Epistatic Interaction between Fibrinogen alpha and gamma Genes and Factor V Leiden in Children with Venous Thrombosis: Results from a Family-Based Association Study.

Background: Fibrinogen, the precursor of fibrin, is an essential component of the hemostatic system. A previous large case-control study showed that genetic variation in the fibrinogen gamma gene (FGG) increased the risk for VT in adults. Here we investigated the association of haplotypes comprising the fibrinogen alpha (FGA) and gamma (FGG) genes, carriership of the Factor V Leiden mutation and risk for VT in a large family-based study sample for pediatric VT.

Methods: We genotyped 188 pediatric VT families for seven single nucleotide polymorphisms (SNPs) (rs6050, rs2070016, rs2070014 and rs2070011, rs1049636, rs2066861, rs2066860) as well as the G1691A Factor V Leiden (FV) polymorphism. Association was assessed using the Transmission Disequilibrium Test (TDT) and corrected for multiple testing using permutation testing as implemented in HAPLOVIEW. Interaction between FV and FGA and FGG, respectively, was assessed by TDT in families stratified for presence or absence of the FV mutation in the affected child.

Results: rs6050, rs2070016, rs2070014 and rs2070011 located in the FGA gene are in tight linkage disequilibrium (LD) and define 5 common haplotypes (HT) and are linked with the neighboring FGG gene (q= 0.91). rs1049636, rs2066861, rs2066860 located in FGG are in tight LD and define 4 HTs. HTs in both, FGA and FGG are significantly overtransmitted from parents to affected offspring (FGA: HT1 (AACT), HT frequency 0.32, T:U 62: 32, p=0.0025; FGG: HT2 (ATC), HT frequency 0.32, T:U 60:32, p=0.0035). When stratifying for FV status, it became apparent that the association between FGA and FGG and VT was more pronounced in FV-negative families (FGA, HT1, T:U 55:24, p=0.0006; FGG, HT2, T:U 55:24, p=0.0005), while absent in FV-positive families.

Conclusion: Our results indicate that genetic variation in FGA and FGG are risk factors for VT in children, and further that an epistatic interaction between FGA/FGG and FV Leiden influences the risk of FGG and FGA on pediatric VT. Our study highlights the complex nature of VT and the necessity to evaluate gene-gene interactions in association studies of complex, polygenic diseases.