Clarification of the risk for venous thrombosis associated with hereditary protein S deficiency by investigation of a large kindred with a characterized gene defect

Atypical presentation of pulmonary embolism and deep vein thrombosis due to protein S deficiency in a young female with chest pain

Venous thromboembolism has many risk factors including protein S deficiency, which poses a significant diagnostic challenge as it presents with atypical complaints. A treatable yet potentially fatal condition, acute pulmonary embolism, is currently third most common cause of cardiovascular death. Clinicians should include pulmonary embolism as differential diagnosis in young adults with atypical symptoms with 2 D ECHO findings of the dilated right atrium, right ventricle, and elevated pulmonary artery pressure, and diagnosis is confirmed by computed tomography pulmonary angiography (CTPA). Anticoagulants including NOACs should be initiated promptly to improve the outcome for patients.

Managing thromboembolic risk in patients with hereditary and acquired thrombophilias

While we are now able to diagnose inherited thrombophilias in a substantial number of patients with venous thromboembolism (VTE), the initial hope that their presence would inform recurrence risk and thus decisions on anticoagulation duration has largely been disappointing. Indeed, the presence or absence of transient provoking risk factors has proven to be the most important determinant of VTE recurrence risk. Thus, particular attention to transient acquired risk factors for VTE remains paramount, as they have generally been shown to carry more prognostic weight than inherited thrombophilias. The presence of other acquired risk factors may require additional management considerations, whether pertaining to anticoagulant choice, as in antiphospholipid antibody syndrome, or to addressing a new predisposing medical condition, as in malignancy. Antithrombin deficiency or the presence of ≥1 thrombophilic defect may be exceptions that can have a role in prognostication; however, as illustrated in this review through several case vignettes, interpretation and clinical application of the results of inherited thrombophilia testing is nuanced. We have chosen to focus on cases in which patients have been identified as having thrombophilic defects rather than the indications for undertaking testing in the first place or the extent of investigation. Management decisions in such cases ultimately hinge on individualized consideration of the benefits and risks of anticoagulation along with patient preference rather than on an algorithmic pathway based on thrombophilia status.

Burden of rare exome sequence variants in PROC gene is associated with venous thromboembolism: a population-based study

BACKGROUND

Rare coding mutations underlying deficiencies of antithrombin and proteins C and S contribute to familial venous thromboembolism (VTE). It is uncertain whether rare variants play a role in the etiology of VTE in the general population.

OBJECTIVES

We conducted a deep whole-exome sequencing (WES) study to investigate the associations between rare coding variants and the risk of VTE in two population-based prospective cohorts.

PATIENTS/METHODS

Whole-exome sequencing was performed in the Longitudinal Investigation of Thromboembolism Etiology (LITE), which combines the Atherosclerosis Risk in Communities (ARIC) study (316 incident VTE events among 3159 African Americans [AAs] and 458 incident VTEs among 7772 European Americans [EAs]) and the Cardiovascular Healthy Study (CHS; 60 incident VTEs among 1751 EAs). We performed gene-based tests of rare variants (allele frequency < 1%, exome-wide significance P < 1.47 × 10-6 ) separately in each study and ancestry group, and meta-analyzed the results for the EAs in ARIC and CHS.

RESULTS

In the meta-analysis of EAs, we identified one gene, PROC, in which the burden of rare, coding variants was significantly associated with increased risk of VTE (HR = 5.42 [3.11, 9.42] for carriers versus non-carriers, P = 2.27 × 10-9 ). In ARIC EAs, carriers of the PROC rare variants had on average 0.75 standard deviation (SD) lower concentrations of plasma protein C and 0.28 SD higher D-dimer (P < .05) than non-carriers. Adjustment for low protein C status did not eliminate the association of PROC burden with VTE. In AAs, rare coding PROC variants were not associated with VTE.

CONCLUSIONS

Rare coding variants in PROC contribute to increased VTE risk in EAs in this general population sample.

Peripartum myocardial infarction associated with coronary spasm and acquired protein S deficiency: A case report

RATIONALE

Coronary angiography (CAG) findings of acute myocardial infarction (AMI) in pregnant women are characterized by a high incidence of normal coronary arteries. This is the first report of AMI with normal coronary arteries during pregnancy, showing coronary spasm and pregnancy-related acquired protein S (PS) deficiency.

PATIENT CONCERNS

A 30-year-old Japanese woman was admitted to an emergency department. One hour before admission, she developed sudden onset of precordial discomfort, back pain, and dyspnea. She was a primigravida at 39 weeks' gestation and had no abnormality in the pregnancy thus far. She had no history of heart disease, diabetes, hypertension, dyslipidemia, deep vein thrombosis (DVT), smoking, or oral contraceptive use and no family history of ischemic heart disease, hemostasis disorder, or DVT. She did not take any medication.

DIAGNOSIS

Electrocardiography showed ST-segment elevations in leads II, III, aVF, and V2-V6. Heart-type fatty acid-binding protein was positive. Echocardiography showed hypokinesis of the anterior interventricular septum and inferior wall. Continuous intravenous infusion of isosorbide dinitrate was initiated. Coronary computed tomography angiography revealed diffuse narrowing of the apical segment of the left anterior descending coronary artery. Three hours after admission, troponin T became positive, and the following enzymes reached their peak levels: creatine kinase (CK), 1,886 U/L; CK-muscle/brain, 130 U/L. She was diagnosed with transmural AMI due to severe coronary spasm and administered benidipine hydrochloride. Five hours after admission, premature membrane rupture occurred.

INTERVENTIONS

Emergency cesarean section was performed. There were no anesthetic or obstetrical complications during the operation. On postpartum day 1, the free PS antigen level was low (29%). On postpartum day 18, she was discharged with no reduction in physical performance.

OUTCOMES

Four months after the infarction, CAG showed normal coronary arteries. Acetylcholine provocation test showed diffuse vasospasm in the coronary artery. She was advised that her next pregnancy should be carefully planned. Two years after delivery, free PS antigen level was within normal range, at 86%. She had not experienced recurrence of angina during the 2-year period. Her child was also developing normally.

LESSONS

In addition to coronary spasm, pregnancy-related acquired PS deficiency may be involved in AMI etiology.

Anticoagulant Protein S Targets the Factor IXa Heparin-Binding Exosite to Prevent Thrombosis

OBJECTIVE

PS (protein S) is a plasma protein that directly inhibits the coagulation FIXa (factor IXa) in vitro. Because elevated FIXa is associated with increased risk of venous thromboembolism, it is important to establish how PS inhibits FIXa function in vivo. The goal of this study is to confirm direct binding of PS with FIXa in vivo, identify FIXa amino acid residues required for binding PS in vivo, and use an enzymatically active FIXa mutant that is unable to bind PS to measure the significance of PS-FIXa interaction in hemostasis.

APPROACH AND RESULTS

We demonstrate that PS inhibits FIXa in vivo by associating with the FIXa heparin-binding exosite. We used fluorescence tagging, immunohistochemistry, and protein-protein crosslinking to show in vivo interaction between FIXa and PS. Importantly, platelet colocalization required a direct interaction between the 2 proteins. FIXa and PS also coimmunoprecipitated from plasma, substantiating their interaction in a physiological milieu. PS binding to FIXa and PS inhibition of the intrinsic Xase complex required residues K132, K126, and R170 in the FIXa heparin-binding exosite. A double mutant, K132A/R170A, retained full activity but could not bind to PS. Crucially, Hemophilia B mice infused with FIXa K132A/R170A displayed an accelerated rate of fibrin clot formation compared with wild-type FIXa.

CONCLUSIONS

Our findings establish PS as an important in vivo inhibitor of FIXa. Disruption of the interaction between PS and FIXa causes an increased rate of thrombus formation in mice. This newly discovered function of PS implies an unexploited target for antithrombotic therapeutics.

Coagulation factor and protease pathways in thrombosis and cardiovascular disease

The biochemical characterisation of the proteolytic pathways that constitute blood coagulation was one of the most relevant achievements in biomedical research during the second half of the 20th century. Understanding these pathways was of crucial importance for improving global health through application in haemostasis and thrombosis pathologies. Immediately after the cloning of the genes corresponding to these proteins, mutations were discovered in them that were associated with imbalances in haemostasis. Later, the importance of coagulation pathways in other pathological processes was demonstrated, such as in atherosclerosis and inflammation, both essential processes involved in vascular disease. In the present review we evaluate the concepts that have allowed us to reach the integrated vision on coagulation that we have today. The thrombo-inflammation model encompassing these aspects includes a pivotal role for the proteases of the coagulation pathway as well as the regulatory proteins thereof. These concepts illustrate the importance of the coagulation cascade in cardiovascular pathology, not only in thrombotic processes, but also in atherosclerotic processes and in the response to ischaemia-reperfusion injury, making it a central mechanism in cardiovascular disease.

PROS1 genotype phenotype relationships in a large cohort of adults with suspicion of inherited quantitative protein S deficiency

Inherited protein S deficiency (PSD) is an established risk factor for venous thromboembolism (VTE). However, data are conflicting concerning risk of VTE associated with decreased free PS level (FPS) and information on PROS1 genotype-phenotype relationship is sparse. In a retrospective cohort of 579 patients with inherited type I/III deficiency suspicion, PROS1 genotyping was performed and the effect of genotype on FPS and on VTE risk was investigated. We found 116 (including 65 novel) detrimental mutations (DM) in 222 (type I/III in 194, type II in 28), PS Heerlen in 74, possibly non DM in 38 and no mutation in 245 subjects. Among DMs, type I/IIIDMs only were found in subjects with FPS< 30 %. Prevalence of type I/III DM decreased with increasing FPS level. Risk of VT associated with FPS level and genotype was studied in the 467 subjects with personal or family history of thrombosis. Only type I/IIIDM carriers presented with an increased risk of VTE [1.41 (95 %CI (1.05-1.89)] compared to subjects with no mutation. Among the group of type I/IIIDM heterozygotes and subjects with no mutation, the optimal FPS cut-off point for identifying subjects at increased VTE risk was searched for. We found that only subjects with FPS< 30 % and type I/IIIDM presented with an increased risk [1.48 (95 %CI 1.08-2.04)]. Our findings confirm the value of a cut-off FPS level for identifying subjects at increased VTE risk far below the lower limit of the normal range and suggest a place for PROS1 genotyping in PSD diagnosis strategy.

TFPI cofactor function of protein S: essential role of the protein S SHBG-like domain

Protein S is a cofactor for tissue factor pathway inhibitor (TFPI), accelerating the inhibition of activated factor X (FXa). TFPI Kunitz domain 3 residue Glu226 is essential for enhancement of TFPI by protein S. To investigate the complementary functional interaction site on protein S, we screened 44 protein S point, composite or domain swap variants spanning the whole protein S molecule for their TFPI cofactor function using a thrombin generation assay. Of these variants, two protein S/growth arrest-specific 6 chimeras, with either the whole sex hormone-binding globulin (SHBG)-like domain (Val243-Ser635; chimera III) or the SHBG laminin G-type 1 subunit (Ser283-Val459; chimera I), respectively, substituted by the corresponding domain in growth arrest-specific 6, were unable to enhance TFPI. The importance of the protein S SHBG-like domain (and its laminin G-type 1 subunit) for binding and enhancement of TFPI was confirmed in FXa inhibition assays and using surface plasmon resonance. In addition, protein S bound to C4b binding protein showed greatly reduced enhancement of TFPI-mediated inhibition of FXa compared with free protein S. We show that binding of TFPI to the protein S SHBG-like domain enables TFPI to interact optimally with FXa on a phospholipid membrane.

Association between pre-eclampsia and inherited thrombophilias

OBJECTIVE

The aim was to determine whether inherited thrombophilia increases the risk of pre-eclampsia (PE) or interferes with its clinical course.

MATERIAL AND METHODS

We included 50 patients with severe PE and 50 healthy pregnant women. Patients were evaluated for inherited thrombophilia.

RESULTS

Fourteen patients in the study group was factor V Leiden (FVL) carrier while it was 12% in the control group. In women with PE, FVL and other inherited thrombophilic factors were not more prevalent than in the controls.

CONCLUSION

The present study failed to demonstrate an association between the inherited thrombophilias and PE.

Protein S

Protein S (PS) is a vitamin K-dependent plasma glycoprotein. Around 60-70% of PS in plasma is noncovalently bound to C4-binding protein (C4BP). Free PS functions as a cofactor that enhances the activity of activated protein C (APC) in the proteolytic degradation of activated factors V and VIII. PS also has a more recently described APC-independent ability to directly inhibit prothrombinase and tenase by direct binding of activated factors V, VIII, and X. Given that PS is one of the major naturally occurring inhibitors of coagulation, acquired or hereditary deficiencies of this protein result in excessive thrombin generation. As a vast array of mutations are responsible for hereditary PS deficiencies, screening for their presence by DNA testing would require sequencing each entire gene involving numerous exons. Moreover, the knowledge of the gene mutation does not offer any benefit in the treatment of thrombophilic families, so the routine molecular characterization is not indicative. These defects are detected by functional or immunological assays for free and total PS forms. Given that functional PS assays may detect some forms of PS deficiency that free PS immunoassays may miss, it is recommended to include them for initial testing along with immunoassays for free PS, although they should be used with caution. Functional PS assays are subject to multiple interference. For example in the presence of lupus anticoagulant (LA), only free PS immunoassays are recommended for initial testing. PS antigen assays are more popular with most laboratories.

TFPI resistance related to inherited or acquired protein S deficiency

BACKGROUND

Protein S (PS) is an essential component of the protein C pathway and PS deficiency can explain a poor response to activated protein C. It has recently been shown that PS also acts as a cofactor of Tissue Factor Pathway Inhibitor (TFPI).

OBJECTIVES

In the present study, we investigated whether PS deficiency could be responsible for a poor response to TFPI.

PATIENTS/METHODS

Thirty-one patients with inherited PS deficiency, seven pregnant women and 36 controls were enrolled in the study. We measured the plasma response to added TFPI using a two-step diluted prothrombin time (dPT) assay. The response of the different plasmas to the anticoagulant activity of TFPI was expressed as TFPI Normalised Ratio (TFPI NR).

RESULTS

The median TFPI NR was statistically significantly lower in patients with inherited PS deficiency (0.5) than in controls (1.0) (p<0.0001). It was statistically significantly lower in patients with type I inherited PS deficiency (0.47) compared to patients with type III inherited PS deficiency (0.58) (p=0.018). In contrast, it did not differ between patients with and without thrombosis. Median TFPI NR values were statistically significantly lower during pregnancy (0.54) than 3 months after delivery (0.71) (p=0.016). TFPI NR values correlated well with PS activity values (R(2)=0.681) whatever the nature of the PS deficiency.

CONCLUSIONS

Our findings confirm that PS deficiency results in a poor anticoagulant response to TFPI, demonstrating again the cofactor role of PS in TFPI activity.

Inhibition of Intrinsic Xase by Protein S

Objective—

Protein S is a vitamin K–dependent plasma protein that functions in the feedback regulation of thrombin generation. Our goal was to determine how protein S regulates the intrinsic pathway of blood coagulation.

Methods and Results—

We used plasma, including platelet-rich plasma, and in vitro methods to determine how the intrinsic pathway of blood coagulation is regulated by protein S. We obtained the following results: (1) activated partial thromboplastin time assays with protein S–supplemented plasma confirmed that protein S prolongs clotting time; (2) a modified activated partial thromboplastin time assay with factor IX (fIX)–deficient plasma confirmed that protein S affects fIX-initiated clotting; (3) a fIXa/factor VIIIa (fVIIIa)–mediated thrombin generation assay with either platelet-rich plasma or factor-deficient plasma, initiated with a limiting amount of tissue factor, was regulated by protein S; (4) in the presence of phosphatidylserine vesicles, protein S inhibited fIXa in the absence and presence of fVIIIa; and (5) protein S altered only the K M for factor X activation by fIXa in the absence of fVIIIa and both k cat and K M in the presence of fVIIIa.

Conclusion—

From our findings, it can be concluded that protein S inhibits fIXa in the presence or absence of fVIIIa in an activated protein C–independent way.

Small and large PROS1 deletions but no other types of rearrangements detected in patients with protein S deficiency

Protein S deficiency is a dominantly inherited disorder that results from mutations in the PROS1 gene. Previous sequencing of the gene failed to detect mutations in eight out of 18 investigated Swedish families, whereas segregation analyses detected large deletions in three out of the eight families. The present study investigates more thoroughly for the presence of deletions but also for other types of rearrangements. FISH analysis confirmed the existence of the three previously identified large deletions, but failed to identify any other type of rearrangement among the eight analysed families. MLPA analysis of the PROS1 gene revealed two smaller deletions covering two and four exons, respectively. Thus, deletions could be found in five out of eight families where no point mutations could be found despite sequencing of the gene. Twelve additional, not previously analysed, families were subsequently analysed using MLPA. The analysis identified two smaller deletions (3 and 4 exons). Including all PS-deficient families, i.e. also the 10 families where sequencing found a causative point mutation, deletions were identified in seven out of 30 PS-deficient families. A strategy of sequencing followed by MLPA analysis in mutation-negative families identified the causative mutation in 15 out of 18 of Swedish PS-deficient families. Most deletions were different as determined by their sizes, locations and flanking haplotypes. FISH (8 families) and MLPA analysis (20 families) failed to identify other types of rearrangements.

Free protein S level as a risk factor for coronary heart disease and stroke in a prospective cohort study of healthy United Kingdom men

Plasma protein S (PS) levels are reportedly low in patients with venous thrombosis but high in coronary heart disease (CHD) patients. The authors examined the association between free PS concentration and CHD or stroke risk and assessed risk in combination with C-reactive protein (CRP) levels. Free PS concentration was determined in 6 annual visits among 3,052 middle-aged (49-64 years) United Kingdom men from the Second Northwick Park Heart Study, with 297 CHD events from 1989 to 2005. The highest (vs. first) quintile was associated with a significantly increased CHD risk after adjustment for all other risk factors and correction for regression dilution bias (hazard ratio = 1.85, 95% confidence interval: 1.08, 3.16; P = 0.024). Models that included all well-known risk factors plus PS quintiles improved prediction of CHD (net reclassification improvement (NRI) = 7.0% (P = 0.007), category-less NRI (>0) = 22.1% (P < 0.001)), and the likelihood ratio statistic increased significantly (P = 0.018). The increase in CHD risk was particularly strong when subjects also had high CRP levels. There was no association between free PS level and stroke risk. This study confirms the independent association of elevated free PS levels with future risk of CHD, although elevated PS levels added only modestly to prediction metrics. The novel finding of increased CHD risk, particularly when CRP and PS levels are high, requires further study.

Activated protein C cofactor function of protein S: a novel role for a γ-carboxyglutamic acid residue

Protein S has an important anticoagulant function by acting as a cofactor for activated protein C (APC). We recently reported that the EGF1 domain residue Asp95 is critical for APC cofactor function. In the present study, we examined whether additional interaction sites within the Gla domain of protein S might contribute to its APC cofactor function. We examined 4 residues, composing the previously reported "Face1" (N33S/P35T/E36A/Y39V) variant, as single point substitutions. Of these protein S variants, protein S E36A was found to be almost completely inactive using calibrated automated thrombography. In factor Va inactivation assays, protein S E36A had 89% reduced cofactor activity compared with wild-type protein S and was almost completely inactive in factor VIIIa inactivation; phospholipid binding was, however, normal. Glu36 lies outside the ω-loop that mediates Ca(2+)-dependent phospholipid binding. Using mass spectrometry, it was nevertheless confirmed that Glu36 is γ-carboxylated. Our finding that Gla36 is important for APC cofactor function, but not for phospholipid binding, defines a novel function (other than Ca(2+) coordination/phospholipid binding) for a Gla residue in vitamin K-dependent proteins. It also suggests that residues within the Gla and EGF1 domains of protein S act cooperatively for its APC cofactor function.

Thrombophilia screening--at the right time, for the right patient, with a good reason

Thrombophilia can be identified in about half of all patients presenting with venous thromboembolism (VTE). Thrombophilia screening for various indications has increased tremendously, but whether the results of such tests help in the clinical management of patients is uncertain. Here, current recommendations for thrombophilia screening in selected groups of patients, and considerations whether other high-risk subjects should be tested are reviewed. The methods for determination of the most common thrombophilic defects (antithrombin, protein C, protein S deficiencies, Factor V Leiden and prothrombin G20210A) associated with strong to moderate risk of VTE are described, indicating the timing and location of thrombophilia screening. Circumstances when a positive result of thrombophilia screening helps clinicians decide if adjustments of the anticoagulant regime are needed are discussed. Finally, psychological, social and ethical dilemmas associated with thrombophilia screening are indicated.

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.

Genetic polymorphisms in venous thrombosis and pulmonary embolism after total hip arthroplasty: a pilot study

Deep venous thrombosis (DVT) after major orthopaedic surgery is a substantial concern. We asked whether the single or combined presence of thrombophilic genetic polymorphisms might further increase the already high risk for venous thrombosis and pulmonary embolism (PE) after THA. We therefore compared the prevalence of factor V Leiden, prothrombin G20210A, methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C, and plasminogen activator inhibitor 4G/5G polymorphisms between 50 patients with symptomatic DVT within 3 weeks after elective THA and an asymptomatic control group of 85 patients. We found no major difference for the presence of a single mutation between the groups. Factor V Leiden and homozygous MTHFR C667T mutations were of borderline significance with odds ratios (95% confidence intervals) of 3.73 (0.89-15.63) and 2.93 (0.92-9.29), respectively. Patients with homozygous or combined heterozygous status of MTHFR C677T and A1298C mutation had a higher frequency of DVT after elective THA (odds ratio, 2.86; 95% confidence interval, 1.32-6.35) than those with wild-type. The presence of a single mutation may not further increase the already high risk for symptomatic DVT after THA, whereas combinations of mutations of distinct polymorphisms might be important. However, prospective studies with a larger number of patients are needed before we would recommend preoperative screening.

LEVEL OF EVIDENCE

Level III, prognostic study. See the Guidelines for Authors for a complete description of levels of evidence.

Advances in understanding pathogenic mechanisms of thrombophilic disorders

Venous thromboembolism is a major medical problem, annually affecting 1 in 1000 individuals. It is a typical multifactorial disease, involving both genetic and circumstantial risk factors that affect a delicate balance between procoagulant and anticoagulant forces. In the last 50 years, the molecular basis of blood coagulation and the anticoagulant systems that control it have been elucidated. This has laid the foundation for discoveries of both common and rare genetic traits that tip the natural balance in favor of coagulation, with a resulting lifelong increased risk of venous thrombosis. Multiple mutations in the genes for anticoagulant proteins such as antithrombin, protein C, and protein S have been identified and constitute important risk factors. Two single mutations in the genes for coagulation factor V (FV Leiden) and prothrombin (20210G>A), resulting from approximately 20,000-year-old mutations with subsequent founder effects, are common in the general population and constitute major genetic risk factors for thrombosis. In celebration of the 50-year anniversary of the American Society of Hematology, this invited review highlights discoveries that have contributed to our present understanding of the systems that control blood coagulation and the genetic factors that are involved in the pathogenesis of venous thrombosis.

Autoantibodies against endothelial protein C receptor and the risk of a first deep vein thrombosis

BACKGROUND

The endothelial protein C receptor (EPCR) binds protein C and enhances its activation. Anti-EPCR autoantibodies are found in patients with antiphospholipid syndrome and may explain the increased risk of thrombosis in these patients. Anti-EPCR autoantibodies have been associated with fetal death and myocardial infarction in young women.

OBJECTIVES

To determine whether anti-EPCR autoantibodies are associated with deep vein thrombosis (DVT).

PATIENTS/METHODS

We measured plasma anti-EPCR autoantibody levels in the Leiden Thrombophilia Study (LETS), a population-based case-control study consisting of 474 patients with a first DVT and 474 control subjects.

RESULTS

The estimated risk of DVT was increased approximately 2-fold in the presence of elevated IgA, IgG or IgM anti-EPCR autoantibodies (i.e. levels above the 90th percentile as measured in the control subjects). The risk conferred by anti-EPCR increased in a dose-dependent manner for IgA and IgG. When anti-EPCR autoantibodies were considered in the co-presence of lupus anticoagulant (LAC) the odds ratio (OR) was 6.1 [95% CI 1.3-27.9]. Anti-EPCR without LAC remained associated with DVT (OR 1.6; 95% CI 1.2-2.1). Anti-EPCR autoantibodies were associated with high levels of D-dimer and soluble EPCR in controls, suggestive of a prothrombotic status induced by the autoantibodies.

CONCLUSIONS

This study demonstrates that the presence of anti-EPCR autoantibodies is a moderate risk factor for DVT in the general population.

Effects of oral and transvaginal ethinyl estradiol on hemostatic factors and hepatic proteins in a randomized, crossover study

CONTEXT

The use of combined hormonal contraceptives with ethinyl estradiol (EE) and a progestin results in alterations in potential biomarkers of venous thromboembolism risk. Evaluation of the impact of delivery route on these changes is difficult due to an interaction between EE and the progestin component.

OBJECTIVE

The aim of the study was to compare the impact of oral and vaginal administration of EE alone on hemostatic variables and estrogen-sensitive liver proteins.

DESIGN

This was a single-center, randomized, crossover study with two treatment cycles separated by a washout cycle.

SETTING

The study was conducted in an academic outpatient center.

PARTICIPANTS

Fourteen healthy postmenopausal women were enrolled; 13 completed the study and were included in the analyses.

INTERVENTION

Participants were randomized to receive EE (15 microg/d) delivered by oral tablet or vaginal ring for 21 d in one of two treatment sequences.

MAIN OUTCOME MEASURES

Changes in plasma concentration or activity of 10 hemostatic variables and six estrogen-sensitive liver proteins between baseline and d 21 of treatment were the primary outcomes.

RESULTS

Prothrombin fragment 1 + 2 plasma level was unaffected by treatment or delivery route. Angiotensinogen (expressed as plasma level of angiotensin I) increased similarly with oral and vaginal delivery; mean (sd) increases were 2757 (1033) and 2864 (893) ng /ml, respectively (P = 0.0002). Alterations in other study variables, except total cholesterol, were similar with oral and vaginal administration.

CONCLUSION

Our results provide evidence that the customary effects of combined hormonal contraceptives on hemostatic variables and estrogen-sensitive liver proteins are largely related to EE and independent of delivery route during short-term treatment.

Hereditary thrombophilia

Thrombophilia can be defined as a predisposition to form clots inappropriately. Thrombotic events during infancy and childhood are increasingly recognized as a significant source of mortality and morbidity. The predisposition to form clots can arise from genetic factors, acquired changes in the clotting mechanism, or, more commonly, an interaction between genetic and acquired factors. Since the turn of the last century, there has been extensive research focusing on both the genetic and acquired causes of thrombophilia, with particular focus on clotting events in the venous circulation. This review describes clinically relevant aspects of genetic venous thrombophilia, which include well-established, lesser known, and suggested causes of inherited thrombophilias.

Thrombophilias—Practical Implications and Testing Caveats

This review summarizes recent information about the major thrombophilic conditions, their clinical relevance, and practical aspects pertaining to testing for these thrombophilias, such as when to test and what assays are appropriate. Conditions covered include factor V Leiden, prothrombin 20210 mutation, proteins C and S, antithrombin, antiphospholipid antibodies, homocysteine, and methylene-tetrahydrofolate-reductase enzyme mutation. Additional comments focus on education of patients and educational resources for patients, such as the National Alliance for Thrombosis and Thrombophilia (www.nattinfo.org).

Difference in absolute risk of venous and arterial thrombosis between familial protein S deficiency type I and type III. Results from a family cohort study to assess the clinical impact of a laboratory test-based classification

Hereditary protein S (PS) deficiency type I is an established risk factor for venous thromboembolism. Contradictionary data on type III deficiency suggests a difference in risk between both types. We studied 156 first degree relatives (90% of eligible relatives) from type I deficient probands (cohort 1) and 268 (88%) from type III deficient probands (cohort 2) to determine the absolute risk of venous and arterial thromboembolism. Annual incidences of venous thromboembolism were 1.47 and 0.17 per 100 person-years in deficient and non-deficient relatives in cohort 1 [relative risk (RR) 8.9; 95% confidence interval (CI) 2.6-30.0], and 0.27 vs. 0.24 in cohort 2 (RR 0.9; 95% CI 0.4-2.2). Type III deficiency was demonstrated in 20% of non-deficient relatives in cohort 1 and the annual incidence in this subgroup was 0.70 (RR 4.3;0.95-19.0). The cut-off level of free PS to identify subjects at risk was 30%, the lower limit of its normal range (65%). PS deficiency was not a risk factor for arterial thromboembolism. In conclusion, type I deficiency was found to be a strong risk factor for venous thromboembolism, in contrast with type III deficiency. This was because of lower free PS levels in type I deficient subjects and a free PS cut-off level far below the lower limit of its normal range.

The γ-carboxyglutamic acid domain of anticoagulant protein S is involved in activated protein C cofactor activity, independently of phospholipid binding

We expressed 2 chimeras between human protein S (PS) and human prothrombin (FII) in which the prothrombin γ-carboxyglutamic acid (Gla) domain replaced the PS Gla domain in native PS (GlaFII-PS) or in PS deleted of the thrombin-sensitive region (TSR) (GlaFII-ΔTSR-PS). Neither PS/FII chimera had activated protein C (APC) cofactor activity in plasma clotting assays or purified systems, but both bound efficiently to phospholipids. This pointed to a direct involvement of the PS Gla domain in APC cofactor activity through molecular interaction with APC. Using computational methods, we identified 2 opposite faces of solvent-exposed residues on the PS Gla domain (designated faces 1 and 2) as potentially involved in this interaction. Their importance was supported by functional characterization of a PS mutant in which the face 1 and face 2 PS residues were reintroduced into GlaFII-PS, leading to significant APC cofactor activity, likely through restored interaction with APC. Furthermore, by characterizing PS mutants in which PS face 1 and PS face 2 were individually replaced by the corresponding prothrombin faces, we found that face 1 was necessary for efficient phospholipid binding but that face 2 residues were not strictly required for phospholipid binding and were involved in the interaction with APC.

Molecular diversity and thrombotic risk in protein S deficiency: The PROSIT study

The Protein S Italian Team (PROSIT) enrolled 79 protein S (PS) deficient families and found 38 PROS1 variations (19 novel) in 53 probands. Of these, 23 variants were selected for expression in'vitro, to evaluate their role as possible causative variants. Transient expression showed high secretion levels (>75%) for three variants, which were considered neutral. Seven missense and five nonsense variants showed low (<or=11%) expression levels and were classified as severe defects. Intermediate expression was observed for eight variants, which were evaluated by factor Va inactivation assay in order to be globally classified as severe or intermediate. Based on the cumulative data, the hazard ratio associated with causative variants was 4.9 (95% CI: 1.4-17.7) for deep vein thrombosis and/or pulmonary embolism, 5.1 (95% CI: 1.1-23.9) for superficial thrombophlebitis, and 4.8 (95% CI: 1.8-13.0) for any venous thrombosis. The hazard ratio for deep vein thrombosis and/or pulmonary embolism in carriers of severe defects only was 7.4 (95% CI: 1.6-24.1). PROSIT showed that dysfunctional variants causing PS deficiency are more common than expected and confirmed that PS deficiency is associated with increased thrombotic risk, although risk assessment is complicated by molecular heterogeneity.

Normal Functional Protein S Activity Does Not Exclude Protein S Deficiency

Protein S (PS) deficiency appears to increase the risk of venous thrombosis. PS deficiency is classified into three phenotypes using antigenic levels and functional activity. By definition, all three phenotypes of PS deficiency should result in low activated protein C cofactor activity. We compared the results of functional PS activity testing to free antigenic PS testing in order to determine if a normal functional PS activity assay result could eliminate the need for free antigenic PS testing. The sensitivity of the functional assay is 45.5% (95% confidence interval, CI, 36-55%), specificity 95.3% (95% CI 93-97%), negative predictive value 88.6% (95% CI 86-91%) with a positive predictive value of 68.5% (95% CI 57-79%). In conclusion, a normal functional PS activity result does not exclude free antigenic PS deficiency. Functional PS activity testing should not be used as a screening test to eliminate free antigenic PS testing for the laboratory diagnosis of PS deficiency.

Coagulation, inflammation, and apoptosis: different roles for protein S and the protein S-C4b binding protein complex

Protein S (PS) has an established role as an important cofactor to activated protein C (APC) in the degradation of coagulation cofactors Va and VIIIa. This anticoagulant role is evident from the consequences of its deficiency, when there is an increased risk of venous thromboembolism. In human plasma, PS circulates approximately 40% as free PS (FPS) and 60% in complex with C4b-binding protein (C4BP). Formation of this complex results in loss of PS cofactor function, and C4BP can then modulate the anticoagulant activity of APC. It had long been predicted that the complex could act as a bridge between coagulation and inflammation due to the involvement of C4BP in regulating complement activation. This prediction was recently supported by the demonstration of binding of the PS-C4BP complex to apoptotic cells. This review aims to summarize recent findings on the structure and functions of PS, the basis and importance of its deficiency, its interaction with C4BP, and the possible physiologic and pathologic importance of the PS-C4BP interaction.

Hypercoagulable state testing and malignancy screening following venous thromboembolic events

Mounting interest in hypercoagulability, increased availability of hypercoagulable state test 'panels' and enhanced ability to identify abnormalities in tested patients have prompted widespread testing. Testing for acquired and inherited hypercoagulable states uncovers an abnormality in over 50% of patients presenting with an initial venous thromboembolic event (VTE) but may have minimal actual impact on management in most of these patients. Such laboratory screening should be reserved for patients in whom the results of individual tests will significantly impact the choice of anticoagulant agent, intensity of anticoagulant therapy, therapeutic monitoring, family screening, family planning, prognosis determination, and most of all duration of therapy. Testing 'just to know' is neither cost-effective nor clinically appropriate. The most important testing in patients following acute VTE may be age- and gender-specific cancer screening. Cancer screening following VTE seems most prudent in older individuals and in those with idiopathic VTE and no laboratory evidence for an inherited hypercoagulable state. Cancer screening should focus on identification of treatable cancers and those where diagnosis in an early stage favorably impacts patient survival. Extensive searches for occult malignancy employing whole-body computed tomography and serum tumor markers may identify more cancers but without affecting patient outcome. We advocate that physicians should focus their attention more on VTE prophylaxis and proper treatment and less on costly and, at times, invasive testing of questionable value.

Prothrombin G20210A mutation, antithrombin, heparin cofactor II, protein C, and protein S defects

These defects are not as common as factor V Leiden, but they are more common than many other hereditary procoagulant defects. The incidence of the prothrombin gene (G20210A) mutation is not yet known with certainty, but it may approach or even exceed that of factor V Leiden. These defects also seem less common than hereditary sticky platelet syndrome; however, they are all common enough that they always should be considered in any individual with unexplained thrombosis and should be part of the work-up for patients with thrombotic disorders. Of the defects discussed herein, prothrombin G20210A mutation seems, thus far, to be more common than AT, protein C, protein S, or HC-II defects. Assessment of prothrombin gene mutation should be part of the primary evaluation of patients with unexplained thrombosis.

Measuring the outcomes and pharmacoeconomic consequences of venous thromboembolism prophylaxis in major orthopaedic surgery

Patients who have undergone orthopaedic surgery represent a high-risk group for venous thromboembolism (VTE). Despite the routine prophylactic use of antithrombotic agents, patients still experience thrombotic events that can result in mortality and acute morbidity and, in significant numbers of patients, may lead to long-term consequences such as the post-thrombotic syndrome. Increasingly, initial VTE events occur after hospital discharge since the length of stay in hospital after major orthopaedic surgery has decreased in many countries. There is a need for further improvement in the prevention of VTE. As well as undergoing extensive safety and efficacy studies for registration purposes, new prophylactic strategies need to be evaluated from a pharmacoeconomic perspective to help guide their introduction into routine clinical practice. Over the past 15 years a number of pharmacoeconomic evaluation studies of VTE prophylaxis have been carried out in orthopaedic patients, most of which evaluated short-term clinical endpoints. During the same period, improvements in our understanding of the natural history of VTE as well as the emergence of new treatments have led to changes in the management of patients with, or at risk of, VTE. The aim of this paper was to address how best to conduct pharmacoeconomic analyses of new antithrombotic agents in light of changes in practice patterns for orthopaedic patients and greater understanding of the disease process. We put forward recommendations for relevant outcome measures, timeframes, endpoints and epidemiologic data sources. We also suggest a structure for a pharmacoeconomic model. In this model, the outcomes and costs of VTE-related care during both the acute and chronic phases of the disease are incorporated. Symptomatic deep vein thrombosis and pulmonary embolism, recurrent VTE, post-thrombotic syndrome, major hemorrhage and all-cause death are included. We also recommend that the relevance of quality-adjusted survival is investigated, and that economic appraisals are presented in both cost-consequence and budget-impact approaches. These recommendations are based on extensive examination of recent advances in the management of VTE combined with a greater understanding of its natural history.

Superior mesenteric venous thrombosis associated with a familial missense mutation (Pro626Leu) in the SHBG-like domain of the protein S molecule

A 76-year-old Japanese woman was hospitalized for ileus symptoms caused by extensive thrombosis of the superior mesenteric vein. Because laboratory test results suggested type III protein S (PS) deficiency, molecular changes in PS were investigated. A single-base transition, CCG to CTG at codon 626 in exon XV, resulting in the missense mutation Pro626Leu, was identified in an allele of the patient and in her son. Reverse transcriptase polymerase chain reaction analysis indicated the presence of both normal and mutant types of PS messages in platelet-derived messenger RNAs. Our findings thus suggest that Pro626 in SHBG-like domain 7 may be crucial for in vivo antithrombotic activity of the PS molecule.

Implication of protein S thrombin-sensitive region with membrane binding via conformational changes in the gamma-carboxyglutamic acid-rich domain

In the vitamin K-dependent protein family, only protein S (PS) contains a thrombin-sensitive region (TSR), located between the domain containing the gamma-carboxyglutamic acid and the first epidermal growth factor-like domain. To better define the role of TSR in the PS molecule, we expressed a recombinant human PS (rHPS) and its analogue lacking TSR (rTSR-less), and prepared factor Xa- and thrombin-cleaved rHPS. A peptide reproducing TSR (TSR-peptide) was also synthesized in an attempt to obtain direct evidence of the domain involvement in PS anticoagulant activity. In a coagulation assay, both rTSR-less and factor Xa-cleaved PS were devoid of activated protein C cofactor activity. The TSR-peptide did not inhibit rHPS activity, showing that TSR must be embedded in the native protein to promote interaction with activated protein C. The binding of rHPS to activated platelets and to phospholipid vesicles was not modified after factor Xa- or thrombin-mediated TSR cleavage, whereas the binding of rTSR-less was markedly reduced. This suggested a role for TSR in conferring to PS a strong affinity for phospholipid membranes. TSR-peptide did not directly bind to activated platelets or compete with rHPS for phospholipid binding. The results of the present study show that TSR may not interact directly with membranes, but probably constrains the gamma-carboxyglutamic acid-rich domain in a conformation allowing optimal interaction with phospholipids.

Laboratory Investigation of Thrombophilia

Until recently, laboratory diagnosis of thrombophilia was based on investigation of the plasmatic anticoagulant pathways to detect antithrombin, protein C, and protein S deficiencies and on the search for dysfibrinogenemia and anti-phospholipid antibodies/lupus anticoagulants. More recently, laboratory investigations have been expanded to include activated protein C (APC) resistance, attributable or not to the presence of the factor V Leiden mutation; hyperprothrombinemia attributable to the presence of the prothrombin gene mutation G20210A; and hyperhomocysteinemia attributable to impairment of the relevant metabolic pathway because of enzymatic and/or vitamin deficiencies. All of the above are established congenital or acquired conditions associated with an increased risk of venous and, more rarely, arterial thrombosis.

Testing is recommended for patients who have a history of venous thrombosis and should be extended to their first-degree family members. Because most of the tests are not reliable during anticoagulation, it is preferable to postpone laboratory testing until after discontinuation of treatment.

Whenever possible, testing should be performed by means of functional assays. DNA analysis is required for the prothrombin gene mutation G20210A. Laboratory diagnosis for anti-phospholipid antibodies/lupus anticoagulant should be performed by a combination of tests, including phospholipid-dependent clotting assays and solid-phase anti-cardiolipin antibodies. Hyperhomocysteinemia can be diagnosed by HPLC methods or by fluorescence polarization immunoassays.

Thrombosis in the intensive care unit: etiology, diagnosis, management, and prevention in adults and children

Venous thromboembolism, a well-recognized complication in postoperative patients, is emerging as a frequent complication in critically ill patients in intensive care units. Diagnosis can be particularly difficult in such patients because underlying systemic illnesses may mask common presenting signs and symptoms. Although numerous independent risk factors have been identified, the critical role of both central venous catheters and prothrombotic disorders as significant risk factors is a common theme in the pediatric and adult literature. Various diagnostic tests exist, with venography remaining the gold standard and newer, less invasive methods such as ultrasonography and impedance plethysmography becoming increasingly popular. Standard unfractionated heparin remains the mainstay of therapy and prophylaxis, although the use of low molecular weight heparins is becoming more commonplace. Thrombolytic therapy continues to be reserved for severe, life-threatening, acute thrombosis. In this article, we review the common risk factors, diagnostic modalities, and treatment options for venous thromboembolism in critically ill adult and pediatric patients.

Clinical evaluation of hypercoagulable states

Although thromboembolic disease associated with primary thrombophilic conditions most often presents before age 50 years, older adults can be affected by such disorders. This article addresses congenital and acquired prothrombotic states, with specific attention to the likelihood of such disorders occurring in the geriatric patient population. Recommendations for testing and therapy are reviewed.

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa–degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits.

Identification of a Drosophila vitamin K-dependent gamma-glutamyl carboxylase

Using reduced vitamin K, oxygen, and carbon dioxide, gamma-glutamyl carboxylase post-translationally modifies certain glutamates by adding carbon dioxide to the gamma position of those amino acids. In vertebrates, the modification of glutamate residues of target proteins is facilitated by an interaction between a propeptide present on target proteins and the gamma-glutamyl carboxylase. Previously, the gastropod Conus was the only known invertebrate with a demonstrated vitamin K-dependent carboxylase. We report here the discovery of a gamma-glutamyl carboxylase in Drosophila. This Drosophila enzyme is remarkably similar in amino acid sequence to the known mammalian carboxylases; it has 33% sequence identity and 45% sequence similarity to human gamma-glutamyl carboxylase. The Drosophila carboxylase is vitamin K-dependent, and it has a K(m) toward a model pentapeptide substrate, FLEEL, of about 4 mm. However, unlike the human gamma-glutamyl carboxylase, it is not stimulated by human blood coagulation factor IX propeptides. We found the mRNA for Drosophila gamma-glutamyl carboxylase in virtually every embryonic and adult stage that we investigated, with the highest concentration evident in the adult head.

Genetic analysis, phenotypic diagnosis, and risk of venous thrombosis in families with inherited deficiencies of protein S

Protein S deficiency is a recognized risk factor for venous thrombosis. Of all the inherited thrombophilic conditions, it remains the most difficult to diagnose because of phenotypic variability, which can lead to inconclusive results. We have overcome this problem by studying a cohort of patients from a single center where the diagnosis was confirmed at the genetic level. Twenty-eight index patients with protein S deficiency and a PROS1 gene defect were studied, together with 109 first-degree relatives. To avoid selection bias, we confined analysis of total and free protein S levels and thrombotic risk to the patients' relatives. In this group of relatives, a low free protein S level was the most reliable predictor of a PROS1gene defect (sensitivity 97.7%, specificity 100%). First-degree relatives with a PROS1 gene defect had a 5.0-fold higher risk of thrombosis (95% confidence interval, 1.5-16.8) than those with a normal PROS1 gene and no other recognized thrombophilic defect. Although pregnancy/puerperium and immobility/trauma were important precipitating factors for thrombosis, almost half of the events were spontaneous. Relatives with splice-site or major structural defects in the PROS1 gene were more likely to have had a thrombotic event and had significantly lower total and free protein S levels than those relatives having missense mutations. We conclude that persons withPROS1 gene defects and protein S deficiency are at increased risk of thrombosis and that free protein S estimation offers the most reliable way of diagnosing the deficiency.

Regulation of blood coagulation

The protein C anticoagulant pathway converts the coagulation signal generated by thrombin into an anticoagulant response through the activation of protein C by the thrombin-thrombomodulin (TM) complex. The activated protein C (APC) thus formed interacts with protein S to inactivate two critical coagulation cofactors, factors Va and VIIIa, thereby dampening further thrombin generation. The proposed mechanisms by which TM switches the specificity of thrombin include conformational changes in thrombin, blocking access of normal substrates to thrombin and providing a binding site for protein C. The function of protein S appears to be to alter the cleavage site preferences of APC in factor Va, probably by changing the distance of the active site of APC relative to the membrane surface. The clinical relevance of this pathway is now established through the identification of deficient individuals with severe thrombotic complications and through the analysis of families with partial deficiencies in these components and an increased thrombotic tendency. One possible reason that even partial deficiencies are a thrombotic risk is that the function of the pathway can be down-regulated by inflammatory mediators. For instance, clinical studies have shown that the extent to which protein C levels decrease in patients with septic shock is predictive of a negative outcome. Initial clinical studies suggest that supplementation with protein C may be useful in the treatment of acute inflammatory diseases such as sepsis.