In vivo selection of hematopoietic stem cells transduced at a low multiplicity-of-infection with a foamy viral MGMTP140K vector

ObjectiveUsing a clinically relevant transduction strategy, we investigated to what extent hematopoietic stem cells in lineage-negative bone marrow (Linneg BM) could be genetically modified with an foamy virus (FV) vector that expresses the DNA repair protein, O6-methylguanine DNA methyltransferase (MGMTP140K) and selected in vivo with submyeloablative or myeloablative alkylator therapy.Materials and MethodsLinneg BM was transduced at a low multiplicity-of-infection with the FV vector, MD9-P140K, which coexpresses MGMTP140K and the enhanced green fluorescent protein, transplanted into C57BL/6 mice, and mice treated with submyeloablative or myeloablative alkylator therapy. The BM was analyzed for the presence of in vivo selected, MD9-P140K–transduced cells at 6 months post-transplantation and subsequently transplanted into secondary recipient animals.ResultsFollowing submyeloablative therapy, 55% of the mice expressed MGMTP140K in the BM. Proviral integration was observed in ∼50% of committed BM-derived progenitors and analysis of proviral insertion sites indicated up to two integrations per transduced progenitor colony. Transduced BM cells selected with submyeloablative therapy reconstituted secondary recipient mice for up to 6 months post-transplantation. In contrast, after delivery of myeloablative therapy to primary recipient mice, only 25% survived. Hematopoietic stem cells were transduced because BM cells from the surviving animals reconstituted secondary recipients with MGMTP140K-positive cells for 5 to 6 months.ConclusionsIn vivo selection of MD9-P140K–transduced BM cells was more efficient following submyeloablative than myeloablative therapy. These data indicate that a critical number of transduced stem cells must be present to produce sufficient numbers of genetically modified progeny to protect against acute toxicity associated with myeloablative therapy.