Identification and Characterization of Retinitis Pigmentosa in a Novel Mouse Model Caused by PDE6B-T592I

The cGMP-phosphodiesterase 6 beta subunit (PDE6B) is an essential component in the phototransduction pathway for light responses in photoreceptor cells. PDE6B gene mutations cause the death of rod photoreceptors, named as hereditary retinitis pigmentosa (RP) in humans and retinal degeneration (RD) in rodents. Here, we report a new RD model, identified from a phenotypic screen of N-ethyl-N-nitrosourea (ENU)-induced mutant mice, which displays retinal degeneration caused by a point mutation in the Pde6b gene that results in PDE6B-T592I mutant protein. The homozygous mutant mice show an extensive loss of rod photoreceptors at the age of 3 weeks; unexpectedly, the loss of rod photoreceptors can be partly rescued by dark rearing. Thus, this RD mutant model displays a light-dependent loss of rod photoreceptors. Both western blot and immunostaining results show very low level of mutant PDE6B-T592I protein in the retina. Structure modeling suggests that the T592I mutation probably affects the function and stability of PDE6B protein by changing intramolecular interactions. We further demonstrate that the expression of wild-type PDE6B delivered by subretinally injected adeno-associated virus (rAAV) prevents photoreceptor cell death in this RD model in vivo. The PDE6B-T592I mutant is, therefore, a valuable RD model for evaluating rAAV-mediated treatment and for investigating the molecular mechanism of light-dependent rod photoreceptor cell death that is related to impaired PDE6B function.

A Novel Intronic Deletion in PDE6B Causes Autosomal Recessive Retinitis Pigmentosa by Interfering with RNA Splicing

INTRODUCTION

Retinitis pigmentosa (RP) is a rare degenerative retinal disease caused by mutations in approximately seventy genes. Currently, despite the availability of large-scale DNA sequencing technologies, ∼30-40% of patients still cannot be diagnosed at the molecular level. In this study, we investigated a novel intronic deletion of PDE6B, encoding the beta subunit of phosphodiesterase 6 in association with recessive RP.

METHODS

Three unrelated consanguineous families were recruited from the northwestern part of Pakistan. Whole exome sequencing was performed for the proband of each family, and the data were analyzed according to an in-house computer pipeline. Relevant DNA variants in all available members of these families were assessed through Sanger sequencing. A minigene-based splicing assay was also performed.

RESULTS

The clinical phenotype for all patients was compatible with rod cone degeneration, with the onset during childhood. Whole exome sequencing revealed a homozygous 18 bp intronic deletion (NM_000283.3:c.1921-20_1921-3del) in PDE6B, which co-segregated with disease in 10 affected individuals. In vitro splicing tests showed that this deletion causes aberrant RNA splicing of the gene, leading to the in-frame deletion of 6 codons and, likely, to disease.

CONCLUSION

Our findings further expand the mutational spectrum of the PDE6B gene.

A novel homozygous missense substitution p.Thr313Ile in the PDE6B gene underlies autosomal recessive retinitis pigmentosa in a consanguineous Pakistani family

BACKGROUND

Retinitis pigmentosa (RP) is one of the most frequent hereditary retinal diseases that often starts with night blindness and eventually leads to legal blindness. Our study aimed to identify the underlying genetic cause of autosomal recessive retinitis pigmentosa (arRP) in a consanguineous Pakistani family.

METHODS

Following a detailed ophthalmological examination of the patients by an ophthalmologist, whole-exome sequencing was performed on the proband's DNA to delineate the genetic cause of RP in the family. In-depth computational methods, in-silico analysis, and familial co-segregation study were performed for variant detection and validation.

RESULTS

We studied an inbred Pakistani family with two siblings affected by retinitis pigmentosa. The proband, a 32 years old female, was clinically diagnosed with RP at the age of 6 years. A classical night blindness symptom was reported in the proband since her early childhood. OCT report showed a major reduction in the outer nuclear layer and the ellipsoid zone width, leading to the progression of the disease. Exome sequencing revealed a novel homozygous missense mutation (c.938C > T;p.Thr313Ile) in exon 12 of the PDE6B gene. The mutation p.Thr313Ile co-segregated with RP phenotype in the family. The altered residue (p.Thr313) was super conserved evolutionarily across different vertebrate species, and all available in silico tools classified the mutation as highly pathogenic.

CONCLUSION

We present a novel homozygous pathogenic mutation in the PDE6B gene as the underlying cause of arRP in a consanguineous Pakistani family. Our findings highlight the importance of missense mutations in the PDE6B gene and expand the known mutational repertoire of PDE6B-related RP.

Quantitative and qualitative characterization of retinal dystrophies in canine models of inherited retinal diseases using spectral domain optical coherence tomography (SD-OCT)

The purpose of this study was to establish spectral domain optical coherence tomography (SD-OCT) assessment data in well-established canine models of inherited retinal dystrophies: PDE6B-rod-cone dysplasia 1 (RCD1: early onset retinitis pigmentosa), PRCD-progressive rod-cone degeneration (PRCD: late onset retinitis pigmentosa), CNGB3-achromatopsia, and RPE65-Leber congenital amaurosis (LCA). High resolution SD-OCT images of the retina were acquired from both eyes in 5 planes: temporal; superotemporal; superior; nasal; and inferior in adult dogs with: RCD1 (n = 4 dogs, median age: 1.5 yrs); PRCD (n = 2, 4.3 yrs); LCA (n = 3, 5.2 yrs); achromatopsia (n = 3, 4.2 yrs); and wild types (wt, n = 6, 5.5 yrs). Total, inner and outer retinal thicknesses and ellipsoid zone were analyzed. In selected animals, histomorphometric evaluations were performed. In dogs with RCD1, PRCD, and LCA, the thickness of the outer retina was, compared to wt, significantly decreased (p ≤ 0.02) in all OCT imaging planes, and in superotemporal and inferior imaging planes in dogs with achromatopsia. No significant thinning was observed in inner retina thickness in any disease model except in the inferior imaging plane in dogs with RCD1. Dogs with RCD1, PRCD, and LCA had significantly more areas with disrupted ellipsoid zone in the presumed area centralis than wt (p ≤ 0.001). OCT findings provide baseline information for research of retinal dystrophies using these canine models.

Expression of exogenous PDE5 rescues photoreceptors in rd1 mice

BACKGROUND

Catalytic hydrolysis of cyclic guanosine monophosphate (cGMP) by phosphodiesterase 6 (PDE6) is a critical step in phototransduction signalling in photoreceptors. Mutations in the genes encoding any of the three PDE6 subunits are associated with retinitis pigmentosa, the most common form of inherited retinal diseases. The rd1 mouse carries a naturally occurring nonsense mutation in the Pde6b gene. The rd1 mouse retina rapidly degenerates and fails to form rod photoreceptor outer segments due to the elevated cGMP level and subsequent excessive Ca2+ influx. In this study, we aim to test whether the expression of PDE5, a non-photoreceptor-specific member of the PDE superfamily, rescues photoreceptors in the rd1 retina.

METHODS

The PDE5 expression plasmid was used to transfect neonatal rd1 mice by electroporation. The degeneration of the mouse retina was assessed by staining of retinal sections with DAPI. The expression and localization of phototransduction proteins in photoreceptors was analyzed by immunostaining. The expression of proteins in cultured cells was analyzed by immunoblotting.

RESULTS

The expression of exogenous PDE5, a non-photoreceptor-specific member of the PDE superfamily, prevents photoreceptor degeneration in rd1 mice. Unlike endogenous photoreceptor-specific PDE6 that was localised in the outer segments of photoreceptors, ectopically-expressed PDE5 was distributed in inner segments and synaptic terminals. PDE5 also promoted the development of the outer segments in rd1 mice. Co-expression of PDE5 and rhodopsin in cultured cells showed enhanced rhodopsin expression.

CONCLUSION

Lowering the cGMP level in photoreceptor by PDE5 is sufficient to rescue photoreceptors in rd1 retinas. cGMP may also be involved in the regulation of rhodopsin expression in photoreceptors.

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.

Patient-Specific Retinal Organoids Recapitulate Disease Features of Late-Onset Retinitis Pigmentosa

Although an increasing number of disease genes have been identified, the exact cellular mechanisms of retinitis pigmentosa (RP) remain largely unclear. Retinal organoids (ROs) derived from the induced pluripotent stem cells (iPSCs) of patients provide a potential but unvalidated platform for deciphering disease mechanisms and an advantageous tool for preclinical testing of new treatments. Notably, early-onset RP has been extensively recapitulated by patient-iPSC-derived ROs. However, it remains a challenge to model late-onset disease in a dish due to its chronicity, complexity, and instability. Here, we generated ROs from late-onset RP proband-derived iPSCs harboring a PDE6B mutation. Transcriptome analysis revealed a remarkably distinct gene expression profile in the patient ROs at differentiation day (D) 230. Changes in the expression genes regulating cGMP hydrolysis prompted the elevation of cGMP levels, which was verified by a cGMP enzyme-linked immunosorbent assay (ELISA) in patient ROs. Furthermore, significantly higher cGMP levels in patient ROs than in control ROs at D193 and D230 might lead to impaired formation of synaptic connections and the connecting cilium in photoreceptor cells. In this study, we established the first late-onset RP model with a consistent phenotype using an in vitro cell culture system and provided new insights into the PDE6B-related mechanism of RP.

Multiple genetic mutations implicate spectrum of phenotypes in Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a clinically and genetically heterogeneous ciliopathy with several clinical features including retinitis pigmentosa, obesity, kidney dysfunction, postaxial polydactyly, behavioral dysfunction and hypogonadism with wide spectrum of additional features. With multiple phenotypes and heterogeneous distribution, it is unlikely that BBS is caused by single gene defect. We have performed clinical and genetic diagnosis of two individuals from an Indian family with classical BBS symptoms. Whole exome sequencing identified homozygous missense mutation in BBS10 gene, hemizygous missense AR and homozygous missense PDE6B mutations in the proband and affected sibling with BBS. Identification of BBS10 mutation along with AR and PDE6B gene mutation will expand the genetic and phenotypic spectrum in individuals with BBS.

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.

Next-generation sequencing targeted disease panel in rod-cone retinal dystrophies in Māori and Polynesian reveals novel changes and a common founder mutation

IMPORTANCE

This study identifies unique genetic variation observed in a cohort of Māori and Polynesian patients with rod-cone retinal dystrophies using a targeted next-generation sequencing retinal disease gene panel.

BACKGROUND

With over 250 retinal disease genes identified, genetic diagnosis is still only possible in 60-70% of individuals and even less within unique ethnic groups.

DESIGN

Prospective genetic testing in patients with rod-cone retinal dystrophies identified from the New Zealand Inherited Retinal Disease Database, PARTICIPANTS: Sixteen patients of Māori and Polynesian ancestry.

METHODS

Next-generation sequencing of a targeted retinal gene panel. Sanger sequencing for a novel PDE6B mutation in subsequent Māori patients.

MAIN OUTCOME MEASURES

Genetic diagnosis, genotype-phenotype correlation.

RESULTS

Thirteen unique pathogenic variants were identified in 9 of 16 (56.25%) patients in 10 different genes. A definitive genetic diagnosis was made in 7/16 patients (43.7%). Six changes were novel and not in public databases of human variation. In four patients, a homozygous, novel pathogenic variant (c.2197G > C, p.(Ala 733Pro)) in PDE6B was identified and also present in a further five similarly affected Māori patients.

CONCLUSIONS AND RELEVANCE

Over half of the Māori and Polynesian patients with inherited rod-cone diseases have no pathogenic variant(s) detected with a targeted retinal next-generation sequencing strategy, which is supportive of novel genetic mechanisms in this population. A novel PDE6B founder variant is likely to account for 16% of recessive inherited retinal dystrophy in Māori. Careful characterization of the clinical presentation permits identification of further Māori patients with a similar phenotype and simplifies the diagnostic algorithm.

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.