104. Stable High Level Gene Expression by Minimum Sized Murine Leukemia Virus (MLV)-Based Retroviral Vectors in the Mouse Model
Authors: Seung Shin Yu, Youngtae Hong, Sujeong Kim, Chang Wan Joo, Nam Kyung Yoon, Sunyoung Kim
Journal: Molecular Therapy (2005)

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Despite the frequent use of MLV-based retroviral vectors in gene therapy trials, there have been three major criticisms, often relative to the lentivirus-based retroviral vector. First is the claim that MLV-based vector drives a lower level and shorter period gene expression in vivo. However, there is no firm basis for this claim as there have been no serious comparative experiments done thus far between two retroviral vectors. Second, MLV-based vectors cannot deliver a gene to the resting cells. Although this currently holds true, considering the given speed of progress in cell biological technique, the number of cell type to which MLV-based vector can deliver the gene would increase. Third is the possibility of insertional mutagenesis. However, it should not be considered any disadvantage over lentiviral vector, because it also integrates into the host genome like any other retroviruses. Contrary to what has been perceived in the community, MLV-based vectors still have distinctive advantages over lentiviral vectors, especially when the easiness of overall handling, including the production process, is concerned. To study how MLV-based vectors perform in vivo, we tested a series of vectors derived from MT in mouse model. MT is a minimum sized vector that contains no viral coding sequences, but can efficiently drive gene expression and produce high viral titer (Kim et al, J. Virol, 1998, 994-1004; Yu et al, J. Virol. 2000, 8775-8780; Gene Ther., 2000, 797-804; Hong et al, J. Gen. Med., 2003, 18-29). Bone marrow (BM) cells were isolated from mice, and Lin- cells were enriched and transduced with MT-based vectors expressing GFP. 1 |[times]|105 Lin- cells were pre-stimulated with a specially formulated medium for 48 hours and transduced three times at MOI=5 over the period of another two days. At the end, 50|[ndash]|60% of these BM-derived cells became positive to GFP. Immediately after the gene delivery procedure, 1|[times]|106 cells were injected into the lethally irradiated recipient mice through the tail vein (i.v.) or the BM directly (i.BM). During the 40 weeks of follow-up analysis, some differences were found between i.v. (14|[sim]|23%) and i.BM (5|[sim]|14%) injections in terms of the percentage of GFP+ cells in PB cells. Our in vivo comparative analysis indicated that the vector containing the MSCV LTR gave a higher GFP expression than the one with the MoMuLV LTR. Similar long term gene expression was also observed in the mouse model using a MoMuLV-LTR driven vector expressing gp91 (whose defect leads to chronic granulomatous disease in humans). When the integration sites of MT-based vector were analyzed in transduced cells, it appears that there is no strong preference for any particular site as expected from the previous reports. Our data indicated that MLV-based vectors can drive high level gene expression during a substantially long period of time. Given the fact that an MLV-based vector is the most scrutinized gene delivery vehicle with a proven record of safety and efficiency, it should remain a strong candidate as a gene transfer system in the actual clinical settings.