Taboo engraved in human genes - prions

Have you seen "Train to Busan"? This film, with its twists and turns and thought-provoking meaning, has attracted many fans to stop and watch. The emergence of the zombie virus leads to the plot of the survivors escaping and helping each other in the zombie tide. The protagonist awakens his humanity during the escape, and unfortunately gets infected while helping a mother and daughter escape, jumps out of the car and dies. In it, we see humanity and solidarity, but we also have to fear the "tyranny" of the zombie virus. So, does the zombie virus really exist? What does it look like?

Here, let us take a closer look at this real-life version of the “zombie virus” – prion.

What is the chemical nature of prions?

Perhaps, if we understand it literally, we can understand it more or less: "prion" means protein; so, in simple terms, it is a virus made up of protein. But some people may ask: viruses are generally made up of protein and nucleic acid (DNA or RNA), so what is special about it? Of course there is.

Prions are also known as "prion particles", "protein infection factors", "toxin prions" and "infectious proteins". In other words, they are "viruses" composed only of proteins. So, strictly speaking, they are not viruses, but are just commonly called "prion viruses".

Why is it called the "zombie virus"?

When talking about the "zombie virus", let's take a look at the diseases it can cause.

As early as three hundred years ago, humans discovered such a disease among sheep and goats: they usually feel extremely itchy and often rub against rough tree trunks and stone surfaces, causing all the hair on their bodies to fall off, hence the name "sheep pruritus".

Mad cow disease was first discovered in 1958: the related infectious agent (prion) can destroy brain cell tissue, causing cavitation; sick cows have disordered autonomic movement and disrupted biological clocks. Mad cow disease not only caused tens of billions of pounds in economic losses to the UK, but also caused great social panic.

In addition to them, there are also infectious mink encephalomalacia, chronic wasting disease of red deer and deer, spongiform encephalopathy of cats, kuru disease and Creutzfeldt-Jakob disease, etc. It is not difficult to find that these diseases all cause patients to have abnormal behaviors and symptoms similar to zombies. Now, do you understand why it is called "zombie virus"?

What is the structure of prions and their relationship to genes?

We who have studied high school biology and chemistry know that the factors that affect protein are roughly the type and number of amino acids, the order of amino acids and their spatial structure, etc. Genes guide protein synthesis. So, here we can start from these two knowledge points to explain its "true face".

Perhaps you will be surprised by the fact that we have genes that synthesize it in our bodies! Of course, genes in the body generally do not produce substances that are so harmful to the human body. So, what is the reason here?

Humans have 23 pairs of chromosomes, and the 20th pair of chromosomes has the gene encoding the prion protein (PrP). The difference is that the prion we mentioned above is the pathogenic prion protein (PrPSc), while this gene encodes the normal prion protein (PrPc)! What does it mean? Simply put, the prion produced by the human body is usually the normal protein PrP (Note: It is a glycoprotein of normal human cells), but the prion we are talking about is the pathogenic protein PrP.

Because the gene "went wrong" during protein synthesis, the synthesized PrP polypeptide chain was "processed" incorrectly due to its negligence, forming a "defective product" that is harmful to us - an abnormal PrP protein - PrPSc. Compared with the normal protein PrPc, it has several more abnormal folding phenomena, which makes PrPSc low in solubility and will not be hydrolyzed by protease catalysis.

In general, prions are pathogenic proteins encoded by the 20th pair of chromosomes in the human body, with abnormal spatial structures and produced by the denaturation of normal proteins. Now, do you understand the "taboos engraved in human genes"?

What is the pathogenic mechanism of prions?

As mentioned above, prions are abnormal proteins produced by the human body, but it is undoubtedly doomed to fail when it uses the power of just a few proteins to fight against our immune system. Therefore, it uses a "dirty trick" - assimilation.

The role of assimilation can be compared to the "chain reaction" of nuclear fission, and can also be associated with the "domino effect". The abnormal protein comes into "close contact" with the normal protein in our body, and then induces it to transform into the same abnormal protein, and then they continue the next "chain reaction". How about it, insidious?

How to prevent prion diseases?

Since it is so vicious and insidious, how can we prevent it from happening? Here are some explanations and suggestions.

Although we don't have drugs that can kill prions, we can still prevent them. First, don't eat meat that is of unknown origin or has not been quarantined. Second, the medical equipment used in neurological surgeries must be strictly disinfected. Finally, if there are people with related diseases around you, please be careful not to contact them to prevent infection.

(Take a small notebook and write it down, the key points!)

Postscript: Tribute to the scientists who worked hard for it

Many scientists have made great contributions to the fight against soft virus diseases.

Daniel Carlton Gajdusek went deep into the primitive tribe alone, lived with the natives, and explored the mystery of Kru disease;

British biologist Alps creatively proposed that the pathogenic factor of "scrapie" does not contain nucleic acid. Even though it was regarded as heresy, he still insisted on the truth and proved everything with experiments;

American virologist Prusiner discovered prions in 1982, solving the mystery of numerous prion diseases;

......

To this day, countless scientists are still struggling to eliminate prions. Their efforts and persistence, courage and wisdom allow us to understand the mystery step by step, trace the source, and save humanity.

Please express our highest respect to them!

This article is a work of Science Popularization China-Starry Sky Project

Produced by: Science Popularization Department of China Association for Science and Technology

Producer｜China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Author: Fu Chunrui, popular science creator

Reviewer: Tao Ning, Associate Researcher, Institute of Biophysics, Chinese Academy of Sciences