Pathogenic variants in IMPG1 cause autosomal dominant and autosomal recessive retinitis pigmentosa

Clinical manifestations of dual-gene variants involving ABCA4 in retinal dystrophies

BACKGROUND

This study investigates the clinical manifestations of inherited retinal diseases (IRD) associated with dual-gene variant constellations involving biallelic ABCA4 variants.

METHODS

We assess four cases for their unique phenotypic outcomes due to biallelic ABCA4 variants and additional genotypes in CACNA1F, IMPG1, HK1 and MYO7A, respectively.

RESULTS

This study investigates the phenotypic impact of dual-gene variants, including biallelic ABCA4 variants and additional retinal gene variants in CACNA1F, IMPG1, HK1 and MYO7A. In MST465-II:1, the ABCA4-CACNA1F constellation led to progressive macular atrophy and night blindness, with nystagmus linked to CACNA1F. In MST448-II:1, ABCA4 variants primarily contributed to a macular dystrophy, while the IMPG1 variant had no obvious impact, suggesting it may be a benign polymorphism. In SRP1400-II:1, a de novo HK1 variant caused retinitis pigmentosa (RP)-like retinal degeneration and intellectual disability and in USHI105-II:1, MYO7A variants primarily resulted in an Usher syndrome 1 phenotype. In both latter cases, ABCA4 variants play a more subtle role. These findings illustrate the importance of critical phenotype and genotype assessment and how complex interactions between ABCA4 and other genetic variants can configure the phenotype, making it challenging to distinguish the contributions of each gene.

CONCLUSIONS

This study underscores the importance of advanced diagnostic tools and careful genotype evaluation to accurately identify and understand potential complex genetic interactions in IRDs. The observed phenotypes enhance our understanding of how these genes contribute to human retinal function and dysfunction. Furthermore, these insights can impact clinical decision-making, as patients with dual-gene variant constellations might experience questionable benefit from potential future gene therapies. Thus, careful patient selection and complete genotype and phenotype assessment before treatment is essential to manage potential risks and costs effectively.

Unveiling the therapeutic potential and mechanisms of stanniocalcin-1 in retinal degeneration

Retinal degeneration (RD) is a group of ocular diseases characterized by progressive photoreceptor apoptosis and visual impairment. Mitochondrial malfunction, excessive oxidative stress, and chronic activation of neuroglia collectively contribute to the development of RD. Currently, there is a lack of efficacious therapeutic interventions for RD. Stanniocalcin-1 (STC-1) is a promising candidate molecule to decelerate photoreceptor cell death. STC-1 is a secreted calcium/phosphorus regulatory protein that exerts diverse protective effects. Accumulating evidence suggests that STC-1 protects retinal cells from ischemic injury, oxidative stress, and excessive apoptosis through enhancing the expression of uncoupling protein-2 (UCP-2). Furthermore, STC-1 exerts its antiinflammatory effects by inhibiting the activation of microglia and macrophages, as well as the synthesis and secretion of proinflammatory cytokines, such as TNF-α, IL-1, and IL-6. By employing these mechanisms, STC-1 effectively shields the retinal photoreceptors and optic nerve, thereby slowing down the progression of RD. We summarize the STC-1-mediated therapeutic effects on the degenerating retina, with a particular focus on its underlying mechanisms. These findings highlight that STC-1 may act as a versatile molecule to treat degenerative retinopathy. Further research on STC-1 is imperative to establish optimal protocols for its clinical use.

Prevalence of IMPG1 and IMPG2 Mutations Leading to Retinitis Pigmentosa or Vitelliform Macular Dystrophy in a Cohort of Patients with Inherited Retinal Dystrophies

BACKGROUND/OBJECTIVES

The interphotoreceptor matrix proteoglycans 1 and 2 (IMPG1 and IMPG2) are two interdependent proteoglycans of the interphotoreceptor matrix (IPM). Mutations in IMPG1 or IMPG2 are linked to retinal diseases such as retinitis pigmentosa (RP) and vitelliform macular dystrophy (VMD), yet the specific mutations responsible for each condition remain undefined. This study identifies mutations in IMPG1 and IMPG2 linked to either RP or VMD. It also provides an in-depth in silico analysis of these mutations' structural and functional impact on protein domains, alongside a detailed examination of the corresponding disease phenotypes.

METHODS

From a cohort of 480 patients with inherited retinal diseases (IRDs), we identified seven patients with mutations in IMPG1 or IMPG2. Multimodal imaging was performed to assess the clinical phenotypes, including fundus photography, fundus autofluorescence, fluorescein angiography, and spectral domain optical coherence tomography (SD-OCT). We provide structure modeling and analysis of each variant.

RESULTS

Our findings indicate a prevalence of 1.45% of IRD patients being affected by IMPG mutations; two were diagnosed with RP and five with VMD. One VMD patient carried a novel IMPG1 p.Asp423Glu mutation. Most patients exhibited heterozygous mutations, and one RP patient presented a compound heterozygous mutation in IMPG2.

CONCLUSIONS

This work describes a novel mutation and expands our understanding of the specific IMPG protein domains implicated in RP and VMD. Furthermore, it establishes, for the first time, the prevalence of IMPG mutations in an IRD population.

Critical Roles of SEA Domains

This mini-review delves into the multifaceted roles of SEA (sea urchin sperm protein, enterokinase, and agrin) domains, ubiquitous protein modules critical to the structure, and function of a wide range of membrane-associated and secreted proteins in organisms from yeast to humans. We explore the structural and functional characteristics of SEA domains based on their two types of fundamental characteristics: proteolytic and non-proteolytic SEA domains. We also examine the significance of SEA domains in different protein families, particularly in mucins and extracellular matrix proteins, emphasizing their roles in glycosylation, cell adhesion, and signal transduction. The review also highlights the crucial impact of SEA domains in health and disease contexts, with a focus on their implications in cancer progression and retinal health. Mutations within these domains are linked to a range of pathologies, including various cancers and congenital disorders, underscoring their clinical importance. Through this review, we aim to provide a deeper understanding of SEA domains, shedding light on their diverse biological functions and their potential as targets for therapeutic interventions in diseases where they play a pivotal role.

Dual inheritance patterns: A spectrum of non-syndromic inherited retinal disease phenotypes with varying molecular mechanisms

Inherited retinal diseases (IRDs) encompass a variety of disease phenotypes and are known to display both clinical and genetic heterogeneity. A further complexity is that for several IRD-associated genes, pathogenic variants have been reported to cause either autosomal dominant (AD) or autosomal recessive (AR) diseases. The possibility of dual inheritance can create a challenge for variant interpretation as well as the genetic counselling of patients. This review aims to determine whether the molecular mechanisms behind the dual inheritance of each IRD-associated gene is well established, not yet properly understood, or if the association is questionable. Each gene is discussed individually in detail due to different protein structures and functions, but there are overlapping characteristics. For example, eight genes only have a limited number of reported pathogenic variants or a hotspot region implicated in the second inheritance pattern. Whereas CRX and RP1 display distinct spatial patterns for AR and AD pathogenic variants based on the variant type and/or location. The genes with a questionable dual inheritance, namely AIPL1, CRB1, and RCBTB1 highlight the importance of carefully considering allele frequency data. Finally, the crucial role relevant functional studies in animal and cell models play in validating a variant's biochemical or molecular effect is emphasised.

Clinician-Driven Reanalysis of Exome Sequencing Data From Patients With Inherited Retinal Diseases

Importance

Despite advances in next-generation sequencing (NGS), a significant proportion of patients with inherited retinal disease (IRD) remain undiagnosed after initial genetic testing. Exome sequencing (ES) reanalysis in the clinical setting has been suggested as one method for improving diagnosis of IRD.

Objective

To investigate the association of clinician-led reanalysis of ES data, which incorporates updated clinical information and comprehensive bioinformatic analysis, with the diagnostic yield in a cohort of patients with IRDs in Korea.

Design, Setting, and Participants

This was a multicenter prospective cohort study involving 264 unrelated patients with IRDs, conducted in Korea between March 2018 and February 2020. Comprehensive ophthalmologic examinations and ES analyses were performed, and ES data were reanalyzed by an IRD specialist for single nucleotide variants, copy number variants, mobile element insertions, and mitochondrial variants. Data were analyzed from March to July 2023.

Main Outcomes and Measures

Diagnostic rate of conventional bioinformatic analysis and clinician-driven ES reanalysis.

Results

A total of 264 participants (151 [57.2%] male; mean [SD] age at genetic testing, 33.6 [18.9] years) were enrolled, including 129 patients (48.9%) with retinitis pigmentosa and 26 patients (9.8%) with Stargardt disease or macular dystrophy. Initial bioinformatic analysis diagnosed 166 patients (62.9%). Clinician-driven reanalysis identified the molecular cause of diseases in an additional 22 patients, corresponding to an 8.3-percentage point increase in diagnostic rate. Key factors associated with new molecular diagnoses included clinical phenotype updates (4 patients) and detection of previously overlooked variation, such as structural variants (9 patients), mitochondrial variants (3 patients), filtered or not captured variants (4 patients), and noncanonical splicing variants (2 patients). Among the 22 patients, variants in 7 patients (31.8%) were observed in the initial analysis but not reported to patients, while those in the remaining 15 patients (68.2%) were newly detected by the ES reanalysis.

Conclusions and Relevance

In this cohort study, clinician-centered reanalysis of ES data was associated with improved molecular diagnostic yields in patients with IRD. This approach is important for uncovering missed genetic causes of retinal disease.

Posterior Polar Annular Choroidal Dystrophy: Genetic Insights and Differential Diagnosis in Inherited Retinal Diseases

Posterior polar annular choroidal dystrophy (PPACD) is a rare ocular disorder and presents as symmetric degeneration of the retinal pigment epithelium (RPE) and the underlying choriocapillaris, encircling the retinal vascular arcades and optic disc. This condition distinctively preserves the foveal region, optic disc, and the outermost regions of the retina. Despite its distinct clinical presentation, due to the infrequency of its occurrence and the limited number of reported cases, the pathophysiology, and the genetic foundations of PPACD are still largely uncharted. This review aims to bridge this knowledge gap by investigating potential genetic contributors to PPACD, assessing current findings, and identifying genes that warrant further study. Emphasis is also placed on the crucial role of multimodal imaging in diagnosing PPACD, highlighting its importance in understanding disease pathophysiology. By analyzing existing case reports and drawing comparisons with similar retinal disorders, this paper endeavors to delineate the possible genetic correlations in PPACD, providing a foundation for future genetic research and the development of targeted diagnostic strategies.

Vitelliform maculopathy in MELAS syndrome

Purpose

We present a unique case of foveomacular vitelliform lesions in a patient with metabolic encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS).

Observations

After performing large panel next generation sequencing genetic testing, there was no likely alternative genetic etiology for vitelliform maculopathy in this patient.

Conclusions and Importance

We present a rare case of a visually asymptomatic pediatric patient with MELAS and vitelliform maculopathy, which may be part of the spectrum of retinal manifestations in MELAS. Pediatric-onset vitelliform maculopathy in MELAS may be under-diagnosed due to its asymptomatic nature. Given the known risk of choroidal neovascularization in vitelliform maculopathy, it is important to identify these patients for proper surveillance.

IMPG2-associated unilateral adult onset vitelliform macular dystrophy

Purpose

To present a case of unilateral IMPG2-associated adult onset vitelliform macular dystrophy (AVMD).

Observations

A 68 year-old female presented with best corrected visual acuity (BCVA) of 20/20 and 20/40 for the right and left eye respectively. The patient had a left subfoveal yellow lesion on dilated fundus examination. Optical coherence tomography showed hyper-reflective material accumulation below the fovea in the left eye only. The patient was followed for 10 years with stable BCVA, and evolution of the subretinal vitelliform lesion to a “vitelliruptive” stage. The right eye did not develop vitelliform lesion. Genetic testing identified a heterozygous likely disease-causing variant in IMPG2; c.3423–7_3423-4del.

Conclusions and importance

This is the first report of unilateral AVMD associated with IMPG2, expanding the phenotypic spectrum of IMPG2 retinopathy. We provide further evidence that IMPG2 variants can cause both autosomal recessive rod-cone dystrophy and autosomal dominant AVMD, with implications for patient counselling.

Interphotoreceptor matrix proteoglycans IMPG1 and IMPG2 proteolyze in the SEA domain and reveal localization mutual dependency

The interphotoreceptor matrix (IPM) is a specialized extracellular mesh of molecules surrounding the inner and outer segments of photoreceptor neurons. Interphotoreceptor matrix proteoglycan 1 and 2 (IMPG1 and IMPG2) are major components of the IPM. Both proteoglycans possess SEA (sperm protein, enterokinase and agrin) domains, which may support proteolysis. Interestingly, mutations in the SEA domains of IMPG1 and IMPG2 are associated with vision disease in humans. However, if SEA domains in IMPG molecules undergo proteolysis, and how this contributes to vision pathology is unknown. Therefore, we investigated SEA-mediated proteolysis of IMPG1 and IMPG2 and its significance to IPM physiology. Immunoblot analysis confirmed proteolysis of IMPG1 and IMPG2 in the retinas of wildtype mice. Point mutations mimicking human mutations in the SEA domain of IMPG1 that are associated with vision disease inhibited proteolysis. These findings demonstrate that proteolysis is part of the maturation of IMPG1 and IMPG2, in which deficits are associated with vision diseases. Further, immunohistochemical assays showed that proteolysis of IMPG2 generated two subunits, a membrane-attached peptide and an extracellular peptide. Notably, the extracellular portion of IMPG2 trafficked from the IPM around the inner segment toward the outer segment IPM by an IMPG1-dependent mechanism. This result provides the first evidence of a trafficking system that shuttles IMPG1 and IMPG2 from the inner to outer IPM in a co-dependent manner. In addition, these results suggest an interaction between IMPG1-IMPG2 and propose that mutations affecting one IMPG could affect the localization of the normal IMPG partner, contributing to the disease mechanism of vision diseases associated with defective IMPG molecules.

Bioinformatic analysis identifies potential key genes in the pathogenesis of age-related macular degeneration

Purpose

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in older individuals. More studies focused on screening the genes, which may be correlated with the development of AMD. With advances in various technologies like multiple microarray datasets, researchers could identify differentially expressed genes (DEGs) more accurately. Exploring abnormal gene expression in disease status can help to understand pathophysiological changes in complex diseases. This study aims to identify the key genes and upstream regulators in AMD and reveal factors, especially genetic association, and the prognosis of the development of this disease.

Methods

Data from expression profile GSE125564 and profile GSE29801 were obtained from the Gene Expression Omnibus (GEO) database. We analyzed DEGs using R software (version 3.6.3). Functional enrichment and PPI network analysis were performed using the R package and online database STRING (version 11.0).

Results

We compared AMD with normal and found 68 up-regulated genes (URGs) and 25 down-regulated genes (DRGs). We also compared wet AMD with dry AMD and found 41 DRGs in dry AMD. Further work including PPI network analysis, GO classification, and KEGG analysis was done to find connections with AMD. The URGs were mainly enriched in the biological process such as DNA replication, nucleoplasm, extracellular exosome, and cadherin binding. Besides, DRGs were mainly enriched in these functions such as an integral component of membrane and formation of the blood-aqueous barrier (BAB).

Conclusion

This study implied that core genes might involve in the process of AMD. Our findings may contribute to revealing the pathogenesis, developing new biomarkers, and raising strategies of treatment for AMD.

SPACR Encoded by IMPG1 Is Essential for Photoreceptor Survival by Interplaying between the Interphotoreceptor Matrix and the Retinal Pigment Epithelium

Several pathogenic variants have been reported in the IMPG1 gene associated with the inherited retinal disorders vitelliform macular dystrophy (VMD) and retinitis pigmentosa (RP). IMPG1 and its paralog IMPG2 encode for two proteoglycans, SPACR and SPACRCAN, respectively, which are the main components of the interphotoreceptor matrix (IPM), the extracellular matrix surrounding the photoreceptor cells. To determine the role of SPACR in the pathological mechanisms leading to RP and VMD, we generated a knockout mouse model lacking Impg1, the mouse ortholog. Impg1-deficient mice show abnormal accumulation of autofluorescent deposits visible by fundus imaging and spectral-domain optical coherence tomography (SD-OCT) and attenuated electroretinogram responses from 9 months of age. Furthermore, SD-OCT of Impg1^−/− mice shows a degeneration of the photoreceptor layer, and transmission electron microscopy shows a disruption of the IPM and the retinal pigment epithelial cells. The decrease in the concentration of the chromophore 11-cis-retinal supports this loss of photoreceptors. In conclusion, our results demonstrate the essential role of SPACR in maintaining photoreceptors. Impg1^−/− mice provide a novel model for mechanistic investigations and the development of therapies for VMD and RP caused by IMPG1 pathogenic variants.

Genetic dissection of non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) belongs to a group of pigmentary retinopathies. It is the most common form of inherited retinal dystrophy, characterized by progressive degradation of photoreceptors that leads to nyctalopia, and ultimately, complete vision loss. RP is distinguished by the continuous retinal degeneration that progresses from the mid-periphery to the central and peripheral retina. RP was first described and named by Franciscus Cornelius Donders in the year 1857. It is one of the leading causes of bilateral blindness in adults, with an incidence of 1 in 3000 people worldwide. In this review, we are going to focus on the genetic heterogeneity of this disease, which is provided by various inheritance patterns, numerosity of variations and inter-/intra-familial variations based upon penetrance and expressivity. Although over 90 genes have been identified in RP patients, the genetic cause of approximately 50% of RP cases remains unknown. Heterogeneity of RP makes it an extremely complicated ocular impairment. It is so complicated that it is known as “fever of unknown origin”. For prognosis and proper management of the disease, it is necessary to understand its genetic heterogeneity so that each phenotype related to the various genetic variations could be treated.

Identification of a Complex Allele in IMPG2 as a Cause of Adult-Onset Vitelliform Macular Dystrophy

Purpose

Inherited retinal diseases are a group of clinically and genetically heterogeneous disorders with approximately 270 genes involved. IMPG2 is associated with adult-onset vitelliform macular dystrophy. Here, we investigated two unrelated patients with vitelliform macular dystrophy to identify the underlying genetic cause.

Methods

Whole-exome sequencing identified a putative causal complex allele consisting of c.3023-15T>A and c.3023G>A (p.(Gly1008Asp)) in IMPG2 in both individuals. To assess its effect, in vitro splice assays in HEK293T and further characterization in patient-derived photoreceptor precursor cells (PPCs) were conducted.

Results

The results of the midigene splice assays in HEK293T showed that the complex allele causes a variety of splicing defects ranging from a small deletion to (multiple-)exon skipping. This finding was further validated using patient-derived PPCs that showed a significant increase of out-of-frame transcripts lacking one or multiple exons compared to control-derived PPCs. Overall, control PPCs consistently showed low levels of aberrantly spliced IMPG2 transcripts that were highly elevated in patient-derived PPCs. These differences were even more obvious upon inhibition of nonsense-mediated decay with cycloheximide.

Conclusions

We report a heterozygous complex allele in IMPG2 causative for adult-onset vitelliform macular dystrophy in two unrelated individuals with mild visual loss and bilateral vitelliform lesions. The predicted causal missense mutation c.3023G>A, located in the consensus splice acceptor site, enhances the splicing effect of the upstream variant c.3023-15T>A, leading to the generation of aberrant transcripts that decrease the full-length IMPG2 levels. These results suggest a haploinsufficiency mechanism of action and highlight the complementarity of using different models to functionally assesses splicing defects.