Pitx2 is a critical early regulatory gene in normal cecal development

The murine cecum is a critical digestive structure. Morphogenesis of the cecum involves several key genes, including Homeobox (Hox) d12. Ectopic expression of Hoxd12 has been shown to result in cecal agenesis and a down-regulation of both Fibroblast growth factor 10 (Fgf10) and the Pituitary homeobox 2 gene (Pitx2). Homozygous null mutation of Fgf10 or its cognate receptor Fgfr2IIIb results in severe cecal defects where there is the initiation of mesodermal budding, but a failure of the endoderm to grow and extend into this structure. We examined the expression of Pitx2 in the cecum and hypothesized that homozygous null mutation of Pitx2 would result in cecal agenesis.

IACUC approval was obtained for these studies. Whole mount in situ hybridizations for Pitx2 were performed on wild-type embryos between embryonic d (E)11.0 and E12.5. Pitx2 -/- and Fgfr2IIIb -/- embryos were generated from n/+ heterozygote breedings and harvested at E10.5, E11.5, and E13.5. Genotypes were confirmed by PCR. Morphology of Pitx2 -/- cecae were compared with those of wild-type littermates and Fgfr2IIIb -/- embryos at identical stages. Embryos were fixed overnight and photographed the following day.

Pitx2 is expressed in the cecal mesoderm and endoderm as early as E11.0. Expression becomes increasingly more robust by E12.5. Homozygous null mutation of Pitx2 results in agenesis of the cecum. In contrast to Fgfr2IIIb -/- embryos, which demonstrate a persistent mesodermal bud as late as E18.5, no mesodermal bud is present in Pitx2 -/- embryos.

Our findings demonstrate that Pitx2 is a critical regulatory gene in cecal morphogenesis and suggest that Pitx2 is required for initiation of mesodermal budding and likely resides upstream of Fgf10-Fgfr2IIIb signaling in the normal development of this structure.