Huntington's disease Flashcards
LO
- Describe the pathology associated with Huntington’s disease.
- Understand the principle of the CAG tract (poly-glutamine (Q) expansion) and its consequence for Huntingtin (the Huntington disease protein).
- Describe the key determinants and potential consequences of different mis-folding intermediates.
- Use Huntington disease to introduce the idea of loss or gain of function mutation.
- List routes by which the misfolded protein affects cellular function.
- Address the potential for selective degeneration of neuronal type and compartment.
- Consider the evidence for huntingtin protein propagation.
What are the main characteristics of HD?
- Autosomal dominant (50% of children from infected parents will get it)
- Hereditary
- Neurodegenerative
Tell me 4 main characteristics/ facts about HD?
- everyone with the mutated gene will get HD
- probability of each offspring inheriting the affected gene in 50%
- inheritance is independent of gender
- characterised by cognitive, behavioural, and motor dysfunction
Brief history
Recognised as an inherited disorder in 1872 when a 22-year-old American doctor, George Huntington, wrote a paper called: On Chorea.
(https://en.wikisource.org/wiki/On_Chorea)
“Chorea” comes from the Latin and Greek words meaning dances (involuntary/uncontrollable movements/muscle jerks and twitches)
What is the gene responsible in HD?
IT15 gene
everybody expresses this gene but those who inherit the expansion of the gene are the ones with HD
IT15 gene found on chromosome 4 and this gene expresses the Hungtingtin protein
When was the IT15 gene identified and when did genetic tests become available?
It was first identified in 1983
Predictive genetic tests become available from 1993
Tell me about the expansion of the IT15 gene that leads to HD
Poly Q repeats (repeats of glutamine (Q))
Normal conditions: 10-35 repeats
Huntington’s disease: > 36 repeats
Tell me about the poly Q expansion and anticipation
repeat expansion
effect replication –> anticipation. When DNA polymerase replicates DNA there can be an error due to glutamine repeats which can lead to expansion of that region
therefore, child could inherit more repeats than parent due to expansion of CAG repeat
this is more prevalent in male’s parents due to sperm generation, which leads to a high chance of expansion of this region being given to offspring
Tell me about the age of onset with HD
Strong inverse relationship between the age of onset of HD and the number of CAG repeats.
more repeats= earlier age of onset
mild be able to modify process, if direct linear relationship, then you’d think you could modify, so as not linear then suggests there could be factors (genetic/ environmental) that could be modified
cannot map age an individual will show symptoms of disease
Explain the different interpretations of genetic testing for CAG repeats
HD has 100% penetrance: if individuals have ≥ 40 repeats they have the disease.
Symptoms start between the ages of 30-50 years (40+ CAG), although late onset (36-39 CAG) and juvenile manifestation (60+ CAG) also occur.
Prognosis usually between 15-20 years from onset of symptoms.
>60 juvenile form of HD and this is more aggressive and leads to EOHD
people don’t die due to HD they die because of other factors that HD cause
Tell me about the prevalence of HD
Increase in the prevalence of HD over the past two decades.
Family history - likely excluded sporadic or de novo cases (5-8%).
Prevalence studies (genetic and clinical) show:
Higher prevalence:
- America, Australia & most European & Western countries:
- (10.6-13.7:100,000)
Lower prevalence:
- Asia & Africa (0.5:100,000 in Japan & China)
Tell me about the symptoms of HD, what are the main areas infected?
The symptoms usually vary widely between people, even families
this sympton variety means that some people may not be aware they have HD
Changes usually affect three main areas:
1. Movement: Involuntary & Voluntary (physical)
2. Behaviour: Changes in behaviour and personality (physical)
3. Cognitive: Difficulties with planning and thinking (overlaps with other ND diseases so can often be misdiagnosed)
What sympton is usually the most obvious first symptom?
Movement disorder
What symptom is the one which gives patients and carers the most concern?
Behavioural disorder
What is the symptom that people find effects them most in daily life?
Cognitive disorder
Tell me some of the physical symptoms
The symptoms of HD are like having ALS, PD & AD simultaneously.
- Motor deficits (jerky/fidgety motor).
- May seem clumsy or stumble more than usual.
- Voluntary movements are affected.
- Abnormal eye movement.
- Speech becomes slurred.
- As disease progresses, swallowing problems become common.
- Weight loss (excessive movements and malnutrition through dysphagia) and central effects on appetite.
- Incontinence.
- Involuntary movements cannot be consciously suppressed and stop only with sleep.
Tell me some of the cognitive symptoms of HD
- Memory and concentration problems
- Hard to plan and think ahead, difficult to switch between tasks.
- Lack of motivation – appear lazy.
- Reduced ability to read facial expression.
- Emotional changes –subtle changes to mood/behaviour.
- Aggressive, demanding, stubborn and self-centred.
- Impulsive or irrational, behaving in a disinhibited way or obsessive with things. depression, anxiety, and anger.
- Relationships at high risk.
- May lead to social isolation.
- Respiratory/cardiac/suicide (major reasons for mortality – 3-13%).
Tell me about the neuropathology of the disease
basal ganglia is the main site of neuronal loss in HD. mainly the striatal part
all work together and communicate with cortex (in red box and two below). neurons within striatum which are affected early on
Normal conditions
putamen is an inhibitory nucleus which releases the inhibitory NT GABA
GABA has dampening effect on globus pallidus which leads to release of LESS GABA which leads to thalamus having an increased activity, which sends signals to increase movement
Huntington’s disease
These inter-connected areas are associated with different types of activity including movement, learning, thinking, planning, motivation & emotion. As the cells in these parts of the brain reduce in number, changes occur in how they function, resulting in the various symptoms of HD.
if cells loss due to HD.
less neurons to interact with in cortex
so, putamen effect decreases
so, inhibition onto globus pallidus is reduced which means that this remains more active and its activity increases
more GABA onto thalamus which reduces this activity and therefore movement
What aspects of the basal ganglia are mostly affected by HD?
- striatum (caudate nucleus and Putamen)
- Globas pallidus
- subthalamic nucleus
- Substantia nigra
What are two cell types which are affected in HD?
MSNs (medium spiny neurons)
ASNs (aspiny striatal neurons)
Tell me about MSNs
- main and earliest striatal cell type affected in HD
- Vulnerable projection neurons
- long axons
- medium cell body
- prone to excitotoxicity
- inhibitory
Tell me the NT associated with MSNs
GABA
Calbindin
Substance P
Enkephalin
Dynorphin
TGF alpha
Tell me about ASNs
- lack spines
- located only in striatum
- project locally
- spared neurons
Tell me about the NT associated with ASNs
Acetylcholinesterase
NADPH-diaphorase
Parvalbumin
Somatostatin
Neuropeptide Y
Cholecystokinin
Vasointestinal peptide
Tell me about the different grades of neurodegeneration in HD
Pre-symptomatic white and grey matter changes
Tell me about the predictable clinical progression and HD
Tell me some PTMs and associated physiological roles of the HTT protein
PTMs
- Phosphorylation
- SUMOylation
- Ubiquitination
- Acetylation
- Proteolytic cleavage
- Palmitoylation
What does HEAT stand for?
How many potential repeat regions are there?
What do these have roles in?
HEAT (huntingtin, elongation factor 3, protein phosphatase 2A, and TOR1)
37 potential HEAT repeat regions
so all those alongside PTM has huge cellular roles and is expressed throughout whole body
Tell me about the genetic bases of HD
Tell me about the HD aggregates
processed by proteostasis in healthy individuals
in HD, QQQ Pro becomes misfolded and unstable and isn’t degraded as it should be so leads to impairment of proteostasis
Production of Exon 1 fragment is important for pathogenesis of HD
fragments can be different sizes of N-terminal fragments (this is known due to animal models)
What are found in (>40 Q’s)?
inclusion bodies: insoluble protein aggregates
consequence of N-terminal fragments are inclusion bodies
accumulation in cell bodies in humans
accumulation in cytoplasm in mice
expression level of HTT is not proportional on levels of ND- pathology is not due to aggregation
Dysfunction and death: protein misfolding and oligomerisation
inclusion bodies act as a protective mechanism? inadvertently also harming the cell
disruption of aggregates and inclusion bodies can increase toxicity in cells
The misfolding pathway can be used to explain neurotoxicity, how?
Loss of function (LOF) = mutant protein no longer able to perform normal functions. This occurs in the context of a mix of normal and mutant protein (e.g., sequestering protein/s into aggregates away from where it acts; modifying protein interaction making them weaker so binding is lost).
Gain of function (GOF) = extra activity imparted by the mutant protein (e.g., expanded polyQ creates protein conformers which are toxic and create new activity/interactions (albeit dysfunctional).
working but doing things it shouldn’t be doing
Tell me about some of the pathological roles of the mtHTT protein
proteolytic cleavage
nuclear translocation which can lead to: aggregation, oligomerisation inclusion and disruption of transcription
also occur in cytoplasm same as above
generally, UQ tagged. CELL TRIES TO TAG FOR DEGRADATION
process of degradation is not efficient and not working
effect other protein synthesis which leads to disruption of key processes like synaptic dysfunction, mitochondrial toxicity and energy imbalance and axonal transport impairment
What does the mtHTT inhibit and provide an example?
Inhibition of transcription (75%).
Inhibits histone modifications, reducing transcription of genes (acetylation enzymes).
Examples include inhibition of CREB dependent transcription.
What are further examples of transcriptional dysregulation with the mtHTT?
Give an example of a LOF in HD
Tell me about this
Vesicle/ axonal transport abnormalities
mtHTT binds to Huntingtin associated protein1 (HAP1) sequestering it along with other proteins into inclusion bodies (GOF).
mtHTT impairs axonal transport of mitochondria, other organelles, and neurotrophic factors (e.g., BDNF (important for neuronal survival)) (LOF).
Experiments in cultured cells and Drosophila neurones further document reductions in axonal transport and accumulation of axonal vesicle cargos in association with polyQ-mHTT expression. (Sinadinoset al. 2009, Lee et al. 2004)
Give an example of a GOF in HD
Tell me about this
Mitochondrial dysfunction and HD (GOF)
Mitochondria from HD patients and genetic models of HD show:
- Enhanced sensitivity of mPTP to Ca2+-inducing pore opening and release of CytC.
- Reduced membrane potential (ΔΨ).
- Decreased Ca2+ buffering capacity.
- Increase in ROS production- leads to cellular damage to DNA and proteins
Give another example of LOF in HD
Anti-apoptotic (IAP) activity lost (LOF)
normal HTT is anti-apoptotic by binding to procaspase-9
mtHTT cant bind as efficiently to procaspase-9
Give another example of GOF in HD
Ubiquitin-proteasome system: impaired proteostasis (GOF)
What is another pathway that is impaired in HD?
Autophagy pathway (GOF/LOF)
There are two pathways in HD;
Direct and indirect
Tell me about each of these
direct= excitatory on thalamus via inhibition
indirect= inhibitory on thalamus via excitation
- Opposite net effect of both pathways on thalamus
- Indirect pathway effected first
excitation of direct –> cortex –> striatum –> GPI
indirect –> GPE –> STN
What is the roles of the basal ganglia?
- Balance
- Posture
- Balance of activity of both pathways
Tell me some cell-type specific characteristics seen in HD between MSNs and ASNs
different NT expressed seen in diagram
vulnerable (MSN) vs spared neurons (ASN)
- anatomical projections (MSN)
- NT expression
- MSN receive BDNF and glutamate from cortical neurons
- also receive dopaminergic input from SN
- express enkephalin peptides and D2 dopaminergic receptors (MSN)
- express substance P and have D1 receptors (ASN)
What degeneration is seen in HD?
Synapse
Tell me all of the things that contribute to HD
Tell me some properties of neurons that are relatively susceptible (+++/+) or not (+)
Tell me about the toxic effects of mtHTT
No common biochemical characteristic among the main neuronal populations affected in HD.
Increased vulnerability of neurons bearing longer and more prominent axons.
Other cell type-specific properties: afferents, targets, and biochemical content could modulate mtHTT toxicity.
The toxic effects of mtHTT do not appear to selectively affect specific neuronal populations. Instead, cell type-specific features may differentially affect vulnerability of specific neuronal populations to mtHTT-induced toxicity.
Tell me the 9 different polyQ diseases
What does the pattern of spread of HD suggest?
Pathology is not only propagated between neighboutng cell bodies, but also spreads along axonal pathways either away from (anterogradely) or towards (retrogradely) the cell body.
What are the regions of the brain affected by HD?
The basal ganglia are affected early in the disease.
Cortical regions are also affected and undergo thinning in asymptomatic people.
Cortical areas related to more advanced brain functions are affected later.
Thus, cortical degeneration in Huntington’s disease also follows a topographically predictable pattern like other ND conditions.
HD
Non-cell autonomous transmission
Infection of mammalian cells by PolyQ aggregates
Symptomatic treatments of HD
What are some therapeutic targets/ stratergies for HD?
Gene silencing and HD
ASO: antisense oligonucleotides
Allele-selective drugs, that bind to and target for degradation, mutant HTT mRNA via single nucleotide polymorphisms.
2017, Phase1/2: In an announcement likely to stand as one of the biggest breakthroughs in Huntington’s disease since the discovery of the HD gene in 1993, Ionis and Roche today announced that the first human trial of a huntingtin-lowering drug, IONIS-HTTRx, demonstrates that it reduces mutant huntingtin in the nervous system, and is safe and well-tolerated.
Jan 2019: first patients recruited onto Phase 3 trial- Halted now as wanted to assess risks
potential problem with this type of therapy: have to keep giving drugs continuously as mRNA is going to keep being produced
