The Common PAR4 Ala120Thr Variant Has a Major Effect on Platelet Reactivity to Thrombin and These Effects Are Enhanced with PAR1 and P2Y12 Inhibition

• Published in: Blood 2016-12, Vol.128 (22), p.709-709
• Main Authors: Whitley, Michael J., Vesci, Joanne, Holinstat, Michael, Edelstein, Leonard C., Bray, Paul F.
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V128.22.709.709

▪ Introduction: Human platelets express two thrombin receptors, protease activated receptor (PAR) 1 and PAR4 with different affinities for thrombin and different signaling properties. PAR1 and PAR4 have non-identical tissue expression patterns and repertoire of protease activators. We have shown a significant difference in activation kinetics and calciummobilization between platelets from healthy white and black subjects stimulated with PAR4-AP (Nat Med 2013). Part of these racial differences appears to be mediated by PAR4 signaling through Gq to Rap1 and PKC (ATVB 2014). Given the importance of platelet thrombus formation in ischemic arterial disease, these findings may contribute to the known worse outcomes in blacks compared to whites after acute coronary syndromes. Genomic approaches demonstrated that ~50% of the racial variance in PAR4-AP-induced platelet aggregation and calcium flux is caused by an Ala120Thr substitution in PAR4 (encoded by rs773902) and that the Thr120 variant is the "hyperreactive" compared to Ala120 (Blood 2014). Importantly, the Thr120 allele frequency is 63% in blacks and 19% in whites. However, the effect of the physiologic activator, thrombin, on this platelet PAR4 variant (independent of race) has not been characterized. The goals of this work were to perform a detailed characterization of the PAR4 Ala120Thr effect on thrombin-induced platelet function, and to assess pharmacogenetic effects of the PAR4 Ala120Thr variant on standard anti-platelet agents and novel PAR4 inhibitors. Results:We recruited 130 healthy, multi-racial donors and genotyped for rs773902. Subjects were excluded who had reduced arachidonic acid aggregation or who had the rare PAR4 Val296 variant. Preliminary studies showed heterozygotes had intermediate phenotypes to all assays, so most comparisons were between washed platelets homozygous for Thr120 or Ala120. The thrombin dose response curve for Thr120 homozygotes was left-shifted relative to Ala120 homozygotes (p=0.022 for genotype effect; n=10 per genotype). The concentration of thrombin required to produce 50% maximal aggregation (ED50) was 17% lower for Thr120 homozygotes than for Ala120 homozygotes (0.041 ± 0.002 U/mL vs. 0.049 ± 0.002 U/mL, respectively; p