Prions: Definition & Examples

Instructor: Paula Jacoby-Garrett
In this lesson, discover the real life, disease-causing yet non-living prions. We will discuss their interesting history and their real life dangers, as well as the different types of diseases caused by prions.

What is a Prion?

What's smaller than a virus, isn't alive, and leaves your brain riddled with holes? It sounds like the start to a bad joke or a B-rated horror movie, but it is in fact real, and it gets worst.

We are talking about prions - a small piece of protein that isn't alive so isn't classified as an organism and doesn't have genetic material (i.e. DNA). It is considered the smallest infectious particle known. The term prion originated from Dr. Stanley Prusiner, who used the term as a shortened version of the name; proteinaceous infectious particle. He has done extensive work on prions and received the Nobel Prize for his work in 1997.

A Brief History of Prions

Our understanding of the disease started in the 1950s, when a visiting medical officer observed a native tribe in New Guinea eating the brains of their deceased relatives. What might sound disgusting and unthinkable to us, is considered an honor in this particular tribe. By eating the remains of an elder, you gain their wisdom. Individuals participating in this practice would become weak and uncoordinated and lose the ability to eat. This sickness, called Kuru, would eventually lead to death.

People of New Guinea that practiced the custom of cannibalism.
Kuru
Cannibalism spread the disease to other members of the tribe.
Cannibalism

When observing the brains of these victims, researchers found the brain was riddled with holes. They did not know what could cause this effect, but it was similar to a disease in sheep discovered in the 1700s, called scrapie, which also results in death and holes in the brain. To test the disease, researchers tried transmitting the disease to chimpanzees and found it not only could be transmitted, but it would cause a similar death.

Microscope image of brain tissue with open areas of holes where prions infected.
Histology

Fast-forward to the mid-1980s, and a new disease began affecting cattle in Europe. The cattle would begin to have uncoordinated movements and tremors. These cattle would become uncharacteristically aggressive, quit eating and eventually die. Inspections of the brain showed the characteristic holes throughout. The question arose - how did these cattle get this disease.

As common practice in the beef industry, cattle are fed rendered sheep parts to supplement their diet. The rendering process takes left over material of the animal slaughter process and processes it for use as a feed supplement. This practice spread the disease from sheep to cattle, and subsequently, to the people who ate the beef. This form of prion disease was called Mad Cow Disease for the aggressive traits the cattle obtained once infected.

By 2000, hundreds of thousands of cattle were affected, and sadly, 155 people died in Europe from eating tainted beef. Regulations and inspections were put in place to reduce, and hopefully, eliminate the risk of infection spread through the food industry.

Cow affected with Mad Cow Disease
Mad Cow

Who Does It Affect?

Prions affect mammals by affecting their nervous system and impairing brain function. A variety of brain diseases can occur, but they all end with death.

There are three types of prion diseases: familial, acquired and sporadic. The familial form is responsible for a small percent of cases (10-15%). It runs in families (is inherited), and those who carry the gene produce the prion protein. This type is manifested in the disease CJD or Creutzfeldt-Jakob Disease affecting a very small portion of adults over the age of 60 (approximately five in one million).

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