Correlating the Expression and Functional Activity of ABCA4 Disease Variants With the Phenotype of Patients With Stargardt Disease

Abstract

PurposeStargardt disease (STGD1), the most common early-onset recessive macular degeneration, is caused by mutations in the gene encoding the ATP-binding cassette transporter ABCA4. Although extensive genetic studies have identified more than 1000 mutations that cause STGD1 and related ABCA4-associated diseases, few studies have investigated the extent to which mutations affect the biochemical properties of ABCA4. The purpose of this study was to correlate the expression and functional activities of missense mutations in ABCA4 identified in a cohort of Canadian patients with their clinical phenotype.MethodsEleven patients from British Columbia were diagnosed with STGD1. The exons and exon-intron boundaries were sequenced to identify potential pathologic mutations in ABCA4. Missense mutations were expressed in HEK293T cells and their level of expression, retinoid substrate binding properties, and ATPase activities were measured and correlated with the phenotype of the STGD1 patients.ResultsOf the 11 STGD1 patients analyzed, 7 patients had two mutations in ABCA4, 3 patients had one detected mutation, and 1 patient had no mutations in the exons and flanking regions. Included in this cohort of patients was a severely affected 11-year-old child who was homozygous for the novel p.Ala1794Pro mutation. Expression and functional analysis of this variant and other disease-associated variants compared favorably with the phenotypes of this cohort of STGD1 patients.ConclusionsAlthough many factors contribute to the phenotype of STGD1 patients, the expression and residual activity of ABCA4 mutants play a major role in determining the disease severity of STGD1 patients.

Genetic characterization of Stargardt clinical phenotype in South Indian patients using sanger and targeted sequencing

“…Patients exhibiting this variant (homozygous and compound heterozygous) were clinically classified as moderate severity or late-onset disease phenotype [33]. However, in vitro studies revealed a severe dysfunction due to this missense variant [11]. In the current study, fundus imaging of the variant-associated patients (Case IDs: 19, 25) who were in the early onset of disease progression revealed a severity of stages III and IV disease category.…”

“…Consequently, the waste product, alltrans-N-ret-PE, reacts with all-trans-retinal forming dihydropyridinium compounds, which undergo auto-oxidation and thereby generate phosphatidyl-pyridinium bisretinoid A2PE in photoreceptors. So far, more than 1000 mutations have been reported in ABCA4 across different cohorts leading to STGD1 and other retinal disorders like autosomal recessive cone-rod dystrophies, age macular degeneration and retinitis pigmentosa [11]. To our knowledge, only one study reported the clinical and genetic correlation of STGD1 disease in five families belonging to of Indian origin [12].…”

Genetic Spectrum of ABCA4-Associated Retinal Degeneration in Poland

“…The pathogenic effect of p.(Leu541Pro), residing in extracellular domain 1, was reported to result from misfolding. Garces et al described this variant earlier as having a negative impact on the ATP-binding capacity and abolishing of retinal-stimulated ATP hydrolysis [29]. The second variant of the complex allele, p.(Ala1038Val), located in nucleotide binding domain 1, seemed to be a milder alteration.…”

“…The second variant of the complex allele, p.(Ala1038Val), located in nucleotide binding domain 1, seemed to be a milder alteration. Nevertheless, it still contributed to the severe outcome [13,29,30].…”

Pathogenic Mechanisms Underlying Stargardt Macular Degeneration Linked to Mutations in the Transmembrane Domains of ABCA4

“…A number of disease-associated mutations in the exocytoplasmic domains (ECDs), the nucleotide-binding domains (NBDs), and the C-terminal segment of ABCA4 have been previously characterized (Sun, Smallwood et al 2000, Wiszniewski, Zaremba et al 2005, Quazi and Molday 2013, Zhang, Tsybovsky et al 2015, Garces, Jiang et al 2018). Many of these variants result in a significant reduction in cellular expression, mislocalization of ABCA4, and a loss in functional properties including substrate stimulated ATPase activity and ATP-dependent substrate transport activity.…”