Why patients with THBD c.1611C>A (p.Cys537X) nonsense mutation have high levels of soluble thrombomodulin?

BACKGROUND Recently our group has described a new autosomal dominant bleeding disorder characterized by very high plasma levels of soluble thrombomodulin (TM). The THBD c.1611C>A (p.Cys537X) mutation in heterozygous state was found in the propositus. This mutation leads to the synthesis of a truncated TM which has lost the last three amino-acids of the transmembrane domain and the cytoplasmic tail. OBJECTIVE We investigated the mechanism responsible for TM shedding in endothelial cells with THBD c.1611C>A mutation. METHODS Complementary DNA of TM wild type (TM-WT) was incorporated into a pcDNA3.1 vector for transient transfection in COS-1 cells. Mutagenesis was performed to create the c.1611C<A (TM1-536) mutant and 4 other TM mutants (TM1-515, TM1-525, TM1-533 and TM1-537) with a transmembrane domain having different lengths. The effect of shear stress, metalloprotease inhibitor, certain proteases and reducing agents were tested on TM shedding. RESULTS Western blot and immunofluorescent analysis showed that TM1-536 was produced and a certain amount of TM1-536 was anchored on the cell membrane. A significantly higher levels of soluble TM was observed in the TM1-536 cell medium in comparison with TM-WT (56.3 +/- 5.2 vs 8.8 +/- 1.6 ng/mL, respectively, p = 0.001). The shedding of TM1-536 was 75% decreased in cells cultured in the presence of a metalloprotease inhibitor. No difference was observed between TM1-536 and TM-WT shedding after cell exposure to cathepsin G, elastase, several reducing agents and high shear stress (5000 s-1). Significantly higher levels of soluble TM were observed in the cell media of TM1-533, TM1-525, TM1-515 in comparison with TM-WT (p < 0.05). CONCLUSION The mechanism responsible for TM shedding is complex and is not completely understood: higher sensitivity of the TM1-536 to the proteolysis by metalloproteases and a defect of synthesis due to the decreased size of the transmembrane domain might explain the high levels of soluble TM in plasma of the carriers.