Glycoprotein VI deficiency is a bleeding disorder associated with a decreased ability to form blood clots. Normally, blood clots protect the body after an injury by sealing off damaged blood vessels and preventing further blood loss. Because people with glycoprotein VI deficiency cannot form blood clots normally, they have an increased risk of nosebleeds (epistaxis) and may experience abnormally heavy or prolonged bleeding following minor injury or surgery. In some affected individuals, spontaneous bleeding under the skin causes areas of discoloration (ecchymosis). Women with glycoprotein VI deficiency often have heavy or prolonged menstrual periods (menorrhagia).

Glycoprotein VI deficiency can be caused by mutations in the GP6 gene, which provides instructions for making a protein called glycoprotein VI (GPVI). This protein is embedded in the outer membrane of blood cells called platelets, which are an essential component of blood clots. In response to an injury that causes bleeding, the GPVI protein begins clot formation by attaching (binding) to another protein called collagen that is found on blood vessel walls. The binding of GPVI to collagen also signals additional platelets to come together to increase the size of the clot.

GP6 gene mutations can lead to the production of no GPVI protein; an abnormally short, nonfunctional GPVI protein; or a protein that is less able to bind to collagen. Without GPVI binding to collagen, platelets cannot come together efficiently to form a clot, leading to the bleeding problems associated with glycoprotein VI deficiency.

Some cases of glycoprotein VI deficiency are not caused by GP6 gene mutations; instead these cases are acquired, which means they do not appear to be caused by inherited gene mutations. These acquired cases of glycoprotein VI deficiency are associated with autoimmune disorders such as immune thrombocytopenia purpura, Graves' disease, or systemic lupus erythematosus (SLE). Autoimmune disorders occur when the immune system malfunctions and attacks the body's own cells and tissues. Some individuals with these autoimmune disorders produce immune proteins called antibodies that attack and destroy the GPVI protein. As a result, there is a shortage (deficiency) of functional GPVI protein on the surface of platelets, which leads to bleeding problems characteristic of glycoprotein VI deficiency.

Normal Function

The GP6 gene provides instructions for making a protein called glycoprotein VI (GPVI). GPVI is a receptor protein that is embedded in the outer membrane of blood cells called platelets, which are an essential component of blood clots. Normally, blood clots protect the body after an injury by sealing off damaged blood vessels and preventing further blood loss.

Receptor proteins, like GPVI, have specific sites into which certain other proteins, called ligands, fit like keys into locks. Together, ligands and their receptors trigger signals that affect cell function. The main ligand for GPVI is a protein called collagen, which is found on blood vessel walls. In response to an injury that causes bleeding, the GPVI protein attaches (binds) to collagen, which begins clot formation and signals additional platelets to come together to increase the size of the clot. The GPVI protein can also bind to a protein called fibrin, which is the main protein that forms blood clots.

Health Conditions Related to Genetic Changes

Glycoprotein VI deficiency

At least five mutations in the GP6 gene have been found to cause glycoprotein VI deficiency, a bleeding disorder associated with a decreased ability to form blood clots. The mutations lead to the production of no GPVI protein; an abnormally short, nonfunctional GPVI protein; or a protein that is less able to bind to collagen. Without GPVI binding to collagen, platelets cannot come together efficiently at the site of an injury to form a clot, leading to an increased risk of nosebleeds, abnormally heavy or prolonged bleeding following minor injury or surgery, or other bleeding problems associated with glycoprotein VI deficiency.

Other Names for This Gene

• glycoprotein 6
• GPVI
• GPVI collagen receptor
• platelet collagen receptor
• platelet glycoprotein VI
• platelet membrane glycoprotein VI

Genomic Location

When this condition is caused by mutations in the GP6 gene, it is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

The prevalence of glycoprotein VI deficiency is unknown. At least 15 cases have been described in the scientific literature.