The observation of mitotic division aberrations in mammalian cells exposed to chemical and radiation treatments

Heat Stress-Induced Multiple Multipolar Divisions of Human Cancer Cells

Multipolar divisions of heated cells has long been thought to stem from centrosome aberrations of cells directly caused by heat stress. In this paper, through long-term live-cell imaging, we provide direct cellular evidences to demonstrate that heat stress can promote multiple multipolar divisions of MGC-803 and MCF-7 cells. Our results show that, besides facilitating centrosome aberration, polyploidy induced by heat stress is another mechanism that causes multipolar cell divisions, in which polyploid cancer cells engendered by mitotic slippage, cytokinesis failure, and cell fusion. Furthermore, we also find that the fates of theses polyploid cells depend on their origins, in the sense that the polyploid cells generated by mitotic slippage experience bipolar divisions with a higher rate than multipolar divisions, while those polyploid cells induced by both cytokinesis failure and cell fusion have a higher frequency of multipolar divisions compared with bipolar divisions. This work indicates that heat stress-induced multiple multipolar divisions of cancer cells usually produce aneuploid daughter cells, and might lead to genetically unstable cancer cells and facilitate tumor heterogeneity.

Targets and mechanisms of chemically induced aneuploidy. Part 1 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

An aneuploidy workgroup was established as part of the 7th International Workshops on Genotoxicity Testing. The workgroup conducted a review of the scientific literature on the biological mechanisms of aneuploidy in mammalian cells and methods used to detect chemical aneugens. In addition, the current regulatory framework was discussed, with the objective to arrive at consensus statements on the ramifications of exposure to chemical aneugens for human health risk assessment. As part of these efforts, the workgroup explored the use of adverse outcome pathways (AOPs) to document mechanisms of chemically induced aneuploidy in mammalian somatic cells. The group worked on two molecular initiating events (MIEs), tubulin binding and binding to the catalytic domain of aurora kinase B, which result in several adverse outcomes, including aneuploidy. The workgroup agreed that the AOP framework provides a useful approach to link evidence for MIEs with aneuploidy on a cellular level. The evidence linking chemically induced aneuploidy with carcinogenicity and hereditary disease was also reviewed and is presented in two companion papers. In addition, the group came to the consensus that the current regulatory test batteries, while not ideal, are sufficient for the identification of aneugens and human risk assessment. While it is obvious that there are many different MIEs that could lead to the induction of aneuploidy, the most commonly observed mechanisms involving chemical aneugens are related to tubulin binding and, to a lesser extent, inhibition of mitotic kinases. The comprehensive review presented here should help with the identification and risk management of aneugenic agents.

Risks of aneuploidy induction from chemical exposure: Twenty years of collaborative research in Europe from basic science to regulatory implications

Although Theodor Boveri linked abnormal chromosome numbers and disease more than a century ago, an in-depth understanding of the impact of mitotic and meiotic chromosome segregation errors on cell proliferation and diseases is still lacking. This review reflects on the efforts and results of a large European research network that, from the 1980's until 2004, focused on protection against aneuploidy-inducing factors and tackled the following problems: 1) the origin and consequences of chromosome imbalance in somatic and germ cells; 2) aneuploidy as a result of environmental factors; 3) dose-effect relationships; 4) the need for validated assays to identify aneugenic factors and classify them according to their modes of action; 5) the need for reliable, quantitative data suitable for regulating exposure and preventing aneuploidy induction; 6) the need for mechanistic insight into the consequences of aneuploidy for human health. This activity brought together a consortium of experts from basic science and applied genetic toxicology to prepare the basis for defining guidelines and to encourage regulatory activities for the prevention of induced aneuploidy. Major strengths of the EU research programmes on aneuploidy were having a valuable scientific approach based on well-selected compounds and accurate methods that allow the determination of precise dose-effect relationships, reproducibility and inter-laboratory comparisons. The work was conducted by experienced scientists stimulated by a fascination with the complex scientific issues surrounding aneuploidy; a key strength was asking the right questions at the right time. The strength of the data permitted evaluation at the regulatory level. Finally, the entire enterprise benefited from a solid partnership under the lead of an inspired and stimulating coordinator. The research programme elucidated the major modes of action of aneugens, developed scientifically sound assays to assess aneugens in different tissues, and achieved the international validation of relevant assays with the goal of protecting human populations from aneugenic chemicals. The role of aneuploidy in tumorigenesis will require additional research, and the study of effects of exposure to multiple agents should become a priority. It is hoped that these reflections will stimulate the implementation of aneuploidy testing in national and OECD guidelines.

Surface Reactivity, Cytotoxic, and Morphological Transforming Effects of Diatomaceous Earth Products in Syrian Hamster Embryo Cells

In order to evaluate the effect of thermal treatments on the surface reactivity and carcinogenic potential of diatomaceous earth (DE) products, the physicochemical features of some specimens--derived by heating the same original material--were compared with their cytotoxic and transforming potency. The samples were an untreated DE (amorphous) progressively heated in the laboratory at 900 degrees C (DE 900) and 1200 degrees C (DE 1200) and a commercial product manufactured from the same DE (Chd) from which the finer fraction (< 10-microm diameter) was separated (Chd-F). Quartz (Min-U-Sil 5) and a vitreous silica (amorphous) smoothed up with hydrofluoric acid and were used as positive and negative controls, respectively. All samples were analyzed for their degree of crystallization, for their ability to release free radicals and reactive oxygen species, and for their cytotoxic and transforming potencies in Syrian hamster embryo (SHE) cells. X-ray diffractometry showed that DE 900, like DE, was still amorphous, whereas DE 1200 as well as the commercial product (Chd) were partially crystallized into cristobalite. The ability of the dust to release hydroxyl (*OH) radicals in the presence of hydrogen peroxide, as revealed by the spin-trapping technique, was as follows: Chd-F, DE 1200 > Chd > DE 900 > DE, suggesting that on heating, the surface acquires a higher potential for free radical release. Most of the silica samples generated COO* radicals from the formate ion, following homolytic rupture of the carbon-hydrogen bond, in the presence of ascorbic acid. A concentration-dependent decrease in cell proliferation and colony-forming efficiency was observed in SHE cultures treated with Chd-F, Chd, and DE. Heating abolished DE cytotoxicity but conferred a transforming ability to thermal treated particles. DE was the only sample that did not induce morphological transformation of cells. According to their transformation capacity, the samples were classified as follows: Chd-F > Chd, DE 1200 > DE 900 >> DE. Taken together, the reported results suggest that (1) the transforming potential of a biogenic amorphous silica is related to the thermal treatment that transforms the original structure in cristobalite and generates surface active sites; (2) the reactivity of samples in releasing *OH radicals correlates to their transforming ability; (3) the finer fraction of the commercial product is significantly more toxic and transforming than the coarse dust; and (4) opposite to silica dusts of mineral origin, which loose both cytotoxicity and transforming ability upon heating, heated diatomite acquires a cell-transforming potency. DE products should be thus considered a set apart of silica-based potentially toxic materials.

Chromosome nondisjunction and loss induced by protons and X rays in primary human fibroblasts: role of centromeres in aneuploidy

To study the origin of micronuclei induced in human primary fibroblasts by low-energy protons (7.7 and 28.5 keV/microm) and X rays, we have developed a combined antikinetochore-antibody (CREST) and FISH staining with pancentromeric probes. This technique allowed us to analyze the integrity of the kinetochore and centromeric DNA structures and to assess their role in induced aneuploidy. The effect of LET on radiation-induced chromosome nondisjunction was studied in binucleated cells with centromeric-specific DNA probes for chromosomes 7 and 11. Our results indicate that, though more than 90% of radiation-induced micronuclei were CREST(-)/FISH(-), 28.5 keV/microm protons and X rays were also able to induce statistically significant increases in the number of micronuclei that were CREST(-)/FISH(+) and CREST(+)/FISH(+), respectively. One interpretation of these results could be that the protons induced chromosome loss by kinetochore detachment or by breakage in the centromeric DNA region, whereas X rays induced aneuploidy through a non-DNA damage mechanism. Nondisjunction appears to be a far more important mechanism leading to radiation-induced aneuploidy. Irrespective of the higher frequency of micronuclei induced by 28.5 keV/microm protons, the frequency of chromosome loss was markedly higher for X rays than for 28.5 keV/microm protons, strengthening the hypothesis that non-DNA targets, such as components of the mitotic spindle apparatus, may be involved in aberrations in chromosome segregation after X irradiation.

Cell cycle dependent aneuploidy induction by X-rays in vitro in human lymphocytes

Although ionising radiation mainly induces DNA strand breaks leading to chromosomal aberrations, there are indications that it also might induce numerical chromosome aberrations (aneuploidy). The existing data, however, do not provide evidence for a mechanism. To assess the relative sensitivity of the G1 vs. G2 cellular targets, whole blood cultures of lymphocytes were irradiated in vitro with different doses of X-rays (0.5, 1 and 2 Gy). The lymphocytes were harvested after cytochalasin-B blockade to allow the selective study of binucleated cells, having undergone only one division in culture. Harvesting was performed at different sampling times (70, 74, and 78 hours). To evaluate the micronuclei, regarding whole chromosomes or acentric fragments, an oligonucleotide probe that recognises the centromeric region of all human chromosomes was used. The relative percentage of centromere-positive micronuclei ranged from 5 up to 18% depending on the cell cycle stage and on the received dose. Cells exposed during the G1 phase exhibited a slightly higher frequency of centromere-positive micronuclei than cells that were in G2 at the time of exposure. G1 exposure induced a centromere-positive micronuclei dose-effect relationship that was not observed after G2 exposure. The observed difference in response of both phases on the centromere-positive micronuclei yields may be due to the involvement of different targets.

Relationship between locations of chromosome breaks induced by vinyl chloride monomer and lymphocytosis

The distribution of vinyl chloride monomer (VCM)-induced chromosome breaks was studied in cultured lymphocytes of subjects occupationally exposed to this gas. In the examined subjects, the mean group value of chromosome aberrations is 6.5% and for sister chromatid exchange (SCE) frequencies, the mean value per cell is 7.9. These values are significantly higher than in the control population. Occupational exposure to VCM caused lymphocytosis together with disturbances of mitogenic activity in lymphocytes stimulated by phytohaemagglutinin. The results of G-banding showed that sites of chromosome breakpoints caused by VCM can be related to the lymphatic tissue disorders.

Aneuploidy in somatic cells of in utero exposed A-bomb survivors in Hiroshima

Cytogenetic data on cultured lymphocytes of the in utero exposed A-bomb survivors in the RERF Adult Health Study cohort have been analyzed using the G-banding technique to determine the frequency of aneuploid cells. The data consist of blood samples collected between 1985 and 1987 from 264 Hiroshima individuals for whom DS86 maternal uterine dose estimates are available: 124 proximally exposed (74 males and 50 females) with an estimated dose of 0.005 Sv or more, and 140 distally exposed (76 males and 64 females) with a dose estimate of 0 Sv, assuming the neutron relative biological effectiveness (RBE) of 10. A main feature of aneuploidy was that aneuploid frequency in autosomes depended generally on chromosome length; aneuploidies were significantly more frequent in shorter chromosomes than in longer chromosomes. The frequency of aneuploidies also depended on type, with chromosome loss approximately five times more frequent than chromosome gain. However, chromosome 21, as well as the sex chromosomes, were notable in that aneuploidy was much more frequent for these chromosomes than would be predicted from a simple relationship with length. X chromosome aneuploidies were significantly more frequent in females than in males. There was no dependence of aneuploid frequencies on dose when measured 40 years after the exposure.

A comparison of two in vitro mammalian cell cytogenetic assays for the detection of mitotic aneuploidy using 10 known or suspected aneugens

Two in vitro cytogenetic assays were evaluated for their ability to detect aneugenic and polyploidy-inducing agents using a battery of 10 known or suspected aneugens supplied as part of the EEC 4th Environmental Research and Development Programme. The compounds tested were colchicine, vinblastine, chloral hydrate, thiabendazole, hydroquinone, thimerosal, cadmium chloride, econazole nitrate, pyrimethamine and diazepam. The cell division aberration assay employed a differential chromosome/spindle staining procedure to detect perturbations of the mitotic division apparatus. This assay was carried out in two pulmonary-derived Chinese hamster cell lines; the immortal DON:Wg3h culture and a low passage LUC2 culture. The second assay involved quantification of metaphase chromosomes, for which only the LUC2 cell line was used, due to the stability of its diploid karyotype. All the chemicals induced spindle disturbances in the immortal line. In addition, all the compounds except cadmium chloride yielded positive results in the LUC2 culture, although many were not as potent. In the low passage line, 8 of the compounds (colchicine, vinblastine, chloral hydrate, thiabendazole, thimerosal, econazole nitrate, pyrimethamine and diazepam) induced aneuploidy and/or tetraploidy. Cadmium chloride was negative in the chromosome enumeration assay and hydroquinone yielded inconclusive results. The study of cell division aberrations was much less time-consuming and technically complex than the counting of metaphase chromosomes. In addition, it provided a degree of mechanistic understanding of the mode of action of some aneugenic and polyploidy-producing agents. However, the enumeration of chromosomes provides a more definitive data set for the evaluation of a chemical's aneugenic potential.

Methyl vinyl sulphone: a new class of Michael-type genotoxin

Methyl vinyl sulphone (MVS) is a labile, Michael-reactive chemical, similar in structure to acrylamide (AA). Given that acrylamide is a reference mammalian mutagen and a rodent carcinogen, studies were undertaken to evaluate the potential genotoxicity of MVS. In common with AA, MVS was non-mutagenic to Salmonella but active as an aneugen to cultured mammalian cells. It is concluded that vinyl sulphones should be regarded as representative of a new class of genotoxic chemical whose mode of action is probably primarily dependent upon Michael reactivity to proteins.

Detection of aneuploidy and aneuploidy-inducing agents in human lymphocytes using fluorescence in situ hybridization with chromosome-specific DNA probes

The feasibility of utilizing fluorescence in situ hybridization with chromosome-specific DNA probes as the basis of an assay to detect aneuploidy and aneuploidy-inducing agents in interphase human lymphocytes has been investigated. The assay involves counting the number of hybridization regions in interphase cells to determine the number of copies of a specific chromosome of interest, 22,000 interphase nuclei from untreated 72-h lymphocyte cultures were examined following hybridization with probes for chromosomes 1, 7, 9, 17, X or Y. The combined frequencies of nuclei containing 0, 1, 2, 3 and 4 hybridization regions for the various autosomal chromosomes were 0.004, 0.084, 0.909, 0.003 and 0.001, respectively. Based on these frequencies, scoring 1000-2000 cells should allow detection of aneuploid cells with a 0.012 frequency of hyperdiploidy or a 0.11 frequency of hypodiploidy for a specific chromosome of interest (alpha = 0.05, beta = 0.80). This difference in test sensitivity is related to the higher frequency of cells with one apparent spot. A comparison of the ratio of hybridization region to nuclear area in the two-dimensional images used for this analysis indicates that an overlap of the two regions probably accounts for the high frequency of apparent monosomy observed in normal cells. Treatment with the aneuploidy-inducing chemicals, colchicine, vincristine sulfate and diethylstilbestrol resulted in significant dose-related increases in the number of nuclei containing 3 or more hybridization regions. Treatment with the clastogen sodium arsenite produced only a minor increase in apparently hyperdiploid cells whereas treatment with ionizing radiation, another potent clastogen, resulted in a significant increase in nuclei containing multiple hybridization regions. These results suggest that ionizing radiation is an aneuploidy-inducing agent under these conditions although chromosomal breakage within the hybridization region may account for a portion of the increased frequency of nuclei with multiple hybridization regions. These results indicate that the use of fluorescence in situ hybridization with DNA probes is capable of detecting aneuploid cells occurring at relatively low frequencies within a population of cells. Assays based on these techniques should facilitate a more rapid identification of aneuploidy-inducing environmental and therapeutic agents.

Damage to lymphocytes by X-ray and bleomycin measured with the cytokinesis-block micronucleus technique

Chromosome damage induced by X-irradiation or bleomycin was measured using the cytokinesis-block micronucleus assay in the peripheral blood lymphocytes of 6 newborn, 8 young and 10 elderly individuals. An increase in the frequency of spontaneous micronuclei with age was observed. There was no difference in the X-irradiation-induced micronucleus frequency between the 3 groups. There was a significant increase with age in the number of micronuclei induced by bleomycin. Kinetochore-labelling studies revealed that the percentage of kinetochore-positive induced micronuclei was higher for bleomycin (36.2-43.3%) than for X-irradiation (17.1-19.7%). The age-related increase in frequency of spontaneous or bleomycin-induced micronuclei was due to increases in both kinetochore-positive and kinetochore-negative micronuclei. The frequency of kinetochore-positive or -negative micronuclei induced by X-irradiation was not different between the 3 age groups. These results suggest that bleomycin is more potent in inducing whole-chromosome loss than X-rays, and that lymphocytes from aged individuals are more sensitive to bleomycin in terms of both chromosome breakage and whole chromosome loss.

The effects of 1,6-dinitropyrene on spindle morphology in transformed human cells

The effects of 1,6-dinitropyrene (1,6-DNP) on the fidelity of cell division were studied in the transformed human fibroblast cell line MRC5VA. Over a dose range of 0.1-10 micrograms/ml of 1.6-DNP, we observed significant increases in the levels of abnormal division stages, associated with damage to the spindle apparatus of the cell. Qualitative changes in spindle morphology and a quantitative decrease in pole-to-pole spindle length were also observed with increasing doses of 1.6-DNP. Such changes in the size and morphology of the spindle corresponded with an accumulation of cells blocked at metaphase. The presence of catalase did not modify the response, suggesting that the effects on the spindle apparatus and cell division were not caused by the generation of radicals but by the direct action of 1.6-DNP.

Comparisons of tests for aneuploidy

The fundamental problems that face us in the development of suitable assay systems for the detection of potentially aneugenic (aneuploidy-inducing) chemicals include: (a) the diversity of cellular targets and mechanisms where perturbations of structure and function may give rise to changes in chromosome number, and (b) the phylogenetic differences that exist between species in their mechanism and kinetics of cell division and their metabolic profiles. A diverse range of assay systems have been developed, which have been shown to have potential for use in the detection of either changes in chromosome number or of perturbations of the events which may be causal in the induction of aneuploidy. Chromosome number changes may be detected cytologically by karyotypic analysis, or by the use of specialised strains in which aneuploid progeny may be observed due to phenotypic differences with aneuploid parental cells or whole organisms. Techniques for the detection of cellular target modifications range from in vitro studies of tubulin polymerisation to observations of the behaviour of various cellular organelles and their fidelity of action during the division cycle. The diversity of mechanisms which may give rise to aneuploidy and the qualitative relevance of events observed in experimental organisms compared to man make it unlikely that the detection and risk assessment of the aneugenic activity of chemicals will be possible using a single assay system. Optimal screening and assessment procedures will thus be dependent upon the selection of an appropriate battery of predictive tests for the measurement of the potentially damaging effects of aneuploidy induction.

Cytogenetic evaluations in human lymphocytes exposed to methyl acetimidate, a lysine-specific protein crosslinking agent

The cytogenetic effects of methyl acetimidate (MAI), a lysine-specific protein crosslinking reagent, were investigated using human peripheral lymphocytes in culture. Lymphocytes were treated with the chemical either prior to PHA exposure or 2-3 days following mitogenic stimulation and assessed for perturbations in cellular proliferation and induction of SCEs. Severe reductions in the mitotic index (MI) and pronounced decreases in the proportion of metaphases proceeding beyond M(1) were observed following G0 exposure to MAI concentrations of as low as 2 mM; with complete suppression of mitotic activity in all cultures exposed to levels of 3 mM MAI or greater. Concentrations resulting in severe depression in MI caused only moderate increases in SCEs. Cells exposed to less than 10 mM MAI during the late S-G2 stages of the cell cycle and harvested at the first metaphase following treatment exhibited profound mitotic delay, impaired prophase to metaphase transitions and abnormal mitotic configurations. These findings demonstrate that protein-specific crosslinking agents may induce a wide spectrum of adverse cytogenetic outcomes in both cycling and noncycling lymphocytes.

Diphenylhydantoin and mitotic spindle abnormalities in cultured mouse and human cells

The present study demonstrates that the antiepileptic drug diphenylhydantoin (DPH) is capable of inducing aneuploidy but not structural aberrations in cultured mouse embryonic fibroblasts. A high concentration of 200 micrograms/ml was found to increase the percentage of hyperdiploidy from 4.8 (control) to 16.0. The treatment was found to increase mitotic indices as a consequence of a mitotic-arresting action of the drug. These effects are probably due to the effect of the drug on the structure of the mitotic apparatus. Abnormal cell divisions and mitotic disturbance were found to increase in a dose-dependent manner after DPH treatment. In a parallel study, human amnion cells were found to show similar response to DPH treatment.