Retinal Pigment Epithelium Atrophy in Recessive Stargardt Disease as Measured by Short-Wavelength and Near-Infrared Autofluorescence

Detailed phenotype and long-term follow-up of RAB28-associated cone-rod dystrophy

PURPOSE

To gain an insight into the pathophysiology of RAB28-associated inherited retinal degeneration through detailed phenotyping and long-term longitudinal follow-up.

METHODS

The patient underwent complete ophthalmic examinations. Visual function was assessed with microperimetry, full-field electroretinography (ffERG), imaging with optical coherence tomography (OCT), short-wave (SW), and near-infrared (NIR) fundus autofluorescence (FAF).

RESULTS

A healthy Haitian woman with homozygous pathogenic variants (c.68C > T; p.Ser23Phe) in RAB28 presented at 16 years of age with a four-year history of blurred vision. Visual acuities were 20/125 in each eye, which remained relatively stable since. At age 27, cone ffERGs were non-detectable and borderline for rod-mediated responses. Kinetic fields were full to a V-4e target, undetectable to a small I-4e stimulus. Microperimetry showed an absolute central scotoma surrounded by a pericentral relative scotoma. SD-OCT showed an undetectable or barely detectable foveal and parafoveal photoreceptor outer nuclear layer (ONL), photoreceptor outer segment (POS), and retinal pigment epithelium (RPE) signals and loss of the SW- and NIR-FAF signals. This atrophic region was separated from a normally laminated retina by a narrow transition zone (TZ) of hyper SW- and NIR-FAF that co-localized with preserved ONL but abnormally thinned POS and RPE. There was minimal centrifugal (<100 μ m) expansion over a six-year period.

CONCLUSION

The cone-rod dystrophy phenotype documented herein supports a critical role of RAB28 for cone function and POS maintenance. Severe central photoreceptor and RPE loss with a predilection for POS loss in TZs suggests possible disruptions of complex mechanisms that maintain central cone photoreceptor and RPE homeostasis.

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.

Nahinfrarot-Autofluoreszenz: klinische Anwendung und diagnostische Relevanz

Die Nahinfrarot-Autofluoreszenz (NIA) ist ein nicht-invasives Verfahren zur Untersuchung des retinalen Pigmentepithels (RPE) basierend auf der Darstellung des antioxidativen Schutzfaktors Melanin in den RPE-Zellen. Die NIA verbessert die Früherkennung chorioretinaler Erkrankungen, da bei vielen dieser Erkrankungen mit der NIA Strukturveränderungen des RPE nachweisbar sind, bevor sich in anderen Untersuchungen Krankheitszeichen erkennen lassen.

[Near-infrared Fundus Autofluorescence: Clinical Application and Diagnostic Relevance]

Near-infrared autofluorescence (NIA) is a non-invasive retinal imaging technique for examination of the retinal pigment epithelium (RPE) based on the autofluorescence of melanin. Melanin has several functions within the RPE cells, in one of them it serves as a protective antioxidative factor within the RPE cells and is involved in the phagocytosis of photoreceptor outer segments. Disorders that affect the photoreceptor-RPE complex result in alterations of RPE cells which are detectable by alterations of NIA. Therefore, NIA allows to detect early alterations in inherited and acquired chorioretinal disorders, frequently prior to ophthalmoscopical visualisation and often prior to alterations in lipofuscin associated fundus autofluorescence (FAF) or optical coherence tomography (OCT). Although NIA and FAF relate to disorders affecting the RPE, findings between both imaging methods differ and the area involved has been demonstrated to be larger in NIA compared to FAF in several disorders (e.g., age-related macular degeneration, retinitis pigmentosa, ABCA4-gene associated Stargardt disease and cone-rod dystrophy, light damage), indicating that NIA detects earlier alterations compared to FAF. In addition, due to the absence of blue-light filtering which limits foveal visualisation in FAF, foveal alterations can be much better detected using NIA. A reduced subfoveal NIA intensity is the earliest sign of autosomal dominant BEST1-associated disease, when FAF and OCT are still normal. In other disorders, a normal subfoveal NIA intensity is associated with good visual acuity. This review summarizes the present knowledge on NIA and demonstrates biomarkers for various chorioretinal disorders.

Comparisons Among Optical Coherence Tomography and Fundus Autofluorescence Modalities as Measurements of Atrophy in ABCA4-Associated Disease

Purpose

In ABCA4-associated retinopathy, central atrophy was assessed by spectral domain optical coherence tomography (SD-OCT) and by short-wavelength (SW-AF) and near-infrared (NIR-AF) autofluorescence.

Methods

Patients exhibited a central atrophic lesion characterized by hypoautofluorescence (hypoAF) surrounded either by hyperautofluorescent (hyperAF) rings in both AF images (group 1, 4 patients); or a hyperAF ring in SW-AF but not in NIR-AF images (group 2, 11 patients); or hyperAF rings in neither AF images (group 3, 11 patients). Choroidal hypertransmission and widths of ellipsoid zone (EZ) loss were measured in foveal SD-OCT scans, and in AF images hypoAF and total hypo+hyperAF widths were measured along the same axis. Bland-Altman and repeated measures analysis of variance with Tukey post hoc were applied.

Results

For all groups, hypertransmission widths were significantly smaller than EZ loss widths. In Groups 1 and 2, hypertransmission width was not significantly different than SW-hypoAF width, but hypertransmission was narrower than the width of SW-hypo+hyperAF (groups 1, 2) and NIR-hypo+hyperAF (group 1). In group 3, the hypertransmission width was also significantly less than the width of SW-hypoAF and NIR-hypoAF. The EZ loss widths were not significantly different than measurements of total lesion size, the latter being the widths of SW-hypo+hyperAF and NIR-hypo+hyperAF (group 1); widths of NIR-hypoAF and SW-hypo+hyperAF (group 2); and widths of NIR-hypoAF and SW-hypoAF (group 3).

Conclusions

Hypertransmission and SW-hypoAF (except when reflecting total lesion width) underestimate lesion size detected by EZ loss, SW-hypoAF+hyperAF, and NIR-hypo+hyperAF.

Translational Relevance

The findings are significant to the selection of outcome measures in clinical studies.

Stargardt disease and progress in therapeutic strategies

Background: Stargardt disease (STGD1) is an autosomal recessive retinal dystrophy due to mutations in ABCA4, characterized by subretinal deposition of lipofuscin-like substances and bilateral centrifugal vision loss. Despite the tremendous progress made in the understanding of STGD1, there are no approved treatments to date. This review examines the challenges in the development of an effective STGD1 therapy.Materials and Methods: A literature review was performed through to June 2021 summarizing the spectrum of retinal phenotypes in STGD1, the molecular biology of ABCA4 protein, the in vivo and in vitro models used to investigate the mechanisms of ABCA4 mutations and current clinical trials.Results: STGD1 phenotypic variability remains an challenge for clinical trial design and patient selection. Pre-clinical development of therapeutic options has been limited by the lack of animal models reflecting the diverse phenotypic spectrum of STDG1. Patient-derived cell lines have facilitated the characterization of splice mutations but the clinical presentation is not always predicted by the effect of specific mutations on retinoid metabolism in cellular models. Current therapies primarily aim to delay vision loss whilst strategies to restore vision are less well developed.Conclusions: STGD1 therapy development can be accelerated by a deeper understanding of genotype-phenotype correlations.

Stargardt disease: Multimodal imaging: A review

Stargardt disease (STGD1) is an autosomal recessive retinal dystrophy, characterised by bilateral progressive central vision loss and subretinal deposition of lipofuscin-like substances. Recent advances in molecular diagnosis and therapeutic options are complemented by the increasing recognition of new multimodal imaging biomarkers that may predict genotype and disease progression. Unique non-invasive imaging features of STDG1 are useful for gene variant interpretation and may even provide insight into the underlying molecular pathophysiology. In addition, pathognomonic imaging features of STGD1 have been used to train neural networks to improve time efficiency in lesion segmentation and disease progression measurements. This review will discuss the role of key imaging modalities, correlate imaging signs across varied STGD1 presentations and illustrate the use of multimodal imaging as an outcome measure in determining the efficacy of emerging STGD1 specific therapies.