Insights into ADAMTS13 structure: impact on thrombotic thrombocytopenic purpura diagnosis and management

Immune and Hereditary Thrombotic Thrombocytopenic Purpura: Can ADAMTS13 Deficiency Alone Explain the Different Clinical Phenotypes?

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathy caused by a hereditary or immune-mediated deficiency of the enzyme ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). TTPs are caused by the following pathophysiological mechanisms: (1) the presence of inhibitory autoantibodies against ADAMTS13; and (2) hereditary mutations of the ADAMTS13 gene, which is present on chromosome 9. In both syndromes, TTP results from a severe deficiency of ADAMTS13, which is responsible for the impaired proteolytic processing of high-molecular-weight von Willebrand factor (HMW-VWF) multimers, which avidly interact with platelets and subendothelial collagen and promote tissue and multiorgan ischemia. Although the acute presentation of the occurring symptoms in acquired and hereditary TTPs is similar (microangiopathic hemolytic anemia, thrombocytopenia, and variable ischemic end-organ injury), their intensity, incidence, and precipitating factors are different, although, in both forms, a severe ADAMTS13 deficiency characterizes their physiopathology. This review is aimed at exploring the possible factors responsible for the different clinical and pathological features occurring in hereditary and immune-mediated TTPs.

An update on the pathogenesis and diagnosis of thrombotic thrombocytopenic purpura

INTRODUCTION

Severe ADAMTS13 deficiency defines thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is responsible for VWF cleavage. In the absence of this enzyme, widespread thrombi formation occurs, causing microangiopathic anemia and thrombocytopenia and leading to ischemic organ injury. Understanding ADAMTS13 function is crucial to diagnose and manage TTP, both in the immune and hereditary forms.

AREAS COVERED

The role of ADAMTS13 in coagulation homeostasis and the consequences of its deficiency are detailed. Other factors that modulate the consequences of ADAMTS13 deficiency are explained, such as complement system activation, genetic predisposition, or the presence of an inflammatory status. Clinical suspicion of TTP is crucial to start prompt treatment and avoid mortality and sequelae. Available techniques to diagnose this deficiency and detect autoantibodies or gene mutations are presented, as they have become faster and more available in recent years.

EXPERT OPINION

A better knowledge of TTP pathophysiology is leading to an improvement in diagnosis and follow-up, as well as a customized treatment in patients with TTP. This scenario is necessary to define the role of new targeted therapies already available or coming soon and the need to better diagnose and monitor at the molecular level the evolution of the disease.

ADAMTS13 conformations and mechanism of inhibition in immune thrombotic thrombocytopenic purpura

ADAMTS13, a plasma metalloprotease that cleaves von Willebrand factor, is crucial for normal hemostasis. Acquired autoantibody-mediated deficiency of plasma ADAMTS13 results in a potentially fatal blood disorder, immune thrombotic thrombocytopenic purpura (iTTP). Plasma ADAMTS13 protease appears to exist in multiple conformations. Under physiological conditions, plasma ADAMTS13 exists predominantly in its "closed" conformation (or latent form), which may be activated by lowering pH, ligand binding, and binding of an antibody against the distal domains of ADAMTS13. In patients with iTTP, polyclonal antibodies target at various domains of ADAMTS13. However, nearly all inhibitory antibodies bind the spacer domain, whereas antibodies that bind the distal C-terminal domains may activate ADAMTS13 through removing its allosteric inhibition. Additionally, the anti-C-terminal antibodies may alter the potency of inhibitory antibodies towards ADAMTS13 activity. This review summarizes some of the most recent knowledge about the ADAMTS13 conformation and its mechanism of inhibition by its autoantibodies.

Response to belimumab in thrombotic thrombocytopenic purpura associated with systemic lupus erythematosus: a case-based review

Thrombotic thrombocytopenic purpura (TTP), a life-threatening syndrome characterized by acute microangiopathic hemolytic anemia, thrombocytopenia, and visceral ischemia, can be classified as congenital TTP (inherited due to a mutation in ADAMTS13) and acquired TTP. The acquired TTP is further classified as idiopathic and secondary TTP. Systemic lupus erythematosus (SLE) is regarded as one of the most common causes of secondary TTP (SLE-TTP). In contrast to patients with idiopathic TTP, some patients with SLE-TTP, especially those diagnosed with refractory TTP, are resistant to plasma exchange and high-dose corticosteroids and usually require second-line drugs, including newly developed biologicals. Belimumab, a B-lymphocyte stimulator-specific inhibitor, was the first approved new therapy for SLE in the past 50 years. Only two cases of SLE-TTP using belimumab have been reported; however, detailed information has not been made available. Herein, we describe a 28-year-old female patient who presented with palm petechiae, strong tawny urine, and yellow stained skin and sclera, and was diagnosed with SLE-TTP supported by high anti-ANA titers; positive anti-SSA/SM; pleural effusion; decreased platelet count, hemoglobin, and complement C3/C4 counts; increased lactate dehydrogenase level, along with increased schistocytes; and a significant deficiency of ADAMTS13 activity. Belimumab (10 mg/kg) was administered after six plasma exchanges. Good efficiency and outcomes without any adverse events, SLE, or TTP relapse were observed during 12 months of follow-up. Therefore, belimumab is a promising choice for SLE-TTP management. In addition, we provide a focused review of the existing literature on the pathogenesis, diagnosis, and therapeutic strategies for SLE-TTP.

TTP: From empiricism for an enigmatic disease to targeted molecular therapies

The 100th anniversary of the first description of Thrombotic Thrombocytopenic Purpura (TTP) as a disease by Dr. Eli Moschcowitz approaches. For many decades, TTP remained mostly a mysterious fatal condition, where diagnosis was often post-mortem. Initially a pentad of symptoms was identified, a pattern that later revealed to be fallible. Sporadic observations led to empiric interventions that allowed for the first impactful breakthrough in TTP treatment, almost 70 years after its first description: the introduction of plasma exchange and infusions as treatments. The main body of knowledge within the field was gathered in the latest three decades: patient registries were set and proved crucial for advancements; the general mechanisms of disease have been described; the diagnosis was refined; new treatments and biomarkers with improvements on prognosis and management were introduced. Further changes and improvements are expected in the upcoming decades. In this review, we provide a brief historic overview of TTP, as an illustrative example of the success of translational medicine enabling to rapidly shift from a management largely based on empiricism to targeted therapies and personalized medicine, for the benefit of patients. Current management options and present and future perspectives in this still evolving field are summarized.

Conformational plasticity of ADAMTS13 in hemostasis and autoimmunity

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) is a multidomain metalloprotease for which until now only a single substrate has been identified. ADAMTS13 cleaves the polymeric force-sensor von Willebrand factor (VWF) that unfolds under shear stress and recruits platelets to sites of vascular injury. Shear force–dependent cleavage at a single Tyr–Met peptide bond in the unfolded VWF A2 domain serves to reduce the size of VWF polymers in circulation. In patients with immune-mediated thrombotic thrombocytopenic purpura (iTTP), a rare life-threatening disease, ADAMTS13 is targeted by autoantibodies that inhibit its activity or promote its clearance. In the absence of ADAMTS13, VWF polymers are not adequately processed, resulting in spontaneous adhesion of blood platelets, which presents as severe, life-threatening microvascular thrombosis. In healthy individuals, ADAMTS13–VWF interactions are guided by controlled conversion of ADAMTS13 from a closed, inactive to an open, active conformation through a series of interdomain contacts that are now beginning to be defined. Recently, it has been shown that ADAMTS13 adopts an open conformation in the acute phase and during subclinical disease in iTTP patients, making open ADAMTS13 a novel biomarker for iTTP. In this review, we summarize our current knowledge on ADAMTS13 conformation and speculate on potential triggers inducing conformational changes of ADAMTS13 and how these relate to the pathogenesis of iTTP.

Gold nanoparticle enhanced multiplexed biosensing on a fiber optic surface plasmon resonance probe

We present an innovative multiplexing concept on a fiber optic surface plasmon resonance (FO-SPR) platform and demonstrate for the first time the simultaneous detection of two targets using the same FO sensor probe. Co(III)-NTA chemistry was used for oriented and stable co-immobilization of two different His₆-tagged bioreceptors. T2C2 and MDTCS (i.e. fragments of the ADAMTS13 metalloprotease linked to the thrombotic thrombocytopenic purpura disorder) served as model system bioreceptors together with their respective targets (4B9 and II-1 antibodies). Gold nanoparticles were used here in an original way for discriminating the two targets in the same sample, in addition to their traditional signal amplification-role. After verifying the specificity of the selected model system, we studied the bioreceptor surface density and immobilization order. Innovative approach to lower the bioreceptor concentration below surface saturation resulted in an optimal detection of both targets, whereas the order of immobilization of the two bioreceptors did not give any significant difference. By sequentially immobilizing the T2C2 and MDTC bioreceptors, we established calibration curves in buffer and 100-fold diluted human blood plasma. This resulted in calculated limits of detection of 3.38 and 2.31 ng/mL in diluted plasma for 4B9 and II-1, respectively, indicating almost the same sensitivity as in buffer. Importantly, we also proved the applicability of the established calibration curves for quantifying the targets at random and more realistic ratios, directed by the design of experiments. This multiplexing study further expands the repertoire of applications on the FO-SPR biosensing platform, which together with its intrinsic features opens up great opportunities for diagnostics and life sciences.

A human monoclonal antibody against the distal carboxyl terminus of ADAMTS-13 modulates its susceptibility to an inhibitor in thrombotic thrombocytopenic purpura

BACKGROUND

Immune thrombotic thrombocytopenic purpura (iTTP) is a potentially fatal thrombotic microangiopathy, resulting from a severe deficiency of plasma ADAMTS-13 (A Disintegrin And Metalloproteinase with ThromboSpondin type 1 motif, member 13) activity. IgG-type autoantibodies are primarily responsible for the inhibition of plasma ADAMTS-13 activity. However, the mechanism underlying autoantibody-mediated inhibition is not fully understood.

OBJECTIVE

The purpose of the present study is to determine the role of IgG autoantibodies against various carboxyl-terminal domains of ADAMTS-13 in regulating ADAMTS-13 activity and its inhibition.

METHOD

Various human monoclonal antibodies isolated by phage display, recombinant protein expression and purification, and biochemical analyses were employed for the study.

RESULTS

Our results demonstrate for the first time that a human monoclonal antibody fragment, the single chain fragment of the variable region (scFv) isolated from a patient with acute iTTP that binds the distal carboxyl-terminus of ADAMTS-13, is able to activate ADAMTS-13 and increase the proteolytic cleavage of a FRETS-VWF73 substrate; moreover, binding of such a human monoclonal antibody against the carboxyl-terminus of ADAMTS-13 to plasma ADAMTS-13 appears to modulate inhibition by another human monoclonal antibody (i.e., scFv4-20), also isolated from an iTTP patient, that targets the spacer domain of ADAMTS-13. These results provide new insights into our understanding of the pathogenesis of iTTP.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.