A minimal genome simian foamy virus type 1 vector system with efficient gene transfer.

Foamy viruses have several inherent features for the opportunity to develop efficient and versatile vectors for gene therapy. We have constructed a series of vectors and helper plasmids based on simian foamy virus type 1 (SFV-1) to establish the minimum vector genome required for efficient gene transduction. To characterize the efficiency of gene transduction by these vectors, the green fluorescent protein (GFP) coding sequence is linked to the human cytomegalovirus immediate gene promoter. Several deletion analyses of SFV-1 vectors revealed that the minimum genome with efficient GFP transduction contained the 5' untranslated region extending to the first 637 nucleotides of the gag gene, a 596 nucleotides of pol sequence from position 3137-3733, the 3' pol region at position 5200-5693, the 3' end polypurine tract, and the 3' LTR. An additional 1131 nucleotides can be removed from the 3' end LTR without affecting the efficiency of vector transduction. SFV-1 vector can therefore accommodate a minimum 8930 base-size heterologous DNA fragment. Furthermore, the efficiency of SFV-1 vector transduction was analyzed using different packaging plasmids. GFP transduction with packaging plasmid that contained the 5' R-U5 region of the LTR was compared with helper plasmids that had deletions in this region except for 22 nucleotides (positions 21-41), the first 61, 77, or 140 nucleotides of the R of the LTR. Transduction efficiencies were significantly reduced with the deletion mutations implicating that for optimum SFV-1 vector productions a packaging construct that includes the 5' R-U5 is required.