The micronucleus assay with mouse peripheral blood reticulocytes using acridine orange-coated slides

Organ variation in the mutagenicity of dimethylnitrosamine in Big Blue mice

Organ specificity in the lacI mutant frequency (MF) induced by dimethylnitrosamine (DMN) was analyzed in lung, liver, kidney, bone marrow, urinary bladder, and testis of Big Blue mice. Cell proliferative activity was also analyzed in some of these tissues by immunohistochemical staining of proliferating cell nuclear antigen (PCNA). Clastogenicity of DMN was concomitantly analyzed by the peripheral blood micronucleus assay with the same animals used for the lacI mutation assay. Five daily intraperitoneal (i.p.) treatments with DMN (1 mg/kg) increased MF in liver (6.2 x control), kidney (2.4 x control), and lung (2.1 x control). These are known target organs for DMN carcinogenesis. No MF increase was observed in nontarget organs studied, i.e., bone marrow, bladder, and testis. Single ip treatment with DMN also increased lacI MF in liver but the increases were smaller than in a 5-daily-treatment regimen. This result suggests that multiple dosing is more effective in the transgenic mutation assay. The enhancement of cell proliferation observed was in bronchial epithelia 7 days after treatment. No micronucleus induction in peripheral blood was observed 24 hours after 2 and 3 daily ip treatments with 1 mg/kg DMN. An increase in the incidence of micronucleated reticulocytes in peripheral blood was observed 48 hours after single ip treatment with 5 or 10 mg/kg DMN. The present study demonstrated organ-specific induction of gene mutations by DMN which suggests a relevance of this assay for the prediction of organ-specific carcinogenesis.

The micronucleus assay using peripheral blood reticulocytes from X-ray-exposed mice

A new 'fluorescence' variant of the micronucleus assay using supravital staining of peripheral blood reticulocytes with an acridine orange coated slide was recently developed. In this study the application of this method to detect a mutagenic response to low-dose exposure of X-irradiation is reported. The mice were exposed to a single dose of 2.5, 5, 10, 15, 25, and 50 cGy. The induction of micronuclei in peripheral blood reticulocytes was recorded with sampling times of 0, 24, 48, 72, and 96 h after exposure without killing of mice. Dose-dependent effects were observed at sampling times of 24-72 h reaching maximum levels at 48 h after X-ray exposure. A highly significant (p < 0.01) increase of the micronucleated reticulocytes was found for doses of 10, 15, 25, and 50 cGy. Neither the dose of 2.5 nor 5 cGy induced the significant increase in the incidence of micronucleated reticulocytes compared with the respective control group.

Effect of aging on spontaneous micronucleus frequencies in peripheral blood of nine mouse strains: the results of the 7th collaborative study organized by CSGMT/JEMS.MMS. Collaborative Study Group for the Micronucleus Test. Environmental Mutagen Society of Japan. Mammalian Mutagenesis Study Group

The spontaneous frequencies of micronucleated reticulocytes (MNRETs) were examined monthly over the life spans of animals belonging to nine mouse strains for the 7th collaborative study organized by the CSGMT/JEMS.MMS. Both sexes of the BDF1 strain and females of the A/J strain showed a statistically significant increase in mean spontaneous MNRET frequency in their last month of life, suggesting the possibility of strain-specific, age-dependent chromosomal instability. SAMP6/Tan, an accelerated senescence-prone strain, showed the same tendency, although it was not statistically significant. The other strains studied, ddY, CD-1, B6C3F1, SAMR1, and MS/Ae, did not show significant age-related differences in mean of MNRET frequencies. More extensive statistical analyses are underway, and the outcomes will be reported separately.

The induction of micronuclei in mice hepatocytes and reticulocytes by tetrachloroethylene

The clastogenicity of tetrachloroethylene (tetra) was detected by means of the micronucleus assay using hepatocytes and reticulocytes from ddY male mice, to understand its effects in upon hepatocellular carcinomas in mice. The frequency of micronucleated hepatocytes of mice that received a single injection of tetra after partial hepatectomy increased to levels that were significantly higher than those of controls treated with solvent. However, the micronucleus assay using peripheral blood reticulocytes from ddY male mice, revealed that tetra did not induce to a statistically significant increase in micronucleus frequency. These results suggested that tetra metabolites have a clastogenic effect in vivo upon mouse liver but not upon bone marrow cells.

The suitability of rat peripheral blood in subchronic studies for the micronucleus assay

To examine the suitability of using rat peripheral blood from animals used in subchronic toxicity studies for micronucleus analysis, we orally administered phenacetin or 6-mercaptopurine for 14 days to groups of six rats and compared their micronucleus frequencies to the bone marrow micronucleus frequencies of rats similarly treated for only 2 days. In the 14-day test, phenacetin significantly increased the frequency of micronucleated reticulocytes in peripheral blood at 500 mg/kg starting from day 9, and at 750 and 1500 mg/kg starting from day 6; 6-mercaptopurine gave a positive response at 20 mg/kg starting from day 6. Positive responses in the bone marrow assay were obtained at the same dose levels. In the 2-day test, micronucleated polychromatic erythrocyte frequencies increased significantly at 1000 and 2000 mg/kg for phenacetin, and at 50, 100, and 200 mg/kg for 6-mercaptopurine. These results suggest that micronucleus assays using peripheral blood from rats in subchronic animal studies of phenacetin and 6-mercaptopurine are feasible and at least as sensitive for the assessment of micronuclei as an acute bone marrow micronucleus test.

Regulatory Genotoxicity Testing: A Critical Appraisal

This review considers current approaches to regulatory genotoxicity testing, focusing on how the use of animals can be further replaced, reduced and refined. The complementary roles of in vitro and in vivo testing, and the justification for using animals, are discussed in detail. Recommendations are made for improvements and further work, in the light of the considerable current controversy surrounding the composition and deployment of testing strategies, and the interpretation of the data generated, particularly for carcinogenicity prediction. The major problems are the oversensitivity of in vitro tests and the insensitivity of in vivo assays. On the basis of an analysis of some published databases, it is concluded that there is insufficient support for using in vivo genotoxicity assays for screening. Also, it is questionable whether the scientific benefits of using such assays always outweigh the costs to the animals involved. The considerable efforts being made to harmonise in vivo protocols and to develop improved methods for detecting genotoxicity are discussed. It is recommended that more emphasis be placed on characterising genotoxins in vitro, especially for mechanisms of activity, to optimise the benefits of any confirmatory animal tests.. Also, regulatory agencies are urged to require better-designed and more-scientifically sound protocols, in which animal numbers are minimised and data interpretation, particularly that of negative results, is facilitated. Lastly, in the development and validation of transgenic rodent systems, emphasis should be placed on developing protocols in which other acute toxicity and metabolism endpoints can be measured simultaneously with in vivo mutagenesis, while minimising animal numbers.

Micronuclei in mice treated with monocrotaline with and without phenobarbital pretreatment

Monocrotaline is a very potent toxin, producing significant effects of pneumotoxicity, hepatotoxicity, and teratogenicity, as well as carcinogenicity. In addition, the compound has been clearly shown to be mutagenic after metabolic activation. The goal of the experiments reported here was to confirm the reported clastogenesis induced by this agent in vivo and to evaluate the impact of modulation of metabolic activity by phenobarbital, a potent P-450 inducer (both Phase I and Phase II enzymes). The method used in addressing this problem relied on a new technique for monitoring clastogenesis in vivo, i.e., the acridine orange micronucleus assay method originally exploited by Hayashi et al. [1990]. The result of our experiments confirmed monocrotaline to be an effective clastogen in vivo, using the acridine orange method of assessment. The peak in induction of micronuclei occurred on the second day following intraperitoneal administration of the drug. Administration of phenobarbital prior to monocrotaline did appear to modulate the micronucleus induction. At 30 mg/kg bw monocrotaline, the pretreatment with phenobarbital appears to increase the intensity of monocrotaline clastogenesis, while the effect at higher doses (60 and 125 mg/kg bw) is a reduction in potency, presumably reflecting increased importance of Phase II metabolism for monocrotaline at these doses. Thus the study reported here confirms the potent in vivo clastogenesis of monocrotaline, and provides evidence for a dose-related shift in mechanism for the phenomenon.

Effect of carboxymethyl-chitin-glucan on cyclophosphamide induced mutagenicity

The effect of high molecular carboxymethyl-chitin-glucan (CMCG), administered either intraperitoneally, intravenously or orally prior to cyclophosphamide injection, on the frequency of micronucleated reticulocytes was evaluated in peripheral blood of female ICR mice. Both intraperitoneal and intravenous administration of CMCG decreased the clastogenic effect of cyclophosphamide. The protective effect of CMCG was concentration dependent, with a higher decrease achieved by 100 mg/kg than by 50 mg/kg body weight. On the other hand, not even five peroral pretreatments with CMCG in the dose of 200 mg/kg body weight during the week prior to simultaneous administration of CMCG and cyclophosphamide induced a decrease of micronucleated reticulocytes in peripheral blood. It is therefore conceivable that CMCG failed to pass through the gastrointestinal tract, probably due to its high molecular weight. The antimutagenic effect of CMCG against cyclophosphamide was manifested by its intraperitoneal and intravenous administration to female ICR mice.

DNA strand breaking by the carbon-centered radical generated from 4-(hydroxymethyl) benzenediazonium salt, a carcinogen in mushroom Agaricus bisporus

4-(Hydroxymethyl)benzenediazonium salt (HMBD), a carcinogen in mushroom Agaricus bisporus, was found to generate a carbon-centered radical, 4-(hydroxymethyl)phenyl radical, during incubation at pH 7.4 and 37 degrees C, when estimated by Electron Spin Resonance (ESR) spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butylphenyl-alpha-nitrone (PBN) and 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS). Formation of a substantial amount of benzyl alcohol during incubation of HMBD in the presence of a hydrogen donor, ethanol, supported the generation of the carbon-centered radical. When plasmid supercoiled DNA was incubated with HMBD at pH 7.4 and 37 degrees C for 30 min, the supercoiled DNA was converted into a nicked circular relaxed form and subsequently into a linear form. Sequence analysis indicated that the compound cleaved the plasmid DNA strand non-specifically. The intracellular double stranded DNA of Escherichia coli was fragmented by the compound, which may be responsible for its cytotoxicity. The compound induced mouse micronucleated peripheral reticulocytes. The compound was active in breaking DNA strands in the absence of molecular oxygen and in the presence of superoxide dismutase and catalase, indicating that no oxygen-derived radicals participated in the breaking. DNA breaking was inhibited by hydrogen donors butyl hydroxyanisole and ethanol, thiol compounds L-cysteine and 2-mercaptoethanol, and spin-trapping agents DMPO and PBN, indicating the direct contribution of the carbon-centered radical to the breaking.

Induction of micronuclei in peripheral blood and bone marrow erythrocytes of rats and mice exposed to 1,3-butadiene by inhalation

Female CB6F1 mice and male Wistar rats were exposed to different concentrations of 1,3-butadiene (1,3-BD) by inhalation. Micronucleus tests using both peripheral blood erythrocytes and femoral marrow cells of these animals were performed. Cells were stained either using conventional acridine orange (AO) staining or supravitally using AO-coated slides. Dose-dependent increases in the frequency of micronuclei (MN) were observed both in blood and in bone marrow cells in mice. 1,3-BD did not, however, increase the frequency of MN in either blood or bone marrow cells of rats at any of the tested concentrations.

Mutagenicity of 1,3-butadiene inhalation in somatic and germinal cells of mice

Inhalation exposure of mice to 50, 200, 500 or 1300 ppm of 1,3-butadiene for 6 h per day for 5 consecutive days caused micronuclei in mouse bone marrow and peripheral blood erythrocytes. The dose response was non-linear. The slope of the curve flattened with increasing exposure concentration. Coat color spots were found in the mouse spot test after exposure of pregnant females on pregnancy days 8-12 to 500 ppm of 1,3-butadiene. Dominant lethal mutations were induced in spermatozoa and late spermatids after exposure of male mice to 1300 ppm with the 5-day exposure regimen. Thus, in the mouse 1,3-butadiene is a somatic and germ cell mutagen.

Weak genotoxicity of acrylamide on premeiotic and somatic cells of the mouse

The effects of acrylamide (AA) were evaluated, under the EEC/STEP project 'Detection of Germ Cell Mutagens', by carrying out several cytogenetic assays on mouse germ and somatic cells. The spermatid micronucleus (MN) test was applied after treatment of meiotically dividing or premeiotic S phase cells. Acute treatments (50 and 100 mg/kg i.p.) as well as subchronic exposure to AA (4 x 50 mg/kg, 4 i.p. injections at 24-h intervals) were performed. A weak increase of MN was induced only by treatment with AA of cells in S phase. Sister-chromatid exchange (SCE) analysis in differentiating spermatogonia treated i.p. with 50 and 100 mg/kg confirmed the weak genotoxicity of AA in the premeiotic stages of spermatogenesis. The application of the MN test in peripheral blood reticulocytes of the same animals used for the spermatid MN assay indicated that the cytogenetic effects induced by AA in the somatic and the germ cell lines are comparable in magnitude. The results obtained in this study by applying the spermatid micronucleus assay are in very good agreement with those reported by two other laboratories with the same technique.

Radioprotective effect of stobadine in the mouse micronucleus test

Induction of micronuclei was studied 40 h post irradiation in peripheral blood reticulocytes of male mice treated or not with stobadine dipalmitate (70.07 mg/kg body weight) at two time intervals (2 h or 1 h) prior to and immediately after 6.5 Gy 60Co exposure. A significant decrease of micronucleated reticulocytes was observed in the group of mice pretreated 2 h (P < 0.05) or 1 h (P < 0.01) before irradiation. 60Co irradiation followed by treatment with stobadine did not lead to the same protective effect in the micronucleus assay. It is therefore assumed that a radical-scavenging mechanism must be involved in radioprotection by stobadine.

Initial experiences and future directions for transgenic mouse mutation assays

Transgenic mice have introduced new possibilities in the field of mutation research and safety testing. Using lacZ transgenic mice (Muta Mouse), we have combined the peripheral blood micronucleus assay with the transgenic mouse mutation assay, enabling the concomitant detection of gene mutations and micronucleus induction in vivo in the same animals (Suzuki et al., 1993). Several mutagens, i.e., mitomycin C (MMC), ethyl nitrosourea (ENU), ethyl methanesulfonate (EMS) and diethyl nitrosamine (DEN), were tested in this combined assay. All of them increased the lacZ mutant frequency in bone marrow or liver, and all except DEN induced micronuclei in peripheral blood. These initial studies demonstrated that genotoxicity in vivo could be detected with these two endpoints and, more importantly, that some specificity exists among these tissues analyzed. Although transgenic mouse mutation assays have many potential applications in in vivo mutation research, several problems stand in the way of wider use. Paramount among these are cost and labor intensiveness. The color screening systems for lacZ or lacI mutation detection require large numbers of plates and tedious scoring processes. In order to make significant advances in this field, it will be necessary to use positive selection for induced mutants, such as has been described recently for the lacZ and lacI transgenic mouse models.

In vivo rodent erythrocyte micronucleus assay

The following summary represents a consensus of the working group except where noted. The items discussed are listed in the order in which they appear in the OECD guideline (474) for easy reference. Introduction, purpose, scope, relevance, application and limits of test. The analysis of immature erythrocytes in either bone marrow or peripheral blood is equally acceptable for those species in which the spleen does not remove micronucleated erythrocytes. In the mouse, mature erythrocytes are also an acceptable cell population for micronucleus analysis when the exposure duration exceeds 4 weeks. Test substances. Organic solvents such as DMSO are not recommended. Freshly prepared solutions or suspensions should be used unless stability data demonstrate the acceptability of storage. Vegetable oils are acceptable as solvents or vehicles. Suspension of the test chemicals is acceptable for p.o. or i.p. administration but not for i.v. injection. The use of any unusual solvent should be justified. Selection of species. Any commonly used laboratory rodent species is acceptable. There is no strain preference. Number and sex. The size of experiment (i.e., number of cells per animal, number of animals per group) should be finalized based on statistical considerations. Although a consensus was not achieved, operationally it was agreed that 2000 cells per animal and four animals per group was a minimum requirement. In general, the available database suggests that the use of one gender is adequate for screening. However, if there is evidence indicating a significant difference in the toxicity between male and female, then both sexes should be used. Treatment schedule. No unique treatment schedule can be recommended. Results from extended dose regimens are acceptable as long as positive. For negative studies, toxicity should be demonstrated or the limit dose should be used, and dosing continued until sampling. Dose levels. At least three dose levels separated by a factor between 2 and square root of 10 should be used. The highest dose tested should be the maximum tolerated dose based on mortality, bone marrow cell toxicity, or clinical symptoms of toxicity. The limit dose is 2 g/kg/day for treatment periods of 14 days or less and 1 g/kg/day for treatment periods greater than 14 days. A single dose level (the limit dose) is acceptable if there is no evidence of toxicity. Controls. Concurrent solvent (vehicle) controls should be included at all sampling times. A pretreatment sample, however, may also be acceptable only in the short treatment period peripheral blood studies. A concurrent positive control group should be included for each experiment.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of the DNA breaking activity of mutagenic 5-diazouracil

5-Diazouracil in monohydrated form showed mutagenicity and cytotoxicity on Salmonella typhimurium TA98 and TA100 strains without metabolic activation, and induced mouse micronucleated peripheral reticulocytes. Incubation of a plasmid supercoiled DNA with the compound caused DNA single-strand breaking: the supercoiled form was transformed into an open circular relaxed form and then into a linear form. The breaking was similarly caused in the absence of molecular oxygen. The breaking was not inhibited by superoxide dismutase and catalase, but inhibited by ethanol, butyl hydroxyanisole and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), suggesting the involvement of radical species other than oxygen-derived radical species. Sequencing analysis of the singly 5'-end-labeled DNA fragment showed that the phosphodiester breaking was not site-specific. When Escherichia coli cells were incubated with the compound, the intracellular double-strand DNA was fragmented. The fragmentation was inhibited by ethanol, DMPO, N-tert.-butyl-alpha-phenylnitrone (PBN) and thiol compounds. Generation of the carbon-centered radical was confirmed by the electron spin resonance spin-trapping technique using DMPO and PBN. The mutagenicity and the DNA breaking activity of 5-diazouracil can be ascribed to the carbon-centered radical.

An assessment of the in vivo clastogenicity of erythrosine

In an investigation of the in vivo clastogenic potential of the food colouring erythrosine (ER), male B6C3F1 mice were treated by ip injection at doses of 50, 100 and 200 mg/kg, repeated 24 hr apart. Signs of toxicity were observed at the highest dose of ER administered. The three cytogenetic endpoints analysed were sister chromatid exchanges (SCEs) in peripheral blood lymphocytes (PBLs), micronuclei in bone marrow polychromatic erythrocytes (PCEs), and micronuclei in peripheral blood reticulocytes (PBRs). SCE frequencies in PBLs were 4.13, 4.58, 4.33 and 4.60 SCE/cell at 0, 50, 100 and 200 mg ER/kg, respectively. At the same doses, the frequencies of micronucleated PCEs were 3.5, 3.2, 2.0 and 2.5/1000 PCEs. Micronuclei in PBRs ranged from 1.2 to 3.6 and from 1.4 to 3.0/1000 PBRs in control and treated mice, respectively. These results indicate that ER is inactive as a clastogen in mouse blood and marrow cells. This result supports the hypothesis of a non-genotoxic mechanism for ER carcinogenicity.

Effect of beta-carotene on spontaneous and X-ray-induced chromosomal damage in bone marrow cells of mice

The effect of beta-carotene on spontaneous and X-ray-induced chromosomal damage in bone marrow cells of mice was studied. As a source of beta-carotene, dried Dunaliella bardawil (containing 6% beta-carotene) or oil suspension of Dunaliella beta-carotene was used. In Experiment 1, mice were given a basal diet, a 0.5% Dunaliella diet, or a 4% Dunaliella diet for four weeks. In Experiment 2, mice were given an oil suspension of Dunaliella beta-carotene (300 mg/kg body wt) by gavage for seven days while being fed a fat-rich diet. After beta-carotene treatment for the indicated time, spontaneous and X-ray (0.3 Gy, whole-body)-induced chromosomal damage in bone marrow cells was evaluated in terms of the percentages of micronucleated reticulocytes in their peripheral blood. The beta-carotene treatment slightly lowered the spontaneous and X-ray-induced chromosomal damage in bone marrow cells. Despite the higher doses of beta-carotene, the concentrations of beta-carotene in bone marrow, liver, and serum were much lower than those of vitamin E. In addition, the beta-carotene treatment markedly lowered the concentration of vitamin E in the tissues.

Bone marrow micronucleus assay: a review of the mouse stocks used and their published mean spontaneous micronucleus frequencies

We have examined published negative control data from 581 papers on micronucleated bone marrow polychromatic erythrocytes (mnPCE) for differences in mean frequency and the frequency distribution profile among the mouse stocks used with the bone marrow micronucleus assay. For the 55 mouse stocks with published micronucleus assay data, the overall mean frequency is 1.95 mnPCE/1,000 PCE (1.95 mnPCE/1,000); for the 13 stocks most commonly used in the assay, it is 1.88 mnPCE/1,000. During the last 5 years, the mnPCE rate for these 13 major stocks has been 1.74 mnPCE/1,000. This current mean frequency is a substantial decrease from the mean of 3.07 mnPCE/1,000 observed for these 13 stocks for data published prior to 1981. Of the major stocks, the highest mean mnPCE negative control frequencies were observed for MS/Ae > BALB/c > C57Bl/6, and the lowest for CD-1 < Swiss Webster. We note that hybrid mouse stocks appear to have lower and less variable negative control frequencies than either of their parent strains and that the negative control frequency for some progeny stocks have diverged significantly from that of the parent stocks. Overall mean negative control frequencies appear to be correlated with breadth of the frequency distribution profile of published mean negative control values. Furthermore, a possible correlation between negative control frequency in the micronucleus assay and sensitivity to clastogens of different mouse strains may be indicated. The databases generated here allow us to define a range of norms for both the historical mean frequency and individual experimental mean frequencies for most stocks, but in particular, for the more commonly used mouse stocks. Our analysis, for the most part, bears out the recommendation of the first Gene-Tox Report on the micronucleus assay that the historical negative control frequency for a mouse stock should fall between 1 and 3 mnPCE/1,000. Eighty-six percent of the most commonly used mouse stocks have historical mean frequencies within this range. Though individual experimental mean values would not necessarily be expected to fall within the 1-3.00 mnPCE/1,000 range, 65.3% of the 2,327 published negative control values do, and 83.5% are < 3 mnPCE/1,000. The frequency with which an individual experimental mean value lies outside the 1.00 to 3.00 mnPCE/1,000 range differs among stocks and appears related to the mouse mean frequency. We suggest that the recommended range for historical mean frequency be extended slightly, to approximately 3.4 mnPCE/1,000, to accommodate some commonly used strains with overall mean negative control frequencies just above 3.00 mnPCE/1,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of micronucleated cells by flow cytometry. 3. Advanced technology for detecting clastogenic activity

Under optimum conditions, flow cytometry (FCM) can provide a powerful technology for analyzing rare micronucleated cells in the peripheral blood. Our efforts in this endeavor have been directed toward a careful and meticulous optimization of experimental conditions, in order to achieve high resolution and high accuracy before introducing biological variation. We have achieved high resolution (Tometsko et al., 1993a) wherein the micronucleus signal is moved 100-fold upfield and away from the DNA deficient red blood cell (RBC) peak. In addition, we have demonstrated the high accuracy of our flow cytometry method in scoring rare micronucleated cells (Tometsko et al., 1993b). In the course of our studies, we rigorously pursued conditions which minimized experimental noise, demanding that FCM-scoring accuracy should approach theoretical limits. Thus, we laid the foundation for detecting clastogen activity with great sensitivity. The experiments described herein extend the previous studies by using high-speed flow cytometry to detect a clastogen-induced increase in MN cells in the total erythrocyte population. Methyl methanesulfonate (MMS) served as a model clastogen in these studies. This manuscript describes our development of a suitable blood-sampling regimen, the advantages of obtaining initial blood samples before dosing, the sex-linked difference in background micronucleus levels in BALB/c mice, and the analysis of a clastogen-induced biological response in male and female mice. As described, our flow cytometry method is able to provide a quantitative analysis of the net change in micronucleated cells (delta MN) for each mouse.

Micronucleus induction in mouse peripheral blood reticulocytes by 2,7-diaminophenazine and related compounds

The peripheral blood micronucleus test using acridine orange (AO) supravital staining was performed with phenazine (Pz), 1-aminophenazine (1-APz), 2-aminophenazine (2-APz) and 2,7-diaminophenazine (2,7-DAPz). Male ICR mice and BDF1 mice were treated intraperitoneally once or twice 24 h apart with each chemical. Irrespective of the number of administrations, Pz and 1-APz were not clastogenic in either mouse strain. In the single treatment regime, 2-APz slightly increased micronucleated reticulocytes (MNRETs) 48 h after 200 mg/kg dosing in ICR mice and 72 h after 400 mg/kg dosing in BDF1 mice. The MNRET frequencies were not enhanced after double treatment in either mouse strain. The single treatment with 2,7-DAPz strongly and dose-dependently induced MNRETs in BDF1 mice, and the incidences of MNRETs, which peaked 48 or 72 h after treatment, were much higher than those in ICR mice. In both mouse strains, the frequencies of MNRETs observed after double treatment with 2,7-DAPz were higher than those after single treatment. The MNRET incidence induced 72 h after first 200 mg/kg dosing in BDF1 mice was 2.21%.

The clastogen-suppressing effects of green tea, Po-lei tea and Rooibos tea in CHO cells and mice

The suppressing effects of crude extracts of three kinds of tea-green tea (GT) from Japan, Po-lei tea (PT) from China, and Rooibos tea (RT) from South Africa-on the induction of chromosome aberrations in cultured CHO cells and mice were studied. When CHO cells were exposed to each tea extract in the presence of rat liver microsomal enzymes (S9 mix) together with benzo[a]pyrene (B(a)P) or mitomycin C (MMC), a decrease in the frequency of chromosome aberrations was observed. PT and RT, but not GT, also suppressed the induction of chromosome aberrations by MMC in the absence of S9 mix. When cells were treated with tea extract after B(a)P or MMC treatment, RT suppressed the induction of chromosome aberrations in the presence and absence of S9 mix whereas GT and PT showed suppressing effects only in the presence of S9 mix. These data suggest that catechines, well-known antimutagens in tea samples, might account for the inhibitory effect in the case of GT and PT. Since RT contains few catechines, several unknown antimutagenic components could be responsible for its effect. The antimutagenic effects of tea extracts at concentration levels consumed by humans were examined in mice using micronucleus induction with B(a)P or MMC. When mice received oral gavage of 0.2% GT, 0.1% PT, and 0.1% RT at 1.0 ml/mouse 6 h before intraperitoneal injection of MMC, a decrease in the frequency of micronuclei was observed. The induction of micronuclei by B(a)P was suppressed by oral dosage of GT, PT and RT at 1.0 ml/mouse/day for 28 days. This was not due to a delay in the maturation of micronucleated reticulocytes. In conclusion, intake of tea might suppress the mutagenic activity of certain potent mutagens in human beings.

Effects of extended low-dose-rate exposure to 137Cs detected by flow-cytometric enumeration of micronucleated erythrocytes in mouse peripheral blood

An automated variant the of micronucleus assay for erythrocytes in mouse peripheral blood has recently been developed. The flow-cytometric technique used allows very large numbers of erythrocytes to be analysed with a relatively small effort. Here we report the potential of this method to detect a response to extended low-dose-rate exposure to gamma-irradiation. The mice were irradiated with a 137Cs source at a dose rate of 4.8 cGy/day for 26 days. Sampling was continued for another 39 days after irradiation. Elevated frequencies compared with the control group were found at days 2, 9 and 20 after the start of the irradiation for micronucleated polychromatic erythrocytes, and at days 9, 20, 29, 42, 51 and 65 for micronucleated normochromatic erythrocytes.

Organotins are co-clastogens in a whole mammalian system

The frequency of micronuclei induced by mitomycin C (MMC) in mouse peripheral reticulocytes was enhanced by treatment with organotins: bis(tri-n-butyltin) oxide (TBTO) and triphenyltin chloride (TPTC). TBTO and TPTC did not themselves induce micronucleated reticulocytes (MNRETs) in mouse peripheral blood. When 50 mg/kg TBTO or 100 mg/kg TPTC was given orally to mice simultaneously with an intraperitoneal (i.p.) administration of 1 mg/kg MMC, however, TBTO and TPTC caused about 55% and 51% increases, respectively, in MMC-induced a synergistic MNRET frequency. Organotin administration 3 or 6 h before and 3 h after MMC injection also caused enhancement of MNRET frequency. These results demonstrate that TBTO and TPTC act as co-clastogens in a whole mammalian system.

Induction of micronuclei in rat bone marrow and peripheral blood following acute and subchronic administration of azathioprine

The frequency of micronuclei was assessed in polychromatic erythrocytes of bone marrow and in polychromatic and normochromatic erythrocytes in peripheral blood of rats following exposure to azathioprine for 28 days. This was compared with the incidence of micronuclei in bone-marrow following exposure to a single dose of azathioprine. The incidence of micronuclei in bone-marrow polychromatic erythrocytes at the maximum tolerated dose (10 mg/kg) following exposure for 28 days was 29.5/1000. The incidence of micronucleated polychromatic erythrocytes in the peripheral blood at this dose was 4.4/1000. At the maximum tolerated dose in the single-dose study (40 mg/kg) the incidence obtained at 48 h post-treatment was 15.7/1000. This supports the view that the use of animals in a subchronic toxicity study is at least as sensitive for assessing in vivo clastogenic activity as an acute study and could reduce animal usage in toxicology assessments.

The concomitant detection of gene mutation and micronucleus induction by mitomycin C in vivo using lacZ transgenic mice

A new assay system that can simultaneously provide gene mutagenicity and clastogenicity data in vivo is described. Transgenic mice (Muta Mouse) harboring the lacZ gene as a target for mutation analysis were injected intraperitoneally with mitomycin C (MMC), either once or on 5 successive days. Micronucleus assays were performed with small amounts of peripheral blood collected from a tail vessel. The spontaneous frequency of micronucleated reticulocytes was 0.42%. For the mutation analysis, DNA was extracted from bone marrow and liver cells at several harvest times. The lacZ gene was rescued by lambda packaging and infection of E. coli C (lac-), followed by plating on agarose plates containing X-gal. The spontaneous lacZ mutant frequencies were 37 and 29 x 10(-6) in bone marrow and liver, respectively. In the micronucleus assay, single treatments with 1.0 and 2.0 mg/kg of MMC induced micronuclei in 3.6 and 5.8% of reticulocytes, respectively, peaking 48 h after treatment. Muta Mouse sensitivity to micronucleus induction was similar to nontransgenic strains used routinely for the micronucleus test. On the other hand, single treatments with MMC at 1.0 and 2.0 mg/kg did not induce any significant increases in the frequency of lacZ- mutants in bone marrow or liver. N-Ethyl-N-nitrosourea, used at 100 mg/kg as a positive control, yielded a 5-fold increase in mutant frequency above untreated animals in bone marrow only. After 5-day treatments, MMC induced approximately a 2-fold increase in mutant frequency in bone marrow only for the sublethal dose of 2 mg/kg. Therefore, this study indicated that the strong clastogenic activity of MMC in bone marrow was not accompanied by significant gene mutagenic activity.

Effects of antioxidants on induction of micronuclei in rat peripheral blood reticulocytes by potassium bromate

Micronucleus induction in male F344 rat peripheral blood by potassium bromate (KBrO3), a rat renal carcinogen, and its inhibition by several antioxidants were studied using the acridine orange supravital staining method. The frequency of micronucleated reticulocytes (MNRETs) peaked 32 h after a single i.p. treatment of rats with KBrO3 at a dose of 60 mg/kg. Co-treatment with glutathione (GSH) or cysteine (Cys) i.p. at doses of 800 mg/kg and 400 mg/kg, respectively, 30 min before and 30 min after the KBrO3 treatment significantly inhibited the micronucleus induction by KBrO3. Daily i.g. administration of vitamin C for 5 days at a dose of 200 mg/kg/day was also effective in protecting against micronucleus induction by KBrO3 given on the 4th day. However, co-treatment with superoxide dismutase in liposome-encapsulated form by i.p. injection at a dose of 18,000 U/kg 30 min before and 30 min after the KBrO3 application exerted no effect. The results indicate that antioxidants, especially sulfhydryl compounds, have protective potential against the clastogenicity of KBrO3, also suggesting that active oxygen species may play an important role in its clastogenicity.

Induction of micronucleated reticulocytes by potassium bromate and potassium chromate in CD-1 male mice

Micronucleus tests of potassium bromate (KBrO3) and potassium chromate (K2CrO4) were conducted with peripheral blood reticulocytes (PB-RETs) of CD-1 male mice dose intraperitoneally. Peripheral blood cells collected from the tail were stained supravitally with acridine orange (AO) using AO-coated glass slides. Both KBrO3 and K2CrO4 induced micronuclei in PB-RETs in the same manner as in polychromatic erythrocytes of bone marrow.

Bio-antimutagenic activities of vitamin B6 in E. coli and mouse peripheral blood cells

Pyridoxal (PL) and pyridoxal 5'-phosphate (PLP) showed a marked bio-antimutagenic effect on UV-induced mutagenesis in E. coli B/r WP2, but not in the DNA excision repair-deficient strain WP2suvrA under the condition where no cellular toxicity was observed. No delay in the first cell division was seen on post-treatment with PL after UV irradiation. PL reduced not only UV- but 4-nitroquinoline-1-oxide-induced mutation, while it was ineffective in N-methyl-N'-nitro-N-nitrosoguanidine- or gamma-ray-treated cells. These results suggest that PL promotes DNA excision repair directly or indirectly and the decrease in the amount of unrepaired DNA damage might cause the reduction of UV-induced mutations in E. coli B/r WP2. In addition to the above observation, PLP reduced the frequency of mitomycin C- (2 mg/kg, i.p.) induced micronuclei in mouse peripheral blood cells. Simultaneous or subsequent oral administration of PLP (25 mg/kg) decreased the frequency of micronucleated peripheral reticulocytes.

The micronucleus assay using peripheral blood reticulocytes from mitomycin C- and cyclophosphamide-treated rats

It used to be believed that the use of rat peripheral blood for the micronucleus assay would be difficult because micronucleated erythrocytes are captured and destroyed by the spleen quickly. We have applied an acridine orange (AO) supravital staining method to rat peripheral blood using AO-coated glass slides. Normal and splenectomized SD rats were treated once with mitomycin C (i.p.) or cyclophosphamide (p.o.), and 5 microliters of blood was collected at intervals from the tail vein between 0 and 72 h after treatment. For comparison, bone marrow cells were smeared conventionally 30 h after treatment. Although the frequencies of spontaneous and chemically induced micronucleated reticulocytes (MNRETs) from normal rats were lower on average in the highest dose group than those of splenectomized rats, the incidence of micronuclei among type I and II reticulocytes in normal rats at 48 h was almost identical to the incidence of RNA-containing erythrocytes with micronucleus in bone marrow. Thus, we suggest that rat peripheral reticulocytes can be used as target cells for the micronucleus assay.

Micronucleus induction in mouse peripheral reticulocytes by 7,12-dimethylbenz[a]anthracene

Micronucleus assays using mouse peripheral blood stained vitally on acridine orange (AO)-coated slides were evaluated at two laboratories with 7,12-dimethylbenz[a]anthracene (DMBA) and compared with the standard bone marrow assay. DMBA was administered by single intraperitoneal injection to CD-1 mice at doses ranging from 5 to 80 mg/kg, then 5 microliters of peripheral blood was sampled from a tail vein at 24, 48, 72, 96, and 120 h after treatment. Similar incidences of micronucleated young erythrocytes were observed in peripheral blood reticulocytes and bone marrow polychromatic erythrocytes. The dose response of micronucleated reticulocytes was delayed compared to that of micronucleated polychromatic erythrocytes. The dose-response curves after treatment with DMBA differed depending on the sampling times, which revealed the difficulty of obtaining accurate dose-response relations in the micronucleus assay. The present result demonstrated that the simple and rapid AO supravital staining method is a valuable and easier method for obtaining dose- and time-response data for quantification of micronucleus induction by chemicals.

The micronucleus test of methyl methanesulfonate with mouse peripheral blood reticulocytes using acridine orange-coated slides

The usefulness of the micronucleus assay using mouse peripheral blood erythrocytes and acridine orange (AO)-coated slides was evaluated with methyl methanesulfonate (MMS). The micronucleus test was carried out at doses ranging from 20 to 80 mg/kg body weight in CD-1 mice by intraperitoneal injection. Peripheral blood cells were examined from 0 to 72 h after treatment at 12- or 24-h intervals. Bone marrow cells from other mice treated with 80 mg/kg MMS were also sampled at the same times. The frequency of micronucleated reticulocytes (MNRETs) increased dose-dependently at every sampling time except 72 h, and the maximum frequency of MNRETs was observed at about 36 h after treatment. Micronucleated polychromatic erythrocytes (MNPCEs) in bone marrow after a dose of 80 mg/kg were significantly induced at 12 h to 36 h, and the maximum frequency of MNPCEs was observed at 24 h after treatment. The induction of MNRETs was delayed by about 12 h compared to that of MNPCEs in bone marrow, and the maximum frequencies of MNRETs were lower than those of MNPCEs, but the induction of MNRETs by MMS was significant and dose-dependent. It is concluded, therefore, that bone marrow cells could be replaced by peripheral blood cells as material for the micronucleus assay using AO-coated slides.

Micronucleus evaluation in peripheral blood reticulocytes of mice treated with procarbazine hydrochloride or mitomycin C

The usefulness of the acridine orange (AO) supravital staining technique for the mouse peripheral blood reticulocyte micronucleus test was investigated independently by three laboratories using the known clastogens procarbazine hydrochloride (PCZ) and mitomycin C (MMC). In all three laboratories the highest frequencies of micronucleated peripheral blood reticulocytes were observed 48 h after treatment of mice with a single dose of either MMC or PCZ. The animals responded to both chemicals in a dose-dependent manner. Although similar qualitative results were observed, mean micronucleus frequencies induced by a particular dose of a given test chemical did vary quantitatively among the three laboratories. This was most probably due to the use of slightly different scoring criteria by each examiner. This aspect needs special attention. To minimize inter-laboratory variability, therefore, we recommend establishing unequivocal criteria to distinguish the subclass of reticulocytes. These should then be used consistently by all investigators using this method. The most striking advantages of the AO supravital staining technique were the ease of slide preparation, the ease with which reticulocytes and mature erythrocytes could be distinguished by the examiners, and the occurrence of numerous scorable reticulocytes in each microscopic field, which greatly speeded up the manual counting process. The disadvantages of the staining technique were the limited scoring time due to the rapid fading of the fluorescence stain, the degradation of the cells with time, and the frequent need to search for adequate scoring areas within a microscopic field. Based on the data of this study the authors conclude that the AO supravital staining technique is highly suitable for the micronucleus assay in erythrocytic cells of mouse peripheral blood. In addition, we consider the mouse peripheral blood reticulocyte micronucleus test to be a useful tool with which to investigate the clastogenic potential of chemicals in vivo. As pretreatment of mice with Aroclor 1254 markedly increased the effect of PCZ on micronucleus induction, we suggest that the inclusion of inducers of drug metabolizing enzymes in the micronucleus test would be useful for the detection of the clastogenic potential of promutagenic chemicals.

Micronucleated reticulocyte induction by ethylating agents in mice

Six model ethylating agents were tested for clastogenic potency by means of a new technique of the micronucleus assay with mouse peripheral blood cells using acridine orange (AO)-coated slides, to evaluate the test. The alkylating agents were: N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-ethyl-N-nitrosourea (ENU), diethylsulfate (DES), ethyl methanesulfonate (EMS), epichlorohydrin (ECH) and ethylene dibromide (EDB). The animals were given a single intraperitoneal injection of the following doses of the chemicals: ENNG and ENU, 25, 50 and 100 mg/kg; EMS and DES, 100, 200 and 400 mg/kg body weight. For EDB and ECH, the doses were 50, 100 and 200 mg/kg, given twice, 24 h apart. Before and after the injection, blood samples were taken from the tails at 24-h intervals up to 72 h and preparations were made on AO-coated slides. For each dose group, 4 animals were used and 1000 reticulocytes were examined per slide for the presence of micronuclei. At the optimum induction time of 48 h, ENU induced micronucleated reticulocytes (MNRETs) at all 3 doses. ENNG and EMS induced MNRETs significantly at 2 dose levels each and DES only at the highest dose. ECH and EDB failed to induce MNRETs. On the basis of the dose of chemical needed to double the spontaneous frequency, the order of clastogenic potency was ENU greater than ENNG greater than EMS greater than DES. The results obtained compared favorably with those from other in vivo methods. The present technique proves to be simple, flexible and relatively sensitive. Shifts in the optimum induction peak in individual animals and by some chemicals can be picked up easily which is important when testing weak mutagens and chemicals with an unknown mechanism of action.

Micronuclei as an index of cytogenetic damage: past, present, and future

The workshop was designed to present what is known about the production of micronuclei, what protocols are now accepted or proposed internationally, what new results have been obtained, and what new methods and protocols are likely to be forthcoming. This report is designed to convey the flavour of the workshop and to provide the essence of the new information. After the workshop an effort was made to determine what single protocol would satisfy the requirements set for the micronucleus test by as many regulatory agencies as possible. The result, reported here, includes the requirements of six regulatory authorities in Canada, the European Economic Community, the Organization for Economic Co-operation and Development, Japan, and the United States.