rs6063 — FGG Gly191Arg

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.