From the S.F. Chronicle:

Scientists have identified a defective protein that appears to be a central element -- if not the cause -- of two mysterious nervous system disorders: amyotrophic lateral sclerosis, better known as ALS or Lou Gehrig's disease, and one of the most common types of dementia.

The findings suggest that ALS, which attacks the nerve circuits that control movement, and the far more widespread brain disorder known as frontotemporal dementia are essentially different forms of the same disease.

A report on the findings appears in the latest issue of the journal Science. Virginia Lee, director of the Center for Neurodegenerative Disease Research at the University of Pennsylvania, was the senior investigator of a multi-center team that included Dr. Bruce Miller, a neurologist and director of the Memory and Aging Center at UCSF.

[...]

"It suggests that maybe if a treatment can be found that works for ALS, it might work for FTD, too," Miller said. "It takes two relatively understudied diseases and puts them under a common banner."

[...]

Although symptoms are much different, with the new findings researchers now suspect one culprit -- a progressive buildup of the same malformed molecule toxic to nerve cells.

During a telephone interview Thursday, Lee said it's unclear how the critical protein, known as TDP-43, becomes crumpled, or why it causes the death of only certain nerve cells, such as the motor neurons in cases of ALS, or the brain cells that succumb in cases of FTD.

"We don't know why certain populations of cells are selectively vulnerable," Lee said. "What we can say is that TDP-43 is the disease protein that is responsible for motor neuron death you see in ALS, and also is responsible for other neurons to die in FTD."

UPDATE:
magnetbabe points out:
"'It takes two relatively understudied diseases and puts them under a common banner."' = $$$+ mucho incentive for drug companies"

Now there's a market!