A Novel Role for Histone Deacetylase 11 (HDAC11) in B Cell Lymphopoiesis and Plasma Cell Survival in Multiple Myeloma

While multiple myeloma (MM) remains incurable presently, expanded therapeutic options over the past decade have improved patient survival markedly. Proteasome inhibitors have redefined the treatment paradigm for myeloma, often serving as the backbone of front-line treatment. Histone deacetylase (HDAC) inhibitors (HDI), although only marginally active as single agent therapy in hematological malignancies, have demonstrated an ability to salvage bortezomib responsiveness in refractory patients, prompting heightened interest in this class of targeted therapeutics in myeloma. HDAC’s represent a family of enzymes, currently with 11 known members in the classical HDAC family, and subdivided into 4 sub-classes. HDAC11 is currently the only member of the sub-class IV and, as the newest member of the HDAC family, its impact on B cell lymphopoiesis and myeloma development is only starting to be unveiled. Intriguingly, we show that mice with germ-line silencing of HDAC11 (HDAC11KO mice) exhibit a 50% decrease in plasma cells in both the bone marrow and peripheral blood plasma cell compartments relative to wild-type mice. Consistent with this, Tg-HDAC11-eGFP mice, a transgenic strain engineered to express GFP under control of the HDAC11 promoter (Heinz, N Nat. Rev. Neuroscience 2001) reveals that HDAC11 expression is increased in the plasma cell population and to a lesser extent B1 B cells, as compared to earlier lineage stages. Similar observations based on measurements of HDAC11 mRNA were seen in normal human plasma cells. Significant increases in HDAC11 mRNA expression were observed in 7 of 11 primary human multiple myeloma samples and 11 of 12 human myeloma cell lines as compared to normal plasma cells, further emphasizing the potential relevance of HDAC11 to the underlying pathologic processes driving myeloma development and/or survival. Targeted silencing of HDAC11 in RPMI-8226 cells lines using siRNA results in a modest decrease in cell viability as measured by Annexin/PI staining and detection of activated caspase-3. Quisinostat, a second generation pan-HDI, has previously demonstrated activity against human myeloma cell lines in vitro (Stuhmer, Brit J Haematol, 2010), and suppressed bone destruction in an in vivo murine myeloma model (Deleu, Cancer Res, 2009). We similarly observe dose-dependent survival impairment in 10 human myeloma cell lines when cultured in the presence of quisinostat, with EC50’s consistently in the 1-10nM range. Importantly, quisinostat acts synergistically with proteasome inhibitiors (bortezomib and carfilzomib) in RPMI-8226 cells; more importantly, the degree of synergism is amplified in the RPMI-6226-B25 bortezomib-resistant cell line. Although a clear mechanism of action remains to be elucidated, preliminary data suggests that RPMI-8226 cells exposed to quisinostat appear to exhibit a decrease nuclear, but not cytosolic HDAC11. Collectively, these data illustrate a previously unknown role for HDAC11 in plasma cell differentiation and survival. Increased HDAC11 expression seen in myeloma patient specimens and primary myeloma cell lines highlights the potential of HDAC11 as a therapeutic target. Furthermore, we show that quisinostat, a pan-HDI with selectivity towards HDAC11 at lower dosing, acts synergistically with proteasome inhibitors in vitro in proteasome inhibitor sensitive and resistant cell lines. Future work will focus on further elucidating the role of HDAC11 in myeloma survival and drug response, with particular emphasis on proteasome inhibitors.