In the current systematic review and meta-analysis that covered six case-control association studies, we found rs644242 in the PAX6 gene to have a suggestive association with both high and extreme myopia, whereas there was no association between SNPs rs667773, rs3026354, rs2071754, or rs3026393 and myopia. Interestingly, the ORs of rs644242 for extreme myopia (OR = 0.79 and 0.80 in the dominant and heterozygous models, respectively) were lower than for high myopia (OR = 0.87 and 0.85, respectively), indicating that the protective effect might be stronger for extreme myopia. Moreover, the allelic association was significant in extreme myopia (OR = 0.81; p = 0.01) but not in high myopia (OR = 0.90; p = 0.39), and the pooled OR for extreme myopia as compared with high myopia only was 0.93, adding evidence that the A allele of rs644242 is a stronger protective allele for extreme myopia. Because such a difference in ORs is small, a study on a much larger sample size, with more than 10,000 patients and 10,000 controls (assuming α = 0.05, power = 80%, and OR = 0.93 between extreme and high myopia), is needed to confirm the heterogeneity in the ORs. Moreover, to account for multiple testing, we adopted Bonferroni correction, which helps eliminate false-positive findings. After correction, the association of rs644242 with both high and extreme myopia could not withstand (p > 0.005). Therefore, further studies are warranted to consolidate the association between rs644242 and myopia.
Of note, family-based studies, which were not included in the meta-analysis, also showed an evident link between PAX6 and high myopia. In 2004, Hammond et al.35 first reported a significant linkage (with a maximum LOD of 6.1) of the PAX6 locus to refractive errors in a twins-based study, thus, PAX6 was suggested to be a susceptibility gene for myopia. Later, Hewitt et al.36 recruited four pedigrees known to have different mutations in the PAX6 gene and found them to be significantly associated with high myopia. Single-nucleotide polymorphisms rs3026390 and rs3026393 of PAX6 were also found to be associated with high myopia in the family-based study of Han et al.37 These familial studies, together with our meta-analysis of association studies, indicate that PAX6 is a susceptibility gene for high myopia and/or extreme myopia.
The PAX6 protein is a transcriptional factor that regulates the development of the eyeball. It is closely associated with developmental ocular disorders such as aniridia and foveal hyperplasia.32 In a case series of patients with PAX6 mutations, the patient carriers were not only associated with aniridia but also had other manifestations like high myopia.49PAX6 plays a key role in oculogenesis. The dosage of PAX6 could influence the oculogenesis in mice.50 Insufficiency and overexpression of PAX6 can both influence embryonic eyeball development. Therefore, in mice, the extremely abnormal dosage of PAX6 could induce multiple ocular defects, such as microphthalmia and anophthalmia.50 However, because of the variances among different species, a human in embryonic stage suffering from overexpression or insufficiency of PAX6 has a high rate of stillbirth or a low survival rate after birth, resulting from multiple-organ defects.51 In animal studies, myopic models were related to an abnormal expression of PAX6 in the postnatal stage. Researchers found that the expression of PAX6 in the retina was elevated in hyperopic defocused baby monkeys,52 and the expression of PAX6 was reduced in chicken with form-deprivation myopia.53 In addition, chronic hyperinsulinemia was one pathogenesis of juvenile-onset myopia, and insulin was known to act as a strong stimulus of axial length in myopia chicken,54 whereas PAX6 could transactivate the insulin promoters; therefore, elevated insulin levels by overexpressed PAX6 might stimulate elongation of the eyeball.55 Despite all these findings in animal models, it should be noted that myopia is a complex disease resulting from the interaction of multiple genetic and environmental risk factors and usually develops after birth. PAX6 is only one of the associated genes for myopia, and as indicated in our study, the effect size of the PAX6 SNP in myopia is small. Therefore, the PAX6 gene may, if any, contribute to a small proportion of myopia pathogenesis. However, how the SNP rs644242 works to alter the function of PAX6 still needs further investigations.
For the other three SNPs in our meta-analysis, rs2071754, rs3026354, and rs3026393, none was associated with high myopia in respective original studies.39–41,44 Our analysis also confirmed a lack of significant association. In addition, there are a group of SNPs that have been studied and some of them showed a significant association with high myopia. For example, rs662702 was found to be associated with extreme myopia in the study of Liang et al.39 Also, Han et al.37 had detected associations between SNPs rs3026390 and rs3026393 and high myopia in a family-based study, with the haplotypes with the T allele of rs3026393 demonstrating an increased transmission. However, no replication data have been reported for these associations. Therefore, whether they are genuine myopia-associated SNPs have yet to be further investigated.
In the current meta-analysis, we adopted a standard and stringent strategy for study inclusion. The test for heterogeneity and potential biases should render our interpretation more solid. Moreover, our study revealed several limitations in the understanding of PAX6 in myopia genetics. First, the results were generated from a limited number of studies, resulting in borderline pooled p values. Therefore, the association of PAX6 with high and extreme myopia should be validated in more study cohorts. Nevertheless, although the family-based studies could not be included in the meta-analysis, their findings did provide supportive evidence for a link between PAX6 and myopia.35,37,42 Second, a high heterogeneity in some models was detected, which was likely caused by various definitions of the control group and from the innate differences in minor allele frequencies across study populations. In this situation, our interpretation of the findings has been strengthened by the use of the random-effects model, yielding more conservative ORs. Third, the existing studies were mainly based on cohorts of Asians. It may restrict our conclusions within the Asian population and indicates the need for studies in other ethnic populations.
In conclusion, this systematic review and meta-analysis suggested an association of the PAX6 SNP rs644242 with extreme and high myopia. However, the association could not withstand Bonferroni correction and the effect size of PAX6 is small. Therefore, in view of the fact that myopia is a polygenic disease, PAX6 may, if any, confer a small effect on myopia development.
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