Impaired polysaccharide responsiveness without agammaglobulinaemia in three patients with hypomorphic mutations in Bruton Tyrosine Kinase—No detection by newborn screening for primary immunodeficiencies

Hypomorphic mutations in the gene encoding Bruton tyrosine kinase (BTK) may result in milder phenotypes and delayed diagnosis of B‐cell related immunodeficiencies due to residual BTK function. Newborn screening for kappa‐deleting‐recombination‐excision circles (KRECs) reliably identifies classical X‐linked agammaglobulinaemia (XLA) patients with profound B‐cell lymphopenia at birth but has not been evaluated in patients with residual BTK function. We aimed to evaluate clinical findings, BTK function and KREC copy numbers in three patients with BTK mutations presenting with impaired polysaccharide responsiveness without agammaglobulinaemia. One patient had an invasive pneumococcal infection at the age of 4 years. All three patients (two brothers) had visible tonsils, normal to slightly decreased immunoglobulin G levels, undetectable pneumococcal antibodies despite pneumococcal conjugate vaccinations, no antibody response after a diagnostic polysaccharide vaccination as well as profound B‐cell lymphopenia with residual B‐cell differentiation. BTK mutations were identified by Sanger sequencing. BTK staining and phosphorylation assays were performed on peripheral B cells. KREC copy numbers were determined from dried blood spots obtained within the first week of life as well as once at the age of 8, 6 and 3 years, respectively. BTK staining showed residual protein expression. Also, residual BTK activity could be demonstrated. KREC copy numbers from dried blood spots were above the threshold set for detection of patients with profound B‐cell lymphopenia. Male patients with impaired polysaccharide responsiveness should be evaluated for B‐cell lymphopenia followed by BTK analyses irrespective of immunoglobulin levels or tonsil size.