Vacuoles are physiological intracellular organelles in plants, fungi, and bacteria. The presence of cytoplasmic vacuoles in human cells indicates a pathologic process (5). Almost all VEXAS syndrome patients have vacuolation in the promyelocytes, myelocytes, and erythroid precursors in the bone marrow. Vacuoles can also be found in eosinophils, monocytes, megakaryocytes, and plasma cells, but not in mature lymphocytes likely due to the location of the mutated UBA1 gene (4). Lymphoid cells are found to have increased apoptosis in VEXAS syndrome patients (9). Dense vacuolation in more than 10% neutrophilic precursors is both sensitive and specific to diagnose VEXAS syndrome (10).

Nonetheless, the evaluation of cytoplasmic vacuolation should be taken into careful consideration as it can also be observed in other conditions such as alcohol use, copper deficiency, zinc toxicity, and MDS. Cherniawsky et al. reviewed bone marrow biopsies from 210 patients with identified cytoplasmic vacuolation in myeloid and erythroid precursors but only 6 patients tested positive for UBA1 mutations (2.9%) (11). The most common causes of vacuolation among the group were MDS and acute myeloblastic leukemia (AML). Thorough evaluation and exclusion of these conditions are necessary to pursue a diagnosis of VEXAS syndrome. The mechanism of cell vacuolation in VEXAS syndrome is still under further investigation.

The E1 enzyme coded by the UBA1 gene is detected in myeloid progenitor cells and circulating myeloid cells, but not in mature lymphoid cells. The E1 enzyme is responsible for the initiation of the protein ubiquitination (5). Ubiquitination is a post-translational modification essential to protein turnover and plays an important role in the assembly of multiprotein complexes, intracellular signaling, inflammatory signaling, and DNA repair (5). This process also facilitates protein destruction via the proteasome complex or lysosome and is key in maintaining cellular homeostasis as well as facilitating autophagy when needed (8).

Three enzymes (E1, E2, and E3) are required for the binding of ubiquitin to target proteins. The E1 enzyme initializes the process of ubiquitination by forming a thioester bond with the terminal region of ubiquitin, followed by the transfer of ubiquitin to a sulfide group of the E2 conjugating enzyme. The E3 ligase enzyme then binds the ubiquitin to the target protein to initiate the proteasomal degradation (5). Mutations in UBA1 lead to malfunctioning E1 enzymes resulting in reduced ubiquitylation, decreased protein degradation, and increased inflammation (12).

The UBA1 gene is located on the X chromosome (Xp11.3). As such the prevalence of VEXAS syndrome is largely predominant in male patients, particularly those in the latter decades of life. Studies have demonstrated female patients are protected from the manifestation of VEXAS largely due to the presence of additional alleles (5). Rare presentations in female patients can be seen however in patients with acquired X-chromosome monosomy or structural deletion. Presentation is usually a milder form of the disease (13).

A recent study was conducted to address the relationship between UBA1 mutation and inflammation in VEXAS patients using transcriptome sequencing of single bone marrow mononuclear cells, hematopoietic stem cells, and progenitor cells (9). In UBA1 mutated myeloid cells, upregulated inflammatory pathways, active precursor cell cycling, and increased granulocytic differentiation were observed.

The decreased ubiquitination/proteasome pathway activity leads to the accumulation of unfolded proteins. The increased protein response contributes to the elevation of inflammatory cytokines such as interferon-gamma (IFN- γ), and Tumor necrosis factor alpha (TNF)- α, and the activation of multiple inflammatory pathways (14). Meanwhile, neutrophils may also contribute to exacerbating the inflammatory process by promoting the formation of neutrophil extracellular traps. Interestingly, there is an increased apoptosis of lymphoid cells in VEXAS patients (8). Systemic inflammation affects the skin, lungs, vessels, and cartilage. Patients typically present with fever, polychondritis, skin rashes, pulmonary involvement, and vasculitis (4).

Somatic mutations accumulate with age. It can be seen in healthy aging people and patients with various diseases. The UBA1 mutation is a somatic mutation that occurs later in life (1).

UBA1 has two protein isoforms: a nuclear form initiated at p. Met1 and a cytoplasmic form initiated at p.Met41. Loss of the cytoplastic isoform is found in most VEXAS patients (2), which causes the disruption of cytoplasmic E1 enzymes and results in systemic inflammation. The timing between the mutation onset and the development of the disease is still under investigation.

MDS is observed in more than half of the VEXAS patients. There are two large retrospective case series that report MDS diagnosis in 31-50% of cases in VEXAS patients (8). This is much higher than the MDS incidence in paroxysmal nocturnal hemoglobinuria over 10 years, which is approximately 2-6% (18). Compared to VEXAS patients, VEXAS-MDS patients have a higher incidence of non-infectious recurrent fever, respiratory symptoms, and gastrointestinal involvement. They also have lower platelet count and higher medullar blasts (4). However, the difference in overall survival rate between VEXAS patients without MDS and VEXAS-MDS patients was insignificant (19).

MDS is caused by clonal alterations of hematopoietic stem cells, causing ineffective hematopoiesis and progressive cytopenias, bone marrow dysplasia, and eventual leukemic transformation (20). The mutational profile of VEXAS-MDS appears to be less complex, classic MDS-associated mutations are not frequently detected in VEXAS patients (2).

Previous studies have indicated that MDS is associated with a systemic inflammatory process (21). Inflammatory status in VEXAS syndrome plays a critical role in the development of MDS because of UBA1 gene mutation. However, UBA1 has not been included in the MDS scoring system (8).

Macrocytic anemia can be observed in VEXAS syndrome patients with normal B12, folate, and copper levels. Two large case series reported that approximately 32% of VEXAS patients developed transfusion-dependent anemia and had increased the risk of death by 4.5-fold (15). Thrombocytopenia, lymphopenia, and monocytopenia were also reported. Monoclonal gammopathy of unknown significance (MGUS) and multiple myeloma can also be seen in VEXAS syndrome patients (6).

Thromboembolic events usually occur in the early stage of VEXAS syndrome. The venous thrombosis incidence is higher than arterial thrombosis (8). The overactive immune cells, platelets, and endothelial cells contribute to the dysregulation of hemostasis. Chronic inflammation also plays a role in the development of thrombosis by inappropriate formation of fibrin, reduced synthesis, and increased consumption of the natural anticoagulant (5).

In one large-scale study with 119 VEXAS patients, 49% of patients developed thrombotic events, majorly venous thromboembolism, and approximately two-thirds of them were unprovoked. At a median follow-up time of 4.8 years, the overall survival (OS) of the entire cohort was 88% and there was no significant overall survival difference between VEXAS patients with and without thrombosis (22). Thromboembolic prophylaxis should be administered to VEXAS patients unless there are contraindications.

Auricular and nasal chondritis is commonly observed in VEXAS syndrome patients (3). Lung involvement is often seen, associated with pleural effusion. The common gastrointestinal tract symptoms include abdominal pain, diarrhea, and gastrointestinal bleeding. Renal involvement can present with proteinuria, hematuria, and renal insufficiency. Epididymitis is frequently reported, along with orchitis and prostatitis. Lymphadenopathy and splenomegaly are observed (19).

Hypomethylating agents such as azacytidine have shown promising results in managing VEXAS patients. A French cohort study of VEXAS syndrome patients with concomitant MDS showed clinical response after 4 cycles of azacytidine (AZA) treatment in 5/11 (46%) patients (23).

A phase II prospective trial study in 2022 reported that 9/12 (75%) VEXAS patients achieved complete responses or partial responses after 6 cycles of AZA treatment, with improvement in inflammatory manifestations and a decrease in the requirement of steroids (23). However, cytopenia or myelodysplastic features were not improved significantly. Hypomethylating agents may be preferentially considered in patients with concomitant MDS.

The available studies show that high doses of glucocorticoids are effective in ameliorating systemic inflammation, however, attempts to taper steroids have resulted in poor symptom control and even recurrence (4). Prolonged use of high-dose steroids is associated with serious side effects and dependence. Further research needs to be conducted on steroids-sparing agents for the long-term treatment of VEXAS treatment.

JAKi has served a role in treating VEXAX syndrome patients. By suppressing the inflammatory pathway, JAKi decreases the production of inflammatory cytokines such as IL-6, which improves systemic inflammation and related symptoms.

JAKi are effective in relieving inflammatory symptoms, especially skin manifestations. Significant regression of skin lesions has been observed with topical ruxolitinib without increasing the dose of steroids, suggesting the potential effectiveness of JAKi. Heiblig et al. reported complete clinical and biological responses in 7/11 (64%) and 6/11 (54%) patients treated with ruxolitinib for 1 month (21). Subsequently, 87% of patients achieved complete clinical remission after 6 months of treatment with ruxolitinib, with a significant increase in hemoglobin and platelet levels, decreased use of steroids, and even discontinuation of steroids.

These patients were found to have persistent clonal amplification of UBA1 with the treatment, indicating that ruxolitinib aims to alleviate inflammation instead of eradicating UBA1 mutations.

However, side effects of JAKi are common, including severe opportunistic infections, and thromboembolism, which can also occur in the already vulnerable patients with VEXAS syndrome, making the safety of JAK inhibitors challenging to assess (24). Prospective studies are still needed to confirm its efficacy further.

Tocilizumab is a monoclonal antibody against the inflammatory cytokine IL-6 receptor. Within short-term follow-up, tocilizumab showed alleviation of inflammatory symptoms and reduced steroid requirement (5), but the long-term efficacy and side effects need to be investigated.

Tocilizumab has been associated with neutropenia, viral infection reactivation (herpes zoster virus), and severe gastrointestinal side effects in VEXAS syndrome patients. Intestinal perforation is one of the severe comorbidities, especially perforation of the jejunum or ileum at the site of diverticulitis (25).

The use of IL-1 receptor inhibitors like anakinra has been noted to lead to stabilization of the inflammatory symptoms in some patients for 1-2 years, but it caused severe skin reactions at the site of injection (8).

For cases with multisystemic involvement, including severe hematological abnormalities, hematopoietic stem cell transplantation (HSCT) may be considered as a curative option. A few studies investigated the outcome of HSCT in VEXAS syndrome patients. The results were mixed.

In 2021, the Mayo Case Series first reported that a VEXAS syndrome patient with coexisting plasma cell myeloma received bortezomib, lenalidomide, and dexamethasone induction therapy, followed by autologous stem cell transplant (26). The patient demonstrated the resolution of arthritis and the discontinuation of steroids and IL-6 inhibitors. A French case series of six VEXAS syndrome patients reported that all patients were in complete remission after HSCT.

In a UK study in 2022, four patients underwent HSCT, and only one patient achieved remission, with one patient developing a significant infection and extensive Graft-Versus-Host Disease and two patients dying of infection (27).

Abhishek et al. published a prospective series in which five patients underwent reduced-intensity conditioning allogeneic stem cell transplant with a median follow-up of 9.6 months (26). Four patients had normalization of bone marrow morphology with eradication of vacuoles and did not require more steroid treatment. Inflammatory markers normalized in at least two patients. UBA1 mutation disappeared in 2 patients. None of those patients experienced the recurrence of inflammatory symptoms or worsened cytopenia.

HSCT evaluation should be conducted in the early stage, as VEXAS-related complications may limit the efficacy of transplantation. It is recommended to control inflammation before HSCT to reduce post-transplant complications (8). Currently, HSCT studies are under conducted and will provide valuable guidance in treating VEXAS syndrome in the future.