Evidence of shared genes in refraction and axial length: the Genes in Myopia (GEM) twin study.

PURPOSE: Axial length has been shown to explain up to 50% of the total variance in refraction, with axial length and refraction having a major genetic component. However, no study has attempted to determine whether the correlation between axial length and refraction is explained by shared genetic or environmental factors.

METHODS: All twins from Victoria aged 18 years or older were invited to participate in the Genes in Myopia (GEM) twin study through the Australian Twin Registry (ATR). Each twin completed a general questionnaire and underwent dilated objective refraction assessment and measurement of axial length.

RESULTS: A total of 612 twin pairs (1224 twins) aged from 18 to 86 years were examined in the GEM twin study. Axial length correlated negatively with refraction (r = -0.64 in the men, r = -0.68 in the women; P < 0.01). The sex limitation ADE (A, additive genetic; D, dominant genetic; E, unique environmental factors) model provided the best-fit genetic model for both measures. Of the variation in spherical equivalence in both the men and the women, approximately 50% were due to genetic factors influencing axial length.

CONCLUSIONS: From these findings, it is likely that axial length and refraction share common genes in their etiology. The GEM twin study has provided a basis and direction for future research into identifying the gene(s) in axial length that will ultimately improve our understanding of the etiology of refractive error, particularly myopia.