To most scientists, life is all about reproduction. On a biological level, organisms like humans, fungi, plants, and bacteria can be thought of as elaborate protein-based ways for bits of deoxyribonucleic acid (DNA) to copy themselves more effectively.
In fact, the drive to reproduce even extends outside of living organisms. Viruses are an example of a strange limbo between the living and nonliving. In a way, a virus is little more than a reproductive machine. In the cases of some viruses, like human immunodeficiency virus (HIV), DNA isn't even the molecule that drives reproduction. Another nucleotide, RNA (ribonucleic acid), is the driving factor.
Prions (pronounced pree-ons in the US, pry-ons in the UK) are even further removed from the better-understood mechanisms of reproduction involving DNA and RNA. DNA and RNA are nucleotides, a chemical structure used to make proteins, the building blocks of the living organisms designed to ensure successful reproduction. A prion is a protein that doesn't require a nucleotide to reproduce—the prion is more than capable of taking care of itself.
When an abnormally folded prion protein runs into a normal prion protein, the normal protein transforms into another abnormally folded disease-causing prion. The result is a relentless cascade of mutated protein. In cases of inherited prion disease, it's the gene mutation that causes abnormal folding of prion protein. Unfortunately, these are the same proteins used by brain cells in order to function properly, and so nerve cells die as a result, leading to a rapidly progressive dementia. While a disease-causing prion may lay dormant for years, when symptoms finally become apparent, death may follow in as quickly as a few months.
There are five main types of prion diseases currently recognized in humans: Creutzfeldt-Jakob disease (CJD), variant Creutzfeldt-Jakob disease (vCJD), kuru, Gerstmann-Straussler-Scheinker syndrome (GSS), and fatal familial insomnia (FFI). However, newer forms of prion disease are being discovered.
How Prion Diseases Are Spread
Prion diseases can be acquired in three ways: familial, acquired, or sporadic. The most common way to develop a prion disease seems to be spontaneous, with no source of infection or inheritance. Even so, about one in a million people develop this most common form of prion disease. Some prion diseases, like CJD, GSS, and FFI, can be inherited. Others are spread by close contact with the prion protein. For example, kuru was spread by cannibalistic rituals in New Guinea. When brains were eaten as part of the ritual, the prions were ingested, and the disease would spread. A less exotic example is vCJD, which is known to spread from animals to people when we ingest the meat. This is commonly known as "mad cow disease," when the prion is still in the living cow. Other animals, such as elk and sheep, have also been found to sometimes harbor prion diseases. While it's uncommon, prion diseases can also be spread on surgical instruments.
Symptoms of Prion Disease
While all of the prion diseases cause slightly different symptoms, all prions seem to have a unique fondness for the nervous system. Whereas bacterial or viral infections are commonly heard of in many different parts of the body—including the brain—prion diseases seem to exclusively cause neurological symptoms in humans, though the proteins themselves may be found in a wide range of human tissue. Time may show that a prion-like mechanism is behind diseases outside the brain.
The impact on the nervous system is dramatic. Most prion diseases cause what is known as a spongiform encephalopathy. The word spongiform means that the disease erodes brain tissue, creating microscopic holes that make the tissue look like a sponge. Usually, the end result is a rapidly progressive dementia, meaning that the victim loses her ability to think as she used to in a matter of months to a few years. Other symptoms include clumsiness (ataxia), abnormal movements like chorea or tremor, and altered sleep patterns.
One of the frightening things about prion disease is that there can be a prolonged period of incubation between when someone is exposed to a prion and when he develops symptoms. People may go for years before the prions they've been carrying become obvious, with typical neurological problems.
Treatment of Prion Disease
Unfortunately, there's no cure for prion disease. At best, physicians can try to help control symptoms that cause discomfort. In a small European study, a medication caused Flupirtine (not available in the United States) mildly improved thinking in patients with CJD, but did not improve their lifespans. A trial of the drugs chlorpromazine and quinacrine showed no improvement. At this time, prion diseases remain universally fatal.
Geschwind MD, Shu H, Haman A, Sejvar JJ, Miller BL. Rapidly progressive dementia. Annals of Neurology 2008;64:97-108.
AH Ropper, Samuels MA. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009