Gene therapy using hematopoietic stem cells (HSCs) as the target cell population has great potential to improve treatment of a wide range of inherited and acquired blood diseases. Replication-defective retroviruses have been the vehicles of choice for gene delivery and expression in HSCs because of their ability to stably integrate into the genome of target cells. For more than a decade, our laboratory has been designing and optimizing retroviral vectors for gene transfer studies of HSC biology. In particular, our MSCV (murine stem cell virus) retroviral vector has proven to be highly efficient at delivering functional genes to the murine hematopoietic system. For this reason, the MSCV platform was chosen for use in two HSC gene therapy trials currently underway in the United States. To date, however, the outcomes of most clinical trials with retroviral vectors have been disappointing. This is believed to be due in part to low surface density of the amphotropic envelope receptor and the fact that retroviral vectors such as MSCV, which are derived from oncoretroviruses, can only integrate into cells undergoing mitosis. Thus it has been proposed that pantropic vectors developed from the lentivirus, human immunodeficiency virus (HJV), which can readily transfer genes into various types of stationary cells, may be more suitable for gene delivery to HSCs, which reside almost exclusively in the G0/G1 phase of the cell cycle. Even if efficient lentivirus-based gene transfer in HSCs is achieved, accumulated data indicate that in vivo transgene expression is frequently subject to transcriptional silencing and position effects. We propose therefore to develop next-generation HIV-based lentiviral vectors expressly for human HSC gene transfer applications. Our hypothesis is that utilization of transcriptional regulatory elements permissive for expression in HSCs in conjunction with chromatin insulator sequences and scaffold/matrix attachment regions will lead to maintenance of high-level transgene expression in HSCs and their differentiated progeny. To this end, the performance of next-generation lentiviral vectors utilizing the MSCV long terminal repeat as an internal promoter and harboring the chicken b-globin 5' constitutive hypersensitive site (5' HS4) insulator and/or the human interferon-b scaffold attachment region (IFN-SAR) will be assessed in human hematopoietic repopulating cells using a surrogate non-obese diabetic/severe combined immunodeficient (NOD/SCID) xenograft assay and in a murine hemophilia A model.
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