Sensitive and Label-Free DNA Methylation Detection by Ligation-Mediated Hyperbranched Rolling Circle Amplification

Sensitive and specific detection of DNA methylation in CpG sites of genomic DNA is imperative for rapid epigenetic evaluation and early cancer diagnosis. Here, we employ for the first time the thermostable ligation for methylated DNA discrimination and hyperbranched rolling circle amplification (HRCA) for signal enhancement, without the need for restriction enzymes, PCR amplification, or fluorescence-labeled probes. After bisulfite treatment of methylated DNA, the methylation-specific linear padlock probe can be circularized only in the presence of methylated DNA and serves subsequently as a template for HRCA, whose products are easily detected using SYBR Green I and a standard fluorometer. While in the presence of unmethylated DNA, the linear padlock probe cannot be circularized because of the defectively matched substrate, and no HRCA occurs. This ligation-mediated HRCA-based method exhibits excellent specificity and high sensitivity with a detection limit of 0.8 fM and a detection range of 4 orders of magnitude, and it can even distinguish as low as 0.01% methylation level from the mixture, which is superior to most currently used methods for DNA methylation assay. This method can be further applied to analyze genomic DNA in human lung cancer cells.