Prions are small proteinacious, infectious particle that causes transmissible spongiform encephalopathies (TSEs). Prions cause a number of diseases in animals that include Mad Cow Disease, Kuru, Fatal familial insomnia, and chronic wasting disease. In this article, prions and diseases associated with them will be discussed. More emphasis will be given to Mad Cow Disease and Chronic Wasting Disease. The history of prion discovery will also be reviewed in this article.
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tonsil biopsy in variant CJD

Image: “Tonsil biopsy in variant CJD. Prion protein immunostaining.” by Sbrandner – Own work. License: GFDL


Infectious Protein With No Nuclear Acid

A prion is considered an abnormally folded protein that can cause progressive neurodegenerative diseases. They are not able to multiply inside their host organism; however, they affect the brain structure by inducing conversion of proteins with normal folding to an abnormal prion form. The more mis-folded proteins are present, the rate of conversion of normal proteins to abnormal ones become exponential.

Prions are smaller than viruses and can only be seen using an electron microscope as a cluster. Because prion particles are just abnormal versions of normal proteins, they are hard to recognize by the immune system and so they do not trigger any immune response unlike other pathogens.

Pathogenic prions are conformational isoform of a normal host protein PrPc. This protein is found predominantly in the outer surface of neurons. What prions do is to convert the normal PrPc to its abnormal form PrPsc. The more PrPsc present, the faster is the infection spread within the organism

Prions in Medical History

In the 1960s, scientists showed that the agents causing transmissible spongiform encelopathy in sheep, also called scrapie, was highly resistant to ionizing radiation and UV rays. It was noted that these two therapies are the ones usually employed to destroy pathogens containing nucleic acids. Because of these, several suggestions were made as to what type of particle is causing the disease. What the scientists are certain about is that these disease-causing particles are not dependent on nucleic acids.

In 1982, Stanley Prusiner of University of California described the disease-causing particle as a protein. This property of the agent discovered distinguishes it from the other infectious agents like viruses, bacteria and fungi. Because of it being small, proteinaceous, and infectious, a term was developed and the agent was eventually called Prion.

Transmissible Spongiform Encephalopathies (TSEs)

Transmissible spongiform encephalopathies are diseases caused by prions and involve neurodegeneration. Most of the time, these diseases are asymptomatic and become fatal when clinical signs already developed.

Transmission of this disease occurs when healthy animals consume tissues containing the disease causing particles of infected animals. Prions cannot be transmitted through casual contact and inhalation. They may be transmitted also using contaminated medical instruments.

Different animals suffer from different prion diseases. One of the most common prion diseases is bovine spongiform encephalopathy (BSE), also known as Mad Cow Disease. For deers, elk and moose, a common prion disease is CWD or chronic wasting disease. For felines, feline spongiform encelopathy is a common prion disease.

Prion diseases are also common in humans. These diseases include the Creutzfeldt-Jakob Disease (CJD), Kuru, fatal familial insomnia (FFI), Gerstmann-Straussler Syndrome (GSS) and variant CJD (vCJD).

Diseases caused by prions are serious diseases because of the difficulty of detecting the conditions because symptoms only show when the infection is already severe. Unlike most diseases caused by other pathogens, infections caused by prions are untreatable. Worse, these diseases cause invariable fatal neurodegeneration.

There are four features in the tissue damage caused by prions to humans. These include neuronal loss, amyloid plaque formation and astrocytosis. The most common feature is the formation of spongiform where the brains of infected individuals appear to have sponge-like holes.

Because of the tissue damages caused by the prions, infected individuals suffer from psychomotor dysfunctions like involuntary jerking movements, mental impairments, and loss of ability to speak. Other clinical signs of prion infections in humans include personality changes, insomnia, depression, lack of coordination, and ataxia.

Scrapie

Scrapie is a neurodegenerative prion disease that affects sheep and goats. It is one type of TSEs that involves converting normal proteins in sheeps and goats to copies of the prion particles.

Scrapie is transmitted between animals via the environment or directly. Classical scrapie is transmitted through exposure to scrapie-infected animals or contaminated environment. Pregnant female carrying the agent transfer the disease to its offspring. Eating and drinking the prion-rich fluid and tissues from the placenta may also cause infection in healthy animals.

Cases of scrapie occurred in many countries around the world. Two of the countries recognized to be scrapie-free are New Zealand and Australia. Countries develop programs and strategies to reduce occurrence of this disease.

Scrapie is one type of disease that slowly develops. The clinical signs of this disease are only evident in older animals. Animals suffering from this disease show changes in behavior like tremors, incoordination, aggression and abnormal gaits. When these symptoms appear, the infected animal typically dies within a few months. There is no treatment currently available for this disease.

Types of Spongiform Encephalopathies (SE)

Spongiform encephalopathies are various degenerative diseases of the brain that are characterized by the development of porous, spongelike lesions in the tissues of the brain leading to deterioration of neurological functions.

There are three types of spongiform encephalopathies. These include infectious or transmissible spongiform encephalopathy, sporadic spongiform encephalopathy, and familial spongiform encephalopathy.

Infectious or transmissible SEs

Infectious or Transmissible SEs are the types of SEs, which can readily be transmitted by direct exposure to the prions causing the diseases. This accounts for most of the prion diseases reported. Direct transmission includes consumption of tissues of other animals containing the infectious agent.

Forced cannibalism is one of the causes of the fast transmission and spread of the bovine spongiform encephalopathies (BSE) or the Mad Cow disease. Cows were forced to consume animal by products as protein supplements. When sheep with scrapie was used as the material to produce the supplement, it allowed the scrapie proteins to survive and infect also cows. This practice was eventually stopped because of the threat that it may also infect eventually humans who consume beef from infected cows.

Sporadic spongiform encephalopathies

Sporadic spongiform encephalopathies are types of SEs which randomly occur in the population. The cause of the infection is unknown and the disease just occurs spontaneously in the brain. This commonly occurs to people between the age of 45-75. An example of this is the sporadic Creutzfeldt-Jakob disease. Sporadic CJD affects only 1-2 per million of humans worldwide. This type of CJD is not transmittable to others.

Familial spongiform encephalopathies

Familial spongiform encephalopathies are types of SEs, which are transferred via inherited mutations of the PRNP gene. This affects a small percentage of families. The PRNP gene contains instruction to make the prion protein (PRP). Point mutations in this gene cause production of the abnormal protein PrPsc. The disease is inherited in an autosomal dominant pattern which means one copy of the altered gene is needed from both parents to cause the disorder.

Review Questions on Prions

The answers are below the references.

  1. What was discovered by Stanley Pusiner which he described as proteinateous infectious agent that caused scarpie in sheep?
    1. Viruses
    2. Bacteria
    3. Viroids
    4. Prions
  2. Which of the following is not caused by prions?
    1. Mad Cow Disease
    2. Kuru
    3. Psoriasis
    4. Gerstman Straussler Syndrome
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