History and Metabolic Context

Creutzfeldt-Jakob Disease (CJD) is a Transmissible Spongiform Encephalopathy (TSE) caused in part by a prion. TSEs a group of neurodegenerative disorders that can be transferred between infected individuals (usually by transference of an abnormal conformation state prion protein). They create vacuoles in the tissues they affect and act primarily in the central nervous system (CNS). Although researchers do not have a complete understanding of the CJD disease state, there are a number of symptoms that characterize CJD and many factors that have been linked to pathology.

Figure 1. Colorized brain slices showing the similar vacuolization of brain tissue in response to prion infection compared to normal tissue. [Genetics @ Utah.edu via Google Images]
            The symptoms of CJD are likely due in part to the aggregation of prions and to the loss of function of the regular form of Prion Protein (PRNP). They include insomnia, personality changes, memory loss, ataxia, blindness and progressive dementia. The symptoms generally cause death within a year of the onset of the disease (Johnson & Gibbs, 1998)

In the early 1920s two scientists, Hans Creutzfeldt and Alfons Jakob, made separate characterizations of a “spastic pseudo-sclerosis, disseminated encephalomyelopathy” (Creutzfeldt, 1920; Jakob, 1921) in patients in Eastern Europe. Post-autopsy of these and some similar cases, they and several of their contemporaries noted the characteristic vacuolization of cerebral tissues, which leant the “spongiform” label to what would become the category of “TSE”. Though they didn’t know it, these couple of cases of a new, uncharacterized neurodegenerative disorder were the first recorded cases of Creutzfeldt-Jakob disease. Fast forward to 1957, Carleton Gajdusek is studying a rare neurodegenerative disease in the Fore people of New Guinea, and notes striking similarities to both CJD and to a disease in sheep, Scrapie, which has similar symptoms to CJD in humans (Gajdusek, 1957).

Several experiments were done by both Gajdusek and others that showed the ability for Kuru and CJD to be transferred from human subjects to chimpanzee (Gibbs et al., 1968; Gajdusek et al., 1966). Brain tissue from CJD infected individuals was used to inoculate chimpanzees. Following a 13-month incubation period, the animals began to show symptoms of a similar encephalopathy and eventually succumbed to it.

This set the stage for a long search for the infectious agent of Scrapie/CJD/Kuru, which was believed to be the same or similar, given the transmissibility of the three and their shared symptoms. Numerous attempts were made to pin down a virus as the causative agent, but no small pathogen could be found. It was known that whatever the infectious agent was, it wouldn’t be killed (this is when they thought it was alive in the first place) by heat shocks or nucleases. That put the infectious agent of CJD in an entirely new class. It was not until the early 80s that Prusiner, perhaps the most famous name in prion science, proposed the prion hypothesis as well as provided some evidence for its truth (Prusiner, 1982). Prusiner showed via incubation of mouse brain with the Scrapie infectious agent that much more of the agent could be recovered, which implied replication (of course, Prusiner didn’t realize at this time that the Scrapie prion was converting endogenous mouse protein to itself). Prusiner also exposed the “infectious agent” to varying conditions that would kill viruses (it had already been shown at this point by size-exclusion experiments that the infectious agent was much smaller than any known bacteria, so they were working mostly with viruses). He also struck the infectious agent with UV radiation at 254nm, which had been shown to destroy nucleic acids. Proteins, however, were much more resistant to destruction by irradiation at this wavelength.

Figure 2. Prusiner collects data showing the varying susceptibility of infectious agents to 254nm UV radiation. D37 is a measurement of the dose of irradiation that permits 37 percent survival. Note that the dosage required to destroy all but 37 percent of the "scrapie agent" is significantly higher than that of the other small infectious agents. [Adapted from Prusiner, 1982]
Figure 2. Prusiner collects data showing the varying susceptibility of infectious agents to 254nm UV radiation. D37 is a measurement of the dose of irradiation
that permits 37 percent survival. Note that the dosage required to destroy all but 37 percent of the “scrapie agent” is significantly higher than that of the other small infectious agents. [Adapted from Prusiner, 1982]
Following irradiation, the infectious agent was still able to cause a disease-state. It was also shown that several types of viruses and viroids (small bare nucleic acid pathogens usually found in plants which can act similarly to viruses) were essentially completely destroyed by this process. Thus, Prusiner provided evidence for a protein pathogen that could replicate itself, a heretical proposition at the time.

For a long time after the discovery that a prion was the infectious agent of Scrapie/CJD/Kuru, there were no diagnostic tests available. Most CJD cases could be reasoned out based on the symptoms of patients, but only post-mortem autopsies of the brain could reveal the trademark accumulated plaques of prions and the spongiform cerebral tissue that indicated CJD. Because PRNP, the protein which is susceptible to conversion, is expressed primarily in the central nervous system, it was very difficult to find evidence of the prions in other parts of the body and brain sampling is not exactly an ideal diagnostic test. Recently, however, a highly sensitive blood test for the prion was developed which was capable of detecting CJD in 70% of patients (Jackson, 2014). It is 7000 times more likely that a positive readout is a true positive rather than a false positive. Additionally, genetic screening can indicate whether or not an individual has one of the numerous SNPs associated with a less stable (and therefore more vulnerable) form of PRNP (Soldevila, 2005; Beck et al., 2010; Sanchez-Juan et al., 2012). There are also tentative methods for MRI diagnosis of CJD (Tian et al., 2010).

As we’ve mentioned, CJD arises when a misfolded version of PRNP begins to rampantly convert normal versions of PRNP into the misfolded version, but what is PRNP’s normal role in the cell? We know that PRNP is expressed primarily in the CNS and that it has a genetic link to long-term memory (Papassotiropoulos, 2005). Another study of a PRNP homolog in mice indicated that KO mice suffered from altered circadian rhythms (Tobler et al. 1996). Importantly, however, the mice were able to survive at least for a reasonable span of time. The homolog was not absolutely vital to brain function. This being said, short of knowing the structure of the protein and that it is a glycoprotein anchored to the membrane (Zahn et al., 2000; Knaus et al., 2001), the community is still short of an actual functional mechanism for its action and its role in the cell.