Researchers in Wisconsin and Japan said yesterday that they have turned ordinary human skin cells into what are effectively embryonic stem cells without using embryos or women's eggs -- the previously essential ingredients that have embroiled the medically promising field in a nearly decade-long political and ethical debate.
The ability to turn adult cells into embryo-like ones capable of morphing into virtually every kind of cell or tissue, described in two scientific journal articles yesterday, has been a major goal of researchers for years. In theory, it would allow people to grow personalized replacement parts for their bodies from their skin cells and give researchers a powerful means of understanding and treating diseases.
Until now, only human egg cells and embryos, both difficult to obtain and laden with legal and ethical issues, had the mysterious power to turn ordinary cells into stem cells. And until this summer, the challenge of mimicking that process in the lab seemed almost insurmountable, leading many to wonder whether stem cell research would ever unload its political baggage.
Their enthusiasm notwithstanding, scientists warned that medical treatments are not immediately available. The new method uses genetically engineered viruses to transform adult cells into embryo-like ones, and those viruses can cause tumors. But the cells will be instantly useful for research -- "to move a patient's disease into a petri dish," as Daley put it.
Many teams had been racing to be first to create human embryonic stem cell equivalents without embryos after researchers in June succeeded with mice. Yet scientists around the world agreed that nobody deserved to win that race more than the two competing scientists who did: James Thomson of the University of Wisconsin at Madison, who first isolated stem cells from 5-day-old human embryos, and Shinya Yamanaka of Kyoto University, who led the recent effort to obtain mouse stem cells without embryos.
Human embryonic stem cells, from days-old embryos, can multiply without limit and also develop into all of the 200 or so types of cells that make up the body. But because extracting them typically destroys the embryo, experiments with them have been attacked by those who believe that even the earliest stages of human life have moral standing.
An alternative way of making the cells, in which scientists fuse a skin cell to an egg cell whose own DNA has been removed, proved that egg cells harbor chemicals that can turn adult cells into embryonic ones, apparently by turning key genes on or off. But this method, too, raised concerns because large-scale harvesting of eggs from women can be medically risky and exploitative.
The dream of doing in a lab dish what an egg cell does naturally began to come true in June, when Yamanaka's team identified four genes in mouse skin cells that, when operating at high levels together, can turn countless other genes on and off in just the right pattern to make skin cells almost indistinguishable from embryonic stem cells. Yamanaka put copies of those four genes into retroviruses, Trojan-horse-like viruses that insert their genetic payloads into the DNA of cells they infect. Once infected, the skin cells took on virtually all the characteristics of embryonic ones.
More generally, retroviruses are a problem because they disrupt a cell's DNA in random locations, which can trigger tumor growth.
Both Thomson and Yamanaka said they are now testing methods that don't involve retroviruses. Among them are adenoviruses and fatty bubbles called liposomes, which deliver genes to cells without harming DNA, or even direct-injecting the biochemicals that the added genes produce inside cells.
» "Political controversy set the field back four or five years"
» "It also provides a great way to test potential treatments"