Researchers have found a new way to turn adult cells into embryonic stem (ES) cells: using human eggs, or oocytes. The feat comes after more than a decade of failed attempts, and it is still a work in progress. The resulting stem cells are not normal; they carry the genomes of both the adult cell and the oocyte, so they have three copies of each chromosome instead of the usual two. But they seem, in initial tests, to act like other pluripotent stem cells, cells that scientists prize because they can form all of the body's tissue types.
"The authors are to be congratulated," says Ian Wilmut of the University of Edinburgh in the United Kingdom, who with his colleagues cloned Dolly the sheep in 1996 using the same technique, which is known as nuclear transfer.
The work, published online today in Nature, could help scientists better understand "cellular reprogramming," the process that can bestow pluripotency onto an adult cell. Researchers hope to use the technique to make patient-specific stem cell lines, which can help them better understand certain diseases. Ultimately, they hope to be able to treat patients with such cells.
At the same time, the result is sure to spark controversy. Many people fear that such research will prompt demand for human oocytes. Others oppose work such as this because it involves creating a human embryo and then destroying it. And some worry that a version of the technique could be used to generate a viable baby, i.e., to clone a human being. In the near term, at least, the fact that the procedure produces abnormal cells should dampen some of those concerns. The resulting embryos developed for about a week, but they would likely not be viable much beyond that point.
In 2006, a team led by Shinya Yamanaka of Kyoto University in Japan found a way to reprogram cells without oocytes. The team members found that by turning on a handful of genes, they could transform adult cells into pluripotent cells dubbed induced pluripotent stem (iPS) cells. That technique seemed to offer a way around the difficult and controversial research on human nuclear transfer (the scientific term for replacing an oocyte's DNA with that of an adult cell).
But the question has remained: Are iPS cells equivalent to ES cells? There is some evidence, for example, that mouse ES cells produced via nuclear transfer may be more thoroughly reprogrammed than iPS cells, some of which retain traces of the adult tissue type they came from. Many researchers argue that to fully understand reprogramming, they need to be able to study human cells reprogrammed by oocytes.
Since the 1996 announcement that researchers had used nuclear transfer to clone Dolly the sheep from an adult cell—the first such success in mammals—scientists have been trying to use a similar technique to clone human and monkey cells. But primate egg cells have proved very difficult to work with. (Most famously, Woo Suk Hwang at Seoul National University in South Korea claimed to have made a dozen stem cell lines from human nuclear transfer-derived embryos. Those claims turned out to be fraudulent.) In the vast majority of attempts, the human embryos created by nuclear transfer seem to stop developing after about 3 days, when they have just six to eight cells—too early to procure stem cells.
The kind of work described in the Nature study has also been difficult because it requires a scarce resource: eggs from young, fertile, healthy women. Dieter Egli of the New York Stem Cell Foundation laboratory in New York City was able to cooperate with a fertility clinic associated with Columbia University. That partnership gave him a steady supply of oocytes donated by women specifically for research. (The donors received the clinic's standard $8000 payment.) Egli used the chance to set up a systematic study of human nuclear transfer.
At first, the researchers ran into the familiar roadblock. When they removed the egg's nucleus, fused the genome-free oocyte with a skin cell, and triggered cell division, the eggs divided once or twice. But after 2 or 3 days, the cells stopped dividing and ultimately died.
Egli and colleagues noticed that development stopped at the time when the embryonic nucleus would usually start expressing genes. And they saw that the green fluorescent protein (GFP) that marked the donor skin cells was not expressed in the arrested cells. When they looked more closely, they found several lines of evidence that, for some reason, the new nucleus wasn't able to turn on any of its genes.
As a control experiment, the researchers fused a GFP-tagged donor skin cell with an intact oocyte—without removing the oocyte nucleus—and triggered the cell to divide. Immediately, they saw a difference. After a few days, the embryos started to express the GFP. And, out of 63 tries, the researchers produced 13 blastocysts, the hollow ball of cells that forms around day 5 of development. From those 13 blastocysts, the researchers were able to derive two stem cell lines. One carries the genome of a male who has type 1 diabetes, and the other carries the genome of a healthy male adult. Despite their extra chromosomes, the cells expressed genes typical of pluripotent cells, and they were able to form tissues from all three embryonic germ layers—a basic test of pluripotency. An initial analysis also suggests the skin cell "memory" had been erased.
The experiments take researchers much closer to understanding the obstacles in primate nuclear transfer experiments—and ultimately overcoming them, Wilmut says. "There is clearly something missing that you have to have at the start of transcription," he says. "It should be possible to a), identify it, and b), supply it."
Egli and his team are now looking for the missing factor, he says, as well as testing to see whether using a different kind of adult cell—perhaps a stem cell from blood or neural tissue—might get around the problem. They will also continue to characterize the nuclear transfer stem cell lines, he says, to see how they compare with ES and iPS cells.
Despite the ethical, legal, and practical hurdles that complicate his work, Egli says the effort is worth it. "It's not about determining which is the easier approach," he says. "It is about determining which is the better approach."