Antithrombin III Glasgow: a variant with increased heparin affinity and reduced ability to inactivate thrombin, associated with familial thrombosis

Thrombophilia Screening: Not So Straightforward

Although inherited thrombophilias are lifelong risk factors for a first thrombotic episode, progression to thrombosis is multifactorial and not all individuals with inherited thrombophilia develop thrombosis in their lifetimes. Consequently, indiscriminate screening in patients with idiopathic thrombosis is not recommended, since presence of a thrombophilia does not necessarily predict recurrence or influence management, and testing should be selective. It follows that a decision to undertake laboratory detection of thrombophilia should be aligned with a concerted effort to identify any significant abnormalities, because it will inform patient management. Deficiencies of antithrombin and protein C are rare and usually determined using phenotypic assays assessing biological activities, whereas protein S deficiency (also rare) is commonly detected with antigenic assays for the free form of protein S since available activity assays are considered to lack specificity. In each case, no single phenotypic assay is capable of detecting every deficiency, because the various mutations express different molecular characteristics, rendering thrombophilia screening repertoires employing one assay per potential deficiency, of limited effectiveness. Activated protein C resistance (APCR) is more common than discrete deficiencies of antithrombin, protein C, and protein S and also often detected initially with phenotypic assays; however, some centres perform only genetic analysis for factor V Leiden, as this is responsible for most cases of hereditary APCR, accepting that acquired APCR and rare F5 mutations conferring APCR will go undetected if only factor V Leiden is evaluated. All phenotypic assays have interferences and limitations, which must be factored into decisions about if, and when, to test, and be given consideration in the laboratory during assay performance and interpretation. This review looks in detail at performance and limitations of routine phenotypic thrombophilia assays.

Familial Thrombophilia: A Review Analysis

The correct approach to the management of the asymptomatic carrier with a recognized inherited thrombophilic disorder is uncertain because reliable in formation of the risk of spontaneous (unprovoked) throm bosis in these disorders is not available. To determine the best available estimate of the annual incidence of spon taneous thrombosis in asymptomatic carriers of disorders that have been linked to familial thrombophilia, we per formed a literature review. Using Medline search from 1965 to 1992, supplemented by manual searches, we re trieved all articles that presented data on antithrombin III, protein C, protein S, dysfibrinogenemia, plasmino gen, histidine-rich glycoprotein, heparin cofactor II, and fibrinolysis in relation to thrombosis. Publications were included in the analysis if they (1) reported one or more probands with thrombotic disease and a heterozygous biochemical abnormality of the hemostatic system, (2) assessed the presence of this abnormality in family mem bers independent of the presence or absence of a history of thrombotic disease, and (3) assessed the presence of a history of thrombotic disease in all available family mem bers. The biochemical status and clinical details of all family members reported were extracted from each eligi ble article. For each abnormality the odds ratio for throm bosis was compared in family members with and without the biochemical abnormality. If applicable, thrombosis- free survival and age-specific incidences of thrombosis were calculated. The thrombotic episodes were classified as spontaneous or secondary to a recognized risk factor, and the proportion of spontaneous episodes was calcu lated. The influence of diagnostic suspicion bias in symp tomatic patients with a family history of thrombosis was reduced by recalculating the absolute incidence of throm bosis from the odds ratio after adjusting the incidence of venous thrombosis in nonaffected family members to that observed in the general population. Statistically signifi cant associations between the presence of a biochemical abnormality and a history of venous thrombosis were found for antithrombin III deficiency types 1 and 2a and 2b, protein C deficiency type 1, and protein S deficiency type I. Dysfibronogenemia was statistically significantly associated with venous as well as arterial thrombosis. Thirty-five to 67% of the events were classified as being provoked, as they occurred following exposure to a rec ognized risk factor for thrombosis. The recalculated an nual incidence of spontaneous thrombosis was 0.6 to 1.6%/year. It is concluded that this relatively low inci dence does not warrant life-long continuous use of anti coagulant prophylaxis since the reported risk of major and fatal bleeding associated with the use of oral antico agulants is 2-3 and 0.4%/year, respectively.

Conformational changes in serpins: II. The mechanism of activation of antithrombin by heparin

Antithrombin, uniquely among plasma serpins acting as proteinase inhibitors in the control of the blood coagulation cascade, circulates in a relatively inactive form. Its activation by heparin, and specifically by a pentasaccharide core of heparin, has been shown to involve release of the peptide loop containing the reactive centre from partial insertion in the A sheet of the molecule. Here we compare the structures of the circulating inactive form of antithrombin with the activated structure in complex with heparin pentasaccharide. We show that the rearrangement of the reactive centre loop that occurs upon activation is part of a widespread conformational change involving a realignment of the two major domains of the molecule. We also examine natural mutants that possess high affinity for heparin pentasaccharide, and relate the kinetics of their interaction with heparin pentasaccharide to the structural transitions occuring in the activation process.

Heparin-dependent modification of the reactive center arginine of antithrombin and consequent increase in heparin binding affinity

Antithrombin, the principal plasma inhibitor of coagulation proteinases, circulates in a form with low inhibitory activity due to partial insertion of its reactive site loop into the A-beta-sheet of the molecule. Recent crystallographic structures reveal the structural changes that occur when antithrombin is activated by the heparin pentasaccharide, with the exception of the final changes, which take place at the reactive center itself. Here we show that the side chain of the P1 Arg of alpha-antithrombin is only accessible to modification by the enzyme peptidylarginine deiminase on addition of the heparin pentasaccharide, thereby inactivating the inhibitor, whereas the natural P1 His variant, antithrombin Glasgow, is unaffected, indicating that only the P1 Arg becomes accessible. Furthermore, the deimination of P1 Arg converts antithrombin to a form with 4-fold higher affinity for the heparin pentasaccharide, similar to the affinity found for the P1 His variant, due to a lowered dissociation rate constant for the antithrombin-pentasaccharide complex. The results support the proposal that antithrombin circulates in a constrained conformation, which when released, in this study by perturbation of the bonding of P1 Arg to the body of the molecule, allows the reactive site loop to take up the active inhibitory conformation with exposure of the P1 Arg.

Molecular genetics of antithrombin deficiency

Antithrombin is the major proteinase inhibitor of thrombin and other blood coagulation proteinases. Antithrombin has two functional domains, a heparin binding site and a reactive centre (that complexes and inactivates the proteinase). Its deficiency results in an increased risk of venous thromboembolism. Appreciable progress has been made in recent years in understanding the structure and function of this protein, the genetic cause of inherited deficiency and its clinical consequence. The structure of antithrombin is now considered in terms of the models derived from X-ray crystallography, which have provided explanations for the function of its heparin interaction site and of its reactive loop. The structural organization of the antithrombin gene has been defined and numerous mutations have been identified that are responsible for antithrombin deficiency: these may reduce the level of the protein (Type I deficiency), alter the function of the protein (Type II deficiency, altering heparin binding or reactive sites), or even have multiple or 'pleiotropic effects' (Type II deficiency, altering both functional domains and the level of protein).

Antithrombin III Kumamoto II; a single mutation at Arg393-His increased the affinity of antithrombin III for heparin

Abnormal antithrombin III (AT III) was found in a 30-year-old woman who suffered from recurrent thrombosis during pregnancy and the postpartum period. Among her family members, only her father had recurrent episodes of deep vein thrombosis of the lower extremities, from his youth. The antithrombin and antifactor Xa heparin cofactor activities of the proposita's plasma were 61% and 42% of normal, respectively. The progressive antithrombin and antifactor Xa activities were also decreased to 55% and 58% of normal, respectively. The immunoreactive level of AT III was within the normal range (23.1 mg/dl). Analysis of the proposita's plasma by crossed immunoelectrophoresis in the presence or absence of heparin and by affinity chromatography on heparin-Sepharose revealed that the proposita's AT III had apparently normal affinity for heparin. Nucleotide sequencing of 7 exons of the proposita's AT III gene amplified by polymerase chain reaction (PCR) disclosed that the second base of codon 393 comprised both G and A, indicating Arg393-His conversion. The base sequences of exons 1, 2, 3a, 3b, 4, and 5 were normal, excluding any other mutation. These findings indicated that the proposita's AT III was a variant of AT III at the thrombin binding site and that the proposita was a heterozygote for the abnormality. Heparin affinity of purified abnormal AT III from the proposita's plasma was demonstrated to be increased upon affinity chromatography using heparin-Sepharose, suggesting that the mutation (Arg393-His) per se could possibly increase the affinity of antithrombin III for heparin. For this variant AT III (Arg393-His), the name AT III Kumamoto II is proposed.

Pleiotropic effects of antithrombin strand 1C substitution mutations

Six different substitution mutations were identified in four different amino acid residues of antithrombin strand 1C and the polypeptide leading into strand 4B (F402S, F402C, F402L, A404T, N405K, and P407T), and are responsible for functional antithrombin deficiency in seven independently ascertained kindreds (Rosny, Torino, Maisons-Laffitte, Paris 3, La Rochelle, Budapest 5, and Oslo) affected by venous thromboembolic disease. In all seven families, variant antithrombins with heparin-binding abnormalities were detected by crossed immunoelectrophoresis, and in six of the kindreds there was a reduced antigen concentration of plasma antithrombin. Two of the variant antithrombins, Rosny and Torino, were purified by heparin-Sepharose and immunoaffinity chromatography, and shown to have greatly reduced heparin cofactor and progressive inhibitor activities in vitro. The defective interactions of these mutants with thrombin may result from proximity of s1C to the reactive site, while reduced circulating levels may be related to s1C proximity to highly conserved internal beta strands, which contain elements proposed to influence serpin turnover and intracellular degradation. In contrast, s1C is spatially distant to the positively charged surface which forms the heparin binding site of antithrombin; altered heparin binding properties of s1C variants may therefore reflect conformational linkage between the reactive site and heparin binding regions of the molecule. This work demonstrates that point mutations in and immediately adjacent to strand 1C have multiple, or pleiotropic, effects on this serpin, leading ultimately to failure of its regulatory function.

Antithrombin Glasgow II: alanine 382 to threonine mutation in the serpin P12 position, resulting in a substrate reaction with thrombin

A female with recurrent thrombosis was found to have a functional abnormality of antithrombin, with a ratio of functional to immunological activity in plasma of approximately 50%. Crossed immunoelectrophoresis in the presence of heparin was normal, indicating an abnormality of the reactive site, rather than the heparin binding domain. Accordingly, the antithrombin was isolated by heparin-Sepharose chromatography: this produced a mixture of normal and variant antithrombin, as the patient was heterozygous for the abnormality. To remove the normal component, the antithrombin was passed through a column of thrombin-Sepharose. On sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), prior to its application to thrombin-Sepharose, the antithrombin migrated as a single band with identical mobility to that of normal antithrombin. After thrombin-Sepharose, the purified variant component was proteolysed, and migrated as two components, one with a reduced and one with enhanced mobility under non-reducing conditions. This demonstrated that the variant was unable to form stable inhibitor-thrombin complexes and was cleaved in a substrate reaction with thrombin. One site of cleavage was unambiguously ascertained to be the Arg 393-Ser 394 reactive site bond, by NH2 terminal sequencing of the cleaved variant antithrombin: 10 steps beginning at the P1' position, Ser-Leu-Asn-Pro-Asn-Arg,..., were clearly identified. The mutation responsible for this defect was studied by polymerase chain reaction (PCR) amplification of exon 6 of the antithrombin gene and direct sequencing of the amplified product. The presence of both a G and A in the first position of codon 382, identified the mutation GCA to ACA, which results in the substitution of Ala 382 to Thr. This is identical to that reported for antithrombin Hamilton (Devraj-Kizuk et al, 1988), although antithrombin gene polymorphism analysis suggests that the antithrombin Glasgow II mutation has arisen independently. We have recently shown (Caso et al, 1991) that mutation at a nearby position, Ala 384 to Pro, also transforms another variant, antithrombin Vicenza/Charleville, into a substrate for thrombin. The present results with antithrombin Glasgow II suggest that all the alanine residues at the base of the reactive site loop in positions P12-10 may be important for the formation of a stabilized inhibitor-thrombin complex.

Antithrombin Vicenza, Ala 384 to Pro (GCA to CCA) mutation, transforming the inhibitor into a substrate

Antithrombin (AT) Vicenza has been previously identified as a functionally abnormal antithrombin associated with familial thrombosis (Finazzi et al, 1985). It binds normally to heparin, but loses its affinity following interaction with thrombin: it is a poor inhibitor of thrombin. AT Vicenza was isolated from plasma by heparin-Sepharose and thrombin-Sepharose chromatography, fragmented with cyanogen bromide (CNBr) and its tryptic peptides were analysed by fast atom bombardment mass spectrometry mapping. An abnormal peptide mass 1112 was identified. Edman degradation confirmed a substitution of Ala to Pro in the sequence Ala 383-Arg 393. Polymerase chain reaction amplification of exon 6 of the gene followed by genomic sequencing, localized the mutation to codon 384, GCA to CCA. The same mutation has recently been reported in AT Charleville (Mohlo-Sabatier et al, 1989). Sodium dodecyl-sulphate polyacrylamide gel electrophoresis of AT Vicenza (/Charleville) under non-reducing conditions revealed an apparent increase in mol. wt following interaction with thrombin: under reducing conditions the mol. wt was less than that of normal AT. This indicated cleavage and unfolding of the molecule. The site of cleavage was determined by incubation of AT Vicenza (/Charleville) with thrombin-Sepharose, reduction and S-carboxymethylation and reverse phase FPLC. A peptide was identified with the NH2-terminal sequence beginning Ser-Leu-Asn, demonstrating the cleavage had occurred at the reactive site of the variant. It is concluded that the Ala 384 to Pro substitution transforms AT Vicenza (/Charleville) from an inhibitor into a substrate.

Antithrombin III: structural and functional aspects

Antithrombin III is a plasma glycoprotein responsible for thrombin inhibition in the blood coagulation cascade. The X-ray structure of its cleaved form has been determined and refined to 3.2 A resolution. The overall topology is similar to that of alpha 1-antitrypsin, another member of the serpin (serine protease inhibitor) superfamily. The biological activity of antithrombin III is mediated by a polysaccharide, heparin. The binding site of this effector is described. A possible structural transition from the native to the cleaved structure is discussed.

Identification of a new P1 residue mutation (444Arg----Ser) in a dysfunctional C1 inhibitor protein contained in a type II hereditary angioedema plasma

A new reactive-centre P1 residue mutation (444Arg----Ser), has been identified in a dysfunctional C1 inhibitor protein, C1 inhibitor(Ba), contained in a type II hereditary angioedema plasma. This substitution is compatible with a point mutation of the 444Arg codon (CGC----AGC), and represents the first non-histidine, non-cysteine P1 residue mutant described for C1 inhibitor.

Antithrombin: structure, genomic organization, function and inherited deficiency

Antithrombin is a major plasma protein inhibitor of proteinases generated during blood coagulation; it plays an important role in the regulation of thrombin in blood. The anticoagulant heparin greatly accelerates the rate of inactivation of proteinases by antithrombin, predominantly through its well defined, highly specific binding reaction with the inhibitor, but also through a less strictly defined interaction with some of the proteinases (such as thrombin). There is evidence for an analogous acceleratory mechanism in vivo, that functions by the binding of antithrombin to a subpopulation of heparan sulphate proteoglycans intercalated in the surface of endothelial cells. The location and structure of the gene for antithrombin are known. Both its overall organization and the structure of the subdomains of the expressed protein can be considered in terms of their relationships to a serine proteinase inhibitor superfamily, which is believed to have evolved from a common ancestor. The region of the antithrombin gene 5' to the coding region has been characterized. Unlike other members of the serpin family, there is no TATA-like promoter sequence. Two enhancer sequences have been identified that are homologous to enhancer regions of other genes. There are two polymorphisms: an intragenic polymorphism arising from a translationally silent A to G transition in codon 305, and a length polymorphism arising from the presence of 32 bp or 108 bp non-homologous sequences 345 bp upstream from the translation initiation codon. Inherited deficiency of antithrombin is associated with familial thromboembolism. The molecular genetic basis of some subtypes of deficiency is increasingly yielding to investigation. It is interesting to note that a number of mutations have been identified in CpG dinucleotides, supporting the suggestion that this dinucleotide sequence may represent a mutation hotspot in the human genome.

Antithrombin Chicago, amino acid substitution of arginine 393 to histidine

Antithrombin Chicago is a functionally inactive antithrombin variant whose inheritance is associated with thrombotic disease. The variant antithrombin was isolated from plasma of the propositus by chromatography on heparin-Sepharose, followed by passage through thrombin-Sepharose to remove the normal antithrombin component that is present. A pool of fragments ("CNBr pool 4") containing the reactive site region was prepared from the reduced and S-carboxymethylated variant by cleavage with cyanogen bromide followed by reverse-phase HPLC. Sequential treatment of CNBr pool 4 with trypsin and V8 protease produced peptides whose molecular masses were then determined by fast atom bombardment mass spectrometry. The variant protein digests were characterised by a reduction of a peptide of mass 1086, corresponding to the normal antithrombin sequence Ala382-Arg393. However, they contained a peptide of mass 1748, which arises when Arg393 is replaced by His in the sequence Ala382-Arg399. It is concluded that the functional and clinical abnormalities of antithrombin Chicago are all probably caused by a single amino acid substitution, Arg393 to His.

Antithrombin Sheffield: amino acid substitution at the reactive site (Arg393 to His) causing thrombosis

A Sheffield family with a predisposition towards thrombosis has been shown to have a functional abnormality of antithrombin. The abnormality was detected as reduced heparin cofactor activity, with normal antigenic levels of antithrombin. Crossed immunoelectrophoresis performed in the absence and presence of heparin was normal. The antithrombin was isolated by heparin Sepharose affinity chromatography. It had normal mobility on SDS polyacrylamide gel electrophoresis. However, the second order rate constant of inhibition of thrombin was about half that of normal, and this was compatible with a heterozygous abnormality involving the reactive site. The antithrombin was further purified by chromatography on thrombin-Sepharose (to remove the normal component), reduced, S-carboxymethylated and fragmented with cyanogen bromide. A pool containing the reactive site region was digested with trypsin and the molecular size of peptides generated determined by fast atom bombardment mass spectrometry. The two peptides adjacent to the Arg393-Ser394 bond of mass 2290 and 700 were almost absent from the mass spectrum, but an additional peptide of mass 2952 was present. Subdigestion with V8 protease reduced the mass of this peptide to 1748. These peptides generated by trypsin and V8 protease were almost identical to those obtained when another variant, antithrombin Glasgow, was treated in the same way (Erdjument et al, 1988). It is concluded that the molecular abnormality of antithrombin Sheffield is identical to that of antithrombin Glasgow, Arg393 to His.

Antithrombin Glasgow, 393 Arg to His: a P1 reactive site variant with increased heparin affinity but no thrombin inhibitory activity

Antithrombin Glasgow is a hereditary abnormal antithrombin that has lost thrombin inhibitory activity. It was isolated from the plasma of a 41-year-old male with a history of thrombotic events. Antithrombin Glasgow was purified from plasma using heparin-Sepharose chromatography at pH 7.4 eluting with increasing concentrations of NaCl. The normal protein eluted with 0.9 mol/l NaCl and Glasgow with 1.05 mol/l NaCl. Electrophoresis in agarose at pH 8.6 showed the variant to migrate more anodally than normal. The C-terminal small fragment resulting from catalytic cleavage with elastase between P3 and P4 of the reactive loop was isolated and sequenced. This showed the replacement of the arginine at residue 3 by a histidine. This is residue 393 in the intact molecule. The findings suggest that heparin, on binding, interacts indirectly with the reactive centre region of antithrombin.