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By Maggie Fox

A rare, incurable brain disorder that resembles Parkinson’s disease is caused by a misfolded brain protein called a prion, similar to the prions that cause mad cow disease, researchers reported on Monday.

The discovery adds to a very short list of human diseases linked to mutant prions, and suggests anyone doing brain surgery should take extra care, the researchers said.

The disease is called multiple system atrophy or MSA, and it affects somewhere between 15,000 and 50,000 Americans at any given time.

“Symptoms tend to appear in a person’s 50s and advance rapidly over the course of 5 to 10 years, with progressive loss of motor function and eventual confinement to bed,” the National Institute for Neurological Disorders and Stroke, part of the National Institutes of Health, says on its website.

“Now we’ve conclusively shown that a new type of prion causes MSA."

“People with MSA often develop pneumonia in the later stages of the disease and may suddenly die from cardiac or respiratory issues.”

There’s no cure, and MSA can often be confused for Parkinson’s disease.

Dr. Stanley Prusiner of the University of California San Francisco and colleagues wanted to see if it might be caused by a prion, similar to the misfolded prions that cause Creutzfeldt Jakob Disease (CJD); a relative in cattle called bovine spongiform encephalopathy or BSE; and a variant of CJD (vCJD) that has been linked to eating cattle affected by BSE.

Prusiner won the 1997 Nobel Prize in Medicine for his discovery of prions. They spread disease by touching other brain proteins and causing them to malfunction.

He and his team started with the brains of 14 people who had died with MSA. It can only be definitively diagnosed by examining the brain after death, and is marked by the inappropriate buildup of a protein called alpha synuclein.

Prions from their brains infected cells in lab dishes, changing their function, and caused symptoms resembling Parkinson’s in mice, they reported in the Proceedings of the National Academies of Science.

They examined the brains of the mice and found a buildup of alpha-synuclein.

“Now we’ve conclusively shown that a new type of prion causes MSA,” said Kurt Giles, a neurology expert at UCSF who worked on the study.

This doesn’t necessarily mean that MSA is transmissible in the same way that CJD and BSE can be, outside experts cautioned.

“It is important to state that this study does not demonstrate human-to-human transmission. In fact, it suggests MSA does not transmit easily,” Dr. Valerie Sim, a prion expert at the University of Alberta in Canada, said in a statement.

Cattle got BSE from eating sheep infected with scrapie, a similar brain condition. People can get vCJD from eating infected beef but it is extremely rare – just over 200 cases worldwide have been reported since the first case in the mid-1990s. Cannibals who eat human brains get a similar disease called kuru, and deer and elk are vulnerable to a condition called chronic wasting disease.

“It is important to state that this study does not demonstrate human-to-human transmission."

Knowing there’s another disease spread by prions is worrying because it is extremely difficult to destroy the mutant prions. Standard disinfection does not remove them, and people have been infected with CJD from contaminated surgical instruments.

“You can’t kill a protein,” Giles said. “And it can stick tightly to stainless steel, even when the surgical instrument is cleaned.”

Some experimental therapies for Parkinson’s involve implanting cells or electrodes into the brain, and because MSA can be confused for Parkinson’s, surgeons should take extra care, the researchers said.

There’s also a theory that has yet to be proved that Parkinson’s might be caused by mutant prions, Prusiner’s team noted.

And, they said, their findings show that MSA is distinct from Parkinson’s. Sometimes doctors try the standard Parkinson’s treatment levodopa in MSA patients. This study suggests researchers need to take a new approach.

However, there’s no known treatment for any of the prion diseases, including BSE and CJD.