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Myocilin-Associated Glaucoma: a Historical Perspective and Recent Research Progress

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Myocilin-Associated Glaucoma: a Historical Perspective and Recent Research Progress Molecular Vision 2021; 27:480-493 <http://www.molvis.org/molvis/v27/480> © 2021 Molecular Vision Received 3 June 2020 | Accepted 18 August 2021 | Published 20 August 2021 Myocilin-associated Glaucoma: A Historical Perspective and Recent Research Progress Ritika Sharma, Abhinav Grover School of Biotechnology, Jawaharlal Nehru University (JNU), Delhi, India Glaucoma a debilitating disease, is globally the second most common kind of permanent blindness. Primary open-angle glaucoma (POAG) is its most prevalent form and is often linked with alterations in the myocilin gene (MYOC). MYOC encodes the myocilin protein, which is expressed throughout the body, but primarily in trabecu- lar meshwork (TM) tissue in the eyes. TM is principally involved in regulating intraocular pressure (IOP), and elevated IOP is the main risk factor associated with glaucoma. The myocilin protein’s function remains unknown; however, mutations compromise its folding and processing inside TM cells, contributing to the glaucoma phe- notype. While glaucoma is a complex disease with various molecules and factors as contributing causes, the role played by myocilin has been the most widely studied. The current review describes the present understanding of myocilin and its association with glaucoma and aims to shift the focus toward developing targeted therapies for treating glaucoma patients with variations in MYOC. Introduction of which are associated with the risk of developing secondary Glaucoma: Glaucomas are a group of visual impairment glaucoma [6,7]. disorders distinguished by the gradual atrophy of retinal Primary OAG (POAG) is the most prevalent form of ganglion cells (RGCs). The axons of RGCs form the optic glaucoma, constituting approximately 74% of glaucoma cases nerve, which transmits visual stimuli from the eye to the globally [4,5,8]. In the US, >80% of glaucoma patients are brain. RGC degradation results in the thinning and gradual classified as having OAG [9,10]. Several population-based cupping of the neuroretinal rim, eventually leading to an studies have recognized both physiologic and genetic predis- enlarged optic disc, a hallmark for the diagnosis of glau- posing factors behind POAG, such as elevated intra-ocular coma [1,2]. Most patients affected by glaucoma display no pressure (IOP), age, race, and family history [5,11-18]. The symptoms until advanced vision loss has occurred, making risk of developing POAG increases with advancing age, it the second most common cause of irreversible blindness but some subgroups of patients are diagnosed with a rather after cataracts worldwide [3]. Currently, approximately 80 exceptional form of the disorder known as juvenile-onset million people suffer from glaucoma [4,5]. The latest study OAG (JOAG). This is an early onset form of glaucoma that assessing the global incidence and future prognosis of glau- displays a Mendelian pattern of inheritance; this is in contrast coma estimates that by 2040 the number of people suffering to POAG, which is a complex genetic disease with multifac- with glaucoma will increase to almost 112 million [5]. torial risk factors [19]. Genetic alterations in myocilin are Glaucoma is generally classified into two major subtypes, responsible for nearly 4% of POAG cases and >10% of JOAG open-angle glaucoma (OAG) and closed-angle glaucoma cases [20,21]. (CAG), determined by the appearance of the iridocorneal While myocilin has been studied often due to its asso- angle formed between the iris and the cornea in the anterior ciation with inherited cases of glaucoma, its biologic func- compartment of the eye. Both types of glaucoma are further tion remains enigmatic. A comprehensive analysis of past categorized into primary and secondary forms based on the and recent research efforts focusing on the physiologic role underlying cause of the disease. The origin of primary forms myocilin plays in causing the disease phenotype is attempted of glaucoma is not discernible, while secondary forms of in this review. The pathophysiology of glaucoma is complex, glaucoma are attributable to an identifiable cause, such as eye and recent studies have revealed new insights that have injury, cataracts, diabetes, or the prolonged use of steroids, all helped in gaining a better understanding of this disease. A clear understanding of the structure of and physiologic role played by myocilin can help in developing targeted therapies Correspondence to: Abhinav Grover, School of Biotechnology, to treat glaucoma. Jawaharlal Nehru University, New Delhi, India; Phone: +91- 8130738032; email: [email protected] 480 Molecular Vision 2021; 27:480-493 <http://www.molvis.org/molvis/v27/480> © 2021 Molecular Vision Identification of myocilin as a gene associated with OAG: Myocilin: IOP is instrumental in preserving the shape of the eye and The MYOC gene—The human myocilin gene (MYOC) allowing the precise projection of images onto the retina. is approximately 17kb in size and is comprised of three IOP levels are maintained by an equilibrium between the exons and two introns. MYOC is positioned on the long arm formation and drainage of aqueous humor (AH) in the ante- of chromosome 1, specifically on segment 1q24.3–1q25.2 rior region of the eye. AH is formed in the ciliary body and [35,36]. Several mRNAs extending in size from 1.8–2.3 kb secreted into the anterior chamber of the eye from which it are transcribed from this gene. The variation in transcript is drained into the bloodstream via two distinct routes. In length arises due to the discrepancy in the use of three poly- humans, most of the drainage occurs through the sieve- adenylation sites present at the 3′ end of the gene [37-39]. like trabecular meshwork (TM) into the Schlemm’s canal, A typical TATA-box promoter and several transcription which is the conventional route. A relatively minor amount regulatory elements, such as NF-kB and E-box, are found in of AH exits via the unconventional uveal sclera route [22]. the region proximately upstream of the myocilin transcrip- The effective regulation of AH discharge across the TM is tion initiation site [35,37,40,41]. Myocilin is transcribed in necessary to maintain normal IOP. Elevated IOP levels are numerous tissues within and outside the eye, but the highest observed in most OAG patients, making it the key predis- mRNA level is observed in the TM tissue [31,37,38,41,42]. posing factor behind this disease. In OAG eyes, there is The cornea, iris, ciliary body, and retinal epithelium also increased resistance to AH outflow, which causes elevated exhibit significant expression of the MYOC gene [36-38]. In IOP, and IOP remains the only modifiable risk factor for addition to the eyes, skeletal muscle and heart tissues display slowing and treating glaucoma [23-25]. However, a third of major myocilin expression levels [35,38,40]. Myocilin expres- OAG patients are diagnosed without elevated IOP (<21 mm sion is influenced by several molecules, including steroids, of Hg) and are referred to as having normal tension glaucoma transforming growth factor-ꞵ1 (TGF-ꞵ1), and the protein (NTG) [26,27]. Interestingly, advancement of the disease has optineurin in cultured TM cells [41-43]. Stress, principally decreased via treatment focused on reducing IOP levels, even mechanical and oxidative stress, also induce myocilin expres- in NTG patients [25,28,29]. sion [25,33,42]. Efforts to ascertain the genes implicated in the initia- The myocilin protein: The MYOC gene yields a secreted tion of ocular hypertension were first undertaken in the early glycoprotein build of 504 amino acids [35,38,44]. The 1990s, and genetic linkage and cellular protein expression myocilin protein exhibits an isoelectric point (pI) of 5.2, has studies were conducted in parallel by different research a predicted molecular weight of 55.3 kDa, and is visible as groups. Linkage analysis of a family with a history of JOAG a doublet on denaturing PAGE flanked by 53 and 57 kDa led to the detection of the first locus associated with OAG. [35,36]. The 57 kDa fragment is an outcome of N-linked This locus was termed GLC1A and was mapped to the q glycosylation occurring at the amino acid positions 57–59 arm of chromosome 1 [30]. Cell culture studies conducted (Asn-Glu-Ser) in the polypeptide chain [41,44]. Structurally, by Fauss et al. in 1993 and by Polansky et al. in 1997 led to the protein is composed of three major homology regions: the identification of a protein progressively expressed upon the N-terminal coiled-coil (CC) domain, which includes the long-term treatment with dexamethasone, a drug associated leucine zipper (LZ) motifs (amino acids 33–201); the interme- with steroid-induced glaucoma [31-33]. This polypeptide was diate linker region (amino acids 202–243); and the C-terminal formerly called the trabecular meshwork-inducible gluco- olfactomedin (OLF) domain (amino acids 244–504). The corticoid response (TIGR), as its expression was induced by protein also includes an N-terminal signal sequence (amino treatment with the glucocorticoid dexamethasone. Subse- acids 1–32), which is partly responsible for the extracellular quently, the coding sequence for TIGR was mined from a secretion of myocilin [35,45]. While myocilin is known to be cDNA library created from the mRNA of human TM cells secreted in vitro in cultured cell lines expressing this protein treated with the same drug [31,34,35]. Simultaneously, in [44,46-49] and in vivo by its presence in AH [46,48,50- an independent study, an analogous gene was cloned from 52], it has also been reported to be present inside the cells a human retina cDNA library. The protein produced by this expressing it. Intracellularly, myocilin is localized within the gene displayed homology to the non-muscle protein myosin endoplasmic reticulum (ER) [47,49,53-55], the golgi apparatus and was therefore termed myocilin [36].
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