Targeting of DNMT3B in the plasma cell cancer multiple myeloma disturbs cancer cell growth and clonogenicity and boosts the anti-tumor activity of the proteasome inhibitor bortezomib.

Objectives
Multiple myeloma (MM) is an incurable plasma cell cancer due to the development of drug resistance (DR). Epigenetic defects are well-known to play a major role in MM progression and relapse. However, the role of most epigenetic modifiers (epiplayers) in MM cell DR development remains unknown. We found that the epiplayer DNMT3B is significantly increased in the relapsed setting from the MMRF CoMMpass study, suggesting a role in MM relapse. Here, we explored the role of DNMT3B in MM cell biology and drug response.

Results
Using publicly available gene expression profiling data, we found that DNMT3B mRNA levels increase during disease progression, correlating with a worse disease outcome in both newly diagnosed and relapsed patients, further indicating its role in MM progression and DR. In line, the DNMT3B specific inhibitor Nanaomycin A (NA) led to a significant and dose-dependent decrease in cell viability and proliferation and increase in apoptosis in the XG-2, XG-7, and AMO-1 human cell lines. We also validated the anti-MM activity of NA on primary human MM cells. Next, the effect of DNMT3B targeting on clonogenicity was evaluated using a colony formation assay. A significant and dose-dependent decrease in the number of colonies was observed when low doses of NA were added to AMO-1 (100 and 200 nM) and XG-2 (30 and 50 nM) on the day of plating, but not when added 7 days after plating. In contrast, treatment with a high dose (800 nM) significantly reduced the number of colonies at both timepoints, indicating that high doses of NA are cytotoxic whereas low doses impair MM cell proliferation and clonogenicity. The anti-clonogenic effect of DNMT3B targeting was also confirmed by DNMT3B knockdown using inducible shRNA lentiviral vectors in AMO-1 cells. Finally, combining NA (100 nM) with bortezomib (4 nM) resulted in a significant decrease in colony formation compared to both single agents.

Conclusion
Together, our findings indicate that DNMT3B is a novel promising target to overcome or delay relapse in MM. In the near future, the anti-myeloma activity of DNMT3B targeting will be validated in vivo and the underlying mechanisms will be further determined.