Subtype-Independent Dysregulation of the Notch Signaling Pathway and Its miRNA Regulators in Breast Cancer

<b>Background/Objectives</b>: The Notch signaling pathway regulates cell fate, proliferation, and differentiation, and its dysregulation has been implicated in various cancers, including breast cancer. MicroRNAs (miRNAs) are critical post-transcriptional regulators that can modulate Notch pathway components. The aim of this study was to identify miRNAs that may potentially regulate the expression of Notch pathway-related genes across five molecular subtypes of breast cancer in Polish women. <b>Methods</b>: Tumor and adjacent normal tissue samples were collected from 405 patients with five breast cancer subtypes: luminal A (<i>n</i> = 130), HER2-negative luminal B (<i>n</i> = 100), HER2-positive luminal B (<i>n</i> = 96), non-luminal HER2-positive (<i>n</i> = 36), and triple-negative breast cancer (<i>n</i> = 43). Gene expression was profiled using mRNA microarrays and validated with RT-qPCR and ELISA. Candidate regulatory miRNAs were identified by miRNA microarrays and confirmed using the miRDB database. <b>Results</b>: <i>APH1A</i>, <i>CTBP1</i>, <i>DTX1</i>, <i>HEY1</i>, <i>HEY2</i>, <i>JAG2</i>, <i>NOTCH4</i>, <i>TLE2</i>, and <i>TLE4</i> were consistently dysregulated across all breast cancer subtypes. Overexpression of <i>HEY1</i> and <i>JAG2</i> may be driven by decreased levels of miR-145, miR-98, and miR-381. Conversely, downregulation of <i>TLE4</i> may be associated with elevated expression of miR-196a and miR-155. No regulatory miRNAs meeting the selection criteria were identified for <i>APH1A</i>, <i>CTBP1</i>, <i>DTX1</i>, <i>HEY2</i>, <i>NOTCH4</i>, or <i>TLE2</i>. <b>Conclusions</b>: The consistent alterations suggest the presence of a shared Notch-driven oncogenic signature in breast cancer, potentially driving cell proliferation, stemness, and resistance to therapy. These findings enhance our understanding of Notch signaling in breast cancer and propose novel miRNA-Notch interactions as candidate targets for therapeutic intervention.