title: Transgenic expression of oncogenic BRAF induces loss of stem cells in the mouse intestine, which is antagonized by β-catenin activity.
abstract: Colon cancer cells frequently carry mutations that activate the β-catenin and mitogen-activated protein kinase (MAPK) signaling cascades. Yet how oncogenic alterations interact to control cellular hierarchies during tumor initiation and progression is largely unknown. We found that oncogenic BRAF modulates gene expression associated with cell differentiation in colon cancer cells. We therefore engineered a mouse with an inducible oncogenic BRAF transgene, and analyzed BRAF effects on cellular hierarchies in the intestinal epithelium in vivo and in primary organotypic culture. We demonstrate that transgenic expression of oncogenic BRAF in the mouse strongly activated MAPK signal transduction, resulted in the rapid development of generalized serrated dysplasia, but unexpectedly also induced depletion of the intestinal stem cell (ISC) pool. Histological and gene expression analyses indicate that ISCs collectively converted to short-lived progenitor cells after BRAF activation. As Wnt/β-catenin signals encourage ISC identity, we asked whether β-catenin activity could counteract oncogenic BRAF. Indeed, we found that intestinal organoids could be partially protected from deleterious oncogenic BRAF effects by Wnt3a or by small-molecule inhibition of GSK3β. Similarly, transgenic expression of stabilized β-catenin in addition to oncogenic BRAF partially prevented loss of stem cells in the mouse intestine. We also used BRAF(V637E) knock-in mice to follow changes in the stem cell pool during serrated tumor progression and found ISC marker expression reduced in serrated hyperplasia forming after BRAF activation, but intensified in progressive dysplastic foci characterized by additional mutations that activate the Wnt/β-catenin pathway. Our study suggests that oncogenic alterations activating the MAPK and Wnt/β-catenin pathways must be consecutively and coordinately selected to assure stem cell maintenance during colon cancer initiation and progression. Notably, loss of stem cell identity upon induction of BRAF/MAPK activity may represent a novel fail-safe mechanism protecting intestinal tissue from oncogene activation.
authors.affiliation: 1] Laboratory of Molecular Tumor Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany [2] Department of Developmental Genetics, Max-Planck-Institute for Molecular Genetics, Berlin, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Riemer
authors.affiliation: 1] Laboratory of Molecular Tumor Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany [2] Department of Developmental Genetics, Max-Planck-Institute for Molecular Genetics, Berlin, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Sreekumar
authors.affiliation: Department of Developmental Genetics, Max-Planck-Institute for Molecular Genetics, Berlin, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Reinke
authors.affiliation: 1] Department of Medicine II, Klinikum Rechts der Isar, Technische Universität München, München, Germany [2] German Cancer Consortium (DKTK), German Cancer Research Center, Heidelberg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Rad
authors.affiliation: 1] Laboratory of Molecular Tumor Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany [2] German Cancer Consortium (DKTK), German Cancer Research Center, Heidelberg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Schäfer
authors.affiliation: 1] Laboratory of Molecular Tumor Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany [2] German Cancer Consortium (DKTK), German Cancer Research Center, Heidelberg, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Sers
authors.affiliation: Department of General Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Bläker
authors.affiliation: Department of Developmental Genetics, Max-Planck-Institute for Molecular Genetics, Berlin, Germany.
authors.forename: B G
authors.initials: BG
authors.lastname: Herrmann
authors.affiliation: 1] Laboratory of Molecular Tumor Pathology, Institute of Pathology, Charité Universitätsmedizin Berlin, Berlin, Germany [2] Department of Developmental Genetics, Max-Planck-Institute for Molecular Genetics, Berlin, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Morkel
mesh_terms: D000818:Animals; D018938:Caco-2 Cells; D063646:Carcinogenesis / Q000235:genetics*; D002452:Cell Count; D049109:Cell Proliferation / Q000235:genetics; D003110:Colonic Neoplasms / Q000235:genetics*; D015870:Gene Expression / Q000502:physiology; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism / Q000473:pathology; D007422:Intestines / Q000473:pathology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D013234:Stem Cells / Q000473:pathology*; D060449:Wnt Signaling Pathway / Q000235:genetics; D051176:beta Catenin / Q000502:physiology*
chemical_list: D051176:beta Catenin; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf
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