Alzheimer’s, Parkinson’s, more -- due to infectious proteins?

This post has been corrected. See note at the bottom for details.

Who hasn’t heard of mad cow disease? Maybe there are a lot more diseases like that than we recognize -- such as Parkinson’s, Alzheimer’s and Huntington’s -- that are caused by a rogue, mis-folded piece of protein that seeds other bits of protein to mis-fold as well.

So argues Stanley Prusiner, a UC San Francisco professor, in a commentary in the journal Science.

Prusiner won a Nobel Prize for finding that a class of neurodegenerative diseases (of which mad cow is one) is caused by such infectious proteins, dubbed prions. Diseases in this class include Creutzfeldt-Jakob Disease, which occurs sporadically, as well as a few inherited disorders, such as Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia.

In animals, the class includes mad cow (more properly called bovine spongiform encephalopathy or BSE), scrapie in sheep and chronic wasting disease in elk.

And then -- in humans again -- there’s variant Creutzfeldt-Jakob Disease, the type scientists think was linked to consumption of BSE-infected beef.

In all those cases, the rogue is a mis-folded protein called PrP.

But there are many other diseases of the brain in which plaques or tangles of proteins build up. Could some of these also be caused by rogue, self-propagating proteins?

In Alzheimer’s, for example, plaques of a protein called beta-amyloid accumulate in brain cells.

In Parkinson’s, bits of a different protein, alpha-synuclein, clump together to form structures called Lewy bodies.

In Huntington’s disease, bits of a protein called huntingtinclump up.

There are protein clumps in amyotrophic lateral sclerosis (Lou Gehrig’s disease) and in some other diseases as well.

Some studies suggest that under the right conditions, the abnormally folded proteins can contort the shapes of normally folded versions of the protein, Prusiner notes -- in other words, they can spread. Here, for example, is an article that describes such experiments for the protein deposits that are seen with ALS.

And in the case of Alzheimer’s, scientists have shown that beta-amyloid plaque taken from brains of people who had Alzheimer’s can cause normal beta-amyloid in marmosets and rodents to form plaques too, when the material is injected into their brains.

The point of Prusiner’s opinion piece is not to say that these disorders were picked up from something people ate or from another person. He is arguing that a broader range of brain diseases than we have appreciated may be due to prions. The protein culprit in each case could be different -- but all of them could mis-fold and make other, normally shaped proteins in a cell mis-fold as well, in a disease-causing cascade.

And if this is true, that implies a new approach to treating the disorders.

Here are Centers for Disease Control and Prevention websites where you can read about mad cow disease (or BSE); variant CJD, the human disease that scientists think was linked to BSE via consumption of BSE-infected beef; and classic CJD.

[For the record, 2:45 p.m. June 29: An earlier version of this post referred to Lou Gehrig’s disease as amyloid lateral sclerosis instead of amyotrophic lateral sclerosis.]