In vivo CRISPR screening links NFKB1 to endocrine resistance in ER⁺ breast cancer

Resistance to endocrine therapy (ET) remains a major clinical challenge in the treatment of estrogen receptor-positive (ER) breast cancer, underscoring the need for novel therapeutic targets. To identify genetic drivers of ET resistance, we conducted an in vivo genome-wide CRISPR-Cas9 screen in MCF7 cells implanted into ovariectomized nude mice under estrogen-deprived conditions. NFKB1 emerged as a top candidate whose loss promoted estrogen-independent tumor growth and recurrence. Functional studies confirmed that NFKB1 deficiency enhanced tumorigenicity and conferred resistance to tamoxifen and fulvestrant both in vitro and in vivo. Mechanistically, transcriptomic and biochemical analyses revealed that NFKB1 loss activated canonical NF-{kappa}B signaling, leading to inflammatory gene induction and hyperactivation of ER signaling. Importantly, pharmacologic inhibition of NF-{kappa}B signaling restored ET sensitivity in NFKB1-deficient cells. Clinically, NFKB1 downregulation was enriched in ER breast tumors and associated with poor patient outcomes. Collectively, these findings establish NFKB1 as a key suppressor of ET resistance, uncover a mechanistic link between inflammation and ER reactivation, and highlight NF-{kappa}B signaling as a therapeutic vulnerability in NFKB1-deficient ER breast cancer.