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Cell reprogramming leaves a "footprint" behind

Reprogramming adult cells to recapture their youthful "can-do-it-all" attitude appears to leave an indelible mark, a team of Salk researchers including Joseph Ecker and Ronald Evans has found. When the scientists scoured the epigenomes of so-called induced pluripotent stem (iPS) cells base by base, they discovered a consistent pattern of reprogramming errors. What's more, these incompletely or inadequately reprogrammed hotspots are maintained when iPS cells are differentiated into a more specialized cell type, providing what the researchers dubbed an iPS cell- specific signature.

"We can tell by looking at these hotspots whether a cell is an iPS cell or an embryonic stem cell," says Ecker. "But we don't know yet what it means for their self-renewal or differentiation potential."

These findings, published in Nature, confirm that iPS cells, which by all appearances look and act like embryonic stem (ES) cells, differ in certain aspects from their embryonic cousins, emphasizing that further research will be necessary before they can rightfully take embryonic stem cells' place.

The fact that reprogramming of somatic (body) cells does not pose the same ethical quandaries as working with stem cells isolated from eggs or embryos prompted Japanese and U.S. scientists to develop human iPS cells that are just as potent as human embryonic stem cells, with the hope that one day, iPS cell technology can be applied to regenerative medicine. But Ecker's study makes it clear that before cells derived from iPS cells can be used to repair tissue damaged through disease or injury, some remaining questions have to be solved. "Embryonic stem cells are considered the gold standard for pluripotency," says Ecker. "So we need to know whether—and if so, how—iPS cells differ from ES cells."