In unicellular organisms such as yeasts, the cell division cycle is coupled to the nutrient availability. However, it is unclear how tight this coupling is and how the intrinsic time scales of the different cell cycle processes respond to varying nutrient conditions. Here we study the cell cycle behavior of the budding yeast Saccharomyces cerevisiae in response to periodically modulated nutrient availability, using a microfluidic platform which allows for longtime cultivation, programmed medium switching, and automated time-lapse image acquisition. We observe that the division cycle of the yeast cells can follow a periodically modulated medium so that the whole population can be driven into synchrony. When the period of the nutrient modulation is optimized, as many as 80% of the cells in a population are continuously synchronized. The degree of synchronization as a function of the nutrient modulation period can be qualitatively captured by a stochastic phenomenological model. Our work may shed light on the coupling between the cell growth and cell division as well as provide a nontoxic and non-invasive method to continuously synchronize the cell cycle.

The chemotherapeutic agents nocodazole and amsacrine cause meiotic delay and non-disjunction in spermatocytes of mice

Aneuploidy of germ cells contributes to reduced fertility, foetal wastage and genetic defects. The possible risk of aneuploidy induction by the cancer chemotherapeutic drugs amsacrine (AMSA) and nocodazole (NOC) was investigated in male mice. Two molecular cytogenetic approaches were used: (1) the BrdU-incorporation assay to test the altered duration of meiotic divisions and (2) the sperm-FISH assay to determine aneuploidy induction during meiosis by observing hyperhaploid and diploid sperm. Sperm were sampled from the Caudae epididymes of treated and solvent control males. Single intraperitoneal injections with NOC (35 mg/kg) and AMSA (15 mg/kg) caused a meiotic delay of 24h. The timing of sperm sampling for the sperm-FISH assay was adjusted accordingly, i.e. 23 days after treatment. Mice were treated with 18, 35 and 50 mg/kg of NOC, or 5, 10, 15 and 20 mg/kg of AMSA. Significant dose-dependent increases above the concurrent controls in the frequencies of hyperhaploid sperm were found with both agents. Significant increases in the frequencies of diploid sperm were found only with AMSA. These results provide a basis for genetic counselling of patients under AMSA or NOC chemotherapy. During a period of 3-4 months after the end of chemotherapy, they may stand a higher risk of siring chromosomally abnormal offspring.

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.

Glycogen synthase kinase-3 regulation of chromatin segregation and cytokinesis in mouse preimplantation embryos

Glycogen synthase kinase-3 (GSK-3) is a highly conserved serine/threonine protein kinase implicated in diverse cellular processes. Activity of GSK-3 is essential for meiotic chromatin segregation in oocytes, yet expression and/or function of GSK-3 have not been reported in mammalian preimplantation embryos. Objectives of this study were to characterize GSK-3 protein expression/phosphorylation in mouse preimplantation embryos, to assess the effect of GSK-3 activity inhibition on early mitotic events, and to differentiate nuclear and cytoplasmic anomalies in GSK-3 inhibited embryos. Both GSK-3 isoforms were expressed during embryo development, with a differential expression of alpha versus beta. Phosphorylation of GSK-3alpha/beta at residues Y279/Y216 indicated constitutive activation throughout preimplantation development. Phosphorylation at N-terminal residues S21/S9 indicated inhibition of GSK-3alpha/beta activity that was differentially regulated during early development; both alpha and beta isoforms were phosphorylated during early divisions, whereas at the blastocyst stage, only beta was phosphorylated. Cytoplasmic microinjection of zygotes with anti-GSK-3alpha/beta antibody significantly compromised embryonic development past the two-cell stage compared to controls. Reversibility of developmental block was tested via pharmacological inhibitors of GSK-3, lithium chloride (LiCl) and alsterpaullone. Similar to immunoneutralization, significantly fewer zygotes cultured with either LiCl or alsterpaullone developed past the two-cell stage compared to controls and this mitotic block was not reversible. Inhibition of GSK-3 activity significantly compromised timing of pronuclear membrane breakdown and mitosis initiation, nuclear development, and cytokinesis. Inhibition of GSK-3 also resulted in abnormal chromatin segregation, evidenced by incomplete karyokinesis and micronuclei formation. These results suggest that GSK-3 activity is critical for early preimplantation embryonic development.

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.

Exposure of bovine oocytes to the endogenous metabolite 2-methoxyestradiol during in vitro maturation inhibits early embryonic development

Catecholestrogens are endogenous metabolites that have been shown to modulate granulosa, theca, and luteal cell function in some species. The present study was aimed at determining the possible role of these steroids on oocyte maturation. Cumulus-enclosed bovine oocytes were matured for 24 h, fertilized, and then cultured for 8 days. Whereas estradiol was without effect, addition of catecholestrogens (2-hydroxyestradiol, 4-hydroxyestradiol, and 2-methoxyestradiol [2-MOE2]) to the maturation medium did not affect the cleavage rate but was associated with a decrease in blastocyst production on Day 8. Although 2-MOE2 was also able to inhibit blastocyst formation when added during embryo culture, the effects were less pronounced than those seen when the steroid was added only during maturation. In agreement with the known ability of 2-MOE2 to bind tubulin at the colchicine site, marked alterations were observed in the spindle assembly of oocytes exposed to 2-MOE2 during maturation, which lead to gross chromosomal aberrations after fertilization and consequent developmental arrest at the morula stage. Moreover, that the blastocyst rate was not affected when meiosis was blocked with roscovitine during 2-MOE2 exposure is consistent with the idea that altered nuclear maturation is the cause of the low developmental competence. Because 2-MOE2 could be increased in follicular fluid in response to aryl hydrocarbon-receptor ligands, such as some environmental contaminants, our results show that abnormally high intraovarian levels of catecholestrogens could have a deleterious effect on oocyte maturation and early embryonic development arising from the alterations in the meiotic spindle.

Manipulation of the oocyte: possible damage to the spindle apparatus

Oocytes are structured, polarized cells. For high developmental potential, it is essential that the distribution of organelles and molecules, and the function of meiotic spindles remain intact during handling of oocytes in assisted reproduction. Spindles are dynamic cell organelles. Spindle formation depends on activity of motor proteins, energy supply and temperature. Disturbances in spindle function may predispose oocytes to aneuploidy or maturation arrest. Thus, perturbation of the cytoskeletal integrity of oocytes may critically influence the fate of the embryo. Recently, enhanced polarizing microscopy has been developed for non-invasive analysis of spindle morphology in living mammalian oocytes. Chemically induced dynamic alterations have been characterized in the spindle in individual mouse oocytes and it has been shown that spindle aberrations are predictive of risks for non-disjunction. Spindle imaging identified adverse, irreversible effects of handling in living human oocytes (for instance, the extreme susceptibility of human oocytes to cooling). Also, oocyte immaturity may be detected. Selection of metaphase II oocytes and an injection site for intracytoplasmic sperm injection (ICSI) that avoids spindle damage may increase the yield of euploid embryos. The detection of genetic, environmentally induced, or treatment-related defects in oocyte maturation by non-invasive spindle imaging can improve quality control and assist in the selection of morphologically normal oocytes for assisted reproduction.

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.

Analysis of cytogenetic and developmental effects on pre-implantation, mid-gestation and near-term mouse embryos after treatment with trichlorfon during zygote stage

Trichlorfon has been widely used in agriculture as a broad spectrum insecticide. We examined cytogenetic and developmental effects on early mouse zygotes exposed to trichlorfon in vivo. Pregnant female mice were intraperitoneally administered a single dose of trichlorfon (100 or 200mg/kg) at 6h post presumed conception and either sacrificed on day of gestation (dg) 3, 9 or 17 to assess the developmental toxicity and mutagenic effects on embryos. Mean cell number (dg 3) and somite number (dg 9) of embryos in the two trichlorfon-treated groups were significantly fewer than in the control group and the mean micronucleus (MN) number (dg 3) and the frequency of mosaic aneuploidies including monosomic or trisomic cell lines (dg 9) was significantly increased in both trichlorfon-treated groups compared with the control group. However, there was no difference in fetal body weight (dg 17) between the control and trichlorfon-treated groups and no increased incidence of external malformations was observed in the trichlorfon-treated groups. These findings suggest that acute exposure of trichlorfon around fertilization induces a high frequency of MN, mosaic aneuploidies and developmental retardation in pre-implantation and mid-gestation embryos, and thereafter these embryos with MN or chromosome damage appear to develop past mid-gestation and catch up with normal embryos by near-term.

Acute exposure of female hamsters to carbendazim (MBC) during meiosis results in aneuploid oocytes with subsequent arrest of embryonic cleavage and implantation

A single oral dose of the fungicide and microtubule poison, MBC, administered to female hamsters at proestrus, results in infertility and early pregnancy loss (1). To characterize the site and mode of action of this effect, direct assessments of oocyte chromosomes, fertilization, and preimplantation embryo development were made. Female hamsters were given a single dose of MBC (1,000 mg/kg) on the afternoon of proestrus (to coincide with meiotic maturation of the oocytes) and either killed shortly after ovulation (day 1) to recover oocytes, or bred and killed on gestation day (gd) 1 to 5 of pregnancy to assess fertilization and preimplantation embryo development and enumerate early implantation sites. Chromosome analysis in unfertilized oocytes revealed an MBC-induced increase in aneuploidy (37 vs. 14% in controls). When animals were bred after dosing, MBC had no effect on the number of oocytes recovered or fertilized. However, significant increases were found in the proportion of embryos that failed to reach the expected stage of development, namely, the eight-cell stage on the afternoon of gd 3, the morula stage by the morning of gd 4, and the blastocyst stage by the afternoon of gd 4 (a time when some embryos have implanted). The mean number of implantation sites, revealed by Evans Blue staining, was also significantly lower in treated females on the afternoon of gd 4 and the morning of gd 5. These simple direct assessments elucidated a mechanism of MBC-induced early pregnancy loss, induction of aneuploidy in oocytes. They also ruled out an effect on fertilization, but demonstrated a subsequent arrest of preimplantation embryonic development accompained by a decrease in the likelihood of implantation.

Aneuploidy in germ cells: disruption of chromosome mover components

The task of the Workgroup on "Disruption of Chromosome Mover Components" was to establish what cellular structures are involved in chromosome segregation and how disruption of these could occur. Recent research on the mechanism of action of the cellular components that segregate chromosomes accurately during mitosis or meiosis has served to highlight the number of potential targets for disruption. The process of chromosome segregation represents an orchestrated chain of events centered on the activities of cellular motors, kinesins and dyneins. These motors are involved in arranging chromosomes at the metaphase plate, providing the spindle tension necessary for progression, and the actual segregation of the chromosomes to the poles. The Workgroup determined that there is a lack of information on the effects of chemical exposure to cell motors and other chromosome mover components, and that there is a clear need for further research. This article describes the discussions of the Workgroup and highlights areas of future research into chromosome movement, particularly in human meiotic and mitotic cells. The Workgroup emphasized that obtaining mechanistic data on the induction of aneuploidy will allow for extrapolation of the dose response curves for chemical exposures below the level of observation and for using aneuploidy data for quantitative risk assessment for adverse health effects.

Carbendazim (MBC) disrupts oocyte spindle function and induces aneuploidy in hamsters exposed during fertilization (meiosis II)

Peri-fertilization exposure to Carbendazim (MBC; a microtubule poison) induces infertility and early pregnancy loss in hamsters. Presently, both in vivo and in vitro techniques were employed to characterize the effects of MBC on cellular aspects of fertilization in hamsters. Exposure to MBC during either in vivo or in vitro fertilization (IVF) induced identical morphological abnormalities in the maternal chromatin of zygotes and embryos. These abnormalities included either multiple second polar bodies (PB2), and/or multiple small female pronuclei (PN), or meiotic arrest. Multiple PB2, multiple female PN, multiple PB2 with multiple female PN, or meiotic arrest were exhibited by approximately 31%, 15%, 12%, and 2% of the in vivo zygotes; and 3%, 16%, 36%, and 20% of IVF zygotes, respectively. The effects of MBC persisted to day 2 of pregnancy as indicated by decreased (P < 0.05) embryo development to the two-cell stage and the presence of micronuclei in 6% of two-cell embryos from MBC-treated females. Immunofluorescence analysis of microtubules (MTs) confirmed that MBC disrupted spindle MTs during IVF. Numerical chromosome analysis revealed that a single dose of MBC administered during in vivo fertilization induced aneuploidy in the resulting pronuclear-stage zygotes. The present data point to two mechanisms by which peri-fertilization MBC exposure may induce early pregnancy loss: 1) arrested meiosis with no zygotic cleavage; or 2) induction of zygotic aneuploidy with subsequent developmental arrest.

Pattern and frequency of nocodazole induced meiotic nondisjunction in oocytes of mice carrying the 'tobacco mouse' metacentric Rb(16.17)7Bnr

Oocytes from (C3H/HeH x 101/H)F1 and Rb(16.17)7Bnr homozygous females were exposed to a range of doses of nocodazole in vitro. The spindle poison caused a dose dependent increase in metaphase I (MI) arrest and hyperploidy. A concentration of 0.03 microgram/ml was found to induce a maximum hyperploid frequency of 3.1% and 11.6% respectively without a high level of MI arrest. Between 0.03 and 0.05 microgram/ml MI arrest increased substantially and reached a frequency of approximately 90%. In a further experiment oocytes from Rb7 homozygous, heterozygous and 3H1 females were exposed to 0.03 microgram/ml nocodazole 4, 6 or 8 h after the onset of maturation. The phase at which the spindle was inhibited resulted in a specific pattern of nondisjunction which in turn was dependent on whether the female carried an Rb metacentric. 3H1 oocytes gave a normally distributed pattern of increase in aneuploid frequency (over the spontaneous value) centering around a 6 h application. This was thought to be due to the interaction of chromosomes with the microtubules of the spindle during attachment and/or alignment. In contrast both Rb homozygotes and heterozygotes gave the same biphasic response, with a high frequency of aneuploidy in the oocytes when nocodazole was applied 4 and 8 h after the onset of maturation. In Rb homozygotes we demonstrated that the Rb bivalent underwent nondisjunction more frequently than the average acrocentric, when nocodazole was administered early.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemically-induced aneuploidy in mammalian oocytes

The ability of certain chemicals to elevate the frequency of aneuploidy above spontaneous levels in mammalian experimental models prompts the concern that a similar situation might exist in humans. Validation of experimental models for aneuploidy studies is in progress since there is much to be learned about the causes and mechanisms of chemically-induced aneuploidy. Several biological variables have been shown to influence the results from aneuploidy assays. In this review, we examine these variables as they relate to female germ cell aneuploid assays. Also, we have found that the aneuploidy results obtained from different cell types, sexes, and experimental models cannot necessarily be expected to agree due to certain anatomic and physiologic differences and the end points measured.

Fertility, reproduction, and genetic disease: studies on the mutagenic effects of environmental agents on mammalian germ cells

Because genetically based diseases have a major impact on human health, the National Institute of Environmental Health Sciences (NIEHS) has conducted a research and testing program for more than a decade to address chemical induction of heritable genetic damage in the germ cells of mammals. Although most genetic disease results from preexisting mutations, a portion is due to the occurrence of new mutations. The supposition that exposure to mutagenic chemicals contributes to the occurrence of new mutations in the human population is strongly supported by the results from animal models. Such studies clearly demonstrate the potential of environmental chemicals to induce mutations in both somatic and reproductive cells of mammals. This NIEHS program has become a leader in the identification of genetic hazards in the environment and in the acquisition of animal model data used by regulatory agencies in assessing genetic risks to human health.

Maternal age effect: the enigma of Down syndrome and other trisomic conditions

Aneuploidy is the most frequently observed chromosome abnormality in human liveborn, abortuses and oocytes. The only etiological factor that has been established is advanced maternal age for the occurrence of trisomies, particularly trisomy 21 which causes Down syndrome. The maternal age effect remains an enigma. Recent molecular data bearing on this question are reviewed as are the hypotheses that have been proposed linking nondisjunction and maternal age. Rationale is presented for a compromised microcirculation hypothesis that explains the cause of nondisjunction and why its occurrence changes with maternal age from menarche to menopause. It takes into account two facts: (1) 95% of Down syndrome children receive their extra chromosome from their mother, and in 80% or more of these the nondisjunction occurred in the first meiotic division, which is completed in the ovary. (2) The ovarian follicle containing the primary oocyte has no internal circulation. The hypothesis proposes that aneuploid oocytes arise from a concatenation of events. It begins with hormonal imbalance that causes a less-than-optimal microvasculature to develop around the maturing and mature follicles. The resulting decrease in the size of the perifollicular capillary bed reduces the volume of blood flow through the area, leading to an oxygen deficit and a concomitant increase inside the follicle of carbon dioxide and anaerobic products, such as lactic acid. This in turn causes a decrease in the intracellular pH of the oocyte that diminishes the size of the spindle, with consequent displacement and nondisjunction of a chromosome. The compromised microcirculation hypothesis explains the occurrence of aneuploidy in primary and secondary oocytes, sperm precursor cells, tumor and embryonic cells. It also explains why women of all reproductive ages may have a Down syndrome child.

Chromatin remodeling in mammalian zygotes

With sperm-egg fusion at the time of fertilization the gamete nuclei are remodeled from genetically quiescent structures into pronuclei capable of DNA synthesis. Features of this process that are critical to insure the genetic integrity of the zygote and the success of subsequent embryonic development include: oocyte responses that prevent polyspermy; completion of the 2nd meiotic division by the oocyte; exchange of proteins in the sperm nucleus; and, remodelling of the oocyte chromosomes and sperm nucleus into functional pronuclei. Elucidation of the biological and molecular mechanisms underlying zygote formation and chromatin remodeling should enhance our understanding of the potential vulnerability of the zygote to toxicant-induced damage.

Use of the fungicide carbendazim as a model compound to determine the impact of acute chemical exposure during oocyte maturation and fertilization on pregnancy outcome in the hamster

Early pregnancy loss due to acute chemical exposure is difficult to detect and essentially impossible to characterize in humans. Here we use a hamster animal model to identify early pregnancy loss due to an acute chemical exposure to the female during the perifertilization interval. The fungicide carbendazim (methyl 1H-benzimidazole-2-carbamate), a microtubule poison with antimitotic activity, was selected as a model compound because it would be expected to perturb microtubule-dependent events occurring in the oocyte during meiotic maturation and fertilization. Such effects would likely lead to aneuploidy in the zygote with subsequent early pregnancy loss. Female hamsters were given a single oral dose of carbendazim during meiosis I (the afternoon of proestrus) prior to breeding, or during meiosis II (the morning of estrus) following overnight breeding. Pregnancy outcome was assessed on Day 15 (the afternoon before parturition). When given during during meiosis I, carbendazim treatment (750 or 1000 mg/kg body weight) significantly reduced the percentage of pregnant hamsters. In those animals that became pregnant, the average number of live pups was significantly lower at all dosages of carbendazim used (250, 500, 750, and 1000 mg/kg), an effect attributable to both preimplantation and early postimplantation losses. When given early on the morning of estrus, shortly before and during fertilization (0500 or 0600 hr), carbendazim treatment (1000 mg/kg) produced a similar decrease in litter size. This effect disappeared when carbendazim was administered at a slightly later time (0800 or 0900 hr), after the microtubule-dependent events of fertilization have occurred. These results demonstrate that a single exposure to a microtubule poison such as carbendazim at critical times, coincident with microtubule-dependent meiotic events, can result in very early pregnancy loss. Such loss was readily measurable in this animal model and serves as the basis for further mechanistic studies which would be impossible to conduct in humans.

Effects of benzimidazole analogs on cultures of differentiating rodent embryonic cells

Micromass cell culture systems for rat embryo midbrain (CNS) and limb bud (LB) cells were employed to assess the in vitro developmental toxicity of the benzimidazole analogs, mebendazole (MBZ), thiabendazole (TBZ), and nocodazole (NCZ), in addition to the classic microtubule inhibitor, colchicine. Comparison was made to albendazole (ABZ), a previously studied benzimidazole anthelmintic. Two parameters for assessing developmental toxicity were measured: differentiation and cytotoxicity. The relative potencies of the benzimidazole analogs in the micromass system (NCZ greater than MBZ approximately ABZ much greater than TBZ) mirrored their effectiveness in an assay for in vitro inhibition of mammalian tubulin polymerization. Colchicine also exhibits a high affinity for mammalian tubulin and was a potent inhibitor of cell proliferation, chondrogenesis, and neuronal differentiation. Immunofluorescent staining of Day 1 LB cultures with a monoclonal antibody to beta-tubulin revealed that these agents elicited mitotic arrest. Many anti-tubulin agents are teratogenic in rats and their in vivo developmental toxicity may reflect perturbation of microtubular structure or function. With the exception of TBZ, these agents should be considered potential developmental toxicants since they inhibit cell growth and differentiation of micromass cultures at nanomolar concentrations.

Cytogenetic analysis of mouse oocytes and one-cell zygotes as a potential assay for heritable germ cell aneuploidy

Assays are needed for detecting chemically-induced aneuploidy, for investigating the mechanisms of aneuploidy production, and for obtaining heritable germ cell data that can be used to formulate human risk estimates. In this report, we describe the results of experiments designed to study aneuploidy in metaphase II (MII) oocytes induced by intraperitoneal (i.p.) or oral dosages of colchicine, and to investigate the proportion of aneuploid oocytes transmitted to one-cell (1C) zygotes following oral administration of colchicine immediately following HCG. The proportions (and percentages) of hyperploid MII oocytes were: 1/606 (0.2), 37/504 (7.3), 152/731 (20.8) and 75/319 (23.5) for control, 0.2, 0.3 and 0.4 mg/kg, respectively for i.p. administration of colchicine; and 3/216 (1.4), 8/539 (1.5), 81/511 (15.9), 71/398 (17.8) and 98/391 (25.1) for control, 1.0, 2.0, 3.0 and 4.0 mg/kg, respectively for oral administration of colchicine. The proportions of hyperploid 1C zygotes were 2/327 (0.6), 21/389 (5.4), 62/435 (14.3) and 69/438 (15.8) for control, 2.0, 3.0 and 4.0 mg/kg, respectively for oral colchicine. The proportions of hyperploid MII oocytes and 1C zygotes were significantly higher (Chi-square, P less than 0.01) at each i.p. or oral dose (except 1.0 mg/kg oral) than in the controls. The frequencies of hyperploidy induced by oral doses of colchicine were greater in MII oocytes than in 1C zygotes. We also found that the frequency of developmentally delayed and polyploid 1C zygotes increased with the dose of oral colchicine. Developmentally delayed zygotes contained male-derived chromosomes and female-derived fragmented pronuclei and pronuclei with decondensed chromosomes. These results indicate that higher doses of oral colchicine are needed to induce comparable levels of aneuploidy found after i.p. administration, and that aneuploid oocytes are fertilized and reach first cleavage metaphase. In addition, colchicine induces a spectrum of events including aneuploidy, polyploidy and developmentally delayed oocytes and zygotes.