Next-generation sequencing revealed a novel mutation in the gene encoding the beta subunit of rod phosphodiesterase

Identification of a novel large multigene deletion and a frameshift indel in PDE6B as the underlying cause of early-onset recessive rod-cone degeneration

A family, with two affected identical twins with early-onset recessive inherited retinal degeneration, was analyzed to determine the underlying genetic cause of pathology. Exome sequencing revealed a rare and previously reported causative variant (c.1923_1969delinsTCTGGG; p.Asn643Glyfs*29) in the PDE6B gene in the affected twins and their unaffected father. Further investigation, using genome sequencing, identified a novel ∼7.5-kb deletion (Chr 4:670,405-677,862del) encompassing the ATP5ME gene, part of the 5' UTR of MYL5, and a 378-bp (Chr 4:670,405-670,782) region from the 3' UTR of PDE6B in the affected twins and their unaffected mother. Both variants segregated with disease in the family. Analysis of the relative expression of PDE6B, in peripheral blood cells, also revealed a significantly lower level of PDE6B transcript in affected siblings compared to a normal control. PDE6B is associated with recessive rod-cone degeneration and autosomal dominant congenital stationary night blindness. Ophthalmic evaluation of these patients showed night blindness, fundus abnormalities, and peripheral vision loss, which are consistent with PDE6B-associated recessive retinal degeneration. These findings suggest that the loss of PDE6B transcript resulting from the compound heterozygous pathogenic variants is the underlying cause of recessive rod-cone degeneration in the study family.

Novel variants in PDE6A and PDE6B genes and its phenotypes in patients with retinitis pigmentosa in Chinese families

BACKGROUND

Retinitis pigmentosa (RP) is a genetically heterogeneous disease with 89 causative genes identified to date. However, only approximately 60% of RP cases genetically solved to date, predicating that many novel disease-causing variants are yet to be identified. The purpose of this study is to identify novel variants in PDE6A and PDE6B genes and present its phenotypes in patients with retinitis pigmentosa in Chinese families.

METHODS

Five retinitis pigmentosa patients with PDE6A variants and three with PDE6B variants were identified through a hereditary eye disease enrichment panel (HEDEP), all patients' medical and ophthalmic histories were collected, and ophthalmological examinations were performed, followed by an analysis of the possible causative variants. Sanger sequencing was used to verify the variants.

RESULTS

We identified 20 variants in eight patients: 16 of them were identified in either PDE6A or PDE6B in a compound heterozygous state. Additional four heterozygous variants were identified in the genes ADGRA3, CA4, OPTN, RHO. Two novel genetic changes in PDE6A were identified (c.1246G > A and c.1747 T > A), three novel genetic changes in PDE6B were identified (c.401 T > C, c.2293G > C and c.1610-1612del), out of the novel identified variants one was most probably non-pathogenic (c.2293G > C), all other novel variants are pathogenic. Additional variant was identified in CA4 and RHO, which can cause ADRP (c.243G > A, c.688G > A). In addition, a novel variant in ADGRA3 was identified (c.921-1G > A).

CONCLUSIONS

This study reveals novel and known variants in PDE6A and PDE6B genes in Chinese families with autosomal recessive RP, and expands the clinical and genetic findings of photoreceptor-specific enzyme deficiencies.

Correlation between B-scan optical coherence tomography, en face thickness map ring and hyperautofluorescent ring in retinitis pigmentosa patients

OBJECTIVE

To evaluate and compare the B-scan OCT loss of ellipsoid zone, OCT en face thickness map constriction, and hyperautofluorescent ring constriction in RP patients.

METHODS

Retrospective case series study. Forty-eight eyes of 24 RP patients with a parafoveal hyperautofluorescent ring were studied. The diagnosis of RP was established by the presence of rod response impairment and a prevalent decrease of scotopic over photopic responses on electroretinography. The FAF and spectral-domain optical coherence tomography (SD-OCT) images were obtained from 24 patients with RP. The measurements of the EZ line width on B-scan OCT, hyperautofluorescent ring area on FAF, and hyperautofluorescent ring area on en face thickness map were performed by two independent graders. The measurements of these three parameters were correlated.

RESULTS

The mean age of study patients was 46 years old (sd = 19). The external and internal FAF rings involving the fovea were identified in all study eyes. The area of the thickness ring decreased at an average rate of 0.5 (sd 0.4) mm² per year (P < 0.001). The average rate of EZ-line constriction was estimated to be 123 (sd 63) μm per year (P < 0.001). The hyperautofluorescent ring area decreased at an average rate of 0.9 (sd 0.98) mm² per year (P < 0.001). The strongest correlation was observed between hyperautofluorescent ring area and EZ-line width (r = 0.78).

CONCLUSIONS

We observed that the hyperautofluorescent ring area exhibits a faster progression rate than the area of the thickness ring. In addition, we found that the EZ-line width had a high positive correlation with the hyperautofluorescent ring area and a moderate positive correlation with area of the thickness ring.

Lutein and Zeaxanthin Isomers Reduce Photoreceptor Degeneration in the Pde6b rd10 Mouse Model of Retinitis Pigmentosa

PURPOSE

Lutein, RR-zeaxanthin, and RS-zeaxanthin (L-Z) are antioxidants which can reduce endoplasmic reticulum stress (ERS) and oxidative stress (OS), and ameliorate neurodegenerative diseases. However, their treatment effect in the Pde6b rd10 (rd10) mouse model of retinitis pigmentosa (RP) and the underlying cellular mechanisms have not been studied. ERS is an important factor which causes photoreceptor apoptosis. The aim of the current project is to test the treatment effect of L-Z in rd10 mice and to investigate the underlying molecular mechanisms of ERS.

METHODS

L-Z (Lutemax 2020, 10 mg/kg) diluted in sunflower oil (SFO, 1 mg/ml) or the same volume of SFO was administrated via gavage from postnatal day 6 (P6) to P20 daily in L-Z group (n=5) or SFO group (n=6) of rd10 mice. At P21, electroretinography (ERG) was performed to show the functional change of retinas. 78 kDa glucose-regulated protein (GRP78) and endoplasmic reticulum protein 29 (ERp29) were tested by western blot and immunostaining.

RESULTS

The ERG amplitudes were larger in the L-Z group than those of the SFO group in all flash luminances of dark-adapted and light-adapted ERG (all p < 0.01). Western blot revealed that GRP78 in the retinas of the L-Z group was significantly downregulated compared to that of the SFO group (p < 0.01). Meanwhile, the retinal ERp29 protein was significantly upregulated in the L-Z treatment group than that of the SFO group (p < 0.01).

CONCLUSIONS

L-Z provide protection to the photoreceptors of rd10 mouse model of RP, which is probably associated with the reduction of ERS.

Rates of Bone Spicule Pigment Appearance in Patients With Retinitis Pigmentosa Sine Pigmento

PURPOSE

To determine rate of bone spicule pigmentation appearance in patients with retinitis pigmentosa (RP).

DESIGN

Retrospective, observational case series.

PARTICIPANTS

A total of 240 patients were analyzed for this study.

METHODS

A retrospective analysis was conducted at the Electrodiagnostic Clinic at Columbia University Medical Center of all patients' medical records with a diagnosis of RP between July 2017 and January 2018. The medical records of these patients were analyzed to determine whether the patients presented with pigment migration on their first and last visit to our clinic. Among those who did not have bone spicule at first visit, we examined the time to appearance of newly formed bone spicule. The survival distribution was then estimated using the Kaplan-Meier estimator, where the event is bone spicule and time starts at first visit.

RESULTS

From the 240 patients analyzed, 213 patients presented with intraretinal pigmentation on the first visit to our clinic, and 27 patients presented without intraretinal pigmentation. Of these 27 patients, 10 patients developed pigmentation by their follow-up, with a median time to appearance of bone spicule of 5.4 years from first visit, according to the Kaplan-Meier estimates.

CONCLUSIONS

The timeline of bone spicule pigment appearance in RP has important implications in the natural history characterization of disease progression and application as a biomarker for interventional trials.

Structural disease progression in PDE6-associated autosomal recessive retinitis pigmentosa

BACKGROUND AND OBJECTIVE

To evaluate the progression of retinitis pigmentosa (RP) caused by mutations in either PDE6A or PDE6B by measuring the progressive constriction of the hyperautofluorescent ring and shortening of the ellipsoid zone (EZ)-line width.

PATIENTS AND METHODS

Fundus autofluorescence (FAF) and spectral-domain optical coherence tomography (SD-OCT) images were obtained from seven patients with autosomal recessive RP caused by mutations in either PDE6A or PDE6B. Measurements of the EZ line width on SD-OCT images and horizontal, vertical diameter, and ring area on FAF images were performed by two independent graders. The measurements of these four parameters were correlated with one another.

RESULTS

We observed that the EZ line width decreased by an average of 91 ± 64 µm per year, while the horizontal and vertical diameters decreased by 103 ± 53 µm and 92 ± 49 µm per year, respectively. The ring area decreased by a rate of 0.3 ± 0.18 mm² per year. Progression rates were similar for the left eye.

CONCLUSIONS

We observed a progressive loss of EZ line width and Short-wavelength fundus autofluorescence (SW-AF) ring constriction over time. These results may serve as reference for better prognostic prediction and patients selection for clinical trials promoting cone rescue.

Reprogramming the metabolome rescues retinal degeneration

Metabolomics studies in the context of ophthalmology have largely focused on identifying metabolite concentrations that characterize specific retinal diseases. Studies involving mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy have shown that individuals suffering from retinal diseases exhibit metabolic profiles that markedly differ from those of control individuals, supporting the notion that metabolites may serve as easily identifiable biomarkers for specific conditions. An emerging branch of metabolomics resulting from biomarker studies, however, involves the study of retinal metabolic dysfunction as causes of degeneration. Recent publications have identified a number of metabolic processes-including but not limited to glucose and oxygen metabolism-that, when perturbed, play a role in the degeneration of photoreceptor cells. As a result, such studies have led to further research elucidating methods for prolonging photoreceptor survival in an effort to halt degeneration in its early stages. This review will explore the ways in which metabolomics has deepened our understanding of the causes of retinal degeneration and discuss how metabolomics can be used to prevent retinal degeneration from progressing to its later disease stages.

Genetic rescue models refute nonautonomous rod cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited neurodegenerative disease, in which the death of mutant rod photoreceptors leads secondarily to the non-cell autonomous death of cone photoreceptors. Gene therapy is a promising treatment strategy. Unfortunately, current methods of gene delivery treat only a fraction of diseased cells, yielding retinas that are a mosaic of treated and untreated rods, as well as cones. In this study, we created two RP mouse models to test whether dying, untreated rods negatively impact treated, rescued rods. In one model, treated and untreated rods were segregated. In the second model, treated and untreated rods were diffusely intermixed, and their ratio was controlled to achieve low-, medium-, or high-efficiency rescue. Analysis of these mosaic retinas demonstrated that rescued rods (and cones) survive, even when they are greatly outnumbered by dying photoreceptors. On the other hand, the rescued photoreceptors did exhibit long-term defects in their outer segments (OSs), which were less severe when more photoreceptors were treated. In summary, our study suggests that even low-efficiency gene therapy may achieve stable survival of rescued photoreceptors in RP patients, albeit with OS dysgenesis.

MULTIMODAL IMAGING OF DISEASE-ASSOCIATED PIGMENTARY CHANGES IN RETINITIS PIGMENTOSA

PURPOSE

Using multiple imaging modalities, we evaluated the changes in photoreceptor cells and retinal pigment epithelium (RPE) that are associated with bone spicule-shaped melanin pigmentation in retinitis pigmentosa.

METHODS

In a cohort of 60 patients with retinitis pigmentosa, short-wavelength autofluorescence, near-infrared autofluorescence (NIR-AF), NIR reflectance, spectral domain optical coherence tomography, and color fundus images were studied.

RESULTS

Central AF rings were visible in both short-wavelength autofluorescence and NIR-AF images. Bone spicule pigmentation was nonreflective in NIR reflectance, hypoautofluorescent with short-wavelength autofluorescence and NIR-AF imaging, and presented as intraretinal hyperreflective foci in spectral domain optical coherence tomography images. In areas beyond the AF ring outer border, the photoreceptor ellipsoid zone band was absent in spectral domain optical coherence tomography and the visibility of choroidal vessels in short-wavelength autofluorescence, NIR-AF, and NIR reflectance images was indicative of reduced RPE pigmentation. Choroidal visibility was most pronounced in the zone approaching peripheral areas of bone spicule pigmentation; here RPE/Bruch membrane thinning became apparent in spectral domain optical coherence tomography.

CONCLUSION

These findings are consistent with a process by which RPE cells vacate their monolayer and migrate into inner retina in response to photoreceptor cell degeneration. The remaining RPE spread undergo thinning and consequently become less pigmented. An explanation for the absence of NIR-AF melanin signal in relation to bone spicule pigmentation is not forthcoming.

Reprogramming metabolism by targeting sirtuin 6 attenuates retinal degeneration

Retinitis pigmentosa (RP) encompasses a diverse group of Mendelian disorders leading to progressive degeneration of rods and then cones. For reasons that remain unclear, diseased RP photoreceptors begin to deteriorate, eventually leading to cell death and, consequently, loss of vision. Here, we have hypothesized that RP associated with mutations in phosphodiesterase-6 (PDE6) provokes a metabolic aberration in rod cells that promotes the pathological consequences of elevated cGMP and Ca2+, which are induced by the Pde6 mutation. Inhibition of sirtuin 6 (SIRT6), a histone deacetylase repressor of glycolytic flux, reprogrammed rods into perpetual glycolysis, thereby driving the accumulation of biosynthetic intermediates, improving outer segment (OS) length, enhancing photoreceptor survival, and preserving vision. In mouse retinae lacking Sirt6, effectors of glycolytic flux were dramatically increased, leading to upregulation of key intermediates in glycolysis, TCA cycle, and glutaminolysis. Both transgenic and AAV2/8 gene therapy-mediated ablation of Sirt6 in rods provided electrophysiological and anatomic rescue of both rod and cone photoreceptors in a preclinical model of RP. Due to the extensive network of downstream effectors of Sirt6, this study motivates further research into the role that these pathways play in retinal degeneration. Because reprogramming metabolism by enhancing glycolysis is not gene specific, this strategy may be applicable to a wide range of neurodegenerative disorders.

Novel mutations in PDE6B causing human retinitis pigmentosa

AIM

To identify the genetic defects of a Chinese patient with sporadic retinitis pigmentosa (RP).

METHODS

Ophthalmologic examinations were performed on the sporadic RP patient, 144 genes associated with retinal diseases were scanned with capture next generation sequencing (CNGS) approach. Two heterozygous mutations in PDE6B were confirmed in the pedigree by Sanger sequencing subsequently. The carrier frequency of PDE6B mutations of reported PDE6B mutations based on the available two public exome databases (1000 Genomes Project and ESP6500 Genomes Project) and one in-house exome database was investigated.

RESULTS

We identified compound heterozygosity of two novel nonsense mutations c.1133G>A (p.W378X) and c.2395C>T (p.R799X) in PDE6B, one reported causative gene for RP. Neither of the two mutations in our study was presented in three exome databases. Two mutations (p.R74C and p.T604I) in PDE6B have relatively high frequencies in the ESP6500 and in-house databases, respectively, while no common dominant mutation in each of the database or across all databases.

CONCLUSION

We demonstrates that compound heterozygosity of two novel nonsense mutations in PDE6B could lead to RP. These results collectively point to enormous potential of next-generation sequencing in determining the genetic etiology of RP and how various mutations in PDE6B contribute to the genetic heterogeneity of RP.

The Time Course of Deafness and Retinal Degeneration in a Kunming Mouse Model for Usher Syndrome

Usher syndrome is a group of autosomal recessive diseases characterized by congenital deafness and retinitis pigmentosa. In a mouse model for Usher syndrome, KM^ush/ush_, discovered in our laboratory, we measured the phenotypes, characterized the architecture and morphology of the retina, and quantified the level of expression of pde6b and ush2a between postnatal (P) days 7, and 56. Electroretinograms and auditory brainstem response were used to measure visual and auditory phenotypes. Fundus photography and light microscopy were used to measure the architecture and morphology of the retina. Quantitative real-time PCR was used to measure the expression levels of mRNA. KM^ush/ush mice had low amplitudes and no obvious waveforms of Electroretinograms after P14 compared with controls. Thresholds of auditory brainstem response in our model were higher than those of controls after P14. By P21, the retinal vessels of KM^ush/ush mice were attenuated and their optic discs had a waxy pallor. The retinas of KM^ush/ush mice atrophied and the choroidal vessels were clearly visible. Notably, the architecture of each retinal layer was not different as compared with control mice at P7, while the outer nuclear layer (ONL) and other retinal layers of KM^ush/ush mice were attenuated significantly between P14 and P21. ONL cells were barely seen in KM^ush/ush mice at P56. As compared with control mice, the expression of pde6b and ush2a in KM^ush/ush mice declined significantly after P7. This study is a first step toward characterizing the progression of disease in our mouse model. Future studies using this model may provide insights about the etiology of the disease and the relationships between genotypes and phenotypes providing a valuable resource that could contribute to the foundation of knowledge necessary to develop therapies to prevent the retinal degeneration in patients with Usher Syndrome.

Halting progressive neurodegeneration in advanced retinitis pigmentosa

Hereditary retinal degenerative diseases, such as retinitis pigmentosa (RP), are characterized by the progressive loss of rod photoreceptors followed by loss of cones. While retinal gene therapy clinical trials demonstrated temporary improvement in visual function, this approach has yet to achieve sustained functional and anatomical rescue after disease onset in patients. The lack of sustained benefit could be due to insufficient transduction efficiency of viral vectors ("too little") and/or because the disease is too advanced ("too late") at the time therapy is initiated. Here, we tested the latter hypothesis and developed a mouse RP model that permits restoration of the mutant gene in all diseased photoreceptor cells, thereby ensuring sufficient transduction efficiency. We then treated mice at early, mid, or late disease stages. At all 3 time points, degeneration was halted and function was rescued for at least 1 year. Not only do our results demonstrate that gene therapy effectively preserves function after the onset of degeneration, our study also demonstrates that there is a broad therapeutic time window. Moreover, these results suggest that RP patients are treatable, despite most being diagnosed after substantial photoreceptor loss, and that gene therapy research must focus on improving transduction efficiency to maximize clinical impact.

Reduced rod electroretinograms in carrier parents of two Japanese siblings with autosomal recessive retinitis pigmentosa associated with PDE6B gene mutations

PURPOSE

To present the clinical and genetic findings in two siblings with autosomal recessive retinitis pigmentosa (RP) and their non-symptomatic parents.

METHODS

We studied two siblings, a 48-year-old woman and her 44-year-old brother, and their parents. They had general ophthalmic examinations including ophthalmoscopy, perimetry, and electroretinography (ERG). Their whole exomes were analyzed by the next-generation sequence technique.

RESULTS

The two siblings had night blindness for a long time, and clinical examinations revealed diffuse retinal degeneration with bone spicule pigmentation, constriction of the visual field, and non-recordable ERGs. Their parents were non-symptomatic and had normal fundi; however, their rod ERGs were reduced. Genetic examination revealed compound heterozygous mutations of I535N and H557Y in the PDE6B gene in the siblings, and the parents were heterozygous carriers of the mutations.

CONCLUSIONS

Heterozygous mutation in the PDE6B gene can cause a reduction in the rod function to different degrees. The retinal function of non-symptomatic carriers of autosomal recessive RP should be evaluated with care.

Retinal dystrophies, genomic applications in diagnosis and prospects for therapy

Retinal dystrophies (RDs) are degenerative diseases of the retina which have marked clinical and genetic heterogeneity. Common presentations among these disorders include night or colour blindness, tunnel vision and subsequent progression to complete blindness. The known causative disease genes have a variety of developmental and functional roles with mutations in more than 120 genes shown to be responsible for the phenotypes. In addition, mutations within the same gene have been shown to cause different disease phenotypes, even amongst affected individuals within the same family highlighting further levels of complexity. The known disease genes encode proteins involved in retinal cellular structures, phototransduction, the visual cycle, and photoreceptor structure or gene regulation. This review aims to demonstrate the high degree of genetic complexity in both the causative disease genes and their associated phenotypes, highlighting the more common clinical manifestation of retinitis pigmentosa (RP). The review also provides insight to recent advances in genomic molecular diagnosis and gene and cell-based therapies for the RDs.