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From: TSS ()
Subject: Sheep study calls for closer look at prion hypothesis
Date: April 27, 2006 at 6:46 am PST

Nature Medicine
Published online: 27 April 2006; | doi:10.1038/nm0506-484a
Sheep study calls for closer look at prion hypothesis
Prions may not cause mad cow disease or scrapie, controversial report claims.
Michael Hopkin





Don't be a sheep: People don't question the prion hypothesis enough, say British researchers.

Narelle Towie

The infectious agent behind diseases such as mad cow, scrapie and variant Creutzfeld-Jakob disease may not necessarily be rogue prion proteins, say British researchers—a suggestion that flies in the face of current dogma about how these diseases are spread.

Although deformed prions are a characteristic of these diseases, they may not be the initial infectious agent, says lead researcher Martin Jeffrey of the Veterinary Laboratories Agency–Lasswade near Edinburgh. The researchers are basing their theory on how these proteins are absorbed in the sheep gut.

Experts contacted by Nature Medicine declined to comment on the controversial paper.

The scientists inoculated sheep intestines with brain extracts containing the abnormal form of the prion protein (PrP), the hallmark of the killer neurodegenerative disease. But when they later examined the infected sheep, the rogue protein had congregated at entirely different sites (J. Pathol. 209, 4–14; 2006).

Despite the fact that the Nobel Prize was awarded for the prion hypothesis, there remains the possibility that it's not the correct explanation.

Martin Jeffrey
Veterinary Laboratories Agency



"Despite the fact that the Nobel Prize was awarded for the prion hypothesis, there remains the possibility that it's not the correct explanation," says Jeffrey. "There needs to be a more assiduous investigation of the causes of this group of diseases."

The study also suggests that the mechanism of resistance against these diseases does not operate at the level of gut absorption, as some researchers had said. The 50 sheep studied displayed different levels of resistance to scrapie, but all absorbed the prions equally readily.

Although the study suggests that prion proteins can be absorbed by the gut, this seems to happen only rarely, at least in sheep. If the same is true of humans, then scientists may need to rethink exactly how the consumption of tainted meat leads to disease.

Jeffrey's team predigested a mixture of abnormal PrP and typical sheep stomach contents. But when they then injected this into the gut, almost no rogue protein was absorbed, and subsequent testing found that the mixture contained almost no abnormal PrP. Jeffrey says that sheep in the field are probably exposed to very little prion protein in their gut, because their stomachs would dispose of even the miniscule amounts ingested.

Another possibility is that absorption occurs higher in the gut, before proteins are digested—perhaps in the mouth, suggests Nicole Sales, who studies prion disease at the Scripps Research Institute in Jupiter, Florida.

If prions are not the infectious agent, the real culprit remains a mystery. "This experiment doesn't detect the infectious agent," says Jeffrey, but suggests that a fragment of the prion protein, perhaps too small to detect using standard detection methods, may be the guilty party.

The new discovery does not rule out the possibility that prion proteins, if absorbed in sufficient amounts, can also cause disease, Jeffrey says. But he contends that these proteins are not the whole story. "It's opening up a doorway into another biological paradigm," he says.


London

Article brought to you by: Nature Medicine

http://www.nature.com/news/2006/060424/full/nm0506-484a.html

tss



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