A single adeno-associated virus (AAV)-murine factor VIII vector partially corrects the hemophilia A phenotype

Viral vectors engineered for gene therapy

Gene therapy has seen major progress in recent years. Viral vectors have made a significant contribution through efficient engineering for improved delivery and safety. A large variety of indications such as cancer, cardiovascular, metabolic, hematological, neurological, muscular, ophthalmological, infectious diseases, and immunodeficiency have been targeted. Viral vectors based on adenoviruses, adeno-associated viruses, herpes simplex viruses, retroviruses including lentiviruses, alphaviruses, flaviviruses, measles viruses, rhabdoviruses, Newcastle disease virus, poxviruses, picornaviruses, reoviruses, and polyomaviruses have been used. Proof-of-concept has been demonstrated for different indications in animal models. Therapeutic efficacy has also been achieved in clinical trials. Several viral vector-based drugs have been approved for the treatment of cancer, and hematological, metabolic, and neurological diseases. Moreover, viral vector-based vaccines have been approved against COVID-19 and Ebola virus disease.

Viral Vectors in Gene Therapy: Where Do We Stand in 2023?

Viral vectors have been used for a broad spectrum of gene therapy for both acute and chronic diseases. In the context of cancer gene therapy, viral vectors expressing anti-tumor, toxic, suicide and immunostimulatory genes, such as cytokines and chemokines, have been applied. Oncolytic viruses, which specifically replicate in and kill tumor cells, have provided tumor eradication, and even cure of cancers in animal models. In a broader meaning, vaccine development against infectious diseases and various cancers has been considered as a type of gene therapy. Especially in the case of COVID-19 vaccines, adenovirus-based vaccines such as ChAdOx1 nCoV-19 and Ad26.COV2.S have demonstrated excellent safety and vaccine efficacy in clinical trials, leading to Emergency Use Authorization in many countries. Viral vectors have shown great promise in the treatment of chronic diseases such as severe combined immunodeficiency (SCID), muscular dystrophy, hemophilia, β-thalassemia, and sickle cell disease (SCD). Proof-of-concept has been established in preclinical studies in various animal models. Clinical gene therapy trials have confirmed good safety, tolerability, and therapeutic efficacy. Viral-based drugs have been approved for cancer, hematological, metabolic, neurological, and ophthalmological diseases as well as for vaccines. For example, the adenovirus-based drug Gendicine® for non-small-cell lung cancer, the reovirus-based drug Reolysin® for ovarian cancer, the oncolytic HSV T-VEC for melanoma, lentivirus-based treatment of ADA-SCID disease, and the rhabdovirus-based vaccine Ervebo against Ebola virus disease have been approved for human use.

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.

Minimal Essential Human Factor VIII Alterations Enhance Secretion and Gene Therapy Efficiency

One important limitation for achieving therapeutic expression of human factor VIII (FVIII) in hemophilia A gene therapy is inefficient secretion of the FVIII protein. Substitution of five amino acids in the A1 domain of human FVIII with the corresponding porcine FVIII residues generated a secretion-enhanced human FVIII variant termed B-domain-deleted (BDD)-FVIII-X5 that resulted in 8-fold higher FVIII activity levels in the supernatant of an in vitro cell-based assay system than seen with unmodified human BDD-FVIII. Analysis of purified recombinant BDD-FVIII-X5 and BDD-FVIII revealed similar specific activities for both proteins, indicating that the effect of the X5 alteration is confined to increased FVIII secretion. Intravenous delivery in FVIII-deficient mice of liver-targeted adeno-associated virus (AAV) vectors designed to express BDD-FVIII-X5 or BDD-FVIII achieved substantially higher plasma FVIII activity levels for BDD-FVIII-X5, even when highly efficient codon-optimized F8 nucleotide sequences were employed. A comprehensive immunogenicity assessment using in vitro stimulation assays and various in vivo preclinical models of hemophilia A demonstrated that the BDD-FVIII-X5 variant does not exhibit an increased immunogenicity risk compared to BDD-FVIII. In conclusion, BDD-FVIII-X5 is an effective FVIII variant molecule that can be further developed for use in gene- and protein-based therapeutics for patients with hemophilia A.

Translational Potential of Immune Tolerance Induction by AAV Liver-Directed Factor VIII Gene Therapy for Hemophilia A

Hemophilia A (HA) is an X-linked bleeding disorder due to deficiencies in coagulation factor VIII (FVIII). The major complication of current protein-based therapies is the development of neutralizing anti-FVIII antibodies, termed inhibitors, that block the hemostatic effect of therapeutic FVIII. Inhibitors develop in about 20-30% of people with severe HA, but the risk is dependent on the interaction between environmental and genetic factors, including the underlying F8 gene mutation. Recently, multiple clinical trials evaluating adeno-associated viral (AAV) vector liver-directed gene therapy for HA have reported promising results of therapeutically relevant to curative FVIII levels. The inclusion criteria for most trials prevented enrollment of subjects with a history of inhibitors. However, preclinical data from small and large animal models of HA with inhibitors suggests that liver-directed gene therapy can in fact eradicate pre-existing anti-FVIII antibodies, induce immune tolerance, and provide long-term therapeutic FVIII expression to prevent bleeding. Herein, we review the accumulating evidence that continuous uninterrupted expression of FVIII and other transgenes after liver-directed AAV gene therapy can bias the immune system toward immune tolerance induction, discuss the current understanding of the immunological mechanisms of this process, and outline questions that will need to be addressed to translate this strategy to clinical trials.

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.

Long-term correction of hemophilia A mice following lentiviral mediated delivery of an optimized canine factor VIII gene

Current therapies for hemophilia A include frequent prophylactic or on-demand intravenous factor treatments which are costly, inconvenient and may lead to inhibitor formation. Viral vector delivery of factor VIII (FVIII) cDNA has the potential to alleviate the debilitating clotting defects. Lentiviral-based vectors delivered to murine models of hemophilia A mediate phenotypic correction. However, a limitation of lentiviral-mediated FVIII delivery is inefficient transduction of target cells. Here, we engineer a feline immunodeficiency virus (FIV) -based lentiviral vector pseudotyped with the baculovirus GP64 envelope glycoprotein to mediate efficient gene transfer to mouse hepatocytes. In anticipation of future studies in FVIII-deficient dogs, we investigated the efficacy of FIV-delivered canine FVIII (cFVIII). Codon-optimization of the cFVIII sequence increased activity and decreased blood loss as compared to the native sequence. Further, we compared a standard B-domain deleted FVIII cDNA to a cDNA including 256 amino acids of the B-domain with 11 potential asparagine-linked oligosaccharide linkages. Restoring a partial B-domain resulted in modest reduction of endoplasmic reticulum (ER) stress markers. Importantly, our optimized vectors achieved wild-type levels of phenotypic correction with minimal inhibitor formation. These studies provide insights into optimal design of a therapeutically relevant gene therapy vector for a devastating bleeding disorder.

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.

Ethical and regulatory aspects of genome editing

Gene editing is a rapidly developing area of biotechnology in which the nucleotide sequence of the genome of living cells is precisely changed. The use of genome-editing technologies to modify various types of blood cells, including hematopoietic stem cells, has emerged as an important field of therapeutic development for hematopoietic disease. Although these technologies offer the potential for generation of transformative therapies for patients suffering from myriad disorders of hematopoiesis, their application for therapeutic modification of primary human cells is still in its infancy. Consequently, development of ethical and regulatory frameworks that ensure their safe and effective use is an increasingly important consideration. Here, we review a number of issues that have the potential to impact the clinical implementation of genome-editing technologies, and suggest paths forward for resolving them such that new therapies can be safely and rapidly translated to the clinic.

Efficient production of dual recombinant adeno-associated viral vectors for factor VIII delivery

Recombinant adeno-associated viral (rAAV) vectors have gained attention for human gene therapy because of their high safety and clinical efficacy profile. For factor VIII gene delivery, splitting the coding region between two AAV vectors remains a viable strategy to avoid the packaging capacity limitation (∼5.0 kb). However, it is time-consuming and labor-intensive to produce two rAAV vectors in separate batches. Here we demonstrated successful production of dual rAAV vectors for hemophilia A gene therapy in a single preparation. When the AAV vector plasmids carrying the human factor VIII heavy chain (hHC) and the light chain (hLC) expression cassettes were cotransfected into 293 cells along with the AAV rep&cap and mini-adenovirus helper plasmids, both rAAV-hHC and rAAV-hLC were produced at the desired ratio and in high titer. Interestingly, the rAAV-hHC vectors always yielded higher titers than rAAV-hLC vectors as a result of more efficient replication of rAAV-hHC genomes. The resulting vectors were effective in transducing the tissue culture cells in vitro. When these vectors were administered to hemophilia A mice, factor VIII was detected in the mouse plasma by both the activated partial thromboplastin time assay and enzyme-linked immunosorbent assay. The functional activity as well as the antigen levels of secreted factor VIII were similar to those of vectors produced by the traditional method. The dual-vector production method has been successfully extended to both AAV2 and AAV8 serotypes. In conclusion, cotransfection of vector plasmids presents an efficient method for producing dual or multiple AAV vectors at significantly reduced cost and labor.

Gene therapy for hemophilia

Hemophilia is an X-linked inherited bleeding disorder consisting of two classifications, hemophilia A and hemophilia B, depending on the underlying mutation. Although the disease is currently treatable with intravenous delivery of replacement recombinant clotting factor, this approach represents a significant cost both monetarily and in terms of quality of life. Gene therapy is an attractive alternative approach to the treatment of hemophilia that would ideally provide life-long correction of clotting activity with a single injection. In this review, we will discuss the multitude of approaches that have been explored for the treatment of both hemophilia A and B, including both in vivo and ex vivo approaches with viral and nonviral delivery vectors.

Portal vein delivery of viral vectors for gene therapy for hemophilia

The liver is a very complex organ with a large variety of functions, making it an attractive organ for gene replacement therapy. Many genetic disorders can be corrected by delivering gene products directly into the liver using viral vectors. In this chapter, we will describe gene delivery via portal vein administration in mice and dogs to correct the blood coagulation disorder hemophilia B. Although there are multiple delivery routes for both viral and non-viral vectors in animals, portal vein administration delivers vectors directly and efficiently into the liver. Complete correction of murine hemophilia B and multi-year near-correction of canine hemophilia B have been achieved following portal vein delivery of adeno-associated viral (AAV) vectors expressing factor IX from hepatocyte-specific promoters. Peripheral vein injection can lead to increased vector dissemination to off-target organ such as the lung and spleen. Below, we will describe portal vein injection delivery route via laparotomy.

Liver-Directed Adeno-Associated Viral Gene Therapy for Hemophilia

Hemophilia A and B are monogenic bleeding disorders resulting from loss of functional coagulation factors VIII or IX, respectively. Prophylactic treatment requires frequent intravenous injections of exogenous factor VIII (F.VIII) or factor IX (F.IX), due to the short half-life of both factors. Hemophilia patients are at risk of developing neutralizing antibodies to F.VIII (~25-30%) or F.IX (~2-4%), which require the use of expensive bypass agents and immune tolerance induction protocols. Viral vector mediated liver gene transfer of F.VIII or F.IX offers an alternative treatment for hemophilia with easily defined clinical endpoints and no need for strict regulation of coagulation factor expression, as both proteins circulate as inactive zymogens. Adeno-associated viral (AAV) vectors are derived from a non-pathogenic human virus that efficiently transduce non-dividing cells, such as hepatocytes, and provide stable transgene expression. In vivo liver gene transfer of AAV-F.VIII and -F.IX vectors has restored hemostasis in murine and canine hemophilia models long-term, and has also been shown to induce immune tolerance. Consequently, two Phase I/II clinical trials have been conducted, based on hepatic AAV-FIX gene transfer to patients with severe hemophilia B. The first trial, utilizing serotype 2, demonstrated transient correction, which was limited by a cellular immune response against the viral capsid. However, sustained therapeutic expression has been achieved in a second trial, using AAV8 for expression of a codon-optimized F.IX transgene. Translation of F.VIII gene transfer studies into the clinic may require additional optimization of gene transfer and vector to effectively express the larger cDNA of F.VIII.

Transient B cell depletion or improved transgene expression by codon optimization promote tolerance to factor VIII in gene therapy

The major complication in the treatment of hemophilia A is the development of neutralizing antibodies (inhibitors) against factor VIII (FVIII). The current method for eradicating inhibitors, termed immune tolerance induction (ITI), is costly and protracted. Clinical protocols that prevent rather than treat inhibitors are not yet established. Liver-directed gene therapy hopes to achieve long-term correction of the disease while also inducing immune tolerance. We sought to investigate the use of adeno-associated viral (serotype 8) gene transfer to induce tolerance to human B domain deleted FVIII in hemophilia A mice. We administered an AAV8 vector with either human B domain deleted FVIII or a codon-optimized transgene, both under a liver-specific promoter to two strains of hemophilia A mice. Protein therapy or gene therapy was given either alone or in conjunction with anti-CD20 antibody-mediated B cell depletion. Gene therapy with a low-expressing vector resulted in sustained near-therapeutic expression. However, supplementary protein therapy revealed that gene transfer had sensitized mice to hFVIII in a high-responder strain but not in mice of a low-responding strain. This heightened response was ameliorated when gene therapy was delivered with anti-murine CD20 treatment. Transient B cell depletion prevented inhibitor formation in protein therapy, but failed to achieve a sustained hypo-responsiveness. Importantly, use of a codon-optimized hFVIII transgene resulted in sustained therapeutic expression and tolerance without a need for B cell depletion. Therefore, anti-CD20 may be beneficial in preventing vector-induced immune priming to FVIII, but higher levels of liver-restricted expression are preferred for tolerance.

Development of Novel Recombinant AAV Vectors and Strategies for the Potential Gene Therapy of Hemophilia

Recombinant vectors based on a non-pathogenic human parvovirus, the adeno-associated virus (AAV), have gained attention as a potentially safe and useful alternative to the more commonly used retroviral and adenoviral vectors. AAV vectors are currently in use in Phase I/II clinical trials for gene therapy of a number of diseases such as cystic fibrosis, α-1 antitrypsin deficiency, muscular dystrophy, Batten's disease, and Parkinson's disease, and have shown efficacy in patients with Leber's congenital amaurosis, and hemophilia B. For patients with hemophilia B, however, relatively large vector doses are needed to achieve therapeutic benefits. Large vector doses also trigger an immune response as significant fraction of the vectors fails to traffic efficiently to the nucleus, and is targeted for degradation by the host cell proteasome machinery. With a better understanding of the various steps in the life cycle of AAV vectors, strategies leading to the development of novel AAV vectors that are capable of high-efficiency transduction at lower doses are needed. In this review, we summarize our strategies to develop novel AAV vectors for the potential gene therapy of both hemophilia B and hemophilia A, based on our recent studies on the basic molecular biology of AAV. These strategies, including the development of novel AAV vectors by site-directed mutagenesis of critical surface-exposed tyrosine residues on AAV2 capsids to circumvent the ubiquitination step and the use of different AAV serotypes and self-complementary (sc) AAV2 vectors, and their use as helper vectors to circumvent the obstacles of second-strand DNA synthesis of single-stranded (ss) AAV, should dramatically accelerate the progress towards the potential gene therapy of both hemophilia A and hemophilia B.

Engineering Factor Viii for Hemophilia Gene Therapy

Current treatment of hemophilia A by intravenous infusion of factor VIII (fVIII) concentrates is very costly and has a potential adverse effect of developing inhibitors. Gene therapy, on the other hand, can potentially overcome these limitations associated with fVIII replacement therapy. Although hemophilia B gene therapy has achieved promising outcomes in human clinical trials, hemophilia A gene therapy lags far behind. Compared to factor IX, fVIII is a large protein which is difficult to express at sustaining therapeutic levels when delivered by either viral or non-viral vectors. To improve fVIII gene delivery, numerous strategies have been exploited to engineer the fVIII molecule and overcome the hurdles preventing long term and high level expression. Here we reviewed these strategies, and discussed their pros and cons in human gene therapy of hemophilia A.

A simple method to increase the transduction efficiency of single-stranded adeno-associated virus vectors in vitro and in vivo

We have recently shown that co-administration of conventional single-stranded adeno-associated virus 2 (ssAAV2) vectors with self-complementary (sc) AAV2-protein phosphatase 5 (PP5) vectors leads to a significant increase in the transduction efficiency of ssAAV2 vectors in human cells in vitro as well as in murine hepatocytes in vivo. In the present study, this strategy has been further optimized by generating a mixed population of ssAAV2-EGFP and scAAV2-PP5 vectors at a 10:1 ratio to achieve enhanced green fluorescent protein (EGFP) transgene expression at approximately 5- to 10-fold higher efficiency, both in vitro and in vivo. This simple coproduction method should be adaptable to any ssAAV serotype vector containing transgene cassettes that are too large to be encapsidated in scAAV vectors.

Protein trans-splicing based dual-vector delivery of the coagulation factor VIII gene

A dual-vector system was explored for the delivery of the coagulation factor VIII gene, using intein-mediated protein trans-splicing as a means to produce intact functional factor VIII post-translationally. A pair of eukaryotic expression vectors, expressing Ssp DnaB intein-fused heavy and light chain genes of B-domain deleted factor VIII (BDD-FVIII), was constructed. With transient co-transfection of the two vectors into 293 and COS-7 cells, the culture supernatants contained (137+/-23) and (109+/-22) ng mL(-1) spliced BDD-FVIII antigen with an activity of (1.05+/-0.16) and (0.79+/-0.23) IU mL(-1) for 293 and COS-7 cells, respectively. The spliced BDD-FVIII was also detected in supernatants from a mixture of cells transfected with inteinfused heavy and light chain genes. The spliced BDD-FVIII protein bands from cell lysates were visualized by Western blotting. The data demonstrated that intein could be used to transfer the split factor VIII gene and provided valuable information on factor VIII gene delivery by dual-adeno-associated virus in hemophilia A gene therapy.

Optimized adeno-associated virus (AAV)-protein phosphatase-5 helper viruses for efficient liver transduction by single-stranded AAV vectors: therapeutic expression of factor IX at reduced vector doses

Abstract Our studies have shown that coinjection of conventional single-stranded adeno-associated virus 2 (ssAAV2) vectors carrying the enhanced green fluorescent protein (EGFP) gene with self-complementary (sc) AAV2-T cell protein tyrosine phosphatase (TC-PTP) and scAAV2-protein phosphatase-5 (PP5) vectors resulted in an approximately 16-fold increase in EGFP expression in primary murine hepatocytes in vivo [Jayandharan, G.R., Zhong, L., Li, B., Kachniarz, B., and Srivastava, A. (2008). Gene Ther. 15, 1287-1293]. In the present studies, this strategy was further optimized to achieve transgene expression at reduced vector/helper virus doses. These included the use of scAAV helper viruses containing (1) hepatocyte-specific promoters, (2) tyrosine-mutant AAV2 capsids, and (3) additional AAV serotype vectors known to efficiently transduce hepatocytes. The hepatocyte-specific transthyretin (TTR) promoter was approximately 6- to 7-fold more efficient than the Rous sarcoma virus (RSV) promoter; tyrosine-mutant AAV2 capsids were approximately 6- to 11-fold more efficient than the wild-type AAV2 capsids; and the AAV8 serotype helper virus was approximately 16-fold more efficient than AAV2 serotype helper virus. With these modifications, the vector dose of the helper virus could be further reduced by approximately 50-fold. Last, coadministration of scAAV8-PP5 helper virus increased coagulation factor IX expression from an ssAAV2 vector by approximately 7- to 10-fold, thereby achieving therapeutic levels at lower vector doses. No adverse effect on hepatocytes was observed under any of these experimental conditions. The strategy presented here should be adaptable to any ssAAV transgene cassette and, specifically, liver-directed applications of ssAAV2 vectors containing larger genes that cannot be encapsidated in scAAV vectors.

Phosphatidylinositol containing lipidic particles reduces immunogenicity and catabolism of factor VIII in hemophilia a mice

Factor VIII (FVIII) is an important cofactor in blood coagulation cascade. It is a multidomain protein that consists of six domains, NH2-A1-A2-B-A3-C1-C2-COOH. The deficiency or dysfunction of FVIII causes hemophilia A, a life-threatening bleeding disorder. Replacement therapy using recombinant FVIII (rFVIII) is the first line of therapy, but a major clinical complication is the development of inhibitory antibodies that abrogate the pharmacological activity of the administered protein. FVIII binds to anionic phospholipids (PL), such as phosphatidylinositol (PI), via lipid binding region within the C2 domain of FVIII. This lipid binding site not only consists of immunodominant epitopes but is also involved in von Willebrand factor binding that protects FVIII from degradation in vivo. Thus, we hypothesize that FVIII-PL complex will influence immunogenicity and catabolism of FVIII. The biophysical studies showed that PI binding did not alter conformation of the protein but improved intrinsic stability as measured by thermal denaturation studies. ELISA studies confirmed the involvement of the C2 domain in binding to PI containing lipid particles. PI binding prolonged the in vivo circulation time and reduced catabolism of FVIII in hemophilia A mice. FVIII-PI complex reduced inhibitor development in hemophilia A mice following intravenous and subcutaneous administration. The data suggest that PI binding reduces catabolism and immunogenicity of FVIII and has potential to be a useful therapeutic approach for hemophilia A.

The treatment of hemophilia A: from protein replacement to AAV-mediated gene therapy

Factor VIII (FVIII) is an essential component in blood coagulation, a deficiency of which causes the serious bleeding disorder hemophilia A. Recently, with the development of purification level and recombinant techniques, protein replacement treatment to hemophiliacs is relatively safe and can prolong their life expectancy. However, because of the possibility of unknown contaminants in plasma-derived FVIII and recombinant FVIII, and high cost for hemophiliacs to use these products, gene therapy for hemophilia A is an attractive alternative to protein replacement therapy. Thus far, the adeno-associated virus (AAV) is a promising vector for gene therapy. Further improvement of the virus for clinical application depends on better understanding of the molecular structure and fate of the vector genome. It is likely that hemophilia will be the first genetic disease to be cured by somatic cell gene therapy.

Enhanced factor VIII heavy chain for gene therapy of hemophilia A

Hemophilia A gene therapy using recombinant adenovirus-associated virus (AAV) vectors has been hampered by the size of the factor VIII (FVIII) cDNA. Previously, splitting the FVIII coding sequence into a heavy-chain (HC) fragment and a light-chain (LC) fragment for dual recombinant AAV vector delivery has been successfully explored. However, the main disadvantage of this approach is a "chain imbalance" problem in which LC secretion is approximately 1-2 logs higher than that of HC, and therefore, the majority of protein synthesized is nonfunctional. To improve HC secretion, we constructed alternate FVIII HCs based on our observation that LC facilitates HC secretion. To our surprise, most of the new HC molecules exhibited enhanced expression over the traditional HC molecule (HC(745)). The optimized HC mutein, HC(HL), including additional acidic-region-3 (ar3) sequences, exhibited three- to fivefold higher activity in both enzyme-linked immunosorbent assay (ELISA) and activated partial thromboplastin time (aPTT) assay in in vitro testing. Further characterization suggested ar3 sequences increased HC secretion, rather than promoting HC synthesis. Intravenous delivery of AAV8-HC(HL)+AAV8-LC or AAV8-HC(745)+AAV8-LC achieved phenotypic correction in hemophilia A mice. Mice receiving AAV8-HC(HL)+AAV8-LC achieved three- to fourfold higher HC expression than AAV8-HC(745)+AAV8-LC, consistent with the FVIII functional assays. HC(HL) should be substituted for HC(745) in a dual AAV vector strategy due to its enhanced expression.

Protein trans-splicing as a means for viral vector-mediated in vivo gene therapy

Inteins catalyze protein splicing in a fashion similar to how self-splicing introns catalyze RNA splicing. Splitinteins catalyze precise ligation of two separate polypeptides through trans-splicing in a highly specific manner. Here we report a method of using protein trans-splicing to circumvent the packaging size limit of gene therapy vectors. To demonstrate this method, we chose a large dystrophin gene and an adeno-associated viral (AAV) vector, which has a small packaging size. A highly functional 6.3-kb Becker-form dystrophin cDNA was broken into two pieces and modified by adding appropriate split-intein coding sequences, resulting in splitgenes sufficiently small for packaging in AAV vectors. The two split-genes, after codelivery into target cells, produced two polypeptides that spontaneously trans-spliced to form the expected Becker-form dystrophin protein in cell culture in vitro. Delivering the split-genes by AAV1 vectors into the muscle of a mouse model of Duchenne muscular dystrophy rendered therapeutic gene expression and benefits.

Complete correction of hemophilia A with adeno-associated viral vectors containing a full-size expression cassette

Hemophilia A is caused by a deficiency in the factor VIII (FVIII) gene. Constrained by limited packaging capacity, even the 4.3-kb B domain-deleted FVIII remained a challenge for delivery by a single adeno-associated viral (AAV) vector. Studies have shown that up to a 6.6-kb vector sequence may be packaged into AAV virions, which suggested an alternative strategy for hemophilia A gene therapy. To explore the usefulness of AAV vectors carrying an oversized FVIII gene, we constructed the AAV-FVIII vector under the control of a beta-actin promoter with a cytomegalovirus enhancer (CB) and a bovine growth hormone (bGH) poly(A) sequence. The CB promoter plus bGH signal was shown to be 3- to 5-fold more potent than the mini-transthyretin (TTR) promoter with a synthetic poly(A) sequence for directing FVIII expression in the liver. Despite the 5.75-kb genome size of pAAV-CB-FVIII, sufficient AAV vectors were produced for in vivo testing. Approximately 3- to 5-fold more FVIII secretion was observed in animals receiving AAV-CB-FVIII vectors than in those receiving standard-sized AAV-TTR-FVIII vectors. Both the activated partial thromboplastin time assay and the whole blood thromboelastographic analysis confirmed that AAV-FVIII vectors fully corrected the bleeding phenotype of hemophilia mice. These results suggest that AAV vectors with an oversized genome should be useful for not only hemophilia A gene therapy but also other diseases with large cDNA such as muscular dystrophy and cystic fibrosis.

Gene transfer to hemophilia A mice via oral delivery of FVIII-chitosan nanoparticles

Effective oral delivery of a non-viral gene carrier would represent a novel and attractive strategy for therapeutic gene transfer. To evaluate the potential of this approach, we studied the oral gene delivery efficacy of DNA polyplexes composed of chitosan and Factor VIII DNA. Transgene DNA was detected in both local and systemic tissues following oral administration of the chitosan nanoparticles to hemophilia A mice. Functional factor VIII protein was detected in plasma by chromogenic and thrombin generation assays, reaching a peak level of 2-4% FVIII at day 22 after delivery. In addition, a bleeding challenge one month after DNA administration resulted in phenotypic correction in 13/20 mice given 250-600 microg of FVIII DNA in chitosan nanoparticles, compared to 1/13 mice given naked FVIII DNA and 0/6 untreated mice. While further optimization would be required to render this type of delivery system practical for hemophilia A gene therapy, the findings suggest the feasibility of oral, non-viral delivery for gene medicine applications.

Phosphatidylserine containing liposomes reduce immunogenicity of recombinant human factor VIII (rFVIII) in a murine model of hemophilia A

Factor VIII (FVIII) is a multidomain protein that is deficient in hemophilia A, a clinically important bleeding disorder. Replacement therapy using recombinant human FVIII (rFVIII) is the main therapy. However, approximately 15-30% of patients develop inhibitory antibodies that neutralize rFVIII activity. Antibodies to epitopes in C2 domain, which is involved in FVIII binding to phospholipids, are highly prevalent. Here, we investigated the effect of phosphatidylserine (PS)-containing liposomes, which bind to C2 domain with high affinity and specificity, upon the immunogenicity of rFVIII. Circular dichroism studies showed that PS-containing liposomes interfered with aggregation of rFVIII. Immunogenicity of free- versus liposomal-rFVIII was evaluated in a murine model of hemophilia A. Animals treated with s.c. injections of liposomal-rFVIII had lower total- and inhibitory titers, compared to animals treated with rFVIII alone. Antigen processing by proteolytic enzymes was reduced in the presence of liposomes. Animals treated with s.c. injections of liposomal-rFVIII showed a significant increase in rFVIII plasma concentration compared to animals that received rFVIII alone. Based on these studies, we hypothesize that specific molecular interactions between PS-containing bilayers and rFVIII may provide a basis for designing lipidic complexes that improve the stability, reduce the immunogenicity of rFVIII formulations, and permit administration by s.c. route.

Bio-nanocapsule conjugated with liposomes for in vivo pinpoint delivery of various materials

Bio-nanocapsules (BNCs) consisting of hepatitis B virus (HBV) surface antigen (HBsAg) are approximately 50-nm hollow particles displaying a human hepatocyte-recognizing molecule (pre-S1 peptide). They have been used as an HB vaccine for the last two decades. Original BNC can incorporate various payloads (e.g., drugs, genes) by electroporation and deliver them to human hepatocytes specifically by utilizing the HBV infection mechanism. Here, we developed a new BNC conjugated with liposomes and succeeded in incorporating large materials (100-nm fluorescence-labeled polystyrene beads and >30 kbp plasmids) into the BNC-liposome complex. The complex delivered these large materials to human hepatocytes specifically ex vivo and in vivo. The transfection efficiency of the BNC-liposome complex was significantly higher than that of the original BNC. These results indicated that BNC confers the tissue- and cell-specificity on the conventional liposomes and raises new possibilities for drug delivery systems, gene delivery systems, and bio-imaging systems in vivo.

Optimization of self-complementary AAV vectors for liver-directed expression results in sustained correction of hemophilia B at low vector dose

Self-complementary adeno-associated virus (scAAV) vectors can significantly minimize the vector load required to achieve sustained transgene expression. In this study, transcriptional regulatory elements were systematically screened to produce constitutive and liver-specific scAAV factor IX (FIX) expression cassettes. In addition, optimization of GC content, cis- regulatory elements, and codon usage in the human FIX (hFIX) transgene increased expression 4-20-fold. A vector was developed that was capable of expressing high FIX levels in comparison with the single-stranded (ss) AAV vector used in a recent clinical trial. The ssAAV and scAAV vectors display different transgene expression and genome stability patterns in the liver, as determined by immunohistochemical staining, in situ messenger RNA (mRNA) hybridization and vector genome quantitation. The ssAAV2 vector promoted strong FIX expression in only a subset of hepatocytes. The scAAV2-hFIX vector showed widespread ( approximately 80% of hepatocytes), moderate FIX expression levels similar to normal livers with correction of coagulation function in FIX-deficient mice. The ability of low dose scAAV-FIX vectors to achieve near-physiological expression may circumvent inflammatory responses in the liver. In addition to providing an improved scAAV vector for potential application in future hemophilia B clinical trials and liver-directed gene delivery, these studies underscore the need for rigorous analysis and optimization of vector genome cassettes.

Gene therapy for treatment of inherited haematological disorders

Gene therapy, a molecular medicine based on vector-mediated transfer of therapeutic genes, holds promise for a cure of monogenetic inherited diseases. In recent years, tremendous progress has been reported in the treatment of haematological disorders: clinical trials in severe combined immune deficiencies have been successful by using retroviral vectors to express target genes in haematopoietic stem cells, which after transplantation efficiently reconstituted the immune system concomitant with substantial improvement in the clinical status of patients. Conversely, unexpected adverse events were also encountered. In other work, progress towards clinical studies on ex vivo gene transfer for Fanconi anaemia and haemoglobinopathies has been made. Each approach features a unique treatment strategy and also faces various impediments to success. In the case of the X-linked bleeding disorder haemophilia, several Phase I/II clinical trials were conducted, including in vivo administration of viral vectors to skeletal muscle and liver. Adeno-associated viral gene transfer of coagulation Factor IX has been documented in human subjects, reaching therapeutic levels after infusion into a hepatic blood vessel. However, sustained expression of therapeutic levels (as shown in large animal models of haemophilia) has not yet been achieved in humans. In general, long-term follow-up will be important for assessment of the safety of all existing gene therapy strategies.

The enhancing effects of the light chain on heavy chain secretion in split delivery of factor VIII gene

Coagulation factor VIII (FVIII) is secreted as a heterodimer consisting of a heavy chain (HC) and a light chain (LC), which can be expressed independently and reassociate with recovery of biological activity. Because of the size limitation of adeno-associated virus (AAV) vectors, a strategy for delivering the HC and LC separately has been developed. However, the FVIII HC is secreted 10-100-fold less efficiently than the LC. In this study, we demonstrated that the F309S mutation and enhanced B-domain glycosylations alone are not sufficient to improve FVIII HC secretion, which suggested a role of the FVIII LC in regulating HC secretion. To characterize this role of the FVIII LC, we compared FVIII HC secretion with and without the LC via post-translational protein trans-splicing. As demonstrated in vitro, ligation of the LC to the HC significantly increased HC secretion. Such HC secretion increases were also confirmed in vivo by hydrodynamic injection of FVIII intein plasmids into hemophilia A mice. Moreover, similar enhancement of HC secretion can also be observed when the LC is supplied in trans, which is probably due to the spontaneous association of the HC and the LC in the secretion pathway. In sum, enhancing the secretion of the FVIII HC polypeptide may require the proper association of the FVIII LC polypeptide in cis or in trans. These results may be helpful in designing new strategies to improve FVIII gene delivery.

Genetic engineering for haemophilia A

At first sight, haemophilia A would appear to be an ideal candidate for treatment by gene therapy. There is a single gene defect; cells in different parts of the body, but especially the liver, produce Factor VIII, and only 5% of normal levels of Factor VIII are necessary to prevent the serious symptoms of bleeding. This review attempts to outline the status of gene therapy at present and efforts that have been made to overcome the difficulties and remaining problems that require solving. Undoubtedly, success will be achieved, but it is likely that considerably more work will be necessary before experimental models can be introduced into the clinic with any likelihood of success. The most successful results in animals that may have clinical application were from introducing the Factor VIII gene to newborn animals before antibodies are produced, presumably inducing a state of tolerance.

Treatment of human disease by adeno-associated viral gene transfer

During the past decade, in vivo administration of viral gene transfer vectors for treatment of numerous human diseases has been brought from bench to bedside in the form of clinical trials, mostly aimed at establishing the safety of the protocol. In preclinical studies in animal models of human disease, adeno-associated viral (AAV) vectors have emerged as a favored gene transfer system for this approach. These vectors are derived from a replication-deficient, non-pathogenic parvovirus with a single-stranded DNA genome. Efficient gene transfer to numerous target cells and tissues has been described. AAV is particularly efficient in transduction of non-dividing cells, and the vector genome persists predominantly in episomal forms. Substantial correction, and in some instances complete cure, of genetic disease has been obtained in animal models of hemophilia, lysosomal storage disorders, retinal diseases, disorders of the central nervous system, and other diseases. Therapeutic expression often lasted for months to years. Treatments of genetic disorders, cancer, and other acquired diseases are summarized in this review. Vector development, results in animals, early clinical experience, as well as potential hurdles and challenges are discussed.

Transplantation of endothelial cells corrects the phenotype in hemophilia A mice

BACKGROUND

The deficiency of factor VIII, a co-factor in the intrinsic coagulation pathway results in hemophilia A. Although FVIII is synthesized largely in the liver, the specific liver cell type(s) responsible for FVIII production is controversial.

OBJECTIVE

This study aimed to determine the cellular origin of FVIII synthesis and release in mouse models.

METHODS

We transplanted cells into the peritoneal cavity of hemophilia A knockout mice. Plasma FVIII activity was measured using a Chromogenix assay 2-7 days after cell transplantation, and phenotypic correction was determined with tail-clip challenge 7 days following cell transplantation. Transplanted cells were identified by histologic and molecular assays.

RESULTS

Untreated hemophilia A mice, as well as mice treated with the hepatocyte-enriched fraction, showed extensive mortality following tail-clip challenge. In contrast, recipients of unfractionated liver cells (mixture of hepatocytes, liver sinusoidal endothelial cells (LSEC), Kupffer cells, and hepatic stellate cells) or of the cell fraction enriched in LSECs survived tail-clip challenge (P < 0.001). FVIII was secreted in the blood stream in recipients of unfractionated liver cells, LSECs and pancreatic islet-derived MILE SVEN 1 (MS1) endothelial cells. Although transplanted hepatocytes maintained functional integrity in the peritoneal cavity, these cells did not produce detectable plasma FVIII activity.

CONCLUSIONS

The assay of cell transplantation in the peritoneal cavity showed that endothelial cells but not hepatocytes produced phenotypic correction in hemophilia A mice. Therefore, endothelial cells should be suitable additional targets for cell and gene therapy in hemophilia A.

Lower inhibitor development in hemophilia A mice following administration of recombinant factor VIII-O-phospho-L-serine complex

Factor VIII is a multidomain protein composed of A1, A2, B, A3, C1, and C2 domains. Deficiency or dysfunction of factor VIII causes hemophilia A, a bleeding disorder. Administration of exogenous recombinant factor VIII as a replacement leads to development of inhibitory antibodies against factor VIII in 15-30% of hemophilia A patients. Hence, less immunogenic preparations of factor VIII are highly desirable. Inhibitory antibodies against factor VIII are mainly directed against immunodominant epitopes in C2, A3, and A2 domains. Further, several universal epitopes for CD4+ T-cells have been identified within the C2 domain. The C2 domain is also known to interact specifically with phosphatidylserine-rich lipid vesicles. Here, we have investigated the hypothesis that complexation of O-phospho-l-serine, the head group of phosphatidylserine, with the C2 domain can reduce the overall immunogenicity of factor VIII. The biophysical (circular dichroism and fluorescence) and biochemical studies (ELISA and size exclusion chromatography) showed that O-phospho-l-serine binds to the phospholipid-binding region in the C2 domain, and this interaction causes subtle changes in the tertiary structure of the protein. O-Phospho-l-serine also prevented aggregation of the protein under thermal stress. The immunogenicity of the factor VIII-O-phospho-l-serine complex was evaluated in hemophilia A mice. The total and inhibitory antibody titers were lower for factor VIII-O-phospho-l-serine complex compared with factor VIII alone. Moreover, factor VIII administered as a complex with O-phospho-l-serine retained in vivo activity in hemophilia A mice. Our results suggest that factor VIII-O-phospho-l-serine complex may be beneficial to increase the physical stability and reduce immunogenicity of recombinant factor VIII preparations.

Gene therapy of the hemophilias

Development of hemophilia gene therapy depends on testing gene transfer vectors in hemophilic and nonhemophilic animals. Available animal models include factor VIII or factor IX knockout mice as well as dogs with spontaneous hemophilia A or B. Large animals (particularly dogs) more closely replicate the requirements for correction of human hemophilia than do mice. Small animals are more convenient to maintain and require significantly less vector for testing than do large animals. Nonhemophilic animals (mice or nonhuman primates), whose endogenous factor VIII and factor IX complicate analysis of the human proteins, have utility for safety testing of vectors; some assays can discriminate between human coagulation factors and the endogenous coagulation factors. Most animal models suffer the limitations imposed by the immune response to human factor VIII or IX protein. Clinical trials have failed to achieve significant factor VIII expression in hemophilia A patients, while one clinical trial in hemophilia B patients showed only transient therapeutic increments of factor IX expression. Gene therapy remains an investigational method with many obstacles to overcome before it can be widely used as treatment for hemophilia.

Total correction of hemophilia A mice with canine FVIII using an AAV 8 serotype

Despite the popularity of adeno-associated virus 2 (AAV2) as a vehicle for gene transfer, its efficacy for liver-directed gene therapy in hemophilia A or B has been suboptimal. Here we evaluated AAV serotypes 2, 5, 7, and 8 in gene therapy of factor VIII (FVIII) deficiency in a hemophilia A mouse model and found that AAV8 was superior to the other 3 serotypes. We expressed canine B domain-deleted FVIII cDNA either in a single vector or in 2 separate AAV vectors containing the heavy- and light-chain cDNAs. We also evaluated AAV8 against AAV2 in intraportal and tail vein injections. AAV8 gave 100% correction of plasma FVIII activity irrespective of the vector type or route of administration.