Gene conversion between red and defective green opsin gene in blue cone monochromacy

Dyschromatopsia: a comprehensive analysis of mechanisms and cutting-edge treatments for color vision deficiency

Color blindness is a retinal disease that mainly manifests as a color vision disorder, characterized by achromatopsia, red-green color blindness, and blue-yellow color blindness. With the development of technology and progress in theory, extensive research has been conducted on the genetic basis of color blindness, and various approaches have been explored for its treatment. This article aims to provide a comprehensive review of recent advances in understanding the pathological mechanism, clinical symptoms, and treatment options for color blindness. Additionally, we discuss the various treatment approaches that have been developed to address color blindness, including gene therapy, pharmacological interventions, and visual aids. Furthermore, we highlight the promising results from clinical trials of these treatments, as well as the ongoing challenges that must be addressed to achieve effective and long-lasting therapeutic outcomes. Overall, this review provides valuable insights into the current state of research on color blindness, with the intention of informing further investigation and development of effective treatments for this disease.

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.

The landscape of submicroscopic structural variants at the OPN1LW/OPN1MW gene cluster on Xq28 underlying blue cone monochromacy

Blue cone monochromacy (BCM) is an X-linked retinal disorder characterized by low vision, photoaversion, and poor color discrimination. BCM is due to the lack of long-wavelength-sensitive and middle-wavelength-sensitive cone photoreceptor function and caused by mutations in the OPN1LW/OPN1MW gene cluster on Xq28. Here, we investigated the prevalence and the landscape of submicroscopic structural variants (SVs) at single-base resolution in BCM patients. We found that about one-third (n = 73) of the 213 molecularly confirmed BCM families carry an SV, most commonly deletions restricted to the OPN1LW/OPN1MW gene cluster. The structure and precise breakpoints of the SVs were resolved in all but one of the 73 families. Twenty-two families-all from the United States-showed the same SV, and we confirmed a common ancestry of this mutation. In total, 42 distinct SVs were identified, including 40 previously unreported SVs, thereby quadrupling the number of precisely mapped SVs underlying BCM. Notably, there was no "region of overlap" among these SVs. However, 90% of SVs encompass the upstream locus control region, an essential enhancer element. Its minimal functional extent based on deletion mapping in patients was refined to 358 bp. Breakpoint analyses suggest diverse mechanisms underlying SV formation as well as in one case the gene conversion-based exchange of a 142-bp deletion between opsin genes. Using parsimonious assumptions, we reconstructed the composition and copy number of the OPN1LW/OPN1MW gene cluster prior to the mutation event and found evidence that large gene arrays may be predisposed to the occurrence of SVs at this locus.

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.

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.

Gene-based Therapy in a Mouse Model of Blue Cone Monochromacy

Cones are responsible for daylight, central, high acuity and color vision. Three proteins found in human cones, i.e. long-wavelength (L)-, middle-wavelength (M)-, and short-wavelength sensitive (S)-opsins, are responsible for red, green and blue color recognition, respectively. Human blue cone monochromacy (BCM) is characterized by functional loss of both L- and M-cone opsins due to mutations in the OPN1LW/OPN1MW gene cluster on the X chromosome. BCM patients, who rely on their vision from only S-cones and rods, suffer severely reduced visual acuity and impaired color vision. Recent studies show that there is sufficient cone structure remaining in the central fovea of BCM patients to consider AAV-mediated gene augmentation therapy. In contrast, mouse retina has only two opsins, S-opsin and M-opsin, but no L-opsin. We generated an M-opsin knockout mouse (Opn1mw -/-) expressing only S-opsin as a model for human BCM. We show that recombinant M-opsin delivered by AAV5 vectors rescues M-cone function in Opn1mw -/- mice. We also show that AAV delivered M-opsin localizes in the dorsal cone outer segments, and co-localizes with S-opsin in the ventral retina. Our study demonstrates that cones without M-opsin remain viable and respond to gene augmentation therapy, thereby providing proof-of-concept for cone function restoration in BCM patients.

De novo intrachromosomal gene conversion from OPN1MW to OPN1LW in the male germline results in Blue Cone Monochromacy

X-linked cone dysfunction disorders such as Blue Cone Monochromacy and X-linked Cone Dystrophy are characterized by complete loss (of) or reduced L- and M- cone function due to defects in the OPN1LW/OPN1MW gene cluster. Here we investigated 24 affected males from 16 families with either a structurally intact gene cluster or at least one intact single (hybrid) gene but harbouring rare combinations of common SNPs in exon 3 in single or multiple OPN1LW and OPN1MW gene copies. We assessed twelve different OPN1LW/MW exon 3 haplotypes by semi-quantitative minigene splicing assay. Nine haplotypes resulted in aberrant splicing of ≥20% of transcripts including the known pathogenic haplotypes (i.e. ‘LIAVA’, ‘LVAVA’) with absent or minute amounts of correctly spliced transcripts, respectively. De novo formation of the ‘LIAVA’ haplotype derived from an ancestral less deleterious ‘LIAVS’ haplotype was observed in one family with strikingly different phenotypes among affected family members. We could establish intrachromosomal gene conversion in the male germline as underlying mechanism. Gene conversion in the OPN1LW/OPN1MW genes has been postulated, however, we are first to demonstrate a de novo gene conversion within the lineage of a pedigree.

The cone dysfunction syndromes

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

Acquired color vision deficiency

Acquired color vision deficiency occurs as the result of ocular, neurologic, or systemic disease. A wide array of conditions may affect color vision, ranging from diseases of the ocular media through to pathology of the visual cortex. Traditionally, acquired color vision deficiency is considered a separate entity from congenital color vision deficiency, although emerging clinical and molecular genetic data would suggest a degree of overlap. We review the pathophysiology of acquired color vision deficiency, the data on its prevalence, theories for the preponderance of acquired S-mechanism (or tritan) deficiency, and discuss tests of color vision. We also briefly review the types of color vision deficiencies encountered in ocular disease, with an emphasis placed on larger or more detailed clinical investigations.

High-resolution microarray analysis unravels complex Xq28 aberrations in patients and carriers affected by X-linked blue cone monochromacy

The human X chromosome contains ∼ 1600 genes, about 15% of which have been associated with a specific genetic condition, mainly affecting males. Blue cone monochromacy (BCM) is an X-linked condition caused by a loss-of-function of both the OPN1LW and OPN1MW opsin genes. The cone opsin gene cluster is composed of 2-9 paralogs with 99.8% sequence homology and is susceptible to deletions, duplications, and mutations. Current diagnostic tests employ polymerase chain reaction (PCR)-based technologies; however, alterations remain undetermined in 10% of patients. Furthermore, carrier testing in females is limited or unavailable. High-resolution X chromosome-targeted CGH microarray was applied to test for rearrangements in males with BCM and female carriers from three unrelated families. Pathogenic alterations were revealed in all probands, characterized by sequencing of the breakpoint junctions and quantitative real-time PCR. In two families, we identified a novel founder mutation that consisted of a complex 3-kb deletion that embraced the cis-regulatory locus control region and insertion of an additional aberrant OPN1MW gene. The application of high-resolution X-chromosome microarray in clinical diagnosis brings significant advantages in detection of small aberrations that are beyond the resolution of clinically available aCGH analysis and which can improve molecular diagnosis of the known conditions and unravel previously unrecognized X-linked diseases.

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.

Opsin gene duplication and divergence in ray-finned fish

Opsin gene sequences were first reported in the 1980s. The goal of that research was to test the hypothesis that human opsins were members of a single gene family and that variation in human color vision was mediated by mutations in these genes. While the new data supported both hypotheses, the greatest contribution of this work was, arguably, that it provided the data necessary for PCR-based surveys in a diversity of other species. Such studies, and recent whole genome sequencing projects, have uncovered exceptionally large opsin gene repertoires in ray-finned fishes (taxon, Actinopterygii). Guppies and zebrafish, for example, have 10 visual opsin genes each. Here we review the duplication and divergence events that have generated these gene collections. Phylogenetic analyses revealed that large opsin gene repertories in fish have been generated by gene duplication and divergence events that span the age of the ray-finned fishes. Data from whole genome sequencing projects and from large-insert clones show that tandem duplication is the primary mode of opsin gene family expansion in fishes. In some instances gene conversion between tandem duplicates has obscured evolutionary relationships among genes and generated unique key-site haplotypes. We mapped amino acid substitutions at so-called key-sites onto phylogenies and this exposed many examples of convergence. We found that dN/dS values were higher on the branches of our trees that followed gene duplication than on branches that followed speciation events, suggesting that duplication relaxes constraints on opsin sequence evolution. Though the focus of the review is opsin sequence evolution, we also note that there are few clear connections between opsin gene repertoires and variation in spectral environment, morphological traits, or life history traits.

Gene conversion in human genetic disease

Gene conversion is a specific type of homologous recombination that involves the unidirectional transfer of genetic material from a ‘donor’ sequence to a highly homologous ‘acceptor’. We have recently reviewed the molecular mechanisms underlying gene conversion, explored the key part that this process has played in fashioning extant human genes, and performed a meta-analysis of gene-conversion events known to have caused human genetic disease. Here we shall briefly summarize some of the latest developments in the study of pathogenic gene conversion events, including (i) the emerging idea of minimal efficient sequence homology (MESH) for homologous recombination, (ii) the local DNA sequence features that appear to predispose to gene conversion, (iii) a mechanistic comparison of gene conversion and transient hypermutability, and (iv) recently reported examples of pathogenic gene conversion events.

X-linked cone dystrophy caused by mutation of the red and green cone opsins

X-linked cone and cone-rod dystrophies (XLCOD and XLCORD) are a heterogeneous group of progressive disorders that solely or primarily affect cone photoreceptors. Mutations in exon ORF15 of the RPGR gene are the most common underlying cause. In a previous study, we excluded RPGR exon ORF15 in some families with XLCOD. Here, we report genetic mapping of XLCOD to Xq26.1-qter. A significant LOD score was detected with marker DXS8045 (Z(max) = 2.41 [theta = 0.0]). The disease locus encompasses the cone opsin gene array on Xq28. Analysis of the array revealed a missense mutation (c. 529T>C [p. W177R]) in exon 3 of both the long-wavelength-sensitive (LW, red) and medium-wavelength-sensitive (MW, green) cone opsin genes that segregated with disease. Both exon 3 sequences were identical and were derived from the MW gene as a result of gene conversion. The amino acid W177 is highly conserved in visual and nonvisual opsins across species. We show that W177R in MW opsin and the equivalent W161R mutation in rod opsin result in protein misfolding and retention in the endoplasmic reticulum. We also demonstrate that W177R misfolding, unlike the P23H mutation in rod opsin that causes retinitis pigmentosa, is not rescued by treatment with the pharmacological chaperone 9-cis-retinal. Mutations in the LW/MW cone opsin gene array can, therefore, lead to a spectrum of disease, ranging from color blindness to progressive cone dystrophy (XLCOD5).

Gene conversion and DNA sequence polymorphism in the sex-determination gene fog-2 and its paralog ftr-1 in Caenorhabditis elegans

Gene conversion, a form of concerted evolution, bears enormous potential to shape the trajectory of sequence and functional divergence of gene paralogs subsequent to duplication events. fog-2, a sex-determination gene unique to Caenorhabditis elegans and implicated in the origin of hermaphroditism in this species, resulted from the duplication of ftr-1, an upstream gene of unknown function. Synonymous sequence divergence in regions of fog-2 and ftr-1 (excluding recent gene conversion tracts) suggests that the duplication occurred 46 million generations ago. Gene conversion between fog-2 and ftr-1 was previously discovered in experimental fog-2 knockout lines of C. elegans, whereby hermaphroditism was restored in mutant obligately outcrossing male-female populations. We analyzed DNA-sequence variation in fog-2 and ftr-1 within 40 isolates of C. elegans from diverse geographic locations in order to evaluate the contribution of gene conversion to genetic variation in the two gene paralogs. The analysis shows that gene conversion contributes significantly to DNA-sequence diversity in fog-2 and ftr-1 (22% and 34%, respectively) and may have the potential to alter sexual phenotypes in natural populations. A radical amino acid change in a conserved region of the F-box domain of fog-2 was found in natural isolates of C. elegans with significantly lower fecundity. We hypothesize that the lowered fecundity is due to reduced masculinization and less sperm production and that amino acid replacement substitutions and gene conversion in fog-2 may contribute significantly to variation in the degree of inbreeding and outcrossing in natural populations.

Colour vision deficiency

Colour vision deficiency is one of the commonest disorders of vision and can be divided into congenital and acquired forms. Congenital colour vision deficiency affects as many as 8% of males and 0.5% of females--the difference in prevalence reflects the fact that the commonest forms of congenital colour vision deficiency are inherited in an X-linked recessive manner. Until relatively recently, our understanding of the pathophysiological basis of colour vision deficiency largely rested on behavioural data; however, modern molecular genetic techniques have helped to elucidate its mechanisms. The current management of congenital colour vision deficiency lies chiefly in appropriate counselling (including career counselling). Although visual aids may be of benefit to those with colour vision deficiency when performing certain tasks, the evidence suggests that they do not enable wearers to obtain normal colour discrimination. In the future, gene therapy remains a possibility, with animal models demonstrating amelioration following treatment.

Footprints of X-to-Y gene conversion in recent human evolution

Different X-homologous regions of the male-specific portion of the human Y chromosome (MSY) are characterized by a different content of putative single nucleotide polymorphisms (SNPs), as reported in public databases. The possible role of X-to-Y nonallelic gene conversion in contributing to these differences remains poorly understood. We explored this issue by analyzing sequence variation in three regions of the MSY characterized by a different degree of X-Y similarity and a different density of putative SNPs: the PCDH11Y gene in the X-transposed (X-Y identity 99%, high putative SNP content); the TBL1Y gene in the X-degenerate (X-Y identity 86-88%, low putative SNP content); and VCY genes-containing region in the P8 palindrome (X-Y identity 95%, low putative SNP content). Present findings do not provide any evidence for gene conversion in the PCDH11Y and TBL1Y genes; they also strongly suggest that most putative SNPs of the PCDH11Y gene (and possibly the entire X-transposed region) are most likely X-Y paralogous sequence variants, which have been entered in the databases as SNPs. On the other hand, clear evidence for the VCY genes in the P8 palindrome having acted as an acceptor of X-to-Y gene conversion was obtained. A rate of 1.8 x 10(-7) X-to-Y conversions/bp/year was estimated for these genes. These findings indicate that in the VCY region of the MSY, X-to-Y gene conversion can be highly effective to increase the level of diversity among human Y chromosomes and suggest an additional explanation for the ability of the Y chromosome to retard degradation during evolution. Present data are expected to pave the way for future investigations on the role of nonallelic gene conversion in double-strand break repair and the maintenance of Y chromosome integrity.

The opsin repertoire of Jenynsia onca: a new perspective on gene duplication and divergence in livebearers

Background

Jenynsia onca, commonly known as the one sided livebearer, is a member of the family Anablepidae. The opsin gene repertoires of J. onca's close relatives, the four-eyed fish (Anableps anableps) and the guppy (Poecilia reticulata), have been characterized and each found to include one unique LWS opsin. Currently, the relationship among LWS paralogs and orthologs in these species are unclear, making it difficult to test the hypotheses that link vision to morphology or life history traits. The phylogenetic signal appears to have been disrupted by gene conversion. Here we have sequenced the opsin genes of J. onca in order to resolve these relationships.

Findings

We identified nine visual opsins; LWS S180r, LWS S180, LWS P180, SWS1, SWS2A, SWS2B, RH1, RH2-1, and RH2-2. Key site analysis revealed only one unique haplotype, RH2-2, although this is unlikely to shift λ_max significantly. LWS P180 was found to be a product of a gene conversion event with LWS S180, followed by convergence to a proline residue at the 180 site.

Conclusion

Jenynsia onca has at least 9 visual opsins: three LWS, one RH1, two RH2, one SWS1 and two SWS2. The presence of LWS P180 moves the location of the LWS P180-S180 tandem duplication event back to the base of the Poeciliidae-Anablepidae clade, expanding the number of species possessing this unusual blue shifted LWS opsin. The presence of the LWS P180 gene also confirms that gene conversion events have homogenized opsin paralogs in fish, just as they have in humans.

Sexy gene conversions: locating gene conversions on the X-chromosome

Gene conversion can have a profound impact on both the short- and long-term evolution of genes and genomes. Here, we examined the gene families that are located on the X-chromosomes of human (Homo sapiens), chimpanzee (Pan troglodytes), mouse (Mus musculus) and rat (Rattus norvegicus) for evidence of gene conversion. We identified seven gene families (WD repeat protein family, Ferritin Heavy Chain family, RAS-related Protein RAB-40 family, Diphosphoinositol polyphosphate phosphohydrolase family, Transcription Elongation Factor A family, LDOC1-related family, Zinc Finger Protein ZIC, and GLI family) that show evidence of gene conversion. Through phylogenetic analyses and synteny evidence, we show that gene conversion has played an important role in the evolution of these gene families and that gene conversion has occurred independently in both primates and rodents. Comparing the results with those of two gene conversion prediction programs (GENECONV and Partimatrix), we found that both GENECONV and Partimatrix have very high false negative rates (i.e. failed to predict gene conversions), which leads to many undetected gene conversions. The combination of phylogenetic analyses with physical synteny evidence exhibits high resolution in the detection of gene conversions.

Gene conversion: mechanisms, evolution and human disease

Gene conversion, one of the two mechanisms of homologous recombination, involves the unidirectional transfer of genetic material from a 'donor' sequence to a highly homologous 'acceptor'. Considerable progress has been made in understanding the molecular mechanisms that underlie gene conversion, its formative role in human genome evolution and its implications for human inherited disease. Here we assess current thinking about how gene conversion occurs, explore the key part it has played in fashioning extant human genes, and carry out a meta-analysis of gene-conversion events that are known to have caused human genetic disease.

Blue cone monochromatism: a phenotype and genotype assessment with evidence of progressive loss of cone function in older individuals

AIM

To perform a detailed clinical and psychophysical assessment of the members of three British families affected with blue cone monochromatism (BCM), and to determine the molecular basis of disease in these families.

METHODS

Affected and unaffected members of three families with BCM were examined clinically and underwent electrophysiological and detailed psychophysical testing. Blood samples were taken for DNA extraction. The strategy for molecular analysis was to amplify the coding regions of the long wavelength-sensitive (L) and middle wavelength-sensitive (M) cone opsin genes and the upstream locus control region by polymerase chain reaction, and to examine these fragments for mutations by direct sequencing.

RESULTS

We have confirmed the reported finding of protan-like D-15 arrangements of patients with BCM. In addition, we have demonstrated that the Mollon-Reffin (MR) Minimal test is a useful colour-discrimination test to aid in the diagnosis of BCM. Affected males were shown to fail the protan and deutan axes, but retained good discrimination on the tritan axis of the MR test, a compelling evidence for residual colour vision in BCM. This residual tritan discrimination was also readily detected with HRR plates. In two families, psychophysical testing demonstrated evidence for progression of disease. In two pedigrees, BCM could be linked to unequal crossovers within the opsin gene array that resulted in a single 5'-L/M-3' hybrid gene, with an inactivating Cys203Arg mutation. The causative mutations were not identified in the third family.

CONCLUSIONS

The MR test is a useful method of detecting BCM across a wide range of age groups; residual tritan colour discrimination is clearly demonstrated and allows BCM to be distinguished from rod monochromatism. BCM is usually classified as a stationary cone dysfunction syndrome; however, two of our families show evidence of progression. This is the first report of progression associated with a genotype consisting of a single 5'-L/M-3' hybrid gene carrying an inactivating mutation. We have confirmed that the Cys203Arg inactivating mutation is a common sequence change in blue cone monochromats.

A homozygous nonsense mutation in SOX9 in the dominant disorder campomelic dysplasia: a case of mitotic gene conversion

Campomelic dysplasia (CD; MIM 114290), an autosomal dominant skeletal malformation syndrome with XY sex reversal, is caused by heterozygous de novo mutations in and around the SOX9 gene on 17q. We report a patient with typical signs of CD, including sex reversal, who was, surprisingly, homozygous for the nonsense mutation Y440X. Since neither parent carried the Y440X mutation, possible mechanisms explaining the homozygous situation were a de novo mutation followed by uniparental isodisomy, somatic crossing over, or gene conversion. As the patient was heterozygous for six microsatellite markers flanking SOX9, uniparental isodisomy and somatic crossing over were excluded. Analysis of intragenic single-nucleotide polymorphisms suggested that the homozygous mutation arose by a mitotic gene conversion event involving exchange of at least 440 nucleotides and at most 2,208 nucleotides between a de novo mutant maternal allele and a wild-type paternal allele. Analysis of cloned alleles showed that homozygous mutant cells constituted about 80% of the leukocyte cell population of the patient, whereas about 20% were heterozygous mutant cells. Heterozygous Y440X mutations, previously described in three CD cases, have been identified in seven additional cases, thus constituting the most frequent recurrent mutations in SOX9. These patients frequently have a milder phenotype with longer survival, possibly because of the retention of some transactivation activity of the mutant protein on SOX9 target genes, as shown by cell transfection experiments. The fact that the patient survived for 3 months may thus be explained by homozygosity for a hypomorphic rather than a complete loss-of-function allele, in combination with somatic mosaicism. This is, to our knowledge, the first report of mitotic gene conversion of a wild-type allele by a de novo mutant allele in humans.

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.

The cone dysfunction syndromes

The cone dystrophies comprise a heterogeneous group of disorders characterised by visual loss, abnormalities of colour vision, central scotomata, and a variable degree of nystagmus and photophobia. They may be stationary or progressive. The stationary cone dystrophies are better described as cone dysfunction syndromes since a dystrophy often describes a progressive process. These different syndromes encompass a wide range of clinical and psychophysical findings. The aim is to review current knowledge relating to the cone dysfunction syndromes, with discussion of the various phenotypes, the currently mapped genes, and genotype-phenotype relations. The cone dysfunction syndromes that will be discussed are complete and incomplete achromatopsia, oligocone trichromacy, cone monochromatism, blue cone monochromatism, and Bornholm eye disease. Disorders with a progressive cone dystrophy phenotype will not be discussed.

FXY2/MID2, a gene related to the X-linked Opitz syndrome gene FXY/MID1, maps to Xq22 and encodes a FNIII domain-containing protein that associates with microtubules

Opitz G/BBB syndrome (OS) is a genetically heterogeneous disorder with an X-linked locus and an autosomal locus linked to 22q11.2. OS affects multiple organ systems with often variable severity even between siblings. The clinical features, which include hypertelorism, cleft lip and palate, defects of cardiac septation, hypospadias, and anorectal anomalies, indicate an underlying disturbance of the developing ventral midline of the embryo. The gene responsible for X-linked OS, FXY/MID1, is located on the short arm of the human X chromosome within Xp22.3 and encodes a protein with both an RBCC (RING finger, B-box, coiled coil) and a B30.2 domain. The Fxy gene in mice is also located on the X chromosome but spans the pseudoautosomal boundary in this species. Here we describe a gene closely related to FXY/MID1, called FXY2, which also maps to the X chromosome within Xq22. The mouse Fxy2 gene is located on the distal part of the mouse X chromosome within a region syntenic to Xq22. Analysis of genes flanking both FXY/MID1 and FXY2 (as well as their counterparts in mouse) suggests that these regions may have arisen as a result of an intrachromosomal duplication on an ancestral X chromosome. We have also identified in both FXY2 and FXY/MID1 proteins a conserved fibronectin type III domain located between the RBCC and B30.2 domains that has implications for understanding protein function. The FXY/MID1 protein has previously been shown to colocalize with microtubules, and here we show that the FXY2 protein similarly associates with microtubules in a manner that is dependent on the carboxy-terminal B30.2 domain.

Rods induce transient tritanopia in blue cone monochromats

Transient tritanopia is a cone-cone post-receptoral interaction between short-wavelength (S) cones and medium (M) and long (L) wavelength cones. Blue cone monochromats have rods and S cones of normal sensitivity but lack functional M/L cones. All blue cone monochromats tested (n = 8) show significant amounts of transient tritanopia mediated by rods. Attempts to find a similar rod-S cone interaction while silencing the L/M cones in normals yielded only a small amount of S cone sensitivity loss. The results suggest an exaggerated influence of rods on the S cone pathway in the retina of blue cone monochromats.

Origins and antiquity of X-linked triallelic color vision systems in New World monkeys

It is known that the squirrel monkey, marmoset, and other related New World (NW) monkeys possess three high-frequency alleles at the single X-linked photopigment locus, and that the spectral sensitivity peaks of these alleles are within those delimited by the human red and green pigment genes. The three alleles in the squirrel monkey and marmoset have been sequenced previously. In this study, the three alleles were found and sequenced in the saki monkey, capuchin, and tamarin. Although the capuchin and tamarin belong to the same family as the squirrel monkey and marmoset, the saki monkey belongs to a different family and is one of the species that is most divergent from the squirrel monkey and marmoset, suggesting the presence of the triallelic system in many NW monkeys. The nucleotide sequences of these alleles from the five species studied indicate that gene conversion occurs frequently and has partially or completely homogenized intronic and exonic regions of the alleles in each species, making it appear that a triallelic system arose independently in each of the five species studied. Nevertheless, a detailed analysis suggests that the triallelic system arose only once in the NW monkey lineage, from a middle wavelength (green) opsin gene, and that the amino acid differences at functionally critical sites among alleles have been maintained by natural selection in NW monkeys for >20 million years. Moreover, the two X-linked opsin genes of howler monkeys (a NW monkey genus) were evidently derived from the incorporation of a middle (green) and a long wavelength (red) allele into one chromosome; these two genes together with the (autosomal) blue opsin gene would immediately enable even a male monkey to have trichromatic vision.

The cone dystrophies

The cone dystrophies are a heterogeneous group of inherited disorders that result in dysfunction of the cone photoreceptors and sometimes their post-receptoral pathways. The major clinical features of cone dystrophy are photophobia, reduced visual acuity and abnormal colour vision. Ganzfeld electroretinography shows reduced or absent cone responses. On the basis of their natural history, the cone dystrophies may be broadly divided into two groups: stationary and progressive cone dystrophies. The stationary cone dystrophies have received more attention, and subsequently our knowledge of their molecular genetic, psychophysical and clinical characteristics is better developed. Various methods of classification have been proposed for the progressive cone dystrophies, but none is entirely satisfactory, largely because the underlying disease mechanisms are poorly understood. Multidisciplinary studies involving clinical assessment, molecular genetics, electrophysiology and psychophysics should lead to an improved understanding of the pathogenesis of these disorders.