PB2295: EVANS SYNDROME AND PHENOTYPIC HETEROGENEITY OF SASH3 GERMLINE LOSS-OF-FUNCTION MUTATIONS BEYOND X-LINKED IMMUNODEFICIENCY

Abstract

Background: “Evans syndrome (ES)” denotes a heterogeneous disease entity, defined by two or more autoimmune cytopenias, due to mostly unknown pathogenic mechanisms. Further, ES is increasingly recognized in pediatric patients with defined inborn errors of immunity (IEIs) including pathogenic variants in CTLA4, STAT3, TNFRSF6, PIK3CD, and KRAS. Loss-of-function mutations in Sterile alpha motif (SAM) and Src homology-3 (SH3) domain-containing 3 (SASH3) have recently been reported as a novel IEI entity in four patients with X-linked combined immunodeficiency with immune dysregulation including immune cytopenias. SASH3 is an X-chromosomal encoded adaptor protein that is exclusively expressed in lymphocytes. Although knowledge of the exact molecular function of this protein is scarce, humans and mice lacking expression of SASH3 show defects in proximal and distal T-cell receptor signalling. Aims: We aimed to dissect aberrant immune cell functionality in SASH3-deficient patients to further enhance our understanding of pathogenic mechanisms in patients with ES. Methods: We performed whole exome sequencing, deep clinical phenotyping and immunophenotyping in a patient with ES and his clinically asymptomatic brother. Results: Our index patient presented with severe ES and recurrent infections at the age of 8 years. Immune phenotyping of his peripheral mononuclear cells (PBMCs) showed transient CD4+ T- and B-cell lymphopenia and reduced T-cell proliferation upon PHA and SEB stimulation. Furthermore, the patient had hypogammaglobulinemia, and the ‘transitory CD21 low B-cells’ were significantly increased, reflecting impaired lymphocyte maturation possibly causing the autoreactive phenotype of the disease. Interestingly, the patient developed large hemorrhagic splenic cysts during ITP associated bleeding diathesis, and thus underwent splenectomy, which led to a complete remission of ES. Using whole-exome sequencing, we discovered a germline nonsense mutation in SASH3 (c.862C>T;p.Arg288Ter) in a now 16 years-old patient with ES. Surprisingly, this maternally inherited mutation was also identified in the hitherto clinically inconspicuous younger brother. The identified mutation is identical to the one reported in two males in the discovery of SASH3 deficiency. Divergently, the now 8-year-old brother who is also carrying the same variant has no clinically apparent phenotype until now. Notably, disease onset occurred earlier (i.e., between age 2 and 5 years) in the previously reported patients compared to our patients. Summary/Conclusion: We present the 5th and 6th patient with germline hemizygous deleterious variants in SASH3. Thus, this study expands the clinical phenotype and underlines a marked phenotypic heterogeneity as illustrated in siblings bearing the same mutation. Future studies will need to address comprehensively the phenotypic spectrum of the disease and genetic and non-genetic modifiers of clinical phenotypes and severity.