Transmissible spongiform encephalopathy

Tiny bits in the brain can change shape.

These bits make other bits change too. This makes the brain look like a sponge. It can make it hard to move. It can also make it hard to speak.

Many animals can get these diseases. Sheep, cows, and deer can get sick. Even humans can get sick. It is very rare for humans.

These diseases can be very fast. Some people stay sick for a long time. It is a very serious thing.

Scientists study these tiny bits. They want to learn how they work. It is a big mystery.

Some diseases change the way the brain works. These are called prion diseases. They are caused by tiny bits called prions. Prions are parts of a protein.

Normal proteins have a specific shape. But prions are misshapen. When a misshapen prion meets a normal protein, it changes that protein too. The new protein becomes misshapen as well. This starts a chain reaction. These bad proteins build up in the brain. This buildup harms the brain cells. It can make the brain look like a sponge under a microscope. We call this spongiform change.

Many animals can get these diseases. Sheep can get scrapie. Cattle can get BSE, or "mad cow disease." Deer and elk can get chronic wasting disease. Humans can get prion diseases too. One example is Creutzfeldt-Jakob disease. Most human cases happen for no known reason. Some cases come from genes passed down in families. Other cases come from contact with infected material. These diseases are very serious and always fatal. They make it hard to move, speak, or think.

Prion diseases are a group of very serious conditions. They affect the nervous system in many living things. This includes humans, cattle, and sheep. These diseases are progressive, which means they get worse over time. They are also incurable and always fatal.

Scientists once had an unusual idea about how these diseases spread. They found that the cause is not a virus or bacteria. Instead, the cause is a tiny, misshapen protein called a prion.

To understand how it works, we must look at proteins. Every cell makes a normal protein called PrP. These normal proteins have a specific, healthy shape. A prion is a version of this protein that is folded the wrong way. When a misshapen prion meets a normal PrP molecule, it forces that molecule to change shape too. This creates a chain reaction called a seeding process. Because cells keep making new proteins, the bad proteins build up in the brain. This buildup harms neurons, which are the cells in the nervous system. Eventually, the brain tissue develops tiny holes. These holes make the brain look like a sponge under a microscope.

Researchers spent many years studying these strange proteins. In the 1930s, Jean Cuillé and Paul-Louis Chelle showed that scrapie in sheep was infectious. Later, in 1959, William Hadlow saw similarities between scrapie and a human disease called kuru. D. Carleton Gajdusek then proved these diseases could move from humans to primates. His team reported on kuru in 1966 and Creutzfeldt-Jakob disease in 1968. Finally, Stanley Prusiner helped prove that prions are just proteins in 1982.

There are many different names for these diseases depending on the animal. In sheep, the disease is called scrapie. Cattle can get bovine spongiform encephalopathy, or BSE. People often call BSE "mad cow disease." In deer and elk, it is called chronic wasting disease. Humans can face diseases like kuru or Gerstmann-Sträussler-Scheinker syndrome. One common human version is Creutzfeldt-Jakob disease, or CJD. CJD can be sporadic, which means it happens for no known reason. It can also be hereditary, meaning it comes from a mutation in the PRNP gene.

Prion diseases are different from the common colds we usually catch. Most infections are caused by things with DNA or RNA. Prions are unique because they are made only of abnormal protein. You can think of them like a single broken piece in a machine. If that piece touches a working piece, it might break that one too. This can happen through eating infected food or through medical treatments. While these diseases are rare in humans, they show how much shape matters in biology. Even a tiny change in a protein can change how a whole body works.

Transmissible spongiform encephalopathies, or TSEs, are a group of serious conditions. These diseases affect the nervous system in many animals. This includes humans, cattle, and sheep. TSEs are progressive, meaning they worsen over time. They are also incurable and always fatal. They are often called prion diseases. These diseases cause the brain and nervous system to break down. This happens because of the behavior of specific proteins.

To understand TSEs, we must look at the prion protein, known as PrP. Most cells produce a normal form called cellular PrP, or PrPC. This protein has a specific, healthy shape. A prion is a misshapen version called PrPSc. PrPSc causes damage through a crystallization-like seeding process. When a misshapen PrPSc molecule meets a normal PrPC molecule, it forces the normal one to change shape. This creates a chain reaction of misfolding. Because cells constantly produce new PrP, the abnormal proteins build up. This accumulation harms neurons in the brain.

The damage to the brain creates very specific physical changes. One major sign is spongiform change. This means tiny holes, or vacuoles, appear in the central nervous system. Under a microscope, these holes make the brain tissue look like a sponge. Other changes include the death of neurons and gliosis. Gliosis is an abnormal increase in the number of astrocytes, which are support cells. There is also a buildup of abnormal PrP. In some cases, these proteins form amyloid plaques. These features help scientists diagnose the disease during an autopsy.

Prion diseases are categorized by their cause and host. In non-human mammals, several types exist. Scrapie affects sheep and goats. Bovine spongiform encephalopathy, or BSE, affects cattle. People often call BSE "mad cow disease." Chronic wasting disease, or CWD, affects deer and elk. In humans, the diseases are more varied. Creutzfeldt–Jakob disease, or CJD, is a major human prion disease. Other human types include kuru and Gerstmann–Sträussler–Scheinker syndrome. There is also fatal familial insomnia and variably protease-sensitive prionopathy.

Humans can acquire these diseases in different ways. Most human cases are sporadic, or idiopathic. This means they happen for no known reason. About 10% to 15% of cases are hereditary. These result from mutations in the PRNP gene. This gene provides the instructions for making PrP. Mutations can cause the protein to be too short or too long. It can also change a single amino acid. Other cases are iatrogenic, meaning they are spread through medical treatments. This can happen via contaminated surgical instruments or certain biological materials.

Scientists have worked for decades to understand these proteins. In the 1930s, Jean Cuillé and Paul-Louis Chelle proved scrapie was infectious. In 1959, William Hadlow noticed similarities between scrapie and kuru. D. Carleton Gajdusek then showed these diseases could move to primates. He reported on kuru in 1966 and CJD in 1968. For a long time, people called the agent a "slow virus." It was not until 1982 that Stanley Prusiner proved the cause was a protein. This discovery established the concept of the prion.

Prion diseases are unique compared to other infections. Most infectious diseases are caused by viruses or bacteria. Those agents use DNA or RNA to replicate. Prions have no DNA or RNA. They are made solely of abnormal protein. This makes them very difficult to destroy. Prions can survive heating in an autoclave, which is a common sterilization method. Because of this, doctors must use special precautions during brain surgery. This highlights how much the shape of a molecule matters in biology. A single change in a protein's fold can change a whole organism.