Abstract
Reversing abnormal gene silencing in cancer cells due to DNA hypermethylation of promoter CpG islands may offer new cancer prevention or therapeutic approaches. Moreover, such approaches may be broadly applicable to enhance the efficacy of radiotherapy, chemotherapy, or immunotherapy. Here, we demonstrate the powerful utility of a novel gene reporter system to permit studies of the dynamics, mechanisms, and translational relevance of candidate therapies of this type in human colon cancer cells. The reporter system is based on in situ modification of the endogenous locus of the tumor-suppressor gene SFRP1, a pivotal regulator of the Wnt pathway that is silenced by DNA hypermethylation in many colon cancers. The modified SFRP1-GFP reporter allele used remained basally silent, like the unaltered allele, and it was activated only by drug treatments that derepress gene silencing by reversing DNA hypermethylation. We used the established DNA methyltransferase inhibitor (DNMTi) 5-aza-deoxycitidine (DAC) to show how this system can be used to address key questions in the clinical development of epigenetic cancer therapies. First, we defined conditions for which clinically relevant dosing could induce sustained induction of RNA and protein. Second, we found that, in vivo, a more prolonged drug exposure than anticipated was essential to derepress gene silencing in significant cell numbers, and this has implications for generating effective anticancer responses in patients with hematopoietic or solid tumors. Finally, we discovered how histone deacetylase inhibitors (HDACi) alone, when administered to cells actively replicating DNA, can robustly reexpress the silenced gene with no change in promoter methylation status. Taken together, our findings offer a new tool and insights for devising optimal clinical experiments to evaluate DNMTi and HDACi, alone or in combination, and with other cancer treatments, as agents for the epigenetic management and prevention of cancer.