Beneficial effects on vision in patients undergoing retinal gene therapy for choroideremia

Parafoveal cone function in choroideremia assessed with adaptive optics optoretinography

Choroideremia (CHM) is an X-linked retinal degeneration leading to loss of the photoreceptors, retinal pigment epithelium (RPE), and choroid. Adaptive optics optoretinography is an emerging technique for noninvasive, objective assessment of photoreceptor function. Here, we investigate parafoveal cone function in CHM using adaptive optics optoretinography and compare with cone structure and clinical assessments of vision. Parafoveal cone mosaics of 10 CHM and four normal-sighted participants were imaged with an adaptive optics scanning light ophthalmoscope. While acquiring video sequences, a 2 s 550Δ10 nm, 450 nW/deg2 stimulus was presented. Videos were registered and the intensity of each cone in each frame was extracted, normalized, standardized, and aggregated to generate the population optoretinogram (ORG) over time. A gamma-pdf was fit to the ORG and the peak was extracted as ORG amplitude. CHM ORG amplitudes were compared to normal and were correlated with bound cone density, ellipsoid zone to RPE/Bruch's membrane (EZ-to-RPE/BrM) distance, and foveal sensitivity using Pearson correlation analysis. ORG amplitude was significantly reduced in CHM compared to normal (0.22 ± 0.15 vs. 1.34 ± 0.31). In addition, CHM ORG amplitude was positively correlated with cone density, EZ-to-RPE/BrM distance, and foveal sensitivity. Our results demonstrate promise for using ORG as a biomarker of photoreceptor function.

Retention Rates of Genetic Therapies Based on AAV Serotypes 2 and 8 Using Different Drug-Delivery Materials

The purpose of this study was to compare the retention rate of Adeno-associated viral vector (AAV) gene therapy agents within different subretinal injection systems. The retention of AAV serotype 2-based voretigene neparvovec (VN) and a clinical-grade AAV serotype 8 vector within four different subretinal cannulas from two different manufacturers was quantified. A standardized qPCR using the universal inverted terminal repeats as a target sequence was developed. The instruments compared were the PolyTip® cannula 25 g/38 g by MedOne Surgical, Inc., Sarasota, FL, USA, and three different subretinal injection needles by DORC, Zuidland, The Netherlands (1270.EXT Extendible 41G subretinal injection needle (23G), DORC 1270.06 23G Dual bore injection cannula, DORC 27G Subretinal injection cannula). The retention rate of VN and within the DORC products (10-28%) was comparable to the retention rate (32%) found for the PolyTip® cannula that is mentioned in the FDA-approved prescribing information for VN. For the AAV8 vector, the PolyTip® cannula showed a retention rate of 14%, and a similar retention rate of 3-16% was found for the DORC products (test-retest variability: mean 4.5%, range 2.5-20.2%). As all the instruments tested showed comparable retention rates, they seem to be equally compatible with AAV2- and AAV8-based gene therapy agents.

Proof-of-concept for multiple AON delivery by a single U7snRNA vector to restore splicing defects in ABCA4

The high allelic heterogeneity in Stargardt disease (STGD1) complicates the design of intervention strategies. A significant proportion of pathogenic intronic ABCA4 variants alters the pre-mRNA splicing process. Antisense oligonucleotides (AONs) are an attractive yet mutation-specific therapeutic strategy to restore these splicing defects. In this study, we experimentally assessed the potential of a splicing modulation therapy to target multiple intronic ABCA4 variants. AONs were inserted into U7snRNA gene cassettes and tested in midigene-based splice assays. Five potent antisense sequences were selected to generate a multiple U7snRNA cassette construct, and this combination vector showed substantial rescue of all of the splicing defects. Therefore, the combination cassette was used for viral synthesis and assessment in patient-derived photoreceptor precursor cells (PPCs). Simultaneous delivery of several modified U7snRNAs through a single AAV, however, did not show substantial splicing correction, probably due to suboptimal transduction efficiency in PPCs and/or a heterogeneous viral population containing incomplete AAV genomes. Overall, these data demonstrate the potential of the U7snRNA system to rescue multiple splicing defects, but also suggest that AAV-associated challenges are still a limiting step, underscoring the need for further optimization before implementing this strategy as a potential treatment for STGD1.

A Prospective, Observational, Non-interventional Clinical Study of Participants With Choroideremia: The NIGHT Study

PURPOSE

The NIGHT study aimed to assess the natural history of choroideremia (CHM), an X-linked inherited chorioretinal degenerative disease leading to blindness, and determine which outcomes would be the most sensitive for monitoring disease progression.

DESIGN

A prospective, observational, multicenter cohort study.

METHODS

Males aged ≥18 years with genetically confirmed CHM, visible active disease within the macular region, and best-corrected visual acuity (BCVA) ≥34 Early Treatment Diabetic Retinopathy Study (ETDRS) letters at baseline were assessed for 20 months. The primary outcome was the change in BCVA over time at Months 4, 8, 12, 16, and 20. A range of functional and anatomical secondary outcome measures were assessed up to Month 12, including retinal sensitivity, central ellipsoid zone (EZ) area, and total area of fundus autofluorescence (FAF). Additional ocular assessments for safety were performed.

RESULTS

A total of 220 participants completed the study. The mean BCVA was stable over 20 months. Most participants (81.4% in the worse eye and 77.8% in the better eye) had change from baseline > -5 ETDRS letters at Month 20. Interocular symmetry was low overall. Reductions from baseline to Month 12 were observed (worse eye, better eye) for retinal sensitivity (functional outcome; -0.68 dB, -0.48 dB), central EZ area (anatomical outcome; -0.276 mm2, -0.290 mm2), and total area of FAF (anatomical outcome; -0.605 mm2, -0.533 mm2). No assessment-related serious adverse events occurred.

CONCLUSIONS

Retinal sensitivity, central EZ area, and total area of FAF are more sensitive than BCVA in measuring the natural progression of CHM.

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.

iOCT-guided simulated subretinal injections: a comparison between manual and robot-assisted techniques in an ex-vivo porcine model

The purpose of this study is to compare robot-assisted and manual subretinal injections in terms of successful subretinal blistering, reflux incidences and damage of the retinal pigment epithelium (RPE). Subretinal injection was simulated on 84 ex-vivo porcine eyes with half of the interventions being carried out manually and the other half by controlling a custom-built robot in a master-slave fashion. After pars plana vitrectomy (PPV), the retinal target spot was determined under a LUMERA 700 microscope with microscope-integrated intraoperative optical coherence tomography (iOCT) RESCAN 700 (Carl Zeiss Meditec, Germany). For injection, a 1 ml syringe filled with perfluorocarbon liquid (PFCL) was tipped with a 40-gauge metal cannula (Incyto Co., Ltd., South Korea). In one set of trials, the needle was attached to the robot's end joint and maneuvered robotically to the retinal target site. In another set of trials, approaching the retina was performed manually. Intraretinal cannula-tip depth was monitored continuously via iOCT. At sufficient depth, PFCL was injected into the subretinal space. iOCT images and fundus video recordings were used to evaluate the surgical outcome. Robotic injections showed more often successful subretinal blistering (73.7% vs. 61.8%, p > 0.05) and a significantly lower incidence of reflux (23.7% vs. 58.8%, p < 0.01). Although larger tip depths were achieved in successful manual trials, RPE penetration occurred in 10.5% of robotic but in 26.5% of manual cases (p > 0.05). In conclusion, significantly less reflux incidences were achieved with the use of a robot. Furthermore, RPE penetrations occurred less and successful blistering more frequently when performing robotic surgery.

Using Goldmann Visual Field Volume to Track Disease Progression in Choroideremia

Purpose

Choroideremia is an X-linked choroidopathy caused by pathogenic variants in the CHM gene. It is characterized by the early appearance of multiple scotomas in the peripheral visual field that spread and coalesce, usually sparing central vision until late in the disease. These features make quantitative monitoring of visual decline particularly challenging. Here, we describe a novel computational approach to convert Goldmann visual field (GVF) data into quantitative volumetric measurements. With this approach, we analyzed visual field loss in a longitudinal, retrospective cohort of patients with choroideremia.

Design

Single-center, retrospective, cohort study.

Participants

We analyzed data from 238 clinic visits of 56 molecularly-confirmed male patients with choroideremia from 41 families (range, 1-27 visits per patient). Patients had a median follow up of 4 years (range, 0-56 years) with an age range of 5 to 76 years at the time of their visits.

Methods

Clinical data from molecularly-confirmed patients with choroideremia, including GVF data, were included for analysis. Goldmann visual field records were traced using a tablet-based application, and the 3-dimensional hill of vision was interpolated for each trace. This procedure allowed quantification of visual field loss from data collected over decades with differing protocols, including different or incomplete isopters. Visual acuity (VA) data were collected and converted to logarithm of the minimum angle of resolution values. A delayed exponential mixed-effects model was used to evaluate the loss of visual field volume over time.

Main Outcome Measures

Visual acuity and GVF volume.

Results

The estimated mean age at disease onset was 12.6 years (standard deviation, 9.1 years; 95% quantile interval, 6.5-36.4 years). The mean field volume loss was 6.8% per year (standard deviation, 4.5%; 95% quantile interval, 1.9%-18.8%) based on exponential modeling. Field volume was more strongly correlated between eyes (r2 = 0.935) than best-corrected VA (r2 = 0.285).

Conclusions

Volumetric analysis of GVF data enabled quantification of peripheral visual function in patients with choroideremia and evaluation of disease progression. The methods presented here may facilitate the analysis of historical GVF data from patients with inherited retinal disease and other diseases associated with visual field loss. This work informs the creation of appropriate outcome measures in choroideremia therapeutic trials, particularly in trial designs.

Financial Disclosures

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

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.

Gene Augmentation of CHM Using Non-Viral Episomal Vectors in Models of Choroideremia

Choroideremia (CHM) is an X-linked chorioretinal dystrophy leading to progressive retinal degeneration that results in blindness by late adulthood. It is caused by mutations in the CHM gene encoding the Rab Escort Protein 1 (REP1), which plays a crucial role in the prenylation of Rab proteins ensuring correct intracellular trafficking. Gene augmentation is a promising therapeutic strategy, and there are several completed and ongoing clinical trials for treating CHM using adeno-associated virus (AAV) vectors. However, late-phase trials have failed to show significant functional improvements and have raised safety concerns about inflammatory events potentially caused by the use of viruses. Therefore, alternative non-viral therapies are desirable. Episomal scaffold/matrix attachment region (S/MAR)-based plasmid vectors were generated containing the human CHM coding sequence, a GFP reporter gene, and ubiquitous promoters (pS/MAR-CHM). The vectors were assessed in two choroideremia disease model systems: (1) CHM patient-derived fibroblasts and (2) chmru848 zebrafish, using Western blotting to detect REP1 protein expression and in vitro prenylation assays to assess the rescue of prenylation function. Retinal immunohistochemistry was used to investigate vector expression and photoreceptor morphology in injected zebrafish retinas. The pS/MAR-CHM vectors generated persistent REP1 expression in CHM patient fibroblasts and showed a significant rescue of prenylation function by 75%, indicating correction of the underlying biochemical defect associated with CHM. In addition, GFP and human REP1 expression were detected in zebrafish microinjected with the pS/MAR-CHM at the one-cell stage. Injected chmru848 zebrafish showed increased survival, prenylation function, and improved retinal photoreceptor morphology. Non-viral S/MAR vectors show promise as a potential gene-augmentation strategy without the use of immunogenic viral components, which could be applicable to many inherited retinal disease genes.

Subretinal timrepigene emparvovec in adult men with choroideremia: a randomized phase 3 trial

Choroideremia is a rare, X-linked retinal degeneration resulting in progressive vision loss. A randomized, masked, phase 3 clinical trial evaluated the safety and efficacy over 12 months of follow-up in adult males with choroideremia randomized to receive a high-dose (1.0 × 1011 vector genomes (vg); n = 69) or low-dose (1.0 × 1010 vg; n = 34) subretinal injection of the AAV2-vector-based gene therapy timrepigene emparvovec versus non-treated control (n = 66). Most treatment-emergent adverse events were mild or moderate. The trial did not meet its primary endpoint of best-corrected visual acuity (BCVA) improvement. In the primary endpoint analysis, three of 65 participants (5%) in the high-dose group, one of 34 (3%) participants in the low-dose group and zero of 62 (0%) participants in the control group had ≥15-letter Early Treatment Diabetic Retinopathy Study (ETDRS) improvement from baseline BCVA at 12 months (high dose, P = 0.245 versus control; low dose, P = 0.354 versus control). As the primary endpoint was not met, key secondary endpoints were not tested for significance. In a key secondary endpoint, nine of 65 (14%), six of 35 (18%) and one of 62 (2%) participants in the high-dose, low-dose and control groups, respectively, experienced ≥10-letter ETDRS improvement from baseline BCVA at 12 months. Potential opportunities to enhance future gene therapy studies for choroideremia include optimization of entry criteria (more preserved retinal area), surgical techniques and clinical endpoints. EudraCT registration: 2015-003958-41 .

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.

Oxidative and Endoplasmic Reticulum Stress Represent Novel Therapeutic Targets for Choroideremia

Choroideremia (CHM) is a rare X-linked chorioretinal dystrophy, affecting the photoreceptors, retinal pigment epithelium (RPE) and choroid, with no approved therapy. CHM is caused by mutations in the CHM gene, which encodes the ubiquitously expressed Rab escort protein 1 (REP1). REP1 is involved in prenylation, a post-translational modification of Rab proteins, and plays an essential role in intracellular trafficking. In this study, we examined oxidative and endoplasmic reticulum (ER) stress pathways in chmru848 zebrafish and CHMY42X patient fibroblasts, and screened a number of neuroprotectants for their ability to reduce stress. The expression of the oxidative stress markers txn, cat and sod3a, and the ER stress markers bip, atf4 and atf6, were dysregulated in chmru848 fish. The expression of SOD2 was also reduced in CHMY42X fibroblasts, along with reduced BIP and increased CHOP expression. The lack of REP1 is associated with defects in vesicular trafficking, photoreceptor outer segment phagocytosis and melanosome transport, leading to increased levels of stress within the retina and RPE. Drugs targeting oxidative and ER stress pathways represent novel therapeutic avenues.

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-Mediated Gene Therapy for Choroideremia: 5-Year Results and Alternate Anti-sense Oligonucleotide Therapy

PURPOSE

To assess the long-term safety and efficacy of AAV2-REP1 in choroideremia (CHM) patients, and to test a potential antisense oligonucleotide therapy for CHM.

DESIGN

Extended, prospective phase 1/2 clinical trial and laboratory investigation.

METHODS

Five patients who received a single subfoveal injection of AAV2-REP1 were studied. The long-term safety was evaluated by ophthalmic examination, spectral domain optical coherence tomography, and fundus autofluorescence (FAF) for up to 5 years. Functional and structural changes were determined by different test modalities. Four antisense oligonucleotides (ASOs) were designed to treat the CHM c.1245-521A>G mutation, which was present in 2 patients within this trial.

RESULTS

Subject P3 experienced a localized intraretinal immune response that resulted in a significant loss of preserved retinal pigment epithelium (RPE). P4 experienced an exacerbation of peripheral retinoschisis. P2 had a constant ≥15-letter best-corrected visual acuity (BCVA) gain in the treated eye, whereas P5 had ≥15-letter BCVA improvement once in the untreated eye. The preserved FAF areas declined more rapidly in the treated eyes compared to the untreated eyes (P = .043). A customized 25-mer ASO recovered 83.2% to 95.0% of the normal RNA and 57.5% of the normal protein in fibroblasts from 2 trial patients.

CONCLUSIONS

Intraretinal inflammation triggered by AAV2-REP1 subretinal injection stabilized after 2 years but resulted in permanent damage to the retinal structure. Long-term progression of the disease was seen in both treated and untreated eyes, casting doubt as to the effectiveness of this approach in late-stage CHM. Alternative approaches such as ASO may have a therapeutic effect in a subgroup of CHM patients.

Bioengineering strategies for restoring vision

Late-stage retinal degenerative disease involving photoreceptor loss can be treated by optogenetic therapy, cell transplantation and retinal prostheses. These approaches aim to restore light sensitivity to the retina as well as visual perception by integrating neuronal responses for transmission to the cortex. In age-related macular degeneration, some cell-based therapies also aim to restore photoreceptor-supporting tissue to prevent complete photoreceptor loss. In the earlier stages of degeneration, gene-replacement therapy could attenuate retinal-disease progression and reverse loss of function. And gene-editing strategies aim to correct the underlying genetic defects. In this Review, we highlight the most promising gene therapies, cell therapies and retinal prostheses for the treatment of retinal disease, discuss the benefits and drawbacks of each treatment strategy and the factors influencing whether functional tissue is reconstructed and repaired or replaced with an electronic device, and summarize upcoming technologies for enhancing the restoration of vision.

Assessment of Scotopic Function in Rod-Cone Inherited Retinal Degeneration With the Scotopic Macular Integrity Assessment

Purpose

The scotopic macular integrity assessment (S-MAIA) can perform scotopic assessment to detect localized changes to scotopic rod and cone function. This study is an exploratory investigation of the feasibility of using the S-MAIA in a rod-cone dystrophy population to identify the pattern of loss in scotopic photoreceptor function.

Methods

Twenty patients diagnosed with a rod-cone dystrophy underwent visual acuity testing, full-field stimulus threshold assessment, and multiple S-MAIA tests after dark adaptation periods of 20 minutes and 45 minutes performed separately. Only right eyes were tested. Three tests were performed following a learning test. A Bland-Altman analysis was used to assess repeatability and agreement between tests after the two time periods. Spatial interpolation maps were created from the group plots to display the pattern of rod and cone loss.

Results

Learning effects took place between testing sessions 1 and 2 but not 2 and 3. Limits of agreement were larger in the patient eyes than control eyes, but within previously reported values. Using longer adaptation time of 45 minutes did not offer a significant advantage over 20 minutes. Patterns for the cyan and red sensitivities were different, indicating different patterns of loss for rods and cones.

Conclusions

A dark adaptation time of 20 minutes before testing is sufficient for thresholding. The S-MAIA is suitable for use in patients with a logarithm of the minimum angle of resolution vision of at least 0.7 and provides a viable outcome measure for patients with rod-cone dystrophies and preserved central vision. The spatial information about scotopic function from the S-MAIA provides information about disease processes and progression.

Translational Relevance

There is a need for scotopic measures for use in clinical trials. Scotopic microperimetry works well in patients with early disease, allowing the extension of recruitment criteria for novel therapies of rod-cone dystrophies.

MERTK missense variants in three patients with retinitis pigmentosa

BACKGROUND

MERTK (MER proto-oncogene, tyrosine kinase) is a transmembrane protein essential in regulating photoreceptor outer segment phagocytosis. Biallelic mutations in MERTK cause retinal degeneration. Here we present the retinal phenotype of three patients with missense variants in MERTK.

MATERIALS AND METHODS

All patients underwent a full clinical examination, fundus photography, short-wavelength fundus autofluorescence and optical coherence tomography imaging. Two patients also underwent Goldmann visual field testing and electroretinography was undertaken for the third patient. Molecular genetic testing was undertaken using next generation or whole-exome sequencing with all variants confirmed by Sanger sequencing.

RESULTS

The first patient was a 29-year-old female heterozygous for a missense variant (c.1133C>T, p.Thr378 Met) and a nonsense variant (c.1744_1751delinsT, p.Ile582Ter) in MERTK. The second patient was a 26-year-old male homozygous for a c.2163T>A, p.His721Gln variant in MERTK. The third patient was an 11-year-old female heterozygous for a deletion of exons 5-19 and a missense variant (c.1866 G>C, p.Lys622Asn) in MERTK. Reduced night vision was the initial symptom in all patients. Fundoscopy revealed typical signs of retinitis pigmentosa (RP) with early-onset macular atrophy. All three MERTK missense variants affect highly conserved residues within functional domains, have low population frequencies and are predicted to be pathogenic in silico.

CONCLUSIONS

We report three missense variants in MERTK and present the associated phenotypic data, which are supportive of non-syndromic RP. MERTK is a promising candidate for viral-mediated gene replacement therapy. Moreover, one variant represents a single nucleotide transition, which is theoretically targetable with CRISPR-Cas9 base-editing.

Quantitative measurements of intraocular structures and microinjection bleb volumes using intraoperative optical coherence tomography

Intraoperative optical coherence tomography (OCT) systems provide high-resolution, real-time visualization and/or guidance of microsurgical procedures. While the use of intraoperative OCT in ophthalmology has significantly improved qualitative visualization of surgical procedures inside the eye, new surgical techniques to deliver therapeutics have highlighted the lack of quantitative information available with current-generation intraoperative systems. Indirect viewing systems used for retinal surgeries introduce distortions into the resulting OCT images, making it particularly challenging to make calibrated quantitative measurements. Using an intraoperative OCT system based in part on the Leica Enfocus surgical microscope interface, we have devised novel measurement procedures, which allowed us to build optical and mathematical models to perform validation of quantitative measurements of intraocular structures for intraoperative OCT. These procedures optimize a complete optical model of the sample arm including the OCT scanner, viewing attachments, and the patient's eye, thus obtaining the voxel pitch throughout an OCT volume and performing quantitative measurements of the dimensions of imaged objects within the operative field. We performed initial validation by measuring objects of known size in a controlled eye phantom as well as ex vivo porcine eyes. The technique was then extended to measure other objects and structures in ex vivo porcine eyes and in vivo human eyes.

Advances in Vitreoretinal Surgery

Advances in vitreoretinal surgery provide greater safety, efficacy, and reliability in the management of the several vitreoretinal diseases that benefit from surgical treatment. The advances are divided into the following topics: scleral buckling using chandelier illumination guided by non-contact visualization systems; sclerotomy/valved trocar diameters; posterior vitrectomy systems and ergonomic vitrectomy probes; chromovitrectomy; vitreous substitutes; intraoperative visualization systems including three-dimensional technology, systems for intraoperative optical coherence tomography, new instrumentation in vitreoretinal surgery, anti-VEGF injection before vitrectomy and in eyes with proliferative diabetic retinopathy, and new surgical techniques; endoscopic surgery; the management of subretinal hemorrhages; gene therapy; alternative techniques for refractory macular hole; perspectives for stem cell therapy and the prevention of proliferative vitreoretinopathy; and, finally, the Port Delivery System. The main objective of this review is to update the reader on the latest changes in vitreoretinal surgery and to provide an understanding of how each has impacted the improvement of surgical outcomes.

Advancements in ocular gene therapy delivery: vectors and subretinal, intravitreal, and suprachoroidal techniques

INTRODUCTION

: Ocular gene therapy represents fertile ground for rapid innovation, with ever-expanding therapeutic strategies, molecular targets, and indications.

AREAS COVERED

: Potential indications for ocular gene therapy have classically focused on inherited retinal disease (IRD), but more recently include acquired retinal diseases, such as neovascular age-related macular degeneration, geographic atrophy and diabetic retinopathy. Ocular gene therapy strategies have proliferated recently, and include gene augmentation, gene inactivation, gene editing, RNA modulation, and gene-independent gene augmentation. Viral vector therapeutic constructs include adeno-associated virus and lentivirus and continue to evolve through directed evolution and rationale design. Ocular gene therapy administration techniques have expanded beyond pars plana vitrectomy with subretinal injection to intravitreal injection and suprachoroidal injection.

EXPERT OPINION

: The success of treatment for IRD, paired with the promise of clinical research in acquired retinal diseases and in administration techniques, has raised the possibility of in-office gene therapy for common retinal disorders within the next five to ten years.

Emerging Gene Manipulation Strategies for the Treatment of Monogenic Eye Disease

Genetic eye diseases, representing a wide spectrum of simple and complex conditions, are one of the leading causes of visual loss in children and working adults, and progress in the field has led to changes in disease investigation, diagnosis, and management. The past 15 years have seen the emergence of novel therapies for these previously untreatable conditions to the extent that we now have a licensed therapy for one form of genetic eye disease and many more in clinical trial. This is a systematic review of published and ongoing clinical trials of gene therapies for monogenic eye diseases. Databases of clinical trials and the published literature were searched for interventional studies of gene therapies for eye diseases. Standard methodological procedures were used to assess the relevance of search results. A total of 59 registered clinical trials are referenced, showing the significant level of interest in the potential for translation of these therapies from bench to bedside. The breadth of therapy design is encouraging, providing multiple possible therapeutic mechanisms. Some fundamental questions regarding gene therapy for genetic eye diseases remain, such as optimal dosing, the relative benefits of adeno-associated virus (AAV)-packaging and the potential for a significant inflammatory response to the therapy itself. As a result, despite the promise of the eye as a target, it has proven difficult to deliver clinically effective gene therapies to the eye. Despite setbacks, the licensing of Luxturna (voretigene neparvovec, Novartis) for the treatment of RPE65-mediated Leber congenital amaurosis (LCA) is a major advance in efforts to treat these rare, but devastating, causes of visual loss.

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.

Multimodal imaging reveals retinoschisis masquerading as retinal detachment in patients with choroideremia

Purpose

To report three cases of retinoschisis in patients with intermediate to advanced choroideremia.

Observations

Three patients were referred for evaluation of retinal detachment in the context of an inherited retinal degenerative disease. In all three cases, patients carried variants in the CHM gene suspected to be pathogenic and exhibited the characteristic findings of choroideremia, including pigment clumping and chorioretinal atrophy with scleral exposure and prominent choroidal vessels. Interestingly, these patients were also found to have areas of typical retinoschisis and cystoid degeneration located in the outer plexiform layer of the mid periphery or macula. Retinoschisis was confirmed by spectral domain optical coherence tomography (SD-OCT).

Conclusions/Importance

This paper draws attention to the occurrence of retinoschisis in patients with choroideremia. OCT can be used to confirm the presence of retinoschisis rather than retinal detachment, as the clinical exam findings that distinguish the two conditions are not helpful in the setting of advanced chorioretinal atrophy. Although it remains unclear whether patients with choroideremia as a group are at increased risk of retinoschisis, it is possible that abnormal vesicular traffic in the RPE and photoreceptors could contribute to abnormalities in cell adhesion and the extracellular matrix. As gene therapy by subretinal injection of adeno-associated virus becomes the standard of care to slow down or arrest retinal degeneration in choroideremia, it will be critical to carefully screen these patients for retinoschisis prior to surgical intervention and to incorporate any such findings into surgical planning.

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.

First-in-Human Robot-Assisted Subretinal Drug Delivery Under Local Anesthesia

PURPOSE

To report the results of a first-in-human study using a robotic device to assist subretinal drug delivery in patients undergoing vitreoretinal surgery for macular hemorrhage.

DESIGN

Double-armed, randomized controlled surgical trial (ClinicalTrials.gov identifier: NCT03052881).

METHODS

The study was performed at the Oxford Eye Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom. In total, 12 participants were recruited-6 in the robot-assisted and 6 in the control manual surgery arm according to the prespecified inclusion and exclusion criteria. All subjects presented with acute loss of vision owing to a subfoveal hemorrhage secondary to neovascular age-related macular degeneration. After standard vitrectomy, intraoperative optical coherence tomography-guided subretinal injection of tissue plasminogen activator (TPA) was performed by either robot-assisted or conventional manual technique under local anesthesia. The robotic part of the procedure involved advancement of a cannula through the retina and stabilizing it during foot-controlled injection of up to 100 µL of TPA solution. We assessed surgical success, duration of surgery, adverse events, and tolerability of surgery under local anesthesia.

RESULTS

The procedure was well tolerated by all participants and safely performed in all cases. Total duration of surgery, time taken to complete the injection, and retinal microtrauma were similar between the groups and not clinically significant. Subretinal hemorrhage was successfully displaced at 1 month postintervention, except for 1 control subject, and the median gain in visual acuity was similar in both arms.

CONCLUSIONS

This first-in-human study demonstrates the feasibility and safety of high-precision robot-assisted subretinal drug delivery as part of the surgical management of submacular hemorrhage, simulating its potential future application in gene or cell therapy.

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.

Conversion of the liver into a biofactory for DNaseI using adeno-associated virus vector gene transfer reduces neutrophil extracellular traps in a model of Systemic Lupus Erythematosus

Adeno-associated virus (AAV) vectors are proving to be clinically transformative tools in the treatment of monogenic genetic disease. Rapid ongoing development of this technology promises to not only increase the number of monogenic disorders amenable to this approach, but also to bring diseases with complex multigenic and non-genetic aetiologies within therapeutic reach. Here we explore the broader paradigm of converting the liver into a biofactory for systemic output of therapeutic molecules using AAV-mediated delivery of DNaseI as an exemplar. DNaseI can clear neutrophil extracellular traps (NETs), which are nuclear-protein structures possessing anti-microbial action that are also involved in the pathophysiology of clinically troubling immune-mediated diseases. However, a translational challenge is short half-life of the enzyme in vivo (<5 hours). The current study demonstrates that AAV-mediated liver-targeted gene transfer stably induces serum DNaseI activity to >190-fold above physiological levels. In lupus-prone mice (NZBWF1) activity was maintained for longer than 6 months, the latest time point tested, and resulted in a clear functional effect with reduced renal presence of neutrophils, NETs, IgG and complement C3. However, treatment in this complex disease model did not extend life-span, improve serological endpoints or preserve renal function indicating there are elements of pathophysiology not accessible to DNaseI in the NZBWF1 model. We conclude that a translational solution to the challenge of short half-life of DNaseI is AAV-mediated gene delivery and that this may be efficacious in treating disease where NETs are a dominant pathological mechanism.

Investigation of Cas9 antibodies in the human eye

Preexisting immunity against Cas9 proteins in humans represents a safety risk for CRISPR–Cas9 technologies. However, it is unclear to what extent preexisting Cas9 immunity is relevant to the eye as it is targeted for early in vivo CRISPR–Cas9 clinical trials. While the eye lacks T-cells, it contains antibodies, cytokines, and resident immune cells. Although precise mechanisms are unclear, intraocular inflammation remains a major cause of vision loss. Here, we used immunoglobulin isotyping and ELISA platforms to profile antibodies in serum and vitreous fluid biopsies from human adult subjects and Cas9-immunized mice. We observed high prevalence of preexisting Cas9-reactive antibodies in serum but not in the eye. However, we detected intraocular antibodies reactive to S. pyogenes-derived Cas9 after S. pyogenes intraocular infection. Our data suggest that serum antibody concentration may determine whether specific intraocular antibodies develop, but preexisting immunity to Cas9 may represent a lower risk in human eyes than systemically.

Pre-existing antibodies against Cas9 proteins represent a potential issue for gene therapies, including those targeting the eye. Here the authors assess the presence of intraocular antibodies, and show that Cas9 antibodies were prevalent in human serum but not the eye, unless prior bacterial infection occurred.

Advances in Ophthalmic Optogenetics: Approaches and Applications

Recent advances in optogenetics hold promise for vision restoration in degenerative eye diseases. Optogenetics refers to techniques that use light to control the cellular activity of targeted cells. Although optogenetics is a relatively new technology, multiple therapeutic options are already being explored in pre-clinical and phase I/II clinical trials with the aim of developing novel, safe, and effective treatments for major blinding eye diseases, such as glaucoma and retinitis pigmentosa. Optogenetic approaches to visual restoration are primarily aimed at replacing lost or dysfunctional photoreceptors by inserting light-sensitive proteins into downstream retinal neurons that have no intrinsic light sensitivity. Such approaches are attractive because they are agnostic to the genetic causes of retinal degeneration, which raises hopes that all forms of retinal dystrophic and degenerative diseases could become treatable. Optogenetic strategies can also have a far-reaching impact on translational research by serving as important tools to study the pathogenesis of retinal degeneration and to identify clinically relevant therapeutic targets. For example, the CRY-CIBN optogenetic system has been recently applied to animal models of glaucoma, suggesting a potential role of OCRL in the regulation of intraocular pressure in trabecular meshwork. As optogenetic strategies are being intensely investigated, it appears crucial to consider the opportunities and challenges such therapies may offer. Here, we review the more recent promising optogenetic molecules, vectors, and applications of optogenetics for the treatment of retinal degeneration and glaucoma. We also summarize the preliminary results of ongoing clinical trials for visual restoration.

Review of gene therapies for age-related macular degeneration

Gene therapies aim to deliver a therapeutic payload to specified tissues with underlying protein deficiency. Since the 1990s, gene therapies have been explored as potential treatments for chronic conditions requiring lifetime care and medical management. Ocular gene therapies target a range of ocular disorders, but retinal diseases are of particular importance due to the prevalence of retinal disease and the current treatment burden of such diseases on affected patients, as well as the challenge of properly delivering these therapies to the target tissue. The purpose of this review is to provide an update on the most current data available for five different retinal gene therapies currently undergoing clinical trials for use against age-related macular degeneration (AMD) and the development of novel delivery routes for the administration of such therapies. Research has been performed and compiled from PubMed and the select authors of this manuscript on the treatment and effectiveness of five current retinal gene therapies: Luxturna, ADVM-022, RGX-314, GT-005, and HMR59. We present the available data of current clinical trials for the treatment of neovascular and dry age-related macular degeneration with different AAV-based gene therapies. We also present current research on the progress of developing novel routes of administration for ocular gene therapies. Retinal gene therapies offer the potential for life-changing treatment for chronic conditions like age-related macular degeneration with a single administration. In doing so, gene therapies change the landscape of treatment options for these chronic conditions for both patient and provider.

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.

In Silico Analysis of Pathogenic CRB1 Single Nucleotide Variants and Their Amenability to Base Editing as a Potential Lead for Therapeutic Intervention

Mutations in the Crumbs homolog 1 (CRB1) gene cause both autosomal recessive retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA). Since three separate CRB1 isoforms are expressed at meaningful levels in the human retina, base editing shows promise as a therapeutic approach. This retrospective analysis aims to summarise the reported pathogenic CRB1 variants and investigate their amenability to treatment with currently available DNA base editors. Pathogenic single nucleotide variants (SNVs) were extracted from the Leiden open-source variation database (LOVD) and ClinVar database and coded by mutational consequence. They were then analyzed for their amenability to currently available DNA base editors and available PAM sites from a selection of different Cas proteins. Of a total of 1115 unique CRB1 variants, 69% were classified as pathogenic SNVs. Of these, 62% were amenable to currently available DNA BEs. Adenine base editors (ABEs) alone have the potential of targeting 34% of pathogenic SNVs; 19% were amenable to a CBE while GBEs could target an additional 9%. Of the pathogenic SNVs targetable with a DNA BE, 87% had a PAM site for a Cas protein. Of the 33 most frequently reported pathogenic SNVs, 70% were targetable with a base editor. The most common pathogenic variant was c.2843G>A, p.Cys948Arg, which is targetable with an ABE. Since 62% of pathogenic CRB1 SNVs are amenable to correction with a base editor and 87% of these mutations had a suitable PAM site, gene editing represents a promising therapeutic avenue for CRB1-associated retinal degenerations.

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

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

The Next Generation of Molecular and Cellular Therapeutics for Inherited Retinal Disease

Inherited retinal degenerations (IRDs) are a diverse group of conditions that are often characterized by the loss of photoreceptors and blindness. Recent innovations in molecular biology and genomics have allowed us to identify the causative defects behind these dystrophies and to design therapeutics that target specific mechanisms of retinal disease. Recently, the FDA approved the first in vivo gene therapy for one of these hereditary blinding conditions. Current clinical trials are exploring new therapies that could provide treatment for a growing number of retinal dystrophies. While the field has had early success with gene augmentation strategies for treating retinal disease based on loss-of-function mutations, many novel approaches hold the promise of offering therapies that span the full spectrum of causative mutations and mechanisms. Here, we provide a comprehensive review of the approaches currently in development including a discussion of retinal neuroprotection, gene therapies (gene augmentation, gene editing, RNA modification, optogenetics), and regenerative stem or precursor cell-based therapies. Our review focuses on technologies that are being developed for clinical translation or are in active clinical trials and discusses the advantages and limitations for each approach.

Molecular Therapy for Choroideremia: Pre-clinical and Clinical Progress to Date

Choroideremia is an inherited retinal disease characterised by a degeneration of the light-sensing photoreceptors, supporting retinal pigment epithelium and underlying choroid. Patients present with the same symptoms as those with classic rod-cone dystrophy: (1) night blindness early in life; (2) progressive peripheral visual field loss, and (3) central vision decline with a slow progression to legal blindness. Choroideremia is monogenic and caused by mutations in CHM. Eight clinical trials (three phase 1/2, four phase 2, and one phase 3) have started (four of which are already finished) to evaluate the therapeutic efficacy of gene supplementation mediated by subretinal delivery of an adeno-associated virus serotype 2 (AAV2/2) vector expressing CHM. Furthermore, one phase 1 clinical trial has been initiated to evaluate the efficiency of a novel AAV variant to deliver CHM to the outer retina following intravitreal delivery. Lastly, a non-viral-mediated CHM replacement strategy is currently under development, which could lead to a future clinical trial. Here, we summarise the rationale behind these various studies, as well as any results published to date. The diversity of these trials currently places choroideremia at the forefront of the retinal gene therapy field. As a consequence, the trial outcomes, regardless of the results, have the potential to change the landscape of gene supplementation for inherited retinal diseases.

CRISPR Systems Suitable for Single AAV Vector Delivery

CRISPR (clustered regularly interspaced short palindromic repeats)/Cas gene editing is a revolutionary technology that can enable the correction of genetic mutations in vivo, providing great promise as a therapeutic intervention for inherited diseases. Adeno-associated viral (AAV) vectors are a potential vehicle for delivering CRISPR/Cas. However, they are restricted by their limited packaging capacity. Identifying smaller Cas orthologs that can be packaged, along with the required guide RNA elements, into a single AAV would be an important optimization for CRISPR/Cas gene editing. Expanding the options of Cas proteins that can be delivered by a single AAV not only increases translational application but also expands the genetic sites that can be targeted for editing. This review considers the benefits and current scope of small Cas protein orthologs that are suitable for gene editing approaches using single AAV vector delivery.

Assessing Photoreceptor Status in Retinal Dystrophies: From High-Resolution Imaging to Functional Vision

Purpose

To describe the value of integrating phenotype/genotype data, disease staging, and evaluation of functional vision in patient-centered management of retinal dystrophies.

Methods

(1) Cross-sectional structure-function and retrospective longitudinal studies to assess the correlations between standard fundus autofluorescence (FAF), optical coherence tomography, visual acuity (VA), and perimetry (visual field [VF]) examinations to evaluate photoreceptor functional loss in a cohort of patients with rod-cone dystrophy (RCD); (2) flood-illumination adaptive optics (FIAO) imaging focusing on photoreceptor misalignment and orientation of outer segments; and (3) evaluation of the impact of visual impairment in daily life activities, based on functional (visual and mobility) vision assessment in a naturalistic environment in visually impaired subjects with RCD and subjects treated with Luxturna^Ⓡ for RPE65-related Leber congenital amaurosis before and after therapy.

Results

The results of the cross-sectional transversal study showed that (1) VA and macular sensitivity were weakly correlated with the structural variables; and (2) functional impairment (VF) was correlated with reduction of anatomical markers of photoreceptor structure and increased width of autofluorescent ring. The dimensions of the ring of increased FAF evolved faster. Other criteria that differed among groups were the lengths of the ellipsoid zone, the external limiting membrane, and the foveal thickness. FIAO revealed a variety of phenotypes: paradoxical visibility of foveal cones; heterogeneous brightness of cones; dim, inner segment–like, and RPE-like mosaic. Directional illumination by varying orientation of incident light (Stiles-Crawford effect) and the amount of side illumination (gaze-dependent imaging) affected photoreceptor visibility. Mobility assessment under different lighting conditions showed correlation with VF, VA, contrast sensitivity (CS), and dark adaptation, with different predictive values depending on mobility study paradigms and illumination level. At high illumination level (235 lux), VF was a predictor for all mobility performance models. Under low illumination (1 and 2 lux), VF was the most significant predictor of mobility performance variables, while CS best explained the number of collisions and segments. In subjects treated with Luxturna^Ⓡ, a very favorable impact on travel speed and reduction in the number of collisions, especially at low luminance, was observable 6 months following injection, in both children and adults.

Conclusions

Our results suggest the benefit of development and implementation of quantitative and reproducible tools to evaluate the status of photoreceptors and the impact of both visual impairment and novel therapies in real-life conditions. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Gene therapy for inherited retinal diseases

Inherited retinal diseases (IRDs) are a genetically variable collection of devastating disorders that lead to significant visual impairment. Advances in genetic characterization over the past two decades have allowed identification of over 260 causative mutations associated with inherited retinal disorders. Thought to be incurable, gene supplementation therapy offers great promise in treating various forms of these blinding conditions. In gene replacement therapy, a disease-causing gene is replaced with a functional copy of the gene. These therapies are designed to slow disease progression and hopefully restore visual function. Gene therapies are typically delivered to target retinal cells by subretinal (SR) or intravitreal (IVT) injection. The historic Food and Drug Administration (FDA) approval of voretigene neparvovec for RPE65-associated Leber's congenital amaurosis (LCA) spurred tremendous optimism surrounding retinal gene therapy for various other monogenic IRDs. Novel disease-causing mutations continue to be discovered annually, and targeted genetic therapy is now under development in clinical and preclinical models for many IRDs. Numerous clinical trials for other IRDs are ongoing or have recently completed. Disorders being targeted for genetic therapy include retinitis pigmentosa (RP), choroideremia (CHM), achromatopsia (ACHM), Leber's hereditary optic neuropathy, usher syndrome (USH), X-linked retinoschisis, and Stargardt disease. Here, we provide an update of completed, ongoing, and planned clinical trials using gene supplementation strategies for retinal degenerative disorders.

Precision Medicine Trials in Retinal Degenerations

The beginning of the twenty-first century was marked by the innovative use of pharmacochemical interventions, which have since expanded to include gene-based molecular therapies. For years, treatment has focused on tackling the pathophysiology of monogenic orphan diseases, and one of the first applications of these novel genome editing technologies was the treatment of rare inherited retinal dystrophies. In this review, we present recent, ongoing, and future gene therapy-based treatment trials for choroideremia, X-linked retinitis pigmentosa, Stargardt disease, and age-related macular degeneration. As these trials pave the way toward halting the progression of such devastating diseases, we will begin to see the exciting development of newer, cutting-edge strategies including base editing and prime editing, ushering in a new era of precision medicine.

Characterizing the cellular immune response to subretinal AAV gene therapy in the murine retina

Although adeno-associated viral (AAV) vector-mediated retinal gene therapies have demonstrated efficacy, the mechanisms underlying dose-dependent retinal inflammation remain poorly understood. Here, we present a quantitative analysis of cellular immune response to subretinal AAV gene therapy in mice using multicolor flow cytometry with a panel of key immune cell markers. A significant increase in CD45+ retinal leukocytes was detected from day 14 post-subretinal injection of an AAV8 vector (1 × 109 genome copies) encoding green fluorescent protein (GFP) driven by a ubiquitous promoter. These predominantly consisted of infiltrating peripheral leukocytes including macrophages, natural killer cells, CD4 and CD8 T cells, and natural killer T cells; no significant change in resident microglia population was detected. This cellular response was persistent at 28 days and suggestive of type 1 cell-mediated effector immunity. High levels (80%) of GFP fluorescence were found in the microglia, implicating their role in viral antigen presentation and peripheral leukocyte recruitment. When compared against AAV.GFP in paired eyes, an equivalent dose of an otherwise identical vector encoding the human therapeutic transgene Rab-escort protein 1 (REP1) elicited a significantly diminished cellular immune response (4.2-fold; p = 0.0221). However, the distribution of immune cell populations remained similar, indicating a common mechanism of AAV-induced immune activation.

Mirtron-mediated RNA knockdown/replacement therapy for the treatment of dominant retinitis pigmentosa

Rhodopsin (RHO) gene mutations are a common cause of autosomal dominant retinitis pigmentosa (ADRP). The need to suppress toxic protein expression together with mutational heterogeneity pose challenges for treatment development. Mirtrons are atypical RNA interference effectors that are spliced from transcripts as short introns. Here, we develop a novel mirtron-based knockdown/replacement gene therapy for the mutation-independent treatment of RHO-related ADRP, and demonstrate efficacy in a relevant mammalian model. Splicing and potency of rhodopsin-targeting candidate mirtrons are initially determined, and a mirtron-resistant codon-modified version of the rhodopsin coding sequence is validated in vitro. These elements are then combined within a single adeno-associated virus (AAV) and delivered subretinally in a Rho^P23H knock-in mouse model of ADRP. This results in significant mouse-to-human rhodopsin RNA replacement and is associated with a slowing of retinal degeneration. This provides proof of principle that synthetic mirtrons delivered by AAV are capable of reducing disease severity in vivo.

Therapy Approaches for Stargardt Disease

Despite being the most prevalent cause of inherited blindness in children, Stargardt disease is yet to achieve the same clinical trial success as has been achieved for other inherited retinal diseases. With an early age of onset and continual progression of disease over the life course of an individual, Stargardt disease appears to lend itself to therapeutic intervention. However, the aetiology provides issues not encountered with the likes of choroideremia and X-linked retinitis pigmentosa and this has led to a spectrum of treatment strategies that approach the problem from different aspects. These include therapeutics ranging from small molecules and anti-sense oligonucleotides to viral gene supplementation and cell replacement. The advancing development of CRISPR-based molecular tools is also likely to contribute to future therapies by way of genome editing. In this we review, we consider the most recent pre-clinical and clinical trial data relating to the different strategies being applied to the problem of generating a treatment for the large cohort of Stargardt disease patients worldwide.

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.

Engineering adeno-associated viral vectors to evade innate immune and inflammatory responses

Nucleic acids are used in many therapeutic modalities, including gene therapy, but their ability to trigger host immune responses in vivo can lead to decreased safety and efficacy. In the case of adeno-associated viral (AAV) vectors, studies have shown that the genome of the vector activates Toll-like receptor 9 (TLR9), a pattern recognition receptor that senses foreign DNA. Here, we engineered AAV vectors to be intrinsically less immunogenic by incorporating short DNA oligonucleotides that antagonize TLR9 activation directly into the vector genome. The engineered vectors elicited markedly reduced innate immune and T cell responses and enhanced gene expression in clinically relevant mouse and pig models across different tissues, including liver, muscle, and retina. Subretinal administration of higher-dose AAV in pigs resulted in photoreceptor pathology with microglia and T cell infiltration. These adverse findings were avoided in the contralateral eyes of the same animals that were injected with the engineered vectors. However, intravitreal injection of higher-dose AAV in macaques, a more immunogenic route of administration, showed that the engineered vector delayed but did not prevent clinical uveitis, suggesting that other immune factors in addition to TLR9 may contribute to intraocular inflammation in this model. Our results demonstrate that linking specific immunomodulatory noncoding sequences to much longer therapeutic nucleic acids can "cloak" the vector from inducing unwanted immune responses in multiple, but not all, models. This "coupled immunomodulation" strategy may widen the therapeutic window for AAV therapies as well as other DNA-based gene transfer methods.

Deep phenotyping of the Cdhr1-/- mouse validates its use in pre-clinical studies for human CDHR1-associated retinal degeneration

PURPOSE

To validate the Cdhr1^-/- mouse as a model for human CDHR1-associated retinal degeneration, which may present as cone-rod dystrophy or geographic atrophy.

METHODS

Deep phenotyping of Cdhr1^-/-(n = 56) and C57BL6J wildtype control mice (n = 45) was undertaken using in vivo multimodal retinal imaging and dark- and light-adapted electroretinography (ERG) over 15 months to evaluate rod- and cone-photoreceptor responses and retinal morphology.

RESULTS

Cdhr1^-/- retinas exhibited outer retinal thinning on optical coherence tomography (OCT) at 1-month versus C57BL6J (mean 14.6% reduction; P < 0.0001), with progressive degeneration to 15 months. The OCT layer representing photoreceptor outer segments was more significantly shortened in Cdhr1^-/- eyes at 1 month (mean 33.7% reduction; P < 0.0001), remained stable to 3 months and was not identifiable at later timepoints. Outer retinal thinning was more pronounced at inferior versus superior retinal locations in Cdhr1^-/- eyes (P < 0.002 at 3-9 months). Dark-adapted ERG identified severe functional deficits in Cdhr1^-/- mice at 1 month of age versus C57BL6J (mean 62% reduction) that continued to decline to 15 months (P < 0.0001). Light-adapted flicker identified severe deficits in cone function at 1 month (mean 70% reduction), with improved function to 3 months followed by progressive decline (P < 0.0001).

CONCLUSIONS

The Cdhr1^-/- mouse exhibits structural and functional evidence of progressive outer retinal degeneration at a slow rate. Early functional deficits affecting both rod and cone photoreceptors in the context of relatively mild structural changes reflect the human phenotype. This study validates the use of the Cdhr1^-/- mouse for the pre-clinical evaluation of therapeutics for human CDHR1-associated retinal degeneration.