Prions are infectious proteins thought to cause disease as viruses do, though they lack the DNA or RNA that forms the command structure of a virus. Prions have been implicated in scrapie, kuru, Creutzfeldt-Jakob disease, and in Bovine Spongiform Encephalopathy (BSE), also known as Mad Cow Disease. These mysterious and powerful proteins seem to contradict a central tenet of molecular biology: genes affect proteins, but proteins do not affect DNA.
Any known disease transmitted by prion is untreatable. Prions attack the nervous system, particularly the brain. In sheep, scrapie-infected animals stagger and die as their brains disintegrate. With BSE, humans suffer a hideously similar fate.
We are used to thinking of disease-causing agents as living invaders. They seem to function as any visible organism does, eating, growing, and reproducing. Even viruses, once they are in a cell, seem to live, grow, and reproduce. Prions work differently, almost the way that crystals do.
When certain minerals solidify, dissolved materials condense in a particular pattern around a seed crystal. The seed provides a nucleus, a template, for a particular shape of crystal to grow. It is possible to construct crystals of alternate forms from the same solution by using differently shaped seeds.
Analogously, prions are misfolded, misshapen, proteins, and once in an animal’s brain cells, proteins near the prions apparently begin to deform and refold themselves to match the defective prion. Normal proteins distort themselves and become abnormal ones. The misfolded proteins finally destroy the cells they inhabit, spreading and leaving behind debris. Eventually, infected brains are spongy and full of cavities.
Once, viruses were thought to be the smallest possible infectious agent. In the 1960s, John Stanley Griffith and Tikvah Alper developed the idea that certain diseases were transmitted by some agent that did not contain genetic material, as viruses do. They noticed that UV light, which destroys DNA by breaking its linkages, did not seriously harm agents causing certain encephalopathies.
Stanley B. Prusiner built on their research and won the Nobel Prize for his work on prions, which he named. He formed the word from the first syllables of the words proteinaceous and infectious, with “on” added add the end to show that a prion is a tiny particle like a virion. Dr. Prusiner found that the protein fragment he studied came in two forms, the misfolded infectious type, and the common, non-infectious form.
The common form of the protein he studied is found throughout animal nervous systems. It does no harm, and possibly helps memory storage. A distorted form of the protein has caused scrapie in sheep for centuries. At some point, scrapie-causing prions crossed over to cattle, probably when cows were fed improperly processed sheep offal, and from there some humans got the prions, and thus a disease, by eating beef.
Actually, humans can acquire distorted proteins in several ways. Some families carry a genetic tendency to form misfolded protein fragments as they age. Some unfortunate people were accidentally given malformed protein bits in organ transplants or pituitary gland extracts before the existence of prions was known. Some people got a form of encephalopathy called kuru from religious practices which involved respectfully handling, and sometimes eating, parts of the remains of dead relatives.
The study of prions is very new. Some scientists, in fact, are not certain that prions completely explain certain encephalopathies. The study of prions has, at least, helped promote methods that defend humankind from the spread of BSE.