The photoreceptor rim protein is an ABC transporter encoded by the gene for recessive Stargardt's disease (ABCR)

ABCR unites what ophthalmologists divide(s)

Over the last years, the molecular causes of monogenic chorioretinal diseases have been elucidated at an increasing pace. In contrast, only recently have genetic factors been found that contribute to multifactorial eye disorders such as age-related macular degeneration (AMD). Mutations in the retina-specific ATP-binding cassette transporter gene (ABCR) cause recessive Stargardt's disease (STGD) and fundus flavimaculatus (FFM), and were also found in 16% of patients with AMD. In addition, ABCR mutations were identified in families with recessive retinitis pigmentosa (RP), cone dystrophy (COD), and cone-rod dystrophy (CRD). In this review, we summarize these findings and propose a model which provides a framework to explain the observed genotypes and phenotypes. We hypothesize that most ABCR mutations can be classified in different classes of severity, and that, depending on the remaining total activity of ABCR, the phenotype can range from AMD at the mild end to RP at the severe end of the spectrum. This model allows us to make several predictions on the type and/or severity of ABCR mutations that are present in patients with AMD, STGD/FFM, COD, CRD, and RP.

A unique pattern of photoreceptor degeneration in cyclin D1 mutant mice

Cyclin D1-deficient mice have small eyes with thin retinas. We observed that there was a lower level of retinal cell proliferation and a unique pattern of photoreceptor cell death. Death was first observed in scattered clusters of cells in the retina. It then appeared to spread from these few cells to nearby photoreceptors, eventually producing extensive holes in the photoreceptor layer. These holes appeared to be filled with interneurons from the inner nuclear layer. The death mainly occurred during the second to fourth postnatal weeks. Other models of photoreceptor degeneration in rodents differ in that they occur more uniformly across the retina, with death proceeding over a longer period of time until all, or nearly all, of the photoreceptors degenerate. We also tested whether expression of a bcl-2 transgene could prevent the death and found that it could not.

ATP-binding-cassette (ABC) transport systems: functional and structural aspects of the ATP-hydrolyzing subunits/domains

Members of the superfamily of adenosine triphosphate (ATP)-binding-cassette (ABC) transport systems couple the hydrolysis of ATP to the translocation of solutes across a biological membrane. Recognized by their common modular organization and two sequence motifs that constitute a nucleotide binding fold, ABC transporters are widespread among all living organisms. They accomplish not only the uptake of nutrients in bacteria but are involved in diverse processes, such as signal transduction, protein secretion, drug and antibiotic resistance, antigen presentation, bacterial pathogenesis and sporulation. Moreover, some human inheritable diseases, like cystic fibrosis, adrenoleukodystrophy and Stargardt's disease are caused by defective ABC transport systems. Thus, albeit of major significance, details of the molecular mechanism by which these systems exert their functions are still poorly understood. In this review, recent data concerning the properties and putative role of the ATP-hydrolyzing subunits/domains are summarized and compared between bacterial and eukaryotic systems.

Genetic blindness: current concepts in the pathogenesis of human outer retinal dystrophies

Outer retinal dystrophies are the major causes of incurable blindness in the Western world. Understanding the etiology of retinal dystrophies has improved remarkably over the past decade. A number of genes, such as RHO, PDE-beta, RDS, TIMP3, MYO7A, RETGC1, RPGR, CRX and ABCR, are now known to be particularly important. Characterization of the genetic basis for disease is leading to new concepts of pathogenesis at the molecular and cellular levels. Such detailed understanding of disease processes is also stimulating a renewed interest in therapeutic strategies.

Mutation of the Stargardt disease gene (ABCR) in age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of severe central visual impairment among the elderly and is associated both with environmental factors such as smoking and with genetic factors. Here, 167 unrelated AMD patients were screened for alterations in ABCR, a gene that encodes a retinal rod photoreceptor protein and is defective in Stargardt disease, a common hereditary form of macular dystrophy. Thirteen different AMD-associated alterations, both deletions and amino acid substitutions, were found in one allele of ABCR in 26 patients (16%). Identification of ABCR alterations will permit presymptomatic testing of high-risk individuals and may lead to earlier diagnosis of AMD and to new strategies for prevention and therapy.