Allelic and phenotypic heterogeneity in ABCA4 mutations

Retinoic acid related orphan receptor α is a genetic modifier that rescues retinal degeneration in a mouse model of Stargardt disease and Dry AMD

Degeneration of the macula is associated with several overlapping diseases including age-related macular degeneration (AMD) and Stargardt Disease (STGD). Mutations in ATP Binding Cassette Subfamily A Member 4 (ABCA4) are associated with late-onset dry AMD and early-onset STGD. Additionally, both forms of macular degeneration exhibit deposition of subretinal material and photoreceptor degeneration. Retinoic acid related orphan receptor α (RORA) regulates the AMD inflammation pathway that includes ABCA4, CD59, C3 and C5. In this translational study, we examined the efficacy of RORA at attenuating retinal degeneration and improving the inflammatory response in Abca4 knockout (Abca4-/-) mice. AAV5-hRORA-treated mice showed reduced deposits, restored CD59 expression and attenuated amyloid precursor protein (APP) expression compared with untreated eyes. This molecular rescue correlated with statistically significant improvement in photoreceptor function. This is the first study evaluating the impact of RORA modifier gene therapy on rescuing retinal degeneration. Our studies demonstrate efficacy of RORA in improving STGD and dry AMD-like disease.

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.

ABCA4 variant screening in a Turkish cohort with Stargardt disease

PURPOSE

This study aims to evaluate the ABCA4 variants in patients diagnosed with Stargardt disease.

METHODS

This is a retrospective study designed to investigate variants in the ABCA4 in Stargardt disease and the clinical findings of the cases. Sex, age, age of onset of symptoms, best-corrected visual acuity, color fundus photography, optical coherence tomography, and visual field test of the patients were recorded. Genetic analyses were screened, and patients with at least two variants in the ABCA4 were included in this study.

RESULTS

Twenty-seven patients diagnosed with Stargardt disease with the ABCA4 variants were included in this study. Twelve of them (44.4%) were female and fifteen (55.5%) were male. The mean age of the cases was 27.44 years (ranging from 8 to 56 years). Thirty different variants were detected in 54 ABCA4 alleles of 27 patients. The two most common pathogenic variants were c.5882 G>A p.(Gly1961Glu) and c.52C>T p.(Arg18Trp) in this cohort. Two novel variants were identified (c.3855_3856dup, c.1554 + 3_1554 + 4del) and the patient with the c.1554 + 3_1554 + 4del variant additionally had a different ABCA4 variant in trans. The other novel variant was homozygous.

CONCLUSIONS

In this study, two novel variants were described in a Turkish cohort with Stargardt disease. The variant c.52C>T p.(Arg18Trp) was the most common disease-causing variant besides the c.5882 G>A p.(Gly1961Glu) which was identified frequently in the previous studies. A larger sample size is necessary for describing different pathogenic variants and understanding the phenotype-genotype correlations.

Stargardt macular dystrophy and therapeutic approaches

Stargardt macular dystrophy (Stargardt disease; STGD1; OMIM 248200) is the most prevalent inherited macular dystrophy. STGD1 is an autosomal recessive disorder caused by multiple pathogenic sequence variants in the large ABCA4 gene (OMIM 601691). Major advances in understanding both the clinical and molecular features, as well as the underlying pathophysiology, have culminated in many completed, ongoing and planned human clinical trials of novel therapies.The aims of this concise review are to describe (1) the detailed phenotypic and genotypic characteristics of the disease, multimodal imaging findings, natural history of the disease, and pathogenesis, (2) the multiple avenues of research and therapeutic intervention, including pharmacological, cellular therapies and diverse types of genetic therapies that have either been investigated or are under investigation and (3) the exciting novel therapeutic approaches on the translational horizon that aim to treat STGD1 by replacing the entire 6.8 kb ABCA4 open reading frame.

ABCA4 Variant c.5714+5G>A in Trans With Null Alleles Results in Primary RPE Damage

Purpose

To determine the disease pathogenesis associated with the frequent ABCA4 variant c.5714+5G>A (p.[=,Glu1863Leufs*33]).

Methods

Patient-derived photoreceptor precursor cells were generated to analyze the effect of c.5714+5G>A on splicing and perform a quantitative analysis of c.5714+5G>A products. Patients with c.5714+5G>A in trans with a null allele (i.e., c.5714+5G>A patients; n = 7) were compared with patients with two null alleles (i.e., double null patients; n = 11); with a special attention to the degree of RPE atrophy (area of definitely decreased autofluorescence and the degree of photoreceptor impairment (outer nuclear layer thickness and pattern electroretinography amplitude).

Results

RT-PCR of mRNA from patient-derived photoreceptor precursor cells showed exon 40 and exon 39/40 deletion products, as well as the normal transcript. Quantification of products showed 52.4% normal and 47.6% mutant ABCA4 mRNA. Clinically, c.5714+5G>A patients displayed significantly better structural and functional preservation of photoreceptors (thicker outer nuclear layer, presence of tubulations, higher pattern electroretinography amplitude) than double null patients with similar degrees of RPE loss, whereas double null patients exhibited signs of extensive photoreceptor ,damage even in the areas with preserved RPE.

Conclusions

The prototypical STGD1 sequence of events of primary RPE and secondary photoreceptor damage is congruous with c.5714+5G>A, but not the double null genotype, which implies different and genotype-dependent disease mechanisms. We hypothesize that the relative photoreceptor sparing in c.5714+5G>A patients results from the remaining function of the ABCA4 transporter originating from the normally spliced product, possibly by decreasing the direct bisretinoid toxicity on photoreceptor membranes.

Natural History of Stargardt Disease: The Longest Follow-Up Cohort Study

Long-term natural history studies are important in rare disease research. This study aimed to assess electrophysiological and fundus autofluorescence (FAF) progression rate in 18 genetically confirmed Stargardt disease (STGD1) patients with a minimum follow-up of 10 years. Age at the first and last exams, age at onset, Snellen decimal visual acuity (VA), electroretinography (ERG), and FAF images were evaluated. Patients were classified into four Fishman stages and three electroretinography groups, and areas of definitely decreased autofluorescence (DDAF) were measured. Patients were further substratified based on genotype, and phenotype-genotype correlations were performed. The median follow-up was 18 (range 10-26) years. The median yearly VA loss was 0.009 (range 0.002-0.071), while the median progression rate of the DDAF area was 0.354 (range 0.002-4.359) mm² per year. Patients harbouring p.(Gly1961Glu) or p.(Asn1868Ile) allele had significantly slower DDAF area progression when compared to patients with other genotypes (0.07 mm² vs. 1.03 mm², respectively), as well as significantly later age at onset (20 years vs. 13 years, respectively). Results showed that structural and functional parameters, together with genotype, should be considered when counselling patients regarding prognosis and monitoring disease progression. Patients harbouring hypomorphic variants p.(Gly1961Glu) or p.(Asn1868Ile) presented with overall milder disease than patients with other genotypes.

Monitoring Lesion Area Progression in Stargardt Disease: A Comparison of En Face Optical Coherence Tomography and Fundus Autofluorescence

Purpose

To compare longitudinal changes in en face spectral domain-optical coherence tomography (SD-OCT) measurements of ellipsoid zone (EZ) and retinal pigment epithelium (RPE) loss to changes in the hypoautofluorescent and hyperautofluorescent (AF) areas detected with short-wavelength (SW)-AF in ABCA4-associated retinopathy.

Methods

SD-OCT volume scans were obtained from 20 patients (20 eyes) over 2.6 ± 1.2 years (range 1-5 years). The EZ, and RPE/Bruch's membrane boundaries were segmented, and en face slab images generated. SubRPE and EZ slab images were used to measure areas of atrophic RPE and EZ loss. These were compared to longitudinal measurements of the hypo- and abnormal AF (hypoAF and surrounding hyperAF) areas.

Results

At baseline, the en face area of EZ loss was significantly larger than the subRPE atrophic area, and the abnormal AF area was significantly larger than the hypoAF area. The median rate of EZ loss was significantly greater than the rate of increase in the subRPE atrophic area (1.2 mm2/yr compared to 0.5 mm2/yr). The median rate of increase in the abnormal AF area was significantly greater than the increase in the hypoAF area (1.6 mm2/yr compared to 0.6 mm2/yr).

Conclusions

En face SD-OCT can be used to quantify changes in RPE atrophy and photoreceptor integrity. It can be a complementary or alternative technique to SW-AF with the advantage of monitoring EZ loss. The SW-AF results emphasize the importance of measuring changes in the hypo- and abnormal AF areas.

Translational Relevance

The findings are relevant to the selection of outcome measures for monitoring ABCA4-associated retinopathy.

NOVEL PRPH2/RDS MUTATION IDENTIFIED IN A FAMILY WITH VARYING CLINICAL MANIFESTATIONS: A CASE REPORT

PURPOSE

To present the case of a family with a novel PRPH2/RDS mutation.

METHODS

A case report of a 44-year-old woman and her immediate family, including the father and a sister who shared her PRPH2/RDS mutation.

RESULTS

A 44-year-old woman presented with examination findings consistent with a butterfly-type pattern dystrophy. A sister had a similar butterfly-type dystrophy, whereas their father had a severe cone-rod dystrophy. Genetic testing revealed the same novel PRPH2/RDS mutation in all three affected individuals, suggesting that this single mutation can produce at least two disparate retinal disease phenotypes.

CONCLUSION

This case describes a novel p.Y225X nonsense mutation in the PRPH2/RDS gene and demonstrates that it is both pathologic and capable of significant phenotypic variability.

A five-year follow-up of ABCA4 carriers showing deterioration of retinal function and increased structural changes

PURPOSE

To investigate whether the reduced retinal function and morphological retinal changes previously demonstrated in ABCA4 carriers had remained stationary or had deteriorated over time at 5-year follow-up to further explore if carriers of an autosomal recessive trait also express a weak phenotype, although this is not expected for an autosomal recessive disorder.

METHODS

Thirteen ABCA4 carriers from a previous study that included parents to patients with well known genetically verified ABCA4-associated retinal degenerations were reexamined 5 years after the initial examination. As novel genes and new variants in already established genes are continuously reported, all subjects underwent renewed genetic testing with a next-generation sequencing (NGS) panel that included 288 genes associated with retinal dystrophies and an analysis of deep intronic mutations and copy number variations in the ABCA4 gene. Moreover, to evaluate any changes in retinal function and/or structure over time, clinical reassessment with Goldmann perimetry, visual acuity testing, fundus photography, fundus autofluorescence (FAF) imaging, optical coherence tomography (OCT), full-field electroretinography (ffERG), and multifocal ERG (mfERG) were performed 5 years after the initial investigation. The values of the ffERG parameters were compared between the two time points (the measurements obtained in the initial study versus the measurements at 5-year follow-up) and with the controls. The mfERG results of the carriers were compared with those of the controls.

RESULTS

The renewed genetic testing confirmed the previously established ABCA4 mutations but also revealed the hypomorph ABCA4 variant c.5603A>T in five ABCA4 carriers. In three of them, the variant was found to be associated with known disease-causing alleles that always carry the c.5603A>T in cis. According to recent publications, the subjects could still be considered ABCA4 carriers because both variants are on the same allele. In the remaining two subjects, c.5603A>T could be in trans with the previously known ABCA4 variant, and the subjects were therefore excluded from the study since they could no longer be considered as carriers only. Statistical comparison of ffERG parameters showed significant reduction of the isolated rod, -as well as the combined rod-cone amplitudes over the five years of follow-up, but not compared with the controls. Concerning macular function, mfERG amplitudes were reduced for all rings in the carriers compared with the controls. Fundus photographs demonstrated morphological changes in 64% of the carriers, and 36% of them had further changes at follow-up. FAF images showed alterations in 55% of the carriers, with increased changes in 36% of them. Abnormalities on OCT were observed in 82% of the carriers, of whom 9% had newly found abnormalities at follow-up.

CONCLUSIONS

At 5-year follow-up, the ABCA4 carriers, who previously demonstrated reduced macular function, presented with deterioration of general retinal function, including reduced isolated rod and mixed rod-cone ffERG responses combined with a slight increase in morphological changes in some subjects. This indicates that carriership of at least some ABCA4 variants may cause a condition similar to a subgroup of dry age-related macular degeneration (AMD). In the long run, this might be of importance concerning the possibilities to also treat this subgroup of AMD patients with future gene-based and pharmacological drugs targeting ABCA4-associated disorders.

Inherited retinal diseases: Linking genes, disease-causing variants, and relevant therapeutic modalities

Inherited retinal diseases (IRDs) are a clinically complex and heterogenous group of visual impairment phenotypes caused by pathogenic variants in at least 277 nuclear and mitochondrial genes, affecting different retinal regions, and depleting the vision of affected individuals. Genes that cause IRDs when mutated are unique by possessing differing genotype-phenotype correlations, varying inheritance patterns, hypomorphic alleles, and modifier genes thus complicating genetic interpretation. Next-generation sequencing has greatly advanced the identification of novel IRD-related genes and pathogenic variants in the last decade. For this review, we performed an in-depth literature search which allowed for compilation of the Global Retinal Inherited Disease (GRID) dataset containing 4,798 discrete variants and 17,299 alleles published in 31 papers, showing a wide range of frequencies and complexities among the 194 genes reported in GRID, with 65% of pathogenic variants being unique to a single individual. A better understanding of IRD-related gene distribution, gene complexity, and variant types allow for improved genetic testing and therapies. Current genetic therapeutic methods are also quite diverse and rely on variant identification, and range from whole gene replacement to single nucleotide editing at the DNA or RNA levels. IRDs and their suitable therapies thus require a range of effective disease modelling in human cells, granting insight into disease mechanisms and testing of possible treatments. This review summarizes genetic and therapeutic modalities of IRDs, provides new analyses of IRD-related genes (GRID and complexity scores), and provides information to match genetic-based therapies such as gene-specific and variant-specific therapies to the appropriate individuals.

Association of ABCA4 Gene Polymorphisms with Cleft Lip with or without Cleft Palate in the Polish Population

Background: Non-syndromic cleft lip with/without cleft palate (NSCL/P) is a common congenital condition with a complex aetiology reflecting multiple genetic and environmental factors. Single nucleotide polymorphisms (SNPs) in ABCA4 have been associated with NSCL/P in several studies, although there are some inconsistent results. This study aimed to evaluate whether two SNPs in ABCA4, namely rs4147811 and rs560426, are associated with NSCL/P occurrence in the Polish population. Methods: The study included 627 participants: 209 paediatric patients with NSCL/P and 418 healthy newborn controls. DNA was isolated from the saliva of NSCL/P patients and from umbilical cord blood in the controls. Genotyping of rs4147811 and rs560426 was performed using quantitative PCR. Results: The rs4147811 (AG genotype) SNP in ABCA4 was associated with a decreased risk of NSCL/P (odds ratio (OR) 0.57; 95% confidence interval (CI) 0.39–0.84; p = 0.004), whereas the rs560426 (GG genotype) SNP was associated with an increased risk of NSCL/P (OR 2.13; 95% CI 1.31–3.48; p = 0.002). Limitations: This study—based on the correlation between single genetic variants and the occurrence of different phenotypes—might have limited power in detecting relevant, complex inheritance patterns. ORs are often low to moderate when investigating the association of single genes with the risk of a complex trait. Another limitation was the small number of available NSCL/P samples. Conclusions: The results suggest that genetic variations in ABCA4 are important risk markers of NSCL/P in the Polish population. Further investigation in a larger study group is warranted.

Systemic complement activation levels in Stargardt disease

Purpose

Preclinical research provides evidence for the complement system as a potential common pathway in Stargardt disease (STGD1) and age-related macular degeneration (AMD) leading to retinal pigment epithelium (RPE) loss. However, systemic complement activation has not yet been assessed in STGD1 patients. We conducted a cross-sectional case-control study to assess systemic complement activation in STGD1 patients and its association with disease severity.

Methods

Systemic concentrations of complement component C3 and its degradation product C3d were compared between 80 STGD1 patients and 80 controls that were frequency matched for age and sex. The C3d/C3 ratio was used as parameter of systemic complement activation. Within the STGD1 cohort, we additionally examined the association between the C3d/C3 ratio, demographic and behavioural factors (age, sex, smoking and BMI), and measures of disease severity (age at onset, visual acuity, and area of atrophy).

Results

The C3d/C3 ratio did not significantly differ between patients (mean C3d/C3 ratio 3.5±1.4) and controls (mean C3d/C3 ratio 3.6±1.0), mean difference -0.156 (p = 0.804, independent samples t-test). The overall effect size was 8% (95% confidence interval, 3–15%). Elevated C3d/C3 ratios (>8.1) were found in three patients who all had a concomitant inflammatory condition at the time of blood draw. Within the patient cohort, C3 levels were associated with sex (mean difference -134, p = 0.001, independent samples t-test) and BMI (correlation coefficient 0.463, p<0.001, Spearman’s Correlation).

Conclusions

Systemic complement levels were not elevated in STGD1 patients compared to age and sex matched controls and was not associated with STGD1 severity. Considering the continued absent proof of a systemic contribution of the complement system to RPE loss in STGD1 patients, we hypothesize that complement activation in STGD1 is more likely a local process. In light of upcoming complement-targeted therapies, further studies are needed that measure complement levels in the eye of STGD1 patients.

Perspectives of people with inherited retinal diseases on ocular gene therapy in Australia: protocol for a national survey

INTRODUCTION

Voretigene neparvovec-rzyl (Luxturna) was approved by the Australian Therapeutic Goods Administration on 4 August 2020 for the treatment of biallelic mutations in the RPE65 gene, a rare cause of congenital and adult-onset retinal dystrophy (predominantly Leber congenital amaurosis). Previous studies have shown that individuals who might participate in gene therapy trials overestimate clinical effect and underestimate risks. However, little is known about the perspectives of patients who may be offered approved gene therapy treatment for ocular conditions (as distinct from participating in clinical trials of gene therapy). The main objective of this study is to develop a tool to assess knowledge, attitudes and perceptions of approved and future genetic therapies among potential recipients of ocular gene therapy. In addition, we aim to assess the quality of life, attitudes towards clinical trials and vision-related quality of life among this cohort.

METHODS AND ANALYSIS

A new 'Attitudes to Gene Therapy for the Eye' tool will be developed following consultation with people with inherited retinal disease (IRD) and content matter experts. Australians with IRD or their guardians will be asked to complete an internet-based survey comprising existing quality of life and visual function instruments and items for the newly proposed tool. We expect to recruit 500 survey participants from patient support groups, the practices of Australian ophthalmologists who are specialists in IRD and Australian ophthalmic research institutions. Launch is anticipated early 2021. Responses will be analysed using item response theory methodology.

ETHICS AND DISSEMINATION

This study has received ethics approval from the University of Melbourne (#2057534). The results of the study will be published in a peer-reviewed journal and will be presented at relevant conferences. Organisations involved in recruitment, and the Patient Engagement Advisory committee will assist the research team with dissemination of the study outcomes.

Contribution of Optical Coherence Tomography to the Diagnosis of Childhood-Onset Stargardt Disease in Its Early Stages

Stargardt disease may present with alterations in optical coherence tomography before symptoms and fundus abnormalities appear. With this non-invasive test, a presumptive diagnosis can be made and genetically confirmed in the subclinical stages of the disease. [J Pediatr Ophthalmol Strabismus. 2021;58(3):e5-e8.].

Identification of Four Novel Variants and Determination of Genotype-Phenotype Correlations for ABCA4 Variants Associated With Inherited Retinal Degenerations

Purpose

The purpose of the study is to describe the genetic and clinical features of 17 patients with ABCA4-related inherited retinal degenerations (IRDs) and define the phenotype–genotype correlations.

Methods

In this multicenter retrospective study, 17 patients from 16 families were enrolled, and ABCA4 gene variants were detected using targeted next-generation sequencing using a custom designed panel for IRDs. Sanger sequencing and co-segregation analysis of the suspected pathogenic variants were performed with the family members. The pathogenicities of variants were evaluated according to the American College of Medical Genetics and Genomics guidelines (ACMG). Protein structure modifications mediated by the variants were studied using bioinformatic analyses.

Results

The probands were diagnosed with Stargardt disease 1 (7), cone-rod dystrophy type 3 (8), cone dystrophy (1), and retinitis pigmentosa 19 (1). Onset of symptoms occurred between 5 and 27 years of age (median age = 12.4 years). A total of 30 unique ABCA4 suspicious pathogenic variations were observed, including 18 missense mutations, seven frameshift mutations, two nonsense mutations, one canonical splice site mutation, one small in-frame deletion, and one insertion. Four novel ABCA4 variants were identified. Two novel frameshift variants, c.1290dupC (p.W431fs), and c.2967dupT (G990fs), were determined to be pathogenic. A novel missense variant c.G5761T (p.V1921L) was likely pathogenic, and another novel missense c.C170G (p.P57R) variant was of undetermined significance. All ABCA4 variants tested in this study inordinately changed the physico-chemical parameters and structure of protein based on in silico analysis.

Conclusion

ABCA4-related IRD is genetically and clinically highly heterogeneous. Four novel ABCA4 variants were identified. This study will expand the spectrum of disease-causing variants in ABCA4, which will further facilitate genetic counseling.

Imaging in inherited retinal disorders

Inherited retinal diseases, which results from mutations in over 260 identified genes, affect more than 2 million people globally. The diseases mostly cause severe vision loss in young working population and have severe impact on social economic status of the population. Advances in retinal imaging techniques along with developments in gene identification and cell biology techniques have yielded to a better understanding of the genetic and biochemical mechanisms causing these diseases. Retinal imaging along with through ophthalmological examination is essential to make an accurate diagnosis, to decrease the burden of unneccessary anciliary tests and to select the potential patients that can get benefit from the gene treatment. The purpose of the review is to yield an update on inherited retinal diseases by highlighting microstructural changes in retina and to summarize the retinal changes detected by currently available multimodal imaging techniques.

The role of multimodal imaging and vision function testing in ABCA4-related retinopathies and their relevance to future therapeutic interventions

The aim of this review article is to describe the specific features of Stargardt disease and ABCA4 retinopathies (ABCA4R) using multimodal imaging and functional testing and to highlight their relevance to potential therapeutic interventions. Standardised measures of tissue loss, tissue function and rate of change over time using formal structured deep phenotyping in Stargardt disease and ABCA4R are key in diagnosis, and prognosis as well as when selecting cohorts for therapeutic intervention. In addition, a meticulous documentation of natural history will be invaluable in the future to compare treated with untreated retinas. Despite the familiarity with the term Stargardt disease, this eponymous classification alone is unhelpful when evaluating ABCA4R, as the ABCA4 gene is associated with a number of phenotypes, and a range of severity. Multimodal imaging, psychophysical and electrophysiologic measurements are necessary in diagnosing and characterising these differing retinopathies. A wide range of retinal dystrophy phenotypes are seen in association with ABCA4 mutations. In this article, these will be referred to as ABCA4R. These different phenotypes and the existence of phenocopies present a significant challenge to the clinician. Careful phenotypic characterisation coupled with the genotype enables the clinician to provide an accurate diagnosis, associated inheritance pattern and information regarding prognosis and management. This is particularly relevant now for recruiting to therapeutic trials, and in the future when therapies become available. The importance of accurate genotype-phenotype correlation studies cannot be overemphasised. This approach together with segregation studies can be vital in the identification of causal mutations when variants in more than one gene are being considered as possible. In this article, we give an overview of the current imaging, psychophysical and electrophysiological investigations, as well as current therapeutic research trials for retinopathies associated with the ABCA4 gene.

Unique combination of clinical features in a large cohort of 100 patients with retinitis pigmentosa caused by FAM161A mutations

FAM161A mutations are the most common cause of autosomal recessive retinitis pigmentosa in the Israeli-Jewish population. We aimed to characterize the spectrum of FAM161A-associated phenotypes and identify characteristic clinical features. We identified 114 bi-allelic FAM161A patients and obtained clinical records of 100 of these patients. The most frequent initial symptom was night blindness. Best-corrected visual acuity was largely preserved through the first three decades of life and severely deteriorated during the 4th–5th decades. Most patients manifest moderate-high myopia. Visual fields were markedly constricted from early ages, but maintained for decades. Bone spicule-like pigmentary changes appeared relatively late, accompanied by nummular pigmentation. Full-field electroretinography responses were usually non-detectable at first testing. Fundus autofluorescence showed a hyper-autofluorescent ring around the fovea in all patients already at young ages. Macular ocular coherence tomography showed relative preservation of the outer nuclear layer and ellipsoid zone in the fovea, and frank cystoid macular changes were very rare. Interestingly, patients with a homozygous nonsense mutation manifest somewhat more severe disease. Our clinical analysis is one of the largest ever reported for RP caused by a single gene allowing identification of characteristic clinical features and may be relevant for future application of novel therapies.

The genetic architecture of Stargardt macular dystrophy (STGD1): a longitudinal 40-year study in a genetic isolate

Stargardt disease (STGD1) is a form of inherited retinal dystrophy attributed to variants affecting function of the large ABCA4 gene and is arguably the most complex monogenic disease. Therapeutic trials in patients depend on identifying causal ABCA4 variants in trans, which is complicated by extreme allelic and clinical heterogeneity. We report the genetic architecture of STGD1 in the young genetically isolated population of Newfoundland, Canada. Population-based clinical recruitment over several decades yielded 29 STGD1 and STGD1-like families (15 multiplex, 14 singleton). Family interviews and public archival records reveal the vast majority of pedigree founders to be of English extraction. Full gene sequencing and haplotype analysis yielded a high solve rate (38/41 cases; 92.7%) for STGD1 and identified 16 causative STGD1 alleles, including a novel deletion (NM_000350.3: ABCA4 c.67-1delG). Several STGD1 alleles of European origin (including NM_000350.3: ABCA4 c.5714 + 5G>A and NM_000350.3: ABCA4 c.5461-10T>C) have drifted to a relatively high population frequency due to founder effect. We report on retinal disease progression in homozygous patients, providing valuable allele-specific insights. The least involved retinal disease is seen in patients homozygous for c.5714 + 5G>A variant, a so-called "mild" variant which is sufficient to precipitate a STGD1 phenotype in the absence of other pathogenic variants in the coding region and intron/exon boundaries of ABCA4. The most severe retinal disease is observed in cases with ABCA4 c.[5461-10T>C;5603A>T] complex allele. We discuss the advantages of determining genetic architecture in genetic isolates in order to begin to meet the grand challenge of human genetics.

Phenogenon: Gene to phenotype associations for rare genetic diseases

As high-throughput sequencing is increasingly applied to the molecular diagnosis of rare Mendelian disorders, a large number of patients with diverse phenotypes have their genetic and phenotypic data pooled together to uncover new gene-phenotype relations. We introduce Phenogenon, a statistical tool that combines, Human Phenotype Ontology (HPO) annotated patient phenotypes, gnomAD allele population frequency, and Combined Annotation Dependent Depletion (CADD) score for variant pathogenicity, in order to jointly predict the mode of inheritance and gene-phenotype associations. We ran Phenogenon on our cohort of 3,290 patients who had undergone whole exome sequencing. Among the top associations, we recapitulated previously known, such as "SRD5A3—Abnormal full-field electroretinogram—recessive" and "GRHL2 –Nail dystrophy—recessive", and discovered one potentially novel, “RRAGA–Abnormality of the skin—dominant”. We also developed an interactive web interface available at https://phenogenon.phenopolis.org to visualise and explore the results.

Prospective Cohort Study of Childhood-Onset Stargardt Disease: Fundus Autofluorescence Imaging, Progression, Comparison with Adult-Onset Disease, and Disease Symmetry

PURPOSE

To determine the reliability and repeatability of quantitative evaluation of areas of decreased autofluorescence (DAF) from fundus autofluorescence (FAF) images and track disease progression in children with Stargardt disease (STGD1), and to investigate clinical and genotype correlations, disease symmetry, and intrafamilial variability.

DESIGN

Prospective cohort study.

METHODS

Children and adults with molecularly confirmed STGD1 (n = 90) underwent longitudinal FAF imaging with subsequent semiautomated measurement of the area of DAF and calculation of the annual rate of progression. The age of disease onset was recorded for all subjects, as well as the electroretinography (ERG) group at baseline (n = 86). Patients were grouped for analysis based on the age at baseline and age of onset, into children (n = 56), adults with childhood-onset STGD1 (n = 15), and adults with adult-onset (n = 19). Fifty FAF images were selected randomly and analyzed by 2 observers to evaluate repeatability and reproducibility. Differences between groups, interocular symmetry, genotype-phenotype correlations, and intrafamilial variability were also investigated both for baseline measurements as well as progression rates. We measured visual acuity, molecular genetics, ERG group, FAF metrics, and their correlations.

RESULTS

The mean age of onset ± SD was 9.6 ± 3.4 years for childhood-onset (n = 71) and 28.3 ± 7.8 years for adult-onset STGD1 (n = 19). The intra- and interobserver reliability of DAF quantification was excellent (intraclass correlation coefficients 0.995 and 0.987, respectively). DAF area was symmetric between eyes and the mean rate of progression (SD) was 0.69 (0.72), 0.78 (0.48), and 0.40 (0.36) mm²/year for children, adults with childhood-onset, and adults with adult-onset disease, respectively. Patients belonging to a group 3 ERG phenotype (generalized cone and rod dysfunction) had a significantly greater progression rate. Limited intrafamilial variability was observed.

CONCLUSIONS

This is the first large prospective study of FAF in a cohort of molecularly confirmed children with STGD1. DAF area quantification was highly reliable and may thereby serve as a robust structural endpoint. A high rate of progression was observed in childhood-onset disease, making this subtype of STGD1 ideally suited to be considered for prioritization in clinical trials.

Macular dystrophies: clinical and imaging features, molecular genetics and therapeutic options

Macular dystrophies (MDs) consist of a heterogeneous group of disorders that are characterised by bilateral symmetrical central visual loss. Advances in genetic testing over the last decade have led to improved knowledge of the underlying molecular basis. The developments in high-resolution multimodal retinal imaging have also transformed our ability to make accurate and more timely diagnoses and more sensitive quantitative assessment of disease progression, and allowed the design of optimised clinical trial endpoints for novel therapeutic interventions. The aim of this review was to provide an update on MDs, including Stargardt disease, Best disease, X-linked r etinoschisis, pattern dystrophy, Sorsby fundus dystrophy and autosomal dominant drusen. It highlights the range of innovations in retinal imaging, genotype-phenotype and structure-function associations, animal models of disease and the multiple treatment strategies that are currently in clinical trial or planned in the near future, which are anticipated to lead to significant changes in the management of patients with MDs.

Cross-Sectional and Longitudinal Assessment of the Ellipsoid Zone in Childhood-Onset Stargardt Disease

Purpose

To evaluate the reliability of ellipsoid zone (EZ) loss width and area measurements from spectral-domain optical coherence tomography (SD-OCT) images and track disease progression in childhood-onset Stargardt disease (STGD1).

Methods

Children with molecularly confirmed STGD1 (n = 46, mean age 12.4 years) underwent SD-OCT for the measurement of the transverse (width) loss of the EZ and en face analysis to quantify the area of EZ loss. All scans were analyzed twice by two graders to evaluate reliability. The annual rate of EZ width and area loss were calculated.

Results

The intra- and intergrader reliability of transverse EZ loss and area of EZ loss measurements at baseline for both graders was 0.99. The mean annual rate of transverse EZ loss (±standard deviation) was 279.5 ± 259.9 μm/y. The mean rate of area of EZ loss (±standard deviation) was 1.20 ± 1.29 mm²/y. The percentage transverse EZ loss was 10.2 ± 9.9%/y, which was significantly lower than the area of EZ loss at 19.4 ± 16.3%/y. High degree of interocular symmetry was observed.

Conclusions

This is a prospective study on the quantification of EZ loss in children with STGD1 and highlights the reliability of SD-OCT in measuring EZ loss. High intra- and intergrader reliability was observed, with good ability to detect changes over time.

Translational Relevance

Measuring the area of EZ loss was more sensitive compared with transverse EZ width loss measurements and will be valuable for natural history studies and clinical trials requiring sensitive and reliable structural endpoints.

Monitoring and Management of the Patient with Stargardt Disease

Stargardt disease (STGD1) represents one of the major common causes of inherited irreversible visual loss. Due to its high phenotypic and genotypic heterogeneity, STGD1 is a complex disease to understand. Non-invasive imaging, biochemical, and genetic advances have led to substantial improvements in unveiling the disease processes and novel promising therapeutic landscapes have been proposed. This review recapitulates the modalities for monitoring patients with STGD1 and the therapeutic options currently under investigation for the different stages of the disease.

Cross-Sectional and Longitudinal Assessment of Retinal Sensitivity in Patients With Childhood-Onset Stargardt Disease

Purpose

We assess cross-sectional and longitudinal microperimetry and full-field static perimetry-derived retinal sensitivity with conventional and volumetric indices of retinal function in childhood-onset Stargardt disease (STGD1).

Methods

Subjects with molecularly confirmed childhood-onset STGD1 underwent full-field static perimetry and/or microperimetry using custom designed grids. Mean sensitivity (MS) and total volume (V_TOT) were computed for each microperimetry test. MS, V_TOT, and central field volume (V₃₀) were computed for each full-field static perimetry test. Subjects under 18 years old at baseline were classified as children and subjects 18 years or older as adults.

Results

A total of 43 children (mean age at baseline, 13.0 years; range, 8–17) and 13 adults (mean age at baseline, 23.1 years; range, 18–32) were included in the analysis. For full-field static perimetry and microperimetry for both subgroups, intraclass correlation coefficient results for MS and volumetric indices were good to excellent, indicating strong test–retest reliability. Interocular symmetry in terms of baseline measurements and the annual rate of progression was observed. A greater rate of progression was observed in childhood.

Conclusions

To our knowledge, this is the first prospective study of retinal sensitivity in a large cohort of molecularly confirmed subjects with childhood-onset STGD1 demonstrating that children with STGD1 can reliably undertake detailed functional testing. Moreover, using custom designed grids and subsequent topographic analysis, volumetric indices of retinal function provide a reliable measure of retinal sensitivity.

Translational Relevance

This study highlights the use of microperimetry and full-field static perimetry, as well as volumetric indices of retinal function, in monitoring disease progression.

Variants in the ABCA4 gene in a Brazilian population with Stargardt disease

PURPOSE

The aim of this study was to analyze and report pathogenic variants in the ABCA4 gene in Brazilian patients with a clinical diagnosis of Stargardt disease.

METHODS

This retrospective study evaluated variants in the ABCA4 gene in Brazilian patients with Stargardt disease. The patients' visual acuity and age of symptom onset were obtained from previous medical records. The patients were classified according to the autofluorescence patterns.

RESULTS

Fifty patients aged between 10 and 65 years from 44 families were included in the study. Among these cases, the mean age of symptom onset was 14 years (range, 5-40 years). ABCA4 gene sequencing was conclusive in 40 patients (80%), negative in two patients (4%), and inconclusive in eight patients (16%). Four families carried homozygous pathogenic variants. Segregation analysis results were available for 23 families. One novel variant was found: p.Ala2084Pro. The most frequent pathogenic variant in this group was p.Arg602Trp (12/100 alleles). Based on the phenotypic characteristics assessed with fundus autofluorescence imaging, 12 patients were classified as having type I phenotype, 16 as having type II, and 18 patients as having type III. The cases classified as type III phenotype included patients who were homozygous for the p.Asn96Asp and p.Arg2030* variants. One patient with a type I phenotype carried the homozygous intronic variant c.3862+1G>A.

CONCLUSIONS

Next-generation sequencing was effective for the molecular diagnosis of genetic diseases and specifically allowed a conclusive diagnosis in 80% (40/50) of the patients. As the ABCA4 gene does not show a preferential region for pathogenic variants, the diagnosis of Stargardt disease depends on broader analysis of the gene. The most common pathogenic variants in the ABCA4 gene described in the literature were also found in these Brazilian patients. Although some genotype-phenotype correlations were found, more studies regarding the progression of Stargardt disease will help increase our understanding of the pathogenicity of these gene variants.

Gene therapy in inherited retinal degenerative diseases, a review

Hereditary diseases of the retina represent a group of diseases with several heterogeneous mutations that have the common end result of progressive photoreceptor death leading to blindness. Retinal degenerations encompass multifactorial diseases such as age-related macular degeneration, Leber congenital amaurosis, Stargardt disease, and retinitis pigmentosa. Although there is currently no cure for degenerative retinal diseases, ophthalmology has been at the forefront of the development of gene therapy, which offers hope for the treatment of these conditions. This article will explore an overview of the clinical trials of gene supplementation therapy for retinal diseases that are underway or planned for the near future.

Detailed genetic characteristics of an international large cohort of patients with Stargardt disease: ProgStar study report 8

Background/aims

To describe the genetic characteristics of the cohort enrolled in the international multicentre progression of Stargardt disease 1 (STGD1) studies (ProgStar) and to determine geographic differences based on the allele frequency.

Methods

345 participants with a clinical diagnosis of STGD1 and harbouring at least one disease-causing ABCA4 variant were enrolled from 9 centres in the USA and Europe. All variants were reviewed and in silico analysis was performed including allele frequency in public databases and pathogenicity predictions. Participants with multiple likely pathogenic variants were classified into four national subgroups (USA, UK, France, Germany), with subsequent comparison analysis of the allele frequency for each prevalent allele.

Results

211 likely pathogenic variants were identified in the total cohort, including missense (63%), splice site alteration (18%), stop (9%) and others. 50 variants were novel. Exclusively missense variants were detected in 139 (50%) of 279 patients with multiple pathogenic variants. The three most prevalent variants of these patients with multiple pathogenic variants were p.G1961E (15%), p.G863A (7%) and c.5461-10 T>C (5%). Subgroup analysis revealed a statistically significant difference between the four recruiting nations in the allele frequency of nine variants.

Conclusions

There is a large spectrum of ABCA4 sequence variants, including 50 novel variants, in a well-characterised cohort thereby further adding to the unique allelic heterogeneity in STGD1. Approximately half of the cohort harbours missense variants only, indicating a relatively mild phenotype of the ProgStar cohort. There are significant differences in allele frequencies between nations, although the three most prevalent variants are shared as frequent variants.

Gene therapy and genome surgery in the retina

Precision medicine seeks to treat disease with molecular specificity. Advances in genome sequence analysis, gene delivery, and genome surgery have allowed clinician-scientists to treat genetic conditions at the level of their pathology. As a result, progress in treating retinal disease using genetic tools has advanced tremendously over the past several decades. Breakthroughs in gene delivery vectors, both viral and nonviral, have allowed the delivery of genetic payloads in preclinical models of retinal disorders and have paved the way for numerous successful clinical trials. Moreover, the adaptation of CRISPR-Cas systems for genome engineering have enabled the correction of both recessive and dominant pathogenic alleles, expanding the disease-modifying power of gene therapies. Here, we highlight the translational progress of gene therapy and genome editing of several retinal disorders, including RPE65-, CEP290-, and GUY2D-associated Leber congenital amaurosis, as well as choroideremia, achromatopsia, Mer tyrosine kinase- (MERTK-) and RPGR X-linked retinitis pigmentosa, Usher syndrome, neovascular age-related macular degeneration, X-linked retinoschisis, Stargardt disease, and Leber hereditary optic neuropathy.

Caring for Hereditary Childhood Retinal Blindness

Inherited retinal diseases (IRDs) are a major cause of incurable familial blindness in the Western world. In the pediatric population, IRDs are a major contributor to the 19 million children worldwide with visual impairment. Unfortunately, the road to the correct diagnosis is often complicated in the pediatric population, as typical diagnostic tools such as fundus examination, electrodiagnostic studies, and other imaging modalities may be difficult to perform in the pediatric patient. In this review, we describe the most significant IRDs with onset during the pediatric years (ie, before the age of 18). We describe the pathogenesis, clinical presentation, and potential treatment of these diseases. In addition, we advocate the use of a pedigree (family medical history), electroretinography, and genetic testing as the 3 most crucial tools for the correct diagnosis of IRDs in the pediatric population.

Long-term follow-up of autosomal dominant Stargardt macular dystrophy (STGD3) subjects enrolled in a fish oil supplement interventional trial

BACKGROUND

Earlier studies have raised the notion that docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) supplementation could be a useful intervention in autosomal dominant Stargardt macular dystrophy (STGD3). We sought to assess whether fish oil supplementation has a beneficial effect on the clinical course of STGD3 secondary to a mutation in the ELOVL4 gene.

MATERIALS AND METHODS

Eleven patients with STGD3 were enrolled in an 8-year open-label, clinical interventional study of over-the-counter fish oil supplements at a recommended daily dose of 650 mg EPA and 350 mg DHA (NCT00420602). Subjects had annual eye examinations with complete imaging, visual function testing, and blood lipid analyses. Compliance with therapy was measured by periodic patient self-report and with serum and red blood cell biomarkers of lipid consumption. Paired sample t-tests were used to measure differences in mean values of visual acuity, lipid biomarkers, and contrast sensitivity obtained at baseline and the last follow-up.

RESULTS

All subjects showed progression of their maculopathy, and we could not discern a beneficial effect of the intervention. Compliance with the recommended fish oil supplement intervention was poor as assessed by patient self-report and biomarkers of lipid consumption.

CONCLUSIONS

Our inability to detect a benefit of fish oil could be the result of small subject numbers, poor compliance, or intervention too late in the course of the disease. We still advise STGD3 patients to consume fish or fish oil regularly, and we recommend that pre-symptomatic children with ELOVL4 mutations should be especially targeted for these interventions.

Full-field ERG as a predictor of the natural course of ABCA4-associated retinal degenerations

PURPOSE

To assess retinal function in combination with the retinal structure in ABCA4-associated retinal degenerations. Moreover, to evaluate the possibility of predicting the natural course of these disorders.

METHODS

34 patients with Stargardt disease or cone rod dystrophy carrying confirmed mutations in ABCA4 were selected from our retinitis pigmentosa (RP) register. Sequence analysis of the entire coding region of the ABCA4 gene was performed. The patients were subdivided into three groups based on their most recent visual fields. Group 1 included ten patients with central scotomas within 10°, group 2 included 19 patients with larger central scotomas of 10-35°, and group 3 included five patients with mere temporal residues. The patients underwent slit-lamp and fundus examinations, visual acuity testing, optical coherence tomography (OCT), fundus photography (color, red-free, and autofluorescence (AF) images), full-field electroretinography (ffERG), and multifocal electroretinography (mERG). FfERG and mERG results were analyzed statistically. Total rod and cone function, as well as macular function, was compared between the three groups and of each group to a normal material. In 23 patients who had undergone ffERG on a previous occasion, the 30 Hz flicker implicit time (IT) from the first visit was also analyzed.

RESULTS

The ffERG statistics revealed significant differences between the groups regarding cone and rod function with group 1 showing the highest amplitudes and the shortest ITs while group 3 demonstrated the lowest amplitudes and the most delayed ITs. When compared to controls, group 1 did not show any significant changes while groups 2 and 3 demonstrated reduced amplitudes and delayed 30 Hz ITs. Regarding estimation of the natural course, identical results of the 30 Hz IT were encountered for the groups also at the first visit early in the course of disease. Comparison of the mERGs showed significant differences with group 1 demonstrating the highest amplitudes and group 3 the lowest for all rings but rings 2 and 3 in the right eye for which the amplitudes were the second highest. The mERGs for each group were also compared to controls showing reduced mERG amplitudes for all rings in all groups, except group 1, left eye. OCT showed macular attenuation in all patients. Evaluation of the inner and outer photoreceptor junction (IS/OS) morphology revealed alterations related to macular function measured with mERG in all eyes. Eight patients in group 1 showed foveal IS/OS junction loss, one had foveal IS/OS junction disorganization, and one had IS/OS loss also beyond the fovea. In group 2, one patient had IS/OS junction loss confined to the fovea, and the rest showed total loss of IS/OS junctions. Group 3 was devoid of IS/OS junctions. Concerning the AF images, group 1 showed small areas of absent AF in the macula, peripapillary sparing, and flecks of increased and reduced AF in the posterior pole. In group 2, the central areas of absent AF were larger. Flecks of reduced AF were the most dominant and reached beyond the posterior pole. Seven of 19 patients had peripapillary sparing. In group 3, large confluent areas of reduced AF were found in the posterior pole and beyond with small areas of increased AF in the far periphery. No peripapillary sparing was seen.

CONCLUSIONS

The current study demonstrates a significant difference in total retinal function, as well as macular function, between patients with ABCA4-associated retinal degeneration and a different degree of visual field defects with gradual deterioration of function along with increased visual field constriction. Likewise, the morphological changes, including the deviant AF pattern and loss of IS/OS junctions, that were related to macular function measured with mERG worsened with the degree of visual field defects. Moreover, in these groups of patients with ABCA4-associated retinal degenerations, full-field cone 30 Hz flicker IT seems to be a predictor of the natural course of the disease also on long-term follow-up.

Early impairment of the full-field photopic negative response in patients with Stargardt disease and pathogenic variants of the ABCA4 gene

BACKGROUND

To study the photopic negative response of the full-field photopic electroretinography (ERG) in Stargardt patients with pathogenic variants in the ABCA4 gene.

METHODS

A retrospective analysis of 35 Stargardt patients with ABCA4 gene pathogenic variants, compared to normal age-matched controls. Patients were clinically followed at the Ophthalmology Department of Fondazione Policlinico Universitario A. Gemelli/Università Cattolica del Sacro Cuore, Rome, Italy.

RESULTS

The photopic negative response of the full-field photopic ERG was compromised in most Stargardt patients. In the presence of a normal B-wave, the amplitude ratio between the photopic negative response and the B-wave displayed a 97% accuracy in detecting diseased eyes (receiver operating characteristic curves).

CONCLUSIONS

In Stargardt patients with ABCA4 pathogenic mutations, the photopic negative response of the full-field photopic ERG is a very sensitive disease read-out. Its inclusion in standard ERG analysis would be a no-cost addition of practical consequence in the follow-up of Stargardt disease. The early impairment of the photopic negative response suggests that inner retinal function might be affected in Stargardt disease earlier than previously acknowledged.

Bull's eye maculopathy and subfoveal deposition in two mucopolysaccharidosis type I patients on long-term enzyme replacement therapy

Purpose

To report retinal findings in two patients with mucopolysaccharidosis type I (MPS I) receiving human recombinant alpha-l-iduronidase (Laronidase) as enzyme replacement therapy.

Observations

Patient 1 had visual acuity 20/20 right eye, 20/25 left eye and unremarkable anterior segment and retinal examination. Optical coherence tomography (OCT) scanning demonstrated parafoveal thinning and subfoveal hyperreflectant material. Patient 2 had visual acuity 20/20 both eyes, with dense nuclear cataract both eyes. Retinal examination demonstrated bull's eye maculopathy both eyes. OCT scanning confirmed parafoveal atrophy and demonstrated similar appearing subfoveal hyperreflectant material, more prominent than in case 1.

Conclusions and importance

These two patients with MPS I receiving Laronidase treatment have developed bull's eye maculopathy changes and subfoveal deposition of hyperreflectant material despite excellent compliance and good tolerance of the standard dose of enzyme therapy for this disorder. Further studies are required to determine the nature of the material, the incidence and the effect of enzyme replacement therapy on these findings in patients with MPS I.

Stem Cell Therapies in Retinal Disorders

Stem cell therapy has long been considered a promising mode of treatment for retinal conditions. While human embryonic stem cells (ESCs) have provided the precedent for regenerative medicine, the development of induced pluripotent stem cells (iPSCs) revolutionized this field. iPSCs allow for the development of many types of retinal cells, including those of the retinal pigment epithelium, photoreceptors, and ganglion cells, and can model polygenic diseases such as age-related macular degeneration. Cellular programming and reprogramming technology is especially useful in retinal diseases, as it allows for the study of living cells that have genetic variants that are specific to patients’ diseases. Since iPSCs are a self-renewing resource, scientists can experiment with an unlimited number of pluripotent cells to perfect the process of targeted differentiation, transplantation, and more, for personalized medicine. Challenges in the use of stem cells are present from the scientific, ethical, and political realms. These include transplant complications leading to anatomically incorrect placement, concern for tumorigenesis, and incomplete targeting of differentiation leading to contamination by different types of cells. Despite these limitations, human ESCs and iPSCs specific to individual patients can revolutionize the study of retinal disease and may be effective therapies for conditions currently considered incurable.

Clinical and genetic analyses reveal novel pathogenic ABCA4 mutations in Stargardt disease families

Stargardt disease (STGD1) is a juvenile macular degeneration predominantly inherited in an autosomal recessive pattern, characterized by decreased central vision in the first 2 decades of life. The condition has a genetic basis due to mutation in the ABCA4 gene, and arises from the deposition of lipofuscin-like substance in the retinal pigmented epithelium (RPE) with secondary photoreceptor cell death. In this study, we describe the clinical and genetic features of Stargardt patients from four unrelated Chinese cohorts. The targeted exome sequencing (TES) was carried out in four clinically confirmed patients and their family members using a gene panel comprising 164 known causative inherited retinal dystrophy (IRD) genes. Genetic analysis revealed eight ABCA4 mutations in all of the four pedigrees, including six mutations in coding exons and two mutations in adjacent intronic areas. All the affected individuals showed typical manifestations consistent with the disease phenotype. We disclose two novel ABCA4 mutations in Chinese patients with STGD disease, which will expand the existing spectrum of disease-causing variants and will further aid in the future mutation screening and genetic counseling, as well as in the understanding of phenotypic and genotypic correlations.

Test-Retest Variability of Functional and Structural Parameters in Patients with Stargardt Disease Participating in the SAR422459 Gene Therapy Trial

Purpose

The goal of this analysis was to determine the test–retest variability of functional and structural measures from a cohort of patients with advanced forms of Stargardt Disease (STGD) participating in the SAR422459 (NCT01367444) gene therapy clinical trial.

Methods

Twenty-two participants, aged 24 to 66, diagnosed with advanced forms of STGD, with at least one pathogenic ABCA4 mutation on each chromosome participating in the SAR422459 (NCT01367444) gene therapy clinical trial, were screened over three visits within 3 weeks or less. Functional visual evaluations included: best-corrected visual acuity (BCVA) Early Treatment Diabetic Retinopathy Study (ETDRS) letter score, semiautomated kinetic perimetry (SKP) using isopters I4e, III4e, and V4e, hill of vision (HOV) calculated from static visual fields (SVF) by using a 184n point centrally condensed grid with the stimulus size V test target. Retinal structural changes such as central macular thickness and macular volume were assessed by spectral-domain optical coherence tomography (SD-OCT). Repeatability coefficients (RC) and 95% confidential intervals (CI) were calculated for each parameter using a hierarchical mixed-effects model and bootstrapping.

Results

Criteria for statistically significant changes for various parameters were found to be the following: BCVA letter score (8 letters), SKP isopters I4e, III4e, and V4e (3478.85; 2488.02 and 2622.46 deg², respectively), SVF full volume HOV (V_TOT, 14.62 dB-sr), central macular thickness, and macular volume (4.27 μm and 0.15 mm³, respectively).

Conclusions

This analysis provides important information necessary to determine if significant changes are occurring in structural and functional assessments commonly used to measure disease progression in this cohort of patients with STGD. Moreover, this information is useful for future trials assessing safety and efficacy of treatments in STGD.

Translational Relevance

Determination of variability of functional and structural measures in participants with advanced stages of the STGD is necessary to assess efficacy and safety in treatment trials involving STGD patients.

Isolated and Syndromic Retinal Dystrophy Caused by Biallelic Mutations in RCBTB1, a Gene Implicated in Ubiquitination

Inherited retinal dystrophies (iRDs) are a group of genetically and clinically heterogeneous conditions resulting from mutations in over 250 genes. Here, homozygosity mapping and whole-exome sequencing (WES) in a consanguineous family revealed a homozygous missense mutation, c.973C>T (p.His325Tyr), in RCBTB1. In affected individuals, it was found to segregate with retinitis pigmentosa (RP), goiter, primary ovarian insufficiency, and mild intellectual disability. Subsequent analysis of WES data in different cohorts uncovered four additional homozygous missense mutations in five unrelated families in whom iRD segregates with or without syndromic features. Ocular phenotypes ranged from typical RP starting in the second decade to chorioretinal dystrophy with a later age of onset. The five missense mutations affect highly conserved residues either in the sixth repeat of the RCC1 domain or in the BTB1 domain. A founder haplotype was identified for mutation c.919G>A (p.Val307Met), occurring in two families of Mediterranean origin. We showed ubiquitous mRNA expression of RCBTB1 and demonstrated predominant RCBTB1 localization in human inner retina. RCBTB1 was very recently shown to be involved in ubiquitination, more specifically as a CUL3 substrate adaptor. Therefore, the effect on different components of the CUL3 and NFE2L2 (NRF2) pathway was assessed in affected individuals’ lymphocytes, revealing decreased mRNA expression of NFE2L2 and several NFE2L2 target genes. In conclusion, our study puts forward mutations in RCBTB1 as a cause of autosomal-recessive non-syndromic and syndromic iRD. Finally, our data support a role for impaired ubiquitination in the pathogenetic mechanism of RCBTB1 mutations.

Stargardt disease: clinical features, molecular genetics, animal models and therapeutic options

Stargardt disease (STGD1; MIM 248200) is the most prevalent inherited macular dystrophy and is associated with disease-causing sequence variants in the gene ABCA4. Significant advances have been made over the last 10 years in our understanding of both the clinical and molecular features of STGD1, and also the underlying pathophysiology, which has culminated in ongoing and planned human clinical trials of novel therapies. The aims of this review are to describe the detailed phenotypic and genotypic characteristics of the disease, conventional and novel imaging findings, current knowledge of animal models and pathogenesis, and the multiple avenues of intervention being explored.

Phenotypic features of CRB1-associated early-onset severe retinal dystrophy and the different molecular approaches to identifying the disease-causing variants

PURPOSE

The aim of this study was to determine the molecular genetic basis of an early-onset severe retinal dystrophy in three unrelated consecutive patients of Czech origin and to describe their ocular phenotype.

METHODS

DNA samples from two probands were analyzed using a genotyping microarray (Asper) followed by either target analysis of 43 genes implicated in retinal disorders by next generation sequencing or whole-exome sequencing, respectively. The third proband underwent conventional Sanger sequencing of CRB1 based on her ocular findings.

RESULTS

All three probands harboured a known disease-causing mutation c.2843G>A; p.(Cys948Tyr) in the CRB1 gene. One individual was homozygous for this mutation, while in the other two probands c.2308G>A; p.(Gly770Ser) and c.3121A>G; p.(Met1041Val) were also identified in the heterozygous state, respectively. Both variants were novel and evaluated by in silico analysis as pathogenic. A false-negative result was observed in one of the two samples examined by the genotyping microarray. Disease onset in all patients was before the age of 7 years. Hypermetropic refractive error, bilateral nummular retinal pigmentation, retinal thickening and cystoid spaces in the macula were observed in two probands, aged 6 and 7 years. The third proband, aged 28 years, had bone spicule-like pigmentary changes associated with increased retinal nerve fiber layer.

CONCLUSIONS

The first study reporting on the molecular genetic cause of non-syndromic early-onset severe retinal dystrophy in Czech patients identified one homozygous and two compound heterozygote probands with CRB1 mutations. Retina nerve fibre layer measurements should be considered an integral part of the clinical evaluation of retinal dystrophies. Detailed clinical examination and imaging can both direct molecular screening and help to confirm or refute disease causation of identified variants.

Correlating Photoreceptor Mosaic Structure to Clinical Findings in Stargardt Disease

Purpose

To demonstrate a method for correlating photoreceptor mosaic structure with optical coherence tomography (OCT) and microperimetry findings in patients with Stargardt disease.

Methods

A total of 14 patients with clinically diagnosed Stargardt disease were imaged using confocal and split-detection adaptive optics scanning light ophthalmoscopy. Cone photoreceptors were identified manually in a band along the temporal meridian. Resulting values were compared to a normative database (n = 9) to generate cone density deviation (CDD) maps. Manual measurement of outer nuclear layer plus Henle fiber layer (ONL+HFL) thickness was performed, in addition to determination of the presence of ellipsoid zone (EZ) and interdigitation zone (IZ) bands on OCT. These results, along with microperimetry data, were overlaid with the CDD maps.

Results

Wide variation in foveal structure and CDD maps was seen within this small group. Disruption of ONL+HFL and/or IZ band was seen in all patients, with EZ band preservation in regions with low cone density in 38% of locations analyzed. Normality of retinal lamellar structure on OCT corresponded with cone density and visual function at 50/78 locations analyzed. Outer retinal tubulations containing photoreceptor-like structures were observed in 3 patients.

Conclusions

The use of CDD color-coded maps enables direct comparison of cone mosaic local density with other measures of retinal structure and function. Larger normative datasets and improved tools for automation of image alignment are needed.

Translational Relevance

The approach described facilitates comparison of complex multimodal data sets from patients with inherited retinal degeneration, and can be expanded to incorporate other structural imaging or functional testing.

Molecular Heterogeneity Within the Clinical Diagnosis of Pericentral Retinal Degeneration

PURPOSE

To characterize in detail the phenotype and genotype of patients with pericentral retinal degeneration (PRD).

METHODS

Patients were screened for an annular ring scotoma ranging from 3° to 40° (n = 28, ages 24-71) with kinetic perimetry. All patients had pigmentary retinopathy in the region of the dysfunction. Further studies included cross-sectional and en face imaging, static chromatic perimetry, and electroretinography. Molecular screening was performed.

RESULTS

Genotypes of 14 of 28 PRD patients were identified: There were mutations in eight different genes previously associated with autosomal dominant or autosomal recessive RDs. Kinetic fields monitored in some patients over years to more than a decade could be stable or show increased extent of the scotoma. Electroretinograms were recordable but with different severities of dysfunction. Patterns of photoreceptor outer nuclear layer (ONL) loss corresponded to the distribution of visual dysfunction. Outer nuclear layer thickness topography and en face imaging indicated that the greatest disease expression was in the area of known highest rod photoreceptor density.

CONCLUSIONS

Molecular heterogeneity was a feature of the PRD phenotype. Many of the molecular causes were also associated with other phenotypes, such as maculopathies, typical retinitis pigmentosa (RP) and cone-rod dystrophy. The pericentral pattern of retinal degeneration is thus confirmed to be an uncommon phenotype of many different genotypes rather than a distinct disease entity.

New insights into Stargardt disease with multimodal imaging

A 20-year-old woman with bilateral mild blurring of vision presented with a bull's eye maculopathy and was diagnosed with Stargardt disease, confirmed with genetic testing. The authors present several novel multimodal imaging findings including multicolor and multi-spectral imaging that enhanced visualization of perifoveal flecks, fundus autofluorescence that revealed both perifoveal and perimacular rings of hyperautofluorescence, adaptive optics imaging that revealed unprecedented visualization of cones at the fovea due to decreased cone density, and spectral-domain optical coherence tomography that identified thickening and increased hyperreflectivity of the external limiting membrane as a possible transient biomarker of early Stargardt disease.

Reduced macular function in ABCA4 carriers

PURPOSE

To study retinal function and morphology in ABCA4 carriers to investigate if ABCA4 carriership is associated with any functional or morphological changes and, if so, to explore whether certain mutations may be associated with particularly severe alterations.

METHODS

Eighteen subjects were recruited by means of being the parents of 10 teenagers/young adults with genetically confirmed ABCA4-associated retinal degenerations. The teenagers/young adults are well-known patients and have been followed in our clinic for many years. The eighteen subjects underwent careful ophthalmological examinations, including fundus photography and autofluorescence imaging, Goldmann perimetry, optical coherence tomography (OCT), full-field electroretinography (ffERG), multifocal electroretinography (mERG), and ABCA4 gene sequencing. The ffERG and mERG results were compared with those of healthy controls.

RESULTS

All subjects carried at least one ABCA4 mutation. Two subjects were compound heterozygous and therefore were excluded from the group-wise statistical analysis. Thirteen different ABCA4 mutations were found. C.2894 A>G (5/18) and c.768 G>T (4/18) were most common. Fourteen of 16 ABCA4 carriers demonstrated significantly altered mERG parameters (reduced amplitudes and/or delayed implicit times (ITs)) compared to normal values. In addition, the two subjects with compound heterozygous ABCA4 mutations had altered mERG parameters. A statistical comparison to the control group showed significantly reduced amplitudes and delayed ITs; p≤0.003 for all mERG parameters. FfERG parameters were altered in two ABCA4 carriers and one of the subjects with compound heterozygous ABCA4 mutations (reduced amplitude and delayed IT for the 30 Hz flicker ERG). No significant alterations were found for the whole group of ABCA4 carriers compared to the ffERG control group. Fundus photographs showed subtle to extensive pigmentary changes in several ABCA4 carriers.

CONCLUSIONS

In this study, ABCA4 carriers demonstrated reduced macular function measured by mERG along with none to subtle and even extensive morphological retinal changes. The c.768 G>T, c.5461-10T>C, and c.319 C>T mutations were associated with the most deviant ERGs, including both significant reduction of mERG amplitudes and prolongation of mERG ITs, as well as with reduced amplitude or delayed IT for the 30 Hz flicker ffERG in a few cases. They may therefore be considered serious mutations. The c.5917delG and c.4469 G>A mutations were associated with milder or no macular alteration. Long-term follow-up of these and other ABCA4 carriers may be of importance to elucidate the role of ABCA4 mutations in age-related macular degeneration. Moreover, improved knowledge of separate ABCA4 mutations may help us to better understand their role in ABCA4-associated retinal degenerations.

Identification of Genetic Defects in 33 Probands with Stargardt Disease by WES-Based Bioinformatics Gene Panel Analysis

Stargardt disease (STGD) is the most common hereditary macular degeneration in juveniles, with loss of central vision occurring in the first or second decade of life. The aim of this study is to identify the genetic defects in 33 probands with Stargardt disease. Clinical data and genomic DNA were collected from 33 probands from unrelated families with STGD. Variants in coding genes were initially screened by whole exome sequencing. Candidate variants were selected from all known genes associated with hereditary retinal dystrophy and then confirmed by Sanger sequencing. Putative pathogenic variants were further validated in available family members and controls. Potential pathogenic mutations were identified in 19 of the 33 probands (57.6%). These mutations were all present in ABCA4, but not in the other four STGD-associated genes or in genes responsible for other retinal dystrophies. Of the 19 probands, ABCA4 mutations were homozygous in one proband and compound heterozygous in 18 probands, involving 28 variants (13 novel and 15 known). Analysis of normal controls and available family members in 12 of the 19 families further support the pathogenicity of these variants. Clinical manifestation of all probands met the diagnostic criteria of STGD. This study provides an overview of a genetic basis for STGD in Chinese patients. Mutations in ABCA4 are the most common cause of STGD in this cohort. Genetic defects in approximately 42.4% of STGD patients await identification in future studies.