Understanding how a highly prevalent GRK5 polymorphism affects platelets and enhances thrombotic risk.

Inherited genetic variants that modulate platelet function contribute significantly to thrombotic disorders, yet their mechanisms and clinical implications remain underexplored. Two genome-wide association studies identified an A→G variant (rs10886430) in the first intron of G protein-coupled receptor kinase 5 (GRK5), found in homozygosity in ∼5 million Americans. The homozygous GRK5 GG genotype is associated with an increased risk of stroke and venous thromboembolism, but the mechanistic link between this variant and thrombotic risk has remained unclear. To investigate this, we identified 3 GG individuals. GRK5 protein levels in GG platelets were 90% lower than in AA controls. The significant reduction in GRK5 levels in GG platelets led to elevated platelet responsiveness to thrombin and a protease-activated receptor 1 (PAR1) agonist but not a PAR4 agonist. These findings were corroborated in GRK5-/- induced pluripotent stem cell-derived megakaryocytes, transgenic Grk5-deficient murine platelets, and AA platelets exposed to a GRK5 inhibitor. We demonstrated that PAR1 internalization was reduced in GG platelets, leading to enhanced PAR1 signaling. Under venous shear in an endothelialized microfluidic system, GG platelets exhibited increased accumulation, which was reversed by PAR1 inhibition with vorapaxar. In an arterial murine thrombosis model following human platelet infusion, GG platelets also showed enhanced thrombus formation in vivo. This study provides, to our knowledge, the first experimental evidence directly linking a highly prevalent human GRK5 variant to defective PAR1 regulation and increased thrombotic risk. Together, these findings establish that the GRK5 GG genotype confers increased thrombotic potential through impaired PAR1 desensitization, providing mechanistic insight that connects human genetics, thrombin receptor signaling, and thrombotic disease.