Phenotypic Heterogeneity in Patients and Mice with Immunodeficiency Caused by SASH3 Mutations

Abstract

Abstract SASH3 encodes a putative adaptor protein designated SAM and SH3 domain-containing 3 (SASH3), also known as SH3-containing Lymphocyte Protein 1 (SLY1), that is expressed in the cytoplasm and nucleus of lymphocytes. Recently we reported mutations in SASH3 in 4 unrelated male patients as the cause of X-linked recessive immunodeficiency presenting with B, NK and CD4+ T cell lymphopenia; recurrent sinopulmonary, cutaneous and mucosal infections; and refractory autoimmune cytopenias. Patient P1 was hemizygous for a cysteine substitution at the highly conserved Arg347 located in the C-terminus resulting in a protein-positive phenotype, while the other three patients were hemizygous for nonsense protein-negative variants. P1’s immunophenotype was confounded by concomitant large granular lymphocyte leukemia (LGL) and was clinically associated with severe neutropenia, low IgM, significant wart burden on all four extremities, recurrent aphthous stomatitis, and a history of pneumonia. To study the effects p.Arg347Cys, we developed a knock in (KI) mouse model in the C57BL/6 strain designated SASH3KI by CRISPR-Cas9 gene editing. Similar to the patient, SASH3KI mice displayed significant T cell lymphopenia in blood, whereas B cell numbers were normal. CD8+ T cells in SASH3KI mice were reduced in frequency and absolute number in bone marrow, spleen and both mesenteric and inguinal lymph nodes, but were normal in thymus. After infection with LCMV, T cells in SASH3KI mice proliferated normally and produced normal levels of effector molecules. These findings contrast with those reported previously in Sly1−/− and Sly1d/d mutant mice, which both exhibit thymic and splenic hypocellularity and B and T cell lymphopenia.