The detection of mitotic and meiotic chromosome gain in the yeast Saccharomyces cerevisiae: effects of methyl benzimidazol-2-yl carbamate, methyl methanesulfonate, ethyl methanesulfonate, dimethyl sulfoxide, propionitrile and cyclophosphamide monohydrate

Hormesis commonly observed in the assessment of aneuploidy in yeast

Extensive dose response studies have assessed the potential of toxic chemical agents to induce aneuploidy in the yeast model. An assessment of such findings revealed that hormetic-like biphasic dose responses were commonly observed. A preliminary estimate of the frequency of the hormetic responses using a priori entry and evaluative criteria was approximately 65-80%.

Chemical Exposure Generates DNA Copy Number Variants and Impacts Gene Expression

DNA copy number variation is long associated with highly penetrant genomic disorders, but it was not until recently that the widespread occurrence of copy number variation among phenotypically normal individuals was realized as a considerable source of genetic variation. It is also now appreciated that copy number variants (CNVs) play a role in the onset of complex diseases. Many of the complex diseases in which CNVs are associated are reported to be influenced by yet to be identified environmental factors. It is hypothesized that exposure to environmental chemicals generates CNVs and influences disease onset and pathogenesis. In this study a proof of principle experiment was completed with ethyl methanesulfonate (EMS) and cytosine arabinoside (Ara-C) to investigate the generation of CNVs using array comparative genomic hybridization (CGH) and the zebrafish vertebrate model system. Exposure to both chemicals resulted in CNVs. CNVs were detected in similar genomic regions among multiple exposure concentrations with EMS and five CNVs were common among both chemicals. Furthermore, CNVs were correlated to altered gene expression. This study suggests that chemical exposure generates CNVs with impacts on gene expression warranting further investigation of this phenomenon with environmental chemicals.

Simultaneous measurement of the frequencies of intrachromosomal recombination and chromosome gain using the yeast DEL assay

The yeast DEL assay measures the frequency of intrachromosomal recombination between two partially-deleted his3 alleles on chromosome XV. The his3Delta alleles share approximately 400bp of overlapping homology, and are separated by an intervening LEU2 sequence. Homologous recombination between the his3Delta alleles results in deletion of the intervening LEU2 sequence (DEL), and reversion to histidine prototrophy. In this study we have attempted to further extend the use of the yeast DEL assay to measure the frequency of chromosome XV gain events. Reversion to His(+)Leu(+) in the haploid yeast DEL tester strain RSY6 occurs upon non-disjunction of chromosome XV sister chromatids, coupled with a subsequent DEL event. Here we have tested the ability of the yeast DEL assay to accurately predict the aneugenic potential of the diversely-acting, known or suspected aneugens actinomycin D, benomyl, chloral hydrate, ethyl methanesulfonate (EMS), methyl methanesulfonate (MMS), and methotrexate. Actinomycin D and benomyl strongly induced aneuploidy. EMS and methotrexate modestly induced aneuploidy, while chloral hydrate and MMS failed to illicit any significant induction. In addition, by FACS-analysis of DNA content it was shown that the majority of both spontaneous- and chemically-induced His(+)Leu(+) revertants were heterodiploid. Thus, our results indicate endoreduplication of almost entire chromosome sets as a major mechanism of aneuploidy induction in haploid Saccharomyces cerevisiae.

Evaluation of benomyl and carbendazim in the in vivo aneuploidy/micronucleus assay in BDF1 mouse bone marrow

Benomyl and its active metabolite carbendazim were investigated in BDF1 mouse bone marrow to establish whether micronuclei induced by these fungicides are caused by clastogenic or aneugenic events. Micronuclei were evaluated for kinetochores using immunofluorescent antikinetochore antibodies. Kinetochore positive (K+) micronuclei are likely to arise from chromosome loss since they presumably contain intact kinetochores and are indicative of aneuploidy. Conversely, kinetochore negative (K-) micronuclei are mostly likely to contain acentric chromosome fragments arising primarily from clastogenic damage. Benomyl and carbendazim were administered as single oral doses of 0.3, 8.6 or 17.2 mmol/kg (for benomyl, equivalent to 100, 2500 or 5000 mg/kg; for carbendazim, equivalent to 66, 1646 or 3293 mg/kg). Both compounds were positive in the micronucleus test at doses of 8.6 and 17.2 mmol/kg, and an average of 82% (benomyl) and 87% (carbendazim) of the total micronucleated polychromatic erythrocytes were K+. No effects were seen with either fungicide at 0.3 mmol/kg. These results are analogous to findings with known aneugens such as vincristine but are in contrast to results with classical clastogens such as cyclophosphamide. Thus, benomyl and carbendazim induce micronuclei in mouse bone marrow cells primarily through an aneugenic mechanism.

Formaldehyde, glyoxal, urethane, methyl carbamate, 2,3-butanedione, 2,3-hexanedione, ethyl acrylate, dibromoacetonitrile and 2-hydroxypropionitrile induce chromosome loss in Saccharomyces cerevisiae

Induction of mitotic chromosome loss could be demonstrated for the dialdehyde glyoxal, the diketones 2,3-butanedione and 2,3-hexanedione, ethyl and methyl carbamate, ethyl acrylate, dibromoacetonitrile, 2-hydroxypropionitrile and formaldehyde, but only when they were combined with subacute concentrations of propionitrile, which is a strong inducer of chromosomal malsegregation. The same chemicals did not induce mitotic chromosome loss when applied in pure form. However, glyoxal, ethyl acrylate, dibromoacetonitrile and formaldehyde when applied in pure form also induced mitotic recombination. Respiratory deficiency was induced, in the absence of propionitrile, by these recombinogenic agents and also by 2,3-hexanedione and 2-hydroxypropionitrile which are not recombinogenic.

Mitotic hyperploidy for chromosomes VIII and III in Saccharomyces cerevisiae

The arg4-8 and cup1s markers comprise a copy-number-dependent signal device in the yeast Saccharomyces cerevisiae. These alleles permit reliable discrimination between euploid and disomic haploids as well as between euploid and trisomic diploids. To investigate and compare inherent inter-chromosomal differences as regards propensity for hyperploidy, we transplaced arg4-8 and cup1s by deleting them from chromosome VIII and then re-introducing them at the leu2 locus on chromosome III. The rate of chromosome gain was significantly greater for the chromosome III construct compared to the native chromosome VIII, in both diploid and haploid strains. In addition, more coincident aneuploidy for other chromosomes was found among chromosome VIII hyperploids compared to chromosome III hyperploids.

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.