Pre-clinical evaluation of an enhanced-function factor VIII variant for durable hemophilia A gene therapy in male mice
Anna R. Sternberg, Cristina Martos-Rus, Robert J. Davidson, Xueyuan Liu, Lindsey A. George
Abstract
Durable factor VIII expression that normalizes hemostasis is an unrealized goal of hemophilia A adeno-associated virus-mediated gene therapy. Trials with initially normal factor VIII activity observed unexplained year-over-year declines in expression while others reported low-level, stable expression inadequate to restore normal hemostasis. Here we demonstrate that male mice recapitulate expression-level-dependent loss of factor VIII levels due to declines in vector copy number. We show that an enhanced function factor VIII variant (factor VIII-R336Q/R562Q), resistant to activated protein C-mediated inactivation, normalizes hemostasis at below-normal expression without evidence of prothrombotic risk in male hemophilia A mice. These data support that factor VIII-R336Q/R562Q may restore normal factor VIII function at low levels of expression to permit durability using low vector doses to minimize dose-dependent adeno-associated virus toxicities. This work informs the mechanism of factor VIII durability after gene transfer and supports that factor VIII-R336Q/R562Q may safely overcome current hemophilia A gene therapy limitations. A major limitation in hemophilia A gene therapy is the progressive loss of factor VIII expression. Here, the authors demonstrate that low-level expression of a gain-of-function factor VIII variant may safely restore normal hemostasis and permit durability of expression.