Delayed dark-adaptation and lipofuscin accumulation in abcr+/- mice: implications for involvement of ABCR in age-related macular degeneration.

PURPOSE: To examine the ocular phenotype in mice heterozygous for a null mutation in the abcr gene. METHODS: Retinas and retinal pigment epithelia (RPE) were prepared from wild-type, abcr+/-, and abcr-/- mice. Fresh tissues were homogenized and analyzed by normal phase high-performance liquid chromatography (HPLC) for the presence of retinoids and phospholipids. In another study, fixed tissues were sectioned and analyzed by light and electron microscopy. Finally, anesthetized mice were studied by electroretinography (ERG) at different times after exposure to strong light. RESULTS: A2E, the major fluorophore of lipofuscin, and its precursors, A2PE-H(2) and A2PE, were approximately fourfold more abundant in 8-month-old abcr+/- than in the wild-type retina and RPE. The levels of these substances in abcr+/- mice were approximately 40% those in abcr-/- mice. Lipofuscin pigment-granules were also visible in abcr+/- RPE cells by electron microscopy. Accumulation of A2PE-H(2) and A2E in abcr+/- retina and RPE, respectively, was strongly dependent on light exposure. Heterozygous mutants also exhibited delayed recovery of rod sensitivity by ERG. This delay was correlated with elevated levels of all-trans-retinaldehyde (all-trans-RAL) in retina after a photobleach and was not caused by a reduction in quantum-catch due to depletion of 11-cis-retinaldehyde (11-cis-RAL). CONCLUSIONS: Partial loss of the ABCR or rim protein is sufficient to cause a phenotype in mice similar to recessive Stargardt's disease (STGD) and age-related macular degeneration (AMD) in humans. These data are consistent with the suggestion that the STGD carrier-state may predispose to the development of AMD.

Autosomal dominant retinal degeneration and bone loss in patients with a 12-bp deletion in the CRX gene.

PURPOSE: To define the phenotypic expression of a deletion in the gene encoding the transcription factor CRX in a large, seven-generation, white family. METHODS: Fourteen affected individuals, all heterozygous for the Leu146del12 mutation in the cone-rod homeobox gene (CRX), and four nonaffected relatives from the same family were examined with visual function tests, and 10 underwent bone mineral density (BMD) measurement. RESULTS: The ability of the mutated CRX protein to transactivate rhodopsin promoter was decreased by approximately 25%, and its ability to react synergistically with neural retinal leucine zipper (NRL) was reduced by more than 30%. The affected members of the family had an autosomal dominant ocular condition most closely resembling Leber congenital amaurosis (LCA) with severe visual impairment at an early age. Depending on age, affected members showed varying degrees of significant visual acuity loss, elevated dark-adaptation thresholds, significantly reduced cone and rod electroretinogram (ERG) amplitudes, and progressive constriction of the visual fields, in most cases leading to complete blindness. Six affected members had reduced levels of BMD in the spine and the hip (osteopenia). Four affected female members who were receiving long-term hormonal replacement therapy (HRT) demonstrated normal values of BMD. CONCLUSIONS: This large deletion of the CRX gene is associated with a severe form of autosomal dominant retinal degeneration. Affected members not receiving HRT showed reduced BMD (osteopenia). This phenotype may reflect the abnormal influence of mutant CRX on both retinal and pineal development.

Visual phenotype in patients with Arg41Gln and ala196+1bp mutations in the CRX gene.

Our aim was to describe the visual function characteristics of affected members from two unrelated families with different dominant mutations in the CRX gene. Standard full-field ERGs and high-intensity a-wave series were obtained. In addition, in most subjects, dark-adapted (DA) thresholds, color vision function (arrangement tests), and static perimetry were assessed. A point mutation in codon 41 of the CRX gene (Arg41Gln) was identified in family members from the RFS087 family who were tested on several occasions since 1983. Depending on age, affected members showed varying degrees of acuity loss, normal or slightly elevated DA thresholds, reduced cone a- and b-wave amplitudes, normal or minimally delayed cone b-wave implicit times, and normal rod and cone phototransduction gain parameters. An insertion mutation (Ala196+1bp) was found in two members of another family (RFS014). Affected members showed reduced visual acuity, normal or slightly elevated DA thresholds, relatively preserved rod ERG and substantially reduced or undetectable cone ERG, and normal rod phototransduction gain parameters. The Arg41Gln was associated with a late-onset, slowly progressing mild form of cone-rod dystrophy with cone loss but preserved rod and cone sensitivity until later in life. The Ala196+1bp mutation was associated with an early-onset, severe form of cone-rod dystrophy similar to that described in the original CORD2 family (Evans et al., Arch Ophthalmol 1995;113:195-201).

Insights into the function of Rim protein in photoreceptors and etiology of Stargardt's disease from the phenotype in abcr knockout mice.

Rim protein (RmP) is an ABC transporter of unknown function in rod outer segment discs. The human gene for RmP (ABCR) is affected in several recessive retinal degenerations. Here, we characterize the ocular phenotype in abcr knockout mice. Mice lacking RmP show delayed dark adaptation, increased all-trans-retinaldehyde (all-trans-RAL) following light exposure, elevated phosphatidylethanolamine (PE) in outer segments, accumulation of the protonated Schiff base complex of all-trans-RAL and PE (N-retinylidene-PE), and striking deposition of a major lipofuscin fluorophore (A2-E) in retinal pigment epithelium (RPE). These data suggest that RmP functions as an outwardly directed flippase for N-retinylidene-PE. Delayed dark adaptation is likely due to accumulation in discs of the noncovalent complex between opsin and all-trans-RAL. Finally, ABCR-mediated retinal degeneration may result from "poisoning" of the RPE due to A2-E accumulation, with secondary photoreceptor degeneration due to loss of the RPE support role.