Motor neurone disease (ALS)

Question 1

What does ALS stand for?

Answer 1

Amyotrophic lateral sclerosis.

Question 2

In which cell types are mutations expressed?

Answer 2

All cell types, but only motor neurones affected.

Question 3

Where are the upper motor neurons found?

Answer 3

Brain

Question 4

Where are the lower motor neurons found?

Answer 4

Brainstem and spinal cord.

Question 5

What is the incidence of ALS?

Answer 5

1-4/100,000 per year

Question 6

Which age range is the highest risk factor for ALS?

Answer 6

40-60 years

Question 7

What percentage of cases are familial?

Answer 7

10%

Question 8

Which genes are most commonly involved?

Answer 8

SOD1

C90ORF72

Question 9

What are the symptoms of ALS?

Answer 9

Twitching
Cramping
Stiffness or weakness of muscles
Slurred and nasal speech
Difficulty in chewing or swallowing. 
Muscle weakness and atrophy spread to progressive degeneration of all upper and lower motor neurons and their connections.

Question 10

What are the upper motor neuron symptoms?

Answer 10

Muscle stiffness and exaggerated reflexes.

Question 11

What are the lower motor neuron symptoms?

Answer 11

Muscle weakness, cramps and transitory contractions and fasciculations.

Question 12

What are the later signs of ALS?

Answer 12

Dysarthria (poor articulation of speech)
Spasticity
Hyperreflexia
Dysphagia
Eventual paralysis due to denervation.

Question 13

Does ALS impair cognitive ability?

Answer 13

No, does not affect ssensory faculties, impair thinking or other cognitive abilities, apart from a minority showing symptoms of dementia, or problems with memory or decision making.

Question 14

What is the mortality rate?

Answer 14

Death usually occurs due to respiratory failure within three to five years from the onset, although about 10% of all ALS cases survive >10 years.

Question 15

What affect does glutamate have?

Answer 15

Glutamate and aberrant protein impair MAP kinase signalling.

Question 16

How many genetic loci have been identified to date?

Answer 16

14: 8 with typical adult onset, 3 with juvenile onset and slow progression.

Question 17

What percentage of fALS is due to mutations in the SOD1 gene?

Answer 17

15-25%

Question 18

What are 40-50% of fALS linked to?

Answer 18

Repeat nucleotide expansions in the gene encoding C9ORF72

Question 19

What is SOD1?

Answer 19

Ubiquitiously expressed 153-amino acid protein involved in conversion of superoxide radicals to H202.

Question 20

What happens in SOD1 knockout mice?

Answer 20

Appear normal, but later develop muscle denervation.

Question 21

What happens in mouse lines expressing mutant human SOD1?

Answer 21

Develop adult-onset progressive motor neuron disease similar to human condition, with varying ages of onset and disease progression rates.

Pathological hallmarks are early-onset astrogliosis and microgliosis, glutamate-mediated excitotoxicity, axonal transport deficits, mitochondrial vacuolization, aberrant neurofilament processing, and reduced metabolic support of the motor neurons by surrounding glia.

These models do not develop cortical motor neuronal degeneration; a fundamental feature of the human disorder that is required for ALS.

Question 22

What are reduced in numbers in the CNS of ALS patients?

Answer 22

Astroglial glutamate GLT-1 transporters.

Question 23

What is the consequence of the reduction in astroglial glutamate transporters?

Answer 23

Increase in glutamate content at the synaptic cleft and overstimulation of the postsynaptic glutamate receptors on motor neurons.
Leads to an enhanced Ca2+ influx and excitotoxic cell death.
Pathogenic mechanism confirmed in several mutant SOD1 transgenic mouse models.

Question 24

In what instance does cell death in mutant SOD1 containing motor neurons not occur?

Answer 24

When they’re surrounded by wildtype glial cells.

Question 25

What does mutant SOD1 do that normal SOD1 does not?

Answer 25

Aggregate.

Question 26

Where do aberrant SOD1 accumulate?

Answer 26

Mitochondria of spinal cord.

Question 27

Other than SOD1 which mutation is associated with ALS?

Answer 27

TDP-43 (causal in 3% of ALS patients)

Question 28

What is TDP-43 invovled in?

Answer 28

RNA processing, transport and splicing.

Question 29

Which mutation is found in C9ORF72?

Answer 29

GGCCCC repeat expansion in first intron of C9ORF72 gene.

Question 30

What is the consequence of the repeat expansion in C9ORF72?

Answer 30

It leads to loss of one alternatively spliced transcript and to formation of nuclear RNA foci and aberrant RNA metabolism in ALS.

Question 31

What is the structure of the protein encoded by the TDP-43 gene?

Answer 31

414 amino acids.
N-terminal nuclear localisation signal followed by two RNA-recognition motifs and a C-terminal glycine-rich domain where the majority of TDP-43-associated mutations occur.

Question 32

What is the inheritance of TDP-43.

Answer 32

Dominantly inherited, could cause toxic gain or loss of function.

Question 33

How could TDP-43 cause gain of function?

Answer 33

By binding several RNAs, altering RNA metabolism in the cytoplasm through a toxic gain of function.

Question 34

How could TDP-43 cause loss of function?

Answer 34

Nuclear depletion of TDP-43 could lead to a change in RNA metabolism through loss of function.

Question 35

What happened in human mutant TDP-43 expressing mice?

Answer 35

Over expressed around 3-fold under PrP promoter.
Mice appear normal up to 3 months, then developed a swimming gait with weight loss followed by death at about 154 days.
About 20% of spinal motor neurons were lost, with up to 50% axonal loss in the corticospinal tracts.
Developed cytoplasmic ubiquitinated inclusions in specific neuronal subsets, some with mis-localisation of TDP-43.

Question 36

What is TDP-43?

Answer 36

Transactive DNA binding protein 43.
A nuclear protein that binds DNA and RNA.

It is also known to regulate mRNA splicing - mutants in TDP-43 are involved in generating incorrect splicing of CFTR mRNA leading to cystic fibrosis.

Also involved in microRNA biogenesis, cell division, apoptosis.

Question 37

Where is FUS found in ALS?

Answer 37

Aggregates in cytoplasm (rather than its normal nuclear location)

Question 38

What happens to ANG in ALS?

Answer 38

Loss of function

Question 39

What is ANG?

Answer 39

Codes for angiogenin - a protein with RNAse activity that stimulates vascularisation.

Strongly expressed in foetal and adult spinal motor neurons.
In cell lines it promotes neurite extension and protects from hypoxia: mutant forms lose both functions.

Question 40

What do mutations in SEXT cause?

Answer 40

Autosomal dominant juvenile-onset ALS with slow progression.

Question 41

What does SEXT encode?

Answer 41

Senataxin - has RNA and DNA helicase activity.

Question 42

Are all genes associated with ALS involved in RNA/DNA function?

Answer 42

No:
ALS2 (alsin) (l.o.f) - involved in endosome trafficking and nurite extension.
VAPB - targets certain proteins to the cytoplasmic surface of the ER
FIG4 - phosphoinositide 5-phosphatase, regulates the abundance of phospholipids on the membranes of late endosome vesicles.
OPTN - multifunctional protein involved in several aspects of vesicular trafficking.
Profilin 1 - involved in regulation of actin dynamics
VCP - dysfunction impairs degradation of certain proteins by the ubiquitin-proteasome system
UBQLN2 - involved in protein degradation via the ubiquitin-proteasome system.
DCTN1 - modulaes binding of dynein to cell organelles
UNC13A - involved in vesicle priming for transmitter release.

Question 43

Have you studied 3 ALS genes in detail?

Answer 43

DO IT, IT WAS MENTIONED IN REFERENCE TO THE EXAM.

Question 44

What is currently the only licensed drug for ALS?

Answer 44

Riluzole - it reduces glutamate excitotoxicity.