EZH2-mediated functional Rb inactivation drives cell survival and defines a selective vulnerability in CDK4/6 inhibitor-resistant metastatic breast cancer

Overcoming resistance to first-line treatment remains a major clinical challenge for metastatic breast cancer (MBC) patients affected by ER+/HER2- tumors. Although RB1 loss-of-function mutations/deletions have been linked to a lack of response to CDK4/6 inhibitors and endocrine treatment, over 90% ER+/HER2- tumors retain wild-type RB1 . Using an integrated approach that combines analyses of clinical biospecimens and mechanistic in vitro and in vivo approaches, this study provides evidence of a novel actionable signaling axis wherein EZH2 accumulates in the cytosol of resistant cells and, through non-canonical functions, participates in the functional inactivation of Rb, bypassing CDK4/6 inhibition in these tumors. Pharmacological degradation of EZH2 restored Rb function, suppressed E2F activity, and selectively impaired the growth of resistant cells. This work offers a biologically grounded framework for the rational selection of therapeutic strategies targeting EZH2 in RB1 -proficient ER+/HER2- MBCs that are resistant to CDK4/6 inhibitors, offering a promising pathway to overcome resistance.