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Abstract NF1 is a key tumor suppressor that represses both RAS and estrogen receptor-α (ER) signaling in breast cancer. Blocking both pathways by fulvestrant (F), a selective ER degrader (SERD), together with binimetinib (B), a MEK inhibitor, promotes tumor regression in NF1-depleted ER+ models. We aimed to establish approaches to determine how NF1 protein levels impact B+F treatment response to improve our ability to identify B+F sensitive tumors. We examined a panel of ER+ PDX models by DNA and mRNA sequencing and found that more than half of these models carried an NF1 shallow deletion and generally have low mRNA levels. Consistent with RAS and ER activation, RET and MEK levels in NF1-depleted tumors were elevated when profiled by mass spectrometry (MS) after kinase inhibitor bead pull-down. MS showed that NF1 can also directly and selectively bind to palbociclib-conjugated beads, aiding quantification. An immunohistochemistry (IHC) assay was also established to measure NF1, but the MS-based approach was more quantitative. Combined IHC and MS analysis defined a threshold of NF1 protein loss in ER+ breast PDX, below which tumors regressed upon treatment with B+F. These results suggest that we now have a MS-verified NF1 IHC assay that can be used for patient selection as a complement to somatic genomic analysis. Citation Format: Beom-Jun Kim, Ze-Yi Zheng, Jonathan Lei, Matthew Holt, Anran Chen, Jianheng Peng, Diana Fandino, Purba Singh, Hilda Kennedy, Yongchao Dou, M Rosario Chica-Parrado, Emmanuel Bikorimana, Dan Ye, Yunguan Wang, Ariella Hanker, Nada Mohamed, Susan Hilsenbeck, Bora Lim, Jaya Ruth Asirvatham, Arun Sreekumar, Bing Zhang, George Miles, Meenakshi Anurag, Matthew Ellis, Eric Chang. Proteogenomic approaches for the identification of NF1/neurofibromin-depleted estrogen receptor positive breast cancers for targeted treatment 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 PO2-14-11.
Kim et al. (Thu,) studied this question.