Ionizing Radiation Greatly Improves Gene Transfer Efficiency in Mammalian Cells

Abstract

The vast majority of clinical protocols involving gene therapy today rely on viral vectors for gene transduction. The primary reason that plasmid vectors have not been widely used for gene therapy trials is their relatively low rate of stable gene transfer. We show here that ionizing radiation can improve plasmid transfection efficiency in both normal and neoplastic human and mouse cells. As high as 1,400-fold improvement in transfection efficiency can be seen in primary human fibroblasts treated with 9 Gy. Radiation improves transfection efficiency in a dose-dependent manner of only linearized plasmid DNA in transformed or immortalized cells, but of both linearized and supercoiled plasmid in normal human fibroblasts. The gene transfer dose–response curves are linear for neoplastic cell lines and exponential for primary cell lines. This suggests that radiation can improve gene integration by at least two mechanisms, one that may require free DNA ends and one that does not. The 2-hr delay described here, from the time of irradiation to the beginning of enhanced gene integration, suggests an inducible process that becomes active after the bulk of the radiation damage has been repaired. Our data further suggest that radiation may be useful to target human gene therapy using plasmid vectors. The major drawback to the use of plasmid vectors for gene therapy is their relatively poor transduction efficiency compared with viral vectors. We show that modest doses of ionizing radiation can greatly improve plasmid transduction efficiency. Neoplastic and primary human cells have very different dose response curves, suggesting that at least two mechanisms may exist. Because transduction of Eco RI-linearized plasmid is enhanced much more than supercoiled plasmid, double-strand break rejoining may be important in the recombination event. The time course of enhancement suggests a radiation-inducible process, with a delay of approximately 2 hr between irradiation and gene integration. Thus, ionizing radiation can be used to target plasmid-mediated gene transfer.