Phenotyping and genotyping inherited retinal diseases: Molecular genetics, clinical and imaging features, and therapeutics of macular dystrophies, cone and cone-rod dystrophies, rod-cone dystrophies, Leber congenital amaurosis, and cone dysfunction syndromes

Inherited retinal diseases (IRD) are a leading cause of blindness in the working age population and in children. The scope of this review is to familiarise clinicians and scientists with the current landscape of molecular genetics, clinical phenotype, retinal imaging and therapeutic prospects/completed trials in IRD. Herein we present in a comprehensive and concise manner: (i) macular dystrophies (Stargardt disease (ABCA4), X-linked retinoschisis (RS1), Best disease (BEST1), PRPH2-associated pattern dystrophy, Sorsby fundus dystrophy (TIMP3), and autosomal dominant drusen (EFEMP1)), (ii) cone and cone-rod dystrophies (GUCA1A, PRPH2, ABCA4, KCNV2 and RPGR), (iii) predominant rod or rod-cone dystrophies (retinitis pigmentosa, enhanced S-Cone syndrome (NR2E3), Bietti crystalline corneoretinal dystrophy (CYP4V2)), (iv) Leber congenital amaurosis/early-onset severe retinal dystrophy (GUCY2D, CEP290, CRB1, RDH12, RPE65, TULP1, AIPL1 and NMNAT1), (v) cone dysfunction syndromes (achromatopsia (CNGA3, CNGB3, PDE6C, PDE6H, GNAT2, ATF6), X-linked cone dysfunction with myopia and dichromacy (Bornholm Eye disease; OPN1LW/OPN1MW array), oligocone trichromacy, and blue-cone monochromatism (OPN1LW/OPN1MW array)). Whilst we use the aforementioned classical phenotypic groupings, a key feature of IRD is that it is characterised by tremendous heterogeneity and variable expressivity, with several of the above genes associated with a range of phenotypes.

A natural history study of autosomal dominant GUCY2D-associated cone-rod dystrophy

PURPOSE

To describe the natural history of autosomal dominant (AD) GUCY2D-associated cone-rod dystrophies (CRDs), and evaluate associated structural and functional biomarkers.

METHODS

Retrospective analysis was conducted on 16 patients with AD GUCY2D-CRDs across two sites. Assessments included central macular thickness (CMT) and length of disruption to the ellipsoid zone (EZ) via optical coherence tomography (OCT), electroretinography (ERG) parameters, best corrected visual acuity (BCVA), and fundus autofluorescence (FAF).

RESULTS

At first visit, with a mean age of 30 years (range 5-70 years), 12 patients had a BCVA below Australian driving standard (LogMAR ≥ 0.3 bilaterally), and 1 patient was legally blind (LogMAR ≥ 1). Longitudinal analysis demonstrated a deterioration of LogMAR by - 0.019 per year (p < 0.001). This accompanied a reduction in CMT of - 1.4 µm per year (p < 0.0001), lengthened EZ disruption by 42 µm per year (p =  < 0.0001) and increased area of FAF by 0.05 mm2 per year (p = 0.027). Similarly, cone function decreased with increasing age, as demonstrated by decreasing b-wave amplitude of the light-adapted 30 Hz flicker and fused flicker (p = 0.005 and p = 0.018, respectively). Reduction in CMT and increased EZ disruption on OCT were associated with functional changes including poorer BCVA and decreased cone function on ERG.

CONCLUSION

We have described the natural long-term decline in vision and cone function associated with mutations in GUCY2D and identified a set of functional and structural biomarkers that may be useful as outcome parameters for future therapeutic clinical trials.

Occult Macular Dysfunction Syndrome: Identification of Multiple Pathologies in a Clinical Spectrum of Macular Dysfunction with Normal Fundus in East Asian Patients: EAOMD Report No. 5

Occult macular dystrophy (OMD) is the most prevalent form of macular dystrophy in East Asia. Beyond RP1L1, causative genes and mechanisms remain largely uncharacterised. This study aimed to delineate the clinical and genetic characteristics of OMD syndrome (OMDS). Patients clinically diagnosed with OMDS in Japan, South Korea, and China were enrolled. The inclusion criteria were as follows: (1) macular dysfunction and (2) normal fundus appearance. Comprehensive clinical evaluation and genetic assessment were performed to identify the disease-causing variants. Clinical parameters were compared among the genotype groups. Seventy-two patients with OMDS from fifty families were included. The causative genes were RP1L1 in forty-seven patients from thirty families (30/50, 60.0%), CRX in two patients from one family (1/50, 2.0%), GUCY2D in two patients from two families (2/50, 4.0%), and no genes were identified in twenty-one patients from seventeen families (17/50, 34.0%). Different severities were observed in terms of disease onset and the prognosis of visual acuity reduction. This multicentre large cohort study furthers our understanding of the phenotypic and genotypic spectra of patients with macular dystrophy and normal fundus. Evidently, OMDS encompasses multiple Mendelian retinal disorders, each representing unique pathologies that dictate their respective severity and prognostic patterns.

Development of an AAV-CRISPR-Cas9-based treatment for dominant cone-rod dystrophy 6

Cone-rod dystrophy 6 (CORD6) is caused by gain-of-function mutations in the GUCY2D gene, which encodes retinal guanylate cyclase-1 (RetGC1). There are currently no treatments available for this autosomal dominant disease, which is characterized by severe, early-onset visual impairment. The purpose of our study was to develop an adeno-associated virus (AAV)-CRISPR-Cas9-based approach referred to as "ablate and replace" and evaluate its therapeutic potential in mouse models of CORD6. This two-vector system delivers (1) CRISPR-Cas9 targeted to the early coding sequence of the wild-type and mutant GUCY2D alleles and (2) a CRISPR-Cas9-resistant cDNA copy of GUCY2D ("hardened" GUCY2D). Together, these vectors knock out ("ablate") expression of endogenous RetGC1 in photoreceptors and supplement ("replace") a healthy copy of exogenous GUCY2D. First, we confirmed that ablation of mutant R838S GUCY2D was therapeutic in a transgenic mouse model of CORD6. Next, we established a proof of concept for "ablate and replace" and optimized vector doses in Gucy2e+/-:Gucy2f-/- and Gucy2f-/- mice, respectively. Finally, we confirmed that the "ablate and replace" approach stably preserved retinal structure and function in a novel knockin mouse model of CORD6, the RetGC1 (hR838S, hWT) mouse. Taken together, our results support further development of the "ablate and replace" approach for treatment of CORD6.

Preclinical studies in support of phase I/II clinical trials to treat GUCY2D-associated Leber congenital amaurosis

Mutations in GUCY2D are associated with severe early-onset retinal dystrophy, Leber congenital amaurosis type 1 (LCA1), a leading cause of blindness in children. Despite a high degree of visual disturbance stemming from photoreceptor dysfunction, patients with LCA1 largely retain normal photoreceptor structure, suggesting that they are good candidates for gene replacement therapy. The purpose of this study was to conduct the preclinical and IND-enabling experiments required to support clinical application of AAV5-hGRK1-GUCY2D in patients harboring biallelic recessive mutations in GUCY2D. Preclinical studies were conducted in mice to evaluate the effect of vector manufacturing platforms and transgene species on the therapeutic response. Dose-ranging studies were conducted in cynomolgus monkeys to establish the minimum dose required for efficient photoreceptor transduction. Good laboratory practice (GLP) studies evaluated systemic biodistribution in rats and toxicology in non-human primates (NHPs). These results expanded our knowledge of dose response for an AAV5-vectored transgene under control of the human rhodopsin kinase (hGRK1) promoter in NHPs with respect to photoreceptor transduction and safety and, in combination with the rat biodistribution and mouse efficacy studies, informed the design of a first-in-human clinical study in patients with LCA1.

The Natural History of Leber Congenital Amaurosis and Cone-Rod Dystrophy Associated with Variants in the GUCY2D Gene

OBJECTIVE

To describe the spectrum of Leber congenital amaurosis (LCA) and cone-rod dystrophy (CORD) associated with the GUCY2D gene and to identify potential end points and optimal patient selection for future therapeutic trials.

DESIGN

International, multicenter, retrospective cohort study.

SUBJECTS

Eighty-two patients with GUCY2D-associated LCA or CORD from 54 families.

METHODS

Medical records were reviewed for medical history, best-corrected visual acuity (BCVA), ophthalmoscopy, visual fields, full-field electroretinography, and retinal imaging (fundus photography, spectral-domain OCT [SD-OCT], fundus autofluorescence).

MAIN OUTCOMES MEASURES

Age of onset, evolution of BCVA, genotype-phenotype correlations, anatomic characteristics on funduscopy, and multimodal imaging.

RESULTS

Fourteen patients with autosomal recessive LCA and 68 with autosomal dominant CORD were included. The median follow-up times were 5.2 years (interquartile range [IQR] 2.6-8.8 years) for LCA and 7.2 years (IQR 2.2-14.2 years) for CORD. Generally, LCA presented in the first year of life. The BCVA in patients with LCA ranged from no light perception to 1.00 logarithm of the minimum angle of resolution (logMAR) and remained relatively stable during follow-up. Imaging for LCA was limited but showed little to no structural degeneration. In patients with CORD, progressive vision loss started around the second decade of life. The BCVA declined annually by 0.022 logMAR (P < 0.001) with no difference between patients with the c.2513G>A and the c.2512C>T GUCY2D variants (P = 0.798). At the age of 40 years, the probability of being blind or severely visually impaired was 32%. The integrity of the ellipsoid zone (EZ) and that of the external limiting membrane (ELM) on SD-OCT correlated significantly with BCVA (Spearman ρ = 0.744, P = 0.001, and ρ = 0.712, P < 0.001, respectively) in those with CORD.

CONCLUSIONS

Leber congenital amaurosis associated with GUCY2D caused severe congenital visual impairment with relatively intact macular anatomy on funduscopy and available imaging, suggesting long preservation of photoreceptors. Despite large variability, GUCY2D-associated CORD generally presented during adolescence, with a progressive loss of vision, and culminated in severe visual impairment during mid-to-late adulthood. The integrity of the ELM and EZ may be suitable structural end points for therapeutic studies of GUCY2D-associated CORD.

New Insights on the Regulatory Gene Network Disturbed in Central Areolar Choroidal Dystrophy-Beyond Classical Gene Candidates

Central areolar choroidal dystrophy (CACD) is a rare hereditary disease that mainly affects the macula, resulting in progressive and usually profound visual loss. Being part of congenital retinal dystrophies, it may have an autosomal dominant or recessive inheritance and, until now, has no effective treatment. Given the shortage of genotypic information about the disease, this work systematically reviews the literature for CACD-causing genes. Three independent researchers selected 33 articles after carefully searching and filtering the Scielo, Pubmed, Lilacs, Web of Science, Scopus, and Embase databases. Mutations of six genes (PRPH2, GUCA1A, GUCY2D, CDHR1, ABCA4, and TTLL5) are implicated in the monogenic dominant inheritance of CACD. They are functionally related to photoreceptors (either in the phototransduction process, as in the case of GUCY2D, or the recovery of retinal photodegradation in photoreceptors for GUCA1A, or the formation and maintenance of specific structures within photoreceptors for PRPH2). The identified genetic variants do not explain all observed clinical features, calling for further whole-genome and functional studies for this disease. A network analysis with the CACD-related genes identified in the systematic review resulted in the identification of another 20 genes that may influence CACD onset and symptoms. Furthermore, an enrichment analysis allowed the identification of 13 transcription factors and 4 long noncoding RNAs interacting with the products of the previously mentioned genes. If mutated or dysregulated, they may be directly involved in CACD development and related disorders. More than half of the genes identified by bioinformatic tools do not appear in commercial gene panels, calling for more studies about their role in the maintenance of the retina and phototransduction process, as well as for a timely update of these gene panels.

Retinal degeneration-3 protein attenuates photoreceptor degeneration in transgenic mice expressing dominant mutation of human retinal guanylyl cyclase

Different forms of photoreceptor degeneration cause blindness. Retinal degeneration-3 protein (RD3) deficiency in photoreceptors leads to recessive congenital blindness. We proposed that aberrant activation of the retinal membrane guanylyl cyclase (RetGC) by its calcium-sensor proteins (guanylyl cyclase-activating protein [GCAP]) causes this retinal degeneration and that RD3 protects photoreceptors by preventing such activation. We here present in vivo evidence that RD3 protects photoreceptors by suppressing activation of both RetGC1 and RetGC2 isozymes. We further suggested that insufficient inhibition of RetGC by RD3 could contribute to some dominant forms of retinal degeneration. The R838S substitution in RetGC1 that causes autosomal-dominant cone-rod dystrophy 6, not only impedes deceleration of RetGC1 activity by Ca²⁺GCAPs but also elevates this isozyme's resistance to inhibition by RD3. We found that RD3 prolongs the survival of photoreceptors in transgenic mice harboring human R838S RetGC1 (R838S⁺). Overexpression of GFP-tagged human RD3 did not improve the calcium sensitivity of cGMP production in R838S⁺ retinas but slowed the progression of retinal blindness and photoreceptor degeneration. Fluorescence of the GFP-tagged RD3 in the retina only partially overlapped with immunofluorescence of RetGC1 or GCAP1, indicating that RD3 separates from the enzyme before the RetGC1:GCAP1 complex is formed in the photoreceptor outer segment. Most importantly, our in vivo results indicate that, in addition to the abnormal Ca²⁺ sensitivity of R838S RetGC1 in the outer segment, the mutated RetGC1 becomes resistant to inhibition by RD3 in a different cellular compartment(s) and suggest that RD3 overexpression could be utilized to reduce the severity of cone-rod dystrophy 6 pathology.

A reinterpretation of critical flicker-frequency (CFF) data reveals key details about light adaptation and normal and abnormal visual processing

Our ability to see flicker has an upper frequency limit above which flicker is invisible, known as the "critical flicker frequency" (CFF), that typically grows with light intensity (I). The relation between CFF and I, the focus of nearly 200 years of research, is roughly logarithmic, i.e., CFF ∝ log(I)-a relation called the Ferry-Porter law. However, why this law should occur, and how it relates to the underlying physiology, have never been adequately explained. Over the past two decades we have measured CFF in normal observers and in patients with retinal gene defects. Here, we reanalyse and model our data and historical CFF data. Remarkably, CFF-versus-I functions measured under a wide range of conditions in patients and in normal observers all have broadly similar shapes when plotted in double-logarithmic coordinates, i.e., log (CFF)-versus-log(I). Thus, the entire dataset can be characterised by horizontal and vertical logarithmic shifts of a fixed-shape template. Shape invariance can be predicted by a simple model of visual processing built from a sequence of low-pass filters, subtractive feedforward stages and gain adjustment (Rider, Henning & Stockman, 2019). It depends primarily on the numbers of visual processing stages that approach their power-law region at a given intensity and a frequency-independent gain reduction at higher light levels. Counter-intuitively, the CFF-versus-I relation depends primarily on the gain of the visual response rather than its speed-a conclusion that changes our understanding and interpretation of human flicker perception. The Ferry-Porter "law" is merely an approximation of the shape-invariant template.

Leber's Congenital Amaurosis: Current Concepts of Genotype-Phenotype Correlations

Leber’s congenital amaurosis (LCA), one of the most severe inherited retinal dystrophies, is typically associated with extremely early onset of visual loss, nystagmus, and amaurotic pupils, and is responsible for 20% of childhood blindness. With advances in molecular diagnostic technology, the knowledge about the genetic background of LCA has expanded widely, while disease-causing variants have been identified in 38 genes. Different pathogenetic mechanisms have been found among these varieties of genetic mutations, all of which result in the dysfunction or absence of their encoded proteins participating in the visual cycle. Hence, the clinical phenotypes also exhibit extensive heterogenicity, including the course of visual impairment, involvement of the macular area, alteration in retinal structure, and residual function of the diseased photoreceptor. By reviewing the clinical course, fundoscopic images, optical coherent tomography examination, and electroretinogram, genotype-phenotype correlations could be established for common genetic mutations in LCA, which would benefit the timing of the diagnosis and thus promote early intervention. Gene therapy is promising in the management of LCA, while several clinical trials are ongoing and preliminary success has been announced, focusing on RPE65 and other common disease-causing genes. This review provides an update on the genetics, clinical examination findings, and genotype-phenotype correlations in the most well-established causative genetic mutations of LCA.

Novel variants in GUCY2D causing retinopathy and the genotype-phenotype correlation

Leber congenital amaurosis (LCA) is the most severe form of retinopathy and cone/cone-rod dystrophy (CORD) is a common form of inherited retinopathy. Variants in GUCY2D constitute the most common cause of LCA and autosomal dominant CORD (ADCORD). The purpose of this study was to reveal novel variants and document associated phenotypes of patients with GUCY2D-associated retinopathy. Fifty-two potentially pathogenic variants (PPVs), including 12 novel ones (p.Gly144_Ala164del, p.Trp154Glyfs*12, p.Leu186Pro, p.Ala207Pro, p.Ala229Asp, p.Ala353Glu, p.Trp372*, p.Arg528*, p.Arg660Pro, p.Ile682Thr, p.Trp788Cys, and c.1026 + 171_*486del), were identified in 16 families with ADCORD and 34 families with autosomal recessive LCA (ARLCA). The novel variant c.1026 + 171_*486del is a large-scale (16.3 kb) deletion involving exons 4-20 of GUCY2D, and was identified in an ARLCA family in heterozygous status mimicking a homozygous p.Trp788Cys variant. Among the detected 52 PPVs, 32 (61.5%) were missense, seven (13.5%) were splicing, six (11.5%) were nonsense, four (7.7%) were inframe indel, and three (5.8%) were frameshift deletion. The median age of examination in 27 patients with ADCORD was 21.0 years (ranges 3-54) with a median visual acuity (VA) of 0.10 (ranges 0.02-0.90). There were 48.0% of patients with macular atrophy, 86.4% with severe reduced or extinguished cone responses, 77.3% with normal or mildly reduced rod responses, and 60.9% with high myopia. Visual impairment, macular dystrophy, and cone dysfunction deteriorated with age. The median age of examination in 34 patients with ARLCA was 1.1 years (ranges 0.3-25). There were 55.9% of patients with roving nystagmus, 68.2% with VA of worse than hand motion, 59.4% with almost normal fundus, 90.6% with extinguished rod and cone responses, and 50.0% with high hyperopia. In conclusions, twelve novel PPVs in GUCY2D (including a novel large-scale deletion) were identified. Most (32/52, 61.5%) of causative GUCY2D variants were missense. Progressive development of macular atrophy, cone dysfunction, visual impairment, and myopia are four major characteristics of GUCY2D-associated ADCORD. Normal fundus, roving nystagmus, and hypermetropia in early age are common findings specific to GUCY2D-associated ARLCA. The obtained data in this study will be of value in counselling patients and designing future therapeutic approaches.

Regulation of retinal membrane guanylyl cyclase (RetGC) by negative calcium feedback and RD3 protein

This article presents a brief overview of the main biochemical and cellular processes involved in regulation of cyclic GMP production in photoreceptors. The main focus is on how the fluctuations of free calcium concentrations in photoreceptors between light and dark regulate the activity of retinal membrane guanylyl cyclase (RetGC) via calcium sensor proteins. The emphasis of the review is on the structure of RetGC and guanylyl cyclase activating proteins (GCAPs) in relation to their functional role in photoreceptors and congenital diseases of photoreceptors. In addition to that, the structure and function of retinal degeneration-3 protein (RD3), which regulates RetGC in a calcium-independent manner, is discussed in detail in connections with its role in photoreceptor biology and inherited retinal blindness.

GUCY2D mutations in retinal guanylyl cyclase 1 provide biochemical reasons for dominant cone-rod dystrophy but not for stationary night blindness

Mutations in the GUCY2D gene coding for the dimeric human retinal membrane guanylyl cyclase (RetGC) isozyme RetGC1 cause various forms of blindness, ranging from rod dysfunction to rod and cone degeneration. We tested how the mutations causing recessive congenital stationary night blindness (CSNB), recessive Leber's congenital amaurosis (LCA1), and dominant cone-rod dystrophy-6 (CORD6) affected RetGC1 activity and regulation by RetGC-activating proteins (GCAPs) and retinal degeneration-3 protein (RD3). CSNB mutations R666W, R761W, and L911F, as well as LCA1 mutations R768W and G982VfsX39, disabled RetGC1 activation by human GCAP1, -2, and -3. The R666W and R761W substitutions compromised binding of GCAP1 with RetGC1 in HEK293 cells. In contrast, G982VfsX39 and L911F RetGC1 retained the ability to bind GCAP1 in cyto but failed to effectively bind RD3. R768W RetGC1 did not bind either GCAP1 or RD3. The co-expression of GUCY2D allelic combinations linked to CSNB did not restore RetGC1 activity in vitro The CORD6 mutation R838S in the RetGC1 dimerization domain strongly dominated the Ca2+ sensitivity of cyclase regulation by GCAP1 in RetGC1 heterodimer produced by co-expression of WT and the R838S subunits. It required higher Ca2+ concentrations to decelerate GCAP-activated RetGC1 heterodimer-6-fold higher than WT and 2-fold higher than the Ser838-harboring homodimer. The heterodimer was also more resistant than homodimers to inhibition by RD3. The observed biochemical changes can explain the dominant CORD6 blindness and recessive LCA1 blindness, both of which affect rods and cones, but they cannot explain the selective loss of rod function in recessive CSNB.

Variants at codon 838 in the GUCY2D gene result in different phenotypes of cone rod dystrophy

BACKGROUND

The GUCY2D gene encodes the photoreceptor guanylate cyclase (GC-E) and different pathogenic variants can lead to Leber congenital amaurosis (LCA) or cone-rod dystrophy (CRD). In this study, we describe three unrelated families who carried different mutations at codon 838 of the GUCY2D gene, and presented different phenotypes of retinal degeneration.

MATERIALS AND METHODS

Family and personal histories were collected, and the patients underwent best corrected visual acuity (BCVA), fundus photography (FP), electroretinography (ERG), optical coherence tomography (OCT) and fundus autofluorescence (FAF). Venous blood was drawn from patients and family members, and genomic DNA was extracted. Next-generation sequencing of known ocular genes was applied to the proband to find pathogenic variants. Polymerase chain reaction (PCR) and Sanger sequencing were conducted for validation and segregation.

RESULTS

Six patients from three unrelated families were enrolled. All the patients manifested decreased vision, photophobia and myopia from childhood. ERG recordings demonstrated a significant reduction in cone responses for all patients, while rod responses ranged widely from normal to moderately reduced. All patients were diagnosed with CRD, but the disease severity and progression rates in the three families were significantly different. Three pathogenic variants in the GUCY2D gene (c.2512 C > T (p.R838C), c.2512 C > A (p.R838S) and c.2513 G > A (p.R838H)) were identified.

CONCLUSIONS

We presented the phenotypes of three Chinese adCRD families carrying different variants at codon 838 of the GUCY2D gene. The R838S variant is a novel genotype associated with GUCY2D-CRD. The R838H variant can cause severe retinal features. Our findings enhance the understanding of GUCY2D phenotypic diversity.

Novel GUCY2D Variant (E843Q) at Mutation Hotspot Associated with Macular Dystrophy in a Japanese Patient

BACKGROUND

The GUCY2D (guanylate cyclase 2D) gene encodes a photoreceptor guanylate cyclase (GC-E), that is predominantly expressed in the cone outer segments. Mutations in the GUCY2D lead to severe retinal disorders such as autosomal dominant cone-rod dystrophy (adCRD) and autosomal recessive Leber congenital amaurosis type 1. The purpose of this study was to identify the phenotype of a Japanese patient with a probably pathogenic GUCY2D variant.

METHODS

Detailed ophthalmic examinations were performed, and whole exome sequencing was performed on DNA obtained from the patient. The variants identified by exome sequencing and targeted analysis were further confirmed by direct sequencing.

RESULTS

A 47-year-old man had atrophic and pigmentary changes in the macula of both eyes. Amplitudes and implicit times on full-field electroretinograms (ERGs) were within normal limits; however, the densities of multifocal ERGs in the central area were reduced in both eyes. Whole exome sequencing identified heterozygous variant c.2527G>C, p.Glu843Gln in the GUCY2D gene within the mutation hot spot for adCRD. The allelic frequencies of this variant are extremely low and, according to American College of Medical Genetics and Genomics standards and guidelines, the variants are classified as likely pathogenic.

CONCLUSIONS

This is the first report of a heterozygous variant, c.2527G>C, p.Glu843Gln, in the GUCY2D, in a patient presenting with mild macular dystrophy without a general reduction in cone function. Our findings expand the spectrum of the clinical phenotypes of GUCY2D-adCRD and help clarify the morphological and functional changes caused by defects of dimerization of GC-E in the phototransduction cascade.

Clinical and Genetic Characteristics of 15 Affected Patients From 12 Japanese Families with GUCY2D-Associated Retinal Disorder

Purpose

To determine the clinical and genetic characteristics of patients with GUCY2D-associated retinal disorder (GUCY2D-RD).

Methods

Fifteen patients from 12 families with inherited retinal disorder (IRD) and harboring GUCY2D variants were ascertained from 730 Japanese families with IRD. Comprehensive ophthalmological examinations, including visual acuity (VA) measurement, retinal imaging, and electrophysiological assessment were performed to classify patients into three phenotype subgroups; macular dystrophy (MD), cone-rod dystrophy (CORD), and Leber congenital amaurosis (LCA). In silico analysis was performed for the detected variants, and the molecularly confirmed inheritance pattern was determined (autosomal dominant/recessive [AD/AR]).

Results

The median age of onset/examination was 22.0/38.0 years (ranges, 0-55 and 1-73) with a median VA of 0.80/0.70 LogMAR units (ranges, 0.00-1.52 and 0.10-1.52) in the right/left eye, respectively. Macular atrophy was identified in seven patients (46.7%), and two had diffuse fundus disturbance (13.3%), and six had an essentially normal fundus (40.0%). There were 11 patients with generalized cone-rod dysfunction (78.6%), two with entire functional loss (14.3%), and one with confined macular dysfunction (7.1%). There were nine families with ADCORD, one with ARCORD, one with ADMD, and one with ARLCA. Ten GUCY2D variants were identified, including four novel variants (p.Val56GlyfsTer262, p.Met246Ile, p.Arg761Trp, p.Glu874Lys).

Conclusions

This large cohort study delineates the disease spectrum of GUCY2D-RD. Diverse clinical presentations with various severities of ADCORD and the early-onset severe phenotype of ARLCA are illustrated. A relatively lower prevalence of GUCY2D-RD for ADCORD and ARLCA in the Japanese population was revealed.

Translational Relevance

The obtained data help to monitor and counsel patients, especially in East Asia, as well as to design future therapeutic approaches.

Possible dual contribution of a novel GUCY2D mutation in the development of retinal degeneration in a consanguineous population

Molecular characterization of novel mutations in Leber Congenital Amaurosis (LCA) disease improves the disease diagnosis and contributes to the development of preventive and therapeutic approaches. We studied an isolated inbred population in Iran with a high prevalence of retinal degeneration with clinical variability. The clinical examinations were performed on eight patients belonging to three consanguineous families. The identical-by-descent (IBD) mapping technique was employed to identify the shared loci in patients. Subsequently, Sanger sequencing of the GUCY2D gene, in silico analysis, as well as segregation study were conducted. The whole-exome sequencing method was applied for negative cases of GUCY2D mutation, followed by segregation study in suspected variants among families. A novel deletion mutation in the GUCY2D gene can explain the emergence of LCA-1 in most patients but not all. Besides, a heterozygous variant of uncertain significance (VUS) was observed in the BEST1 gene in some healthy and participant patients. These results further support inter/intra-familial clinical heterogeneity in retinal dystrophy and suggest that screening the GUCY2D gene would be needed for the diagnosis of LCA in Iranian people living in the central regions. The variant in the BEST1 gene might be considered a benign heterozygous variant; however, we hypothesized a possible double heterozygosity in both GUCY2D and BEST1 genes that may cause the pathogenesis of cone-rod dystrophy-6 (CRD-6) disease. This would propose a new scenario for the pathogenesis of a monogenic disorder such as CRD-6 disease in which other genetic elements may be involved in the development of the disease.

Somatic Gene Editing of GUCY2D by AAV-CRISPR/Cas9 Alters Retinal Structure and Function in Mouse and Macaque

Mutations in GUCY2D, the gene encoding retinal guanylate cyclase-1 (retGC1), are the leading cause of autosomal dominant cone-rod dystrophy (CORD6). Significant progress toward clinical application of gene replacement therapy for Leber congenital amaurosis (LCA) due to recessive mutations in GUCY2D (LCA1) has been made, but a different approach is needed to treat CORD6 where gain of function mutations cause dysfunction and dystrophy. The CRISPR/Cas9 gene editing system efficiently disrupts genes at desired loci, enabling complete gene knockout or homology directed repair. Here, adeno-associated virus (AAV)-delivered CRISPR/Cas9 was used specifically to edit/disrupt this gene's early coding sequence in mouse and macaque photoreceptors in vivo, thereby knocking out retGC1 expression and demonstrably altering retinal function and structure. Neither preexisting nor induced Cas9-specific T-cell responses resulted in ocular inflammation in macaques, nor did it limit GUCY2D editing. The results show, for the first time, the ability to perform somatic gene editing in primates using AAV-CRISPR/Cas9 and demonstrate the viability this approach for treating inherited retinal diseases in general and CORD6 in particular.

Long term follow-up of a family with GUCY2D dominant cone dystrophy

AIM

To describe long term follow-up in a family with GUCY2D dominant cone dystrophy.

METHODS

Optical coherence tomography scans and fundus autofluorescence images were obtained. Flash and pattern electroretinograms (ERGs) and occipital pattern reversal visual evoked potentials were recorded.

RESULTS

Two members of the same family (father and son) were identified to have the heterozygous R838C mutation in the GUCY2D gene. The father presented at the age of 45 with bilateral bull's eye maculopathy and temporal disc pallor. Over 13y of serial follow up visits, the bull's eye maculopathy progressed gradually into macular atrophy. Electrophysiological tests were significantly degraded suggesting poor macular function. Spectral-domain optical coherence tomography (SD-OCT) scans showed progressive loss and disruption of the ellipsoid layer at the foveal level. His son presented at the age of 16 with bilateral granular retinal pigment epithelial changes in both maculae. Electrophysiological testing was initially borderline normal but has gradually deteriorated to show reduced cone ERGs and macula function. SD-OCT demonstrated gradual macular thinning and atrophy bilaterally. Unlike his father, there was no disruption of the ellipsoid layer.

CONCLUSION

Both family members exhibited gradual changes in their fundi, electrophysiological testing and multimodal imaging. Changes were milder than those observed in other mutations of the same gene.

Novel GUCY2D mutation causes phenotypic variability of Leber congenital amaurosis in a large kindred

Background

Leber congenital amaurosis (LCA) is a severe retinal degenerative disease that manifests as blindness or poor vision in infancy. The purpose of this study was to clinically characterize and identify the cause of disease in a large inbred Bedouin Israeli tribe with LCA.

Methods

Thirty individuals of a single kindred, including eight affected with LCA, were recruited for this study. Patients’ clinical data and electroretinography (ERG) findings were collected. Molecular analysis included homozygosity mapping with polymorphic markers and Sanger sequencing of candidate genes.

Results

Of the eight affected individuals of the kindred, nystagmus was documented in five subjects and keratoconus in three. Cataract was found in 5 of 16 eyes. Photopic and scotopic ERG performed in 5 patients were extinguished. All affected subjects were nearly blind, their visual acuity ranged between finger counting and uncertain light perception. Assuming autosomal recessive heredity of a founder mutation, studies using polymorphic markers excluded homozygosity of affected individuals at the genomic loci of all previously known genes associated with LCA, except GUCY2D. Sequencing of GUCY2D identified a novel missense mutation (c.2129C>T; p.Ala710Val) resulting in substitution of alanine by valine at position 710 within the protein kinase domain of the retina-specific enzyme guanylate cyclase 1 (GC1) encoded by GUCY2D. Molecular modeling implied that the mutation changes the conformation of the regulatory segment within the kinase styk-domain of GC1 and causes loss of its helical structure, likely inhibiting phosphorylation of threonine residue within this segment, which is needed to activate the catalytic domain of the protein.

Conclusions

This is the first documentation of the p.Ala710Val mutation in GC1 and the second ever described mutation in its protein kinase domain. Our findings enlarge the scope of genetic variability of LCA, highlight the phenotypic heterogeneity found amongst individuals harboring an identical LCA mutation, and possibly provide hope for gene therapy in patients with this congenital blinding disease. As the Bedouin kindred studied originates from Saudi Arabia, the mutation found might be an ancient founder mutation in that large community.

A Mini-review: Animal Models of GUCY2D Leber Congenital Amaurosis (LCA1)

GUCY2D encodes retinal guanylate cylase-1 (retGC1), a protein that plays a pivotal role in the recovery phase of phototransduction. Mutations in GUCY2D are associated with a leading cause of recessive Leber congenital amaurosis (LCA1). Patients present within the first year of life with aberrant or unrecordable electroretinogram (ERG), nystagmus and a relatively normal fundus. Aside from abnormalities in the outer segments of foveal cones and, in some patients, foveal cone loss, LCA1 patients retain normal retinal laminar architecture suggesting they may be good candidates for gene replacement therapy. Several animal models of LCA1, both naturally occurring and engineered, have been characterized and provide valuable tools for translational studies. This mini-review will summarize the phenotypes of these models and describe how each has been instrumental in proof of concept studies to develop a gene replacement therapy for GUCY2D-LCA1.

Insights gained from gene therapy in animal models of retGC1 deficiency

Vertebrate species possess two retinal guanylate cyclases (retGC1 and retGC2) and at least two guanylate cyclase activating proteins (GCAPs), GCAP1 and GCAP2. GCAPs function as Ca²⁺ sensors that regulate the activity of guanylate cyclases. Together, these proteins regulate cGMP and Ca²⁺ levels within the outer segments of rod and cone photoreceptors. Mutations in GUCY2D, the gene that encodes retGC1, are a leading cause of the most severe form of early onset retinal dystrophy, Leber congenital amaurosis (LCA1). These mutations, which reduce or abolish the ability of retGC1 to replenish cGMP in photoreceptors, are thought to lead to the biochemical equivalent of chronic light exposure in these cells. In spite of this, the majority of LCA1 patients retain normal photoreceptor laminar architecture aside from foveal cone outer segment abnormalities, suggesting they may be good candidates for gene replacement therapy. Work began in the 1980s to characterize multiple animal models of retGC1 deficiency. 34 years later, all models have been used in proof of concept gene replacement studies toward the goal of developing a therapy to treat GUCY2D-LCA1. Here we use the results of these studies as well as those of recent clinical studies to address specific questions relating to clinical application of a gene therapy for treatment of LCA1.