The SOD1 gene provides instructions for making copper-zinc superoxide dismutase, an enzyme that neutralizes one of the most common toxic byproducts of cellular metabolism. Sitting on chromosome 21, this gene became famous in the early 1990s as the first gene linked to a heritable form of amyotrophic lateral sclerosis (ALS), but its biological reach extends well beyond motor neuron disease. SOD1 touches aging, muscle wasting, Down syndrome, and even cancer, and it is now the target of the first gene-specific therapy ever approved for ALS.
What the SOD1 Enzyme Actually Does
Every cell that uses oxygen generates superoxide, a reactive molecule that can damage DNA, proteins, and the fatty membranes surrounding cells. The SOD1 enzyme converts superoxide into molecular oxygen and hydrogen peroxide through a rapid chemical reaction that cycles a copper atom between two states.1PubMed. SOD1, more than just an antioxidant Hydrogen peroxide is itself reactive, but other enzymes like catalase and glutathione peroxidase quickly break it down into water. The net result is a defense system that keeps superoxide from accumulating to dangerous levels inside the cell.
SOD1 lives mostly in the cytoplasm, the gel-like interior of the cell, though smaller amounts end up inside mitochondria and in the space between cells. The body also has two other superoxide dismutases: SOD2, which sits inside mitochondria and uses manganese instead of copper, and SOD3, which operates outside cells. SOD1 handles the bulk of superoxide cleanup in most tissues, and its loss has consequences that the other two cannot fully compensate for.
How the Protein Holds Together
The SOD1 protein normally exists as a pair of identical subunits locked together in a structure called a homodimer. Each subunit binds one copper atom, which does the catalytic work, and one zinc atom, which holds the protein in the right shape. Zinc stabilizes each monomer, promotes dimerization, and fine-tunes the active site so that copper can cycle rapidly.2PubMed Central. Mutations in Superoxide Dismutase 1 Linked to Familial Amyotrophic Lateral Sclerosis Can Disrupt High-Affinity Zinc-Binding Promoted by the Copper Chaperone for Sod A disulfide bond within each subunit acts as an additional internal brace. Simulations confirm that metal ions stabilize both the monomer and the dimer, and losing metals makes the protein dramatically more prone to unfolding.3PubMed Central. Dynamical roles of metal ions and the disulfide bond in Cu, Zn superoxide dismutase folding and aggregation
This structural fragility turns out to be central to disease. When SOD1 loses its metals or its disulfide bond breaks, the protein destabilizes, and destabilized SOD1 tends to misfold and clump together. The metal-binding state and disulfide status of SOD1 significantly affect its aggregation behavior, which is directly relevant to ALS.4PubMed Central. Posttranslational modifications in Cu,Zn-superoxide dismutase and mutations associated with amyotrophic lateral sclerosis
SOD1 and ALS
SOD1 was the first gene identified as a cause of familial ALS, the inherited form of the disease that accounts for roughly 5 to 10 percent of all ALS cases.5PubMed Central. Variability in SOD1-associated amyotrophic lateral sclerosis: geographic patterns, clinical heterogeneity, molecular alterations, and therapeutic implications Since the initial discovery in 1993, researchers have catalogued over 200 different mutations scattered across the gene. The mutations do not all behave the same way. Some severely destabilize the protein. Others barely change its shape but still cause disease. What they share is a tendency to make the protein misfold, and mutant SOD1 can adopt a whole spectrum of misfolded states that differ depending on which mutation is present.6PubMed Central. Superoxide dismutase 1 encoding mutations linked to ALS adopts a spectrum of misfolded states
An early and reasonable guess was that the mutations simply knocked out the enzyme’s ability to scavenge superoxide. That turned out to be wrong. Many ALS-causing mutant SOD1 proteins retain partial or even full enzymatic activity. The problem is not a loss of function but a toxic gain of function: the misfolded protein actively harms motor neurons through several converging mechanisms.
How Mutant SOD1 Kills Motor Neurons
The toxicity of mutant SOD1 does not reduce to a single pathway. Research over the past three decades has identified several distinct mechanisms, and they likely act together to overwhelm the cell.
One surprising finding is that the toxic form of mutant SOD1 is not necessarily the large visible clumps. In cell culture, the more soluble a mutant SOD1 protein is, the more toxic it tends to be. These soluble misfolded forms are probably cleared by the cell’s protein-disposal system, and when that system is blocked, toxicity in cells expressing mutant SOD1 increases sharply.7PubMed Central. Cellular toxicity of mutant SOD1 protein is linked to an easily soluble, non-aggregated form in vitro The big aggregates may actually be a cell’s last-ditch effort to quarantine the dangerous material. In yeast models, cells expressing mixtures of normal and mutant SOD1 showed decreased antioxidant activity, increased oxidative damage, and reduced survival, with mutant SOD1 accumulating in intracellular inclusions under stress.8PubMed Central. Characterization of the activity, aggregation, and toxicity of heterodimers of WT and ALS-associated mutant Sod1
Mitochondrial Damage
Motor neurons are energy-intensive cells with long axons, so healthy mitochondria are essential. Mutant SOD1 accumulates inside mitochondria, and this buildup is an important driver of mitochondrial dysfunction and motor neuron death.9PubMed Central. SOD1 misplacing and mitochondrial dysfunction in amyotrophic lateral sclerosis pathogenesis One way this happens is through direct physical interference: misfolded mutant SOD1 binds to VDAC1, a channel protein in the outer mitochondrial membrane that controls what passes in and out. When mutant SOD1 plugs this channel, the flow of ADP across the membrane drops, choking the cell’s energy supply.10PubMed Central. Misfolded mutant SOD1 directly inhibits VDAC1 conductance in a mouse model of inherited ALS
Disrupted Axonal Transport
Motor neurons can extend axons over a meter long in humans. Those axons depend on an active transport system to shuttle mitochondria, proteins, and other cargo from the cell body to the distant nerve terminals and back. In mice carrying mutant SOD1, deficits in axonal transport appear well before any visible symptoms of disease. Both anterograde transport (outbound, toward the nerve terminal) and retrograde transport (inbound, back to the cell body) are impaired early on.11PubMed Central. Deficits in axonal transport precede ALS symptoms in vivo 12PubMed. Selective impairment of fast anterograde axonal transport in the peripheral nerves of asymptomatic transgenic mice with a G93A mutant SOD1 gene
The mechanism behind this transport failure has become clearer. Mutant SOD1 triggers the degradation of a protein called Miro1, which anchors mitochondria to the molecular motors that move them along axons. With less Miro1 available, mitochondria stall in the cell body instead of reaching the parts of the axon that need them most.13PubMed Central. Amyotrophic lateral sclerosis-associated mutant SOD1 inhibits anterograde axonal transport of mitochondria by reducing Miro1 levels A neuron whose nerve terminals are starved of mitochondria cannot sustain normal signaling to muscles, and the connection eventually dies.
Prion-Like Spreading
One of the more alarming discoveries about mutant SOD1 is that its misfolded aggregates can spread from cell to cell in a way that resembles prion diseases. Aggregates of mutant SOD1 enter new cells, escape into the cytoplasm, and seed the misfolding of otherwise soluble mutant SOD1 already present. This process is self-perpetuating and does not require direct cell-to-cell contact; the aggregates are released into the surrounding space and taken up by neighboring cells.14PubMed Central. Prion-like propagation of mutant superoxide dismutase-1 misfolding in neuronal cells This prion-like behavior offers a plausible explanation for why ALS symptoms typically start in one region of the body and progressively spread outward.15PubMed Central. Prion Properties of SOD1 in Amyotrophic Lateral Sclerosis and Potential Therapy
Damage From Neighboring Cells
Motor neurons do not die in isolation. Glial cells, especially astrocytes, play an active role in the degenerative process. In ALS, these support cells become dysfunctional and begin contributing to motor neuron death rather than protecting against it.16PubMed Central. Non-cell-autonomous pathogenic mechanisms in amyotrophic lateral sclerosis Experiments have shown that expressing mutant SOD1 only in motor neurons is not sufficient to cause full-blown ALS in mice; the surrounding glia must also be involved. This “non-cell-autonomous” toxicity is a major reason the disease is so difficult to stop once it starts.
Could Normal SOD1 Also Cause Problems?
The link between SOD1 mutations and familial ALS is well established, but a more controversial question is whether the normal, non-mutated version of SOD1 might contribute to sporadic ALS, the form of the disease with no clear genetic cause. Sporadic ALS accounts for about 90 percent of cases. Several lines of evidence now suggest this is at least possible.
The normal SOD1 protein can be driven into toxic misfolded shapes by non-genetic insults such as losing its metals, breaking its disulfide bond, or undergoing oxidative modifications. When this happens, wild-type SOD1 adopts conformations similar to those seen in familial ALS-linked mutant variants.17PubMed Central. An emerging role for misfolded wild-type SOD1 in sporadic ALS pathogenesis Using antibodies that specifically recognize these misfolded forms, researchers have detected misfolded wild-type SOD1 in spinal cord tissue from sporadic ALS patients.18Scientific Reports. Misfolded SOD1 pathology in sporadic Amyotrophic Lateral Sclerosis Misfolded wild-type SOD1 has also been found in the cerebrospinal fluid of sporadic ALS cases.19PubMed Central. Wild-type Cu/Zn-superoxide dismutase is misfolded in cerebrospinal fluid of sporadic amyotrophic lateral sclerosis This does not prove that wild-type SOD1 causes sporadic ALS, and the idea remains debated. But it raises the possibility that the toxic gain-of-function story is not limited to the rare patients who carry SOD1 mutations.
The Down Syndrome Connection
The SOD1 gene sits on chromosome 21, in the region 21q22.20PubMed Central. Nucleotide sequence and expression of human chromosome 21-encoded superoxide dismutase mRNA People with Down syndrome have three copies of chromosome 21 instead of the usual two, which means they carry an extra copy of SOD1. This extra dose leads to measurably elevated SOD1 levels in the brain: significantly increased SOD1 has been found in the temporal, parietal, and occipital cortex of people with Down syndrome.21PubMed. Superoxide dismutase SOD1, encoded on chromosome 21, but not SOD2 is overexpressed in brains of patients with Down syndrome
At first glance, having more of an antioxidant enzyme sounds beneficial. In practice, it backfires. The extra SOD1 pumps out more hydrogen peroxide than the downstream enzymes can clear, creating oxidative stress rather than relieving it. Cells overexpressing SOD1 show increased lipid damage, elevated protein oxidation, and slower growth rates, reproducing many of the biochemical abnormalities seen in Down syndrome.22PubMed. Effect of overexpression of wild-type and mutant Cu/Zn-superoxide dismutases on oxidative damage and antioxidant defences: relevance to Down’s syndrome and familial amyotrophic lateral sclerosis More recent work confirms that Down syndrome cells show elevated hydrogen peroxide and lipid peroxidation as toxic consequences of SOD1 overexpression.23PubMed Central. SOD1 Is an Integral Yet Insufficient Oxidizer of Hydrogen Sulfide in Trisomy 21 B Lymphocytes and Can Be Augmented by a Pleiotropic Carbon Nanozyme The SOD1 gene dosage imbalance is now considered one contributor to the oxidative stress and accelerated brain aging seen in Down syndrome.
What Happens When SOD1 Is Missing Entirely
Knockout mice engineered to lack SOD1 altogether provide a stark picture of the gene’s importance beyond ALS. These mice have about a 30 percent shorter lifespan than normal mice and develop a dramatic acceleration of age-related muscle loss, a condition called sarcopenia.24PubMed Central. Accelerated sarcopenia in Cu/Zn superoxide dismutase knockout mice They also show a sharp increase in liver cancer.25PubMed Central. Liver specific expression of Cu/ZnSOD extends the lifespan of Sod1 null mice
The sarcopenia in these mice is not simply a muscle problem. When researchers deleted SOD1 only from muscle cells, or only from neurons, neither deletion alone reproduced the severe wasting seen in full knockouts.26PubMed Central. Comparison of Whole Body SOD1 Knockout with Muscle-Specific SOD1 Knockout Mice Reveals a Role for Nerve Redox Signaling in Regulation of Degenerative Pathways in Skeletal Muscle The severe phenotype seems to require a vicious cycle between nerve and muscle: loss of SOD1 in motor neurons disrupts the connections between nerves and muscles, which leads to mitochondrial dysfunction and excess reactive oxygen in the muscle, which feeds back to destabilize more nerve-muscle connections.27PubMed Central. Accelerated sarcopenia in Cu/Zn superoxide dismutase knockout mice The takeaway is that SOD1 is not just protecting cells from oxidative rust; it is actively maintaining the communication between the nervous system and muscles throughout life.
Therapies Targeting SOD1
Understanding that mutant SOD1 causes ALS through toxic gain of function rather than loss of function led to an obvious therapeutic strategy: reduce the amount of mutant SOD1 protein in the nervous system.
Tofersen
Tofersen is an antisense oligonucleotide, a short strand of synthetic DNA that binds to SOD1 messenger RNA and marks it for destruction, preventing the cell from making SOD1 protein. Injected into the spinal fluid, it is the first and only therapy approved specifically for ALS in adults who carry a SOD1 mutation.28JAMA Neurology. Long-Term Tofersen in SOD1 Amyotrophic Lateral Sclerosis In clinical trials, tofersen reduced SOD1 protein levels in cerebrospinal fluid and lowered plasma neurofilament light chains, a marker of nerve damage.29PubMed. Trial of Antisense Oligonucleotide Tofersen for SOD1 ALS The drug’s approval was a landmark moment because it represented the first time any ALS treatment was designed around a specific genetic cause.
CRISPR Gene Editing
While tofersen requires ongoing infusions because it suppresses rather than eliminates SOD1 production, gene editing aims for a more permanent fix. In mouse models of SOD1-linked ALS, CRISPR-Cas9 delivered via a viral vector has been used to directly disrupt the mutant SOD1 gene. One study reported that this approach reduced mutant SOD1 protein by more than 2.5-fold in the spinal cord, preserved about 50 percent more motor neurons at end stage, and extended survival by roughly 25 percent.30PubMed Central. In vivo genome editing improves motor function and extends survival in a mouse model of ALS Another study in SOD1-mutant mice found that CRISPR-based deletion of the SOD1 gene improved lifespan by about 55 percent and markedly reduced muscle wasting.31PubMed. The deletion of mutant SOD1 via CRISPR/Cas9/sgRNA prolongs survival in an amyotrophic lateral sclerosis mouse model These results are still preclinical, and the usual caveats about translating mouse results to humans apply, but the numbers are striking enough that gene editing is being actively pursued for future human trials.
Copper-Delivery Compounds
A different strategy takes the opposite approach: instead of removing mutant SOD1, make it more stable. The small molecule CuATSM delivers copper into the central nervous system, loading it into SOD1 and helping the protein achieve its properly folded, metal-bound state. In ALS mice, CuATSM treatment extended survival and reduced the fraction of copper-deficient SOD1 in the spinal cord.32PubMed Central. Copper delivery to the CNS by CuATSM effectively treats motor neuron disease in SOD G93A mice co-expressing the Copper-Chaperone-for-SOD A polytherapy approach combining CuATSM (for copper delivery) with ebselen (which promotes disulfide bond formation) aims to mimic the full activity of the natural chaperone that normally helps SOD1 fold correctly.33PubMed Central. A copper chaperone-mimetic polytherapy for SOD1-associated amyotrophic lateral sclerosis CuATSM has advanced to human clinical trials, though definitive results are still pending.
SOD1, Oxidative Stress, and Epigenetic Changes
Beyond neurodegeneration, SOD1 has been linked to broader effects on gene regulation. In animal studies, knocking down SOD1 and letting oxidative stress rise led to measurable losses in DNA methylation, a chemical tag that controls gene activity, in prostate tissue.34PubMed Central. Superoxide dismutase 1 knockdown induces oxidative stress and DNA methylation loss in the prostate DNA methylation loss is a hallmark of aging and cancer, so the finding suggests that SOD1 may quietly help preserve the epigenetic landscape of cells. When it is absent or insufficient, the resulting oxidative stress does not just damage proteins and membranes; it reshapes which genes are turned on and off, potentially pushing cells toward malignant transformation. This connects to the observation that SOD1-knockout mice develop liver cancer at high rates.
An Ancient Enzyme
SOD1’s importance is reflected in how far back it goes. Copper-zinc superoxide dismutases are found across nearly all eukaryotes, from single-celled organisms to humans. Phylogenetic analyses show that eukaryotic-like copper-zinc SOD sequences also exist in certain bacteria, and these bacterial versions appear to share a common ancestor with the eukaryotic enzymes rather than having evolved independently.35Molecular Biology and Evolution. Bacterial Evolutionary Precursors of Eukaryotic Copper–Zinc Superoxide Dismutases The enzyme’s evolutionary history includes multiple rounds of gene duplication and diversification, even within relatively narrow groups of organisms.36Scientific Reports. Antarctic marine ciliates under stress: superoxide dismutases from the psychrophilic Euplotes focardii are cold-active yet heat tolerant enzymes The fact that life has maintained and repeatedly duplicated this enzyme across vastly different environments underscores how fundamental the problem of superoxide toxicity is. It is one of the oldest chemical threats that oxygen-using life faces, and SOD1 is one of the oldest solutions.
How SOD1 Was Discovered
The protein now known as SOD1 had a quiet early life in science. It was first isolated from red blood cells in the 1930s under the name “haemocuprein,” a copper-containing protein of unknown function. For decades, nobody knew what it did. Then in 1969, Joe McCord and Irwin Fridovich demonstrated that this same protein had enzymatic activity: it catalyzed the removal of superoxide radicals. The “haemocuprein” turned out to be superoxide dismutase, and the discovery launched the entire field of free-radical biology.37PubMed. From haemocuprein to copper-zinc superoxide dismutase: a history on the fiftieth anniversary of the discovery of haemocuprein and the twentieth anniversary of the discovery of superoxide dismutase It is one of those stories where a protein sat on the shelf for 30 years because the question it answered had not yet been asked.