NF1-depleted ER+ breast cancers are differentially sensitive to CDK4/6 inhibitors
Neurofibromin/NF1-depletion (NF1low) is associated with endocrine therapy resistance in approximately 20% of ER+/HER2– early-stage breast cancer but specific treatments for NF1low tumors are not established.  Proteogenomic analyses on ER+/HER2– breast cancer demonstrated that NF1low tumors exhibit elevated CDK4/6 activity.  NF1-silencing in cell lines promoted ER recruitment to CCND1, thus increasing cyclin-D1 levels.  Additionally, RAF is demonstrated to directly phosphorylate the CDK4 activation-loop (pT172) thus providing a direct connection between NF1 loss and an increase in CDK4 activity.  Consequently, NF1low cancer cells are differentially sensitive to a fulvestrant plus CDK4/6 inhibitor (CDK4/6i) combination, with cell-death induction in vitro, and durable tumor regressions in ER+ NF1low PDX models in vivo. Furthermore, NF1low ER+/HER2– tumors treated neoadjuvantly with 4 weeks of anastrozole exhibited a minimal reduction of a multigene mRNA proliferation score vs NF1high tumors but exhibited marked sensitivity to the subsequent addition of palbociclib. In conclusion, dual ER and downstream RAF activation following NF1-loss drives endocrine therapy resistance and CDK4/6i sensitivity in NF1low ER+ breast cancer.  NF1 status should therefore be prioritized for biomarker investigations in adjuvant CDK4/6i trials to determine whether the NF1low status identifies a particularly sensitive subset where CDK4/6i therapy is critical. Overall design: ER-ChIP-seq of doxycycline inducible knockdown of NF1 in hormone-deprived MCF7 breast cancer cell lines  -/+ E2.