Protein S Deficiency

Etiology

Protein S deficiency can be congenital or acquired. Mutations in the PROS1 gene cause congenital protein S deficiency.[4] Most PROS mutations are point mutations, such as transversion mutations, that produce a premature stop codon and thus result in a truncated protein S molecule.[5][6] More than 200 PROS mutations have been described and may result in 3 different forms of protein S deficiency:

• Type 1: Quantitative defect presenting with 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 protein S activity, with normal levels of TPS and FPS antigens
• Type 3 (also known as Type 2a): Quantitative defect presenting with normal levels of TPS but reduced levels of FPS and protein S activity

Protein S deficiency is an autosomal dominant pathology. Mutations in a single copy in heterozygous individuals cause mild protein S deficiency, whereas individuals with homozygous mutations present with severe protein S deficiency.

Causes of acquired fluctuations in protein S levels may include:

• Vitamin K-antagonist therapy
• Chronic infections
• Severe hepatic disease
• Systemic lupus erythematosus
• Myeloproliferative disorders
• Nephritic syndrome
• Disseminated intravascular coagulation (DIC) [7]
• The risk of VTE is also increased in patients using oral contraceptives and pregnancy [8][9]

Epidemiology

Congenital protein S deficiency is autosomal dominant, with variable penetrance. 50% of patients who are heterozygous for protein S deficiency develop VTE; the remaining 50% are asymptomatic and never develop VTE. The annual incidence of venous thrombosis is 1.90%, with 29 years being the median age of presentation. Protein S deficiency can occur as a homozygous state, and these individuals develop purpura fulminans. Purpura fulminans appear during the neonatal period and are characterized by small-vessel thrombosis with cutaneous and subcutaneous necrosis. Estimates of mild congenital protein S deficiency incidence are between 1 in 500 individuals. Severe protein S deficiency is rare, and its prevalence in the general population remains unknown due to difficulty diagnosing the condition.

Protein S deficiency rarely occurs in healthy people without VTE. In a study on healthy blood donors, the prevalence of the familial form of protein S deficiency was found to be 0.03 to 0.13%.[10] When a select group of patients with a history of recurrent thrombosis or a family history significant for thrombosis is assessed, the frequency of protein S deficiency rises to between 3 and 5%.[11][12]

Studies reporting on the clinical significance of the association between protein S levels and VTE risk suggest lowering the cutoff of protein S levels required for the diagnosis. This would, in turn, change the prevalence of the disease.[13] Data from the US and European studies reveal no difference in the prevalence of protein S deficiency. However, protein S deficiency is higher in the Japanese population, at 12.7% in patients with VTE and around 0.48% to 0.63% in the general population.[14]

Protein S deficiency is rare in the healthy population. In a study of 3788 persons, the prevalence of familial protein S deficiency was 0.03 to 0.13%.[10] In patients with a family history of thrombosis or recurrent thrombosis, the frequency of protein S deficiency increases to 3% to 5%.

Race

Protein S deficiency is 5 to 10 times more common in Japanese populations than in whites. The prevalence of protein S deficiency is 0.48% to 0.63% in the general Japanese population and 12.7% in patients with thrombosis.

Sex

Men have a higher level of protein S antigen.

Age

The age of onset of thrombosis varies by heterozygous versus homozygous state. Heterozygous protein S deficiency causes thrombosis in persons younger than 40 to 45. The rare homozygous patients have onset in early infancy.

Pathophysiology

Protein S is a vitamin K-dependent protease that circulates in plasma at low concentrations and serves a crucial role in the regulation of coagulation. In normal circumstances, the anticoagulant proteins keep the blood in a liquid, non-thrombotic state. In circulation, approximately 40% of protein S is free, and about 60% is in a high-affinity complex with the complement regulatory factor C4b-binding protein (C4BP).[4] The anticoagulant activity of protein S is expressed in 2 ways:

1. Protein S is a cofactor for activated protein C (APC) and inactivating coagulation factors 5a and 8a. This process is designed to stop clotting by switching off the cofactor proteins F5a and F8a. Protein S and APC can inactivate FVa. However, for the inactivation of factor 8a, APC and protein S need factor 5.
2. Protein S is also a cofactor for the tissue factor pathway inhibitor (TFPI) protein, resulting in the inactivation of factor 10a and tissue factor (TF)/factor 7a.[1][15][16]

Protein S is a complex protein with multiple structural moieties. Protein S is a single-chain glycoprotein, and it is dependent on vitamin K action for posttranslational modification of the protein to a normal functional state. The 3-dimensional structure is yet to be resolved but is expected to contribute to understanding the complex functional nature of PROS1 mutations.

History and Physical

History

The symptoms in patients with heterozygous protein S deficiency and mild reductions in protein S activity can range in severity. Almost half of all individuals with protein S deficiency become symptomatic before age 55.[17] Venous thrombotic events (VTE), including parenchymal thrombi, deep vein thrombosis (DVT), pulmonary emboli (PE), and a propensity to DIC are common clinical manifestations, with some patients also experiencing cerebral, visceral, or axillary vein thrombosis.[18] Some women may have fetal loss as their only manifestation of protein S deficiency. Approximately half of these recurrent VTE episodes occur in the absence of common risk factors for thrombosis. The variability in risk of thrombotic events in carriers of protein S mutations may be due to different functional consequences of PROS1 mutations, incomplete gene penetrance, exposure to thrombotic risk factors, and environmental or other genetic influences.[19] A family history of thrombosis suggests inherited thrombophilia. Thrombosis before the age of 55 or recurrent thrombosis indicates an inherited thrombophilic state like protein S deficiency.

Severe protein S deficiency resulting from congenital homozygous mutations presents in neonates soon after birth and has a characteristic presentation of purpura fulminans (PF). Affected individuals rarely survive childhood without early diagnosis and treatment.

Physical

The results of the physical examination are generally nonspecific and often misleadingly lead to the diagnosis of DVT. Uncommon sites of thrombosis, such as a mesenteric vein, cerebral sinuses, etc., are rare.[20]

Deep Vein Thrombosis

Venous thrombosis is the most common presentation in almost 90% of cases. The classic presentation of DVT includes calf pain, edema, and pain on dorsiflexion of the foot (i.e., Homan sign). All these 3 findings are present in less than a third of DVT cases. The most common finding is unilateral leg or calf swelling with mild or moderate pain. Superficial thrombophlebitis can also be seen in some cases, with or without DVT.

Pulmonary Embolism

Patients with pulmonary embolism present with dyspnea, chest pain, syncope, and palpitations. Tachypnea is the most frequent sign. Massive pulmonary embolism can present with syncope or cyanosis. Massive embolism also presents with acute right-sided heart failure. Patients have distended neck veins, a left parasternal lift, and an accentuated pulmonic component of the second heart sound.

Evaluation

Diagnostic testing for protein S deficiency is performed using functional assays, including clotting assays and enzyme-linked immunosorbent assays (ELISA), to determine levels of protein S activity.[21]

Protein S Antigen

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 protein S assays are indirect and based on the prolongation of blood clotting by forming activated protein C (APC) and its function in the assay.

Many conditions reduce the blood levels of protein S on both antigenic and functional assays. These include:

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

Plasma protein S levels fluctuate with age, gender, and genetic or acquired influences such as hormonal status or lipid metabolism.[22] 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 deviations in hormone levels. Free protein S levels are not affected by age. Most importantly, a falsely low functional protein S can be seen in patients with factor V Leiden, a disorder that interferes with protein C function. There are some new commercial methods available for determining protein S deficiency in factor V Leiden accurately after dilution of test plasma.[23][24]

Protein S deficiency is classified into 3 phenotypes based on free and total protein S antigen and functional protein S activity by the International Society on Thrombosis and Hemostasis (ISTH), as discussed in the etiology section. Type 2 deficiency is rare. The most common are types 1 and 3.

Total protein S tests perform excellently but cannot detect type 2 and 3 protein S deficiency. Free protein S assays may be a useful alternative, although they lack reproducibility. Measurement of APC cofactor activity could be used as a proxy indicator of protein S deficiency, although these assays have a high false-positive rate.

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

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

Treatment / Management

Protein S deficiency is managed in case of acute venous thromboembolism (VTE). In asymptomatic carriers without thrombotic events, prophylaxis may be used. Management of acute thrombosis is the same as for all acute VTE episodes, based on disease severity and hemodynamic stability. VTE management is by the administration of anticoagulation therapies such as heparin (low-molecular-weight heparin or unfractionated), vitamin K antagonist (VKA), or direct oral anticoagulant (DOAC). Initial heparin treatment may be with intravenous unfractionated heparin or subcutaneous low molecular weight heparin (LMWH). Heparin should be given for a minimum of 5 days, followed by vitamin K antagonist (VKA) or direct oral anticoagulant (DOAC). The choice between a DOAC and VKA depends on patient preference and convenience. In the past, VKA was the drug of choice for VTE, but it has changed with the advent of DOACs. Due to their efficacy and safety profile, DOACs are now increasingly used for VTE. In a cohort study of patients with inherited thrombophilias, DOACs had the same efficacy as heparin/VKAs, but there was an increased risk of non-major bleeding, while VKAs had a slightly increased risk of significant bleeding.[25]

Patients with congenital protein S deficiency normally receive anticoagulation therapy for a longer duration until coagulation activity has stabilized for at least 2 consecutive days.[18] Prophylactic anticoagulation therapy with warfarin is sustained for 3 to 6 months following a thrombotic episode and should be for longer durations in patients with coexisting coagulation conditions.[18] Lifelong therapy is recommended if the first thrombotic event 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 strong event (trauma, surgery) and the thrombosis is not life-threatening or involves multiple or unusual sites.(B3)

Prophylactic treatment should also be administered to patients with protein S deficiency exposed to thrombotic risk factors such as air travel, surgery, pregnancy, or long 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.[18][26](B3)

Differential Diagnosis

Patients with thrombophilia without other risk factors may suffer from protein S deficiency. Alternative causes of thrombophilia include other congenital coagulation abnormalities or a combination of protein S deficiency with other VTE risk factors. Protein S deficiency may also be an acquired condition rather than congenital due to conditions including pregnancy, vitamin K deficiency, oral contraceptives, severe hepatic dysfunction, and chronic infections. Differential diagnoses that must be considered in patients with protein S deficiency include:

• Antiphospholipid syndrome
• Antithrombin deficiency
• Factor 5 Leiden mutation
• Protein C deficiency
• Malignancy
• Prothrombin gene mutation
• Paroxysmal nocturnal hemoglobinuria [27][28]

Prognosis

Patients with mild protein S deficiency are prone to recurrent episodes of VTEs, including DVT. VTE induces significant morbidity and mortality. However, little evidence suggests that thrombophilia related to protein S deficiency results in a deteriorated prognosis for VTE. The development of recurrent thrombotic events in individuals with thrombophilia can contribute to increased morbidity. Furthermore, extended periods of anticoagulation treatment with warfarin can lead to an increased risk of bleeding.

Neonates presenting with severe protein S deficiency have a poor prognosis. Complications from frequent infusions of plasma, such as fluid overload, contribute to a high infant death rate. There is limited data regarding the long-term outcome of patients with severe congenital protein S deficiency.

Complications

Complications of protein S deficiency are categorized into 2 groups, ie, complications due to protein S deficiency and complications due to the anticoagulation therapy.

Complications of Protein S Deficiency

• Post-thrombotic phlebitis
• Recurrent pulmonary embolism can cause cor pulmonale
• Early fetal loss
• Purpura Fulminas

Complications of Anticoagulation

• In adolescents and adults, long-term anticoagulation therapy increases the cumulative likelihood of severe bleeding complications.
• Skin necrosis is a complication associated with warfarin treatment and is manageable with short-term heparin administration.[18]