Induced pluripotent stem cells: A new era for hepatology

Progress towards safer iPS cells and application of differentiated hepatocytes. Methods have evolved from conventional viral integration to virus-free strategies by transgene removal or the avoidance of viral integration. iPS cells have first been generated using integrating retroviral and lentiviral vectors to deliver reprogramming factors such as: OCT4, SOX2, KLF4, and c-MYC (A). Alternatively, iPS cells can be produced by the integration of reprogramming factors using the LoxP site containing plasmids, lentiviruses, or piggyBac transposon mediated gene transfer system followed by removal of transgene sequences from the host genome using Cre or transposase enzymes (B). Another strategy to generate iPS cells without viral integration used non-integrating viruses or plasmids to introduce reprogramming factors (C). Mouse and human iPS cell lines were also derived using vector or adenoviruses that transiently expressed Oct4, Sox2, Klf4, and c-Myc (C). It was also shown that virus-free mouse iPS cells could be generated using repeated plasmid transfections and by nucleofection of a single plasmid construct expressing Oct4, Sox2, Klf4, and c-Myc as a single polycistronic unit (C). Human-iPS cells were also recently generated completely free of vector and transgene sequences by transfection of a non-integrating episomal vector. Another recent strategy to create iPSCs without viral integration has been reported by protein transduction of the four reprogramming proteins fused with a cell-penetrating peptide (CPP) (D). Somatic or iPS cells can be used to treat disease by correction of the underlying genetic defect. The wild-type gene can be used to replace the defective gene by homologous recombination of somatic cells or iPS cells. The differentiated hepatic cell lineages provide fascinating possibilities and tools for the study of disease models, tissue engineering, and creation of bio-artificial livers, in addition to their use for drug discovery and regenerative medicine.