In 2024, Biogen announced that its antisense oligonucleotide drug Tofersen met the primary endpoint in a Phase III clinical trial for SOD1-mutated amyotrophic lateral sclerosis (ALS). This achievement, published in the New England Journal of Medicine, marks the entry of SOD1-targeted therapeutic strategies into a new clinical stage, demonstrating that reducing mutant SOD1 protein levels via intrathecal administration can significantly delay disease progression.
 

SOD1 gene mutation is a major causative factor in familial amyotrophic lateral sclerosis, with recent breakthroughs in mechanism research and therapeutic strategies. Let's explore the latest developments of this classic target!
 

SOD1 is a highly conserved homodimeric metalloenzyme encoded by five exons and containing 153 amino acids. Its active center relies on copper ions (Cu²⁺) and zinc ions (Zn²⁺) to maintain structure, with the classical function of catalyzing the disproportionation of superoxide anions (O₂•⁻) to generate hydrogen peroxide (H₂O₂) and molecular oxygen (O₂), constituting the first line of defense in the cellular antioxidant system. SOD1 is primarily localized in the cytoplasm, but is also distributed in the mitochondrial intermembrane space, nucleus, and lysosomes.
 

I) Metabolic Regulation and Energy Balance: Key Regulator of Cellular Metabolic Reprogramming
 

Beyond its classical antioxidant role, SOD1 participates in cellular metabolic reprogramming. SOD1 inhibits mitochondrial electron transport chain (ETC) activity to reduce cellular oxygen consumption rate and regulates substrate phosphorylation status via casein kinase signaling pathways, thereby shifting energy metabolism from oxidative phosphorylation to glycolysis. Simultaneously, SOD1 serves as an intracellular signaling integration node, converting changes in oxygen partial pressure and glucose concentration into fluctuations in mitochondrial superoxide levels, generating hydrogen peroxide as a second messenger through catalysis, which in turn regulates the activity of key metabolic regulators such as HIF-1 (hypoxia-inducible factor-1) and AMPK (AMP-activated protein kinase), guiding cellular metabolic adaptation.
 

II) Familial ALS: Multiple Toxic Cascades Driven by SOD1 Mutations
 

In familial amyotrophic lateral sclerosis (Familial ALS), SOD1 gene mutation is one of the primary causative factors. To date, over 155 mutations have been identified, accounting for approximately 20% of familial ALS cases. In ALS pathology, mutant SOD1 protein drives motor neuron degeneration through dual mechanisms of "toxic gain-of-function" and "loss-of-function."

 

Model of the vicious cycle of "loss-of-function" and "toxic gain-of-function" of SOD1 in ALS. Figure source: Is SOD1 loss of function involved in amyotrophic lateral sclerosis?
 

Gene mutations disrupt SOD1 conformational stability, leading to β-barrel structure unfolding and formation of amyloid fibrillar aggregates, causing loss of antioxidant function in catalyzing superoxide anion disproportionation, and abnormal accumulation in the mitochondrial intermembrane space. Within mitochondria, mutant proteins abnormally interact with the voltage-dependent anion channel (VDAC1), inducing mitochondrial permeability transition pore (mPTP) opening, leading to membrane potential collapse and cytochrome c release; simultaneously, unfolded protein accumulation in the endoplasmic reticulum activates the unfolded protein response (UPR), triggering endoplasmic reticulum stress-induced apoptosis. Loss of enzymatic activity and protein aggregation further impair glutamate transporter GLT1 function, causing elevated synaptic glutamate concentration and NMDA receptor-mediated calcium influx overload, and disrupting dynein-mediated retrograde axonal transport. Additionally, abnormal aggregates activate the NLRP3 inflammasome in microglia, promoting IL-1β release, while activated astrocytes secrete toxic factors such as TNF-α and NO, collectively forming a neuroinflammatory microenvironment that ultimately leads to progressive motor neuron death.
 

Gene Therapy
 

Antisense Oligonucleotide (ASO) Technology: ASOs are short-chain synthetic DNA molecules that can bind to specific mRNAs, reducing target mRNA levels through RNase H-mediated degradation mechanisms. In ALS treatment, ASOs are designed to target mutant SOD1 mRNA, reducing toxic protein production. For example, Tofersen has been approved for treating SOD1-mutated ALS patients, reducing toxic protein via intrathecal injection. Jacifusen (targeting FUS mutation) has also shown some efficacy in clinical trials.

 

Schematic diagram of antisense oligonucleotides and other RNA-targeted therapies. Figure source: CellPress
 

Gene Editing Therapy: Gene editing technology can target and excise mutant SOD1 genes. Both in vitro experiments and animal model studies have shown that this technology can effectively reduce mutant SOD1 expression with minimal impact on wild-type SOD1, demonstrating therapeutic potential.
 

Mouse Models
 

SOD1−/− Mice: Exhibit progressive muscle atrophy and motor dysfunction, with significant motor neuron loss at 12-16 months of age. This model is primarily used to study oxidative stress damage mechanisms caused by SOD1 loss-of-function, particularly in age-related neurodegenerative changes and hepatocellular carcinoma development.
 

SOD1G93A Mice: The standard model for studying familial ALS, developing hindlimb paralysis symptoms at 3-4 months of age with an average lifespan of 4-5 months. This model recapitulates major pathological features of human ALS, including motor neuron degeneration, mitochondrial dysfunction, and neuroinflammatory responses, and is widely used in preclinical evaluation of therapeutic drugs.
 

SOD1 Humanized Mice: Replace the endogenous mouse Sod1 gene with the human SOD1 gene, normally expressing human SOD1 protein with full enzymatic activity, and do not exhibit significant motor dysfunction or neurodegenerative pathology. Primarily used to evaluate the in vivo efficacy and safety of gene therapy strategies targeting human SOD1.

References:
 

1.Saccon RA, Bunton-Stasyshyn RK, Fisher EM, Fratta P. Is SOD1 loss of function involved in amyotrophic lateral sclerosis? Brain. 2013 Aug;136(Pt 8):2342-58. doi: 10.1093/brain/awt097. Epub 2013 May 17. PMID: 23687121; PMCID: PMC3722346.

2.Papa L, Manfredi G, Germain D. SOD1, an unexpected novel target for cancer therapy. Genes Cancer. 2014 Apr;5(1-2):15-21. doi: 10.18632/genesandcancer.4. PMID: 24955214; PMCID: PMC4063254.

3.Kim G, Gautier O, Tassoni-Tsuchida E, Ma XR, Gitler AD. ALS Genetics: Gains, Losses, and Implications for Future Therapies. Neuron. 2020 Dec 9;108(5):822-842. doi: 10.1016/j.neuron.2020.08.022. Epub 2020 Sep 14. PMID: 32931756; PMCID: PMC7736125.

4.Bonafede R, Mariotti R. ALS Pathogenesis and Therapeutic Approaches: The Role of Mesenchymal Stem Cells and Extracellular Vesicles. Front Cell Neurosci. 2017 Mar 21;11:80. doi: 10.3389/fncel.2017.00080. PMID: 28377696; PMCID: PMC5359305.