Dominant genetic effects on corneal astigmatism: The genes in myopia (GEM) twin study

PURPOSE. This study was conducted to assess the relative influence of genetics and environment on corneal astigmatism and corneal curvature in a large sample of twins. METHODS. A total of 612 twin pairs (345 monozygotic [MZ] and 267 dizygotic [DZ]) aged between 18 and 86 years (mean age, 52.11 +/- 15.85 years) were recruited from the Australian Twin Registry (ATR). Each subject completed a general questionnaire, undertook a dilated eye examination, including ocular biometric measurements, and contributed a blood sample. Corneal astigmatism was defined as the absolute difference between the K1 and K2 meridians and corneal curvature as the average of K1 and K2. RESULTS. Intrapair correlations were significantly higher (P < 0.001) in MZ twin pairs compared with those in DZ twin pairs for both corneal astigmatism (CA; r(mz) = 0.48 vs. r(dz) = 0.13) and corneal curvature (CC; r(mz) = 0.84 vs. r(dz) = 0.41). A sex-limited model with parameters estimating additive genetic, nonadditive genetic, and unique environmental influences (denoted ADE) was the most parsimonious model explaining both measures. Heritability estimates were as high as 60% and 71% for CA and CC, respectively. CONCLUSIONS. This study provides evidence that genetic factors explain interindividual variation in CA and CC, with nonadditive genetic factors explaining most of the variance due to those genetic factors. Heritability estimates were sex specific and indicate the need for future linkage studies for the identification of genes involved in the etiology of CA and CC.