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

Characterization of two novel mutations of the antithrombin gene observed in Japanese thrombophilic patients

We investigated the molecular basis of reduced functional levels of antithrombin (AT) in two individuals suffering from thromboembolic events. In each case direct sequencing of amplified DNA revealed 13,260-13,262 del in one patient and 2511C>A in the other patient, predicting a heterozygous E381del and P16H, respectively. Both patients had no 20210A allele and factor V Leiden mutation. To understand the molecular mechanism responsible for antithrombin deficiency, stable expression experiments were performed using HEK293 cells transfected with the expression vector containing the wild-type or the mutated recombinant cDNA. In these experiments, the media levels of the two mutated antithrombins were the same as that of wild type, but the specific activity of the E381del mutant decreased significantly compared with that of wild type. These results showed that the E381del mutation was responsible for type II deficiency, whereas the other mutation, P16H, did not produce any definite abnormality which could contribute to antithrombin deficiency.

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.

Arginine substitutions in the hinge region of antichymotrypsin affect serpin beta-sheet rearrangement

A hallmark of serpin function is the massive beta-sheet rearrangement involving the insertion of the cleaved reactive loop into beta-sheet A as strand s4A. This structural transition is required for inhibitory activity. Small hydrophobic residues at P14 and P12 positions of the reactive loop facilitate this transition, since these residues must pack in the hydrophobic core of the cleaved serpin. Despite the radical substitution of arginine at the P12 position, the crystal structure of cleaved A347R antichymotrypsin reveals full strand s4A insertion with normal beta-sheet A geometry; the R347 side chain is buried in the hydrophobic protein core. In contrast, the structure of cleaved P14 T345R antichymotrypsin reveals substantial yet incomplete strand s4A insertion, without burial of the R345 side chain.

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).

Molecular cloning, expression, and partial characterization of two novel members of the ovalbumin family of serine proteinase inhibitors

A human placental lambda gt11 cDNA library was screened for sequences encoding proteins related to human proteinase inhibitor 6 (PI6), and two plaques were identified that displayed weak hybridization at high stringency. Isolation and characterization of the DNA inserts revealed two novel sequences encoding proteins composed of 376 and 374 amino acids with predicted molecular masses of approximately 42 kDa. The novel proteins displayed all of the structural features unique to the ovalbumin family of intracellular serpins including the apparent absence of a cleavable N-terminal signal sequence. The degree of amino acid sequence identity between the novel serpins and PI6 (63-68%) significantly exceeds that of any other combination of known intracellular serpins. The two novel serpins encoded by the two novel cDNA sequences have been designated as proteinase inhibitor 8 (PI8) and proteinase inhibitor 9 (PI9). The putative reactive center P1-P1' residues for PI8 and PI9 were identified as Arg339-Cys340 and Glu340-Cys341, respectively. PI9 appears to be unique in that it is the first human serpin identified with an acidic residue in the reactive center P1 position. In addition, the reactive center loop of PI9 exhibits 54% identity with residues found in the reactive center loop of the cowpox virus CrmA serpin. Two PI8 transcripts of 1.4 kilobases (kb) and 3.8 kb were detected by Northern analysis in equal and greatest abundance in liver and lung, while the 1.4-kb mRNA was in excess over the 3.8-kb mRNA in skeletal muscle and heart. Two PI9 transcripts of 3.4 and 4.4 kb were detected in equal and greatest abundance in lung and placenta and were weakly detected in all other tissues. PI8 and PI9 were expressed in baby hamster kidney and yeast cells, respectively. Immunoblot analyses using rabbit anti-PI6 IgG indicated the presence of PI8 in the cytosolic fraction of stably transfected cells that formed an SDS-stable 67-kDa complex with human thrombin. PI9 was purified to homogeneity from the yeast cell lysate by a combination of heparin-agarose chromatography and Mono Q fast protein liquid chromatography and migrated as a single band in SDS-polyacrylamide gel electrophoresis with an apparent molecular mass of 42 kDa. Purified recombinant PI9 failed to inhibit the amidolytic activities of trypsin, papain, thrombin, or Staphylococcus aureus endoproteinase Glu-C and did not form an SDS-stable complex when incubated with thrombin. The cognate intracellular proteinases that interact with PI8 and PI9 are unknown.

Antithrombin-TRI (Ala382 to Thr) causing severe thromboembolic tendency undergoes the S-to-R transition and is associated with a plasma-inactive high-molecular-weight complex of aggregated antithrombin

An antithrombin (AT) variant Ala382 to Thr (AT-TRI) was identified by mass spectrometric techniques. The variant behaved as a substrate rather than a thrombin inhibitor, but, contrary to previously described P12 AT variants, AT-TRI, expressed as a heterozygous dominant trait, caused severe thromboembolic tendency beginning in their teens in affected members of an English family. In addition, it underwent the S-to-R conformational state transition as evidenced by an increased resistance to thermal denaturation on active centre cleavage, but did not react with a monoclonal antibody, 4C9, directed against a neoepitope that is present on complexed and cleaved normal AT. Antithrombin-TRI, in plasma, was also associated with an abnormal high molecular weight (M(r)) 194,000) component composed of non-covalently-linked antithrombin molecules. This component (D194) showed low affinity for heparin and was devoid of antithrombin progressive activity. D194, isolated by ammonium sulphate precipitation and three chromatographic steps (heparin Sepharose, ion exchange and immunoaffinity), migrated as a single band of M(r) 60,000 on SDS-PAGE under both reducing and non-reducing conditions and was recognized by monospecific anti-human antithrombin antibodies, but did not immunoreact with antibodies raised against a number of proteins including albumin and thrombin. The above data and the fact that the 15 N-terminal amino acids of this M(r) 60,000 band were identical to that of normal antithrombin indicated that the inactive D194 component was composed of aggregated antithrombin molecules, possibly antithrombin trimers. In conclusion, early adulthood severe thromboembolic tendency, failure to expose the 4C9 epitope, and presence of aggregated AT molecules in the plasma are characteristic features of AT-TRI not previously described in other ALA-382 THR mutations.

The molecular genetics of familial venous thrombosis

This study of naturally occurring mutations predisposing to venous thrombosis has led to a number of important advances in our understanding of protein structure and function relationships and the molecular basis of gene mutation. It has also potentiated the accurate and reliable presymptomatic and antenatal detection of predisposing gene lesions. Perhaps the major challenge facing us is the probabilistic nature of thromboembolism; only a certain proportion of patients with recognized gene defects predisposing to thrombosis will actually suffer from thrombotic episodes. Environmental insults of various kinds, and perhaps epistatic effects resulting from the influence of other loci, are likely to be contributory factors and will help to determine whether a thrombotic event occurs in individuals already compromised by a defect in a gene whose malfunction is known to predispose to thrombosis. Since molecular genetic techniques allow us to dissect the allelioheterogeneity of the different deficiency states by characterizing the wide spectrum of gene mutations giving rise to thrombosis, it may eventually prove possible to relate specific gene lesions to the probability of thromboembolism as well as to the severity and frequency of thrombotic episodes. The multifactorial nature of thrombosis demands a multidisciplinary approach to the analysis of its causation, early detection, treatment and prevention. The application of the new and powerful techniques of molecular genetics promises to make a substantial contribution to all aspects of thrombosis research.

Proteolytically cleaved mutant antithrombin-Hamilton has high stability to denaturation characteristic of wild type inhibitor serpins

The serpin family of proteins consists primarily of proteinase inhibitors which form tight complexes with target proteinases. Inhibitor serpins are cleaved by proteinase and undergo a large conformational change in which the polypeptide segment terminating at the target reactive site, at which cleavage takes place, inserts itself as an additional strand, s4A, in the center of a preexisting beta-sheet. This change in conformation increases the stability towards denaturation of the cleaved serpin relative to the native uncleaved form. Mutant serpins with single amino acid changes in the s4A strand have been identified, and in most cases these are proteinase substrates but not inhibitors. We have measured the stability to denaturation of one of these non-inhibitor substrate mutants, antithrombin-Hamilton, which has an Ala-->Thr change at position P12 in strand s4A. We find that it undergoes the transformation to the more stable form which is observed for inhibitor serpins, from which we conclude that the Ala-->Thr change in antithrombin-Hamilton does not prevent insertion of s4A into beta-sheet A in the cleaved form.

Effects of mutations in the hinge region of serpins

An expression system for alpha 1-antitrypsin in Escherichia coli was developed using a T7 RNA polymerase promoter. Addition of rifampicin to inhibit the E. coli RNA polymerase after induction of the T7 RNA polymerase gene resulted in about 30% of newly synthesized protein being alpha 1-antitrypsin. This expression system was then used to examine the effect of mutations in the hinge region of alpha 1-antitrypsin on its activity. The mutations were based on ones in antithrombin III that had previously been shown to have adverse effects on activity. Mutation of Ala347 to threonine in alpha 1-antitrypsin did not affect the kinetic behavior of the protein with trypsin or human leukocyte elastase. In contrast, mutation of Gly349 to proline converted the majority of the protein into a substrate for both proteinases. The small fraction of this mutant that was active, however, had kinetic parameters that were indistinguishable from wild-type alpha 1-antitrypsin. Cleavage within the reactive-site loop of wild-type alpha 1-antitrypsin causes a conformational change in the molecules (the S-to-R transition) and results in a marked increase in heat stability. This increase in heat stability was also seen upon cleavage within the reactive-site loops of both of the alpha 1-antitrypsin mutants. The results are discussed in terms of a kinetic mechanism for serpin-proteinase interactions, in which after the formation of an initial complex the serpin partitions between the formation of a stable complex and a cleavage reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of conformational change in serpin structure and function

Serpins are members of a family of structurally related protein inhibitors of serine proteinases, with molecular masses between 40 and 100kDa. In contrast to other, simpler, proteinase inhibitors, they may interact with proteinases as inhibitors, as substrates, or as both. They undergo conformational interconversions upon complex formation with proteinase, upon binding of some members to heparin, upon proteolytic cleavage at the reactive center, and under mild denaturing conditions. These conformational changes appear to be critical in determining the properties of the serpin. The structures and stabilities of these various forms may differ significantly. Although the detailed structural changes required for inhibition of proteinase have yet to be worked out, it is clear that the serpin does undergo a major conformational change. This is in contrast to other, simpler, families of protein inhibitors of serine proteinases, which bind in a substrate-like or product-like manner. Proteolytic cleavage of the serpin can result in a much more stable protein with new biological properties such as chemo-attractant behaviour. These structural transformations in serpins provide opportunities for regulation of the activity and properties of the inhibitor and are likely be important in vivo, where serpins are involved in blood coagulation, fibrinolysis, complement activation and inflammation.

Glycosaminoglycans and the regulation of blood coagulation

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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.

Novel point mutations leading to type 1 antithrombin deficiency and thrombosis

Direct sequencing of antithrombin III (AT) gene fragments specifically amplified by the polymerase chain reaction was utilized to identify the molecular basis of type 1 AT deficiency in two unrelated kindreds, both with thrombotic disease. Two novel point mutations were identified, deletion of a T from the second position of codon 81 in one propositus and insertion of a G in codon 424 in the second kindred. The AT 81(-T) frameshift mutation leads to a premature stop signal in codon 89, while the AT 424(+G) allele has a premature stop only one codon short of the normal gene. The latter mutation changes the eight carboxy terminal residues of AT, including 429Cys, and increases the proportion of polar amino acids in this region. We suggest that altered folding of the mutant protein may explain the AT deficiency.