Abstract 3264: Targeting bone marrow-resident dormant CTCs to overcome metastasis
Authors: Boral Debasish , Liu Haowen N. , Yin Wei , Vishnoi Monika , Scamardo Antonio T. , Hong David S. , Marchetti Dario 
Published: August 2018

Abstract
Metastatic “seeding” is an early event initiated when cancer cells are shed from the primary tumor, enter into the blood stream as circulating tumor cells (CTCs), and travel to distant organs where they can remain dormant as disseminated cancer cells (DTCs) for indefinite lengths of time. The organ serving as the foremost reservoir for DTCs is the bone marrow (BM). Despite widespread knowledge that bone marrow-resident breast cancer cells (BMRCs) affect tumor progression, signaling mechanisms of BMRCs implicated in maintaining long-term dormancy have not been characterized. This is caused by the lack of biomarkers that can identify and isolate viable but dormant BMRCs not expressing neoplastic and/or proliferation markers.
To overcome these hurdles, we developed a patient-derived CTC model of clinical dormancy. We hypothesized that a significant distinction exists between signaling pathways of BM-homing vs metastasis-competent CTCs upon transplantation in xenografts. The model was established by three sequential steps: (a) isolation of CTC-enriched cell populations from the peripheral blood of patients with metastatic breast cancer, followed by CTC implantation in NOD-SCID-gamma mice; (b) in vivo depletion of “normal” human cell lineages over 4-8 months (before onset of overt metastasis), and (c) capture and characterization of viable human CTCs from blood and BM of mice via multi-parametric flow cytometry. This strategy was implemented on a set of breast cancer patients stratified according to their primary tumor subtype (ER+/PR+, HER2+, and ER-/PR-/HER2-), and gene-signatures of putative ex vivo CTCs and BMRCs were obtained by whole genome transcriptomic arrays.
First, comparative analyses of ex vivo vs de novo CTC-gene signatures identified increased mTORC2 along with decreased mTORC1 signaling activity as the most significant characteristic of human BM-resident CTCs. mTORC2/mTORC1 represent the two complementary arms of mTOR signaling - a critical pathway frequently dysregulated in breast cancer and implicated in cell survival and dormancy. Second, heightened mTORC2 downstream targets augmented quiescent CTC populations (Ki67-/RBL2+ cells) in paired metastatic vs primary breast cancer tissues. Third, IHC analyses of breast cancer CTC xenograft tissues showed that solitary BM and tissue-resident breast cancer CTCs had high mTORC2 activity. Finally, shRNA knockdown of Rictor-the essential component of mTORC2 actions, increased Ki67/PCNA expression and proliferation.
Collectively, these findings suggest that the balance between mTORC1 vs mTORC2 signaling pathways regulate CTC-associated mitotic and/or dormancy characteristics. Further elucidation of mTOR-mediated CTC dormancy will provide novel strategies for therapeutic interventions in breast cancer patients having metastatic “seeding” but yet to develop overt metastasis.
Citation Format: Debasish Boral, Haowen N. Liu, Wei Yin, Monika Vishnoi, Antonio T. Scamardo, David S. Hong, Dario Marchetti. Targeting bone marrow-resident dormant CTCs to overcome metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3264.