Question
Prions
Answer
Prions are unique infectious agents composed entirely of protein, with no associated nucleic acid (DNA or RNA) — a fundamental departure from all other known infectious agents (bacteria, viruses, fungi, parasites), which all contain genetic material.
Nature and mechanism: prions are misfolded, abnormal isoforms (PrP^Sc) of a normal host cellular protein (PrP^C, prion protein, normally present on neuronal cell surfaces, encoded by the host’s own PRNP gene). The abnormal PrP^Sc conformation is able to directly convert normal PrP^C into more of the abnormal PrP^Sc form, through a self-propagating conformational templating mechanism, without requiring replication via nucleic acid — this is the basis of prion “infectivity” and represents an entirely novel biological principle of disease transmission.
Pathogenesis: accumulated abnormal PrP^Sc protein aggregates in neural tissue, resistant to normal cellular protein degradation, progressively causing neuronal dysfunction and death, producing the characteristic spongiform (vacuolated) degeneration of brain tissue seen in prion diseases, without eliciting an inflammatory or immune response (since the abnormal protein is host-derived and not recognized as foreign).
Diseases caused by prions:
- Creutzfeldt-Jakob disease (CJD) — the most common human prion disease, causing rapidly progressive dementia; can occur sporadically, be inherited (familial CJD, associated with PRNP mutations), or be acquired (iatrogenic, e.g., from contaminated neurosurgical instruments or cadaveric growth hormone; or variant CJD, linked to consumption of BSE-contaminated beef).
- Kuru — historically described among the Fore people of Papua New Guinea, associated with ritualistic cannibalism.
- Bovine Spongiform Encephalopathy (BSE, “mad cow disease”) — the animal prion disease linked to variant CJD in humans.
- Fatal familial insomnia and Gerstmann-Sträussler-Scheinker syndrome — rarer inherited human prion diseases.
Distinctive properties: prions are remarkably resistant to conventional sterilization methods — including standard autoclaving, formaldehyde, and most chemical disinfectants — requiring special decontamination protocols (extended high-temperature autoclaving, or specific chemical treatments such as sodium hydroxide or sodium hypochlorite at high concentration) for instruments used on suspected/confirmed cases, an important infection-control consideration in neurosurgery and pathology.