The activation of PTA (factor XI) and Hageman factor (factor XII).
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Blood Coagulation Factors↗
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Activation of factor XI in plasma is dependent on factor XII.
The activation of factor XI initiates the intrinsic coagulation pathway. Until recently it was believed that the main activator of factor XI is factor XIIa in conjunction with the cofactor high molecular weight kininogen on a negatively charged surface. Two recent reports have presented evidence that in a purified system factor XI is activatable by thrombin together with the soluble polyanion dextran sulfate. To assess the physiological relevance of these findings we studied the activation of factor XI in normal and factor XII-deficient plasma. We used either kaolin/cephalin or dextran sulfate as a surface for the intrinsic coagulation pathway, tissue factor to generate thrombin via the extrinsic pathway, or the addition of alpha-thrombin directly. 125I-factor XI, added to factor XI-deficient plasma at physiologic concentrations (35 nmol/L), is rapidly cleaved on incubation with kaolin. The kinetics appear to be exponential with half the maximum cleavage at 5 minutes. Similar kinetics of factor XI cleavage are seen when 40 nmol/L factor XIIa (equal to 10% of factor XII activation) is added to factor XII-deficient plasma if an activating surface is provided. Tissue factor (1:500) added to plasma did not induce cleavage of factor XI during a 90-minute incubation, although fibrin formation within 30 seconds indicated that thrombin was generated via the extrinsic pathway. Adding 1 mumol/L alpha-thrombin (equivalent to 50% prothrombin activation) directly to factor XII deficient or normal plasma (with or without kaolin/cephalin/Ca2+ or dextran sulfate) led to instantaneous fibrinogen cleavage, but again no cleavage of factor XI was observable. We conclude that in plasma surroundings factor XI is not activated by thrombin, and that proposals of thrombin initiation of the intrinsic coagulation cascade are not supportable.
Autoradiography↗
Mutation of surface residues to promote crystallization of activated factor XI as a complex with benzamidine: an essential step for the iterative structure-based design of factor XI inhibitors.
Activated factor XI (FXIa) is a key enzyme in the amplification phase of the blood-coagulation cascade. Thus, a selective FXIa inhibitor may have lesser bleeding liabilities and provide a safe alternative for antithrombosis therapy to available drugs on the market. In a previous report, the crystal structures of the catalytic domain of FXIa (rhFXI(370-607)) in complex with various ecotin mutants have been described. However, ecotin forms a matrix-like interaction with rhFXI(370-607) and is impossible to displace with small-molecule inhibitors; ecotin crystals are therefore not suitable for iterative structure-based ligand design. In addition, rhFXI(370-607) did not crystallize in the presence of small-molecule ligands. In order to obtain the crystal structure of rhFXI(370-607) with a weak small-molecule ligand, namely benzamidine, several rounds of surface-residue mutation were implemented to promote crystal formation of rhFXI(370-607). A quadruple mutant of rhFXI(370-607) (rhFXI(370-607)-S434A,T475A,C482S,K437A) readily crystallized in the presence of benzamidine. The benzamidine in the preformed crystals was easily exchanged with other FXIa small-molecule inhibitors. These crystals have facilitated the structure-based design of small-molecule FXIa inhibitors.
Benzamidines↗
The molecular genetics of factor XI deficiency.
Hereditary factor XI deficiency is characterised by a functional deficiency of factor XI and the absence of factor XI-related antigen in circulation. It occurs with a high frequency in the Ashkenazi Jewish population. Cloning of abnormal factor XI genes and studies on the molecular genetics of factor XI deficiency show that the cause for factor XI deficiency is heterogeneous. So far, two independent single base substitutions, one at the conserved intron donor consensus dinucleotide of intron N (type I) and a nonsense mutation at the codon for Glu117 (type II), have been identified. These two types of mutation together account for approximately half of the genetic changes in abnormal factor XI genes. At least one or more types of genetic change has yet to be defined. In the course of these studies, rapid methods that utilize the polymerase chain reaction and subsequent restriction endonuclease analysis have been developed.
Amino Acid Sequence↗
Factor XI deficiency.
That factor XI has a role in normal blood coagulation is evidenced by the fact that patients with deficiency are prone to excessive bleeding after haemostatic challenge. The role of factor XI in physiological processes has become clearer since the discovery that it is activated by thrombin; this fact has contributed to a revised model of blood coagulation. Factor XI deficiency is particularly common in Ashkenazi Jews. Bleeding is typically provoked by surgery in areas of increased fibrinolysis, and is not restricted to individuals with severe deficiency. The bleeding tendency is variable and the reasons for this are not fully understood, although in severe deficiency there is some correlation between phenotype and genotype. The factor XI gene is 23 kb long, and two mutations are responsible for most factor XI deficiency in the Ashkenazi population. A total of 13 mutations have thus far been published. Factor XI deficient patients may need specific therapy to cover surgery and dental extractions. Although a factor XI concentrate is available there have been recent reports of coagulation activation and thrombosis indicating that it should be used cautiously. Fresh frozen plasma may be an acceptable alternative in some situations.
Amino Acid Sequence↗
Plasma replacement therapy during labor is not mandatory for women with severe factor XI deficiency.
Severe factor XI deficiency is an injury-related bleeding disorder. The risk of excessive post-partum hemorrhage in affected women has so far been evaluated in a relatively small number of patients and it is uncertain whether prophylactic treatment with fresh frozen plasma or factor XI concentrate is needed during or after vaginal or cesarean delivery. We retrospectively analyzed bleeding manifestations related to vaginal and/or cesarean deliveries in a cohort of 62 women with factor XI activity < 17 U/dl and evaluated whether replacement therapy is essential. Fifty-one women had 139 vaginal deliveries, six women had 13 cesarean deliveries, and five women had seven vaginal as well as five cesarean deliveries. Forty-three of the 62 women (69.4%) never experienced post-partum hemorrhage during 93 deliveries (85 vaginal, eight cesarean). Hemorrhage occurred in 19 women, which in six women accompanied each one of their 17 vaginal deliveries. Post-partum hemorrhage had no relationship with the abnormal genotype that caused factor XI deficiency nor with factor XI level. These observations suggest that the use of fresh frozen plasma or factor XI concentrate during and/or after vaginal delivery is not mandatory in women with severe factor XI deficiency and can be reserved for patients who develop excessive hemorrhage. For women requiring cesarean section it appears that the same policy can be advocated but more observations are needed.
Blood Component Transfusion↗
Factor XI deficiency.
Factor XI (FXI) deficiency leads to an injury-related bleeding diathesis, which is notable for the variability in the bleeding tendency and the lack of a clear relationship between bleeding and FXI coagulant activity. Bleeding in this disorder occurs especially in areas of high fibrinolytic activity. Although a rare disorder, the frequency of FXI deficiency is high in certain populations, notably persons of Ashkenazi descent and the Basque population of Southern France. In these populations, five mutations of the FXI gene have been identified and a founder effect has been confirmed for three of these. This paper reviews the role of FXI in coagulation and documents factors known to modify the bleeding tendency. Treatment of surgical bleeding in patients with FXI deficiency is reviewed with emphasis on the combined use of recombinant activated factor VII (rFVIIa; NovoSeven(R), Novo Nordisk, Bagsvaerd, Denmark) and the antifibrinolytic agent, tranexamic acid.
Antifibrinolytic Agents↗
A common ancestral mutation (C128X) occurring in 11 non-Jewish families from the UK with factor XI deficiency.
Factor XI (FXI) deficiency is a mild bleeding disorder that is particularly common in Ashkenazi Jews, but has been reported in all populations. In Jews, two FXI gene (F11) mutations (a stop codon in exon 5, E117X, type II, and a point mutation in exon 9, F283L, type III) are particularly common, but in other populations a variety of different mutations have been described. In the Basque region of France one mutation, C38R in exon 3, was found in eight of 12 families studied, haplotype analysis suggesting a founder effect. In the course of screening 78 unrelated individuals (including 15 Jewish and 12 Asian) we have found 10 Caucasian non-Jewish patients with the mutation C128X in exon 5. Individuals were investigated because of a personal or family history of bleeding, or finding a prolonged activated partial thromboplastin time. Individuals negative for the type II and type III mutations were screened by a combination of SSCP and heteroduplex analysis. The C128X mutation was found in 10 families (one previously described). Among three individuals with severe FXI deficiency, one was homozygous for the C128X mutation, and two were compound heterozygotes for the C128X and another mutation; other individuals were carriers of the C128X mutation. This is a nonsense mutation producing a truncated protein; individuals have FXI antigen levels concordant with FXI coagulant activity. Haplotype analysis of 11 families, including a further kindred previously reported from the USA, but which originally came from the UK (in which the index patient was homozygous for C128X), suggests a founder effect.
Adolescent↗
Identification of a defective factor XI cross-reacting material in a factor XI-deficient patient.
A homozygous factor XI-deficient girl, who appeared to be positive for cross-reacting material (CRM+) was studied for clarification. Factor XI antigen (F XI:Ag) was measured by radial immunodiffusion using monospecific, heterologous anti-factor XI antibodies. Factor XI coagulant activity (F XI:C) was determined in a modified activated partial thromboplastin time (APTT) test. The ratio of F XI:C to F XI:Ag was 0.04 for the proposita, as compared with 0.7 to 0.74 in the other family members. In contrast, 12 normal individuals had ratios of F XI:C to F XI:Ag of 1.04 +/- 0.15. F XI esterolytic activity was clearly higher than F XI:C in the proband, but not in her relatives. Immunoblotting studies demonstrated F XI CRM in the patient's plasma. Chromatography on diethylaminoethanol (DEAE)-Sephadex at pH 8.4 led to an almost complete removal of F XI from the plasma. The defective F XI was not bound to a negatively charged kaolin surface due to an abnormal interaction with high-mol-wt kininogen (HMWK).
Antigens↗
New observations on factor XI deficiency.
Factor (F) XI is an injury-related bleeding tendency that commonly occurs when trauma involves tissues rich in fibronolytic activators. Severe FXI deficiency is defined when the activity of FXI in plasma is less than 15 U dL(-1). The disorder is inherited as an autosomal recessive trait manifesting in homozygotes or compound heterozygotes, and infrequently in heterozygotes. So far 53 mutations in the gene of FXI have been described and four of them were found to be prevalent in Ashkenazi Jews, Iraqi Jews, Basques or the English population. For each of the four mutations a founder effect was discerned. Inhibitors can develop in patients with FXI level < 1U dL(-1) who were exposed to plasma which seriously complicates their management during surgery. No correction of a prolonged aPTT by normal plasma is indicative of the presence of an inhibitor. In contrast to patients with haemophilia A, severe FXI deficiency provides no protection against myocardial infarction. In patients with severe FXI deficiency undergoing surgery, fresh frozen plasma is the treatment of choice. FXI concentrates can also be used but cause thrombosis in approximately 10% of patients, particularly those with cardiovascular disease. Recombinant FVIIa has successfully prevented bleeding during or after surgery in patients with FXI inhibitors.
Blood Loss, Surgical↗
Coronary artery bypass in hereditary factor XI deficiency.
Hereditary factor XI deficiency is a disorder of coagulation that has been associated with postoperative bleeding. Because cardiopulmonary bypass itself induces transient abnormalities in hemostasis, the patient with factor XI deficiency could be at increased risk for bleeding after cardiac surgical procedures. We report the successful management of a 61-year-old man with hereditary factor XI deficiency who had coronary artery bypass. Treatment with low-dose aspirin, begun 24 hours postoperatively for graft patency, was well tolerated. Once recognized and aggressively treated, factor XI deficiency does not appear to be a contraindication to potentially life-saving procedures like coronary revascularization.
Blood Transfusion↗
Use of a factor XI concentrate in three severe factor XI-deficient patients.
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Adult↗
Thrombin activates factor XI on activated platelets in the absence of factor XII.
Thrombin can activate factor XI in the presence of dextran sulfate or sulfatides. However, a physiological cofactor for thrombin activation of factor XI has not been identified. We examined this question in a cell-based, tissue factor-initiated model system. In the absence of factor XII, factor XI enhanced thrombin generation in this model. The effect on thrombin generation was reproduced by 2 to 5 pmol/L factor XIa. A specific inhibitor of factor XIIa did not diminish the effect of factor XI. Thus, factor XI can be activated in a model system that does not contain factor XIIa or nonphysiological cofactors. Preincubation of factor XI with activated platelets and thrombin or factor Xa enhanced subsequent thrombin generation in the model system. Preincubation of factor XI with thrombin or factor Xa, but without platelets, did not enhance thrombin generation, suggesting that these proteases might activate factor XI on platelet surfaces. Thrombin and factor Xa were then directly tested for their ability to activate factor XI. In the presence of dextran sulfate, thrombin or factor Xa activated factor XI. Thrombin, but not factor Xa, also cleaved detectable amounts of factor XI in the presence of activated platelets. Thus, thrombin activates enough factor XI to enhance subsequent thrombin generation in a model system. Platelet surfaces might provide the site for thrombin activation of functionally significant amounts of factor XI in vivo.
Anticoagulants↗
Preparation, characterization, and activation of a highly purified factor XI: evidence that a hitherto unrecognized plasma activity participates in the interaction of factors XI and XII.
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Arginine↗
Delayed hemorrhage after cervical conization unmasking severe factor XI deficiency.
BACKGROUND: Factor XI deficiency, a rare bleeding disorder found most commonly in patients of Ashkenazi Jewish background, may be present in patients with a history of abnormal bleeding after elective surgery. CASE: A patient of Ashkenazi Jewish descent presented 12 days after cervical conization for adenocarcinoma in situ with severe vaginal bleeding requiring multiple transfusions and uterine artery embolization. After a thorough workup, a severe factor XI deficiency was found. The patient ultimately required modified radical hysterectomy for treatment of early cervical cancer. With appropriate perioperative management, the patient underwent abdominal surgery without further bleeding complications. CONCLUSION: Factor XI deficiency can present with severe bleeding episodes after elective surgery. Adequate preoperative assessment and perioperative management are necessary to prevent bleeding complications in these patients.
Adenocarcinoma↗
Bleeding problems in factor XI deficient women.
Factor XI deficiency affects both sexes. Some bleeding problems are specific to women, and have been overlooked in the past. Women with factor XI deficiency are prone to menorrhagia and to bleeding complications after childbirth. These may occur in women with partial deficiency, as well as in those with severe deficiency. Instrumental delivery and gynaecological surgery need careful planning in such women with close collaboration between the obstetrician and haematologist.
Adult↗
Factor XI deficiency and its management.
Factor XI deficiency has a more variable bleeding tendency than haemophilia A or B. Individuals with severe deficiency have only a mild bleeding tendency, which is typically provoked by surgery, but the risk of bleeding is not restricted to individuals with severe deficiency. The bleeding tendency varies between individuals with similar factor XI levels, and sometimes the bleeding tendency of an individual may vary. The reasons for this are not fully understood, although in cases of severe deficiency there is some correlation between phenotype and genotype. Factor XI is activated by thrombin. The role of factor XI in physiological processes has become clearer since this fact was discovered, and the discovery has contributed to a revised model of blood coagulation. Factor XI deficiency occurs in all racial groups, but is particularly common in Ashkenazi Jews. The factor XI gene is 23 kilobases long. Two mutations are responsible for most factor XI deficiency in the Ashkenazi population, but a number of other mutations have now been reported in other racial groups. Individuals with factor XI deficiency may need specific therapy for surgery, accidents, and dental extractions. Several therapies are available which include fresh frozen plasma, factor XI concentrates, fibrin glue, antifibrinolytic drugs, and desmopressin. Each has advantages and risks to be considered. Factor XI concentrate may be indicated for procedures with a significant risk of bleeding especially in younger patients with severe deficiency, but its use in older patients has been associated with thrombotic phenomena. If fresh frozen plasma is to be used it is preferable to obtain one of the virally inactivated products. Fibrin glue is a useful treatment which deserves further study.
Blood Coagulation↗
Hageman-factor-dependent fibrinolysis: generation of fibrinolytic activity by the interaction of human activated factor XI and plasminogen.
Human coagulation factor XI has been purified, and upon activation with Hageman factor fragments, was found to convert the fibrinolytic proenzyme plasminogen to plasmin. This proactivator activity was shown to be functionally and antigenically distinct from prekallikrein. When the gamma-globulin fractions of plasma deficient in Hageman factor, prekallikrein and factor XI were isolated, factor-XI-deficient plasma possessed two-thirds of the plasminogen proactivator activity of the Hageman-factor-deficient plasma, while prekallikrein deficient plasma had only one-third of the plasminogen proactivator activity. Thus, the Hageman-factor-dependent plasminogen proactivator previously reported to be present in the gamma-globulin fraction of normal human plasma is a function of prekallikrein and factor XI, while the activity observed in prekallikrein-deficient plasma is attributable to factor XI. When compared utilizing digestion of iodinated fibrin, prekallikrein and factor XIa had similar potency per active site; they were, however, far less active than urokinase.
Blood Coagulation Disorders↗