24.11.5 Human prion diseases 6109 Simon Mead and R

24.11.5 Human prion diseases 6109 Simon Mead and R.G. Will

24.11.5  Human prion diseases 6109 Stoll M, Schmidt RE (2003). Immune restoration inflammatory
syndromes:  the dark side of successful antiretroviral treatment. Curr Infect Dis Rep, 5, 266–​76. Swartz MN, Healy BP, Musher DM (1999). Late syphilis. In:
Holmes KK, et al. (eds) Sexually transmitted diseases, pp. 487–​509. McGraw-​Hill, New York, NY. Tan IL, et al. (2012). HIV-​associated opportunistic infections of the CNS. Lancet Neurol, 11, 605–​17. 24.11.5  Human prion diseases Simon Mead and R.G. Will ESSENTIALS Prion (for proteinacious infectious particle) protein is a membrane-​ associated glycoprotein present in all mammalian species. Its normal function is unknown, but in prion diseases (also known as transmis- sible spongiform encephalopathies) a misfolded polymer form of the protein, partially resistant to protease digestion, is deposited in the brain and associated—​typically after long incubation periods—​with neuronal dysfunction and death. Prion disease aetiologies and clinical syndromes Several forms are recognized: (1) Sporadic prion diseases, principally, (a) Creutzfeldt–​Jakob disease—​a rare condition typically presenting in late middle age with a rapidly progressive dementia associated with a range of neurological signs, most commonly myoclonus of the limbs, cerebellar ataxia, and rigidity. Few patients survive for more than 2 years. Also, (b) variably protease sensitive prionopathy—​a seem- ingly rare type of sporadic prion disease associated with psychiatric signs, language output impairment, and ataxia. (2)  Inherited prion disease—​dominant pattern of inheritance; heterogeneous condition; in many cases age at clinical onset is younger and clinical duration is longer than sporadic disease; can be indistinguishable from sporadic Creutzfeldt–​Jakob disease. (3) Acquired forms including (a) iatrogenic Creutzfeldt–​Jakob disease—​exposures in or adjacent to the nervous system (e.g. neurosurgical instruments, dura mater grafts, corneal transplants) typically present in a similar manner to sporadic disease; peripheral exposure to infection (pituitary hormones) usually mani- fests with progressive cerebellar ataxia; can have very long incuba- tion times. (b) Variant Creutzfeldt–​Jakob disease—​bovine spongiform encephalopathy was identified in 1986 as a prion disease in cattle, with the favoured hypothesis being that contamination of feed, prob- ably with tissues from the central nervous system of affected animals. Variant Creutzfeldt–​Jakob disease is caused by transmission of bovine spongiform encephalopathy to humans. Typical presentation is with psychiatric symptoms, followed after a period of months by progres- sive ataxia, dementia, and choreiform or dystonic involuntary move- ments which often evolve into myoclonus. By 2018 there were 178 deaths from variant Creutzfeldt–​Jakob disease in the United Kingdom, with the most recent death occurring in 2016. (c) Kuru—​in Papua New Guinea this disease was transmitted by ritual cannibalism, which ceased by 1960, hence there have been no cases in people born after 1959. Typical presentation was with headache and limb pain, progressing to a cerebellar syndrome, with eventual dementia and immobility. Investigation, treatment, and prevention Investigation—​suspicion of the disorder depends on the recognition of clinical characteristics, then supported by (1) electroencephalography—​ periodic triphasic complexes may be seen in 60–​70% of cases of spor- adic Creutzfeldt–​Jakob disease and in some cases of its iatrogenic variant; (2) cerebrospinal fluid analysis—​elevation of 14-​3-​3 protein in the cerebrospinal fluid is about 90% sensitive and specific for sporadic Creutzfeldt–​Jakob disease, but less useful for its variant version; protein amplification technologies such as rt-​QUIC are increasingly useful and are over 90% sensitive and almost 100% specific; (3) MRI—​diffusion-​ weighted imaging protocols usually show abnormalities in the caudate nucleus, putamen, thalamus and cortex in sporadic Creutzfeldt–​Jakob disease, and the pulvinar region of the posterior thalamus in variant Creutzfeldt–​Jakob disease; (4) tissue biopsy of brain or tonsil. Treatment and prevention—​there is nothing that influences the clinical course of human prion diseases, nor any treatment to pre- vent the development of neurological disease after infection. Several symptomatic therapies may be helpful. Introduction Prion diseases, also known as transmissible spongiform encephal- opathies, are fatal disorders of the central nervous system affecting both humans and animals. The clinical features and patterns of oc- currence of these diseases vary, but they are linked by several charac- teristics including experimental and natural transmissibility, shared neuropathological features, prolonged incubation periods meas- ured in years, and the deposition of prion protein in the brain of the host. Prion diseases have become the subject of intense scien- tific and public interest because they are caused by a biologically dis- tinct disease mechanism and because of the implications for public health following the identification of a new human prion disease, variant Creutzfeldt–​Jakob disease (vCJD), and the evidence that it is caused by the transmission to humans of a cattle prion disease, bovine spongiform encephalopathy (BSE). There have been remarkable scientific advances in the under- standing of prion diseases and it is hoped that this may lead to improved diagnostic tests in life for the presence of infection and to therapies to prevent the development of disease. Human prion diseases have attained a public notoriety disproportionate to the overall burden of disease caused by these rare conditions. However, the transmission of an animal prion disease to humans has been a tragedy and the prolonged incubation periods characteristic of this group of diseases indicate that the eventual consequences of BSE for public health both in the United Kingdom and in other countries remain unpredictable. Historical perspective Scrapie was first transmitted experimentally from sheep to sheep in 1936 and to laboratory mice in 1961, but laboratory transmission of

section 24  Neurological disorders 6110 human prion diseases was not achieved until 1966 (kuru) and 1968 (CJD). The seminal discovery that neurodegenerative diseases were transmissible stimulated extensive research into the nature of the infectious agent and attempts to identify the source of infection in CJD. Table 24.11.5.1 sets out the principal prion diseases known to affect humans and animals. Aetiology, genetics, pathogenesis, and pathology No bacterium or virus has been isolated in these diseases and there is no immunological response to infection. In 1982 Prusiner proposed that the protein deposited in the central nervous system (CNS) in these diseases was itself the causal agent. Purified infectious fractions of brain contain ‘prion’ protein, which is the major, and possibly the only, component of the infectious agent. Prions are remarkably re- sistant to inactivation procedures, including those that disrupt nu- cleic acids. Prion protein is a membrane-​anchored glycoprotein, and is present in all mammalian species (Fig. 24.11.5.1a). The normal function of prion protein is unknown. In prion diseases, a misfolded and multimeric form of the protein, partially resistant to protease digestion, is deposited in the brain and is associated with neuronal dysfunction and death. Prions propagate by a process of seeded polymerization whereby the normal cellular form of prion protein is misfolded on binding with the disease form (Fig. 24.11.5.1b). There is a range of experimental evidence supporting the hypoth- esis that the disease-​associated form of prion protein is the causal agent in prion diseases, most notably a series of elegant studies in transgenic rodents. Cellular expression of prion protein is neces- sary for the development of the neuropathological changes and the disease. Hereditary forms of human prion disease, which can be shown to be associated with prion infection of brain tissue, are caused by mutations of the prion protein gene. The occurrence of multiple strains of the infectious agent and the stability of the trans- mission characteristics of the bovine spongiform encephalopathy (BSE) agent in the laboratory after cross-​species transmission was seen by some as a challenge to the prion hypothesis. Most now hold the view that the multiple strains of prions disease are encoded by the structure of the disease-​associated form. The phenotype of dif- ferent clinicopathological subtypes of CJD is strongly determined by prion strain. In experimental transmission of prion diseases there are several key determinants of the efficiency of transmission, as judged by the incubation periods in recipient animals and the proportion of these animals that develop disease. The intracerebral route is the most effi- cient. Intravenous, intraperitoneal, and oral routes are decreasingly efficient. The incubation period is inversely related to the infective dose, while the strain of the infectious agent influences both the incubation period and whether recipient animals develop disease. Table 24.11.5.1  The prion diseases or spongiform encephalopathies Disorder Species Sporadic Creutzfeldt–​Jakob disease Human Inherited prion disease (includes Gerstmann–​Straussler–​Scheinker, fatal familial insomnia, and PrP systemic amyloidosis) Human Iatrogenic Creutzfeldt–​Jakob disease Human Kuru Human Variant Creutzfeldt–​Jakob diseasea Human Variably protease sensitive prionopathy Human Scrapie Sheep/​goat/​moufflon Transmissible mink encephalopathy Mink Chronic wasting disease Deer/​elk/moose/reindeer Bovine spongiform encephalopathya Cattle Feline spongiform encephalopathya Cat/​cheetah/​puma/​ocelot/​tiger Spongiform encephalopathy of captive exotic ungulatesa Kudu/​nyala/​oryx/​gemsbok/​eland a These disorders are associated with the same infectious agent (bovine spongiform encephalopathy). glycans Monomers (a) (b) Seed Fibre Fragmentation Clearance helix 1 helix 2 membrane anchor Fig. 24.11.5.1  (a) The structure of the normal cell-​surface prion protein (PrP). (b) A model of prion replication invoking seeded protein polymerization. In this model the normal prion protein binds to the end of a prion protein ‘seed’ (a ‘prion’), as a result the normal protein is misfolded into the abnormal conformation. The model also envisages that prion seeds can both fragment to generate more seeds, or be cleared. In CJD, the fragmentation (propagation) pathway overwhelms clearance.

24.11.5  Human prion diseases 6111 In some transmission studies (e.g. transmission of BSE to hamsters), recipient animals do not develop disease even after intracerebral in- oculation of high levels of infectivity. Within-​species transmission is generally more efficient than cross-​species transmission and this ‘barrier’ to transmission is influenced by characteristics of both the host and the infective agent. The relative homology of amino acid sequences of prion proteins between species is crucial, but is not the only determinant of the transmission barrier. After oral exposure, prions may replicate in the lymphoreticular system, including the spleen and lymph nodes, before entering the thoracic spinal cord or brainstem, probably via the autonomic ner- vous system, and then spreading caudally to the brain. Moderate levels of infectivity plateau in the lymphoreticular system and are not associated with organ dysfunction, while in the spinal cord and brain high levels of infectivity develop (e.g. 1012 infectious units per gram of brain in one model of hamster scrapie), leading to neuronal death and clinical disease. In some experimental and natural prion diseases, infectivity in the lymphoreticular system can be detected at about a third of the total incubation period by inoculation of tissues of the lymphoreticular system, such as spleen, into recipient ani- mals. The implication is that animals or humans incubating a prion disease may harbour significant infectivity in peripheral organs or tissues for long periods, if not lifelong. This has important implica- tions for the control and public health implications of prion diseases. Human prion diseases Human prion diseases may be classified as sporadic, inherited, or acquired (Table 24.11.5.2). Sporadic CJD Epidemiology Sporadic CJD is a rare disease, with an annual incidence of about 1–​2 cases per million population and a lifetime risk of 1:5000. The disease occurs worldwide and the cause is unknown, with no con- vincing evidence of an environmental source of infection and in par- ticular no proven link with the animal prion diseases. The regional clusters of cases identified in some countries are unusual and may reflect the chance aggregation of a rare phenomenon. Overall the geographical and temporal distribution of cases of sporadic CJD ap- pear to be random and case–​control studies have demonstrated no consistent risk factors for the development of disease, with no good evidence of an increased risk through occupation, dietary factors, or animal contact. The currently favoured hypotheses are that sporadic CJD is caused by a spontaneous mutation of prion protein to the abnormal form, which acts as a template for protein self-​replication and eventual disease, or alternatively there is a mutation of the prion protein gene in somatic cell(s) leading to the expression of mutant prion protein predisposed to convert into an abnormal form. It is also plausible that a small proportion of sporadic CJD is an unrecog- nized zoonotic or iatrogenic condition. Clinical features Clinically, sporadic CJD presents with a rapidly progressive de- mentia associated with a range of neurological signs, most com- monly myoclonus of the limbs, cerebellar ataxia, and rigidity. Less common features include dysphasia, pyramidal or extrapyramidal signs, primitive reflexes, cortical blindness, and lower motor neuron signs. Despite the predominantly cortical neuropathology epilepsy is rare. The rapidity of the progression of neurological deficits and cog- nitive decline is distinct from most other causes of dementia and the mean survival is only about 4 months from clinical onset, although in about 10% of cases the illness is more prolonged and a small minority of patients survive for 2 years or more (Fig. 24.11.5.2). Terminally there is often a state of akinetic mutism. Although the clinical presentation in sporadic CJD is relatively stereotyped, a minority of cases present atypically (e.g. acutely mimicking stroke), with cortical blindness, or with an initially pure cerebellar syndrome. The neuropathological characteristics of sporadic CJD include spongiform change, neuronal loss, and astrocytosis in the cerebral and cerebellar cortex, in accordance with the neurological signs seen in life. Neuropathological changes are widespread and deposition of prion protein can be detected with immunocytochemical tech- niques. In about 10% of cases there are cortical deposits of prion protein in the form of amyloid plaques. There is heterogeneity in the distribution and morphology of the neuropathological changes, which correlate in part with the clinical phenotype and with types of prion protein that can be distinguished on Western blot of brain tissue (Fig. 24.11.5.3 and Fig. 24.11.5.4). Sporadic CJD is mainly a disease of late middle age (Fig. 24.11.5.5) with a mean age at death of 67 years. In most systematic studies males and females are affected with equal frequency. The human prion protein gene is situated on chromosome 20 and contains a polymorphic nucleotide at codon 129, which expresses ei- ther methionine or valine. Homozygosity (MM or VV) at codon 129 increases susceptibility to sporadic CJD. The genotype distribution in sporadic cases in the United Kingdom is MM 70%, valine homo- zygous (VV) 17%, and heterozygous (MV) 13% in contrast to the genotype distribution in the healthy UK population (MM 40%, MV 49%, and VV 11%). There is accumulating evidence that the disease phenotype in sporadic CJD, as well as susceptibility, is influenced by an interplay between the codon 129 genotype and the prion protein strain type. The most rapidly progressive forms of CJD are associ- ated with the MM genotype, followed by the VV genotype, with the slowest progression associated with the MV genotype. Inherited prion diseases Epidemiology Familial occurrence of prion disease accounts for about 10% of all cases with a dominant pattern of inheritance. The paradox of a transmissible disease that is also inherited was clarified by the Table 24.11.5.2  Human prion diseases Sporadic Creutzfeldt–​Jakob disease Variably protease sensitive prionopathy Inherited Familial Creutzfeldt–​Jakob disease Gerstmann–​Straussler–​Scheinker syndrome Fatal familial insomnia PrP systemic amyloidosis Acquired Iatrogenic Creutzfeldt–​Jakob disease Variant Creutzfeldt–​Jakob disease Kuru

section 24  Neurological disorders 6112 identification of an insertional mutation of six extra octapeptide re- peats and a missense mutation at codon 102 of the prion protein gene in families affected by the Gerstmann–​Straussler–​Scheinker syndrome, a condition known to be a human prion disease on the basis of the neuropathology and laboratory transmissibility. More than 60 prion protein gene mutations, including point and insertional mutations, have now been identified in inherited prion disease (Fig. 24.11.5.6). All clearly familial cases are associated with PRNP mutations. Fatal familial insomnia was first identified as a prion disease following the identification of a mutation at codon 178 of the prion protein gene in affected family members, and it was only later that transmission in the laboratory confirmed the status of fatal familial insomnia as a prion disease. PrP systemic amyloid- osis was first described in 2014 and does not seem to be trans- missible. The current hypothesis is that mutations of the prion protein gene predispose the protein to adopt the prion disease-​ associated form. Several inherited prion diseases, typically those which cause a CJD like phenotype show evidence of partial pene- trance in old age. The incidence of CJD in localized areas of Slovakia and in Libyan-​ born Israelis was discovered many years ago to be 60 to 100 times greater than expected. Following the identification of the mutations of prion protein in human disease, genetic studies have shown that in both clusters there is a high population frequency of mutations at codon 200 of the prion protein gene, and that the excess of cases of CJD is due to an excess of familial cases, with an expected back- ground incidence of sporadic cases. Clinical features Clinical presentation, age of clinical onset, and clinical dur- ation are highly variable. With some mutations, notably the most common worldwide, E200K, the clinical course is similar to spor- adic CJD, but cases of inherited prion disease may present with ataxia (e.g. Gerstmann–​Straussler–​Scheinker syndrome), or with a 0.15 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0 0–30 30–60 60–90 90–120 120–150 Clinical duration from first symptom (days) Clinical duration of sporadic CJD Proportion 150–180 180–210 210–240 240–270 270–300 300–330 330–360 360–720

720 Fig. 24.11.5.2  Survival of patients with sporadic Creutzfeldt–​Jakob disease recruited in the National Prion Monitoring Cohort 2008–​2015. alternative classification 1 1 2A sCJD kDa 36 30 16 1 2 3 4 London PrPSc type vCJD 2B Fig. 24.11.5.3  Western blot of brain tissue showing multiple types of disease-​associated prion protein (termed PrPSc). The three bands in each lane represent different glycosylation states of PrP, either unglycosylated (bottom band, lowest molecular weight), monoglycosylated, or diglycosylated. Lanes 1–​3 are sporadic Creutzfeldt–​Jakob disease, whereas lane 4 is variant CJD. Two classifications are in use, the ‘London classification’ and an alternative system with fewer types. Types are distinguished in both systems by the mobility and the relative predominance of different bands. Differing mobility of types of sporadic CJD is thought to be related to the access of protease to the N-​terminal amino acids of PrPSc and is thought to correlate with prion strains.

24.11.5  Human prion diseases 6113 highly atypical phenotype such as fatal familial insomnia in which the early clinical features include dysautonomia and insomnia. PrP systemic amyloidosis causes a syndrome of diarrhoea and hereditary sensory and autonomic neuropathy with onset ranging from ages 25–​60. There may be variation in the clinical and patho- logical phenotype both within and between families even if these are associated with the same underlying mutation in the prion protein gene. Fig. 24.11.5.4  Examples of prion disease pathology. (a) spongiform change in sCJD (H&E), (b) gliosis and spongiform change in sCJD (GFAP), (c) kuru-like plaques in sCJD (ICSM35), (d) perineuronal PrP staining in sCJD (ICSM35), (e) perivacuolar PrP staining in sCJD (ICSM35), (f) synpatic PrP staining in sCJD (ICSM35), (g, h) florid plaques in vCJD (H&E, ICSM35), (i) PrP deposition in a tonsillar biopsy specimen in vCJD (ICSM35). Images were provided by Professor Sebastian Brandner, UCL Institute of Neurology and UCLH NHS Foundation Trust. 0.5 0.4 0.3 0.2 0.1 0 20–30 30–40 40–50 50–60 60–70 70–80 80–90 90–100 Age at clinical onset (years) Age at clinical onset in sporadic CJD Proportion 100–110 Fig. 24.11.5.5  Age-​distribution of sporadic Creutzfeldt–​Jakob disease recruited in the National Prion Monitoring Cohort 2008–​2015.

section 24  Neurological disorders 6114 Iatrogenic CJD Epidemiology CJD has been transmitted accidentally in the course of medical treatment by neurosurgical instruments, corneal grafts, cadav- eric dura mater grafts, and human pituitary-​derived hormones (Table 24.11.5.3). The presumption is that infection from individ- uals with CJD was transmitted to uninfected individuals via these procedures and there is strong circumstantial evidence that this has occurred. In the two transmissions by corneal grafts the donors died of sporadic CJD, and in the neurosurgical transmissions there was a clear temporal link between surgical procedures on CJD cases and patients operated on using the same instruments who subsequently developed CJD. It is presumed that some human dura mater grafts and human pituitary hormones came from individuals with CJD and there may have been cross-​contamination in the production process, leading to dissemination of infection. Infection via human pituitary growth hormone has been demonstrated in laboratory 5- 0PRI 8- 0PRI 4- OPRI P102L P105L G114V G131V Y145X Q160X D178N E196K D202N Q217R Y218N V210I Y226X Q227X A117V Y163X T183A H187R E200K E211Q F198S P105T P84S P105S S132I R148H D178DEL2BP E211D V203I D202G A133V 2-0PRD S39L S17T P39S G54S 1-0PRI H85Y G142S G127V M129V E219K M154I N171S G195R V209M P238S 1-0PRD T188M M232T 3-0PRI R25H S97V 2-0PRI D167N F198V Q212P T188K T188A D167G T193I T188R T201SS I215V R208H R208C E196A V180I M232R E200D 129INSLGGLGGYV 6- 0PRI 9- 0PRI Definite Probable Possible Risk factor Polymorphism Resistance 7- 0PRI 12- 0PRI Fig. 24.11.5.6  Inherited prion diseases—​mutations of the prion protein gene. Over 60 definite, probable, or possibly causal mutations of the prion protein gene are known. There are also variants with increase or reduce the risk of sporadic prion disease, and variants which appear to confer no alteration of risk. Table 24.11.5.3  Iatrogenic Creutzfeldt–​Jakob disease worldwide Mode of infection No. of patients Mean incubation period (years) Clinical signs on presentation Neurosurgery 4 1.4 Visual/​dementia/​cerebellar Depth electrodes 2 1.3, 1.7 Dementia Corneal transplant 2 1.5, 27 Dementia Dura mater graft 228 12 Visual/​cerebellar/​dementia Growth hormone 226 17 Cerebellar Gonadotrophin 4 13.5 Cerebellar Packed red blood cellsa 3 6.5, 7.8, 8.3 Psychiatric, sensory, dementia, cerebellar a An additional asymptomatic but infected red-​cell recipient died of an unrelated illness: another asymptomatic infected haemophilia patient who had been exposed to potentially contaminated factor VIII also died of an unrelated illness (neither is included in the table).

24.11.5  Human prion diseases 6115 transmission studies. All cases of iatrogenic transmission of spor- adic CJD have involved surgical instruments, grafts, or hormonal products potentially contaminated by CNS tissue and, by implica- tion, high levels of infectivity. Clinical features There is a distinction between the clinical features in iatrogenic CJD which depends on the route of inoculation. In exposures in or adja- cent to the nervous system (neurosurgical instruments, dura mater grafts, and corneal transplants) most cases present with a progres- sive dementia similar to sporadic CJD. With a peripheral route of exposure to infection (cadaver derived human growth hormone, c-​ hGH) there is a progressive cerebellar ataxia and cognitive impair- ment develops late in the clinical course, if at all. The incubation period also varies according to the route of ex- posure to infection. With central exposure the mean incubation period ranges from about 18 months, similar to the incubation periods in primates after experimental intracerebral inoculation, to 12 years with dura mater grafts. With a peripheral route of ex- posure, the mean incubation period may extend to over 30 years, which is similar to the extended incubation periods in kuru, a human prion disease also caused by a peripheral route of exposure to infection. The polymorphism at PRNP codon 129 is a strong modifier of the incubation time of iatrogenic CJD; however, the experiences differ related to different exposures. For example, in c-​hGH iatrogenic CJD in France, individuals with an MM genotype have a shorter mean incubation time than those with genotype MV, whereas in the United Kingdom, those with the shortest mean incubation time are genotype VV, with MV and MM having longer incubations. These differences probably relate to the strain of prion which contamin- ated the c-​hGH. Despite incubation times approaching 40 years in the United Kingdom, those with iCJD related to c-​hGH and geno- type MM have rapid clinical courses. Measures to reduce the risk of iatrogenic transmission of CJD have been introduced in many countries. There are strict selection criteria for obtaining corneal grafts, recombinant growth hormone replaced human growth hormone in 1985, and human dura mater grafts have not been licensed in the United Kingdom since the early 1990s. Variant CJD Epidemiology Bovine spongiform encephalopathy was identified in 1986 as a novel prion disease in cattle in the United Kingdom, and is thought to have been caused by feeding cattle material contaminated with sheep scrapie or, perhaps, a previously unrecognized endemic prion disease of cattle. Bovine-​to-​bovine recycling of infection through cattle feed amplified the epidemic and there have now been over 180 000 documented cases of BSE in the United Kingdom. Small numbers of cases of BSE have been identified in other countries, mainly in Europe. In 1996, 10 cases of a novel form of human prion disease, variant Creutzfeldt–​Jakob disease (vCJD), were identified in the United Kingdom and a causal link with BSE was proposed as this was a new disease occurring only in the United Kingdom, the country with the greatest potential human exposure to BSE. Up to August 2015 there have been 174 primary cases of vCJD in the United Kingdom, 27 in France, and a limited number of cases in some other coun- tries. The mean age at death in vCJD is 30 years (range 14–​75 years) contrasting with a mean age at death in sporadic CJD of 67 years. The hypothesis that vCJD is caused by the BSE agent has been sup- ported by the consistent disease phenotype and, in particular, the neuropathology which is distinct from other human prion diseases, the failure to identify similar cases in the past either in the United Kingdom or elsewhere, and laboratory transmission studies that have shown a remarkable similarity between the transmission char- acteristics of BSE and vCJD in mice. Cases of vCJD have been identified from throughout the United Kingdom, and risk factors include residence in the United Kingdom and an MM genotype at codon 129 of the prion protein gene. All the United Kingdom cases and some of the cases in other countries had been resident in the United Kingdom during the 1980s to early 1990s, when human exposure to BSE was likely to have been max- imal. However, many of the cases outside the United Kingdom had never visited the United Kingdom, implying that exposure to BSE must have occurred in the country of origin to indigenous BSE or export from the United Kingdom of cattle or food products. The favoured hypothesis is that transmission of BSE to humans was through contamination of food, probably with tissues from the CNS such as brain or spinal cord which are known to contain high levels of infectivity in cattle infected with BSE. All tested cases of definite or probable vCJD to date have been MM homozygotes at codon 129 of the prion protein gene. This genotype is also present in about 70% of cases of sporadic CJD and may represent a sus- ceptibility factor for the development of vCJD. Variation at this locus can, however, influence the incubation period and disease phenotype and it is possible that cases of human infection with BSE may yet be identified in individuals with a VV or MV genetic back- ground. Indeed, a possible UK case of variant CJD with an MV gen- etic background was described in 2009. The potential future number of cases of vCJD is unknown, but the outbreak in the United Kingdom peaked in 1999/​2000 with a subsequent decrease in the annual number of deaths. Early pre- dictions estimated a total of 100 to over 136 000 cases of vCJD in the United Kingdom, but recent estimates are more conservative, predicting a maximum of no more than a few hundred cases There are, however, several uncertainties that make accurate predic- tion problematic, including the mean incubation period of BSE in humans, the level of the species barrier between bovines and hu- mans, and the possibility of future cases in a non-​MM genetic background. The identification of three cases of vCJD and one sub- clinical infection caused by transmission of the infectious agent through blood transfusion has raised concerns about the possibility of other routes of secondary transmission (e.g. through contamin- ated surgical instruments). Clinical features Presentation of vCJD is with behavioural and psychiatric disturb- ances and, in some cases, sensory disturbance. Initial referral is often to a psychiatrist with depression, anxiety, withdrawal, and behavioural change. Suicidal ideation is, however, infrequent and response to antidepressants poor. Delusions, which are complex and unsustained, are common. Other features include emotional la- bility, aggression, insomnia, and auditory and visual hallucinations.

section 24  Neurological disorders 6116 Dysaesthesiae, or pain in the limbs or face, which was persistent ra- ther than intermittent and unrelated to anxiety levels is a frequent early feature. Typically, a progressive cerebellar syndrome then de- velops with gait and limb ataxia followed with dementia and pro- gression to akinetic mutism. Myoclonus is frequent, and may be preceded by chorea. Cortical blindness develops in a minority of pa- tients in late disease. Upgaze paresis, an uncommon feature of clas- sical CJD, has been noted in some patients. Kuru Epidemiology The transmissibility of human prion diseases was first demon- strated in 1966 with the transmission of a spongiform encephalop- athy to chimpanzees 18–​21 months after intracerebral inoculation of a brain extract from a patient who had died of kuru. This sem- inal experiment followed years of clinical, epidemiological, and anthropological research in the Fore region of Papua New Guinea where kuru was endemic. In the early 1960s kuru caused over half of all deaths in the affected population and there have been more than 3000 deaths from kuru in the at-​risk population of 30 000 people. Kuru predominantly affected women and children. The disease was transmitted at cannibalistic feasts where tissues with the greatest concentration of prions, principally the brain, were preferentially eaten by the children and females, the males older than 7 consuming predominantly muscle. The disease is a progressive ataxia and subsequent dementia developing over one to two years the patient ultimately becoming moribund. There is often a prodrome of headache. Banning canni- balistic practices has resulted in a dramatic decline in the prevalence of kuru, although a few cases may still occur. Interestingly, while the early cases were predominantly 129MM and 129VV, in the most re- cent analysis heterozygotes are the majority some with extremely long incubation times (over 50 years). As a result of the extreme selection pressures caused by the severity of the kuru epidemic, the Fore population show evidence of a population genetic evolutionary response. Survivors of the kuru epidemic, who were alive at the time of cannibalistic feasts, show marked Hardy-​Weinberg diserquilibrium at codon 129, with a pre- dominance of the resistant MV genotype. Remarkably, some elderly women who attended cannibalistic feasts but did not get kuru pos- sess a novel genetic resistance factor, G127V, unique to the Fore. In transgenic mice, this human gene variant in the homozygous state confers complete resistance to all prion diseases. The diagnosis of human prion diseases Human prion diseases are rare, but the high public profile of CJD and vCJD has resulted in an increase in the number of cases in which the diagnosis of one of these diseases is suspected. Accurate diag- nosis of any condition, including patients suffering from a human prion disease, is essential but the exclusion of a diagnosis is also im- portant, particularly for a fatal and untreatable condition. Although symptomatic treatment (e.g. for involuntary movements), can be helpful in human prion diseases, there is currently no available treat- ment that influences the clinical course or any treatment to prevent the development of neurological disease after infection. An im- portant objective is to improve diagnostic accuracy in human prion diseases and in particular to allow early diagnosis. In the absence of a test for the presence of the infectious agent, diagnosis depends on the recognition of the clinical characteristics of human prion dis- eases supported by a range of investigations, some of which have been developed in recent years. Diagnostic criteria for sporadic, iatrogenic, familial, and vCJD have been formulated and validated (Tables 24.11.5.4 and 24.11.5.5). In all human prion diseases, a def- inite diagnosis can be made only by the examination of tissue sam- ples, usually post-​mortem. In most cases of sporadic CJD, the diagnosis is suspected in life because of the multifocal neurological deficits, the development of myoclonus, and in particular the rapidity in the progression of cognitive impairment. The clinical picture is usually distinct from more common forms of dementia. In forms of sporadic CJD with early focal neurological features, such as a cerebellar syndrome, the rapid evolution of other neurological deficits and dementia suggests the diagnosis of CJD. Diagnosis may not be suspected in cases of sporadic CJD with atypical features such as long duration of illness, and/​or those with only or predominantly cognitive features. MRI, particularly with diffusion-​weighted and fluid-​attenuated inversion recovery (FLAIR) imaging, which in more than 90% shows specific high signal abnormalities in sporadic CJD, is now the key initial diagnostic investigation. Inherited prion diseases are often suspected because of a family history of a similar disorder, but in a significant proportion of cases of CJD associated with a prion protein gene mutation there is a family history of another neurodegenerative disorder or no relevant family history. The gradual clinical progression in many forms of hereditary human prion disease makes accurate diagnosis difficult and the diagnosis may be recognized in life only after prion protein gene analysis. Genetic testing should be carried only out with fully informed consent. Table 24.11.5.4  Diagnostic criteria for sporadic Creutzfeldt–​Jakob disease I Rapidly progressive dementia IIA Myoclonus IIB Visual or cerebellar problems IIC Pyramidal or extrapyramidal features IID Akinetic mutism III Typical electroencephalogram IV High signal in caudate/​putamen on MRI brain scan Definite Neuropathologically/​immunocytochemically confirmed Probable I + two of II + III or I + two of II + IV or I + two of II + positive 14-​3-​3 or Progressive neurological syndrome and positive RT-QuIC in CSF or other tissues Possible I + two of II + duration less than 2 years

24.11.5  Human prion diseases 6117 The diagnosis of iatrogenic CJD depends on the identification of a relevant risk factor (e.g. previous treatment with human growth hormones), and an assessment of the neurological presentation. Most patients with growth hormone-​related CJD present with a cerebellar syndrome, whereas after central iatrogenic exposure to infection the clinical picture is usually similar to that of sporadic CJD. The utility of specialist investigation in iatrogenic CJD is un- certain because of their rarity, but positive findings on MRI of the brain and/​or the 14-​3-​3 cerebrospinal fluid (CSF) test may support the diagnosis. The clinical picture in the later stages of vCJD is similar to that of sporadic CJD and, although the recognition of the diagnosis in the first cases of this new disease was difficult, the clinical pheno- type is now well known and the diagnosis is usually apparent after neurological signs develop, often in young patients in an age group in which dementia is very unusual. Diagnosis in the early stages is, however, difficult as there is a period of many months in which the clinical picture is dominated by psychiatric symptoms, including de- pression, anxiety, and withdrawal. Clues to the possibility of vCJD include cognitive impairment, subtle gait ataxia, and persistent painful sensory symptoms in combination with the psychiatric symptoms. The clinical features of sporadic and vCJD are compared in Table 24.11.5.6. Investigations in human prion disease Many of the investigations carried out in suspected cases of human prion disease do not show any specific disease-​related abnormality, but help to exclude other diagnoses, some potentially treatable. The interpretation of the results of investigations depends on the clinical picture because the sensitivity and specificity of surrogate markers for prion disease, such as 14-​3-​3 CSF analysis (see next), depend on clearly defining the characteristics of the patients in which the test has been carried out. Routine haematological and biochemical tests are usually normal. About a third of cases of sporadic or vCJD may have minor abnor- malities in liver function tests. The electroencephalogram (EEG) shows periodic triphasic com- plexes at about 1/​s in 60–​70% of cases of sporadic CJD (Fig. 24.11.5.7) and in some cases of iatrogenic CJD after central exposure to infec- tion. These EEG changes are relatively specific, but similar appear- ances can be seen in hepatic encephalopathy, lithium, or metrizamide toxicity, metabolic disturbance, and rarely in other forms of dementia such as Alzheimer’s disease. There is no CSF pleocytosis in any form of human prion disease, but CSF protein is elevated in about a third of cases. Elevation of the 14-​3-​3 CSF protein, a marker for neuronal damage, has a sen- sitivity and specificity of about 90% in the diagnosis of sporadic CJD, but is less useful in the diagnosis of vCJD. A new test has become available in recent years termed the real-​time quaking induced conversion assay (rtQUIC). This assay detects prion seeds by using cycles of shaking and incubation of a CSF sample in an excess of recombinant prion protein. The recombinant prion protein binds the seed, in a similar way to what is thought to happen during prion propagation, and misfolds, generating prion protein aggregates. Shaking (or ‘quaking’) then breaks up Table 24.11.5.5  Diagnostic criteria for variant Creutzfeldt–​Jakob disease IA Progressive neuropsychiatric disorder IB Duration of illness more than 6 months IC Routine investigations do not suggest an alternative diagnosis ID No history of potential iatrogenic exposure IE No evidence of a familial form of transmissible spongiform encephalopathy (TSE) IIA Early psychiatric symptomsa IIB Persistent painful sensory symptomsb IIC Ataxia IID Myoclonus or chorea or dystonia IIE Dementia IIIA EEG does not show the typical appearance of sporadic Creutzfeldt–​Jakob diseasec (or no EEG performed)c IIIB Bilateral pulvinar high signal on MRI scan IVA Positive tonsil biopsyd Definite IA + neuropathological confirmation of variant
Creutzfeldt–​ Jakob diseasee Probable I + four out of five of II + IIIA + IIIB Probable I + IVAd Possible I + four out of five of II + IIIA a Depression, anxiety, apathy, withdrawal, delusions. b This includes both frank pain and/​or unpleasant dysaesthesia. c The typical appearance of the EEG in sporadic CJD consists of generalized triphasic periodic complexes at approximately one per second. These may occasionally been seen in the late stages of vCJD. d Tonsil biopsy is not recommended routinely, nor in cases with EEG appearances typical of sporadic CJD, but may be useful in suspect cases in which the clinical features are compatible with vCJD and MRI does not show bilateral pulvinar high signal. e Spongiform change and extensive deposition of prion protein with florid plaques, throughout the cerebrum and cerebellum. Table 24.11.5.6  Clinical features of sporadic and variant Creutzfeldt–​Jakob disease Feature Sporadic CJD Variant CJD Mean age at death 67 years 30 years Median illness duration 4 months 14 months Symptoms at onset: Cognitive impairment 70% 10% Psychiatric (depression, anxiety,
and so on) <5% 70% Painful sensory symptoms <1% 20% Signs during clinical course: Dementia 100% 100% Ataxia 80% 100% Involuntary movements: Myoclonus 95% 70% Chorea <5% 50% Dystonia <5% 25%

section 24  Neurological disorders 6118 the aggregates to generate more seeds for the reaction. The aggre- gates generated in this way can be labelled with the fluorescent thioflavin T. In some laboratories the rtQUIC test is more than 90% sensitive with sporadic CJD CSF samples, and is extremely specific (close to 100%). The test is particularly useful in atyp- ical clinical and imaging cases. rtQUIC is not at present opti- mized for variant CJD. Amplification assays using urine and/​or nasal mucosa have shown promising initial results, but further research is needed. MRI is the most useful modality of imaging in CJD (Fig. 24.11.5.8). High signal return from grey matter is characteristic of CJD and is usually most apparent on diffusion-​weighted images, less so on FLAIR, and least on T2 weighted images. Diffusion-​ weighted imaging can be done at various b-​values but conven- tionally 1000 s/​mm2. Apparent diffusion coefficient maps should be calculated to confirm true restricted diffusion and remove T2-​weighted ‘shine through’. Enhancement with gadolinium does not occur in any type of CJD. The distribution of the abnormal signal varies between different types of CJD. In sporadic CJD there is high signal return from the basal ganglia, typically the caudate and anterior putamen in around 70%. This may be asymmetrical. In addition, thalamic signal is often abnormal and can be focal, patchy, or diffuse. The abnormality can include the posterior complex, leading to consideration of variant CJD, but in sporadic CJD the thalamic signal is less intense than that from the caudate nuclei. Cortical ‘ribboning’ is found in many patients usually in addition to the basal ganglia abnormality. This is best seen on diffusion-​weighted imaging and apparent diffusion coefficient maps. The distri- bution can be focal involving any part of the cortex; care must be taken in determining abnormality in areas of allocortex par- ticularly the anterior cingulate and insula with 3T scanning and with frontal cortex adjacent to the frontal sinuses. Nevertheless, the cingulate abnormality often extends caudally and can be the sole abnormal cortical region. The cortical signal abnormality is usually asymmetrical. In a few patients cortical ribboning is the only abnormality. Taken together with the subcortical changes, the MRI signal is abnormal in over 90% of patients with sporadic CJD. In vCJD about 90% of patients in a prospective series have high signal return from the pulvinar and medial areas of the thalamus particularly adjacent to the ventricle, the so called ‘hockey stick’ sign. This is apparent on FLAIR and diffusion-​weighted imaging in most cases. However, it is unclear when this sign develops and it is not infrequent that the initial scan is reported as normal, but be- comes clearly abnormal over a few months. Brain biopsy can allow the confirmation of the diagnosis of a human prion disease in life, but this investigation has risks and is mainly carried out when there is a realistic possibility of an alterna- tive diagnosis. Tonsil biopsy in vCJD is a more accessible procedure for tissue diagnosis and may be important for the relatives of the patient and for clinicians. Fig. 24.11.5.7  Typical electroencephalogram in sporadic Creutzfeldt–​Jakob disease. Image was provided by Dr Tim Wehner of UCLH NHS Foundation Trust.

24.11.5  Human prion diseases 6119 FURTHER READING Atarashi R, et al. (2011). Ultrasensitive human prion detection in cerebrospinal fluid by real-​time quaking-​induced conversion. Nat Med, 17, 175–​8. Bradley R, et al. (2006). Variant CJD (vCJD) and bovine spongiform encephalopathy (BSE): 10 and 20 years on: part 1. Folia Neuropathol, 44, 93–​101. Brown P (2008). Transmissible spongiform encephalopathy in the 21st century. Neurology, 70, 713–​22. Cali I, et al. (2006). Classification of sporadic Creutzfeldt–​Jakob dis- ease revisited. Brain, 129, 2266–​77. Collee JG, et al. (2006). Variant CJD (vCJD) and bovine spongiform encephalopathy (BSE): 10 and 20 years on: part 2. Folia Neuropathol, 44, 102–​10. Collinge J, et al. (2006). Kuru in the 21st century—​an acquired human prion disease with very long incubation periods. Lancet, 367, 2068–​74. Collins SJ, et  al. (2006). Determinants of diagnostic investigation
sensitivities across the clinical spectrum of sporadic Creutzfeldt–​ Jakob disease. Brain, 129, 2278–​87. de Armond SJ, et  al. (2003). Perspectives on prion biology, prion
disease pathogenesis, and pharmacologic approaches to treatment. Clin Lab Med, 23, 1–​41. Mead S, et al. (2009). A novel protective prion protein variant that colocalizes with kuru exposure. N Engl J Med, 361, 2056–​65. Wadsworth JDF, et  al. (2007). Update on human prion disease. Biochim Biophys Acta, 1772, 598–​609. Fig. 24.11.5.8  MR image of sporadic Creutzfeldt–​Jakob disease showing (a) ‘cortical ribbon’ on diffusion-​weighted images, (b) high signal from the caudate, putamen, thalamus, and cortex on FLAIR images, (c) high signal from caudate, putamen and less so the thalamus on diffusion-​weighted images, and (d) ‘pulvinar’ sign in variant CJD on FLAIR images. Images were provided by Dr Harpreet Hyare of the National Prion Clinic and UCLH NHS Foundation Trust.