UBA1 Mutation: VEXAS Syndrome and Spinal Muscular Atrophy

Mutations in the UBA1 gene sit behind two strikingly different diseases: VEXAS syndrome, a severe autoinflammatory condition discovered only in 2020, and X-linked infantile spinal muscular atrophy (SMAX2), a rare and usually fatal neurodegenerative disorder that appears at birth. Both diseases trace back to the same gene, but the type of mutation, when it arises, and which cellular functions it disrupts determine whether someone develops runaway inflammation in middle age or progressive motor neuron loss in infancy. The shared genetic origin and divergent outcomes make UBA1 one of the more fascinating case studies in how a single gene can shape disease in radically different ways.

What UBA1 Actually Does

UBA1 encodes the main enzyme that kicks off ubiquitination, the process cells use to tag proteins for degradation, relocation, or signaling. Think of it as the master switch at the top of a quality-control cascade. UBA1 activates ubiquitin in an energy-dependent step and hands it off to downstream enzymes that attach it to target proteins.¹ Because nearly every protein in the cell eventually depends on this tagging system for proper turnover and regulation, a defective UBA1 enzyme has outsized consequences.² Which consequences depend on the nature and location of the mutation.

VEXAS Syndrome and How It Was Found

VEXAS stands for Vacuoles, E1 enzyme, X-linked, Autoinflammatory, and Somatic. The name is an acronym built from the disease’s defining features, and the “somatic” part matters: the mutations are not inherited but arise spontaneously in blood-forming stem cells during a person’s lifetime. VEXAS was identified through a genotype-first approach, meaning researchers started with genomic sequencing in large groups of people with treatment-resistant inflammatory conditions and worked backward to find a shared genetic cause. That search uncovered somatic UBA1 mutations in aging men whose symptoms had previously been categorized under various separate rheumatologic and hematologic diagnoses.³ Before VEXAS had a name, many of these patients had been diagnosed with relapsing polychondritis, Sweet syndrome, polyarteritis nodosa, or myelodysplastic syndrome, with no one realizing the conditions were linked.⁴

The discovery was a genuine paradigm shift. Rather than grouping patients by symptoms and then hunting for a cause, clinicians grouped them by mutation and found a unifying disease. This genotype-first method has since influenced how researchers think about undiagnosed inflammatory syndromes more broadly.

What Goes Wrong in VEXAS at the Molecular Level

The mutations most commonly seen in VEXAS cluster at a single amino acid position, methionine-41 (p.Met41), which serves as the start codon for the cytoplasmic form of UBA1. Normally, UBA1 exists in both nuclear and cytoplasmic versions, each with distinct roles. When p.Met41 is mutated, the cell loses the normal cytoplasmic isoform and instead produces a shorter, catalytically impaired version that starts further downstream at position 67. The result is decreased ubiquitination in the affected blood cells and activation of innate immune pathways.⁵

The three most common VEXAS variants at this codon are p.Met41Leu, p.Met41Thr, and p.Met41Val. These are not equally severe. The p.Met41Val variant supports the least production of the cytoplasmic UBA1 isoform, and patients carrying it tend to have worse outcomes, including higher rates of transfusion dependence and lower survival. A study of 83 patients found that the Val variant was independently associated with decreased survival, while ear chondritis (inflammation of cartilage in the ear, a hallmark VEXAS symptom) was associated with better outcomes.⁶ The molecular logic tracks: less residual UBA1b production means more severe disruption of ubiquitin-dependent processes, which means more disease.

The Inflammatory Cascade in VEXAS

What makes VEXAS feel so systemic is that the failing ubiquitin machinery triggers a broad inflammatory response. Blood samples from VEXAS patients show elevated levels of many proinflammatory signaling molecules, with two in particular standing out: IL-1β and IL-18. Both are regulated by the inflammasome, a cellular alarm system that, once tripped, drives intense inflammation. Gene expression analysis has confirmed that TNF-α and NF-κB signaling pathways, which mediate cell death and inflammation, are strongly upregulated in VEXAS.⁷ Multi-omics single-cell analysis has painted a picture of broad immune system activation, with multiple inflammatory response pathways switched on simultaneously.⁸

Clinically, this manifests as fevers, skin lesions (often resembling Sweet syndrome or neutrophilic dermatoses), blood clots, ear and nose cartilage inflammation, and progressive blood count problems. The hematologic side is often what proves most dangerous: many patients develop transfusion-dependent anemia or overt myelodysplastic syndrome. A hallmark finding on bone marrow biopsy is cytoplasmic vacuoles in myeloid and erythroid precursor cells, the “V” in VEXAS.⁴

How Clonal Hematopoiesis Feeds Into VEXAS

Because VEXAS mutations arise in blood stem cells, the disease involves clonal hematopoiesis, where a single mutated stem cell multiplies and eventually dominates blood production. Prospective single-cell analysis has shown that the UBA1-mutant clone is typically the dominant one, and clonal expansion follows one of two broad patterns. In the first, other age-related mutations in blood cells precede the UBA1 mutation, and the UBA1-mutant cell then gains a selective advantage within that already-mutated lineage. In the second pattern, the UBA1 mutation arises as a subclone or in an independent lineage alongside other clonal changes.⁹ Either way, the mutant cells progressively take over the bone marrow, which explains why the disease is chronic and progressive rather than episodic.

Why VEXAS Mainly Strikes Older Men

UBA1 sits on the X chromosome, so males (with one X) are hemizygous: a single somatic mutation is enough to knock out normal UBA1 function in whatever blood cell lineage carries it. This is why the vast majority of VEXAS patients are men, typically over 50.¹⁰ The late onset likely reflects the time it takes for a somatic mutation to arise and for the mutant clone to expand enough to cause problems.

Women were initially thought to be mere carriers, but that has changed. The twist is that UBA1 is not subject to X-chromosome inactivation, the process by which one of a woman’s two X chromosomes is silenced in each cell. Because both copies of UBA1 stay active, a heterozygous woman (one mutant copy, one normal copy) still has enough functional UBA1 to avoid disease. For a woman to develop VEXAS, she needs both the UBA1 mutation and loss of the other X chromosome in the affected cells. This can happen through acquired mosaic X monosomy in the bone marrow, a phenomenon that becomes more common with aging.¹¹ When women do develop the syndrome, their symptoms can be just as severe as those in men.¹²

Diagnosing VEXAS

VEXAS is still widely underdiagnosed, partly because its symptoms overlap with so many other conditions. A patient might see a rheumatologist for cartilage inflammation, a dermatologist for skin lesions, and a hematologist for anemia, with none of them connecting the dots. The critical diagnostic steps are a bone marrow biopsy showing characteristic vacuolization in myeloid and erythroid precursors, followed by genetic sequencing of UBA1. In one illustrative case, targeted sequencing of UBA1 revealed the p.Met41Thr missense mutation at a variant allele frequency of 74%, establishing the diagnosis only after reinvestigation of the bone marrow prompted by clinical suspicion.¹³ The high allele frequency in that case underscores how thoroughly the mutant clone can take over marrow production.

Awareness is the biggest diagnostic bottleneck. Any adult, particularly a man over 50, with unexplained systemic inflammation, steroid-dependent symptoms, blood count abnormalities, and no clear autoimmune diagnosis should prompt consideration of VEXAS. The genotype-first lesson that led to VEXAS’s discovery applies at the bedside too: when the phenotype is confusing, sometimes sequencing clarifies what clinical reasoning cannot.

Treatment Options for VEXAS

There is no approved therapy specifically for VEXAS, and management remains challenging. Most patients initially respond to high-dose glucocorticoids, but relapse when doses are tapered. Several drug classes have shown varying degrees of promise. JAK inhibitors appear to be among the more effective medical options so far, with a systematic review of 186 cases concluding they showed more efficacy than other available drugs and had tolerable side effects, though long-term data remain limited.¹⁴ Interleukin inhibitors targeting IL-1 and IL-6, as well as hypomethylating agents (drugs used in myelodysplastic syndrome), have also been tried with mixed results.

The only potentially curative treatment is allogeneic hematopoietic cell transplantation, essentially replacing the patient’s blood-forming stem cells with donor cells that carry a normal UBA1 gene.¹⁵ This approach carries significant risks, especially in the older population that VEXAS typically affects, but for patients with severe or life-threatening disease it may be the best option. The decision involves weighing transplant-related morbidity against the reality that untreated or poorly controlled VEXAS is progressive and often fatal.

X-Linked Infantile Spinal Muscular Atrophy

At the other end of the UBA1 disease spectrum sits SMAX2, an X-linked form of infantile spinal muscular atrophy that is among the most severe neuromuscular diseases. Unlike VEXAS, which involves somatic mutations acquired in adulthood, SMAX2 is caused by germline (inherited or de novo) mutations in UBA1 that are present from conception and affect every cell in the body.¹⁶ Because it is X-linked, SMAX2 almost exclusively affects boys.

Affected infants typically present with profound low muscle tone, absent reflexes, facial muscle weakness, tongue fasciculations, and severe respiratory insufficiency. Some have joint contractures at birth, though that feature is not universal.¹⁷ The disease is usually fatal in infancy. It is distinct from the more common autosomal form of spinal muscular atrophy caused by SMN1 mutations, though the two share some clinical features, which can create diagnostic confusion.

Why the Same Gene Produces Such Different Diseases

This is arguably the most interesting question about UBA1 pathology. A somatic mutation in blood stem cells of a 60-year-old man produces a systemic inflammatory syndrome. A germline mutation in a newborn boy produces motor neuron degeneration. How does one gene do both?

Research comparing the molecular effects of these mutations has revealed that they break UBA1 in fundamentally different ways. VEXAS-causing mutations at p.Met41 specifically alter expression of the cytoplasmic isoform without necessarily destroying the enzyme’s catalytic core. Other VEXAS mutations elsewhere in the gene reduce catalytic activity through mechanisms like aberrant oxyester formation. SMA-causing mutations, by contrast, render UBA1 thermolabile, meaning the enzyme becomes structurally unstable and loses function at normal body temperature.¹⁸ The E2-charging bottleneck seen in some VEXAS and lung cancer-associated UBA1 mutations does not appear in SMA-causing mutations, reinforcing that these are biochemically distinct defects in the same protein.

Tissue vulnerability also matters. Even though SMAX2 mutations affect every cell, motor neurons bear the brunt. Large motor neurons with long axons and energy-demanding synaptic terminals appear to be particularly sensitive to disruptions in ubiquitin homeostasis. Work in animal models supports this: motor neurons show outsized vulnerability to UBA1 perturbation compared to other cell types.¹⁹ In VEXAS, the somatic mutation is confined to the blood lineage, so the disease plays out in the hematopoietic and immune compartments rather than the nervous system.

Gene Therapy Research for UBA1-Related Motor Neuron Disease

Preclinical work on restoring UBA1 levels has produced encouraging results. In a mouse model of spinal muscular atrophy, systemic delivery of the UBA1 gene using an adeno-associated virus (AAV9) vector increased UBA1 protein levels throughout the body. The treatment reduced weight loss, extended survival, improved motor performance, and ameliorated pathology in both the neuromuscular system and other organs.²⁰ In a related set of experiments, the same gene therapy approach completely restored the number of sensory inputs onto motor neurons in SMA mice, correcting a connectivity defect that contributes to motor dysfunction.²¹

These results are in mice, and the leap from preclinical success to human therapy is always uncertain. But the AAV9 delivery platform has already been used successfully in humans for SMN1-related spinal muscular atrophy (the approved gene therapy onasemnogene abeparvovec), which provides a template for how a similar approach might eventually be adapted for UBA1-related disease. The challenge with SMAX2 specifically is its rarity. Extremely rare diseases struggle to attract the investment needed for clinical trials, and the rapid progression of SMAX2 means the therapeutic window is very narrow.

UBA1 Mutations Beyond VEXAS and SMA

The picture is getting more complicated as researchers look harder. UBA1 mutations have been identified in some lung cancers, where the enzyme’s impairment appears to involve a similar E2-charging bottleneck to what is seen in certain VEXAS mutations.¹⁸ The idea that a single gene can produce autoinflammatory disease, neurodegeneration, or cancer depending on the specific mutation and the tissue context underscores how central ubiquitin homeostasis is to normal cell function. UBA1 also has relevance in the broader field of age-related clonal hematopoiesis, where somatic mutations accumulate in blood stem cells with aging and raise the risk of blood cancers and cardiovascular disease. VEXAS may represent one particularly severe manifestation of a phenomenon that is far more common in milder forms.

One area of emerging interest is whether milder UBA1 variants, not severe enough to cause VEXAS or SMA, might contribute to chronic inflammatory conditions that are currently classified under other names. The genotype-first approach that uncovered VEXAS has already reshaped diagnostic thinking in rheumatology and hematology. As sequencing becomes cheaper and more routine, the full spectrum of UBA1-related disease may turn out to be broader than what is currently recognized. A recent adult-onset case of X-linked spinal-bulbar muscular atrophy was linked to a UBA1 mutation, suggesting the neurological phenotype is not limited to infants.²² That finding opens the question of whether there are adult patients with progressive motor neuron symptoms whose disease is actually driven by UBA1 mutations that have gone unidentified.