5caC

5-carboxylcytosine (5caC) is a modified form of cytosine in DNA in which a carboxyl group is added to the fifth carbon of the pyrimidine ring. In mammals, 5caC is generated through the action of TET family dioxygenases that oxidize 5-methylcytosine (5mC) in a stepwise process: 5mC → 5-hydroxymethylcytosine (5hmC) → 5-formylcytosine (5fC) → 5caC. Among these oxidation products, 5caC is the least abundant under normal conditions but can accumulate under certain circumstances, such as inhibition of downstream repair steps.

Biologically, 5caC is considered an intermediate in active DNA demethylation. It can be removed from DNA by thymine DNA glycosylase (TDG) during base excision repair, creating an abasic site that is subsequently repaired to unmodified cytosine. This makes 5caC part of a dynamic pathway that modulates DNA methylation status and gene regulation rather than a static epigenetic mark. The distribution of 5caC in genomes has been observed to vary with development and tissue type, and it is often associated with gene regulatory regions such as promoters and enhancers in some cell types, though patterns are not universal.

Detection of 5caC relies on specialized sequencing and chemical labeling techniques capable of distinguishing it from other cytosine forms; conventional bisulfite sequencing cannot. Research on 5caC continues to explore its precise genomic localization, regulatory roles, and potential involvement in development and disease, including cancer, as part of the broader study of TET-mediated DNA oxidation and active demethylation.