Congenital stationary night blindness and a "Schubert-Bornschein" type electrophysiology in a family with dominant inheritance

BACKGROUND/AIMS

To present the clinical, psychophysical, and electrophysiological characteristics of a family with dominantly inherited congenital stationary night blindness (CSNB).

METHODS

Five affected family members from three generations were ascertained. Four affected individuals underwent ophthalmic examination and electrodiagnostic investigations. Three affected individuals also underwent scanning laser ophthalmoscopy and psychophysical testing.

RESULTS

Affected individuals reported night blindness from an early age. Visual acuities were normal. Fundal appearances were normal apart from one older patient showing areas of peripheral chorioretinal atrophy. Autofluorescence images showed no gross abnormality. International Society for Clinical Electrophysiology of Vision (ISCEV) standard electroretinography (ERG) showed undetectable rod specific responses and electronegative maximal responses, but normal ISCEV cone responses. Additional S-cone specific ERG recordings were of reduced amplitude in all patients studied. There was no apparent rod component to the dark adaptation curve. Central 30 degrees thresholds were normal under photopic conditions but showed increased thresholds under scotopic conditions for both red and blue stimuli.

CONCLUSION

Results from investigation of this family are consistent with an impairment of rod photoreceptor signalling. The ERG findings suggest an abnormality occurring after phototransduction with rod and S-cone pathway involvement. These findings differ from those rare families reported previously with dominant CSNB.

Photopic and scotopic VEPs in patients with congenital stationary night-blindness

Extended set of visual evoked potentials (VEPs) using pattern-reversal (PREPs), linear motion-onset and radial (expansion) motion-onset stimuli (M-VEPs) (detailed specification at http://www.lfhk.cuni.cz/elf) was used to verify congenital stationary night-blindness (CSNB) characteristics in 7 patients (compared to 7 age matched controls) in photopic conditions (luminance of 17 cd/m2). No differences were found in any of the M-VEPs, whilst PREPs displayed prolonged latencies in 3 of 7 CSNB patients. Additionally, the PREPs and M-VEPs were tested in 3 normal and 3 CSNB subjects (the only available ones from the original group) over large range of scotopic, mesopic and photopic luminances (from 0.0001 to 65.4 cd/m2). Both types of low luminance VEPs had distinctly increased luminance threshold needed for reliable VEPs eliciting in CSNB patients (0.06 cd/m2) when compared with controls (0.003 cd/m2); the VEP appearance threshold was almost identical with the perceptual threshold in both groups. Thus, our pilot study proved that CSNB can be objectively detected also via scotopic VEP examination. Since the prolonged PREP latencies at 17 cd/m2 normalised with luminance increase, it indicates that the lower luminance stimuli (compared to the standard recommended by ISCEV) can be more sensitive for some visual disorders detection.

ERGs in female carriers of incomplete congenital stationary night blindness (I-CSNB). A family report

ERG findings in five sisters are reported. By pedigree analysis, four of the five must be obligate carriers for I-CSNB since their sons were affected (impaired night vision, reduced visual acuity, variable ametropia, congenital nystagmus and ERG with both scotopic and photopic b-wave reduced amplitude). The fifth was childless at the time of examination and her ERG analysis was normal. Three of the four obligate carriers showed significant reduction in the sum of the OPs amplitude as previously reported as being an electrophysiological signs in female carriers: two without alteration in other ERG components and the third with association with a flicker ERG amplitude significantly increased. The fourth female carrier showed a normal sum of the OPs amplitude whereas the other b-wave ERG or flicker amplitudes were significantly decreased. These last two ERG results suggest a possible modifications of synaptic transmission at a post-receptoral site (outer plexiform layer or involvement of the bipolar pathways) in these two carriers.

[Congenital stationary night blindness]

Authors present diagnostic procedures and dinical features of the congenital stationary night blindness (CSNB) in 19 patients. Effective therapy of the concomitant ophthalmological pathologies usually improves patient's vision ability.

Abnormalities of the scotopic threshold response correlated with gene mutation in X-linked retinoschisis and congenital stationary night blindness

STRs and dark-adapted ERGs were recorded in nine normal subjects, nine patients with XLRS, 11 patients with CSNB1 and one patient with CSNB2. In XLRS STR amplitude was significantly lower than normal at every intensity, but the response could be recorded in every patient and the maximum amplitude response was outside the 95% confidence limits in only four of the nine patients. STRs were significantly poorer in patients with CSNB and a responses was not measurable at any intensity in nine of the 11 patients with CSNB1. In both CSNB and XLRS the STR could only be recorded at higher stimulus intensities, suggesting reduced sensitivity of the STR. In XLRS onset and peak latencies were also significantly prolonged and the slope of the intensity-response functions for amplitude and onset latency differed significantly from normal. Maximum STR amplitude did not correlate with the maximum dark-adapted ERG response. The finding of abnormal STRs and dark adapted ERGs in all three dystrophies indicates that the different causative genes must have similar effects on the rod On-bipolar cell pathway. But there were also differences between the three clinical groups, particularly in the greater severity of the abnormality in CSNB1, which suggests that there may be multiple sites of abnormality.

Coats-like lesions in Usher syndrome type II

BACKGROUND

An unusual case of Usher syndrome type II associated with bilateral Coats-like exudative retinopathy is described.

METHODS

A 14-year-old boy with congenital sensorineural deafness and normal vestibular functions presented with a recent history of night blindness. He was followed for 3 years with fundus photography, intravenous fluorescein angiography, electroretinography and audiometric testings. His parents refused any form of treatment.

RESULTS

Fundoscopy showed bilateral retinitis pigmentosa and a single focus of subretinal exudation and overlying telangiectatic retinal vessels inferotemporal to the vascular arcade in the right eye. He had bilateral mild macular edema. A year later, a similar lesion developed inferotemporally in the left fundus. Electroretinography responses, particularly the rod-mediated signals, were significantly reduced. Audiometric studies documented hearing loss in high frequencies. His visual acuity declined from 20/40 to 20/80 RE and from 20/80 to 20/100 LE during follow-up. No new lesions developed.

CONCLUSIONS

Coats-type exudative lesions may develop in patients with Usher syndrome type II. Although left untreated, only a minimal increase in exudation occurred over 3 years.

Specific isomerization of rhodopsin-bound 11-cis-retinal to all-trans-retinal under thermal denaturation

The natural ligand of the retinal photoreceptor rhodopsin, 11-cis-retinal, is isomerized to its all-trans configuration as a consequence of light absorption in the first step of the visual phototransduction process. Here we show, by means of difference spectroscopy and high-performance liquid chromatography analysis, that thermal denaturation of rhodopsin induces the same type of isomerization. This effect is likely due to thermally induced conformational rearrangements of amino acid residues in the retinal-binding pocket--possibly implying helical movements--and highlights the tight coupling between 11-cis-retinal and opsin. This effect could have implications in the instability and functional changes seen for certain mutations in rhodopsin associated with retinal disease, and in the stability of the different conformers induced by mutations in other G protein-coupled receptors.

NYX (nyctalopin on chromosome X), the gene mutated in congenital stationary night blindness, encodes a cell surface protein

PURPOSE

The complete type of X-linked congenital stationary night blindness (CSNB1) in human and mouse is caused by mutations in the NYX gene. The human NYX protein has been predicted to contain an N-terminal endoplasmic reticulum (ER) signaling sequence and a C-terminal glycosylphosphatidylinositol (GPI) anchor. In the current study, these computer predictions were verified experimentally by expression of domain-specific cDNA constructs in COS-7 and HeLa cells. Moreover, computer-based analysis of the orthologue mouse amino acid sequence did not reveal a GPI anchor, which may result in a different protein localization compared with human NYX. Therefore, the cellular localization for the mouse Nyx protein was also examined.

METHODS

A new method was established that differentially visualizes both the protein at the surface of the living cell and subsequently in intracellular compartments. The localization of the human and mouse V5-tagged wild-type and mutant NYX protein were studied.

RESULTS

Human and mouse V5-NYX proteins were dispersed in the form of speckles over the entire cell surface. Subsequent staining of the same cells after detergent extraction revealed that V5-NYX located to the ER and Golgi apparatus. Deletion of the GPI anchor domain of NYX resulted in a time-dependent loss of V5-NYX from the surface of living cells and accumulation of this truncated protein in the ER and Golgi apparatus. Deletion of the ER signal sequence in Nyx delocalized the intracellular V5-Nyx protein and caused its dispersion in the cytosol. Furthermore, mutations introduced in the leucine-rich repeat (LRR)-region, which has been described as a pathogenic variant of NYX, had no effect on subcellular localization of the protein.

CONCLUSIONS

These data provide evidence that human and mouse nyctalopin are membrane-bound extracellular proteins and are functionally conserved.

Opsin activation as a cause of congenital night blindness

Three different mutations of rhodopsin are known to cause autosomal dominant congenital night blindness in humans. Although the mutations have been studied for 10 years, the molecular mechanism of the disease is still a subject of controversy. We show here, using a transgenic Xenopus laevis model, that the photoreceptor cell desensitization that is a hallmark of the disease results from persistent signaling by constitutively active mutant opsins.

Macular dystrophy in a Japanese family with fundus albipunctatus

PURPOSE

To report a Japanese family with fundus albipunctatus and macular dystrophy associated with a mutation in the 11-cis retinol dehydrogenase (RDH5) gene.

DESIGN

Observational case report.

METHOD

Ophthalmic examinations and DNA analysis were performed.

RESULTS

The fundi of a 56-year-old man and his 51-year-old sister showed numerous yellow-white punctata. He also had bull's-eye maculopathy and prepappillary arterial loops, whereas she did not, and his best-corrected visual acuity was impaired, whereas hers was normal. Their kinetic visual fields did, however, show central or paracentral scotoma, and both had tritanomalous color vision. Their scotopic electroretinograms were typical of fundus albipunctatus, and photopic electroretinograms were significantly reduced. A homozygous Gly107Arg mutation in the RDH5 gene was detected in both siblings.

CONCLUSIONS

We suggest that the macular dystrophy is caused by the RDH5 gene mutation as a phenotype variation in fundus albipunctatus.

A novel CACNA1F mutation in a french family with the incomplete type of X-linked congenital stationary night blindness

PURPOSE

To describe a French family with the incomplete type of X-linked congenital stationary night blindness (CSNB2) associated with a novel mutation in the retina-specific calcium channel alpha(1) subunit gene (CACNA1F).

DESIGN

Interventional case report.

METHODS

Two family members with a history of nonprogressive night blindness and subnormal visual acuity were clinically examined and the genotype determined by molecular genetic analysis.

RESULT

Both patients had clinical manifestations characteristic of CSNB2. Electrophysiologically, we found a predominant reduction of the ERG B-wave in the maximal response. Both rod and cone function were subnormal, with the latter tending to be more attenuated. We identified a C deletion at nucleotide position 4548, resulting in a frameshift with a predicted premature termination at codon 1524.

CONCLUSIONS

The clinical and genetic study of a novel mutation in the CACNA1F gene adds further support to the contention that CSNB2 represents a genetically distinct retinal disorder of a calcium channel.

Isolation of the mouse nyctalopin gene nyx and expression studies in mouse and rat retina

PURPOSE

It has been shown recently that mutations in NYX (nyctalopin on chromosome X), encoding a novel protein associated with the leucine-rich repeat (LRR) protein superfamily, are responsible for the complete form of X-linked congenital stationary night blindness (CSNB1). This study describes the isolation and molecular characterization of the mouse orthologue Nyx and its expression pattern in the retina.

METHODS

Nyx was isolated by conventional DNA library screening and polymerase chain reaction (PCR)-based approaches. Gene expression in different mouse tissues was studied by RT-PCR. Subsequently, the expression pattern of Nyx and its gene product in mouse and rat retinas was investigated by RNA in situ hybridization and immunohistochemistry with Nyx-specific antibodies.

RESULTS

The Nyx gene encodes a protein of 476 amino acids that contain 11 consecutive LRR motifs flanked by amino- and carboxyl-terminal cysteine-rich LRRs. At the amino acid level, Nyx is highly homologous to its human orthologue (86% identity). The gene is expressed in the eye but also, at lower levels, in brain, lung, spleen, and testis. Nyx expression was found during all stages of postnatal retinal development and was confined to cells of the inner nuclear layer and the ganglion cell layer in adult mouse and rat retinas.

CONCLUSIONS

These data suggest an important function of the Nyx protein in the inner retina and provide evidence that CSNB1 is based on a defect in the inner retinal circuitry.

Characterization of rhodopsin congenital night blindness mutant T94I

The Thr94 --> Ile mutation in the second transmembrane segment of rhodopsin has been reported to be associated with a congenital night blindness phenotype in a large Irish pedigree. Previously, two other known rhodopsin mutants that cause congenital night blindness, A292E and G90D, have been shown in vitro to constitutively activate the G protein transducin in the absence of a chromophore. The proposed mechanism of constitutive activation of these two mutants is an electrostatic disruption of the active site salt bridge between Glu113 and Lys296 that contributes to stabilization of the protein in the inactive state. Here, the T94I rhodopsin mutant is characterized and compared to the two other known rhodopsin night blindness mutants. The T94I mutant opsin is shown also to constitutively activate transducin. The T94I mutant pigment (with a bound 11-cis-retinal chromophore), like the other known rhodopsin night blindness mutants, is not active in the dark and has wild-type activity upon exposure to light. Similar to the Gly90 --> Asp substitution, position 94 is close enough to the Schiff base nitrogen that an Asp at this position can functionally substitute for the Glu113 counterion. However, in contrast to the other night blindness mutants, the T94I MII intermediate decays with a half-life that is approximately 8-fold slower than in the wild-type MII intermediate. Thus, the one phenotype shared by all congenital night blindness mutants that is different from the wild-type protein is constitutive activation of the apoprotein.

Unusual thermal and conformational properties of the rhodopsin congenital night blindness mutant Thr-94 --> Ile

Naturally occurring point mutations in the opsin gene cause the retinal diseases retinitis pigmentosa and congenital night blindness. Although these diseases involve similar mutations in very close locations in rhodopsin, their progression is very different, with retinitis pigmentosa being severe and causing retinal degeneration. We report on the expression and characterization of the recently found T94I mutation associated with congenital night blindness, in the second transmembrane helix or rhodopsin, and mutations at the same site. T94I mutant rhodopsin folded properly and was able to bind 11-cis-retinal to form chromophore, but it showed a blue-shifted visible band at 478 nm and reduced molar extinction coefficient. Furthermore, T94I showed dramatically reduced thermal stability, extremely long lived metarhodopsin II intermediate, and highly increased reactivity toward hydroxylamine in the dark, when compared with wild type rhodopsin. The results are consistent with the location of Thr-94 in close proximity to Glu-113 counterion in the vicinity of the Schiff base linkage and suggest a role for this residue in maintaining the correct dark inactive conformation of the receptor. The reported results, together with previously published data on the other two known congenital night blindness mutants, suggest that the molecular mechanism underlying this disease may not be structural misfolding, as proposed for retinitis pigmentosa mutants, but abnormal functioning of the receptor by decreased thermal stability and/or constitutive activity.

Mutations in the CACNA1F and NYX genes in British CSNBX families

X-linked congenital stationary night blindness (CSNBX) is a genetically and phenotypically heterogeneous non-progressive disorder, characterised by impaired night vision but grossly normal retinal appearance. Other more variable features include reduction in visual acuity, myopia, nystagmus and strabismus. Genetic mapping studies by other groups, and our own studies of British patients, identified key recombination events indicating the presence of at least 2 disease genes on Xp11. Two causative genes (CACNA1F and NYX) for CSNBX have now been identified through positional cloning strategies. In this report, we present the results of comprehensive mutation screening in 14 CSNBX families, three with mutations in the CACNA1F gene and 10 with mutations in the NYX gene. In one family we failed to identify the mutation after testing RP2, RPGR, NYX and CACNA1F. NYX gene mutations are a more frequent cause of CSNBX, although there is evidence for founder mutations. Our report of patient population mutation screening for both CSNBX genes, and our exclusion of RP2 and RGPR, indicates that mutations in CACNA1F and NYX are likely to account for all CSNBX.

[Establishment of the concept of new clinical entities--complete and incomplete form of congenital stationary night blindness]

I summarized our long-term study to prove that the complete and incomplete types of congenital stationary night blindness (CSNB) are different clinical entities and that the latter is a newly identified disease which has never been reported in the past. CSNB with normal fundi and negative electroretinogram (ERG) showing selective reduction of the b-wave was previously known as the "Schubert-Bornschein type". For the sake of convenience, we classified the disease into two types according to the absence or the presence of rod function: complete CSNB and incomplete CSNB. The hereditary mode of the former is X-linked recessive and autosomal recessive, while that of the latter is X-linked recessive. They are never found together in a single family. We found several additional differences between the two types, including ERG oscillatory potentials, cone mediated ERG, and refractive errors, all leading us to hypothesize that the two types are not variants of a single disease but are the sum of two different clinical entities. Our hypothesis has recently been proven true by molecular genetical analysis. Namely, the mutated gene in X-linked recessive complete CSNB was found in the nyctalopin (NYX) gene, while that in incomplete CSNB was found in the calcium channel (CANCA1F) gene which encodes the retina-specific calcium channel alpha 1-subunit. These results proved that complete and incomplete CSNB are different clinical entities and that the latter is the first disease of the eye which discloses mutation of this region. We classified 90 patients to include 49 complete and 41 incomplete types. Fifteen incomplete CSNB patients underwent gene analysis and they all showed mutation of the CACNA1F gene. We also examined for gene mutation in several patients who had progressive retinal disease and negative ERG and found two siblings with CANA1F gene mutation. This finding indicates that the mutation of the CACNA1F gene can also cause progressive retinal disease in addition to incomplete CSNB. Gene analysis of 11 patients with complete CSNB was performed and 6 revealed mutation of the NYX gene. The remaining 5 patients showed neither NYX nor CACNA1F gene mutation, suggesting they are of autosomal recessive complete CSNB where gene mutation has not been identified. The comparison of our phenotype and genotype diagnosis indicated that a precise ERG analysis can provide correct differentiation between complete and incomplete types. Other clinical findings include moderately low visual acuity in both types, high or moderate myopia in complete CSNB, and wide distribution from myopia to hyperopia in incomplete CSNB. Pathophysiology studies using clinical patients and animal models suggested that complete CSNB has a complete defect of the ON-bipolar cells or their synapses in the rod and cone visual pathways, leaving the OFF pathway intact (OFF-retina). On the other hand, the incomplete CSNB has an incomplete defect of the ON and OFF bipolar cells or their synapses in the rod and cone visual pathways. The macular function is relatively well preserved in both types, which was shown by focal macular ERG. The incomplete CSNB patients seldom complain of night blindness, which causes us to overlook this disease because we then tend not to perform ERG testing. This disease is not so rare and clinicians should be more aware of its existence. The incomplete CSNB is a new hereditary retinal disease detected by Japanese investigators just like the Oguchi disease, and it has much unknown pathophysiology which needs to be identified in the future. Since the namings of complete and incomplete CSNB may be misunderstood as indicating functional classification of one disease, it has been proposed internationally to change the name "complete type" to CSNB1 and that of "incomplete type" to CSNB2.

The eye photoreceptor protein rhodopsin. Structural implications for retinal disease

Rhodopsin is the membrane receptor responsible for photoreception in the vertebrate retina. Its characteristic seven-transmembrane helical structural motif is today widely recognised as a paradigm in signal transduction. Rhodopsin and the phototransduction system are frequently used as structural and mechanistic models for the G-protein coupled receptor superfamily. Recent advances in the activation mechanism (as derived from the structural available data) and the implications for normal and pathological - in retinal disorders - visual function will be reviewed.

Incomplete congenital stationary night blindness associated with symmetrical retinal atrophy

PURPOSE

To describe a Japanese patient with incomplete congenital stationary night blindness (iCSNB) with atypical retinal atrophy and kinetic visual field defects.

METHODS

An ophthalmologic examination was performed, and the CACNA1F gene was analyzed by direct genomic sequencing.

RESULTS

The patient had a hemizygous Arg913stop mutation in CACNA1F and had electroretinographic changes that were typical of iCSNB. The fundus had atrophic retinal lesions around the inferior vascular arcades OU, and Goldmann kinetic perimetry showed relative scotomas in the corresponding areas.

CONCLUSIONS

Although most patients with iCSNB show essentially normal fundi without visual field defects, this case demonstrated retinal atrophy associated with visual field defects indicating a phenotypic heterogeneity induced by the CACNA1F mutation.

Abnormalities of the long flash ERG in congenital stationary night blindness of the Schubert-Bornschein type

We investigated abnormalities of the retinal cone ON- and OFF-pathways in 24 males with Schubert-Bornschein congenital stationary night blindness (CSNB). Substantial differences were found between both CSNB types. In incomplete type, a-, b- and d-waves were reduced and delayed, whereas in complete type only the b-wave showed significant changes. Oscillatory potentials (OPs) were not discernible from noise in incomplete CSNB and showed significant peak alterations of the ON-OPs only in complete CSNB. In the complete CSNB type, the ON pathway appeared to be mainly affected. In the incomplete CSNB form marked involvement of both the ON and the OFF pathways was noted.

Role of the beta(2) subunit of voltage-dependent calcium channels in the retinal outer plexiform layer

PURPOSE

Mutations in the alpha(1F) subunit of voltage-dependent calcium channels (VDCCs) have been shown to cause incomplete congenital stationary night blindness (CSNB2). The purpose of this study was to dentify which of the four beta subunits of VDCCs participates in the formation of this channel at the photoreceptor synapse and to determine how its absence affects visual processing.

METHODS

Mice without each of the four known beta subunits of VDCCs were generated by gene targeting and transgenic rescue (CNS-beta(1), -beta(2)) or by gene targeting alone (beta(3)) or were obtained from a commercial provider (beta(4)). Retinal function and visual sensitivity were examined by electroretinography and an active avoidance behavioral test, respectively. The structure of the retina and expression of the alpha(1F) subunit were examined at the light microscopic level and by immunohistochemistry.

RESULTS

Under dark-adapted conditions, CNS-beta(2)-null mice had a normal ERG a-wave, but did not have a normal b-wave. In addition, these mice showed decreased sensitivity to light. Both the a- and b-waves appear normal in the CNS-beta(1)-, beta(3)-, and beta(4)-null mice. Histologic analyses of all four mouse lines indicated that only the CNS-beta(2)-null mice had altered retinal morphology. Eyes of these mice had a thinner outer plexiform layer (OPL) than eyes of control animals. In addition, the labeling pattern of the alpha(1F) subunit in the OPL was altered in CNS-beta(2)-null mice.

CONCLUSIONS

The normal distribution of the alpha(1F) subunit of the VDCCs in the OPL is dependent on the expression of the beta(2) subunit. The expression of both of these subunits is required for normal maintenance and/or formation of the OPL and synaptic transmission.

A distinctive form of congenital stationary night blindness with cone ON-pathway dysfunction

PURPOSE

To characterize a distinctive form of congenital stationary night blindness (CSNB).

DESIGN

Observational case report.

PARTICIPANTS

A 30-year-old male with a history of night blindness, several members of his family, a patient with "complete" congenital stationary night blindness (CSNB1), and groups of age-similar control subjects.

METHODS

Rod-system function was evaluated by measuring psychophysical dark-adapted thresholds, by recording dark-adapted electroretinograms (ERGs), and by fundus reflectometry. Cone-system function was evaluated by recording light-adapted ERGs, including those to sawtooth flicker, and by recording light-adapted visually evoked potentials (VEPs) to luminance increments and decrements.

MAIN OUTCOME MEASURES

Dark-adapted thresholds, ERGs, rhodopsin double densities, Goldmann visual fields, and VEPs.

RESULTS

The patient's visual acuity, visual fields, and color vision were normal. His peripheral dark-adapted thresholds were rod-mediated but elevated by approximately 3 log units above normal. Rhodopsin double density and bleaching recovery were normal. His dark-adapted maximal-flash ERG showed a "negative" waveform, in which the b-wave was more reduced in amplitude than the a-wave, although the a-wave amplitude was also reduced. The rod photoreceptors contributed to the patient's dark-adapted ERGs, as illustrated by the unequal responses to cone-matched stimuli. The patient's cone-mediated thresholds for long-wavelength stimuli were within the normal range. However, his light-adapted brief-flash b-wave was abnormal in amplitude and implicit time. Selective abnormalities of the ON responses of the cone system were apparent in the patient's reduced b-wave amplitude to rapid-on flicker with a normal response to rapid-off flicker, and his prolonged VEP latencies to increments but not to decrements.

CONCLUSIONS

The overall pattern of findings distinguishes this patient from previously described forms of CSNB. The results suggest that two factors likely contribute to the patient's night blindness: (1) a rod phototransduction defect and (2) a postreceptoral defect. The results also indicate dysfunction within the cone ON pathway.

Macular dystrophy in a 9-year-old boy with fundus albipunctatus

PURPOSE

To report a 9-year-old boy with fundus albipunctatus and macular dystrophy.

DESIGN

Observational case report.

METHODS

A complete ophthalmic examination was performed. The 11-cis retinol dehydrogenase gene (RDH5) was examined by direct genomic sequencing.

RESULTS

The fundi of the 9-year-old boy showed numerous yellow-white punctata as well as foveal atrophic lesions in both eyes. His corrected visual acuity was RE: 0.5 and LE: 0.3. Scotopic full-field electroretinograms were not present after 20 minutes of dark-adaptation but were normal after 3 hours of dark-adaptation. Full-field cone and 30-Hz flicker electroretinograms were normal; however, focal macular cone electroretinograms were significantly reduced. A compound heterozygous mutation of Tyr281His and Leu310GluVal in RDH5 was detected.

CONCLUSION

We suggest that the macular dystrophy is caused by the RDH5 mutation as a phenotype variation in fundus albipunctatus.

Expression of the alpha1F calcium channel subunit by photoreceptors in the rat retina

PURPOSE

The CACNA1F gene encodes a voltage-gated calcium channel alpha1 subunit, alpha1F, which is expressed in the human retina. Mutations in this gene cause incomplete X-linked congenital stationary night blindness (CSNB2). The aim of this study was to obtain the sequence of the rat alpha1F cDNA and localize the encoded polypeptide in the rat retina.

METHODS

The full-length rat alpha1F sequence was compiled from sequencing of overlapping alpha1F PCR fragments amplified from rat retinal cDNA. Antiserum was raised against a human alpha1F peptide. It was found that the human alpha1F peptide used to generate the antiserum was conserved at only 11 out of 19 residues in the cloned rat sequence. Therefore, antibodies were affinity purified against either the human alpha1F peptide or the equivalent rat peptide and used for immunofluorescent staining of rat retina sections.

RESULTS

The rat alpha1F amino acid sequence was found to be 91% and 95% identical to the human and mouse alpha1F sequences, respectively. Antibodies affinity purified against the human alpha1F peptide stained both the outer nuclear layer (ONL) and outer plexiform layer of rat retina sections. In contrast, staining with antibodies affinity purified against the corresponding rat alpha1F peptide was restricted to the ONL.

CONCLUSIONS

The rat alpha1F amino acid sequence is highly homologous to the human and mouse sequences. The immunohistochemical results indicate the existence of distinct alpha1F isoforms or alpha1F-like channels, which are differentially distributed in the cell bodies and synaptic terminals of photoreceptors in the rat retina.

Case populations must match the respective disease model: Genotype diversity causes linkage disequilibrium mapping failure in monogenic disorders

Traditional linkage analysis in large families is the most promising approach for mapping disease genes of monogenic heritable disorders when the number of informative meioses is sufficient. With rare diseases, however, the low availability of informative pedigrees poses a significant limitation. As an adjunct to family linkage methods, association studies based on the investigation of individual haplotypes from a number of unrelated patients (i.e. linkage disequilibrium analysis) have recently been employed in mapping hereditary disease loci. However, such haplotype analysis is hampered by a number of effects that influence statistical evaluation, e.g. i) population history and size, ii) allele and haplotype frequencies in the respective population(s), iii) heterogeneous mutation and natural selection processes, and iv) small sample sizes of patient groups. The purpose of the present study was to determine the utility and limitations of haplotype-based genetic mapping in estimating the location of the NYX gene, which has recently been identified as the causative gene for a rare inherited retinal disorder known as the complete type of X-linked congenital stationary night blindness (CSNB1). For this purpose we recapitulated haplotypes and tested for linkage disequilibrium in 20 unrelated male CSNB1 patients from three European populations and 44 healthy individuals. All subjects were genotyped for 17 polymorphic microsatellite loci covering the Xp11.4 region with an average marker density of approximately 0.29 cM. We found that a precise model to describe mutations at loci that erroneously break up linkage is highly required, and that the case population must match the respective disease model.