Cytosine methylation within the 5'-C-phosphate-G-3' sequence of nucleotides (called CpG methylation) is a well-known epigenetic modification of genomic DNA that plays an important role in gene expression and development. CpG methylation is likely to be altered in the CpG islands. CpG islands are rich in cytosine, forming a structure called the i-motif via cytosine-cytosine hydrogen bonding. However, little is known about the effect of CpG methylation on the i-motif. In this study, The CpG methylation-induced structural changes on the i-motif was examined by thermal stability, circular dichroism (CD) spectroscopy, and native-polyacrylamide gel electrophoresis (Native-PAGE) evaluation of five i-motif-forming DNAs from four cancer-related genes (VEGF, C-KIT, BCL2, and HRAS). This research shows that CpG methylation increased the transitional pH of several i-motif-forming DNAs and their thermal stability. When examining the effect of CpG methylation on the i-motif in the presence of opposite G4-forming DNAs, CpG methylation influenced the proportion of G4 and i-motif formation. This study showed that CpG methylation altered the stability and structure of the i-motif in CpG islands.
Keywords: CpG islands; CpG methylation; G-quadruplex; epigenetics; i-motif; pH dependance; structural change; thermal stability.