K‐ras oncogene activation in lung adenocarcinomas from former smokers evidence that K‐ras mutations are an early and irreversible event in the development of adenocarcinoma of the lung

Background. Point mutations in codon 12 of the K‐ras protooncogene occur more frequently in lung adenocarcinomas from smokers (30%) than they do in lung adenocarcinomas from nonsmokers (7%), suggesting that smoking is an important factor in the induction of these mutations. The lack of well defined “early” premalignant or in situ glandular neoplasms of the lung, however, has not permitted direct evaluation of the chronology of ras activation in the development of lung adenocarcinomas. To circumvent the need to evaluate precursor lesions, we examined lung adenocarcinomas from former smokers for point mutations in K‐ras. Methods. Mutations in codon 12 of K‐ras were detected using polymerase chain reaction amplification and mutation‐specific oligonucleotide probes. The types and frequencies of mutations found in adenocarcinomas obtained from 57 former smokers were compared to those found in 27 adenocarcinomas from patients who never smoked and to those found in 27 adenocarcinomas from patients who were current smokers. Results. The overall prevalence of K‐ras point mutations in lung adenocarcinomas obtained from former smokers (32%) was not different from that seen in adenocarcinomas from patients who were current smokers (30%, P = 0.83), and was greater than that seen in adenocarcinomas from patients who never smoked (7%, P = 0.015). This pattern was independent of the duration of abstinence from smoking. Furthermore, the predominant type of mutation found in tumors from former smokers was a guanine‐to‐thymine transversion, the specific type of mutation induced by benzo(a)pyrene, one of the chemical carcinogens found in tobacco smoke. Conclusions. These findings support previous findings that suggest that codon 12 of the K‐ras oncogene may be a specific target of the mutagenic activity of tobacco smoke, and suggest that DNA alterations at this site can occur early and irreversibly during the development of adenocarcinomas of the lung.