Prion Disease

Article about prion disease.

A prion is a tiny, protein-based infectious particle. Prions transmit diseases that cause degeneration of the central nervous system, including Creutzfeldt–Jakob disease in humans and bovine spongiform encephalopathy (BSE) in cattle. Prions do not contain nucleic acids, unlike viruses, and are difficult to destroy because they are highly resistant to heat and disinfectants.There is currently no treatment available for prion diseases.

Prion diseases are also known as slow virus diseases. They area a group of diseases of the central nervous system that occur many months or even years after infection with a virus. They cause gradual widespread destruction of nerve tissue, with progressive loss of brain function and a fatal outcome. Examples include Creutzfeldt– Jakob disease and kuru.

Creutzfeldt-Jacob disease (CJD) is a rare, progressive degenerative disease of the brain. CJD is thought to be caused by infection with a prion (a type of infectious protein). A similar agent causes scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle.

Types of CJD

One form of CJD largely affects middle aged or elderly people, and appears to have no obvious cause. A second form occurs in younger people. and this is associated with contamination during brain surgery or transplants from infected people, or with treatment using infected human growth hormone or gonadotrophins. A third form, new variant (nv) CJD, was first identified in 1995 and affects people in their teens and 20s. It is thought to be acquired by eating beef infected with bovine spongiform encephalopathy (BSE). This form causes pathological changes in the brain that are similar to the changes found in cattle suffering from BSE.

Symptoms of CJD 

Symptoms are broadly similar for all forms of the disease. Slowly progressive dementia (deterioration in brain function) and myoclonus (sudden muscular contractions) occur; coordination diminishes; the intellect and personality deteriorate; and blindness may develop. As the disease progresses, speech is lost and the body becomes rigid.

Outlook for CJD

There is no treatment, and death usually occurs within two to three years.

Kuru

Kuru is a rare, fatal infection of the brain that affects some inhabitants of New Guinea. Kuru is caused by a “slow” virus, or prion, which has a long incubation period and is spread by cannibalism. Signs include progressive difficulty in controlling movements and dementia. The study of scrapie in sheep led to the identification of prions, which also play a role in Creutzfeldt–Jakob disease and BSE.

Read more: An alternative article about prion disease - Human prion diseases

Prion diseases in detail - technical

Introduction

The human prion diseases, also known as the subacute spongiform encephalopathies, have been traditionally classified into Creutzfeldt– Jakob disease ( CJD), Gerstmann–Sträussler syndrome (GSS) (also known as Gerstmann–Sträussler–Scheinker disease), and kuru.

Although rare, affecting about one person per million worldwide per annum, remarkable attention has been recently focused on these diseases. This is because of the unique biology of the transmissible agent or prion, and also because bovine spongiform encephalopathy ( BSE), an epidemic bovine prion disease, appears to have transmitted to humans as ‘new variant' CJD ( vCJD), opening the possibility of a significant threat to public health through dietary exposure to infected tissues.

The transmissibility of the human diseases was demonstrated with the transmission, by intracerebral inoculation with brain homogenates into chimpanzees, of first kuru and then CJD in 1966 and 1968 respectively. (1,2) Transmission of GSS followed in 1981. The prototypic prion disease is scrapie, a naturally occurring disease of sheep and goats, which has been recognized in Europe for over 200 years (3) and which is present in the sheep flocks of many countries. Scrapie was demonstrated to be transmissible by inoculation in 1936 (4) and the recognition that kuru, and then CJD, resembled scrapie in its histopathological appearances led to the suggestion that these diseases may also be transmissible.(5) Kuru reached epidemic proportions amongst the Fore linguistic group in the Eastern Highlands of Papua New Guinea and was transmitted by ritual cannibalism. Since the cessation of cannibalism in the 1950s the disease has declined but a few cases still occur as a result of the long incubation periods in this condition.

The term Creutzfeldt–Jakob disease was introduced by Spielmeyer in 1922 bringing together the case reports published by Creutzfeldt and Jakob. Several of these cases would not meet modern diagnostic criteria for CJD and indeed it was not until the demonstration of transmissibility allowed diagnostic criteria to be reassessed and refined that a clear diagnostic entity developed. All these diseases share common histopathological features; the classical triad of spongiform vacuolation (affecting any part of the cerebral grey matter), astrocytic proliferation, and neuronal loss may be accompanied by the deposition of amyloid plaques. (6)

Aetiology

Prion diseases of both humans and animals are associated with the accumulation in the brain of an abnormal, partially protease-resistant, isoform of a host-encoded glycoprotein known as prion protein (PrP). The disease-related isoform, PrP Sc, is derived from its normal cellular precursor, PrP C, by a post-translational process that involves a conformational change. PrPC is rich in a-helical structure while PrP Sc appears to be predominantly composed of b-sheet structure. According to the ‘protein-only' hypothesis, (7) an abnormal PrP isoform (8) is the principal, and possibly the sole, constituent of the transmissible agent or prion. PrP Sc is hypothesized to act as a conformational template, promoting the conversion of PrP C to further PrPSc. PrPC appears to be poised between two radically different folding states, and a and b forms of PrP can be interconverted in suitable conditions. (9) Soluble b-PrP aggregates in physiological salt concentrations to form fibrils with morphological and biochemical characteristics closely similar to PrP Sc.

A molecular mechanism for prion propagation can now be proposed.(9) Prion replication, with recruitment of PrP C into the aggregated PrPSc isoform, may be initiated by a pathogenic mutation (resulting in a PrP C predisposed to form b-PrP) in inherited prion diseases, by exposure to a ‘seed' of PrPSc in acquired cases, or as a result of the spontaneous conversion of PrP C to b-PrP (and subsequent formation of aggregated material) as a rare stochastic event in sporadic prion disease.

The human PrP gene (PRNP) is a single-copy gene located on the short arm of chromosome 20 and was an obvious candidate for genetic linkage studies in the familial forms of CJD and GSS, which both showed an autosomal dominant pattern of disease segregation. A turning point in understanding the human prion diseases was the identification of mutations in the prion protein gene in familial CJD and GSS in 1989. The first mutation to be identified in PRNP was in a family with CJD and constituted a 144-bp insertion into the coding sequence. (10) A second mutation was reported in two families with GSS and genetic linkage was confirmed between this missense variant at codon 102 and GSS, confirming that GSS was an autosomal dominant Mendelian disorder. (11) Uniquely, these diseases are therefore both inherited and transmissible. Current evidence suggests that around 15 per cent of prion diseases are inherited and at least 20 coding mutations in PRNP are now recognized. (12)

With the exception of the rare iatrogenic CJD cases mentioned above, most prion disease occurs as sporadic CJD. While, by definition, there will not be a family history in sporadic cases, mutations are seen in occasional apparently sporadic cases, as with a late-onset disease the family history may not be apparent or non-paternity may occur. However, in the majority of sporadic CJD cases there is neither a coding mutation nor a history of iatrogenic exposure. Human prion diseases can therefore be subdivided into inherited, sporadic, and acquired forms. However, a common PrP polymorphism at residue 129, where either methionine or valine can be encoded, is a key determinant of genetic susceptibility to acquired and sporadic prion diseases, the large majority of which occur in homozygous individuals.(13,14) This protective effect of PRNP codon 129 heterozygosity is also seen in some of the inherited prion diseases. (15,16)

The aetiology of sporadic CJD remains unclear. It has been speculated that these cases might arise from somatic mutation of PRNP or spontaneous conversion of PrPC to PrPSc as a rare stochastic event. The alternative hypothesis, that such cases arise as a result of exposure to an environmental source of either human or animal prions, is not supported by epidemiological evidence. (17)

A major problem for the ‘protein-only' hypothesis of prion propagation has been how to explain the existence of multiple isolates or strains of prions which have distinct biological properties. Understanding how a protein-only infectious agent could encode such phenotypic information has been of considerable biological interest. However, it is now clear that prion strains can be distinguished by differences in the biochemical properties of PrP Sc. Prion strain diversity appears to encoded by differences in PrP conformation and pattern of glycosylation. (18) A molecular strain typing approach based on these characteristics has allowed the identification of four main types amongst CJD cases, sporadic and iatrogenic CJD being of PrP Sc types 1–3, while all vCJD cases are associated with a distinctive type 4 PrPSc type. (18,19) A similar PrPSc type to that seen in vCJD is seen in BSE and BSE when transmitted to several other species. (18) Such molecular strain typing strongly supported the hypothesis that vCJD was human BSE. This conclusion was strengthened by subsequent transmission studies of vCJD into both transgenic and conventional mice which argued that cattle BSE and vCJD were caused by the same strain. (20,21) Such studies are allowing a molecular classification of human prion diseases; it is likely that additional PrP Sc types or strains will be identified. This may well open new avenues of epidemiological investigation and offer insights transmission. It would be surprising if this mechanism had not been used more widely during evolution such that prion biology may prove to be of far wider relevance.

Transmission of prion diseases between different mammalian species is limited by a so-called ‘species barrier'. (22) Early studies of the molecular basis of the species barrier argued that it principally resided in differences in PrP primary structure between the species from which the inoculum was derived and the inoculated host. Transgenic mice expressing hamster PrP were, unlike wild-type mice, highly susceptible to infection with hamster prions. (23) That most sporadic and acquired CJD occurred in individuals homozygous at PRNP polymorphic codon 129 supported the view that prion propagation proceeded most efficiently when the interacting PrP Sc and PrPC were of identical primary structure. (13,14) However, it has been long recognized that prion strain type affects ease of transmission to another species. Interestingly, with BSE prions the strain component to the barrier seems to predominate, with BSE not only transmitting efficiently to a range of species, but maintaining its transmission characteristics even when passaged through an intermediate species with a distinct PrP gene. (24,20) The term ‘species–strain barrier' or simply ‘transmission barrier' may be preferable.(25) Both PrP amino acid sequence and strain type affect the three-dimensional structure of glycosylated PrP which will presumably, in turn, affect the efficiency of the protein– protein interactions thought to determine prion propagation. Contribution of other components to the species barrier are possible and may involve interacting cofactors which mediate the efficiency of prion propagation, although no such factors have yet been identified.

The species barrier between cattle BSE and humans cannot be directly measured but can be modelled in transgenic mice expressing human PrP C, which produce human PrPSc when challenged with human prions.(26) When such mice, expressing both human PrP valine 129 (at high levels) and mouse PrP, are challenged with BSE, three possibilities could be envisaged: these mice could produce human prions, murine prions, or both. In fact, only mouse prion replication could be detected. Although there are caveats with respect to this model, particularly that human prion propagation in mouse cells may be less efficient that that of mouse prions, this result would be consistent with the bovine to human barrier being higher than the bovine to mouse barrier for this PRNP genotype. In the second phase of these experiments, mice expressing only human PrP were challenged with BSE. While CJD isolates transmit efficiently to such mice at around 200 days, only infrequent transmissions at over 500 days were seen with BSE, consistent with a substantial species barrier for this human PRNP genotype.(27) However, it is important to repeat these studies in mice expressing only human PrP methionine 129 and in heterozygotes. So far, BSE appears to have transmitted only to humans of PRNP codon 129 methionine homozygous genotype.

Clinical features and diagnosis

The human prion diseases can be divided aetiologically into inherited, sporadic, and acquired forms with CJD, GSS, and kuru now seen as clinicopathological syndromes within a wider spectrum of disease. Kindreds with inherited prion disease have been described with phenotypes of classical CJD, GSS, and also with other neurodegenerative syndromes including fatal familial insomnia. (28) Some kindreds show remarkable phenotypic variability which can encompass both CJD- and GSS-like cases as well as other cases which do not conform to either CJD or GSS phenotypes. (29) Cases diagnosed by PrP gene analysis have been reported which are not only clinically atypical but which lack the classical histological features entirely. (30) Significant clinical overlap exists with familial Alzheimer's disease, Pick's disease, frontal lobe degeneration of non-Alzheimer type, and amyotrophic lateral sclerosis with dementia. Although classical GSS is described below it now seems more sensible to designate the familial illnesses as inherited prion diseases and then to subclassify these according to mutation. Acquired prion diseases include iatrogenic CJD, kuru, and now vCJD. Sporadic prion diseases at present consist of CJD and atypical variants of CJD. Cases lacking the characteristic histological features of CJD have been transmitted. As there are at present no equivalent aetiological diagnostic markers for sporadic prion diseases to those for the inherited diseases, it cannot yet be excluded that more diverse phenotypic variants of sporadic prion disease exist. 

Sporadic prion disease

Creutzfeldt–Jakob disease

The core clinical syndrome of classic CJD is of a rapidly progressive multifocal dementia usually with myoclonus. The onset is usually in the 45- to 75-year age group with peak onset between 60 and 65. The clinical progression is typically over weeks progressing to akinetic mutism and death often in 2 to 3 months. Around 70 per cent of cases die in under 6 months. Prodromal features, present in around a third of cases, include fatigue, insomnia, depression, weight loss, headaches, general malaise, and ill-defined pain sensations. In addition to mental deterioration and myoclonus, frequent additional neurological features include extrapyramidal signs, cerebellar ataxia, pyramidal signs, and cortical blindness. About 10 per cent of cases present initially with cerebellar ataxia.

Routine haematological and biochemical investigations are normal although occasional cases have been noted to have raised serum transaminases or alkaline phosphatase. There are no immunological markers and acute-phase proteins are not elevated. Examination of the cerebrospinal fluid is normal although raised neuronal specific enolase and S-100 have both been proposed as useful markers, although it is clear that they are not specific for CJD and represent markers of neuronal injury.(31,32 and 33) A promising cerebrospinal fluid marker is estimation of 14-3-3 protein, which, while again not a specific disease marker, appears to be a useful adjunct to diagnosis in the appropriate clinical context. (34,35) It is also positive in recent cerebral infarction or haemorrhage and in viral encephalitis, although these conditions do not usually present diagnostic confusion with CJD. Neuroimaging with CT or magnetic resonance imaging ( MRI) is useful to exclude other causes of subacute neurological illness but there are no diagnostic features; cerebral and cerebellar atrophy may be present. The EEG may, however, show characteristic pseudoperiodic sharp wave activity which is helpful in diagnosis but present only in around 70 per cent of cases. To some extent demonstration of a typical EEG is dependent on the number of EEGs performed, and serial EEG is indicated to try and demonstrate this appearance.

Prospective epidemiological studies have demonstrated that cases with a progressive dementia, and two or more of the following: myoclonus, cortical blindness, pyramidal, cerebellar, or extrapyramidal signs, or akinetic mutism, in the setting of a typical EEG, nearly always turn out to be confirmed as histologically definite CJD if neuropathological examination is performed.

Neuropathological confirmation of CJD is by demonstration of spongiform change, neuronal loss, and astrocytosis. PrP amyloid plaques are usually not present in CJD although PrP immunohistochemistry, using appropriate pretreatments,(36) will nearly always be positive. Protease-resistant PrP, seen in all the currently recognized prion diseases, can be demonstrated by immunoblotting of brain homogenates. PRNP analysis is important to exclude pathogenic mutations. Genetic susceptibility to CJD has been demonstrated in that most cases of classical CJD are homozygous with respect to the common 129 polymorphism of PrP (see discussion of aetiology above).

Atypical forms of Creutzfeldt–Jakob disease

Atypical forms of CJD are well recognized. Around 10 per cent of cases of CJD have a much more prolonged clinical course with a disease duration of over 2 years. (37) These cases may represent the occasional occurrence of CJD in individuals heterozygous for PrP polymorphisms. (38) Around 10 per cent of CJD cases present with cerebellar ataxia rather than cognitive impairment, so-called ataxic CJD. (39) Heidenhain's variant of CJD refers to cases in which cortical blindness predominates with severe involvement of the occipital lobes. The panencephalopathic type of CJD refers to cases with extensive degeneration of the cerebral white matter in addition to spongiform vacuolation of the grey matter and has been predominantly reported from Japan. (39) Amyotrophic variants of CJD have been described with prominent early muscle wasting. However, most cases of dementia with amyotrophy are not experimentally transmissible (40) and their relationship with CJD is unclear. Most cases are probably variants of motor neurone disease with associated dementia. Amyotrophic features in CJD are usually seen in late disease when other features are well established.

Acquired prion diseases

While human prion diseases can be transmitted to experimental animals by inoculation, they are not contagious in humans. Documented case-to-case spread has only occurred during ritual cannibalistic practices (kuru) or following accidental inoculation with prions during medical or surgical procedures (iatrogenic CJD).

Kuru

Kuru reached epidemic proportions amongst a defined population living in the Eastern Highlands of Papua New Guinea. The earliest cases are thought to date back to the early part of the century. Kuru affected the people of the Fore linguistic group and their neighbours with whom they intermarried. Kuru predominantly affected women and children (of both sexes), with only 2 per cent of cases in adult males (41) and was the most common cause of death amongst women in affected villages. It was the practice in these communities to engage in consumption of dead relatives as a mark of respect and mourning. Women and children predominantly ate the brain and internal organs, which is thought to explain the differential age and sex incidence.

Preparation of the cadaver for consumption was performed by the women and children such that other routes of exposure may also have been relevant. It is thought that the epidemic related to a single sporadic CJD case occurring in the region some decades earlier. Epidemiological studies provided no evidence for vertical transmission, since most of the children born after 1956 (when cannibalism had effectively ceased) and all of those born after 1959 of mothers affected with or incubating kuru were unaffected. (41) From the age of the youngest affected patient, the shortest incubation period is estimated as 4.5 years, although may have been shorter, since time of infection was usually unknown. Currently, two to three cases are occurring annually, all in individuals aged 40 or more, consistent with exposure prior to 1956 and indicating that incubation periods can be 40 years or more (unpublished data).

Kuru affects both sexes and onset of disease has ranged from age 5 to over 60. The mean clinical duration of illness is 12 months with a range of 3 months to 3 years; the course tends to be shorter in children. The central clinical feature is progressive cerebellar ataxia. In sharp contrast to CJD, dementia is usually absent, even in the latter stages, although in the terminal stages many patients have their faculties obtunded. (41) The occasional case in which gross dementia occurs is in marked contrast to the clinical norm. Detailed clinical descriptions have been given by a number of observers and the disease does not appear to have changed in features at different stages of the epidemic. A prodrome and three clinical stages are recognized as follows.

Prodromal stage of kuru

Kuru typically begins with prodromal symptoms consisting of headache, aching of limbs, and joint pains which can last for several months.

Ambulatory stage

Kuru was frequently self-diagnosed by patients at the earliest onset of unsteadiness in standing or walking, or of dysarthria or diplopia. At this stage there may be no objective signs of disease. Gait ataxia, however, worsens and patients develop a broad-based gait, truncal instability, and titubation. A coarse postural tremor is usually present and accentuated by movement; patients characteristically hold their hands together in the midline to suppress this. Standing with feet together reveals clawing of toes to maintain posture. This marked clawing response is regarded as pathognomonic of kuru. Patients often become withdrawn at this stage and occasionally develop a severe reactive depression.

Prodromal symptoms tend to disappear. Astasia and gait ataxia worsen and the patient requires a stick for walking. Intention tremor, dysmetria, hypotonia, and dysdiadochokinesis develop. Although eye movements are ataxic and jerky, nystagmus is rarely seen. Strabismus, usually convergent, may occur particularly in children. This strabismus does not appear to be concomitant or paralytic and may fluctuate in both extent and type sometimes disappearing later in the clinical course. Photophobia is common and there may be an abnormal cold sensitivity with shivering and piloerection even in a warm environment. Tendon reflexes are reduced or normal and plantar responses are flexor. Dysarthria usually occurs. As ataxia progresses the patient passes from the first (ambulatory) stage to the second (sedentary) stage. The mean clinical duration of the first stage is around 8 months and correlates closely with total duration. (42)

Sedentary stage

At this stage patients are able to sit unsupported but cannot walk. Attempted walking with support leads to a high steppage, wide-based gait with reeling instability, and flinging arm movements in an attempt to maintain posture. Hyper-reflexia is seen although plantar responses usually remain flexor with intact abdominal reflexes. Clonus is characteristically short-lived. Athetoid and choreiform movements and parkinsonian tremors may occur. There is no paralysis, although muscle power is reduced. Obesity is common at this stage but may be present in early disease associated with bulimia. Characteristically, there is emotional lability and bizarre uncontrollable laughter, which has led to the disease being referred to as ‘laughing death'. There is no sensory impairment. In sharp contrast to CJD, myoclonic jerking is rarely seen. EEG is usually normal or may show non-specific changes. (43) This stage lasts around 2 to 3 months. When truncal ataxia reaches the point where the patient is unable to sit unsupported, the third or tertiary stage is reached.

Tertiary stage

Hypotonia and hyporeflexia develop and the terminal state is marked by flaccid muscle weakness. Plantar responses remain flexor and abdominal reflexes intact. Progressive dysphagia occurs, and patients become incontinent of urine and faeces. Inanition and emaciation develop. Transient conjugate eye signs and dementia may occur. Primitive reflexes develop in occasional cases. Brainstem involvement and both bulbar and pseudobulbar signs occur. Respiratory failure and bronchopneumonia eventually lead to death. The tertiary stage lasts 1 to 2 months.

Iatrogenic Creutzfeldt–Jakob disease

Iatrogenic transmission of CJD has occurred by accidental inoculation with human prions as a result of medical procedures. Such iatrogenic routes include the use of inadequately sterilized neurosurgical instruments, dura mater and corneal grafting, and use of human cadaveric pituitary-derived growth hormone or gonadotrophin. It is of considerable interest that cases arising from intracerebral or optic inoculation manifest clinically as classical CJD, with a rapidly progressive dementia, while those resulting from peripheral inoculation, most notably following pituitary-derived growth hormone exposure, typically present with a progressive cerebellar syndrome, and are in that respect somewhat reminiscent of kuru. Unsurprisingly the incubation period in intracerebral cases is short (19–46 months for dura mater grafts) as compared with peripheral cases (typically 15 years or more). There is evidence for genetic susceptibility to iatrogenic CJD with an excess of codon 129 homozygotes (13) (see discussion of aetiology above).

Epidemiological studies have not shown increased risks of particular occupations that may be exposed to human or animal prions, although individual CJD cases in two histopathology technicians, a neuropathologist, and a neurosurgeon have been documented. While there have been concerns that CJD may be transmissible by blood transfusion, extensive epidemiological analysis in the United Kingdom has found that the frequency of blood transfusion and donation was no different in over 200 cases of CJD and a matched control population. (44) Recipients of blood transfusions who developed CJD had clinical presentations similar to those of sporadic CJD patients and not to the more kuru-like iatrogenic cases arising from peripheral exposure to human prions. Furthermore, experimental transmission studies have shown only weak evidence for infectivity in blood, (45) even when inoculated via the most efficient (intracerebral) route. It cannot be assumed that the same picture will hold for vCJD as this is caused by a distinct prion strain (18) from those causing classical CJD and has a distinct pathogenesis. (46) It is also conceivable that many individuals could currently be incubating this disease.

New variant of human prion disease

In late 1995, two cases of sporadic CJD were reported in the United Kingdom in teenagers. (47,48) Only four cases of sporadic CJD had previously been recorded in teenagers, and none of these cases occurred in the United Kingdom. In addition, both cases were unusual in having kuru-type plaques, a finding seen in only around 5 per cent of CJD cases. Soon afterwards a third very young sporadic CJD case occurred. (49) These cases caused considerable concern and the possibility was raised that they might suggest a link with BSE. It was clearly of some importance to see if any further such extraordinarily rare cases occurred in the United Kingdom. By March 1996, further extremely young onset cases were apparent and review of the histology of these cases showed a remarkably consistent and unique pattern. These cases were named ‘new variant' CJD although it was clear that they were also rather atypical in their clinical presentation; in fact most cases did not meet the accepted clinical diagnostic criteria for probable CJD. Extensive studies of archival cases of CJD or other prion diseases failed to show this picture and it seemed that it did represent the arrival of a new form of prion disease in the United Kingdom.

The statistical probability of such cases occurring by chance was vanishingly small and ascertainment bias seemed most unlikely as an explanation. It was clear that a new risk factor for CJD had emerged and appeared to be specific to the United Kingdom. The United Kingdom government Spongiform Encephalopathy Advisory Committee concluded that, while there was no direct evidence for a link with BSE, exposure to specified bovine offal prior to the ban on its inclusion in human foodstuffs in 1989, was the most likely explanation. A case of vCJD was soon after reported in France. (50) Direct experimental evidence that vCJD is caused by BSE was provided by molecular analysis of human prion strains and transmission studies in transgenic and wild-type mice (see aetiology).

While it is now clear that vCJD is human BSE, it is unclear why this particular age group should be affected and why none of these cases had a pattern of unusual occupational or dietary exposure to BSE. However, very little is known of which foodstuffs contained high-titre bovine offal. It is possible that certain foods containing particularly high titres were eaten predominately by younger people. An alternative is that young people are more susceptible to BSE following dietary exposure or that they have shorter incubation periods. It is important to appreciate that BSE-contaminated feed was fed to sheep, pigs, and poultry and that although there is no evidence of natural transmission to these species, it would be prudent to remain open minded about other dietary exposure to novel animal prions.

The clinical presentation is with behavioural and psychiatric disturbances and in some cases with sensory disturbance. (51) Initial referral is usually to a psychiatrist and the most prominent feature is depression, but anxiety, withdrawal, and behavioural change is also frequent. (52) Suicidal ideation is infrequent and response to antidepressants poor. Delusions, which are complex and unsustained, are common. Other features include emotional lability, aggression, insomnia, and auditory and visual hallucinations. A prominent early feature in some was dysaesthesiae or pain in the limbs or face or pain which was persistent rather than intermittent and unrelated to anxiety levels. A minority of cases have been noted to have forgetfulness or mild gait ataxia from an early stage, but in most cases overt neurological features are not apparent until some months into the clinical course. (53) In most patients a progressive cerebellar syndrome develops with gait and limb ataxia.

Dementia usually developed later in the clinical course with progression to akinetic mutism in the majority of cases. Myoclonus was seen in most patients, in some cases preceded by chorea. Cortical blindness develops in a minority of patients in the late stages of disease. Upgaze paresis, an uncommon feature of classical CJD, has been noted in some patients. (53) The age at onset in the initial 14 cases reported ranged from 16 to 48 years (mean 29 years) and the clinical course was unusually prolonged (9–35 months, median 14 months). The EEG is abnormal, most frequently showing generalized slow-wave activity, but without the pseudoperiodic pattern seen in most sporadic CJD cases. Neuroimaging by CT is either normal or shows only mild atrophy. However, a high signal in the posterior thalamus on T2-weighted MRI is seen in a proportion of cases. (46,53) The sensitivity and specificity of this sign is unclear at present. Cerebrospinal fluid 14-3-3 protein may be elevated. PrP gene analysis showed that all cases available for study were homozygous for methionine at codon 129. No known or novel pathogenic mutations were found in the coding sequence. (54)

Recently it has become clear that vCJD can be diagnosed by detection of characteristic PrP immunostaining and PrPSc. (46,55) It has long been recognized that prion replication, in experimentally infected animals, is first detectable in the lymphoreticular system, considerably earlier than the onset of neurological symptoms. Importantly, PrPSc is only detectable in vCJD, and not other forms of human prion disease studied. The PrP Sc type detected on Western blot in vCJD tonsil has a characteristic pattern designated type 4t. (46) A positive tonsil biopsy obviates the need for brain biopsy which may otherwise be considered in such a clinical context to exclude alternative, potentially treatable diagnoses.

The neuropathological appearances of vCJD are striking and consistent. While there is widespread spongiform change, gliosis, and neuronal loss, most severe in the basal ganglia and thalamus, the most remarkable feature was the abundant PrP amyloid plaques in cerebral and cerebellar cortex. These consisted of kuru-like, ‘florid' (surrounded by spongiform vacuoles), and multicentric plaque types. The ‘florid' plaques, seen previously only in scrapie, were a particularly unusual but highly consistent feature. There was also abundant pericellular PrP deposition in the cerebral and cerebellar cortex. A further highly unusual feature was the extensive PrP deposition in the molecular layer of the cerebellum.

Some of the features of vCJD are reminiscent of kuru, in which behavioural changes and progressive ataxia predominate. In addition, peripheral sensory disturbances are well recognized in the kuru prodrome. Kuru plaques are seen in around 70 per cent of cases and are especially abundant in younger kuru cases. The observation that iatrogenic prion disease related to peripheral exposure to human prions has a more kuru-like than CJD-like clinical picture may well be relevant and would be consistent with a peripheral prion exposure.

The relatively stereotyped clinical presentation and neuropathology of vCJD contrasts sharply with sporadic CJD. This may be because vCJD is caused by a single prion strain and may also suggest that a relatively homogeneous genetically susceptible subgroup of the population with short incubation periods to BSE has been selected to date.

Inherited prion diseases

Gerstmann–Sträussler–Scheinker disease

The first case was described by Gerstmann in 1928 and was followed by a more detailed report on seven other affected members of the same family in 1936. (56) The classical presentation of GSS is with a chronic cerebellar ataxia accompanied by pyramidal features, with dementia occurring later in a much more prolonged clinical course than that seen in CJD. The mean duration is around 5 years, with onset usually in either the third or fourth decades. Histologically, the hallmark is the presence of multicentric amyloid plaques. Spongiform change, neuronal loss, astrocytosis, and white-matter loss are also usually present. Numerous GSS kindreds from several countries (including the original Austrian family described by Gerstmann, Sträussler, and Scheinker in 1936) have now been demonstrated to have mutations in the PrP gene. GSS is an autosomal dominant disorder which can now be classified within the spectrum of inherited prion disease.

Classification and clinical features of inherited prion diseases

The identification of one of the pathogenic PrP gene mutations in a case with neurodegenerative disease allows not only molecular diagnosis of an inherited prion disease but also its subclassification according to mutation. Pathogenic mutations reported to date in the human PrP gene consist of two groups:

  1. point mutations within the coding sequence resulting in amino acid substitutions in PrP or in one case production of a stop codon resulting in expression of a truncated PrP;
  2. insertions encoding additional integral copies of an octapeptide repeat present in a tandem array of five copies in the normal protein.

A suggested notation for these diseases is ‘inherited prion disease (PrP mutation)', for instance: inherited prion disease (PrP 144-bp insertion) or inherited prion disease (PrP P102L).

Missense mutations

PrP P102L

This mutation was first reported in 1989 in a British and North American family and has now been demonstrated in many other kindreds worldwide. Progressive ataxia is the dominant clinical feature, with dementia and pyramidal features. However, marked variability both at the clinical and neuropathological level is apparent in some families, and has recently been extensively documented in the original Austrian GSS family. (57) A family with marked amyotrophic features has also been reported. (58) Cases with severe dementia in the absence of prominent ataxia are also recognized. Histological examination reveals PrP immunoreactive plaques in the majority of cases. Transmissibility to experimental animals has been demonstrated.

PrP P105L

The Pro–Leu change at codon 105 has been found in four patients from three Japanese families. (59) It has not been reported outside Japan so far. The patients presented with a history of spastic paraparesis and dementia. The clinical duration from onset to the development of akinetic mutism was around 5 years. There was no periodic synchronous discharge on EEG, but MRI scans showed atrophy of the motor cortex. On pathological examination there were plaques in the cerebral cortex, and neuronal loss but no spongiosis. Neurofibrillary tangles were variably present amongst cases and no plaques were found in the cerebellum.

PrP A117V

This mutation was first described in a French family (60) and subsequently in a North American family of German origin. (61) The clinical features are presenile dementia associated with pyramidal signs, parkinsonism, pseudobulbar features, and cerebellar signs. Neuropathologically, PrP immunoreactive plaques are usually present. This mutation has also been identified in a large family in the United Kingdom. (62)

PrP Y145STOP

This mutation was detected in a Japanese patient who had a clinical diagnosis of Alzheimer's disease. She developed memory disturbance and a slowly progressive dementia at age 38. The duration of illness was 21 years. Histological examination revealed typical Alzheimer pathology without spongiform change. (63) Many amyloid plaques were seen in the cortex along with diffuse neuropil threads of paired helical filaments. However, the plaques were immunoreactive with PrP antisera. A4 immunocytochemistry was negative. The clinicopathological findings in this case emphasize the importance of PrP gene analysis in the differential diagnosis of dementias.

PrP D178N

This mutation was originally described in two Finnish families with a CJD-like phenotype (although without typical EEG appearances) (64) and has since been demonstrated in additional CJD families in Hungary, The Netherlands, Canada, Finland, France, and the United Kingdom. The Finnish pedigree included 15 affected members in four generations. The mean age of onset was 47 and mean duration was 27.5 months. Brain biopsy and autopsy specimens showed spongiform change without amyloid plaques.

This mutation was also reported in two unrelated families with fatal familial insomnia ( FFI). (65,66) The first cases described had a rapidly progressive disease characterized clinically by untreatable insomnia, dysautonomia, and motor signs, and neuropathologically by selective atrophy of the anterior–ventral and mediodorsal thalamic nuclei. There is marked thalamic astrocytosis. Mild spongiform change is seen in some cases and protease-resistant PrP can be demonstrated, albeit weakly, by immunoblotting. Proteinase-K treatment of extracted PrPSc from FFI cases has shown a different sized PrP band on Western blots than PrP Sc from CJD cases(67) suggesting that FFI may be caused by a distinct prion strain type. In a recent study, Goldfarb et al. (68) reported that in all the codon 178 families they studied with a CJD-like disease, the codon 178 mutation was encoded on a valine 129 allele while all FFI kindreds encode the same codon 178 mutation on a methionine 129 allele. They suggested that the genotype at codon 129 determines phenotype. However, they have not demonstrated that the families they describe are unrelated and that therefore their comparison may only be based on two extended families. Insomnia is not uncommon in CJD patients, and FFI and CJD may represent extremes of a spectrum of related disease phenotypes. Recently an inherited case with the E200K mutation, which is normally associated with a CJD-like phenotype, has been reported with an FFI phenotype. (69) An Australian family has also been reported with the FFI genotype but in which affected family members have a range of phenotypes encompassing typical CJD, FFI, and an autosomal dominant cerebellar ataxia-like illness. (70) It is of interest that the CJD-like codon 178 cases have frequently transmitted to experimental animals while the FFI type did not transmit to laboratory primates. (71) Recently, transmission of an FFI case to mice has been reported although this case was unusual in that a single octapeptide repeat deletion was present on the same allele. (72) This individual came from an extensive kindred in which other family members, with the same PRNP genotype, had a CJD-like phenotype. (73) However, two cases of FFI, one a British case and the second an Italian case, both with the usual FFI genotype of D178N, 129M, transmitted to transgenic mice expressing human prion protein. (74)

PrP V180I

This mutation was identified in two Japanese patients with subacute dementia and myoclonus. (59) The period from onset to akinetic mutism was 6 to 10 months. No family history was noted. EEG did not show pseudoperiodic sharp wave activity. Neuropathological examination demonstrated spongiform change, neuronal loss, and astrocytosis. Interestingly, one of the patients with PrP Ile 180 also had PrP Arg 232 (see later). These were on different alleles. This disease has not been transmitted to laboratory animals.

PrP T183A

This mutation was reported in a single Brazilian family with a frontotemporal dementia of mean onset 45 years and duration 4 years. (75) Parkinsonian features were also present in some patients. Neuropathological examination revealed severe spongiform change and neuronal loss in deep cortical layers and putamen while there was relatively little gliosis. PrP immunoreactivity was demonstrated in putamen and cerebellum. No transmission studies have been reported to date.

PrP F198S

A variant form of GSS was described in a large Indiana kindred which has been traced back to 1792. Unlike other GSS patients with presenile onset of neurological disability, the Indiana kindred had widespread Alzheimer-like neurofibrillary tangles composed of paired helical filaments in the cortex and subcortical nuclei in addition to amyloid plaques. The amyloid plaques were composed of PrP and not bA4. Affected individuals in this kindred have a codon 198 T-C transition resulting in a phenylalanine to serine conversion. (76) There is an apparent codon 129 effect with this mutation, in that individuals who were heterozygous at codon 129 had a later age of onset than homozygotes. Transmission of this disease to laboratory animals has not yet been reported.

PrP E200K

This mutation was first described in families with CJD. Affected individuals develop a rapidly progressive dementia with myoclonus and pyramidal, cerebellar, or extrapyramidal signs and a duration of illness usually less than 12 months. The average age of onset for the disease is 55. Histologically these patients are typical of CJD, plaques are absent but PrPSc can be demonstrated by immunoblotting. In marked contrast to other variants of inherited prion disease, the EEG usually shows the characteristic pseudoperiodic sharp-wave activity seen in sporadic CJD. Interestingly, this mutation accounts for the three reported ethnogeographic clusters of CJD where the local incidence of CJD is around 100-fold higher than elsewhere (amongst Libyan Jews and in regions of Slovakia and Chile). (77,78,,79 and 80) Now that cases can be diagnosed by PrP gene analysis, atypical forms of this condition are being detected with phenotypes other than that of classical CJD. Of interest also are reports that peripheral neuropathy can occur in this disease. (81) Elderly unaffected carriers of the mutation have been reported. Chapman et al. (82) have made a detailed analysis on 52 mutation-carrying patients with definite or probable CJD and 34 unaffected mutation carriers. They conclude that the cumulative penetrance reaches 50 per cent at the age of 60, and 80 per cent by the age of 80. However, there was a group of patients, of ages 69 to 82 with possible CJD containing five proven and two obligate carriers of the mutation. That is to say the patients were demented but did not fulfil the clinical criteria for probable CJD. If the analysis was carried out assuming that these possible cases were actually CJD then the penetrance reaches 100 per cent by the age of 80. Individuals homozygous for the mutation have been identified and are phenotypically indistinguishable from heterozygotes, indicating that this condition is a fully dominant disorder. (77) Patients with this condition have now been reported in several other countries outside the well-recognized clusters, including the United Kingdom. At least one of the British cases does not appear to related to the ethnogeographic clusters mentioned above, suggesting a separate United Kingdom focus for this type of inherited prion disease. (83) Goldfarb et al. have found this mutation amongst 46 out of 55 CJD affected families studied at the National Institutes of Health in the United States. (84) The codon 129 genotype does not appear to affect age at onset of this disorder. Transmission to experimental animals has been demonstrated.

PrP R208H

This was reported in a single patient with CJD and confirmed at autopsy. No details of the family history or phenotypic details have yet been published. (85)

PrP V210I

This mutation has been reported only in a single case in France (86) with a rapidly progressive dementia, cerebellar signs, and myoclonus, with age of onset of 63. EEG showed pseudoperiodic sharp-wave activity. The clinical duration was 4 months and neuropathological examination showed spongiform change, neuronal loss, and astrocytosis. No amyloid plaques were seen. Parents had died at ages of 60 and 66 without dementia. A sister with the mutation had died of colon cancer at age 67. It is possible that this mutation produces a very late onset disease or is incompletely penetrant. Transmission to experimental animals has not been reported.

PrP Q217R

Reported to date only in a single Swedish family, the presentation is with dementia followed by gait ataxia, dysphagia, and confusion. (16) As with the inherited prion disease with codon 198 serine mutation (Indiana kindred) there are prominent neurofibrillary tangles. Transmissibility to experimental animals has not yet been demonstrated in this condition.

PrP M232R

This mutation was first found on the opposite allele to a codon 180 mutation in a Japanese patient with prion disease. (59) It was further demonstrated in two additional Japanese patients with dementia. Both of the latter cases appeared to present as sporadic cases with no family history of neurological disease. Both patients had progressive dementia, myoclonus, and periodic synchronous discharges in the EEG. The mean duration of illness was 3 months. Neuropathology showed spongiform change, neuronal loss, and astrocytosis. PrP immunostaining revealed diffuse grey-matter staining, but no plaques.

Insertional mutations

PrP 24-bp insertion (one extra repeat)

A single octapeptide repeat insertion has been reported in a single French individual who presented at age 73 with dizziness. He later developed visual agnosia, cerebellar ataxia, and intellectual impairment, and diffuse periodic activity was noted on EEG. Myoclonus and cortical blindness developed and he progressed to akinetic mutism. Disease duration was 4 months. The patient's father had died at age 70 from an undiagnosed neurological disorder.

No neuropathological information is available.

PrP 48-bp insertion (four extra repeats)

This mutation has been reported in a single family from the United States. (87) The proband had a CJD-like phenotype both clinically and pathologically with a typical EEG and an age at onset of 58. However, the proband's mother had onset of cognitive decline at age 75 with a slow progression to a severe dementia over 13 years. The maternal grandfather had a similar late onset (at age 80) and slowly progressive cognitive decline over 15 years.

PrP 96-bp insertion (four extra repeats)

A 96-bp insertional mutation, encoding four octapeptide elements, was first reported in an individual who died aged 63 of hepatic cirrhosis.(88) There was no history of neurological illness and it is unclear if this finding indicates incomplete penetrance of this mutation. This is the only recorded case of a PRNP insertional mutation other than in an affected individual with a prion disease or an at-risk individual from an affected kindred. Two separate four-octapeptide repeat insertional mutations have been reported in affected individuals, each differing in the DNA sequence from the original four-repeat insertion, although all three of the mutations encode the same PrP. Laplanche et al. (89) reported a 96-bp insertion in an 82-year-old French woman who developed progressive depression and behavioural changes. She progressed over 3 months to akinetic mutism with pyramidal signs and myoclonus. EEG showed pseudoperiodic complexes. Duration of illness was 4 months. There was no known family history of neurological illness. Another 96-bp insertional mutation was seen in a patient with classical clinical and pathological features of CJD with the exception of the unusual finding of pronounced PrP immunoreactivity in the molecular layer of the cerebellum. (90)

PrP 120-bp insertion (five extra repeats)

A five additional octapeptide repeat mutation was reported in a North American family with an illness characterized by progressive dementia, abnormal behaviour, cerebellar signs, tremor, rigidity, hyper-reflexia, and myoclonus. The age at onset was 31 to 45, with a clinical duration of 5 to 15 years.(88) EEG showed diffuse slowing only. Histological features were of spongiosis, neuronal loss, and gliosis. Transmission has been demonstrated.

PrP 144-bp insertion (six extra repeats)

This was the first PrP mutation to be reported and was found in a small British family with familial CJD. (10) The diagnosis in the family had been based on an individual who died in the 1940s with a rapidly progressive illness characteristic of CJD. (91) The reported duration of illness was 6 months. Pathologically there was gross status spongiosis and astrocytosis affecting the entire cerebral cortex, and this case is used to illustrate classic CJD histology in Greenfield's Neuropathology. However, other family members had a much longer duration of GSS-like illness. Histological features were also extremely variable. This observation led to screening of various cases of neurodegenerative disease and to the identification of a case classified on clinical grounds as familial Alzheimer's disease. (92) More extensive screening work identified further families with the same mutation which were then demonstrated by genealogical studies to form part of an extremely large kindred. (29,93) Clinical information has been collected on around 50 affected individuals over seven generations. Affected individuals develop in the third to fourth decade onset of a progressive dementia associated with a varying combination of cerebellar ataxia and dysarthria, pyramidal signs, myoclonus, and occasionally extrapyramidal signs, chorea, and seizures. The dementia is often preceded by depression and aggressive behaviour. A number of cases have a long-standing personality disorder, characterized by aggression, irritability, antisocial and criminal activity, and hypersexuality which may be present from early childhood, long before overt neurodegenerative disease develops. The histological features vary from those of classical spongiform encephalopathy (with or without PrP amyloid plaques) to cases lacking any specific features of these conditions. (30) Age at onset in this condition can be predicted according to genotype at polymorphic codon 129. Since this pathogenic insertional mutation occurs on a methionine 129 PrP allele, there are two possible codon 129 genotypes for affected individuals, methionine 129 homozygotes or methionine 129/valine 129 heterozygotes. Heterozygotes have an age at onset which is about a decade later than homozygotes. (93) Limited transmission studies to marmosets were unsuccessful. Transmission to transgenic mice expressing human prion protein has been achieved (unpublished data). Further families with 144-bp insertions, of different nucleotide sequence, have now been reported in the United Kingdom (94) and Japan. (95)

PrP 168-bp insertion (seven extra repeats)

This mutation has been reported in a North American family. The clinical features described include mood change, abnormal behaviour, confusion, aphasia, cerebellar signs, involuntary movements, rigidity, dementia, and myoclonus. The age at onset was 23 to 35 years and the clinical duration 10 to over 13 years. EEG showed diffuse slowing in two cases; a third showed slow-wave burst suppression. Neuropathological examination showed spongiform change, neuronal loss, and gliosis to varying degrees. (88) Experimental transmission has been demonstrated.

PrP 192-bp insertion (eight extra repeats)

This mutation has been reported in a French family with clinical features which include abnormal behaviour, cerebellar signs, mutism, pyramidal signs, myoclonus, tremor, intellectual slowing, and seizures. The disease duration ranged from 3 months to 13 years. The EEG findings include diffuse slowing, slow-wave burst suppression, and periodic triphasic complexes. Neuropathological examination revealed spongiform change, neuronal loss, gliosis, and multicentric plaques in the cerebellum. (88,96) Experimental transmission has been reported.

PrP 216-bp insertion (nine extra repeats)

The finding of a nine-octapeptide insertional mutation was first reported in a single case from the United Kingdom. (97) The clinical onset was around 54 years with falls, axial rigidity, myoclonic jerks, and progressive dementia. (98) Although there was no clear family history of a similar illness, the mother had died at age 53 with a cerebrovascular event. The maternal grandmother died at age 79 with senile dementia. EEG was of low amplitude but did not show pseudoperiodic sharp-wave activity. Neuropathological examination showed no spongiform encephalopathy but marked deposition of plaques, which in the cerebellum and the basal ganglia showed immunoreactivity with PrP antisera. (98) In the hippocampus there were neuritic plaques positive for both b-amyloid protein and tau. Some neurofibrillary tangles were also seen. In some respects therefore the pathology resembled Alzheimer's disease. Experimental transmission studies have not been attempted. A second (German) family with a nine-octapeptide repeat insertion of different sequence has now been reported. (99)

Presymptomatic and antenatal testing

Since a direct gene test has become available it has been possible to provide an unequivocal diagnosis in patients with inherited forms of the disease. This has also led to the possibility of performing presymptomatic testing of unaffected but at-risk family members, as well as antenatal testing. (100) Because of the effect of codon 129 genotype on the age of onset of disease associated with some mutations it is possible to determine within a family whether a carrier of a mutation will have an early or late onset of disease. Most of the mutations appear to be fully penetrant; however, experience with some is extremely limited. In families with the E200K mutation there are examples of elderly unaffected gene carriers who appear to have escaped the disease.

Genetic counselling in prion disease resembles that of Huntington's disease in many respects and those protocols established for Huntington's disease can be adapted for prion disease counselling. PrP gene analysis may have very important consequences for family members other than the individual tested, and it is preferable to have discussed all the issues with the whole family before testing commences. Following the identification of a mutation the family should be referred for genetic counselling. Testing of asymptomatic individuals should only follow adequate counselling of individuals and will require their full informed consent.

Prognosis and treatment

All forms of prion diseases that are currently recognized are invariably fatal and follow a relentlessly progressive course. No currently available treatment alters the clinical course of the disease and all that can be offered at present is general supportive care for the patient and family with hospitalization in the later stages. The duration of illness in sporadic patients is very short with a mean of 3 to 4 months. However, in some of the inherited cases the duration can be 20 years or more. (30) The prion diseases are now perhaps the best understood of the degenerative brain diseases and the development of rational treatments is appearing realistic. In particular, the development of drugs which selectively bind PrP Sc or which bind to PrPC to inhibit its conversion might be able to block prion propagation. Improved early diagnosis will be crucial.

Secondary prophylaxis after accidental exposure

Certain occupational groups are at risk of exposure to human prions, for instance neurosurgeons and other operating theatre staff, pathologists and morticians, histology technicians, and increasing number of laboratory workers. Because of the prolonged incubation periods to prions following administration to sites other than the central nervous system, which is associated with clinically silent prion replication in the lymphoreticular tissue, (101) treatments inhibiting prion replication in lymphoid organs may represent a viable strategy for rational secondary prophylaxis after accidental exposure. A preliminary suggested regimen is a short course of immunosuppression with oral corticosteroids in individuals with significant accidental exposure to human prions. (102) Urgent surgical excision of the inoculum might also be considered in exceptional circumstances. There is hope that progress in the understanding of the peripheral pathogenesis will identify the precise cell types and molecules involved in colonization of the organism by prions. The ultimate goal will be to target the rate-limiting steps in prion spread with much more focused pharmacological approaches, which may eventually prove useful in preventing disease even after iatrogenic and alimentary exposure. (103)

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