Abstract P1-08-01: Regulation of estrogen receptor-α by NF1

Background. Although great strides have been made in targeting the ER pathway for treating ER + breast cancer, relapse and death is common and is closely linked to resistance to ER-targeting agents. As a result, the majority of deaths from breast cancer still come from ER + tumors. To discover drivers for endocrine resistance, we have sequenced tumor DNAs from a cohort of >600 patients treated with 5-year tamoxifen (Tam) monotherapy with a median 10.4 years follow up. Our preliminary data show that the worst outcome mutations (Hazard Ratio of ∼3 for relapse) were mostly those of the Neurofibromatosis type 1 ( NF1 ) gene ( encoding Neurofibromin) , with nonsense/frame shift mutations creating early stop codons. Germline NF1 mutations cause neurofibromatosis type 1, a common inherited disorder that predisposes individuals to both benign and malignant tumors of the nervous system, as well as an increased risk for breast cancer. Analysis of DNA sequencing data has also shown that the NF1 gene is mutated in a wide range of common cancers (e.g., melanoma, lymphoma, and cancers of the lung, breast, and colon). Thus, NF1 -deficiency underlies the formation and/or progression of a large number of cancers, so that the development of therapies targeted to NF1 -deficient malignancies would have broad impact. These observations support the hypothesis that NF1 gene inactivation is associated with aggressive tumor behaviors, such as endocrine therapy resistance in breast cancer. The key focus of this study is to define how the NF1 protein neurofibromin, regulates endocrine therapy resistance. Although neurofibromin is best known as a negative regulator for Ras, our data show that it may have other functions. Method. Our data suggest that many of the identified nonsense/frame shift create a NF1 null state; thus, we have used gene-silencing to recapitulate the effects of such NF1 mutations on the activities of ER + breast cancer cells. NF1 + and NF1 – ER + breast cancer cells were grown in defined media to measure how estradiol (E 2 ) and Tam impact their growth, transforming activities, and gene expression. The binding between neurofibromin and components of the ER transcriptional pathway was measured biochemically and using the mammalian two-hybrid system. Results. Our data showed that NF1 -silenced cells use Tam as an agonist and can grow with very little E 2 , and these activities are driven by enhanced recruitment of ER to the ERE, leading to efficient expression of many classic ER-responsive genes. Expressing the NF1-GAP domain does not restore normal responses to Tam and E 2 in NF1 -silenced cells, suggesting that neurofibromincan regulate ER activity in a Ras-independent manner. To investigate the possibility that neurofibromin can directly regulate ER, we found that it can bind ER; furthermore, neurofibromin was more strongly recruited to the ERE by Tam than by E2. Conclusion. Our data support a model whereby neurofibromin acts like a co-repressor for ER. As such, NF1 loss may result in more aggressive tumor behaviors by activating, not only the Ras pathways, but also the ER transcriptional pathways. Simultaneous activation of two powerful oncogenic pathways by the loss of a single tumor suppressor may explain why neurofibromin is such a potent tumor suppressor lost in a wide range of cancers. Citation Format: Zheng Z-Y, Cakar B, Lavere P, Cao J, Yao J, Singh P, Lei JT, Toonen JA, Haricharan S, Anurag M, Shah K, Kavuri M, Chan DW, Chen X, Gutmann DH, Foulds CE, Ellis MJ, Chang EC. Regulation of estrogen receptor-α by NF1 [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-08-01.