Abstract A021: Loss-of-function screen for breast tumor initiating cells reveals PRC1 dependence

Abstract

Abstract Breast tumor-initiating cells (BTICs) are a subset of cells within breast tumors thought to be more tumorigenic than non-BTICs and possess both self-renewal and differentiation potential. Gene expression and functional studies have demonstrated a correlation between BTICs and high-grade tumors, poor patient prognosis, and therapy resistance; however, the mechanisms underlying BTIC regulation remains poorly understood. Because epigenetic alterations play a key role in breast cancer progression and in stem cell regulation, we hypothesized that epigenetic factors mediate BTIC function. Using a mouse MMTV-Myc mammosphere (MS) culture system, which we showed to be enriched in BTICs by in vivo tumorigenicity assays and gene expression studies, we performed a pooled shRNA library screen coupled to next generation sequencing (NGS). Using a library of ~450 shRNAs targeting 60 epigenetic regulators, our screen identified multiple members of the Polycomb Repressive Complex 1 (PRC1) as top candidates, with shRNAs targeting PRC1 members significantly depleted in MS cells, but not in control bulk cells. This strongly suggests that PRC1 is required for MS/BTIC function. We have validated that knockdown of PRC1 members reduces MS formation and BTIC ‘markers’ in MMTV-Myc cells and human MDAMB-231Luc cells in vitro, and are currently performing in vivo tumorigenesis assays. Further, RNA-Seq and ChIP-Seq is underway to decipher the mechanisms by which PRC1 mediates BTIC regulation. In accordance with our screen, The Cancer Genome Atlas (TCGA) reports several members of PRC1 to be overexpressed in breast tumors, and we are further exploring the clinical relevance of PRC1 members in breast cancer patient samples. Citation Format: Chi-Yeh Chung, Alexandre Gaspar-Maia, Gavriel Mullokandov, Almudena Bosch-Gutierrez, Eduardo Farias, Brian Brown, Emily Bernstein. Loss-of-function screen for breast tumor initiating cells reveals PRC1 dependence. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A021.