Additional Environmental and/or Genetic Factors Are Required to Trigger TTP in ADAMTS13-Deficient Mice.

Thrombotic Thrombocytopenic Purpura (TTP) is a life threatening systemic illness characterized by the formation of platelet-rich thrombi in the microcirculation and the clinical pentad of fever, hemolytic anemia, thrombocytopenia, neurological symptoms, and renal dysfunction. TTP is associated with ultra-large von Willebrand Factor multimers (UL-VWF) in the circulation due to deficiency of the ADAMTS13 metalloprotease. ADAMTS13 gene mutations account for most if not all cases of familial TTP, and autoantibodies to ADAMTS13 underlie most cases of acquired TTP. To further explore the pathogenesis of TTP in vivo, ADAMTS13 deficient mice were generated by gene targeting to remove exons 1–6 which encode most of the protease domain of ADAMTS13. Although the resulting homozygous null mice (Adamts13−/−) have lost all specific VWF-cleaving protease activity (< 1% of control), embryonic development is normal, and the null mice are born at the expected Mendelian frequency. Analysis of baseline hematologic parameters and peripheral blood smears revealed no difference between Adamts13−/− mice and wild-type littermates, with no evidence for thrombocytopenia or microangiopathic hemolytic anemia. Pathologic survey of multiple tissues revealed only normal histology and no evidence for platelet or VWF-rich thrombi in the vasculature. Despite the absence of VWF-cleaving protease activity, plasma from wild-type and ADAMTS13-deficient mice exhibited identical VWF multimer size distributions. However, VWF multimers from both wild-type and Adamts13−/− mice were observed to be considerably larger than those from normal human plasma, and equivalent in size to UL-VWF seen in plasma from familial TTP patients. Challenge of Adamts13−/− mice by injection with endotoxin, and genetic crosses to mice with markedly elevated VWF levels (CASA/Rk), failed to induce findings consistent with TTP. However, treatment with verotoxin-2 (a bacterial endothelial toxin important in the pathogenesis of the hemolytic uremic syndrome), caused thrombocytopenia in 6 of 11 Adamts13−/− mice (vs. 3 of 11 wild-type controls, p < 0.07) and mortality at 6 days in 9 of 11 Adamts13−/− mice (vs. 6 of 11 controls, p < 0.02). Examination of peripheral blood from one of the Adamts13−/− mice at 8 days following verotoxin administration demonstrated marked microangiopathic changes. In conclusion, mice with targeted disruption of the Adamts13 gene do not develop TTP spontaneously, suggesting the requirement for additional environmental triggers or genetic modifiers. Though some humans with congenital ADAMTS13 deficiency have been reported to remain asymptomatic for many years, the increased size of the baseline VWF multimer distribution in mice may indicate a higher threshold for VWF-ADAMTS13 interaction which may be protective for TTP. Our results also suggest that microbial-derived toxins, or other sources of endothelial injury, may be one of the key environmental triggers responsible for the lack of spontaneous TTP findings in the ADAMTS13-deficient mice, and possibly for the intermittent symptoms seen in humans.