The missense mutation Arg(593)->Cys is related to antibody formation in a patient with mild hemophilia A

The development of inhibitory antibodies to factor VIII in patients affected by a mild form of hemophilia A (factor VIII > 0.05 IU/mL) is considered a rare event, In this study, we evaluated the relationship between genotype and anti-factor VIII antibody formation in a patient with mild hemophilia A. Mutation analysis showed that a missense mutation in the factor VIII gene leading to replacement of Arg(593) by CYS in the A2 domain of factor VIII was associated with hemophilia A in this patient, The anti-factor VIII antibodies present in the patient's plasma were characterized using metabolically labeled factor V111 fragments expressed in insect cells, The anti-factor VIII antibodies, composed of subclasses IgG2 and IgG4, reacted with both the fragment corresponding to the factor VIII heavy chain and the A2 domain, The Arg(593) --> Cys substitution was introduced into the cDNA encoding the A2 domain of factor V111 and the resulting construct was expressed in insect cells. Strikingly, the metabolically labeled A2 domain carrying the Arg(593) --> Cys mutation was not recognized by the anti-factor VIII antibodies present in the plasma of the patient, These data indicate that the antifactor VIII antibodies are exclusively directed against exogenous factor VIII. This strongly suggests that the Arg(593) --> CYS substitution results in recognition of wild-type factor VIII as nonself and is thereby related to the formation of anti-factor VIII antibodies after factor VIII replacement therapy in this particular patient. (C) 1997 by The American Society of Hematology.