Naturally occurring mutations in the thrombomodulin gene leading to impaired expression and function

Genetic factors, risk prediction and AI application of thrombotic diseases

In thrombotic diseases, coagulation, anticoagulation, and fibrinolysis are three key physiological processes that interact to maintain blood in an appropriate state within blood vessels. When these processes become imbalanced, such as excessive coagulation or reduced anticoagulant function, it can lead to the formation of blood clots. Genetic factors play a significant role in the onset of thrombotic diseases and exhibit regional and ethnic variations. The decision of whether to initiate prophylactic anticoagulant therapy is a matter that clinicians must carefully consider, leading to the development of various thrombotic risk assessment scales in clinical practice. Given the considerable heterogeneity in clinical diagnosis and treatment, researchers are exploring the application of artificial intelligence in medicine, including disease prediction, diagnosis, treatment, prevention, and patient management. This paper reviews the research progress on various genetic factors involved in thrombotic diseases, analyzes the advantages and disadvantages of commonly used thrombotic risk assessment scales and the characteristics of ideal scoring scales, and explores the application of artificial intelligence in the medical field, along with its future prospects.

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.

Thrombomodulin (THBD) gene variants and thrombotic risk in a population-based cohort study

BACKGROUND

The protein C anticoagulant system plays a key role in maintaining the hemostatic balance. Although several studies have identified thrombomodulin gene (THBD) variants among venous thromboembolism (VTE) patients, the role of THBD in relation to VTE in humans remains to be clarified.

OBJECTIVES

This study aimed to determine the thrombotic risk of rare and common THBD variants in a large population-based cohort of middle-aged and older adults.

PATIENTS/METHODS

The exome sequence of THBD was analyzed for qualifying variants in 28,794 subjects (born 1923-1950, 60% women), who participated in the Malmö Diet and Cancer study (1991-1996). Patients were followed from baseline until the first event of VTE, death, or 2018. Qualifying variants were defined as loss-of-function or non-benign (PolyPhen-2) missense variants with minor allele frequency <0.1%.

RESULTS

The single common coding variant rs1042579 was not associated with incident VTE. Sixteen rare variants were classified as qualifying and included in collapsing analysis. Seven individuals with VTE compared to 24 individuals without VTE carried one qualifying variant. Cox multivariate regression analysis adjusted for age, sex, body mass index, systolic blood pressure, smoking and alcohol consumption, rs6025, rs1799963, and the top two eigenvectors from a principal components analysis showed a hazard ratio of 3.0 (95% confidence interval 1.4-6.3) for the rare qualifying variants. The distributions of qualifying variants in THBD showed a difference for individuals with and without incident VTE indicating a possible position effect.

CONCLUSIONS

Rare qualifying THBD variants were associated with VTE, suggesting that rare variants in THBD contribute to development of VTE.

Endothelial Dysfunction after Hematopoietic Stem Cell Transplantation: A Review Based on Physiopathology

Endothelial dysfunction (ED) is frequently encountered in transplant medicine. ED is an argument of high complexity, and its understanding requires a wide spectrum of knowledge based on many fields of basic sciences such as molecular biology, immunology, and pathology. After hematopoietic stem cell transplantation (HSCT), ED participates in the pathogenesis of various complications such as sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD), graft-versus-host disease (GVHD), transplant-associated thrombotic microangiopathy (TA-TMA), idiopathic pneumonia syndrome (IPS), capillary leak syndrome (CLS), and engraftment syndrome (ES). In the first part of the present manuscript, we briefly review some biological aspects of factors involved in ED: adhesion molecules, cytokines, Toll-like receptors, complement, angiopoietin-1, angiopoietin-2, thrombomodulin, high-mobility group B-1 protein, nitric oxide, glycocalyx, coagulation cascade. In the second part, we review the abnormalities of these factors found in the ED complications associated with HSCT. In the third part, a review of agents used in the treatment of ED after HSCT is presented.

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.

Significant common environmental effects on leukocyte subpopulations

Major efforts are invested in the analysis of phenotypic variation in a population of individuals. While many of these studies focus on the genetic basis of phenotypic variation via measurements of DNA polymorphic sites, the environmental effects are still elusive. Here we propose a methodology, called CCCE ('Cell Composition Common Environment'), to identify environmental effects on the composition of immune cell functionalities. Specifically, CCCE is focused on the common experiences that are shared between siblings (the 'common environment'), designed to correct for cell subpopulation heterogeneity, and is based on a multicolor flow cytometry analysis across a large cohort of human monozygotic and dizygotic twins. We demonstrate that the CCCE methodology can provide insights on the relations between common environmental effects and the heterogenic functions of several immune cell types, such as NK cells effector functions and coagulation-related capabilities of monocytes. The software described in this article is available at http://csgi.tau.ac.il/CCCE.

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.

Thrombomodulin gene c.1418C>T polymorphism and risk of recurrent venous thromboembolism

Thrombomodulin gene (THBD) is a critical cofactor in protein C anticoagulant system. THBD c.1418C>T polymorphism is reported to be associated with higher risk of primary venous thromboembolism (VTE) but its role in VTE recurrence is unknown. The aim of this study was to investigate the role of THBD polymorphism in VTE recurrence. THBD c.1418C>T polymorphism was genotyped by using Taqman polymerase chain reaction in a prospective population based study of 1465 consecutive objectively verified VTE patients. Uni- and multivariate Cox regression were performed for the risk assessment of VTE recurrence. Patients who had VTE before inclusion or had recurrence or died during anticoagulant treatment were excluded. Among the remaining (N = 1046) patients, 126 (12.05 %) had VTE recurrence during the follow up period (from 1998 to 2008). THBD polymorphism was not significantly associated with risk of VTE recurrence in the univariate [Hazard ratio (HR) 1.11, 95 % confidence interval (CI) 0.78-1.59, p = 0.55] as well as the multivariate analysis adjusted for age, sex and thrombophilia (HR 1.11, 95 % CI 0.78-1.59, p = 0.54). Similarly, in unprovoked first VTE (n = 614), no association was observed between THBD polymorphism and risk of VTE recurrence (HR 1.22 and 95 % CI 0.78-1.89, p = 0.38). In this prospective study, our results do not suggest a predictive role for THBD c.1418C>T polymorphism in VTE recurrence.

Identification of Genetic Aberrations in Thrombomodulin Gene in Patients With Recurrent Venous Thromboembolism

Thrombomodulin (THBD) serves as a cofactor for thrombin-mediated activation of anticoagulant protein C pathway. Genetic aberrations in THBD have been studied in arterial and venous thrombosis. However, genetic changes in THBD and their role in the risk assessment of recurrent venous thromboembolism (VTE) are not well understood. The aim of the present study was to identify the genetic aberrations in THBD and their association with the risk of VTE recurrence in a prospective population-based study. We sequenced the entire THBD gene, first in selected patients with VTE (n = 95) by Sanger sequencing and later validated those polymorphisms with minor allele frequency (MAF) ≥5% in the whole study population (n = 1465 with the follow-up period of 1998-2008) by Taqman polymerase chain reaction. In total, we identified 8 polymorphisms in THBD, and 3 polymorphisms with MAF ≥5% were further validated. No significant association between THBD polymorphisms and risk of VTE recurrence on univariate or multivariate Cox regression analysis was found (hazard ratio [HR] = 0.89, 95% confidence interval [CI] = 0.62-1.28, HR = 1.27, 95% CI = 0.88-1.85, and HR = 1.15, 95% CI = 0.80-1.66 for THBD rs1962, rs1042580, and rs3176123 polymorphisms, respectively), adjusted for family history, acquired risk factors for VTE, location of deep vein thrombosis, and risk of thrombophilia. Subanalysis of patients with unprovoked first VTE also showed no significant association of identified THBD polymorphisms with the risk of VTE recurrence. Our results show that aberrations in the THBD gene may not be useful for the assessment of VTE recurrence; however, further studies with large sample size are needed to confirm these findings.

Retinal artery and vein thrombotic occlusion during pregnancy: markers for familial thrombophilia and adverse pregnancy outcomes

Background

Ocular vascular occlusion (OVO), first diagnosed during or immediately after giving birth, often reflects superposition of the physiologic thrombophilia of pregnancy on previously undiagnosed underlying familial or acquired thrombophilia associated with spontaneous abortion, eclampsia, or maternal thrombosis.

Specific aim

We describe OVO, first diagnosed during pregnancy or immediately postpartum, in three young females (ages 32, 35, 40) associated with previously undiagnosed familial thrombophilia.

Results

Branch retinal artery occlusion (BRAO) occurred at 9 and 13 weeks gestation in two females, aged 32 and 35. Central retinal vein occlusion occurred immediately postpartum in a 40-year-old. One of the two females with BRAO subsequently developed eclampsia, and one had a history of unexplained first trimester spontaneous abortion. All three females were found to have previously unexplained familial thrombophilia. The two females with BRAO had low first trimester free protein S 42 (41%), lower normal limit (50%), and one of these two had high factor VIII (165%, upper normal limit 150%). The woman with central retinal vein occlusion had high factor XI (169%, upper normal limit 150%). Enoxaparin (40–60 mg/day) was started and continued throughout pregnancy in both females with BRAO to prevent maternal–placental thrombosis, and of these two females, one had an uncomplicated pregnancy course and term delivery, and the second was at gestational week 22 without complications at the time of this manuscript. There were no further OVO events in the two females treated with enoxaparin or in the untreated patient with postpartum eclampsia.

Conclusion

OVO during pregnancy may be a marker for familial or acquired thrombophilia, which confers increased thrombotic risk to the mother and pregnancy, associated with spontaneous abortion or eclampsia. OVO during pregnancy, particularly when coupled with antecedent adverse pregnancy outcomes, should prompt urgent thrombophilia evaluation and institution of thromboprophylaxis to prevent adverse maternal and fetal–placental thrombotic events.

Mycolactone-Dependent Depletion of Endothelial Cell Thrombomodulin Is Strongly Associated with Fibrin Deposition in Buruli Ulcer Lesions

A well-known histopathological feature of diseased skin in Buruli ulcer (BU) is coagulative necrosis caused by the Mycobacterium ulcerans macrolide exotoxin mycolactone. Since the underlying mechanism is not known, we have investigated the effect of mycolactone on endothelial cells, focussing on the expression of surface anticoagulant molecules involved in the protein C anticoagulant pathway. Congenital deficiencies in this natural anticoagulant pathway are known to induce thrombotic complications such as purpura fulimans and spontaneous necrosis. Mycolactone profoundly decreased thrombomodulin (TM) expression on the surface of human dermal microvascular endothelial cells (HDMVEC) at doses as low as 2ng/ml and as early as 8hrs after exposure. TM activates protein C by altering thrombin’s substrate specificity, and exposure of HDMVEC to mycolactone for 24 hours resulted in an almost complete loss of the cells’ ability to produce activated protein C. Loss of TM was shown to be due to a previously described mechanism involving mycolactone-dependent blockade of Sec61 translocation that results in proteasome-dependent degradation of newly synthesised ER-transiting proteins. Indeed, depletion from cells determined by live-cell imaging of cells stably expressing a recombinant TM-GFP fusion protein occurred at the known turnover rate. In order to determine the relevance of these findings to BU disease, immunohistochemistry of punch biopsies from 40 BU lesions (31 ulcers, nine plaques) was performed. TM abundance was profoundly reduced in the subcutis of 78% of biopsies. Furthermore, it was confirmed that fibrin deposition is a common feature of BU lesions, particularly in the necrotic areas. These findings indicate that there is decreased ability to control thrombin generation in BU skin. Mycolactone’s effects on normal endothelial cell function, including its ability to activate the protein C anticoagulant pathway are strongly associated with this. Fibrin-driven tissue ischemia could contribute to the development of the tissue necrosis seen in BU lesions.

Buruli ulcer (BU) is a neglected tropical disease that is most common in West Africa and parts of Australia, but has been reported from over 30 countries worldwide. The symptoms are painless ulcers of the skin caused by a bacterial infection. The bacteria, Mycobacterium ulcerans, produce a macrolide toxin called mycolactone. In this manuscript, we have studied the effects of mycolactone on endothelial cells, specialised cells that line blood vessels and form capillaries. One of the most important functions of these cells is to prevent blood from clotting inside the vessels. We show that mycolactone reduces the ability of cultured endothelial cells to anticoagulate blood, by blocking the expression of a protein called thrombomodulin. We went on to examine samples of BU patient skin and found that thrombomodulin is also reduced here, and that in contrast to normal skin large amounts of fibrin (one of the main constituents of blood clots) were present. This means that it may be useful to consider whether anticoagulants might improve the response to antibiotics and thereby improve treatment outcomes for BU patients.

Ethnic diversity in the genetics of venous thromboembolism

Genetic susceptibility is considered as a crucial factor for the development of venous thromboembolism (VTE). Epidemiologic and genetic studies have revealed clear disparities in the incidence of VTE and the distribution of genetic factors for VTE in populations stratified by ethnicity worldwide. While gain-of-function polymorphisms in the procoagulant genes are common inherited factors in European-origin populations, the most prevalent molecular basis for venous thrombosis in Asians is confirmed to be dysfunctional variants in the anticoagulant genes. With the breakthrough of genomic technologies, a set of novel common alleles and rare mutations associated with VTE have also been identified, in different ethnic groups. Several putative pathways contributing to the pathogenesis of thrombophilia in populations of African-ancestry are largely unknown, as current knowledge of hereditary and acquired risk factors do not fully explain the highest risk of VTE in Black groups. In-depth studies across diverse ethnic populations are needed to unravel the whole genetics of VTE, which will help developing individual risk prediction models and strategies to minimise VTE in all populations.

Common genetic risk factors for venous thrombosis in the Chinese population

Venous thrombosis is a major medical disorder caused by both genetic and environmental factors. Little is known about the genetic background of venous thrombosis in the Chinese population. A total of 1,304 individuals diagnosed with a first venous thrombosis and 1,334 age- and sex-matched healthy participants were enrolled in this study. Resequencing of THBD (encoding thrombomodulin) in 60 individuals with venous thrombosis and 60 controls and a functional assay showed that a common variant, c.-151G>T (rs16984852), in the 5' UTR significantly reduced the gene expression and could cause a predisposition to venous thrombosis. Therefore, this variant was genotyped in a case-control study, and results indicated that heterozygotes had a 2.80-fold (95% confidence interval = 1.88-4.29) increased risk of venous thrombosis. The THBD c.-151G>T variant was further investigated in a family analysis involving 176 first-degree relatives from 38 index families. First-degree relatives with this variant had a 3.42-fold increased risk of venous thrombosis, and their probability of remaining thrombosis-free was significantly lower than that of relatives without the variant. In addition, five rare mutations that might be deleterious were also identified in thrombophilic individuals by sequencing. This study is the largest genetic investigation of venous thrombosis in the Chinese population. Further study on genetics of thrombosis should focus on resequencing of THBD and other hemostasis genes in different populations.

Thrombomodulin as a regulator of the anticoagulant pathway: implication in the development of thrombosis

Thrombomodulin is a cell surface-expressed glycoprotein that serves as a cofactor for thrombin-mediated activation of protein C (PC), an event further amplified by the endothelial cell PC receptor. The PC pathway is a major anticoagulant mechanism that downregulates thrombin formation and hedges thrombus formation. The objectives of this review were to review recent findings regarding thrombomodulin structure, its involvement in the regulation of hemostasis and further discuss the implication, if any, of the genetic polymorphisms in the thrombomodulin gene in the risk of development of thrombosis. We performed a literature search by using electronic bibliographic databases. Although the direct evaluation of risk situations associated with thrombomodulin mutations/polymorphisms could be of clinical significance, it appears that mutations that affect the function of thrombomodulin are rarely associated with venous thromboembolism. However, several polymorphisms are reported to be associated with increased risk for arterial thrombosis. Additionally studies on knock out mice as well studies on humans bearing rare mutations suggest that thrombomodulin dysfunction may be implicated in the pathogenesis of myocardial infraction.

Association of thrombomodulin gene polymorphisms and plasma thrombomodulin levels with acute myocardial infarction in north Indian patients

This is the first study from north India that investigated the association of thrombomodulin (TM) polymorphisms with acute myocardial infarction (AMI) in 350 patients (≤ 40 years, n = 184 and ≥ 60 years, n = 166) and 350 matched-controls. The TM polymorphisms were determined by polymerase chain reaction-single-stranded conformational polymorphism and DNA sequencing. The TM 1418TT genotype (odds ratio [OR] 2.8; 95% confidence interval [CI] 1.3-6.4; P = .012) was independent risk predictor of young AMI as were hypertension (OR 3.3; 95% CI 1.8-5.9; P < .001), diabetes mellitus (OR 14.3; 95% CI 2.9-44.6; P = .001), smoking (OR 13.8; 95% CI 7.7-24.7; P < .001), family history (OR 1.8; 95% CI 1.1-3.3; P = .045), high body mass index (OR 2.2; 95% CI 1.3-3.6; P = .002), and high waist-hip ratio (OR 4.1; 95% CI 2.4-7.1; P < .001). Mean plasma TM also showed association with young AMI (P < .001). Smoking carriers of TM 1418CT + TT genotype had significantly higher risk of AMI (OR 12.8; 95% CI 6.0-27.3; P < .001) when compared with nonsmoking noncarriers. In conclusion, TM 1418C/T polymorphism is independent predictor of AMI and synergies with smoking.

Thrombomodulin and its role in inflammation

The goal is to provide an extensive review of the physiologic role of thrombomodulin (TM) in maintaining vascular homeostasis, with a focus on its anti-inflammatory properties. Data were collected from published research. TM is a transmembrane glycoprotein expressed on the surface of all vascular endothelial cells. Expression of TM is tightly regulated to maintain homeostasis and to ensure a rapid and localized hemostatic and inflammatory response to injury. By virtue of its strategic location, its multidomain structure and complex interactions with thrombin, protein C (PC), thrombin activatable fibrinolysis inhibitor (TAFI), complement components, the Lewis Y antigen, and the cytokine HMGB1, TM exhibits a range of physiologically important anti-inflammatory, anti-coagulant, and anti-fibrinolytic properties. TM is an essential cofactor that impacts on multiple biologic processes. Alterations in expression of TM and its partner proteins may be manifest by inflammatory and thrombotic disorders. Administration of soluble forms of TM holds promise as effective therapies for inflammatory diseases, and infections and malignancies that are complicated by disseminated intravascular coagulation.

Human thrombomodulin knock-in mice reveal differential effects of human thrombomodulin on thrombosis and atherosclerosis

OBJECTIVE

We sought to develop a murine model to examine the antithrombotic and antiinflammatory functions of human thrombomodulin in vivo.

METHODS AND RESULTS

Knock-in mice that express human thrombomodulin from the murine thrombomodulin gene locus were generated. Compared with wild-type mice, human thrombomodulin knock-in mice exhibited decreased protein C activation in the aorta (P<0.01) and lung (P<0.001). Activation of endogenous protein C following infusion of thrombin was decreased by 90% in knock-in mice compared with wild-type mice (P<0.05). Carotid artery thrombosis induced by photochemical injury occurred more rapidly in knock-in mice (12±3 minutes) than in wild-type mice (31±6 minutes; P<0.05). No differences in serum cytokine levels were detected between knock-in and wild-type mice after injection of endotoxin. When crossed with apolipoprotein E-deficient mice and fed a Western diet, knock-in mice had a further decrease in protein C activation but did not exhibit increased atherosclerosis.

CONCLUSION

Expression of human thrombomodulin in place of murine thrombomodulin produces viable mice with a prothrombotic phenotype but unaltered responses to systemic inflammatory or atherogenic stimuli. This humanized animal model will be useful for investigating the function of human thrombomodulin under pathophysiological conditions in vivo.

Thrombomodulin mutations in atypical hemolytic-uremic syndrome

BACKGROUND

The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.

METHODS

We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.

RESULTS

Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.

CONCLUSIONS

Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

Fetal gene defects precipitate platelet-mediated pregnancy failure in factor V Leiden mothers

We describe a mouse model of fetal loss in factor V Leiden (FvL) mothers in which fetal loss is triggered when the maternal prothrombotic state coincides with fetal gene defects that reduce activation of the protein C anticoagulant pathway within the placenta. Fetal loss is caused by disruption of placental morphogenesis at the stage of labyrinth layer formation and occurs in the absence of overt placental thrombosis, infarction, or perfusion defects. Platelet depletion or elimination of protease-activated receptor 4 (Par4) from the mother allows normal placentation and prevents fetal loss. These findings establish a cause–effect relationship for the observed epidemiologic association between maternal FvL status and fetal loss and identify fetal gene defects as risk modifiers of pregnancy failure in prothrombotic mothers. Pregnancy failure is mediated by Par4-dependent activation of maternal platelets at the fetomaternal interface and likely involves a pathogenic pathway independent of occlusive thrombosis. Our results further demonstrate that the interaction of two given thrombosis risk factors produces markedly disparate consequences on disease manifestation (i.e., thrombosis or pregnancy loss), depending on the vascular bed in which this interaction occurs.

Haplotype of thrombomodulin gene associated with plasma thrombomodulin level and deep vein thrombosis in the Japanese population

INTRODUCTION

Thrombomodulin (TM) is an essential cofactor in protein C activation by thrombin. Here, we evaluated the contribution of genetic variations in the TM gene to soluble TM (sTM) level and deep vein thrombosis (DVT) in Japanese.

PATIENTS AND METHODS

We sequenced the TM putative promoter, exon, and 3'-untranslated region in DVT patients (n=118). Among 17 genetic variations we identified, two missense mutations (R385K, D468Y) and three common single nucleotide polymorphisms (-202G>A, 2487A>T, 2729A>C) were genotyped in a general population of 2247 subjects (1032 men and 1215 women) whose sTM levels were measured. We then compared the frequency of these mutations in DVT patients with that in the age, body mass index-adjusted population-based controls.

RESULTS

We identified one neutral mutation (H381) and three missense mutations (R385K; n=2, A455V; n=53 heterozygous, n=14 homozygous, D468Y; n=2) of TM in the DVT patients. Age-adjusted mean values of sTM were lower in C-allele carriers of 2729A>C than in noncarriers in the Japanese general population (women: 16.7+/-0.3 U/ml vs. 17.9+/-0.2 U/ml, p<0.01, men: 19.4+/-0.3 U/ml vs. 20.4+/-0.3 U/ml, p=0.03). Additionally, the CC genotype of this mutation was more common in the male DVT patients than in the male individuals of the general population (odds ratio=2.76, 95% confidence interval=1.14-6.67; p=0.02). This mutation was in linkage disequilibrium (r-square>0.9) with A455V mutation.

CONCLUSIONS

TM mutations, especially those with a haplotype consisting of 2729A>C and A455V missense mutation, affect sTM levels, and may be associated with DVT in Japanese.

Relationship between soluble thrombomodulin in patients with intermittent claudication and critical ischemia

INTRODUCTION

Thrombomodulin (TM) has been described as a marker of endothelial injury in atherosclerosis. The role of TM as a predictor of PAD severity is to be proven. The goal of the present study is to compare the level of plasmatic (TMp) in patients with intermittent claudication with patients with critical ischemia in the lower limbs.

MATERIALS AND METHODS

TMp was measured using ELISA in the plasma of 41 patients with intermittent claudication degree 1 and in 40 patients presenting critical ischemia in the lower limbs degrees 2 and 3, according to TASC. The hypotheses of normality and homogeneity of the variance had been proven via Shapiro-Wilk and Levene tests, respectively. The comparison of the TMp between the groups was done using the t-Student test.

RESULTS

No statistically significant difference was observed. The average levels of TMp for intermittent claudication were 5.2 ng/ml (0.78-13.61 ng/ml) and TMp for critical ischemia in the lower limbs were 6.34 (0.82-18.22 ng/ml) where p=0.265.

CONCLUSION

TMp does not seem to be an appropriate marker for PAD severity.

Thrombomodulin gene polymorphisms or haplotypes as potential risk factors for venous thromboembolism: a population-based case-control study

Dysfunction of the protein C anticoagulant system is associated with venous thromboembolism (VTE) and thrombomodulin (TM) is a critical cofactor within the protein C system. The aim of this study was to test the hypotheses that polymorphisms or haplotypes within the TM gene are common risk factors for VTE. We screened the TM putative promoter, exon and 3'-untranslated region for sequence variations in a random sample (n = 266) of consecutive idiopathic, objectively confirmed non-Olmsted County VTE patients referred to the Mayo Clinic. We then genotyped a sample of Olmsted County, MN residents with a first lifetime, objectively confirmed VTE in the 25-year period, 1966-90 (n = 223), and a sample of Olmsted County residents without VTE (n = 237) for polymorphisms either discovered in the screening population or previously published, and tested for an association of VTE with TM genotype or haplotypes using unconditional logistic regression and generalized linear models, respectively. We also genotyped these Olmsted County cases and controls at 20 'null' genetic maker loci and tested for population admixture. Nine novel and three previously described mutations were identified in the screening population. Mutations within the TM promoter, EGF(1-5), serine/threonine-rich, transmembrane, and cytoplasm regions were absent or uncommon. TM845G-->A (Ala25Thr; lectin region), TM2136T-->C (Ala455Val; EGF(6) region), TM2470C deletion (3'-untranslated region), and 4363A-->G (3'-flanking region) were more common, but were not associated with VTE by genotype or haplotype. Null genetic marker allele frequencies did not differ significantly among cases and controls. We conclude that polymorphisms or haplotypes within the TM gene are not common risk factors for incident VTE.

Does activated protein C-resistant factor V contribute to thrombin generation in hemophilic plasma?

In this study we assessed the role of factor V (FV) inactivation in hemophilic plasma with particular reference to the activated protein C (APC)-resistant variants FV-R506Q (FV Leiden) and FV-R306T (FV Cambridge). Purified recombinant full-length FV carrying these single substitutions and FV-R306T/R506Q were used in thrombin generation experiments. Plasma was first immunodepleted of FV, and subsequently of factors VIII, IX, or combinations thereof. Thrombin generation was initiated by low concentrations of recombinant tissue factor. Recombinant soluble thrombomodulin (TM) was used to trigger the APC system. Surprisingly, TM concentrations that reduced thrombin generation in normal plasma by no more than 50% virtually abolished thrombin formation in plasma deficient in the factor VIII/IX complex. This was already apparent at TM levels as low as 0.1 nmol L(-1). By varying the concentrations of purified (activated) protein C to plasma that was additionally depleted of protein C, we confirmed that impaired thrombin generation indeed was the result of the action of APC. In contrast, this did not occur when FV-depleted plasma had been reconstituted with FV-R306T/R506Q. Addition of FV-R306T or FV-R506Q partially reduced prothrombin activation, demonstrating the involvement of both APC cleavage sites. FV inactivation also occurred on the surface of human microvascular endothelial cells. Apparently, these cells express sufficient TM to down-regulate thrombin production via the APC pathway. We further conclude that in hemophilic plasma this pathway can induce a secondary defect because of premature FV inactivation. It therefore seems conceivable that APC-resistant FV has the potential of alleviating hemophilic bleeding.

Thrombomodulin Ala455Val Polymorphism and the risk of cerebral infarction in a biracial population: the Stroke Prevention in Young Women Study

Background

The genes encoding proteins in the thrombomodulin-protein C pathway are promising candidate genes for stroke susceptibility because of their importance in thrombosis regulation and inflammatory response. Several published studies have shown that the Ala455Val thrombomodulin polymorphism is associated with ischemic heart disease, but none has examined the association with stroke. Using data from the Stroke Prevention in Young Women Study, we sought to determine the association between the Ala455Val thrombomodulin polymorphism and the occurrence of ischemic stroke in young women.

Methods

All 59 hospitals in the greater Baltimore-Washington area participated in a population-based case-control study of stroke in young women. We compared 141 cases of first ischemic stroke (44% black) among women 15 to 44 years of age with 210 control subjects (35% black) who were identified by random digit dialing and frequency matched to the cases by age and geographical region of residence. Data on historical risk factors were collected by standardized interview. Genotyping of the thrombomodulin Ala455Val polymorphism was performed by pyrosequencing.

Results

The A allele (frequency = 0.85) was associated with stroke under the recessive model. After adjustment for age, race, cigarette smoking, hypertension, and diabetes, the AA genotype, compared with the AV and VV genotypes combined, was significantly associated with stroke (odds ratio 1.9, 95% CI 1.1–3.3). The AA genotype was more common among black than white control subjects (81% versus 68%) but there was no significant interaction between the risk genotype and race (adjusted odds ratio 2.7 for blacks and 1.6 for whites). A secondary analysis removing all probable (n = 16) and possible (n = 15) cardioembolic strokes demonstrated an increased association (odds ratio 2.2, 95% CI 1.2–4.2).

Conclusions

Among women aged 15 to 44 years, the AA genotype is more prevalent among blacks than whites and is associated with increased risk of early onset ischemic stroke. Removing strokes potentially related to cardioembolic phenomena increased this association. Further studies are needed to determine whether this polymorphism is functionally related to thrombomodulin expression or whether the association is due to population stratification or linkage to a nearby functional polymorphism.

Novel functions of thrombomodulin in inflammation

The objective of this study was to review the mechanisms by which thrombomodulin (TM) may modulate inflammation. The data were taken from published research performed by other laboratories and our own experimental results. TM is a transmembrane glycoprotein receptor and cofactor for thrombin in the protein C anticoagulant system. Recent studies have revealed that TM has activities, both dependent and independent of either protein C or thrombin, that affect biological systems beyond the coagulation pathway. This review highlights recent insights, provided by in vitro and in vivo analyses, into how the unique structural domains of TM effectively modify coagulation, fibrinolysis, and inflammation in health and disease. A paradigm is presented to describe how these apparently distinct functions are integrated to maintain homeostasis under stress conditions. Finally, we explore the potential diagnostic and therapeutic utility of dissecting out the structure-function correlates of TM. We conclude that TM plays a central role in regulating not only hemostasis but also inflammation, thus providing a close link between these processes. Elucidation of the molecular mechanisms by which TM functions will likely provide novel targets for therapeutic intervention.

Genetic variation in the human thrombomodulin promoter locus and prognosis after acute coronary syndrome

INTRODUCTION

Endothelial thrombomodulin (TM) plays a critical role in both anticoagulation and anti-inflammation. An impaired TM cofactor function or reduced TM gene expression could constitute a prethrombotic abnormality leading to acute coronary events. Mutations in the TM gene occur, but their functional consequences on the expression and activity of the gene are not yet fully understood.

MATERIALS AND METHODS

We performed a prospective study investigating the prevalence of TM mutations in the promoter region in 182 patients with acute coronary syndrome as well as in a control group. The patients were followed-up after 30 days and after 2 years for acute myocardial infarction (MI) and mortality.

RESULTS AND CONCLUSIONS

We identified 10 point mutations and 2 small deletions: -1861 C/A, -1852 C/G, -1803 G/C, -1752 G/C, -1213/1212 delTT, -1089 C/G, -1088 C/T, -1083/1082 delCC, -1066 A/C, -801 C/G, -651 A/C and -52 G/A. Two of the mutations, -1752 G/C and -1213/1212 delTT, were frequent in the patients as well as in the controls, while all the others were rare. The only significant finding was that both -1752 G/C and -1213/1212 delTT were associated with a lower than normal risk of suffering a clinical event among smokers at 30 days and 2 years. We did not gain any support for the hypothesis that TM mutations confer an increased risk of MI or mortality.

Thrombomodulin

Since its discovery as a critical cofactor in the initiation of the protein C (PC) anticoagulant pathway [1,2], biochemical and structural investigations, combined with in vivo analyses of genetically engineered mice have revealed new, and in part PC- and thrombin-independent aspects of thrombomodulin (TM) function in fibrinolysis and inflammation, and in embryogenesis. This review summarizes more recent structural and functional investigations of TM, gives an overview of the association of TM gene polymorphisms with human disease, and provides a synopsis of what is know about TM function in disease states of thrombosis, stroke, arteriosclerosis, and cancer. Newly emerging aspects of TM function in inflammation and embryogenesis are presented and discussed in detail.

Mutations in the human Na-K-2Cl cotransporter (NKCC2) identified in Bartter syndrome type I consistently result in nonfunctional transporters

Bartter syndrome (BS) is a heterogeneous renal tubular disorder affecting Na-K-Cl reabsorption in the thick ascending limb of Henle's loop. BS type I patients typically present with profound hypokalemia and metabolic alkalosis. The main goal of the present study was to elucidate the functional implications of six homozygous mutations (G193R, A267S, G319R, A508T, del526N, and Y998X) in the bumetanide-sensitive Na-K-2Cl cotransporter (hNKCC2) identified in patients diagnosed with BS type I. To this end, capped RNA (cRNA) of FLAG-tagged hNKCC2 and the corresponding mutants was injected in Xenopus laevis oocytes and transporter activity was measured after 72 h by means of a bumetanide-sensitive (22)Na(+) uptake assay at 30 degrees C. Injection of 25 ng of hNKCC2 cRNA resulted in bumetanide-sensitive (22)Na(+) uptake of 2.5 +/- 0.5 nmol/oocyte per 30 min. Injection of 25 ng of mutant cRNA yielded no significant bumetanide-sensitive (22)Na(+) uptake. Expression of wild-type and mutant transporters was confirmed by immunoblotting, showing significantly less mutant protein compared with wild-type at the same cRNA injection levels. However, when the wild-type cRNA injection level was reduced to obtain a protein expression level equal to that of the mutants, the wild-type still exhibited a significant bumetanide-sensitive (22)Na(+) uptake. Immunocytochemical analysis showed immunopositive staining of hNKCC2 at the plasma membrane for wild-type and all studied mutants. In conclusion, mutations in hNKCC2 identified in type I BS patients, when expressed in Xenopus oocytes, result in a low expression of normally routed but functionally impaired transporters. These results are in line with the hypothesis that the mutations in hNKCC2 are the underlying cause of the clinical abnormalities seen in patients with type I BS.

Prospective study of the A455V polymorphism in the thrombomodulin gene, plasma thrombomodulin, and incidence of venous thromboembolism: the LITE Study

Plasma thrombomodulin (soluble TM; sTM) is considered to be a marker of endothelial injury, but a recent report indicated that the relationship of sTM with thrombosis is complex. Venous thromboembolic events were identified in adults in two longitudinal cohort studies, the Atherosclerosis Risk in Communities Study and the Cardiovascular Health Study, totaling 21 690 participants. After 8 years of follow-up, sTM was measured in baseline plasma of 305 participants who developed venous thrombosis and 607 who did not. Thrombomodulin A455V genotype was determined in 302 cases and 626 controls. There was no difference in the prevalence of the three TM genotypes between cases and controls and no difference in age-adjusted mean values of sTM by genotype. There were no associations of age-adjusted sTM or TMA455V genotype with overall venous thromboembolism or with thrombosis in any subtype of venous thromboembolism.