Defective colour vision associated with a missense mutation in the human green visual pigment gene

Evolutionarily new genes in humans with disease phenotypes reveal functional enrichment patterns shaped by adaptive innovation and sexual selection

New genes (or young genes) are genetic novelties pivotal in mammalian evolution. Their phenotypic impacts and evolutionary pattern over time, however, remain elusive in humans due to the technical and ethical complexities in functional studies. By combining human gene age dating and Mendelian disease phenotyping, our research reveals a gradual increase in disease gene proportions with gene age. Logistic regression modeling indicates that this increase could be related to longer protein lengths and higher burdens of deleterious de novo germline variants (DNVs) for older genes. We also find a steady integration of new genes with biomedical phenotypes into the human genome over macroevolutionary timescales (~0.07% per million years). Despite this stable pace, we observe distinct patterns in phenotypic enrichment, pleiotropy, and selective pressures across gene ages. Notably, young genes show significant enrichment in diseases related to the male reproductive system, indicating strong sexual selection. Young genes also exhibit disease-related functions in tissues and systems potentially linked to human phenotypic innovations, such as increased brain size, musculoskeletal phenotypes, and color vision. We further reveal a logistic growth pattern of pleiotropy over evolutionary time, indicating a diminishing marginal growth of new functions for older genes due to intensifying selective constraints over time. We propose a "pleiotropy-barrier" model that delineates higher potentials of phenotypic innovation for young genes than for older genes, a process subject to natural selection. Our study demonstrates that evolutionary new genes are critical in influencing human reproductive evolution and adaptive phenotypic innovations driven by sexual and natural selection, with low pleiotropy as a selective advantage.

Expression of red/green-cone opsin mutants K82E, P187S, M273K result in unique pathobiological perturbations to cone structure and function

Long-and middle-wavelength cone photoreceptors, which are responsible for our visual acuity and color vision, comprise ~95% of our total cone population and are concentrated in the fovea of our retina. Previously, we characterized the disease mechanisms of the L/M-cone opsin missense mutations N94K, W177R, P307L, R330Q and G338E, all of which are associated with congenital blue cone monochromacy (BCM) or color-vision deficiency. Here, we used a similar viral vector-based gene delivery approach in M-opsin knockout mice to investigate the pathogenic consequences of the BCM or color-vision deficient associated L-cone opsin (OPN1LW) mutants K82E, P187S, and M273K. We investigated their subcellular localization, the pathogenic effects on cone structure, function, and cone viability. K82E mutants were detected predominately in cone outer segments, and its expression partially restored expression and correct localization of cone PDE6α' and cone transducin γ. As a result, K82E also demonstrated the ability to mediate cone light responses. In contrast, expression of P187S was minimally detected by either western blot or by immunohistochemistry, probably due to efficient degradation of the mutant protein. M273K cone opsin appeared to be misfolded as it was primarily localized to the cone inner segment and endoplasmic reticulum. Additionally, M273K did not restore the expression of cone PDE6α' and cone transducin γ in dorsal cone OS, presumably by its inability to bind 11-cis retinal. Consistent with the observed expression pattern, P187S and M273K cone opsin mutants were unable to mediate light responses. Moreover, expression of K82E, P187S, and M273K mutants reduced cone viability. Due to the distinct expression patterns and phenotypic differences of these mutants observed in vivo, we suggest that the pathobiological mechanisms of these mutants are distinct.

Evolutionarily new genes in humans with disease phenotypes reveal functional enrichment patterns shaped by adaptive innovation and sexual selection

New genes (or young genes) are structural novelties pivotal in mammalian evolution. Their phenotypic impact on humans, however, remains elusive due to the technical and ethical complexities in functional studies. Through combining gene age dating with Mendelian disease phenotyping, our research reveals that new genes associated with disease phenotypes steadily integrate into the human genome at a rate of ~ 0.07% every million years over macroevolutionary timescales. Despite this stable pace, we observe distinct patterns in phenotypic enrichment, pleiotropy, and selective pressures between young and old genes. Notably, young genes show significant enrichment in the male reproductive system, indicating strong sexual selection. Young genes also exhibit functions in tissues and systems potentially linked to human phenotypic innovations, such as increased brain size, bipedal locomotion, and color vision. Our findings further reveal increasing levels of pleiotropy over evolutionary time, which accompanies stronger selective constraints. We propose a "pleiotropy-barrier" model that delineates different potentials for phenotypic innovation between young and older genes subject to natural selection. Our study demonstrates that evolutionary new genes are critical in influencing human reproductive evolution and adaptive phenotypic innovations driven by sexual and natural selection, with low pleiotropy as a selective advantage.

Long wavelength-sensing cones of zebrafish retina exhibit multiple layers of transcriptional heterogeneity

Introduction

Understanding how photoreceptor genes are regulated is important for investigating retinal development and disease. While much is known about gene regulation in cones, the mechanism by which tandemly-replicated opsins, such as human long wavelength-sensitive and middle wavelength-sensitive opsins, are differentially regulated remains elusive. In this study, we aimed to further our understanding of transcriptional heterogeneity in cones that express tandemly-replicated opsins and the regulation of such differential expression using zebrafish, which express the tandemly-replicated opsins lws1 and lws2.

Methods

We performed bulk and single cell RNA-Seq of LWS1 and LWS2 cones, evaluated expression patterns of selected genes of interest using multiplex fluorescence in situ hybridization, and used exogenous thyroid hormone (TH) treatments to test selected genes for potential control by thyroid hormone: a potent, endogenous regulator of lws1 and lws2 expression.

Results

Our studies indicate that additional transcriptional differences beyond opsin expression exist between LWS1 and LWS2 cones. Bulk RNA-Seq results showed 95 transcripts enriched in LWS1 cones and 186 transcripts enriched in LWS2 cones (FC > 2, FDR < 0.05). In situ hybridization results also reveal underlying heterogeneity within the lws1- and lws2-expressing populations. This heterogeneity is evident in cones of mature zebrafish, and further heterogeneity is revealed in transcriptional responses to TH treatments.

Discussion

We found some evidence of coordinate regulation of lws opsins and other genes by exogenous TH in LWS1 vs. LWS2 cones, as well as evidence of gene regulation not mediated by TH. The transcriptional differences between LWS1 and LWS2 cones are likely controlled by multiple signals, including TH.

Blue cone monochromacy and gene therapy

Blue cone monochromacy (BCM) is a congenital vision disorder characterized by complete loss or severely reduced long- and middle-wavelength cone function, caused by mutations in the OPN1LW/OPN1MW gene cluster on the X-chromosome. BCM patients typically suffer from poor visual acuity, severely impaired color discrimination, myopia, and nystagmus. In this review, we cover the genetic causes of BCM, clinical features of BCM patients, genetic testing, and clinical outcome measurements for future BCM clinical trials. However, our emphasis is on detailing the animal models for BCM and gene therapy using adeno-associated vectors (AAV). We describe two mouse models resembling the two most common causes of BCM, current progress in proof-of-concept studies to treat BCM with deletion mutations, the challenges we face, and future directions.

A novel instrument to determine hue discrimination

PURPOSE

We report and verify a novel hue discrimination instrument. We also investigate its efficiency to determine hue discrimination in persons with normal color vision.

STUDY DESIGN

Experimental and clinical investigation.

METHOD AND STUDY PARTICIPANTS

The instrument setup comprises an optical unit and examination unit. The optical unit is composed of the same 2 spectrometers and their controllers, which enables the independent emission of different spectral lights. Two independent bundle fibers connect the optical unit and the examination unit. Two different wavelength lights are illuminated on the bipartite upper and lower circular objectives with a visual angle of 2 degrees in the examination unit. The examinee recognizes the difference in the spectral lights between the bipartite targets. Persons with normal color vision are examined and the findings are confirmed using the Ishihara Test for Colour Deficiency.

RESULTS

The instrument could generate spectral light from 450 to 650 nm within 2-nm accuracy. The spectral light showed a different light intensity according to the spectral centroid, ranging from 450 to 650 nm, but the difference could be adjusted and was negligible in terms of determination of hue discrimination using the power meter. Three width slits, 0.2 mm, 0.5 mm, and 1.0 mm, to homogenize the light path were investigated. The half-width wavelength was accurate on each spectral centroid; however, the 0.5 mm slit was suitable to generate an efficient light path. The hue discrimination differed among the study participants. In general, at short and long wavelength lights, the hue discrimination range was large: about 15 nm at 450 nm and about 10 nm at 650 nm. Between 470 and 620 nm, the hue discrimination showed good sensitivity and specificity between 8 and 2 nm depending on the targeting wavelength lights. Intraindividual variation was small, ranging from 3 to 1 nm, thus indicating good repeatability. The time to examine the hue discrimination was about 20 min.

CONCLUSION

This newly invented instrument using two independent spectrometer units enabled the determination of hue discrimination. The instrument's sensitivity and specificity including its repeatability were confirmed and indicated that the instrument could be a clinically applicable method.

Disease mechanisms of X-linked cone dystrophy caused by missense mutations in the red and green cone opsins

Cone photoreceptors are responsible for the visual acuity and color vision of the human eye. Red/green cone opsin missense mutations N94K, W177R, P307L, R330Q, and G338E have been identified in subjects with congenital blue cone monochromacy or color‐vision deficiency. Studies on disease mechanisms due to these cone opsin mutations have been previously carried out exclusively in vitro, and the reported impairments were not always consistent. Here we expressed these mutants via AAV specifically in vivo in M‐opsin knockout mouse cones to investigate their subcellular localization, the pathogenic effects on cone structure, function, and cone viability. We show that these mutations alter the M‐opsin structure, function, and localization. N94K and W177R mutants appeared to be misfolded since they localized exclusively in cone inner segments and endoplasmic reticulum. In contrast, P307L, R330Q, and G338E mutants were detected predominately in cone outer segments. Expression of R330Q and G338E, but not P307L opsins, also partially restored expression and correct localization of cone PDE6α’ and cone transducin γ and resulted in partial rescue of M‐cone‐mediated light responses. Expression of W177R and P307L mutants significantly reduced cone viability, whereas N94K, R330Q, and G338E were only modestly toxic. We propose that although the underlying biochemical and cellular defects caused by these mutants are distinct, they all seem to exhibit a dominant phenotype, resembling autosomal dominant retinitis pigmentosa associated with the majority of rhodopsin missense mutations. The understanding of the molecular mechanisms associated with these cone opsin mutants is fundamental to developing targeted therapies for cone dystrophy/dysfunction.

Mutations in G Protein-Coupled Receptors: Mechanisms, Pathophysiology and Potential Therapeutic Approaches

There are approximately 800 annotated G protein-coupled receptor (GPCR) genes, making these membrane receptors members of the most abundant gene family in the human genome. Besides being involved in manifold physiologic functions and serving as important pharmacotherapeutic targets, mutations in 55 GPCR genes cause about 66 inherited monogenic diseases in humans. Alterations of nine GPCR genes are causatively involved in inherited digenic diseases. In addition to classic gain- and loss-of-function variants, other aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, pseudogenes, gene fusion, and gene dosage, contribute to the repertoire of GPCR dysfunctions. However, the spectrum of alterations and GPCR involvement is probably much larger because an additional 91 GPCR genes contain homozygous or hemizygous loss-of-function mutations in human individuals with currently unidentified phenotypes. This review highlights the complexity of genomic alteration of GPCR genes as well as their functional consequences and discusses derived therapeutic approaches. SIGNIFICANCE STATEMENT: With the advent of new transgenic and sequencing technologies, the number of monogenic diseases related to G protein-coupled receptor (GPCR) mutants has significantly increased, and our understanding of the functional impact of certain kinds of mutations has substantially improved. Besides the classical gain- and loss-of-function alterations, additional aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, uniparental disomy, pseudogenes, gene fusion, and gene dosage, need to be elaborated in light of GPCR dysfunctions and possible therapeutic strategies.

Genotype determination of the OPN1LW/OPN1MW genes: novel disease-causing mechanisms in Japanese patients with blue cone monochromacy

Blue cone monochromacy (BCM) is characterized by loss of function of both OPN1LW (the first) and OPN1MW (the downstream) genes on the X chromosome. The purpose of this study was to investigate the first and downstream genes in the OPN1LW/OPN1MW array in four unrelated Japanese males with BCM. In Case 1, only one gene was present. Abnormalities were found in the promoter, which had a mixed unique profile of first and downstream gene promoters and a -71A > C substitution. As the promoter was active in the reporter assay, the cause of BCM remains unclear. In Case 2, the same novel mutation, M273K, was present in exon 5 of both genes in a two-gene array. The mutant pigments showed no absorbance at any of the wavelengths tested, suggesting that the mutation causes pigment dysfunction. Case 3 had a large deletion including the locus control region and entire first gene. Case 4 also had a large deletion involving exons 2-6 of the first gene. As an intact LCR was present upstream and one apparently normal downstream gene was present, BCM in Case 4 was not ascribed solely to the deletion. The deletions in Cases 3 and 4 were considered to have been caused by non-homologous recombination.

An epigenetic mechanism for cavefish eye degeneration

Coding and non-coding mutations in DNA contribute significantly to phenotypic variability during evolution. However, less is known about the role of epigenetics in this process. Although previous studies have identified eye development genes associated with the loss-of-eyes phenotype in the Pachón blind cave morph of the Mexican tetra Astyanax mexicanus, no inactivating mutations have been found in any of these genes. Here, we show that excess DNA methylation-based epigenetic silencing promotes eye degeneration in blind cave A. mexicanus. By performing parallel analyses in A. mexicanus cave and surface morphs, and in the zebrafish Danio rerio, we have discovered that DNA methylation mediates eye-specific gene repression and globally regulates early eye development. The most significantly hypermethylated and downregulated genes in the cave morph are also linked to human eye disorders, suggesting that the function of these genes is conserved across vertebrates. Our results show that changes in DNA methylation-based gene repression can serve as an important molecular mechanism generating phenotypic diversity during development and evolution.

Assessment of #TheDress With Traditional Color Vision Tests: Perception Differences Are Associated With Blueness

Based on known color vision theories, there is no complete explanation for the perceptual dichotomy of #TheDress in which most people see either white-and-gold (WG) or blue-and-black (BK). We determined whether some standard color vision tests (i.e., color naming, color matching, anomaloscope settings, unique white settings, and color preferences), as well as chronotypes, could provide information on the color perceptions of #TheDress. Fifty-two young observers were tested. Fifteen of the observers (29%) reported the colors as BK, 21 (40%) as WG, and 16 (31%) reported a different combination of colors. Observers who perceived WG required significantly more blue in their unique white settings than those who perceived BK. The BK, blue-and-gold, and WG observer groups had significantly different color preferences for the light cyan chip. Moreland equation anomaloscope matching showed a significant difference between WG and BK observers. In addition, #TheDress color perception categories, color preference outcomes, and unique white settings had a common association. For both the bright and dark regions of #TheDress, the color matching chromaticities formed a continuum, approximately following the daylight chromaticity locus. Color matching to the bright region of #TheDress showed two nearly distinct clusters (WG vs. BK) along the daylight chromaticity locus and there was a clear cutoff for reporting WG versus BK. All results showing a significant difference involved blue percepts, possibly due to interpretations of the illuminant interactions with the dress material. This suggests that variations in attributing blueness to the #TheDress image may be significant variables determining color perception of #TheDress.

Photoreceptor Cilia and Retinal Ciliopathies

Photoreceptors are sensory neurons designed to convert light stimuli into neurological responses. This process, called phototransduction, takes place in the outer segments (OS) of rod and cone photoreceptors. OS are specialized sensory cilia, with analogous structures to those present in other nonmotile cilia. Deficient morphogenesis and/or dysfunction of photoreceptor sensory cilia (PSC) caused by mutations in a variety of photoreceptor-specific and common cilia genes can lead to inherited retinal degenerations (IRDs). IRDs can manifest as isolated retinal diseases or syndromic diseases. In this review, we describe the structure and composition of PSC and different forms of ciliopathies with retinal involvement. We review the genetics of the IRDs, which are monogenic disorders but genetically diverse with regard to causality.

CRISPR/Cas9-Mediated Three Nucleotide Insertion Corrects a Deletion Mutation in MRP1/ABCC1 and Restores Its Proper Folding and Function

A three-nucleotide deletion in cystic fibrosis transmembrane conductance regulator/ATP-binding cassette transporter C7 (CFTR/ABCC7) resulting in the absence of phenylalanine at 508 leads to mis-fold of the mutated protein and causes cystic fibrosis. We have used a comparable three-nucleotide deletion mutant in another ABCC family member, multidrug resistance-associated protein (MRP1)/ABCC1, to determine whether CRISPR-Cas9-mediated recombination can safely and efficiently knock in three-nucleotide to correct the mutation. We have found that the rate of homology-directed recombination mediated by guideRNA (gRNA) complementary to the deletion mutant is significantly higher than the one mediated by gRNA complementary to the wild-type (WT) donor. In addition, the rate of homology-directed recombination mediated by gRNA complementary to the WT donor is significantly higher than that of gRNAs complementary to the 5' or 3' side of the deletion mutant. Interestingly, the frequency of mutations introduced by gRNA complementary to the deletion mutant is significantly higher than with gRNA complementary to WT donor. However, combination of gRNAs complementary to both WT donor and deletion mutant decreased the rate of homology-directed recombination, but did not significantly decrease the mutation rate introduced by this system. Thus, the data presented here provide guidance for designing of gRNA and donor DNA to do genome editing, especially to correct the mutations with three mismatched nucleotides, such as three-nucleotide deletion or insertion.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

Three different cone opsin gene array mutational mechanisms with genotype-phenotype correlation and functional investigation of cone opsin variants

Mutations in the OPN1LW (L-) and OPN1MW (M-)cone opsin genes underlie a spectrum of cone photoreceptor defects from stationary loss of color vision to progressive retinal degeneration. Genotypes of 22 families with a range of cone disorders were grouped into three classes: deletions of the locus control region (LCR); missense mutation (p.Cys203Arg) in an L-/M-hybrid gene; and exon 3 single-nucleotide polymorphism (SNP) interchange haplotypes in an otherwise normal gene array. Moderate-to-high myopia was observed in all mutation categories. Individuals with LCR deletions or p.Cys203Arg mutations were more likely to have nystagmus and poor vision, with disease progression in some p.Cys203Arg patients. Three disease-associated exon 3 SNP haplotypes encoding LIAVA, LVAVA, or MIAVA were identified in our cohort. These patients were less likely to have nystagmus but more likely to show progression, with all patients over the age of 40 years having marked macular abnormalities. Previously, the haplotype LIAVA has been shown to result in exon 3 skipping. Here, we show that haplotypes LVAVA and MIAVA also result in aberrant splicing, with a residual low level of correctly spliced cone opsin. The OPN1LW/OPN1MW:c.532A>G SNP, common to all three disease-associated haplotypes, appears to be principally responsible for this mutational mechanism.

A new subset of deutan colour vision defect associated with an L/M visual pigment gene array of normal order and -71C substitution in the Japanese population

In 524 Japanese individuals with deutan colour vision defect, 76 had a normal-order pigment gene array, where the L gene is at the first position and the M gene(s) is located downstream. Of these 76 individuals, 69 had a -71A>C substitution in the M gene without any other mutation. Because the expression of L/M genes is up-regulated by thyroid hormone (T3) in human retinoblastoma WERI cells, we examined the effects of T3 on promoter activity; T3 increased the activity of the -71A promoter 2-fold, but it had no effect on the -71C promoter. Similarly, the -71C promoter was much less activated by T3 than the -71A promoter in HEK293 cells expressing thyroid hormone receptor isoform β2. Such a weak response of the -71C promoter to T3 may cause a decrease in the number of M cones and/or the density of M pigment during the differentiation of M cones. The average Rayleigh match midpoint was 18.9 ± 4.1 in 162 ordinary deuteranomaly individuals, but was 37.3 ± 9.1 in 63 deuteranomaly individuals with -71C. The -71A>C substitution was found to be specific to eastern Asia. These results suggest that there may be a new subset of deuteranomaly associated with -71C in the Japanese (and probably eastern Asian) population(s).

Curing color blindness--mice and nonhuman primates

It has been possible to use viral-mediated gene therapy to transform dichromatic (red-green color-blind) primates to trichromatic. Even though the third cone type was added after the end of developmental critical periods, treated animals acquired red-green color vision. What happened in the treated animals may represent a recapitulation of the evolution of trichromacy, which seems to have evolved with the acquisition of a third cone type without the need for subsequent modification to the circuitry. Some transgenic mice in which a third cone type was added also acquired trichromacy. However, compared with treated primates, red-green color vision in mice is poor, indicating large differences between mice and monkeys in their ability to take advantage of the new input. These results have implications for understanding the limits and opportunities for using gene therapy to treat vision disorders caused by defects in cone function.

X-linked cone dystrophy and colour vision deficiency arising from a missense mutation in a hybrid L/M cone opsin gene

In this report, we describe a male subject who presents with a complex phenotype of myopia associated with cone dysfunction and a protan vision deficiency. Retinal imaging demonstrates extensive cone disruption, including the presence of non-waveguiding cones, an overall thinning of the retina, and an irregular mottled appearance of the hyper-reflective band associated with the inner segment ellipsoid portion of the photoreceptor. Mutation screening revealed a novel p.Glu41Lys missense mutation in a hybrid L/M opsin gene. Spectral analysis shows that the mutant opsin fails to form a pigment in vitro and fails to be trafficked to the cell membrane in transfected Neuro2a cells. Extensive sequence and quantitative PCR analysis identifies this mutant gene as the only gene present in the affected subject's L/M opsin gene array, yet the presence of protanopia indicates that the mutant opsin must retain some activity in vivo. To account for this apparent contradiction, we propose that a limited amount of functional pigment is formed within the normal cellular environment of the intact photoreceptor, and that this requires the presence of chaperone proteins that promote stability and normal folding of the mutant protein.

The effect of cone opsin mutations on retinal structure and the integrity of the photoreceptor mosaic

PURPOSE

To evaluate retinal structure and photoreceptor mosaic integrity in subjects with OPN1LW and OPN1MW mutations.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Unique haplotype in exon 3 of cone opsin mRNA affects splicing of its precursor, leading to congenital color vision defect

We have analyzed L/M visual pigment gene arrays in 119 Japanese men with protanopia color vision defect and found that five had a normal gene order of L-M. Among the five men, two (identified as A376 and A642) had apparently normal L genes. To clarify their L gene defect, the whole L or M gene from A376 and control subjects was cloned in an expression vector. Total RNA extracted from the transfected HEK293 cells was analyzed by Northern blot and reverse transcription-polymerase chain reaction. The product from the cloned L gene of A376 was smaller than the normal control due to the absence of exon 3. To investigate such exon-skipping at splicing, minigenes of exon 3 accompanying introns 2 and 3 were prepared from A376, A642, and control subjects. The minigenes of A376 (L) and A642 (L) showed the product lacking exon 3 only, while the minigene of normal control N44 (L) showed the product retaining exon 3 only. Exchanging of introns 2 and 3 between the A376 (L) and N44 (L) minigenes showed that the skipping of exon 3 was caused by the exon itself. Seven differences in exon 3 between A376 (L) and N44 (L) were all within already-known polymorphisms as follows: G(151-3), C(153-1), G(155-3), A(171-1), T(171-3), G(178-1) and G(180-1) in A376 (L) and A642 (L), and A(151-3), A(153-1), C(155-3), G(171-1), G(171-3), A(178-1) and T(180-1) in N44 (L). An in vitro mutagenesis experiment with these nucleotides in the minigenes showed that exon 3 was completely skipped at splicing only in the haplotype observed in A376 (L) and A642 (L). These results suggest that complete skipping of exon 3 at splicing, due to the unique haplotype of the exon, causes loss of expression of L-opsin in these men.

The genetics of normal and defective color vision

The contributions of genetics research to the science of normal and defective color vision over the previous few decades are reviewed emphasizing the developments in the 25years since the last anniversary issue of Vision Research. Understanding of the biology underlying color vision has been vaulted forward through the application of the tools of molecular genetics. For all their complexity, the biological processes responsible for color vision are more accessible than for many other neural systems. This is partly because of the wealth of genetic variations that affect color perception, both within and across species, and because components of the color vision system lend themselves to genetic manipulation. Mutations and rearrangements in the genes encoding the long, middle, and short wavelength sensitive cone pigments are responsible for color vision deficiencies and mutations have been identified that affect the number of cone types, the absorption spectra of the pigments, the functionality and viability of the cones, and the topography of the cone mosaic. The addition of an opsin gene, as occurred in the evolution of primate color vision, and has been done in experimental animals can produce expanded color vision capacities and this has provided insight into the underlying neural circuitry.

Adaptive optics retinal imaging: emerging clinical applications

The human retina is a uniquely accessible tissue. Tools like scanning laser ophthalmoscopy and spectral domain-optical coherence tomography provide clinicians with remarkably clear pictures of the living retina. Although the anterior optics of the eye permit such non-invasive visualization of the retina and associated pathology, the same optics induce significant aberrations that obviate cellular-resolution imaging in most cases. Adaptive optics (AO) imaging systems use active optical elements to compensate for aberrations in the optical path between the object and the camera. When applied to the human eye, AO allows direct visualization of individual rod and cone photoreceptor cells, retinal pigment epithelium cells, and white blood cells. AO imaging has changed the way vision scientists and ophthalmologists see the retina, helping to clarify our understanding of retinal structure, function, and the etiology of various retinal pathologies. Here, we review some of the advances that were made possible with AO imaging of the human retina and discuss applications and future prospects for clinical imaging.

Color-deficient cone mosaics associated with Xq28 opsin mutations: a stop codon versus gene deletions

Our understanding of the etiology of red-green color vision defects is evolving. While missense mutations within the long- (L-) and middle-wavelength sensitive (M-) photopigments and gross rearrangements within the L/M-opsin gene array are commonly associated with red-green defects, recent work using adaptive optics retinal imaging has shown that different genotypes can have distinct consequences for the cone mosaic. Here we examined the cone mosaic in red-green color deficient individuals with multiple X-chromosome opsin genes that encode L opsin, as well as individuals with a single X-chromosome opsin gene that encodes L opsin and a single patient with a novel premature termination codon in his M-opsin gene and a normal L-opsin gene. We observed no difference in cone density between normal trichomats and multiple or single-gene deutans. In addition, we demonstrate different phenotypic effects of a nonsense mutation versus the previously described deleterious polymorphism, (LIAVA), both of which differ from multiple and single-gene deutans. Our results help refine the relationship between opsin genotype and cone photoreceptor mosaic phenotype.

Cone photoreceptor mosaic disruption associated with Cys203Arg mutation in the M-cone opsin

Missense mutations in the cone opsins have been identified as a relatively common cause of red/green color vision defects, with the most frequent mutation being the substitution of arginine for cysteine at position 203 (C203R). When the corresponding cysteine is mutated in rhodopsin, it disrupts proper folding of the pigment, causing severe, early onset retinitis pigmentosa. While the C203R mutation has been associated with loss of cone function in color vision deficiency, it is not known what happens to cones expressing this mutant opsin. Here, we used high-resolution retinal imaging to examine the cone mosaic in two individuals with genes encoding a middle-wavelength sensitive (M) pigment with the C203R mutation. We found a significant reduction in cone density compared to normal and color-deficient controls, accompanying disruption in the cone mosaic in both individuals, and thinning of the outer nuclear layer. The C203R mosaics were different from that produced by another mutation (LIAVA) previously shown to disrupt the cone mosaic. Comparison of these mosaics provides insight into the timing and degree of cone disruption and has implications for the prospects for restoration of vision loss associated with various cone opsin mutations.

A novel mutation in the short-wavelength-sensitive cone pigment gene associated with a tritan color vision defect

Inherited tritan color vision deficiency is caused by defects in the function of the short-wavelength-sensitive (S) cones. This heterozygous group of disorders has an autosomal dominant pattern of inheritance. Amino acid variations of the S cone opsin are rare and all that have been identified thus far are associated with inherited tritan color vision defects. Here we report the identification of a 30-year-old male who made errors on standard color vision tests consistent with the presence of a mild tritan color vision deficiency. We tested the hypothesis that his color vision impairment was due to a mutation in the S cone photopigment gene. He was found to be heterozygous for a mutation that caused the amino acid proline to be substituted in place of a highly conserved leucine at amino acid position 56 in the S cone opsin. This mutation was absent in 564 S cone photopigment genes from 282 subjects who did not make tritan errors. Thus, we conclude that this mutation disrupts the normal function of S cones.

Novel form of a single X-linked visual pigment gene in a unique dichromatic color-vision defect

In normal trichromats, the long- (L) and middle-wavelength-sensitive (M) pigment genes are arranged in a head-to-tandem array on the X chromosome. Two amino acids at positions 277 and 285, encoded by exon 5 of the L and M genes, respectively, are essential for the spectral difference between L and M pigments whose spectral peaks are at approximately 560 and 530 nm. Intragenic or intergenic unequal crossing-over commonly occurs between the highly homologous L and M genes, resulting in red-green color vision deficiencies. The dichromacy is usually associated with a single L gene for deuteranopia or a single 5' L-M 3' hybrid gene with M-gene exon 5 for protanopia. We clinically diagnosed a total of 88 male dichromats using a Nagel model I anomaloscope, which included one unclassified subject in addition to 31 protanopes and 56 deuteranopes. The objective of this study was to characterize the phenotype of the subject and to determine the genotype of his X-linked pigment genes. The subject accepted not only any red-green mixture but also an extended yellow-scale range at each matching point (i.e. 20 to 32 scale units at the green primary and 3.5 to 6 scale units at the red primary). The slopes of regression lines were in the range of -0.34 to -0.23, while the mean slopes for the protanopes and deuteranopes were -0.38 and -0.01, respectively. Spectral sensitivity tests showed that the subject's curve was shifted between the protanope and deuteranope curves. Molecular analysis revealed a novel form of a single pigment gene with a unique arrangement of exon 5 (Y277 from the L gene and A285 from the M gene). The predicted lambdamax (541 to 546 nm) of the unique pigment was closer to the M than to the L pigment. Our outcome suggests that intragenic unequal crossing-over may have occurred between amino acid positions 279 and 283.

Protan color vision deficiency with a unique order of green-red as the first two genes of a visual pigment array

Normal visual pigment gene arrays on the human X chromosome have a red gene at the first and a green gene at the second positions. More than half of the arrays have additional green genes downstream, but only the first two genes of the array are likely to be expressed in the retina. An array consisting of four genes in two Japanese participants, A121 and A447, was detected either by pulsed field gel electrophoresis and subsequent Southern hybridization or by single nucleotide primer extension reaction. In both participants, the first gene of the array was green, downstream genes were red and green, and the fourth gene was green. The red gene was determined to be at the second position by comparison of polymorphic sites among the intergenic regions that had been amplified by long-range PCR. Such an array with a reverse normal order of pigment genes, green-red as the first two, has never been reported before. They were expected to have normal color vision but showed protan deficiency (protanomaly), a phenotype lacking the red pigment. The red gene had no mutations in the exons and exon/intron boundaries, but had an A-71C substitution in the promoter in both participants.

[Inherited colour vision deficiencies--from Dalton to molecular genetics]

In recent years, great advances have been made in our understanding of the molecular basis of colour vision defects, as well as of the patterns of genetic variation in individuals with normal colour vision. Molecular genetic analyses have explained the diversity of types and degrees of severity in colour vision anomalies, their frequencies, pronounced individual variations in test results, etc. New techniques have even enabled the determination of John Dalton's real colour vision defect, 150 years after his death. Inherited colour vision deficiencies most often result from the mutations of genes that encode cone opsins. Cone opsin genes are linked to chromosomes 7 (the S or "blue" gene) and X (the L or "red" gene and the M or "green" gene). The L and M genes are located on the q arm of the X chromosome in a head-to-tail array, composed of 2 to 6 (typically 3) genes--a single L is followed by one or more M genes. Only the first two genes of the array are expressed and contribute to the colour vision phenotype. The high degree of homology (96%) between the L and M genes predisposes them to unequal recombination, leading to gene deletion or the formation of hybrid genes (comprising portions of both the L and M genes), explaining the majority of the common red-green colour vision deficiencies. The severity of any deficiency is influenced by the difference in spectral sensitivity between the opsins encoded by the first two genes of the array. A rare defect, S monochromacy, is caused either by the deletion of the regulatory region of the array or by mutations that inactivate the L and M genes. Most recent research concerns the molecular basis of complete achromatopsia, a rare disorder that involves the complete loss of all cone function. This is not caused by mutations in opsin genes, but in other genes that encode cone-specific proteins, e.g. channel proteins and transducin.

The molecular basis of variation in human color vision

Common variation in red-green color vision exists among both normal and color-deficient subjects. Differences at amino acids involved in tuning the spectra of the red and green cone pigments account for the majority of this variation. One source of variation is the very common Ser180Ala polymorphism that accounts for two spectrally different red pigments and that plays an important role in variation in normal color vision as well as in determining the severity of defective color vision. This polymorphism most likely resulted from gene conversion by the green-pigment gene. Another common source of variation is the existence of several types of red/green pigment chimeras with different spectral properties. The red and green-pigment genes are arranged in a head-to-tail tandem array on the X-chromosome with one red-pigment gene followed by one or more green-pigment genes. The high homology between these genes has predisposed the locus to relatively common unequal recombination events that give rise to red/green hybrid genes and to deletion of the green-pigment genes. Such events constitute the most common cause of red-green color vision defects. Only the first two pigment genes of the red/green array are expressed in the retina and therefore contribute to the color vision phenotype. The severity of red-green color vision defects is inversely proportional to the difference between the wavelengths of maximal absorption of the photopigments encoded by the first two genes of the array. Women who are heterozygous for red and green pigment genes that encode three spectrally distinct photopigments have the potential for enhanced color vision.

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

BACKGROUND

X-linked blue cone monochromatism (BCM) has to be differentiated from x-linked cone dystrophy and autosomal recessive rod monochromatism.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Molecular genetics of color-vision deficiencies

The normal X-chromosome-linked color-vision gene array is composed of a single long-wave-sensitive (L-) pigment gene followed by one or more middle-wave-sensitive (M-) pigment genes. The expression of these genes to form L- or M-cones is controlled by the proximal promoter and by the locus control region. The high degree of homology between the L- and M-pigment genes predisposed them to unequal recombination, leading to gene deletion or the formation of L/M hybrid genes that explain the majority of the common red-green color-vision deficiencies. Hybrid genes encode a variety of L-like or M-like pigments. Analysis of the gene order in arrays of normal and deutan subjects indicates that only the two most proximal genes of the array contribute to the color-vision phenotype. This is supported by the observation that only the first two genes of the array are expressed in the human retina. The severity of the color-vision defect is roughly related to the difference in absorption maxima (lambda(max)) between the photopigments encoded by the first two genes of the array. A single amino acid polymorphism (Ser180Ala) in the L pigment accounts for the subtle difference in normal color vision and influences the severity of red-green color-vision deficiency. Blue-cone monochromacy is a rare disorder that involves absence of L- and M-cone function. It is caused either by deletion of a critical region that regulates expression of the L/M gene array, or by mutations that inactivate the L- and M-pigment genes. Total color blindness is another rare disease that involves complete absence of all cone function. A number of mutants in the genes encoding the cone-specific alpha- and beta-subunits of the cGMP-gated cation channel as well as in the alpha-subunit of transducin have been implicated in this disorder.

Variety of genotypes in males diagnosed as dichromatic on a conventional clinical anomaloscope

The hypothesis that dichromatic behavior on a clinical anomaloscope can be explained by the complement and arrangement of the long- (L) and middle-wavelength (M) pigment genes was tested. It was predicted that dichromacy is associated with an X-chromosome pigment gene array capable of producing only a single functional pigment type. The simplest case of this is when deletion has left only a single X-chromosome pigment gene. The production of a single L or M pigment type can also result from rearrangements in which multiple genes remain. Often, only the two genes at the 5' end of the array are expressed; thus, dichromacy is also predicted to occur if one of these is defective or encodes a defective pigment, or if both of them encode pigments with identical spectral sensitivities. Subjects were 128 males who accepted the full range of admixtures of the two primary lights as matching the comparison light on a Neitz or Nagel anomaloscope. Strikingly, examination of the L and M pigment genes revealed a potential cause for a color-vision defect in all 128 dichromats. This indicates that the major component of color-vision deficiency could be attributed to alterations of the pigment genes or their regulatory regions in all cases, and the variety of gene arrangements associated with dichromacy is cataloged here. However, a fraction of the dichromats (17 out of 128; 13%) had genes predicted to encode pigments that would result in two populations of cones with different spectral sensitivities. Nine of the 17 were predicted to have two pigments with slightly different spectral peaks (usually < or = 2.5 nm) and eight had genes which specified pigments identical in peak absorption, but different in amino acid positions previously associated with optical density differences. In other subjects, reported previously, the same small spectral differences were associated with anomalous trichromacy rather than dichromacy. It appears that when the spectral difference specified by the genes is very small, the amount of residual red-green color vision measured varies; some individuals test as dichromats, others test as anomalous trichromats. The discrepancy is probably partly attributable to testing method differences and partly to a difference in performance not perception, but it seems there must also be cases in which other factors, for example, cone ratio, contribute to a person's ability to extract a color signal from a small spectral difference.

Analysis of L-cone/M-cone visual pigment gene arrays in Japanese males with protan color-vision deficiency

The L-cone/M-cone visual pigment gene arrays were analyzed in 125 Japanese males with protan color-vision deficiency. Arrays were successfully determined in 62/65 subjects with protanopia and 57/60 protanomaly subjects. Among the 62 protanopia subjects, 48 (77%) had an array consisting of a single 5' L-M hybrid gene (PS-array) or a 5' L-M hybrid gene followed by an M gene(s) that was structurally identical to the hybrid gene (PI-array). In the remaining 14 subjects, 11 had an array consisting of a 5' L-M hybrid gene followed by an M gene(s) that was structurally different from the hybrid gene (PD-array) and 3 subjects had an apparently normal array consisting of a single L gene followed by an M gene(s) (PN-array). In the 11 subjects with the PD-array, subject A67 had an 11 bp-deletion in exon 3 of the downstream genes and 6 had an A-71C substitution in the second gene of the array. In the 3 subjects with the PN-array, subject A289 had a missense mutation (Pro231Leu) in exon 4 of the L gene. When the function of the missense mutation was studied by in vitro reconstitution of visual pigments, it was found to be deleterious to both cone opsin and rhodopsin. Among the 57 protanomaly subjects, 49 (86%) had the PD-array, but 25 subjects had a difference only in exon 2 between the first and downstream genes that suggested a contribution of exon 2-encoded difference in the M pigment to color-discrimination. In the remaining 8 subjects, 2 had the PS-array, 2 had the PI-array and the other 4, including subject A89 with a missense mutation (Glu338Gly) in the L gene, had the PN-array. Genotype-phenotype relationships in protan color-vision deficiency are discussed.

Functional photoreceptor loss revealed with adaptive optics: an alternate cause of color blindness

There is enormous variation in the X-linked L/M (long/middle wavelength sensitive) gene array underlying "normal" color vision in humans. This variability has been shown to underlie individual variation in color matching behavior. Recently, red-green color blindness has also been shown to be associated with distinctly different genotypes. This has opened the possibility that there may be important phenotypic differences within classically defined groups of color blind individuals. Here, adaptive optics retinal imaging has revealed a mechanism for producing dichromatic color vision in which the expression of a mutant cone photopigment gene leads to the loss of the entire corresponding class of cone photoreceptor cells. Previously, the theory that common forms of inherited color blindness could be caused by the loss of photoreceptor cells had been discounted. We confirm that remarkably, this loss of one-third of the cones does not impair any aspect of vision other than color.

Topographical cone photopigment gene expression in deutan-type red–green color vision defects

Eye donors were identified who had X-chromosome photopigment gene arrays like those of living deuteranomalous men; the arrays contained two genes encoding long-wavelength sensitive (L) pigments as well as genes to encode middle-wavelength sensitive (M) photopigment. Ultrasensitive methods failed to detect the presence of M photopigment mRNA in the retinas of these deutan donors. This provides direct evidence that deuteranomaly is caused by the complete absence of M pigment mRNA. Additionally, for those eyes with mRNA corresponding to two different L-type photopigments, the ratio of mRNA from the first vs. downstream L genes was analyzed across the retinal topography. Results show that the pattern of first relative to downstream L gene expression in the deuteranomalous retina is similar to the pattern of L vs. M gene expression found in normal retinas.

An A-71C substitution in a green gene at the second position in the red/green visual-pigment gene array is associated with deutan color-vision deficiency

We studied 247 Japanese males with congenital deutan color-vision deficiency and found that 37 subjects (15.0%) had a normal genotype of a single red gene followed by a green gene(s). Two of them had missense mutations in the green gene(s), but the other 35 subjects had no mutations in either the exons or their flanking introns. However, 32 of the 35 subjects, including all 8 subjects with pigment-color defect, a special category of deuteranomaly, had a nucleotide substitution, A-71C, in the promoter of a green gene at the second position in the red/green visual-pigment gene array. Although the -71C substitution was also present in color-normal Japanese males at a frequency of 24.3%, it was never at the second position but always found further downstream. The substitution was found in 19.4% of Chinese males and 7.7% of Thai males but rarely in Caucasians or African Americans. These results suggest that the A-71C substitution in the green gene at the second position is closely associated with deutan color-vision deficiency. In Japanese and presumably other Asian populations further downstream genes with -71C comprise a reservoir of the visual-pigment genes that cause deutan color-vision deficiency by unequal crossing over between the intergenic regions.

Novel missense mutations in red/green opsin genes in congenital color-vision deficiencies

The DNAs from 217 Japanese males with congenital red/green color-vision deficiencies were analyzed. Twenty-three subjects had the normal genotype of a single red gene, followed by a green gene. Four of the 23 were from the 69 protan subject group and 19 of the 23 were from the 148 deutan subject group. Three of the 23 subjects had missense mutations. The mutation Asn94Lys (AAC-->AAA) occurred in the single green gene of a deutan subject (A155). The Arg330Gln (CGA-->CAA) mutation was detected in both green genes of another deutan subject (A164). The Gly338Glu (GGG-->GAG) mutation occurred in the single red gene of a protan subject (A89). Both normal and mutant opsins were expressed in cultured COS-7 cells and visual pigments were regenerated with 11-cis-retinal. The normal red and green opsins showed absorbance spectra with lambda(max) of 560 and 530 nm, respectively, but the three mutant opsins had altered spectra. The mutations in Asn94Lys and Gly338Glu resulted in no absorbance and the Arg330Gln mutation gave a low absorbance spectrum with a lambda(max) of 530 nm. Therefore these three mutant opsins are likely to be affected in the folding process, resulting in a loss of function as a visual pigment.

Autosomal dominant Stargardt-like macular dystrophy

Autosomal dominant Stargardt-like macular dystrophy is one of the early onset macular dystrophies. It is characterized clinically in its early stages by visual loss and by the presence of atrophic macular changes with or without the presence of yellowish flecks. It is an important retinal dystrophy to study, not only because it has implications in the care and treatment of patients with the condition, but because it also provides important information regarding retinal function. Review of the literature suggests that many of the reported families are linked to chromosome 6q. Genetic and genealogical evidence suggests that these families have descended from a common ancestor or founder. The recent identification of a disease-causing gene that is involved in fatty acid metabolism may have implications in the study of the more common age-related macular degeneration. We review the recent clinical, genetic, and genealogical aspects of autosomal dominant Stargardt-like macular dystrophy.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Summary of ocular genetic disorders and inherited systemic conditions with eye findings

Of the close to 10,000 known inherited disorders that affect humankind, a disproportionately high number affect the eye. The total number of genes responsible for the normal structure, function, and differentiation of the eye is unknown, but the list of these genes is rapidly and constantly growing. The objective of this paper is to provide a current list of mapped and/or cloned human eye genes that are responsible for inherited diseases of the eye. The ophthalmologist should be aware of recent advances in molecular technology which have resulted in significant progress towards the identification of these genes. The implications of this new knowledge will be discussed herein.

Cone and rod function in cone degenerations

Progressive cone dystrophy (CD) is usually marked in the initial stages by reduced visual acuity, color vision deficiency and alterations in the photopic electroretinogram, while morphological alterations can be very mild; in some forms rods are affected in a later stage as well. We examined 40 patients with progressive cone dystrophy to determine the extent of functional losses in the cone system with psychophysical tests. A great variety of visual acuity and fundus alterations was found. Myopia was present in 74% of the patients. An autosomal dominant pattern of inheritance predominated (32%). No prevalence of gender was found. The age of onset ranged between 10 and 30 yr. All patients had progression of their symptoms. The total error score in color arrangement tests, the saturated Farnsworth Panel D-15 and the Farnsworth-Munsell 100-hue test, was pathologic with a predominance of confusions along the tritan and scotopic axis. Especially if visual acuity was below 0.5, color vision defects increased, but color vision defects were also found in patients with normal visual acuity. A general decrease of sensitivity in all three cone mechanisms was observed in measurements of spectral sensitivity. Moreover, cone-cone interaction as tested by transient tritanopia measurements was usually disturbed. In the dark adaptation function the threshold of the cone branch was usually elevated. These tests provide a good means to ascertain the correct diagnosis in early stages of the disease and to monitor progression in patients suffering from cone dystrophy.

Increment threshold and purity discrimination spectral sensitivities of X-chromosome-linked color-defective observers

The goal of the study was to evaluate spectral opponency in nine X-chromosome-linked color-defective observers. The tasks included increment threshold spectral sensitivity on an achromatic background, heterochromatic flicker photometry, and colorimetric purity discrimination. With a task of heterochromatic flicker photometry, the anomalous trichromatic observers showed spectral sensitivity of the corresponding dichromat. The increment threshold spectral sensitivity and colorimetric purity discrimination data were analyzed using the concept of standard cone photopigment spectral sensitivities for normal and defective vision, and a model that postulates one cone-additive and two cone-antagonistic systems. The model incorporated a shift of the peak spectral sensitivity of the long-wavelength-sensitive (LWS) pigment (for protan observers) or of the middle-wavelength-sensitive (MWS) pigment (for deutan observers). Two dichromats and two anomalous trichromats did not show clear evidence of LWS vs MWS cone antagonism. Five anomalous trichromats showed such cone antagonism. Molecular genetic analysis of the opsin genes is presented for eight of the observers.

Molecular genetics of human color vision

The significant advances in our understanding of color vision has been due to the convergence of information from behavioral and molecular genetic analyses. The molecular biology of the visual pigments; molecular genetic basis of variation in normal and abnormal color vision, and regulation of the genes at the LWS-MWS pigment gene locus are discussed.

Disruption of conserved rhodopsin disulfide bond by Cys187Tyr mutation causes early and severe autosomal dominant retinitis pigmentosa

PURPOSE

To determine the molecular basis of an early and severe form of autosomal dominant retinitis pigmentosa and to characterize the associated phenotype.

METHODS

Visual function evaluation included electrophysiologic and psychophysical testing. Molecular genetic analysis included determining the DNA sequence of sections of the rhodopsin gene amplified by polymerase chain reaction and screening for changes single-nucleotide by allele-specific oligonucleotide hybridization.

RESULTS

Affected family members are heterozygous for a unique Cys187Tyr rhodopsin mutation which disrupts a highly conserved disulfide bond essential to normal rhodopsin function. The retinitis pigmentosa (RP) phenotype includes early and severe retinal dysfunction. The full-field electroretinogram showed only negligible remaining rod and cone responses by 22 years of age. Visual fields were constricted severely by early middle-age years. Macular dysfunction caused reduced visual acuity in early adult years, and macular atrophy was present in older age. The severity of phenotype generally correlated with age, with the exception of an affected 44-year-old patient who had better visual acuity, fields, electroretinogram, and dark-adapted thresholds than did three younger affected relatives, ranging in age from 22 to 38 years.

CONCLUSION

An early onset, blinding form of autosomal dominant RP results from a rhodopsin Cys187Tyr mutation that eliminates a residue necessary for the formation of a highly conserved disulfide bond essential to normal rhodopsin function. The fact that one family member is significantly less affected than his younger relatives suggests that genetic or environmental factors can modulate the phenotype.