Aneuploidy in mouse metaphase II oocytes exposed in vivo and in vitro in preantral follicle culture to nocodazole

Aneuploidy tests are important in evaluating genetic hazards especially when chemical exposures are suspected to affect the fidelity of chromosome segregation in oocytes and embryos. In the current study, a newly established method, mouse preantral follicle culture, was employed to grow oocytes in vitro within follicles. The sensitivity of in vitro grown follicle enclosed oocytes was compared with oocytes maturing in vivo in the ovary. In both the cases, oocytes were exposed to the cytostatic chemical, nocodazole, from the time of hormonally stimulated resumption of meiosis. The in vivo study revealed a significant decrease in the number of ovulated mouse oocytes and an increase in meiosis I-arrested and hyperploid metaphase II oocytes at a single i.p. dose of 70 mg/kg body weight of nocodazole. A significant increase was also observed in the number of meiosis I-arrested and hyperploid mouse oocytes from preantral follicle culture, when they were cultured in the presence of >or=30 nM nocodazole during the final stages of maturation. This concentration is slightly lower than that previously shown to induce nondisjunction in denuded mouse oocytes or in cultured human lymphocytes. The higher sensitivity of the in vitro matured oocytes from preantral follicle culture than that of denuded oocytes may be related to a synergistic adverse influence of nocodazole on the oocyte, on somatic cell integrity and on cell-cell communication, which possibly also affects ovulation in vivo. When expressed in molarity relative to the mouse weight, the effective dose of the acute exposure in vivo is 3-4 orders of magnitude higher than the lowest effective concentration employed continuously in vitro. Reduced bioavailability of nocodazole to the target cells due to its poor water solubility may contribute to this difference. Preantral follicle culture can be helpful in analysing mechanisms in chemically induced aneuploidy in mammalian oogenesis, and in predicting the consequences of chemical exposures in vivo.

Nondisjunction rates of mouse specific chromosomes involved in heterozygous Rb rearrangements measured by chromosome painting of spermatocytes II. I. The effects of the number of trivalents

Dual-colour FISH painting with alternative fluorescent chromosome-specific probes allowed us to distinguish chromosomes 1, 4, 6 and 14. The purpose was to check whether nondisjunction rates of specific chromosomes involved in heterozygous Robertsonian fusions are independent of the number of trivalents, or an epistatic effect among Rb chromosomes takes place affecting nondisjunction rates. Probes were used on DAPI-stained metaphases of spermatocytes II of laboratory strains of mice with reconstructed karyotypes heterozygous for one, two, three or four Robertsonian metacentrics in an all-acrocentric background. The existence of such epistatic interactions was not verified.

Melphalan-induced DNA damage in p53(+/-) and wild type mice analysed by the comet assay

Melphalan is an alkylating substance used as a therapeutic agent; its mutagenicity is related to its ability to produce monoadducts and to form DNA cross-links. The alkaline comet assay is a useful test for the detection of DNA lesions. However, cross-links are not easily detected under standard conditions. Recently, modifications to the test have been introduced to measure cross-links by evaluating the reduction in induced DNA migration. In this work, the standard comet assay and an assay modified by prolonging the electrophoresis time have been applied to evaluate DNA lesions induced by single, 4 or 26 weekly oral administrations of melphalan to p53(+/-) knockout and to isotype parental mice. Cells were analysed from the liver, bone marrow, peripheral blood and the distal intestine. Moreover, a further protocol in which the presence of cross-links was inferred by the reduction in X-ray-induced DNA migration was applied to bone marrow cells and the sensitivity of the different methods was compared. The majority of groups examined by the standard protocol showed no difference compared to controls, while the modified protocol (prolonged electrophoresis time) could detect a retarded DNA migration in cells from all the organs analysed with the exception of bone marrow cells. Only the protocol based on X-ray in vitro irradiation showed the presence of melphalan-induced cross-links in bone marrow cells exposed to 2mg/kg for 4 weeks, demonstrating that this was the most sensitive approach for detecting this type of lesion. DNA lesions were evident in all the organs analysed. However, results suggest that the kinetics of cross-link repair could be different in bone marrow cells compared to other organs tested. After comparison between genotype-matched treated and control groups, a significant effect was shown more frequently in p53(+/-) than in wild type groups.

Evaluation of chromosome painting to assess the induction and persistence of chromosome aberrations in bone marrow cells of mice treated with benzene

Fluorescence in situ hybridization with chromosome-specific painting probes (FISH painting) has been successfully applied to detect radiation-induced stable aberrations in humans and mice, whereas a few mouse studies with chemicals mostly failed to show any increase in chromosome-painting-detectable changes, especially in bone marrow cells. To further explore the feasibility of the painting approach to detect chemically induced stable aberrations, we treated mice with a single high dose of benzene, a potent bone-marrow-targeting clastogenic chemical and sacrificed them 24, 36 h or 15 days later to collect bone marrow cells and analyze chromatid- and chromosome-type aberrations by FISH painting. In addition, we treated another group of mice with 18 daily low doses to show the potential for aberration induction and accumulation under chronic exposure. Chromatid-type aberrations were significantly increased 24 and 36 h after acute treatment while chromosome-type ones were elevated above control values 36 h and 15 days after exposure, showing that at least part of benzene-induced chromatid exchanges were converted into potentially stable chromosome aberrations. The most common aberration was an extra copy of one painted chromosome in a metaphase with the euploid number of centromeres which was interpreted as the consequence of a symmetric recombination between pericentromeric regions of one painted and one unpainted chromatid. Under chronic exposure, neither chromosome- nor chromatid-type aberrations were significantly elevated over control values, suggesting that the probability of enough primary lesions and secondary DNA double strand breaks occurring close enough together in time to allow chromosome exchanges to form is a critical limiting factor especially in a cycling cell population.

Cosegregation of Robertsonian metacentric chromosomes in the first meiotic division of multiple heterozygous male mice as revealed by FISH analysis of spermatocyte II metaphases

Contrasting results (random segregation or cosegregation of isomorphic chromosomes) have been reported up to now on the segregation pattern of Robertsonian metacentric chromosomes of Mus musculus domesticus in multiple heterozygotes, using different approaches (karyotypical analysis of the progeny or of second meiotic metaphases). In the present contribution data are presented based on FISH (Fluorescence In Situ Hybridisation) analysis with telomeric probes, which allowed us to distinguish metacentric chromosomes from pairs of acrocentric chromosomes with their centromeric regions close to each other. Probes were hybridized to DAPI stained metaphases of spermatocytes II of mice heterozygous for two, three or four Robertsonian metacentrics in an all-acrocentric background, the karyotype of which has been reconstructed starting from laboratory strains. Isomorphic chromosomes tend to cosegregate (metacentrics with metacentrics, acrocentrics with acrocentrics); the values found for cosegregation have a clear even if moderate effect on the reproductive isolation caused by underdominant chromosomal rearrangements.

Effects of lindane on oocyte maturation and preimplantation embryonic development in the mouse

Lindane, an organochlorine insecticide, is suspected of preimplantation embryonic toxicity based on in vitro experiments with bovine and murine embryos. To verify this hypothesis in vivo we tested lindane for developmental alterations during early embryonic cleavage in the mouse. Two treatment schedules were tested: three daily doses of 15 or 25mg/kg b.w. lindane were orally administered to female mice either before mating or immediately after mating. Morphologic alterations (lysis or fragmentation of blastomeres, developmental arrest) of two-cell embryos and morulae were evaluated by inverted microscopy. In addition, cytologic abnormalities and cell proliferation delay, possibly induced during the first four cleavage cycles, were evaluated by fluorescent microscope analysis of the number and morphology of blastomere nuclei. A statistically significant increase of degenerating two-cell embryos was induced by exposure of preovulatory oocytes to the highest tested lindane dose. Early cleavage embryos exposed to the same dose showed a lower average number of blastomeres per morula, as well as a 40% reduction of the mitotic index with respect to matched controls. However, mean values in individual litters were variable and litter analysis did not show a lindane-related effect. One possible mechanism for the observed effects could be the recently demonstrated inhibitory action of lindane on gap junction-mediated cell communication between oocyte and cumulus cells. A comparison between human exposure levels and experimental doses based on measured and predicted blood concentrations suggests that there are ample margins of safety for human embryonic development at the present exposure levels.

Cytogenetic biomonitoring of workers from laboratories of clinical analyses occupationally exposed to chemicals

A cytogenetic monitoring study was carried out on a group of workers in clinical analysis laboratories to investigate the risk of occupational exposure to chronic low levels of chemicals.Thirty-four clinical laboratories have been involved in the study. In these laboratories, toxicants and analytical procedures utilized have been characterized. The individual occupational exposure of workers was assessed by use of a questionnaire concerning the chemical substances utilized. About 300 different chemicals have been identified. Cytogenetic analyses (chromosomal aberration and micronucleus tests) were carried out on a strictly selected group of 50 workers enrolled from these laboratories and compared to 53 controls (healthy blood donors) matched for gender and age. The exposed group shows a significantly higher frequency of genetic damage than the control group. Both chromatid and chromosome aberration frequencies in workers appear significantly higher than in controls. Similarly, comparison between micronucleated cells rates of exposed and unexposed groups show significantly higher frequencies of binucleated cells with micronucleus (BNMN) and of total micronuclei (MN tot) in workers than in controls.

Griseofulvin induces mitotic delay and aneuploidy in bone marrow cells of orally treated mice

Griseofulvin (GF) is a fungicide drug well characterized for its aneugenic activity in vitro. In vivo strong evidence of aneuploidy and polyploidy induction has been obtained in germ cells, especially in oocytes. More controversial are the data on the aneugenicity of griseofulvin in somatic cells. In this paper we provide evidence that GF induces non-disjunction and cell cycle delay in bone marrow cells of orally treated mice. Adult female mice were administered olive oil suspensions of 200, 666 or 2000 mg/kg GF by gavage and killed 18 or 24 h later. To minimize animal-to-animal variation in the absorption and distribution of GF, mice were fasted from the time of GF administration to the time of killing. Two hours before treatment the animals were s.c. implanted with a bromodeoxyuridine tablet to obtain differential chromatid staining and to determine the number of divisions after GF treatment for each metaphase. Mitostatic effects of GF were assessed by the relative proportions of first, second and third generation metaphases and the average generation time (AGT) method. A statistically significant increase with respect to the control AGT value was observed after treatment with 666 and 2000 mg/kg, suggesting that GF, as already shown in meiosis, interfered with cell cycle progression. Hyperploidy was scored in second generation metaphases. Eighteen hours after treatment, the frequencies of hyperploid cells were significantly (P < 0.05) higher in all GF-treated groups than in their matched control group. The effect was not dose-dependent. No further increase in aneuploidy was observed at 24 h, suggesting that cells overcoming mitotic arrest did not have a higher rate of non-disjunction. No induction of polyploidy was demonstrated. We conclude that GF induces mitotic delay and aneuploidy in mouse bone marrow and suggest that the protocol used to formulate the gavage suspensions and the after-treatment fasting of the animals enhanced the bioavailability of GF to bone marrow cells.

Genotoxic effects of butadiene in mouse lung cells detected by an ex vivo micronucleus test

Lung fibroblasts from BD-exposed mice have been analysed for the occurrence of micronuclei. Primary cultures set up 24h after the end of exposure were treated with cytochalasin B and micronuclei scored in binucleate cells. A three-fold statistically significant increase of micronucleated cells was detected after exposure to 500ppm, the lowest tested concentration. A linear dose effect relationship was observed between 500 and 1300ppm. Immunofluorescent staining of kinetochore proteins was applied to distinguish between acentric micronuclei produced by chromosome breaks and micronuclei containing a centromeric region, most likely induced by chromosome loss. A statistically significant increase of both types of MN in 1300ppm-exposed females and a significant increase in centromeric MN in 500ppm-exposed males were detected. These data demonstrate that an intermediate of BD metabolism with a potential for clastogenic and aneugenic effects is active in lung cells after inhalation exposure. These effects can play a role in the initiation and promotion of BD-induced lung tumours.

Butadiene diolepoxide- and diepoxybutane-derived DNA adducts at N7-guanine: a high occurrence of diolepoxide-derived adducts in mouse lung after 1,3-butadiene exposure

Butadiene (BD) is a high production volume chemical and is known to be tumorigenic in rodents. BD is metabolized to butadiene monoepoxide (BMO), diepoxybutane (DEB) and butadiene diolepoxide (BDE). These epoxides are genotoxic and alkylate DNA both in vitro and in vivo, mainly at the N7 position of guanine. In this study, a 32P-post-labeling/thin-layer chromatography (TLC)/high-pressure liquid chromatography (HPLC) assay for BDE and DEB adducts at the N7 of guanine was developed and was used in determining the enantiomeric composition of the adducts and the organ dose of BD exposure in lung. Exposure of 2'-deoxyguanosine (dGuo), 2'-deoxyguanosine-5'-phosphate (5'-dGMP) and 2'-deoxyguanosine-3'-phosphate (3'-dGMP) to racemic BDE followed by neutral thermal hydrolysis gave two products (products 1 and 2) that were identified by MS and UV and NMR spectroscopy as a diastereomeric pair of N7-(2,3,4-trihydroxybutan-1-yl)-guanines. Exposure of dGuo nucleotides to RR/SS DEB (also referred to as dl DEB) followed by thermal depurination resulted in a single product coeluting with the BDE product 1. If the reaction mixture of BDE and 5'-dGMP was analyzed by HPLC before hydrolysis of the glycosidic bond, four major nucleotide alkylation products (A, B, C and D) with identical UV sepectra were detected. The products were isolated and hydrolyzed, after which A and C coeluted with product 1 and B and D coeluted with the product 2. The major adduct of DEB-exposed 5'-dGMP was N7-(2-hydroxy-3,4-epoxy-1-yl)-dGMP (product E). A 32P-post-labeling assay was used to detect BDE- and DEB-derived N7-dGMP adducts in DNA. Levels of adducts increased with a dose of BDE and DEB and exhibited a half life of 30 +/- 3 (r = 0.98) and 31 +/- 4 h (r = 0.95), respectively. Incubation of DEB-modified DNA at 37 degrees C at neutral pH for up to 142 h did not lead to an increase of N7-(2,3,4-trihydroxybutan-1-yl)-dGMP in the DNA. These observations led to the conclusion that the N7-(2,3, 4-trihydroxybutan-1-yl)-dGMP adducts in DNA can be used as a marker of BDE exposure and that N7-(2-hydroxy-3,4-epoxy-1-yl)-dGMP adducts are related to DEB exposure. Dose-related levels of BDE- and DEB-derived adducts were detected in lungs of mice inhaling butadiene. Most of the N7-dGMP adducts (73%; product D) were derived from the 2R-diol-3S-epoxide of 1,3-butadiene. The data presented in this paper indicate that in vivo, 98% of N7-dGMP alkylation after BD exposure is derived from BDE, and approximately 2% of the adducts were derived from DEB and BMO.

Measurement and characterization of micronuclei in cultured primary lung cells of mice following inhalation exposure to benzene

The genotoxic effects of benzene in lung cells of mice exposed to single acute doses by inhalation have been estimated by cytogenetic analysis of micronuclei in primary cultures of lung fibroblasts. Mice were nose-only exposed to 1000 p.p.m. for 30 or 60 min or to 3500 p.p.m. for 30 min and sacrificed 24 h after the end of exposure. Lung fibroblasts were cultured attached to coverslips for 72 h, the last 48 h in the presence of 0.75 microgram/ml cytochalasin B. Micronuclei were scored in binucleate cells. The mechanism(s) of micronucleus induction was characterized by immunofluorescent staining of kinetochore proteins (CREST staining), which allowed micronuclei due to chromosome loss (kinetochore-positive) to be distinguished from those produced by chromosome breakage (kinetochore-negative). Three- and 4-fold statistically significant increases in total micronucleus frequencies were observed in all benzene-exposed mice with respect to unexposed controls. The effect was neither concentration nor time dependent. This is compatible with a plateau dose-effect relationship for the effects on bone marrow, which is explained by saturation of metabolism. Both chromosome loss and chromosome breakage appear to contribute to micronucleus formation, suggesting that in addition to chromosome rearrangements, aneuploidy may be a relevant early genotoxic event associated with benzene carcinogenicity. Under the same treatment conditions no micronucleus induction could be shown in spleen lymphocytes, suggesting that with very short benzene exposures cells at the first contact site with local metabolizing capacity have a higher probability of genetic alterations potentially leading to neoplasia.

Analysis of micronuclei and DNA single-strand breaks in mouse splenocytes and peripheral lymphocytes after oral administration of tetramethylthiuram disulfide (thiram)

The fungicide thiram (tetramethylthiuram disulfide, TMTD) was administered by repeated oral intubations to groups of male B6C3F1 mice at 100, 300 and 900 mg/kg body weight for 4 consecutive days, or at 300 mg/kg for 8 and 12 days. 24 hr after the last treatment animals were killed, and splenocyte cultures were set up for the analysis of micronuclei by the cytokinesis-block method. DNA single strand breaks (ssb) and alkali labile sites were also analysed by the single cell gel electrophoresis (Comet) assay in splenocytes and lymphocytes of animals receiving the 8- and 12-day treatments. Parallel experiments with human peripheral lymphocytes were carried out to assess the ability of thiram to induce micronuclei and DNA ssb and alkaline labile sites under in vitro conditions. No significant increase of micronucleated splenocytes was observed in treated animals, despite some evidence of treatment-related cellular toxicity. A borderline excess of DNA damage was suggested by the Comet assay on circulating lymphocytes, whereas negative results were obtained with splenocytes. In vitro, positive results with both genetic end points were obtained in assays with human lymphocytes in the dose ranges 0.5-24 microg/ml and 0.1-8 microg/ml for micronucleus and Comet assays, respectively. These results suggest that thiram, despite its established genotoxicity in vitro, is devoid of appreciable clastogenic and/or aneugenic activity in vivo after oral administration to mice at the maximum tolerated dose.

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.

Diepoxybutane cytotoxicity on mouse germ cells is enhanced by in vivo glutathione depletion: a flow cytometric approach

Diepoxybutane is one of the key metabolites of butadiene, a compound of high environmental and occupational concern. The effects of diepoxybutane on mouse reproductive cells have been previously characterized by flow cytometry demonstrating a specific, dose-dependent cytotoxicity for differentiating spermatogonia. It is known that butadiene epoxides, deriving from butadiene bioactivation by cytochrome P450-monooxygenase systems, can be enzymatically conjugated to glutathione by glutathione S-transferases. In this paper, we tested the hypothesis whether a pretreatment with phorone, a well-known intracellular glutathione depleter, would enhance the germ cell cytotoxicity of diepoxybutane. Results were consistent with an active role played in vivo by the glutathione-detoxifying system, as diepoxybutane cytotoxicity was increased after chemically induced reduction of glutathione concentration.

Genetic effects of 1,3-butadiene and associated risk for heritable damage

A summary of the results of the studies conducted in the EU Project "Multi-endpoint analysis of genetic damage induced by 1,3-butadiene and its major metabolites in somatic and germ cells of mice, rats and man" is presented. Results of the project are summarized on the detection of DNA and hemoglobin adducts, on the cytotoxic and clastogenic effects in somatic and germinal cells of mice and rats, on the induction of somatic mutations at the hprt locus of experimental rodents and occupationally exposed workers, on the induction of dominant lethal mutations in mice and rats, and on heritable translocations induced in mice, after exposure to butadiene (BD) or its major metabolites, butadiene monoepoxide (BMO), diepoxybutane (DEB) and butadiene diolepoxide (BDE). The primary goal of this project was to collect experimental data on the genetic effects of BD in order to estimate the germ cell genetic risk to humans of exposure to BD. To achieve this, the butadiene exposure are based on data for heritable translocations and bone marrow micronuclei induced in mice and chromosome aberrations observed in lymphocytes of exposed workers. A doubling dose for heritable translocations in human germ cells of 4900 ppm/h is estimated, which, assuming cumulative BD exposure over the sensitive period of spermatogenesis, corresponds to 5-6 weeks of continuous exposure at the workplace to 20-25 ppm. Alternatively, the rate of heritable translocation induction per ppm/h of BD exposure is estimated to be approximately 0.8 per million live born, compared to a spontaneous incidence of balanced translocations in humans of approximately 800 per million live born. These estimates have large confidence intervals and are only intended to indicate orders of magnitude of human genetic risk. These risk estimates are based on data from germ cells of BD-exposed male mice. The demonstration that clastogenic damage was induced by DEB in preovulatory oocytes at doses which were not ovotoxic implies that additional studies on the response of mammalian female germ cells to BD and its metabolites are needed. The basic assumption of the above genetic risk estimates is that experimental mouse data obtained after BD exposure can be extrapolated to humans. Several points exist in the present report and in the literature which contradict this assumption: (1) the level of BMO-hemoglobin adducts was significantly elevated in BD-exposed workers; however, it was considerably lower than would have been predicted from comparable rat and mouse exposures; (2) the concentrations of the metabolites DEB and BMO were significantly higher in mouse than in rat blood after BD exposure. Thus, while metabolism of BD is qualitatively similar in the two species, it is quantitatively different; (3) no increase of HPRT mutations was shown in 19 workers exposed on average to 1.8 ppm of BD, while in a different population of workers from a US plant exposed on average to 3.5 ppm of BD, a significant increase of HPRT variants was detected; and (4) data from cancer bioassays and cancer epidemiology suggest that rat is a more appropriate model than mouse for human cancer risk from BD exposure. However, the dominant lethal study in rats gave a negative result. At present, we do not know which BD metabolite(s) may be responsible for the genetic effects even though the bifunctional alkylating agent DEB is the most likely candidate for the induction of clastogenic events. Unfortunately, methods to measure DEB adducts in hemoglobin or DNA are only presently being developed. Despite these several uncertainties the use of the mouse genetic data is regarded as a justifiable and conservative approach to human genetic risk estimation given the considerable heterogeneity observed in the biotransformation of BD in humans.

DNA adducts in mouse testis and lung after inhalation exposure to 1,3-butadiene

1,3-Butadiene is a widely used industrial chemical and also an environmental contaminant. Recent findings have shown that butadiene can also be a male germ cell mutagen. In this study, DNA adduct formation in testis and lung has been explored by using N7-alkylated guanines as a marker of biological effective dose. The adducts measured were the four structurally different guanine N7-adducts alkylated by butadienemonoepoxide, the main metabolite of 1,3-butadiene. This study demonstrates the dose-dependent adduct formation in lung and testis. At lower exposures (50 and 200 ppm) the adduct levels were about the same in the two organs, but at 500 ppm the adduct level was significantly (p < 0.03) higher in testis than in lung. The enantiomeric composition of the adducts detected was also different. In lung, all 4 possible adducts were present (S-C-1" dominating, 49%), but in testis only two out of four adducts were detected (S-C-2" being the most abundant adduct, 71%). These novel observations indicate that the DNA repair is different in these two organs studied and that heritable genetic effects observed may be mediated through the DNA adducts.

32P-postlabelling/HPLC assay reveals an enantioselective adduct formation in N7 guanine residues in vivo after 1,3-butadiene inhalation exposure

We have established a protocol that allows qualitative and quantitative determination of butadiene monoepoxide-DNA adducts formed as a result of inhalation exposure to 1,3-butadiene. We observed that in this particular case in vivo samples required extensive sample purification to facilitate a low background. Sample preparation included a solid phase extraction carried out with a strong anion exchange column and one-dimensional ion exchange TLC. The ultimate analysis is based on reverse phase HPLC with on-line radioactivity and UV detectors. The qualitative identification and quantitation is based on characterized markers, which are used as external and internal standards. Modified 3'-dGMP markers were used to control labelling efficiency, which varies, and modified 5'-dGMP markers were used as an optical standard to qualitatively assign the products and to determine recovery of the sample preparation. Using this method we were able to demonstrate, for the first time, specific enantio- and regioisomeric adduct formation at the N7 position of guanine residues in liver DNA of rats inhalation-exposed to 1,3-butadiene. The major adduct formed was the C-2 isomer derived from the R enantiomer of butadiene monoepoxide, contributing 47% of all adducts formed at the N7 position of guanine. The relative proportions of the remaining three other adducts detected were 22 (R C-1), 18 (S C-2) and 14% (S C-1) respectively. Inhalation exposure to 200 p.p.m. for 5 days resulted in an alkylation level of 7.2 fmol/10 microg DNA or 2.4 adducts/10(-7) normal nucleotides.

Induction and transmission of chromosome aberrations in mouse oocytes after treatment with butadiene diepoxide

A study was conducted on the genotoxicity of butadiene diepoxide (DEB) in mouse oocytes. Superovulated female mice were injected intraperitoneally with DEB and mated with untreated males. Oocyte exposure occurred approximately 1.5 days before ovulation. DEB doses ranged between 26 and 52 mg/kg. Chromosome aberrations were scored in C-banded metaphases of one-cell embryos. The percentage of mated females, the average number of zygotes harvested per female, the frequencies of unfertilized oocytes and developmentally delayed zygotes did not reveal any overt sign of chemical toxicity which hindered the propensity of animals to mate or affected the ovulation, fertilization, or cell cycle progression of treated oocytes. A dose-dependent induction of chromosome aberrations was observed which was best fitted by a linear-quadratic equation. Half of all the aberrations transmitted by DEB-treated oocytes were chromatid-type breaks or exchanges. Among chromosome-type aberrations, double fragments for exceeded chromosome exchanges. This spectrum of structural aberrations differed markedly from what was previously observed in one-cell embryos conceived by DEB-treated sperm, where 97% were chromosome-type aberrations and 40% were dicentrics or translocations. This difference suggests that chromosome damage in one-cell embryos can be fixed by different mutagenic pathways influenced by the targeted gamete and its specific chromatin configuration. After exposure to the same dose, oocytes transmitted to one-cell embryos between 4 and 8 times fewer aberrations than DEB-treated sperm. While the rate of aberration induction suggests that female germ cells may be less at risk than mature sperm, especially at low-dose levels, the higher threshold for reproductive toxicity observed in female than in male mice may justify inclusion of data on female germ cell mutagenicity in the genetic risk assessment of butadiene exposure.

Synthesis report of the step project detection of germ cell mutagens

The project 'Detection of Germ Cell Mutagens' was designed with three major goals: (1) Detection and characterization of germ-cell mutagens; (2) standardization and validation of new germ-cell tests; and (3) development of a data base on germ-cell mutagenicity. All three goals were achieved. The classical germ-cell tests were applied to characterize the genetic effects of acrylamide (AA), 1,3-butadiene (BD), trophosphamide (TP) and urethane (UR). All but UR were found to cause heritable genetic damage. The experimental data obtained for AA and BD were the basis for genetic risk evaluations during the EC/US Workshop on Risk Assessment 'Human Genetic Risk from Exposure to Chemicals, Focusing on the Feasibility of the Parallelogram Approach'. Nine chemicals were employed to validate the spermatid micronucleus assay with mice and rats: AA, BD and its metabolites 1,2-epoxybutene-3 and 1,2:3,4-diepoxybutane, chlorambucil, mitomycin C, methylnitrosourea, TP and UR. The spermatid micronucleus test was combined with micronucleus tests in somatic cells such as bone marrow or peripheral blood erythrocytes, and splenocytes which allowed a comparison of effects in somatic and germinal cells. Improvements of the spermatid micronucleus test included BrdU-labelling of premeiotic S-phase for the determination of stage sensitivity and fluorescence in situ hybridization with pancentromeric DNA-probes to distinguish between clastogenic and aneugenic events. The results indicate that the spermatid micronucleus test with its improvements is an adequate procedure to detect germ-cell clastogenicity and to compare the activity of chemicals in different tissues and between species, i.e., rats and mice. Other germ cell methods under study were the flow cytometric measurement of testicular sperm DNA and the cytogenetic analysis of preimplantation embryos for chromosomal aberrations and micronuclei. The collection of a reliable germ-cell data base was accomplished through a critical evaluation of the literature and with the data obtained in the present project. Remarkable concordance between responses of germ cell tests to chemical mutagens was the most striking conclusion to be drawn from the present data base.

Flow cytometric assessment of trophosphamide toxicity on mouse spermatogenesis

The effects of trophosphamide on mouse reproductive cells have been investigated by flow cytometric analysis of testicular cell populations and alterations of sperm chromatin structure. Mice were treated with single intraperitoneal injections of TP, the doses ranging between 50 and 150 mg/kg, and were killed after 7, 14, 21, 28, 35, or 49 days. Dose-dependent reductions of tetraploid cells, round spermatids, and elongated spermatids were detected at 7, 21, and 28 days, respectively, reflecting cytotoxic damage to the differentiating spermatogonia compartment. The dose necessary to reduce the number of differentiating spermatogonia to half the control value was approximately 70 mg/kg. Stem cells were not affected by this treatment, and the normal spermatogenic process was restored after 7 weeks. In addition, cauda epididymal sperm were analyzed by the sperm chromatin structure assay, a flow cytometric measurement of the susceptibility of the sperm nuclear DNA to in situ acid denaturation; a statistically significant increase of sperm with altered chromatin structure was detected after a TP treatment of 150 mg/kg. Together with previous findings published in the literature, where the same doses induced heritable genetic damage, this study demonstrates a marked adverse cytotoxic effect of TP on the male reproductive integrity. All this information should be taken into consideration when TP is used in chemotherapeutic regimens.

Flow cytometric and histological assessment of 1,2:3,4-diepoxybutane toxicity on mouse spermatogenesis

The effects of diepoxybutane (DEB) on mouse reproductive cells have been investigated by flow cytometric and histological description of testicular cell populations and alterations of sperm chromatin packaging. Mice were treated with single intraperitoneal injections of DEB, with doses ranging between 8.5 and 78 mg/kg (100-900 microM), and were killed after 7, 14, 21, 28 or 35 d. Dose-dependent reductions of tetraploid cells, round spermatids, and elongated spermatids were detected at 7, 21, and 28 d, respectively, reflecting cytotoxic damage on the differentiating spermatogonia compartment. The dose necessary to reduce the number of differentiating spermatogonia to half the control value was estimated equal to 650 microM or 55 mg/kg. Stem cells were not affected by this treatment. Histological section of seminiferous tubules showed depletion of spermatids and reduction of the secondary spermatocyte layers. In addition, a high although not statistically significant frequency of sperm with altered chromatin packaging was detected after DEB treatment. DEB is one of the key metabolites of butadiene, which is a compound of high environmental and occupational concern. These results contribute to the assessment of the reproductive health impact of butadiene in humans.

The centromere as a target for the induction of chromosome damage in resting and proliferating mammalian cells: assessment of mitomycin C-induced genetic damage at kinetochores and centromeres by a micronucleus test in mouse splenocytes

The cytokinesis-block micronucleus assay (MN) on murine splenocytes was used for the estimation of chromosome damage in a resting cell population in vivo that can be induced to proliferate in vitro. Mitomycin C at different doses (10(-8), 6 x 10(-8), 10(-7), 6 x 10(-7) and 10(-6)M) was used to induce cytogenetic damage in resting and cycling splenocytes. Antikinetochore antibodies (CREST) and two-colour fluorescence in situ hybridization (FISH) with minor and major satellite DNA were applied. These approaches allowed the detailed characterization of the mechanisms by which MN originates, since it was possible to identify breaks induced in pericentric heterochromatic (resulting in MN containing the major but not the minor satellite DNA) or detachment/disruption of kinetochore (resulting in different frequencies of MN containing kinetochore or both probes). Based on the evidence that resting and cycling mouse splenocytes are characterized by different spatial distribution of centromeric regions, the hypothesis was tested that the damage induced by mutagens at centromeres is influenced by the phase of the cell cycle in which the cells are treated. Data presented here show that resting and cycling splenocytes are both sensitive to mitomycin C action, and indicate that this compound has an aneugenic potential, besides its strong clastogenic activity. In particular, results obtained after CREST and FISH characterization of MN differed when cells were treated during proliferation, suggesting a disruption/detachment of kinetochores induced by mitomycin C at this cell stage. Furthermore, under the same treatment condition the proportion of MN containing the major satellite DNA only was greater than expected on the basis of random breakage at this site. Treatment of resting cells produced aneugenic damage, but without evidence of disruption/detachment of kinetochores or preferential breakage at the centromere. These results indicate that the amount and type of chromosome damage induced by mitomycin C in mouse splenocytes differ in relation to the proliferative status of treated cells.

Genotoxicity of trophosphamide in mouse germ cells: assessment of micronuclei in spermatids and chromosome aberrations in one-cell zygotes

The genotoxicity of trophosphamide (TP) in mouse germ cells was assessed by the cytogenetic analysis of micronuclei in spermatids and chromosome aberrations in one-cell zygotes and compared with the genotoxicity in somatic cells evaluated by the micronucleus reticulocyte assay. Single acute doses of 50, 75, 100 and 150 mg/kg were studied after i.p. injection. TP was only weakly mutagenic for preleptotene spermatocytes-differentiating spermatogonia, but clear-cut cytotoxic effects were demonstrated after treatment of these cells by a dose-dependent reduction of the ratio between Golgi and cap phase spermatids. Effects induced in post-meiotic stages were estimated, after mating the treated males with untreated superovulated females, by the frequencies of zygotes with chromosome aberrations: a peak of genetic damage was detected in late spermatids, with as many as 55% zygotes with aberrations, but spermatozoa and early spermatids were also clearly affected. When compared with matched solvent-injected controls, the lowest effective dose in spermatozoa and late spermatids was 100 mg/kg, although the 3- to 4-fold increases detected at 50 mg/kg were also statistically significant when compared with a pool of laboratory controls. In peripheral blood reticulocytes, the micronucleus frequencies were increased by 3-20 times the respective baseline values in the individual animals. A marked cytotoxic effect on bone marrow cells was revealed by the reduction of the proportion of early reticulocyte stages, which dropped to 20% of the control value at 150 mg/kg. Both genotoxic and cytotoxic effects were higher in bone marrow than in germ cells of the same animals, pointing to a generalized higher susceptibility of somatic cells to TP, possibly related to chemical distribution and target organ accessibility. The accurate description of stage- and dose-effect relationships in germ cells of experimental models is crucial for genetic risk assessment after chemical exposure. The approaches applied in this study may contribute to this goal.

Clastogenicity of diepoxybutane in bone marrow cells and male germ cells of mice

The bifunctional metabolite of 1,3-butadiene, 1,2:3,4-diepoxybutane (DEB), was tested in the mouse bone marrow micronucleus assay and in male mouse germ cell tests, namely the analysis of first cleavage divisions and the dominant lethal assay. All experiments were performed with single intraperitoneal treatment of the animals. In the micronucleus test, DEB doses of 4.5, 9.0, 18.0 and 36.0 mg/kg body weight were tested at a sampling interval of 24 h for bone marrow. The dose response for the induction of micronuclei in polychromatic erythrocytes was linear with the lowest effective dose of 9.0 mg/kg body weight. No sensitivity difference was observed between male and female mice. the cytogenetic analysis of first cleavage division chromosomes was performed after treatment of male mice with 17, 26, 34, 43 and 52 mg/kg body weight of DEB and mating the males to hormonally stimulated females on days 7, 14, 21 and 28 after treatment. The two higher doses caused general toxicity evidenced by the poor mating behavior of the males. Only 13 and 20% of the mated females were fertilized on day 7 after treatment of the males with 43 and 52 mg/kg body weight of DEB, respectively. An increased number of unfertilized oocytes was obtained from fertilized females on day 7 after treatment of the males with 34 mg/kg body weight of DEB. With a dose of 26 mg/kg body weight, it was demonstrated that chromosomal aberrations were only induced in spermatozoa (mating on day 7 after treatment) while spermatids (mating on days 14 and 21) and spermatocytes (mating on day 28) were not susceptible to the clastogenic effect of DEB. The response in spermatozoa in the dose range 17-34 mg/kg body weight was linear up to 26 mg/kg body weight and reached a plateau thereafter. The results of the dominant lethal experiments performed in the dose range 18-54 mg/kg body weight gave results similar to the cytogenetic study. With the highest dose tested, the toxicity and cytotoxicity during the first 8 mating days after treatment dramatically reduced the number of pregnant females and, consequently, the total implantations, so that no significant dominant lethal effect could be assessed. During mating days 9-12 (treated late spermatids), a significant dominant lethal effect was observed. With the two lower doses (18 and 36 mg/kg body weight), the dominant lethal effect was restricted to spermatozoa. The good correlation of the chromosomal aberrations with dominant lethal mutations confirms the chromosomal origin of dominant lethal effects. The clastogenic effect of DEB in somatic cells and in germ cells of mice was of the same order of magnitude.