LAUSR

Simple Summary Cancer develops due to the expression of genes that promote cell growth and the repression of genes that limit growth. Epigenetics is a mechanism that regulates gene expression via the chemical modification of DNA and histones. Proteins that regulate this process have emerged as potential therapeutic targets. Here, we investigate the role of the epigenetic regulatory protein CBX2 in aggressive forms of breast cancer, which have few therapeutic options. We show that functioning CBX2 is crucial for cancer cell growth and viability. By analysing gene expression patterns in CBX2-depleted cells, we show that CBX2 activates signalling pathways that promote cell growth (mTORC1 signalling) and inhibits the activity of a protein complex that limits cell growth (the DREAM complex) by repressing the expression of key tumour suppressor genes. We have therefore identified novel mechanisms by which CBX2 promotes breast cancer growth and provide evidence that inhibition of CBX2 may be a novel therapeutic strategy. Abstract Chromobox 2 (CBX2) is a chromatin-binding component of polycomb repressive complex 1, which causes gene silencing. CBX2 expression is elevated in triple-negative breast cancer (TNBC), for which there are few therapeutic options. Here, we aimed to investigate the functional role of CBX2 in TNBC. CBX2 knockdown in TNBC models reduced cell numbers, which was rescued by ectopic expression of wild-type CBX2 but not a chromatin binding-deficient mutant. Blocking CBX2 chromatin interactions using the inhibitor SW2_152F also reduced cell growth, suggesting CBX2 chromatin binding is crucial for TNBC progression. RNA sequencing and gene set enrichment analysis of CBX2-depleted cells identified downregulation of oncogenic signalling pathways, including mTORC1 and E2F signalling. Subsequent analysis identified that CBX2 represses the expression of mTORC1 inhibitors and the tumour suppressor RBL2. RBL2 repression, in turn, inhibits DREAM complex activity. The DREAM complex inhibits E2F signalling, causing cell senescence; therefore, inhibition of the DREAM complex via CBX2 may be a key oncogenic driver. We observed similar effects in oestrogen receptor-positive breast cancer, and analysis of patient datasets suggested CBX2 inhibits RBL2 activity in other cancer types. Therapeutic inhibition of CBX2 could therefore repress mTORC1 activation and promote DREAM complex-mediated senescence in TNBC and could have similar effects in other cancer types.