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Recent successes in treating genetic immunodeficiencies have demonstrated the therapeutic potential of stem cell gene therapy. However, the use of gammaretroviral vectors in these trials led to insertional activation of nearby oncogenes and leukemias in some study subjects, prompting studies of modified or alternative vector systems. Here we describe the use of foamy virus vectors to treat canine leukocyte adhesion deficiency (CLAD). Four of five dogs with CLAD that received nonmyeloablative conditioning and infusion of autologous, CD34+ hematopoietic stem cells transduced by a foamy virus vector expressing canine CD18 had complete reversal of the CLAD phenotype, which was sustained more than 2 years after infusion. In vitro assays showed correction of the lymphocyte proliferation and neutrophil adhesion defects that characterize CLAD. There were no genotoxic complications, and integration site analysis showed polyclonality of transduced cells and a decreased risk of integration near oncogenes as compared to gammaretroviral vectors. These results represent the first successful use of a foamy virus vector to treat a genetic disease, to our knowledge, and suggest that foamy virus vectors will be effective in treating human hematopoietic diseases. [ABSTRACT FROM AUTHOR]

Activation of proto-oncogenes by retroviral insertion is an important issue delaying clinical development of gene therapy. We have reported the nonrandom persistence of hematopoietic clones with vector insertions within the MDS1/EVI1 locus following transplantation of rhesus macaques. We now ask whether prolonged culture of transduced CD34+ cells before transplantation selects for clones with insertions in the MDS1/EVI11 or other proto-oncogene loci. CD34+ cells were transduced with standard retroviral vectors for 4 days and then continued in culture for an additional 6 days before transplantation. A 15% of insertions identified in granulocytes 6 months post-transplant were in MDS1/EVI11, significantly increased compared to the frequency in animals transplanted with cells immediately following transduction. MDS1/EVI1 clones became more dominant over time post-transplantation in one animal that was followed long term, accompanied by an increased overall copy number of vector-containing granulocytes, with one MDS1/EVI1 clone eventually accounting for 100% of transduced granulocytes and marrow colony-forming unit (CFU). This vector insertion increased the expression of Evi1 mRNA. There was no overrepresentation of MDS1/EVI1 insertions contributing to lymphoid lineages. Strategies involving prolonged ex vivo expansion of transduced cells may increase the risk of genotoxicity. [ABSTRACT FROM AUTHOR]