PB0910 - A novel factor V compound heterozygous mutation associated with thrombosis (Y1961C; FV-Kanazawa, and 1982_1983del).

Background: Coagulation factor V (FV) has procoagulant and anticoagulant functions. We found a FV-deficient patient (FV:C 6 IU/dL, FV:Ag 32 IU/dL) with recurrent deep vein thrombosis. The patient demonstrated activated protein C resistance (APCR) with compound heterozygous mutations consisting of FV-Y1961C (FV Kanazawa) and FV-1982_1983del.

Aims: To clarify thrombotic mechanisms associated with this FV abnormality.

Methods: We expressed full-length FV using HEK293T cell. APC-catalyzed inactivation of FVa-Y1961C was examined by a PT-based clotting assay. And a property of FV-Y1961D as the APC cofactor was examined in FVIIIa degradation assay. And the light chain region of FV is thought to inhibit TF-induced procoagulant function. Diluted prothrombin time were measured using normal plasma and normal with FVa.

Results: We targeted on the FV-Y1961C mutation since the levels of FV-1982_1983del were below the detection sensitivity in our expression experiments FVa-WT:C was reduced to ～10% of the initial level at 10 minutes. But inactivation of FVa-Y1961C was delayed and FVa-Y1961C:C persisted at 50～60% of the initial level regardless of the existence of Protein S. FVIIIa was inactivated by APC with FV-WT to ～25% of the initial level at 20 minutes. However FVIIIa was remained to～50% in FV-Y1961C. And compared with normal plasma diluted prothrombin time was prolonged in plasma with FVa-WT. But in plasma with FVa-Y1961C, the result was comparable to normal plasma, and no inhibitory effect was observed. Similar results were observed in Antithrombin, Protein C, and Protein S deficient plasmas.

Conclusion(s): We identified a compound heterozygous. FV-Y1961C mutation in the C1 domain representing a novel FV mutation resulting in not only APCR due to impaired FVa susceptibility and FV cofactor activity for APC function, but impaired inhibition of TF-induced procoagulant function. These defects in anticoagulant function associated with FV in FV-Y1961C contributed to a prothrombotic state.