Spontaneous and ethylnitrosourea-induced mutation fixation and molecular spectra at the lacI transgene in the Big Blue rat-2 embryo cell line.

Big Blue Rat-2 cells were evaluated for mutagenesis and mutational spectra (spontaneous and ethylnitrosourea [ENU]-induced). Survival, mutant frequency, population doubling time, and kinetics of mutant increase (to 120 hr) were determined. Exposures were 100, 200, 400, 600, and 1,000 micrograms ENU/ml. The spontaneous mutant frequency was similar to that previously reported in vivo, i.e., 5 X 10(5). Dose-related increases in mutant frequency were observed following ENU treatment. Kinetics (time course) of mutant frequency increase, population doubling, and mutational spectra were investigated following treatment at 1,000 micrograms ENU/ml. Among 39 spontaneous mutants, 26 independent mutations were found as follows: nine (34.6%) G:C-->A:T transitions (five at CpG sites), six (23%) G:C-->T:A transversions, three (11.5%) G:C-->C:G transversions (two at CpG sites), two (7.7%) frameshifts, five (19%) deletions or insertions, and one (3.8%) complex (deletion+insertion) mutation. Among 46 ENU-induced mutants, 37 independent mutations (all base substitutions) were found as follows: 15 (40.5%) G:C-->A:T transitions (four at CpG sites), five (13.5%) A:T-->G:C transitions, four (10.8%) G:C-->T:A transversions, 11 (30%) A:T-->T:A transversions, and two (5.4%) A:T-->C:G transversions. Nearly 50% of the base substitutions in the ENU-treated cells were at A:T base pairs, in contrast to the spontaneous mutants where none was found. Both the spontaneous and the ENU-induced mutational spectra were similar to that reported in vivo and for other cells. An important aspect of the experiment is that all mutations sequenced following ENU treatment (1,000 micrograms/ml) occurred under conditions which our experiments show corresponded to very little mitotic activity.