The C9orf72 gene hexanucleotide repeat expansion (GGGGCC) is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), accounting for approximately 40% of familial ALS and 25% of familial FTD cases. This causal chain page traces the molecular pathway from the repeat expansion through multiple toxic mechanisms to clinical disease manifestation.
flowchart TD
subgraph Genetic["Genetic Defect"]
Expansion["G4C2 Repeat<br/>Expansion"]
Repeat1000["100-1000+<br/>Repeats"]
Methylation["DNA/RNA<br/>Methylation"]
end
subgraph Toxicity1["RNA-Mediated Toxicity"]
RNAFoci["RNA Foci<br/>Formation"]
Splicing["Alternative<br/>Splicing Defects"]
RBPseq["RBP<br/>Sequestration"]
end
subgraph Toxicity2["Dipeptide Repeat Proteins"]
DPR["DPR Proteins<br/>(GP, GA, GR, PR, PA)"]
Nuclear["Nuclear<br/>Dysfunction"]
Translation["RAN<br/>Translation"]
end
subgraph Toxicity3["Nucleocytoplasmic Transport"]
NCT["Nucleocytoplasmic<br/>Transport Defect"]
Pore["Nuclear Pore<br/>Dysfunction"]
Import["Importin<br/>Dysfunction"]
end
subgraph Disease["Disease Outcome"]
Motor["Motor Neuron<br/>Degeneration"]
Frontotemporal["FTD<br/>Phenotype"]
ALS["ALS<br/>Phenotype"]
end
Expansion --> Repeat1000
Repeat1000 --> Methylation
Repeat1000 --> RNAFoci
RNAFoci --> Splicing
RNAFoci --> RBPseq
Repeat1000 --> DPR
DPR --> Nuclear
DPR --> NCT
NCT --> Pore
NCT --> Import
Import --> Motor
Import --> Frontotemporal
Splicing --> Motor
Splicing --> Frontotemporal
RBPseq --> Motor
Motor --> ALS
Frontotemporal --> FTD
style Expansion fill:#ffcdd2,stroke:#c62828
style DPR fill:#fff3e0,stroke:#e65100
style Disease fill:#ffcdd2,stroke:#c62828
¶ Discovery and Epidemiology
The C9orf72 hexanucleotide repeat expansion was simultaneously discovered by two groups in 2011, representing one of the most significant genetic findings in ALS/FTD research 1 2:
| Parameter |
Value |
| Normal repeat range |
2-30 |
| Pathogenic threshold |
>30-40 (penetrant) |
| Typical affected range |
100-1000+ repeats |
| Maximum observed |
>10,000 repeats |
Population frequencies:
- ~1 in 400 individuals carry the expansion (unknown if all develop disease)
- 40% of familial ALS cases
- 25% of familial FTD cases
- 5-10% of sporadic ALS cases
- ~6% of sporadic FTD cases
The C9orf72 expansion shows:
- Autosomal dominant inheritance with high but incomplete penetrance
- Anticipation: earlier onset in successive generations (controversial)
- Founder effect: common ancestor in many populations
- Variable phenotypic expression: ALS-only, FTD-only, or ALS/FTD combination
The expanded repeat is transcribed into abnormal RNA that forms nuclear foci, sequestering RNA-binding proteins (RBPs):
flowchart LR
subgraph Transcription
DNA["G4C2 Repeat<br/>DNA"]
PreRNA["Pre-mRNA<br/>G4C2 Expansion"]
end
subgraph Foci_Formation
Hairpin["Hairpin<br/>Structure"]
RBP1["TDP-43"]
RBP2["hnRNPA1"]
RBP3["SRSF2"]
RBP4["Pur-α"]
end
subgraph Sequestration
Loss["RBP<br/>Sequestration"]
Gain["Foci<br/>Gain-of-Function"]
end
PreRNA --> Hairpin
Hairpin --> RBP1
Hairpin --> RBP2
Hairpin --> RBP3
Hairpin --> RBP4
RBP1 --> Loss
RBP2 --> Loss
RBP3 --> Loss
RBP4 --> Loss
RBP1 --> Gain
style Transcription fill:#e3f2fd
style Foci_Formation fill:#fff3e0
style Sequestration fill:#ffebee
Sequestered proteins include:
Consequences:
The expansion can be translated in all six reading frames via Repeat-Associated Non-ATG (RAN) translation, producing five dipeptide repeat proteins:
| DPR Type |
Reading Frame |
Toxicity |
Localization |
| Poly-GA (Gly-Ala) |
Sense |
High |
Cytoplasmic |
| Poly-GR (Gly-Arg) |
Sense |
High |
Nuclear |
| Poly-PR (Pro-Arg) |
Sense |
Very High |
Nuclear |
| Poly-PA (Pro-Ala) |
Antisense |
Moderate |
Cytoplasmic |
| Poly-GP (Gly-Pro) |
Antisense |
Moderate |
Cytoplasmic |
The poly-GR and poly-PR proteins are particularly toxic, causing:
- Nucleolar stress
- Ribosome biogenesis defects
- Translation dysregulation
- Stress granule formation
flowchart TD
subgraph RAN_Translation
mRNA["G4C2 mRNA"]
Frame1["+1 Frame: GA"]
Frame2["+2 Frame: GR"]
Frame3["+3 Frame: PR"]
end
subgraph DPR_Toxicity
Nucleolus["Nucleolar<br/>Stress"]
Ribosome["Ribosome<br/>Biogenesis"]
Transport["NCT<br/>Defect"]
Stress["Stress<br/>Granules"]
end
subgraph Cellular_Effects
Dysfunction["Neuronal<br/>Dysfunction"]
Death["Neuronal<br/>Death"]
end
mRNA --> Frame1
mRNA --> Frame2
mRNA --> Frame3
Frame1 --> Transport
Frame2 --> Nucleolus
Frame2 --> Ribosome
Frame3 --> Nucleolus
Frame3 --> Stress
Nucleolus --> Dysfunction
Ribosome --> Dysfunction
Transport --> Dysfunction
Stress --> Death
style RAN_Translation fill:#e8f5e9
style DPR_Toxicity fill:#fff3e0
style Cellular_Effects fill:#ffebee
A landmark 2016 study demonstrated that C9orf72 dipeptide repeats directly disrupt nucleocytoplasmic transport (NCT) 3:
flowchart LR
subgraph DPR_Effect
DPR["DPR Proteins"]
Nup["Nuclear Pore<br/>Complex Proteins"]
Importin["Importin<br/>Complex"]
Exportin["Exportin<br/>Complex"]
end
subgraph Transport_Defect
Import["Nuclear Import<br/>↓"]
Export["Nuclear Export<br/>↓"]
Leak["Nuclear<br/>Leakiness"]
end
subgraph Cellular_Consequence
TDP43["TDP-43<br/>Mislocalization"]
Transcript["Transcriptional<br/>Dysregulation"]
Translation["Translation<br/>Defects"]
end
DPR --> Nup
Nup --> Importin
Nup --> Exportin
Importin --> Import
Exportin --> Export
Import --> Leak
Import --> TDP43
Export --> Transcript
Leak --> Translation
style DPR_Effect fill:#ffcdd2
style Transport_Defect fill:#fff3e0
style Cellular_Consequence fill:#fce4ec
Nuclear pore complex (NPC) components affected:
- Nup107, Nup133 (core scaffold proteins)
- Nup98 (transport receptor binding)
- Nup153 (basket component)
- Importin α/β family
Functional consequences:
- Impaired TDP-43 import (contributing to cytoplasmic TDP-43 aggregates)
- Reduced mRNA export from nucleus
- Disrupted protein quality control
- Enhanced vulnerability to cellular stress
The expansion also reduces C9orf72 protein expression through multiple mechanisms:
- Promoter methylation reducing transcription
- RNA foci sequestering transcripts
- Repeat-associated nonsense-mediated decay
C9orf72 is involved in:
- Autophagosome formation
- Lysosomal function
- Endosomal trafficking
Loss of function contributes to:
- Impaired autophagy-lysosome pathway
- Decreased protein clearance
- Enhanced accumulation of toxic proteins
C9orf72-associated ALS is characterized by:
- Typical onset: 50-60 years
- Site of onset: Limb (most common), bulbar
- Disease duration: 2-5 years (mean ~3 years)
- Cognitive involvement: 30-40% develop FTD
Motor neuron involvement:
- Upper motor neuron signs prominent
- Combined UMN/LMN phenotype
- Rapid progression compared to sporadic ALS
C9orf72-associated FTD often presents as:
- Subtype: Behavioral variant FTD (bvFTD) most common
- Speech: Progressive aphasia variants possible
- Motor features: Some develop ALS
Behavioral features:
- Early disinhibition
- Apathy
- Loss of empathy
- Food preference changes
The C9orf72 expansion causes a continuous spectrum:
- Pure ALS (~40%)
- Pure FTD (~20%)
- ALS+FTD (~40%)
| Strategy |
Approach |
Status |
Notes |
| Antisense oligonucleotides |
Silence C9orf72 expression |
Preclinical |
Reduces DPRs, improves phenotype |
| Small molecule transport |
Restore NCT function |
Preclinical |
Targeting nuclear pore |
| DPR antibodies |
Anti-GA immunotherapies |
Preclinical |
Remove toxic proteins |
| Gene therapy |
AAV-C9orf72 |
Preclinical |
Restore function |
| Biomarker |
Change in C9orf72 Cases |
Utility |
| CSF DPR (poly-GA) |
Elevated |
Diagnostic, tracking |
| CSF NfL |
Elevated |
Progression marker |
| MRI |
Frontotemporal atrophy |
Early detection |
| PET |
Variable |
Under investigation |
- ASOs targeting C9orf72 in development (Wave, Ionis, others)
- Focus on reducing both RNA foci and DPR production
- Challenges: delivery to CNS, off-target effects
¶ Stress Granule Dynamics and Cellular Stress Response
Stress granules (SGs) are membrane-less organelles that form in response to cellular stress, serving as transient repositories for translationally stalled mRNAs and associated proteins. In C9orf72-associated ALS/FTD, stress granule biology is profoundly disrupted through multiple mechanisms:
Key stress granule proteins affected in C9orf72 ALS:
- G3BP1/2 - primary SG nucleators
- TIA1 (T-cell-restricted intracellular antigen-1) - SG structural component
- TDP-43 - abnormally recruited to SGs and sequestered
- FUS - another RNA-binding protein with SG localization
flowchart TD
subgraph Stress_Trigger
Oxidative["Oxidative Stress"]
Heat["Heat Shock"]
ER["ER Stress"]
Viral["Viral Infection"]
end
subgraph SG_Formation
eIF2a["eIF2α<br/>Phosphorylation"]
mRNA["Stalled<br/>mRNAs"]
G3BP["G3BP1/2<br/>Nucleation"]
TIA1["TIA1<br/>Recruitment"]
end
subgraph SG_Dysfunction
Persistent["Persistent<br/>SGs"]
Fusion["Abnormal<br/>Fusion"]
Clearance["Impaired<br/>Clearance"]
end
subgraph Neurodegeneration
Translation["Global Translation<br/>↓"]
Aggregate["TDP-43<br/>Aggregation"]
Death["Neuronal<br/>Death"]
end
Stress_Trigger --> eIF2a
eIF2a --> mRNA
mRNA --> G3BP
G3BP --> TIA1
TIA1 --> Persistent
Persistent --> Translation
Translation --> Aggregate
Aggregate --> Death
Style Stress_Trigger fill:#e3f2fd
Style SG_Formation fill:#e8f5e9
Style SG_Dysfunction fill:#fff3e0
Style Neurodegeneration fill:#ffcdd2
Key stress granule proteins affected in C9orf72 ALS:
- G3BP1/2 (Ras-GAP SH3 domain-binding proteins) - primary SG nucleators
- TIA1 (T-cell-restricted intracellular antigen-1) - SG structural component
- TDP-43 - abnormally recruited to SGs and sequestered
- FUS - another RNA-binding protein with SG localization
C9orf72 protein localizes to stress granules through its interaction with the autophagy adaptor proteins p62/SQSTM1 and OPTN. Loss of C9orf72 function disrupts this regulation:
- Altered SG dynamics: C9orf72-deficient cells show prolonged SG persistence
- Impaired SG clearance: Autophagy-mediated SG dissolution is compromised
- Aberrant SG fusion: Abnormal coalescence of multiple SGs
- Sequestration of essential proteins: Critical translational machinery trapped
Stress granule-modulating strategies are actively being explored:
- eIF2α dephosphorylation to restore translation
- Small molecules targeting SG assembly/disassembly
- Autophagy enhancers to improve SG clearance
- Combination approaches addressing both SG dysfunction and DPR toxicity
¶ Epigenetic Modifications and Biomarkers
The C9orf72 repeat expansion is associated with profound epigenetic alterations, particularly DNA methylation at the repeat region 3:
flowchart LR
subgraph Repeat_Expansion
Repeat["G4C2 Repeat<br/>(100-1000+)"]
end
subgraph Epigenetic_Changes
Methyl["CpG Methylation"]
Histone["Histone<br/>Modifications"]
Chromatin["Chromatin<br/>Remodeling"]
end
subgraph Functional_Consequences
Silencing["Gene<br/>Silencing"]
Expression["C9orf72<br/>Expression ↓"]
end
subgraph Biomarkers
Blood["Blood DNA<br/>Methylation"]
CSF["CSF DPR<br/>Levels"]
Clinical["Clinical<br/>Phenotype"]
end
Repeat --> Methyl
Methyl --> Silencing
Silencing --> Expression
Expression --> Blood
Expression --> CSF
Expression --> Clinical
Style Repeat_Expansion fill:#ffcdd2
Style Epigenetic_Changes fill:#fff3e0
Style Functional_Consequences fill:#e8f5e9
Style Biomarkers fill:#e3f2fd
Key epigenetic findings:
- Hypermethylation of the repeat expansion correlates with reduced C9orf72 expression
- Methylation levels vary with repeat size - larger repeats show more methylation
- Blood-based methylation testing can serve as a surrogate for CNS methylation status
- Methylation patterns may predict clinical phenotype (ALS vs. FTD vs. combined)
Recent studies have identified several promising CSF biomarkers for C9orf72-associated disease 4:
| Biomarker |
Change in C9orf72 ALS/FTD |
Clinical Utility |
| Poly-GA DPR |
Markedly elevated |
Diagnostic, disease progression |
| Poly-GR DPR |
Elevated |
Toxicity marker |
| Neurofilament light chain (NfL) |
Elevated |
Progression, survival |
| Phosphorylated tau (p-tau) |
Variable |
Differentiation from AD |
| Total tau (t-tau) |
Elevated |
Neuronal injury |
| YKL-40 |
Elevated |
Neuroinflammation |
¶ Repeat Size and Phenotype Correlation
The hexanucleotide repeat size correlates with clinical presentation 5:
- Very large repeats (>500): Earlier onset, more severe phenotype
- Intermediate repeats (60-500): Variable presentation
- Small expanded repeats (30-60): Later onset, possibly incomplete penetrance
Somatic mosaicism - different tissues carry different repeat sizes - may explain phenotypic variability.
¶ iPSC Models and Mechanistic Insights
Patient-derived iPSC models have revolutionized understanding of C9orf72 pathobiology 6:
flowchart TD
subgraph iPSC_Generation
Patient["Patient<br/>Fibroblasts"]
Reprogram["Yamanaka<br/>Factors"]
iPSC["iPSC<br/>Lines"]
end
subgraph Neuronal_Differentiation
Neural["Neural<br/>Progenitors"]
Motor["Motor<br/>Neurons"]
Neurons["Cortical<br/>Neurons"]
end
subgraph Phenotypic_Analysis
RNAFoci["RNA Foci<br/>Formation"]
DPR["DPR<br/>Accumulation"]
NCT["NCT<br/>Defect"]
Function["Electrophysiology"]
end
subgraph Therapeutic_Screening
Drug["Drug<br/>Screening"]
ASO["ASO<br/>Testing"]
GeneEdit["Gene<br/>Editing"]
end
Patient --> Reprogram
Reprogram --> iPSC
iPSC --> Neural
Neural --> Motor
Neural --> Neurons
Motor --> RNAFoci
Motor --> DPR
DPR --> NCT
NCT --> Function
Drug --> Therapeutic_Screening
Style iPSC_Generation fill:#e3f2fd
Style Neuronal_Differentiation fill:#e8f5e9
Style Phenotypic_Analysis fill:#fff3e0
Style Therapeutic_Screening fill:#ffcdd2
Key findings from iPSC studies:
- Motor neurons show reduced survival and axonal defects
- RNA foci are present in patient-derived neurons
- DPR proteins accumulate in neurons over time
- Nucleocytoplasmic transport is impaired in neurons
- Stress granule dynamics are altered
iPSC studies have identified several novel therapeutic approaches:
| Target |
Approach |
Status |
| Ran-GAP |
Restore nucleocytoplasmic transport |
Preclinical |
| Importin α/β |
Pharmacological activation |
Discovery |
| eIF2α pathway |
Translation restoration |
Preclinical |
| Autophagy enhancers |
Improve protein clearance |
Preclinical |
| Antisense transcripts |
Block sense/antisense RNAs |
Clinical trials |
¶ Nuclear Pore Pathology and Transport Defects
Recent research has revealed specific Nuclear Pore Complex (NPC) components affected in C9orf72 ALS:
flowchart TD
subgraph NPC_Architecture
Cytoplasmic["Cytoplasmic<br/>Filaments"]
Central["Central<br/>Channel"]
Nuclear["Nuclear<br/>Basket"]
Pore["Pore<br/>Membrane"]
end
subgraph Affected_Nups
Nup107["Nup107<br/>(Core)"]
Nup133["Nup133<br/>(Core)"]
Nup98["Nup98<br/>(Transport)"]
Nup153["Nup153<br/>(Basket)"]
end
subgraph Transport_Consequences
Importin["Importin<br/>Dysfunction"]
Exportin["Exportin<br/>Dysfunction"]
Leak["Nuclear<br/>Leakiness"]
end
subgraph Pathological_Consequences
TDP43["TDP-43<br/>Mislocalization"]
mRNA["mRNA<br/>Export Defect"]
Protein["Protein<br/>Quality Control ↓"]
end
NPC_Architecture --> Affected_Nups
Affected_Nups --> Importin
Affected_Nups --> Exportin
Importin --> TDP43
Exportin --> mRNA
Importin --> Leak
Leak --> Protein
Style NPC_Architecture fill:#e3f2fd
Style Affected_Nups fill:#fff3e0
Style Transport_Consequences fill:#e8f5e9
Style Pathological_Consequences fill:#ffcdd2
Novel strategies to restore nucleocytoplasmic transport include 8:
- Small molecule enhancers of nuclear import receptors
- Ran-GAP restoration through gene therapy
- NPC stabilizing compounds to prevent DPR-induced damage
- Chaperones for nuclear import machinery
- Combination therapies targeting multiple NCT components
Recent studies reveal that DPRs cause extensive nuclear envelope remodeling 9:
- Membrane deformation by poly-GR and poly-PR
- Lamina disruption affecting nuclear structural integrity
- Blebbing of nuclear envelope
- Formation of nuclear invaginations
These structural changes may contribute to:
- Impaired nuclear import/export
- Disrupted chromatin organization
- DNA damage accumulation
- Accelerated cellular senescence
¶ Clinical Trial Landscape
Multiple pharmaceutical companies are developing ASOs targeting C9orf72 10:
| Company |
Target |
Approach |
Development Stage |
| Wave Life Sciences |
C9orf72 mRNA |
Allele-selective |
Phase 1 |
| Ionis Pharmaceuticals |
C9orf72 mRNA |
Non-selective |
Preclinical |
| Biogen |
C9orf72 mRNA |
Various approaches |
Discovery |
| Roche |
DPR production |
Translation inhibition |
Preclinical |
- Delivery to CNS: Requires effective penetration of the blood-brain barrier
- Allele selectivity: Distinguishing mutant from wild-type C9orf72
- Dosing schedule: Determining optimal administration frequency
- Biomarker development: Tracking target engagement and efficacy
- Patient selection: Identifying optimal responders based on genetic/clinical markers
Alternative therapeutic strategies focus on RNA foci 11:
- Small molecules that disperse RNA foci
- RNA-binding protein competitors to release sequestered proteins
- Small interfering RNAs targeting expanded repeat transcripts
- RNA-binding small molecules blocking DPR translation
Microglia play a crucial role in C9orf72-associated ALS/FTD:
- C9orf72 haploinsufficiency alters microglial function
- TREM2 pathway activation in expansion carriers
- Pro-inflammatory cytokine release (IL-1β, TNF-α)
- Altered phagocytic capacity
Therapeutic implications:
- TREM2 modulators: Enhance microglial clearance
- CSF1R inhibition: Reduce microglial proliferation
- Anti-inflammatory approaches: Cytokine pathway targeting
Astrocytes exhibit impaired function in C9orf72 ALS:
- Reduced glutamate uptake capacity
- Impaired trophic support to neurons
- Potential for non-cell autonomous toxicity
- DPR transfer between cell types possible
¶ Poly-GA Pathology and Propagation
Poly-GA dipeptide repeats form distinctive neuronal inclusions in C9orf72 ALS/FTD 12:
flowchart TD
subgraph GA_Production
mRNA["G4C2 mRNA"]
RAN["RAN Translation"]
GA["Poly-GA<br/>Proteins"]
end
subgraph Aggregation
Monomer["Monomers"]
Oligomer["Oligomers"]
Fibril["Fibrils"]
Inclusion["Large<br/>Inclusions"]
end
subgraph Propagation
CellToCell["Cell-to-Cell<br/>Spread"]
Tunneling["Tunneling<br/>Nanotubes"]
Exosomes["Exosome<br/>Release"]
end
subgraph Consequences
Proteostasis["Proteostasis<br/>Disruption"]
Axonal["Axonal<br/>Transport ↓"]
Synaptic["Synaptic<br/>Dysfunction"]
end
mRNA --> RAN
RAN --> GA
GA --> Monomer
Monomer --> Oligomer
Oligomer --> Fibril
Fibril --> Inclusion
Inclusion --> CellToCell
CellToCell --> Proteostasis
Proteostasis --> Axonal
Axonal --> Synaptic
Style GA_Production fill:#e3f2fd
Style Aggregation fill:#fff3e0
Style Propagation fill:#e8f5e9
Style Consequences fill:#ffcdd2
¶ Seeding and Spread
Emerging evidence suggests that poly-GA aggregates may:
- Seed aggregation in neighboring neurons
- Propagate through interconnected neural networks
- Exert toxicity both at sites of formation and remotely
The complete C9orf72 pathogenic cascade integrates all mechanisms described above:
flowchart TD
subgraph GENETIC["Genetic Foundation"]
Expansion["G4C2 Repeat<br/>Expansion"]
Methylation["DNA/RNA<br/>Methylation"]
Expression["C9orf72<br/>Expression ↓"]
end
subgraph RNA_TOXICITY["RNA-Mediated Toxicity"]
Foci["RNA Foci<br/>Formation"]
RBP["RBP<br/>Sequestration"]
Splicing["Splicing<br/>Dysregulation"]
end
subgraph DPR_TOXICITY["DPR Pathogenesis"]
RAN["RAN Translation"]
GA["Poly-GA<br/>Aggregation"]
GR["Poly-GR/PR<br/>Toxicity"]
end
subgraph TRANSPORT["Transport Defects"]
NPC["Nuclear Pore<br/>Dysfunction"]
NCT["NCT<br/>Impairment"]
SG["Stress Granule<br/>Dysfunction"]
end
subgraph CELLULAR["Cellular Consequences"]
Translation["Translation<br/>↓"]
Aggregation["TDP-43<br/>Aggregation"]
Dysfunction["Neuronal<br/>Dysfunction"]
end
subgraph DISEASE["Disease Phenotypes"]
Motor["Motor Neuron<br/>Degeneration"]
Cognitive["Cognitive/Behavioral<br/>Dysfunction"]
ALS["ALS<br/>Phenotype"]
FTD["FTD<br/>Phenotype"]
end
Expansion --> Methylation
Expansion --> Foci
Foci --> RBP
RBP --> Splicing
Expansion --> RAN
RAN --> GA
RAN --> GR
GA --> NPC
GR --> NCT
NPC --> SG
NCT --> Translation
Translation --> Aggregation
Aggregation --> Dysfunction
Dysfunction --> Motor
Dysfunction --> Cognitive
Motor --> ALS
Cognitive --> FTD
style GENETIC fill:#ffcdd2,stroke:#c62828
style RNA_TOXICITY fill:#fff3e0,stroke:#e65100
style DPR_TOXICITY fill:#fff3e0,stroke:#e65100
style TRANSPORT fill:#e8f5e9,stroke:#2e7d32
style CELLULAR fill:#e3f2fd,stroke:#1565c0
style DISEASE fill:#ffcdd2,stroke:#c62828
- Penetrance: Why don't all carriers develop disease?
- Modifier genes: What determines ALS vs. FTD phenotype?
- Therapeutic window: At what disease stage is intervention effective?
- DPR toxicity hierarchy: Which DPR is most critical to target?