Prions: Infectious Proteins Causing Brain Diseases

A prion is an infectious protein containing no DNA or RNA at all. It is simply a misfolded version of a normal protein, called PrP, that is found naturally on the surface of brain and nerve cells. The infectious form is PrP^Sc; the normal form is PrP^C.

Prions matter because they cause a group of always-fatal brain diseases in humans and animals, and they shattered the old rule that all infectious agents must contain nucleic acid. Stanley Prusiner proposed this idea in 1982 and won the 1997 Nobel Prize for it.

Key Concepts

The normal PrP^C protein is rich in alpha-helices and is easily broken down by the cell. When it misfolds into PrP^Sc, it becomes rich in beta-sheets, extremely stable, and resistant to heat, radiation and disinfectants. The misfolded prion then acts as a template: each PrP^Sc converts neighbouring normal PrP^C molecules into the misfolded form, creating a chain reaction. The abnormal proteins clump into amyloid plaques; dying nerve cells leave the brain full of tiny holes, hence spongiform encephalopathy.

• Scrapie: prion disease of sheep and goats, known for centuries.
• BSE (mad cow disease): prion disease of cattle, spread through contaminated feed.
• CJD (Creutzfeldt-Jakob disease): rare human prion disease, occurring in about one person per million per year.
• vCJD: the variant linked to eating BSE-infected beef; kuru affected the Fore people of Papua New Guinea through ritual cannibalism.

How It Works

Prion disease develops in stages. First, misfolded PrP^Sc enters via infected tissue, arises spontaneously, or comes from an inherited PrP gene mutation. Second, in the brain it converts normal PrP^C into more PrP^Sc, doubling the amount each round. Third, the misfolded proteins aggregate into plaques that the cell cannot clear. Fourth, neurons malfunction and die over months or years, causing memory loss, loss of coordination, and eventually death. There is no immune response, because the body sees the prion as its own protein.

Worked Example

Suppose one misfolded prion doubles the total each round: 1 becomes 2, then 4, then 8, and so on. How many misfolded prions exist after 30 such doubling cycles?

1. After n cycles the count is 2 to the power n.
2. 2 to the power 10 = 1,024, so 2 to the power 30 is roughly 1,000 cubed = 1,000,000,000.
3. More precisely, 2 to the power 30 = 1,073,741,824.

Answer: about 1.07 billion misfolded prions from a single starting molecule after 30 cycles. This exponential amplification explains how a tiny initial dose can eventually destroy the brain.

Common Misconceptions

• Misconception: Prions are a type of virus. Correction: Viruses contain nucleic acid; prions are pure protein with no genetic material whatsoever.
• Misconception: Cooking meat destroys prions. Correction: Prions survive normal cooking and even standard autoclaving at 121°C; only extreme treatment, such as 134°C for 18 minutes, reliably inactivates them.
• Misconception: Prion diseases spread like the flu. Correction: They are not contagious through casual contact; transmission needs ingestion of infected tissue, contaminated surgical instruments, or inherited gene mutations.

Real-World Applications

• Food safety: bans on feeding meat-and-bone meal to cattle broke the BSE epidemic in Britain.
• Surgical protocols: hospitals use extra-strict sterilisation for instruments used on brain tissue.
• Diagnostics research: tests that detect misfolded PrP help screen blood and identify at-risk patients early.

Prions are infectious misfolded proteins with no nucleic acid, converting normal brain PrP^C into the stable PrP^Sc form in a chain reaction that causes fatal spongiform brain diseases like scrapie, BSE, CJD and kuru. They resist normal sterilisation, have long incubation periods, and are controlled through feed bans and strict decontamination.