Form-Deprivation Myopia in Chick Induces Limited Changes in Retinal Gene Expression

Despite its major public health impact, the etiology of myopia is poorly understood. Persuasive evidence in animals and humans has localized the controlling mechanism(s) for refractive development in large part to the retina.1-4 Although evidence implicates various specific neurotransmitters or other intercellular signaling molecules, a cohesive understanding of the retinal cells and signaling pathways that account either for normal refractive development or for refractive errors has remained elusive.4 Form deprivation has been a useful biological model for assessing mechanisms potentially pertinent to clinical myopia.1,2 Disturbances of visual input cause myopia in chicks, as well as in tree shrews, primates, and other mammals. Disturbances of visual input also cause myopia in children.2,4 Although form deprivation induces a robust myopic response in newly hatched chicks, as evaluated in the current study, its effects persist to a more limited extent in chickens at ages that correspond developmentally to human adolescence.5 As further validations for the use of this model in the study of signaling pathways potentially pertinent to the clinical disorder, an antimyopia drug first identified in form-deprivation myopia in the chick6 has been shown to be active against human myopia in two multicenter clinical trials,7,8 and risk factors suggested by the pharmacology of this model have been corroborated indirectly through cross-sectional clinical surveys.9 To approach retinal signaling pathways potentially underlying the pathogenesis of experimental myopia, we profiled gene expression patterns in the combined retina and retinal pigment epithelium (RPE) from chicks with form-deprivation myopia, a well-established eye-growth model.4 The recent availability of chicken genome microarrays permits such a transcriptome-level study. We specifically assayed the retina/RPE after 6 hours and 3 days of unilateral goggle wear. Although minor contralateral effects are recognized in chick eye-growth models,10,11 potential spurious differences between birds complicate statistical approaches to interbird comparisons, and our bioinformatics approach stresses the most important experimental-to-contralateral control eye in individual birds. Although choroidal thinning comprises the predominant anatomic change in goggled chick eyes at 6 hours, increased scleral proteoglycan synthesis by that time reveals activation of a retina-to-sclera signaling pathway that will produce later measurable effects on scleral growth.12 Because of potential roles of diurnal cycling of both retinal dopamine and ocular dimensions in refractive development,4 sampling at 6 hours thus is a reasonable time to expect activation and transcription of at least initial genes pertinent to signaling a scleral growth response, but it is shorter than a diurnal cycle and may be less likely to induce potential diurnal– circadian confounding than would sampling at a later time. While expression of immediate-early genes is altered at earlier times,13 such nonspecific genes alone may not reveal specific pathways or mediators of eye growth. With longer times of goggle wear, it becomes increasingly ambiguous whether altered genes identify a pathway generating myopic growth or instead reflect secondary changes such as those necessary for the retina to conform anatomically to the enlarging scleral– choroidal coat. By day 3, chick eyes manifest the growth and refractive effects of goggle wear,12,14 and despite the diurnal– circadian issues just discussed, this time permits characterization of established progressing myopia and minimizes more marked secondary effects that may occur with longer-term visual manipulations and still more pronounced anatomic growth.