動脈血栓症の制圧 : VWF-GPIb軸依存性血小板血栓形成を調節するADAMTS13の基礎・臨床病態解析

Abstract

The mortality due to arterial thrombosis such as myocardial infarction and cerebral infarction exceeds that of malignant neoplasm, is increasing further, and is also becoming a big social issue in our country. Currently, arterial thrombosis is assumed to be established in vivo under rheological conditions where blood flow creates high shear stress. Under such conditions, the von Willebrand factor (VWF)-platelet glycoprotein (GP) Ib interaction plays a definitive role on platelet adhesion and aggregation that could be a key event for the formation of arterial thrombosis. In this regard. the biological activity of VWF under high shear stress is absolutely dependent upon its unique multimeric structure, the size of which is thought to be precisely regulated in vivo by the recently identified specific VWF-cleaving protease (ADAMTS13). Thus, the aim of our research project is to overcome arterial thrombosis targeting the functional relationships between the VWF-GPIb axis and ADAMTS13 in platelet thrombus formation. In the initial stage of our research, we have focused on the structure-function relationships of VWF. We have clarified the several functional sites within the VWF molecule critical for platelet thrornbus formation. In addition, our functional studies of VWF using a perfusion chamber system that can reproduce physiologic blood flow in vitro elucidated the shear-dependent function of VWF in platelet adhesion and aggregation, as well as the molecular mechanisms of the VWF-GPIb axis regulation by ADAMTS13 under blood flow conditions. Thus, our results altogether contributed to extend our understandings of molecular mechanisms for the arterial thrombosis. In the progressed stage of our research. our focus was shifted to the pathophysiology of the ADAMTS13 molecule. In particular, analyzing the phenotype-genotype relationships of congenital deficiency of ADAMTS13, termed Upshaw-Schulman syndrome (USS), due to ADAMTS13 gene mutations, can provide in vivo prototypic model of platelet thrombus formation under high shear stress. But USS is an extremely rare disease or often masqueraded as an isolated thrombocytopenia with mild clinical signs during childhood. During the past 10 years, we have diagnosed 37 patients with USS by assaying ADAMTS13 activity and its inhibitor titers in the laboratory of Nara Medical University. Further, through analyzing the natural history and ADAMTS13 gene mutations in these patients, we found that severe neonatal jaundice that requires exchange blood transfusions, a classic hallmark of USS, was seen in only 16 (43%) of 37 patients. Twenty-nine (79%) of the 37 patients had a history of thrombocytopenia during childhood that was misdiagnosed as idiopathic thrombocytopenic purpura(ITP). Nine women from 6 families were first diagnosed during pregnancy. Further, we documented that thrombocytopenia inevitably developed during the 2nd or 3rd trimesters of all 16 pregnancies in these 9 women. Often the initial isolated thrombocytopenia was followed by overt signs of microangiopathic hemolytic anemia and thrombotic thrombocytopenic purpura (TTP). Notably, of their 16 pregnancies,8 infants were stillborn or died soon after birth; the remaining 8 were all premature but survived. Six of these 9 women had the episodes of severe-to-mild thrombocytopenia during childhood that had been incorrectly diagnosed as ITP. In extension of the abovementioned studies, we have identified that the following disorders including TTP are related to acquired deficiencies of ADAMTS13 activity with an appearance of UL-VWFM; brain infarction, renal insufficiency, habitual abortion, liver cirrhosis，liver transplantation, acute severe pancreatitis, hepatic ven←occlusive disease, cardiac infarction, and sleeping apnea syndrome. Thus, during the past few years, Copernican-like conversion has been made on the understanding these diseases. Evolution of new rapid assays evaluating ADAMTS13 activity developed by our research group certainly accelerated this. Further, the idea of contraindication for platelet transfusions to the patients with severe deficiency of ADAMTS13 activity has also been well established, because of confident observations on platelet hyperaggregability under high shear stress in the presence of UL-VWFM.