"Prions, once dismissed as an impossibility, have now gained wide recognition as extraordinary agents that cause a number of infectious, genetic and spontaneous disorders" (Stanley Prusiner).

Prions
Prions are proteins that are found in the brains of both normal and afflicted individuals. Prion proteins are found in two forms, the wild type form (PrPc) and the mutant form (PrPsc). Even after much research the actual function of these proteins still remains a mystery. Yet, prions (proteinaceous infectious particles), have been linked to many diseases. The origins of these diseases were previously an enigma; the sources could be linked to neither a bacterial agent nor a viral agent. Via some creative thinking by a man named Stanley B. Prusiner, the prion idea was hypothesized. He put forth the idea that the causal agent was a protein.

It is very strange indeed to consider a protein as the inducer of such disorders. How could a simple sequence of amino acids can lead to the neurodegenerative effects commonly associated with different prion diseases? This idea was strongly opposed when it was first suggested. However, after much research Prusiner's stance has been widely accepted and the following diseases have been associated with the build up of mutant prion proteins in the brain.

Sheep: Scrapies Mink: Transmissible Mink Encephalopathy (TME) Elk, Mule deer: Chronic Wasting Disease (CWD) Cows: Bovine Spongiform Encephalopathy (BSE) (Mad Cow Disease!!) Humans: Creutzfeld-Jacob Disease (CJD) Gerstmann-Straussler-Scheinker syndrome (GSS) Fatal Familial Insomnia (FFI) Kuru Alpers Syndrome Alzheimer's disease

It has been hypothesized that a mutant prion protein acts as a chaperone, causing incorrect folding of wild type prion proteins. In other words, mutant prion proteins self-propagate: Incorrectly folded proteins lead to more incorrectly folded proteins. The mutant proteins have a tendency to aggregate forming polymers called "rods", ultimately leading to the "amyloid" plaque formation commonly associated with diseases such as Alzheimer's.

Different Forms of Transmission
Even more perplexing than the origin of mutant prion proteins is the fact that prion diseases can be infectious, genetic, and even sporadic. In the past, sheep parts were ground up and used in cattle feed called bone meal. If the sheep had scrapie (a prion disease), the mutated prion proteins from the infected sheep were transferred to the cows where they propagated, eventually giving the cows Bovine Spongiform Encephalopathy, more commonly known as Mad Cow Disease.

Iatrogenic transmission, inadvertently spreading the disease while attempting to treat a different medical problem, also occurs as a result of the "indestructible" nature of PrPsc. Thus, mutant prion proteins can be spread by way of corneal transplants, implantation of dura mater or electrodes in the brain, use of contaminated instruments (because normal sterilization procedures are inadequate), or injection of growth hormone (before recombinant growth hormone was used).

Kuru is also an infectious prion disease. Kuru, a neurodegenerative disease, ran rampant through the Fore tribe in New Guinea. The Fore tribe honored their dead by consuming the brains of their deceased. Consequently, mutant prions from an infected individual were directly transferred from one person to the next. Not surprisingly, the practice of cannibalism has since stopped.

GSS is a prion disease that is passed along genetically and CJD is most commonly seen as a sporadic disease that affects one in a million people.

Generally, a person with a prion disease will show signs of abnormal behavior, loss of coordination (ataxia), tremors, rigidity, irritability, and progressive dementia. Death usually occurs within 3-12 months from the onset of symptoms.