STAT3 gain-of-function mutation presenting with adult-onset recurrent abdominal pain, fever and lymphadenopathy, managed by JAK inhibitor therapy.

Signal transducer and activator of transcription 3 (STAT3) gain-of-function (GOF) syndrome is a rare genetic immune dysregulation disorder that typically presents in infancy with autoimmunity, lymphoproliferation, susceptibility to infections and growth delay. We report a case of a man in his late 20s admitted to hospital multiple times with abdominal pain, fever and raised C-reactive protein, where imaging revealed progressive mesenteric lymphadenopathy. A definitive diagnosis remained elusive until genetic analysis highlighted a GOF heterozygous mutation in the STAT3 gene. Targeted therapy with Janus kinase inhibitor (tofacitinib) led to significant clinical and radiological improvement. This case underscores the value of a multidisciplinary assessment and early genome sequencing in diagnosing unexplained systemic manifestations, offering promising prospects for more effective management through targeted therapies.