Frequent activation of the Ki‐ras oncogene at codon 12 in N‐methyl‐N‐nitrosourea‐induced rat prostate adenocarcinomas and neurogenic sarcomas

Rat neoplasms induced by methylating carcinogens frequently contain ras genes activated by a single point mutation. Rat prostatic tumors induced by a combination of a single injection of N‐methyl‐N‐nitrosourea (MNU) and long‐term treatment with testosterone were examined for the presence of such activating point mutations in ras genes. These tumors, which arose exclusively in the dorsolateral prostate, included both adenocarcinomas and sarcomas. Activating mutations in codon 12 of the Ki‐ras gene were found in 7 of 10 carcinomas and 4 of 5 sarcomas, using selective oligonucleotide hybridization analysis of DNA amplified by the polymerase chain reaction (PCR). However, no mutated Ha‐ras oncogenes were detected. The presence of PCR‐engineered Hphl restriction sites created by the existence of a G³⁵→A mutation in the rat Ki‐ras oncogene identified the mutation as a GC→A transition at the second position of codon 12. Production of O⁶‐methylguanine adducts in the Ki‐ras codon 12 followed by base mispairing during replicative DNA synthesis is thus the likely molecular mechanism of initiation of prostatic carcinogenesis by MNU in the rat. Three of the four sarcomas positive for the Ki‐ras G³⁵→A mutation were immunohistochemically defined as of Schwann cell origin, indicating that involvement of the ras gene family is possible in tumorigenesis of this cell lineage. Loss of the wild‐type Ki‐ras allele was also observed in all four of these sarcomas.