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Overview of Prion Diseases

by Pierluigi Gambetti, MD

Prion diseases (transmissible spongiform encephalopathies) are rare progressive, fatal, and untreatable degenerative disorders of the brain that result when a protein changes into an abnormal form called 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 normal protein called cellular prion protein (PrP C ) changes shape (misfolds) and becomes abnormal. This abnormal substance is called scrapie prion protein (PrP Sc ), or prion. Scrapie refers to the prion disease first observed in sheep. The newly formed prions resist being broken down by enzymes in the brain. Thus, they slowly accumulate. Prions also trigger other nearby PrP C to change into prions, and the process continues. When prions reach a certain number, disease results. Prions never change back into PrP C .

PrP C is present in all cells of the body, with a high concentration in the brain. Consequently, prion diseases affect the nervous system predominantly or exclusively. When these normal proteins change into prions, they usually cause tiny bubbles to form in brain cells. Gradually, the affected cells die and the brain becomes filled with microscopic holes. When samples of brain tissue are viewed through a microscope, they somewhat resemble Swiss cheese or a sponge (hence, the term spongiform).

Prion diseases may

  • Occur spontaneously, without any known reason (most common)

  • Occur in families

  • Be acquired from contaminated material (uncommon)

Spontaneously occurring (sporadic) prion diseases are the most common of all human prion diseases, accounting for 85 to 90% of all cases. Sporadic Creutzfeldt-Jakob disease is the most common type of the sporadic prion diseases. Worldwide, sporadic Creutzfeldt-Jakob disease occurs in about 1 in a million people each year.

There are three types of sporadic prion disease:

Prion diseases that occur in families (called familial prion diseases) involve a mutation in the gene for PrP C , which can be inherited. The mutation makes PrP C more likely to change into prions. Many different mutations exist. Each mutation typically causes a different prion disease. There are three main groups of familial prion diseases:

Prion diseases can also be acquired, although such cases are very uncommon. They can be acquired

  • When people eat beef from prion-infected cattle—as is the case in variant Creutzfeldt-Jakob disease (sometimes called the human version of mad cow disease)

  • When an organ or tissue contaminated with prions is transplanted

  • When people are injected with a prion-contaminated drug

  • When brain surgery is done with prion-contaminated instruments

  • Rarely, when people receive a prion-contaminated blood transfusion

Kuru is also an acquired prion disease, which used to occur in natives of Papua New Guinea (see Kuru).

There is no cure for prion diseases, which are all fatal, usually within months to a few years after symptoms appear. Treatment focuses on comfort measures and symptom relief. A number of strategies can help caregivers of people with a prion disease cope with the dementia caused by the disease (see ). If possible, people who have a prion disease should establish advance directives (see 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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