Researchers from Japan and Wisconsin reported Tuesday that they had reprogrammed mature human cells to behave almost exactly like embryonic stem cells, a biological breakthrough that instantly recasts the field's ethical, scientific and economic landscape.
By activating a handful of dormant genes, the researchers were able to coax the cells back in time to a point in embryonic development before they had committed to becoming a particular type of tissue.
The rejuvenated cells were able to grow into all the main tissue types in the body, including muscle, gut, cartilage, neurons and heart cells.
The discovery provides a clear road map for creating genetically matched replacement cells that could be used to treat patients for a variety of diseases -- the personalized biological repair kits that are the ultimate goal of regenerative medicine.
For scientists, the method offers a straightforward alternative to the tricky and still unsuccessful cloning process in which a patient's DNA is inserted into a human egg to create a cloned embryo whose stem cells theoretically could be harvested.
The technique also bypasses the thorny debate over the morality of destroying embryos in the cause of alleviating human suffering.
"It's a win-win," said Richard Doerflinger, secretariat for pro-life activities at the U.S. Conference of Catholic Bishops in Washington. "The scientists can get all the benefits they think they might get from embryonic stem cells, and the rest of us can applaud and support it."
Several key hurdles remain before the technique is ready for clinical use. The viruses employed to turn on the genes cause mutations that can lead to cancer, and one of the genes itself also has a tendency to cause tumors.
But scientists said solutions to these problems were in the works.
"This is a tremendous scientific milestone -- the biological equivalent of the Wright brothers' first airplane," said Dr. Robert Lanza, a stem cell researcher at Advanced Cell Technology in Worcester, Mass., who wasn't involved in the research.
The White House praised the work as an example of cutting-edge research that was conducted "within ethical boundaries."
"The president believes medical problems can be solved without compromising either the high aims of science or the sanctity of human life," Press Secretary Dana Perino said.
The discovery has been eagerly anticipated since June, when three research groups achieved the same feat in mice. Scientists expected the experiments to be repeated in humans, but many said it would take years, not months.
Stem cells are coveted for their ability to grow into any kind of cell, such as insulin-secreting islet cells that diabetes patients need or brain tissue that could treat stroke victims.
Until now, the only source of such "pluripotent" cells was the inner cell mass of an early-stage embryo, and the only way to harvest them was by destroying the embryo.
The advance was made by Dr. Shinya Yamanaka, who spearheaded the reprogramming technique in mice, and his colleagues at Kyoto University. They dubbed their cells induced pluripotent stem cells, or iPS cells.
In their new study, published in the journal Cell, they applied essentially the same recipe used with the mice skin cells to human cells taken from the subsurface layer of facial skin belonging to a 36-year-old woman.
The idea was to turn on genes that are active during embryonic development to see whether they would rewind mature adult cells. After testing combinations of 24 candidate genes, the team hit upon a group of four that produces the proteins Oct4, Sox2, c-Myc and Klf4.
Yamanaka's group used a retrovirus to turn on the genes. The proteins, known as transcription factors, initiated a still-unknown biochemical process that returned the cells to an embryonic state.
The researchers grew the iPS cells in dishes and found they behaved almost exactly like embryonic stem cells. Under the right conditions, they became neural cells, or cardiac cells that beat in unison. When injected into mice, the iPS cells formed tumors containing a jumble of body parts.
"They are very, very close, if not functionally identical to human embryonic stem cells," said Owen Witte, director of the Broad Center of Regenerative Medicine and Stem Cell Research at UCLA.
Altogether, the researchers were able to create about 10 lines of iPS cells from each batch of 50,000 skin cells.
"The incredible thing about it is how easy it seems to be," Doerflinger said. "From one little biopsy of a skin sample, you always get enough cells changing to create a stem cell line."
Yamanaka said he could not use human embryonic stem cells in Japan, so he hadn't been able to make a head-to-head comparison between those cells and the new iPS cells. He said he intended to do the tests at UC San Francisco's Gladstone Institute of Cardiovascular Disease.
Even if there were subtle differences in the iPS cells, he said, "I don't think they have to be identical" to embryonic stem cells to be useful in medical applications.
A second team led by Dr. James Thomson at the University of Wisconsin-Madison started with a group of 14 proteins that the researchers suspected would return mature human cells to a pluripotent state. They winnowed down that pool to four proteins: Oct4, Sox2, Nanog and Lin28.
Using their recipe, the Wisconsin scientists were able to reprogram fetal connective tissue cells called fibroblasts. It also worked on fibroblasts from the foreskin of newborns. Their results were published in the journal Science.
Thomson, who isolated the first human embryonic stem cells in 1998, says that the technique is being tested in older cells and that he is optimistic it will work. Other scientists said it might be difficult to adapt the method to cells that were years removed from their embryonic origins.
One benefit of Thomson's approach is that he did not use c-Myc, which has a tendency to cause tumors. About 20% of the mice in Yamanaka's previous study developed tumors attributed to the protein.
The fact that reprogramming could occur without the problematic c-Myc "represents a very important advance," Lanza said.
Both of the discoveries will bring a host of practical benefits. President Bush made most embryonic stem cell research off-limits for federal funding, but the new technique is eligible because it doesn't destroy human embryos. Indeed, Thomson's study was funded in part by the National Institutes of Health.
Money from the California Institute for Regenerative Medicine, which was established to fund embryonic stem cell research, can be used for iPS experiments even though federal support is also available, said Robert Klein, chairman of the agency's oversight board. However, he predicted that studies on embryonic stem cells would win the bulk of the agency's grants.
Scientists, including Yamanaka, stressed the need to continue funding for traditional embryonic stem cell research.
"It would be extremely foolish to place all of your bets on any single approach," said Witte of UCLA.
Some were also concerned that the new approach would prompt a brain drain away from embryonic stem cells, which are still necessary to illuminate the fundamental processes of developmental biology.
Just last week, cloning pioneer Ian Wilmut said he planned to shift his focus from the technology he used to clone Dolly the sheep in 1996 to cellular reprogramming.
"In the long run, reprogramming will be so much more useful and practical," said Wilmut, director of the Scottish Centre for Regenerative Medicine at the University of Edinburgh.
Doerflinger, a longtime opponent of embryonic stem cell research, said he was heartened by Wilmut's decision.
"It would, of course, be ethically unjustifiable to pursue the research using embryos if there were a less morally controversial way to proceed," he said. "Increasingly, it seems that burden has been met."
Though the new technique dodges some significant ethical problems, it also creates new ones, said Insoo Hyun, a bioethicist at Case Western Reserve University in Cleveland. In previous experiments, mouse iPS cells were transformed into sperm and egg cells. If the same can be done in people, he said, "it transforms what we think about human fertility."
Theoretically, the method would even enable people to reproduce after death as long as they banked a tissue sample.
"A person doesn't even have to be alive to create sperm or eggs," Hyun said. "It really is a new technology that brings with it a new set of issues."
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