A year after University of Wisconsin-Madison scientist James Thomson announced a new type of human embryonic stem cells, campus researchers have realized a major promise of the new cells: replicating a disease in a lab dish.

A team led by neuroscientist Clive Svendsen used the new stem cells to create a model of spinal muscular atrophy, the most common genetic cause of infant mortality. Researchers at Harvard University and elsewhere have used the cells to simulate other diseases, but Svendsen is the first to do so and show how a disease process works, said a prominent scientist in the field.

"They have gone a step further," Christopher Henderson of Columbia University said of Svendsen and his colleagues, including Allison Ebert. "It's highly promising."

A similar development this summer by Henderson and a Harvard researcher, involving Lou Gehrig's disease, was named the No. 1 medical breakthrough of the year by Time magazine.

Svenden said his discovery means that spinal muscular atrophy, which paralyzes children and usually kills them by age 2, can now be studied up close in the lab. That has been impossible because animal models don't closely mimic the condition and spinal-cord cells involved in the disorder can't be obtained from patients while they are alive.

"We at last have a model for a neurodegenerative disease in a dish," Svenden said. "This might be a poster child for other genetically inherited diseases."

Ebert, lead author of a report on the findings that is published in the new edition of the journal Nature, said the researchers accomplished two key steps. They showed that spinal muscular atrophy harms cells known as motor neurons and that drugs tested on the diseased cells boost production of a beneficial protein, meaning the cells are useful for screening other drugs.

"The beauty of our system is that we can generate the motor neurons and look at the protein production," Ebert said.

The accomplishment builds on work announced in November 2007 by Thomson, his colleague Junying Yu and a competing team led by Japanese researcher Shinya Yamanaka. Using a virus and four genes, the two groups forced skin cells to go back in time to their embryonic state.

The reprogrammed cells they produced — known as induced pluripotent stem cells, or iPS cells — appear to be identical to stem cells derived from days-old embryos, which Thomson was the first to create in 1998. Both kinds of stem cells continuously multiply and can morph into most or all of the body's 220 cell types, powers scientists are trying to harness to better understand and treat a variety of diseases.

Scientists say iPS cells carry two significant advantages over regular embryonic stem cells. No embryos are destroyed or even used to create the cells, offering a potential solution to an ethical dilemma that has plagued the field. And since the cells are crafted from a donor's skin cells, they can be genetically matched to patients, reducing the risk of immune system rejection of potential therapies developed from the cells.

Thomson and Yu helped Svendsen and Ebert with the latest research. So did Ferrill Rose and Virginia Mattis of UW-Madison and Christian Lorson of the University of Missouri.

First, Svendsen's lab obtained skin cells through a cell bank in New Jersey from a 3-year-old boy with spinal muscular atrophy who has died and from his mother, who doesn't have the disease.

Next, Thomson and Yu, using the same technique that brought them worldwide attention last year, created iPS cells from the skin cells.

Then Svendsen and Ebert converted the iPS cells into motor neurons, which control movement. After a month, the motor neurons from the boy started to shrink and die off while those from his mother kept thriving.

"We were essentially recapitulating the human disease in the dish," Ebert said.

When the boy's cells were treated with two drugs, their production of a helpful protein grew two to three times, proof that the cells could screen other drugs, she said.

Svendsen and Ebert are now trying to figure out how the motor neurons die. They're also trying to create similar cells for Huntington's disease, another brain condition that usually strikes in midlife.

With the UW-Madison cell line and other iPS cell models for diseases — including those for Parkinson's, diabetes, muscular dystrophy and other conditions, created by the Harvard reseachers — stem-cell research seems to be progressing faster than before, said Henderson of Columbia.

"This field is moving very quickly now," he said.