Flow cytometric analysis of micronuclei in the CD2+/- subpopulation of human lymphocytes enriched by magnetic separation

Micronucleus Analysis by Flow Cytometry

During the last two decades the micronucleus (MN) test has been extensively used as a genotoxicity screening tool of chemicals and in a variety of exploratory and mechanistic investigations. The MN is a biomarker for chromosomal damage or mitotic abnormalities since it can originate from chromosome fragments or whole chromosomes that fail to be incorporated into daughter nuclei during mitosis (Fenech et al., Mutagenesis 26: 125-132, 2011; Kirsch-Volders et al., Arch Toxicol 85: 873-899, 2011). The simplicity of scoring, accuracy, amenability to automation by image analysis or flow cytometry and the readiness to be applied to a variety of cell types either in vitro or in vivo made it a versatile tool that contributed to a large extent in our understanding of key toxicological issues related to genotoxins and their effects at the cellular and organism levels. Recently, the final acceptance of the in vitro MN test Organization for Economic Cooperation and Development (OECD) guideline 487 (OECD, Guideline for testing of chemicals: in vitro mammalian cell micronucleus test 487: in vitro mammalian cell micronucleus test (MNVIT). Organization for Economic Cooperation and Development, Paris, 2010) together with the standard in vivo MN test OECD guideline 474 (OECD, Guideline for the testing of chemicals no. 474 mammalian erythrocyte micronucleus test. Organization for Economic Cooperation and Development, Paris, 1997) further positioned the assay as a key driver in the determination of the genotoxicity potential in exploratory research as well as in the regulatory environment. This book chapter covers to some extent the protocol designs and experimental steps necessary for a successful performance of the MN test and an accurate analysis of the MN by the flow cytometry technique.

Follow-up study of genotoxic effects in individuals exposed to oil from the tanker Prestige, seven years after the accident

The accident with the oil tanker Prestige in November 2002 resulted in a major spill of about 63,000 tons of heavy fuel oil. More than 300,000 people participated in the clean-up activities, which lasted for up to 10 months. Previous studies reported increases in genotoxicity endpoints in individuals exposed to Prestige oil, both at the moment of exposure [DNA breakage, micronuclei (MN), sister chromatid exchange] and two years later (chromosomal aberrations). In this work we carried out for the first time the follow-up of genotoxic effects in subjects exposed to an oil spill seven years after the exposure. The main objective was to determine the possible persistence of genotoxic damage in individuals exposed to Prestige oil seven years after the accident. The exposed group was composed of 54 residents of Galician villages in Spain that were heavily affected by the spill. This group was involved in clean-up labor for at least two months in the period November 2002-September 2003. They were compared with 50 matched controls. Primary DNA damage was evaluated by the comet assay, mutagenicity by the T-cell receptor (TCR) mutation assay, and MN frequency was determined both by the cytokinesis-block test and by flow cytometry. The results obtained showed no significant differences between the exposed and the controls in the comet assay, the TCR mutation assay and the cytokinesis-block MN test. An unexpected and significant decrease was observed in the exposed group for the results of the MN test evaluated by flow cytometry, probably influenced by modifying factors - other than age, sex and smoking - not considered in this study. Our results show no evidence of the persistence of genotoxic damage in individuals exposed to Prestige oil seven years later. Nevertheless, the need to plan biomonitoring studies on people participating in clean-up activities in case a new oil spill occurs should be established.

Flow cytometric determination of micronucleus frequency

During the last two decades the micronucleus (MN) test has been extensively used as a genotoxicity screening tool of chemicals and in a variety of exploratory and mechanistic investigations. The MN is a biomarker for chromosomal damage or mitotic abnormalities, since it can originate from chromosome fragments or whole chromosomes that fail to be incorporated into daughter nuclei during mitosis (Fenech et al., Mutagenesis 26:125-132, 2011; Kirsch-Volders et al., Arch Toxicol 85:873-899, 2011). The simplicity of scoring, accuracy, amenability to automation by image analysis or flow cytometry, and readiness to be applied to a variety of cell types either in vitro or in vivo have made it a versatile tool that has contributed to a large extent in our understanding of key toxicological issues related to genotoxins and their effects at the cellular and organism levels. Recently, the final acceptance of the in vitro MN test guideline 487 (OECD Guideline for Testing of Chemicals, In vitro mammalian cell micronucleus test 487. In vitro mammalian cell micronucleus test (MNVIT). Organization for Economic Cooperation and Development, Paris, 2010) together with the standard in vivo MN test OECD guideline 474 (OECD Guideline for The Testing of Chemicals, Mammalian erythrocyte micronucleus test no. 474. Organization for Economic Cooperation and Development, Paris, 1997) will further position the assay as a key driver in the determination of the genotoxicity potential in exploratory research as well as in the regulatory environment. This chapter covers to some extent the protocol designs and experimental steps necessary for a successful performance of the MN test and an accurate analysis of the MN by the flow cytometry technique.

Miniaturized flow cytometry-based in vitro primary human lymphocyte micronucleus assay-validation study

Most in vitro mammalian genotoxicity assays show a low specificity (high rate of irrelevant positive results), and therefore, lead to an increase in follow-up in vivo genotoxicity testing. One of the sources of the high rate of in vitro irrelevant positive results that find no confirmation in in vivo studies may be the characteristics of the test system used. It has been shown that cells that are p53 deficient or carry an alteration in DNA repair genes may be more prone to produce high rate of false/irrelevant positive results. Primary human lymphocytes (HuLy) are considered to show a higher specificity in predicting the in vivo genotoxic potential of a tested compound. We recently developed a flow cytometry-based primary human T-lymphocyte micronucleus test (MNT) and showed that the technology is promising and reliable in detecting genotoxic compounds. The purpose of the present work was to develop and validate a miniaturized format of the assay. For validation purposes of the flow cytometry HuLy MNT a wide selection of compounds with different mechanisms of genotoxicity was used. The evaluation covered 30 compounds: 19 commercially available genotoxicants and nongenotoxicants and 11 early pharmaceutical development compounds. Being faster and less tedious than the microscopic analysis, the miniaturized flow cytometry-based methodology showed very promising results. Conveniently, cell division is verified within the same sample as the MN frequency. Moreover analysis of hypodiploid events may provide an indication for a mode of action, i.e. clastogenic versus aneugenic mechanism, for further follow-up testing.

In vitro primary human lymphocyte flow cytometry based micronucleus assay: simultaneous assessment of cell proliferation, apoptosis and MN frequency

In order to minimise the number of positive in vitro cytogenetic results which are not confirmed in rodent carcinogenicity tests, biological systems that are p53 and DNA repair proficient should be recommended. Moreover, an appropriate cytotoxicity parameter for top dose selection should be considered. Recent International Conference on Harmonisation draft S2 and Organisation for Economic Co-operation and Development (OECD) 487 guideline accepted the in vitro micronucleus test (MNT) as a valid alternative method for in vitro chromosome aberration test within the in vitro cytogenetic test battery. Since mitosis is a prerequisite for expression of the micronuclei, it is compulsory to demonstrate that cell division occurred, and if possible, to identify the cells that completed mitosis. The OECD guideline recommends the use of a cytokinesis block for the assessment of proliferation in primary T-lymphocytes. The work presented in this manuscript was initiated to develop a novel flow cytometry-based primary human lymphocyte MNT method. This new assay is based on a three-step staining procedure: carboxyfluorescein succinimidyl ester as a proliferation marker, ethidium monoazide for chromatin of necrotic and late apoptotic cells discrimination and 4,6-diaminodino-2-phenylindole as a DNA marker. The proof of principle of the method was performed using genotoxic and non-genotoxic compounds: methyl methanesulfonate, mitomycin C, vinblastine sulphate, cyclophosphamide, sodium chloride and dexamethasone. It has been shown that the new flow cytometry-based primary human lymphocyte MNT method is at least equally reliable method as the standard Cytochalasin B MNT. However, further validation of the assay using a wide selection of compounds with a variety of mechanisms of action is required, before it can be used for regulatory purposes. Moreover, a miniaturisation of the technology may provide an additional advantage for early drug development.

A flow cytometry based in vitro micronucleus assay in TK6 cells--validation using early stage pharmaceutical development compounds

The micronucleus test (MNT) is a well established test for detecting clastogenic and aneugenic compounds. Despite the assay's advantages, the MNT may produce false positive and false negative results in some conditions. This fact may be related to the underestimation of apoptosis or necrosis, the p53 status of the cell system or the cytotoxicity assay, and the top dose selection. The purpose of our studies was to contribute to the validation efforts of the flow cytometry based MNT. To identify the most reliable cytotoxicity assay for the top dose selection five parameters for relative survival were tested: relative cell count, relative population doubling, trypan blue supravital staining, relative ratio of scored nuclei to latex beads, and ethidium monoazide staining. For all compounds the least sensitive method was the relative cell count and the most reliable was the nuclei/beads ratio. The comparative evaluation of micronuclei induction in TK6 cells, analyzed with microscopy and flow cytometry, was performed with reference compounds and internal Novartis early development compounds with positive, weak positive, equivocal, and negative genotoxic effects. Our data document a good correlation between the MNT results obtained by flow cytometry and by microscopy. The results confirm that the method may be applied for routine testing in the pharmaceutical industry for the tested group of compounds, including compounds which require metabolic activation. However, further validation and miniaturization may be required.

The in vitro MN assay in 2011: origin and fate, biological significance, protocols, high throughput methodologies and toxicological relevance

Micronuclei (MN) are small, extranuclear bodies that arise in dividing cells from acentric chromosome/chromatid fragments or whole chromosomes/chromatids lagging behind in anaphase and are not included in the daughter nuclei at telophase. The mechanisms of MN formation are well understood; their possible postmitotic fate is less evident. The MN assay allows detection of both aneugens and clastogens, shows simplicity of scoring, is widely applicable in different cell types, is internationally validated, has potential for automation and is predictive for cancer. The cytokinesis-block micronucleus assay (CBMN) allows assessment of nucleoplasmic bridges, nuclear buds, cell division inhibition, necrosis and apoptosis and in combination with FISH using centromeric probes, the mechanistic origin of the MN. Therefore, the CBMN test can be considered as a "cytome" assay covering chromosome instability, mitotic dysfunction, cell proliferation and cell death. The toxicological relevance of the MN test is strong: it covers several endpoints, its sensitivity is high, its predictivity for in vivo genotoxicity requires adequate selection of cell lines, its statistical power is increased by the recently available high throughput methodologies, it might become a possible candidate for replacing in vivo testing, it allows good extrapolation for potential limits of exposure or thresholds and it is traceable in experimental in vitro and in vivo systems. Implementation of in vitro MN assays in the test battery for hazard and risk assessment of potential mutagens/carcinogens is therefore fully justified.

Evaluation of okadaic acid-induced genotoxicity in human cells using the micronucleus test and γH2AX analysis

Marine algal blooms have become a public health concern due to increasing frequency in the environment and severity of exposure consequences. Human intoxications produced by phycotoxins occur globally through consumption of marine fish products containing bioaccumulated toxins. Okadaic acid (OA) is the main representative of diarrheic shellfish poisoning (DSP) toxin. OA was found to inhibit protein phosphatases and to produce oxidative damage, as well as to disturb different cellular functions including cell cycle, gene expression, and DNA repair mechanisms. The aim of this study was to determine whether OA induced genotoxicity by using a micronucleus (MN) test and γH2AX analysis, and to elucidate the underlying mechanisms. Human peripheral blood leukocytes, neuroblastoma cells (SHSY5Y), and hepatoma cells (HepG2) were treated with a range of OA concentrations in the presence and absence of S9 fraction. MN induction was observed in leukocytes at all concentrations tested, and in SHSY5Y and HepG2 cells only at the highest concentration (1000 nM). In contrast, γH2AX analysis was only positive for HepG2 cells. Taking together these data, in addition to the comet assay results obtained in a previous study in this issue, OA was found to exert a either a clastogenic or aneugenic effect dependent upon the cell types examined.

Interlaboratory evaluation of a flow cytometric, high content in vitro micronucleus assay

An international, multi-lab trial was conducted to evaluate a flow cytometry-based method for scoring 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. Mol. Mutagen. 47 (2006) 56-66]. A reference laboratory investigated the potential of six chemicals to induce micronuclei -- the genotoxicants mitomycin C (MMC), etoposide (ETOPO), and vinblastine (VB), and the non-genotoxicants sucrose (SUC), staurosporine (STS), and dexamethasone (DEX). The latter two non-genotoxicants were selected as extreme challenges to the assay because of their potent apoptogenic activity. Three collaborating laboratories were supplied with prototype In Vitro MicroFlow kits, and each was assigned one genotoxicant and one non-genotoxicant. Cells were treated continuously for 24h over a range of concentrations up to 5 mg/ml, or overtly cytotoxic concentrations. Micronuclei were scored via standard microscopy and flow cytometry. In addition to enumerating micronucleus frequencies, a cytotoxicity measurement that is simultaneously acquired with the flow cytometric micronucleus scoring procedure was evaluated (Flow-NBR). With this method, latex particles served as counting beads, and facilitated relative survival measurements that exclude the presence of dead/dying cells. For comparison purposes, additional cytotoxicity endpoints were measured, including several that are based on cell number, and others that reflect compromised membrane integrity, including dye permeability and/or phospholipid distribution. Key findings for this set of compounds include the following: (1) significant discrepancies in top concentration selection were found when cytotoxicity measurements were based on different methods, with the Flow-NBR approach tending to be the most sensitive, (2) both microscopy- and flow cytometry-based scoring methods detected concentration-dependent micronucleus formation for the three genotoxic agents studied, with good agreement between the reference laboratory and the collaborating laboratories, and (3) whereas flow cytometric analyses showed no significant increases for the non-genotoxicants when top concentration selection was based on Flow-NBR, significantly elevated micronucleus frequencies were observed for concentrations that were chosen based on less-sensitive cytotoxicity assays. Collectively, these results indicate that rapid assessment of genotoxicity can be accomplished with a relatively simple flow cytometric technique, and that the scoring system is transferable across laboratories. Furthermore, a concurrent assessment of cytotoxicity, Flow-NBR, may help reduce the occurrence of irrelevant positive results, as it may represent a more appropriate means for choosing top concentration levels. Finally, the data presented herein reinforce concerns about the manner in which cytotoxicity limits are described in guidance documents, since these recommendations tend to cite fixed cut-off values without reference to methodology.

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.

In vitro micronucleus scoring by flow cytometry: differential staining of micronuclei versus apoptotic and necrotic chromatin enhances assay reliability

The in vitro micronucleus test has received considerable attention in recent years for its use in drug safety assessment and toxicological research. The less tedious nature of the assay relative to chromosome aberration analyses is a driving force, and explains why many chemical and drug safety programs have adopted the endpoint. Development of a high-throughput micronucleus scoring system would further enhance the utility of the assay for lead optimization and other early drug development work. Although several variations of a flow cytometric (FCM) method for scoring cell-culture-derived micronuclei (MN) have been described in the literature, they have been unable to distinguish true MN from apoptotic and necrotic chromatin (Nüsse M and Marx K 1997: Mutat Res 392: 109-115). Here, we report advances to this methodology whereby a sequential staining procedure is used to differentially label these types of sub-2n particles. With the use of ethidium monoazide (EMA), the chromatin of dead and dying cells is labeled. After a photoactivation step that covalently binds EMA to chromatin, cytoplasmic membranes are digested and resulting lysates are incubated with RNase plus a pan-nucleic acid dye (SYTOX Green). This process provides a suspension of free nuclei and sub-2n particles that are labeled with either SYTOX or SYTOX and EMA. Preliminary studies with heat-shocked L5178Y mouse cells demonstrated that EMA stains necrotic and mid- through late-stage apoptotic cells. Importantly, the sequential labeling procedure provided reliable micronucleus enumeration, even when cultures contained high percentages of dead cells. Subsequently, experiments with the following diverse genotoxicants were performed: hydroxyurea, methyl methanesulfonate, benzo[a]pyrene, etoposide, cyclophosphamide, and vinblastine. Additionally, the nongenotoxicants sucrose, tributyltin methoxide, and dexamethasone were tested up to 5 mg/ml, or to cytotoxic concentrations. FCM data were found to correspond closely with microscopy-based measurements. Collectively, these data suggest that this sequential EMA/SYTOX staining procedure provides reliable, high-throughput enumeration of in vitro MN.

Comet assay and flow cytometry analysis of DNA repair in normal and cancer cells treated with known mutagens with different mechanisms of action

In order to determine differences in repair after treatment with DNA damaging agents, normal and cancer cells were selected for analysis of single strand breaks and DNA crosslinks using the Comet assay. Normal human lymphocytes, human colorectal adenocarcinoma SW620 cells, lung carcinoma A549, and H460 cell lines were exposed to an ethylating agent (ethylmethane sulfonate [EMS]), and a cross-linking agent (mitomycin C [MMC]). Differences in repair profiles of DNA damage demonstrated using the comet assay were observed in human lymphocytes and tumour cell lines with both mutagens. Results were also indicative that MMC repair is concentration-dependent. It was also apparent that normal cells repair DNA damage more readily than tumour cells. Repair also varied between different cell lines. To investigate the mechanistic differences of these two chemicals, flow cytometry studies were undertaken in tumour cells, namely cell cycle analysis and frequency of micronuclei induction (FMN). A G2M phase block was clearly evident following treatment with EMS at all concentrations tested. With MMC, an initial arrest of cells in G2M was accompanied by a build-up in S-phase over longer exposure periods. Also, at the highest mutagen doses there were different patterns of micronuclei induction. Thus, using the mutagens with different mechanisms of action highlighted the differences in repair patterns between normal and tumour cells.

Flow cytometric analysis of genetic damage, effect on cell cycle progression, and apoptosis by thiophanate-methyl in human lymphocytes

Flow cytometric technique was used to study the effects of the fungicide Thiophanate-methyl on cell proliferation, micronucleus induction, and apoptosis in human peripheral blood lymphocytes treated in vitro. In particular, a combined approach of flow cytometry and fluorescence in situ hybridization (FISH) with a pancentromeric alpha-satellite probe was used to evaluate the mechanism of micronucleus induction by Thiophanate-methyl. Flow sorted micronuclei (MN) induced in human lymphocytes by Thiophanate-methyl were analyzed by FISH and the results were compared with results from FISH analysis on MN in binucleated cells. It could be shown that most MN induced by Thiophanate-methyl did not reveal any centromeric signal, thus demonstrating clastogenic action of this fungicide. Moreover, it was found that as a function of the concentration of Thiophanate-methyl, cellular proliferation was delayed and the frequency of apoptotic cells was increased.

Flow cytometric analysis of micronuclei in cell cultures and human lymphocytes: advantages and disadvantages

Flow cytometric techniques are described to quantify micronucleus (MN) induction in cell cultures and human lymphocytes. The advantages and disadvantages of these techniques are discussed. Because a suspension of nuclei and MN has to be prepared for flow cytometric measurements, care has to be taken to avoid unspecific debris that can influence the results. Using additional flow cytometric parameters, most of the unspecific particles in the suspension can, however, be gated out. Apoptotic cells and apoptotic bodies can overlap the MN during measurement, it is, therefore, proposed not to use the technique if apoptosis is induced by the respective treatment. Advantages of the automated flow cytometric techniques are that results can be obtained in short time intervals, the frequency of MN and the DNA distribution of MN can be measured simultaneously and flow sorting can be used for a further analysis of MN using other techniques.

Automated scoring of micronuclei in binucleated human lymphocytes

Manual and automatic scoring of micronuclei (MN) in binucleated human lymphocytes (BNC) were compared after irradiation of whole blood samples. The blood samples were irradiated with X-ray doses (1, 2 or 3 Gy) and stained with Giemsa. The preparation technique was optimized in such a way that acceptable conditions (cell density, contrast) were obtained for both scoring procedures. To estimate the quality of automatic micronucleus detection, two researchers who had different experience in scoring MN (6 months and 5 years) analysed the samples independently from each other. Automatic scoring was carried out with a digital image analysis system and the recognition procedure was divided into two parts. The BNC positions were detected with low microscope magnification (100x), and the recognition of micronuclei within the cytoplasm of the classified BNC was carried out at high magnification (630x). A fuzzy logic classification system as well as two different segmentation steps (preclassification and postclassification) made it possible that about 94% of all automatically recognized BNC were classified correctly). On the other hand, the classification system was optimized in such a way that false positive decisions were minimized (95% of automatically recognized micronuclei were classified correctly). Failure to recognize micronuclei (8.5%-25% false negatives) was mainly due to extremely small micronuclei, poor contrast with respect to the cytoplasm, and aggregation of micronuclei especially at higher doses.