K-Ras mutant fraction in A/J mouse lung increases as a function of benzo[a]pyrene dose

Abstract

K-Ras mutant fraction (MF) was measured to examine the default assumption of low-dose linearity in the benzo[a]pyrene (B[a]P) mutational response. Groups of 10 male A/J mice (7- to 9-weeks old) received a single i.p. injection of 0, 0.05, 0.5, 5, or 50 mg/kg B[a]P and were sacrificed 28 days after treatment. K-Ras codon 12 TGT and GAT MFs in lung DNAs were measured using Allele-specific Competitive Blocker-PCR (ACB-PCR). The K-Ras codon 12 TGT geometric mean MF was 3.88 × 10-4 in controls, indicating an average of 1 mutation in every ∼1,288 lung cells. The K-Ras codon 12 TGT geometric mean MFs were as follows: 3.56 × 10-4; 6.19 × 10-4; 2.02 × 10-3, and 3.50 × 10-3 for the 0.05, 0.5, 5, and 50 mg/kg B[a]P treatment groups, respectively. The 5 and 50 mg/kg dose groups had TGT MFs significantly higher than did controls. Although 10 -5 is considered as the limit of accurate ACB-PCR quantitation, K-Ras codon 12 GAT geometric mean MFs were as follows: 8.38 × 10-7, 1.47 × 10-6, 2.19 × 10-6, 5.71 × 10 -6, and 8.99 × 10-6 for the 0, 0.05, 0.5, 5, and 50 mg/kg B[a]P treatment groups, respectively. The K-Ras TGT and GAT MFs increased in a B[a]P-dose-dependent manner, with response approximately linear over the 0.05 to 5 mg/kg dose range. K-Ras MF increased with B[a]P adduct burden measured for identical doses in a separate study. Thus, ACB-PCR may be useful in characterizing the shape of a dose-response curve at low doses and establishing relationships between DNA adducts and tumor-associated mutations.