Thrombotic thrombocytopenic purpura and the hemolytic uremic syndrome

Archives of Pathology & Laboratory Medicine,  Nov 2002  by Moake, Joel L

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* Objective-To evaluate the usefulness and feasibility of measuring plasma von Willebrand factor (vWF)-cleaving metalloprotease activity (ADAMTS 13) in the differential diagnosis of thrombotic thrombocytopenic purpura (TTP), the hemolytic uremic syndrome, and other thrombotic microangiopathies.

Data Sources.-Articles published in the medical literature.

Data Extraction and Synthesis.-In TTP, a multimeric form of vWF that is larger than that ordinarily found in the plasma may cause systemic platelet aggregation under the high-shear conditions of the microcirculation. ADAMTS 13 is a divalent cation-activated, vWF-cleaving metalloprotease that converts unusually large vWF multimers derived from endothelial cells into smaller vWF forms in normal plasma. ADAMTS 13 is severely reduced or absent in most patients with TTP. The vWF-cleaving metalloprotease is present in fresh-frozen plasma, cryoprecipitate-depleted plasma (cryosupernatant), and in plasma that has been treated with solvent and detergent. The enzyme is defective in children with chronic relapsing TTP. Infusion of any of the plasma products that contain the vWF-cleaving metalloprotease stops or prevents (for about 3 weeks) TTP episodes in these patients. An immunoglobulin (Ig) G autoantibody to

the vWF-cleaving metalloprotease is found transiently in many adult patients with acquired acute idiopathic, recurrent, and ticlopidine/clopidogrel-associated TTP. Patients with acquired TTP require plasma exchange, that is, both infusion of a plasma product containing vWF-cleaving metalloprotease and removal of autoantibody and/or unusually large vWF multimers by plasmapheresis. The pathophysiology of platelet aggregation in bone marrow transplantation/chemotherapy-associated thrombotic microangiopathy, as well as in hemolytic uremic syndrome, is not established. in neither condition is there a severe decrease in plasma vWF-cleaving metalloprotease activity, as there is in TTP.

Conclusions.-The presently available lengthy and complicated procedure for estimation of plasma vWF-cleaving metalloprotease activity is not yet practical for rapid diagnostic use. This test has supplanted the equally lengthy and difficult, less specific analysis of plasma vWF multimeric pattern. If the clinical distinction between TTP and hemolytic uremic syndrome is uncertain, it is appropriate to acquire (before therapy) a citrate-plasma sample for the ultimate determination of vWF-cleaving metalloprotease activity.

(Arch Pathol Lab Med. 2002;126:1430-1433)

THROMBOTIC THROMBOCYTOPENIC PURPURA

von Willebrand Factor Multimers and von Willebrand

Factor-Cleaving Metalloprotease

After release from endothelial cells, large and "unusually large" von Willebrand factor (vWF) multimers are cleaved at Tyr842/Met843 in domain A2 of monomeric vWF subunits.1,2 This proteolysis is catalyzed by a divalent cation-activated, vWF-cleaving metalloprotease present in normal plasma and may occur optimally under high (arterial-type) shear stress.3 Unusually large vWF multimers are converted by this process into a series of somewhat smaller vWF forms. Unusually large vWF multimers are especially capable of mediating direct shear stress-induced platelet aggregation,4 which is distinct from the platelet aggregation that follows platelet-subendothelial adhesion at sites of vascular injury.

The vWF-cleaving metalloprotease has recently been partially purified and characterized as ADAMTS 13, a disintegrin and metalloprotease with 8 thrombospondin-1like domains and an Arg-Gly-Asp (RGD) segment.5-8 ADAMTS 13 is a Zn^sup 2+^ and Ca^sup 2+^requiring 190000-d glycoprotein that is encoded on chromosome 9q34. The enzyme is produced predominantly by liver cells. von Willebrand factor-cleaving metalloprotease activity in citrate plasma is reduced to about 50% of normal in patients with severe liver disease and in newborns (as a transient developmental process).9 Enzyme activity has also been reported to be somewhat decreased in disseminated malignancies.10 In contrast, patients with incipient TTP episodes have a severe reduction in plasma vWF-cleaving metalloprotease activity (to less than about 5%-10% of the level in normal citrate-pooled plasma).11-17

It is possible that reduction of some of the disulfide bonds in large and unusually large vWF multimers18 by thrombospondin-l-like domains with disulfide bond reductase/isomerase activity, is involved in vWF multimer breakdown.19 Alternatively, the thrombospondin-1 domains may be involved in attaching the ADAMTS 13 enzyme to the surface of endothelial cells, awaiting the release of unusually large vWF multimers.

The ADAMTS 13 vWF-cleaving metalloprotease is functionally defective in patients who have congenital chronic relapsing TTP. These patients also have unusually large vWF multimers in their plasma, which may account for the periodic platelet aggregation in their high-shear microcirculation.20-22 The microvascular platelet thrombi in TTP patients have been demonstrated by immunohistochemical techniques to be vWF positive and fibrinogen negative (the opposite of thrombi in disseminated intravascular coagulation).23 Using flow cytometry, it has been shown that the single platelets of patients with chronic relapsing, acute idiopathic, intermittently recurrent, and ticlopidine- or clopidogrel-associated types of TTP have increased vWF on their surfaces at the onset of TTP episodes.24 This platelet-vWF binding is sometimes associated with disappearance of the largest plasma vWF multimers.25 The divalent cation-activated, vWF-cleaving metalloprotease that proteolyzes unusually large vWF multimers to plasma-type vWF forms in normal plasma is severely reduced or absent in most TTP patients as the episodes commence.11-13,15,17,26,27 The enzyme activity is absent or barely detectable at all times in the plasma of patients with chronic relapsing TTP11,12,14