Prions are misfolded proteins that are responsible for several fatal neurodegenerative diseases, called transmissible spongiform encephalopathies (TSEs).
Prions can penetrate the brain through infection or they can be produced by mutations. Once present in the brain, prions spread through a snowball effect. Indeed, the presence of prions induce normal proteins to refold in the abnormal shape. Prions are resistant to proteases (the enzymes that degrade proteins). Thus, as prions multiply, they are free to accumulate within neurons, thereby destroying them.
In humans, prions are believed to be responsible for a wide range of disease, such as Creutzfeldt-Jakob disease, fatal familial insomnia or Gerstmann-Sträussler-Scheinker disease. There is also evidence that prions may have a major role in Alzheimer’s disease and Parkinson’s disease. Bovine spongiform encephalopathy (commonly called mad cow disease) is one of the most famous prion diseases affecting animals, along with scrapie and chronic wasting disease of mule deer and elk.
In mammals, all known prion diseases affect the structure of the brain or other neural tissue. All prion diseases are progressive, have no known effective treatment and are always fatal.