Introduction: Immune tolerance therapy (ITT), consists of administration of daily high doses of factor VIII concentrates by intravenous infusions. It can induce tolerance to the exogenous protein and eradicate the antibody response to FVIII, which still represents the main complication of replacement therapy in haemophilia A (HA) severe cases.

Methods: We have investigated 71 HA patients with inhibitors (sixty-nine severe, one moderate and one mild cases). We screened the patients for the causative mutations in the F8 gene using Long Range PCR for the intron22 inversion, multiplex-PCR for intron1 inversion or conformation sensitive gel electrophoresis (CSGE) followed by DNA sequencing for other mutation types.

Results: Diverse genetic defects were detected in the severe cases, with a predominance of gross mutations: F8 gene inversions, large deletions and nonsense mutations account for 68% of the mutations, whereas in the two non severe patients specific missense mutations were identified. ITT has been attempted in 16 HR patients of this cohort but failed in 5 cases.

Conclusion: We confirmed that the presence of inhibitors correlates well with the presence of null mutations, as reported by Schwaab et al (Thromb Haemost 1995). Several predictive factors for ITT outcome have been so far taken into consideration but no correlation has been made between F8 gene defect and ITT response. In our cohort 2 large deletions, 2 intron22 inversions and 1 nonsense mutations failed to respond. For the remaining 2 large deletions no ITT were attempted so no information are available. Large deletions appear to be a high risk genetic factor both for inhibitor development and for long term inhibitor persistence and ITT unresponsiveness. The identification of these patients at very high risk of inhibitor development and ITT unresponsiveness by mutation analysis should therefore be strongly recommended soon after the diagnosis, even because of the high cost of the therapy.

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