Creutzfeldt–Jakob disease in the context of "Myoclonic"

A prion (/ˈpriːɒn/ ) is a misfolded protein that induces misfolding in normal variants of the same protein, leading to cellular death. Prions are responsible for prion diseases, known as transmissible spongiform encephalopathy (TSEs), which are fatal and transmissible neurodegenerative diseases affecting both humans and animals. These proteins can misfold sporadically, due to genetic mutations, or by exposure to an already misfolded protein, leading to an abnormal three-dimensional structure that can propagate misfolding in other proteins.

The term prion comes from "proteinaceous infectious particle". Unlike other infectious agents such as viruses, bacteria, and fungi, prions do not contain nucleic acids (DNA or RNA). Prions are mainly twisted isoforms of the major prion protein (PrP), a naturally occurring protein with an uncertain function. They are the hypothesized cause of various TSEs, including scrapie in sheep, chronic wasting disease (CWD) in deer, bovine spongiform encephalopathy (BSE) in cattle (mad cow disease), and Creutzfeldt–Jakob disease (CJD) in humans.

In medicine, proteinopathy ([pref. protein]; -pathy [suff. disease]; proteinopathies pl.; proteinopathic adj), or proteopathy, protein conformational disorder, or protein misfolding disease, is a class of diseases in which certain proteins become structurally abnormal, and thereby disrupt the function of cells, tissues and organs of the body.

Often the proteins fail to fold into their normal configuration; in this misfolded state, the proteins can become toxic in some way (a toxic gain-of-function) or they can lose their normal function. The proteinopathies include such diseases as Creutzfeldt–Jakob disease (and a variant associated with mad cow disease) and other prion diseases, Alzheimer's disease, Parkinson's disease, amyloidosis, multiple system atrophy, and a wide range of other disorders. The term proteopathy was first proposed in 2000 by Lary Walker and Harry LeVine.

Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is an incurable and always fatal neurodegenerative disease of cattle. Symptoms include abnormal behavior, trouble walking, and weight loss. Later in the course of the disease, the cow becomes unable to function normally. There is conflicting information about the time between infection and onset of symptoms. In 2002, the World Health Organization suggested it to be approximately four to five years. Time from onset of symptoms to death is generally weeks to months. Spread to humans is believed to result in variant Creutzfeldt–Jakob disease (vCJD) or Creutzfeldt–Jakob disease (CJD). As of 2024, a total of 233 cases of vCJD had been reported globally.

BSE is thought to occur due to an infection by a misfolded protein, known as a prion. Cattle are believed to have been infected by being fed meat-and-bone meal that contained either the remains of cattle who spontaneously developed the disease or scrapie-infected sheep products. The United Kingdom was afflicted with an outbreak of BSE and vCJD in the 1980s and 1990s. The outbreak increased throughout the UK due to the practice of feeding meat-and-bone meal to young calves of dairy cows. Cases are suspected based on symptoms and confirmed by examination of the brain. Cases are classified as classic or atypical, with the latter divided into H- and L types. It is a type of transmissible spongiform encephalopathy.

The major prion protein (PrP) is encoded in the human body by the PRNP gene also known as CD230 (cluster of differentiation 230). Expression of the protein is most prominent in the nervous system but occurs in many other tissues throughout the body.

The protein can exist in multiple isoforms: the normal PrP form, and the protease-resistant form designated PrP such as the disease-causing PrP (scrapie) and an isoform located in mitochondria. The misfolded version PrP is associated with a variety of uniformly fatal neurodegenerative diseases in humans and nonhuman species. In nonhuman species these include ovine scrapie, bovine spongiform encephalopathy (BSE, mad cow disease), feline spongiform encephalopathy, transmissible mink encephalopathy (TME), exotic ungulate encephalopathy, chronic wasting disease (CWD) which affects deer; human prion diseases include Creutzfeldt–Jakob disease (CJD), fatal familial insomnia (FFI), Gerstmann–Sträussler–Scheinker syndrome (GSS), kuru, and variant Creutzfeldt–Jakob disease (vCJD). Similarities exist between kuru, thought to be due to human ingestion of diseased individuals, and vCJD, thought to be due to human ingestion of BSE-tainted cattle products.

Chronic wasting disease (CWD), sometimes called zombie deer disease, is a transmissible spongiform encephalopathy (TSE) naturally affecting members of the deer family. TSEs are a family of diseases caused by misfolded proteins called prions and include similar diseases such as mad cow disease in cattle, Creutzfeldt–Jakob disease in humans, and scrapie in sheep. In the United States, CWD affects mule deer, white-tailed deer, red deer, sika deer, elk, antelope, caribou, and moose. The transmission of CWD to other species such as squirrel monkeys and humanized mice has been observed in experimental settings.

In 1967, CWD was first identified in mule deer at a government research facility in northern Colorado, United States. It was initially recognized as a clinical "wasting" syndrome and then in 1978, it was identified more specifically as a TSE disease. Since then, CWD has been found in free-ranging and captive animal populations in 33 US states and five Canadian provinces. In addition, CWD has been found in one Minnesota red deer farm, one wild reindeer herd in Norway (March 2016) as well as in wild moose. Single cases of CWD in moose have been found in Finland (March 2018) and in Sweden (March and May 2019, September 2020). CWD was found in South Korea in some deer imported from Canada. CWD is typified by chronic weight loss and clinical signs compatible with brain lesions, aggravated over time, always leading to death.

Alien hand syndrome (AHS) or Dr. Strangelove syndrome is a category of conditions in which a person experiences their limbs acting seemingly on their own, without conscious control over the actions. There are a variety of clinical conditions that fall under this category, most commonly affecting the left hand. There are many similar terms for the various forms of the condition, but they are often used inappropriately. The affected person may sometimes reach for objects and manipulate them without wanting to do so, even to the point of having to use the controllable hand to restrain the alien hand. The occurrence of alien hand syndrome can be usefully conceptualized as a phenomenon reflecting a functional "disentanglement" between thought and action.

Alien hand syndrome is best documented in cases where a person has had the two hemispheres of their brain surgically separated, a procedure sometimes used to relieve the symptoms of extreme cases of epilepsy and epileptic psychosis, e.g., temporal lobe epilepsy. It also occurs in some cases after brain surgery, stroke, infection, tumor, aneurysm, migraine and specific degenerative brain conditions such as Alzheimer's disease, corticobasal degeneration and Creutzfeldt–Jakob disease. Other areas of the brain that are associated with alien hand syndrome are the frontal, occipital, and parietal lobes.