Arg60 to Leu mutation of the human thromboxane A2 receptor in a dominantly inherited bleeding disorder.

Abstract

Recent advances in molecular genetics have revealed the mechanisms underlying a variety of inherited human disorders. Among them, mutations in G protein-coupled receptors have clearly demonstrated two types of abnormalities, namely loss of function and constitutive activation of the receptors. Thromboxane A2(TXA2) receptor is a member of the family of G protein-coupled receptors and performs an essential role in hemostasis by interacting with TXA2 to induce platelet aggregation. Here we identify a single amino acid substitution (Arg4-Leu) in the first cytoplasmic loop of the TXA2 receptor in a dominantly inherited bleeding disorder characterized by defective platelet response to TXA2. This mutation was found exclusively in affected members of two unrelated families with the disorder. The mutant receptor expressed in Chinese hamster ovary cells showed decreased agonist-induced second messenger formation despite its normal ligand binding affinities. These results suggest that the Arg'0 to Leu mutation is responsible for the disorder. Moreover, dominant inheritance of the disorder suggests the possibility that the mutation produces a dominant negative TXA2 receptor. (J. Clin. Invest.

Coactivation of two different G protein-coupled receptors is essential for ADP-induced platelet aggregation

“…The thrombin receptor has been shown to couple to both the activation of phospholipase C and inhibition of adenylyl cyclase, by coupling to different G proteins, G q and G i , (27), and hence thrombin causes platelet aggregation by activation of G q and G i pathways with the same receptor (28). Similarly, thromboxane A 2 receptors also can couple to G i and G q , although it is not clear whether this is achieved by different receptor subtypes (or splice variants) (29,30). Interestingly, platelet aggregation induced by thrombin or U46619, a thromboxane mimetic, was also abrogated in mice lacking G␣ q (26), corroborating the essential role of the G q -mediated signaling pathway in platelet aggregation.…”

Thromboxane A2 Signalling in Humans: a ‘Tail’ of Two Receptors

“…Perturbations in the levels of these two prostanoids, their synthases or their receptors have been implicated in a number of cardiovascular disorders including myocardial infarction, unstable angina, atherosclerosis, pregnancy-induced hypertension and ischemic heart disease [12][13][14][15][16]. Moreover, a number of genetic bleeding disorders have been described in humans where the individuals' platelets were unresponsive to TXA 2 [17]. Both TXA 2 and PGI 2 signal through their signature receptors, each members of the G protein coupled receptor (GPCR) superfamily [1,2,6,7].…”

TBXA2R gene variants associated with bleeding

“…To date, one quantitative defect causing reduced TPα receptor expression (2) and four qualitative defects caused by TPα receptor amino acid substitutions have been reported ((6–9); see Table I and Figure 1). …”

“…The first reported qualitative defect in the TPα receptor was caused by a missense TBXA2R variant predicting the p.Arg60Leu substitution in the TPα receptor at the start of the first intracellular loop (Figure 1) (9). This variant was first described in a patient with a history of postsurgical bleeding (9), and has since been described in a further pedigree with a history of mild bleeding (10).…”

“…This variant was first described in a patient with a history of postsurgical bleeding (9), and has since been described in a further pedigree with a history of mild bleeding (10). Platelets from affected individuals show absent or reduced aggregation to the synthetic TXA 2 analog U46619.…”