prion

prion. A prion is a misfolded, pathogenic protein agent that can transmit its misfolded shape onto normal variants of the same protein, primarily within the brain. These infectious agents lack nucleic acid and are responsible for a group of fatal neurodegenerative disorders known as transmissible spongiform encephalopathies. The discovery of prions, championed by Stanley B. Prusiner who coined the term and later won the Nobel Prize in Physiology or Medicine, challenged the central dogma of biology by demonstrating that a protein alone could act as an infectious agent.

Overview

The prion concept emerged from decades of research into unusual neurodegenerative diseases observed in animals, such as scrapie in sheep and Creutzfeldt-Jakob disease in humans. Key experiments by Carleton Gajdusek on kuru, a disease found in the Fore people of Papua New Guinea, demonstrated the transmissible nature of these conditions, earning him a Nobel Prize. The radical protein-only hypothesis, formalized by Stanley B. Prusiner in the 1980s, faced significant skepticism but was ultimately validated, fundamentally altering understanding of infection and neurodegeneration. The primary prion protein, PrP, is encoded by the PRNP gene in mammals.

Structure and properties

The normal, cellular prion protein (PrP^C) is a glycoprotein found on the surface of neurons, particularly in the central nervous system, and its exact function remains under investigation, with proposed roles in copper binding and cell signaling. The pathogenic form (PrP^Sc) is an isoform with the same amino acid sequence but a profoundly different three-dimensional conformation, rich in beta-sheet structures as opposed to the alpha-helical content of PrP^C. This misfolded form is extremely stable, resisting degradation by protease enzymes and conventional sterilization methods like autoclaving and formaldehyde. The PrP^Sc isoform acts as a template, inducing the conformational conversion of neighboring PrP^C molecules in a chain reaction, leading to aggregation and neuronal damage.

Prion diseases

Prion diseases, or transmissible spongiform encephalopathies, are uniformly fatal and affect both humans and animals. In humans, they include sporadic Creutzfeldt-Jakob disease, familial forms like Gerstmann-Sträussler-Scheinker syndrome and fatal familial insomnia, and acquired forms such as variant Creutzfeldt-Jakob disease linked to bovine spongiform encephalopathy. Animal diseases encompass bovine spongiform encephalopathy in cattle, chronic wasting disease in deer and elk, and scrapie in sheep and goats. These disorders are characterized by a long incubation period, rapid progression of neurological symptoms including ataxia and dementia, and distinctive neuropathological features such as spongiform change, astrogliosis, and amyloid plaque deposition, often detectable via MRI or examination of cerebrospinal fluid.

Transmission and prevention

Transmission routes vary by specific disease. Sporadic cases arise spontaneously, while familial forms are linked to mutations in the PRNP gene. Acquired transmission can occur through ingestion of contaminated tissue, as with kuru from ritualistic cannibalism and variant Creutzfeldt-Jakob disease from bovine spongiform encephalopathy-tainted beef. Iatrogenic transmission has been documented via contaminated human growth hormone extracts, corneal transplants, and inadequately sterilized neurosurgery instruments. Prevention focuses on strict control measures: banning specified risk materials in animal feed, implementing rigorous surveillance programs like those by the World Health Organization and Centers for Disease Control and Prevention, and using enhanced sterilization protocols such as prolonged autoclaving in sodium hydroxide for surgical tools.

Research and treatment

Research efforts are multifaceted, focusing on understanding the precise conversion mechanism, developing early diagnostic tests, and finding effective therapeutics. Promising diagnostic approaches include the real-time quaking-induced conversion assay and protein misfolding cyclic amplification. Therapeutic strategies aim to stabilize the normal PrP^C conformation, inhibit the conversion process, or enhance clearance of aggregates, with compounds like quinacrine and pentosan polysulfate studied in models. Major research is conducted at institutions like the MRC Prion Unit at University College London and the Rocky Mountain Laboratories. While no cure exists, ongoing clinical trials and studies into immunotherapy and gene silencing techniques offer potential future avenues for intervention. Category:Infectious diseases Category:Proteins Category:Neurodegenerative disorders