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Chapter Outlines

Chapter 19      What About Prions and Viroids?

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What About Prions and Viroids? Prions and viroids are unconventional infectious agents Prions - infectious proteins that cause a group of diseases of the brain and nervous system called Transmissible Spongiform Encephalopathies (TSEs) Viroids - small, pathogenic RNAs that cause viruslike diseases in plants Human Prion Diseases. See Table 19-1. Animal Prion Diseases. See Table 19-2. More About TSEs None of the TSEs evoke an immune response Prions cause a noninflammatory process that results in vacuolation or spongiosis in the gray matter of the brain 19.1 The "Mad" Diseases, Transmissible Spongiform Encephalopathies (TSEs): Kuru and Cannibalism Kuru - human TSE Mysterious disease affecting large numbers of the South Fore people of Papua New Guinea in the 1950's and 1960's Investigated by Vin Zigas, Shirley Lindenbaum and Carleton Gadjusek Gadjusek Observed 3 Distinct Stages of Symptoms of Kuru Ambulant stage - unsteady gait, voice, hands and eyes, tremors and shivering, slurred speech, loss of coordination Sedentary stage - patients could no longer walk without support, increased severity of tremors and coordination, jerky movements, outbursts of laughter, depression, mental slowing Terminal stage - patients could not sit without support, increased tremors and speech slurring, incontinence, difficulty swallowing, deep ulcerations Fore Tribe Practiced Endocannibalism Practice of eating dead relatives. Began during the 1900's Sign of love and respect, as part of funeral rites Human flesh was regarded as meat Kuru victims fat resemble pork and an excellent source of food Males ate muscle Women and children ate morsels of the brain and other internal organs Two Theories to Explain Cause of Kuru Kuru was a hereditary disease Kuru was caused by a biological agent transmitted by endocannibalism 19.2 PrP and the "Protein Only" Hypothesis Prions are infectious proteins that cause Kuru and similar diseases such as Creutzfeldt Jakob disease (CJD), scrapie etc. Gajdusek shared the Nobel Prize for Physiology or Medicine in 1976 for research on the origin and dissemination of TSEs. Stanley Prusiner's team isolated the infectious "prion" agent that caused Kuru. In 1984 Prusiner's group showed that the gene encoding the prion was found in all animals tested, including humans. Prion Characteristics PrP stands for "proteinaceous infectious particle" Prions are highly resistant to routine methods of decontamination Not inactivated by proteases, organic solvents, alkaline cleaners, ultraviolet radiation, ethanol, formaldehyde or extremely high temperatures (e.g. greater than 100 oC; sterilization for one hour at 121 oC in an autoclave does not kill prions) Typical Decontamination Protocol that Researchers Use: Tissues, infectious waste, and instruments used in the processing of prion-contaminated samples decontaminate them in: 1 N NaOH or undiluted fresh household bleach followed by autoclaving at 132 oC for 4.5 hours Two Distinct Conformations of the Prion Protein PrPC - "cellular" form found throughout the tissues of the body in healthy people and animals PrPC is sensitive to denaturing agents The ‘protein only' hypothesis proposes that abnormal, misfolded proteins causes PrPC to convert to the highly resistant or stable form termed PrPres. Over time, PrPres accumulates into clumps that damage or destroy nerve cells in the brain PRNP gene encodes PrPC PRNP gene located on chromosome 20 of humans Codes for a 254 AA protein. It is unique (no other proteins of similar homology in the database) PrPC is targeted via a secretory pathway to the cell surface of neurons and other cell types A glycosylinositol phospholipid anchors it into the membrane. PrPC Function? PrPc may bind copper and is then cycled back into the cell via endocytic vesicles where they may be degraded in lysosomes. Within the lysozymes, the infectious PrPres may interact with PrPC, causing the noninfectious form to be converted to the infectious form. Infectious PrPres are resistant to degradation and accumulate, causing neurotoxicity. Hypothetical Model showing PrPc Involvement in the Secretory Pathway of Cells. See Figure 19-4 PrPC is highly conserved in mammals and expressed predominantly in the brain. Exact function is unknown. Possible functions are roles in: Signal transduction Cellular differentiation Cell adhesion Copper transport Resistance to the accumulation of destructive free radicals that can result in neuronal death Three Ways that TSEs Can Arise Infection diet, vCJD Iatrogenic means e.g. surgery) Growth hormone injections Corneal transplants Inherited Genetic CJD Gerstmann-Straussler-Scheinker Disease (GSS) Fatal Familial Insomia (FFI) Sporadic forms CJD 19.3 Oral Transmission: How Do "Eaten" Prions Travel to the Brain to Cause Disease? Why Isn't Variant CJD More Common? Oral transmission of TSEs is very inefficient compared to intracranial injections Infectious dose through ingestion of prion-contaminated food is unknown New research suggests infectious prions enter the brain via the hypglossal nerve of the tongue Food products that contain tongue may be a potential source of prion infection for humans 2005 study-low doses via diet may be enough to cause a subclinical disease 19.4 Other Routes of Transmission: Iatrogenic Transmission, Including Prions in Blood Iatrogenic disease - inadvertedly caused by a physician or surgeon by a contaminated medical or surgical instrument or diagnostic procedure Examples of Iatrogenic Transmission of CJD Corneal grafts from donors who developed CJD Sharing of contaminated deep EEG electrodes implanted into brain Contaminated neurosurgical instruments Receipt of human growth hormone from CJD infected donors Patients who received dura mater grafts from donors who developed CJD Bloodborne Transmission? Bloodborne transmission has been suspected for 2 reasons vCJD can be detected in lymphoid tissues, raising the possibility that it could also be found in circulating lymphocytes present in the blood Prions may exist in the blood as it travels from the original site of the gut to the brain Experimental studies have shown the transmission of BSE to sheep by blood transfusion from asymptomatic infected sheep to healthy sheep Blood Transmission Surveillance At least 48 individuals who received blood components from 15 donors who later developed variant CJD are being monitored as a precautionary step for their "at risk " status Hemophiliacs in the U. K. have been notified that they are at risk for developing variant CJD. 19.5 Clinical Signs and Symptoms of Variant Creutzfeldt-Jakob Disease (Variant CJD) 50% of variant CJD patients die before the age of 30 average age of death is 28 Patients suffering from classic CJD die at an average of 68 years Symptoms of Variant CJD Anxiety Memory loss Mood changes Depression Withdrawal Neurological signs Twitching Spasms (jerky movements) Posture and gait abnormalities (motor difficulties) Final Symptoms of Variant CJD Loss of speech Stupor Persistent vegetative state (coma) Death (14 months after symptoms appear) 19.6 Diagnosis of Variant CJD Most patients referred to a psychiatrist because of behavioral changes Definitive diagnosis - prion positive immunostaining of biopsy material from: Tonsil Spleen Lymph nodes EEG - looking for slow or negative brain wave activity MRI - looking for brain lesions CSF - looking for elevated levels of neuronal, astrocytic and glial proteins Elevated levels are a consequence of damage to the blood brain barrier (extensive brain tissue damage) 19.7 Pathogenesis of TSEs Incubation period of TSEs (with the exception of vCJD) is long (20-56 years) South Fore tribe members still getting Kuru some 39-56 years after the cessation of cannibalism Histological/Brain Changes of TSEs Infected brains become spongiform (the brain has vacuoles - clear zones, similar to a sponge) Neuronal loss Astrocystosis (spread of astrocytes to damaged tissues in the brain) Amyloid plaques - formation of PrPres threadlike aggregates Diagram of the Major Regions of the Human Brain Affected fy the Different TSEs. See Figure 19-6 Brain Changes Depending upon what region of the brain is affected e.g. memory is affected when the cerebral cortex is infected No inflammation or immune defense against prions exists Natural proteins (body does not recognize as foreign antigens) Prion proteins are only harmful when they are converted to PrPres 19.8 Genetic Research and the Function of PrPC Knockout mice that do not express PrPC are resistant to scrapie infection Do changes in copper metabolism cause TSEs? (assuming prions are involved in copper metabolism) PRNP gene lacking octarepeat copper binding motif was introduced into knock-out mice. Mice became susceptible to scrapie infection Copper binding is not important in pathogenesis? Other copper binding sites outside of the octarepeat region which may play a more important role in copper metabolism? Other Copper Experiments Radioactive copper added to cells expressing PRNP gene Radioactive copper bound to the PrPC These cells were more resistant to copper toxicity and oxidative stress compared to cells that did not express PRNP Does PrPC play a role in cellular scavenging mechanisms? Human Genetics: Codon 129 50 known mutations in the PRNP gene PRNP codon 129 appears to act as a genetic susceptibility factor (codes for methionine or valine at position 129 of the PrPC). All people suffering from vCJD acquired through consuming prion-contaminated beef products were homozygous methionine at codon 129 19.9 Steps Toward Treatment and Vaccination No drug therapies available. Treatment is supportive No vaccine available PrPC antibodies injected into the brains of mice cause neurotoxicity 19.10 Species Barrier: Bovine Spongiform Encephalopathy (BSE) and Variant Creutzfeldt-Jakob Disease (Variant CJD) Transmissibility among species is easy Transmission can occur between different species Origin of BSE unclear Accepted hypothesis is that BSE came from cattle ingesting scrapie-contaminated bone meal derived from sheep offal fed to young calves Scrapie Scrapie was considered a rare disease of sheep that did not cause disease in humans Scrapie has been a disease of sheep and goats in Great Britain, Western Europe for more than 250 years First case in the U.S. was in 1947 (Michigan flock of sheep) Australia and New Zealand are still scrapie-free BSE First diagnosed in 1985. Dairy farmer in England notices several cows with abnormal behavior Unsteady gait, Aggressiveness, Kicking during milking The term "raging" or mad cow disease came to be Signs of BSE Difficulty in rising from a lying position Itching Heightened sensory perception Anorexia Excessive licking Decreased milk production Symptoms last for 2-6 months before the animal dies. Great Britain BSE Statistics By June of 1990 14,324 cases of BSE confirmed out of 10 million cattle Since 1986 In 1989 Over 200,000 cattle in the U.K. have developed mad cow disease 1-3 million were likely to have been infected with the BSE agent U.K. implemented control measures that included culling of sick animals and the banning of all bovine materials from entering the food that was fed to cattle BSE cases in North America, 1993-2007. See Figure 19-7 Variant CJD or vCJD Statistics First described in the U.K. in 1996 10 people had died from a new form of CJD Believed to be caused by the ingestion of BSE contaminated cattle products As of October, 2007, 161 deaths of vCJD patients reported in the U.K. 19.11 Plant Viroids: The Smallest Living Fossils of a Former RNA World? Viroids are small RNA molecules that infect plants in the same manner as conventional viruses (Chapter 20) +ssRNA, covalently, closed circular pathogenic molecules Internal base-pairing Viroids do not code for any proteins Viroids depend upon the plant host enzymes for their replication and other functions Two Families of Viroids Avsunviroidae Branched, 'quasi' rodlike structure Accumulate and replicate in chloroplasts Pospiviroidae True rodlike secondary structure Nuclear localization Viroids contain 5 structural or functional domains Central ( C ) Pathogenicity (P) Variable (V) Two terminal domains (T1 and T2) Model of Viroid Structures. See Figure 19-8 Additional Viroid Characteristics Virioids do not replicate in the cytoplasm like conventional plant RNA viruses Viroids traffic within the cell through the nuclear pores using a host nuclear localization protein that binds to viroid RNA Viroids replicate in either the nucleus or chloroplast of the plant The cell's DNA dependent RNA polymerase III synthesizes viroid RNA Viroid Spread Mechanical transmission Vegetative propagation Pollen and seed transfer (Chapter 20) Nothing distinguishes disease symptoms produced by viroids from those caused by plant viruses 19.12 Viroid Pathogenesis Plant viroids activate a plant RNA activated proteins termed PKR This PKR is analogous to the mammalian PKR that is activated by viral RNAs which leads to induction of the interferon pathway (Chapter 7)