Intravenous administration of an AAV-2 vector for the expression of factor IX in mice and a dog model of hemophilia B

Systematic gene therapy derived from an investigative study of AAV2/8 vector gene therapy for Fabry disease

BACKGROUND

Fabry disease (FD) is a progressive multisystemic disease characterized by a lysosomal enzyme deficiency. A lack of α-galactosidase A (α-Gal A) activity results in the progressive systemic accumulation of its substrates, including globotriaosylceramide (Gb3) and globotriaosylsphingosine (Lyso-Gb3), which results in renal, cardiac, and/or cerebrovascular disease and early death. Enzyme replacement therapy (ERT) is the current standard of care for FD; however, it has important limitations, including a low half-life, limited distribution, and requirement of lifelong biweekly infusions of recombinant enzymes.

METHODS

Herein, we evaluated a gene therapy approach using an episomal adeno-associated viral 2/8 (AAV2/8) vector that encodes the human GLA cDNA driven by a liver-specific expression cassette in a mouse model of FD that lacks α-Gal A activity and progressively accumulates Gb3 and Lyso-Gb3 in plasma and tissues.

RESULTS

A pharmacology and toxicology study showed that administration of AAV2/8-hGLA vectors (AAV2/8-hGLA) in FD mice without immunosuppression resulted in significantly increased plasma and tissue α-Gal A activity and substantially normalized Gb3 and Lyso-Gb3 content.

CONCLUSIONS

Moreover, the plasma enzymatic activity of α-Gal A continued to be stably expressed for up to 38 weeks and sometimes even longer, indicating that AAV2/8-hGLA is effective in treating FD mice, and that α-Gal A is continuously and highly expressed in the liver, secreted into plasma, and absorbed by various tissues. These findings provide a basis for the clinical development of AAV2/8-hGLA.

Antiviral Targeting of Varicella Zoster Virus Replication and Neuronal Reactivation Using CRISPR/Cas9 Cleavage of the Duplicated Open Reading Frames 62/71

Varicella Zoster Virus (VZV) causes Herpes Zoster (HZ), a common debilitating and complicated disease affecting up to a third of unvaccinated populations. Novel antiviral treatments for VZV reactivation and HZ are still in need. Here, we evaluated the potential of targeting the replicating and reactivating VZV genome using Clustered Regularly Interspaced Short Palindromic Repeat-Cas9 nucleases (CRISPR/Cas9) delivered by adeno-associated virus (AAV) vectors. After AAV serotype and guide RNA (gRNA) optimization, we report that a single treatment with AAV2-expressing Staphylococcus aureus CRISPR/Cas9 (saCas9) with gRNA to the duplicated and essential VZV genes ORF62/71 (AAV2-62gRsaCas9) greatly reduced VZV progeny yield and cell-to-cell spread in representative epithelial cells and in lytically infected human embryonic stem cell (hESC)-derived neurons. In contrast, AAV2-62gRsaCas9 did not reduce the replication of a recombinant virus mutated in the ORF62 targeted sequence, establishing that antiviral effects were a consequence of VZV-genome targeting. Delivery to latently infected and reactivation-induced neuron cultures also greatly reduced infectious-virus production. These results demonstrate the potential of AAV-delivered genome editors to limit VZV productive replication in epithelial cells, infected human neurons, and upon reactivation. The approach could be developed into a strategy for the treatment of VZV disease and virus spread in HZ.

AAV2/6 Gene Therapy in a Murine Model of Fabry Disease Results in Supraphysiological Enzyme Activity and Effective Substrate Reduction

Fabry disease is an X-linked lysosomal storage disorder caused by mutations in the alpha-galactosidase A (GLA) gene, which encodes the exogalactosyl hydrolase, alpha-galactosidase A (α-Gal A). Deficient α-Gal A activity results in the progressive, systemic accumulation of its substrates, globotriaosylceramide (Gb3) and globotriaosylsphingosine (Lyso-Gb3), leading to renal, cardiac, and/or cerebrovascular disease and early demise. The current standard treatment for Fabry disease is enzyme replacement therapy, which necessitates lifelong biweekly infusions of recombinant enzyme. A more long-lasting treatment would benefit Fabry patients. Here, a gene therapy approach using an episomal adeno-associated viral 2/6 (AAV2/6) vector that encodes the human GLA cDNA driven by a liver-specific expression cassette was evaluated in a Fabry mouse model that lacks α-Gal A activity and progressively accumulates Gb3 and Lyso-Gb3 in plasma and tissues. A detailed 3-month pharmacology and toxicology study showed that administration of a clinical-scale-manufactured AAV2/6 vector resulted in markedly increased plasma and tissue α-Gal A activities, and essentially normalized Gb3 and Lyso-Gb3 at key sites of pathology. Further optimization of vector design identified the clinical lead vector, ST-920, which produced several-fold higher plasma and tissue α-Gal A activity levels with a good safety profile. Together, these studies provide the basis for the clinical development of ST-920.

Translational Potential of Immune Tolerance Induction by AAV Liver-Directed Factor VIII Gene Therapy for Hemophilia A

Hemophilia A (HA) is an X-linked bleeding disorder due to deficiencies in coagulation factor VIII (FVIII). The major complication of current protein-based therapies is the development of neutralizing anti-FVIII antibodies, termed inhibitors, that block the hemostatic effect of therapeutic FVIII. Inhibitors develop in about 20-30% of people with severe HA, but the risk is dependent on the interaction between environmental and genetic factors, including the underlying F8 gene mutation. Recently, multiple clinical trials evaluating adeno-associated viral (AAV) vector liver-directed gene therapy for HA have reported promising results of therapeutically relevant to curative FVIII levels. The inclusion criteria for most trials prevented enrollment of subjects with a history of inhibitors. However, preclinical data from small and large animal models of HA with inhibitors suggests that liver-directed gene therapy can in fact eradicate pre-existing anti-FVIII antibodies, induce immune tolerance, and provide long-term therapeutic FVIII expression to prevent bleeding. Herein, we review the accumulating evidence that continuous uninterrupted expression of FVIII and other transgenes after liver-directed AAV gene therapy can bias the immune system toward immune tolerance induction, discuss the current understanding of the immunological mechanisms of this process, and outline questions that will need to be addressed to translate this strategy to clinical trials.

Gene therapy for glycogen storage diseases

The focus of this review is the development of gene therapy for glycogen storage diseases (GSDs). GSD results from the deficiency of specific enzymes involved in the storage and retrieval of glucose in the body. Broadly, GSDs can be divided into types that affect liver or muscle or both tissues. For example, glucose-6-phosphatase (G6Pase) deficiency in GSD type Ia (GSD Ia) affects primarily the liver and kidney, while acid α-glucosidase (GAA) deficiency in GSD II causes primarily muscle disease. The lack of specific therapy for the GSDs has driven efforts to develop new therapies for these conditions. Gene therapy needs to replace deficient enzymes in target tissues, which has guided the planning of gene therapy experiments. Gene therapy with adeno-associated virus (AAV) vectors has demonstrated appropriate tropism for target tissues, including the liver, heart and skeletal muscle in animal models for GSD. AAV vectors transduced liver and kidney in GSD Ia and striated muscle in GSD II mice to replace the deficient enzyme in each disease. Gene therapy has been advanced to early phase clinical trials for the replacement of G6Pase in GSD Ia and GAA in GSD II (Pompe disease). Other GSDs have been treated in proof-of-concept studies, including GSD III, IV and V. The future of gene therapy appears promising for the GSDs, promising to provide more efficacious therapy for these disorders in the foreseeable future.

Liver depot gene therapy for Pompe disease

Gene therapy for Pompe disease has advanced to early phase clinical trials, based upon proof-of-concept data indicating that gene therapy could surpass the benefits of the current standard of care, enzyme replacement therapy (ERT). ERT requires frequent infusions of large quantities of recombinant human acid α-glucosidase (GAA), whereas gene therapy involves a single infusion of a vector that stably transduces tissues to continuously produce GAA. Liver-specific expression of GAA with an adeno-associated virus (AAV) vector established stable GAA secretion from the liver accompanied by receptor-mediated uptake of GAA, which corrected the deficiency of GAA and cleared the majority of accumulated glycogen in the heart and skeletal muscle. Liver depot gene therapy was equivalent to ERT at a dose of the AAV vector that could be administered in an early phase clinical trial. Furthermore, high-level expression of GAA has decreased glycogen stored in the brain. A unique advantage of liver-specific expression stems from the induction of immune tolerance to GAA following AAV vector administration, thereby suppressing anti-GAA antibodies that otherwise interfere with efficacy. A Phase I clinical trial of AAV vector-mediated liver depot gene therapy has been initiated based upon promising preclinical data (NCT03533673). Overall, gene therapy promises to address limits of currently available ERT, if clinical translation currently underway is successful.

AAV8 Gene Therapy for Crigler-Najjar Syndrome in Macaques Elicited Transgene T Cell Responses That Are Resident to the Liver

Systemic delivery of adeno-associated viral (AAV) vectors has been evaluated for the treatment of several liver diseases, including homozygous familial hypercholesterolemia, ornithine transcarbamylase deficiency, and hemophilia. Here, we evaluated this approach for the treatment of Crigler-Najjar syndrome. We administered wild-type rhesus macaques with 1.0 × 10¹³ or 2.5 × 10¹³ genome copies/kg of an AAV serotype 8 vector expressing a codon-optimized version of human uridine diphosphate glucuronosyl transferase 1A1 (UGT1A1) from a liver-specific promoter. We extensively studied vector biodistribution, transgene expression, and immune responses following vector administration. All rhesus macaques survived until their scheduled necropsy at day 56 and showed no clinical abnormalities during the course of the study. Macaques administered with either vector dose developed a T cell response to the AAV capsid and/or transgene. We mapped the immunodominant epitope in the human UGT1A1 sequence, and we found no correlation between peripheral and tissue-resident lymphocyte responses. Upon further investigation, we characterized CD107a⁺, granzyme B⁺, CD4⁺, and CD8⁺ transgene-specific cellular responses that were restricted to tissue-resident T cells. This study highlights the importance of studying immune responses at the vector transduction site and the limited usefulness of blood as a surrogate to evaluate tissue-restricted T cell responses.

Inhibition of hepatitis B virus replication via HBV DNA cleavage by Cas9 from Staphylococcus aureus

Chronic hepatitis B virus (HBV) infection is difficult to cure due to the presence of covalently closed circular DNA (cccDNA). Accumulating evidence indicates that the CRISPR/Cas9 system effectively disrupts HBV genome, including cccDNA, in vitro and in vivo. However, efficient delivery of CRISPR/Cas9 system to the liver or hepatocytes using an adeno-associated virus (AAV) vector remains challenging due to the large size of Cas9 from Streptococcus pyogenes (Sp). The recently identified Cas9 protein from Staphylococcus aureus (Sa) is smaller than SpCas9 and thus is able to be packaged into the AAV vector. To examine the efficacy of SaCas9 system on HBV genome destruction, we designed 5 guide RNAs (gRNAs) that targeted different HBV genotypes, 3 of which were shown to be effective. The SaCas9 system significantly reduced HBV antigen expression, as well as pgRNA and cccDNA levels, in Huh7, HepG2.2.15 and HepAD38 cells. The dual expression of gRNAs/SaCas9 in these cell lines resulted in more efficient HBV genome cleavage. In the mouse model, hydrodynamic injection of gRNA/SaCas9 plasmids resulted in significantly lower levels of HBV protein expression. We also delivered the SaCas9 system into mice with persistent HBV replication using an AAV vector. Both the AAV vector and the mRNA of Cas9 could be detected in the C3H mouse liver cells. Decreased hepatitis B surface antigen (HBsAg), HBV DNA and pgRNA levels were observed when a higher titer of AAV was injected, although this decrease was not significantly different from the control. In summary, the SaCas9 system accurately and efficiently targeted the HBV genome and inhibited HBV replication both in vitro and in vivo. The system was delivered by an AAV vector and maybe used as a novel therapeutic strategy against chronic HBV infection.

Impact of intravenous infusion time on AAV8 vector pharmacokinetics, safety, and liver transduction in cynomolgus macaques

Systemically delivered adeno-associated viral (AAV) vectors are now in early-phase clinical trials for a variety of diseases. While there is a general consensus on inclusion and exclusion criteria for each of these trials, the conditions under which vectors are infused vary significantly. In this study, we evaluated the impact of intravenous infusion rate of AAV8 vector in cynomolgus macaques on transgene expression, vector clearance from the circulation, and potential activation of the innate immune system. The dose of AAV8 vector in terms of genome copies per kilogram body weight and its concentration were fixed, while the rate of infusion varied to deliver the entire dose over different time periods, including 1, 10, or 90 minutes. Analyses during the in-life phase of the experiment included sequential evaluation of whole blood for vector genomes and appearance of proinflammatory cytokines. Liver tissues were analyzed at the time of necropsy for enhanced green fluorescent protein (eGFP) expression and vector genomes. The data were remarkable with a relative absence of any statistically significant effect of infusion time on vector transduction, safety, and clearance. However, some interesting and unexpected trends did emerge.

Bioengineering of differentiated hepatocytes with human factor IX-expressing plasmids in vitro

For somatic gene therapy of hemophilia B, hepatocytes as the main cellular host for expression of hFIX are attractive targets. In gene therapy protocols, an efficient expression vector equipped with cis-regulatory elements such as introns is required. With this in mind, hFIX-expressing plasmids equipped with different combinations of 2 human β-globin (hBG) introns inside the hFIX-cDNA and Kozak element were used for bioengineering of HepG2 cells as a model for differentiated hepatocytes and CHO cells a cell line generally used to produce recombinant hFIX (rhFIX). In HepG2 cells, the highest hFIX secretion level occurred for the intron-less plasmid with 8.5 to 53.8- fold increases, while in CHO cells, the hBG intron-I containing plasmid induced highest hFIX secretion level with 2.3 to 14.3-fold increases as compared to other plasmids. The first hBG intron appears to be more effective than the second one in both cell lines. The expression level was further increased upon the inclusion of the Kozak element. The highest hFIX activity was obtained from the cells that carrying the intron-less plasmids with 470 mU/ml and 25 mU/ml for HepG2 and CHO cells respectively. Secretion of active hFIX by all constructs was documented except for hBG intron-II containing construct in both cell lines. HepG2 cells were able to secret higher hFIX levels by 0.6 to 112.2-fold increases with activity by 5.3 to 16.4-fold increases compared to CHO cells transfected with the same constructs. Presence of both hBG intron-I and II inside the hFIX-cDNA provides properly spliced hFIX transcripts in both cell lines. In conclusion, the advantages of hBG introns as attractive cis-regulatory elements to obtain higher expression level of hFIX particularly in CHO cells were demonstrated. Hepatocytes could be effectively bioengineered with the use of plasmid vectors and this strategy may provide a potential in-vitro source of functional hepatocytes for ex-vivo gene therapy of hemophilias and production of rhFIX in vitro.

Genetic modification of bone-marrow mesenchymal stem cells and hematopoietic cells with human coagulation factor IX-expressing plasmids

Ex-vivo gene therapy of hemophilias requires suitable bioreactors for secretion of hFIX into the circulation and stem cells hold great potentials in this regard. Viral vectors are widely manipulated and used to transfer hFIX gene into stem cells. However, little attention has been paid to the manipulation of hFIX transgene itself. Concurrently, the efficacy of such a therapeutic approach depends on determination of which vectors give maximal transgene expression. With this in mind, TF-1 (primary hematopoietic lineage) and rat-bone marrow mesenchymal stem cells (BMSCs) were transfected with five hFIX-expressing plasmids containing different combinations of two human β-globin (hBG) introns inside the hFIX-cDNA and Kozak element and hFIX expression was evaluated by different methods. In BMSCs and TF-1 cells, the highest hFIX level was obtained from the intron-less and hBG intron-I,II containing plasmids respectively. The highest hFIX activity was obtained from the cells that carrying the hBG intron-I,II containing plasmids. BMSCs were able to produce higher hFIX by 1.4 to 4.7-fold increase with activity by 2.4 to 4.4-fold increase compared to TF-1 cells transfected with the same constructs. BMSCs and TF-1 cells could be effectively bioengineered without the use of viral vectors and hFIX minigene containing hBG introns could represent a particular interest in stem cell-based gene therapy of hemophilias.

Translational data from adeno-associated virus-mediated gene therapy of hemophilia B in dogs

Preclinical testing of new therapeutic strategies in relevant animal models is an essential part of drug development. The choice of animal models of disease that are used in these studies is driven by the strength of the translational data for informing about safety, efficacy, and success or failure of human clinical trials. Hemophilia B is a monogenic, X-linked, inherited bleeding disorder that results from absent or dysfunctional coagulation factor IX (FIX). Regarding preclinical studies of adeno-associated virus (AAV)-mediated gene therapy for hemophilia B, dogs with severe hemophilia B (<1% FIX) provide well-characterized phenotypes and genotypes in which a species-specific transgene can be expressed in a mixed genetic background. Correction of the hemophilic coagulopathy by sustained expression of FIX, reduction of bleeding events, and a comprehensive assessment of the humoral and cell-mediated immune responses to the expressed transgene and recombinant AAV vector are all feasible end points in these dogs. This review compares the preclinical studies of AAV vectors used to treat dogs with hemophilia B with the results obtained in subsequent human clinical trials using muscle- and liver-based approaches.

AAV-mediated delivery of zinc finger nucleases targeting hepatitis B virus inhibits active replication

Despite an existing effective vaccine, hepatitis B virus (HBV) remains a major public health concern. There are effective suppressive therapies for HBV, but they remain expensive and inaccessible to many, and not all patients respond well. Furthermore, HBV can persist as genomic covalently closed circular DNA (cccDNA) that remains in hepatocytes even during otherwise effective therapy and facilitates rebound in patients after treatment has stopped. Therefore, the need for an effective treatment that targets active and persistent HBV infections remains. As a novel approach to treat HBV, we have targeted the HBV genome for disruption to prevent viral reactivation and replication. We generated 3 zinc finger nucleases (ZFNs) that target sequences within the HBV polymerase, core and X genes. Upon the formation of ZFN-induced DNA double strand breaks (DSB), imprecise repair by non-homologous end joining leads to mutations that inactivate HBV genes. We delivered HBV-specific ZFNs using self-complementary adeno-associated virus (scAAV) vectors and tested their anti-HBV activity in HepAD38 cells. HBV-ZFNs efficiently disrupted HBV target sites by inducing site-specific mutations. Cytotoxicity was seen with one of the ZFNs. scAAV-mediated delivery of a ZFN targeting HBV polymerase resulted in complete inhibition of HBV DNA replication and production of infectious HBV virions in HepAD38 cells. This effect was sustained for at least 2 weeks following only a single treatment. Furthermore, high specificity was observed for all ZFNs, as negligible off-target cleavage was seen via high-throughput sequencing of 7 closely matched potential off-target sites. These results show that HBV-targeted ZFNs can efficiently inhibit active HBV replication and suppress the cellular template for HBV persistence, making them promising candidates for eradication therapy.

AAV-mediated delivery of zinc finger nucleases targeting hepatitis B virus inhibits active replication

Despite an existing effective vaccine, hepatitis B virus (HBV) remains a major public health concern. There are effective suppressive therapies for HBV, but they remain expensive and inaccessible to many, and not all patients respond well. Furthermore, HBV can persist as genomic covalently closed circular DNA (cccDNA) that remains in hepatocytes even during otherwise effective therapy and facilitates rebound in patients after treatment has stopped. Therefore, the need for an effective treatment that targets active and persistent HBV infections remains. As a novel approach to treat HBV, we have targeted the HBV genome for disruption to prevent viral reactivation and replication. We generated 3 zinc finger nucleases (ZFNs) that target sequences within the HBV polymerase, core and X genes. Upon the formation of ZFN-induced DNA double strand breaks (DSB), imprecise repair by non-homologous end joining leads to mutations that inactivate HBV genes. We delivered HBV-specific ZFNs using self-complementary adeno-associated virus (scAAV) vectors and tested their anti-HBV activity in HepAD38 cells. HBV-ZFNs efficiently disrupted HBV target sites by inducing site-specific mutations. Cytotoxicity was seen with one of the ZFNs. scAAV-mediated delivery of a ZFN targeting HBV polymerase resulted in complete inhibition of HBV DNA replication and production of infectious HBV virions in HepAD38 cells. This effect was sustained for at least 2 weeks following only a single treatment. Furthermore, high specificity was observed for all ZFNs, as negligible off-target cleavage was seen via high-throughput sequencing of 7 closely matched potential off-target sites. These results show that HBV-targeted ZFNs can efficiently inhibit active HBV replication and suppress the cellular template for HBV persistence, making them promising candidates for eradication therapy.

Exploiting natural diversity of AAV for the design of vectors with novel properties

Twelve AAV serotypes have been described so far in human and nonhuman primate (NHP) populations while surprisingly high diversity of AAV sequences is detected in tissue biopsies. The analysis of these novel AAV sequences has indicated a rapid evolution of the viral genome both by accumulation of mutations and recombination. This chapter describes how this rich resource of naturally evolved sequences is used to derive gene transfer vectors with a wide array of activities depending on the nature of the cap gene used in the packaging system. AAV2-based recombinant genomes have been packaged in dozens of different capsid types, resulting in a wide array of "pseudotyped vectors" that constitute a rich resource for the development of gene therapy clinical trials. We describe a polymerase chain reaction-based molecular rescue method for novel AAV isolation that uses primers designed to recognize the highly conserved regions in known AAV isolates and generate amplicons across the hypervariable regions of novel AAV genomes present in the analyzed sample.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Evaluation of adeno-associated viral vectors for liver-directed gene transfer in dogs

This study evaluated six adeno-associated viral (AAV) vectors expressing green fluorescent protein (GFP) from the liver-specific thyroid hormone-binding globulin (TBG) promoter made with novel capsids in canine liver-directed gene transfer. Studies in 1.5-month-old dogs, which were administered vector through a peripheral vein, showed that AAV8 capsid vectors had the most favorable performance profiles. Interestingly, the absolute levels of hepatocyte transduction achieved with AAV8 were lower in dogs compared with what had been achieved in mice and nonhuman primates. Additional studies were performed with AAV8 delivered into the hepatic artery in adult dogs, with higher doses of vector used to assess potential dose-limiting toxicities. These studies showed good transduction on day 7 in one dog that apparently was lost by day 28 in another dog through the generation of GFP-specific T cells. Each adult dog was carefully monitored for any hemodynamic changes associated with vector infusion. Both animals demonstrated mild to moderate hypotension and bradycardia, which appeared to be anesthesia-related, making it difficult to evaluate contributions of the vector.

Activin IIB receptor blockade attenuates dystrophic pathology in a mouse model of Duchenne muscular dystrophy

Modulation of transforming growth factor-β (TGF-β) signaling to promote muscle growth holds tremendous promise for the muscular dystrophies and other disorders involving the loss of functional muscle mass. Previous studies have focused on the TGF-β family member myostatin and demonstrated that inhibition of myostatin leads to muscle growth in normal and dystrophic mice. We describe a unique method of systemic inhibition of activin IIB receptor signaling via adeno-associated virus (AAV)-mediated gene transfer of a soluble form of the extracellular domain of the activin IIB receptor to the liver. Treatment of mdx mice with activin IIB receptor blockade led to increased skeletal muscle mass, increased force production in the extensor digitorum longus (EDL), and reduced serum creatine kinase. No effect on heart mass or function was observed. Our results indicate that activin IIB receptor blockade represents a novel and effective therapeutic strategy for the muscular dystrophies.

Gene therapy with helper-dependent adenoviral vectors: current advances and future perspectives

Recombinant Adenoviral vectors represent one of the best gene transfer platforms due to their ability to efficiently transduce a wide range of quiescent and proliferating cell types from various tissues and species. The activation of an adaptive immune response against the transduced cells is one of the major drawbacks of first generation Adenovirus vectors and has been overcome by the latest generation of recombinant Adenovirus, the Helper-Dependent Adenoviral (HDAd) vectors. HDAds have innovative features including the complete absence of viral coding sequences and the ability to mediate high level transgene expression with negligible chronic toxicity. This review summarizes the many aspects of HDAd biology and structure with a major focus on in vivo gene therapy application and with an emphasis on the unsolved issues that these vectors still presents toward clinical application.

Enhancement of the human factor IX expression, mediated by an intron derived fragment from the rat aldolase B gene in cultured hepatoma cells

Combinations of a liver-specific rat aldolase B intronic enhancer (rABE) with either of the hepatocyte-specific human α1-antitrypsin promoter (hAATp) and cytomegalovirus enhancer/promoter (CMVp) were used to construct a number of plasmids expressing non-viral human factor IX (hFIX). The efficacies of the plasmids were evaluated in a hepatocyte cell line (HepG2). Potential of the rABE was evidenced, by 300%--and 800% increase of the hFIX expression levels when it was combined with the CMVp and hAATp, respectively. The highest hFIX expression level was obtained when the rABE was combined with the CMVp for which the maximum intracellular accumulation of hFIX was also evidenced. Therefore, the rABE is suggested as a suitable cis-acting element for protein expression in hepatocytes. Considering the potential of introns during post-transcriptional processes, the function of the human β-globin (hBG) intron-II, within the hFIX coding region, in the second generations of the hFIX expressing plasmids was also examined, which leaded to reduction of the hFIX expression level, probably due to improper splicing of the hBG intron-II.

Systemic myostatin inhibition via liver-targeted gene transfer in normal and dystrophic mice

Background

Myostatin inhibition is a promising therapeutic strategy to maintain muscle mass in a variety of disorders, including the muscular dystrophies, cachexia, and sarcopenia. Previously described approaches to blocking myostatin signaling include injection delivery of inhibitory propeptide domain or neutralizing antibodies.

Methodology/Principal Findings

Here we describe a unique method of myostatin inhibition utilizing recombinant adeno-associated virus to overexpress a secretable dominant negative myostatin exclusively in the liver of mice. Systemic myostatin inhibition led to increased skeletal muscle mass and strength in control C57 Bl/6 mice and in the dystrophin-deficient mdx model of Duchenne muscular dystrophy. The mdx soleus, a mouse muscle more representative of human fiber type composition, demonstrated the most profound improvement in force production and a shift toward faster myosin-heavy chain isoforms. Unexpectedly, the 11-month-old mdx diaphragm was not rescued by long-term myostatin inhibition. Further, mdx mice treated for 11 months exhibited cardiac hypertrophy and impaired function in an inhibitor dose–dependent manner.

Conclusions/Significance

Liver-targeted gene transfer of a myostatin inhibitor is a valuable tool for preclinical investigation of myostatin blockade and provides novel insights into the long-term effects and shortcomings of myostatin inhibition on striated muscle.

A systematic study of the function of the human beta-globin introns on the expression of the human coagulation factor IX in cultured Chinese hamster ovary cells

BACKGROUND

Intronic sequences have the potential to improve gene expression in eukaryotes by a variety of mechanisms. In this context, human beta-globin (hBG) introns were inserted into the human factor IX (hFIX) cDNA in cytomegalovirus (CMV)-regulated plasmids. The resulting construct was then used for further expression analysis in vitro.

METHODS

Seven hFIX-expressing plasmids with different combinations of the two hBG introns and the Kozak element were constructed and used for a systematic expression analysis in cultured Chinese hamster ovary (CHO) cells. In parallel, the hBG intronic sequences were analysed for the presence of possible regulatory elements.

RESULTS

All the constructed plasmids resulted in transient expression of the hFIX. However, the coagulation activities varied according to the particular constructs used. Based on the hFIX antigenic assay, a wide range of variation was observed during persistent expression. The second hBG intron appears to be more effective than the first one. The expression level was further increased upon the inclusion of the Kozak element. Sequence analysis has detected several transcription factor binding (TFB) motifs in both of the introns, but with a higher frequency in the second one.

CONCLUSIONS

Potentials of hBG introns as enhancer-like elements for the expression of the hFIX in cultured CHO cells and a higher activity with respect to the second hBG intron compared to the first one were demonstrated. The larger number of TFBs in the second hBG intron reflects its stronger effect. The results obtained suggest possible synergistic functions of the hBG introns and Kozak on the expression level of hFIX in vitro.

AAV gene therapy as a means to increase apolipoprotein (Apo) A-I and high-density lipoprotein-cholesterol levels: correction of murine ApoA-I deficiency

BACKGROUND

Inherited apolipoprotein (Apo) A-I deficiency is an orphan disorder characterized by high-density lipoprotein (HDL)-cholesterol deficiency and premature atherosclerosis. Constitutive over-expression of ApoA-I might provide a means to treat this disease. The present study provides a comprehensive evaluation of adeno-associated virus (AAV)-mediated ApoA-I gene delivery to express human (h)ApoA-I and correct the low HDL-cholesterol phenotype associated with ApoA-I deficiency.

METHODS

In an effort to maximize AAV-mediated gene expression, we performed head-to-head comparisons of recombinant AAVs with pseudotype capsids 1, 2, 6 and 8 administered by different routes with the use of five different liver-specific promoters in addition to cytomegalovirus as single-stranded or as self-complementary (sc) AAV vectors.

RESULTS

Intravenous administration of 1 x 10(13) gc/kg scAAV8, in combination with the liver-specific promoter LP1, in female ApoA-I(-/-) mice resulted in hApoA-I expression levels of 634 +/- 69 mg/l, which persisted for the duration of the study (15 weeks). This treatment resulted in full recovery of HDL-cholesterol levels with correction of HDL particle size and apolipoprotein composition. In addition, we observed increased adrenal cholesterol content and a significant increase in bodyweight in treated mice.

CONCLUSIONS

The present study demonstrates that systemic delivery of a scAAV8 vector provides a means for efficient liver expression of hApoA-I, thereby correcting the lipid abnormalities associated with murine ApoA-I deficiency. Importantly, the study demonstrates that AAV-based gene therapy can be used to express therapeutic proteins at a high level for a prolonged period of time and, as such, provides a basis for further development of this strategy to treat hApoA-I deficiency.

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

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

Quiescent subpopulations of human CD34-positive hematopoietic stem cells are preferred targets for stable recombinant adeno-associated virus type 2 transduction

We have previously demonstrated recombinant adeno-associated viral (rAAV) transduction of human CD34(+) hematopoietic stem cells (HSCs) capable of serial engraftment in vivo. Here we evaluated the capacity of rAAV2 to mediate gene transfer into nondividing, quiescent, primitive CD34(+) cells subdivided on the basis of metabolic, mitotic, and phenotypic properties. Results revealed that CD34(+)CD38() marrow cells are the most quiescent, exist primarily in G(0) at isolation and are the only population to remain nondividing during the entire exposure to free rAAV. Despite significant differences in the extended clonogenic capacities of CD34(+) subsets in stromal cultures, the frequency of rAAV marking of colonies derived from primitive progenitors was similar in all three populations, suggesting that both primitive and more differentiated progenitors were initially transduced at equal levels. After transduction, episomal and integrated rAAV genomes were detected in all CD34(+) subsets. However, the more quiescent cells displayed higher levels of integrated rAAV than did rapidly dividing cells. Importantly, stable long-term integration was observed only in the most primitive, quiescent CD34(+)CD38(-) subset, indicating that this HSC compartment comprises the preferred substrate for stable rAAV2 transduction. Previously described rate limitations to transgene expression were observed in transduced CD34(+) cells and could be overcome by tyrphostin pretreatment, which resulted in augmented second-strand synthesis. These results represent the first demonstration of rAAV-mediated gene transfer to primitive, quiescent human CD34(+)CD38(-) stem cells and reveal that nondividing CD34(+)CD38(-) HSCs are the optimal CD34(+) targets for rAAV transduction.

Intrahepatic injection of recombinant adeno-associated virus serotype 2 overcomes gender-related differences in liver transduction

The liver is an attractive organ for gene therapy because of its important role in many inherited and acquired diseases. Recombinant adeno-associated viruses (rAAVs) have been shown to be good candidates for liver gene delivery, leading to long-term gene expression. We evaluated the influence of the route of administration on rAAV-mediated liver transduction by comparing levels of luciferase expression in the livers of male and female mice after injection of rAAV serotype 2, using three different routes of administration: intravenous (IV), intraportal (IP), or direct intrahepatic (IH) injection. To determine transgene expression we used a noninvasive optical bioluminescence imaging system that allowed long-term in vivo analysis. After IV injection dramatic differences in liver transgene expression were observed, depending on gender. When IP injection was used the differences were reduced although they were still significant. Interestingly, direct intrahepatic injection of rAAV vectors was associated with the fastest and strongest onset of luciferase expression. Moreover, no gender differences in liver transduction were observed and luciferase expression was confined to the site of injection. Thus, direct intrahepatic injection of rAAV offers specific advantages, which support the potential of this route of administration for future clinical applications.

Comparison of promoter region constructs for in vivo intramuscular expression

BACKGROUND

High transgene expression is generally expected after gene transfer. However, different level, kinetics and localization of expression might be needed for relevant therapeutic applications. Former studies have compared various promoter regions driving gene expression leading to conflicting results. In the present work, two promoter families have been compared using the efficient in vivo intramuscular electrotransfer technique.

METHODS

Three promoter regions were constructed by associating the strong ubiquitous cytomegalovirus (CMV) enhancer-promoter to its homologous intron A or to a heterologous intron, or to a hybrid intron. Promoter regions derived from the muscle creatine kinase (MCK) promoter were also studied. The expression of the same transgene (SeAP or neurotrophin-3) under control of these different promoters was compared after plasmid electrotransfer in mouse tibialis-cranialis skeletal muscle.

RESULTS

Heterologous intron association to the CMV promoter did not modify gene expression kinetics nor increase gene expression level. Usefulness of intron A or hybrid intron association to the CMV promoter depended on the gene. The various MCK promoters drove efficient gene expression but lower than that obtained with the CMV promoter. Furthermore, peak value was reached earlier with MCK promoter regions (14 days).

CONCLUSION

For applications of gene transfer restricted to skeletal muscle, the MCK promoter or a MCK promoter variant would be a promising alternative to the CMV promoter. Indeed, it has been demonstrated that the use of MCK promoter limits humoral and cell-mediated immune responses. Furthermore, the MCK promoter decreases the initial expression peak that may be detrimental, drives a sustained gene expression, and improves gene transfer safety.

Rapamycin control of transgene expression from a single AAV vector in mouse salivary glands

Salivary glands (SGs) appear to be a useful target site for gene therapeutics. The ability to control transgene expression is essential for clinical application. Previously, in a proof-of-concept study, we have shown that the rapamycin-inducible transcriptional regulation system can regulate protein expression after adenoviral-mediated gene transfer to SGs. To evaluate the potential ability to utilize this regulatory system for long-term control of transgene expression in this tissue, we employed a 'third generation', single adenoassociated serotype 2 viral (AAV2) vector encoding human erythropoietin (hEPO) under the control of a rapamycin-inducible promoter. The vector, rAAV-TF2.3-hEPO (10(10) particles/animal), was delivered to mouse SGs. No detectable increase in serum hEPO or hematocrit levels was observed in the absence of rapamycin administration. However, rapamycin induced elevation of serum hEPO levels, as well as concomitant hematocrit changes, that were dose-dependent, completely reversible, and relatively stable over the course of this study (6 months), with no appreciable change in rapamycin responsiveness. Our results suggest that the rapamycin transcriptional regulation system delivered in a single AAV2 vector to SGs may be valuable for systemic protein replacement applications.

Sustained phenotypic correction of canine hemophilia B after systemic administration of helper-dependent adenoviral vector

We have evaluated the potential of liver-directed, helper-dependent adenoviral (HDAd) vector-mediated gene therapy in the hemophilia B dog. Two dogs were injected intravenously with HDAd (3 x 10(12) VP/kg) bearing a liver-restricted canine coagulation factor IX (FIX) expression cassette. After injection, the whole blood clotting time for both dogs declined from >60 min to </=20 min for at least 604 and 446 days, respectively. Peak FIX activities of 34.1 and 129.2% were detected at 12x14 days and then slowly declined to 2 to 5% by 120 days and stabilized at these therapeutic levels for at least 418 and 257 days. For one dog, a peak FIX level of 500 ng/ml was achieved and stabilized at >170 ng/ml for at least 256 days. For the other dog, a peak FIX level of 1258 ng/ml was achieved and stabilized at >400 ng/ml for at least 213 days. Inhibitor formation was not evident in either animal. Importantly, whereas untreated hemophilia B dogs suffer five or six spontaneous bleeds per year, the treated dogs suffered no such bleeds postinjection. Significantly, this study is the first to demonstrate long-term phenotypic correction of a genetic disorder in a large animal with HDAd. Although no evidence of chronic toxicity was observed in either animal, systemic vector administration at 3 x 10(12) VP/kg was accompanied by acute, albeit transient and variable laboratory abnormalities (alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, creatine phosphokinase, and platelet counts). The results of this study highlight both the potential benefit and the risk associated with systemic intravascular delivery of high-dose HDAd for liver-directed gene therapy.

Large animal models and gene therapy

Over the last two decades, gene transfer experiments for the treatment of inherited or acquired diseases have mainly been performed in mice. While mice provide proof of principle and allow testing of a variety of therapeutic modalities, mouse models have some limitations, as only short-term experiments can be performed, their homogenous genetic background is unlike humans, and the knockout models do not always faithfully represent the human disease. Naturally occurring large animal models of human genetic diseases have become increasingly important despite the costs and the extensive clinical attention they require because of their similarities to human patients. Large animals are reasonably outbred, long lived allowing for longitudinal studies, are more similar in size to a neonate or small child providing an opportunity to address issues related to scaling up therapy, and many physiological parameters including the immune system are more similar to those in humans versus those in mice.

Aminoglycoside suppression of nonsense mutations in severe hemophilia

Aminoglycoside antibiotics exhibit their bactericidal effect by interfering with normal ribosomal activity. In this pilot study, we have evaluated the effect of the aminoglycoside antibiotic gentamicin on the factor VIII (FVIII) and IX levels of severe hemophiliacs with known nonsense mutations. Five patients were enrolled and each patient was given 3 consecutive days of gentamicin at a dose of 7 mg/kg intravenously every 24 hours. Two patients (patient no. 1: hemophilia A, Ser1395Stop; and patient no. 5: hemophilia B, Arg333Stop) showed a decrease in their activated partial thromboplastin time (aPTT), an increase in their FVIII (0.016 IU/mL, 1.6%) or FIX (0.02 IU/mL, 2%) levels, and an increase in thrombin generation. The remaining 3 patients (patient no. 2: hemophilia B, Arg252Stop; patient no. 3: hemophilia A, Arg2116Stop; and patient no. 4: hemophilia A, Arg427Stop) showed no response in the aPTTs or factor levels, but one (patient no. 2: hemophilia B, Arg252Stop) showed an increase in the factor IX antigen level (2%-5.5%) that persisted throughout the period of the study and was concordant with an increase in thrombin generation. Gentamicin is unlikely to be an effective treatment for severe hemophilia due to its potential toxicities and the minimal response documented in this report. This study, however, does provide a proof of principle, suggesting that ribosomal interference with a less toxic agent may be a potential therapeutic mechanism for severe hemophilia patients with nonsense mutations.

In utero gene therapy: current challenges and perspectives

Over the past few years, considerable progress in prenatal diagnosis and surgery combined with improvements in vector design vindicate a reappraisal of the feasibility of in utero gene therapy for serious monogenetic diseases. As adult gene therapy gathers pace, several apparent obstacles to its application as a treatment may be overcome by pre- or early postnatal treatment. This review will examine the concepts and practice of prenatal vector administration. We aim to highlight the advantages of early therapeutic intervention focusing on diseases that could benefit greatly from a prenatal gene therapy approach. We will pay special attention to the strategies and vectors that are most likely to be used for this application and will speculate on their expected developments for the near future.

A direct comparison of two nonviral gene therapy vectors for somatic integration: in vivo evaluation of the bacteriophage integrase phiC31 and the Sleeping Beauty transposase

In this study we performed a head-to-head comparison of the integrase phiC31 derived from a Streptomyces phage and the Sleeping Beauty (SB) transposase, a member of the TC1/mariner superfamily of transposable elements. Mouse liver was cotransfused with a vector containing our most robust human coagulation factor IX expression cassette and the appropriate recombinase recognition site and either a phiC31- or a SB transposase-expressing vector. To analyze transgene persistence and to prove somatic integration in vivo we induced cell cycling of mouse hepatocytes and found that the transgene expression levels dropped by only 16 to 21% and 56 to 66% in mice that received phiC31 and SB, respectively. Notably, no difference in the toxicity profile was detected in mice treated with either recombinase. Moreover we observed that with the integrase-mediated gene transfer, transgene expression levels were dependent on the remaining noncoding vector sequences, which also integrate into the host genome. Further analyses of a hot spot of integration after phiC31-mediated integration revealed small chromosomal deletions at the target site and that the recombination process was not dependent on the orientation in which the phiC31 recognition site attached to the pseudo-recognition sites in the host genome. Coupled together with ongoing improvements in both systems this study suggests that both nonviral vector systems will have important roles in achieving stable gene transfer in vivo.

Development and characterization of a cell line for large-scale, serum-free production of recombinant adeno-associated viral vectors

BACKGROUND

One of the major limitations to the use of adeno-associated virus (AAV) vectors for gene therapy has been the difficulty in producing enough vector to supply a clinical trial. More than 20 000 roller bottles may be required to generate AAV by the traditional transient transfection process to treat 50 patients. A scalable AAV producer cell line grown in serum-free media will meet the needs for the manufacture of AAV gene therapeutics.

METHODS

A packaging cell line was generated by introducing the AAV rep and cap genes into A549 cells. From this packaging cell line, a number of producer cell lines were generated by infecting the packaging cell with the appropriate AAV vector. Producer cell lines were then adapted to serum-free suspension conditions for growth in bioreactors.

RESULTS

We report here the development of six AAV producer cell lines that generate > 10(4) particles/cell. The rAAV vector preparations from these cell lines have physical and functional characteristics similar to rAAV vectors prepared by transient transfection. To enable large-scale production, producer cell lines were adapted to serum-free suspension and we demonstrate production of AAV at the 15 L scale. In addition, vector preparations from these cell lines were shown to be free of wild-type AAV.

CONCLUSIONS

AAV producer cell lines can be readily scaled to meet the needs of clinical trials. One 500 L bioreactor of these producer cells can produce the equivalent of 2500 high capacity roller bottles or 25 000 T-175 tissue culture flasks.

Fetal and neonatal gene therapy: benefits and pitfalls

The current approaches to gene therapy of monogenetic diseases into mature organisms are confronted with several problems including the following: (1) the underlying genetic defect may have already caused irreversible pathological changes; (2) the level of sufficient protein expression to ameliorate or prevent the disease requires prohibitively large amounts of gene delivery vector; (3) adult tissues may be poorly infected by conventional vector systems dependent upon cellular proliferation for optimal infection, for example, oncoretrovirus vectors; (4) immune responses, either pre-existing or developing following vector delivery, may rapidly eliminate transgenic protein expression and prevent future effective intervention. Early gene transfer, in the neonatal or even fetal period, may overcome some or all of these obstacles. The mammalian fetus enjoys a uniquely protected environment in the womb, bathed in a biochemically and physically supportive fluid devoid of myriad extra-uterine pathogens. Strong physical and chemical barriers to infection might, perhaps, impede the frenetic cell division. The physical support and the biochemical support provided by the fetal-maternal placental interface may, therefore, minimize the onset of genetic diseases manifest early in life. The fetal organism must prepare itself for birth, but lacking a mature adaptive immune system may depend upon more primordial immune defences. It is the nature of these defences, and the vulnerabilities they protect, that are poorly understood in the context of gene therapy and might provide useful information for approaches to gene therapy in the young, as well as perhaps the mature organism.

Immune responses to gene therapy vectors: influence on vector function and effector mechanisms

Circumventing the immune response to the vector is a major challenge with all vector types. Viral vectors are the most likely to induce an immune response, especially those, like adenovirus and AAV, which express immunogenic epitopes within the organism. The first immune response occurring after vector transfer emerges from the innate immune system, mainly consisting in a rapid (few hours) inflammatory cytokines and chemokines secretion around the administration site. This reaction is high with adenoviral vectors and almost null with AAV. It is noteworthy that plasmid DNA vectors, because of CpG stimulatory islets, also stimulate the innate immunity via the stimulation of TLR receptors on leukocytes. Specific immune response leading to antibodies production and T lymphocytes activation also occurs within a few days after vector introduction. Capsid antigens are mostly responsible for specific immunity toward adenoviruses, and are also involved in the response against AAV. In the former case only, however, viral gene-encoded proteins can also be immunogenic. The pre-existing humoral immunity coming from early infections with wild-type AAV or adenovirus can prevent efficient gene transfer with the corresponding vectors. In all cases, some parameters like route of administration, dose, or promoter type have been extensively described as critical factors influencing vector immunity. Strategies to fight against vector-induced immunity can come from the immunology field, since tolerance induction or immunosuppression are a possibility. Alterations to vector structure have also been extensively performed to circumvent the immune system and thus enhance gene transfer efficiency and safety.

Recombinant adeno-associated viral (rAAV) vectors as therapeutic tools for Duchenne muscular dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a lethal genetic muscle disorder caused by recessive mutations in the dystrophin gene. The size of the gene (2.4 Mb) and mRNA (14 kb) in addition to immunogenicity problems and inefficient transduction of mature myofibres by currently available vector systems are formidable obstacles to the development of efficient gene therapy approaches. Adeno-associated viral (AAV) vectors overcome many of the problems associated with other vector systems (nonpathogenicity and minimal immunogenicity, extensive cell and tissue tropism) but accommodate limited transgene capacity (<5 kb). As a result of these observations, a number of laboratories worldwide have engineered a series of microdystrophin cDNAs based on genotype-phenotype relationship in Duchenne (DMD) and Becker (BMD) dystrophic patients, and transgenic studies in mdx mice. Recent progress in characterization of AAV serotypes from various species has demonstrated that alternative AAV serotypes are far more efficient in transducing muscle than the traditionally used AAV2. This article summarizes the current progress in the field of recombinant adeno-associated viral (rAAV) delivery for DMD, including optimization of recombinant AAV-microdystrophin vector systems/cassettes targeting the skeletal and cardiac musculature.

Salivary glands: novel target sites for gene therapeutics

Directing the local or systemic expression of therapeutic proteins is a potentially important clinical application of gene transfer technology. Gene-based therapeutics theoretically offer many advantages over protein therapeutics. Numerous tissues have been evaluated for this purpose in animal models, most commonly the liver and skeletal muscle. Based on pre-clinical studies, we suggest that salivary glands are a valuable, yet under-appreciated, target tissue for both systemic and upper gastrointestinal tract gene therapeutic applications.