INCREASED FREQUENCY OF K-RAS MUTATIONS IN LUNG NEOPLASMS FROM FEMALE B6C3F1 MICE EXPOSED TO OZONE FOR 24 OR 30 MONTHS

The National Toxicology Program recently completed longterm ozone inha lation studies in B6C3F1 mice and F344/N rats. Mice and rats were expo sed to 0, 0.5 or 1.0 p.p.m. ozone by inhalation for 24 or 30 months. T here was an increased incidence of lung neoplasms in B6C3F1 mice. Howe ver, there was no evidence of carcinogenicity in F344/N rats. The obje ctives of this study were to (i) evaluate benign and malignant lung ne oplasms from B6C3F1 mice for mutations in the K-ras gene at codons 12, 13 and 61, (ii) determine if the frequency and spectra of K-ras mutat ions were unique for ozone-induced lung neoplasms, (iii) determine if specific K-ras mutations were associated with the size and morphologic al patterns of lung neoplasms or ozone exposure concentrations and (iv ) screen lung neoplasms by immunohistochemical methods for the p53 pro tein. K-ras mutations were detected by single-strand conformation anal ysis and identified by direct sequencing of polymerase chain reaction- amplified DNA isolated from formalin-fixed, paraffin-embedded neoplasm s. K-ras mutations were detected in 73% of ozone-induced neoplasms, as compared with 33% of lung neoplasms from controls. The predominant mu tations consisted of A-->T transversions at codon 61 (8/19) and G-->T transversions at codon 12 (7/19). Specific K-ras mutations in lung neo plasms were not associated with various morphological patterns. Our da ta suggests that ozone may cause direct and/or indirect DNA damage in the K-ras proto-oncogene of B6C3F1 mice.