We performed this comprehensive meta-analysis to assess whether rs6885224 and rs634990 are associated with myopia. Our results showed that rs634990 was significantly associated with myopia but that rs6885224 was not. Nevertheless, we do not recommend a conclusion on this point. Indeed, the development of myopia is affected by both genetic and environmental factors. In general, different results will be obtained for people from different regions and ethnic groups due to genetic heterogeneity. This makes it necessary to conduct verification studies among different ethnicities. However, as there are, to our knowledge, no relevant reports on Tujia or Miao populations, we performed a replication study to assess the correlation of two potential SNPs associated with high myopia in the Tujia and Miao populations. These SNPs have been reported to be associated with high myopia in Han Chinese, Singaporean Chinese, Japanese and Dutch populations[49-53, 56, 57]. The Enshi Tujia and Miao Autonomous Prefecture is the only autonomous prefecture with ethnic minorities in Hubei Province. It is an ethnic minority area mainly characterized by Tujia, Miao, Dong, Bai, Mongolian, Hui and other ethnic minorities, with Tujia and Miao accounting for the majority. The Enshi minority region is located in southwest Hubei Province and is considered a low-income area. Although the quality of life in this area has been somewhat improved by the development of railways and highways in recent years, living habits are still relatively conservative, and migration into and out of the region is limited. No studies to date have been conducted in the populations in this area. According to our results, neither rs6885224 nor rs634990 is associated with high myopia in these population.
The SNP rs6885224 is located in region 5p15.2 within the Catenin Delta 2 gene (CTNND2), which belongs to the beta-catenin family, is 932 kb in length, and includes 26 exons (Figure 4-A). This gene encodes an adhesive junction-associated protein in the armadillo/beta-catenin subfamily that is involved in the development of the brain and eyes as well as the development of cancer[59-62]. Expression of the CTNND2-encoded protein is stimulated by hepatocyte growth factor, which then promotes the destruction of E-cadherin-based adherens junctions[63]. It was previously reported that this gene is located in a region on the short arm of chromosome 5, and hemizygosity of CTNND2 is associated with cri du chat syndrome[64]. Furthermore, Matter et al.[65] reported attenuated cortical responses to visual stimulation in 10-week-old mice with homozygous loss of CTNND2. Some have speculated that rs6885224 in CTNND2 might regulate mRNA transcription and affect expression of the gene, thereby affecting the occurrence of myopia[50]. Others have speculated that CTNND2 may regulate the structure and function of the sclera by breaking down E-cadherins in scleral fibroblasts, which may lead to myopia[56].
The SNP rs634990 in region 15q14 has no corresponding nearby gene. It is located in the intergenic region near the gap junction protein delta-2 gene (GJD2) and the golgin A8 family member B gene (GOLGA8B) (Figure 4-B). GJD2, also called connexin-36 (CX36) and formerly called GJA9, is located approximately 39 kb downstream of rs634990 and encodes a member of the connexin protein family. Studies have shown that CX36 is present in a number of retinal neurons, including rod photoreceptors, cone bipolar cells and all amacrine cells[66]. The gap junction containing CX36 plays an important role in normal synaptic transmission in the rod pathway[67]. In addition, expression of CX36 contributes to the survival of retinal cells and resistance to injury, and it has been found to play an important role in the electrophysiology of the chicken retina[68, 69]. GOLGA8B is a protein-coding gene that may be involved in maintaining Golgi structures. Regardless, the specific function of this gene in the eyes has not been reported, though a study by Solouki et al.[25] examined GOLGA8B gene expression in the retinas of postmortem humans and found that GJD2 was highly expressed nut that GOLGA8B was lowly expressed. rs634990 is located in the intergenic region of these two genes and might be related to the expression or function of either gene.
In our study, neither rs634990 nor rs6885224 showed an association with high myopia in the Tujia and Miao populations. Additionally, HWE p values >0.05 were observed for both the experimental and control groups, indicating that the alleles carried by the subjects were in HWE and that the subjects are from a population with random mating and little influx of new genetic material. Although it appears that our sample was reliable, our results differed from those of previous studies[25, 49-52]. The reasons for the conflicting results may be because the pathogenesis of myopia is complex and is influenced by both environmental and genetic factors. First of all, myopia is a multifactorial genetic disease, which is determined by a combination of environment and genetic factors, and neither can be ignored. The genetic backgrounds of the Tujia and Miao populations differ from other ethnic groups in China and the world, and our research is designed to explore these differences in relation to myopia. In fact, few genetic studies have been carried out on the Tujia and Miao populations, and we feel that those differences we have found are worth being further analyzed, even if we cannot completely account for environmental factors due to practical limitations of the study. Secondly, in terms of environmental factors, these environmental factors, such as educational level, near-work, outdoor activities, work in artificial light and the use of digital electronic products, are also important risk factors for myopia. Especially for education level and outdoor activity time, longer education means more near-work and less outdoor activities. Many scholars around the world are doing relevant studies. A large number of studies have shown that overweight learning burden, long hours of near-work and less outdoor activities can increase the risk of disease[70-73]. The Enshi ethnic minority area belongs to poor mountainous area with backward traffic and communication conditions, which prevents us from making return visits to the participants. Nevertheless, we are still trying to get in touch with them. In our efforts, only 52 cases and 26 controls were contacted. The education years of 52 cases are 8.63±3.29 and that of the controls are 6.65±3.71, which means that there is significant difference between the two groups (t=2.402, p=0.019). Unfortunately, this number of responses is not enough to serve as covariates in our study. Thirdly, there are also some studies that have reported the interaction of genetic and environmental factors on the risk of myopia[71, 72, 74-76]. For example, the study of Fan et al. found that three genome-wide associated loci, AREG, GABRR1 and PDE10A, showed strong interaction with education in Asian populations, but this interaction was not significant in European populations[77]. This study not only showed the interaction between genetics and environment, but also showed that the interaction is different in different ethnic groups. Although the SNPs in these studies are different from those we studied, they do suggest that interactions between genetics and environment have impact on the risk of myopia. The study by Pozarickij et al. does show an interaction for the 15q14 region, although not for the specific SNPs we used[74]. For now, no paper has reported on the interaction of genetic and environmental factors in Tujia and Miao populations, so it is worth collecting more data about Tujia and Miao populations for further analysis and research, but that is beyond the scope of this report.
Of course, we have to admit that there are some limitations in our study. First of all, this is a population-based study which requires a large enough sample size to provide more forceful evidence. However, our research involves a relatively small sample size, which leads to the fact that our study evidence seems unconvincing. Secondly, we chose the two SNPs that have been studied on the previous reports. More pathogenic SNPs should be found through GWAS study in the future. What's more, in future studies, we should include environmental factors such as education years, outdoor activity time and electronic product frequency etc. and conduct stratified analysis on them to give a further analysis on the association between SNP and myopia in Tujia and Miao populations.