Prions:
Proteinaceous Infectious Particles"
Recent cases of Mad Cow Disease have focused the public attention on prion diseases and the small proteins that are believed to cause them. The scientific community has been slow to recognize this mechanism of disease, since prion-caused encephalopathies can demonstrate diverse symptoms, and share characteristics with other disorders, such as dementia.
Prions, as the acronym (Proteinaceous Infectious Particles) suggests, are small proteins that are typically expressed in brain tissue, and may exist in a normal or abnormal shape. The prion protein is encoded by a gene found on the human chromosome 20. Usually, the prion protein is translated in neural tissue, folds into its normal conformation, carries out its cellular role, and is eventually degraded by enzymes. The abnormal prion, however, folds differently from its normal counterpart. This different shape makes it more difficult to degrade, and leads to the brain damage that is seen in patients with prion diseases.
The discovery of prions is credited to Stanley Prusiner, a professor at the University of California, San Francisco, School of Medicine. Professor Prusiner coined the term 'prion' to describe an apparently new phenomenon of disease transmission and was awarded the Nobel Prize for Medicine in 1997. Prion disease, however, has been documented for centuries in various species. Scrapie, a disease that affects sheep and goats has been recognized since the 1700s. Similarly, bovine spongiform encephalopathy (BSE) is a progressive neurodegenerative disease that affects cows. Prusiner won the Nobel Prize for his 'prion hypothesis' that was originally published in 1982, although investigation into human prion diseases dates back decades earlier.
Carleton Gajdusek, an American pediatrician, was instrumental in laying the groundwork for Prusiner's 'prion hypothesis'. In the 1950s, Gajdusek studied a rare disease, kuru, that was infecting the Fore tribe of New Guinea highlanders. He found that the devastating neurodegeneration exhibited by certain members of the Fore tribe could be traced to ritual cannibalism. Autopsied brains of the victims showed a similar type of brain damage to the brains of sheep affected with scrapie. Gajdusek also studied patients with Creutzfeld-Jakob disease (CJD) and noted the brain tissue had a similar sponge-like appearance. Based on these apparently diverse diseases, Gajdusek postulated a new type of infectious agent was responsible. Although the term "prion" didn't come into use until the 1980s, Dr. Gajdusek was awarded the Nobel Prize for Medicine in 1976 for the work that linked these disparate afflictions.
The most common human prion disease is Creutzfeld-Jakob Disease, a progressive neurodegenerative disorder that can arise randomly ("sporadic CJD") or as a result of contamination with infected tissue. Because prions are proteins, and proteins are coded for by genes, some prion diseases can be inherited. A small percentage (10-15%) [http://www.albany.net/~tjc/prion.html]of CJD is attributable to an inherited gene mutation. A similar mutation is responsible for other, less common, prion diseases. A fatal form of insomnia caused by progressive prion brain damage (fatal familial insomnia - FFI) is caused by a mutation in the prion gene, as is Gertsman-Straussler-Scheinker disease (GSS), a similar affliction to CJD. An umbrella term that refers to the family of prion diseases is transmissible spongiform encephalopathies (TSE).
The infectious nature of the prion protein is based upon its conformation once it is translated and folded. Usually, the protein product of the prion gene folds into a structure that consists primarily of alpha-helix coils. This shape is known as PrPC (prion protein, cellular) and allows the protein to function normally. Although the role of the normally folded prion protein is not known, it is believed to play a role in synaptic message transmission in brain tissue. When the protein has fulfilled its purpose, it is degraded by brain tissue enzymes. The way in which a protein folds after being translated is critical to the protein's function. This alternate folding means that the same amino acid sequence can have devastating consequences if it folds in a different manner. In the case of the abnormal prion protein, this abnormal conformation consists of more beta-pleated sheets than alpha-helices. (This abnormal form is denoted PrPSc - prion protein, scrapie). The different shape of the infectious prion protein makes it resistant to degradation by enzymatic function. The protein builds up in brain tissue, damaging neurons and causing the sponge-like (spongiform) appearance that is typical of prion disease-infected brains.
Prion diseases progress because the abnormally folded proteins can 'infect' nearby, normal proteins, and cause them to refold into the abnormal conformation. In this...
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