SIX1 cooperates with RUNX1 and SMAD4 in cell fate commitment of Müllerian duct epithelium
Authors: Terakawa Jumpei , Serna Vanida A. , Nair Devi , Sato Shigeru , Kawakami Kiyoshi , Radovick Sally , Maire Pascal , Kurita Takeshi 
Published: September 2018

During female mammal reproductive tract development, epithelial cells of the lower Müllerian duct are committed to become stratified squamous epithelium of vagina and ectocervix, when the expression of ΔNp63 transcription factor is induced by mesenchymal cells. The absence of ΔNp63 expression leads to adenosis, the putative precursor of vaginal adenocarcinoma. Our previous studies with genetically engineered mouse models have established that fibroblast growth factor (FGF)/mitogen-activated protein kinase (MAPK), bone morphogenetic protein (BMP)/SMAD, and activin A/runt related transcription factor 1 (RUNX1) signaling pathways are independently required for ΔNp63 expression in Müllerian duct epithelium (MDE). Here we report that sine oculis homeobox homolog 1 (SIX1) plays a critical role in the activation of ΔNp63 locus in MDE as a downstream transcription factor of mesenchymal signals. In mouse developing reproductive tract, SIX1 expression was restricted to MDE of the future cervix and vagina. SIX1 expression was totally absent in SMAD4 null MDE and was reduced in RUNX1 null and FGFR2 null MDE, indicating that SIX1 is under the control of vaginal mesenchymal factors, BMP4, activin A and FGF7/10. Furthermore,Six1, Runx1andSmad4gene-dose-dependently activated ΔNp63 expression in MDE within vaginal fornix. Using a mouse model of diethylstilbestrol (DES)-associated vaginal adenosis, we found DES action through epithelial estrogen receptor α (ESR1) down-regulates SIX1 and RUNX1 in MDE within the vaginal fornix. This study establishes that the vaginal/ectocervical cell fate of MDE is regulated by a collaboration of multiple transcription factors including SMAD4, SIX1 and RUNX1, and the down-regulation of these key transcription factors leads to vaginal adenosis.

Author Summary

In embryogenesis, differentiation fate of cells is specified through constant communication between neighboring cells. In this study, we investigated the molecular mechanism of epithelial cell fate commitment in the lower female reproductive organs utilizing mouse genetic models. The cell fate of epithelial cells in the uterus, cervix and vagina is directed by signaling from mesenchymal cells. We demonstrated that within the epithelial cells of the developing vagina, signals from mesenchymal cells are integrated into activities of transcription factors including SMAD4, RUNX1 and SIX1, which dose-dependently co-operate in the determination of vaginal epithelial cell fate. Disruption of these processes alters the cell fate from vaginal to uterine epithelium, resulting in a condition called vaginal adenosis, a putative precursor of vaginal adenocarcinoma. Women exposed to diethylstilbestrol (DES) in the womb have about 40 times the risk of developing vaginal adenocarcinoma. We determined that developmental exposure to DES induces vaginal adenosis by repressing SIX1 and RUNX1 through ESR1 in the epithelial cells. This discovery enhances the understanding of how early-life events, such as exposure to endocrine disruptors, causes vaginal adenosis, and thus may contribute to the prevention and therapeutic treatment of idiopathic vaginal adenocarcinoma.