β-Actin Deficiency in Baraitser-Winter Syndrome Type 1 Disrupts T-Cell Function and Immune Regulation: Implications for Targeted Therapy in Actinopathies

PURPOSE: Baraitser-Winter syndrome type 1 (BRWS1) is a rare disorder characterized by intellectual disability, short stature, facial dysmorphism, cortical malformations, macrothrombocytopenia, and recurrent infections. BRWS1 is caused by loss-of-function variants in ACTB, leading to β-actin deficiency. Given the essential role of the actin cytoskeleton in T-cell activation, the immunological consequences of ACTB mutations remain unexplored. Here, we characterize immune dysfunction associated with a novel ACTB variant in a patient with BRWS1. METHODS: Whole-exome sequencing identified a heterozygous ACTB p.Gln360ProfsTer4 variant in a patient with BRWS1 and combined immunodeficiency. Functional studies were performed in HEK293T cells transfected with wild-type and mutant ACTB constructs. Patient-derived T cells were analyzed for immunological synapse formation, cytokine production, activation, and proliferation. The therapeutic effects of exogenous IL-2 and dupilumab were evaluated. RESULTS: The mutant β-actin protein was rapidly degraded and exerted a dominant-negative effect on wild-type β-actin, thereby disrupting cytoskeletal integrity. Patient-derived T cells demonstrated defective immunological synapse formation, reduced intra-synaptic IL-2 levels, and impaired activation and proliferation. Supplementation with exogenous IL-2 partially restored T-cell function in vitro. Notably, dupilumab treatment led to significant clinical and immunological improvement, suggesting a role in restoring immune regulation. CONCLUSION: BRWS1 represents a novel primary immune regulatory disorder. Our findings highlight actinopathy-driven immunodeficiency as a target for therapeutic intervention, with broader implications for cytoskeletal disorders.