Histone deacetylases (HDACs) counterbalance acetylation of lysine residues, a protein modification involved in numerous biological processes. Here, Hdac1 and Hdac2 conditional knock‐out alleles were used to study the function of class I Hdac1 and Hdac2 in cell cycle progression and haematopoietic differentiation. Combined deletion of Hdac1 and Hdac2, or inactivation of their deacetylase activity in primary or oncogenic‐transformed fibroblasts, results in a senescence‐like G1 cell cycle arrest, accompanied by up‐regulation of the cyclin‐dependent kinase inhibitor p21Cip. Notably, concomitant genetic inactivation of p53 or p21Cip indicates that Hdac1 and Hdac2 regulate p53–p21Cip‐independent pathways critical for maintaining cell cycle progression. In vivo, we show that Hdac1 and Hdac2 are not essential for liver homeostasis. In contrast, total levels of Hdac1 and Hdac2 in the haematopoietic system are critical for erythrocyte‐megakaryocyte differentiation. Dual inactivation of Hdac1 and Hdac2 results in apoptosis of megakaryocytes and thrombocytopenia. Together, these data indicate that Hdac1 and Hdac2 have overlapping functions in cell cycle regulation and haematopoiesis. In addition, this work provides insights into mechanism‐based toxicities observed in patients treated with HDAC inhibitors.

Purpose: Replication-selective oncolytic adenoviruses are a promising class of tumor-targeting agents with proven safety in hundreds of patients. However, clinical responses have been limited and viral mutants with higher potency are needed. Here, we report on the generation of a novel set of mutants with improved efficacy in prostate and pancreatic carcinoma models. Currently, no curative treatments are available for late-stage metastatic prostate or rapidly progressing pancreatic cancers.Experimental Design: Adenovirus type 5 mutants were created with deletions in the E1ACR2 region for tumor selectivity and/or the E1B19K gene for attenuated replication in vivo; all constructs retain the E3 genes intact. Cell-killing efficacy, replication, and cytotoxicity in combination with chemotherapeutics were investigated in normal cells (PrEC and NHBE), seven carcinoma cell lines, and human (PC3 and DU145) and murine (TRAMPC, CMT-64, and CMT-93) tumor models in vivo.Results: The double-deleted AdΔΔ (ΔE1ACR2 and ΔE1B19K) mutant had high cell-killing activity in prostate, pancreatic, and lung carcinomas. Replication was similar to wild-type in all tumor cells and was attenuated in normal cells to levels less than the single-deleted AdΔCR2 mutant. AdΔΔ combined with the chemotherapeutics docetaxel and mitoxantrone resulted in synergistically enhanced cell killing and greatly improved antitumor efficacy in prostate xenografts in vivo. In murine immunocompetent in vivo models efficacy was greater for mutants with the E3B genes intact even in the absence of viral replication, indicating attenuated macrophage-dependent clearance.Conclusions: These data suggest that the novel oncolytic mutant AdΔΔ is a promising candidate for targeting of solid tumors specifically in combination with chemotherapeutics. Clin Cancer Res; 16(2); 541-53. ©2010 AACR.

Key Points Hdac1 and Hdac2 are dosage-dependent tumor suppressors. Hdac1 and Hdac2 regulate p53-modulating genes as a barrier to prevent Myc-driven tumorigenesis.

Class I histone deacetylases are critical regulators of gene transcription by erasing lysine acetylation. Targeting histone deacetylases using relative non-specific small molecule inhibitors is of major interest in the treatment of cancer, neurological disorders and acquired immune deficiency syndrome. Harnessing the therapeutic potential of histone deacetylase inhibitors requires full knowledge of individual histone deacetylases in vivo. As hematologic malignancies show increased sensitivity towards histone deacetylase inhibitors we targeted deletion of class I Hdac1 and Hdac2 to hematopoietic cell lineages. Here, we show that Hdac1 and Hdac2 together control hematopoietic stem cell homeostasis, in a cell-autonomous fashion. Simultaneous loss of Hdac1 and Hdac2 resulted in loss of hematopoietic stem cells and consequently bone marrow failure. Bone-marrow-specific deletion of Sin3a, a major Hdac1/2 co-repressor, phenocopied loss of Hdac1 and Hdac2 indicating that Sin3a-associated HDAC1/2-activity is essential for hematopoietic stem cell homeostasis. Although Hdac1 and Hdac2 show compensatory and overlapping functions in hematopoiesis, mice expressing mono-allelic Hdac1 or Hdac2 revealed that Hdac1 and Hdac2 contribute differently to the development of specific hematopoietic lineages.

<div>Abstract<p><b>Purpose:</b> Replication-selective oncolytic adenoviruses are a promising class of tumor-targeting agents with proven safety in hundreds of patients. However, clinical responses have been limited and viral mutants with higher potency are needed. Here, we report on the generation of a novel set of mutants with improved efficacy in prostate and pancreatic carcinoma models. Currently, no curative treatments are available for late-stage metastatic prostate or rapidly progressing pancreatic cancers.</p><p><b>Experimental Design:</b> Adenovirus type 5 mutants were created with deletions in the <i>E1ACR2</i> region for tumor selectivity and/or the <i>E1B19K</i> gene for attenuated replication <i>in vivo</i>; all constructs retain the <i>E3</i> genes intact. Cell-killing efficacy, replication, and cytotoxicity in combination with chemotherapeutics were investigated in normal cells (PrEC and NHBE), seven carcinoma cell lines, and human (PC3 and DU145) and murine (TRAMPC, CMT-64, and CMT-93) tumor models <i>in vivo</i>.</p><p><b>Results:</b> The double-deleted AdΔΔ (ΔE1ACR2 and ΔE1B19K) mutant had high cell-killing activity in prostate, pancreatic, and lung carcinomas. Replication was similar to wild-type in all tumor cells and was attenuated in normal cells to levels less than the single-deleted AdΔCR2 mutant. AdΔΔ combined with the chemotherapeutics docetaxel and mitoxantrone resulted in synergistically enhanced cell killing and greatly improved antitumor efficacy in prostate xenografts <i>in vivo</i>. In murine immunocompetent <i>in vivo</i> models efficacy was greater for mutants with the <i>E3B</i> genes intact even in the absence of viral replication, indicating attenuated macrophage-dependent clearance.</p><p><b>Conclusions:</b> These data suggest that the novel oncolytic mutant AdΔΔ is a promising candidate for targeting of solid tumors specifically in combination with chemotherapeutics. Clin Cancer Res; 16(2); 541–53</p></div>

Supplementary Data from Improved Potency and Selectivity of an Oncolytic E1ACR2 and E1B19K Deleted Adenoviral Mutant in Prostate and Pancreatic Cancers

Supplementary Data from Improved Potency and Selectivity of an Oncolytic E1ACR2 and E1B19K Deleted Adenoviral Mutant in Prostate and Pancreatic Cancers

<div>Abstract<p><b>Purpose:</b> Replication-selective oncolytic adenoviruses are a promising class of tumor-targeting agents with proven safety in hundreds of patients. However, clinical responses have been limited and viral mutants with higher potency are needed. Here, we report on the generation of a novel set of mutants with improved efficacy in prostate and pancreatic carcinoma models. Currently, no curative treatments are available for late-stage metastatic prostate or rapidly progressing pancreatic cancers.</p><p><b>Experimental Design:</b> Adenovirus type 5 mutants were created with deletions in the <i>E1ACR2</i> region for tumor selectivity and/or the <i>E1B19K</i> gene for attenuated replication <i>in vivo</i>; all constructs retain the <i>E3</i> genes intact. Cell-killing efficacy, replication, and cytotoxicity in combination with chemotherapeutics were investigated in normal cells (PrEC and NHBE), seven carcinoma cell lines, and human (PC3 and DU145) and murine (TRAMPC, CMT-64, and CMT-93) tumor models <i>in vivo</i>.</p><p><b>Results:</b> The double-deleted AdΔΔ (ΔE1ACR2 and ΔE1B19K) mutant had high cell-killing activity in prostate, pancreatic, and lung carcinomas. Replication was similar to wild-type in all tumor cells and was attenuated in normal cells to levels less than the single-deleted AdΔCR2 mutant. AdΔΔ combined with the chemotherapeutics docetaxel and mitoxantrone resulted in synergistically enhanced cell killing and greatly improved antitumor efficacy in prostate xenografts <i>in vivo</i>. In murine immunocompetent <i>in vivo</i> models efficacy was greater for mutants with the <i>E3B</i> genes intact even in the absence of viral replication, indicating attenuated macrophage-dependent clearance.</p><p><b>Conclusions:</b> These data suggest that the novel oncolytic mutant AdΔΔ is a promising candidate for targeting of solid tumors specifically in combination with chemotherapeutics. Clin Cancer Res; 16(2); 541–53</p></div>