Rescue of the Stargardt phenotype in Abca4 knockout mice through inhibition of vitamin A dimerization

Abstract

Stargardt disease, an ATP-binding cassette, subfamily A, member 4 (ABCA4)-related retinopathy, is a genetic condition characterized by the accelerated accumulation of lipofuscin in the retinal pigment epithelium, degeneration of the neuroretina, and loss of vision. No approved treatment exists. Here, using a murine model of Stargardt disease, we show that the propensity of vitamin A to dimerize is responsible for triggering the formation of the majority of lipofuscin and transcriptional dysregulation of genes associated with inflammation. Data further demonstrate that replacing vitamin A with vitamin A deuterated at the carbon 20 position (C20-D 3 -vitamin A) impedes the dimerization rate of vitamin A-by approximately fivefold for the vitamin A dimer A2E-and subsequent lipofuscinogenesis and normalizes the aberrant transcription of complement genes without impairing retinal function. Phenotypic rescue by C20-D 3 -vitamin A was also observed noninvasively by quantitative autofluorescence, an imaging technique used clinically, in as little as 3 months after the initiation of treatment, whereas upon interruption of treatment, the age-related increase in autofluorescence resumed. Data suggest that C20-D 3 -vitamin A is a clinically amiable tool to inhibit vitamin A dimerization, which can be used to determine whether slowing the dimerization of vitamin A can prevent vision loss caused by Stargardt disease and other retinopathies associated with the accumulation of lipofuscin in the retina.S targardt disease, first described in 1909, is an autosomal recessive macular dystrophy affecting ∼1 in 10,000 people. The majority of people affected by the disease present with uncorrectable, decreased visual acuity during their teenage years, which most often progresses to legal blindness. To date, there is no approved intervention. Stargardt disease is marked by premature accumulation of lipofuscin in the retinal pigment epithelium (RPE), degeneration of the neuroretina, and subsequent loss of vision. The condition results from mutations in the ATPbinding cassette, subfamily A, member 4 (ABCA4) gene (1), which encodes a transmembrane flippase localized in photoreceptor outer segments. The flippase transports the phosphatidyl-ethanolamineretinaldehyde Schiff base between the cytosol and the cytoplasmic disk surfaces (2). Mutations in ABCA4 also result in retinitis pigmentosa and cone-rod dystrophy and have been linked to age-related macular degeneration (AMD) (3, 4).The accumulation of lipofuscin in the RPE is a common denominator in retinopathies associated with mutations in the ABCA4 gene. Also known as "wear and tear pigment," lipofuscin accumulates as a byproduct of cumulative damage during aging. The brown-yellow, autofluorescent, electron-dense material is also found in cells of the liver, kidney, heart muscle, adrenals, nerve, and ganglion and is considered one of the most consistent morphologic features of aging with a rate of accumulation inversely related to longevity (5, 6). In Stargardt disease, changes in RPE lipof...

A2E Distribution in RPE Granules in Human Eyes

“…It is believed that A2E affects the normal functions of the RPE and is involved in retinal diseases. Strategies designed to inhibit A2E formation for AMD and Stargardt disease by inhibiting key visual cycle enzymes or using deuterated vitamin a (C20-D 3 -Vitamin A), respectively, are in development [16][17][18][19]. Most notably, a phase 2 clinical trial using deuterated vitamin a for Stargardt disease is ongoing [20].…”

Lipofuscin Accumulation into and Clearance from Retinal Pigment Epithelium Lysosomes: Physiopathology and Emerging Therapeutics

“…Long-term, oral administration of ALK-001 to ABCA4 −/− reduced the accumulation of lipofuscin and A2E by 70 and 80%, respectively [109]. Assessment of the retina electric response to light signals (electroretinogram) revealed that ALK-001 treatment prevented the gradual loss of visual function observed in the ABCA4 −/− mouse.…”

Juvenile Macular Degenerations

“…Several interventions for STGD have been proposed, including light deprivation 42 , regulation of Vitamin A 43; 44; 45; 46 , gene-replacement therapy 44 , and human stem cell transplantation. 44; 47 Information about current clinical treatment trials can be accessed at ClinicalTrials.gov.…”