Abstract
BACKGROUND: G-quadruplex (G4) structures are enriched in key genomic regions and regulate gene expression and chromosomal stability. However, their role in de novo DNA methylation remains unclear. DNA methyltransferase 3A (DNMT3A) and DNA methyltransferase 3B (DNMT3B) are vital for cancer initiation and progression. This study investigated the role of G4 structures in regulating DNMT3A and DNMT3B expression and their epigenetic function in breast cancer. METHODS: G4 structures in DNMT3A and DNMT3B were identified using Quadruplex forming G-Rich Sequences Mapper, circular dichroism, and gel mobility shift assays. The effects of the G4 stabilizer pyridostatin on breast cancer 4T1 cell proliferation, migration, DNMT3A and DNMT3B expression, and promoter methylation of Breast Cancer 1 (BRCA1), E-cadherin (CDH1), and Ras Association Domain Family Member 1 (RASSF1) were evaluated. RESULTS: G4-forming sequences were found in the core promoter regions of DNMT3A and DNMT3B. They formed parallel G4 structures in vitro. Pyridostatin enhanced G4 stability, inhibited 4T1 cell proliferation and migration, downregulated DNMT3A and DNMT3B expression, reduced promoter methylation of BRCA1 and RASSF1, and altered target gene expression. CONCLUSION: Promoter G4 structures actively regulate gene expression by modulating de novo DNA methylation, suggesting that targeting G4s may represent a novel therapeutic strategy for breast cancer.