The Light Chain of Rat Factor VIII Confers Its Superior Cofactor Activity

Hemophilia A is caused by genetic defect of human coagulation factor VIII (hFVIII) and patients have to take lifelong replacement therapy to prevent excessive bleedings upon hemostatic challenges. Due to the short half-life of hFVIII, replacement treatment has to be given frequently and inhibitors against infused hFVIII can be developed in about 20-30% of patients. These shortcomings have generated tremendous interest in developing HA gene therapies which is more efficient and long-lasting. However, early preclinical studies have shown FVIII activities were still limited after vector delivery. A Modified hFVIII with higher specific activity and pharmacodynamics properties is highly desirable to overcome the disadvantages of current protein replacement and gene therapy strategies.

In the current study, we successfully constructed a B-domain deleted rat FVIII(rBDDF8) that contained a PACE/furin recognition site (RHQR) within a 14 amino acid linker between A2 and A3 domains. The rBDDF8 displayed significantly higher coagulation activity(~2.5-fold) than hBDDF8 after transfection into HEK 293 cells. In order to explore the mechanism for the observed superior cofactor activity, we constructed heavy chain(rHC) and light chain(rLC) of rFVIII. The rHC and rLC are able to reconstitute 5 times more FVIII activity than their human counter parts. However, when rHC is associated with human FVIII light chain (hLC), the reconstituted FVIII activity is lower that from hHC and hLC, suggesting that high coagulation activity of rFVIII is not mediated by its HC. On the contrary, when FVIII is constituted by hHC with rLC, we found that the activity is increased by 3~5-fold as against hHC and hLC. The hHC antigen level of FVIII reconstituted from hHC and rLC was 1.5-fold higher than that of hHC and hLC, suggesting that higher activity of FVIII with hHC and rLC is not through increased secretion. The specific activity deduced from activity/antigen ratio showed that FVIII with rLC is 3 times higher more than FVIII with hLC. To investigate the potential application of rFVIII in gene therapy, rBDDF8 was delivered in hemophilia A mouse model using AAV8 vectors. The high dose rBDDF8(4*10¹¹vg/mouse) resulted 2.5U FVIII activity at week 17, which is much higher(about 10-fold) than that of hBDDF8. When the rFVIII was delivered by dual chains strategy, i.e, administering vectors carrying only LC or HC simultaneously, it also showed 2-4 fold increased in FVIII activity. Interestingly, the combination of hHC and rLC also generated similar FVIII activity as rHC and rLC, further proving the rLC is the major contributor to the superior coagulation activity of rFVIII.  Our results showed that the rFVIII has higher cofactor activity conferred by its LC. Our results suggest that rFVIII can be further exploited to make an ideal candidate for hemophilia gene therapy using AAV vectors.