Circular RNA dynamics in breast-to-brain metastatic cascade

Breast-to-brain metastases (BTB mets) represent one of the most aggressive and clinically challenging manifestations of breast cancer. The limited efficacy of available therapies and poor patient outcomes reflect the intricate molecular and cellular processes underlying metastatic colonization of the brain. The molecular events driving this process involve extensive transcriptomic reprogramming, encompassing not only protein-coding genes but also diverse classes of non-coding RNAs. Circular RNAs (circRNAs) are a distinct class of stable, non-coding transcripts increasingly recognized as key regulators of cancer biology, yet their role in brain-tropic metastasis remains poorly defined. This study aimed to systematically profile circRNA expression across primary tumors and metastatic sites to uncover regulatory mechanisms and define site-specific circRNA expression patterns associated with BTB mets.

CircRNAs displayed highly site-specific expression patterns during metastatic breast cancer progression. Notably, the majority of differentially expressed circRNAs were uncoupled from changes in their linear host genes, highlighting transcriptionally-independent circularization as a regulatory mechanism. Notably, the majority of circRNAs distinguishing BTB mets from primary breast tumors and pleural effusions likewise distinguished them from primary brain tumors (glioblastoma). Analysis of the circRNA landscape revealed 42 circRNAs that are consistently deregulated and characteristic of BTB mets. Subsequent functional analysis of circRNA-microRNA interactions revealed that these circRNAs are enriched in metastatic signaling pathways and brain-related biological processes, suggesting a potential role for circRNA-mediated regulation in adaptation to the brain microenvironment. Additionally, circRNA landscapes exhibit high stability under conditions that promote either stemness or differentiation. To support data exploration, Breast circScope (circrna.netlify.app), an open-access resource that enables analysis of circRNA expression and host gene associations, along with curated visualization tools, was developed.

Presented findings uncover a previously unrecognized dimension of post-transcriptomic regulation in metastatic breast cancer, demonstrating that circRNAs exhibit site-specific expression patterns associated with metastatic progression. These results highlight circRNAs as integral molecules in the regulatory landscape of breast cancer metastasis and provide a framework for understanding their functional contribution to brain-tropic disease.