MYC hyperactivates WNT signaling in APC/CTNNB1-mutated colorectal cancer cells through miR-92a-dependent repression of DKK3

Activation of Wnt signaling is among the earliest events of colon cancer development. It is achieved either via activating mutations in the CTNNB1 gene encoding {beta}-catenin, the key transcription factor in the Wnt pathway, or most commonly by inactivating mutations in APC, a major {beta}-catenin binding partner and negative regulator. However, our analysis of recent Pan Cancer Atlas data revealed that CTNNB1 mutations significantly co-occur with those affecting Wnt receptor complex components (e.g., Frizzled and LRP6), underscoring the importance of additional regulatory events even in the presence of common APC/CTNNB1 mutations. In our effort to identify non-mutational hyperactivating events, we determined that KRAS-transformed murine colonocytes overexpressing direct {beta}-catenin target MYC show significant upregulation of the Wnt signaling pathway and reduced expression of Dickkopf 3 (DKK3), a reported ligand for Wnt co-receptors. We demonstrate that Myc suppresses Dkk3 transcription through one of mir-17-92 cluster microRNAs, miR-92a. We further examined the role of DKK3 by overexpression and knockdown and discovered that DKK3 suppresses Wnt signaling in APC-null murine colonic organoids and human colon cancer cells despite the presence of downstream activating mutations in the Wnt pathway. Conversely, MYC overexpression in the same cell lines resulted in hyperactive Wnt signaling, acquisition of epithelial-to-mesenchymal transition markers, and enhanced migration and invasion and metastasis in syngeneic orthotopic mouse colon cancer model. Our results suggest that the MYC->miR-92a-|DKK3 axis hyperactivates Wnt signaling, forming a feedforward oncogenic loop. SIGNIFICANCECommon APC and CTNNB1 mutations activate Wnt signaling in colorectal cancers. Here we demonstrate that further potentiation of this pathway involves microRNA-dependent repression of the DKK3 gene by the Myc oncoprotein.