Somatic STAT5B^N642H mutations shape variable immune landscapes resulting in heterogenous immune diseases.

Abstract

Inborn errors of immunity are traditionally understood as monogenic germline disorders. However, somatic mosaicism can also result in immune-mediated diseases, mimicking inborn errors of immunity. While early postzygotic mosaicism is the predominant mechanism, genetic variants causing a selective advantage to hematopoietic progenitors and/or mature immune cells may cause immune dysregulation and initiate disease at any age. Somatic mosaicism for STAT5B^N642H was linked to severe allergic disease in infancy, but its full clinical spectrum and underlying mechanisms remain incompletely defined. We elucidated how somatic N642H mutations of the STAT5B gene shape lineage-specific mosaicism, immune cell function, and clinical phenotypes. We investigated 3 new patients-including one adult-with STAT5B^N642H mosaicism using deep sequencing, flow and mass cytometry, and functional immune assays. Mutant cell distribution was mapped across blood lineages. A mouse model with mosaic STAT5B^N642H mutation in hematopoietic stem cells was generated to study clonal dynamics and immune phenotypes. Patients displayed variable lineage mosaicism correlating with two predominant clinical outcomes: early-onset severe atopy with hypereosinophilia, and autoimmune-lymphoproliferative immunodeficiency with expansions of CD8 and γδ T cells. Functional studies revealed enhanced IL-2-mediated proliferation, effector differentiation, and oligoclonal T-cell expansions. In mice, a few mutant hematopoietic stem cells reproduced the patient lineage-skewed immune landscapes with variable growth advantage of mutant cells across hematopoietic development and recapitulated patient T-cell phenotypes. Targeted mTOR inhibition successfully controlled lymphoproliferation in patients. A single somatic variant in a few stem cells can remodel hematopoiesis, generating variable immune mosaics and heterogeneous immune disease.