By Julie Steenhuysen

CHICAGO (Reuters) - U.S. researchers said on Thursday they have found a safer way to coax human skin cells into becoming powerful embryonic-like stem cells, taking a step closer to their potential use as treatments for diseases.

A team at the University of Wisconsin said they made the so-called induced pluripotent stem cells, or iPS cells, from human cells without using viruses or exotic genes, which leave behind genetic material that might pose risks if the cells were used as medical therapies.

James Thomson of the University of Wisconsin, whose study appears in the journal Science, said the finding represents the first time researchers have made human induced pluripotent stem cells without inserting potentially problematic new genes into their DNA.

Many teams are working on better ways to get ordinary skin cells to behave like embryonic stem cells, the body's master cells that give rise to all 220 cell types in the human body.

Scientists hope to harness the unique qualities of these cells to create new treatments for a variety of medical conditions.

Induced pluripotent stem cells promise many of the possible therapeutic benefits of embryonic stem cells without the ethics controversy because, unlike embryonic stem cells, they can be created without destroying a human embryo.

But earlier methods of making the iPS cells required the use of viruses as a vehicle, or "vector," to carry genes into the cells and trigger cell reprogramming.

Thomson said the new method uses a circle of DNA called a plasmid, which carries the genes needed to transform a skin cell into an iPS cell.

NO HARMFUL GENES

Over time, the plasmid disappears naturally from the cell population, avoiding the danger posed by using viruses, which can insert harmful genes into the cells' genetic material.

"That means they are less likely to form tumors, less likely to destroy the function of some important gene," Thomson said in a telephone interview.

Other teams have used different methods to do the same sort of thing in mouse cells, but not in human cells, said Jeremy Berg, director of the National Institute of General Medical Sciences, part of the U.S. National Institutes of Health.

"What Dr. Thomson has done for the first time in human iPS cells is created methods which don't involve inserting DNA into the host genome at all -- using plasmids which go into the cells but never get incorporated into the DNA," Berg said in a telephone interview.

Ultimately, Thomson thinks there will be several methods for creating iPS cells, and then scientists will start looking for the ones that produce the most consistent results.

Thomson said President Barack Obama's move earlier this month to lift Bush administration restrictions on federal funding of human embryonic stem cell research will make it easier to test the effectiveness of new iPS cells.

While they still hold promise for treatments, Thomson said some of the first benefits of iPS cells will be in testing the effects of new drugs on human tissue, rather than animals.

(Editing by Michael Kahn and Will Dunham)

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