Germ cell mutations in mice.

The most extensive set of information on the factors affecting mutation induction by chemical mutagens in mice has been obtained by use of the dominant lethal assay and the specific locus method. Dominant lethal mutations are caused by chromosomal aberrations, and they occur shortly before or after implantation. The specific locus method can be used to detect a variety of mutational events, ranging from intralocus changes to multilocus deletions. The scoring of recessive specific locus mutations can be combined with the detection of dominant cataract mutations in mice. With these methods, the following factors that influence the induced mutation rates have been identified: differential spermatogenic response, fractionation of doses (low-dose effects), different treatment conditions (combined treatment), different strains, different sexes, and differences in the response of recessive and dominant mutations. The importance of these factors for the evaluation of chemical mutagens has not generally been appreciated in genetic toxicology. However, they are important for the risk assessment of a chemical mutagen. In addition, the induction of specific locus mutations and of inherited dominant cataract mutations or dominant mutations affecting the skeleton of mice can be used to quantify human genetic risk due to a mutagen

Germ-cell mutations in mice: Standards for protecting the human genome.

In my presentation I used some selected examples to demonstrate the importance of germ-cell mutation studies in mice for the evaluation of a chemical mutagen. I presented several examples of the differential spermatogenic response. For statistical evaluation of a chemical mutagen, for example DES, and risk assessment it is critical in which germ-cell stages the mutations are induced. For the fractionation effect an example was selected, in which the effects of multiple small doses were strictly additive. This statement is correct for MMS and a fractionation interval of 24 or 48 h. The time interval is here the critical point. Russell et al. (1982) found a pronounced reduction of the mutation frequency in specific-locus experiments after 10 x 10 mg/kg of ENU given at weekly intervals, whereas we observed no differences in the mutation rate after a single injection of 160 mg/kg of ENU or fractionated injection of 2 x 80 mg/kg of ENU (Favor et al., 1989) or 4 x 40 mg/kg of ENU (Ehling and Neuhauser-Klaus, 1984) when the interval for the i.p. injection was 24 h. The results of pretreatment with cyclophosphamide followed 24 h later by radiation exposure are of special importance, because we have too little information on the effects of combined mutagen exposure. The synergistic effect observed in these experiments is due to the inhibition of the DNA-repair process for radiation-induced specific-locus mutations in mice. The radiation-induced mutation rate of specific loci in mice was independent of the genetic background. However, a significant difference in the mutation rate of the 4 loci was observed in an ENU experiment. In DBA/2 males the mutation rate was 37.1 x 10-5 mutations/locus/gamete and in the (101/El x C3H/El)F1 males the mutation rate was 63.8 x 10-5 mutations/locus/gamete. It is very likely that strain-related different repair capacities of spermatogonia or detoxification are the main causes for the differential mutability. The sex differences are striking. In contrast to radiation, chemical mutagens show different patterns for the induction of mutations. The last point I tried to make concerns the differences in the yields of recessive and dominant mutations. In addition, I used the frequency of dominant mutations to demonstrate the possibility of quantifying the genetic risk and I used the recessive mutations to indicate the usefulness of the doubling-dose concept for genetic toxicology

Mutations in the F1-Generation of Mice.

The occurrence of induced dominant genetic damage can be measured by comparing mutation frequencies in first generation descendants from treated and untreated populations, but for many characters, it is difficult to distinguish between the effects of newly occurring genetic damage and the within-strain variation. This problem has been solved for skeletal abnormalities and for dominant cataract mutations. The skeleton was chosen because it is formed over an extended period of development and is, therefore, presumably subject to modification by gene action expressed during a wide range of time. The direct comparability of the genetic endpoint of mice and men was one aspect which led us to initiate a systematic investigation of the induction of dominant cataracts, an opacity of the lens, in the mouse. Another aspect was that Ehling (1976) developed a concept for the direct estimation of the risk of radiation induced genetic damage to the human population expressed in the first generation, based on the induction of dominant mutations in mammals. Using the direct method we estimated that 20-50 dominant radiation induced mutations would be expected in 19,000 offspring born to parents exposed in Hiroshima and Nagasaki, but only a small proportion of these mutants would have been detected with the techniques used in the population study. The detection of dominant cataract mutations can be combined with the detection of recessive specific locus mutations in mice. The specific locus method consists of mating treated wild-type mice to mice homozygous for seven recessive marker loci and scoring in the first generation offspring for mutations at any of these marked loci. The advantage of a combined investigation of dominant cataract mutations and specific locus mutations is at least three-fold: 1st The number of scorable mutations is increased by a factor of four. 2nd The combined investigation allows the comparison of the mutation frequency of selected and unselected loci. 3rd In the same experiment the frequency of mutations with a dominant and a recessive mode of expression can be compared. The specific locus method was used to investigate the effects of chemicals and radiation on the mutation frequency with respect to the following factors: differential spermatogenic response; changes of the mutation spectrum with different doses of chemicals; comparison of the mutation rates under different treatment conditions; sex and strain differences. Using this method we will demonstrate the possibility to quantify the genetic risk of chemical mutagens. In addition, the per locus ratio of radiation induced dominant to recessive mutations in spermatogonia of the mouse is approximately 1:10.(ABSTRACT TRUNCATED AT 400 WORDS