Myocilin

Myocilin, trabecular meshwork inducible glucocorticoid response (TIGR), also known as MYOC, is a protein which in humans is encoded by the MYOC gene.[5][6] Mutations in MYOC are a major cause of glaucoma.

MYOC
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesMYOC, GLC1A, GPOA, JOAG, JOAG1, TIGR, myocilin
External IDsOMIM: 601652 MGI: 1202864 HomoloGene: 220 GeneCards: MYOC
Orthologs
SpeciesHumanMouse
Entrez

4653

17926

Ensembl

ENSG00000034971

ENSMUSG00000026697

UniProt

Q99972

O70624

RefSeq (mRNA)

NM_000261

NM_010865

RefSeq (protein)

NP_000252

NP_034995

Location (UCSC)Chr 1: 171.64 – 171.65 MbChr 1: 162.47 – 162.48 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Gene location

The cytogenetic location of human MYOC gene is on the long (q) arm of chromosome 1, specifically at position 24.3 (1q24.3).[7] The gene's molecular location starts at 171,635,417 bp and ends at 171,652,63 bp on chromosome 1 (Annotation: GRCh38.p12) (assembly).

Protein characteristics

Myocilin is a protein with a weight of 55 kDa (504 amino acid) and an overall acidic property, the product of the first gene that has been linked to Primary Open Angle Glaucoma (POAG).[5]

Protein structure

The protein is made up of the two folding domains, the leucine zipper-like domain at the N-terminal and an olfactomedin-like domain at the C-terminal. The domain at the N-terminal is known to have 77.6% homology to the myosin heavy chain of Dictyostelium discoideum and 25% homology with the cardiac β-myosin heavy chain.[6][8] The gene encodes three different exons, each consisting of different structural and functional domains.[9]

The N-terminal is encoded by exon 1 and contains the leucine zipper structural motif, which consists of 50 amino acid residues (117-169 amino acids).[8] The motif is found on an α-helix, which enhances the binding of the protein. The name of the domain arises due to the occurrence of leucine as well as, arginine repeats periodically on the α-helix.[9][8] The leucine zipper domain also contains the myocilin-myocilin interactions between amino acid residues 117-166.[10] Exon 2 encodes the central region of the protein at amino acid residues 203-245 however, no structural or functional domains are found in this region. Exon 3 encodes the C-terminal of myocilin and has been found to contain the olfactomedin-like domain.[11] The olfactomedin is an extracellular matrix protein with no defined role but is abundantly found in the olfactory neuroepithelium.[11] In the myocilin protein, the domain consists of a single disulfide bond which connects two cysteine residues (245 and 433 amino acids).[12]

Protein localisation

Myocilin is specifically located in the ciliary rootlet and basal body which connects to the cilium of photoreceptor cells in the rough endoplasmic reticulum. The intracellularly distributed protein is processed in the endoplasmic reticulum (ER) and in secreted into the aqueous humour.[13] It is only imported into the trabecular meshwork of the mitochondria. In the extracellular space, it appears in the trabecular meshwork cells through an unconventional mechanism which is associated with exosome-like vesicles. Myocilin localises in the Golgi apparatus of corneal fibroblasts and Schlemm's canal endothelial cells.[14][15]

Protein processing

Several isoforms are produced due to post-translational modifications processes, including glycosylation and palmitoylation.[16] The gene undergoes N-glycosylation at the Asn-Glu-Ser site (57–59 amino acids) and O-glycosylation throughout the protein at the Ser-Pro, Pro-Ser, Thr-Xaa-Xaa-Pro, Ser-Xaa-Xaa-Xaa-Pro sites.[16][17]

Myocilin also undergoes a proteolytic cleavage in the endoplasmic reticulum at residue Arg-226. The cleavage process is calcium dependant and results in two fragments.[18] One fragment contains the C-terminal olfactomedin-like domain (35 kDa), and the other contains the N-terminal leucine zipper-like domain (20 kDa).[9][18]

Function

MYOC encodes the protein myocilin. The precise function of myocilin is unknown, but it is normally secreted into the aqueous humor of the eye. MYOC mutations, which cause myocilin to accumulate in the cells of the trabecular meshwork are a common cause of glaucoma. Most MYOC mutations identified in glaucoma patients are heterozygous and are confined to the olfactomedin domain, which is encoded by exon 3.[8]

Myocilin is believed to have a role in cytoskeletal function. MYOC is expressed in many ocular tissues, including the trabecular meshwork, and was revealed to be the trabecular meshwork glucocorticoid-inducible response protein (TIGR). The trabecular meshwork is a specialized eye tissue essential in regulating intraocular pressure, and mutations in MYOC have been identified as the cause of hereditary juvenile-onset open-angle glaucoma.[19]

Scientific research has found the function of myocilin to be linked with other proteins, making it part of a protein complex. The isoform of the cytochrome P450 protein, 1B1 (CYP1B1) has shown interaction with myocilin. CYP1B1 is also found in several structures so the eye including, trabecular meshwork and the ciliary body.[20]

Mutations and associated diseases

Differing mutations in the MYOC gene have been reported to associate with glaucoma 1, open angle (GLC1A) and glaucoma 3, primary congenital (GLC3A).

Glaucoma 1, open angle (GLC1A)

Glaucoma 1 is a form of primary open-angle glaucoma (POAG), which is characterized based on a specific pattern of defects in the optic nerve, thus causing visual defects.[5][21] The disease causes an angle in the anterior chamber of the eye to be left open, which in turn causes the intraocular pressure to be increased. Although an increase in the intraocular pressure is a major factor for glaucoma, the disease can occur independently of the intraocular pressure.[21] Furthermore, the damage done to the optical nerve has been classified as irreversible because no symptoms of the disease are apparent (asymptomatic) until its last stages.[21]

Glaucoma 3, primary congenital (GLC3A)

Glaucoma 3 arises due to mutations in the distinct genetic loci of MYOC. This mutation contributes to GLC3A through digenic inheritance with the CYP1B1 protein.[20] The mutation gives rise to an autosomal recessive form of primary congenital glaucoma (PCG). The disease initiates at birth or in early childhood due to the increase in intraocular pressure, large ocular globes (buphthalmos) and corneal edema. The progression of the disease causes defects in the trabecular meshwork and anterior chamber angle of the eye preventing the drainage from the aqueous humor.[20]

Overall frequency of disease-causing mutations at MYOC in different races[22]
RaceOccurrence frequency (%)
African4.44
Asian3.30
Caucasian3.86

Clinical significance

MYOC contains a signal sequence for secretion and is secreted into the aqueous humor of the eye by the trabecular meshwork. Mutations in MYOC are found in 4% of adult-onset primary open-angle glaucoma and >10% of juvenile-onset primary open-angle glaucoma. Overexpression or underexpression of MYOC does not cause glaucoma. However, the MYOC gene also contains a signal sequence, which is normally not functional, that directs intracellular proteins to peroxisomes. Glaucoma-associated mutations activate that signal sequence and direct myocilin to peroxisomes, where they accumulate in the cell, instead of being secreted. Decreased secretion and increased accumulation appear to be the initial steps in myocilin-associated glaucoma.[23]

A study employing an iterative pocket and ligand-similarity based approach to virtual ligand screening predicted small molecule binders for the olfactomedin domain of human myocilin. The predictions were subsequently assessed by differential scanning fluorimetry.[24]

Interactions

MYOC has been shown to interact with the following proteins:[16][25][26]

References
  1. GRCh38: Ensembl release 89: ENSG00000034971 - Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000026697 - Ensembl, May 2017
  3. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. Stone EM, Fingert JH, Alward WL, Nguyen TD, Polansky JR, Sunden SL, et al. (January 1997). "Identification of a gene that causes primary open angle glaucoma". Science. 275 (5300): 668–70. doi:10.1126/science.275.5300.668. PMID 9005853. S2CID 46810363.
  6. Kubota R, Noda S, Wang Y, Minoshima S, Asakawa S, Kudoh J, et al. (May 1997). "A novel myosin-like protein (myocilin) expressed in the connecting cilium of the photoreceptor: molecular cloning, tissue expression, and chromosomal mapping". Genomics. 41 (3): 360–9. doi:10.1006/geno.1997.4682. PMID 9169133.
  7. "MYOC myocilin [Homo sapiens (human)] - Gene - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2018-11-08.
  8. Ortego J, Escribano J, Coca-Prados M (August 1997). "Cloning and characterization of subtracted cDNAs from a human ciliary body library encoding TIGR, a protein involved in juvenile open angle glaucoma with homology to myosin and olfactomedin". FEBS Letters. 413 (2): 349–53. doi:10.1016/S0014-5793(97)00934-4. PMID 9280311. S2CID 45445865.
  9. Aroca-Aguilar JD, Sánchez-Sánchez F, Ghosh S, Coca-Prados M, Escribano J (June 2005). "Myocilin mutations causing glaucoma inhibit the intracellular endoproteolytic cleavage of myocilin between amino acids Arg226 and Ile227". The Journal of Biological Chemistry. 280 (22): 21043–51. doi:10.1074/jbc.m501340200. PMID 15795224.
  10. Fautsch MP, Vrabel AM, Johnson DH (June 2006). "Characterization of the Felix domesticus (cat) glaucoma-associated protein myocilin". Experimental Eye Research. 82 (6): 1037–45. doi:10.1016/j.exer.2005.08.023. PMID 16289048.
  11. Bal RS, Anholt RR (February 1993). "Formation of the extracellular mucous matrix of olfactory neuroepithelium: identification of partially glycosylated and nonglycosylated precursors of olfactomedin". Biochemistry. 32 (4): 1047–53. doi:10.1021/bi00055a008. PMID 8424933.
  12. Nagy I, Trexler M, Patthy L (March 2003). "Expression and characterization of the olfactomedin domain of human myocilin". Biochemical and Biophysical Research Communications. 302 (3): 554–61. doi:10.1016/s0006-291x(03)00198-0. PMID 12615070.
  13. Caballero M, Rowlette LL, Borrás T (November 2000). "Altered secretion of a TIGR/MYOC mutant lacking the olfactomedin domain". Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease. 1502 (3): 447–60. doi:10.1016/s0925-4439(00)00068-5. PMID 11068187.
  14. Gong G, Kosoko-Lasaki O, Haynatzki GR, Wilson MR (April 2004). "Genetic dissection of myocilin glaucoma". Human Molecular Genetics. 13 Spec No 1 (9): R91-102. doi:10.1093/hmg/ddh120. PMID 14764620.
  15. Severs NJ (November 1981). "Localization of cholesterol in the Golgi apparatus of cardiac muscle cells". Experientia. 37 (11): 1195–8. doi:10.1007/bf01989915. PMID 7319008. S2CID 19829695.
  16. Nguyen TD, Chen P, Huang WD, Chen H, Johnson D, Polansky JR (March 1998). "Gene structure and properties of TIGR, an olfactomedin-related glycoprotein cloned from glucocorticoid-induced trabecular meshwork cells". The Journal of Biological Chemistry. 273 (11): 6341–50. doi:10.1074/jbc.273.11.6341. PMID 9497363.
  17. Joe MK, Sohn S, Kim TE, Im JE, Choi YR, Kee C (May 2011). "Analysis of glucocorticoid-induced MYOC expression in human trabecular meshwork cells". Vision Research. 51 (9): 1033–8. doi:10.1016/j.visres.2011.02.014. PMID 21334360. S2CID 15094219.
  18. Sánchez-Sánchez F, Martínez-Redondo F, Aroca-Aguilar JD, Coca-Prados M, Escribano J (September 2007). "Characterization of the intracellular proteolytic cleavage of myocilin and identification of calpain II as a myocilin-processing protease". The Journal of Biological Chemistry. 282 (38): 27810–24. doi:10.1074/jbc.m609608200. PMID 17650508.
  19. "Entrez Gene: MYOC myocilin, trabecular meshwork inducible glucocorticoid response".
  20. Kaur K, Reddy AB, Mukhopadhyay A, Mandal AK, Hasnain SE, Ray K, Thomas R, Balasubramanian D, Chakrabarti S (April 2005). "Myocilin gene implicated in primary congenital glaucoma". Clinical Genetics. 67 (4): 335–40. doi:10.1111/j.1399-0004.2005.00411.x. PMID 15733270. S2CID 19577329.
  21. Adam MF, Belmouden A, Binisti P, Brézin AP, Valtot F, Béchetoille A, Dascotte JC, Copin B, Gomez L, Chaventré A, Bach JF, Garchon HJ (November 1997). "Recurrent mutations in a single exon encoding the evolutionarily conserved olfactomedin-homology domain of TIGR in familial open-angle glaucoma". Human Molecular Genetics. 6 (12): 2091–7. doi:10.1093/hmg/6.12.2091. PMID 9328473.
  22. Hodapp, Elizabeth (2012-12-05). "Faculty of 1000 evaluation for Myocilin polymorphisms and primary open-angle glaucoma: a systematic review and meta-analysis". doi:10.3410/f.717961970.793466645. {{cite journal}}: Cite journal requires |journal= (help)
  23. Kwon YH, Fingert JH, Kuehn MH, Alward WL (March 2009). "Primary open-angle glaucoma". The New England Journal of Medicine. 360 (11): 1113–24. doi:10.1056/NEJMra0804630. PMC 3700399. PMID 19279343.
  24. Srinivasan B, Tonddast-Navaei S, Skolnick J (September 2017). "Pocket detection and interaction-weighted ligand-similarity search yields novel high-affinity binders for Myocilin-OLF, a protein implicated in glaucoma". Bioorganic & Medicinal Chemistry Letters. 27 (17): 4133–4139. doi:10.1016/j.bmcl.2017.07.035. PMC 5568477. PMID 28739043.
  25. Torrado M, Trivedi R, Zinovieva R, Karavanova I, Tomarev SI (May 2002). "Optimedin: a novel olfactomedin-related protein that interacts with myocilin". Human Molecular Genetics. 11 (11): 1291–301. doi:10.1093/hmg/11.11.1291. PMID 12019210.
  26. Polansky JR, Fauss DJ, Zimmerman CC (June 2000). "Regulation of TIGR/MYOC gene expression in human trabecular meshwork cells". Eye. 14 ( Pt 3B) (3b): 503–14. doi:10.1038/eye.2000.137. PMID 11026980.

Further reading
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