Retinal function in Bothnia dystrophy. An electrophysiological study

RDH5 and RLBP1-Associated Inherited Retinal Diseases: Refining the Spectrum of Stationary and Progressive Phenotypes

PURPOSE

To investigate the clinical, functional, and imaging characteristics in patients affected by inherited retinal diseases associated with RDH5 and RLBP1 gene variants, and to report novel genotype-phenotype correlations.

DESIGN

Retrospective single-center cohort study.

METHODS

Twenty-two patients with molecularly confirmed RLBP1-associated retinopathy and 5 with RDH5-associated retinopathy. Medical records were reviewed to obtain data on family history and ophthalmologic examinations, including retinal imaging and full-field electroretinography (ffERG). Genotype was determined by targeted next-generation sequencing followed by confirmation and familial segregation by Sanger sequencing.

RESULTS

The median (interquartile ranges) age at baseline for the RDH5 and RLBP1 cohort was 44.6 (38.2-67.9) years and 36.9 (23.1-45.2) years, respectively. Macular atrophy (MA) was found in approximately 80% of eyes from both cohorts. The RLBP1 genotype was associated with a lower macular volume by 0.28 mm3 (95% CI, -0.46 to -0.11; P = .005) compared to the RDH5 genotype. In both genotypic cohorts, we found a significant annual rate of macular volume loss, estimated at -0.007 mm3/y (95% CI, -0.012 to -0.001; P = .02), without any significant difference between the two genotypes. Three unrelated patients homozygous for the c.361C>T p.(Arg121Trp) RLBP1 variant showed minimal impairment of both the rod and cone systems function on ffERG and absence of MA.

CONCLUSIONS

Progressive MA in addition to congenital night blindness can be identified in adult patients with RDH5-associated retinopathy. Vice versa, hypomorphic RLBP1 variants may cause milder retinal phenotypes rather than the typical severe rod-cone dystrophy with MA. These findings could prove beneficial to improve the prognostication of patients and help in designing future interventional trials.

Retinal Dystrophy Associated With RLBP1 Retinitis Pigmentosa: A Five-Year Prospective Natural History Study

Purpose

To assess the progression in functional and structural measures over a five-year period in patients with retinal dystrophy caused by RLBP1 gene mutation.

Methods

This prospective, noninterventional study included patients with biallelic RLBP1 mutations from two clinical sites in Sweden and Canada. Key assessments included ocular examinations, visual functional measures (best-corrected visual acuity [BCVA], contrast sensitivity [CS], dark-adaptation [DA] kinetics up to six hours for two wavelengths [450 and 632 nm], Humphrey visual fields [HVF], full-field flicker electroretinograms), and structural ocular assessments.

Results

Of the 45 patients enrolled, 38 completed the full five years of follow-up. At baseline, patients had BCVA ranging from -0.2 to 1.3 logMAR, poor CS, HVF defects, and prominent thinning in central foveal thickness. All patients had extremely prolonged DA rod recovery of approximately six hours at both wavelengths. The test-retest repeatability was high across all anatomic and functional endpoints. Cross-sectionally, poorer VA was associated with older age (right eye, correlation coefficient [CC]: 0.606; left eye, CC: -0.578; P < 0.001) and HVF MD values decreased with age (right eye, CC: -0.672, left eye, CC: -0.654; P < 0.001). However, no major changes in functional or structural measures were noted longitudinally over the five-year period.

Conclusions

This natural history study, which is the first study to monitor patients with RLBP1 RD for five years, showed that severely delayed DA sensitivity recovery, a characteristic feature of this disease, was observed in all patients across all age groups (17-69 years), making it a potentially suitable efficacy assessment for gene therapy treatment in this patient population.

Origin of Retinal Oscillatory Potentials in the Mouse, a Tool to Specifically Locate Retinal Damage

To determine the origin of oscillatory potentials (OPs), binocular electroretinogram (ERG) recordings were performed under light and dark adaptation on adult healthy C57BL/6J mice. In the experimental group, 1 μL of PBS was injected into the left eye, while the right eye was injected with 1 μL of PBS containing different agents: APB, GABA, Bicuculline, TPMPA, Glutamate, DNQX, Glycine, Strychnine, or HEPES. The OP response depends on the type of photoreceptors involved, showing their maximum response amplitude in the ERG induced by mixed rod/cone stimulation. The oscillatory components of the OPs were affected by the injected agents, with some drugs inducing the complete abolition of oscillations (APB, GABA, Glutamate, or DNQX), whereas other drugs merely reduced the oscillatory amplitudes (Bicuculline, Glycine, Strychnine, or HEPES) or did not even affect the oscillations (TPMPA). Assuming that rod bipolar cells (RBC) express metabotropic Glutamate receptors, GABA_A, GABA_C, and Glycine receptors and that they release glutamate mainly on Glycinergic AII amacrine cells and GABAergic A17 amacrine cells, which are differently affected by the mentioned drugs, we propose that RBC-AII/A17 reciprocal synapses are responsible for the OP generation in the ERG recordings in the mice. We conclude that the reciprocal synapses between RBC and AII/A17 are the basis of the ERG OP oscillations of the light response, and this fact must be taken into consideration in any ERG test that shows a decrease in the OPs' amplitude.

Disturbed retinoid metabolism upon loss of rlbp1a impairs cone function and leads to subretinal lipid deposits and photoreceptor degeneration in the zebrafish retina

The RLBP1 gene encodes the 36 kDa cellular retinaldehyde-binding protein, CRALBP, a soluble retinoid carrier, in the visual cycle of the eyes. Mutations in RLBP1 are associated with recessively inherited clinical phenotypes, including Bothnia dystrophy, retinitis pigmentosa, retinitis punctata albescens, fundus albipunctatus, and Newfoundland rod–cone dystrophy. However, the etiology of these retinal disorders is not well understood. Here, we generated homologous zebrafish models to bridge this knowledge gap. Duplication of the rlbp1 gene in zebrafish and cell-specific expression of the paralogs rlbp1a in the retinal pigment epithelium and rlbp1b in Müller glial cells allowed us to create intrinsically cell type-specific knockout fish lines. Using rlbp1a and rlbp1b single and double mutants, we investigated the pathological effects on visual function. Our analyses revealed that rlbp1a was essential for cone photoreceptor function and chromophore metabolism in the fish eyes. rlbp1a-mutant fish displayed reduced chromophore levels and attenuated cone photoreceptor responses to light stimuli. They accumulated 11-cis and all-trans-retinyl esters which displayed as enlarged lipid droplets in the RPE reminiscent of the subretinal yellow-white lesions in patients with RLBP1 mutations. During aging, these fish developed retinal thinning and cone and rod photoreceptor dystrophy. In contrast, rlbp1b mutants did not display impaired vision. The double mutant essentially replicated the phenotype of the rlbp1a single mutant. Together, our study showed that the rlbp1a zebrafish mutant recapitulated many features of human blinding diseases caused by RLBP1 mutations and provided novel insights into the pathways for chromophore regeneration of cone photoreceptors.

Function of mammalian M-cones depends on the level of CRALBP in Müller cells

Cone photoreceptors mediate daytime vision in vertebrates. The rapid and efficient regeneration of their visual pigments following photoactivation is critical for the cones to remain photoresponsive in bright and rapidly changing light conditions. Cone pigment regeneration depends on the recycling of visual chromophore, which takes place via the canonical visual cycle in the retinal pigment epithelium (RPE) and the Müller cell-driven intraretinal visual cycle. The molecular mechanisms that enable the neural retina to regenerate visual chromophore for cones have not been fully elucidated. However, one known component of the two visual cycles is the cellular retinaldehyde-binding protein (CRALBP), which is expressed both in the RPE and in Müller cells. To understand the significance of CRALBP in cone pigment regeneration, we examined the function of cones in mice heterozygous for Rlbp1, the gene encoding CRALBP. We found that CRALBP expression was reduced by ∼50% in both the RPE and retina of Rlbp1+/- mice. Electroretinography (ERG) showed that the dark adaptation of rods and cones is unaltered in Rlbp1+/- mice, indicating a normal RPE visual cycle. However, pharmacologic blockade of the RPE visual cycle revealed suppressed cone dark adaptation in Rlbp1+/- mice in comparison with controls. We conclude that the expression level of CRALPB specifically in the Müller cells modulates the efficiency of the retina visual cycle. Finally, blocking the RPE visual cycle also suppressed further cone dark adaptation in Rlbp1-/- mice, revealing a shunt in the classical RPE visual cycle that bypasses CRALBP and allows partial but unexpectedly rapid cone dark adaptation.

Progress in treating inherited retinal diseases: Early subretinal gene therapy clinical trials and candidates for future initiatives

Due to improved phenotyping and genetic characterization, the field of 'incurable' and 'blinding' inherited retinal diseases (IRDs) has moved substantially forward. Decades of ascertainment of IRD patient data from Philadelphia and Toronto centers illustrate the progress from Mendelian genetic types to molecular diagnoses. Molecular genetics have been used not only to clarify diagnoses and to direct counseling but also to enable the first clinical trials of gene-based treatment in these diseases. An overview of the recent reports of gene augmentation clinical trials by subretinal injections is used to reflect on the reasons why there has been limited success in this early venture into therapy. These first-in human experiences have taught that there is a need for advancing the techniques of delivery of the gene products - not only for refining further subretinal trials, but also for evaluating intravitreal delivery. Candidate IRDs for intravitreal gene delivery are then suggested to illustrate some of the disorders that may be amenable to improvement of remaining central vision with the least photoreceptor trauma. A more detailed understanding of the human IRDs to be considered for therapy and the calculated potential for efficacy should be among the routine prerequisites for initiating a clinical trial.

CRALBP supports the mammalian retinal visual cycle and cone vision

Mutations in the cellular retinaldehyde-binding protein (CRALBP, encoded by RLBP1) can lead to severe cone photoreceptor-mediated vision loss in patients. It is not known how CRALBP supports cone function or how altered CRALBP leads to cone dysfunction. Here, we determined that deletion of Rlbp1 in mice impairs the retinal visual cycle. Mice lacking CRALBP exhibited M-opsin mislocalization, M-cone loss, and impaired cone-driven visual behavior and light responses. Additionally, M-cone dark adaptation was largely suppressed in CRALBP-deficient animals. While rearing CRALBP-deficient mice in the dark prevented the deterioration of cone function, it did not rescue cone dark adaptation. Adeno-associated virus-mediated restoration of CRALBP expression specifically in Müller cells, but not retinal pigment epithelial (RPE) cells, rescued the retinal visual cycle and M-cone sensitivity in knockout mice. Our results identify Müller cell CRALBP as a key component of the retinal visual cycle and demonstrate that this pathway is important for maintaining normal cone-driven vision and accelerating cone dark adaptation.

Genotype-phenotype correlations in Bothnia dystrophy caused by RLBP1 gene sequence variations

PURPOSE

To evaluate phenotypes caused by different RLBP1 mutations in autosomal recessive retinitis pigmentosa of Bothnia type.

METHODS

Compound heterozygotes for mutations in the RLBP1 gene [c.677T>A]+[c.700C>T] (p.M226K+p.R234W), n = 10, aged 7-84 years, and homozygotes c.677T>A (p.M226K), n = 2, aged 63 and 73 years, were studied using visual acuity (VA), low-contrast VA, visual fields (VFs) and optical coherence tomography (OCT). Retrospective VA and VFs, standardized dark adaptation and full-field electroretinograms (ERGs) were analysed and prolonged dark adaptometry and ERG (at 24 hr) were performed.

RESULTS

Progressive decline of VA and VF areas was age-dependent. Retinal degenerative maculopathy, peripheral degenerative changes and retinitis punctata albescens (RPA) were present. Early retinal thinning in the central foveal, foveal (Ø 1 mm), and inner ring (Ø 3 mm) in the macular region, with homogenous, high-reflectance RPA changes, was visualized in and adjacent to the retinal pigment epithelium/choriocapillaris using OCT. Reduced dark adaptation and affected ERGs were present in all ages. Prolonged dark adaptation and ERG (at 24 hr), an increase in final threshold, and ERG rod and mixed rod/cone responses were found.

CONCLUSIONS

The two RLBP1 genotypes presented a phenotypical and electrophysiological expression of progressive retinal disease similar to that previously described in homozygotes for the c.700C>T (p.R234W) RLBP1 mutation. The uniform phenotypical expression of RLBP1 mutations is relevant information for the disease and of importance in planning future treatment strategies.

Cis-retinoids and the chemistry of vision

We discuss here principal biochemical transformations of retinoid molecules in the visual cycle. We focus our analysis on the accumulating evidence of alternate pathways and functional redundancies in the cycle. The efficiency of the visual cycle depends, on one hand, on fast regeneration of the photo-bleached chromophores. On the other hand, it is crucial that the cyclic process should be highly selective to avoid accumulation of byproducts. The state-of-the-art knowledge indicates that single enzymatically active components of the cycle are not strictly selective and may require chaperones to enhance their rates. It appears that protein-protein interactions significantly improve the biological stability of the visual cycle. In particular, synthesis of thermodynamically less stable 11-cis-retinoid conformers is favored by physical interactions of the isomerases present in the retina with cellular retinaldehyde binding protein.

Vitamin A metabolism in rod and cone visual cycles

The chromophore of all known visual pigments consists of 11-cis-retinal (derived from either vitamin A1 or A2) or a hydroxylated derivative, bound to a protein (opsin) via a Schiff base. Absorption of a photon results in photoisomerization of the chromophore to all-trans-retinal and conversion of the visual pigment to the signaling form. Regeneration of the 11-cis-retinal occurs in an adjacent tissue and involves several enzymes, several water-soluble retinoid-binding proteins, and intra- and intercellular diffusional processes. Rod photoreceptor cells depend completely on the output of 11-cis-retinal from adjacent retinal pigment epithelial (RPE) cells. Cone photoreceptors cells can use 11-cis-retinal from the RPE and from a second more poorly characterized cycle, which appears to involve adjacent Müller (glial) cells. Recent progress in the characterization of rod and cone visual cycle components and reactions will result in the development of approaches to the amelioration of blinding eye diseases associated with visual cycle defects.

Self-reported quality of life in patients with retinitis pigmentosa and maculopathy of Bothnia type

PURPOSE

To assess vision-related quality-of-life subscales with objective measurements of visual function in patients affected with retinitis pigmentosa of Bothnia type (BD).

METHODS

Forty-nine patients answered the NEI-VFQ-25 questionnaire. High- and low-contrast distance acuity (VA), near VA, and visual fields (VF) were measured. Weighted VA (WVA) and low-contrast (10%) VA (WLCVA), binocular VF areas, and central scotoma were calculated. Adjusted mean subscale scores were calculated and associations analyzed.

RESULTS

Subscale scores for general, far, and near vision, social functioning, and color vision were lowest while general health, ocular pain, and mental health were highest in the BD phenotype. The correlations were substantial and similar for WVA, WLCVA, and near vision. The degree of measured VF impairment had few associations with the different adjusted subscale scores.

CONCLUSION

The NEI VFQ-25 subscales were well associated with clinical vision measures depending on VA. The progression of VF defects typical for the BD phenotype does not seem to affect the self-perceived quality of life, which might indicate adaptability to this type of progressive VF loss. The BD phenotype has a significant impact on multiple domains of daily life, but there are no signs of accelerating depression related to the increasing visual impairment.

Tinted contact lenses in Bothnia dystrophy

PURPOSE

To determine whether tinted contact lenses can improve visual function in patients with Bothnia dystrophy (BD), a genetically defined retinal dystrophy with prolonged dark adaptation.

METHODS

Twelve patients with BD were fitted with the same type of soft contact lenses tinted dark brown. Visual acuity (VA), contrast vision, near vision and visual fields were tested before and 1 month after contact lens fitting. The patients completed a visual function questionnaire. The physical properties of the contact lenses were tested using spectrophotometry.

RESULTS

The patients with the lowest VA described the most obvious improvement in visual function. This group of patients preferred darker contact lenses and continued wearing their contact lenses after the study ended. The patients with the best VA preferred lighter contact lenses and a few patients in this group discontinued contact lens wear upon completion of the study.

CONCLUSIONS

Visual function in BD patients was improved by dark tinted contact lenses. The optimal colour for lenses varies, depending on the season and the individual. Other patient groups with retinal dystrophies associated with prolonged dark adaptation or dysfunction of the cone system, such as cone dystrophies or achromatopsia, may also benefit from this type of contact lens.

Fundus albipunctatus in a 6-year old girl due to compound heterozygous mutations in the RDH5 gene

We report a case study of Fundus Albipunctatus (FA) due to compound heterozygous mutations in RDH5, the gene encoding for the 11-cis-retinal dehydrogenase (RDH). A 6-year old Hispanic American female with a clinical presentation suggestive of FA underwent dark-adapted full-field flash electroretinography (ERG) at 30 and 120 min. The pattern of ERG abnormalities was consistent with the working diagnosis FA. However, only ERG responses to dim stimuli were profoundly affected, and maximal ERG responses were already near normal after only 30 min of dark adaptation. The patient also demonstrated a subnormal maximal ERG response b/a-wave ratio at 30 min that resolved after 120 min of dark adaptation. When measurable, dark-adapted post-receptoral responses were normal in timing under all circumstances, and became increasingly faster after prolonged dark adaptation. Cone-driven responses were completely normal at this young age. Sequencing of the RDH5 gene revealed two distinct missense mutations, a G238W mutation, previously reported in patients with FA, and a D128N mutation, which has not been reported before but is known to cause reduced 11-cis-RDH activity. These findings confirmed the clinical and functional diagnosis of FA and excluded that of retinitis punctata albescens (RPA). The behavior of dark-adapted ERG responses in FA displays characteristics that differ from those of RPA patients, which may be useful to differentiate functionally these two conditions at their common albipunctate stages.

Clinical disorders affecting mesopic vision

Vision in the mesopic range is affected by a number of inherited and acquired clinical disorders. We review these conditions and summarize the historical background, describing the clinical characteristics alongside the genetic basis and molecular biological mechanisms giving rise to rod and cone dysfunction relevant to twilight vision. The current diagnostic gold standards for each disease are discussed and curative and symptomatic treatment strategies are summarized.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.