Abstract
Cytidine deaminase (CDA) overexpression confers susceptibility to epigenetically modified cytidines.
Major finding: Cytidine deaminase (CDA) overexpression confers susceptibility to epigenetically modified cytidines.
Mechanism: CDA deaminates modified cytidines, creating uridine variants that are toxic when incorporated in DNA.
Impact: Cancers that are resistant to cytidine analogues may be responsive to oxidized epigenetic cytidines.
Nucleotides required for DNA replication and repair can be formed by either de novo synthesis or recovery of nucleosides through salvage pathways. Catalyzed by the salvage enzymes, deoxycytidine triphosphate is synthesized by sequential phosphorylation of deoxycytidine. Oxidized epigenetic variants of 5-methyl-2′deoxycytidine (5mdC) have been identified, including 5-hydroxymethyl-2′deoxycytidine (5hmdC), 5-formyl-2′deoxycytidine (5fdC), and 5-carboxyl-2′deoxycytidine (5cadC), but the mechanisms regulating the metabolism of these variants remain unknown. Zauri and colleagues found that DNA polymerases nonselectively incorporated modified and unmodified forms of cytidine into the DNA of human cancer cells, but that the salvage pathway kinase cytidine monophosphate kinase 1 (CMPK1) selectively phosphorylated unmodified cytidine monophosphate, identifying CMPK1 as the main barrier to incorporation of modified cytosine into DNA. When human cancer cell lines were supplemented with 5hmdC or 5fdC, most continued to proliferate at a normal rate, but a subset of cell lines unexpectedly arrested growth. Genomic approaches revealed that the cells that were sensitive to 5hmdC or 5fdC overexpressed cytidine deaminase (CDA), which was necessary and sufficient for the cytotoxicity of modified cytidine variants, and indicated that the level of CDA expression was predictive of this cytotoxicity. In vitro assessment of CDA protein activity determined that CDA deaminates 5hmdC and 5fdC, but not 5cadC, to produce the uridine variants 5hmdU and 5fdU, which, when phosphorylated and incorporated into DNA, led to DNA damage accumulation, cell-cycle arrest, and cell death. In vivo, administration of 5hmdC or 5fdC revealed no toxicity in mice and inhibited the growth of xenograft tumors with CDA overexpression. Overall, these findings suggest that CDA overexpression selectively sensitizes cancer cells to the cytotoxic effects of modified cytidines, and that treatment with these variants may be therapeutically beneficial in cancers that exhibit CDA overexpression or clinical resistance to gemcitabine and other cytidine analogues.