Adeno-Associated Viral Gene Therapy for Inherited Retinal Disease.

Inherited retinal diseases (IRDs) are a group of rare, heterogenous eye disorders caused by gene mutations that result in degeneration of the retina. There are currently limited treatment options for IRDs; however, retinal gene therapy holds great promise for the treatment of different forms of inherited blindness. One such IRD for which gene therapy has shown positive initial results is choroideremia, a rare, X-linked degenerative disorder of the retina and choroid. Mutation of the CHM gene leads to an absence of functional Rab escort protein 1 (REP1), which causes retinal pigment epithelium cell death and photoreceptor degeneration. The condition presents in childhood as night blindness, followed by progressive constriction of visual fields, generally leading to vision loss in early adulthood and total blindness thereafter. A recently developed adeno-associated virus-2 (AAV2) vector construct encoding REP1 (AAV2-REP1) has been shown to deliver a functional version of the CHM gene into the retinal pigment epithelium and photoreceptor cells. Phase 1 and 2 studies of AAV2-REP1 in patients with choroideremia have produced encouraging results, suggesting that it is possible not only to slow or stop the decline in vision following treatment with AAV2-REP1, but also to improve visual acuity in some patients.

Functional rescue of REP1 following treatment with PTC124 and novel derivative PTC-414 in human choroideremia fibroblasts and the nonsense-mediated zebrafish model.

Choroideremia (CHM) is an X-linked chorioretinal dystrophy that is caused by mutations within a single gene, CHM Currently no effective treatment exists for these patients. Since over 30% of patients harbour nonsense mutations in CHM, nonsense suppression therapy using translational readthrough inducing drugs may provide functional rescue of REP1, thus attenuating progressive sight loss. Here, we employed two CHM model systems to systematically test the efficacy and safety of ataluren (PTC124) and its novel analog PTC-414: (1) the chmru848 zebrafish, the only nonsense mutation animal model of CHM harbouring a TAA nonsense mutation, and (2) a primary human fibroblast cell line from a CHM patient harbouring a TAG nonsense mutation. PTC124 or PTC-414 treatment of chmru848 embryos led to a ∼2.0-fold increase in survival, prevented the onset of retinal degeneration with reduced oxidative stress and apoptosis, increased rep1 protein by 23.1% (PTC124) and 17.2% (PTC-414) and restored biochemical function as confirmed through in vitro prenylation assays (98 ± 2% [PTC124] and 68 ± 5% [PTC-414]). In CHMY42X/y fibroblasts, there was a recovery of prenylation activity following treatment with either PTC124 (42 ± 5%) or PTC-414 (36 ± 11%), although an increase in REP1 protein was not detected in these cells, in contrast to the zebrafish model. This comprehensive study on the use of PTC124 and PTC-414 as successful nonsense suppression agents for the treatment of CHM highlights the translational potential of these drugs for inherited retinal disease.

MULTIMODAL IMAGING OF MOSAIC RETINOPATHY IN CARRIERS OF HEREDITARY X-LINKED RECESSIVE DISEASES.

PURPOSE: To investigate the clinical features in carriers of X-linked retinitis pigmentosa, X-linked ocular albinism, and choroideremia (CHM) using multimodal imaging and to assess their diagnostic value in these three mosaic retinopathies. METHODS: We prospectively examined 14 carriers of 3 X-linked recessive disorders (X-linked retinitis pigmentosa, X-linked ocular albinism, and CHM). Details of abnormalities of retinal morphology were evaluated using fundus photography, fundus autofluorescence (FAF) imaging, and spectral domain optical coherence tomography. RESULTS: In six X-linked retinitis pigmentosa carriers, fundus appearance varied from unremarkable to the presence of tapetal-like reflex and pigmentary changes. On FAF imaging, all carriers exhibited a bright radial reflex against a dark background. By spectral domain optical coherence tomography, loss of the ellipsoid zone in the macula was observed in 3 carriers (50%). Regarding the retinal laminar architecture, 4 carriers (66.7%) showed thinning of the outer nuclear layer and a dentate appearance of the outer plexiform layer. All five X-linked ocular albinism carriers showed a characteristic mud-splatter patterned fundus, dark radial streaks against a bright background on FAF imaging, and a normal-appearing retinal structure by spectral domain optical coherence tomography imaging. Two of the 3 CHM carriers (66.7%) showed a diffuse moth-eaten appearance of the fundus, and all 3 showed irregular hyper-FAF and hypo-FAF spots throughout the affected area. In the CHM carriers, the structural changes observed by spectral domain optical coherence tomography imaging were variable. CONCLUSION: Our findings in an Asian cohort suggest that FAF imaging is a practical diagnostic test for differentiating X-linked retinitis pigmentosa, X-linked ocular albinism, and CHM carriers. Wide-field FAF is an easy and helpful adjunct to testing for the correct diagnosis and identification of lyonization in carriers of these three mosaic retinopathies.

Quantitative optical coherence tomography angiography of vascular abnormalities in the living human eye.

Retinal vascular diseases are important causes of vision loss. A detailed evaluation of the vascular abnormalities facilitates diagnosis and treatment in these diseases. Optical coherence tomography (OCT) angiography using the highly efficient split-spectrum amplitude decorrelation angiography algorithm offers an alternative to conventional dye-based retinal angiography. OCT angiography has several advantages, including 3D visualization of retinal and choroidal circulations (including the choriocapillaris) and avoidance of dye injection-related complications. Results from six illustrative cases are reported. In diabetic retinopathy, OCT angiography can detect neovascularization and quantify ischemia. In age-related macular degeneration, choroidal neovascularization can be observed without the obscuration of details caused by dye leakage in conventional angiography. Choriocapillaris dysfunction can be detected in the nonneovascular form of the disease, furthering our understanding of pathogenesis. In choroideremia, OCT's ability to show choroidal and retinal vascular dysfunction separately may be valuable in predicting progression and assessing treatment response. OCT angiography shows promise as a noninvasive alternative to dye-based angiography for highly detailed, in vivo, 3D, quantitative evaluation of retinal vascular abnormalities.

Single-base substitutions in the CHM promoter as a cause of choroideremia.

Although over 150 unique mutations affecting the coding sequence of CHM have been identified in patients with the X-linked chorioretinal disease choroideremia (CHM), no regulatory mutations have been reported, and indeed the promoter has not been defined. Here, we describe two independent families affected by CHM bearing a mutation outside the gene's coding region at position c.-98: C>A and C>T, which segregated with the disease. The male proband of family 1 was found to lack CHM mRNA and its gene product Rab escort protein 1, whereas whole-genome sequencing of an affected male in family 2 excluded the involvement of any other known retinal genes. Both mutations abrogated luciferase activity when inserted into a reporter construct, and by further employing the luciferase reporter system to assay sequences 5' to the gene, we identified the CHM promoter as the region encompassing nucleotides c.-119 to c.-76. These findings suggest that the CHM promoter region should be examined in patients with CHM who lack coding sequence mutations, and reveals, for the first time, features of the gene's regulation.

Clinical and genetic studies in a family with a new splice-site mutation in the choroideremia gene.

PURPOSE: To describe the clinical and molecular findings of an Italian family with a new mutation in the choroideremia (CHM) gene. METHODS: We performed a comprehensive ophthalmologic examination, fundus photography, macular optical coherence tomography, perimetry, electroretinography, and fluorescein angiography in an Italian family. The clinical diagnosis was supported by western blot analysis of lymphoblastoid cell lines from patients with CHM and carriers, using a monoclonal antibody against the 415 C-terminal amino acids of Rab escort protein-1 (REP-1). Sequencing of the CHM gene was undertaken on genomic DNA from affected men and carriers; the RNA transcript was analyzed with reverse transcriptase-PCR. RESULTS: The affected men showed a variability in the rate of visual change and in the degree of clinical and functional ophthalmologic involvement, mainly age-related, while the women displayed aspecific areas of chorioretinal degeneration. Western blot did not show a detectable amount of normal REP-1 protein in affected men who were hemizygous for a novel mutation, c.819+2T>A at the donor splicing site of intron 6 of the CHM gene; the mutation was confirmed in heterozygosity in the carriers. CONCLUSIONS: Western blot of the REP-1 protein confirmed the clinical diagnosis, and molecular analysis showed the new in-frame mutation, c.819+2T>A, leading to loss of function of the REP-1 protein. These results emphasize the value of a diagnostic approach that correlates genetic and ophthalmologic data for identifying carriers in families with CHM. An early diagnosis might be crucial for genetic counseling of this type of progressive and still untreatable disease.

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.

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.

Reduced Retinal Pigment Epithelial Autophagy Due to Loss of Rab12 Prenylation in a Human iPSC-RPE Model of Choroideremia.

Choroideremia is an X-linked chorioretinal dystrophy caused by mutations in CHM, encoding Rab escort protein 1 (REP-1), leading to under-prenylation of Rab GTPases (Rabs). Despite ubiquitous expression of CHM, the phenotype is limited to degeneration of the retina, retinal pigment epithelium (RPE), and choroid, with evidence for primary pathology in RPE cells. However, the spectrum of under-prenylated Rabs in RPE cells and how they contribute to RPE dysfunction remain unknown. A CRISPR/Cas-9-edited CHM-/- iPSC-RPE model was generated with isogenic control cells. Unprenylated Rabs were biotinylated in vitro and identified by tandem mass tag (TMT) spectrometry. Rab12 was one of the least prenylated and has an established role in suppressing mTORC1 signaling and promoting autophagy. CHM-/- iPSC-RPE cells demonstrated increased mTORC1 signaling and reduced autophagic flux, consistent with Rab12 dysfunction. Autophagic flux was rescued in CHM-/- cells by transduction with gene replacement (ShH10-CMV-CHM) and was reduced in control cells by siRNA knockdown of Rab12. This study supports Rab12 under-prenylation as an important cause of RPE cell dysfunction in choroideremia and highlights increased mTORC1 and reduced autophagy as potential disease pathways for further investigation.

Outer retinal and choriocapillaris modifications in choroideremia: three differentially impaired retinal regions and the potential diagnostic role of the external limiting membrane.

BACKGROUND: The external limiting membrane (ELM) is formed by the apical processes of Müller cells attached to the inner segments of the photoreceptor cells. Both cells are implicated in the pathogenesis of choroideremia (CHM). The purpose of this study was to explore the diagnostic role of ELM in CHM. METHODS: The study was designed as observational case series. Sixteen CHM eyes were examined by multimodal imaging and were compared to healthy controls. The main outcome was the measurement of ELM thickness and reflectivity over the follow-up, and its relationship with other multimodal imaging quantitative parameters. RESULTS: Baseline ELM was characterized by 11 ± 1 µm of thickness and 0.68 ± 0.13 of reflectivity, resulting 8 ± 1 µm (p < 0.01) and 0.65 ± 0.14 (p > 0.05) at the last follow-up. Choriocapillaris (CC) analysis revealed 3 regions. The first was characterized by normal vessel density (VD). The second surrounding the partially preserved islet, showing significantly lower baseline VD and undergoing minor changes over the follow-up. The third was localized in the partially preserved islet, showing significantly lower VD at baseline, and resulted atrophic at the last follow-up. ELM reflectivity and ELM thickness correlated both with outer retinal atrophy progression and the CC status. CONCLUSIONS: ELM may be considered a useful imaging biomarker in CHM. Its assessment confirmed a primary role of Müller cells impairment in the pathogenesis of CHM.

Clinical and Genetic Findings in Korean Patients with Choroideremia.

PURPOSE: We share and analyze the clinical presentations and genotypes of Korean male patients and female carriers who visited our clinic. METHODS: Six male patients and three female carriers with comprehensive ophthalmic examinations and next-generation sequencing were included. Detailed clinical features were identified using visual field (VF) test and multimodal imaging including color fundus photography, fundus autofluorescence (FAF), and optical coherence tomography (OCT). RESULTS: Six male patients were diagnosed with choroideremia at the median age of 25 years. Before genetic testing, three patients (50.0%) were clinically diagnosed with choroideremia, while the other three patients (50.0%) with retinitis pigmentosa. Patients showed different types of hemizygous CHM variants, including two nonsense variants, c.715C>T:p.(Arg239*) and c.799C>T:p.(Arg267*); two frameshift variants, c.1584_1587del:p.(Val529Hisfs*7) and c.403_404del:p.(Asp135Phefs*9); one splicing variant c.1511-28_1511-2del; and one exon 2-8 duplication. The latter three variants were novel. Two female carriers had heterozygous exon 2-8 duplication and the other one female carrier had heterozygous nonsense variant c.715C>T:p. (Arg239*). Fundus showed diffuse yellow-whitish scleral reflex and granular pigmented lesions. FAF showed multiple patchy hypofluorescence lesions, sparing macula. OCT showed thinning of outer nuclear layer, ellipsoid zone, retinal pigment epithelium layer, choroid thickness, interlaminar bridges, outer retinal tubulations, and microcysts in the inner nuclear layer. VF showed ring scotoma pattern with small amount of remaining central field. Asymptomatic female carriers showed variable fundus findings and mild changes in OCT. CONCLUSIONS: A detailed description of the genotypes with three novel mutations and phenotypes of six choroideremia patients and three CHM mutation female carriers are presented.

Genetic and phenotypic characteristics of three Mainland Chinese families with choroideremia.

PURPOSE: To describe the phenotype and genotype of three Mainland Chinese families affected by choroideremia (CHM). METHODS: Complete ophthalmic examinations were conducted in three unrelated Chinese families with CHM. Peripheral blood samples were collected from the families for genetic and immunoblot analysis. All exons and flanking intronic regions of the gene encoding Rab escort protein-1 (Rep-1) were amplified with PCR and screened for mutations with Sanger sequencing. The three-dimensional structure of mutated Rep-1 was modeled using sequence homology with rat proteins to analyze the effect of the mutation detected in one family. RESULTS: All affected males had characteristic signs and symptoms of CHM; however, central visual acuity impairment occurred earlier than expected. All female carriers older than 45 years had pigmentary changes, and one female carrier was symptomatic with vision loss. Three different mutations in Rep-1, c.1801-1G>A, c.1130 T>A, and c.612delAG, were detected in the three families. CONCLUSIONS: In Mainland Chinese families, the central visual acuity of male patients with CHM can be affected at an early age (second decade), whereas female CHM carriers may manifest signs and symptoms at a later age (≥ 45 years). One previously reported and two novel Rep-1 mutations were detected in three Chinese patients with CHM.

A generation of human-induced pluripotent stem cell line (MUi032-A) from a Choroideremia disease patient carrying a hemizygous mutation on the CHM gene.

Choroideremia (CHM) is a monogenic, X-linked inherited retinal disease caused by mutations in the CHM gene. CHM patients develop progressive loss of vision due to degeneration of cell layers in the retina. In this report, the human-induced pluripotent stem cell, MUi032-A, was generated from CD34+ hematopoietic stem/progenitor cells of a male CHM patient by co-electroporation of non-integration episomal vectors containing OCT4/shp53, Sox-2/KLF4, and L-MYC/LIN-28. The MUi032-A showed normal karyotype and a hemizygous c.715C > T mutation. They expressed pluripotency markers and differentiated into cells derived from three germ layers. This cell line may be useful for disease mechanisms and gene therapy studies.

A UNILATERAL FOVEAL VITELLIFORM LESION IN A CHOROIDEREMIA CARRIER.

PURPOSE: To describe a unilateral foveal vitelliform lesion associated with subnormal visual acuity in a choroideremia carrier. METHODS: A retrospective case report, assessment of the best-corrected visual acuity, fundus photography, wide-angle scanning laser ophthalmoscopy, optical coherence tomography, and microperimetry. RESULTS: A 37-year-old woman with a pathogenic 907C>T mutation in the choroideremia gene encoding Rab escort protein-1 presented with blurred vision in her left eye.The Snellen best-corrected visual acuity was 20/20 in the right eye and 20/32 in the left eye, a unilateral decrease because it was 20/20 in both eyes at the most recent examination nine years earlier. In the left eye, a large vitelliform lesion with a diameter of 1,300 µ m had developed in the fovea, whereas in the right eye, a smaller similar lesion was seen close to the fovea. Both eyes showed classical radial patterns of multiple bright fundus patches with associated autofluorescence defects and focal drusenoid lesions of the outer retina. CONCLUSION: With its large size and foveal location the vitelliform lesion in this patient's left eye is an unusual manifestation in an otherwise common Rab escort protein-1 mutation carrier state, and its unilaterality fits the assumption of random X-chromosome inactivation.

PERIPHERAL OPTICAL COHERENCE TOMOGRAPHY FINDINGS IN A CHOROIDEREMIA CARRIER.

PURPOSE: To describe the peripheral optical coherence tomography findings in a female choroideremia carrier. METHODS: A 56-year-old woman was referred for visual disturbance complaining of some occasional photopsias and increasing difficulty with her vision at night in both eyes. Best-corrected visual acuity was 20/20 in the right eye and 20/150 in the left eye. Fundus examination revealed mildly tilted disks and peripapillary atrophy with subtle retinal pigment epithelial changes in the periphery. RESULTS: Macular optical coherence tomography in the right eye appeared unremarkable, but the in the left eye, there was diffuse ellipsoid zone band disruption. Green-light fundus autofluorescence revealed mottled areas of decreased autofluorescence in the mid and far periphery creating an irregular mosaic pattern. Peripheral optical coherence tomography scans revealed more diffuse ellipsoid zone alterations than were apparent on the fundus autofluorescence imaging. Genetic testing revealed a heterozygous pathogenic variant in the CHM gene (c.715C>T, p.Arg239). An additional heterozygous mutation was noted in the CNGB1 gene (c.290+2T>C, splice donor). CONCLUSION: Choroideremia carriers may manifest widespread photoreceptor alterations, which may be more extensive than apparent on fundus autofluorescence imaging.

Automated Assessment of Photoreceptor Visibility in Adaptive Optics Split-Detection Images Using Edge Detection.

Purpose: Adaptive optics scanning laser ophthalmoscopy (AOSLO) is a high-resolution imaging modality that allows measurements of cellular-level retinal changes in living patients. In retinal diseases, the visibility of photoreceptors in AOSLO images is affected by pathology, patient motion, and optics, which can lead to variability in analyses of the photoreceptor mosaic. Current best practice for AOSLO mosaic quantification requires manual assessment of photoreceptor visibility across overlapping images, a laborious and time-consuming task. Methods: We propose an automated measure for quantification of photoreceptor visibility in AOSLO. Our method detects salient edge features, which can represent visible photoreceptor boundaries in each image. We evaluate our measure against two human graders and two standard automated image quality assessment algorithms. Results: We evaluate the accuracy of pairwise ordering (PO) and the correlation of ordinal rankings (ORs) of photoreceptor visibility in 29 retinal regions, taken from five subjects with choroideremia. The proposed measure had high association with manual assessments (Grader 1: PO = 0.71, OR = 0.61; Grader 2: PO = 0.67, OR = 0.62), which is comparable with intergrader reliability (PO = 0.76, OR = 0.75) and outperforms the top standard approach (PO = 0.57; OR = 0.46). Conclusions: Our edge-based measure can automatically assess photoreceptor visibility and order overlapping images within AOSLO montages. This can significantly reduce the manual labor required to generate high-quality AOSLO montages and enables higher throughput for quantitative studies of photoreceptors. Translational Relevance: Automated assessment of photoreceptor visibility allows us to more rapidly quantify photoreceptor morphology in the living eye. This has applications to ophthalmic medicine by allowing detailed characterization of retinal degenerations, thus yielding potential biomarkers of treatment safety and efficacy.

Choroideremia Carriers: Dark-Adapted Perimetry and Retinal Structures.

Purpose: In choroideremia (CHM) carriers, scotopic sensitivity was assessed by dark adapted chromatic perimetry (DACP) and outer retinal structure was evaluated by multimodal imaging. Methods: Nine carriers (18 eyes) and 13 healthy controls (13 eyes) underwent DACP testing with cyan and red stimuli. Analysis addressed peripapillary (4 test locations closest to the optic disc), macular (52 locations), and peripheral (60 locations outside the macula) regions. Responses were considered to be rod-mediated when cyan relative to red sensitivity was >5 dB. Fundus imaging included spectral domain optical coherence tomography (SD-OCT), short-wavelength (SW-AF), near-infrared (NIR-AF), ultrawide-field (200 degrees) pseudocolor fundus imaging, and quantitative (qAF) fundus autofluorescence. Results: Detection of the cyan stimulus was rod mediated in essentially all test locations (99.7%). In the macular and peripheral areas, DACP sensitivity values were not significantly different from healthy eyes. In the peripapillary area, sensitivities were significantly decreased (P < 0.05). SD-OCT imaging ranged from hyper-reflective lesions and discontinuities of the outer retinal bands to hypertransmission of signal. SW-AF and NIR-AF images presented with peripapillary atrophy in seven patients (14 eyes). Mosaicism was detectable in SW-AF images in seven patients and in NIR-AF images in five patients. Frank hypo-autofluorescence was visible in eight patients with distinct chorioretinopathy in seven patients. The qAF values were below the 95% confidence interval (CI) of healthy age-matched individuals in 12 eyes. Conclusions: Rod mediated scotopic sensitivity was comparable to that in control eyes in macular and peripheral areas but was decreased in the peripapillary area where changes in retinal structure were also most severe.

Widespread subclinical cellular changes revealed across a neural-epithelial-vascular complex in choroideremia using adaptive optics.

Choroideremia is an X-linked, blinding retinal degeneration with progressive loss of photoreceptors, retinal pigment epithelial (RPE) cells, and choriocapillaris. To study the extent to which these layers are disrupted in affected males and female carriers, we performed multimodal adaptive optics imaging to better visualize the in vivo pathogenesis of choroideremia in the living human eye. We demonstrate the presence of subclinical, widespread enlarged RPE cells present in all subjects imaged. In the fovea, the last area to be affected in choroideremia, we found greater disruption to the RPE than to either the photoreceptor or choriocapillaris layers. The unexpected finding of patches of photoreceptors that were fluorescently-labeled, but structurally and functionally normal, suggests that the RPE blood barrier function may be altered in choroideremia. Finally, we introduce a strategy for detecting enlarged cells using conventional ophthalmic imaging instrumentation. These findings establish that there is subclinical polymegathism of RPE cells in choroideremia.

An In Silica Model for RPE Loss Patterns in Choroideremia.

Purpose: To use empirical data to develop a model of cell loss in choroideremia that predicts the known exponential rate of RPE loss and central, scalloped preservation pattern seen in this disease. Methods: A computational model of RPE loss was created in Python 3.7, which constructed an array of RPE cells clusters, binarized as either live or atrophic. Two rules were applied to this model: the background effect gave each cell a chance of dying defined by a background function, and the neighbor effect increased the chance of RPE cell death if a neighbor were dead. The known anatomic distribution of rods, RPE, choriocapillaris density, amacrine, ganglion, and cone cells were derived from the literature and applied to this model. Atrophy growth rates were measured over arbitrary time units and fit to the known exponential decay model. The main outcome measures: included topography of atrophy over time and fit of simulated residual RPE area to exponential decay. Results: A background effect alone can simulate exponential decay, but does not simulate the central island preservation seen in choroideremia. An additive neighbor effect alone does not simulate exponential decay. When the neighbor effect multiplies the background effect using the rod density function, our model follows an exponential decay, similar to previous observations. Also, our model predicts a residual island of RPE that resembles the topographic distribution of residual RPE seen in choroideremia. Conclusions: The pattern of RPE loss in choroideremia can be predicted by applying simple rules. The RPE preservation pattern typically seen in choroideremia may be related to the underlying pattern of rod density. Further studies are needed to validate these findings.