PiggyBac toolbox.

The PiggyBac (PB) transposon system was originally derived from the cabbage looper moth Trichoplusia ni and represents one of the most promising transposon systems to date. Engineering of the PB transposase enzyme (PBase) and its cognate transposon DNA elements resulted in a substantial increase in transposition activities. Consequently, this has greatly enhanced the versatility of the PB toolbox. It is now widely used for stable gene delivery into a broad variety of cell types from different species, including mammalian cells. This opened up new perspectives for potential therapeutic applications in the fields of gene therapy and regenerative medicine. In particular, we have recently demonstrated that PB transposons could be used to stably deliver genes into human CD34(+) hematopoietic stem cells (HSCs) resulting in sustained transgene expression in its differentiated progeny. The PB transposon system is particularly attractive for the generation of induced pluripotent stem cells (iPS). Typically, this can be accomplished by stable gene transfer of genes encoding one or more reprogramming factors (i.e., c-MYC, KLF-4, OCT-4, and/or SOX-2). We have generated a PB-based nonviral reprogramming toolbox that contains different combinations of these reprogramming genes. The main advantage of using this PB toolbox for iPS generation is that the reprogramming cassette can be excised by de novo transposase expression, without leaving any molecular trace in the target cell genome. This "traceless excision" paradigm obviates potential risks associated with inadvertent re-expression of reprogramming factors in the iPS progeny. These various applications in gene therapy, stem cell engineering, and regenerative medicine underscore the emerging versatility of the PB toolbox.