In a pooled analysis of two nested case-control studies from large prospective cohorts, we observed that the association with hypertension and cigarette smoking in relation to POAG depended on NOS3 SNPs polymorphisms. We observed “cross-over” interactions where the relations between cigarette smoking and hypertension were in the opposite direction depending on the selected NOS3 SNP genotypes. Interactions were not observed with alcohol or red wine intake.
NOS3 codes for nitric oxide (NO) derived from vascular endothelium. In POAG, high NO levels can induce beneficial vasodilation, which leads to increased optic nerve blood flow33
, but NO could also induce hyperperfusion damage and reactions that form peroxynitrite, a free radical that induces retinal ganglion cell death34
. Thus, there may be biological plausibility to the observed significant “cross-over” interaction between hypertension and the promoter T-786C SNP, which influences nitric oxide levels13, 35
. Among those with the wild type TT genotype, hypertension was significantly adversely associated with POAG. Hypertension may exacerbate nerve fiber layer damage caused by compensatory hyperperfusion, which has been observed in ocular hypertension36
and early stage-POAG37
; moreover the presence of abundant NO could also trigger peroxynitrate toxicity with hyperperfusion that leads to retinal ganglion cell apoptosis.34
In contrast, among those with the variant allele (C), who have reduced NO production,13
hypertension could increase blood flow to the optic nerve without peroxynitrate toxicity. Also, in those with normal blood pressure, slight elevations in IOP results in lower ocular perfusion,38–42
which increases the risk of POAG, and this adverse effect may be worse in those with a NOS3 variant that might predispose to reduced retinal vessel diameters; thus hypertension may either have a net null effect or a mildly beneficial effect in this group. Accounting for NOS3 signaling may explain why both low blood pressure 41, 43–45
and high blood pressure has been associated with POAG.46, 47
More confirmatory and mechanistic studies are warranted.
The interaction between the rs7830 tagging SNP and cigarette smoking was similar. The functional significance of the rs7830 NOS3 SNP is unknown, and the reason why other NOS3 SNPs did not interact with cigarette smoking in POAG overall is unclear. Cigarette smoking also may have both adverse and protective effects in POAG, due to effects of nicotine on IOP, circulating endothelin-1 concentrations, vascular tone and alterations in blood pressure. 48–53
Cigarette smoking also contributes to endothelial dysfunction through the uncoupling of the NOS3-mediated synthesis of NO, increasing oxidative stress and reducing plasma antioxidant levels 54, 55
. In a previous study, we observed an overall weak non-significant inverse association between cigarette smoking on POAG risk.56
Given these findings, the relation between cigarette smoking and POAG may be complex.
Previously, we observed weak trends of protective associations with high consumption of alcohol.57
Even though alcohol and resveratrol in red wine is known to upregulate NOS3 expression,58, 59
we did not observe interactions between NOS3 SNP polymorphisms and alcohol or red wine. The levels of alcohol and red wine intake may have been too low to detect potential interactions in this study.
Limitations should be considered. First, our glaucoma definition was based on self-report with confirmation with medical records and visual fields. This definition has very high specificity, as we required documentation of reproducible defect on at least 2 reliable visual field tests. Given the insidiousness of glaucoma, some controls might have had undiagnosed glaucoma. However, it is unlikely to have had a major influence on our results, as the prevalence of glaucoma in adults over age 40 is 1.3% in Caucasians.60
Furthermore, our controls were required to have had an eye exam as of the matched cases’ diagnosis dates, and the average number of eye exams reported as of their selection as controls were three exams, implying that advanced glaucoma, if present, would likely have been detected. Any misclassifications of the disease would have biased the results towards the null. Third, our participants were generally healthy Caucasians and thus we are unable to make inferences to less healthy populations or minorities. Fourth, we lacked IOP data on the controls, and thus we were not able to explore the interactions with perfusion pressure, which might be more etiologically relevant. Finally, it is possible that our results may be due to chance, given the multiple comparisons. We did not explicitly correct for multiple comparisons, and therefore, these findings should be interpreted with caution and confirmed in future studies, particularly with different racial/ethnic groups.
The genetic determinants of the endothelial NO signaling system may affect how other factors may contribute to POAG. Understanding the complex gene-environment interactions in POAG may serve to shed light on the etiology of this disease.