An approach towards to standardization of the mammalian spot test

Synthesis report of the step project detection of germ cell mutagens

The project 'Detection of Germ Cell Mutagens' was designed with three major goals: (1) Detection and characterization of germ-cell mutagens; (2) standardization and validation of new germ-cell tests; and (3) development of a data base on germ-cell mutagenicity. All three goals were achieved. The classical germ-cell tests were applied to characterize the genetic effects of acrylamide (AA), 1,3-butadiene (BD), trophosphamide (TP) and urethane (UR). All but UR were found to cause heritable genetic damage. The experimental data obtained for AA and BD were the basis for genetic risk evaluations during the EC/US Workshop on Risk Assessment 'Human Genetic Risk from Exposure to Chemicals, Focusing on the Feasibility of the Parallelogram Approach'. Nine chemicals were employed to validate the spermatid micronucleus assay with mice and rats: AA, BD and its metabolites 1,2-epoxybutene-3 and 1,2:3,4-diepoxybutane, chlorambucil, mitomycin C, methylnitrosourea, TP and UR. The spermatid micronucleus test was combined with micronucleus tests in somatic cells such as bone marrow or peripheral blood erythrocytes, and splenocytes which allowed a comparison of effects in somatic and germinal cells. Improvements of the spermatid micronucleus test included BrdU-labelling of premeiotic S-phase for the determination of stage sensitivity and fluorescence in situ hybridization with pancentromeric DNA-probes to distinguish between clastogenic and aneugenic events. The results indicate that the spermatid micronucleus test with its improvements is an adequate procedure to detect germ-cell clastogenicity and to compare the activity of chemicals in different tissues and between species, i.e., rats and mice. Other germ cell methods under study were the flow cytometric measurement of testicular sperm DNA and the cytogenetic analysis of preimplantation embryos for chromosomal aberrations and micronuclei. The collection of a reliable germ-cell data base was accomplished through a critical evaluation of the literature and with the data obtained in the present project. Remarkable concordance between responses of germ cell tests to chemical mutagens was the most striking conclusion to be drawn from the present data base.

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.

Mutagenic activity of 2-amino-N6-hydroxyadenine in the mouse spot test

The base analogue 2-amino-N6-hydroxyadenine (AHA) was mutagenic in the spot test in (T x HT)F1 mouse embryos. Females were injected with single doses of 20 or 40 mg AHA per kg body weight on the 9th day of pregnancy. To rank the mutagenic potency of different compounds, the frequencies of genetically relevant spots induced by 1 mg/kg body weight were calculated. The observed somatic mutation frequency for 1 mg/kg AHA was lower (1.95 x 10(-3)) spots of genetic relevance) than that of mitomycin C (16 x 10(-3)), ethylnitrosourea (6.8 x 10(-3)) and cyclophosphamide (6.4 x 10(-3)) and therefore AHA was not classified as a very potent mutagen in this test system. The doubling dose to induce genetically relevant spots was calculated to be 20 mg/kg b.w. Based on these data, AHA is suggested to be a candidate to induce recessive specific-locus mutations in germ cells of mice.

Suppression of developmental anomalies by maternal macrophages in mice

We tested whether nonspecific tumoricidal immune cells can suppress congenital malformations by killing precursor cells destined to cause such defects. Pretreatment of pregnant ICR mice with synthetic (Pyran copolymer) and biological (Bacillus Calmette-Guérin) agents significantly suppressed radiation- and chemical-induced congenital malformations (cleft palate, digit anomalies, tail anomalies, etc.). Such suppressive effects were associated with the activation of maternal macrophages by these agents, but were lost either after the disruption of activated macrophages by supersonic waves or by inhibition of their lysosomal enzyme activity with trypan blue. These results indicate that a live activated macrophage with active lysosomal enzymes can be an effector cell to suppress maldevelopment. A similar reduction by activated macrophages was observed in strain CL/Fr, which has a high spontaneous frequency of cleft lips and palates. Furthermore, Pyran-activated maternal macrophages could pass through the placenta, and enhanced urethane-induced cell killing (but not somatic mutation) in the embryo. It is likely that a maternal immunosurveillance system eliminating preteratogenic cells allows for the replacement with normal totipotent blast cells during the pregnancy to protect abnormal development.

Effects of gamma-radiation on the differentiation of mouse melanocytes in the hair follicles

Pregnant mice were whole-body irradiated with a single acute dose of gamma-rays (60Co) to investigate the effect of gamma-radiation on embryonic melanoblasts. The effect was studied by scoring changes in the differentiation of melanocytes in the hair follicles of mice heterozygous for the recessive coat color mutation pink-eyed dilution (p). Abnormal round melanocytes were found in the hair matrix and the dermal papilla of F1 offspring 3.5 days after birth. However, these round melanocytes possessed a melanin deposition of similar intensity to normal hair follicular melanocytes. The frequency of the abnormal hair follicles increased in a dose-dependent manner. Moreover, higher frequencies were found in the animals irradiated at earlier stages of embryonic development. These results indicate that gamma-radiation affects dendritogenesis and the location of mouse melanocytes in the hair follicles, with greater effects seen at the earlier stages of development.

Simultaneous induction of mutagenic and cancerogenic effects in T x HT mice with transplacental ethylnitrosourea treatment

The relationship between mutagenesis and carcinogenesis was investigated in T x HT crossbred mice using diaplacental application of ethylnitrosourea (ENU) at different stages of embryonal development. Mutagenesis was detected by induction of coat color spots, and the carcinogenic response was investigated in a long-term follow-up study of the F1-generation. The animals were particularly sensitive to induction of tumors at the central nervous system (CNS)-skull/vertebra interface (30% and 20% in ENU-treated male and female offspring, respectively, compared with less than 1% in controls). There was a correlation between the appearance of these tumors and the presence of color spots. This correlation was low but statistically significant in female offspring. Three other types of tumors showed a correlation with the presence of coat color spots. Liver tumors were significantly increased in color spot-positive females but unchanged in males. Lung tumors were reduced in color spot-positive males and appeared earlier in color spot-positive females. There was a lower incidence of lymphoma/leukemia in all spot-positive mice. The reduction in tumor incidence beyond the spontaneous rate in spot-positive animals might be caused by a high cytolethal response to ENU in the relevant organs and tissues.

The mutagenic effect of caprolactam in the spot test with (T x HT) F1 mouse embryos

The mouse spot test was used to investigate the in vivo mutagenicity of caprolactam. Among offspring treated with 500 mg/kg, the frequency of spots was increased over controls in 4 separate experiments, leading to the conclusion that caprolactam is mutagenic in the spot test.

Dominant cataract and recessive specific-locus mutations detected in offspring of procarbazine-treated male mice

The induction of dominant cataract mutations by procarbazine was studied concomitantly with the induction of specific-locus mutations in treated male mice. The most effective dose in the specific-locus test, 600 mg/kg of procarbazine, and a fractionated dose of 5 X 200 mg/kg were used. The frequencies of dominant cataract mutations were higher, but not significantly different from the historical control. The ratio between the number of recovered specific-locus and dominant cataract mutations was in accordance with that found in our experiments with gamma-rays (Ehling et al., 1982; Kratochvilova, 1981) or in experiments with ethylnitrosourea (Favor, 1986). A total of 3 dominant cataract mutations were recovered in the offspring of procarbazine-treated spermatogonial stem cells. Two mutations had complete penetrance while the third exhibited a reduced penetrance of approximately 70%. The viability and fertility of the heterozygotes of all 3 mutations were not affected. Only 1 mutation was shown to be viable as a homozygote.

Effects of X-irradiation at different times during development on the yield of somatic mutations in melanocytes of the mouse

The effect of 2.0 Gy X-irradiation at different times during foetal and early post-natal development on the resultant somatic mutation frequency was investigated by scoring for changes in follicular melanocyte morphology (nucleofugal vs. nucleopetal) in mice heterozygous for the recessive coat colour mutations dilute (d) and leaden (ln). Two peaks were observed following X-irradiation on days 12.5 and 17.5 post coitus (p.c.). The biomodal character of the mutation frequency with time of X-irradiation may be related to changes in the dynamics of the melanocyte population with foetal age. Nonetheless, the results validate the treatment time used in the pilot study (Searle and Stephenson, 1982) as the most sensitive to the induction of somatic mutations within the follicular melanocyte population.

The mouse spot test. Evaluation of its performance in identifying chemical mutagens and carcinogens

The published results on 60 chemicals and X-rays investigated in the mouse spot test were compared with data on the same chemicals tested in the bacterial mutation assay (Ames test) and lifetime rodent bioassays. The performance of the spot test as an in vivo complementary assay to the in vitro bacterial mutagenesis test reveals that of 60 agents, 38 were positive in both systems, 6 were positive only in the spot test, 10 were positive only in the bacterial test and 6 were negative in both assays. The spot test was also considered as a predictor of carcinogenesis; 45 chemicals were carcinogenic of which 35 were detected as positive by the spot test and 3 out of 6 non-carcinogens were correctly identified as negative. If the results are regarded in sequence, i.e. that a positive result in a bacterial mutagenicity test reveals potential that may or may not be realized in vivo, then 48 chemicals were mutagenic in the bacterial mutation assay of which 38 were active in the spot test and 31 were confirmed as carcinogens in bioassays. 12 chemicals were non-mutagenic to bacteria of which 6 gave positive responses in the spot test and 5 were confirmed as carcinogens. These results provide strong evidence that the mouse coat spot test is an effective complementary test to the bacterial mutagenesis assay for the detection of genotoxic chemicals and as a confirmatory test for the identification of carcinogens. The main deficiency at present is the paucity of data from the testing of non-carcinogens. With further development and improvement of the test it is probable that the predictive performance of the assay in identifying carcinogens should improve, since many of the false negative responses may be due to inadequate testing.

The mouse spot test: results with a new cross

The fertility and sensitivity of a new cross using male PDB (a/a b/b d/d p/p) and non-agouti female NMRI (a/a c/c) mice have been assessed. This cross results in embryos which are heterozygous at the same coat colour loci as are present in the embryos from the classical TxC57Bl/6 cross, commonly used in the mammalian spot test. The fertility of this new cross reflects the good breeding performance of the NMRI females. The typical litter size at the age when the F1 are examined for coat colour mosaics is 9-10, and this, together with the high pregnancy rate, results in substantial economies in the number of treated animals. The sensitivity of the cross to treatment with ethylnitrosourea, procarbazine, isoniazid, cyclophosphamide and 4-nitroquinoline oxide has been shown to compare well with other published results. If further evaluation of this cross confirms these preliminary results, then the cross can be used to reduce numbers of experimental animals without loss of sensitivity.

Comparison of types of chemically induced genetic changes in mammals

The present paper reviews the currently available in vivo systems for detection of chemically induced mutations and chromosome aberrations and summarizes the data of the relevant tests for mammalian germ-cell mutations (specific-locus test and heritable translocation test). The value of in vivo screening tests (somatic mutations and sperm abnormalities) for predicting specific-locus mutations is illustrated by comparing doubling doses. The results from the mammalian germ-cell mutation tests (specific-locus test and heritable translocation test) constitute the base-line for an assessment of predictability. Radiation and chemically induced specific-locus mutations differ in a number of respects, suggesting a need for caution in making risk estimates for chemical mutagen exposures in terms of radiation-equivalent doses. In vivo nondisjunction tests are discussed. Finally, unsolved problems and difficulties in generalizing qualitative and quantitative correlations between test systems are outlined. It is concluded that even qualitative predictions from data on somatic cells to germ cells are at best insecure because germ-cell specificity cannot be foretold, not to mention the fact that quantitative extrapolations from the results of in vivo screening tests, in general, are fraught with even more uncertainties. There is an acute need for collection of more data from studies involving germ cells.

Mutagenicity studies with the mouse spot test

The mammalian spot test, which detects somatic gene mutations in mouse embryos, was investigated with selected chemicals to (a) further validate this test system (ENU, EMS, 2AAF, colchicine) and (b) evaluate the mutagenic potential, in a whole-mammal system, of environmental compounds that had been previously recognized as mutagens in other mammalian or submammalian test systems (1,2-dichloroethane, hydroquinone, nitrofurantoin, o-phenylenediamine, fried sausage extract). Of these substances, ENU, EMS and 2AAF were significantly mutagenic, 1,2-dichloroethane was probably weakly mutagenic. The ENU data were used to estimate the number of pigment precursor cells present at the time of treatment (day 9.25). We also describe in this report the use of a fluorescence microscope for classification of hairs from spots on the coat of C57BL/6JHan X T hybrids.

A method to detect tumors and presumed somatic mutations in mice

A method was tested to detect both tumors and somatic mutations in mice. When HT-A/J F1 embryos were treated with ethylnitrosourea on day 11 of gestation, a single injection was enough to induce tumors and presumed somatic mutations of coat color in significantly high incidence. Urethane, treated in a similar way, induced somatic mutations and malformations in the offspring. Tumor incidence was also increased but was not significantly different from controls. Treatment on day 13 was too late to induce somatic mutations and malformations, while high incidence of tumors were induced.