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- 2. Prions Prions are infectious particles that contain no nucleic acids Prions, so-called because they are proteinaceous, are infectious particles, smaller than viruses, that contain no nucleic acids (neither DNA nor RNA). Historically, the idea of an infectious agent that did not use nucleic acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the majority of biologists that such agents do indeed exist.
- 3. Prions Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle (commonly known as “mad cow disease”), were shown to be transmitted by prions. The disease was spread by the consumption of meat, nervous tissue, or internal organs between members of the same species.
- 4. kuru Kuru, native to humans in Papua New Guinea, was spread from human to human via ritualistic cannibalism. BSE, originally detected in the United Kingdom, spread between cattle by the practice of including cattle nervous tissue in feed for other cattle. Individuals with kuru and BSE show symptoms of loss of motor control and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death. Kuru was controlled by inducing the population to abandon its ritualistic cannibalism.
- 5. Bovine spongiform encephalopathy (BSE) On the other hand, BSE was initially thought to affect only cattle. Cattle that died of BSE had developed lesions or “holes” in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak, however, it was shown that a similar encephalopathy in humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired from eating beef from animals with BSE, sparking bans by various countries on the importation of British beef and causing considerable economic damage to the British beef industry. BSE still exists in various areas. Although a rare disease, individuals that acquire CJD are difficult to treat. The disease spreads from human to human by blood, so many countries have banned blood donation from regions associated with BSE.
- 6. Pathogenesis The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural variant of a normal cellular protein called PrP (prion protein). It is this variant that constitutes the prion particle. PrP exists in two forms: PrPc, the normal form of the protein PrPsc, the infectious form Once introduced into the body, the PrPsc contained within the prion binds to PrPc and converts it to PrPsc. This leads to an exponential increase of the PrPsc protein, which aggregates. PrPsc is folded abnormally; the resulting conformation (shape) is directly responsible for the lesions seen in the brains of infected cattle.
- 8. Viriods Viriods are a group of virus-like pathogens much smaller and distinctly different from viruses, consists solely of small single stranded circular RNA molecule and have no protein coat.
- 9. Structure Viriods are covalently closed, circular, ssRNAs, about 250-370 nucleotides long. The circular RNA normally exist as a rod-like shape due to intrastrand base pairing, which forms double stranded regions with single-stranded loops. The RNA of a viriod doesn’t encode any gene products;so they can’t replicate themselves. It is believed to adopt the host RNA Polymerase for this purpose which synthesizes a negative RNA strand. Then it serves as a template for the synthesis of the other viriod RNAs
- 10. DISEASES Caused BY VIRIODS Viriods causes over 20 different plant diseases of which the most studied are 1. Potato spindle-tuber disease 2. Exocortis disease of citrus trees 3. Chrysanthemum stunt disease The only human disease caused by viriods is Hepatitis-D
- 11. Pathogenicity Though the pathogenicity of viriods is not well understood It is presumed that viriods cause disease by triggering a eukaryotic response called RNA SILENCING, which normally functions to protect against infection caused by double stranded viruses, by degrading it. Viriods may usurp this response by hybridizing to specific host mRNA molecules to which they have a complementary sequence forming Hybrid viriod:host mRNA ds molecule which will elicit RNA silencing. This results in destruction of the host message and thus, silencing of the host gene. This leads to disease in the host plant.
- 12. Potato spindle tuber viriod Potato Spindle Tuber Viroid (PSTV) is the first recognized viroid. PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons. The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing. Under the electron microscope, purified but denatured PSTV appears as short strands about 50 nm long and thickness of double stranded DNA PSTVd is a small, circular RNA molecule. The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Lycopersicon esculentum). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance.
- 13. Hepatitis D Hepatitis D is a disease caused by a viriod called Hepatitis delta virus(HDV). HDV is considered to be a subviral satellite because it can propagate only in the presence of HBV. The HDV genome exists as an enveloped negative sense, single stranded, closed circular RNA. Because of a nucleotide sequence that is 70% self-complementary, the HDV genome forms a partially double stranded RNA structure that is described as rod-like. With a genome of approximately 1700 nucleotides, HDV is the smallest viriod known to infect animals.