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 (PrPC) changes shape (misfolds) and becomes abnormal. This abnormal prion protein is called scrapie prion protein (PrPSc), or prion. Scrapie refers to the prion disease first observed in sheep. Scrapie is so named because the sheep scrape themselves against trees, fence posts, or other structures and tear their wool off. The disease causes sheep to behave in other bizarre ways, and it is fatal.
Some of the newly formed prions resist being broken down by enzymes in the brain. Thus, they slowly accumulate. Prions also trigger other nearby PrPC to change into prions, and the process continues. When prions reach a certain number, disease results. Prions never change back into normal PrPC.
PrPC is present in all cells of the body but has a high concentration in the brain. Consequently, most prion diseases affect the nervous system predominantly or exclusively. The most common change caused by prions is the formation of tiny bubbles in brain cells, and the brain becomes filled with microscopic holes. When samples of brain tissue are viewed through the microscope, they somewhat resemble Swiss cheese or a sponge (hence, the term spongiform). After a period of time (which can vary), the affected cells cease to function and die.
Prion diseases may be
Sporadic prion diseases are the most common of all human prion diseases, accounting for 85 to 90% of all cases.
There are three types of sporadic prion disease:
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.
In sporadic prion diseases, how the first prion forms is unknown. But it is suspected that an error occurs in a cell's processes (cell metabolism), and this error causes PrPC to spontaneously change its shape.
Familial prion diseases involve a mutation in the gene for PrPC, which can be inherited. The mutation makes PrPC more likely to change into prions. More than 50 mutations exist. Each mutation typically causes a different prion disease. Familial prion diseases are inherited as an autosomal dominant disorder. That means that the mutation is not on the sex (X or Y) chromosomes and that only one gene for the disease, one from either parent, is required for the disease to develop.
There are three main groups of familial prion diseases:
Another familial prion disease has been recently discovered. It differs from other prion diseases because it causes diarrhea and affects nerves throughout the body years before symptoms of brain malfunction develop. It is described as prion disease associated with diarrhea and autonomic neuropathy.
Acquired prion diseases are rare. They occur
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 a prion-contaminated organ or tissue is transplanted or grafted
When a prion-contaminated substance (such as a hormone) is given by injection
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. It used to occur in natives of Papua New Guinea who practiced ritual cannibalism.
There are no reports of prion disease having been spread through casual contact with people who have the disease.
There is no cure for prion diseases, which are all fatal, usually within months to a few years after symptoms appear. Treatment focuses on symptom relief and comfort measures.
A number of strategies can help caregivers of people with a prion disease cope with the dementia caused by the disease (see sidebar Creating a Beneficial Environment for People With Dementia).
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.