Analysis of results from a collaborative study of the dominant lethal assay

Effect of disulfiram on the genotoxic potential of acetaldehyde in mouse spermatogonial cells

The initial purpose of the study was to determine the potential of acetaldehyde (Ace) to increase the rate of sister-chromatid exchanges (SCEs) in mouse spermatogonia. We tested four doses of Ace (from 0.4 to 400.0 mg/kg), including a negative and a positive control group (distilled water and cyclophosphamide, respectively). The results showed that all tested doses were SCE inducers. The highest tested dose increased the control level more than three times. Also, the cumulative frequencies of SCEs per cell were higher in the Ace-treated animals than in the control cells. Ace is transformed into acetate through the enzyme aldehyde dehydrogenase, a process that may be blocked by disulfiram (Dis) generating the accumulation of Ace. The second purpose of the study was to determine if the administration of Dis (150 mg/kg) could increase the SCE rate produced by non-genotoxic doses of Ace. (0.004 and 0.04 mg/kg). The animals treated with the two doses of Ace alone showed no increase in the frequency of SCEs; also, Dis by itself was not an SCE inducer. However, the groups of animals previously treated with Dis showed an increase of 31 and 60% with respect to the values obtained with the two doses of Ace alone. Furthermore, the cumulative frequencies of SCEs per cell were higher in the animals administered with both compounds together than in those treated with them separately. These results suggest the need to extend this type of study to other models.

Mutagenicity of anticancer drugs in mammalian germ cells

The evidence for mammalian germ cell mutagenicity induced by anticancer drugs is summarized. Primary attention is paid to the three major mouse germ cell mutagenicity tests- the dominant lethal, heritable translocation, and morphological specific locus tests- from which most germ cell mutagenicity data historically have been obtained. Of the 21 anticancer drugs reviewed, 16 have been tested in one or more of these three tests; with all 16 tested in the most common germ cell test, the male dominant lethal test, and 9 of the 16 also tested in the female dominant lethal test. The patterns of germ cell stage specificity for most of the anticancer drugs are similar, and generally resemble the patterns seen with other types of chemicals; however, some of the patterns are unique. For example, 2 of the 8 chemicals shown to induce dominant lethal mutations in female oocytes, do not induce dominant lethal mutations in male germ cells (adriamycin and platinol). Ten of the 16 chemicals tested in the dominant lethal test were positive in post-meiotic stages (spermatids through mature sperm), and seven also induced reciprocal translocations and/or specific locus mutations in post-meiotic stages. This propensity to induce mutations in post-meiotic stages has been observed with most mutagens. However, 5 of the anticancer drugs also induced dominant lethal mutations in spermatocytes (meiotic prophase cells) and one of them, 6-mercaptopurine, uniquely induced dominant lethal mutations exclusively in preleptotene spermatocytes. Finally, three of the anticancer drugs (melphalan, mitomycin C, procarbazine) are members of a very select group of chemicals shown to induce specific locus mutations in spermatogonial stem cells of mice. The implications for human risk are discussed.

Use of cyclophosphamide as a positive control in dominant lethal and micronucleus assays

Analyses of dominant lethal (DL) mutations and micronuclei (MN) are 2 important and widely used genotoxicity assays to measure drug-induced chromosome damage in germ cells and somatic cells, respectively. Cyclophosphamide (CP) has been widely used as a positive control in the single-dose mouse MN assay; however, its utility as a positive control for the DL assay has not been fully studied. In the present study, CP was tested in both assays under similar experimental conditions and MN seen in somatic tissue (bone marrow) were correlated with DL mutations seen in germinal tissue. In a dose-range finding study, groups of 5 male mice were dosed i.p. daily for 5 days at 0, 30 or 40 mg/kg CP and bone marrow was harvested 24 h later for MN assay. CP induced a dose-related increase (7- and 11-fold over control at 30 and 40 mg/kg) in micronucleated polychromatic erythrocytes (MNPCEs) and decreased %PCEs (to 60% and 54% of controls at 30 and 40 mg/kg, respectively). Based on this, a definitive DL and MN study was conducted using separate groups of 30 male mice at 0 and 40 mg/kg CP with a daily times 5 dosing regimen. For the MN assay, bone marrow was collected 24 h after the last dose from 5 animals and evaluated for MNPCEs and %PCEs. For the DL assay, each male was caged with 2 untreated females per week for 8 weeks to cover the postmeiotic germ cell stages. On day 17 after the initiation of breeding, the females were evaluated for the number of implantation sites and live, dead and resorbed implants. The results indicated that CP induced about a 17-fold increase in MNPCEs and a 46% decrease in PCEs in relation to controls. In the DL assay, CP produced a slight (13%) but statistically significant reduction in fertility index at week 7 of mating. Also, the total number of implants was significantly lower during weeks 1, 2, 3, 6 and 7 and the numbers of dead implants and postimplantation loss (PIL) were increased for weeks 1, 2 and 3 (55%, 71% and 34% PIL, respectively) over controls. These data clearly show that CP produced clastogenicity and some toxicity in both somatic tissue and germinal tissue. It was concluded that a dose of 40 mg/kg CP can be used as a positive control compound in the DL assay and in the multiple-dose marrow MN assay.

Cyclophosphamide: review of its mutagenicity for an assessment of potential germ cell risks

Cyclophosphamide (CP) is used to treat a wide range of neoplastic diseases as well as some non-malignant ones such as rheumatoid arthritis. It is also used as an immunosuppressive agent prior to organ transplantation. CP is, however, a known carcinogen in humans and produces secondary tumors. There is little absorption either orally or intravenously and 10% of the drug is excreted unchanged. CP is activated by hepatic mixed function oxidases and metabolites are delivered to neoplastic cells via the bloodstream. Phosphoramide mustard is thought to be the major anti-neoplastic metabolite of CP while acrolein, which is highly toxic and is produced in equimolar amounts, is thought to be responsible for most of the toxic side effects. DNA adducts have been formed after CP treatment in a variety of in vitro systems as well as in rats and mice using 3H-labeled CP. 32P-postlabeling techniques have also been used in mice. However, monitoring of adducts in humans has not yet been carried out. CP has also been shown to induce unscheduled DNA synthesis in a human cell line. CP has produced mutations in base-pair substituting strains of Salmonella tryphimurium in the presence of metabolic activation, but it has been shown to be negative in the E. coli chromotest. It has also been shown to be positive in Saccharomyces cerevisiae in D7 strain for many endpoints but negative in D62.M for aneuploidy/malsegregation. It has produced positive responses in Drosophila melanogaster for various endpoints and in Anopheles stephensi. In somatic cells, CP has been shown to produce gene mutations, chromosome aberrations, micronuclei and sister chromatid exchanges in a variety of cultured cells in the presence of metabolic activation as well as sister chromatid exchanges without metabolic activation. It has also produced chromosome damage and micronuclei in rats, mice and Chinese hamsters, and gene mutations in the mouse spot test and in the transgenic lacZ construct of Muta Mouse. Increases in chromosome damage and gene mutations have been found in the peripheral blood lymphocytes of nurses, pharmacists and female workers occupationally exposured to CP during its production or distribution. Chromosome aberrations, sister chromatid exchanges and gene mutations have been observed in somatic cells of patients treated therapeutically with CP. In general, there is a maximum dose and an optimum time for the detection of genetic effects because the toxicity associated with high doses of CP will affect cell division. In germ cells, CP has been shown to induce genetic damage in mice, rats and hamsters although the vast majority of such studies have used male mice.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on the effect of ethanol on dominant lethal mutations in Swiss, C57BL6 and CBA mice

Swiss, C57BL6 and CBA males were given 0.1 ml of 40% ethanol per mouse per day for three consecutive days, intraperitoneally. These males were mated with untreated virgin Swiss females employing a 4-day mating schedule and three consecutive matings were carried out. In another study, C57BL6 males were given an ascending gradient of 5% to 40% ethanol in drinking water for a total period of 11 weeks. These males were mated with C57BL6 females for 2 weeks. Females were dissected at mid-term pregnancy for the examination of uterine contents including total, live and dead implants. All the investigations comprised at least two or three independent experiments which were evaluated independently as well as after pooling the data. Swiss, C57BL6 and CBA males given 0.1 ml of 40% ethanol, intraperitoneally, gave no evidence of any significant increase in post-implantation lethality in the postmeiotic phase of spermatogenesis attributable to ethanol treatment. A moderate but significant reduction in mean total implants indicating pre-implantation losses was seen in Swiss but not in CBA mice. Prolonged feeding of ethanol up to 40% in drinking water failed to provide any evidence of dominant lethal mutations in C57BL6 males at the pre-implantation level and the post-implantation lethals were also not significantly higher than in controls. In Swiss mice, however, the mutagenic index based on both pre- and post-implantation lethality was consistently positive.

Statistical methods for analyzing developmental toxicity data

A description and review of methods for performing per-litter analyses involving extrabinomial proportion response is provided. It is stressed that the litter should be regarded as the appropriate experimental unit for quantitative analysis in studies for teratogenic or heritable mutagenic effects. Attention is directed at statistical identification of possible treatment effects, such as a positive dose response to a chemical stimulus. The methods range from distribution-free, nonparametric analyses to models involving parametric distributions such as the beta-binomial density. It is seen that most current methods require computer implementation. When concern is raised over misspecification of assumptions critical to the statistical analysis, it is argued that relatively parameter-free methods are appropriate for use. These include statistical bootstrapping and rank-based analyses.

The present lack of evidence for unique rodent germ-cell mutagens

Six chemicals, diethylhexyl phthalate (DEHP), ethanol, cyclohexylamine (CHA), sodium saccharin (NaS), cadmium chloride (CdCl2) and triflupromazine (TFP), were suggested to be unique germ-cell mutagens (Auletta and Ashby, 1988) by the GeneTox Workgroups of the U.S. Environmental Protection Agency (EPA). If this is a correct classification it would have major consequences when screening for mutagenicity and when labelling genotoxic substances. However, our re-evaluation of the GeneTox literature, including some more recent publications, has failed to find substantive evidence that any of these chemicals have been unequivocally established as having unique mutagenic activity in germ cells. For DEHP, NaS and TFP the evidence for genotoxic/mutagenic effects is questionable, in both germinal and somatic cells. Ethanol and CdCl2 showed clastogenic activity, but it was not restricted to germ cells. Both, ethanol and cadmium salts, appear to induce aneuploidy. The unconfirmed clastogenic effect of CHA was restricted to rats, but it occurred in both bone marrow and spermatogonia. Therefore, the general observation that rodent germ-cell mutagens are also genotoxic in somatic cells in vivo (Brusick, 1980; Holden, 1982) remains valid.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 15/1. Genetic effects of ethanol

Alcoholics have a higher frequency of chromosomal aberrations and sister-chromatid exchanges (SCEs) in their peripheral lymphocytes. In human and mammalian cells in vitro, ethanol generally does not induce genetic damage, but it induces SCEs in the presence of an exogenous metabolic system. In human lymphocytes in vitro, ethanol induces SCEs in the presence of alcohol dehydrogenase. In animals in vivo, ethanol induces a variety of genetic effects, including SCEs, micronuclei, dominant lethal mutations and aneuploidy in mouse eggs. There is some indication that ethanol may lead to genetic damage in sperm. In bacteria, ethanol is at best marginally active. Ethanol leads to anomalous chromosome segregation in Aspergillus, to mutations in yeast, to chromosomal aberrations and SCEs in plant root tips and to disturbances of meiosis and micronuclei in tetrads in Zea and Tradescantia respectively. The first metabolite of ethanol, acetaldehyde is mutagenic in a variety of test systems. The mutagenic activity of acetaldehyde in bacteria is questionable, but there is no doubt of its mutagenic activity in a variety of eukaryotic test systems in vitro as well as in vivo.

The use of a battery of strains of mice in a factorial design to study the induction of dominant lethal mutations

The induction of dominant lethality following oral dosing of males with 200 mg/kg of cyclophosphamide was investigated using a factorial experimental design. Males from 3 genotypes, BALB/c, CBA/Ca and CBA/Ca X C57BL/6JF1 hybrid (CBB6F1) were mated to 6 females of the same genotype as the males over 3 weeks. Cyclophosphamide reduced the mating frequency of the BALB/c and CBA/Ca males. The total number of implants/female was reduced in all 3 genotypes with the greatest effect in the first 2 weeks after the males were treated. The proportion of early deaths/litter was significantly increased in CBA/Ca and CBB6F1 but the increase was smaller and non-significant with BALB/c. There was a high incidence (29.8%) of early deaths in the control BALB/c females. Statistical analysis of the ratio of early deaths to total implants in a litter using either the Freeman-Tukey binomial or the arc-sine transformation gave similar and satisfactory results. Analysis of early death data rather then the ratio of early deaths: total implants would have led to misleading conclusions. The implications of the use of a factorial design in dominant lethal assays for the detection of strain variation in mutagenic response without an increase in animal usage is discussed.

Ethanol-induced late fetal death in mice exposed around the time of fertilization

In the mouse, all autosomal monosomies and trisomies are lethal by the time of birth (Searle, 1981). To test whether ethanol ingested by females shortly after mating induces nondisjunction, as reported by Kaufman (1983) on the basis of cytological evidence, we attempted to determine whether the incidence of intrauterine death was affected by this treatment. The incidence of late death (day 11 postconception or later) was found to be significantly increased when ethanol was administered 2 h following a 30-min mating period, but not when the interval was shorter. Measurements of early death were not sensitive enough (because of the high control frequency) to show an effect of ethanol treatment. Limited cytological data showed an induced incidence of trisomy in line with the excess frequency of late death, but the trisomy incidence by itself was not significantly different from control. The overall level of effect in the present experiment was lower than that reported by Kaufman.

A comparison of alternative distributions of postimplantation death in the dominant lethal assay

Statistical analysis of dominant lethal assay data has been the subject of much discussion, a large part of which has been concerned with the distributional form displayed by postimplantation embryonic deaths. The purpose of this study was to compare the fit of the frequently suggested distributions and some others, and to comment on the effect of distribution upon interpretation of results, using a particular set of dominant lethal assay data. The data set used included experiments involving a known mutagen (cyclophosphamide). For these experiments the parameters of the best fitting distributions were reviewed to consider how a dominant lethal effect is displayed.