Recombinant Adeno-Associated Viral Vectors (rAAV)-Vector Elements in Ocular Gene Therapy Clinical Trials and Transgene Expression and Bioactivity Assays

In vivo rescue of genetic dilated cardiomyopathy by systemic delivery of nexilin

BACKGROUND

Dilated cardiomyopathy (DCM) is one of the most common causes of heart failure. Multiple identified mutations in nexilin (NEXN) have been suggested to be linked with severe DCM. However, the exact association between multiple mutations of Nexn and DCM remains unclear. Moreover, it is critical for the development of precise and effective therapeutics in treatments of DCM.

RESULTS

In our study, Nexn global knockout mice and mice carrying human equivalent G645del mutation are studied using functional gene rescue assays. AAV-mediated gene delivery is conducted through systemic intravenous injections at the neonatal stage. Heart tissues are analyzed by immunoblots, and functions are assessed by echocardiography. Here, we identify functional components of Nexilin and demonstrate that exogenous introduction could rescue the cardiac function and extend the lifespan of Nexn knockout mouse models. Similar therapeutic effects are also obtained in G645del mice, providing a promising intervention for future clinical therapeutics.

CONCLUSIONS

In summary, we demonstrated that a single injection of AAV-Nexn was capable to restore the functions of cardiomyocytes and extended the lifespan of Nexn knockout and G645del mice. Our study represented a long-term gene replacement therapy for DCM that potentially covers all forms of loss-of-function mutations in NEXN.

A New Mouse Model for Usher Syndrome Crossing Kunming Mice with CBA/J Mice

BACKGROUND

Previously, we discovered a strain of Kunming mice, referred to as the KMush/ush strain, that exhibited notably abnormal electroretinogram (ERG) readings and elevated thresholds for auditory brainstem responses (ABRs), which resembled the characteristics of Usher Syndrome (USH). We successfully identified the pathogenic genes, Pde6b and Adgrv1, after KMush/ush crossbred with CBA/CaJ mice, referred to as CBA-1ush/ush, CBA-2ush/ush or CBA-2ush/ush. In this investigation, we crossbred KMush/ush and CBA/J mice to establish novel recombinant inbred lines and analysed their phenotypic and genotypic characteristics.

METHODS

ERG readings, ABR testing, fundus morphology, histological examination of the retina and inner ear, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis, western blotting, DNA sequence analysis and behavioural experiments were performed to assess the phenotypes and genotypes of the progeny lines.

RESULTS

No obvious waveforms in the ERG were detected in F1 hybrid mice while normal ABR results were recorded. The F2 hybrids, which were called J1ush/ush or J2ush/ush, exhibited segregated hearing-loss phenotypes. J1ush/ush mice had a retinitis pigmentosa (RP) phenotype with elevated ABR thresholds, whereas J2ush/ush mice exhibited only the RP phenotype. Interestingly, J1ush/ush mice showed significantly higher ABR thresholds than wild-type mice at 28 days post born (P28), and RT-qPCR and DNA-sequencing analysis showed that Adgrv1 gene expression was significantly altered in J1ush/ush mice, but histological analysis showed no significant structural changes in the organ of Corti or spiral ganglia. Further elevation of ABR-related hearing thresholds by P56 manifested only as a reduced density of spiral ganglion cells, which differed significantly from the previous pattern of cochlear alterations in CBA-2ush/ush mice.

CONCLUSIONS

We successfully introduced the hearing-loss phenotype of inbred mice with USH into CBA/J mice, which provides a good animal model for future studies on the important physiological roles of the Adgrv1 gene in inner-ear structure and for therapeutic studies targeting Adgrv1-mutated USH.

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

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

Genotype-Phenotype of CRB1-Associated Early-Onset Retinal Dystrophy: Novel Insights on Retinal Architecture and Therapeutic Window for Clinical Trials

Purpose

The purpose of this study was to investigate the genotypic and phenotypic characteristics of CRB1-associated early onset retinal dystrophy (CRB1-eoRD) and retinal architecture by swept-source optical coherence tomography (SS-OCT).

Methods

Eleven probands with CRB1-eoRD were recruited. Clinical information, genetic analysis, and comprehensive ophthalmic examinations including SS-OCT and SS-OCT angiography (SS-OCTA) were conducted.

Results

A total of 81.8% (9/11) of CRB1-eoRD presented as Leber congenital amaurosis (LCA). Common clinical manifestations included coin-like yellow-white retinal spots (20/22, 90.9%) and para-arteriolar retinal pigment epithelial retention (12/22, 54.5%). Nineteen different CRB1 variants were detected in our case series, including 12 missense, 3 frameshifts, 3 nonsense, and 1 splicing. Of them, 12 variants had been reported, and 7 were novel. SS-OCT showed thinner central macula (the LCA group, P < 0.0001), thicker total retina (P < 0.0001), thinner outer retina (P < 0.05), and thicker inner retina (P < 0.0001) compared with the healthy control. The inner/outer (I/O) retina thickness ratio of CRB1-eoRD was 3.0, higher than the healthy control of 1.2 and other inherited retinal diseases (IRDs) of 2.2 (P < 0.0001 and P = 0.0027, respectively). SS-OCTA revealed an increased vascular density and perfusion area of the superficial vascular complex and deep vascular complex in CRB1-eoRD.

Conclusions

LCA emerges as a frequently occurring phenotype in CRB1-eoRD. The unique features of SS-OCT and SS-OCTA are illustrated, and the novel biomarker, I/O ratio, may facilitate early diagnosis. The insights gained from this study hold significant value in determining the treatment window for potential forthcoming CRB1 gene therapy.

Intravitreal injection of new adeno-associated viral vector: Enhancing retinoschisin 1 gene transduction in a mouse model of X-linked retinoschisis

Adeno-associated virus (AAV) vectors have been widely used in therapy to treat hereditary retinal diseases. But its transduction efficiency by intravitreal injection still needs to be improved. In this study, we investigated the transduction efficiency of AAV-DJ (K137R)-GFP in different retinal cells of normal mice, as well as the therapy effection of AAV-DJ (K137R)-Rs1 on retinal function and structure in Rs1-KO mice. The intravitreal injection of AAV-DJ (K137R)-GFP demonstrated that this vector transduced cells in all layers of the retina, including the inner nuclear layer and photoreceptor layer. The intravitreal injection of AAV-DJ (K137R)-Rs1 found that 3 months post-injection of this vector improved retinal function and structure in Rs1-KO mice. Our conclusion is that AAV-DJ (K137R) vector can efficiently and safely penetrate the inner limiting membrane and transduce different layers of retinal cells in the long term, as well as being able to continuously and efficiently express target therapeutic proteins, making it a candidate therapeutic vector for X-linked retinoschisis (XLRS).

Characterization and AAV-mediated CRB gene augmentation in human-derived CRB1KO and CRB1KOCRB2+/- retinal organoids

The majority of patients with mutations in CRB1 develop either early-onset retinitis pigmentosa as young children or Leber congenital amaurosis as newborns. The cause for the phenotypic variability in CRB1-associated retinopathies is unknown, but might be linked to differences in CRB1 and CRB2 protein levels in Müller glial cells and photoreceptor cells. Here, CRB1KO and CRB1KOCRB2+/- differentiation day 210 retinal organoids showed a significant decrease in the number of photoreceptor nuclei in a row and a significant increase in the number of photoreceptor cell nuclei above the outer limiting membrane. This phenotype with outer retinal abnormalities is similar to CRB1 patient-derived retinal organoids and Crb1 or Crb2 mutant mouse retinal disease models. The CRB1KO and CRB1KOCRB2+/- retinal organoids develop an additional inner retinal phenotype due to the complete loss of CRB1 from Müller glial cells, suggesting an essential role for CRB1 in proper localization of neuronal cell types. Adeno-associated viral (AAV) transduction was explored at early and late stages of organoid development. Moreover, AAV-mediated gene augmentation therapy with AAV.hCRB2 improved the outer retinal phenotype in CRB1KO retinal organoids. Altogether, these data provide essential information for future gene therapy approaches for patients with CRB1-associated retinal dystrophies.

Dual AAV-based PCDH15 gene therapy achieves sustained rescue of visual function in a mouse model of Usher syndrome 1F

Mutations in the PCDH15 gene, encoding protocadherin-15, are among the leading causes of Usher syndrome type 1 (USH1F), and account for up to 12% USH1 cases worldwide. A founder truncating variant of PCDH15 has a ∼2% carrier frequency in Ashkenazi Jews accounting for nearly 60% of their USH1 cases. Although cochlear implants can restore hearing perception in USH1 patients, presently there are no effective treatments for the vision loss due to retinitis pigmentosa. We established a founder allele-specific Pcdh15 knockin mouse model as a platform to ascertain therapeutic strategies. Using a dual-vector approach to circumvent the size limitation of adeno-associated virus, we observed robust expression of exogenous PCDH15 in the retinae of Pcdh15KI mice, sustained recovery of electroretinogram amplitudes and key retinoid oxime, substantially improved light-dependent translocation of phototransduction proteins, and enhanced levels of retinal pigment epithelium-derived enzymes. Thus, our data raise hope and pave the way for future gene therapy trials in USH1F subjects.

Recent Developments in Gene Therapy for Neovascular Age-Related Macular Degeneration: A Review

Age-related macular degeneration (AMD) is a complex and multifactorial disease and a leading cause of irreversible blindness in the elderly population. The anti-vascular endothelial growth factor (anti-VEGF) therapy has revolutionized the management and prognosis of neovascular AMD (nAMD) and is currently the standard of care for this disease. However, patients are required to receive repeated injections, imposing substantial social and economic burdens. The implementation of gene therapy methods to achieve sustained delivery of various therapeutic proteins holds the promise of a single treatment that could ameliorate the treatment challenges associated with chronic intravitreal therapy, and potentially improve visual outcomes. Several early-phase trials are currently underway, evaluating the safety and efficacy of gene therapy for nAMD; however, areas of controversy persist, including the therapeutic target, route of administration, and potential safety issues. In this review, we assess the evolution of gene therapy for nAMD and summarize several preclinical and early-stage clinical trials, exploring challenges and future directions.

Safety and Efficacy of Adeno-Associated Viral Gene Therapy in Patients With Retinal Degeneration: A Systematic Review and Meta-Analysis

Purpose

This systematic review evaluates the safety and efficacy of ocular gene therapy using adeno-associated virus (AAV).

Methods

MEDLINE, Embase, Cochrane Central Register of Controlled Trials, and ClinicalTrials.gov were searched systematically for controlled or non-controlled interventional gene therapy studies using key words related to retinal diseases, gene therapy, and AAV vectors. The primary outcome measure was safety, based on ocular severe adverse events (SAEs). Secondary outcome measures evaluated efficacy of the therapy based on best corrected visual acuity (BCVA) and improvements in visual sensitivity and systemic involvement following ocular delivery. Pooling was done using a DerSimonian Laird random effects model. Risk of bias was assessed using the Cochrane Risk of Bias Tool, version 1.

Results

Our search identified 3548 records. Of these, 80 publications met eligibility criteria, representing 28 registered clinical trials and 5 postmarket surveillance studies involving AAV gene therapy for Leber congenital amaurosis (LCA), choroideremia, Leber hereditary optic neuropathy (LHON), age-related macular degeneration (AMD), retinitis pigmentosa (RP), X-linked retinoschisis, and achromatopsia. Overall, AAV therapy vectors were associated with a cumulative incidence of at least one SAE of 8% (95% confidence intervals [CIs] of 5% to 12%). SAEs were often associated with the surgical procedure rather than the therapeutic vector itself. Poor or inconsistent reporting of adverse events (AEs) were a limitation for the meta-analysis. The proportion of patients with any improvement in BCVA and visual sensitivity was 41% (95% CIs of 31% to 51%) and 51% (95% CIs of 31% to 70%), respectively. Systemic immune involvement was associated with a cumulative incidence of 31% (95% CI = 21% to 42%).

Conclusions

AAV gene therapy vectors appear to be safe but the surgical procedure required to deliver them is associated with some risk. The large variability in efficacy can be attributed to the small number of patients treated, the heterogeneity of the population and the variability in dosage, volume, and follow-up.

Translational Relevance

This systematic review will help to inform and guide future clinical trials.

Reduction of retinal ganglion cell death in mouse models of familial dysautonomia using AAV-mediated gene therapy and splicing modulators

Familial dysautonomia (FD) is a rare neurodevelopmental and neurodegenerative disease caused by a splicing mutation in the Elongator Acetyltransferase Complex Subunit 1 (ELP1) gene. The reduction in ELP1 mRNA and protein leads to the death of retinal ganglion cells (RGCs) and visual impairment in all FD patients. Currently patient symptoms are managed, but there is no treatment for the disease. We sought to test the hypothesis that restoring levels of Elp1 would thwart the death of RGCs in FD. To this end, we tested the effectiveness of two therapeutic strategies for rescuing RGCs. Here we provide proof-of-concept data that gene replacement therapy and small molecule splicing modifiers effectively reduce the death of RGCs in mouse models for FD and provide pre-clinical foundational data for translation to FD patients.

Gene augmentation therapy attenuates retinal degeneration in a knockout mouse model of Fam161a retinitis pigmentosa

Photoreceptor cell degeneration and death is the major hallmark of a wide group of human blinding diseases including age-related macular degeneration and inherited retinal diseases such as retinitis pigmentosa. In recent years, inherited retinal diseases have become the "testing ground" for novel therapeutic modalities, including gene and cell-based therapies. Currently there is no available treatment for retinitis pigmentosa caused by FAM161A biallelic pathogenic variants. In this study, we injected an adeno-associated virus encoding for the longer transcript of mFam161a into the subretinal space of P24-P29 Fam161a knockout mice to characterize the safety and efficacy of gene augmentation therapy. Serial in vivo assessment of retinal function and structure at 3, 6, and 8 months of age using the optomotor response test, full-field electroretinography, fundus autofluorescence, and optical coherence tomography imaging as well as ex vivo quantitative histology and immunohistochemical studies revealed a significant structural and functional rescue effect in treated eyes accompanied by expression of the FAM161A protein in photoreceptors. The results of this study may serve as an important step toward future application of gene augmentation therapy in FAM161A-deficient patients by identifying a promising isoform to rescue photoreceptors and their function.

Comparison of heterologous prime-boost immunization strategies with DNA and recombinant vaccinia virus co-expressing GP3 and GP5 of European type porcine reproductive and respiratory syndrome virus in pigs

Vaccination is principally used to control and treat porcine reproductive and respiratory syndrome virus (PRRSV) infection. This study investigated immunogenicity and protective efficacy of heterologous prime-boost regimens in pigs, including recombinant DNA and vaccinia virus vectors coexpressing PRRSV European genotype (EU) isolate GP3 and GP5: group A, pVAX1-EU-GP3-GP5 prime and rddVTT-EU-GP3-GP5 boost; group B, rddVTT-EU-GP3-GP5 prime and pVAX1-EU-GP3-GP5 boost; group C, empty vector pVAX1; group D, E3L gene-deleted vaccinia virus E3L- VTT. Vaccine efficacy was tested in an EU-type PRRSV (Lelystad virus strain) challenge pig model based on evaluating PRRSV-specific antibody responses, neutralizing antibodies, cytokines, T lymphocyte proliferation, CD4+ and CD8+ T lymphocytes, clinical symptoms, viremia and tissue virus loads. Plasmid DNA was delivered as chitosan-DNA nanoparticles, and Quil A (Quillaja) was used to increase vaccine efficiency. All piglets were boosted 21 days post the initial inoculation (dpi) and then challenged 14 days later. At 14, 21, 28 and 35 dpi, groups A and B developed significantly higher PRRSV-specific antibody responses compared with control groups C and D. Two weeks after the boost, significant differences in neutralizing antibody and IFN-γ levels were observed between groups A, C, D and B. At 49 dpi, groups A and B had markedly increased peripheral blood CD3+CD4+ T cell levels. Following virus challenge, group A showed viremia, but organ virus loads were lower than those in other groups. Thus, a heterologous prime-boost vaccine regimen (rddVTT-EU-GP3-GP5 prime, pVAX1-EU-GP3-GP5 boost) can improve humoral- and cell-mediated immune responses to provide resistance to EU-type PRRSV infection in vivo.

Selection of viral capsids and promoters affects the efficacy of rescue of Tmprss3-deficient cochlea

Adeno-associated virus (AAV)-mediated gene transfer has shown promise in rescuing mouse models of genetic hearing loss, but how viral capsid and promoter selection affects efficacy is poorly characterized. Here, we tested combinations of AAVs and promoters to deliver Tmprss3, mutations in which are associated with hearing loss in humans. Tmprss3tm1/tm1 mice display severe cochlear hair cell degeneration, loss of auditory brainstem responses, and delayed loss of spiral ganglion neurons. Under the ubiquitous CAG promoter and AAV-KP1 capsid, Tmprss3 overexpression caused striking cytotoxicity in vitro and in vivo and failed to rescue degeneration or dysfunction of the Tmprss3tm1/tm1 cochlea. Reducing the dosage or using AAV-DJ-CAG-Tmprss3 diminished cytotoxicity without rescue of the Tmprss3tm1/tm1 cochlea. Finally, the combination of AAV-KP1 capsid and the EF1α promoter prevented cytotoxicity and reduced hair cell degeneration, loss of spiral ganglion neurons, and improved hearing thresholds in Tmprss3tm1/tm1 mice. Together, our study illustrates toxicity of exogenous genes and factors governing rescue efficiency, and suggests that cochlear gene therapy likely requires precisely targeted transgene expression.

Utilization of the retinal organoid model to evaluate the feasibility of genetic strategies to ameliorate retinal disease(s)

Organoid models have quickly become a popular research tool to evaluate novel therapeutics on 3-D recapitulated tissue. This has enabled researchers to use physiologically relevant human tissue in vitro to augment the standard use of immortalized cells and animal models. Organoids can also provide a model when an engineered animal cannot recreate a specific disease phenotype. In particular, the retinal research field has taken advantage of this burgeoning technology to provide insight into inherited retinal disease(s) mechanisms and therapeutic intervention to ameliorate their effects. In this review we will discuss the use of both wild-type and patient-specific retinal organoids to further gene therapy research that could potentially prevent retinal disease(s) progression. Furthermore, we will discuss the pitfalls of current retinal organoid technology and present potential solutions that could overcome these hurdles in the near future.

Insights into adeno-associated virus-based ocular gene therapy: A bibliometric and visual analysis

BACKGROUND

Adeno-associated virus (AAV) plays a vital role in ocular gene therapy and has been widely studied since 1996. This study summarizes and explores the publication outputs and future research trends of AAV-based ocular gene therapy.

METHODS

Publications and data about AAV-based ocular gene therapy were downloaded from the Web of Science Core Collection or ClinicalTrials.gov database. The publications and data were analyzed by Microsoft Excel, CiteSpace, VOS viewer, and a free online platform (http://bibliometric.com).

RESULTS

Totally 832 publications from the Web of Science Core Collection relevant to AAV-based ocular gene therapy were published from 1996 to 2022. These publications were contributed by research institutes from 42 countries or regions. The US contributed the most publications among these countries or regions, notably the University of Florida. Hauswirth WW was the most productive author. "Efficacy" and "safety" are the main focus areas for future research according to the references and keywords analysis. Eighty clinical trials examined AAV-based ocular gene therapy were registered on ClinicalTrials.Gov. Institutes from the US and European did the dominant number or the large proportion of the trials.

CONCLUSIONS

The research focus of the AAV-based ocular gene therapy has transitioned from the study in biological theory to clinical trialing. The AAV-based gene therapy is not limited to inherited retinal diseases but various ocular diseases.

Reduction of retinal ganglion cell death in mouse models of familial dysautonomia using AAV-mediated gene therapy and splicing modulators

Familial dysautonomia (FD) is a rare neurodevelopmental and neurodegenerative disease caused by a splicing mutation in the Elongator Acetyltransferase Complex Subunit 1 ( ELP1 ) gene. The reduction in ELP1 mRNA and protein leads to the death of retinal ganglion cells (RGCs) and visual impairment in all FD patients. Currently, patient symptoms are managed, but there is no treatment for the disease. We sought to test the hypothesis that restoring levels of Elp1 would thwart the death of RGCs in FD. To this end, we tested the effectiveness of two therapeutic strategies for rescuing RGCs. Here we provide proof-of-concept data that gene replacement therapy and small molecule splicing modifiers effectively reduce the death of RGCs in mouse models for FD and provide pre-clinical data foundation for translation to FD patients.

Potency testing of cell and gene therapy products

Potency is one of the critical quality attributes of biological medicinal products, defining their biological activity. Potency testing is expected to reflect the Mechanism of Action (MoA) of the medicinal product and ideally the results should correlate with the clinical response. Multiple assay formats may be used, both in vitro assays and in vivo models, however, for timely release of the products for clinical studies or for commercial use, quantitative, validated in vitro assays are necessary. Robust potency assays are fundamental also for comparability studies, process validation and for stability testing. Cell and Gene Therapy Products (CGTs, also called Advanced Therapy Medicinal Products, ATMPs) are part of biological medicines, having nucleic acids, viral vectors, viable cells and tissues as starting material. For such complex products potency testing is often challenging and may require a combination of methods to address multiple functional mechanisms of the product. For cells, viability and cell phenotype are important attributes but alone will not be sufficient to address potency. Furthermore, if the cells are transduced with a viral vector, potency probably is related to the expression of the transgene but will also be dependent on the target cells and transduction efficiency/copy number of the transgene in the cells. Genome Editing (GE) together with other cell manipulations can result into multiple changes in the characteristics and activity of the cells, which should be all somehow captured by the potency testing. Non-clinical studies/models may provide valuable support for potency testing, especially for comparability testing. However, sometimes lack of suitable potency data may lead to situations where bridging clinical efficacy data are required to solve the problems of the potency testing, for example where comparability of different clinical batches is unclear. In this article the challenges of potency testing are discussed together with examples of assays used for different CGTs/ATMPs and the available guidance addressing differences between the European Union and the United States.

A Review of CRISPR Tools for Treating Usher Syndrome: Applicability, Safety, Efficiency, and In Vivo Delivery

This review considers research into the treatment of Usher syndrome, a deaf-blindness syndrome inherited in an autosomal recessive manner. Usher syndrome mutations are markedly heterogeneous, involving many different genes, and research grants are limited due to minimal patient populations. Furthermore, gene augmentation therapies are impossible in all but three Usher syndromes as the cDNA sequence exceeds the 4.7 kb AAV packaging limit. It is, therefore, vital to focus research efforts on alternative tools with the broadest applicability. The CRISPR field took off in recent years following the discovery of the DNA editing activity of Cas9 in 2012. New generations of CRISPR tools have succeeded the original CRISPR/Cas9 model to enable more sophisticated genomic amendments such as epigenetic modification and precise sequence alterations. This review will evaluate the most popular CRISPR tools to date: CRISPR/Cas9, base editing, and prime editing. It will consider these tools in terms of applicability (in relation to the ten most prevalent USH2A mutations), safety, efficiency, and in vivo delivery potential with the intention of guiding future research investment.

Improving adeno-associated viral (AAV) vector-mediated transgene expression in retinal ganglion cells: comparison of five promoters

Recombinant adeno-associated viral vectors (AAVs) are an effective system for gene transfer. AAV serotype 2 (AAV2) is commonly used to deliver transgenes to retinal ganglion cells (RGCs) via intravitreal injection. The AAV serotype however is not the only factor contributing to the effectiveness of gene therapies. Promoters influence the strength and cell-selectivity of transgene expression. This study compares five promoters designed to maximise AAV2 cargo space for gene delivery: chicken β-actin (CBA), cytomegalovirus (CMV), short CMV early enhancer/chicken β-actin/short β-globulin intron (sCAG), mouse phosphoglycerate kinase (PGK), and human synapsin (SYN). The promoters driving enhanced green fluorescent protein (eGFP) were examined in adult C57BL/6J mice eyes and tissues of the visual system. eGFP expression was strongest in the retina, optic nerves and brain when driven by the sCAG and SYN promoters. CBA, CMV, and PGK had moderate expression by comparison. The SYN promoter had almost exclusive transgene expression in RGCs. The PGK promoter had predominant expression in both RGCs and AII amacrine cells. The ubiquitous CBA, CMV, and sCAG promoters expressed eGFP in a variety of cell types across multiple retinal layers including Müller glia and astrocytes. We also found that these promoters could transduce human retina ex vivo, although expression was predominantly in glial cells due to low RGC viability. Taken together, this promoter comparison study contributes to optimising AAV-mediated transduction in the retina, and could be valuable for research in ocular disorders, particularly those with large or complex genetic cargos.

Develop an efficient and specific AAV-based labeling system for Muller glia in mice

Reprogramming Müller glia (MG) into functional cells is considered a promising therapeutic strategy to treat ocular diseases and vision loss. However, current AAV-based system for MG-tracing was reported to have high leakage in recent studies. Here, we focused on reducing the leakage of AAV-based labeling systems and found that different AAV serotypes showed a range of efficiency and specificity in labeling MG, leading us to optimize a human GFAP-Cre reporter system packaged in the AAV9 serotype with the woodchuck hepatitis virus post-transcriptional regulatory element (WPRE) removed. The leakage ratio of the AAV9-hGFAP-Cre-ΔWPRE decreased by an approximate 40-fold compared with the AAV9-hGFAP-Cre-WPRE labeling system. In addition, we validated the specificity of the AAV-ΔWPRE system for tracing MG reprogramming under Ptbp1-suppression and observed strict non-MG-conversion, similar to previous studies using genetic lineage tracking mouse models. Thus, the AAV9-hGFAP-Cre-ΔWPRE system showed high efficiency and specificity for MG labeling, providing a promising tool for tracing cell fate in vivo.

Prime Editing for the Installation and Correction of Mutations Causing Inherited Retinal Disease: A Brief Methodology

Inherited retinal diseases (IRDs) encompass a large heterogeneous group of rare blinding disorders whose etiology originates from mutations in the 280 genes identified to date. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) systems represent a promising avenue for the treatment of IRDs, as exemplified by FDA clinical trial approval of EDIT-101 (AGN-151587), which removes a deep intronic variant in the CEP290 gene that causes Leber congenital amaurosis (LCA) type 10. Prime editing is a novel double-strand break (DSB) independent CRISPR/Cas system which has the potential to correct all 12 possible transition and transversion mutations in addition to small deletions and insertions. Here, as a proof-of-concept study, we describe a methodology using prime editing for the in vitro installation and correction of the classical Pde6b^rd10 c.1678C > T (p.Arg560Cys) mutation which causes autosomal recessive retinitis pigmentosa (RP) in mice.

Emerging gene therapy products for RPGR-associated X-linked retinitis pigmentosa

INTRODUCTION

Mutations in the RPGR gene are responsible for one of the most prevalent and severe types of retinitis pigmentosa. Gene therapy has shown great promise to treat inherited retinal diseases, and currently, four RPGR gene therapy vectors are being evaluated in clinical trials.

AREAS COVERED

This manuscript reviews the gene therapy products that are in development for X-linked retinitis pigmentosa caused by mutations in RPGR, and the challenges that scientists and clinicians have faced.

EXPERT OPINION

The development of a gene therapy product for RPGR-associated retinal degeneration has been a great challenge due to the incomplete understanding of the underlying genetics and mechanism of action of RPGR, and on the other hand, due to the instability of the RPGR gene. Three of the four gene therapy vectors currently in clinical trials include a codon-optimized version of the human RPGR sequence, and the other vector contains a shortened version of the human RPGR. To date, the only Phase I/II results published in a peer-reviewed journal demonstrate a good safety profile and an improvement in the visual field using a codon optimized version of RPGRORF15.

X-Linked Retinitis Pigmentosa Gene Therapy: Preclinical Aspects

The most common inherited eye disease is retinitis pigmentosa (RP). X-linked RP (XLRP) is one of the most severe types of RP, with a considerable disease burden. Patients with XLRP experience a decrease in their vision and become blind in their 4th decade of life, causing much morbidity after starting a rather normal life. Treatment of XLRP remains challenging, and current treatments are not effective enough in restoring vision. Gene therapy of XLRP, capable of restoring the functional RPGR gene, showed promising results in preclinical studies and clinical trials; however, to date, no approved product has entered the market. The development of a gene therapy product needs through preliminary assessment of the drug in animal models before administration to humans. In this article, we reviewed the genetic pathology of XLRP, along with the preclinical aspects of the XLRP gene therapy, animal models, associated assessments, and future challenges and directions.

Cell-specific regulation of gene expression using splicing-dependent frameshifting

Precise and reliable cell-specific gene delivery remains technically challenging. Here we report a splicing-based approach for controlling gene expression whereby separate translational reading frames are coupled to the inclusion or exclusion of mutated, frameshifting cell-specific alternative exons. Candidate exons are identified by analyzing thousands of publicly available RNA sequencing datasets and filtering by cell specificity, conservation, and local intron length. This method, which we denote splicing-linked expression design (SLED), can be combined in a Boolean manner with existing techniques such as minipromoters and viral capsids. SLED can use strong constitutive promoters, without sacrificing precision, by decoupling the tradeoff between promoter strength and selectivity. AAV-packaged SLED vectors can selectively deliver fluorescent reporters and calcium indicators to various neuronal subtypes in vivo. We also demonstrate gene therapy utility by creating SLED vectors that can target PRPH2 and SF3B1 mutations. The flexibility of SLED technology enables creative avenues for basic and translational research.

Emerging therapeutic potential of adeno-associated virus-mediated gene therapy in liver fibrosis

Liver fibrosis is a wound-healing response that results from various chronic damages. If the causes of damage are not removed or effective treatments are not given in a timely manner, it will progress to cirrhosis, even liver cancer. Currently, there are no specific medical therapies for liver fibrosis. Adeno-associated virus (AAV)-mediated gene therapy, one of the frontiers of modern medicine, has gained more attention in many fields due to its high safety profile, low immunogenicity, long-term efficacy in mediating gene expression, and increasingly known tropism. Notably, increasing evidence suggests a promising therapeutic potential for AAV-mediated gene therapy in different liver fibrosis models, which helps to correct abnormally changed target genes in the process of fibrosis and improve liver fibrosis at the molecular level. Moreover, the addition of cell-specific promoters to the genome of recombinant AAV helps to limit gene expression in specific cells, thereby producing better therapeutic efficacy in liver fibrosis. However, animal models are considered to be powerless predictive of tissue tropism, immunogenicity, and genotoxic risks in humans. Thus, AAV-mediated gene therapy will face many challenges. This review systemically summarizes the recent advances of AAV-mediated gene therapy in liver fibrosis, especially focusing on cellular and molecular mechanisms of transferred genes, and presents prospective challenges.

The E3 ubiquitin ligase HUWE1 acts through the N-Myc-DLL1-NOTCH1 signaling axis to suppress glioblastoma progression

BACKGROUND

Elucidation of the post-transcriptional modification has led to novel strategies to treat intractable tumors, especially glioblastoma (GBM). The ubiquitin-proteasome system (UPS) mediates a reversible, stringent and stepwise post-translational modification which is closely associated with malignant processes of GBM. To this end, developing novel therapeutic approaches to target the UPS may contribute to the treatment of this disease. This study aimed to screen the vital and aberrantly regulated component of the UPS in GBM. Based on the molecular identification, functional characterization, and mechanism investigation, we sought to elaborate a novel therapeutic strategy to target this vital factor to combat GBM.

METHODS

We combined glioma datasets and human patient samples to screen and identify aberrantly regulated E3 ubiquitin ligase. Multidimensional database analysis and molecular and functional experiments in vivo and in vitro were used to evaluate the roles of HECT, UBA and WWE domain-containing E3 ubiquitin ligase 1 (HUWE1) in GBM. dCas9 synergistic activation mediator system and recombinant adeno-associated virus (rAAV) were used to endogenously overexpress full-length HUWE1 in vitro and in glioma orthotopic xenografts.

RESULTS

Low expression of HUWE1 was closely associated with worse prognosis of GBM patients. The ubiquitination and subsequent degradation of N-Myc mediated by HUWE1, leading to the inactivation of downstream Delta-like 1 (DLL1)-NOTCH1 signaling pathways, inhibited the proliferation, invasion, and migration of GBM cells in vitro and in vivo. A rAAV dual-vector system for packaging and delivery of dCas9-VP64 was used to augment endogenous HUWE1 expression in vivo and showed an antitumor activity in glioma orthotopic xenografts.

CONCLUSIONS

The E3 ubiquitin ligase HUWE1 acts through the N-Myc-DLL1-NOTCH1 signaling axis to suppress GBM progression. Antitumor activity of rAAV dual-vector delivering dCas9-HUWE1 system uncovers a promising therapeutic strategy for GBM.

Newer therapeutic options for inherited retinal diseases: Gene and cell replacement therapy

Inherited retinal diseases (IRD) are genotypically and phenotypically varied disorders that lead to progressive degeneration of the outer retina and the retinal pigment epithelium (RPE) eventually resulting in severe vision loss. Recent research and developments in gene therapy and cell therapy have shown therapeutic promise in these hitherto incurable diseases. In gene therapy, copies of a healthy gene are introduced into the host cells via a viral vector. Clinical trials for several genes are underway while treatment for RPE65 called voretigene neparvovec, is already approved and commercially available. Cell therapy involves the introduction of stem cells that can replace degenerated cells. These therapies are delivered to the target tissues, namely the photoreceptors (PR) and RPE via subretinal, intravitreal, or suprachoroidal delivery systems. Although there are several limitations to these therapies, they are expected to slow the disease progression and restore some visual functions. Further advances such as gene editing technologies are likely to result in more precise and personalized treatments. Currently, several IRDs such as retinitis pigmentosa, Stargardt disease, Leber congenital amaurosis, choroideremia, achromatopsia, and Usher syndrome are being evaluated for possible gene therapy or cell therapy. It is important to encourage patients to undergo gene testing and maintain a nationwide registry of IRDs. This article provides an overview of the basics of these therapies and their current status.

Cruciform DNA Structures Act as Legible Templates for Accelerating Homologous Recombination in Transgenic Animals

Inverted repeat (IR) DNA sequences compose cruciform structures. Some genetic disorders are the result of genome inversion or translocation by cruciform DNA structures. The present study examined whether exogenous DNA integration into the chromosomes of transgenic animals was related to cruciform DNA structures. Large imperfect cruciform structures were frequently predicted around predestinated transgene integration sites in host genomes of microinjection-based transgenic (Tg) animals (αLA-LPH Tg goat, Akr1A1eGFP/eGFP Tg mouse, and NFκB-Luc Tg mouse) or CRISPR/Cas9 gene-editing (GE) animals (αLA-AP1 GE mouse). Transgene cassettes were imperfectly matched with their predestinated sequences. According to the analyzed data, we proposed a putative model in which the flexible cruciform DNA structures acted as a legible template for DNA integration into linear DNAs or double-strand break (DSB) alleles. To demonstrate this model, artificial inverted repeat knock-in (KI) reporter plasmids were created to analyze the KI rate using the CRISPR/Cas9 system in NIH3T3 cells. Notably, the KI rate of the 5′ homologous arm inverted repeat donor plasmid (5′IR) with the ROSA gRNA group (31.5%) was significantly higher than the knock-in reporter donor plasmid (KIR) with the ROSA gRNA group (21.3%, p < 0.05). However, the KI rate of the 3′ inverted terminal repeat/inverted repeat donor plasmid (3′ITRIR) group was not different from the KIR group (23.0% vs. 22.0%). These results demonstrated that the legibility of the sequence with the cruciform DNA existing in the transgene promoted homologous recombination (HR) with a higher KI rate. Our findings suggest that flexible cruciform DNAs folded by IR sequences improve the legibility and accelerate DNA 3′-overhang integration into the host genome via homologous recombination machinery.

Tropism of AAV Vectors in Photoreceptor-Like Cells of Human iPSC-Derived Retinal Organoids

Purpose

To expand the use of human retinal organoids from induced pluripotent stem cells (iPSCs) as an in vitro model of the retina for assessing gene therapy treatments, it is essential to establish efficient transduction. To date, targeted transduction of the photoreceptor-like cells of retinal organoids with adeno-associated virus (AAV) vectors has had varied degrees of success, which we have looked to improve in this study.

Methods

Retinal organoids were differentiated from iPSCs of healthy donors and transduced with reporter AAV containing a CAG.GFP, CAG.RFP, GRK1.GFP, or EFS.GFP transgene. Capsid variants assessed were AAV5, AAV2 7m8, AAV2 quad mutant, AAV2 Y444F, and AAV8 Y733F. At 27 days post-transduction, retinal organoids were assessed for reporter expression and viability.

Results

The short intron-less elongation factor 1 alpha (EFS) promoter provided minimal reporter expression, whereas vectors containing the CAG promoter enabled transduction in 1% to 37% of cells depending on the AAV serotype; the AAV2 quad mutant (average 19.4%) and AAV2 7m8 (16.4%) outperformed AAV5 (12%) and AAV8 Y733F (2.1%). Reporter expression from rhodopsin kinase (GRK1) promoter transgenes occurred in ∼5% of cells regardless of the serotype. Positive co-localization with recoverin-expressing cells was achieved from all GRK1 vectors and the CAG AAV2 quad mutant variant. Treatment with the AAV vectors did not influence retinal organoid viability.

Conclusions

Reliable transduction of the photoreceptor-like cells of retinal organoids can be readily achieved. When using a CAG-driven transgene, transduction of a broad range of cell types is observed, and GRK1 transgenes provide a more restricted expression profile locating to the outer layer of photoreceptor-like cells of retinal organoids.

Translational Relevance

This study expands the AAV capsid and transgene options for preclinical testing of gene therapy in iPSC-derived human retinal organoids.

Benefits and Shortcomings of Laboratory Model Systems in the Development of Genetic Therapies

Gene therapeutic approaches promise treatment or even a cure of diseases that were previously untreatable. Retinal gene therapies tested in clinical trials comprise a wide range of different strategies, including gene supplementation therapies, in vivo gene editing, modulation of splicing mechanisms, or the suppression of gene expression. To guarantee efficient transfer of genetic material into the respective target cells while avoiding major adverse effects, the development of genetic therapies requires appropriate in vitro model systems that allow tests of efficacy and safety of the gene therapeutic approach. In this review, we introduce various in vitro models of different levels of complexity used in the development of genetic therapies and discuss their respective benefits and shortcomings using the example of adeno-associated virus-based retinal gene therapy.

Delivery of nVEGFi using AAV8 for the treatment of neovascular age-related macular degeneration

Inhibition of vascular endothelial growth factor (VEGF) is the standard therapy for neovascular age-related macular degeneration (nAMD). However, anti-VEGF agents used in the clinic require repeated injections, causing adverse effects. Gene therapy could provide sustained anti-VEGF levels after a single injection, thereby drastically decreasing the treatment burden and improving visual outcomes. In this study, we developed a novel VEGF Trap, nVEGFi, containing domains 1 and 2 of VEGFR1 and domain 3 of VEGFR2 fused to the Fc portion of human IgG. The nVEGFi had a higher expression level than aflibercept under the same expression cassettes of adeno-associated virus (AAV)8 in vitro and in vivo. nVEGFi was found to be noninferior to aflibercept in binding and blocking VEGF in vitro. AAV8-mediated expression of nVEGFi was maintained for at least 12 weeks by subretinal delivery in C57BL/6J mice. In a mouse laser-induced choroidal neovascularization (CNV) model, 4 × 10⁸ genome copies of AAV8-nVEGFi exhibited a significantly increased reduction in the CNV area compared with AAV8-aflibercept (78.1% vs. 63.9%, p < 0.05), while causing no structural or functional changes to the retina. In conclusion, this preclinical study showed that subretinal injection of AAV8-nVEGFi was long lasting, well tolerated, and effective for nAMD treatment, supporting future translation to the clinic.

Gene Therapy Advances: A Meta-Analysis of AAV Usage in Clinical Settings

Adeno-associated viruses (AAVs) are the safest and most effective gene delivery vehicles to drive long-term transgene expression in gene therapy. While animal studies have shown promising results, the translatability of AAVs into clinical settings has been partly limited due to their restricted gene packaging capacities, off-target transduction, and immunogenicity. In this study, we analysed over two decades of AAV applications, in 136 clinical trials. This meta-analysis aims to provide an up-to-date overview of the use and successes of AAVs in clinical trials, while evaluating the approaches used to address the above challenges. First, this study reveals that the speed of novel AAV development has varied between therapeutic areas, with particular room for improvement in Central Nervous System disorders, where development has been slow. Second, the lack of dose-dependent toxicity and efficacy data indicates that optimal dosing regimes remain elusive. Third, more clinical data on the effectiveness of various immune-modulation strategies and gene editing approaches are required to direct future research and to accelerate the translation of AAV-mediated gene therapy into human applications.

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

Progress in Gene Editing Tools and Their Potential for Correcting Mutations Underlying Hearing and Vision Loss

Blindness and deafness are the most frequent sensory disorders in humans. Whatever their cause - genetic, environmental, or due to toxic agents, or aging - the deterioration of these senses is often linked to irreversible damage to the light-sensing photoreceptor cells (blindness) and/or the mechanosensitive hair cells (deafness). Efforts are increasingly focused on preventing disease progression by correcting or replacing the blindness and deafness-causal pathogenic alleles. In recent years, gene replacement therapies for rare monogenic disorders of the retina have given positive results, leading to the marketing of the first gene therapy product for a form of childhood hereditary blindness. Promising results, with a partial restoration of auditory function, have also been reported in preclinical models of human deafness. Silencing approaches, including antisense oligonucleotides, adeno-associated virus (AAV)-mediated microRNA delivery, and genome-editing approaches have also been applied to various genetic forms of blindness and deafness The discovery of new DNA- and RNA-based CRISPR/Cas nucleases, and the new generations of base, prime, and RNA editors offers new possibilities for directly repairing point mutations and therapeutically restoring gene function. Thanks to easy access and immune-privilege status of self-contained compartments, the eye and the ear continue to be at the forefront of developing therapies for genetic diseases. Here, we review the ongoing applications and achievements of this new class of emerging therapeutics in the sensory organs of vision and hearing, highlighting the challenges ahead and the solutions to be overcome for their successful therapeutic application in vivo.

Promoter CAG is more efficient than hepatocyte‑targeting TBG for transgene expression via rAAV8 in liver tissues

The recombinant adeno‑associated virus 8 (rAAV8) vector is a widely used tool in basic research and clinical trials. The cytomegalovirus immediate‑early enhancer/chicken β‑actin (CAG) promoter is a synthetic promoter used in adenoviral constructs with a wide spectrum and notable efficiency. The thyroxine binding globulin (TBG) promoter is a liver‑specific promoter, which directs transgene expression in hepatocytes. However, the transduction efficiency of the rAAV vector is dependent on both the administration routes and the promoter elements. In the present study, the transduction efficiency in the liver following intraperitoneal (IP) and intravenous (IV) injections of rAAV8 with the CAG, TBG669 and TBG410 promoters was compared. Enhanced green fluorescent protein (EGFP) expression was used as the biomarker to indicate efficiency. Among the three different promoters, CAG exhibited the highest efficiency from both IV and IP injections. Following IV administration, EGFP expression, induced by the CAG promoter, was 67‑fold higher compared with that in the TBG410 promoter group and 26‑fold higher compared with that in the TBG669 promoter group. EGFP protein expression was higher with IV injection compared with that for IP injection for both the CAG and TBG669 promoters (P<0.05). With the CAG promoter, EGFP protein expression was 1.5‑fold higher with the use of IV injection than with IP injection. With the TBG410 promoter, no differences were observed between the two administrations. In conclusion, these findings demonstrated that the CAG promoter was much more efficient at driving gene expression in the liver compared with that for the TBG promoters in rAAV8. In addition, IP administration produced comparable efficiency for gene delivery via the rAAV8 vector, particularly with the promoter TBG410.

Adeno-Associated Viruses (AAV) and Host Immunity - A Race Between the Hare and the Hedgehog

Adeno-associated viruses (AAV) have emerged as the lead vector in clinical trials and form the basis for several approved gene therapies for human diseases, mainly owing to their ability to sustain robust and long-term in vivo transgene expression, their amenability to genetic engineering of cargo and capsid, as well as their moderate toxicity and immunogenicity. Still, recent reports of fatalities in a clinical trial for a neuromuscular disease, although linked to an exceptionally high vector dose, have raised new caution about the safety of recombinant AAVs. Moreover, concerns linger about the presence of pre-existing anti-AAV antibodies in the human population, which precludes a significant percentage of patients from receiving, and benefitting from, AAV gene therapies. These concerns are exacerbated by observations of cellular immune responses and other adverse events, including detrimental off-target transgene expression in dorsal root ganglia. Here, we provide an update on our knowledge of the immunological and molecular race between AAV (the “hedgehog”) and its human host (the “hare”), together with a compendium of state-of-the-art technologies which provide an advantage to AAV and which, thus, promise safer and more broadly applicable AAV gene therapies in the future.

Short-hairpin RNAs delivered by recombinant adeno-associated virus inhibited the replication of influenza A viruses in vitro

Antiviral short-hairpin RNAs (shRNAs) delivered by recombinant adeno-associated virus (rAAV) were investigated for their potential prophylactic and therapeutic applications related to the influenza A virus (IAV). To express shRNAs efficiently, an H1 promoter was inserted into the commercial rAAV2 system. The modified rAAV2 system could express shRNAs, and the purified rAAV was obtained at levels over 10¹³ viral genomes/ml and 10¹⁰ viral infection units/ml. The shNP-1496-n and shM2-925 delivered by rAAV could inhibit the replication of the H1N1 and H5N1 virus by targeting the conserved regions of the IAV nucleoprotein and matrix 2 genes in MDCK cells. The shNP-1496-n and shM2-925 expressed by rAAV could provide potent and long-term anti-H5N1 virus effects in rAAV-shRNA-enriched MDCK cells. Our findings provide a rational basis for developing RNA interference for the prevention and therapy of IAV infection.

Analytical methods to characterize recombinant adeno-associated virus vectors and the benefit of standardization and reference materials

Recombinant adeno-associated virus (rAAV) is an increasingly important gene therapy vector, but its properties present unique challenges to critical quality attribute (CQA) identification and analytics development. Advances in, and ongoing hurdles to, characterizing rAAV proteins, nucleic acids, and vector potency are discussed in this review. For nucleic acids and vector potency, current analytical techniques for defined CQAs would benefit from further optimization, while for proteins, more complete characterization and mapping of properties to safety and efficacy is needed to finalize CQAs. The benefits of leveraging reference vectors to validate analytics and CQA ranges are also proposed. Once defined, CQA specifications can be used to establish target parameters for and inform the development of next generation rAAV processes.

Human stem cell-based retina on chip as new translational model for validation of AAV retinal gene therapy vectors

Gene therapies using adeno-associated viruses (AAVs) are among the most promising strategies to treat or even cure hereditary and acquired retinal diseases. However, the development of new efficient AAV vectors is slow and costly, largely because of the lack of suitable non-clinical models. By faithfully recreating structure and function of human tissues, human induced pluripotent stem cell (iPSC)-derived retinal organoids could become an essential part of the test cascade addressing translational aspects. Organ-on-chip (OoC) technology further provides the capability to recapitulate microphysiological tissue environments as well as a precise control over structural and temporal parameters. By employing our recently developed retina on chip that merges organoid and OoC technology, we analyzed the efficacy, kinetics, and cell tropism of seven first- and second-generation AAV vectors. The presented data demonstrate the potential of iPSC-based OoC models as the next generation of screening platforms for future gene therapeutic studies.

The Ocular Gene Delivery Landscape

The eye is at the forefront of developing therapies for genetic diseases. With the FDA approval of the first gene-therapy drug for a form of congenital blindness, numerous studies have been initiated to develop gene therapies for other forms of eye diseases. These examinations have revealed new information about the benefits as well as restrictions to using drug-delivery routes to the different parts of the eye. In this article, we will discuss a brief history of gene therapy and its importance to the eye and ocular delivery landscape that is currently being investigated, and provide insights into their advantages and disadvantages. Efficient delivery routes and vehicle are crucial for an effective, safe, and longer-lasting therapy.

Genetics, pathogenesis and therapeutic developments for Usher syndrome type 2

Usher syndrome (USH) is a rare, autosomal recessively inherited disorder resulting in a combination of sensorineural hearing loss and a progressive loss of vision resulting from retinitis pigmentosa (RP), occasionally accompanied by an altered vestibular function. More and more evidence is building up indicating that also sleep deprivation, olfactory dysfunction, deficits in tactile perception and reduced sperm motility are part of the disease etiology. USH can be clinically classified into three different types, of which Usher syndrome type 2 (USH2) is the most prevalent. In this review, we, therefore, assess the genetic and clinical aspects, available models and therapeutic developments for USH2. Mutations in USH2A, ADGRV1 and WHRN have been described to be responsible for USH2, with USH2A being the most frequently mutated USH-associated gene, explaining 50% of all cases. The proteins encoded by the USH2 genes together function in a dynamic protein complex that, among others, is found at the photoreceptor periciliary membrane and at the base of the hair bundles of inner ear hair cells. To unravel the pathogenic mechanisms underlying USH2, patient-derived cellular models and animal models including mouse, zebrafish and drosophila, have been generated that all in part mimic the USH phenotype. Multiple cellular and genetic therapeutic approaches are currently under development for USH2, mainly focused on preserving or partially restoring the visual function of which one is already in the clinical phase. These developments are opening a new gate towards a possible treatment for USH2 patients.

Inflammation in Viral Vector-Mediated Ocular Gene Therapy: A Review and Report From a Workshop Hosted by the Foundation Fighting Blindness, 9/2020

On September 14–15, 2020, the Foundation Fighting Blindness convened a virtual workshop to discuss intraocular inflammation during viral vector-mediated gene therapy for inherited retinal diseases. The workshop's goals were to understand immune activation's nature and significance during ocular gene therapy, consider whether ocular inflammation limits gene therapy's potential, and identify knowledge gaps for future research. The event brought together a small group of experienced researchers in the field to present and discuss current data. Collectively, participants agreed that clinical, as well as subclinical, inflammation during ocular gene therapy is common. The severity of inflammation in both animal and clinical studies varied widely but is generally related to vector dose. Severe inflammation was associated with reduced gene therapy efficacy. However, the relationship between outcomes and subclinical inflammation, pre-existing antivector antibodies, or induced adaptive immune responses is still unclear. Uncertainties about the contribution of vector manufacturing issues to inflammation were also noted. Importantly, various immunosuppressive treatment protocols are being used, and this heterogeneity confounds conclusions about optimal strategies. Proposed near-term next steps include establishing an immunological consultant directory, establishing a data repository for pertinent animal and clinical data, and developing a larger meeting. Priority areas for future research include deeper understanding of immune activation during retinal diseases and during ocular gene therapy; better, harmonized application of animal models; and identifying best practices for managing gene therapy vector-related ocular inflammation.

Recombinant adeno-associated virus-based gene therapy combined with tissue engineering for musculoskeletal regenerative medicine

Recombinant adeno-associated viral (rAAV) vector-mediated gene delivery is a novel molecular therapeutic approach for musculoskeletal disorders which achieves tissue regeneration by delivering a transgene to the impaired tissue. In recent years, substantial scientific progress in rAAV gene therapy has led to several clinical trials for human musculoskeletal diseases. Nevertheless, there are still limitations in developing an optimal gene therapy model due to the low transduction efficiency and fast degradation of the gene vectors. To overcome the challenges of rAAV gene therapy, tissue engineering combined with gene therapy has emerged as a more promising alternative. An rAAV viral vector incorporated into a biomaterial has a more controlled gene expression, lower immune response, and higher efficiency. A number of biomaterials and architectures have been combined with rAAV viral vectors, each having its own advantages and limitations. This review aims to give a broad introduction to combinatorial therapy and the recent progress this new technology has offered.

Voretigene Neparvovec and Gene Therapy for Leber's Congenital Amaurosis: Review of Evidence to Date

Gene therapy has now evolved as the upcoming modality for management of many disorders, both inheritable and non-inheritable. Knowledge of genetics pertaining to a disease has therefore become paramount for physicians across most specialities. Inheritable retinal dystrophies (IRDs) are notorious for progressive and relentless vision loss, frequently culminating in complete blindness in both eyes. Leber’s congenital amaurosis (LCA) is a typical example of an IRD that manifests very early in childhood. Research in gene therapy has led to the development and approval of voretigene neparvovec (VN) for use in patients of LCA with a deficient biallelic RPE65 gene. The procedure involves delivery of a recombinant virus vector that carries the RPE65 gene in the subretinal space. This comprehensive review reports the evidence thus far in support of gene therapy for LCA. We explore and compare the various gene targets including but not limited to RPE65, and discuss the choice of vector and method for ocular delivery. The review details the evolution of gene therapy with VN in a phased manner, concluding with the challenges that lie ahead for its translation for use in communities that differ much both genetically and economically.