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Variant Creutzfeldt-Jakob disease (CJD) is a novel form of human prion disease that appears to result from oral infection by the bovine spongiform encephalopathy (BSE) agent. Variant CJD is also unique in human prion diseases in that infectivity and accumulation of the disease-associated isoform of prion protein are readily detectable outside the central nervous system, perhaps reflecting the peripheral pathogenesis of this disorder following an oral infection with BSE. The neuropathological features of variant CJD are unique in terms of the histological features and the biochemical features of the abnormal isoform of prion protein in the brain and in lymphoid tissues. This peripheral accumulation of infectivity has also resulted in the apparent iatrogenic transmission of variant CJD on 2 occasions, following a transfusion with non-leucodepleted red blood cells from donors who subsequently died from variant CJD. All clinical cases of variant CJD have so far occurred in individuals who are methionine homozygotes at codon 129 in the prion protein gene. However, one of the iatrogenic infections occurred in an individual who is heterozygous at this locus, indicating the future clinical cases might also occur in this group. Continuing surveillance of all forms of CJD is required to address this possibility, not just in the UK but in other countries either with or at risk of cases of BSE in the cattle population.

Variant CJD is one of the human prion diseases and has relatively uniform clinico-pathological characteristics, reflecting its presumed single cause and the uniform genotype of affected individuals. Typically, it presents with predominantly psychiatric features (often relatively non-specific ones suggesting depression) and any accompanying neurological features are often interpreted as part of a primarily psychiatric illness; variant CJD may be very difficult to diagnose in the early stages. Psychiatric features may remain as part of the overall picture even following the development of clear-cut neurological signs (after a median of around six months). The obviously neurological clinical picture is typically one of a rapidly progressive dementia with cerebellar ataxia, involuntary movements and painful sensory disturbances leading to death at a median of 14 months from onset. Definite diagnosis requires neuropathology but clinical diagnostic criteria have been developed allowing a very confident pre-mortem diagnosis in the majority of cases. These criteria depend on the exclusion of other diseases, the presence of a typical clinical picture and the results of certain supportive investigations, most notably the cerebral MRI and tonsil biopsy.

A nationwide survey documented 117 cases of Creutzfeldt-Jakob disease (CJD) transmitted from cadaveric dura mater grafts in Japan to September 2004. Of these, 110 patients were identified to have received the same type of lyophilized cadaveric dura mater graft during the period between 1978 and 1991. Incubation period from grafting to onset of symptoms varied from 16 months to 23 years, with most patients developing neurological symptoms after 2 to 15 years. We conducted q retrospective review of the full medical records of 107 of dura-related CJD (dCJD) patients. Patients were divided into two groups by site of neurosurgical or orthopedic procedure (supratentorial vs. infratentorial). Hemiparesis or hemianopsia developed as an initial manifestation in 31.9% of 47 patients with supratentorial grafts but did not develop among any of the infratentorial group (p<0.0001). Conversely, brainstem symptoms (nystagmus, diplopia, ipsilateral hearing loss, facial paresis or paresthesia) were noted in 25.0% of the infratentorial group, but were not seen in the supratentorium group (p<0.0001).

Prion diseases (PrD), also known as transmissible spongiform encephalopathies, are believed to be caused by accumulation of an abnormal isoform of the prion protein (PrP^SC) in the central nervous system. Creutzfeld-Jacob disease (CJD) in its sporadic and variant form is the most frequent and clinically important PrD. At present there is no proven specific or effective treatment available for any form of CJD, although some oral agents, such as quinacrine or flupirtine, are being investigated in clinical trials. Pentosan polysulphate (PPS), a large polyglycoside molecule with weak heparin-like activity, has been shown to prolong the incubation period of PrP^SC infection when administered to the cerebral ventricles in a rodent scrapie model. PPS also prevents the production of further PrP^SC in cell culture models. However, PPS penetrates poorly the blood-brain barrier and only a minor fraction of orally administered drug may reach the CNS. These properties of PPS prompted its cerebroventricular administration in patients with vCJD and other PrD, such as iatrogenic CJD and Gerstmann-Sträussler-Scheinker syndrome (GSS). Long-term continuous infusion of PPS at doses of up to 110 µ g/kg/d did not cause serious drug-related side effects. Follow-up CT and MRI imaging demonstrated that brain atrophy may progress further during PPS administration, while the neurological status may remain stable. Proof of clinical efficacy has not been the aim of the current clinical studies of PPS, however one patient with vCJD survived for 23 months after initial symptoms and 39 months after diagnosis, while the median duration of illness with vCJD is 13 months (range 6–39). Some lessons have been learned from the early studies of application of PPS in PrD patients. Surgery in a brain affected by PrD may result in a higher rate of surgical complications than might be expected in analogous cases with other conditions. Secondly, efficacy of PPS or any other treatment option in advanced PrD cases will be very difficult to assess, due to the lack of specific and objective criteria for measurement of response. Overall survival may remain therefore one of the few objective ways of assessing outcome in treated patients. Finally, if clinically significant benefits to patients are to be expected, PPS administration should start as early as possible in the course of the respective disease and before irreversible loss of neurological function has occurred. Further clinical, neuroradiological and laboratory investigations of cerebroventricular PPS administration in the setting of a prospective clinical study will be essential for the assessment of possible clinical benefits of PPS in PrD.

Prion diseases include a wide range of diseases affecting both humans and animals. One commom feature of this diverse group of diseases is the deposition of PrP^Sc, the abnormally folded form of the host encoded prion protein. The proposed replicative cycle of prions is relatively simple. It encompasses misfolding of a single protein, the cellular prion protein PrP^C, into a disease-associated form termed PrP^Sc. This is followed by PrP^Sc aggregation, and possibly fragmentation of aggregates which may augment the number of replicative units. While there is no formal proof of the correctness of the above model, a wealth of evidence indicates that pathogen-encoded informational nucleic acids are dispensable for prion replication. The detection of prp^Sc in various tissue compartments including the central nervous system, the lymphoreticular system and skeletal muscle is the principle of the majority of diagnostic test aimed to verify the clinical suspicion of a prion disease. This article discusses current concepts surrounding the basic biology of prions and the diagnosis of prion diseases.

Much progress has been made in understanding the molecular basis of phenotypic variability in Creutzfeldt-Jakob disease (CJD) in the last ten years. The most significant advance was the discovery that the genotype at polymorphic codon 129 of PRNP and the “type” of the protease-resistant prion protein fragment, PrP-res, have a major influence on the disease phenotype in all forms of CJD, irrespective of their etiology. The most widely accepted CJD classification includes six clinico-pathological phenotypes and two major types of PrP-res, types 1 and 2, which can be distinguished on the basis of a ∼2 kDa difference in relative molecular mass of the protein fragment. However, alternative classifications of human PrP-res types distinguished three patterns of PrP-res molecular mass instead of two, thereby creating significant confusion in the field. Fortunately, progress has been recently made in clarifying these disparities. Most significant in this regard, has been the finding that pH variation among CJD brain homogenates in standard buffers influences the size of PrP-res. Thus, some of the PrP-res heterogeneity used to identify putative strain-specific PrP-res types simply represents a technical “artefact” related to the experimental conditions. On the other hand, recent data have also shown that PrP-res types 1 and 2 are heterogeneous biological species, which can be further distinguished into molecular subtypes that fit the current histopathological classification of sporadic CJD in 6 subtypes. Finally, novel truncated PrP-res fragments of smaller size than PrP-res types 1 and 2 have recently been identified in CJD. Although more studies are needed to fully characterize the presence, characteristics and biological significance of these peptides, preliminary results indicate that their search and characterization may be useful in the molecular diagnostics of CJD subtypes.

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy of free-ranging and farmed cervids in North America that is distinct from scrapie of domestic sheep, bovine spongiform encephalopathy, and Creutzfeldt-Jacob disease of humans. The purpose of this paper is to review the current status of CWD in North America. The natural host range of CWD includes mule deer ( Odocoileus hemionus ), white-tailed deer ( Odocoileus virginianus ), and Rocky Mountain elk ( Cervus elaphus nelsoni ). Experimentally, by intracerebral or oral exposure, the host range is wider, but there appears to be a significant barrier to infection of cattle and humans. The exact mechanism of CWD transmission is not known but recent studies indicate that direct transmission, indirect transmission via environmental contamination, and transmission associated with carcasses are possible. Maternal transmission does not appear to play a significant role in CWD. Although still under investigation, polymorphisms in the prion protein influence CWD pathogenesis in mule deer and elk. Studies of CWD pathogenesis following oral exposure demonstrate early widespread distribution of abnormal prion protein in the lymphoid tissue prior to invasion of the central nervous system. These data have lead to techniques for CWD surveillance in deer based on testing retropharyngeal lymph node. However, both brain and lymph node must be tested in elk for highest sensitivity due to differences in pathogenesis in this species compared to deer. The unique nature of a transmissible spongiform encephalopathy occurring in free-ranging cervids is a serious challenge to wildlife managers and animal health agencies in North America.

Bovine spongiform encephalopathy (BSE) has become an important problem not only for the animal industry, but also for public health. BSE was first recognized in Japan in September 2001 as a result of fallen stock surveillance. Since October 2001, all cattle slaughtered at abattoirs have been tested for BSE. From April 2004, examination of all dead cattle aged over 24 months has been conducted at livestock hygiene service centers. Samples positive on enzyme linked immunosorbent assay (ELISA) are further subjected to western blot (WB) and immunohistochemistry (IHC). Fourteen BSE cases had been reported by November 2004; thirteen were classified as the typical form while the other was atypical. Variant forms of BSE with atypical histopathological and/or biochemical phenotype have also been reported in Italy and France. Further study is required to characterize the BSE prion, and to this end, brain homogenates from Japanese BSE cases were intracerebrally inoculated into wild-type mice. The first case diagnosed in Japan (BSE/Chiba) has been successfully transmitted to rodents. Mean incubation period (409.0 ± 28.2 days) was considerably longer than that previously reported. PrP^Sc distribution, possibly due to prion titer, recipient susceptibility and/or sample storage conditions. Transgenic mice that overexpress a bovine PrP gene were recently introduced to overcome the species barrier problem, hopefully enabling accelerated transmission of BSE prions. These transgenic mice are also being used in ongoing transmission studies of the atypical BSE case.

PrP is central to TSE disease and has been hypothesised to be the infectious agent. Polymorphisms in the PrP gene are associated with different incubation times of disease following exposure to an infectious agent and mutations in the human PrP gene can apparently lead to spontaneous genetic disease. Strains of TSE agent are proposed to be generated and maintained through differences in glycosylation or conformation of PrP and the barrier to infection between species is thought to be due to the differences in the sequence of PrP between different species. To test these hypotheses, we have introduced specific modifications into the endogenous mouse Prnp gene by gene targeting. The mutated PrP gene is in the correct location under the control of the endogenous Prnp regulatory sequences and thus expressed in the same tissues and amounts as the wild type Prnp gene. By altering the murine PrP coding region to that of another species we have established that increasing overall identity between host and donor PrP can lead to either an increase or a decrease in incubation time of disease in a strain dependent manner. We have introduced a point mutation (101L) into the N-terminus of the host PrP and shown that it dramatically changes the susceptibility of the host to infection from different species. We have in addition demonstrated that polymorphisms in the N terminus (L108T) and C-terminus (F189V) of host PrP both alter the incubation time of disease. We have in addition introduced mutations into the Prnp gene which prevent glycosylation at each or both of the two N-linked glycosylation sites of PrP. Inoculation of these mice with infectivity has established that glycosylation of host PrP can influence incubation time of disease, vacuolar pathology and strain determination.

Many natural infections with transmissible spongiform encephalopathy (TSE) agents are likely to be acquired peripherally for example, following ingestion of contaminated feed. Following peripheral exposure TSE agents accumulate in lymphoid tissues before spreading to the brain. Many studies have attempted to identify the cells and their components that are required for the delivery of the TSE agent from the site of inoculation to the brain, a process termed neuroinvasion . In the lymphoid tissues of TSE-affected hosts these agents, as identified by disease-specific prion protein accumulations, usually accumulate on follicular dendritic cells (FDCs). Studies of mouse TSE models have shown that mature FDCs are critical for replication of infection in lymphoid tissues and subsequent neuroinvasion. Although examples of FDC-independent neuroinvasion have been described, treatments that interfere with the integrity or function of FDCs reduce TSE susceptibility by blocking the spread of disease to the brain. For example, temporary depletion of FDCs before oral inoculation with TSE agents blocks the accumulation of disease-specific PrP in Peyer’s patches and mesenteric lymph nodes, and prevents neuroinvasion. Studies in mice have shown that skin scarification is also an effective means of TSE agent transmission. Following inoculation via the skin the agent accumulates in the draining lymph node in association with FDCs. The accumulation of TSE agents in association with FDCs is also critical for the transmission of disease from the skin to the brain, as disease susceptibility is reduced in their absence. The mechanisms through which TSE agents are transported from the site of inoculation such as the gut or skin to lymphoid tissues are not known. Bone marrow-derived migratory dendritic cells have been proposed as a potential method of TSE agent transport from the gut lumen. Langerhans cells (LCs) reside in the epidermis and migrate to the draining lymph node after encountering antigen, suggesting these cells might play a role in TSE agent transportation from the skin. To investigate the potential role of LCs in TSE agent transportation, mouse models have been utilized in which their migration was blocked. Experiments show that the early accumulation of TSE agents in the draining lymph node and their subsequent neuroinvasion was not impaired in mice with blocked LC migration. Thus although LCs have the potential to acquire TSE agents they are not involved in their transportation to draining lymphoid tissues. The identification of cell populations critical for TSE pathogenesis provides cellular targets to which therapies can be directed. Described below are current understandings of the involvement of the immune system in the neuroinvasion of TSE agents.