Rogue on the loose

Rogue on the loose

June 16, 2001 New Scientist by Anil Ananthaswamy

A PROTEIN found in the blood of cancer patients can behave just like a prion, the infectious agent that causes mad cow disease. Its discoverers think it might help cancers spread throughout the body.

"We always tend to think of cancer as a genetic disease, caused by mutations," says Mike Scott, a prion researcher at the University of California, San Francisco. "But there is this distinct possibility that there might be some prion-like mechanism involved in at least some tumours."

This is the first example, he says, of a prion-like protein in mammals that doesn't belong to the family of proteins called PrP, which cause scrapie and BSE as well as its human variant, vCJD. Prions are an abnormal form of PrP. They persuade normal PrP proteins to turn into the rogue form, which can resist heat and enzymes called proteases that normally break down proteins.

It's this ability to alter other proteins that James Morre and his colleagues at Purdue University in Indiana discovered in a cancer-related protein called tNOX. Morre's team digested proteins from human cancer cells using an enzyme called proteinase K. Afterwards, they found another protein survived as well as tNOX. "We were not looking for it, but there it was," says Morre.

It turned out to be a protein called GAPDH, which is normally digested by proteinase K. Was tNOX making GAPDH resistant to the protease ? To answer that, Morre and his team incubated tNOX and normal GAPDH together. In every case GAPDH became protease-resistant, they report in a paper that will appear in Biochemistry. The tNOX was acting like a prion.

And that was not the only similarity. The tNOX protein also forms insoluble lumps, or plaques, and resembles the prion that causes scrapie. However, the researchers don't know how tNOX itself becomes like a prion.

But unlike known prions, which alter proteins similar to themselves, tNOX modifies an entirely different protein. This is another first, Scott says. "Everyone expected this would happen sometime. But this is the first example where a prion-like protein seems to be able to convert a different protein into a prion-like form," he says.

In addition, tNOX can do its normal job while acting like a prion. "This is tremendously significant," says Scott. "There's an obvious selective advantage in being converted into the prion-like state, because the protein has full activity but is now resistant to proteases."

Morre found the protein's concentration was highest in the blood of people whose cancers had spread through their body. He suspects the protein might help cancers spread. "I believe it does, but there's no hard evidence to support that," he says.

But he stresses that tNOX protein cannot "infect" other people. He's tried injecting the protein into healthy mice. "The mice do not get cancer. This would be required if this were a real disease threat in terms of transmission," says Morre. What is not clear is if the protein, when present, could tip the balance in cells that are already becoming cancerous for other reasons.

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