Human-induced pluripotent stem cells as a source of hepatocyte-like cells: new kids on the block

Abstract

One of the most exciting recent discoveries in the field of biology was the demonstration that both mouse and human somatic cells engineered via epigenetic reprogramming for the expression of combinations of few defined transcription factors (including Oct4, KLF4, Sox2, NANOG, LIN28, and c-Myc) can become pluripotent [1–3]. These cells closely resemble embryonic stem (ES) cells for their pluripotency and have been defined as induced pluripotent stem (iPS) cells. These cells have rapidly emerged as very promising tools for the achievement of significant advancement in different fields without the controversies and ethical concerns associated with the use of human ES cells [4]. In particular, the possibility to get iPS cells from readily obtainable somatic cells theoretically opened the way to a number of perspectives and practical applications, including (1) to design and test patient-customized (i.e., autologous) cell therapy without the need for immune suppression and (2) to increase our knowledge on mechanisms of inherited diseases with a realistic opportunity, as recently shown for human iPS-derived hepatocytes [5–7], for modeling inherited metabolic human diseases, understanding disease pathogenesis, and for drug discovery and testing. It should be anticipated, however, that the latter objective (i.e., the use of human iPS cell-derived hepatocytes for drug discovery and testing) can currently be envisaged as the most useful and safe application. Indeed, a number of relevant limitations have progressively emerged in relation to the clinical use of iPS cells and different laboratories have developed strategies in order to at least partially overcome such limitations. The original methods used in order to derive iPS cells from either human or mouse somatic cells employed viral vectors, a strategy leading to the integration of both desired transgenes and vector backbone into the host cell genome [1–3]. Of relevance, the use of these vectors is at risk to produce insertional mutations and genetic alterations able to interfere with normal functions of cells derived from iPS cells and to favor reactivation of reprogramming factors at later stages. Indeed, residual transgene expression has been reported to affect differentiation into specific lineages [2] or even result in tumorigenesis [8–10]. At present, the oncogenic potential of iPS cells represents a major concern for clinical application [9] since these cells, much as ES cells, can readily form teratomas when injected into immunodeficient mice. This may rely, for both iPS and ES cells, on the presence of residual diploid pluripotent cells that have not undergone differentiation in the population of transplanted human ES or iPS cells [9]. Moreover, tumor formation in iPS cell chimeric mice has been attributed to the expression of c-Myc in iPS cell-derived somatic cells, irrespective of the type of the original somatic cell employed [9–13]. However, as well reviewed [9], several other reasons are likely to underlie iPS cell tumorigenicity. In particular, although iPS cells may behave similarly to ES cells, differences between these two kinds of cells indeed exist and this is particularly significant for human cells, with human iPS cells being more tumorigenic than human ES cells because of genetic and epigenetic causes [9]. Along these lines, human iPS cells have been reported to acquire chromosomal alterations and genetic changes (including aneuploidy, gain or loss of small chromosomes, and point mutations) even more readily than ES cells C. Busletta E. Novo M. Parola (&) Department of Experimental Medicine and Oncology, InterUniversity Center for Hepatic Pathophysiology, University of Torino, Corso Raffaello 30, 10125 Turin, Italy e-mail: maurizio.parola@unito.it