Transmission of endogenous VL30 retrotransposons by helper cells used in gene therapy

Retrovirus-derived vectors and packaging cell lines are basic components used for gene transfer in human gene therapy. To eliminate recombinational and transcriptional problems associated with retroviral vectors, synthetic retrotransposon VL30 vectors were devised. During experimentation with these new vectors, extensive cotransmission of endogenous VL30 retrotransposon sequences was observed, originating within helper cell lines used in gene therapy experiments. The RNA was efficiently packaged into helper virus and was transmitted to recipient human cells, where it was again expressed as RNA. Transmission occurred regardless of whether the vector was retrovirus-derived or VL30, or if no vector was used. Endogenous VL30 RNA was readily detected in unselected recipient cells after a single exposure to helper virus, demonstrating a high efficiency of transmission compared with a cotransmitted VL30 vector, which contained an extensive deletion of internal sequences. These studies indicated that VL30 retrotransposons were ubiquitously transmitted by murine helper cells. Furthermore, the data strongly suggested that improvements in gene transfer may be obtained, both by using nonmurine helper cells (to reduce competitive inhibition by endogenous VL30) and by using VL30-derived vectors with intact packaging sequences.