Molecular mechanisms & clinical consequences of prothrombin mutations A.J. Hauer 07-12-2018
Prothrombin & the coagulation cascade Coagulation factor II, thrombin. Prothrombin is synthesized in the liver as inactive zymogen. Activation to thrombin by factor Xa (cofactor Va & calcium). Thrombin converts fibrinogen to fibrin à clot formation. Thrombin also inhibits coagulation by activating protein C.
The discovery of 20210G>A, and its (genetic) epidemiology (1996) Sequencing (PCR) the prothrombin (F2) gene of 28 probands from families with unexplained familial thrombophilia: 5/28 vs. 1/100 of healthy controls showed sequence variation (20210G>A) in the 3 untranslated region (3'-UTR) of the cdna just before the poly(a) tail. Subsequent confirmation in a large population-based case-control study: 20210G>A was found in 1 2% of healthy controls, 6.2% of consecutive patients with a first deep venous thrombosis (DVT), and in 18% of patients that had been selected for unexplained familial thrombophilia. Association of 20210G>A with an increased risk of a first DVT (OR: 2.8; 95% CI, 1.4 5.6). 2 nd most common prothrombotic mutation (after Factor V Leiden). Overall prevalence 20210G>A: 2.0%. Southern > northern Europe (3.0% vs. 1.7%). Very rare in non-caucasians. Analyses of linkage disequilibria à founder mutation approximately 24.000 years ago. The Hardy Weinberg equilibrium estimates homozygosity for G20210A in 4 in 10.000 individuals.
Genetic & molecular mechanisms Prothromin (F2) gene: spanning 21kb, containing 14 exons, located on 11p11.2. G20210G/A à rs1799963 (SNP ID) The G20210A polymorphism in the 3'-UTR of the F2 gene à efficiency of 3 end processing à concomitant mrna accumulation and protein synthesis à thrombophilic gain-of-function mutations. 20210G>A à 30% higher plasma concentrations of functionally normal prothrombin à hypercoagulability? (Virchow's triad) Elevated prothrombin levels may inhibit factor Va inactivation by activated protein C (APC) à thus inducing APC resistance. Additional mutational vulnerability within the 3 flanking sequence? à e.g. the C20209T polymorphism.
Clinical consequences venous thrombosis Most G20210A carriers remain asymptomatic for their lifetime! Incidence of VTE in heterozygotes: 0.05 0.42/100 person-years (2- to 7-fold risk of VTE). G20210A & Factor V Leiden: 20-fold risk of VTE compared to individuals with neither mutation. Interactions with other genetic & acquired risk factors à important determinants of the overall risk of VTE, e.g. use of oral contraceptives à up to 16-fold risk of VTE. ClinVar : à G20210A pathogenic for venous thromboembolism (VTE) Meta-analysis of 15 studies (776 cases; 2.636 controls): 20210G>A à risk of cerebral sinus thrombosis (OR: 6.05; 95% CI, 4.12 8.90, I2 = 5%; p = 0.39).
Clinical consequences arterial thrombosis? 20210G>A à no substantial risk of arterial thromboembolism (ATE): à associations with coronary artery disease (CAD) and ischaemic stroke (IS) only modest (much weaker than with VTE) or absent. Low cumulative incidence of ATE in asymptomatic 20210A carriers, and in those with prior VTE. Synergistic interaction with traditional CV risk factors? à Smoking?! (by itself prothrombotic) Genome-wide association studies (GWAS) of CAD and IS (subtypes): rs1799963 not significant (thus far). Lack of power? à limited sample size, effect size, not tagged/limited LD, low minor allele frequency. Cross-phenotyping of GWAS à possible association of rs1799963 with peripheral artery disease (of note: lead SNP à mostly not causal).
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