THE SPATIAL TRANSCRIPTOMIC LANDSCAPE OF BREAST CANCER BRAIN METASTASIS

Brain metastases (BM) are responsible for the majority of cancer mortality. Breast cancer is one of the most common primary sites for brain metastasis. Metastatic cancer cells are known to diverge genetically and phenotypically from their primary counterpart. Together with the unique tumor brain microenvironment (TBME), this poses additional challenges in the treatment of BM. METHODS: We identified 30 cases of patient-paired, surgically resected brain metastasis with breast origin. Six cases of non-tumoral brain control were also included. Two tissue microarray (TMA) blocks were constructed to include all cases. Spatial RNAseq was performed while PanCK, CD45, and GFAP were used as morphology markers to annotate the regions of interest (ROIs). For each patient, five ROIs were analyzed: primary breast cancer (BC), primary breast cancer microenvironment (BCME), metastatic tumor cells (MTC), immediate TBME (iTBME), and distant TBME (dTBME). RESULTS: 1) Triple-negative breast cancers (TNBC) demonstrated distinct gene signatures in both primary and metastatic sites. 2) There were 9 BC shifted their genetic profiles and were reclassified into different molecular subtypes at their matched brain metastatic site. Functional enrichment analysis revealed enriched EMT, ECM-receptor interaction, and the complement system in these profile shifting-BC cases. In contrast, the BC cases that preserved their original profiles (non-shifting BC) shared upregulated pathways of ribosome biogenesis and cell cycle. 3) TBME underwent cellular and molecular plasticity characterized by elevated neutrophils, reactive astrocytes, and activated microglia. In the TBME homing TNBC, cancer-associated fibroblasts (CAF) represented a hub in the cellular interaction between MTCs and TBME.