The upstream ectoderm enhancer in Pax6 has an important role in lens induction

The Pax6 gene has a central role in development of the eye. We show, throug h targeted deletion in the mouse, that an ectoderm enhancer in the Pax6 gen e is required for normal lens formation. Ectoderm enhancer-deficient embryo s exhibit distinctive defects at every stage of lens development. These inc lude a thinner lens placode, reduced placodal cell proliferation, and a sma ll lens pit and lens vesicle. In addition, the lens vesicle fails to separa te from the surface ectoderm and the maturing lens is smaller and shows a d elay in fiber cell differentiation. Interestingly, deletion of the ectoderm enhancer does not eliminate Pax6 production in the lens placode but result s in a diminished level that, in central sections, is apparent primarily on the nasal side. This argues that Pax6 expression in the lens placode is co ntrolled by the ectoderm enhancer and at least one other transcriptional co ntrol element. It also suggests that Pax6 enhancers active in the lens plac ode drive expression in distinct subdomains, an assertion that is supported by the expression pattern of a lacZ reporter transgene driven by the ectod erm enhancer. Interestingly, deletion of the ectoderm enhancer causes loss of expression of Foxe3, a transcription factor gene mutated in the dysgenet ic lens mouse. When combined, these data and previously published work allo w us to assemble a more complete genetic pathway describing lens induction. This pathway features (1) a pre-placodal phase of Pax6 expression that is required for the activity of multiple, downstream Pax6 enhancers; (2) a lat er, placodal phase of Pax6 expression regulated by multiple enhancers; and (3) the Foxe3 gene in a downstream position. This pathway forms a basis for future analysis of lens induction mechanism.