An alloantibody recognizing the FVIII A1 domain in a patient with CRM reduced haemophilia a due to deletion of a large portion of the A1 domain DNA sequence

We report the development of a FVIII inhibitor in a patient with seven, cro ss reacting material reduced (CRMR) haemophilia A. The level of Factor VIII antigen (FVIII:Ag) measured by ELISA using anti-C2 monoclonal and alloanti bodies was 1.9 U/dl. This baseline FVIII:Ag level was increased to 8.3 U/dl after administration of DDAVP. The anti-FVIII inhibitor titer was 2.9 Beth esda U/ml. DNA analysis showed a large deletion of the FVIII gene from exon 4 to 7, corresponding to amino acid residues 111-317 included within the A 1 domain. The size of the gene deletion was approximately 28 kb. 5' and 3' breakpoints were identified by sequencing in intron 3 and intron 7, respect ively. FVIII mRNA was detected in the patient's peripheral lymphocytes and the deletion spanning exon 4 to 7 was confirmed at the RNA level. Immunopre cipitation experiments using I-125 labeled A1, A2 and light chain demonstra ted that the inhibitor reacted only with the 54 kDa A1 domain. The inhibito r activity was more than 95% neutralized by A1 domain polypeptide. Our find ings suggest a close relationship between the inhibitor epitope and the spe cific gene deletion with regard to the pathogenesis of the inhibitor in thi s patient.