Shigatoxin Triggers Thrombotic Thrombocytopenic Purpura in Genetically Susceptible ADAMTS13-Deficient Mice.

Abstract

The life threatening disease TTP is associated with ultra-large von Willebrand Factor multimers (UL-VWF) in the circulation due to inherited or acquired deficiency of the ADAMTS13 metalloprotease. Here we show that ADAMTS13-deficient mice generated by gene targeting are viable and exhibit normal survival through 2 years of age. Despite the absence of VWF-cleaving protease activity (<1% of normal), wild-type and ADAMTS13-deficient plasma exhibit identical VWF multimer distributions, and Adamts13−/− mice develop spontaneous TTP at an extremely low rate (2 cases out of 358 mice). However, intravital microscopy demonstrated that VWF-mediated platelet-endothelial interactions are significantly prolonged in Adamts13−/− mice. These observations suggested that additional environmental triggers and/or genetic modifying factors may be required to bring about TTP in the setting of ADAMTS13 deficiency. To address the effect of VWF level on development of TTP, Adamts13−/− mice were crossed to mice of the CASA/Rk strain which exhibit markedly elevated plasma VWF levels. Resulting CASA/Adamts13−/− mice demonstrated plasma VWF ranging from 150% to 600% of C57BL/6 controls, and we found that 21% of these mice were thrombocytopenic at baseline (vs. 0% of controls). Introduction of the CASA/Rk genetic background also resulted in the appearance of UL-VWF in CASA/Adamts13−/− mice, further prolonged VWF-mediated platelet-endothelial cell interactions, increased the rate of spontaneous TTP, and markedly decreased survival. Challenge of CASA/Adamts13−/− mice with shigatoxin (derived from bacterial pathogens associated with the related human disease hemolytic uremic syndrome) resulted in a striking syndrome closely resembling human TTP, with thrombocytopenia, profound microangiopathic hemolytic anemia, and platelet- and VWF-thrombi seen in multiple organs. Surprisingly, we observed no correlation between plasma VWF level and severity of TTP, implying the existence of TTP-modifying genes distinct from VWF. Our laboratory is pursuing the identification of these genes which may provide insight into the pathogenesis and treatment of TTP in humans. Finally, our data also suggest that microbial-derived toxins, or possibly other sources of endothelial injury, may be among the key factors required to trigger acute TTP in the setting of ADAMTS13 deficiency.