Dept. of Molecular and Medical Genetics
Oregon Health & Science University
Fibrinogen is the zymogen precursor of fibrin, the protein that makes up blood clots. Fibrinogen is synthesized in the liver and circulates in plasma at a concentration of approximately 3 mg/mL. Fibrinogen is comprised of two sets of three polypeptide chains, which are encoded by the fibrinogen Î± (FGA), Î² (FGB) and Î³ (FGG) genes. Plasma fibrinogen concentration is an independent predictor of cardiovascular disease (CVD) including myocardial infarction (MI), coronary artery disease (CAD) and stroke. Recently, a gamma chain variant of fibrinogen, Î³â fibrinogen, was discovered as an additional independent risk factor for the development of MI and CAD. However, the mechanisms that regulate the production of this variant isoform have not been elucidated. The aim of this dissertation was to examine novel mechanisms that regulate the production of total and Î³â fibrinogen. Additionally, we sought to examine the molecular defect in a patient that presented with hemorrhage and a diagnosed dysfibrinogenemia. Interferon-Î³ (IFN-Î³) was identified as a novel regulator of total fibrinogen synthesis. We identified an IFN-Î³ activated sequence (GAS) within the promoter of FGG. We have shown that IFN-Î³ signals to activate signal transducer and activator of transcription-1 (STAT1), which dimerizes and binds to the novel GAS element within the FGG promoter to decrease transcription. We also show that IFN-Î³ is able to inhibit interleukin-6 (IL-6) induced fibrinogen synthesis by interfering with the ability of IL-6 induced STAT3 to bind to known response elements within the FGG promoter. We have also identified IL-6, IFN-Î³ and tumor necrosis factor-Î± (TNF-Î±) as molecules that differentially regulate the production of the fibrinogen Î³ chain isoforms. IL-6 acts by increasing total fibrinogen while decreasing Î³â fibrinogen production, thereby lowering the ratio of Î³â/total fibrinogen. TNF-Î± decreases Î³â fibrinogen while not affecting the levels of total fibrinogen produced, again decreasing the Î³â/total fibrinogen ratio. IFN-Î³ decreases total fibrinogen synthesis, but not Î³â synthesis, to increase the Î³â/total fibrinogen ratio. Additionally, we have discovered that the fibrin degradation product D-dimer is able to enter HepG2 cells and travel to the perinuclear space. Subsequently, we see a decrease in the amount of Î³â fibrinogen produced, suggesting that D-dimer is a negative regulator of Î³â fibrinogen synthesis. While we were not able to identify a receptor for D-dimer on the cell surface, we found ample evidence that D-dimers are taken up via endocytosis. Finally, we identified an R275C mutation in the Î³ chain of fibrinogen in a 54-year-old female. Most patients with this mutation are asymptomatic, and those that do exhibit symptoms usually present with thrombosis. Our patient was rather unique in that their presentation was hemorrhagic, and this work shows that the Î³R275C mutation is enough to sustain a hemorrhagic phenotype.
School of Medicine
Rein, Chantelle, "Genetic regulation of the expression of two fibrinogen gamma chain splice variants by fibrin degeneration products and inflammatory cytokines" (2010). Scholar Archive. 482.