Prion diseases are associated with a protein produced naturally by the cells of humans and many vertebrate animals. PrP, as it is called, is a small protein: about a hundred times smaller than the smallest viruses, it is invisible even for the best electron microscope. Neurons – the cells in the brain and spinal cord – make the most.
However, like the vast majority of proteins, its existence is limited in time. As new PrP proteins are produced, the old proteins are destroyed by other proteins called “proteases” so that they do not accumulate inside cells and hinder the proper functioning of these proteins. past.
But apart from the fact that it is associated with the cell membrane – that is, its envelope – it is unclear exactly what PrP is used for under normal circumstances.
One protein, seven diseases
Yet in humans, the same PrP protein is implicated in at least seven rare diseases, all of which are characterized by memory loss, behavioral disorders and disordered movement, followed by dementia, the outcome of which is rapidly fatal. The best known so far has been Creutzfeldt-Jakob disease, of which there are three forms that produce slightly different neuropathologies.
First, a sporadic form that affects one new person per year per million inhabitants around the world and occurs around the age of 60. Then, a genetic form – familial cases appearing between 45 and 60 years old – due to mutations in the gene coding for the PrP protein.
But this disease can also be transmitted by a contaminated source (iatrogenic form). Children have died of Creutzfeldt-Jakob after receiving growth hormone injections from pituitary glands taken from deceased people. Other possible routes of infection are the use of poorly sterilized surgical instruments or the transplantation of corneas. And then in 1995 appeared what is called the new variant of CreutzfeldtJakob disease (nvCJD).
Characterized by a slower course (14 months on average between the appearance of the first symptoms and death, compared to six weeks to six months in the usual form), but a shorter incubation time, this variant therefore affects people young people (under 40), who have very unusual lesions in their brains. The first cases were detected in England – the country most affected by bovine spongiform encephalopathy or BSE (beef prion disease) – nvCJD was quickly suspected of being the consequence of the transmission to humans of bovine disease by ingestion of contaminated food.
A revolutionary hypothesis
In order for an infection to occur, it was until recently thought that a microorganism had to enter our body and multiply there until it disrupted its proper functioning. By microorganism is meant a microscopic organism possessing DNA, that is to say its own genetic information – such as a virus, a bacteria or certain fungi – which will allow it to produce in our body its own proteins and multiply there.
But here, nothing like it. All attempts to detect the presence of foreign genetic material that could betray the intrusion of a microorganism have so far failed: it seems that the infection is due to the ingestion of a modest protein. A dogma of molecular biology is seriously shaken. But how could a simple protein become infectious and reproduce in an organism (to the point of causing death!) Without genetic material?
The answer is not in the bottom but in the form: PrP can indeed exist in two forms: healthy (we then speak of PrPC) or abnormal (in this case called PrPSc). From a chemical point of view, it is almost impossible to distinguish them, the two forms being almost identical. In reality, the only real difference is in their three-dimensional structure. Imagine a protein like a necklace made up of pearls (amino acids) strung one behind the other.
The order of the pearls and even their number may vary somewhat between different species. For example, the two forms of the human PrP protein consist of 253 pearls, while those of beef have 264. But this necklace is neither rigid nor straight. On the contrary, once manufactured, it will fold in on itself until it adopts a particular shape which will allow it to ensure its function in the cell. However, between the two forms of the PrP protein, there is only one difference in folding: the abnormal PrPSc protein – a prion component – being simply “badly folded” or misshapen.
In addition, and this is where everything plays out, everything happens as if the deformed PrPSc protein is able to “mold” a normal PrPC protein in its image, so that it takes its “twisted” form. For the moment, this stage is still mysterious. We do not understand very well how the transformation takes place and we suspect that another “actor”, still unknown but present in the cell, comes into play. However, it is certain that the PrPSc protein “misfolded” plays an essential role in setting the bad example to its normal neighbors.
This change in the form of the protein has several consequences. First, a chain reaction, since any new “misfolded” protein can in turn cause a normal protein to change shape. On the other hand, as it is still the same protein, the immune system – responsible for defending the body against infections – sees nothing but fire and does not react to destroy proteins that have become dangerous.
Second, its twisted shape gives PrPSc the ability to aggregate as deposits (so-called “amyloids”) in and around neurons, preventing them from functioning properly. In this compact form PrPSc is completely resistant to proteases – these proteins responsible for destroying “outdated” or “abnormal” proteins. Since the cell is unable to eliminate them, deformed proteins accumulate indefinitely.
Ultimately, this situation leads neurons to self-destruct in a sort of collective cell suicide releasing the abnormal proteins that will infect neighboring cells. Gradually, this phenomenon quickly leads to the death of clusters of neurons and creates real holes in the brain, giving it this characteristic sponge-like appearance.
However, the extent of the damage seen in the brains of sick people suggests that the accumulation of the abnormal protein in neurons is not the only factor. It is likely that the abnormal PrPSc protein interacts with other proteins that are directly involved in the destruction of neurons.
From beef to man:
Long criticized by the scientific community, the revolutionary hypothesis of an infectious protein – advanced for the first time by the American biologist Stanley Prusiner in 1982 – has gradually taken hold. However, the discovery of the existence of different “strains” of prions was almost fatal.
The concept of “strains” of prions is based on several observations: first, on different periods of time between infection and the appearance of the first symptoms (incubation time), then on the types of lesions observed – the different strains do not destroy for example the same regions of the brain – and finally on certain different physicochemical properties including a more or less great resistance to proteases.
But, contrary to what one might think, these strains do not necessarily correspond to different species. Indeed, several strains of prions can coexist inside the same species. In this case, the proteins therefore have exactly the same sequence (order and number of pearls on the necklace). For example, the sporadic and iatrogenic forms of Creutzfeld-Jacob disease are caused by two different human strains. But the strongest thing is that each strain is able to “transmit” its characteristics to normal PrPC proteins.
Take for example a hamster infected with a particular strain of hamster prions. He will start producing PrPSc prions from his own PrPC proteins which will have the same characteristics as the strain that infected him. That is to say that when injected into a third hamster, its PrPSc proteins will produce the same type of lesions after the same incubation time, and will have the same biochemical properties as the strain from the first hamster!
A real headache for researchers: how to explain that the same sequence of “pearls” can produce various abnormal proteins each having different properties and transmissible to normal PrP proteins? For some researchers, this is proof that the infection is ultimately due to a microorganism with DNA, probably a virus. Indeed, the DNA of viruses evolves very quickly, that is to say that it easily undergoes mutations which change the information it carries, which would perfectly explain the existence of the different strains observed.
“No”, replies Prusiner who once again suggests that the solution is hidden in the structure of the protein: each strain would correspond to a different conformation of PrPSc. However, how to explain that the same protein can adopt as many different forms as there are strains? You should know that once produced, a protein will still undergo a certain number of modifications, which do not modify its sequence, but can, among other things, influence its form. And indeed, recent results seem to indicate that some of these modifications vary according to the strains, which would explain how, starting from the same protein (PrP), several different infectious agents or prions would be born.
But even if this hypothesis holds true, it remains to be explained how an abnormal PrPSc protein is capable of “transmitting” its own modifications to the normal PrP proteins it encounters, which remains completely obscure for the moment. To date, only one strain of prions has been identified from the brains of cattle suffering from BSE. Interesting point: all the cases of nvCJD listed are also due to a single strain which causes lesions in the brain similar to those observed in animals infected naturally or experimentally with BSE and this after an identical incubation time.
This finding supports the bovine CMV hypothesis of bovine origin. Other similarities in the biochemical and biological properties of the two infectious agents have successfully convinced the medical community that the bovine prion is capable of infecting humans.
Emergence of a “super” prion
In the laboratory, when you inject a prion of one species into an animal of another species, you see a much lower transmission efficiency than if it were the same species. This resistance to infection represents what is known as the “species barrier”. The small sequence differences (order and number of pearls on the necklace) that exist between normal PrPC proteins in many mammals cause variations in their “normal” three-dimensional structure which certainly play a central role in this resistance to infection .
In the laboratory, this barrier can be overcome by using very high infectious doses, a direct route of administration – by injecting prions into the brains of animals – and at the cost of a much longer incubation time. This is why, it was initially thought that in nature a protein “badly folded” could only influence has form of a protein of the same species, ie that a sheep prion could only infect another sheep, a beef prion another beef and a human prion another human. As a rule, this is true.
However, laboratory studies have confirmed what a number of observations already suggested: although the disease initially known in sheep as “scrapie” in sheep is incapable of infecting humans, it may well have been transmitted to beef through animal meal. On the other hand, it seems today to be capable of transmitting it to humans as well as to many other species.
It is now believed that the change in the method of preparing animal meal that took place from the end of the 1970s may have led to the emergence of a “super” prion, capable of bridging the barrier of species .
Should we be afraid of eating beef?
When ingesting contaminated food, the abnormal protein PrPSc – which has become resistant to proteases – remains intact while the normal protein PrPC will be completely degraded by our digestion enzymes. A mechanism that is still poorly understood would then allow the deformed protein to pass from the intestine to the nervous system and in particular to the brain.
Despite the confusion around it, be aware that the skeletal muscle of beef, in other words meat or beef steak, does not contain a prion. Indeed, it would seem that the normal PrPC protein present in the cells of this tissue somehow resists the transformation into abnormal PrPSc protein. This is what laboratory tests indicate, although the reason is not very clear.
On the other hand, the infectious agent is present in the peripheral nerves and the blood of an infected animal and these cannot be eliminated during the preparation of the meat. However, the risk of infection through this route is probably very low, even if it remains difficult to estimate for the time being until the minimum infectious dose for humans has not been determined, nor if this dose should be single or can be cumulative.
With prion diseases, researchers are faced with a radically new concept. Monitoring the course of the disease through epidemiological studies and developing sensitive and rapid screening tests to detect infection while the patient is alive are two priorities for current research. In the therapeutic area, everything remains to be done, since there is currently no treatment. Recent advances in understanding the spread of prions are encouraging, however.
Much hope is placed in the elucidation of the three-dimensional structure of the abnormal protein PrPSc. Once obtained, this can be compared to that of the normal protein already known. Using bioinformatics, molecular modeling should then make it possible to design synthetic molecules capable of either preventing or reversing the observed change in conformation. “Pray” that by then we will succeed, through strict measures, to contain and possibly eradicate this disease …