Factor XI Deficiency Reveals Weak Link Between Gene Mutations and Bleeding Risk
A 93-person Italian cohort study finds FXI activity levels alone cannot predict bleeding severity, exposing complex genotype-phenotype gaps.
Summary
Researchers from four southern Italian hospitals analyzed 93 individuals with congenital Factor XI (FXI) deficiency over 15 years, identifying 24 distinct F11 gene variants including three novel mutations. Most patients were heterozygous with moderate deficiency, while only 12 had severe deficiency. Bleeding symptoms occurred in just 31% of subjects, and even among those with severe deficiency, spontaneous bleeding was uncommon. Critically, no clear genotype-phenotype correlation was found—FXI activity levels alone proved insufficient to predict bleeding risk. The study highlights the clinical and genetic heterogeneity of this rare coagulation disorder and underscores the need for individualized patient assessment incorporating age, sex, and clinical history.
Detailed Summary
Factor XI (FXI) deficiency, also called hemophilia C, is a rare autosomal recessive bleeding disorder caused by mutations in the F11 gene on chromosome 4. Unlike hemophilia A and B, its relationship between laboratory values and clinical bleeding is notoriously unpredictable, creating management challenges for clinicians. This study aimed to clarify that relationship in a real-world southern Italian cohort.
The research team recruited 93 individuals (37 men, 56 women; mean age 31.6 years) from 39 unrelated families across four reference hospitals between 2007 and 2022. FXI coagulant activity (FXI:C) was measured, and the entire F11 gene was sequenced using Sanger sequencing. Variant pathogenicity was evaluated with multiple in silico tools including MISSENSE3D, Varsome, and Franklin. The cohort included 39 index cases and 54 relatives identified through family screening.
Sanger sequencing uncovered 24 distinct F11 variants, predominantly missense mutations (17 of 24), along with three nonsense mutations, three splicing variants, and one small deletion. Three variants were entirely novel: p.Val89* (a premature stop codon), p.Leu306Pro (a structurally damaging missense), and p.Trp515Gly (which disrupts buried hydrogen bonds and expands cavity volume). Variants were distributed throughout the gene with no domain-specific clustering, though the Apple 2, Apple 4, and serine protease domains were most frequently affected. The most common mutations were p.Glu135* and p.Glu315Lys.
The majority of subjects (81/93, 87%) showed moderate FXI deficiency consistent with heterozygosity, with a mean FXI:C of 39 IU/dL. The 14 homozygous or compound heterozygous individuals had substantially lower mean levels (10.6 IU/dL). Despite this, only 31% of all subjects experienced bleeding symptoms. Among index cases with severe deficiency, only three of nine bled clinically. Bleeding was most often triggered by surgery, trauma, or—in women—menorrhagia, rather than occurring spontaneously. No statistically significant genotype-phenotype correlation was detected, and bleeding tendency was distributed evenly across subjects carrying variants in different protein domains.
These findings reinforce that FXI:C levels, while informative, are insufficient as standalone predictors of hemorrhagic risk. Additional modifiers—including patient age, sex (notably, women face menorrhagia risk), concurrent hemostatic stressors, and possibly other genetic modifiers—must be factored into clinical decision-making. The identification of three novel pathogenic variants expands the known mutational spectrum and contributes to the broader effort to populate variant databases for this undercharacterized disorder.
Key Findings
- 24 distinct F11 variants identified; three were novel (p.Val89*, p.Leu306Pro, p.Trp515Gly) with predicted structural damage.
- Only 31% of FXI-deficient subjects experienced bleeding, even among those with severe deficiency.
- Heterozygotes had ~4-fold higher FXI:C levels than homozygotes/compound heterozygotes (39 vs. 10.6 IU/dL).
- No genotype-phenotype correlation detected; variant location across protein domains did not predict bleeding tendency.
- FXI activity levels alone are insufficient to predict individual bleeding risk in clinical practice.
Methodology
Retrospective cohort study of 93 individuals from 39 families across four southern Italian hospitals (2007–2022). FXI coagulant activity was measured in plasma, and all F11 coding regions were sequenced by Sanger sequencing. Novel variant pathogenicity was assessed using multiple in silico tools (MISSENSE3D, Varsome, Franklin, gnomAD, ClinVar, EAHAD, HGMD databases).
Study Limitations
Single-country cohort from a geographically limited region limits generalizability to other ethnic populations. Functional assays to confirm the pathogenicity of novel variants in vitro were not performed. The study could not fully account for other genetic modifiers that may independently influence bleeding phenotype.
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