Contributor: Pierluigi Gambetti, MD
Prion diseases (transmissible spongiform encephalopathies) are very rare degenerative diseases of the brain thought to be caused by a protein that converts to an abnormal form called a prion.
Before prions were identified, diseases such as Creutzfeldt-Jakob disease and other spongiform encephalopathies were thought to be caused by viruses. Prions are much smaller than viruses and differ from viruses, bacteria, and all living cells because they do not contain any genetic material. In prion diseases, a specific protein molecule called cellular prion protein (PrPc) changes shape and becomes an abnormal protein molecule called scrapie prion protein (PrPsc)--a prion. (Scrapie refers to a prion disease first observed in sheep.) The newly formed prion then converts other nearby PrPc into prions, and the process continues. When the prions reach a certain number, disease results. Prions never convert back into PrPc.
PrPc occurs in all cells of the body, with a high concentration in the brain. However, prion diseases affect the nervous system predominantly or exclusively. When these proteins are converted into prions, they usually cause tiny bubbles to develop in brain cells. Gradually, the affected cells die and the brain becomes filled with holes. When samples of brain tissue are viewed through a microscope, they somewhat resemble Swiss cheese or a sponge (hence the term "spongiform").
Prion disease may occur in families because of an inherited susceptibility that makes PrPc molecules more likely to convert to prions. The susceptibility results from a mutation in the gene for PrPc. Many different mutations exist. Each mutation generally causes different prion diseases, which, however, fit into three groups: fatal familial insomnia, familial Creutzfeldt-Jakob disease, and Gerstmann-Sträussler-Scheinker disease.
Prion disease may occur when prions are acquired from an external source, such as contaminated beef--as occurs in the so-called variant Creutzfeldt-Jakob disease. Or, prion disease may occur on its own (spontaneously).
There is no cure for prion diseases. All are fatal. The only available treatments are directed at keeping people comfortable and treating symptoms. A number of strategies can help caregivers of people with a prion disease cope with the dementia caused by the disease. If possible, people who have a prion disease should establish advance directives about what kind of medical care they want at the end of life. Family members of people who develop the hereditary form of the disease may benefit from genetic counseling.
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