RAC2 mutations and immune deficiency – functional spectrum of an international cohort

Abstract

Mutations in RAC2 were first identified in neonates carrying RAC2 dominant negative p.D57N, who presented with severe neutrophil defects and T-cell lymphopenia. Subsequently, dominant-activating mutations were reported in patients with combined immunodeficiency/immunodysregulation (CIID) or severe combined immunodeficiency (SCID). We assembled an international cohort of 48 published and unpublished patients with RAC2 mutations to characterize the spectrum of disease.Clinical and immunologic phenotypes of 48 germline mutant RAC2 patients were obtained from referring physicians and literature reports. All patients had significant T lymphopenia with several patients detected by newborn screening, most patients exhibited B and NK lymphopenia as well. Respiratory infections were common, affecting 90% of patients while 56% had significant viral infections and 15% had malignancies. Heterologous RAC2 expression assays including superoxide production, PAK1 binding, and AKT activation were used to characterize RAC2 variant effects on downstream signaling. Protein stability was determined by Western blot. Patient presentation varied by mutation: five patients presented as SCID with reticular dysgenesis, five as neonatal neutrophil deficiency (D57N), and 38 as CIID. Patients with dominant, active RAC2 variants with high protein stability (Q61R, Q61K) presented in the first days of life with SCID and absent lymphocytes. Activating variants with lower stability (C18W, G30R, E62K, N92T, R174) presented as CIID with lymphopenia, detectable by newborn screening but presenting later with recurrent sinopulmonary and viral infections. A third class of mutations with unstable transcript or protein were phenotypic only in homozygosity (W56*, R68W). Consistent with the role of RAC2 in the NADPH oxidase complex, patients with mutations causing reduced superoxide production exhibited bacterial infections. Severity of phenotype and initial clinical presentation were correlated with protein stability such that high stability led to neonatal presentation whereas dominant mutations with decreased stability had later clinical onset.Clinical presentation of RAC2 mutation-bearing patients is determined by a combination of protein activity and stability. Heterologous expression together with RAC2 functional analysis is useful for deciphering the molecular basis of the clinical phenotype related to these immunodeficiencies.