Researchers Report Stem Cell Advance
THURSDAY Sept. 25, 2008 -- Researchers report that they have sidestepped a major technical hurdle in the generation of pluripotent stem cells from adult cells.
A team of Boston scientists developed a way to generate induced pluripotent stem cells (iPS) -- which are functionally similar to embryonic stem cells, but which can be produced from adult cells, rather than via the creation or destruction of an embryo -- more safely than ever.
Should the findings, which involved mouse cells, be repeated with humans, they could pave the way for using iPS to delve into the biology of a wide range of genetic diseases. Longer term, they could lead to patient-specific stem-cell therapies.
"I think it's a really important, landmark study," said Kevin Eggan, an assistant professor of Stem Cell and Regenerative Biology and an assistant investigator of the Stowers Medical Institute at Harvard University. He was not involved in the study.
The results were published in the Sept. 25 online edition of Science.
Shinya Yamanaka, of Kyoto University, Japan, first demonstrated in 2006 that adult mouse cells -- for instance, skin cells -- could be reprogrammed into something akin to an embryonic stem cell by the introduction of four specific genes. According to the lead author of this latest study, Matthias Stadtfeld, that "was like a gigantic, essentially quantum leap for biology." The following year, Yamanaka and James Thomson, of the University of Wisconsin, Madison, demonstrated the same approach could create human iPS cells.
Normally, the four genes -- all of which can induce cancer if left unchecked -- are delivered using retroviruses, which integrate their viral DNA into the cells' chromosomes; the worry is that these random insertions will introduce mutations into the cells that would alter their behavior, thus minimizing the cells' potential usefulness as research tools. Should these cells ever be used to generate tissues that were transplanted into human patients, researchers fear they could inadvertently lead to cancer.
Konrad Hochedlinger, of Massachusetts General Hospital and the Harvard Stem Cell Institute, his postdoctoral fellow Stadtfeld, and their colleagues circumvented this problem by delivering the genes using adenoviruses instead, which do not insert their viral DNA into a cell's chromosomes. iPS cells generated by this new approach appear indistinguishable from other iPS cells, carry some of the molecular hallmarks of embryonic stem cells, and can form multiple cell types when transplanted into mice (that is, they are pluripotent).
"My conclusion is that viral integration is not necessary for reprogramming to a pluripotent state, which is an important step toward safer patient-specific iPS cells, if it can be translated into humans," said Stadtfeld.
Human iPS cells have several potential applications. On a research level, they may be used to study how particular genetic defects lead to disease. Pharmaceutical companies might be able to use these cells to design drugs that alter, circumvent or repair these behaviors. Ultimately, iPS cells could be used clinically to develop patient-specific transplants, for instance, of genetically repaired neurons in patients with neurodegenerative diseases.
Before any of that can happen, however, this new method must be optimized. Only about one in 1 million skin cells actually developed into an iPS by Stadtfeld's method, compared to one in 10,000 using retroviruses. Using liver cells (hepatocytes), the efficiency was about one in 50,000 -- better, but still worse than with retroviruses.
"For this to be translated into humans, we have to find ways to make the process more efficient and properly make it work in cell types which are more easily accessible than hepatocytes, such as skin cells, for example," said Stadtfeld.
Dr. Rudolf Jaenisch, of the Whitehead Institute and Massachusetts Institute of Technology, who studies iPS cells, called the findings "clearly an advance."
"They show that, although very inefficiently, they can get iPS cells, which apparently don't have any genetic alterations," he said.
Though he praised the study, Eggan noted several caveats. First, though adenoviruses do not normally integrate their DNA into the host cell's chromosomes, sometimes they do. "There's always going to be this lingering possibility that it can happen in some cases," he said.
Yet Eggan also said that, given the success of this study, researchers may ultimately find that they can reprogram adult cells into iPS cells without using viruses at all, for instance, with chemicals or by delivering pure RNAs instead.
"All sorts of things like this become more of a reality due to the success of this experiment," he said.
Eggan's second caveat was that, despite their apparent similarity, iPS cells are not embryonic stem cells. It remains an open question as to whether the two are functionally equivalent.
"There are a variety of fundamental issues which haven't been well addressed about the equivalence of iPS and embryonic stem cells," he said, "I cannot say that strongly enough."
For instance, no one has yet been able to grow a mouse from a single iPS cell -- what Eggan calls the "gold standard" assay of embryonic stem cell pluripotency -- and not for lack of trying, he said.
Questions aside, Stadtfeld said he anticipates no problems translating these findings to human cells, aside from the fact that human embryonic stem cell culture is inherently trickier than its murine counterpart.
"I believe it will be a technical hurdle, but I would expect people would take the hurdle in the next half-year or year -- probably half-year," he said. Then he added, "I think maybe people have taken the hurdle already. It's possible."
For more about stem cells, visit the National Institutes of Health.
Posted: September 2008