GENETIC CONTROL OF EYE AND CENTRAL NERVOUS SYSTEM DEVELOPMENT

Abstract

Aniridia, a congenital ocular disorder caused by haploinsufficiency of transcription factor PAX6, is characterized by complete or partial iris hypoplasia with associated foveal hypoplasia. Brain imaging performed in patients heterozygous for PAX6 mutations often reveal absence of the brain anterior or posterior commissure, absence of the pineal gland, and a present but reduced in size corpus callosum. Renal coloboma syndrome, another autosomal dominant inherited disease, is characterized by hypodysplastic kidneys and optic nerve defects, and is caused by haploinsufficiency of transcription factor PAX2. In the first part of this thesis we investigated the role of these Pax genes in neural development, by generating an allelic series of knock-in models at the Pax6 locus. We showed that Pax6(5a) and Pax2 could not replace Pax6 for its auto-regulation in lens induction or for neural differentiation in retina. In brain development, however, we demonstrated that cell proliferation in the cerebral cortex and dorsoventral patterning of the telencephalon and neural tube was partially rescued in either knock-in mutant. We believe our novel findings not only reveal Pax-protein functional specificity during neural development, but may also be utilized to understand the aberrant molecular mechanism that result in aniridia and/or renal coloboma syndrome. Aphakia (lack of lens) is a rare human congenital disorder with its genetic etiology largely unknown. In the second part of this thesis, we show that homozygous deletion of Nf1, the Ras GTPase gene underlying human neurofibromatosis type 1 syndrome, caused lens dysgenesis in mouse. While early lens specification proceeded normally in Nf1 mutants, lens induction was disrupted due to deficient cell proliferation. Further analysis showed that ERK signaling was initially elevated in invaginating lens placode, but by lens vesicle stage, Ras signaling antagonist Sprouty2 was up regulated, followed by rapid decrease in ERK phosphorylation. Only after intraperitoneal treatment of U0126, an inhibitor of ERK phosphorylation, was lens development restored in Nf1 mutants. Hyperactive RAS-MAPK signaling is known to cause neuro-cardiofacial-cutaneous (NCFC) syndromes in human. As a member of NCFC family genes, Nf1 represents the first example that attenuation of Ras-MAPK kinase signaling pathway is essential for normal lens development.