Modular affinity-labeling of the cytosine demethylation base elements in DNA

5-methylcytosine is the most studied DNA epigenetic modification, having been linked to diverse biological processes and disease states. The elucidation of cytosine demethylation has drawn added attention the three additional intermediate modifications involved in that pathway—5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine—each of which may have distinct biological roles. Here, we extend a modular method for labeling base modifications in DNA to recognize all four bases involved in demethylation. We demonstrate both differential insertion of a single affinity tag (biotin) at the precise position of target elements and subsequent repair of the nicked phosphate backbone that remains following the procedure. The approach enables affinity isolation and downstream analyses without inducing widespread damage to the DNA.