Identification and characterisation of an antithrombin III mutant (AT Dublin 2) with marginally decreased heparin reactivity.

The antithrombin III (ATIII) isoform patterns of plasma and serum samples from cancer patients and controls were analysed by isoelectric focusing and immunoblotting. A novel ATIII banding pattern was identified in two individuals: a patient with breast carcinoma who developed deep venous thrombosis and a blood donor. Family studies in the patient showed the abnormal pattern to be due to a mutant form of ATIII (AT Dublin 2). The coagulation properties of AT Dublin 2 heterozygotes were normal. Immunologic and activity levels of ATIII, measured by standard techniques, were normal. Mutant plasma ATIII showed reduced thrombin reactivity at low concentrations of thrombin and demonstrated decreased reactivity with heparin over a range of heparin concentrations. This was confirmed using a modified ATIII heparin cofactor activity assay with varying heparin concentrations. The abnormal ATIII was also found to elute from heparin agarose at a lower ionic strength than normal ATIII. Two dimensional gel electrophoresis showed the abnormal ATIII to have similar molecular size distribution to normal ATIII. Neuraminidase treatment of normal and mutant plasma reduced the ATIII isoforms to one in both samples. The possible role of AT Dublin 2 in predisposing to hypercoagulation is discussed.