Monochromacy Research Articles

Genetische Ursachen erblicher Erkrankungen der Zapfen-Photorezeptoren

Cone and cone-rod dystrophies belong to the genetically and phenotypically very heterogeneous group of retinal degenerations. This article aims to review the current knowledge of genes and mutations involved in these rare disorders that primarily affect the cone photoreceptor system. Literature and database search and summary of our own molecular genetic analyses in patients affected by achromatopsia, blue cone monochromatism, and cone and cone-rod dystrophy. Cone and cone-rod dystrophies can be divided according to the disease course into stationary and progressive disorders or by the genetic mode of inheritance into autosomal-recessive, autosomal-dominant, and X-linked traits. To date, seven genes for autosomal-recessive and nine for autosomal-dominant inherited forms of cone and cone-rod dystrophy, as well as two underlying genes on the X chromosome, have been identified. Linkage analyses imply two additional loci for autosomal-dominant, autosomal-recessive, and X-linked forms of these disorders. Reliable data on the prevalence and incidence of hereditary cone and cone-rod dystrophies and the underlying genetic defects exist only for distinct clinical and genetic entities. Analysis of the known genes results in identification of the genetic defect and mutation in only a subset of patients.

Rod and Rod-Driven Function in Achromatopsia and Blue Cone Monochromatism

To evaluate rod photoreceptor and postreceptor retinal function in pediatric patients with achromatopsia (ACHR) and blue cone monochromatism (BCM) using contemporary electroretinographic (ERG) procedures. Fifteen patients (age range, 1-20 years) with ACHR and six patients (age range, 4-22 years) with BCM were studied. ERG responses to full-field stimuli were obtained in scotopic and photopic conditions. Rod photoreceptor (S(rod), R(rod)) and rod-driven postreceptor (log sigma, V(max)) response parameters were calculated from the a-wave and b-wave. ERG records were digitally filtered to demonstrate the oscillatory potentials (OPs); a sensitivity parameter, log SOPA(1/2), and an amplitude parameter, SOPA(max), were used to characterize the OP response. Response parameters were compared with those of 12 healthy control subjects. As expected, photopic responses were nondetectable in patients with ACHR and BCM. In addition, mean scotopic photoreceptor (R(rod)) and postreceptor (V(max) and SOPA(max)) amplitude parameters were significantly reduced compared with those in healthy controls. The flash intensity required to evoke a half-maximum b-wave amplitude (log sigma) was significantly increased. Results of this study provide evidence that deficits in rod and rod-mediated function occur in the primary cone dysfunction syndromes ACHR and BCM.

In vivo imaging of the photoreceptor mosaic of a rod monochromat

Complete achromatopsia (i.e., rod monochromacy) is a congenital vision disorder in which cone function is absent or severely diminished, often due to mutations in one of two components of the cone phototransduction cascade (transducin or the cyclic-nucleotide gated channel). Previous histological data concerning cone structure are conflicting; suggesting anywhere from normal numbers of foveal cones to a complete absence of foveal cones. Here, we used an adaptive optics ophthalmoscope to obtain in vivo retinal images from a rod monochromat for whom the genetic basis of the disorder consists of a homozygous mutation in the CNGB3 gene. Behavioral data from the patient were consistent with an absence of cone function. Retinal images revealed a severely disrupted photoreceptor mosaic in the fovea and parafovea, where the size and density of the visible photoreceptors resembled that of normal rods. Imaging of additional rod monochromats to characterize differences in the photoreceptor mosaic between genetically classified patients will be required to determine which, if any, might be receptive to restorative gene therapy procedures.

Nystagmus characteristics in congenital stationary night blindness (CSNB)

Aim:To analyse nystagmus characteristics in patients with congenital stationary night blindness (CSNB) for differentiation from other forms of early childhood nystagmus.Methods:Horizontal and vertical eye movements of 10 patients (6–46 years,...

Functional analysis of rod monochromacy-associated missense mutations in the CNGA3 subunit of the cone photoreceptor cGMP-gated channel

Thirty-nine missense mutations, which had been identified in rod monochromacy or related disorders, in the CNGA3 subunit of cone photoreceptor cGMP-gated channels were analyzed. HEK293 cells were transfected with cDNA of the human CNGA3 subunit harboring each of these mutations in an expression vector. Patch–clamp recordings demonstrated that 32 of the 39 mutants did not show cGMP-activated current, suggesting that these 32 mutations cause a loss of function of the channels. From the remaining 7 mutants that showed cGMP-activated current, two mutations in the cyclic nucleotide-binding domain, T565M or E593K, were further studied. The half-maximal activating concentration (K1/2) for cGMP in the homomeric CNGA3-T565M channels (160μM) was 17.8-fold higher than that of the homomeric wild-type CNGA3 channels (9.0μM). Conversely, the K1/2 for cGMP in the homomeric CNGA3-E593K channels (3.0μM) was 3-fold lower than that of the homomeric wild-type CNGA3 channels. These results suggest that the T565M and E593K mutations alter the apparent affinity for cGMP of the channels to cause cone dysfunction, resulting in rod monochromacy.

Achromatopsia: the CNGB3 p.T383fsX mutation results from a founder effect and is responsible for the visual phenotype in the original report of uniparental disomy 14

Achromatopsia (ACHM) or rod monochromacy is an autosomal recessive and genetically heterogeneous retinal disorder. It is characterized by a lack of color discrimination, poor visual acuity, photodysphoria, pendular infantile nystagmus, and abnormal photopic electroretinographic (ERG) recordings with preservation of rod-mediated function. Mutations in three known genes are causative; including genes for the alpha and beta subunits of the cyclic nucleotide-gated cation channel (CNGA3 and CNGB3, respectively) and cone photoreceptor transducin--GNAT2. We investigated the prevalence of mutations in achromatopsia-causing genes in a cohort of 16 families with both clinical and electrophysiologic evidence consistent with autosomal recessive transmission, including one subject with achromatopsia and maternal isodisomy for chromosome 14. The most frequent mutation, p.T383fsX in CNGB3, accounted for 75% (18/24) of disease-associated alleles; intragenic SNPs in unrelated patients revealed transmission of a common haplotype consistent with a founder effect. Homozygous p.T383fsX mutation in CNGB3 that maps to chromosome 8 was detected in a patient with achromatopsia and systemic features associated with uniparental disomy (UPD) of chromosome 14. Two novel variants, p.R223G and p.A621E were found in CNGA3. We conclude that CNGA3 and CNGB3 mutations are responsible for the substantial majority of achromatopsia. Furthermore, the CNGB3 mutation p.T383fsX is a predominant mutation, results from a founder effect, and is responsible for the ACHM in the original clinical report of UPD 14.

Quantitative Analysis of OCT Characteristics in Patients with Achromatopsia and Blue-Cone Monochromatism

To quantify optical coherence tomography (OCT) images of the central retina in patients with blue-cone monochromatism (BCM) and achromatopsia (ACH) compared with healthy control individuals. The study included 15 patients with ACH, 6 with BCM, and 20 control subjects. Diagnosis of BCM and ACH was established by visual acuity testing, morphologic examination, color vision testing, and Ganzfeld ERG recording. OCT images were acquired with the Stratus OCT 3 (Carl Zeiss Meditec AG, Oberkochen, Germany). Foveal OCT images were analyzed by calculating longitudinal reflectivity profiles (LRPs) from scan lines. Profiles were analyzed quantitatively to determine foveal thickness and distances between reflectivity layers. Patients with ACH and BCM had a mean visual acuity of 20/200 and 20/60, respectively. Color vision testing results were characteristic of the diseases. The LRPs of control subjects yielded four peaks (P1-P4), presumably representing the RPE (P1), the ovoid region of the photoreceptors (P2), the external limiting membrane (ELM) (P3), and the internal limiting membrane (P4). In patients with ACH, P2 was absent, but foveal thickness (P1-P4) did not differ significantly from that in the control subjects (187 +/- 20 vs. 192 +/- 14 microm, respectively). The distance from P1 to P3 did not differ significantly (78 +/- 10 vs. 82 +/- 5 microm) between ACH and controls subjects. In patients with BCM, P3 was lacking, and P2 advanced toward P1 compared with the control subjects (32 +/- 6 vs. 48 +/- 4 microm). Foveal thickness (153 +/- 16 microm) was significantly reduced compared with that in control subjects and patients with ACH. Quantitative OCT image analysis reveals distinct patterns for controls subjects and patients with ACH and BCM, respectively. Quantitative analysis of OCT imaging can be useful in differentiating retinal diseases affecting photoreceptors. Foveal thickness is similar in both normal subjects and patients with ACH but is decreased in patients with BCM.

Pupil shapes and lens optics in the eyes of terrestrial vertebrates

Animal eyes that are primarily used under low-light conditions usually have optical systems of short depth of focus, such that chromatic defocus may lead to considerable blurring of the images. In some vertebrates, the problem is solved by multifocal lenses having concentric zones of different focal lengths, each of which focuses a different relevant spectral range onto the retina. A partially constricted circular pupil would shade the peripheral zones of the lens, leading to the loss of well-focused images at relevant wavelengths. The slit pupil, however, allows for use of the full diameter of the lens even in bright light. We studied species of terrestrial vertebrates from a variety of phylogenetic groups to establish how widespread multifocal lenses are and how pupil shapes are adapted to the optical systems. We found that multifocal lenses are common from amphibians to mammals, including primates. Slit pupils were only present in animals having multifocal optical systems. Among the felids, small species have multifocal lenses and slit pupils, while large species have monofocal lenses and round pupils. The Eurasian lynx, a cat of intermediate size, has an intermediate eye design. The functional significance of the absence of multifocal optical systems in large felids remains mysterious, because such systems are present in other large-eyed terrestrial vertebrates. Multifocal optical systems in nocturnal prosimians suggest that those animals have colour vision despite being described as cone monochromats.

Absorptive and tinted contact lens for reduction of glare

PurposeReport four cases of fitness of absorptive and tinted contact lenses as an aid for glare reduction, in subjects with retinal dystrophy, when the use of filter in glasses is not possible or enough. MethodsAbsorptive and tinted contact lenses were fitted in four subjects; two with rod monochromacy and the others with cone dystrophy, with complaints of severe photophobia indoors and outdoors. They were fit with soft absorptive lens, Optolens, polymer polymacon DK 10, 46% water, refraction index of 1.428, dark brown with a amber pupil of 3 mm, in both eyes. ResultsThe subjects immediately reported a positive effect on comfort in seeing, indoors and outdoors. The visual acuity remained unaffected. The color vision and contrast sensitivity showed discreet alterations. ConclusionIn spite of the limited cases, the findings suggest the use of absorptive and tinted contact lenses as an important aid to reduce disability glare for patients with retinal dystrophy.

Monocromatismo de conos azules ligado a X: Una familia afecta

A family affected by X-linked blue cone monochromatism is presented. There are 4 male affected individuals and 9 female carriers. The diagnosis of blue cone monochromatism is based on severely affected color vision with preserved blue function, poor visual acuity, nystagmus, nearly absent photopic ERG, and a family pedigree compatible with X-linked inheritance. The female carriers showed normal visual function and ocular motility. It is important to be familiar with non progressive cone dysgenesis in order to make a genetic diagnosis of the illnesses in this group.

Full-field ERG responses recorded with skin electrodes in paediatric patients with retinal dystrophy

Assess ERG responses recorded with skin electrodes in children with retinal dystrophies. ERG responses were recorded using skin electrodes in 17 healthy children and 43 paediatric patients with retinal dystrophy. Subjects were aged 4-14 years. ERG responses were recorded to full-field stimuli similar to those recommended in the ISCEV standard. The type of retinal dystrophy was classified on the basis of standard clinical criteria and the ERG responses were compared with those of the age-matched controls. ERG responses were abnormal in every patient. The specific type of ERG abnormality was also consistent with the clinical findings in the majority of patients. Rod responses were abnormal in every patient with a rod-cone dystrophy and cone responses were also abnormal in the majority of patients. Those patients with cone dystrophy or rod monochromatism had normal or near normal rod responses but sub-normal or absent cone responses. Patients with CSNB or XLRS had a sub-normal b-wave but normal amplitude a-wave. ERGs can be recorded successfully with skin electrodes in paediatric patientsand responses can aid the diagnosis of the type of retinal dystrophy.

Characterization of a novel form of X-linked incomplete achromatopsia

X-linked incomplete achromatopsia (XIA), also called blue-cone monochromacy (BCM), is a rare cone disorder that most commonly results either from one of two conditions. The first condition is a deletion of the locus control region (LCR) which is a critical DNA element that lies upstream of the L and M photopigment gene array on the X-chromosome and is necessary for expression of the photopigment genes. The second condition is an inactivating point mutation within the coding sequence of the remaining photopigment gene in an array from which all but one gene has been deleted. Many previous studies have concluded that affected individuals either have only rods and S-cones (Blackwell & Blackwell, 1957, 1961; Daw & Enoch, 1973; Hess et al., 1989) or have rods, S-cones, and another cone type that contains the rod pigment (Pokorny et al., 1970; Alpern et al., 1971). However, Smith et al. (1983) described individuals with XIA who had residual L-cone function. Here we report results for a subject with XIA who appears to have residual M-cone function. Genetic analysis revealed that he had apparently normal genes for M-cone photopigment thus leaving open the possibility that he has a contribution to vision based on expression of these genes at a very low level.

Blue cone monochromatism: clinical findings in patients with mutations in the red/green opsin gene cluster.

X-linked blue cone monochromatism (BCM) has to be differentiated from x-linked cone dystrophy and autosomal recessive rod monochromatism. In nine male patients with congenital cone dysfunction (one family, six single cases; age range: 9-55 years), mutations in the red/green opsin gene cluster were confirmed. Clinical findings were analyzed retrospectively. In one family and three single cases, a single red-green hybrid gene was found carrying a Cys203Arg mutation. Two patients had multiple opsin genes, a red/green hybrid gene and at least one green pigment gene, all carrying the Cys203Arg mutation. In one patient, a large deletion of the locus control region and parts of the red pigment gene were detected. Two patients (ages: 45 and 55 years) complained about progression. Two patients presented with nystagmus. Refractive errors (+8.0 and -11.0 D) and visual acuity were variable (0.05-0.3). Only four patients had a visual acuity > or = 0.1. In two patients, visual acuity could be improved using blue filter glasses. Four of five patients > or = 25 years had dystrophic alterations in the macula. Severe color vision defects and relative central scotoma were present in all patients. In the electroretinogram, residual cone responses were detected in 2/8 patients. Hybrid red/green opsin genes carrying the Cys203Arg mutation are a frequent cause of BCM in German patients. Molecular genetic evaluation is mandatory for adequate diagnosis of patients since from the clinical data only two patients were diagnosed as having BCM. In the other patients, either rod monochromatism or cone-rod dystrophy could not be excluded with certainty. The patients should be cautioned that macular dystrophy may develop in adults older than 30 years.

ERGs, cone-isolating VEPs and analytical techniques in children with cone dysfunction syndromes.

Photoreceptor and post-receptoral function in children with congenital and acquired cone disorders was measured by full-field electroretinogram (ERG) and transient visual evoked potentials (VEPs). Subjects were five rod monochromats (RM), five with cone dystrophy (CD), and 30 controls. Patients were diagnosed by clinical findings, ERGs, and standard color vision tests. VEP stimuli were check reversals and color grating onsets that stimulated each photoreceptor type (L-, M-, or S-cones) or post-receptoral pathways (L-M, white/black). VEP signal-to-noise ratios (S/N) were calculated by Fourier analysis of VEP epochs. All RM patients showed extinguished cone ERGs. A near normal S-cone VEP was recorded from a blue-cone rod monochromat without any signal from the L- or M-cone stimuli. Two other RM patients were classified as incomplete RM based on a low-level VEP signal from either L- or M-cone stimuli. CD patients had mildly to severely reduced ERGs and VEPs were abnormal to all cone-isolating stimuli. The VEP S/N ratio was not significantly correlated with the amount of rod contrast in the color stimuli. Color VEPs provide an objective assessment of macular cone function in children with cone dysfunction syndromes that is more sensitive to residual central cone function than standard full-field ERGs. VEP techniques may be useful in the early detection of cone loss in children, especially in children who do not tolerate ERG testing.

Genetics of color vision deficiencies.

The normal X-chromosome-linked color vision gene array is composed of a single red pigment gene followed by one or more green pigment genes. The high degree of homology between these genes predisposed them to unequal recombination, leading to gene deletions or the formation of red-green hybrid genes that explain the majority of the common red-green color vision deficiencies. Gene expression studies suggest that only the two most proximal genes of the array are expressed in the retina. The severity of the color vision defect is roughly related to the difference in absorption maxima of the photopigments encoded by the first two genes of the array. A single amino acid polymorphism (Ser180Ala) in the red pigment accounts for the subtle difference in normal color vision and influences the severity of color vision deficiency. Blue cone monochromacy is a rare disorder that involves absence of red and green cone function. It is caused either by deletion of a critical region that regulates expression of the red/green gene array, or by mutations that inactivate the red and green pigment genes. Total color blindness is another rare disease that involves complete absence of all cone function. A number of mutations in the genes encoding the cone-specific alpha- and beta-subunits of the cation channel and the alpha-subunit of transducin have been implicated in this disorder.

Colour vision in aquatic mammals—facts and open questions

Aquatic mammals had to adapt to visual environments that are very diVerent from those encountered by terrestrial mammals. Herein, we review the current knowledge on the colour-vision capabilities of aquatic mammals and discuss the puzzles that emerge from those studies. The common pattern in terrestrial mammals is dichromatic colour vision based on two spectral types of cone photoreceptors in the retina (commonly green and blue cones). Behavioural studies in a few pinnipeds and one dolphin species suggest a similar type of dichromatic colour vision in marine mammals. In contrast, recent immunocytochemical, physiological, and molecular genetic studies of the cone visual pigments in a larger sample of species show that whales and seals generally lack the blue cones and presumably are green cone monochromats. This challenges the behavioural Ž ndings, because cone monochromacy usually is tantamount to colour blindness. Furthermore, a loss of the blue cones seems a rather inadequate adaptation to the bluedominated underwater light Ž eld in the open ocean. Other aquatic and amphibiousmammals (sirenians, otters, hippopotamuses and polar bear) appear to show the more common pattern of cone dichromacy. The present review summarizes available data on the various aquatic mammalian taxa, assesses the reliability of these data, and discusses the potential adaptive pressures involved in blue cone loss. It is suggested that blue cones were lost in an early, ‘coastal’ period of cetacean and pinniped evolution; many coastal waters preferentially absorb blue light and constitute a long-wavelengthdominated environment. Residual colour vision in these cone monochromats could be achieved in mesopic lighting conditions by exploiting the signal diVerences between the remaining green cones and the rods.

Reorganization of human cortical maps caused by inherited photoreceptor abnormalities.

We describe a compelling demonstration of large-scale developmental reorganization in the human visual pathways. The developmental reorganization was observed in rod monochromats, a rare group of congenitally colorblind individuals who virtually lack cone photoreceptor function. Normal controls had a cortical region, spanning several square centimeters, that responded to signals initiated in the all-cone foveola but was inactive under rod viewing conditions; in rod monochromats this cortical region responded powerfully to rod-initiated signals. The measurements trace a causal pathway that begins with a genetic anomaly that directly influences sensory cells and ultimately results in a substantial central reorganization.

Clinical features of achromatopsia in Swedish patients with defined genotypes

Purpose: To describe the clinical phenotype, with emphasis on the electrophysiological findings, of patients with autosomal recessive rod monochromacy (RM) and defined mutations in the CNGA3 / CNGB3 genes. Methods: RM patients from eight different families were included in the study. Their genotypes were determined by DNA sequencing and/or RFLP analysis of PCR-amplified genomic segments of the CNGA3 and CNGB3 genes. For comparison, we investigated one patient with blue-cone monochromacy (BCM). The clinical examination included best-corrected visual acuity, fundus examination, and full-field ERG. In six patients, the examination was complemented by multifocal ERG (MERG). Results: Three patients had three different CNG3A genotypes. Five patients were homozygous and one patient compound heterozygous for a 1-bp deletion (1148delC) in the CNGB3 gene. All patients examined presented with a visual acuity of 0.1–0.15. Small residual cone responses were noted in four young RM patients. The oldest patient examined (age 47 years) presented with pigmentary changes in the mid-peripheral retina and concentric constrictions of the visual fields. Conclusions: Patients with RM and mutations in the CNGA3 / CNGB3 genes presented a similar clinical phenotype, confirming the essential function of both the a- and ß-subunits of the cGMP-gated cation channel in cone photoreceptor function. Small remaining cone responses in a few of the younger patients and mid-peripheral pigmentary degenerations in the oldest patient examined indicate that there could be some degree of progression in retinal dysfunction in at least some patients with RM.

Nystagmus.

This article reviews the recent literature on nystagmus and various aspects of the pathophysiology of congenital idiopathic nystagmus and nystagmus treatment. One paper shows a new classification of latent/manifest latent nystagmus based on eye movement recordings. Nystagmus associated with complex syndromes and with onset in childhood represents the subject of several important recent articles, as does acquired nystagmus. Nystagmus as a manifestation of the toxicity of pharmacological agents is becoming increasingly recognized. Important contributions have been made to the genetics of various forms of nystagmus that represent an essential feature of retinal diseases, such as congenital stationary night blindness, albinism, blue cone monochromatism, and achromatopsia.

For whales and seals the ocean is not blue: a visual pigment loss in marine mammals.

Most terrestrial mammals have colour vision based on two spectrally different visual pigments located in two types of retinal cone photoreceptors, i.e. they are cone dichromats with long-to-middle-wave-sensitive (commonly green) L-cones and short-wave-sensitive (commonly blue) S-cones. With visual pigment-specific antibodies, we here demonstrate an absence of S-cones in the retinae of all whales and seals studied. The sample includes seven species of toothed whales (Odontoceti) and five species of marine carnivores (eared and earless seals). These marine mammals have only L-cones (cone monochromacy) and hence are essentially colour-blind. For comparison, the study also includes the wolf, ferret and European river otter (Carnivora) as well as the mouflon and pygmy hippopotamus (Artiodactyla), close terrestrial relatives of the seals and whales, respectively. These have a normal complement of S-cones and L-cones. The S-cone loss in marine species from two distant mammalian orders strongly argues for convergent evolution and an adaptive advantage of that trait in the marine visual environment. To us this suggests that the S-cones may have been lost in all whales and seals. However, as the spectral composition of light in clear ocean waters is increasingly blue-shifted with depth, an S-cone loss would seem particularly disadvantageous. We discuss some hypotheses to explain this paradox.