IB Biology/Option F - Microbes and Biotechnology/Prion hypothesis

The idea that protein could be the solo infectious agent for prion disease was first proposed 43 years ago.Since then, the topic has been controversy. Although there is no evidence that proved such hypothesis yet, many evidences had indicated the infectious agent is not nucleus acid base. On the other hand, there are also evidences that support the idea protein is at least not the solo component responsible for prion disease. There are still many outstanding questions waiting to be answered and thus the prion hypothesis still remains inconclusive.

What is Prion diseases?

Prion diseases, also previously referred to as subacute spongiform encephalopathies, slow virus diseases and transmissible dementias, are transmissible neurodegenerative conditions found in human and animals. Some prion diseases found in animals are known as spongiform encephalopathy (BSE) and scrapie. Scrapie, commonly seen on sheep and goats, is one of the most well known prion diseases that has been recognized in Europe for over 200 years and is presence in many counties today. The type of prion disease found in human can be classified into Creutzfeldt-Jakob Disease (CJD), Gerstmann-Straussler syndrome (GSS) and kuru. Such disease, however, is very rare in human, affecting approximately one person per million. Yet, researchers have put special focus on such disease due to the possibility that prion disease can be transferred to different species by dietary exposure to infected tissues. In fact, in 1986, an outbreak of a prion disease BSE in cattle in Great Britain drew more attention to such topic. It is likely that a majority of UK population have consumed the infected products. Such concern was confirmed when researchers discovered that Creutzfeldt-Jakob disease was linked to BSE.,

In addition, around 70 years ago, scrapie was found to be transmissible between sheep and later passed on to mice. The causation of such transmission was most likely due to an accidental vaccine derived from an animal with scrapie. Researchers assumed that virus was the infectious agents. However, certain evidences did not agree with such hypothesis. The hypothesis was challenged as the result of the failure to directly identity such virus and the fact the infectious agent showed resistance to treatment expected in inactivate nucleic acid. Thus, prion hypothesis was proposed as an alternative explanation of how prion disease was transmitted. The prion hypothesis, also known as the protein only hypothesis, states that protein, rather than virus or bacteria, is the infectious agent of the prion disease., ,

What is protein hypothesis?

Protein hypothesis states that the misfolded prion protein, rather than virus or bacteria, is the infectious agent that results in the transmission of prion diseases.

Evidences Support Prion Hypothesis

Even from the initial discoveries of the prion diseases, it was clear that the infectious agent for prion disease was different from conventional micro-organism (such as virus and bacteria). Such observation was supported by the experiments done by Alper. Alper subjected mouse brain affected by scrapie with ultraviolet light at multiple wavelengths, a condition that will inactivate all nucleic acid,to observe the UV light effects on infectious agent of scrapie. The level of infectious agent, however, did not change. Such result indicates that the infectious agent is most likely not DNA or RNA based since the UV radiation near to peak of the absorption spectrum seems to have no effect on the agent. Alper believes that even with the most efficient repair system, a declination should still be detected if the infectious agent is nucleus based. However, the same studies also did not support the prion hypothesis. According to the study, if the infectious agent is protein based, a decline of infectious agent should appear when subjected to UV light at about 280mu. A declination was observed, however, was not enough to make a statistical difference. In other words, the finding of the study remains inconclusive whether the infectious agent is protein based.

Recent research seems to have a significant breakthrough in identifying the infectious agent. According to the article “identification of a protein that purifies with the scrapie prion”, a protease resistant prion protein (PrPsc), was found in the scrapie infected hamster brain that did not exist in health hamster brain. The molecular size of such protein was approximately 27,000 to 30,000 Daltons in sodium dodcyl sulfate polyacrylamide gel. Researchers were able to distinguish such protein from others due to its resistance to protease K. the initial result indicates the amount of this protein correlates with the tier of the infectious agent. , In fact, “immunoaffinity purification and neutralization of scrapie prion infectivity” by Gabizon R shows the infectivity remained constant throughout the purification of PrPsc. During the study, PrPsc extracted from scrapie infected hamster brain was subjected to detergent lipid protein complex, which destroyed the structure of PrPsc protein, the scrapie infectivity then decreased by a factor of 100. The direct correlation between the PrPsc and the infectivity indicates that PrPsc is most like involved in the infectious process.,

Furthermore, researches show that the gene encoded PrP is the same in prion infected brain and healthy brain. Such result indicates the difference between PrPsc and PrPc (normal PrP protein) was due to post-translational activities., Another strong evidence that supports the prion hypothesis comes from the demonstration of PrnP-/- gene. Mice with such gene were lack of both normal and the mutated type of PrP proteins and did not show signs of scrapie ,, which supports the hypothesis PrP protein must be presence during prion diseases. Furthermore, in 2001, Soto and his team successfully converted PrPc into PrPsc like structure with a very small amount of PrPsc in vitro. Such process is known as Protein Misfolding Cyclic Amplification (PMCA). PMCA allows prion to be produced in an accelerated rate and are widely used to study prion diseases today. Such finding is also evidence that PrPsc trigger further misfolding and serves as a catalyst for prion replication.,

Evidences against Prion Hypothesis

Although many evidences have shown the infectious agent for prion diseases is most likely not nucleus based, skeptics argue that there is no proof the infectious material is a protein. They argue the prion hypothesis is not proven because infectious agent produced in vitro did not originated from pure, normal prion protein. Another question that is often brought up about prion hypothesis is whether prion replication process requires co- factors? Many studies shown that purified PrPc collected from the hamsters did not convert into PrPsc when mixed with purified PrPsc . However, when purified PrPc was mixed with brain homogenate, the conversion occurred.,,, These results suggest that other factors in the brain might contribute to the prion conversion process. In fact, researchers have several hypotheses of the possible functions of co factors, which is listed as followed.

• Contributes for biological information to the infectious materials
• Acts as a catalyst for prion replication
• Helps to stabilize the conformation of PrPsc
• Participate in the fragmentation of PrPsc polymers
• Increase the biological stability of prion

Outstanding question about prion hypothesis

Whether prion hypothesis is valid still remain questionable. The following are outstanding questions about prion hypothesis suggested by Soto in “Prion Hypothesis: the end of Controversy”.

• Is PrPsc the only component of the infectious prion agent?
• What is the nature and role of cellular co-factors in prion replication?
• What is the 3D structure PrPsc?
• What is the normal function of PrPc? does it have anything to do with the conformational flexibility of the prtotein?
• How does PrPsc induce brain degeneration?
• To what extend does the prion principle operate in other disease of protein misfolding?
• How common is the prion phenomenon in nature?
• Do prion like proteins exist in mammals which confer advantageous properties to their infected host?