Transcriptional regulation of the c-MYC (MYC) proto-oncogene by oncogenic Wnt/β-catenin signaling in colorectal carcinomas
Open Access
- Author:
- Rennoll, Sherri Ann
- Graduate Program:
- Cell and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 28, 2015
- Committee Members:
- Gregory Yochum, Dissertation Advisor/Co-Advisor
Gregory Yochum, Committee Chair/Co-Chair
Laura Carrel, Committee Member
Sergei A Grigoryev, Committee Member
Lisa M Shantz, Committee Member - Keywords:
- MYC
beta-catenin
AXIN2
colorectal cancer - Abstract:
- Over 90% of sporadic colorectal cancers (CRCs) are associated with initiating mutations in components of the Wnt/β-catenin signaling pathway. These mutations result in elevated nuclear levels of the β-catenin transcriptional co-activator. In the nucleus, β-catenin associates with the T-cell factor/Lymphoid enhancer factor (TCF/Lef) family of sequence-specific transcription factors at Wnt-responsive DNA elements (WREs) to increase target gene expression. Aberrant expression of these genes by oncogenic Wnt/β-catenin signaling leads to the formation of small, benign adenomas. The c-MYC (MYC) proto-oncogene is a direct β-catenin target gene that is required for Wnt/β-catenin-mediated intestinal tumorigenesis. However, the molecular mechanisms underlying β-catenin regulation of MYC expression are not fully defined. The work described within this thesis aimed to further define these molecular mechanisms by addressing the involvement of nuclear AXIN2 and the MYC 3’ WRE in controlling oncogenic MYC gene expression. AXIN2, a scaffolding protein and component of the Wnt/β-catenin signaling pathway, was recently demonstrated to localize to the nucleus. The function of AXIN2 within this subcellular compartment, however, was unknown. By constitutively localizing AXIN2 to the nucleus, we demonstrate that nuclear AXIN2 forms a complex with β-catenin/TCF and represses MYC gene expression by binding the MYC gene locus. These results indicate that nuclear AXIN2 functions as a molecular rheostat to maintain a “just right” level of MYC necessary for tumorigenesis in CRC. Similar to nuclear AXIN2, we find that the MYC 3’ WRE, which maps 1.4-kb downstream from the MYC transcription stop site, drives oncogenic MYC gene expression in CRC cells. Using CRISPR/Cas9 genome editing technology, we generated a knockout CRC cell line in which a single TCF binding element within the MYC 3’ WRE was deleted. Deletion of this element decreased β-catenin/TCF occupancy at the MYC 3’ WRE and MYC gene expression. The observed decrease in MYC gene expression corresponded to diminished cellular proliferation and growth of the knockout cells. Together, our studies have uncovered new mechanisms for regulation of MYC gene expression by oncogenic Wnt/β-catenin signaling, and suggest that approaches to target nuclear AXIN2 or the MYC 3’ WRE would be effective strategies for the treatment of individuals suffering from CRC.