In this large, population-based, case-control study, we found no relation between NOS3 gene variants and POAG overall. However, several SNPs, including the functional T –786C SNP in the promoter region, were associated with high-tension glaucoma, particularly in the women. The interactions between NOS3 and their sex and NOS3 and current PMH use among the postmenopausal women for high-tension POAG suggests that the biology of the sexes and gene–environment interactions play a role in POAG pathogenesis. As this is the first study to evaluate these gene–environment interactions, these findings should be interpreted with caution and confirmed in future studies in different racial/ethnic groups.
The functional SNP Glu298Asp, which has been linked to ischemic heart disease33
and ischemic stroke,34
was not associated with POAG overall or with the POAG subtypes. Furthermore, polymorphisms in this SNP were not associated with POAG of either sex, and this SNP did not interact with attributes of female reproductive aging. A relation between the promoter region functional SNP (T −786C) polymorphism has been reported with coronary vasospasm in a Japanese study35
; however, this finding has not been replicated elsewhere.
In our study, we observed associations between NOS3
gene variants and high-tension POAG, particularly among the women. Similar associations have been found in other studies. In a study of 56 cases of familial high-tension POAG and 100 controls, Tunny et al.17
found that a NOS3
variant close to the functional T −786C variant was positively associated with familial high-tension POAG; the associations by sex were not provided. In another study that included 58 patients with high-tension POAG, 76 with normal-tension glaucoma, and 38 control subjects, Logan et al.18
failed to find an association between NOS3
allelic variants and POAG overall, but did find an association between NOS3
allelic haplotypes, including the T −786C and POAG with migraine.18
The associations by sex were also not presented, but it is known that migraine, which is characterized by a dysfunction in the vasodilatory response, is a female-predominant condition.36
Our results suggest that reproductive hormones play a role in modulating IOP and the risk of POAG in women. An Australian study demonstrated that the incidence of POAG in women was lower than that in men up to the sixth decade of life, implying that before menopause, women are less likely to develop POAG.37
PMH use among postmenopausal women produces modest reductions of IOP and may enhance optic nerve blood flow.38–44
In the present study, the percentage of POAG cases with elevated IOP at diagnosis was lower in current PMH users than in non-PMH users (56% among the users versus 71% among the nonusers), supporting the role of PMH in IOP modulation. The Rotterdam Eye Study found that early menopause was associated with an increased risk of POAG,20
which was confirmed in the NHS.21
Furthermore, in the NHS, current use of estrogen with progestin was associated with a reduced risk of high-tension POAG.21
Circulating estrogen may influence NOS3
, and current PMH use in postmenopausal women may systemically upregulate NOS3
. Yang et al.45
found that estrogen enhances the release of NO from cultured human coronary artery endothelial cells. Furthermore, brachial artery vasoconstriction was documented within 1 week of surgical menopause, supporting the role of estrogen in modulating endothelial NOS activity.46
In postmenopausal women not treated with PMHs, the endothelial NOS immunoreactivity in the uterine wall was significantly lower when compared to tissue from PMH users.47
Perara et al.48
found that small arteries from patients with type 2 diabetes exhibited enhanced vascular relaxation ex vivo after 6 months of PMH exposure. Presumably, these changes are mimicked in other tissue beds, including the eye, and these data support the interaction between NOS3
variants and PMH in relation to high-tension POAG.
Limitations of our study should be considered. First, our definition of glaucoma was based on self-report and confirmation with medical records and visual fields. This definition has very high specificity, as we required documentation of reproducible defect on at least two reliable visual field tests. It is possible that given the insidiousness of glaucoma, some control subjects may 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.49
Furthermore, our control subjects had to have undergone an eye examination as of the matched cases' diagnosis dates, and the average number of eye examinations reported as of their selection as controls was three, implying that advanced glaucoma, if present, would most likely have been detected. Any misclassifica-tions of the disease in the control group or in the POAG group would have biased the results toward the null. Third, our participants were generally healthy Caucasians, and thus we are unable to make inferences to less healthy or minority populations. Finally, it is possible that our results may be due to chance, as several NOS3
polymorphisms as well as several interactions were examined in relation to three outcomes. Therefore, these findings should be confirmed in future studies, particularly with different racial/ethnic groups, and the results should be interpreted with caution.
To summarize the major findings of this study, we observed the following:
- None of the NOS3 SNPs investigated was significantly associated with overall POAG
- For high-tension POAG, the association with one tagging SNP (rs3918188) was significantly inversely associated among the women but not among the men (P-heterogeneity by sex = 0.02).
- In the women, four of the five NOS3 SNPs showed significant interactions with current PMH use in relation to high-tension POAG. For example, PMH use was inversely associated (RR = 0.41; 95% CI, 0.22–0.76) only among the women with the TT genotype in T −786C.
In conclusion, NOS3 gene polymorphisms may interact with sex and PMH use in women in relation to high-tension POAG, suggesting that sex-based biology and reproductive hormones play a role in POAG pathogenesis.