Development of Improved Factor VIII Molecules and New Gene Transfer Approaches for Hemophilia A

Abstract

Hemophilia A, the most common inherited bleeding disorder, is caused by deficiency or functional defects in coagulation factor VIII (fVIII). Conventional treatment for this disease involves intravenous infusions of plasma-derived or recombinant fVIII products. Although replacement therapy effectively stops the bleeding episodes, it has a risk of transmission of viral blood-borne diseases and development of neutralizing antibodies that inactivate the administered fVIII protein. Hemophilia A is an attractive candidate for application of gene therapy approaches because the therapeutic window is wide and even modest elevation of fVIII levels will correct the hemophilic phenotype. Ongoing preclinical investigations utilize animal models of hemophilia A, including genetically fVIII-deficient mice and naturally fVIII-deficient dogs, to optimize vectors, transgenes and target cell populations for Phase I clinical trials. In this review, we outline the progress in understanding the mechanisms of fVIII turnover, which provides a basis for development of improved fVIII molecules with prolonged half-life in the circulation. We discuss the possibility of incorporating these improved fVIII molecules as transgenes into self-inactivating lentiviral vectors carrying chromatin insulator sequences, representing a new generation of gene delivery vehicle, to target hematopoietic stem cells and endothelial cells. The use of hematopoietic stem cells as the target cell population may prevent inhibitor formation to transduced fVIII by induction of immune tolerance. Alternatively, endothelial cells may support optimal synthesis of fVIII and myeloablative conditioning of patients with radiation or chemotherapy may not be required for efficient engraftment of the engineered cells. Collectively, these proposed advances represent promising prophylactic strategies toward long-term correction of the coagulation defect in this progressively debilitating, life-threatening disease.

Sustained Expansion and Transgene Expression of Coagulation Factor VIII–Transduced Cord Blood–Derived Endothelial Progenitor Cells

“…The deletion of the B domain increases the expression level of recombinant FVIII and does neither impair the procoagulant activity nor result in enhanced immunogenicity of the FVIII protein (see the study by Saenko et al 18 and the references therein). We show that lentiviral transduction yields CBEC populations that have retained phenotypic characteristics of endothelial cells and that are readily expandable to large numbers of transplantable EPCs secreting high amounts of the FVIII transgene protein.…”

Factor VIII ectopically targeted to platelets is therapeutic in hemophilia A with high-titer inhibitory antibodies

“…The development of inhibitory antibodies to the FVIII transgene product in plasma remains a significant barrier to some patient candidates. Many groups have developed various strategies for directing FVIII synthesis (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15), although inadequacies of gene delivery and expression and inhibitor formation remain clinical problems (7,(16)(17)(18).…”

Syngeneic transplantation of hematopoietic stem cells that are genetically modified to express factor VIII in platelets restores hemostasis to hemophilia A mice with preexisting FVIII immunity

“…There has been substantial progress in gene therapy of hemophilia A in preclinical trials. [18][19][20][21][22][23][24][25][26][27][28][29] Gene therapy of hemophilia A with preexisting FVIII immunity is especially challenging because circulating inhibitory antibodies in plasma may inactivate functional FVIII if it is constitutively secreted into plasma. Therefore, developing a mechanism for secure cellular delivery to sequester and protect FVIII from inhibitor inactivation is a more promising approach than plasma delivery of FVIII to the site of injury when inhibitory antibodies are present in the plasma.…”