Oncogenes and Tumor Suppressor Genes in the Carcinogenicity of Fibers and Particles

Asbestos fibers and crystalline silica are carcinogenic to humans when inhaled into the lungs. Asbestos fibers and cigarette smoke most likely act as cofactors in the induction of lung cancer. Point mutations in the K-ras oncogene and the p53 tumor-suppressor gene are frequent in lung cancers and are consistent with the known mutagenic spectrum of tobacco-smoke carcinogens. The FHIT tumor suppressor gene is also frequently inactivated in lung cancers of smokers and in workers who were exposed to asbestos. Recent molecular studies of p53 tumor suppressor gene mutations and p53 protein expression in the lungs of patients with lung cancer and occupational exposure to crystalline silica and other dusts have been conducted. Mutations in the p53 gene were detected at a frequency similar to those in smoking-related lung cancers. Expression of p53 protein can be detected by immunohisto-chemistry in preneoplastic epithelial lesions in the lungs of smokers and workers. Human malignant mesotheliomas frequently show overexpression of p53 protein; however, point mutations at the p53 tumor suppressor gene or ras oncogene locus are rare. Most cases of malignant mesotheliomas have codeletions of the p15 and p16 tumor suppressor genes and alterations at the NF2 tumor suppressor gene locus with monosomy of chromosome 22. The molecular alterations characteristic of malignant mesotheliomas may develop during later stages of tumor progression and may not reflect the direct genotoxic effects of fibers on the target cell population.