The methyltransferases METTL7A and METTL7B confer resistance of cancer cells to thiol-based histone deacetylate inhibitors (ATAC-Seq)

Histone deacetylase inhibitors (HDI) are among the growing class of epigenetic therapies used for the treatment of cancer. Although histone deacetylase inhibitors (HDIs) are effective in the treatment of T-cell lymphomas, solid tumors are largely resistant, and few mechanisms of resistance have been described. Overexpression of the multi-drug resistance gene ABCB1 that encodes P-glycoprotein (P-gp) is known to confer resistance to the HDI romidepsin, yet it is not associated with resistance in patients, suggesting other mechanisms of resistance arise in patients. To identify alternative romidepsin resistance mechanisms, we selected MCF-7 breast cancer cells with romidepsin in the presence of verapamil to reduce the chance of P-gp over-expression developing as a resistance mechanism. The resulting subline, MCF-7 DpVp300, does not express P-gp and was found to be selectively resistant to romidpesin but not other HDIs such as belinostat, panobinostat, or vorinostat. RNA sequencing analysis demonstrated upregulation of the putative methyltransferase, METTL7A, a paralog of which, METTL7B, was found to methylate thiol groups on hydrogen sulfide and captopril. We hypothesized that METTL7A could methylate and inactivate romidepsin as well as other HDIs with a thiol as the zinc binding group. Here we show that expression of METTL7A is necessary for thiol-based HDI resistance in the MCF-7 DpVp300 cell line, and that expression of METTL7A or METTL7B in sensitive cells confers resistance to thiol based HDIs. We thus propose that METTL7A and METTL7B confer resistance to thiol-based HDIs by methylating and inactivating the zinc-binding thiol.