[Show abstract][Hide abstract] ABSTRACT: Mutations in the myocilin gene (MYOC) leading to a perturbed outflow of aqueous in the trabecular meshwork (TM) has been associated with the pathophysiology of glaucoma. This study examines the expression of normal and mutant myocilin (Gly367Arg) in cultured TM cells.
Normal and mutant MYOC cDNA constructs were used to transfect the TM cells. In order to confirm the method of transfection, reverse transcriptase polymerase chain reaction (RT-PCR) was carried out. Further, confocal microscopic analysis was used to determine the cellular localization of myocilin protein. The extracellular nature of myocilin in the culture supernatant and cell lysates of the transfected cells was analyzed by western blot. Molecular modeling was done earlier using a knowledge based consensus method which involved threading the protein into the identified pentein fold for the COOH-terminal part. Molecular dynamics was carried out using GROMACS for the mutant model which was built using the native myocilin model.
The Gly367Arg mutation causes accumulation of myocilin protein within TM cells with extensively reduced secretion contrary to wild type myocilin being characterized by intracellular localization and extracellular secretion. Further, Gly367Arg mutation occurs in a hydrophobic region which leads to burial of a charged residue. The dynamics suggests large conformational change is required to accommodate the mutation favoring aggregation of the protein.
Our results suggest that Gly367Arg is a potential mutation that causes malfunction of TM cells either by dominant negative effect or gain of function of mutant myocilin. The structural model suggests that the mutated myocilin could aggregate, implying the possible role of Gly367Arg in causing Primary open angle glaucoma (POAG).
[Show abstract][Hide abstract] ABSTRACT: Primary open angle glaucoma (POAG) is the most common form of glaucoma and the second leading cause of blindness in the world. Discovery of the candidate gene MYOC (TIGR/MYOC) encoding the protein myocilin, believed to have a role in cytoskeletal function, might play a key role in understanding the pathogenesis of POAG. MYOC is expressed in many ocular tissues, including trabecular meshwork (TM), a specialised eye tissue essential in regulating intraocular pressure (IOP). Later it was shown to be the trabecular meshwork inducible-glucocorticoid response protein (TIGR). Mutations in MYOC have been identified as the cause of hereditary juvenile-onset open-angle glaucoma (JOAG). The unprocessed myocilin with signal peptide is a 55-kDa protein with 504 amino acids. Mature myocilin is known to form multimers. Wild type myocilin protein is normally secreted into the trabecular extracellular matrix (ECM) and there appears to interact with various ECM materials. It is believed that the deposition of high amounts of myocilin in trabecular ECM could affect aqueous outflow either by physical barrier and/or through cell-mediated process leading to elevation of IOP. The N-terminal region of the myocilin has sequence similarity to myosin (muscle protein) and the C-terminal of the protein has an olfactomedin-like domain. Structural and genetic studies of the MYOC gene and its protein product along with molecular modeling could lead to better understanding of the pathogenesis of POAG. This review highlights the current understanding of myocilin and the relevance of genetic and structural work.
Indian Journal of Ophthalmology 01/2005; 52(4):271-80. · 1.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To screen for mutations in the MYOC gene of patients with Primary Open Angle Glaucoma (POAG) in India and to better understand the mutations using a possible model of myocilin.
We analyzed DNA for mutations in 107 subjects with POAG and 90 normal control subjects. The exonic sequences of the MYOC gene from all subjects were amplified by Polymerase Chain Reaction (PCR). We carried out Single Strand Conformation Polymorphism (SSCP) for all the PCR products. The DNA samples which showed mobility shift in the banding pattern in SSCP gel were sequenced. We also analyzed the presence of the common mutation Gln368Stop using a specific restriction enzyme Taa 1. The mutations observed here and elsewhere have been mapped onto a possible model built for myocilin using a knowledge-based consensus modeling approach.
Two heterozygous mutations Gly367Arg (1099G>A) and Thr377Met (1130C>T) were identified in exon3 of the MYOC gene of probands 40-1 and 51-1 respectively, from material obtained from the 107 unrelated subjects with POAG. These two mutations were not present in the normal controls studied. We identified a Single Nucleotide Polymorphism (SNP) Gly122Gly (366C>T) in exon1 of proband 57-1 as a non-disease causing variation. The common mutation Gln368Stop found in the Western population was not observed in the POAG cases screened in Indian population. The possible structural model for myocilin suggests a predominantly [beta]-strand rich C-terminal region (181-504) which is connected by the [alpha]-helical mid-region (111-180) to the N-terminal region (34-110) which has low secondary structure content. Both the mutations, Gly367Arg and Thr377Met identified in our study, map on to the C-terminal region. These mutations disfavor burial of this region during oligomer formation due to the charged or bulky nature of the mutants. Most of the other mutations known for myocilin also are surface exposed on the C-terminal region.
Our findings indicate that the mutation frequency of the MYOC gene is 2% in the Indian population affected with POAG, which is not a well-studied ethnic group of the Asian continent. The variations identified in our study have been previously reported in the Western population. The nonsense mutation Gln368Stop was not observed in the present study and thereby suggests that it may not be a common disease-causing mutation in the Indian population. Amongst other Asian populations, studies in Japan also didn't report this nonsense mutation. The location of these mutations suggest that a plausible mode of action could be by disruption of dimer or oligomer formation by the C-terminal region allowing greater chances of nucleation of aggregation by the N-terminal region.
[Show abstract][Hide abstract] ABSTRACT: Human myocilin is a 55kDa protein that is implicated in primary open angle glaucoma (POAG). Understanding the structure and
folding of the native protein and the mutants that increase aggregation could lead to possible prevention of the condition.
We report here the over expression and purification of the human myocilin in E. coli. The initial expression of recombinant myocilin in E. coli was found to be low. The problem of low yield was found to be due to multiple causes and was overcome using a suitable combination
of vectors, tags, host background and expression protocols. The overexpressed human myocilin was purified by affinity column
chromatography to yield about 8mg of protein from 1l of culture. The protein purity and folding were confirmed using electrophoresis,
immunoblotting and fluorescence spectroscopy. Further biophysical characterization and crystallization trials using the recombinant
human myocilin will pave the way for better understanding of the structure–aggregation relationship that is involved in causing
World Journal of Microbiology and Biotechnology 24(6):903-907. · 1.35 Impact Factor