Novel mutations and electrophysiologic findings in RGS9- and R9AP-associated retinal dysfunction (Bradyopsia)

Phenotyping and genotyping inherited retinal diseases: Molecular genetics, clinical and imaging features, and therapeutics of macular dystrophies, cone and cone-rod dystrophies, rod-cone dystrophies, Leber congenital amaurosis, and cone dysfunction syndromes

Inherited retinal diseases (IRD) are a leading cause of blindness in the working age population and in children. The scope of this review is to familiarise clinicians and scientists with the current landscape of molecular genetics, clinical phenotype, retinal imaging and therapeutic prospects/completed trials in IRD. Herein we present in a comprehensive and concise manner: (i) macular dystrophies (Stargardt disease (ABCA4), X-linked retinoschisis (RS1), Best disease (BEST1), PRPH2-associated pattern dystrophy, Sorsby fundus dystrophy (TIMP3), and autosomal dominant drusen (EFEMP1)), (ii) cone and cone-rod dystrophies (GUCA1A, PRPH2, ABCA4, KCNV2 and RPGR), (iii) predominant rod or rod-cone dystrophies (retinitis pigmentosa, enhanced S-Cone syndrome (NR2E3), Bietti crystalline corneoretinal dystrophy (CYP4V2)), (iv) Leber congenital amaurosis/early-onset severe retinal dystrophy (GUCY2D, CEP290, CRB1, RDH12, RPE65, TULP1, AIPL1 and NMNAT1), (v) cone dysfunction syndromes (achromatopsia (CNGA3, CNGB3, PDE6C, PDE6H, GNAT2, ATF6), X-linked cone dysfunction with myopia and dichromacy (Bornholm Eye disease; OPN1LW/OPN1MW array), oligocone trichromacy, and blue-cone monochromatism (OPN1LW/OPN1MW array)). Whilst we use the aforementioned classical phenotypic groupings, a key feature of IRD is that it is characterised by tremendous heterogeneity and variable expressivity, with several of the above genes associated with a range of phenotypes.

Application of Electrophysiology in Non-Macular Inherited Retinal Dystrophies

Inherited retinal dystrophies encompass a diverse group of disorders affecting the structure and function of the retina, leading to progressive visual impairment and, in severe cases, blindness. Electrophysiology testing has emerged as a valuable tool in assessing and diagnosing those conditions, offering insights into the function of different parts of the visual pathway from retina to visual cortex and aiding in disease classification. This review provides an overview of the application of electrophysiology testing in the non-macular inherited retinal dystrophies focusing on both common and rare variants, including retinitis pigmentosa, progressive cone and cone-rod dystrophy, bradyopsia, Bietti crystalline dystrophy, late-onset retinal degeneration, and fundus albipunctatus. The different applications and limitations of electrophysiology techniques, including multifocal electroretinogram (mfERG), full-field ERG (ffERG), electrooculogram (EOG), pattern electroretinogram (PERG), and visual evoked potential (VEP), in the diagnosis and management of these distinctive phenotypes are discussed. The potential for electrophysiology testing to allow for further understanding of these diseases and the possibility of using these tests for early detection, prognosis prediction, and therapeutic monitoring in the future is reviewed.

Human cone elongation responses can be explained by photoactivated cone opsin and membrane swelling and osmotic response to phosphate produced by RGS9-catalyzed GTPase

Optical coherence tomography has established that human cone photoreceptor outer segments elongate in response to stimuli bleaching large fractions of their visual pigment. Elongation responses are completely described over their 200-fold bleaching range as the sum of two exponentially rising components differing 13-fold in time constants and 4-fold in light sensitivity. Bleaching measurements of individual cones with adaptive optics scanning laser ophthalmoscopy (SLO) suggest that component 2 arises from cone opsin and disk membrane swelling triggered by photoactivation. Application of a model of phototransduction suggests that component 1 corresponds to free phosphate generated by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in the α-subunit of G protein complexed with phosphodiesterase.

Human cone outer segment (COS) length changes in response to stimuli bleaching up to 99% of L- and M-cone opsins were measured with high resolution, phase-resolved optical coherence tomography (OCT). Responses comprised a fast phase (∼5 ms), during which COSs shrink, and two slower phases (1.5 s), during which COSs elongate. The slower components saturated in amplitude (∼425 nm) and initial rate (∼3 nm ms⁻¹) and are well described over the 200-fold bleaching range as the sum of two exponentially rising functions with time constants of 80 to 90 ms (component 1) and 1,000 to 1,250 ms (component 2). Measurements with adaptive optics reflection densitometry revealed component 2 to be linearly related to cone pigment bleaching, and the hypothesis is proposed that it arises from cone opsin and disk membrane swelling triggered by isomerization and rate-limited by chromophore hydrolysis and its reduction to membrane-localized all-trans retinol. The light sensitivity and kinetics of component 1 suggested that the underlying mechanism is an osmotic response to an amplified soluble by-product of phototransduction. The hypotheses that component 1 corresponds to G-protein subunits dissociating from the membrane, metabolites of cyclic guanosine monophosphate (cGMP) hydrolysis, or by-products of activated guanylate cyclase are rejected, while the hypothesis that it corresponds to phosphate produced by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in G protein–phosphodiesterase complexes was found to be consistent with the results. These results provide a basis for the assessment with optoretinography of phototransduction in individual cone photoreceptors in health and during disease progression and therapeutic interventions.

Extended Phenotyping and Functional Validation Facilitate Diagnosis of a Complex Patient Harboring Genetic Variants in MCCC1 and GNB5 Causing Overlapping Phenotypes

Identifying multiple ultra-rare genetic syndromes with overlapping phenotypes is a diagnostic conundrum in clinical genetics. This study investigated the pathogenicity of a homozygous missense variant in GNB5 (GNB5L; NM_016194.4: c.920T > G (p. Leu307Arg); GNB5S; NM_006578.4: c.794T > G (p. Leu265Arg)) identified through exome sequencing in a female child who also had 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency (newborn screening positive) and hemoglobin E trait. The proband presented with early-onset intellectual disability, the severity of which was more in keeping with GNB5-related disorder than 3-MCC deficiency. She later developed bradycardia and cardiac arrest, and upon re-phenotyping showed cone photo-transduction recovery deficit, all known only to GNB5-related disorders. Patient-derived fibroblast assays showed preserved GNB5S expression, but bioluminescence resonance energy transfer assay showed abolished function of the variant reconstituted Gβ5S containing RGS complexes for deactivation of D2 dopamine receptor activity, confirming variant pathogenicity. This study highlights the need for precise phenotyping and functional assays to facilitate variant classification and clinical diagnosis in patients with complex medical conditions.

The electroretinogram in the genomics era: outer retinal disorders

The inherited retinal diseases (IRDs) have traditionally been described phenotypically with the description evolving to incorporate more sophisticated structural and functional assessments. In the last 25 years there has been considerable advances in the understanding of underlying genetic aetiologies. The role of the ophthalmologist is now to work in a multi-disciplinary team to identify the disease-causing genotype, which might be amenable to gene-directed intervention. Visual electrophysiology is an important tool to assist the ophthalmologist in guiding the clinical geneticist to reach a final molecular diagnosis. This review outlines the physiological basis for the ISCEV standard electrophysiology tests, the role of electrophysiology in localising the functional deficit, correlation with structural findings to guide diagnosis and finally management of IRDs in the era of genomics with emphasis on the outer retina.

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

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

Clinical vision and molecular loss: Integrating visual psychophysics with molecular genetics reveals key details of normal and abnormal visual processing

Over the past two decades we have developed techniques and models to investigate the ways in which known molecular defects affect visual performance. Because molecular defects in retinal signalling invariably alter the speed of visual processing, our strategy has been to measure the resulting changes in flicker sensitivity. Flicker measurements provide not only straightforward clinical assessments of visual performance but also reveal fundamental details about the functioning of both abnormal and normal visual systems. Here, we bring together our past measurements of patients with pathogenic variants in the GNAT2, RGS9, GUCA1A, RPE65, OPA1, KCNV2 and NR2E3 genes and analyse the results using a standard model of visual processing. The model treats flicker sensitivity as the result of the actions of a sequence of simple processing steps, one or more of which is altered by the genetic defect. Our analyses show that most defects slow down the visual response directly, but some speed it up. Crucially, however, other steps in the processing sequence can make compensatory adjustments to offset the abnormality. For example, if the abnormal step slows down the visual response, another step is likely to speed up or attenuate the response to rebalance system performance. Such compensatory adjustments are probably made by steps in the sequence that usually adapt to changing light levels. Our techniques and modelling also allow us to tease apart stationary and progressive effects, and the localised molecular losses help us to unravel and characterise individual steps in the normal and abnormal processing sequences.

Retinal imaging in inherited retinal diseases

Inherited retinal diseases (IRD) are a leading cause of blindness in the working age population. The advances in ocular genetics, retinal imaging and molecular biology, have conspired to create the ideal environment for establishing treatments for IRD, with the first approved gene therapy and the commencement of multiple therapy trials. The scope of this review is to familiarize clinicians and scientists with the current landscape of retinal imaging in IRD. Herein we present in a comprehensive and concise manner the imaging findings of: (I) macular dystrophies (MD) [Stargardt disease (ABCA4), X-linked retinoschisis (RS1), Best disease (BEST1), pattern dystrophy (PRPH2), Sorsby fundus dystrophy (TIMP3), and autosomal dominant drusen (EFEMP1)], (II) cone and cone-rod dystrophies (GUCA1A, PRPH2, ABCA4 and RPGR), (III) cone dysfunction syndromes [achromatopsia (CNGA3, CNGB3, PDE6C, PDE6H, GNAT2, ATF6], blue-cone monochromatism (OPN1LW/OPN1MW array), oligocone trichromacy, bradyopsia (RGS9/R9AP) and Bornholm eye disease (OPN1LW/OPN1MW), (IV) Leber congenital amaurosis (GUCY2D, CEP290, CRB1, RDH12, RPE65, TULP1, AIPL1 and NMNAT1), (V) rod-cone dystrophies [retinitis pigmentosa, enhanced S-Cone syndrome (NR2E3), Bietti crystalline corneoretinal dystrophy (CYP4V2)], (VI) rod dysfunction syndromes (congenital stationary night blindness, fundus albipunctatus (RDH5), Oguchi disease (SAG, GRK1), and (VII) chorioretinal dystrophies [choroideremia (CHM), gyrate atrophy (OAT)].

Unique retinal signaling defect in GNB5-related disease

OBJECTIVE

To report a unique retinal signaling defect in GNB5-related disease.

METHODS

A 3-year-old female child underwent detailed systemic and ophthalmological evaluation. The eye examination included fundus photography, spectral domain optical coherence tomography and an extended protocol full-field electroretinography (ERG) including the ISCEV recommended standard steps. The dark-adapted (DA) ERGs were performed to a series of white flashes (range 0.006-30.0 cd s m^-2) and two red flashes. The DA ERGs to higher stimulus intensities (3.0, 10.0 and 30.0 cd s m^-2) were tested using a range of inter-stimulus intervals (ISI) of up to 60 s. In addition to standard light-adapted (LA) ERGs, a short-duration (0.5 s) LA 3.0 30-Hz flicker ERG and a long-duration LA ON-OFF ERG were also performed. Genetic testing included microarray, mitochondrial genome testing and whole exome sequencing.

RESULTS

The child was diagnosed to have status epilepticus and bradycardia at 6 months of age. Subsequently, she was diagnosed to have global developmental delay and hypotonia. On ophthalmological evaluation, the child fixes and follows light. Fundus evaluation showed mild optic disk pallor; macular SD-OCT was normal. The dim flash DA ERGs (DA 0.006 and DA 0.01 cd s m^-2) were non-detectable. DA red flash ERGs showed the presence of an x-wave (cone component) and no rod component. The DA 3.0, 10.0 and 30.0 ERGs showed electronegative configuration regardless of the ISI; the averaged a-wave amplitude (4 flashes) was smaller at shorter ISI but became normal at a prolonged ISI (60 s). The LA 30-Hz flicker ERG was severely reduced but detectable for the initial 0.5 s; this became non-detectable after 5 s of averaging. The LA 3.0 2-Hz ERG showed markedly reduced a- and b-wave amplitudes and a reduced b:a ratio; the LA ON-OFF ERGs were non-detectable. WES identified a homozygous null mutation in G protein subunit beta 5 (GNB5; c.1032C>A/p.Tyr344*).

CONCLUSION

This report identifies for the first time a unique retinopathy associated with biallelic mutations in GNB5. The observed phenotype is consistent with a dual retinal signaling defect reminiscent of features of bradyopsia and rod ON-bipolar dysfunction.

Genetic Analysis of Rare Human Variants of Regulators of G Protein Signaling Proteins and Their Role in Human Physiology and Disease

Regulators of G protein signaling (RGS) proteins modulate the physiologic actions of many neurotransmitters, hormones, and other signaling molecules. Human RGS proteins comprise a family of 20 canonical proteins that bind directly to G protein-coupled receptors/G protein complexes to limit the lifetime of their signaling events, which regulate all aspects of cell and organ physiology. Genetic variations account for diverse human traits and individual predispositions to disease. RGS proteins contribute to many complex polygenic human traits and pathologies such as hypertension, atherosclerosis, schizophrenia, depression, addiction, cancers, and many others. Recent analysis indicates that most human diseases are due to extremely rare genetic variants. In this study, we summarize physiologic roles for RGS proteins and links to human diseases/traits and report rare variants found within each human RGS protein exome sequence derived from global population studies. Each RGS sequence is analyzed using recently described bioinformatics and proteomic tools for measures of missense tolerance ratio paired with combined annotation-dependent depletion scores, and protein post-translational modification (PTM) alignment cluster analysis. We highlight selected variants within the well-studied RGS domain that likely disrupt RGS protein functions and provide comprehensive variant and PTM data for each RGS protein for future study. We propose that rare variants in functionally sensitive regions of RGS proteins confer profound change-of-function phenotypes that may contribute, in newly appreciated ways, to complex human diseases and/or traits. This information provides investigators with a valuable database to explore variation in RGS protein function, and for targeting RGS proteins as future therapeutic targets.

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.

ISCEV extended protocol for the dark-adapted red flash ERG

The International Society for Clinical Electrophysiology of Vision (ISCEV) standard for full-field electroretinography (ERG) describes a minimum procedure, but encourages more extensive testing. This ISCEV extended protocol describes an extension to the ERG standard, namely the dark-adapted (DA) red flash ERG. The DA red flash ERG can be incorporated conveniently within the ISCEV standard ERG protocol after a minimum of 20-min DA and recorded after the DA 0.01 ERG to a flash strength of 0.3 phot cd s m⁻², eliciting a waveform with two positive peaks in healthy individuals. The first positive component is the cone-mediated x-wave with a peak at 30–50 ms; the second is a rod-mediated b-wave with a peak time of approximately 100 ms. Shorter DA times may be desirable to shorten the recording time or to alter the prominence of the early cone-mediated x-wave relative to the rod-mediated b-wave. The DA red flash ERG is used to aid the diagnosis of achromatopsia (rod monochromacy), cone dystrophy and other forms of cone system dysfunction, including “Bradyopsia” (RGS9/R9AP-retinopathy), when the DA red flash ERG x-wave is preserved in the absence of ISCEV standard LA ERGs. The DA red flash ERG can also help determine the origin of residual DA ERGs in cases of severe rod dysfunction, for example in disorders such as vitamin A deficiency, fundus albipunctatus (RDH5-retinopathy), Oguchi disease (SAG- or GRK1-retinopathy) and some rod-cone dystrophies. To shorter DA periods, the x-wave may be elicited without the following rod b-wave, shown to be helpful in abbreviated protocols for children.

The clinical presentation of bradyopsia in children

Diagnosing bradyopsia can be challenging in young children because structural ophthalmic examination is typically normal and visual acuity can improve with pinhole despite no significant refractive error. This case series highlights the clinical presentations and features of 5 affected children (3 Arab families) who harbored the same homozygous RGS9 frameshift mutation, which seems to represent a founder effect for the Arabian Peninsula.

Adaptive optics imaging of inherited retinal diseases

Adaptive optics (AO) ophthalmoscopy allows for non-invasive retinal phenotyping on a microscopic scale, thereby helping to improve our understanding of retinal diseases. An increasing number of natural history studies and ongoing/planned interventional clinical trials exploit AO ophthalmoscopy both for participant selection, stratification and monitoring treatment safety and efficacy. In this review, we briefly discuss the evolution of AO ophthalmoscopy, recent developments and its application to a broad range of inherited retinal diseases, including Stargardt disease, retinitis pigmentosa and achromatopsia. Finally, we describe the impact of this in vivo microscopic imaging on our understanding of disease pathogenesis, clinical trial design and outcome metrics, while recognising the limitation of the small cohorts reported to date.

Prediction of Human Phenotype Ontology terms by means of hierarchical ensemble methods

BACKGROUND

The prediction of human gene-abnormal phenotype associations is a fundamental step toward the discovery of novel genes associated with human disorders, especially when no genes are known to be associated with a specific disease. In this context the Human Phenotype Ontology (HPO) provides a standard categorization of the abnormalities associated with human diseases. While the problem of the prediction of gene-disease associations has been widely investigated, the related problem of gene-phenotypic feature (i.e., HPO term) associations has been largely overlooked, even if for most human genes no HPO term associations are known and despite the increasing application of the HPO to relevant medical problems. Moreover most of the methods proposed in literature are not able to capture the hierarchical relationships between HPO terms, thus resulting in inconsistent and relatively inaccurate predictions.

RESULTS

We present two hierarchical ensemble methods that we formally prove to provide biologically consistent predictions according to the hierarchical structure of the HPO. The modular structure of the proposed methods, that consists in a "flat" learning first step and a hierarchical combination of the predictions in the second step, allows the predictions of virtually any flat learning method to be enhanced. The experimental results show that hierarchical ensemble methods are able to predict novel associations between genes and abnormal phenotypes with results that are competitive with state-of-the-art algorithms and with a significant reduction of the computational complexity.

CONCLUSIONS

Hierarchical ensembles are efficient computational methods that guarantee biologically meaningful predictions that obey the true path rule, and can be used as a tool to improve and make consistent the HPO terms predictions starting from virtually any flat learning method. The implementation of the proposed methods is available as an R package from the CRAN repository.

The cone dysfunction syndromes

The cone dysfunction syndromes are a heterogeneous group of inherited, predominantly stationary retinal disorders characterised by reduced central vision and varying degrees of colour vision abnormalities, nystagmus and photophobia. This review details the following conditions: complete and incomplete achromatopsia, blue-cone monochromatism, oligocone trichromacy, bradyopsia and Bornholm eye disease. We describe the clinical, psychophysical, electrophysiological and imaging findings that are characteristic to each condition in order to aid their accurate diagnosis, as well as highlight some classically held notions about these diseases that have come to be challenged over the recent years. The latest data regarding the genetic aetiology and pathological changes observed in the cone dysfunction syndromes are discussed, and, where relevant, translational avenues of research, including completed and anticipated interventional clinical trials, for some of the diseases described herein will be presented. Finally, we briefly review the current management of these disorders.

Retinal Architecture in ​RGS9- and ​R9AP-Associated Retinal Dysfunction (Bradyopsia)

PURPOSE

To characterize photoreceptor structure and mosaic integrity in subjects with ​RGS9- and R9AP-associated retinal dysfunction (bradyopsia) and compare to previous observations in other cone dysfunction disorders such as oligocone trichromacy.

DESIGN

Observational case series.

METHODS

setting: Moorfields Eye Hospital (United Kingdom) and Medical College Wisconsin (USA).

STUDY POPULATION

Six eyes of 3 subjects with disease-causing variants in ​RGS9 or R9AP.

MAIN OUTCOME MEASURES

Detailed retinal imaging using spectral-domain optical coherence tomography and confocal adaptive-optics scanning light ophthalmoscopy.

RESULTS

Cone density at 100 μm from foveal center ranged from 123 132 cones/mm(2) to 140 013 cones/mm(2). Cone density ranged from 30 573 to 34 876 cones/mm(2) by 600 μm from center and from 15 987 to 16,253 cones/mm(2) by 1400 μm from center, in keeping with data from normal subjects. Adaptive-optics imaging identified a small, focal hyporeflective lesion at the foveal center in both eyes of the subject with RGS9-associated disease, corresponding to a discrete outer retinal defect also observed on spectral-domain optical coherence tomography; however, the photoreceptor mosaic remained intact at all other observed eccentricities.

CONCLUSIONS

Bradyopsia and oligocone trichromacy share common clinical symptoms and cannot be discerned on standard clinical findings alone. Adaptive-optics imaging previously demonstrated a sparse mosaic of normal wave-guiding cones remaining at the fovea, with no visible structure outside the central fovea in oligocone trichromacy. In contrast, the subjects presented in this study with molecularly confirmed bradyopsia had a relatively intact and structurally normal photoreceptor mosaic, allowing the distinction between these disorders based on the cellular phenotype and suggesting different pathomechanisms.

Long-term follow-up of two patients with oligocone trichromacy

INTRODUCTION

Oligocone trichromacy (OT) is an uncommon cone dysfunction disorder, the mechanism of which remains poorly understood. OT has been thought to be non-progressive, but its long-term visual outcome has been seldom reported in the literature. Our aim was to present two OT patients followed at our institution over 18 years.

MATERIALS AND METHODS

Complete ocular examination, color vision, visual fields, and full-field electroretinography (ERG) were performed at initial presentation and follow-up. Spectral-domain optical coherence tomography (OCT) was performed during follow-up when available at our institution.

RESULTS

Initial ocular examination showed satisfactory visual acuities with normal fundus examination and near-to-normal color vision. However, computerized perimetry demonstrated a ring-shaped scotoma around fixation, and ERG showed a profound cone dysfunction. The discrepancy between preserved color vision and profound cone dysfunction leads to the diagnosis of OT. Subsequent follow-ups over 18 years showed subtle degradation of visual acuities along with progression of the myopia in both patients and slight worsening of color vision in one patient. Initial OCT revealed a focal interruption of the ellipsoid line along with decreased thickness of the perifoveal macula. Subsequent OCT imaging performed 2 years later did not show any macular changes.

CONCLUSION

Although OT is known to be a non-progressive cone dysfunction, our results suggest that subtle degradation of the visual function might happen over time.

RGS Protein Regulation of Phototransduction

First identified in yeast and worm and later in other species, the physiological importance of Regulators of G-protein Signaling (RGS) in mammals was first demonstrated at the turn of the century in mouse retinal photoreceptors, in which RGS9 is needed for timely recovery of rod phototransduction. The role of RGS in vision has also been established a synapse away in retinal depolarizing bipolar cells (DBCs), where RGS7 and RGS11 work redundantly and in a complex with Gβ5-S as GAPs for Goα in the metabotropic glutamate receptor 6 pathway situated at DBC dendritic tips. Much less is known on how RGS protein subserves vision in the rest of the visual system. The research into the roles of RGS proteins in vision holds great potential for many exciting new discoveries.

Photophobia and abnormally sustained pupil responses in a mouse model of bradyopsia

PURPOSE

Mutations in the RGS9 gene cause the visual disorder bradyopsia, which includes difficulty adapting to changes in light and photophobia. The purpose of this study was to determine whether lack of Rgs9 also caused photophobia-like behavior in Rgs9 knockout (Rgs9-/-) mice and to identify useful diagnostic measures of Rgs9 dysfunction.

METHODS

We measured two behavioral responses to light and the pupillary light reflex to determine the form and basis of photophobia in Rgs9-/- mice.

RESULTS

Rgs9-/- mice spent less time than wild-type mice in both dim and bright light. The mice also showed increased sensitivity to light in negative masking behavior, with a half maximal response at 0.08 lux (0.01 μW·cm(-2)) in Rgs9-/- mice compared to 5.0 lux (0.85 μW·cm(-2)) in wild-type mice. These behaviors were not due to increased anxiety or increased pupil size causing more light to enter the eye. Rather, constriction of the pupil showed that Rgs9-/- mice had an abnormally sustained response to light across multiple irradiance measurement pathways.

CONCLUSIONS

Rgs9-/- mice recapitulate a photophobia phenotype of bradyopsia, and the pupil light reflex identifies a simple means to screen for irradiance measurement abnormalities in bradyopsia and potentially other genetic disorders involving photophobia.

Pathognomonic (diagnostic) ERGs. A review and update

PURPOSE

To review three inherited retinal disorders associated with diagnostic or pathognomonic electroretinogram (ERG) abnormalities: cone dystrophy with supernormal rod ERG (KCNV2), enhanced S-cone syndrome (NR2E3), and bradyopsia (RGS9/R9AP).

METHODS

A review of clinical details, genetic basis, and electrophysiological features in these disorders and a brief summary of the standard and nonstandard ERG techniques required to identify the disorders.

RESULTS

The electrophysiological features in each of these three disorders are pathognomonic such that the responsible gene can be specified. The results from nonstandard electrophysiological testing in excess of international standards are necessary to describe the pathognomonic changes in cone dystrophy with supernormal rod ERG and bradyopsia. The clinical phenotype in the disorders can be variable. Mutations in NR2E3 may additionally be associated with phenotypes other than enhanced S-cone syndrome.

CONCLUSION

: Characteristic ERG changes enable the diagnosis of cone dystrophy with supernormal rod ERG, enhanced S-cone syndrome, and bradyopsia and accurate genetic screening. This review highlights the need for additional nonstandard ERGs to make the diagnosis in two of these disorders.

The relationship between slow photoresponse recovery rate and temporal resolution of vision

The rate at which photoreceptors recover from excitation is thought to be critical for setting the temporal resolution of vision. Indeed, mutations in RGS9 (regulator of G-protein signaling 9) and R9AP (RGS9 anchor protein) proteins mediating rapid photoresponse recovery impair patients' ability to see moving objects. In this study, we analyzed temporal properties of retinal sensitivity and spatiotemporal aspects of visual behavior in R9AP knock-out mice. Surprisingly, we have found that this knock-out does not affect dim-light vision mediated by rods acting as single-photon counters. Under these conditions, vision was also unaffected in mice overexpressing R9AP in rods, which causes accelerated photoresponse recovery. However, in brighter light, slow photoresponse recovery in rods and cones impaired visual responses to high temporal frequency stimuli, as reported for the daylight vision of human patients. Therefore, the speed of photoresponse recovery can affect temporal resolution and motion detection when photoreceptors integrate signals from multiple photons but not when they act as single-photon counters.

Differential assembly of GPCR signaling complexes determines signaling specificity

Recent proteomic and biochemical evidence indicates that cellular -signaling is organized in protein modules. G protein-coupled receptors (GPCRs) are privileged entry points for extracellular signals that are transmitted through the plasma membrane into the cell. The adequate cellular response and signaling specificity is regulated by GPCR-associated protein modules. The composition of these modules is dynamic and might depend on receptor stimulation, the proteome of a given cellular context, the subcellular localization of receptor-associated modules, the formation of GPCR oligomers and the variation of expression levels of components of these modules under physiological, for example circadian rhythm, or pathological conditions. The current article will highlight the importance of GPCR-associated protein modules as a biochemical basis for signaling specificity.

Integrity of the cone photoreceptor mosaic in oligocone trichromacy

PURPOSE

Oligocone trichromacy (OT) is an unusual cone dysfunction syndrome characterized by reduced visual acuity, mild photophobia, reduced amplitude of the cone electroretinogram with normal rod responses, normal fundus appearance, and normal or near-normal color vision. It has been proposed that these patients have a reduced number of normal functioning cones (oligocone). This paper has sought to evaluate the integrity of the cone photoreceptor mosaic in four patients previously described as having OT.

METHODS

Retinal images were obtained from two brothers (13 and 15 years) and two unrelated subjects, one male (47 years) and one female (24 years). High-resolution images of the cone mosaic were obtained using high-speed adaptive optics (AO) fundus cameras. Visible structures were analyzed for density using custom software. Additional retinal images were obtained using spectral domain optical coherence tomography (SD-OCT), and the four layers of the photoreceptor-retinal pigment epithelium complex (ELM, IS/OS, RPE1, RPE2) were evaluated. Cone photoreceptor length and the thickness of intraretinal layers were measured and compared to previously published normative data.

RESULTS

The adult male subject had infantile onset nystagmus while the three other patients did not. In the adult male patient, a normal appearing cone mosaic was observed. However, the three other subjects had a sparse mosaic of cones remaining at the fovea, with no structure visible outside the central fovea. On SD-OCT, the adult male subject had a very shallow foveal pit, with all major retinal layers being visible, and both inner segment (IS) and outer segment (OS) length were within normal limits. In the other three patients, while all four layers were visible in the central fovea and IS length was within normal limits, the OS length was significantly decreased. Peripherally the IS/OS layer decreased in intensity, and the RPE1 layer was no longer discernable, in keeping with the lack of cone structure observed on AO imaging outside the central fovea.

CONCLUSIONS

Findings are consistent with the visual deficits being caused by a reduced number of healthy cones in the two brothers and the adult female. In the unrelated adult subject, no structural basis for the disorder was found. These data suggest two distinct groups on the basis of structural imaging. It is proposed that the former group with evidence of a reduction in cone numbers is more in keeping with typical OT, with the latter group representing an OT-like phenotype. These two groups may be difficult to readily discern on the basis of phenotypic features alone, and high-resolution imaging may be an effective way to distinguish between these phenotypes.

Lessons from photoreceptors: turning off g-protein signaling in living cells

Phototransduction in retinal rods is one of the most extensively studied G-protein signaling systems. In recent years, our understanding of the biochemical steps that regulate the deactivation of the rod's response to light has greatly improved. Here, we summarize recent advances and highlight some of the remaining puzzles in this model signaling system.