Different Prevalence of Thromboembolism in the Subtypes of Congenital Antithrombin III Deficiency:Review of 404 Cases

Impact of SERPINC1 mutation on thrombotic phenotype in children with congenital antithrombin deficiency-first analysis of the International Society on Thrombosis and Haemostasis pediatric antithrombin deficiency database and biorepository

BACKGROUND

The natural history and genotype-phenotype correlation of congenital antithrombin (AT) deficiency in children are unknown.

OBJECTIVES

To describe the clinical presentation of congenital AT deficiency in children and evaluate its correlation to specific mutations in SERPINC1.

METHODS

In 2017, a prospective pediatric database and DNA biorepository for congenital AT deficiency was established. During the pilot phase, the database was opened at 4 tertiary care centers in Canada and US. Approval from research ethics board was obtained at each participating center. Written consent/assent was obtained from guardians/subjects who met eligibility. Demographic/clinical data were uploaded into a database. DNA extraction and SERPINC1 sequencing were centralized for US centers. Standard statistical methods were used to summarize parameters. Probability of VTE-free survival was assessed using the Kaplan-Meier method.

RESULTS

Overall, 43 participants (25 females) from 31 unique kindreds were enrolled. Median age (range) at enrollment was 14.8 years (1-21 years). Median AT activity was 52% (24%-87%), and median AT antigen (n = 20) was 55% (38%-110%). Nineteen (44%) participants had a history of venous thromboembolism (VTE). Median age at VTE diagnosis was 12.8 years (0.1-19.2 years). SERPINC1 sequencing was completed for 31 participants and 21 unique mutations were identified, including 5 novel variants. Probability of 5-year VTE-free survival (95% CI) for carriers of missense mutations (92.0% [95% CI: 71.6%-97.9%]) was significantly higher compared with carriers of null mutations (66.7% [95% CI: 19.5%-90.4%]); p = .0012.

CONCLUSION

To our knowledge, this is the first pediatric study to document a severe thrombotic phenotype in carriers of null mutations in SERPINC1, when compared with carriers of missense mutations; underscoring the importance of genetic testing.

Anticoagulant SERPINs: Endogenous Regulators of Hemostasis and Thrombosis

Appropriate activation of coagulation requires a balance between procoagulant and anticoagulant proteins in blood. Loss in this balance leads to hemorrhage and thrombosis. A number of endogenous anticoagulant proteins, such as antithrombin and heparin cofactor II, are members of the serine protease inhibitor (SERPIN) family. These SERPIN anticoagulants function by forming irreversible inhibitory complexes with target coagulation proteases. Mutations in SERPIN family members, such as antithrombin, can cause hereditary thrombophilias. In addition, low plasma levels of SERPINs have been associated with an increased risk of thrombosis. Here, we review the biological activities of the different anticoagulant SERPINs. We further consider the clinical consequences of SERPIN deficiencies and insights gained from preclinical disease models. Finally, we discuss the potential utility of engineered SERPINs as novel therapies for the treatment of thrombotic pathologies.

Treatment of Homozygous Type II Antithrombin Heparin-Binding Site Deficiency in Pregnancy

Pregnancy is associated with an increased risk of venous thromboembolism (VTE). Previous VTE and severe thrombophilia are important risk factors. Our case was a 36-year-old woman, gravida 6, para 0, with antithrombin (AT) deficiency caused by a homozygous mutation in the heparin-binding site (HBS). Her history included seven prior VTEs, three early and two late pregnancy losses. She was prophylactically treated with both human plasma-derived AT concentrate (hpATC) and low molecular weight heparin (LMWH), resulting in a successful 6^th pregnancy and a healthy live born baby. There is limited evidence and guidance on the management of AT deficiency in pregnancy. Dosing and monitoring of anticoagulants, alone or together with hpATC, must be based on individual risk assessment. The severity of clinical manifestations varies with the type of AT deficiency. Characterization of the AT mutation may aid in the decision-making process and optimize pregnancy outcomes.

Thrombophilia, risk factors and prevention

INTRODUCTION

Fifty-three years after the first description of an inherited prothrombotic condition (antithrombin deficiency), our knowledge on hereditary and acquired causes of hypercoagulability that can predispose carriers to venous thromboembolism (VTE) has greatly improved. Areas covered: Main causes of hereditary thrombophilia are summarized alongside new prothrombotic mutations recently discovered. The main causes of acquired thrombophilia, and namely, antiphospholipid antibody syndrome and hyperhomocysteinemia, are also discussed together with other common acquired prothrombotic states characterized by an increase of procoagulant factors and/or a decrease of natural anticoagulants. Finally, suggestions for thromboprophylaxis in carriers of hereditary thrombophilia according to current guidelines/evidence are made for the most challenging high-risk situations (i.e. surgery, pregnancy, contraception, cancer, economy class syndrome) as well as for the prevention of post-thrombotic syndrome. Expert opinion: A carrier of inherited thrombophilia should be evaluated in the framework of other (genetic and/or acquired) coexisting risk factors for first or recurrent VTE when assessing the need and duration of prevention (primary prophylaxis). Prevention strategies should be tailored to each patient and every situational risk factor. The knowledge of the carriership status of severe thrombophilia in the proband can be important to provide asymptomatic relatives with adequate counseling on thrombophilia screening or primary thromboprophylaxis.

Classic thrombophilic gene variants

Thrombophilia is defined as a condition predisposing to the development of venous thromboembolism (VTE) on the basis of a hypercoagu-lable state. Over the past decades, great advances in the pathogenesis of VTE have been made and nowadays it is well established that a thrombophilic state may be associated with acquired and/or inherited factors. The rare loss-of-function mutations of the genes encoding natural anticoagulant proteins (i. e. protein C, protein S and antithrombin) and the more common gain-of-function polymorphisms factor V Leiden and prothrombin G20210A are the main genetic determinants of thrombophilia. In addition, non-O blood group has been consistently demonstrated to be the most frequent inherited marker of an increased risk of VTE. The mechanism role of these inherited thrombophilia markers will be discussed in this narrative review.

How I treat heterozygous hereditary antithrombin deficiency in pregnancy

Untreated hereditary antithrombin deficiency in pregnancy is associated with maternal venous thromboembolism (VTE) and possibly with fetal loss. Thromboprophylaxis during pregnancy is recommended, but dosages remain controversial. Our objective was to perform a retrospective assessment of thrombotic events and pregnancy outcomes in women with hereditary antithrombin deficiency managed according to a standard protocol. Pregnancies in individuals with hereditary antithrombin deficiency were identified from a hospital database. Women with no prior VTE received enoxaparin 40 mg daily until 16 weeks gestation and thereafter 40 mg twice daily. Women with prior VTE received intermediate dose enoxaparin (1 mg/kg) once daily, increased to twice daily at 16 weeks and anti-Xa monitored dosing. Thromboprophylaxis was stopped at initiation of labour or 12 hours prior to caesarean and 50 IU/kg antithrombin concentrate given. Thromboprophylaxis was restarted after delivery. Eighteen pregnancies in 11 women with antithrombin deficiency were identified. Seventeen pregnancies (94%) were successful. Median gestation was 39 weeks (range 30–41) and median birth-weight was 2,995 g (910–4,120 g),but 6/17 infants (35%) were small for gestational age (p=0.01). Estimated blood loss at delivery was median 375 ml (200–600 ml). Four pregnancies were complicated by VTE; one newly presented with a thrombotic event, two patients were not taking thromboprophylaxis and one occurred despite thromboprophylaxis. Two novel mutations(p.Leu317Ser and p.His33GInfsX32) are described. In conclusion, in antithrombin deficiency the use of low-molecular-weight heparin in pregnancy and puerperium with antithrombin concentrate predelivery was associated with successful pregnancy outcome; rates of VTE appear to be lower than previously reported, but remain elevated

Early onset of abdominal venous thrombosis in a newborn with homozygous type II heparin-binding site antithrombin deficiency

: The overall incidence of thromboembolic events in the neonatal period is 5 per 100 000 births, wherein more than 40% of all such manifestations are symptomatic renal vein thromboses. We describe the case of a newborn female who developed extensive thrombosis, which filled the inferior vena cava and renal vein and was diagnosed in the first weeks of life. A homozygous type II heparin-binding site antithrombin deficiency (c. 391C>T, p. Leu131Phe) was detected in the background. Despite the timely diagnosis and appropriate treatment, clinical signs of renal insufficiency, because of left kidney atrophy and arterial hypertension, were observed. Our case demonstrates the seriousness of the consequences arising after early onset of venous thrombosis caused by homozygous type II heparin-binding site antithrombin deficiency. In addition to prompt diagnosis, of huge importance is the determination of inherited thrombophilia, as it significantly affects therapeutic treatment and indicates that long-term follow-up is mandatory.

Thrombotic risk according to SERPINC1 genotype in a large cohort of subjects with antithrombin inherited deficiency

Inherited quantitative (type I) or qualitative (type II) antithrombin deficiency (ATD) due to mutations in the SERPINC1 gene is a well-known risk factor for venous thromboembolism. ATD may also increase risk for arterial thrombosis. Few studies have investigated risk for thrombosis according to mutations. We addressed this topic in a large retrospective cohort study of 540 heterozygous carriers of SERPINC1 mutations and compared risk for first venous or arterial thrombosis associated with carrying of different type II or type I mutations. No clear difference in risk for first venous thrombotic event was observed among type I (missense or null), type IIRS or type IIPE mutation carriers except for a few variants that displayed lower risk [all events, adjusted relative risk: Cambridge II: 0.42 (95 %CI 0.25-0.70), Dublin: 0.35 (95 %CI 0.13-0.99)]. IIHBS mutation carrying was associated with a clearly lower risk than type I mutation carrying [0.28 (95 %CI 0.20-0.40)]. These differences in risk were observed for both all venous thrombotic events and pulmonary embolism associated with deep venous thrombosis. The HBS group was also heterogeneous, with AT Budapest 3 carriers displaying a non-significantly different risk [0.61 (95 %CI 0.31-1.20)] compared to type I mutation carriers. We also studied risk for arterial thrombosis and found no significant influence of mutation type. Altogether, our findings suggest a place for SERPINC1 genotyping in the diagnosis of ATD.

Syndromes of Thrombosis and Hypercoagulability: Congenital and Acquired Thrombophilias

This article stresses the common hereditary and acquired blood protein defects associated with thrombosis. The most common of the hereditary defects apear to be APC-R, SPS, antithrombin, protein C, and protein S deficiency, and the most common acquired defects are anticardiolipin antibodies and the lupus anticoagulant (antiphospholipid antibodies). Therefore, these are the defects that should first be looked for in an individual with unexplained thrombosis. If these more common defects are not found, then the rarer defects including HC II, plasminogen or TPA deficiency, dysfibrinogenemia, el evated PAI-1 and hyperhomocysteinemia should be sought. The importance of finding these defects has significant impli cations for therapy of the individual patient and for institutions of family studies to identify, inform, and possibly treat others at risk. It is expected that as knowledge of hemostasis expands, more hereditary and acquired defects, such as elevated lipopro tein (a) or defects of extrinsic (tissue factor) pathway inhibitor (EPI, TFPI), may be associated with enhanced risks of throm bosis. Finally, it must be recalled that a diagnosis of thrombo sis, like that of anemia, is only a generic and partial diagnosis; just as in the anemic patient, the etiology must be clearly de fined. Only in this manner can cost-effective and appropriate therapy for both primary treatment and secondary prevention be designed. In addition, the demonstration of a hereditary defect will allow primary prevention in afflicted family mem bers by allowing the choice of appropriate therapy.

Founder effect is responsible for the p.Leu131Phe heparin-binding-site antithrombin mutation common in Hungary: phenotype analysis in a large cohort

BACKGROUND

Antithrombin (AT) is a key regulator of the coagulation. In type II deficiency, the heparin-binding-site defect (type II HBS) is considered to be relatively low thrombosis risk.

OBJECTIVES

Our aims were to search for SERPINC1 mutation(s) and to describe the clinical and laboratory phenotype of a large number of AT Budapest3 (ATBp3, p.Leu131Phe) carriers and confirm the presence of a founder effect.

PATIENTS/METHODS

AT-deficient patients were recruited and carriers of ATBp3, n = 102 (63 families) were selected. To investigate the founder effect, eight intragenic single nucleotide polymorphisms, a 5'-length dimorphism, and five microsatellite markers were detected. Clinical and laboratory data of the patients were collected and analyzed.

RESULTS

In AT deficiency, 16 different causative mutations were found, and the great majority of patients were of type II HBS subtype. Most of them (n = 102/118, 86.5%) carried the ATBp3 mutation. The ATBp3 mutant allele was associated with one single haplotype, while different haplotypes were detected in the case of normal allele. The anti-factor Xa-based AT activity assay that we used could detect all ATBp3 patients with high sensitivity in our cohort. ATBp3 homozygosity (n = 26) was associated with thrombosis at a young age and conferred a high thrombotic risk. Half of the heterozygotes (n = 41/76, 53.9%) also had venous and/or arterial thrombosis, and pregnancy complications were also recorded.

CONCLUSION

In Hungary, the founder mutation, ATBp3, is the most common AT deficiency. Our study is the first in which the clinical characterization of ATBp3 mutation was executed in a large population.

Impact of the type of SERPINC1 mutation and subtype of antithrombin deficiency on the thrombotic phenotype in hereditary antithrombin deficiency

Mutations in the antithrombin (AT) gene can impair the capacity of AT to bind heparin (AT deficiency type IIHBS), its target proteases such as thrombin (type IIRS), or both (type IIPE). Type II AT deficiencies are almost exclusively caused by missense mutations, whereas type I AT deficiency can originate from missense or null mutations. In a retrospective cohort study, we investigated the impact of the type of mutation and type of AT deficiency on the manifestation of thromboembolic events in 377 patients with hereditary AT deficiencies (133 from our own cohort, 244 reported in the literature). Carriers of missense mutations showed a lower risk of venous thromboembolism (VTE) than those of null mutations (adjusted hazard ratio [HR] 0.39, 95% confidence interval [CI] 0.27-0.58, p<0.001), and the risk of VTE was significantly decreased among patients with type IIHBS AT deficiency compared to patients with other types of AT deficiency (HR 0.23, 95%CI 0.13-0.41, p<0.001). The risk of pulmonary embolism complicating deep-vein thrombosis was lower in all type II AT deficiencies compared to type I AT deficiency (relative risk 0.69, 95%CI 0.56-0.84). By contrast, the risk of arterial thromboembolism tended to be higher in carriers of missense mutations than in those with null mutations (HR 6.08-fold, 95%CI 0.74-49.81, p=0.093) and was 5.9-fold increased (95%CI 1.22-28.62, p=0.028) in type IIHBS versus other types of AT deficiency. Our data indicate that the type of inherited AT defect modulates not only the risk of thromboembolism but also the localisation and encourage further studies to unravel this phenomenon.

The superiority of anti-FXa assay over anti-FIIa assay in detecting heparin-binding site antithrombin deficiency

OBJECTIVES

Antithrombin is a progressive inhibitor of active factor X (FXa) and thrombin (FIIa). Its effect is 500- to 1,000-fold accelerated by heparin or heparan sulfate. Heterozygous type I (quantitative) and most type II (qualitative) antithrombin deficiencies highly increase the risk of venous thromboembolism (VTE), while homozygous mutations are lethal. The functional defect affecting the heparin-binding site confers moderate risk of VTE to heterozygous and high risk of VTE to homozygous individuals.

METHODS

Antithrombin activity assays based on the inhibition of FIIa and FXa were compared for their efficiency in detecting heparin-binding site defects.

RESULTS

With a single exception, in heterozygotes for heparin-binding site defects (n = 20), anti-FIIa activities remained in the reference interval, while anti-FXa activities were uniformly decreased. In individuals who were homozygous for heparin-binding site mutations (n = 9), anti-FIIa activities were in the range of 48% to 80%; the range of anti-FXa activities was 9% to 25%. Anti-FIIa and anti-FXa activities in type I deficiencies and type II pleiotropic deficiency did not differ significantly.

CONCLUSIONS

Anti-FXa antithrombin assay is recommended as a first-line test to detect type II heparin-binding site antithrombin deficiency.

Prevalence of hereditary antithrombin mutations is higher than estimated in patients with thrombotic events

Antithrombin is the major inhibitor of blood coagulation, primarily inactivating thrombin and factor Xa. Inherited antithrombin deficiency is associated with an increased risk of thromboembolism. Its prevalence is estimated to be about 0.2% in the general healthy population, whereas it is 2% in patients with a history of thrombosis. We previously demonstrated thrombosis patients receiving therapeutic dosage of low-molecular-weight heparin having a lower antifactor Xa activity than originally estimated. In those patients, mutations in the AT gene (SERPINC1) were detected despite normal antithrombin activity. Thus we concluded that prevalence of antithrombin mutations might be higher than previously calculated. The aim of the present study was to investigate the prevalence of hereditary antithrombin mutations by analyzing the antithrombin gene in patients with a history of thromboembolism. Mutation analyses were performed by direct sequencing of SERPINC1 in 150 patients (aged <40 years) with thromboembolism and normal antithrombin levels. Patients with thrombophilic defects [factor V Leiden (G1691A), prothrombin (G20210T) mutation, protein C deficiency, protein S deficiency, and antiphospholipid antibodies] were excluded. The results were compared with those of 150 healthy controls without any personal history of thrombosis. Mutation analyses of all seven antithrombin exons revealed five (3.5%) patients with antithrombin mutations: three different heterozygous missense mutations were found in exon 2 and one (in two patients) in exon 7. Prevalence of inherited antithrombin mutations in thrombosis patients is higher than previously estimated. Functional antithrombin tests are unable to detect all clinically relevant antithrombin defects.

Clinical multicenter evaluation of a new FXa-based Antithrombin assay

INTRODUCTION

The determination of functional Antithrombin is a central part of thrombophilia screening. In this multicenter study, a new FXa-based method (INNOVANCE® Antithrombin) was evaluated on four different analyzers.

METHODS

The INNOVANCE Antithrombin method was evaluated by precision and reference interval studies and by comparing the new method with established methods through parallel measurement of samples from 249 patients and 151 apparently healthy individuals.

RESULTS

The INNOVANCE Antithrombin assay demonstrated on all analyzers repeatability coefficients of variation (CVs) ≤ 3.2% and within-device and between-run CVs ≤ 6.9%. The reference intervals of all analyzers are comparable with 2.5th percentiles between 80% and 85% of normal. The INNOVANCE Antithrombin and the FIIa-based Berichrom® AT III (A) methods demonstrated good concordance with correlation coefficients of r = 0.908 or higher. The INNOVANCE Antithrombin method demonstrated furthermore an excellent comparability with the STA® Antithrombin III assay and an acceptable comparability with the Coamatic® LR Antithrombin assay. The patients with congenital deficiency (n = 31) were identified with all assays except for the patients carrying the P41L heparin-binding site mutation, which was only identified with the INNOVANCE Antithrombin and the STA Antithrombin III methods.

CONCLUSION

The INNOVANCE Antithrombin assay has high sensitivity for Antithrombin deficiencies and is reliable, precise and suitable for routine clinical use.

Molecular basis of antithrombin deficiency

Antithrombin (AT) is the most important physiological inhibitor of coagulation proteases. It is activated by glycosaminoglycans such as heparin. Hereditary antithrombin deficiency is a rare disease that is mainly associated with venous thromboembolism. So far, more than 200 different mutations in the antithrombin gene (SERPINC1) have been described. The aim of our study was to characterise the molecular background in a large cohort of patients with AT deficiency. Mutation analysis was performed by direct sequencing of SERPINC1 in 272 AT-deficient patients. Large deletions were identified by multiplex PCR coupled with liquid chromatography or multiplex ligation-dependent probe amplification (MLPA) analysis. To predict the effect of SERPINC1 sequence variations on the pathogenesis of AT deficiency, in silico assessments, multiple sequence alignment, and molecular graphic imaging were performed. The mutation profile consisted of 59% missense, 10% nonsense, 8% splice site mutations, 15% small deletions/insertions/duplications, and 8% large deletions. Altogether 87 different mutations, including 42 novel mutations (22 missense and 20 null mutations), were identified. Of the novel missense mutations, nine are suspected to impair the conformational changes that are needed for AT activation, two to affect the central reactive loop or the heparin binding site, and six to impair the structural integrity of the molecule. Despite the heterogeneous background of AT deficiency, 10 AT variants occurred in multiple index patients. Characterisation of the SERPINC1 mutation profile in large cohorts of patients may help to further elucidate the pathogenesis of AT deficiency and to establish genotype-phenotype associations.

Antithrombin deficiency and its laboratory diagnosis

Antithrombin (AT) belongs to the serpin family and is a key regulator of the coagulation system. AT inhibits active clotting factors, particularly thrombin and factor Xa; its absence is incompatible with life. This review gives an overview of the protein and gene structure of AT, and attempts to explain how glucosaminoglycans, such as heparin and heparan sulfate accelerate the inhibitory reaction that is accompanied by drastic conformational change. Hypotheses on the regulation of blood coagulation by AT in physiological conditions are discussed. Epidemiology of inherited thrombophilia caused by AT deficiency and its molecular genetic background with genotype-phenotype correlations are summarized. The importance of the classification of AT deficiencies and the phenotypic differences of various subtypes are emphasized. The causes of acquired AT deficiency are also included in the review. Particular attention is devoted to the laboratory diagnosis of AT deficiency. The assay principles of functional first line laboratory tests and tests required for classification are discussed critically, and test results expected in various AT deficiency subtypes are summarized. The reader is provided with a clinically oriented algorithm for the correct diagnosis and classification of AT deficiency, which could be useful in the practice of routine diagnosis of thrombophilia.

Inherited Thrombophilia

Inherited thrombophilia can be defined as a genetically determined predisposition to the development of thromboembolic complications. Since the discovery of activated protein C resistance in 1993, several additional disorders have been described and, at present, it is possible to identify an inherited predisposition in about 60 to 70% of patients with such complications. These inherited prothrombotic risk factors include qualitative or quantitative defects of coagulation factor inhibitors, increased levels or function of coagulation factors, defects of the fibrinolytic system, altered platelet function, and hyperhomocysteinemia. In this review, the main inherited prothrombotic risk factors are analyzed from epidemiological, laboratory, clinical, and therapeutic points of view. Finally, we discuss the synergism between genetic and acquired prothrombotic risk factors in particular conditions such as childhood and pregnancy.

Hereditary and acquired antithrombin deficiency: epidemiology, pathogenesis and treatment options

Antithrombin is a glycoprotein critical to the regulation of coagulation. Its primary action is the inhibition of the activated coagulation factors IIa (thrombin) and Xa. In addition there is growing evidence to suggest that antithrombin also plays a role in the inhibition of inflammation within the environment of the vascular endothelium. Reduced plasma antithrombin may result from congenital deficiency or arise secondarily from a range of disorders such as liver dysfunction, premature infancy and sepsis, or as a result of interventions such as major surgery or cardiopulmonary bypass. Congenital antithrombin deficiency is the most clinically important of the inherited thrombophilias resulting in thrombosis in the majority of those affected. The challenge in managing these patients is preventing potentially life-threatening thrombosis, while minimising the equally significant risk of haemorrhage associated with long-term anticoagulation. This is achieved in the first instance by identifying high-risk episodes such as surgery, immobility and pregnancy for which prophylactic anticoagulation can be used in the short term. Prophylaxis for such periods is best provided by the use of low molecular weight heparin (LMWH) with substitution by or addition of antithrombin concentrate in particularly high-risk circumstances. In the case of pregnancy, antithrombin concentrate is often used around the time of birth when LMWH may increase the risk of post-partum haemorrhage. As patients with congenital antithrombin deficiency get older so their thrombotic risk gradually increases and for many patients long-term anticoagulation becomes unavoidable because of recurrent episodes of venous thromboembolism. There has been much interest in the role of antithrombin deficiency in the setting of sepsis and the critically ill patient where there is a clear correlation between severity of illness and degree of antithrombin reduction. It is not clear yet, however, to what extent the depletion of antithrombin affects the clinical condition of such patients. A number of trials have investigated the use of antithrombin as a treatment in the intensive care setting with the overall conclusion being that there is some benefit to its use but only if large supra-physiological doses are used. It has also become clear that the concurrent use of any form of heparin removes whatever benefit may be derived from antithrombin treatment in this setting. Until recently, antithrombin replacement was only available as a pooled plasma-derived product, which despite effective viral inactivation still carries an uncertain risk of transfusion transmitted infection. A recombinant antithrombin product now under investigation, and recently licensed in Europe, may provide a useful alternative treatment option.

Antithrombin Nagasaki (Ser 116 to Pro): a rare antithrombin variant with abnormal heparin binding presenting during pregnancy

Quantitative antithrombin deficiency constitutes an important risk factor for venous thromboembolism, stillbirth, and other complications of pregnancy. Studies suggest, however, that individuals heterozygous for missense mutations involving the heparin-binding site of antithrombin do not have a significantly increased thrombotic risk. Owing to the rarity of such mutations, it remains unclear whether any specific heparin-binding site defects might be associated with thrombotic potential. We report here the case of a pregnant woman with an exceptionally rare Type II heparin-binding site antithrombin variant. This case highlights the difficult issues that are associated with the management of Type II antithrombin deficiency during pregnancy.

Inherited thrombophilias and anticoagulation in pregnancy

Thromboprophylaxis, primary or secondary, should be considered in selected pregnant women with inherited thrombophilias; such women may be divided into high-, medium- and low-risk categories on the basis of the specific thrombophilic defect and any personal or family history of venous thromboembolism (VTE). Women at high risk of VTE should receive treatment doses of low-molecular-weight heparin (LMWH) throughout pregnancy and should remain on anticoagulation for 6 weeks postpartum, or, where appropriate, long-term. Women at moderate risk should be treated with prophylactic fixed-dose LMWH throughout pregnancy and for 6 weeks postpartum. Women at low risk should receive prophylactic fixed-dose LMWH for 6 weeks postpartum, and low-dose aspirin LDA should be considered during pregnancy. LWMH offers important advantages over unfractionated heparin (UFH); heparin-induced thrombocytopaenia (HIT) and osteopaenia are rarely seen. For treatment doses of LMWH, dosage adjustment based on anti-Xa levels is usually required as pregnancy progresses. Warfarin should be avoided throughout pregnancy. LMWH, UFH and warfarin are safe for breast-feeding mothers.

Clinical and laboratory evaluation of thrombophilia

Thrombophilia is the predisposition to venous thromboembolism and is caused by inherited and acquired factors, alone or in combination. With the discovery of APC resistance and the prothrombin gene mutation, more than half of all patients with clinical characteristics of thrombophilia are now diagnosed with an inherited disorder. The hypercoagulable work-up of patients with venous thromboembolism is important, because the causes can influence the duration and management of anticoagulation therapy, as well as affect other decisions regarding life and health issues.

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.

Antithrombin deficiency: issues in laboratory diagnosis

OBJECTIVE

To review the current understanding of the pathophysiology of antithrombin deficiency and its role in congenital thrombophilia. Recommendations for diagnostic testing of antithrombin function and concentration, derived from the medical literature and consensus opinions of recognized experts in the field, are included. These recommendations specify whom, how, and when to test.

DATA SOURCES

Review of the published medical literature.

DATA EXTRACTION AND SYNTHESIS

A summary of the medical literature and proposed testing recommendations were prepared and presented at the College of American Pathologists Conference XXXVI: Diagnostic Issues in Thrombophilia. After discussion at the conference, consensus recommendations presented in this article were accepted after a two-thirds majority vote by the participants.

CONCLUSIONS

Antithrombin deficiency is an infrequent genetic abnormality that may be a significant contributing cause of thrombophilia. Antithrombin deficiency also may be observed in conjunction with other genetic or acquired risk factors. Assay of antithrombin plasma levels is appropriate in the laboratory evaluation of individuals with thrombophilia, preferably using a functional, amidolytic antithrombin assay. The diagnosis of antithrombin deficiency should be established only after other acquired causes of antithrombin deficiency, such as liver disease, consumptive coagulopathy, or heparin therapy, are excluded. A low antithrombin level should be confirmed with a subsequent assay on a fresh specimen, and family studies may be helpful to establish the diagnosis. Antigenic antithrombin assays may be of benefit in subclassification of the type of antithrombin deficiency and to confirm the decreased antithrombin level in patients with type I deficiency.

Pathogenesis of venous thromboembolism

Three factors are related with the pathogenesis of venous thrombosis: (1) blood stasis, (2) hypercoagulability, and (3) vessel damage. Local and systemic factors are implicated in blood stasis. Remarkable advances have been recently achieved regarding the understanding of the concept of hypercoagulability, with special emphasis to thrombophilic molecular abnormalities. Increased thromboembolic risk has been described in patients with antithrombin III, protein C, or protein S deficiencies as well as factor V Leiden, prothrombin mutation G20210A, or hyperhomocystinemia. Vessel wall has a remarkable role in protecting against and in promoting thrombosis. The role of inflammation on venous thrombosis is under investigation.

Regulation of vascular bed-specific prothrombotic potential

Hemostasis is the result of interdependent and complex systemic and local endothelial pathways that govern vascular integrity and rheology. A striking feature of hypercoagulable conditions is the focal nature of the resultant thrombotic pathology. Such disorders in hemostasis may be associated with distinct vascular beds, thus implying that the relative combined contribution of individual regulatory pathways may be specific and/or unique to a particular locale in the vasculature. Systemic factors and platelets mediate the formation of fibrin deposition; however, it is the diverse interrelationships in the interaction of these systemic elements with the local endothelial components that dictate vascular bed-specific hemostatic regulation. Indeed, the local activation of coagulation cascades, rather than increases in systemic thrombotic potential, is what leads to fibrin formation in different vascular beds. Hence, the propensity for congenital or acquired disorders to result in local thrombotic pathology is based on the relative contribution of the various hemostatic regulatory pathways in individual vascular beds. The present review highlights the role of local endothelial regulation in the interaction between local and systemic elements that contribute to vascular bed-specific prothrombotic potential.

Recurrent leg ulcers and arterial thrombosis in a 33-year-old homozygous variant of antithrombin

We report here a homozygous variant case of antithrombin (AT) associated with arterial thrombosis and recurrent leg ulcers. The deep vein thrombosis was recognized by the venogram of his pelvic veins. His leg ulcers were scattered around his left ankle and accompanied by lipodermatosclerosis, which was evident in venous insufficiency. The propositus had developed cerebral infarction 12 years prior to his leg ulcers. Coagulation study showed low heparin cofactor activity of his AT with a normal level of immunoreactive AT. Nucleotide sequence analysis of the exon 2 of his AT gene showed Arg47-Cys mutation, leading to the lack of affinity of AT for heparin. The propositus is a homozygote for this abnormality.

Anticoagulant proteins in childhood venous and arterial thrombosis: a review

Thrombotic disease is less frequent in children than in adults, but may result in severe morbidity and mortality. The coagulation system is balanced to provide rapid activation to stop bleeding and appropriate inhibition to prevent unwanted clot extension. It is regulated by fibrinolysis and by three major anticoagulant pathways: the protein C, antithrombin, and tissue factor pathway inhibitor systems. Acquired or inherited abnormalities of coagulation proteins or hemostatic regulatory mechanisms, particularly when combined with dehydration or the presence of indwelling catheters, may pose a high risk for thrombosis. Thrombosis in a child warrants investigation of potential underlying prothrombotic conditions. These include acquired antiphospholipid antibodies or the lupus anticoagulant as well as abnormalities of the inherited anticoagulant factors including protein C, protein S, antithrombin, and Factor V Leiden. Other abnormalities may result in heightened levels of otherwise normal coagulation proteins such as hyperprothrombinemia due to the prothrombin 20210 mutation. A large survey of children with thrombosis indicated that Factor V Leiden, protein C deficiency, and increased lipoprotein(a) were found most commonly. The most severe predisposition occurs with homozygous protein S or protein C deficiency with resultant purpura fulminans in the newborn. The risk of thrombosis in children with heterozygous deficiencies of anticoagulant proteins is not well defined, although it is clear that combined heterozygotes or a combination of an inherited and an acquired defect heightens the risk for thrombosis. Treatment of thrombosis primarily involves a rapidly acting anticoagulant such as heparin or low-molecular-weight heparin to prevent extension, and long-term anticoagulation with warfarin may be instituted to prevent recurrence. Fibrinolytic therapy is infrequently used because of the risk of serious bleeding complications and is reserved for selected cases of arterial thrombosis to initiate rapid reperfusion of ischemic tissue or used in those patients with a large venous thrombosis and pulmonary emboli causing hemodynamic compromise.

Thrombophilia in pregnancy

Thrombophilia can be defined as a predisposition to thrombosis. Abnormalities in haemostasis that are associated with clinical thrombophilia include heritable defects, such as mutations in the genes encoding the natural anticoagulants antithrombin, protein C, and protein S, or clotting factors prothrombin and factor V, and acquired defects, such as antiphospholipids. Women with thrombophilic defects have been shown to be at increased risk, not only of pregnancy associated thromboembolism, but also of other vascular complications of pregnancy, including pre-eclampsia and fetal loss. Routine thrombophilia screening of all women attending antenatal clinics is not recommended. Because some thrombophilic defects--for example, type 1 antithrombin deficiency and antiphospholipids--are associated with a high risk of recurrent thrombosis or other pregnancy complications, it is suggested that selected women (those with a personal or confirmed family history of venous thromboembolism or with a history of recurrent fetal loss) are screened for these defects to allow pregnancy management planning.

Rapid genetic diagnosis in neonatal pulmonary artery thrombosis caused by homozygous antithrombin Budapest 3

We report a case of spontaneous left pulmonary artery thrombosis in a 3-day-old male neonate. The presentation of heparin resistance and thrombosis raised the possibility of a type II heparin binding site antithrombin deficiency. A continuous infusion of antithrombin concentrate was used successfully, following failure of plasma, to correct the heparin resistance. Rapid genetic analysis allowed sequencing of the antithrombin gene within 5 working days. This showed the infant to be homozygous for the substitution of C to T at nucleotide 2759. This base change causes mutation of the native leucine at codon 99 to a phenylalanine. This antithrombin variant has been previously reported (antithrombin Budapest 3) and results in reduced binding of heparin to antithrombin. Such a molecular diagnostic approach is feasible and warranted in such cases of neonatal thrombosis because of the diagnostic difficulties encountered.

Management of patients with hereditary hypercoagulable disorders

The inherited hypercoagulable states can be divided into those that are common and associated with a modest risk of thrombosis (i.e. factor V Leiden and G20210A prothrombin gene) and those that are uncommon but associated with a high risk of thrombosis. There is no convincing evidence that, independent of other clinical factors, the presence of factor V Leiden or the prothrombin gene mutation should influence the use of primary prophylaxis or the duration of anticoagulant therapy following an episode of thrombosis. Indirect evidence suggests that the presence of antithrombin, protein C deficiency, or protein S deficiency justifies avoiding additional risk factors for thrombosis, such as estrogen therapy, and justifies use of more aggressive primary prophylaxis when additional risk factors cannot readily be avoided (e.g. pregnancy). The presence of one of these three abnormalities also favors more prolonged anticoagulant therapy following venous thrombosis. However, their presence or absence appears to have less influence on the risk of recurrent venous thromboembolism than whether thrombosis was provoked by a major reversible risk factor, such as surgery.

Protein S secretion differences of missense mutants account for phenotypic heterogeneity

To elucidate the molecular background for the heterogeneity in protein S plasma concentrations observed in protein S deficient individuals, the in vitro synthesis of recombinant protein S missense mutants was investigated. Six different naturally occurring mutations identified in the protein S gene (PROS1) of thrombosis patients were reproduced in protein S cDNA by site directed mutagenesis. Two mutants, G441C and Y444C (group A), were associated with low total plasma concentration of protein S. Modestly low protein S was found in families with R520G and P626L (group B) mutants. T57S and I518M (group C), which was associated with marginally low protein S, did not segregate with protein S deficiency in the respective families, raising doubts as to whether they were causative mutations or rare neutral variants. The 6 protein S mutants were transiently expressed in COS 1 cells. The Y444C mutant showed the lowest level of secretion (2.5%) followed by the G441C mutant (40%). Group B demonstrated around 50% reduction in secretion, whereas group C mutants showed normal secretion. Pulse-chase experiments demonstrated impaired protein S processing with intracellular degradation and decreased secretion into the culture media of group A and B mutants. Interestingly, there was a good correlation between in vitro secretion and the concentration of free protein S in the plasma of heterozygous carriers. These results demonstrate impaired protein S secretion to be an important mechanism underlying hereditary protein S deficiency and that variations in protein secretion is a major determinant of the phenotypic heterogeneity observed in protein S deficiency. (Blood. 2000;95:173-179)

Protein S secretion differences of missense mutants account for phenotypic heterogeneity

To elucidate the molecular background for the heterogeneity in protein S plasma concentrations observed in protein S deficient individuals, the in vitro synthesis of recombinant protein S missense mutants was investigated. Six different naturally occurring mutations identified in the protein S gene (PROS1) of thrombosis patients were reproduced in protein S cDNA by site directed mutagenesis. Two mutants, G441C and Y444C (group A), were associated with low total plasma concentration of protein S. Modestly low protein S was found in families with R520G and P626L (group B) mutants. T57S and I518M (group C), which was associated with marginally low protein S, did not segregate with protein S deficiency in the respective families, raising doubts as to whether they were causative mutations or rare neutral variants. The 6 protein S mutants were transiently expressed in COS 1 cells. The Y444C mutant showed the lowest level of secretion (2.5%) followed by the G441C mutant (40%). Group B demonstrated around 50% reduction in secretion, whereas group C mutants showed normal secretion. Pulse-chase experiments demonstrated impaired protein S processing with intracellular degradation and decreased secretion into the culture media of group A and B mutants. Interestingly, there was a good correlation between in vitro secretion and the concentration of free protein S in the plasma of heterozygous carriers. These results demonstrate impaired protein S secretion to be an important mechanism underlying hereditary protein S deficiency and that variations in protein secretion is a major determinant of the phenotypic heterogeneity observed in protein S deficiency. (Blood. 2000;95:173-179)

Heritable coagulopathies in pregnancy

Heritable coagulopathies are leading causes of maternal thromboembolism and are associated with an increased risk of maternal and perinatal morbidity and mortality. The most common of these disorders are antithrombin III deficiency, protein C deficiency, protein S deficiency, activated protein C resistance resulting from the factor V Leiden mutation, elevated prothrombin activity associated with a mutation in the prothrombin gene, and hyperhomocystinemia. The maternal risk of a thromboembolic episode is increased by a factor of eight in the presence of any of these heritable states. In addition, the relative risk for a stillbirth in the presence of one of these disorders is 3.6. These conditions are also associated with intrauterine growth retardation and preeclampsia. Proper management of heritable coagulopathies during pregnancy is essential to reduce the risk of these serious sequelae. Patients with newly diagnosed deep-vein thromboses or pulmonary emboli should be treated with therapeutic levels of unfractionated or low molecular weight heparin, followed by subsequent prophylactic heparin therapy. All patients with a history of thromboembolism before pregnancy or evidence of any of these coagulopathies may be offered prophylactic therapy with low molecular weight heparin. Patients with antithrombin III deficiency should receive full therapeutic heparin therapy for the entire pregnancy, irrespective of their thromboembolic history. Postpartum therapy with either heparin or warfarin is required in all cases.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the various heritable coagulopathies that can complicate pregnancy, to state the potential adverse effects of heritable coagulopathies in pregnancy, and to explain the management of heritable coagulopathies during pregnancy.

The molecular genetics of familial venous thrombosis

In the past few years, important advances have been made in the identification of factors predisposing to familial thrombophilia. Particular attention has been paid to the characterization of known inherited defects and their genotype-phenotype relationship, and to studying the interaction between single or multiple inherited conditions and acquired risk factors for venous thrombosis. The recent discovery of 'new' and very common genetic lesions predisposing to thrombosis has greatly expanded the interest in this field. Hereditary predisposition to venous thrombosis may be related to lesions in one or more of 10-15 genes encoding antithrombin, Protein C, Protein S, Factor V, prothrombin, enzymes of the homocysteine metabolic pathway, fibrinogen, heparin cofactor II, plasminogen and thrombomodulin. About 500 different gene lesions (substitutions, deletions, insertions) have so far been reported to affect these genes in patients with thrombotic disease. Because there are potentially multiple interactions between genetic and environmental factors, familial thrombophilia is now considered to be a multifactorial disease. The aim of this chapter is to review aspects of the molecular genetics of familial thrombophilia. In particular, those gene/protein defects for which there is convincing evidence of an association with familial thrombosis will be examined in detail.

Familial Overexpression of β Antithrombin Caused by an Asn135Thr Substitution

We have investigated the basis of antithrombin deficiency in an asymptomatic individual (and family) with borderline levels (≈70% antigen and activity) of antithrombin. Direct sequencing of amplified DNA showed a mutation in codon 135, AAC to ACC, predicting a heterozygous Asn135Thr substitution. This substitution alters the predicted consensus sequence for glycosylation, Asn-X-Ser, adjacent to the heparin interaction site of antithrombin. The antithrombin isolated from plasma of the proband by heparin-Sepharose chromatography contained amounts of β antithrombin (the very high affinity fraction) greatly increased (≈20% to 30% of total) above the trace levels found in normals. Expression of the residue 135 variant in both a cell-free system and COS-7 cells confirmed altered glycosylation arising as a consequence of the mutation. Wild-type and variant protein were translated and exported from COS-7 cells with apparently equal efficiency, in contrast to the reduced level of variant observed in plasma of the affected individual. This case represents a novel cause of antithrombin deficiency, removal of glycosylation concensus sequence, and highlights the potentially important role of β antithrombin in regulating coagulation.