Evaluation and understanding of myocilin mutations in Indian primary open angle glaucoma patients.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Kanagavalli
Janakaraj
J
Department of Genetics, Aravind Medical Research Foundation, Aravind Eye Hospital, Madurai, Tamilnadu, India.
Krishnadas
Subbaiah Ramasamy
SR
Pandaranayaka
Eswari
E
Krishnaswamy
Sankaran
S
Sundaresan
Periasamy
P
D000293:Adolescent; D000328:Adult; D000368:Aged; D000959:Antihypertensive Agents / Q000627:therapeutic use; D002648:Child; D003598:Cytoskeletal Proteins; D004252:DNA Mutational Analysis; D005136:Eye Proteins / Q000235:genetics*; D005260:Female; D005787:Gene Frequency; D005902:Glaucoma, Open-Angle / Q000235:genetics* / Q000473:pathology / Q000628:therapy; D006023:Glycoproteins / Q000235:genetics*; D006801:Humans; D007194:India; D007429:Intraocular Pressure; D008297:Male; D008875:Middle Aged; D009154:Mutation; D016133:Polymerase Chain Reaction; D020641:Polymorphism, Single Nucleotide; D018807:Polymorphism, Single-Stranded Conformational; D014130:Trabeculectomy; D000099179:Myocilin
D000959:Antihypertensive Agents; D003598:Cytoskeletal Proteins; D005136:Eye Proteins; D006023:Glycoproteins; D000099179:Myocilin