This Article Discusses About:
- What Are TSEs Or Prion Diseases ?
- History Of Transmissible Spongiform Encephalopathies
- Etiology Of Transmissible Spongiform Encephalopathies
- Pathophysiology Of Transmissible Spongiform Encephalopathies
- The Infectious Agent That Causes TSEs
- Transmission Modes Of TSEs- Causing Prions
- Routes Of Delivery Of Prions To The Brain In TSEs
- The Molecular Basis Of Brain Damage By Prions In TSEs
- Diagnosis Of TSEs
- Understanding Defences Against Prions Could Lead To TSEs
- Advances In Prevention And Cure Of TSEs
- Epidemiology Of TSEs
What Are TSEs Or Prion Diseases ?
Transmissible spongiform encephalopathies (TSEs) or Prion Diseases are infections transmitted to man and animal, caused by the infectious particle prion. Transmissible spongiform encephalopathies (TSEs) or Prion Diseases are always lethal diseases that affect the entire central nervous system (CNS), causing widespread neurodegeneration. There is no cure for Transmissible spongiform encephalopathies (TSEs) or Prion Diseases as yet. Clinical symptoms of Transmissible spongiform encephalopathies (TSEs) or Prion Diseases include cognitive and motor dysfunction. Propagation of the prion particles with extensive formation of amyloid plaques in the brain are common. The human prion diseases include Kuru, Creutzfeldt-Jakob disease (CJD), variant CJD (vCJD), Gerstmann-Stra¨ussler-Scheinker (GSS) disease and fatal familial insomnia (FFI) ( Imran and Mahmood. An overview of human prion diseases. Virology Journal 2011, 8:559; Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009 ref ).
The animal prion diseases include scrapie, which infects sheep and goats, bovine spongiform encephalopathy (BSE) or "mad cow" disease, and chronic wasting disease (CWD), which infects deer and elk.
All infectious diseases are caused by bacteria, viruses and parasites with the exception of the Transmissible spongiform encephalopathies (TSEs) or Prion Diseases. Unlike these organisms which contain either protein and nucleic acid or just nucleic acid, the prion is a unique, self-propagating protein particle, that contains no nucleic acid. This is all the more intriguing as nucleic acid encodes genetic material and is a pre-requisite for replication/propagation. The prion is an abnormal counterpart of a normal cellular protein called the cellular prion protein (PrPC). The PrPC protein is encoded by the PRNP gene. The prion is a misfolded protein, with an abnormal conformation, and prone to conspicuous aggregation. The prion imposes its abnormal conformation on the host PrPC, thus replicating itself. ( Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009 ref ). Hence PrPC is required for prion propagation, transmission, and neurodegeneration. The mechanism of prion toxicity is not understood, but the prion likely interferes with the cellular function of the normal PrPC .
History Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
The first human TSE/prion disease was discovered by Gajdusek who observed that the TSE Kuru occurred in the spread among the Fore people of Papua New Guinea Eastern Highlands and the neighboring peoples through ritual cannibalism ( Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009, Imran and Mahmood. An overview of human prion diseases. Virology Journal 2011, 8:559 ref ). The first case of Kuru was discovered in 1920. Human-to-human transmission of Kuru was endemic to Papua New Guinea aborigines due to the ritual of eating the brains and viscera of the deceased by women and children as a sign of respect to the dead. Men who consumed the muscles were much less exposed to Kuru. A ban on ritualistic cannibalism imposed by Australian authorities in the 1950s, resulted in the decline of Kuru. Kuru was the first human prion disease to be transmitted experimentally to animals by intracerebral injection of infected human brain homogenates to chimpanzee. Subsequently this pattern of experimental transmission of other human prion diseases to animals was shown. At first it was believed that Kuru and the animal disease scrapie were both caused by viruses. However the discovery of the autosomal dominant inheritance of CJD about 90 years ago, and subsequent identification of mutations in the protein encoding sequence of the PRNP gene (which codes for the PrPC) established CJD as a genetic disease. Experimental transmission of the disease to animals was demonstrated by injection of brain extracts of patients who died of familial TSEs. This established the genetic basis of the prion diseases in the mutations of the PRNP gene, and the "faulty prion" as the infectious agent.
Etiology Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
There are sixteen variant TSEs reported to date including nine humans and seven in animals ( Imran and Mahmood. An overview of human prion diseases. Virology Journal 2011, 8:559). Human prion diseases can arise sporadically, be hereditary or be acquired (Imran and Mahmood. An overview of human prion diseases. Virology Journal 2011, 8:559 ref ). Sporadic human prion diseases include Cruetzfeldt-Jacob disease (CJD), fatal insomnia and variably protease-sensitive prionopathy. Familial or hereditary prion diseases are caused by autosomal dominant genetic mutations in the PRNP gene, with >20 mutations linked to inheritance of prion disease ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ).They include familial CJD, fatal familial insomnia and Gerstmann-Sträussler- Scheinker syndrome. Only 5% of cases are due to acquired human prion diseases. These include kuru, iatrogenic CJD; and the variant form of CJD (vCJD) that was transmitted to humans by consumption of infected beef. The prions enter the body through various non-neural routes. Cerebral surgery has unfortunately resulted in direct brain infection with prions ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ).
Pathophysiology Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
The TSEs destroy the gray matter of the central nervous system, resulting in loss of neurons, gliosis, and spongiform appearance of the central nervous system tissue due to vacuolation or plaque formation ( medscape: Prion-Related Diseases. Author: Tarakad S Ramachandran, MBBS, FRCP, FRCPC; Chief Editor: Niranjan N Singh, MD, DM ref ). The plaques are aggregates of abnormal prion protein, and are revealed by antibodies specific to the prion. Antibodies against prions do not cross react to other amyloid plaques such as those caused by beta-amyloid protein which is a characteristic pathogenic protein of Alzheimer's disease plaques. The prion plaques show characteristic amyloid staining such as apple-green birefringence after Congo Red staining under polarized light. In approximately 10% of patients with CJD, amyloid plaques are seen in cerebral and cerebellum areas of the brain. Multicentric cerebellar plaques are always seen in Gerstmann-Stra¨ussler-Scheinker disease- infected brains.
The Infectious Agent That Causes Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
As mentioned earlier the pathogenic or abnormal conformer PrPSc is the misfolded form of the cellular prion protein PrPC, encoded by the gene PRNP. The PRNP gene occurs on the short arm of chromosome 20 (20p13) as a 16 kilo base single copy gene ( Imran and Mahmood. An overview of human prion diseases. Virology Journal 2011, 8:559 ref ). The PRNP gene consists of two exons i.e. coding regions of the gene, where the second exon contains the open reading frame (whole sequence of the PrPC or PrPSc protein). The primary amino acid sequence of both PrPC and PrPSc is identical, but the secondary and tertiary conformations differ in that PrPC is alpha helical while PrPSc has a beta-sheet structure. The PrPSc isoform is extremely resistant to proteolysis and degradation by both chemical and physical disinfecting agents, while the normal PrPC is susceptible to both, and easily degraded. PrPC dissolves in detergents, and is proteolyzed easily, while PrPSc is insoluble and protease-resistant. PrPSc is also extremely thermally stable unlike PrPC. PrPSc stands for PrPscrapie and is used to indicate pathogenic nature of the particle, scrapie being the first animal TSE known. The seeding-nucleation model, based on experimental studies, suggests that PrPSc oligomers catalyze the conversion of the normal PrPC protein to the misfolded, aberrant beta-sheet conformation of PrPSc, thus propagating PrPSc. The newly created PrPSc is prone to oligomerization into fibrils, and its eventual degradation supplies further PrPSc templates for conversion of PrPC to PrPSc. Depending on the etiology of the particular disease, the PrPSc is either preexisting i.e. endogenous, or acquired through de novo infection. The pathogenesis of TSEs is due to the propagation of prions in the central nervous system, especially in the brain and the resultant destructive plaque formation. In fact researchers have developed a protein misfolding cyclic ampliﬁcation assay or PMCA ( Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009 ref ) where PrPSc mediated autocatalytic conversion and replication of PrPC is shown to occur in vitro. Small amounts of PrPSc- infected brain homogenates are mixed with PrpC- containing uninfected brain homogenate. Upon coincubation, PrPC is converted to PrPSc in a cyclic manner resulting in amplification of PrPSc. The amplified PrPSc aggregates, and when the aggregates are broken down by sonication into smaller fibrils, the fibrils in turn, when coincubated with fresh PrPC, again cause amplification of PrPSc i.e act as seed for forming new PrPSc aggregates. The cycle of PrPSc amplification can thus be repeated endlessly.
Transmission Modes Of Transmissible Spongiform Encephalopathies (TSEs) - Causing Prions
Oral uptake of prions has caused major epidemics such as Kuru and variant CJD; and epizootics such as scrapie ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref). Kuru epidemic resulting from ritualistic cannibalism has already been described earlier Bovine spongiform encephalopathy is a prion disease of cattle which spread due to the practice of recycling foodstuff to cattle which was contaminated with prions. Bovine spongiform encephalopathy more commonly known as mad cow disease killed more than 280,000 cattle worldwide. Variant Creutzfeldt-Jacob Disease, which spread in humans due to consumption of prion-contaminated beef from cows suffering from bovine spongiform encephalopathy, has killed over 200 humans.
Prions are efficiently transmitted parenterally. Iatrogenic parenteral transmission of prions has occurred at high incidence in the past. Variant CJD has been transmitted through transfusion of prion- contaminated blood products originating from variant CJD infected donors, such as non-leukocyte-reduced red cells, and anticoagulant purified factor VIII preparations. Other therapeutics derived from human sources such as human pituitary hormones: growth hormone (used to treat dwarfism) and fertility hormones, have resulted in parenteral prion transmission. Prior to recombinant DNA technology biologic therapeutics were developed by extracting the hormones from human cadaver pituitary gland. Such pituitary extracts resulted in more than 160 cases of death due to CJD. In experimental animal models, parenteral administration of prions is very efficient in establishing infection, with replication of prions both extraneural, lymphoid and invasion of central nervous system. Iatrogenic intracerebral transmission of prions has also occurred in the past. As it is prions are efficient at evading the host immune defences, replicating in lymphoid organs, invading the central nervous system and crossing the blood-brain barrier to enter the brain. Prions are transmitted very efficiently by direct administration to the brain. Iatrogenic transmission of CJD (iCJD) have taken place during neurosurgery and dura mater grafting. The earliest such instances were in Zürich in the 1970s when stereotactic electroencephalographic (EEG) recordings were made with electrodes that were reused after sterilization with ethanol and formaldehyde vapors (that kill viruses and bacteria). Prions are not affected by these sterilizing agents. Two patients died of the infection. Later transmission of CJD to chimpanzee via the electrodes established them as the source of the infectious particle. Aerosols transmit prions effectively to lab mice compelling a revision of current prion-related practices and guidelines on biologic safety in diagnostic and research laboratories.
Routes Of Delivery Of Prions To The Brain In Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
Prions enter the lymphoid organs ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ) and replicate particularly in the follicular dendritic cells that reside within the lymphoid tissue. Ablating the follicular dendritic cells therefore, can discourage prion propagation within the body. B cells of the immune system secrete lymphotoxins and tumor necrosis factor that are required for the maturation of follicular dendritic cells. Hence depletion of B cells should deplete follicular dendritic cells and result in resistance to prion infection. Congruent to this observation, B cell–deficient mice (μMT, Rag1−/−, Rag2−/−) are resistant to extraneural infection by prions, and lack follicular dendritic cells. Prion replication in the follicular dendritic cells depends on PrPC expression in these cells, as observed for the ME7 prion strain. Contradictory to these observations mice lacking tumor necrotic factor receptor 1, and hence lacking mature follicular dendritic cells- develop high titers of prions in extraneural lymph node tissue when challenged with prion. Moreover, prions replicate in a lymphotoxin-dependent manner in inflammatory granulomas lacking follicular dendritic cells. Taken together all these observations indicate that not only follicular dendritic cells, but other cell types also harbor and replicate prions in the extraneural tissues. Following replication in the lymph organs, prions enter the sympathetic and parasympathetic nerves and invade the central nervous system. The route of prion invasion upon oral administration of prions was followed by tracking the temporal sequence of prion accumulation. Upon intraperitoneal challenge, ablation of the sympathetic nerves either transiently or permanently by chemical or immunologic intervention, delays or prevents scrapie, while hyperinnervation increases prion invasion and pathogenesis. These results strongly suggest that after replication in the lymph node, the prions travel through the nerves to enter the central nervous system. The rate of invasion of the nervous system depends on the distance between follicular dendritic cells and nerves.
The Molecular Basis Of Brain Damage By Prions In Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
In order to develop successful therapeutic measures to control the massive damage to brain caused by spongiform encephalopathy, it is basic to understand the mechanisms exerted by prions that result in this horrible pathology. Prion-induced neurotoxicity requires PrPC. ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ). One hypothesis is that the PrPC act as receptors for prion mediated signaling that causes neurotoxic effects. This hypothesis is based on findings in Alzheimer's disease which although not a TSE, exhibits characteristic brain pathology similar to the prion diseases, with extensive plaque formation. In vitro PrPC mediates synaptic toxicity of amyloid-β (Aβ) oligomers and also in Aβ transgenic mice (APPswe/PSen1ΔE9). Anti-PrP antibodies or their PrP-binding regions alone not only blocked the interaction between PrP and amyloid beta oligomers, but also blocked the amyloid beta-dependent synaptic toxicity, suggesting that PrPC is involved in the pathogenesis of Alzheimer's disease. However, intracerebral injection with amyloid beta, in absence of PrPC still caused deficient hippocampal responses. Also, the finding of PrP involvement in amyloid beta mediated synaptic toxicity could not be reproduced. While the hypothesis remains inconclusive and controversial, it has been suggested that the effect could depend on copper availability. There are too many unknowns in the mechanism of prion based toxicity to the brain. PrPC variants consisting of the prion related protein with different internal regions missing- referred to as internal deletions (Δ32–134; Δ94–134), induce neurodegeneration in Shmerling's and Baumann's disease. The condition can be rescued by expressing the full-length PrPC by introducing the normal PRNP gene (without any deletion). Therefore it is thought that the variant PrPC competes with PrPC-like molecules for binding a common receptor, and perhaps inhibits the function of the PrPC ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ). Further, since deletion of residues Δ32–134 and Δ94–134 results in neurodegeneration, but deletion of residues Δ23–134 has no negative or toxic effect, it is believed that residues 23-31 are involved in causing Shmerling's and Baumann's disease. These residues span the amino-terminal tail of the PrPC protein. It is therefore suggested that toxicity may be induced by the amino-terminal tail making pores in, and disrupting the cellular plasma membrane in case of the PrPC deletion variants, causing the pathology of the disease. In case of the full length PrPC, the internal globular regions of the protein create a steric structure that holds the amino-terminal tail away from the membrane. Expression of a fourteen octapeptide repeats insertion (PG14) in the PrPC protein in transgenic mice triggered neurodegeneration in both mice possessing, as well as mice lacking, PRNP gene. The pathology was similar to that seen in humans carrying a similar mutation ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ). This pathology could not be rescued by introducing PrPC. Along with the fact that the octapeptides repeat induced neurodegeneration in PRNP possessing animals that would express normal PrPC, this finding indicates that the octapeptides repeat of PG14 PrP causes neurodegeneration via an irreversible mechanism.
Diagnosis Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
The molecular diagnosis of TSEs relies on the differential cleavage by the enzyme proteinase K of PrPC compared to PrPSc, which is highly resistant to proteolysis, and only the NH2 terminus is cleaved. A recent advance is the use of the protease enzyme thermolysin for diagnosis of prion diseases. Thermolysin hydrolyzes PrPC but does not cleave PrPSc at all (leaves the NH2 terminus intact) ( Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009 ref ). Highly sensitive immunoreagents for detection of PrPSc in tissues and body ﬂuids are required for diagnosis of TSEs. High quality, highly PrPSc-specific, high affinity diagnostics are available such as the "POM" antibody series that recognize conformational epitopes unique to PrPC in the COOH-terminal region of PrPC, and linear epitopes in the NH2-terminal region. Some antibodies have very high afﬁnities in the femtomolar range for prion protein. Antibodies that only bind PrPSc without binding PrPC have also been developed. PrP-derived peptides that specifically bind PrPSc have been discovered, and are useful for highly sensitive detection of prions. In an immunoassay called the sandwich ELISA, the PrP derived peptides, bind and detect PrPSc in the nanoliter range, in variant CJD brain homogenate diluted in plasma. This is a highly sensitive assay specially useful for detection of pathogenic prions in patient blood. The protein misfolding cyclic amplification or PMCA method is also a highly sensitive and specific method for pathogenic prion detection, with a sensitivity six thousand,six hundered- fold greater than standard methods for prion detection. PrPSc was amplified and detected by the protein misfolding cyclic amplification assay during the presymptomatic phase of disease from blood of hamsters infected with scrapie prions. Luminescent conjugated polymers are a unique class of amyloidotropic dyes. These dyes possess a thiophene backbone, and the geometry of the backbone governs the optical properties such as dye fluorescence. When the luminescent conjugated polymers interacts with and binds to the amyloid deposits of the prion protein it generates a unique optical ﬁngerprint for each protein conformation. Thus various prion aggregates within a heterogenous mixture can be identified using luminescent conjugated polymers staining. Luminescent conjugated polymers show specific binding to prion protein deposits, even when these are not stained by other amyloidotropic dyes like Congo red and ThT. Moreover the staining pattern differs depending on the prion strains, and therefore these strains can be differentiated by luminescent conjugated polymers with distinct ionic side chains. Various combinations of the above diagnostic methods can be envisaged, and are under exploration, for example, luminescent conjugated polymers, and protein misfolding cyclic amplification together in a pathogenic prion detection assay permitting real time visualization of prion folding and multiplication.
Surrogate biomarkers representing specific host reactions to prion infection can make useful diagnostic tools for identification of patients at risk, especially for purposes of blood transfusion- whether donor or recipient. The surrogate markers should be detectable at presymptomatic stage of infection and be easily accessible for detection e.g. in body fluids like blood or urine. S-100, neuron-speciﬁc enolase, 14-3-3 protein and cysteine proteinase inhibitor cystatin C are examples of biomarkers that increase during prion infection in cerebrospinal ﬂuid e.g. in individuals infected with sporadic CJD. Urinary alpha1-antichymotrypsin is also a biomarker unique to prion infection.
The best proof for detection of prion is prion infectivity. Prion infectivity assays in animals such as transgenic PrPC(tga20)- expressing mice, transgenic mice expressing the human immune system, and the bank vole (Clethrionomys glareolus) are efficient models for detecting various strains of prions from TSEs of human, sheep, goat, mouse, hamster, and other species. Unfortunately it takes six to seven moths to obtain the complete readout from such animal assays. They are also very expensive. Neural cell line clones: PK1 N2a are highly susceptible to prion infectivity and provide a good in vitro model for prion detection, and may be adapted for high throughput screens. However, these cell lines are susceptible to infection by only murine prions. Early disease diagnosis improves chances of successful treatment of Transmissible spongiform encephalopathies (TSEs) or Prion Diseases. So far TSEs are only diagnosed based on clinical symptoms. Presymptomatic diagnosis does not exist, and the symptomatic stage when diagnosis is done, occurs at a considerably advanced stage of the disease when the infection has progressed well.
In 1997 researchers proposed that tonsil biopsy could be a suitable diagnostic approach for variable CJD upon finding that protease-resistant PrPSc could be detected in tonsillar tissue of variant CJD patients. Detectable amounts of PrPSc in the tonsil and appendix at preclinical stages of variable CJD have been reported suggesting that biopsy of these lymphoid tissues, and other lymph organs could be useful for diagnosis of the prion diseases at asymptomatic stages. PrPSc was found in many skeletal muscle, spleen and the olfactory epithelium samples taken from patients with sporadic CJD. Therefore there is potential for development of less invasive diagnostic methods than brain biopsy for detection of prions and TSE.
Understanding Defences Against Prions Could Lead To Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases Treatment
Researchers believe that rapid clearance of prions from the body may be a critical defence against prion infection ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ). There is evidence to suggest that an efficient prion clearance mechanism does exist as PRNP negative mice (mice that do not possess the PRNP gene and hence cannot produce PrPC) which cannot replicate prions due to lack of PrPC, clear experimentally introduced prions within four days. While the cellular mechanism and the molecules involved in prion clearance are not known there is evidence pointing towards possible mechanistics of prion clearance. Microglia are specialized cells of the brain and central nervous system, that are involved in phagocytosis: these cells engulf and dispose off cellular garbage. Notably, in cerebellar slices depleted of microglial cells by pharmacogenetic ablation, prion levels increased fifteen-fold, compared to cerebellar slices with intact microglia. This strongly indicates a role for microglia to remove prions from the brain. Further evidence for the role of microglia in prion clearance from the central nervous system, comes from the role of cellular molecules involved in phagocytosis in clearing prions: milk fat globule epidermal growth factor 8 (Mfge8) is molecule is involved in phagocytosis of apoptotic cells. Mice lacking milk fat globule epidermal growth factor 8 were highly susceptible to prion infection and pathogenesis. Clearance of apoptotic bodies decreased, PrPSc accumulation and prion titers increased in the brain of these mice. Thus, milk fat globule epidermal growth factor 8 is required for prion clearance. It is likely that other molecules mediating phagocytosis of apoptotic cells are required for clearance of prions from the brain.
Advances In Prevention And Cure Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
The one hundred percent efficient prevention of contracting a TSE is by not having the prion protein PrP ( Adriano Aguzzi* and Caihong Zhu. Five Questions on Prion Diseases. PLoS Pathog. 2012 May; 8(5): e1002651 ref ). This approach has been used by eliminating the PRNP gene which encodes for PrP by genetic manipulation. Goats and sheep suffer from scrapie. The first attempt to erase PRNP gene was made by scientists who cloned a PRNP lacking sheep. However the cloned animals died after birth. Cattle suffer from bovine spongiform encephalitis. In 2007, scientists attempted ablation of PRNP gene in somatic (non-reproductive) cells, followed by transfer of the nuclear contents in cattle. This approach was successful as viable cattle were established. Later targeted gene disruption of PRNP in goats was successful, with resultant viable animals. Apart from its obvious value in livestock agriculture, the PRNP lacking animals are a safe source of biologic therapeutics. Many biologic drugs such as proteins and antibodies are produced in animals, or in cell cultures, the latter are nevertheless derived from animal or human origin. Thus there is a looming risk of prion infection while being treated with biotherapeutics- that are otherwise becoming increasingly valuable in treating many serious diseases and health conditions. For example variant CJD is transmitted through blood transfusion and purified blood products. The advent of PrP- free animals for production of prion- free biotherapeutics is therefore a significant advance in curbing Transmissible spongiform encephalopathies (TSEs) or Prion Diseases. Also, these animals would prevent unfortunate epidemics such as the CJD breakout from ingestion of infected beef. The advances in treatment of prion diseases are slow. Many compounds exhibit anti- prion properties in vitro, e.g. in cell culture screens, including Congo red, amphotericin B, anthracyclins, sulfated polyanions, porphyrins, branched polyamines, beta sheetbreakers, the spice curcumin, and the single-stranded phosphorothiolated analogs of natural nucleic acids ( Aguzzi A, Calella AM. Prions: Protein Aggregation and Infectious Diseases. Physiol Rev 89: 1105–1152, 2009; doi:10.1152/physrev.00006.2009 ref ). However most fail to show any anti-prion effects in vivo. The lack of efficacy may be due to factors such as insufficient activity/ efficacy in vivo, pharmacokinetics, drug metabolism and bioavailability, and safety/ toxicity of the potential anti- prion drugs. As an example, quinacrine treated neuroblastoma cells infected with scrapie in culture become prion- free. However quinacrine does not control prion infection in scrapie- infected mice and CJD patients, and is also hepatotoxic. The control of prion infection can be achieved in theory by leveraging the host immune system- by enhancing or suppressing certain immune parameters depending on the role they play in prion pathogenesis and control. Both the acquired and innate immune system can be adapted to not only control prion infection, but even prevent it. Essentially four basic approaches are underway experimentally:
- The follicular dendritic cells in the lymphoid organs host and promote prion-replication, hence removal of the follicular dendritic cells would decrease or eliminate prion- infectivity
- Enhancing the innate immune activity against prion infection
- The removal of the PrPSc and/ or the PrPC by using anti- PrP antibodies, so that the PrPC cannot be converted to PrPSc
- The removal of PrPSc and/ or the PrPC by using agents that bind PrPSc and PrP, making them unavailable for conversion and prion replication.
The laboratory mouse has been adapted for a scrapie infection model, and since all Transmissible spongiform encephalopathies (TSEs) or Prion Diseases show the same pathogenic mechanism and involvement of lymph organs in prion propagation, findings from the scrapie mouse model are considered to be applicable for potential control of all Transmissible spongiform encephalopathies (TSEs) or Prion Diseases. Besides innate, and acquired immune system adaption, prophylaxis vaccination is also being developed. The acquired immune approaches include both active and passive immunization. Exposure to recombinant prion protein resulted in active immunization in mice, with delay in onset of prion disease, although the infection remained lethal. Protective immunity against prions by generating intolerance to PrP is another popular approach. Attempts include introducing the antigenic portion of the PrP protein along with bacterial chaperons. The bacterial chaperons are required to induce immunogenic response against the prion antigen. Oral vaccine consisting of the PrP protein along with attenuated (decreased virulence) Salmonella induced mucosal anti- PrP immunoglobulins Ig A in the gut and systemic anti- PrP Ig G in mice. High titres of mucosal anti-PrP Ig A and high titres of serum Ig G titer were achieved. The mice were challenged with PrPSc scrapie strain 139A by oral administration, and remained symptom- free at 400 days. It has been observed in several instances that concomitant presence of two PrPC moieties that differ subtly antagonizes prion replication. The molecular mechanism behind this phenomenon remains to be deciphered. It is possible that the slightly variant PrPC binds incoming PrPSc and prevents it from being available for replication. This hypothesis was tested in a transgenic mouse model expressing soluble, dimeric (two units linked together by fusion with the Fc portion of human Ig G1) mouse PrP (refered to as PrP- Fc2). Upon challenge with prion infection, the mice remained disease-free. The PrP- Fc2 did not cause disease, nor did it transform into a disease- causing prion isoform. When these PrP- Fc2-expressing mice were back-crossed with wild- type mice and the progeny challenged with prions, the progeny resisted development of the disease, with a hundred thousand-fold reduction in prion titer. This control of prion infection was observed upon both intra- cerebral or intra- peritoneal challenge with scrapie prion, and in two different lines of transgenic mice expressing PrP- Fc2, suggesting that the PrP-Fc2 neutralizes prion in both brain and spleen. The PrP- Fc2 cannot be converted to the protease-resistant, insoluble, pathogenic prion particle PrPSc, and therefore the most likely explanation for prion control by PrP- Fc2 is that it binds the scrapie prion PrPSc , and thus makes PrPSc unavailable for binding to and converting PrPC to PrPSc. A parallel experiment was carried out where the gene expressing PrP-Fc2 was transferred by lentiviral gene transfer to mouse brain. When these mice were challenged with scrapie prion, infection onset and progression was very slow as the PrPSc replication decreased significantly. Therefore somatic gene transfer PrP Fc-2 and other prion antagonist molecules may effectively neutralize prion infection post- exposure and control prion diseases. Further studies will reveal the therapeutic benefit of prion antagonist drugs in the clinic.
Epidemiology Of Transmissible Spongiform Encephalopathies (TSEs) Or Prion Diseases
CJD is the most common TSE occurring at one in million both in the U.S. and worldwide ( medscape: Prion-Related Diseases. Author: Tarakad S Ramachandran, MBBS, FRCP, FRCPC; Chief Editor: Niranjan N Singh, MD, DM ref ). Five percent TSEs are of acquired nature. Prion diseases arising from autosomal dominant inheritance of mutations in the PRNP gene cause familial TSEs. Familial or hereditary TSEs like Gerstmann-Stra¨ussler-Scheinker and fatal familial insomnia are much rarer. Ten percent of CJD cases are of familial nature. Varaint CJD was first reported in 1996. To date two hundred and twenty nine variant CJD patients from twelve countries have been reported. As of June 2, 2014, the vast majority of the cases, amounting to one hundred and seventy seven, are from the United Kingdom, with almost all the remaining cases from across Europe, and one each from Japan, Saudi Arabia, and Taiwan. Two of four cases in the United States of America are thought to arise from beef consumption ( http://www.cdc.gov/ncidod/dvrd/vcjd/epidemiology.htm ref) As already mentioned, as of now there is no cure or preventive method available for prion diseases, and TSEs are invariably progressive and lethal. The duration of the disease and its acuteness depends on the type of TSE: sporadic, acquired or familial. Sporadic CJD occurs over an average period of 8 months, while variant CJD occurs over an average period of 14 months. Familial CJD occurs over an average period of 26 months. The mean duration of Gerstmann-Stra¨ussler-Scheinker is comparatively longer at 60 months. All races are susceptible to TSEs. Prion disease susceptibility is also based on race origin. Two populations are exceptionally susceptible to CJD. Israelis from Libya and certain populations of Slovakian origin report sixty to hundred- fold higher incidence of CJD than usual. Case controlled studies disprove the original reasoning that the high CJD incidence in these populations was due to diet containing prions. Subsequently the real explanation was found: these persons carry codon 200 mutations in the PRNP gene, making them highly susceptible to prion infection. In people carrying this mutation, CJD is characterized by peripheral neuropathy besides the normal pathophysiology of CJD seen in people of all races. Acquired prion disease, such as variant CJD arose in United Kingdom from consuming beef from cows suffering bovine spongiform encephalopathy, and is found almost exclusively in Europe. Biologically both men and women are equally susceptible to Transmissible spongiform encephalopathies (TSEs) or Prion Diseases. TSEs can occur in persons of a wide age range, from 17 years to 83 years. The mean age of onset for CJD has been reported as 62 years. The incidence of sporadic CJD is one in million in the overall population, but is higher in older populations between 60 years to 74 years , at five cases in a million. The average age of onset of variant CJD is 28 years. The mean age of onset of familial prion diseases like familial CJD, Gerstmann-Stra¨ussler-Scheinker and fatal familial insomnia is 45 years to 49 years.