Impaired Cellular Trafficking of a Novel Thrombomodulin Mutant Causes Severe Bleeding, Thrombosis, and Atypical HUS

Thrombomodulin is an endothelial membrane protein that has anticoagulant and anti-inflammatory actions, inhibits fibrinolysis, and modulates innate immunity and complement activation. We report the characterization of a novel thrombomodulin (THBD) mutation, C256S, homozygously expressed in an adolescent male. The patient presented with a wide range of symptoms, including life-threating epistaxis, thrombotic microangiopathy, massive intraoperative bleeding, venous thromboses and skin necrosis. THBD-C256S is predicted to prevent the formation of a critical disulfide bond within EGF-like region 1. The mutant was non-functional when expressed in a zebrafish model. In patient-derived endothelial cells, THBD-C256S exhibited impaired glycosylation, increased intracellular retention, and reduced cell surface levels of thrombomodulin. Expression of the mutant led to decreased levels of other endothelial proteins, including PECAM-1 and VE-cadherin. Our data suggest that misfolding of THBD-C256S disrupts normal thrombomodulin functions leading to the broad constellation of clinical abnormalities.