Classification of micronuclei in murine erythrocytes: immunofluorescent staining using CREST antibodies compared to in situ hybridization with biotinylated gamma satellite DNA

Chromosome-specific induction of micronuclei and chromosomal aberrations by mitomycin C: Involvement of human chromosomes 9, 1 and 16

Cytogenetic studies have shown that human chromosomes 1, 9, and 16, with a large heterochromatic region of highly methylated classical satellite DNA, are prone to induction of chromatid breaks and interchanges by mitomycin C (MMC). A couple of studies have indicated that material from chromosome 9, and possibly also from chromosomes 1 and 16, are preferentially micronucleated by MMC. Here, we further examined the chromosome-specific induction of micronuclei (MN; with and without cytochalasin B) and chromosomal aberrations (CAs) by MMC. Cultures of isolated human lymphocytes from two male donors were treated (at 48 h of culture, for 24 h) with MMC (500 ng/ml), and the induced MN were examined by a pancentromeric DNA probe and paint probe for chromosome 9, and by paint probes for chromosomes 1 and 16. MMC increased the total frequency of MN by 6-8-fold but the frequency of chromosome 9 -positive (9+) MN by 29-30-fold and the frequency of chromosome 1 -positive (1+) MN and chromosome 16 -positive (16+) MN by 12-16-fold and 10-17-fold, respectively. After treatment with MMC, 34-47 % of all MN were 9+, 17-20 % 1+, and 3-4 % 16+. The majority (94-96 %) of the 9+ MN contained no centromere and thus harboured acentric fragments. When MMC-induced CAs aberrations were characterized by using the pancentromeric DNA probe and probes for the classical satellite region and long- and short- arm telomeres of chromosome 9, a high proportion of chromosomal breaks (31 %) and interchanges (41 %) concerned chromosome 9. In 83 % of cases, the breakpoint in chromosome 9 was just below the region (9cen-q12) labelled by the classical satellite probe. Our results indicate that MMC specifically induces MN harbouring fragments of chromosome 9, 1, and 16. CAs of chromosome 9 are highly overrepresented in metaphases of MMC-treated lymphocytes. The preferential breakpoint is below the region 9q12.

Effect of the mechanical properties of carbon-based coatings on the mechanics of cell-material interactions

The paper presents an influence of the surface mechanical properties of thin-film materials on blood cell adhesion under shear stress conditions. Physical vapour deposited (PVD) coatings i.e. hydrogenated amorphous carbon (a-C:H) doped with nitrogen or silicon have been investigated. The mechanical properties of materials, namely their microhardness and Young's modulus were measured using indentation test with Rockwell indenter. The adhesion efficiency of blood cells in dynamic conditions were analysed using a radial flow chamber. Red blood cells (RBC) were used as representative cells to analyse cell-material interactions. The biomaterial examinations were performed under physiological flow conditions at the single-cell level. The 3D FVM (finite volume method) model of multi-phase radial flow test was developed to reproduce the physical test and to predict distributions of shear stresses and velocity during blood washout with PBS. Cell-material interactions were found to be strongly associated with the mechanical properties of the thin-film material. The decrease in the hardness of the coatings translated into a weaker cell - material interactions.

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.

Mitigating effect of Indian propolis against mitomycin C induced bone marrow toxicity

A major drawback with cancer chemotherapy is its severe toxic effects on non-target tissues. Assessment of natural products for their protective effect against anticancer drugs-induced toxicity is gaining importance in cancer biology. The present study was aimed at assessing the protective effect of hydroethanolic extract of Indian propolis (HEIP) against mitomycin C (MMC)-induced genotoxicity and cytotoxicity. Swiss albino mice were injected with various doses of HEIP (100, 200, 300, 400, 600 and 800 mg/kg b. wt., i.p) 1 h prior to MMC (8 mg/kg, i.p.) injection. The geno- and cyto-toxicities were evaluated in mice by performing bone marrow micronucleus and TUNEL assays. In vitro antioxidant and lipid peroxidation inhibitory assays were carried out to understand the mechanism of the protective effects. The significant increase in the frequency of micronculeated cells (12.51 ± 0.48), apoptotic cells (23.43 ± 1.86) and reduction in P/N ratio (0.69 ± 0.04) compared with control indicated the potential geno- and cytotoxic effects of MMC in bone marrow. Pretreatment with HEIP resulted in the significant recovery of the toxic effects induced by MMC. HEIP at 400 mg/kg b. wt. was found to be the optimum dose imparting the maximum protective effects. The in vitro antioxidant and lipid peroxidation inhibitory assays suggest that the extract possesses substantial free radical scavenging activities. In conclusion, HEIP possesses substantial geno- and cyto-protective properties against MMC, which could be mediated through efficient free radical scavenging and inhibitory effect on lipid peroxidation.

Modulation of Ras signaling alters the toxicity of hydroquinone, a benzene metabolite and component of cigarette smoke

Background

Benzene is an established human leukemogen, with a ubiquitous environmental presence leading to significant population exposure. In a genome-wide functional screen in the yeast Saccharomyces cerevisiae, inactivation of IRA2, a yeast ortholog of the human tumor suppressor gene NF1 (Neurofibromin), enhanced sensitivity to hydroquinone, an important benzene metabolite. Increased Ras signaling is implicated as a causal factor in the increased pre-disposition to leukemia of individuals with mutations in NF1.

Methods

Growth inhibition of yeast by hydroquinone was assessed in mutant strains exhibiting varying levels of Ras activity. Subsequently, effects of hydroquinone on both genotoxicity (measured by micronucleus formation) and proliferation of WT and Nf1 null murine hematopoietic precursors were assessed.

Results

Here we show that the Ras status of both yeast and mammalian cells modulates hydroquinone toxicity, indicating potential synergy between Ras signaling and benzene toxicity. Specifically, enhanced Ras signaling increases both hydroquinone-mediated growth inhibition in yeast and genotoxicity in mammalian hematopoetic precursors as measured by an in vitro erythroid micronucleus assay. Hydroquinone also increases proliferation of CFU-GM progenitor cells in mice with Nf1 null bone marrow relative to WT, the same cell type associated with benzene-associated leukemia.

Conclusions

Together our findings show that hydroquinone toxicity is modulated by Ras signaling. Individuals with abnormal Ras signaling could be more vulnerable to developing myeloid diseases after exposure to benzene. We note that hydroquinone is used cosmetically as a skin-bleaching agent, including by individuals with cafe-au-lait spots (which may be present in individuals with neurofibromatosis who have a mutation in NF1), which could be unadvisable given our findings.

Bioinformatic Tools Identify Chromosome-Specific DNA Probes and Facilitate Risk Assessment by Detecting Aneusomies in Extra-embryonic Tissues

Despite their non-diseased nature, healthy human tissues may show a surprisingly large fraction of aneusomic or aneuploid cells. We have shown previously that hybridization of three to six non-isotopically labeled, chromosome-specific DNA probes reveals different proportions of aneuploid cells in individual compartments of the human placenta and the uterine wall. Using fluorescence in situ hybridization, we found that human invasive cytotrophoblasts isolated from anchoring villi or the uterine wall had gained individual chromosomes. Chromosome losses in placental or uterine tissues, on the other hand, were detected infrequently. A more thorough numerical analysis of all possible aneusomies occurring in these tissues and the investigation of their spatial as well as temporal distribution would further our understanding of the underlying biology, but it is hampered by the high cost of and limited access to DNA probes. Furthermore, multiplexing assays are difficult to set up with commercially available probes due to limited choices of probe labels. Many laboratories therefore attempt to develop their own DNA probe sets, often duplicating cloning and screening efforts underway elsewhere. In this review, we discuss the conventional approaches to the preparation of chromosome-specific DNA probes followed by a description of our approach using state-of-the-art bioinformatics and molecular biology tools for probe identification and manufacture. Novel probes that target gonosomes as well as two autosomes are presented as examples of rapid and inexpensive preparation of highly specific DNA probes for applications in placenta research and perinatal diagnostics.

Cdk2 and Cdk4 regulate the centrosome cycle and are critical mediators of centrosome amplification in p53-null cells

The two mitotic centrosomes direct spindle bipolarity to maintain euploidy. Centrosome amplification-the acquisition of > or =3 centrosomes-generates multipolar mitoses, aneuploidy, and chromosome instability to promote cancer biogenesis. While much evidence suggests that Cdk2 is the major conductor of the centrosome cycle and that it mediates centrosome amplification induced by various altered tumor suppressors, the role played by Cdk4 in a normal or deregulated centrosome cycle is unknown. Using a gene knockout approach, we report that Cdk2 and Cdk4 are critical to the centrosome cycle, since centrosome separation and duplication are premature in Cdk2(-)(/)(-) mouse embryonic fibroblasts (MEFs) and are compromised in Cdk4(-)(/)(-) MEFs. Additionally, ablation of Cdk4 or Cdk2 abrogates centrosome amplification and chromosome instability in p53-null MEFs. Absence of Cdk2 or Cdk4 prevents centrosome amplification by abrogating excessive centriole duplication. Furthermore, hyperactive Cdk2 and Cdk4 deregulate the licensing of the centrosome duplication cycle in p53-null cells by hyperphosphorylating nucleophosmin (NPM) at Thr199, as evidenced by observations that ablation of Cdk2, Cdk4, or both Cdk2 and Cdk4 abrogates that excessive phosphorylation. Since a mutant form of NPM lacking the G(1) Cdk phosphorylation site (NPM(T199A)) prevents centrosome amplification to the same extent as ablation of Cdk2 or Cdk4, we conclude that the Cdk2/Cdk4/NPM pathway is a major guardian of centrosome dysfunction and genomic integrity.

Factors Influencing the DNA Content of Radiation-induced Micronuclei

The distribution of the DNA content of radiation-induced micronuclei was analysed in several cell lines (Chinese hamster, Syrian hamster and mouse NIH-3T3 cells) by flow cytometry. Frequency and DNA content of micronuclei were measured simultaneously using fluorescence and forward scatter signals of micronuclei and nuclei in suspension stained with ethidium bromide. Computerized random breakage of chromosomes and random combination of fragments was performed to compare the measured micronucleus distributions in synchronized cells irradiated during G1-phase with calculated distributions. The measured DNA distribution of radiation-induced micronuclei was found to be influenced by several factors: (1) the DNA distribution and the centromeric index of the chromosomes in the various cell lines; (2) the cell cycle phase at time of micronucleus measurement due to DNA synthesis in micronuclei; (3) the presence of chromosome fragments in micronuclei; and (4) the presence of whole chromosomes in micronuclei. These factors were shown to be responsible for the previously found large radiation-induced micronuclei which could not be explained by the classic assumption only that radiation-induced micronuclei are mainly produced by single acentric fragments.

Hydroquinone: an evaluation of the human risks from its carcinogenic and mutagenic properties

The toxicology of hydroquinone has been reviewed on a number of previous occasions. This review targets its potential for carcinogenicity and possible modes of carcinogenic action. The evaluation made by IARC (1999) of its carcinogenic risk to humans was that hydroquinone is not classifiable as to its carcinogenicity to humans (Group 3). This evaluation was based on inadequate evidence in humans and limited evidence in experimental animals. The epidemiological information comes from four cohort studies involving occupational exposures. A cohort of lithographers, some of whom had worked with hydroquinone, had an excess of malignant melanoma based on five cases, but only two of the cases had reported exposure to hydroquinone. In a study of photographic processors the number of exposed individuals was uncertain and the numbers of cases of individual cancer sites were small. In view of the statistical power limitations of these studies for individual diagnostic categories of cancers, they are not considered to be informative with regard to the carcinogenicity of hydroquinone. A cohort of workers with definite and lengthy exposure to hydroquinone, during either its manufacture or its use, had low cancer rates compared with two comparison populations; the reason for the lower than expected rates is unclear. In a motion picture film processing cohort there were significant excess malignancies of the respiratory system among workers engaged in developing, where there was exposure to hydroquinone as well as other chemicals. There was no information on tobacco smoking habits and no dose-response relationship. Hydroquinone has been shown reproducibly to induce benign neoplasms in the kidneys of male F344 rats dosed orally either by gavage (25 and 50 mg/kg body weight) or diet (0.8%). The gavage study has been evaluated in considerable detail. This evaluation showed that all renal tubule adenomas and all cases of renal tubule atypical hyperplasia occurred in areas of severe or end-stage chronic progressive nephropathy and that the neoplasms were not otherwise confined to any particular part of the kidney. It is likely that the mode of carcinogenic action of hydroquinone is exacerbation of this natural disease process. Hydroquinone is mutagenic in vitro and in vivo, having caused genotoxicity or chromosomal aberrations in rodent bone-marrow cells. At least a portion, if not all, of the chromosomal effects are caused by interference by hydroquinone or its metabolites with chromosomal segregation, probably due to interaction with mitotic spindle proteins. However, the dose routes used to demonstrate these effects in almost all of the studies in vivo were intraperitoneal or subcutaneous injection, which were considered inappropriate. There were five studies by the oral route. These included a mouse bone-marrow cell micronucleus test in which a weak, marginally positive response was obtained following a single oral dose of 80 mg/kg body weight. The remaining oral route studies all showed no significant effect. They included a mouse bone-marrow cell micronucleus test in which there was no genotoxic activity after exposure to a diet containing 0.8% hydroquinone for 6 days; two (32)P-post-labeling assays, one with targets of Zymbal gland, liver, and spleen in Sprague-Dawley rats, the other with the kidney as target in F344 rats; and the last oral assay was for 8-hydroxydeoxyguanosine adducts in F344 rat kidney DNA. Thus, the evidence (and the database) for any genotoxic effect in vivo is sparse and none has been observed in kidney. While glutathione conjugates could be responsible for the tumor induction, careful histology seems to show that the most actively toxic of several glutathione compounds tested, 2,3,5-triglutathion-S-yl hydroquinone, targets a very specific region of the kidney, the outer stripe of the outer medulla (OSOM), whereas hydroquinone-associated adenomas are more randomly distributed and occur in the cortex as well as the medulla. A nongenotoxic mode of action that involves exacerbation of a spontaneously occurring rodent renal disease, chronic progressive nephropathy (CPN), is proposed and evaluated. This disease is particularly prominent in male rats and the evidence is consistent with an absence of any human counterpart; therefore, the increased incidence of renal tubule adenomas in hydroquinone-dosed male rats is without human consequence.

Genotoxicity of the marine toxin okadaic acid, in human Caco-2 cells and in mice gut cells

Micronucleus induction by the diarrhetic shellfish toxin okadaic acid (OA) was investigated in two intestinal models, cultured human Caco-2 cells and colon epithelial cells of mice treated in vivo. Exposure to OA for 4 and 24 h induced dose-responsive increases in the frequency of micronucleated Caco-2 cells; the minimum OA doses increasing micronucleus frequency were 20 nM for the 4 h treatment and 5 nM for the 24 h treatment. OA treatment of Caco-2 cells also resulted in dose- and time-dependent increases in mitotic arrest and multinucleated cells. Two experiments were conducted in which mice were treated with single oral gavages of 435-610 and 115-1341 microg/kg OA. In the first experiment, samples were taken 24 h after the treatment, and the frequencies of both micronucleated and mitotic gut cells were increased after treatment with 525 microg/kg OA. In the second experiment, no increases in micronucleus frequency were detected at 24, 36, or 48 h following OA doses of 230 and 115 microg/kg; however, an increase in the mitotic index was observed 36 h after a gavage with 115 microg/kg OA. In this experiment, doses higher than 230 microg/kg were rapidly lethal to the mice. Immunohistology with monoclonal OA antibodies showed that OA was distributed into the liver at all the sampling times and in the small intestine at 24 and 36 h; OA was not detected in the colon. In addition, the TUNEL assay indicated that OA induced apoptosis in mouse ileum, liver, and kidney. The results of our investigations suggest that OA is aneugenic in Caco-2 cells, whereas the in vivo data were inconclusive. Further studies should be performed in mice using intragastric doses of 230-525 microg/kg OA. Moreover, the apoptosis and cell proliferation results indicate that OA can reach organs other than colon, indicating further evaluation of the genotoxic potential of OA in these organs is warranted.

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.

Chromosome breakage is primarily responsible for the micronuclei induced by 1,4-dioxane in the bone marrow and liver of young CD-1 mice

1,4-Dioxane, a widely used industrial chemical and rodent hepatocarcinogen, has produced mixed, largely negative results in the mouse erythrocyte micronucleus assay. In contrast, a recent report has indicated that 1,4-dioxane induces micronuclei in mouse hepatocytes following in vivo treatment. The objective of this study was to confirm these earlier results and identify the origin of the induced micronuclei. Following an initial range-finding study, mice were administered 1,4-dioxane by gavage at doses ranging from 1500 to 3500 mg/kg. The test animals were also implanted with BrdU-releasing osmotic pumps to allow cell proliferation to be measured in the liver and to increase the sensitivity of the hepatocyte assay. Upon sacrifice, the frequency of micronuclei in the bone marrow erythrocytes and in the proliferating BrdU-labeled hepatocytes was determined. Significant dose-related increases in micronuclei were seen in both the liver and the bone-marrow with significant increases being detected at all the tested doses in the bone marrow and at the 2500 and 3500 mg/kg doses in the liver. Using CREST staining or pancentromeric FISH to determine the origin of the induced micronuclei, it was determined that 80-90% of the micronuclei in both tissues originated from chromosomal breakage. Small increases in centromere-containing micronuclei were also seen in the hepatocytes. Decreases in hepatocyte proliferation as well as in the ratio of bone marrow PCE:NCE were also observed. Based on these results, we conclude that at high doses: (i) dioxane exerts genotoxic effects in both the mouse bone marrow and liver; (ii) the induced micronuclei are formed primarily from chromosomal breakage; and (iii) dioxane can interfere with cell proliferation in both the liver and bone marrow.

Genotoxicity of benzene and its metabolites

The potential role of genotoxicity in human leukemias associated with benzene (BZ) exposures was investigated by a systematic review of over 1400 genotoxicity test results for BZ and its metabolites. Studies of rodents exposed to radiolabeled BZ found a low level of radiolabel in isolated DNA with no preferential binding in target tissues of neoplasia. Adducts were not identified by 32P-postlabeling (equivalent to a covalent binding index <0.002) under the dosage conditions producing neoplasia in the rodent bioassays, and this method would have detected adducts at 1/10,000th the levels reported in the DNA-binding studies. Adducts were detected by 32P-postlabeling in vitro and following high acute BZ doses in vivo, but levels were about 100-fold less than those found by DNA binding. These findings suggest that DNA-adduct formation may not be a significant mechanism for BZ-induced neoplasia in rodents. The evaluation of other genotoxicity test results revealed that BZ and its metabolites did not produce reverse mutations in Salmonella typhimurium but were clastogenic and aneugenic, producing micronuclei, chromosomal aberrations, sister chromatid exchanges and DNA strand breaks. Rodent and human data were compared, and BZ genotoxicity results in both were similar for the available tests. Also, the biotransformation of BZ was qualitatively similar in rodents, humans and non-human primates, further indicating that rodent and human genotoxicity data were compatible. The genotoxicity test results for BZ and its metabolites were the most similar to those of topoisomerase II inhibitors and provided less support for proposed mechanisms involving DNA reactivity, mitotic spindle poisoning or oxidative DNA damage as genotoxic mechanisms; all of which have been demonstrated experimentally for BZ or its metabolites. Studies of the chromosomal translocations found in BZ-exposed persons and secondary human leukemias produced by topoisomerase II inhibitors provide some additional support for this mechanism being potentially operative in BZ-induced leukemia.

Resolution of anaphase bridges in cancer cells

Chromosomal instability is a key step in the generation of the cancer cell karyotype. An indicator of unstable chromosomes is the presence of chromatin bridges during anaphase. We examined in detail the fate of anaphase bridges in cultured oral squamous cell carcinoma cells in real-time. Surprisingly, chromosomes in bridges typically resolve by breaking into multiple fragments. Often these fragments give rise to micronuclei (MN) at the end of mitosis. The formation of MN is shown to have important consequences for the cell. We found that MN have incomplete nuclear pore complex (NPC) formation and nuclear import defects and the chromatin within has greatly reduced transcriptional activity. Thus, a major consequence of the presence of anaphase bridges is the regular sequestration of chromatin into genetically inert MN. This represents another source of ongoing genetic instability in cancer cells.

Marine toxin okadaic acid induces aneuploidy in CHO-K1 cells in presence of rat liver postmitochondrial fraction, revealed by cytokinesis-block micronucleus assay coupled to FISH

Okadaic acid (OA), a major polyether toxin involved in diarrhetic shellfish poisoning (DSP), is a potent tumor promoter in rodent skin and glandular stomach and a specific inhibitor of the serine/threonine protein phosphatases PP1 and PP2A. A previous study, which used the cytokinesis-block micronucleus (CBMN) assay in CHO-K1 cells, showed that OA induced chromosome damage in the presence of a rat liver metabolic activation system (S9). To support OA biotransformation by S9, the same test system was performed, and DNA damage induced by OA was measured with and without metabolic activation as well as in the presence of heat-inactivated S9 fraction. The results showed that only in the presence of active S9 did OA significantly increased the frequency of micronucleated binucleated (MNBN) cells. After a 4-h treatment a 2- to 5-fold increase of MNBN cells was observed at 30 nM and at 50 nM of OA. However, without S9 or in the presence of heat-inactivated S9, OA did not induce any chromosome damage. We concluded that OA can be metabolically activated in vitro into metabolites that are more genotoxic. The CBMN assay coupled with fluorescence in situ hybridization (FISH) using a DNA probe for centromere detection was performed to discriminate between clastogenic (chromosome breakage) and aneugenic (chromosome loss) effects. FISH analysis showed that OA metabolites increased in a dose-dependent manner in centromere positive micronuclei (CEN+): 60% of CEN+ at 30 nM and 75% of CEN+ at 50 nM of OA. The uptake of OA into CHO-K1 cells and the biotransformation of the toxin are discussed.

Report from the in vitro micronucleus assay working group

At the Washington "2nd International Workshop on Genotoxicity Testing" (25-26 March 1999) current methodologies and data for the in vitro micronucleus test were reviewed. As a result, guidelines for the conduct of specific aspects of the protocol were developed. Agreement was achieved on the following topics: choice of cells, slide preparation, analysis of micronuclei, toxicity, use of cytochalasin-B, number of doses, and treatment/harvest times [Environ. Mol. Mutagen. 35 (2000) 167]. Because there were a number of important in vitro micronucleus validation studies in progress, it was not possible to design a definitive, internationally harmonized protocol at that time. These studies have now been completed and the data were reviewed at the Plymouth "3rd International Workshop on Genotoxicity Testing" (28-29 June 2002). Data from studies coordinated by the French Society of Genetic Toxicology, Japanese collaborative studies, European pharmaceutical industry validation studies, along with data from Lilly Research Laboratories were used to prepare conclusions on the main aspects of the in vitro micronucleus protocol. In this paper, the consensus agreements on the protocol for performing the in vitro micronucleus assay are presented. The major recommendations concern: 1. Demonstration of cell proliferation: both cell lines and lymphocytes can be used, but demonstration of cell proliferation in both control and treated cells is compulsory for the acceptance of the test. 2. Assessment of toxicity and dose range finding: assessment of toxicity should be performed by determining cell proliferation, e.g. increased cell counts (CC) or population doubling (PD) without cytochalasin-B, or e.g. cytokinesis-block proliferation index with cytochalasin-B; and by determining other markers for cytotoxicity (confluency, apoptosis, necrosis) which can provide valuable additional information. 3. Treatment schedules for cell lines and lymphocytes. 4. Choice of positive controls: without S9-mix both a clastogen (e.g. mitomycin C or bleomycin) and an aneugen (e.g. colchicine) should be included as positive controls and a clastogen that requires S9 for activity when S9-mix is used (e.g. dimethylnitrosamine, or cyclophosphamide in those cell types that cannot activate this agent directly). 5. Duplicate cultures and number of cells to be scored. 6. Repeat experiments: in lymphocytes, for each experiment blood from 2 different healthy young and non-smoking donors should be compared. In cell lines, the experiments need only to be repeated if the first one is negative. 7.

STATISTICS

statistical significance should not be the sole factor for determining positive results. Biological meaning should serve as a guideline. Examples of statistical analyses are given.

Comparative genotoxic effect of vincristine, vinblastine, and vinorelbine in somatic cells of Drosophila melanogaster

In this study, the vinca alkaloids vincristine (VCR), vinblastine (VBL) and vinorelbine (VNR) were investigated for genotoxicity in the wing Somatic Mutation and Recombination Test (SMART) of Drosophila. Our in vivo experiments demonstrated that all drugs assessed induced genetic toxicity, causing increments in the incidence of mutational events, as well as in somatic recombination. Another point to be considered is the fact that VNR was able to induce, respectively, approximately 13.0 and 1.7 times more mutant clones per millimolar exposure unit as their analogues VCR and VBL. The replacement of a CH(3) attached to vindoline group in VBL by a CHO in VCR seems to be responsible for the approximately seven times higher potency of the former. In contrast, the structural modifications on VNR's catharantine group could be related to its higher genotoxic potency, as well as its similar mutagenic and recombinagenic action.

Genotoxic ability of cadmium, chromium and nickel salts studied by kinetochore staining in the cytokinesis-blocked micronucleus assay

The aneugenic and clastogenic ability of cadmium chloride(II), cadmium sulfate(II), nickel chloride(II), nickel sulfate(II), chromium chloride(III) and potassium dichromate(IV) have been evaluated through kinetochore-stained micronucleus test. Traditional genotoxicity assays evaluate DNA damage, gene mutations and chromosome breakage. However, these tests are not adequate to detect aneugenic agents that do not act directly on DNA. Staining kinetochores in the cytokinesis-blocked micronucleus assay is a useful way to discriminate between clastogens and aneuploidogens and may allow a rapid identification of aneuploidy-inducing environmental compounds. Human diploid fibroblasts (MRC-5) were employed. All compounds increased micronuclei frequency in a statistically significant way. However, increases in kinetochore-positive micronuclei frequencies were higher than in kinetochore-negative ones. The present work demonstrates the genotoxic ability of the cadmium and chromium salts studied. Aneugenic as well as clastogenic ability could be observed with this assay. Nickel salts, as it was expected because of their known weak mutagenicity, showed lower genotoxic effects than the other metal salts studied. As the test employed only allows the detection of malsegregation, it is proposed that this mechanism is at least one of those by which the tested metal salts induced aneuploidy. On the other hand, visualization of kinetochores in all experiments suggests that the compounds studied did not act by damaging these structures.

In vivo rodent micronucleus assay: protocol, conduct and data interpretation

In vivo rodent micronucleus assay has been widely used to detect genotoxicity. Evaluation of micronucleus induction is the primary in vivo test in a battery of genotoxicity tests and is recommended by the regulatory agencies around the globe to be conducted as part of product safety assessment. The assay, when performed appropriately, detects both clastogenicity and aneugenicity. Methods for performing micronucleus evaluation have evolved since its initial description in the 1970s. In recent years, the focus has been directed toward improving micronucleus detection with high efficiency by proposing data-based recommendations to the standard initial protocol design. Such improvements include, e.g., the use of appropriate harvest time(s), inclusion of one or both sexes, number of doses tested, limit dose, integrating micronucleus assessment into the routine toxicology studies, use of fluorescent staining, automation of micronucleus detection and assessment of micronuclei in multiple tissues. This protocol paper describes: the mechanism of micronucleus formation, a generalized protocol for manual detection, enumeration of micronuclei, and data interpretation in light of published information thus far, on the regulatory aspects of this assay. Certain recent protocol issues that are practical in nature are equally valid in relation to standard manual method and provide robust database, which are also included for consideration. It is expected that such improvements of the protocol will continue to drive the utility of this assay in the product safety assessment.

Human cytogenetic biomonitoring using flow-cytometric analysis of micronuclei in transferrin-positive immature peripheral blood reticulocytes

We have developed a method to isolate and analyze nascent human reticulocytes in peripheral blood for the presence of micronuclei (MN). For a very short time peripheral reticulocytes show residual expression of the transferrin receptor. Using immunomagnetic separation of cells expressing the transferrin receptor, a population of immature reticulocytes (Trf-Ret) was isolated from peripheral blood. In humans, the spleen actively removes micronucleated erythrocytes but during the short lifetime of the isolated Trf-Ret only a fraction (less than about 20%) of the MN-containing reticulocytes will have been eliminated. Cells were stained with the fluorescent dyes Thiazole Orange for RNA and Hoechst 33342 for DNA and analyzed by flow cytometry and fluorescence microscopy. Baseline frequencies of MN-Trf-Ret on a group of healthy donors were found to be 1.1% for males and 1.4% for females; however, the gender difference was not significant. The frequency of MN-Trf-Ret in the studied group increased with age, and was dependent on blood group. In three donors studied over 4 months, the baseline level remained stable. In cancer patients treated with radiation or chemotherapy, the frequency of MN-Trf-Ret increased 10- to 20-fold after 1-4 days, depending on the treatment. A high correlation between flow and manual analysis of MN-Trf-Ret was seen. We believe the method has a high potential as a sensitive and rapid method for biological monitoring in presumed exposed groups and individuals.

In vivo rodent erythrocyte micronucleus assay. II. Some aspects of protocol design including repeated treatments, integration with toxicity testing, and automated scoring

An expert working group on the in vivo micronucleus assay, formed as part of the International Workshop on Genotoxicity Test Procedures (IWGTP), discussed protocols for the conduct of established and proposed micronucleus assays at a meeting held March 25-26, 1999 in Washington, DC, in conjunction with the annual meeting of the Environmental Mutagen Society. The working group reached consensus on a number issues, including: (1) protocols using repeated dosing in mice and rats; (2) integration of the (rodent erythrocyte) micronucleus assay into general toxicology studies; (3) the possible omission of concurrently-treated positive control animals from the assay; (4) automation of micronucleus scoring by flow cytometry or image analysis; (5) criteria for regulatory acceptance; (6) detection of aneuploidy induction in the micronucleus assay; and (7) micronucleus assays in tissues (germ cells, other organs, neonatal tissue) other than bone marrow. This report summarizes the discussions and recommendations of this working group. In the classic rodent erythrocyte assay, treatment schedules using repeated dosing of mice or rats, and integration of assays using such schedules into short-term toxicology studies, were considered acceptable as long as certain study criteria were met. When the micronucleus assay is integrated into ongoing toxicology studies, relatively short-term repeated-dose studies should be used preferentially because there is not yet sufficient data to demonstrate that conservative dose selection in longer term studies (longer than 1 month) does not reduce the sensitivity of the assay. Additional validation data are needed to resolve this point. In studies with mice, either bone marrow or blood was considered acceptable as the tissue for assessing micronucleus induction, provided that the absence of spleen function has been verified in the animal strains used. In studies with rats, the principal endpoint should be the frequency of micronucleated immature erythrocytes in bone marrow, although scoring of peripheral blood samples gives important supplementary data about the time course of micronucleus induction. When dose concentration and stability are verified appropriately, concurrent treatment with a positive control agent is not necessary. Control of staining and scoring procedures can be obtained by including appropriate reference samples that have been obtained from a separate experiment. For studies in rats or mice, treatment/sampling regimens should include treatment at intervals of no more than 24 hr (unless the test article has a half-life of more than 24 hr) with sampling of bone marrow or blood, respectively, within 24 or 40 hr after the last treatment. The use of a DNA specific stain is recommended for the identification of micronuclei, especially for studies in the rat. In the case of a negative assay result with a non-toxic test article, it is desirable that systemic exposure to the test article is demonstrated. The group concluded that successful application of automated scoring by both flow cytometry and image analysis had been achieved, and defined criteria that should be met if automated scoring is employed. It was not felt appropriate to attempt to define specific recommended protocols for automated scoring at the present time. Other issues reviewed and discussed by the working group included micronucleus assays that have been developed in a number of tissues other than bone marrow. The group felt that these assays were useful research tools that could also be used to elucidate mechanisms in certain regulatory situations, but that these assays had not yet been standardized and validated for routine regulatory application.

MAPK mediates RAS-induced chromosome instability

The generation of micronuclei is a reflection of DNA damage, defective mitosis, and loss of genetic material. The involvement of the MAPK pathway in mediating v-ras-induced micronuclei in NIH 3T3 cells was examined by inhibiting MAPK activation. Conversely, the MAPK pathway was constitutively activated by infecting cells with a v-mos retrovirus. Micronucleus formation was inhibited by the MAPK kinase inhibitors PD98059 and U0126, but not by wortmannin, an inhibitor of the Ras/phosphatidylinositol 3-kinase pathway. Transduction of cells with v-mos resulted in an increase in micronucleus formation, also consistent with the involvement of the MAPK pathway. Staining with the anti-centromeric CREST antibody revealed that instability induced by constitutive activation of MAPK is due predominantly to aberrant mitotic segregation, since most of the micronuclei were CREST-positive, reflective of lost chromosomes. A significant fraction of the micronuclei were CREST-negative, reflective of lost acentric chromosome fragments. Some of the instability observed was due to mitotic events, consistent with the increased formation of bi-nucleated cells, which result from perturbations of the mitotic spindle and failure to undergo cytokinesis. This chromosome instability, therefore, is a consequence of mitotic aberrations, mediated by the MAPK pathway, including centrosome amplification and formation of mitotic chromosome bridges.

Validation study of the in vitro micronucleus test in a Chinese hamster lung cell line (CHL/IU)

We conducted a collaborative validation study, under the auspices of the Japanese Ministry of Labour, on the in vitro micronucleus test to see if it could be used as an alternative to the in vitro chromosome aberration test for evaluation of chemical safety. We used the Chinese hamster lung cell line (CHL/IU), which is the most widely used system for the latter test in Japan, and evaluated 66 chemicals, including clastogens and polyploidy inducers. The cytochalasin B cytokinesis blocking method, which is commonly used in human lymphocyte culture, was applied to the established cell line, but did not improve the detection of chemically-induced micronuclei in continuously growing cells. The highest micronucleus frequencies were obtained at 48 or 72 h continuous treatments. In short treatments (6 h), a 42 h recovery time yielded the best responses. Concordance between the results of the micronucleus test and the chromosomal aberration test was satisfactorily high (88.7%), and we concluded that the in vitro micronucleus test could be used in place of the chromosomal aberration test as a simple and rapid method for detecting clastogens and aneugens in vitro. We also propose a protocol for the test.

Induction of micronuclei in bone marrow by two pesticides and their differentiation with CREST staining: an in vivo study in mice

Two pesticides, organophosphate phosphamidon (PHO) and organochlorine dieldrin (DED) were assayed by the mouse bone marrow micronucleus test, to ascertain whether they showed genotoxic activity in vivo. Two doses, sub-lethal (PHO=3 mg/kg b.wt.; DED=60 mg/kg b.wt.) and lethal (PHO=5 mg/kg b.wt.; DED=90 mg/kg b.wt.), of each substance were administered intraperitoneally to 9-10-week old CBA male mice, in acute and repeated exposure. The sub-lethal dose was also administered at two different times and twice at 24-h intervals. Both PHO and DED proved able to induce a dose-dependent increase of micronucleated polychromatic erythrocytes (PCE). The two pesticides also showed a different detoxification time. Furthermore, the CREST staining with antikinetochore antibodies allowed us to conclude that the two chemicals are clastogens.

Low dose effects of chemicals as assessed by the flow cytometric in vivo micronucleus assay

Using flow cytometric automation of the mouse in vivo, micronucleus assay increases the sensitivity of the test. This is achieved through a very large increase in the number of cells scored, by a factor of 100x, which in turn greatly reduces the sampling error. With this method, dose-response relationships of in vivo micronucleus induction for four model agents mitomycin C (MMC), diepoxybutane (DEB), cyclophosphamide (CPA), and colchicine (COL) were studied at low dose levels. For the three clastogens MMC, DEB and CPA, linear dose-response relationships were found over the dose ranges studied, even in the very low dose region (defined as the dose region where the frequency of micronucleated erythrocytes is less than twice the baseline frequency). This is consistent with the view that no threshold should exist for genotoxic agents which target DNA. For COL a dose range was found, in which the frequency of micronucleated erythrocytes did not increase with dose, possibly indicating an in vivo threshold. The flow cytometric in vivo micronucleus assay represents one possibility for in vivo low dose-response studies.

The position of the in vitro micronucleus test within the battery of screening for genotoxic potential determination and the regulatory guidelines

The in vitro micronucleus test is available to demonstrate the clastogenic and aneugenic potencies of chemicals. This test is easier to evaluate than metaphase analysis and, although used routinely by some laboratories as a prescreening test for the determination of chromosomal mutation induction potency, it has not been recognised by regulatory authorities as a test to be included in the battery of assays for human risk assessment. This limitation is due to the lack of a fixed protocol, a limited data bank, particularly in the case of clastogens, and to the lack of sufficient robust interlaboratory validation studies. If we aim to recognize this test in the form of an OECD guideline and to introduce it as an alternative to in vitro metaphase analysis in cell culture, it is necessary to begin a collaborative study in order to determine the best protocol which is valid for the detection of all clastogens as well as to determine clearly the limits of the test. The use of this in vitro test as a prescreening assay could be very useful as it takes structural and numerical chromosomal abnormalities into account and is simpler to perform than in vitro metaphase analysis.

A critical evaluation of centromeric labeling to distinguish micronuclei induced by chromosomal loss and breakage in vitro

The in vitro micronucleus assay in conjunction with CREST-staining and fluorescence in situ hybridization (FISH) with centromere-specific DNA probes is being increasingly utilized for the detection of clastogenic and aneuploidy-inducing agents. Although potentially powerful techniques, both methods have unique characteristics that can influence sample processing and the interpretation of results. In this article, the use of the CREST and the FISH modifications of the in vitro micronucleus assay have been used to characterize the origin of the micronuclei induced by cyclophosphamide, 4,4'-methylene-bis(2-chloroaniline), 4-nitroquinoline N-oxide and ionizing radiation in metabolically competent MCL-5 cells or a derived cell line lacking metabolic activation. Using these results and our previous experiences with these techniques, a detailed comparison including the strengths and limitations of each technique as well as potential problems in performing each assay and in analyzing the data is discussed. In spite of their limitations, our results to date indicate that CREST-staining as well as FISH with centromere-specific DNA probes can be used to accurately distinguish micronuclei formed from chromosome loss from those originating from chromosome breakage and that these techniques can be valuable complements to the in vitro micronucleus assay.

Analysis of DES-induced micronuclei in binucleated rat fibroblasts: comparison between FISH with a rat satellite I probe and immunocytochemical staining with CREST serum

The usefulness of fluorescence in situ hybridization (FISH) with rat satellite I DNA was compared with immunocytochemical staining with CREST serum for the analysis of the content of micronuclei from primary rat fibroblasts. We analyzed micronuclei induced in vitro by the aneugenic compound diethylstilbestrol (DES) or the clastogenic compound mitomycin C (MMC). Since a centromeric probe was not available for the rat, we isolated rat satellite I DNA by PCR with primers designed on the basis of the known rat satellite I DNA sequence. The PCR products obtained as well as the cloned PCR products showed hybridization to the centromeric regions of a large number of chromosomes, but not of chromosome 1, 19, 20, X and Y. Clone 18-5 was further analyzed and was shown to contain at least 4 repeats of the rat satellite I family. This probe, which hybridizes in the centromeric region of 34 of the 42 chromosomes, was used throughout the study as a probe for the FISH analysis of the micronuclei. For the immunocytochemical staining, the commonly used commercial anti-centromeric antibodies could not be used because of the weakness of the fluorescent signals given. Consequently, CREST serum of a single patient was used, which showed bright and distinct signals on the kinetochores of each chromosome. After treatment of the cells with the aneugen DES an increase in centromere (FISH) and kinetochore (CREST) positive micronuclei was found, whereas after treatment with the clastogen MMC, the percentage of centromere-positive micronuclei was similar to that observed in controls. Analysis of a large number of DES-induced micronuclei showed that the immunocytochemical method is equally as or slightly less sensitive for the detection of chromosomes in micronuclei and we therefore recommend FISH with probe 18-5 for the detection of chromosome loss in rat cells.

The in vitro micronucleus test: a multi-endpoint assay to detect simultaneously mitotic delay, apoptosis, chromosome breakage, chromosome loss and non-disjunction

Genotoxicity testing aims to detect a large range of genetic damage endpoints and evaluate such results in context of cell survival. The cytokinesis block micronucleus test offers the advantage to provide simultaneously information on both cell cycle progression and chromosome/genome mutations. Indeed, 1. frequencies of cytokinesis-blocked binucleated cells (and polynucleated) are good estimators of the mitotic rate; 2. frequencies of apoptotic figures in mononucleated and binucleated cells provide a measure for cell death before or after cell division; 3. combination of fluorescence in situ hybridization (FISH) for centromere/telomeres and micronucleus scoring allows discrimination between clastogenic and aneugenic events; 4. detection of FISH signals for chromosome specific sequences in both macronuclei and micronuclei, discriminates between aneuploidy due to chromosome non-disjunction or to chromosome loss. The cytokinesis block in vitro micronucleus test is thus a cytogenetic multi-test providing mechanistic information with a simple, rapid, objective, microscopical analysis.

Treatment with the anti-tumor drugs, cis-platin and mafosfamide, does not affect the structure of prekinetochores in a human breast cancer cell line. An immunofluorescence study using human anti-centromere autoantibodies

The goal of the present article was to determine whether a nuclear parameter, centromere structure of interphase cells, could serve as an indicator to assess cellular damage caused by anti-tumor drugs. These were cis-platin and mafosfamide, which are widely used for the management of solid tumors. To visualize the centromeres, we probed treated and untreated cells of a human breast cancer cell line, MX-1, with a human anti-centromere serum. The serum was obtained from a scleroderma patient and detects antigens associated with prekinetochores of the decondensed chromosomes. The DNA was simultaneously displayed by a specific fluorescent dye. The cells were grown on coverslips, incubated for 1 h in a drug-containing medium, transferred into a drug-free medium and observed 24 h later. Since the efficiency of many anti-tumor drugs increases with the temperature, two different temperatures, 37 and 42 degrees C, were used. The analysis revealed that the treatment did not visibly alter the labeling pattern. We conclude that chromosome structure remains largely intact and is not suitable for the cytological evaluation of the efficiency of anti-tumor drugs. This is in contrast with, for example, the microtubular cytoskeleton and mitochondria, which were extensively damaged under the conditions applied here (compare Wolf et al. 1995). Independent of the drug and the temperature selected, the nuclear lumen of mononucleated and multinucleated cells contained small fluorescent spots. Double dots corresponding to the sister centromeres in the G2 phase of the cell cycle were rare. In addition to the voluminous nuclei, some cells possessed micronuclei in the lateral cytoplasm and these were regularly labeled by the autoantibodies. A small subset of the mononucleated MX-1 cells had unusually large nuclei. It is reasonable to assume that they are polyploid. The fluorescent spots marking the prekinetochores were very large in these cells. This may indicate that the chromosomes remain associated after replication.

Spermatid micronucleus analysis of aging effects in hamsters

Spermatid micronuclei (MN) from Armenian hamsters in different age groups were compared with regard to frequencies and kinetochore status (presence or absence) as determined with immunofluorescent staining. Six thousand cells analyzed from each of fifteen young animals (3 months) revealed a group mean frequency of 0.45 MN/1000 spermatids; kinetochore staining was uniformly negative. Six thousand cells scored from each of fifteen older animals (2 years) revealed a group mean frequency of 1.00 MN/1000 spermatids. Most of the MN in these animals were negative for kinetochore staining, although a significant representation of MN with positive kinetochore staining was also observed. The results indicate that frequencies of spermatid MN increase with advancing age, and suggest that the increase is due to significant elevations in both chromosome breakage and chromosome loss.

Micronuclei induced in round spermatids of mice after stem-cell treatment with chloral hydrate: evaluations with centromeric DNA probes and kinetochore antibodies

The chromosomal effects of chloral hydrate (CH) on germ cells of male mice were investigated using two methods to detect and characterize spermatid micronuclei (SMN); (a) anti-kinetochore immunofluorescence (SMN-CREST) and (b) multicolor fluorescence in situ hybridization with DNA probes for centromeric DNA and repetitive sequences on chromosome X (SMN-FISH). B6C3F1 mice received single intraperitoneal (i.p.) injections of 82.7, 165.4, or 413.5 mg/kg and round spermatids were sampled at three time intervals representing cells treated in late meiosis, early meiosis, or as spermatogonial stem cells. No increases in the frequencies of SMN were detected for cells treated during meiosis using either SMN-CREST or SMN-FISH methods. After spermatogonial stem-cell treatment, however, elevated frequencies of SMN were detected by both methods. With SMN-FISH, dose trends were observed both in the frequencies of spermatids containing micronuclei and in the frequency of spermatids carrying centromeric label. These findings corroborate the recent report by Allen and colleagues [Allen JW et al.(1994): Mutat. Res. 323:81-88] that CH treatment of spermatogenic stem cells induced SMN. Furthermore, our findings suggest that chromosomal malsegregation or loss may occur in spermatids long after CH treatment of stem cells. Further studies are needed to understand the mechanism of action of the CH effect on stem cells and to determine whether similar effects are induced in human males treated with CH.

In vivo genotoxicity of dichloroacetic acid: evaluation with the mouse peripheral blood micronucleus assay and the single cell gel assay

Chlorination is a widely used method for disinfection of drinking water supplies. Reaction of chlorine with naturally present organic compounds can result in toxic by-products. One major disinfection by-product from the chlorination of drinking water is dichloroacetic acid (DCA). This chemical has been shown to be carcinogenic in rodents, yet little genotoxicity data are available to assess the possible role of DNA and/or chromosomal damage in this process. We have used the peripheral blood erythrocyte micronucleus (MN) assay and the alkaline single cell gel electrophoresis (SCG) technique to investigate the in vivo genotoxicity of DCA in bone marrow and blood leukocytes, respectively. The MN assay detects chromosome breakage and/or malsegregation, while the SCG assay detects DNA damage (e.g., single strand breaks, alkali-labile sites, crosslinking). Mice were exposed to this compound in drinking water, available ad libitum, for up to 31 weeks. Our results show a small but statistically significant dose-related increase in the frequency of micronucleated polychromatic erythrocytes (PCEs) after subchronic exposure to DCA for 9 days. In addition, at the highest dose of DCA tested (3.5 g/l), a small but significant increase in the frequency of micronucleated normochromatic erythrocytes (NCE) was detected following exposure for > or = 10 weeks. Coadministration of the antioxidant vitamin E did not affect the ability of DCA to induce this damage, indicating that the small induction of MN by DCA was probably not due to oxidative damage. Based on the lack of any difference observed in the proportion of kinetochore-positive micronuclei between the treated and control animals, we interpret MN as arising from clastogenic events. The SCG technique suggested the presence of DNA crosslinking in blood leukocytes in mice exposed to 3.5 g/l DCA for 28 days. These data provide evidence that DCA may be an extremely weak inducer of chromosome damage when provided to mice in drinking water under conditions which lead to increased levels of tumors.

Advantages and limitations of using fluorescence in situ hybridization for the detection of aneuploidy in interphase human cells

Fluorescence in situ hybridization with chromosome-specific DNA probes is being increasingly utilized for the detection of chromosome aberrations induced in vitro and in vivo by chemical and physical agents. Although potentially a powerful technique, FISH studies for aneuploidy can be heavily influenced by cellular phenomena and hybridization artifacts which make the performance and interpretation of the results difficult. As a consequence, frequently hyperdiploid frequencies are reported in the literature which are substantially higher than one would expect based upon frequencies seen in conventional metaphase analyses. In this article, a number of the potential pitfalls that we have encountered while performing FISH analyses for aneuploidy are discussed and their potential impact on the observed hybridization frequencies is described. After considering these factors, the frequencies of lymphocyte nuclei containing 3 and 4 chromosome copies are compared between metaphase values obtained from published human population studies and interphase values obtained from similar studies using FISH. It is concluded that by using caution in the evaluation of slides, interphase studies using FISH to detect hyperdiploidy and polyploidy can provide estimates of numerical alterations which closely reflect those seen during metaphase analysis using either FISH or conventional approaches. However, due to the inability of interphase analysis to distinguish hyperdiploidy from polyploidy as well as other potential problems, frequencies of aneuploid nuclei obtained using single label FISH should only be considered approximations of absolute frequencies. For additional accuracy, multi-color FISH with two or more different probes should be performed.

Important biological variables that can influence the degree of chemical-induced aneuploidy in mammalian oocyte and zygotes

The ability of certain chemicals to increase the frequency of aneuploidy in mammalian oocytes elicits concern about human health and well-being. This concernment exists because aneuploidy is the most prevalent class of human genetic disorders, and very little information exists about the etiology of aneuploidy. Although there are experimental models for studying aneuploidy in female germ cells and zygotes, these models are still being validated because insufficient information exists about the biological variables that can influence the degree of chemical-induced aneuploidy. In this regard, variables such as dose, solvent, use of gonadotrophins, mode and preovulatory time of chemical administration, time of cell harvest relative to the possibility of chemical-induced meiotic delay, criteria for cytogenetic analysis and data reporting, and an introduction to differences between cell types and sexes are presented. Besides these variables, additional information is needed about the various molecular mechanisms associated with oocyte meiotic maturation and the genesis of aneuploidy. Also, differences between the results from selected chromosome analysis and DNA-hybridization studies are presented. Based upon the various biologic endpoints measured and the differences in cellular physiology and biochemical pathways, agreement among the results from different aneuploidy assays cannot necessarily be expected. To gain further insight into the etiology of aneuploidy in female germ cells, information is needed about the chemical interactions between endogenous and exogenous compounds and those involved with oocyte meiotic maturation.

Aneuploidies and micronuclei in the germ cells of male mice of advanced age

The objective of this research was to determine whether the frequencies of chromosomally defective germ cells increased with age in male laboratory mice. Two types of chromosomal abnormalities were characterized: (1) testicular spermatid aneuploidy (TSA) as measured by a new method of multi-color fluorescence in situ hybridization (FISH) with DNA probes specific for mouse chromosomes X, Y and 8, and (2) spermatid micronucleus (SMN) analyses using anti-kinetochore antibodies. B6C3F1 mice (aged 22.5 to 30.5 months, heavier than controls but otherwise in good health) showed significant approximately 2.0 fold increases in the aneuploidy phenotypes X-X-8, Y-Y-8, 8-8-X and 8-8-Y with the greatest effects appearing in animals aged greater than 28 months. No age effect was observed, however, in X-Y-8 hyperhaploidy. Major age-related increases were seen in Y-Y-8 and X-X-8 hyperhaploidies suggesting that advanced paternal age is associated primarily with meiosis II rather than meiosis I disjunction errors. A approximately 5 fold increase was also found in the frequency of micronucleated spermatids in aged mice when compared with young controls. All micronuclei detected in the aged animals lacked kinetochore labeling, suggesting that they either did not contain intact chromosomes or the chromosomes lacked detectable kinetochores. The findings of the TSA and SMN assays are consistent with meiotic or premeiotic effects of advanced age on germ cell chromosomes, but there were differences in the age dependencies of aneuploidy and micronuclei. In summary, advanced paternal age may be a risk factor for chromosomal abnormalities (both aneuploidy and structural abnormalities) in male germ cells.

Genotoxicity of quercetin in the micronucleus assay in mouse bone marrow erythrocytes, human lymphocytes, V79 cell line and identification of kinetochore-containing (CREST staining) micronuclei in human lymphocytes

Quercetin, a mutagenic flavonoid widely distributed in edible plants, was studied for the induction of micronuclei (MN). We have carried out the MN assay in bone marrow polychromatic erythrocytes in mice, in cytokinesis-blocked human lymphocytes and in cytokinesis-blocked V79 cells. MN assay in vitro was performed in the presence and in the absence of S9. To further extend the study, an antikinetochore antibody (CREST staining) was used to distinguish MN containing whole chromosomes (kinetochore positive) from those containing acentric fragments (kinetochore negative). When tested in vivo quercetin failed to induce micronuclei, a result which is in agreement with other published reports. When tested in vitro in V79 cells quercetin clearly induces micronuclei in the absence of S9 and also in the presence of S9 for the highest dose used. When tested in vitro in human lymphocytes quercetin shows a significant induction of micronuclei in the absence and in the presence of S9. The presence of S9 compared to its absence is not significant for any of the systems used. Both in the presence and absence of S9, quercetin appears to behave as a clastogenic agent in human lymphocytes inducing a significant majority of kinetochore-negative MN.

Vanadium salts induce cytogenetic effects in in vivo treated mice

Three vanadium salts, vanadyl sulfate (SVO5), sodium orthovanadate (Na3VO4) and ammonium metavanadate (NH4VO3), were tested for induction of genotoxic effects in bone marrow of mice following intragastric treatment. Micronucleus (MN) induction in polychromatic erythrocytes (PCEs), structural (sCA) and numerical (nCA) chromosome aberrations in bone marrow cells were evaluated. The micronucleus test, performed at different harvesting times (0-72 h), was found to be positive for all compounds tested. In contrast, except for vanadyl sulfate, no difference was found between controls and treated animals in the sCA test performed 24 and 36 h after treatment. At the same sampling intervals, second metaphases (M II) were positively scored for nCA induction for all three vanadium salts. In addition, the frequency of hypoploid and hyperploid cells was shown to be statistically different from the control value. Polyploid cells were also induced by all compounds, but their frequency was not statistically significant. The positive results obtained by nCA analysis support the finding of a significant presence of types of micronuclei that are probably aneuploidy-related. This finding was further supported by the successful classification of such micronuclei on the basis of shape and size according to Tinwell and Ashby (1991) during microscope analysis.

Simplified and optimized kinetochore detection: cytogenetic marker for late-G2 cells

Cytogenetic detection of kinetochore proteins using the CREST antibody coupled with secondary antibodies labeled with different fluorescent probes has been optimized for several in vitro mammalian cell lines. This study investigated selected parameters including the influence of common fixatives (methanol, ethanol, methanol:acetic acid (3:1)), detergents (Triton-X100, Tween), fluorescent probes (CY3, BODIPY, FITC), washing protocols, and an antifading agent (paraphenylenediamine) on the detection of kinetochore proteins in control and X-ray (240 kVp)-irradiated cells. Utilizing an optimized fixation and staining protocol, a brilliant visualization of kinetochores in interphase cells was obtained in control as well as X-ray-irradiated interhase cells. Application of this improved kinetochore staining methodology readily permits discriminating cells containing either single or paired kinetochores, the latter of which are characteristic of late-G2 phase and prophase cells.

Detection of chromosome malsegregation to the daughter nuclei in cytokinesis-blocked transgenic mouse splenocytes

Most of the recently developed tests for detecting aneugenic activity of chemicals are based on the induction of micronuclei (MN) in cytokinesis-blocked (CB) binucleated cells. In such a test, aneugens can be discriminated from clastogens by checking for the presence of centromeres in the MN, indicating the loss of whole chromosomes. Tracing particular chromosomes in interphase nuclei using fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes is another method used for detecting numerical chromosome aberrations. Here, we describe a method using a cytokinesis-blocked MN assay in combination with identifying specific chromosomes of mice. For this purpose transgenic mice with foreign DNA inserted in three pairs of their chromosomes were generated. Splenocytes of these mice were cultured and treated in vitro with vinblastine (VBL) or X-rays, followed by recovery in medium containing cytochalasin B. By tracing the marker chromosomes in binucleated splenocytes, reciprocal products of chromosome malsegregation to the daughter nuclei could be easily traced. The results showed that besides clastogenic activity, X-rays also exhibited aneugenic activity. Treatment with vinblastine showed a close relationship between micronuclei induction and chromosome malsegregation, although at higher doses malsegregation processes became more prominent. Simultaneous malsegregation of more than one chromosome was observed frequently, but the three marker chromosomes were found to be randomly involved in this process.

Effects of vinblastine and colchicine on male rat meiosis in vivo: disturbances in spindle dynamics causing micronuclei and metaphase arrest

We have studied the effects of vinblastine sulfate (VBL) and colchicine (COL) on male rat in vivo and in vitro meiosis. A novel methodology based on isolating a segment of seminiferous tubules containing meiotically dividing spermatocytes was applied. During meiotic divisions at stage XIV of rat spermatogenesis, both chemicals induced only low frequencies of micronuclei (MN), 0.8-3.2 MN/1,000 spermatids. Fluorescence in situ hybridization experiments in mice with the mouse centromere-specific gamma-satellite DNA probe showed that 50.7% of VBL-induced MN and 56.6% of COL-induced MN were centromere positive, indicating that the MN induced by both chemicals contained detached chromosomes. The inhibition of cell proliferation was determined by counting the number of cells arrested at metaphase during the first meiotic (MI) or the second meiotic (MII) division. VBL was found to be a potent inducer of cell death while COL was not. The direct effects of VBL and COL on the meiotic spindles were evaluated using immunohistochemistry with anti-alpha-tubulin and confocal microscopy. In the control animals a significant difference was observed between the mean length of metaphase spindles of MI and MII. Both were dramatically decreased 6 hr after treatment with 2.0 mg/kg of VBL and 0.8 mg/kg of COL, respectively. At 18 hr after COL injection the spindles had about the same length as in the controls. However, the VBL-induced shortening was even more evident at 18 hr for both MI and MII. The possible reasons for observed differences between the two chemicals and between meiosis and mitosis are discussed.