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Mamata Manne, Padma Gunda, Ravi K Kondreddy, Nagaraju Thurlapati, Padma Tirunilai; Novel Indoleamine 2, 3-Dioxygenase; (IDO1) gene mutation in the Pathogenesis of age-related cataract.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6418.
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Exposure to UV light is considered as the major risk factor for the development of age-related cataract. UV filters that are produced during tryptophan catabolism maintain the transparency of the lens and also protect retina from photodamage. Indoleamine 2, 3 dioxygenase (IDO) is the first rate limiting enzyme in the tryptophan catabolism which is encoded by IDO gene localized on chromosome 8p12-p11 region. Mutations in IDO gene can affect synthesis of UV filters qualitatively or quantitatively. Hence the present study was planned to screen for mutations in IDO gene and to evaluate their role in the causation of age-related cataract.
Genomic DNA from 331 age-related cataract cases [110-Nuclear cataract, 110-Cortical cataract, 111-Posterior Subscapular types and 210 normal controls were PCR amplified for all regions covering the entire coding sequence and splice junctions of IDO gene. Amplified PCR products were purified and sequenced directly in both directions with an automated genetic analysis system (3100; ABI) and were analyzed with a commercial software program package. Structure prediction and energetic analysis of wild-type IDO compared with mutants were predicted using various Bioinformatic tools.
Mutational screening in exon-7 of IDO gene showed the presence of a novel insertion deletion variation (c.596_597 delins TT; rs267606590) in heterozygous pattern in 2 (1 NC & 1 CC) of the cases studied and none among controls, leading to substitution of alanine (A) at position 199 by valine (V) in mutants. SIFT and polyphen tools predicted possible damaging effect of mutation on protein. Protein modeling by Triton package indicated wide variation between the wild type and mutant protein structure. Km values of wild type (68.66±0.26 μM) and mutant IDO proteins (74.92±0.40 μM) showed a significant difference indicating reduced affinity between the enzyme and substrate for mutant protein as compared to wild type.
The novel variation p.A199V in exon 7 of IDO gene among patients with NC and CC cataracts resulting in the low affinity between mutant IDO enzyme and substrate tryptophan is likely to reduce the rate of UV filter synthesis in lens. The reduced levels of UV filters may cause exposure of lens to UV light resulting in aggregation of lens proteins and development of cataract.
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