Introduction: Circulating activated factor VII (FVIIa; ~ 1-2% of total FVII) needs to bind tissue factor (TF) to initiate coagulation. This is a rare event in a healthy person, with the FVIIa-TF complex almost immediately inactivated by antithrombin (AT). Therefore, the FVIIa-AT complex reflects the amount of recent TF-FVIIa formation and may be a biomarker of thrombotic risk. We hypothesized that FVIIa-AT would be associated with single nucleotide polymorphisms (SNPs) in genes involved in the activation and regulation of coagulation and with incident cardiovascular disease (CVD) events.
Methods: FVIIa-AT, FVIIa (ELISA and truncated TF-based clot rate assay, both Diagnostica Stago, Inc.), and FVII clotting activity (FVIIc) (in-house clot rate assay) were measured in citrated plasma samples from ~3,500 older adults from the Cardiovascular Health Study (CHS) during the Year 5 CHS exam. Associations of FVIIa-AT with ~50K SNPs in CVD-related genes were evaluated using the IBCv2 genotyping array. Cox proportional hazards models were used to evaluate associations of FVIIa-AT with incident coronary heart disease (CHD), ischemic stroke, and CVD event-related mortality over a median 14.5 years of follow-up.
Results: FVIIa-AT was moderately correlated with FVIIa (Pearson r2=0.48) and FVIIc (r2=0.28; both p-values <0.0001). Mean FVIIa-AT was higher in women (152 pM, standard deviation (SD)= 54 pM) than men (128 pM, SD= 49 pM) and higher in European-Americans (EAs) (142 pM, SD= 53 pM) than African-Americans (124 pM, SD= 57 pM) (both p-values <0.0001). Among EAs, 11 SNPs in the F7/F10 locus and 5 SNPs in the PROCR locus, which encodes the gene for endothelial protein C receptor (EPCR), were significantly associated with FVIIa-AT. The most significant association was rs488703 located in an intron of the F7 gene (p=3.7x10-85); each copy of the minor allele was associated with 24.6 pM lower FVIIa-AT. The most significant SNP in the PROCR locus was rs867186 (3.3x10-10) associated with 9.0 pM higher FVIIa-AT per minor allele. After adjustment for CVD-risk factors, the hazard ratio for CVD-related mortality among participants in the highest FVIIa-AT quintile was 1.24 (95% confidence interval=1.01, 1.54; p=0.04) compared with those in the lowest quintile. FVIIa-AT was not significantly associated with risk of incident CHD or ischemic stroke independently of traditional CVD risk factors.
Conclusions: These results are consistent with a role of common F7/F10 and PROCR genetic variants on the extent of FVII activation and support a role for recent TF-FVIIa formation in CVD-related mortality among older adults.