Understanding variable disease severity in X-linked retinoschisis: Does RS1 secretory mechanism determine disease severity?

Disparate X-linked retinoschisis phenotypes in fraternal twins with the same pathogenic variant in the RS1 gene

INTRODUCTION

In X-linked retinoschisis (XLRS), the RS1 pathogenic variant and the patient's age might be the most important determinants of the XLRS phenotype. In this case report, we present fraternal twins with the same RS1 pathogenic mutation who were examined at the same age yet exhibited significantly different phenotypes.

METHODS

This is a retrospective case report. Both patients underwent best-corrected visual acuity (BCVA) testing, slit-lamp examination, wide-field fundus imaging, optical coherence tomography (OCT), and genetic testing on the same day.

RESULTS

Fraternal twins, aged 21, were found to be hemizygous for the c.267T>A p. (Tyr89*) mutation in the RS1 gene. The first patient presented with a spoke-wheel pattern in the macula, extensive intraretinal cystoid cavities (ICC), and peripheral retinoschisis inferiorly and temporally, accompanied by breaks in the inner retinal layers. The second patient exhibited only tiny ICCs in the macula with mild disruption of the ellipsoid zone at the fovea and no peripheral retinoschisis.

CONCLUSION

Family members with the same pathogenic variant and of the same age can present with significantly different XLRS phenotypes. This highlights the importance of other factors, such as genetic modifiers, epigenetic regulatory events, and environmental influences, in contributing to phenotypic heterogeneity in XLRS patients.

Genotype-Phenotype Correlations in 83 Korean X-linked Retinoschisis Patients: Impact of Retinoschisin 1 Secretion Profiles on Clinical Phenotypes

PURPOSE

To assess the correlation between genotype and phenotype severity in X-linked juvenile retinoschisis (XLRS) by examining clinical and genetic features of a cohort of Korean XLRS patients.

DESIGN

Retrospective, observational study.

PARTICIPANTS

Data from 83 consecutive male patients with molecularly confirmed XLRS were collected retrospectively.

METHODS

Clinical evaluation included best-corrected visual acuity (BCVA), fundus photography, spectral domain OCT (SD-OCT), and full-field electroretinography (ERG).

MAIN OUTCOME MEASURES

The phenotypic characteristics of a cohort of pediatric Korean patients with XLRS, based on mutation types (truncating vs. missense) and secretory profile (secretion vs. nonsecretion), were assessed.

RESULTS

A total of 166 eyes of 83 patients were included. The mean age at diagnosis was 6.1 ± 8.8 years (range, 0.5-20.7 years), with a mean follow-up time of 9.2 ± 7.0 years (range, 0.6-24.3 years). The BCVA at first and last examination ranged from light perception to 0.1 logarithm of the minimum angle of resolution (mean ± standard deviation, 0.75 ± 0.59 and 0.82 ± 0.65, respectively). No significant differences were observed between the truncating (0.71 ± 0.51 and 0.75 ± 0.44) and missense (0.77 ± 0.59 and 0.84 ± 0.66) variants (P = 0.678 and 0.551). Clinical parameters from fundus photography, SD-OCT, and ERG showed no differences. However, BCVA was better for the secretion group (0.51 ± 0.24 and 0.61 ± 0.30) than for the nonsecretion group (0.65 ± 0.71 and 0.87 ± 0.81), with a significant difference in the last BCVA (P = 0.021). OCT revealed a higher frequency of ellipsoid zone disruption in the nonsecretion group (P = 0.030), with no significant differences in other parameters.

CONCLUSIONS

The secretion profile of Retinoschisin 1 (RS1) could influence the severity of XLRS phenotypes. Patients with RS1-secreted mutants, particularly with intact octamerization, exhibit more homogeneous phenotypes and better visual acuity than the RS1-nonsecreted group. This data provide insights for studying genotype and phenotype correlations in both clinical and research fields.

FINANCIAL DISCLOSURE(S)

The authors have no proprietary or commercial interest in any materials discussed in this article.

The D126G mutation contributes to the early-onset X-linked juvenile retinoschisis

X-linked juvenile retinoschisis (XLRS) is an inherited retinal disease caused by mutations in the RS1 gene, resulting in splitting of the retinal layers and visual disturbances. To provide insights on this disease in our cohort, genetic examination, clinical presentation, and functional analysis were performed. We observed three main RS1 mutations in our cohort of six unrelated patients: RS1-D126G, RS1-R209H, and RS1-R213W. The RS1-D126G mutation, exclusively reported in Thai patients so far, showed the highest prevalence. Phenotypically, the D126G mutation manifested early (0.3-4 years old), presenting as asymmetrical visual acuity and schisis. Functional analysis revealed that the molecular pathomechanism of D126G was the failure of protein secretion attributable to endoplasmic reticulum retention. The understanding of the genotype-phenotype relationship and the pathomechanisms of specific mutations in a particular population could immensely benefit the pipeline of personalised treatment design for XLRS.

Genotypic and phenotypic diversity in X-linked retinoschisis: Findings from a South Indian patient cohort

PURPOSE

Retinoschisis is a distinctive condition characterized by intraretinal layer clefts, primarily associated with X-linked recessive inheritance due to RS1 gene mutations. This study aims to uncover the RS1 mutation spectrum in a cohort of 22 X-linked retinoschisis cases from South India and emphasizes the genotypic and phenotypic associations within patients harboring only RS1 mutations.

METHODS

A total of 22 probands were suspected of having X-linked retinoschisis. All study subjects underwent ophthalmic investigations, including assessments of visual acuity, fundus examination, optical coherence tomography (OCT), and electroretinogram (ERG). RS1 gene screening was conducted using Sanger sequencing, and the pathogenicity of the variants was assessed through Sorting Intolerant from Tolerant (SIFT) and PolyPhen-2 in silico tools.

RESULTS

The study found that the probands had an average visual acuity of 0.79 ± 0.39 log of minimum angle of resolution (logMAR), ranging from 0.17 to 1.77. During fundus examination, the probands exhibited a characteristic spoke wheel-like pattern in the macular region. Furthermore, OCT analysis revealed distinct alterations in the inner retinal microstructure, and ERG results consistently showed a reduction in b-wave amplitude. Eventually, Sanger sequencing results showed hemizygous mutations in the RS1 gene in only 12 probands, including a novel missense mutation in the RS1 gene's signal sequence.

CONCLUSION

This study provides valuable insights into the spectrum of RS1 mutations in X-linked retinoschisis probands from South India. It reveals distinct genotypic-phenotypic associations and highlights the clinical manifestations associated with the disease pathogenesis.

Genotype-Phenotype Associations in an X-Linked Retinoschisis Patient Cohort: The Molecular Dynamic Insight and a Promising SD-OCT Indicator

Purpose

This study investigated a three-dimensional indicator in spectral-domain optical coherence tomography (SD-OCT) and established phenotype-genotype correlation in X-linked retinoschisis (XLRS).

Methods

Thirty-seven patients with XLRS underwent comprehensive ophthalmic examinations, including visual acuity (VA), fundus examination, electroretinogram (ERG), and SD-OCT. SD-OCT parameters of central foveal thickness (CFT), cyst cavity volume (CCV), and photoreceptor outer segment length were assessed. CCV was defined as the sum of the areas of cyst cavities in uential B-scans, measured automatically by self-developed software (OCT-CCSEG). Structural changes of the protein associated with missense variants were quantified by molecular dynamics (MD). The correlation between genotype and phenotype was analyzed.

Results

Twenty-seven different RS1 variants were identified, including a novel variant c.336_337insT(p.L113Sfs*8). The average age of onset was 14.76 ± 15.75 years, and the mean VA was 0.84 ± 0.43 logMAR. The mean CCV was 1.69 ± 1.87 mm3, correlating significantly with CFT (R = 0.66; P < 0.01). In the genotype-phenotype analysis of missense variants, CCV significantly correlated with the structural effect on the protein of mutational changes referred to as wild type, including root-mean-square deviation (R = 0.34; P = 0.04), solvent accessible surface area (R = 0.38; P = 0.02), and surface hydrophobic area (R = 0.37; P = 0.03). The amplitude of scotopic 3.0 ERG a-waves and b-waves significantly correlated with the percentage change of the β-strand in the secondary structure (a-wave: R = -0.58, P < 0.01; b-wave: R = -0.53, P < 0.01).

Conclusions

CCV is a promising indicator to quantify the structural disorganization of XLRS retina. The OCT-CCSEG software calculated CCV automatically, potentially facilitating prognosis assessment and development of personalized treatment. Moreover, MD-involved genotype-phenotype analysis suggests an association between protein structural alterations and XLRS severity measured by CCV and ERG.

Identification and molecular characterization of two recurrent missense mutations in the RS1 gene in two families with X-linked retinoschisis from North India

X-linked retinoschisis (XLR) is a rare medical condition that involves in the splitting of neurosensory layers and the impairment of vision in the retina. In majority of the XLR cases, pathogenic variants in Retinoschisin 1 (RS1) gene have been implicated in males with an early age of onset during early childhood. In the present study, we have recruited two North Indian families having multiple affected male members, who were diagnosed with XLR. The entire protein-coding region of RS1 was screened by PCR-Sanger sequencing and two recurrent pathogenic variants (p.I81N and p.R102Q) were unraveled. The in vitro study of these variants demonstrated the aggregation of mutant RS1 within the endoplasmic reticulum. Furthermore, mutant forms of this protein showed significant intracellular retention, which was evident by the absence of retinoschisin protein fractions in the extracellular media. These inferences were also supported by extensive bioinformatics analysis of the mutants, which showed dramatic conformational changes in the local structure of retinoschisin. Thus, our study suggests that the identified pathogenic variants interfere with proper protein folding, leading to anomalous structural changes ultimately resulting in intracellular retention of retinoschisin within the retina.

Advances in understanding the molecular structure of retinoschisin while questions remain of biological function

Retinoschisin (RS1) is a secreted protein that is essential for maintaining integrity of the retina. Numerous mutations in RS1 cause X-linked retinoschisis (XLRS), a progressive degeneration of the retina that leads to vision loss in young males. A key manifestation of XLRS is the formation of cavities (cysts) in the retina and separation of the layers (schisis), disrupting synaptic transmission. There are currently no approved treatments for patients with XLRS. Strategies using adeno-associated viral (AAV) vectors to deliver functional copies of RS1 as a form of gene augmentation therapy, are under clinical evaluation. To improve therapeutic strategies for treating XLRS, it is critical to better understand the secretion of RS1 and its molecular function. Immunofluorescence and immunoelectron microscopy show that RS1 is located on the surfaces of the photoreceptor inner segments and bipolar cells. Sequence homology indicates a discoidin domain fold, similar to many other proteins with demonstrated adhesion functions. Recent structural studies revealed the tertiary structure of RS1 as two back-to-back octameric rings, each cross-linked by disulfides. The observation of higher order structures in vitro suggests the formation of an adhesive matrix spanning the distance between cells (∼100 nm). Several studies indicated that RS1 readily binds to other proteins such as the sodium-potassium ATPase (NaK-ATPase) and extracellular matrix proteins. Alternatively, RS1 may influence fluid regulation via interaction with membrane proteins such as the NaK-ATPase, largely inferred from the use of carbonic anhydrase inhibitors to shrink the typical intra-retinal cysts in XLRS. We discuss these models in light of RS1 structure and address the difficulty in understanding the function of RS1.

X-linked retinoschisis: mutation spectrum and genotype-phenotype relationship in an Italian pediatric cohort

BACKGROUND

X-linked juvenile retinoschisis (×LRS) is an X-linked vitreoretinal degenerative disease that consists of variable phenotypes ranging from severe early-onset defects to subtle abnormalities diagnosed in elderly patients. XLRS is caused by a loss of function of the protein Retinoschisin (RS1), which is essential to preserve retinal integrity and function of photoreceptor-bipolar synapse. The literature data so far mostly agree on the absence of a clear genotype-phenotype correlation in XLRS. We reviewed clinical and molecular characteristics of a cohort of Italian pediatric XLRS patients to assess the presence of a correlation between genotype and phenotype severity.

MATERIALS AND METHODS

We retrospectively examined clinical and genetic features of a cohort of 27 XLRS patients. In this study we included patients with a diagnosis of XLRS confirmed by fundus photography, spectral domain optical coherence tomography, and molecular analysis and with an onset of less than 10 years of age. We sorted RS1 variants according to their effect of RS1 structure and function in three separate groups.

RESULTS

According to previous studies, we did not observe a conclusive genotype-phenotype correlation in our cohort; nevertheless, we noticed that patients harboring RS1 variants leading to RS1-secreted mutants show a more homogeneous phenotype, with an overall good visual acuity, compared to the other two groups.

CONCLUSIONS

Our data support the hypothesis that secretion profile of RS1 could influence the severity of the phenotype. More extensive and functional studies are needed to acquire notions in view of the opportunity of gene replacement therapy for XLRS patients.

Genotype and phenotype characteristics of X-linked retinoschisis: the first report of a Turkish population

BACKGROUND

X-linked retinoschisis is an inherited retinal disease caused by mutations in the RS1 gene; however, a genotype-phenotype correlation regarding the mutation type or location within the RS1 gene and clinical characteristics of the patients has not been established yet. This is the first report documenting the genotypes and ophthalmological findings in a Turkish population with confirmed RS1 mutations.

MATERIALS AND METHODS

Fifty eyes of 25 male patients were included in the study. RS1 mutation analysis was performed by DNA sequencing. Retrospective analysis of ocular examinations and SD-OCT scans were applied.

RESULTS

The major mutation was c.422 G > A (p.Arg141His, exon 5) affecting 14 patients (56%) and c.531 T > G was the only non-sense mutation out of 7 pathogenic variants. At presentation; the mean age was 24.6 ± 16.2 (4-72) years, mean visual acuity (VA) was 0.61 ± 0.32 (logMAR, 0.10-1.30). Forty-six (92%) eyes had macular, 16 eyes (32%) had peripheral retinoschisis. None of the eyes had macular scar, whereas 7 eyes (14%) had macular atrophy. The most frequent location of schisis was inner nuclear layer (37.5%). The eyes with disruption of ellipsoid zone (EZ) or external limiting membrane (ELM) had worse VA (for EZ, 0.65 ± 0.25 versus 0.45 ± 0.34, logMAR, 31 versus 17 eyes, p = .013; for ELM, 0.66 ± 0.27 versus 0.45 ± 0.31, logMAR, 30 versus 18 eyes, p = .008).

CONCLUSIONS

Seven different pathogenic variants in the RS1 gene were identified; with c.422 G > A (p.Arg141His) as the most frequent variant and c.531 T > G as only non-sense mutation. Having EZ or ELM disruption were the significant factors affecting VA.

Of men and mice: Human X-linked retinoschisis and fidelity in mouse modeling

X-linked Retinoschisis (XLRS) is an early-onset transretinal dystrophy, often with a prominent macular component, that affects males and generally spares heterozygous females because of X-linked recessive inheritance. It results from loss-of-function RS1 gene mutations on the X-chromosome. XLRS causes bilateral reduced acuities from young age, and on clinical exam and by ocular coherence tomography (OCT) the neurosensory retina shows foveo-macular cystic schisis cavities in the outer plexiform (OPL) and inner nuclear layers (INL). XLRS manifests between infancy and school-age with variable phenotypic presentation and without reliable genotype-phenotype correlations. INL disorganization disrupts synaptic signal transmission from photoreceptors to ON-bipolar cells, and this reduces the electroretinogram (ERG) bipolar b-wave disproportionately to photoreceptor a-wave changes. RS1 gene expression is localized mainly to photoreceptors and INL bipolar neurons, and RS1 protein is thought to play a critical cell adhesion role during normal retinal development and later for maintenance of retinal structure. Several independent XLRS mouse models with mutant RS1 were created that recapitulate features of human XLRS disease, with OPL-INL schisis cavities, early onset and variable phenotype across mutant models, and reduced ERG b-wave to a-wave amplitude ratio. The faithful phenotype of the XLRS mouse has assisted in delineating the disease pathophysiology. Delivery to XLRS mouse retina of an AAV8-RS1 construct under control of the RS1 promoter restores the retinal structure and synaptic function (with increase of b-wave amplitude). It also ameliorates the schisis-induced inflammatory microglia phenotype toward a state of immune quiescence. The results imply that XLRS gene therapy could yield therapeutic benefit to preserve morphological and functional retina particularly when intervention is conducted at earlier ages before retinal degeneration becomes irreversible. A phase I/IIa single-center, open-label, three-dose-escalation clinical trial reported a suitable safety and tolerability profile of intravitreally administered AAV8-RS1 gene replacement therapy for XLRS participants. Dose-related ocular inflammation occurred after dosing, but this resolved with topical and oral corticosteroids. Systemic antibodies against AAV8 increased in dose-dependent fashion, but no antibodies were observed against the RS1 protein. Retinal cavities closed transiently in one participant. Technological innovations in methods of gene delivery and strategies to further reduce immune responses are expected to enhance the therapeutic efficacy of the vector and ultimate success of a gene therapy approach.

Surgical management of a large retinal cyst in X-linked retinoschisis with internal drainage: Report of an unusual case

Although X-linked retinoschisis is a common retinal degeneration condition, the presence of a large retinal cyst obscuring the visual axis in an infant is a rare presentation. Herein, we describe such a case of a child who presented to us with the diagnosis of retinal detachment in both the eyes. However, following multimodal imaging and electrophysiology, the child was found to have bilateral juvenile retinoschisis with a large retinoschisis cyst involving the visual axis seen intraoperatively in the left eye. A limbal approach followed by lensectomy was used to excise the inner retinal layer of the cyst. The intracystic fluid was then drained and the stretched retinal vessels were endocauterized and severed without causing any iatrogenic outer retinal breaks and retinal detachment. The correct diagnosis and meticulous preoperative planning of the surgical procedure helped us manage this challenging case with a favorable anatomical and functional outcome.