Functional validation of a novel STAT3 'variant of unknown significance' identifies a new case of STAT3 GOF Syndrome and reveals broad immune cell defects.

Introduction: STAT3 orchestrates crucial immune responses through its pleiotropic functions as a transcription factor. Patients with germline monoallelic dominant negative or hypermorphic STAT3 variants, who present with immunodeficiency and/or immune dysregulation, have revealed the importance of balanced STAT3 signaling in lymphocyte differentiation and function, and immune homeostasis. Here, we report a novel missense variant of unknown significance in the DNA binding domain of STAT3 in a patient who experienced hypogammaglobulinemia, lymphadenopathy, hepatosplenomegaly, immune thrombocytopenia, eczema and enteropathy over a 35-year period.

Methods: In vitro demonstration of prolonged STAT3 activation due to delayed de-phosphorylation, and enhanced transcriptional activity, confirmed this to be a novel pathogenic STAT3 gain-of-function variant. Peripheral blood lymphocytes from this patient, and patients with confirmed STAT3 Gain-of-function Syndrome, were collected to investigate mechanisms of disease pathogenesis.

Results: B cell dysregulation was evidenced by a loss of class-switched memory B cells and a significantly expanded CD19hiCD21lo B cell population, likely influenced by a skewed CXCR3+ TFH population. Interestingly, unlike STAT3 dominant negative variants, cytokine secretion by activated peripheral blood STAT3 GOF CD4+ T cells and frequencies of Treg cells were intact, suggesting CD4+ T cell dysregulation likely occurs at sites of disease rather than the periphery.

Conclusion: This study provides an in-depth case study in confirming a STAT3 gain-of-function variant and identifies lymphocyte dysregulation in peripheral blood of patients with STAT3 Gain-of-function Syndrome. Identifying cellular biomarkers of disease provide a flow cytometric based screen to guide validation of additional novel STAT3 gain-of-function variants as well as provide insights into putative mechanisms of disease pathogenesis.