Spectra of gpt mutations in ethylnitrosourea-treated and untreated transgenic mice

We have established a new transgenic mouse mutagenicity assay for the efficient detection of point mutations and deletions in vivo (Nohmi et al. [1996] Env. Mol. Mutagen. 28:465-470). In this assay, the gpt gene of Escherichia coli is used as a reporter for the detection of point mutations. Treatment of mice with ethylnitrosourea (ENU, 150 mg/kg) enhances by several-fold the mutant frequency of gpt in bone marrow. Here, we report the mutation spectra of the gpt gene recovered from bone marrow of ENU-treated and untreated transgenic mice. In the gpt mutants rescued from ENU-treated mice, more than 90% of the mutations were base change mutations; the predominant types were A:T to T:A transversions and G:C to A:T transitions. On the contrary, in the mutants rescued from untreated mice, 54% were base substitutions and the remainders were short deletions and insertions. Among untreated mice, the most frequently observed base substitution was G:C to A:T transitions (7/14 mutants). Three of these occurred at 5'-CpG-3' sites. Interestingly, the mutation spectra of the gpt gene were different from those of the gpt gene in ENU-treated and untreated E.coli, whereas they were similar to those of the lacZ and lacI genes in ENU-treated and untreated other transgenic mice or cultured mammalian cells. We also report the establishment of homozygous transgenic mice that have transgene lambdaEG10 DNA in both chromosome 17 of C57BL/6J mouse.