Integration-free reprogramming of human somatic cells to induced pluripotent stem cells (iPSCs) without viral vectors, recombinant DNA, and genetic modification.

Stem cells are envisaged to be integral components of multicellular systems engineered for therapeutic applications. The reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) via recombinant expression of a limited number of transcription factors, which was first achieved by Yamanaka and colleagues in 2007, heralded a major breakthrough in the stem cell field. Since then, there has been rapid progress in the field of iPSC generation, including the identification of various small molecules that can enhance reprogramming efficiency and reduce the number of different transcription factors required for reprogramming. Nevertheless, the major obstacles facing clinical applications of iPSCs are safety concerns associated with the use of viral vectors and recombinant DNA for expressing the appropriate transcription factors during reprogramming. In particular, permanent genetic modifications to newly reprogrammed iPSCs have to be avoided in order to meet stringent safety requirements for clinical therapy. These safety challenges can be overcome by new technology platforms that enable cellular reprogramming to iPSCs without the need to utilize either recombinant DNA or viral vectors. The use of recombinant cell-penetrating peptides and direct transfection of synthetic mRNA encoding appropriate transcription factors have both been shown to successfully reprogram somatic cells to iPSCs. It has also been shown more recently that the direct transfection of certain miRNA species can reprogram somatic cells to pluripotency without the need for any of the transcription factors commonly utilized for iPSC generation. This chapter describes protocols for iPSC generation with these new techniques, which would obviate the use of recombinant DNA and viral vectors in cellular reprogramming, thus avoiding permanent genetic modification to the reprogrammed cells.