Prion, an abnormal form of a normally harmless protein found in the brain that is responsible for a variety of fatal neurodegenerative diseases of animals, including humans, called transmissible spongiform encephalopathies.

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In the early 1980s American neurologist Stanley B. Prusiner and colleagues identified the “proteinaceous infectious particle,” a name that was shortened to “prion” (pronounced “pree-on”). Prions can enter the brain through infection, or they can arise from mutations in the gene that encodes the protein. Once present in the brain prions multiply by inducing benign proteins to refold into the abnormal shape. This mechanism is not fully understood, but another protein normally found in the body may also be involved. The normal protein structure is thought to consist of a number of flexible coils called alpha helices. In the prion protein some of these helices are stretched into flat structures called beta strands. The normal protein conformation can be degraded rather easily by cellular enzymes called proteases, but the prion protein shape is more resistant to this enzymatic activity. Thus, as prion proteins multiply, they are not broken down by proteases and instead accumulate within neurons, destroying them. Progressive neuron destruction eventually causes brain tissue to become filled with holes in a spongelike, or spongiform, pattern.

Diseases caused by prions that affect humans include: Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, fatal familial insomnia, and kuru. Prion diseases affecting animals include scrapie, bovine spongiform encephalopathy (commonly called mad cow disease), and chronic wasting disease of mule deer and elk. For decades physicians thought that these diseases resulted from infection with slow-acting viruses, so-called because of the lengthy incubation times required for the illnesses to develop. These diseases were, and sometimes still are, referred to as slow infections. The pathogenic agent of these diseases does have certain viral attributes, such as extremely small size and strain variation, but other properties are atypical of viruses. In particular, the agent is resistant to ultraviolet radiation, which normally inactivates viruses by destroying their nucleic acid.

Prions are unlike all other known disease-causing agents in that they appear to lack nucleic acid—i.e., DNA or RNA—which is the genetic material that all other organisms contain. Another unusual characteristic of prions is that they can cause hereditary, infectious, and sporadic forms of disease—for example, Creutzfeldt-Jakob disease manifests in all three ways, with sporadic cases being the most common. Prion proteins can act as infectious agents, spreading disease when transmitted to another organism, or they can arise from an inherited mutation. Prion diseases also show a sporadic pattern of incidence, meaning that they seem to appear in the population at random. The underlying molecular process that causes the prion protein to form in these cases is unknown. Other neurodegenerative disorders, such as Alzheimer disease or Parkinson disease, may arise from molecular mechanisms resulting in protein misfolding that are similar to those that cause the prion diseases.