Efficient Packaging of a Specific VL30 Retroelement by Ψ2 Cells Which Produce MoMLV Recombinant Retroviruses

Abstract

FTO-2B rat hepatoma cells acquired mouse VL30 retrotransposon(s) when infected with Moloney murine leukemia virus (MoMLV) recombinant retroviruses produced from Ψ2 cells. The VL30 provirus was integrated into the rat genome, expressed at high levels, and its transcription induced 40-fold by dexamethasone. VL30 RNA was detected in hepatoma cells even without selection for the expression of the amino-3′-glycosyl phosphotransferase (neo) gene, which was co-transferred with a MoMLV retrovirus. However, the extent of transfer of the VL30 RNA was inversely related to the titer of the MoMLV recombinant retrovirus. The restriction map analysis of the transferred VL30 provirus was identical to the mouse VL30s of the NVL subfamily which is known to be a significant fraction of the transcriptionally active VL30 subset. Additionally, the regenerating liver from an adult rat, which was infected with a defective MoMLV-derived retrovirus, expressed VL30 RNA. These results indicate that great care should be given to the transfer of unwanted passengers, like VL30, present in retroviral packaging cell lines like the Ψ2 cells, which are currently being used for gene therapy.

Overview summary

One of the safety concerns in retroviral-mediated gene transfer is that the virions carrying the vector might also contain other genetic material (derived from the producer cells) that could be harmful. One type of genetic material postulated to be present is the VL30 retrotransposon element. Hatzoglou and her colleagues demonstrate that VL30 are indeed present in murine retroviral virions, but that the level of VL30 transfer decreases as the titer of the retroviral virion increases. This is an encouraging result for gene therapy because it suggests that very high titer vectors, which are the goal for clinical use, may be safer.