SEQUENCE SPECTRA OF SPONTANEOUS LACZ GENE-MUTATIONS IN TRANSGENIC MOUSE SOMATIC AND GERMLINE TISSUES

As a critical step in determining whether transgenic mouse gene mutati on systems are suitable models for the detection and quantification of induced gene mutations in vivo, spontaneous mutant frequencies and mu tation spectra have been characterized for liver, bone marrow, and mal e germ cells of the lacZ transgenic mouse strain 40.6. The lacZ transg ene is carried on a recombinant bacteriophage lambda shuttle vector th at is recovered from mouse genomic DNA, and analysed in vitro for muta tions that occurred in the mouse tissues. Mutations are detected visua lly as clear or pale blue plaques when X-gal is the substrate for beta -galactosidase; whereas, the wild-type plaques are dark blue. There wa s no statistical difference in the mutant frequency among the three ti ssues studied, the pooled mutant frequency being 2.23 +/- 0.41 per 10( 5) pfu. The predominant type of mutation was GC --> AT transitions, wi th most occurring in 5'-CpG dinucleotides, suggesting that the deamina tion of 5-methylcytosine is the main mechanism of mutagenesis. There w as, however, a statistically significant difference in the base pair s ubstitution mutation spectrum for the liver and bone marrow when mutat ions were grouped according to GC or AT base-pairs. The proportion of transition versus transversion mutations was also statistically differ ent among the three tissues, resulting mainly from the fact that germ cells were different from both bone marrow and liver. A lower number o f spontaneous transitions in male germ cells was accompanied by an inc rease in transversions, with the proportion of GC --> AT transitions i n 5'-CpG sites also declining. Only nine of the 66 independent mutatio ns sequenced were deletions, ranging from 1 to 553 bp. The relatively low proportion of deletions appears to reflect the deletion size limit ations of bacteriophage-based systems. Most deletions had short repeat or duplicate base sequences at their termini or were in monotonic bas e repeats, which suggests strand slippage as the mechanism for their o ccurrence.