Mild Phenotype in Functionally Validated STAT1 Gain‐of‐Function Disease Due to a Novel DNA ‐Binding Variant

ABSTRACT Signal transducer and activator of transcription 1 (STAT1) gain‐of‐function (GOF) disease is associated with chronic mucocutaneous candidiasis, infections, autoimmunity, and immune dysregulation; however, mutation‐level determinants of phenotypic variability remain incompletely defined. We aimed to describe the clinical, immunologic, and functional features of four patients with STAT1‐GOF disease, including three related patients carrying a novel DNA‐binding domain (DBD) variant associated with a mild phenotype. Clinical manifestations, immunologic findings, and immune deficiency and dysregulation activity scores were analysed. Whole‐exome sequencing (WES) with Sanger confirmation was performed, and the functional activity of the novel p.Leu400Pro variant was assessed using a gamma‐activated sequence luciferase reporter assay after interferon‐ γ stimulation. All patients presented with early‐onset chronic mucocutaneous candidiasis. Three related patients carrying p.Leu400Pro showed a mild, stable clinical course without severe infections or multisystem autoimmunity during a median follow‐up of 8 years and had low disease activity scores. Functional analysis demonstrated increased IFN‐ γ –induced transcriptional activity compared with wild‐type STAT1, reaching levels comparable to the established R274Q GOF control under the experimental conditions used and supporting a GOF effect. In contrast, an unrelated patient with p.Glu353Lys developed severe multisystem autoimmunity, lymphoproliferation, and bronchiectasis with high disease activity scores. Ruxolitinib led to clinical improvement and was used as bridging therapy before haematopoietic stem cell transplantation (HSCT). These findings identify a novel, functionally validated DNA‐binding domain STAT1‐GOF variant associated with a mild phenotype, supporting mutation‐level risk stratification and individualized management. Targeted Janus kinase inhibition may help stabilize severe disease before transplantation.