Abstract 1294: Loss of bromodomain-7 (BRD7) promotes breast cancer lung metastasis by reprogramming the tumor immune microenvironment
Authors: Mondal Jayanta , Zhang Junfeng , Giancotti Filippo , Huse Jason 
Published: April 2023

Abstract
Introduction: Thwarting metastasis is widely regarded as the “holy-grail” in the treatment of cancer. Despite epigenetic alterations being a major causal factor in promoting tumorigenesis, relatively little is known about key epigenetic events that drive breast cancer lung metastasis. Additionally, the molecular mechanism(s) by which the loss of SWI/SNF complex subunits-like Bromodomain-7 (BRD7)- induces incipient breast cancer cells to metastasize in the lung remains to be elucidated.
Methods: We established a novel in vivo screening platform which enabled the identification of specific epigenetic entities that regulate metastatic reactivation. RNA-seq enabled the identification of the differential gene expression/signaling pathway profiles that govern metastatic reactivation upon loss of BRD7. Furthermore, tumor spheres and migration/invasion assays were performed. To decipher the modifications in the chromatin accessibility upon loss of BRD7, ChIP-seq and ATAC-seq were conducted. A magnetic bead-based Luminex cytokine assay informed us about the regulation of cytokines upon loss-of-BRD7. In vivo experiments were performed in the 4TO7-TGL and D2A1-d dormancy models and the NSG immunodeficient mice. FACS immune-phenotyping coupled with scRNA-seq was performed to comprehend the changes in the tumor immune microenvironment upon loss of BRD7.
Results: The in vivo loss-of-function epigenetic screen revealed that loss of BRD7 led to breast cancer lung metastasis. Interestingly, RNA-seq revealed that BRD7 knockout promotes the expression of genes involved in inflammation, hypoxia and EMT. Intriguingly, the top signatures enriched in BRD7-silenced cells were IL6-JAK-STAT3 signaling and TNF-α signaling. Further, ATAC-seq and ChIP-seq experiments indicated that inactivation of BRD7 causes an increased chromatin accessibility at enhancer sites that were enriched for interferon-regulated immune response elements. Additionally, we found that BRD7-inactivation induces expression of pre-metastatic cytokines such as IL6, IL33, and CXCL12. Finally, the loss of BRD7 led to the up-regulation of the tumor-promoting neutrophils and CD8+ T exhausted cells while down-regulating the tumor-suppressive M1 macrophages, CD8+ Teff and Tmemory cells and the dendritic cell population in vivo.
Conclusion: Our novel genomic screening platform shall enable the identification of specific genes that mediate breast cancer lung metastasis. Additionally, our findings that BRD7 is a suppressor of breast cancer lung metastasis, and a predictive cancer biomarker could have major implications in the formulation of myriad chemo-therapeutic strategies for metastatic cancers. Taken together, we strongly anticipate that our study will bring forth a paradigm shift in our understanding of how epigenetic regulators, like BRD7, mechanistically regulate breast cancer lung metastasis.
Citation Format: Jayanta Mondal, Junfeng Zhang, Filippo Giancotti, Jason Huse. Loss of bromodomain-7 (BRD7) promotes breast cancer lung metastasis by reprogramming the tumor immune microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1294.