Dose-dependent ras mutation spectra in N-nitrosodiethylamine induced mouse liver tumors and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone induced mouse lung tumors

In a previous study, the spectrum of H-ras mutations detectedin B6C3F1 mouse liver tumors induced by 5, 50 or 150 µmol/kgbody wt of N-nitrosodiethylamine (NDEA) was similar to thatin spontaneous B6C3F1 mouse liver tumors, suggesting that activationof the H-ras gene in NDEA-Induced mouse liver tumors may notbe the direct result of the chemical interaction with the H-rasgene. In the present study, mutations in the H-ras oncogenefrom B6C3F1 mouse liver tumors induced by 5 or 50 µmol/kgbody wt of NDEA were characterized by DNA amplification withpolymerase chain reaction (PCR), single-strand conformationpolymorphism (SSCP) and direct sequence analysis. Twenty-oneof 66 NDEA-induced B6C3F1 mouse liver tumors contained activatedH-ras gene with 2 of 21 having a CG to AT transversion at thefirst base of codon 61, 17 of 21 having AT to GC transitionand 2 of 21 having an AT to TA transversion at the second baseof codon 61 in the H-ras gene. The predominant mutation, ATto GC transition (17/21, 81%) is consistent with the formationof O⁴-ethylthymine adduct, and is distinct from the predominantCG to AT transversion (50%) at the first base of codon 61 detectedin H-ras gene from NDEA-induced B6C3F1 mouse liver tumors ina previous study by Stowers et al. Mutations in the K-ras oncogenefrom 59 A/J mouse lung tumors induced by 0.53 mmol/kg body wtof 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) werealso characterized by using the above mentioned methods. Forty-sixof 59 NNK-induced A/J mouse lung tumors contained activatedK-ras genes. All 46 (100%) of the activated K-ras gene had GCto AT transitions at the second base of codon 12. The same mutationwas observed in 70% (7/10) of the K-ras oncogene from A/J lungtumors induced by 4.8 mmol/kg body wt (given in 21 doses) ofNNK. These data suggest that other factors in addition to genotoxiceffect might be involved in the induction of rodent tumors bysome carcinogens when given at higher doses. Therefore, furtherstudies to compare the dose-dependent differences in the profileof ras mutations induced by chemical carcinogens may help toassess human cancer risk. Mutation(s) in exons 5-8 of the p53gene was not found in these NDEA-induced mouse liver tumorsand NNK-induced mouse lung tumors.