BSBM-05 CIRCULAR <i>MALAT1</i> FUELS BREAST-TO-BRAIN METASTASES: A NEW AVENUE FOR THERAPEUTIC INTERVENTION

Abstract Breast-to-brain metastatic (BTB-met) cancer remains a formidable challenge for modern medicine and science. As research on protein-coding genes has led to only incremental advances in developing anticancer therapies, the focus has increasingly shifted toward non-coding RNAs, particularly the recently discovered circular RNAs (circRNAs). Importantly, metastatic lesions in distant organs such as the brain are the main cause of breast cancer-related deaths rather than the primary tumor itself. To investigate the role of circRNAs in BTB-met cancer, an analysis of the circular RNA expression profile in patient-derived BTB-met cancer cells (n=4) and neural progenitor cells (NPCs) (n=3) using an Arraystar™ microarray approach was performed. Out of a total of 12,660 circRNAs detected, there were 547 circRNAs identified as significantly deregulated in BTB-met cells compared to NPCs upon applying a cutoff of fold change ≥2 and p-value &amp;lt;0.05. CircMALAT1 was significantly upregulated in BTB-met compared to NPC (p&amp;lt;0.0001) and normal breast epithelial cells (p&amp;lt;0.001, qPCR). The deregulation of the cell cycle, altered cell shape, decreased cell migration and invasion, and lower average speed of cells suspended in the extracellular matrix were apparent in BTB-met cells with stable circMALAT1 knockdown. Next-generation sequencing further identified significant deregulation of 30 mRNAs and 420 long non-coding RNAs after circMALAT1 knockdown, including the upregulation of key tumor and metastasis suppressors such as CLDN6, FRZB, ABI3BP, and LINC-PINT. Additionally, circMALAT1 knockdown increased BTB-met cell sensitivity to low-dose chemotherapy. NSG mice implanted with circMALAT1-knockdown BTB-met cells exhibited a delayed onset of tumor formation compared to controls. In turn, overexpression of circMALAT1 in normal breast epithelial cells reduced Ki67-negative cells (11.5% vs. 1.9%), enhanced migration and invasion, and amplified TGFβ-induced epithelial-to-mesenchymal transition. In summary, the findings demonstrate that circMALAT1 drives the metastasis of breast cancer to the brain, highlighting the potential of non-coding RNA-based approaches for innovative treatment strategies.