Transmissible spongiform encephalopathies in humans

Transmissible spongiform encephalopathies (TSE) are dementing diseases and have been known to affect humans for over 90 years. The most common of these is the sporadic form of Creutzfeldt-Jakob disease (sCJD), followed by its familial (fCJD) and an iatrogenic (iCJD) form. 1996 a variant of CJD (vCJD) has been described in the UK, of which so far 131 cases have been observed worldwide. Specific biochemical and neuropathological signatures allow to distinguish between vCJD and sCJD and lead to the hypothesis that vCJD is due to transmission of BSE prions to humans. Although promising therapeutical approaches are being investigated, human TSE remain untreatable entities. Thus preventive measures are essential. In Switzerland the population has been exposed to BSE prions, too, but no case of vCJD as described in the UK has been observed until now. Since 2001, however, a so far unexplained increase of sCJD cases is being observed.     

Transmissible spongiform encephalopathies in ruminants.

Transmissible spongiform encephalopathies (TSEs) have been observed in North American sheep, cervidae, and cattle. The causative agent of TSEs seems to be proteins that induce a conformational change in normal host proteins. Common clinical signs ofTSEs include chronic weight loss and deteriorating neurologic function. Mechanisms of TSE transmission vary among host species.The diagnostic approach for TSEs of ruminants is presented,and the history and justification of regulatory programs to control and eradicate TSEs are outlined.     

Transmissible spongiform encephalopathies in sheep and goats

Scrapie has been known to occur in sheep and goats for over 250 years. Sheep and goats can be experimentally infected orally with BSE, however BSE infection can only be distinguished from scrapie using costly and extensive laboratory procedures. An overview of both the current state of knowledge on Transmissible Spongiform Encephalopathies (TSE) in small ruminants as well as the TSE monitoring and control strategies in place in Switzerland is presented.     

Transmissible spongiform encephalopathies (TSE): old diseases with new explosive force

In view of the first 64 BSE cases (date: 11.5.01) in German cattle herds an overview on TSE and their similarities and differences regarding clinic, pathogenesis and pathology is given. The mechanism of the unconventional agent, an infectious protein (prion), is explained based on the prion model of Stanley Prusiner. The knowledge on transmission, incubation time, host specificity as well as resistance and immunity drawn from experimentally infected animals is discussed. Thus, after oral infection prions are transported by lymphocytes from the stomach-intestinal tract to the spleen. The way to the CNS is still unknown. The presumption for crossing the species barrier is twofold: first the prions of different species have to be biochemically homologous and a genetical disposition has to exist. This is the case for BSE and the new variant of Creutzfeldt-Jakob-Disease (vCJD). There is evidence that in Great Britain so far 97 (date: 30.3.01) young people acquired vCJD due to consumption of food that contained bovine risk material. Regarding the infectious prion dosis brain, spinal cord and lymphoid tissues are regarded to be most dangerous. The principle of the BSE-test, its evidence as well as steps for prevention and control of BSE are presented.     

Approaches to investigating transmission of spongiform encephalopathies in domestic animals using BSE as an example

Bovine spongiform encephalopathy was a novel spongiform encephalopathy, in an hitherto unaffected species, that had characteristics of a point source epidemic, with an agent that could have been incorporated into a wide variety of feedstuffs and iatrogenically administered to na?ve populations, and there was early evidence that it was not restricted to bovines. It was vital to establish, albeit experimentally, which other species might be affected, and whether the epidemic could be maintained by natural transmission, if the source was removed. In contrast, scrapie has been endemic throughout Great Britain for centuries, is maintained naturally (even if we don't know exactly how) and has a known host range. The principles, process and integration of evidence from different types of studies, however, are similar for both of these transmissible spongiform encephalopathies (TSE) and can be applied to any emerging or suspected spongiform encephalopathy. This review discusses the experimental approaches used to determine TSE transmissibility and infectivity and how they relate to natural disease and control measures.     

Transmissible encephalopathies in animals.

Scrapie in sheep and goats is the best known of the transmissible encephalopathies of animals. The combination of maternal transmission of infection and long incubation periods effectively maintains the infection in flocks. A single sheep gene (Sip) controls both experimental and natural scrapie and the discovery of allelic markers could enable the use of sire selection in the control of the natural disease. Studies of experimental rodent scrapie show that neuroinvasion occurs by spread of infection from visceral lymphoreticular tissues along nerve fibers to mid-thoracic cord. The slowness of scrapie is due to restrictions on replication and cell-to-cell spread of infection affecting neuroinvasion and subsequent neuropathogenesis. Probably both stages in mice are controlled by Sinc gene, the murine equivalent of Sip. The glycoprotein PrP may be the normal product of Sinc gene. Posttranslationally modified PrP forms the disease specific "scrapie associated fibrils" and may also be a constituent of the infectious agent. Scrapie-like diseases have been reported in mink and several species of ruminants including cattle. All of them may be caused by the recycling of scrapie infected sheep material in animal feed. The human health implications are discussed.     

PrP genetics in ruminant transmissible spongiform encephalopathies

Scrapie, bovine spongiform encephalopathy (BSE), and chronic wasting disease (CWD) are prion diseases in ruminants with considerable impact on animal health and welfare. They can also pose a risk to human health and control is therefore an important issue. Prion protein (PrP) genetics may be used to control and eventually eradicate animal prion diseases. The PrP gene in sheep and other representatives of the order Artiodactyles has many polymorphisms of which several are crucial determinants of susceptibility to prion diseases, also known as transmissible spongiform encephalopathies (TSE). This review will present the current understanding of PrP genetics in ruminants highlighting similarity and difference between the species in the context of TSE.     

Diagnosis of transmissible spongiform encephalopathies in animals

Histopathological examination of the central nervous system is essential for the confirmation of a TSE diagnosis. Typical lesions are spongiform changes of the grey matter, intraneuronal vacuoles in particular nuclei of the brain stem, gliosis and neuronal degeneration. The nature of the lesions is similar between species. However, the variation in the distribution and severity of the changes is striking. Even more reliable than histopathology is the detection of disease-specific protease-resistant prion protein (PrPSc) using immunohistochemistry. The so-called "rapid tests" allow detection of PrPSc in unfixed tissues and are mostly used for the screening of risk populations and slaughtered animals.     

Glycosylation of prion strains in transmissible spongiform encephalopathies

This is a review of prion replication in the context of the cell biology of membrane proteins especially folding quality control in the endoplasmic reticulum (ER). Transmissible spongiform encephalopathies, such as scrapie and BSE, are infectious lethal diseases of mammalian neurons characterised by conversion of the normal membrane protein PrPC to the disease-associated conformational isomer called PrPSc. PrPSc, apparently responsible for infectivity, forms a number of different conformations and specific N-glycosylation site occupancies that correlate with TSE strain differences. Dimerisation and specific binding of PrPc and PrPSc seems critical in PrPSc biosynthesis and is influenced by N-glycosylation and disulfide bond formation. PrPsc can be amplified in vitro but new glycosylation cannot occur in cell free environments without the special conditions of microsome mediated in vitro translation, thus strain specific glycosylation of PrPSc formed in vitro in the absence of these conditions must take place by imprintation of PrPc from existing glycosylation site-occupancies. PrPSc formed in cell free homogenates is not infectious pointing to events necessary for infectivity that only occur in intact cells. Such events may include glycosylation site occupancy and ER folding chaperone activity. In the biosynthetic pathway of PrPSc, early acquisition of sensitivity of the GPI anchor to phospholipase C can be distinguished from the later acquisition of protease resistance and detergent insolubility. By analogy to the co-translational formation of the MHC I loading complex, it is postulated that PrPSc or its specific peptides could imprint nascent PrPc chains thereby ensuring its own folds and the observed glycosylation site occupancy ratios of strains.     

Diagnosis of transmissible spongiform encephalopathies in animals: a review.

Transmissible spongiform encephalopathies (TSEs) in animals include, among others, bovine spongiform encephalopathy (BSE), scrapie, chronic wasting disease, and atypical forms of prion diseases. Diagnosis of TSEs is based on identification of characteristic lesions or on detection of the abnormal prion proteins in tissues, often by use of their partial proteinase K resistance property. Correctly sampling of target tissues is of utmost importance as this has a considerable effect on test sensitivity. Most of the rapid or screening tests are based on ELISA or Western immunoblot (WB) analysis, and many are officially approved. Confirmatory testing is normally performed by use of histologic examination, immunohistochemical analysis, certain WB protocols, or detection of prion fibrils by use of electron microscopy (scrapie-associated fibril). The discriminatory methods for diagnostic use are mostly based on WB technology and provide initial identification of the prion strain, particularly for differentiation of BSE from scrapie in small ruminants. Definitive prion strain characterization is performed by use of bioassays, usually in mice. A burgeoning number of transgenic mice have been developed for TSE studies. Development of new tests with higher sensitivity and of more reliable diagnostic applications for live animals tested for food safety reasons is a rapidly developing field. Ultimately, the choice of a test for TSE diagnosis depends on the rationale for the testing.     

Prions as the driving force in transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies are degenerative disorders affecting the central nervous system (CNS) occurring in a variety of species. The causative agent is thought to be composed of an abnormal form of the host encoded prion protein (PrPC), termed PrPSc. The conformational change of PrPC into PrPSc can occur spontaneously, however, it can also be induced by PrPSc. Prion diseases such as bovine spongiform encephalopathy (BSE), scrapie and variant Creutzfeldt-Jakob-Disease (vCJD) are most likely caused by peripheral uptake of prions. The process by which prions proceed to the CNS following peripheral uptake is referred to as neuroinvasion. Infection with prions is thought to occur in two phases: After ingestion prions first replicate in lymphatic tissue and then gain access to the CNS via peripheral nerves. Studies looking at the biochemical and clinical characteristics of BSE and vCJD demonstrated that BSE is most likely responsible for vCJD in humans.     

Emerging therapeutic agents for transmissible spongiform encephalopathies: a review

Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative disorders associated with misfolding of prion protein, from PrPC to PrPSc. Different types of experimental studies have resulted in a better understanding of the pathogenesis of the prion diseases. Genetic and molecular properties of PrP isoforms have been explained but the conformational conversion of the PrPC isoform to the PrPSc isoform has not yet been entirely elucidated. However, a number of possible therapeutic agents have been tried and some have proven to be effective against TSEs but most have limitations in terms of toxicity and pharmacokinetics. Congo red (CR), anthracyclines, and polyanionic dextran sulfate have limited ability to cross the blood-brain barrier and may be toxic. The efficacy of polyene antibiotics seems to be restricted to certain scrapie strains. Tetrapyrroles and tetracyclines with low toxicities and favorable pharmacokinetics could be useful in preventing PrPSc accumulation. Compounds like branched polyamines, Cp-60, analogs of CR, quinacrine and chlorpromazine, beta-sheet breaker peptides and inhibitory peptides, active immunization using recombinant PrP and passive immunization with anti-PrP antibodies, have potential use as therapeutic agents but would need further research and clinical trials.     

The transmissible spongiform encephalopathies: disease risks for North America.

Transmissible spongiform encephalopathies exotic to North America (BSE and associated diseases) are unlikely to be introduced or to persist should they be introduced into the United States [2]. Domestic TSEs (scrapie, CWD, and TME) seem to be relatively restricted in their host range, and none of these diseases is known to naturally cause disease in cattle. It is important that surveillance for TSEs continues, however, particularly in cattle because of the extreme consequences to the livestock industries, and potentially, public health, if BSE becomes established. Because the TSEs have implications beyond effects on their natural host species, adequate surveillance, control, and even eradication of these diseases should be goals for the livestock industries, wildlife managers, and animal health agencies in the United States.     

Polymorphisms of the prion gene promoter region that influence classical bovine spongiform encephalopathy susceptibility are not applicable to other transmissible spongiform encephalopathies in cattle

Two regulatory region polymorphisms in the prion gene of cattle have been reported to have an association with resistance to classical bovine spongiform encephalopathy (BSE). However, it is not known if this association also applies to other transmissible spongiform encephalopathies (TSE) in cattle. In this report, we compare the relationship between these 2 polymorphisms and resistance in cattle affected with naturally occurring atypical BSE as well as in cattle experimentally inoculated with either scrapie, chronic wasting disease, or transmissible mink encephalopathy. Our analysis revealed no association between genotype and resistance to atypical BSE or experimentally inoculated TSE. This indicates the promoter polymorphism correlation is specific to classical BSE and that atypical BSE and experimentally inoculated TSE are bypassing the site of influence of the polymorphisms. This genetic discrepancy demonstrates that atypical BSE progresses differently in the host relative to classical BSE. These results are consistent with the notion that atypical BSE originates spontaneously in cattle.     

Bovine spongiform encephalopathy (BSE) safety measures in Japan

Since the first identification of bovine spongiform encephalopathy (BSE) in Japan in September 2001, a series of safety measures was introduced by the Ministry of Agriculture, Forestry and Fisheries, the Ministry of Health, Labour and Welfare and the Food Safety Commission of the Cabinet Office. These measures included blanket BSE testing and removal of specified risk materials at slaughterhouses, surveillance of risk animals and a ban on the use of meat-and-bone meals and traceability on all farms. The Japanese experience over the past five years has shed light on several issues in countries that have a low BSE incidence.     

一种简便快速的体外点突变方法

体外点突变是研究蛋白质结构 -功能关系、基因表达、载体修饰等的重要技术之一。已报道的一些体外突变方法 [1～ 4 ] ,一般需以单链 DNA为模板 ,并要求对突变基因进行亚克隆和对单链 DNA拯救 ,技术上较复杂 ,多数需 1周以上时间。Stratagen公司研制的 Quickchange试剂盒 ,可以双链 DNA质粒为模板 ,只需 4步操作 ,在 1～ 2 d内即可完成点突变过程 [5] ,但该试剂盒价格昂贵 ,使用次数有限。我们根据 Pfu和 Dpn 等酶的特性 ,自行设计了突变反应及方法 ,对 6kb、8kb和1 4.6kb的目的质粒分别进行了点突变 ,成功地在质粒上创造了相应的酶切…     

疯牛病的研究动态和我国防控体系建设

目前,欧洲、北美洲、亚洲都不同程度发生疯牛病疫情,文章重点介绍世界各国在疯牛病预防、监测、基础理论方面的研究动态,以及我国疯牛病防控体系现状。