CHANGES IN RETINAL SENSITIVITY AFTER GENE THERAPY IN CHOROIDEREMIA

Current developments of gene therapy in human diseases

Gene therapy has witnessed substantial advancements in recent years, becoming a constructive tactic for treating various human diseases. This review presents a comprehensive overview of these developments, with a focus on their diverse applications in different disease contexts. It explores the evolution of gene delivery systems, encompassing viral (like adeno-associated virus; AAV) and nonviral approaches, and evaluates their inherent strengths and limitations. Moreover, the review delves into the progress made in targeting specific tissues and cell types, spanning the eye, liver, muscles, and central nervous system, among others, using these gene technologies. This targeted approach is crucial in addressing a broad spectrum of genetic disorders, such as inherited lysosomal storage diseases, neurodegenerative disorders, and cardiovascular diseases. Recent clinical trials and successful outcomes in gene therapy, particularly those involving AAV and the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated proteins, are highlighted, illuminating the transformative potentials of this approach in disease treatment. The review summarizes the current status of gene therapy, its prospects, and its capacity to significantly ameliorate patient outcomes and quality of life. By offering comprehensive analysis, this review provides invaluable insights for researchers, clinicians, and stakeholders, enriching the ongoing discourse on the trajectory of disease treatment.

Comparison of SNCG and NEFH Promoter-Driven Expression of Human SIRT1 Expression in a Mouse Model of Glaucoma

Purpose

Adeno-associated virus (AAV) demonstrates promise in delivering therapeutic genes to retinal ganglion cells (RGCs). Delivery of neuroprotective genes is constrained by packaging size and/or cell selectivity. This study compares the ability of the RGC-selective gamma-synuclein (SNCG) promoter and the smaller RGC-selective neurofilament heavy chain (NEFH) promoter, as well as portions of the RGC-selective atonal bHLH transcription factor 7 (ATOH7) enhancer, to drive gene expression in RGCs.

Methods

AAV2 constructs with green fluorescent protein (GFP) or human sirtuin 1 (hSIRT1) driven by cytomegalovirus (CMV) enhancer and NEFH promoter (AAV2-eCMV-NEFH) or distal active sequences of the ATOH7 enhancer (DiATOH7) with the SNCG promoter (AAV2-DiATOH7-SNCG) were intravitreally injected into C57BL/6J mice. RGCs were immunolabeled with Brn3a antibodies and counted. AAV constructs with the utmost transduction efficiency were used to test the therapeutic efficacy of the hSIRT1 gene in 12-week-old C57BL/6J mice subjected to microbead (MB)-induced intraocular pressure (IOP) elevation. Visual function was measured using optokinetic responses (OKRs).

Results

The eGFP transduction efficiency of AAV2-eCMV-NEFH was similar to that of AAV2-eCMV-SNCG and AAV2-DiATOH7-SNCG. When combined with the SNCG promoter, a larger ATOH7 enhancer was less efficient than the shorter DiATOH7 enhancer. Similarly, the hSIRT1 efficiency of AAV2-eCMV-NEFH was similar to that of AAV2-eCMV-SNCG. The latter two vectors were equally efficient in increasing RGC survival and improving visual function in the mouse model of MB-induced IOP elevation.

Conclusions

SNCG and NEFH promoters represent two equally efficient and comparable RGC selective promoter sequences; however, the NEFH promoter offers a smaller packaging size.

Translational Relevance

Smaller enhancer-promoter combinations can be used to deliver larger genes in human cells and for treatment in optic neuropathies including glaucoma.

Retinal Imaging Findings in Inherited Retinal Diseases

Inherited retinal diseases (IRDs) represent one of the major causes of progressive and irreversible vision loss in the working-age population. Over the last few decades, advances in retinal imaging have allowed for an improvement in the phenotypic characterization of this group of diseases and have facilitated phenotype-to-genotype correlation studies. As a result, the number of clinical trials targeting IRDs has steadily increased, and commensurate to this, the need for novel reproducible outcome measures and endpoints has grown. This review aims to summarize and describe the clinical presentation, characteristic imaging findings, and imaging endpoint measures that are being used in clinical research on IRDs. For the purpose of this review, IRDs have been divided into four categories: (1) panretinal pigmentary retinopathies affecting rods or cones; (2) macular dystrophies; (3) stationary conditions; (4) hereditary vitreoretinopathies.

Toward low-cost gene therapy: mRNA-based therapeutics for treatment of inherited retinal diseases

Inherited retinal diseases (IRDs) stem from genetic mutations that result in vision impairment. Gene therapy shows promising therapeutic potential, exemplified by the encouraging initial results with voretigene neparvovec. Nevertheless, the associated costs impede widespread access, particularly in low-to-middle income countries. The primary challenge remains: how can we make these therapies globally affordable? Leveraging advancements in mRNA therapies might offer a more economically viable alternative. Furthermore, transitioning to nonviral delivery systems could provide a dual benefit of reduced costs and increased scalability. Relevant stakeholders must collaboratively devise and implement a research agenda to realize the potential of mRNA strategies in equitable access to treatments to prevent vision loss.

Beyond Vision: An Overview of Regenerative Medicine and Its Current Applications in Ophthalmological Care

Regenerative medicine (RM) has emerged as a promising and revolutionary solution to address a range of unmet needs in healthcare, including ophthalmology. Moreover, RM takes advantage of the body's innate ability to repair and replace pathologically affected tissues. On the other hand, despite its immense promise, RM faces challenges such as ethical concerns, host-related immune responses, and the need for additional scientific validation, among others. The primary aim of this review is to present a high-level overview of current strategies in the domain of RM (cell therapy, exosomes, scaffolds, in vivo reprogramming, organoids, and interspecies chimerism), centering around the field of ophthalmology. A search conducted on clinicaltrials.gov unveiled a total of at least 209 interventional trials related to RM within the ophthalmological field. Among these trials, there were numerous early-phase studies, including phase I, I/II, II, II/III, and III trials. Many of these studies demonstrate potential in addressing previously challenging and degenerative eye conditions, spanning from posterior segment pathologies like Age-related Macular Degeneration and Retinitis Pigmentosa to anterior structure diseases such as Dry Eye Disease and Limbal Stem Cell Deficiency. Notably, these therapeutic approaches offer tailored solutions specific to the underlying causes of each pathology, thus allowing for the hopeful possibility of bringing forth a treatment for ocular diseases that previously seemed incurable and significantly enhancing patients' quality of life. As advancements in research and technology continue to unfold, future objectives should focus on ensuring the safety and prolonged viability of transplanted cells, devising efficient delivery techniques, etc.

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.

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.

Choroideremia: The Endpoint Endgame

Choroideremia is an X-linked retinal degeneration resulting from the progressive, centripetal loss of photoreceptors and choriocapillaris, secondary to the degeneration of the retinal pigment epithelium. Affected individuals present in late childhood or early teenage years with nyctalopia and progressive peripheral visual loss. Typically, by the fourth decade, the macula and fovea also degenerate, resulting in advanced sight loss. Currently, there are no approved treatments for this condition. Gene therapy offers the most promising therapeutic modality for halting or regressing functional loss. The aims of the current review are to highlight the lessons learnt from clinical trials in choroideremia, review endpoints, and propose a future strategy for clinical trials.

Correlation Between Fundus Autofluorescence Pattern and Retinal Function on Microperimetry in Choroideremia

Purpose

In patients with choroideremia, it is not known how smooth and mottled patterns on short-wavelength fundus autofluorescence (AF) imaging relate to retinal function.

Methods

A retrospective case-note review was undertaken on 190 patients with choroideremia at two specialist centers for retinal genetics. Twenty patients with both smooth and mottled zones on short-wavelength AF imaging and concurrent mesopic microperimetry assessments were included. Mean retinal sensitivities within the smooth and mottled zones were compared between choroideremia patients, and identical points on mesopic microperimetry collected from 12 age-matched controls. Longitudinal analyses were undertaken at 2 and 5 years in a subset of patients.

Results

In patients with choroideremia, mean retinal sensitivities at baseline were significantly greater in the smooth zone (26.1 ± 2.0 dB) versus the mottled zone (20.5 ± 4.2 dB) (P < 0.0001). Mean retinal sensitivities at baseline were similar in the smooth zone between choroideremia patients and controls (P = 0.054) but significantly impaired in the mottled zone in choroideremia compared to controls (P < 0.0001). The rate of decline in total sensitivity over 5 years was not significant in either the smooth or mottled zone in a small subset of choroideremia patients (n = 7; P = 0.344).

Conclusions

In choroideremia, retinal sensitivity as determined by microperimetry correlates with patterns on AF imaging: retinal function in the smooth zone, where the retinal pigment epithelium is anatomically preserved, is similar to controls, but retinal sensitivity in the mottled zone is impaired.

Translational Relevance

Patterns on AF imaging may represent a novel, objective outcome measure for clinical trials in choroideremia as a surrogate for retinal function.

Development of retinal atrophy after subretinal gene therapy with voretigene neparvovec

BACKGROUND/AIMS

Voretigene neparvovec (VN) is the first and only subretinal gene therapy approved by the Food and Drug Administration and European Medicines Agency. Real-world application has started in 2018 in patients with vision impairment due to biallelic retinal pigment epithelium (RPE) 65 mutation-associated inherited retinal degenerations. Herein, we evaluated the development of retinal atrophy within in a single-centre patient cohort treated with VN.

METHODS

13 eyes of eight patients treated with VN were retrospectively analysed for areas of retinal atrophy over a period of 6-24 months following surgery. Ultrawide field images were used to measure the area of atrophy. Fundus autofluorescence imaging is presented as an instrument for early detection of signs of retinal atrophy in these patients.

RESULTS

Atrophic changes beyond the retinotomy site were observed in all eyes. Areas of atrophy developed within the area of detachment (bleb) in all eight patients and outside the bleb in three patients. Changes in autofluorescence preceded the development of retinal atrophy and were already evident 2 weeks after surgery in the majority of patients. The areas of atrophy increase with time and progression continued over year 1. Functional outcomes remained stable (VA, FST, visual field).

CONCLUSION

Subretinal injection of VN can lead to RPE atrophy with consequent photoreceptor loss in and outside of the bleb area. Fundus autofluorescence is an important tool to monitor atrophic changes in patients after gene therapy. Interestingly, while areas of atrophy also included central areas, the functional benefits of the treatment did not appear to be affected and remained stable.

Choroideremia: Toward Regulatory Approval of Retinal Gene Therapy

Choroideremia is an X-linked inherited retinal degeneration characterized by primary centripetal degeneration of the retinal pigment epithelium (RPE), with secondary degeneration of the choroid and retina. Affected individuals experience reduced night vision in early adulthood with blindness in late middle age. The underlying CHM gene encodes REP1, a protein involved in the prenylation of Rab GTPases essential for intracellular vesicle trafficking. Adeno-associated viral gene therapy has demonstrated some benefit in clinical trials for choroideremia. However, challenges remain in gaining regulatory approval. Choroideremia is slowly progressive, which presents difficulties in demonstrating benefit over short pivotal clinical trials that usually run for 1-2 years. Improvements in visual acuity are particularly challenging due to the initial negative effects of surgical detachment of the fovea. Despite these challenges, great progress toward a treatment has been made since choroideremia was first described in 1872.

AAV2-hCHM Subretinal Delivery to the Macula in Choroideremia: Two Year Interim Results of an Ongoing Phase I/II Gene Therapy Trial

PURPOSE

To assess the safety of the subretinal delivery of a recombinant adeno-associated virus serotype 2 (AAV2) vector carrying a human CHM-encoding cDNA in choroideremia (CHM).

DESIGN

Prospective, open-label, non-randomized, dose-escalation, phase 1/2 clinical trial.

SUBJECTS, PARTICIPANTS, AND/OR CONTROLS

Fifteen CHM patients (ages 20-57 years at dosing).

METHODS, INTERVENTION, OR TESTING

Patients received uniocular subfoveal injections of low dose (up to 5x10¹⁰ vector genome (vg) per eye, n=5) or high dose (up to 1x10¹¹ vg per eye, n=10) AAV2-hCHM. Patients were evaluated pre- and post-operatively for two years with ophthalmic examinations, multimodal retinal imaging and psychophysical testing.

MAIN OUTCOME

Measures: visual acuity (VA), perimetry (10-2 protocol), spectral-domain optical coherence tomography (SD-OCT) and short-wavelength fundus autofluorescence (SW-FAF).

RESULTS

We detected no vector-related or systemic toxicities. VA returned to within 15 letters of baseline in all but two patients (one developed acute foveal thinning, another patient, a macular hole); the rest showed no gross changes in foveal structure at two years. There were no significant differences between intervention and control eyes in mean light-adapted sensitivity by perimetry, or in the lateral extent of retinal pigment epithelium (RPE) relative preservation by SD-OCT and SW-FAF. Microperimetry showed non-significant (<3SD of the intervisit variability) gains in sensitivity in some locations and participants in the intervention eye. There were no obvious dose-dependent relationships.

CONCLUSIONS

VA was within 15 letters of baseline after the subfoveal AAV2-hCHM injections in 13/15 (87%) of the patients. Acute foveal thinning with unchanged perifoveal function in one patient and macular hole in a second suggests foveal vulnerability to the subretinal injections. Longer observation intervals will help establish the significance of the minor differences in sensitivities and rate of disease progression observed between intervention and control eyes.

Choroideremia: molecular mechanisms and therapies

Choroideremia (CHM) is a monogenic X-linked chorioretinal dystrophy affecting the photoreceptors, retinal pigment epithelium (RPE), and choroid; it is caused by mutations involving the CHM gene. CHM is characterized by night blindness in early childhood, progressing to peripheral visual field loss and eventually to complete blindness from middle age. CHM encodes the ubiquitously expressed Rab escort protein 1 (REP1), which is responsible for prenylation of Rab proteins and is essential for intracellular trafficking of vesicles. In this review we explore the role of REP1 in the retina and its newly discovered systemic manifestations, and discuss the therapeutic strategies for tackling this disease, including the outcomes from recent clinical trials.

Short-term Assessment of Subfoveal Injection of Adeno-Associated Virus-Mediated hCHM Gene Augmentation in Choroideremia Using Adaptive Optics Ophthalmoscopy

Importance

Subretinal injection for gene augmentation in retinal degenerations forcefully detaches the neural retina from the retinal pigment epithelium, potentially damaging photoreceptors and/or retinal pigment epithelium cells.

Objective

To use adaptive optics scanning light ophthalmoscopy (AOSLO) to assess the short-term integrity of the cone mosaic following subretinal injections of adeno-associated virus vector designed to deliver a functional version of the CHM gene (AAV2-hCHM) in patients with choroideremia.

Design, Setting, and Participants

This longitudinal case series study enrolled adult patients with choroideremia from February 2015 to January 2016 in the US. To be included in the study, study participants must have received uniocular subfoveal injections of low-dose (5 × 1010 vector genome per eye) or high-dose (1 × 1011 vector genome per eye) AAV2-hCHM. Analysis began February 2015.

Main Outcomes and Measures

The macular regions of both eyes were imaged before and 1 month after injection using a custom-built multimodal AOSLO. Postinjection cone inner segment mosaics were compared with preinjection mosaics at multiple regions of interest. Colocalized spectral-domain optical coherence tomography and dark-adapted cone sensitivity was also acquired at each time point.

Results

Nine study participants ranged in age from 26 to 50 years at the time of enrollment, and all were White men. Postinjection AOSLO images showed preservation of the cone mosaic in all 9 AAV2-hCHM-injected eyes. Mosaics appeared intact and contiguous 1 month postinjection, with the exception of foveal disruption in 1 patient. Optical coherence tomography showed foveal cone outer segment shortening postinjection. Cone-mediated sensitivities were unchanged in 8 of 9 injected and 9 of 9 uninjected eyes. One participant showed acute loss of foveal optical coherence tomography cone outer segment-related signals along with cone sensitivity loss that colocalized with disruption of the mosaic on AOSLO.

Conclusions and Relevance

Integrity of the cone mosaic is maintained following subretinal delivery of AAV2-hCHM, providing strong evidence in support of the safety of the injections. Minor foveal thinning observed following surgery corresponds with short-term cone outer segment shortening rather than cone cell loss. Foveal cone loss in 1 participant raises the possibility of individual vulnerability to the subretinal injection.

Importance of Nonhuman Primates as a Model System for Gene Therapy Development in Ophthalmology

Gene therapy is a treatment concept that uses, in most cases, viral vectors to deliver a therapeutic transgene to target cells. Although the idea of gene therapy dates back over 50 years ago, due to the complexity of the treatment concept, it took until the last decade for the responsible agencies like FDA and EMA to recommend the first gene therapy products for clinical use. The development of these therapies relies on molecular engineering of specifically designed vectors and models to test the effectiveness and safety of the treatment. Despite an increasing effort to find effective surrogates, animal models are still irreplaceable in gene therapy development. Rodents are important for exploring pathways and disease mechanisms and identifying potential treatment targets. However, only the primate eye resembles the human eye to a degree where most structures are nearly identical. Some research questions can therefore only be answered using a nonhuman primate (NHP) model. In this review, we want to summarize these key features and highlight the importance of the NHP model for gene therapy development in ophthalmology.

Bilateral visual acuity decline in males with choroideremia: a pooled, cross-sectional meta-analysis

BACKGROUND

Choroideremia is a rare inherited retinal disease that leads to blindness. Visual acuity (VA) is a key outcome measure in choroideremia treatment studies, but VA decline rates change with age. An accurate understanding of the natural deterioration of VA in choroideremia is important to assess the treatment effect of new therapies in which VA is the primary outcome measure. We conducted a meta-analysis of data on individuals with choroideremia to determine the rate of VA deterioration between the better- and worse-seeing eye (BSE and WSE, respectively).

METHODS

Data were collected from the prospective Natural History of the Progression of Choroideremia (NIGHT) study (613 eyes, baseline data only), studies included in a recent meta-analysis, and studies identified in a targeted literature search performed on March 25, 2020, including individual best-corrected VA (BCVA) and age data in male individuals with choroideremia. Best-corrected VA decline rates (measured by logMAR units) by age and trends in BCVA decline rates in the BSE and WSE were evaluated.  RESULTS: Data from 1037 males (1602 eyes; mean age, 41.8 years) were included. Before and after an age cutoff of 33.8 years, BCVA decline rates for the WSE were 0.0086 and 0.0219 logMAR per year, respectively. Before and after an age cutoff of 39.1 years, BCVA decline rates for the BSE were 0.00001 and 0.0203 logMAR per year, respectively. Differences in absolute BCVA and decline rates increased between the 2 eyes until age ~ 40; thereafter, differences in absolute BCVA and decline rates were similar between eyes.

CONCLUSIONS

Using the largest choroideremia data set to date, this analysis demonstrates accelerated BCVA decline beginning between 30 and 40 years of age. Disparate interocular progression rates were observed before the transition age, with similar interocular progression rates after the transition age.

The safety and efficacy of gene therapy treatment for monogenic retinal and optic nerve diseases: A systematic review

PURPOSE

This study aimed to systematically review and summarize gene therapy treatment for monogenic retinal and optic nerve diseases.

METHODS

This review was prospectively registered (CRD42021229812). A comprehensive literature search was performed in Ovid MEDLINE, Ovid Embase, Cochrane Central, and clinical trial registries (February 2021). Clinical studies describing DNA-based gene therapy treatments for monogenic posterior ocular diseases were eligible for inclusion. Risk of bias evaluation was performed. Data synthesis was undertaken applying Synthesis Without Meta-analysis guidelines.

RESULTS

This study identified 47 full-text publications, 50 conference abstracts, and 54 clinical trial registry entries describing DNA-based ocular gene therapy treatments for 16 different genetic variants. Study summaries and visual representations of safety and efficacy outcomes are presented for 20 unique full-text publications in RPE65-mediated retinal dystrophies, choroideremia, Leber hereditary optic neuropathy, rod-cone dystrophy, achromatopsia, and X-linked retinoschisis. The most common adverse events were related to lid/ocular surface/cornea abnormalities in subretinal gene therapy trials and anterior uveitis in intravitreal gene therapy trials.

CONCLUSION

There is a high degree of variability in ocular monogenic gene therapy trials with respect to study design, statistical methodology, and reporting of safety and efficacy outcomes. This review improves the accessibility and transparency in interpreting gene therapy trials to date.

CRISPR-Based Genome Editing as a New Therapeutic Tool in Retinal Diseases

Retinal diseases are the primary reasons for severe visual defects and irreversible blindness. Retinal diseases are also inherited and acquired. Both of them are caused by mutations in genes or disruptions in specific gene expression, which can be treated by gene-editing therapy. Clustered regularly interspaced short palindromic repeats (CRISPR-Cas9) system is a frontier of gene-editing tools with great potential for therapeutic applications in the ophthalmology field to modify abnormal genes and treat the genome or epigenome-related retinal diseases. The CRISPR system is able to edit and trim the gene include deletion, insertion, inhibition, activation, replacing, remodeling, epigenetic alteration, and modify the gene expression. CRISPR-based genome editing techniques have indicated the enormous potential to treat retinal diseases that previous treatment was not available for them. Also, recent CRISPR genome surgery experiments have shown the improvement of patient's vision who suffered from severe visual loss. In this article, we review the applications of the CRISPR-Cas9 system in human or animal models for treating retinal diseases such as retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), age-related macular degeneration (AMD), proliferative diabetic retinopathy (PDR), and proliferative vitreoretinopathy (PVR), then we survey limitations of CRISPR system for clinical therapy.

Expression of Rab Prenylation Pathway Genes and Relation to Disease Progression in Choroideremia

Purpose

Choroideremia results from the deficiency of Rab Escort Protein 1 (REP1), encoded by CHM, involved in the prenylation of Rab GTPases. Here, we investigate whether the transcription and expression of other genes involved in the prenylation of Rab proteins correlates with disease progression in a cohort of patients with choroideremia.

Methods

Rates of retinal pigment epithelial area loss in 41 patients with choroideremia were measured using fundus autofluorescence imaging for up to 4 years. From lysates of cultured skin fibroblasts donated by patients (n = 15) and controls (n = 14), CHM, CHML, RABGGTB and RAB27A mRNA expression, and REP1 and REP2 protein expression were compared.

Results

The central autofluorescent island area loss in patients with choroideremia occurred with a mean half-life of 5.89 years (95% confidence interval [CI] = 5.09-6.70), with some patients demonstrating relatively fast or slow rates of progression (range = 3.3-14.1 years). Expression of CHM mRNA and REP1 protein were significantly decreased in all patients. No difference in expression of CHML, RABGGTB, RAB27A, or REP2 was seen between patients and controls. No correlation was seen between expression of the genes analyzed and rates of retinal degeneration. Non-sense induced transcriptional compensation of CHML, a CHM-like retrogene, was not observed in patients with CHM variants predicted to undergo non-sense mediated decay.

Conclusions

Patients with choroideremia, who are deficient for REP1, show normal levels of expression of other genes involved in Rab prenylation, which do not appear to play any modifying role in the rate of disease progression.

Translational Relevance

There remains little evidence for selection of patients for choroideremia gene therapy based on genotype.

Focused Update on AAV-Based Gene Therapy Clinical Trials for Inherited Retinal Degeneration

Inherited retinal diseases (IRDs) comprise a clinically and genetically heterogeneous group of disorders that can ultimately result in photoreceptor dysfunction/death and vision loss. With over 270 genes known to be involved in IRDs, translation of treatment strategies into clinical applications has been historically difficult. However, in recent years there have been significant advances in basic research findings as well as translational studies, culminating in an increasing number of clinical trials with the ultimate goal of reducing vision loss and associated morbidities. The recent approval of Luxturna^® (voretigene neparvovec-rzyl) for Leber congenital amaurosis type 2 (LCA2) prompts a review of the current clinical trials for IRDs, with a particular focus on the importance of adeno-associated virus (AAV)-based gene therapies. The present article reviews the current state of AAV use in gene therapy clinical trials for IRDs, with a brief background on AAV and the reasons behind its dominance in ocular gene therapy. It will also discuss pre-clinical progress in AAV-based therapies aimed at treating other ocular conditions that can have hereditable links, and what alternative technologies are progressing in the same therapeutic space.

Gene Therapy for Inherited Retinal Disorders: Update on Clinical Trials

Within the last decade, continuous advances in molecular biological techniques have made it possible to develop causative therapies for inherited retinal disorders (IRDs). Some of the most promising options are gene-specific approaches using adeno-associated virus-based vectors to express a healthy copy of the disease-causing gene in affected cells of a patient. This concept of gene supplementation therapy is already advocated for the treatment of retinal dystrophy in RPE65-linked Leber's congenital amaurosis (LCA) patients. While the concept of gene supplementation therapy can be applied to treat autosomal recessive and X-linked forms of IRD, it is not sufficient for autosomal dominant IRDs, where the pathogenic gene product needs to be removed. Therefore, for autosomal dominant IRDs, alternative approaches that utilize CRISPR/Cas9 or antisense oligonucleotides to edit or deplete the mutant allele or gene product are needed. In recent years, research retinal gene therapy has intensified and promising approaches for various forms of IRD are currently in preclinical and clinical development. This review article provides an overview of current clinical trials for the treatment of IRDs.

Is subretinal AAV gene replacement still the only viable treatment option for choroideremia?

INTRODUCTION

Choroideremia is an X-linked inherited retinal degeneration resulting from mutations in the CHM gene, encoding Rab escort protein-1 (REP1), a protein regulating intracellular vesicular transport. Loss-of-function mutations in CHM lead to progressive loss of retinal pigment epithelium (RPE) with photoreceptor and choriocapillaris degeneration, leading to progressive visual field constriction and loss of visual acuity. Three hundred and fifty-four unique mutations have been reported in CHM. While gene augmentation remains an ideal therapeutic option for choroideremia, other potential future clinical strategies may exist.

AREAS COVERED

The authors examine the pathophysiology and genetic basis of choroideremia. They summarize the status of ongoing gene therapy trials and discuss CHM mutations amenable to other therapeutic approaches including CRISPR/Cas-based DNA and RNA editing, nonsense suppression of premature termination codons, and antisense oligonucleotides for splice modification. The authors undertook a literature search in PubMed and NIH Clinical Trials in October 2020.

EXPERT OPINION

The authors conclude that AAV-mediated gene augmentation remains the most effective approach for choroideremia. Given the heterogeneity of CHM mutations and potential risks and benefits, genome-editing approaches currently do not offer significant advantages. Nonsense suppression strategies and antisense oligonucleotides are exciting novel therapeutic options; however, their clinical viability remains to be determined.

Surgical Aspects in Gene Therapy for Inherited Retinal Diseases

Inherited retinal dystrophies (IRD) have been studied since their recognition by Franz Donders and Albrecht von Graefe. It nevertheless took 100 years for a causal therapy to take shape in the form of gene therapy: The approval of Voretigen Neparvovec (VN) for the treatment of hereditary retinal dystrophies due to RPE65 mutations was thus a significant milestone - for the era of personalised medicine in general and ophthalmology in particular. The clinical management around gene therapy applications is complex and requires the cooperation of various experts as a multidisciplinary team. This article describes the requirements, challenges, approaches, and open questions regarding the surgical aspects of gene therapy for retinal dystrophies. The first part outlines the standard surgical treatment. Based on this standard, alternative approaches are indicated for each individual step and their value discussed. Knowledge gaps are defined and in the outlook we speculate on future developments.

Clinical Perspectives and Trends: Microperimetry as a trial endpoint in retinal disease

Endpoint development trials are underway across the spectrum of retinal disease. New validated endpoints are urgently required for the assessment of emerging gene therapies and in preparation for the arrival of novel therapeutics targeting early stages of common sight-threatening conditions such as age-related macular degeneration. Visual function measures are likely to be key candidates in this search. Over the last two decades, microperimetry has been used extensively to characterize functional vision in a wide range of retinal conditions, detecting subtle defects in retinal sensitivity that precede visual acuity loss and tracking disease progression over relatively short periods. Given these appealing features, microperimetry has already been adopted as an endpoint in interventional studies, including multicenter trials, on a modest scale. A review of its use to date shows a concurrent lack of consensus in test strategy and a wealth of innovative disease and treatment-specific metrics which may show promise as clinical trial endpoints. There are practical issues to consider, but these have not held back its popularity and it remains a widely used psychophysical test in research. Endpoint development trials will undoubtedly be key in understanding the validity of microperimetry as a clinical trial endpoint, but existing signs are promising.

Update on Gene Therapy Clinical Trials for Choroideremia and Potential Experimental Therapies

Background and objectives: Choroideremia (CHM) is an X-linked recessive chorioretinal dystrophy caused by mutations involving the CHM gene. Gene therapy has entered late-phase clinical trials, although there have been variable results. This review gives a summary on the outcomes of phase I/II CHM gene therapy trials and describes other potential experimental therapies. Materials and Methods: A Medline (National Library of Medicine, Bethesda, MD, USA) search was performed to identify all articles describing gene therapy treatments available for CHM. Results: Five phase I/II clinical trials that reported subretinal injection of adeno-associated virus Rab escort protein 1 (AAV2.REP1) vector in CHM patients were included. The Oxford study (NCT01461213) included 14 patients; a median gain of 5.5 ± 6.8 SD (-6 min, 18 max) early treatment diabetic retinopathy study (ETDRS) letters was reported. The Tubingen study (NCT02671539) included six patients; only one patient had an improvement of 17 ETDRS letters. The Alberta study (NCT02077361) enrolled six patients, and it reported a minimal vision change, except for one patient who gained 15 ETDRS letters. Six patients were enrolled in the Miami trial (NCT02553135), which reported a median gain of 2 ± 4 SD (-1 min, 10 max) ETDRS letters. The Philadelphia study (NCT02341807) included 10 patients; best corrected visual acuity (BCVA) returned to baseline in all by one-year follow-up, but one patient had -17 ETDRS letters from baseline. Overall, 40 patients were enrolled in trials, and 34 had 2 years of follow-up, with a median gain of 1.5 ± 7.2 SD (-14 min, 18 max) in ETDRS letters. Conclusions: The primary endpoint, BCVA following gene therapy in CHM, showed a marginal improvement with variability between trials. Optimizing surgical technique and pre-, peri-, and post-operative management with immunosuppressants to minimize any adverse ocular inflammatory events could lead to reduced incidence of complications. The ideal therapeutic window needs to be addressed to ensure that the necessary cell types are adequately transduced, minimizing viral toxicity, to prolong long-term transgenic potential. Long-term efficacy will be addressed by ongoing studies.

Current Clinical Applications of In Vivo Gene Therapy with AAVs

Hereditary diseases are caused by mutations in genes, and more than 7,000 rare diseases affect over 30 million Americans. For more than 30 years, hundreds of researchers have maintained that genetic modifications would provide effective treatments for many inherited human diseases, offering durable and possibly curative clinical benefit with a single treatment. This review is limited to gene therapy using adeno-associated virus (AAV) because the gene delivered by this vector does not integrate into the patient genome and has a low immunogenicity. There are now five treatments approved for commercialization and currently available, i.e., Luxturna, Zolgensma, the two chimeric antigen receptor T cell (CAR-T) therapies (Yescarta and Kymriah), and Strimvelis (the gammaretrovirus approved for adenosine deaminase-severe combined immunodeficiency [ADA-SCID] in Europe). Dozens of other treatments are under clinical trials. The review article presents a broad overview of the field of therapy by in vivo gene transfer. We review gene therapy for neuromuscular disorders (spinal muscular atrophy [SMA]; Duchenne muscular dystrophy [DMD]; X-linked myotubular myopathy [XLMTM]; and diseases of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, Canavan disease, aromatic l-amino acid decarboxylase [AADC] deficiency, and giant axonal neuropathy), ocular disorders (Leber congenital amaurosis, age-related macular degeneration [AMD], choroideremia, achromatopsia, retinitis pigmentosa, and X-linked retinoschisis), the bleeding disorder hemophilia, and lysosomal storage disorders.

Immune responses to retinal gene therapy using adeno-associated viral vectors - Implications for treatment success and safety

Recombinant adeno-associated virus (AAV) is the leading vector for gene therapy in the retina. As non-pathogenic, non-integrating, replication deficient vector, the recombinant virus efficiently transduces all key retinal cell populations. Successful testing of AAV vectors in clinical trials of inherited retinal diseases led to the recent approval of voretigene neparvovec (Luxturna) for the treatment of RPE65 mutation-associated retinal dystrophies. However, studies applying AAV-mediated retinal gene therapy independently reported intraocular inflammation and/or loss of efficacy after initial functional improvements. Both observations might be explained by targeted removal of transduced cells via anti-viral defence mechanisms. AAV has been shown to activate innate pattern recognition receptors (PRRs) such as toll-like receptor (TLR)-2 and TLR-9 resulting in the release of inflammatory cytokines and type I interferons. The vector can also induce capsid-specific and transgene-specific T cell responses and neutralizing anti-AAV antibodies which both limit the therapeutic effect. However, the target organ of retinal gene therapy, the eye, is known as an immune-privileged site. It is characterized by suppression of inflammation and promotion of immune tolerance which might prevent AAV-induced immune responses. This review evaluates AAV-related immune responses, toxicity and inflammation in studies of retinal gene therapy, identifies influencing variables of these responses and discusses potential strategies to modulate immune reactions to AAV vectors to increase the safety and efficacy of ocular gene therapy.

Fundus-controlled perimetry (microperimetry): Application as outcome measure in clinical trials

Fundus-controlled perimetry (FCP, also called 'microperimetry') allows for spatially-resolved mapping of visual sensitivity and measurement of fixation stability, both in clinical practice as well as research. The accurate spatial characterization of visual function enabled by FCP can provide insightful information about disease severity and progression not reflected by best-corrected visual acuity in a large range of disorders. This is especially important for monitoring of retinal diseases that initially spare the central retina in earlier disease stages. Improved intra- and inter-session retest-variability through fundus-tracking and precise point-wise follow-up examinations even in patients with unstable fixation represent key advantages of these technique. The design of disease-specific test patterns and protocols reduces the burden of extensive and time-consuming FCP testing, permitting a more meaningful and focused application. Recent developments also allow for photoreceptor-specific testing through implementation of dark-adapted chromatic and photopic testing. A detailed understanding of the variety of available devices and test settings is a key prerequisite for the design and optimization of FCP protocols in future natural history studies and clinical trials. Accordingly, this review describes the theoretical and technical background of FCP, its prior application in clinical and research settings, data that qualify the application of FCP as an outcome measure in clinical trials as well as ongoing and future developments.

Genetic defects of CHM and visual acuity outcome in 24 choroideremia patients from 16 Japanese families

Choroideremia (CHM) is an incurable progressive chorioretinal dystrophy. Little is known about the natural disease course of visual acuity in the Japanese population. We aimed to investigate the genetic spectrum of the CHM gene and visual acuity outcomes in 24 CHM patients from 16 Japanese families. We measured decimal best-corrected visual acuity (BCVA) at presentation and follow-up, converted to logMAR units for statistical analysis. Sanger and/or whole-exome sequencing were performed to identify pathogenic CHM variants/deletions. The median age at presentation was 37.0 years (range, 5–76 years). The mean follow-up interval was 8.2 years. BCVA of the better-seeing eye at presentation was significantly worsened with increasing age (r = 0.515, p < 0.01), with a high rate of BCVA decline in patients > 40 years old. A Kaplan–Meier survival curve suggested that a BCVA of Snellen equivalent 20/40 at follow-up remains until the fifties. Fourteen pathogenic variants, 6 of which were novel [c.49 + 5G > A, c.116 + 5G > A, p.(Gly176Glu, Glu177Ter), p.Tyr531Ter, an exon 2 deletion, and a 5.0-Mb deletion], were identified in 15 families. No variant was found in one family only. Our BCVA outcome data are useful for predicting visual prognosis and determining the timing of intervention in Japanese patients with CHM variants.

The new landscape of retinal gene therapy

Novel therapeutics for inherited retinal dystrophies (IRDs) have rapidly evolved since groundbreaking clinical trials for LCA due to RPE65 mutations led to the first FDA-approved in vivo gene therapy. Since then, advancements in viral vectors have led to more efficient AAV transduction and developed other viral vectors for gene augmentation therapy of large gene targets. Furthermore, significant developments in gene editing and RNA modulation technologies have introduced novel capabilities for treatment of autosomal dominant diseases, intronic mutations, and/or large genes otherwise unable to be treated with current viral vectors. We highlight strategies currently being evaluated in gene therapy clinical trials and promising preclinical developments for IRDs.

Long-term natural history of visual acuity in eyes with choroideremia: a systematic review and meta-analysis of data from 1004 individual eyes

BACKGROUND/AIMS

Best-corrected visual acuity (BCVA) is the most common primary endpoint in treatment trials for choroideremia (CHM) but the long-term natural history of BCVA is unclear.

METHODS

We searched in seven databases to identify studies that reported BCVA of untreated eyes with CHM. We sought individual-level data and performed segmented regression between BCVA and age. For eyes followed longitudinally, we introduced a horizontal translation factor to each dataset to account for different ages at onset of a rapid BCVA decline.

RESULTS

We included 1004 eyes from 23 studies. BCVA of the right and left eyes was moderately correlated (r=0.60). BCVA as a function of age followed a 2-phase decline (slow followed by rapid decline), with an estimated transition age of 39.1 years (95% CI 33.5 to 44.7). After the introduction of horizontal translation factors to longitudinal datasets, BCVA followed a 2-phase decline until it reached 0 letters (r²=0.90). The BCVA decline rate was 0.33 letters/year (95% CI -0.38 to 1.05) before 39 years, and 1.23 letters/year (95% CI 0.55 to 1.92) after 39 years (p=0.004).

CONCLUSION

BCVA in eyes with CHM follows a 2-phase linear decline with a transition age of approximately 39 years. Future trials enrolling young patients may not be able to use BCVA as a primary or sole endpoint, but rather, may need to employ additional disease biomarkers that change before age 39. BCVA may still have utility as a primary endpoint for patients older than 39 years who have measurable BCVA decline rates.

Retinal Dystrophies and the Road to Treatment: Clinical Requirements and Considerations

: Retinal dystrophies (RDs) comprise relatively rare but devastating causes of progressive vision loss. They represent a spectrum of diseases with marked genetic and clinical heterogeneity. Mutations in the same gene may lead to different diagnoses, for example, retinitis pigmentosa or cone dystrophy. Conversely, mutations in different genes may lead to the same phenotype. The age at symptom onset, and the rate and characteristics of peripheral and central vision decline, may vary widely per disease group and even within families. For most RD cases, no effective treatment is currently available. However, preclinical studies and phase I/II/III gene therapy trials are ongoing for several RD subtypes, and recently the first retinal gene therapy has been approved by the US Food and Drug Administration for RPE65-associated RDs: voretigene neparvovec-rzyl (Luxturna). With the rapid advances in gene therapy studies, insight into the phenotypic spectrum and long-term disease course is crucial information for several RD types. The vast clinical heterogeneity presents another important challenge in the evaluation of potential efficacy in future treatment trials, and in establishing treatment candidacy criteria. This perspective describes these challenges, providing detailed clinical descriptions of several forms of RD that are caused by genes of interest for ongoing and future gene or cell-based therapy trials. Several ongoing and future treatment options will be described.

Long-term Natural History of Atrophy in Eyes with Choroideremia-A Systematic Review and Meta-analysis of Individual-Level Data

PURPOSE

To conduct a systematic review and meta-analysis of the natural history of atrophy secondary to choroideremia (CHM).

CLINICAL RELEVANCE

A sensitive and reliable anatomic measure to monitor disease progression is needed in treatment trials for CHM. However, the long-term natural history of the residual retinal pigment epithelium (RPE) is unclear, with reported RPE area decline rates varying widely among patients.

METHODS

We searched in 7 literature databases up through July 17, 2019, to identify studies that assessed the residual RPE area in untreated eyes with CHM using fundus autofluorescence (FAF). We sought individual-eye data and investigated the RPE decline pattern using 3 models: the area linear model (ALM), radius linear model (RLM), and area exponential model (AEM), in which the area, radius, and log-transformed area of RPE change linearly with time, respectively. To account for different eyes' entry times into the studies, we added a horizontal translation factor to each dataset. The RPE decline rate was estimated using a 2-stage random-effects meta-analysis. We assessed the risk of bias using the Quality In Prognosis Studies tool.

RESULTS

Of 807 articles screened, we included 9 articles containing cross-sectional data (257 eyes) from 6 studies and longitudinal data (229 visits from 68 eyes) from 5 studies. The residual RPE area followed a trend of exponential decay as a function of patient age. After the introduction of horizontal translation factors to longitudinal datasets of individual eyes, the datasets fit along a straight line in the AEM over nearly 60 years (r² = 0.997). The decline rate of log-transformed RPE area was 0.050 (95% confidence interval, 0.046-0.055) log(mm²)/year and was independent of the baseline RPE area (r = -0.18; P = 0.15) and age (r = 0.06; P = 0.63). In contrast, the decline rates of the area and effective radius of residual RPE strongly correlated with the baseline RPE area (r = 0.90 and 0.61, respectively; P < 0.001).

CONCLUSIONS

The loss of residual RPE area in untreated eyes with CHM follows the AEM over approximately 60 years. Log-transformed residual RPE area measured by FAF can serve as an anatomic endpoint to monitor CHM.

Perspectives on Gene Therapy: Choroideremia Represents a Challenging Model for the Treatment of Other Inherited Retinal Degenerations

Purpose

To report combined viewpoints on ocular gene therapy from a select group of clinician scientists and a patient advocacy group.

Methods

With the support of Randy Wheelock and Dr. Chris Moen from the Choroideremia Research Foundation (CRF), a special interest group at the 2019 Annual meeting of the Association for Research in Vision and Ophthalmology in Vancouver, Canada, shared their knowledge, experience, concepts, and ideas and provided a forum to discuss therapeutic strategies for the treatment of inherited retinal disorders, using experience in choroideremia (CHM) as a model.

Results

A member of the CRF presented the patient perspective and role in clinical trials. Five clinician scientists presented reasons for limited long-term visual improvement in many gene therapy trials, including challenges with dose, incomplete understanding of photoreceptor metabolism, vector delivery, inflammation, and identification of patients likely to benefit from treatment.

Conclusions

The shared experience of the five clinician scientists indicates that the results of ocular gene therapy for choroideremia have been less successful than for RPE65-related Leber congenital amaurosis. Improvement in vector delivery and developing a better understanding of gene expression in target tissues, treatment dose and side effects, and inflammation, as well as identifying patients who are most likely to benefit without suffering excessive risk, are necessary to advance the development of effective therapies for inherited retinal degenerations.

Translational Relevance

Additional long-term data are required to determine if ocular gene therapy will be sufficient to alter natural progression in choroideremia. Combination therapies may have to be considered, as well as alternative vectors that minimize risk.

Inclusion of PF68 Surfactant Improves Stability of rAAV Titer when Passed through a Surgical Device Used in Retinal Gene Therapy

Recent advances in recombinant adeno-associated virus (rAAV) gene therapy for choroideremia show gene replacement to be a promising approach. It is, however, well known that contact of vector solution with plastic materials in the surgical device may result in non-specific adsorption with resulting loss of physical titer and/or level of protein expression and activity. Here we assessed the biocompatibility and stability of rAAV2-REP1 (Rab Escort Protein-1) before and following passage through the injection device over a period of time to mimic the clinical scenario. Three identical devices were screened using two concentrations of vector: high (1E+12 DNase-resistant particles [DRP]/mL) and low (1E+11 DRP/mL), to mimic high- and low-dose administrations of vector product. The low dose was prepared using either formulation buffer that contained 0.001% of a non-ionic surfactant (PF68) or balanced salt solution (BSS). We observed significant losses in the genomic titer of samples diluted with BSS for all time points. The addition of 0.001% PF68 did not, however, affect rAAV physical titer, or REP1 protein expression and biological activity. Hence we observed that neither the genomic titer nor the biological activity of a rAAV2-REP1-containing solution was affected following passage through the surgical device when PF68 was present as a surfactant and this was maintained over a period up to 10 h.

Spectrum of Disease Severity and Phenotype in Choroideremia Carriers

PURPOSE

To characterize and bring awareness to the disease spectrum of female choroideremia patients, as severity can vary from mild to severe disease, comparable to that observed in male patients.

DESIGN

Retrospective cohort study.

METHODS

Twelve female carriers of disease-causing variants in the CHM gene confirmed by molecular genetic sequencing were characterized clinically and imaged with short-wave fundus autofluorescence (SW-FAF), spectral-domain optical coherence tomography (OCT), and color fundus imaging.

RESULTS

Twelve unrelated female patients with a clinical and genetic diagnosis of choroideremia carriers were included in this study. Disease severity among these phenotypes ranged from mild to severe, resembling the typical presentation of choroideremia in male patients. Mild disease presented with retinal pigment epithelium mottling, a patchy pattern of hypoautofluorescent speckles on SW-FAF, and intact retinal layers on spectral-domain OCT. Severe disease presented with widespread chorioretinal atrophy as shown by SW-FAF and spectral-domain OCT. Each of the identified genetic variants in CHM was predicted to be disease-causing according to in silico prediction software. Disease progression analysis of 4 patients with follow-up showed a decline in visual acuity for 2 patients, with progression observed on spectral-domain OCT in 1 of the patients. No significant disease progression on SW-FAF was observed for any of the patients.

CONCLUSIONS

Female carriers of choroideremia can present with a wide range of clinical phenotypes and disease severity, from mild to severe disease, similar to male subjects. Symptomatic female subjects should be considered for current and upcoming gene replacement therapy clinical trials.

Efficacy and Safety of Retinal Gene Therapy Using Adeno-Associated Virus Vector for Patients With Choroideremia: A Randomized Clinical Trial

Importance

Choroideremia (CHM) is a rare, degenerative, genetic retinal disorder resulting from mutation of the CHM gene, leading to an absence of functional ras-associated binding escort protein 1 (REP1). There is currently no approved treatment for CHM.

Objective

To assess the safety and efficacy of retinal gene therapy with an adeno-associated virus vector (AAV2) designed to deliver a functional version of the CHM gene (AAV2-REP1) for treatment of patients with choroideremia.

Design, Setting, and Participants

Tübingen Choroideremia Gene Therapy (THOR) was a single-center, phase 2, open-label randomized clinical trial. Data were collected from January 11, 2016, to February 26, 2018. Twenty-four-month data are reported for 6 men with a molecularly confirmed diagnosis of CHM. Intention-to-treat analysis was used.

Interventions

Patients received AAV2-REP1 by a single, 0.1-mL subretinal injection of 1011 genome particles during vitrectomy into 1 eye randomly assigned to receive treatment.

Main Outcomes and Measures

Primary end point was change in best-corrected visual acuity (BCVA) on the Early Treatment Diabetic Retinopathy Study chart from baseline to month 24 in the treated eye vs the control eye. Secondary end points included microperimetry variables, change in fundus autofluorescence, and spectral-domain optical coherence tomographic evaluations from baseline to month 24 in the treated eye vs the control eye.

Results

On enrollment, the mean (SD) age of the 6 men included in the study was 54.9 (4.1) years. The mean (SD) BCVA score was 60.3 (13.4) (approximately 20/63 Snellen equivalent) in the study eyes and 69.3 (20.6) (approximately 20/40 Snellen equivalent) in the control eyes. At 24 months, the BCVA change was 3.7 (7.5) in the treated eyes and 0.0 (5.1) in the control eyes (difference, 3.7; 95% CI, -7.2 to 14.5; P = .43). Mean change in retinal sensitivity was 10.3 (5.5) dB in the treated eyes and 9.7 (4.9) dB in the control eyes (difference, 0.6; 95% CI, -10.2 to 11.4; P = .74). A total of 28 adverse events were reported; all were consistent with the surgical procedure (eg, conjunctival hyperemia, foreign body sensation), and none were regarded as severe.

Conclusions and Relevance

Among 6 participants, gene therapy with AAV2-REP1 was associated with maintenance or improvement of visual acuity, although no significant difference was found from control eyes. All safety issues were associated with the surgical procedure and none were judged severe. Continued investigations could more precisely define the efficacy and safety of gene therapy with AAV2-REP1 in CHM.

Trial Registration

ClinicalTrials.gov identifier: NCT02671539.

Molecular Therapies for Choroideremia

Advances in molecular research have culminated in the development of novel gene-based therapies for inherited retinal diseases. We have recently witnessed several groundbreaking clinical studies that ultimately led to approval of Luxturna, the first gene therapy for an inherited retinal disease. In parallel, international research community has been engaged in conducting gene therapy trials for another more common inherited retinal disease known as choroideremia and with phase III clinical trials now underway, approval of this therapy is poised to follow suit. This chapter discusses new insights into clinical phenotyping and molecular genetic testing in choroideremia with review of molecular mechanisms implicated in its pathogenesis. We provide an update on current gene therapy trials and discuss potential inclusion of female carries in future clinical studies. Alternative molecular therapies are discussed including suitability of CRISPR gene editing, small molecule nonsense suppression therapy and vision restoration strategies in late stage choroideremia.

Outcome Measures Used in Ocular Gene Therapy Trials: A Scoping Review of Current Practice

Multiple gene therapy trials are occurring for a variety of ophthalmic diseases around the world. The safety of gene therapy in the eye has been established, and the next step is to reliably assess efficacy. This is primarily done through the use of imaging techniques and visual function measures. Standardized visual function assessments, however, were originally developed for a clinical setting and may not be suitable for detecting and quantifying therapeutic changes. This scoping review takes a comprehensive look at current practice in terms of the outcome measures defined at trial registration. These were compared to the outcome measures reported in the literature. All published trials reported the pre-registered primary outcome measure. A range of additional secondary outcomes were reported that were not originally planned. Gaps in gene therapy assessment exist and further discussion are required to find a way forward, particularly as more conditions progress to phase 2 and 3 trials. Several factors impacting on trial design and outcome measure choice are discussed.

Rescue of Rod Synapses by Induction of Cav Alpha 1F in the Mature Cav1.4 Knock-Out Mouse Retina

Purpose

Cav1.4 is a voltage-gated calcium channel clustered at the presynaptic active zones of photoreceptors. Cav1.4 functions in communication by mediating the Ca2+ influx that triggers neurotransmitter release. It also aids in development since rod ribbon synapses do not form in Cav1.4 knock-out mice. Here we used a rescue strategy to investigate the ability of Cav1.4 to trigger synaptogenesis in both immature and mature mouse rods.

Methods

In vivo electroporation was used to transiently express Cav α1F or tamoxifen-inducible Cav α1F in a subset of Cav1.4 knock-out mouse rods. Synaptogenesis was assayed using morphologic markers and a vision-guided water maze.

Results

We found that introduction of Cav α1F to knock-out terminals rescued synaptic development as indicated by PSD-95 expression and elongated ribbons. When expression of Cav α1F was induced in mature animals, we again found restoration of PSD-95 and elongated ribbons. However, the induced expression of Cav α1F led to diffuse distribution of Cav α1F in the terminal instead of being clustered beneath the ribbon. Approximately a quarter of treated animals passed the water maze test, suggesting the rescue of retinal signaling in these mice.

Conclusions

These data confirm that Cav α1F expression is necessary for rod synaptic terminal development and demonstrate that rescue is robust even in adult animals with late stages of synaptic disease. The degree of rod synaptic plasticity seen here should be sufficient to support future vision-restoring treatments such as gene or cell replacement that will require photoreceptor synaptic rewiring.

Progress in the development of novel therapies for choroideremia

INTRODUCTION

There are no currently approved treatments for choroideremia, an X-linked progressive inherited retinal degeneration that leads to blindness by middle age. Several treatment options are being explored, but with major advances in adeno-associated vector (AAV) gene replacement therapy that has reached phase III clinical trials.

AREAS COVERED

In this review we discuss new insights into the clinical phenotyping and genetic testing of choroideremia patients, that aid disease characterisation, progression and patient inclusion into clinical trials. Recent advances in in-vitro studies have resulted in the development of functional assays that can be used to confirm the diagnosis in challenging cases and to quantify vector potency for use in clinical trials. We review the progress in current gene therapy trials and some considerations towards gene therapy approval for the treatment of choroideremia. Lastly, we discuss developments in alternative therapies including optogenetics.

EXPERT COMMENTARY

AAV gene replacement therapy is the most promising treatment strategy for choroideremia, that has developed exponentially over the last few years with a phase III clinical trial now underway. Optogenetics is a promising alternative strategy that might be applicable in late stages of degeneration.