MadSci Network: Medicine |
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
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