Abstract B014: Mutant p53-mediated CSF1/CSF1R signaling promotes tumor invasion and lung metastasis in esophageal squamous cell carcinoma

Introduction: Mutations of the tumor suppressor p53 are detected up to 80% of esophageal squamous cell carcinoma (ESCC) cases, which in turn correlate with high metastatic rates and poor prognosis. To understand the mutant p53-mediated mechanisms in promoting ESCC metastasis, we conducted RNA-Seq and cytokine array on isogenic primary and metastatic tumor cells harvested from our mouse model of esophageal cancer harboring Trp53R172H/-, in which we have identified Colony stimulating factor 1 (Csf1) to be upregulated. Our goal is to investigate the role and mediators of CSF1 signaling through its cognate receptor CSF1R by which missense p53 mutations can promote tumor invasion and lung metastasis in ESCC. Methods: We have utilized novel L2-Cre; LSL-Trp53R172H; Rosa26LSL-YFP mice and isolated tumor cells to model metastatic ESCC. We conducted ChIP-Seq analysis for p53 on tumor cells derived from lung metastases in our mouse models. Furthermore, we have genetically and pharmacologically targeted CSF1/CSF1R signaling axis to assess its role in ESCC tumor invasion and lung metastasis. In addition to studying tumor intrinsic mechanisms, we have established quantitative multiplex immunofluorescence (qmIF) and flow cytometry approaches to characterize the changes in tumor microenvironment (TME). Results And Discussion: We demonstrate that metastatic ESCC has increased Csf1 expression compared to primary tumors, and this is dependent upon p53 mutation status, which is reinforced by the TCGA data and patient-derived tissue microarrays (TMAs). The overlay of the RNA-Seq with the ChIP-Seq analysis indicates that Csf1 is a direct gene target of p53-R172H with enriched binding motifs. Furthermore, based on the proximity ligation assay (PLA) with murine ESCC tumor cells, bromodomain and extra-terminal motif (BET) protein BRD4 interacts with p53-R172H to induce Csf1 expression. We show that the BRD4-CSF1 axis fosters tumor invasion, subcutaneous tumor growth and metastatic burden in a mutant p53 background. In accordance, upon inhibiting this signaling pathway in the tail-vein injection lung metastasis models, we have identified reduced expression of CD31+ cells and decreased infiltration of F4/80+CD163+ and F4/80+CD206+ M2-polarized macrophages at the metastatic tumor sites, indicating that the CSF1/CSF1R pathway plays a critical role in shaping the pro-metastatic and immunosuppressive TME. Finally, analysis of the esophageal cancer datasets reveals that specific p53 muta