Chronically shortened rod outer segments accompany photoreceptor cell death in Choroideremia

Lytic photoreceptor cell death caused by Rab escort protein deficiency in Drosophila

Choroideremia (CHM) is a rare X-linked recessive form of inherited retinal degeneration caused by the deficiency of the Rab escort protein 1 (REP1)-encoding CHM gene. REP1 is essential for the post-translational prenylation of the key players in intracellular membrane trafficking, the Rab GTPases. In this study, we aimed to analyze the mechanisms of retinal degeneration caused by Rep deficiency using the Drosophila retina as a model system. Rab GTPases lost their membrane association ability and diffused into the cytoplasm, and the accumulation of unprenylated Rab6 and Rab7 was observed in Rep-deficient photoreceptors. Notably, Rep-deficient photoreceptors underwent progressive cell death via cell swelling and rupture rather than apoptosis. These findings provide new insight to seek a therapeutic approach to CHM. Impact statement Choroideremia is an inherited retinal degeneration caused by a deficiency of Rab escort protein 1 (Rep-1). We used the Drosophila retina as a model to study the mechanism of retinal degeneration in Rep-deficiency and found that Rep-deficient photoreceptors undergo progressive cell death via cell swelling and rupture rather than apoptosis.

Genetic Analysis of Choroideremia-Related Rab Escort Proteins

Choroideremia is a rare X-linked recessive retinal disorder characterized by progressive vision loss caused by retinal degeneration resulting from mutations in the CHM gene, which encodes Rab escort protein 1 (REP-1). In humans and mice, the Rab escort protein (REP) family consists of two members, REP-1 and REP-2, with REP-2 hypothesized to compensate for REP-1 deficiency in tissues outside the eye in choroideremia. In this study, we conducted a systematic mutational analysis of the mouse orthologs of REP-1 and REP-2. Blood analyses revealed metabolic abnormalities in the mutant mice deficient for REP-1, resembling the systemic metabolic disturbances observed in individuals with choroideremia, such as altered lipid and hemoglobin metabolism. We also observed an elevation in systemic inflammatory biomarkers in these mutant mice. Interestingly, these systemic abnormalities emerged before retinal degeneration became detectable in REP-1-deficient mice. Transcriptomic analysis of retinas isolated from REP-1 deficient mice revealed enrichment of proinflammatory signaling pathways. These results suggest important similarities between choroideremia and some forms of retinitis pigmentosa. While engineered loss of REP-2 alone caused no detectable phenotypic changes, dual deficiency in REP-1 and REP-2 resulted in lethality under both in vivo and in vitro conditions. Our findings offer novel insights into REPs and deepen our understanding of choroideremia, which may contribute to the development of new treatments for this genetic condition.

Gene therapy for choroideremia: progress, potential and pitfalls

INTRODUCTION

Choroideremia is a rare disease with a significant disease burden. Gene-supplementation methods for choroideremia gene therapy have been the most successful form of gene therapy thus far.

AREAS COVERED

The aim of the current review is to provide an overview of current progress of gene therapy trials to date, with a focus on potential and pitfalls of such trials. We propose a novel end point that may be clinically meaningful for obtaining regulatory approval in subsequent clinical trials. Additionally, we offer recommendations for further optimization of surgical techniques.

EXPERT OPINION

Lessons learnt from this phase 3 clinical trial, encompassing optimal vector design, delivery techniques, patient selection criteria, and long-term safety profiles can be used in the development of treatments for polygenic retinal disorders, which may necessitate a more nuanced approach due to genetic complexity.

The USH3A causative gene clarin1 functions in Müller glia to maintain retinal photoreceptors

Mutations in CLRN1 cause Usher syndrome type IIIA (USH3A), an autosomal recessive disorder characterized by hearing and vision loss, and often accompanied by vestibular dysfunction. The identity of the cell types responsible for the pathology and mechanisms leading to vision loss in USH3A remains elusive. To address this, we employed CRISPR/Cas9 technology to delete a large region in the coding and untranslated (UTR) region of zebrafish clrn1. The retinas of clrn1 mutant larvae exhibited sensitivity to cell stress, along with age-dependent loss of function and degeneration in the photoreceptor layer. Investigation revealed disorganization in the outer retina in clrn1 mutants, including actin-based structures of the Müller glia and photoreceptor cells. To assess cell-specific contributions to USH3A pathology, we specifically re-expressed clrn1 in either Müller glia or photoreceptor cells. Müller glia re-expression of clrn1 prevented the elevated cell death observed in larval clrn1 mutant zebrafish exposed to high-intensity light. Notably, the degree of phenotypic rescue correlated with the level of Clrn1 re-expression. Surprisingly, high levels of Clrn1 expression enhanced cell death in both wild-type and clrn1 mutant animals. However, rod- or cone-specific Clrn1 re-expression did not reduce the extent of cell death. Taken together, our findings underscore three crucial insights. First, clrn1 mutant zebrafish exhibit key pathological features of USH3A; second, Clrn1 within Müller glia plays a pivotal role in photoreceptor maintenance, with its expression requiring controlled regulation; third, the reliance of photoreceptors on Müller glia suggests a structural support mechanism, possibly through direct interactions between Müller glia and photoreceptors mediated in part by Clrn1 protein.

A hypomorphic variant of choroideremia is associated with a novel intronic mutation that leads to exon skipping

INTRODUCTION

Molecular confirmation of pathogenic sequence variants in the CHM gene is required prior to enrolment in retinal gene therapy clinical trials for choroideremia. Individuals with mild choroideremia have been reported. The molecular basis of genotype-phenotype associations is of clinical relevance since it may impact on selection for retinal gene therapy.

METHODS AND MATERIALS

Genetic testing and RNA analysis were undertaken in a patient with mild choroideremia to confirm the pathogenicity of a novel intronic variant in CHM and to explore the mechanism underlying the mild clinical phenotype.

RESULTS

A 42-year-old male presented with visual field loss. Fundoscopy and autofluorescence imaging demonstrated mild choroideremia for his age. Genetic analysis revealed a variant at a splice acceptor site in the CHM gene (c.1350-3C > G). RNA analysis demonstrated two out-of-frame transcripts, suggesting pathogenicity, without any detectable wildtype transcripts. One of the two out-of-frame transcripts is present in very low levels in healthy controls.

DISCUSSION

Mild choroideremia may result from +3 or -3 splice site variants in CHM. It is presumed that the resulting mRNA transcripts may be partly functional, thereby preventing the development of the null phenotype. Choroideremia patients with such variants may present challenges for gene therapy since there may be residual transcript activity which could result in long-lasting visual function which is atypical for this disease.

The USH3A causative gene clarin1 functions in Müller glia to maintain retinal photoreceptors

Mutations in CLRN1 cause Usher syndrome type IIIA (USH3A), an autosomal recessive disorder characterized by hearing and vision loss, and often accompanied by vestibular balance issues. The identity of the cell types responsible for the pathology and mechanisms leading to vision loss in USH3A remains elusive. To address this, we employed CRISPR/Cas9 technology to delete a large region in the coding and untranslated (UTR) region of zebrafish clrn1. Retina of clrn1 mutant larvae exhibited sensitivity to cell stress, along with age-dependent loss of function and degeneration in the photoreceptor layer. Investigation revealed disorganization in the outer retina in clrn1 mutants, including actin-based structures of the Müller glia and photoreceptor cells. To assess cell-specific contributions to USH3A pathology, we specifically re-expressed clrn1 in either Müller glia or photoreceptor cells. Müller glia re-expression of clrn1 prevented the elevated cell death observed in larval clrn1 mutant zebrafish exposed to high-intensity light. Notably, the degree of phenotypic rescue correlated with the level of Clrn1 re-expression. Surprisingly, high levels of Clrn1 expression enhanced cell death in both wild-type and clrn1 mutant animals. However, rod- or cone-specific Clrn1 re-expression did not rescue the extent of cell death. Taken together, our findings underscore three crucial insights. First, clrn1 mutant zebrafish exhibit key pathological features of USH3A; second, Clrn1 within Müller glia plays a pivotal role in photoreceptor maintenance, with its expression requiring controlled regulation; third, the reliance of photoreceptors on Müller glia suggests a structural support mechanism, possibly through direct interactions between Müller glia and photoreceptors mediated in part by Clrn1 protein.

Loss of REP1 impacts choroidal melanogenesis and vasculogenesis in choroideremia

Choroideremia (CHM) is a rare X-linked chorioretinal dystrophy affecting the photoreceptors, retinal pigment epithelium (RPE) and choroid, however, the involvement of the choroid in disease progression is not fully understood. CHM is caused by mutations in the CHM gene, encoding the ubiquitously expressed Rab escort protein 1 (REP1). REP1 plays an important role in intracellular trafficking of vesicles, including melanosomes. In this study, we examined the ultrastructure of the choroid in chmru848 fish and Chmnull/WT mouse models using transmission electron and confocal microscopy. Significant pigmentary disruptions were observed, with lack of melanosomes in the choroid of chmru848 fish from 4 days post fertilisation (4dpf), and a reduction in choroidal blood vessel diameter and interstitial pillars suggesting a defect in vasculogenesis. Total melanin and expression of melanogenesis genes tyr, tryp1a, mitf, dct and pmel were also reduced from 4dpf. In Chmnull/WT mice, choroidal melanosomes were significantly smaller at 1 month, with reduced eumelanin at 1 year. The choroid in CHM patients were also examined using spectral domain optical coherence tomography (SD-OCT) and OCT-angiography (OCT-A) and the area of preserved choriocapillaris (CC) was found to be smaller than that of overlying photoreceptors, suggesting that the choroid is degenerating at a faster rate. Histopathology of an enucleated eye from a 74-year-old CHM male patient revealed isolated areas of RPE but no associated underlying CC. Pigmentary disruptions in CHM animal models reveal an important role for REP1 in melanogenesis, and drugs that improve melanin production represent a potential novel therapeutic avenue.

The first synapse in vision in the aging mouse retina

Vision is our primary sense, and maintaining it throughout our lifespan is crucial for our well-being. However, the retina, which initiates vision, suffers from an age-related, irreversible functional decline. What causes this functional decline, and how it might be treated, is still unclear. Synapses are the functional hub for signal transmission between neurons, and studies have shown that aging is widely associated with synaptic dysfunction. In this study, we examined the first synapse of the visual system - the rod and cone photoreceptor ribbon synapse - in the mouse retina using light and electron microscopy at 2-3 months, ~1 year, and >2 years of age. We asked, whether age-related changes in key synaptic components might be a driver of synaptic dysfunction and ultimately age-related functional decline during normal aging. We found sprouting of horizontal and bipolar cells, formation of ectopic photoreceptor ribbon synapses, and a decrease in the number of rod photoreceptors and photoreceptor ribbon synapses in the aged retina. However, the majority of the photoreceptors did not show obvious changes in the structural components and protein composition of their ribbon synapses. Noteworthy is the increase in mitochondrial size in rod photoreceptor terminals in the aged retina.

Cone Photoreceptor Morphology in Choroideremia Assessed Using Non-Confocal Split-Detection Adaptive Optics Scanning Light Ophthalmoscopy

Purpose

Choroideremia (CHM) is an X-linked inherited retinal degeneration causing loss of the photoreceptors, retinal pigment epithelium, and choriocapillaris, although patients typically retain a central island of relatively preserved, functioning retina until late-stage disease. Here, we investigate cone photoreceptor morphology within the retained retinal island by examining cone inner segment area, density, circularity, and intercone space.

Methods

Using a custom-built, multimodal adaptive optics scanning light ophthalmoscope, nonconfocal split-detection images of the photoreceptor mosaic were collected at 1°, 2°, and 4° temporal to the fovea from 13 CHM and 12 control subjects. Cone centers were manually identified, and cone borders were segmented. A custom MATLAB script was used to extract area and circularity for each cone and calculate the percentage of intercone space in each region of interest. Bound cone density was also calculated. An unbalanced two-way ANOVA and Bonferroni post hoc tests were used to assess statistical differences between the CHM and control groups and along retinal eccentricity.

Results

Cone density was lower in the CHM group than in the control group (P < 0.001) and decreased with eccentricity from the fovea (P < 0.001). CHM cone inner segments were larger in area (P < 0.001) and more circular (P = 0.042) than those of the controls. Intercone space in CHM was also higher than in the controls (P < 0.001).

Conclusions

Cone morphology is altered in CHM compared to control, even within the centrally retained, functioning retinal area. Further studies are required to determine whether such morphology is a precursor to cone degeneration.

Deficits in mitochondrial TCA cycle and OXPHOS precede rod photoreceptor degeneration during chronic HIF activation

BACKGROUND

Major retinal degenerative diseases, including age-related macular degeneration, diabetic retinopathy and retinal detachment, are associated with a local decrease in oxygen availability causing the formation of hypoxic areas affecting the photoreceptor (PR) cells. Here, we addressed the underlying pathological mechanisms of PR degeneration by focusing on energy metabolism during chronic activation of hypoxia-inducible factors (HIFs) in rod PR.

METHODS

We used two-photon laser scanning microscopy (TPLSM) of genetically encoded biosensors delivered by adeno-associated viruses (AAV) to determine lactate and glucose dynamics in PR and inner retinal cells. Retinal layer-specific proteomics, in situ enzymatic assays and immunofluorescence studies were used to analyse mitochondrial metabolism in rod PRs during chronic HIF activation.

RESULTS

PRs exhibited remarkably higher glycolytic flux through the hexokinases than neurons of the inner retina. Chronic HIF activation in rods did not cause overt change in glucose dynamics but an increase in lactate production nonetheless. Furthermore, dysregulation of the oxidative phosphorylation pathway (OXPHOS) and tricarboxylic acid (TCA) cycle in rods with an activated hypoxic response decelerated cellular anabolism causing shortening of rod photoreceptor outer segments (OS) before onset of cell degeneration. Interestingly, rods with deficient OXPHOS but an intact TCA cycle did not exhibit these early signs of anabolic dysregulation and showed a slower course of degeneration.

CONCLUSION

Together, these data indicate an exceeding high glycolytic flux in rods and highlight the importance of mitochondrial metabolism and especially of the TCA cycle for PR survival in conditions of increased HIF activity.

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.

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.