MERTK — MER Proto-Oncogene Tyrosine Kinase

The MERTK gene provides instructions for making a protein that acts like a specialized sensor and cleanup crew in the eye. This protein is primarily found in a layer of cells called the retinal pigment epithelium (RPE), which sits just behind the light-sensing cells (photoreceptors) of the retina. Every day, the photoreceptors shed their worn-out tips, and the MERTK protein signals the RPE cells to swallow up and clear away this debris. This daily cleanup is absolutely essential to keep the photoreceptors healthy and functioning properly. When there is a mutation in the MERTK gene, this cleanup process breaks down. The RPE cells can no longer effectively clear away the shed photoreceptor tips, causing toxic debris to build up in the retina. This buildup eventually damages and kills the light-sensing cells, leading to vision loss. For patients, this typically manifests as a condition called retinitis pigmentosa (RP). Symptoms often begin in childhood or adolescence with night blindness, followed by a gradual loss of peripheral (side) vision, creating a "tunnel vision" effect. Unlike some other forms of RP, MERTK mutations often affect central vision early on as well. MERTK-related retinal disease 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 condition. The parents, who each carry one mutated copy and one normal copy, are typically unaffected and have normal vision. For families, understanding this inheritance pattern is important for genetic counseling, as each sibling of an affected individual has a 25% chance of also having the condition. While there is currently no cure, early genetic diagnosis is crucial because researchers are actively developing and testing new treatments, including gene therapies, specifically for MERTK-related vision loss.

Gene description: Encodes a receptor tyrosine kinase critical for the phagocytosis of photoreceptor outer segment (POS) tips by the RPE.

Patient and family guide: The MERTK gene provides instructions for making a protein that acts like a specialized sensor and cleanup crew in the eye. This protein is primarily found in a layer of cells called the retinal pigment epithelium (RPE), which sits just behind the light-sensing cells (photoreceptors) of the retina. Every day, the photoreceptors shed their worn-out tips, and the MERTK protein signals the RPE cells to swallow up and clear away this debris. This daily cleanup is absolutely essential to keep the photoreceptors healthy and functioning properly. When there is a mutation in the MERTK gene, this cleanup process breaks down. The RPE cells can no longer effectively clear away the shed photoreceptor tips, causing toxic debris to build up in the retina. This buildup eventually damages and kills the light-sensing cells, leading to vision loss. For patients, this typically manifests as a condition called retinitis pigmentosa (RP). Symptoms often begin in childhood or adolescence with night blindness, followed by a gradual loss of peripheral (side) vision, creating a "tunnel vision" effect. Unlike some other forms of RP, MERTK mutations often affect central vision early on as well. MERTK-related retinal disease 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 condition. The parents, who each carry one mutated copy and one normal copy, are typically unaffected and have normal vision. For families, understanding this inheritance pattern is important for genetic counseling, as each sibling of an affected individual has a 25% chance of also having the condition. While there is currently no cure, early genetic diagnosis is crucial because researchers are actively developing and testing new treatments, including gene therapies, specifically for MERTK-related vision loss.

Gene function: MERTK is a receptor tyrosine kinase expressed in the retinal pigment epithelium (RPE). It is essential for the daily phagocytosis of shed photoreceptor outer segment (POS) tips, a process vital for photoreceptor health and survival. Defective MERTK leads to an accumulation of POS debris, causing RPE dysfunction and progressive retinal degeneration.

Protein structure: The MERTK gene encodes a transmembrane receptor tyrosine kinase that is 999 amino acids in length. The protein structure is characterized by a distinct modular architecture typical of the TAM (Tyro3, Axl, Mer) family of receptors. The extracellular portion of the protein contains two immunoglobulin (Ig)-like domains followed by two fibronectin type III (FNIII) domains. These extracellular domains are responsible for recognizing and binding to specific ligands, such as GAS6 and PROS1, which are often bound to phosphatidylserine on the surface of apoptotic cells or shed photoreceptor outer segments. Following the extracellular domains is a single transmembrane helix that anchors the receptor in the cell membrane. The intracellular portion of the protein consists of a highly conserved tyrosine kinase domain. Upon ligand binding to the extracellular domains, the receptor undergoes dimerization and subsequent autophosphorylation of specific tyrosine residues within the intracellular kinase domain. This autophosphorylation activates the kinase, creating docking sites for downstream signaling molecules and initiating the intracellular signaling cascades necessary for processes like phagocytosis.

Molecular function: The MERTK gene encodes the MER proto-oncogene, tyrosine kinase, a transmembrane receptor that plays a critical role in various cellular processes, most notably the phagocytosis of apoptotic cells (efferocytosis). In the retina, MERTK is essential for the daily phagocytosis of shed photoreceptor outer segments by the retinal pigment epithelium (RPE). This process is vital for the survival and function of photoreceptors, as the accumulation of shed outer segments is highly toxic. MERTK acts as the primary signaling receptor that triggers the ingestion phase of this phagocytic process. Upon binding to its ligands, such as Growth Arrest-Specific 6 (GAS6) or Protein S (PROS1), which act as bridging molecules between the RPE and the phosphatidylserine exposed on the shed outer segments, MERTK undergoes autophosphorylation. This activation initiates a cascade of intracellular signaling events, including the activation of focal adhesion kinase (FAK) and the reorganization of the actin cytoskeleton, ultimately leading to the engulfment of the outer segment debris. Beyond its role in phagocytosis, MERTK signaling is also involved in regulating cell survival, migration, and differentiation. In the immune system, it helps maintain tolerance by promoting the clearance of apoptotic cells and dampening inflammatory responses. In the retina, the failure of MERTK-mediated phagocytosis not only leads to the toxic buildup of debris but also triggers secondary inflammation and microglial activation, further contributing to the rapid degeneration of photoreceptors seen in MERTK-related diseases.

Expression pattern: The MERTK gene is widely expressed across various tissues, but its expression in the eye is particularly critical for retinal health. Within the retina, MERTK is predominantly expressed in the retinal pigment epithelium (RPE), a monolayer of cells situated just outside the neurosensory retina. The RPE plays a vital role in maintaining the health and function of photoreceptors, and MERTK is localized to the apical microvilli of the RPE cells, where it interacts directly with the photoreceptor outer segments. Beyond the eye, MERTK is expressed in cells of the immune system, particularly macrophages and dendritic cells, where it is involved in the clearance of apoptotic cells (efferocytosis) and the regulation of immune responses. It is also found in the reproductive system, nervous system, and various other tissues. Despite this widespread expression, mutations in MERTK primarily manifest as isolated retinal disease, suggesting that other redundant pathways may compensate for its loss in systemic tissues, whereas the retina is uniquely dependent on MERTK function for survival.

Mutation spectrum: The mutation spectrum of the MERTK gene is diverse, encompassing a wide range of pathogenic variants that lead to inherited retinal diseases. To date, over 80 distinct pathogenic variants have been identified. These include missense mutations, which are the most common, as well as nonsense mutations, splice-site defects, small insertions and deletions (indels), and large genomic rearrangements. The mutations are distributed throughout the gene, affecting various functional domains of the protein, including the extracellular fibronectin and immunoglobulin-like domains, as well as the intracellular tyrosine kinase domain. While many mutations are private to specific families, some founder mutations have been identified in certain populations. A notable example is a large 91-kb deletion encompassing exons 1-7 of the MERTK gene, which is a common founder mutation in the Faroe Islands. This specific deletion accounts for a significant proportion of retinitis pigmentosa cases in that population. The variety of mutation types highlights the importance of comprehensive genetic testing, including methods capable of detecting large structural variations, for accurate diagnosis.

Pathogenic variants: 1. p.Arg844Cys - A well-characterized missense mutation located in the kinase domain, frequently associated with severe autosomal recessive retinitis pigmentosa. 2. p.Arg651X - A nonsense mutation leading to premature truncation of the protein, resulting in a complete loss of function and severe early-onset retinal dystrophy. 3. c.2189+1G>T - A canonical splice-site mutation that disrupts normal mRNA splicing, leading to a non-functional protein and typical MERTK-related RP. 4. 91-kb deletion (exons 1-7) - A large structural deletion that is a major founder mutation in the Faroe Islands, causing a high proportion of RP cases in that population. 5. p.Gly705Arg - Another significant missense variant affecting the kinase domain, known to impair the autophosphorylation and signaling capability of the receptor.

Clinical significance: Mutations in the MERTK gene primarily cause autosomal recessive retinitis pigmentosa (AR-RP), specifically designated as RP38. This condition is characterized by a severe and early-onset rod-cone dystrophy. Patients typically present with nyctalopia (night blindness) within the first two decades of life, followed by progressive visual field constriction. A distinguishing feature of MERTK-related RP is the early involvement of the macula, which can manifest as macular atrophy, bull's eye maculopathy, or yellowish-white deposits. This early macular involvement often leads to a more rapid decline in central vision compared to other forms of RP, with many patients reaching legal blindness by the fourth to sixth decade of life. While RP is the most common presentation, MERTK mutations have also been associated with other severe inherited retinal diseases, including Leber congenital amaurosis (LCA) and severe early-onset retinal dystrophy (EORD). The clinical spectrum can vary, but the underlying hallmark is the progressive loss of photoreceptors due to RPE dysfunction. Systemic features are generally not associated with MERTK-related retinal dystrophies, as the disease is typically confined to the eye despite the gene's expression in other tissues.

Inheritance: Autosomal Recessive

Chromosomal location: 2q14.1

Genotype-phenotype correlations: Genotype-phenotype correlations in MERTK-related retinal dystrophies are complex and not fully elucidated, but some patterns have emerged. In general, biallelic loss-of-function mutations, such as nonsense, frameshift, or large deletions that result in a complete absence of functional MERTK protein, tend to cause a more severe phenotype. These patients often present with early-onset retinitis pigmentosa or Leber congenital amaurosis, characterized by rapid progression and early macular involvement leading to severe visual impairment early in life. Missense mutations, which may result in a partially functional protein, can sometimes be associated with a slightly milder or later-onset phenotype, although this is highly variable. The specific location of the missense mutation within the protein domains (e.g., the kinase domain versus the extracellular domains) can influence the degree of functional impairment. However, even with identical mutations, there can be significant phenotypic variability among individuals, suggesting that genetic modifiers or environmental factors may also play a role in determining disease severity and progression.

Research and therapeutic approaches: Therapeutic approaches for MERTK-related inherited retinal diseases are an area of active research, with gene therapy being the most promising strategy. The goal of gene therapy is to deliver a functional copy of the MERTK gene to the retinal pigment epithelium (RPE) cells to restore their phagocytic capability and halt the secondary degeneration of photoreceptors. Because the MERTK gene is relatively small, it can be efficiently packaged into adeno-associated virus (AAV) vectors, which are the standard delivery vehicles for retinal gene therapy. A Phase 1 clinical trial (NCT01482195) has evaluated the safety and preliminary efficacy of subretinal injection of an AAV2 vector carrying the human MERTK gene (rAAV2-VMD2-hMERTK) in patients with MERTK-associated retinitis pigmentosa. The results indicated that the treatment was generally safe and well-tolerated, with some patients showing transient improvements or stabilization in visual function, although long-term efficacy remains to be fully established. More recently, Opus Genetics has launched a new clinical development program for an investigational gene therapy, OPGx-MERTK, aimed at treating MERTK-related RP, indicating continued investment in this approach. Currently, there are no FDA-approved therapies specifically for MERTK-related retinal diseases (unlike Luxturna, which is approved for RPE65 mutations). Management remains supportive, focusing on maximizing remaining vision through low vision aids and monitoring for complications such as cataracts or macular edema. However, the ongoing advancements in AAV-mediated gene delivery and the clear understanding of the disease mechanism in the RPE provide a strong foundation for the future development of effective treatments.

Diagnostic testing: Diagnosis of MERTK-related inherited retinal diseases is typically achieved through comprehensive genetic testing. Given the clinical overlap with other forms of retinitis pigmentosa and early-onset retinal dystrophies, targeted next-generation sequencing (NGS) panels that include MERTK and other known IRD genes are the standard of care. Whole exome sequencing (WES) or whole genome sequencing (WGS) may be employed if panel testing is inconclusive, particularly to identify deep intronic variants or complex structural changes like large deletions that might be missed by standard panels. Genetic counseling is essential for patients and families affected by MERTK mutations. Since the condition follows an autosomal recessive inheritance pattern, both parents of an affected individual are obligate carriers of a pathogenic variant. Carriers are typically asymptomatic. Each sibling of an affected individual has a 25% chance of inheriting the disease, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Early genetic diagnosis is crucial not only for family planning but also for determining eligibility for emerging gene therapies and clinical trials.

Animal models: The Royal College of Surgeons (RCS) rat is the classic and most extensively studied animal model for MERTK-related retinal degeneration. This naturally occurring mutant has a deletion in the Mertk gene, leading to a complete failure of the retinal pigment epithelium (RPE) to phagocytose shed photoreceptor outer segments. This defect results in the accumulation of outer segment debris in the subretinal space, followed by rapid and severe photoreceptor cell death. The RCS rat has been instrumental in elucidating the function of MERTK in the retina and has served as the primary preclinical model for testing gene therapy approaches. In addition to the RCS rat, Mertk knockout mice have been generated and exhibit a similar phenotype of defective RPE phagocytosis and progressive retinal degeneration. These mouse models have allowed for more targeted genetic studies, including the investigation of tissue-specific modifier alleles that influence the severity of the retinal phenotype. Studies in these models have confirmed that MERTK is essential for the ingestion phase of RPE phagocytosis and that its loss leads to secondary photoreceptor degeneration due to the toxic accumulation of unphagocytosed outer segments.

Population genetics: MERTK mutations are a relatively rare cause of inherited retinal diseases globally, accounting for approximately 1-3% of all autosomal recessive retinitis pigmentosa cases. The carrier frequency in the general population is low. However, population-specific founder effects can significantly alter this prevalence. The most striking example is in the Faroe Islands, where a specific 91-kb deletion in the MERTK gene is a common founder mutation. In this isolated population, the carrier frequency for this specific deletion is estimated to be around 3%, and it is responsible for approximately 30% of all retinitis pigmentosa cases in the region. This highlights the importance of considering ethnic and geographic background when evaluating patients for IRDs.

Selected references: 1. Audo I, et al. MERTK mutation update in inherited retinal diseases. Hum Mutat. 2018;39(7):887-913. PMID: 29659094 2. D'Cruz PM, et al. Mutation of the receptor tyrosine kinase gene Mertk in the retinal dystrophic RCS rat. Hum Mol Genet. 2000;9(4):645-651. PMID: 10699188 3. Gal A, et al. Mutations in MERTK, the human orthologue of the RCS rat retinal dystrophy gene, cause retinitis pigmentosa. Nat Genet. 2000;26(3):270-271. PMID: 11062461 4. Feng W, et al. Mertk triggers uptake of photoreceptor outer segments during phagocytosis by cultured retinal pigment epithelial cells. J Biol Chem. 2002;277(19):17016-17022. PMID: 11861639 5. Ostergaard E, et al. A novel MERTK deletion is a common founder mutation in the Faroe Islands and is responsible for a high proportion of retinitis pigmentosa cases. Mol Vis. 2011;17:1485-1492. PMID: 21677792 6. Ghazi NG, et al. Treatment of retinitis pigmentosa due to MERTK mutations by ocular subretinal injection of adeno-associated virus gene vector: results of a phase I trial. Hum Genet. 2016;135(3):327-343. PMID: 26825853