Vertebrate opsins belonging to different classes vary in constitutively active properties resulting from salt-bridge mutations

Vertebrate opsins are classified into one of five classes on the basis of amino acid similarity. These classes are short wavelength sensitive 1 and 2 (SWS1, SWS2), medium/long wavelength sensitive (M/LWS), and rod opsin like 1 and 2 (RH1, RH2). In bovine rod opsin (RH1), two critical amino acids form a salt bridge in the apoprotein that maintains the opsin in an inactive state. These residues are K296, which functions as the chromophore binding site, and E113, which functions as the counterion to the protonated Schiff base. Corresponding residues in each of the other vertebrate opsin classes are believed to play similar roles. Previous reports have demonstrated that mutations in these critical residues result in constitutive activation of transducin by RH1 class opsins in the absence of chromophore. Additionally, recent reports have shown that an E113Q mutation in SWS1 opsin is constitutively active. Here we ask if the other classes of vertebrate opsins maintain activation characteristics similar to that of bovine RH1 opsin. We approach this question by making the corresponding substitutions which disrupt the K296/E113 salt bridge in opsins belonging to the other vertebrate opsin classes. The mutant opsins are tested for their ability to constitutively activate bovine transducin. We demonstrate that mutations disrupting this key salt bridge produce constitutive activation in all classes. However, the mutant opsins differ in their ability to be quenched in the dark state by the addition of chromophore as well as in their level of constitutive activation. The differences in constitutive activation profiles suggest that structural differences exist among the opsin classes that may translate into a difference in activation properties.

Progressive cone and cone-rod dystrophies: phenotypes and underlying molecular genetic basis

The cone and cone-rod dystrophies form part of a heterogeneous group of retinal disorders that are an important cause of visual impairment in children and adults. There have been considerable advances made in recent years in our understanding of the pathogenesis of these retinal dystrophies, with many of the chromosomal loci and causative genes having now been identified. Mutations in 12 genes, including GUCA1A, peripherin/RDS, ABCA4 and RPGR, have been described to date; and in many cases detailed functional assessment of the effects of the encoded mutant proteins has been undertaken. This improved knowledge of disease mechanisms has raised the possibility of future treatments for these disorders, for which there are no specific therapies available at the present time.

Evolution of the Cichlid Visual Palette through Ontogenetic Subfunctionalization of the Opsin Gene Arrays

The evolution of cone opsin genes is characterized by a dynamic process of gene birth and death through gene duplication and loss. However, the forces governing the retention and death of opsin genes are poorly understood. African cichlid fishes have a range of ecologies, differing in habitat and foraging style, which make them ideal for examining the selective forces acting on the opsin gene family. In this work, we present data on the riverine cichlid, Oreochromis niloticus, which is an ancestral outgroup to the cichlid adaptive radiations in the Great African lakes. We identify 7 cone opsin genes with several instances of gene duplication. We also characterize the spectral sensitivities of these genes through reconstitution of visual pigments. Peak absorbances demonstrate that each tilapia cone opsin gene codes for a spectrally distinct visual pigment: SWS1 (360 nm), SWS2b (423 nm), SWS2a (456 nm), Rh2b (472 nm), Rh2a beta (518 nm), Rh2a alpha (528 nm), and LWS (561 nm). Furthermore, quantitative reverse transcription polymerase chain reaction at 3 ontogenetic time points demonstrates that although only 4 genes (SWS2a, Rh2a alpha and beta, and LWS) are expressed in adults, mRNAs for the other genes are all expressed during ontogeny. Therefore, subfunctionalization through differential ontogenetic expression may be a key mechanism for preservation of opsin genes. The distinct peak absorbances of these preserved opsin genes provide a palette from which selection creates the diverse visual sensitivities found among the cichlid species of the lacustrine adaptive radiations.

A study of the nuclear trafficking of the splicing factor protein PRPF31 linked to autosomal dominant retinitis pigmentosa (ADRP)

In this study the mechanism of nuclear importation of the splicing factor PRPF31 is examined and the impact of two disease-linked mutations, A194E and A216P, assessed. Using pull-down assays with GST-tagged importin proteins, we demonstrate that His-tagged PRPF31 interacts with importin beta1 for translocation to the nucleus, with no requirement for importin alpha1. The A194E and A216P mutations have no affect on this interaction. Fluorescence recovery after photobleaching (FRAP) was used to estimate the rate of movement of EGFP-tagged PRPF31 into the nuclei of live cells. The kinetics indicated a two-component recovery process; a fast component with tau approximately 6 s and a slow component with tau approximately 80 s. The mutations affected neither component. We conclude that the two mutations have no negative effect on interaction with the nuclear importation machinery. Reduced mutant protein solubility resulting in an insufficiency of splicing activity in cells with a very high metabolic demand remains the most likely explanation for the disease pathology in ADRP patients.

A novel GJA8 mutation is associated with autosomal dominant lamellar pulverulent cataract: further evidence for gap junction dysfunction in human cataract

PURPOSE

To identify the gene responsible for autosomal dominant lamellar pulverulent cataract in a four-generation British family and characterise the functional and cellular consequences of the mutation.

METHODS

Linkage analysis was used to identify the disease locus. The GJA8 gene was sequenced directly. Functional behaviour and cellular trafficking of connexins were examined by expression in Xenopus oocytes and HeLa cells.

RESULTS

A 262C>A transition that resulted in the replacement of proline by glutamine (P88Q) in the coding region of connexin50 (Cx50) was identified. hCx50P88Q did not induce intercellular conductance and significantly inhibited gap junctional activity of co-expressed wild type hCx50 RNA in paired Xenopus oocytes. In transfected cells, immunoreactive hCx50P88Q was confined to the cytoplasm but showed a temperature sensitive localisation at gap junctional plaques.

CONCLUSIONS

The pulverulent cataract described in this family is associated with a novel GJA8 mutation and has a different clinical phenotype from previously described GJA8 mutants. The cataract likely results from lack of gap junction function. The lack of function was associated with improper targeting to the plasma membrane, most probably due to protein misfolding.

Colour vision and speciation in Lake Victoria cichlids of the genus Pundamilia

Lake Victoria cichlids are one of the most speciose groups of vertebrates. Selection on coloration is likely playing an important role in their rapid speciation. To test the hypothesis that sensory biases could explain species differences in mating preferences and nuptial coloration, we studied seven populations of four closely related species of the genus Pundamilia that differ in visual environment and male nuptial colour. Microspectrophotometry determined that the wavelength of maximum absorption (lambdamax) of the rod pigment and three cone pigments were similar in all four species. Only the long wavelength sensitive (LWS) pigment varied among species, with 3-4 nm shifts in lambdamax that correlated with differences in the LWS opsin sequence. These subtle shifts in lambdamax coincided with large shifts in male body colour, with red species having longer LWS pigments than blue species. Furthermore, we observed within and between species a correlation between water transparency and the proportion of red/red vs. red/green double cones. Individuals from turbid water had more red/red double cones than individuals from clear water. The variation in LWS lambdamax and in the proportion of red/red double cones could lead to differences in perceived brightness that may explain the evolution of variation in male coloration. However, other factors, such as chromophore shifts and higher order neural processing, should also be investigated to fully understand the physiological basis of differential responses to male mating hues in cichlid fish.

Isolation and characterization of murine Cds (CDP-diacylglycerol synthase) 1 and 2

Phototransduction in Drosophila is a phosphoinositide-mediated signalling pathway. Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in this process, and its levels are tightly regulated. A photoreceptor-specific form of the enzyme CDP-diacylglycerol synthase (CDS), which catalyzes the formation of CDP-diacylglycerol from phosphatidic acid, is a key regulator of the amount of PIP2 available for signalling. cds mutants develop light-induced retinal degeneration. We report here the isolation and characterization of two murine genes encoding this enzyme, Cds1 and Cds2. The genes encode proteins that are 73% identical and 92% similar but exhibit very different expression patterns. Cds1 shows a very restricted expression pattern but is expressed in the inner segments of the photoreceptors whilst Cds2 shows a ubiquitous pattern of expression. Using fluorescent in situ hybridization we have mapped Cds1 and Cds2 to chromosomes 5E3 and 2G1 respectively. These are regions of synteny with the corresponding human gene localization (4q21 and 20p13). Transient transfection experiments with epitope tagged proteins have also demonstrated that both are associated with the endoplasmic reticulum.

Mix and Match Color Vision: Tuning Spectral Sensitivity by Differential Opsin Gene Expression in Lake Malawi Cichlids

Cichlid fish of the East African Rift Lakes are renowned for their diversity and offer a unique opportunity to study adaptive changes in the visual system in rapidly evolving species flocks. Since color plays a significant role in mate choice, differences in visual sensitivities could greatly influence and even drive speciation of cichlids. Lake Malawi cichlids inhabiting rock and sand habitats have significantly different cone spectral sensitivities. By combining microspectrophotometry (MSP) of isolated cones, sequencing of opsin genes, and spectral analysis of recombinant pigments, we have established the cone complements of four species of Malawi cichlids. MSP demonstrated that each of these species predominately expresses three cone pigments, although these differ between species to give three spectrally different cone complements. In addition, rare populations of spectrally distinct cones were found. In total, seven spectral classes were identified. This was confirmed by opsin gene sequencing, expression, and in vitro reconstitution. The genes represent the four major classes of cone opsin genes that diverged early in vertebrate evolution. All four species possess a long-wave-sensitive (LWS), three spectrally distinct green-sensitive (RH2), a blue-sensitive (SWS2A), a violet-sensitive (SWS2B), and an ultraviolet-sensitive (SWS1) opsin. However, African cichlids determine their spectral sensitivity by differential expression of primarily only three of the seven available cone opsin genes. Phylogenetic analysis suggests that all percomorph fish have similar potential.

Cone-rod dystrophy, intrafamilial variability, and incomplete penetrance associated with the R172W mutation in the peripherin/RDS gene

PURPOSE

To determine the underlying molecular genetic basis of a retinal dystrophy identified in a 5-generation family, and to examine the phenotype and degree of intrafamilial variability.

DESIGN

Family genetic study.

PARTICIPANTS

Nine affected individuals from a nonconsanguineous British family.

METHODS

Ophthalmologic examination, color vision testing, fundus photography, autofluorescence imaging, and electrophysiological assessment were performed. The clinical notes of 2 additional deceased affected family members were also reviewed. Blood samples were taken for DNA extraction, with linkage analysis being performed, and subsequent mutation screening of the peripherin/RDS gene.

RESULTS

Linkage analysis established a disease interval on chromosome 6p, which harbored the retinal candidate gene, peripherin/RDS. The 3 coding exons of the peripherin/RDS gene were subsequently screened for mutations in affected and unaffected family members. A nonconservative missense substitution, Arg172Trp (R172W), segregated uniquely in all affected subjects. The majority of subjects carrying the R172W peripherin/RDS mutation complained of reduced central vision starting in the second or third decade, with subsequent gradual deterioration of visual acuity and color vision. Three affected individuals complained of nyctalopia. A range of macular appearances were seen, varying from a typical granular appearance to extensive macular atrophy. Autofluorescence imaging in the majority of individuals identified a highly characteristic speckled macular appearance. All affected subjects had abnormal pattern electroretinograms (ERGs) consistent with macular dysfunction and 4 subjects showed additional full-field ERG abnormalities, providing evidence of generalized retinal dysfunction. There was marked variation in the clinical phenotype in those individuals who carried the R172W peripherin/RDS mutation, ranging from severe cone-rod dystrophy to asymptomatic individuals with normal retinal function.

CONCLUSIONS

The Arg172Trp (R172W) peripherin/RDS mutation has been previously reported to cause a fully penetrant progressive macular dystrophy with high intrafamilial and interfamilial consistency of phenotype. This is the first report describing marked intrafamilial variation associated with this mutation, including nonpenetrance. These findings are clinically important in relation to advice on prognosis and accurate genetic counseling.

Adaptations to an extreme environment: retinal organisation and spectral properties of photoreceptors in Antarctic notothenioid fish

The Notothenioid suborder of teleosts comprises a number of species that live below the sea ice of the Antarctic. The presence of 'antifreeze' glycoproteins in these fish as an adaptation to freezing temperature has been well documented but little is known about the adaptations of the visual system of these fish to a light environment in which both the quantity and spectral composition of downwelling sunlight has been reduced by passage through ice and snow. In this study, we show that the red/long-wave sensitive (LWS) opsin gene is not present in these fish but a UV-sensitive short-wave sensitive (SWS1) pigment is expressed along with blue-sensitive (SWS2) and green/middle-wave sensitive (Rh2) pigments. The identity and spectral location of maximal absorbance of the SWS1 and Rh2 pigments was confirmed by in vitro expression of the recombinant opsins followed by regeneration with 11-cis retinal. Only the SWS2 pigment showed interspecific variations in peak absorbance. Expression of the Rh2 opsin is localised to double cone receptors in both the central and peripheral retina, whereas SWS2 opsin expression is present only in the peripheral retina. SWS1 cones could not be identified by either microspectrophotometry or in situ hybridisation, presumably reflecting their low number and/or uneven distribution across the retina. A study of photoreceptor organisation in the retina of two species, the shallower dwelling Trematomus hansoni and the deeper dwelling Dissostichus mawsoni, identified a square mosaic in the former, and a row mosaic in the latter species; the row mosaic in Dissostichus mawsoni with less tightly packed cone photoreceptors allows for a higher rod photoreceptor density.

Mutation in the Gene GUCA1A, Encoding Guanylate Cyclase-Activating Protein 1, Causes Cone, Cone-Rod, and Macular Dystrophy

PURPOSE

To determine the underlying molecular genetic basis of a retinal dystrophy identified in a 4-generation family and to examine the phenotype and the degree of intrafamilial variability.

DESIGN

Prospective case series.

PARTICIPANTS

Six affected individuals from a nonconsanguineous British family.

METHODS

Detailed ophthalmologic examination, color fundus photography, autofluorescence imaging, and electrophysiologic assessment were performed. Blood samples were taken for DNA extraction, and mutation screening of GUCA1A, the gene encoding guanylate cyclase-activating protein 1 (GCAP1), was undertaken.

RESULTS

All affected subjects complained of mild photophobia and reduced central and color vision. Onset was between the third and fifth decade, with subsequent gradual deterioration of visual acuity and color vision. Visual acuity ranged between 6/9 and counting fingers. Color vision was either absent or markedly reduced along all 3 color axes. A range of macular appearances was seen, varying from mild retinal pigment epithelial disturbance to extensive atrophy. Electrophysiologic testing revealed a range of electrophysiologic abnormalities: isolated cone electroretinography abnormalities, reduced cone and rod responses (with cone loss greater than rod), and isolated macular dysfunction. The 4 coding exons of GUCA1A were screened for mutations in affected and unaffected family members. A single transition, A319G, causing a nonconservative missense substitution, Tyr99Cys, segregated uniquely in all affected subjects.

CONCLUSIONS

The Tyr99Cys GUCA1A mutation has been previously shown to cause autosomal dominant progressive cone dystrophy. This is the first report of this mutation also causing both cone-rod dystrophy and isolated macular dysfunction. The phenotypic variation described here exemplifies the intrafamilial heterogeneity of retinal dysfunction that can be observed in persons harboring the same mutation and chromosomal segment.

X-Linked Cone Dysfunction Syndrome with Myopia and Protanopia

PURPOSE

To perform a detailed clinical, psychophysical, and molecular assessment of members of 4 families with an unusual X-linked cone dysfunction syndrome associated with myopia.

PARTICIPANTS

Affected and unaffected members of 4 British nonconsanguineous families.

METHODS

Subjects underwent both detailed clinical examination and psychophysical testing. After informed consent was obtained, blood samples were taken for DNA extraction, and molecular genetic analysis was performed. The strategy for molecular analysis was to amplify the coding regions of the long and middle wavelength-sensitive cone opsin genes and the upstream locus control region by polymerase chain reaction and to examine these fragments for mutations by sequencing of DNA.

RESULTS

The phenotype was almost identical in all 4 families, consisting of moderate to high myopia, astigmatism, moderately reduced acuity, and normal fundi. Electroretinography showed abnormal cone but normal rod responses. Psychophysical testing showed a selective impairment of long cones in combination with well-preserved middle cone and short cone function. There was no evidence to suggest that the phenotype was progressive. Molecular analysis of the X-linked opsin gene array in the 4 families indicated that affected males have inherited the same X-chromosome from their mother. In 2 families, a long/middle hybrid gene was detected. In a third family, the commonly described deleterious Cys203Arg amino acid substitution was identified in both the long and middle opsin genes. In the fourth family, the only abnormality was absence of a middle opsin exon 2; the cause of the protanopia in this family is uncertain.

CONCLUSIONS

The X-linked cone dysfunction syndrome associated with myopia and dichromacy described here has many similarities to Bornholm eye disease, a condition previously mapped to Xq28. Except for the Cys203Arg substitution in one family, no alterations in the opsin gene array were identified that could underlie the cone dysfunction. It is therefore possible that the cone dysfunction may have a genetic origin different from that of the dichromacy.

Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus)

The potential for trichromacy in mammals, thought to be unique to primates, was recently discovered in two Australian marsupials. Whether the presence of three cone types, sensitive to short- (SWS), medium- (MWS) and long- (LWS) wavelengths, occurs across all marsupials remains unknown. Here, we have investigated the presence, distribution and spectral sensitivity of cone types in two further species, the quokka (Setonix brachyurus) and quenda (Isoodon obesulus). Immunohistochemistry revealed that SWS cones in the quokka are concentrated in dorso-temporal retina, while in the quenda, two peaks were identified in naso-ventral and dorso-temporal retina. In both species, MWS/LWS cone spatial distributions matched those of retinal ganglion cells. Microspectrophotometry (MSP) confirmed that MWS and LWS cones are spectrally distinct, with mean wavelengths of maximum absorbance at 502 and 538 nm in the quokka, and at 509 and 551 nm, in the quenda. Although small SWS cone outer segments precluded MSP measurements, molecular analysis identified substitutions at key sites, accounting for a spectral shift from ultraviolet in the quenda to violet in the quokka. The presence of three cone types, along with previous findings in the fat-tailed dunnart and honey possum, suggests that three spectrally distinct cone types are a feature spanning the marsupials.

Chromosomal localization, genomic organization and evolution of the genes encoding human phosphatidylinositol transfer protein membrane-associated (PITPNM) 1, 2 and 3

Eukaryotic proteins containing a phosphatidylinositol transfer (PITP) domain can be divided into two groups, one consisting of small soluble 35-kDa proteins and the other those that are membrane-associated and show sequence similarities to the Drosophila retinal degeneration B (rdgB) protein. The rdgB protein consists of four domains, an amino terminal PITP domain, a Ca2+-binding domain, a transmembrane domain and a carboxyl terminal domain that interacts with the protein tyrosine kinase PYK2. Three mammalian phosphatidylinositol transfer protein membrane-associated genes (PITPNM1, 2 and 3) with homology to Drosophila rdgB have previously been described and shown to be expressed in the mammalian retina. These findings and the demonstration that the rdgB gene plays a critical role in the invertebrate phototransduction pathway have led to the mammalian genes being considered as candidate genes for human eye diseases. In order to facilitate the analysis of these genes we have used radiation hybrid mapping and fluorescence in situ hybridization to localize the PITPNM2 and 3 genes to human chromosomes 12p24 and 17p13 respectively and hybrid mapping to confirm the localization of PITPNM1 to chromosome 11q13. We have also determined the genomic organization of both the soluble and membrane-associated Drosophila and human PITP domain-containing genes. Phylogenetic analysis indicates that the two groups arose by gene duplication that occurred very early in animal evolution.

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 detailed phenotypic study of "cone dystrophy with supernormal rod ERG"

AIMS

To characterise the detailed phenotype of "cone dystrophy with supernormal rod ERG" in a case series of 10 patients.

METHODS

10 affected patients were examined clinically and underwent colour fundus photography, with nine undergoing detailed electrophysiological testing. Five patients were assessed further with fundus autofluorescence (AF) imaging, automated photopic and dark adapted perimetry, and dark adaptometry. Detailed colour vision assessment was performed in six subjects. Blood samples were taken from four patients for DNA extraction and mutation screening of NR2E3 was undertaken.

RESULTS

The onset of symptoms was in the first and second decades of life. Subjects presented with reduced central vision and marked photophobia. All individuals were myopic and colour vision testing revealed severely reduced colour discrimination predominantly along the red-green axes; tritan colour vision was relatively well preserved. Nyctalopia is a later feature of the disorder. Funduscopy and AF imaging revealed a range of macular appearances. There was electrophysiological evidence of marked macular dysfunction, reduced and delayed cone responses, and supernormal and delayed rod responses. Photopic and dark adapted perimetry revealed central scotomata with widespread peripheral sensitivity loss. No disease causing sequence variants in NR2E3 were identified.

CONCLUSIONS

The largest case series to date has been described of the clinical, psychophysical and electrophysiological characteristics of this unusual cone dystrophy with supernormal rod responses. Electrophysiological data were consistent with a post-phototransduction, but pre-inner nuclear layer, site of dysfunction. While the definitive diagnosis can only be made with electrophysiological testing, several characteristics that may increase suspicion of this diagnosis are presented.

A detailed study of the phenotype of an autosomal dominant cone-rod dystrophy (CORD7) associated with mutation in the gene for RIM1

AIM

To characterise the phenotype of an autosomal dominant cone-rod dystrophy (CORD7) associated with the Arg844His mutation in RIM1.

METHODS

Eight members of a four generation, non-consanguineous British family were examined clinically and underwent electrophysiological testing, automated dark adapted perimetry, dark adaptometry, colour vision assessment, colour fundus photography, fundus fluorescein angiography (FFA), and fundus autofluorescence (AF) imaging.

RESULTS

The majority of affected individuals described a progressive deterioration of central vision, night vision, and peripheral visual field usually between the third and fourth decades. The visual acuity ranged from 6/6 to 3/60. Colour vision testing showed mild to moderate dyschromatopsia in the majority of individuals. Fundus changes comprised a range of macular appearances varying from mild retinal pigment epithelial (RPE) disturbance to extensive atrophy and pigmentation. In some individuals retinal vessels were attenuated and in two subjects peripheral areas of retinal atrophy were present. An absent or severely reduced PERG was detected in all subjects, indicative of marked macular dysfunction. Full field ERG showed abnormal rod and cone responses. AF imaging revealed decreased macular AF centrally surrounded by a ring of increased AF in the majority of individuals. "Bull's eye" lesions were present in two individuals, comprising of a ring of decreased perifoveal AF bordered peripherally and centrally by increased AF. Photopic sensitivity testing demonstrated elevated central visual field thresholds with additional superior greater than inferior peripheral field loss. There were rod and cone sensitivity reductions in the central and peripheral visual fields, with the inferior retina being more affected than the superior.

CONCLUSIONS

The detailed phenotype is described of the autosomal dominant cone-rod dystrophy, CORD7, which is associated with a point mutation in RIM1, a gene encoding a photoreceptor synaptic protein. The pattern of disease progression and long term visual outcome facilitates improved genetic counselling and advice on prognosis. Such phenotypic data will be invaluable in the event of future therapy.

A review of sleeve fractures of the patella in children

Patella fractures are rare in children and the sleeve fracture is a particular form of fracture that only occurs in children where it is the most common patella fracture. It is caused by rapid muscle contraction. Diagnosis may be difficult both clinically and on the X-ray which may look normal if there is no bony fragment. Patella alta is the best sign and ultrasound is very helpful. Awareness of the existence of the injury is all important. This fracture differs from straightforward avulsion because of the "sleeve" of periosteum which is pulled off the patella and will continue to form bone if not treated thus enlarging or even duplicating the patella. Treatment involves prompt reduction and, usually, internal fixation of the disrupted patella tendon.

Purification, characterisation and intracellular localisation of aryl hydrocarbon interacting protein-like 1 (AIPL1) and effects of mutations associated with inherited retinal dystrophies

Mutations in AIPL1 are associated with Leber Congenital Amaurosis (LCA), a major cause of childhood blindness, yet the cellular function of the encoded protein has yet to be fully elucidated. In order to investigate the biochemistry of AIPL1, we have developed a system for the expression of the recombinant protein in bacteria and its subsequent purification. The secondary structure and thermostability of wild-type and mutant proteins have been examined by circular dichroism (CD) spectroscopy. Some of the variants, notably W278X and P376S, had markedly different secondary structure compositions, indicating that the proteins had not folded properly, whilst W278X and T114I were particularly thermolabile. When eukaryotic cells were transfected with the AIPL1 expression constructs, we show by immunofluorescence microscopy that wild-type protein is distributed throughout the nucleus and cytoplasm. Several of the mutants give similar results. With two of the disease-associated variants (W278X and A336Delta2), however, the protein remains in the cytoplasm in aggresome-like particles. These particles were shown to be ubiquitinated, indicating that the mutant protein had been tagged for proteosomal degradation. On this basis, we can conclude that wild-type protein is expressed in a soluble and folded manner, and that some of the disease-associated mutant proteins are nonfunctional because they are insoluble and are degraded by the cell. Other mutations appear to have a more localised effect on secondary structure, which does not result in insolubility or affect protein targeting, but reduces the stability of the protein at human body temperature.

Progressive cone dystrophy associated with mutation in CNGB3

PURPOSE

To determine the molecular basis for phenotypic variability in a three-generation consanguineous family containing a single individual with complete achromatopsia and three individuals with progressive cone dystrophy.

METHODS

Four affected individuals underwent ophthalmic examination, electrophysiological assessment, color fundus photography, and psychophysical testing. Blood samples were obtained for DNA extraction and mutation screening of the cone-specific cGMP-gated (CNG) channel protein gene CNGB3 was undertaken.

RESULTS

The clinical findings in one family member were consistent with a diagnosis of complete achromatopsia, with nystagmus, photophobia, and poor visual acuity from early infancy and complete color-blindness, normal fundi, and absent cone responses with normal rod responses on electroretinography (ERG). Mutation analysis revealed her to be homozygous for the common CNGB3 achromatopsia mutation, 1148delC (Thr383fs). In contrast, the three other symptomatic individuals in the family had findings consistent with progressive cone dystrophy. Their visual problems began later in childhood (ranging from 3 to 14 years of age) and there was evidence of progressive deterioration in cone function. All three had a marked tritanopic color vision defect and fundoscopy revealed bilateral macular atrophy. Electrophysiological testing of these three subjects demonstrated clear evidence of progressive deterioration of cone responses over time; rod responses were normal. All three individuals with this progressive phenotype were found to be compound heterozygotes for the 1148delC (Thr383fs) frameshift mutation and a novel Arg403Gln missense mutation in CNGB3.

CONCLUSIONS

Mutations in CNGB3, which have been shown to cause achromatopsia, are now shown to be associated with autosomal recessive progressive cone dystrophy. In this study, a novel Arg403Gln mutation was identified, located in the middle of the pore domain of the cone CNG cation channel beta-subunit, which when associated with the nonsense mutation Thr383fs, resulted in progressive cone dystrophy.

A Novel Amino Acid Substitution Is Responsible for Spectral Tuning in a Rodent Violet-Sensitive Visual Pigment

Cone short-wave (SWS1) visual pigments can be divided into two categories that correlate with spectral sensitivity, violet sensitive above 390 nm and ultraviolet sensitive below that wavelength. The evolution and mechanism of spectral tuning of SWS1 opsins are proving more complex than those of other opsin classes. Violet-sensitive pigments probably evolved from an ancestral ultraviolet-sensitive opsin, although in birds ultraviolet sensitivity has re-evolved from violet-sensitive pigments. In certain mammals, a single substitution involving the gain of a polar residue can switch sensitivity from ultraviolet to violet sensitivity, but where such a change is not involved, several substitutions may be required to effect the switch. The guinea pig, Cavia porcellus, is a hystricognathous rodent, a distinct suborder from the Sciurognathi, such as rats and mice. It has been shown by microspectrophotometry to have two cone visual pigments at 530 and 400 nm. We have ascertained the sequence of the short-wave pigment and confirmed its violet sensitivity by expression and reconstitution of the pigment in vitro. Moreover, we have shown by site-directed mutagenesis that a single residue is responsible for wavelength tuning of spectral sensitivity, a Val86Phe causing a 60 nm short-wave shift into the ultraviolet and a Val86Tyr substitution shifting the pigment 8 nm long wave. The convergent evolution of this mammalian VS pigment provides insight into the mechanism of tuning between the violet and UV.

Oligocone trichromacy: a rare and unusual cone dysfunction syndrome

AIM

To describe the phenotype of a case series of six patients with oligocone trichromacy.

METHODS

The six affected individuals underwent an ophthalmological examination, electrophysiological testing and detailed psychophysical assessment.

RESULTS

All six affected patients had a history of moderately reduced visual acuity (6/12 to 6/24) from infancy, not improved by full spectacle correction. They complained of mild photophobia and they were not aware of any colour vision deficiency. They had no nystagmus and fundi were normal. Electrophysiological testing revealed either absent/profoundly reduced cone flicker responses or preserved but delayed and mildly reduced flicker responses. Colour vision was found to be within normal limits, but some patients showed mildly elevated discrimination thresholds along all axes.

CONCLUSION

The largest case series to date of patients with oligocone trichromacy is presented. The electrophysiological findings suggest that there may be more than one disease mechanism. The mode of inheritance is likely to be autosomal recessive, and while previous reports have suggested that this disorder is stationary, in one of these families there is clinical evidence of progression.