RADIATION-INDUCED POINT MUTATIONS, DELETIONS AND MICRONUCLEI IN LACI TRANSGENIC MICE

Ionizing radiation induces gene mutations (point mutations, deletions and insertions) as well as chromosome damage in mammalian cells. Altho ugh these effects have been studied extensively in cells in culture, u ntil recently it has not been possible to analyze the mutagenic potent ial of ionizing radiation in vivo, especially at the molecular level. The development of transgenic mutagenesis systems has now made it poss ible to study the effects of ionizing radiation at both the molecular and chromosomal levels in the same animal. In this report we present p reliminary data on the response of Big Blue(TM) lad transgenic mice to ionizing radiation as measured by lad mutations and micronuclei. C57B l/6 transgenic mice were irradiated with Cs-137 gamma-rays at doses ra nging from 0.1 to 14 Gy, and expression times ranging from 2 to 14 day s. Dose-related increases in the mutant frequency were observed after irradiations with longer expression times. Mutant plaques were analyze d by restriction enzyme digestion to detect large structural changes i n the target sequence. Of 34 gamma-ray-induced mutations analyzed, 4 w ere large-scale rearrangements. 3 of these rearrangements were deletio ns within the lad gene characterized by the presence of short regions of homology at the breakpoint junctions. The fourth rearrangement was a deletion that extended from within the alpha lacZ gene into downstre am sequences and that had 43 bp of homology at the junction. These dat a indicate that the Big Blue(TM) lacI transgenic mouse system is sensi tive to the types of mutations induced by ionizing radiation. To deter mine whether the presence of the transgene affects micronucleus induct ion we compared the response of nontransgenic to hemizygous transgenic B6C3F1 mice and the response of nontransgenic to hemizygous and homoz ygous transgenic C57Bl/6 mice. The presence or absence of the lacI tra nsgene had no effect on spontaneous micronucleus frequencies for eithe r strain. However, radiation-induced micronucleus frequencies were sig nificantly higher in hemizygous lad B6C3F1 mice than in nontransgenic litter mates; the converse was true in C57Bl/6 mice. These data sugges t that the lad transgene does not cause chromosome instability as meas ured by spontaneous micronucleus levels. However, the response of thes e transgenic mice to a variety of clastogenic agents needs to be inves tigated before they are integrated into standard in vivo assays for ch romosome damage.