Genomic organisation and alternative splicing of human RIM1, a gene implicated in autosomal dominant cone-rod dystrophy (CORD7)

Progressive Keratoconus in a Patient With Severe Pectus Excavatum and a Cartilage Oligomeric Matrix Protein Gene Mutation: A Case Report

INTRODUCTION

Keratoconus is a progressive ocular disorder associated with numerous systemic diseases, many of which affect the musculoskeletal system. Although the etiology and pathophysiology of the disorder remain elusive, recent studies suggest a significant role of genetic predisposition in the pathogenesis of keratoconus. This case report aims to elucidate a potential genetic association in a patient presenting with keratoconus, severe pectus excavatum, generalized muscular weakness, and skeletal deformities.

CASE DESCRIPTION

A 31-year-old Iranian man presented with progressively diminishing vision in both eyes over the years, eventually diagnosed with keratoconus. The patient's history and further examination indicated generalized muscular weakness, skeletal deformities, and severe pectus excavatum with cardiac and large vessel displacement. Whole-exome sequencing identified two heterozygous gene variants: one in the Cartilage Oligomeric Matrix Protein (COMP) gene and another in the Regulating Synaptic Membrane Exocytosis 1 gene. The patient's systemic and ocular symptoms, combined with the gene variants identified, suggested a connective tissue systemic disorder, potentially within the clinical spectrum of COMPopathies.

CONCLUSION

This is the first documented case of bilateral progressive keratoconus associated with severe pectus excavatum, generalized musculoskeletal dystrophy, and a COMP gene mutation. It highlights the necessity of continued search into the pathogenic genes of keratoconus, particularly in cases with coexisting systemic manifestations, to further our understanding of the etiology and pathogenesis of this complex disease.

Four different gene-related cone-rod dystrophy: clinical and genetic findings in six Chinese families with diverse modes of inheritance

Purpose: To investigate pathogenic variants in six families with cone-rod dystrophy (CORD) presenting various inheritance patterns by using whole-exome sequencing (WES) and analyzing phenotypic features. Methods: A total of six families with CORD were enrolled in Ningxia Eye Hospital for this study. The probands and their family members received comprehensive ophthalmic examinations, and DNA was abstracted from patients and family members. Whole-exome sequencing was performed on probands to screen the causative variants, and all suspected pathogenic variants were determined via Sanger sequencing. Furthermore, co-segregation analysis was performed on available family members. The pathogenicity of novel variants was predicted using in silico analysis and evaluated according to the American College of Medical Genetics and Genomics (ACMG) guidelines. Results: Of the six families, two families were assigned as X-linked recessive (XL), two families were assigned as autosomal recessive (AR), and two families were assigned as autosomal dominant (AD). Pathogenic variants were detected in CACNA1F in two X-linked recessive probands, among which family 1 had a hemizygous frameshift variant c.2201del (p.Val734Glyfs*17) and family 2 had a hemizygous missense variant c.245G>A (p.Arg82Gln). Both probands had high myopia, with fundus tessellation accompanied by abnormalities in the outer structure of the macular area. The homozygous splice variant c.2373 + 5G>T in PROM1 and the homozygous nonsense variant c.604C>T (p.Arg202Ter) in ADAM9 were detected in two autosomal recessive families of the probands. Both probands showed different degrees of atrophy in the macular area, and the lesions showed hypofluorescence changes in autofluorescence. The heterozygous variation in CRX c.682C>T (p.Gln228Ter) was detected in two autosomal dominant families. The onset age of the two probands was late, with better vision and severe macular atrophy. According to ACMG guidelines and the analysis of online in silico tools, all variations were labeled as potentially harmful or pathogenic. Conclusion: Pathogenic variants in CACNA1F, PROM1, ADAM9, and CRX genes were identified in six families affected by the diverse inheritance patterns of CORD. Furthermore, the potential impact of the nonsense-mediated decay (NMD) mechanism on the manifestation of CORD phenotypes was examined and addressed. Simultaneously, the spectrum of pathogenic variants and clinical phenotypes associated with the CORD gene was extended.

Losing, preserving, and restoring vision from neurodegeneration in the eye

The retina is a part of the brain that sits at the back of the eye, looking out onto the world. The first neurons of the retina are the rod and cone photoreceptors, which convert changes in photon flux into electrical signals that are the basis of vision. Rods and cones are frequent targets of heritable neurodegenerative diseases that cause visual impairment, including blindness, in millions of people worldwide. This review summarizes the diverse genetic causes of inherited retinal degenerations (IRDs) and their convergence onto common pathogenic mechanisms of vision loss. Currently, there are few effective treatments for IRDs, but recent advances in disparate areas of biology and technology (e.g., genome editing, viral engineering, 3D organoids, optogenetics, semiconductor arrays) discussed here enable promising efforts to preserve and restore vision in IRD patients with implications for neurodegeneration in less approachable brain areas.

Retinitis pigmentosa-associated mutations in mouse Prpf8 cause misexpression of circRNAs and degeneration of cerebellar granule cells

A subset of patients with retinitis pigmentosa (RP) carry mutations in several spliceosomal components including the PRPF8 protein. Here, we established two alleles of murine Prpf8 that genocopy or mimic aberrant PRPF8 found in RP patients-the substitution p.Tyr2334Asn and an extended protein variant p.Glu2331ValfsX15. Homozygous mice expressing the aberrant Prpf8 variants developed within the first 2 mo progressive atrophy of the cerebellum because of extensive granule cell loss, whereas other cerebellar cells remained unaffected. We further show that a subset of circRNAs were deregulated in the cerebellum of both Prpf8-RP mouse strains. To identify potential risk factors that sensitize the cerebellum for Prpf8 mutations, we monitored the expression of several splicing proteins during the first 8 wk. We observed down-regulation of all selected splicing proteins in the WT cerebellum, which coincided with neurodegeneration onset. The decrease in splicing protein expression was further pronounced in mouse strains expressing mutated Prpf8. Collectively, we propose a model where physiological reduction in spliceosomal components during postnatal tissue maturation sensitizes cells to the expression of aberrant Prpf8 and the subsequent deregulation of circRNAs triggers neuronal death.

The role of RIM in neurotransmitter release: promotion of synaptic vesicle docking, priming, and fusion

There are many special sites at the end of a synapse called active zones (AZs). Synaptic vesicles (SVs) fuse with presynaptic membranes at these sites, and this fusion is an important step in neurotransmitter release. The cytomatrix in the active zone (CAZ) is made up of proteins such as the regulating synaptic membrane exocytosis protein (RIM), RIM-binding proteins (RIM-BPs), ELKS/CAST, Bassoon/Piccolo, Liprin-α, and Munc13-1. RIM is a scaffold protein that interacts with CAZ proteins and presynaptic functional components to affect the docking, priming, and fusion of SVs. RIM is believed to play an important role in regulating the release of neurotransmitters (NTs). In addition, abnormal expression of RIM has been detected in many diseases, such as retinal diseases, Asperger's syndrome (AS), and degenerative scoliosis. Therefore, we believe that studying the molecular structure of RIM and its role in neurotransmitter release will help to clarify the molecular mechanism of neurotransmitter release and identify targets for the diagnosis and treatment of the aforementioned diseases.

Genetic disorders of neurotransmitter release machinery

Synaptic neurotransmitter release is an evolutionarily conserved process that mediates rapid information transfer between neurons as well as several peripheral tissues. Release of neurotransmitters are ensured by successive events such as synaptic vesicle docking and priming that prepare synaptic vesicles for rapid fusion. These events are orchestrated by interaction of different presynaptic proteins and are regulated by presynaptic calcium. Recent studies have identified various mutations in different components of neurotransmitter release machinery resulting in aberrant neurotransmitter release, which underlie a wide spectrum of psychiatric and neurological symptoms. Here, we review how these genetic alterations in different components of the core neurotransmitter release machinery affect the information transfer between neurons and how aberrant synaptic release affects nervous system function.

Ribbon Synapses and Retinal Disease: Review

Synaptic ribbons are presynaptic protein complexes that are believed to be important for the transmission of sensory information in the visual system. Ribbons are selectively associated with those synapses where graded changes in membrane potential drive continuous neurotransmitter release. Defective synaptic transmission can arise as a result of the mutagenesis of a single ribbon component. Visual diseases that stem from malfunctions in the presynaptic molecular machinery of ribbon synapses in the retina are rare. In this review, we provide an overview of synaptopathies that give rise to retinal malfunction and our present understanding of the mechanisms that underlie their pathogenesis and discuss muscular dystrophies that exhibit ribbon synapse involvement in the pathology.

Dominant Cone Rod Dystrophy, Previously Assigned to a Missense Variant in RIMS1, Is Fully Explained by Co-Inheritance of a Dominant Allele of PROM1

Purpose

Autosomal dominant cone rod dystrophy 7 (CORD7) was initially linked to the gene RIMS1 and reported in a 4-generation British family in 1998. The purpose of this study was to investigate the legitimacy of this association, and to correctly characterize the genetic cause of this condition.

Methods

The allele frequency of RIMS1 c.2459G>A, p.Arg820His, was investigated in the Genomes Aggregation Dataset (gnomAD) datasets and whole genome sequencing (WGS) was performed for 4 members of the CORD7 family with filtering of rare pathogenic variants in a virtual gene panel comprising all genes known to be associated with inherited retinal dystrophy (IRD). Cytogenetic analysis was performed to rule out interchromosomal translocation.

Results

RIMS1 p.Arg820His has a maximal carrier frequency of >1:5000 in Europeans. A previously well-characterized PROM1 variant: c.1118C>T, p.Arg373Cys, was detected in 9 affected members of the CORD7 family who underwent WGS or direct sequencing. One affected family member is now known to have macular dystrophy in the absence of RIMS1 p.Arg820His. Clinical analysis of affected family members and 27 individuals with retinopathy associated with the same - PROM1 - variant showed consistent phenotypes.

Conclusions

The case for pathogenicity of RIMS1 p.Arg820His is not strong based on its presence on 10 alleles in the gnomAD dataset and absence from additional CORD affected individuals. The finding of a known pathogenic variant in PROM1 correlates well with the phenotypic characteristics of the affected individuals, and is likely to account for the condition. Clear evidence of association between RIMS1 and a retinal dystrophy is yet to be described.

Exploring the mutational landscape of genes associated with inherited retinal disease using large genomic datasets: identifying loss of function intolerance and outlying propensities for missense changes

Background

Large databases permit quantitative description of genes in terms of intolerance to loss of function (‘haploinsufficiency’) and prevalence of missense variants. We explored these parameters in inherited retinal disease (IRD) genes.

Methods

IRD genes (from the ‘RetNet’ resource) were classified by probability of loss of function intolerance (pLI) using online Genome Aggregation Database (gnomAD) and DatabasE of genomiC varIation and Phenotype in Humans using Ensembl Resources (DECIPHER) databases. Genes were identified having pLI ≥0.9 together with one or both of the following: upper bound of CI <0.35 for observed to expected (o/e) ratio of loss of function variants in the gnomAD resource; haploinsufficiency score <10 in the DECIPHER resource. IRD genes in which missense variants appeared under-represented or over-represented (Z score for o/e ratio of <−2.99 or >2.99, respectively) were also identified. The genes were evaluated in the gene ontology Protein Analysis THrough Evolutionary Relationships (PANTHER) resource.

Results

Of 280 analysed genes, 39 (13.9%) were predicted loss of function intolerant. A greater proportion of X-linked than autosomal IRD genes fulfilled these criteria, as expected. Most autosomal genes were associated with dominant disease. PANTHER analysis showed >100 fold enrichment of spliceosome tri-snRNP complex assembly. Most encoded proteins were longer than the median length in the UniProt database. Fourteen genes (11 of which were in the ‘haploinsufficient’ group) showed under-representation of missense variants. Six genes (SAMD11, ALMS1, WFS1, RP1L1, KCNV2, ADAMTS18) showed over-representation of missense variants.

Conclusion

A minority of IRD-associated genes appear to be ‘haploinsufficient’. Over-representation of spliceosome pathways was observed. When interpreting genetic tests, variants found in genes with over-representation of missense variants should be interpreted with caution.

Rebuilding essential active zone functions within a synapse

Presynaptic active zones are molecular machines that control neurotransmitter secretion. They form sites for vesicle docking and priming and couple vesicles to Ca²⁺ entry for release triggering. The complexity of active zone machinery has made it challenging to determine its mechanisms in release. Simultaneous knockout of the active zone proteins RIM and ELKS disrupts active zone assembly, abolishes vesicle docking, and impairs release. We here rebuild docking, priming, and Ca²⁺ secretion coupling in these mutants without reinstating active zone networks. Re-expression of RIM zinc fingers recruited Munc13 to undocked vesicles and rendered the vesicles release competent. Action potential triggering of release was reconstituted by docking these primed vesicles to Ca²⁺ channels through attaching RIM zinc fingers to Ca_Vβ4-subunits. Our work identifies an 80-kDa β4-Zn protein that bypasses the need for megadalton-sized secretory machines, establishes that fusion competence and docking are mechanistically separable, and defines RIM zinc finger-Munc13 complexes as hubs for active zone function.

The human cognition-enhancing CORD7 mutation increases active zone number and synaptic release

Humans carrying the CORD7 (cone-rod dystrophy 7) mutation possess increased verbal IQ and working memory. This autosomal dominant syndrome is caused by the single-amino acid R844H exchange (human numbering) located in the 310 helix of the C2A domain of RIMS1/RIM1 (Rab3-interacting molecule 1). RIM is an evolutionarily conserved multi-domain protein and essential component of presynaptic active zones, which is centrally involved in fast, Ca2+-triggered neurotransmitter release. How the CORD7 mutation affects synaptic function has remained unclear thus far. Here, we established Drosophila melanogaster as a disease model for clarifying the effects of the CORD7 mutation on RIM function and synaptic vesicle release.

To this end, using protein expression and X-ray crystallography, we solved the molecular structure of the Drosophila C2A domain at 1.92 Å resolution and by comparison to its mammalian homolog ascertained that the location of the CORD7 mutation is structurally conserved in fly RIM. Further, CRISPR/Cas9-assisted genomic engineering was employed for the generation of rim alleles encoding the R915H CORD7 exchange or R915E,R916E substitutions (fly numbering) to effect local charge reversal at the 310 helix. Through electrophysiological characterization by two-electrode voltage clamp and focal recordings we determined that the CORD7 mutation exerts a semi-dominant rather than a dominant effect on synaptic transmission resulting in faster, more efficient synaptic release and increased size of the readily releasable pool but decreased sensitivity for the fast calcium chelator BAPTA. In addition, the rim CORD7 allele increased the number of presynaptic active zones but left their nanoscopic organization unperturbed as revealed by super-resolution microscopy of the presynaptic scaffold protein Bruchpilot/ELKS/CAST.

We conclude that the CORD7 mutation leads to tighter release coupling, an increased readily releasable pool size and more release sites thereby promoting more efficient synaptic transmitter release. These results strongly suggest that similar mechanisms may underlie the CORD7 disease phenotype in patients and that enhanced synaptic transmission may contribute to their increased cognitive abilities.

Astrocytic hamartoma in a patient heterozygous for RIM1 mutation associated-retinal dystrophy

BACKGROUND

Autosomal-dominant cone-rod dystrophy 7 (CORD7) has been documented in association with RIM1 mutation (c.2459 G>A). We report a patient with retinal dystrophy who was heterozygous for RIM1 missense variant with a newly found point mutation (c.4036 G>T). Clinical findings of this genetic variant manifested differently from a typical CORD7. In addition, astrocytic hamartomas at bilateral optic discs are also a unique feature, which has not been described in CORD previously.

MATERIALS AND METHODS

Medical records of this patient were retrospectively reviewed. Genetic testing with whole exon sequencing was performed.

RESULTS

This 43-year-old female with history of decreased night vision since childhood came to our hospital complaining of blurred vision in both eyes for more than half a year. Her best-corrected visual acuity was 20/200 in both eyes. Dilated fundoscopic examination revealed symmetric diffuse atrophy of retinal pigment epithelium with peripheral pigmentary clumps. Also, optic disc astrocytic hamartomas were found bilaterally. Optical coherence tomography revealed extensive disruption of inner segment/outer segment junction in both eyes. Visual field test showed severe peripheral defect sparing central vision. Electroretinogram demonstrated both rod and cone cells abnormalities. Subsequent genetic testing reported heterozygosity for the RIM1 (c.4036 G>T) mutation.

CONCLUSIONS

This is the first reported case of RIM1 mutation-associated retinal dystrophy with a newly found point mutation (c.4036 G>T), which presented differently from a typical CORD7 and more similarly to the phenotype of RP. Furthermore, our finding of bilateral optic disc astrocytic hamartomas has not been reported in association with CORD previously.

Piccolo is essential for the maintenance of mouse retina but not cochlear hair cell function

Piccolo is a presynaptic protein with high conservation among different species, and the expression of Piccolo is extensive in vertebrates. Recently, a small fragment of Piccolo (Piccolino), arising due to the incomplete splicing of intron 5/6, was found to be present in the synapses of retinas and cochleae. However, the comprehensive function of Piccolo in the retina and cochlea remains unclear. In this study, we generated Piccolo knockout mice using CRISPR-Cas9 technology to explore the function of Piccolo. Unexpectedly, whereas no abnormalities were found in the cochlear hair cells of the mutant mice, significant differences were found in the retinas, in which two layers (the outer nuclear layer and the outer plexiform layer) were absent. Additionally, the amplitudes of electroretinograms were significantly reduced and pigmentation was observed in the fundoscopy of the mutant mouse retinas. The expression levels of Bassoon, a homolog of Piccolo, as well as synapse-associated proteins CtBP1, CtBP2, Kif3A, and Rim1 were down-regulated. The numbers of ribbon synapses in the retinas of the mutant mice were also reduced. Altogether, the phenotype of Piccolo-/- mice resembled the symptoms of retinitis pigmentosa (RP) in humans, suggesting Piccolo might be a candidate gene of RP and indicates Piccolo knockout mice are a good model for elucidating the molecular mechanisms of RP.

SNAREopathies: Diversity in Mechanisms and Symptoms

Neuronal SNAREs and their key regulators together drive synaptic vesicle exocytosis and synaptic transmission as a single integrated membrane fusion machine. Human pathogenic mutations have now been reported for all eight core components, but patients are diagnosed with very different neurodevelopmental syndromes. We propose to unify these syndromes, based on etiology and mechanism, as "SNAREopathies." Here, we review the strikingly diverse clinical phenomenology and disease severity and the also remarkably diverse genetic mechanisms. We argue that disease severity generally scales with functional redundancy and, conversely, that the large effect of mutations in some SNARE genes is the price paid for extensive integration and exceptional specialization. Finally, we discuss how subtle differences in components being rate limiting in different types of neurons helps to explain the main symptoms.

Loss of Function of RIMS2 Causes a Syndromic Congenital Cone-Rod Synaptic Disease with Neurodevelopmental and Pancreatic Involvement

Congenital cone-rod synaptic disorder (CRSD), also known as incomplete congenital stationary night blindness (iCSNB), is a non-progressive inherited retinal disease (IRD) characterized by night blindness, photophobia, and nystagmus, and distinctive electroretinographic features. Here, we report bi-allelic RIMS2 variants in seven CRSD-affected individuals from four unrelated families. Apart from CRSD, neurodevelopmental disease was observed in all affected individuals, and abnormal glucose homeostasis was observed in the eldest affected individual. RIMS2 regulates synaptic membrane exocytosis. Data mining of human adult bulk and single-cell retinal transcriptional datasets revealed predominant expression in rod photoreceptors, and immunostaining demonstrated RIMS2 localization in the human retinal outer plexiform layer, Purkinje cells, and pancreatic islets. Additionally, nonsense variants were shown to result in truncated RIMS2 and decreased insulin secretion in mammalian cells. The identification of a syndromic stationary congenital IRD has a major impact on the differential diagnosis of syndromic congenital IRD, which has previously been exclusively linked with degenerative IRD.

Synaptic ribbons foster active zone stability and illumination-dependent active zone enrichment of RIM2 and Cav1.4 in photoreceptor synapses

Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark. Since ribbons are physically connected to the active zones, we asked whether illumination-dependent changes of ribbons influence Cav1.4/RIM2 protein clusters at the active zone and whether these illumination-dependent effects at the active zone require the presence of the synaptic ribbon. We found that synaptic ribbon length and the length of presynaptic Cav1.4/RIM2 clusters are tightly correlated. Dark-adaptation did not change the number of ribbons and active zone puncta. However, mean ribbon length and length of presynaptic Cav1.4/RIM2 clusters increased significantly during dark-adaptation when tonic exocytosis is highest. In the present study, we identified by the analyses of synaptic ribbon-deficient RIBEYE knockout mice that synaptic ribbons are (1) needed to stabilize Cav1.4/RIM2 at rod photoreceptor active zones and (2) are required for the darkness-induced active zone enrichment of Cav1.4/RIM2. These data propose a role of the ribbon in active zone stabilization and suggest a homeostatic function of the ribbon in illumination-dependent active zone remodeling.

Sensory Processing at Ribbon Synapses in the Retina and the Cochlea

In recent years, sensory neuroscientists have made major efforts to dissect the structure and function of ribbon synapses which process sensory information in the eye and ear. This review aims to summarize our current understanding of two key aspects of ribbon synapses: 1) their mechanisms of exocytosis and endocytosis and 2) their molecular anatomy and physiology. Our comparison of ribbon synapses in the cochlea and the retina reveals convergent signaling mechanisms, as well as divergent strategies in different sensory systems.

Natural Genetic Variation Screen in Drosophila Identifies Wnt Signaling, Mitochondrial Metabolism, and Redox Homeostasis Genes as Modifiers of Apoptosis

Apoptosis is the primary cause of degeneration in a number of neuronal, muscular, and metabolic disorders. These diseases are subject to a great deal of phenotypic heterogeneity in patient populations, primarily due to differences in genetic variation between individuals. This creates a barrier to effective diagnosis and treatment. Understanding how genetic variation influences apoptosis could lead to the development of new therapeutics and better personalized treatment approaches. In this study, we examine the impact of the natural genetic variation in the Drosophila Genetic Reference Panel (DGRP) on two models of apoptosis-induced retinal degeneration: overexpression of p53 or reaper (rpr). We identify a number of known apoptotic, neural, and developmental genes as candidate modifiers of degeneration. We also use Gene Set Enrichment Analysis (GSEA) to identify pathways that harbor genetic variation that impact these apoptosis models, including Wnt signaling, mitochondrial metabolism, and redox homeostasis. Finally, we demonstrate that many of these candidates have a functional effect on apoptosis and degeneration. These studies provide a number of avenues for modifying genes and pathways of apoptosis-related disease.

Allele frequency analysis of variants reported to cause autosomal dominant inherited retinal diseases question the involvement of 19% of genes and 10% of reported pathogenic variants

BACKGROUND

Next generation sequencing (NGS) generates a large amount of genetic data that can be used to better characterise disease-causing variants. Our aim was to examine allele frequencies of sequence variants reported to cause autosomal dominant inherited retinal diseases (AD-IRDs).

METHODS

Genetic information was collected from various databases, including PubMed, the Human Genome Mutation Database, RETNET and gnomAD.

RESULTS

We generated a database of 1223 variants reported in 58 genes, including their allele frequency in gnomAD that contains NGS data of over 138 000 individuals. While the majority of variants are not represented in gnomAD, 138 had an allele count of >1 and were examined carefully for various aspects including cosegregation and functional analyses. The analysis revealed 122 variants that were reported pathogenic but unlikely to cause AD-IRDs. Interestingly, in some cases, these unlikely pathogenic variants were the only ones reported to cause disease in AD inheritance pattern for a particular gene, therefore raising doubt regarding the involvement of 11 (19%) of the genes in AD-IRDs.

CONCLUSION

We predict that these data are not limited to a specific disease or inheritance pattern since non-pathogenic variants were mistakenly reported as pathogenic in various diseases. Our results should serve as a warning sign for geneticists, variant database curators and sequencing panels' developers not to automatically accept reported variants as pathogenic but cross-reference the information with large databases.

RIM C2B Domains Target Presynaptic Active Zone Functions to PIP2-Containing Membranes

Rapid and efficient synaptic vesicle fusion requires a pool of primed vesicles, the nearby tethering of Ca²⁺ channels, and the presence of the phospholipid PIP₂ in the target membrane. Although the presynaptic active zone mediates the first two requirements, it is unclear how fusion is targeted to membranes with high PIP₂ content. Here we find that the C₂B domain of the active zone scaffold RIM is critical for action potential-triggered fusion. Remarkably, the known RIM functions in vesicle priming and Ca²⁺ influx do not require RIM C₂B domains. Instead, biophysical experiments reveal that RIM C₂ domains, which lack Ca²⁺ binding, specifically bind to PIP₂. Mutational analyses establish that PIP₂ binding to RIM C₂B and its tethering to the other RIM domains are crucial for efficient exocytosis. We propose that RIM C₂B domains are constitutive PIP₂-binding modules that couple mechanisms for vesicle priming and Ca²⁺ channel tethering to PIP₂-containing target membranes.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

Retinal Diseases Caused by Mutations in Genes Not Specifically Associated with the Clinical Diagnosis

Purpose

When seeking a confirmed molecular diagnosis in the research setting, patients with one descriptive diagnosis of retinal disease could carry pathogenic variants in genes not specifically associated with that description. However, this event has not been evaluated systematically in clinical diagnostic laboratories that validate fully all target genes to minimize false negatives/positives.

Methods

We performed targeted next-generation sequencing analysis on 207 ocular disease-related genes for 42 patients whose DNA had been tested negative for disease-specific panels of genes known to be associated with retinitis pigmentosa, Leber congenital amaurosis, or exudative vitreoretinopathy.

Results

Pathogenic variants, including single nucleotide variations and copy number variations, were identified in 9 patients, including 6 with variants in syndromic retinal disease genes and 3 whose molecular diagnosis could not be distinguished easily from their submitted clinical diagnosis, accounting for 21% (9/42) of the unsolved cases.

Conclusion

Our study underscores the clinical and genetic heterogeneity of retinal disorders and provides valuable reference to estimate the fraction of clinical samples whose retinal disorders could be explained by genes not specifically associated with the corresponding clinical diagnosis. Our data suggest that sequencing a larger set of retinal disorder related genes can increase the molecular diagnostic yield, especially for clinically hard-to-distinguish cases.

Brg1 coordinates multiple processes during retinogenesis and is a tumor suppressor in retinoblastoma

Retinal development requires precise temporal and spatial coordination of cell cycle exit, cell fate specification, cell migration and differentiation. When this process is disrupted, retinoblastoma, a developmental tumor of the retina, can form. Epigenetic modulators are central to precisely coordinating developmental events, and many epigenetic processes have been implicated in cancer. Studying epigenetic mechanisms in development is challenging because they often regulate multiple cellular processes; therefore, elucidating the primary molecular mechanisms involved can be difficult. Here we explore the role of Brg1 (Smarca4) in retinal development and retinoblastoma in mice using molecular and cellular approaches. Brg1 was found to regulate retinal size by controlling cell cycle length, cell cycle exit and cell survival during development. Brg1 was not required for cell fate specification but was required for photoreceptor differentiation and cell adhesion/polarity programs that contribute to proper retinal lamination during development. The combination of defective cell differentiation and lamination led to retinal degeneration in Brg1-deficient retinae. Despite the hypocellularity, premature cell cycle exit, increased cell death and extended cell cycle length, retinal progenitor cells persisted in Brg1-deficient retinae, making them more susceptible to retinoblastoma. ChIP-Seq analysis suggests that Brg1 might regulate gene expression through multiple mechanisms.

Summary: The SWI/SNF protein Brg1 controls cell cycle length, cell cycle exit and cell survival, and is required for cell differentiation and retinal lamination, in the developing mouse retina.

Retinitis pigmentosa and bilateral cystoid macular oedema in a patient heterozygous for the RIM1 mutation previously associated with cone-rod dystrophy 7

BACKGROUND

Autosomal dominant cone-rod dystrophy 7 (CORD7) has been previously associated with the RIM1 c.2459G>A (Arg820His) mutation. Cystoid macular oedema (CMO) is a rare feature of CORD and has not been described in CORD7. We report a patient who was heterozygous for the RIM1 mutation with bilateral CMO and who manifested a retinitis pigmentosa phenotype.

MATERIALS AND METHODS

The patient's medical notes were retrospectively reviewed over an 18-month period. Genetic testing was performed by next generation sequencing for a panel of 176 genes associated with retinal dystrophy.

RESULTS

A 34-year-old man presented with a 5-year history of bilateral floaters and blurred vision. Visual acuity was 20/23 and 20/33 in the right and left eyes, respectively. Optical coherence tomography scans revealed bilateral CMO. Goldmann visual field tests detected mid-peripheral ring scotomas. Electrodiagnostic testing was overall consistent with a primary photoreceptor abnormality involving both rods and cones. Subsequent genetic testing identified heterozygosity for the RIM1 c.2459G>A (Arg820His) mutation. Various treatments for CMO were trialled unsuccessfully. However, at his latest clinic appointment the CMO had partially improved following topical brinzolamide therapy. Most recent visual acuity was 20/25 in the right eye and 20/24 in the left eye.

CONCLUSIONS

This is the first reported case of bilateral CMO in association with the RIM1 mutation. Overall, our findings were more consistent with a phenotype of retinitis pigmentosa. This could imply that the RIM1 mutation causes diverse retinal dystrophies, or that the previously described CORD7 phenotype resulted from a different variant on the same haplotype.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

Biology and therapy of inherited retinal degenerative disease: insights from mouse models

Retinal neurodegeneration associated with the dysfunction or death of photoreceptors is a major cause of incurable vision loss. Tremendous progress has been made over the last two decades in discovering genes and genetic defects that lead to retinal diseases. The primary focus has now shifted to uncovering disease mechanisms and designing treatment strategies, especially inspired by the successful application of gene therapy in some forms of congenital blindness in humans. Both spontaneous and laboratory-generated mouse mutants have been valuable for providing fundamental insights into normal retinal development and for deciphering disease pathology. Here, we provide a review of mouse models of human retinal degeneration, with a primary focus on diseases affecting photoreceptor function. We also describe models associated with retinal pigment epithelium dysfunction or synaptic abnormalities. Furthermore, we highlight the crucial role of mouse models in elucidating retinal and photoreceptor biology in health and disease, and in the assessment of novel therapeutic modalities, including gene- and stem-cell-based therapies, for retinal degenerative diseases.

Advances in imaging ultrastructure yield new insights into presynaptic biology

Synapses are the fundamental functional units of neural circuits, and their dysregulation has been implicated in diverse neurological disorders. At presynaptic terminals, neurotransmitter-filled synaptic vesicles are released in response to calcium influx through voltage-gated calcium channels activated by the arrival of an action potential. Decades of electrophysiological, biochemical, and genetic studies have contributed to a growing understanding of presynaptic biology. Imaging studies are yielding new insights into how synapses are organized to carry out their critical functions. The development of techniques for rapid immobilization and preservation of neuronal tissues for electron microscopy (EM) has led to a new renaissance in ultrastructural imaging that is rapidly advancing our understanding of synapse structure and function.

Retinal dystrophies, genomic applications in diagnosis and prospects for therapy

Retinal dystrophies (RDs) are degenerative diseases of the retina which have marked clinical and genetic heterogeneity. Common presentations among these disorders include night or colour blindness, tunnel vision and subsequent progression to complete blindness. The known causative disease genes have a variety of developmental and functional roles with mutations in more than 120 genes shown to be responsible for the phenotypes. In addition, mutations within the same gene have been shown to cause different disease phenotypes, even amongst affected individuals within the same family highlighting further levels of complexity. The known disease genes encode proteins involved in retinal cellular structures, phototransduction, the visual cycle, and photoreceptor structure or gene regulation. This review aims to demonstrate the high degree of genetic complexity in both the causative disease genes and their associated phenotypes, highlighting the more common clinical manifestation of retinitis pigmentosa (RP). The review also provides insight to recent advances in genomic molecular diagnosis and gene and cell-based therapies for the RDs.

Diagnostic application of an extensive gene panel for Leber congenital amaurosis with severe genetic heterogeneity

Leber congenital amaurosis (LCA) is a genetically heterogeneous disorder and the most severe form of inherited retinal dystrophy. We report results of a diagnostic application of an extensive gene panel composed of 204 retinal dystrophy-related genes and discuss its feasibility as a diagnostic tool. Nineteen unrelated LCA patients were included in the study: two patients for validation purposes of our gene panel, 15 previously analyzed patients with no identified mutations, and two previously unanalyzed patients. Genetic diagnosis for each patient was conducted according to whether the variants were consistent with the known inheritance pattern of each gene. We identified two heterozygous or homozygous pathogenic variants in seven of 19 patients. On the basis of mutation information, clinical features were re-reviewed, and clinical diagnoses for two patients were revised from LCA to LCA-related disorders. In addition, a coverage simulation was performed to determine the optimal depth of coverage of the gene panel. Using our gene panel, we diagnosed LCA and LCA-related disorders in 36.8% of patients and one or more deleterious variants or variants of unknown significance in 89.5% of patients. Molecular diagnosis using this extensive gene panel is expected to facilitate diagnosis of retinal dystrophy and help provide proper treatment to patients, although further analyses is needed for a complete clinical validation.

Presynaptic [Ca(2+)] and GCAPs: aspects on the structure and function of photoreceptor ribbon synapses

Changes in intracellular calcium ions [Ca²⁺] play important roles in photoreceptor signaling. Consequently, intracellular [Ca²⁺] levels need to be tightly controlled. In the light-sensitive outer segments (OS) of photoreceptors, Ca²⁺ regulates the activity of retinal guanylate cyclases thus playing a central role in phototransduction and light-adaptation by restoring light-induced decreases in cGMP. In the synaptic terminals, changes of intracellular Ca²⁺ trigger various aspects of neurotransmission. Photoreceptors employ tonically active ribbon synapses that encode light-induced, graded changes of membrane potential into modulation of continuous synaptic vesicle exocytosis. The active zones of ribbon synapses contain large electron-dense structures, synaptic ribbons, that are associated with large numbers of synaptic vesicles. Synaptic coding at ribbon synapses differs from synaptic coding at conventional (phasic) synapses. Recent studies revealed new insights how synaptic ribbons are involved in this process. This review focuses on the regulation of [Ca²⁺] in presynaptic photoreceptor terminals and on the function of a particular Ca²⁺-regulated protein, the neuronal calcium sensor protein GCAP2 (guanylate cyclase-activating protein-2) in the photoreceptor ribbon synapse. GCAP2, an EF-hand-containing protein plays multiple roles in the OS and in the photoreceptor synapse. In the OS, GCAP2 works as a Ca²⁺-sensor within a Ca²⁺-regulated feedback loop that adjusts cGMP levels. In the photoreceptor synapse, GCAP2 binds to RIBEYE, a component of synaptic ribbons, and mediates Ca²⁺-dependent plasticity at that site. Possible mechanisms are discussed.

Disease progression in autosomal dominant cone-rod dystrophy caused by a novel mutation (D100G) in the GUCA1A gene

PURPOSE

To document longitudinal fundus autofluorescence (FAF) and electroretinogram (ERG) findings in a family with cone-rod dystrophy (CRD) caused by a novel missense mutation (D100G) in the GUCA1A gene.

METHODS

Observational case series.

RESULTS

Three family members 26-49 years old underwent complete clinical examinations. In all patients, funduscopic findings showed intraretinal pigment migration, loss of neurosensory retinal pigment epithelium, and macular atrophy. FAF imaging revealed the presence of a progressive hyperautofluorescent ring around a hypoautofluorescent center corresponding to macular atrophy. Full-field ERGs showed a more severe loss of cone than rod function in each patient. Thirty-hertz flicker responses fell far below normal limits. Longitudinal FAF and ERG findings in one patient suggested progressive CRD. Two more advanced patients exhibited reduced rod response consistent with disease stage. Direct sequencing of the GUCA1A gene revealed a new missense mutation, p.Asp100Gly (D100G), in each patient.

CONCLUSION

Patients with autosomal dominant CRD caused by a D100G mutation in GUCA1A exhibit progressive vision loss early within the first decade of life identifiable by distinct ERG characteristics and subsequent genetic testing.

Novel GUCA1A mutations suggesting possible mechanisms of pathogenesis in cone, cone-rod, and macular dystrophy patients

Here, we report two novel GUCA1A (the gene for guanylate cyclase activating protein 1) mutations identified in unrelated Spanish families affected by autosomal dominant retinal degeneration (adRD) with cone and rod involvement. All patients from a three-generation adRD pedigree underwent detailed ophthalmic evaluation. Total genome scan using single-nucleotide polymorphisms and then the linkage analysis were undertaken on the pedigree. Haplotype analysis revealed a 55.37 Mb genomic interval cosegregating with the disease phenotype on chromosome 6p21.31-q15. Mutation screening of positional candidate genes found a heterozygous transition c.250C>T in exon 4 of GUCA1A, corresponding to a novel mutation p.L84F. A second missense mutation, c.320T>C (p.I107T), was detected by screening of the gene in a Spanish patients cohort. Using bioinformatics approach, we predicted that either haploinsufficiency or dominant-negative effect accompanied by creation of a novel function for the mutant protein is a possible mechanism of the disease due to c.250C>T and c.320T>C. Although additional functional studies are required, our data in relation to the c.250C>T mutation open the possibility that transacting factors binding to de novo created recognition site resulting in formation of aberrant splicing variant is a disease model which may be more widespread than previously recognized as a mechanism causing inherited RD.

Ocular phenotype of CORD5, an autosomal dominant retinal dystrophy associated with PITPNM3 p.Q626H mutation

PURPOSE

To describe in detail the phenotype of CORD5 in two families segregating a mutation c.1878G>C (p.Q626H) in the PITPNM3 gene.

METHODS

The study included 35 individuals from two different families of Swedish origin, all heterozygous for a PITPNM3 p.Q626H mutation. All participants underwent ophthalmological examination including kinetic perimetry, and in selected cases adaptometry, colour vision tests and optical coherence tomography (OCT). Electrophysiological studies were also performed. In some cases, the data were obtained from medical records.

RESULTS

The majority of patients showed subnormal visual acuity and light sensitivity from childhood. Early signs of macular degeneration were also observed. There was a progressive decrease in visual acuity leading to legal blindness in early adulthood. Electrophysiological testing showed a progressive loss of photoreceptor function restricted mainly to the cones. OCT revealed decreased macular thickness with flattened and enlarged fovea.

CONCLUSION

Our observations of the PITPNM3 p.Q626H mutation carriers confirm that CORD5 is a disease not to mix with other retinal degenerations mapped to 17p13. The results of our clinical evaluation so far indicate that CORD5 is characterized by predominant cone dysfunction without signs of general involvement of the retinal pigment epithelium. The rod system also seems to be unaffected. In this sense, CORD5 is different from other autosomal dominant CORDs where rod involvement is present to some degree in a late phase of the disease. Some intra- and inter-familial differences regarding the severity of the clinical picture were observed.

Detection of novel genetic variation in autosomal dominant retinitis pigmentosa

We explored an approach to detect disease-causing sequence variants in 448 candidate genes from five index cases of autosomal dominant retinitis pigmentosa (adRP) by sequence DNA capture and next-generation DNA sequencing (NGS). Detection of sequence variants was carried out by sequence capture NimbleGen and NGS in a SOLiD platform. After filtering out variants previously reported in genomic databases, novel potential adRP-causing variants were validated by dideoxy capillary electrophoresis (Sanger) sequencing and co-segregation in the families. A total of 55 novel sequence variants in the coding or splicing regions of adRP candidate genes were detected, 49 of which were confirmed by Sanger sequencing. Segregation of these variants in the corresponding adRP families showed three variants present in all the RP-affected members of the family. A novel mutation, p.L270R in IMPDH1, was found to be disease causing in one family. In another family a variant, p.M96T in the NRL gene was detected; this variant was previously reported as probably causing adRP. However, the previously reported p.A76V mutation in NRL as a cause of RP was excluded by co-segregation in the family. We discuss the benefits and limitations of our approach in the context of mutation detection in adRP patients.

Screening for variants in 20 genes in 130 unrelated patients with cone-rod dystrophy

Cone-rod dystrophy (CORD) is a hereditary retinal disorder with primary cone impairment and subsequent rod involvement. To date, mutations responsible for CORD have been reported in 24 genes. However, the systemic evaluation of variants in these genes in a cohort of patients is rare, particularly in East Asia. In this study, 58 coding exons from 20 CORD genes, including 35 exons with previously identified mutations in 17 genes and all 23 coding exons for the other 3 genes (GUCY2D, PRPH2 and KCNV2), were analyzed by cycle sequencing on 130 unrelated probands with CORD. Four heterozygous mutations, 1 novel and 3 known, were detected in 4/130 patients, including c.259G>A (p.Asp87Asn) in UNC119, c.2512C>T (p.Arg838Cys) and c.2513G>A (p.Arg838His) in GUCY2D and c.946T>G (p.Trp316Gly) in PRPH2. The result implies a comparatively low rate of mutations in these exons in Chinese patients. These data suggest that in Chinese patients, CORD may be caused by mutations in exons that have not yet been screened or in genes that have yet to be identified. Further analysis of these patients may provide clarification.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Mechanism and genotype-phenotype correlation of two proximal 6q deletions characterized using mBAND, FISH, array CGH, and DNA sequencing

Proximal 6q deletions have a milder phenotype than middle and distal 6q deletions. We describe 2 patients with non-overlapping deletions of about 15 and 19 Mb, respectively, which subdivide the proximal 6q region into 2 parts. The aberrations were identified using karyotyping and analysed using mBAND and array CGH. The unaffected mother of the first patient carried a mosaic karyotype with the deletion in all metaphases analysed and a small supernumerary marker formed by the deleted material in about 77% of cells. Her chromosome 6 centromeric signal was split between the deleted chromosome and the marker, suggesting that this deletion arose through the centromere fission mechanism. In this family the location of the proximal breakpoint in the centromere prevented cloning of the deletion junction, but the junction of the more distal deletion in the second patient was cloned and sequenced. This analysis showed that the latter aberration was most likely caused by non-homologous end joining. The second patient also had a remarkably more severe phenotype which could indicate a partial overlap of his deletion with the middle 6q interval. The phenotypes of both patients could be partly correlated with the gene content of their deletions and with phenotypes of other published patients.

A genome-wide association study of aging

Human longevity and healthy aging show moderate heritability (20%-50%). We conducted a meta-analysis of genome-wide association studies from 9 studies from the Cohorts for Heart and Aging Research in Genomic Epidemiology Consortium for 2 outcomes: (1) all-cause mortality, and (2) survival free of major disease or death. No single nucleotide polymorphism (SNP) was a genome-wide significant predictor of either outcome (p < 5 × 10(-8)). We found 14 independent SNPs that predicted risk of death, and 8 SNPs that predicted event-free survival (p < 10(-5)). These SNPs are in or near genes that are highly expressed in the brain (HECW2, HIP1, BIN2, GRIA1), genes involved in neural development and function (KCNQ4, LMO4, GRIA1, NETO1) and autophagy (ATG4C), and genes that are associated with risk of various diseases including cancer and Alzheimer's disease. In addition to considerable overlap between the traits, pathway and network analysis corroborated these findings. These findings indicate that variation in genes involved in neurological processes may be an important factor in regulating aging free of major disease and achieving longevity.

Spondylometaphyseal dysplasia with cone-rod dystrophy

The co-occurrence of skeletal dysplasia and ophthalmologic abnormality is extremely rare. We report on a boy of a unique form of spondylometaphyseal dysplasia associated with cone-rod dystrophy. He presented with postnatal severe short stature, progressive lower limb deformity with rhizomelic shortening of the long bones, prominent joints with limited mobility in knees and elbows, mild thoracic scoliosis, and vision impairment due to cone dystrophy. Correction of deformity and simultaneous limb lengthening was performed in bilateral femora and tibiae without major complications. Skeletal manifestations in addition to comprehensive ophthalmologic examinations were described in this patient who had been followed from infancy to 16 years of age.

PITPNM3 is an uncommon cause of cone and cone-rod dystrophies

The first mutation in PITPNM3, a human homologue of the Drosophila retinal degeneration (rdgB not not) gene was reported in two large Swedish families with autosomal dominant cone dystrophy. To establish the global impact that PITPNM3 has on retinal degenerations we screened 163 patients from Denmark, Germany, the UK, and USA. Four sequence variants, two missence mutations and two intronic changes were identified in the screen. Thus, mutations in PITPNM3 do not appear to be a major cause of cone or cone-rod dystrophy.

Rab3-interacting molecule gamma isoforms lacking the Rab3-binding domain induce long lasting currents but block neurotransmitter vesicle anchoring in voltage-dependent P/Q-type Ca2+ channels

Assembly of voltage-dependent Ca(2+) channels (VDCCs) with their associated proteins regulates the coupling of VDCCs with upstream and downstream cellular events. Among the four isoforms of the Rab3-interacting molecule (RIM1 to -4), we have previously reported that VDCC beta-subunits physically interact with the long alpha isoform of the presynaptic active zone scaffolding protein RIM1 (RIM1alpha) via its C terminus containing the C(2)B domain. This interaction cooperates with RIM1alpha-Rab3 interaction to support neurotransmitter exocytosis by anchoring vesicles in the vicinity of VDCCs and by maintaining depolarization-triggered Ca(2+) influx as a result of marked inhibition of voltage-dependent inactivation of VDCCs. However, physiological functions have not yet been elucidated for RIM3 and RIM4, which exist only as short gamma isoforms (gamma-RIMs), carrying the C-terminal C(2)B domain common to RIMs but not the Rab3-binding region and other structural motifs present in the alpha-RIMs, including RIM1alpha. Here, we demonstrate that gamma-RIMs also exert prominent suppression of VDCC inactivation via direct binding to beta-subunits. In the pheochromocytoma PC12 cells, this common functional feature allows native RIMs to enhance acetylcholine secretion, whereas gamma-RIMs are uniquely different from alpha-RIMs in blocking localization of neurotransmitter-containing vesicles near the plasma membrane. Gamma-RIMs as well as alpha-RIMs show wide distribution in central neurons, but knockdown of gamma-RIMs attenuated glutamate release to a lesser extent than that of alpha-RIMs in cultured cerebellar neurons. The results suggest that sustained Ca(2+) influx through suppression of VDCC inactivation by RIMs is a ubiquitous property of neurons, whereas the extent of vesicle anchoring to VDCCs at the plasma membrane may depend on the competition of alpha-RIMs with gamma-RIMs for VDCC beta-subunits.

GCAP1 mutations associated with autosomal dominant cone dystrophy

We discuss the heterogeneity of autosomal dominant cone and cone-rod dystrophies (adCD, and adCORD, respectively). As one of the best characterized adCD genes, we focus on the GUCA1A gene encoding guanylate cyclase activating protein 1 (GCAP1), a protein carrying three high affinity Ca(2+) binding motifs (EF hands). GCAP1 senses changes in cytoplasmic free [Ca(2+)] and communicates these changes to GC1, by either inhibiting it (at high free [Ca(2+)]), or stimulating it (at low free [Ca(2+)]). A number of missense mutations altering the structure and Ca(2+) affinity of EF hands have been discovered. These mutations are associated with a gain of function, producing dominant cone and cone rod dystrophy phenotypes. In this article we review these mutations and describe the consequences of specific mutations on GCAP1 structure and GC stimulation.We discuss the heterogeneity of autosomal dominant cone and cone-rod dystrophies (adCD, and adCORD, respectively). As one of the best characterized adCD genes, we focus on the GUCA1A gene encoding guanylate cyclase activating protein 1 (GCAP1), a protein carrying three high affinity Ca(2+) binding motifs (EF hands). GCAP1 senses changes in cytoplasmic free [Ca(2+)] and communicates these changes to GC1, by either inhibiting it (at high free [Ca(2+)]), or stimulating it (at low free [Ca(2+)]). A number of missense mutations altering the structure and Ca(2+) affinity of EF hands have been discovered. These mutations are associated with a gain of function, producing dominant cone and cone rod dystrophy phenotypes. In this article we review these mutations and describe the consequences of specific mutations on GCAP1 structure and GC stimulation.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Molecular Genetic Analysis of Two Functional Candidate Genes in the Autosomal Recessive Retinitis Pigmentosa, RP25, Locus

PURPOSE

To identify the disease gene in five Spanish families with autosomal recessive retinitis pigmentosa (arRP) linked to the RP25 locus. Two candidate genes, EEF1A1 and IMPG1, were selected from the region between D6S280 and D6S1644 markers where the families are linked. The genes were selected as good candidates on the basis of their function, tissue expression pattern, and/or genetic data.

METHODS

A molecular genetic study was performed on DNA extracted from one parent and one affected member of each studied family. The coding exons, splice sites, and the 5' UTR of the genes were amplified by polymerase chain reaction (PCR). For mutation detection, direct sequence analysis was performed using the ABI 3100 automated sequencer. Segregation of an IMPG1 single nucleotide polymorphism (SNP) in all the families studied was analyzed by restriction enzyme digest of the amplified gene fragments.

RESULTS

In total, 15 SNPs were identified of which 7 were novel. Of the identified SNPs, one was insertion, two were deletions, five were intronic, six were missense, and one was located in the 5' UTR. These changes, however, were also identified in unaffected members of the families and/or 50 control Caucasians. The examined known IMPG1 SNP was not segregating with the disease phenotype but was correlating with the genetic data in all families studied.

CONCLUSIONS

Our results indicate that neither EEF1A1 nor IMPG1 could be responsible for RP25 in the studied families due to absence of any pathogenic variants. However, it is important to notice that the methodology used in this study cannot detect larger deletions that lie outside the screened regions or primer site mutations that exist in the heterozygous state. A role of both genes in other inherited forms of RP and/or retinal degenerations needs to be elucidated.

A novel GCAP1(N104K) mutation in EF-hand 3 (EF3) linked to autosomal dominant cone dystrophy

The GUCA1A gene encodes a guanylate cyclase activating protein (GCAP1) that is involved in regulation of phototransduction in the vertebrate retina. We discovered a novel C312A transversion in exon 2 of the human GUCA1A gene, replacing Asn-104 (N104) in GCAP1 with Lys (K), in two affected members of a family with dominant cone dystrophy. The mutation N104K is located in the third EF-hand motif (EF3) shown previously to be instrumental in converting Ca2+-free GCAP1 to a GC inhibitor in the Ca2+-bound form. In one patient, rod ERGs were fairly stable over a 12-year-period whereas 30 Hz flicker ERG and single-flash cone ERGs declined. In both patients, double-flash ERGs showed that rod recovery from an intense test flash was significantly delayed. The EC(50) for GC stimulation shifted from approximately 250 nM in wild-type GCAP1 to approximately 800 nM in the GCAP1(N104K) mutant suggesting inability of the mutant to assume an inactive form under physiological conditions. The replacement of N104 by K in GCAP1 is the first naturally occurring mutation identified in the EF3 loop. The rod recovery delays observed in double-flash ERG of affected patients suggest a novel dominant-negative effect that slows GC stimulation.

Interaction of alpha-actinin-4 with class I PxxP motif-containing OK/SW-CL.16 protein

BACKGROUND

The aim of this study was to identify novel binding partners for alpha-actinin-4 (actinin-4), an essential component of the glomerular filtration barrier.

METHODS

We performed bacterial two-hybrid screenings of a human kidney cDNA library using as a bait human actinin-4 containing the spectrin-like repeat R1. The identified interactions were further verified by in vitro affinity assays. To investigate the expression of the identified molecules in the kidney and other tissues, a human tissue cDNA panel screening and in situ hybridization were performed.

RESULTS

One isolated cDNA from the library encoded OK/SW-CL.16 protein with a segment similar to known actinin-interacting regions. OK/SW-CL.16 protein also contained the class I PxxP motif, which can participate in binding to Src homology 3 (SH3) domain-containing signaling proteins. In vitro affinity assays showed interactions of recombinant actinin-4 R1 and full-length renal actinin-4 with recombinant OK/SW-CL.16 protein. A tissue cDNA panel screening revealed a ubiquitous expression of OK/SW-CL.16 mRNA. In situ hybridization showed glomerular expression of OK/SW-CL.16 mRNA, mainly in podocytes and mesangial cells.

CONCLUSION

Our results suggest that OK/SW-CL.16 protein is a novel binding partner for actinin-4. OK/SW-CL16 protein might act as a linker between actinin-4 and some SH3 domain-containing signaling proteins in podocytes.