Key Points• Genetic studies suggest HDAC3-selective suppression may prove useful for treatment of hematological tumors but will not induce apoptosis.• Genetic and pharmacological cosuppression of HDAC1 with HDAC2 induces a potent proapoptotic response of tumor cells.Histone deacetylase (HDAC) inhibitors (HDACis) have demonstrated activity in hematological and solid malignancies. Vorinostat, romidepsin, belinostat, and panobinostat are Food and Drug Administration-approved for hematological malignancies and inhibit class II and/or class I HDACs, including HDAC1, 2, 3, and 6. We combined genetic and pharmacological approaches to investigate whether suppression of individual or multiple Hdacs phenocopied broad-acting HDACis in 3 genetically distinct leukemias and lymphomas. Individual Hdacs were depleted in murine acute myeloid leukemias (MLL-AF9;Nras
G12D; PML-RARa acute promyelocytic leukemia [APL] cells) and Em-Myc lymphoma in vitro and in vivo. Strikingly, Hdac3-depleted cells were selected against in competitive assays for all 3 tumor types. Decreased proliferation following Hdac3 knockdown was not prevented by BCL-2 overexpression, caspase inhibition, or knockout of Cdkn1a in Em-Myc lymphoma, and depletion of Hdac3 in vivo significantly reduced tumor burden. Interestingly, APL cells depleted of Hdac3 demonstrated a more differentiated phenotype. Consistent with these genetic studies, the HDAC3 inhibitor RGFP966 reduced proliferation of Em-Myc lymphoma and induced differentiation in APL. Genetic codepletion of Hdac1 with Hdac2 was pro-apoptotic in Em-Myc lymphoma in vitro and in vivo and was phenocopied by the HDAC1/2-specific agent RGFP233. This study demonstrates the importance of HDAC3 for the proliferation of leukemia and lymphoma cells, suggesting that HDAC3-selective inhibitors could prove useful for the treatment of hematological malignancies. Moreover, our results demonstrate that codepletion of Hdac1 with Hdac2 mediates a robust pro-apoptotic response. Our integrated genetic and pharmacological approach provides important insights into the individual or combinations of HDACs that could be prioritized for targeting in a range of hematological malignancies. (Blood. 2015;126(21):2392-2403