Increased RhoA pathway activation downstream of αIIbβ3/SRC contributes to heterozygous Bernard Soulier syndrome

Bernard Soulier syndrome (BSS) is a severe bleeding disorder with moderate to severe thrombocytopenia, giant platelets, and platelet dysfunction, caused by biallelic mutations in GP1BA, GP1BB, or GP9 genes. We generated induced pluripotent stem cells (iPSC) from a BSS patient with a novel heterozygous GP1BA p.N103D mutation, resulting in moderate macrothrombocytopenia. The mutation does not affect megakaryocyte (MK) differentiation or GPIb-GPIX complex expression but reduces affinity to von Willebrand factor (VWF). It induces increased signaling independent of VWF and αIIbβ3-mediated outside-in signaling, causing a profound defect in proplatelet formation after adhesion on fibrinogen. Pre-activation of αIIbβ3 integrin and heightened stress fiber formation linked to RhoA pathway overactivation were observed, likely due to increased phosphorylation of SRC at Y419 downstream of GPIbα. Dasatinib, a SRC inhibitor, restored stress fiber formation. Using a 3D bone marrow model to mimic platelet release under flow, we demonstrated that the ROCK1/2 inhibitor Y27632 increased platelet number and restored platelet size in GPIbαN103D MK, as well as in MK from two other patients with heterozygous GP1BA mutations (p.L160P and p.N150S). However, Y27632 had no additional effect on platelet generation from MK of two patients with biallelic BSS, suggesting a distinct molecular mechanism in biallelic cases.