Induced reprogramming of human somatic cells into pluripotency: a new way how to generate pluripotent stem cells

Abstract

It is now possible to create human cells that exhibit morphology and properties of human pluripotent embryonic stem (ES) cells by induced expression of only four factors. ES cells are remarkable cells. They can self-renew and maintain pluripotency indefinitely or can differentiate into any cellular types in the body. Therefore human ES (hES) cells provide a great promise as a source of cells use for treating various diseases or for use in toxicology testing of new drugs. However, there is one huge obstacle to their use. Derivation of hES cells is achieved by destroying pre-implantation embryo, an action that presents ethical and religious issues. The question if pluripotent ES stem cells could be obtained without destroying embryos had been raised and many scientists were eager to find new ways of creating pluripotent stem cells (for a review, see Hochedlinger and Jaenisch, 2006). The breakthrough work was done by Takahashi and Yamanaka (2006) last year when they demonstrated that induced expression of only four factors, Oct-4, Sox-2, c-Myc, and Klf-4, in mouse fibroblast cells can form ES-like cells, termed as induced pluripotent stem (iPS) cells. This year Yamanaka’s group (Okita et al., 2007) published an improved protocol and they went on to demonstrate that iPS cells are truly pluripotent stem cells because they not only differentiate into a large variety of tissues but also make germline chimeras. The technique has since been reproduced by other laboratories (Maherali et al., 2007; Meissner et al., 2007; Qin et al., 2007; Wernig et al., 2007). The question remained. Could the same technique be applied in human or were more factors needed to reprogram human somatic cells into pluripotency? We did not need to wait long for the answer. It worked. Two teams, Yamanaka’s and Thompson’s, have demonstrated that human somatic cells can be reprogrammed into iPS cells (Table 1). Yamanaka’s group successfully applied the same technique they used in mice (Takahashi et al., 2007). They used human dermal fibroblasts and two other human fibroblast populations from different human donors and transduced them with retroviral vectors carrying human cDNAs of Oct-4, Sox-2, c-Myc, and Klf-4 genes. This time they did not use selection transgene as they did in their previous works and cultured cells in hES culture medium supplemented with FGF-2. Thirty days after infection, they identified, by morphology, human iPS colonies resembling hES colonies among other colonies. Colonies were further expanded and established iPS lines (10 iPS lines derived from 50,000 cells) were subjected to many assays to compare them with hES cells derived from embryos. They studied morphology, expression of cell-surface markers, epigenetic status, and their potential to differentiate in vitro and in vivo (teratoma formation) in newly derived iPS cells. Cell lines exhibited properties of hES cells, and, moreover, DNA microarray analysis revealed the similarity between the global gene expression of human iPS cells with hES cells. They also conformed the genetic origin of iPS lines with their original parental fibroblast cells, thus excluding any cross-contamination. Chromosomal analysis revealed that iPS cells had a normal karyotype; however, minor genetic alterations could not be observed using this technique. Unlike Yamanaka’s team, Thompson’s team first decided to perform a screen to identify the genes that were highly enriched in hES cells relative to myeloid precursors and compiled a list that did not contain c-myc and Klf-4. Subsequently they showed that 14 hES cellenriched genes, transduced by lentiviral vectors, reprogrammed somatic cells to iPS cells. Moreover, they showed that out of 14 genes Oct-4, Sox-2, nanog, and Lin28 are core genes able to reprogram human somatic cells with a mesenchymal phenotype to iPS cells (Yu et al., 2007). They also found that although Gabriela Durcova-Hills ( . *) Wellcome Trust Cancer Research UK Gurdon Institute, University of Cambridge, Tennis Court Road Cambridge CB2 1QN, U.K. Tel: 144 1223 334137 Fax: 144 1223 334089 E-mail: gd225@cam.ac.uk