Effect of single nucleotide polymorphisms on expression of the gene encoding thrombin-activatable fibrinolysis inhibitor: a functional analysis

Abstract

Thrombin-activable fibrinolysis inhibitor (TAFI) is a plasma zymogen that acts as a molecular link between coagulation and fibrinolysis. Numerous single nucleotide polymorphisms (SNPs) have been identified in CPB2, the gene encoding TAFI, and are located in the 5-flanking region, in the coding sequences, and in the 3-untranslated region (UTR) of the CPB2 mRNA transcript. Associations between CPB2 SNPs and variation in plasma TAFI antigen concentrations have been described, but the identity of SNPs that are causally linked to this variation is not known. In the current study, we investigated the effect of the SNPs in the 5-flanking region on CPB2 promoter activity and SNPs in the 3-UTR on CPB2 mRNA stability. Whereas the 5-flanking region SNPs (with 2 exceptions) did not have a significant effect on promoter activity, either alone or in haplotypic combinations seen in the human population, all of the 3-UTR SNPs substantially affected mRNA stability. We speculate that these SNPs, in part, contribute to variation in plasma TAFI concentrations via modulation of CPB2 gene expression through an effect on mRNA stability. IntroductionThrombin-activable fibrinolysis inhibitor (TAFI), also known as procarboxypeptidase U or R or plasma procarboxypeptidase B, is a human plasma zymogen that may play a role in mediating the balance between blood coagulation and fibrinolysis. 1 Upon cleavage of TAFI by thrombin, 2 thrombin-thrombomodulin, 3 or plasmin, 4 an enzyme is formed (TAFIa) that possesses basic carboxypeptidase activity. TAFIa has been demonstrated to attenuate plasminogen activation, and thus fibinolysis, by removing from partially degraded fibrin the carboxyl-terminal lysine residues that mediate positive feedback in the fibrinolytic cascade. 5 In addition, TAFIa has been shown to remove the carboxyl-terminal arginine residues from bradykinin and the anaphylatoxins C3a and C5a, thereby implicating the TAFI pathway as a link between coagulation and inflammation. [6][7][8][9] Plasma concentrations of TAFI vary significantly in the human population. 10,11 The vast majority of individuals have TAFI antigen levels between 50% and 150% of the mean population value, 12,13 thereby ranging from approximately 100 to 200 nM. Importantly, these concentrations of TAFI fall below the K m for activation of TAFI by thrombin or thrombomodulin (1 M), 3 indicating that individuals with higher plasma TAFI concentrations would exhibit a higher rate of TAFIa production following a procoagulant stimulus. Indeed, variation in "functional" TAFI concentrations has been observed using a clot lysis assay of plasma samples. 14 Therefore, it is reasonable to consider the gene encoding TAFI (CPB2) as a candidate gene for thrombotic disorders. Indeed, elevated concentrations of TAFI have been shown to be a mild risk factor for first venous thrombosis 15 as well as recurrent venous thrombosis, 16 and to be more common in carriers of Factor V Leiden with venous thromboembolism than in asymptomatic carriers. 17 High functional TAFI concentrations hav...

Low thrombin activatable fibrinolysis inhibitor activity levels are associated with an increased risk of a first myocardial infarction in men

“…24 Similarly, the -438G/A might itself not be functional. 37 However, the linkage between SNP in the TAFI gene is so strong that the results of the SNP measured in our study are reliable indicators of the effects of the putative functional SNP.…”

Structural effects of linkage disequilibrium on the transcriptome

“…In particular, the 59 and 39 UTRs of genes are central components of the regulatory machinery (Lai et al 1998;Pesole et al 1999;Boffa et al 2008;Halvorsen et al 2010;Kilty et al 2010). In bacteria, Riboswitches in 59 UTRs will bind small molecules (often metabolite precursors) that alter their conformation to regulate protein expression levels (Tucker and Breaker 2005;Edwards and Ferre-D'Amare 2006;Weinberg et al 2007;Stoddard et al 2008;Wang et al 2008a).…”

A 3′-untranslated region polymorphism in the TBX21 gene encoding T-bet is a risk factor for genital herpes simplex virus type 2 infection in humans

“…Although it is premature to imply any function without proper experimental proof, one may speculate that the location of this SNP in the non-translated 39UTR could affect the stability of the TBX21 mRNA and subsequently influence downstream activity related to T-bet function. Such effects on mRNA stability by single-nucleotide changes in the 39UTR were recently demonstrated for several other genes, including thymidylate synthase (Pullmann et al, 2006), RET (Griseri et al, 2007), CD24 (Wang et al, 2007), NCALD (Kamiyama et al, 2007) and CPB2 (Boffa et al, 2008), and such variations are considered to be one of the possible mechanisms whereby non-coding sequences may affect downstream events. In the case of the rs17244587 SNP, it is therefore tempting to speculate that an A allele in this position reduces the TBX21 mRNA stability and thus the translation of T-bet, leading to reduced IFN-c-driven cell-mediated immune responses and thereby an increased susceptibility to HSV-2 infection.…”