Prion

Prion is an acronym for "proteinaceous infectious particle," a term coined by Prusiner in the early 1980s to describe the nature of the agent causing the fatal brain disorders known as transmissible spongiform encephalopathies (TSE), also called prion diseases. Well-known examples of prion diseases include scrapie in sheep and goats, bovine spongiform encephalopathy (BSE, or "mad cow" disease) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. Prion diseases are infectious and can also be transmitted to healthy animals by inoculating them with extracts of diseased brain.

In the mid-1960s, Tikvah Alper and colleagues reported that nucleic acid was unlikely to be a component of the infectious agent that causes scrapie. In 1967 J. S. Griffith speculated that the scrapie agent might be a protein capable of "self replication" without nucleic acid. However, Prusiner was the first, in the early 1980s, to successfully purify the infectious agent and to show that it consisted mostly of protein (technically speaking it is a glycoprotein, because it has a sugar group attached). He chose to name the new agent "prion" to distinguish it from viruses or viroids.

The essential protein component of prion was later identified in 1984 as prion protein (PrP), which is encoded by a chromosome gene in the host genome. Researchers concluded that the prion is a new infectious agent that consists mostly of PrP. This view is often referred to as the "protein only" or prion hypothesis. Some scientists find this notion hard to accept and have argued that nucleic acid is needed to carry information necessary for infection. However, no one has been able to demonstrate that either DNA or RNA play a direct role in prion replication.

In 1992 Charles Weissmann and colleagues obtained conclusive evidence for the central role of PrP in the transmission of prion diseases, when they created transgenic mice devoid of the PrP gene. These so-called PrP knockout mice were found to be completely resistant to infection when inoculated with scrapie brain preparations. When the PrP gene was reintroduced into the knockout mice, they once again became susceptible to prion infection.