PLASMOVIRUSES - NONVIRAL VIRAL VECTORS FOR GENE-THERAPY

We have generated a chimeric gene transfer vector that combines the si mplicity of plasmids with the infectivity and long-term expression of retroviruses. We replaced the env gene of a Moloney murine leukemia vi rus-derived provirus by a foreign gene, generating a plasmid that upon transfer to tumor cells generates noninfectious retroviral particles carrying the transgene. We added to this plasmid an independent expres sion cassette comprising a cytomegalovirus promoter, an amphotropic re troviral envelope, and a polyadenylylation signal from simian virus 40 . These constructs were designed to minimize the risk of recombination generating replication-competent retroviruses. Their only region of h omology is a 157-bp sequence with 53% identity. We show that the sole transfection of this plasmid in various cell lines generates infectiou s but defective retroviral particles capable of efficiently infecting and expressing the transgene. The formation of infectious particles al lows the transgene propagation in vitro. Eight days after transfection in vitro, the proportion of cells expressing the transgene is increas ed by 10-60 times. There was no evidence of replication-competent retr ovirus generation in these experiments. The intratumoral injection of this plasmid, but not of the control vector lacking the env gene, led to foci of transgene-expressing cells, suggesting that the transgene h ad propagated in situ, Altogether, these ''plasmoviruses'' combine adv antages of viral and non-viral vectors. They should be easy to produce in large quantity as clinical grade materials and should allow effici ent and safe in situ targeting of tumor cells.