Jurnal Internasional Sebuah mutasi gen EPHB2 manusia mengarah ke defek fungsional trombosit utama
The thyrotine transmembrane receptor family of tyrosine kinase is involved in platelet function. We report the first variant of EPHB2 affecting platelets in 2 siblings (P1 and P2) from relatives of families with recurrent bleeding and normal platelet counts. The whole sequence identified a c.2233C> variant T (missense p.R745C) from the gene EPHB2 . P1 and P2 are homozygous for this variant, while their asymptomatic parents are heterozygous. The p.R745C variant in the tyrosine kinase domain is associated with a defect in platelet aggregation, activation of αIIbβ3, and granule secretion induced by G-protein-coupled receptor (GPCR) agonists and convulxin, and in thrombus formation in collagen under flow. In contrast, clot retraction, flow-dependent platelet adhesion, and spread on fibrinogen are only slightly affected, suggesting a limited effect on internal αIIbβ3 signaling. Most importantly, Lyn, Syk, and FcR phosphorylation, the initial steps in glycoprotein VI (GPVI) platelet signaling are drastically impaired in the absence of platelet contact, showing a positive role for EPHB2 in GPVI activation. Likewise platelet activation by PAR4-AP shows damaged Src activation, which is contrary to the normal activity of protein kinase C and Ca mobilization 2+ . Excessive expression of wild-type variants and R745C EPHB2 in RBL-2H3 cells (basophilic leukemia mice) that expressly express humans GPVI confirms that EPHB2 R745C mutations damage the autophosphorylation of EPHB2 but have no effect on ephrin ligand-induced EPHB2 clustering, suggesting it does not interfere with EPHB2 definition of cell-cell-mediator. In conclusion, this novel inheriting platelet disorders affecting EPHB2 shows that tyrosine kinase receptors play an important role in platelet function through crosstalk with GPVI and GPCR signaling.