The spectrum of factor XI deficiency in Southeast China: four recurrent variants can explain most of the deficiencies.

BACKGROUND: Factor XI (FXI) deficiency is an autosomal hemorrhagic disorder characterized by reduced plasma FXI levels. Multiple ancestral variants in the F11 gene have been identified in Ashkenazi Jews and other selected European populations. However, there are few reports of predominant variants in Chinese and/or East Asian populations. The aim of this study is to characterize the genotypes and phenotypes of FXI deficiency and identify the predominant variants. RESULTS: Of the 41 FXI-deficient patients, 39 exhibited severe FXI defects, considerably more than those with partial defects. The APTT levels showed a negative correlation with FXI activity levels (coefficient=-0.584, P < .001). Only nine patients experienced mild bleeding, including one partially defective patient and eight severely defective patients. The majority of patients were referred for preoperative screenings (n = 22) and checkups (n = 14). Genetic analysis revealed that 90% of the patients had genetic defects, with 2, 16, and 19 cases of heterozygous, homozygous, and compound heterozygous patients, respectively. Seventeen variants were detected in the F11 gene (6 novel), including eleven missense variants, four nonsense variants, and two small deletions scattered throughout the F11. Of the 11 missense variants, six have not yet been studied for in vitro expression. Protein modeling analyses indicated that all of these variants disrupted local structural stability by altering side-chain orientation and hydrogen bonds. Nine variants, consisting of three missense and six null variants, were detected with a frequency of two or more. The highest allele frequency was observed in p.Q281* (21.25%), p.W246* (17.50%), p.Y369* (12.50%), and p.L442Cfs*8 (12.50%). The former two were variants specific to East Asia, while the remaining two were southeast China-specific variants. CONCLUSION: Our population-based cohort demonstrated that no correlation between the level of FXI activity and the bleeding severity in FXI deficiency. Additionally, the prevalence of FXI deficiency may have been underestimated. The nonsense p.Q281* was the most common variant in southeast China, suggesting a possible founder effect.

Hereditary coagulation factor XI deficiency: a rare or neglected disease? Results from a retrospective, single-centre cohort in northern Italy.

To examine real-life clinical data regarding hereditary factor XI (FXI) deficiency from a secondary care centre. Retrospective review of clinical records for every FXI:C 0.7 IU/ml or less reported from 2012 to 2020. Seventy-nine patients were included. Six (7.6%) had a severe deficiency (FXI:C <0.2 IU/ml). Only 55 (69.6%) patients were referred to the Haemostasis Centre. Among them, six (15%) were subsequently not identified at increased haemorrhagic risk before a surgical/obstetrical procedure. Thirty-three (41.8%) experienced at least one bleeding event, minor (25 patients) and/or major (16 patients). Minor bleedings were predominantly spontaneous and more frequent in women, major events were mainly provoked. No correlation was found between FXI:C and risk of bleeding ( P  = 0.9153). Lower FXI:C, but not a positive bleeding history, was related with higher likelihood of being referred to the Haemostasis Centre ( P  = 0.0333). Hereditary FXI deficiency prevalence is likely underestimated, real-life clinical practices outside reference centres could be suboptimal.

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.

Causative alleles for chondrodysplastic dwarfism, factor XI deficiency, and factor XIII deficiency in the Kumamoto sub-breed of Japanese Brown cattle.

Japanese Brown cattle are the second most popular Wagyu breed, and the Kumamoto sub-breed shows better daily gain and carcass weight. One of the breeding objectives for this sub-breed is to reduce genetic defects. Chondrodysplastic dwarfism and factor VIII deficiency have been identified as genetic diseases in the Kumamoto sub-breed. Previously, we detected individuals in the Kumamoto sub-breed with causative alleles of genetic diseases identified in Japanese Black cattle. In the current study, 11 mutations responsible for genetic diseases in the Wagyu breeds were analyzed to evaluate the risk of genetic diseases in the Kumamoto sub-breed. Genotyping revealed the causative mutations of chondrodysplastic dwarfism, factor XI deficiency, and factor XIII deficiency and suggested the appearance of affected animals in this sub-breed. DNA testing for these diseases is needed to prevent economic loses in beef production using the Kumamoto sub-breed.

[Analysis of a Chinese pedigree affected with Hereditary coagulation factor â ª deficiency due to variant of F11 gene].

OBJECTIVE: To explore the molecular pathogenesis of a Chinese pedigree affected with Hereditary coagulation factor Ⅺ (FⅪ) deficiency due to variants of the F11 gene. METHODS: A male proband with Hereditary coagulation factor Ⅺ deficiency who was admitted to the First Affiliated Hospital of Wenzhou Medical University due to urinary calculi on November 30, 2020 and his family members (7 individuals from 3 generations in total) were selected as the study subjects. Clinical data of the proband were collected, and relevant coagulation indices of the proband and his family members were determined. Genomic DNA of peripheral blood samples was extracted for PCR amplification. All exons, flanking sequences, and 5' and 3' untranslated regions of the F11 gene of the proband were analyzed by direct sequencing. And the corresponding sites were subjected to sequencing in other family members. The conservation of amino acid variation sites was analyzed by bioinformatic software, and the effect of the variant on the protein function was analyzed. Variants were graded based on the guidelines from the American College of Medical Genetics and Genomics (ACMG). RESULTS: The proband was a 36-year-old male. His activated partial thromboplastin time (APTT) was 89.2s, which was significantly prolonged. The FⅪ activity (FⅪ:C) and FⅪ antigen (FⅪ:Ag) were 2.0% and 3.5%, respectively, which were extremely reduced. Both the proband and his sister were found to harbor compound heterozygous variants of the F11 gene, including a c.689G>T (p.Cys230Phe) missense variant in exon 7 from their father and a c.1556G>A (p.Trp519*) nonsense variant in exon 13 from their mother. Conservation analysis indicated the Cys230 site to be highly conserved. The c.1556G>A (p.Trp519*) variant was known to be pathogenic, whilst the c.689G>T variant was classified as likely pathogenic (PM2+PM5+PP1+PP3+PP4) based on the ACMG guidelines. CONCLUSION: The c.689G>T and c.1556G>A compound heterozygous variants of the F11 gene probably underlay the pathogenesis of FⅪ deficiency in this pedigree.

A focus on dominant negative variants in a series of 170 heterozygous FXI-deficient patients.

INTRODUCTION: Dominant-negative effects have been described for 10 F11 variants in the literature. AIM: The current study aimed at identifying putative dominant-negative F11 variants. MATERIAL AND METHODS: This research consisted in a retrospective analysis of routine laboratory data. RESULTS: In a series of 170 patients with moderate/mild factor XI (FXI) deficiencies, we identified heterozygous carriers of previously reported dominant-negative variants (p.Ser243Phe, p.Cys416Tyr, and p.Gly418Val) with FXI activities inconsistent with a dominant-negative effect. Our findings also do not support a dominant-negative effect of p.Gly418Ala. We also identified a set of patients carrying heterozygous variants, among which five out of 11 are novel, with FXI activities suggesting a dominant-negative effect (p.His53Tyr, p.Cys110Gly, p.Cys140Tyr, p.Glu245Lys, p.Trp246Cys, p.Glu315Lys, p.Ile421Thr, p.Trp425Cys, p.Glu565Lys, p.Thr593Met, and p.Trp617Ter). However, for all but two of these variants, individuals with close to half normal FXI coagulant activity (FXI:C) were identified, indicating an inconstant dominant effect. CONCLUSION: Our data show that for some F11 variants recognized has having dominant-negative effects, such effects actually do not occur in many individuals. The present data suggest that for these patients, the intracellular quality control mechanisms eliminate the variant monomeric polypeptide before homodimer assembly, thereby allowing only the wild-type homodimer to assemble and resulting in half normal activities. In contrast, in patients with markedly decreased activities, some mutant polypeptides might escape this first quality control. In turn, assembly of heterodimeric molecules as well as mutant homodimers would result in activities closer to 1:4 of FXI:C normal range.

Impact of genetic structural variants in factor XI deficiency: identification, accurate characterization, and inferred mechanism by long-read sequencing.

BACKGROUND: Congenital factor XI (FXI) deficiency is a probably underestimated coagulopathy that confers antithrombotic protection. Characterization of genetic defects in F11 is mainly focused on the identification of single-nucleotide variants and small insertion/deletions because they represent up to 99% of the alterations accounting for factor deficiency, with only 3 gross gene defects of structural variants (SVs) having been described. OBJECTIVES: To identify and characterize the SVs affecting F11. METHODS: The study was performed in 93 unrelated subjects with FXI deficiency recruited in Spanish hospitals over a period of 25 years (1997-2022). F11 was analyzed by next-generation sequencing, multiplex ligand probe amplification, and long-read sequencing. RESULTS: Our study identified 30 different genetic variants. Interestingly, we found 3 SVs, all heterozygous: a complex duplication affecting exons 8 and 9, a tandem duplication of exon 14, and a large deletion affecting the whole gene. Nucleotide resolution obtained by long-read sequencing revealed Alu repetitive elements involved in all breakpoints. The large deletion was probably generated de novo in the paternal allele during gametogenesis, and despite affecting 30 additional genes, no syndromic features were described. CONCLUSION: SVs may account for a high proportion of F11 genetic defects implicated in the molecular pathology of congenital FXI deficiency. These SVs, likely caused by a nonallelic homologous recombination involving repetitive elements, are heterogeneous in both type and length and may be de novo. These data support the inclusion of methods to detect SVs in this disorder, with long-read-based methods being the most appropriate because they detect all SVs and achieve adequate nucleotide resolution.

Gene Variants in Two Families with Inherited Coagulation Factor XI Deficiency and Identification of Mutations.

INTRODUCTION: Mutations in the F11 gene can cause factor XI (FXI) deficiency, leading to abnormal coagulation activity and injury-related bleeding tendency. Therefore, identifying F11 gene mutations and studying the molecular basis will help us understand the pathogenesis of FXI deficiency. METHODS: Coagulation tests and gene sequencing analysis of all members were performed. FXI wild-type and mutant expression plasmids were constructed and transfected into HEK293FT cells. The FXI protein expression level was evaluated by ELISA and Western blot. RESULTS: The FXI activity (FXI:C) and FXI antigen (FXI:Ag) of proband-1 were decreased to 2% and 5%, respectively. FXI:C and FXI:Ag of proband-2 were reduced to 15% and 32%, respectively. Four mutations were found in the two unrelated families, including c.536C>T (p.T179M), c.1556G>A (p.W519*), c.434A>G (p.H145R), and c.1325_1325delT (p.L442Cfs*8). In vitro studies in transiently transfected HEK293FT cells demonstrated that p.T179M, p.W519*, and p.L442Cfs*8 mutations significantly lowered the FXI levels in the culture media. The FXI levels in the culture media and cell lysates of p.H145R mutation were similar to the wild type. CONCLUSION: Our results confirm that the four mutations in the F11 gene are causative in the 2 FXI deficiency families. Moreover, the p.H145R mutation is a cross-reactive material (CRM)-positive phenotype. The other three mutations are CRM-negative phenotypes and lead to FXI protein secretion disorder.

Global epidemiology of factor XI deficiency: A targeted review of the literature and foundation reports.

INTRODUCTION: Hereditary factor XI (FXI) deficiency is a rare coagulation disorder that may result in excessive bleeding requiring intervention to restore haemostasis. AIM: The aim of this review was to report the current knowledge of the worldwide incidence and prevalence of FXI deficiency. METHODS: A targeted PubMed search using terms related to FXI deficiency was conducted to identify studies published from April 2002 through April 2022. A manual search supplemented the electronic search. Studies were eligible for data abstraction if they reported population-based incidence proportions/rates or prevalence proportions for FXI deficiency. RESULTS: The electronic and manual searches returned 253 publications. After applying exclusion criteria, seven publications were included in the analysis, including a global report from the World Federation of Haemophilia (WFH). Six publications provided information on the prevalence of FXI deficiency that included 74 countries and regions. The estimated prevalence of FXI in the WFH report ranged from 0/100,000 in several countries to 55.85/100,000 individuals in the United Kingdom. Prevalence estimates in the PubMed findings ranged from .1 to 246.2/1,000,000 inhabitants with varying methods of case identification and time periods of analysis. One study estimated the incidence of FXI deficiency in Yecla, Spain at 2% of blood donors and .09% of hospital inpatients/outpatients with activated partial thromboplastin time (aPTT) tests. CONCLUSION: FXI deficiency is rare across the world, but additional steps could be taken to improve incidence and prevalence estimation, for example, development of a consistent FXI deficiency definition and incorporating genetic testing into a clinical routine to better identify and characterise cases.

Reinvestigation of unidentified causative variants in FXI-deficient patients: Focus on gene segment deletions.

INTRODUCTION: Data on failure to identify the molecular mechanism underlying FXI deficiency by Sanger analysis and the contribution of gene segment deletions are almost inexistent. AIMS AND METHODS: Prospective and retrospective analysis was conducted on FXI-deficient patients' DNA via Next Generation Sequencing (NGS), or Sanger sequencing and Multiplex Probe Ligation-dependent Assay (MLPA) to detect cryptic causative gene variants or gene segment deletions. RESULTS: Sanger analysis or NGS enabled us to identify six severe and one partial (median activity 41 IU/dl) FXI deficient index cases with deletions encompassing exons 11-15, the whole gene, or both. After Sanger sequencing, retrospective evaluation using MLPA detected seven additional deletion cases in apparently homozygous cases in non-consanguineous families, or in previously unsolved FXI-deficiency cases. Among the 504 index cases with a complete genetic investigation (Sanger/MLPA, or NGS), 23 remained unsolved (no abnormality found [n = 14] or rare intronic variants currently under investigation, [n = 9]). In the 481 solved cases (95% efficiency), we identified F11 gene-deleted patients (14 cases; 2.9%). Among these, whole gene deletion accounted for four heterozygous cases, exons 11-15 deletion for five heterozygous and three homozygous ones, while compound heterozygous deletion and isolated exon 12 deletion accounted for one case each. CONCLUSION: Given the high incidence of deletions in our population (2.9%), MLPA (or NGS with a reliable bioinformatic pipeline) should be systematically performed for unsolved FXI deficiencies or apparently homozygous cases in non-consanguineous families.

Analysis of a Chinese pedigree affected with Hereditary coagulation factor Ⅺ deficiency due to variant of F11 gene / 中华医学遗传学杂志

OBJECTIVE@#To explore the molecular pathogenesis of a Chinese pedigree affected with Hereditary coagulation factor Ⅺ (FⅪ) deficiency due to variants of the F11 gene.@*METHODS@#A male proband with Hereditary coagulation factor Ⅺ deficiency who was admitted to the First Affiliated Hospital of Wenzhou Medical University due to urinary calculi on November 30, 2020 and his family members (7 individuals from 3 generations in total) were selected as the study subjects. Clinical data of the proband were collected, and relevant coagulation indices of the proband and his family members were determined. Genomic DNA of peripheral blood samples was extracted for PCR amplification. All exons, flanking sequences, and 5' and 3' untranslated regions of the F11 gene of the proband were analyzed by direct sequencing. And the corresponding sites were subjected to sequencing in other family members. The conservation of amino acid variation sites was analyzed by bioinformatic software, and the effect of the variant on the protein function was analyzed. Variants were graded based on the guidelines from the American College of Medical Genetics and Genomics (ACMG).@*RESULTS@#The proband was a 36-year-old male. His activated partial thromboplastin time (APTT) was 89.2s, which was significantly prolonged. The FⅪ activity (FⅪ:C) and FⅪ antigen (FⅪ:Ag) were 2.0% and 3.5%, respectively, which were extremely reduced. Both the proband and his sister were found to harbor compound heterozygous variants of the F11 gene, including a c.689G>T (p.Cys230Phe) missense variant in exon 7 from their father and a c.1556G>A (p.Trp519*) nonsense variant in exon 13 from their mother. Conservation analysis indicated the Cys230 site to be highly conserved. The c.1556G>A (p.Trp519*) variant was known to be pathogenic, whilst the c.689G>T variant was classified as likely pathogenic (PM2+PM5+PP1+PP3+PP4) based on the ACMG guidelines.@*CONCLUSION@#The c.689G>T and c.1556G>A compound heterozygous variants of the F11 gene probably underlay the pathogenesis of FⅪ deficiency in this pedigree.

Molecular heterogeneity of factor XI deficiency in Tunisia.

Factor XI (FXI) deficiency is a rare inherited bleeding disorder that is highly prevalent in Ashkenazi Jewish ancestry but sporadically observed in most ethnic groups. It is heterogeneous both in clinical presentation and in genetic causality. Although a large spectrum of mutations associated with this disorder has been reported in several populations, genetic data of FXI deficiency in Tunisia are poorly described. The purpose of this study was to determine the molecular basis of FXI deficiency among Tunisian patients. Fourteen index cases from nine unrelated families with FXI deficiency, referred to Hemophilia Treatment Center of Aziza Othmana Hospital, were included in this study. The patients' F11 genes were amplified by PCR and subjected to direct DNA sequencing analysis. Sequencing analysis of F11 genes identified three distinct mutations; the Jewish type II nonsense mutation E117X, one previously reported missense mutation E602Q and one novel missense mutation V271M, which led to the disruption of the third apple domain structure of FXI. Furthermore, seven polymorphisms previously described, were also detected: C321F, c. 294A>G, -138 A>C, p.D125D, p.T249T, p.G379G, p.D551D. This report represents the first genetic study analyzing the molecular characteristics of factor XI deficiency within Tunisian population. Identification of the Jewish type II mutation in two families, as well as one missense previously reported mutation and one novel mutation confirmed the genetic heterogeneity of this disorder. Screening a large number of Tunisian factor XI deficient would reveal the spectrum mutations causing factor XI deficiency in Tunisia.

[Analysis of hereditary coagulation factor â ª deficiency in a Chinese pedigree with compound heterozygous mutations].

OBJECTIVE: To explore the molecular mechanisms of a Chinese pedigree with hereditary factor Ⅺ (FⅪ) deficiency. METHODS: All of the 15 exons, flanking sequences of the FⅪ gene and the corresponding mutation sites of family members were analyzed by the Sanger sequencing, followed by the extraction of the peripheral blood genomic DNA. And all the results were verified by the reverse sequencing. The conservation of the mutated sites was analyzed by the ClustalX-2.1-win. Three online bioinformatics software tools, including Mutation Taster, PolyPhen2 and the PROVEAN, were used to assess the possible impact of the mutations. Swiss-pdbviewer software was used to analyze the effects of mutant amino acids on protein structure. RESULTS: Genetic analysis revealed that the proband had compound heterozygous mutations including a nonsense mutation of c.1107C>A (Tyr369stop) in exon 10 and missense mutation of c.1562A>G (Tyr521Cys) in exon 13. The same c.1107C>A (Tyr369stop) was present in her father, the same c.1562A>G (Tyr521Cys) was present in both her mother and daughter. Conservation analysis indicated that Tyr521 was a highly conserved site during evolution. The prediction of pathogenicity showed that both c.1107C>A and c.1562A>G were pathogenic mutations. Protein structure prediction showed that in the wild type FⅪ protein structure, Tyr521 formed a hydrogen bond with the Lys572 and Ile388, respectively. When Tyr521 was replaced by Cys521, the original benzene ring structure disappeared, and side chains of Lys572 added a hydrogen bond with the Cys521, which may change protein catalytic domain structure. When Tyr369 was mutated to a stop codon, resulting in the truncated protein. CONCLUSION: The compound heterozygous mutations including the c.1107C>A heterozygous missense variant in exon 10 and the c.1562A>G heterozygous nonsense mutation in exon 13 may be responsible for the hereditary factor Ⅺ deficiency in this Chinese pedigree.

[Two cases of coagulation factor â ª deficiency caused by compound heterozygous mutations].

OBJECTIVE: To investigate the molecular pathogenesis of two coagulation factor Ⅺ (FⅪ) deficiency patients. METHODS: Coagulant assays: activated partial thromboplastin time (APTT), normal pooled-plasma corrected APTT test, PT, PT-INR and one-stage assay of coagulation factors activities were validated to diagnose coagulation factor Ⅺ deficiency. The patients' DNA samples were extracted and all exons and flanking sequences of F11 gene were amplified using PCR. After purified, the products of PCR were sequenced directly, the mutations were detected by comparing with wild sequences and analyzed using some bio-informatics softwares. RESULTS: The two patients were diagnosed with coagulation factor Ⅺ deficiency due to prolonged APTT, corrected APTT and low activities of coagulation factor FⅪ. The results of APTT, FⅪ: C were 88.1s, 1.1% and 107.1s, 3.8%, and the prolonged APTT could be corrected to normal range 32.9 s and 31.5 s, respectively. Through genetic analysis, we discovered compound heterozygous mutations g.1305-1G>A and g.1325delT in patient 1 and the sequencing results of TA plasmid clones showed that the two mutations were located on different strands of chromosomes. Compound heterozygous mutations g.1124A>G and g.1550C>G were detected in patient 2 resulting in Lys357Arg and Cys482Trp. Software analysis indicated the mutations probably brought amino acid sequence changed, protein features affected and splice site changed. CONCLUSION: Compound heterozygous mutations g.1305-1G>A, g.1325delT and g.1124A>G, g.1550C>G had been identified in two coagulation factor Ⅺ deficiency patients which might be responsible for their prolonged APTT and low FⅪ: C. To the best of our knowledge, g.1325delT and g.1550C>G have been reported, while g.1124A>G and g.1305-1G>A are reported for the first time in the literature.

Detection and Gene Mutation Analysis of Three Variations in Two Unrelated Chinese Hereditary Coagulation Factor XI Deficiency Families.

INTRODUCTION: Three variations including a novel F11 gene variation were detected in two unrelated Chinese families with coagulation factor XI deficiency, and their possible pathogenesis was elucidated. METHODS: The genomic DNA of the probands' pedigrees was extracted, and all exons and flanking sequences of F11 gene were subjected to PCR amplification and Sanger sequencing. ClustalX-2.1-win, Mutation Taster, and Swiss-Pdb Viewer software were used to analyze the conservation and impact of the variations on protein function and structure. RESULTS: DNA sequencing showed that the proband one had p.Gly350Glu and p.Trp501stop complex heterozygous variations, while the proband two took p.Pro338Leu and p.Trp501stop compound heterozygous variations. Conservation, structural, and functional analysis of variant amino acids indicated that these three variations were harmful and probably affected the structure and function of the variable protein. CONCLUSIONS: Three variations including p.Pro338Leu, p.Gly350Glu, and p.Trp501stop responsible for the reduction of the FXI activities were herein detected. Notably, the p.Pro338Leu variation was discovered for the first time in the world. Furthermore, the p.Gly350Glu was first reported in China.

A Common Missense Variant Causing Factor XI Deficiency and Increased Bleeding Tendency in Maine Coon Cats.

Hereditary factor XI (FXI) deficiency is characterized as an autosomal mild to moderate coagulopathy in humans and domestic animals. Coagulation testing revealed FXI deficiency in a core family of Maine Coon cats (MCCs) in the United States. Factor XI-deficient MCCs were homozygous for a guanine to adenine transition resulting in a methionine substitution for the highly conserved valine-516 in the FXI catalytic domain. Immunoblots detected FXI of normal size and quantity in plasmas of MCCs homozygous for V516M. Some FXI-deficient MCCs experienced excessive post-operative/traumatic bleeding. Screening of 263 MCCs in Europe revealed a mutant allele frequency of 0.232 (23.2%). However, V516M was not found among 100 cats of other breeds. Recombinant feline FXI-M516 (fFXI-M516) expressed ~4% of the activity of wild-type fFXI-V516 in plasma clotting assays. Furthermore, fFXIa-M516 cleaved the chromogenic substrate S-2366 with ~4.3-fold lower catalytic efficacy (kcat/Km) than fFXIa-V516, supporting a conformational alteration of the protease active site. The rate of FIX activation by fFXIa-M516 was reduced >3-fold compared with fFXIa-V516. The common missense variant FXI-V516M causes a cross-reactive material positive FXI deficiency in MCCs that is associated with mild-moderate bleeding tendencies. Given the prevalence of the variant in MCCs, genotyping is recommended prior to invasive procedures or breeding.