Ask and Answer Paper: Understanding Prions

Ask and Answer Paper:
Understanding Prions

Steven English

Ask and Answer Paper: Understanding Prions 1

As its name suggests, Pasteur's germ theory of disease simply states that microbes are the cause of disease. Though it seems obvious now, it was revolutionary at the time of its inception and is central to our modern understanding and treatment of diseases. It replaced the commonly held and relatively complex 'humoral theory' with a simpler concept that boiled down to a relationship between microorganism and host (“Germ Theory,” n.d.). But as our understanding of the disease process grows with more and more diseases continuing to be discovered, we find that while viruses and microorganisms might be the main cause they aren't the only way in which disease can spread.
Prions are one such challenge to this simple germ-disease paradigm. In this paper, the following questions about prion diseases (focusing primarily on kuru and Creutzfeldt-Jakob disease) will be examined: “What are they and what does their discovery mean for our understanding of disease and evolution?”
The prion protein (PrPC) is a protein seemingly found in the brains of almost all vertebrate animals which leads to the conclusion that it serves an evolutionarily important role. Due to its affinity for copper and location in the brain, the role PrPC plays in the brain is possibly related to the absorption of excess copper released after a neuron fires. However, studies with mice genetically engineered to lack the PrPC protein suggest that it is not necessary for normal development and that there are other pathways through which whatever function it serves may be accomplished (Yam, 2003). The term, 'prion' is primarily used to describe a PrPC protein, which normally exists in a mainly α-helical structure, that has abnormally folded into a form that has abundant β-sheets (Mayo Clinic, 1998)[1].
It is the result of this abnormal refolding that causes disease in the host organism. Occurring in inherited, sporadic or infectious forms, the abnormal proteins (PrPSc) then encourage other proteins to 'misfold' and take on the variant shape eventually leading to the formation of sponge-like holes and often a build up of amyloid plaques that are in some ways similar to plaques seen in the brains of Ask and Answer Paper: Understanding Prions 2 people who suffer from Alzheimer's disease (Rhodes, 1997). The prion, is both the causative agent of the disease and the way in which the disease is transmitted (Ridley, 1999). The early research into prion-diseases centered on Carlton Gajdusek's research into kuru, which was an epidemic noted in certain populations of aboriginal Papua-New Guineans beginning in the 1950's. The disease afflicted mostly women and children, was 100% fatal and was associated with dementia-like symptoms but no observable inflammatory immune response (Rhodes, 1997). At its height in 1961, kuru resulted in the death of as many as 200 individuals in a single year, which devastated the small isolated population skewing the gender imbalance to a 3-to-1 male/female ratio in some villages (Rhodes, 1997).
The spread of the disease was found to be caused by the practice of familial ritualistic cannibalism in which the relatives would eat the remains of the dead. The brain matter, however, was left to the women and child-age relatives. The ingestion of the infected brain matter caused the spread of the disease to an estimated 2,700 individuals since it was first identified by researchers (Chandler, 2012). The practice was outlawed in the sixties and the incidence of the disease has fallen to just nine deaths since 2000, almost certainly all of which were caused by infections that occurred decades earlier due to the long incubation of the disease (Chandler, 2012).
Where the kuru came from remained a mystery for years until study of the brains of those who died from kuru by Gajdusek and his collaborators found that similar damage, most notably the sponge-like perforations of the brain, was found in the brains of sheep who suffered from another mysterious neurological disease known as scrapie (Yam, 2003). Scrapie was well known to sheep farmers and was characterized by sheep exhibiting strange behavior and often rubbing themselves against objects until they scraped the wool off, hence the name. Over time, more comparisons were made between the brain damage seen in kuru and other fatal diseases in humans and even mink. Eventually, the similarities between kuru and the very rare Creutzfeldt-Jakob disease were noticed. Ask and Answer Paper: Understanding Prions 3
The sporadic form of Creutzfeldt-Jakob disease has a natural incidence rate of about 1 in 1,000,000 and accounts for about 85% of total cases (Missert, et al, 2012). In the sporadic form, a non-transferable genetic mutation results in the accumulation of misfolded PrPSc, which in turn causes the symptoms.
However, CJD also comes in an inheritable familial form, as well as iatrogenic and transmissible forms (Ridley 1999). Though it seems as if there is much fluidity between the transmission routes of CJD. For example, genetic evidence suggests that a single case of sporadic CJD is to blame for the spread of kuru, which was transmitted orally, in the Papua-New Guinea aborigine population (Chandler, 2012). It was an outbreak of a transmissible form of CJD, often called variant CJD or vCJD, that caused world wide panic in the early 1990's. It arose after people in England ate or otherwise used beef products that were infected with bovine spongiform encephalopathy (BSE), which is the bovine version of CJD (Yam, 2003). The infection pattern likely followed that of the kuru outbreak in that a small number of cows, or perhaps even an individual, was infected with BSE and, because of changing farming practices, their carcasses were turned into feed for other cows which infected more and more livestock until a sufficient amount were infected and it caused a large scale transfer of infectious prions to enter the food supply (Yam, 2003). Recent studies into lateral transmission also suggest that prions can survive the passage through the digestive system of some animals, suggesting that cows living in close quarters could pass the disease between individuals in the same herds (VerCauten, et al, 2012).
At its height in January of 1993, 1000 new cases of mad cow, as it became known due to the neurological effect it had on cows suffering from it, were reported. This resulted in a total of 184,500 cows being infected between 1986 and 2010 (CDC, 2013). The ban of animal protein in ruminant feed implemented in 1997 caused the gradual decline back to sporadic rates of BSE and curtailed a larger epidemic among both bovine and humans (Yam, 2003). Though it is difficult to pinpoint the exact number of human deaths caused by mad cow, the CDC estimates 140 people died from vCJD in the UK Ask and Answer Paper: Understanding Prions 4 between 1995 and 2003 (2010). The vCJD prevalence estimates for the UK stands at about one in 2000 individuals (Jackson, et al, 2013). Among all prion diseases, a neurological decline is noted and, depending on the disease, that is often accompanied by spongiform damage to the brain and the accumulation of plaques and fibrils, though there are certain inheritable diseases like fatal familial insomnia that have some additional symptoms, like the inability to sleep in the case of FFI (Rhodes, 1997). In terms of how the disease builds up, it seems to be uniform among all forms across all species. A single PrPSc serves as a seed influencing normally folded PrPC to take the abnormal form during their creation in ribosomes (Yam, 2003). Studies have shown that misfolded PrP from yeast can go on to influence rat PrP to misfold, but synthetically created PrPSc has yet to promote misfolding, suggesting that PrPSc is necessary, but that there are likely cofactors involved that have yet to be identified (Yam, 2003). It is worth pointing out that though that different routes of transmission in CJD can have marked differences in pathologies, as highlighted in figure 2, though the reasons are not yet known. In both versions of the disease, the incubation time can often be measured in decades, thought the variant version often shows a much shorter incubation. As noted earlier, there is no discernible inflammatory reaction, meaning there is no known host defenses; although some research suggests that the conversion does not go on indefinitely and that genetic disposition determines if one is even susceptible to the conversion process (Yam, 2003).
Prevention of certain types of prion diseases, including the genetic and sporadic forms, seems to be impossible. However, avoiding products made from livestock in areas with high incidences of any transmissible spongiform encephalopathy can significantly decrease the chances of becoming infected with a transmissible variant (Rhodes, 1997). This is evident in the differences between number of deaths in the UK, where the BSE outbreak occurred, and the US where few cows have been infected with BSE (CDC, 2013). Much early research suggested that eating ground meat, which often contains Ask and Answer Paper: Understanding Prions 5 some brain and CNS matter, was the most likely route of oral transmission and that muscle meat likely doesn't have prion proteins in concentrations high enough to cause disease (Yam, 2003). But recent research indicates that prions might be in present in body fluids, like milk, and in high enough concentrations to possibly cause disease (Franscini, 2006).
There is currently no treatment or diagnostic tests, and current treatment strategies have focused mainly on soothing symptoms. However, there are some recent developments on both of those fronts. In one study, it was shown that the injection of a genetically modified viron into mice infected with prions not only increased survival time by 20%, but resulted in an improvement in behavior and a reversal in spongiform damage in some brain regions (Toupet, 2008). This is an important advancement because the treatment was administered in the late stage of the disease process, which is very likely necessary for success in any human treatment due to the long incubation and difficulty in detecting the disease at an early stage, short of genetic testing that could verify a mutation of the PrP gene on chromosome 20. Depending on the codon location of the mutation, or in some cases two mutations, one can determine if they are susceptible to a particular inheritable or sporadic types of CJD (Ridley, 1997). Most detection tests were done by bioassay, but a recent study found that the actual detection of prions in cerebral spinal fluid can be done quickly and with greater accuracy and sensitivity than current methods (Sano, et al, 2012). Some suggest that blood tests are the most likely method for a diagnostic test to be developed (Jackson, et al, 2013).
The thing about prion diseases that causes the most debate is the fact that they seem to come in different strains and, most puzzling, seem to be possible of evolving, or at least to be involved in driving the evolution process. A 2012 study of yeast prions showed that wild type yeast strains showed an array of prion-dependent phenotypes in nature (Chang, et al, 2012). In the study, it was found that 40% of the prion derived phenotypes were beneficial to growth under the 12 conditions tested. Perhaps then, the presence of prions in human and other animal brains are a hold over from early bacterial Ask and Answer Paper: Understanding Prions 6 ancestors, where they were beneficial for certain environments. It is known that human prions maintain a core that is resistant to protease K, which completely dissolves normal PrP, and that these cores come in different sizes depending on the type of disease it causes (Yam, 2003). This suggests that the damaged proteins themselves serve as the elements of inheritance and answers the question of how different strains of the disease can be present. A prion can misfold in a variety of ways dependent on mutations to different codons on the PrP gene, which in turn can either be passed on to offspring or possibly passed on to another organism that ingests biological matter infected with that specific prion. Recent research suggests that all cases of vCJD all have the same causative agent, meaning that there is one type of misfolded protein that causes that disease, giving more credence to the theory that each prion corresponds to a specific disease pathology (Diack, et al. 2012).
This presents us with a completely different version of an infectious agent that does not conform to the virus or bacteria path of disease. These proteins are smaller than any known true virus and can withstand stresses that would kill almost any other bacteria (Yam, 2003). Even referring to a prion infection does not entirely tell the whole picture as they can originate in the brains of the host organism without any outside input. They are damaging to humans, but can infer great advantages to a yeast. The fact that the protein is not necessary for normal life functions and that mutations in the genes of animals confers a great evolutionary disadvantage, one can conclude that it is the long incubation of the disease that has kept the gene around in animals and would have been lost if the genetic variations of the disease were more quickly fatal. Evidence for this lies in the fact that among the populations who suffered from kuru, which often manifested itself much more quickly in many of those infected, have shown a marked increase in genetic resistance to the disease (Chandler, 2012).
The curious case of the prion is far from being solved, but as we come to understand them we can possibly gain insights to other neurological diseases. A likely unfortunate genetic leftover from our prokaryotic ancestors might some day lead to a better understanding of Alzheimer's disease and Ask and Answer Paper: Understanding Prions 7 possibly even a treatment, which makes their study so important.

Ask and Answer Paper: Understanding Prions 8

REFERENCES

Centers for Disease Control (2013). BSE (Bovine spongiform encephalopathy, or mad cow disease). Retreived from: http://www.cdc.gov/ncidod/dvrd/bse/
Centers for Disease Control (2013). vCJD (Variant Crutzfeldt-Jakob disease). http://www.cdc.gov/ncidod/dvrd/vcjd/index.htm
Chandler, J. (2012, November, 13). The last laughing death. The Daily Mail. Retrieved from: http://www.theglobalmail.org/feature/the-last-laughing-death/470/
Diack, A. B., Ritchie, D., Bishop M., Pinion, V., Brandel, et al. (2012) Constant transmission properties of variant Creutzfeldt-Jakob disease in 5 countries. Emerging Infectious Diseases 18.10 (Oct. 2012): p1574. doi: http://dx.doi.org.libproxy.howardcc.edu/10.3201/eid1810.120792
Franscini N, Gedaily AE, Matthey U, Franitza S, Sy M-S, et al. (2006) Prion Protein in Milk. PLoS ONE 1(1): e71. doi:10.1371/journal.pone.0000071. Retrieved from: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000071
Harvard University Library Open Collections Program. Germ theory. Retrieved from: http://ocp.hul.harvard.edu/contagion/germtheory.html Jackson, G., Meade, S., Collinge, J. (2013) Developing early diagnostics for prion diseases. Neurodegenerative Disease Management, 3.1 (Feb. 2013): p53. doi: http://dx.doi.org.libproxy.howardcc.edu/10.2217/nmt.12.76
Missert, M., Qazi, K. J., Ionita, C. C. (2009) Utilising Therapeutic Hypothermia in the Control of Non- convulsive Status Epilepticus in a Patient with Creutzfeldt-Jakob Encephalopathy. British Journal of Medical Practicioners 2012;5(2):a515. Retrieved from: http://www.bjmp.org/content/utilising-therapeutic-hypothermia-control-non-convulsive-status- epilepticus-patient-creutzfe
Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. New York: HarperCollins.
Rhodes, R. (1997). Deadly feasts: Tracking the secrets of a terrifying new plague. New York: Simon & Schuster
Sano K, Satoh K, Atarashi R, Takashima H, Iwasaki Y, et al. (2013) Early Detection of Abnormal Prion Protein in Genetic Human Prion Diseases Now Possible Using Real-Time QUIC Assay. PLoS ONE 8(1): e54915. doi:10.1371/journal.pone.0054915. Retrieved from: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0054915
Toupet K, Compan V, Crozet C, Mourton-Gilles C, Mestre-Francés N, et al. (2008) Effective Gene Therapy in a Mouse Model of Prion Diseases. PLoS ONE 3(7): e2773. doi:10.1371/journal.pone.0002773. Retrieved from: http://www.plosone.org/article/info%3Adoi %2F10.1371%2Fjournal.pone.0002773#s4
VerCauteren KC, Pilon JL, Nash PB, Phillips GE, Fischer JW (2012) Prion Remains Infectious after Passage through Digestive System of American Crows (Corvus brachyrhynchos). PLoS ONE 7(10): e45774. doi:10.1371/journal.pone.0045774 Retrieved from: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0045774
Yam, P. (2003). The pathological protein. New York: Copernicus Books

Ask and Answer Paper: Understanding Prions 9
Fig. 1
Source: http://www.mayoclinic.com/health/medical/IM02291

Fig. 2
Clinical and Pathologic Characteristics Distinguishing Classic CJD from variant CJD

|Characteristic |Classic CJD |Variant CJD |
|Median age at death |68 years |28 years |
|Median duration of illness |4-5 months |13-14 months |
|Clinical signs and symptoms |Dementia; early neurologic |Prominent psychiatric/behavioral symptoms; painful |
| |signs |dyesthesiasis; delayed neurologic signs |
|Periodic sharp waves on electroencephalogram |Often present |Often absent |
|"Pulvinar sign" on MRI* |Not reported |Present in >75% of cases |
|Presence of "florid plaques" on neuropathology |Rare or absent |Present in large numbers |
|Immunohitochemical analysis of brain tissue |Variable accumulation |Marked accumulation of protease-resistance prion protein|
|Presence of agent in lymphoid tissue |Not readily detected |Readily detected |
|Increased glycoform ratio on immunoblot analysis of |Not reported |Marked accumulation of protease-resistance prion protein|
|protease-resistance prion protein | | |
|Source: Adapted from Belay E., Schonberger L. Variant Creutzfeldt-Jakob Disease and Bovine Spongiform Encephalopathy. Clin Lab Med 2002;22:849-62. |
|*An abnormal signal in the posterior thalami on T2- and diffusion-weighted images and fluid-attenuated inversion recovery sequences on brain |
|magnetic resonance imaging (MRI); in the appropriate clinical context, this signal is highly specific for vCJD. |

Source: http://www.cdc.gov/ncidod/dvrd/vcjd/index.htm

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[1] See figure 1