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prions

prions (rhyming with aeons) is an acronym for ‘proteinaceous infectious particles’. The term was coined in 1982 by Stanley B. Prusiner, a neurologist at the University of California at San Francisco, who proposed that a new type of pathogen consisting solely of protein is responsible for a school of deadly neurodegenerative diseases called Transmissible Spongiform Encephalopathies (TSEs). These include scrapie in sheep, bovine spongiform encephalopathy (BSE or ‘mad cow disease’) in cattle and Creutzfeldt-Jakob Disease (CJD) in people.

Inheritance and infection

TSEs come in an array of strains and types, each causing a distinct pattern of brain damage and clinical signs. The ‘drowsy’ and ‘hyper’ lines of sheep scrapie first alerted researchers to such variations in the 1950s. In the late 1970s a type was discovered in North American captive elk that causes a wasting disease. In some human strains, such as CJD, symptoms progress from disturbances of balance and co-ordination to blindness and deep dementia. Others produce sleep disorders.

Some TSEs look like genetic conditions. For example, the very rare human TSE Gerstmann– Straussler–Scheinker Syndrome (GSS) appears to be strictly familial, striking distant cousins on opposite sides of the globe with eerie similarity. By contrast, other TSEs can clearly be the result of infection. Pioneering work by French veterinarians in the 1930s and 1940s illustrated that scrapie could be spread between sheep and goats by injection.

Among humans, the disease kuru, found in the south Pacific, was also shown to be transmissible. In the 1950s, it was the leading cause of death in the Fore-speaking tribe of the Eastern Highlands of Papua New Guinea, until an international team of researchers discovered that it was spread by funeral rites in which the dead were revered by eating or handling their organs. The West suffered cases of what experts came to dub ‘high tech cannibalism’: since the 1970s, corneal transplants, dural grafts and contaminated human growth hormone extracted from cadaveric pituitary glands have all been shown to be potential vectors for the spread of CJD.

Mechanism of infection

TSE infection has some very odd features. Victims mount no obvious immune response, and the agent responsible is extraordinarily resilient. The solvents used for the storage of pituitary glands for production of growth hormone should have killed all known pathogens. The infectivity of brain matter from scrapie-positive sheep survives exposure to formaldehyde and even ultraviolet radiation. The latter observation prompted a suggestion by British researchers that the scrapie agent, unlike viruses and bacteria, might not contain nucleic acid (DNA or RNA), since this would have been destroyed by ultraviolet radiation. Prusiner cited this evidence when he proposed the prion model in 1982.

Since then, Prusiner and supporters of his ideas around the world have tackled TSEs with a series of dramatic experiments using the latest molecular techniques. They have treated diseased brain tissue with detergents and centrifuges and harvested the encrusted, suspect prion. After several groups determined the genetic sequence of that protein, Prusiner realized that it was a fragment of a normal protein (prion protein or PrP), the function of which is still uncertain, which is found in healthy nerve cells. They have gone on to argue, but not to prove, that once mutated, this protein becomes an aberrant prion, e.g. PrP (Scrapie), which might then convert similar healthy protein to the diseased form through a domino-style process that Prusiner calls ‘conformation’. This conversion can be sparked, Prusiner speculates, in three different ways: a person can inherit ‘weak’ proteins genetically inclined to mutate; a person's natural prion protein can spontaneously mutate; or the mutated form can be transmitted through food, surgery, or drugs, seeding the transformation of the host animal's natural protein.

Continuing controversy

In 1997, Prusiner was awarded the Nobel Prize for Physiology or Medicine for elucidating an ‘entirely new genre of disease-causing agents’. However, for UK government scientists at the coalface of the British BSE crisis, and other TSE specialists in the United States, the award was premature.

Prions, they observed, had never been shown to cause disease. Only four days before Prusiner's Nobel Prize was announced, the prion scarcely merited a mention in an article in the journal Nature by the leading researcher Moira Bruce of the Neuropathogenesis Unit (NPU) in Edinburgh. Bruce described evidence that the same agent that had infected more than a million British cattle was responsible for the variant form of CJD (vCJD), which had started to strike young Britons. Two groups of test mice experimentally infected with either diseased cattle brain or human tissue from victims of vCJD, had very similar patterns of brain damage after very similar incubation times. In presenting her evidence, Bruce only once mentioned the word prion, and couched it in a distancing pair of quotation marks.

Bruce insists that the TSE agent ‘behaves exactly like a virus’, though her group thinks that it may be an unconventional sort, which they call a ‘virino’. The argument between the virus/virino and prion camps is built on styles of investigation that could scarcely be more different. Bruce's group is inheritor of a line of research founded on traditional biological observation. Much of what we know about the pathogenesis of these diseases comes from this group, and the Institute for Animal Health in Compton, Berkshire.

They inoculated mice with extracts of brain tissue from sheep with scrapie, then observed the emergence of infection over two years or more. They established that the strains of scrapie can be recognized by the incubation time and pattern of brain damage in such host mice. They detected the presence in the test mice of a gene that clearly affects incubation times, which they named Sinc, for ‘Scrapie Incubation’. It turns out that Sinc is the PrP gene.

They also discovered that the host animal must have a healthy immune system for infection to take hold. Infection somehow rides the organs of the immune system and eventually floods out into the central nervous system, proceeding up the spinal cord to the brain, causing holes and protein deposits (plaques).

In 1993, presenting her findings to the Royal Society in London, Bruce demonstrated that when another species (monkey, sheep, antelope, cat) has been infected with material from a cow with BSE, the infectious material from that new species still exhibits the characteristics of BSE in her strain-typing tests. The prion conformation model has yet to cope adequately with this finding. All the other species have very different natural prion proteins. The conformation and thus progression of the disease should logically vary according to the particular chemical composition of the victim's own prion protein, which is supposed to be transformed into an aberrant prion by the initial infection. To Bruce, the obvious explanation for the persistent properties of BSE in so many different species was that the BSE agent is a virus-like agent, possessing its own DNA or RNA, which, as in a viral infection, causes the production of more infectious agent just like itself.

To Prusiner, the failure of the opposition to isolate a virus or a nucleic acid is critical. He and his collaborators have shown that mice genetically engineered to stop them producing their own PrP cannot be infected with TSEs from other animals. To them, this is evidence that the protein is the agent. The virus camp sees PrP as a receptor for a foreign agent.

Prusiner and his supporters come back to the toughness of the aberrant protein and its resilience in the face of enzymes, radiation, formaldehyde, and heat. However, prion-sceptics point to work from 1991 indicating that TSE agents are probably not indestructible, just devilishly tricky to get at. And other viruses can survive formaldehyde. During rendering, autoclaving and hormone extraction, protein fragments toughen and aggregate. Whether it is this toughening, or native impenetrability, that protects the TSE agents, their inactivation remains a key challenge in agriculture and medicine.

The most important inroads paved so far by prionism have come in the field of molecular genetics. Prionists have found mutations in the PrP gene that point to genetic susceptibility to TSEs. Neurologist John Collinge, of Imperial College London, with Prusiner in the early 1990s, discovered the PrP mutation involved in the seemingly familial prion disease GSS.

It turns out that the natural prion protein usually carries two delicate tree-like carbohydrate structures, called glycoforms. Prusiner and collaborators in Oxford and Ohio have observed that glycoforms change during disease. Prionists construe the change as a destabilizing part of the protein conformation process. The viral camp sees it as a classic side-effect of a foreign agent getting inside a cell and disrupting protein glycosylation.

Whatever causes the change, pathologists around the world now use glycoform analysis to help them determine the strain of TSE they are seeing in patients. Not enough is known about the protein-sugar variation to determine whether or not it can serve as a stand-alone test to, say, distinguish scrapie from BSE.

The prionists even claim to have demonstrated the conformation process in a test tube, by mixing normal PrP with the aberrant scrapie version, although only limited amounts were converted before the process fizzled out.

Despite impressive progress for the prion model the scientific case is not proven. Bruce's group still believes that the tough protein revealed by Prusiner's research simply cloaks and protects an independent nucleic acid, making up a virino. The prionists, they argue, have not adequately accounted for strain variation in scrapie, or the persistence of a particular TSE's characteristics, whatever its host.

What began as an obscure argument over a rare class of neurological diseases, and continues as an intense scientific controversy, is now at the heart of a world-wide public health crisis. Estimates of the number of Britons likely to succumb to vCJD now swing from hundreds to hundreds of thousands. And the rest of Europe is now battling to stem BSE in its own herds. A current challenge is development of reliable tests that can quickly detect the difference between normal and diseased prions, for screening of food and blood.

Emily Green, and Colin Blakemore

Bibliography

Collinge, J. (2000) Concise Oxford Textbook of Medicine Chapter 13.17 Oxford University Press, Oxford, 855.
Farquhar, C. F.,, Somerville, R. A., & and Bruce, M. E. (1998) Straining the prion hypothesis, Nature 391, 345–346
Prusiner, S. B. (1982) Novel Proteinaceous Infectious Particles Cause Scrapie, Science, 216: 136–144.


See also dementia; infection; microorganisms; virus.

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Prion

Prion

In 1997 Stanley Prusiner was awarded the Nobel Prize in physiology or medicine for a revolutionary theory about the mechanisms of infection. His theory, the "prion hypothesis," concerns an unusual protein, the prion, which occurs in the complete absence of DNA and RNA. According to Prusiner's theory, the prion differs from other well-known infections agents including bacteria and viruses. While the latter rely on nucleic acid for survival and replication, the prion is made of a protein and lacks nucleic acid. Both the existence of the prion and the underlying mode of infection are unprecedented in medical sciences. While several critical issues remain to be addressed, the prion hypothesis may furnish a plausible framework to understand the pathogenesis of several deadly brain diseases of the central nervous system.

A New Infectious Agent

Prion is an acronym for "proteinaceous infectious particle," a term coined by Prusiner in the early 1980s to describe the nature of the agent causing the fatal brain disorders known as transmissible spongiform encephalopathies (TSE), also called prion diseases. Well-known examples of prion diseases include scrapie in sheep and goats, bovine spongiform encephalopathy (BSE, or "mad cow" disease) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. Prion diseases are infectious and can also be transmitted to healthy animals by inoculating them with extracts of diseased brain.

In the mid-1960s, Tikvah Alper and colleagues reported that nucleic acid was unlikely to be a component of the infectious agent that causes scrapie . In 1967 J. S. Griffith speculated that the scrapie agent might be a protein capable of "self replication" without nucleic acid. However, Prusiner was the first, in the early 1980s, to successfully purify the infectious agent and to show that it consisted mostly of protein (technically speaking it is a glycoprotein , because it has a sugar group attached). He chose to name the new agent "prion" to distinguish it from viruses or viroids.

The essential protein component of prion was later identified in 1984 as prion protein (PrP), which is encoded by a chromosome gene in the host genome. Researchers concluded that the prion is a new infectious agent that consists mostly of PrP. This view is often referred to as the "protein only" or prion hypothesis. Some scientists find this notion hard to accept and have argued that nucleic acid is needed to carry information necessary for infection. However, no one has been able to demonstrate that either DNA or RNA play a direct role in prion replication.

In 1992 Charles Weissmann and colleagues obtained conclusive evidence for the central role of PrP in the transmission of prion diseases, when they created transgenic mice devoid of the PrP gene. These so-called PrP knockout mice were found to be completely resistant to infection when inoculated with scrapie brain preparations. When the PrP gene was reintroduced into the knockout mice, they once again became susceptible to prion infection.

Role of Protein Conformation

How can a protein such as PrP made by a cellular gene become an infectious agent? Prusiner and associates had found that PrP could exist in two forms, a normal or cellular form (PrPC) normally expressed at low levels in neurons and other cell types, and an abnormal or scrapie form (PrPSc) built up in diseased brain. PrPC is a cell-surface glycoprotein, the function of which has yet to be established. PrPC consists of a single polypeptide chain folded into predominantly spiral conformations known as α-helices. These structures give rise to a globular shape that is soluble and can be cleared from the cell by degrading enzymes called proteases.

In contrast, PrPSc that has been isolated from diseased brain is rich in an alternative conformation that resembles extended strands. These structures are known as β-sheets. The β-sheet rich PrPSc tends to aggregate and is resistant to heat and degradation by proteases. It is assumed that PrPSc can initiate the infection process by binding to predominantly-helical PrPC and converting it into more stable PrPSc with β-sheet conformation. This will set off a chain reaction leading to accumulation of large amounts of PrPSc to levels that result in brain tissue damage. The conformational conversion from α-helices to β-sheets transforms the benign PrPC into disease-causing PrPSc. This model of conformational conversion provides useful insights into the pathogenesis of prion diseases.

Prion Diseases

Historically, prion diseases have been given distinct names. Scrapie is a naturally occurring prion disease of sheep and goats that was first documented in Iceland during the eighteenth century. BSE or mad cow disease is a prion disease of cattle and is believed to be acquired through scrapie-contaminated foodstuffs. Kuru, a prion disease found among the Fore tribe of New Guinea, was shown by D. Carleton Gajdusek to be transmitted by the consumption of human tissue, particularly brain tissue, during funerary rituals. Gajdusek was awarded the 1976 Nobel Prize in physiology or medicine for this contribution. The early symptom of Kuru is a loss of coordination, followed by mental confusion and, ultimately, death. It has virtually disappeared since 1958, when the practice of eating human tissue was more or less eradicated in New Guinea.

CJD is the most common human prion disease, affecting about one in a million people. The main symptom is dementia , along with other neurological signs. There are three forms of CJD. Sporadic CJD, the cause of which has yet to be found, is a spontaneous disease that accounts for a majority of CJD cases. Familial CJD affects people who carry a mutation in the PrP gene on chromosome 20. The third form, called iatrogenic CJD, is the result of accidental transmission during medical treatments. A newly emerged CJD phenotype, commonly called variant CJD, has occurred in the United Kingdom since 1985. Variant CJD has a unique disease profile, and may result from the consumption of BSE-contaminated meat products. It has been diagnosed mostly in young people who initially seek treatment for psychiatric symptoms. Gertsmann-Sträussler-Scheinker (GSS) syndrome is a familial prion disease resulting from a mutation in the PrP gene. The main symptom of GSS is the loss of coordination and dementia. Fatal familial insomnia (FFI) is another a familial prion disease in which fatal dementia follows the loss of physiological sleep.

Although human prion diseases manifest as three etiologically different formsspontaneously (sporadic CJD), through inheritance (familial CJD, GSS, and FFI), and by infection (iatrogenic CJD, kuru, and possibly the new variant CJD), they nonetheless share a common pathogenetic event. Within the framework of "protein only" hypothesis, they all involve the protein conformational change that converts PrPC to pathogenic PrPSc. Such a structural change in PrP may be triggered by a rare spontaneous event leading to a sporadic disease, a mutation that causes a familial disease, or exposure to foreign PrPSc, leading to an acquired disease. The "protein only" hypothesis provides a plausible mechanism underlying the pathogenesis of all forms of prion diseases. Moreover, it also helps explain the tremendous variability in prion-associated disease phenotypes . Structurally distinct variants of PrPSc may accumulate in different regions of the brain and initiate pathogenic changes that may eventually lead to distinct pathology in different areas of the brain, and subsequently the particular disease symptoms.

The concept of the prion and the role of protein conformation in disease pathogenesis have renewed inquiry into the causes of other and more common neurodegenerative disorders, such as Alzheimer's disease, Hunt-ington's disease, and Parkinson's disease. A common hallmark of all these diseases, as in prion diseases, is the conversion of an otherwise soluble and functional neuronal protein into a β-sheet rich and protease-resistant protein that has a higher tendency to aggregate and is harmful to the brain. These common pathogenetic features raise the hope that therapeutic interventions based on the same principles may be effective in all these diseases.

see also Protein.

Pierluigi Gambetti

and Shu G. Chen

Bibliography

Cohen, F. E., and S. B. Prusiner. "Pathologic Conformations of Prion Proteins."Annual Review in Biochemistry 67 (1998): 793-819.

Prusiner, S. B. "Molecular Biology of Prion Diseases." Science 252 (1991): 1515-1522.

. "The Prion Diseases." Scientific American (1995): 48-57.

. Prion Biology and Diseases. New York: Cold Spring Harbor Laboratory Press,1999.

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Prions

Prions

Forensic investigations can often be focused on an illness outbreak or death that is suspected of being of infectious origin. Then, a critical task of forensic scientists is to identify the source of the illness and, if it is determined to be contagious, to track the pattern of the infection in order to help quell the present and future outbreaks.

Bacteria, viruses, fungi, and protozoa are the usual causes of infections. However, within the past several decades, a protein found in the brain has been determined to be the cause of one or more similar diseases of humans and animals (variant Creutzfeld-Jacob disease in humans; Bovine Spongiform Encephalopathy [BSE] or "mad cow" in cattle) that produce a progressive destruction of brain tissue.

The determination of the involvement of the protein, dubbed prion, is an example of forensic science . Post-mortem examinations of tissue samples are geared toward unearthing the indications of prion activity and in detecting the presence of the abnormal form of the protein. As in other infectious disease investigations, establishing the origin of the infection becomes a priority.

Prions are proteins that are infectious. Indeed, the name prion is derived from "proteinaceous infectious particles." The forensically relevant investigations that have implicated prions in degenerative brain diseases have been revolutionary. The discovery of prions and confirmation of their infectious nature overturned a central dogma that infections were caused by intact organisms, particularly microorganisms such as bacteria, fungi, parasites, or viruses. Since prions lack genetic material, the prevailing attitude was that a protein could not cause disease.

Prions were discovered and their role in brain degeneration was proposed by Stanley Pruisner. This work earned him the Nobel Prize in medicine or physiology in 1997.

In contrast to infectious agents that are not normal residents of a host, prion proteins are a normal constituent of brain tissue in humans and in all mammals studied thus far. The prion normally is a constituent of the membrane that surrounds the cells. The protein is also designated PrP (for the aforementioned proteinaceous infectious particle). PrP is a small protein, being only some 250 amino acids in length. The protein is arranged with regions that have a helical conformation and other regions that adopt a flatter, zigzag arrangement of the amino acids. The normal function of the prion is still not clear. Studies from mutant mice that are deficient in prion manufacture indicate that the protein may help protect the brain tissue from destruction that occurs with increasing frequency as someone ages. The normal prions may aid in the survival of brain cells known as Purkinje cells, which predominate in the cerebellum, a region of the brain responsible for movement and coordination.

The so-called prion theory states that PrP is the only cause of the prion-related diseases, and that disease results when a normally stable PrP is "flipped" into a different shape that causes disease. Regions that are helical and zigzag are still present, but their locations in the protein are altered. This confers a different three-dimensional shape to the protein.

As of 2005, the mechanism by which a normally functioning protein is first triggered to become infectious is not known. One hypothesis, known as the virino hypothesis, proposes that the infectious form of a prion is formed when the PrP associates with nucleic acid from some infectious organism. Efforts to find prions associated with nucleic acid have, as of 2005, been unsuccessful.

If the origin of the infectious prion is unclear, the nature of the infectious process following the creation of an infectious form of PrP is becoming clearer. The altered protein is able to stimulate a similar structural change in surrounding prions. The change in shape may result from the direct contact and binding of the altered and infectious prion with the unaltered and still-normally functioning prions. The altered proteins also become infective and encourage other proteins to undergo the conformational change. The cascade produces proteins that adversely effect neural cells, and the cells lose their ability to function and ultimately die.

The death of regions of the brain cells produces holes in the tissue. This appearance led to the designation of the disease as spongiform encephalopathy. This appearance is a hallmark of forensic examinations.

The weight of evidence now supports the contention that prion diseases of animals, such as scrapie in sheep and BSE in cattle, can cross the species barrier to humans. In humans, the progressive loss of brain function is clinically apparent as Creutzfeld-Jacob disease, kuru, and Gerstmann-Ströussler-Scheinker disease. Other human diseases that are candidates (but as yet not definitively proven) for a prion origin are Alzheimers disease and Parkinsons disease.

In the past several years, a phenomenon that bears much similarity to prion infection has been discovered in yeast. The prion-like protein is not involved in a neurological degeneration. Rather, the microorganism is able to transfer genetic information to the daughter cell by means of a shape-changing protein, rather than by the classical means of genetic transfer. The protein is able to stimulate the change of shape of other proteins in the interior of the daughter cell, which produces proteins having a new function.

The recent finding of a prion-related mechanism in yeast indicates that prions may be ubiquitous features of many organisms and that the protein may have other functions than promoting disease.

see also Animal evidence; Mad cow disease investigation.

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Prions

Prions

Prions are proteins that are infectious. Indeed, the name prion is derived from "proteinaceous infectious particles." The discovery of prions and confirmation of their infectious nature overturned a central dogma that infections were caused by intact organisms, particularly microorganisms such as bacteria , fungi , parasites , or viruses . Since prions lack genetic material, the prevailing attitude was that a protein could not cause disease.

Prions were discovered and their role in brain degeneration was proposed by Stanley Pruisner. This work earned him the 1997 Nobel Prize in medicine or physiology.

In contrast to infectious agents that are not normal residents of a host, prion proteins are a normal constituent of brain tissue in humans and in all mammals studied thus far. The prion normally is a constituent of the membrane that surrounds the cells. The protein is also designated PrP (for proteinaceous infectious particle). PrP is a small protein, being only some 250 amino acids in length. The protein is arranged with regions that have a helical conformation and other regions that adopt a flatter, zigzag arrangement of the amino acids. The normal function of the prion is still not clear. Studies from mutant mice that are deficient in prion manufacture indicate that the protein may help protect the brain tissue from destruction that occurs with increasing frequency as someone ages. The normal prions may aid in the survival of brain cells known as Purkinje cells, which predominate in the cerebellum, a region of the brain responsible for movement and coordination.

The so-called prion theory states that PrP is the only cause of the prion-related diseases, and that these disease results when a normally stable PrP is "flipped" into a different shape that causes disease. Regions that are helical and zigzag are still present, but their locations in the protein are altered. This confers a different three-dimensional shape to the protein.

As of 2002, the mechanism by which normally functioning protein is first triggered to become infectious is not known. One hypothesis, known as the virino hypothesis, proposes that the infectious form of a prion is formed when the PrP associates with nucleic acid from some infectious organism. Efforts to find prions associated with nucleic acid have, as of 2001, been unsuccessful.

If the origin of the infectious prion is unclear, the nature of the infectious process following the creation of an infectious form of PrP is becoming clearer. The altered protein is able to stimulate a similar structural change in surrounding prions. The change in shape may result from the direct contact and binding of the altered and infectious prion with the unaltered and still-normally functioning prions. The altered proteins also become infective and encourage other proteins to undergo the conformational change. The cascade produces proteins that adversely effect neural cells and the cells lose their ability to function and die.

The death of regions of the brain cells produces holes in the tissue. This appearance leads to the designation of the disease as spongiform encephalopathy.

The weight of evidence now supports the contention that prion diseases of animals, such as scrapie in sheep and bovine spongiform encephalopathy (BSEpopularly known as Mad cow disease) can cross the species barrier to humans. In humans, the progressive loss of brain function is clinically apparent as Creutzfeld-Jacob disease, kuru, and Gerstmann-Sträussler-Scheinker disease. Other human disease that are candidates (but as yet not definitively proven) for a prion origin are Alzheimer's disease and Parkinson's disease.

In the past several years, a phenomenon that bears much similarity to prion infection has been discovered in yeast . The prion-like protein is not involved in a neurological degeneration. Rather, the microorganism is able to transfer genetic information to the daughter cell by means of a shape-changing protein, rather than by the classical means of genetic transfer. The protein is able to stimulate the change of shape of other proteins in the interior of the daughter cell, which produces proteins having a new function.

The recent finding of a prion-related mechanism in yeast indicates that prions my be a ubiquitous feature of many organisms and that the protein may have other functions than promoting disease.

See also BSE and CJD disease; BSE and CJD disease, advances in research

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Prion

Prion

Unlike all other infectious agents, prions contain no deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). This radical difference has slowed the understanding and acceptance of the infectious properties of prions since their discovery. Prions are infectious agents composed of protein that cause fatal brain diseases. Prion diseases include scrapie in sheep, "mad cow disease" (bovine spongiform encephalopathy, or BSE) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans. Prion diseases can be transmitted when an organism consumes infected brain material from another organism. This occurred in England (and elsewhere) when cows were fed processed remains of infected livestock. While the cause of most cases of CJD is unknown, a small number of European cases have been correlated with the consumption of contaminated beef.

First called "slow viruses," the unusual nature of these infectious agents became clear from experiments performed in the 1960s. For example, the agents were particularly resistant to sterilization procedures that inactivated bacteria and viruses.

In the early 1980s American neurologist Stanley Prusiner published biochemical purification studies suggesting that these pathogens were composed mainly of one type of protein and were thus fundamentally differentand by implication, far simpler chemicallythan conventional infectious pathogens of animals and plants. Prusiner coined the term prion (derived from pro- teinaceous in fectious pathogen) to highlight this distinction. The single protein implicated as the causative agent was named the prion protein, PrP for short. Although the theory was first greeted with skepticism, Prusiner was vindicated by receiving the 1997 Nobel Prize in Biology or Medicine.

Generally, and as first suggested by Norwegian-American chemist Christian Anfinsen, the linear sequence of amino acids in a protein determines its unique three-dimensional structure, or "conformation." This conformation arises from folding of the peptide chain driven by thermodynamic considerations. A normal form of PrP made in healthy animals is called PrPC and follows a predetermined pattern of folding. The folding results in three corkscrew ("α -helical") segments that compact down upon each other to form a globular core region. Surprisingly, analysis of the infectious form of the PrP referred to as PrPSc reveals a different shape. Compared to PrPC, PrPSc has a diminished amount of α-helix and an increased amount of another folding pattern called α-sheet, despite the fact that they have the same amino acid sequence.

These findings defined a new mechanism of disease resulting from proteins adopting alternative, inappropriate conformations. The exact means whereby PrPSc molecules are formed from PrPC molecules is not fully understood. Nonetheless, it appears to involve a templating reaction where PrPC molecules are first unfolded and then refolded into the shape characteristic of PrPSc using preexisting PrPSc molecules as templates . Since the generation of new PrPSc molecules is equated with (and perhaps the same as) the generation of new infectious particles, it can be seen that prions "replicate" in a strange and novel manner, namely by subverting the folding of a normal cell-surface protein.

see also Neurologic Diseases; Protein Structure

David Westaway

Bibliography

Prusiner, S. B. Scientific American 272, no. 1 (January 1995): 4857.

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prion

prion (prē´ŏn), abnormal form of a protein found in mammals, believed to cause a group of diseases known as prion diseases or transmissible spongiform encephalopathies. Well-known prion diseases are Creutzfeldt-Jakob disease (CJD) and kuru in humans, scrapie in sheep, bovine spongiform encephalopathy (BSE), also called "mad cow disease," in cattle, and chronic wasting disease in deer and elk (wapiti). There is no effective treatment for any prion disease.

Sometimes taking more than 30 years to display symptoms, the diseases slowly attack brain tissue, often leaving spongelike holes. They are characterized by accumulations of prions, abnormal forms of a protein called prion protein. Unlike viruses or bacteria, prions contain no genetic material and have no known ability to reproduce themselves. Normal prion proteins occur naturally in brain tissue. Prions differ in shape from normal prion proteins due to misfolding, and are not susceptible to enzymes that normally break down proteins. In the brain, prions appear to increase their number by directly converting normal prion proteins.

Prion diseases have both infectious and hereditary components. The gene that codes for prion proteins can mutate and be passed on to the next generation. Most of the diseases also can be acquired directly by infection with prions, but unlike other infectious agents, prions provoke no immune response. Most prion diseases, however, are not highly transmissable; chronic wasting disease is the exception because infected deer that have not developed the disease shed prions from lymph tissue in their intestines, contaminating the soil and plants on which other deer graze with the prions in their feces.

An epidemic of BSE in Great Britain that was diagnosed in 1986 and infected some 178,000 cows appears to have been caused by a protein feed supplement that contained rendered remains of scrapie-infected sheep brains. In 1996 a suspicion that BSE had been transmitted to humans who died of a variant of CJD in Britain caused a scientific and economic furor as the European Union imposed a ban (1996) on the export of British beef, which was partially lifted in 1999 and fully lifted in 2006. The U.S. Dept. of Agriculture banned the import of cattle and many cattle byproducts from most European nations because of BSE. Instances of BSE in cattle have also occurred in many other European countries, Canada, the United States, and Japan, but the vast majority of cases occurred in Britain in the 1980s. There is now compelling evidence that BSE is the same disease as variant CJD (vCJD), which has killed less than 200 people, but it is not yet known exactly how the disease is passed from animals to humans.

The idea of disease-causing protein particles was first put forward in 1981 by Stanley B. Prusiner, the neurologist who coined the term prion (from proteinaceous infectious particle). The prion theory was controversial from the beginning, and although scientific evidence for the existence of such infectious particles has increased, an exact causal link between prions and the diseases they are believed to cause remains to be established. Critics believe that these diseases are caused by unidentified viruses.

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Prions

PRIONS

Prions are infectious proteinaceous particles or, more simply, proteins that lack nucleic acid. They were discovered by Stanley Prusiner, who received the Nobel Prize in medicine in 1997 for his work on them. Prions are biologically unique, existing somewhere in the border zone between living things and nonliving matter. Although they show none of the characteristics associated with life, such as the need to metabolize and the capacity to reproduce, they are in some manner capable of replication in the body of a human or certain other mammals.

Prions apparently gain entry to the body mainly by ingestion, or else in contaminated human growth hormone, or, possibly, in contaminated blood or blood products. They selectively attack the central nervous system, causing a relentless and progressive destruction of neural tissue, leaving in its place microscopic vesicular globules. The pathological name for this is spongiform encephalopathy. Conditions in this category, all of them invariably fatal, are all transmissible. They include kuru, Creutzfeldt-Jakob disease, scrapie (a degenerative neural disease of sheep), bovine spongiform encephalopathy (mad cow disease), and variant Creutzfeldt-Jakob disease, which appears to be acquired by ingesting beef contaminated by the prions that cause mad cow disease.

As of September 2000, it remains unknown what other mammalian species are vulnerable to prions; in research laboratories they have been shown to infect rodents and primates. It is possible that all domestic farm animals are at risk, though so far only sheep, beef and dairy cattle, and wild ungulates such as deer and elk have been confirmed as vulnerable. There is no vaccine or serum to protect against infection, and no agent that can arrest or retard the progress of the spongiform degeneration once it begins.

John M. Last

(see also: Transmissible Spongiform Encephalopathy )

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prion

prion An abnormal form of a normal cell protein (PrP) found in the brain of mammals that is believed to be the agent responsible for the diseases scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt–Jakob disease in humans. Produced by mutation of the normal PrP gene, the abnormal prion protein induces the normal protein to fold incorrectly, causing it to form aggregates. These accumulate in the brain and progressively damage and destroy brain cells. Prion protein can be transmitted to other individuals of the same or closely related species, by injection or ingestion of infected tissue, and appears to be transmissible between species that are not closely related, e.g. between cattle and humans.

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prion

prion Infective agent that appears to consist simply of a protein. Prions are thought to cause diseases such as Creutzfeldt-Jakob disease (CJD) and kuru in humans, bovine spongiform encephalopathy (BSE) in cattle, and scrapie in sheep. It is not yet understood how prions work; unlike viruses and bacteria, they do not contain dna or rna. A virus is a hundred times larger than a prion. The US neurologist Stanley Prusiner won the 1997 Nobel Prize for developing the prion theory in 1982.

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prion

prion (pree-on) n. a constituent protein of brain cells that, in an abnormal form, accumulates in and destroys brain tissue. This form is very stable and resistant to radiation and sterilization; it is thought to interact with normal prion protein in such a way as to convert it to abnormal prion. Prions are now widely accepted as being the causal agents of Creutzfeldt-Jakob disease and other spongiform encephalopathies.

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prion

prion The infective agent(s) responsible for Creutzfeld‐Jacob disease, kuru and possibly other degenerative diseases of the brain in human beings, scrapie in sheep, and bovine spongiform encephalopathy (BSE). They are simple proteins, and unlike viruses do not contain any nucleic acid. Transmission occurs by ingestion of infected tissue.

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prions

prions (Pachyptila) See PROCELLARIIDAE.

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prion

prionaide-de-camp, aides-de-camp, anon, Asunción, au courant, begone, Bonn, bon vivant, Caen, Canton, Carcassonne, Ceylon, chaconne, chateaubriand, ci-devant, Colón, colon, Concepción, con (US conn), cretonne, don, Duchamp, Evonne, foregone, fromage blanc, Gabon, Garonne, gone, guenon, hereupon, Inchon, Jean, john, Jon, Le Mans, León, Luzon, Mont Blanc, Narbonne, odds-on, on, outgone, outshone, Perón, phon, piñon, Pinot Blanc, plafond, Ramón, Saigon, Saint-Saëns, Sand, Schwann, scone, shone, side-on, sine qua non, Sorbonne, spot-on, swan, thereon, thereupon, ton, Toulon, undergone, upon, Villon, wan, whereon, whereupon, won, wonton, yon, Yvonne •crayon, rayon •Leon, Lyons, neon, prion •Ceredigion • Mabinogion • nucleon •Amiens • dupion • parathion •Laocoon •gluon, Rouen •bon-bon • Audubon •Brian, cyan, Gaian, Geminian, Hawaiian, ion, iron, Ixion, lion, Lyon, Mayan, Narayan, O'Brien, Orion, Paraguayan, prion, Ryan, scion, Uruguayan, Zion •andiron •gridiron, midiron •dandelion • anion • Bruneian •cation, flatiron •gowan, Palawan, rowen •anthozoan, bryozoan, Goan, hydrozoan, Minoan, protozoan, protozoon, rowan, Samoan, spermatozoon •Ohioan • Chicagoan • Virgoan •Idahoan •doyen, Illinoisan, IroquoianEwan, Labuan, McEwan, McLuhan, Siouan •Saskatchewan • Papuan • Paduan •Nicaraguan • gargantuan •carbon, chlorofluorocarbon, graben, hydrocarbon, Laban, radiocarbon •ebon • Melbourne • Theban •gibbon, ribbon •Brisbane, Lisbon •Tyburn •auburn, Bourbon •Alban • Manitoban • Cuban •stubborn •Durban, exurban, suburban, turban, urban

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