AAV cis-regulatory sequences are correlated with ocular toxicity

Subretinal AAV delivery of RNAi-therapeutics targeting VEGFA reduces choroidal neovascularization in a large animal model

Neovascular age-related macular degeneration (nAMD) is a frequent cause of vision loss among the elderly in the Western world. Current disease management with repeated injections of anti-VEGF agents accumulates the risk for adverse events and constitutes a burden for society and the individual patient. Sustained suppression of VEGF using gene therapy is an attractive alternative, which we explored using adeno-associated virus (AAV)-based delivery of novel RNA interference (RNAi) effectors in a porcine model of choroidal neovascularization (CNV). The potency of VEGFA-targeting, Ago2-dependent short hairpin RNAs placed in pri-microRNA scaffolds (miR-agshRNA) was established in vitro and in vivo in mice. Subsequently, AAV serotype 8 (AAV2.8) vectors encoding VEGFA-targeting or irrelevant miR-agshRNAs under the control of a tissue-specific promotor were delivered to the porcine retina via subretinal injection before CNV induction by laser. Notably, VEGFA-targeting miR-agshRNAs resulted in a significant and sizable reduction of CNV compared with the non-targeting control. We also demonstrated that single-stranded and self-complementary AAV2.8 vectors efficiently transduce porcine retinal pigment epithelium cells but differ in their transduction characteristics and retinal safety. Collectively, our data demonstrated a robust anti-angiogenic effect of VEGFA-targeting miR-aghsRNAs in a large translational animal model, thereby suggesting AAV-based delivery of anti-VEGFA RNAi therapeutics as a valuable tool for the management of nAMD.

Targeting miR-181a/b in retinitis pigmentosa: implications for disease progression and therapy

BACKGROUND

Retinitis pigmentosa (RP) is a genetically heterogeneous group of degenerative disorders causing progressive vision loss due to photoreceptor death. RP affects other retinal cells, including the retinal pigment epithelium (RPE). MicroRNAs (miRs) are implicated in RP pathogenesis, and downregulating miR-181a/b has shown therapeutic benefit in RP mouse models by improving mitochondrial function. This study investigates the expression profile of miR-181a/b in RPE cells and the neural retina during RP disease progression. We also evaluate how miR-181a/b downregulation, by knocking out miR-181a/b-1 cluster in RPE cells, confers therapeutic efficacy in an RP mouse model and explore the mechanisms underlying this process.

RESULTS

Our findings reveal distinct expression profiles, with downregulated miR-181a/b in RPE cells suggesting a protective response and upregulated miR-181a/b in the neural retina indicating a role in disease progression. We found that miR-181a/b-2, encoded in a separate genomic cluster, compensates for miR-181a/b-1 ablation in RPE cells at late time points. The transient downregulation of miR-181a/b in RPE cells at post-natal week 6 (PW6) led to improved RPE morphology, retarded photoreceptor degeneration and decreased RPE aerobic glycolysis.

CONCLUSIONS

Our study elucidates the underlying mechanisms associated with the therapeutic modulation of miR-181a/b, providing insights into the metabolic processes linked to its RPE-specific downregulation. Our data further highlights the impact of compensatory regulation between miR clusters with implications for the development of miR-based therapeutics.

Massively parallel in vivo Perturb-seq reveals cell-type-specific transcriptional networks in cortical development

Leveraging AAVs' versatile tropism and labeling capacity, we expanded the scale of in vivo CRISPR screening with single-cell transcriptomic phenotyping across embryonic to adult brains and peripheral nervous systems. Through extensive tests of 86 vectors across AAV serotypes combined with a transposon system, we substantially amplified labeling efficacy and accelerated in vivo gene delivery from weeks to days. Our proof-of-principle in utero screen identified the pleiotropic effects of Foxg1, highlighting its tight regulation of distinct networks essential for cell fate specification of Layer 6 corticothalamic neurons. Notably, our platform can label >6% of cerebral cells, surpassing the current state-of-the-art efficacy at <0.1% by lentivirus, to achieve analysis of over 30,000 cells in one experiment and enable massively parallel in vivo Perturb-seq. Compatible with various phenotypic measurements (single-cell or spatial multi-omics), it presents a flexible approach to interrogate gene function across cell types in vivo, translating gene variants to their causal function.

Overexpression of miR-96 leads to retinal degeneration in mice

Double knockout of miR-183 and miR-96 results in retinal degeneration in mice; however, single knockout of miR-96 leads to developmental delay but not substantial retinal degeneration. To further explore the role of miR-96, we overexpressed this miRNA in mouse retinas. Interestingly, we found that overexpression of miR-96 at a safe dose results in retinal degeneration in the mouse retina. The retinal photoreceptors dramatically degenerated in the miR-96-overexpressing group, as shown by OCT, ERG and cryosectioning at one month after subretinal injection. Degenerative features such as TUNEL signals and reactive gliosis were observed in the miR-96-overexpressing retina. RNA-seq data revealed that immune responses and microglial activation occurred in the degenerating retina. Further qRT‒PCR and immunostaining experiments verified the microglial activation. Moreover, the number of microglia in the miR-96-overexpressing retinas was significantly increased. Our findings demonstrate that appropriate miR-96 expression is required for mouse retinal homeostasis.

Gene Therapy in Cardiology: Is a Cure for Hypertrophic Cardiomyopathy on the Horizon?

Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiomyopathy worldwide, affecting approximately 1 in 500 individuals. Current therapeutic interventions include lifestyle optimisation, medications, septal reduction therapies, and, rarely, cardiac transplantation. Advances in our understanding of disease-causing genetic variants in HCM and their associated molecular mechanisms have led to the potential for targeted therapeutics and implementation of precision and personalised medicine. Results from preclinical research are promising and raise the question of whether cure of some subtypes of HCM may be possible in the future. This review provides an overview of current genetic therapy platforms, including 1) genome editing, 2) gene replacement, 3) allelic-specific silencing, and 4) signalling pathway modulation. The current applicability of each of these platforms within the paradigm of HCM is examined, with updates on current and emerging trials in each domain. Barriers and limitations within the current landscape are also highlighted. Despite recent advances, translation of genetic therapy for HCM to clinical practice is still in early development. In realising the promises of genetic HCM therapies, ethical and equitable access to safe gene therapy must be prioritised.

Macrophage activation contributes to diabetic retinopathy

Diabetic retinopathy (DR) is recognized as a neurovascular complication of diabetes, and emerging evidence underscores the pivotal role of inflammation in its pathophysiology. Macrophage activation is increasingly acknowledged as a key contributor to the onset and progression of DR. Different populations of macrophages originating from distinct sources contribute to DR-associated inflammation. Retinal macrophages can be broadly categorized into two main groups based on their origin: intrinsic macrophages situated within the retina and vitreoretinal interface and macrophages derived from infiltrating monocytes. The former comprises microglia (MG), perivascular macrophages, and macrophage-like hyalocytes. Retinal MG, as the principal population of tissue-resident population of mononuclear phagocytes, exhibits high heterogeneity and plasticity while serving as a crucial connector between retinal capillaries and synapses. This makes MG actively involved in the pathological processes across various stages of DR. Activated hyalocytes also contribute to the pathological progression of advanced DR. Additionally, recruited monocytes, displaying rapid turnover in circulation, augment the population of retinal macrophages during DR pathogenesis, exerting pathogenic or protective effect based on different subtypes. In this review, we examine novel perspectives on macrophage biology based on recent studies elucidating the diversity of macrophage identity and function, as well as the mechanisms influencing macrophage behavior. These insights may pave the way for innovative therapeutic strategies in the management of DR.

An innate immune response to adeno-associated virus genomes decreases cortical dendritic complexity and disrupts synaptic transmission

Recombinant adeno-associated viruses (AAVs) allow rapid and efficient gene delivery to the nervous system, are widely used in neuroscience research, and are the basis of FDA-approved neuron-targeting gene therapies. Here we find that an innate immune response to the AAV genome reduces dendritic length and complexity and disrupts synaptic transmission in mouse somatosensory cortex. Dendritic loss is apparent 3 weeks after injection of experimentally relevant viral titers, is not restricted to a particular capsid serotype, transgene, promoter, or production facility, and cannot be explained by responses to surgery or transgene expression. AAV-associated dendritic loss is accompanied by a decrease in the frequency and amplitude of miniature excitatory postsynaptic currents and an increase in the proportion of GluA2-lacking, calcium-permeable AMPA receptors. The AAV genome is rich in unmethylated CpG DNA, which is recognized by the innate immunoreceptor Toll-like receptor 9 (TLR9), and acutely blocking TLR9 preserves dendritic complexity and AMPA receptor subunit composition in AAV-injected mice. These results reveal unexpected impacts of an immune response to the AAV genome on neuronal structure and function and identify approaches to improve the safety and efficacy of AAV-mediated gene delivery in the nervous system.

Could internal limiting membrane peeling before Voretigen neparvovec-ryzl subretinal injection prevent focal chorioretinal atrophy?

Purpose

To report the effect of internal limiting membrane (ILM) peeling prior to Voretigen Neparvovec-ryzl (VN) subretinal injection on focal chorioretinal atrophy development in patients presenting with RPE65-mediated Leber congenital amaurosis (LCA).

Design

Retrospective case series.

Methods

Three patients who underwent bilateral subretinal VN injection for RPE65-mediated LCA were followed up for 18-24 months. ILM peeling was performed unilaterally in patients 1 and 2 and bilaterally in patient 3. Chorioretinal atrophy was identified on fundus biomicroscopy, non-mydriatic retinography and/or ultrawide field fundus imaging. Best corrected visual acuity (BCVA), spectral-domain optical coherence tomography (SD-OCT), visual fields, full-field stimulus threshold (FST) and visual functioning questionnaire score (NEI-VFQ-25) were reported. Outcome measures were changes in BCVA, visual fields, FST, NEI-VFQ-25, and chorioretinal atrophy location.

Results

Chorioretinal atrophy at the injection site exclusively developed in eyes which did not undergo prior ILM peeling. In patient 3, bilateral pre-operative nummular chorioretinal alterations progressed toward epithelial atrophic patches in the mid and extreme retinal periphery 18 months after VN injection. BCVA and visual fields improved bilaterally. NEI_VFQ 25 remained stable in patient 1 and improved in patient 2 and 3. FST test improved bilaterally in patient 3.

Conclusions

ILM peeling prior to VN injection seems to be a smoother and safer technique to administer VN treatment and may prevent secondary focal atrophy development at the injection site. However, another type of more extended chorioretinal atrophy might exist and could be related to LCA evolution or to incompletely understood adverse effect of VN product.

rAAV-compatible human mini promoters enhance transgene expression in rat retinal ganglion cells

Recombinant adeno-associated viral vectors (rAAV) are the safest and most effective gene delivery platform to drive the treatment of many inherited eye disorders in well-characterized animal models. The use in rAAV of ubiquitous promoters derived from viral sequences such as CMV/CBA (chicken β-actin promoter with cytomegalovirus enhancer) can lead to unwanted side effects such as pro-inflammatory immune responses and retinal cytotoxicity, thus reducing therapy efficacy. Thus, an advance in gene therapy is the availability of small promoters, that potentiate and direct gene expression to the cell type of interest, with higher safety and efficacy. In this study, we used six human mini-promoters packaged in rAAV2 quadruple mutant (Y-F) to test for transduction of the rat retina after intravitreal injection. After four weeks, immunohistochemical analysis detected GFP-labeled cells in the ganglion cell layer (GCL) for all constructs tested. Among them, Ple25sh1, Ple25sh2 and Ple53 promoted a widespread reporter-transgene expression in the GCL, with an increased number of GFP-expressing retinal ganglion cells when compared with the CMV/CBA vector. Moreover, Ple53 provided the strongest levels of GFP fluorescence in both cell soma and axons of retinal ganglion cells (RGCs) without any detectable adverse effects in retina function. Remarkably, a nearly 50-fold reduction in the number of intravitreally injected vector particles containing Ple53 promoter, still attained levels of transgene expression similar to CMV/CBA. Thus, the tested MiniPs show great potential for protocols of retinal gene therapy in therapeutic applications for retinal degenerations, especially those involving RGC-related disorders such as glaucoma.

Long-term labeling and imaging of synaptically connected neuronal networks in vivo using double-deletion-mutant rabies viruses

Rabies-virus-based monosynaptic tracing is a widely used technique for mapping neural circuitry, but its cytotoxicity has confined it primarily to anatomical applications. Here we present a second-generation system for labeling direct inputs to targeted neuronal populations with minimal toxicity, using double-deletion-mutant rabies viruses. Viral spread requires expression of both deleted viral genes in trans in postsynaptic source cells. Suppressing this expression with doxycycline following an initial period of viral replication reduces toxicity to postsynaptic cells. Longitudinal two-photon imaging in vivo indicated that over 90% of both presynaptic and source cells survived for the full 12-week course of imaging. Ex vivo whole-cell recordings at 5 weeks postinfection showed that the second-generation system perturbs input and source cells much less than the first-generation system. Finally, two-photon calcium imaging of labeled networks of visual cortex neurons showed that their visual response properties appeared normal for 10 weeks, the longest we followed them.

Neuronal Replenishment via Hydrogel-Rationed Delivery of Reprogramming Factors

The central nervous system's limited capacity for regeneration often leads to permanent neuronal loss following injury. Reprogramming resident reactive astrocytes into induced neurons at the site of injury is a promising strategy for neural repair, but challenges persist in stabilizing and accurately targeting viral vectors for transgene expression. In this study, we employed a bioinspired self-assembling peptide (SAP) hydrogel for the precise and controlled release of a hybrid adeno-associated virus (AAV) vector, AAVDJ, carrying the NeuroD1 neural reprogramming transgene. This method effectively mitigates the issues of high viral dosage at the target site, off-target delivery, and immunogenic reactions, enhancing the vector's targeting and reprogramming efficiency. In vitro, this vector successfully induced neuron formation, as confirmed by morphological, histochemical, and electrophysiological analyses. In vivo, SAP-mediated delivery of AAVDJ-NeuroD1 facilitated the trans-differentiation of reactive host astrocytes into induced neurons, concurrently reducing glial scarring. Our findings introduce a safe and effective method for treating central nervous system injuries, marking a significant advancement in regenerative neuroscience.

Immunology of Retinitis Pigmentosa and Gene Therapy-Associated Uveitis

The underlying immune state of inherited retinal degenerations (IRDs) and retinitis pigmentosa (RP) has been an emerging area of interest, wherein the consequences have never been greater given the widespread recognition of gene therapy-associated uveitis (GTU) in gene therapy clinical trials. Whereas some evidence suggests that the adaptive immune system may play a role, the majority of studies indicate that the innate immune system is likely the primary driver of neuroinflammation in RP. During retinal degeneration, discrete mechanisms activate resident microglia and promote infiltrating macrophages that can either be protective or detrimental to photoreceptor cell death. This persistent stimulation of innate immunity, overlaid by the introduction of viral antigens as part of gene therapy, has the potential to trigger a complex microglia/macrophage-driven proinflammatory state. A better understanding of the immune pathophysiology in IRD and GTU will be necessary to improve the success of developing novel treatments for IRDs.

Preclinical Efficacy And Safety Evaluation of AAV-OTOF in DFNB9 Mouse Model And Nonhuman Primate

OTOF mutations are the principal causes of auditory neuropathy. There are reports on Otof-related gene therapy in mice, but there is no preclinical research on the drug evaluations. Here, Anc80L65 and the mouse hair cell-specific Myo15 promoter (mMyo15) are used to selectively and effectively deliver human OTOF to hair cells in mice and nonhuman primates to evaluate the efficacy and safety of OTOF gene therapy drugs. A new dual-AAV-OTOF-hybrid strategy to transfer full-length OTOF is generated, which can stably restore hearing in adult OTOFp.Q939*/Q939* mice with profound deafness, with the longest duration being at least 150 days, and the best therapeutic effect without difference in hearing from wild-type mice. An AAV microinjection method into the cochlea of cynomolgus monkeys without hearing impairment is further established and found the OTOF can be safely and effectively driven by the mMyo15 promoter in hair cells. In addition, the therapeutic dose of AAV drugs has no impact on normal hearing and does not cause significant systemic toxicity both in mouse and nonhuman primates. In summary, this study develops a potential gene therapy strategy for DFNB9 patients in the clinic and provides complete, standardized, and systematic research data for clinical research and application.

Classification and Growth Rate of Chorioretinal Atrophy after Voretigene Neparvovec-Rzyl for RPE65-Mediated Retinal Degeneration

PURPOSE

Classify the appearance and quantify the growth rate of chorioretinal atrophy in patients who received voretigene neparvovec-rzyl (VN) for RPE65-mediated retinal degeneration.

DESIGN

Multicenter retrospective analysis.

SUBJECTS

Patients who underwent subretinal VN injection at 5 institutions and demonstrated posterior-pole chorioretinal atrophy.

METHODS

Ultrawidefield scanning laser ophthalmoscopy or color fundus photos were assessed before and after subretinal VN. Atrophy was defined as regions with ≥ 2 of the following: (1) partial or complete retinal pigment epithelial depigmentation; (2) round shape; (3) sharp margins; and (4) increased visibility of choroidal vessels. Atrophy was qualitatively classified into different subtypes. All atrophy was manually segmented. Linear mixed-effects models with random slopes and intercepts were fit using atrophy area and square root of atrophy area.

MAIN OUTCOME MEASURES

Number of eyes with each atrophy pattern, and slopes of linear mixed-effects models.

RESULTS

Twenty-seven eyes from 14 patients across 5 centers developed chorioretinal atrophy after subretinal VN. A mean of 5.8 ± 2.7 images per eye obtained over 2.2 ± 0.8 years were reviewed, and atrophy was categorized into touchdown (14 eyes), nummular (15 eyes), and perifoveal (12 eyes) subtypes. Fifteen eyes demonstrated > 1 type of atrophy. Thirteen of 14 patients demonstrated bilateral atrophy. The slopes of the mixed-effects models of atrophy area and square root of atrophy area (estimate ± standard error) were 1.7 ± 1.3 mm2/year and 0.6 ± 0.2 mm/year for touchdown atrophy, 5.5 ± 1.3 mm2/year and 1.2 ± 0.2 mm/year for nummular atrophy, and 16.7 ± 1.8 mm2/year and 2.3 ± 0.2 mm/year for perifoveal atrophy. The slopes for each type of atrophy were significantly different in the square root of atrophy model, which best fit the data (P < 0.05).

CONCLUSIONS

Chorioretinal atrophy after subretinal VN for RPE65-mediated retinal degeneration developed according to a touchdown, nummular, and/or perifoveal pattern. Perifoveal atrophy grew the most rapidly, while touchdown atrophy grew the least rapidly. Understanding the causes of these findings, which are present in a minority of patients, merits further investigation.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

AAV for Gene Therapy in Ocular Diseases: Progress and Prospects

Owing to the promising therapeutic effect and one-time treatment advantage, gene therapy may completely change the management of eye diseases, especially retinal diseases. Adeno-associated virus (AAV) is considered one of the most promising viral gene delivery tools because it can infect various types of tissues and is considered as a relatively safe gene delivery vector. The eye is one of the most popular organs for gene therapy, since its limited volume is suitable for small doses of AAV stably transduction. Recently, an increasing number of clinical trials of AAV-mediated gene therapy are underway. This review summarizes the biological functions of AAV and its application in the treatment of various ocular diseases, as well as the characteristics of different AAV delivery routes in clinical applications. Here, the latest research progresses in AAV-mediated gene editing and silencing strategies to modify that the genetic ocular diseases are systematically outlined, especially by base editing and prime editing. We discuss the progress of AAV in ocular optogenetic therapy. We also summarize the application of AAV-mediated gene therapy in animal models and the difficulties in its clinical transformation.

AAV Immunotoxicity: Implications in Anti-HBV Gene Therapy

Hepatitis B virus (HBV) has afflicted humankind for decades and there is still no treatment that can clear the infection. The development of recombinant adeno-associated virus (rAAV)-based gene therapy for HBV infection has become important in recent years and research has made exciting leaps. Initial studies, mainly using mouse models, showed that rAAVs are non-toxic and induce minimal immune responses. However, several later studies demonstrated rAAV toxicity, which is inextricably associated with immunogenicity. This is a major setback for the progression of rAAV-based therapies toward clinical application. Research aimed at understanding the mechanisms behind rAAV immunity and toxicity has contributed significantly to the inception of approaches to overcoming these challenges. The target tissue, the features of the vector, and the vector dose are some of the determinants of AAV toxicity, with the latter being associated with the most severe adverse events. This review discusses our current understanding of rAAV immunogenicity, toxicity, and approaches to overcoming these hurdles. How this information and current knowledge about HBV biology and immunity can be harnessed in the efforts to design safe and effective anti-HBV rAAVs is discussed.

High-titer AAV disrupts cerebrovascular integrity and induces lymphocyte infiltration in adult mouse brain

The brain is often described as an "immune-privileged" organ due to the presence of the blood-brain-barrier (BBB), which limits the entry of immune cells. In general, intracranial injection of adeno-associated virus (AAV) is considered a relatively safe procedure. In this study, we discovered that AAV, a popular engineered viral vector for gene therapy, can disrupt the BBB and induce immune cell infiltration in a titer-dependent manner. First, our bulk RNA sequencing data revealed that injection of high-titer AAV significantly upregulated many genes involved in disrupting BBB integrity and antiviral adaptive immune responses. By using histologic analysis, we further demonstrated that the biological structure of the BBB was severely disrupted in the adult mouse brain. Meanwhile, we noticed abnormal leakage of blood components, including immune cells, within the brain parenchyma of high-titer AAV injected areas. Moreover, we identified that the majority of infiltrated immune cells were cytotoxic T lymphocytes (CTLs), which resulted in a massive loss of neurons at the site of AAV injection. In addition, antagonizing CTL function by administering antibodies significantly reduced neuronal toxicity induced by high-titer AAV. Collectively, our findings underscore potential severe side effects of intracranial injection of high-titer AAV, which might compromise proper data interpretation if unaware of.

Viral Vectors in Gene Replacement Therapy

Throughout the years, several hundred million people with rare genetic disorders have been receiving only symptom management therapy. However, research and development efforts worldwide have led to the development of long-lasting, highly efficient, and safe gene therapy for a wide range of hereditary diseases. Improved viral vectors are now able to evade the preexisting immunity and more efficiently target and transduce therapeutically relevant cells, ensuring genome maintenance and expression of transgenes at the relevant levels. Hematological, ophthalmological, neurodegenerative, and metabolic therapeutic areas have witnessed successful treatment of hemophilia and muscular dystrophy, restoration of immune system in children with immunodeficiencies, and restoration of vision. This review focuses on three leading vector platforms of the past two decades: adeno-associated viruses (AAVs), adenoviruses (AdVs), and lentiviruses (LVs). Special attention is given to successful preclinical and clinical studies that have led to the approval of gene therapies: six AAV-based (Glybera® for lipoprotein lipase deficiency, Luxturna® for retinal dystrophy, Zolgensma® for spinal muscular atrophy, Upstaza® for AADC, Roctavian® for hemophilia A, and Hemgenix® for hemophilia B) and three LV-based (Libmeldy® for infantile metachromatic leukodystrophy, Zynteglo® for β-thalassemia, and Skysona® for ALD). The review also discusses the problems that arise in the development of gene therapy treatments, which, nevertheless, do not overshadow the successes of already developed gene therapies and the hope these treatments give to long-suffering patients and their families.

High temporal frequency light response in mouse retina is mediated by ON and OFF bipolar cells and requires FAT3 signaling

Vision is initiated by the reception of light by photoreceptors and subsequent processing via parallel retinal circuits. Proper circuit organization depends on the multi-functional tissue polarity protein FAT3, which is required for amacrine cell connectivity and retinal lamination. Here we investigated the retinal function of Fat3 mutant mice and found decreases in physiological and perceptual responses to high frequency flashes. These defects did not correlate with abnormal amacrine cell wiring, pointing instead to a role in bipolar cell subtypes that also express FAT3. Indeed, similar deficits were observed in mice lacking the bipolar cell glutamate receptors GRIK1 (OFF-bipolar cells) and GRM6 (ON-bipolar cells). Mechanistically, FAT3 binds to the synaptic protein PTPσ and is required to localize GRIK1 to OFF-cone bipolar cell synapses with cone photoreceptors. How FAT3 impacts ON-cone bipolar cell function at high temporal frequency remains to be uncovered. These findings expand the repertoire of FAT3's functions and reveal the importance of both ON- and OFF-bipolar cells for high frequency light response.

Patient stem cell-derived in vitro disease models for developing novel therapies of retinal ciliopathies

Primary cilia are specialized organelles on the surface of almost all cells in vertebrate tissues and are primarily involved in the detection of extracellular stimuli. In retinal photoreceptors, cilia are uniquely modified to form outer segments containing components required for the detection of light in stacks of membrane discs. Not surprisingly, vision impairment is a frequent phenotype associated with ciliopathies, a heterogeneous class of conditions caused by mutations in proteins required for formation, maintenance and/or function of primary cilia. Traditionally, immortalized cell lines and model organisms have been used to provide insights into the biology of ciliopathies. The advent of methods for reprogramming human somatic cells into pluripotent stem cells has enabled the generation of in vitro disease models directly from patients suffering from ciliopathies. Such models help us in investigating pathological mechanisms specific to human physiology and in developing novel therapeutic approaches. In this article, we review current protocols to differentiate human pluripotent stem cells into retinal cell types, and discuss how these cellular and/or organoid models can be utilized to interrogate pathobiology of ciliopathies affecting the retina and for testing prospective treatments.

Inflammatory Mediators of Axon Regeneration in the Central and Peripheral Nervous Systems

Although most pathways in the mature central nervous system cannot regenerate when injured, research beginning in the late 20th century has led to discoveries that may help reverse this situation. Here, we highlight research in recent years from our laboratory identifying oncomodulin (Ocm), stromal cell-derived factor (SDF)-1, and chemokine CCL5 as growth factors expressed by cells of the innate immune system that promote axon regeneration in the injured optic nerve and elsewhere in the central and peripheral nervous systems. We also review the role of ArmC10, a newly discovered Ocm receptor, in mediating many of these effects, and the synergy between inflammation-derived growth factors and complementary strategies to promote regeneration, including deleting genes encoding cell-intrinsic suppressors of axon growth, manipulating transcription factors that suppress or promote the expression of growth-related genes, and manipulating cell-extrinsic suppressors of axon growth. In some cases, combinatorial strategies have led to unprecedented levels of nerve regeneration. The identification of some similar mechanisms in human neurons offers hope that key discoveries made in animal models may eventually lead to treatments to improve outcomes after neurological damage in patients.

Massively parallel in vivo Perturb-seq reveals cell type-specific transcriptional networks in cortical development

Systematic analysis of gene function across diverse cell types in vivo is hindered by two challenges: obtaining sufficient cells from live tissues and accurately identifying each cell's perturbation in high-throughput single-cell assays. Leveraging AAV's versatile cell type tropism and high labeling capacity, we expanded the resolution and scale of in vivo CRISPR screens: allowing phenotypic analysis at single-cell resolution across a multitude of cell types in the embryonic brain, adult brain, and peripheral nervous system. We undertook extensive tests of 86 AAV serotypes, combined with a transposon system, to substantially amplify labeling and accelerate in vivo gene delivery from weeks to days. Using this platform, we performed an in utero genetic screen as proof-of-principle and identified pleiotropic regulatory networks of Foxg1 in cortical development, including Layer 6 corticothalamic neurons where it tightly controls distinct networks essential for cell fate specification. Notably, our platform can label >6% of cerebral cells, surpassing the current state-of-the-art efficacy at <0.1% (mediated by lentivirus), and achieve analysis of over 30,000 cells in one experiment, thus enabling massively parallel in vivo Perturb-seq. Compatible with various perturbation techniques (CRISPRa/i) and phenotypic measurements (single-cell or spatial multi-omics), our platform presents a flexible, modular approach to interrogate gene function across diverse cell types in vivo, connecting gene variants to their causal functions.

In-depth comparison of Anc80L65 and AAV9 retinal targeting and characterization of cross-reactivity to multiple AAV serotypes in humans

Anc80L65 is a synthetic, ancestral adeno-associated virus that has high tropism toward retinal photoreceptors after subretinal injection in mice and non-human primates. We characterized, for the first time, the post-intravitreal cell-specific transduction profile of Anc80L65 compared with AAV9. Here we use Anc80L65 and AAV9 to intravitreally deliver a copy of the gene encoding GFP into WT C57Bl/6J mice. GFP expression was driven by one of two clinically relevant promoters, chicken β actin (CB) or truncated MECP2 (P546). After qualitative assessment of relative GFP expression, we found Anc80L65 and AAV9 to have similar transduction profiles. Through the development of a novel method for quantifying GFP-positive retinal cells, we found Anc80L65 to have higher tropism in Müller glia and AAV9 to have higher tropism in horizontal cells. In addition, we found P546 to promote GFP expression at a more moderate level compared with the high levels seen under the CB promoter. Finally, for the first time, we characterized Anc80L65 cross-reactivity in human sera; 83% of patients with AAV2 pre-existing antibodies were found to be seropositive for Anc80L65. This study demonstrates the expanded therapeutic applications of Anc80L65 to treat retinal disease and provides the first insights to Anc80L65 pre-existing immunity in humans.

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.

rAAV-PHP.B escapes the mouse eye and causes lethality whereas rAAV9 can transduce aniridic corneal limbal stem cells without lethality

Recently safety concerns have been raised in connection with high doses of recombinant adeno-associated viruses (rAAV). Therefore, we undertook a series of experiments to test viral capsid (rAAV9 and rAAV-PHP.B), dose, and route of administration (intrastromal, intravitreal, and intravenous) focused on aniridia, a congenital blindness that currently has no cure. The success of gene therapy for aniridia may depend on the presence of functional limbal stem cells (LSCs) in the damaged aniridic corneas and whether rAAV can transduce them. Both these concerns were unknown, and thus were also addressed by our studies. For the first time, we report ataxia and lethality after intravitreal or intrastromal rAAV-PHP.B virus injections. We demonstrated virus escape from the eye and transduction of non-ocular tissues by rAAV9 and rAAV-PHP.B capsids. We have also shown that intrastromal and intravitreal delivery of rAAV9 can transduce functional LSCs, as well as all four PAX6-expressing retinal cell types in aniridic eye, respectively. Overall, lack of adverse events and successful transduction of LSCs and retinal cells makes it clear that rAAV9 is the capsid of choice for future aniridia gene therapy. Our finding of rAAV lethality after intraocular injections will be impactful for other researchers developing rAAV-based gene therapies.

Emerging Gene Therapy Technologies for Retinal Ganglion Cell Neuroprotection

ABSTRACT

Optic neuropathies encompass a breadth of diseases that ultimately result in dysfunction and/or loss of retinal ganglion cells (RGCs). Although visual impairment from optic neuropathies is common, there is a lack of effective clinical treatments. Addressing a critical need for novel interventions, preclinical studies have been generating a growing body of evidence that identify promising new drug-based and cell-based therapies. Gene therapy is another emerging therapeutic field that offers the potential of specifically and robustly increasing long-term RGC survival in optic neuropathies. Gene therapy offers additional benefits of driving improvements following a single treatment administration, and it can be designed to target a variety of pathways that may be involved in individual optic neuropathies or across multiple etiologies. This review explores the history of gene therapy, the fundamentals of its application, and the emerging development of gene therapy technology as it relates to treatment of optic neuropathies.

Soluble CX3CL1-expressing retinal pigment epithelium cells protect rod photoreceptors in a mouse model of retinitis pigmentosa

BACKGROUND

Retinitis pigmentosa (RP) is an inherited retinal disease that results in photoreceptor degeneration, leading to severe vision loss or blindness. Due to its genetic heterogeneity, developing a new gene therapy to correct every genetic mutation contributing to its progression is infeasible. Photoreceptor transplantation can be harnessed to restore vision; however, this approach is limited by poor cell survival and synaptic integration into the neural retina. Thus, we developed a combined cell and gene therapy that is expected to protect photoreceptors in most, if not all, cases of RP.

METHODS

Human embryonic stem cells (hESCs) modified with our FailSafe™ system were genetically engineered to overexpress sCX3CL1, an inhibitor of microglia activation that has been shown to preserve photoreceptor survival and function in mouse models of RP, independent of the genetic cause. These cells were differentiated into human retinal pigment epithelium (hRPE) cells and used as therapeutic cells due to their longevity and safety, both of which have been demonstrated in preclinical and clinical studies. Transgenic hRPE were delivered into the subretinal space of immunodeficient mice and the rd10 mouse model of RP to evaluate donor cell survival and retention of transgene expression. The outer nuclear layer was quantified to assess photoreceptor protection.

RESULTS

Transgenic FailSafe™ hRPE (FS-hRPE) cells can survive for at least four months in the retina of immunodeficient mice and retain transgene expression. However, these cells do not persist beyond two weeks post-injection in the retina of immunocompetent rd10 recipients, despite Cyclosporine A treatment. Nevertheless, sCX3CL1-expressing FailSafe™ hRPE cells prevented photoreceptor degeneration in a local acting manner during the duration of their presence in the subretinal space.

CONCLUSIONS

Transgenic hESCs differentiate into hRPE cells and retain sCX3CL1 transgene expression both in vitro and in vivo. Moreover, hRPE cells delivered to the subretinal space of rd10 mice prevented photoreceptor degeneration in a local-acting manner, suggesting that this approach could have applications for preserving photoreceptors in specific subregions of the retina, such as the macula. Overall, our study not only reveals the potential of a combined cell and gene therapy for the treatment of RP, but also the possibility of using hRPE cells to deliver therapeutic biologics in situ to treat diseases over long-term.

Genome editing in the treatment of ocular diseases

Genome-editing technologies have ushered in a new era in gene therapy, providing novel therapeutic strategies for a wide range of diseases, including both genetic and nongenetic ocular diseases. These technologies offer new hope for patients suffering from previously untreatable conditions. The unique anatomical and physiological features of the eye, including its immune-privileged status, size, and compartmentalized structure, provide an optimal environment for the application of these cutting-edge technologies. Moreover, the development of various delivery methods has facilitated the efficient and targeted administration of genome engineering tools designed to correct specific ocular tissues. Additionally, advancements in noninvasive ocular imaging techniques and electroretinography have enabled real-time monitoring of therapeutic efficacy and safety. Herein, we discuss the discovery and development of genome-editing technologies, their application to ocular diseases from the anterior segment to the posterior segment, current limitations encountered in translating these technologies into clinical practice, and ongoing research endeavors aimed at overcoming these challenges.

AAV-GRN partially corrects motor deficits and ALS/FTLD-related pathology in Tmem106b-/-Grn-/- mice

Loss of function of progranulin (PGRN), encoded by the granulin (GRN) gene, is implicated in several neurodegenerative diseases. Several therapeutics to boost PGRN levels are currently in clinical trials. However, it is difficult to test the efficacy of PGRN-enhancing drugs in mouse models due to the mild phenotypes of Grn-/- mice. Recently, mice deficient in both PGRN and TMEM106B were shown to develop severe motor deficits and pathology. Here, we show that intracerebral ventricle injection of PGRN-expressing AAV1/9 viruses partially rescues motor deficits, neuronal loss, glial activation, and lysosomal abnormalities in Tmem106b-/-Grn-/- mice. Widespread expression of PGRN is detected in both the brain and spinal cord for both AAV subtypes. However, AAV9 but not AAV1-mediated expression of PGRN results in high levels of PGRN in the serum. Together, these data support using the Tmem106b-/-Grn-/- mouse strain as a robust mouse model to determine the efficacy of PGRN-elevating therapeutics.

A comparison of the impact on neuronal transcriptome and cognition of rAAV5 transduction with three different doses in the mouse hippocampus

Introduction

Recombinant adeno-associated viruses (rAAVs) are widely used in genetic therapeutics. AAV5 has shown superior transduction efficiency, targeting neurons and glial cells in primate brains. Nonetheless, the comprehensive impact of AAV5 transduction on molecular and behavioral alterations remains unexplored. This study focuses on evaluating the effects of AAV5 transduction in the hippocampus, a critical region for memory formation and emotional processes.

Methods

In this experiment, fluorescence-activated cell sorting (FACS) was utilized to isolate the mCherry-labeled pyramidal neurons in the hippocampus of CaMkIIα-cre mice following three different doses rAAV5-mCherry infusion after 3 weeks, which were then subjected to RNA sequencing (RNA-seq) to assess gene expression profiles. The cytokines concentration, mRNA expression, and glial response in hippocampi were confirmed by ELASA, digital droplet PCR and immunohistochemistry respectively. Locomotion and anxiety-like behaviors were elevated by Open Field Test and Elevated Plus Maze Test, while the Y-Maze were used to assessed spatial working memory. Recognition memory and fear responses were examined by the Novel Object Recognition Test and Fear Conditioning Test, respectively.

Results

We found that 2.88 × 10¹⁰ v.g rAAV5 transduction significantly upregulated genes related to the immune response and apoptosis, and downregulated genes associated with mitochondrial function and synaptic plasticity in hippocampal pyramidal neurons, while did not induce neuronal loss and gliosis compared with 2.88 × 10⁹ v.g and 2.88 × 10⁸ v.g. Furthermore, the same doses impaired working memory and contextual fear memory, without effects on locomotion and anxiety-related behaviors.

Discussion

Our findings highlight the detrimental impact of high-dose administration compared to median-dose or low-dose, resulting in increased neural vulnerability and impaired memory. Therefore, when considering the expression effectiveness of exogenous genes, it is crucial to also take potential side effects into account in clinical settings. However, the precise molecular mechanisms underlying these drawbacks of high-dose rAAV5-mCherry still require further investigation in future studies.

AAV Gene Augmentation of Truncated Complement Factor H Differentially Rescues Ocular Complement Dysregulation in a Mouse Model

Purpose

Complement dysregulation in the eye has been implicated in the pathogenesis of age-related macular degeneration (AMD), and genetic variants of complement factor H (CFH) are strongly associated with AMD risk. We therefore aimed to untangle the role of CFH and its splice variant, factor H-like 1 (FHL-1), in ocular complement regulation derived from local versus circulating sources. We assessed the therapeutic efficacy of adeno-associated viruses (AAVs) expressing human FHL-1 and a truncated version of CFH (tCFH), which retains the functional N- and C-terminal ends of the CFH protein, in restoring the alternative complement pathway in Cfh-/- mouse eyes and plasma.

Methods

Using Cfh-/- mice as a model of complement dysregulation, AAV vectors expressing tCFH or FHL-1 were injected subretinally or via tail vein, and the efficacy of the constructs was evaluated.

Results

Following subretinal injections, tCFH expression rescued factor B (FB) retention in the eye, but FHL-1 expression did not. By contrast, both constructs restored FB detection in plasma following tail vein injections. Both tCFH and FHL-1 proteins accumulated in the posterior eyecup from the circulation following liver transduction; however, neither was able to significantly regulate local ocular complement.

Conclusions

Our findings demonstrate that the C-terminus of human CFH is necessary for complement regulation in the murine eye. Furthermore, exogenous CFH must be synthesized locally to maximize complement regulation in the retina. These findings establish a critical foundation for development of CFH augmentation-based gene therapies for the eye.

Improved gene therapy for MFRP deficiency-mediated retinal degeneration by knocking down endogenous bicistronic Mfrp and Ctrp5 transcript

The membrane frizzled-related protein (Mfrp) and C1-tumor necrosis factor related protein 5 (Ctrp5) genes are transcribed as a bicistronic unit and dysregulation of either gene is associated with retinal degeneration in the retinal pigment epithelium (RPE) cells. However, the mechanisms that regulate the expression of the bicistronic transcript remain controversial. Here, we identified a microRNA-based negative feedback loop that helps maintain a normal expression level of the bicistronic Mfrp and Ctrp5 transcript. Specifically, miR-149-3p, a conserved microRNA, binds to the 3'UTR of the Mfrp gene. In MFRP-deficient rd6 mice, the miR-149-3p levels were compromised compared with those in WT mice, resulting in an increase in the bicistronic transcript. We also report a capsid-modified rAAVDJ-3M vector that is capable of robustly and specifically transducing RPE cells following subretinal delivery. Compared with the parental vector, the modified vector elicited similar levels of serum anti-rAAV antibodies, but recruited fewer microglial infiltrations. Most significantly, we also demonstrate that simultaneous overexpressing of MFRP and knockdown of the bicistronic transcript was more effective in rescuing vision than MFRP overexpression alone. Our findings offer new insights into the function of MFRP and provide a promising therapeutic strategy for the treatment of MFRP-associated ocular diseases.

Genome editing, a superior therapy for inherited retinal diseases

Gene augmentation and genome editing are promising strategies for the treatment of monogenic inherited retinal diseases. Although gene augmentation treatments are commercially available for inherited retinal diseases, there are many shortcomings that need to be addressed, like progressive retinal degeneration and diminishing efficacy over time. Innovative CRISPR-Cas9-based genome editing technologies have broadened the proportion of treatable genetic disorders and can greatly improve or complement treatment outcomes from gene augmentation. Progress in this relatively new field involves the development of therapeutics including gene disruption, ablate-and-replace strategies, and precision gene correction techniques, such as base editing and prime editing. By making direct edits to endogenous DNA, genome editing theoretically guarantees permanent gene correction and long-lasting treatment effects. Improvements to delivery modalities aimed at limiting persistent gene editor activity have displayed an improved safety profile and minimal off-target editing. Continued progress to advance precise gene correction and associated delivery strategies will establish genome editing as the preferred treatment for genetic retinal disorders. This commentary describes the applications, strengths, and drawbacks of conventional gene augmentation approaches, recent advances in precise genome editing in the retina, and promising preclinical strategies to facilitate the use of robust genome editing therapies in human patients.

Gene-agnostic approaches to treating inherited retinal degenerations

Most patients with inherited retinal degenerations (IRDs) have been waiting for treatments that are "just around the corner" for decades, with only a handful of seminal breakthroughs happening in recent years. Highlighting the difficulties in the quest for curative therapeutics, Luxturna required 16 years of development before finally obtaining United States Food and Drug Administration (FDA) approval and its international equivalents. IRDs are both genetically and phenotypically heterogeneous. While this diversity offers many opportunities for gene-by-gene precision medicine-based approaches, it also poses a significant challenge. For this reason, alternative (or parallel) strategies to identify more comprehensive, across-the-board therapeutics for the genetically and phenotypically diverse IRD patient population are very appealing. Even when gene-specific approaches may be available and become approved for use, many patients may have reached a disease stage whereby these approaches may no longer be viable. Thus, alternate visual preservation or restoration therapeutic approaches are needed at these stages. In this review, we underscore several gene-agnostic approaches that are being developed as therapeutics for IRDs. From retinal supplementation to stem cell transplantation, optogenetic therapy and retinal prosthetics, these strategies would bypass at least in part the need for treating every individual gene or mutation or provide an invaluable complement to them. By considering the diverse patient population and treatment strategies suited for different stages and patterns of retinal degeneration, gene agnostic approaches are very well poised to impact favorably outcomes and prognosis for IRD patients.

Developing New Vectors for Retinal Gene Therapy

Since their discovery over 55 years ago, adeno-associated virus (AAV) vectors have become powerful tools for experimental and therapeutic in vivo gene delivery, particularly in the retina. Increasing knowledge of AAV structure and biology has propelled forward the development of engineered AAV vectors with improved abilities for gene delivery. However, major obstacles to safe and efficient therapeutic gene delivery remain, including tropism, inefficient and untargeted gene delivery, and limited carrying capacity. Additional improvements to AAV vectors will be required to achieve therapeutic benefit while avoiding safety issues. In this review, we provide an overview of recent methods for engineering-enhanced AAV capsids, as well as remaining challenges that must be overcome to achieve optimized therapeutic gene delivery in the eye.

Full-field scotopic threshold improvement following voretigene neparvovec-rzyl treatment correlates with chorioretinal atrophy

PURPOSE

To analyze demographic and ophthalmologic data in patients with and without chorioretinal atrophy after voretigene neparvovec-rzyl (VN) in order to identify possible causes for this phenomenon.

DESIGN

Retrospective cohort study with longitudinal follow-up.

SUBJECTS

71 eyes of 38 patients aged 2 to 44 years with RPE65-mediated retinal dystrophy treated with VN across two large gene therapy centers in the USA and Germany.

METHODS

VN-treated patients who developed atrophy were compared to those who did not.

MAIN OUTCOME MEASURES

Gender, age, surgical center, spherical equivalent refraction, BCVA, baseline FST, and post-treatment change in full-field scotopic threshold testing (FST).

RESULTS

20 eyes of 12 patients developed atrophy following treatment with VN (28% of all eyes). There was no significant difference in gender, age, surgical center, or spherical equivalent refraction between the Atrophy group and the No Atrophy group. However, patients between school age and young adulthood were predominantly affected, whereas the youngest and the oldest patients did not develop atrophy. Baseline BCVA was better in patients who developed atrophy than those who did not (P=0.006). The postoperative improvement in FST at 1 month was significantly higher in the Atrophy than the No Atrophy group (P=0.0005), and this difference remained statistically significant at 1 year (P=0.0001). There was no correlation to baseline FST, to inflammation, or to which eye was treated first.

CONCLUSIONS

The degree of FST improvement following VN appears to be strongly correlated with the development of VN-related chorioretinal atrophy. This finding raises the possibility that atrophy may develop as a toxic or metabolic sequela of vector-mediated RPE65 expression. In light of the expanding number of retinal gene therapy clinical trials, this complication warrants further study as it may not be limited to VN.

Dual-AAV split prime editor corrects the mutation and phenotype in mice with inherited retinal degeneration

The prime editor (PE) can edit genomes with almost any intended changes, including all 12 possible types of base substitutions, small insertions and deletions, and their combinations, without the requirement for double strand breaks or exogenous donor templates. PE demonstrates the possibility of correcting a variety of disease-causing mutations and might expand the therapeutic application of gene editing. In this study, PE was optimized based on a dual-adeno-associated virus (AAV) split-intein system in vitro by screening different split sites and split inteins. We found that splitting PE before amino acid 1105(Ser) of SpCas9 with Rma intein resulted in the highest on-target editing. The orientations of pegRNA and nicking sgRNA in the AAV vector were further optimized. To test the in vivo performance of the optimized dual-AAV split-PE3, it was delivered by subretinal injection in rd12 mice with inherited retinal disease Leber congenital amaurosis. The prime editors corrected the pathogenic mutation with up to 16% efficiency in a precise way, with no detectable off-target edits, restored RPE65 expression, rescued retinal and visual function, and preserved photoceptors. Our findings establish a framework for the preclinical development of PE and motivate further testing of PE for the treatment of inherited retinal diseases caused by various mutations.

Gene targeting as a therapeutic avenue in diseases mediated by the complement alternative pathway

The complement alternative pathway (AP) is implicated in numerous diseases affecting many organs, ranging from the rare hematological disease paroxysmal nocturnal hemoglobinuria (PNH), to the common blinding disease age-related macular degeneration (AMD). Critically, the AP amplifies any activating trigger driving a downstream inflammatory response; thus, components of the pathway have become targets for drugs of varying modality. Recent validation from clinical trials using drug modalities such as inhibitory antibodies has paved the path for gene targeting of the AP or downstream effectors. Gene targeting in the complement field currently focuses on supplementation or suppression of complement regulators in AMD and PNH, largely because the eye and liver are highly amenable to drug delivery through local (eye) or systemic (liver) routes. Targeting the liver could facilitate treatment of numerous diseases as this organ generates most of the systemic complement pool. This review explains key concepts of RNA and DNA targeting and discusses assets in clinical development for the treatment of diseases driven by the alternative pathway, including the RNA-targeting therapeutics ALN-CC5, ARO-C3, and IONIS-FB-LRX, and the gene therapies GT005 and HMR59. These therapies are but the spearhead of potential drug candidates that might revolutionize the field in coming years.

The Presence of Hyperreflective Foci Reflects Vascular, Morphologic and Metabolic Alterations in Retinitis Pigmentosa

Background: The presence of hyperreflective foci (HRF) in retinitis pigmentosa (RP) is a potentially new finding. We investigated the presence of HRF in SD-OCT images in eyes with RP and its relation to vascular, morphologic and metabolic findings in RP. Methods: The study was performed on 42 RP patients and 24 controls. Using SD-OCT, we calculated the amount of HRF within the entire retina (HRF-ER) and the outer nuclear layer (HRF-ONL). Retinal vessel diameters (μm) and oxygen saturation (%) values were measured using Oxymap T1. We evaluated the mean diameter in retinal arterioles (D-A) and venules (D-V), the corresponding oxygen saturation values (A-SO₂, V-SO₂) and the oxygen saturation difference (A-V SO₂). Results: RP differed from controls by HRF-ER, HRF-ON and EZ-length (p < 0.001). D-A and D-V were narrower and A-SO₂ and V-SO₂ were higher in RP (p ≤ 0.001). Within RP, significant interactions were found between the HRF-ER* group and: BCVA, EZ length, D-A, A-SO₂ and A-V SO₂ (p ≤ 0.018). The HRF-ONL* group interactions were significant for: BCVA, EZ length, D-A, A-SO₂ and A-V SO₂ (p ≤ 0.014). Conclusion: The present study highlights the presence of HRF to reflect the vascular, morphologic and metabolic alterations in RP. These biomarkers seem to be associated with remodeling and apoptosis that occur with the progression of degeneration.

Minicircle Delivery to the Neural Retina as a Gene Therapy Approach

Non-viral gene therapy has the potential to overcome several shortcomings in viral vector-based therapeutics. Methods of in vivo plasmid delivery have developed over recent years to increase the efficiency of non-viral gene transfer, yet further improvements still need to be made to improve their translational capacity. Gene therapy advances for inherited retinal disease have been particularly prominent over the recent decade but overcoming physical and physiological barriers present in the eye remains a key obstacle in the field of non-viral ocular drug delivery. Minicircles are circular double-stranded DNA vectors that contain expression cassettes devoid of bacterial DNA, thereby limiting the risks of innate immune responses induced by such elements. To date, they have not been extensively used in pre-clinical studies yet remain a viable vector option for the treatment of inherited retinal disease. Here, we explore the potential of minicircle DNA delivery to the neural retina as a gene therapy approach. We consider the advantages of minicircles as gene therapy vectors as well as review the challenges involved in optimising their delivery to the neural retina.

Gene Therapy for Inherited Retinal Disease: Long-Term Durability of Effect

The recent approval of voretigene neparvovec (Luxturna®) for patients with biallelic RPE65 mutation-associated inherited retinal dystrophy with viable retinal cells represents an important step in the development of ocular gene therapies. Herein, we review studies investigating the episomal persistence of different recombinant adeno-associated virus (rAAV) vector genomes and the preclinical and clinical evidence of long-term effects of different RPE65 gene replacement therapies. A targeted review of articles published between 1974 and January 2021 in Medline®, Embase®, and other databases was conducted, followed by a descriptive longitudinal analysis of the clinical trial outcomes of voretigene neparvovec. Following an initial screening, 14 publications examining the episomal persistence of different rAAV genomes and 71 publications evaluating gene therapies in animal models were included. Viral genomes were found to persist for at least 22 months (longest study follow-up) as transcriptionally active episomes. Treatment effects lasting almost a decade were reported in canine disease models, with more pronounced effects the earlier the intervention. The clinical trial outcomes of voretigene neparvovec are consistent with preclinical findings and reveal sustained results for up to 7.5 years for the full-field light sensitivity threshold test and 5 years for the multi-luminance mobility test in the Phase I and Phase III trials, respectively. In conclusion, the therapeutic effect of voretigene neparvovec lasts for at least a decade in animal models and 7.5 years in human subjects. Since retinal cells can retain functionality over their lifetime after transduction, these effects may be expected to last even longer in patients with a sufficient number of outer retinal cells at the time of intervention.

Changing trends in the development of AAV-based gene therapies: a meta-analysis of past and present therapies

Gene therapy has seen a transformation from a proof-of-concept approach to a clinical reality over the past several decades, with adeno-associated virus (AAV)-mediated gene therapy emerging as the leading platform for in vivo gene transfer. A systematic review of AAV-based gene therapies in clinical development was conducted herein to determine why only a handful of AAV-based gene therapy products have achieved market approval. The indication to be treated, route of administration and vector design were investigated as critical factors and assessed for their impact on clinical safety and efficacy. A shift in recent years towards high-dose systemic administration for the treatment of metabolic, neurological and haematological diseases was identified, with intravenous administration demonstrating the highest efficacy and safety risks in clinical trials. Recent years have seen a decline in favour of traditional AAV serotypes and promoters, accompanied by an increase in favour and higher clinical success rate for novel capsids and tissue-specific promoters. Furthermore, a meta-analysis was performed to identify factors that may inhibit the translation of therapeutic efficacy from preclinical large animal studies to first-in-human clinical trials and a detrimental effect on clinical efficacy was associated with alterations to administration routes.

Inhibition of experimental autoimmune uveitis by intravitreal AAV-Equine-IL10 gene therapy

Equine recurrent uveitis (ERU) is a spontaneous, painful, and vision threatening disease affecting up to 25% of equine populations worldwide. Current treatments of ERU are non-specific and have many side effects which limits them to short-term use. In order to develop an effective therapy for ERU, we investigated the use of adeno-associated virus (AAV) gene therapy, exploiting a natural immune tolerance mechanism induced by equine interleukin-10 (Equine-IL10). The purpose of this study was to evaluate the therapeutic efficacy of a single intravitreal (IVT) dose of AAV8-Equine-IL10 gene therapy for inhibition of experimental autoimmune uveitis (EAU) in rats. Each rat was dosed intravitreally (IVT) in both eyes with either balanced salt solution (BSS) (control; n = 4), AAV8-Equine-IL10 at a low dose (2.4x10⁹ vg; n = 5) or high dose (2.4x10¹⁰ vg; n = 5). EAU was induced in all groups of rats 7 days after IVT injections and euthanized 21 days post-injection. Ophthalmic examination and aqueous humor (AH) cell counts were recorded with the observer blinded to the treatment groups. Histopathology and qPCR were performed on selected ocular tissues. Data presented herein demonstrate that AAV8-Equine-IL10 treated rats exhibited a significant decrease in clinical inflammatory scores and AH cell counts compared to BSS-treated EAU eyes on days 10, 12 and 14 post EAU induction at both administered vector doses. Mean cellular histologic infiltrative scores were also significantly less in AAV8-Equine-IL10 dosed rats compared to the BSS group. Intravitreal injection of AAV8-Equine-IL10 resulted in Equine-IL10 cDNA expression in the ciliary body, retina, cornea, and optic nerve in a dose-dependent manner. A single IVT injection of AAV8-Equine-IL10 appeared to be well-tolerated and inhibited EAU even at the lowest administered dose. These results demonstrate safety and efficacy of AAV8-Equine-IL10 to prevent EAU and support continued exploration of AAV gene therapy for the treatment of equine and perhaps human recurrent uveitis.

Postnatal eye size in mice is controlled by SREBP2-mediated transcriptional repression of Lrp2 and Bmp2

Eye size is a key parameter of visual function, but the precise mechanisms of eye size control remain poorly understood. Here, we discovered that the lipogenic transcription factor sterol regulatory element-binding protein 2 (SREBP2) has an unanticipated function in the retinal pigment epithelium (RPE) to promote eye size in postnatal mice. SREBP2 transcriptionally represses low density lipoprotein receptor-related protein 2 (Lrp2), which has been shown to restrict eye overgrowth. Bone morphogenetic protein 2 (BMP2) is the downstream effector of Srebp2 and Lrp2, and Bmp2 is suppressed by SREBP2 transcriptionally but activated by Lrp2. During postnatal development, SREBP2 protein expression in the RPE decreases whereas that of Lrp2 and Bmp2 increases as the eye growth rate reduces. Bmp2 is the key determinant of eye size such that its level in mouse RPE inversely correlates with eye size. Notably, RPE-specific Bmp2 overexpression by adeno-associated virus effectively prevents the phenotypes caused by Lrp2 knock out. Together, our study shows that rapid postnatal eye size increase is governed by an RPE-derived signaling pathway, which consists of both positive and negative regulators of eye growth.

Tropism of the Novel AAVBR1 Capsid Following Subretinal Delivery

A serious limitation of current adeno-associated viral (AAV) capsids employed for subretinal delivery is achieving adequate lateral spread beyond the injection site, required for the efficient delivery of gene therapy to the outer retina and/or RPE. AAVBR1 is a unique AAV with exceptional tropism for CNS microvasculature following systemic delivery. Here, we used in vivo and ex vivo analysis to show that subretinal delivery of AAVBR1.GFP in mice achieves superior tropism to RPE and outer retina than either AAV2.GFP or AAV8.GFP, two of the most common capsids used for subretinal delivery. At a low (5 × 108 vg) subretinal dose, the AAVBR1.GFP signal was visible by 48 h and significantly surpassed peak fluorescence of other AAVs in retina and RPE. The co-injection of AAVBR1.GFP with the AAVBR1-specific heptapeptide, NRGTEWD, significantly blocked the AAVBR1.GFP signal, but had no effect on AAV2.GFP fluorescence, confirming that AAVBR1’s enhanced tropism for RPE and outer retina derives from this 7AA modification within the capsid-binding motif. Enhanced dispersal and consequent transduction suggest that AAVBR1 can be employed at a lower dosage than the standard AAV2 capsid to achieve equivalent expression for gene therapy, warranting further evaluation of its utility as a therapeutic vehicle for subretinal delivery.

AAV-PHP.eB transduces both the inner and outer retina with high efficacy in mice

Recombinant adeno-associated virus (AAV) vectors are one of the main gene delivery vehicles used in retinal gene therapy approaches; however, there is a need to further improve the efficacy, tropism, and safety of these vectors. In this study, using a CMV-EGFP expression cassette, we characterize the retinal utility of AAV-PHP.eB, a serotype recently developed by in vivo directed evolution, which can cross the blood-brain barrier and target neurons with high efficacy in mice. Systemic and intravitreal delivery of AAV-PHP.eB resulted in the high transduction efficacy of retinal ganglion and horizontal cells, with systemic delivery providing pan-retinal coverage of the mouse retina. Subretinal delivery transduced photoreceptors and retinal pigment epithelium cells robustly. EGFP expression (number of transduced cells and mRNA levels) were similar when the retinas were transduced systemically or intravitreally with AAV-PHP.eB or intravitreally with AAV2/2. Notably, in photoreceptors, EGFP fluorescence intensities and mRNA levels were 50–70 times higher, when subretinal injections with AAV-PHP.eB were compared to AAV2/8. Our results demonstrate the pan-retinal transduction of ganglion cells and extremely efficient transduction of photoreceptor and retinal pigment epithelium cells as the most valuable features of AAV-PHP.eB in the mouse retina.

Optogenetics for visual restoration: From proof of principle to translational challenges

Degenerative retinal disorders are a diverse family of diseases commonly leading to irreversible photoreceptor death, while leaving the inner retina relatively intact. Over recent years, innovative gene replacement therapies aiming to halt the progression of certain inherited retinal disorders have made their way into clinics. By rendering surviving retinal neurons light sensitive optogenetic gene therapy now offers a feasible treatment option that can restore lost vision, even in late disease stages and widely independent of the underlying cause of degeneration. Since proof-of-concept almost fifteen years ago, this field has rapidly evolved and a detailed first report on a treated patient has recently been published. In this article, we provide a review of optogenetic approaches for vision restoration. We discuss the currently available optogenetic tools and their relative advantages and disadvantages. Possible cellular targets will be discussed and we will address the question how retinal remodelling may affect the choice of the target and to what extent it may limit the outcomes of optogenetic vision restoration. Finally, we will analyse the evidence for and against optogenetic tool mediated toxicity and will discuss the challenges associated with clinical translation of this promising therapeutic concept.

Rescue of behavioral and electrophysiological phenotypes in a Pitt-Hopkins syndrome mouse model by genetic restoration of Tcf4 expression

Pitt-Hopkins syndrome (PTHS) is a neurodevelopmental disorder caused by monoallelic mutation or deletion in the transcription factor 4 (TCF4) gene. Individuals with PTHS typically present in the first year of life with developmental delay and exhibit intellectual disability, lack of speech, and motor incoordination. There are no effective treatments available for PTHS, but the root cause of the disorder, TCF4 haploinsufficiency, suggests that it could be treated by normalizing TCF4 gene expression. Here, we performed proof-of-concept viral gene therapy experiments using a conditional Tcf4 mouse model of PTHS and found that postnatally reinstating Tcf4 expression in neurons improved anxiety-like behavior, activity levels, innate behaviors, and memory. Postnatal reinstatement also partially corrected EEG abnormalities, which we characterized here for the first time, and the expression of key TCF4-regulated genes. Our results support a genetic normalization approach as a treatment strategy for PTHS, and possibly other TCF4-linked disorders.

Envisioning the development of a CRISPR-Cas mediated base editing strategy for a patient with a novel pathogenic CRB1 single nucleotide variant

BACKGROUND

Inherited retinal degeneration (IRD) associated with mutations in the Crumbs homolog 1 (CRB1) gene is associated with a severe, early-onset retinal degeneration for which no therapy currently exists. Base editing, with its capability to precisely catalyse permanent nucleobase conversion in a programmable manner, represents a novel therapeutic approach to targeting this autosomal recessive IRD, for which a gene supplementation is challenging due to the need to target three different retinal CRB1 isoforms.

PURPOSE

To report and classify a novel CRB1 variant and envision a possible therapeutic approach in form of base editing.

METHODS

Case report.

RESULTS

A 16-year-old male patient with a clinical diagnosis of early-onset retinitis pigmentosa (RP) and characteristic clinical findings of retinal thickening and coarse lamination was seen at the Oxford Eye Hospital. He was found to be compound heterozygous for two CRB1 variants: a novel pathogenic nonsense variant in exon 9, c.2885T>A (p.Leu962Ter), and a likely pathogenic missense change in exon 6, c.2056C>T (p.Arg686Cys). While a base editing strategy for c.2885T>A would encompass a CRISPR-pass mediated "read-through" of the premature stop codon, the resulting missense changes were predicted to be "possibly damaging" in in-silico analysis. On the other hand, the transversion missense change, c.2056C>T, is amenable to transition editing with an adenine base editor (ABE) fused to a SaCas9-KKH with a negligible chance of bystander edits due to an absence of additional Adenines (As) in the editing window.

CONCLUSIONS

This case report records a novel pathogenic nonsense variant in CRB1 and gives an example of thinking about a base editing strategy for a patient compound heterozygous for CRB1 variants.

Gene therapy for neuromuscular disorders: prospects and ethics

Most childhood neuromuscular disorders are caused by mutations causing abnormal expression or regulation of single genes or genetic pathways. The potential for gene therapy, gene editing and genetic therapies to ameliorate the course of these conditions is extraordinarily exciting, but there are significant challenges associated with their use, particularly with respect to safety, efficacy, cost and equity. Engagement with these novel technologies mandates careful assessment of the benefits and burdens of treatment for the patient, their family and their society. The examples provided by spinal muscular atrophy and Duchenne muscular dystrophy illustrate the potential value and challenges of gene and genetic therapies for paediatric neurological conditions. The cost and complexity of administration of these agents is a challenge for all countries. Jurisdictional variations in availability of newborn screening, genetic diagnostics, drug approval and reimbursement pathways, treatment and rehabilitation will affect equity of access, nationally and internationally. These challenges will best be addressed by collaboration by governments, pharma, clinicians and patient groups to establish frameworks for safe and cost-effective use of these exciting new therapies.