The Types of Prion Diseases

What are Prion Diseases?

Prion diseases (PrDs) are triggered by misfolded proteins called prions. PrDs are neurodegenerative diseases with symptoms including brain disorder and dementia, which often ultimately lead to death. Newly formed prions are uniquely troublesome because they can misfold nearby healthy proteins. These diseases are extremely hard to identify and cure because the prions often damage the brain’s core functions before symptoms can even begin to show. Although prions are simple proteins with no genetic material like DNA or RNA, they are highly resistant to extreme conditions. One paper describes prions as “remaining active at temperatures exceeding 600°C and resisting conventional sterilization methods such as autoclaving”1. Autoclaving uses heat and steam to destroy microorganisms such as bacteria, viruses, and fungi, but it cannot completely remove prions.

PrDs, which affect both animals and humans, fall into three major categories of causation: sporadic, genetic, and acquired. The most common form of PrD is sporadic, where prions appear randomly. Genetic PrDs occur when a mutated form of the PRNP gene, which encodes prion proteins, gets passed down. Acquired PrDs, on the other hand, come from external sources as they are transmitted by contaminated food or medical equipment; transmission is uncommon but it is still a major cause of serious outbreaks. Although there are no fundamental cures to these diseases, scholars are researching ways to prolong the lives of patients by slowing down the progression of symptoms.

Creutzfeldt-Jakob Disease

First discovered in the 1920s, Creutzfeldt-Jakob disease (CJD) is a type of PrD that is only found in humans. Its symptoms vary from behavioral changes to mental and motor impairment. CJD is a fatal disease that progresses quickly. In fact, roughly 70% of people with CJD die within one year of getting the disease3.

Although most cases of CJD are sporadic, CJD outbreaks are also often caused by the reuse of contaminated surgical equipment and transplantation of tissue containing prions, a testament to the poor medical practices of some clinics. However, because of CJD’s long latency period, which is estimated to be 10 years4, the detection of the source of transmission is delayed. New medical practices such as replacing the transplantation of pituitary glands with DNA-derived hormones avoid prion contamination, reducing acquired CJD prevalence.

Bovine Spongiform Encephalopathy (BSE), also referred to as "mad cow disease," is a variant of CJD that led to a major outbreak in the U.K. in the 1980s and 1990s. Scientists believe that the cause of this disease was cattle being fed prion-infected meat. The impact of this epidemic was devastating as “more than 184,000 cows in the U.K. died from BSE between 1986 and 2015”5. To combat these outbreaks among animals, regulators imposed widespread bans on recycling spinal columns and infected livestock tissue into animal feed, an action that drastically reduced the number of BSE cases reported.

Fatal Familial Insomnia

Fatal Familial Insomnia (FFI) is a rare genetic PrD that damages the thalamus, a part of the brain that regulates sleep. Its symptoms include sleep difficulties, memory loss, and muscle twitching. FFI is a life-threatening hereditary disease that leads to progressive brain degeneration. The prions drive the nervous system into a state of overactivity, inducing anxiety, high blood pressure, and increased heart rate. In addition, since sleep plays a vital role in the human body, the lack of sleep caused by FFI negatively affects one’s overall well-being as the brain is unable to recharge the energy required for proper bodily function.

The Kuru Epidemic

Cannibalism is known to be the origin of the Kuru epidemic, which “wiped out about 30% (at least 3000 individuals) of the Fore people of Papua New Guinea in the 20th century”1. This epidemic started among the Fore people, an ethnic group of New Guinea, who practiced cannibalism as part of their funerary rituals. The consumption of the brains of their ancestors was a holy act of commemorating their lives and passing down their wisdom to younger generations. However, this tradition triggered the transmission of prions to the bodies of the Fore people, spreading the disease. Although the ritual was discontinued by the Australian government during the 1950s and 1960s, new cases continued to be reported because of kuru’s long latency period, which ranged from approximately five to forty years.

The Search for a CJD Cure

Researchers Sonia Vallabh and Eric Minikel found out that the introduction of small interfering RNA (siRNA) can be effective as a cure for prion diseases. The siRNA cure functions by cutting the messenger RNA that contains the genetic instructions for making prion proteins, preventing or delaying the misfolding of proteins. Minikel explains that they “want to go upstream and target the RNA molecule that makes the protein” because the cells that create prions are hard to detect7. This treatment is entering clinical trials, and Vallabh and Minikel hope that their research data will contribute to new discoveries in the whole field.

Although prion diseases are rare, their highly resistant nature has troubled the pursuit for an effective treatment. Researchers continue to seek approaches that could provide a cure for these diseases for patients in the near future.