Bisulfite-free whole-genome mapping of 5-methylcytosine at single-base resolution by NTD-seq

5-Methylcytosine (5mC) is a crucial epigenetic modification which plays a significant role in the regulation of gene expression. Accurate and quantitative detection of 5mC at single-base resolution is essential for understanding its epigenetic functions within genomes. In this study, we develop a novel Naegleria TET-assisted deaminase sequencing (NTD-seq) method for the base-resolution and quantitative detection of 5mC in genomic DNA. The NTD-seq method utilizes a Naegleria TET-like dioxygenase (nTET) to oxidize 5mC, generating 5-methylcytosine oxidation products (5moC). We also engineered a variant of the human apolipoprotein B mRNA-editing catalytic polypeptide-like 3A (A3A), creating an A3A mutant (A3Am). Treatment with A3Am results in the conversion of cytosine to uracil, while 5moC remains unchanged. Consequently, NTD-seq enables the direct deamination of cytosine to uracil by A3Am, which is sequenced as thymine, whereas 5mC, once oxidized to 5moC by nTET, resists deamination and is sequenced as cytosine. Therefore, the cytosines that persist in the sequencing data represent the original 5mC sites. We applied NTD-seq to HEK293T cells, generating a base-resolution map of 5mC that exhibits strong concordance with maps generated by conventional BS-seq. NTD-seq emerges as a powerful, bisulfite-free approach for the single-base resolution mapping of 5mC stoichiometry in genomic DNA.