MUTAGENESIS OF A SINGLE AT BASEPAIR IN MICE TRANSGENIC FOR PHIX174-AM3 PHIX174-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 ethylnitroso urea (ENU) as the mutagen. The transgenic mice were produced on the C5 7BL6/J background. The line (am54), which carries 50 copies of PhiX pe r haploid genome integrated in a tandem array, was selected for experi mental use and was maintained by random breeding. The animals for muta genesis studies were produced by mating homozygous am54 males to wildt ype C57BL6/J females. Hemizygous male offspring (8 to 10 weeks old) fr om this cross were injected i.p. with 150 mg ENU per kg and were eutha nized 3, 10 or 110 days after treatment. The spontaneous revertant fre quency in the spleen was 1.42 x 10(-6) per plaque forming unit (pfu) a nd in the testis it was 1.41 x 10(-6) per pfu. There was no significan t difference between the two tissues. In spleen, it was not until 110 days after ENU treatment that the average revertant frequency among tr eated animals was significantly higher than the revertant frequency am ong 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 rever tant frequencies in the spleen was not different from control variance . In testis, the average revertant frequency 110 days post ENU injecti on was not significantly different from the control. However, two impo rtant observations were made regarding the testis data. First, one ani mal had a significantly increased revertant frequency 110 days after E NU 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 incr eased. Taken together, these observations may indicate that the revert ants formed large clusters in one testis sample. (C) 1998 Elsevier Sci ence B.V.