Dose-related changes in the profile of ras mutations in chemically induced CD-1 mouse liver tumors

We investigated the role od dosing regimen on ras mutationsin chemically induced CD-1 mouse liver tumors. The spectra ofras gene mutations in liver tumors that were induced by 15 dailyi.p. injections of 7,12-dimethyl-benz[a]anthracene (DMBA), 4-aminoazobenzene(AAB), N-hydroxy-2-acetylaminofluorene (N-OH-AAF) or N-nitrosodiethylamine(DEN) were compared to those previously obtained for tumorsinduced by a single but higher dose of each carcinogen. Theprincipal assay used was a direct tumor analysis involving sequencingof polumerase chain reaction (PCR)-amplified tumor DNA; additionalmutations that were present in only a small fraction of tumorcells were detected using a transfection assay or a PCR-engineeredrestriction fragment length polymorphism method. Spontaneousliver tumors had a relatively low frequency of ras mutations,all found in Ha-ras codon 61, and most of these mutations werepresent in only a small fraction of tumor cells. With the exceptionof multiple-dose DEN, each group of single- and multiple-dosecarcinogen-induced tumors exhibited a higher frequency of rasmutations compared with spontaneous tumors. For AAB, N-OH-AAFand DEN, the dosing regimen was found to affect significantlythe profile of rasmutations. For each of these carcinogens,the multiple-dose tumor group (versus single-dose group) hadfewer Ki-ras codon 61 (c