Mutagenesis of a single AT basepair in mice transgenic for PhiX174 am3 cs70 I. Spleen and testis

Mutations induced in a single AT base pair were studied in spleen and testis by using mice transgenic for PhiX174 am3, cs70 and ethylnitrosourea (ENU) as the mutagen. The transgenic mice were produced on the C57BL6/J background. The line (am54), which carries 50 copies of PhiX per haploid genome integrated in a tandem array, was selected for experimental use and was maintained by random breeding. The animals for mutagenesis studies were produced by mating homozygous am54 males to wildtype C57BL6/J females. Hemizygous male offspring (8 to 10 weeks old) from this cross were injected i.p. with 150 mg ENU per kg and were euthanized 3, 10 or 110 days after treatment. The spontaneous revertant frequency in the spleen was 1.42 x 10(-6) per plaque forming unit (pfu) and in the testis it was 1.41 x 10(-6) per pfu. There was no significant difference between the two tissues. In spleen, it was not until 110 days after ENU treatment that the average revertant frequency among treated animals was significantly higher than the revertant frequency among the control animals. In spleen, the induced frequency of basepair substitutions in the center AT basepair in the am3 nonsense codon was 2 x 10(-6). Also at this post-injection interval the variance of revertant frequencies in the spleen was not different from control variance. In testis, the average revertant frequency 110 days post ENU injection was not significantly different from the control. However, two important observations were made regarding the testis data. First, one animal had a significantly increased revertant frequency 110 days after ENU treatment in comparison to the other four animals in the group that had revertant frequencies equal to or lower than the average control frequency. Second, the variance of revertant frequencies in the testis among the treated animals increased as the post injection period increased. Taken together, these observations may indicate that the revertants formed large clusters in one testis sample.