Platelet dysfunction associated with the novel Trp29Cys thromboxane A₂ receptor variant.

BACKGROUND: Genetic variations that affect the structure of the thromboxane A2 receptor (TP receptor) provide insights into the function of this key platelet and vascular receptor, but are very rare in unselected populations.
OBJECTIVES: To determine the functional consequences of the TP receptor Trp29Cys (W29C) substitution. PATIENTS/
METHODS: We performed a detailed phenotypic analysis of an index case (P1) with reduced platelet aggregation and secretion responses to TP receptor pathway activators, and a heterozygous TP receptor W29C substitution. An analysis of the variant W29C TP receptor expressed in heterologous cells was performed.
RESULTS: Total TP receptor expression in platelets from P1 was similar to that of controls, but there was reduced maximum binding and reduced affinity of binding to the TP receptor antagonist [(3) H]SQ29548. HEK293 cells transfected with W29C TP receptor cDNA showed similar total TP receptor expression to wild-type (WT) controls. However, the TP receptor agonist U46619 was less potent at inducing rises in cytosolic free Ca(2+) in HEK293 cells expressing the W29C TP receptor than in WT controls, indicating reduced receptor function. Immunofluorescence microscopy and cell surface ELISA showed intracellular retention and reduced cell surface expression of the W29C TP receptor in HEK293 cells. Consistent with the platelet phenotype, both maximum binding and the affinity of binding of [(3) H]SQ29548 to the W29C TP receptor were reduced compared to WT controls.
CONCLUSION: These findings extend the phenotypic description of the very rare disorder TP receptor deficiency, and show that the W29C substitution reduces TP receptor function by reducing surface receptor expression and by disrupting ligand binding.