Hamster and rat fetal cells have low spontaneous mutation frequencies and rates

Somatic cells of whole Syrian hamster fetuses (gestation day 13) were isola ted and tested by an in vivo/in vitro mutation assay for spontaneous mutati on frequencies using independent 6-thioguanine (6-TG), diphtheria toxin (DT ), and ouabain mutation selection systems. Optimum conditions were ascertai ned. For 6-TG mutants, a total of 21 mutants were found in cells from 24 li tters on 1993 plates, for an overall mutant frequency of 1.8 x 10(-7) per v iable cell with 12 positive litters. In all, 26 litters were tested using D T; 77 mutants were found in 840 plates, yielding an overall mutant frequenc y of 2.6 x 10(-7), with 20 positive Litters. No correlations or familial ef fects were found among 23 Litters tested for both DT and 6-TG. Of 14 litter s which were tested for ouabain mutants, 4 were positive, with a total of 5 mutants found on 988 plates, for an overall mutant frequency of 7.6 x 10(- 8). For 14 F344 rat fetuses, the overall 6-TG spontaneous mutation frequenc y was determined to be 1.6 x 10(-7). From the data, estimates of mutation r ates were calculated. For mutation to 6-TG resistance the rate was 8.3 x 10 (-8) for mutation to DT resistance the rate was 8.1 x 10(-8) and for ouabai n, the spontaneous mutation rate was 5.7 x 10(-8). For F344 rat, the sponta neous mutation rate was 1.1 x 10(-7). Induced mutant frequencies after in u tero exposure to 1 mmol/kg N-ethyl-N-nitrosourea (ENU) were 311, 135 and 20 0 times the spontaneous value for 6-TG, DT and ouabain, respectively, for S yrian hamster fetal cells and 125 times the spontaneous 6-TG value for feta l F344 rat cells. Both spontaneous mutation frequencies and underlying spon taneous mutation rates are low, consistent with the view that fetal cells e xercise extremely tight control over DNA fidelity. Published by Elsevier Sc ience B.V.