Prions: The Misfolded Proteins Behind Mad Cow Disease and CJD

An Infectious Agent With No Nucleic Acid

In 1982, neurologist Stanley Prusiner published a paper in Science proposing that a novel class of infectious agents — which he named "prions" (from "proteinaceous infectious particle") — consisted entirely of protein, with no DNA or RNA. The proposal was heretical. Every known infectious agent — bacteria, viruses, fungi, parasites — encoded its own genetic information to replicate. Prusiner's claim that a misfolded protein could spread infection by inducing normal copies of itself to adopt the abnormal conformation violated every established principle of molecular biology. He received the Nobel Prize in Physiology or Medicine in 1997. His critics received fifteen more years to accept the evidence.

Prion diseases — formally called transmissible spongiform encephalopathies (TSEs) — are a group of progressive, invariably fatal neurodegenerative disorders that affect humans and other mammals. They are characterized by sponge-like holes in brain tissue, accumulation of abnormal prion protein, and neurological deterioration. They have no treatment. They cannot be destroyed by cooking, autoclaving, or most standard sterilization methods. They can incubate for decades before symptoms appear. The most dramatic human prion disease — variant Creutzfeldt-Jakob disease, linked to the bovine spongiform encephalopathy (BSE) epidemic in British cattle — killed 178 people in the United Kingdom between 1995 and 2024, and surveillance for new cases remains ongoing.

The Prion Protein: Normal and Abnormal

One protein. Two shapes. Completely different consequences.

The cellular prion protein (PrP^C) is a normal protein expressed at high levels in neurons throughout the brain and at lower levels in other tissues. Its precise function remains incompletely understood, but evidence suggests roles in copper metabolism, synaptic function, and neuroprotection. PrP^C is encoded by the PRNP gene and is a component of normal cellular physiology.

The disease-causing form, PrP^Sc (named after scrapie, the sheep prion disease that provided the original experimental model), is a misfolded conformational variant of the same protein. PrP^C is rich in alpha-helices; PrP^Sc has a much higher proportion of beta-sheets. This structural difference makes PrP^Sc resistant to protease enzymes that would normally degrade damaged proteins. When PrP^Sc contacts PrP^C, it can catalyze the conversion of PrP^C to the abnormal conformation — a template-directed misfolding process. The aggregated misfolded protein cannot be cleared and accumulates in neurons, leading to cell death.

Human Prion Diseases

Kuru and the Discovery of Transmissibility

Cannibalism revealed the mechanism.

Kuru was epidemic among the Fore people of the Eastern Highlands of Papua New Guinea in the 1950s and 1960s. The disease caused progressive cerebellar ataxia (loss of coordination), dementia, and death within 3–24 months of symptom onset. Physician D. Carleton Gajdusek investigated the epidemic beginning in 1957 and demonstrated in 1966 — by inoculating chimpanzees with brain material from kuru victims and observing the development of the disease after a 21-month incubation — that kuru was a transmissible neurological disease. The transmission route was mortuary cannibalism: the Fore practiced consumption of deceased relatives' brain and nervous tissue as a funeral rite, transmitting the infectious agent from generation to generation.

When Australian authorities ended the practice in the late 1950s, new kuru cases progressively declined and eventually stopped, though the extremely long incubation period of prion diseases meant cases continued to appear for decades afterward. The last confirmed kuru death occurred in 2009. Gajdusek received the Nobel Prize in 1976 for his work demonstrating the transmissibility of kuru and Creutzfeldt-Jakob disease.

BSE and the British Epidemic

Feeding cattle to cattle created a prion disaster.

Bovine spongiform encephalopathy emerged in British cattle in 1986 and reached epidemic proportions by the early 1990s, peaking at approximately 37,000 confirmed cases in 1992. The epidemic originated from the practice of including meat-and-bone meal (MBM) — rendered from the carcasses of other cattle and sheep, including possibly scrapie-infected sheep — in cattle feed. Recycling infected nervous tissue through the food chain amplified the prion agent through successive generations of cattle.

• 1986: First confirmed BSE case in UK cattle
• 1988: UK bans use of ruminant-derived MBM in ruminant feed
• 1996: UK government acknowledges likely link between BSE and the new variant CJD cases in humans; global panic over beef safety
• 1997: Prusiner awarded Nobel Prize; prion hypothesis achieves mainstream scientific acceptance
• 2001: Peak of variant CJD deaths in UK (28 deaths in one year)
• 178: Total UK vCJD deaths as of 2024; global total approximately 232

Prion Resistance and Decontamination

Standard sterilization fails. Completely.

PrP^Sc's resistance to degradation is a direct consequence of its structural stability. The protein is:

• Not inactivated by temperatures up to 600°C in dry heat
• Not inactivated by standard autoclave cycles (121°C for 15 minutes) — though extended cycles at higher temperatures (134°C for 18+ minutes) provide partial inactivation
• Not inactivated by formaldehyde, alcohol, or most disinfectants
• Not inactivated by ultraviolet or ionizing radiation at doses that destroy nucleic acids

Effective decontamination requires treatment with 2M sodium hydroxide (lye) or sodium hypochlorite (bleach) at high concentrations combined with autoclaving, or incineration. Surgical instruments that contact high-risk prion tissues (brain, spinal cord, eye) in neurosurgical procedures require specific decontamination protocols. At least 4 iatrogenic CJD transmissions through neurosurgical instruments have been documented.

Prusiner's prize came 15 years after his initial proposal and required overcoming the resistance of an entire field trained to think that information flows only from nucleic acids to proteins. The prion, which carries no nucleic acid at all, transmits information through shape alone — a category of biological information transfer that nobody predicted and that, once accepted, raised new questions about the boundary between infection and disease that biology is still working out.