The retinitis pigmentosa GTPase regulator, RPGR, interacts with the delta subunit of rod cyclic GMP phosphodiesterase

Updates on protein-prenylation and associated inherited retinopathies

Membrane-anchored proteins play critical roles in cell signaling, cellular architecture, and membrane biology. Hydrophilic proteins are post-translationally modified by a diverse range of lipid molecules such as phospholipids, glycosylphosphatidylinositol, and isoprenes, which allows their partition and anchorage to the cell membrane. In this review article, we discuss the biochemical basis of isoprenoid synthesis, the mechanisms of isoprene conjugation to proteins, and the functions of prenylated proteins in the neural retina. Recent discovery of novel prenyltransferases, prenylated protein chaperones, non-canonical prenylation-target motifs, and reversible prenylation is expected to increase the number of inherited systemic and blinding diseases with aberrant protein prenylation. Recent important investigations have also demonstrated the role of several unexpected regulators (such as protein charge, sequence/protein-chaperone interaction, light exposure history) in the photoreceptor trafficking of prenylated proteins. Technical advances in the investigation of the prenylated proteome and its application in vision research are discussed. Clinical updates and technical insights into known and putative prenylation-associated retinopathies are provided herein. Characterization of non-canonical prenylation mechanisms in the retina and retina-specific prenylated proteome is fundamental to the understanding of the pathogenesis of protein prenylation-associated inherited blinding disorders.

Retinitis pigmentosa GTPase regulator-related retinopathy and gene therapy

Retinitis pigmentosa GTPase regulator (RPGR)-related retinopathy is a retinal dystrophy inherited in a X-linked recessive manner that typically causes progressive visual loss starting in childhood with severe visual impairment by the fourth decade of life. It manifests as an early onset and severe form of retinitis pigmentosa. There are currently no effective treatments for RPGR-related retinopathy; however, there are multiple clinical trials in progress exploring gene augmentation therapy aimed at slowing down or halting the progression of disease and possibly restoring visual function. This review focuses on the molecular biology, clinical manifestations, and the recent progress of gene therapy clinical trials.

RPGR: Deep Phenotyping and Genetic Characterization With Findings Specific to the 3'-end of ORF15

Purpose

To describe a group of patients with retinitis pigmentosa GTPase regulator (RPGR)-related retinopathy with a tapetal-like retinal sheen and corresponding changes in the reflectivity of the ellipsoid zone on optical coherence tomography (OCT) imaging.

Methods

A retrospective case series of 66 patients with a disease-causing variant in RPGR was performed. An expert examiner, masked to patient demographics, clinical evaluations, and specific RPGR variant, analyzed color fundus photographs for the presence of a tapetal-like retinal sheen and assessed OCT images for the presence of an abnormally broad hyper-reflective band in the outer retina. Longitudinal reflectivity profiles were generated and compared with healthy controls.

Results

Twelve patients (18.2%) had a retinal sheen on color images that cosegregated with an abnormally broad hyper-reflective ellipsoid zone band on OCT imaging. Three-fourths of these patients were male, had a cone-rod dystrophy, and had pathogenic RPGR variants located toward the 3'-end of ORF15. This group had a different longitudinal reflectivity profile signature compared with controls. After a period of prolonged dark adaptation, the abnormal hyper-reflective band on OCT became less apparent, and the outer retinal layers adopted a more normal appearance.

Conclusions

RPGR-related retinopathy should be considered for males presenting with retinal sheen, abnormal ellipsoid zone hyper-reflectivity, and cone or cone-rod dysfunction on ERG, and pursued with molecular testing. Our results have implications for understanding the role of the C-terminal domain encoded by RPGR ORF15 in the phototransduction cascade. Further, the findings may be important to incorporate into both inclusion criteria and outcome measure developments in future RPGR-related cone or cone-rod dystrophy clinical trials.

PDE6D Mediates Trafficking of Prenylated Proteins NIM1K and UBL3 to Primary Cilia

Mutations in PDE6D impair the function of its cognate protein, phosphodiesterase 6D (PDE6D), in prenylated protein trafficking towards the ciliary membrane, causing the human ciliopathy Joubert Syndrome (JBTS22) and retinal degeneration in mice. In this study, we purified the prenylated cargo of PDE6D by affinity proteomics to gain insight into PDE6D-associated disease mechanisms. By this approach, we have identified a specific set of PDE6D-interacting proteins that are involved in photoreceptor integrity, GTPase activity, nuclear import, or ubiquitination. Among these interacting proteins, we identified novel ciliary cargo proteins of PDE6D, including FAM219A, serine/threonine-protein kinase NIM1 (NIM1K), and ubiquitin-like protein 3 (UBL3). We show that NIM1K and UBL3 localize inside the cilium in a prenylation-dependent manner. Furthermore, UBL3 also localizes in vesicle-like structures around the base of the cilium. Through affinity proteomics of UBL3, we confirmed its strong interaction with PDE6D and its association with proteins that regulate small extracellular vesicles (sEVs) and ciliogenesis. Moreover, we show that UBL3 localizes in specific photoreceptor cilium compartments in a prenylation-dependent manner. Therefore, we propose that UBL3 may play a role in the sorting of proteins towards the photoreceptor outer segment, further explaining the development of PDE6D-associated retinal degeneration.

Emerging gene therapy products for RPGR-associated X-linked retinitis pigmentosa

INTRODUCTION

Mutations in the RPGR gene are responsible for one of the most prevalent and severe types of retinitis pigmentosa. Gene therapy has shown great promise to treat inherited retinal diseases, and currently, four RPGR gene therapy vectors are being evaluated in clinical trials.

AREAS COVERED

This manuscript reviews the gene therapy products that are in development for X-linked retinitis pigmentosa caused by mutations in RPGR, and the challenges that scientists and clinicians have faced.

EXPERT OPINION

The development of a gene therapy product for RPGR-associated retinal degeneration has been a great challenge due to the incomplete understanding of the underlying genetics and mechanism of action of RPGR, and on the other hand, due to the instability of the RPGR gene. Three of the four gene therapy vectors currently in clinical trials include a codon-optimized version of the human RPGR sequence, and the other vector contains a shortened version of the human RPGR. To date, the only Phase I/II results published in a peer-reviewed journal demonstrate a good safety profile and an improvement in the visual field using a codon optimized version of RPGRORF15.

Retrospective Natural History Study of RPGR-Related Cone- and Cone-Rod Dystrophies While Expanding the Mutation Spectrum of the Disease

Variants in the X-linked retinitis pigmentosa GTPase regulator gene (RPGR) and, specifically, in its retinal opening reading frame-15 isoform (RPGRORF15) may cause rod-cone (RCD), cone, and cone-rod dystrophies (CDs and CRDs). While RPGR-related RCDs have been frequently evaluated, the characteristics and progression of RPGR-related CD/CRDs are largely unknown. Therefore, the goal of our work was to perform genotype-phenotype correlations specifically in RPGRORF15-related CD/CRDs. This retrospective longitudinal study included 34 index patients and two affected relatives with a molecular diagnosis of RPGR-related CD/CRDs. Patients were recruited at the "Quinze-Vingts" Hospital, Paris, France and screened for mutations in RPGRORF15 at the Institut de la Vision, Paris, France. We identified 29 distinct variants, of which 27 were truncating. All were located in the 3' half of the RPGRORF15 transcript. Twenty of them were novel. Fifteen subjects were affected by CD, the remaining had CRD. When analyzing the longitudinal data, a progressive decline in visual acuity (VA) was noted, with more than 60% of the patients reaching VA ≥ 1 LogMar in the best eye after the fifth decade of life. To our knowledge, this is the largest described study of a cohort of CD/CRD patients affected by RPGRORF15 variants. Longitudinal data showed a rapidly progressive disease, possibly locating an optimal window of intervention for future therapies in younger ages.

Delivery Strategies for CRISPR/Cas Genome editing tool for Retinal Dystrophies: challenges and opportunities

CRISPR/Cas, an adaptive immune system in bacteria, has been adopted as an efficient and precise tool for site-specific gene editing with potential therapeutic opportunities. It has been explored for a variety of applications, including gene modulation, epigenome editing, diagnosis, mRNA editing, etc. It has found applications in retinal dystrophic conditions including progressive cone and cone-rod dystrophies, congenital stationary night blindness, X-linked juvenile retinoschisis, retinitis pigmentosa, age-related macular degeneration, leber's congenital amaurosis, etc. Most of the therapies for retinal dystrophic conditions work by regressing symptoms instead of reversing the gene mutations. CRISPR/Cas9 through indel could impart beneficial effects in the reversal of gene mutations in dystrophic conditions. Recent research has also consolidated on the approaches of using CRISPR systems for retinal dystrophies but their delivery to the posterior part of the eye is a major concern due to high molecular weight, negative charge, and in vivo stability of CRISPR components. Recently, non-viral vectors have gained interest due to their potential in tissue-specific nucleic acid (miRNA/siRNA/CRISPR) delivery. This review highlights the opportunities of retinal dystrophies management using CRISPR/Cas nanomedicine.

The Major Ciliary Isoforms of RPGR Build Different Interaction Complexes with INPP5E and RPGRIP1L

X-linked retinitis pigmentosa (XLRP) is frequently caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene. A complex splicing process acts on the RPGR gene resulting in three major isoforms: RPGR^ex1-19, RPGR^ORF15 and RPGR^skip14/15. We characterized the widely expressed, alternatively spliced transcript RPGR^skip14/15 lacking exons 14 and 15. Using the CRISPR/eSpCas9 system, we generated HEK293T cell lines exclusively expressing the RPGR^skip14/15 transcript from the endogenous RPGR gene. RPGR^ex1-19 and RPGR^ORF15 were knocked out. Immunocytochemistry demonstrated that the RPGR^skip14/15 protein localizes along primary cilia, resembling the expression pattern of RPGR^ex1-19. The number of cilia-carrying cells was not affected by the absence of the RPGR^ex1-19 and RPGR^ORF15 isoforms. Co-immunoprecipitation assays demonstrated that both RPGR^ex1-19 and RPGR^skip14/15 interact with PDE6D, further supporting that RPGR^skip14/15 is associated with the protein networks along the primary cilium. Interestingly, interaction complexes with INPP5E or RPGRIP1L were only detectable with isoform RPGR^ex1-19, but not with RPGR^skip14/15, demonstrating distinct functional properties of the major RPGR isoforms in spite of their similar subcellular localization. Our findings lead to the conclusion that protein binding sites within RPGR are mediated through alternative splicing. A tissue-specific expression ratio between RPGR^skip14/15 and RPGR^ex1-19 seems required to regulate the ciliary concentration of RPGR interaction partners.

Cone Dystrophy Associated with a Novel Variant in the Terminal Codon of the RPGR-ORF15

Mutations in RPGR^ORF15 are associated with rod-cone or cone/cone-rod dystrophy, the latter associated with mutations at the distal end. We describe the phenotype associated with a novel variant in the terminal codon of the RPGR^ORF15 c.3457T>A (Ter1153Lysext*38), which results in a C-terminal extension. Three male patients from two families were recruited, aged 31, 35, and 38 years. Genetic testing was performed by whole exome sequencing. Filtered variants were analysed according to the population frequency, ClinVar database, the variant’s putative impact, and predicted pathogenicity; and were classified according to the ACMG guidelines. Examination included visual acuity (Snellen), colour vision (Ishihara), visual field, fundus autofluorescence (FAF), optical coherence tomography (OCT), and electrophysiology. All patients were myopic, and had central scotoma and reduced colour vision. Visual acuities on better eyes were counting fingers, 0.3 and 0.05. Electrophysiology showed severely reduced cone-specific responses and macular dysfunction, while the rod-specific response was normal. FAF showed hyperautofluorescent ring centred at the fovea encompassing an area of photoreceptor loss approximately two optic discs in diameter (3462–6342 μm). Follow up after 2–11 years showed enlargement of the diameter (avg. 100 μm/year). The novel c.3457T>A (Ter1153Lysext*38) mutation in the terminal RPGR^ORF15 codon is associated with cone dystrophy, which corresponds to the previously described phenotypes associated with mutations in the distal end of the RPGR^ORF15. Minimal progression during follow-up years suggests a relatively stable disease after the initial loss of the central cones.

X-Linked Retinitis Pigmentosa Caused by Non-Canonical Splice Site Variants in RPGR

We aimed to validate the effect of non-canonical splice site variants in the RPGR gene in five patients from four families diagnosed with retinitis pigmentosa. Four variants located in intron 2 (c.154 + 3_154 + 6del), intron 3 (c.247 + 5G>A), intron 7 (c.779-5T>G), and intron 13 (c.1573-12A>G), respectively, were analyzed by means of in vitro splice assays. Splicing analysis revealed different aberrant splicing events, including exon skipping and intronic nucleotide addition, which are predicted to lead either to an in-frame deletion affecting relevant protein domains or to a frameshift of the open reading frame. Our data expand the landscape of pathogenic variants in RPGR, thereby increasing the genetic diagnostic rate in retinitis pigmentosa and allowing patients harboring the analyzed variants to be enrolled in clinical trials.

The X-linked retinopathies: Physiological insights, pathogenic mechanisms, phenotypic features and novel therapies

X-linked retinopathies represent a significant proportion of monogenic retinal disease. They include progressive and stationary conditions, with and without syndromic features. Many are X-linked recessive, but several exhibit a phenotype in female carriers, which can help establish diagnosis and yield insights into disease mechanisms. The presence of affected carriers can misleadingly suggest autosomal dominant inheritance. Some disorders (such as RPGR-associated retinopathy) show diverse phenotypes from variants in the same gene and also highlight limitations of current genetic sequencing methods. X-linked disease frequently arises from loss of function, implying potential for benefit from gene replacement strategies. We review X-inactivation and X-linked inheritance, and explore burden of disease attributable to X-linked genes in our clinically and genetically characterised retinal disease cohort, finding correlation between gene transcript length and numbers of families. We list relevant genes and discuss key clinical features, disease mechanisms, carrier phenotypes and novel experimental therapies. We consider in detail the following: RPGR (associated with retinitis pigmentosa, cone and cone-rod dystrophy), RP2 (retinitis pigmentosa), CHM (choroideremia), RS1 (X-linked retinoschisis), NYX (complete congenital stationary night blindness (CSNB)), CACNA1F (incomplete CSNB), OPN1LW/OPN1MW (blue cone monochromacy, Bornholm eye disease, cone dystrophy), GPR143 (ocular albinism), COL4A5 (Alport syndrome), and NDP (Norrie disease and X-linked familial exudative vitreoretinopathy (FEVR)). We use a recently published transcriptome analysis to explore expression by cell-type and discuss insights from electrophysiology. In the final section, we present an algorithm for genes to consider in diagnosing males with non-syndromic X-linked retinopathy, summarise current experimental therapeutic approaches, and consider questions for future research.

Myopia with X-linked retinitis pigmentosa results from a novel gross deletion of RPGR gene

AIM

To identify mutations with whole exome sequencing (WES) in a Chinese X-linked retinitis pigmentosa (XLRP) family.

METHODS

Patients received the comprehensive ophthalmic evaluation. Genomic DNA was extracted from peripheral blood and subjected to SureSelect Human All Exon 6+ UTR exon capture kit. The exons were sequenced as 100 base paired reads on Illumina HiSeq2500 system. Only mutations that resulted in a change in amino acid sequence were selected. A pattern of inheritance of the RP family was aligned to identified causal mutation.

RESULTS

We analysed the data of WES information from XLRP family. The analysis revealed a hemizygous large genomic deletion of RPGR c.29_113del was responsible for this XLRP. The gross deletion lead to a frame-shift mutation and generate stop codon at 7 animo acid behind Asp (D10Afs*7), which would serious truncate RPGR protein. The novel frame-shift mutation was found to segregate with retinitis pigmentosa (RP) phenotype in this family. Bilateral myopia was present on the male patients, but carrier female showed unilateral myopia without RP.

CONCLUSION

Our study identifies a novel frame-shift mutation of RPGR in a Chinese family, which would expand the spectrum of RPGR mutations. The geno-phenotypic analysis reveals a correlation between RP and myopia. Although exact mechanism of RP related myopia is still unknown, but the novel frame-shift mutation will give our hit on studying the molecular pathogenesis of RP and myopia.

Integrated approaches to unravel the impact of protein lipoxidation on macromolecular interactions

Protein modification by lipid derived reactive species, or lipoxidation, is increased during oxidative stress, a common feature observed in many pathological conditions. Biochemical and functional consequences of lipoxidation include changes in the conformation and assembly of the target proteins, altered recognition of ligands and/or cofactors, changes in the interactions with DNA or in protein-protein interactions, modifications in membrane partitioning and binding and/or subcellular localization. These changes may impact, directly or indirectly, signaling pathways involved in the activation of cell defense mechanisms, but when these are overwhelmed they may lead to pathological outcomes. Mass spectrometry provides state of the art approaches for the identification and characterization of lipoxidized proteins/residues and the modifying species. Nevertheless, understanding the complexity of the functional effects of protein lipoxidation requires the use of additional methodologies. Herein, biochemical and biophysical methods used to detect and measure functional effects of protein lipoxidation at different levels of complexity, from in vitro and reconstituted cell-like systems to cells, are reviewed, focusing especially on macromolecular interactions. Knowledge generated through innovative and complementary technologies will contribute to comprehend the role of lipoxidation in pathophysiology and, ultimately, its potential as target for therapeutic intervention.

Disruption of RPGR protein interaction network is the common feature of RPGR missense variations that cause XLRP

Retinitis pigmentosa (RP) is an inherited retinal degenerative disease with severe vision impairment leading to blindness. About 10-15% of RP cases are caused by mutations in the RPGR gene, with RPGR mutations accounting for 70% of X-linked RP cases. The mechanism by which RPGR mutations cause photoreceptor cell dysfunction is not well understood. In this study, we show that the two isoforms of RPGR (RPGR^1-19 and RPGR^ORF15) interact with endogenous PDE6D, INPP5E, and RPGRIP1L. The RPGR^1-19 isoform contains two PDE6D binding sites with the C-terminal prenylation site being the predominant PDE6D binding site. The C terminus of RPGR^1-19 that contains the prenylation site regulates its interaction with PDE6D, INPP5E, and RPGRIP1L. Only the RPGR^1-19 isoform localizes to cilia in cultured RPE1 cells. Missense variations found in RPGR patients disrupt the interaction between RPGR isoforms and their endogenous interactors INPP5E, PDE6D, and RPGRIP1L. We evaluated a RPGR missense variation (M58K) found in a family with X-linked retinitis pigmentosa (XLRP) and show that this missense variation disrupts the interaction of RPGR isoforms with their endogenous interactors. The M58K variation also disrupts the ciliary localization of the RPGR^1-19 isoform. Using this assay, we also show that some of the RPGR missense variants reported in the literature might not actually be disease causing. Our data establishes an in vitro assay that can be used to validate the potential pathogenicity of RPGR missense variants.

Gene therapy and genome surgery in the retina

Precision medicine seeks to treat disease with molecular specificity. Advances in genome sequence analysis, gene delivery, and genome surgery have allowed clinician-scientists to treat genetic conditions at the level of their pathology. As a result, progress in treating retinal disease using genetic tools has advanced tremendously over the past several decades. Breakthroughs in gene delivery vectors, both viral and nonviral, have allowed the delivery of genetic payloads in preclinical models of retinal disorders and have paved the way for numerous successful clinical trials. Moreover, the adaptation of CRISPR-Cas systems for genome engineering have enabled the correction of both recessive and dominant pathogenic alleles, expanding the disease-modifying power of gene therapies. Here, we highlight the translational progress of gene therapy and genome editing of several retinal disorders, including RPE65-, CEP290-, and GUY2D-associated Leber congenital amaurosis, as well as choroideremia, achromatopsia, Mer tyrosine kinase- (MERTK-) and RPGR X-linked retinitis pigmentosa, Usher syndrome, neovascular age-related macular degeneration, X-linked retinoschisis, Stargardt disease, and Leber hereditary optic neuropathy.

Gene therapy for the treatment of X-linked retinitis pigmentosa

INTRODUCTION

X-linked retinitis pigmentosa caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene is the most common form of recessive RP. The phenotype is characterised by its severity and rapid disease progression. Gene therapy using adeno-associated viral vectors is currently the most promising therapeutic approach. However, the construction of a stable vector encoding the full-length RPGR transcript has previously proven to be a limiting step towards gene therapy clinical trials. Recently however, a codon optimised version of RPGR has been shown to increase the stability and fidelity of the sequence, conferring a therapeutic effect in murine and canine animal models.

AREAS COVERED

This manuscript reviews the natural history of X-linked retinitis pigmentosa and the research performed from the discovery of the causative gene, RPGR, to the preclinical testing of potential therapies that have led to the initiation of three clinical trials.

EXPERT OPINION

X-linked retinitis pigmentosa is an amenable disease to be treated by gene therapy. Codon optimisation has overcome the challenge of designing an RPGR vector without mutations, and with a therapeutic effect in different animal models. With the RPGR gene therapy clinical trials still in the early stages, the confirmation of the safety, tolerability and potency of the therapy is still ongoing.

Prenylated retinal ciliopathy protein RPGR interacts with PDE6δ and regulates ciliary localization of Joubert syndrome-associated protein INPP5E

Ciliary trafficking defects underlie the pathogenesis of severe human ciliopathies, including Joubert Syndrome (JBTS), Bardet-Biedl Syndrome, and some forms of retinitis pigmentosa (RP). Mutations in the ciliary protein RPGR (retinitis pigmentosa GTPase regulator) are common causes of RP-associated photoreceptor degeneration worldwide. While previous work has suggested that the localization of RPGR to cilia is critical to its functions, the mechanism by which RPGR and its associated cargo are trafficked to the cilia is unclear. Using proteomic and biochemical approaches, we show that RPGR interacts with two JBTS-associated ciliary proteins: PDE6δ (delta subunit of phosphodiesterase; a prenyl-binding protein) and INPP5E (inositol polyphosphate-5-phosphatase 5E). We find that PDE6δ binds selectively to the C-terminus of RPGR and that this interaction is critical for RPGR’s localization to cilia. Furthermore, we show that INPP5E associates with the N-terminus of RPGR and trafficking of INPP5E to cilia is dependent upon the ciliary localization of RPGR. These results implicate prenylation of RPGR as a critical modification for its localization to cilia and, in turn suggest that trafficking of INPP5E to cilia depends upon the interaction of RPGR with PDE6δ. Finally, our results implicate INPP5E, a novel RPGR-interacting protein, in the pathogenesis of RPGR-associated ciliopathies.

Disease mechanisms of X-linked retinitis pigmentosa due to RP2 and RPGR mutations

Photoreceptor degeneration is the prominent characteristic of retinitis pigmentosa (RP), a heterogeneous group of inherited retinal dystrophies resulting in blindness. Although abnormalities in many pathways can cause photoreceptor degeneration, one of the most important causes is defective protein transport through the connecting cilium, the structure that connects the biosynthetic inner segment with the photosensitive outer segment of the photoreceptors. The majority of patients with X-linked RP have mutations in the retinitis pigmentosa GTPase regulator (RPGR) or RP2 genes, the protein products of which are both components of the connecting cilium and associated with distinct mechanisms of protein delivery to the outer segment. RP2 and RPGR proteins are associated with severe diseases ranging from classic RP to atypical forms. In this short review, we will summarise current knowledge generated by experimental studies and knockout animal models, compare and discuss the prominent hypotheses about the two proteins' functions in retinal cell biology.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

Gene and cell-based therapies for inherited retinal disorders: An update

Retinal degenerations present a unique challenge as disease progression is irreversible and the retina has little regenerative potential. No current treatments for inherited retinal disease have the ability to reverse blindness, and current dietary supplement recommendations only delay disease progression with varied results. However, the retina is anatomically accessible and capable of being monitored at high resolution in vivo. This, in addition to the immune-privileged status of the eye, has put ocular disease at the forefront of advances in gene- and cell-based therapies. This review provides an update on gene therapies and randomized control trials for inherited retinal disease, including Leber congenital amaurosis, choroideremia, retinitis pigmentosa, Usher syndrome, X-linked retinoschisis, Leber hereditary optic neuropathy, and achromatopsia. New gene-modifying and cell-based strategies are also discussed. © 2016 Wiley Periodicals, Inc.

Shuttling and sorting lipid-modified cargo into the cilia

Primary cilia are hair-like microtubule-based organelles that can be found on almost all human cell types. Although the cilium is not separated from the cell by membranes, their content is different from that of the cell body and their membrane composition is distinct from that of the plasma membrane. Here, we will introduce a molecular machinery that shuttles and sorts lipid-modified proteins to the cilium, thus contributing in maintaining its distinct composition. The mechanism involves the binding of the GDI-like solubilising factors, uncoordinated (UNC)119a, UNC119b and PDE6D, to the lipid-modified ciliary cargo and the specific release of the cargo in the cilia by the ciliary small G-protein Arl3 in a GTP-dependent manner.

RPGR, a prenylated retinal ciliopathy protein, is targeted to cilia in a prenylation- and PDE6D-dependent manner

RPGR (retinitis pigmentosa GTPase regulator) is a ciliary protein associated with several forms of inherited retinal degenerative diseases. PDE6D is a ubiquitously expressed prenyl-binding protein and involved in ciliary targeting of prenylated proteins. The current working model for the RPGR function depicts that RPGR acts as a scaffold protein to recruit cargo-loaded PDE6D to primary cilia. Here, we present evidence demonstrating an alternative relationship between RPGR and PDE6D, in which RPGR is a cargo of PDE6D for ciliary targeting. We found that the constitutive isoform of RPGR, which is prenylated, requires prenylation for its ciliary localization. We also found that there are at least two independent ciliary targeting signals in RPGR: one within the N-terminal region that contains the RCC1-like domain and the other near the prenylation site at the C-terminus. Ablation of PDE6D blocked ciliary targeting of RPGR. Our study indicates that prenylated RPGR is one of the cargos of PDE6D for ciliary trafficking and provides insight into the mechanisms by which RPGR is targeted to cilia.

Summary: RPGR is a ciliary protein that functions as a scaffold to recruit cargo-loaded PDE6D to cilia. Our study shows that RPGR is also a cargo of PDE6D.

RPGR-associated retinopathy: clinical features, molecular genetics, animal models and therapeutic options

Retinitis pigmentosa GTPase regulator (RPGR) gene sequence variants account for the vast majority of X linked retinitis pigmentosa (RP), which is one of the most severe forms of RP. Symptoms of nyctalopia typically begin in childhood, with increasing loss of peripheral visual field during teenage years, and progressive central visual loss during the second to fourth decade of life. There is however marked intrafamilial and interfamilial phenotypic heterogeneity in affected males and carrier females. There is now a far greater understanding of the range of phenotypes associated with variants in this gene; including rod-cone dystrophy, cone-rod dystrophy, cone dystrophy, macular dystrophy and non-ocular phenotypes. There are also increasingly established genotype-phenotype associations and structure-function correlations. RPGR is involved in ciliary function, with ciliary dysfunction now recognised as the mechanism underlying a large proportion of inherited retinal disease. There has been significant progress in identifying naturally occurring animal models and developing novel models to define the underlying disease mechanisms and to test gene replacement therapy, in addition to advances in human retinal imaging, culminating in completed and planned clinical trials. These significant developments will be discussed.

The N- and C-terminal ends of RPGR can bind to PDE6δ

This study shows that the prenylated C‐terminus of RPGR can bind to PDE6δ with high affinity, suggesting two distinct binding sites of the RPGR/PDE6δ complex. The serine residue at the −3 position relative to the prenylated cysteine seems to play a key role in defining the selectivity of PDE6δ towards ciliary prenylated cargo.

RPGR: Its role in photoreceptor physiology, human disease, and future therapies

Mammalian photoreceptors contain specialised connecting cilia that connect the inner (IS) to the outer segments (OS). Dysfunction of the connecting cilia due to mutations in ciliary proteins are a common cause of the inherited retinal dystrophy retinitis pigmentosa (RP). Mutations affecting the Retinitis Pigmentosa GTPase Regulator (RPGR) protein is one such cause, affecting 10-20% of all people with RP and the majority of those with X-linked RP. RPGR is located in photoreceptor connecting cilia. It interacts with a wide variety of ciliary proteins, but its exact function is unknown. Recently, there have been important advances both in our understanding of RPGR function and towards the development of a therapy. This review summarises the existing literature on human RPGR function and dysfunction, and suggests that RPGR plays a role in the function of the ciliary gate, which controls access of both membrane and soluble proteins to the photoreceptor outer segment. We discuss key models used to investigate and treat RPGR disease and suggest that gene augmentation therapy offers a realistic therapeutic approach, although important questions still remain to be answered, while cell replacement therapy based on retinal progenitor cells represents a more distant prospect.

The Role of RPGR and Its Interacting Proteins in Ciliopathies

Ciliopathies encompass a group of genetic disorders characterized by defects in the formation, maintenance, or function of cilia. Retinitis pigmentosa (RP) is frequently one of the clinical features presented in diverse ciliopathies. RP is a heterogeneous group of inherited retinal disorders, characterized by the death of photoreceptors and affecting more than one million individuals worldwide. The retinitis pigmentosa GTPase regulator (RPGR) gene is mutated in up to 20% of all RP patients. RPGR protein has different interacting partners to function in ciliary protein trafficking. In this review, we specifically focus on RPGR and its two interacting proteins: RPGRIP1 and RPGRIP1L. We summarize the function of the three proteins and highlight recent studies that provide insight into the cellular function of those proteins.

Membrane protein transport in photoreceptors: the function of PDEδ: the Proctor lecture

This lecture details the elucidation of cGMP phosphodiesterase (PDEδ), discovered 25 years ago by Joe Beavo at the University of Washington. PDEδ, once identified as a fourth PDE6 subunit, is now regarded as a promiscuous prenyl-binding protein and important chaperone of prenylated small G proteins of the Ras superfamily and prenylated proteins of phototransduction. Alfred Wittinghofer's group in Germany showed that PDEδ forms an immunoglobulin-like β-sandwich fold that is closely related in structure to other lipid-binding proteins, for example, Uncoordinated 119 (UNC119) and RhoGDI. His group cocrystallized PDEδ with ARL (Arf-like) 2(GTP), and later with farnesylated Rheb (ras homolog expressed in brain). PDEδ specifically accommodates farnesyl and geranylgeranyl moieties in the absence of bound protein. Germline deletion of the Pde6d gene encoding PDEδ impeded transport of rhodopsin kinase (GRK1) and PDE6 to outer segments, causing slowly progressing, recessive retinitis pigmentosa. A rare PDE6D null allele in human patients, discovered by Tania Attié-Bitach in France, specifically impeded trafficking of farnesylated phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase (INPP5E) to cilia, causing severe syndromic ciliopathy (Joubert syndrome). Binding of cargo to PDEδ is controlled by Arf-like proteins, ARL2 and ARL3, charged with guanosine-5'-triphosphate (GTP). Arf-like proteins 2 and 3 are unprenylated small GTPases that serve as cargo displacement factors. The lifetime of ARL3(GTP) is controlled by its GTPase-activating protein, retinitis pigmentosa protein 2 (RP2), which accelerates GTPase activity up to 90,000-fold. RP2 null alleles in human patients are associated with severe X-linked retinitis pigmentosa (XLRP). Germline deletion of RP2 in mouse, however, causes only a mild form of XLRP. Absence of RP2 prolongs the activity of ARL3(GTP) that, in turn, impedes PDE6δ-cargo interactions and trafficking of prenylated protein to the outer segments. Hyperactive ARL3(GTP), acting as a hyperactive cargo displacement factor, is predicted to be key in the pathobiology of RP2-XLRP.

Mistrafficking of prenylated proteins causes retinitis pigmentosa 2

The retinitis pigmentosa 2 polypeptide (RP2) functions as a GTPase-activating protein (GAP) for ARL3 (Arf-like protein 3), a small GTPase. ARL3 is an effector of phosphodiesterase 6 Δ (PDE6D), a prenyl-binding protein and chaperone of prenylated protein in photoreceptors. Mutations in the human RP2 gene cause X-linked retinitis pigmentosa (XLRP) and cone-rod dystrophy (XL-CORD). To study mechanisms causing XLRP, we generated an RP2 knockout mouse. The Rp2h(-/-) mice exhibited a slowly progressing rod-cone dystrophy simulating the human disease. Rp2h(-/-) scotopic a-wave and photopic b-wave amplitudes declined at 1 mo of age and continued to decline over the next 6 mo. Prenylated PDE6 subunits and G-protein coupled receptor kinase 1 (GRK1) were unable to traffic effectively to the Rp2h(-/-) outer segments. Mechanistically, absence of RP2 GAP activity increases ARL3-GTP levels, forcing PDE6D to assume a predominantly "closed" conformation that impedes binding of lipids. Lack of interaction disrupts trafficking of PDE6 and GRK1 to their destination, the photoreceptor outer segments. We propose that hyperactivity of ARL3-GTP in RP2 knockout mice and human patients with RP2 null alleles leads to XLRP resembling recessive rod-cone dystrophy.

Gene augmentation for X-linked retinitis pigmentosa caused by mutations in RPGR

X-linked retinitis pigmentosa (XLRP) caused by mutations in the RPGR gene is a severe and early onset form of retinal degeneration, and no treatment is currently available. Recent evidence in two clinically relevant canine models shows that adeno-associated viral (AAV)-mediated RPGR gene transfer to rods and cones can prevent disease onset and rescue photoreceptors at early- and mid-stages of degeneration. There is thus a strong incentive for conducting long-term, preclinical efficacy and safety studies, while concomitantly pursuing the detailed phenotypic characterization of XLRP disease in patients that may benefit from such corrective therapy.

A homozygous PDE6D mutation in Joubert syndrome impairs targeting of farnesylated INPP5E protein to the primary cilium

Joubert syndrome (JS) is characterized by a distinctive cerebellar structural defect, namely the << molar tooth sign >>. JS is genetically heterogeneous, involving 20 genes identified to date, which are all required for cilia biogenesis and/or function. In a consanguineous family with JS associated with optic nerve coloboma, kidney hypoplasia, and polydactyly, combined exome sequencing and mapping identified a homozygous splice-site mutation in PDE6D, encoding a prenyl-binding protein. We found that pde6d depletion in zebrafish leads to renal and retinal developmental anomalies and wild-type but not mutant PDE6D is able to rescue this phenotype. Proteomic analysis identified INPP5E, whose mutations also lead to JS or mental retardation, obesity, congenital retinal dystrophy, and micropenis syndromes, as novel prenyl-dependent cargo of PDE6D. Mutant PDE6D shows reduced binding to INPP5E, which fails to localize to primary cilia in patient fibroblasts and tissues. Furthermore, mutant PDE6D is unable to bind to GTP-bound ARL3, which acts as a cargo-release factor for PDE6D-bound INPP5E. Altogether, these results indicate that PDE6D is required for INPP5E ciliary targeting and suggest a broader role for PDE6D in targeting other prenylated proteins to the cilia. This study identifies PDE6D as a novel JS disease gene and provides the first evidence of prenyl-binding-dependent trafficking in ciliopathies.

C2 domains as protein-protein interaction modules in the ciliary transition zone

RPGR-interacting protein 1 (RPGRIP1) is mutated in the eye disease Leber congenital amaurosis (LCA) and its structural homolog, RPGRIP1-like (RPGRIP1L), is mutated in many different ciliopathies. Both are multidomain proteins that are predicted to interact with retinitis pigmentosa G-protein regulator (RPGR). RPGR is mutated in X-linked retinitis pigmentosa and is located in photoreceptors and primary cilia. We solved the crystal structure of the complex between the RPGR-interacting domain (RID) of RPGRIP1 and RPGR and demonstrate that RPGRIP1L binds to RPGR similarly. RPGRIP1 binding to RPGR affects the interaction with PDEδ, the cargo shuttling factor for prenylated ciliary proteins. RPGRIP1-RID is a C2 domain with a canonical β sandwich structure that does not bind Ca(2+) and/or phospholipids and thus constitutes a unique type of protein-protein interaction module. Judging from the large number of C2 domains in most of the ciliary transition zone proteins identified thus far, the structure presented here seems to constitute a cilia-specific module that is present in multiprotein transition zone complexes.

Pathogenesis of X-linked RP3: insights from animal models

Retinitis Pigmentosa (RP) is a genetically heterogeneous disorder characterized by rod and cone photoreceptor cell dysfunction. X-linked RP (XLRP) is one of the most severe forms of human retinal degeneration, as determined by age-of-set and progression, and accounts for six to 20 % of all RP cases. At least six XLRP loci have been identified, but RP3 is the major subtype of XLRP, accounting for 70 to 80 % of affected families. The RPGR gene is responsible for the RP3 form of XLRP and is mutated in 10-20 % of all RP patients. The pathogenesis of retinitis pigmentosa GTPase regulator (RPGR) mutant-causing RP is not clear, different animal models have been used to understand the pathogenesis of these diseases. In this brief review, we will summarize the functional characterization of RPGR and highlight recent studies in animal models, which will not only shed light on the disease mechanisms in XLRP but will also provide therapeutic strategies for RP treatment.

The interplay between RPGR, PDEδ and Arl2/3 regulate the ciliary targeting of farnesylated cargo

Defects in primary cilia result in human diseases known as ciliopathies. The retinitis pigmentosa GTPase regulator (RPGR), mutated in the most severe form of the eye disease, is located at the transition zone of the ciliary organelle. The RPGR-interacting partner PDEδ is involved in trafficking of farnesylated ciliary cargo, but the significance of this interaction is unknown. The crystal structure of the propeller domain of RPGR shows the location of patient mutations and how they perturb the structure. The RPGR·PDEδ complex structure shows PDEδ on a highly conserved surface patch of RPGR. Biochemical experiments and structural considerations show that RPGR can bind with high affinity to cargo-loaded PDEδ and exposes the Arl2/Arl3-binding site on PDEδ. On the basis of these results, we propose a model where RPGR is acting as a scaffold protein recruiting cargo-loaded PDEδ and Arl3 to release lipidated cargo into cilia.

RPGR mutations might cause reduced orientation of respiratory cilia

RPGR gene encodes retinitis pigmentosa guanosine triphosphatase regulator protein, mutations of which cause 70% of the X-linked retinitis pigmentosa (XLRP) cases. Rarely, RPGR mutations can also cause primary ciliary dyskinesia (PCD), a multisystem disorder characterized by recurrent respiratory tract infections, sinusitis, bronchiectasis, and male subfertility. Two patients with PCD_RP and their relatives were analyzed using DNA sequencing, transmission electron microscopy (TEM), immunofluorescence (IF), photometry, and high-speed videomicroscopy. The Polish patient carried a previously known c.154G>A substitution (p.Gly52Arg) in exon 2 (known to affect splicing); the mutation was co-segregating with the XLRP symptoms in his family. The c.824 G>T mutation (p. Gly275Val) in the Australian patient was a de novo mutation. In both patients, TEM and IF did not reveal any changes in the respiratory cilia structure. However, following ciliogenesis in vitro, in contrast to the ciliary beat frequency, the ciliary beat coordination in the spheroids from the Polish proband and his relatives carrying the c.154G>A mutation was reduced. Analysis of the ciliary alignment indicated severely disturbed orientation of cilia. Therefore, we confirm that defects in the RPGR protein may contribute to syndromic PCD. Lack of ultrastructural defects in respiratory cilia of the probands, the reduced ciliary orientation and the decreased coordination of the ciliary bundles observed in the Polish patient suggested that the RPGR protein may play a role in the establishment of the proper respiratory cilia orientation.

Zebrafish model for the genetic basis of X-linked retinitis pigmentosa

Retinitis pigmentosa (RP) affects 1/4000 individuals in most populations, and X-linked RP (XLRP) is one of the most severe forms of human retinal degeneration. Mutations in both the retinitis pigmentosa GTPase regulator (RPGR) gene and retinitis pigmentosa 2 (RP2) gene account for almost all cases of XLRP. The functional roles of both RPGR and RP2 in the pathogenesis of XLRP are unclear. Due to the surprisingly high degree of functional conservation between human genes and their zebrafish orthologues, the zebrafish has become an important model for human retinal disorders. In this brief review, we summarize the functional characterization of XLRP-causing genes, RPGR and RP2, in zebrafish, and highlight recent studies that provide insight into the cellular functions of both genes. This will not only shed light on disease mechanisms in XLRP but will also provide a solid platform to test RP-causing mutants before proposing XLRP gene therapy trials.

The prenyl-binding protein PrBP/δ: a chaperone participating in intracellular trafficking

Expressed ubiquitously, PrBP/δ functions as chaperone/co-factor in the transport of a subset of prenylated proteins. PrBP/δ features an immunoglobulin-like β-sandwich fold for lipid binding, and interacts with diverse partners. PrBP/δ binds both C-terminal C15 and C20 prenyl side chains of phototransduction polypeptides and small GTP-binding (G) proteins of the Ras superfamily. PrBP/δ also interacts with the small GTPases, ARL2 and ARL3, which act as release factors (GDFs) for prenylated cargo. Targeted deletion of the mouse Pde6d gene encoding PrBP/δ resulted in impeded trafficking to the outer segments of GRK1 and cone PDE6 which are predicted to be farnesylated and geranylgeranylated, respectively. Rod and cone transducin trafficking was largely unaffected. These trafficking defects produce progressive cone-rod dystrophy in the Pde6d(-/-) mouse.

Ciliary transition zone (TZ) proteins RPGR and CEP290: role in photoreceptor cilia and degenerative diseases

INTRODUCTION

Primary cilia are microtubule-based extensions of the plasma membrane in nearly all cell types. In vertebrate photoreceptors, the sensory cilium develops as outer segment (OS) that contains the photopigment rhodopsin and other proteins necessary for phototransduction. The distinct composition of proteins and lipids in the OS membrane is maintained by the selective barrier located at the border between the basal body and the ciliary compartment, called the transition zone (TZ).

AREAS COVERED

In this review, we will discuss the identification and function of two ciliary TZ proteins, RPGR (retinitis pigmentosa GTPase regulator) and CEP290. Mutations in these proteins account for a majority of retinopathies due to ciliary dysfunction. We will also discuss the potential of such information in designing therapeutic approaches to treat cilia-dependent photoreceptor degenerative diseases.

EXPERT OPINION

RPGR and CEP290 perform overlapping yet distinct functions in regulating trafficking of cargo via the TZ of photoreceptors. While RPGR modulates the trafficking by acting as a GEF for the small GTPase RAB8A, CEP290 may be involved in maintaining the polarized distribution of proteins in the OS by modulating intracellular levels of selected proteins involved in inhibiting OS formation.

Structural and functional plasticity of subcellular tethering, targeting and processing of RPGRIP1 by RPGR isoforms

Mutations affecting the retinitis pigmentosa GTPase regulator-interacting protein 1 (RPGRIP1) interactome cause syndromic retinal dystrophies. RPGRIP1 interacts with the retinitis pigmentosa GTPase regulator (RPGR) through a domain homologous to RCC1 (RHD), a nucleotide exchange factor of Ran GTPase. However, functional relationships between RPGR and RPGRIP1 and their subcellular roles are lacking. We show by molecular modeling and analyses of RPGR disease-mutations that the RPGR-interacting domain (RID) of RPGRIP1 embraces multivalently the shared RHD of RPGR_1–19 and RPGR_ORF15 isoforms and the mutations are non-overlapping with the interface found between RCC1 and Ran GTPase. RPGR disease-mutations grouped into six classes based on their structural locations and differential impairment with RPGRIP1 interaction. RPGRIP1α₁ expression alone causes its profuse self-aggregation, an effect suppressed by co-expression of either RPGR isoform before and after RPGRIP1α₁ self-aggregation ensue. RPGR_1–19 localizes to the endoplasmic reticulum, whereas RPGR_ORF15 presents cytosolic distribution and they determine uniquely the subcellular co-localization of RPGRIP1α₁. Disease mutations in RPGR_1–19, RPGR_ORF15, or RID of RPGRIP1α₁, singly or in combination, exert distinct effects on the subcellular targeting, co-localization or tethering of RPGRIP1α₁ with RPGR_1–19 or RPGR_ORF15 in kidney, photoreceptor and hepatocyte cell lines. Additionally, RPGR_ORF15, but not RPGR_1–19, protects the RID of RPGRIP1α₁ from limited proteolysis. These studies define RPGR- and cell-type-dependent targeting pathways with structural and functional plasticity modulating the expression of mutations in RPGR and RPGRIP1. Further, RPGR isoforms distinctively determine the subcellular targeting of RPGRIP1α_1, with deficits in RPGR_ORF15-dependent intracellular localization of RPGRIP1α₁ contributing to pathomechanisms shared by etiologically distinct syndromic retinal dystrophies.

Novel RPGR-ORF15 mutations in X-linked retinitis pigmentosa patients

X-linked retinitis pigmentosa (XLRP) is the most severe type of retinitis pigmentosa (RP), with patients consistently showing early onset and rapid deterioration. Obtaining a genetic diagnosis for a family with XLRP is important for counseling purposes. In this study, we aimed to identify disease-causing mutations in two unrelated XLRP families. Genetic analysis was performed on two unrelated XLRP families. Genomic DNA was extracted from peripheral blood or amniotic fluid samples. The coding regions and intron/exon boundaries of the Retinitis Pigmentosa GTPase Regulator (RPGR) and RP2 genes were amplified by PCR and then sequenced directly. A clinically unaffected pregnant female and the four month old fetus were found to have a hemizygous 2 base pair deletion (g.ORF15+484_485delAA) in the exon ORF15 of RPGR gene. In another XLRP family, a nonsense mutation (g.ORF15+810G>T) was identified. Neither mutation has been reported previously. Both are predicted to cause premature termination of the protein. In conclusion, we identified a micro-deletion through prenatal genetic diagnosis and another novel nonsense mutation in RPGR-ORF15. Identifying a disease-causing mutation facilitated early diagnosis and genetic counseling for the patients. Discovery of novel mutations also broadens knowledge of XLRP and the spectrum of its pathogenic genotypes.

Gene therapeutic approach using mutation-adapted U1 snRNA to correct a RPGR splice defect in patient-derived cells

Retinitis pigmentosa (RP) is a disease that primarily affects the peripheral retina and ultimately causes visual impairment. X-chromosomal forms of RP are frequently caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene. We show that the novel splice donor site (SDS) mutation c.1245+3A>T in intron 10 of RPGR cosegregates with RP in a five-generation Caucasian family. The mutation causes in-frame skipping of exon 10 from RPGR transcripts in patient-derived primary fibroblasts. To correct the splice defect, we developed a gene therapeutic approach using mutation-adapted U1 small nuclear RNA (U1). U1 is required for SDS recognition of pre-mRNAs and initiates the splice process. The mutation described herein interferes with the recognition of the SDS by U1. To overcome the deleterious effects of the mutation, we generated four U1 isoforms with increasing complementarity to the SDS. Lentiviral particles were used to transduce patient-derived fibroblasts with these U1 variants. Full complementarity of U1 corrects the splice defect partially and increases recognition of the mutant SDS. The therapeutic effect is U1-concentration dependent as we show for endogenously expressed RPGR transcripts in patient-derived cells. U1-based gene therapeutic approaches constitute promising technologies to treat SDS mutations in inherited diseases including X-linked RP.

Phosphodiesterase 6D, cGMP-specific rod delta

Network Map States Transitions Functions Protein Classes Sequence Interactions Pathways Domains & Motifs Protein Structure Orthologs Sequence Interactions Pathways Domains & Motifs Protein Structure Orthologs Blast Data.

RPGR: role in the photoreceptor cilium, human retinal disease, and gene therapy

Cilia are specialized dynamic organelles extending from the surface of almost all mammalian cells. Since proteins and protein precursors are transported across the ciliary compartments via intraflagellar transport (IFT), mutations in genes encoding proteins that participate in IFT can cause a spectrum of different ciliopathies. Photoreceptors of the mammalian retina contain ciliary structures that connect the inner (IS) with the outer segments (OS). This structure, the connecting cilium (CC), serves as the only junction between OS and IS, the correct passage of proteins through the CC is crucial for the functioning and maintenance of the cells. Therefore, any impairment of the IFT leads to severe malfunction of photoreceptors, and may induce apoptosis ultimately leading to the degeneration of the retina. The Retinitis Pigmentosa GTPase Regulator (RPGR), which is located in the CC, participates in the IFT and interacts with a variety of proteins, including RPGRIP-1, CEP290, NPM, SMC1 and 3 and IFT88. However, the function of RPGR through its interaction with these proteins is not yet entirely understood. Mutations in the RPGR gene lead to X-linked Retinitis pigmentosa (XLRP), one of the most severe and early onset forms of RP. Gene therapy is considered a potential therapeutic option and is currently under investigation in several animal models of XLRP. However, some of the currently available mouse models are only partially suitable for the development of therapeutic strategies and the quest for more appropriate small animal models is still an issue.

Interaction of retinitis pigmentosa GTPase regulator (RPGR) with RAB8A GTPase: implications for cilia dysfunction and photoreceptor degeneration

Defects in biogenesis or function(s) of primary cilia are associated with numerous inherited disorders (called ciliopathies) that may include retinal degeneration phenotype. The cilia-expressed gene RPGR (retinitis pigmentosa GTPase regulator) is mutated in patients with X-linked retinitis pigmentosa (XLRP) and encodes multiple protein isoforms with a common N-terminal domain homologous to regulator of chromosome condensation 1 (RCC1), a guanine nucleotide exchange factor (GEF) for Ran GTPase. RPGR interacts with several ciliopathy proteins, such as RPGRIP1L and CEP290; however, its physiological role in cilia-associated functions has not been delineated. Here, we report that RPGR interacts with the small GTPase RAB8A, which participates in cilia biogenesis and maintenance. We show that RPGR primarily associates with the GDP-bound form of RAB8A and stimulates GDP/GTP nucleotide exchange. Disease-causing mutations in RPGR diminish its interaction with RAB8A and reduce the GEF activity. Depletion of RPGR in hTERT-RPE1 cells interferes with ciliary localization of RAB8A and results in shorter primary cilia. Our data suggest that RPGR modulates intracellular localization and function of RAB8A. We propose that perturbation of RPGR–RAB8A interaction, at least in part, underlies the pathogenesis of photoreceptor degeneration in XLRP caused by RPGR mutations.

Transcriptional profile analysis of RPGRORF15 frameshift mutation identifies novel genes associated with retinal degeneration

PURPOSE

To identify genes and molecular mechanisms associated with photoreceptor degeneration in a canine model of XLRP caused by an RPGR exon ORF15 microdeletion. Methods. Expression profiles of mutant and normal retinas were compared by using canine retinal custom cDNA microarrays. qRT-PCR, Western blot analysis, and immunohistochemistry (IHC) were applied to selected genes, to confirm and expand the microarray results.

RESULTS

At 7 and 16 weeks, respectively, 56 and 18 transcripts were downregulated in the mutant retinas, but none were differentially expressed (DE) at both ages, suggesting the involvement of temporally distinct pathways. Downregulated genes included the known retina-relevant genes PAX6, CHML, and RDH11 at 7 weeks and CRX and SAG at 16 weeks. Genes directly or indirectly active in apoptotic processes were altered at 7 weeks (CAMK2G, NTRK2, PRKCB, RALA, RBBP6, RNF41, SMYD3, SPP1, and TUBB2C) and 16 weeks (SLC25A5 and NKAP). Furthermore, the DE genes at 7 weeks (ELOVL6, GLOD4, NDUFS4, and REEP1) and 16 weeks (SLC25A5 and TARS2) are related to mitochondrial functions. qRT-PCR of 18 genes confirmed the microarray results and showed DE of additional genes not on the array. Only GFAP was DE at 3 weeks of age. Western blot and IHC analyses also confirmed the high reliability of the transcriptomic data.

CONCLUSIONS

Several DE genes were identified in mutant retinas. At 7 weeks, a combination of nonclassic anti- and proapoptosis genes appear to be involved in photoreceptor degeneration, whereas at both 7 and 16 weeks, the expression of mitochondria-related genes indicates that they may play a relevant role in the disease process.

RPGR-containing protein complexes in syndromic and non-syndromic retinal degeneration due to ciliary dysfunction

Dysfunction of primary cilia due to mutations in cilia-centrosomal proteins is associated with pleiotropic disorders. The primary (or sensory) cilium of photoreceptors mediates polarized trafficking of proteins for efficient phototransduction. Retinitis pigmentosa GTPase regulator (RPGR) is a cilia-centrosomal protein mutated in >70% of X-linked RP cases and 10%-20% of simplex RP males. Accumulating evidence indicates that RPGR may facilitate the orchestration of multiple ciliary protein complexes. Disruption of these complexes due to mutations in component proteins is an underlying cause of associated photoreceptor degeneration. Here, we highlight the recent developments in understanding the mechanism of cilia-dependent photoreceptor degeneration due to mutations in RPGR and PGR-interacting proteins in severe genetic diseases, including retinitis pigmentosa, Leber congenital amaurosis (LCA), Joubert syndrome, and Senior-Loken syndrome, and explore the physiological relevance of photoreceptor ciliary protein complexes.

Multiprotein complexes of Retinitis Pigmentosa GTPase regulator (RPGR), a ciliary protein mutated in X-linked Retinitis Pigmentosa (XLRP)

Mutations in Retinitis Pigmentosa GTPase Regulator (RPGR) are a frequent cause of X-linked Retinitis Pigmentosa (XLRP). The RPGR gene undergoes extensive alternative splicing and encodes for distinct protein isoforms in the retina. Extensive studies using isoform-specific antibodies and mouse mutants have revealed that RPGR predominantly localizes to the transition zone to primary cilia and associates with selected ciliary and microtubule-associated assemblies in photoreceptors. In this chapter, we have summarized recent advances on understanding the role of RPGR in photoreceptor protein trafficking. We also provide new evidence that suggests the existence of discrete RPGR multiprotein complexes in photoreceptors. Piecing together the RPGR-interactome in different subcellular compartments should provide critical insights into the role of alternative RPGR isoforms in associated orphan and syndromic retinal degenerative diseases.

RPGR and RP2: targets for the treatment of X-linked retinitis pigmentosa?

Retinitis pigmentosa is the most important hereditary eye disease and there is currently no cure available. Although mutations were found in more than 40 genes in patients with retinitis pigmentosa, only two genes have thus far been found to be responsible for one of the most severe forms of the disease, X-linked retinitis pigmentosa. In this review, we highlight the current knowledge about the two gene products RPGR and RP2 and try to link genetic data from patients with functional data on the corresponding proteins. Based on the fact that recent gene therapeutic approaches for eye diseases are at a very promising stage, we discuss the potential of RPGR and RP2 as drug targets to treat retinitis pigmentosa.