Overexpression of pairedless Pax6 in the retina disrupts corneal development and affects lens cell survival

The Pax6 transcription factor is required for multiple aspects of vertebrate eye development. The Pax6 gene encodes isoforms that either contain (Pax6 + PD) or lack (Pax6 Delta PD) the N-terminal paired-box DNA-binding domain, in addition to the homeodomain. Alternative promoters control the expression of Pax6 + PD and Pax6 Delta PD in the eye. Using a modified bacterial artificial chromosome (BAC) transgene that specifically expresses Pax6 Delta PD, but not paired-containing Pax6, in the normal endogenous pattern, we show that overexpression of Pax6 Delta PD causes a severe microphthalmic phenotype in both wild-type and Pax6-deficient (Sey(/+)) mice in a dosage-dependent manner. The microphthalmic phenotype is due to lens degeneration during embryonic development. Lens development initiates correctly, but cells in the lens undergo apoptotic cell death between E12 and E13. Concomitantly, in these mice, changes in Bmp4, Msx1, and Wnt2b expression were observed in the mesenchymal cells of the developing cornea. To visualize Pax6 Delta PD expression, we developed a dual-reporter Pax6 BAC transgene in which EGFP and DsRed demonstrate paired-containing and pairedless transcripts, respectively. In BAC transgenic mice, DsRed is predominantly expressed in the peripheral neural retina during early eye development, but not in the developing lens or cornea. Later DsRed is strongly expressed in the developing ciliary body, but not in the iris. We suggest that the ratio of Pax6+PD and Pax6 Delta PD isoforms in the distal retina is important for both cornea and lens development, either directly by controlling transcription of necessary growth factors or indirectly by controlling development of the distal neural retina. (c) 2007 Elsevier Inc. All rights reserved.