Lecture 4: Huntington's and Motor Neuron Disease

Question 1

What are some of the clinical signs and symptoms of HD?

Answer 1

Involuntary bowing, twisting, grimacing movements often with facial and vocal tics

slow or abnormal eye movements

difficulty with speech and swallowing

sometimes memory problems/dementia

psychiatric symptoms (hallucinations and delusions)

Question 2

What is usually the cause of death in HD?

Answer 2

patients often die 15-20 years after signs first appear from a secondary cause (such as heart failure)

Question 3

Describe the pathology behind HD?

Answer 3

severe loss of neurons in the sub-cortical regions - largely in the caudate-putamen of the basal ganglia = brain weight reduced by about 30%

enlargement of lateral ventricles (CSF in lateral ventricles expands to fill the space and compensate due to loss of neurons)

Cells in cerebral cortex may also be affected (variable between people)

Gliosis (inflammation associated with neuronal death)

Question 4

What are the alterations to neurotransmitter systems in HD (motor circuit)?

Answer 4

Glutamate excitotoxicity (primarily in the caudate putamen that drives the death of cholinergic and GABAergic neurons here)

Question 5

How does dopamine neurotransmission differ in the early and later stages of HD?

Answer 5

biphasic changes in dopamine neurotransmission:
- early stages = increased dopamine release = contributes to hyperactivation of thalamus = increased stimulation of motor cortex that increases movement
- later stages = lose dopaminergic input

Question 6

How does the biphasic changes in dopamine neurotransmission in HD affect response to treatments?

Answer 6

giving L-dopa (used to compensate for loss of dopamine) in early stages will aggravate symptoms but dopamine antagonists will alleviate symptoms

as the disease progresses, you might see a reversal of this

Question 7

How is HD inherited?

Answer 7

autosomal dominant inheritance

Question 8

What is the age of onset?

Answer 8

usually 40-50 (although children constitute about 10% of the cases)

Question 9

What phenomenon is displayed by HD?

Answer 9

genetic anticipation (worsening of the disease phenotype over generations)

Question 10

True or false: inheritance from mother is likely to result in more severe disease?

Answer 10

False: inheritance from father is likely to result in more severe disease

Question 11

How does the number of CAG repeats relate to age of onset?

Answer 11

Individuals with higher number of CAG repeats have an earlier onset of disease

Question 12

What is the name of the gene and protein involved in HD?

Answer 12

gene: IT-15

protein: huntingtin (Htt)

Question 13

Where is huntingtin found?

Answer 13

throughout brain and in the testes

Question 14

What is the function of huntingtin?

Answer 14

unknown but may act as a transcription factor or contribute to protein trafficking or endocytosis/membrane cycling in neurons

Question 15

What evidence shows huntingtin is essential for normal embryonic development?

Answer 15

Huntingtin KO mice is embryonic lethal

Question 16

True or false: HD is a repeat expansion disorder?

Answer 16

True

Question 17

What is the nucleotide repeat implicated in HD?

Answer 17

trinucleotide CAG repeat within exon 1 of coding region of huntingtin near N-terminus

Question 18

What three things does the length of the CAG repeat determine in HD?

Answer 18

Whether an individual will develop HD (E.g. 36 repeats and above = affected, 40 repeats and above results in adult-juvenile onset)
degree of penetrance
age of onset
disease severity

Question 19

Describe the Huntingtin protein (Htt)

Answer 19

cytosolic protein with molecular weight around 350 KDa
- N-terminal PolyQ region begins at amino acid 18
- PolyQ region followed by PolyP (polyproline) region which is thought to keep the Htt in solution
- multiple HEAT domains that are thought to play a role in vesicle trafficking
- Nuclear export signal
- nuclear localisation signal

Question 20

How is the wildtype huntingtin protein post-translationally modified to form non-toxic fragments?

Answer 20

don’t have the long polyQ repeat = cleavage of n-terminal fragment by caspases
- N-terminal fragment interacts with C-terminal fragment

Question 21

How is the mutant huntingtin protein post-translationally modified to form toxic fragments?

Answer 21

• long polyQ repeat domain = alternate cleavage of Htt to generate a larger N-terminal fragment that is not able to interact with the C-terminus (mis-folded) resulting in an aggregate property.

Question 22

How do expanded polyQ Htt proteins aggregate?

Answer 22

form detergent-insoluble protein aggregates with characteristics of amyloid fibrils

• aggregate into oligomers, intermediates then fibrils but still debate over which species is most toxic

Question 23

How does the distribution of Htt change in HD?

Answer 23

changes from cytoplasm to peri/nuclear (forms nuclear inclusions)

Question 24

Which neurons predominantly contain the Htt nuclear inclusions?

Answer 24

Neurons of the caudate-putamen and cortex

Question 25

What experimental evidence has suggested that Htt amyloid fibrils may be the more toxic species?

Answer 25

>30 CAG repeats results in fibril formation with longer repeats aggregating faster and at lower concentrations Htt inclusions form before the onset of neurological symptoms and neurodegeneration so questions whether these aggregates are the cause of the disease

Question 26

What experimental evidence has suggested that Htt amyloid fibrils may be the least toxic species?

Answer 26

cultured human neurons and developed a robotic microscope that could track the fate of each cell in the dish - expressed the GFP tagged Htt protein to see where the Htt distributed - found that in some cells, they developed red Htt inclusions but the neuron was healthier than the neurons surrounding it suggested that the neurons with intranuclear inclusions were actually protected from death and suggesting that perhaps the fibrillar species was actually the least toxic

Question 27

What are the potential targets of disease-modifying treatments for HD?

Answer 27

- depletion or stabilisation of dopamine, serotonin and other monoamines involved in pathogenesis and biphasic responses - antioxidants to reduce free-radicals - anti-sense oligonucleotide treatment to silence the expression of the IT-15 gene (risky bc don't understand the physiological role of the protein so maybe just target repeat domains in exon 1) - stem cell therapy

Question 28

What are some of the current treatments for HD?

Answer 28

Symptomatic treatments: - physical therapy to treat motor issues - rebalance the neurotransmitter systems (GABA, glutamate, acetylcholine, dopamine)

Question 29

What is motor neuron disease also called?

Answer 29

Amyotrophic lateral sclerosis

Question 30

What is MND?

Answer 30

progressive, fatal disease that attacks nerve cells controlling voluntary muscles

Question 31

What are some of the clinical features and symptoms of MND?

Answer 31

muscle atrophy, weakness and muscle twitch that signify disease of the lower motor neurons glial scar in the lateral columns of the spinal cord at autopsy following degeneration of the corticospinal tracts loss of brain's ability to initiate and control voluntary movement difficulty speaking, swallowing and eventually breathing

Question 32

What is the typical cause of death in patients with MND?

Answer 32

respiratory failure

Question 33

True or false: dementia is often seen in individuals with MND nearer the end of disease?

Answer 33

False: cognitive function is often spared but about 10% of cases develop a type of dementia

Question 34

What is the difference between upper and lower motor neurons?

Answer 34

Upper motor neurons originate in the motor cortex in the cerebral cortex and travel down to, and synapse in, the brain stem and spinal cord lower motor neurons begin in the spinal cord and brain stem and innervate the muscles throughout the body

Question 35

True or false: men are more likely to develop MND than women?

Answer 35

True

Question 36

What is the typical age of MND onset?

Answer 36

late middle age, peak 60-70 years

Question 37

What is the mean duration of survival for individuals diagnosed with MND?

Answer 37

3 to 5 years

Question 38

True or false: most cases of MND are familial?

Answer 38

False: most are sporadic (around 90%)

Question 39

What is one of the first genes identified that, when mutated, increases risk of developing familial MND?

Answer 39

mutations in Cu/Zn superoxide mismutase (SOD1)

Question 40

what mutations in which gene accounts for around 40% of familial MND cases?

Answer 40

expanded hexanucleotide repeat (GGGGCC) in C9ORF72

Question 41

Other than SOD1 and C9ORF72, which other mutations have been identified associated with familial MND?

Answer 41

mutations in DNA/RNA-binding proteins: TAR DNA binding protein (TDP-43) and Fused in sarcoma (FUS

Question 42

What are three environmental risk factors of MND?

Answer 42

1. consumption of beta-methylaminoalanine (BMAA) 2. exposure to pesticides 3. exposure to viruses

Question 43

Give three occupations that are risk factors for MND?

Answer 43

professional athletes served in the military farmers

Question 44

Give three lifestyle risk factors of developing MND?

Answer 44

1. head trauma 2. smoking 3. diet/obesity

Question 45

What evidence suggests that glutamate excitotoxicity may have a pathological role in MND?

Answer 45

elevated levels of glutamate have been found in CSF and serum of those with MND treatment with glutamate antagonist Riluzole delays disease progression

Question 46

What factors may contribute to the development MND?

Answer 46

glutamate excitotoxicity failure of UPS mitochondrial dysfunction oxidative damage

Question 47

Describe the pathology of MND

Answer 47

degeneration and loss of motor neurons with astrocyte gliosis intraneuronal inclusions - main component in the pathology is TDP-43 but may also see ubiquitin deposits, Bunina bodies, SOD1 or FUS aggregates, poly-(Gly-Ala), poly-(Gly-Pro) and poly-(Gly-Arg) dipeptide repeat proteins in C9orf72

Question 48

What is the most common protein aggregate observed in intraneuronal inclusions in MND?

Answer 48

TDP-43

Question 49

What are the two approved treatments for MND?

Answer 49

Riluzole and Edaravone

Question 50

What is Riluzole and what does it do for treatment of MND?

Answer 50

a glutamate antagonist so targets excessive glutamate in excitotoxicity that delays death or time to tracheostomy

Question 51

What is Edaravone and what does it do for treatment of MND?

Answer 51

an antioxidant and free-radical scavenger that delays motor deterioration

Question 52

What are potential targets in the pipeline for MND treatment?

Answer 52

- reduce oxidative stress and mitochondrial dysfunction - reduce excitotoxicity - reduce neuroinflammation (astrogliosis) - apoptosis and stem cell therapies - replace the motor neuron - nucleocytoplasmic transport - DNA damage and RNA splicing/metabolism

Question 53

polymorphisms in which gene encoding an anti-inflammatory protein is a risk factor for AD, MND and PD?

Answer 53

TREM2