Mechanism: C9orf72 Hexanucleotide Repeat Expansion in ALS/FTD

Overview

This experiment investigates the pathogenic mechanisms of the C9orf72 hexanucleotide repeat expansion, the most common genetic cause of both ALS and FTD. Understanding why this single mutation produces either ALS, FTD, or ALS/FTD will reveal disease mechanisms and therapeutic targets.

Related: [C9orf72/FTD Phenotype Divergence Gap](/gaps/c9orf72-els-ftd-phenotype-divergence) | [ALS Cure Roadmap](/therapeutics/als-cure-roadmap) | [TDP-43 Pathology](/gaps/progranulin-tdp43-ftd)

Hypothesis

The C9orf72 expansion causes disease through three parallel mechanisms:

Loss of function: Reduced C9orf72 protein disrupts autophagy/lysosomal function

RNA toxicity: Expanded repeat RNA sequesters RNA-binding proteins

Dipeptide repeat proteins (DPRs): Translation of toxic poly-GA, poly-GP, poly-PR aggregates

The phenotype (ALS vs FTD vs ALS/FTD) is determined by the relative contribution of these mechanisms and neuronal vulnerability.

Experimental Design

Cohort

• N=300: Carriers of C9orf72 expansion from:
• ALS patients (n=100)
• FTD patients (n=100)
• Asymptomatic carriers (n=50)
• Non-carrier controls (n=50)

Primary Endpoints

| Endpoint | Measurement | Rationale |
|----------|-------------|-----------|
| Repeat size | Southern blot, Amplicon size | Correlation with phenotype |
| DPR burden | CSF poly-GA, poly-GP ELISA | Direct toxicity measurement |
| Autophagy function | LC3 flux, p62 turnover | Loss of function effect |
| RNA foci | smFISH in patient tissue | RNA toxicity burden |

Study Arms

...

Overview

This experiment investigates the pathogenic mechanisms of the C9orf72 hexanucleotide repeat expansion, the most common genetic cause of both ALS and FTD. Understanding why this single mutation produces either ALS, FTD, or ALS/FTD will reveal disease mechanisms and therapeutic targets.

Related: [C9orf72/FTD Phenotype Divergence Gap](/gaps/c9orf72-els-ftd-phenotype-divergence) | [ALS Cure Roadmap](/therapeutics/als-cure-roadmap) | [TDP-43 Pathology](/gaps/progranulin-tdp43-ftd)

Hypothesis

The C9orf72 expansion causes disease through three parallel mechanisms:

Loss of function: Reduced C9orf72 protein disrupts autophagy/lysosomal function

RNA toxicity: Expanded repeat RNA sequesters RNA-binding proteins

Dipeptide repeat proteins (DPRs): Translation of toxic poly-GA, poly-GP, poly-PR aggregates

The phenotype (ALS vs FTD vs ALS/FTD) is determined by the relative contribution of these mechanisms and neuronal vulnerability.

Experimental Design

Cohort

• N=300: Carriers of C9orf72 expansion from:
• ALS patients (n=100)
• FTD patients (n=100)
• Asymptomatic carriers (n=50)
• Non-carrier controls (n=50)

Primary Endpoints

| Endpoint | Measurement | Rationale |
|----------|-------------|-----------|
| Repeat size | Southern blot, Amplicon size | Correlation with phenotype |
| DPR burden | CSF poly-GA, poly-GP ELISA | Direct toxicity measurement |
| Autophagy function | LC3 flux, p62 turnover | Loss of function effect |
| RNA foci | smFISH in patient tissue | RNA toxicity burden |

Study Arms

Phenotype comparison: ALS vs FTD vs ALS/FTD carriers

Progression analysis: Asymptomatic → symptomatic conversion

iPSC neurons: Generate from carriers with different phenotypes

Validation Protocol

Phase 1: Clinical Cohort Analysis (Month 1-12)

• Collect CSF from all 300 participants
• Measure DPR levels, NfL, biomarkers
• Correlate with repeat size, phenotype

Phase 2: Mechanistic Studies (Month 12-24)

• iPSC neurons from carriers with different phenotypes
• Test whether reducing DPR production improves function
• Assess autophagy modulation

Phase 3: Therapeutic Target Validation (Month 24-36)

• Test antisense oligonucleotides in patient-derived neurons
• Validate biomarkers for clinical trials
• Develop genotype-phenotype prediction model

Expected Outcomes

Mechanistic insight: Determine relative contribution of three mechanisms to disease

Phenotype predictors: Biomarkers distinguishing ALS vs FTD risk

Therapeutic targets: Validate ASO approach for C9orf72 carriers

Biomarkers: DPR levels as pharmacodynamic marker

Model Systems

| System | Use | Strength |
|--------|-----|----------|
| Human CSF/tissue | Primary analysis | Direct measurement of disease mechanisms |
| iPSC neurons | Mechanism validation | Patient-specific, functional |
| Mouse models (C9 BAC) | In vivo validation | Established model |

Feasibility Assessment

• Data availability: Multiple cohorts (GENESIS, CADRE, ALSA) have C9orf72 carriers
• Cost: Medium — CSF assays and iPSC ($800K)
• Timeline: 36 months for complete analysis

Risk Analysis

| Risk | Mitigation |
|------|------------|
| Repeat size variability | Use multiple measurement techniques |
| Phenotype heterogeneity | Large cohort, stratified analysis |
| iPSC differentiation | Use established protocols |

Cross-Links

• [Autophagy Enhancement Drug Screening](/experiments/autophagy-enhancement-drug-screening)](/experiments)
• [ALS Immune Signature Stratification](/gaps/als-immune-signature-stratification)
• [TDP-43 Pathology Mechanisms](/gaps/progranulin-tdp43-ftd)

Scoring

| Dimension | Score | Rationale |
|-----------|-------|-----------|
| Scientific Value | 10 | Most common genetic cause of ALS/FTD |
| Feasibility | 8 | Established cohorts available |
| Novelty | 8 | Phenotype prediction is unresolved |
| Disease Impact | 10 | Enables precision medicine approach |
| Cure Proximity | 9 | Direct therapeutic implications (ASO) |
| Total | 45/50 | |

References

[Miller et al., Tofersen for SOD1 ALS (2022)](https://pubmed.ncbi.nlm.nih.gov/35709026/)

[Chio et al., ALS disease-modifying therapies (2020)](https://pubmed.ncbi.nlm.nih.gov/32877554/)

[Ling et al., Converging mechanisms in ALS and FTD (2013)](https://pubmed.ncbi.nlm.nih.gov/24315312/)

[Ravits et al., ALS motor phenotype heterogeneity (2009)](https://pubmed.ncbi.nlm.nih.gov/19575058/)