Thrombosis

In the Western world, thrombotic disease as an underlying cause in cardiovascular disease, is a major cause of mortality. The clinical definition of thrombosis is that of the pathological presence of a clot (thrombus) in a blood vessel or in the heart that causes the obstruction of blood flow through the circulatory system. Depending on the location where thrombus formation takes place (i.e. in the venous or arterial part of the vessel tree), thrombotic disease can be classified into venous and arterial thrombosis. One of the most frequent forms of arterial thrombosis is in heart attacks, where one of the coronary arteries, is occluded, which leads to hypoxia of the heart tissue that is nourished by the artery that is occluded. Another well-known and commonly seen form of thrombosis is deep vein thrombosis (DVT) which is a form of venous thromboembolism, caused by one or more thrombi that occlude the normal blood flow in the lower leg (usually the calf).

Cause of death in the Netherlands for the years 1980-2004.

Source: Hart- en Vaatziekten in Nederland 2006, Nederlandse Hartstichting

 

Arterial and venous thromboses are considered as distinct disease states that are characterized by different pathogenic mechanisms and underlying risk factors. However, central to the pathogenesis of both venous and arterial thrombosis is the perturbation of the normal haemostatic balance. A commonly accepted view of homeostasis is that in healthy individuals, haemostasis is carefully balanced by several anticoagulant mechanisms that counteract the procoagulant forces and thus prevent inappropriate vascular blood clotting. In other words, under physiological conditions, homeostasis is in fact a dynamic equilibrium between the pro- and anticoagulant forces that are ongoing at a low level. Several observations are even in favor of a slight dominance of the anticoagulant forces during homeostasis. The equilibrium between pro- and anticoagulant factors can be rapidly shifted in favor of coagulation in case of a physiological need for cessation of blood loss. However, this proneness towards clot formation, necessary to prevent excessive blood loss by a swift formation of a blood clot also implies a risk to develop thrombosis.

It has been observed that thrombosis in general is not a spontaneous process, in fact, it only occurs when several predisposing (risk) factors or situations coincide. Factors that contribute to thrombosis a commonly divided into two groups: the acquired (environmental) and inherited thrombosis risk factors. Acquired risk factors for thrombosis include changes induced by various influences such as pregnancy, surgery, malignancy, immobilization, hypobaric travel, autoimmune disease, diet, smoking and the use of oral contraceptives, or by intercurrent disorders such as diabetes mellitus, dyslipidemia, hypertension and hyperhomocystinemia. Perturbation of the haemostatic balance towards coagulation may also be influenced by genetic changes, i.e. when idividuals are carriers for mutations in one of the proteins that are involved in coagulation. Most of the mutations affect the protein C anticoagulant system, such as activated protein C (APC) resistance caused by the factor VLeiden mutation, or other polymorphic forms of FV, deficiencies of protein S, protein C or antithrombin or by increased levels of procoagulant factors such as FVIII or prothrombin; all of these can be caused by variations at the genetic level. Other distinct factors are age, sex or bloodgroup type, each affecting an individuals’ cumulative thrombosis risk.

Thrombosis has for a long time been regarded as a single gene disorder. This view was conditioned largely by a seemingly obvious thrombosis etiology that was concluded from studies with small numbers of patients or families with inherited single deficiencies in the anticoagulant proteins antithrombin, protein C and protein S and the associated high prevalence or severity of thrombosis. Over the last decade however, additional risk factors for thrombosis have been described and our understanding of the underlying mechanisms that contribute to the actual development of thrombosis has increased. Currently, it has become widely accepted that thrombosis is a multifactorial disease that may occur as the result of the interplay between two or more genetic, environmental or behavioral risk factors, or combinations of these. These factors together are capable of synergistically passing a certain anticoagulant threshold, thereby tipping the natural haemostatic balance between pro- and anticoagulant forces. When the threshold is passed, the natural anticoagulant systems are insufficient to balance the procoagulant factors, resulting in the development of a thrombotic event. Since thrombin is the dominant enzyme in so many humoral and cellular processes and since for the formation of thrombin in plasma there is an absolute requirement for the presence of coagulation factor V (FV), regulation of FV activities is a major regulatory mechanism for the control of thrombin formation. FV is a protein that has been described as a Janus faced protein: it not only plays a role in procoagulant processes, FV also plays an important role in the anticoagulant protein C pathway. For a recent review of this see: PMID: 17849041 or PMID: 11950687.

Blood coagulation

Initiation of blood coagulation (also known as secondary haemostasis) in vivo is triggered by the exposure of the transmembrane glycoprotein tissue factor (TF) to blood. Exposed TF from subendothelial layers binds with high affinity and specificity to both the zymogen and activated forms of coagulation factor VII (FVII and FVIIa respectively). A small fraction of FVII circulates in blood as an active protease (FVIIa) and activates factors IX and X when it is bound to TF. Under normal conditions, when the endothelial lining of the vessel wall is intact, TF is not exposed to the blood flow, which will ensure that no initiation of coagulation will occur. Activated FIX (FIXa) subsequently interacts with its nonenzymatic cofactor FVIIIa on the activated platelet surface to form the tenase complex, which efficiently activates FX. After its formation by the tenase complex, FXa assembles on the platelet membrane with its cofactor FVa to form the prothrombinase complex. The prothrombinase complex efficiently activates prothrombin to thrombin, which feedback amplifies its own formation by activating FV, FVIII and FXI. Activation of FXI results in the generation of additional FIXa, which in turn activates FX, thus enforcing thrombin formation. Most importantly, thrombin converts soluble fibrinogen into insoluble fibrin fibers which aggregate to a soft fibrin clot.

The formation of a stable plug by platelet aggregation and fibrin polymerization following vascular injury is a very rapid and important process that prevents excessive blood loss. However, intrinsic to the capacity of the blood clotting system to respond quickly to vascular injury is the danger of excessive clotting that may contribute to thrombosis. Therefore, several anticoagulant mechanisms exist that control the coagulation pathway at different levels. Two major systems can be discriminated that provide efficient control of the activity of pro- and anticoagulant pathways. A first system comprises circulating inhibitors (e.g. TFPI, antithrombin, ?2 macroglobulin , antitrypsin) which can directly neutralize activated coagulation factors. A second important negative regulatory pathway of the coagulation cascade is the protein C (PC) pathway. In contrast to the inhibitors, where in general, enzymes are being inhibited by virtue of the formation of a complex between the enzyme and an inhibitor, the PC pathway depends on proteolysis of key cofactor proteins in the coagulation pathway.

 

The PC pathway is initiated when traces of thrombin disperse into the vasculature. A key event in the initiation of the PC pathway is the formation of the thrombin-thrombomodulin complex on the surface of endothelial cells. Thrombomodulin (TM) is a transmembrane protein present on intact endothelium, primarily that of the smaller vessels, that acts as a thrombin receptor by binding to thrombin exosite I and provides a binding platform for the substrate protein C.

The thrombin exosite I plays an important role in the thrombin-mediated recognition and activation of the procoagulant factors FV, FVIII and fibrinogen. As a consequence of the binding to TM, the procoagulant properties of thrombin molecules, that escape a site of ongoing coagulation and migrate into the microvasculature, are lost. In addition, TM-binding causes conformational changes near the active site of thrombin which alter the substrate specificity of thrombin such that thrombin becomes an anticoagulant protein that can efficiently activate PC in a process that is enhanced by the endothelial cell protein C receptor (EPCR).

Proteolytic inactivation of FVIIIa and FVa is accomplished by activated protein C (APC) and is stimulated by the nonenzymatic cofactor protein S. Also, membrane components appear to have specific effects on the activity of APC towards its substrates. In addition to protein S, during the degradation of FVIIIa, FV acts as a cofactor to APC and the stimulating effect of FV and protein S on FVIIIa inactivation was found to be synergistic rather than additive.

Under physiological conditions, the presence of the circulating inhibitors and the protein C pathway together form a thrombosis threshold that prevents the initiation and propagation of thrombin formation. When the trigger of coagulation is such that sufficient amounts of proteolytically active enzymes are formed that cannot be controlled by the inhibitors or the protein C pathway, the formation of large amounts of thrombin cannot be prevented anymore. At this stage the threshold is surpassed. The physiological importance of the PC pathway is illustrated by the observation that deficiencies of protein C or protein S result in an increased risk for venous thrombosis. In fact, deficiency of protein C can result in a life-threatening condition that is treated in newborns by the administration of protein C concentrates.

Text adapted from: PMID: 17849041