Genetic Variants of Hemostasis: Clinical Significance and Limitations

Hemostasis includes coagulation, platelet function, natural anticoagulant mechanisms, and fibrinolysis. Variants in these systems have different implications: some are established factors in hereditary venous thrombophilia, while others are common research markers. A positive result does not mean thrombosis will inevitably occur.

The best-established hereditary thrombophilias include factor V Leiden, prothrombin G20210A, and antithrombin, protein C, and protein S deficiencies. Testing is selected when it may change management. Screening every variant listed below before surgery, pregnancy, or contraceptive use is not recommended.

FGB: G(-455)A

FGB is located at 4q31.3 and encodes the fibrinogen beta chain. Thrombin converts fibrinogen to fibrin, and activated factor XIII stabilizes the fibrin network through covalent bonds.

G(-455)A is in the regulatory region; genotypes are GG, GA, and AA. Studies have reported associations with fibrinogen concentration. However, genotype does not determine the actual protein concentration or establish hypercoagulability. It is not an independently recommended marker for thrombosis, stroke, or pregnancy complications.

F2: G20210A

F2 is located at 11p11.2. It encodes prothrombin, a vitamin-K-dependent thrombin precursor synthesized predominantly in the liver. Within the prothrombinase complex, activated factor X converts prothrombin to thrombin, which cleaves fibrinogen and contributes to clot formation.

G20210A is the historical designation of a variant in the F2 3′ untranslated region; its current designation is c.*97G>A. Traditional genotypes are GG, GA, and AA. The variant is associated with increased prothrombin production and increased venous thromboembolism risk. It represents predisposition, not inevitable disease.

Interpretation considers venous thrombosis episodes, family history, other risk factors, and planned treatment. Venous risk cannot automatically be extrapolated to myocardial infarction or arterial stroke, nor should every carrier automatically receive anticoagulation.

F5: Factor V Leiden

F5 is located at 1q24.2 and encodes factor V. Activated factor Va is a cofactor for Xa in the prothrombinase complex, increasing thrombin generation. At the same time, thrombin bound to thrombomodulin activates protein C, which, with protein S, inactivates Va and VIIIa. This limits the spread of coagulation.

Factor V Leiden is historically designated G1691A, Arg506Gln; current nomenclature is c.1601G>A, p.Arg534Gln. Traditional GG, GA, and AA labels indicate absence of the variant, heterozygosity, and homozygosity. Altered factor Va is less readily inactivated by activated protein C. It is a cofactor for Xa, not an enzyme directly activating Xa.

The variant increases venous thromboembolism risk, particularly with additional factors such as estrogen-containing medication. Risk depends on copy number and clinical circumstances. Genotype alone cannot predict miscarriage, preeclampsia, arterial stroke, or myocardial infarction; pregnancy management is individualized.

F7: G10976A (Historically Arg353Gln)

F7 is located at 13q34 and encodes vitamin-K-dependent factor VII. Activated VIIa complexed with tissue factor initiates coagulation by activating factors IX and X.

G10976A is a historical designation for a common variant; genotypes are GG, GA, and AA. Studies have linked it with differences in factor VII level or activity. It is not equivalent to rare inherited factor VII deficiency and is not a reliable independent scale for protection against myocardial infarction or delivery-related bleeding risk. This genotype does not determine anticoagulant dosing.

F13A1: G103T (Val34Leu)

F13A1 is located at 6p25.1 and encodes the catalytic A subunit of factor XIII. Plasma XIII is an A2B2 complex; the B subunits have carrier and stabilizing roles. Following thrombin activation and in the presence of calcium, XIIIa cross-links fibrin, increasing clot resistance to mechanical stress and fibrinolysis.

Traditional Val34Leu genotypes are GG, GT, and TT. This common variant affects XIII activation and fibrin properties; it cannot simply be described as reduced enzyme activity. Its effect depends on conditions, including fibrinogen concentration.

Val34Leu is not equivalent to severe XIII deficiency. Its independent use to diagnose a bleeding disorder, predict pregnancy loss, or change anticoagulation is not established.

SERPINE1 (PAI-1): -675 4G/5G

The gene is located at 7q22.1. Its product PAI-1 inhibits plasminogen activators, regulating plasmin formation and fibrin breakdown. The promoter variant has 5G/5G, 4G/5G, and 4G/4G genotypes; studies have associated it with differences in PAI-1 expression.

This common variant is not an established basis for clinical thrombophilia diagnosis. Finding it does not prove the cause of infertility, pregnancy loss, or prostate cancer and is not an indication for anticoagulation.

ITGA2: C807T

ITGA2 is located at 5q11.2 and encodes the integrin alpha-2 subunit involved in platelet binding to collagen in an injured vessel wall. Possible genotypes are CC, CT, and TT.

C807T is a common synonymous coding variant, not simply a regulatory-region variant. Reported associations with receptor density and aggregation do not establish it as an independent clinical test for thrombosis, atherosclerosis, or pregnancy complications. Genotype does not replace platelet function assessment when indicated.

ITGB3: T1565C (Leu59Pro)

ITGB3 is located at 17q21.32 and encodes integrin beta-3. With alpha-IIb on platelets, it forms the fibrinogen receptor needed for aggregation. Traditional genotypes are TT, TC, and CC; variant names depend on the numbering system.

This variant is also associated with the HPA-1 platelet antigen system. Immune incompatibility involving platelet antigens and ordinary carriage of a variant are different clinical situations. Genotype alone cannot diagnose neonatal thrombocytopenia, post-transfusion purpura, or autism, or predict myocardial infarction. HPA investigation for a specific clinical suspicion is a separate task.

JAK2: V617F (c.1849G>T)

JAK2 encodes a tyrosine kinase involved in signaling from cytokine receptors through the JAK–STAT pathway. The acquired V617F variant sustains abnormal pathway activation and occurs in certain myeloproliferative neoplasms.

This is usually a somatic mutation in a cell clone, not a common hereditary thrombophilia. Results are described in relation to assay sensitivity and, for quantitative testing, variant allele fraction. The previously stated frequency of 16–40% in the general European population is incorrect.

Testing is used when a myeloproliferative disorder is suspected and in selected cases of unexplained thrombosis at unusual sites. It is not a routine test for miscarriage. A negative result does not exclude other molecular causes; repeat testing depends on clinical developments, rather than automatically being performed after a year.

General Limitations

A clinician interprets results according to the exact variant, method, and clinical context. Common polymorphisms should not be labeled as independent autosomal dominant or recessive diseases. A variant does not replace diagnosis of current thrombosis or determine prevention and treatment by itself.