Congenital microangiopathic hemolytic anemia and thrombocytopenia with unusually large von Willebrand factor multimers and von Willebrand factor-cleaving protease

Thrombotic thrombocytopenic purpura directly linked with ADAMTS13 inhibition in the baboon (Papio ursinus)

Thrombotic thrombocytopenic purpura (TTP) is the prototypical microangiopathy characterized by disseminated microthromboses, hemolytic anemia, and ultimately organ dysfunction. A link with deficiency of the von Willebrand factor–cleaving protease (ADAMTS13) has been demonstrated, but additional genetic and/or environmental triggers are thought to be required to incite acute illness. Here we report that 4 days of ADAMTS13 functional inhibition is sufficient to induce TTP in the baboon (Papio ursinus), in the absence of inciting triggers because injections with an inhibitory monoclonal antibody (mAb) consistently (n = 6) induced severe thrombocytopenia (< 12 × 109/L), microangiopathic hemolytic anemia, and a rapid rise in serum lactate dehydrogenase. Immunohistochemical staining revealed the characteristic disseminated platelet- and von Willebrand factor–rich thrombi in kidney, heart, brain, and spleen but not lungs. Prolonged inhibition (14 days, n = 1) caused myocardial ischemic damage and asplenia but not death. Control animals (n = 5) receiving equal doses of a noninhibitory anti-ADAMTS13 mAb remained unaffected. Our results provide evidence for a direct link between TTP and ADAMTS13 inhibition and for a mild disease onset. Furthermore, we present a reliable animal model of this disease as an opportunity for the development and validation of novel treatment strategies.

Pathogenesis of thrombotic microangiopathies

Profound thrombocytopenia and microangiopathic hemolytic anemia characterize thrombotic microangiopathy, which includes two major disorders: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). TTP has at least three types: congenital or familial, idiopathic, and nonidiopathic. The congenital and idiopathic TTP syndromes are caused primarily by deficiency of ADAMTS13, owing to mutations in the ADAMTS13 gene or autoantibodies that inhibit ADAMTS13 activity. HUS is similar to TTP, but is associated with acute renal failure. Diarrhea-associated HUS accounts for more than 90% of cases and is usually caused by infection with Shiga-toxin-producing Escherichia coli (O157:H7). Diarrhea-negative HUS is associated with complement dysregulation in up to 50% of cases, caused by mutations in complement factor H, membrane cofactor protein, factor I or factor B, or by autoantibodies against factor H. The incomplete penetrance of mutations in either ADAMTS13 or complement regulatory genes suggests that precipitating events or triggers may be required to cause thrombotic microangiopathy in many patients.

Platelets: thrombotic thrombocytopenic purpura

Abnormalities of plasma von Willebrand factor (VWF) have been recognized to be associated with thrombotic thrombocytopenic purpura (TTP) for over 20 years. Patients with chronic, relapsing TTP have VWF multimers that are larger than normal, similar in size to those secreted by cultured endothelial cells. Recent observations have documented that a deficiency of a VWF-cleaving protease (termed ADAMTS13) may be responsible for the presence of these unusually large VWF multimers. Multiple mutations of the ADAMTS13 gene can result in ADAMTS13 deficiency and cause congenital TTP; autoantibodies neutralizing ADAMTS13 protease activity have been associated with acquired TTP. In Section I, Dr. Evan Sadler reviews the structure, biosynthesis, and function of the ADAMTS13 protease. He describes the mutations that have been identified in congenital TTP and describes the relationship of ADAMTS13 deficiency to the development of both congenital and acquired TTP. Dr. Sadler postulates that the development of TTP may be favored by conditions that combine increased VWF secretion, such as during the later stages of pregnancy, and decreased ADAMTS13 activity. In Section II, Dr. Bernhard Lämmle describes the assay methods for determining ADAMTS13 activity. Understanding the complexity of these methods is essential for understanding the difficulty of assay performance and the interpretation of assay data. Dr. Lämmle describes his extensive experience measuring ADAMTS13 activity in patients with TTP as well as patients with acute thrombocytopenia and severe illnesses not diagnosed as TTP. His data suggest that a severe deficiency of ADAMTS13 activity (< 5%) is a specific feature of TTP. However, he emphasizes that, although severe ADAMTS13 deficiency may be specific for TTP, it may not be sensitive enough to identify all patients who may be appropriately diagnosed as TTP and who may respond to plasma exchange treatment. In Section III, Dr. James George describes the evaluation and management of patients with clinically suspected TTP, as well as adults who may be described as having hemolytic-uremic syndrome (HUS). Dr. George presents a classification of TTP and HUS in children and adults. Appropriate evaluation and management are related to the clinical setting in which the diagnosis is considered. A clinical approach is described for patients in whom the diagnosis of TTP or HUS is considered (1) following bone marrow transplantation, (2) during pregnancy or the postpartum period, (3) in association with drugs which may cause TTP either by an acute immune-mediated toxicity or a dose-related toxicity, (4) following a prodrome of bloody diarrhea, (5) in patients with autoimmune disorders, and (6) in patients with no apparent associated condition who may be considered to have idiopathic TTP. Patients with idiopathic TTP appear to have the greatest frequency of ADAMTS13 deficiency and appear to be at greatest risk for a prolonged clinical course and subsequent relapse. Management with plasma exchange has a high risk of complications. Indications for additional immunosuppressive therapy are described.

ADAMTS13 gene mutation in congenital thrombotic thrombocytopenic purpura with previously reported normal VWF cleaving protease activity

Deficiency of von Willebrand factor (VWF) cleaving protease ADAMTS13 is associated with the development of thrombotic thrombocytopenic purpura (TTP). A case of congenital TTP that was previously reported to have normal ADAMTS13 activity was analyzed at the molecular level. Reanalysis of plasma VWF cleaving protease activity using a different assay revealed that the patient had less than 0.1 U/L ADAMTS13 protease activity, while the parents were both partially deficient. Sequence analysis of DNA amplified by polymerase chain reaction showed that the patient was homozygous for a novel TT deletion in exon 15 of the ADAMTS13 gene resulting in a frameshift, while both parents were heterozygous for the same mutation. Taken together with other recent reports, all the cases of hereditary TTP studied by DNA sequence analysis to date appear to be due to mutations within the ADAMTS13 gene.

Advances in the pathogenesis, diagnosis, and treatment of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) and the hemolytic uremic syndrome (HUS) are both characterized by thrombocytopenia, microangiopathic hemolysis, and organ dysfunction. Other disorders occasionally present with similar manifestations. Recent studies have demonstrated that deficiency in the von Willebrand factor cleaving protease ADAMTS13, due to genetic mutations or autoimmune inhibitors, causes TTP. Molecular cloning of ADAMTS13 elucidates the structure of the protease, raising the prospect for advances in diagnosis and treatment of the disease. Assay of ADAMTS13 activity distinguishes TTP from HUS and other types of thrombotic microangiopathy (TMA); therefore, the term TTP/HUS should be avoided because it obscures the known or potential differences among the various types of TMA.

Case series of thrombotic thrombocytopenic purpura in children and adolescents

Thrombotic thrombocytopenic purpura (TTP) is a well-described entity in adults but is rarely observed in children. The authors describe a series of seven children with suspected acquired TTP. Clinical findings included petechiae, purpura, or jaundice ( 6), central nervous system events ( 5), fever ( 3), diarrhea ( 3), renal insufficiency ( 2), and hematuria ( 2). Significant central nervous system events included cerebral vascular accidents ( 2), altered mental status ( 2), seizures ( 1), and hemiparesis ( 1). Patients were treated with daily plasma infusions (1/7) or plasma exchange (5/7). Response was prompt, although relapses were frequent. Decreased vWF-protease activity was found in four of five cases and vWF-protease inhibitors were found in three of five cases. Although rare, TTP is a life-threatening illness that does occur in children and should be considered in the differential diagnosis of thrombocytopenia with hemolytic anemia.

Platelet activation and the formation of the platelet plug: deficiency of ADAMTS13 causes thrombotic thrombocytopenic purpura

In the circulation, a plasma metalloprotease, ADAMTS13, cleaves von Willebrand factor (vWF) in a shear-dependent manner. This article reviews the role of this cleavage in regulating vWF-platelet interaction and proposes a scheme for understanding how a deficiency of ADAMTS13 results in the development of microthrombi in patients with thrombotic thrombocytopenic purpura.

ADAMTS-13 rapidly cleaves newly secreted ultralarge von Willebrand factor multimers on the endothelial surface under flowing conditions

Thrombotic thrombocytopenic purpura (TTP) is a devastating thrombotic disorder caused by widespread microvascular thrombi composed of platelets and von Willebrand factor (VWF). The disorder is associated with a deficiency of the VWF-cleaving metalloprotease, ADAMTS-13, with consequent accumulation of ultralarge (UL) VWF multimers in the plasma. ULVWF multimers, unlike plasma forms of VWF, attach spontaneously to platelet GP Ibalpha, a component of the GP Ib-IX-V complex. We have found that ULVWF multimers secreted from stimulated endothelial cells (ECs) remained anchored to the endothelial surface where platelets and Chinese hamster ovary cells expressing the GP Ib-IX-V complex attached to form long beads-on-a-string structures in the presence of fluid shear stresses in both the venous (2.5 dyne/cm(2)) and arterial (20 and 50 dyne/cm(2)) ranges. Although measurement of the activity of the ADAMTS-13 VWF-cleaving metalloprotease in vitro requires prolonged incubation of the enzyme with VWF under nonphysiologic conditions, EC-derived ULVWF strings with attached platelets were cleaved within seconds to minutes in the presence of normal plasma (containing approximately 100% ADAMTS-13 activity) or in the presence of partially purified ADAMTS-13. By contrast, the strings persisted for the entire period of perfusion (10 minutes) in the presence of plasma from patients with TTP containing 0% to 10% ADAMTS-13 activity. These results suggest that cleavage of EC-derived ULVWF multimers by ADAMTS-13 is a rapid physiologic process that occurs on endothelial cell surfaces.

ADAMTS13 and TTP

Thrombotic thrombocytopenic purpura (TTP) has been a mysterious and deadly disease that often could be treated effectively by plasma exchange, but without real understanding of the underlying pathophysiology. Recent advances now suggest that deficiency of a specific von Willebrand factor (VWF) cleaving protease promotes tissue injury in TTP. VWF multimers participate in the formation of platelet thrombi. Proteolytic cleavage of VWF multimers normally limits platelet thrombus growth, and failure to cleave VWF appears to encourage microvascular thrombosis. The VWF cleaving protease proves to be a new member of the ADAMTS family of metalloproteases, designated ADAMTS13. Autoantibodies that inhibit ADAMTS13 cause sporadic TTP, and mutations in the ADAMTS13 gene cause an autosomal recessive form of chronic relapsing TTP. Further studies of ADAMTS13 seem likely to change our approach to the diagnosis and treatment of TTP and other thrombotic microangiopathies.