Abstract 2490: Optimizing treatment strategy for NF1-depleted estrogen receptor positive breast cancer
Authors: Zheng Zeyi , Lei Jonathan T. , Anurag Meenakshi , Feng Long , Singh Purba , Kennedy Hilda , Cao Jin , Chen Xi , Ellis Matthew J. , Chang Eric C. 
Published: July 2021

Abstract
Background: Germline mutations in the NF1 gene are responsible for neurofibromatosis type 1, which is the world's most common genetic disorder. NF1 is also a key tumor suppressor gene that is frequently somatically mutated in a wide range of cancers. Approximately 80% of breast cancer is driven by the estrogen receptor α (ER), encoded by the ESR1 gene, and ER-positive (ER+) breast cancer can be treated by endocrine therapy targeting the ER transcriptional pathway. NF1 encodes neurofibromin, which is best known as a GTPase Activating Protein (GAP) for repressing Ras signaling. However, in a recent study we presented evidence supporting the model that NF1 has a GAP-independent activity by also acting as a transcriptional co-repressor for ER. NF1 loss enhanced ER transcription causing resistance to tamoxifen and aromatase inhibition.
Approach and Results: In this study, we examined patient data from TCGA cohort and found that low NF1 mRNA levels associated with recurrence in luminal breast cancer, particularly in the luminal B subtype. Using purified components, we showed that full-length NF1 can directly bind ER. The ESR1-pE380Q mutation is a recurrent event in metastatic ER+ breast cancer. Our two-hybrid data showed that NF1 interacted less with the ER-E380Q than wild type ER, which agrees with structural analysis predicting co-repressors binding to be mediated by the ER-E380 residue. To assess how NF1-loss impacts ER-dependent gene expression in ER+ breast cancer cells, our ER ChIP-seq data showed that in the presence of estradiol, NF1-depletion promoted global ER recruitment to estrogen response elements (EREs) on chromatin. Expression of ERE-bound genes showed concordant expression changes by RNA-seq, confirming genome-wide transcriptional dysregulation of ER targeted genes by NF1 loss. ER+ NF1-depleted breast cancer cells responded initially to a selective ER degrader (SERD), such as fulvestrant and an oral SERD AZD9496, but acquired resistance with prolonged treatment. Resistance may be dependent on CDK4/6, a common growth pathway controlled by both Ras and ER. We showed that fulvestrant together with a CDK4/6 inhibitor Palbociclib can efficiently inhibit the growth of ER+ NF1-depleted breast cancer leading to tumor regression in a patient derived xenograft model.
Conclusion: The loss of the full length NF1 can stimulate both ER and Ras signaling, and it is possible to efficiently treat ER+ NF1-depleted breast cancer by a SERD, in combination with CDK4/6 inhibitor.
Citation Format: Zeyi Zheng, Jonathan T. Lei, Meenakshi Anurag, Long Feng, Purba Singh, Hilda Kennedy, Jin Cao, Xi Chen, Matthew J. Ellis, Eric C. Chang. Optimizing treatment strategy for NF1-depleted estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2490.