A cross-reactive material positive variant of coagulation factor XI (FXIP520L) with a catalytic defect

Biology of factor XI

ABSTRACT

Unique among coagulation factors, the coagulation factor XI (FXI) arose through a duplication of the gene KLKB1, which encodes plasma prekallikrein. This evolutionary origin sets FXI apart structurally because it is a homodimer with 2 identical subunits composed of 4 apple and 1 catalytic domain. Each domain exhibits unique affinities for binding partners within the coagulation cascade, regulating the conversion of FXI to a serine protease as well as the selectivity of substrates cleaved by the active form of FXI. Beyond serving as the molecular nexus for the extrinsic and contact pathways to propagate thrombin generation by way of activating FIX, the function of FXI extends to contribute to barrier function, platelet activation, inflammation, and the immune response. Herein, we critically review the current understanding of the molecular biology of FXI, touching on some functional consequences at the cell, tissue, and organ level. We conclude each section by highlighting the DNA mutations within each domain that present as FXI deficiency. Together, a narrative review of the structure-function of the domains of FXI is imperative to understand the etiology of hemophilia C as well as to identify regions of FXI to safely inhibit the pathological function of activation or activity of FXI without compromising the physiologic role of FXI.

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.

A family with factor-XI deficiency due to a compound heterozygosis between Gln 47 Pro (new mutation) in exon 3 and Leu 619 Pro in exon 15

A new factor XI mutation (Gln 47 Pro) has been found in combination with another known mutation (Leu 619 Pro) in a female patient with FXI deficiency and a moderate bleeding tendency. FXI activity and antigen in the proposita were 2% activity and less than 5% of normal, respectively. The parents are not consanguineous and are asymptomatic. The father is heterozygote for the new mutation whereas the mother is heterozygote for the known mutation. Other family members are heterozygotes for either one of the two mutations. The new mutation is not a polymorphism as it was not found in the population of the area. The geographical area, north-east of Italy, of the present family is the same area where a cluster of another new mutation (Ile 436 Lys) was recently reported. No relation was found between the present family and those with the previous mutation.

Factor XI: hemostasis, thrombosis, and antithrombosis

Coagulation factor FXI (FXI), a plasma serine protease zymogen, has important roles in both intrinsic and extrinsic coagulation pathways and bridges the initiation and amplification phases of plasmatic hemostasis. Recent studies have provided new insight into the molecular structure and functional features of FXI and have demonstrated distinct structural and biological differences between activated factor XII (FXIIa)-mediated FXI activation and tissue factor/thrombin-mediated FXI activation. The former is important in thrombosis; the latter is more essential in hemostasis. Activated partial thromboplastin tine (aPTT) artificially reflects FXIIa-initiated intrinsic coagulation pathway in vitro. Conversely, FXIIa-inhibited diluted thromboplastin time assay may reflect tissue factor/thrombin-mediated FXI activation in vivo. Further explication of the genetic mutations of FXI deficiency has improved the understanding of the structure-function relationship of FXI. Besides its procoagulant activity, the antifibrinolytic activity of FXI was well documented in a wealth of literature. Finally, the new emerging concept of inhibiting FXI as a novel antithrombotic approach with an improved benefit-risk ratio has been supported through observations from human FXI deficiency and various animal models. Large- and small-molecule FXI inhibitors have shown promising antithrombotic effects. The present review summarizes the recent advancements in the molecular physiology of FXI and the molecular pathogenesis of FXI deficiency and discusses the evidence and progress of FXI-targeting antithrombotics development.

Molecular characterization of FXI deficiency

Factor XI (FXI) deficiency is a rare autosomal bleeding disease associated with genetic defects in the FXI gene. It is a heterogeneous disorder with variable tendency in bleeding and variable causative FXI gene mutations. It is characterized as a cross-reacting material-negative (CRM-) FXI deficiency due to decreased FXI levels or cross-reacting material-positive (CRM+) FXI deficiency due to impaired FXI function. Increasing number of mutations has been reported in FXI mutation database, and most of the mutations are affecting serine protease (SP) domain of the protein. Functional characterization for the mutations helps to better understand the molecular basis of FXI deficiency. Prevalence of the disease is higher in certain populations such as Ashkenazi Jews. The purpose of this review is to give an overview of the molecular basis of congenital FXI deficiency.

Update on the physiology and pathology of factor IX activation by factor XIa

Factor IX is a key component of the plasma system that forms a fibrin clot at a site of vascular injury. Activation of factor IX by factor XIa is required in certain situations to prevent bleeding from premature clot degradation. Factor XIa is a coagulation protease comprised of two identical subunits. The biochemical and physiologic implications of this unusual structural feature are being actively investigated. Congenital factor XI deficiency causes a mild-to-moderate bleeding disorder, with hemorrhage typically involving the oral/nasal cavities and the urinary tract. Current treatment recommendations take this tissue-specific bleeding pattern into account and target factor replacement to certain types of procedures and clinical situations. Results from animal models and human population studies indicate that factor XI contributes to thromboembolic disease. This protease may therefore be a legitimate therapeutic target.