Abstract GS01-08: CDK4/6 inhibition is a potential vulnerability in NF1-depleted ER+ breast cancer

Abstract CDK4/6 control a key checkpoint by phosphorylating Rb, leading to Rb degradation and subsequent S-phase entry. CDK4/6 inhibition, together with endocrine therapy, is standard of care for advanced ER+ breast cancer; however, CDK4/6 inhibition typically is cytostatic. This study aims to identify breast cancer patients who may better respond to treatments targeting CDK4/6. NF1/neurofibromin is a key tumor suppressor that we have shown can represses not only RAS as a GAP but also ER as a transcriptional co-repressor. Dual activation of RAS and ER promotes endocrine therapy resistance. We computed kinase activity scores from phosphor-proteomic data in the CPTAC breast cancer cohort and found that both CDK4/6 activity scores and Rb-pS780 levels negatively correlated with NF1 protein levels. Furthermore, NF1 mutations co-occur with mutations in CDK4/6 and CCND1-3 in breast tumors. These results suggest a functional dependency between NF1 loss and activation of the CDK4/6-Rb pathway. To delineate how NF1 and CDK4 interact molecularly, we depleted NF1 in ER+ breast cancer cells and found an increase in Rb-pS780 and in phosphorylation in CDK4’s activation loop (CDK4-pT172). Cyclin-D binding is required for CDK4 activation. CCND1 expression is under the direct control by ER, and upon NF1-depletion, Cyclin D1 expression was increased, partly due to enhanced ER recruitment to an ERE in CCND1. Cyclin-D-bound CDK4 must be phosphorylated at T172 to be fully active; however, what kinase is responsible for this is largely unknown. We have evidence that CDK4-T172 phosphorylation is RAF, but not ERK, dependent. When the NF1-depleted ER+ cells were seeded in the presence of fulvestrant together with either palbociclib or abemaciclib, apoptosis readily occurred. Remarkably, fulvestrant plus palbociclib efficiently caused durable tumor regression in two ER+ PDX models, in which NF1 is undetectable by IHC. In contrast, no tumor regression was seen in a PDX model with detectable NF1. Finally, greater growth inhibition and apoptosis were detected in NF1-low tumors in the neoadjuvant NeoPalAna trial when palbociclib was added after anastrozole. These data support a model whereby ER and RAS signaling converge upon CDK4/6, and CDK4/6 activation is a key survival mechanism when ER signaling is attenuated by treatment in NF1-depleted ER+ breast cancer cells. This apparent addiction for CDK4/6 activity makes NF1-depleted ER+ breast tumors vulnerable to CDK4/6 inhibition, thus creating a potential therapeutic opportunity to match CDK4/6 inhibition with patients who can benefit the most. Citation Format: Ze-Yi Zheng, Anran Chen, Eric Jaehnig, Meenakshi Anurag, Jonathan Lei, Chenwei Wang, Long Feng, Purba Singh, Hilda Kennedy, Jin Cao, Ghazal Yadav, Jill Tsai, Xi Chen, Charles Foulds, Bora Lim, Matthew Ellis, Bing Zhang, Eric Chang. CDK4/6 inhibition is a potential vulnerability in NF1-depleted ER+ breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr GS01-08.