Encyclopedia
Please note this is a comparison between Version 1 by Liang Wang and Version 3 by Lindsay Dong.
Autosomal recessive Stargardt disease (STGD1) is an inherited retinal degenerative disease associated with a mutated ATP-binding cassette, subfamily A, member 4 (ABCA4) gene. STGD1 is the most common form of juvenile macular degeneration with onset in late childhood to early or middle adulthood and causes progressive, irreversible visual impairment and blindness.
- Stargardt disease
- ABCA4
- gene therapy
- visual cycle modulators
- stem cell therapy
- adeno-associated viral vectors
1. Introduction
Autosomal recessive Stargardt disease (STGD1, OMIM#248200) is an inherited macular dystrophy also known as fundus flavimaculatus (the variant with more widespread flecks and less macular involvement), associated with disease-causing genetic variants in the ATP-binding cassette, sub-family A, member 4 (ABCA4) gene [1]. It is one of the most common inherited retinal diseases with a prevalence estimated at 1 in 8000–10,000 [2]. The carrier frequency is as high as 1 in 20 in some populations. The disorder is characterized by progressive central vision loss and may be accompanied by peripheral vision loss in later stages or more severe cases of the disease [3][4][3,4]. STGD1 commonly manifests during childhood or early adulthood, and less frequently in later adulthood, when later onset confers a better visual prognosis with a slower rate of visual loss [5]. The classical clinical features include variable pigmentary changes and pisciform flecks in the macula progressing to a variable extent of chorioretinal atrophy, which depends on the severity of the underlying pathogenic ABCA4 variants, with more severe variants seen in childhood-onset STGD1 [4]. Over 2000 variants within the ABCA4 gene have been described [6]. Patients report decreased visual acuity, central scotomas, and color vision abnormalities, with variable degrees of nyctalopia and visual field constriction in later stages or more severe cases. Diagnosis involves ophthalmic evaluation, multimodal imaging, and genetic testing [7]. The ABCA4 gene encodes a transmembrane protein, a member of the ATP-binding cassette transporter superfamily, sub-family A, member 4, which plays an essential role in the retinoid recycling in the visual cycle and localizes to the disc membranes in the outer segments of rods and cones. Lack of ABCA4 protein leads to accumulation of visual cycle byproducts (lipofuscin) and dysfunction of the retinal pigment epithelium (RPE) and subsequent photoreceptor degeneration [8][9][8,9]. Although no cure currently exists, promising treatment strategies including genetic therapy, pharmacological treatments, and stem cell therapy, are being explored. Advancing our understanding of the underlying mechanisms and developing effective interventions are essential for managing this debilitating retinal disorder and preventing irreversible vision loss.
FAF is a useful non-invasive imaging modality. It may show an increased signal from excessive lipofuscin and highlights the flecks with both increased and decreased autofluorescence in some patients and reduced central autofluorescence surrounded by a brighter signal similar to a bull’s eye appearance [1][10][1,10] (Figure 1). Areas of a decreased FAF signal may appear later and correspond to areas of RPE atrophy with secondary photoreceptor loss. A significant correlation has been shown between FAF subtype and genotype, with milder pathological variants producing a localized low signal at the fovea with a surrounding homogeneous background, whereas more severe variants (including nonsense) correlate with areas of hypofluorescent atrophic areas [10].
OCT typically shows outer retinal loss in the central macula (Figure 1) with possible peripapillary sparing of the retina and RPE, which if present strongly supports the diagnosis of autosomal recessive STGD1 and may also show the earliest abnormality in young children consisting of external limiting membrane thickening [10][12][14][10,12,14].
1.1. Clinical Presentation and Diagnostic Testing
STGD1 presents with a spectrum of clinical features. On initial ophthalmoscopy, many children with STGD1 can have a normal-appearing fundus with or without vision loss; therefore, diagnosis can be delayed until the appearance of flecks or visual symptoms becomes significant enough to warrant deeper investigation with multimodal imaging including a combination of fundus fluorescein autofluorescence (FAF), optical coherence tomography (OCT), and electroretinography (ERG) [4][10][11][12][13][4,10,11,12,13]. Very young children may also present with symmetrical yellowish fine dots in the central macula. Later on, the characteristic key clinical features appear and include bilateral, progressive central vision loss, which encompasses both dyschromatopsia and central scotomata, macular atrophy, and yellow-white flecks within the RPE at the posterior pole [3]. The yellow-white flecks (Figure 1) represent the accumulation of lipofuscin, a yellowish autofluorescent pigment in the RPE cells that blocks the underlying choroidal fluorescence and causes the classic dark appearance of the choroid on FAF in 65–86% of patients [10][11][10,11].Figure 1. Typical presentation of Stargardt Type 1 at diagnosis (A1–A3) and 3-year follow-up (B1–B3). Color fundus (A1,B1) shows macular atrophy with yellow-white retinal flecks. Fundus autofluorescence (A2,B2) shows patches of hypoautofluorescence surrounded by an increased signal with flecks of both increased and decreased autofluorescence. Optical coherence tomography B-scans (A3,B3) show outer retinal and retinal pigment epithelium loss with hypertransmission defects corresponding to atrophy. Atrophy growth is significant at 3-year follow-up (B1–B3) compared to presentation at diagnosis (A1–A3). Best corrected visual acuity (BCVA) remains stable at 0.9 Snellen decimals (20/160). Reprinted from [1], used under an open-access license agreement distributed under the terms of the Creative Commons CC-BY license.
1.2. Genetics
STGD1 is caused by an autosomal recessive mutation of ABCA4 gene, located in the rim of outer segments of the photoreceptor cells and involved in the visual cycle. Photoreceptors during the visual cycle are constantly producing new disc membranes, pushing the older membranes further out towards the RPE [15][16]. One of the roles of the RPE is to phagocytose the most distal disc membranes with retinoid waste derivatives. ABCA4 proteins are “flippase” transporters, located in the outer segment disc membranes of rod and cone photoreceptors, which move retinoids from the lumen to the cytoplasmic side of the disc membranes mainly in the form of N-11-cis-retinylidene-phosphatidylethanolamine (NrPE) [8][16][8,17]. This allows the continuation of the visual cycle and decreases the production of bisretinoid compounds such as phosphatidyl-pyridium bisretinoid (A2PE) in the photoreceptor outer segment. As new disc membranes are produced, the older ones assume a more distal position, and the distal discs are phagocytosed by the RPE cells. After phagocytosis, the A2PE is converted to N-retinylidene-N retinylethanolamine (A2E) compound, which is insoluble and accumulates in the RPE cells to form lipofuscin (Figure 2) if it is not regularly managed by ABCA4 and other mechanisms [15][16]. With a mutated ABCA4, the accumulation of A2E and other bisretinoids leads to accumulation of toxic lipofuscin within the RPE, which with time will cause RPE cell death and photoreceptor dysfunction from a lack of RPE support [8][16][8,17]. The severity of phenotype correlates with the remaining ABCA4 function where more severe mutations, such as two null variants, result in a more severe phenotype with earlier onset and worse visual prognosis [13].Figure 2. Potential pharmaceutical interventions for Stargardt disease (STGD1). Treatment options (blue “x”) are shown in a summarized representation of the visual cycle with mutated ATP-binding cassette, subfamily A, member 4 (ABCA4) gene resulting in the accumulation of lipofuscin in the retinal pigment epithelium (RPE). Treatment for STGD1 includes several strategies. Gene therapy includes vector delivery of human ABCA4 gene. Pharmaceutical therapies include visual cycle modulators (VCM), metformin, avacincaptad pegol, and docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). VCM represented are retinol binding protein 4 (RBP4) antagonists, deuterated vitamin A, and retinal pigment epithelium-specific 65 kDa protein (RPE65) inhibitor (emixustat). Stem cell therapy includes human pluripotent stem cell-derived retinal pigment epithelium (hESC-RPE) transplantation for regenerating RPE layer. Created with BioRender.com.
1.3. Adeno-Associated Viral Vectors
2.3. Adeno-Associated Viral Vectors
AAV is a gene therapy delivery system that has shown safety with minimal adverse events in various trials; however, a major limitation of AAVs as vectors is their limited cargo capacity which is estimated to be around 4.7 kb [17][26]. Given the large coding region of the ABCA4 gene (50 exons and 150 kb), several in vitro models have been studied. The first attempt included creating an “oversized” transgene with the complete 6.8kb coding sequence yet transduction of these genes did not lead to the production of full-length protein [18][27]. Another model includes dual vector AAV strategies which can include fusing overlapping gene regions to create a transgene by fragmenting the original gene size. An alternative technique involves trans-splicing which provides no overlap between two transgenes since repeated sequences are removed, leaving an intact full-length coding sequence. Pre-clinical studies have shown early signs of success in delivering full-length ABCA4 coding regions using dual AAV and trans-splicing techniques (Figure 2) [19][20][28,29]. Given the positive outcomes obtained from animal studies, several studies are forthcoming using AAV as a gene therapy delivery system for individuals with STGD1.
AB0-504 (Abeona Therapeutics) is a dual AAV vector delivery system created by the Cre-LoxP recombinase system to reconstitute the full-length ABCA4 gene and is currently being tested in pre-clinical models. Preliminary results presented at the 26th Annual Meeting of the American Society of Gene & Cell Therapy provided evidence of the dual AAV vector system’s capability to generate the complete ABCA4 protein in cell culture [21][30]. Recent proof-of-concept investigations have further expanded upon these results, showcasing the expression of both ABCA4 messenger RNA (mRNA) and intact ABCA4 protein within the retinal tissue of ABCA4−/− knockout mice following subretinal administration. Notably, the observed levels of ABCA4 mRNA and full-length protein in the treated mice closely resemble those found in the naturally occurring ABCA4 of wild-type animals [21][22][30,31].
Moreover, other companies such as ViGeneron have also tapped into a novel dual AAV technology platform, achieving efficient expression of large genes in photoreceptors after intravitreal injection. They created VG-801 by using REconstitution Via mRNA Trans-splicing (REVeRT) technology enabling split genes encoding the 5′ and 3′ portions of human ABCA4 to be packaged into dual vgAAV vectors, which led to the generation of full-length protein. Notably, the REVeRT technology demonstrated high efficiency in reconstituting both transcript and protein levels of ABCA4 [23][32].
