EYS — eyes shut homolog

The EYS gene provides instructions for making a protein called "eyes shut homolog," which is essential for the health and survival of the light-sensitive cells in the retina, known as photoreceptors. These cells, specifically the rods and cones, are responsible for capturing light and sending visual signals to the brain. The EYS protein helps maintain the delicate structure of these cells, particularly a part called the ciliary axoneme, which is crucial for their proper function. When the EYS gene is mutated, it cannot produce a functional protein. Without this protein, the photoreceptor cells gradually break down and die. This leads to a condition called retinitis pigmentosa (RP), specifically a type known as RP25. Patients typically first notice problems with their vision in dim light or at night (night blindness). Over time, they lose their peripheral (side) vision, creating a "tunnel vision" effect, and eventually, their central vision may also be affected, leading to severe visual impairment or blindness. EYS-associated retinitis pigmentosa is inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, typically do not show any symptoms and are known as carriers. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit the condition. Understanding this inheritance pattern is important for family planning and genetic counseling.

Gene description: EYS encodes a large extracellular matrix protein crucial for photoreceptor outer segment development and maintenance.

Patient and family guide: The EYS gene provides instructions for making a protein called "eyes shut homolog," which is essential for the health and survival of the light-sensitive cells in the retina, known as photoreceptors. These cells, specifically the rods and cones, are responsible for capturing light and sending visual signals to the brain. The EYS protein helps maintain the delicate structure of these cells, particularly a part called the ciliary axoneme, which is crucial for their proper function. When the EYS gene is mutated, it cannot produce a functional protein. Without this protein, the photoreceptor cells gradually break down and die. This leads to a condition called retinitis pigmentosa (RP), specifically a type known as RP25. Patients typically first notice problems with their vision in dim light or at night (night blindness). Over time, they lose their peripheral (side) vision, creating a "tunnel vision" effect, and eventually, their central vision may also be affected, leading to severe visual impairment or blindness. EYS-associated retinitis pigmentosa is inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, typically do not show any symptoms and are known as carriers. If both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit the condition. Understanding this inheritance pattern is important for family planning and genetic counseling.

Gene function: EYS is essential for the structural integrity and function of photoreceptor outer segments, playing a key role in the formation of the interphotoreceptor matrix. It is involved in maintaining the proper environment for photoreceptor survival and light perception, thus directly impacting the visual cycle and overall retinal health. Mutations disrupt this critical support, leading to photoreceptor degeneration and vision loss.

Protein structure: The EYS gene encodes a large extracellular protein, with the major isoforms (isoform 1 and isoform 4) consisting of 3,144 and 3,165 amino acids, respectively. The protein has a complex, repetitive domain architecture that is characteristic of extracellular matrix proteins. It begins with an N-terminal signal peptide, which directs its secretion or localization to specific cellular compartments. The core structure of the EYS protein is composed of 28 epidermal growth factor (EGF) and EGF-like domains, a putative coiled-coil domain, and five Laminin G-like (LamG) domains. These domains are interspersed throughout the protein sequence. The EGF-like and LamG domains are known to mediate protein-protein interactions and are crucial for the structural integrity and function of the protein in the extracellular space. The specific assembly and interacting partners of the human EYS protein are still being investigated, but it is thought to form a network that supports the photoreceptor ciliary axoneme.

Molecular function: The exact molecular function of the EYS protein in humans is not completely understood, but it is known to be crucial for the maintenance and survival of photoreceptor cells. EYS is an extracellular or secreted protein that localizes to the ciliary axoneme of both rod and cone photoreceptors. It is believed to play a structural role in stabilizing the ciliary axoneme and maintaining the integrity of the photoreceptor ciliary pocket. In Drosophila, the EYS ortholog (Spacemaker) interacts with Prominin to promote the formation of the inter-rhabdomeral space, a fluid-filled cavity essential for proper photoreceptor function. It is hypothesized that a similar interaction may occur in the human retina, where EYS could be involved in organizing the extracellular matrix surrounding the photoreceptor outer segments. The loss of functional EYS protein leads to the destabilization of the ciliary structure, mislocalization of essential outer segment proteins, and ultimately, the degeneration of photoreceptor cells, which is the hallmark of retinitis pigmentosa.

Expression pattern: The EYS gene is predominantly expressed in the retina, specifically within the photoreceptor cells. Its expression is localized to the region of the photoreceptor ciliary axoneme in both rod and cone photoreceptors. Additionally, EYS protein has been detected in the cytoplasm of retinal ganglion cells. The precise developmental timing and tissue-specific isoforms of EYS are still being elucidated, but the gene produces multiple transcripts. The major isoforms, such as isoform 1 and isoform 4, are highly expressed in the retina. Smaller variants, such as isoforms 2 and 3, have also been identified in the retina and other tissues like the testis. The specific roles of these different isoforms in various tissues remain an area of active investigation.

Mutation spectrum: The mutation spectrum of the EYS gene is highly diverse, with hundreds of pathogenic variants identified to date. These include missense, nonsense, frameshift, and splice-site mutations, as well as large genomic deletions and duplications. The mutations are distributed throughout the gene, affecting various domains of the protein, including the EGF-like and Laminin G-like domains. There are notable founder mutations in specific populations. For example, in the Japanese population, the c.4957dupA and c.8805C>A mutations are particularly common and account for a significant proportion of EYS-associated RP cases. In European populations, other specific variants may be more prevalent. The high allelic heterogeneity and the presence of deep intronic mutations that affect splicing complicate the genetic diagnosis and highlight the need for comprehensive sequencing approaches.

Pathogenic variants: 1. c.4957dupA (p.Ser1653Lysfs*2) - A common founder mutation in the Japanese population, leading to a frameshift and premature truncation of the protein, associated with severe retinitis pigmentosa. 2. c.8805C>A (p.Tyr2935*) - Another frequent nonsense mutation in the Japanese population, resulting in a truncated protein and causing autosomal recessive retinitis pigmentosa. 3. c.8648G>A (p.Trp2883*) - A nonsense mutation reported in various populations, leading to premature protein termination and associated with RP25. 4. c.6416G>A (p.Cys2139Tyr) - A missense mutation affecting a conserved cysteine residue in an EGF-like domain, likely disrupting protein folding and function. 5. c.2259+1G>A - A canonical splice-site mutation that disrupts normal pre-mRNA splicing, leading to an aberrant transcript and loss of functional EYS protein.

Clinical significance: Mutations in the EYS gene are a major cause of autosomal recessive retinitis pigmentosa (arRP), specifically designated as RP25. EYS mutations are estimated to account for 5% to 10% of arRP cases globally, with a significantly higher prevalence in certain populations, such as the Japanese, where they are responsible for 18% to 23.5% of cases. The clinical presentation of EYS-associated RP typically involves night blindness (nyctalopia) as the initial symptom, often starting in adolescence or early adulthood. As the disease progresses, patients experience a gradual constriction of their visual field, leading to "tunnel vision," and eventually, a decline in central visual acuity. The severity and rate of progression can vary, but many patients experience significant visual impairment by their fourth or fifth decade of life. In addition to typical RP, EYS mutations have occasionally been associated with autosomal recessive cone-rod dystrophy (arCRD), which presents with earlier central vision loss and color vision abnormalities. The disease is generally non-syndromic, meaning it affects only the eyes without systemic involvement.

Inheritance: Autosomal Recessive

Chromosomal location: 6q12

Genotype-phenotype correlations: Genotype-phenotype correlations in EYS-associated retinitis pigmentosa are complex and not fully understood, partly due to the large number of unique mutations and the variability in clinical presentation even among individuals with the same genotype. However, some general trends have been observed. Patients with two truncating mutations (e.g., nonsense or frameshift mutations) often present with a more severe phenotype and an earlier onset of symptoms compared to those with missense mutations. Specific founder mutations, such as those prevalent in the Japanese population, have been associated with distinct clinical trajectories. For instance, certain combinations of mutations may lead to a faster rate of visual field loss or earlier macular involvement. Ongoing natural history studies and large-scale genomic analyses are essential to further refine these correlations, which will be critical for predicting disease prognosis and selecting appropriate candidates for future clinical trials.

Research and therapeutic approaches: Currently, there are no approved therapies specifically targeting EYS-associated retinitis pigmentosa. Management primarily focuses on supportive care, including low-vision aids, orientation and mobility training, and regular ophthalmological monitoring. However, several therapeutic strategies are under active investigation in preclinical and early clinical stages. Gene therapy is a major area of focus, but the large size of the EYS gene (over 9 kb coding sequence) exceeds the packaging capacity of standard adeno-associated virus (AAV) vectors, which are commonly used for ocular gene therapy (e.g., Luxturna for RPE65). To overcome this, researchers are exploring alternative delivery methods, such as dual-AAV systems, lentiviral vectors, or non-viral nanoparticles. Another promising approach is CRISPR/Cas9-based gene editing, including exon skipping strategies aimed at removing mutated exons while preserving the reading frame and partial protein function. Antisense oligonucleotides (ASOs) are also being investigated to correct specific splicing defects caused by deep intronic mutations. Natural history studies (e.g., NCT04127006) are currently underway to establish clinical endpoints for future therapeutic trials.

Diagnostic testing: Diagnostic testing for EYS-associated inherited retinal diseases typically involves comprehensive genetic screening. Given the large size of the EYS gene and the clinical overlap with other forms of retinitis pigmentosa, targeted next-generation sequencing (NGS) panels that include EYS and other known IRD genes are the standard approach. Whole exome sequencing (WES) or whole genome sequencing (WGS) may also be employed, particularly when panel testing is inconclusive or to identify complex structural variants and deep intronic mutations. Genetic counseling is a crucial component of the diagnostic process. Since EYS-associated RP follows an autosomal recessive inheritance pattern, both parents of an affected individual are typically obligate carriers of one pathogenic variant. The recurrence risk for siblings of an affected individual is 25%. Genetic counselors can help families understand the inheritance pattern, the implications of the genetic test results, and the potential risks for future offspring, as well as provide information on available clinical trials and support resources.

Animal models: Currently, there is no mammalian model of human EYS disease, as the EYS gene was lost in several lineages of mammals, including rodents, during evolution. Therefore, traditional mouse models cannot be used to study EYS-associated retinitis pigmentosa. To overcome this limitation, researchers have turned to alternative animal models, particularly the zebrafish (Danio rerio) and Drosophila melanogaster. In Drosophila, the EYS ortholog (Spacemaker) is essential for the formation of the inter-rhabdomeral space, which isolates individual photoreceptor cells. In zebrafish, which possess an ortholog of the human EYS gene, knockout models have been developed using CRISPR/Cas9 technology. These zebrafish models exhibit mislocalization of outer segment proteins and progressive retinal degeneration, mimicking the human phenotype. They have proven valuable for studying the function of EYS in maintaining the ciliary axoneme and for evaluating potential therapeutic strategies, such as exon skipping and midigene therapies.

Population genetics: The prevalence of EYS mutations varies significantly across different populations. It is one of the most common causes of autosomal recessive retinitis pigmentosa worldwide, accounting for approximately 5% to 10% of cases in European and North American populations. However, the carrier frequency and disease prevalence are markedly higher in the Japanese population, where EYS mutations are the leading cause of arRP, responsible for 18% to 23.5% of cases. This high prevalence in Japan is largely driven by specific founder mutations, such as c.4957dupA and c.8805C>A. Understanding these population-specific genetic landscapes is crucial for designing targeted screening panels and developing population-specific therapeutic strategies.

Selected references: 1. Abd El-Aziz MM, et al. EYS, encoding an ortholog of Drosophila spacemaker, is mutated in autosomal recessive retinitis pigmentosa. Nat Genet. 2008;40(11):1285-1287. PMID: 18836446 2. Collin RW, et al. Mutations in EYS are a major cause of autosomal recessive retinitis pigmentosa. Hum Mutat. 2008;29(10):1246-1254. PMID: 18781615 3. Alfano G, et al. EYS Is a Protein Associated with the Ciliary Axoneme in Rods and Cones. PLoS One. 2016;11(11):e0166397. PMID: 27846257 4. Arai Y, et al. EYS is a major gene involved in retinitis pigmentosa in Japan: genetic landscapes revealed by stepwise genetic screening. PLoS One. 2015;10(3):e0118738. PMID: 25742131 5. Schellens R, et al. Zebrafish as a Model to Evaluate a CRISPR/Cas9-Based Exon Excision Approach as a Future Treatment Option for EYS-Associated Retinitis Pigmentosa. Int J Mol Sci. 2021;22(17):9154. PMID: 34502064 6. Placidi G, et al. Retinitis Pigmentosa Associated with EYS Gene Mutations: Disease Severity Staging and Central Retina Atrophy. Diagnostics (Basel). 2023;13(5):850. PMID: 36899994