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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.

Insertional mutagenesis continues to be a major concern in hematopoietic stem-cell gene therapy. Nonconventional gene transfer vectors with more favorable integration features in comparison with conventional retrovirus and lentivirus vectors are being developed and optimized. In this study, we report for the first time a systematic analysis of 198 avian sarcoma leukosis virus (ASLV) insertion sites identified in rhesus long-term repopulating cells, and a comparison of ASLV insertion patterns to Moloney murine leukemia virus (MLV) (n = 396) and simian immunodeficiency virus (SIV) (n = 289) using the newly released rhesus genome databank. Despite a weak preference toward gene-coding regions, ASLV integration is nonclustered, does not favor gene-rich regions, transcription start sites, or CpG islands. There was no propensity for ASLV insertions within or near proto-oncogenes, and most importantly, no insertions close to or within the Mds1-Evi1 locus, which is in contrast to the significant over-representation of this insertion site for MLV vectors in the same transplantation model. Furthermore, ASLV long terminal repeats (LTRs) do not have detectable promoter and enhancer activity in a quantitative luciferase assay to measure neighboring gene activation. The combination of these features is unique for ASLV and suggests that optimized vectors based on this virus could be useful and safe for gene transfer to hematopoietic stem cells and progenitor cells.