Conditionally replicating lentiviral-hybrid episomal vectors for suicide gene therapy

Lentiviral vectors have been shown to be good candidates for gene transfer protocols; however, prevention of insertional mutagenesis remains problematic. Here we report on the design of a conditionally replicating integrase (IN)-defective lentiviral-hybrid episomal vector in which the insertion of the SV40 promoter/origin of replication provides long-term persistence of the extrachromosomal DNA in the presence of the corresponding trans-acting T antigen (Tag) for targeted suicide gene therapy. SV40-driven GFP expression from the IN-defective lentiviral-hybrid vector was sustained only in the Tag positive 293T cell line, while expression was transient in the parental Tag deficient cell line 293. Quantitative PCR for the 2-LTR circular forms indicated that the unintegrated forms remained stable in 293T for up to 56 days post-transduction, while they were undetectable in the cell line 293 after day 14. Transduction of 293T cells with the IN-defective lentiviral-hybrid episomal vector containing the thymidine kinase (TK) gene rendered the Tag expressing cells highly susceptible to ganciclovir (GCV) treatment, as opposed to the cells infected with the control vector or in Tag negative cells. These data suggest that conditionally replicating IN-defective lentiviral-hybrid episomal vectors could prove useful as vehicles for suicide gene therapy, in particular in cells transformed by SV40.