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Prions

Prions are proteins which are unique infectious agents lacking genetic material. These proteins can induce transmittable spongiform encephalopathy, a degenerative neurological disease. These agents have an incubation period of up to 30 years, leading to progressive dementia and death within 7-8 months of symptom onset. It took a large amount of time for scientists to identify prions as a source of disease, as they do not fit any definition of life and it was assumed that diseases originating from an external source (rather than a defect in ones own organs or cells) must come from a living or pseudo living source such as a bacteria or a virus. The notion that proteins could propagate disease independently of other mechanisms was novel and took a while to be embraced by parts of the scientific community, with some claiming that there was simply an as of yet not identified virus present in those patients affected by prion diseases, and it was this virus that was causing both the disease and the protein aggregates that are appreciated to be linked to prion disease.

The infectious prion protein is an alternative form of a protein normally found on the surface of neurons (PrPc; the infectious form is PrPsc). When PrPsc encounters PrPC, it converts it to PrPsc and releases it in a positive feedforward loop, causing the disease to worsen rapidly as time passes. PrPsc then aggregates and is taken up by neurons, wherein it forms vacuoles that disrupt normal cell function and lead to neuron death. The PrPc protein also has normal biological functions that are disrupted by prion disease, and this disruption likely contributes to and worsens disease severity. For example, the protein is believed to play a protective role in shielding cells from various stressors such as oxidative stress. Thus, if a person has the prion protein in their neurons then it will physically disrupt neuronal function, and i will interfere with normal PrPc function thus causing affected cells to be more susceptible to the stress induced by the formation of PrPsc aggregates. This dual functionality of PrPsc makes it all the more dangerous and helps to explain why the disease rapidly increases in severity over time in those that are infected by this unique pathogenic agent.

Disease Causes

Sporadic Creutzfeldt-Jakob disease is a form of prion disease which is genetic in origin and rarely occurs in humans, but transmissible forms also exist from eating contaminated beef infected with bovine spongiform encephalopathy. This is also known as “mad cow disease”, and is a major public health concern as it may not be obvious that beef being eaten is contaminated, and the negative health effects may not be apparent for decades after the consumption of contaminated beef.

Another cause of prion-induced disease is the direct consumption of the brain matter of an infected individual. As cannibalism is rare, this is not a common source of disease, however until the end of the 20th century certain tribes in Papua New Guinea were known to suffer from a prion disease known as Kuru due to cannibalism. These tribes would eat the brains of those they defeated in combat, and as a result a large number of members of these tribes suffered from the neurological dementia that this infection caused. Indeed, if one lists certain ailments on the online web MD symptom checker website that are consistent with a degenerative mental condition, the site will ask you whether you have engaged in cannibalism in Papua New Guinea. While this option may be partly tongue in cheek, it is a fact that Kuru was a problem that spread rapidly through cannibalistic societies in these areas and needed to be cut off at the source by preventing the consumption of contaminated brain matter.

Since they lack any form of nucleic acid, prions are very resistant to stress and can survive for a long time in soil making it difficult to contain and eliminate outbreaks of the disease.

Treatment

No treatment exists for prion diseases. Infected animals are killed and incinerated to prevent infection of other parts of the food supply. Humans that have contracted the disease will inevitably suffer from slow but steady neurological decay leading to eventual death. There is currently research underway that aims to determine whether there are faster ways to treat and diagnose prion disease in an efficient manner. Currently all such efforts are at the highly experimental and largely theoretical stage, though some progress has been made, particularly towards advance diagnosis. Evidently, the PRP protein folds in a unique way when it is found in the infectious prion associated form, and when it does so it reveals a specific chain of amino acids that it does not normall reveal and that is fairly unique to this protein. With special antibodies that are optimized to recognize this sequence, it is possible to identify it much more readily in biopsies. As antibodies are currently used as treatments for certain autoimmune diseases, it is feasible that at some point in the future a similar approach will be taken for prion like diseases. Patients that were diagnosed with prion disease would be given a course of antibodies intravenously, which would bind to the prion protein in the brain. This binding would target the pathogenic proteins for destruction, while ideally leaving the other cells intact. This treatment would be difficult to implement as getting things into the brain is complex since the body tries to suppress inflammation at this site above all else, but progress is being made in that direction.

References

  • 1. Prusiner, S.B., et al., Transgenetic studies implicate interactions between homologous PrP isoforms in scrapie prion replication. Cell, 1990. 63(4): p. 673-686.
  • 2. Bessen, R.A., et al., Non-genetic propagation of strain-specific properties of scrapie prion protein. 1995.
  • 3. Telling, G.C., et al., Prion propagation in mice expressing human and chimeric PrP transgenes implicates the interaction of cellular PrP with another protein. Cell, 1995. 83(1): p. 79-90.
  • 4. Forloni, G., et al., Neurotoxicity of a prion protein fragment. 1993.
  • 5. Riek, R., et al., NMR structure of the mouse prion protein domain PrP (121-231). Nature, 1996. 382(6587): p. 180-182.
  • 6. Pan, K.-M., et al., Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins. Proceedings of the National Academy of Sciences, 1993. 90(23): p. 10962-10966.
  • 7. McKinley, M.P., D.C. Bolton, and S.B. Prusiner, A protease-resistant protein is a structural component of the scrapie prion. Cell, 1983. 35(1): p. 57-62.
  • 8. Hill, A.F., et al., The same prion strain causes vCJD and BSE. Nature, 1997. 389(6650): p. 448-450.
  • 9. Palmer, M.S., et al., Homozygous prion protein genotype predisposes to sporadic Creutzfeldt–Jakob disease. 1991.
  • 10. Collinge, J., et al., Molecular analysis of prion strain variation and the aetiology of'new variant'CJD. 1996.

Biology


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