Estrogen receptor 1 (ESR1) XbaI polymorphisms may affect breast cancer susceptibility; however, the results of previously published studies are inconsistent. This meta-analysis aimed to investigate the relationship between ESR1 XbaI polymorphism and breast cancer risk.  Methods: Articles from the PubMed, Embase, Cochrane Library, WoS, Scopus, Wanfang Data, CNKI, CBM and CQVIP databases were systematically searched to determine the association between ESR1 XbaI polymorphism and breast cancer risk. The pooled results were assessed using odds ratios (ORs) and 95% confidence intervals (CIs), followed by subgroup analysis.  Results: Twenty-two studies involving 12,821 cases and 14,739 control subjects were analyzed. The pooled results indicated that ESR1 XbaI polymorphism may decrease risk of breast cancer in AG vs. AA (co-dominant model: OR = 0.88, 95% CI = 0.79-0.97, P = 0.015) and AG + GG vs. AA models (dominant model: OR = 0.89, 95% CI = 0.80-0.98, P = 0.022). Subgroup analysis indicated significant associations between the ESR1 XbaI polymorphism and breast cancer risk were observed in Asian subjects, non-Hardy-Weinberg equilibrium study, post-menopausal status and hospital-based subgroups under the AG vs. AA and AG + GG vs. AA models (all P < 0.05).  Conclusions: Our analysis of pooled data indicated that AG genotype in ESR1 XbaI may be a protective factor for breast cancer patients in some subgroups.

(1) Background: The prevalence of knee osteoarthritis (OA) in women is significantly higher than in men. The estrogen receptor α (ERα) has been considered to play a key role due to a large gender difference in its expression. ERα is encoded by the gene estrogen receptor 1 (ESR1), which is widely studied to explore the gender difference in knee OA. Several polymorphisms in ESR1 [PvuII (rs2234693) and BtgI (rs2228480)] were confirmed as the risk factors of OA. However, the evidence of the last widely investigated polymorphism, ESR1 Xbal (rs9340799), is still insufficient for concluding its effect on knee OA. (2) Objective: This study proposed a case-control study to investigate the association between ESR1 Xbal and knee OA. Moreover, a meta-analysis and trial sequential analysis (TSA) were conducted to enlarge the sample size for obtaining a conclusive evidence. (3) Methods: In total, 497 knee OA cases and 473 healthy controls were recruited between March 2015 and July 2018. The Kellgren-Lawrence grading system was used to identify the knee OA cases. To improve the evidence level of our study, we conducted a meta-analysis including the related studies published up until December 2018 from PubMed, Embase, and previous meta-analysis. The results are expressed as odds ratios (ORs) with corresponding 95% confidence intervals (CI) for evaluating the effect of this polymorphism on knee OA risk. TSA was used to estimate the sample sizes required in this issue. (4) Results: We found non-significant association between the G allele and knee OA [Crude-OR: 0.97 (95% CI: 0.78-1.20) and adjusted-OR: 0.90 (95% CI: 0.71-1.15) in allele model] in the present case-control study, and the analysis of other genetic models showed a similar trend. After including six published studies and our case-control studies, the current evidence with 3174 Asians showed the conclusively null association between ESR1 XbaI and knee OA [OR: 0.78 (95% CI: 0.59-1.04)] with a high heterogeneity (I2: 78%). The result of Caucasians also concluded the null association [OR: 1.05 (95% CI: 0.56-1.95), I2: 87%]. (5) Conclusions: The association between ESR1 XbaI and knee OA was not similar with other polymorphisms in ESR1, which is not a causal relationship. This study integrated all current evidence to elaborate this conclusion for suggesting no necessity of future studies.