A Novel Lysine to Arginine Substitution at Position 301 Enhances Activity of Factor IX

Introduction: AAV-mediated gene transfer of blood coagulation Factor IX (FIX) has been established as a safe and long-term treatment for patients suffering from severe hereditary Haemophilia B. A gain-of-function F9 transgene (F9-R338L; Padua) has recently been used to achieve higher functional levels of FIX, effectively eliminating the need for regular prophylaxis. The naturally-occurring R338L Padua mutation is situated in the catalytic domain of FIX on a helical side loop (region 332-339) that is involved in FVIIIa-mediated stimulation of substrate turnover. Here, we examined if a single amino acid substitution of a lysine at position 301 leads to gain of function. This basic residue sits adjacent to the 332-339 loop on an exposed helical segment (292-303) that has been implicated to interact with the FVIIIa A2 domain in the FIXa-FVIIIa tenase complex. Methods: We examined the lysine at position 301 (numbering based on mature polypeptide chain) in more detail by conservative mutation to arginine (K301R) and non-conservative mutation to leucine (K301L). To assess specific FIX activity, F9-K301 variants were transiently expressed in HEK293T cells and tested for antigenic FIX levels and chromogenic activity 48 hours post transfection. To assess specific activity in plasma, AAV-mediated gene transfer (1x1010vg/mouse) of F9-K301 variants in hemophilia B knock-out mice (CL57B6) was carried out. In addition, we investigated whether the F9-K301R mutation enhances specific activity in combination with the F9-R338L Padua mutation via site-specific genome integration. Results: Transient transfection of F9-K301 variants in HEK293T cells showed a 25% increase in specific activity with F9-K301R but a 50% reduction in activity with F9-K301L as compared to wild type F9 (WT-F9). Validation of gain-of-function was done by AAV-mediated gene transfer in hemophilia B knock-out mice. Four weeks post injection, plasma FIX antigen levels were similar in mice transduced with either F9-K301R (0.91±0.3 U/ml; N=3), F9-K301L (0.93±0.0 U/ml; N=2) or WT-F9 (0.94±0.19 U/ml; N=4) constructs. Interestingly, specific chromogenic activity in plasma from F9-K301R mice (2.71±0.66 U/ml) was more than 2-fold higher compared to plasma from mice in the WT-F9 cohort (1.25±0.2 U/ml). On the other hand, specific activity in the F9-K301L cohort (0.37±0.07 U/ml) was reduced compared to wild type F9, consistent with a haemophilic phenotype. Next, we investigated whether the F9-K301R mutation enhances