Dysfunction of ubiquitin protein ligase MYCBP2 leads to cell resilience in human breast cancers

Breast cancer is the most common type of cancer among women worldwide, and it is estimated that 294 000 new diagnoses and 37 000 deaths will occur each year in the United States alone by 2030. Large-scale genomic studies have identified a number of genetic loci with alterations in breast cancer. However, identification of the genes that are critical for tumorgenicity still remains a challenge. Here, we perform a comprehensive functional multi-omics analysis of somatic mutations in breast cancer and identify previously unknown key regulators of breast cancer tumorgenicity. We identify dysregulation of MYCBP2, an E3 ubiquitin ligase and an upstream regulator of mTOR signaling, is accompanied with decreased disease-free survival. We validate MYCBP2 as a key target through depletion siRNA using in vitro apoptosis assays in MCF10A, MCF7 and T47D cells. We demonstrate that MYCBP2 loss is associated with resistance to apoptosis from cisplatin-induced DNA damage and cell cycle changes, and that CHEK1 inhibition can modulate MYCBP2 activity and caspase cleavage. Furthermore, we show that MYCBP2 knockdown is associated with transcriptomic responses in TSC2 and in apoptosis genes and interleukins. Therefore, we show that MYCBP2 is an important genetic target that represents a key node regulating multiple molecular pathways in breast cancer corresponding with apparent drug resistance in our study.

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

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.

Abstract 4262: NR2F1 and SOX9 mediate reprogramming of tumor cells into dormancy: Potential role in dormant bone marrow DTCs

Single disseminated tumor cells (DTCs) found in secondary organs (i.e. bone marrow BM, lung, liver) appear to activate a quiescence program and undergo a dormancy phase triggered by specific tissue microenvironment signals. The timing of this asymptomatic clinical phase may rely on specific signals that convert a previous non-permissive/non-proliferative environment into a permissive one, fueling metastasis development. The mechanisms underlying dormancy and means to maintain this state remain unknown. Here we show that the orphan nuclear receptor NR2F1/COUP-TF1 is silenced in breast and HNSCC tumors, cancer cell lines and MMTV-Neu and -Myc mouse tumor models. Surprisingly, retinoid signaling, stress signaling and microenvironmental cues from target organs (i.e. bone marrow) can induce re-expression of NR2F1 and dormancy of HNSCC cells. Forced NR2F1 re-expression resulted in induction of tumor cell quiescence in vivo and this effect depended on the expression of the transcription factor SOX9 and p27 induction. In contrast, RNAi-mediated silencing of NR2F1 or SOX9 interrupted the quiescence of dormant tumor cells. Further, quiescence induction was associated with p27, p15, p16 and HES-1. Mechanistic analysis revealed that NR2F1 is primarily nuclear and that its expression is activated by MKK6-p38α stress signaling and is negatively regulated by DNA promoter methylation. Lastly, we showed that atRA treatment followed 5-Aza-C treatment reprograms tumorigenic cells into dormancy in an NR2F1-dependent fashion. Our data suggest that re-expression of NR2F1 may promote reprogramming of aggressive tumor cells into dormancy identifying a new target for maintenance therapies for residual disease.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4262. doi:1538-7445.AM2012-4262