Tests for urethane induction of germ-cell mutations and germ-cell killing in the mouse

Efficacy of crocin and safranal as protective agents against genotoxic stress induced by gamma radiation, urethane and procarbazine in mice

Crocin (CRO) and safranal (SAF) are bioactive constituents of saffron (dried stigma of Crocus sativus flower), an expensive spice with medicinal properties. Aqueous extract of saffron is known for its antigenotoxic effect against environmental genotoxins/carcinogens. However, there is need to identify saffron constituents responsible for this antigenotoxic effect. The aim of our investigation was to ascertain the role of CRO and SAF as inhibitors of in vivo genotoxic stress. For this purpose, Swiss albino mice were pretreated with CRO (50-mg/kg body weight (bw))/SAF (0.025- and 0.25-ml/kg bw) by gavage for 2 days. Thereafter, the pretreated mice were exposed to the genotoxic agents: (1) gamma radiation (GR; 2 Gy), (2) urethane (URE; 800 mg/kg) and (3) procarbazine (PCB; 60 mg/kg). In addition, CRO (50 mg/kg) was co-administered with the nitrosation reaction mixture of methylurea (MU; 300-mg/kg bw) + sodium nitrite (15 mg/kg) which can form N-nitroso-N-MU in the stomach. Genotoxic damage was measured by performing the bone marrow micronucleus test. Results obtained demonstrated significant reductions in the incidence of micronucleated polychromatic erythrocytes in the bone marrow of mice pretreated with CRO/SAF before exposure to the above DNA damaging agents, GR, URE and PCB. Co-administration of CRO with the nitrosation reaction mixture led to significant decrease in genotoxicity when compared to nitrosation reaction mixture alone. Histopathological studies revealed that these saffron constituents reduced testicular cell damage induced by the test genotoxins. The cell-free DNA-nicking assay using pBR322 DNA showed significant protective effects of CRO against hydroxyl radical-induced strand breaks.

Effects of Male Germ-Cell Stage on the Frequency, Nature, and Spectrum of Induced Specific-Locus Mutations in the Mouse

By means of the mouse specific-locus test (SLT) with visible markers, which is capable of detecting intragenic mutations as well as larger lesions, about 20 mutagens have been studied comparatively across arrays of male germ-cell stages. In addition, a very large historical control, accumulated over decades, provides data on spontaneous mutations in males. Each mutagen has a characteristic germ-cell-stage sensitivity pattern. Although most chemicals yield their maximum numbers of mutations following exposure of spermatozoa and late spermatids, mutagens have now been identified that peak in each of the major stages of spermatogenesis and spermiogenesis, including those in which effects on recombination can also be induced. Stem-cell spermatogonia have yielded positive results with only five of 15 mutagenic chemicals. In postspermatogonial stages, all chemicals, as well as radiations, induce primarily large lesions (LL). By contrast, in spermatogonia (either stem-cell or differentiating) all chemicals except one (bleomycin) produce very few such lesions. The spectrum of relative mutation frequencies at the seven loci of the SLT is characteristic for treated germ-cell stage and mutagen. Treatments that induce primarily LL are characterized by a great preponderance of s (Ednrb)-locus mutations (possibly due to a paucity of haplo-insufficient genes in the surrounding region); and those that induce very few, if any, LL by a great preponderance of p-locus mutations. Spontaneous locus-spectra differ from both types of treatment-induced spectra; moreover, there are two distinct types of spontaneous spectra, depending on whether mutations occurred in mitotic cells or during the perigametic interval.

Investigation of lung tumour induction in C3H/HeH mice, with and without tumour promotion with urethane, following paternal X-irradiation

In series of papers Nomura has reported that parental irradiation can lead to an enhanced incidence of lung and other tumours. However, in a recent study with BALB/cJ mice, using optimum conditions as defined by Nomura, we were unable to confirm this. We have now repeated the investigation using a different inbred strain, C3H/HeH, with and without tumour promotion in the F1 by urethane, again using protocols defined by Nomura. In a series of replicate studies spanning over 2 years, males were exposed to single, acute doses of 0, 250 and 500 cGy X-rays and thereafter placed with two females each in each of two consecutive weeks. Half the offspring from each treatment group and each week of mating were given 5 mmol/kg body weight of the urethane, while the remainder remained untreated. Most of the offspring produced were killed and scored for lung tumours at 6 months of age, while the rest were examined at 12 months of age. The proportion of fertile females and litter size provided evidence of a dose-dependent mutational response to the paternal irradiation, but no trace of a radiation-enhanced lung tumour incidence was detected among the progeny, whether in the urethane or non-urethane groups at 6 or 12 months of age, and whether assessed by numbers of mice with tumours, clusters of tumours, or cluster size. As seen in the BALB/cJ study, significant differences among different replicates were found, again suggesting a cyclical or seasonal variation in tumour incidence, but the variations seen with the two strains were not the same. The need for concurrent controls for tumour work was, nevertheless, again indicated. The overall findings do not therefore accord with those of Nomura. Furthermore, they do not support the causal association between the raised incidence of childhood leukaemia and non-Hodgkins lymphoma near Sellafield and the father's recorded radiation exposure during employment in the nuclear industry, as suggested by the Gardner report.

Comparative study on germ cell mutation induced by urethane (ethyl carbamate) gas and X-rays in Drosophila melanogaster

Although its mutagenicity has not been confirmed in mouse germ cells, urethane (ethyl carbamate) gas induces a significant increase of X-linked recessive lethal mutations in the germ cells of Drosophila melanogaster. The mutation frequency increased as the exposure time was changed from 3.5 to 5.5 h. Mutations were also induced by X-rays (20 to 40 Gy) and N-methyl-N-nitrosourea (MNU) (0.06 to 0.10%). However, no significant increase of chromosomal changes (partial loss of the Y chromosome, total loss of X or Y, and translocations) was produced by urethane, although these were readily induced by X-rays. There were large and significant increase in chromosomal changes caused by X-rays (20 Gy) compared to urethane (5.5 h) or MNU (0.06%). In contrast, there were no substantial differences among these three treatments as regards recessive lethal mutations. Urethane-induced DNA lesions detected as recessive lethals appear to be intragenic mutations. Complementation analysis with 15 reference single-site loci (cistrons) in the zeste-white region of the X chromosome revealed that 29 of 723 urethane-induced recessive lethals were located in the zeste-white region and all were restricted to a single locus. However, among 28 of 890 X-ray-induced lethals, 2 were non-complementary to 2 or 3 adjacent loci, indicating deletions encompassing 2 or 3 loci. In addition, 3 of these lethal chromosomes included mutations outside the zeste-white region. Another difference between urethane and X-rays was in the distribution of mutation sites. Urethane-induced mutations were strikingly non-random with two hot spots at zw-1 and zw-2, whereas the distribution of X-ray-induced mutations was more nearly random.

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.

Induction of DNA damage by urethane in mouse testes: DNA binding and unscheduled DNA synthesis

The extent and persistence of DNA damage and repair were investigated in mouse spermatogenic cells exposed in vivo to urethane (ethyl carbamate, EC). Adult male mice exposed to [3H]EC at 10-1,000 mg/kg were sacrificed 12 hr later. EC/metabolite binding to liver and testicular DNA and to sperm heads from the vasa deferentia was measured. Other male mice were exposed to EC at 50-750 mg/kg, and unscheduled DNA synthesis (UDS) induction was investigated in early spermatid stages. Similar experiments were conducted with vinyl carbamate (VC; putative EC metabolite) at 10-75 mg/kg. [3H]EC bound to liver and testicular DNA and to whole sperm heads. Testicular DNA binding increased linearly with dose, although binding was at least 2 orders of magnitude lower than with liver DNA. Sperm head binding also increased linearly with dose. Dose response studies with the UDS assay showed that EC and VC induced a small but significant increase of the UDS response in early spermatid stages. However, the induced UDS responses were quite variable and did not consistently increase with the administered dose. To determine the time kinetics of UDS induction, [3H]dThd was injected at various times after treatment with 500 mg/kg of EC or 60 mg/kg of VC. A slight but significant UDS increase was observed 4 hr after treatment with EC but not with VC. Overall, these results suggest that EC metabolites bind to testis DNA and cause low-level DNA damage in mouse spermatogenic cells. This type of DNA damage apparently does not have significant genetic consequences.

Assessing the risk of heritable gene mutation in mammals: drosophila sex-linked recessive lethal test and tests measuring DNA damage and repair in mammalian germ cells

The former U.S. EPA OPPT tiered test scheme for heritable gene mutations included the Drosophila sex-linked recessive lethal (SLRL) test in which positive results triggered the mouse specific locus (MSL) test. However, review of available literature indicated that the evaluation of mutations in the germ cells of this insect is not a good predictor of the risk of heritable gene mutations in mammals. The database contained 29 compounds for which there were conclusive MSL test results in either spermatogonial and/or postspermatogonial cells. Results in the SLRL test were available for 27 of those compounds. Of the 24 SLRL-positive chemicals, only 13 (54%) induced heritable mutations in mice; the three SLRL-negative compounds were nonmutagenic in mouse germ cells. The overall concordance between the two tests was 59%. In contrast, results of unscheduled DNA synthesis (UDS: 18 chemicals) and alkaline elution (AE: 14 chemicals) assays in rodent testicular cells following in vivo exposure correlated well with results in the MSL test (83% and 86%, respectively). MSL test results in spermatogonia and postspermatogonia were also compared separately to the SLRL, UDS, and AE assays. The concordances for the two cell types in the SLRL relative to the MSL test were 36% and 79%, respectively, indicating that the SLRL test is extremely poor in predicting heritable gene mutations in mammalian spermatogonia. Concordances for UDS and AE assays relative to MSL test results in spermatogonia (53% and 54%, respectively) and postspermatogonia (91% and 100%, respectively) were greater. Based on these analyses, the U.S. EPA OPPT has revised its tiered test scheme using assays for interaction with gonadal DNA (e.g., UDS and AE) in place of the SLRL test.

Fertility, reproduction, and genetic disease: studies on the mutagenic effects of environmental agents on mammalian germ cells

Because genetically based diseases have a major impact on human health, the National Institute of Environmental Health Sciences (NIEHS) has conducted a research and testing program for more than a decade to address chemical induction of heritable genetic damage in the germ cells of mammals. Although most genetic disease results from preexisting mutations, a portion is due to the occurrence of new mutations. The supposition that exposure to mutagenic chemicals contributes to the occurrence of new mutations in the human population is strongly supported by the results from animal models. Such studies clearly demonstrate the potential of environmental chemicals to induce mutations in both somatic and reproductive cells of mammals. This NIEHS program has become a leader in the identification of genetic hazards in the environment and in the acquisition of animal model data used by regulatory agencies in assessing genetic risks to human health.

Activity of urethane and N,N-dimethylurethane in the mouse bone-marrow micronucleus assay: equivalence of oral and intraperitoneal routes of exposure

Urethane is shown to be active in the mouse bone-marrow micronucleus assay when administered as a single dose by either gavage or intraperitoneal injection. The magnitude of the response using the two routes was not statistically significantly different. N,N-Dimethylurethane (DMU) is shown to be mutagenic to Salmonella and active in the bone-marrow micronucleus assay by both routes of administration. The activity of DMU in the bone marrow precludes elimination of ethanol, yielding cyanate ion, as an explanation for the micronucleus-inducing activity of urethane.

Induction of micronuclei in bone marrow of mice exposed to 1, 2 or 3 daily doses of urethane

Urethane was studied for its potential to induce micronuclei in bone marrow of CD-1 mice following various dosing and sampling schedules. It was found that: (1) mice dosed for 3 days by gavage with urethane at daily doses of 300, 600 and 1200 mg.kg-1 showed no increases in micronuclei compared to normal control values; (2) mice injected with 3 daily intraperitoneal (i.p.) doses of urethane at 400 and 1200 mg.kg-1 demonstrated substantial and dose-related increases in micronuclei; (3) mice given urethane at i.p. doses of 400 mg.kg-1 for 1, 2 and 3 days showed protocol related micronucleus yields with triple greater than single greater than double dosing, while mice given urethane at i.p. doses of 1200 mg.kg-1 showed a different trend with double greater than single dosing, the triple-dosing regimen proving cytotoxic to the bone marrow; (4) mice exposed i.p. to 400 mg.kg-1 of urethane had more micronuclei when sampled at 24 h than at 48 h after 1 or 2 daily treatments, while mice dosed i.p. with 1200 mg.kg-1 of urethane showed more micronuclei at 48 h than at 24 h after a single treatment, the double-dosing protocol being toxic to the marrow; (5) female mice were more susceptible to bone-marrow micronucleus induction than males with both quantitative and qualitative sex differences noted, depending on dose, regimen and sample.

Mutagenicity, metabolism, and DNA interactions of urethane

Urethane, a known animal carcinogen, has been the subject of intensive research efforts spanning 40 years. Recent concerns have focused on the presence of urethane in a variety of fermented foods and alcoholic beverages, although no epidemiological studies or human case reports have been published. Much information is available about the mutagenesis, metabolism, and DNA interactions of urethane in experimental systems. Urethane is generally not mutagenic in bacteria although in some instances it acts as a weak mutagen. Urethane is not mutagenic in Nuerospora but is weakly mutagenic in Saccharomyces. Drosophila appear to be the only organisms that consistently give positive mutagenic results with urethane, but its mutagenicity is weak and in many cases shows no clear dose dependence. Urethane is a good clastogen in mammalian somatic cells in vivo, but it shows variable results with cells in vitro. It efficiently induces sister chromatid exchanges in a variety of cells. Mammalian spermatogenic cells are insensitive to the induction of specific locus and dominant lethal mutations by urethane. Mutational synergism has been reported to occur between ethyl methanesulfonate and urethane when administered two generations apart, and some investigators have suggested possible synergism for cancer-causing mutations in mice exposed to X-rays and urethane one generation apart. These studies are controversial and have not been confirmed. Studies on the induction of cancer-causing dominant mutations by urethane are at variance with results from extensive studies with the specific locus test in mice. Urethane studies with the unscheduled DNA synthesis assay in mouse spermatogenic cells and with the sperm abnormality test have given negative results. Urethane is rapidly and evenly distributed in the body. The rate of elimination of urethane from plasma is a saturable process and varies according to the strain and age of the animal. Recent studies have concentrations similar to those in wine, ethanol inhibits the tissue distribution of urethane in mice. These results are important because they suggest a lower carcinogenic/mutagenic risk than expected from exposure to urethane in alcoholic beverages. Although research on the metabolic activation of urethane has been extensive, no conclusive results have been obtained about its active metabolite, at one time thought to be N-hydroxyurethane. More recently, it has been postulated that urethane is activated to vinyl carbamate and that this metabolite is capable of reacting with DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Chlorambucil effectively induces deletion mutations in mouse germ cells

The chemotherapeutic agent chlorambucil was found to be more effective than x-rays or any chemical investigated to date in inducing high yields of mouse germ-line mutations that appear to be deletions or other structural changes. Induction of mutations involving seven specific loci was studied after exposures of various male germ-cell stages to chlorambucil at 10-25 mg/kg. A total of 60,750 offspring was scored. Mutation rates in spermatogonial stem cells were not significantly increased over control values; this negative result is not attributable to selective elimination of mutant cells. Mutations were, however, clearly induced in treated post-stem-cell stages, among which marked variations in mutational response were found. Maximum yield occurred after exposure of early spermatids, with approximately 1% of all offspring carrying a specific-locus mutation in the 10 mg/kg group. The stage-response pattern for chlorambucil differs from that of all other chemicals investigated to date in the specific-locus test. Thus far, all but one of the tested mutations induced by chlorambucil in post-stem-cell stages have been proved deletions or other structural changes by genetic, cytogenetic, and/or molecular criteria. Deletion mutations have recently been useful for molecular mapping and for structure-function correlations of genomic regions. For generating presumed large-lesion germ-line mutations at highest frequencies, chlorambucil may be the mutagen of choice.