Mutations in liver DNA of lacI transgenic mice (Big Blue) following subchronic exposure to 2-acetylaminofluorene

2-Acetylaminofluorene (2-AAF) was administered at levels of 0, 300 and 600 ppm in the diet for 28 days to female transgenic mice bearing the lacI gene in a lambda vector (Big Blue mice). The lambda vector was excised from liver DNA and packaged in vitro into bacteriophage particles which were allowed to infect E. coli bacteria, forming plaques on agar plates. Approximately 10(5) plaques were screened per animal for the appearance of a blue colour, indicative of mutations in the lacI gene which had resulted in an inactive gene product. Background mutation rate was 2.7 x 10(-5) (pooled results of two animals, 8 mutant plaques/289,530 plaques). At 300 ppm in the diet, the rate of 3.5 x 10(-5) (8/236,300) was not significantly increased over background. At 600 ppm in the diet, the rate increased approximately 3 fold to 7.7 x 10(-5) (17/221,240). In comparison to the usual single or 5-day carcinogen exposure regimes, the 4-week exposure protocol allowed the use of much lower dose levels (10-1000 fold lower). Overt toxicity could thus be avoided. The daily doses used were somewhat higher than those required in 2-year carcinogenicity studies with 2-AAF.