Whole genome sequencing to elucidate novel genetic modifiers of FVIII clearance.

Despite significant insights into factor VIII (FVIII) biosynthesis, the mechanisms involved in FVIII clearance remain poorly defined. To investigate genetic modifiers of FVIII clearance, we studied 106 SNPs across 43 different loci previously reported to influence plasma FVIII and/or von Willebrand (VWF) levels and reviewed their association with FVIII pharmacokinetic (PK) parameters in 52 patients with hemophilia A. Marked inter-individual variability in FVIII clearance was observed. Plasma VWF levels, ABO blood group and age all influenced FVIII PK parameters. Collectively however, linear regression analysis demonstrated that these explained less than 40% of the observed variance in FVIII clearance. Consistent with previous reports, SNPs mapped to the FVIII/VWF clearance receptors STAB2, ASGR2, and CLEC4M were associated with FVIII clearance. Importantly however, we identified four additional genetic signals marked by SNPs rs4904820 (TC2N), rs3124768 (ADAMTS13), rs12979891 (RASIP1/MAMSTR/FUT2) and rs6494314 (C2CD4B) that were also significantly associated with FVIII PK parameters. Of particular interest, the FUT2 gene on chromosome 19q13 determines Secretor blood group status and thus determines secretion of ABO(H) glycan determinants into plasma. In addition to the association between the rs12979891 SNP and FVIII clearance, we further observed an effect of Secretor genotype on FVIII PK parameters, with prolonged FVIII half-life in non-secretor individuals compared to homozygous secretors. Furthermore, plasma FVIII levels were significantly increased in Fut2 knockout mice. Cumulatively, these data identify novel modifiers of FVIII clearance in vivo. Given that plasma FVIII levels constitute a dose-dependent risk factor for cardiovascular disease, these findings are of direct clinical importance.