Role of the lncRNA BORG in Breast Cancer Metastatic Progression and Recurrence

Case Western Reserve University, Cleveland OH

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Abstract

Although metastasis is the most lethal characteristic of breast cancer (BC), our understanding of the molecular mechanisms that govern this event remains incomplete. Interestingly, many BCs disseminate long before their primary tumors become symptomatic. In fact, ~50% of women diagnosed with small tumors of the breast (4 mm) already harbor disseminated carcinoma cells in their bone marrow. Moreover, these micrometastases escape clinical detection by remaining latent for years before reemerging as incurable secondary tumors that are insensitive to chemotherapies that were originally effective against the primary tumor. A major barrier to eradicating BC reflects the paucity of knowledge related to how mammary tumors acquire metastatic and recurrent phenotypes, which underlies the inability of science and medicine to detect and treat latent micrometastases. These knowledge deficits are especially problematic for triple-negative breast cancers (TNBCs), which are highly aggressive and prone to rapid relapse; they also lack FDA-approved targeted therapies necessary to improve their dismal overall survival rates. Long noncoding RNAs (lncRNAs) have recently emerged as powerful global regulators of chromatin remodeling and gene expression in diverse physiological settings, including cell and tissue homeostasis, embryogenesis and development. Moreover, an ever expanding array of scientific evidence related to the pathophysiology of lncRNAs in human disease led us to postulate that developing and progressing TNBCs hijack the global chromatin reprogramming ability of lncRNAs, thereby eliciting emergence from metastatic latency and initiating lethal disease recurrence. Accordingly, we identified BORG (BMP/OP-Responsive Gene (BORG), as a powerful oncogenic lncRNA whose aberrant expression correlated with the acquisition of EMT (epithelial-mesenchymal transition) and metastatic phenotypes in (a) human and murine TNBCs cells; (b) human breast tumors and their corresponding CNS metastases; and (c) patient-derived xenograft (PDX) models of human BC as compared to normal breast epithelial cells. Additionally, BORG is sufficient in driving latent disseminated TNBCs cells to reactivate proliferative programs both in vitro and in vivo, events associated with epigenomic reprogramming operant in repressing cellular senescence programs, and in activating cell survival programs. Based on these and other preliminary findings, we hypothesize that BORG drives TNBC metastasis and recurrence by (i) remodeling the epigenome to reactivate proliferative programs that circumvent quiescence- and senescence-associated transcriptomes, and (ii) promoting the induction of survival signaling systems coupled to the acquisition of chemoresistant phenotypes. These hypotheses will be addressed by two Specific Aims. Aim 1 will determine the mechanisms whereby BORG:TRIM28 complexes form and drive TNBC metastasis and recurrence. We will identify the minimal BORG determinants necessary to bind TRIM28, as well as the domains in TRIM28 that bind BORG. Additionally, we will map the chromatin alterations provoked by BORG by performing TRIM28 and H3K4me1 ChIP-seq analyses in BORG-proficient and -deficient TNBCs, findings that will be validated in human breast cancer specimens. Aim 2 will determine the mechanisms whereby BORG:RPA1 complexes and NF-kB promote TNBC survival and chemoresistance. Similar to Aim 1, we will identify the minimal BORG sequences necessary to bind RPA1, and conversely, the regions in RPA1 that interact with BORG. The impact of preventing BORG:RPA1 complex formation on TNBC survival and chemoresistance will be determined, as will the role of ATM in regulating the interplay between TRIM28 and RPA1 when bound to BORG. Finally, the impact of these events in mediating TNBC resistance to cytotoxic chemotherapy will be assessed in vitro, and in preclinical therapy trials. Collectively, the findings obtained in this innovative application will provide novel molecular insights into how BORG drives TNBC to acquire metastatic, recurrent, and chemoresistant phenotypes.