DNA methylation as an oncogenic driver in breast cancer: Therapeutic targeting via epigenetic reprogramming of DNA methyltransferases.

The malignant manifestation of breast cancer is driven by complex molecular alterations that extend beyond genetic mutations to include epigenetic dysregulation. Among these, DNA methylation is a critical and reversible epigenetic modification that significantly influences breast cancer initiation, progression, and therapeutic resistance. This process, mediated by DNA methyltransferases (DNMTs), involves the addition of methyl groups to cytosine residues within CpG dinucleotides, resulting in transcriptional repression of genes. Aberrant methylation patterns are characterized by promoter hypermethylation of tumor suppressor genes (BRCA1, CDH1, PTEN, RARβ), which leads to their suppression, and global hypomethylation that activates oncogenes and promotes genomic instability. Central to these events is the dysregulation of DNA methyltransferases which drive maladaptive epigenetic programming. Nucleoside DNMT inhibitors (DNMTis) incorporate into DNA to trap and deplete DNMTs, while non-nucleoside DNMTis avoid DNA integration and directly target DNMT through catalytic site binding, cofactor competition, DNMT-DNA interaction disruption, or DNMT degradation to achieve improved selectivity and reduced toxicity. Preclinical and clinical studies demonstrate that DNMTis not only restore the expression of silenced tumor suppressor genes but also enhance the efficacy of chemotherapy, sensitize homologous recombination-deficient tumors to PARP inhibitors, and synergize with histone deacetylase inhibitors and immunotherapies. Notably, DNMTi monotherapies have shown significant antitumor effects in breast cancer models. Furthermore, methylation profiling holds translational potential as predictive biomarker, particularly in triple-negative breast cancer, where DNMT inhibition may expand therapeutic opportunities. This review highlights the mechanistic underpinnings of DNMT dysregulation, the therapeutic landscape of DNMT inhibitors, and the integration of methylation profiling into precision oncology.