Huntington 2

Goal of HD treatment

Increase during of the GABAeric Enk+ (D2) neurons in the striatum to reduce dyskinesia

Three broad symptoms treatments

• Relief of D inhibition by dopamine agonists (classic antipsychotics)

• Atypical antipsychotics- less side effects

• Tetrabenazine (TBZ) presynaptic monoamine-depleting agent

TBZ (Xenazine)

• Benzoquinolizine derivative

• Treat dyskinetic disorders

• Reversible inhibitor of vesicular monoamine transporter (VMAT-2) 

• Depletes DA stores

• Weak inhibitor of D2 receptor

TBZ side effects

• Depression

• Suicidal thoughts

• Akayhisia (restlessness) 

• Dizziness

• Sedation

• Parkinsonism (less with antipsychotics)

Deuterated tetrabenazine (Deutetrabenazine)

• Analogue of tetrabenazine 

• Hydrogen in two methods groups replaced with deuterium analogue

• Slows oxidative metabolism

• Decreased metabolism (longer-half life)

• Decreased dosage, better tolerability

GM1

• Modulator of cell signalling

• 30% of the total brain ganglioside pool

• Lots in membrane microdomains in cell signalling

• Modulated various membrane receptors and ion channels

• Makes HTT less toxic

GM1 drug

Ceramide

Gangliosides

• Lipid chaperones

• Myelin-axon interactions

• Cell adhesion

• Neuritogenesis and axon sprouting 

• Cell-cell communication

• Modulation of receptor activity 

3 diseases that come from a loss of function of gangliosides

• Infantile-onset symptomatic epilepsy syndrome

  • Progressive brain atrophy, epilepsy, motor symptoms

• Complex hereditary spastic paraplegia

  • Epilepsy, cognitive impairement, motor symptoms, deafness

• Loss of GM1 in Th+ in PD substantia nigra

How does HTT get modified with GM1

Phosphorylation of serine 13 and 16

Decrease amount of misfolded mutant HTT therefore less aggregates (soluble mutant HTT)

Motor test to test if movement is restored

Narrow beam

What aspects does GM1 treat

• Motor

• Non-motor: depression, anxiety, cognitive

• Slows down neurodegeneration 

• Reduces levels of mutant HTT

Mechanism of action of GM1

• Likely pleiotropic

• Anti-inflammatory effects

• Gangliosides increase secretion of misfolded proteins through extracellular vesicles

• Might increase their clearance by microglia 

• Packs it up for secretion by EV

Extracellular vesicles (EVs)

• Cell-cell communication

• Immune system modulation

• Elimination of misfolded proteins

Is GM1 HTT specific

No

• Tau too

Cells expressing a-syn and Tau that GM1 effects

• N2a (a-syn)

• HEK293 (tau)

Three challenges of GM1 treatment

• Mode of administration- doesn’t cross BBB

• Animal studies are done chronically infusing it into the ventricles

• Need to test intrathecal or nasal

What is the ultimate therapeutic approach

Silencing mutant huntingtin

What are the two approaches to silencing HTT

• Antisense oligonucleotide (ASO): short synthetic modified nucleic acids

• Small interfering RNA (siRNA) and microRNA

ASOs

• Traget RNA in cytoplasm and nucleus

• Can be taken up by cells in naked form

• Poor passage through BBB

• Find its target alone (single strand)

siRNA and micro RNA

• Target mRNA in cytoplasm only

• Must be delivered with viral vectors

• Poor passage though BBB

Steps of siRNA and microRNA

• Duplex associates with AGO

• Passenger strand is removed

• Guide strand. leads AGO to target

ASO modification needed

• Rapidly degraded by nucleases

• Modify phosphate backbone to increase stability

• Phosphorothioate DNA (PS): substitution of a non-bridging oxygen in the phosphate backbone with a sulfur atom

• Resistant to exo and endonuclease

• Recognition and cleavage of target sequence by RNase H preserved

Delivery of RNAi and ASO

Poor intestinal absorption and poor BBB permeability

• intraparenchimal/cerevroventricle/thecal

• Continuous or repeated administration

What clinical trials are siRNA being tested in

• Silencing of apoB in familial hypercholesterolemia

• Silencing of XIAP in acute myeloid leukemia

• Silencing of mutant SOD1 in familial ALS

Specific silencing

Don’t want to silence normal huntingtin

75% have targetable SNPs

Efficacy of ASOs

10-50%

Potential off-target effects

Hypothesis for failure of genetic trial

• Tominersen

• ASO affected levels of beneficial Huntingtin

• Intrathecal administration didn’t reach the right brain regions

Advantages of AVV-delivered micro-RNA

• Viral DNA constructs remain episomal inside the cells and keep producing miRNA

• One single injection results in miRNA production for years (maybe lifetime)

• Can spread beyond site of viral infection (part by EVs)

Current trial mentioned

AMT-130

Study limitations

• No in-trial control group 

• Very small sample size

• Target engagement (mHTT levels) not yet verified

• Procedure cost and invasiveness

• Long-term effects remain to be determined