A systematic approach to understanding how patient variants affect the activity of Wiskott-Aldrich syndrome protein

Wiskott-Aldrich syndrome (WAS) and X-linked neutropenia (XLN) are caused by genetic variants in the WAS gene. How WAS variants lead to clinical disease remains unsolved in many cases. We expressed human WASp using a spider silk inspired solubility tag (NT*-tag) and inserted patients variants. Native mass spectrometry and pyrene actin assays showed that five variants (L270P, F271S, S272P, I290T, I294T) predicted to cause XLN led to open protein conformation and high actin polymerization rate in the absence of the WASp activator, Cdc42. One previously reported XLN variant (R268W), two loss-of-function WAS variants (A236G, D485N), and one variant of unknown significance (R431W) behaved similarly to wildtype WASp in terms of structural conformation and actin polymerization. Patient CD4+ T cells were used for analysis of WASp expression and phosphorylation, actin polymerization, anti-CD3 induced proliferation capacity, and upregulation of high affinity LFA-1, distinguishing loss-of-function and gain-of-function variants from benign WAS variants. This systematic approach reveals how WAS genetic variants cause severe human disease and stratify variants to guide clinical decision for definitive therapy. Key PointsO_LIGain-of-function WASp variant has extended protein conformation probed by native mass spectrometry and raised pyrene actin polymerization. C_LIO_LIFunctional analysis of patients CD4+ T cells classifies WASp variants as loss-of-function, reduced-function, gain-of-function, and benign. C_LI