5-aza-2’-deoxycytidine (DAC) is approved by the US Food and Drug Administration for the treatment of myelodysplastic syndrome. It is also being tested in Phase I and II clinical trials for efficacy against other malignancies, including brain, prostate, breast, and lung cancers. Identifying modifier genes that could confer resistance to DAC treatments is, therefore, essential in realizing the full potential of DAC as a chemotherapeutic agent. DAC is a cytosine analog that is incorporated into newly synthesized DNA, where it can prevent propagation of DNA methylation by irreversibly trapping DNA methyltransferases. The consequent reversal of epigenetic silencing of tumor suppressor genes contributes to tumor cell death in the therapeutic context. Previous studies have identified relatively few determinants of DAC resistance, including the equilibrative nucleoside transporters, deoxycytidine kinase, and cytosine deaminase. While these genes are logical candidates because of their functions in cellular pyrimidine uptake and metabolism, our large-scale, unbiased random insertional mutagenesis screen in human cancer cell lines have identified additional, unexpected loci involved in DAC resistance. Our initial screen produced 9 mutant clones resistant to DAC at concentrations of 0.1μM and 1μM. In particular, several of the insertional mutants targeted components of the DNA damage response pathway, including ubiquitin conjugating enzymes and Fanconi anemia complementation group genes, indicating that the primary mechanism of killing by DAC may not be DNA demethylation. This is consistent with the observation that DNA double-strand breaks are among the consequences of DAC treatments. Ongoing functional studies of the mutant clones also suggest that cancer cells with increased efficiency at repairing genomic DNA damage may survive DNA demethylation in the presence of DAC. These results not only inform us of the mechanism of action of DAC-mediated cytotoxicity, our findings also have important clinical implications as these genetic modifiers are also valuable biomarkers and therapeutic targets for achieving personalized cancer treatments involving DAC.

Citation Format: Angela H. Ting, Lars Matkin. Unconventional genetic determinants of resistance to 5-aza-2’-deoxycytidine. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4620. doi:10.1158/1538-7445.AM2013-4620