PRMT5-regulated splicing of DNA repair genes drives chemoresistance in breast cancer stem cells

Breast cancer stem cells (BCSCs) are a rare cell population that is responsible for tumour initiation, metastasis and chemoresistance.Despite this, the mechanism by which BCSCs withstand genotoxic stress is largely unknown. Here, we uncover a pivotal role for thearginine methyltransferase PRMT5 in mediating BCSC chemoresistance by modulating DNA repair efficiency. Mechanistically, weidentify PRMT5 as a major regulator of DNA damage response (DDR) gene splicing in BCSCs, particularly those integral to theFanconi Anaemia and homologous recombination pathways, with PRMT5 inhibition synergising with chemotherapy to promoteBCSC apoptosis. A comparison of BCSCs and their bulk cell progeny identified some shared (ATM, DDX11, EXO1, FAN1, SLX4) butmany unique (ATR, RAD17, RAD51D, RUVBL1) PRMT5-dependent alternative DDR splicing events. Surprisingly, these skipped exonsand retained intron events rarely lead to substantial gene expression repression, suggesting that PRMT5 inhibition predominantlyresults in nuclear detention of intron-containing transcripts and the production of non-canonical isoforms with compromisedprotein function. Since many genes within the same DDR pathway undergo deregulated splicing, this study thus reveals additionalpoints of vulnerability and alternative combination drug strategies that could improve the therapeutic efficacy of PRMT5 inhibitorsto promote BCSC eradication.