Spongiform Encephalopathies

Dr. Cathal Collins14/02/05

Introduction Prion disease Neurodegenerative Long incubation period Progresses inexorably 5 human prion diseases:

Kuru Creutzfeldt-Jokob disease (CJD) New variant CJD Gerstmann-Straussler-Scheinker syndrome Fatal familial insomnia

Neuropathology Neuronal loss Proliferation of glial cells Absence of an inflammatory

response Presence of small vacuoles which

produces a spongiform appearance

Animal prion diseases Scrapie: sheep, goats; first described

1738 Transmissible Mink Encephalopathy:

minks; 1964 Chronic Wasting Disease: mule, deer,

elk; 1980 Bovine Spongiform Encephalopathy:

cattle; 1986 Feline Spongiform Encephalopathy:

cats; 1990

BSE History in UK 1985: first cases of BSE 1988: June BSE made notifiable

July Ruminant feed bannedAugust Compulsory slaughterand destruction of suspect cattle

1989February Southwood reportNovember Ban of sale of bovine offal for human consumption

BSE History in UK 1990November Bovine offal

feed ban for all animals and birds

1992: Peak incidence of BSE 1996: First cases of nvCJD

Further restriction of bovine products from food

and food chains

Biology of Prions Dr. Stanley Prusiner: coined the term “prion” in

1982: proteinacious infectious particle Small infectious pathogen containing protein

and lacking nucleic acid One characteristic feature is their resistance to

a number of normal decontaminating procedures

Resistant to: Aldehydes e.g formaldehydes Nucleases Heat (80C) UV and ionising radiation Non-ionic detergents

Biology of Prions Inacticated by:

Prolonged autoclaving (at 121C and 15psi for 4.5h) Immersion in 1M NaOH (for 30 min, repeat 3 times) Immersion in strong organic solvents

Inadequate autoclaving can establish heat resistant subpopulations

Stainless steel instruments may also retain infectivity even after treatment with 10% formaldehyde

Prion Protein (PrP) The gene (PRPN) is located on the short arm

of chromosome 20 Encodes a protein (PrPc) found in normal

brain (alpha helical); normal function unknown

PrPsc (PrP in scrapie infected anmals) is a conformational isomer of PrPc (B-pleated sheet)

PrPsc is resistant to digestion with proteases and has a tendency to polymerise into scrapie-associated fibrils or prion rods

Biosynthesis of PrPc Key step is modification of amino and

carboxy terminals with the addition of a phosphatidylinositol glycolipid which serves to anchor the protein to the cell surface

PrPc is found attached to plasma membranes of neurons and may be concentrated at synaptic membranes

PrPc has transmembranous domains Degraded after endocytosis in acidic

vesicles

PrPsc Accumulates within cells; does not normally

appear on the cell surface Found predominantly in cytoplasmic

vacuoles and secondary lysosomes Studies with mice either devoid of PrPc or

with abnormal isoforms indicate that host PrPc must be present for the development of prion disease

Prion diseases result from accumulation of abnormal isoforms of the PrP which is dependent upon conversion of PrPc to PrPsc

PrPsc How the first molecule of PrPsc appears

in the host remains a mystery Exogenous source in sporadic and

iatrogenic CJD Mutation in PRNP gene in familial forms The initial appearance (?de novo)

probably triggers the replication of PrPsc; PrPc acts as a nidus for the formation of PrPsc on challenge

PrPsc Prior to transport to the nervous system,

follicular dendritic cells within germinal centres of lymhoid tissue appear to act as a reservoir for the protein

Two reports suggest that complement plays a role in early pathogenesis ( C3, C1q, Bf/C2, or complement receptors)

Transport of PrPsc to the nervous system occurs via axons

Neurotoxicity of prion protein PrPsc causes apoptosis and cell

death Misfolded PrP is transported to the

cytosol for degradation- even small amounts of this protein in the cytosol are highly neurotoxic

Accumulation of this protein may be an important step in prion disease pathogenesis

Genetics PRNP encodes PrP and is located on the

short arm of chromosome 20 A strong link has been established between

mutations in the PRNP gene and forms of prion disease with a familial predisposition (fCJD, GSS, FFI)

More than 50 different mutations have been identified

A single mutation may produce different clinical phenotypes in different individuals or families

Genetics The phenotype of a particular mutation may

be influenced by the nature of the amino acids present at codon 129

Normal individuals have either valine or methionine at this site

PRNP is an autosomal dominant gene; can be homozygous or heterozygous

D178N mutation (asparagine for aspartic acid in codon 178): Homozygous for valine at codon 129 appear to develop CJD; those homozygous for methionine tend to have FFI

Genetics: Codon 129 Molecular classification scheme for

sporadic CJD based upon codon 129 polymorphism and characterisation of the properties of PrPsc

A pattern of type 1 PrPsc plus at least 1 methionine at codon 129 was demonstrated in 70%

Type 2 PrPsc plus codon 129 homozygous or heterozygous for valine was present in 25% and associated with ataxia

Genetics: CJD Familial CJD: most common mutation is a

substitution of lysine for glutamine in codon 200; phenotype may depend on codon 129

Sporadic CJD and iatrogenic CJD are not associated with PRNP gene mutations; however in these forms and nvCJD, phenotyping at codon 129 appears to affect susceptibility and perhaps expression of the clinical illness

Genetics: GSS and FFI GSS

- P102L mutation is the most common PRNP- large degree of phenotypic

heterogeneity- polymorphism at codon 129 may play

a modulating role FFI

-D178N mutation predominant in those homozygous for methionine at codon

129

Kuru First transmissible

neurodegenerative disease to be identified and well studied

Has served as the prototype of human prion diseases

Kuru- Epidemiology Was endemic in Papua New Guinea among

the Fore tribes Felt to be transmitted from person to person

by ritual cannibalism Still remains uncertain as to whether the

brain was actually eaten, but it was handled after death, particularly by females and children

No cases observed since these practices discontinued

Primary cause of death in the tribes in 1960s

Kuru- Clinical Features Ambulatory phase: tremors (kuru=shivering),

ataxia and postural instability Sedentary stage: loss of ambulation resulting

from increased tremors and ataxia; involuntary movements

Late stage: dementia, indifference Terminal stage: frontal release signs,

cerebellar type dysarthria and inability to get out of bed

Death typically due to pneumonia occurring within 9-24 months form onset of disease

Kuru- Diagnosis&Pathology CSF unremarkable, EEG not

characteristic Pathological hallmark is PrPsc-

reactive plaques occurring with the greatest frequency in the cerebellum; neuronal loss and hypertrophy of astrocytes is also observed

Creutzfeld-Jakob Disease Most frequent of the human prion

diseases Still very rare Sporadic (sCJD), familial (fCJD),

iatrogenic (iCJD) and new variant (nvCJD)

CJD- Epidemiology Approx 1 case per 1 million population/year World-wide distribution Mean age of onset is 57-62 Patients with nvCJD and iCJD tend to be much

younger No gender predilection Incidence increased 30-100 fold in certain

areas of North Africa, Israel and Slovakia, due primarily to clusters of fCJD

Vast majoriy sporadic (85-95%), 5-15% fCJD, <5% iCJD

Iatrogenic CJD Following administration of cadaveric

human pituitary hormones, dural graft transplants, use of dural mater in radiographic embolisation procedures, corneal transplants, liver transplants, and the use of contaminated neurosurgical instruments or stereotactic depth electrodes

Hx of preceding infusion does not increase risk of developing CJD (epidemiology studies); however low levels of infectivity in in vitro studies

CJD- Clinical Features Rapidly progressive mental deterioration and

myoclonus are two cardinal manifestations of sCJD

Number of variants or subtypes of disease based based upon area of involvement of the brain

Visual Cerebellar Thalamic Striatal

Variants of sCJD also classified based on genotype of PRNP and the molecular properties of the pathological PrPsc

CJD- Clinical Features Mental deterioration may manifest as

dementia, behavioural abnormalities and deficits involving higher cortical function

Concentration, memory and judgement difficulties are frequent early signs

Mood changes such as apathy and depression are common

Dementia becomes dominant and can advance rapidly

Death usually occurs within one year

CJD- Clinical Features Myoclonus, especially provoked by startle, is

present in more than 90% sCJD should always be considered in a patient

with a combination of a rapidly progressive dementia and myoclonus

Extrapyramidal signs such as hypokinesia and cerebellar manifestations including nystagmus and ataxia occur in 2/3

Corticospinal tract involvement in 40-80% Sensory signs and symptoms are common in

nvCJD; otherwise extremely atypical

Subtypes of sCJD Clinical phenotypes of sCJD associated with

molecular subtypes determined by the PRNP gene codon 129 genotype and the pathologic prion protein (PrPsc) type

PRNP genotype homozygous or heterozygous for methionine (M) or valine (V) at codon 129

The PrPsc type is determined by Western blot analysis amd classified in the Parchi/Gambetti nomenclature as type 1 or type 2

Alternate Collinge nomenclature distinguishes 4 PrPsc subtypes: types 1 and 2 correspond with P&G PrPsc 1; 3 and 4 with P&G PrPsc 2

Subtypes of sCJD 6 clinical phenotypes of sCJD MM1 and MV1 (myoclonic, Heidenhain

variant) account for about 70% of cases and correlate with the “classic CJD” phenotype

VV2 (ataxic variant) accounts for 16% MV2 (Kuru plaque variant): 9% MM2 thalamic, MM2 cortical and VV1

account for the rest

CJD- Diagnosis Clinical and laboratory features generally are

sufficient for a ‘probable’ diagnosis of sCJD WHO criteria for ‘probable’ diagnosis:

Progressive dementia >/=2 of 4 of: myoclonus; visual or cerebellar

disturbance; pyramidal/extrapyramidal dysfunction; akinetic mutism

A typical EEG during an illness of any duration and/or positive 14-3-3 CSF assay with a clinical duration to death in less than 2 years

Routine investigations should not suggest an alternative diagnosis

CJD-Diagnosis A definitive diagnosis requires these

features in combination with one or more of the following:

Loss of neurons, gliosis, spongiform degeneration, or plaqes postive for PrPsc on histopathlogy of brain tissue

Positive PrPsc staining following pretreatment of brain tissue to destroy PrPc reactivity

Positive histoblotting of brain tissue extracts for PrPsc after treatment to destroy PrPc reactivity

Transmission of characteristc neurodegenerative disease to experimental animals

Demonstration of PRNP gene mutations

CJD- Diagnosis Neuroimaging: diffusion weighted MRI- can

detect abnormalities as early as 3 weeks of symptom duration, CT generally normal

EEG: aids diagnosis: characteristic pattern of periodic synchronous bi or triphasic sharp wave complexes (PSWCs)

Protein markers: 14-3-3 protein in CSF- especially in those with classical subtypes of sCJD

Pathological studies of brain material to detect protease resistant PrPsc remains gold standard

New Variant CJD Initial reports of nvCJD

in 1996 quickly focused intense interest on the human prion diseases

Linked with bovine spongiform encephalopathy

Unique epidemiological features of this illness led to early recognition that this was indeed a ”new variant”

nvCJD- Epidemiology First report of a case of nvCJD in a 16 year

old from the UK appeared in 1995 was quickly followed in 1996 by 22 other cases

All bar one of the initial cases were from the UK

January 2004- 155 cases world-wide 145 from UK, 6 from France, 1 each from

Ireland, Italy, Canada and the US All except the italian had resided in

countries with known BSE

nvCJD V sCJD nvCJD is distinguished from sCJD by

A considerably younger age of onset (mean age of onset 29 versus 65)

Less rapid progression of disease (duration 14 months versus 4-5)

Differences in clinical presentation (sensory and psychiatric symptoms prominent in nvCJD)

Type 2 (P&G)/ type 4 (Collinge) PrPsc in nvCJD Differences in neuropathology

nvCJD- Clinical features 63/100 of the 1st cases presented initially

with psychiatric symptoms Neurological symptoms preceded psychiatric

in 15 Both were present in 22 Psychiatric symptoms include depression,

apathy, anxiety, psychosis and intermittent delusions

Sensory abnoramalities include dysaesthesias and paraesthesia of the face, hands, feet, legs or even hemibody

nvCJD- Clinical features Once neurological symptoms, typically

ataxia, becomes evident, progression is more rapid

Cognitive impairment, involuntary movements, immobility, unresponsiveness, and mutism are common signs as the disease progresses

Paresis of upward gaze may be present (uncommon in other forms of CJD)

nvCJD- Diagnosis CSF studies rarely helpful: 14-3-3 is not a

sensitive marker Combination of 14-3-3 and tau protein in

CSF may be useful MRI better than CT: MRI may show signal

hyperintensity in the pulvinar (pulvinar sign) or in both pulvinar and dorsomedial thalamus (hockey stick sign)

EEG: abnormal in 70% but only slow wave pattern

nvCJD- Diagnosis PRNP gene mutations are not present in

nvCJD, but all patients with clinically expressed nvCJD have been homozygous for methionine at codon 129

Type 2 PrPsc (P&G nomenclature) or type 4 PrPsc in the Collinge nomenclature has been found in patients with nvCJD (not characteristic of other human prion diseases)

nvCJD- Neuropathology A number of neuropathological feature

distinguish nvCJD from sCJD: Presence of plaques, which stain intensely for

PrPsc, distributed throughout the cerebrum and cerebellum and to a lesser extent the basal ganglia and thalamus

The plaques have an eosinophilic centre and pale periphery with surrounding spongiform changes

Cases of kuru and GSS have similar but not identical plaques

The cerebellum is characteristically involved in nvCJD

nvCJD has distinct type 4 PrPsc

nvCJD- Links 2 cases of possible transfusion

transmisssion of nvCJD have been reported

There is increasing evidence supporting the possibility that nvCJD represents bovine-to-human transmission of BSE

nvCJD and BSE The appearance of nvCJD followed an

epidemic of BSE in the UK The removal of organic solvents, which

inactivate PrPsc, from the rendering process for bovine offal and the subsequent use of the offal as a component of feed for cattle has been suggested as a mechanism for amplifying the epidemic in animals

Approx 50,000 infected cattle are estimated to have entered the food chain

nvCJD and BSE The prohibition of ruminant-derived

proteins in feeds for all animals and poultry in november 1990 and the banning of consumption of animals over the age of 30 months in March 1996 has led to a dramatic decline in cases of BSE

Evidence in the favour of the association between nvCJD and BSE includes the type 4 pattern of PrPsc which has not been seen in other prion diseases

nvCJD and BSE Despite the apparent link between nvCJD

and BSE, the number of cases of nvCJD has remained small. Possible reasons include: Low levels of PrPsc in milk and meat Inefficiency of oral route of infection Restriction of spread based upon a species

barrier Low incidence of host genetic factors such as

the frequency of homozygosity at codon 129

Gerstmann-Straussler-Scheinker Syndrome A rare human prion disease 1-10 cases per 100 million population/year Autosomal dominant pattern with virtual

complete penetrance Hallmark: progressive cerebellar

degeneration accompanied by different degrees of dementia in patients entering mid-life (mean 43-48)

Course of illness advances for 5 years before culminating in death

Myoclonus is typically absent

GSS Phenotypic variability due to differences in

underlying PRNP mutation or in the polymorphism in codon 129

Lab or imaging studies not useful Demonstration of PRNP mutation useful means

for diagnosis Neuropathological features consistent with

other forms of prion disease: however kuru-like plaques especially in the cerebellum are common findings; also neurofibrillary tangles and neuropil threads identical to those seen in Alzheimers

Fatal Familial Insomnia Rapidly fatal with a mean duration of 13

months; midlife (35-61 years) Develop progressive insomnia with loss of

normal circadian sleep-activity pattern Impaired concentration and memory,

confusion, inattention and behavioural changes occur but overt dementia is rare

Myoclonus, ataxia and spasticity occur with disease progression

FFI Only prion disease to produce

dysautonomia (increased T, BP, HR) and endocrine disturbances (ACTH, cortisol, GH, PRL)

Genetic studies are diagnostic procedure of choice

Most cases associated with D178N PRNP mutation

Spongiform degeneration rarely detected; neuronal loss and gliosis is maximal within the thalamus

Treatment of Prion Diseases

No effective treatment has been identified for human prion diseases which are universally fatal; supportive treatment mainstay

Flupirtine maleate is a centrally acting, non-opioid analgesic that has displayed cytoprotective activity in vitro in neurons treated with a prion protein fragment: better MMSe but survival not enhanced

Chlorpromazine and quinacrine: inhibit PrPsc formation in vitro: studies needed

Potential targets will include the steps in the conversion of PrPc to PrPsc

Summary Prions: do not have nucleic acid Conversion of PrPc to PrPsc Codon 129 in PRNP gene Neurodegenerative diseases Long incubation period Progress inexorably; no treatment 5 types of human prion disease nvCJD and links to BSE