Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive loss of motor neurons. Mutations in the SOD1 gene (Superoxide Dismutase 1) were the first genetic cause identified in familial ALS, accounting for approximately 12-20% of familial ALS cases. Over 190 SOD1 mutations have been identified, providing critical insights into ALS pathogenesis through toxic gain-of-function mechanisms.
SOD1 is a 32 kDa metalloenzyme that catalyzes the dismutation of superoxide radical (O₂⁻) to hydrogen peroxide (H₂O₂) and molecular oxygen:
2 O₂⁻ + 2 H⁺ → H₂O₂ + O₂
This reaction protects cells from oxidative damage caused by reactive oxygen species (ROS).
SOD1 consists of:
| Mutation | Location | Frequency | Phenotype |
|---|---|---|---|
| A4V | N-terminus | ~50% (US) | Aggressive, rapid progression |
| G93A | β-strand 4 | Common | Moderate progression |
| G37R | β-strand 2 | Common | Intermediate |
| H46R | Loop 4 | Asian populations | Slow progression |
| L126Z | C-terminus | Rare | Unknown |
Mutations affect:
SOD1 mutations cause disease through loss of dismutase activity-independent mechanisms:
Wild-type SOD1 → Normal dismutase → Cellular protection
↓
Mutant SOD1 → Toxic gain-of-function → Multiple pathogenic pathways
Mutant SOD1 forms toxic aggregates through multiple mechanisms:
SOD1 mutants cause mitochondrial damage through [1]:
Motor neurons have long axons requiring efficient transport:
Non-cell autonomous toxicity in ALS:
Mutant SOD1 accumulation in the ER triggers:
Motor neurons are selectively vulnerable due to:
| Factor | Contribution |
|---|---|
| Size | Protein homeostasis burden |
| Axon length | Transport dependence |
| Calcium handling | Excitotoxicity |
| Mitochondria | Energy demand |
| Neurotrophin dependency | Support dependence |
| Approach | Mechanism | Status |
|---|---|---|
| Antisense oligonucleotides | Knockdown SOD1 | Phase 3 (BIIB067) |
| CRISPR gene editing | Correct mutations | Preclinical |
| RNAi | Silence mutant expression | Preclinical |
| Target | Compound | Mechanism |
|---|---|---|
| Aggregation | Arimoclomol | Hsp90 co-inducer |
| Oxidative stress | Edaravone | ROS scavenger |
| Glutamate | Riluzole | Anti-excitotoxic |
| Mitochondria | Olesoxime | Mitochondrial stabilizer |
| Trial | Compound | Outcome |
|---|---|---|
| NCT02623699 | BIIB067 (tofersen) | Primary endpoint missed, but showed benefits |
| NCT00706147 | Arimoclomol | Failed Phase 2/3 |
| NCT01908791 | Edaravone | Approved (IV formulation) |
Nagai M et al. Astrocyte toxicity in SOD1-ALS. Neuron. 2023. ↩︎