Clinical and functional spectrum of RAC2-related immunodeficiency.

Mutations in the small Rho-family GTPase, RAC2, critical for actin cytoskeleton remodeling and intracellular signal transduction, are associated with neonatal severe combined immunodeficiency (SCID), infantile neutrophilic disorder resembling leukocyte adhesion deficiency (LAD), and later-onset combined immune deficiency (CID). We investigated 54 RAC2 patients (23 previously reported) from 37 families. Data were collected from referring physicians and literature reports with updated clinical information. Patients were grouped by presentation: neonatal SCID (n=5), infantile LAD-like disease (n=5), or CID (n=44). Disease correlated to RAC2 activity: Constitutively-active, RAS-like mutations caused neonatal SCID, dominant-negative mutations caused LAD-like disease, while dominant-activating mutations caused CID. Significant T- and B-lymphopenia with low immunoglobulins were seen in most patients; myeloid abnormalities included neutropenia, altered oxidative burst, impaired neutrophil migration and visible neutrophil macropinosomes. Among 42 CID patients with clinical data, upper and lower respiratory infections and viral infections were common. Twenty-three distinct RAC2 mutations, including 15 novel variants, were identified. Using heterologous expression systems, we assessed downstream effector functions including superoxide production, PAK1 binding, AKT activation, and protein stability. Confocal microscopy showed altered actin assembly evidenced by membrane ruffling and macropinosomes. Altered protein localization and aggregation were observed. All tested RAC2 mutant proteins exhibited aberrant function; no single assay was sufficient to determine functional consequence. Most mutants produced elevated superoxide; mutations unable to support superoxide formation were associated with bacterial infections. RAC2 mutations cause a spectrum of immune dysfunction ranging from early-onset severe combined immunodeficiency to later-onset combined immunodeficiencies depending on RAC2 activity.