Notes on Prions and Viroids

Prions and Viroids

• Definition:
  • Prions: Infectious proteins causing transmissible spongiform encephalopathies (TSEs).
  • Viroids: Small pathogenic RNAs causing virus-like diseases in plants.

Human Prion Diseases

• Types:
  • Kuru: Transmitted through cannibalism.
  • Variant Creutzfeldt-Jakob disease (vCJD): From contaminated beef.
  • Other diseases: Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker disease, Fatal familial insomnia.
• Lifespan after symptoms: Varies from 3 months to 6 years.

Symptoms of Kuru

• Stages:
  • Ambulant: Unsteady gait, tremors, slurred speech.
  • Sedentary: Severe tremors, mental slowing, outbursts of laughter.
  • Terminal: Inability to sit, increased tremors, severe weakness.

Pathogenesis of Prion Diseases

• Prions induce abnormal folding of the PrPC protein into the PrPSc form, causing cell damage.
• PrPSc aggregates lead to neuron death and brain damage, producing a spongiform appearance.

Viroids Characteristics

• Composition: Small, circular RNA molecules, do not code for proteins.
• Replication: Requires host plant enzymes, occurs in nucleus or chloroplasts.

Viroid Pathogenesis

• Activate the plant RNA-activated protein kinase (PKR), inducing an immune-like response in the plant.

Transmission

• Prion Transmission: Through consumption of infected tissue, iatrogenic means, or hereditary mutations.
• Viroid Transmission: Via mechanical means, and through seeds and pollen.