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

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.

Preclinical evaluation of NG101, a potential AAV gene therapy for wet age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of vision loss in individuals over the age of 55. Approximately 10%-15% of AMD patients develop choroidal neovascularization (CNV), leading to wet AMD (wAMD), which accounts for nearly 90% of AMD-related blindness. Inhibition of vascular endothelial growth factor (VEGF) is the standard treatment for wAMD. However, the frequent administration of the current treatment imposes a significant burden on wAMD patients. Therefore, there is an unmet need for treatments that require less-frequent administration. Here, we present findings on the safety and efficacy of NG101, a recombinant adeno-associated virus (rAAV) vector encoding aflibercept, an anti-VEGF agent, for wAMD therapy. A single subretinal injection of NG101 effectively reduced CNV lesion leakage and size at doses as low as 1 × 106 in mouse and 3 × 109 viral genomes per eye in cynomolgus monkeys. In cynomolgus monkeys, NG101-derived aflibercept expression in ocular tissues persisted for 1 year post-injection, indicating sustained therapeutic potential. Biodistribution analysis revealed that NG101 was primarily localized in ocular tissues. Only mild and transient ocular inflammatory responses were observed. Overall, these findings suggest that NG101, with its efficacy at low doses and sustained expression, is a promising therapeutic candidate for wAMD.

Adeno-associated viral delivery of Env-specific antibodies prevents SIV rebound after discontinuing antiretroviral therapy

An alternative to lifelong antiretroviral therapy (ART) is needed to achieve durable control of HIV-1. Here we show that adeno-associated virus (AAV)-delivery of two rhesus macaque antibodies to the SIV envelope glycoprotein (Env) with potent neutralization and antibody-dependent cellular cytotoxicity can prevent viral rebound in macaques infected with barcoded SIVmac239M after discontinuing suppressive ART. Following AAV administration, sustained antibody expression with minimal anti-drug antibody responses was achieved in all but one animal. After ART withdrawal, SIV replication rebounded within two weeks in all of the control animals but remained below the threshold of detection in plasma (<15 copies/mL) for more than a year in four of the eight animals that received AAV vectors encoding Env-specific antibodies. Viral sequences from animals with delayed rebound exhibited restricted barcode diversity and antibody escape. Thus, sustained expression of antibodies with potent antiviral activity can afford durable, ART-free containment of pathogenic SIV infection.

Recent Advancements in AAV-Vectored Immunoprophylaxis in the Nonhuman Primate Model

Monoclonal antibodies (mAbs) are important treatment modalities for preventing and treating infectious diseases, especially for those lacking prophylactic vaccines or effective therapies. Recent advances in mAb gene cloning from naturally infected or immunized individuals has led to the development of highly potent human mAbs against a wide range of human and animal pathogens. While effective, the serum half-lives of mAbs are quite variable, with single administrations usually resulting in short-term protection, requiring repeated doses to maintain therapeutic concentrations for extended periods of time. Moreover, due to their limited time in circulation, mAb therapies are rarely given prophylactically; instead, they are generally administered therapeutically after the onset of symptoms, thus preventing mortality, but not morbidity. Adeno-associated virus (AAV) vectors have an established record of high-efficiency in vivo gene transfer in a variety of animal models and humans. When delivered to post-mitotic tissues such as skeletal muscle, brain, and heart, or to organs in which cells turn over slowly, such as the liver and lungs, AAV vector genomes assume the form of episomal concatemers that direct transgene expression, often for the lifetime of the cell. Based on these attributes, many research groups have explored AAV-vectored delivery of highly potent mAb genes as a strategy to enable long-term expression of therapeutic mAbs directly in vivo following intramuscular or intranasal administration. However, clinical trials in humans and studies in nonhuman primates (NHPs) indicate that while AAVs are a powerful and promising platform for vectored immunoprophylaxis (VIP), further optimization is needed to decrease anti-drug antibody (ADA) and anti-capsid antibody responses, ultimately leading to increased serum transgene expression levels and improved therapeutic efficacy. The following review will summarize the current landscape of AAV VIP in NHP models, with an emphasis on vector and transgene design as well as general delivery system optimization. In addition, major obstacles to AAV VIP, along with implications for clinical translation, will be discussed.

Rescue of GM3 synthase deficiency by spatially controlled, rAAV-mediated ST3GAL5 delivery

GM3 synthase deficiency (GM3SD) is an infantile-onset epileptic encephalopathy syndrome caused by biallelic loss-of-function mutations in ST3GAL5. Loss of ST3GAL5 activity in humans results in systemic ganglioside deficiency and severe neurological impairment. No disease-modifying treatment is currently available. Certain recombinant adeno-associated viruses (rAAVs) can cross the blood-brain barrier to induce widespread, long-term gene expression in the CNS and represent a promising therapeutic strategy. Here, we show that a first-generation rAAV-ST3GAL5 replacement vector using a ubiquitous promoter restored tissue ST3GAL5 expression and normalized cerebral gangliosides in patient-derived induced pluripotent stem cell neurons and brain tissue from St3gal5-KO mice but caused fatal hepatotoxicity when administered systemically. In contrast, a second-generation vector optimized for CNS-restricted ST3GAL5 expression, administered by either the intracerebroventricular or i.v. route at P1, allowed for safe and effective rescue of lethality and behavior impairment in symptomatic GM3SD mice up to a year. These results support further clinical development of ST3GAL5 gene therapy.

AAV- based vector improvements unrelated to capsid protein modification

Recombinant adeno-associated virus (rAAV) is the leading platform for delivering genetic constructs in vivo. To date, three AAV-based gene therapeutic agents have been approved by the FDA and are used in clinical practice. Despite the distinct advantages of gene therapy development, it is clear that AAV vectors need to be improved. Enhancements in viral vectors are mainly associated with capsid protein modifications. However, there are other structures that significantly affect the AAV life cycle and transduction. The Rep proteins, in combination with inverted terminal repeats (ITRs), determine viral genome replication, encapsidation, etc. Moreover, transgene cassette expression in recombinant variants is directly related to AAV production and transduction efficiency. This review discusses the ways to improve AAV vectors by modifying ITRs, a transgene cassette, and the Rep proteins.

AAV vectors: The Rubik's cube of human gene therapy

Defective genes account for ∼80% of the total of more than 7,000 diseases known to date. Gene therapy brings the promise of a one-time treatment option that will fix the errors in patient genetic coding. Recombinant viruses are highly efficient vehicles for in vivo gene delivery. Adeno-associated virus (AAV) vectors offer unique advantages, such as tissue tropism, specificity in transduction, eliciting of a relatively low immune responses, no incorporation into the host chromosome, and long-lasting delivered gene expression, making them the most popular viral gene delivery system in clinical trials, with three AAV-based gene therapy drugs already approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA). Despite the success of AAV vectors, their usage in particular scenarios is still limited due to remaining challenges, such as poor transduction efficiency in certain tissues, low organ specificity, pre-existing humoral immunity to AAV capsids, and vector dose-dependent toxicity in patients. In the present review, we address the different approaches to improve AAV vectors for gene therapy with a focus on AAV capsid selection and engineering, strategies to overcome anti-AAV immune response, and vector genome design, ending with a glimpse at vector production methods and the current state of recombinant AAV (rAAV) at the clinical level.

AAV Vector-Mediated Antibody Delivery (A-MAD) in the Central Nervous System

In the last four decades, monoclonal antibodies and their derivatives have emerged as a powerful class of therapeutics, largely due to their exquisite targeting specificity. Several clinical areas, most notably oncology and autoimmune disorders, have seen the successful introduction of monoclonal-based therapeutics. However, their adoption for treatment of Central Nervous System diseases has been comparatively slow, largely due to issues of efficient delivery resulting from limited permeability of the Blood Brain Barrier. Nevertheless, CNS diseases are becoming increasingly prevalent as societies age, accounting for ~6.5 million fatalities worldwide per year. Therefore, harnessing the full therapeutic potential of monoclonal antibodies (and their derivatives) in this clinical area has become a priority. Adeno-associated virus-based vectors (AAVs) are a potential solution to this problem. Preclinical studies have shown that AAV vector-mediated antibody delivery provides protection against a broad range of peripheral diseases, such as the human immunodeficiency virus (HIV), influenza and malaria. The parallel identification and optimization of AAV vector platforms which cross the Blood Brain Barrier with high efficiency, widely transducing the Central Nervous System and allowing high levels of local transgene production, has now opened a number of interesting scenarios for the development of AAV vector-mediated antibody delivery strategies to target Central Nervous System proteinopathies.

In vivo Gene Therapy to the Liver and Nervous System: Promises and Challenges

In vivo genetic engineering has recently shown remarkable potential as a novel effective treatment for an ever-growing number of diseases, as also witnessed by the recent marketing authorization of several in vivo gene therapy products. In vivo genetic engineering comprises both viral vector-mediated gene transfer and the more recently developed genome/epigenome editing strategies, as long as they are directly administered to patients. Here we first review the most advanced in vivo gene therapies that are commercially available or in clinical development. We then highlight the major challenges to be overcome to fully and broadly exploit in vivo gene therapies as novel medicines, discussing some of the approaches that are being taken to address them, with a focus on the nervous system and liver taken as paradigmatic examples.

Modulating immune responses to AAV by expanded polyclonal T-regs and capsid specific chimeric antigen receptor T-regulatory cells

Immune responses to adeno-associated virus (AAV) capsids limit the therapeutic potential of AAV gene therapy. Herein, we model clinical immune responses by generating AAV capsid-specific chimeric antigen receptor (AAV-CAR) T cells. We then modulate immune responses to AAV capsid with AAV-CAR regulatory T cells (Tregs). AAV-CAR Tregs in vitro display phenotypical Treg surface marker expression, and functional suppression of effector T cell proliferation and cytotoxicity. In mouse models, AAV-CAR Tregs mediated continued transgene expression from an immunogenic capsid, despite antibody responses, produced immunosuppressive cytokines, and decreased tissue inflammation. AAV-CAR Tregs are also able to bystander suppress immune responses to immunogenic transgenes similarly mediating continued transgene expression, producing immunosuppressive cytokines, and reducing tissue infiltration. Taken together, AAV-CAR T cells and AAV-CAR Tregs are directed and powerful immunosuppressive tools to model and modulate immune responses to AAV capsids and transgenes in the local environment.

CRISPR/Cas System and Factors Affecting Its Precision and Efficiency

The diverse applications of genetically modified cells and organisms require more precise and efficient genome-editing tool such as clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas). The CRISPR/Cas system was originally discovered in bacteria as a part of adaptive-immune system with multiple types. Its engineered versions involve multiple host DNA-repair pathways in order to perform genome editing in host cells. However, it is still challenging to get maximum genome-editing efficiency with fewer or no off-targets. Here, we focused on factors affecting the genome-editing efficiency and precision of CRISPR/Cas system along with its defense-mechanism, orthologues, and applications.

Recombinant adeno-associated viruses as a gene delivery vehicle for the use in molecular medicine

Viral mechanisms for the delivery of genetic material are widely used in molecular medicine. Recombinant adeno-associated viruses (rAAV) represent a promising tool for in vivo gene delivery. The review considers nosological spectrum, molecular mechanisms, the choice of drug administration route depending on target structures, the choice of serotype, and the methods of active ingredient manufacturing for rAAV-mediated gene therapy.

Delivery Platforms for CRISPR/Cas9 Genome Editing of Glial Cells in the Central Nervous System

Glial cells (astrocytes, oligodendrocytes, and microglia) are emerging as key players in several physiological and pathological processes of the central nervous system (CNS). Astrocytes and oligodendrocytes are not only supportive cells that release trophic factors or regulate energy metabolism, but they also actively modulate critical neuronal processes and functions in the tripartite synapse. Microglia are defined as CNS-resident cells that provide immune surveillance; however, they also actively contribute to shaping the neuronal microenvironment by scavenging cell debris or regulating synaptogenesis and pruning. Given the many interconnected processes coordinated by glial cells, it is not surprising that both acute and chronic CNS insults not only cause neuronal damage but also trigger complex multifaceted responses, including neuroinflammation, which can critically contribute to the disease progression and worsening of symptoms in several neurodegenerative diseases. Overall, this makes glial cells excellent candidates for targeted therapies to treat CNS disorders. In recent years, the application of gene editing technologies has redefined therapeutic strategies to treat genetic and age-related neurological diseases. In this review, we discuss the advantages and limitations of clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-based gene editing in the treatment of neurodegenerative disorders, focusing on the development of viral- and nanoparticle-based delivery methods for in vivo glial cell targeting.

The use of miR122 and its target sequence in adeno-associated virus-mediated trichosanthin gene therapy

OBJECTIVE

Plant-derived cytotoxic transgene expression, such as trichosanthin (tcs), regulated by recombinant adeno-associated virus (rAAV) vector is a promising cancer gene therapy. However, the cytotoxic transgene can hamper the vector production in the rAAV producer cell line, human embryonic kidney (HEK293) cells. Here, we explored microRNA-122 (miR122) and its target sequence to limit the expression of the cytotoxic gene in the rAAV producer cells.

METHODS

A miR122 target (122T) sequence was incorporated into the 3' untranslated region of the tcs cDNA sequence. The firefly luciferase (fluc) transgene was used as an appropriate control. Cell line HEK293-mir122 was generated by the lentiviral vector-mediated genome integration of the mir122 gene in parental HEK293 cells. The effects of miR122 overexpression on cell growth, transgene expression, and rAAV production were determined.

RESULTS

The presence of 122T sequence significantly reduced transgene expression in the miR122-enriched Huh7 cell line (in vitro), fresh human hepatocytes (ex vivo), and mouse liver (in vivo). Also, the normal liver physiology was unaffected by delivery of 122T sequence by rAAV vectors. Compared with the parental cells, the miR122-overexpressing HEK293-mir122 cell line showed similar cell growth rate and expression of transgene without 122T, as well as the ability to produce liver-targeting rAAV vectors. Fascinatingly, the yield of rAAV vectors carrying the tcs-122T gene was increased by 77.7-fold in HEK293-mir122 cells. Moreover, the tcs-122T-containing rAAV vectors significantly reduced the proliferation of hepatocellular carcinoma cells without affecting the normal liver cells.

CONCLUSION

HEK293-mir122 cells along with the 122T sequence provide a potential tool to attenuate the cytotoxic transgene expression, such as tcs, during rAAV vector production.

Overcoming innate immune barriers that impede AAV gene therapy vectors

The field of gene therapy has made considerable progress over the past several years. Adeno-associated virus (AAV) vectors have emerged as promising and attractive tools for in vivo gene therapy. Despite the recent clinical successes achieved with recombinant AAVs (rAAVs) for therapeutics, host immune responses against the vector and transgene product have been observed in numerous preclinical and clinical studies. These outcomes have hampered the advancement of AAV gene therapies, preventing them from becoming fully viable and safe medicines. The human immune system is multidimensional and complex. Both the innate and adaptive arms of the immune system seem to play a concerted role in the response against rAAVs. While most efforts have been focused on the role of adaptive immunity and developing ways to overcome it, the innate immune system has also been found to have a critical function. Innate immunity not only mediates the initial response to the vector, but also primes the adaptive immune system to launch a more deleterious attack against the foreign vector. This Review highlights what is known about innate immune responses against rAAVs and discusses potential strategies to circumvent these pathways.

Adeno-Associated Viral Vector Mediated Expression of Broadly- Neutralizing Antibodies Against HIV-Hitting a Fast-Moving Target

The vast genetic variability of HIV has impeded efforts towards a cure for HIV. Lifelong administration of combined antiretroviral therapy (cART) is highly effective against HIV and has markedly increased the life expectancy of HIV infected individuals. However, the long-term usage of cART is associated with co-morbidities and the emergence of multidrug-resistant escape mutants necessitating the development of alternative approaches to combat HIV/AIDS. In the past decade, the development of single-cell antibody cloning methods has facilitated the characterization of a diverse array of highly potent neutralizing antibodies against a broad range of HIV strains. Although the passive transfer of these broadly neutralizing antibodies (bnAbs) in both animal models and humans has been shown to elicit significant antiviral effects, long term virologic suppression requires repeated administration of these antibodies. Adeno-associated virus (AAV) mediated antibody gene transfer provides a long-term expression of these antibodies from a single administration of the recombinant vector. Therefore, this vectored approach holds promises in the treatment and prevention of a chronic disease like HIV infection. Here, we provide an overview of HIV genetic diversity, AAV vectorology, and anti-HIV bnAbs and summarize the promises and challenges of the application of AAV in the delivery of bnAbs for HIV prevention and therapy.

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.

An Analysis of Mechanisms for Cellular Uptake of miRNAs to Enhance Drug Delivery and Efficacy in Cancer Chemoresistance

Up until recently, it was believed that pharmaceutical drugs and their metabolites enter into the cell to gain access to their targets via simple diffusion across the hydrophobic lipid cellular membrane, at a rate which is based on their lipophilicity. An increasing amount of evidence indicates that the phospholipid bilayer-mediated drug diffusion is in fact negligible, and that drugs pass through cell membranes via proteinaceous membrane transporters or carriers which are normally used for the transportation of nutrients and intermediate metabolites. Drugs can be targeted to specific cells and tissues which express the relevant transporters, leading to the design of safe and efficacious treatments. Furthermore, transporter expression levels can be manipulated, systematically and in a high-throughput manner, allowing for considerable progress in determining which transporters are used by specific drugs. The ever-expanding field of miRNA therapeutics is not without its challenges, with the most notable one being the safe and effective delivery of the miRNA mimic/antagonist safely to the target cell cytoplasm for attaining the desired clinical outcome, particularly in miRNA-based cancer therapeutics, due to the poor efficiency of neo-vascular systems revolting around the tumour site, brought about by tumour-induced angiogenesis. This acquisition of resistance to several types of anticancer drugs can be as a result of an upregulation of efflux transporters expression, which eject drugs from cells, hence lowering drug efficacy, resulting in multidrug resistance. In this article, the latest available data on human microRNAs has been reviewed, together with the most recently described mechanisms for miRNA uptake in cells, for future therapeutic enhancements against cancer chemoresistance.

Viral vector platforms within the gene therapy landscape

Throughout its 40-year history, the field of gene therapy has been marked by many transitions. It has seen great strides in combating human disease, has given hope to patients and families with limited treatment options, but has also been subject to many setbacks. Treatment of patients with this class of investigational drugs has resulted in severe adverse effects and, even in rare cases, death. At the heart of this dichotomous field are the viral-based vectors, the delivery vehicles that have allowed researchers and clinicians to develop powerful drug platforms, and have radically changed the face of medicine. Within the past 5 years, the gene therapy field has seen a wave of drugs based on viral vectors that have gained regulatory approval that come in a variety of designs and purposes. These modalities range from vector-based cancer therapies, to treating monogenic diseases with life-altering outcomes. At present, the three key vector strategies are based on adenoviruses, adeno-associated viruses, and lentiviruses. They have led the way in preclinical and clinical successes in the past two decades. However, despite these successes, many challenges still limit these approaches from attaining their full potential. To review the viral vector-based gene therapy landscape, we focus on these three highly regarded vector platforms and describe mechanisms of action and their roles in treating human disease.

The Challenge of Gene Therapy for Neurological Diseases: Strategies and Tools to Achieve Efficient Delivery to the Central Nervous System

For more than 10 years, gene therapy for neurological diseases has experienced intensive research growth and more recently therapeutic interventions for multiple indications. Beneficial results in several phase 1/2 clinical studies, together with improved vector technology have advanced gene therapy for the central nervous system (CNS) in a new era of development. Although most initial strategies have focused on orphan genetic diseases, such as lysosomal storage diseases, more complex and widespread conditions like Alzheimer's disease, Parkinson's disease, epilepsy, or chronic pain are increasingly targeted for gene therapy. Increasing numbers of applications and patients to be treated will require improvement and simplification of gene therapy protocols to make them accessible to the largest number of affected people. Although vectors and manufacturing are a major field of academic research and industrial development, there is a growing need to improve, standardize, and simplify delivery methods. Delivery is the major issue for CNS therapies in general, and particularly for gene therapy. The blood-brain barrier restricts the passage of vectors; strategies to bypass this obstacle are a central focus of research. In this study, we present the different ways that can be used to deliver gene therapy products to the CNS. We focus on results obtained in large animals that have allowed the transfer of protocols to human patients and have resulted in the generation of clinical data. We discuss the different routes of administration, their advantages, and their limitations. We describe techniques, equipment, and protocols and how they should be selected for safe delivery and improved efficiency for the next generation of gene therapy trials for CNS diseases.

Of rAAV and Men: From Genetic Neuromuscular Disorder Efficacy and Toxicity Preclinical Studies to Clinical Trials and Back

Neuromuscular disorders are a large group of rare pathologies characterised by skeletal muscle atrophy and weakness, with the common involvement of respiratory and/or cardiac muscles. These diseases lead to life-long motor deficiencies and specific organ failures, and are, in their worst-case scenarios, life threatening. Amongst other causes, they can be genetically inherited through mutations in more than 500 different genes. In the last 20 years, specific pharmacological treatments have been approved for human usage. However, these "à-la-carte" therapies cover only a very small portion of the clinical needs and are often partially efficient in alleviating the symptoms of the disease, even less so in curing it. Recombinant adeno-associated virus vector-mediated gene transfer is a more general strategy that could be adapted for a large majority of these diseases and has proved very efficient in rescuing the symptoms in many neuropathological animal models. On this solid ground, several clinical trials are currently being conducted with the whole-body delivery of the therapeutic vectors. This review recapitulates the state-of-the-art tools for neuron and muscle-targeted gene therapy, and summarises the main findings of the spinal muscular atrophy (SMA), Duchenne muscular dystrophy (DMD) and X-linked myotubular myopathy (XLMTM) trials. Despite promising efficacy results, serious adverse events of various severities were observed in these trials. Possible leads for second-generation products are also discussed.

AAV-Mediated ApoC2 Gene Therapy: Reversal of Severe Hypertriglyceridemia and Rescue of Neonatal Death in ApoC2-Deficient Hamsters

Apolipoprotein C2 (ApoC2) is a key activator of lipoprotein lipase for plasma triglyceride metabolism. ApoC2-deficient patients present with severe hypertriglyceridemia and recurrent acute pancreatitis, for whom the only effective treatment is the infusion of normal plasma containing ApoC2. However, since ApoC2 has a fast catabolic rate, a repeated infusion is required, which limits its clinical use. To explore a safe and efficient approach for ApoC2 deficiency, we herein established an adeno-associated virus expressing human ApoC2 (AAV-hApoC2) to evaluate the efficacy and safety of gene therapy in ApoC2-deficient hypertriglyceridemic hamsters. Administration of AAV-hApoC2 via jugular or orbital vein in adult and neonatal ApoC2-deficient hamsters, respectively, could prevent the neonatal death and effectively improve severe hypertriglyceridemia of ApoC2-deficient hamsters without side effects in a long-term manner. Our novel findings in the present study demonstrate that AAV-hApoC2-mediated gene therapy will be a promising therapeutic approach for clinical patients with severe hypertriglyceridemia caused by ApoC2 deficiency.

Liver-Directed but Not Muscle-Directed AAV-Antibody Gene Transfer Limits Humoral Immune Responses in Rhesus Monkeys

A number of publications have described the use of adeno-associated virus (AAV) for the delivery of anti-HIV and anti-simian immunodeficiency virus (SIV) monoclonal antibodies (mAbs) to rhesus monkeys. Anti-drug antibodies (ADAs) have been frequently observed, and long-term AAV-mediated delivery has been inconsistent. Here, we investigated different AAV vector strategies and delivery schemes to rhesus monkeys using the rhesus monkey mAb 4L6. We compared 4L6 immunoglobulin G1 (IgG1) delivery using the AAV1 versus the AAV8 serotype with a cytomegalovirus (CMV) promoter and the use of a muscle-specific versus a liver-specific promoter. Long-term expression levels of 4L6 IgG1 following AAV8-mediated gene transfer were comparable to those following AAV1-mediated gene transfer. AAV1-mediated gene transfer, using a muscle-specific promoter, showed robust ADAs and transiently low 4L6 IgG1 levels that ultimately declined to below detectable levels. Intravenous AAV8-mediated gene transfer, using a liver-specific promoter, also resulted in low levels of delivered 4L6 IgG1, but those low levels were maintained in the absence of any detectable ADAs. Booster injections using AAV1-CMV allowed for increased 4L6 IgG1 serum levels in animals that were primed with AAV8 but not with AAV1. Our results suggest that liver-directed expression may help to limit ADAs and that re-administration of AAV of a different serotype can result in successful long-term delivery of an immunogenic antibody.

Next-generation AAV vectors—do not judge a virus (only) by its cover

Recombinant adeno-associated viruses (AAV) are under intensive investigation in numerous clinical trials after they have emerged as a highly promising vector for human gene therapy. Best exemplifying their power and potential is the authorization of three gene therapy products based on wild-type AAV serotypes, comprising Glybera (AAV1), Luxturna (AAV2) and, most recently, Zolgensma (AAV9). Nonetheless, it has also become evident that the current AAV vector generation will require improvements in transduction potency, antibody evasion and cell/tissue specificity to allow the use of lower and safer vector doses. To this end, others and we devoted substantial previous research to the implementation and application of key technologies for engineering of next-generation viral capsids in a high-throughput ‘top-down’ or (semi-)rational ‘bottom-up’ approach. Here, we describe a set of recent complementary strategies to enhance features of AAV vectors that act on the level of the recombinant cargo. As examples that illustrate the innovative and synergistic concepts that have been reported lately, we highlight (i) novel synthetic enhancers/promoters that provide an unprecedented degree of AAV tissue specificity, (ii) pioneering genetic circuit designs that harness biological (microRNAs) or physical (light) triggers as regulators of AAV gene expression and (iii) new insights into the role of AAV DNA structures on vector genome stability, integrity and functionality. Combined with ongoing capsid engineering and selection efforts, these and other state-of-the-art innovations and investigations promise to accelerate the arrival of the next generation of AAV vectors and to solidify the unique role of this exciting virus in human gene therapy.

Circumventing cellular immunity by miR142-mediated regulation sufficiently supports rAAV-delivered OVA expression without activating humoral immunity

Recombinant adeno-associated virus (rAAV)-mediated gene delivery can efficiently target muscle tissues to serve as "biofactories" for secreted proteins in prophylactic and therapeutic scenarios. Nevertheless, efficient rAAV-mediated gene delivery is often limited by host immune responses against the transgene product. The development of strategies to prevent anti-transgene immunity is therefore crucial. The employment of endogenous microRNA (miRNA)-mediated regulation to detarget transgene expression from antigen presenting cells (APCs) has shown promise for reducing immunogenicity. However, the mechanisms underlying miRNA-mediated modulation of anti-transgene immunity by APC detargeting are not fully understood. Using the highly immunogenic ovalbumin (OVA) protein as a proxy for foreign antigens, we show that rAAV vectors containing miR142 binding sites efficiently repress co-stimulatory signals in dendritic cells, significantly blunt the cytotoxic T cell response, allow for sustained transgene expression in skeletal myoblasts, and attenuate clearance of transduced muscle cells in mice. Furthermore, the blunting of humoral immunity against circulating OVA correlates with detargeting of OVA expression from APCs. This demonstrates that incorporating APC-specific miRNA binding sites into rAAV vectors provides an effective strategy for reducing transgene-specific immune response. This approach holds promise for clinical applications where the safe and efficient delivery of a prophylactic or therapeutic protein is desired.

Adeno-associated virus vector as a platform for gene therapy delivery

Adeno-associated virus (AAV) vectors are the leading platform for gene delivery for the treatment of a variety of human diseases. Recent advances in developing clinically desirable AAV capsids, optimizing genome designs and harnessing revolutionary biotechnologies have contributed substantially to the growth of the gene therapy field. Preclinical and clinical successes in AAV-mediated gene replacement, gene silencing and gene editing have helped AAV gain popularity as the ideal therapeutic vector, with two AAV-based therapeutics gaining regulatory approval in Europe or the United States. Continued study of AAV biology and increased understanding of the associated therapeutic challenges and limitations will build the foundation for future clinical success.

Adeno-Associated Virus Delivery of Anti-HIV Monoclonal Antibodies Can Drive Long-Term Virologic Suppression

Long-term delivery of anti-HIV monoclonal antibodies (mAbs) using adeno-associated virus (AAV) vectors holds promise for the prevention and treatment of HIV infection. We describe a therapy trial in which four rhesus monkeys were infected with SHIV-AD8 for 86 weeks before receiving the AAV-encoded mAbs 3BNC117, 10-1074, and 10E8. Although anti-drug antibody (ADA) responses restricted mAb delivery, one monkey successfully maintained 50-150 μg/mL of 3BNC117 and 10-1074 for over 2 years. Delivery of these two mAbs to this monkey resulted in an abrupt decline in plasma viremia, which remained undetectable for 38 successive measurements over 3 years. We generated two more examples of virologic suppression using AAV delivery of a cocktail of four mAbs in a 12-monkey study. Our results provide proof of concept for AAV-delivered mAbs to produce a "functional cure." However, they also serve as a warning that ADAs may be a problem for practical application of this approach in humans.

Intravenous Infusion of AAV for Widespread Gene Delivery to the Nervous System

The central nervous system (CNS) is a fascinating and intricate set of biological structures that we have yet to fully understand. Studying the in vivo function of the CNS and finding novel methods for treating neurological disorders have been particularly challenging. One difficulty is correcting genetic disorders afflicting the CNS in a targeted manner. Recombinant adeno-associated viruses (rAAVs) have emerged as promising therapeutic tools for treating genetic defects of the CNS, due to their excellent safety profile and ability to cross the blood-brain barrier (BBB). While stereotactic injection of AAV is promising for localized gene delivery, it is less desirable for some applications because of the technique's invasiveness and limited intraparenchymal spread. Alternatively, intravascular administration can achieve widespread delivery of rAAV to the CNS. In this chapter, we will discuss the prevalent routes of administration to deliver rAAV to the CNS via intravenous (IV) injection in mice. We will highlight key considerations for using rAAV, and the advantages and disadvantages of each administration method. We will also briefly discuss intravenous delivery in larger animal models, factors that may impact experimental interpretations, and outlooks for clinical translation.

Systemic AAV Micro-dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin gene mutation. Conceptually, replacing the mutated gene with a normal one would cure the disease. However, this task has encountered significant challenges due to the enormous size of the gene and the distribution of muscle throughout the body. The former creates a hurdle for viral vector packaging and the latter begs for whole-body therapy. To address these obstacles, investigators have invented the highly abbreviated micro-dystrophin gene and developed body-wide systemic gene transfer with adeno-associated virus (AAV). Numerous microgene configurations and various AAV serotypes have been explored in animal models in many laboratories. Preclinical data suggests that intravascular AAV micro-dystrophin delivery can significantly ameliorate muscle pathology, enhance muscle force, and attenuate dystrophic cardiomyopathy in animals. Against this backdrop, several clinical trials have been initiated to test the safety and tolerability of this promising therapy in DMD patients. While these trials are not powered to reach a conclusion on clinical efficacy, findings will inform the field on the prospects of body-wide DMD therapy with a synthetic micro-dystrophin AAV vector. This review discusses the history, current status, and future directions of systemic AAV micro-dystrophin therapy.

Improving miRNA Delivery by Optimizing miRNA Expression Cassettes in Diverse Virus Vectors

The RNA interference pathway is an evolutionary conserved post-transcriptional gene regulation mechanism that is exclusively triggered by double-stranded RNA inducers. RNAi-based methods and technologies have facilitated the discovery of many basic science findings and spurred the development of novel RNA therapeutics. Transient induction of RNAi via transfection of synthetic small interfering RNAs can trigger the selective knockdown of a target mRNA. For durable silencing of gene expression, either artificial short hairpin RNA or microRNA encoding transgene constructs were developed. These miRNAs are based on the molecules that induce the natural RNAi pathway in mammals and humans: the endogenously expressed miRNAs. Significant efforts focused on the construction and delivery of miRNA cassettes in order to solve basic biology questions or to design new therapy strategies. Several viral vectors have been developed, which are particularly useful for the delivery of miRNA expression cassettes to specific target cells. Each vector system has its own unique set of distinct properties. Thus, depending on the specific application, a particular vector may be most suitable. This field was previously reviewed for different viral vector systems, and now the recent progress in the field of miRNA-based gene-silencing approaches using lentiviral vectors is reported. The focus is on the unique properties and respective limitations of the available vector systems for miRNA delivery.

Engineering antibody-like inhibitors to prevent and treat HIV-1 infection

Purpose of review

Here we discuss recently developed HIV-1 entry inhibitors that can target multiple epitopes on the HIV-1 envelope glycoprotein (Env), with an emphasis on eCD4-Ig. Some of these inhibitors are more potent and broader than any single antibody characterized to date. We also discuss the use of recombinant adeno-associated virus (rAAV) vectors as a platform for long-term expression of these inhibitors.

Recent findings

Much of the exterior of HIV-1 Env can be targeted by broadly neutralizing antibodies (bNAbs). Recent studies combine the variable regions or Fabs from different bNAbs, often with the receptor-mimetic components, to create broad, potent, and hard-to-escape inhibitors. rAAV vectors can express these inhibitors for years in vivo, highlighting their ability to prevent or treat HIV-1 infection.

Summary

By targeting multiple epitopes on Env, bispecific and antibody-like inhibitors can be broader and more potent than bNAbs. These inhibitors can provide long-term protection from, and perhaps suppression of, HIV-1 if they are administered by a delivery platform, like rAAV vectors, but only after rAAV limitations are addressed.

Complexity of immune responses to AAV transgene products - Example of factor IX

After two decades of research, in vivo gene transfer with adeno-associated viral (AAV) vectors has now resulted in successful treatments and even cures for several human diseases. However, the potential for immune responses against the therapeutic gene products remains one of the concerns as this approach is broadened to more patients, diverse diseases, and target organs. Immune responses following gene transfer of coagulation factor IX (FIX) for the treatment of the bleeding disorder hemophilia B has been extensively investigated in multiple animal models. Findings from these studies have not only influenced clinical trial design but have broader implications for other diseases. The impact of vector design and dose, as well as target organ/route of administration on humoral and cellular immune responses are reviewed. Furthermore, the potential for tolerance induction by hepatic gene transfer or combination with immune modulation is discussed.

Promise and problems associated with the use of recombinant AAV for the delivery of anti-HIV antibodies

Attempts to elicit antibodies with potent neutralizing activity against a broad range of human immunodeficiency virus (HIV) isolates have so far proven unsuccessful. Long-term delivery of monoclonal antibodies (mAbs) with such activity is a creative alternative that circumvents the need for an immune response and has the potential for creating a long-lasting sterilizing barrier against HIV. This approach is made possible by an incredible array of potent broadly neutralizing antibodies (bnAbs) that have been identified over the last several years. Recombinant adeno-associated virus (rAAV) vectors are ideally suited for long-term delivery for a variety of reasons. The only products made from rAAV are derived from the transgenes that are put into it; as long as those products are not viewed as foreign, expression from muscle tissue may continue for decades. Thus, use of rAAV to achieve long-term delivery of anti-HIV mAbs with potent neutralizing activity against a broad range of HIV-1 isolates is emerging as a promising concept for the prevention or treatment of HIV-1 infection in humans. Experiments in mice and monkeys that have demonstrated protective efficacy against AIDS virus infection have raised hopes for the promise of this approach. However, all published experiments in monkeys have encountered unwanted immune responses to the AAV-delivered antibody, and these immune responses appear to limit the levels of delivered antibody that can be achieved. In this review, we highlight the promise of rAAV-mediated antibody delivery for the prevention or treatment of HIV infection in humans, but we also discuss the obstacles that will need to be understood and solved in order for the promise of this approach to be realized.

Methods for Optimizing CRISPR-Cas9 Genome Editing Specificity

Advances in the development of delivery, repair, and specificity strategies for the CRISPR-Cas9 genome engineering toolbox are helping researchers understand gene function with unprecedented precision and sensitivity. CRISPR-Cas9 also holds enormous therapeutic potential for the treatment of genetic disorders by directly correcting disease-causing mutations. Although the Cas9 protein has been shown to bind and cleave DNA at off-target sites, the field of Cas9 specificity is rapidly progressing, with marked improvements in guide RNA selection, protein and guide engineering, novel enzymes, and off-target detection methods. We review important challenges and breakthroughs in the field as a comprehensive practical guide to interested users of genome editing technologies, highlighting key tools and strategies for optimizing specificity. The genome editing community should now strive to standardize such methods for measuring and reporting off-target activity, while keeping in mind that the goal for specificity should be continued improvement and vigilance.

Adeno-Associated Virus-Based Gene Therapy for CNS Diseases

Gene therapy is at the cusp of a revolution for treating a large spectrum of CNS disorders by providing a durable therapeutic protein via a single administration. Adeno-associated virus (AAV)-mediated gene transfer is of particular interest as a therapeutic tool because of its safety profile and efficiency in transducing a wide range of cell types. The purpose of this review is to describe the most notable advancements in preclinical and clinical research on AAV-based CNS gene therapy and to discuss prospects for future development based on a new generation of vectors and delivery.

MicroRNA-regulated viral vectors for gene therapy

Safe and effective gene therapy approaches require targeted tissue-specific transfer of a therapeutic transgene. Besides traditional approaches, such as transcriptional and transductional targeting, microRNA-dependent post-transcriptional suppression of transgene expression has been emerging as powerful new technology to increase the specificity of vector-mediated transgene expression. MicroRNAs are small non-coding RNAs and often expressed in a tissue-, lineage-, activation- or differentiation-specific pattern. They typically regulate gene expression by binding to imperfectly complementary sequences in the 3' untranslated region (UTR) of the mRNA. To control exogenous transgene expression, tandem repeats of artificial microRNA target sites are usually incorporated into the 3' UTR of the transgene expression cassette, leading to subsequent degradation of transgene mRNA in cells expressing the corresponding microRNA. This targeting strategy, first shown for lentiviral vectors in antigen presenting cells, has now been used for tissue-specific expression of vector-encoded therapeutic transgenes, to reduce immune response against the transgene, to control virus tropism for oncolytic virotherapy, to increase safety of live attenuated virus vaccines and to identify and select cell subsets for pluripotent stem cell therapies, respectively. This review provides an introduction into the technical mechanism underlying microRNA-regulation, highlights new developments in this field and gives an overview of applications of microRNA-regulated viral vectors for cardiac, suicide gene cancer and hematopoietic stem cell therapy, as well as for treatment of neurological and eye diseases.

Engineered AAV vectors for improved central nervous system gene delivery

Adeno-associated viruses (AAV) are non-pathogenic members of the Parvoviridae family that are being harnessed as delivery vehicles for both basic research and increasingly successful clinical gene therapy. To address a number of delivery shortcomings with natural AAV variants, we have developed and implemented directed evolution—a high-throughput molecular engineering approach to generate novel biomolecules with enhanced function—to create novel AAV vectors that are designed to preferentially transduce specific cell types in the central nervous system (CNS), including astrocytes, neural stem cells, and cells within the retina. These novel AAV vectors—which have enhanced infectivity in vitro and enhanced infectivity and selectivity in vivo—can enable more efficient studies to further our understanding of neurogenesis, development, aging, and disease. Furthermore, such engineered vectors may aid gene or cell replacement therapies to treat neurodegenerative disease or injury.

Early interaction of adeno-associated virus serotype 8 vector with the host immune system following intramuscular delivery results in weak but detectable lymphocyte and dendritic cell transduction

Following in vivo recombinant adeno-associated virus (rAAV)-based gene transfer, adaptive immune responses specific to the vector or the transgene product have emerged as a potential roadblock to successful clinical translation. The occurrence of such responses depends on several parameters, including the route of vector administration as well as the viral serotype and the genome configuration, either self-complementary (sc) or single-stranded (ss). These parameters influence rAAV vector-associated immunity by modulating the crosstalk between the vector and the host immune system, including vector ability to interact or even transduce lymphoid tissues in general and antigen-presenting cells (APCs) in particular. Little is known about immune cell populations that are targeted in vivo by rAAV vectors. Moreover, the transduction of dendritic cells is still controversial and not directly demonstrated. Here, we show that intramuscular administration of an sc rAAV8 vector in the mouse leads to a rapid distribution of viral genomes in the lymphoid tissues that is associated with transgene expression. Transduced cells were detected in follicular areas of the spleen and the draining lymph nodes. In addition to B and T lymphocytes, transduced professional APCs were detected although at very low frequency. In addition, viral genomes and transgene transcripts were also detected in these cell populations after ss rAAV8 vector administration. Although the functional significance of those observations needs further explorations, our results highlight an early and intricate interaction between the rAAV vector upon its in vivo delivery and the host immune system.

MicroRNAs in skin tissue engineering

35.2 million annual cases in the U.S. require clinical intervention for major skin loss. To meet this demand, the field of skin tissue engineering has grown rapidly over the past 40 years. Traditionally, skin tissue engineering relies on the "cell-scaffold-signal" approach, whereby isolated cells are formulated into a three-dimensional substrate matrix, or scaffold, and exposed to the proper molecular, physical, and/or electrical signals to encourage growth and differentiation. However, clinically available bioengineered skin equivalents (BSEs) suffer from a number of drawbacks, including time required to generate autologous BSEs, poor allogeneic BSE survival, and physical limitations such as mass transfer issues. Additionally, different types of skin wounds require different BSE designs. MicroRNA has recently emerged as a new and exciting field of RNA interference that can overcome the barriers of BSE design. MicroRNA can regulate cellular behavior, change the bioactive milieu of the skin, and be delivered to skin tissue in a number of ways. While it is still in its infancy, the use of microRNAs in skin tissue engineering offers the opportunity to both enhance and expand a field for which there is still a vast unmet clinical need. Here we give a review of skin tissue engineering, focusing on the important cellular processes, bioactive mediators, and scaffolds. We further discuss potential microRNA targets for each individual component, and we conclude with possible future applications.

The Skeletal Muscle Environment and Its Role in Immunity and Tolerance to AAV Vector-Mediated Gene Transfer

Since the early days of gene therapy, muscle has been one the most studied tissue targets for the correction of enzyme deficiencies and myopathies. Several preclinical and clinical studies have been conducted using adeno-associated virus (AAV) vectors. Exciting progress has been made in the gene delivery technologies, from the identification of novel AAV serotypes to the development of novel vector delivery techniques. In parallel, significant knowledge has been generated on the host immune system and its interaction with both the vector and the transgene at the muscle level. In particular, the role of underlying muscle inflammation, characteristic of several diseases affecting the muscle, has been defined in terms of its potential detrimental impact on gene transfer with AAV vectors. At the same time, feedback immunomodulatory mechanisms peculiar of skeletal muscle involving resident regulatory T cells have been identified, which seem to play an important role in maintaining, at least to some extent, muscle homeostasis during inflammation and regenerative processes. Devising strategies to tip this balance towards unresponsiveness may represent an avenue to improve the safety and efficacy of muscle gene transfer with AAV vectors.

AAV-Mediated Gene Therapy for Research and Therapeutic Purposes

Adeno-associated virus (AAV) is a small, nonenveloped virus that was adapted 30 years ago for use as a gene transfer vehicle. It is capable of transducing a wide range of species and tissues in vivo with no evidence of toxicity, and it generates relatively mild innate and adaptive immune responses. We review the basic biology of AAV, the history of progress in AAV vector technology, and some of the clinical and research applications where AAV has shown success.

A dystrophic muscle broadens the contribution and activation of immune cells reacting to rAAV gene transfer

Recombinant adeno-associated viral vectors (rAAVs) are used for therapeutic gene transfer in skeletal muscle, but it is unclear if immune reactivity to gene transfer and persistence of transgene are affected by pathologic conditions such as muscular dystrophy. Thus, we compared dystrophic mice devoid of α-sarcoglycan with healthy mice to characterize immune cell activation and cellular populations contributing to the loss of gene-modified myofibers. Following rAAV2/1 delivery of an immunogenic α-sarcoglycan reporter transgene in the muscle, both strains developed strong CD4 and CD8 T-cell-mediated immune responses in lymphoid organs associated with muscle CD3+ T and CD11b+ mononuclear cell infiltrates. Selective cell subset depletion models revealed that CD4+ T cells were essential for transgene rejection in both healthy and pathologic mice, but macrophages and CD8+ T cells additionally contributed as effector cells of transgene rejection only in dystrophic mice. Vectors restricting transgene expression in antigen-presenting cells showed that endogenous presentation of transgene products was the sole mechanism responsible for T-cell priming in normal mice, whereas additional and protracted antigenic presentation occurred in dystrophic animals, leading to secondary CD4+ T-cell activation and failure to maintain transgene expression. Therefore, the dystrophic environment diversifies cellular immune response mechanisms induced by gene transfer, with a negative outcome.

The potential of adeno-associated viral vectors for gene delivery to muscle tissue

INTRODUCTION

Muscle-directed gene therapy is rapidly gaining attention primarily because muscle is an easily accessible target tissue and is also associated with various severe genetic disorders. Localized and systemic delivery of recombinant adeno-associated virus (rAAV) vectors of several serotypes results in very efficient transduction of skeletal and cardiac muscles, which has been achieved in both small and large animals, as well as in humans. Muscle is the target tissue in gene therapy for many muscular dystrophy diseases, and may also be exploited as a biofactory to produce secretory factors for systemic disorders. Current limitations of using rAAVs for muscle gene transfer include vector size restriction, potential safety concerns such as off-target toxicity and the immunological barrier composing of pre-existing neutralizing antibodies and CD8(+) T-cell response against AAV capsid in humans.

AREAS COVERED

In this article, we will discuss basic AAV vector biology and its application in muscle-directed gene delivery, as well as potential strategies to overcome the aforementioned limitations of rAAV for further clinical application.

EXPERT OPINION

Delivering therapeutic genes to large muscle mass in humans is arguably the most urgent unmet demand in treating diseases affecting muscle tissues throughout the whole body. Muscle-directed, rAAV-mediated gene transfer for expressing antibodies is a promising strategy to combat deadly infectious diseases. Developing strategies to circumvent the immune response following rAAV administration in humans will facilitate clinical application.

Recombinant AAV as a platform for translating the therapeutic potential of RNA interference

RNA interference has become a ubiquitous biological tool, and is being harnessed for therapeutic purposes as well. Therapeutic posttranscriptional gene silencing takes advantage of the endogenous RNAi pathway through delivery of either chemically synthesized siRNAs, or transgenes expressing hairpin-based inhibitory RNAs (e.g., shRNAs and artificial miRNAs). RNAi has expanded the field of viral gene therapy from gene replacement to gene knockdown. Here, we review various noncoding RNAs such as shRNAs, miRNAs, and miRNA decoys which can be utilized for therapeutic applications when expressed from recombinant adeno-associated vectors (AAV), and present examples of their basic design. In addition the basis of exploiting cellular miRNA profiles for detargeting AAV expression from specific cells is described. Finally, an overview of AAV-mediated RNAi preclinical studies is presented, and current RNAi-based clinical trials are reviewed.