Epigenetic Plasticity in Triple-Negative Breast Cancer: Mechanisms of Therapy Resistance, Biomarkers, and Therapeutic Vulnerabilities

Triple-negative breast cancer (TNBC) is an aggressive and clinically heterogeneous breast cancer subtype characterized by the absence of estrogen receptor, progesterone receptor, and HER2 overexpression, limited targeted treatment options, early relapse, and frequent development of therapy resistance. Although TNBC often shows initial sensitivity to chemotherapy, durable responses are commonly undermined by the emergence of adaptive resistant cell states rather than solely by fixed genetic mutations. This review synthesizes the role of epigenetic plasticity as a central mechanism that enables TNBC cells to dynamically reprogram transcriptional identity, survive therapeutic stress, and transition between epithelial, mesenchymal, stem-like, immune-evasive, and drug-tolerant persister phenotypes. Key epigenetic mechanisms include aberrant DNA methylation, histone acetylation and methylation, BET/BRD4-dependent transcriptional regulation, EZH2-mediated repression, SWI/SNF-dependent chromatin remodeling, non-coding RNA networks, and three-dimensional genome reorganization. These processes regulate tumor suppressor silencing, DNA-damage repair, epithelial–mesenchymal plasticity, cancer stem-cell maintenance, metabolic adaptation, immune-checkpoint regulation, and minimal residual disease. The review also highlights the translational relevance of epigenetic biomarkers, including DNA methylation signatures, circulating epigenetic markers, chromatin-accessibility profiles, and single-cell epigenomic approaches for diagnosis, prognosis, therapy prediction, and monitoring resistance evolution. Finally, therapeutic strategies targeting epigenetic plasticity are discussed, including DNMT, HDAC, BET, EZH2, KDM, and LSD1 inhibitors, with emphasis on rational combination approaches involving chemotherapy, PARP inhibitors, immunotherapy, and metabolic targeting. Overall, epigenetic plasticity represents both a major driver of TNBC resistance and a therapeutically exploitable vulnerability, provided those future strategies account for tumor heterogeneity, adaptive cell-state transitions, biomarker-guided patient selection, and combination-based treatment design.