Genetic heterogeneity of primary open-angle glaucoma in Pakistan

Abstract

Background: Glaucoma is a neurodegenerative ophthalmic disorder and is considered among the leading causes of irreversible blindness. Primary open-angle glaucoma (POAG) is the most common type of glaucoma that affects after 30 years of life, progressing slowly, and manifests as decreased visual acuity leading to blindness if not treated. POAG is genetically heterogeneous, inherited most commonly in autosomal dominant mode. Several genes have been reported for POAG with myocilin (Myoc) being most common. The present study has been conducted to screen 25 POAG families with 2 or more affected members for their association with Myoc and CYP1B1 (the most common gene in primary congenital glaucoma).

Methods: After approval from Institutional Ethical Review Committee (ERC), 25 POAG families were enrolled from the southern province (Sindh) of Pakistan. Written informed consent was obtained from all participating individuals and diagnosis was confirmed by consultant ophthalmologists using various instruments and means. Venous blood was obtained from affected individuals and their normal family members for DNA extraction and subsequent analysis.

Results: All samples were initially screened for the Myoc gene followed by CYP1B1. Screening for Myoc revealed one previously reported variant c.144G>T in POAG-06 whereas screening for CYP1B1 in all 25 families showed a novel variant c.649G>A in POAG-02. The pathogenicity of the novel variant was confirmed using various bioinformatics tools.

Conclusion: This is the first report of any POAG family found associated with a novel variant in CYP1B1 from the southern province of Pakistan whereas one family found associated with a reported variant in Myoc. The remaining 23 POAG families did not found to be associated with either Myoc or CYP1B1 indicating genetic heterogeneity of the population in this part of the world.

Keywords: CYP1B1; Glaucoma; Myocilin; POAG.

Fig. 1

A- Family pedigree of POAG06 showing and affected individuals (filled squares and circles). Females are represented by circles and males by squares. B- Shows Myoc protein topology with known variants (highlighted in blue). C- Clustal Omega multiple alignment sequence showing conservation of glutamine in various species. D- Molecular modeling prediction of glutamine replacement with histidine showing bigger size than wild-type residue.

Fig. 2

A- Family pedigree of POAG02 showing and affected individuals (filled squares and circles). Females are represented by circles and males by squares. B- Shows CYP1B1 protein topology with known variants (highlighted in blue). C- Clustal Omega multiple alignment sequence showing conservation of aspartate in various species. D- Molecular modeling prediction of aspartic acid replacement with asparagine resulting in loss of normal charge and interactions among neighboring amino acids.

Fig. 3

Hydrophobicity plot showed CYP1B1 protein to contain hydrophobic amino terminal with −2.122 score. Ramachandran plots calculated protein’s backbone bond angles, interestingly 93 and 98% of CYP1B1 mutant protein residues were found in favored and allowed regions respectively versus wild type protein (77 and 91%). MYOC protein’shydropathy plot analysis showed hydrophobicity score to be −0.744 for wild type versus mutant protein −0.711. Ramachandran plots analysis revealed 73 and 87% of wild type protein residues to lie in favored and allowed regions respectively with 64 outliers. However, MYOC mutant protein 72 and 85% residues lie in favored and allowed regions with 75 outliers.