Mechanisms of Dynamic Transcriptional Reprogramming in Metastasis Stem Cells

Metastasis causes >90% of cancer death. The persistence and lethality of metastasis is driven by cells capable of self-renewal, slow cell-cycling, tumor re-initiation, and therapy resistance, termed metastasis stem cells (MetSCs). Development of effective strategies for eliminating metastasis requires a better understanding of the mechanisms that MetSCs exploit for survival. We recently demonstrated that (1) disseminating colorectal cancers (CRC) undergo a dynamic phenotypic switch from an LGR5+ tumor-initiating cancer stem cell (CSC) state to a distinct LGR5lowL1CAM+ state required for metastasis. (2) This phenotypic plasticity of MetSCs is retained in ex vivo patient derived organoids, which can be used to dissect mechanisms of plasticity. (3) L1CAM+ MetSCs are functionally distinct from intestinal tumor-initiating LGR5+ CSCs: L1CAM is required for organoid formation, the regeneration of intestinal epithelium after colitis, and tumor formation after metastatic dissemination. But unlike LGR5, it is dispensable for epithelial homeostasis or intestinal tumor initiation. In contrast to tumor initiation, where homeostatic stem cells undergo oncogene-driven hyper proliferation in intact tissues, metastasis subverts a regenerative mechanism to survive and regrow outside an intact epithelial niche. (4) We have shown that the principal driver of L1CAM expression is loss of epithelial integrity itself, acting via loss of E-cadherin intercellular adherens junctions to transiently displace the transcriptional silencer REST/NSRF from chromatin in quiescent MetSCs, in turn derepressing expression of L1CAM and other genes required for tissue regeneration1. Proliferation, restoration of epithelial structures, and macrometastatic outgrowth, on the other hand, require high REST levels. Our evidence suggests that MetSCs cells are regenerative stem cells that emerge directly in response to loss of epithelial integrity to drive repair, a phenotype of physiological wound healing that is redeployed in MetSCs. In this proposal, we will define the molecular mechanism by which REST chromatin binding is dynamically regulated in MetSCs, and how this in turn enables cell fate plasticity from stemness to proliferation. Our preliminary data implicates the mRNA binding protein ZFP36L1/2 in REST- mediated metastatic plasticity. Project hypothesis: The ZFP36L1/2REST axis is a master regulator of cell fate plasticity in intestinal epithelial progenitors. Aim 1: Define the function of the ZFP36L1/2-REST axis in normal and neoplastic intestinal stem cell self-renewal, differentiation, and proliferation. Aim 2: Dissect the molecular mechanism of ZFP36L1/2-mediated cell fate plasticity. Aim 3: Determine the functional consequences of ZFP36L1/2-REST dynamic regulation in metastatic seeding and colonization. Results will define mechanisms of cancer progression and identify putative therapeutic targets to limit regenerative plasticity, with potential to impact clinical outcomes.