Transmissible spongiform encephalopathy

Transmissible spongiform encephalopathy
Classification and external resources
Specialty Infectious disease
ICD-10 A81
ICD-9-CM 046
DiseasesDB 25165
eMedicine neuro/662
MeSH D017096

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of progressive conditions that affect the brain (encephalopathies) and nervous system of many animals, including humans. According to the most widespread hypothesis, they are transmitted by prions, though some other data suggest an involvement of a Spiroplasma infection.[1] Mental and physical abilities deteriorate and myriad tiny holes appear in the cortex causing it to appear like a sponge (hence spongiform) when brain tissue obtained at autopsy is examined under a microscope. The disorders cause impairment of brain function, including memory changes, personality changes and problems with movement that worsen over time. Prion diseases of humans include classic Creutzfeldt–Jakob disease, new variant Creutzfeldt–Jakob disease (nvCJD, a human disorder related to bovine spongiform encephalopathy), Gerstmann–Sträussler–Scheinker syndrome, fatal familial insomnia, kuru, and the recently discovered variably protease-sensitive prionopathy. These conditions form a spectrum of diseases with overlapping signs and symptoms.

Unlike other kinds of infectious disease, which are spread by microbes, the infectious agent in TSEs is believed to be a type of protein, called the prion protein. Misshapen prion proteins carry the disease between individuals and cause deterioration of the brain. TSEs are unique diseases in that their aetiology may be genetic, sporadic, or infectious via ingestion of infected foodstuffs and via iatrogenic means (e.g., blood transfusion).[2] Most TSEs are sporadic and occur in an animal with no prion protein mutation. Inherited TSE occurs in animals carrying a rare mutant prion allele, which expresses prion proteins that contort by themselves into the disease-causing conformation. Transmission occurs when healthy animals consume tainted tissues from others with the disease. In recent times, a type of TSE called bovine spongiform encephalopathy (BSE) spread in cattle in an epidemic fashion. This occurred because cattle were fed the processed remains of other cattle, a practice now banned in many countries.

Prions cannot be transmitted through the air or through touching or most other forms of casual contact. However, they may be transmitted through contact with infected tissue, body fluids, or contaminated medical instruments. Normal sterilization procedures such as boiling or irradiating materials fail to render prions non-infective.

Classification

Known spongiform encephalopathies
ICTVdb Code Disease name Natural host Prion name PrP isoform
Non-human mammals
90.001.0.01.001. Scrapie Sheep and goats Scrapie prion OvPrPSc
90.001.0.01.002. Transmissible mink encephalopathy (TME) Mink TME prion MkPrPSc
90.001.0.01.003. Chronic wasting disease (CWD) Elk, White-tailed deer, Mule Deer and Red Deer CWD prion MDePrPSc
90.001.0.01.004. Bovine spongiform encephalopathy (BSE)
commonly known as "Mad Cow Disease"		 Cattle BSE prion BovPrPSc
90.001.0.01.005. Feline spongiform encephalopathy (FSE) Cats FSE prion FePrPSc
90.001.0.01.006. Exotic ungulate encephalopathy (EUE) Nyala and greater kudu EUE prion NyaPrPSc
Human diseases
90.001.0.01.007. Kuru Humans Kuru prion HuPrPSc
90.001.0.01.008. Creutzfeldt-Jakob disease (CJD) CJD prion
(New) Variant Creutzfeldt-Jakob disease (vCJD, nvCJD) vCJD prion[3]
90.001.0.01.009. Gerstmann-Sträussler-Scheinker syndrome (GSS) GSS prion
90.001.0.01.010. Fatal familial insomnia (FFI) FFI prion

History

In the 5th century BCE, Hippocrates described a disease like TSE in cattle and sheep, which he believed also occurred in man.[4] Publius Flavius Vegetius Renatus records cases of a disease with similar characteristics in the 4th and 5th centuries AD.[5] In 1755, an outbreak of scrapie was discussed in the British House of Commons and may have been present in Britain for some time before that.[6] Although there were unsupported claims in 1759 that the disease was contagious, in general it was thought to be due to inbreeding and countermeasures appeared to be successful. Early-20th-century experiments failed to show transmission of scrapie between animals, until extraordinary measures were taken such as the intra-ocular injection of infected nervous tissue. No direct link between scrapie and disease in man was suspected then or has been found since. TSE was first described in man by Alfons Maria Jakob in the 1921.[7] Daniel Carleton Gajdusek's discovery that Kuru was transmitted by cannibalism accompanied by the finding of scrapie-like lesions in the brains of Kuru victims strongly suggested an infectious basis to TSE.[8] The priority given the search for a viral infectious agent almost cost Stanley Prusiner tenure when his research showed that a protein transferred the disease.[9] A paradigm shift to a non-nucleic infectious entity was required when the results were validated with an explanation of how a prion protein might transmit spongiform encephalopathy.[10] It wasn't until 1988 that the neuropathology of spongiform encephalopathy was properly described in cows.[11] The alarming amplification of BSE in the British cattle herd heightened fear of transmission to humans and reinforced the belief in the infectious nature of TSE. This was confirmed with the identification of a Kuru-like disease, called new variant Creutzfeldt–Jakob disease, in humans exposed to BSE.[12] Although the infectious disease model of TSE has been questioned in favour of a prion transplantation model that explains why cannibalism favours transmission,[13] the search for a viral agent is being continued in some laboratories.[14]

Features of TSE

The degenerative tissue damage caused by human prion diseases (CJD, GSS, and kuru) is characterised by four features: spongiform change, neuronal loss, astrocytosis, and amyloid plaque formation. These features are shared with prion diseases in animals, and the recognition of these similarities prompted the first attempts to transmit a human prion disease (kuru) to a primate in 1966, followed by CJD in 1968 and GSS in 1981. These neuropathological features have formed the basis of the histological diagnosis of human prion diseases for many years, although it was recognized that these changes are enormously variable both from case to case and within the central nervous system in individual cases.[15]

The clinical signs in humans vary, but commonly include personality changes, psychiatric problems such as depression, lack of coordination, and/or an unsteady gait (ataxia). Patients also may experience involuntary jerking movements called myoclonus, unusual sensations, insomnia, confusion, or memory problems. In the later stages of the disease, patients have severe mental impairment (dementia) and lose the ability to move or speak.[16]

Early neuropathological reports on human prion diseases suffered from a confusion of nomenclature, in which the significance of the diagnostic feature of spongiform change was occasionally overlooked. The subsequent demonstration that human prion diseases were transmissible reinforced the importance of spongiform change as a diagnostic feature, reflected in the use of the term "spongiform encephalopathy" for this group of disorders.

Prions appear to be most infectious when in direct contact with affected tissues. For example, Creutzfeldt-Jakob disease has been transmitted to patients taking injections of growth hormone harvested from human pituitary glands, from cadaver dura allografts and from instruments used for brain surgery (Brown, 2000) (prions can survive the "autoclave" sterilization process used for most surgical instruments). It is also believed that dietary consumption of affected animals can cause prions to accumulate slowly, especially when cannibalism or similar practices allow the proteins to accumulate over more than one generation. An example is kuru, which reached epidemic proportions in the mid 20th century in the Fore people of Papua New Guinea, who used to consume their dead as a funerary ritual.[17] Laws in developed countries now ban the use of rendered ruminant proteins in ruminant feed as a precaution against the spread of prion infection in cattle and other ruminants.

Note that not all encephalopathies are caused by prions, as in the cases of PML (caused by the JC virus), CADASIL (caused by abnormal NOTCH3 protein activity), and Krabbe disease (caused by a deficiency of the enzyme galactosylceramidase). Progressive Spongiform Leukoencephalopathy (PSL)—which is a spongiform encephalopathy—is also probably not caused by a prion, although the adulterant that causes it among heroin smokers has not yet been identified.[18][19][20][21] This, combined with the highly variable nature of prion disease pathology, is why a prion disease cannot be diagnosed based solely on a patient's symptoms.

Genetics

Mutations in the PRNP gene cause prion disease. Familial forms of prion disease are caused by inherited mutations in the PRNP gene. Only a small percentage of all cases of prion disease run in families, however. Most cases of prion disease are sporadic, which means they occur in people without any known risk factors or gene mutations. In rare circumstances, prion diseases also can be transmitted by exposure to prion-contaminated tissues or other biological materials obtained from individuals with prion disease.

The PRNP gene provides the instructions to make a protein called the prion protein (PrP). Under normal circumstances, this protein may be involved in transporting copper into cells. It may also be involved in protecting brain cells and helping them communicate. 24 Point-Mutations in this gene cause cells to produce an abnormal form of the prion protein, known as PrPSc. This abnormal protein builds up in the brain and destroys nerve cells, resulting in the signs and symptoms of prion disease.

Familial forms of prion disease are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person inherits the altered gene from one affected parent.

In some people, familial forms of prion disease are caused by a new mutation in the PRNP gene. Although such people most likely do not have an affected parent, they can pass the genetic change to their children.

Competing hypotheses

Protein-only hypothesis

Protein could be the infectious agent, inducing its own replication by causing conformational change of normal cellular PrPC into PrPSc. Evidence for this theory:

  • infectivity titre correlates with PrPSc levels. However, this is disputed.[22]
  • PrPSc is an isomer of PrPC
  • Denaturing PrP removes infectivity[23]
  • PrP-null mice cannot be infected[24]
  • PrPC knockout in mice following inoculation with PrPSc reverses early spongeosis and behavioural deficits, halts further disease progression and increases life-span [25]

Multi-component hypothesis

While not containing a nucleic acid genome, prions may be composed of more than just a protein. Purified PrPC appears unable to convert to the infectious PrPSc form, unless other components are added, such as RNA and lipids.[26] These other components, termed cofactors, may form part of the infectious prion, or they may serve as catalysts for the replication of a protein-only prion.

Viral hypothesis

This hypothesis postulates that an infectious viral agent is the cause of the disease. Evidence for this hypothesis is as follows:

  • Incubation time is comparable to a lentivirus
  • Strain variation of different isolates of PrPSc[27]
  • An increasing titre of PrPSc as the disease progresses suggests a replicating agent.

Epidemiology

These spontaneous disorders in humans are very rare, affecting only about one person per million worldwide each year. However, transmissible spongiform encephalopathies can reach epidemic proportions, as was seen in the UK BSE outbreak of the 1980s and 1990s. It is very hard to map the spread of the disease due to the difficulty of identifying individual strains of the prions. This means that, if animals at one farm begin to show the disease after an outbreak on a nearby farm, it is very difficult to determine whether it is the same strain affecting both herds—suggesting transmission—or if the second outbreak came from a completely different source.

Possible cure or vaccine and diagnosis

There continues to be a very practical problem with diagnosis of prion diseases, including BSE and CJD. They have an incubation period of months to decades during which there are no symptoms, even though the pathway of converting the normal brain PrP protein into the toxic, disease-related PrPSc form has started. At present, there is virtually no way to detect PrPSc reliably except by examining the brain using neuropathological and immunohistochemical methods after death. Accumulation of the abnormally folded PrPSc form of the PrP protein is a characteristic of the disease, but it is present at very low levels in easily accessible body fluids like blood or urine. Researchers have tried to develop methods to measure PrPSc, but there are still no fully accepted methods for use in materials such as blood.

In 2010, a team from New York described detection of PrPSc even when initially present at only one part in a hundred billion (10−11) in brain tissue. The method combines amplification with a novel technology called Surround Optical Fiber Immunoassay (SOFIA) and some specific antibodies against PrPSc. After amplifying and then concentrating any PrPSc, the samples are labelled with a fluorescent dye using an antibody for specificity and then finally loaded into a micro-capillary tube. This tube is placed in a specially constructed apparatus so that it is totally surrounded by optical fibres to capture all light emitted once the dye is excited using a laser. The technique allowed detection of PrPSc after many fewer cycles of conversion than others have achieved, substantially reducing the possibility of artefacts, as well as speeding up the assay. The researchers also tested their method on blood samples from apparently healthy sheep that went on to develop scrapie. The animals’ brains were analysed once any symptoms became apparent. The researchers could therefore compare results from brain tissue and blood taken once the animals exhibited symptoms of the diseases, with blood obtained earlier in the animals’ lives, and from uninfected animals. The results showed very clearly that PrPSc could be detected in the blood of animals long before the symptoms appeared.[28][29]

Recent research from the University of Toronto and Caprion Pharmaceuticals has discovered one possible avenue that might lead to quicker diagnosis, a vaccine or possibly even treatment for prion diseases. The abnormally folded proteins that cause the disease have been found to expose a side chain of amino acids that the properly folded protein does not expose. Antibodies specifically coded to this side-chain amino acid sequence have been found to stimulate an immune response to the abnormal prions and leave the normal proteins intact.[30]

Another idea involves using custom peptide sequences. Since some research suggests prions aggregate by forming beta barrel structures, work done in vitro has shown that peptides made up of beta barrel-incompatible amino acids can help break up accumulations of prion.

A third idea concerns genetic therapy, whereby the gene for encoding protease-resistant protein is considered to be an error in several species, and therefore something to be inhibited.

See also

Further reading

References

  1. ↑ Bastian FO, Sanders DE, Forbes WA, Hagius SD, Walker JV, Henk WG, Enright FM, Elzer PH; Sanders; Forbes; Hagius; Walker; Henk; Enright; Elzer (2007). "Spiroplasma spp. from transmissible spongiform encephalopathy brains or ticks induce spongiform encephalopathy in ruminants". Journal of Medical Microbiology 56 (9): 1235–1242. doi:10.1099/jmm.0.47159-0. PMID 17761489.
  2. ↑ Brown P, Preece M, Brandel JP, Sato T, McShane L, Zerr I, Fletcher A, Will RG, Pocchiari M, Cashman NR, d'Aignaux JH, Cervenakova L, Fradkin J, Schonberger LB, Collins SJ; Preece; Brandel; Sato; McShane; Zerr; Fletcher; Will; Pocchiari; Cashman; d'Aignaux; Cervenáková; Fradkin; Schonberger; Collins (2000). "Iatrogenic Creutzfeldt-Jakob disease at the millennium". Neurology 55 (8): 1075–81. doi:10.1212/WNL.55.8.1075. PMID 11071481.
  3. ↑ Believed to be identical to the BSE prion.
  4. ↑ McAlister, V (June 2005). "Sacred disease of our times: failure of the infectious disease model of spongiform encephalopathy". Clin Invest Med 28 (3): 101–4. PMID 16021982. Retrieved 2011-06-20.
  5. ↑ Digesta Artis Mulomedicinae, Publius Flavius Vegetius Renatus
  6. ↑ Brown P, Bradley R; Bradley (December 1998). "1755 and all that: a historical primer of transmissible spongiform encephalopathy". BMJ 317 (7174): 1688–92. doi:10.1136/bmj.317.7174.1688. PMC 1114482. PMID 9857129.
  7. ↑ Katscher F. (May 1998). "It's Jakob's disease, not Creutzfeldt's". Nature 393 (6680): 11. Bibcode:1998Natur.393Q..11K. doi:10.1038/29862. PMID 9590681.
  8. ↑ Gajdusek DC (Sep 1977). "Unconventional viruses and the origin and disappearance of kuru". Science 197 (4307): 943–60. Bibcode:1977Sci...197..943C. doi:10.1126/science.197.4307.943 (inactive 2015-02-01). PMID 142303.
  9. ↑ Prusiner S. "Autobiography". Nobel Prize in Physiology or Medicine 1997. Retrieved 2011-11-20.
  10. ↑ Collins SJ, Lawson VA, Masters CL.; Lawson; Masters (Jan 2004). "Transmissible spongiform encephalopathies". Lancet 363 (9204): 51–61. doi:10.1016/S0140-6736(03)15171-9. PMID 14723996.
  11. ↑ Hope J, Reekie LJ, Hunter N, Multhaup G, Beyreuther K, White H, Scott AC, Stack MJ, Dawson M, Wells GA.; Reekie; Hunter; Multhaup; Beyreuther; White; Scott; Stack; Dawson; et al. (Nov 1988). "Fibrils from brains of cows with new cattle disease contain scrapie-associated protein". Nature 336 (6197): 390–2. Bibcode:1988Natur.336..390H. doi:10.1038/336390a0. PMID 2904126.
  12. ↑ Will RG, Ironside JW, Zeidler M, Cousens SN, Estibeiro K, Alperovitch A, Poser S, Pocchiari M, Hofman A, Smith PG.; Ironside; Zeidler; Cousens; Estibeiro; Alperovitch; Poser; Pocchiari; Hofman; Smith (April 1996). "A new variant of Creutzfeldt-Jakob disease in the UK". Lancet 347 (9006): 921–5. doi:10.1016/S0140-6736(96)91412-9. PMID 8598754.
  13. ↑ McAlister, V (June 2005). "Sacred disease of our times: failure of the infectious disease model of spongiform encephalopathy". Clin Invest Med 28 (3): 101–4. PMID 16021982. Retrieved 2011-06-20.
  14. ↑ Manuelidis L, Yu ZX, Barquero N, Banquero N, Mullins B; Yu; Banquero; Mullins (February 2007). "Cells infected with scrapie and Creutzfeldt-Jakob disease agents produce intracellular 25-nm virus-like particles". Proceedings of the National Academy of Sciences of the United States of America 104 (6): 1965–70. Bibcode:2007PNAS..104.1965M. doi:10.1073/pnas.0610999104. PMC 1794316. PMID 17267596.
  15. ↑ Jeffrey M, Goodbrand IA, Goodsir CM; Goodbrand; Goodsir (1995). "Pathology of the transmissible spongiform encephalopathies with special emphasis on ultrastructure". Micron 26 (3): 277–98. doi:10.1016/0968-4328(95)00004-N. PMID 7788281.
  16. ↑ Collinge J (2001). "Prion diseases of humans and animals: their causes and molecular basis". Annu Rev Neurosci 24: 519–50. doi:10.1146/annurev.neuro.24.1.519. PMID 11283320.
  17. ↑ Collins S, McLean CA, Masters CL; McLean; Masters (2001). "Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia, and kuru: a review of these less common human transmissible spongiform encephalopathies". J Clin Neurosci 8 (5): 387–97. doi:10.1054/jocn.2001.0919. PMID 11535002.
  18. ↑ "hafci.org". Archived from the original on November 1, 2004. Retrieved 2007-12-02.
  19. ↑ Kriegstein AR; Shungu DC; Millar WS; et al. (1999). "Leukoencephalopathy and raised brain lactate from heroin vapor inhalation ("chasing the dragon")". Neurology 53 (8): 1765–73. doi:10.1212/WNL.53.8.1765. PMID 10563626.
  20. ↑ Chang YJ, Tsai CH, Chen CJ; Tsai; Chen (1997). "Leukoencephalopathy after inhalation of heroin vapor". J. Formos. Med. Assoc. 96 (9): 758–60. PMID 9308333.
  21. ↑ Koussa S, Zabad R, Rizk T, Tamraz J, Nasnas R, Chemaly R; Zabad; Rizk; Tamraz; Nasnas; Chemaly (2002). "[Vacuolar leucoencephalopathy induced by heroin: 4 cases]". Rev. Neurol. (Paris) (in French) 158 (2): 177–82. PMID 11965173.
  22. ↑ Barron RM; Campbell SL; King D; et al. (December 2007). "High titers of transmissible spongiform encephalopathy infectivity associated with extremely low levels of PrPSc in vivo". The Journal of Biological Chemistry 282 (49): 35878–86. doi:10.1074/jbc.M704329200. PMID 17923484.
  23. ↑ Supattapone S; Wille H; Uyechi L; et al. (April 2001). "Branched Polyamines Cure Prion-Infected Neuroblastoma Cells". Journal of Virology 75 (7): 3453–61. doi:10.1128/JVI.75.7.3453-3461.2001. PMC 114138. PMID 11238871.
  24. ↑ Sakudo A; Lee DC; Saeki K; et al. (August 2003). "Impairment of superoxide dismutase activation by N-terminally truncated prion protein (PrP) in PrP-deficient neuronal cell line". Biochemical and Biophysical Research Communications 308 (3): 660–7. doi:10.1016/S0006-291X(03)01459-1. PMID 12914801.
  25. ↑ Mallucci G; Dickinson A; Lineham J; et al. (October 2003). "Depleting Neuronal PrP in Prion Infection Prevents Disease and Reverses Spongiosis". Science 302 (5646): 871–874. Bibcode:2003Sci...302..871M. doi:10.1126/science.1090187. PMID 14593181.
  26. ↑ Deleault NR, Harris BT, Rees JR, Supattapone S; Harris; Rees; Supattapone (June 2007). "Formation of native prions from minimal components in vitro". Proc. Natl. Acad. Sci. U.S.A. 104 (23): 9741–6. Bibcode:2007PNAS..104.9741D. doi:10.1073/pnas.0702662104. PMC 1887554. PMID 17535913.
  27. ↑ Bruce ME (2003). "TSE strain variation". British Medical Bulletin 66: 99–108. doi:10.1093/bmb/66.1.99. PMID 14522852.
  28. ↑ "Detecting Prions in Blood" (PDF). Microbiology Today.: 195. August 2010. Retrieved 2011-08-21.
  29. ↑ "SOFIA: An Assay Platform for Ultrasensitive Detection of PrPSc in Brain and Blood" (PDF). SUNY Downstate Medical Center. Retrieved 2011-08-19.
  30. ↑ Paramithiotis E; Pinard M; Lawton T; et al. (July 2003). "A prion protein epitope selective for the pathologically misfolded conformation". Nature Medicine 9 (7): 893–9. doi:10.1038/nm883. PMID 12778138. Lay summary – ScienceDaily (2003-06-02).
  31. ↑ Deadly Feasts: The "Prion" Controversy and the Public's Health, Richard Rhodes, 1998, Touchstone, ISBN 0-684-84425-7

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