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Author Topic: PRIONS  (Read 4960 times)
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« on: February 20, 2011, 12:14:20 am »

Please can someone help with the reason why there is barrier in interspecific trans mission of prion diseases but easy in intra species
Francis Umeoguaju
Expert in Bioscience Issues
Posts: 657

« Reply #1 on: February 20, 2011, 06:29:35 am »

I believe this excerpt would answer your question. You can read the full article from this link>>

"Alan G. Dickinson, Hugh Fraser and Moira E. Bruce of the Institute for Animal Health in Edinburgh, who have examined the differential effects of varied isolates in mice, are among those who note that only pathogens containing nucleic acids are known to occur in multiple strains. Hence, they and others assert, the existence of prion "strains" indicates the prion hypothesis must be incorrect; viruses must be at the root of scrapie and its relatives. Yet because efforts to find viral nucleic acids have been unrewarding, the explanation for the differences must lie elsewhere. One possibility is that prions can adopt multiple conformations. Folded in one way, a prion might convert normal PrP to the scrapie form highly efficiently, giving rise to short incubation times. Folded another way, it might work less efficiently. Similarly, one "conformer" might be attracted to neuronal populations in one part of the brain, whereas another might be attracted to neurons elsewhere, thus producing different symptoms. Considering that PrP can fold in at least two ways, it would not be surprising to find it can collapse into other structures as well.

Since the mid-1980s we have also sought insight into a phenomenon known as the species barrier. This concept refers to the fact that something makes it difficult for prions made by one species to cause disease in animals of another species. The cause of this difficulty is of considerable interest today because of the epidemic of mad cow disease in Britain. We and others have been trying to find out whether the species barrier is strong (6) enough to prevent the spread of prion disease from cows to humans.

Breaking the Barrier

The barrier was discovered by Pattison, who in the 1960s found it hard to transmit scrapie between sheep and rodents. To determine the cause of the trouble, my colleague Michael R. Scott and I later generated transgenic mice expressing the PrP gene of the Syrian hamster--that is, making the hamster PrP protein. The mouse gene differs from that of the hamster gene at 16 codons out of 254. Normal mice inoculated with hamster prions rarely acquire scrapie, but the transgenic mice became ill within about two months. We thus concluded that we had broken the species barrier by inserting the hamster genes into the mice. Moreover, on the basis of this and other experiments, we realized that the barrier resides in the amino acid sequence of PrP: the more the sequence of a scrapie PrP molecule resembles the PrP sequence of its host, the more likely it is that the host will acquire prion disease.

In one of those other experiments, for example, we examined transgenic mice carrying the Syrian hamster PrP gene in addition to their own mouse gene. Those mice make normal forms of both hamster and mouse PrP. When we inoculated the animals with mouse prions, they made more mouse prions. When we inoculated them with hamster prions, they made hamster prions. From this behavior, we learned that prions preferentially interact with cellular PrP of homologous, or like, composition. The attraction of scrapie PrP for cellular PrP having the same sequence probably explains why scrapie managed to spread to cows in England from food consisting of sheep tissue: sheep and bovine PrP differ only at seven positions. In contrast, the sequence difference between human and bovine PrP is large: the molecules diverge at more than 30 positions. Because the variance is great, the likelihood of transmission from cows to people would seem to be low. "

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