CHICAGO - Using a new type of stem cells made from ordinary skin cells, U.S. researchers said on Thursday they treated mice with sickle cell anemia, proving in principle that such cells could be used as a therapy.

U.S. and Japanese researchers last month reported they had reprogrammed human skin cells into behaving like embryonic stem cells, the body's master cells. They call the cells induced pluripotent stem cells, or iPS cells for short.

The Japanese team had previously done the reprogramming work in mouse skin cells.

A team at the Whitehead Institute of Biomedical Research in Cambridge, Massachusetts, has now used the new cells to treat mice engineered to have sickle cell anemia, a disease of the blood caused by a defect in a single gene.

"This is the first evaluation of these cells for therapy," said Dr. Jacob Hanna, who worked on the study. "The field has been working for years on strategies to generate customized stem cells," he added in a telephone interview.

Creating stem cell therapies from a person's own cells would make them genetically identical, eliminating the need for immune suppression or donor matching, Hanna said.

"Now, with the breakthrough of this new method for generating stem cell-like cells, can we try to substitute a diseased tissue in a living animal?"

Hanna and colleagues working in Rudolf Jaenisch's lab at Whitehead Institute took skin cells from diseased mice and inserted four genes that reprogram the cells into becoming iPS cells.

"We call it the magic four factor," Hannah said.

Pluripotent or multipurpose cells, such as embryonic stem cells and the new cells, can morph into any type of cell in the human body.

The researchers then coaxed these mouse master cells into becoming blood-forming stem cells and substituted the faulty gene that causes sickle cell anemia with a working one.

FAR FROM PERFECTED

When they transplanted these cells into the diseased mice, tests showed normal blood and kidney function, they report in Friday's issue of the journal Science.

"This demonstrates that iPS cells have the same potential for therapy as embryonic stem cells, without the ethical and practical issues raised in creating embryonic stem cells," Jaenisch said in a statement.

But the technique is far from perfected.

The four genes needed to turn skin cells into master cells are delivered using a type of virus called a retrovirus.

"Once they enter the genome, there is the danger that they can silence some genes that are important or they can activate some dangerous genes that shouldn't be activated," Hanna said.

Another obstacle is that one of the four genes used is c-Myc, which is known to cause cancer.

Hanna and colleagues got around that by removing the c-Myc gene after it had done its job of converting the skin cells into iPS cells. "It is far from solving the problem," he said.

Scientists hope to use stem cells to treat a host of diseases like diabetes, Parkinson's disease and spinal injuries. And the new technique for making stem cells will make them easier to study.

But many researchers including Hanna say human embryonic stem research paved the way for such discoveries and should continue.

"They are the gold standard for what is normal and how a stem cell should behave," he said.