Investigation and management of heritable thrombophilia

Abstract

There is no internationally accepted definition of thrombophilia. The British Committee for Standards in Haematology (1990) suggested that the term thrombophilia be used to describe ‘disorders of the haemostatic mechanisms which are likely to predispose to thrombosis’. This definition is widely used but has disadvantages. Firstly, as new thrombophilic conditions have been described, it has become evident that many individuals who carry these defects remain asymptomatic. Secondly, even though there is an increase in the number of abnormalities recognized as likely to enhance the risk of thrombosis, detailed laboratory investigations fail to detect any abnormality in at least 50% of patients who present with a history of thrombosis. In North America, the term thrombophilic is frequently used by clinicians to describe patients who have developed venous thrombosis either spontaneously or of a severity out of proportion to any recognized stimulus, patients who have recurrent venous thrombotic events and patients who develop venous thrombosis at an early age. This may be a more clinically useful concept. Individuals who have a thrombophilic defect identified on laboratory testing and who have a family history of proven venous thrombosis are at greater risk of thrombosis than individuals who have a thrombophilic defect but no personal or family history of venous thrombosis (Lensen et al, 1996). Inevitably, with the developing interest in the role of prothrombotic abnormalities in thrombosis risk, haematologists and other clinicians have come under pressure to initiate laboratory tests on an increasing number of patients. Performance of a comprehensive range of laboratory tests for thrombophilia has thus become commonplace in subjects presenting with deep vein thrombosis (DVT) or pulmonary embolism. Detection of a heritable prothrombotic state may also lead to testing of family members in an attempt to identify asymptomatic relatives who may be at increased risk of venous thromboembolism (VTE). There are several important issues relevant to the clinical utility and cost effectiveness of this approach. Clinicians may tend to overestimate the risk of thrombosis associated with thrombophilias and to underestimate the risks associated with anticoagulation. Frequently, this has led to the belief that prophylactic anticoagulation is a safer option than clinical surveillance. As evidence about the risk of venous thrombosis associated with thrombophilias accumulates, it is becoming clear that for most patients this is not the case. Furthermore, there is often a lack of recognition that failure to identify a defect in an individual is not proof that no defect exists, only that the particular defects for which tests have been performed are probably not present. The patient with ‘negative’ test results may well have as yet unidentifiable prothrombotic abnormalities, which increase his or her risk of thrombosis. Reassuring patients with normal test results may constitute false reassurance, which may present a real risk for the patient if negative laboratory results lead to ignoring or underestimating the clinical history. Prothrombotic states may be heritable, acquired or mixed – the result of the environment (e.g. oestrogen use, obesity or other lifestyle factors) interacting with genetic background. To date, a limited number of genetic variants are proven to be independent risk factors for venous thromboembolism. These include mutations in the genes encoding the natural anticoagulants antithrombin, protein C and protein S, and the clotting factors fibrinogen, prothrombin and factor V. Antithrombin deficiency Antithrombin (previously called antithrombin III) is synthesized by the liver. Its inhibitory effect is not confined to thrombin. It also inhibits the activated clotting factors IXa, Xa, XIa, XIIa and tissue factor-bound factor VIIa. Heparins markedly accelerate the rate of complex formation between antithrombin and the serine proteases. Within the last decade, our understanding of the basis of familial antithrombin deficiency has been greatly facilitated by advances made in the molecular biology and functional characterization of this inhibitory glycoprotein. Of particular significance has been the recognition that the antithrombin molecule possesses two important functional regions – a heparin-binding domain and a thrombin-binding domain. Two major types of heritable antithrombin deficiency are recognized. Type I is characterized by a quantitative reduction of qualitatively (functionally) normal antithrombin protein. Type II deficiency is due to the production of a qualitatively abnormal protein. In both types of antithrombin deficiency, antithrombin activity is reduced to a variable extent. Type II deficiency is subclassified according to the site of the molecular defect: (a) Reactive site (RS) – abnormalities residing in the reactive (thrombin binding) site. (b) Heparin binding site (HBS) – abnormalities residing in the heparin binding site. (c) Pleiotropic effect (PE) – abnormalities residing in both reactive and heparin binding sites. Antithrombin assays Only functional assays of heparin cofactor activity will detect both type I and type II antithrombin deficiencies. For routine clinical purposes it is recommended that a heparin cofactor activity assay be used in the initial screen. The distinction between the subtypes of antithrombin deficiency is of clinical relevance as the incidence of thrombosis is higher in association with type I deficiency and type II deficiency in which the mutation affects the reactive site than in type II deficiency in which the mutation affects the heparin binding site (Finazzi et al, 1987). An initial classification into type I or type II can be made by comparing the result of an immunological assay with the result of the heparin cofactor assay. Type II heparin-binding variants are associated with a lower risk of thrombosis than type II reactive site defects. However, a heparin-binding variant may increase the attributable risk of an additional thrombophilic defect, such as the factor V Leiden mutation. A short incubation, of 30 s or less, with a low concentration of heparin is required for detection. As most currently used antithrombin activity assays utilize a long incubation, heparin-binding defects are not detected. The distinction between type II defects is therefore only an issue for those centres using assays specifically to detect heparin-binding defects. Crossed immunoelectrophoresis with heparin is a simple tool that may be used to detect type II heparin-binding site variants in those centres that need to identify these defects. Although many mutations associated with antithrombin deficiency have been described, identification of the mutation is not usually necessary for clinical purposes. Normal ranges and variations Age- and sex-related variations in antithrombin activity (Tait et al, 1993a) and antigen levels are minor, so the reference ranges in healthy populations are narrow. Antithrombin levels are slightly lower in premenopausal women than in men of similar age or post-menopausal women and are slightly lower in women using combined oral contraceptive pills than in non-pill-using women (Tait et al, 1993a). More significant decreases in antithrombin activity are observed in patients on heparin treatment (Marciniak & Gockerman, 1977) and in those with current thrombosis. Profound decreases in plasma antithrombin are seen in disseminated intravascular coagulation, liver disease and the nephrotic syndrome. Prevalence of antithrombin deficiency and risk of thrombosis The prevalence of type I antithrombin gene mutations in the general population is of the order of 0·02% (Tait et al, 1994). Family studies suggest that antithrombin deficiency is a more severe disorder than deficiencies of protein C or protein S with the majority of patients suffering thrombosis before the age of 25 years (Thaler & Lechner, 1981; Hirsh et al, 1989; Demers et al, 1992). In studies of unselected patients with thrombosis, antithrombin deficiency was reported in 1% (Heijboer et al, 1990) and 0·5% (Mateo et al, 1997). The relative risk of venous thromboembolism is around 25–50-fold for individuals with type I antithrombin deficiency (Rosendaal, 1999). Protein C deficiency Protein C is a vitamin K-dependent glycoprotein that is synthesized in the liver. Before activation by the thrombin–thrombomodulin complex on the endothelial cell surface, it circulates as a two-chain zymogen. By degrading activated clotting factors Va and VIIIa, activated protein C (APC) functions as one of the major inhibitors of the coagulation system. Activated protein C also reduces platelet prothrombinase activity by degrading platelet-bound factor Va at the receptor for factor Xa. The inhibitory effects of activated protein C are facilitated through the cofactor activity of protein S. As with antithrombin deficiency, familial protein C deficiency can be classified into two types on the basis of phenotypic analysis using functional and immunological assays. Type I is characterized by parallel reductions of functional and immunoreactive protein C. In type II the functional level is substantially lower than that of the antigen. In contrast to antithrombin deficiency, in which type II deficiency is more common than type I, type I protein C deficiency is more common than type II. The anticipation that the underlying genetic variant and associated phenotype might be predictive of the degree of thrombotic risk has not been realized and phenotypic classification of protein C deficiency therefore serves no useful clinical purpose. Protein C assays Most functional assays of protein C use the specific activator Protac which is derived from snake venom. The activated protein C formed can be quantified by clotting or chromogenic methods. Both are available in kit form from commercial manufacturers. A standard calibrated against the current

Keywords

Affiliated Institutions

• British Society for Haematology GB

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