Increased phase lag of the fundamental harmonic component of the 30 Hz flicker ERG in Schubert-Bornschein complete type CSNB

The 30 Hz flicker electroretinogram (ERG) response was studied in seven patients with Schubert-Bornschein complete-type congenital stationary night blindness (SB-CSNB) by measuring conventional peak implicit times and by harmonic analysis. Responses were elicited with xenon photostrobe flashes. The fundamental flicker component showed a significant increased phase lag (P = 0.002), even though the peak implicit times were not delayed (P = 0.22). Although others have noted a "squared-off" flicker waveform in SB-CSNB, this is the first quantitative demonstration of a temporal abnormality in the flicker ERG and is the first report that a retinal disease can delay the fundamental flicker harmonic while the peak implicit time remains normal. Similarly, the phase of the flicker fundamental harmonic in monkey was also delayed after intravitreal 2-amino-4-phosphonobutyric acid (APB) that blocks the activity of depolarizing bipolar cells (DBC), whereas peak implicit time remained virtually unchanged. These observations, coupled with the normal amplitude and slope of the photopic a-wave (which was recorded under conditions that elicit a substantial contribution from hyperpolarizing bipolar cells) supports the hypothesis that the abnormality in the DBC pathway in SB-CSNB lies post-synaptic to the cones and not in the presynaptic glutamate release by cone photoreceptors.

Defects in the rhodopsin kinase gene in the Oguchi form of stationary night blindness

Oguchi disease is a recessively inherited form of stationary night blindness due to malfunction of the rod photoreceptor mechanism. Patients with this disease show a distinctive golden-brown colour of the fundus that occurs as the retina adapts to light, called the Mizuo phenomenon. Recently a defect in arrestin, a member of the rod phototransduction pathway, was found to cause this disease in some Japanese patients. As rhodopsin kinase works with arrestin in shutting off rhodopsin after it has been activated by a photon of light, it is reasonable to propose that some cases of Oguchi disease might be caused by defects in rhodopsin kinase. This report describes an analysis of the arrestin and rhodopsin kinase genes in three unrelated cases of Oguchi disease. No defects in arrestin were detected, but all three cases had mutations in the rhodopsin kinase gene. Two cases were found to be homozygous for a deletion encompassing exon 5, predicted to lead to a nonfunctional protein. The third case was a compound heterozygote with two allelic mutations, a missense mutation (Val380Asp) affecting a residue in the catalytic domain, and a frameshift mutation (Ser536(4-bp del)) resulting in truncation of the carboxy terminus. Our results indicate that null mutations in the rhodopsin kinase gene are a cause of Oguchi disease and extend the known genetic heterogeneity in congenital stationary night blindness.

The molecular basis of disorders caused by defects in G proteins

G proteins couple receptors for many hormones to effectors that regulate second messenger metabolism. G protein dysfunction could involve gain or loss of function. For Gs, the G protein that couples receptors to stimulation of cAMP formation, examples of both types have already been defined. Germline loss of function mutations in the Gs(alpha) gene are responsible for a form of inherited hormone resistance termed pseudohypoparathyroidism (Albright hereditary osteodystrophy). Conversely, somatic gain of function mutations cause constitutive stimulation of cAMP, independent of receptor activation, in acromegaly, in hyperfunctional thyroid nodules, and in the McCune-Albright syndrome. Future work is likely to uncover additional disorders caused by defective G proteins with implications for diagnosis and treatment.

Spectroscopic evidence for altered chromophore--protein interactions in low-temperature photoproducts of the visual pigment responsible for congenital night blindness

The replacement of Gly90 by Asp in human rhodopsin causes congenital night blindness. It has been suggested that the molecular origin for the trait is an altered electrostatic environment of the protonated retinal Schiff base chromophore. We have investigated the corresponding recombinant bovine rhodopsin mutant G90D, as well as the related mutants E113A and G90D/E113A, using spectroscopy at low temperature. This allows the assessment of chromophore-protein interactions under conditions where conformational changes are mainly restricted to the retinal-binding site. Each of the mutant pigments formed bathorhodopsin- and isorhodopsin-like intermediates, but the concomitant visible absorption changes reflected differences in the electrostatic environment of the protonated Schiff base in each pigment. Fourier transform infrared-difference spectroscopy revealed effects on the chromophore fingerprint and hydrogen-out-of-plane vibrational modes, which were indicative of the removal of an electrostatic perturbation near C12 of the retinal chromophore in all three mutants. A comparison of the UV-visible and infrared-difference spectra of the mutant pigments strongly suggests that Glu113 is stably protonated in G90D. The corresponding carbonyl-stretching mode is assigned to a band at 1727 cm-1. In contrast to the case in native bathorhodopsin, the all-trans-retinal chromophores in the primary photoproducts of the mutant pigments are essentially relaxed. The peptide carbonyl vibrational changes in mutants G90D and G90D/ E113A suggest that this is due to a more flexible retinal-binding site. Therefore, the steric strain exerted on the chromophore in native bathorhodopsin may be caused by electrostatic forces that specifically involve glutamate 113.

Conclusive evidence for a distinct congenital stationary night blindness locus in Xp21.1

X linked congenital stationary night blindness (CSNBX) is a non-progressive retinal disorder characterised by decreased visual acuity and disturbance of night vision. CSNBX appears to be not only clinically but also genetically heterogeneous. On studying a single large family, we recently suggested the presence of a distinct locus for CSNBX in Xp21.1. Here, we describe the results of a linkage analysis in another large CSNBX family, which confirms this finding. Thus, the data presented here provide conclusive evidence for a distinct CSNBX locus in Xp21.1, closely linked to the X linked retinitis pigmentosa type 3 gene. The results combined with other published results indicate the order Xpter-DXS451-DMD-DYS1-(DXS1110, CSNBX1, XLRP3)-DXS7-(CSNBX2, XLRP2)-DXS14-Xcen.

[Congenital stationary night blindness]

The clinical and electrophysiological data in 48 cases of congenital stationary night blindness (CSNB) are analysed. The existence of the complete (with only the cone part of the dark adaptation curve) and incomplete (some rod activity also present) form of this anomaly has been confirmed. The Schubert-Bornschein's type of ERG responses corresponds to the complete CSNB. After 12 years of observation period in some patients the diminution of both the a and b potential of the ERG curves but not the extinction of the ERG potentials has been found. PERG curves and PVEP responses in CSNB are normal. Two pedigrees of CSNB are presented. The first reflects the autosomal dominant mode of heredity in 4 generations, in the second pedigree (5 generations) it is not possible to estimate the mode of heredity.

Undetectable S cone electroretinogram b-wave in complete congenital stationary night blindness

AIMS

The short wavelength sensitive (S) cone electroretinograms (ERGs) were examined in two patients with the complete type of congenital stationary night blindness (CSNB).

METHODS

Ganzfeld spectral flashes in the presence of strong white adapting fields were used to elicit the S cone ERGs.

RESULTS

The S cone ERG b-wave was not detectable to short wavelength stimuli, while the mixed long (L) and middle (M) wavelength sensitive cone responses appeared normal in waveforms with normal amplitude in both patients. Both patients had normal colour vision on the Farnsworth Panel D-15.

CONCLUSIONS

These ERG results indicated that the S cone system as well as rod system is more impaired in complete CSNB than the L and M cone system and that normal colour vision may not depend on a normal S cone ERG to full field stimuli.

Missense mutation in the gene encoding the alpha subunit of rod transducin in the Nougaret form of congenital stationary night blindness

Patients with congenital stationary night blindness enjoy normal daytime vision, which is mediated by cone photoreceptors, but are blind when ambient light is so dim that a normal individual would utilize only rod photoreceptors to see without colour discrimination. The disease is genetically heterogeneous. One form of dominantly inherited congenital night blindness is eponymously named "Nougaret' because pedigree analysis reveals that the disease originated in Jean Nougaret (1637-1719), a butcher who lived in Vendémian in southern France. Here we report that his affected descendants carry a missense mutation in the gene encoding the alpha subunit of rod transducin the G-protein that couples rhodopsin to cGMP-phosphodiesterase in the phototransduction cascade. Based on these results, rod transducin joins rhodopsin and the beta subunit of rod cGMP-phosphodiesterase to become the third component of the rod phototransduction cascade where a defect is implicated as a cause of stationary night blindness. Interestingly, the amino acid residue in transducin affected by the Nougaret mutation is in the position homologous to that affected by the oncogenic mutation originally reported in p21ras, a distant relative in the G-protein superfamily.

Electrophysiological findings in patients with Oguchi's disease

Seven patients with Oguchi's disease were examined by conventional full-field rod and cone electroretinograms (ERGs), spectral characteristic studies with photopically matched short- and long-wavelength stimuli (3 of the 7 patients) and electrooculogram (EOG) (6 patients). ON and OFF photopic ERG responses, recorded with rectangular stimuli, in 3 patients were compared with those of 13 normal subjects and 13 patients with complete congenital stationary night blindness (CSNB). Conventional full-field ERGs recorded after 30 minutes of dark adaptation revealed absent rod, but essentially normal cone and flicker ERGs in all patients. With the dark-adapted standard flash ERG, the a-wave amplitude was significantly lower than normal in all patients. The spectral characteristic study indicated that the ERG recorded after 30 minutes of dark adaptation represented cone activity. The EOG was abnormal in all patients examined. The ON and OFF responses of the photopic ERG were normal in Oguchi's disease patients; however the ON responses were reduced in patients with complete CSNB. The absence of a rod a-wave, with the abnormal EOG, strongly suggests that the rod itself is abnormal in Oguchi's disease. This finding differs from the report by Carr and Gouras (1965) and Carr and Ripps (1967). The normal ON and OFF responses in the photopic ERGs of Oguchi's disease patients contrast with those of the complete CSNB patients in whom only the ON response is reduced.

[Electrophysiologic tests for diagnosis of congenital night blindness]

Electrophysiological study in diagnosis of congenital stationary night blindness.

MATERIAL AND METHODS

The goal of this study was to describe a family with X-linked congenital stationary night blindness (CSNB) in which 3 brothers had similar symptoms: night blindness, reduced visual acuity and "negative" type of Flash ERG.

RESULTS

Electrophysiological study permitted to differentiate CSNB from generalized progressive retinal degeneration, what has a prognostic value for these patients.

On- and off-responses in the photopic electroretinogram in complete-type congenital stationary night blindness

We examined the on- and off-responses of the photopic electroretinogram in patients with complete congenital stationary night blindness. Standard flash electroretinograms as well as those produced in a ganzfeld modified for long-duration light stimuli (500 msec) permitted the separation of on- and off-responses in four patients and four normal subjects. The amplitude and latency of the elctroretinogram on-response (a- and b-waves) and off-response (d-wave) in addition to the oscillatory potentials of the off-response in normal subjects and patients were compared. The abnormal on-response was demonstrated in all the patients, and the off-response with its oscillatory potentials were preserved. We showed that the second portion of the off-response (of inner retinal origin) is normal. If congenital stationary night blindness is a defect of depolarizing bipolar cells, these results preclude input of the depolarizing bipolar cells and support the hyperpolarizing bipolar cells as the cellular origin of the off-response electroretinogram.

Congenital stationary nightblindness in a patient with osteopetrosis

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The electroretinographic diagnosis of the incomplete form of congenital stationary night blindness

Fifteen patients with the incomplete form of congenital stationary night blindness (iCSNB) were reviewed to better characterize their electroretinographic (ERG) findings in view of differential diagnosis with other retinal conditions also presenting with negative bright-flash ERG responses. In all 15 patients, in dark-adapted conditions, the bright-flash ERG response had a normal a-wave followed by a subnormal b-wave. Oscillatory potentials (OPs) observed on the ascending limb of the b-wave, although delayed in implicit time, were of large amplitude. The response to a long-wavelength stimulus showed cone-related components and some well-delineated OPs. On the other hand, the photopically elicited cone responses were strongly abnormal, with a subnormal a-wave followed by a barely recordable b-wave. No OPs could be elicited under photopic conditions. The cone related components and the OP characteristics clearly distinguish iCSNB from the complete form of CSNB and other retinal conditions presenting with minimal fundus abnormalities but with negative bright-flash ERG responses, such as found in Duchenne muscular dystrophy and Aland Island eye disease. The severely abnormal post-synaptic components in the photopic recordings contrast with the well-differentiated cone activity evoked in scotopic conditions. We propose a cone system that does not respond optimally under the normal operating range (photopic) but rather under mesopic or scotopic conditions. In spite of the severe cone-ERG deficits, visual acuity was only slightly reduced. We propose that the defect, which interferes marginally with the neuronal flow of information, lies in the structures responsible for the building of the b-wave.

[Suppressive rod-cone interaction in complete and incomplete congenital stationary night blindness]

Dark-adapted rods have a suppressive effect on cone response (suppressive rod-cone interaction: SRCI). This phenomenon has been applied to analyze the pathological mechanism of congenital stationary night blindness (CSNB), but the results have been discrepant among the investigators. Alexander et al reported that 2 individuals with CSNB exhibited a normal SRCI, but Arden and Hogg reported an absence of SRCI in 4 cases of CSNB. We evaluated SRCI in complete and incomplete CSNB. SRCI was tested by measuring luminance threshold for flicker detection (685nm, 1.7 degrees in diameter, 20Hz) across the horizontal meridian of the visual field without and with full-field background illumination. In twelve normal subjects, a reduction of the flicker threshold was observed during presentation of the background. Five cases with complete CSNB showed normal SRCI, but in one case we failed to detect SRCI except at one tested locus. Similarly 5 cases of incomplete CSNB showed normal SRCI, but in one case we failed to detect SRCI except at one tested locus. These results may indicate that SRCI is normal in both complete and incomplete CSNB, but in some cases, SRCI is absent because of localized retinal dysfunction.

A homozygous 1-base pair deletion in the arrestin gene is a frequent cause of Oguchi disease in Japanese

Oguchi disease is a rare autosomal recessive form of congenital stationary night blindness with all other visual functions, including visual acuity, visual field, and colour vision being usually normal. A typical clinical feature of the disorder is a golden or gray-white discolouration of the fundus which disappears in the dark-adapted state and reappears shortly after the onset of light ('Mizuo phenomenon'; Fig. 1). The course of dark adaptation of rod photoreceptors is extremely retarded in Oguchi disease while that of cones appears to proceed normally. The locus for Oguchi disease was recently mapped between D2S172 and D2S345 on distal chromosome 2q by linkage analysis. Interestingly, the gene for arrestin, an intrinsic rod photoreceptor protein implicated in the recovery phase of light transduction, also maps to this region of chromosome 2q (refs 6, 7). Here we report that in five out of six unrelated Japanese patients with Oguchi disease, we have identified a homozygous deletion of nucleotide 1147 (1147delA) in codon 309 of the arrestin gene, predicting a shift in the reading frame and a premature termination of translation which may result in 'functional null alleles.'

Oguchi disease: suggestion of linkage to markers on chromosome 2q

Oguchi disease is a rare autosomal recessive form of congenital stationary night blindness. The condition is associated with fundus discolouration and abnormally slow dark adaptation. Earlier studies suggested that the 48 kD protein S antigen may be involved in the recovery phase of light transduction. Previous cytogenetic and linkage studies have localised the S antigen gene (SAG) to chromosome 2q37.1. In the present study markers which map to distal chromosome 2q were typed in an inbred Oguchi pedigree. The segregation data obtained suggested that the affected subjects are homozygous by descent for a region between D2S172 and D2S345. An intragenic SAG polymorphism was homozygous in all affected people and a recombination event suggested that SAG maps proximal to D2S345. Collectively, these findings support the hypothesis that a defect in S antigen may be responsible for Oguchi disease.

Duchenne muscular dystrophy: negative electroretinograms and normal dark adaptation. Reappraisal of assignment of X linked incomplete congenital stationary night blindness

Aland Island eye disease (AIED) and X linked congenital stationary night blindness (CSNB) have been mapped to Xp11.3. Patients have been described with deletions of the Duchenne muscular dystrophy (DMD) gene who also had a negative electroretinogram (ERG) similar to that seen in patients with CSNB and AIED. This seems to confirm that some cases of AIED and CSNB map to Xp21. We examined 16 boys with DMD/BMD (Becker muscular dystrophy) of whom 10 had negative ERGs, eight of them having deletions downstream from exon 44. Normal dark adaptation thresholds were observed in all patients and there were no anomalous visual functions. Hence, CSNB cannot be assigned to Xp21 and negative ERG in DMD/BMD is not associated with eye disease. Six boys with DMD/BMD had normal ERGs. We speculate that a retinal or glial dystrophin may be truncated or absent in the boys with negative ERGs.

Aland island eye disease: clinical and electrophysiological studies of a Welsh family

Clinical and molecular genetic studies were performed on a single, large, white family, in which congenital nystagmus and moderate to high refractive error segregated as a sex linked trait with manifestation in some female carriers. In this family, affected males demonstrate myopia, but a high proportion of female carriers, and some of the possibly affected males, show hypermetropia. Clinical ophthalmic examination and electrodiagnostic studies of retinal function were fully compatible with a diagnosis of either incomplete congenital stationary night blindness or of Aland island eye disease. Previous studies have mapped both disorders to the proximal short arm of the X chromosome: our molecular studies support this localisation. Incomplete congenital stationary nightblindness and Aland Island eye disease could be considered as a single entity.

[Histological examination of the human retina with congenital stationary night blindness]

We report histological findings of the retina from an eye with congenital stationary night blindness. The case was an 82-year-old woman who showed a Schubert-Bornschein type electroretinogram, and whose eye was enucleated due to orbital invasion of paranasal malignant melanoma. The retina had a disorganized outer plexiform layer with displaced nuclei of the outer and inner nuclear layer. There was also outward displacement of the cone nuclei, formation of hyaline and serous drusen, and disorganized retinal pigment epithelium as the histological changes due to aging. The histological abnormality seen in the outer nuclear layer could be responsible for the electrophysiological abnormality in this case.

Quantitative eye and head movement recordings of retinal disease mimicking spasmus nutans

PURPOSE/METHODS

To investigate whether quantitative head and eye movement recordings can distinguish patients with spasmus nutans from patients with retinal diseases mimicking spasmus nutans. A patient with congenital stationary night blindness was followed up for seven years with electro-oculographic eye movement recordings.

RESULTS/CONCLUSIONS

Rhythmic head movements and fine, intermittent, asymmetric, disconjugate, high-frequency, out-of-phase pendular nystagmus were recorded. Eye and head movement recordings of patients with congenital stationary night blindness can mimic spasmus nutans.

Linkage analysis in a Dutch family with X-linked recessive congenital stationary night blindness (XL-CSNB)

Linkage analysis has been performed in a large Dutch pedigree with X-linked recessive congenital stationary night blindness (CSNB) by utilizing 16 DNA markers from the proximal short arm of the human X chromosome (Xp21.1-11.2). Thirteen polymorphic markers are at least partially informative and have enabled pairwise and multipoint linkage analysis. For three loci, i.e. DXS228, the monoamine oxidase B gene and the Norrie disease gene (NDG), multipoint linkage studies have yielded maximum lod scores of > 3.0 at a recombination fraction of zero. Analysis of recombination events has enabled us to rule out the possibility that the underlying defect in this family is allelic to RP3; the gene defect could also be excluded from the proximal part of the region known to carry RP2. Linkage data are consistent with a possible involvement of the NDG but mutations in the open reading frame of this gene have not been found.

Slowly progressive changes of the retina and retinal pigment epithelium in Briard dogs with hereditary retinal dystrophy. A morphological study

Seven eyes from 2 generations of Briard dogs (5 weeks--7 years old) with congenital night blindness and (in the second generation) impairment of day vision to varying degrees, were examined by light and electron microscopy. Specimens from 4 locations were studied: the central area, the midperiphery of the tapetal area, the upper periphery and the lower periphery. Disorientation of rod outer segment disc membranes was seen in the 5-week-old dog. Large electron-lucent inclusions were found in the RPE at 3.5 months of age. These inclusions occurred most frequently in the central and midperipheral-tapetal areas and seemed to increase in numbers and spread towards the periphery with increasing age. The content of these inclusions is not elucidated. Rod photoreceptor degeneration was apparent from 7 months of age and was most prominent in the peripheral areas. The cones were better preserved. The 7-year-old dog showed reduction of photoreceptors in the central and midperipheral-tapetal areas and almost complete photoreceptor degeneration in the periphery. This dog also showed severe changes of the inner retina in the peripheral fundus. It appears that these Briard dogs suffer from a very slowly progressive retinal degeneration, in which the photoreceptor degenerative changes do not correlate anatomically to the changes in the RPE cells. The disease seems to be different from the retinopathy described in the English Briards. It is not clear yet whether the lipid type of retinopathy found in American Briards is identical to the present disease.

Duchenne muscular dystrophy: negative scotopic bright-flash electroretinogram but not congenital stationary night blindness

Patients with Duchenne muscular dystrophy (DMD) have recently been reported to have an abnormal scotopic electroretinogram (ERG) showing weak rod-related responses along with a negative configuration of the bright-flash response, which has been described as being similar to the one in congenital stationary night blindness (CSNB). We compared qualitatively and quantitatively the ERGs of 6 subjects with DMD, 10 subjects with the complete form of CSNB (cCSNB), 13 subjects with the incomplete form of CSNB (iCSNB) and 1 subject with complex glycerol kinase deficiency (CGKD). The rod-related activity and the bright-flash responses were abnormal and similar in all four groups. The cone-related activity, however, was within normal limits only in the DMD group; the b-wave was subnormal in CGKD, truncated in cCSNB and nearly absent in iCSNB. The electrophysiologic signature in DMD clearly distinguishes the retinal function of these patients from any other retinal condition so far described.

Scotopic threshold response in complete and incomplete types of congenital stationary night blindness

PURPOSE

To study the function of the rod visual pathway in the complete and incomplete types of congenital stationary night blindness (CSNB), with special reference to the scotopic threshold response (STR) of electroretinograms (ERGs)

METHODS

Using full-field stimuli with light intensities ranging from near absolute threshold to bright, ERG intensity series from two patients with complete CSNB, four patients with incomplete CSNB, and four normal subjects were recorded.

RESULTS

Neither the rod b-wave nor the STR was recordable from the patients with complete CSNB. In the patients with incomplete CSNB, the STR was clearly recorded, although the absolute threshold was elevated in accordance with elevation of the psychophysical absolute threshold. The b-wave stimulus threshold was not elevated, and the b-wave amplitude near the threshold was normal. The peak time of the STR was delayed by approximately 80 msec, whereas that of the b-wave was normal.

CONCLUSIONS

These STR results indicate that the rod system abnormality in complete CSNB differs from that in incomplete CSNB. Furthermore, the greatly delayed peak time of STR in the patients with incomplete CSNB made the interaction between b-wave and STR different from that in normal subjects.

[Congenital non-progressing night blindness--report of cases]

The authors present 5 cases of nonprogressing congenital disturbances of adaptation. Electroretinogram, typical of this entity can be useful in the differential diagnosis of disorders of scotopic adaptation in other diseases.