Amplification of the c-myc oncogene during progression of radiation-induced rat skin tumors

Evaluation of a large panel of radiation-induced rat skin tumors of diverse size and histological type revealed a correlation between c-myc copy number and tumor size. Both the frequency and degree of c-myc gene amplification were increased in large compared to small carcinomas, but none of the sarcomas examined showed c-myc amplification. Serial biopsies of individual tumors exhibited similar trends of increasing c-myc copy number in later biopsies. In one regressing tumor, the c-myc gene copy number paralleled the growth rate of the tumor during growth and regression. The average time required from tumor appearance to significant gene amplification was close to the average period between tumor appearance and the onset of rapid growth. The data suggest that, rather than being a target gene for the direct early effects of ionizing radiation, c-myc functions as a late-stage progression-related oncogene in this model system.

An experimental model for oncogene activation during tumor progression in vivo

Tumor progression usually involves a complex pattern of molecular alterations. In many human tumors oncogene amplification or activation has been associated with advanced stages of cancer. Transfection studies have demonstrated the ability of several cellular oncogenes to induce a more malignant phenotype in transformed cells. We have examined the role of c-myc in tumor progression in rat tracheal cell culture, and in rat skin tumors induced by ionizing radiation. In the latter in vivo model, c-myc amplification was found to occur as a function of tumor size. Serial biopsies of growing tumors confirmed the trend toward increased gene copy number with time and stage of progression. This effect was specific for the c-myc gene, in epithelial tumors. Evidence was found for a role of tumor heterogeneity and evolution of tumor cell subpopulations in determining the oncogene activation profile of individual tumors.