EZH2 CO-ACTIVATES NF-κB TARGETS WITH RELA IN A PRC2-INDEPENDENT MANNER IN TRIPLE NEGATIVE BREAST CANCER CELLS

Epigenetic modifications of DNA that modulate gene expression are centrally important in both biological and pathological processes. EZH2 is the methyltransferase component of epigenetic silencing complex Polycomb Repressive Complex 2 (PRC2), responsible for catalyzing histone H3 lysine 27 trimethylation (H3K27me3), which ultimately promotes chromatin condensation and gene silencing. EZH2 is essential for embryonic development and facilitates lineage commitment, and its expression typically decreases as cells terminally differentiate. However, EZH2 is overexpressed in many hematological and solid cancers, strongly associated with more aggressive subtypes and malignant phenotypes, and is considered a bona fide oncogene. Thus, targeting EZH2 is of clinical interest, and multiple small molecule catalytic inhibitors and compounds that degrade or disrupt PRC2 are in various stages of pre-clinical and clinical evaluation. Importantly, “epidrugs” as a monotherapy may only exert partial responses and face primary resistance in solid tumors. Further, accumulating evidence supports non-canonical histone methyltransferase- and PRC2-independent EZH2 functions, especially in cancer contexts, which would not be targeted with current therapeutics. EZH2 is a demonstrated oncogene in breast cancer, especially in aggressive subtypes including Triple Negative Breast Cancer (TNBC). Non-canonical EZH2 activities have been reported in TNBC cells including interaction with NF-kappaB (NF-κB) transcription factors and activation of NF-κB inflammatory target genes, which are independently implicated in the same aggressive subtypes of breast cancer. However, the global signaling, mechanism of activation, and oncogenic implications of the EZH2-NF-κB axis in TNBC cells remained incompletely understood, as did the broader contributions of EZH2 to TNBC in both canonical and non-canonical roles. Here, we present evidence supporting EZH2 transcriptional co-activation of a subset of NF-κB target genes via physical interaction with NF-κB subunits RelA and RelB, which contributes to malignant phenotypes and appears independent of PRC2 subunits and histone methyltransferase activity. Further, we suggest EZH2 catalytic activity is independently important for TNBC phenotypes, and that signaling crosstalk between EZH2, NF-κB, and other axes may also contribute to disease. In all, these data both support and expand upon previous reports, and importantly underscore the necessity of considering non-canonical EZH2 when developing and evaluating therapeutic strategies.