Oncogenic Transformation Of The Intestinal Epithelium By Musashi2Protein

Colorectal caner (CRC) is one the leading causes of cancer related deaths in the word. Shedding on the mechanism of intestinal tumorigenesis is very important. Musashi (Msi), an RNA-binding protein, plays an important role in regulating neural cell asymmetric division in Drosophia. Msi is a conserved RNA-binding protein whose primary structure and expression pattern have been conserved among species in nematodes (C. elegans), the fruit fly (Drosophila), ascidians (Ciona intestinalis), and vertebrates as a whole. There are two members in mammalian Musashi family, which are Musashi1(Msi1) and Musashi2(Msi2), whose amino acid sequence and RNA-binding motif are highly homologous. Recently, multiple reports demonstrated that Msi1and Msi2are expressed in neural stem cell, mammary stem cell and hematopoietic stem cell and aberrantly activated in neuroglioma, breast cancer cell and leukemia cells. All these suggest that Msil and Msi2contribute to stem cell homeostasis and tumorigenesis in variety of tissues. However, the function of Msi2in intestinal stem cell (ISC) and intestinal tumorigenesis is remaining to be uncovered.Msi2is broadly expressed in most of crypt cells, including intestinal stem cells, in wild type mice. In human, Msi2is overexpressed in all type of gastrointestinal tumors. We use drug inducible Msi2overexpressing transgene mice, TRE-Msi2/Rosa26-rtTA, to investigate the role of Msi2/MSI2in intestinal epithelium. Interestingly, we observed that the Doxycycline (Dox) treated TRE-Msi2mice show similar phenotype with acute APC (Adenomatous Polyposis Coli) loss mice, including expansion of proliferation cells, blocking of differentiation, crypts fission and increasing apoptosis. Consistently to the phenotype, we find that the epithelium in Msi2inducible mice have similar gene expression signature with APC-loss mice by Gene set enrichment analysis (GSEA). APC is a tumor suppressor by inhibiting Wnt signaling pathway and most kinds of gut cancer are caused by APC mutation. Meanwhile, aberrant expression of Msi2arises the adenoma genesis in APCmin/+mouse model of intestinal tumorigenesis. All these data suggest that Msi2/MSI2play an important role in tumorigenesis. However, no change of APC expression and activation of Wnt signaling pathway were found in the Msi2transgenic mice, although TRE-Msi2mice exhibit similar morphological and molecular consequences with APC loss mouse.Transcriptome-wide RNA-binding analysis by Clip-seq suggests that Msi2/MSI2acts as a pleiotropic inhibitor of known intestinal tumor suppressors including Irigl, bmpr1a, p21, and pten. Combining with the data from GESA, we find that Msi2/MSI2blocks the translation of PTEN by binding to3’UTR in mRNA. Thus lower level of Pten protein triggers the activation of PI3K-Akt-mTORCl axis pathway and mimicked the APC-loss phenotypes. In summary, our findings identify Msi2/MSI2as a potent oncogene that acts in a pathway parallel to the canonical Wnt/β-catenin axis to promote intestinal transformation.