MILWAUKEE — Researchers are one step closer to reprogramming skin cells into tailor-made, healthy replacements for diseased cells.

Applying the technique first developed by James Thomson of the University of Wisconsin-Madison and Shinya Yamanaka of Kyoto University, scientists at Harvard and Columbia universities reported online Thursday in the journal Science that they had turned skin cells from two elderly patients with the neurodegenerative disorder amyotrophic lateral sclerosis into motor neurons, the nerve cells that become damaged in ALS.

This is the first time that scientists have coaxed embryonic-like cells from adult patients suffering from a genetic-based disease, then induced the cells to form the specific cell types that would be needed to study and treat the disease.

"It's a big step forward," said Stephen Duncan, a stem cell researcher at the Medical College of Wisconsin in Wauwatosa, who was not involved in the research. "It opens the door for people to go confidently into generating (reprogrammed stem) cells for other disease models."

A team led by Kevin Eggan of the Harvard Stem Cell Institute in Boston started with skin cells from two sisters, ages 82 and 89, who both carried a rare gene associated with a slowly progressing form of ALS — Lou Gehrig's disease.

Around the world, one or two people per 100,000 develop ALS each year, which is marked by a wasting away of certain spinal cord nerve cells called motor neurons. The single-gene form of ALS studied by Eggan's team affects only about 2 percent of ALS sufferers, while the vast majority of ALS cases are sporadic.

In their laboratory, the researchers inserted four genes into the cells using a virus that rewound the cell's developmental clock into an embryonic-like state.

The researchers then bathed the cells in signaling molecules to produce motor neurons and the nerve cells that support and protect the neurons, called glial cells.

"It's possible to use these cells to make the actual cell type that is destroyed in that person's disease," Eggan said.