Stable Transgene Expression in Primitive Human CD34+Hematopoietic Stem/Progenitor Cells, Using theSleeping BeautyTransposon System

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Abstract
Sleeping Beauty (SB) transposon-mediated integration has been shown to achieve long-term transgene expression in a wide range of host cells. In this study, we improved the SB transposon-mediated gene transfer system for transduction of human CD34+ stem/progenitor cells by two approaches: (1) to increase the transposition efficacy, a hyperactive mutant of SB, HSB, was used; (2) to improve the expression of the SB transposase and the transgene cassette carried by the transposon, different viral and cellular promoters were evaluated. SB components were delivered in trans into the target cells by Nucleoporation. The SB transposon-mediated integration efficacy was assessed by integrated transgene (enhanced green fluorescent protein [eGFP]) expression both in vitro and in vivo. In purified human cord blood CD34+ cells, HSB achieved long-term transgene expression in nearly 7-fold more cells than the original SB transposase. Significantly brighter levels of eGFP expression (5-fold) were achieved with the human elongation factor 1α (EF1-α) promoter in Jurkat human T cells, compared with that achieved with the modified myeloproliferative sarcoma virus long terminal repeat enhancer–promoter (MNDU3); in contrast, the MNDU3 promoter expressed eGFP at the highest level in K-562 myeloid cells. In human CD34+ cord blood cells studied under conditions directing myeloid differentiation, the highest transgene integration and expression were achieved using the EF1-α promoter to express the SB transposase combined with the MNDU3 promoter to express the eGFP reporter. Stable transgene expression was achieved at levels up to 27% for more than 4 weeks of culture after improved gene transfer to CD34+ cells (average, 17%; n = 4). In vivo studies evaluating engraftment and differentiation of the SB-modified human CD34+ cells demonstrated that SB-modified human CD34+ cells engrafted in NOD/SCID/γ chainnull (NSG) mice and differentiated into multilineage cell types with eGFP expression. More importantly, secondary transplantation studies demonstrated that the integrated transgene was stably expressed in more primitive CD34+ hematopoietic stem cells (HSCs) with long-term repopulating capability. This study demonstrates that an improved HSB gene transfer system can stably integrate genes into primitive human HSCs while maintaining the pluripotency of the stem cells, which shows promise for further advancement of non-virus-based gene therapy using hematopoietic stem cells.