Our study showed that OPRK

Our study showed that OPRK1 and OPRM1 were more methylated in Xinjiang (Northwest China) than Zhejiang (Southeast China) Han Chinese healthy controls. The above region-related methylation changes of OPRK1 and OPRM1 suggested that their DNA methylation could be altered by environmental factors [31]. Xinjiang province is located in the Northwestern China, and it TIC10 receptor is a multi-ethnic-populated region where Han and Uygur Chinese consist of two largest Xinjiang ethnicities [32]. The genetic backgrounds, lifestyles and diets are distinct between the two populations [33]. Epidemiological study revealed that the prevalence of AD in Uygur and Han populations was 3.24% and 4.19%, respectively [33]. Zhejiang province with simple ethnic composition was in the Southeastern China [34]. As shown in Fig. 3, Xinjiang is located in inland region and Zhejiang was in the east coast of China. Hence, distinct geographic locations rendered various external factors including climates, diet habits and lifestyles which may contribute to different DNA methylation outcomes of the two genes. Our results also showed that Xinjiang Han male controls tended to have higher OPRK1 methylation levels than female controls. The prevalence of MCI in males was higher than females [35]. Male MCI had a higher rate of progression to AD [36], and a higher mortality risk [37]. Female MCI benefited more through cognition training than male MCI [38]. The gender-related difference in OPRK1 methylation may help elaborate the gender difference in the susceptibility and prognosis of MCI. Interestingly, we found OPRK1 methylation was significantly correlated with aging in Xinjiang Uygur male controls. Given its higher methylation in MCI, Uygur males might face a higher risk of MCI along with aging. We also found a significantly inverse correlation of OPRM1 CpG2-4 methylation level with HDL-C in Han male controls. Lipids are known to involve the inflammation pathway, Aβ formation and stability of neuron, whereby their abnormalities have been shown to increase the risk of AD [39], [40], [41]. HDL-C, as the TC transporters, was decreased in MCI [42]. The current findings revealed their negative associations of HDL-C with OPRM1 methylation in controls, mostly in males, this result might help explain the relationship of blood TC-related indexes with DNA methylation. Our study has several limitations. Firstly, our study only involved a moderate number of MCI patients, and future studies with larger sample size may help confirm our findings. Secondly, the present case-control study only covers two populations from Xinjiang region. The current findings needed further validation in other ethnic populations. Thirdly, although promoter fragments of OPRK1 and OPRM1 were able to upregulate gene expression, the exact epigenetic mechanism of gene expression regulation required to be explored. In conclusion, our findings suggested that OPRK1 and OPRM1 methylation might serve as blood biomarkers of MCI in Xinjiang Chinese. Specifically, OPRK1 hypermethylation was a MCI risk factor in Xinjiang female Han. OPRM1 CpG1 hypermethylation and CpG2-4 hypomethylation were associated with MCI risk in Xinjiang Uygur and Han, respectively. In addition, our results showed that OPRK1 promoter methylation was related to gender, ethnicity, aging and environmental changes, while OPRM1 promoter methylation was related to blood lipids and living regions.