Molecular mechanisms governing lung tumor pathogenesis in mice with differing susceptibility to polycyclic hydrocarbon-mediated tumorigenesis

A pharmacogenetic mouse model was utilized to determine the role of cytochrome P4501al expression on the formation of Ki-ra5 mutations in lung tumors following transplacental exposure to polycyclic aromatic hydrocarbons. A backcross between Ah responsive male B6D2F1 mice and nonresponsive female DBA mice resulted in a litter in which both responsive and nonresponsive fetuses resided in the same nonresponsive maternal environment. Pregnant mothers received a single ip injection of either 10 or 30 mg/kg of 3- methylcholanthrene or olive oil vehicle on day 17 of gestation. At the higher dose of 3- methylcholanthrene, the responsive offspring of both sexes had significantly (P < 0.05) higher incidences of lung tumors than their nonresponsive litter mates. The male responsive mice also exhibited a significantly increased liver tumor incidence over the nonresponsive mice at the P < 0.05 level. Administration of 10 mg/kg of 3-methylcholanthrene caused a very low incidence of lung tumors and did not result in the appearance of macroscopically visible liver tumors. Exons 1 and 2 of the Ki-ras gene were amplified from paraffin-embedded tissue samples. The polymerase chain reaction products were screened by allele-specific hybridization. Thirteen of 16 lung tumors (81%) exhibited point mutations in the 12th or 13th codon, including 7 tumors that contained GGT-GTT (GLY¹²→VAL¹²) transversions, 4 which exhibited GGT-TGT (GLY¹²→CYS¹²) transversions, and 2 which contained GGC-CGC (GLY¹³→ARG¹³) transversions. None of the tumors had mutations at codon 61. These results are consistent with a key role for Cyp1a1 in modulating individual susceptibility to cancer formation through the formation of reactive intermediates that bind to DNA and result in activating mutations in key regulatory molecules.