Congenital Stationary Night Blindness

A systematic review of inherited retinal dystrophies in Pakistan: updates from 1999 to April 2023

BACKGROUND

Inherited retinal degenerations (IRDs) are a group of rare genetic conditions affecting retina of the eye that range in prevalence from 1 in 2000 to 1 in 4000 people globally. This review is based on a retrospective analysis of research articles reporting IRDs associated genetic findings in Pakistani families between 1999 and April 2023.

METHODS

Articles were retrieved through survey of online sources, notably, PubMed, Google Scholar, and Web of Science. Following a stringent selection criterion, a total of 126 research articles and conference abstracts were considered. All reported variants were cross-checked and validated for their correct genomic nomenclature using different online resources/databases, and their pathogenicity scores were explained as per ACMG guidelines.

RESULTS

A total of 277 unique sequence variants in 87 distinct genes, previously known to cause IRDs, were uncovered. In around 70% cases, parents of the index patient were consanguineously married, and approximately 88.81% of the detected variants were found in a homozygous state. Overall, more than 95% of the IRDs cases were recessively inherited. Missense variants were predominant (41.88%), followed by Indels/frameshift (26.35%), nonsense (19.13%), splice site (12.27%) and synonymous change (0.36%). Non-syndromic IRDs were significantly higher than syndromic IRDs (77.32% vs. 22.68%). Retinitis pigmentosa (RP) was the most frequently observed IRD followed by Leber's congenital amaurosis (LCA). Altogether, mutations in PDE6A gene was the leading cause of IRDs in Pakistani families followed by mutations in TULP1 gene.

CONCLUSION

In summary, Pakistani families are notable in expressing recessively inherited monogenic disorders including IRDs likely due to the highest prevalence of consanguinity in the country that leads to expression of rare pathogenic variants in homozygous state.

CACNA1F-related synaptic dysfunction: challenges diagnosing congenital stationary night blindness presenting without night blindness

OBJECTIVE

To describe pediatric patients with CACNA1F-associated incomplete X-linked congenital stationary night blindness presenting without nyctalopia, and review the causes leading to diagnosis delay.

DESIGN

Retrospective cohort.

METHODS

This was set in a single institution between 2004 and 2019. There were12 patients. The intervention or observation procedures used were clinical course, visual acuity, refractive error, images, electrophysiology, genetic testing, pedigree. The main outcome measures were cohort description and causes of diagnosis delay.

RESULTS

For these 12 cases, the referring diagnosis was congenital nystagmus (7), reduced best-corrected visual acuity (BCVA, 4), and progressive myopia (1). Nyctalopia was not a presenting symptom and developed in 4 patients during follow-up. Seven patients presented with nystagmus. All patients developed early-onset myopia. Myopia progressed more rapidly before age 6 than after (average 1.14 D vs 0.25 D) (p = 0.0033). The average final BCVA was 20/50 (20/30-20/150). Vision at presentation was correlated with final visual acuity (r2 = 0.87, p = 5.4E-06). The first cycloplegic refraction was correlated to the final refractive error (r2 = 0.49, p = 0.009). Patients with nystagmus had worse BCVA on average. Full-field electroretinogram was abnormal and diagnostic in all cases, as confirmed by genetic testing. The average time to diagnosis was 4.2 years, and the average age at diagnosis was 7.9 years. The delay in diagnosis was due to the absence of nyctalopia, not performing an electroretinogram and/or an alternative diagnosis.

CONCLUSIONS

In children, CACNA1F-associated synaptic dysfunction does not usually present with night blindness. It should be suspected in male patients with early-onset myopia, especially with a history of nystagmus.

The Value of Electroretinography in Identifying Candidate Genes for Inherited Retinal Dystrophies: A Diagnostic Guide

Inherited retinal dystrophies (IRDs) are a group of heterogeneous diseases caused by genetic mutations that specifically affect the function of the rod, cone, or bipolar cells in the retina. Electroretinography (ERG) is a diagnostic tool that measures the electrical activity of the retina in response to light stimuli, and it can help to determine the function of these cells. A normal ERG response consists of two waves, the a-wave and the b-wave, which reflect the activity of the photoreceptor cells and the bipolar and Muller cells, respectively. Despite the growing availability of next-generation sequencing (NGS) technology, identifying the precise genetic mutation causing an IRD can be challenging and costly. However, certain types of IRDs present with unique ERG features that can help guide genetic testing. By combining these ERG findings with other clinical information, such as on family history and retinal imaging, physicians can effectively narrow down the list of candidate genes to be sequenced, thereby reducing the cost of genetic testing. This review article focuses on certain types of IRDs with unique ERG features. We will discuss the pathophysiology and clinical presentation of, and ERG findings on, these disorders, emphasizing the unique role ERG plays in their diagnosis and genetic testing.

Congenital Stationary Night Blindness: Clinical and Genetic Features

Congenital stationary night blindness (CSNB) is an inherited retinal disease (IRD) that causes night blindness in childhood with heterogeneous genetic, electrophysical, and clinical characteristics. The development of sequencing technologies and gene therapy have increased the ease and urgency of diagnosing IRDs. This study describes seven Taiwanese patients from six unrelated families examined at a tertiary referral center, diagnosed with CSNB, and confirmed by genetic testing. Complete ophthalmic exams included best corrected visual acuity, retinal imaging, and an electroretinogram. The effects of identified novel variants were predicted using clinical details, protein prediction tools, and conservation scores. One patient had an autosomal dominant CSNB with a RHO variant; five patients had complete CSNB with variants in GRM6, TRPM1, and NYX; and one patient had incomplete CSNB with variants in CACNA1F. The patients had Riggs and Schubert-Bornschein types of CSNB with autosomal dominant, autosomal recessive, and X-linked inheritance patterns. This is the first report of CSNB patients in Taiwan with confirmed genetic testing, providing novel perspectives on molecular etiology and genotype-phenotype correlation of CSNB. Particularly, variants in TRPM1, NYX, and CACNA1F in our patient cohort have not previously been described, although their clinical significance needs further study. Additional study is needed for the genotype-phenotype correlation of different mutations causing CSNB. In addition to genetic etiology, the future of gene therapy for CSNB patients is reviewed and discussed.

ON and OFF Signaling Pathways in the Retina and the Visual System

Visual processing starts at the retina of the eye, and signals are then transferred primarily to the visual cortex and the tectum. In the retina, multiple neural networks encode different aspects of visual input, such as color and motion. Subsequently, multiple neural streams in parallel convey unique aspects of visual information to cortical and subcortical regions. Bipolar cells, which are the second order neurons of the retina, separate visual signals evoked by light and dark contrasts and encode them to ON and OFF pathways, respectively. The interplay between ON and OFF neural signals is the foundation for visual processing for object contrast which underlies higher order stimulus processing. ON and OFF pathways have been classically thought to signal in a mirror-symmetric manner. However, while these two pathways contribute synergistically to visual perception in some instances, they have pronounced asymmetries suggesting independent operation in other cases. In this review, we summarize the role of the ON-OFF dichotomy in visual signaling, aiming to contribute to the understanding of visual recognition.

Photoreceptor Phosphodiesterase (PDE6): Structure, Regulatory Mechanisms, and Implications for Treatment of Retinal Diseases

The photoreceptor phosphodiesterase (PDE6) is a member of large family of Class I phosphodiesterases responsible for hydrolyzing the second messengers cAMP and cGMP. PDE6 consists of two catalytic subunits and two inhibitory subunits that form a tetrameric protein. PDE6 is a peripheral membrane protein that is localized to the signal-transducing compartment of rod and cone photoreceptors. As the central effector enzyme of the G-protein coupled visual transduction pathway, activation of PDE6 catalysis causes a rapid decrease in cGMP levels that results in closure of cGMP-gated ion channels in the photoreceptor plasma membrane. Because of its importance in the phototransduction pathway, mutations in PDE6 genes result in various retinal diseases that currently lack therapeutic treatment strategies due to inadequate knowledge of the structure, function, and regulation of this enzyme. This review focuses on recent progress in understanding the structure of the regulatory and catalytic domains of the PDE6 holoenzyme, the central role of the multi-functional inhibitory γ-subunit, the mechanism of activation by the heterotrimeric G protein, transducin, and future directions for pharmacological interventions to treat retinal degenerative diseases arising from mutations in the PDE6 genes.

Photoreceptor phosphodiesterase (PDE6): activation and inactivation mechanisms during visual transduction in rods and cones

Rod and cone photoreceptors of the vertebrate retina utilize cGMP as the primary intracellular messenger for the visual signaling pathway that converts a light stimulus into an electrical response. cGMP metabolism in the signal-transducing photoreceptor outer segment reflects the balance of cGMP synthesis (catalyzed by guanylyl cyclase) and degradation (catalyzed by the photoreceptor phosphodiesterase, PDE6). Upon light stimulation, rapid activation of PDE6 by the heterotrimeric G-protein (transducin) triggers a dramatic drop in cGMP levels that lead to cell hyperpolarization. Following cessation of the light stimulus, the lifetime of activated PDE6 is also precisely regulated by additional processes. This review summarizes recent advances in the structural characterization of the rod and cone PDE6 catalytic and regulatory subunits in the context of previous biochemical studies of the enzymological properties and allosteric regulation of PDE6. Emphasis is given to recent advances in understanding the structural and conformational changes underlying the mechanism by which the activated transducin α-subunit binds to-and relieves inhibition of-PDE6 catalysis that is controlled by its intrinsically disordered, inhibitory γ-subunit. The role of the regulator of G-protein signaling 9-1 (RGS9-1) in regulating the lifetime of the transducin-PDE6 is also briefly covered. The therapeutic potential of pharmacological compounds acting as inhibitors or activators targeting PDE6 is discussed in the context of inherited retinal diseases resulting from mutations in rod and cone PDE6 genes as well as other inherited defects that arise from excessive cGMP accumulation in retinal photoreceptor cells.

Sequence features, structure, ligand interaction, and diseases in small leucine rich repeat proteoglycans

Small leucine rich repeat proteoglycans (SLRPs) are a group of active components of the extracellular matrix in all tissues. SLRPs bind to collagens and regulate collagen fibril growth and fibril organization. SLRPs also interact with various cytokines and extracellular compounds, which lead to various biological functions such cell adhesion and signaling, proliferation, and differentiation. Mutations in SLRP genes are associated with human diseases. Now crystal structures of five SLRPs are available. We describe some features of amino acid sequence and structures of SLRPs. We also review ligand interactions and then discuss the interaction surfaces. Furthermore, we map mutations associated with human diseases and discuss possible effects on structures by the mutations.

Structural Analysis of the Regulatory GAF Domains of cGMP Phosphodiesterase Elucidates the Allosteric Communication Pathway

Regulation of photoreceptor phosphodiesterase (PDE6) activity is responsible for the speed, sensitivity, and recovery of the photoresponse during visual signaling in vertebrate photoreceptor cells. It is hypothesized that physiological differences in the light responsiveness of rods and cones may result in part from differences in the structure and regulation of the distinct isoforms of rod and cone PDE6. Although rod and cone PDE6 catalytic subunits share a similar domain organization consisting of tandem GAF domains (GAFa and GAFb) and a catalytic domain, cone PDE6 is a homodimer whereas rod PDE6 consists of two homologous catalytic subunits. Here we provide the x-ray crystal structure of cone GAFab regulatory domain solved at 3.3 Å resolution, in conjunction with chemical cross-linking and mass spectrometric analysis of conformational changes to GAFab induced upon binding of cGMP and the PDE6 inhibitory γ-subunit (Pγ). Ligand-induced changes in cross-linked residues implicate multiple conformational changes in the GAFa and GAFb domains in forming an allosteric communication network. Molecular dynamics simulations of cone GAFab revealed differences in conformational dynamics of the two subunits forming the homodimer and allosteric perturbations on cGMP binding. Cross-linking of Pγ to GAFab in conjunction with solution NMR spectroscopy of isotopically labeled Pγ identified the central polycationic region of Pγ interacting with the GAFb domain. These results provide a mechanistic basis for developing allosteric activators of PDE6 with therapeutic implications for halting the progression of several retinal degenerative diseases.