Schematic representation of selected platelet responses to activation and platelet function abnormalities associated with RUNX1 mutations. Platelet receptor activation results in the formation of intracellular mediators that regulate the end responses, such as aggregation and secretion from AGs and DGs, and from vesicles bearing acid hydrolases. Receptor activation leads to hydrolysis of PIP₂ by phospholipase C to form diacylglycerol, which activates protein kinase C, and IP₃, which mediates the rise in cytoplasmic Ca²⁺ levels. Protein kinase C phosphorylates numerous proteins including pleckstrin. The increase in Ca²⁺ levels leads to other responses, such as activation of MYC kinase to phosphorylate MLC and activation of PLA₂, which mediates the release of free arachidonic acid from phospholipids. Arachidonic acid is converted by CO and TS to thromboxane A₂. Numerous defects in platelet function have been described in platelets with RUNX1 haplodeficiency. These are shown with downward arrows (blue). Included below in this legend in parenthesis are some of the relevant genes that are RUNX1 targets and downregulated in RUNX1 haplodeficiency. The abnormalities include reduction in the surface receptors for TPO (MPL); defects in signaling mechanisms, including impaired pleckstrin and MLC phosphorylation, and decreased PRKCQ and MLC (MYL9); decreased ALOX12 and 12-HETE production; impaired activation of GPIIb-IIIa and aggregation on platelet activation; DG (PLDN) and AG (PF4) deficiency; and impaired secretion of AG and DG contents and from vesicles containing acid hydrolases. Other genes shown to be downregulated and not shown in the figure include PCTP and NFE2. 12-HETE, 12-hydroxyeicosatetraenoic acid; ADP, adenosine 5′-diphosphate; CO, cyclooxygenase; IP₃, inositoltrisphosphate; LO, lipoxygenase; PLA₂, phospholipase A₂; PGG2, prostaglandin G2; PGH2, prostaglandin H2; PIP₂, phosphatidylinositol bisphosphate; PLC, phospholipase C; TS, thromboxane synthase; TxA2, thromboxane A2.