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Prions

Priones

For medical students2 min readUpdated 2026-10-10

Prions are unique non-cellular infectious agents consisting exclusively of protein particles completely devoid of nucleic acids. They differ fundamentally from canonical viruses and cause specific conformational diseases affecting the nervous system in humans and animals.

Nature of agentInfectious protein without RNA or DNA
TargetNeurons (formation of a 'spongiform' structure)
GeneticsEncoded by the PRNP gene on chromosome 20
MechanismAlteration of tertiary or quaternary protein structure

Non-cellular Infectious Agents

In microbiology, alongside classical (canonical) viruses, a distinct group of infectious molecules exists, which includes prions and viroids. The main feature of these pathogens is their extremely simple and atypical structure.

Normally, human and many animal cells constantly express a normal cellular prion protein (designated as $PrP^c$). This normal protein is not pathogenic and performs important regulatory functions within cells. However, under certain conditions, this endogenous protein can transform into a lethal infectious agent.

Pathogenesis Mechanism (Conformational Diseases)

Diseases caused by prions belong to the category of so-called conformational diseases. This means that pathogenesis is based not on cell destruction by toxins or replication of a classical virus, but on changes in the spatial structure of the protein itself.

The process of pathology development includes the following stages:

  1. The infectious agent is a protein with an altered spatial configuration, involving a major disruption of normal tertiary or quaternary structure.
  2. The pathological prion enters the body or is spontaneously generated within it.
  3. Physical contact occurs between the altered infectious prion and the normal cellular protein $PrP^c$.
  4. This contact triggers a chain reaction: the normal protein changes its conformation and transforms into a pathological prion itself.

Thus, the infectious process is sustained by the continuous structural conversion of the organism's own proteins based on the template of the pathological agent.

Genetic Aspects and Etiology

Despite lacking their own nucleic acids, prions are closely linked to the host's genetic apparatus.

The transition of the normal protein into the infectious form can be triggered by two main causes. First, it can occur as a result of a genetic mutation within the PRNP gene itself. Second, the transformation process can begin due to external infection, when an already altered pathological molecule enters the body from the outside.

Clinical Presentation and Cellular Consequences

Clinical classification groups prion diseases under transmissible spongiform encephalopathies (TSEs). This name accurately reflects the nature of tissue damage.

As the disease progresses, the pathological protein accumulates in large quantities directly inside neurons. The cytopathic effect of prion infection is highly specific: affected cells of the nervous system acquire a characteristic 'spongiform' appearance (vacuolization). The consequence of such massive neuronal destruction is the development of severe, rapidly progressive neurological disorders.

The best-known examples of human prion pathologies include:

Viroids: Fundamental Differences

To fully understand non-cellular agents, it is important to distinguish prions from viroids. Viroids are infectious agents close to viruses but possessing an even more primitive structure.

Mnemonic

For quick recall: PRIONS are PROteins (causing spongiform encephalopathy in humans and animals), while VIROIDS are naked VIRus-like RNA (causing diseases in plants only).

Frequently asked questions

What human diseases are caused by prions?

Human prion diseases belong to the group of transmissible spongiform encephalopathies. They include:

  • Creutzfeldt-Jakob disease, presenting in sporadic, hereditary, and iatrogenic forms.
  • Kuru.
  • Gerstmann-Sträussler-Scheinker syndrome — a hereditary pathology characterized by dementia, muscle hypotonia, and bulbar signs.
  • Fatal familial insomnia — an autosomal dominant condition featuring progressive intractable insomnia.
  • Amyotrophic leukospongiosis.
What are the routes of transmission for prion infections in humans?

Prion transmission to humans occurs via acquired and hereditary pathways. The main mechanisms of infection include:

  • Alimentary route — consumption of infected animal products and dietary supplements derived from raw cattle organs.
  • Iatrogenic route — transmission via medical procedures, including blood transfusions, organ and tissue transplants, administration of animal-derived medications, and the use of inadequately sterilized surgical instruments.
  • Hereditary route — transmission of a mutant gene in an autosomal dominant manner.
What methods are used for the laboratory diagnosis of prion infections?

Laboratory diagnosis of prion infections relies on a combination of pathomorphological, immunochemical, and molecular genetic methods. Definite diagnosis confirmation is only possible by examining CNS tissue biopsies.

  • Pathohistological method — detection of spongiform changes in brain tissue, astrogliosis, and amyloid staining in the absence of inflammatory infiltrates.
  • Immunochemical methods — detection of protein markers in cerebrospinal fluid using enzyme-linked immunosorbent assay (ELISA) and immunoblotting with monoclonal antibodies.
  • Molecular genetic methods — genetic analysis of the prion gene and PCR for detecting the prion protein.
What is the resistance of prions to physical and chemical sterilization methods?

Prions are extremely resistant to standard physical and chemical agents, including high heat, radiation, formaldehyde, $\beta$-propiolactone, and proteolysis. Standard sterilization cycles are ineffective against them. To inactivate prions, rigorous conditions are required:

  • Autoclaving — at 134 °C for 30 minutes or at 121 °C for 4 hours (with the addition of 1N NaOH).
  • Chemical treatment — using bleach combined with 1N NaOH with a 1-hour exposure.
  • Physical method — complete incineration of infected materials.
What specific functions does the normal prion protein ($PrP^c$) perform in a healthy body?

In a healthy organism, the normal cellular prion protein ($PrP^c$) performs several important regulatory functions, including:

  • Regulation of nerve impulse transmission — participation in the normal functioning of the central nervous system.
  • Regulation of circadian rhythms — control of the body's biological clock.
  • Regulation of copper metabolism — maintaining copper homeostasis in CNS tissues.
  • Regulation of stem cell division — controlling the proliferation of bone marrow stem cells.
Do prions contain genetic material (DNA or RNA)?

No, prions are completely devoid of nucleic acids. They are exclusively infectious protein particles with an altered spatial configuration.

What is the main mechanism of prion disease development?

Pathogenesis is based on conformational changes. Contact between an altered infectious prion and the body's normal protein ($PrP^c$) leads to the transformation of the normal protein into the pathological form.

Where is the gene encoding the normal prion protein located?

The normal protein is encoded by the PRNP gene, which is localized on the short arm of human chromosome 20.

What cytopathic effect is characteristic of prion infections?

The accumulation of pathological protein inside neurons gives the cells a characteristic 'spongiform' appearance, leading to severe neurological disorders.

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