A Retinoic Acid-Rfx6 Cascade Modulates Wnt/Bmp-dependent Gut Tube Patterning: Insight into the Molecular Basis of Mitchell-Riley Syndrome.

Multiple signaling pathways and transcription factors (TFs) establish organ domains in the developing gastrointestinal tract. How these are integrated into spatial-temporal networks to regulate organogenesis and how disruptions to those networks lead to congenital syndromes remain poorly understood. Using human pluripotent stem cell cultures and Xenopus embryos, we demonstrate that retinoic acid (RA) from the lateral plate mesoderm directly activates expression of the TF rfx6 in posterior foregut endoderm. Rfx6 subsequently promotes posterior foregut identity while suppressing Wnt-dependent hindgut and Bmp-dependent pharyngeal fates through direct and indirect mechanisms. Rfx6 can directly activate the expression of several key foregut TFs (onecut1 and pdx1) and Wnt antagonists (sfrp2/5) while indirectly restricting expression of Wnt and Bmp ligands. Rfx6 also directly suppresses transcription of the Wnt-dependent hindgut TF cdx2 and the Bmp-dependent pharyngeal TFs nkx2-5 and nkx2-6. Thus, Rfx6 acts at multiple levels to integrate RA, Wnt, and Bmp activity into a network with lineage-promoting TFs to control gut tube patterning. These results provide insight into the molecular basis of Mitchell-Riley Syndrome congenital anomalies, which are caused by RFX6 mutations.