Functional analysis of human CNGA3 mutations associated with colour blindness suggests impaired surface expression of channel mutants A3(R427C) and A3(R563C)

Unlocking cellular traffic jams: olive oil-mediated rescue of CNG mutant channels

One of the reasons to suggest olive oil consumption for a healthy life is its potential to induce robust lipidomic remodeling through membrane modification by dietary lipids. This remodeling might, in turn, modulate essential lipid-protein interactions while maintaining accurate transmembrane protein/domain orientation. Oleic acid, the primary compound in olive oil, has been suggested as a modulator of ion channel function. In this study, we explored whether this lipid could rescue the trafficking of mutated transmembrane proteins. In our initial approach, we supplemented the cell culture medium of HEK-293 cells expressing cyclic nucleotide channels tagged using green fluorescent protein (CNG-GFP) with olive oil or oleic acid. In addition to wild-type channels, we also expressed R272Q and R278W mutant channels, two non-functional intracellularly retained channels related to retinopathies. We used fluorescence microscopy and patch-clamp in the inside-out configuration to assess changes in the cell localization and function of the tested channels. Our results demonstrated that olive oil and oleic acid facilitated the transport of cyclic nucleotide-gated R272Q mutant channels towards the plasma membrane, rendering them electrophysiologically functional. Thus, our findings reveal a novel property of olive oil as a membrane protein traffic inductor.

Functional evaluation allows ACMG/AMP-based re-classification of CNGA3 variants associated with achromatopsia

PURPOSE

CNGA3 encoding the main subunit of the cyclic nucleotide-gated ion channel in cone photoreceptors is one of the major disease-associated genes for achromatopsia. Most CNGA3 variants are missense variants with the majority being functionally uncharacterized and therefore hampering genetic diagnosis. In light of potential gene therapy, objective variant pathogenicity assessment is essential.

METHODS

We established a medium-throughput aequorin-based luminescence bioassay allowing mutant CNGA3 channel function assessment via quantification of CNGA3 channel-mediated calcium influx in a cell culture system, thereby enabling American College of Medical Genetics and Genomics/Association for Molecular Pathology-based variant re-classification.

RESULTS

We provide functional read-out obtained for 150 yet uncharacterized CNGA3 missense substitutions of which 55 were previously categorized as variants of uncertain significance (VUS) identifying 25 as functionally normal and 125 as functionally abnormal. These data enabled the American College of Medical Genetics and Genomics/ Association for Molecular Pathology-based variant re-classification of 52/55 VUS as either benign, likely benign, or likely pathogenic reaching a VUS re-classification rate of 94.5%.

CONCLUSION

Our aequorin-based bioassay allows functionally ensured clinical variant interpretation for 150 CNGA3 missense variants enabling and supporting VUS re-classification and assuring molecular diagnosis to patients affected by CNGA3-associated achromatopsia, hereby identifying patients eligible for future gene therapy trials on this disease.

Biology, Pathobiology and Gene Therapy of CNG Channel-Related Retinopathies

The visual process begins with the absorption of photons by photopigments of cone and rod photoreceptors in the retina. In this process, the signal is first amplified by a cyclic guanosine monophosphate (cGMP)-based signaling cascade and then converted into an electrical signal by cyclic nucleotide-gated (CNG) channels. CNG channels are purely ligand-gated channels whose activity can be controlled by cGMP, which induces a depolarizing Na⁺/Ca²⁺ current upon binding to the channel. Structurally, CNG channels belong to the superfamily of pore-loop cation channels and share structural similarities with hyperpolarization-activated cyclic nucleotide (HCN) and voltage-gated potassium (KCN) channels. Cone and rod photoreceptors express distinct CNG channels encoded by homologous genes. Mutations in the genes encoding the rod CNG channel (CNGA1 and CNGB1) result in retinitis-pigmentosa-type blindness. Mutations in the genes encoding the cone CNG channel (CNGA3 and CNGB3) lead to achromatopsia. Here, we review the molecular properties of CNG channels and describe their physiological and pathophysiological roles in the retina. Moreover, we summarize recent activities in the field of gene therapy aimed at developing the first gene therapies for CNG channelopathies.

Comprehensive variant spectrum of the CNGA3 gene in patients affected by achromatopsia

Achromatopsia (ACHM) is a congenital cone photoreceptor disorder characterized by impaired color discrimination, low visual acuity, photosensitivity, and nystagmus. To date, six genes have been associated with ACHM (CNGA3, CNGB3, GNAT2, PDE6C, PDE6H, and ATF6), the majority of these being implicated in the cone phototransduction cascade. CNGA3 encodes the CNGA3 subunit of the cyclic nucleotide-gated ion channel in cone photoreceptors and is one of the major disease-associated genes for ACHM. Herein, we provide a comprehensive overview of the CNGA3 variant spectrum in a cohort of 1060 genetically confirmed ACHM patients, 385 (36.3%) of these carrying "likely disease-causing" variants in CNGA3. Compiling our own genetic data with those reported in the literature and in public databases, we further extend the CNGA3 variant spectrum to a total of 316 variants, 244 of which we interpreted as "likely disease-causing" according to ACMG/AMP criteria. We report 48 novel "likely disease-causing" variants, 24 of which are missense substitutions underlining the predominant role of this mutation class in the CNGA3 variant spectrum. In addition, we provide extensive in silico analyses and summarize reported functional data of previously analyzed missense, nonsense and splicing variants to further advance the pathogenicity assessment of the identified variants.

Delineating the Molecular and Phenotypic Spectrum of the CNGA3-Related Cone Photoreceptor Disorder in Pakistani Families

Cone photoreceptor dysfunction represents a clinically heterogenous group of disorders characterized by nystagmus, photophobia, reduced central or color vision, and macular dystrophy. Here, we described the molecular findings and clinical manifestations of achromatopsia, a partial or total absence of color vision, co-segregating with three known missense variants of CNGA3 in three large consanguineous Pakistani families. Fundus examination and optical coherence tomography (OCT) imaging revealed myopia, thin retina, retinal pigment epithelial cells loss at fovea/perifovea, and macular atrophy. Combination of Sanger and whole exome sequencing revealed three known homozygous missense variants (c.827A>G, p.(Asn276Ser); c.847C>T, p.(Arg283Trp); c.1279C>T, p.(Arg427Cys)) in CNGA3, the α-subunit of the cyclic nucleotide-gated cation channel in cone photoreceptor cells. All three variants are predicted to replace evolutionary conserved amino acids, and to be pathogenic by specific in silico programs, consistent with the observed altered membrane targeting of CNGA3 in heterologous cells. Insights from our study will facilitate counseling regarding the molecular and phenotypic landscape of CNGA3-related cone dystrophies.

Structural and functional characterization of an achromatopsia-associated mutation in a phototransduction channel

Numerous missense mutations in cyclic nucleotide-gated (CNG) channels cause achromatopsia and retinitis pigmentosa, but the underlying pathogenic mechanisms are often unclear. We investigated the structural basis and molecular/cellular effects of R410W, an achromatopsia-associated, presumed loss-of-function mutation in human CNGA3. Cryo-EM structures of the Caenorhabditis elegans TAX-4 CNG channel carrying the analogous mutation, R421W, show that most apo channels are open. R421, located in the gating ring, interacts with the S4 segment in the closed state. R421W disrupts this interaction, destabilizes the closed state, and stabilizes the open state. CNGA3_R410W/CNGB3 and TAX4_R421W channels are spontaneously active without cGMP and induce cell death, suggesting cone degeneration triggered by spontaneous CNG channel activity as a possible cause of achromatopsia. Our study sheds new light on CNG channel allosteric gating, provides an impetus for a reevaluation of reported loss-of-function CNG channel missense disease mutations, and has implications for mutation-specific treatment of retinopathy.

Achromatopsia: Genetics and Gene Therapy

Achromatopsia (ACHM), also known as rod monochromatism or total color blindness, is an autosomal recessively inherited retinal disorder that affects the cones of the retina, the type of photoreceptors responsible for high-acuity daylight vision. ACHM is caused by pathogenic variants in one of six cone photoreceptor-expressed genes. These mutations result in a functional loss and a slow progressive degeneration of cone photoreceptors. The loss of cone photoreceptor function manifests at birth or early in childhood and results in decreased visual acuity, lack of color discrimination, abnormal intolerance to light (photophobia), and rapid involuntary eye movement (nystagmus). Up to 90% of patients with ACHM carry mutations in CNGA3 or CNGB3, which are the genes encoding the alpha and beta subunits of the cone cyclic nucleotide-gated (CNG) channel, respectively. No authorized therapy for ACHM exists, but research activities have intensified over the past decade and have led to several preclinical gene therapy studies that have shown functional and morphological improvements in animal models of ACHM. These encouraging preclinical data helped advance multiple gene therapy programs for CNGA3- and CNGB3-linked ACHM into the clinical phase. Here, we provide an overview of the genetic and molecular basis of ACHM, summarize the gene therapy-related research activities, and provide an outlook for their clinical application.

Sleep and circadian phenotype in people without cone-mediated vision: a case series of five CNGB3 and two CNGA3 patients

Light exposure entrains the circadian clock through the intrinsically photosensitive retinal ganglion cells, which sense light in addition to the cone and rod photoreceptors. In congenital achromatopsia (prevalence 1:30-50 000), the cone system is non-functional, resulting in severe light avoidance and photophobia at daytime light levels. How this condition affects circadian and neuroendocrine responses to light is not known. In this case series of genetically confirmed congenital achromatopsia patients (n = 7; age 30-72 years; 6 women, 1 male), we examined survey-assessed sleep/circadian phenotype, self-reported visual function, sensitivity to light and use of spectral filters that modify chronic light exposure. In all but one patient, we measured rest-activity cycles using actigraphy over 3 weeks and measured the melatonin phase angle of entrainment using the dim-light melatonin onset. Owing to their light sensitivity, congenital achromatopsia patients used filters to reduce retinal illumination. Thus, congenital achromatopsia patients experienced severely attenuated light exposure. In aggregate, we found a tendency to a late chronotype. We found regular rest-activity patterns in all patients and normal phase angles of entrainment in participants with a measurable dim-light melatonin onset. Our results reveal that a functional cone system and exposure to daytime light intensities are not necessary for regular behavioural and hormonal entrainment, even when survey-assessed sleep and circadian phenotype indicated a tendency for a late chronotype and sleep problems in our congenital achromatopsia cohort.

Identification of Chemical and Pharmacological Chaperones for Correction of Trafficking-Deficient Mutant Cyclic Nucleotide-Gated A3 Channels

Trafficking deficiency caused by missense mutations is a well known phenomenon that occurs for mutant, misfolded proteins. Typically, the misfolded protein is retained by the protein quality-control system and degraded by the endoplasmic reticulum-associated protein degradation pathway and thus does not reach its destination, although residual function of the protein may be preserved. Chemical and pharmacological chaperones can improve the targeting of trafficking-deficient proteins and thus may be promising candidates for therapeutic applications. Here, we report the application of a cellular bioassay based on the bioluminescent calcium reporter aequorin to quantify surface expression of mutant CNGA3 channels associated with the autosomal recessively inherited retinal disease achromatopsia. A screening of 77 compounds enabled the identification of effective chemical and pharmacological chaperones that result in a 1.5- to 4.8-fold increase of surface expression of mutant CNGA3. Using selected compounds, we confirmed that the rescue of the defective trafficking is not limited to a single mutation in CNGA3. Active compounds and our structure-activity correlated data for the dihydropyridine compound class may provide valuable information for developing a treatment of the trafficking defect in achromatopsia. SIGNIFICANCE STATEMENT: This study describes a novel luminescence-based assay to detect the surface expression of mutant trafficking-deficient CNGA3 channels based on the calcium-sensitive photoprotein aequorin. Using this assay for a compound screening, this study identifies novel chemical and pharmacological chaperones that restore the surface localization of mutant trafficking-deficient CNGA3 channels. The results from this work may serve as starting point for the development of potent compounds that rescue trafficking deficiencies in the autosomal recessively inherited retinal disease achromatopsia.

Long-Range PCR-Based NGS Applications to Diagnose Mendelian Retinal Diseases

The purpose of this study was to develop a flexible, cost-efficient, next-generation sequencing (NGS) protocol for genetic testing. Long-range polymerase chain reaction (PCR) amplicons of up to 20 kb in size were designed to amplify entire genomic regions for a panel (n = 35) of inherited retinal disease (IRD)-associated loci. Amplicons were pooled and sequenced by NGS. The analysis was applied to 227 probands diagnosed with IRD: (A) 108 previously molecularly diagnosed, (B) 94 without previous genetic testing, and (C) 25 undiagnosed after whole-exome sequencing (WES). The method was validated with 100% sensitivity on cohort A. Long-range PCR-based sequencing revealed likely causative variant(s) in 51% and 24% of proband from cohorts B and C, respectively. Breakpoints of 3 copy number variants (CNVs) could be characterized. Long-range PCR libraries spike-in extended coverage of WES. Read phasing confirmed compound heterozygosity in 5 probands. The proposed sequencing protocol provided deep coverage of the entire gene, including intronic and promoter regions. Our method can be used (i) as a first-tier assay to reduce genetic testing costs, (ii) to elucidate missing heritability cases, (iii) to characterize breakpoints of CNVs at nucleotide resolution, (iv) to extend WES data to non-coding regions by spiking-in long-range PCR libraries, and (v) to help with phasing of candidate variants.

High-Throughput Ca2+ Flux Assay To Monitor Cyclic Nucleotide-Gated Channel Activity and Characterize Achromatopsia Mutant Channel Function

Cone photoreceptor cyclic-nucleotide gated channels (CNG) are tetrameric proteins composed of subunits from CNGA3 and CNGB3. These channels transduce light information into electrical signals carried by both Na+ and Ca2+ ions. More than 100 mutations in the CNGA3 gene are associated with the inherited retinal disorder, achromatopsia 2 (ACHM2), which results in attenuation or loss of color vision, daylight blindness, and reduced visual acuity. Classical techniques to measure CNG channel function utilize patch clamp electrophysiology measuring Na currents in the absence of divalent cations, yet intracellular Ca2+ regulates both light and dark adaptation in photoreceptors. We developed a fluorescence-based, high-throughput Ca2+ flux assay using yellow fluorescent protein (YFP) tagged CNGA3 channels expressed in HEK293 cells which allow monitoring for folding defects in mutant channels. The cell permeant cGMP analog, 8-(4-chlorophenylthio)-cGMP (CPT-cGMP), was used to activate Ca2+ flux. The assay was validated using wild-type CNGA3 homomeric and heteromeric channels and ACHM2-associated homomeric mutant CNG channels, CNGA3-R427C, CNGA3-E590K, and CNGA3-L633P. Additionally, we examined two naturally occurring canine mutations causing day-blindness previously studied by patch clamp. We compared the CPT-cGMP K0.5 values of the channels with patch clamp values from previous studies. The assay provides a screen for modulation of gating and/or rescue of trafficking and/or misfolding defects in ACHM2-associated CNG channels. Importantly, the calcium flux assay is advantageous compared to patch clamp as it allows the ability to monitor CNG channel activity in the presence of calcium.

Accessory heterozygous mutations in cone photoreceptor CNGA3 exacerbate CNG channel-associated retinopathy

Mutations in CNGA3 and CNGB3, the genes encoding the subunits of the tetrameric cone photoreceptor cyclic nucleotide-gated ion channel, cause achromatopsia, a congenital retinal disorder characterized by loss of cone function. However, a small number of patients carrying the CNGB3/c.1208G>A;p.R403Q mutation present with a variable retinal phenotype ranging from complete and incomplete achromatopsia to moderate cone dysfunction or progressive cone dystrophy. By exploring a large patient cohort and published cases, we identified 16 unrelated individuals who were homozygous or (compound-)heterozygous for the CNGB3/c.1208G>A;p.R403Q mutation. In-depth genetic and clinical analysis revealed a co-occurrence of a mutant CNGA3 allele in a high proportion of these patients (10 of 16), likely contributing to the disease phenotype. To verify these findings, we generated a Cngb3R403Q/R403Q mouse model, which was crossbred with Cnga3-deficient (Cnga3-/-) mice to obtain triallelic Cnga3+/- Cngb3R403Q/R403Q mutants. As in human subjects, there was a striking genotype-phenotype correlation, since the presence of 1 Cnga3-null allele exacerbated the cone dystrophy phenotype in Cngb3R403Q/R403Q mice. These findings strongly suggest a digenic and triallelic inheritance pattern in a subset of patients with achromatopsia/severe cone dystrophy linked to the CNGB3/p.R403Q mutation, with important implications for diagnosis, prognosis, and genetic counseling.

Diseases associated with mutations in CNGA3: Genotype-phenotype correlation and diagnostic guideline

Along with the molecular and functional characterization of CNGA3, knowledge about diseases associated with CNGA3 mutations has made great progress. So far, CNGA3 mutations are not only one of the most common causes of achromatopsia and cone dystrophy or cone-rod dystrophy but also one of the most commonly mutated genes among various forms of retinopathy. Understanding the clinical characteristics of CNGA3-associated retinal diseases may help clinical practice of infants or children with related diseases. Recognizing the importance of CNGA3 in inherited retinal diseases may enhance related research in searching for functional restoration or repair of CNGA3 defects.

An early nonsense mutation facilitates the expression of a short isoform of CNGA3 by alternative translation initiation

The cyclic nucleotide-gated (CNG) channel - composed of CNGA3 and CNGB3 subunits - mediates the influx of cations in cone photoreceptors after light stimulation and thus is a key element in cone phototransduction. Mutations in CNGA3 and CNGB3 are associated with achromatopsia, a rare autosomal recessive retinal disorder. Here, we demonstrate that the presence of an early nonsense mutation in CNGA3 induces the usage of a downstream alternative translation initiation site giving rise to a short CNGA3 isoform. The expression of this short isoform was verified by Western blot analysis and DAB staining of HEK293 cells and cone photoreceptor-like 661W cells expressing CNGA3-GST fusion constructs. Functionality of the short isoform was confirmed by a cellular calcium influx assay. Furthermore, patients carrying an early nonsense mutation were analyzed for residual cone photoreceptor function in order to identify a potential role of the short isoform to modify the clinical outcome in achromatopsia patients. Yet the results suggest that the short isoform is not able to compensate for the loss of the long isoform leaving the biological role of this variant unclear.

Achromatopsia: clinical features, molecular genetics, animal models and therapeutic options

Achromatopsia is an autosomal recessive condition, characterised by reduced visual acuity, impaired colour vision, photophobia and nystagmus. The symptoms can be profoundly disabling, and there is no cure currently available. However, the recent development of gene-based interventions may lead to improved outcomes in the future. This article aims to provide a comprehensive review of the clinical features of the condition, its genetic basis and the underlying pathogenesis. We also explore the insights derived from animal models, including the implications for gene supplementation approaches. Finally, we discuss current human gene therapy trials.

Gene Augmentation Therapy for a Missense Substitution in the cGMP-Binding Domain of Ovine CNGA3 Gene Restores Vision in Day-Blind Sheep

Purpose

Applying CNGA3 gene augmentation therapy to cure a novel causative mutation underlying achromatopsia (ACHM) in sheep.

Methods

Impaired vision that spontaneously appeared in newborn lambs was characterized by behavioral, electroretinographic (ERG), and histologic techniques. Deep-sequencing reads of an affected lamb and an unaffected lamb were compared within conserved genomic regions orthologous to human genes involved in similar visual impairment. Observed nonsynonymous amino acid substitutions were classified by their deleteriousness score. The putative causative mutation was assessed by producing compound CNGA3 heterozygotes and applying gene augmentation therapy using the orthologous human cDNA.

Results

Behavioral assessment revealed day blindness, and subsequent ERG examination showed attenuated photopic responses. Histologic and immunohistochemical examination of affected sheep eyes did not reveal degeneration, and cone photoreceptors expressing CNGA3 were present. Bioinformatics and sequencing analyses suggested a c.1618G>A, p.Gly540Ser substitution in the GMP-binding domain of CNGA3 as the causative mutation. This was confirmed by genetic concordance test and by genetic complementation experiment: All five compound CNGA3 heterozygotes, carrying both p.Arg236* and p.Gly540Ser mutations in CNGA3, were day-blind. Furthermore, subretinal delivery of the intact human CNGA3 gene using an adeno-associated viral vector (AAV) restored photopic vision in two affected p.Gly540Ser homozygous rams.

Conclusions

The c.1618G>A, p.Gly540Ser substitution in CNGA3 was identified as the causative mutation for a novel form of ACHM in Awassi sheep. Gene augmentation therapy restored vision in the affected sheep. This novel mutation provides a large-animal model that is valid for most human CNGA3 ACHM patients; the majority of them carry missense rather than premature-termination mutations.

Achromatopsia caused by novel missense mutations in the CNGA3 gene

AIM

To identify the genetic defects in a Chinese family with achromatopsia.

METHODS

A 2.5-year-old boy, who displayed nystagmus, photophobia, and hyperopia since early infancy, was clinically evaluated. To further confirm and localize the causative mutations in this family, targeted region capture and next-generation sequencing of candidate genes, such as CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H were performed using a custom-made capture array.

RESULTS

Slit-lamp examination showed no specific findings in the anterior segments. The optic discs and maculae were normal on fundoscopy. The unaffected family members reported no ocular complaints. Clinical signs and symptoms were consistent with a clinical impression of autosomal recessive achromatopsia. The results of sequence analysis revealed two novel missense mutations in CNGA3, c.633T>A (p.D211E) and c.1006G>T (p.V336F), with an autosomal recessive mode of inheritance.

CONCLUSION

Genetic analysis of a Chinese family confirmed the clinical diagnosis of achromatopsia. Two novel mutations were identified in CNGA3, which extended the mutation spectrum of this disorder.

Novel CNGA3 mutations in Chinese patients with achromatopsia

OBJECTIVE

To study the clinical features and to identify the pathogenic mutations in Chinese patients with achromatopsia (ACHM).

DESIGN

Fifteen patients from 10 unrelated families were included in this study. Detailed ocular examinations were performed for the affected subjects, including best-corrected visual acuity (BCVA), colour vision, slit lamp, fundus, electroretinography, perimetry, and spectral domain optical coherent topography (SD-OCT). Peripheral blood samples were obtained from all of the patients and their family members for genomic DNA extraction. All exons of CNGA3, CNGB3, GNAT2, PDE6C and PDE6H were amplified by a PCR and screened for mutation by direct Sanger sequencing. The sequences were analysed using the Blat tool and then compared with the gene transcript. A segregation test was conducted in the patients' parents if they were available. The variants were compared with the database of the 1000 Genomes Project to exclude polymorphism.

RESULTS

Nystagmus, photophobia, and impaired colour discrimination were observed in all patients. The BCVA of the affected subjects ranged from 0.05-0.2. Severely depressed and non-recordable cone electroretinograms were observed. Noticeable structural changes including disruption or loss of the macular inner/outer segments (IS/OS) junction of the photoreceptors were observed with SD-OCT. CNGA3 mutations were identified in 13 patients from eight families. Sequencing revealed seven novel missense mutations, three novel deletion mutations, and four previously reported mutations among those patients.

CONCLUSIONS

CNGA3 mutation is the most frequent cause of ACHM in this cohort of patients. Ten novel mutations were identified in CNGA3. Genetic characterisation of patients with ACHM is important for genetic counselling and future gene therapies. This study reports the comprehensive clinical and genetic features of Chinese patients with ACHM.

Homozygous missense variant in the human CNGA3 channel causes cone-rod dystrophy

We assessed a large consanguineous Pakistani family (PKAB157) segregating early onset low vision problems. Funduscopic and electroretinographic evaluation of affected individuals revealed juvenile cone-rod dystrophy (CRD) with maculopathy. Other clinical symptoms included loss of color discrimination, photophobia and nystagmus. Whole-exome sequencing, segregation and haplotype analyses demonstrated that a transition variant (c.955T>C; p.(Cys319Arg)) in CNGA3 co-segregated with the CRD phenotype in family PKAB157. The ability of CNGA3 channel to influx calcium in response to agonist, when expressed either alone or together with the wild-type CNGB3 subunit in HEK293 cells, was completely abolished due to p.Cys319Arg variant. Western blotting and immunolocalization studies suggest that a decreased channel density in the HEK293 cell membrane due to impaired folding and/or trafficking of the CNGA3 protein is the main pathogenic effect of the p.Cys319Arg variant. Mutant alleles of the human cone photoreceptor cyclic nucleotide-gated channel (CNGA3) are frequently associated with achromatopsia. In rare cases, variants in CNGA3 are also associated with cone dystrophy, Leber's congenital amaurosis and oligo cone trichromacy. The identification of predicted p.(Cys319Arg) missense variant in CNGA3 expands the repertoire of the known genetic causes of CRD and phenotypic spectrum of CNGA3 alleles.

Causes and consequences of inherited cone disorders

Hereditary cone disorders (CDs) are characterized by defects of the cone photoreceptors or retinal pigment epithelium underlying the macula, and include achromatopsia (ACHM), cone dystrophy (COD), cone-rod dystrophy (CRD), color vision impairment, Stargardt disease (STGD) and other maculopathies. Forty-two genes have been implicated in non-syndromic inherited CDs. Mutations in the 5 genes implicated in ACHM explain ∼93% of the cases. On the contrary, only 21% of CRDs (17 genes) and 25% of CODs (8 genes) have been elucidated. The fact that the large majority of COD and CRD-associated genes are yet to be discovered hints towards the existence of unknown cone-specific or cone-sensitive processes. The ACHM-associated genes encode proteins that fulfill crucial roles in the cone phototransduction cascade, which is the most frequently compromised (10 genes) process in CDs. Another 7 CD-associated proteins are required for transport processes towards or through the connecting cilium. The remaining CD-associated proteins are involved in cell membrane morphogenesis and maintenance, synaptic transduction, and the retinoid cycle. Further novel genes are likely to be identified in the near future by combining large-scale DNA sequencing and transcriptomics technologies. For 31 of 42 CD-associated genes, mammalian models are available, 14 of which have successfully been used for gene augmentation studies. However, gene augmentation for CDs should ideally be developed in large mammalian models with cone-rich areas, which are currently available for only 11 CD genes. Future research will aim to elucidate the remaining causative genes, identify the molecular mechanisms of CD, and develop novel therapies aimed at preventing vision loss in individuals with CD in the future.

A cyclic nucleotide-gated channel mutation associated with canine daylight blindness provides insight into a role for the S2 segment tri-Asp motif in channel biogenesis

Cone cyclic nucleotide-gated channels are tetramers formed by CNGA3 and CNGB3 subunits; CNGA3 subunits function as homotetrameric channels but CNGB3 exhibits channel function only when co-expressed with CNGA3. An aspartatic acid (Asp) to asparagine (Asn) missense mutation at position 262 in the canine CNGB3 (D262N) subunit results in loss of cone function (daylight blindness), suggesting an important role for this aspartic acid residue in channel biogenesis and/or function. Asp 262 is located in a conserved region of the second transmembrane segment containing three Asp residues designated the Tri-Asp motif. This motif is conserved in all CNG channels. Here we examine mutations in canine CNGA3 homomeric channels using a combination of experimental and computational approaches. Mutations of these conserved Asp residues result in the absence of nucleotide-activated currents in heterologous expression. A fluorescent tag on CNGA3 shows mislocalization of mutant channels. Co-expressing CNGB3 Tri-Asp mutants with wild type CNGA3 results in some functional channels, however, their electrophysiological characterization matches the properties of homomeric CNGA3 channels. This failure to record heteromeric currents suggests that Asp/Asn mutations affect heteromeric subunit assembly. A homology model of S1-S6 of the CNGA3 channel was generated and relaxed in a membrane using molecular dynamics simulations. The model predicts that the Tri-Asp motif is involved in non-specific salt bridge pairings with positive residues of S3/S4. We propose that the D262N mutation in dogs with CNGB3-day blindness results in the loss of these inter-helical interactions altering the electrostatic equilibrium within in the S1-S4 bundle. Because residues analogous to Tri-Asp in the voltage-gated Shaker potassium channel family were implicated in monomer folding, we hypothesize that destabilizing these electrostatic interactions impairs the monomer folding state in D262N mutant CNG channels during biogenesis.

Erbliche Ionenkanalerkrankungen der Netzhaut

Retinale Ionenkanalerkrankungen sind klinisch und genetisch sehr heterogen. Die bisher identifizierten krankheitsassoziierten Ionenkanäle umfassen zyklisch nukleotidgesteuerte (CNG-)Kanäle, spannungsgesteuerte Kalium- und Kalziumkanäle, einen einwärtsrektifizierenden Kaliumkanal, einen kalziumaktivierten Chloridkanal und den transienten Rezeptorpotenzialionenkanal TRPM1. Dieses breite Spektrum spiegelt sich auch in der resultierenden Pathophysiologie wieder. Mutationen in retinalen Ionenkanälen können die Detektion von Lichtreizen bzw. deren Umwandlung in ein elektrisches Signal oder die Weiterleitung des Signals von den Fotorezeptoren zu nachgeschalteten Neuronen beeinträchtigen. Einige Erkrankungen werden auch durch Mutationen in Ionenkanälen, die im retinalen Pigmentepithel lokalisiert sind, hervorgerufen. Dieses ist mit seinen unterstützenden Aufgaben für eine normale Netzhautfunktion essenziell.

Diagnostic fundus autofluorescence patterns in achromatopsia

PURPOSE

To describe the unique diagnostic fundus autofluorescence (FAF) patterns in patients with achromatopsia and the associated findings on optical coherence tomography (OCT).

DESIGN

Observational case series.

METHODS

We evaluated 10 patients with achromatopsia by means of best-corrected visual acuity (BCVA), ophthalmoscopy, Goldmann visual field, full-field electroretinography (ffERG), OCT, and FAF photography. FAF patterns were compared with patient age and foveal changes on OCT.

RESULTS

Patients fell into two dichotomous age groups at the time of evaluation: six patients ranged from 11 to 23 years of age, and 3 patients ranged from 52 to 63 years of age. All patients had severely reduced photopic ffERG responses, including those exhibiting preserved foveal structure on OCT. The younger patients had absent to mild foveal atrophy on OCT, and four of the six demonstrated foveal and parafoveal hyperfluorescence on FAF. In addition, a 7-month-old child with compound heterozygous mutations in CNGA3 demonstrated similar foveal hyperfluorescence. The older patients demonstrated advanced foveal atrophy and punched-out foveal hypofluorescence with discrete borders on FAF imaging corresponding to the area of outer retinal cavitation on OCT.

CONCLUSIONS

Foveal hyperfluorescence is an early sign of achromatopsia that can aid in clinical diagnosis. In our cohort, patients with achromatopsia demonstrated age-dependent changes in FAF, which are likely to be progressive and to correlate with foveal atrophy and cavitation on OCT. This finding may be useful in charting the natural course of the disease and in defining a therapeutic window for treatment.

Restoration of proper trafficking to the cell surface for membrane proteins harboring cysteine mutations

A common phenotype for many genetic diseases is that the cell is unable to deliver full-length membrane proteins to the cell surface. For some forms of autism, hereditary spherocytosis and color blindness, the culprits are single point mutations to cysteine. We have studied two inheritable cysteine mutants of cyclic nucleotide-gated channels that produce achromatopsia, a common form of severe color blindness. By taking advantage of the reactivity of cysteine’s sulfhydryl group, we modified these mutants with chemical reagents that attach moieties with similar chemistries to the wild-type amino acids’ side chains. We show that these modifications restored proper delivery to the cell membrane. Once there, the channels exhibited normal functional properties. This strategy might provide a unique opportunity to assess the chemical nature of membrane protein traffic problems.

Genetic analysis of four Pakistani families with achromatopsia and a novel S4 motif mutation of CNGA3

BACKGROUND

To identify the causative variants of achromatopsia (ACHM) in four Pakistani families presenting autosomal recessive ACHM.

METHODS

Four families (50, 55, 70 and 74) exhibiting features of achromatopsia were subjected to homozygosity mapping with STS markers flanking known ACHM loci. Mutation screening was done for two of the families linked to CNGA3 and CNGB3 by direct sequencing of the coding regions and exon-intron boundaries of genes to find the pathogenic variant.

RESULTS

Homozygosity mapping showed co-segregation of CNGA3 in family 50 and CNGB3 in family 74. Sequencing of coding regions of CNGA3 in family 50 revealed a novel missense mutation, c.827A>G, in exon 7, which results in p.N276S substitution. N276S is located in the S4 motif of the CNGA3 protein and is conserved in all channel proteins. Bioinformatics analysis showed that the N276S substitution altered the channel conformation by shifting the helix. No pathogenic variation was identified in any affected members of family 74 in the coding sequence of CNGB3. The other two families, 55 and 70, were not linked to any known ACHM loci, indicating further heterogeneity of the ACHM phenotype.

CONCLUSIONS

We describe a novel S4 motif mutation of CNGA3 in a Pakistani family.

Long-term and age-dependent restoration of visual function in a mouse model of CNGB3-associated achromatopsia following gene therapy

Mutations in the CNGB3 gene account for >50% of all known cases of achromatopsia. Although of early onset, its stationary character and the potential for rapid assessment of restoration of retinal function following therapy renders achromatopsia a very attractive candidate for gene therapy. Here we tested the efficacy of an rAAV2/8 vector containing a human cone arrestin promoter and a human CNGB3 cDNA in CNGB3 deficient mice. Following subretinal delivery of the vector, CNGB3 was detected in both M- and S-cones and resulted in increased levels of CNGA3, increased cone density and survival, improved cone outer segment structure and normal subcellular compartmentalization of cone opsins. Therapy also resulted in long-term improvement of retinal function, with restoration of cone ERG amplitudes of up to 90% of wild-type and a significant improvement in visual acuity. Remarkably, successful restoration of cone function was observed even when treatment was initiated at 6 months of age; however, restoration of normal visual acuity was only possible in younger animals (e.g. 2–4 weeks old). This study represents achievement of the most substantial restoration of visual function reported to date in an animal model of achromatopsia using a human gene construct, which has the potential to be utilized in clinical trials.

Dissecting the pathogenic mechanisms of mutations in the pore region of the human cone photoreceptor cyclic nucleotide-gated channel

The CNGA3 gene encodes the A3 subunit of the cone photoreceptor cyclic nucleotide-gated (CNG) channel, an essential component of the phototransduction cascade. Certain mutations in CNGA3 cause autosomal recessive achromatopsia, a retinal disorder characterized by severely reduced visual acuity, lack of color discrimination, photophobia, and nystagmus. We identified three novel mutations in the pore-forming region of CNGA3 (L363P, G367V, and E376K) in patients diagnosed with achromatopsia. We assessed the expression and function of channels with these three new and two previously described mutations (S341P and P372S) in a heterologous HEK293 cell expression system using Western blot, subcellular localization on the basis of immunocytochemistry, calcium imaging, and patch clamp recordings. In this first comparative functional analysis of disease-associated mutations in the pore of a CNG channel, we found impaired surface expression of S341P, L363P, and P372S mutants and reduced macroscopic currents for channels with the mutations S341P, G367V, and E376K. Calcium imaging and patch clamp experiments after incubation at 37 degrees C revealed nonfunctional homo- and heteromeric channels in all five mutants, but incubation at 27 degrees C combined with coexpression of the B3 subunit restored residual function of channels with the mutations S341P, G367V, and E376K.

The disease-causing mutations in the carboxyl terminus of the cone cyclic nucleotide-gated channel CNGA3 subunit alter the local secondary structure and interfere with the channel active conformational change

The cone photoreceptor cyclic nucleotide-gated (CNG) channel plays a pivotal role in phototransducton. Mutations in the channel subunits are associated with achromatopsia and progressive cone dystrophy in humans. More than 50 mutations have been identified in the channel CNGA3 subunit, with 50% of them located in the carboxyl (C) terminus. This study investigates the defects of the two frequently occurring mutations, R377W and F488L, in the C-terminus of CNGA3. Ratiometric measurement of the intracellular Ca(2+) concentration and electrophysiological recordings showed the loss of functional activity of the mutant channels in an HEK293 heterologous expression system. Immunofluorescence labeling revealed an apparent cytosolic aggregation of the mutant channels compared to the wild type (WT). The R377W and F488L mutants, expressed and purified from Escherichia coli as glutathione S-transferase (GST) fused to the CNGA3 C-terminal domain, showed no negative effects on interactions with the channel subunits. Circular dichroism spectrum analyses were performed to examine the structural impact of the mutations. Although the R377W and F488L C-termini mutants retained stable, folded structures, the secondary structures of both mutants differed from the WT protein. Furthermore, the WT C-terminus exhibited a significant decrease in alpha-helical content in response to the channel ligands, while this allosteric transition was diminished in the two mutants. This is the first study showing the structural impact of the disease-causing mutations in the cone CNG channel subunit. The observed alterations in the local secondary structure and active conformational change may confer an adverse effect on the channel's activity and cellular processing.