Prion

• 1. Introduction

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  A prion is a misfolded protein, but that simple description conceals something deeply unsettling: it can force other proteins of the same type to misfold, spreading corruption through the body without carrying a single strand of DNA or RNA. That fact alone set the scientific world on edge when it was first proposed. Every other infectious agent known to medicine, every virus, every bacterium, every fungus, depends on genetic material to replicate. Prions do not. They hijack the body's own proteins and turn them into agents of destruction. The diseases they cause attack the brain, are always fatal, and have no effective treatment. How does a protein, one of the most fundamental building blocks of life, become a killer? And what does its existence tell us about the other diseases quietly destroying brains around the world?

• 2. A Name Born in 1982

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  Stanley B. Prusiner of the University of California, San Francisco, announced in 1982 that his team had purified the hypothetical infectious protein. The name he coined, prion, was short for "proteinaceous infectious particle". In his 1982 paper, Prusiner specified that it should be "pronounced pree-on". The announcement came before the protein was fully isolated; that took another two years after Prusiner's initial claim. For his research, Prusiner was awarded the Nobel Prize in Physiology or Medicine in 1997. The road to that prize, however, ran through decades of skepticism, because the idea that a protein alone could be infectious contradicted one of biology's most fundamental assumptions.

  In the 1960s, two London-based researchers had laid the groundwork. Radiation biologist Tikvah Alper and biophysicist John Stanley Griffith developed the hypothesis that transmissible spongiform encephalopathies are caused by an infectious agent consisting solely of proteins. Their motivation was a puzzling experimental finding: the mysterious agent causing scrapie and Creutzfeldt-Jakob disease resisted ionizing radiation, which normally destroys nucleic acids. Griffith proposed three distinct ways a protein could act as a pathogen. His second hypothesis, that an abnormal form of a cellular protein can convert normal proteins of the same type into its own abnormal form, became the foundation of modern prion theory. Francis Crick noted in the 1970 second edition of his "central dogma of molecular biology" that Griffith's hypothesis represented a potential contradiction to the flow of information from protein back to protein. History sided with Griffith.

• 3. The Protein Itself

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  PrP, the prion protein, exists in every healthy human body. The normal form, called PrPC, sits on the membranes of cells and has 209 amino acids in humans, one disulfide bond, and a molecular mass of 35. It binds copper(II) ions with high affinity, and this property is thought to underpin its anti-oxidative function. It also plays a role in cell-cell adhesion and intracellular signaling, and may be involved in cell-cell communication in the brain. Platelets constitute the largest reservoir of PrPC in human blood components.

  The infectious form, PrPSc, takes its name from scrapie, one of the first diseases linked to prions. Where PrPC is mainly alpha-helical in structure, PrPSc has a higher proportion of beta-sheet structure. That structural shift is everything. PrPSc always causes prion disease when present. Several highly infectious, brain-derived PrPSc structures have been discovered by cryo-electron microscopy, including a fibril structure isolated from humans with Gerstmann-Straussler-Scheinker syndrome. In the prion amyloids, individual PrP molecules stack via intermolecular beta sheets, building fibers whose ends act as templates onto which free protein molecules attach and are refolded into the infectious conformation. Under most circumstances, only PrP molecules with an identical amino acid sequence to the infectious PrPSc are incorporated into the growing fiber, though cross-species transmission does happen rarely.

• 4. How Prions Spread and Replicate

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  The primary method of prion infection in animals is ingestion of PrPSc. Prions may be deposited in the environment through the remains of dead animals and through urine, saliva, and other body fluids. Once in the soil, they can linger by binding to clay and other minerals. A University of California research team provided evidence that infection can occur from prions in feces. Because animal excrement is present near water reservoirs and manure fertilizes crop fields, the possibility of widespread environmental transmission is a genuine concern. In 2015, researchers at The University of Texas Health Science Center at Houston found that plants can serve as a vector: hamsters fed grass grown on ground where a deer died with chronic wasting disease became ill with CWD.

  At the molecular level, prion replication follows a fibril model. Fibril ends bind PrPC and convert it into PrPSc. If fibrils only grew longer, the quantity of prions would increase linearly. What scientists actually observe is exponential growth. The explanation is fibril breakage: when a fiber breaks, two growing ends become four, allowing the infectious population to expand rapidly. The exponential growth rate depends largely on the square root of the PrPC concentration, and data from transgenic mice match this mathematical prediction. Researchers at Dartmouth College discovered that cofactor molecules, including the phospholipid phosphatidylethanolamine and polyanions such as single-stranded RNA, are necessary to form PrPSc molecules with high levels of specific infectivity in vitro. Protein-only PrPSc molecules appear to lack significant biological infectivity without these cofactors.

• 5. The Diseases They Cause

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  Scrapie in sheep may be the oldest recorded transmissible spongiform encephalopathy. In the 18th and 19th centuries, the exportation of sheep from Spain was observed to coincide with the disease, which caused affected animals to lie down, bite at their feet and legs, rub their backs against posts, fail to thrive, stop feeding, and finally go lame. In humans, the prion diseases include Creutzfeldt-Jakob disease in its several forms, Gerstmann-Straussler-Scheinker syndrome, fatal insomnia, kuru, and familial spongiform encephalopathy. Variant Creutzfeldt-Jakob disease is thought to be caused by the prion responsible for bovine spongiform encephalopathy in cattle, transmitted through infected meat. Chronic wasting disease affects white-tailed deer, elk, mule deer, and moose.

  All known prion diseases are untreatable and fatal. The incubation period in humans is relatively long, running from 5 to 20 years or more, but once symptoms appear the disease progresses rapidly. Neurodegenerative symptoms can include convulsions, dementia, ataxia, and behavioral or personality changes. Prions destroy the brain by forming plaques of amyloid that disrupt normal tissue structure, producing a characteristic spongy appearance with holes in the tissue caused by vacuole formation in neurons. This is accompanied by astrogliosis and, notably, an absence of inflammatory reaction, which distinguishes prion pathology from many other infectious diseases. In the early 1950s, Carleton Gajdusek began research that eventually showed kuru could be transmitted to chimpanzees, work for which he received the 1976 Nobel Prize.

• 6. Sterilization and Containment

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  Prions resist proteases, heat, ionizing radiation, and formaldehyde treatments. This makes them a serious challenge for hospitals and laboratories. Ordinary sterilization procedures that destroy viruses and bacteria cannot be assumed to neutralize prions. Effective decontamination requires protein hydrolysis or the destruction of protein tertiary structure. Sodium hypochlorite, sodium hydroxide, and strongly acidic detergents such as LpH are among the agents that work.

  The World Health Organization has issued specific protocols for heat-resistant surgical instruments. One procedure calls for immersing instruments in 1N sodium hydroxide and then placing them in a gravity-displacement autoclave at 121 degrees Celsius for 30 minutes, followed by cleaning, rinsing, and routine sterilization. Another calls for sodium hypochlorite at 20,000 parts per million available chlorine for one hour, followed by autoclaving at 121 degrees Celsius for an additional hour. Heating at 134 degrees Celsius for 18 minutes in a pressurized steam autoclave has been found to be somewhat effective. Ozone sterilization has been studied as a potential approach, alongside thiourea-urea treatment, guanidinium chloride treatment, and special heat-resistant subtilisin combined with heat and detergent. A complicating factor is that a method sufficient for sterilizing prions on one material may fail on another. Partially denatured prions can be renatured to an infective status under certain artificial conditions, meaning even degraded material requires caution.

• 7. Prions Beyond the Brain

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  Genetic susceptibility shapes who develops prion disease. The majority of human prion diseases are classified as sporadic Creutzfeldt-Jakob disease. A polymorphism at codon 129 in the PRNP gene has been linked to susceptibility. Individuals with a homozygous methionine/methionine genotype at this position are approximately five times more likely to develop sporadic CJD than those with a heterozygous methionine/valine genotype.

  The relevance of prions extends well beyond the diseases named for them. In 2015, a prion form of alpha-synuclein was linked to multiple system atrophy, expanding the category of prion diseases beyond PrP for the first time. Misfolded proteins operating by a prion-like mechanism have been implicated in Alzheimer's disease, Parkinson's disease, ALS, frontotemporal lobar degeneration, and Huntington's disease. The protein TDP-43, an RNA-binding protein, forms cytoplasmic inclusions in neurons in ALS patients; its misfolding is largely directed by a prion-like domain. Bioinformatic screens have predicted that over 250 human proteins contain prion-like domains. Of the 210 known proteins with an RNA recognition motif, 29 also carry a putative prion domain. A 2006 article from the Whitehead Institute for Biomedical Research found that PrP expression on stem cells is necessary for self-renewal of bone marrow, and that hematopoietic tissues with PrP-null stem cells show increased sensitivity to cell depletion, a reminder that the same protein causing fatal disease also serves essential functions in the healthy body.

• 8. Fungal Prions and Treatments

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  Proteins showing prion-type behavior are found in some fungi, and they have proven useful for understanding mammalian prions. Fungal proteins that exhibit templated structural change were discovered in the yeast Saccharomyces cerevisiae by Reed Wickner in the early 1990s. A prion has also been found in the fungus Podospora anserina. Unlike their mammalian counterparts, fungal prions are generally nontoxic to their hosts. Susan Lindquist's group at the Whitehead Institute argued that some fungal prions are not associated with any disease state and may have a useful role, while researchers at the NIH offered arguments suggesting they could be considered a diseased state. Within yeasts, prions can act as vectors of epigenetic inheritance, transferring traits to offspring without any genomic change. Purified protein extracted from cells with a prion state has been demonstrated to convert the normal form of the protein into a misfolded form in vitro, lending strong support to the protein-only concept.

  As of 2018, there are no effective treatments for prion diseases. Clinical trials in humans have not met with success and have been hampered by the rarity of the diseases. One treatment that prolongs the incubation period in lab mice failed in human patients diagnosed with variant Creutzfeldt-Jakob disease. Another treatment attempted in six human patients, all of whom died, showed no significant increase in lifespan, though autopsy results suggested the drug was safe and reached what researchers described as "encouraging" concentrations in the brain and cerebrospinal fluid. Drug design research points toward a specific target: a drug that binds to fibril ends and blocks further growth could slow the exponential expansion of prions using the lowest possible dose, since an effective drug would not need to eliminate all prions, only reduce their growth rate.