Persistent Systemic Production of Human Factor IX in Mice by Skeletal Myoblast-Mediated Gene Transfer: Feasibility of Repeat Application to Obtain Therapeutic Levels

Use of Repeating Dispensers to Increase the Efficiency of the Intramuscular Myogenic Cell Injection Procedure

Intramuscular myoblast transplantation in humans and nonhuman primates requires precise repetitive cell injections very close to each other. Performed with syringes operated manually throughout large regions, this procedure takes a lot of time, becoming tiring and thus imprecise. We tested two repetitive dispensers with Hamilton syringes as cell injection devices to facilitate this procedure. Monkeys received intramuscular allotransplantations of β-galactosidase-labeled myoblasts, using either a monosyringe or a multisyringe repeating dispenser. The monosyringe repeating dispenser allowed performing cell injections faster and easier than with a manually operated syringe. The multisyringe dispenser accelerated the procedure still more, but it was not ergonomic. Biopsies of the myoblast-injected sites 1 month later showed abundant β-galactosidase-positive myofibers, with the same density and morphological pattern observed following myoblast transplantation with a syringe operated manually. We recommend the monosyringe repeating dispenser for myoblast transplantation in skeletal muscles and maybe in the heart.

A First Semimanual Device for Clinical Intramuscular Repetitive Cell Injections

Intramuscular cell transplantation in humans requires so far meticulous repetitive cell injections. Performed percutaneously with syringes operated manually, the procedure is very time consuming and requires a lot of concentration to deliver the cells exactly in the required region. This becomes impractical and inaccurate for large volumes of muscle. In order to accelerate this task, to render it more precise, and to perform injections more reproducible in large volumes of muscle, we developed a specific semimanual device for intramuscular repetitive cell injections. Our prototype delivers very small quantities of cell suspension, homogeneously throughout several needles, from a container in the device. It was designed in order to deliver the cells as best as possible only in a given subcutaneous region (in our case, skeletal muscles accessible from the surface), avoiding wasting in skin and hypodermis. The device was tested in monkeys by performing intramuscular allotransplantations of β-galactosidase-labeled myoblasts. During transplantations, it was more ergonomic and considerably faster than manually operated syringes, facilitating the cell graft in whole limb muscles. Biopsies of the myoblast-injected muscles 1 month later showed abundant β-galactosidase-positive myofibers with homogeneous distribution through the biopsy sections. This is the first device specifically designed for the needs of intramuscular cell transplantation in a clinical context.

Angiogenesis Enhances Factor IX Delivery and Persistence from Retrievable Human Bioengineered Muscle Implants

Human muscle progenitor cells transduced with lentiviral vectors secreted high levels of blood clotting factor IX (FIX). When bioengineered into postmitotic myofibers as human bioartificial muscles (HBAMs) and subcutaneously implanted into immunodeficient mice, they secreted FIX into the circulation for >3 months. The HBAM-derived FIX was biologically active, consistent with the cells' ability to conduct the necessary posttranslational modifications. These bioengineered muscle implants are retrievable, an inherent safety feature that distinguishes this "reversible" gene therapy approach from most other gene therapy strategies. When myofibers were bioengineered from human myoblasts expressing FIX and vascular endothelial growth factor, circulating FIX levels were increased and maintained long term within the therapeutic range, consistent with the generation of a vascular network around the HBAM. The present study implicates an important role for angiogenesis in the efficient delivery of therapeutic proteins using tissue engineered stem cell-based gene therapies.

Cardiotropic effect of extracardiac transplantation of embryonic human myoblasts to mice with bradycardia: various effects of cell material

Animals with bradycardia were detected in reproductive colony of mdx mice. Low pulse rate was associated with poor survival and predisposition to sudden death, but did not directly depend on the presence of dystrophin mutant gene or animal age. Heart rate increased in old mice with bradycardia after extracardial, intramuscular, and intravenous injection of human embryonic myoblasts. Stable normalization of the pulse was observed 2 weeks after transplantation, but early peak of heart rate was observed as early as 24 h after cell transplantation. Cell suspensions, which could contain stem cells (blood mononuclears and CD34+ lymphocytes), also corrected heart rhythm. Unlike the effect of myoblasts, cardiotropic effect of mononuclears was preceded by a period of tachycardia, while the effect of CD34+ lymphocytes was very unstable. The cardiotropic effect of myoblasts was combined with life span prolongation and certain rejuvenation in some animals. Erythrocytes and supernatant obtained during blood cell fractionation did not modify the heart rhythm in mice with bradycardia. After injection of myoblasts to mice with rare and normal pulses serum creatine kinase activity decreased with different rates. These data attest to a variety of biological effects of stem cells and/or their derivatives and to ambiguous mechanisms of these effects.

Tissue-Specific Characteristics of in Vivo Electric Gene: Transfer by Tissue and Intravenous Injection of Plasmid DNA

PURPOSE

To evaluate the tissue-specific characteristics of electric gene transfer after tissue and intravenous injection of naked plasmid DNA (pDNA).

METHODS

pDNA encoding firefly luciferase was injected directly into the liver, kidney, spleen, skin and muscle, or into the tail vein of mice, and electric pulses were then applied to one of these organs. The distribution of transgene expressing cells was evaluated using pDNA encoding beta-galactosidase.

RESULTS

Tissue injection of pDNA produced a significant degree of transgene expression in any tissue with the greatest amount in the liver, followed by kidney and spleen. The expression in these organs decreased quickly with time, and muscle showed the greatest expression at 7 days. Electroporation significantly increased the expression, and the expression level was comparable among the organs. Intravenous injection of pDNA followed by electroporation resulted in a significant expression in the liver, spleen, and kidney but not in the skin or muscle.

CONCLUSIONS

Electric gene transfer to the liver, kidney, and spleen can be an effective approach to obtain significant amounts of transgene expression by either tissue or intravenous injection of pDNA, whereas it is only effective after tissue injection as far as skin- or muscle-targeted gene transfer is concerned.

Myoblast-mediated gene transfer for therapeutic angiogenesis and arteriogenesis

Therapeutic angiogenesis aims at generating new blood vessels by delivering growth factors such as VEGF and FGF. Clinical trials are underway in patients with peripheral vascular and coronary heart disease. However, increasing evidence indicates that the new vasculature needs to be stabilized to avoid deleterious effects such as edema and hemangioma formation. Moreover, a major challenge is to induce new vessels that persist following cessation of the angiogenic stimulus. Mature vessels may be generated by modulating timing and dosage of growth factor expression, or by combination of 'growth' factors with 'maturation' factors like PDGF-BB, angiopoietin-1 or TGF-beta. Myoblast-mediated gene transfer has unique characteristics that make it a useful tool for studying promising novel approaches to therapeutic angiogenesis. It affords robust and long-lasting expression, and can be considered as a relatively rapid form of 'adult transgenesis' in muscle. The combined insertion of different gene constructs into single myoblasts and their progeny allows the simultaneous expression of different 'growth' and 'maturation' factors within the same cell in vivo. The additional insertion of a reporter gene makes it possible to analyze the phenotype of the vessels surrounding the transgenic muscle fibers into which the myoblasts have fused. The effects of timing and duration of gene expression can be studied by using tetracycline-inducible constructs, and dosage effects by selecting subpopulations consistently expressing distinct levels of growth factors. Finally, the autologous cell-based approach using transduced myoblasts could be an alternative gene delivery system for therapeutic angiogenesis in patients, avoiding the toxicities seen with some viral vectors.

Nucleofection of muscle-derived stem cells and myoblasts with ϕC31 integrase: stable expression of a full-length-dystrophin fusion gene by human myoblasts

Ex vivo gene therapy offers a potential treatment for Duchenne muscular dystrophy by transfection of the dystrophin gene into the patient's own myogenic precursor cells, followed by transplantation. We used nucleofection to introduce DNA plasmids coding for enhanced green fluorescent protein (eGFP) or eGFP-dystrophin fusion protein and the phage phiC31 integrase into myogenic cells and to integrate these genes into a limited number of sites in the genome. Using a plasmid expressing eGFP, we transfected 50% of a mouse muscle-derived stem cell line and 60% of normal human myoblasts. Co-nucleofection of a plasmid expressing the phiC31 integrase and an eGFP expression plasmid containing an attB sequence produced 15 times more frequent stable expression, because of site-specific integration of the transgene. Co-nucleofection of the phiC31 integrase plasmid and a large plasmid containing the attB sequence and the gene for an eGFP-full-length dystrophin fusion protein produced fluorescent human myoblasts that were able to form more intensely fluorescent myotubes after 1 month of culture. A nonviral approach combining nucleofection and the phiC31 integrase may eventually permit safe autotransplantation of genetically modified cells to patients.

FGF6 mediated expansion of a resident subset of cells with SP phenotype in the C2C12 myogenic line

Fibroblast growth factor 6 (FGF6) is selectively expressed during muscle development and regeneration. We examined its effect on muscle precursor cells (mpc) by forcing stable FGF6 expression in C2C12 cells in vitro. FGF6 produced in genetically engineered mpc was active, inducing strong morphological changes, altering cell adhesion and compromising their ability to differentiate into myotubes. Expression of MyoD and myogenin, but not of Myf5, was abrogated in FGF6 engineered mpc. These effects were reversed by FGF inhibitors. Ectopic expression of MyoD also restored fiber formation indicating that FGF6 interferes with the myogenic differentiation pathway upstream of MyoD. We also report that in the presence of FGF6, the minor (0.5-2%) subpopulation of cells actively excluding Hoechst 33342 in a verapamil-dependent manner (SP phenotype) was increased to 15-20% and the expression of the mdr1a gene (but not mdr1b) was upregulated by 400-fold. Our data establish a previously undescribed link between FGF6--a muscle specific growth factor--and a multidrug resistance gene expressed in stem cells, and suggest a role for FGF6 in the maintenance of a reserve pool of progenitor cells in the skeletal muscle.

Dynamics of the early immune cellular reactions after myogenic cell transplantation

The role of immune cells in the early donor cell death/survival following myoblast transplantation is confusing, one of the reasons being the lack of data about the immune reactions following cell transplantation. We used outbred mice as hosts for transplantation of primary cultured muscle cells and T-antigen-immortalized myoblasts. The host muscles were analyzed 1 h to 7 days after cell injection. No net loss of the donor primary cultured cell population was observed in this period. The immune cellular reaction in this case was: 1) a brief (<48 h) neutrophil invasion; 2) macrophage infiltration from days 1 to 7; 3) a specific response involving CTL and few NK cells (days 6 and 7), preceded by a low CD4+ cell infiltration starting at day 3. In contrast, donor-immortalized myoblasts completely disappeared during the 7-day follow-up. In this case, an intense infiltration of CTL and macrophages, with moderate CD4+ infiltration and lower amounts of NK cells, was observed starting at day 2. The nonspecific immune response at days 0 and 1 was similar for both types of donor cells. The present observations set a basis to interpret the role of immune cells on the early death/survival of donor cells following myoblast transplantation.

Bone marrow stromal cells as a genetic platform for systemic delivery of therapeutic proteins in vivo: human factor IX model

BACKGROUND

Hemophilia B is an X-linked bleeding disorder that results from a deficiency in functional coagulation factor IX (hFIX). In patients lacking FIX, the intrinsic coagulation pathway is disrupted leading to a lifelong, debilitating and sometimes fatal disease.

METHODS

We have developed an ex vivo gene therapy system using genetically modified bone marrow stromal cells (BMSCs) as a platform for sustained delivery of therapeutic proteins into the general circulation. This model exploits the ability of BMSCs to form localized ectopic ossicles when transplanted in vivo. BMSCs were transduced with MFG-hFIX, a retroviral construct directing the expression of hFIX. The biological activity of hFIX expressed by these cells was assessed in vitro and in vivo.

RESULTS

Transduced cells produced biologically active hFIX in vitro with a specific activity of 90% and expressed hFIX at levels of approximately 497 ng/10(6) cells/24 h and 322 ng/10(6) cells/24 h for human and porcine cells, respectively. The secretion of hFIX was confirmed by Western blot analysis of the conditioned medium using a hFIX-specific antibody. Transduced BMSCs (8 x 10(6) cells per animal) were transplanted within scaffolds into subcutaneous sites in immunocompromised mice. At 1 week post-implantation, serum samples contained hFIX at levels greater than 25 ng/ml. Circulating levels of hFIX gradually decreased to 11.5 ng/ml at 1 month post-implantation and declined to a stable level at 6.1 ng/ml at 4 months.

CONCLUSIONS

These findings demonstrate that genetically modified BMSCs can continuously secrete biologically active hFIX from self-contained ectopic ossicles in vivo, and thus represent a novel delivery system for releasing therapeutic proteins into the circulation.

Complete and sustained phenotypic correction of hemophilia B in mice following hepatic gene transfer of a high-expressing human factor IX plasmid

Therapeutic correction of hemophilia B was achieved by rapid infusion of a large-volume solution containing a high-expressing human factor IX (hFIX) plasmid into the tail vein of hemophilia B mice. hFIX circulated at therapeutic levels (1-5 micro g mL-1) in all animals for more than 1 year as determined by both species-specific antigen assay and an activated partial thromboplastin time (APTT)-based clotting assay. There was acute, transient hepatic tissue damage by the infusion procedure and no significant inhibitory anti-hFIX antibodies developed. No bleeding episode was observed during or after treatment. Immunohistochemical studies indicated that the hFIX gene was exclusively expressed in hepatocytes, and that transduced cells had readily detectable hFIX protein at 4 h postinfusion, and stainable protein persisted for up to 1 year. Repeated infusions of hFIX plasmids boosted the hFIX expression to higher levels. These results demonstrate that hemophilia B can be treated by gene transfer of naked hFIX plasmids.

Treatment of hemophilia B in mice with nonautologous somatic gene therapeutics

The implantation of nonautologous cells encapsulated in immunoprotective microcapsules provides an alternative nonviral method for gene therapy. This strategy was successful in reversing the disease phenotypes of dwarfism and a lysosomal storage disease, mucopolysaccharidosis VII, in murine models. In this article we implanted transgenic hemophilic B mice with microcapsules enclosing factor IX-secreting C2C12 myoblasts to study the clinical potential of this approach in the treatment of hemophilia. Treated mice showed increased plasma factor IX levels as high as 28 ng of human factor IX per milliliter of plasma and decreased activated thromboplastin times (reduced by 20% to 29%). However, the level of factor IX decreased to baseline levels by day 7, coinciding with emergence of anti-human factor IX antibody, the titer of which increased greater than 10-fold by day 28. Monoclonal anti-CD4 antibodies were used to deplete CD4+ T cells to suppress the immune response against the recombinant factor IX. In the treated hemophilic mice, the anti-factor IX antibody response was totally suppressed to beyond day 28 accompanied by a significant decrease in activated thromboplastin time compared with that seen in untreated hemophilic mice. When the microcapsules were recovered from the intraperitoneal cavity after 38 days of implantation, the encapsulated cells continued to secrete factor IX at preimplantation levels, but both cell viability and microcapsule mechanical stability were reduced. Hence although the polymer chemistry of the microcapsules and cell viability may need to be improved for long-term delivery, nonautologous gene therapy with microencapsulated cells has been shown to be effective, at least for the short-term, in alleviating the hemophilic hemostatic anomaly. Coadministration of an immunosuppressant is effective in inhibiting antibody development against the delivered factor IX and should be considered for recipients at risk of inhibitor development.

Efficient non-viral DNA-mediated gene transfer to human primary myoblasts using electroporation

Gene transfer of human primary myoblasts with various non-viral methods has been hampered by low yield of transfection. We report here an efficient, simple and reproducible non-viral DNA-mediated gene transfer procedure for transfecting human myoblasts. We found that electroporation promotes a highly efficient DNA uptake by human primary cultures of myogenic cells. Under optimal conditions, 60-70% of human myoblasts transfected with the enhanced green fluorescent gene expressed the enhanced green fluorescent protein. Electroporated myoblasts behaved normally as judged by their ability to synthesize and express developmentally regulated proteins and to undergo terminal differentiation, i.e. to fuse and form myotubes. We showed, in addition, that a subpopulation of cultured human myoblasts with self-renewing properties and equivalent to native muscle satellite cells were as efficiently transfected by electroporation as proliferating myoblasts. Thus, the development of gene therapies based on the engineering and transplantation of human myoblasts may greatly benefit from gene transfer by electroporation.

Sustained and therapeutic delivery of factor IX in nude haemophilia B mice by encapsulated C2C12 myoblasts: concurrent tumourigenesis

This study reports the generation of an immunodeficient murine model for haemophilia B, obtained by breeding factor IX-deficient mice with an immunodeficient mouse strain, and use of this mouse model to evaluate the long-term efficacy and safety of a gene therapy strategy for treating haemophilia B. Nude haemophilic mice were implanted with biocompatible microcapsules enclosing recombinant myoblasts secreting human factor IX. The activated partial thromboplastin time (APTT) of plasma of mice thus treated was invariably shortened 3 weeks after microcapsule implantation, and remained shortened for at least 77 days. Shortening of the APTT of the haemophilia mice coincided with the appearance of human factor IX in mice plasmas (up to 600 ng mL(-1) on day 77), and normalization of the tail-bleeding time. Thus, the microencapsulated myoblasts reversed the clinical phenotype of haemophilia B. In contrast, plasmas of immunocompetent haemophilic mice similarly implanted with microcapsules only showed a transient shortening of APTT, and coincident transient delivery of human factor IX antigen. Rapid disappearance of human factor IX from plasmas of immunocompetent mice also coincided with production of antibodies to the human transgene. Significantly, 86% of the nude haemophilia mice developed tumours of myoblast origin. Thus, while this study revealed the feasibility of this gene therapy approach to treat severe haemophilia B, it also highlights the importance of using safer cell lines to prevent tumour development.

Systemic production of human granulocyte colony-stimulating factor in nonhuman primates by transplantation of genetically modified myoblasts

Clinical use of human granulocyte-colony stimulating factor (hG-CSF) to treat various diseases involving neutropenia has been previously shown to (1) successfully increase circulating neutrophils, (2) reduce condition-related infections, and (3) cause few side effects in patients. To alleviate the symptoms of neutropenia, the patient must receive frequent injections of recombinant hG-CSF. Permanent ways to deliver stable levels of the molecule to the patient are being investigated. Among them, the transplantation of hG-CSF-secreting cells has been proposed and performed successfully in rodents, using fibroblast cell lines and primary muscle cells. We thus investigated whether similar results could be obtained by intramuscular myoblast transplantation in a large animal model. When 1-3 x 10(8) myoblasts were injected into three Macaca mulatta, hG-CSF was detected at high levels (300-900 pg/ml), which in turn led to a four- to fivefold increase in circulating neutrophils. However, both the concentrations of hG-CSF and neutrophil levels were found to decrease over time. Nonetheless, neutrophils were found at higher levels from the fourth week until the end the experiment (up to 29 weeks) in G-CSF monkeys compared with control animals. These results show that transplantation of hG-CSF-secreting myoblasts may indeed be a therapeutic option for the treatment of neutropenic patients.

Kinetics of recombinant adeno-associated virus-mediated gene transfer

Recombinant adeno-associated virus (rAAV) vectors have been shown to be useful for efficient gene delivery to a variety of dividing and nondividing cells. Mechanisms responsible for the long-term, persistent expression of the rAAV transgene are not well understood. In this study we investigated the kinetics of rAAV-mediated human factor IX (hFIX) gene transfer into human primary myoblasts and myotubes. Transduction of both myoblasts and myotubes occured with a similar and high efficiency. After 3 to 4 weeks of transduction, rAAV with a cytomegalovirus (CMV) promoter showed 10- to 15-fold higher expression than that with a muscle-specific creatine kinase enhancer linked to beta-actin promoter. Factor IX expression from transduced myoblasts as well as myotubes reached levels as high as approximately 2 microgram of hFIX/10(6) cells/day. Southern blot analyses of high-molecular-weight (HMW) cellular genomic and Hirt DNAs isolated from rAAV/CMVhFIXm1-transduced cells showed that the conversion of single-stranded vector genomes to double-stranded DNA forms, but not the level of the integrated forms in HMW DNA, correlated with increasing expression of the transgene. Together, these results indicate that rAAV can transduce both proliferating and terminally differentiated muscle cells at about the same efficiency, that expression of transgenes increases linearly over their lifetime with no initial lag phase, and that increasing expression correlates with the appearance of double-stranded episomal rAAV genomes. Evidence showing that the rAAV virions can copackage hFIX, presumably nonspecifically, was also obtained.

Progress in myoblast transplantation: a potential treatment of dystrophies

Myoblast transplantation (MT) consists of injecting normal or genetically modified myogenic cells into muscles, where they are expected to fuse and form mature fibers. As an experimental approach to treat severe genetic muscle diseases, MT was tested in dystrophic patients at the beginning of the 1990s. Although these early clinical trials were unsuccessful, MT has progressed through the research on animal models. Many factors that may condition the success of MT were identified in the last years. The present review updates our knowledge on MT and describes the different problems that have limited its success. Factors that were first underestimated, like the specific immune response after MT, are presently well characterized. Destruction of the hybrid fibers by activated T-lymphocytes and production of antibodies against the transplanted myoblasts take place after MT and are responsible for the graft rejection. The choice of the immunosuppression seems to be very important, and FK506 is the best agent known to allow the best results after MT. Under FK506 immunosuppression, very efficient MT were obtained both in mice and monkeys. Moreover, in dystrophic mice it was demonstrated that MT ameliorates some phenotypical characteristics of the disease. The improvement of the survival of the transplanted cells and the increase of their migration into the injected tissue are presently under investigation. Some of the present research is directed also to bypass the immunosuppression by using the patient's own cells for MT. In this sense, efforts are conducted to introduce the normal gene into the patient's myoblasts before MT and to improve the ability of these cells to proliferate in vitro. Micros. Res. Tech. 48:213-222, 2000.

Gene therapy for hemophilia

Hemophilia A and B are X-linked genetic disorders caused by deficiency of the coagulation factors VIII and IX, respectively. Because of the health hazards and costs of current product replacement therapy, much effort is devoted to the development of gene therapy for these disorders. Approaches to gene therapy for the hemophilias include: ex vivo gene therapy in which cells from the intended recipients are explanted, genetically modified to secrete Factor VIII or IX, and reimplanted into the donor; in vivo gene therapy in which Factor VIII or IX encoding vectors are directly injected into the recipient; and non-autologous gene therapy in which universal cell lines engineered to secrete Factor VIII or IX are enclosed in immuno-protective devices before implantation into recipients. Research into these approaches is aided by the many murine and canine models available. While problems of achieving high and sustained levels of factor delivery, and issues related to efficacy, safety and cost are still to be resolved, progress in gene therapy for the hemophilias has been encouraging and is likely to reach human clinical trial in the foreseeable future.

Differential expression and secretion of alpha 1 anti-trypsin between direct DNA injection and implantation of transfected myoblast

Muscle can be used for systemic delivery of non-muscle proteins. In order to investigate the relative effectiveness of direct DNA plasmid injection versus implantation of genetically modified myogenic cell lines, we have used the human alpha 1 anti-trypsin (alpha1AT) cDNA driven by either cytomegalovirus (CMV) or the muscle creatine kinase 3.3 kb (MCK) promoter in immunodeficient mice. We demonstrate that the implantation of transfected myoblasts stably expressing the human alpha1AT cDNA generates a more persistent production of alpha1AT than does direct intramuscular injection of the same construct as plasmid DNA. Moreover, immunohistological labelling of muscle sections implanted with myoblasts show that the newly formed muscle fibres are those containing the human protein.

Persistent delivery of factor IX in mice: gene therapy for hemophilia using implantable microcapsules

Severe hemophilia B is a life-threatening, life long condition caused by absence of or defective coagulation factor IX. Gene therapy could provide an alternative treatment to repeated injection of plasma-derived concentrate or recombinant factor IX. We have previously described the use of implantable microcapsules containing recombinant myoblasts to deliver human factor IX in mice. This study reports the generation of improved myoblast-specific expression vectors. Mouse myoblast clones transfected with the various vectors secreted factor IX in vitro, at rates between 70 and 1000 ng/10(6) cells/day. The recombinant myoblast clones were then encapsulated and implanted into mice. Immunocompetent mice implanted with encapsulated myoblasts had up to 65 ng of factor IX per milliliter in their plasma for up to 14 days, after which antibodies to human factor IX became detectable, and this coincided with decreased factor IX in mouse plasma. In immunodeficient mice, however, factor IX delivery was maintained at a constant level for at least 6 weeks (end of experiment). Interestingly, the highest-secreting myoblast clone in vitro did not deliver the highest level of hFIX in vivo. This discrepancy observed between performance in vitro and in vivo may have important implications for the development of gene therapy protocols based on recombinant cells.

The Rhesus Macaque as an Animal Model for Hemophilia B Gene Therapy

We have determined the 2905 nucleotide sequence of the rhesus macaque factor IX complementary DNA (cDNA) and found it to be greater than 95% identical to that of the human factor IX cDNA. The cDNA has a large 3′ untranslated region like the human cDNA, but unlike the human cDNA has two polyadenylation sites 224 nucleotides apart that are used for transcription of the messenger RNA. The deduced amino acid sequence is greater than 97% identical to that of human factor IX, differing in only 11 of 461 amino acids in the complete precursor protein. We found a single silent polymorphism in the nucleotide sequence at the third position of the codon for asparagine at position 167 in the secreted protein (AAC/AAT). All residues subject to posttranslational modifications in the human protein are also found in the rhesus factor IX sequence. The high degree of homology between the rhesus and human factor IX proteins suggested the possibility that the human factor IX protein might be nonimmunogenic in the rhesus. We tested the immunogenicity of human factor IX in three rhesus macaques by repeated intravenous injections of monoclonal antibody–purified, plasma-derived human factor IX over the course of more than a year and assessed the recovery and half-life of the infused protein, as well as in vitro indicators of antihuman factor IX antibodies. Human factor IX recovery and half-life remained unchanged over the course of a year in the three animals studied, and aPTT mixing studies showed no evidence for neutralizing antihuman factor IX antibodies. An outbred, nonhuman primate model that permits assessment of the level and duration of factor IX expression as well as vector safety would complement the use of other (mouse and canine) hemophilia B animal models in current use for the development of gene therapy for hemophilia B.

Ex vivo fibroblast transduction in rabbits results in long-term (>600 days) factor IX expression in a small percentage of animals

Delivery of human factor IX to the circulation was analyzed in rabbits by ex vivo fibroblast transduction followed by subcutaneous implantation. Kinetic studies of human factor IX in rabbits demonstrated a half-life of approximately 16 hr and a volume distribution of 22%, where intraperitoneal and subcutaneous bioavailability was three- to sevenfold lower than by intravenous administration. Ex vivo retroviral transduction of autologous fibroblasts was performed on 15 animals. After subcutaneous injection of fibroblast-collagen mixtures, the expression of human factor IX in rabbit plasma was followed by ELISA. Of 15 rabbits injected, expression of human factor IX was detected in 2 animals, and expression was long term (>600 days). One animal had stable levels of human factor IX, at 20 ng/ml, while the second animal had lower and gradually decreasing levels of human factor IX. There were no gross differences in pathology at the injection sites, when comparing animals with human factor IX in plasma and those without. Immunological studies demonstrated antibody formation in response to injection mixture components (including human factor IX), but again there was no correlation with immune response and long-term factor IX production in animals. Tissues at the implantation sites were positive for factor IX DNA by PCR analysis, regardless of whether there was detectable plasma factor IX or not. Small numbers of PCR-positive cells were detected in the internal organs of the long term-expressing rabbits while similar tissues were negative in nonexpressing animals.

Adeno-Associated Viral Vector-Mediated Gene Transfer of Human Blood Coagulation Factor IX Into Mouse Liver

Recombinant adeno-associated virus vectors (AAV) were prepared in high titer (1012 to 1013 particles/mL) for the expression of human factor IX after in vivo transduction of murine hepatocytes. Injection of AAV-CMV-F.IX (expression from the human cytomegalovirus IE enhancer/promoter) into the portal vein of adult mice resulted in no detectable human factor IX in plasma, but in mice injected intravenously as newborns with the same vector, expression was initially 55 to 110 ng/mL. The expression in the liver was mostly transient, and plasma levels decreased to undetectable levels within 5 weeks. However, long-term expression of human F.IX was detected by immunofluorescence staining in 0.25% of hepatocytes 8 to 10 months postinjection. The loss of expression was likely caused by suppression of the CMV promoter, because polymerase chain reaction data showed no substantial loss of vector DNA in mouse liver. A second vector in which F.IX expression was controlled by the human EF1α promoter was constructed and injected into the portal vein of adult C57BL/6 mice at a dose of 6.3 × 1010 particles. This resulted in therapeutic plasma levels (200 to 320 ng/mL) for a period of at least 6 months, whereas no human F.IX was detected in plasma of mice injected with AAV-CMV-F.IX. Doses of AAV-EF1α-F.IX of 2.7 × 1011particles resulted in plasma levels of 700 to 3,200 ng/mL. Liver-derived expression of human F.IX from the AAV-EF1α-F.IX vector was confirmed by immunofluorescence staining. We conclude that recombinant AAV can efficiently transduce hepatocytes and direct stable expression of an F.IX transgene in mouse liver, but sustained expression is critically dependent on the choice of promoter.