Thrombotic thrombocytopenic purpura (TTP) is a rare but increasingly encountered disorder associated with abnormalities of plasma von Willebrand factor (vWF). Moschcowitz reported the first case in 1924, and a major review of TTP in 1966 described the clinical features of 271 patients. TTP was characterized by a classic pentad of signs, present to variable degrees: microangiopathic hemolytic anemia, thrombocytopenia, neurological dysfunction, renal failure, and fever. No effective treatment was known, and mortality was an impressive 90%. Today, plasma exchange therapy has reduced the mortality to about 20%, although why it works is not certain.
Many etiologies for TTP have been proposed, including endothelial damage and defects of platelets or plasma components. The first association with vWF was made in 1982, when Moake discovered that “ultra-large” vWF (UL-vWF) multimers occurred in patients with relapsing TTP. The absence of large vWF multimers impairs platelet adhesion and causes bleeding, particularly in von Willebrand disease type 2A. Conversely, UL-vWF multimers were proposed to mediate platelet aggregation, thrombosis, and microangiopathic hemolysis in TTP.
The UL-vWF theory of TTP gained considerable support with the discovery in 1997 that most patients with congenital relapsing TTP lack a plasma protease that cleaves vWF and converts large multimers into smaller forms. The following year, 2 studies documented that acquired TTP is associated with autoimmune IgG inhibitors of this protease. These observations suggested that plasma exchange is beneficial because it removes inhibitors (if present) and infuses a specific protease. But plasma exchange is expensive and cumbersome and has significant risks. Furthermore, the proposed role of UL-vWF multimers remained dependent on correlation rather than direct evidence. Clearly, identifying the vWF-cleaving protease would open new scientific, diagnostic, and therapeutic opportunities for this enigmatic disorder.
In this issue, 2 groups report the purification and amino-terminal sequence of the vWF protease. Gerritsen and colleagues (page 1654) used the IgG fraction from a patient with TTP containing an antibody protease inhibitor to purify the protease from plasma mainly by immunoaffinity chromatography. Fujikawa and coworkers (page 1662) used friendlier starting material than plasma, factor VIII/vWF concentrates, and employed conventional chromatography on heparin, ion exchange, and size exclusion columns. From the products of these independent purification methods, both groups obtained the same protein sequence.
The vWF protease is a 150-kd single-chain glycoprotein that requires divalent metal ions. Exploring the human genomic database, Fujikawa and colleagues found a match between their N-terminal protein sequence and an EST cluster from chromosome 9q34. This composite cDNA sequence encodes a metalloprotease belonging to the ADAMTS subfamily, although the expected thrombospondin domain is not part of this partial sequence. ADAMTS proteases typically contain both zinc and calcium ions, and the congruence between the known properties of the vWF protease and ADAMTS proteases provides additional compelling evidence that the vWF protease has been identified.
To what use might this initial information be applied? It is likely that polyclonal and monoclonal antibodies can be prepared against the deduced protein sequence, making quantitation of plasma protease antigen and a rapid diagnostic test for TTP no more than a clinical ELISA away. But a significant fraction of patients with microangiopathy have normal levels of this protease, and new tools based on this new protease should facilitate the dissection of these potentially distinct entities that converge on a common TTP-like phenotype. Ultimately, completion of the cDNA sequence will lead to production of recombinant protein, which could enable specific therapy for the rare patient with congenital TTP and possibly for patients with low-titer autoantibodies. Eventually we should be able to prove or refute the UL-vWF multimer theory of TTP, thereby moving us one step closer to a cure.
