Epigenetic silencing of the tumor suppressor RASSF4 favors multiple myeloma progression

RAS is frequently mutated in multiple myeloma (MM) leading to aberrant signaling. Next to their classical oncogenic effects, RAS proteins also mediate anti-tumor effects through the Ras-Association Domain Family (RASSF) proteins. Currently, no data about the role of RASSF proteins in MM disease is available. Here we report that RASSF4 is downregulated during MM progression, correlating with a bad prognosis. Epigenetic modulating agents significantly increased RASSF4 expression, indicating that the RASSF4 downregulation is due to epigenetic silencing. Forced RASSF4 expression induced a G2-phase arrest and caspase-3 mediated apoptosis in human MM cell lines, strongly reduced viability of primary CD138+ MM cells and significantly reduced in vivo tumor growth. Moreover, we demonstrated RASSF4-MST1 binding and the involvement of the downstream signaling pathways JNK/Jun, p38 and p53. RASSF4 overexpression sensitized MM cells to the proteasome inhibitor bortezomib, the specific MEK1/2 inhibitor trametinib and the ROS inducer Prima-1Met. Consequently, combining trametinib with histone deacetylase inhibitors (HDACi) revealed very strong synergistic anti-MM activity. In conclusion, our study identified RASSF4 as a new potent tumor suppressor in MM that is epigenetically silenced during MM progression and provides the basis for novel therapeutic strategies enhancing RASSF4 expression (using HDACi) in combination with MEK/ERK inhibitors and bortezomib. Overall design: Paired-end RNA sequencing analysis of the RASSF4 transduced cell lines cultured for 5 days in the presence of doxycycline