Upregulation of TET2 and Resistance to DNA Methyltransferase (DNMT) Inhibitors in DNMT1 -Deleted Cancer Cells.

Simple Summary: Ten-eleven-translocation (TET) 2 is implicated in human cancers such as lung, breast, skin, and kidney cancers, T-cell lymphomas, and leukemia. It is unclear whether TET2 is involved in the re-expression of the p16 tumor suppressor, which was partially silenced by CDKN2A gene methylation. The effect of DNA methyltransferase (DNMT) 1 gene status on TET2 expression following DNMT inhibitor treatment remains unknown in cancer. We found that deleting the DNMT1 gene made cancer cells prone to increased TET2 expression and re-expression of p16 after drug treatment. TET2 is involved in the demethylation of the CDKN2A gene and activation of p16 expression, and concomitant resistance to DNMT inhibitors when DNMT1 is obliterated. The long-sought data for the first time revealed a link between TET2 expression and the activation of p16 expression. The findings should have an impact on reactivating tumor suppressors and cancer treatment. Background: Ten-eleven-translocation (TET) 2 is a member of the TET family of proteins (TET1-3). DNMT1 gene deletion confers resistance to DNA methyltransferase (DNMT) inhibitors in colorectal, breast, and ovarian cancer cells. Currently, the effect of DNMT1 gene status on TET2 phenotype following DNMT inhibitor treatment is unclear in human malignancies. Methods: Human colorectal carcinoma HCT116 cells (DNMT^+/+) and their isogenic DNMT1 knockout (DNMT1^–/–) counterpart were treated with DNMT inhibitors. Expression of TET2 and tumor suppressor (p16^ink4A and p15^ink4B) proteins were examined by Western blot. Apoptosis and CDKN2A promoter demethylation following drug treatment were detected by Annexin-V apoptosis assay and methylation-specific PCR. Results: TET2 expression was robustly increased in DNMT1^−/− cells by 0.5 µM and 5 µM decitabine and azacitidine treatment. Augmentation of TET2 expression was accompanied by re-expression of p16^ink4A and p15^ink4B proteins and CDKN2A promoter demethylation. TET2 upregulation and tumor suppressor re-expression were associated with resistance conferred by DNMT1 deletion. Treatment with 5-aza-4′-thio-2′-deoxycytidine at a low 0.5 µM dose only upregulated TET2 and reduced CDKN2A promoter methylation, and re-expression of p16^ink4A in DNMT1^−/− cells. DNMT inhibitors showed minimal effects on TET2 upregulation and re-expression of tumor suppressor proteins in cells with intact DNMT1. Conclusions: DNMT1 gene deletion made cancer cells prone to TET2 upregulation and activation of tumor suppressor expression upon DNMT inhibitor challenge. TET2 augmentation is concomitant with resistance to DNMT inhibitors in a DNMT1-deleted state. [ABSTRACT FROM AUTHOR]