FACTOR-VIII ISE (R2159C) IN A PATIENT WITH MILD HEMOPHILIA-A, AN ABNORMAL FACTOR-VIII WITH RETENTION OF FUNCTION BUT MODIFICATION OF C2 EPITOPES

We found a patient with mild hemophilia A who had no detectable factor Vm antigen (FVIII:Ag), as shown by two-site ELISA using inhibitor all oantibodies (TK). We then analyzed A2, A2/B, and C2 antigen of the pat ient's DDAVP-induced FVIII using several anti-FVIII monoclonal antibod ies. Factor Vm activity (FVIII:C) was increased from 12 to 42 U/dl by the administration of DDAVP. The DDAVP-induced increases in the A2 and A2/B antigens were 40 and 36 U/dl, respectively. However, the increas e in the C2 antigen was only 7.5 U/dl. SSCP analysis and subsequent se quencing demonstrated an Arg to Cys transition at codon 2159. The anti -FVIII:C titer of monoclonal antibody, NMC-VIII/5 which recognized the C2 domain, against normal plasma was 450 Bethesda U/mg of IgG. Howeve r, the titer against DDAVP-treated patient's plasma was only 15 Bethes da U/mg. We also tested DDAVP-induced increase in the FVIII:Ag in anot her mild hemophilia A patient with the same mutation at Arg(2159). Inc rease in his C2 antigen levels was only 19% of those in the A2 and A2/ B antigen. We designate this abnormal FVIII as FVIII Ise. Our results show that a missense mutation at Arg(2159) to CYS modifies the antigen icity of the C2 domain.