In the latest milestone on the road toward reprogramming cells to pluripotency without permanent genetic modification, researchers have successfully turned the clock back on adult stem cells using only a single transcription factor, according to a study published today (Feb. 5) in Cell.

Ever since Kyoto University's Shinya Yamanaka showed in 2006 that the overexpression of just four genes -- c-Myc, Sox2, Oct4, and Klf4 -- could effectively turn adult skin cells into embryonic-like induced pluripotent stem (iPS) cells, researchers have been in a race to simplify the recipe. Last year, a team led by Hans Schöler of the Max Planck Institute for Molecular Biomedicine in Münster, Germany, found that just two of the standard four genes -- Oct4 and Klf4 -- were necessary to reprogram mouse adult neural stem cells. Now, Schöler has cut that number in half again by fine-tuning his protocol to only a single factor: Oct4.

Schöler's team inserted Oct4 into mouse neural stem cells using retroviruses, and created iPS cells that could efficiently turn back into neural stem cells or become heart muscle cells and germ cells in vitro. What's more, when they mixed the cells with mouse embryos, the reprogrammed cells contributed to the developing gonads and could be transmitted to the next generation. Although the reprogramming efficiency was about ten-fold lower than Schöler's two-factor approach, it was on par with the four-factor technique when starting with skin cells, also known as fibroblasts.

Konrad Hochedlinger of the Massachusetts General Hospital and the Harvard Stem Cell Institute, who was not involved in the study, said it was "amazing" that only a single transcription factor was sufficient to induce pluripotency in a somatic cell, although he was not surprised by the study's findings. "I think it was just a matter of time before we found the optimum conditions to go down in the number of factors," he told The Scientist.

The study shows the utility of starting with tissue-specific cells, which naturally express some of the markers in Yamanaka's original reprogramming toolkit and are often easier to reprogram than fibroblasts, said Sheng Ding, a stem cell researcher at the Scripps Research Institute in La Jolla, Calif., who was not involved in the research. "It's very important to explore these tissue-specific precursor cells," he told The Scientist. "That will allow you to use a minimal number of genes to reprogram a cell."

The one-factor reprogramming of neural stem cells "will now focus attention on whether Oct4 alone can [induce pluripotency] in other cells," Schöler told The Scientist. "Now we have to see if that works for fibroblasts." Last year, Douglas Melton's group at Harvard showed that a two-factor approach, involving Oct4 and Sox2, in combination with a small-molecule inhibitor was sufficient to reprogram human fibroblasts. Schöler suspects a similar approach should work with just Oct4, too.

Schöler's one-ingredient recipe eliminates the two known oncogenes, c-Myc and Klf4. However, it still involves viruses that permanently modify the cells' DNA, which precludes using the cells in any potential clinical applications. Still, fewer factors mean fewer genetic manipulations. And if Oct4 can be induced by non-retroviral means, the countdown to virus-free reprogramming may be complete.

"If you can now find a way to go down to zero [factors], that would be the last final step," said Schöler. "It certainly will come," said Hochedlinger. "It's a matter of time."


Related stories:

Safer iPS cells
[25th September 2008]

Stem cell alchemy
[27th August 2008]

Epithelial cells made pluripotent
[14th February 2008]