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Chromatin-associated functions of the APC tumor suppressor protein

Hankey, William C, IV
http://orcid.org/0000-0001-5935-2391
http://rave.ohiolink.edu/etdc/view?acc_num=osu1480198247672881

Abstract Details

2016, Doctor of Philosophy, Ohio State University, Biomedical Sciences.
Biallelic mutation of the APC tumor suppressor gene occurs in a high percentage of colorectal tumors and is considered the critical event driving tumor initiation in the large intestine. The APC protein performs multiple functions, including negative regulation of the canonical WNT signaling pathway by both cytoplasmic and nuclear mechanisms. As a result, APC suppresses proliferation, cell cycle progression, and genomic instability, while facilitating differentiation, normal directional migration, and apoptosis. The contribution of APC to these phenotypes is not mediated exclusively through its effects on canonical WNT signaling, but also through WNT-independent functions of the APC protein. Intriguing reports that APC interacts with chromatin to repress key WNT-activated targets prompted this study’s initial hypothesis that the chromatin-associated fraction of APC regulates gene transcription through multiple mechanisms, both WNT-dependent and WNT-independent. Chromatin immunoprecipitation and next-generation sequencing identified more than 6,000 genomic peaks associated with the APC protein. Target selection was performed by comparison to whole transcriptome sequencing data from APC-deficient and APC wild- type colon cancer cell lines and mouse tumors. 175 transcripts whose expression changes upon APC loss are linked to genomic regions physically associated with the APC protein. Motif analysis of APC-associated genomic peaks found that binding sites for the TCF7L2 and AP-1 transcription factors are overrepresented and occur in many of the same peaks. Luciferase reporter assays indicate that APC antagonizes canonical WNT signaling not only at WNT-activated genes such as PHLDB2 (Pleckstrin Homology Like Domain Family B Member 2), but also at WNT-repressed genes such as MALL (Mal, T-Cell Differentiation Protein-Like). A transcriptional element within the first intron of the MALL gene mediates transcriptional repression by the canonical WNT signaling pathway and transcriptional activation by AP-1. These findings demonstrate that canonical WNT signaling and the AP-1 transcription factor collaborate to fine-tune the expression of a large number of shared target genes in the colorectal epithelium. Future therapeutic strategies for APC-deficient colorectal cancers might be expanded to include emerging agents targeting the AP-1 signaling pathway. New insights into mechanisms of transcriptional control by the canonical WNT signaling pathway indicate that known target genes should be grouped according to whether their transcription increases or decreases, as well as the presence or absence of co-regulation by AP-1. Transcriptional changes in Apc-deficient colon tumors (from ApcMin/+ mice) and colon tumors with wild-type Apc and a degradation-resistant point mutant of ß-catenin (from mice treated with azoxymethane and dextran sulfate sodium) largely overlap, with only 31-32% unique to each tumor type. Expression changes unique to Apc-deficient tumors are hypothesized to result from loss of ß-catenin-independent functions of chromatin- associated APC. Further study will reveal their role in regulating apoptosis and formation of cell protrusions, as well as their relationship to colorectal cancer prognosis.
Joanna Groden, PhD (Advisor)
Albert de la Chapelle, MD, PhD (Committee Member)
Kay Huebner, PhD (Committee Member)
Mark Parthun, PhD (Committee Member)
Jeffrey Parvin, MD, PhD (Committee Member)
119 p.
Biomedical Research
APC; tumor suppressor; colorectal cancer; canonical WNT signaling; chromatin; transcription; AP-1; transcription factor; ChIP-seq; beta-catenin;

Recommended Citations

Citations

  • Hankey, IV, W. C. (2016). Chromatin-associated functions of the APC tumor suppressor protein [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480198247672881

    APA Style (7th edition)

  • Hankey, IV, William. Chromatin-associated functions of the APC tumor suppressor protein. 2016. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1480198247672881.

    MLA Style (8th edition)

  • Hankey, IV, William. "Chromatin-associated functions of the APC tumor suppressor protein." Doctoral dissertation, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480198247672881

    Chicago Manual of Style (17th edition)

osu1480198247672881
326
© 2016, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.