Stem Cells without Side Effects

Last year, researchers announced one of the most promising methods yet for creating ethically neutral stem cells: reprogramming adult human cells to act like embryonic stem cells. This involved using four transcription factor proteins to turn specific genes on and off. But the resulting cells, called induced pluripotent stem (iPS) cells for their ability to develop into just about any tissue, have one huge flaw. They're made with a virus that embeds itself into the cells' DNA and, over time, can induce cancer. Now, scientists at Harvard University have found a way to effect the same reprogramming without using a harmful virus--a method that paves the way for tissue transplants made from a patient's own cells.

The first generation of iPS cells was created using a retrovirus to insert the four transcription factors into skin cells. Because a retrovirus, by definition, inserts itself permanently into its host's DNA, this ensured that the transcription factors were transferred,, but it also led to the propagation of the virus itself. Furthermore, since the virus confers self-renewal capabilities to its new host cell, many believed that the retrovirus might be required for iPS cells to reproduce.

New research by Konrad Hochedlinger and his colleagues at Harvard University, the Harvard Stem Cell Institute, and the MGH Center for Regenerative Medicine shows that a different type of virus--an adenovirus--can make the transfer in mouse cells without permanently integrating itself. The resulting iPS cells can divide indefinitely but show no trace of the virus--just a temporary infection that disappears within a short time. "That means that the four transcription factors themselves are sufficient to induce pluripotency in adult cells," Hochedlinger says.

Many view the creation of genetically unmodified iPS cells as regenerative medicine's magic bullet. The cells are not derived from embryos, so researchers can circumnavigate the ethical gray areas. And if these cells turn out to be as potent as embryonic stem cells, they could be used to help regrow tissues damaged in conditions ranging from paralysis to Parkinson's disease to diabetes. If they can be grown from a patient's own cells, they could furthermore be transplanted without triggering immune rejection.

Until now, however, creating iPS cells without integrated viruses had been a major hurdle for stem-cell researchers. Although Hochedlinger has overcome that hurdle, he says there is still some distance to travel. While retroviral techniques allow scientists to turn about one in every 1,000 skin cells into an iPS cell, the adenovirus is far less efficient: only one in every 10,000 to 100,000 fetal liver cells can be converted. "It may be that people have tried adenoviruses before but missed the iPS cells because the efficiency is so low," Hochedlinger says. "We ourselves tried to use adenoviruses a year ago, and it didn't work."

The team's first attempt had been with skin cells. Upon hearing that liver cells required less viral integration for effective reprogramming, however, they changed their approach.

The efficiency, as described in latest edition of the journal Science, is still incredibly low. Out of 1 million adenovirus-infected cells, the researchers ultimately produced just one stable line of stem cells. But the line was genetically unaltered, and when the cells were implanted in mice, they formed a cluster of cells that had differentiated into multiple tissue types (a standard test for pluripotency). When the researchers injected the cells into mouse embryos, the resulting mice had integrated the stem cells into a number of different types of tissue, including tissue in the brain, lungs, and heart. And mice as old as 13 weeks remained tumor-free.

Until now, iPS cells couldn't be compared to embryonic stem cells, since the effects of the integrated virus were unknown. "It was like comparing apples and oranges," Hochedlinger says. Now, however, the potency of the two cell types can be evaluated head to head. "You can really think about doing this in a human setting now, and about making genetically unmodified human cells for modeling or even for therapy."

The finding already has other stem-cell experts thinking about the possibilities. "The paper represents a major breakthrough in reprogramming research and proves to the field that we can reprogram cells directly without viral contamination," says George Daley, a Harvard biologist and stem-cell researcher who was not involved with the research. "It is a major step towards making clinical transplantation of patient-specific cells feasible."

Hochedlinger and his colleagues are now working to increase the efficiency of their adenovirus technique and to repeat their methods to create human iPS cells. "Once we do that," Hochedlinger says, "we can figure out whether [embryonic stem] cells and unmodified iPS cells are really identical to each other or not. I don't know the answer yet."