Prions Explained: The Misfolded Proteins That Cause Disease

What Are Prions

The term "prion" was coined by Stanley Prusiner in 1982, derived from "proteinaceous infectious particle." Prusiner proposed that certain neurodegenerative diseases are caused not by bacteria, viruses, or any organism containing genetic material, but by a misfolded form of a normal cellular protein. This hypothesis was deeply controversial when first introduced because it contradicted the central dogma that all infectious agents must contain nucleic acid to replicate. Prusiner was awarded the Nobel Prize in Physiology or Medicine in 1997 after decades of experimental evidence confirmed the protein-only model of prion transmission.

The normal form of the prion protein, designated PrPC (for "cellular"), is a glycoprotein found on the surface of neurons and other cells throughout the body. The gene encoding PrPC, called PRNP in humans, is present in all mammals and has homologs in birds, reptiles, and fish. The normal protein is predominantly alpha-helical in structure and is anchored to the cell membrane by a glycosylphosphatidylinositol (GPI) linkage. Although PrPC has been extensively studied, its precise physiological function remains incompletely understood. Evidence suggests roles in copper metabolism, cell signaling, neuronal development, and protection against oxidative stress.

The disease-causing form, designated PrPSc (for "scrapie," the prion disease of sheep), has the same amino acid sequence as PrPC but a radically different three-dimensional structure. PrPSc is rich in beta-sheet content rather than alpha-helices, and this structural difference gives it dramatically different physical properties. While PrPC is soluble and easily degraded by proteases, PrPSc is insoluble, aggregation-prone, and remarkably resistant to enzymatic digestion. These aggregates accumulate in the brain, forming plaques and causing progressive neuronal death.

How Prions Replicate

Prion replication occurs through a process called templated misfolding or conformational conversion. When a molecule of PrPSc comes into contact with a normally folded PrPC molecule, the misfolded protein acts as a template, inducing the normal protein to refold into the pathological beta-sheet-rich conformation. This newly converted PrPSc molecule can then convert additional PrPC molecules, creating an exponential chain reaction of misfolding.

The precise molecular mechanism of this conversion is still being studied, but two models have been proposed. The template-directed refolding model suggests that PrPSc directly interacts with PrPC and catalyzes its conversion, with a high energy barrier that explains why spontaneous prion disease is rare. The seeded nucleation model proposes that PrPC and PrPSc exist in equilibrium, with PrPSc monomers being unstable but stabilized when they join an existing aggregate (seed). Once a seed reaches a critical size, it can fragment into smaller seeds that each recruit additional PrPSc molecules, leading to exponential growth of the aggregate population.

In vitro experiments have confirmed that purified PrPSc can convert recombinant PrPC into an infectious form, and that synthetic prions generated in the laboratory can cause disease when inoculated into animals. These experiments provide strong evidence that protein misfolding alone, without any nucleic acid involvement, is sufficient for prion propagation and infectivity.

Prion Diseases in Humans

Human prion diseases are rare but uniformly fatal. They can arise through three distinct mechanisms: sporadic (spontaneous), inherited (genetic), and acquired (infectious). All result in progressive, irreversible destruction of brain tissue.

Sporadic Creutzfeldt-Jakob disease (sCJD) accounts for approximately 85 percent of human prion disease cases. It occurs at a rate of roughly one to two cases per million people per year worldwide. The disease typically appears in individuals over age 60, and its cause is unknown. It may result from a rare spontaneous misfolding event in which a single PrPC molecule converts to PrPSc and initiates a chain reaction. Symptoms begin with rapidly progressive dementia, often accompanied by myoclonus (involuntary muscle jerks), visual disturbances, and cerebellar ataxia (loss of coordination). The disease progresses to akinetic mutism and death within an average of five months from symptom onset.

Genetic prion diseases account for approximately 10 to 15 percent of cases and are caused by mutations in the PRNP gene. These mutations destabilize the normal PrPC protein, making it more likely to adopt the misfolded PrPSc conformation. Genetic prion diseases include familial CJD, Gerstmann-Straussler-Scheinker syndrome (GSS), and fatal familial insomnia (FFI). FFI is a particularly devastating condition characterized by progressively worsening insomnia, autonomic dysfunction, and dementia. The disease is caused by a specific point mutation (D178N) in PRNP, combined with a methionine at codon 129 on the same allele.

Acquired prion diseases result from exposure to exogenous PrPSc. Kuru, once prevalent among the Fore people of Papua New Guinea, was transmitted through the ritualistic consumption of the brains of deceased relatives. The epidemic declined after cannibalistic practices ceased in the 1960s, but cases continued to appear for decades because of the extremely long incubation period, which can exceed 50 years. Variant CJD (vCJD) emerged in the United Kingdom in the 1990s and was linked to consumption of beef contaminated with bovine spongiform encephalopathy (BSE) prions. Iatrogenic CJD has resulted from medical procedures including contaminated human growth hormone preparations, dura mater grafts, and corneal transplants.

Prion Diseases in Animals

Several prion diseases affect domesticated and wild animals, some of which have significant agricultural and ecological consequences. Scrapie, the oldest known prion disease, has been recognized in sheep and goats for over 250 years. The disease is named for the characteristic behavior of affected animals, which scrape their fleece against fences and other objects due to intense itching. Scrapie spreads through direct contact between animals and through environmental contamination, as PrPSc can persist in soil for years.

Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, was first identified in the United Kingdom in 1986. The epidemic was caused by the practice of feeding cattle with meat and bone meal derived from the rendered carcasses of other cattle and sheep, inadvertently recycling prion-contaminated material through the food chain. At its peak in 1992, over 37,000 cases of BSE were confirmed in UK cattle in a single year. The BSE crisis led to the slaughter of millions of cattle, massive economic losses to the beef industry, and fundamental changes in animal feed regulations worldwide.

Chronic wasting disease (CWD) affects deer, elk, moose, and reindeer, and it is currently spreading through wild cervid populations in North America and Scandinavia. CWD is particularly concerning because it is the only prion disease known to be epidemic in free-ranging wildlife. Infected animals shed prions in saliva, urine, feces, and blood, contaminating the environment and facilitating horizontal transmission. CWD prions can persist in soil for years, binding to clay minerals and humic substances in ways that maintain or even enhance infectivity.

Why Prions Are So Difficult to Destroy

Prions are extraordinarily resistant to decontamination procedures that effectively destroy bacteria, viruses, and other conventional pathogens. Standard autoclaving at 121 degrees Celsius for 15 minutes, which sterilizes against all known bacteria and viruses, does not reliably inactivate prions. Prions resist ultraviolet radiation, ionizing radiation, formaldehyde fixation, and most chemical disinfectants at standard concentrations.

This resistance stems from the physical properties of PrPSc aggregates. The dense beta-sheet-rich structure of misfolded prion protein resists unfolding by heat and resists cleavage by proteases and chemical agents. Even after treatments that would denature virtually any other protein, enough PrPSc may retain its templating ability to initiate new rounds of misfolding.

Effective prion decontamination requires extreme measures. The World Health Organization recommends immersion in sodium hydroxide (1N NaOH) or sodium hypochlorite (20,000 ppm available chlorine) for one hour, followed by autoclaving at 134 degrees Celsius for 18 minutes. For contaminated surfaces that cannot be immersed, repeated application of concentrated bleach or NaOH is recommended. Surgical instruments that may have contacted prion-infected tissue pose a particular challenge, as prions can adhere tightly to stainless steel surfaces.

Current Research and Future Directions

There is currently no effective treatment for any prion disease. Several compounds have shown anti-prion activity in cell culture and animal models, including quinacrine, pentosan polysulfate, and various anti-PrP antibodies, but none has demonstrated clear clinical benefit in human trials. The rapid and relentless progression of symptomatic prion disease makes therapeutic intervention extremely difficult, as extensive neuronal damage has typically occurred before diagnosis.

Research is focusing on several promising approaches. Immunotherapy strategies aim to develop antibodies that bind to PrPC or PrPSc and either prevent conversion or promote clearance of misfolded protein. Gene silencing approaches, including antisense oligonucleotides (ASOs) that reduce PRNP gene expression, have shown dramatic results in animal models and are being evaluated in clinical trials for human prion disease. If PrPC production can be reduced sufficiently, the chain reaction of misfolding may be slowed or halted.

The prion concept has also expanded beyond the original infectious diseases. Research has revealed that prion-like mechanisms of templated protein misfolding may contribute to the progression of common neurodegenerative diseases including Alzheimer disease, Parkinson disease, and amyotrophic lateral sclerosis (ALS). In these conditions, misfolded forms of proteins such as amyloid-beta, tau, alpha-synuclein, and TDP-43 appear to spread through the brain in a manner reminiscent of prion propagation. Understanding prion biology may therefore have implications far beyond the rare classical prion diseases.