Monochromacy Research Articles

Eyes underground: Regression of visual protein networks in subterranean mammals

Regressive evolution involves the degeneration of formerly useful structures in a lineage over time, and may be accompanied by the molecular decay of phenotype-specific genes. The mammalian eye has repeatedly undergone degeneration in taxa that occupy dim-light environments including subterranean habitats. Here we assess whether a decrease in the amount of light that reaches the retina is associated with increased regression of retinal genes, whether the phototransduction and visual cycle pathways degrade in a predictable pattern, and if the timing of retinal gene loss is associated with the entrance of mammalian lineages into subterranean environments. Sequence data were obtained from the publically available genomes of the Cape golden mole (Chrysochloris asiatica), naked mole-rat (Heterocephalus glaber) and star-nosed mole (Condylura cristata) for 65 genes associated with phototransduction, the visual cycle, and other retinal functions. Gene sequences were inspected for inactivating mutations and, when present, pseudogene sequences were compared to sequences from subaerial outgroup species. To test whether retinal degeneration is correlated with historical entrances into subterranean environments, estimated dates of retinal gene inactivation were compared to the fossil record and phylogenetic inferences of ancestral fossoriality. Our results show that (1) lower levels of light available to the retina correspond with an increase in the number of retinal pseudogenes, (2) retinal protein networks generally degrade in a predictable manner, although the extensive loss of cone phototransduction genes in Heterocephalus raises further questions regarding SWS1-cone monochromacy versus functional rod monochromacy in this species, and (3) inactivation dates of retinal genes usually post-date inferred entrances into subterranean habitats.

Optical coherence tomography in the diagnosis of achromatopsia

Case reportThe case of a fifty-five year-old male with nyctalopia, photophobia, poor color vision and nystagmus is presented. The initial suspected diagnoses were achromatopsia and blue-cone monochromatism, since both are clinically indistinguishable. Optical coherence tomography (OCT) showed the characteristic foveal reflectivity pattern of achromatopsia. This diagnosis was subsequently confirmed by genetic study. DiscussionOCT is a non-invasive diagnostic imaging method that allows tissue morphology to be observed with high resolution. Its use might be of great help to distinguish clinically similar diseases.

S-opsin knockout mice with the endogenous M-opsin gene replaced by an L-opsin variant.

Specific variants of human long-wavelength (L) and middle-wavelength (M) cone opsin genes have recently been associated with a variety of vision disorders caused by cone malfunction, including red-green color vision deficiency, blue cone monochromacy, myopia, and cone dystrophy. Strikingly, unlike disease-causing mutations in rhodopsin, most of the cone opsin alleles that are associated with vision disorders do not have deleterious point mutations. Instead, specific combinations of normal polymorphisms that arose by genetic recombination between the genes encoding L and M opsins appear to cause disease. Knockout/knock-in mice promise to make it possible to study how these deleterious cone opsin variants affect the structure, function, and viability of the cone photoreceptors. Ideally, we would like to evaluate different variants that cause vision disorders in humans against a control pigment that is not associated with vision disorders, and each variant should be expressed as the sole photopigment in each mouse cone, as is the case in humans. To evaluate the feasibility of this approach, we created a line of mice to serve as the control in the analysis of disease-causing mutations by replacing exon 2 through 6 of the mouse M-opsin gene with the corresponding cDNA for a human L-opsin variant that is associated with normal vision. Experiments reported here establish that the resulting pigment, which differs from the endogenous mouse M opsin at 35 amino acid positions, functions normally in mouse cones. This pigment was evaluated in mice with and without coexpression of the mouse short wavelength (S) opsin. Here, the creation and validation of two lines of genetically engineered mice that can be used to study disease-causing variants of human L/M-opsins, in vivo, are described.

Human Cone Visual Pigment Deletions Spare Sufficient Photoreceptors to Warrant Gene Therapy

Human X-linked blue-cone monochromacy (BCM), a disabling congenital visual disorder of cone photoreceptors, is a candidate disease for gene augmentation therapy. BCM is caused by either mutations in the red (OPN1LW) and green (OPN1MW) cone photoreceptor opsin gene array or large deletions encompassing portions of the gene array and upstream regulatory sequences that would predict a lack of red or green opsin expression. The fate of opsin-deficient cone cells is unknown. We know that rod opsin null mutant mice show rapid postnatal death of rod photoreceptors. Using in vivo histology with high-resolution retinal imaging, we studied a cohort of 20 BCM patients (age range 5-58) with large deletions in the red/green opsin gene array. Already in the first years of life, retinal structure was not normal: there was partial loss of photoreceptors across the central retina. Remaining cone cells had detectable outer segments that were abnormally shortened. Adaptive optics imaging confirmed the existence of inner segments at a spatial density greater than that expected for the residual blue cones. The evidence indicates that human cones in patients with deletions in the red/green opsin gene array can survive in reduced numbers with limited outer segment material, suggesting potential value of gene therapy for BCM.

Rod monochromacy and the coevolution of cetacean retinal opsins.

Cetaceans have a long history of commitment to a fully aquatic lifestyle that extends back to the Eocene. Extant species have evolved a spectacular array of adaptations in conjunction with their deployment into a diverse array of aquatic habitats. Sensory systems are among those that have experienced radical transformations in the evolutionary history of this clade. In the case of vision, previous studies have demonstrated important changes in the genes encoding rod opsin (RH1), short-wavelength sensitive opsin 1 (SWS1), and long-wavelength sensitive opsin (LWS) in selected cetaceans, but have not examined the full complement of opsin genes across the complete range of cetacean families. Here, we report protein-coding sequences for RH1 and both color opsin genes (SWS1, LWS) from representatives of all extant cetacean families. We examine competing hypotheses pertaining to the timing of blue shifts in RH1 relative to SWS1 inactivation in the early history of Cetacea, and we test the hypothesis that some cetaceans are rod monochomats. Molecular evolutionary analyses contradict the “coastal” hypothesis, wherein SWS1 was pseudogenized in the common ancestor of Cetacea, and instead suggest that RH1 was blue-shifted in the common ancestor of Cetacea before SWS1 was independently knocked out in baleen whales (Mysticeti) and in toothed whales (Odontoceti). Further, molecular evidence implies that LWS was inactivated convergently on at least five occasions in Cetacea: (1) Balaenidae (bowhead and right whales), (2) Balaenopteroidea (rorquals plus gray whale), (3) Mesoplodon bidens (Sowerby's beaked whale), (4) Physeter macrocephalus (giant sperm whale), and (5) Kogia breviceps (pygmy sperm whale). All of these cetaceans are known to dive to depths of at least 100 m where the underwater light field is dim and dominated by blue light. The knockout of both SWS1 and LWS in multiple cetacean lineages renders these taxa rod monochromats, a condition previously unknown among mammalian species.

Visual function and cortical organization in carriers of blue cone monochromacy.

Carriers of blue cone monochromacy have fewer cone photoreceptors than normal. Here we examine how this disruption at the level of the retina affects visual function and cortical organization in these individuals. Visual resolution and contrast sensitivity was measured at the preferred retinal locus of fixation and visual resolution was tested at two eccentric locations (2.5° and 8°) with spectacle correction only. Adaptive optics corrected resolution acuity and cone spacing were simultaneously measured at several locations within the central fovea with adaptive optics scanning laser ophthalmoscopy (AOSLO). Fixation stability was assessed by extracting eye motion data from AOSLO videos. Retinotopic mapping using fMRI was carried out to estimate the area of early cortical regions, including that of the foveal confluence. Without adaptive optics correction, BCM carriers appeared to have normal visual function, with normal contrast sensitivity and visual resolution, but with AO-correction, visual resolution was significantly worse than normal. This resolution deficit is not explained by cone loss alone and is suggestive of an associated loss of retinal ganglion cells. However, despite evidence suggesting a reduction in the number of retinal ganglion cells, retinotopic mapping showed no reduction in the cortical area of the foveal confluence. These results suggest that ganglion cell density may not govern the foveal overrepresentation in the cortex. We propose that it is not the number of afferents, but rather the content of the information relayed to the cortex from the retina across the visual field that governs cortical magnification, as under normal viewing conditions this information is similar in both BCM carriers and normal controls.

The Effect of Cone Opsin Mutations on Retinal Structure and the Integrity of the Photoreceptor Mosaic

To evaluate retinal structure and photoreceptor mosaic integrity in subjects with OPN1LW and OPN1MW mutations. Eleven subjects were recruited, eight of whom have been previously described. Cone and rod density was measured using images of the photoreceptor mosaic obtained from an adaptive optics scanning light ophthalmoscope (AOSLO). Total retinal thickness, inner retinal thickness, and outer nuclear layer plus Henle fiber layer (ONL+HFL) thickness were measured using cross-sectional spectral-domain optical coherence tomography (SD-OCT) images. Molecular genetic analyses were performed to characterize the OPN1LW/OPN1MW gene array. While disruptions in retinal lamination and cone mosaic structure were observed in all subjects, genotype-specific differences were also observed. For example, subjects with "L/M interchange" mutations resulting from intermixing of ancestral OPN1LW and OPN1MW genes had significant residual cone structure in the parafovea (∼25% of normal), despite widespread retinal disruption that included a large foveal lesion and thinning of the parafoveal inner retina. These subjects also reported a later-onset, progressive loss of visual function. In contrast, subjects with the C203R missense mutation presented with congenital blue cone monochromacy, with retinal lamination defects being restricted to the ONL+HFL and the degree of residual cone structure (8% of normal) being consistent with that expected for the S-cone submosaic. The photoreceptor phenotype associated with OPN1LW and OPN1MW mutations is highly variable. These findings have implications for the potential restoration of visual function in subjects with opsin mutations. Our study highlights the importance of high-resolution phenotyping to characterize cellular structure in inherited retinal disease; such information will be critical for selecting patients most likely to respond to therapeutic intervention and for establishing a baseline for evaluating treatment efficacy.

Tomografía de coherencia óptica en el diagnóstico de la acromatopsia

Case reportThe case of a fifty five year-old male with nyctalopia, photophobia, poor colour vision and nystagmus, is presented. The initial suspected diagnoses were achromatopsia and blue-cone monochromatism, since both are clinically indistinguishable. Optical coherence tomography (OCT) showed the characteristic foveal reflectivity pattern of achromatopsia. This diagnosis was subsequently confirmed by genetic study. DiscussionOCT is a non-invasive diagnostic imaging method that allows tissue morphology to be observed with high resolution. Its use might be of great help to distinguish clinically similar diseases.

The contribution of the rod/melanopsin driven ganglion cells to the dynamic pupil light reflex response

AbstractPurpose Intrinsically photosensitive retinal ganglion cells have been shown to contribute to the control of the steady‐state pupil response. The extent to which melanopsin contributes to rapid pupil responses elicited with brief stimuli remains less clear. The principal objective of this study was to examine how the dynamic pupil light reflex response changes with the level of light adaptation for stimuli that isolate luminance, colour or combined rod and melanopsin. The range of light levels that yield useful rod signals was also investigated.Methods The study involved ten normal subjects, one patient with rod deficiency, one patient with retinitis pigmentosa (RP) and one rod monochromat. The pupil responses were measured to stimuli that isolate photopic luminance, colour or combined rod and melanopsin at three light levels (456, 74.4 and 4.8cd/m2). Visual acuity, contrast sensitivity and chromatic sensitivity were also measured in all subjects.Results The patients with rod deficiency and RP exhibit pupil responses to luminance and colour modulation, but much reduced or absent response to rod/melanopsin modulation. Conversely, the rod monochromat shows complete absence of pupil colour and luminance responses, but exhibits large pupil rod/melanopsin responses at all light levels.Conclusion The findings from this study suggest that pupil responses to briefly presented stimuli are mediated largely by rod and cone signals with no contribution from melanopsin. The results also suggest that rod signals remain unsaturated and can contribute to dynamic pupil responses at much higher light levels.

Clinical significance of red target increment perimetry

AbstractPurpose To elucidate the significance of red target increment perimetry in diagnostic and occupational assessmentMethods Red target increment perimetry does not rely on color discrimination but merely on red target detection on a white background. In a retrospective study, ten exemplary cases of congenital and acquired color vision deficiencies are evaluated by kinetic, static and microperimetric techniquesResults Daltonism: Red brightness loss is common to protan observers. In marked protanomaly and protanopia severe constriction of red target field is a rule; i.e. within the peripheral visual field of protan observers red objects may remain entirely undetected. Female carriers of protan defects: Red target fields may show constriction to variable extent, but less pronounced than in male protan observers. Rod monochromacy: Since rhodopsin is virtually unable to absorb red light, red target fields are entirely lacking. Stargardt’s disease and cone dystrophies: Red target fields are affected earlier and more pronounced than standard white fields. Central serous retinopathy: Red desaturation and red brightness loss are common findings on confrontation testing. Scotomas are detected on microperimetry by red targets while white stimuli still fail to pick up pathology. CAR: Cancer associated retinopathy: Compared to RP, in CAR the cone system shows more involvement. CAR scotomas show up more distinctly in red than in white perimetry.Conclusion In congenital protan defects, red target increment fields have a significant bearing on occupational demands. In various retinal diseases, field defects for red target increments exceed those for white targets and so, may contribute to early diagnosis.

Clinical utility gene card for: blue cone monochromatism

Clinical utility gene card for: blue cone monochromatism

Microspectrophotometric evidence for cone monochromacy in sharks

Sharks are apex predators, and their evolutionary success is in part due to an impressive array of sensory systems, including vision. The eyes of sharks are well developed and function over a wide range of light levels. However, whilst close relatives of the sharks-the rays and chimaeras-are known to have the potential for colour vision, an evolutionary trait thought to provide distinct survival advantages, evidence for colour vision in sharks remains equivocal. Using single-receptor microspectrophotometry, we measured the absorbance spectra of visual pigments located in the retinal photoreceptors of 17 species of shark. We show that, while the spectral tuning of the rod (wavelength of maximum absorbance, λ(max) 484-518nm) and cone (λ(max) 532-561nm) visual pigments varies between species, each shark has only a single long-wavelength-sensitive cone type. This suggests that sharks may be cone monochromats and, therefore, potentially colour blind. Whilst cone monochromacy on land is rare, it may be a common strategy in the marine environment: many aquatic mammals (whales, dolphins and seals) also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual design by convergent evolution. The spectral tuning of the rod and cone pigments of sharks is also discussed in relation to their visual ecology.

Dark Light, Rod Saturation, and the Absolute and Incremental Sensitivity of Mouse Cone Vision

Visual thresholds of mice for the detection of small, brief targets were measured with a novel behavioral methodology in the dark and in the presence of adapting lights spanning ∼8 log(10) units of intensity. To help dissect the contributions of rod and cone pathways, both wild-type mice and mice lacking rod (Gnat1(-/-)) or cone (Gnat2(cpfl3)) function were studied. Overall, the visual sensitivity of mice was found to be remarkably similar to that of the human peripheral retina. Rod absolute threshold corresponded to 12-15 isomerized pigment molecules (R*) in image fields of 800 to 3000 rods. Rod "dark light" (intrinsic retinal noise in darkness) corresponded to that estimated previously from single-cell recordings, 0.012 R* s(-1) rod(-1), indicating that spontaneous thermal isomerizations are responsible. Psychophysical rod saturation was measured for the first time in a nonhuman species and found to be very similar to that of the human rod monochromat. Cone threshold corresponded to ∼5 R* cone(-1) in an image field of 280 cones. Cone dark light was equivalent to ∼5000 R* s(-1) cone(-1), consistent with primate single-cell data but 100-fold higher than predicted by recent measurements of the rate of thermal isomerization of mouse cone opsins, indicating that nonopsin sources of noise determine cone threshold. The new, fully automated behavioral method is based on the ability of mice to learn to interrupt spontaneous wheel running on the presentation of a visual cue and provides an efficient and highly reliable means of examining visual function in naturally behaving normal and mutant mice.

Blue cone nonochromacy gene mutation in Asia: phenotype variability

Abstract Purpose A far East asian family with 4 affected maternal cousin males with congenital nystagmus, low vision and dyschromatopsia was investigated for a genetic cause after informed consent. Blue cone monochromacy is a rare form of X‐linked visual handicap with dyschromatopsia. Methods Family members had ophthalmologic examination including visual acuity, fundoscopy , slit lamp, biomicroscopy,colour vision testing and ERG and VEP recordings.DNA analysis of the composition of the cone ospin gene cluster was performed by PCR and PCR/RFLP as well as direct sequencing of LWS opsin gene. Results A novel nonsense Mutation in the single Long wave sensitive opsin gene was identified in all affected males and carrier females. The variability of the phenotype as well as the added role of parental myopia transmission in the phenotype will be discussed. Conclusion This is the first reported molecular diagnosis of blue cone monochromacy in the Asian population.The compound effect of dominantly inherited myopia offers insight of the effect of the added mutational load in these patients.

Deletion of the X-linked opsin gene array locus control region (LCR) results in disruption of the cone mosaic

Blue cone monochromacy (BCM) is an X-linked condition in which long- (L) and middle- (M) wavelength-sensitive cone function is absent. Due to the X-linked nature of the condition, female carriers are spared from a full manifestation of the associated defects but can show visual symptoms, including abnormal cone electroretinograms. Here we imaged the cone mosaic in four females carrying an L/M array with deletion of the locus control region, resulting in an absence of L/M opsin gene expression (effectively acting as a cone opsin knockout). On average, they had cone mosaics with reduced density and disrupted organization compared to normal trichromats. This suggests that the absence of opsin in a subset of cones results in their early degeneration, with X-inactivation the likely mechanism underlying phenotypic variability in BCM carriers.

Achromatopsia

Hereditary cone diseases manifest as progressive or stationary disorders. Among the stationary cone disorders autosomal recessive achromatopsia occurs most frequently and begins within the first months of life with nystagmus and photophobia. Color discrimination is not possible, and visual acuity is severely reduced. In addition to a thorough ophthalmic examination, color vision tests and electrophysiology are prerequisites to establish a diagnosis of achromatopsia. A genetic examination is very helpful to distinguish achromatopsia from other stationary cone disorders like X-linked recessive blue cone monochromatism and from progressive cone and cone-rod dystrophies. It is the correct clinical and genetic diagnosis that eventually will allow an individual prognosis, accurate genetic counseling, and the optimal choice of low vision aids.

The relationship between the cone photoreceptor mosaic and visual acuity in normal observers and blue cone monochromat carriers

The relationship between the cone photoreceptor mosaic and visual acuity in normal observers and blue cone monochromat carriers

A study of the phenotypic differences manifested in blue-cone monochromacy

A study of the phenotypic differences manifested in blue-cone monochromacy

Variable Retinal Phenotypes Caused by Mutations in the X-Linked Photopigment Gene Array

To examine the involvement of the long (L) and middle (M) wavelength-sensitive cone opsin genes in cone-dominated phenotypes. Clinical and molecular analyses included family history, color vision testing, full-field electroretinography (ERG), linkage analysis, and mutation detection. Eighteen families were recruited that had X-linked retinal disease characterized by cone impairment in which affected males usually had nystagmus, reduced visual acuity, normal to subnormal rod ERG, and reduced or extinguished cone ERG responses. A search for mutations in the L-M pigment gene array revealed disease-causing mutations in six families. In two of them, novel mutations were identified: a large deletion affecting both opsin genes and a single L opsin gene harboring a likely pathogenic mutation, p.Val120Met. A third family carried a single hybrid gene with the p.Cys203Arg mutation. Patients from the three remaining families carried a single opsin gene harboring two similar rare haplotypes. Although the phenotype of members in one of the families was compatible with blue cone monochromacy (BCM), patients from the two other families, who shared an identical haplotype, had only reduced or even normal full-field cone ERGs, but maculopathy was evident. Novel and known mutations affecting the L-M opsin gene array were identified in families with X-linked cone-dominated phenotypes. The results show that different mutations in this gene array can cause a variety of phenotypes, including BCM, cone dystrophy, and maculopathy. Males with X-linked cone-dominated diseases should be routinely analyzed for mutations in the L-M opsin gene array.

Reconstructing foveal pit morphology from optical coherence tomography imaging

Aim:The aim of this study was to describe an automated method for extracting quantitative measures of foveal morphology from optical coherence tomography (OCT) images of the human retina.Methods:We performed a...