Abigail B. Diack

Variant CJD: 18 years of research and surveillance

It is now 18 years since the first identification of a case of vCJD in the UK. Since that time, there has been much speculation over how vCJD might impact human health. To date there have been 177 case reports in the UK and a further 51 cases worldwide in 11 different countries. Since establishing that BSE and vCJD are of the same strain of agent, we have also shown that there is broad similarity between UK and non-UK vCJD cases on first passage to mice. Transgenic mouse studies have indicated that all codon 129 genotypes are susceptible to vCJD and that genotype may influence whether disease appears in a clinical or asymptomatic form, supported by the appearance of the first case of potential asymptomatic vCJD infection in a PRNP 129MV patient. Following evidence of blood transfusion as a route of transmission, we have ascertained that all blood components and leucoreduced blood in a sheep model of vCJD have the ability to transmit disease. Importantly, we recently established that a PRNP 129MV patient blood recipient with an asymptomatic infection and limited PrPSc deposition in the spleen could readily transmit disease into mice, demonstrating the potential for peripheral infection in the absence of clinical disease. This, along with the recent appendix survey which identified 16 positive appendices in a study of 32 441 cases, underlines the importance of continued CJD surveillance and maintaining control measures already in place to protect human health.

Post-translational changes to PrP alter transmissible spongiform encephalopathy strain properties

The agents responsible for transmissible spongiform encephalopathies (TSEs), or prion diseases, contain as a major component PrPSc, an abnormal conformer of the host glycoprotein PrPC. TSE agents are distinguished by differences in phenotypic properties in the host, which nevertheless can contain PrPSc with the same amino‐acid sequence. If PrP alone carries information defining strain properties, these must be encoded by post‐translational events. Here we investigated whether the glycosylation status of host PrP affects TSE strain characteristics. We inoculated wild‐type mice with three TSE strains passaged through transgenic mice with PrP devoid of glycans at the first, second or both N‐glycosylation sites. We compared the infectious properties of the emerging isolates with TSE strains passaged in wild‐type mice by in vivo strain typing and by the standard scrapie cell assay in vitro. Strain‐specific characteristics of the 79A TSE strain changed when PrPSc was devoid of one or both glycans. Thus infectious properties of a TSE strain can be altered by post‐translational changes to PrP which we propose result in the selection of mutant TSE strains.

Prion infectivity in the spleen of a PRNP heterozygous individual with subclinical variant Creutzfeldt–Jakob disease

Blood transfusion has been identified as a source of human-to-human transmission of variant Creutzfeldt–Jakob disease. Three cases of variant Creutzfeldt–Jakob disease have been identified following red cell transfusions from donors who subsequently developed variant Creutzfeldt–Jakob disease and an asymptomatic red cell transfusion recipient, who did not die of variant Creutzfeldt–Jakob disease, has been identified with prion protein deposition in the spleen and a lymph node, but not the brain. This individual was heterozygous (MV) at codon 129 of the prion protein gene (PRNP), whereas all previous definite and probable cases of variant Creutzfeldt–Jakob disease have been methionine homozygotes (MM). A critical question for public health is whether the prion protein deposition reported in peripheral tissues from this MV individual correlates with infectivity. Additionally it is important to establish whether the PRNP codon 129 genotype has influenced the transmission characteristics of the infectious agent. Brain and spleen from the MV blood recipient were inoculated into murine strains that have consistently demonstrated transmission of the variant Creutzfeldt–Jakob disease agent. Mice were assessed for clinical and pathological signs of disease and transmission data were compared with other transmission studies in variant Creutzfeldt–Jakob disease, including those on the spleen and brain of the donor to the index case. Transmission of variant Creutzfeldt–Jakob disease was observed from the MV blood recipient spleen, but not from the brain, whereas there was transmission from both spleen and brain tissues from the red blood cell donor. Longer incubation times were observed for the blood donor spleen inoculum compared with the blood donor brain inoculum, suggesting lower titres of infectivity in the spleen. The distribution of vacuolar pathology and abnormal prion protein in infected mice were similar following inoculation with both donor and recipient spleen homogenates, providing initial evidence of similar transmission properties after propagation in PRNP codon 129 MV and MM individuals. These studies demonstrate that spleen tissue from a PRNP MV genotype individual can propagate the variant Creutzfeldt–Jakob disease agent and that the infectious agent can be present in the spleen without CNS involvement.

The Glycosylation Status of PrPC Is a Key Factor in Determining Transmissible Spongiform Encephalopathy Transmission between Species

ABSTRACT The risk of transmission of transmissible spongiform encephalopathies (TSE) between different species has been notoriously unpredictable because the mechanisms of transmission are not fully understood. A transmission barrier between species often prevents infection of a new host with a TSE agent. Nonetheless, some TSE agents are able to cross this barrier and infect new species, with devastating consequences. The host PrPC misfolds during disease pathogenesis and has a major role in controlling the transmission of agents between species, but sequence compatibility between host and agent PrPC does not fully explain host susceptibility. PrPC is posttranslationally modified by the addition of glycan moieties which have an important role in the infectious process. Here, we show in vivo that glycosylation of the host PrPC has a significant impact on the transmission of TSE between different host species. We infected mice carrying different glycosylated forms of PrPC with two human agents (sCJDMM2 and vCJD) and one hamster strain (263K). The absence of glycosylation at both or the first PrPC glycosylation site in the host results in almost complete resistance to disease. The absence of the second site of N-glycan has a dramatic effect on the barrier to transmission between host species, facilitating the transmission of sCJDMM2 to a host normally resistant to this agent. These results highlight glycosylation of PrPC as a key factor in determining the transmission efficiency of TSEs between different species. IMPORTANCE The risks of transmission of TSE between different species are difficult to predict due to a lack of knowledge over the mechanisms of disease transmission; some strains of TSE are able to cross a species barrier, while others do not. The host protein, PrPC, plays a major role in disease transmission. PrPC undergoes posttranslational glycosylation, and the addition of these glycans may play a role in disease transmission. We infected mice that express different forms of glycosylated PrPC with three different TSE agents. We demonstrate that changing the glycosylation status of the host can have profound effects on disease transmission, changing host susceptibility and incubation times. Our results show that PrPC glycosylation is a key factor in determining risks of TSE transmission between species.

Variably Protease-Sensitive Prionopathy, a Unique Prion Variant with Inefficient Transmission Properties

Transmission properties of this prion disease are biologically distinct, and the disease has limited potential for human-to-human transmission.

Insights into Mechanisms of Chronic Neurodegeneration.

Chronic neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and prion diseases are characterised by the accumulation of abnormal conformers of a host encoded protein in the central nervous system. The process leading to neurodegeneration is still poorly defined and thus development of early intervention strategies is challenging. Unique amongst these diseases are Transmissible Spongiform Encephalopathies (TSEs) or prion diseases, which have the ability to transmit between individuals. The infectious nature of these diseases has permitted in vivo and in vitro modelling of the time course of the disease process in a highly reproducible manner, thus early events can be defined. Recent evidence has demonstrated that the cell-to-cell spread of protein aggregates by a “prion-like mechanism” is common among the protein misfolding diseases. Thus, the TSE models may provide insights into disease mechanisms and testable hypotheses for disease intervention, applicable to a number of these chronic neurodegenerative diseases.

Constant Transmission Properties of Variant Creutzfeldt-Jakob Disease in 5 Countries

Current diagnostic criteria should be sufficient to detect new cases of vCJD.

Ablation of Prion Protein in Wild Type Human Amyloid Precursor Protein (APP) Transgenic Mice Does Not Alter The Proteolysis of APP, Levels of Amyloid-β or Pathologic Phenotype.

The cellular prion protein (PrPC) has been proposed to play an important role in the pathogenesis of Alzheimer’s disease. In cellular models PrPC inhibited the action of the β-secretase BACE1 on wild type amyloid precursor protein resulting in a reduction in amyloid-β (Aβ) peptides. Here we have assessed the effect of genetic ablation of PrPC in transgenic mice expressing human wild type amyloid precursor protein (line I5). Deletion of PrPC had no effect on the α- and β-secretase proteolysis of the amyloid precursor protein (APP) nor on the amount of Aβ38, Aβ40 or Aβ42 in the brains of the mice. In addition, ablation of PrPC did not alter Aβ deposition or histopathology phenotype in this transgenic model. Thus using this transgenic model we could not provide evidence to support the hypothesis that PrPC regulates Aβ production.

Public health risks from subclinical variant CJD

Variant Creutzfeldt-Jakob disease (vCJD) is a zoonotic prion disease thought to have been transmitted to humans through the consumption of food products contaminated with bovine spongiform encephalopathy (BSE) in the 1980s and/or early 1990s. As with all prion disorders, it is a fatal neurodegenerative disease arising from conversion of the normal cellular form of the prion protein PrP, encoded by the prion gene (PRNP), to an abnormal form associated with disease (PrP). Prion diseases have long asymptomatic incubation periods ranging from years to decades. Extensive studies involving both natural and experimental animal diseases have demonstrated that infectivity is present during the subclinical phase and may be transmitted between individuals. While the cases of clinical vCJD currently appear to be in decline, one of the current challenges is defining the prevalence of subclinical disease and the risk this poses to both individuals and the general population.

The cellular form of the prion protein PrPC is processed to varying degrees in different species and PRNP genotypes

Creutzfeldt-Jakob disease (CJD) is characterized by a variety of symptoms including rapidly progressive neurocognitive decline, ataxic gait, and myoclonus. Sporadic CJD (sCJD) has a median disease duration of 6 months. Based on the prion protein gene (PRNP) codon 129 polymorphism and prion protein type, seven different molecular subtypes of sCJD have been identified (MM1, MM2-C, MM2-T, MV1, MV2, VV1, VV2). The VV1 subtype is the rarest molecular subtype of sCJD (only about 1% of cases). To date, reported cases of sCJD VV1 have been characterized by young age at disease onset, an increased length of disease duration, and progressive neurocognitive degeneration. We describe a unique case of sCJD VV1 with older age at disease onset and short illness duration. VV1 is a challenging subtype to diagnose and this report adds further complexity and variation to its clinical characterization. Although PRNP codon 129 polymorphism and prion protein type exert a large influence on clinical and neuropathologic phenomenology in sCJD, other factors remain unknown and require further study. P.02: A transfectant RK13 cell line permissive to caprine scrapie prion infection