Prion (pronounced “pree-on”) diseases are rare neurodegenerative diseases affecting about one in a million people worldwide.
Despite this low prevalence, prion diseases constitute an important disease class as they represent the only known neurodegenerative disease which, in addition to sporadic and familial modes of manifestation (approximately 10% of all cases), can spread horizontally among humans or livestock through an infectious mode of propagation. Prions are also unique amongst infectious pathogens in their ability to direct disease replication without the apparent need for nucleic acids.
Prion diseases in humans are known as Creutzfeldt-Jakob Disease (CJD). Rarer forms of manifestation are known as Gerstmann-Sträussler-Scheinker Syndrome (GSS) and Fatal Familial Insomnia (FFI). The better known prion diseases in livestock are “Scrapie” in sheep and goats, Bovine Spongiform Encephalopathy (BSE) in cattle and Chronic Wasting Disease (CWD) in elk and mule deer.
In the past decade, public awareness of these diseases has grown in parallel with the occurrence of cases of BSE (also known as “Mad Cow Disease”) and the realization that this disease can in rare instances be transmitted to humans through the consumption of contaminated meat and give rise to a variant form of Creutzfeldt-Jakob disease.
In the past few years it has become apparent that the most common dementias, including Alzheimer’s and Parkinson’s disease, spread within the brains of afflicted individuals by a mechanism that was first established to exist in Prion diseases. As a result, insights gained from the study of Prion diseases and the scientific methods used to investigate these relatively rare diseases have become critical for understanding the most common dementias.
The normal cellular prion protein, denoted PrPC is found in most cell types within the body. In disease, this protein undergoes a structural transition to its disease-causing form (PrPSc) with profoundly different physicochemical properties. One of these properties is a resistance to the digestion by enzymes that normally break down proteins.
The conversion of PrPC to PrPSc appears to require localization of PrPC to lipid rafts, specialized cellular membrane regions rich in a subset of cellular lipids such as cholesterol. However, the precise physiological environment which hosts this event remains elusive. In all familial cases of prion disease the above aberrant processes can be linked to the presence of a mutation in the human gene which encodes the prion protein.
Pathology and clinical manifestation
Prion diseases are characterized by the aggregation of PrPSc within the central nervous system. These aggregates can be visualized with specific stains (such as Congo Red stain) and are often referred to “plaques”. The toxic accumulation of PrPSc in turn causes the formation of large intracellular holes (vacuole formation) and neuronal death. Together these morphological changes create a sponge-like appearance of the brain tissue (the reason why prion diseases are sometimes referred to as Transmissible Spongiform Encephalopathies (TSEs)). Other histological changes include an increase in star-shaped non-neuronal cells (astrogliosis) and the conspicuous absence of an overt inflammatory reaction.
The incubation period for prion diseases generally extends over multiple years. However, once symptoms appear the disease progresses rapidly, leading to widespread brain damage. Neurodegenerative symptoms can include dementia, ataxia (dysfunction of balance and coordination), convulsions and behavioural or personality changes. Death typically occurs within one to three years from the onset of clinical symptoms.
Currently, all known prion diseases are untreatable and fatal. Attempts to generate vaccines and pharmacological treatments are ongoing around the world. In livestock, elimination of the prion gene through genetic engineering has been shown to render animals immune to prion infection. Consequently, scientists at the Tanz CRND and elsewhere are looking for ways to reduce the levels of PrPC, a promising avenue for the treatment of prion diseases. Because PrPC has also been implicated in cellular toxicity that manifests in Alzheimer's disease (AD), a successful PrPC reduction strategy may also be of benefit for individuals afflicted with AD.
Prion Diseases Breakthroughs
Tanz Centre scientists have made some important discoveries in the field of prion diseases, including:
Tanz researchers co-discovered the “Doppel” (PRND) protein (2001 and 2004).
Doppel was the first family members of the PrP family to be discovered. We demonstrated that its biology is linked and antagonistic to PrP.
The discovery helped to shed light on the causes of a mysterious ataxic phenotype observed in a subset of PrP knockout mice. Since this seminal discovery, more than 150 papers have been published on Doppel and its relationship to PrP.
Demonstrating that the SPRN gene encodes a CNS expressed protein “Shadoo” with biochemical resemblance to PrP (2007).
Tanz scientists demonstrated that Shadoo confers protective activity in functional assays in cerebellar neurons. Most remarkably, steady state levels of the shadoo protein are markedly reduced in prion infected animals. A simple hypothesis emerged from this line of work which posits that clinical target areas in prion disease reflect loss of shadoo’s protective action.
Identification of the evolutionary origins and the mechanism of evolution of the prion protein (2009).
Tanz scientists discovered the evolutionary origins of the prion gene, best known for causing invariably fatal diseases in humans and livestock. Our subsequent work revealed that the prion founder gene emerged from a genomic insertion of a spliced and C-terminally truncated transcript of a ZIP metal ion transporter. These discoveries explained characteristics of the prion protein (PrP) as remnants of an ancient function in the sensing and transport of metal ions.
First PrP knockout cell models using CRISRP-Cas9 technology (2014).
Tanz scientists successfully generate a first PrP knockout cell model using a novel genetic engineering technology, known as CRISPR-Cas9. In total we reported the generation of three cell models, in which the ability of the cell to produce PrP was eliminated. These models have are now employed by several groups around in efforts to study the molecular biology of the PrP in health and disease.
Over the years, Tanz scientists have shaped our current understanding of the molecular environment of the prion protein (PrP).
In 2001, we identified the neural cell adhesion molecule (NCAM) as the most prominent next neighbor to PrP in neurons. In 2004, we identified more than two dozen proteins that reside in immediate proximity to PrP in the intact brain. In 2009 we reported the first study that investigated the molecular environment of PrP in a relevant cell model by comparative and quantitative mass spectrometry. Striking about these data was that the majority of proteins we observed in proximity to PrP could be explained by a simple model, thereby providing one of the first glimpses into the determinants that shape the molecular environment of a specific membrane protein.
Elucidation of PrP’s cellular role in the polysialylation of the neural cell adhesion molecule 1 (2016).
This project emerged from our earlier study of the evolutionary origins of the prion gene and our successful generation of a first CRISRP-Cas9 PrP knockout cell model. Recognizing that ZIP transporters most closely related to PrP play a critical role in a program known as epithelial to mesenchymal transition (EMT), led us to investigate if PrP inherited an involvement in this morphogenetic program. Indeed, we discovered that PrP levels are massively upregulated during EMT and its presence is essential for the execution of a signaling loop that controls NCAM1 polysialylation. This insight represents a milestone in efforts to reveal PrP’s elusive function.
Establishment of a framework for assessing protein function and its application to the prion protein (2021).
Tanz scientists proposed a framework for inferring a protein’s function from four data categories: ‘inheritance’, ‘distribution’, ‘interactions’ and ‘phenotypes’ (IDIP). We showed that the functions of proteins emerge at the intersection of inferences drawn from these data categories and emphasized the benefit of considering them in an evolutionary context. The main function of the cellular prion protein (PrPC) has been under intense investigation and debate for more than 30 years. When the IDIP framework is applied to PrPC, available data indicate that a role we had first proposed for this protein in 2016 (see above) may indeed be its main function, namely to control a critical post-translational modification of the neural cell adhesion molecule in the context of epithelial-to-mesenchymal transition and related plasticity programs. This insight contributes to our understanding of whether it is safe to target the prion protein therapeutically, a promising disease intervention strategy for the treatment of prion diseases and, possibly, Alzheimer’s disease.