Autosomal dominant stationary night-blindness. A large family rediscovered

Probing the mechanism by which the retinal G protein transducin activates its biological effector PDE6

Phototransduction in retinal rods occurs when the G protein-coupled photoreceptor rhodopsin triggers the activation of phosphodiesterase 6 (PDE6) by GTP-bound alpha subunits of the G protein transducin (GαT). Recently, we presented a cryo-EM structure for a complex between two GTP-bound recombinant GαT subunits and native PDE6, that included a bivalent antibody bound to the C-terminal ends of GαT and the inhibitor vardenafil occupying the active sites on the PDEα and PDEβ subunits. We proposed GαT-activated PDE6 by inducing a striking reorientation of the PDEγ subunits away from the catalytic sites. However, questions remained including whether in the absence of the antibody GαT binds to PDE6 in a similar manner as observed when the antibody is present, does GαT activate PDE6 by enabling the substrate cGMP to access the catalytic sites, and how does the lipid membrane enhance PDE6 activation? Here, we demonstrate that 2:1 GαT-PDE6 complexes form with either recombinant or retinal GαT in the absence of the GαT antibody. We show that GαT binding is not necessary for cGMP nor competitive inhibitors to access the active sites; instead, occupancy of the substrate binding sites enables GαT to bind and reposition the PDE6γ subunits to promote catalytic activity. Moreover, we demonstrate by reconstituting GαT-stimulated PDE6 activity in lipid bilayer nanodiscs that the membrane-induced enhancement results from an increase in the apparent binding affinity of GαT for PDE6. These findings provide new insights into how the retinal G protein stimulates rapid catalytic turnover by PDE6 required for dim light vision.

Metabolic and Redox Signaling of the Nucleoredoxin-Like-1 Gene for the Treatment of Genetic Retinal Diseases

The loss of cone photoreceptor function in retinitis pigmentosa (RP) severely impacts the central and daily vision and quality of life of patients affected by this disease. The loss of cones follows the degeneration of rods, in a manner independent of the causing mutations in numerous genes associated with RP. We have explored this phenomenon and proposed that the loss of rods triggers a reduction in the expression of rod-derived cone viability factor (RdCVF) encoded by the nucleoredoxin-like 1 (NXNL1) gene which interrupts the metabolic and redox signaling between rods and cones. After providing scientific evidence supporting this mechanism, we propose a way to restore this lost signaling and prevent the cone vision loss in animal models of RP. We also explain how we could restore this signaling to prevent cone vision loss in animal models of the disease and how we plan to apply this therapeutic strategy by the administration of both products of NXNL1 encoding the trophic factor RdCVF and the thioredoxin enzyme RdCVFL using an adeno-associated viral vector. We describe in detail all the steps of this translational program, from the design of the drug, its production, biological validation, and analytical and preclinical qualification required for a future clinical trial that would, if successful, provide a treatment for this incurable disease.

Riggs-type dominant congenital stationary night blindness: ERG findings, a new GNAT1 mutation and a systemic association

PURPOSE

Complete congenital stationary night blindness (CSNB) is most often x-linked or recessive, and associated with a transmission defect from photoreceptors to bipolar cells. This produces a characteristic "negative" Schubert-Bornschein type of scotopic rod-cone electroretinogram (ERG) with a large a-wave and minimal b-wave. CSNB from abnormalities in phototransduction can be recessive or dominant and is much less common. This produces a Riggs type of ERG with loss of the rod a-wave as well as the b-wave. We report the clinical and ERG findings from a family with autosomal dominant CSNB that was shown previously to have a new GNAT1 mutation with a novel mechanism of action. They provide a classic demonstration of the Riggs-type ERG and have an unusual systemic association.

METHODS

Clinical case report of a father and daughter.

RESULTS

A Chinese father and daughter presented with good visual acuity, moderate myopia, and lifelong night blindness. Both show normal fundi except for mild myopia, and fundus autofluorescence and OCT images are normal. Their ERGs illustrate the typical Riggs-type ERG with no rod a-wave (they have only a small cone-dominated combined response). They also have postural orthostatic tachycardia syndrome (POST), which is an autonomic dysfunction disorder thought usually to be sporadic. The retinal gene analyses revealed no abnormalities that might account for POST.

CONCLUSIONS

Our family's ERG showed essentially no rod response, consistent with a Danish GNAT1 pedigree but different from the Nougaret GNAT1 pedigree that shows partial preservation of rod signal. A genetic connection between CSNB and POST would be intriguing, but we found no evidence for this.

GNAT1 associated with autosomal recessive congenital stationary night blindness

PURPOSE

Congenital stationary night blindness is a nonprogressive retinal disorder manifesting as impaired night vision and is generally associated with other ocular symptoms, such as nystagmus, myopia, and strabismus. This study was conducted to further investigate the genetic basis of CSNB in a consanguineous Pakistani family.

METHODS

A consanguineous family with multiple individuals manifesting cardinal symptoms of congenital stationary night blindness was ascertained. All family members underwent detailed ophthalmic examination, including fundus photographic examination and electroretinography. Blood samples were collected and genomic DNA was extracted. Exclusion and genome-wide linkage analyses were completed and two-point LOD scores were calculated. Bidirectional sequencing of GNAT1 was completed, and quantitative expression of Gnat1 transcript levels were investigated in ocular tissues at different postnatal intervals.

RESULTS

The results of ophthalmic examinations were suggestive of early-onset stationary night blindness with no extraocular anomalies. The genome-wide scan localized the critical interval to chromosome 3, region p22.1-p14.3, with maximum two-point LOD scores of 3.09 at θ = 0, flanked by markers D3S3522 and D3S1289. Subsequently, a missense mutation in GNAT1, p.D129G, was identified, which segregated within the family, consistent with an autosomal recessive mode of inheritance, and was not present in 192 ethnically matched control chromosomes. Expression analysis suggested that Gnat1 is expressed at approximately postnatal day (P)7 and is predominantly expressed in the retina.

CONCLUSIONS

These data suggest that a homozygous missense mutation in GNAT1 is associated with autosomal recessive stationary night blindness.

A mutation in SLC24A1 implicated in autosomal-recessive congenital stationary night blindness

Congenital stationary night blindness (CSNB) is a nonprogressive retinal disorder that can be associated with impaired night vision. The last decade has witnessed huge progress in ophthalmic genetics, including the identification of three genes implicated in the pathogenicity of autosomal-recessive CSNB. However, not all patients studied could be associated with mutations in these genes and thus other genes certainly underlie this disorder. Here, we report a large multigeneration family with five affected individuals manifesting symptoms of night blindness. A genome-wide scan localized the disease interval to chromosome 15q, and recombination events in affected individuals refined the critical interval to a 10.41 cM (6.53 Mb) region that harbors SLC24A1, a member of the solute carrier protein superfamily. Sequencing of all the coding exons identified a 2 bp deletion in exon 2: c.1613_1614del, which is predicted to result in a frame shift that leads to premature termination of SLC24A1 (p.F538CfsX23) and segregates with the disorder under an autosomal-recessive model. Expression analysis using mouse ocular tissues shows that Slc24a1 is expressed in the retina around postnatal day 7. In situ and immunohistological studies localized both SLC24A1 and Slc24a1 to the inner segment, outer and inner nuclear layers, and ganglion cells of the retina, respectively. Our data expand the genetic basis of CSNB and highlight the indispensible function of SLC24A1 in retinal function and/or maintenance in humans.

Molecular genetics of infantile-onset retinal dystrophies

Over the last decade there have been major advances in our understanding of the molecular pathology of inherited retinal dystrophies. This paper reviews recent advances in the identification of genetic mutations underlying infantile-onset inherited retinal disorders and considers how this knowledge may lead to novel therapeutic approaches.

Transgenic mice carrying the H258N mutation in the gene encoding the beta-subunit of phosphodiesterase-6 (PDE6B) provide a model for human congenital stationary night blindness

Mutations in the beta-subunit of cGMP-phosphodiesterase (PDE6beta) can lead to either progressive retinal disease, such as human retinitis pigmentosa (RP), or stationary disease, such as congenital stationary night blindness (CSNB). Individuals with CSNB in the Rambusch pedigree were found to carry the H258N allele of PDE6B (MIM# 180072); a similar mutation was not found in RP patients. This report describes an individual carrying the H258N allele, who presented with generalized retinal dysfunction affecting the rod system and a locus of dysfunction at the rod-bipolar interface. Also described are preclinical studies in which transgenic mice with the H258N allele were generated to study the pathophysiological mechanisms of CSNB. While Pde6b(rd1)/Pde6b(rd1) mice have severe photoreceptor degeneration, as in human RP, the H258N transgene rescued these cells. The cGMP-PDE6 activity of dark-adapted H258N mice showed an approximate three-fold increase in the rate of retinal cGMP hydrolysis: from 130.1 nmol x min(-1) x nmol(-1) rhodopsin in wild-type controls to 319.2 nmol x min(-1) x nmol(-1) rhodopsin in mutants, consistent with the hypothesis that inhibition of the PDE6beta activity by the regulatory PDE6gamma subunit is blocked by this mutation. In the albino (B6CBA x FVB) F2 hybrid background, electroretinograms (ERG) from H258N mice were similar to those obtained from affected Rambusch family members, as well as humans with the most common form of CSNB (X-linked), demonstrating a selective loss of the b-wave with relatively normal a-waves. When the H258N allele was introduced into the DBA background, there was no evidence of selective reduction in b-wave amplitudes; rather a- and b-wave amplitudes were both reduced. Thus, factors other than the PDE6B mutation itself could contribute to the variance of an electrophysiological response. Therefore, caution is advisable when interpreting physiological phenotypes associated with the same allele on different genetic backgrounds. Nevertheless, such animals should be of considerable value in further studies of the molecular pathology of CSNB.

Night blindness with depolarizing pattern of ON/OFF response in electroretinogram: a case report

PURPOSE

To present a patient who has night blindness with a depolarizing pattern of ON/OFF response by electroretinography (ERG).

CASE

A 43-year-old woman had had night blindness and poorly corrected visual acuity since childhood. Parental consanguinity was noted. The patient had suffered from mental retardation, epilepsy, and mild cerebellar ataxia. Corrected visual acuity was 20/30 in the right eye and 20/25 in the left. Goldmann perimetry showed no scotoma but slight depression with internal isoptors. No evidence for rod activity was observed by Goldmann-Weekers adaptometry. The ocular fundi appeared normal.

METHODS

Conventional full-field ERGs to scotopic (dim and bright flash) and photopic (bright flash and flicker) stimuli were recorded. Photopic ERG responses to long flash stimulation (200 ms) were also examined.

RESULTS

The scotopic responses to dim flash were non-recordable, while those to bright flash were severely reduced. The photopic responses to bright flash were decreased. The amplitudes of flicker responses also were significantly decreased, and the implicit times of those responses were prolonged. Although the amplitudes of a- and d-waves to long flash stimulation were reduced, those of the b-wave were within normal range. The implicit times of a-, b- and d-waves were significantly prolonged. The patient showed a normal rise for the photopic b-wave but lacked a normal falling slope after the b-wave peak. An OFF-response late-negativity was also noted.

CONCLUSIONS

The abnormal ON/OFF response found in the patient could be diagnosed as depolarizing pattern, characterized by elevation of the plateau to a positive potential above the prestimulus baseline. Since the ERG waveforms and clinical features found in our patient were different from those in previous reports, her ERG findings might reflect another retinal physiological mechanism.

LED-generated Multifocal ERG On- and Off-responses in Complete Congenital Stationary Night Blindness – A Case Report

We report on the application of a light emitting diode (LED) screen to elicit multifocal ERG on- and off-responses in a patient presenting with the complete type of congenital stationary night blindness (cCSNB): A 63-years old woman was diagnosed with cCSNB by means of standard ERG procedures and dark adaptometry. To confirm this diagnosis and to investigate topographical differences of on- and off-responses a multifocal approach employing long-duration stimuli was added. Results of mfERG-testing were averaged in three groups (a central area of 7.5 degrees , a ring area of 7.5-21.9 degrees and a peripheral ring of 21.9-31.1 degrees ). When compared to normal controls (n = 4) on-responses (P1-amplitudes) were severely reduced symmetrically at all eccentricities, while off-responses showed no reduction resulting in an increased off/on-ratio. Furthermore on-latencies of P1 were delayed symmetrically at all eccentricities, whereas off-latencies were normal. To our knowledge this is the first report of multifocal on- and off-responses in a CSNB-patient. Stimulus-generation with a LED-screen provides the advantage of a stable luminance during the long-duration on-phase.

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.

Autosomal dominant inheritance of a negative electroretinogram phenotype in three generations

PURPOSE

We report an abnormal electroretinogram with a negative configuration in a child who presented with moderate myopia, nystagmus, and visual developmental delay. We investigated the electroretinogram and explored the possibility of a metabotropic glutamate receptor subtype 6 mutation in six family members spanning four generations.

METHODS

Case report and family study: Complete eye examinations and Ganzfeld electroretinograms were recorded from the maternal great-grandmother, maternal grandmother, mother, uncle, and sibling of the 7-month-old female proband. The electroretinogram was repeated in the proband at 17 months of age. Dark adaptometry was performed in all adult subjects. Fundus photographs and visual field examinations were administered to the grandmother and mother. The metabotropic glutamate receptor subtype 6 gene was amplified and sequenced in all affected subjects.

RESULTS

The proband had a negative electroretinogram and a normal fundus. The maternal grandmother, uncle, and mother had an abnormal electroretinogram identical to the proband yet had no visual complaints. The ophthalmology examinations in the adult subjects were normal, and subsequent examination of the proband at 17 months, 5 years, and 6.5 years of age showed no changes in the fundus or refractive error. Her nystagmus resolved by 5 years of age. Rod threshold and visual fields were normal in the affected adult subjects. No mutation in the metabotropic glutamate receptor subtype 6 gene was found.

CONCLUSIONS

In this family, a negative electroretinogram was not associated with decreased rod threshold, visual acuity loss, visual field loss, muscle disease, or metabotropic glutamate receptor subtype 6 mutation. Additional study will be required to understand the nature of the negative electroretinogram phenotype in this family.

Molecular genetics of Oguchi disease, fundus albipunctatus, and other forms of stationary night blindness: LVII Edward Jackson Memorial Lecture

PURPOSE

To compare the clinical findings of the various forms of stationary night blindness caused by mutations in identified genes encoding proteins of photoreceptors or the retinal pigment epithelium.

METHODS

Review of the visual acuities, visual fields, fundi, dark-adaptation curves, and electroretinograms from patients with stationary night blindness caused by mutations in the genes RHO, GNAT1, PDE6B, RHOK, SAG, RDH5, and CACNA1F, respectively encoding rhodopsin, the alpha subunit of rod transducin, the beta subunit of rod cGMP-phosphodiesterase, rhodopsin kinase, arrestin, 11-cis retinol dehydrogenase, and a retinal L-type calcium channel.

RESULTS

In the evaluated forms of stationary night blindness, the time course of dark adaptation and the characteristics of the electroretinogram indicate that rod photoreceptors are present and that they function, although abnormally. In night blindness resulting from defects in rhodopsin, the alpha subunit of rod transducin, or the beta subunit of rod cGMP phosphodiesterase, rod photoreceptors respond only to light intensities far brighter than normal, and the sensitivity of rods to light is similar to that of normal individuals who are not dark adapted. In fundus albipunctatus and in Oguchi disease, the rod photoreceptors can achieve normal sensitivity to dim light but only after 2 or more hours of dark adaptation, compared with approximately 0.5 hours for normal individuals. In each of these forms of stationary night blindness, the poor rod sensitivity and the time course of dark adaptation correlate with the known or presumed physiologic abnormalities caused by the identified gene defects. Patients with some forms of stationary night blindness, such as fundus albipunctatus and Oguchi disease, may develop degeneration of the retina leading to severe loss of vision in later life.

CONCLUSIONS

The identification of the mutant genes causing forms of stationary night blindness refines the classification of these diseases and enhances our understanding of the underlying physiologic defects. Ophthalmologists must be aware that although these diseases are traditionally categorized as "stationary," some of them lead to reduced visual acuity or constricted visual fields, especially in older patients. Efforts to develop therapies for these diseases should concentrate on these more severe forms.

A novel mutation within the rhodopsin gene (Thr-94-Ile) causing autosomal dominant congenital stationary night blindness

More than 100 mutations within the rhodopsin gene have been found to be responsible for some forms of retinitis pigmentosa, a progressive retinal degeneration characterized by night blindness and subsequent disturbance of day vision that may eventually result in total blindness. Congenital stationary night blindness (CSNB) is an uncommon inherited retinal dysfunction in which patients complain of night vision difficulties of a nonprogressive nature only and in which generally there is no involvement of day vision. We report the results of molecular genetic analysis of an Irish family segregating an autosomal dominant form of CSNB in which a previously unreported threonine-to-isoleucine substitution at codon 94 in the rhodopsin gene was found to segregate with the disease. Computer modeling suggests that constitutive activation of transducin by the altered rhodopsin protein may be a mechanism for disease causation in this family. Only two mutations within the rhodopsin gene have been previously reported in patients with congenital stationary night blindness, constitutive activation also having been proposed as a possible disease mechanism.

Heterozygous missense mutation in the rod cGMP phosphodiesterase beta-subunit gene in autosomal dominant stationary night blindness

The locus for autosomal dominant congenital stationary night blindness (adCSNB) has recently been assigned to distal chromosome 4p by linkage analysis in a large Danish family. Within the candidate gene encoding the beta-subunit of rod photoreceptor cGMP-specific phosphodiesterase (beta PDE), we have identified a heterozygous C to A transversion in exon 4, predicting a His258Asp change in the polypeptide. We found a perfect cosegregation (Zmax = 22.6 at theta = 0.00) of this mutation with the disease phenotype suggesting that this missense mutation is responsible for the disease in this pedigree. Homozygous nonsense mutations in the beta PDE gene have been found recently in patients with autosomal recessive retinitis pigmentosa, a common hereditary photoreceptor dystrophy.