What are prion diseases?
Prion diseases, also called transmissible spongiform encephalopathies, are progressive neurodegenerative disorders in humans and animals that lead to progressive memory loss, personality changes, and impaired motor control (ataxia). The diseases are almost always fatal. Human prion diseases include Creutzfeldt-Jakob disease (CJD), new variant Creutzfeldt-Jacob disease (vCJD), kuru, Gerstmann-Straussler-Scheinker syndrome, and fatal familial insomnia. Animal prion diseases include scrapie, chronic wasting disease (CWD), transmissible mink encephalopathy, feline spongiform encephalopathy, ungulate spongiform encephalopathy, and bovine spongiform encephalopathy (BSE), popularly known as mad cow disease.
While symptoms of scrapie have been noted in sheep and goats for hundreds of years, it was not shown to be transmissible until the 1930s. CJD was first noted in the 1920s, kuru in the 1950s, and BSE in the mid-1980s. Daniel Carleton Gajdusek began studying kuru in 1955 and won the Nobel Prize in Physiology or Medicine in 1976 for his work. Stanley B. Prusiner coined the term “prion” in 1982; he was awarded the Nobel Prize in 1997. Most scientists believe prions are the sole cause for transmissible spongiform encephalopathies.
The diseases are caused by misshapen versions of proteins called prions. Prions clump together and can proliferate by inducing shape changes in normal proteins. As prions aggregate, they cause spongelike lesions in the brain and disrupt brain function. The resultant illnesses are referred to as spongiform encephalopathies, which can be either inherited or transmissible. Prions are found in all species, from yeast to humans, but their normal role is not known. Their evolutionary persistence in so many species implies that they must serve an important purpose, although knockout mice lacking prions do not appear to be adversely affected.
Inherited spongiform encephalopathies are primarily attributed to mutations in the prion gene, leading to abnormally shaped proteins that aggregate over time to cause the brain damage and neurological symptoms characteristic of the diseases. Transmissible diseases occur when a susceptible animal is inoculated with a fragment or an extract of diseased tissue. Inoculation may occur by intravenous transmission, such as via a blood transfusion, or orally by consumption of infected tissue or bodily fluids, such as saliva.
Prions are unusual in that the protein is the infectious agent; typical infectious agents, such as bacteria or viruses, contain nucleic acids, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA). While the infectious agent’s mode of action is not fully understood—and what is understood is not universally accepted—researchers announced in 2003 that they had discovered a new way of identifying an antibody specific to prions. This discovery has important implications for understanding how prions propagate and, in turn, for manipulating the prions for the creation of a vaccine.
These diseases have been described in humans, cattle, sheep, deer, elk, mink, domestic cats, and wild felines, and they have been experimentally induced in monkeys, hamsters, and mice. Prion diseases are usually species-specific. However, the diseases in mink, domestic cats, and wild felines are attributed to consumption of feed derived from diseased animals (usually hooved animals), and a variant in humans is associated with the consumption of meat from cattle with BSE.
The most common animal prion disease is scrapie, found in sheep and goats. It is named for the intense itching that causes sheep to rub against hard objects, scraping off the wool. It also causes staggering, tremors, and blindness. It has been known for more than 250 years in Britain and other countries of western Europe. It has been reported in most sheep-raising countries with a few notable exceptions, such as Australia and New Zealand. It first appears at two to five years of age, can last longer than six months, and is eventually fatal. It is generally accepted that it is an infectious disease in which genetics also plays an important role. It has not been shown to be transmissible to humans and poses no risk to human health.
The most famous prion disease in animals is bovine spongiform encephalopathy (BSE). First noted in British cattle in 1986, it causes nervousness, aggression, and symptoms similar to scrapie. It appears in adult cattle between two and eight years of age and is fatal. The BSE epidemic, which peaked in 1992 and affected more than 200,000 animals, was apparently caused by feed containing protein from animals with prion disease. A ban on incorporating ruminant-derived protein into cattle feed appeared to be bringing the epidemic to an end in Great Britain by the early twenty-first century, although there was a troubling increase in cases in continental Europe.
The best-known prion disease in humans is Creutzfeldt-Jakob disease (CJD), which occurs worldwide at a rate of about one per million persons. Its symptoms include dementia and ataxia, leading to death. The disease may be classified as sporadic, familial (inherited), iatrogenic (acquired through a medical procedure), or variant. The sporadic form, which accounts for the majority of cases, appears in the elderly and has an unknown etiology. The inherited form (5 to 10 percent of cases) is primarily attributed to mutations in the gene for prions, making them more susceptible to aggregation in the brains of affected individuals. The iatrogenic form can be caused by tissue transplants, contaminated surgical instruments, and pharmaceuticals derived from human cadavers.
The new variant form of CJD (vCJD) was first described in 1996; it differed from the sporadic form by affecting younger individuals and having slightly different symptoms and duration; it has been associated with eating beef from cattle infected with BSE. As of 2011, a total of 176 cases of vCJD had been identified in Great Britain since 1986, all of them fatal; far smaller numbers of cases have been reported in other countries. Because of a long time course and an incompletely known cause, it is uncertain how many will be affected by this new variant form.
No effective treatment for prion diseases existed as of the early twenty-first century. Researchers have speculated that human prion diseases might be treated by preventing normal proteins from adopting the abnormal shape, by interfering with the interaction between abnormal and normal proteins, or by destabilizing the shapes of abnormal proteins. Research has identified hundreds of molecules that seem to deter the formation of prions, prolonging the survival times of laboratory animals, but much more testing remains to be done before human drug therapies become available. Thus far, the only treatments are aimed at symptoms, such as opiates for pain and anticonvulsants to lessen neuromuscular problems.
Concerns over prion diseases have increased in North America since the discovery of BSE in cattle from Canada and the United States. A screening program is in place to help detect infected cattle, but critics of the programs in both countries argue that it is not rigorous enough to provide adequate protection of the nations’ food supplies. Despite the concerns over human health effects, considerable work remains to be done simply to achieve a better understanding of the origins, transmission, and pathogenesis of prion diseases.
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