ADGRV1 — adhesion G protein-coupled receptor V1

The ADGRV1 gene provides instructions for making a very large protein that plays a crucial role in the development and function of the senses of hearing and vision. This protein acts like a structural bridge and a signaling molecule in the specialized cells of the inner ear and the retina (the light-sensitive tissue at the back of the eye). In the ear, it helps organize the tiny hair-like structures needed to detect sound. In the eye, it works with other proteins to maintain the health and function of photoreceptors, the cells that capture light. When there are harmful changes (mutations) in both copies of the ADGRV1 gene, it causes a condition called Usher syndrome type 2C. People with this condition are typically born with mild to severe hearing loss. Later in life, usually during adolescence or early adulthood, they begin to experience vision problems caused by a condition called retinitis pigmentosa. This vision loss often starts as night blindness and a narrowing of the visual field (tunnel vision), eventually affecting central vision as well. Usher syndrome type 2C is inherited in an autosomal recessive pattern. This means that for a person to have the condition, they must inherit two mutated copies of the gene, one from each parent. The parents, who each carry one mutated copy, typically do not show any signs of the condition themselves. Understanding the genetic cause helps families know what to expect, allows for early interventions like hearing aids, and opens the door to participating in clinical trials for future treatments.

Gene description: ADGRV1 encodes a very large G protein-coupled receptor involved in inner ear development and photoreceptor function.

Patient and family guide: The ADGRV1 gene provides instructions for making a very large protein that plays a crucial role in the development and function of the senses of hearing and vision. This protein acts like a structural bridge and a signaling molecule in the specialized cells of the inner ear and the retina (the light-sensitive tissue at the back of the eye). In the ear, it helps organize the tiny hair-like structures needed to detect sound. In the eye, it works with other proteins to maintain the health and function of photoreceptors, the cells that capture light. When there are harmful changes (mutations) in both copies of the ADGRV1 gene, it causes a condition called Usher syndrome type 2C. People with this condition are typically born with mild to severe hearing loss. Later in life, usually during adolescence or early adulthood, they begin to experience vision problems caused by a condition called retinitis pigmentosa. This vision loss often starts as night blindness and a narrowing of the visual field (tunnel vision), eventually affecting central vision as well. Usher syndrome type 2C is inherited in an autosomal recessive pattern. This means that for a person to have the condition, they must inherit two mutated copies of the gene, one from each parent. The parents, who each carry one mutated copy, typically do not show any signs of the condition themselves. Understanding the genetic cause helps families know what to expect, allows for early interventions like hearing aids, and opens the door to participating in clinical trials for future treatments.

Gene function: ADGRV1, also known as USH2A, is a large transmembrane protein localized to the photoreceptor cilium and synaptic regions. It is thought to play a role in maintaining the structural integrity and function of photoreceptor cells, possibly through cell adhesion or signaling pathways. Its involvement in Usher syndrome highlights its importance in both auditory and visual system development and maintenance.

Protein structure: The ADGRV1 gene encodes the Adhesion G protein-coupled receptor V1, which is the largest known cell surface protein, comprising over 6,300 amino acids in humans. The protein features a massive extracellular region, a standard 7-transmembrane (7TM) domain typical of G protein-coupled receptors, and a cytoplasmic tail. The extracellular portion is highly complex and contains 35 Calx-beta domains, which are involved in calcium binding and cell adhesion. Additionally, it contains a block of epilepsy-associated repeat (EAR) domains and a pentraxin (PTX) domain. The structure of ADGRV1 allows it to function both as an adhesion molecule and a signaling receptor. In the sensory organs, it assembles into functional multi-protein complexes. In the inner ear, it forms the ankle links by interacting with other Usher proteins like whirlin (WHRN) and PDZD7. In the retina, it is a core component of the Usher protein network at the periciliary membrane complex, where its long extracellular domain is thought to bridge the gap between adjacent membranes, providing structural support and facilitating intracellular transport.

Molecular function: The ADGRV1 gene encodes the Adhesion G protein-coupled receptor V1, which is the largest known cell surface protein. It functions as a critical component of the Usher protein network, a complex of proteins essential for the development and maintenance of sensory cells in the inner ear and retina. In the inner ear, ADGRV1 is a major structural component of the ankle links, which are fibrous connections between adjacent stereocilia in developing hair cells. These links are vital for the proper organization, maturation, and mechanotransduction capabilities of the auditory hair bundles. In the retina, ADGRV1 localizes to the periciliary membrane complex at the connecting cilium of photoreceptor cells. It interacts with other Usher syndrome-associated proteins, including usherin (USH2A) and whirlin (WHRN), to form a dynamic protein network. This complex is thought to play a role in the structural integrity of the connecting cilium and the intracellular transport of proteins between the inner and outer segments of the photoreceptors. Disruption of this network leads to the progressive degeneration of photoreceptor cells, resulting in retinitis pigmentosa. At the biochemical level, ADGRV1 is a G protein-coupled receptor that couples to G-alpha(i) and G-alpha(q) proteins. It is implicated in intracellular signaling pathways, potentially including the PI3K/PKB/mTOR pathway in the retina. The extensive extracellular domain, which contains numerous calcium-binding Calx-beta motifs, suggests a role in cell adhesion and calcium-dependent signaling, which are crucial for the function of both the central nervous system and sensory organs.

Expression pattern: ADGRV1 is ubiquitously expressed throughout the body but has particularly high expression levels in the central nervous system, inner ear, and retina. In the developing inner ear, the protein is localized to the stereocilia of hair cells, specifically at the ankle links, which are crucial for the proper structural maturation and function of the auditory system. This expression is essential during postnatal development for the correct organization of the hair bundles. In the retina, ADGRV1 is expressed in photoreceptor cells, where it localizes to the periciliary membrane complex at the connecting cilium. Here, it interacts with other Usher syndrome proteins, such as usherin (USH2A) and whirlin (WHRN), to form a functional network necessary for the maintenance and survival of photoreceptors. The gene is also highly expressed in the developing central nervous system, which correlates with its association with certain types of epilepsy, such as familial febrile seizures.

Mutation spectrum: The mutation spectrum of ADGRV1 is diverse, encompassing missense, nonsense, frameshift, and splice-site variants, as well as large genomic deletions. Because ADGRV1 is one of the largest genes in the human genome, spanning 90 exons, it is susceptible to a wide range of mutational events. Loss-of-function mutations, particularly truncating variants like nonsense and frameshift mutations, are the most common cause of ADGRV1-associated Usher syndrome type 2C. While there are no universally recognized hotspot regions, many pathogenic variants are distributed across the extensive extracellular domain, affecting the Calx-beta repeats and epilepsy-associated repeats (EAR). Missense mutations in these regions can disrupt calcium binding or protein-protein interactions essential for the formation of the Usher protein complex. The ClinVar database lists numerous pathogenic and likely pathogenic variants for ADGRV1, reflecting its significant contribution to inherited retinal diseases and hearing loss.

Pathogenic variants: 1. p.Ser2934Ter (c.8801C>A) - A nonsense mutation that leads to a premature stop codon, resulting in a truncated, non-functional protein. This loss-of-function variant is a well-characterized cause of Usher syndrome type 2C. 2. p.Arg2642Ter (c.7924C>T) - Another truncating nonsense mutation frequently reported in patients with Usher syndrome type 2C, leading to the classic phenotype of congenital hearing loss and progressive retinitis pigmentosa. 3. p.Gln4886Ter (c.14656C>T) - A nonsense variant located in the latter half of the massive protein, disrupting the transmembrane and intracellular domains critical for G-protein signaling and interaction with other Usher proteins. 4. c.10073-1G>A - A canonical splice-site mutation that disrupts normal pre-mRNA splicing, likely leading to an altered or truncated protein product, pathogenic for Usher syndrome type 2C. 5. p.Cys2153Tyr (c.6458G>A) - A missense mutation affecting a conserved residue in one of the extracellular domains, potentially disrupting protein folding or interactions within the Usher protein network.

Clinical significance: Mutations in the ADGRV1 gene are the primary cause of Usher syndrome type 2C (USH2C), an autosomal recessive disorder characterized by dual sensory impairment. Clinically, USH2C manifests with congenital, bilateral sensorineural hearing loss that is typically mild to moderate in the low frequencies and severe to profound in the high frequencies. Unlike Usher syndrome type 1, patients with USH2C generally have normal vestibular function and do not experience the severe balance issues associated with other subtypes. The ophthalmic component of USH2C is retinitis pigmentosa (RP), which usually becomes symptomatic in adolescence or early adulthood. Patients first experience nyctalopia (night blindness), followed by progressive constriction of the peripheral visual field. Central vision and color vision are often preserved until later in life. Optical coherence tomography (OCT) typically reveals progressive loss of outer retinal structures with initial foveal sparing, and cystoid macular edema may be present. In addition to USH2C, heterozygous mutations in ADGRV1 have been associated with familial febrile seizures-4 (FEB4) and Rolandic epilepsy. This highlights the gene's pleiotropic effects and its critical role in the central nervous system beyond the sensory organs. The severity and specific manifestations can vary, but the hallmark of biallelic ADGRV1 mutations remains the combined auditory and visual deficits of Usher syndrome.

Inheritance: Autosomal Recessive

Chromosomal location: 5q14.3

Genotype-phenotype correlations: Genotype-phenotype correlations for ADGRV1 are complex due to the large size of the gene and the wide variety of mutations. Generally, biallelic loss-of-function mutations (such as nonsense, frameshift, and canonical splice-site variants) are associated with the classic presentation of Usher syndrome type 2C, characterized by congenital hearing loss and adolescent-onset retinitis pigmentosa. The severity of the retinal degeneration can vary even among individuals with similar truncating mutations, suggesting the influence of genetic modifiers or environmental factors. Interestingly, heterozygous missense mutations in specific domains, particularly the Calx-beta domains, have been linked to familial febrile seizures and other forms of epilepsy. This suggests that certain functional domains of the ADGRV1 protein may have distinct roles in the central nervous system compared to the sensory organs. The presence of digenic inheritance, where a mutation in ADGRV1 combined with a mutation in another Usher gene (like PDZD7) causes the disease, further complicates the genotype-phenotype landscape and highlights the interconnected nature of the Usher protein network.

Research and therapeutic approaches: Currently, there are no FDA-approved gene therapies or pharmacological treatments specifically for ADGRV1-associated Usher syndrome or retinitis pigmentosa. Management focuses on supportive care, including hearing aids or cochlear implants for the auditory deficits, and standard low-vision aids for the progressive visual loss. The massive size of the ADGRV1 gene (over 18 kb of coding sequence) presents a significant challenge for traditional viral vector-based gene replacement therapies, such as the AAV vectors used in Luxturna (which targets the much smaller RPE65 gene), because the ADGRV1 coding sequence far exceeds the packaging capacity of AAV. To overcome this size limitation, researchers are exploring alternative therapeutic strategies. One promising approach is exon excision or exon skipping using CRISPR/Cas9 or antisense oligonucleotides (ASOs). This strategy aims to remove or skip the mutated exon, allowing the production of a slightly shortened but still functional protein. Dual-AAV vector systems, which split a large gene into two halves that recombine inside the cell, are also being investigated for large Usher genes, though this is technically challenging for a gene as large as ADGRV1. While these approaches are still in the preclinical stages (often tested in zebrafish or mouse models), they represent the pipeline of future therapies for this complex condition.

Diagnostic testing: Diagnostic testing for ADGRV1 mutations typically involves next-generation sequencing (NGS) approaches. Targeted gene panels for inherited retinal diseases (IRDs) and deafness are the most common initial tests, as they efficiently screen for mutations in ADGRV1 alongside other Usher syndrome genes like USH2A and MYO7A. If panel testing is inconclusive, whole exome sequencing (WES) or whole genome sequencing (WGS) may be employed to identify rare or complex variants, including deep intronic mutations or large structural variations. Genetic counseling is a critical component of the diagnostic process. Since ADGRV1-associated Usher syndrome type 2C is inherited in an autosomal recessive manner, parents of an affected individual are obligate carriers, and siblings have a 25% chance of inheriting the condition. Early diagnosis through newborn hearing screens and subsequent genetic testing allows for timely interventions, such as hearing aids or cochlear implants, and helps families prepare for the progressive visual loss associated with retinitis pigmentosa.

Animal models: The most prominent animal models for studying ADGRV1 are mouse and zebrafish models. The Adgrv1 knockout mouse models (such as the Mass1/Vlgr1 mutant) have been instrumental in understanding the gene's role in the inner ear, where it is required for the proper maturation of stereocilia and the formation of ankle links in cochlear hair cells. These mice exhibit audiogenic seizures and hearing loss, mirroring some aspects of the human phenotype, though they often do not fully recapitulate the retinal degeneration seen in Usher syndrome type 2C. Recently, a zebrafish model (adgrv1 rmc22) was generated using CRISPR/Cas9 technology. This model is particularly valuable because it displays early retinal dysfunction, providing a better platform for studying the ophthalmic manifestations of ADGRV1 mutations. In zebrafish, ADGRV1 has been shown to co-localize with usherin and whirlin in the retina, shaping the USH2 complex at the periciliary membrane. These models are crucial for testing potential therapeutic interventions, such as gene therapy or exon excision strategies.

Population genetics: ADGRV1 mutations account for approximately 5% to 9% of all Usher syndrome type 2 cases, making it the second or third most common genetic cause of this subtype after USH2A. The carrier frequency in the general population is relatively low, but it can be higher in certain isolated or consanguineous populations due to founder effects. While specific founder mutations have not been as widely documented for ADGRV1 as they have for other Usher genes, the large size of the gene means that private mutations within specific families or ethnic groups are common. The overall prevalence of ADGRV1-associated Usher syndrome is estimated to be a fraction of the total Usher syndrome prevalence, which is roughly 1 in 10,000 to 1 in 30,000 individuals worldwide.

Selected references: 1. Weston MD, et al. Mutations in the VLGR1 gene implicate G-protein signaling in the pathogenesis of Usher syndrome type II. Am J Hum Genet, 2004. PMID: 14740321 2. McMillan DR, et al. Very large G protein-coupled receptor-1, the largest known cell surface protein, is highly expressed in the developing central nervous system. J Biol Chem, 2002. PMID: 11606574 3. Maerker T, et al. A novel Usher protein network at the periciliary reloading point between molecular transport machineries in vertebrate photoreceptor cells. Hum Mol Genet, 2008. PMID: 17908740 4. Varela MD, et al. Detailed Clinical, Ophthalmic, and Genetic Characterization of ADGRV1-Associated Usher Syndrome. Am J Ophthalmol, 2023. PMID: 37422026 5. Stemerdink M, et al. Generation and Characterization of a Zebrafish Model for ADGRV1-Associated Retinal Dysfunction Using CRISPR/Cas9 Genome Editing Technology. Int J Mol Sci, 2023. PMID: 37373245 6. McGee J, et al. The very large G-protein-coupled receptor Vlgr1: A component of the ankle link complex required for the normal development of auditory hair bundles. J Neurosci, 2006. PMID: 16775142