Role of transfection and clonal selection in mediating radioresistance

Transfected oncogenes have been reported to increase the radioresistance of rodent cells. Whether transfected nononcogenic DNA sequences and subsequent clonal selection can result in radioresistant cell populations is unknown. The present set of experiments describe the in vitro radiosensitivity and tumorigenicity of selected clones of primary rat embryo cells and human glioblastoma cells, after transfection with a neomycin-resistance marker (pSV2neo or pCMVneo) and clonal selection. Radiobiological data comparing the surviving fraction at 2 Gy (SF2) and the mean inactivation dose show the induction of radioresistance in two rat embryo cell clones and one glioblastoma clone, as compared to untransfected cells. Wild-type and transfectant clones were injected into three strains of immune-deficient mice (scid, NIH, and nu/nu) to assay for tumorigenicity and metastatic potential. Only the glioblastoma parent line and its transfectant clones were tumorigenic. None of the cells produced spontaneous or experimentally induced metastases. Flow cytometric analyses indicated that the induction of radioresistance could not be attributed to changes in cell kinetics at the time of irradiation. Our results show that transfection of a neomycin-resistance marker and clonal selection can impart radioresistance on both normal and tumor cells. The work also indicates that altered radiation sensitivity does not necessarily correlate with changes in cell-cycle kinetics at the time of irradiation, tumorigenicity, or altered metastatic potential. Our findings have critical implications for transfection studies investigating determinants of cellular radiosensitivity.