Protein S deficiency is a disorder that causes abnormal blood clotting. When someone bleeds, the blood begins a complicated series of rapid chemical reactions involving proteins called blood coagulation factors to stop the bleeding. Other proteins in the blood, such as protein S, usually regulate these chemical reactions to prevent excessive clotting. When protein S is missing (deficient), clotting may not be regulated normally and affected individuals have an increased risk of forming a blood clot called a thrombosis. People at risk to have Protein S deficiency are those with an individual or family history of multiple blood clots in the veins.

• Type I deficiency: Characterized by a decrease in the total protein S antigen and free protein S antigen together (quantitative deficiency).
• Type II deficiency: Characterized by normal total and free antigen levels but reduced protein S activity (functional deficiency).
• Type III deficiency: Characterized by low free protein S levels, whereas the total plasma concentration of protein S is normal.

Protein S deficiency is caused by mutations in the PROS1 gene. This gene provides instructions for making protein S, which is found in the bloodstream and is important for controlling blood clotting. Protein S helps block the activity of (inactivate) certain proteins that promote the formation of blood clots.

Most mutations that cause protein S deficiency change single protein building blocks (amino acids) in protein S, which disrupts its ability to control blood clotting. Individuals with this condition do not have enough functional protein S to inactivate clotting proteins, which results in the increased risk of developing abnormal blood clots. Protein S deficiency can be divided into types I, II and III based on how mutations in the PROS1 gene affect protein S.

Normal Function

The PROS1 gene provides instructions for making a protein called protein S that is important for controlling blood clotting. By itself, protein S cannot carry out the chemical reactions necessary for regulating the formation of blood clots. Instead, protein S attaches to certain enzymes and enhances their function. On the basis of this action, protein S is called a cofactor.

Protein S is made chiefly by cells in the liver. The protein circulates in the bloodstream in two forms; it is either attached (bound) to a specific protein or occurs by itself in a free form. Both forms of proteins S can act as cofactors; however, bound protein S is less effective than the free form. Protein S is a cofactor for an enzyme called activated protein C (APC). APC turns off (inactivates) the blood clotting proteins known as factor Va and factor VIIIa. Protein S also helps an enzyme known as tissue factor pathway inhibitor (TFPI) block the activity of another clotting protein, factor Xa.

Health Conditions Related to Genetic Changes

Protein S deficiency

More than 220 mutations in the PROS1 gene have been found to cause protein S deficiency. Most of these mutations change single protein building blocks (amino acids) in protein S, which disrupts its ability to act as a cofactor. Protein S deficiency can be divided into three types based on the mutation in the PROS1 gene.

Protein S deficiency type I is caused by PROS1 gene mutations that result in reduced levels of protein S. Affected individuals do not have enough protein S to control blood clotting, which causes the increased risk for abnormal blood clots in protein S deficiency.

Mutations that cause protein S deficiency type II result in the production of an altered protein S with reduced activity. Individuals with this form of the condition have normal levels of protein S, but the protein is not able to interact with other molecules involved in blood clotting. If protein S does not function properly, abnormal blood clots may form.

Protein S deficiency type III occurs when there is a low amount of free protein S, but the overall amount of protein S is normal. Because free protein S is a more effective cofactor than bound protein S, reduced levels of free protein S can disrupt the inactivation of blood clotting proteins.

Other Names for This Gene

• PROS
• PROS_HUMAN
• protein S (alpha)
• protein S, alpha
• protein S, alpha preproprotein
• protein Sa
• PS21
• PS22
• PS23
• PS24
• PS25
• PSA
• vitamin K-dependent plasma protein S

Genomic Location

Protein S deficiency is inherited in an autosomal dominant pattern, which means one altered copy of the PROS1 gene in each cell is sufficient to cause mild protein S deficiency. Individuals who inherit two altered copies of this gene in each cell have severe protein S deficiency.

• pulmonary embolism
• recurrent venous clots
• recurrent pregnancy clots
• childhood stroke

Mild protein S deficiency is estimated to occur in approximately 1 in 500 individuals. Severe protein S deficiency is rare; however, its exact prevalence is unknown.