Prion diseases are a newly discovered type of disease caused by strange infectious proteins. The best known disease is "mad cow disease" but there are in fact several diseases that appear to be caused by prions. Almost all of these diseases affect the brain.
Prions are infective proteins. They act somewhat like viruses but do not possess any DNA or RNA material, but appear to be only a protein. This idea is somewhat controversial still, but seems to be gaining acceptance.
The most media attention to prions has been due to the "mad cow disease" in animals, and the subsequent "variant CJD" that occurs in humans. However, there are numerous diseases believed to be related. The main diseases believed to be in the class of prion diseases include:
- Animal prion diseases:
- Bovine spongiform encephalopathy (BSE): the proper name for mad cow disease, causing brain damage.
- Scrapies: in sheep (and other animals), caused by brain damage, leading to the sheep itching and scraping off their coats, hence the name.
- Feline spongiform encephalopathy - in cats
- Others: transmissible mink encephalopathy (TME), chronic wasting disease (CWD) of mule deer and elk
- Human prion diseases:
- Creutzfeldt-Jakob Disease (CJD): a rare 1-in-a-million disease that occurs spontaneously, with 10-15% inheritance, and unrelated to animal diseases. It usually leads to dementia.
- variant CJD: a variant of CJD that may be related to BSE
- Kuru: a brain damaging "laughing disease" in Papua New Guinea, where till recently, tribes ate brains of the dead.
- Gerstmann Sträussler Syndrome (GSS): also called Gerstmann-Sträussler-Scheinker disease; inherited, mainly in midlife.
- Fatal familial insomnia (FFI): a strange inherited disease where people actually die from being unable to fall asleep, usually occurring late in life. Unclear: may be caused by prions, or FFI may be an "opposite" to other prion diseases, possibly caused by a deficiency of good prions, rather than an overload of bad prions.
- Alpers Syndrome: prion diseases in infants.
Contagion of Prion Diseases
Prions are not highly contagious. They are infective via injection of contaminated products, but not by air or touch. In other words, they are transmitted only via body fluids that contain the protein from within the blood or more commonly fluids from the infected brain. For example, CJD has also been transmitted by mistakes in surgery and health procedures, especially brain surgery Prusiner 1 mentions that human CJD has been transmitted by "corneal transplantation, implantation of dura mater or electrodes in the brain, use of contaminated surgical instruments, and injection of growth hormone derived from human pituitaries (before recombinant growth hormone became available)".
Genetics of Prion Diseases
Prion diseases are inherited in some percentage of cases, but not all cases. Hence, they are clearly not pure genetic diseases.
Theories of Prion Infection
There is a lot of research about prions and their related diseases. Some of the possible theories include:
- Protein-based infection by conformation: where a "bad" prion converts existing normal good proteins in the body into prions, so they increase by conformation or conversion of other proteins, not by replication.
- Viruses rather than prions: some researchers still dispute the idea of prions as infective proteins, and claim that some kind of virus is actually involved. This seems doubtful given the difficulty in finding DNA or RNA material in infected samples, but is not yet out of the question.
- Copper-Manganese theory: manganese overload, copper deficiency. A controversial theory about BSE (mad cow disease) that was popularized by farmer-amateur-chemist Mark Purdey. Prions are "metalloglycoprotiens" that should bind copper, but may bind manganese instead if copper is deficient and managanese is excessive. Prions have been shown in laboratories to become dangerous when exposed to high manganese and low copper. There are also numerous amounts of circumstantial evidence of the role of manganese and copper. The UK farmers were treating cows with an insecticide containing organophosphates which deplete copper, and were also feeding cows with supplementary chicken manure which is high in manganese due to chicken feed supplementation. There is also evidence of high manganese levels in areas of both variant-CJD cases (BSE-related) and areas of high CJD rates (non-BSE related). Manganese is also less easily absorbed by pigs and chickens, which do not seem to get BSE. Furthermore, manganese miners in times past used to suffer a "manganese madness" disease that seems similar to CJD. There are various conspiracy theories stating that a coverup has started to prevent lawsuits against the insecticide makers, and perhaps even against the makers of other organophosphate-containing products for humans, such as some head lice and scabies medications. In the worse case, such medications might be related to later neurodegeneration and even Alzheimer's disease.
- Other trace element theories: with the popularization of the copper-manganese theory, various researchers are examining other trace elements. Possibly implicated in the diseases BSE and CJD are not only copper and manganese, but also iron, selenium, and zinc.
Why Do Prions Kill?
Why do these prion diseases kill animals and people? What causes the damage from the prion infection? The brains of victims are clearly riddled with "encepalopathy", spongy swellings within the brain, but is this the actual cause of death, a side effect itself, or even unrelated? As always there are several theories and possibilities:
- Prion clumps cause brain cell damage: perhaps too many bad prions building into spongy deposits create stress within cells, notably brain cells, and eventually damage and kill these brain cells. The idea is that prion clumps become like tumors and damage good cells.
- Prions deplete good proteins: in this theory the prions and their spongy structures are harmless, and brain cells are actually damaged because they are depleted of good prion-like proteins. Brain cells make these good proteins for a reason, and cells die without them, but we don't currently know exactly what these good proteins are needed for. Perhaps the problem with bad prions is that they convert the good proteins into bad prions, thus depleting the needed supply of good proteins. The brain cells would then continually try to create new good protieins, which would increasingly be converted to bad prions, which would explain why there are large deposits of prions. Note also that this theory is not entirely contradictory to the prion damage theory, since the prion clumps might be created through this process of the cells trying to create more good prion-like proteins.
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