Heritability of Myopia as a Function of Age, Near Work, and Sports Activity

Abstract

Purpose:: We recently reported that sports activity in children has a protective effect on the risk of myopia onset (Jones et al., ARVO 2006). We wanted to evaluate whether the number of hours spent in sports activity, in addition to age and near work, affects the estimate of heritability obtained from sibling data from the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study.

Methods:: Subjects were 796 sibling pairs 6 to 14 years of age participating in the CLEERE study. Longitudinal data were used to compare proband data with a sibling at an equivalent age. Heritability was computed by doubling the slope of the regression line for refractive error (cycloplegic autorefraction) between siblings at each age. Estimates of time spent in near work and sports activity were obtained by averaging survey data completed each year by a parent or guardian.

Results:: Heritabilities (± SE) estimated from regression slopes between siblings increased with age from a low of 0.25 ± 0.10 at age 6 years to 0.65 ± 0.09 at age 13 years. There was evidence of a shared environment between siblings with correlations of 0.59 (p<0.0001) for diopter-hours of near work and 0.61 (p<0.0001) for hours of sports activities. Each factor was related to proband refractive error with correlations of -0.12 (p = 0.001) for diopter-hours of near work and 0.12 (p = 0.0006) for hours of sports activity. Across all subjects, the unadjusted heritability was 0.77 ± 0.06. The estimate of heritability adjusted for near work and sports activity was similar at 0.73 ± 0.06. The increase in model R² with the addition of both environmental variables was 0.02.

Conclusions:: The heritability of myopia is related to age, increasing to higher values at older ages when myopia is more prevalent. Environmental exposures to sports activity and near work were shared between siblings and were related to refractive error, but had very little effect on heritability. The results suggest a substantial heritable component to childhood myopia with a relatively small contribution from environmental exposure to sports activity and near work.

Keywords: myopia • clinical (human) or epidemiologic studies: risk factor assessment • genetics