SIX2 Mediates Late-Stage Metastasis via Direct Regulation of SOX2 and Induction of a Cancer Stem Cell Program

Abstract The capacity for tumor cells to metastasize efficiently is directly linked to their ability to colonize secondary sites. Here we identify Six2, a developmental transcription factor, as a critical regulator of a breast cancer stem cell program that enables metastatic colonization. In several triple-negative breast cancer (TNBC) models, Six2 enhanced the expression of genes associated with embryonic stem cell programs. Six2 directly bound the Sox2 Srr2 enhancer, promoting Sox2 expression and downstream expression of Nanog, which are both key pluripotency factors. Regulation of Sox2 by Six2 enhanced cancer stem cell properties and increased metastatic colonization. Six2 and Sox2 expression correlated highly in breast cancers including TNBC, where a Six2 expression signature was predictive of metastatic burden and poor clinical outcome. Our findings demonstrate that a SIX2/SOX2 axis is required for efficient metastatic colonization, underscoring a key role for stemness factors in outgrowth at secondary sites. Significance: These findings provide novel mechanistic insight into stemness and the metastatic outgrowth of triple-negative breast cancer cells.