Prion diseases are also called transmissible spongiform encephalopathies (TSEs) because of the sponge-like holes they leave in infected brains. The infectious agents in prion diseases are prions, or proteinaceous infectious particles, that can reproduce themselves. Prions have the ability to transform normal, benign protein molecules into infectious, deadly ones by altering their structure. These deadly proteins initiate a sequence of events in which many benign proteins are transformed into new deadly ones upon contact. Prions are distinct from all other infectious materials in that they do not contain any genetic material. There are multiple prion diseases, including bovine spongiform encephalopathy (BSE), or "mad cow disease." Some prion diseases are hereditary, and involve a mutation in the gene that encodes for the prion protein. Prion diseases are transmissible within a species and between compatible species.
Research on prion diseases was founded by Dr. Stanley Prusiner, a neurologist at the University of California San Francisco. He spent two decades working on the revolutionary topic of self-reproducing prions. At the time, many other scientists regarded their existence as a preposterous idea. Despite being shunned by the scientific community, Dr. Prusiner was able to prove that prions were truly infectious proteins that could cause brain disease in people and animals. The Nobel Prize for Medicine or Physiology was awarded to Dr. Prusiner in 1997 for discovering this new type of disease-causing agents that contain no DNA.
Prion diseases are transmissible between hosts of a single species and different, compatible species. The term "spongiform" in TSE comes from the spongy appearance of the damaged brain tissue. Some examples of infectious prion diseases include scrapie in sheep and goats, kuru in cannibalistic humans of Papua New Guinea, and BSE, or mad cow disease, which is transmitted to humans through infected beef products. Prion diseases can also be transmitted through injections of infected material from a compatible organism. Because of the ability of prions to cross many species barriers, all organisms that carry prion diseases are potential vectors for human infection.
Prion diseases can also be hereditary, as seen in some cases of Creutzfeldt-Jakob disease (CJD), fatal familial insomnia (FFI), and Gerstmann-Straussler-Scheinker disease (GSS). Hereditary prion diseases occur when the PRNP gene that encodes for the normal human PrPc protein, found on the surface of neurons, is mutated so that the prion PrPSc protein (Sc for scrapie) is formed. The PrPSc protein has a different conformational structure than the normal protein and is the infectious agent. PrPSc proteins can convert similar PrPc proteins upon contact into more infectious agents, thereby reproducing themselves. Prion diseases are inherited when at least one copy of the mutated PRNP gene is present. Nervous tissue from patients with hereditary prion diseases is also infectious.
A third category of prion disease is sporadic. CJD and FFI sometimes occur in people with no known history of the disease in their family and with no known exposure to infectious materials. The cause of disease in these cases is unknown. Patients with sporadic prion diseases may have a susceptibility polymorphism in their PRNP gene, and may have spontaneous mutations forming prion proteins.
Maria Basile PhD, The Gale Group Inc., Gale, Detroit,