Characteristic Ocular Features in Cases of Autosomal Recessive PROM1 Cone-Rod Dystrophy

Deep phenotyping of PROM1-associated retinal degeneration

BACKGROUND/AIMS

The purpose of this study was to investigate retinal structure in detail of subjects with autosomal-dominant (AD) and autosomal-recessive (AR) PROM1-associated retinal degeneration (PROM1-RD), study design: institutional, cross-sectional study.

METHODS

Four eyes from four subjects (three with AD and one with AR) PROM1-RD were investigated by ophthalmic examination including best-corrected visual acuity (BCVA) and multimodal retinal imaging: fundus autofluorescence (FAF), spectral-domain optical coherence tomography (SD-OCT) and adaptive optics scanning light ophthalmoscopy. Quantitative assessment of atrophic lesions determined by FAF, thickness of individual retinal layers and cone photoreceptor quantification was performed.

RESULTS

BCVA ranged from 20/16 to 20/200. Initial pathological changes included the presence of hyperautofluorescent spots on FAF imaging, while later stages demonstrated discrete areas of atrophy. In all patients, thinning of the outer retinal layers on SD-OCT with varying degrees of atrophy could be detected depending on disease-causing variants and age. Cone density was quantified both in central and/or at different eccentricities from the fovea. Longitudinal assessments were possible in two patients.

CONCLUSIONS

PROM1-RD comprises a wide range of clinical phenotypes. Depending on the stage of disease, the cone mosaic in PROM1-RD is relatively preserved and can potentially be targeted by cone-directed interventions.

Next-generation sequencing--based genetic testing and phenotype correlation in retinitis pigmentosa patients from India

Purpose

Inherited retinal dystrophies (IRD) are a heterogeneous group of retinal diseases leading to progressive loss of photoreceptors through apoptosis. Retinitis pigmentosa (RP) is considered the most common form of IRD. Panel-based testing in RP has proven effective in identifying the causative genetic mutations in 70% and 80% of the patients. This is a retrospective, observational, single-center study of 107 RP patients who had undergone next-generation sequencing-based targeted gene panel testing for IRD genes. These patients were inspected for common phenotypic features to arrive at meaningful genotype-phenotype correlation.

Methods

Patients underwent complete ophthalmic examination, and blood was collected from the proband for DNA extraction after documenting the pedigree. Targeted Next Generation Sequencing (NGS) was done by panel-based testing for IRD genes followed by co-segregation analysis wherever applicable.

Results

Of the 107 patients, 72 patients had pathogenic mutations. The mean age of onset of symptoms was 14 ± 12 years (range: 5-55). Mean (Best Corrected Visual Acuity) BCVA was 6/48 (0.9 logMAR) (range 0.0-3.0). At presentation, over one-third of eyes had BCVA worse than 6/60 (<1 logMAR). Phenotype analysis with the gene defects showed overlapping features, such as peripheral well-defined chorioretinal atrophic patches in patients with CERKL, PROM1, and RPE65 gene mutations and large macular lesions in patients with RDH12 and CRX gene mutations, respectively. Nummular or clump-like pigmentation was noted in CRB1, TTC8, PDE6A, and PDE6B.

Conclusion

NGS-based genetic testing can help clinicians to diagnose RP more accurately, and phenotypic correlations can also help in better patient counselling with respect to prognosis and guidance regarding ongoing newer gene-based therapies.

Clinical Phenotypes of CDHR1-Associated Retinal Dystrophies

The retinal dystrophy phenotype associated with CDHR1 retinopathy is clinically heterogenous. In this study, we describe the clinical and molecular findings of a retinal dystrophy cohort (10 patients) attributed to autosomal recessive CDHR1 and report novel variants in populations not previously identified with CDHR1-related retinopathy. Seven patients had evaluations covering at least a three-year period. The mean age of individuals at first symptoms was 36 ± 8.5 years (range 5–45 years). Visual acuity at the last visit ranged from 20/20 to 20/2000 (mean LogMAR 0.8 or 20/125). Three clinical subgroups were identified: rod–cone dystrophy (RCD), cone–rod dystrophy (CRD), and maculopathy. Extinguished scotopic electroretinography responses were noted in the RCD patients. Macular involvement was noted in all patients and documented on color fundus photography, fundus autofluorescence, and optical coherence tomography. Notable asymmetry of the degree of macular atrophy was present in two patients. The possible association between CDHR1 variants and clinical findings was predicted using molecular modeling.

Exocyst-mediated membrane trafficking of the lissencephaly-associated ECM receptor dystroglycan is required for proper brain compartmentalization

To assemble a brain, differentiating neurons must make proper connections and establish specialized brain compartments. Abnormal levels of cell adhesion molecules disrupt these processes. Dystroglycan (Dg) is a major non-integrin cell adhesion receptor, deregulation of which is associated with dramatic neuroanatomical defects such as lissencephaly type II or cobblestone brain. The previously established Drosophila model for cobblestone lissencephaly was used to understand how Dg is regulated in the brain. During development, Dg has a spatiotemporally dynamic expression pattern, fine-tuning of which is crucial for accurate brain assembly. In addition, mass spectrometry analyses identified numerous components associated with Dg in neurons, including several proteins of the exocyst complex. Data show that exocyst-based membrane trafficking of Dg allows its distinct expression pattern, essential for proper brain morphogenesis. Further studies of the Dg neuronal interactome will allow identification of new factors involved in the development of dystroglycanopathies and advance disease diagnostics in humans.

Childhood-onset genetic cone-rod photoreceptor diseases and underlying pathobiology

Inherited retinal diseases (IRDs) were first classified clinically by history, ophthalmoscopic appearance, type of visual field defects, and electroretinography (ERG). ERGs isolating the two major photoreceptor types (rods and cones) showed some IRDs with greater cone than rod retinal dysfunction; others were the opposite. Within the cone-rod diseases, there can be phenotypic variability, which can be attributed to genetic heterogeneity and the variety of visual function mechanisms that are disrupted. Most cause symptoms from childhood or adolescence, although others can manifest later in life. Among the causative genes for cone-rod dystrophy (CORD) are those encoding molecules in phototransduction cascade activation and recovery processes, photoreceptor outer segment structure, the visual cycle and photoreceptor development. We review 11 genes known to cause cone-rod disease in the context of their roles in normal visual function and retinal structure. Knowledge of the pathobiology of these genetic diseases is beginning to pave paths to therapy.

Electrophysiological and Pupillometric Abnormalities in PROM1 Cone-Rod Dystrophy

Purpose

To compare electrophysiological and pupillometric responses in subjects with cone–rod dystrophy due to autosomal recessive (AR) PROM1 mutations.

Methods

Four subjects with AR PROM1 dystrophy and 10 visually normal, age-similar controls participated in this study. Full-field, light- and dark-adapted electroretinograms (ERGs) were obtained using conventional techniques. Full-field, light- and dark-adapted measures of the pupillary light reflex (PLR; pupil constriction elicited by a flash of light) were obtained across a range of stimulus luminance using long- and short-wavelength light. Pupil size as a function of stimulus luminance was described using Naka–Rushton functions to derive P_max (maximum response) and s (pupil response sensitivity).

Results

Light-adapted ERGs were non-detectable in all four PROM1 subjects, whereas dark-adapted ERGs were non-detectable in three subjects and markedly attenuated in the fourth. By contrast, each PROM1 subject had light- and dark-adapted PLRs. P_max ranged from normal to slightly attenuated under all conditions. Light-adapted s was generally normal, with the exception of two subjects who had abnormal s for the long-wavelength stimulus. Dark adapted s was abnormal for each PROM1 subject for the long-wavelength stimulus and ranged from the upper limit of normal to substantially abnormal for the short-wavelength stimulus.

Conclusions

ERG and PLR comparison showed an unanticipated dichotomy: ERGs were generally non-detectable, whereas PLRs were normal for all PROM1 subjects under select conditions. Differences between the measures may be attributed to distinct spatiotemporal summation/gain characteristics.

Translational Relevance

These data highlight the potential usefulness of pupillometry in cases where the ERG is non-detectable.

Clinical and genetic characteristics of 10 Japanese patients with PROM1-associated retinal disorder: A report of the phenotype spectrum and a literature review in the Japanese population

Variants in the PROM1 gene are associated with cone (-rod) dystrophy, macular dystrophy, and other phenotypes. We describe the clinical and genetic characteristics of 10 patients from eight Japanese families with PROM1-associated retinal disorder (PROM1-RD) in a nationwide cohort. A literature review of PROM1-RD in the Japanese population was also performed. The median age at onset/examination of 10 patients was 31.0 (range, 10-45)/44.5 (22-73) years. All 10 patients showed atrophic macular changes. Seven patients (70.0%) had spared fovea to various degrees, approximately half of whom had maintained visual acuity. Generalized cone (-rod) dysfunction was demonstrated in all nine subjects with available electrophysiological data. Three PROM1 variants were identified in this study: one recurrent disease-causing variant (p.Arg373Cys), one novel putative disease-causing variant (p.Cys112Arg), and one novel variant of uncertain significance (VUS; p.Gly53Asp). Characteristic features of macular atrophy with generalized cone-dominated retinal dysfunction were shared among all 10 subjects with PROM1-RD, and the presence of foveal sparing was crucial in maintaining visual acuity. Together with the three previously reported variants [p.R373C, c.1551+1G>A (pathogenic), p.Asn580His (likely benign)] in the literature of Japanese patients, one prevalent missense variant (p.Arg373Cys, 6/9 families, 66.7%) detected in multiple studies was determined in the Japanese population, which was also frequently detected in the European population.

The Interplay between Peripherin 2 Complex Formation and Degenerative Retinal Diseases

Peripherin 2 (Prph2) is a photoreceptor-specific tetraspanin protein present in the outer segment (OS) rims of rod and cone photoreceptors. It shares many common features with other tetraspanins, including a large intradiscal loop which contains several cysteines. This loop enables Prph2 to associate with itself to form homo-oligomers or with its homologue, rod outer segment membrane protein 1 (Rom1) to form hetero-tetramers and hetero-octamers. Mutations in PRPH2 cause a multitude of retinal diseases including autosomal dominant retinitis pigmentosa (RP) or cone dominant macular dystrophies. The importance of Prph2 for photoreceptor development, maintenance and function is underscored by the fact that its absence results in a failure to initialize OS formation in rods and formation of severely disorganized OS membranous structures in cones. Although the exact role of Rom1 has not been well studied, it has been concluded that it is not necessary for disc morphogenesis but is required for fine tuning OS disc size and structure. Pathogenic mutations in PRPH2 often result in complex and multifactorial phenotypes, involving not just photoreceptors, as has historically been reasoned, but also secondary effects on the retinal pigment epithelium (RPE) and retinal/choroidal vasculature. The ability of Prph2 to form complexes was identified as a key requirement for the development and maintenance of OS structure and function. Studies using mouse models of pathogenic Prph2 mutations established a connection between changes in complex formation and disease phenotypes. Although progress has been made in the development of therapeutic approaches for retinal diseases in general, the highly complex interplay of functions mediated by Prph2 and the precise regulation of these complexes made it difficult, thus far, to develop a suitable Prph2-specific therapy. Here we describe the latest results obtained in Prph2-associated research and how mouse models provided new insights into the pathogenesis of its related diseases. Furthermore, we give an overview on the current status of the development of therapeutic solutions.