Metastatic cell plasticity is maintained by SOX9 function during mitosis in breast cancer

Disseminated tumor cells (DTCs) contribute to metastasis formation by adapting to the hostile environmental conditions encountered at the seeded secondary organs. This phenotypic plasticity is supported by the epigenetic reprogramming of cis-regulatory elements (CREs), representing a hallmark of metastatic cells. DTCs can enter a reversible state of cancer dormancy, resulting from their ability to enter quiescence after mitosis and escape immune surveillance. In this respect, mitosis may represent a window of opportunity to increase DTCs fitness by priming CREs, ensuring the reactivation of a dormancy-related transcriptional program while preserving the epigenetic information. By profiling the chromatin changes occurring during the early steps of mitosis, we found that DTCs experience fluctuations in chromatin accessibility at CREs, which are bookmarked by pro-metastatic TFs. Among these, the pro-dormancy TF SOX9 remained associated with mitotic chromatin, facilitating transcriptional reactivation upon exiting mitosis. By dissecting the consequences of degrading SOX9 before mitosis, we found that its priming of dormancy-related CREs, combined with the recruitment of the Mediator kinase module, is instrumental for metastatic cells to enter quiescence and escape NK-mediated immune surveillance. These findings indicate that cancer dormancy is primed during mitosis by pro-metastatic TFs.