Cleaved protein S (PS), total PS, free PS, and activated protein C cofactor activity as risk factors for venous thromboembolism

Revisiting the activated protein C-protein S-thrombomodulin ternary pathway: Impact of new understanding on its laboratory investigation

Although suspected conceptually in the 60 s, Protein C and Protein S activities in hemostasis were investigated and reported from the mid-80 s, followed by the discovery of Thrombomodulin, an endothelial cell membrane associated protein, playing the most important heamostatic role. These 3 proteins act in regulating thrombogenesis and protecting against thrombo-embolic events. When blood is activated, any trace of circulating thrombin is captured by Thrombomodulin in the microcirculation, making thrombin become an anticoagulant through its capacity to activate Protein C to Activated Protein C, which operates as a sentinel in blood coagulation, in the form of a complex with free Protein S, to block any new blood activation site, and more especially circulating activated Factors V and VIII. Protein S not only acts as the Activated Protein C cofactor, but also as the cofactor of Tissue Factor Pathway Inhibitor. In addition, it has some functions in the complement pathway through its binding to C4b-BP. Another capability of activated protein C is to lower fibrinolytic activity, as the Activated Protein C Inhibitor is also known as Plasminogen Activator Inhibitor 3. The Protein C-Protein S system becomes less efficient in the presence of mutated Factor V (Factor V-Leiden or other variants), which is resistant to its inactivating effect. Other pathologies linked to this system concern the development of allo- or auto-antibodies to Protein S or to thrombin, which can generate severe thrombotic complications in affected patients. Some antithrombotic drugs have originated from this regulatory system. Protein C or Protein S concentrates are used for treating deficient patients. Activated Protein C (especially in patients with sepsis) or Thrombomodulin are proposed as antithrombotic medications. Most importantly, congenital or acquired Protein C or Protein S deficiencies are associated with severe recurrent thrombotic events. From the clinical standpoint most of the patients are heterozygous, as homozygosity is almost incompatible with life in the absence of a continuous and efficient treatment. Laboratory investigation of this highly complex system involves many different specialized assays for measuring these 3 proteins' activities, their antigenic content or their genetic sequence. The Protein S in-vitro anticoagulant activity is weak and contrasts with its high antithrombotic role in-vivo, showing that diagnostic assays have not yet succeeded in reproducing all the natural mechanisms for evidencing the anticoagulant role of Protein S. This paradoxal notion is discussed and illustrated in this manuscript as well is a revisit of the major characteristics and pathophysiological functions of the Protein C-Protein S-Thrombomodulin system; the associated pathologies; and the main laboratory tools available for clinical diagnosis. In respect to future perspectives, we also focused on developing more significant and relevant assays, especially for Protein S, thanks to the understanding of its biological roles.

Platelet-mediated proteolytic down regulation of the anticoagulant activity of protein S in individuals with haematological malignancies

The natural anticoagulant protein S contains a so-called thrombin-sensitive region (TSR), which is susceptible to proteolytic cleavage. We have previously shown that a platelet-associated protease is able to cleave protein S under physiological plasma conditions in vitro. The aim of the present study was to investigate the relation between platelet-associated protein S cleaving activity and in vivo protein S cleavage, and to evaluate the impact of in vivo protein S cleavage on its anticoagulant activity. Protein S cleavage in healthy subjects and in thrombocytopenic and thrombocythaemic patients was evaluated by immunological techniques. Concentration of cleaved and intact protein S was correlated to levels of activated protein C (APC)-dependent and APC-independent protein S anticoagulant activity. In plasma from healthy volunteers 25% of protein S is cleaved in the TSR. While in plasma there was a clear positive correlation between levels of intact protein S and both APC-dependent and APC-independent protein S anticoagulant activities, these correlations were absent for cleaved protein S. Protein S cleavage was significantly increased in patients with essential thrombocythaemia (ET) and significantly reduced in patients with chemotherapy-induced thrombocytopenia. In ET patients on cytoreductive therapy, both platelet count and protein S cleavage returned to normal values. Accordingly, platelet transfusion restored cleavage of protein S to normal values in patients with chemotherapy-induced thrombocytopenia. In conclusion, proteases from platelets seem to contribute to the presence of cleaved protein S in the circulation and may enhance the coagulation response in vivo by down regulating the anticoagulant activity of protein S.

Similar hypercoagulable state and thrombosis risk in type I and type III protein S-deficient individuals from families with mixed type I/III protein S deficiency

BACKGROUND

Protein S, which circulates in plasma in both free and bound forms, is an anticoagulant protein that stimulates activated protein C and tissue factor pathway inhibitor. Hereditary type I protein S deficiency (low total and low free protein S) is a well-established risk factor for venous thrombosis, whereas the thrombosis risk associated with type III deficiency (normal total and low free protein S) has been questioned.

DESIGN AND METHODS

Kaplan-Meier analysis was performed on 242 individuals from 30 families with protein S deficiency. Subjects were classified as normal, or having type I or type III deficiency according to their total and free protein S levels. Genetic and functional studies were performed in 23 families (132 individuals).

RESULTS

Thrombosis-free survival was not different between type I and type III protein S-deficient individuals. Type III deficient individuals were older and had higher protein S, tissue factor pathway inhibitor and prothrombin levels than type I deficient individuals. Thrombin generation assays sensitive to the activated protein C- and tissue factor pathway inhibitor-cofactor activities of protein S revealed similar hypercoagulable states in type I and type III protein S-deficient plasma. Twelve PROS1 mutations and two large deletions were identified in the genetically characterized families.

CONCLUSIONS

Not only type I, but also type III protein S deficiency is associated with a hypercoagulable state and increased risk of thrombosis. These findings may, however, be restricted to type III deficient individuals from families with mixed type I/III protein S deficiency, as these represented 80% of type III deficient individuals in our cohort.

Denaturing high-performance liquid chromatography mutation analysis in patients with reduced Protein S levels

BACKGROUND

Patients with congenital Protein S deficiency have increased risk of venous thromboembolism. However, Protein S levels show large intra-individual variation and the biochemical assays have low accuracy and a high interlaboratory variability. Genetic analysis might aid in a more precise diagnosis and risk estimation. The aim was to design a high-throughput genetic analysis based on denaturing high-performance liquid chromatography to identify sequence variations in the gene coding for Protein S.

PATIENTS

In total, 55 patients referred to the Section of Thrombosis and Haemostasis, Odense University Hospital, in the period 1998-2004 were included in the study.

RESULTS

Mutations were found in ten of the 55 patients: Six different variants were identified, of which four were not previously reported: One were a nonsense mutation substituting a glutamine with a stopcodon (c.790C>T) and the rest were missense mutations (c.932T>G; c.1367A>G; c.1378T>C). Furthermore, four patients carried the same mutation (c.1045G>A), while two carried the Heerlen mutation (c.1378T>C).

CONCLUSIONS

The reported method will be useful for rapidly detecting sequence variations in the gene coding for Protein S, giving a precise diagnosis and subsequently a better risk estimation.

Protein S type III deficiency is no risk factor for venous and arterial thromboembolism in 168 thrombophilic families: a retrospective study

Free protein S rather than total protein S levels are currently measured to detect inherited protein S deficiency. Because type III (free protein S) deficiency is still not established as risk factor for thrombosis, we assessed the absolute risk of venous and arterial thromboembolism in a family cohort study. Annual incidences in first-degree relatives with and without protein S deficiency type III were compared. Probands had experienced thrombosis and had either the prothrombin G20210A mutation, increased factor VIII:C levels or hyperhomocysteinemia. Relatives were tested for these thrombophilic disorders and factor V Leiden. Levels of antithrombin, protein C, total and free protein S, and factor XI:C were additionally measured. Of 500 relatives enrolled, 105 were excluded from analysis because they could not be classified, due to acquired conditions. Protein S deficiency type III was demonstrated in 60/395 remaining relatives (15%). Other thrombophilic defects were equally distributed among deficient and non-deficient relatives. Annual incidences of venous thromboembolism were 0.28 per 100 person-years [95% confidence interval (CI), 0.09-0.66] in deficient relatives versus 0.20 per 100 person-years (95% CI, 0.12-0.30) in non-deficient relatives [hazard ratio, 1.4 (95% CI, 0.4-4.0)]. For arterial thromboembolism these values were 0.16 per 100 person-years (95% CI, 0.03-0.46) versus 0.10 per 100 person-years (95% CI, 0.05-0.19) [hazard ratio, 1.5 (95% CI, 0.3-6.0)]. These results suggest that protein S deficiency type III is not associated with an increased risk of either venous or arterial thromboembolism.