The end of the politically explosive, decadelong ethical battle over human embryonic stem cells may finally be in sight. Two groups of researchers report today that washing human skin cells in similar cocktails of four genes enabled them to reprogram the cells to resemble those harvested from embryos. The finding potentially paves the way for scores of labs to generate new stem cell lines without cloned embryos, which had long been considered the only realistic way of making human stem cells in the short run.

"The long run's getting a lot closer," says stem cell biologist James Thomson of the University of Wisconsin School of Medicine and Public Health in Madison, a senior author of one of the studies. "I do believe this is the beginning of a great change."

He is not alone. British newspapers reported this weekend that Ian Wilmut, the University of Edinburgh biologist who led the team that in 1997 cloned Dolly the sheep, is getting out of the cloning business in light of the new findings, which seem to offer researchers a likely new source of stem cell lines for basic research that could one day lead to new treatments and perhaps cures for spinal injuries, diabetes and debilitating disorders such as multiple sclerosis and Parkinson's disease. In the nearer term, reprogrammed cells may improve the screening drug candidates for harmful side effects.

"That's the writing on the wall right now," says biologist Arnold Kriegstein, director of the Institute for Regeneration Medicine at the University of California, San Francisco, who was not involved in the research.

Although both groups' reprogrammed cells were able to differentiate into the three main tissue types when injected into mice, Thomson cautions that they may harbor subtle, yet to be found quirks, and will need considerable tweaking before they could be safely transplanted into humans. Importantly, researchers must still study existing embryonic stem cell lines—the gold standard—to rule out any hidden risks in the lab-made cells, he says. "People want to rush and say, 'we don't need embryonic stem cells anymore,' and over time that might be true, but right now that's premature."

The results help fill in the scientific puzzle kicked off by Dolly's cloning, which proved that mammalian egg cells were capable of dissolving the genetic roadblocks that limit the potential of most adult cells to give rise to only a single type of tissue—that of the organ from which they hail—whereas embryonic stem cells have the potential to become virtually any kind of body tissue.

So researchers began testing genes that were only active in embryonic stem cells to try to pin down those capable of triggering the change. One such group, led by biologist Shinya Yamanaka of Kyoto University in Japan, reported last year that four genes, delivered to mouse cells by a retrovirus, were sufficient to induce pluripotency (the ability to differentiate into a multitude of cell types). The genes—Oct 3/4, Sox2, c-Myc and Klf4—are molecular switches called transcription factors, which activate other genes in series like a power strip.

Yamanaka's group and two others followed up earlier this year with firmer evidence that these induced pluripotent stem (iPS) cells faithfully mimicked the patterns of gene activity and cellular differentiation observed in embryonic stem cells.

Now Yamanaka and his colleagues report in the journal Cell that the same combination of genes induced pluripotency in commercially available human fibroblasts (connective tissue cells that play a crucial role in healing) derived from the facial skin of a 36-year-old woman, the joint tissue of a man, aged 69, and a newborn, respectively.

The researchers report they were able to transform about one in 5,000 cells—enough to get several iPS cells from a single culture dish—and then coax them to become nerve cells or heart tissue on the benchtop. Genetic scans indicated that the cells were more similar to embryonic tissue than to the original fibroblasts.

"What I find remarkable," Kriegstein says, "is essentially the same steps that worked in the mouse were able to work with human cells…. Everybody assumed that it'd be a different story in reprogramming the human cells."

Thomson's team reports producing a similar cellular alchemy using two of the same genes—Oct4 and Sox2—and two different ones—Nanog and Lin28,—making it less likely that the Japanese finding was a fluke.

Thomson, who in 1998 became the first scientist to extract human stem cells from embryos, says his group began seeking these factors four years ago, but chose to work with human cells. As of last spring, he says, his group, led by lab member Junying Yu, had pared an initial list of 100-plus genes to 14.

Then came Yamanaka, who Thomson says beat him to the punch because mouse cells grow much faster than human cells do, allowing more rapid experimentation. "We thought, 'oh no, it's already been done; we've been beaten,'" he recalls.

As many as a dozen major labs, he says, have since tried but failed to make reprogramming work in human cells. His team plugged along, testing gene combinations in four cell types in varying degrees of differentiation, hopeful that this would eventually lead to the correct genetic recipe.

In the online edition of Science, he and his colleagues report that Oct4 and Sox2 were capable of converting neonatal foreskin fibroblasts into cells similar to Yamanaka's, whereas Nanog significantly boosted the frequency of reprogramming and Lin28 upped it by a moderate amount.

Although Oct4 and Sox2 were well-known players in embryonic cells, "we thought this would be such a complicated problem, we never tested those genes up front," Thomson says. "It's kind of remarkably lucky that three or four [genes] are sufficient. I wasn't optimistic it would work."

The new results still leave researchers with the task of double-checking that reprogrammed cells are safe and truly have the same potential as the embryonic variety. They also have to figure out how to circumvent problems with the viral delivery system, which may disrupt important genes, resulting in cancer. Kriegstein notes that the hunt will likely commence for small molecules capable of activating the key genes.

Thomson predicts that companies such as Madison-based Cellular Dynamics International, which he co-founded, that test drug candidates for dangerous heart toxicity, could begin using cells derived from reprogramming in their assays within a year.

Whatever the source of pluripotent cells, Thomson says, researchers face the same scientific challenges—namely, understanding how to convert them into key tissues such as the beta islet cells that are impaired in diabetics, and then how to introduce them safely and effectively into humans.

Opponents of research on human embryos might contend that reprogramming happened because of the federal restrictions on embryonic research, but Thomson believes the stigma on the field made researchers wary and delayed the discovery of reprogramming by several years. "I'm cautiously optimistic" about reprogrammed cells, he says. "The worry is the politics will get involved again and squash caution."