Comparison of gene transfer to the murine liver following intraperitoneal and intraportal delivery of hepatotropic AAV pseudo-serotypes

Integrated vector genomes may contribute to long-term expression in primate liver after AAV administration

The development of liver-based adeno-associated virus (AAV) gene therapies is facing concerns about limited efficiency and durability of transgene expression. We evaluated nonhuman primates following intravenous dosing of AAV8 and AAVrh10 vectors for over 2 years to better define the mechanism(s) of transduction that affect performance. High transduction of non-immunogenic transgenes was achieved, although expression declined over the first 90 days to reach a lower but stable steady state. More than 10% of hepatocytes contained single nuclear domains of vector DNA that persisted despite the loss of transgene expression. Greater reductions in vector DNA and RNA were observed with immunogenic transgenes. Genomic integration of vector sequences, including complex concatemeric structures, were detected in 1 out of 100 cells at broadly distributed loci that were not in proximity to genes associated with hepatocellular carcinoma. Our studies suggest that AAV-mediated transgene expression in primate hepatocytes occurs in two phases: high but short-lived expression from episomal genomes, followed by much lower but stable expression, likely from integrated vectors.

AAV Capsid Screening for Translational Pig Research Using a Mouse Xenograft Liver Model

In gene therapy, delivery vectors are a key component for successful gene delivery and safety, based on which adeno-associated viruses (AAVs) gained popularity in particular for the liver, but also for other organs. Traditionally, rodents have been used as animal models to develop and optimize treatments, but species and organ specific tropism of AAV desire large animal models more closely related to humans for preclinical in-depth studies. Relevant AAV variants with the potential for clinical translation in liver gene therapy were previously evolved in vivo in a xenogeneic mouse model transplanted with human hepatocytes. Here, we selected and evaluated efficient AAV capsids using chimeric mice with a >90% xenografted pig hepatocytes. The pig is a valuable preclinical model for therapy studies due to its anatomic and immunological similarities to humans. Using a DNA-barcoded recombinant AAV library containing 47 different capsids and subsequent Illumina sequencing of barcodes in the AAV vector genome DNA and transcripts in the porcine hepatocytes, we found the AAVLK03 and AAVrh20 capsid to be the most efficient delivery vectors regarding transgene expression in porcine hepatocytes. In attempting to validate these findings with primary porcine hepatocytes, we observed capsid-specific differences in cell entry and transgene expression efficiency where the AAV2, AAVAnc80, and AAVDJ capsids showed superior efficiency to AAVLK03 and AAVrh20. This work highlights intricacies of in vitro testing with primary hepatocytes and the requirements for suitable pre-clinical animal models but suggests the chimeric mouse to be a valuable model to predict AAV capsids to transduce porcine hepatocytes efficiently.

Adeno-associated virus as a delivery vector for gene therapy of human diseases

Adeno-associated virus (AAV) has emerged as a pivotal delivery tool in clinical gene therapy owing to its minimal pathogenicity and ability to establish long-term gene expression in different tissues. Recombinant AAV (rAAV) has been engineered for enhanced specificity and developed as a tool for treating various diseases. However, as rAAV is being more widely used as a therapy, the increased demand has created challenges for the existing manufacturing methods. Seven rAAV-based gene therapy products have received regulatory approval, but there continue to be concerns about safely using high-dose viral therapies in humans, including immune responses and adverse effects such as genotoxicity, hepatotoxicity, thrombotic microangiopathy, and neurotoxicity. In this review, we explore AAV biology with an emphasis on current vector engineering strategies and manufacturing technologies. We discuss how rAAVs are being employed in ongoing clinical trials for ocular, neurological, metabolic, hematological, neuromuscular, and cardiovascular diseases as well as cancers. We outline immune responses triggered by rAAV, address associated side effects, and discuss strategies to mitigate these reactions. We hope that discussing recent advancements and current challenges in the field will be a helpful guide for researchers and clinicians navigating the ever-evolving landscape of rAAV-based gene therapy.

The mitochondrial fusion protein OPA1 is dispensable in the liver and its absence induces mitohormesis to protect liver from drug-induced injury

Mitochondria are critical for metabolic homeostasis of the liver, and their dysfunction is a major cause of liver diseases. Optic atrophy 1 (OPA1) is a mitochondrial fusion protein with a role in cristae shaping. Disruption of OPA1 causes mitochondrial dysfunction. However, the role of OPA1 in liver function is poorly understood. In this study, we delete OPA1 in the fully developed liver of male mice. Unexpectedly, OPA1 liver knockout (LKO) mice are healthy with unaffected mitochondrial respiration, despite disrupted cristae morphology. OPA1 LKO induces a stress response that establishes a new homeostatic state for sustained liver function. Our data show that OPA1 is required for proper complex V assembly and that OPA1 LKO protects the liver from drug toxicity. Mechanistically, OPA1 LKO decreases toxic drug metabolism and confers resistance to the mitochondrial permeability transition. This study demonstrates that OPA1 is dispensable in the liver, and that the mitohormesis induced by OPA1 LKO prevents liver injury and contributes to liver resiliency.

AAV-mediated delivery of secreted acid α-glucosidase with enhanced uptake corrects neuromuscular pathology in Pompe mice

Gene therapy is under advanced clinical development for several lysosomal storage disorders. Pompe disease, a debilitating neuromuscular illness affecting infants, children, and adults with different severity, is caused by a deficiency of lysosomal glycogen-degrading enzyme acid α-glucosidase (GAA). Here, we demonstrated that adeno-associated virus-mediated (AAV-mediated) systemic gene transfer reversed glycogen storage in all key therapeutic targets - skeletal and cardiac muscles, the diaphragm, and the central nervous system - in both young and severely affected old Gaa-knockout mice. Furthermore, the therapy reversed secondary cellular abnormalities in skeletal muscle, such as those in autophagy and mTORC1/AMPK signaling. We used an AAV9 vector encoding a chimeric human GAA protein with enhanced uptake and secretion to facilitate efficient spread of the expressed protein among multiple target tissues. These results lay the groundwork for a future clinical development strategy in Pompe disease.

Virally induced CRISPR/Cas9-based knock-in of fluorescent albumin allows long-term visualization of cerebral circulation in infant and adult mice

Albumin, a protein produced by liver hepatocytes, represents the most abundant protein in blood plasma. We have previously engineered a liver-targeting adeno-associated viral vector (AAV) that expresses fluorescent protein-tagged albumin to visualize blood plasma in mice. While this approach is versatile for imaging in adult mice, transgene expression vanishes when AAV is administered in neonates due to dilution of the episomal AAV genome in the rapidly growing liver. Here, we use CRISPR/Cas9 genome editing to insert the fluorescent protein mNeonGreen (mNG) gene into the albumin (Alb) locus of hepatocytes to produce fluorescently labeled albumin (Alb-mNG). We constructed a CRISPR AAV that includes ∼1 kb homologous arms around Alb exon 14 to express Alb-mNG. Subcutaneous injection of this AAV with AAV-CMV-Cas9 in postnatal day 3 mice resulted in two-photon visualization of the cerebral cortex vasculature within ten days. The expression levels of Alb-mNG were persistent for at least three months and were so robust that vasomotion and capillary blood flow could be assessed transcranially in early postnatal mice. This knock-in approach provides powerful means for micro- and macroscopic imaging of cerebral vascular dynamics in postnatal and adult mice.

Structural and functional characterization of capsid binding by anti-AAV9 monoclonal antibodies from infants after SMA gene therapy

Success in the treatment of infants with spinal muscular atrophy (SMA) underscores the potential of vectors based on adeno-associated virus (AAV). However, a major obstacle to the full realization of this potential is pre-existing natural and therapy-induced anti-capsid humoral immunity. Structure-guided capsid engineering is one possible approach to surmounting this challenge but necessitates an understanding of capsid-antibody interactions at high molecular resolution. Currently, only mouse-derived monoclonal antibodies (mAbs) are available to structurally map these interactions, which presupposes that mouse and human-derived antibodies are functionally equivalent. In this study, we have characterized the polyclonal antibody responses of infants following AAV9-mediated gene therapy for SMA and recovered 35 anti-capsid mAbs from the abundance of switched-memory B (smB) cells present in these infants. For 21 of these mAbs, seven from each of three infants, we have undertaken functional and structural analysis measuring neutralization, affinities, and binding patterns by cryoelectron microscopy (cryo-EM). Four distinct patterns were observed akin to those reported for mouse-derived mAbs, but with early evidence of differing binding pattern preference and underlying molecular interactions. This is the first human and largest series of anti-capsid mAbs to have been comprehensively characterized and will prove to be powerful tools for basic discovery and applied purposes.

Effects of Human LAV-BPIFB4 Gene Therapy on the Epigenetic Clock and Health of Aged Mice

The homozygous genotype of the Longevity-Associated Variant (LAV) in Bactericidal/Permeability-Increasing Fold-Containing Family B member 4 (BPIFB4) is enriched in long-living individuals of three independent populations and its genetic transfer in C57BL/6J mice showed a delay in frailty progression and improvement of several biomarkers of aging and multiple aspects of health. The C57BL/6J strain is a suitable model for studying therapies aimed at extending healthy aging and longevity due to its relatively short lifespan and the availability of aging biomarkers. Epigenetic clocks based on DNA methylation profiles are reliable molecular biomarkers of aging, while frailty measurement tools are used to evaluate overall health during aging. In this study, we show that the systemic gene transfer of LAV-BPIFB4 in aged C57BL/6J mice was associated with a significant reduction in the epigenetic clock-based biological age, as measured by a three CpG clock method. Furthermore, LAV-BPIFB4 gene transfer resulted in an improvement of the Vitality Score with a reduction in the Frailty Index. These findings further support the use of LAV-BPIFB4 gene therapy to induce beneficial effects on epigenetic mechanisms associated with aging and frailty in aged mice, with potential implications for future therapies to prevent frailty in humans.

Characterization of the humanized FRG mouse model and development of an AAV-LK03 variant with improved liver lobular biodistribution

Recent clinical successes have intensified interest in using adeno-associated virus (AAV) vectors for therapeutic gene delivery. The liver is a key clinical target, given its critical physiological functions and involvement in a wide range of genetic diseases. In the present study, we first investigated the validity of a liver xenograft mouse model repopulated with primary hepatocytes using single-nucleus RNA sequencing (sn-RNA-seq) by studying the transcriptomic profile of human hepatocytes pre- and post-engraftment. Complementary immunofluorescence analyses performed in highly engrafted animals confirmed that the human hepatocytes organize and present appropriate patterns of zone-dependent enzyme expression in this model. Next, we tested a set of rationally designed HSPG de-targeted AAV-LK03 variants for relative transduction performance in human hepatocytes. We used immunofluorescence, next-generation sequencing, and single-nucleus transcriptomics data from highly engrafted FRG mice to demonstrate that the optimally HSPG de-targeted AAV-LK03 displayed a significantly improved lobular transduction profile in this model.

Safety of Intravenous Administration of an AAV8 Vector Coding for an Oxidation-Resistant Human α1-Antitrypsin for the Treatment of α1-Antitrypsin Deficiency

α1-antitrypsin (AAT) deficiency is a common autosomal recessive hereditary disorder, with a high risk for the development of early-onset panacinar emphysema. AAT, produced primarily in the liver, functions to protect the lung from neutrophil protease; with AAT deficiency, unimpeded neutrophil proteases destroy the lung parenchyma. AAT is susceptible to oxidative damage resulting in an inability to inhibit its target proteases, neutrophil elastase, and cathepsin G. The major sites of oxidative modification on the AAT molecule are methionine residues 351 and 358. We have previously demonstrated that an engineered variant of AAT that resists oxidation by modifying both protein surface methionines (M351V and M358L) provides antiprotease protection, despite oxidative stress. In mice, intravenous delivery of the modified AAT(AVL) variant by AAV serotype 8, AAV8hAAT(AVL), primarily to the liver resulted in long-term expression of an AAT that resists oxidative inactivation. In this study, we evaluated the safety of intravenous administration of AAV8hAAT(AVL) in a dose-escalating, blinded, placebo-controlled toxicology study in wild-type mice. The study assessed organ histology and clinical pathology findings of mice, intravenously administered AAV8hAAT(AVL) at three doses (5.0 × 1011, 5.0 × 1012, and 5.0 × 1013 genome copies [gc]/kg), compared to control mice injected intravenously with phosphate-buffered saline. As previously demonstrated, administration of AAV8hAAT(AVL) resulted in dose-dependent expression of high, potentially therapeutic, levels of serum human AAT protein that persist for at least 6 months. Antibodies against the AAV8 capsid were elicited as expected, but there was no antibody detected against the AAT(AVL) protein generated by the AAV8hAAT(AVL) vector. There was no morbidity or mortality observed in the study. The data demonstrate that intravenous administration of AAV8hAAT(AVL) is safe with no significant adverse effect attributed to AAV8hAAT(AVL) vector at any dose. This study demonstrates that AAV8hAAT(AVL) has a safety profile consistent with the requirements for proceeding to a clinical study.

Adeno-associated virus-based caveolin-1 delivery via different routes for the prevention of cholesterol gallstone formation

BACKGROUND

Hepatic caveolin-1 (CAV1) is reduced in cholesterol gallstone disease (CGD). Mice with CAV1 deficiency were prone to develop CGD. However, it remains unknown whether restored hepatic CAV1 expression prevents the development of CGD.

METHODS

C57BL/6 mice were injected with adeno-associated virus 2/8 (AAV2/8) vectors carrying the CAV1 gene (^AAV2/8CAV1) via intravenous (i.v.) or intraperitoneal (i.p.) route and then subjected to a lithogenic diet (LD) for 8 weeks. Uninjected mice were used as controls. The functional consequences of rescuing CAV1 expression by either i.v. or i.p. ^AAV2/8CAV1 treatment for CGD prevention and its subsequent molecular mechanisms were examined.

RESULTS

CAV1 expression was reduced in the liver and gallbladder of LD-fed CGD mice. We discovered that ^AAV2/8CAV1 i.p. delivery results in higher transduction efficiency in the gallbladder than tail vein administration. Although either i.v. or i.p. injection of ^AAV2/8CAV1 improved liver lipid metabolic abnormalities in CGD mice but did not affect LD feeding-induced bile cholesterol supersaturation. In comparison with i.v. administration route, i.p. administration of ^AAV2/8CAV1 obviously increased CAV1 protein levels in the gallbladder of LD-fed mice, and i.p. delivery of ^AAV2/8CAV1 partially improved gallbladder cholecystokinin receptor (CCKAR) responsiveness and impeded bile cholesterol nucleation via the activation of adenosine monophosphate-activated protein kinase (AMPK) signaling, which induced a reduction in gallbladder mucin-1 (MUC1) and MUC5ac expression and gallbladder cholesterol accumulation.

CONCLUSION

CGD prevention by i.p. ^AAV2/8CAV1 injection in LD-fed mice was associated with the improvement of gallbladder stasis, which again supported the notion that supersaturated bile is required but not sufficient for the formation of cholesterol gallstones. Additionally, AAV treatment via the local i.p. injection offers particular advantages over the systemic i.v. route for much more effective gallbladder gene delivery, which will be an excellent tool for conducting preclinical functional studies on the maintenance of normal gallbladder function to prevent CGD.

Vector engineering, strategies and targets in cancer gene therapy

Understanding the molecular basis of disease and the design of rationally designed molecular therapies has been the holy grail in the management of human cancers. Gene-based therapies are an important avenue for achieving a possible cure. Focused research in the last three decades has provided significant clues to optimize the potential of cancer gene therapy. The development of gene therapies with a high potential to kill the target cells at the lowest effective dose possible, the development of vectors with significant ability to target cancer-associated antigen, the application of adjunct therapies to target dysregulated microRNA, and embracing a hybrid strategy with a combination of gene therapy and low-dose chemotherapy in a disease-specific manner will be pivotal. This article outlines the advances and challenges in the field with emphasis on the biology and scope of vectors used for gene transfer, newer targets identified, and their outcome in preclinical and clinical studies.

Pharmacokinetic analysis identifies a factor VIII immunogenicity threshold after AAV gene therapy in hemophilia A mice

Advances in the development of novel treatment options for hemophilia A are prevalent. However, the anti-factor VIII (FVIII) neutralizing antibody (inhibitor) response to existing FVIII products remains a major treatment challenge. Although some novel products are designed to function in the presence of inhibitors, they do not specific address the immunogenicity risk or mechanistic causes of inhibitor development, which remain unclear. Furthermore, most preclinical studies supporting clinical gene therapy programs have reported immunogenicity signals in animal models, especially at higher vector doses and sometimes using multiple vector designs. In these settings, immunogenicity risk factor determination, comparative immunogenicity of competing vector designs, and the potential for obtaining meaningful prognostic data remain relatively unexplored. Additionally, there remains the opportunity to investigate clinical gene therapy as an alternative to standard immune tolerance induction therapy. The current study was designed to address these issues through longitudinal dose-response evaluation of 4 adeno-associated viral (AAV) vector candidates encoding 2 different FVIII transgenes in a murine model of hemophilia A. Plasma FVIII activity and anti-FVIII antibody data were used to generate a pharmacokinetic model that (1) identifies initial AAV-FVIII product expression kinetics as the dominant risk factor for inhibitor development, (2) predicts a therapeutic window where immune tolerance is achieved, and (3) demonstrates evidence of gene therapy-based immune tolerance induction. Although there are known limitations to the predictive value of preclinical immunogenicity testing, these studies can uncover or support the development of design principles that can guide the development of safe and effective genetic medicines.

Conversion of the liver into a biofactory for DNaseI using adeno-associated virus vector gene transfer reduces neutrophil extracellular traps in a model of Systemic Lupus Erythematosus

Adeno-associated virus (AAV) vectors are proving to be clinically transformative tools in the treatment of monogenic genetic disease. Rapid ongoing development of this technology promises to not only increase the number of monogenic disorders amenable to this approach, but also to bring diseases with complex multigenic and non-genetic aetiologies within therapeutic reach. Here we explore the broader paradigm of converting the liver into a biofactory for systemic output of therapeutic molecules using AAV-mediated delivery of DNaseI as an exemplar. DNaseI can clear neutrophil extracellular traps (NETs), which are nuclear-protein structures possessing anti-microbial action that are also involved in the pathophysiology of clinically troubling immune-mediated diseases. However, a translational challenge is short half-life of the enzyme in vivo (<5 hours). The current study demonstrates that AAV-mediated liver-targeted gene transfer stably induces serum DNaseI activity to >190-fold above physiological levels. In lupus-prone mice (NZBWF1) activity was maintained for longer than 6 months, the latest time point tested, and resulted in a clear functional effect with reduced renal presence of neutrophils, NETs, IgG and complement C3. However, treatment in this complex disease model did not extend life-span, improve serological endpoints or preserve renal function indicating there are elements of pathophysiology not accessible to DNaseI in the NZBWF1 model. We conclude that a translational solution to the challenge of short half-life of DNaseI is AAV-mediated gene delivery and that this may be efficacious in treating disease where NETs are a dominant pathological mechanism.

Recombinant adeno-associated virus mediated gene delivery in the extracranial nervous system of adult mice by direct nerve immersion

This protocol outlines a minimally invasive and quickly performed approach for transgene delivery in the extracranial nervous system of adult mice using recombinant adeno-associated virus (AAV). The technique, named Sciatic Nerve Direct Immersion (SciNDi), relies on the direct bilateral immersion of the exposed sciatic nerve with AAV. We show that in comparison with intramuscular AAV delivery, SciNDi results in widespread transduction in connected neuroanatomical tracts both in the sciatic nerve trunk and the lumbar spinal cord.

For complete details on the use and execution of this protocol, please refer to Jan et al. (2019) and Richner et al. (2011, 2017).

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A facile approach for AAV delivery in the peripheral nervous system of adult mice

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Transduction of sciatic nerve and modestly in spinal cord ventral horn neurons

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Avoids tissue trauma associated with direct intraparenchymal injection of AAV

Gene therapy in a murine model of chronic eosinophilic leukemia-not otherwise specified (CEL-NOS)

Chronic eosinophilic leukemia-not otherwise specified (CEL-NOS) is a rare, aggressive, fatal disease characterized by blood eosinophilia and dysfunction of organs infiltrated with eosinophils. Clinically, the disease manifests with weight loss, cough, weakness, diarrhea, and multi-organ dysfunction that is unresponsive to therapy. We developed a one-time gene therapy for CEL-NOS using an adeno-associated virus (AAV) expressing an anti-eosinophil monoclonal antibody (AAVrh.10mAnti-Eos) to provide sustained suppression of eosinophil numbers in blood, thus reducing eosinophil tissue invasion and organ dysfunction. A novel CEL-NOS model was developed in NOD-scid IL2rγ^null (NSG) mice by administration of AAV expressing the cytokine IL5 (AAVrh.10mIL5), resulting in marked peripheral and tissue eosinophilia of the heart, lung, liver, and spleen, and eventually death. Mice were administered AAVrh.10mAnti-Eos (10¹¹ genome copies) 4 wk after administration of AAVrh.10mIL5 and evaluated for anti-eosinophil antibody expression, blood eosinophil counts, organ eosinophil invasion, and survival. AAVrh.10mAnti-Eos expressed persistent levels of the anti-eosinophil antibody for >24 wk. Strikingly, CEL-NOS treated mice had markedly lower blood eosinophil levels and reduced mortality when compared with control treated mice. These results suggest that a single treatment with AAVrh.10mAnti-Eos has the potential to provide substantial therapeutic benefit to patients with CEL-NOS, a fatal malignant disorder.

Adeno-Associated Virus Vector Gene Delivery Elevates Factor I Levels and Downregulates the Complement Alternative Pathway In Vivo

The complement system is a key component of innate immunity, but impaired regulation influences disease susceptibility, including age-related macular degeneration and some kidney diseases. While complete complement inhibition has been used successfully to treat acute kidney disease, key unresolved challenges include strategies to modulate rather than completely inhibit the system and to deliver therapy potentially over decades. Elevating concentrations of complement factor I (CFI) restricts complement activation in vitro and this approach was extended in the current study to modulate complement activation in vivo. Sustained increases in CFI levels were achieved using an adeno-associated virus (AAV) vector to target the liver, inducing a 4- to 5-fold increase in circulating CFI levels. This led to decreased activity of the alternative pathway as demonstrated by a reduction in the rate of inactive C3b (iC3b) deposition and more rapid formation of C3 degradation products. In addition, vector application in a mouse model of systemic lupus erythematosus (NZBWF1), where tissue injury is, in part, complement dependent, resulted in reduced complement C3 and IgG renal deposition. Collectively, these data demonstrate that sustained elevation of CFI reduces complement activation in vivo providing proof-of-principle support for the therapeutic application of AAV gene delivery to modulate complement activation.

Rapid generation of mouse model for emerging infectious disease with the case of severe COVID-19

Since the pandemic of COVID-19 has intensely struck human society, small animal model for this infectious disease is in urgent need for basic and pharmaceutical research. Although several COVID-19 animal models have been identified, many of them show either minimal or inadequate pathophysiology after SARS-CoV-2 challenge. Here, we describe a new and versatile strategy to rapidly establish a mouse model for emerging infectious diseases in one month by multi-route, multi-serotype transduction with recombinant adeno-associated virus (AAV) vectors expressing viral receptor. In this study, the proposed approach enables profound and enduring systemic expression of SARS-CoV-2-receptor hACE2 in wild-type mice and renders them vulnerable to SARS-CoV-2 infection. Upon virus challenge, generated AAV/hACE2 mice showed pathophysiology closely mimicking the patients with severe COVID-19. The efficacy of a novel therapeutic antibody cocktail RBD-chAbs for COVID-19 was tested and confirmed by using this AAV/hACE2 mouse model, further demonstrating its successful application in drug development.

Upon the emergence of new infectious disease, animal model becomes a pivotal tool for study of disease mechanism and development of therapeutics. In this study, we propose a versatile approach that allows rapid generation of mouse model for novel infectious disease once the receptor of the pathogen is identified. We demonstrated this approach by generating a mouse model for COVID-19 in a month’s time. These mice were capable of recapitulating severe COVID-19 in patients, and successfully applied in the development of a therapeutic antibody cocktail for the disease. This not only suggests the usefulness of this mouse model for the research on COVID-19, but also exhibit the utility of the proposed approach for establishing animal model for infectious disease.

Single amino acid insertion allows functional transduction of murine hepatocytes with human liver tropic AAV capsids

Recent successes in clinical gene therapy applications have intensified the interest in using adeno-associated viruses (AAVs) as vectors for gene delivery into human liver. An inherent intriguing characteristic of AAVs is that vector variants vary substantially in their ability to transduce hepatocytes from different species. This has historically limited the value of preclinical studies using rodent models for predicting the efficiency of AAV vectors in liver-targeted gene therapy clinical studies. In this work, we aimed to investigate the key determinants of the observed differential interspecies transduction abilities among AAV variants. We took advantage of domain swapping strategies between AAV-KP1, a newly identified variant with enhanced murine liver tropism, and AAV3b, which functions poorly in mice. The systematic in vivo comparison of AAV3b/AAV-KP1 chimeric variants allowed us to identify a threonine insertion at position 265 within variable region I (VR-I) as the key residue that confers murine hepatic transduction to human-derived clade B (AAV2-like) and clade C (AAV3b-like) variants. We propose to use this insertion to generate phylogenetically related AAV surrogates in support of toxicology and dosing studies in the murine liver model.

Gain-of-function factor H-related 5 protein impairs glomerular complement regulation resulting in kidney damage

Genetic variation within the factor H-related (FHR) genes is associated with the complement-mediated kidney disease, C3 glomerulopathy (C3G). There is no definitive treatment for C3G, and a significant proportion of patients develop end-stage renal disease. The prototypical example is CFHR5 nephropathy, through which an internal duplication within a single CFHR5 gene generates a mutant FHR5 protein (FHR5mut) that leads to accumulation of complement C3 within glomeruli. To elucidate how abnormal FHR proteins cause C3G, we modeled CFHR5 nephropathy in mice. Animals lacking the murine factor H (FH) and FHR proteins, but coexpressing human FH and FHR5mut (hFH-FHR5mut), developed glomerular C3 deposition, whereas mice coexpressing human FH with the normal FHR5 protein (hFH-FHR5) did not. Like in patients, the FHR5mut had a dominant gain-of-function effect, and when administered in hFH-FHR5 mice, it triggered C3 deposition. Importantly, adeno-associated virus vector-delivered homodimeric mini-FH, a molecule with superior surface C3 binding compared to FH, reduced glomerular C3 deposition in the presence of the FHR5mut. Our data demonstrate that FHR5mut causes C3G by disrupting the homeostatic regulation of complement within the kidney and is directly pathogenic in C3G. These results support the use of FH-derived molecules with enhanced C3 binding for treating C3G associated with abnormal FHR proteins. They also suggest that targeting FHR5 represents a way to treat complement-mediated kidney injury.

Genome editing in the human liver: Progress and translational considerations

Liver-targeted genome editing offers the prospect of life-long therapeutic benefit following a single treatment and is set to rapidly supplant conventional gene addition approaches. Combining progress in liver-targeted gene delivery with genome editing technology, makes this not only feasible but realistically achievable in the near term. However, important challenges remain to be addressed. These include achieving therapeutic levels of editing, particularly in vivo, avoidance of off-target effects on the genome and the potential impact of pre-existing immunity to bacteria-derived nucleases, when used to improve editing rates. In this chapter, we outline the unique features of the liver that make it an attractive target for genome editing, the impact of liver biology on therapeutic efficacy, and disease specific challenges, including whether the approach targets a cell autonomous or non-cell autonomous disease. We also discuss strategies that have been used successfully to achieve genome editing outcomes in the liver and address translational considerations as genome editing technology moves into the clinic.

AAV Gene Transfer to the Heart

Recombinant adeno-associated virus (rAAV) has been widely used for gene therapy. AAV-mediated gene transfer leads to durable protein expression in non-proliferating targeted tissues, which enables long-term modulation of gene expression. Here we describe a rAAV production protocol based on PEI-mediated triple transfection of HEK293T cells, followed by purification by iodixanol density gradient ultracentrifugation. Viral yield varies, depending on the size of the viral genome, but, typically, a yield of 3E11 viral genome (vg) can be achieved using the described protocol. Our results showed that injection of rAAV9 significantly transduces cardiac cells, which supports rAAV9 being an effective tool for gene delivery in the heart in vivo.

Adeno-associated virus (AAV) capsid engineering in liver-directed gene therapy

Introduction: Gene therapy clinical trials with adeno-associated virus (AAV) vectors report impressive clinical efficacy data. Nevertheless, challenges have become apparent, such as the need for high vector doses and the induction of anti-AAV immune responses that cause the loss of vector-transduced hepatocytes. This fostered research focusing on development of next-generation AAV vectors capable of dealing with these hurdles.Areas Covered: While both the viral vector genome and the capsid are subjects to engineering, this review focuses on the latter. Specifically, we summarize the principles of capsid engineering strategies, and describe developments and applications of engineered capsid variants for liver-directed gene therapy.Expert Opinion: Capsid engineering is a promising strategy to significantly improve efficacy of the AAV vector system in clinical application. Reduction in vector dose will further improve vector safety, lower the risk of host immune responses and the cost of manufacturing. Capsid engineering is also expected to result in AAV vectors applicable to patients with preexisting immunity toward natural AAV serotypes.

Adipose Tissue: An Emerging Target for Adeno-associated Viral Vectors

Adipose tissue is one of the largest organs, playing important roles in physiology and pathologies of multiple diseases. However, research related to adeno-associated virus (AAV) targeting adipose tissue has been left far behind studies carried out in the liver, brain, heart, and muscle. Despite initial reports indicating poor performance, AAV-mediated gene delivery to adipose tissue has continued to rise during the past two decades. AAV8 and a novel engineered hybrid serotype, Rec2, have been shown to transduce adipose tissue more efficiently than other serotypes so far tested and have been applied in most of the in vivo studies. The Rec2 serotype displays high efficacy of gene transfer to both brown and white fat via local and systemic administration. This review summarizes the advances in developing AAV vectors with enhanced adipose tropism and restricting off-target transgene expression. We discuss the challenges and strategies to search for and generate novel serotypes with tropism tailoring for adipose tissue and develop AAV vector systems to improve adipose transgene expression for basic research and translational studies.

Use of a Hybrid Adeno-Associated Viral Vector Transposon System to Deliver the Insulin Gene to Diabetic NOD Mice

Previously, we used a lentiviral vector to deliver furin-cleavable human insulin (INS-FUR) to the livers in several animal models of diabetes using intervallic infusion in full flow occlusion (FFO), with resultant reversal of diabetes, restoration of glucose tolerance and pancreatic transdifferentiation (PT), due to the expression of beta (β)-cell transcription factors (β-TFs). The present study aimed to determine whether we could similarly reverse diabetes in the non-obese diabetic (NOD) mouse using an adeno-associated viral vector (AAV) to deliver INS-FUR ± the β-TF Pdx1 to the livers of diabetic mice. The traditional AAV8, which provides episomal expression, and the hybrid AAV8/piggyBac that results in transgene integration were used. Diabetic mice that received AAV8-INS-FUR became hypoglycaemic with abnormal intraperitoneal glucose tolerance tests (IPGTTs). Expression of β-TFs was not detected in the livers. Reversal of diabetes was not achieved in mice that received AAV8-INS-FUR and AAV8-Pdx1 and IPGTTs were abnormal. Normoglycaemia and glucose tolerance were achieved in mice that received AAV8/piggyBac-INS-FUR/FFO. Definitive evidence of PT was not observed. This is the first in vivo study using the hybrid AAV8/piggyBac system to treat Type 1 diabetes (T1D). However, further development is required before the system can be used for gene therapy of T1D.

Experimental Variables that Affect Human Hepatocyte AAV Transduction in Liver Chimeric Mice

Adeno-associated virus (AAV) vector serotypes vary in their ability to transduce hepatocytes from different species. Chimeric mouse models harboring human hepatocytes have shown translational promise for liver-directed gene therapies. However, many variables that influence human hepatocyte transduction and transgene expression in such models remain poorly defined. Here, we aimed to test whether three experimental conditions influence AAV transgene expression in immunodeficient, fumaryl-acetoactetate-hydrolase-deficient (Fah -/-) chimeric mice repopulated with primary human hepatocytes. We examined the effects of the murine liver injury cycle, human donor variability, and vector doses on hepatocyte transduction with various AAV serotypes expressing a green fluorescent protein (GFP). We determined that the timing of AAV vector challenge in the liver injury cycle resulted in up to 7-fold differences in the percentage of GFP expressing human hepatocytes. The GFP+ hepatocyte frequency varied 7-fold between human donors without, however, changing the relative transduction efficiency between serotypes for an individual donor. There was also a clear relationship between AAV vector doses and human hepatocyte transduction and transgene expression. We conclude that several experimental variables substantially affect human hepatocyte transduction in the Fah -/- chimera model, attention to which may improve reproducibility between findings from different laboratories.

Gene therapy for alpha 1-antitrypsin deficiency with an oxidant-resistant human alpha 1-antitrypsin

Alpha 1-antitrypsin (AAT) deficiency, a hereditary disorder characterized by low serum levels of functional AAT, is associated with early development of panacinar emphysema. AAT inhibits serine proteases, including neutrophil elastase, protecting the lung from proteolytic destruction. Cigarette smoke, pollution, and inflammatory cell–mediated oxidation of methionine (M) 351 and 358 inactivates AAT, limiting lung protection. In vitro studies using amino acid substitutions demonstrated that replacing M351 with valine (V) and M358 with leucine (L) on a normal M1 alanine (A) 213 background provided maximum antiprotease protection despite oxidant stress. We hypothesized that a onetime administration of a serotype 8 adeno-associated virus (AAV8) gene transfer vector coding for the oxidation-resistant variant AAT (A213/V351/L358; 8/AVL) would maintain antiprotease activity under oxidant stress compared with normal AAT (A213/M351/M358; 8/AMM). 8/AVL was administered via intravenous (IV) and intrapleural (IPL) routes to C57BL/6 mice. High, dose-dependent AAT levels were found in the serum and lung epithelial lining fluid (ELF) of mice administered 8/AVL or 8/AMM by IV or IPL. 8/AVL serum and ELF retained serine protease–inhibitory activity despite oxidant stress while 8/AMM function was abolished. 8/AVL represents a second-generation gene therapy for AAT deficiency providing effective antiprotease protection even with oxidant stress.

A gene transfer-based therapeutic to deliver oxidant-resistant alpha 1-antitrypsin (AAT) protects mice with AAT deficiency from lung destruction.

Attenuation of Heparan Sulfate Proteoglycan Binding Enhances In Vivo Transduction of Human Primary Hepatocytes with AAV2

Use of the prototypical adeno-associated virus type 2 (AAV2) capsid delivered unexpectedly modest efficacy in an early liver-targeted gene therapy trial for hemophilia B. This result is consistent with subsequent data generated in chimeric mouse-human livers showing that the AAV2 capsid transduces primary human hepatocytes in vivo with low efficiency. In contrast, novel variants generated by directed evolution in the same model, such as AAV-NP59, transduce primary human hepatocytes with high efficiency. While these empirical data have immense translational implications, the mechanisms underpinning this enhanced AAV capsid transduction performance in primary human hepatocytes are yet to be fully elucidated. Remarkably, AAV-NP59 differs from the prototypical AAV2 capsid by only 11 aa and can serve as a tool to study the correlation between capsid sequence/structure and vector function. Using two orthogonal vectorological approaches, we have determined that just 2 of the 11 changes present in AAV-NP59 (T503A and N596D) account for the enhanced transduction performance of this capsid variant in primary human hepatocytes in vivo, an effect that we have associated with attenuation of heparan sulfate proteoglycan (HSPG) binding affinity. In support of this hypothesis, we have identified, using directed evolution, two additional single amino acid substitution AAV2 variants, N496D and N582S, which are highly functional in vivo. Both substitution mutations reduce AAV2's affinity for HSPG. Finally, we have modulated the ability of AAV8, a highly murine-hepatotropic serotype, to interact with HSPG. The results support our hypothesis that enhanced HSPG binding can negatively affect the in vivo function of otherwise strongly hepatotropic variants and that modulation of the interaction with HSPG is critical to ensure maximum efficiency in vivo. The insights gained through this study can have powerful implications for studies into AAV biology and capsid development for preclinical and clinical applications targeting liver and other organs.

Prevention of Cholestatic Liver Disease and Reduced Tumorigenicity in a Murine Model of PFIC Type 3 Using Hybrid AAV-piggyBac Gene Therapy

Recombinant adeno-associated viral (rAAV) vectors are highly promising vehicles for liver-targeted gene transfer, with therapeutic efficacy demonstrated in preclinical models and clinical trials. Progressive familial intrahepatic cholestasis type 3 (PFIC3), an inherited juvenile-onset, cholestatic liver disease caused by homozygous mutation of the ABCB4 gene, may be a promising candidate for rAAV-mediated liver-targeted gene therapy. The Abcb4-/- mice model of PFIC3, with juvenile mice developing progressive cholestatic liver injury due to impaired biliary phosphatidylcholine excretion, resulted in cirrhosis and liver malignancy. Using a conventional rAAV strategy, we observed markedly blunted rAAV transduction in adult Abcb4-/- mice with established liver disease, but not in disease-free, wild-type adults or in homozygous juveniles prior to liver disease onset. However, delivery of predominantly nonintegrating rAAV vectors to juvenile mice results in loss of persistent transgene expression due to hepatocyte proliferation in the growing liver. Conclusion: A hybrid vector system, combining the high transduction efficiency of rAAV with piggyBac transposase-mediated somatic integration, was developed to facilitate stable human ABCB4 expression in vivo and to correct juvenile-onset chronic liver disease in a murine model of PFIC3. A single dose of hybrid vector at birth led to life-long restoration of bile composition, prevention of biliary cirrhosis, and a substantial reduction in tumorigenesis. This powerful hybrid rAAV-piggyBac transposon vector strategy has the capacity to mediate lifelong phenotype correction and reduce the tumorigenicity of progressive familial intrahepatic cholestasis type 3 and, with further refinement, the potential for human clinical translation.

Combination Suicide Gene Delivery with an Adeno-Associated Virus Vector Encoding Inducible Caspase-9 and a Chemical Inducer of Dimerization Is Effective in a Xenotransplantation Model of Hepatocellular Carcinoma

Current treatment approaches for hepatocellular carcinoma (HCC) have a narrow therapeutic index and alternate modes of treatment are thus required. We have utilized a gene delivery vector containing inducible caspase 9 (iCasp9) gene, which is a synthetic analogue based on the mammalian caspase 9 and fused to a human FK506 binding protein that allows its conditional dimerization to a synthetic, small molecule [chemical inducer of dimerization, AP20187] and results in target cell apoptosis. In our studies, we have tested these synthetic vectors based on an adeno-associated virus platform for their potential anti-tumorigenic effect in human HCC cells in vitro and in a HCC tumor model developed in nude mice. Our data demonstrates that the iCasp9-AP20187 bioconjugate is able to trigger terminal effectors of cellular apoptosis and presents a viable approach for the potential treatment of HCC.

AAV-Mediated Gene Delivery to the Mouse Liver

The liver is an attractive target for gene therapy due to the high incidence of liver disease phenotypes. Adeno-associated viral vectors (AAV) are currently the most popular gene delivery system for targeting the liver, reflecting high transduction efficiency in vivo and the availability of a toolkit of multiple different capsids with high liver tropism. While AAV vectors confer stable gene transfer in the relatively quiescent adult liver, the predominantly episomal nature of AAV vector genomes results in less stable expression in the growing liver as a consequence of episome clearance during hepatocellular replication. This is an important consideration in experimental design involving young animals, particularly mice, where liver growth is rapid. Given the immense value of murine models for dissecting disease pathophysiology, experimental therapeutics and vector development, this technical manuscript focuses on AAV-mediated transduction of the mouse liver. Xenograft models, in which chimeric mouse-human livers can be established, are also amenable to AAV-mediated gene transfer and have proven to be powerful tools for in vivo selection and characterization of novel human-specific capsids. While yet to be confirmed, such models have the potential to more accurately predict transduction efficiency of clinical candidate vectors than nonhuman primate models.

AAV-Mediated Gene Delivery to the Liver: Overview of Current Technologies and Methods

Adeno-associated virus (AAV) vectors to treat liver-specific genetic diseases are the focus of several ongoing clinical trials. The ability to give a peripheral injection of virus that will successfully target the liver is one of many attractive features of this technology. Although initial studies of AAV liver gene transfer revealed some limitations, extensive animal modeling and further clinical development have helped solve some of these issues, resulting in several successful clinical trials that have reached curative levels of clotting factor expression in hemophilia. Looking beyond gene replacement, recent technologies offer the possibility for AAV liver gene transfer to directly repair deficient genes and potentially treat autoimmune disease.

MicroRNA-Regulated Gene Delivery Systems for Research and Therapeutic Purposes

Targeted gene delivery relies on the ability to limit the expression of a transgene within a defined cell/tissue population. MicroRNAs represent a class of highly powerful and effective regulators of gene expression that act by binding to a specific sequence present in the corresponding messenger RNA. Involved in almost every aspect of cellular function, many miRNAs have been discovered with expression patterns specific to developmental stage, lineage, cell-type, or disease stage. Exploiting the binding sites of these miRNAs allows for construction of targeted gene delivery platforms with a diverse range of applications. Here, we summarize studies that have utilized miRNA-regulated systems to achieve targeted gene delivery for both research and therapeutic purposes. Additionally, we identify criteria that are important for the effectiveness of a particular miRNA for such applications and we also discuss factors that have to be taken into consideration when designing miRNA-regulated expression cassettes.

How will the field of gene therapy survive its success?

In August 2017, for the first time, a gene therapy was approved for market release in the United States. That approval was followed by two others before the end of the year. This article cites primary literature, review articles concerning particular biotechnologies, and press releases by the FDA and others in order to provide an overview of the current status of the field of gene therapy with respect to its translation into practice. Technical hurdles that have been overcome in the past decades are summarized, as are hurdles that need to be the subject of continued research. Then, some social and practical challenges are identified that must be overcome if the field of gene therapy, having survived past failures, is to achieve not only technical and clinical but also market success. One of these, the need for an expanded capacity for the manufacturing of viral vectors to be able to meet the needs of additional gene therapies that will be coming soon, is a challenge that the talents of current and future bioengineers may help address.

Promoterless gene targeting without nucleases rescues lethality of a Crigler-Najjar syndrome mouse model

Crigler‐Najjar syndrome type I (CNSI) is a rare monogenic disease characterized by severe neonatal unconjugated hyperbilirubinemia with a lifelong risk of neurological damage and death. Liver transplantation is the only curative option, which has several limitations and risks. We applied an in vivo gene targeting approach based on the insertion, without the use of nucleases, of a promoterless therapeutic cDNA into the albumin locus of a mouse model reproducing all major features of CNSI. Neonatal transduction with the donor vector resulted in the complete rescue from neonatal lethality, with a therapeutic reduction in plasma bilirubin lasting for at least 12 months, the latest time point analyzed. Mutant mice, which expressed about 5–6% of WT Ugt1a1 levels, showed normal liver histology and motor‐coordination abilities, suggesting no functional liver or brain abnormalities. These results proved that the promoterless gene therapy is applicable for CNSI, providing therapeutic levels of an intracellular ER membrane‐bound enzyme responsible for a lethal liver metabolic disease.

Bioengineered AAV Capsids with Combined High Human Liver Transduction In Vivo and Unique Humoral Seroreactivity

Existing recombinant adeno-associated virus (rAAV) serotypes for delivering in vivo gene therapy treatments for human liver diseases have not yielded combined high-level human hepatocyte transduction and favorable humoral neutralization properties in diverse patient groups. Yet, these combined properties are important for therapeutic efficacy. To bioengineer capsids that exhibit both unique seroreactivity profiles and functionally transduce human hepatocytes at therapeutically relevant levels, we performed multiplexed sequential directed evolution screens using diverse capsid libraries in both primary human hepatocytes in vivo and with pooled human sera from thousands of patients. AAV libraries were subjected to five rounds of in vivo selection in xenografted mice with human livers to isolate an enriched human-hepatotropic library that was then used as input for a sequential on-bead screen against pooled human immunoglobulins. Evolved variants were vectorized and validated against existing hepatotropic serotypes. Two of the evolved AAV serotypes, NP40 and NP59, exhibited dramatically improved functional human hepatocyte transduction in vivo in xenografted mice with human livers, along with favorable human seroreactivity profiles, compared with existing serotypes. These novel capsids represent enhanced vector delivery systems for future human liver gene therapy applications.

Live cell imaging of cytosolic NADH/NAD+ ratio in hepatocytes and liver slices

Fatty liver disease (FLD), the most common chronic liver disease in the United States, may be caused by alcohol or the metabolic syndrome. Alcohol is oxidized in the cytosol of hepatocytes by alcohol dehydrogenase (ADH), which generates NADH and increases cytosolic NADH/NAD⁺ ratio. The increased ratio may be important for development of FLD, but our ability to examine this question is hindered by methodological limitations. To address this, we used the genetically encoded fluorescent sensor Peredox to obtain dynamic, real-time measurements of cytosolic NADH/NAD⁺ ratio in living hepatocytes. Peredox was expressed in dissociated rat hepatocytes and HepG2 cells by transfection, and in mouse liver slices by tail-vein injection of adeno-associated virus (AAV)-encoded sensor. Under control conditions, hepatocytes and liver slices exhibit a relatively low (oxidized) cytosolic NADH/NAD⁺ ratio as reported by Peredox. The ratio responds rapidly and reversibly to substrates of lactate dehydrogenase (LDH) and sorbitol dehydrogenase (SDH). Ethanol causes a robust dose-dependent increase in cytosolic NADH/NAD⁺ ratio, and this increase is mitigated by the presence of NAD⁺-generating substrates of LDH or SDH. In contrast to hepatocytes and slices, HepG2 cells exhibit a relatively high (reduced) ratio and show minimal responses to substrates of ADH and SDH. In slices, we show that comparable results are obtained with epifluorescence imaging and two-photon fluorescence lifetime imaging (2p-FLIM). Live cell imaging with Peredox is a promising new approach to investigate cytosolic NADH/NAD⁺ ratio in hepatocytes. Imaging in liver slices is particularly attractive because it allows preservation of liver microanatomy and metabolic zonation of hepatocytes. NEW & NOTEWORTHY We describe and validate a new approach for measuring free cytosolic NADH/NAD⁺ ratio in hepatocytes and liver slices: live cell imaging with the fluorescent biosensor Peredox. This approach yields dynamic, real-time measurements of the ratio in living, functioning liver cells, overcoming many limitations of previous methods for measuring this important redox parameter. The feasibility of using Peredox in liver slices is particularly attractive because slices allow preservation of hepatic microanatomy and metabolic zonation of hepatocytes.

Optimized AAV rh.10 Vectors That Partially Evade Neutralizing Antibodies during Hepatic Gene Transfer

Of the 12 common serotypes used for gene delivery applications, Adeno-associated virus (AAV)rh.10 serotype has shown sustained hepatic transduction and has the lowest seropositivity in humans. We have evaluated if further modifications to AAVrh.10 at its phosphodegron like regions or predicted immunogenic epitopes could improve its hepatic gene transfer and immune evasion potential. Mutant AAVrh.10 vectors were generated by site directed mutagenesis of the predicted targets. These mutant vectors were first tested for their transduction efficiency in HeLa and HEK293T cells. The optimal vector was further evaluated for their cellular uptake, entry, and intracellular trafficking by quantitative PCR and time-lapse confocal microscopy. To evaluate their potential during hepatic gene therapy, C57BL/6 mice were administered with wild-type or optimal mutant AAVrh.10 and the luciferase transgene expression was documented by serial bioluminescence imaging at 14, 30, 45, and 72 days post-gene transfer. Their hepatic transduction was further verified by a quantitative PCR analysis of AAV copy number in the liver tissue. The optimal AAVrh.10 vector was further evaluated for their immune escape potential, in animals pre-immunized with human intravenous immunoglobulin. Our results demonstrate that a modified AAVrh.10 S671A vector had enhanced cellular entry (3.6 fold), migrate rapidly to the perinuclear region (1 vs. >2 h for wild type vectors) in vitro, which further translates to modest increase in hepatic gene transfer efficiency in vivo. More importantly, the mutant AAVrh.10 vector was able to partially evade neutralizing antibodies (~27-64 fold) in pre-immunized animals. The development of an AAV vector system that can escape the circulating neutralizing antibodies in the host will substantially widen the scope of gene therapy applications in humans.

Improvement of Adeno-Associated Virus-Mediated Liver Transduction Efficacy by Regional Administration in Macaca fascicularis

The liver is a central organ in metabolism and can be affected by numerous inherited metabolic disorders. Recombinant adeno-associated virus (AAV)-based gene therapy represents a promising therapeutic approach for such diseases. AAVs have been demonstrated to be safe, and resulted in high and long-term expression in preclinical and clinical studies. However, there are still some concerns regarding the expression levels that can be achieved and the percentage of hepatocytes that can be transduced. Because of the cell-autonomous nature of most metabolic liver disorders, a high percentage of hepatocytes needs to be corrected in order to achieve a therapeutic effect. The goal of our work was to improve transduction efficacy of the liver by conveying the vector directly to hepatic tissue. Interventional radiology procedures were used to administer an AAV5 vector expressing a secreted form of human embryonic alkaline phosphatase (hSEAP) under the control of a liver-specific promoter to a clinically relevant animal model, Macaca fascicularis. Balloon occlusion of the regional hepatic venous flow was performed while injecting the vector either into the hepatic artery (HA) or, against flow, via the suprahepatic vein (SHV). In both cases the vector was injected into the right hepatic lobules, and the two routes were compared with conventional intravenous administration. Higher hSEAP levels were obtained when the vector was administered via SHV or HA than after intravenous injection. Furthermore, higher expression levels correlated with a higher number of vector genomes in the injected lobules. In conclusion, direct administration of AAV vectors via the hepatic blood flow with simultaneous balloon occlusion of the hepatic outflow increases liver transduction efficacy in comparison with systemic delivery and can be further improved in bigger animals or humans, where it would be technically feasible to inject the vector into the hepatic vasculature in the generality of lobules.

Adeno-associated virus serotype rh.10 displays strong muscle tropism following intraperitoneal delivery

Recombinant adeno-associated virus (rAAV) is an attractive tool for basic science and translational medicine including gene therapy, due to the versatility in its cell and organ transduction. Previous work indicates that rAAV transduction patterns are highly dependent on route of administration. Based on this relationship, we hypothesized that intraperitoneal (IP) administration of rAAV produces unique patterns of tissue tropism. To test this hypothesis, we investigated the transduction efficiency of 12 rAAV serotypes carrying an enhanced green fluorescent protein (EGFP) reporter gene in a panel of 12 organs after IP injection. Our data suggest that IP administration emphasizes transduction patterns that are different from previously reported intravascular delivery methods. Using this approach, rAAV efficiently transduces the liver, pancreas, skeletal muscle, heart and diaphragm without causing significant histopathological changes. Of note, rAAVrh.10 showed excellent muscle transduction following IP administration, highlighting its potential as a new muscle-targeting vector.

Adeno-Associated Virus Gene Therapy for Liver Disease

The field of adeno-associated virus (AAV) gene therapy has progressed rapidly over the past decade, with the advent of novel capsid serotype and organ-specific promoters, and an increasing understanding of the immune response to AAV administration. In particular, liver-directed therapy has made remarkable strides, with a number of clinical trials currently planned and ongoing in hemophilia A and B, as well as other liver disorders. This review focuses on liver-directed AAV gene therapy, including historic context, current challenges, and future developments.

In Vivo Zinc Finger Nuclease-mediated Targeted Integration of a Glucose-6-phosphatase Transgene Promotes Survival in Mice With Glycogen Storage Disease Type IA

Glycogen storage disease type Ia (GSD Ia) is caused by glucose-6-phosphatase (G6Pase) deficiency in association with severe, life-threatening hypoglycemia that necessitates lifelong dietary therapy. Here we show that use of a zinc-finger nuclease (ZFN) targeted to the ROSA26 safe harbor locus and a ROSA26-targeting vector containing a G6PC donor transgene, both delivered with adeno-associated virus (AAV) vectors, markedly improved survival of G6Pase knockout (G6Pase-KO) mice compared with mice receiving the donor vector alone (P < 0.04). Furthermore, transgene integration has been confirmed by sequencing in the majority of the mice treated with both vectors. Targeted alleles were 4.6-fold more common in livers of mice with GSD Ia, as compared with normal littermates, at 8 months following vector administration (P < 0.02). This suggests a selective advantage for vector-transduced hepatocytes following ZFN-mediated integration of the G6Pase vector. A short-term experiment also showed that 3-month-old mice receiving the ZFN had significantly-improved biochemical correction, in comparison with mice that received the donor vector alone. These data suggest that the use of ZFNs to drive integration of G6Pase at a safe harbor locus might improve vector persistence and efficacy, and lower mortality in GSD Ia.

Modeling correction of severe urea cycle defects in the growing murine liver using a hybrid recombinant adeno-associated virus/piggyBac transposase gene delivery system

Liver-targeted gene therapy based on recombinant adeno-associated viral vectors (rAAV) shows promising therapeutic efficacy in animal models and adult-focused clinical trials. This promise, however, is not directly translatable to the growing liver, where high rates of hepatocellular proliferation are accompanied by loss of episomal rAAV genomes and subsequently a loss in therapeutic efficacy. We have developed a hybrid rAAV/piggyBac transposon vector system combining the highly efficient liver-targeting properties of rAAV with stable piggyBac-mediated transposition of the transgene into the hepatocyte genome. Transposition efficiency was first tested using an enhanced green fluorescent protein expression cassette following delivery to newborn wild-type mice, with a 20-fold increase in stably gene-modified hepatocytes observed 4 weeks posttreatment compared to traditional rAAV gene delivery. We next modeled the therapeutic potential of the system in the context of severe urea cycle defects. A single treatment in the perinatal period was sufficient to confer robust and stable phenotype correction in the ornithine transcarbamylase-deficient Spf(ash) mouse and the neonatal lethal argininosuccinate synthetase knockout mouse. Finally, transposon integration patterns were analyzed, revealing 127,386 unique integration sites which conformed to previously published piggyBac data.

CONCLUSION

Using a hybrid rAAV/piggyBac transposon vector system, we achieved stable therapeutic protection in two urea cycle defect mouse models; a clinically conceivable early application of this technology in the management of severe urea cycle defects could be as a bridging therapy while awaiting liver transplantation; further improvement of the system will result from the development of highly human liver-tropic capsids, the use of alternative strategies to achieve transient transposase expression, and engineered refinements in the safety profile of piggyBac transposase-mediated integration.

Transient suppression of hepatocellular replication in the mouse liver following transduction with recombinant adeno-associated virus

Recombinant vectors based on adeno-associated virus (AAV) are proving to be powerful tools for genetic manipulation of the liver, for both discovery and therapeutic purposes. The system can be used to deliver transgene cassettes for expression or, alternatively, DNA templates for genome editing via homologous recombination. The replicative state of target cells is known to influence the efficiency of these processes and knowledge of the host-vector interactions involved is required for optimally effective vector deployment. Here we show, for the first time in vivo, that in addition to the known effects of hepatocellular replication on AAV-mediated gene transfer, the vector itself exerts a potent, albeit transient suppressive effect on cell cycle progression that is relieved on a time course that correlates with the known rate of clearance of input single-stranded vector DNA. This finding requires further mechanistic investigation, delineates an excellent model system for such studies and further deepens our insight into the complexity of interactions between AAV vectors and the cell cycle in a clinically promising target tissue.

ACE2 Therapy Using Adeno-associated Viral Vector Inhibits Liver Fibrosis in Mice

Angiotensin converting enzyme 2 (ACE2) which breaks down profibrotic peptide angiotensin II to antifibrotic peptide angiotensin-(1-7) is a potential therapeutic target in liver fibrosis. We therefore investigated the long-term therapeutic effect of recombinant ACE2 using a liver-specific adeno-associated viral genome 2 serotype 8 vector (rAAV2/8-ACE2) with a liver-specific promoter in three murine models of chronic liver disease, including carbon tetrachloride-induced toxic injury, bile duct ligation-induced cholestatic injury, and methionine- and choline-deficient diet-induced steatotic injury. A single injection of rAAV2/8-ACE2 was administered after liver disease has established. Hepatic fibrosis, gene and protein expression, and the mechanisms that rAAV2/8-ACE2 therapy associated reduction in liver fibrosis were analyzed. Compared with control group, rAAV2/8-ACE2 therapy produced rapid and sustained upregulation of hepatic ACE2, resulting in a profound reduction in fibrosis and profibrotic markers in all diseased models. These changes were accompanied by reduction in hepatic angiotensin II levels with concomitant increases in hepatic angiotensin-(1-7) levels, resulting in significant reductions of NADPH oxidase assembly, oxidative stress and ERK1/2 and p38 phosphorylation. Moreover, rAAV2/8-ACE2 therapy normalized increased intrahepatic vascular tone in fibrotic livers. We conclude that rAAV2/8-ACE2 is an effective liver-targeted, long-term therapy for liver fibrosis and its complications without producing unwanted systemic effects.

Polyinosinic acid blocks adeno-associated virus macrophage endocytosis in vitro and enhances adeno-associated virus liver-directed gene therapy in vivo

Adeno-associated virus serotype 8 (AAV8) has been demonstrated to be effective for liver-directed gene therapy in humans. Although hepatocytes are the main target cell for AAV8, there is a loss of the viral vector because of uptake by macrophages and Kupffer cells. Reducing this loss would increase the efficacy of viral gene therapy and allow a dose reduction. The receptor mediating this uptake has not been identified; a potential candidate seems the macrophage scavenger receptor A (SR-A) that is involved in the endocytosis of, for instance, adenovirus. In this study we show that SR-A can mediate scAAV8 endocytosis and that blocking it with polyinosinic acid (poly[i]) reduces endocytosis significantly in vitro. Subsequently, we demonstrate that blocking this receptor improves scAAV-mediated liver-directed gene therapy in a model for inherited hyperbilirubinemia, the uridine diphospho-glucuronyl transferase 1A1-deficient Gunn rat. In male rats, preadministration of poly[i] increases the efficacy of a low dose (1×10¹¹ gc/kg) but not of a higher dose (3×10¹¹ gc/kg) scAAV8-LP1-UT1A1. Administration of poly[i] just before the vector significantly increases the correction of serum bilirubin in female rats. In these, the effect of poly[i] is seen by both doses but is more pronounced in the females receiving the low vector, where it also results in a significant increase of bilirubin glucuronides in bile. In conclusion, this study shows that SR-A mediates the endocytosis of AAV8 in vitro and in vivo and that blocking this receptor can improve the efficacy of AAV-mediated liver-directed gene therapy.