Homozygous APC-resistance Combined with Inherited Type I Protein S Deficiency in a Young Boy with Severe Thrombotic Disease

State-of-the-Art Review: Activated Protein C Resistance: Clinical Implications

The discovery of inherited resistance to activated protein C (APC) as a major risk factor for venous thrombosis has dramatically improved our understanding of the pathogenesis of venous thrombosis. In a majority of cases, APC resistance is associated with a single point mutation in the factor V gene (FV) that results in substitution of arginine, R, at position 506 by glutamine, Q. (FV:Q506). The mutation renders factor Va partially resistant to degradation by APC. A functional APC resistance test, which includes predilution of the patient plasma with factor V-deficient plasma, is found to be 100% sensitive and specific for the presence of FV:Q506and is useful as a screening assay. Carriers of the FV:Q506allele have increased thrombin generation, resulting in hypercoagulability and a lifelong increased risk of venous thrombosis. In Western countries, APC resistance due to the FV mutation is present in 20-60% of thrombosis patients and in 1-15% of healthy controls, whereas the mutation is virtually absent from ethnic groups other than Caucasians. This may explain the high incidence of venous thrombosis in Western countries. The thrombotic risk in APC-resistant individuals may be further increased by other genetic defects, e.g., protein C or protein S deficiency, and by exposure to circumstantial risk factors, e.g., oral contraceptives, pregnancy, immobilization, and surgery. The question is thus raised as to whether general screening for APC resistance before circumstantial risk factors occur is warranted in Western countries. Key Words: Factor V—APC resistance-Protein C-Protein S—Thrombosis—Mutation.

Homozygous hereditary resistance to activated protein C presenting as cerebral venous thrombosis

The factor V Leiden mutation is a specific point mutation in the gene coding for factor V. It renders activated factor V resistant to degradation by activated protein C (APC). This hereditary resistance to APC (HRAPC) is a known risk factor for systemic venous thrombosis. We present a case of homozygous HRAPC presenting as cerebral venous thrombosis (CVT). A 24-year-old woman presented with a dense left hemiplegia and papiledema. A computed tomography scan showed a high ritht parieto-occipital infarct with hemorrhagic conversion. Angiography confirmed the diagnosis of extensive CVT. Treatment included heparin and direct intrathrombus thrombolysis initially as a bolus and then as an infusion for 21 hours. Repeat angiography showed partial recanalization. After 9 days, the patient was discharged on warfarin with minimal residual left weakness but persistent papilledema. Homozygous HRAPC appears to be a risk factor for CVT and should be considered in the evaluation of CVT.

Antithrombotic Therapy in Children*

This article about antithrombotic therapy in children is part of the 7th American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. Grade 1 recommendations are strong and indicate that the benefits do, or do not, outweigh the risks, burden, and costs. Grade 2 suggests that individual patients' values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this article are the following. In neonates with venous thromboembolism (VTE), we suggest treatment with either unfractionated heparin or low-molecular-weight heparin (LMWH), or radiographic monitoring and anticoagulation therapy if extension occurs (Grade 2C). We suggest that clinicians not use thrombolytic therapy for treating VTE in neonates, unless there is major vessel occlusion that is causing the critical compromise of organs or limbs (Grade 2C). For children (ie, > 2 months of age) with an initial VTE, we recommend treatment with i.v. heparin or LMWH (Grade 1C+). We suggest continuing anticoagulant therapy for idiopathic thromboembolic events (TEs) for at least 6 months using vitamin K antagonists (target international normalized ratio [INR], 2.5; INR range, 2.0 to 3.0) or alternatively LMWH (Grade 2C). We suggest that clinicians not use thrombolytic therapy routinely for VTE in children (Grade 2C). For neonates and children requiring cardiac catheterization (CC) via an artery, we recommend i.v. heparin prophylaxis (Grade 1A). We suggest the use of heparin doses of 100 to 150 U/kg as a bolus and that further doses may be required in prolonged procedures (both Grade 2 B). For prophylaxis for CC, we recommend against aspirin therapy (Grade 1B). For neonates and children with peripheral arterial catheters in situ, we recommend the administration of low-dose heparin through a catheter, preferably by continuous infusion to prolong the catheter patency (Grade 1A). For children with a peripheral arterial catheter-related TE, we suggest the immediate removal of the catheter (Grade 2C). For prevention of aortic thrombosis secondary to the use of umbilical artery catheters in neonates, we suggest low-dose heparin infusion (1 to 5 U/h) (Grade 2A). In children with Kawasaki disease, we recommend therapy with aspirin in high doses initially (80 to 100 mg/kg/d during the acute phase, for up to 14 days) and then in lower doses (3 to 5 mg/kg/d for > or = 7 weeks) [Grade 1C+], as well as therapy with i.v. gammaglobulin within 10 days of the onset of symptoms (Grade 1A).

Polymorphisms in coagulation factor genes and their impact on arterial and venous thrombosis

Arterial and venous thromboses, with their clinical manifestations such as stroke, myocardial infarction (MI), or pulmonary embolism, are the major causes of death in developed countries. Several studies in twins and siblings have shown that genetic factors contribute significantly to the development of these diseases. Since the advent of molecular genetics in medicine, it has been a focus of interest to elucidate the role of mutations in various candidate genes and their impact on hemostatic disorders such as arterial and venous thromboses. In this article, we review the current knowledge of the contribution of polymorphisms in coagulation factors to the development of thrombotic diseases. We show that in arterial thrombosis, results are controversial. Only for factor XIII 34Leu a protective effect on the development of myocardial infarction has been demonstrated in several studies. No other single polymorphism in a coagulation factor could be confirmed as a relevant risk factor, although there is evidence for a role of factor V Arg506Gln, factor VII Arg353Gln, and vWF Thr789Ala polymorphisms in patient subgroups. Further studies will be necessary to confirm the value of testing for genetic polymorphisms in arterial thrombosis. A large body of data is available on the role of factor V Arg506Gln and the prothrombin G20210A mutation in venous thrombosis. Some papers already recommend diagnosis and treatment strategies. We will discuss these recent publications on venous thrombosis in our review.

Importance of individual activated protein C cleavage site regions in coagulation factor V for factor Va inactivation and for factor Xa activation

Activated protein C (APC) cleavage of Factor Va (FVa) at residues R506 and R306 correlates with its inactivation. APC resistance and increased thrombotic risk are due to the mutation R506Q in Factor V (FV). To study the effects of individual cleavages in FVa by APC and the importance of regions near the cleavage sites, the following recombinant (r) human FVs were prepared and purified: wild-type, Q306-rFV, Q506-rFV, and Q306Q506-rFV. All had similar time courses for thrombin activation. Q506-rFVa was cleaved by APC at R306 and was moderately resistant to APC in plasma-clotting assays and in prothrombinase assays measuring FVa residual activity, in agreement with studies of purified plasma-derived Q506-FVa. Q306-rFVa was cleaved by APC at R506 and gave a low APC-resistance ratio similar to Q506-rFVa in clotting assays, whereas unactivated Q306-rFV gave a near-normal APC-resistance ratio. When FVa residual activity was measured after long exposure to APC, Q306-rFVa was inactivated by only < or = 40% under conditions where Q506-rFVa was inactivated > 90%, supporting the hypothesis that efficient inactivation of normal FVa by APC requires cleavage at R306. In addition, the heavy chain of Q306-rFVa was cleaved at R506 much more rapidly than activity was lost, suggesting that FVa cleaved at only R506 is partially active. Under the same conditions, Q306Q506-rFVa lost no activity and was not cleaved by APC. Therefore, cleavage at either R506 or R306 appears essential for significant inactivation of FVa by APC. Modest loss of activity, probably due to cleavage at R679, was observed for the single site rFVa mutants, as evidenced by a second phase of inactivation. Q306Q506-rFVa had a low activity-to-antigen ratio of 0.50-0.77, possibly due to abnormal Factor Xa (FXa) binding. Furthermore, Q306Q506-rFV was very resistant to cleavage and activation by FXa. Q306Q506-rFV appeared to bind FXa and inhibit FXa's ability to activate normal FV. Thus, APC may downregulate FV/Va partly by impairing FXa-binding sites upon cleavage at R306 and R506. This study shows that R306 is the most important cleavage site for normal efficient inactivation of FVa by APC and supports other studies suggesting that regions near R306 and R506 provide FXa-binding sites and that FVa cleaved at only R506 retains partial activity.

Effect of anticoagulant therapy on the hypercoagulable state in patients carrying the factor V Arg506Gln mutation

Resistance to activated protein C, caused by a single point mutation in the factor V gene (Arg506Gln or FV Leiden), is the most prevalent single risk factor associated with venous thromboembolic disease. The aim of this study was to investigate the effectiveness of standard oral anticoagulant therapy (OAT) in patients with the Arg506Gln mutation compared with a matched control group. The study compared selected variables in 27 patients carrying the Arg506Gln mutation with 27 sex- and age-matched controls in steady state oral anticoagulant treatment (OAT). The study showed that similar doses of vitamin K antagonists in carriers and noncarriers suppress and generate a uniform distribution of coagulation markers in steady state OAT. Thus, it seems that OAT with standard treatment doses is just as effective in patients with the Arg506Gln mutation as in comparable controls without the mutation.

An unknown genetic defect increases venous thrombosis risk, through interaction with protein C deficiency

We used two-locus segregation analysis to test whether an unknown genetic defect interacts with protein C deficiency to increase susceptibility to venous thromboembolic disease in a single large pedigree. Sixty-seven pedigree members carry a His107Pro mutation in the protein C gene, which reduces protein C levels to a mean of 46% of normal. Twenty-one carriers of the mutation and five other pedigree members had verified thromboembolic disease. We inferred the presence in this pedigree of a thrombosis-susceptibility gene interacting with protein C deficiency, by rejecting the hypothesis that the cases of thromboembolic disease resulted from protein C deficiency alone and by not rejecting Mendelian transmission of the interacting gene. When coinherited with protein C deficiency, the interacting gene conferred a probability of a thrombotic episode of approximately 79% for men and approximately 99% for women, before age 60 years.

Hyperhomocyst(e)inemia and a common methylenetetrahydrofolate reductase mutation (Ala223Val MTHFR) in patients with inherited thrombophilic coagulation defects

To assess whether certain abnormalities of the sulfated amino acid metabolism are associated with the occurrence of thromboembolic events in patients with inherited thrombophilic conditions, the levels of homocyst(e)ine, before or after methionine load, and the presence of the Ala223Val substitution in the 5,10-methylenetetrahydrofolate reductase (MTHFR) were evaluated in 119 subjects with a congenital single thrombophilic condition (type I deficiency of antithrombin n = 10, protein C n = 24, protein S n = 16; activated protein C resistance due to factor V Leiden mutation n = 69). Sixty-three subjects had experienced at least one documented thrombotic event, while the remaining 56 subjects were still free from any thrombotic symptom. Our results show that (1) high homocyst(e)ine levels, either in fasting condition or after methionine load, were not more frequent in subjects with inherited thrombophilic alterations (14.4%) than in normal control subjects (10% by definition) and (2) the frequency of hyperhomocyst(e)inemia was similar in thrombophilic subjects, who already have (14.3%) or have not (14.6%) experienced thrombotic events. As regards the MTHFR mutation, the homozygous condition was present in 23.2% of the thrombophilic patients versus 17.5% in the control subjects, a nonsignificant difference. The mutation was slightly more frequent in those thrombophilic subjects who had suffered a thrombotic episode (25.5%) versus those with no thrombosis (20.8%), with odds ratios of 1.61 (confidence interval (CI) = 0.58-4.52) and 1.24 (CI = 0.42-3.43), respectively. These differences were also nonsignificant. It is concluded that in subjects with inherited thrombophilias, a condition of hyperhomocyst(e)inemia "per se" is not a factor increasing the risk of thrombosis. The risk enhancement conferred by the MTHFR mutation, if any, seems to be slight or limited, and its significance could be ascertained only in a large multicenter trial.

Activated protein C resistance due to a common factor V gene mutation is a major risk factor for venous thrombosis

Inherited resistance to activated protein C (APC) was recently discovered to be a cause of familial thrombophilia and is now known to be the most common genetic risk factor for venous thrombosis. It is caused by a single point mutation in the gene for factor V, which predicts substitution or arginine (R) at position 506 with a glutamine (Q). Accordingly, the activated form of mutated factor V (FVa:Q506) is more slowly degraded by activated protein C than normal FVa (FVa:R506) is, resulting in hypercoagulability and a lifelong 5- to 10-fold increased risk of venous thrombosis. Previously known inherited hypercoagulable states, i.e. deficiencies of the anticoagulant proteins antithrombin III, protein S, and protein C, are found fewer than 10-15% of thrombosis patients in western countries, whereas inherited APC resistance is present in 20-60% of such patients. The FV mutation is common in populations of Caucasian origin, with prevalences ranging from 1-15%, whereas it is not found in certain other ethnic groups such as Japanese and Chinese. The high prevalence of APC resistance, in combination with the availability of simple laboratory tests, will have a profound influence on the development of therapeutic and prophylactic regimens for thrombosis and will, it is hoped, result in a decreased incidence of thromboembolic events.

Factor V:Q506 mutation and anticardiolipin antibodies in systemic lupus erythematosus

Inherited resistance to activated protein C (APC resistance) is an important risk factor of venous thrombosis. It is caused by a point mutation in the gene coding for coagulation factor V, called FV:Q506. Arterio-venous thrombosis is a common and serious medical problem in patients with systemic lupus erythematosus (SLE). We studied the prevalence of the factor V mutation associated with APC resistance and IgG anticardiolipin antibodies (aCLs) in an epidemiological cohort of 78 Swedish SLE patients, to determine their roles as risk factors for thrombosis. In addition, a detailed evaluation of the clinical manifestations in these patients was performed. Totally, 19 (24%) of the 78 SLE patients had thrombosis, 11 (14%) had venous thrombosis and 8 (10%) had a cerebral infarction caused by occlusion of cerebral vessels. Twenty-six (33%) SLE patients were aCL positive and 8 (10%) were heterozygous for the factor V mutation. Only one of the patients with venous thrombosis and one of the patients with cerebral thrombosis had the FV:Q506 mutation, whereas 3 patients with venous thrombosis and 5 patients with cerebral infarction were aCL positive. Eleven of 19 patients with heart valve disease were aCL positive, a statistically significant association (P = 0.01). In conclusion, we found no statistically significant association between venous thrombosis and FV:Q506 mutation or venous thrombosis and aCL positivity. There was, however, an association between heart valve disease and aCL positivity.

Pulmonary embolism and premature labor in a patient with both factor V Leiden mutation and methylenetetrahydrofolate reductase gene C677T mutation

Multiple risk factors for thrombosis were found in a 21 year old female who experienced three episodes of premature labor and developed severe pulmonary embolism during her third pregnancy. This patient is heterozygous for factor V Leiden mutation and homozygous for methylenetetrahydrofolate reductase (MTHFR) gene nucleotide 677 C to T (C677T) point mutation. This is a first report of the concordance of homozygous MTHFR C677T mutation in an individual with factor V Leiden mutation. This new case provides further evidence that synergism of multiple genetic and acquired risk factors is often encountered in young patients with symptomatic venous thrombosis.

Thrombosis in otherwise well children with the factor V Leiden mutation

OBJECTIVE

To determine whether resistance to activated protein C caused by the factor V Leiden mutation (Arg506 to Gln) is associated with thrombosis occurring during childhood.

STUDY DESIGN

Children with thrombosis were screened for activated protein C resistance. Children found resistant to activated protein C had DNA analysis for the factor V Leiden mutation. Family members of the children with activated protein C resistance were similarly studied.

RESULTS

Three of fourteen children examined had abnormal normalized activated protein C sensitivity ratios. One child had protein S deficiency. The children had hyperlipidemia. Molecular confirmation of the factor V Leiden mutation was obtained for all three children. Family members of each of the three children were affected.

CONCLUSIONS

Children have thromboses in association with the factor V Leiden mutation, as do adults. This mutation may be identified as an isolated risk factor or in association with other risk factors for thrombosis.

Resistance to activated protein C: role in venous and arterial thrombosis

Activated protein C resistance is the most prevalent cause of thrombophilia: it is found in 20 to 30% of patients with a deep venous thrombosis history. Activated protein C resistance is due to an arginine 506 to glutamine mutation in factor V. This mutation prevents normal inactivation of activated factor V by activated protein C. The estimated increase in relative risk of venous thrombosis is 5- to 10-fold in heterozygotes, and 50- to 100-fold in homozygotes. Activated protein C resistance does not seem to play a role in arterial thrombosis and in the occurrence of myocardial infarction.

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.

Activated protein C resistance: from phenotype to genotype and clinical practice

The anticoagulant protein C system is an important regulator of the blood coagulation process. Its targets are the procoagulant cofactors factor Va and factor VIIIa, which are cleaved and inactivated by activated protein C, protein S and intact factor V working as cofactors. Genetic defects of protein C or protein S were, together with antithrombin III deficiency, the previously established major causes of familial venous thromboembolism. However, these abnormalities are found in less than 5-10% of patients with thrombosis. Inherited resistance to activated protein C was recently identified as a major risk factor for venous thromboembolism. The activated protein C-resistance phenotype is found in 20-60% of the patients with venous thrombosis, depending on selection criteria and on the prevalence of activated protein C-resistance in the population. The frequency of activated protein C-resistance is 2-10% in the normal populations studied so far. In more than 90% of cases, the molecular background for the activated protein C-resistance is a single point mutation in the factor V gene, which predicts substitution of an arginine at position 506 by a glutamine. Mutated factor V is activated by thrombin or factor Xa in the normal way, but impaired inactivation of mutated factor Va by activated protein C results in a life-long hypercoagulability. Owing to the high prevalence of activated protein C-resistance in the population, it occasionally occurs in patients with deficiency of protein S, protein C or antithrombin III. Individuals with combined defects suffer more severely from thrombosis, and often at a younger age, than those with single defects, suggesting thrombophilia to be a multigenetic disease.