The Molecular Defect Of Factor IX Chapel Hill

We had previously described Factor IX Chapel Hill as an abnormal molecule with 100% Factor IX antigenic level but only 5% functional activity as measured by routine one-stage clotting assay. Clinically the patient has a mild bleeding tendency with hemorrhage only after trauma. Initially, we were unable to demonstrate significant cleavage of Factor IX Chapel Hill by a crude human XIa preparation from celite activated plasma and could not determine the specific activity of the molecule. In the present study we have further investigated the cleavage of Factor IX Chapel Hill using a more purified preparation of human Factor XIa. Under similar incubation conditions Factor IX Chapel Hill was cleaved to Factor IXaβ at approximately one half the rate of normal Factor IX. Clotting activity was generated concomitantly with the cleavage of Factor IX Chapel Hill, reaching a maximum level at a time corresponding to complete cleavage of the molecule. The specific activity of Factor IXaβ Chapel Hill was estimated from a standard curve relating clotting time to normal Factor IXa8 concentration. Four determinations were made on separate days and found to correspond to a specific activity for Factor IXaB Chapel Hill of approximately 30% of normal Factor IXaβ. From these results we conclude that the structure of Factor IX Chapel Hill is sufficiently different from normal Factor IX that the interaction with Factor XIa and/or Ca⁺⁺ is altered resulting in a slower rate of cleavage to Factor IXaβ. In addition, the structure of Factor IXaβ Chapel Hill is also sufficiently different from normal Factor IXaβ that the apparent activity of the molecule in plasma is reduced. Further these results suggest that the molecular defect in Factor IX Chapel Hill is not within the activation peptide region of the molecule as a defect in this region would not have affected the specific activity of the molecule.