The Association Between Genetic Polymorphisms in Estrogen Receptor Genes and the Risk of Ocular Disease: A Meta-Analysis

Objectives: To evaluate the association between estrogen receptor (ER) genes polymorphisms and the risk of ocular disease. Materials and Methods: A meta-analysis was performed of all available studies that investigated the association between ER gene polymorphisms and the risk of ocular disease. Results: Studies that were selected based on inclusion criteria reported 5 and 4 single-nucleotide polymorphisms (SNPs) identified in the ESR1 (ERα) (rs2234093, rs12154178, rs1884054, rs1801132, and rs9340799) and ESR2 (ERβ) (rs1268656, rs7159462, rs1256031, and rs4986938) genes, respectively. The pooled result showed a significant association between ESR2 rs1256031 gene polymorphism and ocular disease (odds ratio: 0.55, 95% confidence interval: 0.41-0.74, p<0.0001). Conclusion: The recessive genotype of ESR2 rs1256031 gene polymorphism had a protective effect against ocular disease, which supports the hypothesis that the estrogen-signaling pathway through ERβ plays a pivotal role in the pathogenesis of ophthalmic disorders.


Introduction
Estradiol (E 2 ) is a female sex steroid hormone and considered a major form of estrogen. 1 E 2 biosynthesis is regulated by ratelimiting enzyme aromatase, which is encoded by the cyp19a1 gene. 2,3,4,5 Although the ovary is known as the main site of E 2 production, local E 2 synthesis is also observed in several tissues including the brain, adipocytes, bone, liver, and retina. 6,7 Some evidence suggests an association between gonadal hormones and several diseases that are not related to the reproductive organs, such as diabetes, obesity, and myocardial infarction. 4,8,9 However, little is known about the role of E 2 in relation to the eye.
In mammals, expression of cyp19a1 is detected in the inner nuclear layer, outer plexiform layer, outer nuclear layer, and photoreceptors, while estrogen receptor (ER) is localized in almost all retina layers, cornea, lens, conjunctiva, lacrimal and meibomian glands 7,10 , implying that E 2 synthesis and its signaling are necessary for the eye. It has been reported that E 2 exerts a neuroprotective effect in the retina and optic nerve. 11 Moreover, E 2 seems to be involved in several eye pathologies such as glaucoma, myopia, age-related maculopathy (ARM), and cataract. 12,13,14,15 Furthermore, the prevalence of dry eye disease is predominantly found in females, particularly in the menopausal and postmenopausal age group. 16 Together, these support the notion that estrogen is a contributing factor to the development of ocular disease.
DNA polymorphisms are different DNA sequences that commonly occur among individuals and populations. The most common type of DNA polymorphism is the single nucleotide polymorphism (SNP), which is characterized by a single nucleotide change in the DNA sequence. Interestingly, however, alterations in DNA sequences may be directly or indirectly correlated with the development of disease through the modification of its expression and functional effect. 17 Estrogen-mediated effects are mainly modulated by ERα and ERβ, which are encoded by the ESR1 and ESR2 genes, respectively. 18 The association between genetic polymorphisms of ER genes and the risk of disease has been reported extensively, particularly in cancer. 19,20 Two common SNPs have been identified in ESR1 (rs223469, rs9340799) and ESR2 (rs1256031, rs4986938). 21,22 Thus, SNPs in ESR genes may directly or indirectly affect normal physiological functions of estrogen and may affect the risk of ocular disease. Therefore, in the present study, a meta-analysis of all eligible studies was performed to provide an accurate estimation of the association between ER polymorphisms and the risk of ocular disease.

Methods
A literature search was conducted from PubMed, Google Scholar, Scopus, and Web of Science. Keywords such as ER, polymorphisms, and ocular disease were used in combination.
The literature search was updated until September 2019. The selection criteria were as follows: (1) evaluating the associations between ER gene polymorphisms and the risk of ocular disease and (2) case-control design. The genotypic frequency for the ER polymorphisms was tested by Hardy-Weinberg equilibrium (HWE). Associations between the ER polymorphisms and risk of ocular disease were estimated by calculating pooled odds ratios (OR) and 95% confidence interval (CI). The randomeffect model was used to allow heterogeneity. Heterogeneity was evaluated with Q-test and I 2 . Egger's regression test was used to assess publication bias. P value of <0.05 was indicative of statistical significance.

Results
From the literature search, 5 studies were selected based on inclusion criteria. The characteristics of the selected studies are shown in Table 1 15 did not provide information regarding the gender and age of the participants. However, the study evaluated Five and 4 SNPs occurred in the ESR1 (rs2234093, rs12154178, rs1884054, rs1801132, and rs9340799) and ESR2 (rs1268656, rs7159462, rs1256031, and rs4986938) genes, respectively. All SNPs complied with the HWE (p>0.05). Finally, only 3 studies were included in our meta-analysis of the association between ESR1 (rs2234093 and rs9340799) and ESR2 (rs1256031 and rs4986938) polymorphisms with ocular disease. The pooled results on the association between ER polymorphisms and the risk of ocular disease are shown in Table 2. There was no significant association between ESR1 gene polymorphisms and risk of ocular disease. However, the recessive model of ESR2 rs1256031 gene polymorphism showed a 45% decrease in odds ratio (OR: 0.55, 95% CI: 0.41-0.74, p<0.0001, with low heterogeneity I 2 =15%), suggesting a protective effect of recessive genotype in ESR2 rs1256031 against ocular disease. No publication bias was observed for the association of ER polymorphisms and risk of ocular disease (P Egger test >0.05).

Discussion
In this study, 4 ocular diseases (primary open-angle glaucoma, dry eye, cataract, and ARM) were included in the analysis. Interestingly, such cases are mostly correlated with increased age. Indeed, low estrogen levels are observed in menopausal women and have been associated with increased cytokine production and ocular disease. 23,24 Moreover, menopausal women treated with hormone replacement therapy show a reduction of intraocular pressure 25,26 , which suggests a protective effect of estrogen. However, there is a conflicting result in regards to cyp19a1 polymorphism. A woman with cyp19a1 (rs10046) polymorphism has a higher susceptibility to myopia 24 , while there is no evidence of cyp19a1 polymorphism being associated with the risk of cataract. 15 Furthermore, Nishikawa et al. 27 provided evidence that there is no change in sex steroid hormone levels in patients with vitreoretinal disease. Nonetheless, the role of estrogen on ocular disease needs further investigation.
Because the action of estrogen depends on the interaction with its receptors, understanding ER genetic variations become important to evaluate the association of ER polymorphisms and the risk of ocular disease. This report indicated that the recessive model of ESR2 rs1256031 gene polymorphism was correlated with a reduction of ocular disease risk, which has been also reported as a protective factor in developing type 2 diabetes mellitus. 28 Mice lacking ERβ are more susceptible to in vivo injury to RPE cells 29 , which supports a protective effect of ERβ. It has been reported that the expression of ERβ is more abundant than ERα in the central nervous system 2,4,30 , including the retina. Moreover, the expression of ERβ is relatively more constant than ERα in the human eye 31 , suggesting a prominent role of ERβ in regulating normal physiological function in the eye.

Study Limitations
There are limitations to this study. First, the number of studies included in the meta-analysis was relatively small due to the limited availability of published papers. From 5 published papers, only 3 were suitable for further analysis, while the rest evaluated different SNPs. Second, major and minor alleles might have different roles in the risk of ocular disease, resulting in heterogeneity of the studies. Third, although this study was mainly generated from the pooled data of menopausal women, some studies enrolled both male and female participants, which may affect the pooled estimate. Thus, these findings should be interpreted with caution.

Conclusion
In summary, the current meta-analysis suggests that ESR2 rs1256031 gene polymorphism is significantly associated with the risk of ocular disease. The recessive genotype of ESR2 rs1256031 gene polymorphism was associated with a reduced risk of developing ocular disease. It is expected that more studies will become available, which may help the accurate estimation of the relationship of ER with ocular disease to verify this conclusion.

Ethics
Peer-review: Internally and externally peer reviewed. Financial Disclosure: The author declared that this study received no financial support.