Concise Review: Patient-Specific Stem Cells to Interrogate Inherited Eye Disease

iPSCs-Based Therapy for Trabecular Meshwork

The trabecular meshwork (TM) of the eye serves as an essential tissue in controlling aqueous humor (AH) outflow and intraocular pressure (IOP) homeostasis. However, dysfunctional TM cells and/or decreased TM cellularity is become a critical pathogenic cause for primary open-angle glaucoma (POAG). Consequently, it is particularly valuable to investigate TM characteristics, which, in turn, facilitates the development of new treatments for POAG. Since 2006, the advancement in induced pluripotent stem cells (iPSCs) provides a new tool to (1) model the TM in vitro and (2) regenerate degenerative TM in POAG. In this context, we first summarize the current approaches to induce the differentiation of TM-like cells from iPSCs and compare iPSC-derived TM models to the conventional in vitro TM models. The efficacy of iPSC-derived TM cells for TM regeneration in POAG models is also discussed. Through these approaches, iPSCs are becoming essential tools in glaucoma modeling and for developing personalized treatments for TM regeneration.

Characterization of a novel Pde6b-deficient rat model of retinal degeneration and treatment with adeno-associated virus (AAV) gene therapy

In humans, mutations in the beta subunit of cGMP-phosphodiesterase type 6 (PDE6B) cause autosomal recessive retinitis pigmentosa (RP), which typically has an aggressive clinical course of early-onset severe vision loss due to rapid photoreceptor degeneration. In this study, we describe the generation of a novel Pde6b-deficient rat model using CRISPR-Cas9 genome editing. We characterize the model at multiple time points using clinical imaging modalities as well as histology with immunohistochemistry to show rapid photoreceptor degeneration compared to wild-type and heterozygous animals. We describe the manufacture of two different adeno-associated viral (AAV) vectors (AAV2/1, AAV2/5) under current Good Manufacturing Practices (cGMP) and demonstrate their ability to drive human PDE6B expression in vivo. We further demonstrate the ability of AAV-mediated subretinal gene therapy to delay photoreceptor loss in Pde6b-deficient rats compared to untreated controls. However, severe progressive photoreceptor loss was noted even in treated eyes, likely due to the aggressive nature of the disease. These data provide useful preclinical data to guide the development of potential human gene therapy for PDE6B-associated RP. In addition, the rapid photoreceptor degeneration of the Pde6b-deficient rat with intact inner retina may provide a useful model for the study of cell replacement strategies.

PRPH2 mutation update: In silico assessment of 245 reported and 7 novel variants in patients with retinal disease

Mutations in PRPH2, encoding peripherin-2, are associated with the development of a wide variety of inherited retinal diseases (IRDs). To determine the causality of the many PRPH2 variants that have been discovered over the last decades, we surveyed all published PRPH2 variants up to July 2020, describing 720 index patients that in total carried 245 unique variants. In addition, we identified seven novel PRPH2 variants in eight additional index patients. The pathogenicity of all variants was determined using the ACMG guidelines. With this, 107 variants were classified as pathogenic, 92 as likely pathogenic, one as benign, and two as likely benign. The remaining 50 variants were classified as variants of uncertain significance. Interestingly, of the total 252 PRPH2 variants, more than half (n = 137) were missense variants. All variants were uploaded into the Leiden Open source Variation and ClinVar databases. Our study underscores the need for experimental assays for variants of unknown significance to improve pathogenicity classification, which would allow us to better understand genotype-phenotype correlations, and in the long-term, hopefully also support the development of therapeutic strategies for patients with PRPH2-associated IRD.

Patient derived stem cells for discovery and validation of novel pathogenic variants in inherited retinal disease

Our understanding of inherited retinal disease has benefited immensely from molecular genetic analysis over the past several decades. New technologies that allow for increasingly detailed examination of a patient's DNA have expanded the catalog of genes and specific variants that cause retinal disease. In turn, the identification of pathogenic variants has allowed the development of gene therapies and low-cost, clinically focused genetic testing. Despite this progress, a relatively large fraction (at least 20%) of patients with clinical features suggestive of an inherited retinal disease still do not have a molecular diagnosis today. Variants that are not obviously disruptive to the codon sequence of exons can be difficult to distinguish from the background of benign human genetic variations. Some of these variants exert their pathogenic effect not by altering the primary amino acid sequence, but by modulating gene expression, isoform splicing, or other transcript-level mechanisms. While not discoverable by DNA sequencing methods alone, these variants are excellent targets for studies of the retinal transcriptome. In this review, we present an overview of the current state of pathogenic variant discovery in retinal disease and identify some of the remaining barriers. We also explore the utility of new technologies, specifically patient-derived induced pluripotent stem cell (iPSC)-based modeling, in further expanding the catalog of disease-causing variants using transcriptome-focused methods. Finally, we outline bioinformatic analysis techniques that will allow this new method of variant discovery in retinal disease. As the knowledge gleaned from previous technologies is informing targets for therapies today, we believe that integrating new technologies, such as iPSC-based modeling, into the molecular diagnosis pipeline will enable a new wave of variant discovery and expanded treatment of inherited retinal disease.

The Use of Induced Pluripotent Stem Cells as a Model for Developmental Eye Disorders

Approximately one-third of childhood blindness is attributed to developmental eye disorders, of which 80% have a genetic cause. Eye morphogenesis is tightly regulated by a highly conserved network of transcription factors when disrupted by genetic mutations can result in severe ocular malformation. Human-induced pluripotent stem cells (hiPSCs) are an attractive tool to study early eye development as they are more physiologically relevant than animal models, can be patient-specific and their use does not elicit the ethical concerns associated with human embryonic stem cells. The generation of self-organizing hiPSC-derived optic cups is a major advancement to understanding mechanisms of ocular development and disease. Their development in vitro has been found to mirror that of the human eye and these early organoids have been used to effectively model microphthalmia caused by a VSX2 variant. hiPSC-derived optic cups, retina, and cornea organoids are powerful tools for future modeling of disease phenotypes and will enable a greater understanding of the pathophysiology of many other developmental eye disorders. These models will also provide an effective platform for identifying molecular therapeutic targets and for future clinical applications.

A 2-Year Longitudinal Study of Normal Cone Photoreceptor Density

Purpose

Despite the potential for adaptive optics scanning light ophthalmoscopy (AOSLO) to quantify retinal disease progression at the cellular level, there remain few longitudinal studies investigating changes in cone density as a measure of disease progression. Here, we undertook a prospective, longitudinal study to investigate the variability of cone density measurements in normal subjects during a 2-year period.

Methods

Fourteen eyes of nine subjects with no known ocular pathology were imaged both at a baseline and a 2-year follow-up visit by using confocal AOSLO at five retinal locations. Two-year affine-registered images were created to minimize the effects of intraframe distortions. Regions of interest were cropped from baseline, 2-year manually aligned, and 2-year affine-registered images. Cones were identified (graded masked) and cone density was extracted.

Results

Mean baseline cone density (cones/mm²) was 87,300, 62,200, 45,500, 28,700, and 18,200 at 190, 350, 500, 900, and 1500 μm, respectively. The mean difference (± standard deviation [SD]) in cone density from baseline to 2-year affine-registered images was 1400 (1700), 100 (1800), 300 (800), 400 (800), and 1000 (2400) cones/mm² at the same locations. The mean difference in cone density during the 2-year period was lower for affine-registered images than manually aligned images.

Conclusions

There was no meaningful change in normal cone density during a 2-year period. Intervisit variability in cone density measurements decreased when intraframe distortions between time points were minimized. This variability must be considered when planning prospective longitudinal clinical trials using changes in cone density as an outcome measure for assessing retinal disease progression.

Visual field mean deviation and relevant factors in 928 Chinese retinitis pigmentosa patients

AIM

To investigate the associations between demographic and clinical factors with the rate of visual field mean derivation (MD) decline in retinitis pigmentosa (RP) patients.

METHODS

Correlations of MDs with the visual acuity and retinal pigmentation were analyzed in 928 RP patients. MD decreasing rate in 10y and potential influences of gender, age, family history and retinal pigmentation on the rate were explored in 201 RP patients.

RESULTS

In the 928 patients, average MD and visual acuity were -14.44±8.61 dB and 0.79±0.35 respectively and when MD was lower than -9.18 dB the visual acuity would be below 1.0 (20/20). The average MD medium between eyes with or without retinal pigmentation was -14.82 dB. In 123 non-pigmented eyes, the average MD were lower than the medium but in 153 pigmented eyes it was higher than that. In the 201 patients, the average decreasing value of MD in 10 years' period was -8.01±3.66 dB and the value were correlated to retinal pigmentation but not to gender, age or RP family history.

CONCLUSION

The rate of MD decline in RP eyes is significantly related to retinal pigmentation. Our study demonstrates the quantitative rate of MD decline in RP patients and the value of MD could well reflect the severity of RP.

Defined Xeno-free and Feeder-free Culture Conditions for the Generation of Human iPSC-derived Retinal Cell Models

The production of specialized cells from pluripotent stem cells provides a powerful tool to develop new approaches for regenerative medicine. The use of human-induced pluripotent stem cells (iPSCs) is particularly attractive for neurodegenerative disease studies, including retinal dystrophies, where iPSC-derived retinal cell models mark a major step forward to understand and fight blindness. In this paper, we describe a simple and scalable protocol to generate, mature, and cryopreserve retinal organoids. Based on medium changing, the main advantage of this method is to avoid multiple and time-consuming steps commonly required in a guided differentiation of iPSCs. Mimicking the early phases of retinal development by successive changes of defined media on adherent human iPSC cultures, this protocol allows the simultaneous generation of self-forming neuroretinal structures and retinal pigmented epithelial (RPE) cells in a reproducible and efficient manner in 4 weeks. These structures containing retinal progenitor cells (RPCs) can be easily isolated for further maturation in a floating culture condition enabling the differentiation of RPCs into the seven retinal cell types present in the adult human retina. Additionally, we describe quick methods for the cryopreservation of retinal organoids and RPE cells for long-term storage. Combined together, the methods described here will be useful to produce and bank human iPSC-derived retinal cells or tissues for both basic and clinical research.

Gene Correction Reverses Ciliopathy and Photoreceptor Loss in iPSC-Derived Retinal Organoids from Retinitis Pigmentosa Patients

Retinitis pigmentosa (RP) is an irreversible, inherited retinopathy in which early-onset nyctalopia is observed. Despite the genetic heterogeneity of RP, RPGR mutations are the most common causes of this disease. Here, we generated induced pluripotent stem cells (iPSCs) from three RP patients with different frameshift mutations in the RPGR gene, which were then differentiated into retinal pigment epithelium (RPE) cells and well-structured retinal organoids possessing electrophysiological properties. We observed significant defects in photoreceptor in terms of morphology, localization, transcriptional profiling, and electrophysiological activity. Furthermore, shorted cilium was found in patient iPSCs, RPE cells, and three-dimensional retinal organoids. CRISPR-Cas9-mediated correction of RPGR mutation rescued photoreceptor structure and electrophysiological property, reversed the observed ciliopathy, and restored gene expression to a level in accordance with that in the control using transcriptome-based analysis. This study recapitulated the pathogenesis of RPGR using patient-specific organoids and achieved targeted gene therapy of RPGR mutations in a dish as proof-of-concept evidence.

•

HiPSC-derived 3D retinae with outer segments and electrophysiological properties

•

RPGR mutation results in diseased photoreceptor in patient iPSC-derived 3D retinae

•

Mutation correction rescues defects in photoreceptor morphology and electrophysiology

•

Ciliogenesis defects appear in RPGR patient-specific iPSCs, iPSC-RPE, and 3D retinae

Autologous stem cell therapy for inherited and acquired retinal disease

The mammalian retina, derived from neural ectoderm, has little regenerative potential. For conditions where irreversible retinal pigment epithelium or photoreceptor cell loss occurs, advanced techniques are required to restore vision. Inherited retinal dystrophies and some acquired conditions, such as age-related macular degeneration, have a similar end result of photoreceptor cell death leading to debilitating vision loss. These diseases stand to benefit from future regenerative medicine as dietary recommendations and current pharmacologic therapy only seek to prevent further disease progression. Cell-based strategies, such as autologously derived induced pluripotent stem cells, have come a long way in overcoming previous technical and ethical concerns. Clinical trials for such techniques are already underway. These trials and the preceding preclinical studies will be discussed in the context of retinal disease.

Non-invasive assessment of human cone photoreceptor function

Vision begins when light isomerizes the photopigments within photoreceptors. Noninvasive cellular-scale observation of the structure of the human photoreceptor mosaic is made possible through the use of adaptive optics (AO) enhanced ophthalmoscopes, but establishing noninvasive objective measures of photoreceptor function on a similar scale has been more difficult. AO ophthalmoscope images acquired with near-infrared light show that individual cone photoreceptor reflectance can change in response to a visible stimulus. Here we show that the intrinsic response depends on stimulus wavelength and intensity, and that its action spectrum is well-matched to the spectral sensitivity of cone-mediated vision. Our results demonstrate that the cone reflectance response is mediated by photoisomerization, thus making it a direct measure of photoreceptor function.

Patient-specific induced pluripotent stem cells to evaluate the pathophysiology of TRNT1-associated Retinitis pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous group of monogenic disorders characterized by progressive death of the light-sensing photoreceptor cells of the outer neural retina. We recently identified novel hypomorphic mutations in the tRNA Nucleotidyl Transferase, CCA-Adding 1 (TRNT1) gene that cause early-onset RP. To model this disease in vitro, we generated patient-specific iPSCs and iPSC-derived retinal organoids from dermal fibroblasts of patients with molecularly confirmed TRNT1-associated RP. Pluripotency was confirmed using rt-PCR, immunocytochemistry, and a TaqMan Scorecard Assay. Mutations in TRNT1 caused reduced levels of full-length TRNT1 protein and expression of a truncated smaller protein in both patient-specific iPSCs and iPSC-derived retinal organoids. Patient-specific iPSCs and iPSC-derived retinal organoids exhibited a deficit in autophagy, as evidenced by aberrant accumulation of LC3-II and elevated levels of oxidative stress. Autologous stem cell-based disease modeling will provide a platform for testing multiple avenues of treatment in patients suffering from TRNT1-associated RP.

Using induced pluripotent stem cells to understand retinal ciliopathy disease mechanisms and develop therapies

The photoreceptor cells in the retina have a highly specialised sensory cilium, the outer segment (OS), which is important for detecting light. Mutations in cilia-related genes often result in retinal degeneration. The ability to reprogramme human cells into induced pluripotent stem cells and then differentiate them into a wide range of different cell types has revolutionised our ability to study human disease. To date, however, the challenge of producing fully differentiated photoreceptors in vitro has limited the application of this technology in studying retinal degeneration. In this review, we will discuss recent advances in stem cell technology and photoreceptor differentiation. In particular, the development of photoreceptors with rudimentary OS that can be used to understand disease mechanisms and as an important model to test potential new therapies for inherited retinal ciliopathies.