Since the outbreak of BSE in cattle and vCJD in humans, scientists have struggled to make sense of how an abnormal variation of a normal protein can trigger an infectious disease. Some are questioning whether this simple relationship exists at all. This paper adds new evidence that can inform the debate.
Firstly, it is known that individual people and animals have different levels of genetic susceptibility to this group of diseases, but no one knows how this resistance is achieved. One option is that resistant people do not absorb the disease-associated prion protein (PrP) from their guts.
To test this, the researchers worked with 50 sheep, with different degrees of genetic resistance to scrapie ?the sheep form of the disease. When they injected material containing abnormal prion protein (PrP) into the sheep's gut, it was equally absorbed by all sheep.
"This clearly shows that resistance is not achieved by blocking uptake of abnormal proteins from the gut ?it must be achieved by some other mechanism," says lead author Dr Martin Jeffrey.
Secondly, they looked in more detail at the route of absorption in the gut. Using surgically modified sheep, they loaded a small area of the gut with a fluid mixture containing 0.5 grams of scrapie infected brain containing a large amount of the disease specific variant of the PrP protein and watched how it was taken up. They saw the abnormal PrP was rapidly taken up by finger-like projections called villi and passed in to the lymph. It was not, however, taken up by structur es called Peyer's nodules, that are believed to be the places where animals amplify the infective agent.
"The fact the PrP isn't taken up by the Peyer's nodules questions whether PrP is really infectious, or whether PrP is really just a secondary marker of the presence of the scrapie agent," says Jeffrey.
His belief in this need to reappraise the fundamental understanding of prion diseases is enhanced by one more observation published in this same paper. The team pre-digested a mixture containing disease specific PrP with standard stomach contents, and then injected the resulting mixture into the gut. No PrP transferred into the villi. When they used a highly sensitive version of Western Blot analysis to examine the contents of this pre-digested mixture, they found only the faintest suggestion that some of the PrP had survived. This was despite the fact that the original mixture had a contained a high level of PrP.
"Think about it ?a sheep grazing in a field is not naturally exposed to highly infected brain and could only pick up a tiny amount of PrP from other tissues. This will then be exposed to 48 hours or more digestion before it arrives in the gut, and our experiments show that after this, the chance of there being more than an unmeasurably small amount of PrP left to absorb is very small," says Jeffrey.
"As sheep can become infected, the theoretical probability of this being due to an invisible sub-fraction of digestion resistant PrP molecules is unlikely. The possibility of there being infectious molecules other than PrP must therefore be seriously considered," says Jeffrey.
"A lot of people are completely wedded to the prion hypothesis of diseases like vCJD, but the more you deal with whole animals as opposed to relying purely on in vitro studies, the more cautious you are about saying that prion proteins alone cause the disease," says Martin Jeffrey.
In a commentary published in the same edition of the jo urnal Dr Nicole Sales of the Department of Infectology, at the Scripps Research Institute Jupiter, Florida, suggests that one possible explanation that keeps with the prion hypothesis is that infection occurs as PrPs are absorbed in the mouth, rather than in the gut.
Dr Jeffrey, however, is not convinced by this argument. "Were infection to be acquired from the mouth then the first tissues to accumulate infectivity would be lymph nodes in the throat or the tonsils. But we don't tend to see this in animals, and have no reason to believe it would be different in humans," comments Jeffrey.