We report an 11-year-old girl
Case#: 11 year old female
DiseaseAssertion: Stargardt’s Disease
ParentalTesting: She was the product of an uncomplicated pregnancy born to a healthy Filipino mother and Italian/Irish father with no known family history of ocular disease. The mother and father were asymptomatic but not examined. Segregation analyses showed that both parents are asymptomatic carriers.
CasePresentingHPOs: HP:0007754, HP:0011462, HP:0008035
CasePhenotypeFreeText: The ABCA4 gene, when mutated, results in a spectrum of retinal degeneration, including Stargardt macular dystrophy, fundus flavimaculatus, autosomal recessive retinitis pigmentosa, and cone-rod dystrophy (1). Over 800 disease-associated ABCA4 gene mutations have been reported.
CaseNotHPOs: N/A
CaseNotPhenotypeFreeText: N/A
CasePreviousTesting: The proband underwent a full consultative ophthalmic examination at the Ocular Genetics Clinic at Wills Eye Hospital, including visual acuity, slit-lamp, and dilated fundus examination. Fundus autofluorescence and spectral-domain optical coherence tomography (Spectralis; Heidelberg Engineering), Goldmann visual field (Octopus 900 perimeter; Haag-Streit International), and intravenous fluorescein angiography were obtained. Full-field electroretinogram (Espion; Diagnosys LLC) and multifocal electroretinogram (Veris V.6.4.3; EDI Inc.) were performed in accordance with the International Society of Clinical Electrophysiology and Vision standards. Best-corrected visual acuity was 20/125 in the right eye and 20/200 in the left eye. The patient demonstrated eccentric fixation. Pupillary responses were normal. Slit-lamp examination was normal. Fundus examination revealed healthy optic nerves and retinal blood vessels, bilateral macular geographic pigmentary stippling with subretinal flecks in and around this area, and a blunted internal limiting membrane reflex (Fig. 1). Peripheral retina was normal.
GenotypingMethod: Genotyping microarray chips for ABCA4 can identify >98% of the most common mutations. In this report, we describe 2 novel ABCA4 variants in a patient with Stargardt disease. Bioinformatic and in silico analysis of the functional consequences of these variants provided compelling evidence for pathogenicity.
Variant: c.850_857delATTCAAGA and c.6184_6187delGTCT
CAID: CA10604079 and CA10604078
MultipleGeneVariants: N/A
PreviouslyPublished: N/A
AdditionalInfo: Bioinformatic assessment of the c.850_857delATTCAAGA mutation showed that it resulted in a truncated 317 amino acid polypeptide, devoid of several essential domains of the ABCA4 transporter. The c.6184_6187delGTCT mutation led to a premature stop codon at the C-terminal end of the protein, resulting in a loss of a total of 161 amino acid residues. Although less than 7% of the protein was absent, the important VFVNFA motif, present within the last 30 amino acids of the NBD2 domain, was deleted (Fig. 2). This motif is known to be critical to ABCA4 protein function, is highly conserved among members of the ABCA transporter subfamily, and has also been linked to Tangier disease in the ABCA1 protein (9). Removal of this motif in ABCA4 leads to a loss of retinal stimulated ATPase in vitro and energy transduction of the transporter (9, 10). Protein modeling predicted a loss of an essential β-sheet, which significantly altered its structure. The NBD domains are sites of ATP hydrolysis that provide energy for transport of R-PE through rod outer segment membranes. Enzymatic studies suggest that the NBD2 domain in particular provides energy necessary for translocation of retinal derivatives generated in the visual cycle. The structural changes in NBD2 would affect ABCA4 transporter’s ability to transport retinoids, leading to accumulation of cytotoxic lipofuscin in RPE cells and ultimately photoreceptor cell death.