Novel Somatic UBA1 Variant in a Patient With VEXAS Syndrome

Abstract

Objective: Somatic mutations in UBA1 have recently been causally linked to a severe adult-onset inflammatory condition referred to as VEXAS (vacuoles, E1 enzyme, X-linked, autoinflammatory, somatic) syndrome. Ubiquitin-activating enzyme E1 (UBA-1) is of fundamental importance to the modulation of ubiquitin homeostasis and to the majority of downstream ubiquitylation-dependent cellular processes. Direct sequencing analysis of exon 3 containing the prevalent variants p.Met41Leu, p.Met41Val, and/or p.Met41Thr is usually used to confirm the disease-associated mutations.

Methods: We studied the clinical, biochemical, and molecular genetic characteristics of a 59-year-old man with a 2-year history of arthritis, fever, night sweats, nonspecific skin rash, lymphadenopathy, and myelodysplastic syndrome with multilineage dysplasia.

Results: The mutational analysis revealed a previously undescribed sequence variant c.1430G>C in exon 14 (p.Gly477Ala) in the gene UBA1. In vitro enzymatic analyses showed that p.Gly477Ala led to both decreased E1 ubiquitin thioester formation and E2 enzyme charging.

Conclusion: We report a case of a patient of European ancestry with clinical manifestations of VEXAS syndrome associated with a newly identified dysfunctional UBA-1 enzyme variant. Due to the patient's insufficient response to various immunosuppressive treatments, allogeneic hematopoietic stem cell transplantation was performed, which resulted in significant improvement of clinical and laboratory manifestations of the disease.

Figure 1:

Clinical manifestations and genetic background of the VEXAS syndrome in our patient.

1. A) and B) Severe asymmetrical polyarthritis.

2. C) Skin lesions included signs of neutrophilic dermatosis and livedo racemosa.

3. D) The representative bone marrow aspirate smears showing dysplasia and characteristic cytoplasmic vacuolation in a promyelocyte (May-Grunwald Giemsa stain).

4. E) Electropherograms of sequence of exon 3 showing reference sequence of UBA1: p.Met41 and of exon 14 showing UBA1: p.Gly477 in reference state and new variant p.Gly477Ala (c.1430G>C) in heterozygous and homozygous state, transcript ENST00000335972.

5. F) Electropherograms of a partial sequence of exon 14 showed mosaicism of variant p.Gly477Ala (c.1430G>C).

Figure 2:

UBA1 G477A has impaired catalytic activity in vitro and in vivo

1. A) Recombinant UBA1b G477A has impaired thioester formation as compared to wildtype (WT). bME used as a reducing agent to eliminate charged species.

2. B) Quantification of 3 biologic replicates for A) with *** p<.05 (.003) with Welsch’s t-test.

3. C) Chinese hamster ovary cells (CHO) either parental (E36) or temperature sensitive (ts20), were grown at 39.5° C for 6 hr where ts20 demonstrates loss of UBA1 and loss E2 charging (UBE2D3 upper band). ts20 were either untreated (X) or transduced with FLAG-HA tagged (FH) UBA1 WT, M41V, or G477A. bME used as a reducing agent to eliminate charged species. Actin used as an immunoblotting control.

4. D) Quantification of 3 biologic replicates for A) with ** p<.05 (.0058) with Welsch’s t-test.