In vitro analysis of transformation potential associated with retroviral vector insertions

While replication-defective retroviral vectors provide excellent vehicles f or the long-term expression of therapeutic genes, they also harbor the pote ntial to induce undesired genetic changes by random insertions into the hos t genome. The rate of insertional mutagenesis for retroviral vectors has be en determined in several different assay systems; however, the rate at whic h such events induce cellular transformation has not been directly determin ed. Such measurements are critical to determining the actual risk of carcin ogenesis resulting from retroviral gene therapy. In this study, the ability of a replication-defective retroviral vector, G1nBgSvNa, to induce cellula r transformation in the BALB/c-3T3 in vitro transformation assay was assess ed. The transformation frequency observed in vector-transduced BALB/c-3T3 c ells, which contained one to six copies of integrated provirus, was not sig nificantly different from that of untreated control cells. The finding that G1nBgSvNa was nontransforming in this assay indicates that the rate of tra nsformation induced by retroviral insertions is less than the spontaneous r ate of cellular transformation by BALB/c-3T3 cells, or less than 1.1 x 10(- 5). These results are the first to define an upper limit for the rate of tr ansformation induced by retroviral vectors.