Embryonic stem cell (ESC) research has been a hot topic, with conservatives saying that such research is morally unacceptable and liberals saying that conservatives value a clump of cells more than people who have serious disabling diseases. Several groups of medical researchers (including James Thomson, the first person to culture ESC) recently showed that normal skin cells can be reprogrammed to an embryonic state, producing what are now called induced pluripotent stem (iPS) cells. Originally performed in mice in June, 2007,2 researchers took four genes OCT3/4, SOX2, KLF4, and c-MYC and incorporated those genes into the nucleus of cells to induce pluripotency. Such lines could be expanded indefinitely and could differentiate to form numerous kinds of different tissues.
Human iPS cells
Just five months after the mouse study was published, the feat was repeated by three separate laboratories using human skin cells.3 One research group used the same genes as those used in the mouse study, whereas a second group used OCT3, SOX2, NANOG and LIN28. The techniques were efficient enough to generate one cell line for every 5-10 thousand cells treated. Although not extremely efficient, it is quite usable, since it is possible to obtain hundreds of thousands to millions of cells to carry out these kinds of studies. The technique was recently replicated for adult human skin cells,4 instead of skin cell lines, demonstrating that it could be used to generate patient-specific cell lines.
Do iPS = ESC?
Studies using iPS cell lines have shown that those cells undergo similar changes compared to what is observed with embryonic stem cells. Cell populations grew at the same rate, telomerase (which preserves the ends of chromosomes) was present in both iPS and ESC. Several genes that are silenced in fibroblasts, but active in ESC, were also active in the iPS cells. The iPS cell lines could be differentiated into heart muscle and neuronal cells, in addition to basic cell types (ectoderm, mesoderm, and endoderm). Gene expression assays showed that 5,000 genes from iPS cells showed a five-fold difference in expression compared to those in fibroblasts, although 1,267 genes had a five-fold difference in expression between ESC and iPS cells. According to the James Thomson study, "The human iPS cells described here meet the defining criteria we originally proposed for human ES cells (14), with the significant exception that the iPS cells are not derived from embryos."3
Originally, the new technique is not without its own set of problems, although within two years, virtually all had been resolved. One of the original genes used for reprogramming (c-MYC) has been shown to produce tumors and cancers. Obviously, it would not be a good choice for patient therapy. However, this gene was eliminated in some of the later techniques.5 The second problem was that the genes were originally introduced through the use of a retrovirus that incorporates into the host cell DNA. Depending upon where the gene sequence inserts, it may cause trouble (including mutations and cancers). Those who watched the I am Legend movie will remember that a retrovirus-derived cancer treatment was responsible for turning the surviving members of the human race into an army of grotesque monsters. Although such a transformation is not possible, the initiation of cancer in even a small number of treated patients would make such treatments unusable for human therapy. Two years later the problem of using a retroviral system for reprogramming was solved by switching to a simple lentivirus reprogramming system.6 Within weeks, other researchers went a step further, eliminating viral reprogramming altogether by using reprogramming genes (OCT4, SOX2, NANOG, LIN28, c-Myc, and KLF4) cloned into a circular piece of DNA called a plasmid.7 Subsequent culture of of the iPS over a period of weeks resulted in the complete loss of the plasmid, but with continued pluripotency. The potential of iPS cells is so great that the researcher who first grew ESC in culture is now one of the leading proponents of iPS stem cell research.
A more recent, but somewhat uncertain potential problem has been identified more recently. Since iPS cells are derived from adult tissues, they tend to harbor some of the same epigenetic profiles as those adult tissues from which they are derived. As cells age or differentiate, certain genes are turned on or off through methylation of those gene's promoters. The process prevents those cells from undergoing additional changes that might cause the cells to lose their differentiated properties. When adults cells are induced to pluripotency, some of those epigenetic profiles are retained in the iPS cells.8 How will these vestiges of adult cells affect iPS ability to differentiate into cells that are useful for disease models or therapy? At this point, we don't know for sure. However, my guess is that different ESC lines will exhibit different epigenetic profiles, as will specific isolates of iPS cells. Although researchers have found no problems in producing differentiated iPS lines, some of these epigenetic changes might interfere with the ultimate function of these cells as differentiated cell lines.
Even with these issues, research institutes are beginning to focus their stem cell research on iPS cells. Cedars-Sinai Medical Center recently opened its Induced Pluripotent Stem Cell Core Production Facility in late 2011, according to their press release.9
Induction of pluripotency to produce embryonic-like stem cells is the hot topic in stem cell research. The fact that human iPS cells have been produced in many different laboratories after the initial animal studies shows that the technique is robust and easily reproducible. In contrast, the competing technique, human somatic cell nuclear transfer (cloning), has never been transferred from animal studies to human application, despite years of attempts. At this point, it seems pretty certain that the iPS technique will soon replace ESC as the preferred means of generating human stem cell lines. However, the disadvantage of iPS cells is that the cell lines produced would be patient specific (only useful for the intended patient), whereas the establishment of ESC lines allows biotech companies to patent the lines in order to make lots of money.
- What is Wrong With Embryonic Stem Cell Research?
- Stem Cell Research/Cloning: Status and Ethics
- Baker, M. 2007. Adult cells reprogrammed to pluripotency, without tumors. Nature Reports Stem Cells doi:10.1038/stemcells.2007.124.
- Benefits of Stem Cells to Human Patients: Adult Stem Cells v. Embryonic Stem Cells
- Peer-Reviewed References
- In response to the announcement Democrat Senator Tom Harkin of Iowa said ,"Our top researchers recognize that this new development does not mean that we should discontinue studying embryonic stem cells. Scientists may yet find that embryonic stem cells are more powerful."
- David Cyranoski. 2007. Simple switch turns cells embryonic Technique removes need for eggs or embryos. Nature doi:10.1038/447618a.
- Gretchen Vogel 2007. Researchers Turn Skin Cells Into Stem Cells.
ScienceNOW Daily News 20 November 2007
Yu, J., M. A. Vodyanik, K. Smuga-Otto, J. Antosiewicz-Bourget, J. L. Frane, S. Tian, J. Nie, G. A. Jonsdottir, V. Ruotti, R. Stewart, I. I. Slukvin and J. A. Thomson. 2007. Induced Pluripotent Stem Cell Lines Derived from Human Somatic Cells. Science DOI: 10.1126/science.1151526.
Takahashi et al. 2007. Induction of Pluripotent Stem Cells from Adult Human Fibroblasts by Defined Factors. Cell doi:10.1016/j.cell.2007.11.019.
- In-Hyun Park, I., R. Zhao, J. A. West, A. Yabuuchi, H. Huo, T. A. Ince, P. H. Lerou, M. W. Lensch and G. Q. Daley. 2007. Reprogramming of human somatic cells to pluripotency with defined factors Nature doi:10.1038/nature06534.
- Nakagawa, M., M. Koyanagi, K. Tanabe, K. Takahashi, T. Ichisaka, T. Aoi1, K. Okita, Y. Mochiduki, N. Takizawa and S. Yamanaka. 2007. Generation of induced pluripotent stem cells without Myc from mouse and human fibroblasts. Nat. Biotechnol. doi: 10.1038/nbt1374.
- Shao, L., W. Feng, Y. Sun, H. Bai, J. Liu, C. Currie, J. Kim, R. Gama, Z. Wang, Z. Qian, L. Liaw, and W.S. Wu. 2009. Generation of iPS cells using defined factors linked via the self-cleaving 2A sequences in a single open reading frame. Cell Res. 19: 296-306.
- Junying Yu, Kejin Hu, Kim Smuga-Otto, Shulan Tian, Ron Stewart, Igor I. Slukvin and James A. Thomson. Human Induced Pluripotent Stem Cells Free of Vector and Transgene Sequences. Science DOI: 10.1126/science.1172482.
- Ryan Lister, Mattia Pelizzola, Yasuyuki S. Kida, R. David Hawkins, Joseph R. Nery, Gary Hon, Jessica Antosiewicz-Bourget, Ronan O'Malley, Rosa Castanon, Sarit Klugman, Michael Downes, Ruth Yu, Ron Stewart, Bing Ren, James A. Thomson, Ronald M. Evans, Joseph R. Ecker. 2011. Hotspots of aberrant epigenomic reprogramming in human induced pluripotent stem cells. Nature DOI: 10.1038/nature09798.
- Cedars-Sinai Opens New Induced Pluripotent Stem Cell Core Production Facility, Press release, September 21, 2011.
Last Modified October 6, 2011