High Myc expression and transcription activity underlies intra-tumoral heterogeneity in triple-negative breast cancer

// Nidhi Gupta 1, * , Karen Jung 2, * , Chengsheng Wu 1 , Abdulraheem Alshareef 1 , Hind Alqahtani 1 , Sambasivarao Damaraju 1, 3 , John R. Mackey 2, 3 , Sunita Ghosh 2, 3 , Siham Sabri 4 , Bassam S. Abdulkarim 4 , Gilbert Bigras 1 , Raymond Lai 1, 2, 5 1 Department of Laboratory Medicine and Pathology, University of Alberta, Edmonton, Alberta, Canada 2 Department of Oncology, University of Alberta, Edmonton, Alberta, Canada 3 Cross Cancer Institute, Alberta Heath Services, Edmonton, Alberta, Canada 4 Department of Oncology, McGill University, Montreal, Quebec, Canada 5 DynaLIFE Dx Medical Laboratories, Edmonton, Alberta, Canada * These authors contributed equally to this work Correspondence to: Raymond Lai, email: rlai@ualberta.ca Keywords: triple-negative breast cancer, Myc, transcription activity, intra-tumoral heterogeneity, MAPK/ERK pathway Received: September 13, 2016      Accepted: February 21, 2017      Published: March 03, 2017 ABSTRACT We have previously identified a novel intra-tumoral dichotomy in triple-negative breast cancer (TNBC) based on the differential responsiveness to a reporter containing the Sox2 regulatory region-2 (SRR2), with reporter responsive (RR) cells being more stem-like than reporter unresponsive (RU) cells. Using bioinformatics, we profiled the protein-DNA binding motifs of SRR2 and identified Myc as one of the potential transcription factors driving SRR2 activity. In support of its role, Myc was found to be highly expressed in RR cells as compared to RU cells. Enforced expression of MYC in RU cells resulted in a significant increase in SRR2 activity, Myc-DNA binding, proportion of cellsexpressing CD44 + /CD24 – , chemoresistance and mammosphere formation. Knockdown of Myc using siRNA in RR cells led to the opposite effects. We also found evidence that the relatively high ERK activation in RR cells contributes to their high expression of Myc and stem-like features. Using confocal microscopy and patient samples, we found a co-localization between Myc and CD44 in the same cell population. Lastly, a high proportion of Myc-positive cells in tumors significantly correlated with a short patient survival. In conclusion, inhibition of the MAPK/ERK/Myc axis may be an effective approach in eliminating stem-like cells in TNBC.