Background/Purpose: Identifying the causes of adult-onset rheumatic diseases remains a challenge, and limits diagnosis, prognosis, and targeted treatment. We hypothesized that mutations in genes regulating the post-translational modification ubiquitin, previously implicated in two autoinflammatory diseases, may define new rheumatic disorders.

Methods: We analyzed peripheral blood exome sequence data from 2,560 individuals with inflammation-related diagnoses for deleterious mutations in >800 ubiquitin-related genes. After discovering three patients with novel UBA1 mutations, we identified additional cases based on clinical similarities. Sanger sequencing, digital droplet PCR, immunoblotting, immunohistochemistry, flow cytometry, and transcriptome/cytokine profiling were performed. CRISPR/Cas9 knockout zebrafish provided an in vivo model to assess UBA1 gene function.

Results: Twenty-eight adult males were identified with somatic mutations at methionine 41 in UBA1, an X-linked gene, encoding the major E1 enzyme that initiates ubiquitylation. Methionine 41 is highly conserved in UBA1, and these somatic mutations were not observed in exome sequences from over 80,000 healthy controls. Among affected individuals, mutations were found in more than half of hematopoietic stem cells, exclusively in peripheral blood myeloid cells, and not in lymphocytes or fibroblasts. Patients developed an often-fatal, treatment-refractory inflammatory syndrome in late adulthood, with fevers, cytopenias, characteristic vacuoles in myeloid and erythroid precursors cells, dysplastic bone marrow, neutrophilic cutaneous and pulmonary inflammation, chondritis, and vasculitis. Patients fulfilled clinical criteria for inflammatory (relapsing polychondritis, Sweet syndrome, polyarteritis nodosa, giant cell arteritis) and hematologic (myelodysplastic syndrome or multiple myeloma) conditions. Mutations at p.Met41 resulted in  loss of the cytoplasmic isoform of UBA1 and decreased ubiquitylation and, unexpectedly, the expression of a novel, catalytically inactive, toxic isoform, in mutant, but not wildtype, lineages. Mutant peripheral blood cells exhibited activated innate immune pathways and evidence for unfolded protein response (UPR). Knockout of the zebrafish UBA1 cytoplasmic isoform homologue caused systemic inflammation.

Conclusion: By querying exomes for mutations in ubiquitylation genes, we have defined a novel disorder, VEXAS (vacuoles, E1 enzyme, X-linked, autoinflammatory, somatic) syndrome, which connects seemingly unrelated adult-onset inflammatory syndromes and establishes a precedent for a new molecular taxonomy of rheumatic diseases. Our work also reveals somatic mutations as an underrecognized cause of adult-onset rheumatic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/somatic-mutations-in-a-single-residue-of-uba1-cause-vexas-a-severe-adult-onset-rheumatic-disease-presenting-as-relapsing-polychondritis-polyarteritis-nodosa-or-giant-cell-arteritis/