An ABCA4 loss-of-function mutation causes a canine form of Stargardt disease

Abstract

Autosomal recessive retinal degenerative diseases cause visual impairment and blindness in both humans and dogs. Currently, no standard treatment is available, but pioneering gene therapy-based canine models have been instrumental for clinical trials in humans. To study a novel form of retinal degeneration in Labrador retriever dogs with clinical signs indicating cone and rod degeneration, we used whole-genome sequencing of an affected sib-pair and their unaffected parents. A frameshift insertion in the ATP binding cassette subfamily A member 4 (ABCA4) gene (c.4176insC), leading to a premature stop codon in exon 28 (p.F1393Lfs*1395), was identified. In contrast to unaffected dogs, no full-length ABCA4 protein was detected in the retina of an affected dog. The ABCA4 gene encodes a membrane transporter protein localized in the outer segments of rod and cone photoreceptors. In humans, the ABCA4 gene is associated with Stargardt disease (STGD), an autosomal recessive retinal degeneration leading to central visual impairment. A hallmark of STGD is the accumulation of lipofuscin deposits in the retinal pigment epithelium (RPE). The discovery of a canine homozygous ABCA4 loss-of-function mutation may advance the development of dog as a large animal model for human STGD.

Functional analysis and classification of homozygous and hypomorphic ABCA4 variants associated with Stargardt macular degeneration

“…including an increase in autofluorescence and accumulation of A2E in RPE cells are also observed in Abca4 knockout mice, transgenic mice homozygous for STGD1-disease-causing missense mutations, and a Labrador retriever with a frameshift mutation in ABCA4 (Boyer et al, 2012;Charbel Issa et al, 2013;Makelainen et al, 2019;Molday et al, 2018;Weng et al, 1999;Zhang et al, 2015).…”

“…These compounds including A2E accumulate in RPE cells of STGD1 patients as fluorescent lipofuscin deposits upon phagocytosis of photoreceptor outer segments leading to the degeneration of RPE cells and photoreceptors (Mata, Weng, & Travis, 2000; Sparrow & Boulton, 2005). Phenotypic characteristics of patients with STGD1 including an increase in autofluorescence and accumulation of A2E in RPE cells are also observed in Abca4 knockout mice, transgenic mice homozygous for STGD1‐disease‐causing missense mutations, and a Labrador retriever with a frameshift mutation in ABCA4 (Boyer et al, 2012; Charbel Issa et al, 2013; Makelainen et al, 2019; Molday et al, 2018; Weng et al, 1999; Zhang et al, 2015).…”

Functional Characterization of ABCA4 Missense Variants Linked to Stargardt Macular Degeneration

“…Loss of ABCA4 transport activity results in the formation and accumulation of bisretinoids, including A2E in the photoreceptors and RPE cells [ 20 ]. These compounds accumulate as fluorescent lipofuscin deposits in STGD1 patients and animal models harboring ABCA4 null alleles and loss of function missense mutations [ 17 , 21 , 22 , 23 , 24 , 25 , 26 ]. To date, however, the effect of disease-causing missense variants in the TMDs on the functional properties of ABCA4 has not been examined in detail.…”

Large Animal Models of Inherited Retinal Degenerations: A Review

“…A 1 bp insertion in Abca4 was identified in a family group of Labrador retriever dogs resulting in a frameshift and premature stop codon [54]. This mutation causes a decrease in the mRNA transcript and the loss of the full-length protein.…”

“…Cone and rod photoreceptors both had abnormal function and were decreased in number in older affected dogs (10+ years). The development of a colony of these dogs as a model for therapy will have a substantial impact on the treatment of humans with Stargardt disease because mouse Abca4 −/− models lack a phenotype [54,55]. The phenotype in the dog appears to be milder than that seen in human subjects, with ABCA4 mutations reflecting species differences [56].…”