Plasma Kallikrein Contributes to Coagulation in the Absence of Factor XI by Activating Factor IX.

OBJECTIVES: FXIa (factor XIa) induces clot formation, and human congenital FXI deficiency protects against venous thromboembolism and stroke. In contrast, the role of FXI in hemostasis is rather small, especially compared with FIX deficiency. Little is known about the cause of the difference in phenotypes associated with FIX deficiency and FXI deficiency. We speculated that activation of FIX via the intrinsic coagulation is not solely dependent on FXI(a; activated FXI) and aimed at identifying an FXI-independent FIX activation pathway. Approach and Results: We observed that ellagic acid and long-chain polyphosphates activated the coagulation system in FXI-deficient plasma, as could be demonstrated by measurement of thrombin generation, FIXa-AT (antithrombin), and FXa-AT complex levels, suggesting an FXI bypass route of FIX activation. Addition of a specific PKa (plasma kallikrein) inhibitor to FXI-deficient plasma decreased thrombin generation, prolonged activated partial thromboplastin time, and diminished FIXa-AT and FXa-AT complex formation, indicating that PKa plays a role in the FXI bypass route of FIX activation. In addition, FIXa-AT complex formation was significantly increased in F11-/- mice treated with ellagic acid or long-chain polyphosphates compared with controls and this increase was significantly reduced by inhibition of PKa. CONCLUSIONS: We demonstrated that activation of FXII leads to thrombin generation via FIX activation by PKa in the absence of FXI. These findings may, in part, explain the different phenotypes associated with FIX and FXI deficiencies.

Epidural/spinal anesthesia during delivery in women with factor XI deficiency, a single center experience.

INTRODUCTION: The safety of neuro-axial anaesthesia (epidural/spinal) at labour of women with partial factor XI (FXI) deficiency is uncertain. Although FXI deficiency is frequent in Ashkenazi Jews, it is not routinely measured before labour. Our institute serves a large Ashkenazi population. We assumed that 10% of them have undiagnosed FXI deficiency. AIM: Assess the incidence, bleeding tendency and coagulation status among Jewish Ashkenazi women with FXI deficiency that underwent neuro-axial anaesthesia at delivery. METHODS: Jewish Ashkenazi women who underwent neuro-axial anaesthesia at labour completed the SSC ISTH bleeding assessment tool (BAT) and had blood drawn for coagulation tests, FXI and thrombin generation after labour. Estimation for 10 years was calculated from the 1-year sample. RESULTS: We recruited 261 women during 12 months. Among them, 39 (15%) had FXI deficiency (<70%) with median FXI levels of 63% (range: 33%-70%). Around 50% of them underwent amniocentesis in the current pregnancy and prior neuro-axial anaesthesia with no bleeding complications. BAT score and thrombin generation did not differ between women regardless of FXI status. aPTT was longer in women with partial FXI deficiency (median - 28.6 sec vs 26.3 sec, P < .001, Table 2), although within the normal range in all women. No bleeding complications after neuro-axial anaesthesia at delivery were reported in our centre in the last decade though, and according to our estimation, at least 2150 women had partial FXI deficiency. CONCLUSIONS: A significant number of Jewish Ashkenazi women with undiagnosed partial FXI deficiency undergo neuro-axial anaesthesia at labour without bleeding complications.

Postpartum haemorrhage in women with mild factor XI deficiency.

INTRODUCTION: With the advent of direct-to-consumer genetic testing, mild factor XI deficiency is increasingly recognized. There are limited data regarding the risk of postpartum haemorrhage (PPH) among women with mild FXI deficiency following obstetrical delivery. AIM: To assess the risk of PPH among women with mild FXI deficiency undergoing vaginal or caesarean delivery. METHODS: We conducted a retrospective, case-control study, in women with FXI levels between 20% and 70% of normal. For a control population, delivery outcomes were analysed in 200 women (between 2016 and 2018) without known bleeding disorders. RESULTS: There was no PPH among 45 vaginal deliveries in women with mild FXI deficiency compared with one PPH among 125 vaginal deliveries in the control cohort. The rate of PPH was significantly higher among the 26 caesarean deliveries in women with mild FXI deficiency relative to 75 control caesarean deliveries (odds ratio 2.73, 95% CI 1.02-7.26, P = .04). Prior history of haemorrhage was a strong predictor of PPH following caesarean delivery. All women who developed PPH following caesarean delivery had either a history of haemorrhage or independent risk factor for PPH. CONCLUSION: Due to the low rates of postpartum haemorrhage following vaginal delivery, routine prophylaxis to prevent postpartum haemorrhage in the setting of mild FXI deficiency does not appear warranted, especially in the absence of a bleeding history. Mild FXI deficiency is associated with an increased risk of PPH following caesarean delivery.

Perioperative anesthetic management in a patient with factor XI deficiency undergoing coronary artery bypass graft surgery.

Spontaneous bleeding is rare in patients with factor XI deficiency and significant bleeding usually occurs after a trauma or a surgical procedure. It is difficult to maintain hemostatic balance in these patients. In the present case report, a 68-year-old male patient with no chronic disease was scheduled for elective cardiopulmonary bypass surgery. Eight units of fresh-frozen plasma (FFP) were slowly infused and the operation was initiated with the activated partial thromboplastin time (aPTT) of 34.5, which was 108.7 in the preoperative period. Tranexamic acid bolus was administered before the skin incision and continued throughout the operation. Intraoperative aPTT was measured intermittently and a total of six units of FFP were administered. After 76 minutes of cross-clamp time, the patient was separated from cardiopulmonary bypass without any problem. There is no consensus regarding the management of bleeding during cardiac surgery in patients with factor XI deficiency. The common approach includes normalizing the factor levels via FFP infusion or factor concentrates in the preoperative period, proceeding with surgery following the replacement, and close monitoring of perioperative factor levels and aPTT values.

Normal aPTT in children with mild factor XI deficiency.

It has been suggested that persons with factor XI deficiency can have a normal activated partial thromboplastin time (aPTT). This notion is based on limited data, especially in children. Because of the central role the aPTT plays in diagnostic algorithms for bleeding disorders, it is important to know if a normal aPTT eliminates the need for factor XI activity testing. Our institutional database contains seven children with factor XI deficiency, of whom four have a normal aPTT. This supports the hypothesis that children with factor XI deficiency can have a normal aPTT. Clinicians may wish to consider this evidence when evaluating children with abnormal bleeding.

The Evolving Dilemma of Factor XI in Pregnancy: Suggestions for Management.

A case of a patient with severe factor XI (FXI) deficiency who presented for her seventh labor and delivery is presented. The nature of FXI deficiency, its prevalence, and issues related to genetic screening are discussed. Published literature on the topic is reviewed, including criteria that were developed to assess bleeding, laboratory tools used to estimate bleeding risk, and available treatments. Within the context of this challenging clinical dilemma, specific recommendations are provided for the antepartum, intrapartum, and postpartum stages of pregnancy. These include recommendations that take into account both FXI levels and history of any abnormal bleeding. While there are effective treatments available, it is important to consider that institutional multidisciplinary protocols are needed to manage this complex disorder. More work is needed to define the best management protocols.

Factor XI deficiency enhances the pulmonary allergic response to house dust mite in mice independent of factor XII.

Asthma is associated with activation of coagulation in the airways. The coagulation system can be initiated via the extrinsic tissue factor-dependent pathway or via the intrinsic pathway, in which the central player factor XI (FXI) can be either activated via active factor XII (FXIIa) or via thrombin. We aimed to determine the role of the intrinsic coagulation system and its possible route of activation in allergic lung inflammation induced by the clinically relevant human allergen house dust mite (HDM). Wild-type (WT), FXI knockout (KO), and FXII KO mice were subjected to repeated exposure to HDM via the airways, and inflammatory responses were compared. FXI KO mice showed increased influx of eosinophils into lung tissue, accompanied by elevated local levels of the main eosinophil chemoattractant eotaxin. Although gross lung pathology and airway mucus production did not differ between groups, FXI KO mice displayed an impaired endothelial/epithelial barrier function, as reflected by elevated levels of total protein and IgM in bronchoalveolar lavage fluid. FXI KO mice had a stronger systemic IgE response with an almost completely absent HDM-specific IgG1 response. The phenotype of FXII KO mice was, except for a higher HDM-specific IgG1 response, similar to that of WT mice. In conclusion, FXI attenuates part of the allergic response to repeated administration of HDM in the airways by a mechanism that is independent of activation via FXII.

Evaluation of the use of rotational thromboelastometry in the assessment of FXI deficency.

INTRODUCTION: The absence of a reliable clinical test to predict bleeding tendency leaves factor XI (FXI)-deficient individuals at risk of overtreatment or under treatment. AIM: To assess whether rotational thromboelastometry has value in detection of FXI deficiency and identification of bleeding tendency. METHODS: Thromboelastometry was measured in whole blood and platelet-rich plasma (PRP) samples containing corn trypsin inhibitor (CTI) from controls (n = 50) and FXI-deficient individuals (n = 93) at tissue factor (TF) 0.12 pm. The effect of tissue plasminogen activator was also assessed. For analysis, FXI-deficient individuals were divided into bleeders (n = 24) and non-bleeders (n = 44) based on experience of tonsillectomy and/or dental extraction prior to diagnosis. RESULTS: In whole blood, thromboelastometry distinguished those with major FXI deficiency (FXI:C ≤ 15 IU dL-1 ) but not partial deficiency from control populations, but did not identify bleeding phenotype. In PRP, bleeders had significantly longer clot formation time [CFT; 434 ± 179 s vs. 277 ± 70 s (mean ± SD); P < 0.05] and smaller α angle [43.8 ± 9.5° vs. 52.4 ± 5.8° (mean ± SD); P < 0.05] compared to non-bleeders. However, these parameters were found to depend on multiple additional variables and on an individual basis, ROC analysis showed test specificity for bleeding phenotype identification to be only 38.5% at 100% sensitivity: CFT (area under first derivative curve: AUC = 0.8091, P = 0.0014), α angle (AUC = 0.7804, P = 0.006). CONCLUSION: Thromboelastometry in PRP with CTI samples triggered with TF 0.12 pm was able to distinguish between bleeders and non-bleeders in FXI deficiency, but poor specificity restricts its clinical application as a test to identify bleeding phenotype. Further technical advances to the assay may allow better discrimination.

High incidence of FXI deficiency in a Spanish town caused by 11 different mutations and the first duplication of F11: Results from the Yecla study.

INTRODUCTION: Factor XI (FXI) deficiency is a rare disorder with molecular heterogeneity in Caucasians but relatively frequent and molecularly homogeneous in certain populations. AIM: To characterize FXI deficiency in a Spanish town of 60 000 inhabitants. METHODS: A total of 324 764 APTT tests were screened during 20 years. FXI was evaluated by FXI:C and by Western blot. Genetic analysis of F11 was performed by sequencing, multiplex ligation-dependent probe amplification and genotyping. RESULTS: Our study identified 46 unrelated cases and 170 relatives with FXI deficiency carrying 12 different genetic defects. p.Cys56Arg, described as founder mutation in the French-Basque population, was identified in 109 subjects from 24 unrelated families. This mutation was also identified in 2% of the general population. p.Cys416Tyr, c.1693G>A and p.Pro538Leu were identified in 7, 6 and 2 unrelated families, respectively. NGS analysis of the whole F11 gene revealed a common haplotype for each of the four recurrent mutations, suggesting a founder effect. The analysis of plasma FXI of four p.Pro538Leu homozygous carriers revealed that this variant was not activated by FXIIa. We identified four mutations previously described in other Caucasian subjects with FXI deficiency (p.Lys536Asn; p.Thr322Ile, p.Arg268Cys and c.325G>A) and four new gene defects: p.(Cys599Tyr) potentially causing a functional deficiency, p.(Ile426Thr), p.(Ile592Thr) and the first worldwide duplication of 1653 bp involving exons 8 and 9. Bleeding was rare and mild. CONCLUSIONS: Our population-cohort study supplies new evidences that FXI deficiency in Caucasians is more common than previously thought and confirmed the wide underlying genetic heterogeneity, caused by both recurrent and sporadic mutations.

Factor XI Deficiency Protects Against Atherogenesis in Apolipoprotein E/Factor XI Double Knockout Mice.

OBJECTIVE: Atherosclerosis and atherothrombosis are still major causes of mortality in the Western world, even after the widespread use of cholesterol-lowering medications. Recently, an association between local thrombin generation and atherosclerotic burden has been reported. Here, we studied the role of factor XI (FXI) deficiency in the process of atherosclerosis in mice. APPROACH AND RESULTS: Apolipoprotein E/FXI double knockout mice, created for the first time in our laboratory. There was no difference in cholesterol levels or lipoprotein profiles between apolipoprotein E knockout and double knockout mice. Nevertheless, in 24-week-old double knockout mice, the atherosclerotic lesion area in the aortic sinus was reduced by 32% (P=0.004) in comparison with apolipoprotein E knockout mice. In 42-week-old double knockout mice, FXI deficiency inhibited atherosclerosis progression significantly in the aortic sinus (25% reduction, P=0.024) and in the aortic arch (49% reduction, P=0.028), with a prominent reduction of macrophage infiltration in the atherosclerotic lesions. CONCLUSIONS: FXI deprivation was shown to slow down atherogenesis in mice. The results suggest that the development of atherosclerosis can be prevented by targeting FXI.

The Management of Factor XI Deficiency in Pregnancy.

Management of factor XI (FXI) deficiency in pregnancy is complicated by lack of correlation between FXI level and bleeding risk. Clinicians should be vigilant about the potential for prolonged or excessive bleeding following miscarriage or termination of pregnancy, or postpartum hemorrhage (PPH). A multidisciplinary approach along with an individual care plan is recommended to prevent bleeding complications. Assessment of bleeding history, FXI level, and global tests of hemostasis can aid management decisions regarding hemostatic prophylaxis. The risk of PPH can be minimized by obstetric measures to avoid uterine atony and genital trauma, in addition to provision of appropriate hemostatic prophylaxis for labor and delivery. Women with FXI deficiency can be given the option of regional anesthesia, provided that prior consideration has been given to assessment of potential bleeding risk and appropriate treatment strategies are implemented. Antifibrinolytic agents are effective for the majority of women with FXI deficiency, but those with severe deficiency/phenotype require FXI concentrate. Recombinant activated factor VII (rFVIIa) has also been used successfully to prevent bleeding in FXI deficiency. However, all treatments should be used with caution in pregnancy due to thrombogenic potential. Neonatal bleeding complications are rare in FXI deficiency; however, hemostatic assessment and cover are important for invasive procedures such as circumcision.

Recombinant factor VIIa enhances platelet deposition from flowing haemophilic blood but requires the contact pathway to promote fibrin deposition.

In prior microfluidic studies with haemophilic blood perfused over collagen, we found that a severe deficiency (<1% factor level) reduced platelet and fibrin deposition, while a moderate deficiency (1-5%) only reduced fibrin deposition. We investigated: (i) the differential effect of rFVIIa (0.04-20 nm) on platelet and fibrin deposition, and (ii) the contribution of the contact pathway to rFVIIa-induced haemophilic blood clotting. Haemophilic or healthy blood with low and high corn trypsin inhibitor (CTI, 4 or 40 µg mL(-1) ) was perfused over collagen at an initial venous wall shear rate of 100 s(-1) . At 100 s(-1) wall shear rate, where FXIIa leads to thrombin production without added tissue factor, FXI-deficient blood (3%) or severely FVIII-deficient blood (<1%) produced no fibrin at either CTI level. Whereas rFVIIa potently enhanced platelet deposition, fibrin generation was not rescued. Distinct from the high CTI condition, engagement of the contact pathway (low CTI) in moderately FVIII-deficient (3%) or moderately FIX-deficient blood (5%) resulted in enhanced platelet and fibrin deposition following 4 nm rFVIIa supplementation. In mildly FVIII-deficient blood (15%) at <24 h since haemostatic therapy, rFVIIa enhanced both platelet and fibrin generation in either CTI condition although fibrin was produced more quickly and abundantly in low CTI. For tissue factor-free conditions of severe haemophilic blood clotting, we conclude that rFVIIa reliably generates low levels of 'signaling' thrombin sufficient to enhance platelet deposition on collagen, but is insufficient to drive fibrin polymerization unless potentiated by the contact pathway.

The spectrum of factor XI deficiency in Italy.

Factor XI (FXI) deficiency is a rare inherited bleeding disorder invariably caused by mutations in the FXI gene. The disorder is rather frequent in Ashkenazi Jews, in whom around 98% of the abnormal alleles is represented by Glu117X and Phe283Leu mutations. A wide heterogeneity of causative mutations has been previously reported in a few FXI deficient patients from Italy. In this article, we enlarge the knowledge on the genetic background of FXI deficiency in Italy. Over 4 years, 22 index cases, eight with severe deficiency and 14 with partial deficiency, have been evaluated. A total of 21 different mutations in 30 disease-associated alleles were identified, 10 of which were novel. Among them, a novel Asp556Gly dysfunctional mutation was also identified. Glu117X was also detected, as previously reported from other patients in Italy, while again Phe283Leu was not identified. A total of 34 heterozygous relatives were also identified. Bleeding tendency was present in very few cases, being inconsistently related to the severity of FXI deficiency in plasma. In conclusion, at variance with other populations, no single major founder effect is present in Italian patients with FXI deficiency.

In vitro differential inhibition of the factor XI activity assay in the setting of a lupus anticoagulant.

Acquired factor XI deficiencies due to factor-specific inhibitors are rare and may be associated with lupus anticoagulant. We report a 63-year-old male with suspected postsurgical bleeding, prior surgical site infection, an isolated prolonged activated partial thromboplastin time, and a positive lupus anticoagulant. Although the factor II assay was normal, factor VIII and IX assays initially demonstrated nonparallelism with factor activity that consistently increased to normal reference ranges with serial dilutions. A discrepancy in factor XI activity results was discovered when the in-house method demonstrated undetectable activity (<3%); send-out testing using different instrument/reagent combinations revealed the presence of factor XI activity between 70% and 76%. The patient received surgical follow-up and was subsequently discharged home. Given the differential in vitro inhibition of factor XI activity on our initial in-house testing, this case highlights the importance of recognizing factor assay interference in the presence of a known lupus anticoagulant inhibitor, with strategies to mitigate potentially erroneous results.

[Factor XI deficiency: actuality and review of the literature]. / Déficit en facteur XI : actualités et revue de littérature.

Although two clusters have been identified in France, constitutional factor XI deficiency is a rare disorder. Acquired factor XI deficiency is extremely rare. The management of factor XI deficiency is not staightforward because of the unpredictable bleeding tendency that does not clearly relate to the factor XI level. Other haemostastis parameters have to be taken into account to evaluate the bleeding tendency. We report the cases of a congenital factor XI deficiency, an acquired factor XI deficiency and a von Willebrand disease associated to a factor XI deficiency. On the other hand, some interferences can lead to underestimation of factor XI and we report the case of an interference by lupus anticoagulant. The objective of this review is to better understand how to manage a reduced factor XI level.

Congenital factor XI deficiency: an update.

Severe factor XI (FXI) deficiency is an injury-related bleeding disorder, common in Ashkenazi Jews (with two mutations prevailing), but rare worldwide (with heterogeneous mutations). In the past two decades, more than 220 mutations in the FXI gene have been reported in patients with FXI deficiency, of which 7 showed a founder effect. Inhibitors to FXI were described in patients with null-allele mutations, following exposure to plasma, FXI concentrates, or anti-RhD immunoglobulin. Treatment of patients with severe FXI deficiency remains challenging because factors influencing bleeding risks are still unknown. The use of lower doses of recombinant activated factor VII in comparison with the doses commonly applied in hemophilia A or B seems promising also when assessed in vitro by thrombin generation test. Recently, FXI has been shown to have a separate role in hemostasis and in thrombosis. In animal models, targeting FXI by knocking out the gene or by using FXI-neutralizing antibodies, antisense oligonucleotides, and peptidomimetic inhibitors, prevents arterial and vein thrombosis. The homology between human and murine FXI and the significant antithrombotic effect of FXI deficiency in animal models resulted in the development of a novel approach of targeting FXI for prevention of thrombosis without impairing hemostasis in high-risk patients. The acceptance of FXI as a risk factor for thrombosis is a new concept, and patients with severe FXI deficiency might gain profit during life course.

Biological determinants of bleeding in patients with heterozygous factor XI deficiency.

Bleeding risk is not predictable in patients with factor XI (FXI; F11) deficiency. In this prospective study, our objectives were to determine the biological determinants for bleeding risk in patients with heterozygous FXI deficiency. Patients were classified as either bleeding patients or non-bleeding patients by calculating the bleeding score (BS) described for von Willebrand disease. Primary haemostasis, thrombin generation, thromboelastometry, procoagulant proteins, inhibitors, fibrinolysis, and F11 gene mutations were compared between bleeding and non-bleeding patients. Thirty-nine patients were included. BS significantly correlated with clinical assessment (P=0·001), and a score over 3 discriminated between bleeding (n=15) and non-bleeding (n=24) patients (P=0·034). Despite normal values, von Willebrand factor (VWF) and thrombomodulin (TM) plasma levels were significantly lower in bleeding patients than non-bleeding patients [ristocetin cofactor activity (VWF:RCo)=80·6±29·7 iu/dl and 101·8±29·5iu/dl respectively, P=0·043; and VWF antigen (VWF:Ag)=84·0±28·0 iu/dl and 106·3±36·1 iu/dl respectively, P=0·035; and TM=17·7±11·7ng/ml and 23·6±9·7ng/ml respectively, P=0·043]. When considering BS as a continuous variable, only VWF:RCo remained significant (P=0·042), which accounted for 11% of the variability in BS.

Structure and function of factor XI.

Factor XI (FXI) is the zymogen of an enzyme (FXIa) that contributes to hemostasis by activating factor IX. Although bleeding associated with FXI deficiency is relatively mild, there has been resurgence of interest in FXI because of studies indicating it makes contributions to thrombosis and other processes associated with dysregulated coagulation. FXI is an unusual dimeric protease, with structural features that distinguish it from vitamin K-dependent coagulation proteases. The recent availability of crystal structures for zymogen FXI and the FXIa catalytic domain have enhanced our understanding of structure-function relationships for this molecule. FXI contains 4 "apple domains" that form a disk structure with extensive interfaces at the base of the catalytic domain. The characterization of the apple disk structure, and its relationship to the catalytic domain, have provided new insight into the mechanism of FXI activation, the interaction of FXIa with the substrate factor IX, and the binding of FXI to platelets. Analyses of missense mutations associated with FXI deficiency have provided additional clues to localization of ligand-binding sites on the protein surface. Together, these data will facilitate efforts to understand the physiology and pathology of this unusual protease, and development of therapeutics to treat thrombotic disorders.