Micronuclei and other nuclear anomalies induced in various organs by diethylnitrosamine and 7,12-dimethylbenz[alpha]anthracene

Further evaluation of an in vivo micronucleus test on rat and mouse skin: results with five skin carcinogens

In a previous paper, we presented a practical in vivo micronucleus (MN) test that used rat skin as the target organ. To evaluate the test, as well as to determine the reproducibility and applicability of the method to mice, we used it to test the effect of five skin carcinogens (N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 4-nitroquinoline 1-oxide (4NQO), 7,12-dimethylbenz[a]anthracene (DMBA), and benzo[a]pyrene (B[a]P)) on rat and mouse skin. All five compounds significantly and dose-dependently increased the MN frequencies in the basal cells of the chemical-treated skin. These results indicated the reproducibility of the test results and also the applicability of the test to mice as well as rats.

Induction of lacZ mutation by 7,12-dimethylbenz[a]anthracene in various tissues of transgenic mice

The induction of gene mutations was examined in MutaMouse after an intraperitoneal injection of 7, 8-dimethylbenz[a]anthracene (DMBA) at 20 mg/kg in a collaborative study participated by four laboratories. Although the DMBA dose used was lower than the level that has been reported to induce micronucleated erythrocytes maximally in several mouse strains, a killing effect appeared after day 9 of the post-treatment interval. Mutations in lacZ transgene were detected by the positive selection assay following in vitro packaging of phage lambda from the genomic DNA of the transgenic animals that survived. The mutant induction was evaluated in the bone marrow, liver, skin, colon, kidney, thymus, and testis 7 to 28 days after the treatment. In the bone marrow, the mutant frequency reached a maximum, approximately a 30-fold increase, 14 days after the treatment and the increased frequency persisted at least up to day 28 of the post-treatment. Induction of mutants was detected in the liver, colon, thymus, and skin to lesser extents. Marginal responses were obtained in the kidney and testis. The slight increases in the mutant frequencies in the kidney and testis observed in some laboratories were within laboratory-to-laboratory or animal-to-animal variations. In contrast to the gene mutation induction in the bone marrow, the frequency of micronucleated reticulocytes increased transiently 3 days after the treatment and returned to a control level before day 8 of the post-treatment. It was suggested that DMBA induced gene mutation is fixed in stem cells depending on cell proliferation while DNA damages responsible for chromosome breakage are not transmitted to progeny cells.

Evaluation of the rodent micronucleus assay in the screening of IARC carcinogens (groups 1, 2A and 2B) the summary report of the 6th collaborative study by CSGMT/JEMS MMS. Collaborative Study of the Micronucleus Group Test. Mammalian Mutagenicity Study Group

To assess the correlation between micronucleus induction and human carcinogenicity, the rodent micronucleus assay was performed on known and potential human carcinogens in the 6th MMS/CSGMT collaborative study. Approximately 100 commercially available chemicals and chemical groups on which there was little or no micronucleus assay data were selected from IARC (International Agency for Research on Cancer) Groups 1 (human carcinogen), 2A (probable human carcinogen) and 2B (possible human carcinogen). As minimum requirements for the collaborative study, 5 male mice were treated by intraperitoneal injection or oral gavage once or twice with each chemical at three dose levels, and bone marrow and/or peripheral blood was analyzed. Five positives and 2 inconclusives out of 13 Group 1 chemicals, 7 positives and 5 inconclusives of 23 Group 2A chemicals, and 26 positives and 6 inconclusives of 67 Group 2B chemicals were found. Such low positive rates were not surprising because of a test chemical selection bias, and we excluded well-known micronucleus inducers. The overall evaluation of the rodent micronucleus assay was based on the present data combined with published data on the IARC carcinogens. After merging, the positive rates for Groups 1, 2A and 2B were 68.6, 54.5 and 45.6%, respectively. Structure-activity relationship analysis suggested that the micronucleus assay is more sensitive to the genetic toxicity of some classes of chemicals. Those to which it is sensitive consist of (1) aziridines and bis(2-chloroethyl) compounds; (2) alkyl sulfonate and sulfates; (3) acyl-type N-nitroso compounds; (4) hydrazines; (5) aminobiphenyl and benzidine derivatives; and (6) azo compounds. Those to which it is less sensitive consist of (1) dialkyl type N-nitroso compounds; (2) silica and metals and their compounds; (3) aromatic amines without other functional groups; (4) halogenated compounds; and (5) steroids and other hormones. After incorporation of structure-activity relationship information, the positive rates of the rodent micronucleus assay became 90.5, 65.2 and 60.0% for IARC Groups 1, 2A and 2B, respectively. Noteworthy was the tendency of the test to be more sensitive to those carcinogens with stronger evidence human carcinogenicity.

Variability of neoplastic parameters in colon tumours, and its significance in diagnostic practice

We have reviewed the value of individual variability in the reaction of tissues to treatment with carcinogens, and the manifestation of this variability in different morphological (histological, morphometric, and ultrastructural), histochemical and immunohistochemical parameters generated in tumorous tissues. Moreover, we also reviewed data in the literature on individual variability in the manifestation of some biochemical and immunochemical markers which are accumulated in the serum of tumour-bearing patients and which are characteristic for the different phases of tumourigenesis. The high variability of different tumorous parameters suggests that none can be utilized alone as a conclusive marker of neoplasia and that only their combined use can give objective information. We also reviewed the impact of this variability in the evaluation of various pathological methods to detect different stages of neoplastic transformation in the colon. It has been concluded that none of the present approaches can be absolutely conclusive and without false results, and that objective information regarding early cancerous changes may be obtained only by use of combined analyses utilizing several laboratory methods. The diagnostic procedures should be based on the complex utilization of all appropriate methods using the quantitative interpretation of the obtained data. Multivariate analysis of many parameters should be very effective for the prediction of therapeutic results.

2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2) is a weak in vivo clastogen as revealed by the micronucleus assay

The in vivo clastogenicity of 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2) was examined in the micronucleus test using peripheral blood from three mouse strains (ICR, CD-1, and MS/Ae) and bone marrow from one rat strain (Sprague-Dawley). Doses up to the maximum tolerated were tested. The chemical was given once, twice, thrice, or four times via either the i.p. or p.o. route. Under some conditions, ICR and CD-1 mice showed an increased frequency of micronucleated reticulocytes, but definite conclusions were difficult to draw because the increases were very slight. MS/Ae mice showed a markedly elevated micronucleated reticulocyte frequency after the double and triple ip treatments. Rats showed a slightly but statistically significantly increased frequency of micronucleated polychromatic erythrocytes after double i.p. treatments. These results indicate that AF-2 is a weak in vivo clastogen.

Lack of predictivity of bone marrow micronucleus test versus testis micronucleus test: comparison with four carcinogens

In vivo somatic chromosome mutation tests are usually carried out using the bone marrow micronucleus test in the mouse. This test is also considered predictive for the study of clastogenic effects in germ cells. However, it has been reported that the sensitivity of the bone marrow micronucleus test is insufficient to detect unstable compounds or short-lived metabolites and the use of target cells with metabolic activity (hepatocytes) has been questioned. In order to analyze in vivo micronucleus induction in cells with metabolic enzyme activity, we compared the sensitivity of somatic and germ cells to four carcinogens in the bone marrow and spermatid micronucleus test in the mouse. Three procarcinogens with a complex metabolic pattern (dimethylnitrosamine, diethylnitrosamine and 1,1-dimethylhydrazine) and one direct unstable mutagen (beta-propiolactone) were tested. All four carcinogens were not detected by the bone marrow micronucleus test but were detected in the mouse spermatid micronucleus test in which they induced clear clastogenic effects, as was the case in a previous study in liver micronucleus test. In conclusion, this study demonstrates that the bone marrow micronucleus test is not sufficient for the prediction of a clastogenic hazard in germ cells. In addition to a second in vivo test in an organ with metabolic enzymes, i.e., the liver, the spermatid micronucleus test can be performed when a specific risk to the testis is likely.

Micronuclei induced in peripheral blood of E mu-PIM-1 transgenic mice by chronic oral treatment with 2-acetylaminofluorene or benzene but not with diethyl-nitrosamine or 1,2-dichloroethane

Micronucleus induction in peripheral blood was examined during carcinogenicity assays of the genotoxic carcinogens 2-acetylaminofluorene (2-AAF), benzene, diethylnitrosamine (DEN) and 1,2-dichloroethane (1,2-DCE) in lymphoma prone E mu-PIM-1 transgenic mice. In both sexes, micronuclei were increased in polychromatic (PCE) and normochromatic (NCE) erythrocytes after 14 weeks of oral treatment with 75 mg/kg 2-AAF or 50 and 100 mg/kg benzene. The micronucleus frequencies induced by benzene were higher in males than in females. There was no apparent treatment related suppression of erythropoiesis by 2-AAF or by benzene. Blood micronucleus frequencies induced by benzene were similar in transgenic mice and their non-transgenic litter mates. There was no micronucleus induction or PCE suppression detected in the blood of either sex after treatment with 1 and 3 mg/kg DEN or 100 to 300 mg/kg 1,2-DCE. At 40 weeks bone marrow was sampled from mice given 100 mg/kg benzene, and it was confirmed that micronucleated PCE frequencies in blood were an accurate reflection of those induced in bone marrow. However, the spontaneous and induced frequencies of micronucleated cells in blood were slightly higher in PCE than in NCE suggesting that a small degree of selective removal of micronucleated cells occurs in this mouse strain. Control micronucleus frequencies in E mu-PIM-1 mice appeared comparable to those in other, non-transgenic mouse strains. Thus micronuclei are readily detectable in blood during chronic exposure to the bone-marrow clastogens 2-AAF and benzene, but not to DEN and 1,2-DCE, probably because active species do not reach the bone marrow in sufficient concentrations to induce increases in micronuclei.

Cytogenetic damage and tumor incidence in mouse skin after single, topical applications of 7,12-dimethylbenz[a]anthracene

Micronucleus induction, chromosomal damage and aneuploidy were evaluated in whole skin keratinocyte cultures derived from HRA/Skh mice after single in vivo applications of 0.256, 2.56 and 25.6 micrograms (1, 10 and 100 nmoles) of the carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA). These genotoxicity end-points were compared with papilloma and carcinoma occurrence at the same dose levels of carcinogen. While the lower 2 doses of DMBA significantly increased the incidence of micronuclei and other chromosomal anomalies in keratinocytes, the two highest doses resulted in a significantly increased papilloma yield (0.297 and 3.895 papillomas/mouse) and incidence (24.3 and 100%). Carcinomas appeared only at the highest dose (0.125 carcinomas/mouse; 5% incidence). Neither papillomas nor carcinomas occurred in solvent-treated control mice. None of the three applied doses induced aneuploidy under conditions leading to an increase in tumors and/or chromosomal damage.

Assessment of the genotoxic potential of Caramel Colour I in four short-term tests

A battery of three short-term tests in vitro and one in vivo was used to determine the genotoxicity of Caramel Colour I. The results of the bacterial mutation assay, using five strains of Salmonella typhimurium, and the mouse micronucleus assay in vivo showed no evidence of genotoxic activity. Results from both the cytogenetics assay in vitro, using CHO cells, and the mouse lymphoma assay indicated that there was some genotoxic activity associated with Caramel Colour I but only in the absence of S-9 and at very high dose levels.

The relationship between colony-forming ability, chromosome aberrations and incidence of micronuclei in V79 Chinese hamster cells exposed to microwave radiation

Cultured V79 Chinese hamster fibroblast cells were exposed to continuous radiation, frequency 7.7 GHz, power density 0.5 mW/cm2 for 15, 30 and 60 min. The effect of microwave radiation on cell survival and on the incidence and frequency of micronuclei and structural chromosome aberrations was investigated. The decrease in the number of irradiated V79 cell colonies was related to the power density applied and to the time of exposure. In comparison with the control samples there was a significantly higher frequency of specific chromosome aberrations such as dicentric and ring chromosomes in irradiated cells. The presence of micronuclei in irradiated cells confirmed the changes that had occurred in chromosome structure. These results suggest that microwave radiation can induce damage in the structure of chromosomal DNA.

Nuclear polymorphism and nuclear size in precarcinomatous and carcinomatous lesions in rat colon and liver

The role of nuclear polymorphism and nuclear size in an analysis of the differences between colon and liver tumors in rats as well as in an analysis of hepatic dysplasia was studied. It was shown that colon tumors which developed following treatment of animals with a direct carcinogen alone or with a carcinogen followed by secondary bile acid were characterized by low incidence of nuclear polymorphism and by an increased nuclear size in epithelial cells. Metastatic liver tumors in rats with colon tumors were characterized by a high value for the coefficient of nuclear form polymorphism and by a significant decrease in nuclear size. Hepatic dysplasia which developed as a result of prolonged treatment with secondary bile acids was characterized by high rate of nuclear form polymorphism and by a significant increase in nuclear size. The obtained results suggest that nuclear polymorphism is dependent upon the type of tissue or organ involved in the cancerous transformation and that it may have significance as a diagnostic marker of precarcinomatous and carcinomatous lesions of digestive organs only when used in combination with other analyses.

The in vivo micronucleus assay in mammalian bone marrow and peripheral blood. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The protocol recommended for the micronucleus assay in mammalian bone marrow has been revised and simplified. The number of sample times has been reduced to one or two, depending upon the dosing protocol. The minimum number of cells to be scored per treatment group has been increased to 20,000 to increase the ability of the assay to detect a doubling of the control micronucleus frequency. Use of both male and female animals is recommended. Scoring of micronuclei in polychromatic erythrocytes of peripheral blood is included as a variation of the bone marrow assay. Published data on chemicals tested by the micronucleus assay have been reviewed and are summarized.

Activity of DMBA, DMH and CP in triple- and single-dose rodent bone-marrow micronucleus assays

1,2-Dimethylhydrazine, 7,12-dimethylbenzanthracene and cyclophosphamide were detected as positive using the triple-dose test protocol described by MacGregor (1989). Determination of the 3 day MLD may complicate the use of this test protocol.

Bronchial micronuclei as a marker of an early stage of carcinogenesis in the human tracheobronchial epithelium

In the bronchial epithelium, smoking initiates a multistep process that first appears histologically as premalignant squamous metaplasia/dysplasia, a biological predecessor of squamous-cell lung cancer. Reflecting chromosomal damage from a carcinogenic insult, micronuclei may reveal earlier events in the carcinogenic sequence. We prospectively evaluated and correlated micronucleus count, histology (index of metaplasia) and smoking exposure in 35 consecutive subjects (9 active smokers, 10 previous smokers and 16 never-smokers) undergoing diagnostic bronchoscopy. Samples for micronuclei and histological evaluation were taken from the main carinal mucosa in each subject for site-specific comparisons. The median and mean micronucleus counts per 1,000 cells were significantly higher in active smokers than in non-smokers (subjects who had never smoked and previous smokers): median counts were 3.7 vs. 1.4, p = 0.03; mean counts were 4.7 vs. 1.9, p = 0.01. There was no significant difference, however, in micronucleus counts between subjects who had never smoked and previous smokers. Bronchial metaplasia and smoking history were not associated. Our findings suggest that micronuclei are a readily quantitated, early intermediate-endpoint marker for detecting tobacco-initiated tracheobronchial carcinogenesis.

In vivo micronucleus test using mouse hepatocytes

The bone-marrow micronucleus (BMM) test is highly specific for clastogenic effects but its sensitivity is determined to a great extent by the substances tested, particularly by their metabolism. Some compounds, such as unstable mutagens or those which generate short-lived metabolites, are not detected in this test because the metabolites produced in the liver do not reach the bone marrow. In an attempt to provide qualitative and quantitative assessments of chromosomal mutations produced in vivo by genotoxic agents not detected in the mouse BMM test, a mouse-liver micronucleus test, adapted from Tates model, was developed. The animals were treated twice, with an interval of 24 h between treatments, and then subjected to partial hepatectomy (PH) 24 h after the second treatment in order to induce mitotic stimulation. The incidence of micronucleated hepatocytes was determined 96 h after PH. The test was evaluated with 5 procarcinogens, each with a complex metabolic pattern: dimethylnitrosamine (DMN), diethylnitrosamine (DEN), 1,1-dimethylhydrazine (1,1-DMH), 4-aminophenol (4-APOL), 4-aminobiphenyl (4-ABPYL) and one direct unstable mutagen, beta-propiolactone (BPL). All these compounds are negative in the mouse BMM test but caused a major increase in the incidence of micronuclei in mouse hepatocytes. This test is simple and can be readily compared with the BMM test. Furthermore, it offers a better assessment of the impact of a compound at the chromosomal level in a metabolically competent cell and can therefore be used for the evaluation of the genotoxic activity of compounds with complex metabolic pathways.

Mutagenic and cytogenetic studies of N'-nitrosonornicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco specific nitrosamines (TSNA) N'-nitrosonornicotine (NNN) and 4-(Methylnitrosoamino)-1-(3-pyridyl)-1-butanone (NNK) were tested for mutagenic and clastogenic effects using a battery of short-term test systems. These test systems include the Ames test, micronucleus test (MNT), induction of chromosomal aberrations and sister chromatid exchange (SCEs). NNN and NNK were tested for their potency in inducing mutations in the Ames Salmonella/microsome assay and their clastogenic action were tested by the micronucleus inducing ability in vivo using Swiss mice. Studies on the induction of chromosomal aberrations and SCE exchange were carried out using human peripheral blood lymphocyte cultures. In the Ames test and MNT, NNN was positive but in comparisons with NNK, NNK was a more potent mutagen. Present studies clearly proves the genotoxic potential of both NNN and NNK and between the two NNK is more potent.

Formation of nuclear anomalies in rat intestine by benzidine and its biliary metabolites

Administration of benzidine (BZ) by intraperitoneal injection (i.p.) to rats (0-100 mg/kg) produced, after 24 h, a dose-dependent formation of nuclear anomalies (micronuclei, pyknotic and karyorrhectic nuclei) in intestinal epithelial cells analysed both in isolated cell suspensions and in the intestinal crypts in tissue sections. When bile collected (0-4 h) from rats treated with BZ (150 mg/kg, i.p.) was infused into the duodenum of recipient rats, nuclear anomalies were observed in mucosal epithelial cells, after 24 h, with a similar distribution to that in rats given BZ by i.p. injection. The formation of nuclear anomalies in the intestine is in accord with the intestinal carcinogenic effect of BZ and is, at least partially, dependent on exposure of epithelial cells to biliary metabolites of BZ.

The cytonucleus test in the rat: a combined metaphase and micronucleus assay

The suitability of the rat as a species choice for the micronucleus assay and the possibility of combining both metaphase and micronucleus analysis using one set of animals were investigated. Cyclophosphamide, trenimon, vinblastine sulphate and dimethyl benzanthracene were used to optimise the study design and experimental procedures. The sample times ranged from 12 to 72 h after a single dose of the compound. A maximal response in both micronucleus and chromosome aberration induction was observed 24 h after dosing. This sample time is recommended as the single sample time for the screening of all classes of compound. Using the optimal conditions, a number of different mutagens/carcinogens were then analysed for the induction of both micronuclei and chromosome aberrations in the same animal. The compounds selected were mitomycin C, methotrexate, 5-fluorouracil, hexamethyl phosphoramide, benzo[a]pyrene, benzidine and diaminoterphenyl. The results show that the rat is a responsive test species and that it is possible to combine both metaphase and micronucleus analysis in the same animal.