High-precision mapping reveals rare N6-deoxyadenosine methylation in the mammalian genome

DNA modification has been characterized as a critical epigenetic regulator in gene expression. N6-deoxyadenosine methylation (6mA) is the most widespread type in prokaryotes and some unicellular eukaryotes. Nevertheless, the lack of a precise and comprehensive mapping method greatly hinders studying the presence and biological significance of 6mA in mammals. Here, we develop a new method MM-seq (Modification-induced Mismatch Sequencing) for genome-wide 6mA mapping in single-base resolution. This method balances accuracy and sensitivity based on a novel detection principle. We find modified DNA bases are likely to flip out of the DNA duplex, forming a mismatch-like structure that an antibody can capture. The mismatch-like structure can be recognized by endonuclease or exonuclease, which marks the exact modified site for readout via sequencing. Using this method, we examine the genomic positions of 6mA in bacteria, green algae, and mammalian cells. We find that human and mouse genomes contain rare and randomly distributed 6mA sites, with almost no overlap between different cell types. After knock-out of the RNA m6A methyltransferase Mettl3, limited 6mA sites in gDNA are diminished. Our results support the perspective that rare 6mA in the mammalian genome is caused by the misincorporation of N6-methyladenine via the nucleotide-salvage pathway. We develop a new method MM-seq (Modification-induced Mismatch Sequencing) for genome-wide 6mA mapping in single-base resolution, and applied it in Chlamydomonas reinhardtii, E. coli, human and mouse cell lines.