5 ANTITHROMBIN VARIANTS, 4 ASSOCIATED WITH THROMBOSIS

We have identified five mutations in antithrombin by direct sequencing of exons amplified using polymerase chain reaction. Four of these mut ations are associated with thrombosis, three cause type I antithrombin deficiency and one has features of a type II deficiency, The fifth va riant appears to have no functional consequences. The type I mutations are in exon 2, exon 3b and exon 4. The first of these is a nonsense m utation causing substitution of a Tyr --> stop at position -16 within the secretion signal sequence. The second is a missense mutation resul ting in the substitution Cys --> Ser at position 247. This disrupts th e disulphide bond with Cys 430 leaving a free cysteine residue and the C-terminus unconstrained. The third type I mutation is an in-frame de letion resulting in the loss of Ile 186. This is a highly conserved re sidue in the serpin superfamily and will predictably result in the dis ruption of the F-helix, The fourth mutation, in exon 3a, results in th e substitution of Ser 162 by Asn. This residue is sited in the E-helix and the replacement of the buried side chain of serine by the larger asparagine side chain will predictably cause structural perturbation. The last example, Val 415 --> Asp, was an incidental finding as a foll ow up investigation of a nephrotic patient. Although one other member of the family also had the mutation there was no linked history of thr ombotic disease.