Clinical Research in Huntington's Disease

Question 1

Huntington’s Disease

Answer 1

Commonest genetic dementia -> movement, cognitive and neuropsychiatric symptoms

Autosomal dominant, affecting 1 in 7,800

Adult HD typically causes chorea whereas juvenile HD causes more Parkinson’s-like movements

Question 2

Cause of Huntington’s Disease

Answer 2

Repeat CAG expansion in HTT gene causes mutated huntingtin protein (expressed in all tissues)

> 39 repeats = HD
36-39 = reduced penetrance
29-35 = not pathogenic to carrier, but can expand to pathogenic forms through paternal line

The more repeats, the earlier the age of onset

Question 3

Pathology of HD

Answer 3

Initial selective neurodegeneration in the MSNs in the striatum

Then generalised brain atrophy and loss of functional and structural connectivity between cortex and striatum

Question 4

HD as a Proof of Concept

Answer 4

HD has a single genetic cause but shares features with other, more common neurodegenerative conditions which makes it a good model for development

Involves selective region-specific neuronal toxicity, protein misfolding and aggregation, cellular pathogenesis and early loss of brain connectivity

Question 5

Biomarkers for HD

Answer 5

Needed to identify disease stage, disease progression and target engagement (by a drug/molecule)

Question 6

CSF Mutant Huntingtin in HD

Answer 6

Is detectable in people with HD (much higher in symptomatic patients) and mutation carriers but not controls

Can predict phenotype of disease

Question 7

NfL in HD

Answer 7

NfL is released by dying or dysfunctioning neurons and axons - it increases in all neurodegenerative conditions

Plasma NfL levels significantly increase at every disease stage (compared to control)

Higher and earlier increases in plasma NfL were associated with higher CAG lengths

Baseline plasma NfL can predict cognitive and functional decline and brain atrophy rates = prognostic biomarker to track and predict clinical progression

Question 8

HTT-Lowering Therapies

Answer 8

Main aim of current therapies in development

Overwhelming preclinical evidence that reducing mHTT early can cure HD

Methods include allele ASOs, RNAi, zinc finger proteins and CRISPR-Cas9

Question 9

Delivery of HD Therapeutics

Answer 9

Biggest challenge is delivery and distribution of therapies to the correct part of the brain
-> intrathecal only reaches cortex
-> gene therapy only reaches area injected
-> small molecules can reach whole brain but must cross BBB
-> ommaya reservoir delivers to intraventricular space

Question 10

ASO Therapy in HD

Answer 10

Single strands of chemical modified DNA hybridise with huntingtin mRNA resulting in RNA degradation in the nucleus = less mutant protein made
BUT non-allele specific

ASOs are diffusible, dose-dependent, stable and reversible BUT must be given intrathecally as cannot cross BBB

Showed target engagement, efficacy and safety in phase I study -> escalated to phase III but terminated early as high dose group were worse than placebo

Question 11

Considerations with ASO

Answer 11

• Doses were too high and given to patients too late in the disease
• Sub-group analyses showed some benefit to younger patients earlier in the disease progression
• Use the lowest dose and infrequently as possible as ASOs are very pro-inflammatory

Question 12

Allele-Specific ASOs

Answer 12

Only differences between WT and mHTT is length of Cag repeats

To target mHTT specifically, need to identify a SNP only on mHTT

ASOs have been designed against SNPs but these are only present in 40% of HD patients

Requires a long read phasing assay to make sure the SNP is only present on mHTT and not WT

Question 13

Gene Therapy for HD - miRNA

Answer 13

Non allele-selective single stranded DNA vector genome - targets exon 1 mRNA
Delivered in replication incompetent AAV5

Injection directly to caudate and putamen -> takes many hours

Results in a reduction in both mHTT and wtHTT protein (could cause issues if wtHTT is lowered too much, particularly in embryonic development)

Question 14

Ideal HD Therapy

Answer 14

• Allele selective
• Target exon 1 and mHTT toxicity
• Target DNA or somatic CAG expansion
• Wide distribution in the brain
• Acceptable side effect profile
• Given as early as possible

It is likely multiple approaches will be required

• Aim is to do clinical trial in HD gene carriers before they are symptomatic to prevent the disease from ever occurring