The association of endothelial nitric oxide synthase (eNOS) gene polymorphisms and diabetic retinopathy among patients with type 2 diabetes: A case-control study.

Abstract

Purpose: Diabetic patients experience chronic hyperglycemia that increases oxidative stress by enhancing free radical formation and nitric oxide (NO) production. Genetic mutations in the endothelial nitric oxide synthase (eNOS) enzyme gene affect the levels of NO formation. These mutations, together with chronic hyperglycemia, may increase the risk of diabetic retinopathy (DR) development and/or DR progression as a complication of diabetes. This study aimed to determine whether the eNOS polymorphisms intron 4ab, exon 7 Glu298Asp variant (G894T), and T-786C are associated with DR severity.

Methods: This case-control study involved 250 subjects (172 with type 2 diabetes mellitus (DM) with or without DR and 78 healthy controls). DR was detected by slit lamp biomicroscopy and its severity was determined with the evidence-based International Clinical Diabetic Retinopathy Disease Severity Scale. The genotyping of eNOS polymorphisms was analyzed by polymerase chain reaction (PCR) only or with restriction fragment length polymorphism. The haplotype analysis was performed using the SNPStats tool.

Results: The genotype distribution for the three polymorphisms was significantly different in patients with diabetes compared to controls (intron 4 ab: a/a, 1.7%; a/b, 20.4%; b/b, 77.9%. G894T: GG, 56.4%; GT, 34.3%; TT, 9.3%. T-786C: TT, 58.2%; TC, 33.5%; CC, 8.3%). Differences in genotype or allele frequency was non-significant between subjects with diabetes and DR compared to those without DR, except the C allele of the T-786C polymorphism was significantly less common in DR patients. DR severity was not affected by any polymorphisms. Haplotypes bTC and aTT were significantly less common or more prevalent in DR than DM patients, respectively.

Conclusions: We demonstrated that type 2 DM patients exhibited a higher prevalence of the three polymorphisms when compared to the control group. The C allele of T-786C polymorphism may have a protective effect against DR. Also, none of the mutations was correlated with a higher DR risk nor with the severity of DR. Haplotype aTT increased the risk for DR, while the bTC haplotype reduced it.