Protein S deficiency

Protein S deficiency is a rare disorder characterized by decreased activity of protein S, a plasma serine protease with complex roles in coagulation, inflammation, and apoptosis.[ 1 ] Protein S is an anticoagulant protein discovered in Seattle, Washington in 1979 and named after the city. Protein S facilitates the action of activated protein C (APC) on activated factor 5 (F5a) and activated factor 8 (F8a). Protein S deficiency characteristically exhibits an inability to control blood clotting, leading to excessive blood clot formation (thrombophilia) and venous thromboembolism (VTE).[ 2 ] Protein S deficiency can be inherited or acquired. Acquired deficiency is usually due to liver disease, nephrotic syndrome, or vitamin K deficiency. Hereditary protein S deficiency is an autosomal dominant trait. Thrombosis is observed in both heterozygous and homozygous genetic deficiency of protein S.

Epidemiology

Congenital protein S deficiency is autosomal dominant with variable penetrance. The annual incidence of venous thrombosis is 1.90%, with a mean age of presentation of 29 years. Protein S deficiency can occur in the homozygous state, and these individuals develop purpura fulminans. Purpura fulminans appears in the neonatal period and is characterized by small vessel thrombosis with cutaneous and subcutaneous necrosis. The incidence of mild congenital protein S deficiency is estimated to be 1 in 500 individuals. Severe protein S deficiency is rare, and its prevalence in the general population remains unknown due to the difficulty in diagnosing this condition.

Protein S deficiency is rare in healthy individuals without a history of venous thromboembolism. In a study of healthy blood donors, the prevalence of the familial form of protein S deficiency was found to be between 0.03 and 0.13%. [ 3 ] When a selected group of patients with a history of recurrent thrombosis or a family history significant for thrombosis were examined, the incidence of protein S deficiency increased to 3–5%. [ 4 ], [ 5 ]

Studies reporting clinical significance of the association between protein S levels and the risk of venous thromboembolism suggest a reduction in the threshold protein S level required for diagnosis. This, in turn, would change the prevalence of the disease. [ 6 ] Data from American and European studies did not reveal differences in the prevalence of protein S deficiency. However, the prevalence of protein S deficiency is higher in the Japanese population: it is 12.7% in patients with VTE and about 0.48-0.63% in the general population. [ 7 ]

Protein S deficiency is rare in the healthy population. In a study of 3,788 individuals, the prevalence of familial protein S deficiency was 0.03 to 0.13%. In patients with a family history of thrombosis or recurrent thrombosis, the incidence of protein S deficiency increases to 3 to 5%.

Causes of S protein deficiency

Protein S deficiency can be congenital or acquired. Mutations in the PROS1 gene cause congenital protein S deficiency. [ 8 ] Most PROS mutations are point mutations, such as transversion mutations, which produce a premature stop codon and thus result in a shortened protein S molecule. [ 9 ], [ 10 ] More than 200 PROS mutations have been described, which can lead to three different forms of protein S deficiency:

• Type 1: A quantitative defect characterized by low levels of total protein S (TPS) and free protein S (FPS), with reduced levels of protein S activity.
• Type 2 (also known as type 2b): decreased S protein activity with normal levels of TPS and FPS antigens.
• Type 3 (also known as type 2a): a quantitative defect characterized by normal TPS levels but reduced FPS levels and protein S activity.

Protein S deficiency is an autosomal dominant disorder. Mutations in one copy in heterozygous individuals cause mild protein S deficiency, while individuals with homozygous mutations have severe protein S deficiency.

The causes of acquired fluctuations in protein S levels may be:

• Vitamin K antagonist therapy.
• Chronic infections.
• Severe liver disease.
• Systemic lupus erythematosus.
• Myeloproliferative diseases.
• Nephritic syndrome.
• Disseminated intravascular coagulation (DIC). [ 11 ]
• The risk of VTE is also increased in patients taking oral contraceptives and in pregnant women.[ 12 ],[ 13 ]

Pathogenesis

Protein S is a non-enzymatic cofactor of protein C in the inactivation of factors Va and VIIIa, and has its own anticoagulant activity independent of protein C.

Protein S, like protein C, is dependent on vitamin K and is synthesized in the liver. In the bloodstream, it exists in two forms: free protein S and protein S bound to the complement component C4. Normally, 60–70% of protein S is bound to the complement component C4, a regulator of the classical complement pathway. The level of protein S binding to the complement component C4 determines the content of free protein S. Only the free form of protein S serves as a cofactor for activated protein C (APC).

Normally, the level of protein S in plasma is 80–120%. During pregnancy, the level of both free and bound protein S is reduced and is 60–80% and lower in the postoperative period.

Protein S deficiency is inherited in an autosomal dominant manner. Carriers of the gene mutation are often heterozygous, homozygous carriers are rare. It has been found that the protein S gene is located on chromosome 3. Currently, up to 70 mutations of the protein S gene are known. Hereditary protein S deficiency can be of 2 types:

• Type I - a decrease in the level of free protein S associated with the C4 component of complement, within normal limits;
• Type II - decreased levels of free and bound protein S. According to researchers, the frequency of pregnancy loss is 16.5%. Stillbirths are more common than early pregnancy losses.

Heterozygous deficiency of plasma protein S predisposes to venous thromboembolism and is similar to protein C deficiency in genetics, prevalence, laboratory testing, treatment, and prevention. Homozygous protein S deficiency can cause neonatal purpura fulminans, which is clinically indistinguishable from homozygous protein C deficiency. Acquired protein S (and protein C) deficiency occurs with disseminated intravascular coagulation, warfarin therapy, and L-asparaginase administration. Diagnosis is by detection of total and free protein S antigen. (Free protein S is the form not associated with C4b protein.)

Symptoms of S protein deficiency

Symptoms in patients with heterozygous protein S deficiency and mildly reduced protein S activity can vary in severity. Almost half of all individuals with protein S deficiency develop symptoms before age 55.[ 14 ] Venous thrombotic events (VTE), including parenchymal thrombi, deep vein thrombosis (DVT), pulmonary embolism (PE), and a predisposition to DIC, are common clinical manifestations, with some patients also experiencing cerebral, splanchnic, or axillary vein thrombosis. In some women, fetal loss may be the only manifestation of protein S deficiency. About half of these recurrent VTE episodes occur in the absence of general risk factors for thrombosis. Variability in the risk of thrombotic events in carriers of protein S mutations may be due to different functional consequences of PROS1 mutations, incomplete penetrance of the gene, exposure to thrombotic risk factors, and environmental or other genetic influences. [ 15 ] A family history of thrombosis suggests hereditary thrombophilia. Thrombosis before age 55 or recurrent thrombosis suggests an inherited thrombophilic condition such as protein S deficiency.

Severe protein S deficiency, which results from congenital homozygous mutations, manifests in neonates shortly after birth and has a characteristic purpura fulminans pattern. Affected individuals rarely survive into childhood without early diagnosis and treatment.

Diagnostics of S protein deficiency

Diagnostic testing for protein S deficiency is performed using functional assays, including coagulation tests and enzyme-linked immunosorbent assay (ELISA), to determine protein S activity levels.[ 16 ]

S-antigen protein

Protein S antigen can be detected as total antigen or free protein S antigen. The free form of protein S is functionally active. Both free and total protein S can be measured by ELISA.

Functional protein S

Functional assays for protein S are indirect and rely on the prolongation of blood coagulation due to the formation of activated protein C (APC) and its function in the assay.

Many conditions reduce the level of protein S in the blood, both in antigen and functional tests. These include:

• Vitamin K deficiency.
• Liver disease.
• Antagonism with warfarin reduces protein S levels.
• Acute thrombosis.
• Pregnancy.

Plasma protein S levels vary with age, gender, and genetic or acquired factors such as hormonal status or lipid metabolism.[ 17 ] Total and free protein S levels are lower in women than in men, although total protein S levels increase with age, and this is more pronounced in women due to hormonal abnormalities. Free protein S levels are not affected by age. Most importantly, falsely low functional protein S can be observed in patients with factor V Leiden, a disorder that impairs protein C function. Several new commercial assays are available to accurately detect protein S deficiency in factor V Leiden after dilution of the test plasma.[ 18 ],[ 19 ]

Protein S deficiency is classified by the International Society on Thrombosis and Haemostasis (ISTH) into three phenotypes based on free and total protein S antigen and functional S protein activity, as discussed in the etiology section.

Type 2 deficiency is rare. Types 1 and 3 are the most common.

Total protein S tests have excellent results but cannot detect protein S deficiency types 2 and 3. Free protein S assays may be a useful alternative, although they lack reproducibility. Measurement of APC cofactor activity can be used as an indirect indicator of protein S deficiency, although these assays have a high false-positive rate.

Mutational analysis of the PROS1 gene may be important in the diagnosis of protein S deficiency, and ISTH maintains a registry of documented mutations.

Hemostasis analysis (according to ISTH): Diagnosis of PROS1 mutations is performed using DNA sequencing or polymerase chain reaction (PCR) amplification and analysis followed by gel electrophoresis.

Treatment of S protein deficiency

Patients with protein C and S deficiency are refractory to sodium heparin and antiplatelet agents. However, in acute thrombotic complications, the use of sodium heparin and then low-molecular heparins is justified. Fresh frozen plasma in combination with sodium heparin is used as a source of proteins C and S. Warfarin is used for a long time outside of pregnancy in thrombophilia.

Protein S deficiency is treated for acute venous thromboembolism. In asymptomatic carriers without thrombotic events, prophylaxis can be used. Treatment of acute thrombosis is the same as for all acute episodes of venous thromboembolism, depending on the severity of the disease and hemodynamic stability. Treatment of VTE consists of anticoagulant therapy such as heparin (low molecular weight heparin or unfractionated), a vitamin K antagonist, or a direct oral anticoagulant (DOAC). Initial heparin treatment may include intravenous unfractionated heparin or subcutaneous low molecular weight heparin (LMWH). Heparin should be given for at least five days, followed by a vitamin K antagonist or a direct oral anticoagulant (DOAC). [ 20 ]

Patients with congenital protein S deficiency usually receive anticoagulant therapy for a longer period until coagulation activity has stabilized for at least two consecutive days. Prophylactic anticoagulation with warfarin is continued for 3–6 months after the thrombotic event and should be prolonged in patients with concomitant bleeding disorders.[ 21 ] Lifelong therapy is recommended if the first thrombotic episode is life-threatening or occurs in multiple or unusual sites (eg, cerebral veins, mesenteric veins). Lifelong anticoagulation is not recommended if the thrombotic event is precipitated by a major event (trauma, surgery) and the thrombosis is not life-threatening or involves multiple or unusual sites.

Prophylactic treatment should also be given to patients with protein S deficiency who are exposed to risk factors for thrombotic events, such as air travel, surgery, pregnancy, or prolonged periods of immobilization. During pregnancy, patients in the first trimester or after 36 weeks should be treated with low-molecular-weight heparin rather than warfarin to reduce the risk of fetal and maternal bleeding.[ 22 ]