Transposons: Moving Forward from Preclinical Studies to Clinical Trials.

Transposons have emerged as promising vectors for gene therapy that can potentially overcome some of the limitations of commonly used viral vectors. Transposons stably integrate into the target cell genome, enabling persistent expression of therapeutic genes. Transposons have evolved from being used as basic tools in biomedical research to bona fide therapeutics. Currently, the most promising transposons for gene therapy applications are derived from Sleeping Beauty (SB) or piggyBac (PB). Stable transposition requires co-delivery of the transposon DNA with the corresponding transposase gene, mRNA, or protein. Stable transposition efficiency can be substantially increased by using "next-generation" transposon systems that combine codon-usage optimization with hyper-activating mutations in the SB or PB transposases. By virtue of their relatively large capacity, gene therapy applications with relatively large therapeutic transgenes, such as full-length dystrophin, can now be envisaged. The authors and others have shown that efficient and stable gene transfer can be achieved with these next-generation transposons in several clinically relevant primary cells, such as CD34+ hematopoietic stem/progenitor cells, T cells, and mesenchymal and myogenic stem/progenitor cells that are amenable for ex vivo transfection. Alternatively, in vivo transposon gene delivery has been explored using non-viral vectors or nanoparticles or in combination with viral vectors. The therapeutic potential of these SB- and PB-based transposons has been demonstrated in preclinical models that mimic the cognate human diseases. However, there are still challenges impeding clinical translation of transposons pertaining mainly to the typical limiting efficiencies of most non-viral transfection methods and the intrinsic DNA toxicity. Nevertheless, it is particularly encouraging that transposons have now been used in gene therapy clinical trials. In particular, transposon-engineered T cells expressing chimeric antigen receptors are starting to yield promising results in patients with hematological malignancies.