Influence of single nucleotide polymorphisms in factor VIII and von Willebrand factor genes on plasma factor VIII activity: the ARIC Study

Dynamic and functional linkage between von Willebrand factor and ADAMTS-13 with aging: an Atherosclerosis Risk in Community study

BACKGROUND

von Willebrand factor (VWF) is a multimeric glycoprotein critically involved in hemostasis, thrombosis, and inflammation. VWF function is regulated by its antigen levels, multimeric structures, and the state of enzymatic cleavage. Population studies in the past have focused almost exclusively on VWF antigen levels in cross-sectional study designs.

OBJECTIVE

To identify subjects in the Atherosclerosis Risk in Community study who had persistently low and high VWF antigen over 10 years and to quantify longitudinal changes in the biological activities and cleavage of VWF in these subjects.

METHODS

We measured VWF antigen, propeptide, adhesive activities, and cleavage by ADAMTS-13 quantified using a mass spectrometry method that detected the cleaved VWF peptide EQAPNLVY, as well as coagulation factor VIII activity.

RESULTS

We determined the mean subject-specific increase in VWF to be 22.0 International Units (IU)/dL over 10 years, with 95% between -0.3 and 59.7 IU/dL. This aging-related increase was also detected in VWF propeptide levels, ristocetin cofactor activity, and VWF binding to collagen. We identified 4.1% and 25.0% of subjects as having persistently low (<50 IU/dL) and high (>200 IU/dL) VWF antigen, respectively. Subjects with persistently low VWF had enhanced ristocetin cofactor activity, whereas those with persistently high VWF had elevated levels of ADAMTS-13, resulting in a comparable rate of VWF cleavage between the 2 groups.

CONCLUSIONS

These results provide new information about the effects of aging on VWF antigens and adhesive activity and identify a functional coordination between VWF and the rate of its cleavage by ADAMTS-13.

Factor VIII: A Dynamic Modulator of Hemostasis and Thrombosis in Trauma

A trace amount of thrombin cleaves factor VIII (FVIII) into an active form (FVIIIa), which catalyzes FIXa-mediated activation of FX on the activated platelet surface. FVIII rapidly binds to von Willebrand factor (VWF) after secretion and becomes highly concentrated via VWF-platelet interaction at a site of endothelial inflammation or injury. Circulating levels of FVIII and VWF are influenced by age, blood type (nontype O > type O), and metabolic syndromes. In the latter, hypercoagulability is associated with chronic inflammation (known as thrombo-inflammation). In acute stress including trauma, releasable pools of FVIII/VWF are secreted from the Weibel-Palade bodies in the endothelium and then augment local platelet accumulation, thrombin generation, and leukocyte recruitment. Early systemic increases of FVIII/VWF (>200% of normal) levels in trauma result in a lower sensitivity of contact-activated clotting time (activated partial thromboplastin time [aPTT] or viscoelastic coagulation test [VCT]). However, in severely injured patients, multiple serine proteases (FXa plasmin and activated protein C [APC]) are locally activated and may be systemically released. Severity of traumatic injury correlates with prolonged aPTT and elevated activation markers of FXa, plasmin, and APC, culminating in a poor prognosis. In a subset of acute trauma patients, cryoprecipitate that contains fibrinogen, FVIII/VWF, and FXIII is theoretically advantageous over purified fibrinogen concentrate to promote stable clot formation, but comparative efficacy data are lacking. In chronic inflammation or subacute phase of trauma, elevated FVIII/VWF contributes to the pathogenesis of venous thrombosis by enhancing not only thrombin generation but also augmenting inflammatory functions. Future developments in coagulation monitoring specific to trauma patients, and targeted to enhancement or inhibition of FVIII/VWF, are likely to help clinicians gain better control of hemostasis and thromboprophylaxis. The main goal of this narrative is to review the physiological functions and regulations of FVIII and implications of FVIII in coagulation monitoring and thromboembolic complications in major trauma patients.

Impact of ABO Blood Group on Thromboembolic and Bleeding Complications in Patients with Left Ventricular Assist Devices

BACKGROUND

 The ABO blood group system is linked to hemostasis via its relationship with von Willebrand factor (VWF) and factor VIII (FVIII). In the current study, we investigated the association of the ABO system with clinical outcomes as well as VWF and platelet function in patients with left ventricular assist devices (LVADs).

METHODS

 Bleeding and thromboembolic complications were assessed in 111 patients during 1 year after LVAD implantation. In 67 LVAD patients, VWF antigen, VWF activity, VWF ristocetin cofactor, VWF collagen-binding, and FVIII activity were assessed. Platelet surface P-selectin and activated glycoprotein IIb/IIIa were determined by flow cytometry, and soluble P-selectin was measured with an enzyme-linked immunoassay. Platelet aggregation was assessed by light transmission and impedance aggregometry.

RESULTS

 Thirty-six patients (32.4%) experienced a bleeding and 22 patients (19.8%) a thromboembolic event. In univariate analyses, patients with blood group O had numerically more bleeding complications and less thromboembolic events as compared to patients with blood group non-O (both p ≥ 0.05). After multivariable adjustment, blood group O was significantly associated with a higher risk of bleeding (hazard ratio 2.42 [95% confidence interval 1.03-5.70], p = 0.044) but not linked to thromboembolic complications.

CONCLUSION

 Patients with blood group O had significantly lower levels of VWF and FVIII (all p < 0.05), whereas P-selectin expression in response to thrombin-receptor activating peptide and soluble P-selectin were higher as compared to patients with blood group non-O (both p < 0.05). LVAD patients with blood group O are at an increased bleeding risk, potentially due to lower VWF and FVIII levels.

Coagulation factor VIII: Relationship to cardiovascular disease risk and whole genome sequence and epigenome-wide analysis in African Americans

BACKGROUND

Prospective studies have suggested higher factor VIII (FVIII) levels are an independent risk factor for coronary heart disease (CHD) and stroke. However, limited information, including on genetic and epigenetic contributors to FVIII variation, is available specifically among African Americans (AAs), who have higher FVIII levels than Europeans.

OBJECTIVES

We measured FVIII levels in ~3400 AAs from the community-based Jackson Heart Study and assessed genetic, epigenetic, and epidemiological correlates of FVIII, as well as incident cardiovascular disease (CVD) associations.

METHODS

We assessed cross-sectional associations of FVIII with CVD risk factors as well as incident CHD, stroke, heart failure, and mortality associations. We additionally assessed associations with TOPMed whole genome sequencing data and an epigenome-wide methylation array.

RESULTS

Our results confirmed associations between FVIII and risk of incident CHD events and total mortality in AAs; mortality associations were largely independent of traditional risk factors. We also demonstrate an association of FVIII with incident heart failure, independent of B-type natriuretic peptide. Two genomic regions were strongly associated with FVIII (ABO and VWF). The index variant at VWF is specific to individuals of African descent and is distinct from the previously reported European VWF association signal. Epigenome-wide association analysis showed significant FVIII associations with several CpG sites in the ABO region. However, after adjusting for ABO genetic variants, ABO CpG sites were not significant.

CONCLUSIONS

Larger sample sizes of AAs will be required to discover additional genetic and epigenetic contributors to FVIII phenotypic variation, which may have consequences for CVD health disparities.

Biologic roles of the ABH and Lewis histo-blood group antigens part II: thrombosis, cardiovascular disease and metabolism

The ABH and Lewis antigens were among the first of the human red blood cell polymorphisms to be identified and, in the case of the former, play a dominant role in transfusion and transplantation. But these two therapies are largely twentieth-century innovations, and the ABH and related carbohydrate antigens are not only expressed on a very wide range of human tissues, but were present in primates long before modern humans evolved. Although we have learned a great deal about the biochemistry and genetics of these structures, the biological roles that they play in human health and disease are incompletely understood. This review and its companion, which appeared in a previous issue of Vox Sanguinis, will focus on a few of the biologic and pathologic processes which appear to be affected by histo-blood group phenotype. The first of the two reviews explored the interactions of two bacteria with the ABH and Lewis glycoconjugates of their human host cells, and described the possible connections between the immune response of the human host to infection and the development of the AB-isoagglutinins. This second review will describe the relationship between ABO phenotype and thromboembolic disease, cardiovascular disease states, and general metabolism.

The common VWF single nucleotide variants c.2365A>G and c.2385T>C modify VWF biosynthesis and clearance

Plasma levels of von Willebrand factor (VWF) vary considerably in the general population and this variation has been linked to several genetic and environmental factors. Genetic factors include 2 common single nucleotide variants (SNVs) located in VWF, rs1063856 (c.2365A>G) and rs1063857 (c.2385T>C), although to date the mechanistic basis for their association with VWF level is unknown. Using genotypic/phenotypic information from a European healthy control population, in vitro analyses of recombinant VWF expressing both SNVs, and in vivo murine models, this study determined the precise nature of their association with VWF level and investigated the mechanism(s) involved. Possession of either SNV corresponded with a significant increase in plasma VWF in healthy controls (P < .0001). In vitro expression confirmed this observation and highlighted an independent effect for each SNV (P < .0001 and P < .01, respectively), despite close proximity and strong linkage disequilibrium between them both. The influence of c.2365A>G on VWF levels was also confirmed in vivo. This increase in VWF protein corresponded to an increase in VWF messenger RNA (mRNA) resulting, in part, from prolonged mRNA half-life. In addition, coinheritance of both SNVs was associated with a lower VWF propeptide-to-VWF antigen ratio in healthy controls (P < .05) and a longer VWF half-life in VWF knockout mice (P < .0001). Both SNVs therefore directly increase VWF plasma levels through a combined influence on VWF biosynthesis and clearance, and may have an impact on disease phenotype in both hemostatic and thrombotic disorders.

Association of Single Nucleotide Polymorphisms in the ST3GAL4 Gene with VWF Antigen and Factor VIII Activity

VWF is extensively glycosylated with biantennary core fucosylated glycans. Most N-linked and O-linked glycans on VWF are sialylated. FVIII is also glycosylated, with a glycan structure similar to that of VWF. ST3GAL sialyltransferases catalyze the transfer of sialic acids in the α2,3 linkage to termini of N- and O-glycans. This sialic acid modification is critical for VWF synthesis and activity. We analyzed genetic and phenotypic data from the Atherosclerosis Risk in Communities (ARIC) study for the association of single nucleotide polymorphisms (SNPs) in the ST3GAL4 gene with plasma VWF levels and FVIII activity in 12,117 subjects. We also analyzed ST3GAL4 SNPs found in 2,535 subjects of 26 ethnicities from the 1000 Genomes (1000G) project for ethnic diversity, SNP imputation, and ST3GAL4 haplotypes. We identified 14 and 1,714 ST3GAL4 variants in the ARIC GWAS and 1000G databases respectively, with 46% being ethnically diverse in their allele frequencies. Among the 14 ST3GAL4 SNPs found in ARIC GWAS, the intronic rs2186717, rs7928391, and rs11220465 were associated with VWF levels and with FVIII activity after adjustment for age, BMI, hypertension, diabetes, ever-smoking status, and ABO. This study illustrates the power of next-generation sequencing in the discovery of new genetic variants and a significant ethnic diversity in the ST3GAL4 gene. We discuss potential mechanisms through which these intronic SNPs regulate ST3GAL4 biosynthesis and the activity that affects VWF and FVIII.

Quantitative Influence of ABO Blood Groups on Factor VIII and Its Ratio to von Willebrand Factor, Novel Observations from an ARIC Study of 11,673 Subjects

ABO blood groups are known to influence the plasma level of von Willebrand factor (VWF), but little is known about the relationship between ABO and coagulation factor VIII (FVIII). We analyzed the influence of ABO genotypes on VWF antigen, FVIII activity, and their quantitative relationship in 11,673 participants in the Atherosclerosis Risk in Communities (ARIC) study. VWF, FVIII, and FVIII/VWF levels varied significantly among O, A (A1 and A2), B and AB subjects, and the extent of which varied between Americans of European (EA) and African (AA) descent. We validated a strong influence of ABO blood type on VWF levels (15.2%), but also detected a direct ABO influence on FVIII activity (0.6%) and FVIII/VWF ratio (3.8%) after adjustment for VWF. We determined that FVIII activity changed 0.54% for every 1% change in VWF antigen level. This VWF-FVIII relationship differed between subjects with O and B blood types in EA, AA, and in male, but not female subjects. Variations in FVIII activity were primarily detected at low VWF levels. These new quantitative influences on VWF, FVIII and the FVIII/VWF ratio help understand how ABO genotypes differentially influence VWF, FVIII and their ratio, particularly in racial and gender specific manners.

Evaluation of von Willebrand factor phenotypes and genotypes in Hemophilia A patients with and without identified F8 mutations

BACKGROUND

Hemophilia A (HA) is an X-linked bleeding disorder caused by a deficiency in factor VIII (FVIII). von Willebrand disease (VWD) is characterized by a quantitative or qualitative defect in von Willebrand factor (VWF). Patients with VWD with severely low VWF or VWD Type 2N (VWD2N), a VWD subtype distinguished by defective VWF binding to FVIII, may have reduced FVIII levels secondary to their VWD. These patients superficially resemble patients with HA and pose a potential for misdiagnosis.

OBJECTIVES

To investigate the unexplained cause of bleeding in HA patients without known FVIII mutations by assessing plasma VWF antigen (VWF:Ag), FVIII binding capacities and VWF genotypes.

PATIENTS/METHODS

Thirty-seven of 1027 patients with HA studied as part of the Hemophilia Inhibitor Research Study lacked identifiable F8 mutations. These patients (cases) and 73 patients with identified F8 mutations (controls) were evaluated for VWF:Ag, a patient's VWF capacity to bind FVIII (VWF:FVIIIB) and VWF sequence.

RESULTS

Four cases had VWF:Ag < 3 IU dL(-1) and VWF mutations consistent with Type 3 VWD. Six cases and one control were heterozygous for mutations previously reported to cause Type 1 VWD (VWD1) (n = five cases and one control) or predicted to be deleterious by Polyphen2 and SIFT prediction tools (n = 1 case). One control had VWF:Ag < 30 IU dL(-1) and seven patients (four cases and three controls), including two cases who were heterozygous for a known VWD2N mutation, had reduced VWF:FVIIIB.

CONCLUSIONS

These data emphasize that some patients diagnosed with HA require VWF assessments in order to achieve a comprehensive diagnosis and an optimal treatment strategy.

Gene-centric approach identifies new and known loci for FVIII activity and VWF antigen levels in European Americans and African Americans

Coagulation factor VIII and von Willebrand factor (VWF) are key proteins in procoagulant activation. Higher FVIII coagulant activity (FVIII :C) and VWF antigen (VWF :Ag) are risk factors for cardiovascular disease and venous thromboembolism. Beyond associations with ABO blood group, genetic determinants of FVIII and VWF are not well understood, especially in non European-American populations. We performed a genetic association study of FVIII :C and VWF:Ag that assessed 50,000 gene-centric single nucleotide polymorphisms (SNPs) in 18,556 European Americans (EAs) and 5,047 African Americans (AAs) from five population-based cohorts. Previously unreported associations for FVIII :C were identified in both AAs and EAs with KNG1 (most significantly associated SNP rs710446, Ile581Thr, Ile581Thr, P = 5.10 × 10(-7) in EAs and P = 3.88 × 10(-3) in AAs) and VWF rs7962217 (Gly2705Arg,P = 6.30 × 10(-9) in EAs and P = 2.98 × 10(-2) in AAs. Significant associations for FVIII :C were also observed with F8/TMLHE region SNP rs12557310 in EAs (P = 8.02 × 10(-10) ), with VWF rs1800380 in AAs (P = 5.62 × 10(-11) ), and with MAT1A rs2236568 in AAs (P51.69 × 10(-6) ). We replicated previously reported associations of FVIII :C and VWF :Ag with the ABO blood group, VWF rs1063856(Thr789Ala), rs216321 (Ala852Gln), and VWF rs2229446 (Arg2185Gln). Findings from this study expand our understanding of genetic influences for FVIII :C and VWF :Ag in both EAs and AAs.

Distribution, genetic and cardiovascular determinants of FVIII:c - Data from the population-based Gutenberg Health Study

BACKGROUND

Elevated levels of

FVIII

c are associated with risk for both venous and arterial thromboembolism. However, no population-based study on the sex-specific distribution and reference ranges of plasma

FVIII

c and its cardiovascular determinants is available.

METHODS

FVIII

c was analyzed in a randomly selected sample of 2533 males and 2440 females from the Gutenberg Health Study in Germany. Multivariable regression analyses for

FVIII

c were performed under adjustment for genetic determinants, cardiovascular risk factors and cardiovascular disease.

RESULTS AND CONCLUSIONS

Females (126.6% (95% CI: 125.2/128)) showed higher

FVIII

c levels than males (121.2% (119.8/122.7)).

FVIII

c levels increased with age in both sexes (ß per decade: 5.67% (4.22/7.13) male, 6.15% (4.72/7.57) female; p<0.001). Sex-specific reference limits and categories indicating the grade of deviation from the reference were calculated, and nomograms for

FVIII

c were created.

FVIII

c was approximately 25% higher in individuals with non-O blood type. Adjusted for sex and age, ABO-blood group accounted for 18.3% of

FVIII

c variation. In multivariable analysis,

FVIII

c was notably positively associated with diabetes mellitus, obesity, hypertension and dyslipidemia and negatively with current smoking. In a fully adjusted multivariable model, the strongest associations observed were of elevated

FVIII

c with diabetes and peripheral artery disease in both sexes and with obesity in males. Effects of SNPs in the vWF, STAB2 and SCARA5 gene were stronger in females than in males. The use of nomograms for valuation of

FVIII

c might be useful to identify high-risk cohorts for thromboembolism. Additionally, the prospective evaluation of

FVIII

c as a risk predictor becomes feasible.

Association of vWA and TPOX polymorphisms with venous thrombosis in Mexican mestizos

Objective. Venous thromboembolism (VTE) is a multifactorial disorder and, worldwide, the most important cause of morbidity and mortality. Genetic factors play a critical role in its aetiology. Microsatellites are the most important source of human genetic variation having more phenotypic effect than many single nucleotide polymorphisms. Hence, we evaluate a possible relationship between VTE and the genetic variants in von Willebrand factor, human alpha fibrinogen, and human thyroid peroxidase microsatellites to identify possible diagnostic markers. Methods. Genotypes were obtained from 177 patients with VTE and 531 nonrelated individuals using validated genotyping methods. The allelic frequencies were compared; Bayesian methods were used to correct population stratification to avoid spurious associations. Results. The vWA-18, TPOX-9, and TPOX-12 alleles were significantly associated with VTE. Moreover, subjects bearing the combination vWA-18/TPOX-12 loci exhibited doubled risk for VTE (95% CI = 1.02–3.64), whereas the combination vWA-18/TPOX-9 showed an OR = 10 (95% CI = 4.93–21.49). Conclusions. The vWA and TPOX microsatellites are good candidate biomarkers in venous thromboembolism diseases and could help to elucidate their origins. Additionally, these polymorphisms could become useful markers for genetic studies of VTE in the Mexican population; however, further studies should be done owing that this data only show preliminary evidence.

ABO blood group polymorphisms and risk for ischemic stroke and peripheral arterial disease

Recent studies have demonstrated association between ABO blood system and thrombosis, indicating that individuals belonging to non-O blood groups (A, B or AB) present an increased risk of venous thrombosis, heart disease, and ischemic stroke (IS) as compared to O blood group carriers. In this study, we investigated the frequency of ABO blood group polymorphisms and its association with IS and peripheral arterial disease. Significant differences were observed for O1 (OR 0.57, 95% CI 0.35-0.95, p < 0.05) and O2 (OR 3.47, 95% CI 1.15-10.28, p < 0.05) alleles among IS patients while significant differences were observed for B phenotype (26.3 vs 9.5%, OR 3.42, 95% CI 1.32-8.76, p = 0.01, patients vs controls, respectively) and alleles A1 (OR 0.31, 95% CI 0.11-0.84, p < 0.05), O2 (OR 4.61, 95% CI 1.59-13.23, p < 0.01) and B (OR 3.42, 95% CI 1.62-7.13, p < 0.001) alleles for PAD patients. O1 allele was an independent variable (OR 0.27, 95% CI 0.12-0.57, p < 0.001) for IS patients. These data suggest the relationship of non-O blood groups in pathogenesis of thrombosis events and a possible protective effect of O blood group.

Possible race and gender divergence in association of genetic variations with plasma von Willebrand factor: a study of ARIC and 1000 genome cohorts

The synthesis, secretion and clearance of von Willebrand factor (VWF) are regulated by genetic variations in coding and promoter regions of the VWF gene. We have previously identified 19 single nucleotide polymorphisms (SNPs), primarily in introns that are associated with VWF antigen levels in subjects of European descent. In this study, we conducted race by gender analyses to compare the association of VWF SNPs with VWF antigen among 10,434 healthy Americans of European (EA) or African (AA) descent from the Atherosclerosis Risk in Communities (ARIC) study. Among 75 SNPs analyzed, 13 and 10 SNPs were associated with VWF antigen levels in EA male and EA female subjects, respectively. However, only one SNP (RS1063857) was significantly associated with VWF antigen in AA females and none was in AA males. Haplotype analysis of the ARIC samples and studying racial diversities in the VWF gene from the 1000 genomes database suggest a greater degree of variations in the VWF gene in AA subjects as compared to EA subjects. Together, these data suggest potential race and gender divergence in regulating VWF expression by genetic variations.

High factor VIII, von Willebrand factor, and fibrinogen levels and risk of venous thromboembolism in blacks and whites

Venous thromboembolism (VTE) affects more than 300,000 people in the United States each year. However, it has been estimated that current diagnostic testing fails to identify prothrombotic risk in 50% of VTE patients. This article examines the relationship between levels of the pro-coagulant proteins factor VIII (FVIII), von Willebrand factor (VWF), and fibrinogen and risk of VTE in order to assess the impact of these novel risk factors. Data were collected from patients enrolled in the matched case-control Genetic Attributes and Thrombosis Epidemiology study. Crude and adjusted conditional logistic regression models were used to assess the impact of FVIII, VWF, and fibrinogen on risk of VTE. Before adjustment for independent predictors of VTE risk, high levels of FVIII, VWF, and fibrinogen were significantly associated with increased risk of VTE in both Blacks and Whites. After adjustment for ABO type, factor VII levels, hypertension, renal disease, recent surgery, diabetes, annual household income, alcohol use, and the other proteins of interest (FVIII, VWF, and/or fibrinogen), high FVIII and VWF levels were associated with increased risk of VTE in Blacks (OR: 1.97 [1.01-3.84] and 3.39 [1.58-7.27], respectively). High FVIII only was significantly associated with risk of VTE in Whites (OR: 2.35 [1.16-4.75]). Future research into the inclusion of these protein levels in risk models for VTE could help identify persons at highest risk.

On the versatility of von Willebrand factor

Von Willebrand factor (VWF) is a large multimeric protein, the function of which has been demonstrated to be pivotal to the haemostatic system. Indeed, quantitative and/or qualitative abnormalities of VWF are associated with the bleeding disorder Von Willebrand disease (VWD). Moreover, increased plasma concentrations of VWF have been linked to an increased risk for thrombotic complications. In the previous decades, many studies have contributed to our understanding of how VWF is connected to the haemostatic system, particularly with regard to structure-function relationships. Interactive sites for important ligands of VWF (such as factor VIII, collagen, glycoprotein Ibα, integrin αIIbβ3 and protease ADAMTS13) have been identified, and mutagenesis studies have confirmed the physiological relevance of the interactions between VWF and these ligands. However, we have also become aware that VWF has a more versatile character than previously thought, given its potential role in various non-hemostatic processes, like intimal thickening, tumor cell apoptosis and inflammatory processes. In the presence review, a summary of our knowledge on VWF structure-function relationships is provided in the context of the “classical” haemostatic task of VWF and in perspective of pathological processes beyond haemostasis.

Characterizing polymorphisms and allelic diversity of von Willebrand factor gene in the 1000 Genomes

BACKGROUND

The von Willebrand factor (VWF) gene is highly polymorphic, with variants correlated with VWF antigen levels, adhesion activity, clearance and factor VIII binding. VWF mutations are detected in patients with von Willebrand disease (VWD), whereas polymorphic variants could be associated with thrombosis. However, information on the ethnic diversity of VWF variants and their association with diseases is limited.

OBJECTIVES

To characterize novel VWF variants from different ethnicities in the general population.

PATIENTS/METHODS

We analyzed samples from 1092 subjects of 14 ethnicities available in the 1000 Genomes database for VWF variants and their potential functional impacts.

RESULTS

We identified 2728 SNPs and 91 insertions and deletions that had a high level of ethnic diversity, with Africans having the highest number of variants. The highest level of diversity was found in the D' and D2 domains. Among 94 non-synonymous variants, 31 were predicted to be deleterious, including 19 that were previously associated with VWD. Most of these 'VWD variants' had allele frequencies consistent with disease incidence in European subjects, but some had a significantly higher frequency in other ethnicities. The mutations R2185Q, H817Q and M740I associated with type 1 and type 2N VWD were present in more than 13% of African subjects.

CONCLUSIONS

These results highlight the complexity of VWF variations in different ethnic groups and emphasize the importance of interrogating variations on multiple ethnic backgrounds for associations with bleeding and thrombosis.

Linkage analysis identifies a locus for plasma von Willebrand factor undetected by genome-wide association

The plasma glycoprotein von Willebrand factor (VWF) exhibits fivefold antigen level variation across the normal human population determined by both genetic and environmental factors. Low levels of VWF are associated with bleeding and elevated levels with increased risk for thrombosis, myocardial infarction, and stroke. To identify additional genetic determinants of VWF antigen levels and to minimize the impact of age and illness-related environmental factors, we performed genome-wide association analysis in two young and healthy cohorts (n = 1,152 and n = 2,310) and identified signals at ABO (P < 7.9E-139) and VWF (P < 5.5E-16), consistent with previous reports. Additionally, linkage analysis based on sibling structure within the cohorts, identified significant signals at chromosome 2q12-2p13 (LOD score 5.3) and at the ABO locus on chromosome 9q34 (LOD score 2.9) that explained 19.2% and 24.5% of the variance in VWF levels, respectively. Given its strong effect, the linkage region on chromosome 2 could harbor a potentially important determinant of bleeding and thrombosis risk. The absence of a chromosome 2 association signal in this or previous association studies suggests a causative gene harboring many genetic variants that are individually rare, but in aggregate common. These results raise the possibility that similar loci could explain a significant portion of the "missing heritability" for other complex genetic traits.

von Willebrand factor: the old, the new and the unknown

von Willebrand factor (VWF) is a protein best known from its critical role in hemostasis. Indeed, any dysfunction of VWF is associated with a severe bleeding tendency known as von Willebrand disease (VWD). Since the first description of the disease by Erich von Willebrand in 1926, remarkable progress has been made with regard to our understanding of the pathogenesis of this disease. The cloning of the gene encoding VWF has allowed numerous breakthroughs, and our knowledge of the epidemiology, genetics and molecular basis of VWD has been rapidly expanding since then. These studies have taught us that VWF is rather unique in terms of its multimeric structure and the unusual mechanisms regulating its participation in the hemostatic process. Moreover, it has become increasingly clear that VWF is a more all-round protein than originally thought, given its involvement in several pathologic processes beyond hemostasis. These include angiogenesis, cell proliferation, inflammation, and tumor cell survival. In the present article, an overview of advances concerning the various structural and functional aspects of VWF will be provided.

Elevated factor VIII levels and risk of venous thrombosis

Modern thrombophilia testing fails to identify any underlying prothrombotic tendency in a significant number of patients presenting with objectively confirmed venous thromboemboembolism (VTE). This observation has led to a search for other novel inherited or acquired human thrombophilias. Although a number of putative mechanisms have been described, the evidence behind many of these candidates remains weak. In contrast, an increasing body of work supports the hypothesis that increased plasma factor VIII (FVIII) levels may be important in this context. An association between elevated plasma FVIII levels and VTE was first described in the Leiden Thrombophilia Study (LETS). Subsequently, these conclusions have been supported by an increasing number of independent case-control studies. Cumulatively, these studies have clearly demonstrated that high FVIII levels constitute a prevalent, dose-dependent risk factor for VTE. Furthermore, more recent studies have shown that the risk of recurrent venous thrombosis is also significantly increased in patients with high FVIII levels. In this review, we present the evidence supporting the hypothesis that elevated FVIII levels constitute a clinically important thrombophilia. In addition, we examine the biological mechanisms that may underlie persistently elevated FVIII levels, and the pathways through which high FVIII may serve to increase thrombotic risk.