Stabilization of G-quadruplex structure on vascular endothelial growth factor gene promoter depends on CpG methylation site and cation type

Background DNA methylation at the 5-position of cytosine is an epigenetic modification of CpG dinucleotides. In addition to CpG methylation, the G-quadruplex (G4) structure has been reported as a regulator of gene expression. The identification of G4 forming sequences in CpG islands suggests an involvement of CpG-methylated G4 structures in biological processes; however, few reports have addressed the effects of CpG methylation on G4 structure. Methods The thermostability of a methylated, 21-mer G4 structure located on the vascular endothelial growth factor (VEGF) gene promoter containing four CpG sites (C1, C6, C11, and C17) were investigated using circular dichroism (CD) spectral analysis. Results CD melting analysis revealed that VEGF G4 was stabilized by a single CpG methylation on C11 in the presence of Na+ and Mg2+. However, either C1 or C11 methylation enhanced VEGF G4 thermal stability in the presence of K+. Conclusions Single CpG methylation appears to enhance VEGF G4 thermostability in a manner dependent on both the CpG methylation site and cation type. General significance These results are expected to contribute to the elucidation of the roles of CpG methylation-stabilized G4 structures in biological processes.