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.

Prions

The term prion (derived from "proteinaceous infectious particle") refers to an infectious agent consisting of a tiny protein that lacks genes, but can proliferate inside the host, causing slowly developing neurodegenerative diseases in animals and humans. Prions are thought to cause several diseases that attack the brain , such as Creutzfeldt-Jakob disease in humans, scrapie in sheep , and bovine spongiform encephalopathy (mad cow disease ) in cows.

The normal form of the prion, PrPc, is a cell-membrane protein that may play a role in nerve signaling in the brain. The very existence of prions has been disputed by researchers ever since these agents were first postulated in 1981 by Stanley B. Prusiner, a neurologist at the University of California at San Francisco, and his collaborators. Since then, however, there has been increasing evidence that it is tiny, virus-like particles lacking genetic material that induce normal proteins to change their shape, causing neurodegenerative diseases in animals and humans. This may explain the onset of diseases previously called "slow viral infections," which are not thought to be caused by viruses.

British radiobiologist Ticvah Alper found the first indication that such an infectious agent might cause disease. In the mid-1970s, Alper found that the infectious agent that causes scrapie, a brain disease of sheep and goats , was extremely small and resistant to ultraviolet radiation , which is known to inactivate genetic material. More evidence accumulated for the existence of prions during the 1980s: for example, the isolation of rods thought to be prion proteins (PrP) from the brains of hamsters infected with scrapie and humans with Creutzfeldt-Jakob disease. The term prion disease now refers to any disease in which there is an accumulation of the abnormal form of PrP, known as PrPsc. The abnormal prion protein has a different shape than the normal protein, and is resistant to enzymes that degrade proteins, such as proteases.

Aggregates of prions appear to compose the amyloid plaques ("clumps") and fibrils (tiny fibers) seen in the brains of infected humans and animals. These insoluble aggregates appear to trap other things, such as nucleic acids, the building blocks of genes. When the abnormal protein gets into the brains of animals or humans, it converts normal prion proteins into the abnormal form. The accumulation of abnormal proteins in the brain is marked by the formation of spongy holes.

In 1994, researchers at the Massachusetts Institute of Technology and the Laboratory of Persistent Viral Diseases at the Rocky Mountain Laboratories of the National Institutes of Health in Hamilton, Montana, reported that, in the test tube, the abnormal form of the prion protein found in hamsters can convert the normal form into the protease-resistant version. In 1993, researchers at the University of California at San Francisco discovered that the normal prion's shape consists of many helical turns, while the abnormal prion has a flatter shape.

Prion diseases can arise by direct infection , by inherited genes that produce the abnormal prion protein, or by genetic mutation . PrPc is encoded by a single gene on human chromosome 20 (chromosome 2 in mice ). The prion is thought to arise during translation of the PrPc gene into the protein, during which time it is modified to the PrPsc form. The abnormal form of the protein appears to share the same amino acid sequence as the normal protein, but the modification causes differences in their biochemical properties. This permits separation of the two proteins by biochemical analytical methods. The modification is rare, occurring only about once in a million times in the general population. The onset of this disorder occurs in middle age. However, some mutations of the PrP gene can cause onset of prion disease earlier than middle age.

Of particular interest is the similarity between prion disease and Alzheimer disease, a more commonly known form of dementia . Alzheimer disease occurs when a cell membrane protein, called amyloid precursor protein (APP), is modified into a form called beta (A4). This modified form is deposited in plaques, whose presence is common in elderly people. And like the PrP gene, certain mutations in the APP gene cause this series of events to occur earlier in life, during later middle age.

In humans, prion diseases can occur in one of several forms. Creutzfeldt-Jakob disease (CJD) is a fatal brain disease lasting less than two years. The symptoms include dementia, myoclonus (muscle spasms), severe spongiform encephalitis (brain deterioration marked by a spongy appearance of tissue caused by the vacuolization of nerve cell bodies and cell processes in the gray matter), loss of nerves, astrocytosis (an increase in the number of astrocytes—brain cells that repair damage), and the presence of abnormal protein plaques in neurons. Gerstmann-Straussler-Scheinker syndrome (GSS) is similar to CJD but lasts for more than two years.

Kuru is a fatal, CJD-like form of spongiform encephalopathy lasting less than three years. The symptoms include loss of nerves, astrocytosis, dementia, and sometimes spongiform encephalopathy. Kuru has been reported in tribes people from Papua New Guinea, who had practiced cannibalism, and therefore were directly exposed to a deceased person's diseased brain tissue.

Atypical prion disease is a form of dementia diagnosed by biochemical tests and genetic criteria, but does not otherwise resemble CJD closely. Finally, fatal familial insomnia (FFI) is an atypical prion disease characterized by degeneration of the thalamus and hypothalamus, leading to insomnia and dysautonomia (abnormal nervous system functioning).

GSS and atypical prion disease (including FFI) are usually inherited. CJD may be inherited, acquired or sporadic; it is usually neither epidemic nor endemic . However, kuru and CJD that arise as a complication of medical treatment are both acquired by contamination of the patient with PrPsc from another infected human. Human prion disease, however, has never been traced to infection from an animal .

With respect to bovine spongiform encephalopathy (BSE), the issue is one of concern regarding transmission from cattle, or from cattle products, to human beings. While no cases are documented that contain conclusive evidence for cross-species contamination, fear is abound that the possibility exists and is therefore a viable threat to public health and safety.

BSE, or mad cow disease, was first identified in a laboratory in Weybridge, England in 1988. Since then, a great deal of public concern has been raised about BSE and beef products. After the initial realization of the prion nature of the infectious agent, the UK government introduced legislation that required destruction and analysis of all cattle suspected of BSE infection. Likewise, all animals to be slaughtered are to be inspected specifically for BSE according to the new legislation. In 1997, an addendum to the laws surrounding BSE stated that specified risk material containing beef matter was to be banned from animal feed, cosmetic and pharmaceutical preparations, as well as including new rules on beef labeling and tracing procedures. While initiated in 1988, the epidemic reached a peak in 1993 with thousands of cows affected, believed to have been caused by contaminated feed. Fear from other countries, including the United States, stemmed from the belief that tainted British beef products held the possibility of causing CJD in humans. In reality, there is only a limited link between BSE and CJD in humans. Since the 1993 epidemic, however, the British Ministry of Agriculture, Fisheries, and Food (BMAFF) reports a steady and continual decline in the number of cases of mad cow disease.

CJD, GSS, and atypical prion dementia are not different diseases; rather, they are descriptions of how prion infection affects individual patients. In fact, members of the same family can have three distinct versions of a prion infection linked to the same mutation. Indeed, it was the demonstration that inherited cases of human transmissible spongiform encephalopathy were linked to PrP gene mutations that confirmed prions are central to these diseases. The concept of PrP gene mutations has subsequently been used for diagnosis and in genetic counseling.

Many specific mutations leading to prion disease have been reported. One example is six point mutations in codons 102, 117, 178, 198, 200, and 217 (a codon is a trio of nucleotides in a gene that codes for a specific amino acid in the protein represented by that gene). Insertional mutations consisting of extra 2, 5, 6, 7, 8, or 9 octapeptide repeats have also been associated with prion disease. The presence of PrP gene mutations does not in itself support a diagnosis of prion disease, however, since not all such mutations produce their characteristic effects in an individual possessing the mutation. Moreover, the presence of such a mutation does not protect the patient from other, much more common neurological diseases. Therefore, in the presence of a PrP gene mutation the patient may not have prion disease, but may have a different brain disease.

Further complicating the picture of prion diseases is the fact that, while spongiform encephalitis is found regularly and extensively in sporadic CJD, in cases of familial CJD it is found only in association with a mutation in codon 200 of the PrP gene. Spongiform encephalitis is not found to any significant extent in other prion diseases.

A particularly notable aspect of prion diseases associated with mutations at codon 198 or 217, is the common occurrence of large numbers of neurofibrillary tangles and amyloid plaques, without spongiform encephalitis. If conventional histological techniques are used, this picture appears indistinguishable from Alzheimer's disease. However, immunostaining of the plaques with antibodies to PrP establishes the diagnosis of prion disease.

One prion disease, CJD, is easily transmissible to animals, especially primates , by injecting homogenates (finely divided and mixed tissues) of brains (rather than pure prions) from cases of acquired, sporadic, or inherited spongiform encephalitis in humans into the cerebrums of animals. However, the disease, which may take 18 months to two years to develop, results from the transformation of PrPc into PrPsc, rather than from the replication of an agent that actually causes the disease.

Moreover, there is experimental evidence for transmission of CJD to humans. The evidence suggests that patients infected by receiving prion-contaminated therapeutic doses of human growth hormone or gonadotropin might pose a threat of infection to recipients of their donated blood .

Critics of the prion hypothesis point out that there is no proof that prions cause neurodegenerative disease. Some researchers point out that very tiny viruses are more likely the agents of what is called prion disease, and that the prion protein serves as a receptor for the virus . In addition, as of 1994, no one had been able to cause disease by injecting prion proteins themselves, rather than brain homogenates.

In 1994, Prusiner received the prestigious Albert Lasker award for basic medical research for his work with prions.

See also Virus.

Resources

periodicals

Pennisi, E. "Prying into Prions: A Twisted Tale of an Ordinary Protein Causing Extraordinary Neurological Disorders." Science News 146 (September 24, 1994): 202-3.

Prusiner, S.B. "Biology and Genetics of Prion Diseases." Annual Review of Microbiology 48 (1994): 655-86.

Prusiner, S.B. "The Prion Diseases." Scientific American 272 (January 1995): 48-51+.

Shaw, I. "Mad Cows and a Protein Poison." New Scientist 140 (October 9, 1993): 50-1.

Wong B-S., D.R. Brown, and M-S Sy. "A Yin-yang Role for Metals in Prion Disease." Panminerva Med. 43 (2001): 283-7.

Marc Kusinitz

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 (PrP^C) normally expressed at low levels in neurons and other cell types, and an abnormal or scrapie form (PrP^Sc) built up in diseased brain. PrP^C is a cell-surface glycoprotein, the function of which has yet to be established. PrP^C 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, PrP^Sc 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 PrP^Sc tends to aggregate and is resistant to heat and degradation by proteases. It is assumed that PrP^Sc can initiate the infection process by binding to predominantly-helical PrP^C and converting it into more stable PrP^Sc with β-sheet conformation. This will set off a chain reaction leading to accumulation of large amounts of PrP^Sc to levels that result in brain tissue damage. The conformational conversion from α-helices to β-sheets transforms the benign PrP^C into disease-causing PrP^Sc. 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 forms—spontaneously (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 PrP^C to pathogenic PrP^Sc. 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 PrP^Sc, 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 PrP^Sc 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.

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 different—and by implication, far simpler chemically—than 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 PrP^C 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 PrP^Sc reveals a different shape. Compared to PrP^C, PrP^Sc 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 PrP^Sc molecules are formed from PrP^C molecules is not fully understood. Nonetheless, it appears to involve a templating reaction where PrP^C molecules are first unfolded and then refolded into the shape characteristic of PrP^Sc using preexisting PrP^Sc molecules as templates . Since the generation of new PrP^Sc 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): 48–57.

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.