Is BSE caused by a protein or a virus?
The following article was taken from the sci.med newsgroup.
On Sun, 10 Dec 2000, I posted a message to this newsgroup querying the currently accepted theory regarding transmissible spongiform encephaolopathies (TSEs), "prion" diseases such as scrapie and BSE. (Ed.: For this theory, Stanley Prusiner was awarded the Nobel Prize for Medicine in 1997. - Adok) According to this theory, TSEs are caused by an abnormally folded version of one of the body's own proteins, with the ability to induce other, correctly folded versions of the protein to adopt the abnormal form. An earlier theory that this group of diseases are caused by a "slow virus" was abandoned, because:
* The disease agent is extremely resistant to radiation, heat and other sterilising treatments, more so than any known virus
* There is no sign of an immune response in infected brain tissue
* Scientists were unable to isolate any virus genetic material
* prion protein is crucial for replication of the disease
However, there is a major shortcoming with the "protein-only" prion theory: the existence of many distinct strains of TSE, each with its own characteristic incubation period and symptomology. For instance, there are known to be more than two dozen distinct strains of scrapie in sheep. Each strain holds on to its identity very tenaciously, to the extent that it can be transmitted into a completely different species (a mouse), and from there back into sheep, where it will go on to produce disease indistinguishable from the original strain. The only way of explaining this under the "protein only" theory is for there to be dozens of distinct abnormal configurations of the prion protein, each with the ability to induce normal prion protein to adopt its form and its form alone. Each of these must also retain its unique structure even when strongly heated. It is difficult to see how a protein consisting of just over 100 amino acids can have dozens of distinct, highly stable configurations, each with the ability to transform other copies of the protein into that particular shape.
Then there are the serial transfer experiments, in which scrapie is transferred into mice (whose prion protein has a different amino acid sequence to that of sheep) and then back into sheep, to produce a disease identical to the original. I don't believe that can be explained at all using the "protein-only" theory.
In my posting a year ago, I proposed a theory which gets around the shortcomings of both the "slow virus" and "protein only" theories. The infective agent is in fact a short sequence of nucleic acid far smaller than any conventional virus genome. It need be no more than a few dozen base pairs long. It doesn't code for any protein, instead it has a particular nucleic acid sequence that produces a charge distribution and distribution of hydrophilic and hydrophobic sites that closely match those on the prion protein. As soon as it comes into contact with prion protein, these sites match up and the protein and nucleic acid bind very tightly together. Part of that binding process would involve a structural change to the prion protein.
Months, years or decades later (perhaps triggered by pH change or just simple thermal dissociation) the two separate once more. The nucleic acid goes on to be replicated by the cell's DNA/RNA replicating machinery much as any virus would be. The prion protein, twisted out of shape, is left behind like an empty husk. Since the cell cannot degrade malformed prion protein very well, considerable quantities of these discarded protein wrappers would accumulate. Any extract of resistant prion protein residue will therefore consist mainly of discarded husks, with only a low percentage having infective nucleic acid bound to them. It must be very easy to overlook a small, tightly bound nucleic acid impurity in a protein residue which is itself highly resistant to most forms of chemical attack. This would explain why scientists have so far failed to isolate any TSE genetic material.
In order for its reproductive strategy to succeed, it is crucial for the TSE nucleic acid sequence to produce a close match to the electrical charge distribution and hydrophilic/hydropobic grouping of the particular prion protein it is targeting. It would have to change its base pair sequence - i.e. mutate - in order to target a different prion protein. That explains the existence of a "species barrier" in TSEs. If transmitted to a new species, then back into the original species, most of the time the most efficient mutations to re-adapt to the original host will probably result in the starting nucleic acid sequence being regained, explaining the serial transfer experiments which produced disease indistinguishable from the original strain.
The TSE disease agent's extreme resistance to radiation and other sterilising treatments can be explained by the extremely small size of its genome, and to it being tightly bound to a protective protein. The absence of inflammation or any other signs of immune activity is due to the fact that no foreign proteins are being produced - the immune system will ignore one of the body's own proteins. The catastrophic brain cell death that is the hallmark of TSEs can be explained as virus-induced apoptosis, or cell suicide. A cell which detects it has been infected by a virus will normally commit suicide, depriving the virus of the opportunity to reproduce. The presence of large numbers of actively replicating nucleic acid fragments is quite likely to make a cell conclude that it has become infected by a virus, triggering the suicide response. The brain is affected rather than other tissues because it manufactures lots of prion protein, and because it is unable to replace lost cells, whereas most other tissues can simply replace any cells that undergo apoptosis.
TSEs are characterised by a very long incubation period, a high rate of hidden, "subclinical" infection, and an infective agent that resists all sterilising treatments and can remain infective for many decades. All these factors mean that exposure to TSEs will almost inevitably occur in areas where they have become established. However, the long incubation period and the fact that infection will often have occurred through contact with an animal that appears to be healthy, means that it is usually not going to be possible to determine exactly how and when infection occurred when an animal later develops a TSE. This can explain supposedly "spontaneous" TSEs. As proof of this, consider Australia and New Zealand. Both these countries have very large sheep populations, and yet are completely free of scrapie. There would unavoidably be a low level of scrapie in these two countries if TSEs could really arise spontaneously.
Just over a year has passed since I posted my earlier message. Since then, there have been some significant developments in TSE research. Most importantly, scientists have found a way of making the TSE agent reproduce in the laboratory. It involves using brain homogenate as the culture medium, and ultrasound to speed up the replication process. A crucial question is, why does replication occur in brain homogenate, yet not in purified extracts of prion proteins? Brain homogenate is basically just fresh brain tissue that has been put through a blender, and as such it contains all the proteins, enzymes etc needed to replicate nucleic acids.
A number of pharmaceuticals are also now being tested as possible therapeutic agents for treating CJD, the TSE that affects humans. It is interesting to note that at least one of these is an apoptosis inhibitor, it appears that the researchers have now ruled out toxicity from prion protein aggregates as the cause of neuronal death in TSEs, and are now looking at apoptosis as a possible cause.