Mutation of the APC tumor suppressor gene occurs in approximately 75% of colorectal cancers (CRC) and is considered the central event driving tumor initiation in the large intestine. The APC protein controls gene expression indirectly through the canonical WNT signaling pathway as a component of a cytoplasmic complex that promotes the proteolysis of the transcriptional co-regulator β-catenin. Recent studies have shown that APC is also a chromatin-associated protein that interacts with the promoter of the MYC proto-oncogene to downregulate its transcription. We have tested the hypothesis that APC similarly regulates other genes relevant to tumorigenesis, and that the 75% of CRCs that are deficient in APC may show abnormal activation or suppression of these genes, in comparison to the other 25% of CRCs. These differences could potentially underlie differences in tumor progression and/or responses to chemotherapy, and could enable the identification of new therapeutic targets.
The APC genotype of two CRC cell lines was manipulated using siRNA to reduce APC expression in one cell line wild-type for APC and using an APC transgene to induce exogenous APC expression in a second cell line mutated for APC. NanoString and RNA-seq technologies have generated gene expression datasets from these cell lines and identify more than 700 genes that could be directly regulated at the transcriptional level by APC. RT-PCR and in silico analyses have validated a smaller subset of these genes. Functional pathway analysis has led to the hypothesis that APC loss facilitates evasion of apoptosis, with a subset of 17 pro- and anti-apoptotic candidate genes implicated in this effect. Critical experiments in progress will confirm the extent to which the aberrant expression of these genes in vivo is specific to colon tumors lacking APC, using Apc-deficient and Apc-wild-type mouse models of intestinal cancer, and will identify direct chromatin binding sites of APC by ChIP-seq. These approaches will validate our gene expression analyses and identify a set of direct gene targets that APC regulates.
Citation Format: William Hankey, Michael A. McIlhatton, Xun Lan, Anil G. Jegga, Victor X. Jin, Bruce J. Aronow, Joanna Groden. Direct transcriptional functions of the APC tumor suppressor in regulating gene expression. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr A21.