Spontaneous and ENU-induced mutation spectra at the cII locus in Big Blue Rat2 embryonic fibroblasts

Big Blue Rat2 embryonic fibroblasts carry the lambda-Liz shuttle vector which is also present in the Big Blue mouse and rat. Mutations in the Big Blue systems have most often been measured at the lacI locus. However, a method for positive selection of mutations at the lambda cII locus was recently described. This assay appears to have many advantages over the use of lacI as a mutational target, but it has yet to be well characterized in mammalian mutagenesis studies. The objective of these studies was to determine the spontaneous and ethylnitrosourea (ENU)-induced mutant frequencies (MFs) and mutational spectra at cII using Big Blue Rat2 embryonic fibroblasts. The average spontaneous MF was 13 +/- 1.4 x 10(-5). The average induced MF was 60 +/- 10 x 10(-5) 10 days following a 30 min treatment with 0.1 mg/ml ENU. Eighty four independent spontaneous mutants were sequenced: 23 (27.4%) were frameshift mutations and 61 (72.6%) were base substitutions. Two spontaneous frameshift hotspots were detected, both in mononucleotide runs. G:C-->A:T transitions were the most common type of base substitution in cII; of these 71% occurred at CpG sites. The ENU-induced mutational spectrum at cII (44 mutants) consisted of 42 base substitutions (95.5%) and two -1 frameshift mutations (4.5%). Compared with the spontaneous spectrum, the ENU-induced spectrum had significantly fewer frameshift mutations (4.5 versus 27%) and base substitutions occurred predominantly at A:T base pairs (71 versus 34%). Overall, the spontaneous cII mutational spectrum reported here differs slightly from spontaneous spectra reported at the Big Blue lacI locus, but the mutational spectra and base substitution MFs following treatment with ENU were comparable at both loci. These data support the continued use of cII as a selectable marker in mutagenesis studies involving cells or tissues that carry a lambda transgene.