It is easy to take for granted the body’s everyday miracles, like blood clotting. It turns out that just a simple little cut causes a hugely complex series of reactions to take place – a cascade of events that quickly seal up a cut in a blood vessel. (Can you imagine the alternative? Literally – death from paper cuts.)
Some people, though, are unique in their ability to form clots more easily. This may have been a superpower in ages past — the whole, not bleeding to death from a sword wound thing. But superpowers have their downsides, and a greater risk of dying from a heart attack, stroke, or pulmonary embolism definitely qualifies as a ‘downside’.
This article covers six different genes and the seven genetic variants that increase the risk of blood clots. It is a timely topic — blood clots seem to be a serious complication for people with COVID-19.
Overview of how blood clots form:
When you get a cut or tear in a blood vessel, the first action that happens is that the platelets flowing through the bloodstream jump into the gap. They seal up the broken blood vessel to prevent too much blood loss. To do so, the platelets link up with the edges of the cut and then with each other.
Next up is the coagulation cascade. This is where various different coagulation factors become activated. The coagulation factors are proteins that are also called clotting factors. They are inactive when flowing through the bloodstream (so that they don’t clog everything up). When the coagulation factors are activated, they can quickly form up the blood clot.
The final step in clotting is fibrin coming in to shore up the clot. Next, the body starts breaking down the clot through a process called fibrinolysis. It becomes a balance of adding fibrin and breaking it down.
Going a little deeper: More about platelets
Platelets form from megakaryocyte cells in the bone marrow and lungs. Tiny in size, platelets only survive for 7 to 10 days. These little clot formers have no nucleus (thus, no nuclear DNA). They contain mitochondria, along with granules containing different chemical signals and factors needed for coagulation.
When platelets are zipping around through the body, they are not ‘sticky’. Instead, they are in an inactivated form that allows them to flow freely through your veins.
On the cell membrane of the platelets are receptors that link to collagen. When a blood vessel is torn, the platelets bind to the collagen exposed at the site of the rupture. This causes the platelets to change shape, and secrete chemical signals, initiating the coagulation cascade. [ref]
In addition to platelets being able to bind to exposed collagen, another molecule called the von Willebrand factor (VWF) can also bind to both collagen and the platelet. In a sense, the von Willebrand factor forms a bridge between platelets and the surface of the blood vessel, and your circulating level of von Willebrand factor is important in how long it takes you to clot. [ref]
When platelets bind to either collagen or the von Willebrand factor, a signaling cascade is activated, releasing granules from the platelets. This release includes fibrinogen, ADP, serotonin, as well as other molecules, and the activation of COX-1 (which is inhibited by aspirin). [ref]
Below is a figure depicting all of the functions of platelets – from coagulation and wound healing to immune functions. Creative Commons Image[ref]
So how many of these little guys are zipping around the body? A healthy adult has between 150,000 and 450,000 platelets per microliter of blood.
- A low platelet count (less than 150,000/ul) is known as thrombocytopenia
- A high platelet count (greater than 450,000/ul) is called thrombocytosis
Let’s dig a little deeper into the coagulation cascade so that the genetic variants (listed below) involved in increased clotting make a little more sense. Basically – this is a whole cascade of events that happens quickly and results in the formation of cross-linked fibrin, which forms a mesh to keep the clot together.
The coagulation factors are numbered and referred to with Roman numerals -for example, factor five is written as factor V. Coagulation factors are found both in the plasma (e.g. factor IV) and the endothelial cells that form blood vessels (factors III and VIII).
The coagulation cascade can be divided into two pathways: the extrinsic and intrinsic pathways [ref]
- In the extrinsic pathway, factor VII is activated by tissue factor. The activated factor VII then activates factor X and factor IX. Factor X binds to activated factor V (along with a calcium molecule) and this causes the activation of prothrombin into thrombin. Thrombin converts fibrinogen into fibrin.
- The intrinsic pathway is also called the contact pathway. When thrombin is formed, factor XI is activated, which causes a further activation of factor X and factor IX. Factor X then binds with activated factor V, creating more thrombin. (This is a minor player in the clotting game.)
Finally, all of this results in a fibrin polymer mesh, which is created through the involvement of activated factor XIII. This mesh then holds the clot together, binding the platelets in with the other factors.
After the clot is formed, it gets balanced out with the activation of plasminogen, which is an enzyme that breaks down some of the fibrin in the clot. Basically, there is a balancing act of formation of the clot and breaking down of the clot while the wound heals.
D-dimer is released as clots are broken down, and it is often measured in the blood to determine if someone has a blood clot.
Thrombosis: blood clots forming inside a blood vessel
While it is easy to visualize the blood clotting process when you think of a paper cut on your finger, this same process can take place in blood vessels deep inside you. Instead of a cut, there are a variety of different ‘insults’ to the blood vessel that can cause a clot to form.
So what is the big deal about ‘thrombosis’? Why do we care about blood clotting in a blood vessel?
The majority of heart attacks and about 80% of strokes are caused by blood clots.
Blood clots forming deep in the leg are called deep vein thrombosis (DVT). The clot can block blood from flowing past it, or it could break free and travel through the veins, lodging in the lungs, causing a pulmonary embolism. Blood clots are treated with anticoagulant medications, which come with risks of excess bleeding. In all, the case-fatality rates for DVT and pulmonary embolisms are between 5 and 10% over the course of 30 days. [ref]
Thrombosis due to viral or bacterial infections:
Blood clots can also form due to an infection caused by bacteria or viruses.
Blood clots, though, aren’t unique to COVID-19. Instead, they are a risk factor in a number of severe infectious diseases including influenza and staph infections. In fact, people with severe complications from the H1N1 strain of the flu were at an 18 to 23-fold greater risk for blood clots. [ref][ref]
When the body is fighting off an infection, a lot of proinflammatory cytokines (IL-6, TNF-alpha, IL-1, IFNγ) are released. These cytokines cause both leukocytes (white blood cells) and the surrounding epithelial cells to release procoagulants, such as tissue factors. These procoagulants can tip the body towards too much coagulation, with platelets clumping together. This causes small clots to form, plugging up small blood vessels. The kidneys, liver, lungs, and brain are particularly vulnerable. [ref] [ref]
Infection by bacteria or viruses increases the risk of thrombosis by 2 to 20-fold. The risk is greatest when the infection is active – and in the first few weeks after the infection. Complications from clots, such as strokes, are common in the first three days after respiratory infections and urinary tract infections. [ref]
ARDS (acute respiratory distress syndrome) is a severe complication from pneumonia due to viral or bacterial infections. Research shows that bacterial pneumonia causes “increased platelet aggregation, pulmonary microvascular thrombosis, endothelial damage and hyper-inflammatory cytokine responses ” [ref]
DIC – disseminated intravascular coagulation:
In severe infections, such as sepsis, a condition called disseminated intravascular coagulation (DIC) can occur. DIC causes increased coagulation in the small blood vessels and micro-clots to form in various organs. [ref]
The increased number of clots then causes a depletion of platelets and clotting factors. So, on one hand, you have a lot of little blood clots, but on the other hand, you eventually get increased bleeding risk due to the decreased platelets and decreased fibrinogen. This also leads to elevated levels of D-dimer, which is released when the fibrin clots are broken down.
Genetic variants that increase the risk of blood clots:
Unsurprisingly, the genetic variants that increase the risk of blood clots are mainly in the genes involved in coagulation factors and platelet stickiness.
The F2 gene codes for factor 2, also known as prothrombin. When activated, prothrombin becomes thrombin, which is important in fibrin creation. The variant listed below is known as G20210A, and it is linked to an increased risk of blood clots and strokes.
About 2% of Caucasians carry the G20210A variant, but it is much less common in Asians, Africans, and Native Americans. A meta-analysis of the data from a bunch of studies found that carrying the G20210A variant increased the risk of thromboembolism by 2.6 to 4.4-fold (depending on the location of the clot). The risk of DVT is also greatly increased in carriers of both G20210A and the factor V Leiden (F5 gene, below) variants.[ref]
Check your genetic data for rs1799963, G20210A (23andMe v4, v5 – i3002432; AncestryDNA):
- A/A: a large increase in the risk of blood clots, increased risk of stroke with PFO
- A/G: increased risk of blood clots, increased risk of stroke with PFO
- G/G: typical
Members: Your genotype for rs1799963 (i3002432) is —.
Factor V, coded for by the F5 gene, is a coagulation protein mainly synthesized in the liver and then circulated through the body. It is activated in clotting by thrombin and can bind to activated platelets. Just as there is a cascade of events to quickly cause a blood clot to form, there are other molecules involved, such as protein C, in turning off the clotting when it is no longer needed. [ref]
The F5 gene variant, known as factor V Leiden, has been linked in many studies to an increased risk of deep vein thrombosis and pulmonary embolism. A meta-analysis combined data from 31 studies estimates that the increase in the risk of thrombosis for people carrying one copy of the variant was 4-fold and the increase for those carrying two copies was 11-fold. [ref][ref][ref][ref]
Check your genetic data for rs6025 (23andMe v4, v5; AncestryDNA):
- C/C: typical
- C/T: one copy of factor V Leiden
- T/T: two copies of factor V Leiden
Members: Your genotype for rs6025 is —.
The ITGB3 Gene – PIA1/A2 mutation
The ITGB3 gene codes for the fibrinogen receptor that is a part of how platelets form clots.
One well studied ITGB3 genetic variant is known as PIA1/A2. Studies show that people who carry the A2 variant have faster clotting times. It is also linked to an increased risk of heart attacks and deep vein thrombosis (blood clots).[ref] [ref][ref][ref]
Check your genetic data for rs5918 (23andMe v4, v5; AncestryDNA):
- C/C: two copies of the A2 variant, increased risk of heart disease, may not benefit from aspirin for heart attack prevention, protection against Hantavirus [ref][ref][ref]
- C/T: somewhat increased risk of heart disease, may not benefit as much from aspirin for heart attack prevention, protection against Hantavirus [ref][ref][ref]
- T/T: normal
Members: Your genotype for rs5918 is —.
The VWF gene codes for the von Willebrand factor (VWF), which is needed for platelets to stick to each other. When circulating through the blood, most of the von Willebrand factor is bound to factor VIII (8). When activated by coagulation factors, the factor VIII is released from VWF and becomes part of the activation of factor X and conversion of prothrombin to thrombin (active form).
Higher levels of VWF and factor VIII are risk factors for blood clotting more easily.[ref] People who have decreased von Willebrand factor, though, tend to bleed more easily and may have easy bruising, nosebleeds, and bleeding gums.
People with type O blood typically have lower levels of von Willebrand factor. [ref] (Read about how people with type O blood are at a lower risk of severe COVID-19 complications)
Below are variants linked with higher VWF levels. There are also mutations (not listed here) that lead to easier bleeding and Von Willebrand disease.
Check your genetic data for rs1063856 (23andMe v4, v5):
- T/T: typical
- C/T: likely to have slightly higher Von Willebrand factor,
- C/C: likely to have increased Von Willebrand factor, slightly increased risk of blood clots.[ref][ref]
Members: Your genotype for rs1063856 is —.
Check your genetic data for rs1063857 (23andMe v4; AncestryDNA):
- A/A: typical
- A/G: likely to have slightly higher von Willebrand factor
- G/G: likely to have increased von Willebrand factor[ref]
Members: Your genotype for rs1063857 is —.
The G6P gene codes for a protein called glycoprotein VI, which plays an important role in the aggregation of platelets. Platelets that stick together too easily can be a problem in coronary artery disease.
Check your genetic data for rs1613662 (23andMe v4, v5; AncestryDNA):
- A/A: typical
- A/G: slightly increased platelet stickiness
- G/G: slightly increased platelet stickiness, increased risk of sticky platelet syndrome, slightly increased risk of heart attack[ref][ref]
Members: Your genotype for rs1613662 is —.
The F11 gene codes for the factor XI protein, one of the coagulation factors needed in the coagulation cascade for creating the fibrin net.
Check your genetic data for rs2036914 (23andMe v5; AncestryDNA):
- C/C: increased risk of venous thrombosis, thromboembolism [ref][ref][ref]
- C/T: typical risk
- T/T: typical
Members: Your genotype for rs2036914 is —.
Take the information about your blood clot genetic risk factors as a ‘heads up’. Don’t ignore the signs of a blood clot! Symptoms of a blood clot in your arm or leg can include swelling, pain, redness, and warmth. If you suspect a clot, head to the doctor to get it checked out. While the risk of blood clots increases with age, people who are genetically prone to clots can get one at any age.
Natural blood thinners may decrease the risk of blood clots. If you are on any prescription medications or under a doctor’s care, check with your doctor before experimenting with natural blood thinners.
Curcumin is a natural compound found in turmeric. Studies show that it decreases platelet adhesion and has possible beneficial effects for preventing cardiovascular disease. [ref]
Aspirin is a natural blood thinner. Talk with your doctor to see if low-dose aspirin is a good fit for you.
Maslinic acid, a component of olive pomace oil, has recently been shown to downregulate one of the coagulation factors (factor Xa) and decrease platelet aggregation.[ref]
Salidroside, the bioactive component of the herb Rhodiola rosea, has been shown in studies to decrease thrombosis and inhibit platelet function.[ref]
Glycyrrhetinic acid, a component of licorice, directly inhibits factor Xa and is an anticoagulant.[ref]
Acute Respiratory Distress Syndrome (ARDS) genes
ARDS is caused by an overwhelming immune response to a virus, bacteria, or lung injury. Learn more about which of your immune system genes are involved in ARDS.
Viral Immunity: Your genes protect you
Your genetic variants shape your immune system and give you superpowers against some pathogens – and perhaps more susceptible to others.
Non-secretors: Norovirus resistance and gut microbiome
A genetic variant in the FUT2 gene controls whether or not you secrete your blood type into your saliva and other bodily fluids, such as your intestinal mucosa. This alters the gut microbiome – and protects you from Norovirus.