The DNA methylation inhibitor decitabine induces DNA damage related apoptosis in multiple myeloma, which is amplified by the HDAC-inhibitor JNJ-26481585.

5-aza-2’-deoxycytidine or decitabine (DAC) is a DNA methylation inhibitor (DNMTi) that has been approved for the treatment of myelodysplastic syndrome. DAC has been shown to induce anti-tumor responses which can be linked to the re-expression of silenced tumor suppressor genes. Moreover, DAC can also induce DNA damage, cell cycle arrest and apoptosis. The effect of DAC on these latter processes have not been thoroughly studied in multiple myeloma (MM), a plasma cell malignancy. Here, we investigated the anti-myeloma effects of DAC on different MM cell lines (OPM-2, RPMI 8226 and 5T33MMvt).
We observed a difference in sensitivity to DAC among the three cell lines tested. In RPMI 8226 and 5T33MMvt cells, DAC led to a dose-dependent decrease of proliferation and increase of annexin-V positivity, already after 24 hours. In contrast, in OPM-2 cells, DAC only mildly decreased thymidine incorporation with almost no increase in annexin-V positivity up to 72 hours. Cell cycle analysis revealed that DAC induced G2-phase arrest at 50nM in 5T33MMvt cells, while an arrest in S-phase was observed at higher doses. This was accompanied with a decrease of cyclin-B1 and cdk-1 expression. In OPM-2 cells however, DAC did not affect cell cycle progression. Furthermore, kinetic experiments demonstrated simultaneous phosphorylation of H2AX and PARP cleavage in 5T33MMvt cells which suggests DNA damage related apoptosis. In agreement, we observed MCL-1 cleavage, p53-S15 phosphorylation and increase of p21, p27 and the pro-apoptotic molecules bim, noxa and bax. In OPM-2 cells, we did not observe H2AX phosphorylation nor extensive cleavage of PARP and MCL-1, confirming that OPM-2 cells can survive DAC treatment up to 48 hours. In addition, we observed DNMT-1 degradation, indicating that DAC worked properly. The use of DNMTi together with histone deacetylase inhibitors (HDACi) has been shown to synergistically induce cell death in leukemic cells. To try to augment DAC mediated cell death in MM, we combined DAC with the HDACi JNJ-26481585. We observed that DAC and JNJ-26481585 synergistically induced cell death in all cell lines tested. In both 5T33MMvt and OPM-2 cells, the combination resulted in increased H2AX phosphorylation, PARP and MCL-1 cleavage compared to single agent treatment. In conclusion, we found that DAC induced DNA damage related cell death which is augmented by JNJ-264181585. Future experiments will focus on the use of a p53 inhibitor and pan-caspase inhibitor to elucidate the exact role of p53 and caspases in DAC mediated cell death in MM.