HNS53, HNS54 Molecular Basis Of Neurological Diseases I and II

Question 1

Neurological diseases

Answer 1

1. ***Huntington’s disease
2. Infection (HIV, meningitis, prion etc.)
3. Injury
4. Autoimmune (Myasthenia gravis)
5. Inflammation (Multiple sclerosis)
6. ***Motor neuron disorders (Parkinson’s disease)
7. Synaptic transmission
8. ***Neurodegenerative (Alzheimer’s disease)

Question 2

Huntington’s disease

Answer 2

Autosomal dominant

• every generation
• both male and female affected
• highly penetrant

Typically begin in mid-life (30-45)

NO current treatment to prevent / delay progression of HD

Question 3

Clinical features of HD

Answer 3

1. Motor
   - Chorea
   - Gait abnormalities
   - In-coordination
   - Dysarthria
   - Eye movement abnormalities
   - Rigidity / Bradykinesia / Dystonia (esp. in young onset cases)
2. Cognitive
   - Subcortical structures
   - Frontal type problems (impaired executive functions): problem solving, planning etc.
   - Mental slowing
   - Poor attention / memory difficulties
   - Language generally well-preserved
   - ALL patients will develop global dementia with time
   - Need to check not depressed esp. they can get worse quickly
3. Psychiatric
   - Personality change
   - Apathy, irritability, disinhibition, emotionally labile
   - Mood swings
   - Social withdrawal
   - Poor self care and alcoholism
   - Criminal record due to mental problem
4. Other features (fits in young onset cases, sleep disturbance, weight loss)

Question 4

Clinical stages of HD

Answer 4

1. Early
   - progressive emotional, psychiatric, cognitive disturbances
2. Later
   - motor signs
   - progressive dementia
3. Advanced
   - unable to walk
   - poor dietary intake
   - long term institutional care
4. Swallowing difficulties —> Aspiration pneumonia

Question 5

Neuropathology of HD

Answer 5

• Brain specific
• ***Striatal atrophy
• Prominent neuronal loss (Caudate nucleus + Putamen)
• Selective for **medium spiny neurons (Special type of **GABAergic inhibitory neurons)

Question 6

Etiology of HD: Mapping HD gene

Answer 6

• Use genetic linkage
• Abnormal gene contain multiple **CAG repeats —> multiple **glutamine (abnormal Huntington (htt) protein)
• > 36 CAG repeats —> HD (>39 definitely pathogenic)
• larger number of repeats —> develop symptoms at earlier age

Huntington gene:

• > 200,000 base pairs, 67 exons
• 348kDa protein (3144 a.a.)
• Polyglutamine sequence (CAG repeats)
• ***Ubiquitous expression (highest levels in CNS neurons)
• ***Widespread subcellular localisation (clathrin-coated vesicles, endosomal compartment, microtubule)

Question 7

Key cellular pathogenic mechanisms in HD

Answer 7

Multiple glutamine repeats (每一個獨立mechanism)

1. Conformational change of protein —> unfolding / ***abnormal folding
2. ***Accumulation in cytoplasm
3. Impairment of calcium signaling in ER + homeostasis
4. ***Mitochondrial dysfunction
5. Translocate to nucleus (***impair BDNF gene transcription, form intranuclear inclusions)
6. Alters ***vesicular transport and recycling

—> ALL eventually lead to Prominent loss of medium spiny neurons (Special type of ***GABAergic inhibitory neurons)
—> Striatal atrophy

Question 8

Gene therapy of HD

Answer 8

AMT-130:

• consist of AAV5 vector
• carrying an **artificial micro-RNA —> tailored to **silence mutant Huntington gene

Question 9

Parkinson’s disease

Answer 9

• 2nd most common neurodegenerative disorder
• ***Majority sporadic, 10-20% family history
• Monogenic rare
• ***Male > Female
• Autosomal Dominant / Recessive (both occur)

Question 10

Clinical features of PD

Answer 10

四寶: Resting tremor, Muscle rigidity, Bradykinesia, Abnormal gait/posture

1. Tremor-at-rest
2. Bradykinesia, Hypokinesia, Akinesia
3. Rigidity
4. Flexed posture of neck, trunk, limbs
5. Loss of postural reflexes
6. Freezing phenomenon

Question 11

***Pathological features

Answer 11

1. Loss of monoamine neurons in Substantia nigra pars compacta + Locus ceruleus
   - Presence of Lewy bodies (inclusion bodies within neuron containing α-Synuclein etc.) + ↑ Glial cells
2. Depigmentation
   - loss of ***Neuromelanin (derived from oxidised dopamine) in Substantia nigra pars compacta + Locus ceruleus

Overall:
Dopamine depletion in Substantia nigra + Nigrostriatal pathway to Caudate + Putamen
—> ↑ inhibition of Thalamus + ↓ excitatory input to Motor cortex
—> Bradykinesia and other parkinsonism signs

Question 12

Lewy bodies

Answer 12

• Intracellular eosinophilic masses made up of ***α-Synuclein and other proteins
• ***Pathologic hallmark of PD
• Not specific to PD, also found in other neurodegenerative diseases and normal elderly people

Question 13

***Gene localisations identified for PD

Answer 13

1. PARK1 gene —> Autosomal ***dominant inheritance —> α-Synuclein mutations and aggregation into Fibrillar forms —> Lewy bodies
2. PARK2 gene —> Autosomal ***recessive inheritance —> inactivation of Ubiquitin ligase (Parkin) —> accumulation of Parkin substrates (targeted protein) in neurons —> Selective toxicity to Dopamine neurons

Question 14

Dynamic aggregation processes of α-Synuclein

Answer 14

• α-Synuclein is unfolded in solution but form helix-like structures in association with membrane
• normally exist in equilibrium between membrane and cytosol
• Pathological conditions (Autosomal Dominant mutation of α-Synuclein gene):
  α-Synuclein **aggregates via small oligomeric intermediates
  —> larger **fibrillar forms (β-sheet)
  —(+ Ubiquitin)—> ***Lewy bodies

Question 15

Parkin protein

Answer 15

• E3 Ubiquitin ligase
• 2 domains: RING-box domain + Ubiquitin domain
• 465 a.a.
• Autosomal Recessive juvenile PD + Early onset Recessive PD

1. Ubiquitin-Proteasome pathway:
   - E1 transfer Ub to E2
   —> E2-Ub attach to RING-box
   —> transfer Ub to targeted protein
   —> attachment of Polyubiquitin chains to targeted proteins (將Ubiquitin (死亡signal) attach落targeted protein度)
   —> degradation
   - ***Inactivation of E3 Ubiquitin ligase (Parkin) —> accumulation of Parkin substrates (targeted protein) in neurons —> Selective toxicity to Dopamine neurons
2. Protective role in mitochondria
3. Dopaminergic neurons may also be more vulnerable because of ***intrinsic exposure to oxidative stress —> protein damage and misfolding

Question 16

PD summary

Answer 16

• Neurodegenerative disease with movement disorder + neuropsychiatric clinical features
• Majority sporadic, some genetic forms
• Progressive, chronic, incurable
• ***Pathology:
  1. Neuronal loss in Substantia nigra pars compacta + Locus coeruleus
  2. Depigmentation (loss of Neuromelanin)

Example of gene mutation:

• **α-Synuclein accumulation —> **Lewy body
• ***Inactivation of Parkin —> E3 Ubiquitin ligase —> Proteins targeted for degradation

Question 17

Alzheimer’s disease

Answer 17

• ***Age-dependent neurodegenerative disorder that results in progressive loss of cognitive function
• Memory loss, unusual behaviour, personality changes, decline in thinking abilities
• Burden of AD: emotional, physical, financial stress on family members
• most common cause of dementia among elderly in HK

Risk factors:

• Age
• Family history dementia esp. early onset
• Down’s syndrome
• Severe head injury
• Genetic factors
• Lifestyles
• Vascular risk factors?

Question 18

Classification of AD

Answer 18

1. By age of onset
   - Early onset: <= 65
   - Late onset: >65
2. By family history
   - ***Sporadic case ~90%
   - Familial case ~10%

Question 19

Cognitive features of AD

Answer 19

1. ***Memory
   - impaired anterograde episodic memory
   - delayed recall of stories
   - Word List learning
   - impaired association (visual memory and new learning)
   - working memory spared
2. Attentional + ***Executive deficits
   - poor concentration
   - problem with complex tasks
   - selective attention
   - executive functions
   - Wisconsin Card Sorting test —> test working memory
3. ***Language and knowledge
   - impaired semantic memory
   - category fluency
   - word definitions
   - word finding esp. proper names
   - later phonological and syntactic deficits
4. ***Visuospatial and Perceptual inabilities
   - impaired drawing esp. 3D
   - apraxia mainly conceptual

Question 20

Neuropsychiatric features of AD

Answer 20

1. Apathy
2. Depression
3. Anxiety
4. Delusion (rare in early stage)
5. Paranoia
6. Phantom lodger (inability to recognize one’s own reflected image)
7. Agitation and disruptive behaviour common later

Question 21

Diagnostic criteria of AD

Answer 21

According to NINCDS-ADRDA alzheimer’s criteria:
“Probable AD”:
1. Deficits in >=2 areas of cognition
2. MMSE score <24, plus neuropsychological test evaluation
3. Progressive worsening of cognitive function

Diagnostic methods:

1. ***Neuropsychological
2. Structural imaging
3. Functional ***imaging (PET, SPECT)
4. ***Biological markers (CSF amyloid, tau protein etc.)

Question 22

***Pathology of AD

Answer 22

1. Shrinkage of Cortex (esp. Superior frontal association cortex)
2. Shrinkage of Hippocampus (+ Entorhinal cortex, Inferior temporal cortex)
   —> enlarged Ventricles

Question 23

***Pathological hallmarks in AD brain

Answer 23

1. **Amyloid plaques
   - Extracellular aggregation of **Amyloid-β protein, other association proteins, non-nerve cells
2. **Neurofibrillary tangles
   - Intracellular abnormal form of **microtubule associated protein: Tau —> Hyperphosphorylated —> captured in NFT —> microtubules destabilised + dissociation —> reduced axonal transport —> AD
3. Significant decrease in ACh transferase + Loss of cholinergic neurons

Question 24

Familial AD

Answer 24

1. ***Autosomal dominant families in Early onset AD
2. ***Familial aggregation in Late onset cases (also Autosomal dominant, with age-dependence but high penetrance)
3. ***MZ twins (~40-50%) vs DZ twins (~10-20%) concordance rate

Question 25

***Genetics of Early onset AD

Answer 25

Three ***autosomal dominant genes responsible for almost ALL large early-onset AD families 1. ***APP 2. ***PSEN1 (most mutations and most families) 3. ***PSEN2 Still, ~50% of early-onset AD cases remain genetically unexplained

Question 26

APP processing and Amyloid plaque

Answer 26

One domain of ***Transmembrane protein - β-secretase —> cleave APP to form C99 —> ***γ-secretase (encoded by PSEN1) —> Aβ40/Aβ42 (—> degraded by Neprilysin, IDE, APOE) - α-secretase —> cleave APP to produce C83 Mutation of APP / PSEN1: - affect cleavage activity —> ↑ Aβ40/Aβ42 —> Amyloid-β accumulation —> Amyloid plaque

Question 27

Neurofibrillary tangles

Answer 27

Tau protein: - structural protein in neuron —> binds and ***stabilise microtubule —> help to ***transport ``` Alzheimer’s disease: - Tau protein ***hyperphosphorylated —> Tau ***sequestered / captured in Neurofibrillary tangles —> loss of Tau binding to microtubules —> microtubule ***dissociation —> reduced ***axonal transport —> AD ```

Question 28

ApoE: Risk factor of AD

Answer 28

Risk factor for ***Sporadic AD Mechanism for association with AD: likely ***Aβ clearance + ***neuronal repair ``` Apolipoprotein E - transport plasma lipids within tissues - ***clearance of Amyloid-β at BBB - immune response - synaptic maintenance - 3 alleles: —> ε3 (most common, normal allele) —> ε2 (protective: ↓ frequency in AD cases but risk factor for CVD) —> ε4 (risk allele: ↑ frequency in AD cases) ``` * **ε4 allele: - ↑ risk of developing AD - ↓ age of onset of AD

Question 29

Potential treatment for AD

Answer 29

Aducanumab - targets aggregated forms of β-amyloid —> reducing its buildup - antibody given IV once a month

Question 30

Amyotrophic lateral sclerosis (ALS)

Answer 30

- ***Sporadic ALS: 90% - Familial ALS: 10% - average age of ALS onset: 55 - 50% patients die after 3 years

Question 31

Symptoms of ALS

Answer 31

1. Tripping, stumbling, falling 2. Loss of muscle control and strength in hands, arms 3. Difficulty speaking, swallowing, breathing 4. Chronic fatigue, muscle twitching / cramping Caused by: 1. Upper motor neuron damage: - Stiffness (spasticity) - Muscle twitching (fasciculations) - Muscle shaking (clonus) 2. Lower motor neuron damage: - Muscle weakness - Muscle shrinking (atrophy)

Question 32

Spinal onset vs Bulbar onset of ALS

Answer 32

Spinal onset (problem begin in limbs): - 75% - 6 months of progressive weakness of hand grip - fasciculations of muscle in affected limb - cramps and spasms - weight loss + Bulbar onset (problem begin with speaking / swallowing): - 25% - commoner in elderly women - 9 months of progressive difficulty in speaking / swallowing - weight loss +++ - inappropriate laughing and crying - breathless when lying flat

Question 33

Diagnosis of ALS

Answer 33

- ***NO diagnostic test - Mainly based on progressive pure motor syndrome - not explained by lesion in a single anatomical site —> mean delay in diagnosis: 1 year 1. EMG: supportive paraclinical test 2. inappropriate laughing / crying

Question 34

Pathology of ALS

Answer 34

1. ***Brain atrophy (some areas more severe) 2. Neuronal loss in ***Corticospinal tract 3. Unique microscopic features - ***Bunina bodies in motor neurons - ***“Skeins”, ***Hyaline bodies, ***Dystrophic neurites in anterior horn cells

Question 35

Genetics of ALS

Answer 35

1. ***SOD1 gene - Autosomal dominant - typical ALS 2. ***Dynactin gene - Autosomal dominant - adult onset, slow progressive, early vocal cord paralysis

Question 36

SOD1 mutations

Answer 36

- 15-20% of Familial ALS (1-2% of all cases) - 100 mutations —> Intracellular aggregate, Protein folding / degradation failure Normal SOD1: - 2 dimers - each bind to Cu + Zn ion - function of SOD1: reduction of Cu2+ —> Cu1+ —> regulate dismutation of superoxide ion into hydrogen peroxide H2O2 SOD1 mutation in Familial ALS: Loss of physiological function 1. ***Impaired dismutase activity Gain of toxic function 2. ***Aberrant redox chemistry: due to changes in conformation of SOD1, that leave the channel (responsible for accepting superoxide ion) able to accept larger molecules 3. ***Protein instability and SOD1 aggregation (characteristic of all SOD1 mutants) —> mitochondrial dysfunction, oxidative stress

Question 37

Dynactin gene

Answer 37

Dynactin: responsible for ***Retrograde transport in axons (Dynein/Dynactin) Mutation in Dynactin: ***impaired Retrograde transport Symptoms: - adult onset, slow progressive, early vocal cord paralysis

Question 38

***Summary of HT, PD, AD, ALS

Answer 38

HD: - Autosomal dominant - Brain specific: Striatum (Caudate + Putamen) - Loss of Medium spiny neurons (GABAergic inhibitory neurons) - ***Huntington CAG repeats >36 —> Multiple glutamine repeats - ***Striatal atrophy - Mutant protein aggregation - Chorea + Psychiatric disturbance + Cognitive impairment PD: - Autosomal dominant / recessive - Substantia nigra pars compacta + Locus ceruleus - Loss of Neuromelanin, Monoamine neurons, Monoamine —> Dopamine depletion - ***α-Synuclein, ***Parkin / E3 Ubiquitin ligase (PARK1/2) - ***Lewy bodies - Mutant protein aggregation - Bradykinesia, Rigidity, Resting tremor, Postural instability, Gait disturbance AD: - Cerebral cortex + Subcortical regions (Hippocampus) atrophy - Loss of neurons and synapses - ***APP, ***Presenilin 1/2, ***ApoE, ***Tau - ***Neurofibrillary tangle (***Hyperphosphorylated Tau): Transentorhinal region + Hippocampus - ***Amyloid plaque: Frontal + Posterior cingulate + Posterior association cortices - Mutant protein aggregation - Memory loss, behavioural abnormalities ALS: - Autosomal dominant - Brain + Brainstem + Spinal cord - Motor neurons loss - ***SOD1, ***Dynactin gene - ***Bunina bodies, ***Hyaline bodies, ***“Skeins” - Mutant protein aggregation - Muscle weakness, muscular atrophy, spasticity, eventually paralysis