Sequenc spectra of spontaneous lacZ gene mutations in trransgenic mouse somatic and germline tissues

As a critical step in determining whether transgenic mouse gene mutation systems are suitable models for the detection and quantification of induced gene mutations in vivo, spontaneous mutant frequencies and mutation spectra have been characterized for liver, bone marrow, and male germ cells of the lacZ transgenic mouse strain 40.6. The lacZ transgene is carried on a recombinant bacteriophage λ shuttle vector that Is recovered from mouse genomic DNA, and analysed in vitro for mutations that occurred in the mouse tissues. Mutations are detected visually as clear or pale blue plaques when X-gal is the substrate for β-galactosidase; whereas, the wild-type plaques are dark blue. There was no statistical difference in the mutant frequency among the three tissues studied, the pooled mutant frequency being 2.23 ± 0.41 per 105 pfu. The predominant type of mutation was GC → AT transitions, with most occurring in 5′-CpG dinucleotides, suggesting that the deamination of 5-methyl-cytosine is the main mechanism of mutagenesis. There was, however, a statistically significant difference in the base pair substitution mutation spectrum for the liver and bone marrow when mutations were grouped according to GC or AT base-pairs. The proportion of transition versus transversion mutations was also statistically different among the three tissues, resulting mainly from the fact that germ cells were different from both bone marrow and liver. A lower number of spontaneous transitions in male germ cells was accompanied by an increase in transversions, with the proportion of GC → AT transitions in 5′-CpG sites also declining. Only nine of the 66 independent mutations sequenced were deletions, ranging from 1 to 553 bp. The relatively low proportion of deletions appears to reflect the deletion size limitations of bacteriophage-based systems. Most deletions had short repeat or duplicate base sequences at their termini or were in monotonic base repeats, which suggests strand slippage as the mechanism for their occurrence.