THROMBOSIS AND HEMOSTASIS Fibrinogen and genes and factor VLeiden in children with thromboembolism: results from 2 family-based association studies

Previous case-control studies showed that genetic variation in the fibrinogen gene (FGG) increased the risk for deep vein thrombosis (VT) in adults. We investigated the association between the fibrinogen (FGA) and FGG haplotypes, the factor VLeiden-mutation, and pediatric VT and thromboembolic stroke (TS) in 2 independent study samples. Association analysis revealed that the FGA-H1 and FGG-H2 haplotypes were significantly overtransmitted to VT patients (FGA-H1, P .05; FGG: H2, P .032). In contrast, the FGG-H3 haplotype was undertransmitted (P .022). In an independent study sample, FGA-H1 (P .008) and FGG-H2 (P .05) were significantly associated with TS. The association of FGA and FGG haplotypes with VT was more pronounced in FVLeiden-negative families (FGA-H1, P .001; FGG-H2, P .001), indicating a genetic interaction between both risk factors. The risk-conferring FGG-H2 and the protective FGG-H3 haplotypes correlated with low (FGG-H2) and high (FGG-H3) levels of the chain variant, respectively. These results provide independent and novel evidence that FGA-H1 and FGG-H2 variants are associated with an increased risk of VT and TS in children. The observed negative correlation of genetic VT risk with the plasma levels of the fibrinogen variant suggests that FGG-H2 and -H3 haplotypes modify thrombosis risk by controlling the level of this FGG splice isoform. (Blood. 2009; 114:1947-1953)