Role of sialylation of N-linked glycans in prion pathogenesis

Mammalian prion or PrPSc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of the prion protein or PrPC. PrPC and PrPSc are posttranslationally modified with N-linked glycans, which are sialylated at the terminal positions. More than 30 years have passed since the first characterization of the composition and structural diversity of N-linked glycans associated with the prion protein, yet the role of carbohydrate groups that constitute N-glycans and, in particular, their terminal sialic acid residues in prion disease pathogenesis remains poorly understood. A number of recent studies shed a light on the role of sialylation in the biology of prion diseases. This review article discusses several mechanisms by which terminal sialylation dictates the spread of PrPSc across brain regions and the outcomes of prion infection in an organism. In particular, relationships between the sialylation status of PrPSc and important strain-specific features including lymphotropism, neurotropism, and neuroinflammation are discussed. Moreover, emerging evidence pointing out the roles of sialic acid residues in prion replication, cross-species transmission, strain competition, and strain adaptation are reviewed. A hypothesis according to which selective, strain-specified recruitment of PrPC sialoglycoforms dictates unique strain-specific disease phenotypes is examined. Finally, the current article proposes that prion strains evolve as a result of a delicate balance between recruiting highly sialylated glycoforms to avoid an "eat-me" response by glia and limiting heavily sialylated glycoforms for enabling rapid prion replication.

Genomic, transcriptomic and RNA editing analysis of human MM1 and VV2 sporadic Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease (CJD) is characterized by a broad phenotypic spectrum regarding symptoms, progression, and molecular features. Current sporadic CJD (sCJD) classification recognizes six main clinical-pathological phenotypes. This work investigates the molecular basis of the phenotypic heterogeneity of prion diseases through a multi-omics analysis of the two most common sCJD subtypes: MM1 and VV2. We performed DNA target sequencing on 118 genes on a cohort of 48 CJD patients and full exome RNA sequencing on post-mortem frontal cortex tissue on a subset of this cohort. DNA target sequencing identified multiple potential genetic contributors to the disease onset and phenotype, both in terms of coding, damaging-predicted variants, and enriched groups of SNPs in the whole cohort and the two subtypes. The results highlight a different functional impairment, with VV2 associated with higher impairment of the pathways related to dopamine secretion, regulation of calcium release and GABA signaling, showing some similarities with Parkinson's disease both on a genomic and a transcriptomic level. MM1 showed a gene expression profile with several traits shared with different neurodegenerative, without an apparent distinctive characteristic or similarities with a specific disease. In addition, integrating genomic and transcriptomic data led to the discovery of several sites of ADAR-mediated RNA editing events, confirming and expanding previous findings in animal models. On the transcriptomic level, this work represents the first application of RNA sequencing on CJD human brain samples. Here, a good clusterization of the transcriptomic profiles of the two subtypes was achieved, together with the finding of several differently impaired pathways between the two subtypes. The results add to the understanding of the molecular features associated with sporadic CJD and its most common subtypes, revealing strain-specific genetic signatures and functional similarities between VV2 and Parkinson's disease and providing preliminary evidence of RNA editing modifications in human sCJD.

Emergence of CWD strains

Chronic wasting disease (CWD) strains present a novel challenge to defining and mitigating this contagious prion disease of deer, elk, moose, and reindeer. Similar to strains of other prion diseases (bovine spongiform encephalopathy, sheep scrapie), CWD strains can affect biochemical and neuropathological properties of the infectious agent, and importantly interspecies transmission. To date, ten CWD strains have been characterized. The expanding range of CWD in North America and its presence in South Korea as well as Scandinavian countries will potentially result in millions of cervids infected with CWD; thus, novel strains will continue to emerge. In this review, we will summarize the characteristics of known CWD strains and describe the impact of prion protein gene polymorphisms on the generation of strains. We will also discuss the evidence that individual cervids can harbor more than one CWD strain, complicating strain analysis, and affecting selection and adaptation of strains in new hosts.

Prion strains viewed through the lens of cryo-EM

Mammalian prions are lethal transmissible pathogens that cause fatal neurodegenerative diseases in humans and animals. They consist of fibrils of misfolded, host-encoded prion protein (PrP) which propagate through templated protein polymerisation. Prion strains produce distinct clinicopathological phenotypes in the same host and appear to be encoded by distinct misfolded PrP conformations and assembly states. Despite fundamental advances in our understanding of prion biology, key knowledge gaps remain. These include precise delineation of prion replication mechanisms, detailed explanation of the molecular basis of prion strains and inter-species transmission barriers, and the structural definition of neurotoxic PrP species. Central to addressing these questions is the determination of prion structure. While high-resolution definition of ex vivo prion fibrils once seemed unlikely, recent advances in cryo-electron microscopy (cryo-EM) and computational methods for 3D reconstruction of amyloids have now made this possible. Recently, near-atomic resolution structures of highly infectious, ex vivo prion fibrils from hamster 263K and mouse RML prion strains were reported. The fibrils have a comparable parallel in-register intermolecular β-sheet (PIRIBS) architecture that now provides a structural foundation for understanding prion strain diversity in mammals. Here, we review these new findings and discuss directions for future research.

Sporadic Creutzfeldt-Jakob disease VM1: phenotypic and molecular characterization of a novel subtype of human prion disease

The methionine (M)—valine (V) polymorphic codon 129 of the prion protein gene (PRNP) plays a central role in both susceptibility and phenotypic expression of sporadic Creutzfeldt-Jakob diseases (sCJD). Experimental transmissions of sCJD in humanized transgenic mice led to the isolation of five prion strains, named M1, M2C, M2T, V2, and V1, based on two major conformations of the pathological prion protein (PrP^Sc, type 1 and type 2), and the codon 129 genotype determining susceptibility and propagation efficiency. While the most frequent sCJD strains have been described in codon 129 homozygosis (MM1, MM2C, VV2) and heterozygosis (MV1, MV2K, and MV2C), the V1 strain has only been found in patients carrying VV. We identified six sCJD cases, 4 in Catalonia and 2 in Italy, carrying MV at PRNP codon 129 in combination with PrP^Sc type 1 and a new clinical and neuropathological profile reminiscent of the VV1 sCJD subtype rather than typical MM1/MV1. All patients had a relatively long duration (mean of 20.5 vs. 3.5 months of MM1/MV1 patients) and lacked electroencephalographic periodic sharp-wave complexes at diagnosis. Distinctive histopathological features included the spongiform change with vacuoles of larger size than those seen in sCJD MM1/MV1, the lesion profile with prominent cortical and striatal involvement, and the pattern of PrP^Sc deposition characterized by a dissociation between florid spongiform change and mild synaptic deposits associated with coarse, patch-like deposits in the cerebellar molecular layer. Western blot analysis of brain homogenates revealed a PrP^Sc type 1 profile with physicochemical properties reminiscent of the type 1 protein linked to the VV1 sCJD subtype. In summary, we have identified a new subtype of sCJD with distinctive clinicopathological features significantly overlapping with those of the VV1 subtype, possibly representing the missing evidence of V1 sCJD strain propagation in the 129MV host genotype.

Transmission of cervid prions to humanized mice demonstrates the zoonotic potential of CWD

Prions cause infectious and fatal neurodegenerative diseases in mammals. Chronic wasting disease (CWD), a prion disease of cervids, spreads efficiently among wild and farmed animals. Potential transmission to humans of CWD is a growing concern due to its increasing prevalence. Here, we provide evidence for a zoonotic potential of CWD prions, and its probable signature using mice expressing human prion protein (PrP) as an infection model. Inoculation of these mice with deer CWD isolates resulted in atypical clinical manifestation with prion seeding activity and efficient transmissible infectivity in the brain and, remarkably, in feces, but without classical neuropathological or Western blot appearances of prion diseases. Intriguingly, the protease-resistant PrP in the brain resembled that found in a familial human prion disease and was transmissible upon second passage. Our results suggest that CWD might infect humans, although the transmission barrier is likely higher compared to zoonotic transmission of cattle prions. Notably, our data suggest a different clinical presentation, prion signature, and tissue tropism, which causes challenges for detection by current diagnostic assays. Furthermore, the presence of infectious prions in feces is concerning because if this occurs in humans, it is a source for human-to-human transmission. These findings have strong implications for public health and CWD management.

Phenotypic diversity of genetic Creutzfeldt-Jakob disease: a histo-molecular-based classification

The current classification of sporadic Creutzfeldt–Jakob disease (sCJD) includes six major clinicopathological subtypes defined by the physicochemical properties of the protease-resistant core of the pathologic prion protein (PrP^Sc), defining two major PrP^Sc types (i.e., 1 and 2), and the methionine (M)/valine (V) polymorphic codon 129 of the prion protein gene (PRNP). How these sCJD subtypes relate to the well-documented phenotypic heterogeneity of genetic CJD (gCJD) is not fully understood. We analyzed molecular and phenotypic features in 208 individuals affected by gCJD, carrying 17 different mutations, and compared them with those of a large series of sCJD cases. We identified six major groups of gCJD based on the combination PrP^Sc type and codon 129 genotype on PRNP mutated allele, each showing distinctive histopathological characteristics, irrespectively of the PRNP associated mutation. Five gCJD groups, named M1, M2C, M2T, V1, and V2, largely reproduced those previously described in sCJD subtypes. The sixth group shared phenotypic traits with the V2 group and was only detected in patients carrying the E200K-129M haplotype in association with a PrP^Sc type of intermediate size (“i”) between type 1 and type 2. Additional mutation-specific effects involved the pattern of PrP deposition (e.g., a “thickened” synaptic pattern in E200K carriers, cerebellar “stripe-like linear granular deposits” in those with insertion mutations, and intraneuronal globular dots in E200K-V2 or -M”i”). A few isolated cases linked to rare PRNP haplotypes (e.g., T183A-129M), showed atypical phenotypic features, which prevented their classification into the six major groups. The phenotypic variability of gCJD is mostly consistent with that previously found in sCJD. As in sCJD, the codon 129 genotype and physicochemical properties of PrP^Sc significantly correlated with the phenotypic variability of gCJD. The most common mutations linked to CJD appear to have a variable and overall less significant effect on the disease phenotype, but they significantly influence disease susceptibility often in a strain-specific manner. The criteria currently used for sCJD subtypes can be expanded and adapted to gCJD to provide an updated classification of the disease with a molecular basis.

The degree of astrocyte activation is predictive of the incubation time to prion disease

In neurodegenerative diseases including Alzheimer’s, Parkinson’s and prion diseases, astrocytes acquire disease-associated reactive phenotypes. With growing appreciation of their role in chronic neurodegeneration, the questions whether astrocytes lose their ability to perform homeostatic functions in the reactive states and whether the reactive phenotypes are neurotoxic or neuroprotective remain unsettled. The current work examined region-specific changes in expression of genes, which report on astrocyte physiological functions and their reactive states, in C57Black/6J mice challenged with four prion strains via two inoculation routes. Unexpectedly, strong reverse correlation between the incubation time to the diseases and the degree of astrocyte activation along with disturbance in functional pathways was observed. The animal groups with the most severe astrocyte response and degree of activation showed the most rapid disease progression. The degree of activation tightly intertwined with the global transformation of the homeostatic state, characterized by disturbances in multiple gene sets responsible for normal physiological functions producing a neurotoxic, reactive phenotype as a net result. The neurotoxic reactive phenotype exhibited a universal gene signature regardless of the prion strain. The current work suggests that the degree of astrocyte activation along with the disturbance in their physiological pathways contribute to the faster progression of disease and perhaps even drive prion pathogenesis.

β-Turn exchanges in the α-synuclein segment 44-TKEG-47 reveal high sequence fidelity requirements of amyloid fibril elongation

The folding of turns and β-hairpins has been implicated in amyloid formation, with diverse potential consequences such as promotion or inhibition of fibril nucleation, fibril elongation, or off-pathway oligomer formation. In the Parkinson's disease-associated protein α-synuclein (αS), a β-hairpin comprised of residues 36-56 was detected in complex with an engineered binding protein, with a turn formed by the αS sequence segment 44-TKEG-47. Molecular dynamics simulations revealed extensive populations of transient β-hairpin conformations in this region in free, monomeric αS. Here, we investigated potential effects of turn formation on αS fibril formation by studying the aggregation kinetics of an extensive set of αS variants with between two and four amino acid exchanges in the 44-TKEG-47 segment. The exchanges were chosen to specifically promote formation of β1-, β1'-, or β2'-turns. All variants assembled into amyloid fibrils, with increased β1'- or β2'-turn propensity associated with faster aggregation and increased β1-turn propensity with slower aggregation compared to wild-type (WT) αS. Atomic force microscopy demonstrated that β-turn exchanges altered fibril morphology. In cross-elongation experiments, the turn variants showed a low ability to elongate WT fibril seeds, and, vice versa, WT monomer did not efficiently elongate turn variant fibril seeds. This demonstrates that sequence identity in the turn region is crucial for efficient αS fibril elongation. Elongation experiments of WT fibril seeds in the presence of both WT and turn variant monomers suggest that the turn variants can bind and block WT fibril ends to different degrees, but cannot efficiently convert into the WT fibril structure. Our results indicate that modifications in the 44-TKEG-47 segment strongly affect amyloid assembly by driving αS into alternative fibril morphologies, whose elongation requires high sequence fidelity.

Role of sialylation in prion disease pathogenesis and prion structure

Mammalian prion or PrP^Sc is a proteinaceous infectious agent that consists of a misfolded, self-replicating state of a sialoglycoprotein called the prion protein or PrP^C. Sialylation of the prion protein, a terminal modification of N-linked glycans, was discovered more than 30 years ago, yet the role of sialylation in prion pathogenesis is not well understood. This chapter summarizes current knowledge on the role of sialylation of the prion protein in prion diseases. First, we discuss recent data suggesting that sialylation of PrP^Sc N-linked glycans determines the fate of prion infection in an organism and control prion lymphotropism. Second, emerging evidence pointing out at the role N-glycans in neuroinflammation are discussed. Thirds, this chapter reviews a mechanism postulating that sialylated N-linked glycans are important players in defining strain-specific structures. A new hypothesis according to which individual strain-specific PrP^Sc structures govern selection of PrP^C sialoglycoforms is discussed. Finally, this chapter explain how N-glycan sialylation control the prion replication and strain interference. In summary, comprehensive review of our knowledge on N-linked glycans and their sialylation provided in this chapter helps to answer important questions of prion biology that have been puzzling for years.

Prion protein post-translational modifications modulate heparan sulfate binding and limit aggregate size in prion disease

Many aggregation-prone proteins linked to neurodegenerative disease are post-translationally modified during their biogenesis. In vivo pathogenesis studies have suggested that the presence of post-translational modifications can shift the aggregate assembly pathway and profoundly alter the disease phenotype. In prion disease, the N-linked glycans and GPI-anchor on the prion protein (PrP) impair fibril assembly. However, the relevance of the two glycans to aggregate structure and disease progression remains unclear. Here we show that prion-infected knockin mice expressing an additional PrP glycan (tri-glycosylated PrP) develop new plaque-like deposits on neuronal cell membranes, along the subarachnoid space, and periventricularly, suggestive of high prion mobility and transit through the interstitial fluid. These plaque-like deposits were largely non-congophilic and composed of full length, uncleaved PrP, indicating retention of the glycophosphatidylinositol (GPI) anchor. Prion aggregates sedimented in low density fractions following ultracentrifugation, consistent with oligomers, and bound low levels of heparan sulfate (HS) similar to other predominantly GPI-anchored prions. Collectively, these results suggest that highly glycosylated PrP primarily converts as a GPI-anchored glycoform, with low involvement of HS co-factors, limiting PrP assembly mainly to oligomers. Since PrP^C is highly glycosylated, these findings may explain the high frequency of diffuse, synaptic, and plaque-like deposits in the brain as well as the rapid conversion commonly observed in human and animal prion disease.

Region-specific glial homeostatic signature in prion diseases is replaced by a uniform neuroinflammation signature, common for brain regions and prion strains with different cell tropism

Chronic neuroinflammation is recognized as a major neuropathological hallmark in a broad spectrum of neurodegenerative diseases including Alzheimer's, Parkinson's, Frontal Temporal Dementia, Amyotrophic Lateral Sclerosis, and prion diseases. Both microglia and astrocytes exhibit region-specific homeostatic transcriptional identities, which under chronic neurodegeneration, transform into reactive phenotypes in a region- and disease-specific manner. Little is known about region-specific identity of glia in prion diseases. The current study was designed to determine whether the region-specific homeostatic signature of glia changes with the progression of prion diseases, and whether these changes occur in a region-dependent or universal manner. Also of interest was whether different prion strains give rise to different reactive phenotypes. To answer these questions, we analyzed gene expression in the thalamus, cortex, hypothalamus and hippocampus of mice infected with 22L and ME7 prion strains using a Nanostring Neuroinflammation panel at the subclinical, early clinical and advanced stages of the disease. We found that at the preclinical stage of the disease, the region-specific homeostatic identities were preserved. However, with the appearance of clinical signs, the region-specific signatures were partially lost and replaced with a neuroinflammation signature. While the same sets of genes were activated by both prion strains, the timing of neuroinflammation and the degree of activation in different brain regions was strain-specific. Changes in astrocyte function scored at the top of the activated pathways. Moreover, clustering analysis suggested that the astrocyte function pathway responded to prion infection prior to the Activated Microglia or Neuron and Neurotransmission pathways. The current work established neuroinflammation gene expression signature associated with prion diseases. Our results illustrate that with the disease progression, the region-specific homeostatic transcriptome signatures are replaced by the region-independent neuroinflammation signature, which is common for prion strains with different cell tropism. The prion-associated neuroinflammation signature identified in the current study overlapped only partially with the microglia degenerative phenotype and the disease-associated microglia phenotype reported for animal models of other neurodegenerative diseases.

The role of prion strain diversity in the development of successful therapeutic treatments

Prions are a self-propagating misfolded conformation of a cellular protein. Prions are found in several eukaryotic organisms with mammalian prion diseases encompassing a wide range of disorders. The first recognized prion disease, the transmissible spongiform encephalopathies (TSEs), affect several species including humans. Alzheimer's disease, synucleinopathies, and tauopathies share a similar mechanism of self-propagation of the prion form of the disease-specific protein reminiscent of the infection process of TSEs. Strain diversity in prion disease is characterized by differences in the phenotype of disease that is hypothesized to be encoded by strain-specific conformations of the prion form of the disease-specific protein. Prion therapeutics that target the prion form of the disease-specific protein can lead to the emergence of drug-resistant strains of prions, consistent with the hypothesis that prion strains exist as a dynamic mixture of a dominant strain in combination with minor substrains. To overcome this obstacle, therapies that reduce or eliminate the template of conversion are efficacious, may reverse neuropathology, and do not result in the emergence of drug resistance. Recent advancements in preclinical diagnosis of prion infection may allow for a combinational approach that treats the prion form and the precursor protein to effectively treat prion diseases.

Cofactors influence the biological properties of infectious recombinant prions

Prion diseases are caused by a misfolding of the cellular prion protein (PrP) to a pathogenic isoform named PrP^Sc. Prions exist as strains, which are characterized by specific pathological and biochemical properties likely encoded in the three-dimensional structure of PrP^Sc. However, whether cofactors determine these different PrP^Sc conformations and how this relates to their specific biological properties is largely unknown. To understand how different cofactors modulate prion strain generation and selection, Protein Misfolding Cyclic Amplification was used to create a diversity of infectious recombinant prion strains by propagation in the presence of brain homogenate. Brain homogenate is known to contain these mentioned cofactors, whose identity is only partially known, and which facilitate conversion of PrP^C to PrP^Sc. We thus obtained a mix of distinguishable infectious prion strains. Subsequently, we replaced brain homogenate, by different polyanionic cofactors that were able to drive the evolution of mixed prion populations toward specific strains. Thus, our results show that a variety of infectious recombinant prions can be generated in vitro and that their specific type of conformation, i.e., the strain, is dependent on the cofactors available during the propagation process. These observations have significant implications for understanding the pathogenesis of prion diseases and their ability to replicate in different tissues and hosts. Importantly, these considerations might apply to other neurodegenerative diseases for which different conformations of misfolded proteins have been described.

Prion Protein and Genetic Susceptibility to Diseases Caused by Its Misfolding

Early genetic studies on scrapie, an infectious neurodegenerative disease of sheep that was adapted to mice, provided evidence in support of the hypothesis that the agent was a slow virus with a nucleic acid genome independent of the host. Particularly compelling support for an independent genome came from the existence of strains of scrapie agent, some of which were true breeding, while others appeared to mutate under selective pressure. Kuru, a neurodegenerative disease in the remote highlands of Papua New Guinea, had pathological changes similar to those in scrapie and also proved to be transmissible. Genetic studies with the tools of molecular biology and transgenic mice forced a reevaluation of earlier work and supported the prion hypothesis of a novel pathogen devoid of nucleic acid. In this chapter, I discuss the contributions of classical and molecular genetics to understanding PrP prion diseases and to determining that heritable information is enciphered in protein conformation.

Gene Targeted Transgenic Mouse Models in Prion Research

The production of transgenic mice expressing different forms of the prion protein (PrP) or devoid of PrP has enabled researchers to study the role of PrP in the infectious process of a prion disease and its normal function in the healthy individual. A wide range of transgenic models have been produced ranging from PrP null mice, normal expression levels to overexpression models, models expressing different species of the Prnp gene and different mutations and polymorphisms within the gene. Using this range of transgenic models has allowed us to define the influence of PrP expression on disease susceptibility and transmission, assess zoonotic potential, define strains of human prion diseases, elucidate the function of PrP, and start to unravel the mechanisms involved in chronic neurodegeneration. This chapter focuses mainly on the use of the gene targeted transgenic models and summarizes the ways in which they have allowed us to study the role of PrP in prion disease and the insights they have provided into the mechanisms of neurodegenerative diseases.

Enhanced neuroinvasion by smaller, soluble prions

Infectious prion aggregates can propagate from extraneural sites into the brain with remarkable efficiency, likely transported via peripheral nerves. Yet not all prions spread into the brain, and the physical properties of a prion that is capable of transit within neurons remain unclear. We hypothesized that small, diffusible aggregates spread into the CNS via peripheral nerves. Here we used a structurally diverse panel of prion strains to analyze how the prion conformation impacts transit into the brain. Two prion strains form fibrils visible ultrastructurally in the brain in situ, whereas three strains form diffuse, subfibrillar prion deposits and no visible fibrils. The subfibrillar strains had significantly higher levels of soluble prion aggregates than the fibrillar strains. Primary neurons internalized both the subfibrillar and fibril-forming prion strains by macropinocytosis, and both strain types were transported from the axon terminal to the cell body in vitro. However in mice, only the predominantly soluble, subfibrillar prions, and not the fibrillar prions, were efficiently transported from the tongue to the brain. Sonicating a fibrillar prion strain increased the solubility and enabled prions to spread into the brain in mice, as evident by a 40% increase in the attack rate, indicating that an increase in smaller particles enhances prion neuroinvasion. Our data suggest that the small, highly soluble prion particles have a higher capacity for transport via nerves. These findings help explain how prions that predominantly assemble into subfibrillar states can more effectively traverse into and out of the CNS, and suggest that promoting fibril assembly may slow the neuron-to-neuron spread of protein aggregates.

Methods of Protein Misfolding Cyclic Amplification

Protein misfolding cyclic amplification (PMCA) amplifies infectious prions in vitro. Over the past decade, PMCA has become an essential tool in prion research. The current chapter describes in detail the PMCA format with beads (PMCAb) and several methods that rely on PMCAb for assessing strain-specific prion amplification rates, for selective amplification of subtypes of PrP^Sc from a mixture, and a PMCAb approach that can replace animal titration of scrapie material. Development of PMCAb-based methodology is important for addressing a number of research topics including prion strain evolution, selection and adaptation, strain-typing, prion detection, and biochemical requirements for prion replication.

Infectious prion diseases in humans: cannibalism, iatrogenicity and zoonoses

In contrast with other neurodegenerative disorders associated to protein misfolding, human prion diseases include infectious forms (also called transmitted forms) such as kuru, iatrogenic Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease. The transmissible agent is thought to be solely composed of the abnormal isoform (PrP(Sc)) of the host-encoded prion protein that accumulated in the central nervous system of affected individuals. Compared to its normal counterpart, PrP(Sc) is β-sheet enriched and aggregated and its propagation is based on an autocatalytic conversion process. Increasing evidence supports the view that conformational variations of PrP(Sc) encoded the biological properties of the various prion strains that have been isolated by transmission studies in experimental models. Infectious forms of human prion diseases played a pivotal role in the emergence of the prion concept and in the characterization of the very unconventional properties of prions. They provide a unique model to understand how prion strains are selected and propagate in humans. Here, we review and discuss how genetic factors interplay with strain properties and route of transmission to influence disease susceptibility, incubation period and phenotypic expression in the light of the kuru epidemics due to ritual endocannibalism, the various series iatrogenic diseases secondary to extractive growth hormone treatment or dura mater graft and the epidemics of variant Creutzfeldt-Jakob disease linked to dietary exposure to the agent of bovine spongiform encephalopathy.