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Prions

Prion diseases, also known as transmissible spingiform encephalopathies, are a group of degenerative brain disease capable of transmission between individuals or by inoculation or ingestion of disesaes brain or other tissues. These diseases include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle and Creutzfeld-Jakob disease (CJD), kuru, Gerstmann-Straussler-Scheinker syndrome (GSSS) and iatrogenic TSE in humans. (Iatrogen means caused by physicians).

They are called transmissible spongiform encephalopathies because of their capacity to produce a degeneration of brain tissue leaving such hles in the brain that the tissue appears like a sponge. Because of their long incubation period, ranging from months to decades, and high resistance to inactivation by irradiation, heat, and harsh chemical treatment they have often been referred to as slow or unconvenial viruses. However, we now know that prions are not slow viruses; they are a new form of agent in which the prion protein with the disease configuration is able to convert normal prions to the diseased structure. No effective therapies exist for clinical TSE in humans. There are 3 types of clinical diseases:

  • Infectious TSE: this is kuru and iatrogenic TSE. Kuru occurs from the handling of brain tissue of relatives who died of kuru. Iatrogenic TSE cases have been induced by transplantation of corneal tissue from patients with TSE, or by neurosurgery using incompletely sterilized instruments.
  • Sporadid TSE: sporadic Creutzfeld-Jakob disease is the main clinical syndrome that falls into this category. It affects 1-2 million people worldwide.
  • Familial TSE: families TSE are associated with presence of an autosomal dominant genetic alteration of the PrP gene. The primary clinical feature is either dementia (CJD) or cereballar ataxia (GSSS).

Scrapie has been recognized as a disease in sheep in Europe for over 200 years. The scrapie extract was shown to be filterable, like a virus. Transmissibility as accidentally shown in 1943 when a population of Scottish sheep was inoculated against a common virus with a formalin extract of lymphoid tissue unknowingly derived from an animal with scrapie. After 2 years, nearly 10% of the flock developed scrapie.

It is now known that prions exist as normal cellular membrane proteins. In scrapie, a diseased form of the prion is able to interact with the normal form and convert it to the diseased form. In this way, the Scrapie-type prions are able to propagate themselves.

Mad Cow Disease

In the U.K., cattle had been fed with animal remains for years. In 1986, Bovin Spongiform Encephalopathy (BSE) was discovered in a herd of cows. As of November 2000, 177,500 cases of BSE had been discovered. The prion disease in humans acquired from contaminated beef has been called the new variant of Creutzfeldt-Jakob Disease (nvCJD). While regular CJD is a spontaneous mutation of the normal cellular gene that affects old people, nvCJD is observed in patients of all ages and in fact has a median age of death of 27.5 years. The brain tissue of these patients has kuru-type amyloid plaques and hig levels of the diseased form of PrP as measure immunohistochemically. Since 1990, the U.S. department of agriculture has examined hundreds of cattle brains each year from animals showing neurological signs on the farm or at slaughter. So far, no BSE has been detected in the U.S. scrapie has been observed in sheep flocks throughout the world, except for Australia and New Zealand.

Prions experiments

Does Scrapie Agent Contain DNA or RNA?

An assay was developed to transfer scrape from sheep extracts to mice. This assay took a very long time to complete. It was subsequently shown that Syrian Hamsters are more sensitive hosts, and in fact there is a species barrier to transmission.

  • Exposing scrapie extracts to UV or ionizing radiation did not reduce infectivity, suggesting that scrapie was devoid of nucleic acid.
  • Exposing agent to DNAse and to RNAse did not reduce infectivity, confirming lack of nucleic acid.
  • The transmissible agent was destroyed by reagents that affected protein, such as proteases and extraction with phenol.
Where was the DNA encoding PrP? Was it in the particle or cellular DNA?

Pruisner purified scrapie agent to homogeneity and obtained a partial amino acid sequence of the amino terminal end of the protein. Using this sequence he constructd a degenerated oligonucleotide and used this to obtain a cDNA clone for the gene. He used this sequence (the predicted mRNA) as a probe and found that the DNA encoding prions is found in the cellular genome, not wrapped around the prion protein.

Amino Acid Order:123456789101112131415
Amino Acid:GlyGlnGlyGlyGlyThrHisAsnGlnTrpAsnLysProSerLys
Predicted mRNA (5'-3'):GGNCARGGNGGnGGnCANCAYAAYCARUGGAAYAARCCNUCNAAR
Do diseased prions differ in sequence from normal prions?

Finding the cDNA clone for scrapie was very useful. It was observed that there is a normal cellular gene encoding prions, hereafter denoted PrPc, contained in every cell. The mutant form of this gene is denoted PrPsc. Results: from these studies, it was observed that the human disease was based on an inherited mutation in the prion DNA sequence. Thus, both patients with GSS and CJD contained mutations in DNA encoding prions. It is conceivable that other many years, these mutated forms of the prion are able to convert themselves into the diseased form more than normal cellular prion forms. Pruisner and other showed prions' mechanism of infectivity using transgennic mice. In these studies, the normal cellular gene is denoted PrPc and the diseased form as PrPsc.

How does the GSS mutation behave in mice?

It was known that many GSS patients contain an amino acid substitution at position 102. Results: when transgenic mice were maade containing this gene, the mice spontaneously developed spontaneous prion disease. Importantly, brain homogenates from these mice can transfer prion disease when inoculated into transgenic mice expressing a low level of the same mutant protein otherwise unable to develop the disease.

Does conversion require an endogenous prion gene?

Logically, prions act upon a substrate. Results: When the PrPc gene was ablated, mice were resistant to prion disease and were unable to generate new infectious particles.

How does sporadic CJD cause disease?

In sporadic CJD, most patients are homozygous for a polymorphism at amino acid residue 129. In other words, these patients all have either both methionine or both valine at a position where most people have 2 completely different amino acids. Similarly, nvCJD patients also are homologous at this same site. Results: these and other mouse studies suggest that the region of amino acids 121-131 serve as a nucleation point for conformational chnge during conversion to the PrPsc form.

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