Abstract 2662: Interrogation of the landscape of long noncoding RNAs in breast cancer to identify an ER-regulated predictor of tamoxifen resistance

Background: We have previously performed a bioinformatics-based analysis on 7,256 RNA sequencing libraries from tumors, normal tissues, and cell lines to delineate the landscape of long noncoding RNAs (lncRNAs) in the human transcriptome. This analysis identified 58,648 lncRNAs, including over 45,000 novel transcripts (Iyer and Niknafs et al, Nature Genetics, 2015). We now interrogate this lncRNA compendium to identify top candidate estrogen receptor (ER)-associated lncRNAs in breast cancer and characterize their association with disease progression. Methods: To identify and prioritize lncRNAs that were differentially expressed in cancer vs normal tissue, and in ER-positive vs ER-negative disease, we performed differential expression testing using a previously described non-parametric tool for RNA-seq count data called Sample Set Enrichment Analysis (SSEA) on over 1000 RNA sequencing libraries including breast cancer and normal tissue samples from the The Cancer Genome Atlas (TCGA) project. The effect of the top prioritized lncRNA on cancer phenotypes was studied via proliferation, colony formation, invasion and tamoxifen resistance assays in MCF7 and T47D cell lines, and via mouse xenograft studies in vivo. The mechanism by which this lncRNA promotes tumor progression was investigated by identifying its top protein interactors and its subdomains responsible for function, and then studying the effects of disrupting function of this lncRNA on cancer phenotypes. Finally, in a “guilt-by-association” study, we developed a signature of 150 protein coding genes most strongly associated with our lncRNA of interest, and investigated the association of this signature with tumor grade, recurrence, and metastases using Oncomine analyses. Results: Differential expression analyses on over 1000 TCGA RNA-seq samples nominated Breast Cancer Associated Transcript (BRCAT431) as the top overexpressed ER-regulated lncRNA in breast cancer. In vitro experiments demonstrate that shRNA-mediated knockdown of BRCAT431 resulted in significantly decreased proliferation, colony formation, and invasion (by >50% in most assays). Tamoxifen resistance was associated with significantly increased BRCAT431 levels in both MCF7 and T47D cells, and knockdown of BRCAT431 reversed tamoxifen resistance. In vivo xenograft studies demonstrate that knockdown of BRCAT431 also significantly decreased xenograft growth and tumor invasion by >50%. RNA pulldown followed by mass spectrometry identified the RNA binding protein hnRNPL as a key protein interacting with BRCAT431. Deletion studies identified a 27 base-pair region of BRCAT431 necessary for its interaction with hnRNPL, and loss of this region abrogated BRCAT431-induced invasion. Finally, guilt-by-association studies demonstrate a strong association between BRCAT431 overexpression and tumor grade, recurrence, and metastases. Conclusion: In this study, we develop the largest reported compendium of breast cancer lncRNAs. We prioritize BRCAT431 as the top lncRNA upregulated in ER-positive breast cancers, and demonstrate that it confers aggressive oncogenic phenotypes in vitro and in vivo. We identify a novel mechanism by which this lncRNA functions through interaction with hnRNPL. Our results suggest that by promoting tamoxifen resistance, this lncRNA increases the clinical risk of recurrence and metastases in breast cancer. Citation Format: Yashar S. Niknafs, Sumin Han, Teng Ma, Chao Zhang, Matthew K. Iyer, Rohit Malik, Anton Poliakov, Corey Speers, Daniel Hayes, James Rae, Felix Y. Feng, Arul M. Chinnaiyan. Interrogation of the landscape of long noncoding RNAs in breast cancer to identify an ER-regulated predictor of tamoxifen resistance. [abstract]. In: Proceedings of the AACR Special Conference on Noncoding RNAs and Cancer: Mechanisms to Medicines ; 2015 Dec 4-7; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2016;76(6 Suppl):Abstract nr B19.