Differential Gene Expression in Pax9+/+ and Pax9-/- mouse tooth buds

Objectives: The paired box transcription factor Pax9 activates Bmp4 expression in dental mesenchyme and causes tooth agenesis when mutated or deleted in humans and mice. Here we investigate the Pax9/Bmp4 regulatory pathway in greater detail using a mouse strain (courtesy: Dr. R. Jiang) in which Pax9 was silenced by insertion of an inactive Osr2 cDNA.

Methods: We mated Pax9^+/- x Pax9^+/- mice in order to obtain Pax9-deficient embryos and isolated RNA from micro-dissected tooth organs at embryonic day E14.5, a critical stage of tooth development. Using expression array studies we have determined major target genes of Pax9 in mouse tooth organs.  The Eurexpress database was used to evaluate the normal expression pattern of these genes.

Results: The mouse phenotype was confirmatory for Pax9 deficiency. Array results revealed that only 48 genes (32 named, 16 unnamed) were more than 2 fold up- or down-regulated in the Pax9-deficient tooth organs. Pax9 expression was 7 fold down-regulated, as expected, but unexpected was the expression level of previously described Pax9 effector genes, Msx1 and Lef1, which were only 1.5 fold lower. Surprisingly, Bmp4 showed no decreased expression while Osr2 was up-regulated. Shh and other enamel knot markers were significantly down-regulated and several mesenchymal genes, including transcription factors, were reduced to a level comparable to Msx1 and Lef1. Other differentially expressed genes were located subjacent to the dental mesenchyme.

Conclusions:  For an array study there were surprisingly few differentially expressed genes and expression differences were relatively low, presumably because tooth organ dissection is not very precise, leading to dilution with RNA from extraneous tissue.  Our results suggest that Pax9 deficiency may not only affect gene expression in the dental epithelium and mesenchyme, but also in the tissue surrounding the dental mesenchyme.