MAK — Male Germ Cell Associated Kinase

The MAK gene provides instructions for making a protein called male germ cell-associated kinase. In the eye, this protein is essential for the health and maintenance of photoreceptors, the specialized light-sensing cells in the retina. Photoreceptors have a hair-like structure called a cilium, which acts as a bridge for transporting vital materials within the cell. The MAK protein helps regulate the length and proper function of this cilium, ensuring that the photoreceptor cells can survive and work correctly over a person's lifetime. When the MAK gene is mutated, the protein it produces is either missing or doesn't work properly. Without functional MAK protein, the cilia on the photoreceptor cells become abnormally long, and the transport of materials within the cell is disrupted. Over time, this causes the photoreceptor cells to slowly deteriorate and die. This gradual loss of photoreceptors leads to a vision disorder called retinitis pigmentosa (RP). Patients with MAK-related RP typically experience night blindness and a slow, progressive loss of their peripheral (side) vision, though their central vision often remains good until later in life. MAK-related retinitis pigmentosa is inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the MAK gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, are called carriers; they typically do not have any vision problems themselves. If both parents are carriers, there is a 25% chance with each pregnancy of having a child with the condition. A specific mutation in the MAK gene is particularly common in individuals of Ashkenazi Jewish descent, making carrier testing an important consideration for family planning in this population.

Gene description: Encodes a serine/threonine kinase primarily expressed in photoreceptors, playing a role in cilia and outer segment development.

Patient and family guide: The MAK gene provides instructions for making a protein called male germ cell-associated kinase. In the eye, this protein is essential for the health and maintenance of photoreceptors, the specialized light-sensing cells in the retina. Photoreceptors have a hair-like structure called a cilium, which acts as a bridge for transporting vital materials within the cell. The MAK protein helps regulate the length and proper function of this cilium, ensuring that the photoreceptor cells can survive and work correctly over a person's lifetime. When the MAK gene is mutated, the protein it produces is either missing or doesn't work properly. Without functional MAK protein, the cilia on the photoreceptor cells become abnormally long, and the transport of materials within the cell is disrupted. Over time, this causes the photoreceptor cells to slowly deteriorate and die. This gradual loss of photoreceptors leads to a vision disorder called retinitis pigmentosa (RP). Patients with MAK-related RP typically experience night blindness and a slow, progressive loss of their peripheral (side) vision, though their central vision often remains good until later in life. MAK-related retinitis pigmentosa is inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the MAK gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, are called carriers; they typically do not have any vision problems themselves. If both parents are carriers, there is a 25% chance with each pregnancy of having a child with the condition. A specific mutation in the MAK gene is particularly common in individuals of Ashkenazi Jewish descent, making carrier testing an important consideration for family planning in this population.

Gene function: MAK is a serine/threonine kinase predominantly found in photoreceptor cells, particularly in the connecting cilium and outer segment. It is involved in the development and maintenance of these structures, which are vital for phototransduction. Mutations disrupt outer segment integrity and lead to progressive photoreceptor degeneration, affecting vision.

Protein structure: The MAK gene encodes a serine/threonine protein kinase. The canonical human MAK protein consists of 283 to 549 amino acids, depending on the specific isoform. The protein's structure is characterized by a highly conserved N-terminal protein kinase domain (approximately amino acids 4-284), which is responsible for its catalytic activity and ATP binding. This kinase domain is structurally related to kinases involved in cell cycle regulation. Following the kinase domain, MAK possesses a variable-length C-terminal tail that lacks recognized functional domains but is likely important for regulatory interactions and subcellular localization. The retina-specific isoform of MAK is unique because it includes an additional sequence encoded by exon 12, which is crucial for its pronounced expression and proper localization throughout the photoreceptor cell body and cilia. The protein functions as part of a complex, potentially interacting with other ciliary proteins like RP1 to exert its regulatory effects on microtubule dynamics.

Molecular function: The MAK (male germ cell-associated kinase) gene encodes a serine/threonine protein kinase that plays a crucial role in the regulation of ciliary length and the long-term survival of photoreceptors. In the retina, MAK is essential for maintaining the proper structural organization of the photoreceptor connecting cilium, which acts as the vital intracellular link between the inner and outer segments of the photoreceptor cell. At the molecular level, MAK regulates the turnaround step of intraflagellar transport (IFT) at the ciliary tip. It is involved in the proper localization and transport of IFT proteins (such as IFT88 and IFT57) and kinesin motors along the ciliary axoneme. By phosphorylating specific targets, potentially including RP1 (retinitis pigmentosa 1 protein), MAK helps control microtubule stability and ciliary subcompartmentalization. Loss of MAK function leads to the abnormal elongation of both the connecting cilia and the outer-segment axonemes, disruption of IFT protein localization, and subsequent progressive apoptotic death of photoreceptor cells.

Expression pattern: The MAK gene is expressed in several tissues, with notable expression in the retina, testis, and respiratory epithelium. In the human retina, MAK is specifically expressed in the inner segments, cell bodies, and axons of both rod and cone photoreceptors. The gene produces multiple transcript variants through alternative splicing. The most abundant transcript in the retina is a retina-specific isoform that includes a 75-bp exon 12, which is absent from the canonical transcript expressed in other tissues. Both the canonical and retina-specific isoforms contain exon 9. During retinal development, there is a developmental switch where the transcript bearing exon 9 and exon 12 becomes predominant as cells differentiate into retinal precursors and mature photoreceptors.

Mutation spectrum: The mutation spectrum of the MAK gene is heavily dominated by a specific structural variant: a 353-bp Alu insertion in exon 9. This insertion causes a translational frameshift and premature termination of the protein, leading to a complete loss of functional MAK protein. This single founder mutation accounts for the vast majority of MAK-associated retinitis pigmentosa cases. In addition to the common Alu insertion, a small number of other pathogenic variants have been identified, including missense and nonsense mutations (e.g., p.Gly16Arg). However, these are exceedingly rare compared to the Alu insertion. The total number of known pathogenic variants in MAK is relatively small, reflecting the strong founder effect of the exon 9 insertion in specific populations.

Pathogenic variants: 1. c.899_900insAlu (Alu insertion in exon 9) - The most common pathogenic variant, a founder mutation in the Ashkenazi Jewish population, causing a frameshift and premature truncation. 2. p.Gly16Arg (c.46G>A) - A missense mutation identified in compound heterozygosity with the Alu insertion in a patient with retinitis pigmentosa. 3. p.Arg265* (c.793C>T) - A nonsense mutation reported to cause autosomal recessive retinitis pigmentosa.

Clinical significance: Mutations in the MAK gene cause an autosomal recessive, non-syndromic form of retinitis pigmentosa (RP), sometimes designated as RP62. Clinically, MAK-associated RP is characterized by adult-onset, slowly progressive loss of photoreceptor function. The disease often begins with visual field loss in the superior-temporal or inferonasal aspects of the retina, eventually progressing to more widespread peripheral vision loss. However, many patients retain good central vision and normal visual acuity well into their seventh or eighth decade of life. The retinal phenotype is generally considered relatively mild compared to other forms of photoreceptor ciliopathies. Patients typically exhibit rod-cone dystrophy, with rod function being more severely affected than cone function. Structural imaging, such as optical coherence tomography (OCT), shows a gradual decrease in the horizontal width of the ellipsoid zone (EZ) and thinning of the outer nuclear layer (ONL). Some patients may also develop cystoid macular edema. Unlike some other ciliopathies, MAK-associated RP does not present with apparent extra-ocular or systemic features, despite the gene's expression in other ciliated tissues like the respiratory epithelium and testis.

Inheritance: Autosomal Recessive

Chromosomal location: 6p21.31

Genotype-phenotype correlations: The most prominent genotype-phenotype correlation for the MAK gene involves the homozygous Alu insertion in exon 9. Patients homozygous for this mutation typically present with a relatively mild, adult-onset, slowly progressive form of non-syndromic retinitis pigmentosa. They often maintain excellent central visual acuity into late adulthood, with visual field loss progressing slowly over decades. Other missense and nonsense mutations in MAK have also been reported to cause a similar non-syndromic recessive RP phenotype. In general, patients with biallelic MAK mutations tend to have a later age of onset and a milder retinal phenotype compared to patients with mutations in other RP-associated genes, such as DHDDS. The lack of extra-ocular symptoms in patients with the exon 9 Alu insertion, despite the gene's expression in other ciliated tissues, is thought to be due to the specific disruption of the retina-specific isoform or potential compensatory mechanisms in other tissues.

Research and therapeutic approaches: Currently, there are no FDA-approved therapies specifically for MAK-associated retinitis pigmentosa. Management primarily focuses on supportive care, including low-vision aids, regular ophthalmologic monitoring, and genetic counseling. However, because MAK-associated RP is a recessive condition caused by a loss of gene function, it is considered an excellent candidate for viral-mediated gene augmentation therapy. The relatively mild and slowly progressive nature of the disease provides a wide therapeutic window for intervention before irreversible photoreceptor loss occurs. Significant progress is being made in the preclinical development of gene therapies for MAK. Researchers have successfully developed adeno-associated virus (AAV) vectors carrying the human MAK transcript. In preclinical studies using patient-derived induced pluripotent stem cells (iPSCs) and Mak-knockdown zebrafish models, these viral vectors have demonstrated the ability to safely restore functional MAK protein, correct primary cilia length defects, and mitigate visual dysfunction. Clinical grade vectors have been manufactured and tested for safety in wild-type animal models, showing no evidence of toxicity. These promising preclinical results are paving the way for the initiation of Phase 1/2 human clinical trials to evaluate the safety and efficacy of subretinal AAV-MAK gene therapy in patients with this specific form of RP.

Diagnostic testing: Mutations in the MAK gene are typically detected through comprehensive inherited retinal disease (IRD) gene panel testing or whole exome sequencing (WES). Because the most common pathogenic variant is a specific 353-bp Alu insertion in exon 9, which can sometimes be missed by standard next-generation sequencing pipelines due to mapping challenges with repetitive elements, specialized bioinformatics algorithms or targeted screening assays are often required to accurately identify this insertion. Genetic counseling is highly recommended for individuals diagnosed with MAK-associated RP and their families. Since the disease follows an autosomal recessive inheritance pattern, both parents of an affected individual are obligate carriers, and siblings have a 25% chance of being affected. Carrier screening is particularly relevant for individuals of Ashkenazi Jewish descent, given the high carrier frequency of the founder Alu insertion in this population. Counseling should emphasize the relatively mild and slowly progressive nature of the visual loss associated with MAK mutations, which can provide some reassurance regarding long-term visual prognosis.

Animal models: The primary animal model used to study MAK function is the Mak-knockout (Mak-KO) mouse. These mice develop a slowly progressive form of retinal degeneration that closely mimics the human disease. Studies in Mak-KO mice have revealed that the absence of Mak leads to marked elongation of photoreceptor connecting cilia and outer-segment axonemes in both rods and cones. Additionally, Mak-KO mice show abnormal accumulation of intraflagellar transport (IFT) proteins (such as IFT88 and IFT57) and kinesin motors in the outer-segment axonemes, as well as rhodopsin accumulation in photoreceptor cell bodies. Zebrafish models with mak knockdown have also been utilized, particularly to demonstrate that overexpression of the retinal MAK transgene can restore the ability to regulate primary cilia length and mitigate visual defects, providing a valuable system for testing gene therapy constructs.

Population genetics: The population genetics of the MAK gene are strongly defined by a significant founder effect in the Ashkenazi Jewish population. The 353-bp Alu insertion in exon 9 is highly prevalent in this group, with a carrier frequency estimated at approximately 1 in 55 (about 1.8% to 2.4%). Due to this high carrier rate, mutations in the MAK gene are responsible for approximately one-third of all cases of autosomal recessive retinitis pigmentosa in individuals of Ashkenazi Jewish ancestry. In contrast, this mutation is extremely rare in the general population and in individuals of mixed or non-Jewish ethnicity. This pronounced population-specific prevalence makes targeted screening for the MAK Alu insertion a highly effective diagnostic approach for RP patients of Ashkenazi Jewish descent.

Selected references: 1. Tucker BA, et al. Exome sequencing and analysis of induced pluripotent stem cells identify the cilia-related gene male germ cell-associated kinase (MAK) as a cause of retinitis pigmentosa. Proc Natl Acad Sci U S A, 2011. PMID: 21825139 2. Stone EM, et al. Autosomal recessive retinitis pigmentosa caused by mutations in the MAK gene. Invest Ophthalmol Vis Sci, 2011. PMID: 22110072 3. Omori Y, et al. Negative regulation of ciliary length by ciliary male germ cell-associated kinase (Mak) is required for retinal photoreceptor survival. Proc Natl Acad Sci U S A, 2010. PMID: 21139045 4. Tucker BA, et al. Development and biological characterization of a clinical gene transfer vector for the treatment of MAK-associated retinitis pigmentosa. Gene Ther, 2022. PMID: 34518651 5. Venturini G, et al. Two specific mutations are prevalent causes of recessive retinitis pigmentosa in North American patients of Jewish ancestry. Genet Med, 2015. PMID: 25255364 6. van Huet RAC, et al. Retinitis pigmentosa caused by mutations in the ciliary MAK gene is relatively mild and is not associated with apparent extra-ocular features. Acta Ophthalmol, 2015. PMID: 25385675