The ABCR gene: a major disease gene in macular and peripheral retinal degenerations with onset from early childhood to the elderly

New Editing Tools for Gene Therapy in Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are a heterogeneous group of diseases that affect more than 2 million people worldwide. Gene therapy (GT) has emerged as an exciting treatment modality with the potential to provide long-term benefit to patients. Today, gene addition is the most straightforward GT for autosomal recessive IRDs. However, there are three scenarios where this approach falls short. First, in autosomal dominant diseases caused by gain-of-function or dominant-negative mutations, the toxic mutated protein needs to be silenced. Second, a number of IRD genes exceed the limited carrying capacity of adeno-associated virus vectors. Third, there are still about 30% of patients with unknown mutations. In the first two contexts, precise editing tools, such as CRISPR-Cas9, base editors, or prime editors, are emerging as potential GT solutions for the treatment of IRDs. Here, we review gene editing tools based on CRISPR-Cas9 technology that have been used in vivo and the recent first-in-human application of CRISPR-Cas9 in an IRD.

Molecular diagnosis of ATP-binding cassette transporter-related diseases

ATP-binding cassette (ABC) transporters are involved in a variety of physiologic processes such as xenobiotic defense, lipid metabolism, ion homeostasis and immune functions. A large number of ABC proteins have been causatively linked to rare and common human genetic diseases including familial high-density lipoprotein deficiency, retinopathies, cystic fibrosis, diabetes and cardiomyopathies. Furthermore, genetic variations in ABC transporter genes and dysregulated expression patterns of these molecules significantly contribute to drug resistance in human cancer cells and alter the pharmacokinetic properties of a variety of drugs. In order to analyze DNA sequence alterations or define disease-associated mRNA expression patterns of the complete ABC transporter superfamily, novel high-throughput molecular methods such as quantitative real-time PCR and DNA microarray analysis are emerging. The aim of this review is to provide an overview and to present some examples of human ABC transporters involved in monogenic diseases, cancer and pharmacogenetics. Methodologic aspects of molecular diagnostics applied to analyze genetic variations, mRNA and protein expression levels and functional characteristics of ABC transporters are discussed.

Test-retest reliability and inter-ocular symmetry of multi-focal electroretinography in Stargardt disease

PURPOSE

To evaluate test-retest reliability and inter-ocular symmetry of multi-focal electroretinography recordings in Stargardt disease and in patients without retinal pathology.

METHODS

Multi-focal electroretinography were recorded with continuous fundus monitoring using the VERIS multifocal recording system using an array of 103 hexagons spanning 50 degrees of central retina. Recordings were made sequentially from one eye (test-retest) followed by recordings from the fellow eye. A departure score, expressed as a percentage difference from the first recording (or from the fellow eye), was calculated for each comparison and parameter.

RESULTS

Multi-focal electroretinography response topographies were similar within and between eyes for a particular individual and parameter. However, local response variability within and between eyes was significantly higher in Stargardt disease than in patients without retinal pathology for both amplitude and timing parameters. Amplitude variability in Stargardt disease decreased with more peripheral targets and with larger stimuli but never to the levels achieved with normal observers.

CONCLUSIONS

Both test-retest reliability and inter-ocular symmetry of multi-focal electroretinography responses were significantly lower in Stargardt disease compared to normal eyes, a finding attributable primarily to unsteady fixation. The greater variability between eyes in Stargardt disease may also reflect pathological differences in the topography of retinal function. This inherent variability must be taken into consideration if the multi-focal electroretinography is to be sufficiently sensitive to reliably detect small differences in retinal function.

The role of the photoreceptor ABC transporter ABCA4 in lipid transport and Stargardt macular degeneration

ABCA4 is a member of the ABCA subfamily of ATP binding cassette (ABC) transporters that is expressed in rod and cone photoreceptors of the vertebrate retina. ABCA4, also known as the Rim protein and ABCR, is a large 2,273 amino acid glycoprotein organized as two tandem halves, each containing a single membrane spanning segment followed sequentially by a large exocytoplasmic domain, a multispanning membrane domain and a nucleotide binding domain. Over 500 mutations in the gene encoding ABCA4 are associated with a spectrum of related autosomal recessive retinal degenerative diseases including Stargardt macular degeneration, cone-rod dystrophy and a subset of retinitis pigmentosa. Biochemical studies on the purified ABCA4 together with analysis of abca4 knockout mice and patients with Stargardt disease have implicated ABCA4 as a retinylidene-phosphatidylethanolamine transporter that facilitates the removal of potentially reactive retinal derivatives from photoreceptors following photoexcitation. Knowledge of the genetic and molecular basis for ABCA4 related retinal degenerative diseases is being used to develop rationale therapeutic treatments for this set of disorders.

ABCA4 disease progression and a proposed strategy for gene therapy

Autosomal recessive retinal diseases caused by mutations in the ABCA4 gene are being considered for gene replacement therapy. All individuals with ABCA4-disease show macular degeneration, but only some are thought to progress to retina-wide blindness. It is currently not predictable if or when specific ABCA4 genotypes will show extramacular disease, and how fast it will progress thereafter. Early clinical trials of focal subretinal gene therapy will aim to arrest disease progression in the extramacular retina. In 66 individuals with known disease-causing ABCA4 alleles, we defined retina-wide disease expression by measuring rod- and cone-photoreceptor-mediated vision. Serial measurements over a mean period of 8.7 years were consistent with a model wherein a normal plateau phase of variable length was followed by initiation of retina-wide disease that progressed exponentially. Once initiated, the mean rate of disease progression was 1.1 log/decade for rods and 0.45 log/decade for cones. Spatio-temporal progression of disease could be described as the sum of two components, one with a central-to-peripheral gradient and the other with a uniform retina-wide pattern. Estimates of the age of disease initiation were used as a severity metric and contributions made by each ABCA4 allele were predicted. One-third of the non-truncating alleles were found to cause more severe disease than premature truncations supporting the existence of a pathogenic component beyond simple loss of function. Genotype-based inclusion/exclusion criteria and prediction of the age of retina-wide disease initiation will be invaluable for selecting appropriate candidates for clinical trials in ABCA4 disease.

Role of the C terminus of the photoreceptor ABCA4 transporter in protein folding, function, and retinal degenerative diseases

ABCA4 is an ATP-binding cassette transporter that is expressed in rod and cone photoreceptor cells and implicated in the removal of retinal derivatives from outer segments following photoexcitation. Mutations in the ABCA4 gene are responsible for a number of related retinal degenerative diseases, including Stargardt macular degeneration, cone-rod dystrophy, retinitis pigmentosa, and age-related macular degeneration. In order to determine the role of the C terminus of ABCA4 in protein structure and function and understand mechanisms by which C-terminal mutations cause retinal degenerative diseases, we have expressed and purified a series of deletion and substitution mutants of ABCA4 and ABCA1 in HEK 293T cells for analysis of their cellular localization and biochemical properties. Removal of the C-terminal 30 amino acids of ABCA4, including a conserved VFVNFA motif, resulted in a loss in N-retinylidene-phosphatidylethanolamine substrate binding, ATP photoaffinity labeling, and retinal-stimulated ATPase activity. This mutant was also retained in the endoplasmic reticulum of cells. Replacement of the VFVNFA motif with alanine residues also resulted in loss in function and cellular mislocalization. In contrast, C-terminal deletion mutants that retain the VFVNFA motif were functionally active and localized to intracellular vesicles similar to wild-type ABCA4. Our studies indicated that the VFVNFA motif is required for the proper folding of ABCA4 into a functionally active protein. This motif also contributes to the efficient folding of ABCA1 into an active protein. Our results provide a molecular based rationale for the disease phenotype displayed by individuals with mutations in the C terminus of ABCA4.

Inherited diseases of photoreceptors and prospects for gene therapy

The photoreceptor cells of the retina are subject to a wide range of genetic diseases. This review summarizes current knowledge regarding an important group of retinal diseases caused by mutations in photoreceptor-enriched genes. In addition, progress toward treatment of a variety of these diseases in animal models via adeno-associated virus gene therapy is described. Although no human trials have yet been initiated to treat diseases caused by mutations in photoreceptor-enriched genes, there is a great deal of optimism regarding the prospects of treating these diseases using adeno-associated virus gene therapy.

ATP-binding cassette transporter ABCA4: molecular properties and role in vision and macular degeneration

ABCA4, also known as ABCR or the rim protein, is a member of the ABCA subfamily of ATP binding cassette (ABC) transporters expressed in vertebrate rod and cone photoreceptor cells and localized to outer segment disk membranes. ABCA4 is organized in two tandem halves, each consisting of a transmembrane segment followed successively by a large exocytoplasmic domain, a multispanning membrane domain, and a nucleotide-binding domain. Over 400 mutations in ABCA4 have been linked to Stargardt macular degeneration and related retinal degenerative diseases that cause severe vision loss in affected individuals. Direct binding studies and ATPase activation measurements have identified N-retinylidene-phosphatidylethanolamine, a product generated from the photobleaching of rhodopsin, as the substrate for ABCA4. Mice deficient in ABCA4 accumulate phosphatidylethanolamine, all-trans retinal, and N-retinylidene-phosphatidylethanolamine in photoreceptors and the diretinal pyridinium compound A2E in retinal pigment epithelial cells. On the basis of these studies, ABCA4 is proposed to actively transport or flip N-retinylidene-phosphatidylethanolamine from the lumen to the cytoplasmic side of disc membranes following the photobleaching of rhodopsin. This transport activity insures that retinoids do not accumulate in disc membranes. Disease-linked mutations in ABCA4 that result in diminished transport activity lead to an accumulation of all-trans retinal and N-retinylidene-PE in disc membranes which react to produce A2E precursors. A2E progressively accumulates as lipofuscin deposits in retinal pigment epithelial cells as a result of phagocytosis of outer segment discs. A2E and photo-oxidation products cause RPE cell death and consequently photoreceptor degeneration resulting in a loss in vision in individuals with Stargardt macular degeneration and other retinal degenerative diseases associated with mutations in ABCA4.

Cone rod dystrophies

Cone rod dystrophies (CRDs) (prevalence 1/40,000) are inherited retinal dystrophies that belong to the group of pigmentary retinopathies. CRDs are characterized by retinal pigment deposits visible on fundus examination, predominantly localized to the macular region. In contrast to typical retinitis pigmentosa (RP), also called the rod cone dystrophies (RCDs) resulting from the primary loss in rod photoreceptors and later followed by the secondary loss in cone photoreceptors, CRDs reflect the opposite sequence of events. CRD is characterized by primary cone involvement, or, sometimes, by concomitant loss of both cones and rods that explains the predominant symptoms of CRDs: decreased visual acuity, color vision defects, photoaversion and decreased sensitivity in the central visual field, later followed by progressive loss in peripheral vision and night blindness. The clinical course of CRDs is generally more severe and rapid than that of RCDs, leading to earlier legal blindness and disability. At end stage, however, CRDs do not differ from RCDs. CRDs are most frequently non syndromic, but they may also be part of several syndromes, such as Bardet Biedl syndrome and Spinocerebellar Ataxia Type 7 (SCA7). Non syndromic CRDs are genetically heterogeneous (ten cloned genes and three loci have been identified so far). The four major causative genes involved in the pathogenesis of CRDs are ABCA4 (which causes Stargardt disease and also 30 to 60% of autosomal recessive CRDs), CRX and GUCY2D (which are responsible for many reported cases of autosomal dominant CRDs), and RPGR (which causes about 2/3 of X-linked RP and also an undetermined percentage of X-linked CRDs). It is likely that highly deleterious mutations in genes that otherwise cause RP or macular dystrophy may also lead to CRDs. The diagnosis of CRDs is based on clinical history, fundus examination and electroretinogram. Molecular diagnosis can be made for some genes, genetic counseling is always advised. Currently, there is no therapy that stops the evolution of the disease or restores the vision, and the visual prognosis is poor. Management aims at slowing down the degenerative process, treating the complications and helping patients to cope with the social and psychological impact of blindness.

Three different ABCA4 mutations in the same large family with several consanguineous loops affected with autosomal recessive cone–rod dystrophy

A large multiplex family presumably affected with autosomal recessive cone-rod dystrophy (CRD) was ascertained from Israel. In this family of Christian Arab ancestry with six consanguineous loops, linkage analysis failed to identify homozygosity in all six nuclear families at any of the three arCORD loci hitherto reported. However, homozygosity was found at the CORD3 locus for two nuclear families and the segregation of three distinct haplotypes at this locus in the whole pedigree suggested the alteration of the ABCA4 gene. This hypothesis was confirmed by the identification of three distinct mutations. Subsequently, with regard to the wide spectrum of autosomal recessive retinal dystrophies related to ABCA4 mutations, the natural history of the disease was revisited in all patients. Although the diagnosis of CRD was confirmed in 8/9 patients, the last one, aged of 34, displayed typical signs of Stargardt disease without extension to the peripheral retina. The results of this study emphasize the pitfalls of homozygosity mapping in highly inbred families when the heterozygote carrier frequency is particularly high in the general population.

Les dystrophies maculaires héréditaires

Hereditary macular dystrophies are degenerative diseases of the central area of the retina associating primary anomalies of the retinal pigment epithelium and sensory retina. These conditions, whose hallmark is a loss of visual acuity, are a major cause of blindness and affect patients at all ages. Macular dystrophies group diseases that are heterogenous at the genetic level, as well as at the clinical, histological and physiopathological levels. Monogenic macular dystrophies are rare autosomal dominant conditions, with the exception of Stargardt disease in its typical form, which is not only relatively frequent but is also inherited as an autosomal recessive trait. During the last few years, the molecular bases of these conditions have begun to be elucidated with the identification of several responsible genes. For some macular dystrophies, this new information has confirmed pre-existing hypotheses on their pathophysiology, but for others, the discovery of the disease gene has added further complexity to the disease process. Two contradictory concepts were particularly highlighted by these genetic studies. Several phenotypes previously described as different clinical entities were brought together by the identification of mutations in the same gene, and converselyome conditions that were clinically assigned the same name, often heterogeneous at the clinical level, appeared genetically and physiopathologically heterogeneous. In addition, it is worth noting that the monogenic macular dystrophy genes were often regarded as potential factors for susceptibility to age-related macular degenerations. However, to date, only ABCA4 mutations have been associated with a minority of this frequent multifactorial condition. The aim of this article is to give a progress report on the monogenic macular dystrophy genes and to review current knowledge concerning the pathophysiology of these conditions.

N-retinylidene-phosphatidylethanolamine is the preferred retinoid substrate for the photoreceptor-specific ABC transporter ABCA4 (ABCR)

ABCA4, a member of the family of ATP binding cassette (ABC) proteins found in rod and cone photoreceptors, has been implicated in the transport of retinoid compounds across the outer segment disk membrane following the photoactivation of rhodopsin. Mutations in the ABCA4 gene are responsible for Stargardt macular dystrophy and related retinal degenerative diseases that cause a loss in vision. To identify the retinoid substrate that interacts with ABCA4, we have isolated ABCA4 from rod outer segment disk membranes on an immunoaffinity matrix and analyzed retinoid compounds that bind to ABCA4 using high performance liquid chromatography and radiolabeling methods. When all-trans-retinal was added to ABCA4 in the presence of phosphatidylethanolamine, approximately 0.9 mol of N-retinylidene-phosphatidylethanolamine and 0.3 mol of all-trans-retinal were bound per mol of ABCA4 with an apparent K(d) of 2-5 microm. ATP and GTP released these retinoids from ABCA4, whereas ADP, GDP, and nonhydrolyzable derivatives, adenosine 5'-(beta,gamma-imido)triphosphate and guanosine 5'-(beta,gamma-imido)triphosphate, were ineffective. One mole of N-retinyl-phosphatidylethanolamine, the reduced form of N-retinylidene-phosphatidylethanolamine, bound per mol of ABCA4, whereas 0.3 mol of all-trans-retinal were bound in the absence of phosphatidylethanolamine. No binding of all-trans-retinol to ABCA4 was observed. Our results indicate that ABCA4 preferentially binds N-retinylidene-phosphatidylethanolamine with high affinity in the absence of ATP. Our studies further suggest that ATP binding and hydrolysis induces a protein conformational change that causes N-retinylidene-phosphatidylethanolamine to dissociate from ABCA4.

Macular coloboma in siblings affected by different phenotypes of retinitis pigmentosa

Purpose To report the clinical association between macular coloboma (early-onset macular dystrophies/atrophic changes) and different phenotypes of retinitis pigmentosa (RP). Methods Three young-adult siblings, two males and one female, were retrospectively studied. These patients underwent two complete ophthalmologic examinations (27-month follow-up), including orthoptic evaluation, colour vision test, visual field, corneal topography, electronystagmography, fluorescein angiography, and electroretinography. Eye check, automated visual field test, and complete electroretinographic study were also conducted on other asymptomatic members of the same family. Results All symptomatic siblings were affected by manifest congenital nystagmus, poor visual acuity, and progressive visual field impairment in both eyes, bilaterally presenting macular coloboma associated with three different RP patterns: classic RP; mild dystrophy of the retinal pigment epithelium, associated with subnormal electroretinographic findings (subclinical form of RP); and sector RP. The ophthalmologic reports regarding their deceased father documented that he had suffered from the same alterations of ocular movements and visual performances diagnosing, in both eyes, extensive atrophic changes of the macular area completely surrounded by pigmented bone spicules (RP-type tapeto-retinal dystrophy). The other investigated relatives did not show any specific and/or significant ocular disorder. Conclusions In these three adult members of the same family, the concomitance between macular coloboma and different intrafamilial RP phenotypes is described. This association represents an autosomal dominant clinical entity, hitherto observed only in non familial sporadic cases.Eye (2004) 18, 421-428. doi:10.1038/sj.eye.6700689

The complexities of ocular genetics

The cliché 'a picture is worth a thousand words' is a testament to the power of the visual system in helping us deal with our physical environment. Rarely do perturbations to the visual system, even minor ones, go unnoticed. Major defects in eye development may occur in the absence of systemic problems which threaten health. Ocular anomalies offer a window into many developmental events which would otherwise be difficult to study.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Age-related changes in the mouse outer retina

PURPOSE

To define the physiological and structural changes that may accompany aging in the normal mouse retina.

METHODS

C57BL/6 mice were maintained under cyclic light for either 2, 6, or 12 months. After rod- and cone-mediated corneal electroretinograms (ERG's) were recorded from anesthetized animals, the retinal structure was quantitatively examined. Photoreceptor cell density was measured within 100-microm regions of the central superior and inferior retina. Cone photoreceptor subtypes were identified by immunocytochemistry.

RESULTS

The amplitudes of rod- and cone-mediated ERG's were reduced in older mice, although the overall ERG wave-form did not change appreciably and implicit times were not changed in an age-dependent fashion. In comparison, there was no significant age-related decline in rod or cone photoreceptor density.

CONCLUSIONS

The amplitude of the mouse ERG declines with age. This change does not appear to reflect a change in the structural integrity of the photoreceptor cells. In functional studies of murine models of late-onset retinal disorders, it will be important to take these changes into consideration.

Reliability assessment of a rod photoreceptor outer segment grading system

The purpose of this study was to assess the reliability of a rod photoreceptor outer segment (PR-OS) grading system based on the analysis of 1 microm thick retinal sections obtained from Xenopus laevis whole-eye organ cultures. Digitally captured images, representative of the entire spectrum of rod PR-OS organization levels, were selected and coded numerically. A total of 102 individual rod PR-OS profiles were graded according to a six-step classification scheme based on the percentage of rod PR-OS membrane organization. Unweighted (exact agreement) and weighted kappa (kappa) coefficients (for use with ordered categorical rating scales) were calculated. Differences between kappa coefficients were tested for by chi-square analysis. To investigate the intra- and inter-rater variability and the possible presence of an interaction of the measurements with time, a repeated-measures analysis of variance was performed. The overall unweighted and weighted intra-rater kappa coefficients were 0.78 and 0.92, respectively. The overall unweighted and weighted inter-rater kappa coefficients were 0.73 and 0.90, respectively. There was no significant difference between raters or between first and second reading, nor was interaction between raters and time of rating documented. Individual kappa coefficients were equivalent both between raters and between sessions. Intra- and inter-rater agreement was within one step in 100% of cases. The estimated values of the kappa coefficients are consistent with a good to excellent degree of reliability and reproducibility of this rod PR-OS grading system. This system will be useful in the assessment of rod PR-OS morphology in studies of photoreceptor physiology and pathology.

Genotype-phenotype correlations and differential diagnosis in autosomal dominant macular disease

In the past few years, great progress has been made in the understanding of macular diseases. A number of disease-causing genes have been cloned, and numerous loci for other conditions have been mapped. The purpose of this article is to provide an overview of the current understanding of the genotype-phenotype correlations in autosomal dominant macular diseases with an emphasis on differential diagnostic issues. Whenever possible, the molecular correlates have been reviewed and the implications for age-related macular degeneration have been discussed. The many similarities of these diseases to age-related macular degeneration of the atrophic or exudative type, which can be misleading in elderly subjects, have also been addressed. While some conditions yield disease truly confined to the macula, others show widespread retinal involvement on functional testing. Clear-cut genotype-phenotype correlations are possible only for some forms of macular diseases. To further complicate the diagnostic process, there is a considerable degree of clinical overlap between many of them, making the differential diagnostic process potentially challenging. Functional testing, careful assessment of family history and extensive family work-up are essential in differentiating at the clinical level most, but not all, of these disease entities. Awareness of all of these conditions is required to direct correctly diagnostic investigations, to formulate an accurate prognosis, and for proper genetic counseling.