Transmissible spongiform encephalopathies (TSEs), also called prion diseases, are rare and progressive disorders affecting the central nervous system of humans and animals. These are untreatable fatal diseases in humans. The incubation period is long with signs and symptoms presenting months or years after the exposure to the disease. Prion diseases occur in 1 in every 1 million people worldwide each year.
Symptoms include rapid and progressive dementia causing memory loss, attention problems, intellectual decline and personality changes. Involuntary movements, changes in posture and balance worsen over time and death occurs in a matter of months to a few years after the start of symptoms.
Inherited or familial forms of prion disease are caused by mutations of the PRNP gene located on chromosome 20. It codes for the normal prion protein. It is believed that the protein is normally involved in moving copper into cells and protects brain cells in some way.
Point mutations in the PRNP gene give rise to familial forms of Creutzfeldt-Jakob Disease and fatal familial insomnia (FFI). Fatal familial insomnia was so named because of the disturbances in sleep that marked the onset of the disease.
Another inherited prion disease is Gerstmann-Straussler-Scheinker Syndrome (GSSS). Transmission of the mutated PRNP gene, which is autosomal dominant, also leads to symptoms similar to other prion diseases. Its progression is slower, but it is still as deadly.
There is also a sporadic form of Creutzfeldt-Jakob Disease which means that it occurs in people with no known gene mutations or risk factors. It is the most common form of prion disease, is not inherited and has a peak occurrence at the 7th decade of life. There have been incidences of accidental transfer of the disease from prion-contaminated tissues in medical procedures such as corneal transplants.
An acquired form of transmissible spongiform encephalopathies, the Variant Creutzfeldt-Jakob Disease (vCJD), has been linked to bovine spongiform encephalopathy or mad cow disease. It is acquired from eating beef products which are infected with the disease.
Kuru is another acquired form of prion disease that was first seen in the South Fore tribe from Papua New Guinea. It is believed that its spread was due to the eating of brain tissues of deceased tribe members in cannibalistic funeral rituals.