Further evidence that activated protein C resistance can be misdiagnosed as inherited functional protein S deficiency

In vitro diagnostics for the medical dermatologist. Part II: Hypercoagulability tests

The skin often provides initial clues of hypercoagulability with features such as livedo reticularis, livedo racemosa, retiform purpura, necrosis, and ulcerations. Because these cutaneous manifestations are nonspecific, laboratory testing is often needed to evaluate for underlying causes of hypercoagulability. Importantly, these disorders are reported to be the most common mimicker, resulting in an erroneous diagnosis of pyoderma gangrenosum. Understanding inherent properties of, and indications for, available tests is necessary for appropriate ordering and interpretation of results. Additionally, ordering of these tests in an indiscriminate manner may lead to inaccurate results, complicating the interpretation and approach to management. This second article in this continuing medical education series summarizes information on methodology, test characteristics, and limitations of several in vitro laboratory tests used for the work up of hypercoagulability and vasculopathic disease as it pertains to dermatologic disease.

Different Anticoagulant Response to Activated Protein C (APC test) and to Agkistrodon Contortix Venom (ACV test) in a Family with FV-R506Q Substitution

To identify the defect(s) responsible for the thrombotic condition affecting a 55-year-old male and his family, we have utilized a new methodological approach (ProC Global®, Istituto Behring, Milan, Italy) to screen the global anticoagulant activity of the protein C pathway, a defect that accounts for the majority of inherited thrombophilias. The test is based on the activation of endogenous protein C in plasma by Protac®, derived from Agkistrodon contortix snake venom (ACV test). Nineteen members of the family were investigated, 11 showed low responsiveness to ACV (normalized ACV ratios < 0.66; normal > 1. 12); in these individuals specific assays of protein C (PC) and protein S (PS) levels and normalized activated protein C ratios (n-APC-r) were performed. A second test evaluating response to APC, using the classic commercial APC test (n-APC-r 1), detected only 10 subjects with abnormal responses : the propositus and two members of the family with n-APC-r 1 values < 0.54, indicating the homozygous state for the R506Q factor V gene mutation, and seven with values ranging 0.69-0.83, consistent with the heterozygous condition (normal > 0.85). Although only ten subjects presented with low n-APC-r 1 values, DNA analysis, in agreement with the ACV test, detected 11 individual with factor V-R506Q substitution (two homozygotes and nine heterozygotes). Thus the classical APC test failed to identify the APC resistance phenotype in two heterozygous subjects whose values were clearly normal (1.05) in the first case and homozygous (0.53) in the second. The ACV test, however, and the modified APC test with test plasma 1/5 diluted in factor V-deficient plasma (n-APC-r 2) completely matched the DNA analysis. A phenotype/genotype correlation was observed in dilutions higher than 1/3 test plasma factor V-deficient plasma. The presence of unknown mechanisms that influence plasma response to exogenous preformed APC (normal at high factor V-deficient plasma dilutions) but not endogenous ACV activated PC was suspected. The suspected low levels of proteins C and S found in several R506Q members of the family were excluded by reassaying the anticoagulant activities at higher plasma dilution ; this supports the known influence of factor V Leiden on functional PC and PS clotting activity. We conclude that the ACV test is appropriate to evaluate the APC resistance condition, but for a firm diagnosis DNA analysis together with the modified APC test are strongly advised even in the presence of unquestionable APC-r values. Key Words: APC resistance-Factor V Leiden-APC test-ACV test-Diagnosis-Inherited thrombophilia.

Interference of factor V Leiden on protein S activity: evaluation of a new prothrombin time-based assay

Protein S activity in plasma from factor V Leiden (FVL)-positive patients may be lower than expected. We investigated a new commercially available method for protein S for such interference. Protein S activity was measured for plasmas from 50 individuals with FVL and their results were compared with those obtained for plasmas from 47 sex-matched and age-matched individuals without FVL. We assumed that the median protein S activity value from a relatively large number of individuals with or without FVL would not be significantly different if there is no influence from FVL. The FVL-positive plasmas gave relatively (albeit not significantly) lower protein S levels than FVL-negative plasmas when both were tested undiluted (86 versus 93 IU/dl, P = 0.06). Those differences were reduced (98 versus 102 IU/dl, P = 0.58) when testing was performed on diluted plasmas. Furthermore, the proportion of patients with FVL identified as low-abnormal on the basis of the specific cut-off values (undiluted = 64 U/dl; diluted = 71 IU/dl), which was 8% when testing was performed on undiluted plasmas, was reduced to 4% when testing was performed on diluted plasmas. Conversely, the corresponding proportions of patients without FVL remained unaltered (4.3 versus 4%). In conclusion, these results indicate that the evaluated method is somewhat affected by FVL and that dilution of plasma prior to testing improves specificity. Protein S activity measurement for FVL-positive patients should be performed on diluted plasma and the results interpreted on the basis of the cut-off value specifically determined for diluted plasmas.

Quality Assurance Issues and Interpretation of Assays

The consequences of an erroneous thrombophilia diagnosis may be serious if it is used to determine clinical management. Therefore careful selection, assessment, and control of laboratory tests for thrombophilia are essential. As for other coagulation tests, the pre-analytical phase must be carefully controlled with attention to the specific problems associated with each type of assay. The investigator must then recognize that for most laboratory tests of thrombophilia, there are a number of assay types available, often based on different principles of analysis. This creates the potential for different users to obtain varying results depending on the technique employed. Such problems can occur in assays of antithrombin activity, depending on whether the assay employs factor Xa, human thrombin, or bovine thrombin. In clot-based assays of protein C and protein S, there can be specificity problems related to interference by factor V Leiden (FVL), antiphospholipid antibodies, and other substances. Even genetic tests can give erroneous results and should not automatically be seen as absolute without supporting evidence and careful quality-control measures. Whatever technique is selected, it is mandatory to incorporate sufficient concurrent quality-control samples to validate the results of thrombophilia tests. These should include assessment of the parameter at normal and abnormal levels to give confidence in results across the measurement range that would normally be encountered in routine practice. This should be used in conjunction with regular participation in external quality assessment (EQA) (which has been linked to improved laboratory performance in thrombophilia testing). Larger EQA programs can provide information concerning the relative performance of analytical procedures, including the method principle, reagents, and instruments. Herein, we describe many of the methodologic effects in detail. We use specific examples to illustrate the general principle that, in performing laboratory testing for thrombophilia, one must always consider the performance characteristics and limitations of the assay in use.

Proteolysis of protein C in pooled normal plasma and purified protein C by activated protein C (APC)

Protein C is a vitamin-K dependent zymogen of the anti-coagulant serine protease activated protein C (APC). In this paper, we report four lines of evidence that APC can activate protein C in pooled normal plasma, and purified protein C. First, the addition of APC to protein C-deficient plasma supplemented with protein C produces a prolongation of the clotting time of plasma that is proportional to the amount of protein C. This behavior was observed with APC from the Chromogenix APC resistance kit (Dia Pharm, Franklin, OH, USA) and from APC derived from the thrombin activation of human protein C (Enzyme Research Laboratories, South Bend, IN, USA). Secondly, using immunoblotting after gel electrophoresis, the disappearance of epitopes for monoclonal antibodies that recognize protein C but not APC indicates a time course for the activation by APC of protein C in pooled normal plasma and protein C purified from plasma. Thirdly, the same time course for the disappearance of protein C specific epitope can be followed using ELISA. Finally, protein C can be activated by APC as indicated by the increase in APC specific synthetic substrate Tryp-Arg-Arg-p nitroaniline hydrolysis. Kinetic data indicate a value of 4.7+/-0.4 mM(-1) s(-1) for the activation of protein C by APC under physiological conditions and in the presence of calcium. These observations document that APC must function not only in the inactivation of activated factors V and VIII, but also in the activation of protein C. This additional action of APC may be important to consider more broadly because of APC in the treatment of sepsis.

Epidemiology of venous thromboembolic disease

From the information presented in this article, it can be concluded that clinical suspicion of VTE should be increased in patients with a history of VTE, recent surgery, spinal cord injury, trauma, or malignancy. A variety of medical illnesses also increase the risk of venous thrombosis, including congestive heart failure, myocardial infarction, stroke with paresis, nephrotic syndrome, cigarette smoking, and obesity. Hypercoagulable states, such as antithrombin III deficiency, protein C deficiency, protein S deficiency, or factor V Leiden mutation should be considered in those patients who develop VTE in the absence of known risk factors. Additionally, the presence of vena caval filters does not exclude the possibility of PE or recurrent DVT. With a careful assessment of risk, physicians can hope to increase the diagnostic yield of VTE and decrease the significant morbidity and mortality of caused by this disease.

Interaction of the protein C/protein S anticoagulant system, the endothelium and pregnancy

Normal pregnancy is associated with significant changes in haemostasis, lipid metabolism and endothelial function. This suggests that maternal adaptation in these systems is required for successful pregnancy outcome. A number of acquired and heritable prothrombotic abnormalities are associated with complications in pregnancy. A common feature of these abnormalities is their ability to alter endothelial function or the protein C/protein S system and increase thrombin generation. In this review the normal function of the endothelium and the protein C/protein S system is detailed. The changes which characterize normal and complicated pregnancies are outlined and the evidence for the impact of heritable and acquired disorders of the protein C/protein S system on pre-eclampsia and fetal loss are discussed.

Activated protein C (APC)-resistance: automated detection of the point mutation at position 1691 in the factor V gene

Proteolytic cleavage of factor Va, caused by activated protein C, is an important mechanism in limiting clot formation in normal haemostasis. A single point mutation in the factor V gene has been demonstrated to cause resistance of factor Va to proteolytic cleavage by activated protein C. With an 8-fold increased risk of thrombosis and a 2 to 13% prevalence in the Caucasian population for the heterozygous state of this mutation, knowledge of the patient's genetic disposition is of great importance in conditions such as pregnancy, surgery, use of oral contraceptives and immobilization. Therefore we have developed an automated test for the detection of the factor V mutation. This PCR based test makes use of the disappearance of an Mnl 1 restriction site if the mutation is present. The assay has been developed for the widely used ES-systems of Boehringer Mannheim. The test discriminates between the heterozygous and the homozygous state. Because of its low costs and easy handling the assay can be used as a screening test and can be performed in routine clinical laboratories.

Identification of two novel point mutations in the human protein S gene associated with familial protein S deficiency and thrombosis

Individuals with thrombosis who were believed to possess associated familial protein S deficiency were analyzed for mutations in the protein S gene by a two-step process. First, the individuals were analyzed for protein S Pro626 A/G dimorphism in both their genomic DNA and reverse-transcribed (RT) polymerase chain reaction (PCR)-amplified cDNA from peripheral blood cell mRNA. If a heterozygote expressed both alleles at the mRNA level at this site in genomic DNA, a search for point mutations was made by direct cDNA sequencing. RT-PCR amplification of exons 1-6 with mRNA from two twin sisters, each of whom has severe type I protein S deficiency, revealed both larger and smaller fragments in addition to the expected 504-base pair fragment in normal individuals. A donor splicesite mutation at position +4 of the 5' end of intron A was subsequently identified in both sisters and their mother. This mutation would lead to incorrect precursor mRNA splicing and the observed cDNA products. Translation of the altered mRNAs would result in a truncated protein without biological activity. In a second family, cDNA sequencing revealed a T-->G mutation at codon 603 (Ile-->Ser) in exon 15 of the protein S gene in an individual with protein S deficiency (mixed type) and a history of thrombosis. The same mutation was also detected in the proband's mother and grandmother, both of whom also exhibit protein S deficiency and thrombotic disease. This mutation occurs within a disulfide loop of protein S that is believed to be responsible for binding to C4b binding protein and may result in greater affinity between protein S and C4b, consequently leading to thrombotic disease.

Prevalence of factor V Leiden in children with thrombo-embolism

Hereditary resistance to the anticoagulatory action of activated protein C (APC resistance, APCR) was identified as a possible new thrombophilic factor in a high percentage (17%-60%) of young adults with thrombotic events. A single missense mutation (R506Q) due to a G/A transition (G1691A) in exon 10 of the factor V gene is regarded as the causative molecular defect, resulting in factor V Leiden which is correlated with APCR. Identification of this mutation by polymerase chain reaction-based methods is easy to perform and prevents pre-analytical and analytical errors in the coagulometric assay for APCR. Since the impact of this mutation in children with thrombo-embolic disease has not been determined to date, we initiated a multi centre prevalence study in two paediatric populations, with and without thrombo-embolic events. We compared 125 paediatric patients with thrombosis, divided into three different age groups (0 to < 0.5 years; > 0.5 to < 10 years; > 10 to < 18 years) with a normal population of 159 children. Although the mutation G1691A was found with an unexpectedly high prevalence of 12% in our normal controls, the prevalence was significantly higher in the age groups; 0 to < 0.5 years (26%) and > 10 to < 18 years (30%). In patients between > 0.5 and < 10 years the overall prevalence was similar to that of the control group (13%). However, in patients of this age with spontaneous thrombosis, G1691A was also a significant risk factor (5/17 approximately equal to 29%). Homozygosity for G1691A was detected in three patients but not in the control group. Including deficiencies of protein C, protein S, antithrombin, and the presence of anti-phospholipid antibodies, thrombosis was correlated with endogenous thrombophilic factors in 38/125 patients (30.4%).

CONCLUSION

Our results emphasize the impact of factor V Leiden on thrombogenesis in children. However, the significance is age-dependent and may reflect the different physiology of haemostasis in the three age groups. The diagnostic workup of children with thrombosis should include tests for factor V Leiden. The correlation of factor V Leiden with the clinical course of thrombo-embolism in children is essential to establish rational guidelines for therapy and prophylaxis of APCR-related thrombosis which are not yet available.

Inherited defects of the protein C anticoagulant system in childhood thrombo-embolism

Childhood thrombo-embolism is mostly the result of inherited thrombophilia or vascular insults combined with risk factors such as peripartal asphyxia, fetopathia diabetica, exsiccosis, septicaemia, central lines, congenital heart disease, cancer, trauma, surgery or elevated antiphospholipid antibodies. Inherited thrombophilia includes mainly defects of the protein C pathway, resistance to activated protein C, protein C or protein S deficiency. Resistance to activated protein C, in the majority of cases caused by the point mutation Arg 506 Gln of the factor V gene, has emerged as the most important hereditary cause of thrombo-embolism in adults and children. However, since an acquired risk of thrombo-embolic complications frequently masks the inherited deficiency in affected children, children with thrombo-embolism should have adequate laboratory evaluation for inherited coagulation disorders, especially the protein C pathway. Until more data on childhood thrombo-embolism are available, treatment recommendations will continue to be extrapolated from guidelines for adults.

Guidelines for the management of thrombophilia. Department of Haematology, The Royal London Hospital, Whitechapel, London, UK

Although there are numerous risk factors for venous thromboembolic disease, the term 'thrombophilia' refers only to those familial or acquired disorders of the haemostatic system that result in an increased risk of thrombosis. The inherited thrombophilias include antithrombin III deficiency, resistance to activated protein C (factor V Leiden), protein C and protein S deficiencies as well as some rare forms of dysfibrinogenaemia. It is possible that other inherited conditions might also predispose to thrombosis. In contrast, when using the above definition, the antiphospholipid syndrome is the only genuine acquired thrombophilic state. Patients who have thromboembolic disease at a young age with no provoking event or who have a positive family history or whose thrombosis involves an unusual site should be investigated for thrombophilia. The management of a patient identified as having a laboratory abnormality associated with thrombophilia will depend on a variety of factors such as the patient's individual and family thrombotic history, the site of the thrombosis and the presence of other prothrombotic risk factors. The use of prophylactic anticoagulation during pregnancy and the puerperium requires particularly careful consideration in thrombophilic women. As more becomes known about the thrombophilias it will become possible to formulate more exact guidelines as to the management of these conditions.

Familial thrombophilia: clinical and molecular analysis of Swedish families with inherited resistance to activated protein C or protein S deficiency

This report describes the characterization of Swedish families with inherited resistance to activated protein C (APC resistance) and/or protein S deficiency, two genetic disorders associated with functional impairment of the protein C anticoagulant pathway. The APC resistance phenotype was linked to the factor V gene locus in a kindred with independent inheritance of APC resistance and protein S deficiency. A point mutation changing Arg506 to a Gln (FV:Q506) in the factor V gene was the cause of APC resistance. In studies of 50 families with hereditary APC resistance, the FV:Q506 mutation was identified in 94% (47/50) of the families, and the thrombotic risk was found to be dependent on the factor V genotype. Moreover, 18 families with hereditary deficiency of free protein S were investigated. Type I protein S deficiency (low free and total protein S) and type III deficiency (low free but normal total protein S) coexisted in 78% (14/18) of the families, suggesting the two types to be phenotypic variants of the same genetic disorder. Deficiency of free protein S was caused by equimolar relationship between protein S and beta-chain containing isoforms of C4BP. Though protein S deficiency was a strong risk factor for thrombosis, the FV:Q506 mutation was identified as an additional genetic risk factor in 39% of the families. Thus, familial thrombophilia is a multiple gene disorder. The thrombophilic tendency associated with APC resistance or protein S deficiency was related to increased levels of prothrombin fragment 1 + 2, reflecting increased activation of the common coagulation pathway.

Coagulation assay with improved specificity to factor V mutants insensitive to activated protein C

The prevalence of a new hereditary defect in the protein C anticoagulant pathway, the factor V-Leiden, has been reported to range between 20% to 60% in familial thrombophilia. In addition to differences in patient groups, these very divergent numbers might also be due to the detection method applied. In most studies a modified APTT was used, where activated protein C (APC) is added simultaneously with the start of the clotting reaction. However, this method is also influenced by other factors like protein S, factor VIII or lupus anticoagulants. Furthermore, heparin or oral anticoagulant therapy might interfere. We tried to develop a coagulation assay dependent only on those mutant forms of factor V stable against proteolytic attack by APC. For this purpose, samples were first diluted with a factor V deficient plasma (f.V-dp). Then, coagulation was initiated either on the intrinsic pathway (APTT) or on the extrinsic pathway (PT) or, by directly activating factor X (RVVT). Additionally, APC was added, which prolongation of the clotting time. Deficiencies in protein S or the presence of factor V-Leiden resulted in a less pronounced clotting time prolongation. Titration of protein S-deficient plasma samples with f.V-dp diminished this effect. In contrast, in samples with factor V-Leiden the difference to the clotting time obtained with normal plasma even increased in the order APTT>>RVVT>PT. In the APTT-based method high concentrations of factor VIII shortened the clotting times, thus mimicking a factor V-Leiden defect. This could be compensated for up to 4 U/ml factor VIII by using a f.V-dp containing factor VIII at physiological concentration. Neither unfractionated nor LMW-heparin (up to 2 U/ml) interfered with the determination. In a brief investigation on 16 plasma samples from patients under oral anticoagulation 5 (30%) showed a similar behaviour as observed with normal plasma from factor V-Leiden carriers. These results let us suggest that by simply mixing the patient sample with a factor V-deficient plasma factor V-Leiden might be detected also in patients under oral anticoagulant therapy. Inherited disorders of protein C or protein S are well known as thrombotic risk factors (1). The recent investigations by Dahlbäck et al. (2) led to the discovery of a new hereditary defect in the protein C anticoagulant pathway: the factor V-Leiden (3). This mutation renders activated factor V stable against proteolytic attack by activated protein C (APC). The reports on the prevalence of this mutation in thrombophilic patients show a considerable variation between 21% (4) and 64% (5).(ABSTRACT TRUNCATED AT 400 WORDS)

Activated protein c resistance (APC) and inherited factor V (FV) mis-sense mutation in patients with venous and arterial thrombosis in a haematology clinic

BACKGROUND

Inherited factor V (FV) mis-sense point mutation has recently been identified as a major cause of familial venous thrombosis. The incidence of this congenital haemostatic disorder in Australia is unknown.

AIM

To examine the incidence of this congenital defect in patients with thrombosis attending a haematology clinic.

METHODS

Individuals investigated or treated for venous and arterial thrombosis over a four month period, as well as those who were on anticoagulant for valvular replacement or arrhythmia were studied for the presence of FV mis-sense point mutation, FV Q506 (G to A at nucleotide position 1691) by a polymerase chain reaction based test, and activated protein C (APC) resistance using an APTT based coagulation assay.

RESULTS

Forty-five patients with venous thromboembolism (VTE), 20 patients with coronary artery disease and 25 patients with valvular replacement or arrhythmia who were on anticoagulant were examined. The frequency of FV mis-sense point mutation in these three groups was 26.7%, 15% and 4% respectively. In this study, patients with FV Q506 were of a younger age and had a higher incidence of extensive thrombosis or recurrence as compared to those with the normal factor V gene. This mutation was found in a diverse group of people (four of the 12 patients were of non-European origin). Nearly 50% of these patients had other risk factors for VTE. The number of patients with a family history of VTE was similar for those with the FV mutation and the normal FV.

CONCLUSION

This study confirms the high incidence of FV Q506 mutation in patients with VTE reported overseas. Several clinical features, i.e. young age of onset of VTE, high recurrence rate, diverse ethnic background and importance of associated risk factors are highlighted. The findings in this study also raise the possibility that this mutation may be a risk factor for arterial thrombosis. Large studies are required to substantiate these findings.

Co-segregation of thrombosis with the factor V Q506 mutation in an extended family with resistance to activated protein C

The activated protein C (APC) resistance phenotype results from a mutation at one of the cleavage sites of factor V by APC (Q506). We describe a large family with an APC resistance phenotype and without any other detectable coagulation defect, including eight subjects who had developed deep venous thrombosis (mean age of the first thrombosis episode 29 years; range 17-55 years). The factor V Q506 mutation was detected in the seven patients with thrombosis who could be tested and in 13 asymptomatic subjects (mean age 17 years; range 5-33 years). The APC resistance was detectable in only 10 heterozygotes among the 19 tested. These data suggest that, in affected families, the risk for the factor V Q506 mutation carriers to develop thrombosis may be very high and that factor V genotyping must be performed in patients with thrombosis even without any detectable APC resistance phenotype.