Mutation of the β1-tubulin gene associated with congenital macrothrombocytopenia affecting microtubule assembly

Abstract

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. We identified the first TUBB1 mutation, R318W, in a patient with congenital macrothrombocytopenia. The patient was heterozygous for Q43P, but this single-nucleotide polymorphism (SNP) did not relate to macrothrombocytopenia. Although no abnormal platelet β1-tubulin localization/marginal band organization was observed, the level of β1-tubulin was decreased by approximately 50% compared with healthy controls. Large and irregular bleb protrusions observed in megakaryocytes derived from the patient's peripheral blood CD34+ cells suggested impaired megakaryocyte fragmentation and release of large platelets. In vitro transfection experiments in Chinese hamster ovary (CHO) cells demonstrated no incorporation of mutant β1-tubulin into microtubules, but the formation of punc- tuated insoluble aggregates. These results suggested that mutant protein is prone to aggregation but is unstable within megakaryocytes/platelets. Alternatively, mutant β1-tubulin may not be transported from the megakaryocytes into platelets. W318 β1-tubulin may interfere with normal platelet production, resulting in macrothrombocytopenia. © 2009 by The American Society of Hematology.