Blood Clot Risk and Your Genetics
What Is Venous Thromboembolism and Factor XI?
Venous thromboembolism (VTE) encompasses deep vein thrombosis (DVT), in which a blood clot forms within a deep vein — most commonly in the leg — and pulmonary embolism (PE), in which a clot dislodges and travels to the pulmonary vasculature. VTE is among the most common preventable causes of hospital-related death and carries substantial long-term morbidity from post-thrombotic syndrome and chronic thromboembolic disease.
Factor XI is a coagulation serine protease central to the intrinsic (contact activation) pathway of blood coagulation. Elevated circulating Factor XI levels are associated with increased VTE risk in epidemiological studies. Genome-wide studies examining VTE and Factor XI levels simultaneously — a pleiotropic GWAS design — can identify genetic loci that influence both phenotypes through shared molecular mechanisms, potentially revealing biology that single-trait analyses would miss.
How Genetics Influence VTE and Factor XI Risk
Genetic factors account for a substantial portion of VTE risk. Known monogenic thrombophilias — such as Factor V Leiden and prothrombin G20210A — are well-established high-risk variants. Beyond these rare high-penetrance variants, common genetic variation at multiple loci contributes to population-level differences in coagulation factor levels and thrombotic tendency. Multi-phenotype GWAS approaches that jointly model VTE events and quantitative coagulation factor levels have improved the power to detect pleiotropic loci compared to studying either phenotype in isolation.
Key Genes and Variants
Variants near F11 have been associated with circulating Factor XI coagulation activity and with VTE risk in genome-wide association studies. F11 encodes coagulation Factor XI, which is activated by Factor XII during contact activation and amplifies thrombin generation by activating Factor IX. Epidemiological data consistently show that higher Factor XI levels track with elevated VTE incidence, while Factor XI deficiency (hemophilia C) reduces thrombosis risk without substantially increasing bleeding at mucosal or surgical sites — making F11 an attractive antithrombotic target under active clinical investigation.
Variants near KNG1 have been associated with hemostatic traits in multi-phenotype GWAS, with KNG1 identified as a novel locus linking the contact activation pathway to coagulation factor levels and VTE susceptibility. KNG1 encodes high-molecular-weight kininogen (HMWK), a plasma cofactor that assembles with prekallikrein and Factor XII on negatively charged surfaces — initiating the contact activation cascade that propagates through Factor XI to drive thrombin generation and fibrin clot formation.
Variants near ABO have been consistently associated with VTE risk across multiple large genome-wide studies. The ABO locus influences circulating levels of von Willebrand factor and Factor VIII, with non-O blood groups associated with higher coagulation factor levels and modestly elevated thrombotic risk. Variants near F5 (coagulation Factor V) and FGG (fibrinogen gamma chain) are also implicated in hemostatic trait variation — FGG variants influence fibrinogen levels, a recognized contributor to thrombotic risk. Additional genes in the associated set include C4BPA (complement C4-binding protein alpha, with pleiotropic functions across immune and coagulation biology) and CREB3L1 (a transcription factor expressed in hepatic and endothelial contexts relevant to coagulation factor production).
What the Research Shows
Temprano-Sagrera et al. (2022) conducted a multi-phenotype GWAS integrating seven hemostatic traits and three cardiovascular event phenotypes across 27 phenotype combinations, applying the metaUSAT statistical framework to detect variants with joint effects across correlated phenotypes (J Thromb Haemost, 2022).1
Four novel genetic loci were identified through multi-phenotype hemostatic GWAS, including a KNG1 locus connecting the contact activation pathway to coagulation factor levels and venous thromboembolism risk. The joint multi-phenotype approach provided detection power beyond single-trait GWAS by exploiting shared genetic architecture across coagulation traits and cardiovascular outcomes (Temprano-Sagrera et al., 2022).1
Understanding Your Result
A higher genetic score for this trait reflects greater inherited susceptibility to venous thromboembolism associated with Factor XI coagulation biology. This score aggregates common variant associations from loci including F11, KNG1, ABO, FGG, F5, and related coagulation pathway genes. This represents a population-level statistical association — it reflects relative susceptibility compared to others in the reference population, not a certain individual forecast.
Many individuals with elevated scores will never develop VTE; conversely, many VTE events occur among individuals without elevated polygenic scores. Environmental and clinical factors — including prolonged immobility, surgical procedures, active malignancy, hormonal therapies, and obesity — are major determinants of VTE risk that operate alongside genetic factors. Genetic susceptibility information is most useful as context for informed decisions about risk awareness and modifiable exposures.
This genetic information is for educational and informational purposes only. Results do not constitute a clinical evaluation.
Lifestyle and Considerations
Several modifiable factors influence venous thromboembolism risk independently of genetic background. Prolonged immobility — during extended travel, bed rest, or hospitalization — is a major precipitant; leg exercises, movement breaks, and compression stockings reduce risk in high-risk settings. Obesity substantially elevates VTE risk and represents one of the most impactful modifiable contributors. Smoking cessation supports vascular health broadly across cardiovascular and thrombotic risk categories.
Hormonal therapies including oral contraceptives and hormone replacement therapy increase VTE risk, which may compound with genetic susceptibility at coagulation factor loci. Adequate hydration, regular physical activity, and maintaining a healthy body weight support vascular health. In the context of planned surgical procedures, disclosing any personal or family history of blood clots to healthcare providers allows for consideration of perioperative thromboprophylaxis approaches.
Frequently Asked Questions
What is the contact activation pathway and how does Factor XI relate to blood clots?
The contact activation (intrinsic) coagulation pathway is initiated when Factor XII binds negatively charged surfaces, forming a complex with high-molecular-weight kininogen (encoded by KNG1) and prekallikrein. This leads to Factor XII autoactivation and subsequent activation of Factor XI, which drives the cascade toward thrombin generation and fibrin clot formation. Variants near F11 that raise circulating Factor XI activity accelerate this intrinsic pathway, increasing thrombotic tendency. Because Factor XI inhibition reduces thrombosis with relatively low bleeding risk, F11 has emerged as a promising therapeutic target in anticoagulant drug development.
How is this trait different from other genetic blood clot risk assessments?
Different VTE-associated GWAS traits capture different aspects of coagulation genetics. This trait examines the pleiotropic overlap between VTE risk and Factor XI levels specifically, highlighting the intrinsic pathway (F11, KNG1) alongside established loci like ABO and FGG. Other hemostatic GWAS may focus on Factor VII (extrinsic pathway), fibrinogen, or VTE events without the Factor XI component. The multi-phenotype design used to derive these associations can identify loci that single-trait analyses miss by exploiting correlated genetic architecture across related coagulation phenotypes.
Does ABO blood type affect blood clot risk?
ABO blood group is one of the most replicated genetic associations with VTE risk. Individuals with non-O blood groups (A, B, or AB) have higher levels of von Willebrand factor and Factor VIII, two coagulation proteins that influence thrombotic tendency. Non-O blood group is associated with approximately two-fold higher VTE risk compared to blood group O in meta-analyses, making ABO among the most consistent common variant contributors to coagulation genetics. This association appears across multiple large hemostatic trait GWAS in diverse ancestry groups.
What does a higher genetic score for this trait mean in practice?
A higher score reflects greater aggregated inherited susceptibility from common genetic variation at coagulation pathway loci. This is a relative, population-level signal — it identifies individuals at statistically higher average risk in a reference population, not those who will certainly develop VTE. Genetic susceptibility at these loci is most relevant when considered alongside personal risk factors such as immobility, surgery, BMI, hormonal therapy use, and family history of thrombosis. Individuals with concerns about clotting risk should discuss them with a healthcare provider who can evaluate the full clinical picture.
Can Factor XI levels be measured and is it clinically relevant?
Factor XI activity can be measured through standard coagulation laboratory testing, expressed as a percentage of normal. Factor XI deficiency (hemophilia C) is a bleeding disorder most common in Ashkenazi Jewish populations, caused by rare pathogenic F11 variants distinct from the common variants captured in this GWAS trait. The common variants identified in multi-phenotype hemostatic GWAS have modest effect sizes on Factor XI levels and VTE risk at the population level. Routine Factor XI testing is not standard in general VTE risk evaluation, but may be considered in specific clinical contexts such as evaluation for recurrent thrombosis or unexplained bleeding history.
References
- Temprano-Sagrera G, Sitlani CM, Bone WP, et al. Multi-phenotype analyses of hemostatic traits with cardiovascular events reveal novel genetic associations. J Thromb Haemost. 2022;20(6):1331-1349. (PMID 35285134)