Phenotypic correction of factor IX deficiency in skin fibroblasts of hemophilic dogs

Platelet-targeted hyperfunctional FIX gene therapy for hemophilia B mice even with preexisting anti-FIX immunity

Gene therapy may lead to a cure for hemophilia B (HB) if it is successful. Data from clinical trials using adeno-associated virus (AAV)-mediated liver-targeted FIX gene therapy are very encouraging. However, this protocol can be applied only to adults who do not have liver disease or anti-AAV antibodies, which occur in 30% to 50% of individuals. Thus, developing a protocol that can be applied to all HB patients is desired. Our previous studies have demonstrated that lentivirus-mediated platelet-specific FIX (2bF9) gene therapy can rescue bleeding diathesis and induce immune tolerance in FIXnull mice, but FIX expression was only ∼2% to 3% in whole blood. To improve the efficacy, we used a codon-optimized hyperfunctional FIX-Padua (2bCoF9R338L) to replace the 2bF9 cassette, resulting in 70% to 122% (35.08-60.77 mU/108 platelets) activity levels in 2bCoF9R338L-transduced FIXnull mice. Importantly, sustained hyperfunctional platelet-FIX expression was achieved in all 2bCoF9R338L-transduced highly immunized recipients with activity levels of 18.00 ± 9.11 and 9.36 ± 12.23 mU/108 platelets in the groups treated with 11 Gy and 6.6 Gy, respectively. The anti-FIX antibody titers declined with time, and immune tolerance was established after 2bCoF9R338L gene therapy. We found that incorporating the proteasome inhibitor bortezomib into preconditioning can help eliminate anti-FIX antibodies. The bleeding phenotype in 2bCoF9R338L-transduced recipients was completely rescued in a tail bleeding test and a needle-induced knee joint injury model once inhibitors dropped to undetectable. The hemostatic efficacy in 2bCoF9R338L-transduced recipients was further confirmed by ROTEM and thrombin generation assay (TGA). Together, our studies suggest that 2bCoF9R338L gene therapy can be a promising protocol for all HB patients, including patients with inhibitors.

Synthetic Vascular Grafts Seeded with Genetically Modified Endothelium in the Dog: Evaluation of the Effect of Seeding Technique and Retroviral Vector on Cell Persistence in Vivo

Unique characteristics of endothelium make it an attractive target cell for gene transfer. Genetically modified endothelial cells (ECs) seeded on synthetic vascular grafts offer the potential to control neointimal hyperplasia, decrease graft thrombogenicity and improve small diameter graft patency. This study addresses the issue of synthetic vascular graft colonization with endothelial cells transduced with noninducible retroviral marker genes in the dog. Autologous endothelial cells were enzymatically harvested and transduced with either the bacterial NeoR gene or human growth hormone gene using retroviral vectors. All transduced cells were positive by polymerase chain reaction (PCR) amplification for the transduced gene sequence prior to graft seeding. Transduced ECs were seeded on Dacron grafts (n = 3) pre-clotted with autologous blood. These grafts exhibited complete endothelialization at times from 250 to 360 days. Recovered DNA, however, was negative for the transduced gene sequence when analyzed by PCR and Southern blotting. Expanded polytetrafluoroethylene (ePTFE) was evaluated (n = 8) using several different cell seeding protocols. Grafts were seeded at 3 densities (ranging from 6 × 103 to 1.5 × 105 cells/cm2) and 2 different adherence times. Seeding substrate was also evaluated. Grafts were either preclotted with whole blood or incubated with 20 or 120 μg/ml fibronectin for 60 min. Graft biopsies were evaluated from 2 to 52 wk. Limited endothelialization was present in 4 dogs as early as 2 wk, but never progressed to full luminal coverage. The remaining dogs failed to ever exhibit any luminal EC adherence. Two dogs with limited EC coverage had positive DNA by PCR for the NeoR gene sequence at 2 and 3 wk. In contrast to transduced EC's, nontransduced EC colonization of ePTFE was complete at 2 wk when seeded under conditions that transduced cells had failed to persist. Neither seeding density, adherence time, seeding substrate or retroviral vector used influenced the uniformly poor graft coverage seen with transduced cells. Results of this study indicate that despite successful gene transfer using 4 different retroviral vectors, transduced endothelial cells seeded under varying conditions appear altered in their ability to stably adhere and colonize synthetic vascular grafts in vivo.

Gene therapy for hemophilia

Hemophilia A and B are X-linked monogenic disorders resulting from deficiencies of factor VIII and FIX, respectively. Purified clotting factor concentrates are currently intravenously administered to treat hemophilia, but this treatment is non-curative. Therefore, gene-based therapies for hemophilia have been developed to achieve sustained high levels of clotting factor expression to correct the clinical phenotype. Over the past two decades, different types of viral and non-viral gene delivery systems have been explored for hemophilia gene therapy research with a variety of target cells, particularly hepatocytes, hematopoietic stem cells, skeletal muscle cells, and endothelial cells. Lentiviral and adeno-associated virus (AAV)-based vectors are among the most promising vectors for hemophilia gene therapy. In preclinical hemophilia A and B animal models, the bleeding phenotype was corrected with these vectors. Some of these promising preclinical results prompted clinical translation to patients suffering from a severe hemophilic phenotype. These patients receiving gene therapy with AAV vectors showed long-term expression of therapeutic FIX levels, which is a major step forwards in this field. Nevertheless, the levels were insufficient to prevent trauma or injury-induced bleeding episodes. Another challenge that remains is the possible immune destruction of gene-modified cells by effector T cells, which are directed against the AAV vector antigens. It is therefore important to continuously improve the current gene therapy approaches to ultimately establish a real cure for hemophilia.

Recent progress in gene therapy for hemophilia

Hemophilia A and B are X-linked monogenic disorders caused by deficiencies in coagulation factor VIII (FVIII) and factor IX (FIX), respectively. Current treatment for hemophilia involves intravenous infusion of clotting factor concentrates. However, this does not constitute a cure, and the development of gene-based therapies for hemophilia to achieve prolonged high level expression of clotting factors to correct the bleeding diathesis are warranted. Different types of viral and nonviral gene delivery systems and a wide range of different target cells, including hepatocytes, skeletal muscle cells, hematopoietic stem cells (HSCs), and endothelial cells, have been explored for hemophilia gene therapy. Adeno-associated virus (AAV)-based and lentiviral vectors are among the most promising vectors for hemophilia gene therapy. Stable correction of the bleeding phenotypes in hemophilia A and B was achieved in murine and canine models, and these promising preclinical studies prompted clinical trials in patients suffering from severe hemophilia. These studies recently resulted in the first demonstration that long-term expression of therapeutic FIX levels could be achieved in patients undergoing gene therapy. Despite this progress, there are still a number of hurdles that need to be overcome. In particular, the FIX levels obtained were insufficient to prevent bleeding induced by trauma or injury. Moreover, the gene-modified cells in these patients can become potential targets for immune destruction by effector T cells, specific for the AAV vector antigens. Consequently, more efficacious approaches are needed to achieve full hemostatic correction and to ultimately establish a cure for hemophilia A and B.

Engineering dynamics of growth factors and other therapeutic ligands

Peptide growth factors and other receptor-binding cytokine ligands are of interest in contemporary molecular health care approaches in applications such as wound healing, tissue regeneration, and gene therapy. Development of effective technologies based on operation of these regulatory molecules requires an ability to deliver the ligands to target cells in a reliable and well-characterizable manner. Quantitative information concerning the fate of peptide ligands within tissues is necessary for adequate interpretation of experimental observations at the tissue level and for truly rational engineering design of ligand-based therapies. To address this need, we are undertaking efforts to elucidate effects of key molecular and cellular parameters on temporal and spatial distribution of cytokines in cell population and cell/matrix systems. In this article we summarize some of our recent findings on dynamics of growth factor depletion by cellular endocytic trafficking, growth factor transport through cellular matrices, and growth factor production and release by autocrine cell systems. (c) 1996 John Wiley & Sons, Inc.

The function of dog models in developing gene therapy strategies for human health

The domestic dog is of great benefit to humankind, not only through companionship and working activities cultivated through domestication and selective breeding, but also as a model for biomedical research. Many single-gene traits have been well-characterized at the genomic level, and recent advances in whole-genome association studies will allow for better understanding of complex, multigenic hereditary diseases. Additionally, the dog serves as an invaluable large animal model for assessment of novel therapeutic agents. Thus, the dog has filled a crucial step in the translation of basic research to new treatment regimens for various human diseases. Four well-characterized diseases in canine models are discussed as they relate to other animal model availability, novel therapeutic approach, and extrapolation to human gene therapy trials.

Protein replacement therapy and gene transfer in canine models of hemophilia A, hemophilia B, von willebrand disease, and factor VII deficiency

Dogs with hemophilia A, hemophilia B, von Willebrand disease (VWD), and factor VII deficiency faithfully recapitulate the severe bleeding phenotype that occurs in humans with these disorders. The first rational approach to diagnosing these bleeding disorders became possible with the development of reliable assays in the 1940s through research that used these dogs. For the next 60 years, treatment consisted of replacement of the associated missing or dysfunctional protein, first with plasma-derived products and subsequently with recombinant products. Research has consistently shown that replacement products that are safe and efficacious in these dogs prove to be safe and efficacious in humans. But these highly effective products require repeated administration and are limited in supply and expensive; in addition, plasma-derived products have transmitted bloodborne pathogens. Recombinant proteins have all but eliminated inadvertent transmission of bloodborne pathogens, but the other limitations persist. Thus, gene therapy is an attractive alternative strategy in these monogenic disorders and has been actively pursued since the early 1990s. To date, several modalities of gene transfer in canine hemophilia have proven to be safe, produced easily detectable levels of transgene products in plasma that have persisted for years in association with reduced bleeding, and correctly predicted the vector dose required in a human hemophilia B liver-based trial. Very recently, however, researchers have identified an immune response to adeno-associated viral gene transfer vector capsid proteins in a human liver-based trial that was not present in preclinical testing in rodents, dogs, or nonhuman primates. This article provides a review of the strengths and limitations of canine hemophilia, VWD, and factor VII deficiency models and of their historical and current role in the development of improved therapy for humans with these inherited bleeding disorders.

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.

Stem cell-derived erythroid cells mediate long-term systemic protein delivery

We demonstrate here the capacity of erythroid cells to mediate long-term, systemic and therapeutic protein delivery in vivo. By targeting human factor IX (hFIX) expression to late-stage erythropoiesis, we achieve long-term hFIX secretion at levels significantly higher (>tenfold) than those obtained with an archetypal ubiquitous promoter in a mouse model of hemophilia B. Erythroid cell-derived hFIX is biologically active, resulting in phenotypic correction of the bleeding disorder. In addition to achieving high expression levels and resistance to transcriptional silencing, red cell-mediated protein delivery offers multiple advantages including immune tolerance induction, reduction of the risk of insertional oncogenesis and relative ease of application by either engrafting transduced hematopoietic stem cells or transfusing ex vivo-generated, stem cell-derived erythroid cells.

Factor IX variants improve gene therapy efficacy for hemophilia B

Intramuscular injection of adeno-associated viral (AAV) vector to skeletal muscle of humans with hemophilia B is safe, but higher doses are required to achieve therapeutic factor IX (F.IX) levels. The efficacy of this approach is hampered by the retention of F.IX in muscle extracellular spaces and by the limiting capacity of muscle to synthesize fully active F.IX at high expression rates. To overcome these limitations, we constructed AAV vectors encoding F.IX variants for muscle- or liver-directed expression in hemophilia B mice. Circulating F.IX levels following intramuscular injection of AAV-F.IX-K5A/V10K, a variant with low-affinity to extracellular matrix, were 2-5 fold higher compared with wild-type (WT) F.IX, while the protein-specific activities remained similar. Expression of F.IX-R338A generated a protein with 2- or 6-fold higher specific activity than F.IX-WT following vector delivery to skeletal muscle or liver, respectively. F.IX-WT and variant forms provide effective hemostasis in vivo upon challenge by tail-clipping assay. Importantly, intramuscular injection of AAV-F.IX variants did not trigger antibody formation to F.IX in mice tolerant to F.IX-WT. These studies demonstrate that F.IX variants provide a promising strategy to improve the efficacy for a variety of gene-based therapies for hemophilia B.

Sustained correction of disease in naive and AAV2-pretreated hemophilia B dogs: AAV2/8-mediated, liver-directed gene therapy

Adeno-associated virus 8 (AAV8), a new member of the AAV family isolated from nonhuman primates, is an attractive candidate for hepatic gene transfer applications because of 10- to 100-fold improved transduction efficiency in mouse liver models. Additionally, AAV8 has lesser frequency of pre-existing immunity in humans. These properties could solve some of the problems associated with AAV2 vectors. The benefits of AAV8 demonstrated in mouse models, however, have not been confirmed in larger animals. In this study, we evaluate the efficacy and safety of AAV2/8 vector in both naive and AAV2-pretreated hemophilia B dogs. Two naive hemophilia B dogs that received a single intraportal administration of AAV2/8 vector have achieved sustained expression of 10% and 26% of normal levels of canine factor IX (cFIX) for more than a year. In an AAV2-pretreated hemophilia B dog, cFIX expression increased from less than 1% to 16% of normal levels when treated with an AAV2/8 vector, and a high level of expression has lasted for more than 2 years. No significant liver toxicity or cFIX-specific antibodies have been detected in these animals. Studies here have demonstrated the safety and improved efficacy of AAV2/8 vector in large-animal models for liver-directed gene therapy.

Noninvasive gene transfer to the lung for systemic delivery of therapeutic proteins

This study evaluates the use of vectors based on adeno-associated viruses (AAVs) to noninvasively deliver genes to airway epithelial cells as a means for achieving systemic administration of therapeutic proteins. We intranasally delivered AAV vectors to mice in which the same AAV2 genome encoding a cellular marker was packaged in capsids from AAV1, 2, or 5 (AAV2/1, AAV2/2, or AAV2/5, respectively). Gene expression levels achieved in both airways and alveoli were higher with AAV2/5 than with AAV2/1 and were undetectable with AAV2/2. The same set of vectors encoding a secreted therapeutic protein, erythropoietin (Epo), under the control of a lung-specific promoter (CC10) was intranasally delivered to mice, resulting in polycythemia with the highest levels of serum Epo obtained with AAV2/5 vectors. After a single intranasal administration of this vector, secretion of Epo was documented for 150 days. Similarly, intranasal administration of an AAV2/5-CC10-factor IX vector resulted in secretion of functional recombinant protein in the bloodstream of hemophiliac, factor IX-deficient mice. In addition, we demonstrate successful readministration of AAV2/5 to the lung 5 months after the first delivery of the same vector. In conclusion, we show that intranasal administration of AAV vectors results in efficient gene transfer to the lung only when the vector contains the AAV5 capsid and that this noninvasive route of administration results in sustained secretion of therapeutic proteins in the bloodstream.

Gene therapy for the hemophilias

Recent advances in the field of gene transfer are producing tantalizing results suggesting that the potential to correct disease at a molecular level may be at hand. Genetic correction of the hemophilias--bleeding disorders that stem from the deficiency of functional factor VIII or IX--represent models for the development of a basic understanding of how gene therapy will be achieved. The goals for hemophilia gene transfer are to produce therapeutic amounts of the coagulant protein while minimizing an immune response or antibody inhibitor. This requires the use of nontoxic vectors to deliver genes that express the protein in a functional form for the life of the patient. Based on a scientific understanding of the molecular and cellular defects leading to the bleeding phenotype, gene transfer studies at the laboratory and clinic have produced exciting results. The author here provides a critical assessment of the state of hemophilia gene transfer and its relevance to the field as a whole.

Evaluation of a short interspersed nucleotide element in the 3' untranslated region of the defective dystrophin gene of dogs with muscular dystrophy

OBJECTIVES

To determine the distribution of a 231-base pair (bp) element in the dystrophin gene 3' untranslated region (UTR) in a colony of Golden Retrievers with muscular dystrophy and other unrelated dogs and to estimate the frequency of recombination for the canine dystrophin gene.

ANIMALS

77 dogs from the Golden Retriever Muscular Dystrophy (GRMD) colony at the Murdoch Veterinary School and 30 unrelated dogs from the Murdoch University Veterinary Clinic.

PROCEDURE

Samples of blood or hair from dogs were used for amplification of DNA, using primers to the canine dystrophin 3' UTR.

RESULTS

The DNA from affected dogs generated a larger PCR product than that obtained from clinically normal dogs. Products were cloned and sequenced, and the difference in size was found to be attributable to a 231-bp short interspersed nucleotide element (SINE). The SINE was found in all affected dogs in the colony but not in most unaffected puppies in the colony. Eighteen of 19 dogs in the colony were heterozygous for the GRMD mutation, and 7 of 30 unrelated dogs also were heterozygous for the SINE.

CONCLUSION AND CLINICAL RELEVANCE

Evidence of recombination between the GRMD mutation and the SINE was observed in only 4 dogs (2 sets of littermates) in the GRMD colony. Incidence of this SINE in a few unrelated dogs suggests that this particular insertion into the dystrophin gene may have been a recent event. The SINE in the dystrophin 3' UTR did not have an apparent influence on dystrophin mRNA concentrations.

Gene therapy

With recent advances in molecular biology, the ability to transfer genes to patients is becoming a reality. Ongoing clinical trials using gene transfer techniques have illustrated the potential and pitfalls of this new therapeutic modality for the treatment of a wide variety of disorders. While these techniques are not currently a part of routine clinical practice, it is only a matter of time until some form of gene therapy is approved for general use in the clinic. This review highlights some of the basic methods used in current gene therapy protocols. The objective of this review is to familiarize practitioners with these concepts so they can more effectively follow the progress of this emerging technology and better inform their patients.

Gene therapy for hemophilia

Hemophilia A and B are X-chromosome linked recessive bleeding disorders that result from a deficiency in factor VIII (FVIII) and factor IX (FIX) respectively. Though factor substitution therapy has greatly improved the lives of hemophiliac patients, there are still limitations to the current treatment that have triggered interest in alternative treatments by gene therapy. Significant progress has recently been made in the development of gene therapy for the treatment of hemophilia A and B. These advances parallel the technical improvements of existing vector systems including MoMLV-based retroviral, adenoviral and AAV vectors, and the development of new delivery methods such as lentiviral vectors, helper-dependent adenoviral vectors and improved non-viral gene delivery methods. Therapeutic and physiologic levels of FVIII and FIX could be achieved in FVIII- and FIX-deficient mice and hemophilia dogs by different gene therapy approaches. Long-term correction of the bleeding disorders and in some cases a permanent cure has been realized in these preclinical studies. However, the induction of neutralizing antibodies often precludes stable phenotypic correction. Another complication is that certain promoters are prone to transcriptional inactivation in vivo, precluding long-term FVIII or FIX expression. Several gene therapy phase I clinical trials are currently ongoing in patients suffering from severe hemophilia A or B. No significant adverse side-effects were reported, and semen samples were negative for vector sequences by sensitive PCR assays. Most importantly, some subjects report fewer bleeding episodes and occasionally have very low levels of clotting factor activity detected. The results from the extensive preclinical studies in normal and hemophilic animal models and encouraging preliminary clinical data indicate that the simultaneous development of different strategies is likely to bring a permanent cure for hemophilia one step closer to reality.

Haemophilias: advances towards genetic engineering replacement therapy

Both haemophilia A and B are X-linked recessive disorders and therefore occur almost exclusively in males. The genes for both factors VIII and IX have been mapped to the distal end of the long arm of the X chromosome, bands Xq28 and Xq27.1, respectively. The Factor VIII gene comprises 186 kb DNA with 9 kb of exon of DNA which encodes an mRNA of nearly 9 kb. The Factor IX gene is 34 kb in length and the essential genetic information is present in eight exons which encode 1.6 kb mRNA. In gene therapy, genetic modification of the target cells can be either ex vivo or in vivo. The advantage of the ex vivo approach is that the genetic modification is strictly limited to the isolated cells. In the in vivo approach, the integrity of the target tissue is maintained but the major challenge is to deliver the gene to the target tissue. The use of improved retroviral and adenovirus-based vectors for gene therapy has produced clinically relevant levels of human factor VIII in mice and haemophilic dogs. If further improvements can increase the persistence of expression and decrease the immunological responses, phase I clinical trials in patients can be considered.

Sustained expression of therapeutic level of factor IX in hemophilia B dogs by AAV-mediated gene therapy in liver

We demonstrate that a single intraportal vein injection of a recombinant adeno-associated virus (rAAV) vector encoding canine factor IX (cFIX) cDNA under the control of a liver-specific enhancer/promoter leads to a long-term correction of the bleeding disorder in hemophilia B dogs. Stable expression of the therapeutic level of cFIX (5% of normal level) was detected in the plasma of a dog injected with an AAV vector at a dose of 4.6 x 10(12) particles/kg for over 7 months. Both whole-blood clotting time (WBCT) and activated partial thromboplastin time (aPTT) of the treated dogs have been greatly decreased since the treatment. No anti-canine factor IX antibodies have been detected in the treated animals. Importantly, no bleeding has been observed in the dog that expresses a therapeutic level of cFIX for 7 months following vector administration. Moreover, no persistent significant hepatic enzyme abnormalities were detected in the treated dogs. Thus, a single intraportal injection of a rAAV vector expressing cFIX successfully corrected the bleeding disorder of hemophilia B dogs, supporting the feasibility of using AAV-based vectors for liver-targeted gene therapy of genetic diseases.

Sustained correction of bleeding disorder in hemophilia B mice by gene therapy

Mice generated by disrupting the clotting factor IX gene exhibit severe bleeding disorder and closely resemble the phenotype seen in hemophilia B patients. Here we demonstrate that a single intraportal injection of a recombinant adeno-associated virus (AAV) vector encoding canine factor IX cDNA under the control of a liver-specific enhancer/promoter leads to a long-term and complete correction of the bleeding disorder. High level expression of up to 15-20 microgram/ml of canine factor IX was detected in the plasma of mice injected with 5.6 x 10(11) particles of an AAV vector for >5 months. The activated partial thromboplastin time of the treated mice was fully corrected to higher than normal levels. Liver-specific expression of canine factor IX was confirmed by immunofluorescence staining, and secreted factor IX protein was identified in the mouse plasma by Western blotting. All treated mice survived the tail clip test without difficulty. Thus, a single intraportal injection of a recombinant adeno-associated virus vector expressing factor IX successfully cured the bleeding disorder of hemophilia B mice, proving the feasibility of using AAV-based vectors for liver-targeted gene therapy of genetic diseases.

Regulation of N-acetylgalactosamine 4-sulfatase expression in retrovirus-transduced feline mucopolysaccharidosis type VI muscle cells

As a preliminary step toward muscle-mediated gene therapy in the mucopolysaccharidosis (MPS) type VI cat, we have analyzed the transcriptional regulation of feline N-acetylgalactosamine 4-sulfatase (f4S) gene expression from various retroviral constructs in primary cultures of muscle cells. Two retroviral constructs were made containing the f4S cDNA under the transcriptional control of the human polypeptide chain-elongation factor 1alpha (EF1alpha) gene promoter or the cytomegalovirus (CMV) immediate-early promoter. Two further retroviral constructs were made with the murine muscle creatine kinase (mck) enhancer sequence upstream of the internal promoter. Virus made from each construct was used to transduce feline MPS VI myoblasts. The mck enhancer significantly upregulated f4S gene expression from both the EF1alpha promoter and the CMV promoter in transduced myoblasts and in differentiated myofibers. The highest level of 4S activity was observed in myoblasts and myofibers transduced with the retroviral construct Lmckcmv4S, in which the f4S gene is under the transcriptional regulation of the mck enhancer and CMV immediate-early promoter. Lmckcmv4S-transduced myofibers demonstrated correction of glycosaminoglycan storage and contained a 58-fold elevated level of 4S activity compared with normal myofibers. Recombinant f4S secreted from Lmckcmv4S-transduced myofibers was endocytosed by feline MPS VI myofibers, leading to correction of the biochemical storage phenotype.

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.

Retroviral vector-modified bone marrow stromal cells secrete biologically active factor IX in vitro and transiently deliver therapeutic levels of human factor IX to the plasma of dogs after reinfusion

Canine bone marrow stromal cells (BMSCs), transduced ex vivo with retroviral vectors, expressed and secreted biologically active human and canine coagulation factor IX (hFIX and cFIX) in vitro, and on autologous reinfusion expressed hFIX into the circulation of normal (nonhemophiliac) dogs. Human FIX, when expressed in vitro by BMSCs of two dogs at 1.22 and 1.39 microg/10(6) cells/24 hr in medium supplemented with vitamin K, respectively, exhibited 28.1 and 27.3% normal biological activity as determined on the basis of a one-stage clotting assay. BMSCs of two additional dogs expressed 1.54 and 4.81 microg of cFIX/10(6) cells/24 hr in vitamin K-supplemented medium and the expressed cFIX possessed 58.4 and 32.9% normal activity, respectively. Between 2.33 and 3.35 x 10(8) transduced BMSCs, expressing 1.22 and 2.61 microg of hFIX/10(6) cells/24 hr or 3.24 and 7.82 microg of cFIX/10(6) cells/24 hr were reintroduced into the four donor dogs by intravenous infusion. Human FIX was detected in plasma for 7 or 12 days after BMSC reinfusion, with peak levels of 85.8 and 233.0 ng/ml observed at 2 days. Canine anti-hFIX antibodies, which were detected as early as 2-4 days after reinfusion of BMSCs expressing hFIX, may have masked potentially longer duration expression in vivo. Peak plasma levels of hFIX represented 2.1 and 5.8% normal human hFIX levels. When adjusted for percent normal one-stage clotting activity determined in vitro, these levels represented 0.6 and 1.6% normal human hFIX activity levels. Thus, we have demonstrated that retroviral vector-modified BMSCs can deliver human therapeutic levels of hFIX to the circulation of dogs.

Gene therapy for the hemophilias

There are many lines of evidence that suggest the eventual success of gene therapy as a treatment strategy for hemophilia. Because current treatment protocols using plasma-derived or recombinant proteins are far from ideal, the safe and efficient substitution of the defective gene by a normal copy of the gene, or at least its addition, would be of great benefit to the patient and may even be a potential cure. However, the construction of efficient gene therapy vehicles has proven quite difficult in the past and, so far, there is no system that promises to have all the desired features without any serious disadvantages. In general, either the levels of transgene expression are too low (because of the low titers achieved during the generation of the virus) or shortlived (e.g., because of the specific shut-off of the transferred promoter) as is often seen with retroviruses, or in the case of adenoviral vectors, expression is limited because of a strong immune response of the host. Clearly, much work remains to be done to optimize these promising though still imperfect vector systems. In the case of adenovirus, the development of less immunogenic vectors or in vivo modulation of the host immune system may hold promise for improvements. Reports by Yang et al. (1995) and Kay et al. (1995) are promising steps in the direction of immunomodulation. Both attenuate the immune reaction to the adenoviral vector by simultaneous application of either an interleukin or an immunoglobulin, respectively. When IL-2 was administered, the amounts of IgA were reduced and successful administration of a second dose of virus was possible. When CTLA4-Ig, an immunoglobulin that blocks the second signal during antigen presentation, was administered, a markedly prolonged expression of the transgene resulted. In vivo trials with AAV vectors have been carried out for some diseases (Flotte et al., 1993; Kaplitt et al., 1994) but not for hemophilia. Advances in high-titer AAV vector preparation will make this approach more feasible. The pace continues to quicken in the development of nonviral modes of gene delivery (Perales et al., 1994). Although these results are encouraging for the future of gene therapy as a treatment for genetic diseases, much work remains to be done to make this potential alternative a reality for treatment of hemophilia.

Genetic immunotherapy for cancer

The application of gene therapy to the treatment of human and veterinary diseases offers an innovative addition to the clinician's treatment options. Gene therapy can potentially be used to (1) replace defective or missing genes, (2) treat cancer, and (3) deliver drugs. The focus of this paper is the use of gene therapy in the treatment of cancer. To be effective, genes must be delivered to target cells which can then serve as the factory to produce the gene product. Delivery systems include retroviral vectors, adenoviral vectors, and direct introduction of plasmid DNA into cells. In the case of cancer immunotherapy, introduced genes produce products that enhance tumor immunosurveillance and tumor cell killing by immune mechanisms.

Genetic therapy. Past, present, and future

The early stages of genetic therapy present challenges for clinicians and basic scientists. Clinicians must become familiar with new terminology and concepts, and must keep a perspective on the new field in the face of inflated claims and high-profile failures. Basic scientists must continually return to disease models and to patients to determine what are the proper safety issues and relevant efficacy questions for specific diseases and vector systems. And in an era of instant information, all concerned parties must be careful about how progress is communicated to colleagues, patients, and the lay public.

Comparison of retroviral and adeno-associated viral vectors designed to express human clotting factor IX

Several different designs for retroviral and adeno-associated virus (AAV) vectors were developed to express human clotting factor IX. Seven separate retroviral vectors were constructed, including chimeric long terminal repeat (LTR)-based designs, vectors containing splice donor/acceptor sites with internal ribosome entry sites (IRES), and vectors with an internal cytomegalovirus (CMV)- or hepatitis B virus (HBV)-derived promoter. Five AAV vectors were produced using the same cassette design where a viral promoter was used to transcribe a bicistronic mRNA containing factor IX and an IRES/neo gene. In the human hepatocyte cell line HepG2, the constructs were tested for factor IX production by ELISA, Northern blot, and Western blot, and for biological activity by normalization of the prolonged activated partial thromboplastin time (APTT) of factor IX-deficient plasma. All of the constructs produced biologically active factor IX in the range of 0.23-152 ng/24 hr per 10(6) cells (the HBV-promoted factor IX AAV vector was the least effective, and the CMV-promoted retroviral vector was the most active). Primary fibroblasts of both human and rabbit origin were also evaluated for factor IX production following transduction with viral vectors. Fibroblasts produced substantially more factor IX than the HepG2 cell line, with the best AAV vector synthesizing > 250 ng/24 hr per 10(6) cells and the best retroviral vector making > 900 ng/24 hr per 10(6) cells. Generally, we observed lower transduction efficiency and poorer expression with the AAV vectors versus retroviral vectors in these cell types.

Platelets and coagulation. An update

Accepted mechanisms of coagulation are being challenged as new proteins and feedback mechanisms are discovered and evaluated. These new proteins, including tissue factor pathway inhibitor, not only change the way we look at the coagulation mechanism, but also give us potential alternatives in treatment of hemophilia. Gene therapy continues to be a challenging research area, with hemophilia serving as the prototypic hereditary disease target. As acquired disorders of coagulation secondary to immune-mediated events are studied and better understood, our approaches to management of these cases will also change to the benefit of our patients.

Myoblast gene therapy in canine mucopolysaccharidosis. I: Abrogation by an immune response to alpha-L-iduronidase

Three dogs with deficiency of the lysosomal enzyme alpha-L-iduronidase were treated by gene replacement therapy targeted at muscle. Direct intramuscular injections of plasmid encoding the alpha-L-iduronidase gene cDNA resulted in no detectable enzyme production, but may have resulted in immunologic sensitization to iduronidase protein, which the dogs lack totally. Myoblasts were grown from skeletal muscle biopsies and transduced with a retroviral vector containing the canine gene under control of the muscle creatine kinase enhancer. Several hundred-fold overexpression of enzyme production occurred in cultured cells; however, following reintroduction of the cultured cells into dogs, enzyme production declined rapidly. Concurrent with the falling enzyme levels, there was production of specific immunoglobulin G (IgG) antibody against iduronidase that was further associated with cellular infiltration of the myoblast injection sites. Most inflammatory cells were lymphocytes and plasma cells, suggesting local humoral and cellular immune responses to the enzyme-producing muscle cells. PCR analysis of tissues collected 2-22 weeks after the final treatment showed the persistence of Neo and canine alpha-L-iduronidase sequences in a progressively decreasing percentage of myoblasts. Results from this study in a canine model of mucopolysaccharidosis I underscore the fact that immunologic reactions to cells producing desirable, normal, but foreign, proteins may be as much an impediment to gene therapy as reactions to the viral vectors used to introduce the foreign gene.

An improved plasmid DNA expression vector for direct injection into skeletal muscle

In previous work, the direct injection of 50 micrograms of a plasmid DNA vector encoding firefly luciferase (VR1205) into murine quadriceps muscle produced an average of 6.5 ng of luciferase per muscle at 7 days postinjection. In this report, various elements of the VR1205 vector were modified to increase gene expression levels or to eliminate undesired viral sequences. Expression of the modified vectors was then compared to VR1205 using the intramuscular injection assay. In general, modifications to promoter, enhancer, and intronic sequences either decreased luciferase expression levels or had no effect. However, modifications to the polyadenylation and transcriptional termination sequences, plasmid backbone elements, and the luciferase gene itself each increased luciferase expression levels. The best-expressing vector, designated VR1255, contained a combination of these incrementally beneficial changes. A single intramuscular injection of 50 micrograms of VR1255 produced 300 ng of luciferase at 7 days postinjection, an expression level 46-fold higher than the VR1205 vector (or 22-fold higher, excluding modifications to the luciferase gene) and 154-fold higher than a commercially available luciferase expression vector. Thus, VR1255 represents an improved plasmid DNA vector that may be useful for gene therapy applications.

Conversion of dermal fibroblasts to a myogenic lineage is induced by a soluble factor derived from myoblasts

The limb and axial skeletal muscles of mammals originate from somitic dermomyotome, which during early development separates to form two discrete structures, the dermatome and the myotome. The latter cell mass gives rise to the muscle-forming lineage while cells of the dermatome will form the skin dermal fibroblast population of the dorsal regions of the body. It has been generally accepted for some time that myotome-derived myoblasts were the sole source of muscle fibre nuclei, but evidence has recently been presented from several laboratories that fibroblasts can fuse with myoblasts to contribute active nuclei to the resulting myotubes. We report here an investigation into the myogenic capacity of fibroblasts. Confluent monocultures of mouse dermal fibroblasts, muscle fibroblasts, and C2C12 myoblasts each retain their individual phenotype when maintained for periods up to 7 days in culture. We also grew isolated colonies of fibroblasts and myoblasts in an arrangement which allowed free exchange of tissue culture medium between the 2 cell types. We found evidence of the conversion of dermal fibroblasts to a myogenic lineage as measured by the appearance of MyoD-positive cells expressing the muscle-specific intermediate filament desmin. In addition, dermal fibroblast cultures contained multinucleate syncytia positive for MyoD and containing sarcomeric myosin heavy chain. In contrast, muscle-derived fibroblasts showed no evidence of myogenic conversion when maintained in identical culture conditions. We prepared conditioned medium from confluent cultures of C2C12 myoblasts and added this material to confluent monocultures of either dermal or muscle fibroblasts. While muscle fibroblasts showed no phenotypic alterations, cultures of dermal fibroblasts responded to myoblast conditioned medium by converting to a myogenic lineage as judged by expression of MyoD and desmin. We conclude that a proportion of dermal fibroblasts retain a myogenic capacity into stages well beyond their early association with myoblasts in the dermomyotome.

Puncture-mediated gene transfer to the skin

Simple and efficient gene transfer to the skin would facilitate many local and systemic gene therapy applications. Here we report a novel approach that allows installation and expression of plasmid DNA without a particulate carrier into the skin for the purposes of gene therapy. A device with a constantly high-frequency oscillating bundle of fine metal needles allows puncturing of the skin and leads to DNA transfer to skin-associated cells and expression of reporter genes in mice. Furthermore, puncture-mediated instillation of an expression vector coding for a single T cell epitope induces specific cellular immune responses. If this approach could be optimized, puncture-mediated gene transfer might be useful for the treatment of large body areas with plasmid DNA for the purpose of somatic gene therapy.

Prospects for gene therapy in sports medicine

For the orthopedic sports medicine physician soft tissue injuries often present the greatest clinical problems. Not only do many of the most frequently injured tissues, such as the cruciate ligaments and articular cartilage, have very limited capabilities for spontaneous repair, but they also respond poorly to surgical or nonsurgical intervention. In this article we try to define the role of growth factors in these conditions and to outline concepts for future treatment based upon modulation of the native repair response. We suggest that gene transfer could improve the management of such injuries, particularly when used as vehicles for the targeted delivery of growth factors. The concept of gene therapy in orthopedic sports medicine can be extended to include disorders that present as laxity or mechanical weakness of ligaments. We speculate that subtle genetic differences between individuals may account for those who appear to be injury prone. In these cases it is likely that genes encoding the structural macromolecules of the matrix are defective. Local gene supplementation in such cases could be useful in the future.

Cellular and humoral immune responses to adenoviral vectors containing factor IX gene: tolerization of factor IX and vector antigens allows for long-term expression

Recombinant adenoviruses containing the canine factor IX (FIX) cDNA were directly introduced in the hind leg muscle of mice. We show that (i) in nude mice, high expression (1-5 micrograms/ml in plasma) of FIX protein can be detected for > 300 days; (ii) in contrast, expression of FIX protein was transient (7-10 days) in normal mice; (iii) CD8+ lymphocytes could be detected within 3 days in the infected muscle tissue; (iv) use of beta 2-microglobulin and immunoglobulin M heavy chain "knockout" mice showed that lack of sustained expression of FIX protein is due to cell-mediated and humoral immune responses; (v) normal mice, once infected with recombinant adenovirus, could not be reinfected efficiently for at least 30 days due to neutralizing viral antibodies; and, finally, (vi) using immunosuppressive drugs, some normal mice can be tolerized to produce and secrete FIX protein for > 5 months. We conclude that currently available adenoviral vectors have serious limitations for use for long-term gene therapy.

Dermal fibroblasts convert to a myogenic lineage in mdx mouse muscle

Duchenne muscular dystrophy is a primary muscle disease that manifests itself in young boys as a result of a defect in a gene located on the X-chromosome. This gene codes for dystrophin, a normal muscle protein that is located beneath the sarcolemma of muscle fibres. Therapies to alleviate this disease have centred on implanting normal muscle precursor cells into dystrophic fibres to compensate for the lack of this gene and its product. To date, donor cells for implantation in such therapy have been of myogenic origin, derived from paternal biopsies. Success in human muscle, however, has been limited and may reflect immune rejection problems. To overcome this problem the patient's own myogenic cells, with the dystrophin gene inserted, could be used, but this could lead to other problems, since these cells are those that are functionally compromised by the disease. Here, we report the presence of high numbers of dystrophin-positive fibres after implanting dermal fibroblasts from normal mice into the muscle of the mdx mouse-the genetic homologue of Duchenne muscular dystrophy. Dystrophin-positive fibres were also abundant in mdx muscle following the implantation of cloned dermal fibroblasts from the normal mouse. Our results suggest the in vivo conversion of these non-myogenic cells to the myogenic pathway resulting in the formation of dystrophin-positive muscle fibres in the deficient host. The use of dermal fibroblasts may provide an alternative approach to the previously attempted myoblast transfer therapy, which in human trials has yielded disappointing results.

Retrovirus-mediated gene transfer and expression of human ornithine delta-aminotransferase into embryonic fibroblasts

Ornithine delta aminotransferase (OAT) is a nuclear-encoded mitochondrial matrix enzyme that catalyzes the reversible transamination of ornithine to glutamate semialdehyde. In humans, genetic deficiency of OAT results in gyrate atrophy of the choroid and retina, a blinding chorioretinal degeneration usually beginning in late childhood. This disorder has been shown to be autosomal recessive, and is often caused by missense, nonsense, and/or frameshift mutations in the OAT gene. With the view of applying gene therapy, a Moloney murine leukemia virus (MoMLV)-based recombinant retrovirus vector has been constructed. The human OAT cDNA was placed under the control of the enhancer-promoter regulatory elements derived from the MoMLV long terminal repeat (LTR). The construct was transfected into the retroviral packaging cell lines GP + E - 86 and psi CRIP to produce virus particles. Supernatant from these OAT retrovirus producer cell lines were used to transduce mouse C57B1/6 embryonal fibroblasts. We showed that the recombinant retrovirus transfers the OAT gene to the recipient cells, which produce an OAT RNA transcript when analyzed by Northern blot. Western blot analysis and enzymatic assays confirmed the presence of an OAT polypeptide that has a high enzymatic activity in the transduced cell lines, even after a long period of time in vitro.

In vivo gene therapy of hemophilia B: sustained partial correction in factor IX-deficient dogs

The liver represents a model organ for gene therapy. A method has been developed for hepatic gene transfer in vivo by the direct infusion of recombinant retroviral vectors into the portal vasculature, which results in the persistent expression of exogenous genes. To determine if these technologies are applicable for the treatment of hemophilia B patients, preclinical efficacy studies were done in a hemophilia B dog model. When the canine factor IX complementary DNA was transduced directly into the hepatocytes of affected dogs in vivo, the animals constitutively expressed low levels of canine factor IX for more than 5 months. Persistent expression of the clotting factor resulted in reductions of whole blood clotting and partial thromboplastin times of the treated animals. Thus, long-term treatment of hemophilia B patients may be feasible by direct hepatic gene therapy in vivo.

Cell transplantation in liver-directed gene therapy

Somatic cell gene therapy is a new field of biomedical research that encompasses a variety of traditional basic research and clinical disciplines. This new approach to therapeutics has the potential to prevent, treat, or cure a variety of inherited and acquired diseases. Two divergent strategies of hepatocyte transplantation are being employed in animal models and clinical trials in an attempt to correct genetic deficiencies. Allogeneic hepatocyte transplantation has two main advantages over autologous cell transplantation. First, invasive surgical procedures are not required in the recipient. Second, allogeneic cells can be administered repetitively, so that multiple harvests are not necessary. The major drawbacks to allogeneic hepatocyte transplants are rejection and the risks of immunosuppression. Although there is no clinical experience with the treatment of genetic disease by allogeneic hepatocyte transplantation, a variety of animal models have been characterized, including the Gunn rat (UDP-glucuronosyl transferase deficient), the Nagase analbuminemic rat, and the Watanabe heritable hyperlipidemic rabbit (LDL receptor deficient). The use of genetically corrected autologous cells represents a different and more elegant approach to the correction of inherited disease. A segment of liver is harvested from the affected individual. Recombinant retroviruses are used to transduce normal genes--with a variety of promoter/enhancer constructs--into the patients own hepatocytes. The genetically corrected hepatocytes are then transplanted back into the patient. This approach, known as ex vivo gene therapy, eliminates the risk of rejection and the need for immunosuppression. The safety and efficacy of this approach has been proven in a variety of preclinical animals models, including Watanabe rabbits, dogs, and Papio spp. A clinical trial for the treatment of familial hypercholesterolemia is currently in progress. A number of approaches for the reintroduction of hepatocytes into the recipient have been proposed, including catheter-mediated delivery into the inferior mesenteric vein, the umbilical vein, or into the spleen. Candidate diseases, which are likely to result in the first clinical trials include familial hypercholesterolemia, ornithine transcarbamylase deficiency, Crigler-Najjar syndrome, alpha 1-antitrypsin deficiency, and phenylketonuria.

Expression of human factor IX by microencapsulated recombinant fibroblasts

Deficiency of clotting factor IX (FIX) causes hemophilia B in humans. We propose a novel approach to its treatment by engineering FIX-secreting cell lines suitable for implantation in different allogeneic hosts. To prevent graft rejection following implantation, the recombinant cells can be protected with biocompatible membranes that permit exit of FIX but not entry of cellular immune mediators. To explore the feasibility of this approach, we now report on the creation of mouse Ltk- fibroblast cell lines that can deliver FIX through such immune-protective membranes. Mouse fibroblasts (Ltk-) were transfected with the cDNA for human FIX and clones secreting high levels of FIX were isolated. About 70% of the secreted FIX was biologically active. Over 98% of the recovered biological activity was precipitable by barium citrate, indicating appropriate. gamma-carboxylation of the secreted FIX. The secreted FIX was similar in molecular weight and immunoreactivity to plasma-derived human FIX. Upon enclosure in microcapsules fabricated from the biocompatible polymers, alginate-polylysine-alginate, the cells survived the encapsulation procedure with about 70-90% viability, proliferated within the microcapsules to twice their original number within 2 weeks, and continued to secrete FIX into the culture medium at similar rates as the unencapsulated cells. The biological activity, degree of post-translational gamma-carboxylation, and immunoreactivity of the FIX recovered from the culture media of the encapsulated cells were identical to those of the FIX secreted by the unencapsulated cells. In conclusion, fibroblasts engineered to secrete recombinant human FIX can proliferate and continue to secrete biologically active FIX through the alginate microcapsules. This demonstrates the feasibility of using microencapsulated recombinant cells to deliver human FIX and the potential for allogeneic somatic gene therapy for hemophilia B.

In vivo gene transfer and expression in normal uninjured blood vessels using replication-deficient recombinant adenovirus vectors

Replication-deficient recombinant adenovirus vectors do not require target cell replication for transfer and expression of exogenous genes and thus may be useful for in vivo gene therapy in the endothelium. To evaluate the feasibility of adenovirus-mediated gene transfer in vivo in normal intact blood vessels, adenovirus vectors containing the Escherichia coli lacZ gene or a human alpha 1-antitrypsin (alpha 1AT) cDNA were injected in vivo into the lumen of an occluded vessel segment of sheep jugular vein and/or carotid artery. After 15 minutes of incubation, circulation was restored; the vessels were harvested 1-28 days later and evaluated for gene transfer and expression. Three days after in vivo exposure to the lacZ adenovirus vector, the endothelium of jugular veins and carotid arteries expressed beta-galactosidase. Exposure of jugular veins and carotid arteries in vivo to the alpha 1AT adenovirus vector resulted in the expression of alpha 1AT mRNA transcripts detected by Northern analysis and in the synthesis and secretion of alpha 1AT detected by ex vivo [35S]methionine labeling. Expression with the adenovirus vectors was efficient and easily detectable 1-14 days after injection, with maximum expression at 7 days. Expression was no longer evident at 28 days. Thus, adenovirus vectors are capable of transferring exogenous genes to the endothelium of normal arteries and veins with expression for at least 2 weeks, suggesting that these vectors have the potential for a variety of cardiovascular experimental and clinical applications.