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Cells reprogrammed to make healthy tissue


By Karen Kaplan
Los Angeles Times
December 09. 2007 12:06AM


Taking the next step in a series of breakthrough stem-cell experiments, scientists have cured sickle-cell anemia in mice by rewinding their skin cells to an embryonic state and manipulating them to create healthy, genetically matched replacement tissue.

After the repaired cells were transfused into the animals, they soon began producing healthy blood cells free of the crippling deformities that deprive organs of oxygen, according to scientists from the Whitehead Institute for Biomedical Research in Cambridge, Mass., and the University of Alabama at Birmingham.

The experiments, published online by the journal Science, confirmed the therapeutic potential of a new class of reprogrammed stem cells, which can be custom-made for patients without creating and destroying human embryos.

The strategy should work to treat hemophilia, thalassemia and severe combined immunodeficiency disease, the "bubble boy" disease, according to researchers, and might also apply to disorders linked to mutations in a single gene, such as muscular dystrophy and cystic fibrosis.

Scientists hope to use a similar approach to create cardiac cells to treat heart attack patients or nerve cells that could cure spinal cord injuries. Finding an abundant source of stem cells that could be used as a personalized biological repair kit is the goal of regenerative medicine.

The technique is a few years away from being used to treat humans, scientists said. Before it could be tried, several rounds of animal experiments would need to be done. Researchers also will need to overcome key technical hurdles, including finding a way to reprogram adult cells without using genes and viruses that could cause cancer.

But as a proof of principle, the study is certain to lure more researchers into studying the new class of induced pluripotent stem cells, called "iPS" cells.

"There's going to be this tsunami," said Paul Simmons, director of the Center for Stem Cell Biology at the University of Texas Health Science Center in Houston. "One would have to predict that the pace of observations made using iPS cells is going to rise exponentially."

The study is the latest in a string of significant experiments published in the past five months involving a new approach of reprogramming adult cells so they are capable of growing into any type of tissue in the body. They have captivated researchers, ethicists and politicians looking for an alternative to embryonic stem cells, which can be difficult to work with and raise moral concerns.

Japanese researchers pioneered the new method, which involves turning on four genes that are dormant in adult cells but active in days-old embryos. The cells essentially "forget" that they have become skin cells and behave like embryonic stem cells. Because they are derived from a patient's own cells, there is no risk of tissue rejection.

In June, three research teams showed that the technique worked reliably in mice. In November, two groups demonstrated that it also worked with human cells. But it remained to be seen whether the cells could serve as the raw material to grow replacement parts for patients.

Misspelled DNA

The researchers started with sickle-cell anemia because it has a simple origin - at a key point on the hemoglobin beta gene, patients have a "misspelling" in the chemical letters of DNA, commonly known as A, C, T and G. Instead of having at least one A, they have a pair of Ts. As a result, the gene makes the wrong amino acid, resulting in red blood cells that are curved instead of round.

Those sickle-shaped blood cells get clogged as they travel through the body, blocking blood flow to the small vessels that feed the brain, kidney and other organs. Tissues die, since sickle cells can't deliver enough oxygen to keep them healthy.

Some patients can be treated with a bone marrow transplant, which allows the body to make normal red blood cells. But only about 5 percent of sickle-cell patients are able to find a donor, said Tim Townes, chairman of the department of biochemistry and molecular genetics at the University of Alabama at Birmingham and one of the study's senior authors.

Townes figured that embryonic stem cells might help the 95 percent of patients who couldn't find donors. But the process would be complicated. First, scientists would have to clone embryos using the patient's own DNA. Then they would switch one of the errant Ts to an A. Stem cells would then have to be harvested from the modified embryo and used to make healthy bone marrow for a transplant.

Better genes

But before scientists were able to do that, the first paper on reprogrammed iPS cells appeared.

Townes teamed with Rudolf Jaenisch, a stem-cell researcher at Whitehead and MIT, to see if iPS cells would work in place of embryonic stem cells. They took cells from the tail of a 12-week-old mouse with sickle-cell anemia and used viruses to turn on four dormant genes that are active in days-old embryos. One of those genes, c-Myc, has a tendency to cause tumors, so after the cells had completed their transition back to an embryonic state, the researchers deleted it.

Then they corrected the genetic flaw that causes sickle-cell anemia by engineering a string of DNA that had an A in place of a T but was otherwise identical to the original. It was swapped into place with the help of an electric shock.

The researchers grew the iPS cells into bone marrow stem cells by exposing them to special growth factors and culture conditions. When the cells were ready, they were transplanted into three sick mice that were genetic twins of the donor mouse.

Twelve weeks later, the mice were producing the normal version of hemoglobin beta protein and virtually all of their red blood cells were round. Their body weight and respiratory capacity improved. Their urine, previously watery due to the disease, had normal levels of electrolytes. None of the mice developed tumors, a sign that the threat from c-Myc had been eliminated.

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By KAREN KAPLAN

Los Angeles Times