Molecular Mechanisms Associated with Brain Metastases in HER2-Positive and Triple Negative Breast Cancers

SIMPLE SUMMARY: Breast cancer, the most common malignant tumor among women worldwide, remains an incurable disease once it has spread to the brain. Past research has shown that a primary breast cancer’s biology is an important determining factor predisposing its ability to form brain metastases. This review summarizes our current understanding of which genes, mutations, and molecules cause this increased ability to spread to and survive in the brain, specifically focusing on the different stages of this process. This knowledge may help us develop more effective, tumor-specific therapies and, as such, increase the chance of recovery for patients with breast cancer brain metastases. ABSTRACT: Breast cancer (BC) is the most frequent cause of cancer-associated death for women worldwide, with deaths commonly resulting from metastatic spread to distant organs. Approximately 30% of metastatic BC patients develop brain metastases (BM), a currently incurable diagnosis. The influence of BC molecular subtype and gene expression on breast cancer brain metastasis (BCBM) development and patient prognosis is undeniable and is, therefore, an important focus point in the attempt to combat the disease. The HER2-positive and triple-negative molecular subtypes are associated with an increased risk of developing BCBM. Several genetic and molecular mechanisms linked to HER2-positive and triple-negative BC breast cancers appear to influence BCBM formation on several levels, including increased development of circulating tumor cells (CTCs), enhanced epithelial-mesenchymal transition (EMT), and migration of primary BC cells to the brain and/or through superior local invasiveness aided by cancer stem-like cells (CSCs). These specific BC characteristics, together with the ensuing developments at a clinical level, are presented in this review article, drawing a connection between research findings and related therapeutic strategies aimed at preventing BCBM formation and/or progression. Furthermore, we briefly address the critical limitations in our current understanding of this complex topic, highlighting potential focal points for future research.