The Effect of the PAR4 Ala120Thr Variant on the Platelet Thrombin Response, Receptor Desensitization, and Platelet Antagonists
The F2RL3 gene encodes protease activated receptor 4 (PAR4) and contains a racially dimorphic single nucleotide polymorphism (rs773902). This SNP alters the amino acid at residue 120 with the A variant, more prevalent in Blacks (Americans of African Ancestry), encoding a threonine and the G variant, more prevalent in Whites (Americans of European Ancestry), encoding an alanine. This PAR4 variant alters activation by the artificial peptide sequence AYPGKF with the threonine variant exhibiting enhanced reactivity. To investigate the effect of the Ala/Thr PAR4 variant on the physiological agonist thrombin, healthy human donors (n=184) were screened and genotyped for rs773902. Platelet function was assessed in response to thrombin in the presence and absence of platelet antagonists. Rs773902 AA homozygotes exhibited increased aggregation when stimulated with subnanomolar concentrations of thrombin compared to rs773902 GG homozygotes. Additionally, rs773902 AA homozygotes had increased thrombin induced Ca2+ mobilization and granule release. Unexpectedly, the increased platelet thrombin response in rs773902 AA homozygotes required both PAR4 and PAR1 activity. There was no observed genotype effect on thrombin induced PAR4 cleavage, PAR1-PAR4 dimerization, or lipid raft localization. However, rs773902 AA homozygotes required 3-fold higher concentrations of AYPGKF for receptor desensitization. Both allosteric P2Y12 antagonism with Ticagrelor, as well as PAR1 antagonism with Vorapaxar eliminated the rs773902 genotype effect at subthreshold concentrations of thrombin. However, PAR1 antagonism did not eliminate the genotype effect at threshold concentrations of thrombin. In addition, competitive antagonism of P2Y12 failed to eliminate the genotype effect. The F2RL3 rs773902 polymorphism alters platelet reactivity to thrombin requiring both PAR4 and PAR1 activity. Allosteric P2Y12 antagonism eliminates the genotype effect suggesting that antiplatelet therapy that includes non-competitive P2Y12 antagonists that do not compete with ADP may be beneficial for individuals with either rs773902 variant. Lastly, the rs773902 A variant is resistant to desensitization and may contribute to the enhanced platelet thrombin response potentially through altered receptor trafficking.
Whitley, Michael, "The Effect of the PAR4 Ala120Thr Variant on the Platelet Thrombin Response, Receptor Desensitization, and Platelet Antagonists" (2018). ETD Collection for Thomas Jefferson University. AAI10934296.