Dominant activating RAC2 mutation with lymphopenia, immunodeficiency and cytoskeletal defects

Abstract

RAC2, through interactions with NADPH component p67^phox, activates neutrophil superoxide production, while interactions with p21-activated kinase (PAK1) are necessary for fMLF-induced actin remodeling. We identified three patients with de novo RAC2[E62K] mutations resulting in severe T- and B-cell lymphopenia, myeloid dysfunction and recurrent respiratory infections. Neutrophils from RAC2[E62K] patients exhibited excessive superoxide production, impaired fMLF-directed chemotaxis, and abnormal macropinocytosis. Cell lines transfected with RAC2[E62K] displayed characteristics of active GTP-bound RAC2 including enhanced superoxide production and increased membrane ruffling. Biochemical studies demonstrated that RAC2[E62K] retains intrinsic GTP hydrolysis, however GTPase-activating protein (GAP) failed to accelerate hydrolysis resulting in prolonged active GTP-bound RAC2. Rac2^+/E62K mice phenocopy the T- and B-cell lymphopenia, increased neutrophil F-actin and excessive superoxide production seen in patients. This gain of function mutation highlights a specific, non-redundant role for RAC2 in hematopoietic cells that discriminates RAC2 from the related, ubiquitous RAC1.