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Xiaohu Ding, Ian George Morgan, Mingguang He; Possible cause of monozygotic twins discordant in refraction: nearwork, outdoor activity and stochastic variation. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3642.
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To evaluate the role of nearwork and outdoor activity variance in the monozygotic(MZ) twins with discordance on refraction.
A longitudinal twin study was launched in 2009, Guangzhou City, China. Standard questionnaire was administered by in-person interview to obtain daily activities, including time spent on near work and outdoor activities. Refraction was measured by auto-refraction under cycloplegia. Cross-sectional analyses on associations of refractive discordance with environmental factors were conducted among monozygotic (MZ) twins, who share identical genetic background.
A total of 490 MZ twin pairs were eligible, the refraction was -1.50±2.14 Diopter (Mean±SD), nearwork time was 4.24±1.45 hours, and outdoor activity time was 1.36±0.86 hours. In the mixed model, we found that nearwork activities conferred increased risk to myopic spherical equivalent (SE) whereas the outdoor activity has a marginal protective effect to myopic SE. The variance on nearwork activities within MZ explained about 2.0% of total phenotypic discordance and outdoor activity explained about 0.5%.
Given the very marked genetic similarity of MZ twins, and the small effects of known risk factors on discordance, we suggest that the discordance between MZ twins largely results from uncollected environmental factors or stochastic variations at the genomic or epigenetic levels.
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