Huntington's Disease

What is Huntington’s disease

• Autosomal dominant  

• Neurodegenerative – loss of function 

• Hereditary – everyone with a mutated gene will get Huntington’s disease but not all will have a clinical presentation  

• Probability of inheriting the gene is 50%  

• Characterised by cognitive, behaviour and motor dysfunction  

• Patients have involuntary symptoms (uncontrolled jerking disease) 

Prevalence

• Mostly impacts Western countries in America, Australia and most euorpean countries  

• Due to family lineage, although there are some de novo mutations, most are hereditary 

• Prevalence increasing over the past 2 decades 

• Lower prevalence in Asia and Africa 

The faulty gene

• Huntingtin (HTT) gene  

• CAG region repeated many times which gives rise to poly Q repeats towards the N terminus 

• The actual gene identified in 1983 and then the first genetic test to identify this in 1993 

• CAG repeats vary in number —> down to the number of repeats which determine the severity and when it will appear in the lifecourse  

  • 10-35 repeat individuals will have a relatively normal life  

  • 36+ repeats will begin to impact the individual 

Anticipation

• Relates to each generation having an earlier symptom onset  

• CAG repeats are prone to errors in DNA replication which leads to more mutation  

• If this falls within the huntingtin gen we will have more repats added to the gene 

Age of onset

• Correlation with no of repeats and age of onset 

• 25 repeats – totally unaffected individuals  

• 25-35 – will not develop symptoms but pass on very highly repeated Huntingtin gene to progeny, so children are at risk.  

• Truly affected individuals have over 36 repeats  

  • 36-39 have reduced penetrance —> may have some of the symptoms or may appear later in life 

  • 40 or more have full penetrance 

  • In very extreme cases with 60+ repeats not only do you have full penetrance but the age of onset may be a lot earlier 

• Not an absolute threshold, not everybody will behave according to this model 

Prognosis

The time between point of diagnosis and death – usually between 15-20 years, but this does vary between individuals 

Clinical progression

• In every neurodegenerative disease (apart from stroke) we don’t know when the pathology starts  

• The brain is not something that we can sample easily – reliant on the point which the individual experiences symptoms which is severe enough to consult a clinician

• Each aspect of symptoms has a different trajectory 

• At some point you cross the threshold which this can be clinically diagnosed but all throughout this time the disease has been progressing 

• Neuron loss of function will preceed neuron cell death but this could be years due to the body’s ability to buffer and won’t manifest into clinical dysfunction  

• However, from a treatment point of view, this is challenging because when you have crossed the threshold which has allowed you to diagnose it, there’s little you can do to reverse the damage 

• Huntingtin gene can impact cognitive function, but they can be compensated so they are not noticed by the family.  

Symptoms

• Occur across 3 main domains: movement, Behaviour and cognitive  

• Movement and behaviour domains are related to physical symptoms while cognitive symptoms related to planning and thinking 

• Movement  

  • Main change is the onset of involuntary movement  

  • Usually the first onset  

  • Distinct from other symptoms 

• Behaviour  

  • Range of behavioural change: apathy, depression, changes in personality, aggression  

  • May be the most significant domain but this can be hard to diagnose from a clinicians perspective 

  • Can really affect family members and those around them  

• Cognitive  

  • Difficulties with planning and thinking  

  • Can impact day to day life most —> E.g: lose independence 

Physical symptoms

• Can be affectation to voluntary movements  

• Motor deficit can include: 

  •  jerky or fidgety motor movements 

  • Clumsiness 

  •  abnormal eye contact 

  • speech becoming slurred 

  • Weightloss 

  • Incontinence 

Cognitive symptoms

• Lots of overlap between Alzheimers and parkinsons 

• Memory and concentration problems 

• Aggression, demanding, stubborn  

• Impulsive behaviour 

• Social isolation   

• Suicide related mortality 

• Lack of motivation  

• Reduced ability to read facial expressions  

• Hard to plan and think ahead 

Normal function of teh basal ganglia

• Made up of the:  

  • Caudate and the putamen making up the striatum  

  • Globus pallidus 

• Usually have the cortex sending Glu (excitatory) projections to the putamen which is inhibitory  

• Putamen neurons are GABAergic and send inhibitory signals to the globus pallidus (more input from the cortex = more inhibition)  

• The globus pallidus also inhibitory nucleus and increased inhibition onto an inhibitor (GP) results in more excitation onto the thalamus 

• The thalamus is an excitatory nucleus which excited the motor cortex and relays directly onto muscles through the spinal chord.  

In Huntington’s disease

• If there’s degeneration in the putamen then there will be less inhibition, due to decreased neurons to project to the GP 

• Less inhibition coming into the GP will result in more inhibition coming out of the GP onto the thalamus 

• Motor cortex will be depressed, muscle movements will be depressed  

Not every neuron affected equally

• Main neurons affected are medium spiny neurons 

• 95% of the neurons 

• In contrast with the aspiny neurons which are different morphologically:  

  • Slightly smaller 

  • Spiny neurons are GABAergic 

Neuropathology

• Can begin to detect with MRI  

  • Enlargement of lateral ventricles 

  • Loss of cortical striatal projection neurons 

  • Striatal atrophy  

  • Severe changes in matter extending to other areas of the cortex, cerebellum, hippocampus in severe cases 

• Despite the fact that the primary site of dysfunction is the basal ganglia, this will have a knock on effect because the lack of input to the connecting neurons will also result in loss of function and death 

Molecular changes

• Expansion in the N terminal section of the protein  

• Normal huntingtin protein will go through normal physiological expression and proteostatsis —> when the protein is not needed to it will be degraded  

• Poly Q fragments leads to misfolding of protein, leading to intracellular aggregates which disrupts proteostasis 

• Either:  

  • Can’t be recognised  

  • Can’t be digested and disposed of 

• These proteins function in many cellular pathways but main part of the pathology is the formation of these large aggregates which molecular mechanisms are ot designed to deal with

The misfolding pathway to explain neurotoxicity

• The dysfunctional system can lead to the pathology in different ways:  

  • The disease that is caused by loss of function = the mutant protein no longer able to perform normal function. This occurs in the context of a mix of normal and mutant protein (e.g: sequestering proteins into aggregates away from where it acts, modifying protein interaction making them weaker so binding is lost) 

  • Disease caused by gain of function = extra activity imparted by the mutant protein (expanded polyQ creates protein conformers which are toxic and create new activity/interactions)  

• So we have lost the normal functioning Huntingtin protein and as a result of this loss of function, it has been replaced by a new protein which has gained new functions.

Pathological roles of the mtHTT protein

• Wt Huntingtin will be cleaved into protein fragments which can be degraded 

• New protein will aggregate and also have new properties like:  

  • Translocation to the nucleus where it functions in a way the wt did not  

  • Impact the mitochondrial mechanisms 

  • Generation of toxic protein inclusions which impairs the function of the neuron  

mtHTT impairment of transcription

• Impairs 75% of transcription via inhibition of expression of pol2  

• Could be directly by inhibiting TFs that control expression of pol 2 or could be via histone modifications 

• A lot of the promoter, transcriptional regulation of the expression of the polymerase is affected by mtHTT which can travel to the nucleus and impact transcription  

• Usually the TFII130 TBP complex can bind to promoter sections but the binding of mtHTT to these proteins will impair their activation so the polymerase will not be expressed.  

• It can also bind to other complexes 

• wtHTT is known to bind to REST which is an inhibitor of transcription of some genes like BDNF and RNA pol2 —> however, the mutated protein cannot complete this function and results in the supressed expression of these genes 

• Also more generally affecting transcription and epigenetic regulation - mutant HTT blocks DNA acetylation 

• However, the neuron does have some compensatory mechanisms to survive despite the significant suppression of tranciption  

Effect on mt function

• Covers a range of functions covered by the mt:  

• Neurons expressing mutant HTT will have:  

  • increased sensititivity to Ca2+ inducing pore opening and release of CytC  

  • Reduced membrane potential  

  • Decreased Ca2+ buffering capacity 

  • Increase in ROS production  

• By maintaining membrane potential is the basis of generating energy in the mt so lots of these greatly impact the function of the mt.  

• E.g: calcium is essential for molecular signaling and altered Ca2+ levele sin the cytoplasm will disrupt these pathways 

• Increase in ROS is toxic to the neurons 

Loss of function driving pathology

• Normal Huntingtin blocks procaspase 9 (inhibition)  

• Procaspase 9 when uninhibited becomes much more available for the engagement of the apoptitic pathway —> still requires this trigger but not as tightly controlled 

Degeneration by dying back

• Dying back is the concept that the first element of a neuron that is affected is the distal element (E.g synaptic contact) which is trying to buffer the pathology from reaching the cell body 

• When the synapse is lost there is the retraction of the axon which leads to changes in the body of the neuron that leads to the death of the neuron  

• Synaptic degeneration before the death of the neuron 

• Areas which are affected by the disease are often not those where the pathology originates due to the loss of these synapses 

• Conserved mechanism across lots of neurodegenerative disorders

Why do MSN neurons selectively die

• Activity related specificity of MSNs: 

  •  high level of glutamate dependent activity  

  • Selective expression of neuropeptides compared to other neurons 

  • High metabolic rate and high firing rate  

  • Long axon projections 

• These neurons particularly susceptible to suffer from this pathology 

Other neurons affected

• Even though the mtHTT is expressed in all neurons, there’s no selection based off the protein present 

• Its more to do with how the neuron can deal with the metabolic/cellular processes being affected which confers selective vulnerability 

Symptomatic treatments

• No approved disease modifying treatment 

• Involves helping the patient with the symptoms but not intervening with the disease pathology 

• usually targets specific dysfunctions —> E.g: manifestation of psychosis or irritability, or depression/anxiety related symptoms 

• Very specific for the patient 

• Lots of the symptoms can be reasonably managed with these treatments —> depends on what is most important to them  

Drugs for treatment

Lots of these drugs are repurposed drugs, initially used for other things 

Therepeutic strategies

• Try to enhance the removal of HTT 

• Treatments that help with inflammation (due to activation of glial cells)  

• Direct lowering of mutant HTT at point of expression (gene therapy)  

Gene silencing

• Treatment with most potential  

• As a patient you carry mutated form of given gene —> we know what mutation is, what the sequence is  

• Can design an antisense oligonucleotide which bind to the expressed mRNA and lead to degradation  

• Requires you to be very precise at which copy you want to target —> still want the normal allele 

• Want to design them in a way that leads to effective degradation  

Timeline of gene silencing therapy

• UK leads these trials, some of the timeline of these trials: 

• Gene therapy became available in the 2000s 

• First clinical trial in 2017 —> demonstrated that in a small no. of patients they can reduce the expression of mtHTT  

• Proved to be safe, so encouraged to phase 3 trial in 2019  

• By 2021 had to halt the trial due to safety issues 

• Back to phase 1 and 2 trials in 2025 —> involves surgical delivery  to the specific affected nuclei via a viral vector 

• Currently in submission to FDA approval  

• Still needs to consider whether there are specifcities within this 12 groups which make them ideal candidates. Also the issue that this is an expensive therapy to roll out for a number of patients  

• Potential to eliminate this within the next 3 of 4 generations