Abstract As modulators of chromatin structure and gene expression, histone deacetylase (HDAC) inhibitors are emerging as an important class of epigenetic therapeutics in cancer treatment. These agents exert their activity by targeting HDAC enzymes, primarily through zinc-binding groups (ZBGs) that chelate the catalytic zinc ion essential for HDAC function. The FDA-approved HDAC inhibitor, romidepsin, and some pre-clinical HDAC inhibitors, including ST7612AA1, KD-5170, and NCH-51, are prodrugs that are converted to their active forms containing thiol as ZBGs. We have previously shown that the sulfhydryl methyltransferases, METTL7A and METTL7B, cause the methylation and subsequent detoxification of thiol-containing compounds, including romidepsin. The goal of this study is to evaluate METTL7A and METTL7B as drivers of resistance to ST7612AA1 in the HepG2 liver cancer cell line. Parental HepG2 cells were selected with ST7612AA1 and valspodar continuously for 3 months to generate ST7612AA1-resistant HepG2 cell lines. The resulting resistant cell lines were characterized using cell proliferation, colony formation, and ATP-based viability assays. The expression of METTL7A and METTL7B by the resistant cells was validated by Western blotting. Our result shows that HepG2-STV50, the ST7612AA1-resistant liver cancer cell line that was selected with 50 micromolar ST7612AA1 and 1 micromolar valspodar, had approximately 80% confluency 6 days post-incubation with 10 micromolar ST7612AA1 and showed 8.7-fold resistance to ST7612AA1, compared to the parent HepG2 cell line. HepG2-STV50 also showed cross-resistance (6.7-fold compared to HepG2) to romidepsin. To compare the effect of METTL7A and METTL7B expression on ST7612AA1 in other cell lines, HT1080 cells transfected with METTL7A and METTL7B were treated with ST7612AA1 and KD5170. Compared to the empty vector-transfected HT1080 cells, the METTL7A-overexpressing cells were more resistant (100-fold) than the METTL7B-overexpressing cells (10-fold). Similarly, MCF-7DpVp300, an MCF-7-derived resistant breast cancer cell line that highly expresses METTL7A, which was selected with romidepsin in the presence of the P-gp inhibitor verapamil, showed approximately 100-fold resistance to ST7612AA1 and KD5170, compared to the parent MCF-7 cell line. The ST7612AA1-resistant HepG2 cell lines overexpressed METTL7B more than METTL7A. Since METTL7B does not confer as much resistance as METTL7A, it may be responsible for the relatively low-fold resistance seen for the ST7612AA1-resistant HepG2-STV50 cells compared to the parental HepG2 cells. While HDAC inhibitors have been effective against T-cell lymphomas, they have not shown success in treating solid tumors. This work establishes METTL7A and METTL7B overexpression as a novel mechanism of resistance to thiol-containing HDAC inhibitors and a potential target for the treatment of solid cancer types. Citation Format: Omotola D. Gbadegesin, Robert W. Robey, Michael M. Gottesman. METTL7A and METTL7B confer resistance to the oral thiol-based histone deacetylase inhibitor ST7612AA1 abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1808.
Gbadegesin et al. (Fri,) studied this question.