Disorders of Blood Coagulation

Question 1

Explain the importance of the clotting cascade and explain the cell-based model of coagulation

Answer 1

The clotting cascade, also known as the coagulation cascade, is a series of reactions that ultimately leads to the formation of a clot, or thrombus

This process is crucial for hemostasis - the cessation of blood loss from a damaged vessel

The cascade is composed of two pathways - the intrinsic and extrinsic pathways - which both converge into the common pathway to complete the coagulation process

Importance of the clotting cascade:

1. Preventing Blood Loss: The cascade results in the formation of a blood clot, which plugs the site of injury to prevent blood loss
2. Initiating Healing: By sealing off the injury, the cascade initiates the healing process of the injured vessel
3. Controlling Clot Spread: The cascade also includes mechanisms to ensure that clotting is localised at the site of injury and doesn’t unnecessarily spread to obstruct blood flow in other areas

Cell-Based Model of Coagulation:

1) Initiation:

• Tissue factor (TF), a protein expressed on the surface of cells surrounding blood vessels, becomes exposed to blood following vascular injury
• TF forms a complex with activated factor 7 (FVIIa)
• This TF-FVIIa complex activates factor IX and factor X, generating a small amount of thrombin

2) Amplification:

• The small amount of thrombin produced during initiation activates platelets and the co-factors V, VIII, and XI on the platelet surface
• The activated platelets provide a negatively charged phospholipid surface for the subsequent reactions
• The activated factor XI further potentiates the activation of factor IX

3) Propagation:

• On the activated platelet surface, the tenase complex (FVIIIa-IXa) and prothrombinase complex (FVa-Xa) assemble to generate a burst of thrombin
• This robust thrombin generation leads to the conversion of fibrinogen to fibrin, the activation of factor XIII (which stabilizes the fibrin clot), and further platelet activation

4) Regulation:

• Various inhibitors control the coagulation process to prevent excessive clotting
• These include tissue factor pathway inhibitor (TFPI), antithrombin III, and activated protein C

This model emphasizes the importance of cell surfaces (especially those of activated platelets) and their role in localising the coagulation process, limiting it to the site of injury

Question 2

Describe the mechanism of action underlying protein C and S anticoagulant pathways and fibrinolysis

Answer 2

The Protein C and Protein S anticoagulant pathway and the process of fibrinolysis are essential regulatory mechanisms in the process of coagulation to prevent unnecessary clot formation and to dissolve clots that have served their purpose, respectively

Protein C and Protein S Pathway:

• Protein C is a vitamin K-dependent serine protease, synthesized in the liver, that circulates in the plasma in an inactivated form
• Upon vascular injury, thrombin, which is generated at the site of injury, binds to the endothelial cell receptor thrombomodulin
• This thrombin-thrombomodulin complex then activates Protein C
• Once activated, Protein C forms a complex with its cofactor, Protein S
• The activated Protein C-Protein S complex then exerts its anticoagulant effects by proteolytically degrading Factor Va and Factor VIIIa, both crucial co-factors in the coagulation cascade
• The degradation of these factors effectively slows down the cascade, reducing further thrombin generation and therefore limiting clot formation

Fibrinolysis:

• Fibrinolysis is the process by which a fibrin clot, the product of coagulation, is broken down
• Its main enzyme is plasmin, a proteolytic enzyme that breaks down fibrin clots into soluble fibrin degradation products
• The conversion of plasminogen (the inactive form of plasmin) into plasmin is catalysed by tissue plasminogen activator (t-PA) and urokinase (u-PA), both serine proteases
• t-PA is released from endothelial cells in response to signals such as increased vascular shear stress or the presence of thrombin
• While plasminogen is incorporated into the forming fibrin clot, t-PA remains in the circulation, when t-PA encounters a fibrin clot, it binds to fibrin which significantly enhances the activation of plasminogen to plasmin
• Plasmin then cleaves the fibrin mesh at various places, leading to the production of circulating fragments that are cleared by other proteases or by the liver and kidneys
• there are also inhibitors of fibrinolysis, including plasminogen activator inhibitors (PAI-1 and PAI-2) and alpha 2-antiplasmin

Question 3

Describe that blood clotting disorders arise from abnormalities of the coagulation pathway, platelets or blood vessels

Answer 3

Blood clotting disorders are often due to abnormalities in the coagulation pathway, platelets, or blood vessels

When they’re not functioning correctly, it can lead to either excessive bleeding or clotting

Abnormalities in the Coagulation Pathway:

1) Haemophilia A and B:

• These are genetic disorders that result from the deficiency of clotting factors
• Haemophilia A arises from a deficiency of factor VIII, while Haemophilia B (also known as Christmas disease) is due to a deficiency of factor IX
• This results in the inability of the blood to clot effectively, leading to excessive bleeding

2) Von Willebrand’s disease:

• This is a genetic disorder caused by a deficiency of von Willebrand factor (vWF)
• important for platelet adhesion and as a carrier protein for factor VIII
• results in a bleeding tendency

3) Factor V Leiden:

• This genetic mutation results in a hypercoagulable state because factor V can’t be inactivated by protein C
• This leads to an increased risk of developing abnormal blood clots in veins, a condition known as venous thromboembolism

Abnormalities in Platelets:

1) Thrombocytopenia:

• This is a condition characterised by abnormally low platelet levels
• It can lead to problems with clot formation and result in excessive bleeding
• It can be caused by a variety of conditions, including certain medications, autoimmune diseases, and bone marrow disorders

2) Thrombotic Thrombocytopenic Purpura (TTP) and Haemolytic Uremic Syndrome (HUS):

• These are conditions where small clots suddenly form throughout the body, using up too many platelets

Abnormalities in Blood Vessels:

1) Vasculitis:

• These are conditions where small clots suddenly form throughout the body, using up too many platelets

2) Hereditary haemorrhagic telangiectasia:

• This is a genetic disorder that results in blood vessel malformations which can lead to bleeding

3) Scurvy:

• A disease resulting from vitamin C deficiency, leading to weakened blood vessels and resulting in excessive bleeding

4) Cushing Syndrome:

• Chronic high levels of cortisol lead to thinning of the blood vessel walls and can lead to easy bruising

Question 4

Explain the molecular basis of haemophilias A and B, von Willebrand disease and disseminated intravascular coagulation

Answer 4

Haemophilia A and B:

• Haemophilia A and B are both X-linked recessive disorders characterised by a deficiency in the production of functional clotting factors
• Haemophilia A is caused by a deficiency in clotting factor VIII.
• The F8 gene, located on the X chromosome, encodes for factor VIII
• Mutations in this gene can cause reductions in the quantity or functionality of this clotting factor, leading to the symptoms of haemophilia A
• Haemophilia B is due to a deficiency in clotting factor IX
• The F9 gene, also located on the X chromosome, encodes for factor IX
• mutations in this gene can cause reductions in the quantity or functionality of clotting factor IX, leading to the symptoms of haemophilia
• In both conditions, because of the lack of these crucial clotting factors, the coagulation cascade cannot proceed effectively, leading to a prolonged bleeding time

Von Willebrand Disease (VWD):

• Von Willebrand Disease (VWD) is a genetic disorder caused by the missing or defective von Willebrand factor (VWF), a clotting protein
• VWF binds factor VIII, a key clotting protein, and platelets in blood vessel walls, which help form a platelet plug during the clotting process
• Caused by a defect in or absence of the VWF gene, mutations in this gene result in reductions in the quantity or functionality of VWF

Disseminated Intravascular Coagulation (DIC):

• Disseminated intravascular coagulation (DIC) is not a genetic disorder but a condition characterised by systemic activation of the blood clotting system
• This widespread clotting, often initiated by severe infection, malignancy, or obstetric complications, uses up platelets and coagulation proteins, leading to widespread microvascular thrombosis
• This is then followed by bleeding due to the consumption of clotting factors and platelets
• On a molecular level, the balance between procoagulant activity, anticoagulant activity, and fibrinolysis is disrupted, leading to the pathological generation of fibrin (the protein that makes up clots) and the consumption of coagulation factors and platelets
• The activation of the clotting cascade also leads to the generation of fibrin degradation products that have anticoagulant properties, which can further contribute to the bleeding observed in DIC

Question 5

Explain the molecular mechanisms of deep vein thrombosis (DVT) and treatment approaches with anticoagulants

Answer 5

Molecular Mechanisms of Deep Vein Thrombosis (DVT):

DVT is the formation of a blood clot within a deep vein, most commonly in the lower leg, thigh, or pelvis

This condition is usually associated with a triad of factors known as Virchow’s triad: endothelial injury, stasis of blood flow, and hypercoagulability

1. Endothelial injury: Damage to the interior surface of blood vessels can expose subendothelial tissues to platelets and clotting factors in the blood, thereby initiating the clotting cascade
2. Stasis of blood flow: Slower or stagnant blood flow, as seen in immobilisation or long periods of inactivity, can lead to an accumulation of activated clotting factors, promoting clot formation
3. Hypercoagulability: Certain conditions, such as genetic disorders, malignancies, and some medications, can increase the blood’s propensity to clot, contributing to DVT

Treatment Approaches with Anticoagulants:

The mainstay of treatment for DVT is anticoagulation

Anticoagulant drugs reduce the blood’s ability to clot, thereby preventing the enlargement of existing clots and the formation of new ones

They do not directly dissolve the clot; rather, they allow the body’s own clot-dissolving mechanisms to work more effectively

1. Heparin: This is a fast-acting anticoagulant often used in the initial treatment of DVT. It works by activating antithrombin III, a protein that inhibits several key clotting factors, primarily factor II (thrombin) and factor X
2. Warfarin (Coumadin): This is an oral anticoagulant that acts by inhibiting the synthesis of vitamin K-dependent clotting factors (II, VII, IX, and X). It takes several days to reach its full effect, so it’s often administered alongside heparin until it becomes effective
3. Direct oral anticoagulants (DOACs): These include factor Xa inhibitors (e.g., rivaroxaban, apixaban) and direct thrombin inhibitors (e.g., dabigatran). They have the advantage of not requiring regular blood tests for dose adjustment, unlike warfarin
4. Fondaparinux: This is a synthetic pentasaccharide factor Xa inhibitor. It is injected subcutaneously and is often used in patients who have or are at risk for heparin-induced thrombocytopenia