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The complex of the serine protease factor IX (FIX) and its cofactor, factor VIII (FVIII), is crucial for propagation of the intrinsic coagulation cascade. Absence of either factor leads to hemophilia, a disabling disorder marked by excessive hemorrhage after minor trauma. FVIII is the more commonly affected protein, either by X-chromosomal gene mutations or in autoimmune-mediated acquired hemophilia. Whereas substitution of FVIII is the mainstay of hemophilia A therapy, treatment of patients with inhibitory Abs remains challenging. In the present study, we report the development of FIX variants that can propagate the intrinsic coagulation cascade in the absence of FVIII. FIX variants were expressed in FVIII-knockout (FVIII-KO) mice using a nonviral genetransfer system. Expression of the variants shortened clotting times, reduced blood loss after tail-clip assay, and reinstalled clot formation, as tested by in vivo imaging of laser-induced vessel injury. In IntroductionThe intrinsic coagulation cascade is a tightly regulated proteaseand cofactor-dependent amplification system that ensures the formation of stable clots after injury. 1 Within this system, deficiencies of the coagulation cofactor, factor VIII (FVIII), or the corresponding coagulation protease, factor IX (FIX), lead to the X-chromosomal inherited bleeding disorders hemophilia A and hemophilia B, respectively. Hemophilia A occurs approximately in 1 of 5000 newborn boys, whereas hemophilia B is less common. 2 Untreated, hemophilia presents with spontaneous bleeding preferentially into large joints and skeletal muscle, and internal and intracranial bleedings can also occur. Substitution of deficient coagulation factors by intravenous infusion of plasma-derived or recombinant coagulation factor concentrates is the therapy of choice. In the last decades, treatment has evolved from so-called on-demand treatment for acute injury/hemorrhage to a secondary preventative approach with regular prophylactic infusions. Prophylactic treatment, securing plasma levels above 1% of normal, already prevents the major long-term consequences of the disease: joint damage and muscular atrophy. 3,4 A major obstacle for protein substitution therapy is the occurrence of neutralizing Abs directed against the FVIII protein. This can either be a result of an immune response after exogenous protein exposure 5 or may appear in adult patients as a spontaneous auto-immune event. 6 In these cases, FVIII infusion is often ineffective and so-called bypassing agents are used. These agents consist of constitutively activated proteases such as activated factor VII (FVIIa), and promote clot formation directly without restoring the intrinsic amplification loop. Although these therapeutics are efficient at stopping acute bleeding, limitations include the relatively short half-lives of activated proteases in the circulation and potential vascular risks in long-term treatment. 7,8 In the present study, we report on the generation of FIX variants with FVIII-independent clotting activity that we...

Chronic granulomatous disease (CGD) is a debilitating primary immunodeficiency affecting phagocyte function due to the absence of nicotinamide dinucleotide phosphate (NADPH) oxidase activity. The vast majority of CGD patients in the Western world have mutations within the X-linked CYBB gene encoding for gp91 (NOX2), the redox center of the NADPH oxidase complex (XCGD). Current treatments of XCGD are not entirely satisfactory, and prior attempts at autologous gene therapy using gammaretrovirus vectors did not provide long-term curative effects. A new strategy was developed based on the use of the lentiviral vector G1XCGD expressing high levels of the gp91 transgene in myeloid cells. As a requisite for a clinical trial approval, standardized non-clinical studies were conducted in vitro and in mice in order to evaluate the pharmacodynamics and biosafety of the vector and the biodistribution of G1XCGD-transduced cells. Transduced CD34 cells derived from XCGD patients engrafted and differentiated similarly to their non-transduced counterparts in xenograft mouse models and generated therapeutically relevant levels of NADPH activity in myeloid cells expressing gp91. Expression of functional gp91 in hematopoietic cells did not affect their homing properties, which engrafted at high levels in mice. Extensive in vitro and in vivo genotoxicity studies found no evidence for adverse mutagenesis related to vector treatment. These studies paved the way for the approval of clinical trials in Europe and in the United States for the treatment of XCGD patients with G1XCGD gene-modified autologous hematopoietic cells.

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