Differentially Expressed Genes in Msx1-/- Versus Msx1+/+ Tooth Buds

Objective: Mutations in the Msx1 homeodomain transcription factor causes tooth agenesis in humans. In Msx1^-/- mice tooth development arrests at the bud stage, due to down-regulation of Bmp4 in dental mesenchyme. Since, Msx1 cannot activate Bmp4 by itself, there must be other mechanisms mediating Msx1-Bmp4 regulation. In order to uncover these mechanisms we sought partner genes of Msx1 using microarray analysis of differentially expressed genes in Msx1 wild-type versus mutant tooth bud mesenchyme.

Method: Molar tooth buds from Msx1-deficient and wild-type E14.5 embryos were micro-dissected. RNA samples were isolated from the dissected tissue from each group and sent to a microarray core lab where Affymetrix Mouse 430 2.0 and Exon microarray analysis was performed. Our array results were compared with Eurexpress, an atlas of E14.5 gene expression patterns.

Result: Both arrays showed quite similar results regarding the identity of the differentially expressed genes in Msx1^-/- and Msx1^+/+mice, but they were dissimilar in their fold changes. 95 genes were more than 1.7 fold up or down regulated in the 430 array, most of them were represented in Eurexpress. Genes such as Shh, Odam, Egr3, Calb1, Mmp13, Krt17, Hspa1a, Sostdc1, and Enpp1 showed more than 3 fold differences in expression. Surprisingly, the expression of Bmp4 was not among the down-regulated genes.  

Conclusion: Overall gene expression differences did not vary greatly between Msx1 genotypes. Data from our arrays and Eurexpress showed that the most down regulated genes were previously described and newly discovered ones in the enamel knot and epithelial tissues. Since array data by themselves cannot reveal which of the mesenchymally expressed transcription factors and growth factors are directly related to Msx1 expression, they require follow up by quantitative PCR, in situ hybridization and chromatin immunoprecipitation (ChIP).