Woodard and Wilson (2015) have recently published an admirable review in Trends in Biotechnology of the diverse applications of the piggyBac transposon system in mammalian cells. Insect biologists working with or interested in piggyBac will be interested in reading this review. The breadth of applications of this transposon demonstrate that it is more than just a ‘gene vector’.
The piggyBac DNA transposon is quite well-known to molecular entomologists. It was originally isolated in 1983 from the genome of the cabbage looper moth Trichoplusia ni and, since then, has gained its reputation as one of the principal vectors for insect transgenesis. However, its effectiveness in genomic engineering of mammalian cells that was demonstrated during the last decade may go unnoticed among insect scientists and may benefit from seeing what others are doing with this system.
The authors illustrate the features of this versatile tool focusing on its unique mechanism of transposition and seamless excision. The ability to excise without leaving so-called footprint mutations >95% of the time, unlike other commonly used vectors, is an exceptional advantage to generate transgene-free iPSC (induced Pluripotent Stem Cells) and, therefore, to develop new molecular therapeutic strategies. Among its outlined features is also the flexibility of the transposase activity when modified to create an inducible enzyme or fused with DNA binding domains to improve the targeting of piggyBac elements to user-defined chromosomal locations. The later was first demonstrated in mosquito embryos, before its application to human cells.
The authors also describe several applications of the system. piggyBac insertional mutagenesis contributed to gene discovery, produced large chromosomal deletions (expressing Cre recombinase) and generated inducible genomic transposition for gene trapping approaches. Besides, piggyBac-engineered cancer gene-discovery lines enabled the identification of novel genes that previously evaded recognition by other transposon screens.
Animal transgenesis was also greatly facilitated by means of efficiency and affordability using piggyBac, in discovering and evaluating disease processes, as well as establishing preclinical models of human diseases. Engineering of cell lines to stably express multiple transgenes or multiproteins through a piggyBac multiplex delivery system offers another alternative to derive novel targets for potential therapies or drug discovery.
piggyBac can be also efficiently applied for in vivo gene transfer in mice to correct phenotypes of inherited diseases, by inducing long term gene expression in the desired tissue.
piggyBac-mediated genetic modification of clinically relevant cells, including cellular reprogramming of iPSCs and modification of HSCs (Human Stem Cells) and human T cells are reported as well. Interestingly, in preclinical models piggyBac gene-modified T cells performed targeted killing of cancer cells in vitro and in vivo. Such high functional activity, coupled with the improved safety due to selective removal of the ex-vivo modified sequences, seem to be a promising and cost effective non-viral gene delivery system for cancer immunotherapy in future clinical trials.
All in all, developing new biotechnology applications for transposon technology implemented initially in insects has now widened the fields of application even to mammals, comprising a challenging endeavor for future research.
However, as pointed out by the authors, questions concerning the safety of piggyBac for clinical gene transfers, as well as the selectivity and efficiency for gene therapy applications should not be disregarded.