The epigenetic repressor BMI1 plays an integral role in promoting the self-renewal and proliferation of many adult stem cell populations, and also tumor types, primarily through silencing the Cdkn2a locus, which encodes the tumor suppressors p16Ink4a and p19Arf . However, in cutaneous melanoma, BMI1 drives epithelial-mesenchymal transition programs, and thus metastasis, while having little impact on proliferation or primary tumor growth. This raised questions about the requirement and role for BMI1 in melanocyte stem cell (McSC) biology. Here, we demonstrate that murine melanocyte-specific Bmi1 deletion causes premature hair greying and gradual loss of melanocyte lineage cells. Depilation enhances this hair greying defect, accelerating depletion of McSCs in early hair cycles, suggesting that BMI1 acts to protect McSCs against stress. RNA-seq of McSCs, harvested before onset of detectable phenotypic defects, revealed that Bmi1 deletion derepresses p16Ink4a and p19Arf , as observed in many other stem cell contexts. Additionally, BMI1 loss downregulated the glutathione S-transferase enzymes, Gsta1 and Gsta2, which can suppress oxidative stress. Accordingly, treatment with the antioxidant N-acetyl cysteine (NAC) partially rescued melanocyte expansion. Together, our data establish a critical function for BMI1 in McSC maintenance that reflects a partial role for suppression of oxidative stress, and likely transcriptional repression of Cdkn2a.

Epithelial-mesenchymal transition (EMT) programs operate within carcinoma cells, where they generate phenotypes associated with malignant progression. In their various manifestations, EMT programs enable epithelial cells to enter into a series of intermediate states arrayed along the E-M phenotypic spectrum. At present, we lack a coherent understanding of how carcinoma cells control their entrance into and continued residence in these various states, and which of these states favour the process of metastasis. Here we characterize a layer of EMT-regulating machinery that governs E-M plasticity (EMP). This machinery consists of two chromatin-modifying complexes, PRC2 and KMT2D-COMPASS, which operate as critical regulators to maintain a stable epithelial state. Interestingly, loss of these two complexes unlocks two distinct EMT trajectories. Dysfunction of PRC2, but not KMT2D-COMPASS, yields a quasi-mesenchymal state that is associated with highly metastatic capabilities and poor survival of patients with breast cancer, suggesting that great caution should be applied when PRC2 inhibitors are evaluated clinically in certain patient cohorts. These observations identify epigenetic factors that regulate EMP, determine specific intermediate EMT states and, as a direct consequence, govern the metastatic ability of carcinoma cells.

The Epithelial–Mesenchymal Transition (EMT) and primary ciliogenesis induce stem cell properties in basal Mammary Stem Cells (MaSCs) to promote mammogenesis, but the underlying mechanisms remain incompletely understood. Here, we show that EMT transcription factors promote ciliogenesis at intermediate EMT transition states by activating ciliogenesis inducers, including FGFR1. The resulting primary cilia promote BBS11-dependent ubiquitination and inactivation of a central signaling node, GLIS2. We show that GLIS2 inactivation promotes MaSC stemness, and GLIS2 is required for normal mammary gland development. Moreover, GLIS2 inactivation is required to induce the proliferative and tumorigenic capacities of the Mammary-Tumor-initiating cells (MaTICs) of claudin-low breast cancers. Claudin-low breast tumors can be segregated from other breast tumor subtypes based on a GLIS2-dependent gene expression signature. Collectively, our findings establish molecular mechanisms by which EMT programs induce ciliogenesis to control MaSC and MaTIC biology, mammary gland development, and claudin-low breast cancer formation.

Tissue regeneration relies on adult stem cells (SCs) that possess the ability to self-renew and produce differentiating progeny. In an analogous manner, the development of certain cancers depends on a subset of tumor cells, called cancer stem cells (CSCs), with SC-like properties. In addition to being responsible for tumorigenesis, CSCs exhibit elevated resistance to therapy and thus drive tumor relapse post-treatment. The epithelial-mesenchymal transition (EMT) programs promote SC and CSC stemness in many epithelial tissues. Here, we provide an overview of the mechanisms underlying the relationship between stemness and EMT programs, which may represent therapeutic vulnerabilities for the treatment of cancers.

The epithelial-mesenchymal transition (EMT) is a key cell-biological program enabling carcinoma cell phenotypic plasticity. Accumulating evidence suggests EMT programs do not operate as a stereotypical program that functions as a binary switch, shifting cells from an epithelial (E) to a mesenchymal (M) state. Instead, EMT programs generate cells that enter into a series of intermediate states arrayed along the E-M phenotypic spectrum. At present, we still lack a coherent understanding of how carcinoma cells control their entrance into and residence in these various intermediate states, and which EMT intermediate state(s) favour the metastatic process. Here we characterize a new level of regulation of EMT, consisting of two chromatin-modifying complexes, PRC2 and KMT2D-COMPASS, that function as critical regulators to maintain stable residence of both normal and neoplastic cells in an epithelial state via regulating the downstream response of EMT-inducing signals. Dysfunction of either of these two complexes causes cells that were previously stably ensconced in an epithelial state to lapse into two distinct quasi-mesenchymal cell states with strongly differing metastatic abilities. These observations uncover a novel mechanism that regulates E-M plasticity, specifies which intermediate EMT state a cell will reside in, and thereby determines the metastatic ability of carcinoma cells. Citation Format: Yun Zhang, Joana Liu Donaher, Sunny Das, Xin Li, Ferenc Reinhardt, Jordan A. Krall, Arthur W. Lambert, Prathapan Thiru, Heather R. Keys, Mehreen Khan, Matan Hofree, Molly M. Wilson, Ozlem Yedier-Bayram, Nathan A. Lack, Tamer T. Onder, Tugba Bagci-Onder, Michael Tyler, Itay Tirosh, Aviv Regev, Jacqueline Lees, Robert A. Weinberg. Loss of PRC2 or KMT2D-COMPASS generates two quasi-mesenchymal cell states with distinct metastatic abilities [abstract]. In: Abstracts: AACR Special Virtual Conference on Epigenetics and Metabolism; October 15-16, 2020; 2020 Oct 15-16. Philadelphia (PA): AACR; Cancer Res 2020;80(23 Suppl):Abstract nr PR05.