Sequencing the sixth base (5-hydroxymethylcytosine): selective DNA oxidation enables base-pair resolution.

the family of ten-eleven-translocation (TET) oxygenases converts 5mC to 5hmC. Last year, the two more highly oxidized congeners of 5hmC, 5-formylcytosine and 5-carboxycytosine (5fC and 5caC respectively), were also discovered as 5mC TET oxidation products. The race to disentangle the roles that these “new” DNA bases play, both as epigenetic markers and as intermediates in demethylation pathways, is on. The bisulfite-mediated deamination of cytosines to uridines has played a crucial role in understanding DNA methylation. Shapiro and Hayatsu independently reported and subsequently performed detailed investigations of this reaction over 40 years ago on single nucleotides (Scheme 1a). They showed that bisulfite readily adds to C