Effects of CpG methylation on the thermal stability of 2, *, and 1 G-quadruplex structures.

In genomic DNA, G-quadruplex (G4)-forming DNA can form either a duplex or G4 structure, suggesting that understanding the factors regulating G4 formation is important for revealing the cellular functions controlled by G4 formation. Cytosine DNA methylation in the CpG islands is known to play an important role in transcriptional regulation. Additionally, CpG methylation increases the thermal stability of G4 structures such as and G4. In this study, we evaluated the effects of CpG methylation in three G4 structures (2, *, and 1) produced by the promoter. Each was analyzed using circular dichroism (CD) melting analysis. The results demonstrate that CpG methylation does not alter the thermal stability of 2 G4 structure when formed in the presence of K; a single-CpG methylation at C1 or C11 decreases the thermal stability of any 2 G4 structure formed in the presence of Na and Mg while methylation at C5 increases the thermal stability; CpG methylation does not alter the thermal stability of 1 or * G4 structures formed in the presence of K; and the 1 and * G4-forming oligonucleotides do not form G4 structures in the presence of Na and Mg. These results provide important clues for understanding the regulatory mechanisms underlying the formation of CpG methylation-induced G4 structures.