Molecular and Cellular Mechanisms of the Aberrant Type I Interferon Response in VEXAS Syndrome - PROJECT SUMMARY/ABSTRACT VEXAS (Vacuoles, E1-ubiquitin-activating enzyme, X-linked, Autoinflammatory, Somatic) is a somatically acquired X-linked disorder characterized by autoinflammatory and hematologic manifestations. First reported in 2020, VEXAS is now known to afflict ~1 in 4,269 men older than 50 years old and causes multiorgan autoinflammation and high rates of myelodysplastic syndrome (MDS). VEXAS is caused by a missense mutation in the ubiquitin-activating enzyme 1 (UBA1) gene that results in aberrant clonal expansion of mutant hematopoietic stem cells (HSPCs) and their myeloid, but not lymphoid, progeny. UBA1 is one of only two E1 enzymes that initiate the ubiquitylation cascade. The most prevalent VEXAS mutations occur at the M41T codon and impair translation initiation at Met41 of the cytosolic UBA1 isoform (UBA1b) and lead to aberrant Met67- initiated translation to generate the enzymatically impaired UBA1 isoform, UBA1c. How this switch in UBA1 isoform usage triggers VEXAS pathogenesis at the molecular and cellular levels remains unknown, owing to the lack of genetically engineered cell culture and animal models. Deploying a newly developed base-editing method, we have succeeded in recreating the most common mutation in UBA1 observed in VEXAS in primary macrophages and HSPCs. Using these systems, we have observed a hyperactive IFN-I axis and spontaneous myeloid bias of UBA1-mutant HSPCs ex vivo and in vivo. Given the links between type I interferon and hematopoiesis, these preliminary results lead us to hypothesize that UBA1 mutation may drive VEXAS pathogenesis through dysregulated interferon signaling and its consequent influence on hematopoiesis. In Aim 1, I propose to dissect the molecular mechanisms by which UBA1 mutation affects type I interferon signaling. Accordingly, we will biochemically dissect IFN-I induction pathways in VEXAS macrophages and test whether Uba1-mutant HSPCs, like their myeloid progeny, display altered IFN-I production. We will further assess the ubiquitylation status of signaling components downstream of innate immune signaling pathways in macrophages bearing a Uba1 mutation to identify mechanisms by which aberrant ubiquitylation may dysregulate IFN-I production in VEXAS. In Aim 2, I will dissect mechanisms of myeloid-biased hematopoiesis in VEXAS syndrome. Accordingly, I will interrogate whether IFN-I signaling differentially affects normal versus UBA1-mutant HSPCs ex vivo. In addition, we will evaluate enhanced myelopoiesis of UBA1-mutant HSPCs in vivo and directly test the contribution of dysregulated interferon to the myeloid-biased hematopoiesis associated with VEXAS. Collectively, our work will provide key insights into VEXAS pathogenesis using state-of-the-art genetic models and may therefore identify novel therapeutic targets in this disease of major unmet medical need.
