Abstract LB213: NF1 loss promotes bone metastasis in estrogen receptor positive breast cancer

Abstract Up to 80% of breast cancer (BCa) expresses estrogen receptor-α (ER). Over 60% ER+ BCa metastasizes to the bone and are challenging to treat. This study investigates what drives bone metastasis by focusing on a key tumor suppressor neurofibromin (NF1), a GTPase-activating protein that represses RAS signaling. Earlier, we reported that NF1 acts as a transcriptional corepressor for ER in BCa. Our group established that combination fulvestrant (selective ER degrader) and binimetinib (MEK-inhibitor) induces tumor regression in NF1lowER+ models. This combination is now in clinical trial, NCT05554354 (NCI ComboMATCH). Numerous studies indicate that the NF1 loss may impact bone health by altering the equilibrium between bone formation and resorption. Since, bone is the primary site of metastasis in 50-70% of ER+ BCa, understanding the role of NF1 loss in bone metastases becomes imperative. An osteolytic vicious cycle may drive bone metastasis, where cancer cells in the bone stimulate the bone-destructing osteoclasts (OC) ; leading to subsequent bone deterioration, which in turn, releases growth factors to further promote tumor growth. OCs and many cells in the bone are ER+. We thus hypothesize that deregulated RAS and ER signaling by NF1 loss can drive ER+ bone metastasis by promoting an osteolytic vicious cycle. To assess the clinical impact of NF1 loss on bone metastasis, we analyzed metastatic BCa patient data from cBioPortal and the MSK-EMC cohort. To define how NF1 loss affect bone metastasis, NF1 knockout (KO) ER+ BCa cells were tested in vitro and in vivo for bone tropism, growth in bone and effect on the vicious cycle. Analyses of clinical data suggest that NF1 copy number (CN) loss in ER+ tumors associates with accelerated metastasis to the bone. Additionally, NF1 CN loss and mutations are more prevalent in bone metastasis BCa patients. Lower levels of NF1 mRNA are correlated with shorter duration of bone metastasis-free survival. Our preclinical studies further supported the driver role of NF1 in bone metastasis. NF1KO ER+ BCa cells migrate more efficiently toward the bone in vitro compared to their NF1+ counterparts. When injected into the mammary fat pads of mice, NF1KO cells more frequently metastasized to the bone compared to NF1+ cells. When NF1KO cells were delivered to the mouse femur by intra-iliac injection, they grew more efficiently and induced more bone degradation. This is consistent with our in vitro results, which show that NF1KO ER+ BCa cells can more efficiently promote OC formation from progenitor cells. For this experiment, Mass-spectrometry based unbiased profiling of the conditioned medium showed that osteolytic factors, such as ICAM1 and OSTF1, were enriched in the NF1KO medium. Hence, we conclude that the loss of NF1 deregulates RAS and ER, leading to increased osteolytic factors, that drive bone metastasis. Co-targeting RAS and ER can be a potential treatment for NF1 loss-induced bone metastasis in ER+ BCa. Citation Format: Zifan Zhao, Ze-Yi Zheng, Jonathan Lei, Matthew J. Baik, Yi-Hsuan Wu, Junkai Wang, Igor Bado, Meenakshi Anurag, Bing Zhang, Xiang Zhang, Eric Chang. NF1 loss promotes bone metastasis in estrogen receptor positive breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr LB213.