During surface-initiated blood coagulation in vitro, activated factor XII (fXIIa) converts factor XI (fXI) to fXIa. Whereas fXI deficiency is associated with a hemorrhagic disorder, factor XII deficiency is not, suggesting that fXI can be activated by other mechanisms in vivo. Thrombin activates fXI, and several studies suggest that fXI promotes coagulation independent of fXII. However, a recent study failed to find evidence for fXII-independent activation of fXI in plasma. Using plasma in which fXII is either inhibited or absent, we show that fXI contributes to plasma thrombin generation when coagulation is initiated with low concentrations of tissue factor, factor Xa, or ␣-thrombin. The results could not be accounted for by fXIa contamination of the plasma systems. Replacing fXI with recombinant fXI that activates factor IX poorly, or fXI that is activated poorly by thrombin, reduced thrombin generation. An antibody that blocks fXIa activation of factor IX reduced thrombin generation; however, an antibody that specifically interferes with fXI activation by fXIIa did not. The results support a model in which fXI is activated by thrombin or another protease generated early in coagulation, with the resulting fXIa contributing to sustained thrombin generation through activation of factor IX. (Blood. 2009;114:452-458) IntroductionThe plasmas of placental and marsupial mammals contain factor XI (fXI), 1 the zymogen of a protease (fXIa) that contributes to fibrin formation and stability through activation of factor IX (fIX). [2][3][4] Congenital fXI deficiency is associated with a variable traumainduced bleeding disorder in humans and other species. [5][6][7][8] The mechanism by which fXI is converted to fXIa during blood coagulation has been a topic of recent debate. 9,10 When blood is exposed to a surface in vitro, the process of contact activation converts factor XII (fXII) to the protease fXIIa, which then activates fXI. 3,4 Substances, such as RNA, 11 polyphosphates, 12 and collagen, 13 induce pathologic coagulation in mice in a fXIIdependent manner 13,14 and may represent physiologic surfaces for fXII activation. However, the contribution of fXIIa-mediated fXI activation to normal hemostasis is unclear, as fXII deficiency, unlike fXI deficiency, is not associated with abnormal bleeding in any species in which it has been identified. 4 This key observation supports hypotheses proposing that fXI is either activated during hemostasis by a protease other than fXIIa or that auxiliary mechanisms for fXI activation compensate in the absence of fXII. 3,[15][16][17] Candidates for fXI-activating proteases include ␣-thrombin, 15,16 meizothrombin, 18 and fXIa (autoactivation). 15,16 Thrombin has received much attention in this regard. Several laboratories have presented evidence suggesting that a protease generated early in coagulation, such as thrombin, converts fXI to fXIa. [19][20][21][22][23] This hypothesis has been challenged by a recent study that did not find evidence for fXI activation in thrombin or tissue facto...

The bleeding diathesis associated with hereditary factor XI (fXI) deficiency is prevalent in Ashkenazi Jews, in whom the disorder appears to be an autosomal recessive condition. The homodimeric structure of fXI implies that the product of a single mutant allele could confer disease in a dominant manner through formation of heterodimers with wild-type polypeptide. We studied 2 unrelated patients with fXI levels less than 20% of normal and family histories indicating dominant disease transmission. Both are heterozygous for single amino acid substitutions in the fXI catalytic domain (Gly400Val and Trp569Ser). Neither mutant is secreted by transfected fibroblasts. In cotransfection experiments with a wild-type fXI construct, constructs with mutations common in Ashkenazi Jews (Glu117Stop and Phe283Leu) and a variant with a severe defect in dimer formation (fXI-Gly350Glu) have little effect on wild-type fXI secretion. In contrast, cotransfection with fXI-Gly400Val or fXITrp569Ser reduces wild-type secretion about 50%, consistent with a dominant negative effect. Immunoprecipitation of cell lysates confirmed that fXI-Gly400Val forms intracellular dimers. The data support a model in which nonsecretable mutant fXI polypeptides trap wild-type polypeptides within cells through heterodimer formation, resulting in lower plasma fXI levels than in heterozygotes for mutations that cause autosomal recessive fXI deficiency. IntroductionCoagulation factor XI (fXI) is the zymogen of a plasma protease (fXIa) that contributes to hemostasis through activation of factor IX. 1 Hereditary fXI deficiency is an autosomal disorder characterized by trauma or surgery-induced hemorrhage and only rarely by "spontaneous" bleeding into soft tissues or joints. 2,3 The condition is prevalent in Ashkenazi Jews, in whom the heterozygote frequency may be as high as 10%. 4,5 Two mutations, Glu117Stop and Phe283Leu, account for most of the abnormal alleles in this population. 2,5 Reports on inheritance patterns for fXI deficiency have been conflicting. First described by Rosenthal and coworkers in 1953 as plasma thromboplastin antecedent (PTA) deficiency, 6 fXI deficiency was initially considered an autosomal dominant disorder with variable expressivity based on clinical symptoms and family histories. 7-9 However, Rapaport et al observed that PTA deficiency exists as major (homozygous) and minor (heterozygous) variants and is best described as following an incompletely recessive or "intermediate" mode of inheritance, with few symptoms occurring in heterozygotes. 10 These conclusions, based on measurements of plasma fXI activity primarily in Jewish kindreds, demonstrated distinct ranges for major (fXI level up to 20% of normal) and minor (fXI 30%-65% of normal) PTA deficiency. Subsequent studies supported this work 4,11 and led to the general opinion that fXI deficiency in Jewish patients is a recessive disorder. 12 Not all cases of fXI deficiency are consistent with a simple autosomal recessive model. Ragni and colleagues studied 25 fXI-deficient kindre...