Viroids, prions and virinos

7. Viroids, prions and virinos




Questions


image What are the different properties of viroids, prions and virinos?

image What are the features of the important spongiform/degenerative encephalopathies?

The viroids, prions and virinos are infectious agents that are simpler than viruses.



Viroids


Viroids cause, predominantly, plant diseases such as potato spindle-tuber disease. They consist of circular, single-stranded RNA, usually 250 to 370 nucleotides long. In plants, they are found mainly in the nucleolus, but little else is known of the pathogenic mechanisms that they employ. Human disease caused by viroids is not yet recognized.



Prions


Prions are putative infectious agents, so-called because they are proteinaceous infectious particles (the originator changed it from proin). Although there is not universal agreement that they exist in vivo, the evidence for them is now considerable. The best studied is the prion that causes a degenerative disorder of the CNS of sheep, scrapie. This agent appears to be a 33–35 kDa hydrophobic protein that has been termed PrP (for prion protein). The gene that encodes for this protein exists in normal sheep, but the product in diseased animals has a mutant peptide sequence and an abnormal isoform with β-pleated sheets replacing α-helical domains; the normal cellular protein is termed PrPc, the abnormal PrPsc. The abnormal protein further differs from the normal cellular protein by being insoluble in detergents, highly resistant to proteases and having a tendency to aggregate. Prion proteins have been shown to be infectious and to have a high but not absolute degree of species specificity.



Virinos


An alternative hypothesis for the infectious agent of scrapie has been the virino. This putative agent has a tiny, as yet undetectable, scrapie-specific nucleic acid coated in PrP. There is currently no evidence for the existence of this agent, but it offers a plausible explanation for the strain variation that is known to occur in scrapie.

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Epidemiology of infectious diseases Infections of the immunocompromised host Pathogenesis of infectious disease Zoonoses Tuberculosis and other mycobacterial diseases Innate host defences to infectious disease

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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Viroids, prions and virinos

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