Significance of F8 missense mutations with respect to inhibitor formation.

We have identified 1,135 haemophilia A patients with missense mutations associated with mild (46%), moderate (22%), severe (16%), and mixed haemophilia phenotypes (11%). Altogether, we detected 374 different missense mutations of which 195 are not listed in the HAMSTeRS database. While missense mutations are strongly under-represented within the factor VIII (FVIII) B-domain, they are evenly distributed throughout the entire F8 cDNA sequence. Only 36 (5%) of 720 patients with missense mutations and known inhibitor status showed an association with inhibitor formation. Inhibitor prevalence was four-fold higher for severe haemophilia compared to mild/moderate phenotypes. Mutations associated with inhibitor formation were especially clustered within the C1/C2 domain compared to the other domains (8.7% C1/C2 domain vs. 3.6% non-C1/C2-domain; p-value: 0.01). Three different missense mutations (T314A [T295A], S2010P [S1991P), R21 69H [R2150H]) were associated twice with inhibitor formation. Importantly, we found that the risk of inhibitor formation in association with FVIII missense mutations is significant higher if the amino acid substitution belongs to another physicochemical class than the original residue (p-value 0.039). For this purpose distinct classes of substitutions were grouped in association with side chains properties: class I, small/hydrophobic; class II, neutral; class III, acidic; class IV, basic. Thus, although missense mutations were associated with an overall lower risk of inhibitor formation compared to other F8 gene mutation types, different missense mutations correlate with specific risks for inhibitor formation. These differences have to be identified in assigning risk profiles to aid in choice of preventative treatments designed to prevent inhibitor formation.