Analysis of Pax6 expression using a BAC transgene reveals the presence of a paired-less isoform of Pax6 in the eye and olfactory bulb.

Pax6, a member of the paired-family of transcription factors, exhibits restricted expression and essential functions in the developing eye, olfactory system, central nervous system, and pancreas. To understand Pax6 function, which critically depends on induction of proper expression levels during development, it is necessary to elucidate the molecular mechanisms governing Pax6 transcription. Although previous studies using classic transgenic approaches have provided a wealth of information about the distribution and types of regulatory elements involved in Pax6 regulation, genetic studies in both humans and mice indicate that these enhancers alone are not sufficient for fully regulated Pax6 expression. We report here our analysis of mice transgenic for a 160 kb mouse Pax6 BAC transgene, which was generated as a necessary first step towards testing the long-range control of Pax6 expression in vivo. We show that this BAC transgene replicates Pax6 expression in the eye. This is the first time that a reporter transgene has been expressed in a normal Pax6-like pattern in all of the tissues of the eye and defines an eye regulatory region within the Pax6 downstream regulatory region (DRR). Second, we show that this BAC transgene contains all of the cis regulatory elements required for normal Pax6 expression within the developing embryo, except for within the diencephalon and olfactory bulb. Third, we show that this transgene is subject to Pax6 autoregulation. Lastly, we identify, for the first time in mammals, an isoform of the Pax6 protein lacking the paired domain. This isoform is expressed in the developing olfactory bulb and eye. Over-expression of Pax6DeltaPD causes a microphthalmic phenotype in both Pax6(+/+) mice and Pax6(+/-) mice. These results demonstrate a role for Pax6DeltaPD in eye development, which appears to be different than that ascribed to either canonical Pax6 or Pax6(5a).