The in vitro micronucleus technique

Chromosome segregation defects contribute to aneuploidy in normal neural progenitor cells

Recent studies based predominantly on nucleotide hybridization techniques have identified aneuploid neurons and glia in the normal brain. To substantiate these findings and address how neural aneuploidy arises, we examined individual neural progenitor cells (NPCs) undergoing mitosis. Here we report the identification of chromosomal segregation defects in normal NPCs of the mouse cerebral cortex. Immunofluorescence in fixed tissue sections revealed the presence of supernumerary centrosomes and lagging chromosomes among mitotic NPCs. The extent of aneuploidy followed the prevalence of supernumerary centrosomes within distinct cell populations. Real-time imaging of live NPCs revealed lagging chromosomes and multipolar divisions. NPCs undergoing nondisjunction were also observed, along with interphase cells that harbored micronuclei or multiple nuclei, consistent with unbalanced nuclear division. These data independently confirm the presence of aneuploid NPCs and demonstrate the occurrence of mitotic segregation defects in normal cells that can mechanistically account for aneuploidy in the CNS.

Chromosomal Aberrations and Micronuclei In Lymphocytes of Breast Cancer Patients after an Accident during Radiotherapy with 8 MeV Electrons1

In February 2001 a radiation accident occurred in a radiotherapy unit of an oncology hospital in Poland. Five breast cancer patients undergoing radiotherapy received a single high dose of 8 MeV electrons. The exact doses are not known, but they were heterogeneous and may have reached about 100 Gy. To assess whether such exposure would be detectable in peripheral blood lymphocytes, chromosomal aberrations and micronuclei were analyzed in lymphocytes from the accident patients and compared to values for lymphocytes from 10 control patients who were not involved in the accident but who received similar radiotherapy treatments. Lymphocytes were harvested for analysis of chromosomal aberrations at three different culture times to determine whether heavily damaged cells reached mitosis with a delay. There was no effect of harvest time on the frequencies of chromosomal aberrations, indicating that there was no delay of heavily damaged cells in entering mitosis. A good correlation was observed between micronuclei and chromosomal aberrations. In lymphocytes from three of the accident patients, significantly enhanced frequencies of both aberrations and micronuclei were found. The great individual variability observed in the frequency of cytogenetic damage in lymphocytes from both control and accident patients precluded the unambiguous identification of all accident patients.

Effects of ALDH2 gene polymorphisms and alcohol-drinking behavior on micronuclei frequency in non-smokers

Alcohol abuse is a serious health problem, leading to life-threatening damage to most of the important organ systems. Genotoxic damage is used as an early effect indicator in the surveillance of human exposure to genotoxic substances. Intra- and inter-individual variations of baseline frequencies of micronuclei (MN) in peripheral blood lymphocytes of human populations have been reported previously. Polymorphisms in a few metabolic enzyme genes seem to account for a proportion of this variability, but the impact of specific genetic variants on MN frequencies has not yet been clarified. In 42 healthy Japanese non-smoking men, we investigated the relationship between the MN frequency levels and genetic polymorphisms in three different genes: aldehyde dehydrogenase 2 (ALDH2), X-ray repair cross-complementing group 1 (XRCC1) and excision repair cross-complementing group 2 (ERCC2). Genotyping was performed by PCR-RFLP analysis. The ALDH2 variant (deficient-type) was significantly associated with increased MN frequency levels in subjects with drinking more than three times per week, whereas the XRCC1 and ERCC2 variants seemed to be unrelated to the MN frequency. The ALDH2-deficient habitual drinkers had an average MN frequency of 5.88+/-0.58 (+/- S.E.) compared with 3.20 +/- 0.80 in the ALDH2-proficient habitual drinkers (P<0.05). The ALDH2-proficient non-habitual drinkers had the lowest MN frequency (1.56 +/- 0.41). Furthermore, subjects with highest levels of mean MN frequency, who consumed more than 100g of alcohol per week and more than three times per week, had A2 genotype of ALDH2. A significant odds ratio (12.25, P<0.05) for the MN frequency levels above the 50th percentile value was observed for the ALDH2-deficient individuals versus the ALDH2-proficient individuals after adjustment for several confounders. These results strongly suggest that human early genotoxic effect studies based on the cytogenetic markers of MN should take into account both the individual ALDH2 polymorphism and the potential confounding effect of the drinking behavior.

Report from the in vitro micronucleus assay working group

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

STATISTICS

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

A review of in vitro methods to assess the biological activity of tobacco smoke with the aim of reducing the toxicity of smoke

In the last few years tobacco companies have been developing several research strategies in order to reduce the risks associated with smoking. These strategies include, for example, the refining of alternative cigarette designs that reduce the amount of hazardous chemicals in the mainstream smoke by introducing modified filters, and/or reducing the amount of biologically significant ingredients in tobacco-burning cigarettes. In the last few decades numerous studies have been published to assess the biological activity of tobacco smoke using in vivo and in vitro test systems. In this scenario a general scientific consensus on how to measure and characterize the risk associated with cigarette smoke is still lacking. Short-term in vitro assays, which are widely accepted by regulatory agencies around the world, are useful tools to evaluate both the biological activity and the progress towards a reduction of tobacco smoke toxicity. These assays could be mainly applied to evaluate cytotoxicity and genotoxicity properties on whole cigarette smoke as well as condensates or fractions of whole smoke. Cytotoxicity induction can be measured as cellular viability and growth rates using different end-points. Otherwise, the target of genotoxicity studies is the DNA molecule. For genotoxicity evaluation, the end-points and cell systems should be chosen from those that are relevant and appropriate as clinical surrogate markers. In this respect, the occurrence of early biological effect markers, such as mutational or clastogenic events (point mutations, frameshifts, micronuclei, SCE, DNA adducts) in bacterial and mammalian cells should be studied in a tiered approach following the guidelines of regulatory agencies. The choice of criteria shall be matter of discussion.

Genetic polymorphisms in folate and homocysteine metabolism as risk factors for DNA damage

Epidemiological studies indicated a role for polymorphisms in genes of folate and homocysteine (Hcy) metabolism in the etiology of neurodegenerative disease, congenital defects and coronary artery disease (CAD). This study investigated the effect of several polymorphisms [C677 T, A1298C of methylenetetrahydrofolate reductase (MTHFR) and A66G of methionine synthase reductase (MTRR) genes] on Hcy levels and DNA damage in 68 patients who underwent coronary angiography. Plasma Hcy concentrations were higher in patients with multivessel disease with respect to monovessel disease and no-CAD patients (19.4+/-2.6 vs 11.6+/-1.2 and 13.7+/-1.4 micromol/l, respectively; P=0.03). 677TT patients had higher Hcy levels than those with 677CC or 677CT genotypes (26.2+/-4.3 vs 13.1+/-1.4 and 13.0+/-1.4 micromol/l, respectively; P=0.0006). No significant associations were found between A1298C and A66G polymorphisms and plasma Hcy levels. Among patients with 677CC genotype, 66GG individuals tended to have higher levels of Hcy than 66AA homozygotes (14.5+/-1.9 vs 8.9+/-0.7 micromol/l, P=0.06). Multivessel disease patients showed an increased frequency of DNA damage, measured by the micronucleus (MN) frequency, as compared to monovessel disease and no-CAD subjects (12.5+/-1.1 vs 8.5+/-0.8 and 8.2+/-0.9, respectively; P=0.006). The MN were positively correlated with Hcy levels (r=0.33, P=0.006) and were significantly higher in subjects with the 677TT genotype compared with the 677CC or 677CT genotypes (14.4+/-2.0 vs 8.8+/-1.2 and 9.5+/-0.7, respectively; P=0.006). A1298C and A66G polymorphisms had no effect on MN frequency. However, among 677TT patients, 66GG subjects tended to have higher levels of MN than those 66AG and 66AA (18.2+/-3.6 vs 13.8+/-4.0 and 10.3+/-1.7, respectively; P=NS). Our results indicate that genetic instability may be associated with increased risk for multiple Hcy-related diseases.

Micronuclei to detect in vivo chemotherapy damage in a p53 mutated solid tumour

Apoptosis induction and micronuclei formation were compared following cytotoxic treatments in two rat glioma differing in p53 integrity. In vitro, micronuclei emergence but not apoptosis was linked to the p53 mutated status. In vivo, micronuclei assays were more sensitive to evaluate DNA damage induced by chemotherapy in a p53-mutated solid tumour.

Micronucleus response of human glioblastoma and neuroblastoma cells toward low-LET photon and high-LET p(66)/Be neutron irradiation

The identification of photon resistant tumors that are sensitive to neutrons is still an unresolved problem, and no radiobiological criteria have been developed that could help the selection of patients for neutron therapy. The micronucleus (MN) assay has been evaluated for this purpose in a panel of human glioblastoma and neuroblastoma cell lines spanning a wide range of photon sensitivities defined by mean inactivation doses ([Latin capital letter D with macron above][gamma]) of 1.25-3.21 Gy. We show that the relative biologic effectiveness (RBE) of the p(66)/Be neutrons is significantly correlated with inherent photon sensitivity (r = 0.89, p < 0.01), indicating that the panel of cell lines used is suitable to study the differential biologic response to neutrons and photons. We find that p(66)/Be neutrons are 1.43 to 5.29 times more effective per unit dose in inducing micronuclei than 60Co [gamma]-rays. Surprisingly, cells that are inherently photon resistant tend to show a higher yield of micronuclei following exposure to either photons or neutrons, but no significant correlation could be demonstrated. However, RBE values based on micronucleus yield were found to strongly correlate with RBE values derived from cell survival data (r = 0.91, p < 0.01). It is concluded that although micronucleus yield does not reflect intrinsic sensitivity to either photons or neutrons, the strong correlation between RBE calculated from micronucleus formation and RBE derived from cell survival demonstrates that the micronucleus endpoint has a potential for detecting photon resistant cells that show increased sensitivity to neutrons.

Mutagenic and cytotoxic effectiveness of diisopropyl xanthogen polysulphide in human lymphocyte cultures

The mutagenic and cytotoxic effectiveness of the new rubber vulcanisation accelerator diisopropyl xanthogen polysulphide (Robac AS 100) was tested in human lymphocyte cultures of four healthy probands. The concentrations of Robac AS 100 were 0.57, 5.7 and 57.0 microg/ml. Higher concentrations showed too high cytotoxicity to be evaluable. Without external activation, incubation time with Robac AS 100 was 21 h. In the presence of rat liver microsomes from aroclor-induced rats (2mg microsomal protein/ml), incubation of the test compound was 2h. Mutagenicity testing was performed by analysis of micronuclei (MN), structural chromosome aberrations (CAs) and sister chromatid exchanges (SCEs). The MN-rate was determined using the cytochalasin B (cyt B) block method. For evaluation of cytotoxicity, mitotic index (MI) and nuclear division index (NDI) were determined. The validity of the test methods was ascertained by positive controls: mitomycin C (MMC) and bleomycin (BLM) were used in experiments without exogenous activation and cyclophosphamide (CP) in experiments with exogenous activation. The presence of rat liver microsomes increased the mutagenic effect of Robac AS 100 in the SCE- and MN-test. But only the highest Robac AS 100-concentration (57.0 microg/ml) showed significantly increased mutagenic activity in all tests. However, cytotoxicity at this concentration was already substantial. Therefore, we consider the evidence for mutagenicity of Robac AS 100 as limited.

Nucleoplasmic bridges are a sensitive measure of chromosome rearrangement in the cytokinesis-block micronucleus assay

We have performed experiments using the WIL2-NS human B lymphoblastoid cell line and primary human lymphocytes to: (i). determine the importance of including measurements of nucleoplasmic bridges (NPB) in the cytokinesis-block micronucleus (CBMN) assay; (ii). provide evidence that NPB originate from dicentric chromosomes and centric ring chromosomes. In addition, we describe theoretical models that explain how dicentric chromosomes and centric ring chromosomes may result in the formation of NPB at anaphase. The results with WIL2-NS showed that it was possible to distinguish genotoxic effects induced by different oxidizing agents in terms of the NPB/micronucleus frequency ratio. The results with lymphocytes indicated a strong correlation: (i). between NPB, centric ring chromosomes and dicentric chromosomes in metaphases (r > 0.93, P < 0.0001); (ii). between micronuclei (MNi), acentric chromosome fragments and acentric ring chromosomes (r > 0.93, P < 0.0001). The dose-response curves with gamma-rays were very similar for NPB, ring chromosomes and dicentric chromosomes, as were the dose-response curves for MNi, acentric rings and fragments. However, not all acentric chromosomes and dicentric chromosomes/centric rings were converted to MNi and NPB respectively, depending on the dose of radiation. Preliminary data, using FISH, suggest that NPB often represent DNA from a structural rearrangement involving only one or two homologous chromosomes. The results from this study validate the inclusion of NPB in the CBMN assay which provides a valuable measure of chromosome breakage/rearrangement that was otherwise not available in the micronucleus assay. The CBMN assay allows NPB measurement to be achieved reliably because inhibition of cytokinesis prevents the loss of NPB that would otherwise occur if cells were allowed to divide.

The principal phenolic and alcoholic components of wine protect human lymphocytes against hydrogen peroxide- and ionizing radiation-induced DNA damage in vitro

We have tested the hypothesis that the alcoholic and phenolic components of wine are protective against the DNA-damaging and cytotoxic effects of hydrogen peroxide and gamma-radiation in vitro. The components of wine tested were ethanol, glycerol, a mixture of the phenolic compounds catechin and caffeic acid and tartaric acid, all at concentrations that were 2.5 or 10.0% of the concentration in a typical Australian white wine (Riesling). These components were tested individually or combined as a mixture and compared to a white wine stripped of polyphenols, as well as a Hanks balanced salt solution control, which was the diluent for the wine components. The effect of the components was tested in lymphocytes, using the cytokinesis-block micronucleus assay, after 30 min incubation in plasma or whole blood for the hydrogen peroxide or gamma-radiation challenge, respectively. The results obtained showed that ethanol, glycerol, the catechin-caffeic acid mixture, the mixture of all components and the stripped white wine significantly reduced the DNA-damaging effects of hydrogen peroxide and gamma-radiation (P = 0.043-0.001, ANOVA). The strongest protective effect against DNA damage by gamma-irradiation was observed for the catechin-caffeic acid mixture and the mixture of all components (30 and 32% reduction, respectively). These two treatments as well as ethanol produced the strongest protective effects against DNA damage by hydrogen peroxide (24, 25 and 18%, respectively). The protection provided by the mixture did not account for the expected additive protective effects of the individual components. Ethanol was the only component that significantly increased baseline DNA damage rate, however, this effect was negated in the mixture. In conclusion, our results suggest that the main phenolic and alcoholic components of wine can reduce the DNA-damaging effects of two important oxidants, i.e. hydrogen peroxide and ionizing radiation, in this physiologically relevant in vitro system.

Implication of nitro group reduction in the mutagenic and chromosome damaging activities of 22 new 5-nitroisoquinolines by the Salmonella mutagenicity test and the cytokinesis-blocked micronucleus assay

The mutagenic (MUT) and chromosome-damaging (CHR) activities of 22 potential antimalarial drugs (5-nitroisoquinoline derivatives) were evaluated by the Salmonella test and the cytokinesis-blocked micronucleus assay (CBMN). The Salmonella mutagenicity test was performed with and without metabolic activation (S9 mix) in S. typhimurium strains TA100 and YG1042 (an overproducing nitroreductase and O-acetyltransferase TA100 strain). The CBMN was carried out on human lymphocytes without metabolic activation. Four concentrations were tested: 1, 10, 100 and 1000 ng/ml. MUT was expressed as minimal mutagenic concentrations (MMC, microM) and CHR was expressed as minimal chromosome-damaging concentrations (MCDC, nM) to compare both activities. All the 5-nitroisoquinoline compounds were mutagenic in TA100. MMC ranged from 0.1 to 52.9 microM in TA100. A statistically significant decrease in MMC was observed in YG1042 (8 x 10(-3) to 3.5 microM), implicating reduction of the nitro group. Modulation of MUT by S9 mix was not significant in TA100 and YG1042. CHR was detected in 13 products for at least one concentration. Among the chromosome-damaging compounds, the MCDC ranged from 2.9 x 10(-3) to 3.6 nM. No relationship was found between MUT and CHR, suggesting two distinct pathways of DNA damage.

Intra- and inter-laboratory variation in the scoring of micronuclei and nucleoplasmic bridges in binucleated human lymphocytes. Results of an international slide-scoring exercise by the HUMN project

One of the objectives of the HUman MicroNucleus (HUMN) project is to identify the methodological variables that have an important impact on micronucleus (MN) or micronucleated (MNed) cell frequencies measured in human lymphocytes using the cytokinesis-block micronucleus assay. In a previous study we had shown that the scoring criteria used were likely to be an important variable. To determine the extent of residual variation when laboratories scored cells from the same cultures using the same set of standard scoring criteria, an inter-laboratory slide-scoring exercise was performed among 34 laboratories from 21 countries with a total of 51 slide scorers involved. The results of this study show that even under these optimized conditions there is a great variation in the MN frequency or MNed cell frequency obtained by individual laboratories and scorers. All laboratories ranked correctly the MNed cell frequency in cells from cultures that were unirradiated, or exposed to 1 or 2Gy of gamma rays. The study also estimated that the intra-scorer median coefficient of variation for duplicate MNed cell frequency scores is 29% for unexposed cultures and 14 and 11% for cells exposed to 1 and 2Gy, respectively. These values can be used as a standard for quality or acceptability of data in future studies. Using a Poisson regression model it was estimated that radiation dose explained 67% of the variance, while staining method, cell sample, laboratory, and covariance explained 0.6, 0.3, 6.5, and 25.6% of the variance, respectively, leaving only 3.1% of the variance unexplained. As part of this exercise, nucleoplasmic bridges were also estimated by the laboratories; however, inexperience in the use of this biomarker of chromosome rearrangement was reflected in the much greater heterogeneity in the data and the unexplained variation estimated by the Poisson model. The results of these studies indicate clearly that even after standardizing culture and scoring conditions it will be necessary to calibrate scorers and laboratories if MN, MNed cell and nucleoplasmic bridge frequencies are to be reliably compared among laboratories and among populations.

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.

Analysis of micronuclei in peripheral blood lymphocytes of traffic wardens: effects of exposure, metabolic genotypes, and inhibition of excision repair in vitro by ARA-C

The cytokinesis-block micronucleus (MN) assay in peripheral lymphocytes was used to assess the genetic effects of the occupational exposure to traffic fumes in policemen from the Municipality of Rome. The study population consisted of 192 subjects engaged in traffic control (exposed, 134 subjects), or in office work (controls, 58 subjects). Groups were balanced for age, gender, and smoking habits. The average benzene exposure during the workshift was 9.5 and 3.8 microg/m(3) in exposed individuals and controls, respectively. All subjects were genotyped for CYP1A1, CYP2E1, GSTM1, GSTT1, and DT-diaphorase polymorphisms. The incidence of micronuclei and micronucleated cells was recorded in 1,000 binucleated cells harvested 66 hr after mitogen stimulation. Regression analysis of data showed that MN frequency was mainly modulated by the age (P = 0.001) and gender (P = 0.001) of the study subjects (relatively higher in the elderly and females), whereas it was unaffected by the occupational exposure to traffic fumes and smoking habits. A weak (P = 0.02) association between lower MN frequency and the GSTM1 null genotype was also observed. In order to improve the sensitivity of the method to excision-repairable lesions, a modified protocol, with exposure of cells to the repair inhibitor cytosine arabinoside (Ara-C) during the first 16 hr of growth, was applied to 78 subjects (46 exposed and 32 controls). The results confirmed the higher MN frequency in females (P < 0.05), but failed to demonstrate any significant effect of chemical exposure (occupational or related to smoking habits). When the frequency of MN induced by Ara-C (i.e., spontaneous values subtracted) was considered, a significant inverse correlation with age was observed (P = 0.005), possibly related to the age-dependent decrease in repair proficiency.

Investigating the safety of plants used in South African traditional medicine: testing for genotoxicity in the micronucleus and alkaline comet assays

Previous studies indicate that traditional botanical remedies can be valuable for treating human disease. The potential risk from long-term use of such remedies has not, however, been fully investigated, especially in terms of their potential carcinogenic activity. In the present study, 51 South African plant species were selected on the basis of their use in traditional medicine and crude extracts were sequentially prepared from different dried plant parts using dichloromethane followed by 90% methanol. These extracts were tested for genotoxic activity in human peripheral blood lymphocytes using the micronucleus test, with further testing of select extracts using the alkaline comet assay. Screening results indicated the induction of significant numbers of micronuclei by many of the plant extracts. Several samples also induced DNA damage in human white blood cells using the alkaline comet assay. Although a number of these plants are recognised as toxic by traditional healers, several plants that are used in common remedies were found to be genotoxic and potentially dangerous. Environ.

Using CTLL-2 and CTLL-2 bcl2 cells to avoid interference by apoptosis in the in vitro micronucleus test

In vitro assays for chromosome aberrations (i.e., in vitro micronucleus and in vitro metaphase analysis tests) frequently produce false-positive or exaggerated-positive results. Our previous work suggested that apoptosis interferes with these tests, producing misleading results. These previous studies were conducted by performing the in vitro micronucleus test in CTLL-2 cells and a CTLL-2 cell derivative stably transfected with the apoptosis inhibitor gene bcl2. In the present study, these previous observations were extended by examining micronucleus induction with a larger number of compounds in both CTLL-2 and CTLL-2 bcl2 cells and measuring apoptosis with annexin V-FITC. Both cell lines were treated with different classes of compounds that were anticipated to be exclusively apoptosis inducers, or compounds known to be clastogens or aneugens, some of which were anticipated to be both genotoxic and apoptotic. We were able to confirm that compounds that are only apoptogenic induced micronuclei in CTLL-2 but not CTLL-2 bcl2 cells, indicating that the positive responses are due to apoptosis in CTLL-2 cells. Some genotoxins (clastogens and aneugens) did not produce apoptosis by the annexin V assay and gave similar responses in CTLL-2 and CTLL-2 bcl2 cells. Finally, higher responses were induced in CTLL-2 cells compared to CTLL-2 bcl2 cells that were treated with aneugens or clastogens that were also apoptosis inducers, suggesting that the greater response in CTLL-2 cells is a consequence of both genotoxicity and apoptosis. Finally, it was demonstrated that just eliminating CTLL-2 cells having three or more micronuclei from scoring was not adequate for correctly evaluating agents that only produce apoptosis. The results indicate that coupling the in vitro micronucleus test in both CTLL-2 and CTLL-2 bcl2 cells with the measurement of apoptosis is able to distinguish the genotoxic effects of a test compound from its apoptotic potential and is able to avoid interference from apoptosis in the in vitro micronucleus test. These observations may provide the basis for a useful genotoxicity assay.

Biomarkers of genetic damage for cancer epidemiology

Cancer is a disease of altered gene expression involving a complex array of epigenetic events, gene mutation, chromosome rearrangements and altered chromosome number. The coincidence of genotoxic events with the induction of cancer has fueled great interest in molecular/cytogenetic epidemiological studies aimed at linking polymorphisms in genes for DNA repair and carcinogen metabolism and biomarkers of DNA/chromosome damage with cancer risk. These studies are now being expanded to include the role of dietary factors that are known to be important in DNA metabolism and repair such as folic acid, vitamin B12 and zinc. The use of DNA damage biomarkers as a surrogate for cancer would greatly facilitate our capacity to identify the most important risk factors for cancer however these biomarkers need validation. The Nordic and Italian prospective cohort studies have confirmed that elevated rates of chromosome aberrations in lymphocytes are predictive of cancer risk and similar studies are now underway to validate other biomarkers such as the micronucleus assay which are more practical to apply in the larger population setting. Validation of these biomarkers requires a thorough understanding of the importance of methodological, demographic, environmental and dietary variables and the ongoing HUMN project is a good example how this can be achieved for the micronucleus assay. The capacity to study human populations has opened up new opportunities to define acceptable DNA damage rates and to establish recommended dietary allowances for genomic stability. Controlling DNA damage rate to its possible minimum is likely to have an important impact in preventing cancer and other DNA damage-related degenerative diseases including ageing.

Micronucleus frequency and proliferation in human lymphocytes after exposure to herbicide 2,4-dichlorophenoxyacetic acid in vitro and in vivo

Widespread use of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and its association with non-Hodgkin's lymphoma (NHL) and other cancers has raised public concern. Here, micronucleus (MN) formation has been used as a biomarker of genotoxicity, and replicative and mitotic indices (MIs) as biomarkers of cell cycle kinetics in human lymphocytes. Cells were cultured either as whole blood or isolated lymphocytes and treated with pure or commercial forms of 2,4-D at doses between 0.001 and 1 mM for 48 h. Exposure to 2,4-D produced a minimal increase in MN in whole blood and even smaller one in isolated lymphocyte cultures. This induction took place only at levels approaching cytotoxicity and was accompanied by a significant inhibition of replicative index (RI). At a low (0.005 mM) dose of commercial 2,4-D, a small, marginally significant increase in RI (12-15%) was found in two independent sets of experiments (P=0.052). Additionally, we found that lymphocyte RI was more affected by commercial 2,4-D containing 9.4% of the chemically pure 2,4-D, than with an equal concentration of the latter suggesting that other ingredients present in the commercial pesticide may be responsible or may enhance the effect of 2,4-D. Mitotic index, however, did not show any significant change with either commercial or pure 2,4-D. The lymphocytes of 12 male applicators exposed solely to 2,4-D during a 3-month period had a significantly higher RI than the same group prior to exposure and than a control group (P<0.01), in accordance with the in vitro finding of increased RI at low doses.

Cytogenetic monitoring of industrial radiographers using the micronucleus assay

Industrial radiography is the process of using either gamma-emitting radionuclide sources or X-ray machines to examine the safety of industrial materials. Industrial radiographers are among the radiation workers who receive the highest individual occupational radiation doses. To assess occupationally induced chromosomal damage, we performed the cytokinesis-block micronucleus (CBMN) assay in peripheral lymphocytes of 29 male industrial radiographers, exposed to ionizing radiation for 12.8 years+/-11.2, in comparison with 24 gender-, age-, and smoking habits-matched controls. The CBMN assay was combined with fluorescent in situ hybridization with a pan-centromeric DNA probe in 17 exposed subjects and 17 controls randomized from the initial populations. The mean cumulative equivalent dose, recorded by film dosimeters, was 67.2 mSv+/-49.8 over the past 5 years. The mean micronucleated binucleated cell rate (MCR) was significantly higher in the industrial radiographers than in the controls (10.7 per thousand +/-5.2 versus 6.6 per thousand +/-3.1, P=0.009); this difference was due to a significantly higher frequency of centromere-negative micronuclei (C-MN) in exposed subjects than in controls (8.5 per thousand +/-4.9 versus 2.2 per thousand +/-1.6, P<0.001). The two populations did not significantly differ in centromere-positive micronuclei (C+MN) frequency. These findings demonstrate a clastogenic effect in lymphocytes of industrial radiographers. MCR significantly positively correlated with age in the two groups. After correction for the age effect, MCR did not correlate with duration of occupational exposure. No correlation between radiation doses and MCR, C-MN, and C+MN frequencies was observed. In addition to physical dosimetry records, the enhanced chromosomal damage in lymphocytes of industrial radiographers emphasizes the importance of radiation safety programs.

Chromosomal biomarkers of genomic instability relevant to cancer

It is generally acknowledged that a crucial event in the initiation and evolution of cancer is the acquisition of a genomic instability phenotype. This review focuses on mechanisms of chromosomal instability including aneuploidy, chromosome rearrangement and breakage-fusion-bridge cycles. The role of micronutrient deficiency, such as folate deficiency, in the causation of chromosomal instability is briefly reviewed and the concept of recommended dietary allowances for genomic stability is introduced. In addition, the techniques for measuring the various chromosomal instability events are discussed with a focus on the cytokinesis-block micronucleus assay as an almost complete system for measuring these various genetic mishaps.

Acentromeric micronuclei are increased in peripheral blood lymphocytes of untreated cancer patients

Increased micronucleated cell rates, dicentric chromosomes, and other chromosomal damages have been reported in lymphocytes of cancer patients prior to the initiation of chemotherapy, and/or radiotherapy. The cause of these chromosomal damages in these lymphocytes remains unclear. In the present work, we investigated whether these micronuclei mainly reflect structural or numerical chromosomal aberrations by applying the cytokinesis-blocked micronucleus (CBMN) assay in combination with fluorescent in situ hybridization (FISH) of a DNA centromeric probe on blood samples of 10 untreated cancer patients (UCPs), and 10 healthy subjects (HSs). Micronucleated binucleated lymphocyte rate was significantly increased in patients (mean+/-S.D.: 19.0 per thousand +/-14.1 versus 9.2 per thousand +/-4.6 in controls). Trinucleated cytokinesis-blocked cells were not significantly higher in patients than in controls. Acentromeric, centromeric, and multicentromeric micronucleus levels were two-fold higher in patients than in controls, but the difference was significant only with acentromeric micronuclei. The percentage of micronuclei containing one or more centromeres averaged 69.2, and 71.5% in patients, and controls, respectively. The percentage of micronuclei containing several centromeres was 44.7% in patients, and 54.6% in controls. Among centromere-positive micronuclei, the percentage of micronuclei containing several centromeres averaged 59.7% in patients, and 75.4% in controls. These results indicate that genetic instability in peripheral blood lymphocytes of UCPs occurs because of enhanced chromosome breakage. However, a substantial proportion of this genetic instability occurs because of defects in chromosome segregation.

Micronuclei, nucleoplasmic bridges and nuclear buds induced in folic acid deficient human lymphocytes-evidence for breakage-fusion-bridge cycles in the cytokinesis-block micronucleus assay

We have validated the analysis of nucleoplasmic bridges (NPBs) and nuclear buds as biomarkers of genomic instability within the cytokinesis-block micronucleus assay in long-term lymphocyte cultures. Lymphocytes from 20 subjects were cultured in medium containing 12-120 nM folic acid for 9 days. Binucleate cells were scored for micronuclei (MN), NPBs and nuclear budding on day nine after 24h incubation in the presence of the cytokinesis inhibitor cytochalasin-B. Folic acid concentration was correlated significantly (P<0.0001) and negatively (r=-0.63 to -0.74) with all these markers of chromosome damage. Chromosome damage was minimised at 60-120 nM folic acid, which is greater than the concentration of folate normally observed in plasma (<30 nM). Current evidence suggests that (a) NPBs originate from dicentric chromosomes in which the centromeres have been pulled to the opposite poles of the cell at anaphase and are therefore, indicative of chromosome rearrangement and (b) that the nuclear budding process is the mechanism by which cells remove amplified DNA and is therefore a marker of gene amplification. The strong correlation between micronucleus formation, nuclear budding and NPBs (r=0.75-0.77, P<0.001) is supportive of the hypothesis that folic acid deficiency causes genomic instability and gene amplification by the initiation of breakage-fusion-bridge (BFB) cycles. These results also suggest that the CBMN assay may be a useful model for the study of the BFB cycle which may be one of the key mechanisms for the hypermutability phenotype required for the rapid evolution of cancer cells.

Genotoxicological characterisation of complex mixtures. Genotoxic effects of a complex mixture of perhalogenated hydrocarbons

Toxicological and genotoxicological investigation of complex mixtures is one of the main focus of the recent research in toxicology. Testing complex mixtures present a formidable scientific problem since most recently available toxicological data has been obtained from single substance studies and is not simply transferable to mixtures of chemicals. Although there are no special strategies and standard protocols available for determining toxic and genotoxic effects of complex mixtures, the fundamental concepts of evaluation are the same as those for single substances. The focus of interest of the submitted paper is the genotoxicological characterisation of a complex mixture of mostly perhalogenated hydrocarbons which is generated as a waste product from the plasma etching process in the semiconductor industry. By use of several in vitro test systems (comet assay and micronucleus test), the clastogenic potency of the mixture was tested in various human cell types (lymphocytes and normal bronchial epithelial cells) and in rat hepatocytes. Results demonstrated that the complex perhalogenated hydrocarbons mixture causes DNA single-strand breaks and micronuclei formation, and direct concentration-to-effect correlations were proved in all experiments. The presence of an external metabolising system (S9 mix from rat hepatocytes) in human cell culture systems did not cause any change of the observed effects when compared to experiments performed in the absence of the S9 mix. Therefore, we conclude that the mixture acts as direct genotoxicant and that there is no detoxification by the external enzyme system.Further, convincing and reproducible results of the in vitro comet assay and the micronucleus assay in primary human cell cultures indicated these tests may be utilized for the genotoxicological analyses of complex mixtures with concern to human health hazard.

Genotoxicity of radiofrequency signals. I. Investigation of DNA damage and micronuclei induction in cultured human blood cells

As part of a comprehensive investigation of the potential genotoxicity of radiofrequency (RF) signals emitted by cellular telephones, in vitro studies evaluated the induction of DNA and chromosomal damage in human blood leukocytes and lymphocytes, respectively. The signals were voice modulated 837 MHz produced by an analog signal generator or by a time division multiple access (TDMA) cellular telephone, 837 MHz generated by a code division multiple access (CDMA) cellular telephone (not voice modulated), and voice modulated 1909.8 MHz generated by a global system of mobile communication (GSM)-type personal communication systems (PCS) cellular telephone. DNA damage (strand breaks/alkali labile sites) was assessed in leukocytes using the alkaline (pH>13) single cell gel electrophoresis (SCG) assay. Chromosomal damage was evaluated in lymphocytes mitogenically stimulated to divide postexposure using the cytochalasin B-binucleate cell micronucleus assay. Cells were exposed at 37+/-1 degrees C, for 3 or 24 h at average specific absorption rates (SARs) of 1.0-10.0 W/kg. Exposure for either 3 or 24 h did not induce a significant increase in DNA damage in leukocytes, nor did exposure for 3 h induce a significant increase in micronucleated cells among lymphocytes. However, exposure to each of the four RF signal technologies for 24 h at an average SAR of 5.0 or 10.0 W/kg resulted in a significant and reproducible increase in the frequency of micronucleated lymphocytes. The magnitude of the response (approximately four fold) was independent of the technology, the presence or absence of voice modulation, and the frequency (837 vs. 1909.8 MHz). This research demonstrates that, under extended exposure conditions, RF signals at an average SAR of at least 5.0 W/kg are capable of inducing chromosomal damage in human lymphocytes.

Micronuclei assay by laser scanning cytometry

BACKGROUND

The micronuclei (MN) assay is used to assess the chromosomal/mitotic spindle damage induced by ionizing radiation or mutagenic agents in vivo or in vitro. Because visual scoring of MN is cumbersome semi-automatic procedures that relay either on flow cytometry or image analysis were developed: both offer some advantages but also have shortcomings.

METHODS

In the present study laser scanning cytometer (LSC), the instrument that combines analytical capabilities of flow and image cytometry, has been adapted for quantitative analysis of MN. The micronucleation of human breast carcinoma MCF-7 and leukemic HL-60 and U-937 cells was induced by in vitro treatment with mitomycin C. Cellular DNA was stained with propidium iodide (PI), protein was counterstained with fluorescein isothiocyanate (FITC). Two approaches were used to detect MN: (a) the threshold contour was set based on the data from the photosensor measuring red fluorescence of PI and MN were identified on the bivariate PI versus PI/FITC fluorescence distributions by their characteristic position; (b) the threshold contour was set on the data from the sensor measuring FITC fluorescence which made it possible, using the LSC software dedicated for FISH analysis, to assay both the frequency and DNA content of individual MN within each measured cell.

RESULTS

The capability of LSC to relocate MN for visual examination was useful to confirm their identification. Visual identification of MN combined with their multiparameter characterization that took into an account their DNA content and protein/DNA ratio made it possible establish the gating parameters that excluded objects that were not MN; 93.3+/-3.3 events within the selected gate were MN. It was also possible to successfully apply FISH software to characterize individual cells with respect to quantity of MN residing in them. The percentage of MN assayed by LSC correlated well with that estimated visually by microscopy, both for MCF-7 (r = 0.93) and HL-60 cells (r = 0.87).

CONCLUSIONS

LSC can be used to obtain unbiased estimate of MN frequencies. Unlike flow cytometry, it also allows one to characterize individual cells with respect to frequency and DNA content of MN residing in these cells. These analytical capabilities of LSC may be helpful not only to score MN but also to study mechanisms by which clastogenic agents induce MN.

Differential sensitivity of Chinese hamster V79 and Chinese hamster ovary (CHO) cells in the in vitro micronucleus screening assay

Both the V79 and CHO cell lines are routinely used in the in vitro MN screening assay for the detection of possible genotoxicants. The CHO cell line is the predominant cell line currently used in the genetic toxicology testing industry. However, some laboratories routinely utilize the V79 cell line since the in vitro MN screening assay was initially developed using V79 cells. Our laboratory has historically used the CHO cell line. Therefore, our laboratory was interested in comparing the two cell lines with regard to possible similarities or differences in MN induction sensitivity after exposure to cyclophosphamide (CPA) and mitomycin C (MMC), the two standard positive control chemicals routinely used in this assay. Three exposure conditions in the presence of CPA and MMC were examined in both cell lines. Replicate cultures of CHO cells in McCoy's 5A and V79 cells in both McCoy's 5A and E-MEM were established and treated with 5 microg CPA/ml (4h exposure with S9), 0.5 microg MMC (4h exposure without S9) and 0.5 microg MMC (24h exposure without S9). A total of 400 cytochalasin B-blocked binucleated cells and 200 consecutive cells were analyzed from each culture for MN and cell cycle kinetics, respectively. Analysis of the data demonstrated that CHO cells were up to approximately five-fold more sensitive to the induction of CPA- and MMC-induced MN than V79 cells. Both cell lines exhibited similar average generation times among identical exposure groups. Therefore, the difference in MN sensitivity cannot be attributed to possible differences in cell cycle kinetics and is possibly related to inherent cellular differences in the processing of and/or repair of CPA- and MMC-induced damage by V79 and CHO cells.

Methylenetetrahydrofolate reductase C677T polymorphism does not alter folic acid deficiency-induced uracil incorporation into primary human lymphocyte DNA in vitro

Methylenetetrahydrofolate reductase (MTHFR) is an enzyme which converts 5,10-methylene tetrahydrofolate (5,10-MnTHF) to 5-methyl tetrahydrofolate. A common C to T transition (C677T) in the MTHFR gene is reported to reduce the risk for colorectal cancer and acute lymphocytic leukemia in homozygotes (TTs). It is hypothesized that because TTs have reduced MTHFR activity, more 5,10-MnTHF is available to provide methyl groups for the conversion of uracil to thymidine. Folic acid deficiency causes the intracellular accumulation of dUMP and the subsequent incorporation of uracil into DNA. The removal of uracil from DNA may result in double-stranded DNA breaks, the accumulation of which is a putative risk factor for cancer. We tested whether human lymphocytes taken from TTs (n = 10) were more able to resist uracil incorporation into DNA than controls (n = 14 CCs and 6 CTs) when cultured in medium containing 12-120 nM folic acid for 9 days. DNA uracil content of these lymphocytes was measured by CG-MS. TTs and controls showed a dose-dependent increase in DNA uracil content during folic acid deficiency (P < 0.0001, R2 = 0.23 for TTs and P < 0.0001, R2 = 0.19 for controls). DNA uracil content was not different between the two groups at any of the folic acid concentrations (two-way ANOVA: media [folic acid], P < 0.0001; genotype, P = 0.4). The results show that, in this in vitro system, the MTHFR C677T polymorphism does not affect the cell's ability to resist uracil incorporation into DNA. Chromosome breakage, as measured by micronuclei, was also shown to correlate with folic acid concentration in a preliminary experiment (P < 0.0001). Although the results appear not to support the hypothesis that a reduced risk for certain cancers in TTs is due to diversion of folic acid to thymidine synthesis, differences between the in vivo and in vitro situation make this conclusion not definitive.

In vitro genotoxicity testing of (1-chloroethenyl)oxirane, a metabolite of beta-chloroprene

(1-Chloroethenyl)oxirane (CEO) is a metabolite of beta-chloroprene (2-chloro-1,3-butadiene, CD). The purpose of this study was to evaluate the in vitro mutagenic and clastogenic (chromosome breaking) potential of CEO. For comparative purposes, the study also included an evaluation of the racemic compounds, 3,4-epoxy-1-butene (EB) and 1,2:3,4-diepoxybutane (DEB). Mutagenicity was evaluated in a bacterial reverse mutation test (Ames), using the pre-incubation method in the presence and absence of an exogenous metabolism system (Aroclor)-induced rat liver S9). Four Salmonella typhimurium tester strains, TA97a, TA98, TA100 and TA1535 were used. The exposure concentrations in the sealed incubation vials ranged from 0 to 69 mM for CEO, 0 to 102 mM for EB, and 0 to 83 mM for DEB. All three compounds showed signs of toxicity, with DEB being substantially more toxic than either CEO or EB. Mutagenic activity was observed with all three chemicals in primarily the base pair substitution strains (S. typhimurium TA100 and TA1535), but some activity was also seen in the frameshift elimination strains (S. typhimurium TA97a and TA98). The observed mutagenic responses after exposure with CEO or EB were greater than the observed response for DEB, most likely because of the higher toxicity of DEB. Generally, the mutagenic responses were unchanged in the frameshift strains and base pair substitution strains in the presence of S9 metabolism. In vitro clastogenicity was evaluated using the cytochalasin-B blocked micronucleus test in cultured Chinese hamster V79 cells. The test was conducted without S9 metabolism because of the absence of substantial changes in the Ames test. Exposure concentrations ranged from 0 to 0.943 mM for CEO, 0 to 3.0 mM for EB, and 0 to 0.035 mM for DEB, with the upper exposure concentrations dictated by cytotoxicity. Cytotoxicity, measured as a reduction in the proportion of binucleated cells and altered cell morphology, was observed for CEO at concentrations > or =0.175 mM. Exposure to EB led to a reduced proportion of binucleated cells at concentrations > or =2.0 mM, and cell death was observed after DEB exposure at concentrations > or =0.025 mM. No clastogenicity was observed in the V79 cells when tested up to cytotoxic concentrations of CEO, whereas an elevated frequency of micronuclei was observed after exposure to either EB (> or =1.0 mM) or DEB (> or =0.0125 mM). These results suggest that CEO-induced mutagenicity, but not clastogenicity, may contribute to the observed beta-chloroprene-induced carcinogenicity in the rodent bioassay studies.