X-linked Thrombocytopenia Caused by a Mutation in the Wiskott-Aldrich Syndrome (WAS) Gene That Disrupts Interaction with the WAS Protein (WASP)-interacting Protein (WIP)

Objective: We studied two adult brothers with severe congenital thrombocytopenia in order to determine the genetic etiology of their inherited disorder. Despite the absence of eczema or immunodeficiency, a mutation of the Wiskott-Aldrich syndrome (WAS) gene was suspected because of the presence of microthrombocytes.

Materials And Methods: Peripheral blood was obtained for characterization of hematopoietic cells and megakaryocyte progenitors. The coding region of the WAS gene was fully sequenced, and expression of the Wiskott-Aldrich syndrome protein, WASP, was evaluated by immunoblotting. The ability of WASP to physically associate with the WASP-interacting protein, WIP, was tested by yeast and mammalian two-hybrid techniques.

Results: In addition to thrombocytopenia, our investigation revealed an increased frequency of peripheral megakaryocyte progenitors (CFU-Mk) and incomplete cytoplasmic maturation by electron microscopy. Sequencing the WAS gene revealed a single base mutation, resulting in substitution of proline for arginine 138 (i.e., Arg138Pro). Immunoblotting demonstrated reduced expression of the mutant WAS protein, and we showed that the Arg138Pro mutation significantly, but incompletely, disrupts WASP-WIP interaction.

Conclusions: In this pedigree, X-linked thrombocytopenia is caused by a rare mutation in the fourth exon of the WAS gene. WASP levels are reduced in lymphocyte cell lines derived from the affected individuals. Furthermore, the mutation significantly but incompletely disrupts WASP-WIP interaction, whereas substitution of alanine or glutamic acid residues at the same position does not. This raises the possibility that protein-protein interaction and WASP stability are related properties.