lecture 22 Flashcards
Motor neuron disease 1. brief description of MND 2. symptoms of MND 3. Diagnosis 4. prognosis 5. treatments 6. causes 7. genetics 8. current research
Give a brief description of MND?
- MND = group of diseases –> most common form = amyotrophic lateral sclerosis
- point of commonality: affects motor neurons
- -> UMN: originate in brain/brainstem and do not directly stimulate muscles
- -> LMN: directly innervate muscles
- both forms affected but depending on which is predominantly affected you’ll get different symptoms
- progressive
- fatal
What are the symptoms of MND?
- doesn’t take long to figure out what the symptoms might be if you think about what cells are affected
- disease affects UMN and LMN in CNS
therefore symptoms include: - muscle twitching
- muscle weakness
- difficulty speaking
- difficulty swallowing
- tripping, stumbling, dropping things
- progressive paralysis
- decreased respiratory factors
– respiratory failure is usually what kills you - peak age of onset = 45 - 60 years
- sinister because if genetic you’ve probably had children before you know you even have the disease yourself
- muscle atrophy
- sensory neurons are spared in this disease
- some evidence that you lose some cognitive capacity as well, but mostly spared
- severity escalates
How do we diagnose MND?
- there is no diagnostic test for MND
- diagnosis is purely clinical
- diagnosis is a process of exclusion
- neurologists will often strongly suspect a diagnosis of MND fairly early in the process of investigating a patient’s problem
- at the first or second consultation
- need to convey this concer to patients before they are able to confirm this diagnosis
- diagnostic uncertainty and its associated emotional distress are therefore common in MND
- this uncertainty is often a protracted process due to the need to observe how the disease unfolds
- performing additional investigations, repeating tests overtime and seeking further neurological opinions is burdensome, expensive and because of its low specificity, often unhelpful
What is the prognosis for someone with MND?
- timeframe for symptom progression is variable
- confined to wheelchair within 1-2 years
- death within 3-5 years
- death usually due to respiratory failure
What treatments are there for MND?
- only one drug on the market for this disease
- Riluzole ‘Rilutek’ from Sanofi-Aventis
- only approved therapy for MND
- moderate clinical efficacy
- not really an increase in efficacy with increasing dose
What are the causes of MND?
- the fundamental causes of MND are not known
- > 90% of cases are sporadic
genetic factors
- Cu/Zn superoxide dismutase
- TDP43
- optineurin
- angiogenin
- C9orf72
don’t know whether these mutations are causing something to stop happening or causing something to start happening
environmental factors
- head trauma
- military service (e.g. gulf war veterans exposed to a very specific environmental toxin in the deserts of kuwait)
- chemical toxins
these connections are not so straight forward nor are they common
even though this is a disease that affects motor neurons it is not solely associated with biological processes that occur within the motor neurons
mechanisms occurring in other cell types may also contribute to loss of motor neuron function e.g. astrocytes and microglia
e.g. astrocytes take up glutamate from the synapse, riluzole supposedly works by enhancing this reuptake
What was one of the first recognised causes of familial ALS?
- mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis
- “here we report tight genetic linkage between FALS and a gene that encodes a cytosolic Cu/Zn-binding superoxide dismutase (SOD1)”
still don’t know how this happens
What is Cu/Zn SODI?
- expressed in every cell in the body (why does the motor neuron specifically succumb to the disease/mutation?)
- major antioxidant enzyme
- detoxifies toxic superoxide radicals
- approx 150 amino acids
- binds one copper ion and one zinc ion
- substitution mutations in SODI cause MND
SOD + O2− + 2H+ –> SOD + H2O2
analysis of the functional effects of a mutation in SOD1 associated with familial ALS
- these results suggest that the essential feature causing this subtype of ALS is either a reduction in Cu/Zn-SOD activity, or a novel gain of function that is particularly deleterious to those cell types expressing SOD1 at high levels
shown that it was not a loss of function
- so then how does mutant SOD1 cause MND?
- aberrant pro-oxidant gain of function
- protein misfolding
- protein aggregation
- mitochondrial dysfunction
What is the genetics of MND?
- approximately 90% of MND cases are sporadic
- no family history
- no identified cause
- approximately 10% of MND cases are familial
– family history
– molecular basis increasingly known
> SOD1
> C9orf72
>TDF43
> optineurin
– most familial cases are autosomal dominant - why would mutations in genes encoding for very different things cause the same disease?
What are the effects of MND?
the effects MND has had on my immediate and extended family has been enormous, not just for those who have suffered and lost their lives but also for those left behind. As I told you today, my mum died of familial MND last year, which was pretty devastating for us all. Although I have been tested and the result was negative, I have three siblings and we know the odds are that two of us out of four will have inherited the gene. We also know that for those two, should there be no treatment or cure, it is not a matter of if, but when symptoms will manifest. We live in this shadow and sometimes it is just too much to comprehend.
What shapes current research into MND?
- we do not know the definitive cause of MND
- we cannot accurately diagnose at an early stage
- there are no valid therapeutic options
- good research could theoretically contribute to all three at once
What are models for studying the disease?
- tissue samples from people who have died of the disease (not very frequent from diseased or healthy spinal cords)
- even though familial forms account for only 10% it gives a basis for developing a genetic model of the disease
- mice models: create a genetic modification that gives mice the mutations that cause the disease (e.g. SOD1)
community at large recognises the idea that more SOD1 = more severe motor neuron pathology = more severe phenotype
What has been shown in mouse models?
- progression of SOD1 on motor neuron function
- rotorod - time to stay on it declines with age
- locomotor ability progressive gets worse
- at about 28 weeks falling off straight away
- hind limbs practically completely paralysed
- shortly after reaching this deficit they die (usually respiratory)
- drug that they are testing shows improved locomotor function but still eventually drops off
- not cured
- increases survival rate
What has their drug achieved so far?
- protection of alpha-motorneurons
- amount of oxidative damage is decreased
- increase of SOD1 activity (on top of overexpression in mutant mice)
*remember loss of SOD1 function is not why people with familal forms of MND due to SOD1 mutations get sick
therefore increasing SOD1 activity is not part of the therapeutic mechanism of action for our drug
- treating with this drug caused levels of mutant SOD1 to go up - But phenotype improved? (surprising)
What states does SOD1 exist in?
- Apo (no metal) (gets degraded almost straight away)
- metal deficient (one copper OR one zinc)
- holo (functional form - one copper AND one zinc) (very stable)
