Resolving the basis of phenotypically variable hereditary abnormalities of eye formation

Our eyes start out as outpocketings of brain tissue during early embryonic development. The cells destined to form the eyes originate within the neural plate, the precursor of the central nervous system. As the neural plate folds up to form the brain, the eye-forming cells bulge out laterally forming optic cups, the structures that later differentiate as eyes. Each optic cup undergoes shape changes and tissue fusion closes a gap (the optic fissure) present on one side of the cup, leading to formation of the intact globe shaped eye. The complex orchestration of cell movements that form the eyes is an example of morphogenesis - the process by which embryonic cells form into tissues and organs. Many of the genes that regulate the formation of the eye have yet to be identified. One reason that this lack of knowledge needs to be addressed is that congenital malformations of the eye, such as anophthalmia (lack of eyes), microphthalmia (small eyes) and coloboma (a failure in optic fissure fusion), being compatible with life and reproduction, are relatively common in the human population. As these are congenital defects, this means that eye problems are present from birth. While anophthalmic patients are blind, microphthalmic and coloboma patients can have severe visual impairment. For instance, colobomas are a common cause of visual problems, can cause retinal detachment and cataracts, and often lead to blindness. In this project, we will use zebrafish embryos to identify genes and genetic interactions important for eye formation. Zebrafish embryos are small, transparent and develop externally, facilitating the study of normal development and disease in the intact animal. Together with their amenability to genetic analysis, these features make fish embryos an excellent model system to study eye formation in normal and pathological conditions. Indeed, we can visualise all of the cells in the developing eye in living embryos both in healthy fish and in fish carrying one or more genetic mutations that compromise eye formation. Consequently, we can use research in fish both to identify those genes needed for eye formation and to understand the mechanisms by which such genes build functional eyes. Although some congenital abnormalities of eye formation are due to mutations in single genes, we suspect that in many cases, such defects are due to disruption of two or more genes. Consequently, in this project, we will use novel, powerful approaches that allow us to systematically analyse the consequences of simultaneous disrupted function of two or more genes that are candidates for causing eye defects when non-functional. To facilitate this research, our current MRC funding has enabled us to develop lines of fish carrying mutations that make the fish more likely to show eye phenoytpes when additional genes are disrupted. We will remove function of one or more additional genes in these "sensitised" fish lines to identify new genes and genetic interactions important for eye formation. We will also study the function of several genes that, when disrupted, give very similar eye defects both in fish and in humans as although we know these genes to be important, we do not understand how they function. Finally, we will study why individuals carrying the same genetic mutations can show quite different eye phenotypes. To facilitate this, we have lines of fish in which we can perform genetic or environmental perturbations that affect the severity of the eye defects. Overall, our research will help to bridge the gap between the highest quality research in model systems and human disease phenotypes. We will improve our understanding of normal eye development and will use new zebrafish models of human eye diseases to gain further insights into the causes of hereditary ocular malformations. Our research also has the potential to be of great value in the diagnosis of congenital abnormalities of eye formation.