Restoration of RPGR expression in vivo using CRISPR/Cas9 gene editing

RPGR is a guanine nucleotide exchange factor for the small GTPase RAB37 required for retinal function via autophagy regulation

Although the small GTPase RAB37 acts as an organizer of autophagosome biogenesis, the upstream regulatory mechanism of autophagy via guanosine diphosphate (GDP)-guanosine triphosphate (GTP) exchange in maintaining retinal function has not been determined. We found that retinitis pigmentosa GTPase regulator (RPGR) is a guanine nucleotide exchange factor that activates RAB37 by accelerating GDP-to-GTP exchange. RPGR directly interacts with RAB37 via the RPGR-RCC1-like domain to promote autophagy through stimulating exchange. Rpgr knockout (KO) in mice leads to photoreceptor degeneration owing to autophagy impairment in the retina. Notably, the retinopathy phenotypes of Rpgr KO retinas are rescued by the adeno-associated virus-mediated transfer of pre-trans-splicing molecules, which produce normal Rpgr mRNAs via trans-splicing in the Rpgr KO retinas. This rescue upregulates autophagy through the re-expression of RPGR in KO retinas to accelerate GDP-to-GTP exchange; thus, retinal homeostasis reverts to normal. Taken together, these findings provide an important missing link for coordinating RAB37 GDP-GTP exchange via the RPGR and retinal homeostasis by autophagy regulation.

AAV-RPGR Gene Therapy Rescues Opsin Mislocalisation in a Human Retinal Organoid Model of RPGR-Associated X-Linked Retinitis Pigmentosa

Variants within the Retinitis Pigmentosa GTPase regulator (RPGR) gene are the predominant cause of X-Linked Retinitis Pigmentosa (XLRP), a common and severe form of inherited retinal disease. XLRP is characterised by the progressive degeneration and loss of photoreceptors, leading to visual loss and, ultimately, bilateral blindness. Unfortunately, there are no effective approved treatments for RPGR-associated XLRP. We sought to investigate the efficacy of RPGRORF15 gene supplementation using a clinically relevant construct in human RPGR-deficient retinal organoids (ROs). Isogenic RPGR knockout (KO)-induced pluripotent stem cells (IPSCs) were generated using established CRISPR/Cas9 gene editing methods targeting RPGR. RPGR-KO and isogenic wild-type IPSCs were differentiated into ROs and utilised to test the adeno associated virus (AAV) RPGR (AAV-RPGR) clinical vector construct. The transduction of RPGR-KO ROs using AAV-RPGR successfully restored RPGR mRNA and protein expression and localisation to the photoreceptor connecting cilium in rod and cone photoreceptors. Vector-derived RPGR demonstrated equivalent levels of glutamylation to WT ROs. In addition, treatment with AAV-RPGR restored rhodopsin localisation within RPGR-KO ROs, reducing mislocalisation to the photoreceptor outer nuclear layer. These data provide mechanistic insights into RPGRORF15 gene supplementation functional potency in human photoreceptor cells and support the previously reported Phase I/II trial positive results using this vector construct in patients with RPGR-associated XLRP, which is currently being tested in a Phase III clinical trial.

Retinitis Pigmentosa: From Pathomolecular Mechanisms to Therapeutic Strategies

Retinitis pigmentosa is an inherited disease, in which mutations in different types of genes lead to the death of photoreceptors and the loss of visual function. Although retinitis pigmentosa is the most common type of inherited retinal dystrophy, a clear line of therapy has not yet been defined. In this review, we will focus on the therapeutic aspect and attempt to define the advantages and disadvantages of the protocols of different therapies. The role of some therapies, such as antioxidant agents or gene therapy, has been established for years now. Many clinical trials on different genes and mutations causing RP have been conducted, and the approval of voretigene nepavorec by the FDA has been an important step forward. Nonetheless, even if gene therapy is the most promising type of treatment for these patients, other innovative strategies, such as stem cell transplantation or hyperbaric oxygen therapy, have been shown to be safe and improve visual quality during clinical trials. The treatment of this disease remains a challenge, to which we hope to find a solution as soon as possible.

Therapeutic applications of CRISPR/Cas9 gene editing technology for the treatment of ocular diseases

Ocular diseases are a highly heterogeneous group of phenotypes, caused by a spectrum of genetic variants and environmental factors that exhibit diverse clinical symptoms. As a result of its anatomical location, structure and immune privilege, the eye is an ideal system to assess and validate novel genetic therapies. Advances in genome editing have revolutionized the field of biomedical science, enabling researchers to understand the biology behind disease mechanisms and allow the treatment of several health conditions, including ocular pathologies. The advent of clustered regularly interspaced short palindromic repeats (CRISPR)-based gene editing facilitates efficient and specific genetic modifications in the nucleic acid sequence, resulting in permanent changes at the genomic level. This approach has advantages over other treatment strategies and is promising for the treatment of various genetic and non-genetic ocular conditions. This review provides an overview of the CRISPR/CRISPR-associated protein 9 (Cas9) system and summarizes recent advances in the therapeutic application of CRISPR/Cas9 for the treatment of various ocular pathologies, as well as future challenges.

What did CRISPR-Cas9 accomplish in its first 10 years?

It's been 10 years now from the debut of clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9) era in which gene engineering has never been so accessible, precise and efficient. This technology, like a refined surgical procedure, has offered the ability of removing different types of disease causing mutations and restoring key proteins activity with ease of outperforming the previous resembling methods: zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). Additionally, CRISPR-Cas9 systems can systematically introduce genetic sequences to the specific sites in the human genome allowing to stimulate desired functions such as anti-tumoral and anti-infectious faculties. The present brief review provides an updated resume of CRISPR-Cas9's top achievements from its first appearance to the current date focusing on the breakthrough research including in vitro, in vivo and human studies. This enables the evaluation of the previous phase 'the proof-of-concept phase' and marks the beginning of the next phase which will probably bring a spate of clinical trials.

Revisiting Retinal Degeneration Hallmarks: Insights from Molecular Markers and Therapy Perspectives

Visual impairment and blindness are a growing public health problem as they reduce the life quality of millions of people. The management and treatment of these diseases represent scientific and therapeutic challenges because different cellular and molecular actors involved in the pathophysiology are still being identified. Visual system components, particularly retinal cells, are extremely sensitive to genetic or metabolic alterations, and immune responses activated by local insults contribute to biological events, culminating in vision loss and irreversible blindness. Several ocular diseases are linked to retinal cell loss, and some of them, such as retinitis pigmentosa, age-related macular degeneration, glaucoma, and diabetic retinopathy, are characterized by pathophysiological hallmarks that represent possibilities to study and develop novel treatments for retinal cell degeneration. Here, we present a compilation of revisited information on retinal degeneration, including pathophysiological and molecular features and biochemical hallmarks, and possible research directions for novel treatments to assist as a guide for innovative research. The knowledge expansion upon the mechanistic bases of the pathobiology of eye diseases, including information on complex interactions of genetic predisposition, chronic inflammation, and environmental and aging-related factors, will prompt the identification of new therapeutic strategies.

Genome editing in the treatment of ocular diseases

Genome-editing technologies have ushered in a new era in gene therapy, providing novel therapeutic strategies for a wide range of diseases, including both genetic and nongenetic ocular diseases. These technologies offer new hope for patients suffering from previously untreatable conditions. The unique anatomical and physiological features of the eye, including its immune-privileged status, size, and compartmentalized structure, provide an optimal environment for the application of these cutting-edge technologies. Moreover, the development of various delivery methods has facilitated the efficient and targeted administration of genome engineering tools designed to correct specific ocular tissues. Additionally, advancements in noninvasive ocular imaging techniques and electroretinography have enabled real-time monitoring of therapeutic efficacy and safety. Herein, we discuss the discovery and development of genome-editing technologies, their application to ocular diseases from the anterior segment to the posterior segment, current limitations encountered in translating these technologies into clinical practice, and ongoing research endeavors aimed at overcoming these challenges.

Nanotechnology-based ocular drug delivery systems: recent advances and future prospects

Ocular drug delivery has constantly challenged ophthalmologists and drug delivery scientists due to various anatomical and physiological barriers. Static and dynamic ocular barriers prevent the entry of exogenous substances and impede therapeutic agents' active absorption. This review elaborates on the anatomy of the eye and the associated constraints. Followed by an illustration of some common ocular diseases, including glaucoma and their current clinical therapies, emphasizing the significance of drug therapy in treating ocular diseases. Subsequently, advances in ocular drug delivery modalities, especially nanotechnology-based ocular drug delivery systems, are recommended, and some typical research is highlighted. Based on the related research, systematic and comprehensive characterizations of the nanocarriers are summarized, hoping to assist with future research. Besides, we summarize the nanotechnology-based ophthalmic drugs currently on the market or still in clinical trials and the recent patents of nanocarriers. Finally, inspired by current trends and therapeutic concepts, we provide an insight into the challenges faced by novel ocular drug delivery systems and further put forward directions for future research. We hope this review can provide inspiration and motivation for better design and development of novel ophthalmic formulations.

CRISPR-Cas System: The Current and Emerging Translational Landscape

CRISPR-Cas technology has rapidly changed life science research and human medicine. The ability to add, remove, or edit human DNA sequences has transformative potential for treating congenital and acquired human diseases. The timely maturation of the cell and gene therapy ecosystem and its seamless integration with CRISPR-Cas technologies has enabled the development of therapies that could potentially cure not only monogenic diseases such as sickle cell anemia and muscular dystrophy, but also complex heterogenous diseases such as cancer and diabetes. Here, we review the current landscape of clinical trials involving the use of various CRISPR-Cas systems as therapeutics for human diseases, discuss challenges, and explore new CRISPR-Cas-based tools such as base editing, prime editing, CRISPR-based transcriptional regulation, CRISPR-based epigenome editing, and RNA editing, each promising new functionality and broadening therapeutic potential. Finally, we discuss how the CRISPR-Cas system is being used to understand the biology of human diseases through the generation of large animal disease models used for preclinical testing of emerging therapeutics.

Retinitis Pigmentosa: Novel Therapeutic Targets and Drug Development

Retinitis pigmentosa (RP) is a heterogeneous group of hereditary diseases characterized by progressive degeneration of retinal photoreceptors leading to progressive visual decline. It is the most common type of inherited retinal dystrophy and has a high burden on both patients and society. This condition causes gradual loss of vision, with its typical manifestations including nyctalopia, concentric visual field loss, and ultimately bilateral central vision loss. It is one of the leading causes of visual disability and blindness in people under 60 years old and affects over 1.5 million people worldwide. There is currently no curative treatment for people with RP, and only a small group of patients with confirmed RPE65 mutations are eligible to receive the only gene therapy on the market: voretigene neparvovec. The current therapeutic armamentarium is limited to retinoids, vitamin A supplements, protection from sunlight, visual aids, and medical and surgical interventions to treat ophthalmic comorbidities, which only aim to slow down the progression of the disease. Considering such a limited therapeutic landscape, there is an urgent need for developing new and individualized therapeutic modalities targeting retinal degeneration. Although the heterogeneity of gene mutations involved in RP makes its target treatment development difficult, recent fundamental studies showed promising progress in elucidation of the photoreceptor degeneration mechanism. The discovery of novel molecule therapeutics that can selectively target specific receptors or specific pathways will serve as a solid foundation for advanced drug development. This article is a review of recent progress in novel treatment of RP focusing on preclinical stage fundamental research on molecular targets, which will serve as a starting point for advanced drug development. We will review the alterations in the molecular pathways involved in the development of RP, mainly those regarding endoplasmic reticulum (ER) stress and apoptotic pathways, maintenance of the redox balance, and genomic stability. We will then discuss the therapeutic approaches under development, such as gene and cell therapy, as well as the recent literature identifying novel potential drug targets for RP.

Recent Advances in Genome-Engineering Strategies

In October 2020, the chemistry Nobel Prize was awarded to Emmanuelle Charpentier and Jennifer A. Doudna for the discovery of a new promising genome-editing tool: the genetic scissors of CRISPR-Cas9. The identification of CRISPR arrays and the subsequent identification of cas genes, which together represent an adaptive immunological system that exists not only in bacteria but also in archaea, led to the development of diverse strategies used for precise DNA editing, providing new insights in basic research and in clinical practice. Due to their advantageous features, the CRISPR-Cas systems are already employed in several biological and medical research fields as the most suitable technique for genome engineering. In this review, we aim to describe the CRISPR-Cas systems that have been identified among prokaryotic organisms and engineered for genome manipulation studies. Furthermore, a comprehensive comparison between the innovative CRISPR-Cas methodology and the previously utilized ZFN and TALEN editing nucleases is also discussed. Ultimately, we highlight the contribution of CRISPR-Cas methodology in modern biomedicine and the current plethora of available applications for gene KO, repression and/or overexpression, as well as their potential implementation in therapeutical strategies that aim to improve patients' quality of life.

In vivo delivery of CRISPR-Cas9 genome editing components for therapeutic applications

Since its mechanism discovery in 2012 and the first application for mammalian genome editing in 2013, CRISPR-Cas9 has revolutionized the genome engineering field and created countless opportunities in both basic science and translational medicine. The first clinical trial of CRISPR therapeutics was initiated in 2016, which employed ex vivo CRISPR-Cas9 edited PD-1 knockout T cells for the treatment of non-small cell lung cancer. So far there have been dozens of clinical trials registered on ClinicalTrials.gov in regard to using the CRISPR-Cas9 genome editing as the main intervention for therapeutic applications; however, most of these studies use ex vivo genome editing approach, and only a few apply the in vivo editing strategy. Compared to ex vivo editing, in vivo genome editing bypasses tedious procedures related to cell isolation, maintenance, selection, and transplantation. It is also applicable to a wide range of diseases and disorders. The main obstacles to the successful translation of in vivo therapeutic genome editing include the lack of safe and efficient delivery system and safety concerns resulting from the off-target effects. In this review, we highlight the therapeutic applications of in vivo genome editing mediated by the CRISPR-Cas9 system. Following a brief introduction of the history, biology, and functionality of CRISPR-Cas9, we showcase a series of exemplary studies in regard to the design and implementation of in vivo genome editing systems that target the brain, inner ear, eye, heart, liver, lung, muscle, skin, immune system, and tumor. Current challenges and opportunities in the field of CRISPR-enabled therapeutic in vivo genome editing are also discussed.

X-Linked Retinitis Pigmentosa Gene Therapy: Preclinical Aspects

The most common inherited eye disease is retinitis pigmentosa (RP). X-linked RP (XLRP) is one of the most severe types of RP, with a considerable disease burden. Patients with XLRP experience a decrease in their vision and become blind in their 4th decade of life, causing much morbidity after starting a rather normal life. Treatment of XLRP remains challenging, and current treatments are not effective enough in restoring vision. Gene therapy of XLRP, capable of restoring the functional RPGR gene, showed promising results in preclinical studies and clinical trials; however, to date, no approved product has entered the market. The development of a gene therapy product needs through preliminary assessment of the drug in animal models before administration to humans. In this article, we reviewed the genetic pathology of XLRP, along with the preclinical aspects of the XLRP gene therapy, animal models, associated assessments, and future challenges and directions.

In vivo application of base and prime editing to treat inherited retinal diseases

Inherited retinal diseases (IRDs) are vision-threatening retinal disorders caused by pathogenic variants of genes related to visual functions. Genomic analyses in patients with IRDs have revealed pathogenic variants which affect vision. However, treatment options for IRDs are limited to nutritional supplements regardless of genetic variants or gene-targeting approaches based on antisense oligonucleotides and adeno-associated virus vectors limited to targeting few genes. Genome editing, particularly that involving clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 technologies, can correct pathogenic variants and provide additional treatment opportunities. Recently developed base and prime editing platforms based on CRISPR-Cas9 technologies are promising for therapeutic genome editing because they do not employ double-stranded breaks (DSBs), which are associated with P53 activation, large deletions, and chromosomal translocations. Instead, using attached deaminases and reverse transcriptases, base and prime editing efficiently induces specific base substitutions and intended genetic changes (substitutions, deletions, or insertions), respectively, without DSBs. In this review, we will discuss the recent in vivo application of CRISPR-Cas9 technologies, focusing on base and prime editing, in animal models of IRDs.

CRISPR/Cas9—A Promising Therapeutic Tool to Cure Blindness: Current Scenario and Future Prospects

CRISPR-based targeted genome editing is bringing revolutionary changes in the research arena of biological sciences. CRISPR/Cas9 has been explored as an efficient therapeutic tool for the treatment of genetic diseases. It has been widely used in ophthalmology research by using mouse models to correct pathogenic mutations in the eye stem cells. In recent studies, CRISPR/Cas9 has been used to correct a large number of mutations related to inherited retinal disorders. In vivo therapeutic advantages for retinal diseases have been successfully achieved in some rodents. Current advances in the CRISPR-based gene-editing domain, such as modified Cas variants and delivery approaches have optimized its application to treat blindness. In this review, recent progress and challenges of the CRISPR-Cas system have been discussed to cure blindness and its prospects.

Gene therapy in hereditary retinal dystrophy

Hereditary retinal dystrophies (HRDs), such as retinitis pigmentosa, Leber's congenital amaurosis (LCA), Usher syndrome, and retinoschisis, are a group of genetic retinal disorders exhibiting both genetic and phenotypic heterogeneity. Symptoms include progressive retinal degeneration and constricted visual field. Some patients will be legal or completely blind. Advanced sequencing technologies improve the genetic diagnosis of HRD and lead to a new era of research into gene-targeted therapies. Following the first Food and Drug Administration approval of gene augmentation therapy for LCA caused by RPE65 mutations, multiple clinical trials are currently underway applying different techniques. In this review, we provide an overview of gene therapy for HRD and emphasize four distinct approaches to gene-targeted therapy that have the potential to slow or even reverse retinal degeneration: (1) viral vector-based and nonviral gene delivery, (2) RNA-based antisense oligonucleotide, (3) genome editing by the Clustered Regularly Interspaced Short Palindromic Repeat/cas9 system, and (4) optogenetics gene therapy.

Gene Therapy for Rhodopsin Mutations

Mutations in RHO, the gene for rhodopsin, account for a large fraction of autosomal-dominant retinitis pigmentosa (adRP). Patients fall into two clinical classes, those with early onset, pan retinal photoreceptor degeneration, and those who experience slowly progressive disease. The latter class of patients are candidates for photoreceptor-directed gene therapy, while former may be candidates for delivery of light-responsive proteins to interneurons or retinal ganglion cells. Gene therapy for RHO adRP may be targeted to the mutant gene at the DNA or RNA level, while other therapies preserve the viability of photoreceptors without addressing the underlying mutation. Correcting the RHO gene and replacing the mutant RNA show promise in animal models, while sustaining viable photoreceptors has the potential to delay the loss of central vision and may preserve photoreceptors for gene-directed treatments.

Gene-independent therapeutic interventions to maintain and restore light sensitivity in degenerating photoreceptors

Neurodegenerative retinal diseases are a prime cause of blindness in industrialized countries. In many cases, there are no therapeutic treatments, although they are essential to improve patients' quality of life. A set of disease-causing genes, which primarily affect photoreceptors, has already been identified and is of major interest for developing gene therapies. Nevertheless, depending on the nature and the state of the disease, gene-independent strategies are needed. Various strategies to halt disease progression or maintain function of the retina are under research. These therapeutic interventions include neuroprotection, direct reprogramming of affected photoreceptors, the application of non-coding RNAs, the generation of artificial photoreceptors by optogenetics and cell replacement strategies. During recent years, major breakthroughs have been made such as the first optogenetic application to a blind patient whose visual function partially recovered by targeting retinal ganglion cells. Also, RPE cell transplantation therapies are under clinical investigation and show great promise to improve visual function in blind patients. These cells are generated from human stem cells. Similar therapies for replacing photoreceptors are extensively tested in pre-clinical models. This marks just the start of promising new cures taking advantage of developments in the areas of genetic engineering, optogenetics, and stem-cell research. In this review, we present the recent therapeutic advances of gene-independent approaches that are currently under clinical evaluation. Our main focus is on photoreceptors as these sensory cells are highly vulnerable to degenerative diseases, and are crucial for light detection.

Update of application of olfactory ensheathing cells and stem cells/exosomes in the treatment of retinal disorders

Age-related macular degeneration, diabetic retinopathy, retinitis pigmentosa and other retinal disorders are the main causes of visual impairment worldwide. In the past, these retinal diseases, especially dry age-related macular degeneration, proliferative diabetic retinopathy and retinitis pigmentosa, were treated with traditional surgery and drugs. However, the effect was moderate. In recent years, researchers have used embryonic stem cells, induced pluripotent stem cells, mesenchymal stem cells, olfactory ensheathing cells and other stem cells to conduct experiments and found that stem cells can inhibit inflammation, regulate immune response, secrete neurotrophic factors, and differentiate into retinal cells to replace and promote restoration of the damaged parts. These stem cells have the potential to treat retinal diseases. Whether it is in animal experiments or clinical trials, the increase in the number of retinal cells, maintenance of function and improvement of visual function all reflect the advanced of stem cells to treat retinal diseases, but its risk preserves the donor's hidden pathogenic genes, immune rejection and tumorigenicity. With the development of exosomes study, researchers have discovered that exosomes come from a wide range of sources and can be secreted by almost all types of cells. Using exosomes with stem cell to treat retinal diseases is more effective than using stem cells alone. This review article summarizes the recent advances in the application of olfactory ensheathing cells and stem cells/exosomes in the treatment of retinal disorders.