Mechanism of H-ras Oncogene Activation in Mouse Squamous Carcinoma Induced by an Alkylating Agent¹

A mouse skin squamous cell carcinoma induced by topical application of the direct-acting alkylating agent β-propiolactone contains an activated H-ras oncogene with an A→T transversion at the second nucleotide of codon 61. The mutation was detected in NIH3T3 transfectant and original tumor DNA by an XbaI restriction enzyme polymorphism and confirmed by oligonucleotide “mismatch” hybridization. The mutation was not seen in the liver of the same animal. The activated oncogene also exhibited several restriction enzyme polymorphisms in transfectant DNA due to a reciprocal translocation 3′ to the coding region of the gene, which occurred during transfection. The activating mutation was found in only 1 of 6 β-propiolactone induced mouse skin tumors examined, the only tumor with a transforming H-ras oncogene. This is a much lower frequency of activation than that previously reported for the same tumor type induced by polycyclic aromatic hydrocarbons. The A→T transversion mutation is consistent with a potentially direct mutagenic effect of a specific β-propiolactone-DNA adduct.