Prions – Definition, Structure, Diseases, and Facts

Prions are infectious proteins that cause disease by inducing abnormal folding of normal proteins in the brain and other tissues. Unlike bacteria, viruses, fungi, or parasites, prions contain no DNA or RNA, yet they replicate by converting normal cellular proteins into the same misfolded form. This unusual mechanism leads to progressive, fatal neurodegenerative disorders known as transmissible spongiform encephalopathies (TSEs). These diseases affect humans and other animals and are notable for their long incubation periods, resistance to sterilization, and characteristic sponge-like damage to brain tissue.

Key Takeaways: Prions

• Prions are misfolded proteins that act as infectious agents.
• They lack nucleic acids, distinguishing them from all other pathogens.
• Prions replicate by templated protein misfolding, converting normal proteins into abnormal forms.
• They cause fatal neurodegenerative diseases, including Creutzfeldt–Jakob disease and mad cow disease.
• Prions are highly resistant to heat, radiation, and disinfectants.
• The normal prion protein (PrP) likely has important cellular functions, especially in the nervous system.
• Diagnosis is challenging and usually involves clinical, imaging, and biochemical tests.
• There is currently no cure, only supportive care.

History of Prion Discovery and Study

The concept of prions emerged from studies of unusual neurodegenerative diseases that did not behave like infections caused by known microorganisms.

• 18th century: Farmers in Europe described scrapie, a fatal disease in sheep.
• 1950s–1960s: Researchers studied kuru, a neurodegenerative disease among the Fore people of Papua New Guinea, linked to ritual cannibalism.
• 1967: British mathematician J. S. Griffith proposed that a protein could act as an infectious agent.
• 1982: American neurologist Stanley B. Prusiner coined the term prion and demonstrated that these diseases were caused by proteins, not viruses.
• 1997: Prusiner received the Nobel Prize in Physiology or Medicine for this work.

This discovery fundamentally changed biology by demonstrating that protein structure alone can encode biological information.

What Is a Prion?

A prion is an infectious, misfolded form of a normal protein, most commonly the prion protein (PrP), found in the brain and other tissues.

Word Origin and Pronunciation

• Origin: From proteinaceous infectious particle
• Pronunciation: PREE-on

Prions vs Other Infectious Agents

Prions differ fundamentally from all other pathogens.

The unique characteristics of prions is that they propagate structure, not genetic code.

Structure of a Prion

The normal prion protein, PrP_C, is a cell-surface glycoprotein primarily found in neurons. It is rich in alpha-helical structures and is soluble.

The infectious form, PrP_SC:

• Has a beta-sheet–rich structure
• Is insoluble and aggregation-prone
• Forms amyloid fibrils
• Resists protease degradation

This structural shift is central to prion disease.

How Prions Replicate

Prions do not reproduce in the traditional sense. Instead, they convert normal proteins into misfolded ones.

Mechanism

1. A misfolded prion (PrP_Scinteracts with normal PrP_C.
2. It induces a conformational change in the normal protein.
3. The newly misfolded protein becomes infectious.
4. Aggregates form, damaging cells.

This process is called templated misfolding or protein-only replication.

Function of the Normal Prion Protein (PrP)

The exact function of PrP remains under study, but evidence suggests roles in:

• Cell signaling, especially in neurons
• Protection against oxidative stress
• Copper ion binding and metabolism
• Synaptic function and plasticity
• Cell survival and development

Loss or alteration of these functions may contribute to disease.

Prions and Disease

Prions cause a group of disorders called transmissible spongiform encephalopathies (TSEs).

Human Prion Diseases

• Creutzfeldt–Jakob disease (CJD)
• Variant CJD (linked to contaminated beef)
• Kuru
• Fatal familial insomnia
• Gerstmann–Sträussler–Scheinker syndrome

Animal Prion Diseases

• Bovine spongiform encephalopathy (BSE)
• Scrapie (sheep and goats)
• Chronic wasting disease (deer and elk)

Common Features

• Progressive neurodegeneration
• Spongiform (sponge-like) brain tissue
• Protein aggregates (amyloid plaques)
• Always fatal

Susceptibility

Prion diseases affect:

• Humans
• Other mammals such as cattle, sheep, deer
• Some species show natural resistance, depending on PrP sequence differences

Diagnosis and Treatment

Diagnosing prion diseases is challenging because their early symptoms resemble other neurodegenerative disorders and definitive confirmation traditionally requires examination of brain tissue. Clinicians rely on a combination of clinical evaluation, brain imaging, electrical activity studies, and specialized biochemical tests to identify likely cases while ruling out other conditions. Despite advances in detection, prion diseases remain incurable. Treatment focuses on supportive care, symptom management, and maintaining quality of life, while research continues to explore therapies that might slow or prevent the underlying protein misfolding process.

Diagnosis

Diagnosis is difficult and often confirmed postmortem, but tools include:

• MRI scans showing characteristic brain changes
• EEG abnormalities
• Detection of prion proteins in cerebrospinal fluid
• Advanced assays such as RT-QuIC (real-time quaking-induced conversion)

Treatment

There is currently no cure.

• Care is supportive
• Experimental therapies target protein misfolding or aggregation
• Research focuses on stabilizing PrP^C or blocking conversion

Minimizing Risk from Prions

Although rare, prion diseases require strict precautions.

• Avoid consumption of high-risk tissues (brain, spinal cord)
• Follow food safety regulations (e.g., BSE controls)
• Use specialized sterilization methods in medical settings
  • High-pressure steam sterilization
  • Sodium hypochlorite or strong alkali treatments
• Screen blood and tissue donations where applicable

Prions resist many conventional sterilization techniques, making infection control critical.

Prion Protein Gene (PRNP) and Genetics

Prion diseases can arise not only from infection but also from inherited mutations in the gene encoding the prion protein. The PRNP gene, located on chromosome 20 in humans, encodes the normal prion protein (PrP_C). Mutations in this gene can destabilize the protein’s structure, increasing its tendency to misfold into the disease-causing form.

More than 30 mutations in PRNP are associated with inherited prion diseases, including familial Creutzfeldt–Jakob disease, fatal familial insomnia, and Gerstmann–Sträussler–Scheinker syndrome. These mutations often alter amino acid sequence or protein folding stability. In addition, a well-known polymorphism at codon 129 (methionine or valine) influences susceptibility and disease progression.

This genetic component helps explain why some prion diseases occur sporadically or run in families, even without exposure to infectious material.

Strains of Prions

Although prions lack genetic material, they exist in different strains, each with distinct structural conformations. These conformations influence:

• Disease incubation time
• Severity and symptoms
• Brain regions affected
• Species transmission efficiency

Each strain represents a unique three-dimensional folding pattern of PrP^Sc. These structural differences are “encoded” in protein shape rather than nucleic acid sequence.

This concept challenges the traditional definition of strains and demonstrates that biological information can be stored in protein structure alone.

Species Barrier and Zoonotic Transmission

Prion transmission between species is limited by the species barrier, which depends on differences in amino acid sequence between prion proteins.

When prions from one species enter another:

• Transmission may fail entirely
• Incubation periods may be long
• Adaptation can occur after repeated transmission

A notable example is the transmission of Bovine spongiform encephalopathy to humans, leading to variant Creutzfeldt–Jakob disease.

Understanding the species barrier is essential for food safety, wildlife management, and predicting emerging risks.

Environmental Persistence of Prions

Prions are remarkably stable in the environment. They can persist in soil for years and remain infectious.

• Bind strongly to soil particles, especially clay
• Resist degradation by microbes and enzymes
• Retain infectivity even after environmental exposure

This persistence plays an important role in diseases such as chronic wasting disease in deer populations, where contaminated environments contribute to ongoing transmission.

Prions Beyond Disease: Functional and Beneficial Roles

Not all prion-like proteins are harmful. In some organisms, prion-like mechanisms provide biological advantages.

• In yeast, prion-like proteins can create heritable phenotypic variation
• Some proteins use prion-like domains for memory and long-term cellular changes
• In neurons, prion-like behavior may contribute to long-term synaptic plasticity

These examples suggest that prion-like folding can serve as a mechanism for biological information storage, not just disease.

Prion-Like Mechanisms in Other Diseases

Many neurodegenerative diseases involve protein misfolding and aggregation similar to prions.

Examples include:

• Alzheimer’s disease (amyloid-beta, tau)
• Parkinson’s disease (alpha-synuclein)
• Huntington’s disease (mutant huntingtin protein)

While these are not traditionally classified as prion diseases, they exhibit prion-like propagation, where misfolded proteins spread between cells and induce further misfolding.

Common Misconceptions

• “Prions are viruses.”
  They are not, prions contain no nucleic acids.
• “Cooking destroys prions.”
  Normal cooking temperatures do not reliably inactivate prions.
• “Prion diseases spread easily.”
  They are rare and typically require specific exposure routes.
• “All prion diseases are infectious.”
  Some arise spontaneously or are inherited due to mutations in the PrP gene.

FAQs

Are prions alive?

No. Prions lack metabolism, genetic material, and cellular structure, so they are not considered living organisms.

How long do prion diseases take to develop?

They often have long incubation periods, ranging from years to decades, before symptoms appear.

Can prion diseases be cured?

No cure exists at present. Treatment focuses on symptom management and supportive care.

Has anyone survived prion disease?

No. Prion diseases are considered uniformly fatal, and there are no confirmed cases of recovery. Some patients may live longer than expected, but the disease continues to progress.

What is the life expectancy of a person with prion disease?

Life expectancy depends on the specific disease, but most cases progress rapidly. For example, people with Creutzfeldt–Jakob disease often die within about a year of symptom onset, while some inherited forms may last longer.

Can prions be in any meat?

Prions are not present in all meat. They occur only in infected animals and are concentrated in high-risk tissues such as the brain, spinal cord, and certain nervous system tissues. Muscle meat is generally considered lower risk when proper food safety controls are in place.

Can you kiss someone with CJD?

Casual contact, including kissing, does not transmit prion diseases. Transmission requires specific exposure to infected tissues, such as through medical procedures or consumption of contaminated material.

Why are prions mostly associated with the brain?

PrP is highly expressed in neural tissue, and neurons are particularly vulnerable to protein aggregation and damage.

Can prions cross species?

Yes, but transmission depends on protein compatibility between species. This is called the species barrier.

Are prions only harmful?

The normal prion protein likely has important physiological roles, and only the misfolded form causes disease.

References and Further Reading

• Dobson, C.M. (2001). “The structural basis of protein folding and its links with human disease”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 356 (1406): 133–145. doi:10.1098/rstb.2000.0758
• Halfmann, R.; Lindquist, S. (2010). “Epigenetics in the extreme: prions and the inheritance of environmentally acquired traits”. Science. 330 (6004): 629–632. doi:10.1126/science.1191081
• Prusiner, S.B. (1982). “Novel proteinaceous infectious particles cause scrapie”. Science. 216 (4542): 136–144. doi:10.1126/science.6801762
• Walker, L.C.; Jucker, M. (2015). “Neurodegenerative diseases: Expanding the prion concept”. Annual Review of Neuroscience. 38: 87–103. doi:10.1146/annurev-neuro-071714-033828
• Weissmann, C. (2004). “The state of the prion”. Nature Reviews. Microbiology. 2 (11): 861–871. doi:10.1038/nrmicro1025