Catalytic domain modification and viral gene delivery of activated factor VII confers hemostasis at reduced expression levels and vector doses in vivo

AAV mediated genome engineering with a bypass coagulation factor alleviates the bleeding phenotype in a murine model of hemophilia B

It is crucial to develop a long-term therapy that targets hemophilia A and B, including inhibitor-positive patients. We have developed an Adeno-associated virus (AAV) based strategy to integrate the bypass coagulation factor, activated FVII (murine, mFVIIa) gene into the Rosa26 locus using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 mediated gene-editing. AAV vectors designed for expression of guide RNA (AAV8-gRNA), Cas9 (AAV2 neddylation mutant-Cas9), and mFVIIa (AAV8-mFVIIa) flanked by homology arms of the target locus were validated in vitro. Hemophilia B mice were administered with AAV carrying gRNA, Cas9 (1 × 1011 vgs/mouse), and mFVIIa with homology arms (2 × 1011 vgs/mouse) with appropriate controls. Functional rescue was documented with suitable coagulation assays at various time points. The data from the T7 endonuclease assay revealed a cleavage efficiency of 20-42 %. Further, DNA sequencing confirmed the targeted integration of mFVIIa into the safe-harbor Rosa26 locus. The prothrombin time (PT) assay revealed a significant reduction in PT in mice that received the gene-editing vectors (22 %), and a 13 % decline in mice that received only the AAV-FVIIa when compared to mock treated mice, 8 weeks after vector administration. Furthermore, FVIIa activity in mice that received triple gene-editing vectors was higher (122.5mIU/mL vs 28.8mIU/mL) than the mock group up to 15 weeks post vector administration. A hemostatic challenge by tail clip assay revealed that hemophilia B mice injected with only FVIIa or the gene-editing vectors had significant reduction in blood loss. In conclusion, AAV based gene-editing facilitates sustained expression of coagulation FVIIa and phenotypic rescue in hemophilia B mice.

Gene-based FVIIa prophylaxis modulates the spontaneous bleeding phenotype of hemophilia A rats

A sizable proportion of hemophilia inhibitor patients fails immune tolerance induction and requires bypass agents for long-term bleed management. Recombinant human-activated coagulation Factor VII (rhFVIIa) is an on-demand bypass hemostatic agent for bleeds in hemophilia inhibitor patients. Prophylactic use of rhFVIIa may enable sustained hemostatic management of inhibitor patients, but the critical relationship of rhFVIIa circulating levels and clinical outcome in that setting remains unclear. To address this in vivo, we used the rat hemophilia A (HA) model that exhibits spontaneous bleeds and allows longitudinal studies with sufficient statistical power. We simulated activated Factor VII (FVIIa) prophylaxis by adeno-associated virus (AAV) gene transfer of a rat FVIIa transgene. Compared with naive HA animals, rat FVIIa continuous expression affected the overall observed bleeds, which were resolved with on-demand administration of recombinant rat FVIIa. Specifically, although 91% of naive animals exhibited bleeds, this was reduced to 83% and 33% in animals expressing less than 708 ng/mL (<14 nM) and at least 708 ng/mL (≥14 nM) rat FVIIa, respectively. No bleeds occurred in animals expressing higher than 1250 ng/mL (>25 nM). Rat FVIIa expression of at least 708 ng/mL was also sufficient to normalize the blood loss after a tail vein injury. Continuous, AAV-mediated rat FVIIa transgene expression had no apparent adverse effects in the hemostatic system of HA rats. This work establishes for the first time a dose dependency and threshold of circulating FVIIa antigen levels for reduction or complete elimination of bleeds in a setting of FVIIa-based HA prophylaxis.

Rotational thromboelastometry can predict the probability of bleeding events in a translational rat model of haemophilia A following gene-based FVIIa prophylaxis

INTRODUCTION

Monitoring of clinical effectiveness of bypassing agents in haemophilia patients is hampered by the lack of validated laboratory assays. Thromboelastography (TEG) and rotational thromboelastometry (ROTEM) have been evaluated for predicting clinical effectiveness of bypassing agents, however, with limited success.

AIM

Application of a longitudinal model-based approach may allow for a quantitative characterization of the link between ROTEM parameters and the probability of bleeding events.

METHODS

We analyse longitudinal data from haemophilia A rats receiving gene-based FVIIa prophylaxis in terms of total circulatory levels of FVII/FVIIa, clotting time (CT) measured using ROTEM and the probability of bleeding events.

RESULTS

Using population pharmacokinetic-pharmacodynamic (PKPD) modelling, a PK-CT-repeated time-to-event (RTTE) model was developed composed of three submodels (a) a FVII/FVIIa PK model, (b) a PK-CT model describing the relationship between predicted FVIIa expression and CT and (c) a RTTE model describing the probability of bleeding events as a function of CT. The developed PK-CT-RTTE model accurately described the vector dose-dependent plasma concentration-time profile of total FVII/FVIIa and the exposure-response relationship between AAV-derived FVIIa expression and CT. Importantly, the developed model accurately described the occurrence of bleeding events over time in a quantitative manner, revealing a linear relationship between predicted change from baseline CT and the probability of bleeding events.

CONCLUSION

Using PK-CT-RTTE modelling, we demonstrated that ROTEM parameters can accurately predict the probability of bleeding events in a translational animal model of haemophilia A.

Protein-Engineered Coagulation Factors for Hemophilia Gene Therapy

Hemophilia A (HA) and hemophilia B (HB) are X-linked bleeding disorders due to inheritable deficiencies in either coagulation factor VIII (FVIII) or factor IX (FIX), respectively. Recently, gene therapy clinical trials with adeno-associated virus (AAV) vectors and protein-engineered transgenes, B-domain deleted (BDD) FVIII and FIX-Padua, have reported near-phenotypic cures in subjects with HA and HB, respectively. Here, we review the biology and the clinical development of FVIII-BDD and FIX-Padua as transgenes. We also examine alternative bioengineering strategies for FVIII and FIX, as well as the immunological challenges of these approaches. Other engineered proteins and their potential use in gene therapy for hemophilia with inhibitors are also discussed. Continued advancement of gene therapy for HA and HB using protein-engineered transgenes has the potential to alleviate the substantial medical and psychosocial burdens of the disease.

The M358R variant of α(1)-proteinase inhibitor inhibits coagulation factor VIIa

The naturally occurring M358R mutation of the plasma serpin α1-proteinase inhibitor (API) changes both its cleavable reactive centre bond to Arg-Ser and the efficacy with which it inhibits different proteases, reducing the rate of inhibition of neutrophil elastase, and enhancing that of thrombin, factor XIa, and kallikrein, by several orders of magnitude. Although another plasma serpin with an Arg-Ser reactive centre, antithrombin (AT), has been shown to inhibit factor VIIa (FVIIa), no published data are available with respect to FVIIa inhibition by API M358R. Recombinant bacterially-expressed API M358R and plasma-derived AT were therefore compared using gel-based and kinetic assays of FVIIa integrity and activity. Under pseudo-first order conditions of excess serpin over protease, both AT and API M358R formed denaturation-resistant inhibitory complexes with FVIIa in reactions accelerated by TF; AT, but not API M358R, also required heparin for maximal activity. The second order rate constant for heparin-independent API M358R-mediated FVIIa inhibition was determined to be 7.8 ± 0.8 × 10(2) M(-1)sec(-1). We conclude that API M358R inhibits FVIIa by forming inhibitory complexes of the serpin type more rapidly than AT in the absence of heparin. The likely 20-fold excess of API M358R over AT in patient plasma during inflammation raises the possibility that it could contribute to the hemorrhagic tendencies manifested by rare individuals expressing this mutant serpin.

Sustained correction of FVII deficiency in dogs using AAV-mediated expression of zymogen FVII

Factor VII (FVII) deficiency is a rare autosomal recessive bleeding disorder treated by infusion of fresh-frozen plasma, plasma-derived FVII concentrates and low-dose recombinant activated FVII. Clinical data suggest that a mild elevation of plasma FVII levels (>10% normal) results in improved hemostasis. Research dogs with a G96E missense FVII mutation (FVII-G96E) have <1% FVII activity. By western blot, we show that they have undetectable plasmatic antigen, thus representing the most prevalent type of human FVII deficiency (low antigen/activity). In these dogs, we determine the feasibility of a gene therapy approach using liver-directed, adeno-associated viral (AAV) serotype 8 vector delivery of a canine FVII (cFVII) zymogen transgene. FVII-G96E dogs received escalating AAV doses (2E11 to 4.95E13 vector genomes [vg] per kg). Clinically therapeutic expression (15% normal) was attained with as low as 6E11 vg/kg of AAV and has been stable for >1 year (ongoing) without antibody formation to the cFVII transgene. Sustained and supraphysiological expression of 770% normal was observed using 4.95E13 vg/kg of AAV (2.6 years, ongoing). No evidence of pathological activation of coagulation or detrimental animal physiology was observed as platelet counts, d-dimer, fibrinogen levels, and serum chemistries remained normal in all dogs (cumulative 6.4 years). We observed a transient and noninhibitory immunoglobulin G class 2 response against cFVII only in the dog receiving the highest AAV dose. In conclusion, in the only large-animal model representing the majority of FVII mutation types, our data are first to demonstrate the feasibility, safety, and long-term duration of AAV-mediated correction of FVII deficiency.

The consensus-based approach for gene/enzyme replacement therapies and crystallization strategies: the case of human alanine-glyoxylate aminotransferase

Protein stability is a fundamental issue in biomedical and biotechnological applications of proteins. Among these applications, gene- and enzyme-replacement strategies are promising approaches to treat inherited diseases that may benefit from protein engineering techniques, even though these beneficial effects have been largely unexplored. In the present study we apply a sequence-alignment statistics procedure (consensus-based approach) to improve the activity and stability of the human AGT (alanine-glyoxylate aminotransferase) protein, an enzyme which causes PH1 (primary hyperoxaluria type I) upon mutation. By combining only five consensus mutations, we obtain a variant (AGT-RHEAM) with largely enhanced in vitro thermal and kinetic stability, increased activity, and with no side effects on foldability and peroxisomal targeting in mammalian cells. The structure of AGT-RHEAM reveals changes at the dimer interface and improved electrostatic interactions responsible for increased kinetic stability. Consensus-based variants maintained the overall protein fold, crystallized more easily and improved the expression as soluble proteins in two different systems [AGT and CIPK24 (CBL-interacting serine/threonine-protein kinase) SOS2 (salt-overly-sensitive 2)]. Thus the consensus-based approach also emerges as a simple and generic strategy to increase the crystallization success for hard-to-get protein targets as well as to enhance protein stability and function for biomedical applications.

The endothelial protein C receptor enhances hemostasis of FVIIa administration in hemophilic mice in vivo

Recombinant activated human factor VII (rhFVIIa) is an established hemostatic agent in hemophilia, but its mechanism of action remains unclear. Although tissue factor (TF) is its natural receptor, rhFVIIa also interacts with the endothelial protein C receptor (EPCR) through its γ-carboxyglutamic acid (Gla) domain, with unknown hemostatic consequences in vivo. Here, we study whether EPCR facilitates rhFVIIa hemostasis in hemophilia using a mouse model system. Mouse activated FVII (mFVIIa) is functionally homologous to rhFVIIa, but binds poorly to mouse EPCR (mEPCR). We modified mFVIIa to gain mEPCR binding using 3 amino acid changes in its Gla domain (L4F/L8M/W9R). The resulting molecule mFVIIa-FMR specifically bound mEPCR in vitro and in vivo and was identical to mFVIIa with respect to TF affinity and procoagulant functions. In macrovascular injury models, hemophilic mice administered mFVIIa-FMR exhibited superior hemostatic activity compared with mFVIIa. This was abolished by blocking mEPCR and was absent in ex vivo whole blood coagulation assays, implicating a specific mFVIIa-FMR and endothelial mEPCR interaction. Because mFVIIa-FMR models the TF-dependent and EPCR binding properties of rhFVIIa, our data unmask a novel contribution of EPCR on the action of rhFVIIa administration in hemophilia, prompting the rational design of improved and safer rhFVIIa therapeutics.

Overexpression of factor VII ameliorates bleeding diathesis of factor VIII-deficient mice with inhibitors

INTRODUCTION

Factor VIII (FVIII) treatment for hemophilia A has difficulties in correcting bleeding diathesis in the presence of inhibitors.

MATERIALS AND METHODS

An adeno-associated virus type 8 (AAV8) vector containing the factor VII (FVII) gene or the activated factor VII (FVIIa) gene was used to investigate the therapeutic effect of FVII or FVIIa overexpression in FVIII-deficient mice with inhibitors.

RESULTS

Following repeated human FVIII injection, FVIII-deficient mice developed anti-human FVIII antibodies that cross-reacted with mouse FVIII. High transgene expression of murine FVII or murine FVIIa was achieved using the AAV8 vector and resulted in increased blood FVII activity greater than 800% of normal murine FVII levels in vector-injected FVIII-deficient mice. Thromboelastography analysis showed significant improvements in clotting time, clot formation time, α angle, and mean clot firmness in AAV8 vector-injected FVIII-deficient mice with inhibitors. Overexpression of FVIIa ameliorated the bleeding phenotype of FVIII-deficient mice with inhibitors and significantly increased the survival rate after tail clipping. In addition, overexpression of FVII increased the survival rate of FVIII-deficient mice with inhibitors after tail clipping though it was not as efficient as FVIIa overexpression.

CONCLUSIONS

These data suggest that FVII overexpression is an alternative strategy for the treatment of hemophilia A with inhibitors.

A zymogen-like factor Xa variant corrects the coagulation defect in hemophilia

Effective therapies are needed to control excessive bleeding in a range of clinical conditions. We describe a surprisingly useful approach to improve hemostasis in vivo using a variant of coagulation factor Xa (FXa^I16L). This conformationally pliant derivative is partially inactive due to a defect in transitioning from zymogen to protease ^1,2. Using mouse models of hemophilia, we show that FXa^I16L has a prolonged half-life, relative to wild-type FXa and does not cause excessive activation of coagulation. Once clotting mechanisms are activated to produce its cofactor FVa, FXa^I16L is driven to the protease state and restores hemostasis in hemophilic animals upon vascular injury. Moreover, using human or murine analogs, we show that FXa^I16L is more efficacious than FVIIa which is used to treat bleeding in hemophilia inhibitor patients³. Because of its underlying mechanism of action, FXa^I16L may provide an effective strategy to enhance blood clot formation and act as a rapid pan-hemostatic agent for the treatment of bleeding conditions.

Gene therapy for haemophilia: a long and winding road

BACKGROUND

Cure, or improvement of disease phenotype, has been a long-term goal in the treatment of haemophilia. An obvious strategy for achieving this goal is the use of gene therapy.

OBJECTIVES

This paper summarises prior and current clinical trials of gene therapy for haemophilia A and B, and briefly describes additional strategies in pre-clinical development.

RESULTS AND CONCLUSIONS

Approximately 50 human subjects with severe haemophilia A or B have been enrolled in seven different trials of gene therapy. These have used plasmids, retroviral, adenoviral, and AAV vectors, directed to autologous fibroblasts, skeletal muscle, liver, and other target cell types accessed by intravenous injection of vector. Four separate trials have used AAV vectors, three of these targeting liver. Data from animal models suggest that several different gene replacement strategies may eventually yield long-term expression of factor at therapeutic levels, and that in situ correction of gene defects in hepatocytes may eventually be a therapeutic option.