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Rishi Ramessur, Katie M Williams, Pirro G Hysi, Christopher J Hammond; Risk factors for refractive error: A cross sectional study in identical twins discordant for refractive error. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4505.
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Refractive error is a complex trait influenced by genetic and environmental factors. Monozygotic (MZ) twin pairs discordant for disease are a powerful tool to examine environmental effects on traits and may be used to search for epigenetic factors altering gene expression.
St Thomas’ Hospital’s TwinsUK cohort in London has refractive error, recorded in spherical equivalent (SphE) by non-cycloplegic autorefraction, on over 6,000 twins. Data from 1547 highly correlated (r=0.8) MZ twin pairs was used to select twin pairs discordant for refractive error, defined as ≥2D difference in SphE and discordant for class of refractive error (high/moderate/low myopia, emmetropia, hyperopia). In a 30-item telephone questionnaire twins were separately asked (and scored) about the risk factors urban/rural residence, occupational status and highest educational level. They responded with more(1), less(-1) or the same(0) as their twin on time spent outside, playing outdoor sport, and on close work aged <16 and 16-25. The lower SphE twin’s score was subtracted from the higher SphE twin’s score, and mean values of the difference calculated for each variable.
Of 133 eligible twin pairs (126F, 7M; mean age 57.1, range 30-89yrs), 68 were unavailable due to death(9), withdrawal(14), refusal(8) or subjects unattainable(37). 13 of the 65 pairs interviewed (mean age 59, range 30-80yrs) were not discordant (due to cataracts, laser, glaucoma or recording error). Qualitative questioning revealed no preexisting ideas of risk factors other than close work. Self-reported higher educational level, occupational status, urban residence and more close work across all ages were associated with more negative SphE- more close work between ages 16-25 was most strongly related to myopia (mean score -1.8; 95% CI -2.2 to -1.4). Spending time outside and doing outdoor sport were associated with more positive SphE- more time outside aged 16-25 had the strongest effect (mean score 0.86; CI 0.49-1.2).
Despite the retrospective recall nature of this study, the discordant monozygotic twin model has confirmed known environmental risk factors in participants lacking prior knowledge of potential modifiers of refractive error. These data will allow selection of discordant twins for epigenetic analysis to examine differentially methylated regions to further advance mechanisms of refractive error development.
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