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:

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:

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

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

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