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

A novel micronucleus in vitro assay utilizing human hematopoietic stem cells

The induction of micronucleated reticulocytes in the bone marrow is a sensitive indicator of chromosomal damage. Therefore, the micronucleus assay in rodents is widely used in genotoxicity and carcinogenicity testing. A test system based on cultured human primary cells could potentially provide better prediction compared to animal tests, increasing patient safety while also implementing the 3Rs principle, i.e. replace, reduce and refine. Hereby, we describe the development of an in vitro micronucleus assay based on animal-free ex vivo culture of human red blood cells from hematopoietic stem cells. To validate the method, five clastogens with direct action, three clastogens requiring metabolic activation, four aneugenic and three non-genotoxic compounds have been tested. Also, different metabolic systems have been applied. Flow cytometry was used for detection and enumeration of micronuclei. Altogether, the results were in agreement with the published data and indicated that a sensitive and cost effective in vitro assay to assess genotoxicity with a potential to high-throughput screening has been developed.

Gene-TEQ--a standardized comparative assessment of effects in the comet assay using genotoxicity equivalents

Existing methods for the comparison of genotoxic effects in the comet assay bear considerable disadvantages such as the problem to link information about concentration dependence and severity of effects. Moreover, given the lack of standardized protocols and the use of various standards, it may be extremely difficult to compare different studies. In order to provide a method for standardized comparative assessment of genotoxic effects, the concept of genotoxicity equivalents (Gene-TEQ) was developed. As potential reference compounds for genotoxic effects, three directly acting (N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methyl-methanesulfonate, and N-methyl-N-nitrosourea) and three indirectly acting (cyclophosphamide, dimethylnitrosamine, and 4-nitroquinoline-oxide) genotoxic substances were compared with respect to their cytotoxic (neutral red) and genotoxic (comet assay) concentration-response profiles in the permanent fish cell line RTL-W1. For further comparison, two sediment extracts from the upper Danube River were investigated as environmental samples. Based on the results of cytotoxicity and genotoxicity testing, MNNG was selected as the reference compound. At several exposure levels and durations, genotoxic effects of both the other pure substances and the environmental samples were calculated as percentages of the maximum MNNG effect and related to the absolute MNNG effect (EC values). Thus, genotoxicity equivalent factors (Gene-TEQs) relative to MNNG could be calculated. Gene-TEQs can easily be applied to pure substances, mixtures and field samples to provide information about their toxicity relative to the reference compound. Furthermore, the Gene-TEQ concept allows a direct comparison of environmental samples from different laboratories.

Evaluation of a modified CD71 MicroFlow method for the flow cytometric analysis of micronuclei in rat bone marrow erythrocytes

The aim of this study was to evaluate a modified flow cytometric method for the quantification of micronuclei in rat bone marrow reticulocytes. The method identified uses the erythrocyte pure fraction from cellulose filtered bone marrow with slight modifications to the widely published MicroFlow(®) method developed by Litron Laboratories, Rochester, NY for the detection of micronuclei in peripheral blood. A number of experiments were conducted to compare the micronucleus induction measured by flow cytometry with traditional microscopic analysis in male rats treated daily for 2 days with appropriate vehicle controls or various doses of cyclophosphamide (CP), mitomycin C (MMC), vinblastine sulfate (VBS), 1,2-dimethylhydrazine (DMH), etoposide (ETO), colchicine (COL), or 4-nitroquinoline-1-oxide (4NQO). In addition, for a subset of chemical we compared the induction of micronuclei in bone marrow and peripheral blood. The results from this study showed a very good correlation of micronucleus frequencies in bone marrow between microscopic analysis and the flow cytometry as well as between blood and bone marrow. In general, micronucleus frequencies of test compound treated animals and inter-animal variability were slightly lower by flow cytometric analysis compared to manual slide analysis. The data presented in this study support the use of the CD71 flow method for the analysis of micronuclei in rat bone marrow and also suggest that peripheral blood may be equally as sensitive as bone marrow in detecting a micronucleus response in short term studies.

Flow cytometry peripheral blood micronucleus test in vivo: determination of potential thresholds for aneuploidy induced by spindle poisons

Non-DNA binding genotoxins (e.g., aneugens), unlike DNA-binding genotoxins, are theoretically expected to show thresholded concentration-effect response curves. This is a major issue in genetic toxicology testing because the identification of thresholds in vivo can provide a safety margin for exposure to a particular compound. In the current study we measured micronucleus induction by flow cytometry to determine the dose-response curves for tubulin interacting agents, a specific class of aneugens. All experiments with aneugens, which include colchicine, vinblastine, vincristine, as well as the clastogen cyclophosphamide (CP) were performed in mice to avoid the splenic elimination of micronucleated reticulocytes, which has been described in rats. Flow cytometry analysis revealed a non-linear dose-dependent increase in micronuclei frequencies for all tested aneugens, and a linear dose response curve for the clastogen, CP. To determine whether micronucleus induction at higher doses was due to chromosome loss (aneuploidy) or chromosome breakage (clastogenicity), flow sorting of the micronucleated reticulocytes and fluorescent in situ hybridization (FISH) with a mouse pan centromeric probe were performed for vinblastine, vincristine, and colchicine. Statistical evaluation of the flow cytometry and FISH data was performed to determine the threshold levels for chromosome loss in vivo. The threshold concentrations for vinblastine, vincristine, and colchicine were found at 0.35, 0.017, and 0.49 mg kg(-1), respectively.

Cyanobacterial extracts and microcystin-LR are inactive in the micronucleus assay in vivo and in vitro

Cyanobacteria are sometimes widespread in lakes and can produce potent toxins, which can be dangerous for animals that drink the water, e.g. cattle and dogs. If the toxins are taken up by fish and other organisms in the food chain, or occur in drinking-water, they may pose a problem also for humans. Microcystin-LR, a hepatotoxic cyclic peptide, is one of the most frequently found cyanobacterial toxins. Data on the genotoxic potential of microcystin-LR and other cyanobacterial toxins are contradictory. Here we report results of the micronucleus assay carried out in vivo and in vitro with these toxins. To increase the sensitivity, we used the flow cytometry-based micronucleus assay in the mouse. In this study both pure microcystin-LR and cyanobacterial extracts originating from four different lakes in Sweden were analysed. Although doses up to near lethality were used and an average of 200,000 young erythrocytes, polychromatic erythrocytes, were analysed from each animal, no genotoxic effect was observed, nor could any effect be shown in the in vitro micronucleus study, using human lymphocytes. These results show that the low concentration of microcystins that now and then occur in drinking-water does not increase the cancer risk through chromosome breaks or mal-distribution of chromosomes.

Improving prediction of chemical carcinogenicity by considering multiple mechanisms and applying toxicogenomic approaches

While scientific knowledge of the potential health significance of chemical exposures has grown, experimental methods for predicting the carcinogenicity of environmental agents have not been substantially updated in the last two decades. Current methodologies focus first on identifying genotoxicants under the premise that agents capable of directly damaging DNA are most likely to be carcinogenic to humans. Emphasis on the distinction between genotoxic and non-genotoxic carcinogens is also motivated by assumed implications for the dose-response curve; it is purported that genotoxicants would lack a threshold in the low dose region, in contrast to non-genotoxic agents. However, for the vast majority of carcinogens, little if any empirical data exist to clarify the nature of the cancer dose-response relationship at low doses in the exposed human population. Recent advances in scientific understanding of cancer biology-and increased appreciation of the multiple impacts of carcinogens on this disease process-support the view that environmental chemicals can act through multiple toxicity pathways, modes and/or mechanisms of action to induce cancer and other adverse health outcomes. Moreover, the relationship between dose and a particular outcome in an individual could take multiple forms depending on genetic background, target tissue, internal dose and other factors besides mechanisms or modes of action; inter-individual variability and susceptibility in response are, in turn, key determinants of the population dose-response curve. New bioanalytical approaches (e.g., transcriptomics, proteomics, and metabolomics) applied in human, animal and in vitro studies could better characterize a wider array of hazard traits and improve the ability to predict the potential carcinogenicity of chemicals.

Sensitivity of the erythrocyte micronucleus assay: dependence on number of cells scored and inter-animal variability

Until recently, the in vivo erythrocyte micronucleus assay has been scored using microscopy. Because the frequency of micronucleated cells is typically low, cell counts are subject to substantial binomial counting error. Counting error, along with inter-animal variability, limit the sensitivity of this assay. Recently, flow cytometric methods have been developed for scoring micronucleated erythrocytes and these methods enable many more cells to be evaluated than is possible with microscopic scoring. Using typical spontaneous micronucleus frequencies reported in mice, rats, and dogs we calculate the counting error associated with the frequency of micronucleated reticulocytes as a function of the number of reticulocytes scored. We compare this counting error with the inter-animal variability determined by flow cytometric scoring of sufficient numbers of cells to assure that the counting error is less than the inter-animal variability, and calculate the minimum increases in micronucleus frequency that can be detected as a function of the number of cells scored. The data show that current regulatory guidelines allow low power of the test when spontaneous frequencies are low (e.g., < or =0.1%). Tables and formulas are presented that provide the necessary numbers of cells that must be scored to meet the recommendation of the International Working Group on Genotoxicity Testing that sufficient cells be scored to reduce counting error to less than the inter-animal variability, thereby maintaining a more uniform power of detection of increased micronucleus frequencies across laboratories and species.

In vivo erythrocyte micronucleus assay III. Validation and regulatory acceptance of automated scoring and the use of rat peripheral blood reticulocytes, with discussion of non-hematopoietic target cells and a single dose-level limit test

The in vivo micronucleus assay working group of the International Workshop on Genotoxicity Testing (IWGT) discussed new aspects in the in vivo micronucleus (MN) test, including the regulatory acceptance of data derived from automated scoring, especially with regard to the use of flow cytometry, the suitability of rat peripheral blood reticulocytes to serve as the principal cell population for analysis, the establishment of in vivo MN assays in tissues other than bone marrow and blood (for example liver, skin, colon, germ cells), and the biological relevance of the single-dose-level test. Our group members agreed that flow cytometric systems to detect induction of micronucleated immature erythrocytes have advantages based on the presented data, e.g., they give good reproducibility compared to manual scoring, are rapid, and require only small quantities of peripheral blood. Flow cytometric analysis of peripheral blood reticulocytes has the potential to allow monitoring of chromosome damage in rodents and also other species as part of routine toxicology studies. It appears that it will be applicable to humans as well, although in this case the possible confounding effects of splenic activity will need to be considered closely. Also, the consensus of the group was that any system that meets the validation criteria recommended by the IWGT (2000) should be acceptable. A number of different flow cytometric-based micronucleus assays have been developed, but at the present time the validation data are most extensive for the flow cytometric method using anti-CD71 fluorescent staining especially in terms of inter-laboratory collaborative data. Whichever method is chosen, it is desirable that each laboratory should determine the minimum sample size required to ensure that scoring error is maintained below the level of animal-to-animal variation. In the second IWGT, the potential to use rat peripheral blood reticulocytes as target cells for the micronucleus assay was discussed, but a consensus regarding acceptability for regulatory purposes could not be reached at that time. Subsequent validation efforts, combined with accumulated published data, demonstrate that blood-derived reticulocytes from rats as well as mice are acceptable when young reticulocytes are analyzed under proper assay protocol and sample size. The working group reviewed the results of micronucleus assays using target cells/tissues other than hematopoietic cells. We also discussed the relevance of the liver micronucleus assay using young rats, and the importance of understanding the maturation of enzyme systems involved in the processes of metabolic activation in the liver of young rats. Although the consensus of the group was that the more information with regard to the metabolic capabilities of young rats would be useful, the published literature shows that young rats have sufficient metabolic capacity for the purposes of this assay. The use of young rats as a model for detecting MN induction in the liver offers a good alternative methodology to the use of partial hepatectomy or mitogenic stimulation. Additional data obtained from colon and skin MN models have been integrated into the data bases, enhancing confidence in the utility of these models. A fourth topic discussed by the working group was the regulatory acceptance of the single-dose-level assay. There was no consensus regarding the acceptability of a single dose level protocol when dose-limiting toxicity occurs. The use of a single dose level can lead to problems in data interpretation or to the loss of animals due to unexpected toxicity, making it necessary to repeat the study with additional doses. A limit test at a single dose level is currently accepted when toxicity is not dose-limiting.

Practical threshold for micronucleated reticulocyte induction observed for low doses of mitomycin C, Ara-C and colchicine

Micronucleus induction was studied for the DNA target clastogens mitomycin C (MMC) and 1-beta-D-arabinofuranosylcytosine (Ara-C), and also the non-DNA target aneugen colchicine (COL) in order to evaluate the dose-response relationship at very low dose levels. The acridine orange (AO) supravital staining method was used for microscopy and the anti-CD71-FITC based method was used for flow cytometric analysis. In the AO method, 2000 reticulocytes were analysed as commonly advised, but in the flow cytometric method, 2000, 20,000, 200,000 and 1,000,000 reticulocytes were analysed for each sample to increase the detecting power (i.e. sensitivity) of the assay. The present data show that increasing the number of cells scored increases the statistical power of the assay when the cell was considered as a statistical unit. Even so, statistically significant differences from respective vehicle controls were not observed at the lowest dose level for MMC and Ara-C, or the lower four dose levels for COL, even after one million cells were analysed. When the animal was considered as a statistical unit, only the top dose group for each chemical showed significant increase of micronucleated reticulocytes frequency. As non-linear dose-response curves were obtained for each of the three chemicals studied, these observations provide evidence for the existence of a practical threshold for the DNA target clastogens as well as the non-DNA target aneugen studied.

Do dose response thresholds exist for genotoxic alkylating agents?

The demonstration and acceptance of dose response thresholds for genotoxins may have substantial implications for the setting of safe exposure levels. Here we test the hypothesis that direct-acting DNA reactive agents may exhibit thresholded dose responses. We examine the potential mechanisms involved in such thresholded responses, particularly in relation to those of alkylating agents. As alkylating agents are representative model DNA reactive compounds with well characterized activities and DNA targets, they could help shed light on the general mechanisms involved in thresholded dose responses for genotoxins. Presently, thresholds have mainly been described for agents with non-DNA targets. We pay particular attention here to the contribution of DNA repair to genotoxic thresholds. A review of the literature shows that limited threshold data for alkylating agents are currently available, but the contribution of DNA repair in thresholded dose responses is suggested by several studies. The existence of genotoxic thresholds for alkylating agents methylmethanesulfonate is also supported here by data from our laboratory. Overall, it is clear that different endpoints induced by the same alkylator, can possess different dose response characteristics. This may have an impact on the setting of safe exposure levels for such agents. The limited information available concerning the dose response relationships of alkylators can nevertheless lead to the design of experiments to investigate the mechanisms that may be involved in threshold responses. Through using paired alkylators inducing different lesions, repaired by different pathways, insights into the processes involved in genotoxic thresholds may be elucidated. Furthermore, as alkyl-guanine-DNA transferase, base excision repair and mismatch repair appear to contribute to genotoxic thresholds for alkylators, cells deficient in these repair processes may possess altered dose responses compared with wild-type cells and this approach may help understand the contribution of these repair pathways to the production of thresholds for genotoxic effects in general. Finally, genotoxic thresholds are currently being described for acute exposures to single agents in vitro, however, dose response data for chronic exposures to complex mixtures are, as yet, a long way off.

Hemoglobin adducts and micronuclei in rodents after treatment with isoprene monoxide or butadiene monoxide

1,3-Butadiene and isoprene (2-methyl-1,3-butadiene) are chemically related substances that are carcinogenic to rodents. The overall aim of this work is to elucidate the role of the genotoxic action of diepoxide metabolites in the carcinogenesis of the dialkenes. In vivo doses of the diepoxide metabolites were measured through reaction products with hemoglobin (Hb adducts) in studies of induced micronuclei (MN) in rodents. In the reaction with N-terminal valine in Hb, diepoxybutane and isoprenediepoxide form ring-closed adducts, pyrrolidines [N,N-(2,3-dihydroxy-1,4-butadiyl)valine and N,N-(2,3-dihydroxy-2-methyl-1,4-butadiyl)valine, respectively]. The method applied for Hb-adduct measurement is based on tryptic degradation of the protein and liquid chromatography electrospray ionisation tandem mass spectrometry (LC-ESI-MS/MS) analysis. Mice were given single i.p. injections of the monoepoxides of butadiene and isoprene, 1,2-epoxy-3-butene or 1,2-epoxy-2-methyl-3-butene, respectively. Rats were treated in the same way with 1,2-epoxy-3-butene. In mice pyrrolidine adduct levels increased with increasing administered doses of the monoepoxides. The in vivo dose of diepoxybutane was on average twice as high (0.29+/-0.059 mMh) as the in vivo dose of isoprenediepoxide (0.15+/-0.053 mMh) per administered dose (mmol/kg body weight) of the monoepoxides. In mice the genotoxic effects of the two monoepoxides, measured as the increase in the frequencies of micronuclei (MN), were approximately linearly correlated to the in vivo doses of the diepoxides (except at the highest dose of diepoxybutane). In rats the pyrrolidine-adduct levels from diepoxybutane were below the limit of quantification at all administered doses of 1,2-epoxy-3-butene and no significant increase was observed in the frequency of MN. Measurement of the ring-closed adducts to N-termini in Hb by the applied method permits analysis of in vivo doses of diepoxybutane and isoprenediepoxide, which may be further used for the elucidation of the mechanisms of carcinogenesis of butadiene and isoprene.

A comparison of genotoxicity between three common heterocyclic amines and acrylamide

Heterocyclic amines (HCAs), a group of genotoxic compounds formed during the heating of proteinaceous food items, have been known since the late 1970s. However, the genotoxic effect of these compounds in the low dose region has not yet been thoroughly studied. Here we used a sensitive flow cytometer-based micronucleus assay in mice to determine the frequency of micronucleated erythrocytes (fMPCE) of the three common HCAs, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), in the low dose region. We especially looked for any deviation from linearity of the dose-response curves. Male Balb/C mice were intra peritoneally injected with different doses of either PhIP (0-36 mg/kg b.w.), MeIQx (0-90 mg/kg b.w.) or IQ (0-40 mg/kg b.w.). In the case of PhIP, we found a significant dose-response relationship, while MeIQx and IQ did not display an increased fMPCE level. This flow cytometer method allows for determination of the DNA content of micronuclei. All three HCAs tested here yielded a low DNA content of micronuclei, indicating that they do not possess aneugenic effects. A comparison between the HCAs and acrylamide (AA), another heat induced genotoxic compound, revealed that the slope of the dose-response curve is about 10 times steeper for PhIP than AA. In spite of this, AA probably constitutes a higher human risk than HCAs since the intake is about a 100- to 1000-fold higher than the intake of HCAs.

Spontaneous and mitomycin C-induced micronuclei in peripheral blood reticulocytes from severely malnourished rats

Severe malnutrition caused by deficiencies in protein, calorie, and micronutrient intake is widely distributed throughout the world and is a particular problem in developing countries. Animal models have been useful for studying the effects of malnutrition under different experimental conditions. In this study, we have evaluated the effect of malnutrition on the frequency of spontaneous and mitomycin C (MMC)-induced micronuclei in the peripheral blood of rats measured using a flow cytometric analysis technique. Neonatal rats were experimentally malnourished during lactation and assayed at weaning (21 days of age). The malnourished rats weighed 49.2% less than well-nourished controls and had lower concentrations of serum protein, triglycerides, and cholesterol. In rats not treated with MMC, the frequency of micronucleated reticulocytes (MN-RETs) was 1.6 times greater in malnourished rats than in well-nourished rats (0.48% +/- 0.16% vs. 0.31% +/- 0.09%). The mean MN-RET frequency measured 32 hr after treatment with single i.p. doses of 0.5, 0.75, or 1.0 mg/kg of MMC was 0.60 +/- 0.10 vs. 0.84 +/- 0.14, 1.21 +/- 0.52 vs. 2.36 +/- 0.47, and 2.50 +/- 0.06 vs. 4.64 +/- 1.14 for well-nourished vs. malnourished rats, respectively. Statistical comparisons indicate significant differences between the two groups of rats at all doses tested. Malnourishment and MMC treatment had no significant effects on the frequencies of RETs or micronucleated normochromatic erythrocytes. The data indicate that protein-calorie malnutrition during lactation is associated with increased frequencies of MN-RETs, which are indicative of chromosome damage. These findings suggest that malnutrition could result in greater susceptibility to environmental damage.

The dose-response relationship at very low doses of acrylamide is linear in the flow cytometer-based mouse micronucleus assay

Acrylamide (AA) is genotoxic and has been classified as a probable human carcinogen. Human exposure to AA may be high by the consumption of starch-based food that has been treated at high temperature, e.g. potato chips and crisps. For risk assessment, extrapolation to the expected low doses to humans will be more reliable when data from low experimental doses can be used. We have registered the effects of a series of low doses in the sensitive flow cytometer-based micronucleus assay in mice, paying special attention to deviations from the expected linear dose-response function. Two experiments were performed with CBA mice, injected i.p. with different doses of AA. In one experiment the effects of 22 doses (two mice per dose) ranging from 0 to 100 mg/kg b.w. were studied. In the second experiment seven doses (five mice per dose) ranging from 0 to 30 mg/kg b.w. were used. In both experiments, a clear increase of the frequency of micronucleated erythrocytes was seen, already at the lowest doses used. The dose-response function was found to be linear with a tendency to have a steeper rise at the lowest doses. The low DNA content of the micronuclei indicated an absence of whole chromosomes, i.e. no aneugenic effect of AA.

Induction of micronuclei in mouse and rat by glycidamide, genotoxic metabolite of acrylamide

Male CBA mice and male Sprague-Dawley rats were treated by i.p. injection of glycidamide (GA), the presumed genotoxic metabolite of acrylamide (AA). GA was obtained through a new way of synthesis. As an endpoint of chromosome damage, micronucleus (MN) induction in erythrocytes was measured. Hemoglobin (Hb) adducts were used as a measure of in vivo dose of GA. GA induced linear dose-dependent increases in adduct levels in both species. Rats exhibit, compared with mice, 30% higher Hb adduct levels per unit of administered amount of GA. The incremental MN frequencies per administered dose of GA in mice showed a linear-quadratic dose-dependent curve. In the rat no positive dose-response relationship was obtained, probably due to toxic effects to the bone marrow. The main result of this study is the finding that after treatment with synthetic GA the MN frequency per unit of the in vivo dose of GA in the mouse is very similar to that obtained in a previous study, where animals were treated with AA and GA as a metabolite. This equality in potency of GA, whether its in vivo dose is established by injection of synthetic GA or through metabolism of AA, supports the view that GA is the predominant genotoxic factor in AA exposure.

Macrocyclic musk compounds—an absence of genotoxicity in the Ames test and the in vivo Micronucleus assay

Three synthetic macrocyclic musks, ethylene dodecanedioate, ethylene brassylate, and cyclopentadecanolide, which are widely used as ingredients of perfume fragrances, were tested for genotoxicity. In this report we present results from two different studies, the flow-cytometer-based micronucleus assay in peripheral blood of mice and the Salmonella/microsome test with TA 97, TA 98, and TA 100. Female NMRI and male CD 1 mice were intraperitoneally injected with one of the three macrocyclic musk compounds. Three different doses (0.1-1.6 g/kg bw) of each of the compounds were tested. Blood samples were collected on two occasions from each mouse and the frequency of micronucleated polychromatic erythrocytes (fMPCE) was determined. Neither of the compounds caused a significant difference from the control fMPCE. No mutagenic effect with and without S9 mix in the tested Salmonella strains was observed. The presence of S9 mix reduced the killing effect of high doses.

Hemoglobin adducts and micronucleus frequencies in mouse and rat after acrylamide or N-methylolacrylamide treatment

The reactive industrial chemicals acrylamide (AA) and N-methylolacrylamide (MAA) are neurotoxic and carcinogenic in animals, MAA showing a lower potency than AA. The causative agent in AA-induced carcinogenesis is assumed to be the epoxy metabolite, glycidamide (GA), which in contrast to AA gives rise to stable adducts to DNA. The causative agent in MAA induced carcinogenesis is so far not studied. The two AAs were studied in mice and rats using analysis of hemoglobin (Hb) adducts as a measure of in vivo doses and the in vivo micronucleus (MN) assay as an end-point for chromosome damage. Male CBA mice were treated by intraperitoneal (i.p.) injection of three different doses and male Sprague-Dawley rats with one dose of each AA. Identical adducts were monitored from the two AAs [N-(2-carbamoylethyl)valine] and the respective epoxide metabolites [N-(2-carbamoyl-2-hydroxyethyl)valine]. Per unit of administered amount, AA gives rise to higher (three to six times) Hb adduct levels than MAA in mice and rats. Mice exhibit, compared with rats, higher in vivo doses of the epoxy metabolites, indicating that AAs were more efficiently metabolized in the mice. In mouse the two AAs induced dose-dependent increases in both Hb adduct level and MN frequency in peripheral erythrocytes. Per unit of administered dose MAA showed only half the potency for inducing micronuclei compared with AA, although the MN frequency per unit of in vivo dose of measured epoxy metabolite was three times higher for MAA than for AA. No increase in MN frequency was observed in rat bone marrow erythrocytes, after treatment with either AA. This is compatible with a lower sensitivity of the rat than of the mouse to the carcinogenic action of these compounds.

Extended exposure of adult and fetal mice to 50 Hz magnetic field does not increase the incidence of micronuclei in erythrocytes

The flow cytometer-based micronucleus assay was used to study the effects on chromosomes in erythroid cells of CBA/Ca mice after extended exposure to 50 Hz magnetic field (MF), 14 microT, peak-to-peak (p-p). The study included two different experiments: (a) mice exposed in utero during 18 days of their prenatal stage, and (b) adult mice exposed for 18 days. In experiment (a) 35 days after exposure was terminated, peripheral blood was drawn from the mice exposed in utero to determine whether the exposure had a genotoxic effect on the pluripotent erythroid stem cells. About 200000 polychromatic erythrocytes (PCE) and 200000 normochromatic erythrocytes (NCE) were analysed from each of 20 exposed mice. The EMF exposure did not significantly change the frequency of micronucleated PCE or NCE in comparison with 20 sham-irradiated mice. There was no difference in the proportion of PCE between exposed and unexposed animals. Similarly, in experiment (b) no differences were seen between EMF exposed and unexposed adult mice when samples of peripheral blood were taken at the end of exposure and analyzed for micronuclei in PCE and NCE. The proportion of PCE was the same in both groups. The results indicate that exposure to EMF does not induce direct or indirect effects on chromosomes in erythroid cells expressed as increased levels of micronucleated erythrocytes of mice. No indications of delayed genetic effects were found.

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

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

The micronucleus test in rat erythrocytes from bone marrow, spleen and peripheral blood: the response to low doses of ionizing radiation, cyclophosphamide and vincristine determined by flow cytometry

The frequency of micronucleated polychromatic erythrocytes (fMPCE) was determined in samples from bone marrow, spleen and peripheral blood of rats exposed to low doses of X-rays, cyclophosphamide or vincristine. The fMPCE values were lower in the peripheral blood than in bone marrow or spleen. This is due to the elimination of MPCE from the circulating blood, which was confirmed by the results from prolonged exposure of rats to gamma-radiation. When the analysis was restricted to the youngest PCE in peripheral blood, the sensitivity of the assay was considerably improved. This can be reproducibly achieved with the flow cytometric analysis.