Expression of a truncated cystic fibrosis transmembrane conductance regulator with an AAV5-pseudotyped vector in primates

Adeno-associated virus therapies: Pioneering solutions for human genetic diseases

Adeno-associated virus (AAV) has emerged as a fundamental component in the gene therapy landscape, widely acknowledged for its effectiveness in therapeutic gene delivery. The success of AAV-based therapies, such as Luxturna and Zolgensma, underscores their potential as a leading vector in gene therapy. This article provides an in-depth review of the development and mechanisms of AAV vector-based therapies, offering a comprehensive analysis of the latest clinical trial outcomes in central nervous system (CNS) diseases, ocular conditions, and hemophilia, where AAV therapies have shown promising results. Additionally, we discusse the selection of administration methods and serotypes tailored to specific diseases. Our objective is to showcase the innovative applications and future potential of AAV-based gene therapy, laying the groundwork for continued clinical advancements.

Amelioration of airway and GI disease in G551D-CF ferrets by AAV1 and AAV6

Gene therapy for CF has concentrated on targeting the lung. Here we took a different approach by injecting into the cephalic vein and spraying into the trachea of G551D, CF ferrets either AAV1 or 6 containing Δ27-264-CFTR, a truncated version of CFTR. Treatment with the potentiator VX-770 was halted for 7 days before instillation to induce a disease phenotype. Indeed, all ferrets were pancreas-insufficient when they entered the study. Four ferrets (three receiving AAV1 and one AAV6) were necropsied 48 days after vector delivery, and four (three receiving AAV6, one AAV1) were euthanized or died prior to the planned necropsy. AAV1 or AAV6 vector genomes, mRNA expression, and CFTR protein were detected in all tracheal and lung samples and in the liver, pancreas, and ileum of the treated ferrets. Surface and basal airway cells, pancreatic and bile ducts, and ileal crypts and villi were successfully transduced. Obstruction of the airways accompanied by pulmonary hemorrhaging, plugged pancreatic and bile ducts as well as mucous plugs in the ileum were noticed in untreated but absent from transduced ferrets necropsied at 48 days. Transduction of G551D ferrets suggests that a combination of systemic and airway application may be the preferred route of delivery for CF.

Gene Therapy for Genetic Syndromes: Understanding the Current State to Guide Future Care

Gene therapy holds promise as a life-changing option for individuals with genetic variants that give rise to disease. FDA-approved gene therapies for Spinal Muscular Atrophy (SMA), cerebral adrenoleukodystrophy, β-Thalassemia, hemophilia A/B, retinal dystrophy, and Duchenne Muscular Dystrophy have generated buzz around the ability to change the course of genetic syndromes. However, this excitement risks over-expansion into areas of genetic disease that may not fit the current state of gene therapy. While in situ (targeted to an area) and ex vivo (removal of cells, delivery, and administration of cells) approaches show promise, they have a limited target ability. Broader in vivo gene therapy trials have shown various continued challenges, including immune response, use of immune suppressants correlating to secondary infections, unknown outcomes of overexpression, and challenges in driving tissue-specific corrections. Viral delivery systems can be associated with adverse outcomes such as hepatotoxicity and lethality if uncontrolled. In some cases, these risks are far outweighed by the potentially lethal syndromes for which these systems are being developed. Therefore, it is critical to evaluate the field of genetic diseases to perform cost-benefit analyses for gene therapy. In this work, we present the current state while setting forth tools and resources to guide informed directions to avoid foreseeable issues in gene therapy that could prevent the field from continued success.

Endobronchial Aerosolized AAV1.SERCA2a Gene Therapy in a Pulmonary Hypertension Pig Model: Addressing the Lung Delivery Bottleneck

A disappointing number of new therapies for pulmonary hypertension (PH) have been successfully translated to the clinic. Adeno-associated viral (AAV) gene therapy has the potential to treat the underlying pathology of PH, but the challenge remains in efficient and safe delivery. The aims of this study were (1) to test the efficacy of endobronchial aerosolization delivery for AAV1-mediated sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2a (SERCA2a) gene therapy in a PH pig model and (2) to identify the most efficient airway administration modality for in-lung gene therapy in PH. We hypothesized that delivery to the distal bronchi increases lung viral uptake and avoids virus loss in off-target compartments. In part 1 of the study, PH was induced in pigs by surgically banding the pulmonary veins. Two months postsurgery, 1 × 1013 viral genomes (vg) of AAV1.SERCA2a or saline was endobronchially aerosolized using a bronchoscope. Two months after aerosolization, high vg copies (vgc) were detected in the lungs, accompanied by functional and morphometrical amelioration of PH. In part 2 of the study, we directly compared the endobronchial aerosolization gene delivery to the intratracheal aerosolization in PH pigs. Endobronchial delivery demonstrated higher viral expression (6,719 ± 927 vs. 1,444 ± 402 vgc/100 ng DNA, p = 0.0017), suggesting this delivery modality is a promising method for clinical AAV gene therapy for PH.

Short-Term Steroid Treatment of Rhesus Macaque Increases Transduction

Repeat dosing poses a major hurdle for the development of an adeno-associated virus (AAV)-based gene therapy for cystic fibrosis, in part because of the potential for development of an immune reaction to the AAV1 capsid proteins. Here, to dampen the immune response to AAV1, we treated Rhesus monkeys with methylprednisolone before and after the instillation of two doses of AAV1Δ27-264-CFTR into their airways at 0 and 30 days, followed by a single dose of AAV1-GFP on day 60. Animals were euthanized on day 90, except for one monkey that was sacrificed at 1 year. No adverse events occurred, indicating that the two AAV1 vectors are safe. rAAV1-CFTR and AAV1-GFP vector genomes and mRNA transcripts were detectable in all lung sections and in the liver and pancreas at day 90 and after 1 year at levels comparable with animals necropsied at 90 days. The numbers of vector genomes for cystic fibrosis transmembrane regulator (CFTR) and green fluorescent protein (GFP) detected here were higher than those found in the monkeys infected without methylprednisolone treatment that we tested previously.¹ Also, lung surface and keratin 5-positive basal cells showed higher CFTR and GFP staining than did the cells from the uninfected monkey control. Positive immunostaining, also detected in the liver and pancreas, remained stable for at least a year. All animals seroconverted for anticapsid antibodies by 90 days post-treatment. The neutralizing antibody titer declined in the animal necropsied at 1 year. Conclusion: AAV1 safely and effectively transduces monkey airway and basal cells. Both the presence of vector genomes and transduction from AAV1-CFTR and AAV1-GFP virus seen in the monkeys 4 months to 1 year after the first instillation suggest that repeat dosing with AAV1-based vectors is achievable, particularly after methylprednisolone treatment.

Transduction of Surface and Basal Cells in Rhesus Macaque Lung Following Repeat Dosing with AAV1CFTR

To test the effectiveness of repeat dosing, we sprayed two doses (10¹³ vg each) of AAV1Δ27-264-CFTR into airways of four rhesus monkeys at 0 and 30 days, followed by a single dose of 10¹³ vg of AAV1GFP on day 60. Monkeys were sacrificed on day 90. No adverse events occurred, indicating that AAV1 vectors are safe. An elevated anti-AAV1 neutralizing titer was established by the third dose. A positive ELISPOT to the adeno-associated virus (AAV) capsid but not to cystic fibrosis transmembrane conductance regulator (CFTR) occurred after the third dose in three monkeys. AAV1-CFTR and GFP vectors were detectable in all lung sections and in the heart, liver, and spleen. The CFTR protein was higher in treated monkeys than in an untreated monkey. GFP protein was detected in treated lungs. Lung surface and keratin 5-positive basal cells showed higher CFTR staining than in the uninfected monkey and were positive for GFP staining, indicating widespread gene transduction by AAV1CFTR and GFP. AAV1 safely and effectively transduces monkey airway and basal cells. Both the significant numbers of vector genomes and transduction from AAV1CFTR and GFP virus seen in the monkeys 3 months after the first instillation suggest that repeat dosing with AAV1-based vectors is achievable.

Gene Therapy for Cystic Fibrosis Paved the Way for the Use of Adeno-Associated Virus in Gene Therapy

Shortly after the cystic fibrosis (CF) gene was identified in 1989, the race began to develop a gene therapy for this condition. Major efforts utilized full-length cystic fibrosis transmembrane conductance regulator packaged into adenovirus, adeno-associated virus (AAV), or liposomes and delivered to the airways. The drive to find a treatment for CF based on gene therapy drove the early stages of gene therapy in general, particularly those involving AAV gene therapy. Since general overviews of CF gene therapy have already been published, this review considers specifically the efforts using AAV and is focused on honoring the contributions of Dr. Barrie Carter.

Towards Human Translation of Lentiviral Airway Gene Delivery for Cystic Fibrosis: A One-Month CFTR and Reporter Gene Study in Marmosets

Gene therapy continues to be a promising contender for the treatment of cystic fibrosis (CF) airway disease. We have previously demonstrated that airway conditioning with lysophosphatidylcholine (LPC) followed by delivery of a HIV-1-based lentiviral (LV) vector functionally corrects the CF transmembrane conductance regulator (CFTR) defect in the nasal airways of CF mice. In our earlier pilot study we showed that our technique can transduce marmoset lungs acutely; this study extends that work to examine gene expression in this nonhuman primate (NHP) 1 month after gene vector treatment. A mixture of three separate HIV-1 vesicular stomatitis virus G (VSV-G)-pseudotyped LV vectors containing the luciferase (Luc), LacZ, and hCFTR transgenes was delivered into the trachea through a miniature bronchoscope. We examined whether a single-dose delivery of LV vector after LPC conditioning could increase levels of transgene expression in the trachea and lungs compared with control (phosphate-buffered saline [PBS]) conditioning. At 1 month, bioluminescence was detected in vivo in the trachea of three of the six animals within the PBS control group, compared with five of the six LPC-treated animals. When examined ex vivo there was weak evidence that LPC improves tracheal Luc expression levels. In the lungs, bioluminescence was detected in vivo in four of the six PBS-treated animals, compared with five of the six LPC-treated animals; however, bioluminescence was present in all lungs when imaged ex vivo. LacZ expression was predominantly observed in the alveolar regions of the lung. hCFTR was detected by qPCR in the lungs of five animals. Basal cells were successfully isolated and expanded from marmoset tracheas, but no LacZ-positive colonies were detected. There was no evidence of an inflammatory response toward the LV vector at 1 month postdelivery, with cytokines remaining at baseline levels. In conclusion, we found weak evidence that LPC conditioning improved gene transduction in the trachea, but not in the marmoset lungs. We also highlight some of the challenges associated with translational lung gene therapy studies in NHPs.

Transduction of Pig Small Airway Epithelial Cells and Distal Lung Progenitor Cells by AAV4

Cystic fibrosis (CF) is caused by genetic mutations of the CF transmembrane conductance regulator (CFTR), leading to disrupted transport of Cl⁻ and bicarbonate and CF lung disease featuring bacterial colonization and chronic infection in conducting airways. CF pigs engineered by mutating CFTR develop lung disease that mimics human CF, and are well-suited for investigating CF lung disease therapeutics. Clinical data suggest small airways play a key role in the early pathogenesis of CF lung disease, but few preclinical studies have focused on small airways. Efficient targeted delivery of CFTR cDNA to small airway epithelium may correct the CFTR defect and prevent lung infections. Adeno-associated virus 4 (AAV4) is a natural AAV serotype and a safe vector with lower immunogenicity than other gene therapy vectors such as adenovirus. Our analysis of AAV natural serotypes using cultured primary pig airway epithelia showed that AAV4 has high tropism for airway epithelia and higher transduction efficiency for small airways compared with large airways. AAV4 mediated the delivery of CFTR, and corrected Cl⁻ transport in cultured primary small airway epithelia from CF pigs. Moreover, AAV4 was superior to all other natural AAV serotypes in transducing ITGα6β4⁺ pig distal lung progenitor cells. In addition, AAV4 encoding eGFP can infect pig distal lung epithelia in vivo. This study demonstrates AAV4 tropism in small airway progenitor cells, which it efficiently transduces. AAV4 offers a novel tool for mechanistical study of the role of small airway in CF lung pathogenesis in a preclinical large animal model.

Gene Therapy in Rare Respiratory Diseases: What Have We Learned So Far?

Gene therapy is an alternative therapy in many respiratory diseases with genetic origin and currently without curative treatment. After five decades of progress, many different vectors and gene editing tools for genetic engineering are now available. However, we are still a long way from achieving a safe and efficient approach to gene therapy application in clinical practice. Here, we review three of the most common rare respiratory conditions-cystic fibrosis (CF), alpha-1 antitrypsin deficiency (AATD), and primary ciliary dyskinesia (PCD)-alongside attempts to develop genetic treatment for these diseases. Since the 1990s, gene augmentation therapy has been applied in multiple clinical trials targeting CF and AATD, especially using adeno-associated viral vectors, resulting in a good safety profile but with low efficacy in protein expression. Other strategies, such as non-viral vectors and more recently gene editing tools, have also been used to address these diseases in pre-clinical studies. The first gene therapy approach in PCD was in 2009 when a lentiviral transduction was performed to restore gene expression in vitro; since then, transcription activator-like effector nucleases (TALEN) technology has also been applied in primary cell culture. Gene therapy is an encouraging alternative treatment for these respiratory diseases; however, more research is needed to ensure treatment safety and efficacy.

Restoration of F508-del Function by Transcomplementation: The Partners Meet in the Endoplasmic Reticulum

BACKGROUND/AIMS

Because of the small size of adeno-associated virus, AAV, the cystic fibrosis conductance regulator, CFTR, cDNA is too large to fit within AAV and must be truncated. We report here on two truncated versions of CFTR, which, when inserted into AAV1 and used to infect airway cells, rescue F508-del CFTR via transcomplementation. The purpose of this study is to shed light on where in the cell transcomplementation occurs and how it results in close association between the endogenous F508-del and truncated CFTR.

METHODS

We treated CF airway cells (CFBE41o^-) with AAV2/1 (AAV2 inverted terminal repeats/AAV1 capsid) containing truncated forms of CFTR, ∆264 and ∆27-264 CFTR, who can restore the function of F508-del by transcomplementation. We addressed the aims of the study using a combination of confocal microscopy and short circuit currents measurements. For the latter, CF bronchial epithelial cells (CFBE) were grown on permeable supports.

RESULTS

We show that both F508del and the truncation mutants colocalize in the ER and that both the rescued F508-del and the transcomplementing mutants reach the plasma membrane together. There was significant fluorescence resonance energy transfer (FRET) between F508-del and the transcomplementing mutants within the endoplasmic reticulum (ER), suggesting that transcomplementation occurs through a bimolecular interaction. We found that transcomplementation could increase the Isc in CFBE41o^- cells stably expressing additional wt-CFTR or F508-del and in parental CFBE41o- cells expressing endogenous levels of F508-del.

CONCLUSION

We conclude that the functional rescue of F508-del by transcomplementation occurs via a bimolecular interaction that most likely begins in the ER and continues at the plasma membrane. These results come at an opportune time for developing a gene therapy for CF and offer new treatment options for a wide range of CF patients.

Targeted Gene Delivery through the Respiratory System: Rationale for Intratracheal Gene Transfer

Advances in DNA- and RNA-based technologies have made gene therapy suitable for many lung diseases, especially those that are hereditary. The main objective of gene therapy is to deliver an adequate amount of gene construct to the intended target cell, achieve stable transduction in target cells, and to produce a clinically therapeutic effect. This review focuses on the cellular organization in the normal lung and how gene therapy targets the specific cell types that are affected by pulmonary disorders caused by genetic mutations. Furthermore, it examines the pulmonary barriers that can compromise the absorption and transduction of viral vectors and genetic agents by the lung. Finally, it discusses the advantages and limitations of direct intra-tracheal gene delivery with different viral vectors in small and large animal models and in clinical trials.

Cystic Fibrosis Gene Therapy: Looking Back, Looking Forward

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a cAMP-regulated anion channel. Although CF is a multi-organ system disease, most people with CF die of progressive lung disease that begins early in childhood and is characterized by chronic bacterial infection and inflammation. Nearly 90% of people with CF have at least one copy of the ΔF508 mutation, but there are hundreds of CFTR mutations that result in a range of disease severities. A CFTR gene replacement approach would be efficacious regardless of the disease-causing mutation. After the discovery of the CFTR gene in 1989, the in vitro proof-of-concept for gene therapy for CF was quickly established in 1990. In 1993, the first of many gene therapy clinical trials attempted to rescue the CF defect in airway epithelia. Despite the initial enthusiasm, there is still no FDA-approved gene therapy for CF. Here we discuss the history of CF gene therapy, from the discovery of the CFTR gene to current state-of-the-art gene delivery vector designs. While implementation of CF gene therapy has proven more challenging than initially envisioned; thanks to continued innovation, it may yet become a reality.

Genetic Modification of the Lung Directed Toward Treatment of Human Disease

Genetic modification therapy is a promising therapeutic strategy for many diseases of the lung intractable to other treatments. Lung gene therapy has been the subject of numerous preclinical animal experiments and human clinical trials, for targets including genetic diseases such as cystic fibrosis and α1-antitrypsin deficiency, complex disorders such as asthma, allergy, and lung cancer, infections such as respiratory syncytial virus (RSV) and Pseudomonas, as well as pulmonary arterial hypertension, transplant rejection, and lung injury. A variety of viral and non-viral vectors have been employed to overcome the many physical barriers to gene transfer imposed by lung anatomy and natural defenses. Beyond the treatment of lung diseases, the lung has the potential to be used as a metabolic factory for generating proteins for delivery to the circulation for treatment of systemic diseases. Although much has been learned through a myriad of experiments about the development of genetic modification of the lung, more work is still needed to improve the delivery vehicles and to overcome challenges such as entry barriers, persistent expression, specific cell targeting, and circumventing host anti-vector responses.

A Preclinical Study in Rhesus Macaques for Cystic Fibrosis to Assess Gene Transfer and Transduction by AAV1 and AAV5 with a Dual-Luciferase Reporter System

Cystic fibrosis (CF) is an autosomal recessive disease that is potentially treatable by gene therapy. Since the identification of the gene encoding CF transmembrane conductance regulator, a number of preclinical and clinical trials have been conducted using the first generation of adeno-associated virus, AAV2. All these studies showed that AAV gene therapy for CF is safe, but clinical benefit was not clearly demonstrated. Thus, a new generation of AAV vectors based on other serotypes is needed to move the field forward. This study tested two AAV serotypes (AAV1 and AAV5) using a dual-luciferase reporter system with firefly and Renilla luciferase genes packaged into AAV1 or AAV5, respectively. Two male and two female Rhesus macaques were each instilled in their lungs with both serotypes using a Penn-Century microsprayer. Both AAV1 and AAV5 vector genomes were detected in all the lung samples when measured at the time of necropsy, 45 days after instillation. However, the vector genome number for AAV1 was at least 10-fold higher than for AAV5. Likewise, luciferase activity was also detected in the same samples at 45 days. AAV1-derived activity was not statistically greater than that derived from AAV5. These data suggest that gene transfer is greater for AAV1 than for AAV5 in macaque lungs. Serum neutralizing antibodies were increased dramatically against both serotypes but were less abundant with AAV1 than with AAV5. No adverse events were noted, again indicating that AAV gene therapy is safe. These results suggest that with more lung-tropic serotypes such as AAV1, new clinical studies of gene therapy using AAV are warranted.

Adeno-Associated Virus (AAV) gene therapy for cystic fibrosis: current barriers and recent developments

INTRODUCTION

Since the cystic fibrosis (CF) gene was discovered in 1989, researchers have worked to develop a gene therapy. One of the most promising and enduring vectors is the AAV, which has been shown to be safe. In particular, several clinical trials have been conducted with AAV serotype 2. All of them detected viral genomes, but identification of mRNA transduction was not consistent; clinical outcomes in Phase II studies were also inconsistent. The lack of a positive outcome has been attributed to a less-than-efficient viral infection by AAV2, a weak transgene promoter and the host immune response to the vector. Areas covered: Herein, the authors focus on AAV gene therapy for CF, evaluating past experience with this approach and identifying ways forward, based on the progress that has already been made in identifying and overcoming the limitations of AAV gene therapy. Expert opinion: Such progress makes it clear that this is an opportune time to push forward toward the development of a gene therapy for CF. Drugs to treat the basic defect in CF represent a remarkable advance but cannot treat a significant cohort of patients with rare mutations. Thus, there is a critical need to develop a gene therapy for those individuals.

Optimization of adeno-associated virus vector-mediated gene transfer to the respiratory tract

An efficient adeno-associated virus (AAV) vector was constructed for the treatment of respiratory diseases. AAV serotypes, promoters and routes of administration potentially influencing the efficiency of gene transfer to airway cells were examined in the present study. Among the nine AAV serotypes (AAV1-9) screened in vitro and four serotypes (AAV1, 2, 6, 9) evaluated in vivo, AAV6 showed the strongest transgene expression. As for promoters, the cytomegalovirus (CMV) early enhancer/chicken β-actin (CAG) promoter resulted in more robust transduction than the CMV promoter. Regarding delivery routes, intratracheal administration resulted in strong transgene expression in the lung, whereas the intravenous and intranasal administration routes yielded negligible expression. The combination of the AAV6 capsid and CAG promoter resulted in sustained expression, and the intratracheally administered AAV6-CAG vector transduced bronchial cells and pericytes in the lung. These results suggest that AAV6-CAG vectors are more promising than the previously preferred AAV2 vectors for airway transduction, particularly when administered into the trachea. The present study offers an optimized strategy for AAV-mediated gene therapy for lung diseases, such as cystic fibrosis and pulmonary fibrosis.

CFTR gene transfer with AAV improves early cystic fibrosis pig phenotypes

The physiological components that contribute to cystic fibrosis (CF) lung disease are steadily being elucidated. Gene therapy could potentially correct these defects. CFTR-null pigs provide a relevant model to test gene therapy vectors. Using an in vivo selection strategy that amplifies successful capsids by replicating their genomes with helper adenovirus coinfection, we selected an adeno-associated virus (AAV) with tropism for pig airway epithelia. The evolved capsid, termed AAV2H22, is based on AAV2 with 5 point mutations that result in a 240-fold increased infection efficiency. In contrast to AAV2, AAV2H22 binds specifically to pig airway epithelia and is less reliant on heparan sulfate for transduction. We administer AAV2H22-CFTR expressing the CF transmembrane conductance regulator (CFTR) cDNA to the airways of CF pigs. The transduced airways expressed CFTR on ciliated and nonciliated cells, induced anion transport, and improved the airway surface liquid pH and bacterial killing. Most gene therapy studies to date focus solely on Cl^- transport as the primary metric of phenotypic correction. Here, we describe a gene therapy experiment where we not only correct defective anion transport, but also restore bacterial killing in CFTR-null pig airways.

Barriers to inhaled gene therapy of obstructive lung diseases: A review

Knowledge of genetic origins of obstructive lung diseases has made inhaled gene therapy an attractive alternative to the current standards of care that are limited to managing disease symptoms. Initial lung gene therapy clinical trials occurred in the early 1990s following the discovery of the genetic defect responsible for cystic fibrosis (CF), a monogenic disorder. However, despite over two decades of intensive effort, gene therapy has yet to help patients with CF or any other obstructive lung disease. The slow progress is due in part to poor understanding of the biological barriers to inhaled gene therapy. Encouragingly, clinical trials have shown that inhaled gene therapy with various viral vectors and non-viral gene vectors is well tolerated by patients, and continued research has provided valuable lessons and resources that may lead to future success of this therapeutic strategy. In this review, we first introduce representative obstructive lung diseases and examine limitations of currently available therapeutic options. We then review key components for successful execution of inhaled gene therapy, including gene delivery systems, primary physiological barriers and strategies to overcome them, and advances in preclinical disease models with which the most promising systems may be identified for human clinical trials.

Development of rAAV2-CFTR: History of the First rAAV Vector Product to be Used in Humans

The first human gene therapy trials using recombinant adeno-associated virus (rAAV) vectors were performed in cystic fibrosis (CF) patients. Over 100 CF patients were enrolled in 5 separate trials of rAAV2-CFTR administration via nasal, endobronchial, maxillary sinus, and aerosol delivery. Recombinant AAV vectors were designed to deliver the CF transmembrane regulator (CFTR) gene and correct the basic CFTR defect by restoring chloride transport and reverting the upregulation of proinflammatory cytokines. However, vector DNA expression was limited in duration because of the low incidence of integration and natural airway epithelium turnover. In addition, repeated administration of AAV-CFTR vector resulted in a humoral immune response that prevented effective gene transfer from subsequent doses of vector. AAV serotype 2 was used in human trials before the comparison with other serotypes and determination that serotypes 1 and 5 not only possess higher tropism for the airway epithelium, but also are capable of bypassing the binding and trafficking processes-both were important hindrances to the effectiveness of rAAV2. Although rAAV-CFTR gene therapy does not appear likely to supplant newer small-molecule CFTR modulators in the near future, early work with rAAV-CFTR provided an important foundation for later use of rAAV in humans.

Complement yourself: Transcomplementation rescues partially folded mutant proteins

Cystic Fibrosis (CF) is an autosomal disease associated with malfunction in fluid and electrolyte transport across several mucosal membranes. The most common mutation in CF is an in-frame three-base pair deletion that removes a phenylalanine at position 508 in the first nucleotide-binding domain of the cystic fibrosis conductance regulator (CFTR) chloride channel. This mutation has been studied extensively and leads to biosynthetic arrest of the protein in the endoplasmic reticulum and severely reduced channel activity. This review discusses a novel method of rescuing ΔF508 with transcomplementation, which occurs when smaller fragments of CFTR containing the wild-type nucleotide binding domain are co-expressed with the ΔF508 deletion mutant. Transcomplementation rescues the processing and channel activity of ΔF508 and reduces its rate of degradation in airway epithelial cells. To apply transcomplementation as a therapy would require that the cDNA encoding the truncated CFTR be delivered to cells. We also discuss a gene therapeutic approach based on delivery of a truncated form of CFTR to airway cells using adeno-associated viral vectors.

Correcting the cystic fibrosis disease mutant, A455E CFTR

Cystic fibrosis is caused by more than 1000 mutations, the most common being the ΔF508 mutation. These mutations have been divided into five classes [1], with ΔF508 CFTR in class II. Here we have studied the class V mutation A455E. We report that the mature and immature bands of A455E are rapidly degraded primarily by proteasomes; the short protein half-life of this mutant therefore resembles that of ΔF508 CFTR. A455E could be rescued by treatment of the cells with proteasome inhibitors. Furthermore, co-transfection of A455E with the truncation mutant Δ264 CFTR also rescued the mature C band, indicating that A455E can be rescued by transcomplementation. We found that Δ264 CFTR bound to A455E, forming a bimolecular complex. Treatment with the compound correctors C3 and C4 also rescued A455E. These results are significant because they show that although ΔF508 belongs to a different class than A455E, it can be rescued by the same strategies, offering therapeutic promise to patients with Class V mutations.

Nanotechnology approaches for inhalation treatment of fibrosis

Cystic fibrosis (CF) is an autosomal recessive monogenetic disease that afflicts nearly 70 000 patients worldwide. The mutation results in the accumulation of viscous mucus in multiple organs especially in the lungs, liver and pancreas. High associated morbidity and mortality is caused by CF due to the lack of effective therapies. It is widely accepted that morbidity and mortality caused by CF is primarily due to the respiratory manifestations of the disease. Consequently, several approaches were recently developed for treatment of lung complications of CF. However, the lack of effective methods for delivery and especially targeted delivery of therapeutics specifically to lung tissues and cells limits the efficiency of the therapy. Local pulmonary delivery of therapeutics has two major advantages over systemic application. First, it enhances the accumulation of therapeutics specifically in the lungs and therefore increases the efficiency of the treatment. Second, local lung delivery substantially prevents the penetration of the delivered drug into the systemic circulation limiting adverse side effects of the treatment on other organs and tissues. This review is focused on different approaches to the treatment of respiratory manifestations of CF as well as on methods of pulmonary delivery of therapeutics.

Transcomplementation by a truncation mutant of cystic fibrosis transmembrane conductance regulator (CFTR) enhances ΔF508 processing through a biomolecular interaction

We previously showed that a truncation mutant of CFTR missing the first four transmembrane segments of TMD1, Δ264 CFTR, binds to key elements in the ER quality control mechanism to increase the amounts of the mature C band of both wt and ΔF508 CFTR through transcomplementation. Here, we created a new construct, Δ27-264 CFTR. Even though Δ27-264 CFTR is rapidly degraded in the proteasome, steady state protein can be detected by Western blot. Δ27-264 CFTR can also increase the amounts of the mature C band of both wt and ΔF508 CFTR through transcomplementation. Electrophysiology experiments show that Δ27-264 CFTR can restore chloride channel currents. Further experiments with the conduction mutant S341A show conclusively that currents are indeed generated by rescued channel function of ΔF508 CFTR. Immunoprecipitation studies show that Δ27-264 binds to ΔF508-CFTR, suggesting a bimolecular interaction. Thus the adeno-associated viral vector, rAAV-Δ27-264 CFTR, is a highly promising CF gene therapy vector, because it increases the amount of mature band C protein both from wt and ΔF508 CFTR, and rescues channel activity of ΔF508 CFTR.

Adeno-associated virus for cystic fibrosis gene therapy

Gene therapy is an alternative treatment for genetic lung disease, especially monogenic disorders such as cystic fibrosis. Cystic fibrosis is a severe autosomal recessive disease affecting one in 2500 live births in the white population, caused by mutation of the cystic fibrosis transmembrane conductance regulator (CFTR). The disease is classically characterized by pancreatic enzyme insufficiency, an increased concentration of chloride in sweat, and varying severity of chronic obstructive lung disease. Currently, the greatest challenge for gene therapy is finding an ideal vector to deliver the transgene (CFTR) to the affected organ (lung). Adeno-associated virus is the most promising viral vector system for the treatment of respiratory disease because it has natural tropism for airway epithelial cells and does not cause any human disease. This review focuses on the basic properties of adeno-associated virus and its use as a vector for cystic fibrosis gene therapy.

Gene and cell therapy for cystic fibrosis: from bench to bedside

Clinical trials in cystic fibrosis (CF) patients established proof-of-principle for transfer of the wild-type cystic fibrosis transmembrane conductance regulator (CFTR) gene to airway epithelial cells. However, the limited efficacy of gene transfer vectors as well as extra- and intracellular barriers have prevented the development of a gene therapy-based treatment for CF. Here, we review the use of new viral and nonviral gene therapy vectors, as well as human artificial chromosomes, to overcome barriers to successful CFTR expression. Pre-clinical studies will surely benefit from novel animal models, such as CF pigs and ferrets. Prenatal gene therapy is a potential alternative to gene transfer to fully developed lungs. However, unresolved issues, including the possibility of adverse effects on pre- and postnatal development, the risk of initiating oncogenic or degenerative processes and germ line transmission require further investigation. Finally, we discuss the therapeutic potential of stem cells for CF lung disease.

Gene therapy for cystic fibrosis lung disease

Cystic fibrosis (CF) is characterised by respiratory and pancreatic deficiencies that stem from the loss of fully functional CFTR (CF transmembrane conductance regulator) at the membrane of epithelial cells. Current treatment modalities aim to delay the deterioration in lung function, Which is mostly responsible for the relatively short life expectancy of CF sufferers; however none have so far successfully dealt with the underlying molecular defect. Novel pharmacological approaches to ameliorate the lack of active CFTR in respiratory epithelial cells are beginning to address more of the pathophysiological defects caused by CFTR mutations. However, CFTR gene replacement by gene therapy remains the most likely option for addressing the basic defects, including ion transport and inflammatory functions of CFTR. In this chapter, We will review the latest preclinical and clinical advances in pharmacotherapy and gene therapy for CF lung disease.

Functional cystic fibrosis transmembrane conductance regulator expression in cystic fibrosis airway epithelial cells by AAV6.2-mediated segmental trans-splicing

Cystic fibrosis is characterized by deficiency of the cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) transporter. The packaging constraints of adeno-associated viral (AAV) vectors preclude delivery of both an active promoter and CFTR cDNA to target cells. We hypothesized that segmental trans-splicing, in which two AAV vectors deliver the 5' and 3' halves of the CFTR cDNA, could mediate splicing of two pre-mRNAs into a full-length, functional CFTR mRNA. Using a segmental trans-splicing 5' donor-3' acceptor pair that split the CFTR cDNA between exons 14a and 14b, cotransfection of donor and acceptor plasmids into CFTR(-) cells resulted in full-length CFTR message and protein. Microinjection of plasmids into CFTR(-) cells produced cAMP-activated Cl(-) conductance. Vectors created with an engineered human serotype, AAV6.2, were used to deliver CFTR donor and acceptor constructs, resulting in full-length CFTR mRNA and protein as well as cAMP-activated Cl(-) conductance in CFTR(-) cells, including human CF airway epithelial IB3-1 cells. Thus, segmental trans-splicing can be used with AAV vectors to mediate expression of CFTR, a strategy potentially applicable to individuals with CF.

Gene transfer to the lung: lessons learned from more than 2 decades of CF gene therapy

Gene therapy is currently being developed for a wide range of acute and chronic lung diseases. The target cells, and to a degree the extra and intra-cellular barriers, are disease-specific and over the past decade the gene therapy community has recognized that no one vector is good for all applications, but that the gene transfer agent (GTA) has to be carefully matched to the specific disease target. Gene therapy is particularly attractive for diseases that currently do not have satisfactory treatment options and probably easier for monogenic disorders than for complex diseases. Cystic fibrosis (CF) fulfils these criteria and is, therefore, a good candidate for gene therapy-based treatment. This review will focus on CF as an example for lung gene therapy, but lessons learned may be applicable to other target diseases.

Gene therapy for cystic fibrosis

Cystic Fibrosis (CF) is an autosomal recessive disorder due to mutations in the CF transmembrane conductance regulator (CFTR) gene that lead to defective ion transport in the conducting pulmonary airways and exocrine glands. Through a process that is not fully understood, CFTR defects predispose affected patients to chronic endobronchial infections with organisms such as Pseudomonas aeruginosa and Staphylococcus aureus. Following the discovery of the CFTR gene in 1989, CF became one of the primary targets for gene therapy research. Early enthusiasm surrounded the new field of gene therapy during most of the 1990s and it led academics and clinicians on a big effort to apply gene therapy for cystic fibrosis. Clinical studies have been pursued using recombinant adenovirus, recombinant adeno-associated virus, cationic liposomes, and cationic polymer vectors. Although to this date no dramatic therapeutic benefits have been observed, a lot of information has been gained from the pre-clinical and clinical studies that were performed. This learning curve has led to the optimization of vector technology and an appreciation of immune and mechanical barriers that have to be overcome for successful delivery.

CFTR function and prospects for therapy

Mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) epithelial anion channel cause cystic fibrosis (CF). The multidomain integral membrane glycoprotein, a member of the adenine nucleotide-binding cassette (ABC) transporter family, conserved in metazoan salt-transporting tissues, is required to control ion and fluid homeostasis on epithelial surfaces. This review considers different therapeutic strategies that have arisen from knowledge of CFTR structure and function as well as its biosynthetic processing, intracellular trafficking, and turnover.

Cystic fibrosis transmembrane regulator missing the first four transmembrane segments increases wild type and DeltaF508 processing

We previously generated an adenoassociated viral gene therapy vector, rAAV-Delta264 cystic fibrosis transmembrane conductance regulator (CFTR), missing the first four transmembrane domains of CFTR. When infected into monkey lungs, Delta264 CFTR increased the levels of endogenous wild type CFTR protein. To understand this process, we transfected Delta264 CFTR plasmid cDNA into COS7 cells, and we noted that protein expression from the truncation mutant is barely detectable when compared with wild type or DeltaF508 CFTR. Delta264 CFTR protein expression increases dramatically when cells are treated with proteasome inhibitors. Cycloheximide experiments show that Delta264 CFTR is degraded faster than DeltaF508 CFTR. VCP and HDAC6, two proteins involved in retrograde translocation from endoplasmic reticulum to cytosol for proteasomal and aggresomal degradation, coimmunoprecipitate with Delta264 CFTR. In cotransfection studies in COS7 cells and in transfection of Delta264 CFTR into cells stably expressing wild type and DeltaF508 CFTR, Delta264 CFTR increases wild type CFTR protein and increases levels of maturation of immature band B to mature band C of DeltaF508 CFTR. Thus the adenoassociated viral vector, rAAV-Delta264 CFTR, is a highly promising cystic fibrosis gene therapy vector because it increases the amount of mature band C protein both from wild type and DeltaF508 CFTR and associates with key elements in quality control mechanism of CFTR.

A hybrid vector system expands adeno-associated viral vector packaging capacity in a transgene-independent manner

The trans-splicing (ts) and overlapping (ov) vectors expand the packaging capacity of adeno-associated virus (AAV). But their application depends on the inherent properties of the target gene. The ts vectors require an optimal gene-splitting site and the ov vectors require a highly recombinogenic domain. In order to overcome these limitations, we developed a hybrid dual (hd) vector system. In the hd vectors, we inserted a highly recombinogenic alkaline phosphatase (AP) sequence in the ts vectors to allow for transgene-independent reconstitution through homologous recombination of the AP sequences. We first tested the hybrid system with the LacZ gene. Both in the cell line (in vitro) and in the mouse muscle (in vivo), the hd vectors significantly outperformed the ts and ov vectors. In muscle, the transduction efficiency of the hybrid vectors reached 80% of that from the single intact vector. Southern blot confirmed AP sequence-mediated transgene reconstitution. In order to validate the hybrid system, we split the 6 kilobase (kb) mini-dystrophin gene at the exon 55/56 junction, a predicted poor site for the ts approach. In dystrophic mdx mouse muscle, the hd vectors yielded 5.6-fold higher transduction than the ts vectors did. Taken together, these data suggest that the hybrid system efficiently expresses large therapeutic genes that are poor candidates for the ts and ov approaches.