After decades of outstanding research, the discovery of common genetic mutations occurring in colorectal cancer is nearing saturation. The next frontier will focus on identifying mechanisms downstream of these mutations that result in cancer progression. This proposal's main objective is to describe the transcriptional regulatory mechanisms by which the transcription factors CDX2 and SMAD4 suppress colon tumor progression. The central hypothesis is that these transcription factors stabilize tumors by maintaining chromatin structure and transcription factor regulatory networks that occur in normal, differentiated tissue. The rationale supporting this hypothesis is that 1) CDX2 and SMAD4 each function in development to establish the normal colon epithelium; and 2) that CDX2 and SMAD4 are each silenced or mutated during colon tumor progression.
Aim 1 of the proposed work will take advantage of new mouse models to inactivate CDX2 in early and late cancer conditions to identify whether CDX2's transcriptional regulatory targets or control of chromatin structure are compromised in a way to support cancer growth. Preliminary evidence demonstrates CDX2-inactivation does indeed exacerbate the phenotype of a common oncogenic mouse model and that enhancer chromatin is altered when CDX2 is inactivated.
Aim 2 will focus on elucidating the function of SMAD4 in colonic polyps. SMAD4 is one of the most frequently mutated genes in colon cancer but its transcriptional regulatory mechanisms in adenomas are unclear. A novel mouse model will be employed to inactivate SMAD4 in colonic polyps and immediately determine the consequences of SMAD4 loss on gene regulation and tumor chromatin structure. Finally, despite having similar functions in promoting tumor suppression and tissue differentiation, cooperativity between CDX2 and SMAD4 has not been investigated.
Aim 3 will use a new mutant mouse to test for a genetic interaction between CDX2 and SMAD4 in the colon and potentially reveal a novel mechanism of tumor suppression. Existing research systems are unable to study the direct consequence of CDX2 or SMAD4 loss on tumor chromatin regulation in vivo. This approach is innovative in that we have assembled multiple animal models of colorectal cancer in which transcription factor knockout can be induced with precise temporal resolution. This system thus enables detailed genome-scale experiments to interrogate tumor transcriptional regulation. The proposed research is significant in that it is expected to identify a direct connection between tumor suppressing transcription factors, regulation of tumor chromatin structure, and novel mechanisms of transcriptional regulation of tumor progression.
The proposed research is relevant to the public health in that 1/3 of patients diagnosed with colorectal cancer will succumb to the disease. Patients with tumors that lose SMAD4 and/or CDX2 have a worse prognosis, indicating that these molecules suppress colon cancers from becoming lethal. By understanding how CDX2 and SMAD4 function in tumors, we hope to reveal opportunities to mimic these molecules and suppress colorectal cancer progression.
