Measurement of micronuclei by cytokinesis-block method in cultured Chinese hamster cells: comparison with types and rates of chromosome aberrations

Micronucleus Assay: The State of Art, and Future Directions

During almost 40 years of use, the micronucleus assay (MN) has become one of the most popular methods to assess genotoxicity of different chemical and physical factors, including ionizing radiation-induced DNA damage. In this minireview, we focus on the position of MN among the other genotoxicity tests, its usefulness in different applications and visibility by international organizations, such as International Atomic Energy Agency, Organization for Economic Co-operation and Development and International Organization for Standardization. In addition, the mechanism of micronuclei formation is discussed. Finally, foreseen directions of the MN development are pointed, such as automation, buccal cells MN and chromothripsis phenomenon.

Scientific Opinion on Flavouring Group Evaluation 49, Revision 1 (FGE.49Rev1): xanthine alkaloids from the priority list

Following a request from the European Commission, the EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF) was requested to deliver a scientific opinion on the safety assessment of the flavouring substances caffeine [FL-no: 16.016] and theobromine [FL-no: 16.032] in the Flavouring Group Evaluation 49, Revision 1. Consequent to the 2015 scientific opinion from the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) on the safety of caffeine from all dietary sources, the CEF Panel considered it inappropriate to evaluate the two substances through the Procedure. For caffeine, the Panel based its assessment on the safety threshold of 5.7 mg/kg body weight (bw) per day for adults, except pregnant/lactating women, and 3 mg/kg bw per day for children, adolescents, pregnant and lactating women, as established by the NDA Panel. The safety evaluation of theobromine takes into account that approximately 11% of an oral dose of caffeine is metabolised to theobromine and that both substances have a similar pharmacological profile. For the exposure assessment, a brand loyalty model was chosen. In this model, it was assumed that a consumer is exposed on a long-term basis to a specific category of food (i.e. non-alcoholic beverages), containing caffeine or theobromine at their respective maximum use levels. For the rest of the categories, normal use levels applied. Daily dietary exposure to caffeine and theobromine (excluding systemic exposure) added as a chemically defined flavouring substance ranged 0-2.3 and 0-0.4 mg/kg bw, respectively, across all population groups. The Panel concluded that caffeine [FL-no: 16.016] and theobromine [FL-no: 16.032] would not be expected to present safety concern based on their estimated levels of intake from their use as flavouring substances.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Acute restraint stress increases the frequency of vinblastine-induced micronuclei in mouse bone marrow cells

Acute physiological stress induces remarkable effects on the nervous, endocrine, and immune systems and also on cellular metabolism and cell division processes. Stress-induced instability of cellular mechanisms might play an important role in increasing cell division disorders. In this study, a relationship between stress and micronucleus (MN) induction in mouse (balb/c) bone marrow cells following vinblastine treatment, or stress or stress and vinblastine treatment in comparison to a non-stressed control group was investigated. In order to test the effects of treatments on MN induction, an in vivo MN assay was performed on bone marrow cells. The results revealed a significantly greater increase in MNs in bone marrow cells (polychromatic erythrocytes) from the stressed/vinblastine treated mice. The data indicate the ability of exposure to an emotional stressor to enhance the damaging actions on bone marrow cells of an aneugenic agent.

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.

Cytokinesis-block micronucleus cytome assay

The cytokinesis-block micronucleus cytome assay is a comprehensive system for measuring DNA damage, cytostasis and cytotoxicity. DNA damage events are scored specifically in once-divided binucleated (BN) cells and include (a) micronuclei (MNi), a biomarker of chromosome breakage and/or whole chromosome loss, (b) nucleoplasmic bridges (NPBs), a biomarker of DNA misrepair and/or telomere end-fusions, and (c) nuclear buds (NBUDs), a biomarker of elimination of amplified DNA and/or DNA repair complexes. Cytostatic effects are measured via the proportion of mono-, bi- and multinucleated cells and cytotoxicity via necrotic and/or apoptotic cell ratios. Further information regarding mechanisms leading to MNi, NPBs and NBUDs formation is obtained using centromere and/or telomere probes. The assay is being applied successfully for biomonitoring of in vivo genotoxin exposure, in vitro genotoxicity testing and in diverse research fields such as nutrigenomics and pharmacogenomics as well as a predictor of normal tissue and tumor radiation sensitivity and cancer risk. The procedure can take up to 5 days to complete.

The micronucleus assay determination of chromosomal level DNA damage

The study of DNA damage at the chromosome level is an essential part of genetic toxicology because chromosomal mutation is an important event in carcinogenesis. Micronucleus assays have emerged as one of the preferred methods for assessing chromosome damage because they enable both chromosome loss and chromosome breakage to be measured reliably. Because micronuclei can only be expressed in cells that complete nuclear, division a special method was developed that identifies such cells by their binucleate appearance when blocked from performing cytokinesis by cytochalasin-B, a microfilament-assembly inhibitor. The cytokinesis-block micronucleus (CBMN) assay allows better precision because the data obtained are not confounded by altered cell division kinetics caused by cytotoxicity of agents tested or suboptimal cell culture conditions. The method is now applied to various cell types for population monitoring of genetic damage, screening of chemicals for genotoxic potential and for specific purposes such as the prediction of the radiosensitivity of tumors and the interindividual variation in radiosensitivity. In its current basic form the CBMN assay can provide, using simple morphological criteria, the following measures of genotoxicity and cytotoxicity: chromosome breakage, chromosome loss, chromosome rearrangement (nucleoplasmic bridges), gene amplification (nuclear buds), cell division inhibition, necrosis and apoptosis. The cytosine arabinoside modification of the CBMN assay allows for measurement of excision repairable lesions. The use of molecular probes enables chromosome loss to be distinguished from chromosome breakage and importantly nondisjunction in nonmicronucleated binucleated cells can be efficiently measured. The CBMN technique therefore provides multiple and complementary measures of genotoxicity and cytotoxicity which can be achieved with relative ease within one system. The basic principles and methods (including detailed scoring criteria for all the genotoxicity and cytotoxicity end points) of the CBMN assay are described and areas for future development identified.

SFTG international collaborative study on in vitro micronucleus test

In this report, are presented the results of an international collaborative study on the in vitro micronucleus assay, using CHL cells. Fourteen laboratories participated in this study which was coordinated by an organizing committee supported by the SFTG (the French branch of the European Environmental Mutagen Society). Nine coded substances, having different modes of action and at different levels were assessed in the in vitro micronucleus test, using a common protocol. Mitomycin C was used as a positive control. In order to help to define a standard protocol on CHL cells, short and long treatment periods followed by various recovery times, with or without cytochalasin B, were compared. After an evaluation of the acceptability of the assays, the tested chemicals were classified as negative, positive or equivocal. Mannitol and clofibrate were judged as negative in all treatment schedules. Bleomycin was positive in all the treatment schedules, with an increase in the number of micronucleated cells in both mononucleate and binucleate cells when using cytochalasin B. This was also shown for the aneugens colchicine, diethylstilboestrol and griseofulvin, as expected. Urethane was judged as equivocal only after long treatment with cytochalasin B, and negative in all other treatment schedules. In any case, no genotoxic compound would have been missed with schedules including a short and a long treatment time, whether the treatment was followed by a recovery period or not and whether cytochalasin B was used or not. Thus, these results show that CHL cells were suitable for accurately detecting clastogenic and aneugenic compounds of various types in the in vitro micronucleus test.

Chitosan Oligosaccharide inhibits203HgCl2-induced genotoxicity in mice: Micronuclei occurrence and chromosomal aberration

The purpose of this study was to investigate the safety of chitosan oligosaccharide and the effects of chitosan oligosaccharide on mercury induced genotoxicity in mice using the micronuclei and chromosome aberration. The micronuclei test was performed by microscopic examination (x1,000, stained using a May-Grunwald solution) after administering 0.01, 0.1, and 1% (10 mg/mL) chitosan oligosaccharide for 7, 60, and 180 days ad libitum in mice. Total micronuclei of 1,000 polychromatic erythrocytes were recorded for each group. There was no difference between the untreated and experimental groups. The intake periods and concentrations of chitosan oligosaccharide did not affect the occurrence of micronuclei in bone marrow cells (P>0.05). The chromosomal aberration test was performed by microscopic examination (x1,000, stained using a 4% Giemsa solution) after administering the same concentration of chitosan oligosaccharide to mice, in F1, F2, F3 generations and parents. The frequency of chromosomal aberrations was defined as [Ydr = (D+R)/total number of counted lymphocytes]. Similar to the micronuclei test, there was no difference between the untreated and treated groups. These results showed that the intake periods and concentrations of chitosan oligosaccharide did not affect chromosomal aberrations in bone marrow cells (P>0.05). To investigate the effect of chitosan oligosaccharide on mercury-induced chromosome aberration, mice in each condition were supplied with 203HgCl2 and chitosan oligosaccharide ad libitum. Chitosan oligosaccharide significantly inhibited 203HgCl2-induced chromosome aberration in mice. Based on the results of this study, it may be concluded that the chitosan oligosaccharide is a nontoxic material that could be used as a suppressor of heavy metal-induced genotoxicity.

Characterization of accelerated iron-ion-induced damage in gap junction-competent and -incompetent thyroid follicular cells

Early- and late-passage cultures of Fischer rat thyroid cells differ in their growth properties and gap junction competency. Previous studies comparing early- and late-passage cultures exposed to gamma rays and proton beams revealed that differences in growth rate did not influence their responses; however, the presence of connexin 32 gap junctions conferred resistance to gamma radiation. To further assess differences in radiation quality, suspension cultures of early- and late-passage cells were exposed to accelerated iron ions, and their comparative biological responses were measured. The iron-ion-irradiated cells displayed sustained levels of incorporated dUTP, reflecting persistent DNA damage. These results were supported by the frequency of chromosomal damage measured by micronucleus formation. Iron-ion irradiation induced micronuclei at a rate of eight per gray per 100 binucleated cells scored in early-passage cells and nine per gray per 100 binucleated cells scored in late-passage cells. Relative to photons, the calculated radiobiological effectiveness for frequency of micronuclei was 5.7 and 6.4 for the early- and late-passage cultures, respectively (P > 0.05). Levels of apoptosis fluctuated as a function of dose, and modest increases above basal levels persisted throughout the 48-h period. The comparison of retained follicular structures revealed differences in the alpha components of the linear-quadratic dose-response curves (0.60 Gy(-1) for early-passage and 0.71 Gy(-1) for late-passage cultures, P < 0.014). Cell cycle phase redistribution resulted in a G2 arrest (P < 0.001) for both early- and late-passage cultures. In conclusion, the response of thyroid follicular cells to high-LET radiation was not influenced by the presence of gap junctions or the proliferative status of the target cells.

A protocol for the in vitro micronucleus test. I. Contributions to the development of a protocol suitable for regulatory submissions from an examination of 16 chemicals with different mechanisms of action and different levels of activity

The in vitro micronucleus (IVM) test is currently used as a screen during the early stages of pharmaceutical development to identify chemicals likely to produce positive outcomes in the in vitro chromosome aberration assay. For several reasons, the assay is being considered as an alternative to the aberration assay, but the current screening protocols are not rigorous enough to fully satisfy concerns about genotoxic safety. This manuscript describes the investigation of several protocol parameters to assist with the development of a regulatory guideline for the IVM test. The parameters investigated are: the effect of cytochalasin B on the outcome of the assay when conducted with continually growing cell lines; the need for an extended exposure in the absence of metabolic activation; and the number of cells to be counted for a valid assay. In addition, two statistical procedures for the analysis of data from the test are described. The results of the investigation indicate that cytochalasin B does not effect the outcome of the test, that the extended exposure treatment is not necessary, that counting 2000 cells is preferable to counting 1000, and that the data can be appropriately analyzed using a trend test.

The effect of 835.62 MHz FDMA or 847.74 MHz CDMA modulated radiofrequency radiation on the induction of micronuclei in C3H 10T(1/2) cells

To determine if radiofrequency (RF) radiation induces the formation of micronuclei, C3H 10T(1/2) cells were exposed to 835.62 MHz frequency division multiple access (FDMA) or 847.74 MHz code division multiple access (CDMA) modulated RF radiation. After the exposure to RF radiation, the micronucleus assay was performed by the cytokinesis block method using cytochalasin B treatment. The micronuclei appearing after mitosis were scored in binucleated cells using acridine orange staining. The frequency of micronuclei was scored both as the percentage of binucleated cells with micronuclei and as the number of micronuclei per 100 binucleated cells. Treatment of cells with cytochalasin B at a concentration of 2 microg/ml for 22 h was found to yield the maximum number of binucleated cells in C3H 10T(1/2) cells. The method used for the micronucleus assay in the present study detected a highly significant dose response for both indices of micronucleus production in the dose range of 0.1-1.2 Gy and it was sensitive enough to detect a significant (P > 0.05) increase in micronuclei after doses of 0.3 Gy in exponentially growing cells and after 0.9 Gy in plateau-phase cells. Exponentially growing cells or plateau-phase cells were exposed to CDMA (3.2 or 4.8 W/kg) or FDMA (3.2 or 5.1 W/kg) RF radiation for 3, 8, 16 or 24 h. In three repeat experiments, no exposure condition was found by analysis of variance to result in a significant increase relative to sham-exposed cells either in the percentage of binucleated cells with micronuclei or in the number of micronuclei per 100 binucleated cells. In this study, data from cells exposed to different RF signals at two SARs were compared to a common sham-exposed sample. We used the Dunnett's test, which is specifically designed for this purpose, and found no significant exposure-related differences for either plateau-phase cells or exponentially growing cells. Thus the results of this study are not consistent with the possibility that these RF radiations induce micronuclei.

Response of thyroid follicular cells to gamma irradiation compared to proton irradiation. I. Initial characterization of DNA damage, micronucleus formation, apoptosis, cell survival, and cell cycle phase redistribution

The RBE of protons has been assumed to be equivalent to that of photons. The objective of this study was to determine whether radiation-induced DNA and chromosome damage, apoptosis, cell killing and cell cycling in organized epithelial cells was influenced by radiation quality. Thyroid-stimulating hormone-dependent Fischer rat thyroid cells, established as follicles, were exposed to gamma rays or proton beams delivered acutely over a range of physical doses. Gamma-irradiated cells were able to repair DNA damage relatively rapidly so that by 1 h postirradiation they had approximately 20% fewer exposed 3' ends than their counterparts that had been irradiated with proton beams. The persistence of free ends of DNA in the samples irradiated with the proton beam implies that either more initial breaks or a quantitatively different type of damage had occurred. These results were further supported by an increased frequency of chromosomal damage as measured by the presence of micronuclei. Proton-beam irradiation induced micronuclei at a rate of 2.4% per gray, which at 12 Gy translated to 40% more micronuclei than in comparable gamma-irradiated cultures. The higher rate of micronucleus formation and the presence of larger micronuclei in proton-irradiated cells was further evidence that a qualitatively more severe class of damage had been induced than was induced by gamma rays. Differences in the type of damage produced were detected in the apoptosis assay, wherein a significant lag in the induction of apoptosis occurred after gamma irradiation that did not occur with protons. The more immediate expression of apoptotic cells in the cultures irradiated with the proton beam suggests that the damage inflicted was more severe. Alternatively, the cell cycle checkpoint mechanisms required for recovery from such damage might not have been invoked. Differences based on radiation quality were also evident in the alpha components of cell survival curves (0.05 Gy(-1) for gamma rays, 0.12 Gy(-1) for protons), which suggests that the higher level of survival of gamma-irradiated cells could be attributed to the persistence of nonlethally irradiated thyrocytes and/or the capacity to repair damage more effectively than cells exposed to equal physical doses of protons. The final assessment in this study was radiation-induced cell cycle phase redistribution. Gamma rays and protons produced a similar dose-dependent redistribution toward a predominantly G(2)-phase population. From our cumulative results, it seems likely that a majority of the proton-irradiated cells would not continue to divide. In conclusion, these findings suggest that there are quantitative and qualitative differences in the biological effects of proton beams and gamma rays. These differences could be due to structured energy deposition from the tracks of primary protons and the associated high-LET secondary particles produced in the targets. The results suggest that a simple dose-equivalent approach to dosimetry may be inadequate to compare the biological responses of cells to photons and protons.

The in vitro micronucleus technique

The study of DNA damage at the chromosome level is an essential part of genetic toxicology because chromosomal mutation is an important event in carcinogenesis. The micronucleus assays have emerged as one of the preferred methods for assessing chromosome damage because they enable both chromosome loss and chromosome breakage to be measured reliably. Because micronuclei can only be expressed in cells that complete nuclear division a special method was developed that identifies such cells by their binucleate appearance when blocked from performing cytokinesis by cytochalasin-B (Cyt-B), a microfilament-assembly inhibitor. The cytokinesis-block micronucleus (CBMN) assay allows better precision because the data obtained are not confounded by altered cell division kinetics caused by cytotoxicity of agents tested or sub-optimal cell culture conditions. The method is now applied to various cell types for population monitoring of genetic damage, screening of chemicals for genotoxic potential and for specific purposes such as the prediction of the radiosensitivity of tumours and the inter-individual variation in radiosensitivity. In its current basic form the CBMN assay can provide, using simple morphological criteria, the following measures of genotoxicity and cytotoxicity: chromosome breakage, chromosome loss, chromosome rearrangement (nucleoplasmic bridges), cell division inhibition, necrosis and apoptosis. The cytosine-arabinoside modification of the CBMN assay allows for measurement of excision repairable lesions. The use of molecular probes enables chromosome loss to be distinguished from chromosome breakage and importantly non-disjunction in non-micronucleated binucleated cells can be efficiently measured. The in vitro CBMN technique, therefore, provides multiple and complementary measures of genotoxicity and cytotoxicity which can be achieved with relative ease within one system. The basic principles and methods (including detailed scoring criteria for all the genotoxicity and cytotoxicity end-points) of the CBMN assay are described and areas for future development identified.

Dose-rate effects for apoptosis and micronucleus formation in gamma-irradiated human lymphocytes

We have compared dose-rate effects for gamma-radiation-induced apoptosis and micronucleus formation in human lymphocytes. Long-term assessment of individual radiation-induced apoptosis showed little intraindividual variation but significant interindividual variation. The effectiveness of radiation exposure to cause apoptosis or micronucleus formation was reduced by low-dose-rate exposures, but the reduction was apparent at different dose rates for these two end points. Micronucleus formation showed a dose-rate effect when the dose rate was lowered to 0.29 cGy/min, but there was no accompanying cell cycle delay. A further increase in the dose-rate effect was seen at 0.15 cGy/min, but was now accompanied by cell cycle delay. There was no dose-rate effect for the induction of apoptosis until the dose rate was reduced to 0.15 cGy/min, indicating that the mechanisms or signals for processing radiation-induced lesions for these two end points must be different at least in part. There appear to be two mechanisms that contribute to the dose-rate effect for micronucleus formation. One of these does not affect binucleate cell frequency and occurs at dose rates higher than that required to produce a dose-rate effect for apoptosis, and one affects binucleate cell frequency, induced only at the very low dose rate which coincidentally produces a dose-rate effect for apoptosis. Since the dose rate at which cells showed reduced apoptosis as well as a further reduction in micronucleus formation was very low, we conclude that the processing of the radiation-induced lesions that induce apoptosis, and some micronuclei, is very slow in quiescent and PHA-stimulated lymphocytes, respectively.

Sensitivity of cytokinesis to hydrophobic interactions. Chemical induction of bi-and multinucleated cells

We have tested whether cytokinesis is as sensitive to hydrophobic interactions as karyokinesis, and evaluated the usefulness of the frequency of binucleated cells as end-point. Treating cultured cells for 2 or 24 h, with different lipophilic alcohols and chlorinated hydrocarbons made this possible. Colcemid and cytochalasin B were applied as positive controls for inhibition of karyokinesis and cytokinesis, respectively. Several-fold increases of binucleated cells could be seen with cytochalasin B after 2 h of treatment, while there was no increase with colcemid, which instead blocked cells in prometaphase/metaphase. The solvent acted primarily through hydrophobic interactions. For each solvent, the blocking of cells in prometaphase/metaphase and a minor increase in binucleated cells, were seen at approximately the same concentration; the binucleated cells probably emanated from cells in anaphase/telophase at the start of treatment. We conclude that the spindle function and cleavage show similar sensitivity to hydrophobic interactions. After prolonged treatment, allowing escape from the metaphase block, the solvents induced binucleated and multinucleated cells. By forming the quotient between multinucleated (MULTI) and binucleated (BIN) cells one could distinguish between effects primarily on the spindle or cytokinesis, respectively. All solvents, and a combination of colcemid and cytochalasin B, showed quotients intermediate between those observed with colcemid (high MULTI/BIN) and cytochalasin B (low MULTI/BIN), respectively. Both protocols revealed the same relationship between lowest active concentration and lipophilicity for the solvents, implying that concentration, not dose were of prime importance. The specific inhibitors acted at low concentrations in relation to lipophilicity, clearly demonstrating their chemical mechanisms. This approach can be used for rapid screening of potential aneugens, distinguishing between routes, and when lipophilicity is known, also reveal the principal mechanism of action, i.e. physico-chemical or chemical.

Paradoxical effects of iodine-125 decays in parent and daughter DNA: a new target model for radiation damage

Chinese hamster ovary cells were synchronized at the G(1)/S-phase boundary of the cell cycle and were pulse-labeled with (125)I-iododeoxyuridine 30 min after they entered the S phase. Cell samples were harvested and frozen for accumulation of (125)I decays during the first and second G(2) phase after labeling. Cell aliquots that had accumulated the desired number of decays were thawed and plated for evaluation micronucleus formation and cell death. Cells subjected to (125)I decays during the first G(2) phase after labeling exhibited single-hit kinetics of cell killing (n = 1, D(0) 41 decays/cell). In contrast, decays accumulated during the second G(2) phase killed cells with dual-hit kinetics (n = 1.9, D(0) 81 decays/cell). A similar divergence in the action of (125)I was noted for micronucleus formation. These findings indicate that the effects of (125)I varied depending on whether the decays occurred in daughter DNA (first G(2) phase) or parent DNA (second G(2) phase). Control studies with external X rays showed no such divergence of the action of radiation. To account for this paradox, a model is proposed that invokes higher-order chromatin structures as radiation targets. This model implies differential spatial arrangements for parent and daughter DNA in the genome, with DNA strands organized such that a single (125)I decay originating in daughter DNA damages two targets during the first G(2) phase, but identical decays occurring during the second G(2) phase damage only one of the targets.

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.

Influence of novobiocin on <FONT FACE=Symbol>g</font>-irradiation G0-lymphocytes as analyzed by cytogenetic endpoints

Experiments with novobiocin (NB) post-treatment were performed to verify its effect on the frequencies of micronuclei (MN) and chromosomal aberrations (CA) induced by <FONT FACE="Symbol">g</font>-irradiation (0.75, 1.5 and 3.0 Gy) in human lymphocytes at G0-phase. The frequencies of MN significantly decreased by 44 and 50%, for the treatment with NB 50 µg/ml (30-min pulse) after radiation doses of 1.5 and 3.0 Gy, respectively. However, CA frequencies were not significantly affected. No significant effect on CA was observed when lymphocyte cultures were exposed to a single dose of 2.0 Gy at the G0-phase and posttreated with 25 µg/ml NB for three hours either immediately after irradiation (G0-phase) or after 24 h (S-phase). The significant suppressive effect of NB on MN frequencies supports the hypothesis that NB interaction with chromatin increases access to DNA repair enzymes.

Induction of micronuclei in vitro by organochlorine compounds in beluga whale skin fibroblasts

Beluga whales (Delphinapterus leucas) inhabiting the St. Lawrence estuary are highly contaminated with environmental pollutants and have a high incidence of cancer. Environmental contaminants may be partly responsible for the high cancer incidence observed in this population. DNA damage plays an important role in the development of cancer. The micronuclei (MN) assay was used to test the genotoxic potential of organochlorine (OC) pesticides with and without external metabolic factor in skin fibroblasts of an Arctic beluga whale. Toxaphene, chlordane and p,p'-DDT induced significant (p<0. 05) concentration-response increases of micronucleated cells (MNCs). Statistically significant increases in MNCs, ranging from 1.7- to 5-folds when compared to control cultures, were observed for 0.05, 0. 5, 5 and 10 microg/ml toxaphene, 2, 5 and 10 microg/ml chlordane and 10 and 15 microg/ml p,p'-DDT. Presence of exogeneous metabolic factor (S9) completely abolished the MN induction potency of chlordane and p,p'-DDT, and toxaphene induced MN formation at higher concentrations (0.5 microg/ml) than without S9 mix. The ecotoxicological significance of MN induction by low concentrations of toxaphene is unknown and do not imply that toxaphene is involved in the etiology of cancer in St. Lawrence beluga whales. However, because of the known genotoxicity of toxaphene and the long lifespan of beluga whales, it cannot be excluded that toxaphene may pose a long-term genetic hazard to the more contaminated whales of this population.

Inhibition of methotrexate-induced chromosomal damage by folinic acid in V79 cells

Methotrexate (MTX), an anticancer compound, is widely used in the treatment of leukemia. It induces cytogenetic damage as well as cytostatic effects on a variety of cell systems. Folinic acid (Leucovorin) is generally administered along with MTX as a rescue agent to decrease MTX-induced toxicity. However, information regarding the inhibitory effect of folinic acid against cytogenetic damage caused by MTX is limited. This study was conducted to assess the cytogenetic effect of MTX and its inhibition by folinic acid (FA) using the micronucleus and chromosomal aberration assays concurrently. Exponentially growing V79 cells were treated with MTX at five different concentrations (5-100 micrograms ml-1) with S9 microsomal fraction for 6 h and post-treated with two concentrations of FA (5 or 50 micrograms) for 40 h. Results indicate that MTX alone induced a concentration-related increase in % micronucleated binucleated cells (MNBN) and % aberrant cells (Abs). There was a decrease in nuclear division index (NDI) with increase in MTX concentration. Similarly, the mitotic index (MI) also decreased in all concentrations of MTX tested. The addition of FA at 50 micrograms ml-1 significantly reduced % MNBN (40-68%) and % Abs (36-77%). Inhibition was also seen at 5 micrograms FA (12 to 54% for MNBN and 20 to 61% for Abs). These results indicate that FA is capable of reducing the cytogenetic damage induced by MTX and appears to be an anticlastogenic agent.

Protective effect of vanillin on radiation-induced micronuclei and chromosomal aberrations in V79 cells

Vanillin (VA), an anticlastogen, has been demonstrated to inhibit gene mutations in both bacterial and mammalian cells. However, the data on its effect against radiation-induced cytogenetic damage are limited. The aim of this study was to investigate the protective effect of VA on radiation-induced chromosomal damage in V79 cells. Exponentially growing cells were exposed to five doses of X-rays (1-12 Gy) and UV radiation (50-800 microJ x 10(2) cm-2 and posttreated with 3 concentrations of VA (5, 50 or 100 micrograms ml-1 for 16 h for micronucleus (MN) and 18 h for structural chromosomal aberration (SCA) analyses. MN and SCA assays were performed concurrently according to standard procedures. Results indicate that there was a dose related increase in the percent of micronucleated binucleated cells (MNBN) (5.6 to 79.6) and percent of aberrant cells (Abs) (12 to 98) with X-ray treatment alone. Inhibition studies showed that the addition of VA at 100 micrograms ml-1 significantly reduced the percent of MNBN (21 to 48) induced by X-ray at 1, 2, and 4 Gy. There was a slight decrease in percent MNBN at 5 and 50 micrograms VA ml-1. All three concentrations of VA decreased percent Abs (15.7 to 57.1) induced by X-rays at all doses. UV radiation alone significantly increased percent MNBN (3.5 to 14.8) and percent Abs (17 to 29). Addition of 50 or 100 micrograms VA ml-1, significantly decreased percent MNBN (31.7 to 86.2) and percent Abs (54.5 to 90.9) at all doses of UV radiation. A decrease in percent MNBN (2.8 to 72.4) and percent Abs (34.8 to 66.7) was also noted at 5 micrograms VA ml-1. These data clearly indicate the protective effect of VA on radiation-induced chromosomal damage, suggesting that VA is an anticlastogenic agent.

The advantages and disadvantages of the cytokinesis-block micronucleus method

There is current interest in adopting the micronucleus test instead of metaphase analysis of chromosomes to assess the in vitro genotoxic potential of chemical and physical agents. In this paper, the advantages and disadvantages of the cytokinesis-block micronucleus technique relative to metaphase analysis of chromosomes and the conventional micronucleus technique that does not distinguish between dividing and non-dividing cells are discussed. Furthermore, additional features, such as the measurement of non-disjunction, excision-repairable DNA lesions and HPRT variants and possibilities for further improving the application of the cytokinesis-block method are proposed.

In vitro induction of micronuclei in lymphocytes: the use of bromodeoxyuridine as a proliferation marker

A simple method to determine the induction of micronuclei in cultured lymphocytes is described as an alternative to the cytochalasin-B method. It is proposed for use in the evaluation of the genotoxic potential of agents in vitro. It allows the recording of events only in the proliferating population of cells and at the same time it eliminates the possibility of recording combined effects with a cytokinesis-blocking agent. 16 microM bromodeoxyuridine (BrdU) was used to label proliferating cells that were treated with colcemid or mitomycin C at different concentrations. A monoclonal antibody against BrdU incorporated in the DNA and a peroxidase-diaminobenzidine brown stain were used to identify those cycling cells in a slide. To obtain the maximum yield of micronuclei, the best time for the addition of bromodeoxyuridine was found to be at 40 h from the initiation of cultures, 8 h before treating cells with the chemicals. Identification of micronuclei was easy, fast and unequivocal. In addition, the formation of structures similar to micronuclei, but that still are part of the nucleus could be observed. It is not clear if these structures are an intermediate stage in the formation of MN, but this methodology provides the possibility of observing and studying them.

B[a]P-DNA adduct formation and induction of human epithelial lung cell transformation

In this study we tested the suitability of the human epithelial lung cell line BEAS-2B for in vitro studies of lung carcinogenesis. The human bronchial epithelial lung cell line BEAS-2B, immortalized with an SV-40/Ad-12 hybrid virus construct, was treated for 24 hours with five different concentrations of the lung carcinogen benzo(a)pyrene (B[a]P) to assess the relationship between DNA adduct levels, cell cycle distribution, micronuclei formation (MN), colony forming efficiency (CFE), and anchorage independent growth (AIG). There appeared to be a strong linear correlation between B[a]P concentration and DNA adduct formation, but no difference in cell cycle distribution was observed after incubation with various concentrations of B[a]P. In the incubation range of 4 to 100 nM B[a]P, the number of DNA adducts was linearly correlated with colony formation in AIG and with the number of cells within individual colonies but not the number of colonies in the CFE test. At higher B[a]P concentrations, the clonal expansion of cells in the CFE and the number of colonies in the AIG declined. Also, the number of micronuclei increased with the formation of DNA adducts. It is concluded that after 24 hours of incubation with 100 nM B[a]P, the formation of BPDE-DNA adducts in the human epithelial lung cells BEAS-2B results in maximal induction of cell transformation. Because of this correlation between DNA adduct formation and lung epithelial cell transformation, the BEAS-2B cells seem suitable for in vitro studies on lung carcinogens.

Categorization of micronuclei by size and measurement of each ratio in cytokinesis-block and conventional cultures of human lymphocytes exposed to mitomycin C and colchicine

Micronuclei (MN) assays are very useful tests for monitoring human exposure to mutagens and carcinogens. We investigated the effects of the culture method (either conventional or cytokinesis-block) and exposure time (48 or 72hr) on the frequency and size distribution of MN in human peripheral lymphocytes exposed to mitomycin C (MMC) or colchicine. To quantitatively analyze the effects of the agents, methods and exposure times, we categorized MN by size into small (MN-1), medium (MN-2), and large (MN-3). MN-1 were less than one fifth, MN-2 one fifth to one third, and MN-3 larger than one third of the diameter of the main nucleus.Both MMC and colchicine induced dose-related increases in the frequency of MN. The number and distribution of the size-categorized MN were influenced by the agent, method and exposure time. The conventional culture method was useful for detecting the induction of MN-1 by MMC, whereas the cytokinesis-block method was useful for detecting the induction of MN-1 by colchicine. The ratios of MN in the various size categories reflect the different mechanisms of MN induction by MMC and colchicine.These findings suggest that categorization of MN by size can allow one to differentiate between a clastogen and an aneuploidogen, and that the ratios of MN in the three size categories may provide a good index for estimating the type of MN induction for human monitoring.

Sex chromosome loss and non-disjunction in women: analysis of chromosomal segregation in binucleated lymphocytes

Chromosomal lagging and non-disjunction are the main mechanisms of chromosomal malsegregation at mitosis. To date, the relative importance of these two events in the genesis of spontaneous or induced aneuploidy has not been fully elucidated. A methodology based on in situ hybridization with centromeric probes in binucleated lymphocytes was previously developed to provide some insight into this matter. With this method, both chromosomal loss and non-disjunction can be simultaneously detected by following the distribution of specific chromosomes in the nuclei and micronuclei of binucleated cells. In this study, this approach was used for studying the role of chromosomal loss and non-disjunction in the age-related malsegregation of sex chromosomes in females. For this purpose, cultures of cytokinesis-blocked lymphocytes were established from 12 healthy women ranging in age from 25 to 56. The occurrence of malsegregation of X chromosomes in vitro was estimated in binucleated cells that contained four signals, which originates from the division of normal disomic cells. In this cell population, the frequencies of X chromosome loss and non-disjunction ranged from 0% to 1.69% (mean 0.75%), and from 0.20% to 1.33% (mean 0.57%), respectively. This indicates that both events contribute to malsegregation of X chromosomes in vitro. Moreover, a small but not negligible fraction of binucleated cells with two or six copies of the X chromosome was noticed in all donors. These cells, which are thought to arise from parental monosomic and trisomic types, may indicate the malsegregation of X chromosomes in vivo. The frequency of X chromosome aneuploidy both in vivo and in vitro significantly correlated with the age of donors. Analysis of chromosomal distribution in unbalanced cells demonstrated that both X homologues were frequently involved. The frequency of such multiple events (0.17%) was far greater than that expected by mere chance, indicating a tendency to multiple malsegregation events in the cell population investigated. Finally, parallel analysis of the segregation of chromosomes X and 1 in five of the donors confirmed the greater (about tenfold) susceptibility of X chromosomes to malsegregate compared with autosomes.

Micronuclei in human lymphocytes: the Co-60 gamma-ray dose-response

Dose-response for micronuclei in cytokinesis-blocked lymphocytes after in vitro irradiation of whole blood from 3 donors with Co-60 gamma-rays in the range 0-5.0 Gy was established. The numerical relationship between radiation induced chromosomal aberrations, and micronuclei is also examined. An increased frequency of micronuclei following low doses of gamma-irradiation is reported from a study of 41 radiation workers.

DNA damage, micronucleus formation, and cell death from 125I decays in DNA

CHO cells were pulse-labeled with 125I-iododeoxyuridine, harvested 30 min or 5 h after labeling, and stored at -196 degrees C for accumulation of 125I decays. The 30- min groups yielded low-LET survival curves (large shoulder, D0 136 decays/cell); 5-h groups showed a high-LET pattern of cell killing (no shoulder, D0 45 decay/cell). Surprisingly, the shift in 125I action was abolished in cells exposed to HAT medium; both 30-min and 5-h cell groups exhibited high-LET-type killing (no shoulder, D0 52 decays/cell). The striking difference in cell death was not accompanied by any change in induction or repair of DNA DSBs, but the pattern of micronucleus formation (and by implication chromosome damage) did parallel 125I-induced cell death. These findings suggest that cell killing may not be directly linked to the absolute number of DNA DSBs and that damage to higher-order genome structures may be an important factor in radiation-induced cell death.

Simultaneous evaluation of dexamethasone-induced apoptosis and micronuclei in rat primary spleen cell cultures

Apoptosis or programmed cell death is a biological event that is biochemically and morphologically distinct from cellular necrosis. Nonetheless, its relationship has not been studied in terms of a cytogenetic endpoint such as micronucleus formation. In the present study, based on cytological observations, the incidence of dexamethasone-induced apoptotic cells was related to the frequency of micronucleated cells in vitro. Rat primary spleen cells were grown in 6-well plates with RPMI 1640 media using concanavalin A and lipopolysaccharide as mitogens. At culture initiation, the test agent dexamethasone (10, 20 or 40 microM) and a cytokinesis inhibitor cytochalasin B (3 micrograms/ml) were added. Cultures were harvested 18 h and 40 h later. Slides were prepared and stained with Diff-Quik stain. Frequencies of apoptotic cells and micronucleated binucleate cells were enumerated cytologically based on 500 cells per treatment from the same slides. The results showed a dose-dependent increase in the number of apoptotic cells in rat spleen cultures treated with dexamethasone. At 18 h, the percentages of apoptotic cells were 0.8, 1.6, 3.4 and 4.4 with 0, 10, 20 and 40 microM dexamethasone, respectively. The corresponding percentages of apoptotic cells at 40 h were: 2.8, 2.6, 5.6 and 10.4. However, at the same concentrations of dexamethasone, the micronucleus frequency in binucleate cells remained relatively unchanged. The phenomenon of apoptosis induced by dexamethasone was confirmed biochemically based on a characteristic DNA 'ladder' pattern by gel electrophoresis. These data suggest that dexamethasone at the concentrations which induced apoptosis did not produce cytogenetic damage. Also, these findings indicate that micronucleus formation and nuclear changes leading to apoptosis are separate events and these endpoints may not be closely correlated for dexamethasone.

An in vivo/in vitro method for assessing micronucleus and chromosome aberration induction in rat bone marrow and spleen. 1. Studies with cyclophosphamide

The mouse micronucleus assay has long been used as an indicator of in vivo genotoxicity. Recently, it was shown that no single protocol is adequate to detect all clastogens. As a first step in developing a potentially more sensitive assay, micronucleus induction by cyclophosphamide (CP) was assessed in an in vivo/in vitro system using rat bone marrow and spleen cells. In each of two independent experiments, two rats/dose were treated i.p. with 0, 20, or 40 mg CP/kg and killed 6 h later. Cultures were then established in the presence of growth stimulants (interleukin-3 and granulocyte-macrophage colony stimulating factor for bone marrow; lipopolysaccharide and concanavalin A for spleen) and cytochalasin B, a cytokinesis inhibitor. Bone marrow cells were harvested and slides prepared 24 h after initiation, while spleen cells were harvested at 48 h. One thousand cells/tissue/group were scored for cell cycle kinetics and 1000 binucleate (BN) cells were scored for micronuclei. In addition, spleen cells were concurrently assayed for chromosome aberrations. A dose-related cell cycle delay was observed in both tissues in both experiments. Bone marrow showed a 6% average background frequency of micronucleated BN cells, while the low dose induced an average of 20%, and the high dose 31%. For spleen, the average control frequency of micronucleated BN cells was 3%, the low dose induced a 40% average frequency, and the high dose 65%. Also in splenocytes, a dose-dependent increase in chromosome aberrations was observed, with an almost 40-fold increase observed over the control value at the high dose. Thus, the in vivo/in vitro approach described here shows great potential in detecting drug induced genotoxicity. Also, spleen appears more sensitive than bone marrow to CP.

A comparative study of TK6 human lymphoblastoid and L5178Y mouse lymphoma cell lines in the in vitro micronucleus test

Micronucleus induction was compared in human lymphoblastoid TK6 and mouse lymphoma L5178Y cell lines treated with model clastogens and spindle poisons, i.e., X-rays, methyl methanesulfonate, ethyl methanesulfonate, mitomycin C, colcemid, and vincristine. The spontaneous micronucleated cell (MNC) frequency was stable and reproducible in both cell lines. All clastogens and spindle poisons studied here induced micronuclei in both cell lines. They increased MNC frequency at lower concentrations or caused a greater increase at the same concentration in TK6 cells. These clastogens and spindle poisons, however, were also more toxic to TK6 than to L5178Y cells and when comparison was based on cytotoxicity, they showed more efficient MNC induction in L5178Y cells. In conclusion, neither cell line was superior to the other, and both of them can be used as target cells in the in vitro micronucleus assay.

Factors affecting formation of micronucleus-like structures after colchicine treatment of Trichoderma reesei

Micronucleus-like structures were produced in Trichoderma reesei only when 0.1% colchicine treatment was used to enhance nuclear division. The average DNA content of these 'small nuclei' was 30% that of the normal nuclei, indicating that they were aneuploid nuclei. Such small nuclei may be useful in transferring small amounts of DNA into protoplasts.

Comparative studies on radiation-induced micronuclei and chromosomal aberrations in V79 cells

Induction of micronuclei (MN) and structural chromosomal aberrations (SCA) by physical agents such as X-rays and UV radiation has been studied extensively in a variety of cell lines for genotoxicity assessment. However, comparative data on the relationship between these two cytogenetic endpoints are limited. This study compares SCA and MN formation in V79 Chinese hamster lung cells treated with X-rays and UV radiation. Four replicate cultures of exponentially growing cells were exposed to four doses of X-rays (100-800 rads). For two replicate cultures, cytochalasin B (3 micrograms/ml) was added and cells were harvested 16 h later for MN and nuclear division index (NDI) assessment. For the remaining two replicate cultures, colcemid (0.025 micrograms/ml) was added 16 h post-treatment and cells were harvested 2 h later for SCA and mitotic index (MI) analyses. This experiment was duplicated using four doses of UV radiation (100-800 microJ x 10(2)/cm2). In the X-ray experiment, generally, a decrease in the NDI and MI was noted with increasing dose. Also, there was a clear dose-related increase in percent micronucleated binucleated (MNBN) and aberrant cells. A similar dose response, but with lower frequencies, was observed in the UV radiation treatment. These data suggest a good correlation between chromosome damage as measured by percent MNBN and aberrant cells and cytotoxicity as measured by NDI and MI.

Assessment of micronucleus induction in SCCVII cells treated with bioreductive agents, WIN 59075 (SR 4233) and mitomycin C, under aerobic and hypoxic conditions

WIN 59075 (SR4233, tirapazamine) is a promising bioreductive antitumor agent preferentially more toxic to hypoxic cells and presently undergoing phase I clinical trials. In this investigation, we have examined the applicability of the cytokinesis-block micronucleus assay to assess the effects of bioreductive agents. SCCVII tumor cells were treated with WIN 59075 or mitomycin C at various concentrations under aerobic and hypoxic conditions. Significant induction of micronuclei in binucleate cells was demonstrated in a dose-dependent fashion and it appeared to be strongly correlated with the loss of clonogenicity in the colony assay. Both agents showed selectively higher toxicity to hypoxic cells than to aerobic cells and the ratios of the concentrations required to obtain the equivalent effects under aerobic and hypoxic conditions could be also estimated by this method as follows: the hypoxic toxicity ratios were 120-130 for WIN 59075 and 3.0-3.3 for mitomycin C. For several favorable characteristics, the cytokinesis-block micronucleus assay can provide an alternative, rapid, and reproducible means for evaluation of antitumor activities from chromosomal breakage caused by the bioreductive agents.

Immunofluorescent and confocal laser cytometric analyses of centromeres in V79 cells

Previously, a modified anticentromere antibody (ACA) technique was established in the V79 Chinese hamster lung cells to simultaneously analyze chromosome damage and aneuploidy induced by various agents. Using this method, cyclophosphamide (CP) was evaluated further in the presence and absence of S9 activation for micronucleus/aneuploidy induction. The specific binding nature of ACA to the centromeric region was also analyzed using a confocal scanning laser cytometry. The results indicated that CP was primarily a clastogen and S9 activation was required for its activity. Vinblastine, the positive control for aneuploidy, produced predominantly centromere containing micronuclei and the addition of S9 was not required for its activity. X-radiation, the positive control for clastogenicity, predominantly produced centromere negative micronuclei confirming its clastogenicity. An evaluation of centromeric region under the standard fluorescence microscope indicated that ACA generally binds to most centromeric regions in a cell. However, by confocal imaging it was found that ACA binds to the central core proteins of the centromere region and not to the peripheral proteins.

Induction of micronuclei in human sperm-hamster egg hybrids at the two-cell stage after in vitro gamma-irradiation of human spermatozoa

The efficiency of the micronucleus test to assess radiation-induced chromosomal damage in human spermatozoa has been investigated. Micronuclei were scored in human sperm-hamster egg hybrids at the two-cell stage, after exposure of human spermatozoa to in vitro gamma-rays at doses of 0.00, 0.10, 0.25, 0.50, 1.00, 2.00, and 4.00 Gy. The relationship between the yield of micronuclei per two-cell stage as well as the percentage of two-cell stages with micronuclei and the different doses of irradiation were fitted to linear equations. To evaluate whether scoring micronuclei is useful for the quantification of chromosomal damage occurring in human spermatozoa, induced micronuclei at the different doses of sperm irradiation were compared to the induction of breaks and fragments in sperm-derived chromosomes. After interspecific fertilization of zona-free hamster oocytes by irradiated spermatozoa, a total of 699 fertilized eggs at the two-cell stage and a total of 387 sperm-derived complements were analyzed. The incidence of fertilized eggs with micronuclei at the two-cell stage coincided well with the incidence of sperm-derived chromosome breaks and fragments (e.g., 8.9% vs. 6.7% in the 0.25 Gy group and 52.8% vs. 58.6% in the 4.00 Gy group). A similar correlation was found between the number of micronuclei per two-cell stage and the number of breaks and fragments per sperm complement (0.09 vs. 0.07 in the 0.25 Gy group and 0.71 vs. 0.81 in the 4.00 Gy group). The results show that this test system can be used for the quantification of spontaneous or induced chromosomal damage in human spermatozoa.

Cytogenetic Effects Induced by Cytochalasin B in Chinese Hamster Ovary Cells

We determined the kinetics of the induction of chromosomal aberrations and micronuclei (MN) by mitomycin C (MMC, 0.1 &mgr;g/ml) in Chinese hamster ovary (CHO) cells treated with cytochalasin B (Cyt-B, 3 &mgr;g/ml). In cells treated with Cyt-B as well as with Cyt-B plus MMC the highest yield of binucleated cells was obtained 24 h after treatment. After 40 h of treatment with Cyt-B the frequency of MN in binucleated cells was significantly higher than that observed at previous times in the same cultures as well as in controls. In cultures treated with MMC the frequency of MN increased with time, reaching the highest value at 24 h. The frequency of chromosomal aberrations was also significantly higher in cells treated both with Cyt-B and Cyt-B plus MMC than in controls and exceeded that of MN in parallel cultures. These data confirm the capacity of MMC to induce chromosomal alterations in mammalian cells; in particular they indicate that Cyt-B is able to induce cytogenetic effects in CHO cells. Using immunofluorescence microscopy, after reaction with CREST antikinetochore antibodies, we found that in cells treated with Cyt-B or Cyt-B plus MMC the frequency of MN without kinetochore was, respectively, about 70 and 85%, indicating that under our experimental conditions MN originate mainly from acentric chromatid fragments. Present data suggest that the method based on the blockage of cytokinesis by Cyt-B normally used in the MN assay should be reconsidered. Copyright 1995 S. Karger AG, Basel

Combined cytokinesis-block micronucleus and chromosomal aberration assay for the evaluation of radiosensitizers at low radiation doses

PURPOSE

Several methods have been tried for evaluating the efficacy of hypoxic cell radiosensitizers at clinically relevant low radiation doses (1-4 Gy). The cytokinesis-block micronucleus assay is known to be useful for both the in vitro and in vivo evaluation of radiosensitizers, while the chromosomal aberration assay has been commonly used to assess the mutagenicity of various agents. In the present study, the chromosomal aberration assay and the cytokinesis-block micronucleus assay were performed simultaneously to assess the radiosensitizing effect of etanidazole and KU-2285 at low radiation doses. The correlation between the two assays was also evaluated.

METHODS AND MATERIALS

In vitro study: EMT-6 cells were irradiated at a dose of 1-3 Gy under hypoxic conditions with or without the drugs at 1 mM. In vivo-in vitro study: EMT-6 tumor-bearing BALB/c mice received 2-4 Gy of radiation with or without administration of the drugs at 200 mg/kg. Single-cell suspensions were then obtained in both studies and were used for the cytokinesis-block micronucleus assay and the chromosomal aberration assay. The micronucleus frequency in binucleate cells was evaluated in the former assay, and the frequency of chromosomal aberrations in metaphase cells was evaluated in the latter assay.

RESULTS

In vitro study: the sensitizer enhancement ratios of etanidazole and KU-2285 were 1.73 and 2.21, respectively, in the micronucleus assay, and 1.41 and 1.79 in the chromosomal aberration assay. In vivo-in vitro study: the sensitizer enhancement ratios of etanidazole and KU-2285 were 1.18 and 1.31, respectively, in the micronucleus assay, and 1.16 and 1.42 in the chromosomal aberration assay. In both studies, a linear correlation was observed between the micronucleus frequency and the chromosomal aberration frequency. The background (i.e., the frequency at 0 Gy) of the latter assay was considerably lower than that of the former assay, especially in the vivo study.

CONCLUSIONS

The chromosomal aberration assay has not yet been established as a method for evaluating the effect of radiosensitizers. However, a combination of the cytokinesis-block micronucleus assay and the chromosomal aberration assay seems to be useful for assessing radiosensitizers at low radiation doses for the following reasons: a) the chromosomal aberration assay is as sensitive as the micronucleus assay and possibly more specific, because chromosomal aberrations can be observed before cells pass through the first metaphase after irradiation and, thus, reflect quite acute damage, even though they reflect only a part of the total damage; b) the combined assay provides relatively more information for the time and labor required.

Induction of binucleation in human lymphocytes by 14 synthetic isoindolone derivatives related to cytochalasins

Fourteen hydrogenated isoindolone derivatives with affinity to cytochalasin B were synthesized and tested for the induction of binucleation in the human lymphocyte micronucleus (MN) assay. The experimental procedure was the one commonly used for the human lymphocyte MN assay. Compounds like cyt-B were added to cultures at 44 h and tested at increasing concentrations, up to 200 mumol/l, in the range commonly used for cyt-B in the MN assay (3 micrograms/ml = 6.25 mumol/l). Induction of cytokinesis-blocked binucleated cells was found for all compounds but, at the same molarity, cyt-B induced a higher percentage of binucleation. Only one of the compounds tested was found to induce micronuclei significantly: MN were induced across the dose range of 25-100 mumol/l.

Micronucleus induction in cytokinesis-blocked mouse bone marrow cells in vitro following in vivo exposure to X-irradiation and cyclophosphamide

A cytokinesis-block micronucleus (MN) method for the simultaneous but separate measurement of chromosome damage in erythroid and myeloid bone marrow cells is described. MN induction in cytokinesis-blocked mouse bone marrow cells in vitro following in vivo exposure to x-ray or cyclophosphamide (CP) was investigated. Immediately after whole body irradiation with acute doses of either 0, 1, 2 or 4 Gy x-rays, or 2 hr after treatment with either 0, 12.5, 25, or 50 mg CP/kg body weight, bone marrow cells were collected and then cultured in medium supplemented with 3.0 micrograms/ml cytochalasin B for 24 hr. The binucleated cells were scored in erythroid, myeloid, lymphoid and other cells. The myeloid/erythroid (M/E) ratio was decreased by x-irradiation or CP treatment in a dose-dependent manner. The dividing index (DI; binucleated cells/binucleated + mononucleated cells; %) was decreased in both erythroid and myeloid cells in the same manner. Dose-dependent increases in MN frequency were observed following x-irradiation in both erythroid and myeloid cells. A similar dose-dependent MN induction was observed with CP. The MN frequency in myeloid cells was much greater than in erythroid cells (about 4-fold following 4 Gy exposure, and more than 10-fold after 50 mg/kg CP). Lymphoid and other cells were not suitable for scoring DI and MN frequency because of insufficient numbers of binucleated cells. These results suggest that micronuclei can be identified in both myeloid and erythroid cells and that myeloid cells are more susceptible to x-ray or CP-induced chromosomal damage than erythroid cells as expressed by MN induction.

Studies on cytotoxic and genotoxic effects of cadmium nitrate and lead nitrate in Chinese hamster ovary cells

Cadmium nitrate decreased the viability of Chinese hamster ovary (CHO) cells in a concentration-dependent manner; 50% inhibition (IC50) was achieved at 0.015 mM. In contrast, lead nitrate appeared to be less toxic. Neither cadmium nitrate nor lead nitrate significantly increased frequencies of binucleated CHO cells with micronuclei (MN). However, both cadmium nitrate and lead nitrate could augment sister chromatid exchanges (SCEs). Cadmium nitrate induced SCEs with a potency approximately equal to that of mitomycin C and more than 10 times higher than lead nitrate. Cadmium nitrate also increased chromosome aberrations (CAs), which included breaks, acentrics, interchanges, and dicentrics of chromosomes. In addition, cadmium nitrate induced a decrease in the mitotic index (MI), but lead nitrate increased it. In summary, it appears that both of these two heavy metal salts have cytogenetic toxicities with different degrees of effects on the cytotoxicity, MN, CAs, and SCEs in CHO cells. However, SCE was the most sensitive endpoint for indicating mutagenetic effects of cadmium and lead in the present study.

Radiation dose-dependent variations of micronuclei production in cytochalasin B-blocked human lymphocytes

Using the cytokinesis-block technique, lymphocytes from healthy volunteers (n = 9) were evaluated for 1) the radiation dose-response curve for micronuclei (MN) expression; 2) technique variables on the yield of MN; and 3) the shortest lymphocyte incubation time required for the MN assay. We found that the best fitting of relationships between increasing MN production and increasing irradiation dose (0-4.0 Gy) was the linear-quadratic model as expressed by the yield equation Y = C+alpha D+beta D2 (P = 0.0003). When lymphocytes were irradiated in vitro with 2.0 Gy and harvested at various time intervals, MN increased during the entire 84 hr culture time. The radiation caused a division delay in lymphocyte as indicated by an increased frequency of mononucleated cells and a decreased number of mitotic indices. The data showed that a shortened culture time (60 hr) for the MN assay is possible and that binucleated cells with > or = 3 MN were found only in cells irradiated at > or = 2.0 Gy. These findings suggest that scoring of MN in lymphocytes may be a practical biological dosimeter for the rapid screening of accidental radiation exposure victims, especially when their clinical manifestations are not obvious.