Summary
Using gain-of-function factor IX (FIX) for replacement therapy for haemophilia B (HB)
is an attractive strategy. We previously reported a high-activity FIX, FIX-Triple
(FIX-V86A/E277A/R338A) as a good substitute for FIX-WT (wild-type) in protein replacement
therapy, gene therapy, and cell therapy. Here we generated a new recombinant FIXTripleL
(FIX-V86A/E277A/R338L) by replacing the alanine at residue 338 of FIX-Triple with
leucine as in FIX-Padua (FIX-R338L). Purified FIX-TripleL exhibited 22-fold higher
specific clotting activity and 15-fold increased binding affinity to activated FVIII
compared to FIXWT. FIX-TripleL increased the therapeutic potential of FIX-Triple by
nearly 100% as demonstrated with calibrated automated thrombogram and thromboelastography.
FIX-TripleL demonstrated a normal clearance rate in HB mice. The clotting activity
of FIX-TripleL was consistently 2- to 3-fold higher in these mice than that of FIX-Triple
or FIXR338L. Gene delivery of adeno-associated virus (AAV) in HB mice showed that
FIX-TripleL had 15-fold higher specific clotting activity than FIX-WT, and this activity
was significantly better than FIX-Triple (10-fold) or FIX-R338L (6-fold). At a lower
viral dose, FIX-TripleL improved FIX activity from sub-therapeutic to therapeutic
levels. Under physiological conditions, no signs of adverse thrombotic events were
observed in long-term AAV-FIX-treated C57Bl/6 mice. Hepatocellular adenomas were observed
in the high- but not the medium- or the lowdose AAV-treated mice expressing FIX-WT
or FIX-Triple, indicating the advantages of using hyperfunctional FIX variants to
reduce viral doses while maintaining therapeutic clotting activity. Thus, incorporation
of the FIX Padua mutation significantly improves the clotting function of FIX-Triple
so as to optimise protein replacement therapy and gene therapy.
Keywords
Coagulation factors - haemophilia therapy - animal models