Therapeutic Levels of Factor IX Expression Using a Muscle-Specific Promoter and Adeno-Associated Virus Serotype 1 Vector

Effective Reduction of Transgene-Specific Immune Response With rAAV Vectors Co-Expressing miRNA-UL112-5p or ERAP1 shRNA

Recombinant adeno-associated virus (rAAV) has emerged as one of the best gene delivery vectors for human gene therapy in vivo. However, the clinical efficacy of rAAV gene therapy is often hindered by the host immune response against its transgene products. Endoplasmic reticulum aminopeptidase 1 (ERAP1) is specialised to process peptides presented by class I molecules of major histocompatibility complex. Therefore, we hypothesise that modulation of the ERAP1 activity in rAAV transduced cells may be favoured to evade immune response against transgene products. In this study, we incorporated either miRNA-UL112-5p or ERAP1 shRNA into rAAV vectors expressing full-length ovalbumin (OVA) as a model antigen, and evaluated their effects for antigen presentation, cellular and humour immune response induced by OVA expression. The results indicated that silencing ERAP1 using miR-UL112-5p or ERAP1 shRNA did not affect the expression of OVA in cells, but inhibited the processing and presentation of OVA antigen peptide SIINFEKL in antigen presenting cells (APCs). Moreover, the rAAV vector co-expressing ERAP1 shRNA maintains stable and high expression of OVA in vivo, while simultaneously suppressing the humoral immunity of OVA. In addition, experimental results demonstrated that rAAV vectors incorporated ERAP1 shRNA efficiently repress costimulatory signals in dendritic cells (DCs), significantly attenuated the cytotoxic T-cell response, allowed for sustained transgene expression and reduced clearance of transduced muscle cells in mice. Moreover, our study suggested that the incorporation of miRNA-UL112-5p or ERAP1 shRNA into rAAV vectors effectively reduced transgene products induced immune response. The proposed method may potentially be applied in clinics to deliver therapeutic proteins safely and efficiently.

Synthetic Promoters in Gene Therapy: Design Approaches, Features and Applications

Gene therapy is a promising approach to the treatment of various inherited diseases, but its development is complicated by a number of limitations of the natural promoters used. The currently used strong ubiquitous natural promoters do not allow for the specificity of expression, while natural tissue-specific promoters have lowactivity. These limitations of natural promoters can be addressed by creating new synthetic promoters that achieve high levels of tissue-specific target gene expression. This review discusses recent advances in the development of synthetic promoters that provide a more precise regulation of gene expression. Approaches to the design of synthetic promoters are reviewed, including manual design and bioinformatic methods using machine learning. Examples of successful applications of synthetic promoters in the therapy of hereditary diseases and cancer are presented, as well as prospects for their clinical use.

Immune Responses to Muscle-Directed Adeno-Associated Viral Gene Transfer in Clinical Studies

Muscle-directed gene therapy with adeno-associated viral (AAV) vectors is undergoing clinical development for treating neuromuscular disorders and for systemic delivery of therapeutic proteins. Although these approaches show considerable therapeutic benefits, they are also prone to induce potent immune responses against vector or transgene products owing to the immunogenic nature of the intramuscular delivery route, or the high doses required for systemic delivery to muscle. Major immunological concerns include antibody formation against viral capsid, complement activation, and cytotoxic T cell responses against capsid or transgene products. They can negate therapy and even lead to life-threatening immunotoxicities. Herein we review clinical observations and provide an outlook for how the field addresses these problems through a combination of vector engineering and immune modulation.

Novel Combinatorial MicroRNA-Binding Sites in AAV Vectors Synergistically Diminish Antigen Presentation and Transgene Immunity for Efficient and Stable Transduction

Recombinant adeno-associated virus (rAAV) platforms hold promise for in vivo gene therapy but are undermined by the undesirable transduction of antigen presenting cells (APCs), which in turn can trigger host immunity towards rAAV-expressed transgene products. In light of recent adverse events in patients receiving high systemic AAV vector doses that were speculated to be related to host immune responses, development of strategies to mute innate and adaptive immunity is imperative. The use of miRNA binding sites (miR-BSs) to confer endogenous miRNA-mediated regulation to detarget transgene expression from APCs has shown promise for reducing transgene immunity. Studies have shown that designing miR-142BSs into rAAV1 vectors were able to repress costimulatory signals in dendritic cells (DCs), blunt the cytotoxic T cell response, and attenuate clearance of transduced muscle cells in mice to allow sustained transgene expression in myofibers with negligible anti-transgene IgG production. In this study, we screened individual and combinatorial miR-BS designs against 26 miRNAs that are abundantly expressed in APCs, but not in skeletal muscle. The highly immunogenic ovalbumin (OVA) transgene was used as a proxy for foreign antigens. In vitro screening in myoblasts, mouse DCs, and macrophages revealed that the combination of miR-142BS and miR-652-5pBS strongly mutes transgene expression in APCs but maintains high myoblast and myocyte expression. Importantly, rAAV1 vectors carrying this novel miR-142/652-5pBS cassette achieve higher transgene levels following intramuscular injections in mice than previous detargeting designs. The cassette strongly inhibits cytotoxic CTL activation and suppresses the Th17 response in vivo. Our approach, thus, advances the efficiency of miRNA-mediated detargeting to achieve synergistic reduction of transgene-specific immune responses and the development of safe and efficient delivery vehicles for gene therapy.

A Muscle Hybrid Promoter as a Novel Tool for Gene Therapy

Gene therapy is a promising strategy to cure rare diseases. The lack of regulatory sequences ensuring specific and robust expression in skeletal and cardiac muscle is a substantial limitation of gene therapy efficiency targeting the muscle tissue. Here we describe a novel muscle hybrid (MH) promoter that is highly active in both skeletal and cardiac muscle cells. It has an easily exchangeable modular structure, including an intronic module that highly enhances the expression of the gene driven by it. In cultured myoblasts, myotubes, and cardiomyocytes, the MH promoter gives relatively stable expression as well as higher activity and protein levels than the standard CMV and desmin gene promoters or the previously developed synthetic or CKM-based promoters. Combined with AAV2/9, the MH promoter also provides a high in vivo expression level in skeletal muscle and the heart after both intramuscular and systemic delivery. It is much more efficient than the desmin-encoding gene promoter, and it maintains the same specificity. This novel promoter has potential for gene therapy in muscle cells. It can provide stable transgene expression, ensuring high levels of therapeutic protein, and limited side effects because of its specificity. This constitutes an improvement in the efficiency of genetic disease therapy.

Bone-Targeted Alkaline Phosphatase Treatment of Mandibular Bone and Teeth in Lethal Hypophosphatasia via an scAAV8 Vector

Hypophosphatasia is an inherited disease caused by mutations in the gene encoding tissue-nonspecific alkaline phosphatase (TNALP), the major symptom of which is hypomineralization of the bones and teeth. We had recently demonstrated that TNALP-deficient (Akp2^−/−) mice, which mimic the phenotype of the severe infantile form of hypophosphatasia, can be treated by intramuscular injection of a self-complementary (sc) type 8 recombinant adeno-associated virus (rAAV8) vector expressing bone-targeted TNALP with deca-aspartates at the C terminus (TNALP-D₁₀) via the muscle creatine kinase (MCK) promoter. In this study, we focused on the efficacy of this scAAV8-MCK-TNALP-D₁₀ treatment on the mandibular bone and teeth in neonatal Akp2^−/− mice. Upon scAAV8-MCK-TNALP-D₁₀ injection, an improvement of mandibular growth was observed by X-ray analysis. Micro-computed tomography analysis revealed progressive mineralization of the molar root in the treated Akp2^−/− mice, and morphometric parameters of the alveolar bone were improved. These results suggest that the mandibular bones and teeth of hypophosphatasia were effectively treated by muscle directed rAAV-mediated TNALP-D₁₀ transduction. Our strategy would be promising for future hypophosphatasia gene therapy because it induces dentoalveolar mineralization and reduces the risk of tooth exfoliation.

Rescue of GSDIII Phenotype with Gene Transfer Requires Liver- and Muscle-Targeted GDE Expression

Glycogen storage disease type III (GSDIII) is an autosomal recessive disorder caused by a deficiency of glycogen-debranching enzyme (GDE), which results in profound liver metabolism impairment and muscle weakness. To date, no cure is available for GSDIII and current treatments are mostly based on diet. Here we describe the development of a mouse model of GSDIII, which faithfully recapitulates the main features of the human condition. We used this model to develop and test novel therapies based on adeno-associated virus (AAV) vector-mediated gene transfer. First, we showed that overexpression of the lysosomal enzyme alpha-acid glucosidase (GAA) with an AAV vector led to a decrease in liver glycogen content but failed to reverse the disease phenotype. Using dual overlapping AAV vectors expressing the GDE transgene in muscle, we showed functional rescue with no impact on glucose metabolism. Liver expression of GDE, conversely, had a direct impact on blood glucose levels. These results provide proof of concept of correction of GSDIII with AAV vectors, and they indicate that restoration of the enzyme deficiency in muscle and liver is necessary to address both the metabolic and neuromuscular manifestations of the disease.

Impact of AAV Capsid-Specific T-Cell Responses on Design and Outcome of Clinical Gene Transfer Trials with Recombinant Adeno-Associated Viral Vectors: An Evolving Controversy

Recombinant adenovirus-associated (rAAV) vectors due to their ease of construction, wide tissue tropism, and lack of pathogenicity remain at the forefront for long-term gene replacement therapy. In spite of very encouraging preclinical results, clinical trials were initially unsuccessful; expression of the rAAV vector-delivered therapeutic protein was transient. Loss of expression was linked to an expansion of AAV capsid-specific T-cell responses, leading to the hypothesis that rAAV vectors recall pre-existing memory T cells that had been induced by natural infections with AAV together with a helper virus. Although this was hotly debated at first, AAV capsid-specific T-cell responses were observed in several gene transfer trials that used high doses of rAAV vectors. Subsequent trials designed to circumvent these T-cell responses through the use of immunosuppressive drugs, rAAV vectors based on rare serotypes, or modified to allow for therapeutic levels of the transgene product at low, non-immunogenic vector doses are now successful in correcting debilitating diseases.

Treatment of hypophosphatasia by muscle-directed expression of bone-targeted alkaline phosphatase via self-complementary AAV8 vector

Hypophosphatasia (HPP) is an inherited disease caused by genetic mutations in the gene encoding tissue-nonspecific alkaline phosphatase (TNALP). This results in defects in bone and tooth mineralization. We recently demonstrated that TNALP-deficient (Akp2 (-/-) ) mice, which mimic the phenotype of the severe infantile form of HPP, can be treated by intravenous injection of a recombinant adeno-associated virus (rAAV) expressing bone-targeted TNALP with deca-aspartates at the C-terminus (TNALP-D10) driven by the tissue-nonspecific CAG promoter. To develop a safer and more clinically applicable transduction strategy for HPP gene therapy, we constructed a self-complementary type 8 AAV (scAAV8) vector that expresses TNALP-D10 via the muscle creatine kinase (MCK) promoter (scAAV8-MCK-TNALP-D10) and examined the efficacy of muscle-directed gene therapy. When scAAV8-MCK-TNALP-D10 was injected into the bilateral quadriceps of neonatal Akp2 (-/-) mice, the treated mice grew well and survived for more than 3 months, with a healthy appearance and normal locomotion. Improved bone architecture, but limited elongation of the long bone, was demonstrated on X-ray images. Micro-CT analysis showed hypomineralization and abnormal architecture of the trabecular bone in the epiphysis. These results suggest that rAAV-mediated, muscle-specific expression of TNALP-D10 represents a safe and practical option to treat the severe infantile form of HPP.

Intrinsic transgene immunogenicity gears CD8(+) T-cell priming after rAAV-mediated muscle gene transfer

Antitransgene CD8(+) T-cell responses are an important hurdle after recombinant adeno-associated virus (rAAV) vector-mediated gene transfer. Indeed, depending on the mutational genotype of the host, transgene amino-acid sequences of foreign origin can elicit deleterious cellular and humoral responses. We compared here two different major histocompatibility complex (MHC) class I epitopes of an engineered ovalbumin transgene delivered in muscle tissue by rAAV1 vector and found very different strength of CD8 responses, muscle destruction being correlated with the course of the immunodominant response. We further demonstrate that robust CD8(+) T-cell priming can occur through the cross-presentation pathway but requires the presence of either a strong MHC class II epitope or antibodies to the transgene product. Finally, manipulating transgene subcellular localization, we found that provided we avoid transgene expression in antigen presenting cells, the poorly accessible cytosolic form of ovalbumin transgene lacking strong MHC II epitope, evades CD8(+) T-cell priming and remains permanently expressed in muscle with no immune cell infiltration. Our results demonstrate that the intrinsic immunogenicity of transgenes delivered with rAAV vector in muscle can be manipulated in a rational manner to avoid adverse immune responses.

Prolonged gene expression in muscle is achieved without active immune tolerance using microrRNA 142.3p-regulated rAAV gene transfer

Gene transfer efficacy is limited by unwanted immunization against transgene products. In some models, immunization may be avoided by regulating transgene expression with mir142.3p target sequences. Yet, it is unclear if such a strategy controls T-cell responses following recombinant adeno-associated viral vector (rAAV)-mediated gene transfer, particularly in muscle. In mice, intramuscular rAAV1 gene delivery of a tagged human sarcoglycan muscle protein is robustly immunogenic and leads to muscle destruction. In this model, the simple insertion of mir142.3p-target sequences in the transgene expression cassette modifies the outcome of gene transfer, providing high and persistent levels of muscle transduction in C57Bl/6 mice. Such regulated vector fails to prime specific CD4 and CD8 T cells; although, transgene tolerance seems to result from ignorance and could be broken by a robust antigenic challenge. While effective in normal mice, the mir142.3p-regulated transgene remains immunogenic in sarcoglycan-deficient dystrophic mice. In these mice, transgene expression is only prolonged but does not persist as effector CD4 and CD8 T-cell responses develop. Thus, using a mir142.3p-regulated transgene can improve rAAV muscle gene transfer results, but the level of efficacy depends on the context of application. In normal muscle, this strategy is sufficient to prevent immunization and functions even more effectively than tissue-specific promoters. In dystrophic models, additional strategies are required to fully control T-cell responses.

Mir-142-3p target sequences reduce transgene-directed immunogenicity following intramuscular adeno-associated virus 1 vector-mediated gene delivery

BACKGROUND

Muscle represents an important tissue target for adeno-associated virus (AAV) vector-mediated gene transfer in muscular, metabolic or blood-related genetic disorders. However, several studies have demonstrated the appearance of immune responses against the transgene product after intramuscular AAV vector delivery that resulted in a limited efficacy of the treatment. Use of microRNAs that are specifically expressed in antigen-presenting cells (APCs) is a promising approach for avoiding those immune responses. Cellular mir-142-3p, which is APC-specific, is able to repress the translation of its target cellular transcripts by binding to a specific target sequences.

METHODS

In the present study, we explored the potential of mir-142-3p specific target sequences with respect to reducing or abolishing immune responses directed against ovalbumin (OVA), a highly immunogenic protein, expressed as transgene and delivered by AAV1 vector administered intramuscularly.

RESULTS

The occurrence of immune responses against OVA transgene following intramuscular delivery by AAV have been described previously and resulted in the loss of OVA protein expression. In the present study, we demonstrate that OVA protein expression was maintained when mir-142-3pT sequences were incorporated into the expression cassette. The sustained expression of OVA protein over time correlated with a reduced increase in anti-OVA antibody levels. Furthermore, no cellular infiltrates were observed in the muscle tissue when AAV1 vectors containing four or eight repeats of mir-142-3p target sequences after the OVA sequence were used.

CONCLUSIONS

The rising humoral and cellular immune responses against OVA protein after intramuscular delivery can be efficiently reduced by the use of mir-142-3p target sequences.

Role of antigen-specific regulatory CD4+CD25+ T cells in tolerance induction after neonatal IP administration of AAV-hF.IX

Neonatal AAV8-mediated Factor IX (F.IX) gene delivery was applied as a model for exploring mechanisms of tolerance induction during immune ontogeny. Intraperitoneal delivery of AAV8/ Factor IX (hF.IX) during weeks 1–4 of life, over a 20-fold dose range, directed stable hF.IX expression, correction of coagulopathy in F.IX-null hemophilia B mice, and induction of tolerance to hF.IX; however, only primary injection at 1–2 days of life enabled increasing AAV8-mediated hF.IX expression after re-administration, due to the absence of anti-viral capsid antibodies. Adoptive splenocyte transfer from tolerized mice demonstrated induction of CD4⁺CD25⁺ T regulatory (T_reg) populations that specifically suppressed anti-hF.IX antibody responses, but not responses to third party antigen. Induction of hF.IX antibodies was only observed in tolerized mice after in vivo CD4⁺CD25⁺ cell depletion and hF.IX challenge. Thus, primary injection of AAV during a critical period in the first week of life does not elicit antiviral responses, enabling re-administration of AAV and augmentation of hF.IX levels. Expansion of hF.IX-specific CD4⁺CD25⁺ T_regs has a major role in tolerance induction early in immune ontogeny. Neonatal gene transfer provides a useful approach for defining the ontogeny of immune responses and may suggest approaches for inducing tolerance in the context of genetic therapies.

Further reduction in adenovirus vector-mediated liver transduction without largely affecting transgene expression in target organ by exploiting microrna-mediated regulation and the Cre-loxP recombination system

In order to detarget undesirable transduction in the liver by an adenovirus (Ad) vector, we previously demonstrated that insertion of sequences perfectly complementary to liver-specific miR-122a into the 3'-untranslated region (UTR) of transgene specifically reduced the transgene expression in the liver by approximately 100-fold; however, a certain level of residual transgene expression was still found in the liver. In order to further suppress the hepatic transduction, we developed a two-Ad vector system that uses the microRNA (miRNA)-regulated transgene expression system and the Cre-loxP recombination system, i.e., insertion of miR-122a target sequences and loxP sites into the transgene expression cassette and coadministration of a Cre recombinase-expressing Ad vector. In addition, to maintain as much as possible the transgene expression in the spleen, which is the target organ of this study, spleen-specific miR-142-3p target sequences were inserted into the 3'-UTR of the Cre recombinase gene to suppress Cre recombinase expression in the spleen. The spleen is an attractive target for immunotherapy because the spleen plays important roles in the immune system. Coadministration of Ad vector possessing CMV promoter-driven Cre recombinase expression cassette with miR-142-3p target sequences resulted in a further 24-fold reduction in the hepatic transgene expression by the Ad vector containing miR-122a target sequences and loxP sites, compared with coadministration of control Ad vector. On the other hand, there was no significant reduction of transgene expression in the spleen.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

Effect of tissue-specific promoters and microRNA recognition elements on stability of transgene expression after hydrodynamic naked plasmid DNA delivery

Intravenous hydrodynamic injections into the liver and skeletal muscle have increased the efficacy of naked DNA delivery to a level that makes therapeutically relevant gene transfer attainable. Although there are no concerns about the immunogenicity of the delivered DNA itself, transgene products that are foreign to the host can trigger an immune response and hamper the therapeutic effect. Our goal was to determine whether and to what extent some known preventive measures are applicable to these delivery methods in order to achieve longterm expression of foreign proteins in immunocompetent mice. We designed plasmid DNA vectors that expressed a marker gene under the control of either a ubiquitous or a tissue-specific promoter. We also included microRNA (miR) target sites in the transcripts in order to silence expression in antigen-presenting cells (APCs). The constructs were delivered either into muscle or liver, using outbred ICR and inbred C57BL=6 mice. The data suggest that firefly luciferase, a potent immunogen, triggered a uniform immune response only in outbred ICR mice, and only when expressed from a ubiquitous promoter. This response could not be prevented by including APC-specific miR target sites in the transcript. In contrast, the probability of immune rejection in ICR mice could be significantly diminished by using tissue-specific promoters, and under these circumstances, the silencing of transgene expression in APCs did confer some benefits. After a single hydrodynamic injection, inbred mice did not reject luciferase under any of the tested conditions for at least 8 weeks. To test whether they became tolerized, they were challenged with a second boost of a cytomegalovirus promoter-driven luciferase construct. This triggered a strong immune response, suggesting that luciferase-reactive cells from the animals' T and B cell repertoire had not been eliminated. This secondary reaction could not be prevented by silencing expression in APCs. In conclusion, for the clinical application of hydrodynamic naked DNA delivery the use of tissue-specific promoters in combination with silencing expression in APCs will increase the probability of long-term expression, but the most desirable outcome, the establishment of transgene tolerance, appears unlikely to be achieved by any of these measures.

Impact of the underlying mutation and the route of vector administration on immune responses to factor IX in gene therapy for hemophilia B

Immune responses to factor IX (F.IX), a major concern in gene therapy for hemophilia, were analyzed for adeno-associated viral (AAV-2) gene transfer to skeletal muscle and liver as a function of the F9 underlying mutation. Vectors identical to those recently used in clinical trials were administered to four lines of hemophilia B mice on a defined genetic background [C3H/HeJ with deletion of endogenous F9 and transgenic for a range of nonfunctional human F.IX (hF.IX) variants]. The strength of the immune response to AAV-encoded F.IX inversely correlated with the degree of conservation of endogenous coding information and levels of endogenous antigen. Null mutation animals developed T- and B-cell responses in both protocols. However, inhibitor titers were considerably higher upon muscle gene transfer (or protein therapy). Transduced muscles of Null mice had strong infiltrates with CD8+ cells, which were much more limited in the liver and not seen for the other mutations. Sustained expression was achieved with liver transduction in mice with crm(-) nonsense and missense mutations, although they still formed antibodies upon muscle gene transfer. Therefore, endogenous expression prevented T-cell responses more effectively than antibody formation, and immune responses varied substantially depending on the protocol and the underlying mutation.

Induction of immune tolerance to FIX following muscular AAV gene transfer is AAV-dose/FIX-level dependent

Direct intramuscular injection (IM) of adeno-associated virus (AAV) has been proven a safe and potentially efficient procedure for gene therapy of many genetic diseases including hemophilia B. It is, however, contentious whether high antigen level induces tolerance or immunity to coagulation factor IX (FIX) following IM of AAV. We recently reported induction of FIX-specific immune tolerance by IM of AAV serotype one (AAV1) vector in mice. We hypothesize that the expression of high levels of FIX is critical to induction of FIX tolerance. In this study, we investigated the correlation among AAV dose, FIX expression, and tolerance induction. We observed that induction of immune tolerance or immunity to FIX was dependent on the dose of AAV1-human FIX (hFIX) given and the level of FIX antigen expressed in both normal and hemophilia mice. We then defined the minimum AAV1-hFIX dose and the lowest level of FIX needed for FIX tolerance. Different from hepatic AAV-hFIX gene transfer, we found that FIX tolerance induced by IM of AAV1 was not driven by regulatory T cells. These results provided further insight into the mechanism(s) of FIX tolerance, contributing to development of hemophilia gene therapy, and optimization of FIX tolerance induction protocols.

Recombinant adeno-associated virus type 8-mediated extensive therapeutic gene delivery into skeletal muscle of alpha-sarcoglycan-deficient mice

Autosomal recessive limb-girdle muscular dystrophy type 2D (LGMD 2D) is caused by mutations in the alpha-sarcoglycan gene (alpha-SG). The absence of alpha-SG results in the loss of the SG complex at the sarcolemma and compromises the integrity of the sarcolemma. To establish a method for recombinant adeno-associated virus (rAAV)-mediated alpha-SG gene therapy into alpha-SG-deficient muscle, we constructed rAAV serotypes 2 and 8 expressing the human alpha-SG gene under the control of the ubiquitous cytomegalovirus promoter (rAAV2-alpha-SG and rAAV8-alpha-SG). We compared the transduction profiles and evaluated the therapeutic effects of a single intramuscular injection of rAAVs into alpha-SG-deficient (Sgca(-/-)) mice. Four weeks after rAAV2 injection into the tibialis anterior (TA) muscle of 10-day-old Sgca(-/-) mice, transduction of the alpha-SG gene was localized to a limited area of the TA muscle. On the other hand, rAAV8-mediated alpha-SG expression was widely distributed in the hind limb muscle, and persisted for 7 months without inducing cytotoxic and immunological reactions, with a reversal of the muscle pathology and improvement in the contractile force of the Sgca(-/-) muscle. This extensive rAAV8-mediated alpha-SG transduction in LGMD 2D model animals paves the way for future clinical application.

Human apolipoprotein E expression from mouse skeletal muscle by electrotransfer of nonviral DNA (plasmid) and pseudotyped recombinant adeno-associated virus (AAV2/7)

Plasma apolipoprotein E (apoE) has multiple atheroprotective actions. However, although liver-directed adenoviral gene transfer of apoE reverses hypercholesterolemia and inhibits atherogenesis in apoE-deficient (apoE(-/-)) mice, safety considerations have revived interest in nonviral DNA (plasmid) and nonpathogenic adeno-associated viral (AAV) vectors. Here, we assess the effectiveness of these two delivery vehicles by minimally invasive intramuscular injection. First, we constructed AAV2-based expression plasmids harboring human apoE3 cDNA, driven by two muscle-specific promoters (CK6 and C5-12) and one ubiquitous promoter (CAG); each efficiently expressed apoE3 in transfected cultured C2C12 mouse myoblasts, although muscle-specific promoters were active only in differentiated multinucleate myotubes. Second, a pilot study verified that electrotransfer of the CAG-driven plasmid (p.CAG.apoE3) into tibialis anterior muscles, pretreated with hyaluronidase, of apoE(-/-) mice significantly enhanced (p < 0.001) local intramuscular expression of apoE3. However, in a 7-day experiment, the CK6- and C5-12-driven plasmids produced less apoE3 in muscle than did p.CAG.apoE3 (0.61 +/- 0.38 and 0.45 +/- 0.38 vs. 13.38 +/- 7.46 microg of apoE3 per muscle, respectively), but plasma apoE3 levels were below our detection limit (<15 ng/ml) in all mice and did not reverse the hyperlipidemia. Finally, we showed that intramuscular injection of a cross-packaged AAV serotype 7 viral vector, expressing human apoE3 from the CAG promoter, resulted in increasing levels of apoE3 in plasma over 4 weeks, although the concentration reached (1.40 +/- 0.35 microg/ml) was just below the threshold level needed to reduce the hypercholesterolemia. We conclude that skeletal muscle can serve as an effective secretory platform to express the apoE3 transgene, but that improved gene transfer vectors are needed to achieve full therapeutic levels of plasma apoE3 protein.

Silencing of T lymphocytes by antigen-driven programmed death in recombinant adeno-associated virus vector-mediated gene therapy

Recombinant adeno-associated virus (rAAV) vectors are considered promising for human gene replacement because they facilitate stable expression of therapeutic proteins in transduced tissues. Whether the success of gene therapy will be influenced by cellular immune responses targeting transgene-encoded proteins that are potentially immunogenic is unknown. Here we characterized CD8(+) T-cell activity against beta-galactosidase and enhanced green fluorescent protein, model antigens containing major histocompatibility complex (MHC) class I epitopes that are constitutively produced in murine skeletal muscle after rAAV vector transduction. Antigen-specific CD8(+) T cells were detected in the spleen and liver of mice within 7 days of muscle transduction. CD8(+) T-cell frequencies in these organs were stable, and effector functions were intact for months despite ongoing antigen production in muscle. CD8(+) T cells also infiltrated transduced muscle, where frequencies were at least 5-fold higher than in untransduced spleen and liver. Significantly, the majority of antigen-specific CD8(+) T cells in vector-transduced muscle were not functional. Loss of function in the muscle was associated with programmed death of the effector cells. Stable gene expression therefore depended on selective death of CD8(+) T cells at the site of antigen production, an effective mechanism for subverting immunity that is also potentially reversible.

Construction and analysis of compact muscle-specific promoters for AAV vectors

Adeno-associated viral (AAV) vectors have been broadly used for gene transfer in vivo for various applications. However, AAV precludes the use of most of the original large-sized tissue-specific promoters for expression of transgenes. Efforts are made to develop highly compact, active and yet tissue-specific promoters for use in AAV vectors. In this study, we further abbreviated the muscle creatine kinase (MCK) promoter by ligating a double or triple tandem of MCK enhancer (206-bp) to its 87-bp basal promoter, generating the dMCK (509-bp) and tMCK (720-bp) promoters. The dMCK promoter is shorter but stronger than some previously developed MCK-based promoters such as the enh358MCK (584-bp) and CK6 (589-bp) in vitro in C2C12 myotubes and in vivo in skeletal muscles. The tMCK promoter is the strongest that we tested here, more active than the promiscuous cytomegalovirus (CMV) promoter. Furthermore, both the dMCK and tMCK promoters are essentially inactive in nonmuscle cell lines as well as in the mouse liver (>200-fold weaker than the CMV promoter). The dMCK promoter was further tested in a few lines of transgenic mice. Expression of LacZ or minidystrophin gene was detected in skeletal muscles throughout the body, but was weak in the diaphragm, and undetectable in the heart and other tissues. Similar to other miniature MCK promoters, the dMCK promoter also shows preference for fast-twitch myofibers. As a result, we further examined a short, synthetic muscle promoter C5-12 (312-bp). It is active in both skeletal and cardiac muscles but lacks apparent preference on myofiber types. Combination of a MCK enhancer to promoter C5-12 has increased its strength in muscle by two- to threefold. The above-mentioned compact muscle-specific promoters are well suited for AAV vectors in muscle-directed gene therapy studies.

Recombinant adeno-associated virus vectors induce functionally impaired transgene product-specific CD8+ T cells in mice

Recombinant adeno-associated virus (rAAV) vectors were used in human trials as carriers of vaccines for HIV-1 after encouraging preclinical results. However, the clinical trials yielded disappointing results. Here we demonstrated that in mice, rAAV vectors expressing the gene encoding HIV-1 gag stimulated gag-specific CD8(+) T cells, but these T cells failed to expand after a booster immunization with a replication-defective adenoviral (Ad) vector also expressing gag. We tested rAAV vectors of different serotypes expressing HIV-1 gag for induction of transgene product-specific CD8(+) T cells and found that the immunoinhibitory effect of rAAV priming observed with different AAV serotypes was transgene product specific, was independent of the interval between prime and boost, and extended to boosts with vaccine modalities other than Ad vectors. rAAV vector-induced CD8(+) T cells proliferated poorly, produced low levels of IFN-gamma in response to gag stimulation, and upregulated immunoinhibitory molecules. These T cells did not protect efficiently against challenge with a surrogate pathogen. Finally, we showed that the impaired proliferative capacity of the T cells was caused by persistence of the antigen-encoding rAAV vectors and could be reversed by placing the CD8(+) T cells in an antigen-free environment. Our data suggest that rAAV vectors induce functionally impaired T cells and could dampen the immune response to a natural infection.

The role of the adeno-associated virus capsid in gene transfer

Adeno-associated virus (AAV) is one of the most promising viral gene transfer vectors that has been shown to effect long-term gene expression and disease correction with low toxicity in animal models, and is well tolerated in human clinical trials. The surface of the AAV capsid is an essential component that is involved in cell binding, internalization, and trafficking within the targeted cell. Prior to developing a gene therapy strategy that utilizes AAV, the serotype should be carefully considered since each capsid exhibits a unique tissue tropism and transduction efficiency. Several approaches have been undertaken in an effort to target AAV vectors to specific cell types, including utilizing natural serotypes that target a desired cellular receptor, producing pseudotyped vectors, and engineering chimeric and mosaic AAV capsids. These capsid modifications are being incorporated into vector production and purification methods that provide for the ability to scale-up the manufacturing process to support human clinical trials. Protocols for small-scale and large-scale production of AAV, as well as assays to characterize the final vector product, are presented here. The structures of AAV2, AAV4, and AAV5 have been solved by X-ray crystallography or cryo-electron microscopy (cryo-EM), and provide a basis for rational vector design in developing customized capsids for specific targeting of AAV vectors. The capsid of AAV has been shown to be remarkably stable, which is a desirable characteristic for a gene therapy vector; however, recently it has been shown that the AAV serotypes exhibit differential susceptibility to proteases. The capsid fragmentation pattern when exposed to various proteases, as well as the susceptibility of the serotypes to a series of proteases, provides a unique fingerprint for each serotype that can be used for capsid identity validation. In addition to serotype identification, protease susceptibility can also be utilized to study dynamic structural changes that must occur for the AAV capsid to perform its various functions during the virus life cycle. The use of proteases for structural studies in solution complements the crystal structural studies of the virus. A generic protocol based on proteolysis for AAV serotype identification is provided here.

Gentherapie

Die Gentherapie ist eine junge Wissenschaft, die Nukleinsäuren zur Therapie einsetzt (Hengge u. Bardenheuer 2004). Die somatische Gentherapie befasst sich mit der Behandlung von somatischen (Körper-)Zellen (⧁ Tab. 4.1.1), wobei das therapeutische Gen ein im Organismus benötigtes Protein kodiert.

Enhanced response to enzyme replacement therapy in Pompe disease after the induction of immune tolerance

Pompe disease, which results from mutations in the gene encoding the glycogen-degrading lysosomal enzyme acid alpha -glucosidase (GAA) (also called "acid maltase"), causes death in early childhood related to glycogen accumulation in striated muscle and an accompanying infantile-onset cardiomyopathy. The efficacy of enzyme replacement therapy (ERT) with recombinant human GAA was demonstrated during clinical trials that prolonged subjects' overall survival, prolonged ventilator-free survival, and also improved cardiomyopathy, which led to broad-label approval by the U.S. Food and Drug Administration. Patients who lack any residual GAA expression and are deemed negative for cross-reacting immunologic material (CRIM) have a poor response to ERT. We previously showed that gene therapy with an adeno-associated virus (AAV) vector containing a liver-specific promoter elevated the GAA activity in plasma and prevented anti-GAA antibody formation in immunocompetent GAA-knockout mice for 18 wk, predicting that liver-specific expression of human GAA with the AAV vector would induce immune tolerance and enhance the efficacy of ERT. In this study, a very low number of AAV vector particles was administered before initiation of ERT, to prevent the antibody response in GAA-knockout mice. A robust antibody response was provoked in naive GAA-knockout mice by 6 wk after a challenge with human GAA and Freund's adjuvant; in contrast, administration of the AAV vector before the GAA challenge prevented the antibody response. Most compellingly, the antibody response was prevented by AAV vector administration during the 12 wk of ERT, and the efficacy of ERT was thereby enhanced. Thus, AAV vector-mediated gene therapy induced a tolerance to introduced GAA, and this strategy could enhance the efficacy of ERT in CRIM-negative patients with Pompe disease and in patients with other lysosomal storage diseases.

Design of tissue-specific regulatory cassettes for high-level rAAV-mediated expression in skeletal and cardiac muscle

Systemic delivery of recombinant adeno-associated virus (rAAV) 6 vectors mediates efficient transduction of the entire striated musculature, making this an attractive strategy for muscle gene therapy. However, owing to widespread transduction of non-muscle tissues, optimization of this method would benefit from the use of muscle-specific promoters. Most such promoters either lack high-level expression in certain muscle types or are too large for inclusion in rAAV vectors encoding microdystrophin. Here, we describe novel regulatory cassettes based on enhancer/promoter regions of murine muscle creatine kinase (CK) and alpha-myosin heavy-chain genes. The strongest cassette, MHCK7 (770 bp), directs high-level expression comparable to cytomegalovirus and Rous sarcoma virus promoters in fast and slow skeletal and cardiac muscle, and low expression in the liver, lung, and spleen following systemic rAAV6 delivery in mice. Compared with CK6, our previous best cassette, MHCK7 activity is approximately 400-, approximately 50-, and approximately 10-fold higher in cardiac, diaphragm, and soleus muscles, respectively. MHCK7 also directs strong microdystrophin expression in mdx muscles. While further study of immune responses to MHCK7-regulated microdystrophin expression is needed, this cassette is not active in dendritic cell lines. MHCK7 is thus a highly improved regulatory cassette for experimental studies of rAAV-mediated transduction of striated muscle.

Persistent expression of hF.IX After tolerance induction by in utero or neonatal administration of AAV-1-F.IX in hemophilia B mice

The major complication associated with protein replacement therapy currently used in the treatment of hemophilia B (HB) is the development of antibodies to the infused human Factor IX (hF.IX). We hypothesized that vector-mediated expression of hF.IX, either at a prenatal stage or early in life may lead to tolerance to hF.IX and long-term transgene expression. Fetal, neonatal, and adult F.IX-deficient mice were injected with AAV-1-hF.IX, and the hF.IX levels as well as antibodies to hF.IX in the circulation were assayed. In utero injection followed by postnatal re-administration of adeno-associated virus 1 (AAV-1) vector achieved persistent expression of hF.IX in all animals, with no cellular or humoral immune response to F.IX. Similar results were seen after initial injection in neonatal mice followed by re-administration, whereas all mice injected at the adult stage developed antibodies to hF.IX. In contrast, after administration of AAV-2-hF.IX in the neonatal period, antibodies to hF.IX were formed in all the injected animals. We conclude that in utero or neonatal-stage injection of AAV-1-hF.IX can lead to long-term expression and absence of immune response. The differences in immune response between the AAV-1 and AAV-2 groups suggests that tolerance may be related to differences in bio-distribution, timing of expression, and/or the initial levels of hF.IX expression. This supports the concept of a narrow "window of opportunity" for tolerance induction.

Efficient induction of immune tolerance to coagulation factor IX following direct intramuscular gene transfer

BACKGROUND

The formation of inhibitory anti-factor IX (anti-FIX) antibodies is a major complication of FIX protein replacement-based treatment for hemophilia B. It is difficult to treat patients with anti-FIX antibodies. Gene therapy is emerging as a potentially effective treatment for hemophilia. Direct i.m. injection of adeno-associated virus (AAV) is a safe and efficient procedure for hemophilia B gene therapy. However, the development of anti-FIX antibodies following i.m. of AAV may impede its application to patients.

OBJECTIVE

We aimed to investigate induction of immune tolerance to human FIX (hFIX) by i.m. of AAV1, further validating i.m. of AAV1 for hemophilia B gene therapy.

METHODS AND RESULTS

Cohorts of hemostatically normal and hemophilia B mice with diverse genetic and MHC backgrounds received i.m. of AAV-hFIX. Human FIX antigen and anti-hFIX antibodies were examined. I.m. of 1 x 10(11) vector genomes (VG) of AAV2 elicits formation of anti-hFIX antibodies comparable to those by hFIX protein replacement. I.m. of 1 x 10(11) VG of AAV1 results in expression of therapeutic levels of hFIX (up to 950 ng mL(-1), mean = 772 ng mL(-1), SEM +/- 35.7) and hFIX-specific immune tolerance in C57BL/6 mice.

CONCLUSIONS

A single i.m. of AAV1 can result in efficient expression of therapeutic levels of hFIX and induction of hFIX tolerance in hemostatically normal and hemophilic B mice. Our results substantiate the prospect of i.m. of AAV1 for hemophilia B gene therapy and FIX tolerance induction.

Functions of AAV-CMV-F.IX And AAV-EF1α-F.IX in Gene Therapy for Hemophilia B

There has been substantial progress in using gene therapy to treat animals with hemophilia. Adeno-associated viral (AAV) gene transfer of coagulation factor IX to skeletal muscle and liver of murine and canine models of hemophilia has resulted in sustained systemic expression and, in several studies, in complete cure of the bleeding disorder. Two AAV vectors widely used at present are AAV-CMV-F.IX and AAV-EF1alpha-F.IX. This work compares the predicted molecular functions of AAV-CMV-F.IX and AAV-EF1alpha -F.IX by sequence docking and gene ontology. It is shown that both AAV-CMV-F.IX and AAV-EF1alpha -F.IX induce coagulation factor IXa activity; however, AAV-CMV-F.IX administration also yields coagulation factor XIa activity and AAV-EF1alpha -F.IX treatment results in coagulation factor Xa activity. Therefore, AAV-CMV-F.IX might be useful for factor XI deficiency. AAV-CMV-F.IX has several additional molecular functions and processes compared with AAV-CMV-F.IX.

Viral-mediated gene therapy for the muscular dystrophies: successes, limitations and recent advances

Much progress has been made over the past decade elucidating the molecular basis for a variety of muscular dystrophies (MDs). Accordingly, there are examples of mouse models of MD whose disease progression has been halted in large part with the use of viral vector technology. Even so, we must acknowledge significant limitations of present vector systems that must be overcome prior to successful treatment of humans with such approaches. This review will present a variety of viral-mediated therapeutic strategies aimed at counteracting the muscle-wasting symptoms associated with muscular dystrophy. We include viral vector systems used for muscle gene transfer, with a particular emphasis on adeno-associated virus. Findings of several encouraging studies focusing on repair of the mutant dystrophin gene are also included. Lastly, we present a discussion of muscle compensatory therapeutics being considered that include pathways involved in the up-regulation of utrophin, promotion of cellular adhesion, enhancement of muscle mass, and antagonism of the inflammatory response. Considering the complexity of the muscular dystrophies, it appears likely that a multilayered approach tailored to a patient sub-group may be warranted in order to effectively contest the progression of this devastating disease.

Transgene expression levels and kinetics determine risk of humoral immune response modeled in factor IX knockout and missense mutant mice

Immune responses leading to antibody-mediated elimination of the transgenic protein are a concern in gene replacement for congenital protein deficiencies, for which hemophilia is an important model. Although most hemophilia B patients have circulating non-functional but immunologically crossreactive factor IX (FIX) protein (CRM+ phenotype), inciting factors for FIX neutralizing antibody (inhibitor) development have been studied in crossreactive material-negative (CRM-) animal models. For this study, determinants of FIX inhibitor development were compared in hemophilia B mice, in which circulating FIX protein is absent (CRM- factor IX knockout (FIXKO) model) or present (CRM+ missense R333Q-hFIX model) modeling multiple potential therapies. The investigations compare for the first time different serotypes of adeno-associated virus (AAV) vectors (AAV2 and AAV1), each at multiple doses, in the setting of two different FIX mutations. The comparisons demonstrate in the FIXKO background (CRM- phenotype) that neither vector serotype nor vector particle number independently determine the inhibitor trigger, which is influenced primarily by the level and kinetics of transgene expression. In the CRM+ missense background, inhibitor development was never stimulated by AAV gene therapy or protein therapy, despite the persistence of lymphocytes capable of responding to FIX with non-inhibitory antibodies. This genotype/phenotype is strongly protective against antibody formation in response to FIX therapy.

Gene therapy: future or flop

Many pediatric diseases have now reached a therapeutic plateau using standard therapy. Gene therapy has emerged as an exciting new means to achieve specific therapeutic benefit. Although there have been important and promising breakthroughs in recent clinical trials, there have been some serious setbacks that have tempered this initial excitement. In this review, we discuss the important developments in the field of gene therapy as it applies to various pediatric diseases and relate the recent successes and failures to the future potential of gene therapy as a medical therapeutic application.

Gene Therapy Progress and Prospects – Vectorology: design and production of expression cassettes in AAV vectors

Adeno-associated virus (AAV) derived vectors are considered highly eligible vehicles for human gene therapy. Not only do they possess many great potential for clinical applications due to their wide range of tissue targets but also their excellent preclinical safety profile makes them particularly suitable candidates for treating serious diseases. Initial clinical trials have yielded encouraging results and prompted further improvements in their design and methods of production. Many studies have been performed to modify the tropism of recombinant (r)AAV by capsid modification. However, the precise control of spatial and temporal gene expression, which may be important in determining the safety and efficacy of gene transfer, lies in a rational choice and a subtle combination of various regulatory genetic elements to be inserted into the expression cassette. Moreover, new strategies based on such genetic sequences open new perspectives for enhancing vector genome persistence, disrupting or reducing pathogenic gene expression and even targeting genes.

Major role of local immune responses in antibody formation to factor IX in AAV gene transfer

The risk of an immune response to the coagulation factor IX (F.IX) transgene product is a concern in gene therapy for the X-linked bleeding disorder hemophilia B. In order to investigate the mechanism of F.IX-specific lymphocyte activation in the context of adeno-associated viral (AAV) gene transfer to skeletal muscle, we injected AAV-2 vector expressing human F.IX (hF.IX) into outbred immune-competent mice. Systemic hF.IX levels were transiently detected in the circulation, but diminished concomitant with activation of CD4+ T and B cells. ELISPOT assays documented robust responses to hF.IX in the draining lymph nodes of injected muscle by day 14. Formation of inhibitory antibodies to hF.IX was observed over a wide range of vector doses, with increased doses causing stronger immune responses. A prolonged inflammatory reaction in muscle started at 1.5-2 months, but ultimately failed to eliminate transgene expression. By 1.5 months, hF.IX antigen re-emerged in circulation in approximately 70% of animals injected with high vector dose. Hepatic gene transfer elicited only infrequent and weaker immune responses, with higher vector doses causing a reduction in T-cell responses to hF.IX. In summary, the data document substantial influence of target tissue, local antigen presentation, and antigen levels on lymphocyte responses to F.IX.

Systemic protein delivery by muscle-gene transfer is limited by a local immune response

Adeno-associated viral (AAV) vectors have been successfully used for therapeutic expression of systemic transgene products (such as factor IX or erythropoietin) following in vivo administration to skeletal muscle of animal models of inherited hematologic disorders. However, an immune response may be initiated if the transgene product represents a neoantigen. Here, we use ovalbumin (OVA) as a model antigen and demonstrate immune-mediated elimination of expression on muscle-directed AAV-2 gene transfer. Administration to immune competent mice resulted in transient systemic OVA expression. Within 10 days, OVA-specific T-helper cells had been activated in draining lymph nodes, an inflammatory immune response ensued, and OVA-expressing muscle fibers were destroyed by a cytotoxic CD8(+) T-cell response. Use of a muscle-specific promoter did not prevent this immune response. Adoptively transferred CD4(+) cells transgenic for a T-cell receptor specific to OVA peptide-major histocompatibility complex class II showed antigen-specific, vector dose-dependent proliferation confined to the draining lymph nodes of AAV-OVA-transduced muscle within 5 days after gene transfer and subsequently participated in lymphocytic infiltration of transduced muscle. This study documents that a local immune response limits sustained expression of a secreted protein in muscle gene transfer, a finding that may have consequences for design of clinical protocols.