ANTITHROMBIN AND ITS DEFICIENCY STATES

Antithrombin is the most important physiological proteinase inhibitor of thrombin and other coagulation proteinases. It is a single chain glycoprotein of MW 58 200 which has sequence homology with alpha-1-antitrypsin and other members of the serpin superfamily of inhibitors. Two functional domains of importance have been identified, the reactive centre that interacts with the proteinase and a heparin binding domain. Failure to maintain an adequate level of functional antithrombin in plasma results in an increased risk of thromboembolism: deficiency can be inherited or acquired. There is still uncertainty regarding the prevalence of inherited deficiency and the prevalence of thrombosis in affected individuals. The production of antithrombin is under the control of a single gene which is localized on chromosome 1q 23-25. Characterization of the coding sequence, which is distributed over seven exons, has allowed the analysis of the molecular basis for inherited antithrombin deficiency. To date more than 100 cases have been successfully investigated at the gene and/or protein sequence level and 40 novel mutations have been identified. Mutations causing amino acid substitutions solely affecting the heparin binding site have thus far been located primarily at the N-terminal region of the molecule, residues 7-129; this region has been postulated to align as a positive groove in the molecule that forms the primary contact region for the essential antithrombin binding pentasaccharide of heparin. Not all the residues in which substitutions have been found are basic and some serve to maintain the conformation of nearby basic regions. Examples of this arc provided by the Pro-41 to Leu mutation and a recently investigated mutant, Leu-99 to Phe. The reactive site defects are an interesting group, including those that alter P1, P1' and P12-P10 residues. Perhaps more remote mutations can also be included such as Pro-429 to Leu. The P1 and P1' mutations directly block interaction of the proteinase with antithrombin, while P12-P10 mutants (which have mutations affecting serpin strand s4A) enable the substrate reaction to proceed to completion, i.e. the antithrombin-thrombin complex is not stabilized and the mutant inhibitor is transformed into a substrate. The effect of the Pro-429 to Leu substitution is impairment of the reactive site and heparin binding, and the finding that this variant is not completely recognized by some MAbs implies a conformational change at the C terminus. Another group (nine cases) of interesting mutations is emerging, that has its primary defect in or near serpin strand 1C, amino acid sequence 402-407. These mutations are adjacent to an invariant region in the serpin family that appears to be important for their overall structure. The identified amino acid substitutions in antithrombin produce pleiotropic effects on the inhibitor, altering the reactive site, heparin binding properties and in many cases, the plasma level of protein. Appreciable progress is now also being made on the identification of cases of 'classical' antithrombin deficiency. While a few partial gene deletions are currently being characterized, most cases that have been identified are point mutations (including small deletions and insertions) in the coding region. There is one common mutation at position 129 (Arg to Stop, six cases), a small number of mRNA splice site abnormalities and the remainder are frameshifts or critical amino acid sequence changes distributed throughout the coding region of antithrombin.