rs6063 — FGG Gly191Arg

Rare missense variant in the fibrinogen gamma chain that disrupts fibrin polymerization, producing structurally abnormal clots and a substantially elevated risk of venous thromboembolism

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FGG Gly191Arg — A Rare Fibrinogen Variant With High VTE Risk

Fibrinogen is the central protein of blood clot formation. When a vessel is damaged, thrombin cleaves fibrinogen into fibrin monomers that spontaneously polymerize into a mesh-like scaffold, which factor XIIIa then cross-links into a stable clot. The fibrinogen gamma chain (FGG)11 fibrinogen gamma chain (FGG)
encoded by the FGG gene on chromosome 4q32; the gamma chain forms the outer D-domain of fibrinogen, which mediates the polymerization contacts between fibrin monomers plays a critical structural role in this process. rs6063 introduces an arginine in place of glycine at position 191 of the gamma chain — a change with measurable consequences for how fibrin clots form.

The Mechanism

The p.Gly191Arg substitution replaces a small, electrically neutral amino acid with a large, positively charged one in a region of the gamma chain involved in fibrin monomer-to-monomer contacts. This class of structural defect can disrupt the 'B:b' polymerization knob-hole interactions that drive fibrin assembly, resulting in abnormally structured clots22 This class of structural defect can disrupt the 'B:b' polymerization knob-hole interactions that drive fibrin assembly, resulting in abnormally structured clots
In dysfibrinogenemias caused by gamma-chain missense variants, the polymerization defect typically produces clots that are denser, have thinner fibers, and are more resistant to fibrinolytic breakdown compared to normal fibrin. Structurally abnormal fibrin clots are harder for plasmin to dissolve, shifting the hemostatic balance toward thrombosis.

ClinVar classifies the rs6063 T allele as pathogenic for Fibrinogen Milano XII in a digenic context — meaning this variant can act in concert with a second fibrinogen mutation to produce a named congenital fibrinopathy. In the heterozygous state (one T allele, one C allele), the abnormal gamma chain is incorporated into some fraction of circulating fibrinogen molecules, producing a quantitative but not complete disruption of normal fibrin assembly.

The Evidence

A prospective population-based cohort study (the Malmö Thrombophilia Study) genotyped common missense variants in fibrinogen genes in 1,465 VTE patients followed for approximately 10 years and 429 healthy controls33 A prospective population-based cohort study (the Malmö Thrombophilia Study) genotyped common missense variants in fibrinogen genes in 1,465 VTE patients followed for approximately 10 years and 429 healthy controls
Memon AA, Zöller B, Svensson PJ, Sundquist J, Sundquist K. Fibrinogen genotypes and their impact on recurrence of VTE and family history. Br J Haematol. 2025;206(2):657-665.. rs6063 was significantly associated with primary VTE: OR 8.2 (95% CI 1.05–63.6) after adjustment for age and sex. The wide confidence interval reflects the rarity of the T allele (~0.5% globally) and the resulting small number of T carriers in even large cohorts — but the direction of effect is clear and statistically significant.

A 2024 multicenter analysis of 166 congenital fibrinogen disorder patients across 16 countries documented the clinical heterogeneity of dysfibrinogenemia44 A 2024 multicenter analysis of 166 congenital fibrinogen disorder patients across 16 countries documented the clinical heterogeneity of dysfibrinogenemia
Mohsenian et al. found VTE rates around 10–11% across fibrinogen disorder subtypes, with striking obstetric impact in dysfibrinogenemic women (86% spontaneous abortion rate in affected pregnancies). FGG variants account for a meaningful proportion of dysfibrinogenemia cases.

Compound fibrinogen genotypes — where an index mutation interacts with common SNPs in the same or different fibrinogen genes — can amplify thrombogenic risk55 Compound fibrinogen genotypes — where an index mutation interacts with common SNPs in the same or different fibrinogen genes — can amplify thrombogenic risk
Bor et al. 2022 demonstrated that fibrin structural measures (fiber thickness, mass-to-length ratios) explain thrombotic phenotype variation that neither the index mutation alone nor SNPs alone predict. This is consistent with the ClinVar digenic classification of Fibrinogen Milano XII: the pathogenic phenotype requires rs6063 together with a second fibrinogen-gene variant.

Evidence level is moderate: a large prospective cohort demonstrating a significant association (OR 8.2), supported by consistent mechanistic understanding of dysfibrinogenemia, but limited to a single primary study specifically addressing this rsID with a wide confidence interval due to variant rarity.

Practical Actions

T allele carriers — virtually all of whom are heterozygous CT — have elevated VTE risk and should be aware of high-risk situations: prolonged immobility (long flights, hospitalization), surgery, oral contraceptives or hormone replacement therapy, pregnancy, and dehydration. Anticoagulation decisions require clinical evaluation by a hematologist or thrombosis specialist; anticoagulation after a first unprovoked VTE is typically extended if a thrombophilic variant is confirmed.

The raw OR of 8.2 should be understood in context: the baseline lifetime VTE risk is roughly 5–8% in the general population, so elevated relative risk remains rare in absolute terms. Thrombotic risk is also modified by concurrent thrombophilic variants (Factor V Leiden, prothrombin G20210A), lifestyle factors, and body weight.

Interactions

rs6063 has been classified as pathogenic for Fibrinogen Milano XII in a digenic context — meaning a second fibrinogen-gene variant (in trans) is required for the full named phenotype. The Memon et al. 2025 study also examined rs6050 (FGA Thr312Ala) and rs2066865 (FGG 3' downstream), which may interact with rs6063 to amplify overall fibrinogen dysfunction and VTE risk.

Concurrent Factor V Leiden (rs6025) or prothrombin G20210A (rs1799963) would compound thrombotic risk substantially — the intersection of a dysfibrinogenemia variant and an established thrombophilia warrants hematology evaluation rather than routine monitoring.

Genotype Interpretations

What each possible genotype means for this variant.

CC Normal

Common genotype — standard fibrinogen gamma chain function

You carry two copies of the common C allele at rs6063. This is the reference genotype, present in approximately 99% of people globally. Your fibrinogen gamma chain has the standard glycine at position 191, and fibrin polymerization proceeds normally from this variant's perspective. Your VTE risk is not elevated by rs6063.

CT Dysfibrinogenemia Carrier High Risk

Rare risk variant — significantly elevated primary VTE risk from abnormal fibrin structure

The p.Gly191Arg substitution exchanges a small neutral glycine for a bulky positively-charged arginine at a structurally sensitive position in the fibrinogen gamma chain D-domain. Because fibrinogen is a hexamer (two copies each of Aalpha, Bbeta, and gamma chains), heterozygous carriers incorporate a mixture of normal and Gly191Arg gamma chains into circulating fibrinogen molecules. When clotting is activated, fibrin monomers with the abnormal gamma chain form contacts with reduced fidelity, producing a clot with altered fiber diameter and packing density. Denser, thinner-fibered clots are characteristically more resistant to plasmin- mediated fibrinolysis — the mechanism by which thrombosis risk is elevated.

The ClinVar classification "pathogenic for Fibrinogen Milano XII (digenic)" indicates that the full dysfibrinogenemia syndrome requires a second fibrinogen-gene variant in addition to rs6063 T. The heterozygous CT state alone is classified as uncertain significance for dysfibrinogenemia — meaning it can produce laboratory evidence of abnormal fibrinogen (discrepancy between functional and antigenic fibrinogen levels) but does not consistently cause clinical disease without cofactors. The OR of 8.2 for primary VTE, while derived from a single prospective cohort (Malmö Thrombophilia Study, PMID 39828282), suggests the heterozygous state is clinically actionable even without a co-occurring second fibrinogen mutation.

Pregnancy represents a specific high-risk period: dysfibrinogenemia variants in FGG have been associated with spontaneous abortion rates of up to 86% in severely affected women, and thrombotic complications during pregnancy and the puerperium are recognized risks in fibrinogen disorders generally.

TT Homozygous Risk High Risk

Two copies of the rare risk allele — very rare genotype with expected severe dysfibrinogenemia

You carry two copies of the rare T allele at rs6063. The T allele frequency is approximately 0.5% globally, making the homozygous TT genotype vanishingly rare (estimated frequency under 0.003%). Both copies of your fibrinogen gamma chain carry the Gly191Arg substitution, meaning virtually all circulating fibrinogen molecules contain the abnormal gamma chain. This is expected to produce severe dysfibrinogenemia — substantially impaired fibrin polymerization — with a high risk of both thrombotic complications and, paradoxically, bleeding events, as the abnormal clot structure may be mechanically fragile despite being resistant to fibrinolysis. Urgent hematology evaluation is warranted.