Overview of prion diseases

[Julian]

Prion diseases are rare progressive, fatal, and currently untreatable degenerative disorders of the brain (and rarely of other organs) 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 (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: Occurring spontaneously, without any known reason (most common)
Familial: Occurring in families
Acquired: Acquired from contaminated material (uncommon)