Re: What evidence is there that proves that prion causes BSE? Thanks.

Dear Elizabeth,

That is a great question! Koch's postulates (rules) for determining whether 
something is the cause of an infectious disease is to: a) isolate the 
suspected organism from an infected person (or animal); b) purify the 
organism; c) infect a healthy person (or animal) with the purified 
organisms; and d) demonstrate the same disease in the newly infected 
person.  Keep these rules in mind whenever you think of infectious 
diseases.  But the story of prions is also a very exciting story because it 
is about a scientist named Stanley Prusiner who came up with the theory 
behind prion-related diseases and stuck to his beliefs for years even 
though many scientists didn't believe him and even ridiculed him.

Before scientists knew about prions, they saw under the microscope that the 
brains of patients or animals that died of diseases called BSE, kuru, 
scrapie, and Creutzfeld-Jacob disease (CJD), all had a similar appearance 
(called spongiform encephalopathy--it means the brain had a "spongy"-like 
appearance).  Early studies with kuru showed that it was due to certain 
tribes of humans eating the brains of their deceased loved ones.  They 
found that only those who ate the brain of someone who had a neurologic 
disorder developed the same disorder.  When the scientists discouraged the 
ritual of eating brains, the number of people with kuru dropped.   For 
scrapie and bovine spongiform encephalopathy (BSE) which is a brain 
disorder in sheep and cows, it was discovered that feeding the animals with 
sheep's brains was associated with the disease.  So, the evidence was clear 
that there was something infectious in the brain.  

The question was, what is it?  One of the common findings was that the 
diseased brains had an abnormal protein (it was folded funny).  Stanley 
Prusiner made the hypothesis that it was these abnormally folded proteins, 
termed "prions" that was the cause of the disease.  These prions are no 
different than a specific protein normally found in the brain except that 
it was folded differently.  Dr. Prusiner hypothesized that the prions 
caused the normal proteins to become abnormally folded like the prions as 
well.  His scientists and others showed that introducing prions isolated 
from infected animals into healthy animals caused the same disease.  These 
results therefore met Koch's postulates. However, alot of scientists and 
doctors could not believe that abnormally folded proteins could cause 
disease.  Up until this point, all infectious organisms had to have DNA or 
RNA to make more of themselves.  No one had ever heard of or thought of 
abnormally folded proteins as the cause of disease.  These critics said it 
was impossible for proteins to be able to cause disease because you need to 
have DNA or RNA to spread disease.  These critics believed that maybe there 
was DNA or RNA associated with the prions that Dr. Prusiner did not find.  

What they also showed was that certain variations of the normal protein 
(called PrP) were more likely to become abnormally folded.  When scientists 
genetically modified mice, so that they made the specific variation of the 
normal protein (PrP) that easily becomes abnormally folded, they saw that 
many of the mice became sick with the brain disease with similar 
appearances under the microscope. Dr. Prusiner and his colleagues began to 
make a chart of all the variations of PrP that were likely to become 
abnormally folded and form prions.  So now, we have lists of certain prions 
associated with certain types of prion-related diseases.

For Dr. Prusiner's pioneering efforts, he was awarded the Nobel Prize and 
several other prizes in medicine and science.

Below are some useful links with more info:
 http://www-micro.msb.le.ac.uk/335/Prions.html
 http://www.accessexcellence.org/WN/NM/madcow96.html 
 http://www.the-scientist.library.upenn.edu/yr1997/dec/
opin_971208.html
 http://www.the-scientist.library.upenn.edu/yr1997/
dec/smaglik_p1_971208.htm
l

The original reference:
S.B. Prusiner, Science 216:136-144 (1982)

I hope this helps!  And thanks again for asking a great question.
....mark fung, md,ph