Prion diseases are a class of degenerative central nervous system disorders. They are unique in that while a genetic component of the syndrome exists, prion diseases may also be transmitted, and the infectious agent of the disease is a protein. Dr. Stanley Prusiner coined the term "prion," meaning "proteinaceous infectious particle," in 1982. Dr. Prusiner's controversial, but finally accepted, research in the area of prion diseases led to his winning the Nobel Prize in Medicine in 1997.
As of early 2001, there are five forms of prion disease known to occur in humans: kuru, Creutzfeldt-Jakob disease (CJD), Gertsmann-Straussler-Scheinker disease (GSS), fatal familial insomnia (FFI), and new variant Creutzfeld-Jakob disease, popularly known as "mad cow disease." The prion diseases are also called transmissible spongiform encephalopathies because they can be transmitted between unrelated individuals and they sometimes cause a sponge-like encephalopathy, or degeneration of the brain tissue, in which holes and other abnormal structures are formed in the brain. Prion diseases have also been identified in animals and include scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cows, feline spongiform encephalopathy in cats, and chronic wasting disease in mule, deer, and elk.
Prion diseases have all been associated with the function of a specific cellular protein named the prion protein (PrP). Like all cellular proteins, PrP is a long-chain molecule consisting of linked amino acids. A protein can assume many shapes: twisted into a spiral helix as in DNA, extended into linear strands, or folded into sheets of aligned strands. Different shapes of the same molecular sequence are called isomers. It is theorized that the normal form of cellular PrP is a compact shape consisting mainly of four helix regions, whereas in the abnormal isomer (the prion), the protein is refolded into a sheet-helix combination. Furthermore, this prion, through an unknown mechanism, triggers the conversion of normal PrP to the abnormal shape. The abnormal isomer acts as a template to change more and more normal PrP to the abnormal structure. Therefore, once the protein exists in the abnormal form, it is an infectious agent that can be transmitted from one person to another.
Normal proteins are processed by proteases, which are enzymes present in the body that act to break down excess proteins. However, the abnormal isomer of the prion protein is protease resistant and cannot be broken down by the body's protease enzymes. Therefore, the abnormal prion protein continues to be produced without being processed. This leads to an accumulation of the abnormal protein in the body. In several forms of prion disease, the abnormal prion protein aggregates in deposits, or plaques, in the brain tissue. It is believed that once the abnormal prion protein accumulates to a certain level in the body, the physical symptoms of impaired mental and physical functioning begin to show themselves. However, the exact mechanisms by which the abnormal isomer causes disease are not known. The onset of symptoms often does not occur until the patient is elderly, suggesting that either the rate of accumulation of the abnormal protein is initially slow, or that some triggering event late in life causes the initial formation of the abnormal isomer, after which the disease can spread.
The normal function of the prion protein is not completely understood, but it is known to be involved with the functions of the synapses (nerve connections) in the brain. PrP is found in the highest concentrations in the brain. PrP is also found in the eyes, lungs, heart, kidney, pancreas, testes, blood, and in the neuromuscular junction. The conversion of normal PrP to the infectious, abnormal isomer form may disrupt the normal functions of the prion protein, and this may be another cause of the degenerative symptoms of the disease.
Body tissues containing the abnormal isomer are a source of transmission of the disease between people and even across species. Prion diseases are also found in animals, and in animal studies it has been shown that the disease can be spread through the ingestion of infected brain tissue. The transmission can also cross the species barrier; it has been found that cows became affected by prion disease after eating feed contaminated with infected sheep brain tissue. It is widely speculated that the outbreak of mad cow disease, the most publicized prion disease, was caused by infection from affected cows in the United Kingdom, although the mode of transmission has still not been determined. Transmission through oral ingestion was also shown to be the cause of kuru in the Fore tribe of New Guinea. After the Fore abandoned their practice of ritual cannibalism in which they consumed the brain tissue of ancestors, the incidence of kuru all but disappeared. Other cases of human infection have been shown to be iatrogenic; in other words, transmitted inadvertently during medical treatment. Most of these iatrogenic cases involve direct contact with brain and nervous system tissue. For example, CJD has been reported to result from the use of contaminated surgical instruments, corneal implants, implantation of dura matter or electrodes in the brain, and from the injection of human growth hormones derived from cadaverous pituitary glands.
Other modes of transmission have been shown to be less efficient in animal studies. The recent outbreak of new variant CJD caused increased concern about possible transmission through blood transfusions and plasma-derived products, but no case of new variant CJD has been proven to result from blood transfusion as of 2001. Laboratory and epidemiological evidence supporting a strong risk of the spread of prion disease through blood transfusion is not present, even though this area has been intensively studied.
The gene that encodes the prion protein has been mapped to chromosome 20p12. This gene has been named the PRNP gene. Mutations in the PRNP gene can cause alterations in the chemical sequence of amino acids in the prion protein, and this change is believed to make the protein more susceptible to assuming the abnormal conformation. Over 20 different mutations of the gene have been identified, encompassing point mutations (one base pair substituted for another in the gene sequence), insertions (additions to the gene sequence), and deletions (missing parts of the gene sequence). Depending on the specific mutation, different types of prion disease can appear in the patient.
It is estimated that 10-15% of prion disease cases are caused by inherited mutations of the PRNP gene. Because of the delayed onset of the disease and the wide variation in symptoms, more exact statistics are difficult to determine. The inheritance pattern is autosomal dominant, meaning that if either parent passes the mutated gene to their offspring, the child will be affected by the disease. A parent with prion disease has a 50% probability of passing on the mutated gene to his or her child.
Another genetic factor important in prion disease is the genetic sequence of the PRNP gene. Like all genes, the PRNP gene is made up of two strands of DNA. Each DNA strand consists of a sequence of chemical structures called bases, and the two strands together form a sequence of base pairs. Three base pairs together form a unit called a codon, and each codon codes for a specific amino acid. At codon 129 of the PRNP gene, either the amino acid methionine (Met) or valine (Val) can be encoded. Since one gene is inherited from each parent, an individual may either be homozygous, having two of the same amino acids (Met-Met or Val-Val) at this position; or, heterozygous, having different amino acids (Met-Val). Individuals who are homozygous appear to be more susceptible to infection of prion diseases, because it has been shown that a greater percentage of those infected are homozygous than in the general population. Also, the clinical symptoms, or phenotype, of the prion disease can differ based on whether the individual is Met-Met or Val-Val homozygous, and whether the individual has Met or Val on the same gene as another mutation. For example, a specific point mutation at codon 178 has been found to cause familial CJD if the individual has Val at codon 129 on the mutated gene, while the same mutation causes FFI when the individual has Met at codon 129.
Prion diseases occur worldwide with a rate of one to two cases per one million. CJD is the most common of the prion diseases, while GSS and FFI are extremely rare, and kuru is now virtually nonexistant due to the abandonment of the practice of cannibalism. There is no gender link to the disease. Several forms of prion disease usually do not cause identifiable symptoms until the individual is more than 60 years old, although other forms have an earlier onset and are seen in teenagers and young adults.
Since prion disease can be either inherited or transmitted through infection, the demographics of the disease have both familial and environmental patterns. Inherited CJD is found with high frequency in Libyan Jews and also in other descendants of Sephardic Jews in Greece, Tunisia, Israel, Italy, Spain, and perhaps South America. Other genetic clusters have been identified in Slovakia, Poland, France, and Germany. Fatal familial insomnia has been linked to family pedigrees in Italy, Australia, and the United States, among others. Families with GSS syndrome have been found in several countries in North America and Europe.
The environmental clusters of the disease include the Fore, a remote tribe of New Guinea in which kuru was transmitted through the practice of ritual cannibalism; a group of over 80 cases in Japan resulting from dura mater (brain membrane) grafts from a single surgical supply company; over 100 cases resulting from cadaveric human growth hormone injections in Europe and the United States; and, most famously, the 40-plus cases of new variant CJD or mad cow disease in the United Kingdom.
Prion disease primarily affects the brain and central nervous system, so the symptoms associated with the disease are all related to neurological function. These may include loss of muscular coordination and uncontrollable body movements (ataxia), visual problems, hallucinations, behavioral changes, difficulty in thinking clearly or remembering, sleep disturbances, speech impairment, and insanity. General complaints such as headache, diminished appetitie, and fatigue may occur prior to the onset of the more serious symptoms. The exact combination and severity of these symptoms varies widely between cases and types of prion disease.
CJD, the most common of the human prion diseases, is characterized by a rapid deterioration in mental function from confusion and memory loss into severe dementia, accompanied by loss of muscular control (ataxia) and twitching or spasmodic motion (myoclonus). Other symptoms include vision and speech impairment. A scan of electrical activity in the brain, called an electroencephalogram (EEG), will often show an abnormal periodic spike pattern. The onset of symptoms usually occurs when the patient is over age 50 and death follows within one to five years. Microsopic holes, or vacuoles, in the brain tissue, which give it a "spongiform" appearance, are characteristic of CJD.
Gertsmann-Straussler-Scheinker syndrome (GSS) encompasses a variety of disorders. One form of the disease (the ataxic form) is first characterized by an unsteady walk sometimes accompanied by leg pains. These motor problems get worse over several years and are finally accompanied by mental and behavioral breakdown. By contrast, dementia is the main characteristic of the telencephalic form of GSS, accompanied by rigidity, the inability to make facial expressions, tremors, and stuttering or stammering. In another form of the disease, GSS with neurofibrillary tangles, the main features are loss of muscle coordination (ataxia), tremors, and progressive insanity. As in CJD, the affected individuals are usually in their fifties or older, and the progression of the disease may take from two to six years.
The most noticeable sign of fatal familial insomnia (FFI) is the untreatable and progressively worse difficulty in sleeping. The affected individual then begins to experience complex hallucinations which are often enacted dreams. Excessive sweating, irregular heartbeat, high blood pressure, and hyperventilation are other symptoms. Motor impairment may be present. Shortened attention span and memory loss has been observed. In the terminal stages, stupor and coma precede death. The average age at onset of symptoms is the mid-forties, and the disease progresses rapidly with death resulting after about one year. Autopsy reveals the formation of dense tangles of neural fibers and astrocytes in the thalamus region of the brain.
Kuru was called the "shivering" disease by the Fore tribe members because its primary symptom was twitching and shaking of the body. This twitching began slowly and was not present when the person was completely still, but then progressively worsened until any attempt at motion led to drastic and uncontrollable body movements, and the individual could no longer stand or walk. Mental insanity usually did not appear until the terminal stages of the disease. The onset of symptoms usually occurred in middle-aged individuals and the course of the disease was short: three to 12 months.
The early signs of new variant CJD are most often psychiatric disturbances. Abnormal sensations of prickling or itching (paresthesia) or pain even from light touches (dysesthesia) are often present. So far, individuals affected with new variant CJD are much younger in age, typically teenagers and young adults. The duration of the disease is one to two years. As in CJD, vacuoles are present in the brain, but they are associated with dense deposits, or plaques, of the abnormal PrP isomer.
Because of the many different forms of the disease and the overlap in symptoms with other common syndromes such as Alzheimer disease, prion diseases are often difficult to diagnosis. A diagnosis of prion disease should be considered in any adult patient with signs of neurological impairment such as uncontrollable body movements, confusion, loss of memory, and cognitive degeneration, or psychiatric abnormality. Periodic discharges of brain waves, as observed on an electroencephalogram (EEG), are present in many, but not all, cases of prion disease. A magnetic resonance imaging (MRI) scan of the brain can rule out other causes of brain disease and potentially identify some abnormalities associated with prion disease. A biopsy, or sampling, of brain tissue can reveal the presence of abnormal PrP, although this procedure is generally not used in elderly patients. Genetic testing can reveal those cases of prion disease that are caused by mutations. Bismuth, mercury, or lithium poisoning result in symptoms that are quite similar to prion disease. These poisonings can be differentially diagnosed by blood tests.
The diagnosis of prion disease can be definitively confirmed by the transmission of the disease to an animal host such as a genetically engineered mouse. However, the transmission period may be quite long, as much as six to seven months. After death, prion disease can also be validated by autopsy of the brain tissue.
Patients eventually identified with prion disease have been initially diagnosed with many other diseases including Alzheimer disease, Huntington disease, Parkinson disease, schizophrenia, multiple sclerosis, and myoclonic epilepsy. This illustrates the difficulty in identifying the disease and the importance of careful diagnosis to avoid unnecessary treatments.
At present, there is no known treatment that can prevent or reverse the transformation of the prion protein into its aberrant form. All treatments for prion disease are directed towards management of the symptoms. These treatments may include psychoactive drugs, electroconvulsive therapy (ECT), and professional care to ensure that the loss of physical and mental functions do not lead to accidental injury or death. Research into more advanced treatments is focusing on the application of gene therapies to block the formation of infectious PrP and drugs, which could act to stabilize the normal PrP structure. As with any inherited disease, genetic counseling is important in the management of the familial forms of prion disease.
Since the forms of prion diseases vary widely, the age at onset and rate of worsening of symptoms are also quite variable, but all prion diseases are incurable and fatal with a duration anywhere from a few months to several years after onset.
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Paul A. Johnson