Lack of induction of dominant lethals in mice by orally administered AF-2

Assessing overdispersion and dose-response in the male dominant lethal assay

In dominant lethal studies the primary variables of interest are typically expressed as discrete counts or proportions (e.g., live implants, resorptions, percent pregnant). Simple statistical sampling models for discrete data such as binomial or Poisson generally do not fit this type of data because of extra-binomial or extra-Poisson departures from variability predicted under these simple models. Extra-variability in the fetal response may originate from parental contributions. These can lead to over- or under-dispersion seen as, e.g., extra-binomial variability in the proportion response. Utilizing a large control database, we investigated the relative impact of extra-variability from male or female contributions on the endpoints of interest. Male-related effects did not seem to contribute to overdispersion in our database; female-related effects were, however, evidenced. Various statistical methods were considered to test for significant treatment differences under these forms of sampling variability. Computer simulations were used to evaluate these methods and to determine which are most appropriate for practical use in the evaluation of dominant lethal data. Our results suggest that distribution-free statistical methods such as a nonparametric permutation test or rank-based tests for trend can be recommended for use.

X-ray- and chemically induced germ-line mutation causing phenotypical anomalies in mice

A large and significant increase of phenotypical anomalies was observed in the progeny of ICR parent mice treated before mating with X-rays, urethane, 7,12-dimethylbenz[a]anthracene, ethylnitrosourea (ENU), and 4-nitroquinoline 1-oxide, but the increase was not significant with furylfuramide. Major types of induced anomalies were cleft palate, dwarf, open eyelid, tail anomalies, and exencephalus. Dwarf, open eyelid and tail anomalies were predominant types of viable anomalies and were inherited as if they were dominant mutations with varying expressivity or penetrance. Incidence of prenatal anomalies increased with treated doses of X-rays, urethan, or ENU for both spermatozoa and spermatogonia. Spermatogonia were less sensitive to X-rays and urethane than spermatozoa, while ENU induced a very high incidence of prenatal anomalies by the spermatogonial treatment. In contrast to the previous works with X-rays, there was a clear, almost linear increase of anomalies in the dose range from 0 to 216 rad after spermatogonial exposure. For treatment of oocytes, there was also a clear increase with doses of X-rays and urethane. Doubling doses of X-rays for prenatal anomalies were 12 rad for spermatozoa, 27 rad for spermatogonia, and 19 rad for mature oocytes. These values are similar to those for ordinary mouse mutations. However, the mean rate of prenatal anomalies per rad (1.2 X 10(-4), 6.6 X 10(-5) and 9.1 X 10(-5) for spermatozoa, spermatogonia and mature oocytes, respectively) and that for 1 micrograms/g of ENU (3.4 X 10(-4) for spermatogonia) were 4-40 times higher than that of ordinary mutation in mice, because overall phenotypical abnormalities were scored in this study. Information obtained from the work on phenotypical anomalies is valuable to assess genetic risk of radiation and chemicals, because a majority of human genetic diseases show this kind of irregular and uncertain inheritance and most of the induced anomalies are similar to those found in humans.

Deactivation of furyl furamide (AF-2) by rat-liver microsomes and its implication in short-term tests for mutagenicity/carcinogenicity

The genetic activity of AF-2 in both bacteria and yeast rapidly disappeared in the presence of rat-liver microsomal fraction (S9 mix). Incubation of AF-2 with S9 mix even for 10 min at 37 degrees C was sufficient to inactivate it completely. Data available in the literature suggest that activation of AF-2 is necessary for its geno-toxic effect. The activation step may involve reduction of the nitrofuran to an amino group probably by the enzyme reductase I. Most cultured cell systems, such as bacteria, yeast, Neurospora, mammalian cells and human lymphocytes, can probably bring about this reduction. However, the rapid disappearance of the genetic activity of AF-2 in the presence of rat-liver homogenate suggests that rat-liver microsomes may further metabolize the reduction products to inactive forms. Thus, it becomes necessary to test even those chemicals that are mutagenic per se, with mammalian microsomal preparations before their mutagenic/carcinogenic potentialities can be assessed in short-term tests.

Determination of radiation equivalence of the chemical furylfuramide (AF-2) for the induction of gene conversion in diploid yeast and estimation of genetic risk to the Japanese population

Dose--effect relationships for the induction of gene conversion by AF-2 in diploid yeast have been established. On the basis of these, the rec value for AF-2 for induction of gene conversion has been calculated to be 0.085 microgram/ml.h. This compares well with the rec values obtained with other test systems and other genetic end-points. The genetic burden to the Japanese population due to consumption of AF-2 is estimated to be equivalent to 55 millirec. However, uncertainties in the consumption of AF-2 per person and its metabolic detoxication tend to lower this value to, probably, a small fraction of that due to natural background ionizing radiation.

The distribution of fetal death in control mice and its implications on statistical tests for dominant lethal effects

In dominant lethal testing fetal death is generally assumed to follow either a Poisson or binomial distribution. However, both of these models were found to be inappropriate when three large sets of mouse control data and other data sets from the literature were examined. The validity of statistical test procedures based on these inappropriate models was then studied in detail. It was found that chi-square tests (which assume an underlying binomial distribution) may seriously exaggerate the level of significance and hence should not be used. In contrast, the inappropriateness of the underlying Poisson or binomial model appeared to have little effect on the validity of pairwise comparisons by analysis of variance procedures. Unlike chi-square, these procedures regard the pregnant female rather than the individual implant as the experimental unit. However, a statistical analysis of dominant lethal data generally involves more than a series of pairwise comparisons, and it is unclear how an invalid underlying model may affect statistical test procedures in this more complex situation. Moreover, it is difficult to justify the use of statistical models that are demonstrably invalid when a reasonable alternative exists. Thus, until a satisfactory parametric model can be found and appropriate test procedures derived, we prefer to analyze dominant lethal data by non-parametric (distribution-free) methods. Proportion of dead implants per female appears to be a more meaningful measure of fetal death than number of dead implants per female for several seasons which include (1) analyses based on proportions take the total number of implants per female into account and (2) analyses based on proportions make more reasonable assumptions concerning pre-implantation losses and are more powerful when such losses occur. Despite our concern with the appropriateness of the underlying model, in practice we have found few instances in which non-parametric and analysis of variance procedures have led to markedly different conclusions.

Genetic activity of the antimicrobial food additives AF-2 and H-193 in Saccharomyces cerevisiae

The genetic activity of the antimicrobial food additives AF-2 and H-193 has been investigated in Saccharomyces cerevisiae. The strains chosen for the present studies were D5 for the induction of mitotic recombinational events and XV185-14C for the induction of reversion of the mutants lys1-1, his1-7 and homs3-7. When three concentrations (25, 50 and 100 mug/ml) of AF-2 were used in the reversion system of strain XV185-14C, there was an increase in the frequency of hom+ and his+ revertants as a function of incubation time, while the lysine mutant exhibited a very low frequency of induced reversion. When AF-2 and H-193 were compared at the same concentration and exposure time, AF-2 exhibited a higher genetic activity in both systems than H-193. However, H-193 was genetically more active in inducing revertants than AF-2, when the comparison was made at the same survival level. Cells of both haploid and diploid strains were found to be more sensitive to inactivation by AF-2 than by H-193 It should be pointed out that the solubility of H-193 was lower (about 4 mug/ml saturation) than the solubility of AF-2 (120 mug/ml saturation). The haploid strain was more sensitive to both compounds than the diploid strain.