Tumor radiosensitivity prediction by the cytokinesis-block micronucleus assay

An in vivo to in vitro cytokinesis-block micronucleus assay technique using cytochalasin B (Cyt-B) was established in xenografted human and murine tumors, and the correlation between radiosensitivity measured by this assay and that measured by a colony-forming assay was investigated. Tumors were irradiated in situ, excised immediately, and disaggregated to single cells that were plated for the micronucleus and colony-forming assays. Some of the tumor cells were irradiated in vitro rather than in vivo. For the micronucleus assay, Cyt-B (0.5-3 micrograms/ml) was added to dishes soon after plating or in vitro irradiation and the cells were subsequently fixed and stained at intervals (12-144 h). The micronucleus frequency in binucleate cells was evaluated under conditions of maximum yield of the binucleate cells. The micronucleus frequency after irradiation was quite variable depending on the tumor type and the average number of micronuclei per single binucleate cell after 4 Gy ranged from 0.2 to 1.4. The results of in vitro irradiation were not significantly different from those of in vivo irradiation for all tumors. A good correlation was found between the radiosensitivity determined by the micronucleus assay and that found with the colony-forming assay in six human tumors (r = 0.94 approximately 0.98) but not in four murine tumors because of one exceptional tumor. When this tumor was excluded, a correlation was also found for the remaining nine tumors (r = 0.62 approximately 0.96). These results indicated that the cytokinesis-block micronucleus assay has some promise as a rapid predictive assay of radiosensitivity.     

Cytogenetic status of healthy children assessed with the alkaline comet assay and the cytokinesis-block micronucleus cytome assay

In the present study the alkaline comet assay and the cytokinesis-block micronucleus cytome (CBMN Cyt) assay were used to evaluate the baseline frequency of cytogenetic damage in peripheral blood lymphocytes (PBLs) of 50 healthy children from the general population in Croatia (age, 11.62 ± 1.81 years). Mean values of tail length, tail intensity and tail moment, as comet assay parameters, were 12.92 ± 0.10, 0.73 ± 0.06 and 0.08 ± 0.01, respectively. The mean frequency of micronuclei (MN) for all subjects was 2.32 ± 0.28 per 1000 bi-nucleated cells, while the mean frequency of nucleoplasmic bridges (NPBs) was 1.72 ± 0.24 and of nuclear buds (NBUDs) 1.44 ± 0.19. The mean nuclear division index (NDI) was 1.70 ± 0.05. When comet-assay parameters were considered, higher mean values for all three were found for the female population. According to the Mann–Whitney U test applied on the results of the comet assay, the only statistically significant difference between the male and female populations was found for tail length. Similar to the results obtained by the comet assay, girls showed higher mean values of all three measured parameters of the CBMN Cyt assay. This difference was statistically significant for total number of NPBs only. In the case of the NDI, a higher mean value was also obtained in girls, but this difference was not statistically significant. The results obtained present background data that could be considered as normal values for healthy children living in urban areas, and can later on serve as baseline values for further toxicological monitoring. Additionally, the usefulness of both techniques in measuring cytogenetic damage during bio-monitoring of children is confirmed.     

A sequential study on the use of the cytokinesis-block micronucleus method in mouse spelenocytes

Several known clastogens and mutagens have been tested for their ability to induce micronucleis (MN) using the cytokinesis-block method in mouse splenocytes. The chemicals were harringtonine, cisplatin, cytosine arabinoside, vincristine sulfate, colchicine, potassium chromate, methyl methanesulfonate and 2-acetylaminofluorence. All chemicals tested induced a dose-dependent increase in MN and a delay in cell-cycle progression. The results suggest that the cytokinesis-block micronucleus method in mouse splenocytes in reliable, economical and sensitive enough for detecting mutagenic agents in vivo and in vitro.     

Genotoxicity testing of PLGA-PEO nanoparticles in TK6 cells by the comet assay and the cytokinesis-block micronucleus assay

The in vitro genotoxicity of PLGA-PEO (poly-lactic-co-glycolic acid-polyethylene oxide copolymer) nanoparticles was assessed in TK6 cells using the comet assay as well as cytokinesis-block micronucleus (CBMN) assay. The cells were exposed to 0.12-75μg/cm2 of PLGA-PEO nanoparticles during 2 and 24h for analysis in the comet assay, and to 3-75μg/cm2 of these nanoparticles during 4, 24, 48 and 72h, respectively, for analysis in the CBMN assay. Two different protocols for treatment with cytochalasin B were used. We found that PLGA-PEO was neither cytotoxic (measured by relative cell growth activity and cytokinesis-block proliferation index (CBPI)), nor did it induce DNA strand-breaks (detected by the comet assay) or oxidative DNA lesions (measured by the comet assay modified with lesion-specific enzyme formamidopyrimidine-DNA-glycosylase). There were no statistically significant differences in the frequencies of micronucleated binucleated cells (MNBNCs) between untreated and treated cells in either of the conditions used. This suggests that PLGA-PEO did not have potential genotoxicity. However, using two experimental protocols of the micronucleus assay, PLGA-PEO nanoparticles showed a weak but significant increase in the level of MN in mononucleated cells, in cells treated for 48h with PLGA-PEO nanoparticles when cytochalasin B was added for the last 24h (1st protocol), and in cells treated for 24h with PLGA-PEO nanoparticles followed by washing of NPs and addition of cytochalasin B for another 24h (2nd protocol). It remains unclear whether the increase of MNMNC after treatment with PLGA-PEO nanoparticles is the effect of a possible, weak aneugenic potential or early effect of these particles, or due to another reason. These results suggest that aneugenicity in addition to clastogenicity may be considered as an important biomarker when assessing the genotoxic potential of polymeric nanoparticles.     

Automated analysis of the cytokinesis-block micronucleus assay for radiation biodosimetry using imaging flow cytometry

The cytokinesis-block micronucleus (CBMN) assay is employed in biological dosimetry to determine the dose of radiation to an exposed individual from the frequency of micronuclei (MN) in binucleated lymphocyte cells. The method has been partially automated for the use in mass casualty events, but it would be advantageous to further automate the method for increased throughput. Recently, automated image analysis has been successfully applied to the traditional, slide-scoring-based method of the CBMN assay. However, with the development of new technologies such as the imaging flow cytometer, it is now possible to adapt this microscope-based assay to an automated imaging flow cytometry method. The ImageStream^X is an imaging flow cytometer that has adequate sensitivity to quantify radiation doses larger than 1 Gy while adding the increased throughput of traditional flow cytometry. The protocol and analysis presented in this work adapts the CBMN assay for the use on the ImageStream^X. Ex vivo-irradiated whole blood samples cultured for CBMN were analyzed on the ImageStream^X, and preliminary results indicate that binucleated cells and MN can be identified, imaged and enumerated automatically by imaging flow cytometry. Details of the method development, gating strategy and the dose response curve generated are presented and indicate that adaptation of the CBMN assay for the use with imaging flow cytometry has potential for high-throughput analysis following a mass casualty radiological event.     

Use of the cytokinesis-block micronucleus assay to measure radiation-induced chromosome damage in lymphoblastoid cell lines

To establish the optimal experimental conditions for the use of the micronuclei (MN) test to determine the level of chromosomal damage induced by ionising radiation (IR) exposure in lymphoblastoid cell lines, a time-course study was performed comparing a normal and an ataxia telangiectasia (AT) cell line, the latter being characterised by an extreme radiation sensitivity. Several parameters were analysed: the use of cytochalasin-B (Cyt-B) to quantify MN, the optimum fixation time to measure radiation-induced MN, the most appropriate treatment dose of IR to distinguish between the normal and the radiosensitive cells and the cell-cycle distribution after irradiation. The results obtained showed that the spontaneous as well as the radiation-induced levels of MN were significantly higher in the AT cell compared to the normal cells (P < 0.001 and P = 0.005, respectively). In both cell types the number of radiation-induced MN were lower in the cultures without Cyt-B than those with Cyt-B (P < 0.001), with the AT cells being distinguished in terms of IR-induced MN from the normal cells only with the addition of Cyt-B. The level of MN formation was independent of the dose of Cyt-B used (3 or 6 microg/ml). The optimum time for radiation-induced MN measured was found to be between 48 and 72 h post-irradiation, with 2 and 4 Gy exposures inducing similar levels of MN. However, as the higher dose caused a greater delay in the cell-cycle, treatment with 2 Gy with MN measurement at 48 h in the presence of 3 microg/ml Cyt-B were chosen as the optimum experimental conditions. This choice was validated using two additional normal and AT cell lines. In conclusion, our results show that the use of Cyt-B increases the sensitivity of the MN test for comparing differences in radiosensitivity between lymphoblastoid cell lines.     

Use of the cytokinesis-block micronucleus method in mouse splenocytes

Mouse splenocytes have been used in the cytokinesis-block method for the evaluation of micronuclei induced by mutagenic agents in vitro as well as in vivo. Stimulation with concanavalin A for 48 h followed by 16-24-h treatment with 5 micrograms/ml cytochalasin B was found to be an optimum condition to obtain micronuclei in the binucleated splenocytes after the cells were cultured in vitro. Under the above conditions splenocytes from mice pretreated with a single i.p. injection of cyclophosphamide gave a significant increase in micronucleus production. This increase was dependent on the dose of cyclophosphamide (r = 0.99). A dose of 50 mg/kg resulted in 22% of the binucleated cells producing micronuclei, more than 20 times the level in the untreated control. The increase was also dependent on the time of cyclophosphamide injection before removal of the spleen. A duration of 4-8 h after cyclophosphamide injection gave rather sharp optimum values for the production of micronuclei. When splenocytes from non-treated mice were treated with mitomycin C together with cytochalasin B in the above in vitro condition, there was a significant increase in micronucleus production in the binucleated cells. It was also dependent on the dose of mitomycin C (r = 0.975) and a dose of 0.5 micrograms/ml resulted in a more than 20-fold increase over the untreated control. Thus, the use of mouse splenocytes in the cytokinesis-block micronucleus assay was shown to be sensitive enough for testing mutagenic agents in vivo as well as in vitro.     

HUMN project: detailed description of the scoring criteria for the cytokinesis-block micronucleus assay using isolated human lymphocyte cultures

Criteria for scoring micronuclei and nucleoplasmic bridges in binucleated cells in the cytokinesis-block micronucleus assay for isolated human lymphocyte cultures are described in detail. Morphological characteristics of mononucleated cells, binucleated cells, and multinucleated cells as well as necrotic and apoptotic cells and nuclear buds are also described. These criteria are illustrated by a series of schematic diagrams as well as a comprehensive set of colour photographs that are of practical assistance during the scoring of slides. These scoring criteria, diagrams and photographs have been used in a HUman MicronNucleus (HUMN) project inter-laboratory slide-scoring exercise to evaluate the extent of variability that can be attributable to individual scorers and individual laboratories when measuring the frequency of micronuclei and nucleoplasmic bridges in binucleated cells as well as the nuclear division index. The results of the latter study are described in an accompanying paper. It is expected that these scoring criteria will assist in the development of a procedure for calibrating scorers and laboratories so that results from different laboratories for the cytokinesis-block micronucleus assay may be more comparable in the future.     

Biomonitoring of the cytogenetic effect of antitumor therapy by means of micronucleus assay in exfoliated epithelial cells

Data are presented concerning the possibility of using the micronuclei (MN) level as a biomarker for cytogenetic effects in exfoliated epithelial cells of cancer patients under therapy. The number of MN in buccal cells of cancer patients under chemotherapy are very contradictory. A significant dose-dependent increment of MN in tumor and normal epithelial cells due to radiotherapy was shown in most investigations. Evaluation of MN induced by radiotherapy in exfoliated tumor cells can potentially identify the radiosensitivity of tumors and the outcome of treatment after the first fractions of irradiation. This technique is almost completely noninvasive and easily done in accessible primary cancers (oral cavity and uterine cervix). The text was submitted by the author in English.     

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

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

The in vitro micronucleus assay for detection of cytogenetic effects induced by mutagen-carcinogens: comparison with the in vitro sister-chromatid exchange assay

The sensitivity of a cytogenetic assay, as expressed by the in vitro induction of micronuclei (MN), was compared to the in vitro induction of sister-chromatid exchanges (SCEs). Chinese hamster lung (V79) cells were exposed to 3 known alkylating agents: methyl methanesulphonate (MMS), ethyl methanesulphonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and to 5 newly synthesized naphthofurans: 2-nitro-7-methoxynaphtho[2,1-b]furan (A), 2-nitro-8-methoxynaphtho[2,1-b]furan (B), 2-nitronaphtho[2,1-b]furan (C), 2-nitro-7-bromonaphtho[2,1-b]furan (D) and 7-methoxynaphtho[2,1-b]furan (E). The induction of MN only was also analysed after exposure of the cells to 4 alcohols: ethanol, methanol, butanol and propanol. The lowest dose at which a significant effect could be observed was determined. In both assays, MNNG, MMS and EMS were equally active with the following order of potency: MNNG greater than MMS greater than EMS, the latter being a very weak inducer of MN and SCE. Compounds A and B were also very effective in both assays. Compound C was a more active inducer of SCE than MN. Compounds D and E were not active in either assay. None of the 4 alcohols induced MN. Our results are compared with the previously published data on in vitro and in vivo induction of SCE and MN. We conclude that the MN in vitro assay which detects clastogens as well as agents affecting the spindle apparatus, is a good indicator of genotoxicity, though slightly less sensitive than the in vitro SCE test. It could provide a rapid, simple and inexpensive complementary short-term test for the evaluation of potentially mutagenic chemicals.     

Evaluation of genotoxicity, cytotoxicity and cytostasis in human lymphocytes exposed to patulin by using the cytokinesis-block micronucleus cytome (CBMN cyt) assay

Patulin (PAT) is a fungal secondary metabolite commonly present in apples and apple products. In the present study, PAT was evaluated for its genotoxic, cytotoxic and cytostatic effects to human peripheral blood lymphocytes by using the cytokinesis-block micronucleus cytome (CBMN Cyt) assay. Lymphocyte cultures were treated with PAT at the following concentrations, 0.1, 0.3, 0.5, 1.0, 2.5, 5.0, and 7.5 μM, as well as 0.5 μM mitomycin c (MMC) as a positive control and dimethyl sulfoxide (DMSO) as a vehicle control. PAT was found to induce nucleoplasmic bridges (NPBs) at 5.0 and 7.5 μM concentrations (P?<?0.05), apoptotic cells at 0.1, 1.0, 5.0 μM (P?<?0.05), 7.5 μM concentrations (P?<?0.01) and necrotic cells at 0.3 and 2.5 μM (P?<?0.05), 5.0 and 7.5 μM (P?<?0.01) concentrations in human lymphocytes. The 2.5, 5.0, and 7.5 μM PAT concentrations also led to a clear decrease in the nuclear division index (NDI) (P?<?0.05). PAT caused a significant dose-dependent increase in the number cells of NPBs, in the frequency of apoptotic and necrotic cells, and a significant dose-dependent decrease in the NDI values in lymphocytes. These results indicate that PAT at high concentrations is genotoxic, cytotoxic and cytostatic in cultured human lymphocytes.     

In vitro micronucleus assay scored by flow cytometry provides a comprehensive evaluation of cytogenetic damage and cytotoxicity

This laboratory has previously reported on the development of a flow cytometry-based method for scoring in vitro micronuclei in mouse lymphoma (L5178Y) cells [S.L. Avlasevich, S.M. Bryce, S.E. Cairns, S.D. Dertinger, In vitro micronucleus scoring by flow cytometry: differential staining of micronuclei versus apoptotic and necrotic chromatin enhances assay reliability, Environ. Molec. Mutagen. 47 (2006) 56-66]. With this method, necrotic and mid/late stage apoptotic cells are labeled with the fluorescent dye ethidium monoazide. Cells are then washed, stripped of their cytoplasmic membranes, and incubated with RNase plus a pan-nucleic acid dye (SYTOX Green). This process provides a suspension of free nuclei and micronuclei that are differentially stained relative to chromatin associated with dead/dying cells. The current report extends this line of investigation to include the human cell line TK6. Additionally, methods are described that facilitate simultaneous quantitative analysis of cytotoxicity, perturbations to the cell cycle, and what we hypothesize is aneuploidization. This comprehensive cytogenetic damage assay was evaluated with the following diverse agents: etoposide, ionizing radiation, methyl methanesulfonate, vinblastine, ethanol, and staurosporine. Cells were harvested after 30h of continuous treatment (in the case of chemicals), or following graded doses of radiation up to 1Gy. Key findings include the following: (1) Significant discrepancies in top dose selection were found for five of the six agents studied when relative survival measurements were based on Coulter counting versus flow cytometry. (2) Both microscopy- and flow cytometry-based scoring methods detected dose-dependent micronucleus formation for the four genotoxic agents studied, whereas no significant increases were observed for the presumed non-genotoxicants ethanol and staurosporine when top dose selection was based on flow cytometric indices of cytotoxicity. (3) SYTOX and ethidium monoazide fluorescence signals conveyed cell cycle and cell death information, respectively, and appear to represent valuable aids for interpreting micronucleus data. (4) The frequency of hypodiploid nuclei increased in response to each of the genotoxic agents studied, but not following exposure to ethanol or staurosporine. Collectively, these results indicate that a comprehensive assessment of genotoxicity and other test article-induced toxicities can be acquired simultaneously using a simple two-color flow cytometry-based technique.     

Detection of the cytogenetic effect of inhaled aerosols by the micronucleus test

The induction of micronuclei in mice exposed to aerosols of the following 6 genotoxic chemicals by inhalation was examined: cyclophosphamide (CP), methyl methanesulfonate (MMS), mitomycin C (MMC), dimethylnitrosamine (DMN), ethylcarbamate and colchicine. Exposure of mice to CP aerosols at a theoretical concentration of 2426 mg/m3 for 29, 81 and 139 min induced 0.6, 1.0 and 2.3% micronucleated polychromatic erythrocytes (MNPCEs) in bone marrow 24 h after the termination of exposure. The other chemicals except for DMN showed a similar exposure-response relationship following in vivo exposures to their aerosols. The results obtained in this study suggest that the cytogenetic effect of inhaled aerosols can be detected by the micronucleus test, and the method described in the present report is useful as a rapid in vivo test for atmospheric aerosols.     

The cytokinesis-block micronucleus assay as a biological dosimeter in spleen and peripheral blood lymphocytes of the mouse following acute whole-body irradiation

To evaluate the application of the cytokinesis-block (CB) micronucleus (MN) assay as a biological dosimeter following in vivo exposure to ionising radiation we determined the micronucleus frequency in spleen and peripheral blood lymphocytes of the mouse, serially, for 14 days following acute whole-body irradiation. The baseline MN frequency of spleen lymphocytes (7.86 +/- 0.68, mean +/- 1 SD) was significantly (p less than 0.001) elevated when compared to that for peripheral blood lymphocytes (4.10 +/- 0.53). Immediately after irradiation there was a substantial dose-related increase in MN, but the MN frequencies in spleen lymphocytes (120.2 +/- 9.4 for 1 Gy; 409.5 +/- 38.4 for 2 Gy) were significantly (p less than 0.009) elevated compared to those in peripheral blood lymphocytes (78.0 +/- 7.0 for 1 Gy; 200.2 +/- 10.9 for 2 Gy). During the 14 days after irradiation, the MN frequency in spleen lymphocytes declined gradually to approximately half of the value observed immediately after irradiation. By contrast the MN frequency in peripheral blood lymphocytes increased during the week after irradiation, but ultimately MN frequencies in blood and spleen became approximately the same by day 14. Study of isolated murine lymphocytes irradiated in vitro showed that the number of MN generated by a given dose of radiation was approximately 2-3 times greater than the number generated by in vivo irradiation. These results suggest that measurement of MN in vivo after irradiation can be used as an in vivo dosimeter. However, precise dosimetry is probably affected by factors such as kinetic changes in different lymphocyte populations and possibly by in vivo factors which influence sensitivity of cells to radiation.     

The interactive effect of alcohol and folic acid on genome stability in human WIL2-NS cells measured using the cytokinesis-block micronucleus cytome assay

Chromosomal mutations are commonly found in cancer cells, and can be caused by several factors including dietary insufficiency and exposure to environmental and life-style genotoxins. Folate (vitamin B9), one of the essential micronutrients, is required for DNA repair and synthesis and to maintain genome stability. Since excessive alcohol (ethanol) consumption may alter folate status and low folate might alter susceptibility to alcohol toxicity, a study was performed to investigate the individual and interactive impacts of folic acid (FA) and ethanol on genome stability in vitro. The experiments were performed using WIL2-NS cells cross-tested at three FA (20, 200 and 2000 nM) and four ethanol concentrations (0, 0.09, 0.36 and 1.34%, v/v) over a two-week culture time. Chromosomal damage and cytotoxicity were measured using the cytokinesis-block micronucleus cytome assay. The present study showed dose-related genotoxic effects of both decreasing folic acid concentration and increased ethanol on day 15 resulting in significant induction of micronuclei, nuclear buds and nucleoplasmic bridges which are biomarkers of chromosome breakage or loss, gene amplification and chromosomal rearrangement, respectively. Increased ethanol and FA deficiency interacted to further significantly increase micronuclei and nucleoplasmic bridges. However there was no evidence showing alcohol's ability to cleave FA. The findings from this study suggest a protective effect of FA against alcohol-induced DNA damage and that FA deficiency in the physiological range has a stronger impact on genome stability than exposure to cytotoxic doses of ethanol achievable in binge drinking.     

A non-invasive micronucleus assay in the rat liver

The potent rat-liver mitogen 4-acetylaminofluorene (4AAF) is shown here to provide an effective replacement for the surgical procedure of 2/3 partial hepatectomy (2/3PH) in the in vivo rat-liver micronucleus assay described by Tates and his colleagues. This protocol modification enables the assay to be conducted on a routine basis. Control observations for both 2/3PH and 4AAF-treated rats are presented, together with evidence indicating 4AAF itself to be without activity in the assay, irrespective of the mitogenic stimulus. The activities of the rat carcinogens DMN, 2AAF, DMH and 6BT, and of the non-carcinogens 4AAF and 4N are demonstrated. Recommendations for the conduct of the modified assay are made.