Bruton's Tyrosine Kinase (BTK) is present in normal platelets, and its absence identifies patients with X-Linked Agammaglobulinemia (XLA) and carrier females

Platelets contain substantial amounts of Btk that can be readily demonstrated by staining permeabilized platelets with monoclonal antibody 48-2H followed by flow cytometry. To test the usefulness of this technique for establishing the diagnosis of XLA and to identify carrier females, we quantified Btk by staining platelets from normal controls, XLA patients with known Btk mutations, and female relatives of XLA patients with ascertained carrier status. Platelets isolated from fresh or up-to-two-day-old citrated blood were permeabilized with Saponin, incubated with mAb 48-2H, and staining intensity measured by flow cytometry. The staining patterns observed identified affected boys and carrier females in 12 of 14 XLA families studied. In eight families, patient platelets failed to bind mAb 48-2H (A) and carrier platelets showed two distinct peaks (D); in four families the mAb binding by XLA platelets was decreased (B) and carrier females showed two peaks forming a shoulder (E); two families showed a normal pattern (C, F). These results suggest that the majority (∼ 85%) of Btk mutations observed in XLA families result in the absence of Btk or in a mutated protein that is poorly recognized by mAb 48-2H, and that megakaryocytes from female carriers for XLA undergo random X-inactivation. (Graph Presented).