Mammalian Genetics Laboratory / Shiroishi Group

Interaction between the epithelium and mesenchyme coordinates patterning and differentiation of oral cavity structures including teeth, palatal rugae and tongue papillae. SHH is one of the key signaling molecules for this interaction. Epithelial expression of Shh in the tooth buds and tongue papillae is regulated by at least two enhancers, MRCS1 and MFCS4. However, it is unclear how the two enhancers cooperate to regulate Shh. Here, we found that simultaneous deletion of MRCS1 and MFCS4 results in the formation of a supernumerary tooth in front of the first molar. Since deletion of either single enhancer barely affects tooth development, MRCS1 and MFCS4 evidently act in a redundant fashion. Binding motifs for WNT signaling mediators are shared by MRCS1 and MFCS4, and play a central role in regulating Shh expression, indicating that the two redundant enhancers additively exert their Shh regulation by responding to WNT signal input.

This study was carried out as a collaboration of Tomoko Sagai, Takanori Amano and Toshihiko Shiroishi of National Institute of Genetics, Hiroshi Kiyonari of RIKEN Center for Life Science Technologies, and Hyejin Seo and Sung-Won Cho of Yonsei University in Korea. This study was supported by JSPS KAKENHI 24247002.

Supernumerary tooth formation in the mouse with simultaneous deletion (double knockout) of the MRCS1 and MFCS4 enhancers. (A and B) X-ray micro-CT images of transverse section of mandible in the wild type (A) and the mouse with the combinatorial deletion of MRCS1 and MFCS4 (B). Yellow arrow indicates supernumerary tooth. (C) A model of redundant action of the Shh enhancers. MRCS1 and MFCS4 share three common regulatory motifs that are located in the same order (arrowheads) and have redundant roles for the Shh expression in tooth buds. Expression levels of Shh are additively regulated by MRCS1, MFCS4, and probably unidentified enhancers. When reduction of Shh expression level falls below a threshold, supernumerary molars arise. MRCS1 and MFCS4 control different expression domains of Shh in the oropharyngeal tissues in addition to the tooth bud. Thus, the two redundant enhancers may cooperatively regulate the tooth development, while they have undergone sub-functionalization.