Prions are protein-containing particles



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Chapter 44: Slow Viruses and Prions

SLOW VIRUSES & PRIONS: INTRODUCTION

"Slow" infectious diseases are caused by a heterogeneous group of agents containing both conventional viruses and unconventional agents that are not viruses, e.g., prions. Prions are protein-containing particles with no detectable nucleic acid that are highly resistant to inactivation by heat, formaldehyde, and ultraviolet light at doses that will inactivate viruses. Note that prions are resistant to the temperatures usually employed in cooking, a fact that may be important in their suspected ability to be transmitted by food (see variant Creutzfeldt-Jakob disease [CJD] below). Prions are, however, inactivated by protein- and lipid-disrupting agents such as phenol, ether, NaOH, and hypochlorite (see Chapter 28).

The prion protein is encoded by a normal cellular gene and is thought to function in a signal transduction pathway in neurons. The normal prion protein (known as PrPC, or prion protein cellular) has a significant amount of alpha-helical conformation. When the alpha-helical conformation changes to a beta-pleated sheet (known as PrPSC, or prion protein scrapie), these abnormal forms aggregate into filaments, which disrupt neuron function and cause cell death. Prions, therefore, "reproduce" by the abnormal beta-pleated sheet form recruiting normal alpha-helical forms to change their conformation. Note that the normal alpha-helical form and the abnormal beta-pleated sheet form have the same amino acid sequence. It is only their conformation that differs. A specific cellular RNA enhances this conformational change. Prions are described in more detail in Chapter 28.

In humans, the "slow" agents cause central nervous system diseases characterized by a long incubation period, a gradual onset, and a progressive, invariably fatal course. There is no antimicrobial therapy for these diseases. Note that the term "slow" refers to the disease, not to the rate of replication of those viruses that cause these "slow" diseases. The replication rate of these viruses is similar to that of most other viruses.

The human prion-mediated diseases, e.g., kuru and CJD, are called transmissible spongiform encephalopathies (TSE). The term "spongiform" refers to the spongy, Swiss cheese-like holes seen in the brain parenchyma that are caused by death of the neurons (see Color Plate 30). No virus particles are seen in the brain of people with these diseases.

Color Plate 30



Prion-mediated spongiform encephalopathy (mad cow disease)—Two arrows point to the spongiform appearance ("swiss cheese-like holes") in the brain of a cow with "mad cow" disease. The brain of a patient with Creutzfeldt-Jakob disease (CJD) has a similar appearance. Provider: CDC/Dr. Al Jenny.


The term "encephalopathy" refers to a pathologic process in the brain without signs of inflammation. In contrast, "encephalitis" refers to an inflammatory brain process in which either neutrophils or lymphocytes are present. In TSEs, there are no inflammatory changes in the brain.

The transmissibility of the agent of kuru and CJD ("prions") was initially established by inoculation of material from the brains of infected patients into the brains of primates followed by serial transfer to the brains of other primates.

Note, however, that both kuru and variant CJD (and bovine spongiform encephalopathy [BSE]—"mad cow" disease) are acquired by ingestion. In this route, the prion protein must survive digestion in the intestinal tract and then penetrate the gut mucosa. The prion protein is then amplified within follicle dendritic cells in lymphatic tissue, such as Peyer's patches. Prions then spread to the spleen, carried by migrating dendritic cells. From the spleen, prions spread to the central nervous system probably via the sympathetic nerves.

It is also possible that prions reach the brain within lymphocytes, as there is a documented case of CJD that was acquired by transfused blood. In addition, CJD has been transmitted iatrogenically, i.e., in a medical context, via corneal transplants, dura mater grafts, implanted brain electrodes, and growth hormone extracts made from human pituitary glands.

There is evidence that quinacrine and other acridine analogues inhibit the formation of the pathologic PrPC form in cell culture. These drugs are currently being tested in animal models for their ability to treat or prevent prion diseases.

Prion-caused diseases can be classified into three categories: some are clearly transmissible (infectious) such as kuru, some are clearlyhereditary (genetic) such as fatal familial insomnia, and others are sporadic (neither infectious nor hereditary) such as most cases of CJD. The sporadic cases seem likely to be due to spontaneous somatic mutations in the affected individual.




SLOW DISEASES CAUSED BY CONVENTIONAL VIRUSES

Progressive Multifocal Leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease of the white matter and involves multiple areas of the brain. The clinical picture includes visual field defects, mental status changes, and weakness. The disease rapidly progresses to blindness, dementia, and coma, and most patients die within 6 months. It occurs primarily in individuals with compromised cell-mediated immunity, especially AIDS patients and those who are receiving cancer chemotherapy and immunosuppressive drugs following organ transplantation. Some patients undergoing treatment for multiple sclerosis with the monoclonal antibody natalizumab developed PML (see Chapter 62 for a description of monoclonal antibodies in clinical use). Table 44–1 describes some important features of slow viral diseases in humans caused by conventional viruses.

Disease__Virus__Virus_Family'>Diseases_Caused_by_Conventional_Viruses.'>Table 44–1. Important Features of Slow Viral Diseases Caused by Conventional Viruses.





Disease

Virus

Virus Family

Important Characteristic

Progressive multifocal leukoencephalopathy

JC virus

Papovavirus

Infection widespread; disease only in immunocompromised

Subacute sclerosing panencephalitis

Measles virus

Paramyxovirus

Disease in young children with defective virus in brain

Acquired immunodeficiency syndrome (AIDS)

Human immunodeficiency virus (HIV)

Retrovirus

HIV infects CD4-positive cells, e.g., brain macrophages




PML is caused by JC virus, a papovavirus antigenically distinct from other human papovaviruses such as human papillomavirus. JC virus infects and kills oligodendroglia and causes syncytia in astrocytes. Antibodies to JC virus are found in approximately 75% of normal human sera, indicating that infection is widespread. Disease occurs when latent JC virus is activated in an immunocompromised patient. The virus persists in the kidney and is excreted in the urine. The diagnosis is typically made by polymerase chain reaction assay of a brain biopsy specimen or spinal fluid. There is no effective antiviral treatment, but cidofovir may be beneficial.

Subacute Sclerosing Panencephalitis

Subacute sclerosing panencephalitis (SSPE) is a slowly progressive disease characterized by inflammatory lesions in many areas of the brain. It is a rare disease of children who were infected by measles virus several years earlier. SSPE begins with mild changes in personality and ends with dementia and death.

SSPE is a persistent infection by a variant of measles virus that cannot complete its replication. The evidence for this is as follows:


  1. Inclusion bodies containing helical nucleocapsids, which react with antibody to measles virus, are seen in the affected neurons.

  2. A virus very similar to measles virus can be induced from these cells by cocultivation with permissive cells in culture. The induced virus has a different matrix protein; this protein is important in viral assembly.

  3. Patients have high titers of measles antibody in the blood and spinal fluid.

  4. SSPE has virtually disappeared in the United States since the onset of widespread immunization with measles vaccine.

A progressive panencephalitis can also occur in patients with congenital rubella.

Acquired Immunodeficiency Syndrome (AIDS)

AIDS is caused by human immunodeficiency virus (HIV), a member of the lentivirus group of retroviruses. AIDS is a disease with a long latent period and a progressive course and can involve the central nervous system. See Chapter 45 for more information.


SLOW DISEASES CAUSED BY PRIONS

There are five human transmissible spongiform encephalopathies caused by prions: kuru, CJD, variant CJD, Gerstmann-Sträussler-Scheinker syndrome (GSS), and fatal familial insomnia. Table 44–2 describes some important features of slow viral diseases in humans caused by prions.



Table 44–2. Important Features of Slow Viral Diseases Caused by Prions.





Disease

Pathogenesis

Important Feature

Kuru

Transmissible/infectious

Caused by ingesting or handling brain tissue. Occurred in New Guinea tribespeople

Creutzfeldt-Jakob disease

1. Transmissible/infectious

Iatrogenic transmission by corneal transplant, brain electrodes, and growth hormone

 

2. Hereditary/genetic

Mutation in germ cells

 

3. Sporadic

No relationship to any known cause; possible new mutation in somatic cells; most common form

Variant Creutzfeldt-Jakob disease

Transmissible/infectious

Probably acquired by eating meat or nervous tissue from animals with "mad cow" disease

Gerstmann-Sträussler-Scheinker syndrome

Hereditary/genetic

Mutation in germ cells

Fatal familial insomnia

Hereditary/genetic

Mutation in germ cells




Kuru

This fatal disease is characterized by progressive tremors and ataxia but not dementia. It occurs only among the Fore tribes in New Guinea. It was transmitted during a ritual in which the skulls of the dead were opened and the brains eaten. There are two ways the disease could have been acquired: either by eating the brains or via cuts in the skin that occurred during the preparation of the brains at which time brain tissue was introduced into the body. Since the practice was stopped, kuru has almost disappeared. The agents of kuru and CJD (see below) have been transmitted serially in primates.

Creutzfeldt-Jakob Disease

Pathologic examination of the brains of patients with CJD and kuru reveals a spongiform (sponge or Swiss-cheese) appearance similar to that associated with scrapie in sheep (see below). The spongiform changes are the result of neuronal vacuolation and neuronal loss rather than demyelination. No inflammatory cells are seen in the brains. Prions cause scrapie and have been found in the brains of CJD patients.

In contrast to kuru, CJD is found sporadically worldwide and affects both sexes. The incidence of CJD is approximately 1 case per 1 million population, and there is no increased risk associated with dietary habits, occupation, or animal exposure. Vegetarians and meat eaters have the same rate. The rate of CJD is the same in countries whose animals have scrapie and those whose animals do not. There is no evidence for person-to-person or transplacental transmission.

There is no increased risk for medical caregivers; therefore, gowns and masks are unnecessary. The standard precautions for obtaining infectious specimens should be observed. It has been transmitted iatrogenically, e.g., in a corneal transplant, via intracerebral electrodes, in hormones extracted from human pituitaries, and in grafts of cadaveric dura mater. There is only one confirmed case of CJD being transmitted by blood transfusion, and intravenous drug use does not increase the risk. Proper sterilization of CJD agent-contaminated material consists of either autoclaving or treating with sodium hypochlorite.

The main clinical findings of CJD are dementia (including behavioral changes, memory loss, and confusion) and myoclonic jerking. Additional findings include ataxia, aphasia, visual loss, and hemiparesis. The symptoms typically appear gradually and progress inexorably. In the terminal stage, the patient becomes mute and akinetic, then comatose. About 80% of those affected die within 1 year. Most cases occur in people who are 50–70 years of age.

A presumptive diagnosis of CJD can be made pathologically by detecting spongiform changes in a brain biopsy specimen. Neuronal loss and gliosis are seen. Amyloid plaques are also seen in some cases of CJD. In variant CJD, "florid" plaques composed of flower-like amyloid plaques surrounded by a halo of vacuoles are seen. Brain imaging and the electroencephalogram may show characteristic changes. There is no evidence of inflammation, i.e., no neutrophils or lymphocytes are seen. The blood count and routine spinal fluid test results are normal. The finding of a normal brain protein called 14-3-3 in the spinal fluid supports the diagnosis.

The specific diagnosis of CJD is typically made by immunohistochemistry in which labeled anti-prion antibodies are used to stain the patient's brain specimen. Because we do not make antibodies to prion proteins, there are no serologic diagnostic tests. No antibodies are made in humans because humans are tolerant to our prion proteins. (The antibodies used in the immunohistochemical lab tests are made in other animals in which the human prions are immunogenic.) Unlike viruses, prions cannot be grown in culture, so there are no culture-based diagnostic tests.

Tonsillar tissue obtained from patients with variant CJD was positive for prion protein using monoclonal antibody–based assays. The use of tonsillar or other similar lymphoid tissue may obviate the need for a brain biopsy. Pathologic prion proteins have also been detected in the olfactory epithelium of patients with CJD.

There is no treatment for CJD, and there is no drug or vaccine available for prevention.

Although most cases of CJD are sporadic, about 10% are hereditary. The hereditary (familial) form is inherited as an autosomal dominant trait. In these patients, 12 different point mutations and several insertion mutations in the prion protein gene have been found. One of these, a point mutation in codon 102, is the same mutation found in patients with GSS syndrome, another slow central nervous system disease of humans. The main clinical features of GSS are cerebellar ataxia and spastic paraparesis. The hereditary forms of these diseases may be prevented by the detection of carriers and genetic counseling.

The origin of these spongiform encephalopathies is three-fold: infectious, hereditary, and sporadic. The infectious forms are kuru and probably variant CJD (see below). Transmission of the infectious agent was documented by serial passage of brain material from a person with CJD to chimpanzees. The hereditary form is best illustrated by GSS syndrome described above and by a disease called fatal familial insomnia. The term "sporadic" refers to the appearance of the disease in the absence of either an infectious or a hereditary cause.

Fatal familial insomnia is a very rare disease characterized by progressive insomnia, dysautonomia (dysfunction of the autonomic nervous system) resulting in various symptoms, dementia, and death. A specific mutation in the prion protein is found in patients with this disease.


SLOW DISEASES OF ANIMALS

The "slow" transmissible diseases of animals that are important models for human diseases. Scrapie and visna are diseases of sheep, and BSE ("mad cow" disease) is a disease of cattle that appears to have arisen from the ingestion of sheep tissue by the cattle. Chronic wasting disease occurs in deer and elk. Visna is caused by a virus, whereas the other three are prion-mediated diseases.

Scrapie

Scrapie is a disease of sheep, characterized by tremors, ataxia, and itching, in which the sheep scrape off their wool against fence posts. It has an incubation period of many months. Spongiform degeneration without inflammation is seen in the brain tissue of affected animals. It has been transmitted to mice and other animals via a brain extract that contained no recognizable virus particles. Studies of mice revealed that the infectivity is associated with a 27,000-molecular-weight protein known as a prion (see Atypical Viruslike Agents).

Visna

Visna is a disease of sheep that is characterized by pneumonia and demyelinating lesions in the brain. It is caused by visna virus, a member of the lentivirus subgroup of retroviruses. As such, it has a single-stranded, diploid RNA genome and an RNA-dependent DNA polymerase in the virion. It is thought that integration of the DNA provirus into the host cell DNA may be important in the persistence of the virus within the host and, consequently, in its long incubation period and prolonged, progressive course.

Bovine Spongiform Encephalopathy

BSE is also known as "mad cow" disease. The cattle become aggressive, ataxic, and eventually die. Cattle acquire BSE by eating feed supplemented with organs, e.g., brains, obtained from sheep infected with scrapie prions. (It is also possible that BSE arose in cattle by a mutation in the gene encoding the prion protein.)

BSE is endemic in Great Britain. Supplementation of feed with sheep organs was banned in Great Britain in 1988 and thousands of cattle were destroyed, two measures that have led to a marked decline in the number of new cases of BSE. BSE has been found in cattle in other European countries such as France, Germany, Italy, and Spain, and there is significant concern in those countries that variant CJD may emerge in humans. Two cases of BSE in cattle in the United States have been reported.

In 1996, several cases of CJD occurred in Great Britain that are attributed to the ingestion of beef. These cases are a new variant of CJD (vCJD, also called nvCJD) because they occurred in much younger people than usual and had certain clinical and pathologic findings different from those found in the typical form of the disease. None of those affected had consumed cattle or sheep brains, but brain material may have been admixed into processed meats such as sausages.

Only people whose native prion protein is homozygous for methionine at amino acid 129 contract vCJD. People whose native prion protein is homozygous for valine at amino acid 129 or who are heterozygotic do not contract vCJD. These findings indicate that prion proteins with methionine are more easily folded into the pathologic beta-pleated sheet form.

The prions isolated from the "variant CJD" cases in humans chemically resemble the prions isolated from "mad cow" disease more than they resemble other prions, which is evidence to support the hypothesis that variant CJD originated by eating beef. There is no evidence that eating lamb is associated with variant CJD. As of June 2002, vCJD has killed more than 120 people in Europe, 117 of whom live in Great Britain. No cases of variant CJD have occurred in North America.

It is unknown how many people harbor the pathogenic prion in a latent (asymptomatic) form. The possibility that there may be people who are asymptomatic carriers of the vCJD prion and who could be a source for infection of others, e.g., via blood transfusions, has led blood banks in the United States to eliminate from the donor pool people who have lived in Great Britain for more than 6 months.

Chronic Wasting Disease



Chronic wasting disease (CWD) of deer and elk is a prion-mediated disease that exists in the United States. Because vCJD is strongly suspected to be transmitted by ingesting meat, there is concern regarding the consequences of eating deer and elk meat (venison). In 2002, it was reported that neurodegenerative diseases occurred in three men who ate venison in the 1990s. One of these diseases was confirmed as CJD. Whether there is a causal relationship is unclear and surveillance continues. This concern was heightened in 2006 when prions were detected in the muscle of deer with CWD but not in the muscle of normal deer.



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