The in vivo inactivation of MASTL kinase results in thrombocytopenia

OBJECTIVE: A missense mutation in the Microtubule Associated Serine/Threonine Like kinase gene (MASTL, FLJ14813) on human chromosome 10 was previously linked to a novel form of autosomal dominant inherited thrombocytopenia in a single pedigree. The mutation results in an amino acid change from glutamic acid at position 167 to aspartic acid and segregates perfectly with thrombocytopenic individuals within this extended family. The phenotype is characterized by mild thrombocytopenia with an average platelet count of 60,000 platelets per microliter of blood. We wanted to determine the expression and localization of MASTL, as well as its role in developing thrombocytes using an in vivo model system. METHODS: Northern blot analysis allowed us to examine expression patterns. Morpholino knockdown assays in zebrafish (Danio rerio) were employed to determine in vivo contribution to thrombocyte development. Transient expression in BHK cells resulted in localization of both the wild type and E167D mutant forms of MASTL kinase to the nucleus. RESULTS: Northern blot analysis indicates that MASTL mRNA is restricted in its expression to hematopoietic and cancer cell lines. A transient knockdown of MASTL in zebrafish results in deficiency of circulating thrombocytes. Transient expression of recombinant MASTL kinase in vitro demonstrates localization to the nucleus. CONCLUSIONS: Functional studies presented here demonstrate a direct relationship between the transient knockdown of the MASTL kinase expression and the reduction of circulating thrombocytes in zebrafish. This transient knockdown of MASTL in zebrafish correlates with a decrease in the expression of the thrombopoietin receptor, c-mpl, and the CD41 platelet adhesion protein, GpIIb, but has no effect on essential housekeeping zebrafish gene, EF1alpha.