Genetic and Environmental Effects on Myopia : The Healthy Twin Study

Abstract

Background: Despite of the recent increasing prevalence of myopia in Asia, the investigation of genetic and environmental factors for myopia in Asian populations has been limited. Aim: The aim of this study was to identify environmental and genetic influences on myopia in the Healthy Twin Study comprised of twins and their family members. Methods: Among the Healthy Twin Study, a total of 240 monozygotic twin pairs, and 45 dizygotic twin pairs, and 938 singleton adult family members who were first degree relatives of twins in 345 families were included in the study. Spherical equivalent, axial length, anterior chamber depth, and corneal astigmatism were measured by refraction, corneal topography, and A- scan ultrasonography. To estimate the effect of education and income on ocular traits, multiple linear regression analyses were performed including variables - such as age, sex, education groups, income groups and height. To assess the effect of education and income on the risk of myopia defined as ≤ -0.75 diopters, multiple logistic regression was used. Both linear and logistic regressions were done with the mixed model. To see the degree of resemblance among the different types of family relationships, intraclass correlation coefficients (ICC) were calculated with the mixed model. Variance-component methods were applied to estimate the genetic contributions to eye phenotypes. Results: After adjusting for covariates, education was associated with a lower spherical equivalent of 0.21 diopters (P=0.01) and a longer anterior chamber depth by 0.03 mm (P=0.05), and income was associated with a lower spherical equivalent of 0.18 diopters (P=0.04), a longer axial length by 0.19 mm (P=0.0007) and a longer anterior chamber depth by 0.04 mm (P=0.01). Education level was associated with the risk of myopia (OR=1.84, 95% CI: 1.12-3.01). The ICCs of spherical equivalent and ocular biometrics were significantly higher in monozygotic twins compared to other relative pairs, with greater consistency and conformity. The estimated narrow sense heritability (95% CI) was 0.67 (0.59-0.76) for spherical equivalent, 0.78 (0.69-0.87) for axial length, 0.77 (0.69-0.84) for anterior chamber depth, and 0.18 (0.02-0.34) for corneal astigmatism. Conclusion: Educational level was significantly associated with the risk of myopia. The compelling evidence for the estimated heritability of spherical equivalent and ocular biometrics in the Korean population suggests that all traits are highly heritable. Given the high heritability of myopia, the recent myopia epidemic in East Asia might be due to gene-by-environment interactions. Further studies are required to identify genetic factors and to examine gene-environment interactions, and then personalized prevention and treatment should be implemented in the future.