Minimal modification in the factor VIII B-domain sequence ameliorates the murine hemophilia A phenotype

Characterization of zebrafish coagulation cofactors Fviii and Fv mutants and modeling hemophilia A and factor V deficiency

The aim of this study is to characterize zebrafish coagulation cofactors fviii and fv mutant fish and assess if they phenocopy classical hemophilia A and factor V deficiency in humans. The embryos from fviii and fv zebrafish heterozygote mutants generated by ENU mutagenesis were purchased from the ZIRC repository. They were reared to adulthood and genotyped. The heterozygote male and female were crossed to get homozygote, heterozygote, and wild-type fish. Functional kinetic coagulation assays and bleeding assays were performed on normal and mutant adult fish, and venous laser injury assays were performed on the larvae. The DNA from fviii and fv mutants were sequenced to confirm if they have a premature stop codon in exon 19, and in exon 2, respectively, and in both mutants, the amino acid glutamine is replaced with a stop codon. Homozygous and heterozygous 5 days post fertilization (dpf) larvae for fviii and fv deficient mutants exhibited prolonged time to occlusion after venous laser injury compared to wild-type controls. The homozygous and heterozygous fviii adult mutants showed modest bleeding and delayed fibrin formation in the kinetic partial thromboplastin time (kPTT) assay with their plasma. fv homozygous larvae had poor survival beyond 12 dpf. However, heterozygous fv mutants exhibited heavy bleeding and prolonged fibrin formation in the kPTT and kPT assay compared with wild-type siblings. Our characterization showed fviii and fv mutants from ZIRC phenocopied to a considerable extent classical hemophilia A and factor V deficiency in humans, respectively. These models should be useful in studying and developing novel drugs that reverse the phenotype and in generating suppressor mutations to identify novel factors that compensate for these deficiencies.

AAV gene therapy in companion dogs with severe hemophilia: Real-world long-term data on immunogenicity, efficacy, and quality of life

The hemophilias are the most common severe inherited bleeding disorders and are caused by deficiency of clotting factor (F) VIII (hemophilia A) or FIX (hemophilia B). The resultant bleeding predisposition significantly increases morbidity and mortality. The ability to improve the bleeding phenotype with modest increases in clotting factor levels has enabled the development and regulatory approval of adeno-associated viral (AAV) vector gene therapies for people with hemophilia A and B. The canine hemophilia model has proven to be one of the best predictors of therapeutic response in humans. Here, we report long-term follow-up of 12 companion dogs with severe hemophilia that were treated in a real-world setting with AAV gene therapy. Despite more baseline bleeding than in research dogs, companion dogs demonstrated a 94% decrease in bleeding rates and 61% improvement in quality of life over a median of 4.1 years (range 2.6-8.9). No new anti-transgene immune responses were detected; one dog with a pre-existing anti-FVIII inhibitor achieved immune tolerance with gene therapy. Two dogs expressing 1%-5% FVIII post gene therapy experienced fatal bleeding events. These data suggest AAV liver-directed gene therapy is efficacious in a real-world setting but should target expression >5% and closely monitor those with levels in the 1%-5% range.

Targeted genome engineering based on CRISPR/Cas9 system to enhance FVIII expression in vitro

BACKGROUND

Hemophilia A is caused by a deficiency of coagulation factor VIII in the body due to a defect in the F8 gene. The emergence of CRISPR/Cas9 gene editing technology will make it possible to alter the expression of the F8 gene in hemophiliacs, while achieving a potential cure for the disease.

METHODS

Initially, we identified high-activity variants of FVIII and constructed donor plasmids using enzymatic digestion and ligation techniques. Subsequently, the donor plasmids were co-transfected with sgRNA-Cas9 protein into mouse Neuro-2a cells, followed by flow cytometry-based cell sorting and puromycin selection. Finally, BDD-hF8 targeted to knock-in the mROSA26 genomic locus was identified and validated for FVIII expression.

RESULTS

We identified the p18T-BDD-F8-V3 variant with high FVIII activity and detected the strongest pX458-mROSA26-int1-sgRNA1 targeted cleavage ability and no cleavage events were found at potential off-target sites. Targeted knock-in of BDD-hF8 cDNA at the mROSA26 locus was achieved based on both HDR/NHEJ gene repair approaches, and high level and stable FVIII expression was obtained, successfully realizing gene editing in vitro.

CONCLUSIONS

Knock-in of exogenous genes based on the CRISPR/Cas9 system targeting genomic loci is promising for the research and treatment of a variety of single-gene diseases.

Analysis of vector genome integrations in multicentric lymphoma after AAV gene therapy in a severe hemophilia A dog

Adeno-associated viral (AAV) vectors have traditionally been viewed as predominantly nonintegrating, with limited concerns for oncogenesis. However, accumulating preclinical data have shown that AAV vectors integrate more often than previously appreciated, with the potential for genotoxicity. To understand the consequences of AAV vector integration, vigilance for rare genotoxic events after vector administration is essential. Here, we investigate the development of multicentric lymphoma in a privately owned dog, PC9, with severe hemophilia A that was treated with an AAV8 vector encapsidating a B domain-deleted canine coagulation F8 gene. PC9 developed an aggressive B cell lineage multicentric lymphoma 3.5 years after AAV treatment. Postmortem analysis of the liver, spleen, and lymph nodes showed the expected biodistribution of the AAV genome. Integration events were found both in PC9 and a second privately owned hemophilia A dog treated similarly with canine F8 gene transfer, which died of a bleeding event without evidence of malignancy. However, we found no evidence of expanded clones harboring a single integration event, indicating that AAV genome integrations were unlikely to have contributed to PC9's cancer. These findings suggest AAV integrations occur but are mostly not genotoxic and support the safety profile of AAV gene therapy.

Cellular stress and coagulation factor production: when more is not necessarily better

Remarkably, it has been 40 years since the isolation of the 2 genes involved in hemophilia A (HA) and hemophilia B (HB), encoding clotting factor (F) VIII (FVIII) and FIX, respectively. Over the years, these advances led to the development of purified recombinant protein factors that are free of contaminating viruses from human pooled plasma for hemophilia treatments, reducing the morbidity and mortality previously associated with human plasma-derived clotting factors. These discoveries also paved the way for modified factors that have increased plasma half-lives. Importantly, more recent advances have led to the development and Food and Drug Administration approval of a hepatocyte-targeted, adeno-associated viral vector-mediated gene transfer approach for HA and HB. However, major concerns regarding the durability and safety of HA gene therapy remain to be resolved. Compared with FIX, FVIII is a much larger protein that is prone to misfolding and aggregation in the endoplasmic reticulum and is poorly secreted by the mammalian cells. Due to the constraint of the packaging capacity of adeno-associated viral vector, B-domain deleted FVIII rather than the full-length protein is used for HA gene therapy. Like full-length FVIII, B-domain deleted FVIII misfolds and is inefficiently secreted. Its expression in hepatocytes activates the cellular unfolded protein response, which is deleterious for hepatocyte function and survival and has the potential to drive hepatocellular carcinoma. This review is focused on our current understanding of factors limiting FVIII secretion and the potential pathophysiological consequences upon expression in hepatocytes.

Suppression of anti-drug antibody formation against coagulation factor VIII by oral delivery of anti-CD3 monoclonal antibody in hemophilia A mice

Active tolerance to ingested dietary antigens forms the basis for oral immunotherapy to food allergens or autoimmune self-antigens. Alternatively, oral administration of anti-CD3 monoclonal antibody can be effective in modulating systemic immune responses without T cell depletion. Here we assessed the efficacy of full length and the F(ab')2 fragment of oral anti-CD3 to prevent anti-drug antibody (ADA) formation to clotting factor VIII (FVIII) protein replacement therapy in hemophilia A mice. A short course of low dose oral anti-CD3 F(ab')2 reduced the production of neutralizing ADAs, and suppression was significantly enhanced when oral anti-CD3 was timed concurrently with FVIII administration. Tolerance was accompanied by the early induction of FoxP3+LAP-, FoxP3+LAP+, and FoxP3-LAP+ populations of CD4+ T cells in the spleen and mesenteric lymph nodes. FoxP3+LAP+ Tregs expressing CD69, CTLA-4, and PD1 persisted in spleens of treated mice, but did not produce IL-10. Finally, we attempted to combine the anti-CD3 approach with oral intake of FVIII antigen (using our previously established method of using lettuce plant cells transgenic for FVIII antigen fused to cholera toxin B (CTB) subunit, which suppresses ADAs in part through induction of IL-10 producing FoxP3-LAP+ Treg). However, combining these two approaches failed to improve suppression of ADAs. We conclude that oral anti-CD3 treatment is a promising approach to prevention of ADA formation in systemic protein replacement therapy, albeit via mechanisms distinct from and not synergistic with oral intake of bioencapsulated antigen.

Biological activity of a new recombinant human coagulation factor VIII and its efficacy in a small animal model

Deficiency in human coagulation factor VIII (FVIII) causes hemophilia A (HA). Patients with HA may suffer from spontaneous bleeding, which can be life-threatening. Recombinant FVIII (rFVIII) is an established treatment and prevention agent for bleeding in patients with HA. Human plasma-derived FVIII (pdFVIII), commonly used in clinical practice, is relatively difficult to prepare. In this study, we developed a novel B-domain-deleted rFVIII, produced and formulated without the use of animal or human serum-derived components. rFVIII promoted the generation of activated factor X and downstream thrombin, and, similar to that of other available FVIII preparations, its activity was inhibited by FVIII inhibitors. In addition, rFVIII has ideal binding affinity to human von Willebrand factor. Activated FVIII (FVIIIa) could be degraded by activated protein C and lose its procoagulant activity. In vitro, commercially available recombinant FVIII (Xyntha) and pdFVIII were used as controls, and there were no statistical differences between rFVIII and commercial FVIII preparations, which demonstrates the satisfactory efficacy and potency of rFVIII. In vivo, HA mice showed that infusion of rFVIII rapidly corrected activated partial thromboplastin time, similar to Xyntha. Moreover, different batches of rFVIII were comparable. Overall, our results demonstrate the potential of rFVIII as an effective strategy for the treatment of FVIII deficiency.

Molecular therapeutics of Hemophilia A and B

INTRODUCTION

Hemophilia A (HA) or B (HB) is an X-linked recessive disorder caused by a defect in the factor VIII (FVIII) or factor IX (FIX) gene which leads to the dysfunction of blood coagulation. Protein replacement therapy (PRT) uses recombinant proteins and plasma-derived products, which incurs high cost and inconvenience requiring routine intravenous infusions and life-time treatment. Understanding of detailed molecular mechanisms on FVIII gene function could provide innovative solutions to amend this disorder. In recent decades, gene therapeutics have advanced rapidly and a one-time cure solution has been proposed.

AREAS COVERED

This review summarizes current understanding of molecular pathways involved in blood coagulation, with emphasis on FVIII's functional role. The existing knowledge and challenges on FVIII gene expression, from transcription, translation, post-translational modification including glycosylation to protein processing and secretion, and co-factor interactions are deciphered and potential molecular interventions discussed.

EXPERT OPINION

This article reviews the potential treatment targets for HA and HB, including antibodies, small molecules and gene therapeutics, based on molecular mechanisms of FVIII biosynthesis, and further, assessing the pros and cons of these various treatment strategies. Understanding detailed FVIII protein synthesis and secretory pathways could provide exciting opportunities in identifying novel therapeutics to ameliorate hemophilia state.

Pharmacokinetic analysis identifies a factor VIII immunogenicity threshold after AAV gene therapy in hemophilia A mice

Advances in the development of novel treatment options for hemophilia A are prevalent. However, the anti-factor VIII (FVIII) neutralizing antibody (inhibitor) response to existing FVIII products remains a major treatment challenge. Although some novel products are designed to function in the presence of inhibitors, they do not specific address the immunogenicity risk or mechanistic causes of inhibitor development, which remain unclear. Furthermore, most preclinical studies supporting clinical gene therapy programs have reported immunogenicity signals in animal models, especially at higher vector doses and sometimes using multiple vector designs. In these settings, immunogenicity risk factor determination, comparative immunogenicity of competing vector designs, and the potential for obtaining meaningful prognostic data remain relatively unexplored. Additionally, there remains the opportunity to investigate clinical gene therapy as an alternative to standard immune tolerance induction therapy. The current study was designed to address these issues through longitudinal dose-response evaluation of 4 adeno-associated viral (AAV) vector candidates encoding 2 different FVIII transgenes in a murine model of hemophilia A. Plasma FVIII activity and anti-FVIII antibody data were used to generate a pharmacokinetic model that (1) identifies initial AAV-FVIII product expression kinetics as the dominant risk factor for inhibitor development, (2) predicts a therapeutic window where immune tolerance is achieved, and (3) demonstrates evidence of gene therapy-based immune tolerance induction. Although there are known limitations to the predictive value of preclinical immunogenicity testing, these studies can uncover or support the development of design principles that can guide the development of safe and effective genetic medicines.

Preclinical assessment of an optimized AAV-FVIII vector in mice and non-human primates for the treatment of hemophilia A

Extensive clinical data from liver-mediated gene therapy trials have shown that dose-dependent immune responses against the vector capsid may impair or even preclude transgene expression if not managed successfully with prompt immune suppression. The goal of this preclinical study was to generate an adeno-associated viral (AAV) vector capable of expressing therapeutic levels of B-domain deleted factor VIII (FVIII) at the lowest possible vector dose to minimize the potential Risk of a capsid-mediated immune response in the clinical setting. Here, we describe the studies that identified the investigational agent SPK-8011, currently being evaluated in a phase 1/2 study (NCT03003533) in individuals with hemophilia A. In particular, the potency of our second-generation expression cassettes was evaluated in mice and in non-human primates using two different bioengineered capsids (AAV-Spark100 and AAV-Spark200). At 2 weeks after gene transfer, primates transduced with 2 × 1012 vg/kg AAV-Spark100-FVIII or AAV-Spark200-FVIII expressed FVIII antigen levels of 13% ± 2% and 22% ± 6% of normal, respectively. Collectively, these preclinical results validate the feasibility of lowering the AAV capsid dose for a gene-based therapeutic approach for hemophilia A to a dose level orders of magnitude lower than the first-generation vectors in the clinic.

B cell-activating factor modulates the factor VIII immune response in hemophilia A

Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell development could help identify therapeutic targets. The B cell-activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody-mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI.

Molecular coevolution of coagulation factor VIII and von Willebrand factor

Ancestral sequence reconstruction provides a unique platform for investigating the molecular evolution of single gene products and recently has shown success in engineering advanced biological therapeutics. To date, the coevolution of proteins within complexes and protein-protein interactions is mostly investigated in silico via proteomics and/or within single-celled systems. Herein, ancestral sequence reconstruction is used to investigate the molecular evolution of 2 proteins linked not only by stabilizing association in circulation but also by their independent roles within the primary and secondary hemostatic systems of mammals. Using sequence analysis and biochemical characterization of recombinant ancestral von Willebrand factor (VWF) and coagulation factor VIII (FVIII), we investigated the evolution of the essential macromolecular FVIII/VWF complex. Our data support the hypothesis that these coagulation proteins coevolved throughout mammalian diversification, maintaining strong binding affinities while modulating independent and distinct hemostatic activities in diverse lineages.

Activated protein C has a regulatory role in factor VIII function

Mechanisms thought to regulate activated factor VIII (FVIIIa) cofactor function include A2-domain dissociation and activated protein C (APC) cleavage. Unlike A2-domain dissociation, there is no known phenotype associated with altered APC cleavage of FVIII, and biochemical studies have suggested APC plays a marginal role in FVIIIa regulation. However, the in vivo contribution of FVIIIa inactivation by APC is unexplored. Here we compared wild-type B-domainless FVIII (FVIII-WT) recombinant protein with an APC-resistant FVIII variant (FVIII-R336Q/R562Q; FVIII-QQ). FVIII-QQ demonstrated expected APC resistance without other changes in procoagulant function or A2-domain dissociation. In plasma-based studies, FVIII-WT/FVIIIa-WT demonstrated dose-dependent sensitivity to APC with or without protein S, whereas FVIII-QQ/FVIIIa-QQ did not. Importantly, FVIII-QQ demonstrated approximately fivefold increased procoagulant function relative to FVIII-WT in the tail clip and ferric chloride injury models in hemophilia A (HA) mice. To minimize the contribution of FV inactivation by APC in vivo, a tail clip assay was performed in homozygous HA/FV Leiden (FVL) mice infused with FVIII-QQ or FVIII-WT in the presence or absence of monoclonal antibody 1609, an antibody that blocks murine PC/APC hemostatic function. FVIII-QQ again demonstrated enhanced hemostatic function in HA/FVL mice; however, FVIII-QQ and FVIII-WT performed analogously in the presence of the PC/APC inhibitory antibody, indicating the increased hemostatic effect of FVIII-QQ was APC specific. Our data demonstrate APC contributes to the in vivo regulation of FVIIIa, which has the potential to be exploited to develop novel HA therapeutics.

Enhancing therapeutic efficacy of in vivo platelet-targeted gene therapy in hemophilia A mice

Our previous studies demonstrated that intraosseous (IO) infusion of lentiviral vectors (LVs) carrying a modified B domain-deleted factor VIII (FVIII) transgene driven by a megakaryocyte-specific promoter (GP1Bα promoter; G-F8/N6-LV) successfully transduced hematopoietic stem cells (HSCs) to produce FVIII stored in the platelet α-granules. Platelet FVIII corrected the bleeding phenotype with limited efficacy in hemophilia A (HemA) mice with and without preexisting anti-FVIII inhibitors. The present study sought to further enhance the therapeutic efficacy of this treatment protocol by increasing both the efficiency of LV transduction and the functional activity of platelet FVIII. A combined drug regimen of dexamethasone and anti-CD8α monoclonal antibody enhanced the percentage of transduced bone marrow and HSCs over time. In G-F8/N6-LV-treated HemA mice, significant improvement in phenotypic correction was observed on day 84. To improve platelet FVIII functionality, genes encoding FVIII variant F8X10K12 with increased expression or F8N6K12RH with increased functional activity compared with F8/N6 were incorporated into LVs. Treatment with G-F8X10K12-LV in HemA mice produced a higher level of platelet FVIII but induced anti-FVIII inhibitors. After treatment with combined drugs and IO infusion of G-F8/N6K12RH-LV, HemA mice showed significant phenotypic correction without anti-FVIII inhibitor formation. These results indicate that new human FVIII variant F8/N6K12RH combined with immune suppression could significantly enhance the therapeutic efficacy of in vivo platelet-targeted gene therapy for murine HemA via IO delivery. This protocol provides a safe and effective treatment for hemophilia that may be translatable to and particularly beneficial for patients with preexisting inhibitory antibodies to FVIII.

Gene Therapy for Hemophilia A: Where We Stand

Hemophilia A (HA) is a hereditary hemorrhagic disease caused by a deficiency of coagulation factor VIII (FVIII) in blood plasma. Patients with HA usually suffer from spontaneous and recurrent bleeding in joints and muscles, or even intracerebral hemorrhage, which might lead to disability or death. Although the disease is currently manageable via delivery of plasma-derived or recombinant FVIII, this approach is costly, and neutralizing antibodies may be generated in a large portion of patients, which render the regimens ineffective and inaccessible. Given the monogenic nature of HA and that a slight increase in FVIII can remarkably alleviate the phenotypes, HA has been considered to be a suitable target disease for gene therapy. Consequently, the introduction of a functional F8 gene copy into the appropriate target cells via viral or nonviral delivery vectors, including gene correction through genome editing approaches, could ultimately provide an effective therapeutic method for HA patients. In this review, we discuss the recent progress of gene therapy for HA with viral and nonviral delivery vectors, including piggyBac, lentiviral and adeno-associated viral vectors, as well as new raising issues involving liver toxicity, pre-existing neutralizing antibodies of viral approach, and the selection of the target cell type for nonviral delivery.

Functionalized lipid-like nanoparticles for in vivo mRNA delivery and base editing

Messenger RNA (mRNA) therapeutics have been explored to treat various genetic disorders. Lipid-derived nanomaterials are currently one of the most promising biomaterials that mediate effective mRNA delivery. However, efficiency and safety of this nanomaterial-based mRNA delivery remains a challenge for clinical applications. Here, we constructed a series of lipid-like nanomaterials (LLNs), named functionalized TT derivatives (FTT), for mRNA-based therapeutic applications in vivo. After screenings on the materials, we identified FTT5 as a lead material for efficient delivery of long mRNAs, such as human factor VIII (hFVIII) mRNA (~4.5 kb) for expression of hFVIII protein in hemophilia A mice. Moreover, FTT5 LLNs demonstrated high percentage of base editing on PCSK9 in vivo at a low dose of base editor mRNA (~5.5 kb) and single guide RNA. Consequently, FTT nanomaterials merit further development for mRNA-based therapy.

Infused factor VIII-expressing platelets or megakaryocytes as a novel therapeutic strategy for hemophilia A

B-domainless factor VIII (FVIII) ectopically expressed in megakaryocytes (MKs) is stored in α granules of platelets (pFVIII) and is capable of restoring hemostasis in FVIII^null mice, even in the presence of circulating inhibitors. However, our prior studies have shown that this ectopically expressed pFVIII can injure developing MKs. Moreover, the known risks of prolonged thrombocytopenia after bone marrow transplantation are significant challenges to the use of this strategy to treat individuals with severe hemophilia A and particularly those with intractable clinically relevant inhibitors. Because of these limitations, we now propose the alternative therapeutic pFVIII strategy of infusing pFVIII-expressing MKs or platelets derived from induced pluripotent stem cells (iPSCs). pFVIII-expressing iPSC-derived MKs, termed iMKs, release platelets that can contribute to improved hemostasis in problematic inhibitor patients with hemophilia A. As proof of principle, we demonstrate that hemostasis can be achieved in vitro and in vivo with pFVIII-expressing platelets and show prolonged efficacy. Notably, pFVIII-expressing platelets are also effective in the presence of inhibitors, and their effect was enhanced with recombinant FVIIa. Human pFVIII-expressing iMKs improved hemostasis in vitro, and derived platelets from infused human pFVIII-expressing iMKs improved hemostasis in FVIII^null mice. These studies indicate the potential therapeutic use of recurrent pFVIII-expressing MK or platelet infusions with prolonged hemostatic coverage that may be additive with bypassing agents in hemophilia A patients with neutralizing inhibitors.

Platelet-inspired therapeutics: current status, limitations, clinical implications, and future potential

Recent research has been successful in demonstrating the importance of the addition of platelets to the field of cell-mediated therapeutics, by making use of different platelet forms to design modalities able to positively impact a wide range of diseases. A key obstacle hindering the success of conventional therapeutic interventions is their inability to produce targeted treatment, resulting in a number of systemic side effects and a longer duration for the onset of action to occur. An additional challenge facing current popular therapeutic interventions is biocompatibility of the system, resulting in the decline of patient compliance to treatment. In an attempt to address these challenges, the past few decades have been witness to the discovery and innovation of precision therapy, in order to achieve targeted treatment for an array of conditions, thereby superseding alternative mechanisms of treatment. Platelet-mediated therapeutics, as well as employing platelets as drug delivery vehicles, are key components in advancing precision therapy within research and in clinical settings. This novel approach is designed with the objective that the platelets retain their original structure and functions within the body, thereby mitigating biocompatibility challenges. In this article, we review the current significant impact that the addition of platelet-inspired systems has made on the field of therapeutics; explore certain limitations of each system, together with ideas on how to overcome them; and discuss the clinical implications and future potential of platelet-inspired therapeutics. Graphical abstract.

FVIII expression by its native promoter sustains long-term correction avoiding immune response in hemophilic mice

Here we describe a successful gene therapy approach for hemophilia A (HA), using the natural F8 promoter (pF8) to direct gene replacement to factor VIII (FVIII)-secreting cells. The promoter sequence and the regulatory elements involved in the modulation of F8 expression are still poorly characterized and biased by the historical assumption that FVIII expression is mainly in hepatocytes. Bioinformatic analyses have highlighted an underestimated complexity in gene expression at this locus, suggesting an activation of pF8 in more cell types than those previously expected. C57Bl/6 mice injected with a lentiviral vector expressing green fluorescent protein (GFP) under the pF8 (lentiviral vector [LV].pF8.GFP) confirm the predominant GFP expression in liver sinusoidal endothelial cells, with a few positive cells detectable also in hematopoietic organs. Therapeutic gene delivery (LV.pF8.FVIII) in hemophilic C57/Bl6 and 129-Bl6 mice successfully corrected the bleeding phenotype, rescuing up to 25% FVIII activity, using a codon-optimized FVIII, with sustained activity for the duration of the experiment (1 year) without inhibitor formation. Of note, LV.pF8.FVIII delivery in FVIII-immunized HA mice resulted in the complete reversion of the inhibitor titer with the recovery of therapeutic FVIII activity. Depletion of regulatory T cells (Tregs) in LV-treated mice allowed the formation of anti-FVIII antibodies, indicating a role for Tregs in immune tolerance induction. The significant blood loss reduction observed in all LV.pF8.FVIII-treated mice 1 year after injection confirmed the achievement of a long-term phenotypic correction. Altogether, our results highlight the potency of pF8-driven transgene expression to correct the bleeding phenotype in HA, as well as potentially in other diseases in which an endothelial-specific expression is required.

A Molecular Revolution in the Treatment of Hemophilia

For decades, the monogenetic bleeding disorders hemophilia A and B (coagulation factor VIII and IX deficiency) have been treated with systemic protein replacement therapy. Now, diverse molecular medicines, ranging from antibody to gene to RNA therapy, are transforming treatment. Traditional replacement therapy requires twice to thrice weekly intravenous infusions of factor. While extended half-life products may reduce the frequency of injections, patients continue to face a lifelong burden of the therapy, suboptimal protection from bleeding and joint damage, and potential development of neutralizing anti-drug antibodies (inhibitors) that require less efficacious bypassing agents and further reduce quality of life. Novel non-replacement and gene therapies aim to address these remaining issues. A recently approved factor VIII-mimetic antibody accomplishes hemostatic correction in patients both with and without inhibitors. Antibodies against tissue factor pathway inhibitor (TFPI) and antithrombin-specific small interfering RNA (siRNA) target natural anticoagulant pathways to rebalance hemostasis. Adeno-associated virus (AAV) gene therapy provides lasting clotting factor replacement and can also be used to induce immune tolerance. Multiple gene-editing techniques are under clinical or preclinical investigation. Here, we provide a comprehensive overview of these approaches, explain how they differ from standard therapies, and predict how the hemophilia treatment landscape will be reshaped.

Hemophilia therapy: the future has begun

The success story of hemophilia care first began in the 1970s, when the availability of plasma-derived concentrates of coagulation factor VIII (FVIII) and factor IX (FIX) provided efficacious treatment of bleeding in patients with hemophilia A and B. This positive scenario was consolidated in terms of greater safety and availability in the 1990s, when the first recombinant coagulation factors were produced. This meant that, instead of only treating episodic bleeding events, prophylaxis regimens could be implemented as a preventive measure. Following the demonstration of its superiority in the frame of two randomized clinical trials, prophylaxis became evidence-based standard of care. In high-income countries, these achievements have led to a patients’ life expectancy being extended to close to that of the general male population. Alongside this, the last decade has witnessed further spectacular therapeutic progress, such as the availability of coagulation factors with a longer plasma half-life that allow for wider intervals between treatment. Moreover, new therapeutic products based on new mechanisms other than the replacement of the deficient factor, have become available (emicizumab) or are at an advanced stage of development. This review celebrates the success story of hemophilia care, while also discussing current limitations, issues and as yet unmet needs. The prospects of cure by means of gene therapy are also outlined.

Hemophilia A ameliorated in mice by CRISPR-based in vivo genome editing of human Factor VIII

Hemophilia A is a monogenic disease with a blood clotting factor VIII (FVIII) deficiency caused by mutation in the factor VIII (F8) gene. Current and emerging treatments such as FVIII protein injection and gene therapies via AAV-delivered F8 transgene in an episome are costly and nonpermanent. Here, we describe a CRISPR/Cas9-based in vivo genome editing method, combined with non-homologous end joining, enabling permanent chromosomal integration of a modified human B domain deleted-F8 (BDD-F8) at the albumin (Alb) locus in liver cells. To test the approach in mice, C57BL/6 mice received tail vein injections of two vectors, AAV8-SaCas9-gRNA, targeting Alb intron 13, and AAV8-BDD-F8. This resulted in BDD-F8 insertion at the Alb locus and FVIII protein expression in the liver of vector-, but not vehicle-, treated mice. Using this approach in hemophilic mice, BDD-F8 was expressed in liver cells as functional human FVIII, leading to increased plasma levels of FVIII and restoration of blood clotting properties in a dose-dependent manor for at least 7 months, with no detectable liver toxicity or meaningful off-target effects. Based on these findings, our BDD-F8 genome editing approach may offer an efficacious, long-term and safe treatment for patients with hemophilia A.

Clinical advances in gene therapy updates on clinical trials of gene therapy in haemophilia

Gene therapy is rapidly becoming a new therapeutic strategy for haemophilia A and B treatment. In the 1990s, studies in animal models showed that adeno-associated vectors (AAV) exhibited an efficient expression of factor IX (FIX). In the first clinical trial in patients with haemophilia B, therapeutic levels of FIX were documented but the expression remained only for few weeks. Subsequently, improvements in vector design, such as the use of different AAV serotypes, the development of the self-complementary vector, the engineering of the transgene with codon optimization and liver-specific expression cassette resulted in circulating FIX level between 2% and 5% for long-lasting period. Recently, a natural gain of function FIX variant (Padua) inserted in the F9 cDNA improved the expression of FIX achieving a level of more than 30% resulting in cessation of infusions and in a greatly reduction of bleeding events. Encouraging clinical progresses have been also obtained from trials of gene therapy for haemophilia A. Transgene expression persisted for three years with circulating FVIII activity levels of 52.3% in patients treated with AAV vector containing a codon-optimized F8 cDNA. A complication, reported in both clinical trials for haemophilia A and B, was the elevation of liver enzymes, which resolved with steroid treatment in a large group of patients. However, to date, the pathophysiological mechanism for the liver toxicity remains still unclear. Clinical trials with adeno-associated vectors have documented a significant success for haemophilia gene therapy demonstrating potential to transform haemophilia treatment offering hope for a long-term expression.

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.

Evaluation of the activity levels of rat FVIII and human FVIII delivered by adeno-associated viral vectors both in vitro and in vivo

The development of a novel coagulation factor VIII (FVIII) expression cassette with an enhanced activity for gene therapy of hemophilia A (HA) is essential. The biological properties of several non-human FVIII sequences, such as porcine and canine, have been evaluated. Here, we compared the activity level of rat FVIII (rFVIII) and human FVIII (hFVIII) by using single-chain and dual-chain strategies in 293 T cells and the HA mice. In both in vitro and hydrodynamic injection studies, the activity of rFVIII detected by the activated partial thromboplastin time assay was higher than that of hFVIII both by single-chain (~2.96-fold and ~1.72-fold, respectively) and dual-chain (~7.69-fold and ~2.35-fold, respectively). Moreover, the dual chain exerted a potentially higher delivery efficacy compared with the single chain (~4.96-fold and ~2.99-fold, respectively). The blood loss of HA mice administrated with rFVIII was less than those with hFVIII. AAV-delivered rFVIII and hFVIII also exerted long-term therapeutic effects on HA mice and caused a transient ALT elevation. These data might help to the development of novel, optimized FVIII expression cassettes based on the amino acid difference between rFVIII and hFVIII. These data indicate that the dual-chain strategy would likely enhance the delivery efficiency of the AAV-mediated FVIII gene therapy.

Gene therapy for hemophilia: what does the future hold?

Recent phase I/II adeno-associated viral vector-mediated gene therapy clinical trials have reported remarkable success in ameliorating disease phenotype in hemophilia A and B. These trials, which highlight the challenges overcome through decades of preclinical and first in human clinical studies, have generated considerable excitement for patients and caregivers alike. Optimization of vector and transgene expression has significantly improved the ability to achieve therapeutic factor levels in these subjects. Long-term follow-up studies will guide standardization of the approach with respect to the combination of serotype, promoter, dose, and manufacturing processes and inform safety for inclusion of young patients. Certain limitations preclude universal applicability of gene therapy, including transient liver transaminase elevations due to the immune responses to vector capsids or as yet undefined mechanisms, underlying liver disease from iatrogenic viral hepatitis, and neutralizing antibodies to clotting factors. Integrating vectors show promising preclinical results, but manufacturing and safety concerns still remain. The prospect of gene editing for correction of the underlying mutation is on the horizon with considerable potential. Herein, we review the advances and limitations that have resulted in these recent successful clinical trials and outline avenues that will allow for broader applicability of gene therapy.

Emerging Issues in AAV-Mediated In Vivo Gene Therapy

In recent years, the number of clinical trials in which adeno-associated virus (AAV) vectors have been used for in vivo gene transfer has steadily increased. The excellent safety profile, together with the high efficiency of transduction of a broad range of target tissues, has established AAV vectors as the platform of choice for in vivo gene therapy. Successful application of the AAV technology has also been achieved in the clinic for a variety of conditions, including coagulation disorders, inherited blindness, and neurodegenerative diseases, among others. Clinical translation of novel and effective "therapeutic products" is, however, a long process that involves several cycles of iterations from bench to bedside that are required to address issues encountered during drug development. For the AAV vector gene transfer technology, several hurdles have emerged in both preclinical studies and clinical trials; addressing these issues will allow in the future to expand the scope of AAV gene transfer as a therapeutic modality for a variety of human diseases. In this review, we will give an overview on the biology of AAV vector, discuss the design of AAV-based gene therapy strategies for in vivo applications, and present key achievements and emerging issues in the field. We will use the liver as a model target tissue for gene transfer based on the large amount of data available from preclinical and clinical studies.

Protein-anchoring therapy to target extracellular matrix proteins to their physiological destinations

Endplate acetylcholinesterase (AChE) deficiency is a form of congenital myasthenic syndrome (CMS) caused by mutations in COLQ, which encodes collagen Q (ColQ). ColQ is an extracellular matrix (ECM) protein that anchors AChE to the synaptic basal lamina. Biglycan, encoded by BGN, is another ECM protein that binds to the dystrophin-associated protein complex (DAPC) on skeletal muscle, which links the actin cytoskeleton and ECM proteins to stabilize the sarcolemma during repeated muscle contractions. Upregulation of biglycan stabilizes the DPAC. Gene therapy can potentially ameliorate any disease that can be recapitulated in cultured cells. However, the difficulty of tissue-specific and developmental stage-specific regulated expression of transgenes, as well as the difficulty of introducing a transgene into all cells in a specific tissue, prevents us from successfully applying gene therapy to many human diseases. In contrast to intracellular proteins, an ECM protein is anchored to the target tissue via its specific binding affinity for protein(s) expressed on the cell surface within the target tissue. Exploiting this unique feature of ECM proteins, we developed protein-anchoring therapy in which a transgene product expressed even in remote tissues can be delivered and anchored to a target tissue using specific binding signals. We demonstrate the application of protein-anchoring therapy to two disease models. First, intravenous administration of adeno-associated virus (AAV) serotype 8-COLQ to Colq-deficient mice, resulting in specific anchoring of ectopically expressed ColQ-AChE at the NMJ, markedly improved motor functions, synaptic transmission, and the ultrastructure of the neuromuscular junction (NMJ). In the second example, Mdx mice, a model for Duchenne muscular dystrophy, were intravenously injected with AAV8-BGN. The treatment ameliorated motor deficits, mitigated muscle histopathologies, decreased plasma creatine kinase activities, and upregulated expression of utrophin and DAPC component proteins. We propose that protein-anchoring therapy could be applied to hereditary/acquired defects in ECM and secreted proteins, as well as therapeutic overexpression of such factors.

Platelet-Targeted Gene Therapy for Hemophilia

Gene therapy is an attractive approach for disease treatment. Since platelets are abundant cells circulating in blood with the distinctive abilities of storage and delivery and fundamental roles in hemostasis and immunity, they could be a unique target for gene therapy of diseases. Recent studies have demonstrated that ectopic expression of factor VIII (FVIII) in platelets under control of the platelet-specific promoter results in FVIII storage together with its carrier protein von Willebrand factor (VWF) in α-granules and the phenotypic correction of hemophilia A. Importantly, the storage and sequestration of FVIII in platelets appears to effectively restore hemostasis even in the presence of functional-blocking inhibitory antibodies. This review summarizes studies on platelet-specific gene therapy of hemophilia A as well as hemophilia B.

Novel alternate hemostatic agents for patients with inhibitors: beyond bypass therapy

Inhibitor formation is among the most severe complications of hemophilia treatment. With a cumulative incidence of ∼30% in those with severe hemophilia A and ∼3% in those with severe hemophilia B, inhibitors are caused by a T-cell response directed against infused coagulation factor; these inhibitors neutralize factor VIII or IX activity and disrupt normal hemostasis. Inhibitor patients become unresponsive to standard factor treatment and, as an alternative, use bypass treatment (eg, recombinant factor VIIa or factor VIII inhibitor bypass activity). However, response to bypass agents is poorer and the burden of disease is higher, with greater morbidity, hospitalization, cost, and mortality, than in noninhibitor patients. Furthermore, inhibitor formation interferes with prophylaxis to prevent bleeding episodes and is a contraindication to gene therapy. Thus, more effective therapies for inhibitor patients are greatly needed. In the last several years, there has been an explosion of novel alternative hemostatic agents for hemophilia patients with and without inhibitors. These agents take advantage of technologic manipulation of coagulation factors and natural anticoagulants to promote hemostasis. The approaches include the following: (1) mutants or mimics of coagulation factors, rendering them resistant to natural anticoagulants; or (2) knock-down or disruption of natural anticoagulants, preventing degradation of coagulation factors. The purpose of this article was to review these novel alternative hemostatic agents and their mechanisms of action, as well as the preliminary pharmacokinetic, safety, and efficacy data available from early-phase clinical trials.

A Novel Platform for Immune Tolerance Induction in Hemophilia A Mice

Hemophilia A (HA) is an X-linked bleeding disease caused by factor VIII (FVIII) deficiency. We previously demonstrated that FVIII is produced specifically in liver sinusoid endothelial cells (LSECs) and to some degree in myeloid cells, and thus, in the present work, we seek to restrict the expression of FVIII transgene to these cells using cell-specific promoters. With this approach, we aim to limit immune response in a mouse model by lentiviral vector (LV)-mediated gene therapy encoding FVIII. To increase the target specificity of FVIII expression, we included miRNA target sequences (miRTs) (i.e., miRT-142.3p, miRT-126, and miRT-122) to silence expression in hematopoietic cells, endothelial cells, and hepatocytes, respectively. Notably, we report, for the first time, therapeutic levels of FVIII transgene expression at its natural site of production, which occurred without the formation of neutralizing antibodies (inhibitors). Moreover, inhibitors were eradicated in FVIII pre-immune mice through a regulatory T cell-dependent mechanism. In conclusion, targeting FVIII expression to LSECs and myeloid cells by using LVs with cell-specific promoter minimized off-target expression and immune responses. Therefore, at least for some transgenes, expression at the physiologic site of synthesis can enhance efficacy and safety, resulting in long-term correction of genetic diseases such as HA.

Novel factor VIII variants with a modified furin cleavage site improve the efficacy of gene therapy for hemophilia A

Essentials Factor (F) VIII is an inefficiently expressed protein. Furin deletion FVIII variants were purified and characterized using in vitro and in vivo assays. These minimally modified novel FVIII variants have enhanced function. These variants provide a strategy for increasing FVIII expression in hemophilia A gene therapy.

SUMMARY

Background The major challenge for developing gene-based therapies for hemophilia A is that human factor VIII (hFVIII) has intrinsic properties that result in inefficient biosynthesis. During intracellular processing, hFVIII is predominantly cleaved at a paired basic amino acid cleaving enzyme (PACE) or furin cleavage site to yield a heterodimer that is the major form of secreted protein. Previous studies with B-domain-deleted (BDD) canine FVIII and hFVIII-R1645H, both differing from hFVIII by a single amino acid at this site, suggested that these proteins are secreted mainly in a single polypeptide chain (SC) form and exhibit enhanced function. Objective We hypothesized that deletion(s) of the furin site modulates FVIII biology and may enhance its function. Methods A series of recombinant hFVIII-furin deletion variants were introduced into hFVIII-BDD [Δ1645, 1645-46(Δ2), 1645-47(Δ3), 1645-48(Δ4), or Δ1648] and characterized. Results In vitro, recombinant purified Δ3 and Δ4 were primarily SC and, interestingly, had 2-fold higher procoagulant activity compared with FVIII-BDD. In vivo, the variants also have improved hemostatic function. After adeno-associated viral (AAV) vector delivery, the expression of these variants is 2-4-fold higher than hFVIII-BDD. Protein challenges of each variant in mice tolerant to hFVIII-BDD showed no anti-FVIII immune response. Conclusions These data suggest that the furin deletion hFVIII variants are superior to hFVIII-BDD without increased immunogenicity. In the setting of gene-based therapeutics, these novel variants provide a unique strategy to increase FVIII expression, thus lowering the vector dose, a critical factor for hemophilia A gene therapy.

Circumventing furin enhances factor VIII biological activity and ameliorates bleeding phenotypes in hemophilia models

Processing by the proprotein convertase furin is believed to be critical for the biological activity of multiple proteins involved in hemostasis, including coagulation factor VIII (FVIII). This belief prompted the retention of the furin recognition motif (amino acids 1645-1648) in the design of B-domain-deleted FVIII (FVIII-BDD) products in current clinical use and in the drug development pipeline, as well as in experimental FVIII gene therapy strategies. Here, we report that processing by furin is in fact deleterious to FVIII-BDD secretion and procoagulant activity. Inhibition of furin increases the secretion and decreases the intracellular retention of FVIII-BDD protein in mammalian cells. Our new variant (FVIII-ΔF), in which this recognition motif is removed, efficiently circumvents furin. FVIII-ΔF demonstrates increased recombinant protein yields, enhanced clotting activity, and higher circulating FVIII levels after adeno-associated viral vector-based liver gene therapy in a murine model of severe hemophilia A (HA) compared with FVIII-BDD. Moreover, we observed an amelioration of the bleeding phenotype in severe HA dogs with sustained therapeutic FVIII levels after FVIII-ΔF gene therapy at a lower vector dose than previously employed in this model. The immunogenicity of FVIII-ΔF did not differ from that of FVIII-BDD as a protein or a gene therapeutic. Thus, contrary to previous suppositions, FVIII variants that can avoid furin processing are likely to have enhanced translational potential for HA therapy.

Current animal models of hemophilia: the state of the art

Hemophilia is the most well-known hereditary bleeding disorder, with an incidence of one in every 5000 to 30,000 males worldwide. The disease is treated by infusion of protein products on demand and as prophylaxis. Although these therapies have been very successful, some challenging and unresolved tasks remain, such as reducing bleeding rates, presence of target joints and/or established joint damage, eliminating the development of inhibitors, and increasing the success rate of immune-tolerance induction (ITI). Many preclinical trials are carried out on animal models for hemophilia generated by the hemophilia research community, which in turn enable prospective clinical trials aiming to tackle these challenges. Suitable animal models are needed for greater advances in treating hemophilia, such as the development of better models for evaluation of the efficacy and safety of long-acting products, more powerful gene therapy vectors than are currently available, and successful ITI strategies. Mice, dogs, and pigs are the most commonly used animal models for hemophilia. With the advent of the nuclease method for genome editing, namely the CRISPR/Cas9 system, it is now possible to create animal models for hemophilia other than mice in a short period of time. This review presents currently available animal models for hemophilia, and discusses the importance of animal models for the development of better treatment options for hemophilia.

Hemophilia A gene therapy via intraosseous delivery of factor VIII-lentiviral vectors

Current treatment of hemophilia A (HemA) patients with repeated infusions of factor VIII (FVIII; abbreviated as F8 in constructs) is costly, inconvenient, and incompletely effective. In addition, approximately 25 % of treated patients develop anti-factor VIII immune responses. Gene therapy that can achieve long-term phenotypic correction without the complication of anti-factor VIII antibody formation is highly desired. Lentiviral vector (LV)-mediated gene transfer into hematopoietic stem cells (HSCs) results in stable integration of FVIII gene into the host genome, leading to persistent therapeutic effect. However, ex vivo HSC gene therapy requires pre-conditioning which is highly undesirable for hemophilia patients. The recently developed novel methodology of direct intraosseous (IO) delivery of LVs can efficiently transduce bone marrow cells, generating high levels of transgene expression in HSCs. IO delivery of E-F8-LV utilizing a ubiquitous EF1α promoter generated initially therapeutic levels of FVIII, however, robust anti-FVIII antibody responses ensued neutralized functional FVIII activity in the circulation. In contrast, a single IO delivery of G-FVIII-LV utilizing a megakaryocytic-specific GP1bα promoter achieved platelet-specific FVIII expression, leading to persistent, partial correction of HemA in treated animals. Most interestingly, comparable therapeutic benefit with G-F8-LV was obtained in HemA mice with pre-existing anti-FVIII inhibitors. Platelets is an ideal IO delivery vehicle since FVIII stored in α-granules of platelets is protected from high-titer anti-FVIII antibodies; and that even relatively small numbers of activated platelets that locally excrete FVIII may be sufficient to promote efficient clot formation during bleeding. Additionally, combination of pharmacological agents improved transduction of LVs and persistence of transduced cells and transgene expression. Overall, a single IO infusion of G-F8-LV can generate long-term stable expression of hFVIII in platelets and correct hemophilia phenotype for long term. This approach has high potential to permanently treat FVIII deficiency with and without pre-existing anti-FVIII antibodies.

Potential for cellular stress response to hepatic factor VIII expression from AAV vector

Hemophilia A and B are coagulation disorders resulting from the loss of functional coagulation factor VIII (FVIII) or factor IX proteins, respectively. Gene therapy for hemophilia with adeno-associated virus vectors has shown efficacy in hemophilia B patients. Although hemophilia A patients are more prevalent, the development of therapeutic adeno-associated virus vectors has been impeded by the size of the F8 cDNA and impaired secretion of FVIII protein. Further, it has been reported that over-expression of the FVIII protein induces endoplasmic reticulum stress and activates the unfolded protein response pathway both in vitro and in hepatocytes in vivo, presumably due to retention of misfolded FVIII protein within the endoplasmic reticulum. Engineering of the F8 transgene, including removal of the B domain (BDD-FVIII) and codon optimization, now allows for the generation of adeno-associated virus vectors capable of expressing therapeutic levels of FVIII. Here we sought to determine if the risks of inducing the unfolded protein response in murine hepatocytes extend to adeno-associated virus gene transfer. Although our data show a mild activation of unfolded protein response markers following F8 gene delivery at a certain vector dose in C57BL/6 mice, it was not augmented upon further elevated dosing, did not induce liver pathology or apoptosis, and did not impact FVIII immunogenicity.

Overexpression of factor VIII after AAV delivery is transiently associated with cellular stress in hemophilia A mice

Factor VIII (FVIII) is a large glycoprotein that is challenging to express both in vitro and in vivo. Several studies suggest that high levels of FVIII expression can lead to cellular stress. After gene transfer, transgene expression is restricted to a subset of cells and the increased FVIII load per cell may impact activation of the unfolded protein response. We sought to determine whether increased FVIII expression in mice after adeno-associated viral liver gene transfer would affect the unfolded protein response and/or immune response to the transgene. The FVIII gene was delivered as B-domain deleted single chain or two chain (light and heavy chains) at a range of doses in hemophilia A mice. A correlation between FVIII expression and anti-FVIII antibody titers was observed. Analysis of key components of the unfolded protein response, binding immunoglobulin protein (BiP), and C/EBP homologous protein (CHOP), showed transient unfolded protein response activation in the single chain treated group expressing >200% of FVIII but not after two chain delivery. These studies suggest that supraphysiological single chain FVIII expression may increase the likelihood of a cellular stress response but does not alter liver function. These data are in agreement with the observed long-term expression of FVIII at therapeutic levels after adeno-associated viral delivery in hemophilia A dogs without evidence of cellular toxicity.

Canine models of inherited bleeding disorders in the development of coagulation assays, novel protein replacement and gene therapies

Animal models of inherited bleeding disorders are important for understanding disease pathophysiology and are required for preclinical assessment of safety prior to testing of novel therapeutics in human and veterinary medicine. Experiments in these animals represent important translational research aimed at developing safer and better treatments, such as plasma-derived and recombinant protein replacement therapies, gene therapies and immune tolerance protocols for antidrug inhibitory antibodies. Ideally, testing is done in animals with the analogous human disease to provide essential safety information, estimates of the correct starting dose and dose response (pharmacokinetics) and measures of efficacy (pharmacodynamics) that guide the design of human trials. For nearly seven decades, canine models of hemophilia, von Willebrand disease and other inherited bleeding disorders have not only informed our understanding of the natural history and pathophysiology of these disorders but also guided the development of novel therapeutics for use in humans and dogs. This has been especially important for the development of gene therapy, in which unique toxicities such as insertional mutagenesis, germ line gene transfer and viral toxicities must be assessed. There are several issues regarding comparative medicine in these species that have a bearing on these studies, including immune reactions to xenoproteins, varied metabolism or clearance of wild-type and modified proteins, and unique tissue tropism of viral vectors. This review focuses on the results of studies that have been performed in dogs with inherited bleeding disorders that closely mirror the human condition to develop safe and effective protein and gene-based therapies that benefit both species.

Evaluation of the biological differences of canine and human factor VIII in gene delivery: implications in human hemophilia treatment

The canine is the most important large animal model for testing novel hemophilia A (HA) treatment. It is often necessary to use canine factor VIII (cFIII) gene or protein for the evaluation of HA treatment in the canine model. However, different biological properties between cFVIII and human FVIII (hFVIII) indicated that the development of novel HA treatment may require careful characterization of non-human FVIII. To investigate whether the data obtained using cFVIII can translate to HA treatment in human, we analyzed the differential biological properties of canine heavy chain (cHC) and light chain (cLC) by comparing with human heavy chain (hHC) and light chain (hLC). The secretion of cHC was 5-30-fold higher than hHC, with or without light chains (LCs). cHC+hLC group exhibited ~18-fold increase in coagulation activity compared with hHC+hLC delivery by recombinant adeno-associated viral vectors. Unlike hHC, the secretion of cHC was independent of LCs. cLC improves the specific activity of FVIII by two- to threefold compared with hLC. Moreover, the cLC, but not cHC, contributes to the higher stability of cFVIII. Our results suggested that the cFVIII expression results in the canine model should be interpreted with caution as the cHC secreted more efficiently than hHC and cLC exhibited a more active and stable phenotype than hLC.

Successful Phenotype Improvement following Gene Therapy for Severe Hemophilia A in Privately Owned Dogs

Severe hemophilia A (HA) is an inherited bleeding disorder characterized by <1% of residual factor VIII (FVIII) clotting activity. The disease affects several mammals including dogs, and, like humans, is associated with high morbidity and mortality. In gene therapy using adeno-associated viral (AAV) vectors, the canine model has been one of the best predictors of the therapeutic dose tested in clinical trials for hemophilia B (factor IX deficiency) and other genetic diseases, such as congenital blindness. Here we report our experience with liver gene therapy with AAV-FVIII in two outbred, privately owned dogs with severe HA that resulted in sustained expression of 1–2% of normal FVIII levels and prevented 90% of expected bleeding episodes. A Thr62Met mutation in the F8 gene was identified in one dog. These data recapitulate the improvement of the disease phenotype in research animals, and in humans, with AAV liver gene therapy for hemophilia B. Our experience is a novel example of the benefits of a relevant preclinical canine model to facilitate both translational studies in humans and improved welfare of privately owned dogs.

The impact of minimally oversized adeno-associated viral vectors encoding human factor VIII on vector potency in vivo

Recombinant adeno-associated viral (rAAV) vectors containing oversized genomes provide transgene expression despite low efficiency packaging of complete genomes. Here, we characterized the properties of oversized rAAV2/8 vectors (up to 5.4 kb) encoding human factor VIII (FVIII) under the transcriptional control of three liver promoters. All vectors provided sustained production of active FVIII in mice for 7 months and contained comparable levels of vector genomes and complete expression cassettes in liver. Therefore, for the 5.4 kb genome size range, a strong expression cassette was more important for FVIII production than the vector genome size. To evaluate the potency of slightly oversized vectors, a 5.1 kb AAVrh8R/FVIII vector was compared to a 4.6 kb (wild-type size) vector with an identical expression cassette (but containing a smaller C1-domain deleted FVIII) for 3 months in mice. The 5.1 kb vector had twofold to threefold lower levels of plasma FVIII protein and liver vector genomes than that obtained with the 4.6 kb vector. Vector genomes for both vectors persisted equally and existed primarily as high molecular weight concatemeric circular forms in liver. Taken together, these results indicate that the slightly oversized vectors containing heterogeneously packaged vector genomes generated a functional transgene product but exhibited a twofold to threefold lower in vivo potency.

Megakaryocyte- and megakaryocyte precursor-related gene therapies

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

In vivo genome editing of the albumin locus as a platform for protein replacement therapy

Site-specific genome editing provides a promising approach for achieving long-term, stable therapeutic gene expression. Genome editing has been successfully applied in a variety of preclinical models, generally focused on targeting the diseased locus itself; however, limited targeting efficiency or insufficient expression from the endogenous promoter may impede the translation of these approaches, particularly if the desired editing event does not confer a selective growth advantage. Here we report a general strategy for liver-directed protein replacement therapies that addresses these issues: zinc finger nuclease (ZFN) -mediated site-specific integration of therapeutic transgenes within the albumin gene. By using adeno-associated viral (AAV) vector delivery in vivo, we achieved long-term expression of human factors VIII and IX (hFVIII and hFIX) in mouse models of hemophilia A and B at therapeutic levels. By using the same targeting reagents in wild-type mice, lysosomal enzymes were expressed that are deficient in Fabry and Gaucher diseases and in Hurler and Hunter syndromes. The establishment of a universal nuclease-based platform for secreted protein production would represent a critical advance in the development of safe, permanent, and functional cures for diverse genetic and nongenetic diseases.

Obstacles and future of gene therapy for hemophilia

INTRODUCTION

The recent success of early-phase clinical trials for adeno-associated viral (AAV) liver-directed gene therapy for hemophilia B (HB) demonstrates the potential for gene therapy, in the future, to succeed protein-based prophylaxis therapy for HB. Significant obstacles, however, need to be overcome prior to widespread adoption. The largest obstacles include immune responses to the AAV capsid including preexisting neutralizing antibodies (NAbs) and a delayed cellular immune response. Emerging evidence suggests that the latter is vector-dose dependent. Furthermore, the development and eradication of inhibitors remains a significant safety concern. Similarly, biological differences between Factor VIII and Factor IX (FIX) impose challenges to direct adoption of the successes for HB to hemophilia A (HA).

AREAS COVERED

The advantages and limitations of the current strategies addressing these obstacles for gene therapy for HB and HA are discussed, as well as vector manufacturing issues relevant to widespread adoption. Alternative strategies including both ex-vivo and in-vivo lentiviral-based methods are discussed, though we focus on AAV-based approaches because of their recent clinical success and potential.

EXPERT OPINION

Our opinion is that these obstacles can be overcome with current approaches, and AAV-based gene therapy for HB will likely translate into future clinical care. Innovative approaches are, however, likely needed to solve the current problems obstructing HA gene therapy.

Platelet-delivered therapeutics

We have proposed that modified platelets could potentially be used to correct intrinsic platelet defects as well as for targeted delivery of therapeutic molecules to sights of vascular injury. Ectopic expression of proteins within α-granules prior to platelet activation has been achieved for several proteins, including urokinase, factor (F) VIII, and partially for FIX. Potential uses of platelet-directed therapeutics will be discussed, focusing on targeted delivery of urokinase as a thromboprophylactic agent and FVIII for the treatment of hemophilia A patients with intractable inhibitors. This presentation will discuss new strategies that may be useful in the care of patients with vascular injury as well as remaining challenges and limitations of these approaches.

Profile of efraloctocog alfa and its potential in the treatment of hemophilia A

Hemophilia care has improved dramatically over the past 50 years, evolving from plasma concentrates, to purified plasma proteins, to recombinant clotting factors. These collective developments allowed for home delivery of on-demand and prophylactic treatment, resulting in the reduction of hemophilia morbidity and mortality and improved quality of life. Although efficacious in treating bleeding, conventional factor products’ half-lives require frequent venipuncture, which remains a significant burden to patients. Despite the remarkable advances in hemophilia care, no improvements have, until now, been made to the pharmacokinetic properties of factor products. Multiple strategies have more recently been employed to generate novel bioengineered products that, with great hope, represent the next wave of progress in hemophilia care. The use of these products will undoubtedly raise important discussion about choosing conventional factor over new long-acting factor products. Incorporation of these therapies into clinical care is accompanied by unanswered safety questions that will likely be evaluated only in postmarketing surveillance analysis. Further, these products may change current treatment paradigms with unclear cost repercussions and feasibility. This paper will review efraloctocog alfa (FVIII-Fc) and its role in the treatment of hemophilia A.