RETROVIRAL VECTOR DESIGN FOR LONG-TERM EXPRESSION IN MURINE HEMATOPOIETIC-CELLS IN-VIVO

A series of retroviral vectors containing the human glucocerebrosidase (GC) cDNA driven by various promoters have been constructed in an attempt to discover which vector design can most efficiently transduce murine hematopoietic stem cells (HSCs) and drive expression of the transferred gene in hematopoietic cells of mice reconstituted with the transduced stem cells. The simplest vector, LG, in which the GC gene is driven by the viral LTR, was the most efficient vector at infecting HSCs, with an average viral copy number in hematopoietic tissues of 3 copies/cell in recipient mice. In general, the viral vectors that contained any additional promoters or enhancers to drive expression of either the GC gene or a selectable marker gene (Neo(R)) had lower titers and/or transduced HSCs at a lower efficiency. This was seen most markedly when the human phosphoglycerate (PGK) promoter was used to drive the human GC cDNA. Despite repeated attempts to obtain a high titer producer clone, this virus consistently produced low titers and subsequently resulted in the lowest proviral copy numbers in long-term reconstituted mice. Only the viral LTR and the PGK promoter were capable of driving significant levels of human GC RNA in hematopoietic cells of long-term reconstituted mice, with a much lower level of RNA generated by an internal herpes TK or SV40 immediate early promoter. Insertion of the internal transcription unit in the opposite orientation relative to the viral LTRs had a detrimental effect on gene expression. The levels of RNA generated by a hybrid LTR containing the myeloproliferative sarcoma virus enhancer were higher in bone marrow-derived macrophages than in nonadherent cells of the bone marrow when compared with the LG vector. The presence of an internal promoter to drive expression of the human GC cDNA did not seem to have a detrimental effect on expression levels from the viral LTR. In fact, in the presence of an internal TK or PGK promoter expression from the LTR was increased despite the presence of lower proviral copy numbers. Insertion of a second gene (Neo(R)) into the vector had a negative impact on long-term expression in hematopoietic cells in vivo; however, this seems to be due solely to the lower transduction efficiency of this vector. Overall, the highest levels of GC activity in macrophages of long-term reconstituted mice were generated by the LG vector; however, these levels were variable. Vectors containing an internal SV40, TK, or PGK promoter produced consistent levels of GC activity in these cells, but because of the lower transduction efficiency obtained in the presence of these promoters, simple vectors containing a single gene driven by the viral LTR currently remain the most promising viruses for gene therapy of human hematopoietic disorders.