Tumor cells of epithelial origin present rampant DNA damage but do not undergo apoptosis due to inactivation of TP53. Understanding the mechanisms by which tumor cells are protected from cell death is crucial for identifying new therapeutic targets. Cottini et al. report that hematological malignancies also present DNA damage and a distinct, TP53-independent mechanism for evading cell death: low levels of the Hippo pathway coactivator YAP1 prevent apoptosis induced by nuclear relocalization of ABL1 triggered by DNA damage. A serine-threonine kinase, STK4, downregulates YAP1 levels; notably, STK4 inhibition increases YAP1 and related p73-mediated apoptosis in hematological malignancies. Oncogene-induced DNA damage elicits genomic instability in epithelial cancer cells, but apoptosis is blocked through inactivation of the tumor suppressor p53. In hematological cancers, the relevance of ongoing DNA damage and the mechanisms by which apoptosis is suppressed are largely unknown. We found pervasive DNA damage in hematologic malignancies, including multiple myeloma, lymphoma and leukemia, which leads to activation of a p53-independent, proapoptotic network centered on nuclear relocalization of ABL1 kinase. Although nuclear ABL1 triggers cell death through its interaction with the Hippo pathway coactivator YAP1 in normal cells, we show that low YAP1 levels prevent nuclear ABL1-induced apoptosis in these hematologic malignancies. YAP1 is under the control of a serine-threonine kinase, STK4. Notably, genetic inactivation of STK4 restores YAP1 levels, triggering cell death in vitro and in vivo. Our data therefore identify a new synthetic-lethal strategy to selectively target cancer cells presenting with endogenous DNA damage and low YAP1 levels.

Monoclonal B-cell lymphocytosis (MBL) is classified as chronic lymphocytic leukemia (CLL)–like, atypical CLL, and CD5− MBL. The number of B cells per microliter divides CLL-like MBL into MBL associated with lymphocytosis (usually detected in a clinical setting) and low-count MBL detected in the general population (usually identified during population screening). After a median follow-up of 34 months we reevaluated 76 low-count MBLs with 5-color flow cytometry: 90% of CLL-like MBL but only 44.4% atypical CLL and 66.7% CD5− MBL persisted over time. Population-screening CLL-like MBL had no relevant cell count change, and none developed an overt leukemia. In 50% of the cases FISH showed CLL-related chromosomal abnormalities, including monoallelic or biallelic 13q deletions (43.8%), trisomy 12 (1 case), and 17p deletions (2 cases). The analysis of the T-cell receptor β (TRBV) chains repertoire showed the presence of monoclonal T-cell clones, especially among CD4highCD8low, CD8highCD4low T cells. TRBV2 and TRBV8 were the most frequently expressed genes. This study indicates that (1) the risk of progression into CLL for low-count population-screening CLL-like MBL is exceedingly rare and definitely lower than that of clinical MBL and (2) chromosomal abnormalities occur early in the natural history and are possibly associated with the appearance of the typical phenotype.

Histone deacetylases (HDACs) represent novel molecular targets for the treatment of various types of cancers, including multiple myeloma (MM). Many HDAC inhibitors have already shown remarkable antitumor activities in the preclinical setting; however, their clinical utility is limited because of unfavorable toxicities associated with their broad range HDAC inhibitory effects. Isoform-selective HDAC inhibition may allow for MM cytotoxicity without attendant side effects. In this study, we demonstrated that HDAC3 knockdown and a small-molecule HDAC3 inhibitor BG45 trigger significant MM cell growth inhibition via apoptosis, evidenced by caspase and poly (ADP-ribose) polymerase cleavage. Importantly, HDAC3 inhibition downregulates phosphorylation (tyrosine 705 and serine 727) of signal transducers and activators of transcription 3 (STAT3). Neither interleukin-6 nor bone marrow stromal cells overcome this inhibitory effect of HDAC3 inhibition on phospho-STAT3 and MM cell growth. Moreover, HDAC3 inhibition also triggers hyperacetylation of STAT3, suggesting crosstalk signaling between phosphorylation and acetylation of STAT3. Importantly, inhibition of HDAC3, but not HDAC1 or 2, significantly enhances bortezomib-induced cytotoxicity. Finally, we confirm that BG45 alone and in combination with bortezomib trigger significant tumor growth inhibition in vivo in a murine xenograft model of human MM. Our results indicate that HDAC3 represents a promising therapeutic target, and validate a prototype novel HDAC3 inhibitor BG45 in MM.

Significance: Proteasome inhibitors show remarkable anti-multiple myeloma (MM) activity in preclinical and clinical studies. However, resistance develops in the majority of patients, and novel treatments are urgently needed. Histone deacetylase 6 (HDAC6) has been shown to mediate aggresomal protein degradation and could be a potential target for combination treatment to overcome drug resistance. Here we designed and developed an HDAC6-selective small molecule inhibitor, WT161, and used this compound to define mechanisms of anti-MM activity, both alone and in combination with proteasome inhibitors in in vitro and in vivo studies. This study has established the framework for combination treatment of HDAC6 inhibitors with proteasome inhibitors in MM and validates an in vivo quality chemical probe for broad use by the research community.

Malignant cells have a higher nicotinamide adenine dinucleotide (NAD⁺) turnover rate than normal cells, making this biosynthetic pathway an attractive target for cancer treatment. Here we investigated the biologic role of a rate-limiting enzyme involved in NAD⁺ synthesis, Nampt, in multiple myeloma (MM). Nampt-specific chemical inhibitor FK866 triggered cytotoxicity in MM cell lines and patient MM cells, but not normal donor as well as MM patients PBMCs. Importantly, FK866 in a dose-dependent fashion triggered cytotoxicity in MM cells resistant to conventional and novel anti-MM therapies and overcomes the protective effects of cytokines (IL-6, IGF-1) and bone marrow stromal cells. Nampt knockdown by RNAi confirmed its pivotal role in maintenance of both MM cell viability and intracellular NAD⁺ stores. Interestingly, cytotoxicity of FK866 triggered autophagy, but not apoptosis. A transcriptional-dependent (TFEB) and independent (PI3K/mTORC1) activation of autophagy mediated FK866 MM cytotoxicity. Finally, FK866 demonstrated significant anti-MM activity in a xenograft-murine MM model, associated with down-regulation of ERK1/2 phosphorylation and proteolytic cleavage of LC3 in tumor cells. Our data therefore define a key role of Nampt in MM biology, providing the basis for a novel targeted therapeutic approach.

Ongoing DNA damage is a common feature of epithelial cancers. Here, we show that tumor cells derived from multiple myeloma, a disease of clonal plasma cells, demonstrate DNA-replicative stress, leading to DNA damage. We identified a poor-prognosis subset of multiple myeloma with extensive chromosomal instability and replicative stress, which rely on ATR to compensate for DNA-replicative stress; conversely, silencing of ATR or treatment with a specific ATR inhibitor triggers multiple myeloma cell apoptosis. We show that oncogenes, such as MYC, induce DNA damage in multiple myeloma cells not only by increased replicative stress, but also via increased oxidative stress, and that reactive oxygen species–inducer piperlongumine triggers further DNA damage and apoptosis. Importantly, ATR inhibition combined with piperlongumine triggers synergistic multiple myeloma cytotoxicity. This synthetic lethal approach, enhancing oxidative stress while concomitantly blocking replicative stress response, provides a novel combination targeted therapy to address an unmet medical need in this subset of multiple myeloma. Significance: Multiple myeloma remains an incurable disease. We have identified a subset of multiple myeloma patients with poor prognosis, whose tumors present chromosomal instability, replicative and oxidative stress, and DNA damage. We define a synthetic lethal approach enhancing oxidative stress while targeting replicative stress response, inducing tumor cell apoptosis in this patient subset. Cancer Discov; 5(9); 972–87. ©2015 AACR. This article is highlighted in the In This Issue feature, p. 893