A hereditary bleeding disorder resulting from a premature stop codon in thrombomodulin (p.Cys537Stop)

MEK1/2 and ERK1/2 mediated lung endothelial injury and altered hemostasis promote diffuse alveolar hemorrhage in murine lupus

Objective

About 3% of lupus patients develop severe diffuse alveolar hemorrhage (DAH) with pulmonary vasculitis. B6 mice with pristane-induced lupus also develop DAH, but BALB/c mice are resistant. DAH is independent of TLR signaling and other inflammatory pathways. This study examined the role of the mitogen-activated protein kinase pathway (MEK1/2-ERK1/2, JNK, p38).

Methods

B6 and BALB/c mice were treated with pristane ± inhibitors of MEK1/2 (trametinib/GSK1120212, "GSK"), ERK1/2 (SCH772984, "SCH"), JNK, or p38. Effects on lung hemorrhage and hemostasis were determined.

Results

GSK and SCH abolished DAH, whereas JNK and p38 inhibitors were ineffective. Apoptotic cells were present in lung from pristane-treated mice, but not mice receiving pristane+GSK and endothelial dysfunction was normalized. Expression of the ERK1/2-regulated transcription factor Egr1 increased in pristane-treated B6, but not BALB/c, mice and was normalized by GSK. Pristane also increased expression of the anticoagulant genes Tfpi (tissue factor pathway inhibitor) and Thbd (thrombomodulin) in B6 mice. The ratio of tissue factor ( F3 ) to Tfpi increased in B6 (but not BALB/c) mice and was normalized by GSK. Circulating Thbd protein increased in B6 mice and returned to normal after GSK treatment. Consistent with augmented endothelial anticoagulant activity, pristane treatment increased tail bleeding in B6 mice.

Conclusion

Pristane treatment promotes lung endothelial injury and DAH in B6 mice by activating the MEK1/2-ERK1/2 pathway and impairing hemostasis. The hereditary factors determining susceptibility to lung injury and bleeding in pristane-induced lupus are relevant to the pathophysiology of life-threatening DAH in SLE and may help to optimize therapy.

How to investigate mild to moderate bleeding disorders and bleeding disorder of unknown cause

A bleeding tendency is one of the most common complaints observed by hematologists. It is challenging to differentiate a clinically insignificant bleeding from a bleeding phenotype that requires hemostatic evaluation and medical intervention. A thorough review of personal and familial history, objective assessment of bleeding severity using a bleeding assessment tool, and a focused physical examination are critical to correctly identifying suspected patients with mild to moderate bleeding disorders (MBDs). A basic laboratory work-up should be performed in all patients referred for a bleeding tendency. If a hemostatic abnormality is found such as evidence of von Willebrand disease, a platelet function disorder, or a coagulation factor deficiency, more extensive testing should be performed to further characterize the bleeding disorder. Conversely, if all results are normal the patient is considered to have bleeding disorder of unknown cause (BDUC). For patients with BDUC, further evaluation may include non-routine testing to look for rare bleeding disorders not detected by routine hemostasis tests, such as thrombomodulin-associated coagulopathy, tissue factor pathway inhibitor-related bleeding disorder, hyperfibrinolytic-bleeding disorders or impaired tissue factor production. In this review, we summarize the stepwise diagnostic procedure in MBDs and provide some insights into the biological features of BDUC.

Activated protein C and free protein S in patients with mild to moderate bleeding disorders

BACKGROUND

Underlying mechanisms for bleeding and impaired thrombin generation (TG) and plasma clot formation (PCF) in patients with mild to moderate bleeding disorders (MBDs) are still to be elucidated, especially in bleeding disorder of unknown cause (BDUC). The role of the natural anticoagulants activated protein C (APC) and free protein S (PS) has not yet been investigated in this patient population.

AIMS

To analyze antigen levels of APC and PS in patients with MBDs and BDUC and investigate associations to clinical bleeding phenotype and severity as well as and hemostatic capacity.

METHODS

Antigen levels of APC and free PS were measured in 262 patients from the Vienna Bleeding Biobank (VIBB), a single-center cohort study, by ELISA and compared to 61 healthy controls (HC).

RESULTS

Antigen levels of APC were higher in MBD patients than in HC when adjusted for age, sex and BMI (median (IQR) 33.1 (20.6-52.6) and 28.6 (16.4-47.2) ng/mL). This was most pronounced in patients with BDUC (35.3 (21.7-54.3) ng/mL). No differences in PS antigen levels between patients and HC were seen overall, or according to specific diagnoses. Further, no association between APC or PS and bleeding severity or global tests of hemostasis or TG were identified, while paradoxically APC weakly correlated with shorter lag time and time to peak of PCF in BDUC.

CONCLUSION

Our data demonstrate increased antigen levels of APC in BDUC, which might contribute to the bleeding tendency in some patients and could be a future therapeutic target in BDUC.

Investigating patients for bleeding disorders when most of the "usual" ones have been ruled out

A State of the Art lecture titled "Investigating Patients for Bleeding Disorders When Most of the Usual Ones Have Been Ruled Out" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. Mild to moderate bleeding disorders (MBDs) in patients in whom no diagnosis of an established disorder, such as platelet function defect, von Willebrand disease, or a coagulation factor deficiency, can be identified are classified as bleeding disorders of unknown cause (BDUCs). Prospective data from the Vienna Bleeding Biobank and other studies have revealed a high proportion of BDUCs of up to 70% among patients with MBD who have a similar bleeding phenotype as other MBDs. As BDUC is a diagnosis of exclusion, the accuracy of the diagnostic workup is essential. For example, repeated testing for von Willebrand disease should be considered if von Willebrand factor values are <80 IU/dL. Current evidence does not support the clinical use of global assays such as thromboelastography, platelet function analyzer, or thrombin generation potential. Rare and novel bleeding disorders due to genetic variants in fibrinolytic factors or natural anticoagulants are rare and should only be analyzed in patients with specific phenotypes and a clear family history. In BDUC, blood group O was identified as a risk factor for increased bleeding severity and bleeding risk after hemostatic challenges. Future studies should improve the phenotypical characterization and ideally identify novel risk factors in BDUC, as a multifactorial pathogenesis is suspected. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Management of rare inherited bleeding disorders: Proposals of the French Reference Centre on Haemophilia and Rare Coagulation Disorders

INTRODUCTION

The rare coagulation disorders may present significant difficulties in diagnosis and management. In addition, considerable inter-individual variation in bleeding phenotype is observed amongst affected individuals, making the bleeding risk difficult to assess in affected individuals. The last international recommendations on rare inherited bleeding disorders (RIBDs) were published by the United Kingdom Haemophilia Centre Doctors' Organisation in 2014. Since then, new drugs have been marketed, news studies on surgery management in patients with RIBD have been published, and new orphan diseases have been described.

AIM

Therefore, the two main objectives of this review, based on the recent recommendations published by the French Reference Centre on Haemophilia and Rare Bleeding Disorders, are: (i) to briefly describe RIBD (clinical presentation and diagnostic work-up) to help physicians in patient screening for the early detection of such disorders; and (ii) to focus on the current management of acute haemorrhages and long term prophylaxis, surgical interventions, and pregnancy/delivery in patients with RIBD.

A missense mutation in lectin domain of thrombomodulin causing functional deficiency

Thrombomodulin (TM) functions in coagulation, fibrinolysis and inflammation by its cofactor activity for protein C, thrombin-activatable fibrinolysis inhibitor (TAFI) activation and high mobility group box 1 (HMGB1) degradation induced by thrombin. It has been widely reported that mutations in TM are related to thromboembolic diseases but hardly in lectin domain. Here we report our findings about the functional deficiencies in TM caused by substitution of aspartate with tyrosine at residue 126. Three patients suffering from recurrent thromboembolic diseases were identified with this mutation and their plasma soluble TM levels were decreased. Transfected cells expressing wild-type TM or the variant and corresponding proteins were used to examine TM functions in vitro. The cofactor activity of the mutant for protein C, TAFI activation was reduced to approximately 50% and 60% respectively. Loss in anti-inflammation due to weakened HMGB1 degradation was also observed. And the study with thrombosis models of mice suggested the decreased inhibition of thrombus development of the mutant. Together the results showed deleterious changes on TM function caused by this mutation, which may explain the thrombophilia tendency of the patients. This work provided supportive evidence that mutation in lectin domain of TM might be related to thrombotic diseases and may help us better understand the physiological roles of TM.

Circulating Thrombomodulin: Release Mechanisms, Measurements, and Levels in Diseases and Medical Procedures

Thrombomodulin (TM) is a type-I transmembrane protein that is mainly expressed on endothelial cells and plays important roles in many biological processes. Circulating TM of different forms are also present in biofluids, such as blood and urine. Soluble TM (sTM), comprised of several domains of TM, is the major circulating TM which is generated by either enzymatic or chemical cleavage of the intact protein under different conditions. Under normal conditions, sTM is present in low concentrations (<10 ng/mL) in the blood but is elevated in several pathological conditions associated with endothelial dysfunction such as cardiovascular, inflammatory, infection, and metabolic diseases. Therefore, sTM level has been examined for monitoring disease development, such as disseminated intravascular coagulation (DIC), sepsis and multiple organ dysfunction syndrome in patients with novel coronavirus disease 2019 (COVID-19) recently. In addition, microvesicles (MVs) that contain membrane TM (MV-TM) have been found to be released from activated cells which also contribute to levels of circulating TM in certain diseases. Several release mechanisms of sTM and MV-TM have been reported, including enzymatic, chemical, and TM mutation mechanisms. Measurements of sTM and MV-TM have been developed and explored as biomarkers in many diseases. In this review, we summarize all these advances in three categories as follows: (1) release mechanisms of circulating TM, (2) methods for measuring circulating TM in biological samples, and (3) correlation of circulating TM with diseases. Altogether, it provides a whole picture of recent advances on circulating TM in health and disease.

Identification of the novel G250R variant indicates a role for Thrombomodulin in modulating the risk for venous thromboembolism

Thrombomodulin displays anticoagulant properties through thrombin binding and protein C activation. In close relatives who had a history of recurrent VTE, a missense variant (G250R) in the EGF-1 domain was identified. In an asymptomatic 250R carrier, higher thrombin generation plasma values were found. Evaluation of cells transfected with the wild-type or the mutant construct using imaging technologies and flow cytometry provided evidences that the 250R variant significantly decreased protein localization on cell membrane. The different pattern of cell distribution suggested that the 250R variant is not able to sufficiently target the plasma membrane. Present findings add information to characterize protein structure-function relationships, further suggesting that misfolding of the protein modulates the expression of mature protein. Functional analyses of the 250R variant increase our understanding of the role of Thrombomodulin in coagulation and support the hypothesis that gene variants reducing protein functionality cause deficiency and behave as a thrombophilic risk factor.

Thrombin spatial distribution determines protein C activation during hemostasis and thrombosis

Rebalancing the hemostatic system by targeting endogenous anticoagulant pathways, like the protein C (PC) system, is being tested as a means of improving hemostasis in patients with hemophilia. Recent intravital studies of hemostasis demonstrated that, in some vascular contexts, thrombin activity is sequestered in the extravascular compartment. These findings raise important questions about the context-dependent contribution of activated PC (APC) to the hemostatic response, because PC activation occurs on the surface of endothelial cells. We used a combination of pharmacologic, genetic, imaging, and computational approaches to examine the relationships among thrombin spatial distribution, PC activation, and APC anticoagulant function. We found that inhibition of APC activity, in mice either harboring the factor V Leiden mutation or infused with an APC-blocking antibody, significantly enhanced fibrin formation and platelet activation in a microvascular injury model, consistent with the role of APC as an anticoagulant. In contrast, inhibition of APC activity had no effect on hemostasis after penetrating injury of the mouse jugular vein. Computational studies showed that differences in blood velocity, injury size, and vessel geometry determine the localization of thrombin generation and, consequently, the extent of PC activation. Computational predictions were tested in vivo and showed that when thrombin generation occurred intravascularly, without penetration of the vessel wall, inhibition of APC significantly increased fibrin formation in the jugular vein. Together, these studies show the importance of thrombin spatial distribution in determining PC activation during hemostasis and thrombosis.

A novel homozygous variant of the thrombomodulin gene causes a hereditary bleeding disorder

We report a 19-year-old Vietnamese woman who experienced several life-threatening bleeding events, including ovarian hemorrhage. Blood analysis revealed a decreased fibrinogen level with markedly elevated fibrinogen/fibrin degradation products and D-dimer levels. Despite hemostatic surgery and administration of several medications, such as nafamostat mesylate, tranexamic acid, and unfractionated heparin, the coagulation abnormalities were not corrected, and the patient experienced repeated hemorrhagic events. We found that administration of recombinant human thrombomodulin (rhTM) remarkably improved the patient's pathophysiology. Screening and sequencing of the TM gene (THBD) revealed a previously unreported homozygous variation: c.793T>A (p.Cys265Ser). Notably, the Cys265 residue forms 1 of 3 disulfide bonds in the epidermal growth factor (EGF)-like domain 1 of TM. Transient expression experiments using COS-1 cells demonstrated markedly reduced expression of TM-Cys265Ser on the plasma membrane relative to wild-type TM. The TM-Cys265Ser mutant was intracellularly degraded, probably because of EGF-like domain 1 misfolding. The reduced expression of TM on the endothelial cell membrane may be responsible for the disseminated intravascular-coagulation-like symptoms observed in the patient. In summary, we identified a novel TM variant, c.793T>A (p.Cys265Ser). Patients homozygous for this variant may present with severe bleeding events; rhTM should be considered a possible treatment option for these patients.

Thrombomodulin in patients with mild to moderate bleeding tendency

Introduction

A massive increase of soluble thrombomodulin (sTM) due to variants in the thrombomodulin gene (THBD) has recently been identified as a novel bleeding disorder.

Aim

To investigate sTM levels and underlying genetic variants as a cause for haemostatic impairment and bleeding in a large number of patients with a mild to moderate bleeding disorder (MBD), including patients with bleeding of unknown cause (BUC).

Patients and methods

In 507 MBD patients, sTM levels, thrombin generation and plasma clot formation were measured and compared to 90 age‐ and sex‐matched healthy controls. In patients, genetic analysis of the THBD gene was performed.

Results

No difference in sTM levels between patients and controls was found overall (median ([IQR] 5.0 [3.8‐6.3] vs. 5.1 [3.7‐6.4] ng/ml, p = .762), and according to specific diagnoses of MBD or BUC, and high sTM levels (≥95th percentile of healthy controls) were not overrepresented in patients. Soluble TM levels had no impact on bleeding severity or global tests of haemostasis, including thrombin generation or plasma clot formation. In the THBD gene, no known pathogenic or novel disease‐causing variants affecting sTM plasma levels were identified in our patient cohort.

Conclusion

TM‐associated coagulopathy appears to be rare, as it was not identified in our large cohort of patients with MBD. Soluble TM did not arise as a risk factor for bleeding or altered haemostasis in these patients.

How I treat. Bleeding Disorder of Unknown Cause (BDUC)

Recent studies have demonstrated that only 30% of patients referred for assessment of a possible bleeding tendency will eventually be diagnosed with a mild bleeding disorder (MBD) such as von Willebrand disease (VWD) or platelet function defect (PFD). Rather, the majority of such patients will be diagnosed with Bleeding Disorder of Unknown Cause (BDUC). There remains an important unmet need to define consensus regarding the clinical and laboratory criteria necessary for a formal BDUC diagnosis. Nevertheless, BDUC already accounts for more than 10% of patients registered in some Haemophilia Comprehensive Care centres. Accumulating recent data suggest that BDUC is also being diagnosed with increasing frequency. Increased BAT scores are widely utilized to differentiate significant from trivial symptoms in patients with mucocutaneous bleeding. Objective assessment of bleeding phenotype using a standardised bleeding assessment tool (BAT) therefore represents a fundamental first step in the diagnosis of BDUC. Since BDUC is a diagnosis by exclusion, accurate quantification of bleeding phenotype is critical as this will be the primary determinant upon which a diagnosis of BDUC is reached. Importantly, BAT scores suggest that patients with BDUC display bleeding phenotypes comparable to those seen in patients with VWD or PFD respectively. Despite the prevalence of BDUC, diagnosis and management of these patients commonly pose significant clinical dilemmas. In this manuscript, we consider these challenges in the context of a number of typical case studies, discuss the available evidence and outline our approach to the management of these patients.

Cerebral Cavernous Malformation: From Mechanism to Therapy

Cerebral cavernous malformations are acquired vascular anomalies that constitute a common cause of central nervous system hemorrhage and stroke. The past 2 decades have seen a remarkable increase in our understanding of the pathogenesis of this vascular disease. This new knowledge spans genetic causes of sporadic and familial forms of the disease, molecular signaling changes in vascular endothelial cells that underlie the disease, unexpectedly strong environmental effects on disease pathogenesis, and drivers of disease end points such as hemorrhage. These novel insights are the integrated product of human clinical studies, human genetic studies, studies in mouse and zebrafish genetic models, and basic molecular and cellular studies. This review addresses the genetic and molecular underpinnings of cerebral cavernous malformation disease, the mechanisms that lead to lesion hemorrhage, and emerging biomarkers and therapies for clinical treatment of cerebral cavernous malformation disease. It may also serve as an example for how focused basic and clinical investigation and emerging technologies can rapidly unravel a complex disease mechanism.

Natural anticoagulants: A missing link in mild to moderate bleeding tendencies

Introduction

There is a growing interest in natural anticoagulants as a cause of mild to moderate bleeding disorders (MBDs), particularly in patients with bleeding of unknown cause (BUC), which is defined as having a mild to moderate bleeding phenotype without a definite diagnosis despite exhaustive and repeated laboratory investigations. Recently, abnormalities in two natural anticoagulant pathways, thrombomodulin (TM), and tissue factor pathway inhibitor (TFPI), were identified in single patients or families as the underlying cause for a bleeding tendency.

Aim

The objective of this review is to discuss the current understanding of the role of natural anticoagulants in MBDs using available clinical and translational data.

Methods

A Cochrane Library and PubMed (MEDLINE) search focusing on selected natural anticoagulants and their role in MBDs was conducted.

Results

Data on the influence of natural anticoagulants including protein C, protein S, antithrombin, TM, and TFPI or factors with anticoagulant properties like fibrinogen gamma prime (γ’) on MBDs are scarce. Observations from sepsis treatment and from translational research highlight their importance as regulators of the haemostatic balance, especially via the activated protein C‐related pathway, and suggest a role in some MBDs.

Conclusion

Similar to the distinct genetic variants of natural anticoagulants linked to thrombosis, we hypothesize that novel variants may be associated with a bleeding tendency and could be identified using next generation sequencing.

Heterogeneity in Bleeding Tendency and Arthropathy Development in Individuals with Hemophilia

People with hemophilia (PWH) have an increased tendency to bleed, often into their joints, causing debilitating joint disease if left untreated. To reduce the incidence of bleeding events, PWH receive prophylactic replacement therapy with recombinant factor VIII (FVIII) or FIX. Bleeding events in PWH are typically proportional to their plasma FVIII or IX levels; however, in many PWH, bleeding tendency and the likelihood of developing arthropathy often varies independently of endogenous factor levels. Consequently, many PWH suffer repeated bleeding events before correct dosing of replacement factor can be established. Diagnostic approaches to define an individual's bleeding tendency remain limited. Multiple modulators of bleeding phenotype in PWH have been proposed, including the type of disease-causing variant, age of onset of bleeding episodes, plasma modifiers of blood coagulation or clot fibrinolysis pathway activity, interindividual differences in platelet reactivity, and endothelial anticoagulant activity. In this review, we summarize current knowledge of established factors modulating bleeding tendency and discuss emerging concepts of additional biological elements that may contribute to variable bleeding tendency in PWH. Finally, we consider how variance in responses to new gene therapies may also necessitate consideration of patient-specific tailoring of treatment. Cumulatively, these studies highlight the need to reconsider the current "one size fits all" approach to treatment regimens for PWH and consider therapies guided by the bleeding phenotype of each individual PWH at the onset of therapy. Further characterization of the biological bases of bleeding heterogeneity in PWH, combined with the development of novel diagnostic assays to identify those factors that modulate bleeding risk in PWH, will be required to meet these aspirations.

Characterisation of a novel thrombomodulin c.1487delC,p.(Pro496Argfs*10) variant and evaluation of therapeutic strategies to manage the rare bleeding phenotype

INTRODUCTION

A novel variant in the thrombomodulin (TM) gene, c.1487delC,p.(Pro496Argfs*10), referred to as Pro496Argfs*10, was identified in a family with an unexplained bleeding disorder. The Pro496Argfs*10 variant results in loss of the transmembrane and intracellular segments of TM and is associated with an increase in soluble TM (sTM) in the plasma. The aim of this study was to characterise the effect of elevated sTM on thrombin generation (TG) and fibrinolysis, and to evaluate therapeutic strategies to manage the patients.

METHODS

Plasma samples were obtained from two patients carrying the variant. TG was triggered using 5 pM tissue factor and measured using the Calibrated Automated Thrombogram. A turbidity clot lysis assay was used to monitor fibrinolysis. TM antigen was quantified by ELISA.

RESULTS

Patients with the Pro496Argfs*10 variant had significantly elevated plasma sTM compared to controls (372.6 vs. 6.0 ng/ml). TG potential was significantly lower in patients but was restored by inhibition of activated protein C (APC) or addition of activated Factor VII (FVIIa) or platelet concentrates. In vitro experiments suggested that activated prothrombin complex concentrates (APCC) posed a risk of thrombosis. The time to 50% lysis was significantly prolonged in patients compared to controls, 69.7 vs. 42.3 min. Clot lysis time was shortened by inhibition of activated thrombin activatable fibrinolysis inhibitor (TAFIa).

CONCLUSIONS

Our data demonstrate that increased sTM enhances APC generation and reduces TG. Simultaneously, the rate of fibrinolysis is delayed due to increased TAFI activation by sTM. Treatment with platelet or FVIIa concentrates may be beneficial to manage this rare bleeding disorder.

Bleeding of unknown cause and unclassified bleeding disorders; diagnosis, pathophysiology and management

Bleeding of unknown cause (BUC), also known as unclassified bleeding disorders (UBD), has been defined as a clear bleeding tendency in the presence of normal haemostatic tests. There are challenges in the diagnosis and management of these patients. BUC/UBD encompasses a heterogenous group of disorders which may include undiagnosed rare monogenic diseases, polygenic reasons for bleeding; and patients without a clear bleeding disorder but with a previous bleeding event. Nevertheless, these patients may have heavy menstrual bleeding or be at risk of bleeding when undergoing surgical procedures, or childbirth; optimizing haemostasis and establishing a mode of inheritance is important to minimize morbidity. The bleeding score has been used to clinically assess and describe these patients, but its value remains uncertain. In addition, accurate distinction between normal and pathological bleeding remains difficult. Several studies have investigated cohorts of these patients using research haemostasis tests, including thrombin generation and fibrinolytic assays, yet no clear characteristics have consistently emerged. Thus far, detailed genetic analysis of these patients has not been fruitful in unravelling the cause of bleeding. There is a need for standardization of diagnosis and management guidelines for these patients. This review gives an overview of this field with some suggestions for future research.

A new pedigree with thrombomodulin-associated coagulopathy in which delayed fibrinolysis is partially attenuated by co-inherited TAFI deficiency

BACKGROUND

Thrombomodulin-associated coagulopathy (TM-AC) is a rare bleeding disorder in which a single reported p.Cys537* variant in the thrombomodulin gene THBD causes high plasma thrombomodulin (TM) levels. High TM levels attenuate thrombin generation and delay fibrinolysis.

OBJECTIVES

To report the characteristics of pedigree with a novel THBD variant causing TM-AC, and co-inherited deficiency of thrombin-activatable fibrinolysis inhibitor (TAFI).

PATIENTS/METHODS

Identification of pathogenic variants in hemostasis genes by next-generation sequencing and case recall for deep phenotyping.

RESULTS

Pedigree members with a previously reported THBD variant predicting p.Pro496Argfs*10 and chain truncation in TM transmembrane domain had abnormal bleeding and greatly increased plasma TM levels. Affected cases had attenuated thrombin generation and delayed fibrinolysis similar to previous reported TM_AC cases with THBD p.Cys537*. Coincidentally, some pedigree members also harbored a stop-gain variant in CPB2 encoding TAFI. This reduced plasma TAFI levels but was asymptomatic. Pedigree members with TM-AC caused by the p.Pro496Argfs*10 THBD variant and also TAFI deficiency had a partially attenuated delay in fibrinolysis, but no change in the defective thrombin generation.

CONCLUSIONS

These data extend the reported genetic repertoire of TM-AC and establish a common molecular pathogenesis arising from high plasma levels of TM extra-cellular domain. The data further confirm that the delay in fibrinolysis associated with TM-AC is directly linked to increased TAFI activation. The combination of the rare variants in the pedigree members provides a unique genetic model to develop understanding of the thrombin-TM system and its regulation of TAFI.

Investigation of patients with unclassified bleeding disorder and abnormal thrombin generation for physiological coagulation inhibitors reveals multiple abnormalities and a subset of patients with increased tissue factor pathway inhibitor activity

INTRODUCTION

We have routinely used thrombin generation to investigate patients with unclassified bleeding disorder (UBD).

AIMS

To investigate haemostatic abnormalities in patients with UBD that had abnormal thrombin generation on at least one occasion.

METHODS

Investigation of 13 known UBD patients with thrombin generation and detailed haemostatic testing was undertaken including TFPI assays but also thrombomodulin and fibrinogen-γ.

RESULTS

12 females and 1 male were included. No patient had a platelet function disorder or coagulation factor deficiency that explained the bleeding phenotype, though 2 patients had factor deficiencies; a factor X of 0.41 IU/mL and a factor XI of 0.51 IU/mL. ThromboGenomics revealed variants for these factors but no other abnormalities. Patients were included who previously had either prolonged lag time or decreased endogenous thrombin potential (ETP) via high dose tissue factor (5 pmol/L) or low dose tissue factor (1.5 pmol/L) with corn trypsin inhibitor (CTI). Tissue factor pathway inhibitor (TFPI) activity was significantly increased (P < .001; increased in 8 patients) compared with controls and abnormalities in soluble thrombomodulin (2 patients), fibrinogen-γ (1 patient) and tPA (4 patients for each) were seen. Total and free TFPI levels were not increased. Mixing studies of patient plasma with 50:50 normal plasma for thrombin generation via low dose tissue factor failed to correct the ETP consistent with ongoing inhibition. Addition of an anti-TFPI antibody partially corrected thrombin generation to normal levels. TFPI sequencing was unremarkable.

CONCLUSION

TFPI activity may be increased in a subset of UBD patients. Further research studies are warranted in UBD patients for coagulation inhibitor abnormalities.

New data on FII, FV, FIX and thrombomodulin defects: blood keeps clotting in normal and in peculiar ways

Objective: To present the clinical and laboratory implications of defects or variants of some clotting factors and of thrombomodulin that were discovered during the past few years.Methods: Data concerning new aspects of FII, FV, FIX and thrombomodulin defects were investigated. This involved the dysprothrombinemias, the East Texas or short FV disorder, a FIX defect and a thrombomodulin abnormality.Results: the recently reported clotting defects or variants are: (1) the thrombophilic dysprothrombinemias due to Arg596 mutations (Prothrombin Yukuhashi, Belgrade and Padua 2) which are characterized by absence of bleeding and presence of venous thrombosis; (2) the short FV defects due to Ser356Gly (FV East Texas) or Ala863Gly (FV Amsterdam) mutations characterized by a mild bleeding tendency with normal FV and other clotting factors, increased TFPI and no thrombosis; (3) the abnormal FIX (FIX Padua) due to the Arg338Leu mutation which is associated with high levels of FIX activity, lack of bleeding and venous thrombosis; (4) the thrombomodulin Cys537Stop mutation associated with a mild bleeding tendency despite normal clotting factors but increased plasma levels of soluble thrombomodulin and no thrombosis.Conclusions: these new coagulation defects have great implications in the clinical and laboratory approach to the coagulation disorders. They have demonstrated that a prothrombin defect may be associated with thrombosis, that a mild bleeding tendency may occur despite normal Factor V levels and that high levels of plasmatic thrombomodulin may be associated with mild bleeding.

The transcription factor ERG regulates a low shear stress-induced anti-thrombotic pathway in the microvasculature

Endothelial cells actively maintain an anti-thrombotic environment; loss of this protective function may lead to thrombosis and systemic coagulopathy. The transcription factor ERG is essential to maintain endothelial homeostasis. Here, we show that inducible endothelial ERG deletion (ErgiEC-KO) in mice is associated with spontaneous thrombosis, hemorrhages and systemic coagulopathy. We find that ERG drives transcription of the anticoagulant thrombomodulin (TM), as shown by reporter assays and chromatin immunoprecipitation. TM expression is regulated by shear stress (SS) via Krüppel-like factor 2 (KLF2). In vitro, ERG regulates TM expression under low SS conditions, by facilitating KLF2 binding to the TM promoter. However, ERG is dispensable for TM expression in high SS conditions. In ErgiEC-KO mice, TM expression is decreased in liver and lung microvasculature exposed to low SS but not in blood vessels exposed to high SS. Our study identifies an endogenous, vascular bed-specific anticoagulant pathway in microvasculature exposed to low SS.

Fundamentals for a Systematic Approach to Mild and Moderate Inherited Bleeding Disorders: An EHA Consensus Report

Healthy subjects frequently report minor bleedings that are frequently ‘background noise’ of normality rather than a true disorder. Nevertheless, unexpected or unusual bleeding may be alarming. Thus, the distinction between normal and pathologic bleeding is critical. Understanding the underlying pathologic mechanism in patients with an excessive bleeding is essential for their counseling and treatment. Most of these patients with significant bleeding will result affected by non-severe inherited bleeding disorders (BD), collectively denominated mild or moderate BD for their relatively benign course. Unfortunately, practical recommendations for the management of these disorders are still lacking due to the current state of fragmented knowledge of pathophysiology and lack of a systematic diagnostic approach. To address this gap, an International Working Group (IWG) was established by the European Hematology Association (EHA) to develop consensus-based guidelines on these disorders. The IWG agreed that grouping these disorders by their clinical phenotype under the single category of mild-to-moderate bleeding disorders (MBD) reflects current clinical practice and will facilitate a systematic diagnostic approach. Based on standardized and harmonized definitions a conceptual unified framework is proposed to distinguish normal subjects from affected patients. The IWG proposes a provisional comprehensive patient-centered initial diagnostic approach that will result in classification of MBD into distinct clinical-pathological entities under the overarching principle of clinical utility for the individual patient. While we will present here a general overview of the global management of patients with MBD, this conceptual framework will be adopted and validated in the evidence-based, disease-specific guidelines under development by the IWG.

Ebola Virus Disease in Humans: Pathophysiology and Immunity

Viruses of the Ebolavirus genus cause sporadic epidemics of severe and systemic febrile disease that are fueled by human-to-human transmission. Despite the notoriety of ebolaviruses, particularly Ebola virus (EBOV), as prominent viral hemorrhagic fever agents, and the international concern regarding Ebola virus disease (EVD) outbreaks, very little is known about the pathophysiology of EVD in humans and, in particular, about the human immune correlates of survival and immune memory. This lack of basic knowledge about physiological characteristics of EVD is probably attributable to the dearth of clinical and laboratory data gathered from past outbreaks. The unprecedented magnitude of the EVD epidemic that occurred in West Africa from 2013 to 2016 has allowed, for the first time, evaluation of clinical, epidemiological, and immunological parameters in a significant number of patients using state-of-the-art laboratory equipment. This review will summarize the data from the literature regarding human pathophysiologic and immunologic responses to filoviral infection.

Cerebral cavernous malformations form an anticoagulant vascular domain in humans and mice

Cerebral cavernous malformations (CCMs) are common brain vascular dysplasias that are prone to acute and chronic hemorrhage with significant clinical sequelae. The pathogenesis of recurrent bleeding in CCM is incompletely understood. Here, we show that central nervous system hemorrhage in CCMs is associated with locally elevated expression of the anticoagulant endothelial receptors thrombomodulin (TM) and endothelial protein C receptor (EPCR). TM levels are increased in human CCM lesions, as well as in the plasma of patients with CCMs. In mice, endothelial-specific genetic inactivation of Krit1 (Krit1 ^ECKO ) or Pdcd10 (Pdcd10 ^ECKO ), which cause CCM formation, results in increased levels of vascular TM and EPCR, as well as in enhanced generation of activated protein C (APC) on endothelial cells. Increased TM expression is due to upregulation of transcription factors KLF2 and KLF4 consequent to the loss of KRIT1 or PDCD10. Increased TM expression contributes to CCM hemorrhage, because genetic inactivation of 1 or 2 copies of the Thbd gene decreases brain hemorrhage in Pdcd10 ^ECKO mice. Moreover, administration of blocking antibodies against TM and EPCR significantly reduced CCM hemorrhage in Pdcd10 ^ECKO mice. Thus, a local increase in the endothelial cofactors that generate anticoagulant APC can contribute to bleeding in CCMs, and plasma soluble TM may represent a biomarker for hemorrhagic risk in CCMs.

Regulation of plasminogen activation on cell surfaces and fibrin

The fibrinolytic system dissolves fibrin and maintains vascular patency. Recent advances in imaging analyses allowed visualization of the spatiotemporal regulatory mechanism of fibrinolysis, as well as its regulation by other plasma hemostasis cofactors. Vascular endothelial cells (VECs) retain tissue-type plasminogen activator (tPA) after secretion and maintain high plasminogen (plg) activation potential on their surfaces. As in plasma, the serpin, plasminogen activator inhibitor type 1 (PAI-1), regulates fibrinolytic potential via inhibition of the VEC surface-bound plg activator, tPA. Once fibrin is formed, plg activation by tPA is initiated and effectively amplified on the surface of fibrin, and fibrin is rapidly degraded. The specific binding of plg and tPA to lytic edges of partly degraded fibrin via newly generated C-terminal lysine residues, which amplifies fibrin digestion, is a central aspect of this pathophysiological mechanism. Thrombomodulin (TM) plays a role in the attenuation of plg binding on fibrin and the associated fibrinolysis, which is reversed by a carboxypeptidase B inhibitor. This suggests that the plasma procarboxypeptidase B, thrombin-activatable fibrinolysis inhibitor (TAFI), which is activated by thrombin bound to TM on VECs, is a critical aspect of the regulation of plg activation on VECs and subsequent fibrinolysis. Platelets also contain PAI-1, TAFI, TM, and the fibrin cross-linking enzyme, factor (F) XIIIa, and either secrete or expose these agents upon activation in order to regulate fibrinolysis. In this review, the native machinery of plg activation and fibrinolysis, as well as their spatiotemporal regulatory mechanisms, as revealed by imaging analyses, are discussed.

Why patients with THBD c.1611C>A (p.Cys537X) nonsense mutation have high levels of soluble thrombomodulin?

Background

Recently our group has described a new autosomal dominant bleeding disorder characterized by very high plasma levels of soluble thrombomodulin (TM). The THBD c.1611C>A (p.Cys537X) mutation in heterozygous state was found in the propositus. This mutation leads to the synthesis of a truncated TM which has lost the last three amino-acids of the transmembrane domain and the cytoplasmic tail.

Objective

We investigated the mechanism responsible for TM shedding in endothelial cells with THBD c.1611C>A mutation.

Methods

Complementary DNA of TM wild type (TM-WT) was incorporated into a pcDNA3.1 vector for transient transfection in COS-1 cells. Mutagenesis was performed to create the c.1611C<A (TM_1-536) mutant and 4 other TM mutants (TM_1-515, TM_1-525, TM_1-533 and TM_1-537) with a transmembrane domain having different lengths. The effect of shear stress, metalloprotease inhibitor, certain proteases and reducing agents were tested on TM shedding.

Results

Western blot and immunofluorescent analysis showed that TM_1-536 was produced and a certain amount of TM_1-536 was anchored on the cell membrane. A significantly higher levels of soluble TM was observed in the TM_1-536 cell medium in comparison with TM-WT (56.3 +/- 5.2 vs 8.8 +/- 1.6 ng/mL, respectively, p = 0.001). The shedding of TM_1-536 was 75% decreased in cells cultured in the presence of a metalloprotease inhibitor. No difference was observed between TM_1-536 and TM-WT shedding after cell exposure to cathepsin G, elastase, several reducing agents and high shear stress (5000 s^-1). Significantly higher levels of soluble TM were observed in the cell media of TM_1-533, TM_1-525, TM_1-515 in comparison with TM-WT (p < 0.05).

Conclusion

The mechanism responsible for TM shedding is complex and is not completely understood: higher sensitivity of the TM_1-536 to the proteolysis by metalloproteases and a defect of synthesis due to the decreased size of the transmembrane domain might explain the high levels of soluble TM in plasma of the carriers.

Targeting activated protein C to treat hemophilia

PURPOSE OF REVIEW

Hemophilia is a debilitating disease, marked by frequent, painful bleeding events, joint deterioration and early death. All current treatments consist of i.v. infusions of replacement factor or other procoagulant factors, and are incompletely effective, due in part to the short half-lives of the proteins. An alternative approach is to rebalance hemostasis by inhibiting natural anticoagulant mechanisms. In this article, we explain why activated protein C (APC) is an appropriate and safe target for the treatment of hemophilia.

RECENT FINDINGS

A serpin (serine protease inhibitor) was engineered to specifically inhibit APC and was found to rescue hemostasis in a hemophilia mouse model, even after a severe tail clip injury. However, APC is also anti-inflammatory and has cytoprotective activities, raising safety concerns over the use of an APC inhibitor to treat hemophilia. We summarize the molecular basis of the anticoagulant and signaling activities of APC to assess the potential impact of targeting APC.

SUMMARY

We conclude that the signaling and anticoagulant functions of APC are in spatially and kinetically distinct compartments, and that it is possible to specifically inhibit the anticoagulant activity of APC. Targeting APC with a serpin is remarkably effective and may be safe for long-term prophylactic use in the treatment of hemophilia.

Thrombotic and Hemorrhagic Conditions Due to a Gain of Function of Coagulation Proteins: A Special Type of Clotting Disorders

Coagulation disorders can be classified into 2 types, namely, type I and type II. In the former, there is a concomitant decrease in factor activity and antigen (activity-antigen ratio is 1), whereas in the latter, there is a discrepancy between factor activity which is always low and antigen which is normal or near normal (activity-antigen ratio is <1, eg, 0.5). Recently, several gain-of-function disorders have been described. These are characterized by an increased activity with respect to the antigen level. The condition involves polymorphisms of factor V and factor II, factor IX, von Willebrand disease, thrombomodulin, tissue factor pathway inhibitor, and thrombin activatable fibrinolysis inhibitor. The conditions could be subdivided into prothrombotic and prohemorrhagic. They should also be distinguished as cases of true gain of function (intrinsic increase activity without concomitant increase in protein level) and of "pseudo" gain of function (increase in both activity and protein level). This is a new concept of coagulation defects that has considerably enhanced our knowledge of blood coagulation and that should be familiar to all those interested in the mechanism of blood clotting and its disorders.

Variable phenotypic penetrance of thrombosis in adult mice after tissue-selective and temporally controlled Thbd gene inactivation

Thrombomodulin (Thbd) exerts pleiotropic effects on blood coagulation, fibrinolysis, and complement system activity by facilitating the thrombin-mediated activation of protein C and thrombin-activatable fibrinolysis inhibitor and may have additional thrombin- and protein C (pC)-independent functions. In mice, complete Thbd deficiency causes embryonic death due to defective placental development. In this study, we used tissue-selective and temporally controlled Thbd gene ablation to examine the function of Thbd in adult mice. Selective preservation of Thbd function in the extraembryonic ectoderm and primitive endoderm via the Meox2Cre-transgene enabled normal intrauterine development of Thbd-deficient (Thbd^-/-) mice to term. Half of the Thbd^-/- offspring expired perinatally due to thrombohemorrhagic lesions. Surviving Thbd^-/- animals only rarely developed overt thrombotic lesions, exhibited low-grade compensated consumptive coagulopathy, and yet exhibited marked, sudden-onset mortality. A corresponding pathology was seen in mice in which the Thbd gene was ablated after reaching adulthood. Supplementation of activated PC by transgenic expression of a partially Thbd-independent murine pC zymogen prevented the pathologies of Thbd^-/- mice. However, Thbd^-/- females expressing the PC transgene exhibited pregnancy-induced morbidity and mortality with near-complete penetrance. These findings suggest that Thbd function in nonendothelial embryonic tissues of the placenta and yolk sac affects through as-yet-unknown mechanisms the penetrance and severity of thrombosis after birth and provide novel opportunities to study the role of the natural Thbd-pC pathway in adult mice and during pregnancy.

Whole exome sequencing identifies a mutation in thrombomodulin as the genetic cause of a suspected platelet disorder in a family with normal platelet function

Here, we describe a mother and son with a lifelong bleeding tendency and posttraumatic bleeding who were recruited to the UK Genotyping and Phenotyping of Platelets (GAPP) study with a suspected platelet function disorder. However, despite a clinically significant bleeding score, both had normal platelet counts and normal platelet function. The patients’ blood was analyzed by light transmission aggregometry and genotyping by whole exome sequencing, as outlined by the GAPP study. Approximately 25 000 genetic variants were found for each patient as a result of sequencing and were filtered using a specialized bioinformatics pipeline. A heterozygous variant displaying autosomal dominant inheritance (c.1611 C>A) was found in the gene THBD which encodes the glycoprotein thrombomodulin. This sequence change results in a stop codon (p.Cys537Stop) and truncation of the protein and has been previously described in two other families with bleeding events which suggests it may be a recurrent mutation. In summary, this study shows that patients with a suspected platelet disorder but who present with a normal pattern of platelet aggregation should be investigated for defects in nonplatelet genes.

Genetic basis of ethnic disparities in VTE risk

In this issue of Blood, Hernandez et al identify and replicate single-nucleotide polymorphisms on chromosome 20 (and putatively on the THBD gene for thrombomodulin) that increased the risk of venous thromboembolism (VTE) by about 2.3-fold in African Americans (AAs) in the United States.

Altered fibrinolysis in autosomal dominant thrombomodulin-associated coagulopathy

Thrombomodulin-associated coagulopathy (TM-AC) is a newly recognized dominant bleeding disorder in which a p.Cys537Stop variant in the thrombomodulin (TM) gene THBD, results in high plasma TM levels and protein C-mediated suppression of thrombin generation. Thrombin in complex with TM also activates thrombin-activatable fibrinolysis inhibitor (TAFI). However, the effect of the high plasma TM on fibrinolysis in TM-AC is unknown. Plasma from TM-AC cases and high-TM model control samples spiked with recombinant soluble TM showed reduced tissue factor-induced thrombin generation. Lysis of plasma clots from TM-AC cases was significantly delayed compared with controls but was completely restored when TM/thrombin-mediated TAFI activation was inhibited. Clots formed in blood from TM-AC cases had the same viscoelastic strength as controls but also showed a TAFI-dependent delay in fibrinolysis. Delayed fibrinolysis was reproduced in high-TM model plasma and blood samples. Partial restoration of thrombin generation with recombinant activated factor VII or activated prothrombin complex concentrate did not alter the delayed fibrinolysis in high-TM model blood. Our finding of a previously unrecognized fibrinolytic phenotype indicates that bleeding in TM-AC has a complex pathogenesis and highlights the pivotal role of TM as a regulator of hemostasis.

A high-throughput sequencing test for diagnosing inherited bleeding, thrombotic, and platelet disorders

Inherited bleeding, thrombotic, and platelet disorders (BPDs) are diseases that affect ∼300 individuals per million births. With the exception of hemophilia and von Willebrand disease patients, a molecular analysis for patients with a BPD is often unavailable. Many specialized tests are usually required to reach a putative diagnosis and they are typically performed in a step-wise manner to control costs. This approach causes delays and a conclusive molecular diagnosis is often never reached, which can compromise treatment and impede rapid identification of affected relatives. To address this unmet diagnostic need, we designed a high-throughput sequencing platform targeting 63 genes relevant for BPDs. The platform can call single nucleotide variants, short insertions/deletions, and large copy number variants (though not inversions) which are subjected to automated filtering for diagnostic prioritization, resulting in an average of 5.34 candidate variants per individual. We sequenced 159 and 137 samples, respectively, from cases with and without previously known causal variants. Among the latter group, 61 cases had clinical and laboratory phenotypes indicative of a particular molecular etiology, whereas the remainder had an a priori highly uncertain etiology. All previously detected variants were recapitulated and, when the etiology was suspected but unknown or uncertain, a molecular diagnosis was reached in 56 of 61 and only 8 of 76 cases, respectively. The latter category highlights the need for further research into novel causes of BPDs. The ThromboGenomics platform thus provides an affordable DNA-based test to diagnose patients suspected of having a known inherited BPD.

Characterization of an autosomal dominant bleeding disorder caused by a thrombomodulin mutation

We describe a family with an autosomal dominant disorder characterized by severe trauma- and surgery-related bleeding. The proband, who experienced life-threatening bleeding during a routine operation, had normal clotting times, but markedly reduced prothrombin consumption. Plasma levels of all coagulation factors and of the main coagulation inhibitors were normal. Thrombin generation at low triggers was severely impaired and mixing experiments suggested the presence of a coagulation inhibitor. Using whole exome sequencing, the underlying genetic defect was identified as the THBD c.1611C>A mutation (p.Cys537Stop), which predicts a truncated form of thrombomodulin that is shed from the vascular endothelium. The patient had decreased expression of endothelium-bound thrombomodulin, but extremely elevated levels of soluble thrombomodulin in plasma, impairing the propagation phase of coagulation via rapid activation of protein C and consequent inactivation of factors Va and VIIIa. The same thrombomodulin mutation has been recently described in an unrelated British family with strikingly similar features.