Researchers Turn Skin Cells Into Stem Cells
By Gretchen Vogel
ScienceNOW Daily News
20 November 2007
The work builds on a study published last year by Shinya Yamanaka of Kyoto University in Japan, which showed that mouse tail cells could be transformed into ES-like cells by inserting four genes (ScienceNOW, 3 July 2006). Those genes are normally switched off after embryonic cells differentiate into the various cell types. In June this year, Yamanaka and another group reported that the cells were truly pluripotent, meaning that they had the potential to grow into any tissue in the body (ScienceNOW, 6 June).
Now the race to repeat the feat in human cells has ended in a tie: Two groups report today that they have reprogrammed human skin cells into so-called induced pluripotent cells (iPSs). In a paper published online in Cell, Yamanaka and his colleagues show that their mouse technique works with human cells as well. And in a paper published online in Science, James Thomson of the University of Wisconsin, Madison, and his colleagues report success in reprogramming human cells, again by inserting just four genes, two of which are different from those Yamanaka uses.
In the new work, Yamanaka and his colleagues used a retrovirus to ferry into adult cells the same four genes they had previously used to reprogram mouse cells: OCT3/4, SOX2, KLF4, and c-MYC. They reprogrammed cells taken from the facial skin of a 36-year-old woman and from connective tissue from a 69-year-old man. Roughly one iPS cell line was produced for every 5000 cells the researchers treated using the technique, an efficiency that enabled them to produce several cell lines from each experiment.
Thomson's team started from scratch, identifying its own list of 14 candidate reprogramming genes. Like Yamanaka's group, the team used a systematic process of elimination to identify four factors: OCT3 and SOX2, as Yamanaka used, and two different genes, NANOG and LIN28. The group reprogrammed cells from fetal skin and from the foreskin of a newborn boy. The researchers were able to transform about one in 10,000 cells, less than Yamanaka's technique achieved, Thomson says, but still enough to create several cell lines from a single experiment.
Although promising, both techniques share a downside. The retroviruses used to insert the genes could cause tumors in tissues grown from the cells. The crucial next step, everyone agrees, is to find a way to reprogram cells by switching on the genes rather than inserting new copies. The field is moving quickly toward that goal, says stem cell researcher Douglas Melton of Harvard University. "It is not hard to imagine a time when you could add small molecules that would tickle the same networks as these genes" and produce reprogrammed cells without genetic alterations, he says.
Once the kinks are worked out, "the whole field is going to completely change," says stem cell researcher Jose Cibelli of Michigan State University in East Lansing. "People working on ethics will have to find something new to worry about."
For a more in-depth news story on this topic, see this week's issue of Science, available online 22 November.
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Adult Cells Relive Their Youth
By Gretchen Vogel
ScienceNOW Daily News
3 July 2006
Yamanaka and his colleagues wondered whether the factors that give ES cells their unique properties might also be able to reprogram adult cells to behave like ES cells. They identified 24 genes that are specifically expressed in mouse ES cells and used viral vectors to introduce extra copies of the genes into skin cells taken from mouse tail tips. When they inserted extra copies of all 24 genes, they found that a small percentage of cells that took up the genes did indeed seem to take on characteristics of ES cells. But no single gene introduced alone was able to manage the transformation.
Through a process of elimination, the team whittled down the candidates to a suite of just four genes that, when introduced together into the tail-tip cells, could produce colonies of ES-like cells. As Yamanaka described, three of the four factors are old friends: Oct4, Sox2, and c-Myc are all key genes in both early embryos and ES cells. Yamanaka did not name the fourth gene, but he said it is a transcription factor that until now has not been recognized as playing a major role in ES cells.
The ES-like cells the group produced with the four introduced genes seemed to have almost all the key properties of ES cells derived from embryos. They formed several kinds of tissue in the culture dish and produced tumors called teratomas when they were injected under the skin of immune-compromised mice--both classic characteristics of ES cells.
Yamanaka says his group has not yet tried the technique with human cells. Because of differences in human and mouse embryo development, he says, it's possible that a different set of genes would be required to reprogram human cells.
Other researchers at the meeting were impressed. "It's huge," says Kevin Eggan of Harvard University, who also works on cell reprogramming. Still, he notes that the process is not yet very efficient; the four introduced genes managed to reprogram just 1 out of 1000 cells that received them. That suggests that the four genes are perhaps not the whole story, and that another factor could improve the efficiency of the process. "But this is the litmus test" for finding the genes that are essential for reprogramming, he says.
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Stem Cells Without the Fuss?
By Constance Holden
ScienceNOW Daily News
6 June 2007
The advances, reported in two papers in tomorrow's issue of Nature and in another paper in the July issue of Cell Stem Cell, extend a finding made last year by Shinya Yamanaka of Kyoto University in Japan. By inserting various combinations of genes related to pluripotency active in mouse ES cells, the researchers discovered a combination of four genes that, when introduced into cells from mice's tails, conferred ES-like properties upon them (Science, 7 July 2006, p. 27).
The three teams, headed by Yamanaka, the Massachusetts Institute of Technology's Rudolf Jaenisch, and Harvard's Konrad Hochedlinger, all began by following Yamanaka's procedure, using a viral vector to introduce copies of genes for four transcription factors active in ES cells: Oct4, Sox2, c-Myc, and Klf4. Because the reprogramming works for only one in every 1000 cells, the researchers needed to weed out the nonstarters. Yamanaka did this by looking for the activity of a gene that, as it turned out, selected for cells that were incompletely reprogrammed. In the new studies, the scientists used the expression of Oct4 and Nanog--well-known pluripotency markers.
The cells selected using these markers appear to have all the same traits as ES cells. To test this hypothesis, the researchers tagged the reprogrammed cells, called induced pluripotent stem (iPS) cells, with a fluorescent dye and injected them into early-stage mouse embryos. Some of the resulting chimeric animals had descendents of the iPS cells throughout their bodies. The researchers confirmed this by successfully breeding the chimeras to normal mice. This showed that iPS cells had made it to the germ line in the chimeras.
Together, the three papers give a convincing picture of the reprogramming phenomenon, says Harvard stem cell researcher Chad Cowan. But Yamanaka's study showed a downside as well. The only author to study the offspring of the chimeras after birth, he observed that 20% of the 121 mice developed tumors. That finding, Yamanaka notes, shows the danger of using retroviral vectors, which can turn on cancer-causing genes.
The drawback highlights the long road to potential therapies with reprogrammed adult cells. But Cowan is optimistic: "The most amazing thing about these papers is you now take this whole idea of reprogramming out of the hands of cloning specialists and put it into the hands of anyone who can do molecular and cell biology."
In this environment, the reprogramming studies, preliminary as they are, are likely to be seized on by critics of ES cell research as further evidence that there is no need for the contentious practice of destroying early embryos to obtain stem cells. Hochedlinger and others hasten to point out that research needs to progress on all fronts because all systems "have their limitations."
For a more in-depth version of this story, see the Friday, 8 June, issue of Science.
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