Senescence-associated reprogramming promotes cancer stemness.

Cellular senescence is a stress-responsive cell-cycle arrest program that terminates the further expansion of (pre-)malignant cells. Key signalling components of the senescence machinery, such as p16INK4a, p21CIP1 and p53, as well as trimethylation of lysine 9 at histone H3 (H3K9me3), also operate as critical regulators of stem-cell functions (which are collectively termed 'stemness'). In cancer cells, a gain of stemness may have profound implications for tumour aggressiveness and clinical outcome. Here we investigated whether chemotherapy-induced senescence could change stem-cell-related properties of malignant cells. Gene expression and functional analyses comparing senescent and non-senescent B-cell lymphomas from Eµ-Myc transgenic mice revealed substantial upregulation of an adult tissue stem-cell signature, activated Wnt signalling, and distinct stem-cell markers in senescence. Using genetically switchable models of senescence targeting H3K9me3 or p53 to mimic spontaneous escape from the arrested condition, we found that cells released from senescence re-entered the cell cycle with strongly enhanced and Wnt-dependent clonogenic growth potential compared to virtually identical populations that had been equally exposed to chemotherapy but had never been senescent. In vivo, these previously senescent cells presented with a much higher tumour initiation potential. Notably, the temporary enforcement of senescence in p53-regulatable models of acute lymphoblastic leukaemia and acute myeloid leukaemia was found to reprogram non-stem bulk leukaemia cells into self-renewing, leukaemia-initiating stem cells. Our data, which are further supported by consistent results in human cancer cell lines and primary samples of human haematological malignancies, reveal that senescence-associated stemness is an unexpected, cell-autonomous feature that exerts its detrimental, highly aggressive growth potential upon escape from cell-cycle blockade, and is enriched in relapse tumours. These findings have profound implications for cancer therapy, and provide new mechanistic insights into the plasticity of cancer cells.

Synthetic lethal metabolic targeting of cellular senescence in cancer therapy.

Activated oncogenes and anticancer chemotherapy induce cellular senescence, a terminal growth arrest of viable cells characterized by S-phase entry-blocking histone 3 lysine 9 trimethylation (H3K9me3). Although therapy-induced senescence (TIS) improves long-term outcomes, potentially harmful properties of senescent tumour cells make their quantitative elimination a therapeutic priority. Here we use the Eµ-myc transgenic mouse lymphoma model in which TIS depends on the H3K9 histone methyltransferase Suv39h1 to show the mechanism and therapeutic exploitation of senescence-related metabolic reprogramming in vitro and in vivo. After senescence-inducing chemotherapy, TIS-competent lymphomas but not TIS-incompetent Suv39h1(-) lymphomas show increased glucose utilization and much higher ATP production. We demonstrate that this is linked to massive proteotoxic stress, which is a consequence of the senescence-associated secretory phenotype (SASP) described previously. SASP-producing TIS cells exhibited endoplasmic reticulum stress, an unfolded protein response (UPR), and increased ubiquitination, thereby targeting toxic proteins for autophagy in an acutely energy-consuming fashion. Accordingly, TIS lymphomas, unlike senescence models that lack a strong SASP response, were more sensitive to blocking glucose utilization or autophagy, which led to their selective elimination through caspase-12- and caspase-3-mediated endoplasmic-reticulum-related apoptosis. Consequently, pharmacological targeting of these metabolic demands on TIS induction in vivo prompted tumour regression and improved treatment outcomes further. These findings unveil the hypercatabolic nature of TIS that is therapeutically exploitable by synthetic lethal metabolic targeting.

Opposing roles of NF-κB in anti-cancer treatment outcome unveiled by cross-species investigations.

In malignancies, enhanced nuclear factor-κB (NF-κB) activity is largely viewed as an oncogenic property that also confers resistance to chemotherapy. Recently, NF-κB has been postulated to participate in a senescence-associated and possibly senescence-reinforcing cytokine response, thereby suggesting a tumor-restraining role for NF-κB. Using a mouse lymphoma model and analyzing transcriptome and clinical data from lymphoma patients, we show here that therapy-induced senescence presents with and depends on active NF-κB signaling, whereas NF-κB simultaneously promotes resistance to apoptosis. Further characterization and genetic engineering of primary mouse lymphomas according to distinct NF-κB-related oncogenic networks reminiscent of diffuse large B-cell lymphoma (DLBCL) subtypes guided us to identify Bcl2-overexpressing germinal center B-cell-like (GCB) DLBCL as a clinically relevant subgroup with significantly superior outcome when NF-κB is hyperactive. Our data illustrate the power of cross-species investigations to functionally test genetic mechanisms in transgenic mouse tumors that recapitulate distinct features of the corresponding human entity, and to ultimately use the mouse model-derived genetic information to redefine novel, clinically relevant patient subcohorts.

Passenger Gene Coamplifications Create Collateral Therapeutic Vulnerabilities in Cancer.

DNA amplifications in cancer do not only harbor oncogenes. We sought to determine whether passenger coamplifications could create collateral therapeutic vulnerabilities. Through an analysis of >3,000 cancer genomes followed by the interrogation of CRISPR-Cas9 loss-of-function screens across >700 cancer cell lines, we determined that passenger coamplifications are accompanied by distinct dependency profiles. In a proof-of-principle study, we demonstrate that the coamplification of the bona fide passenger gene DEAD-Box Helicase 1 (DDX1) creates an increased dependency on the mTOR pathway. Interaction proteomics identified tricarboxylic acid (TCA) cycle components as previously unrecognized DDX1 interaction partners. Live-cell metabolomics highlighted that this interaction could impair TCA activity, which in turn resulted in enhanced mTORC1 activity. Consequently, genetic and pharmacologic disruption of mTORC1 resulted in pronounced cell death in vitro and in vivo. Thus, structurally linked coamplification of a passenger gene and an oncogene can result in collateral vulnerabilities. SIGNIFICANCE: We demonstrate that coamplification of passenger genes, which were largely neglected in cancer biology in the past, can create distinct cancer dependencies. Because passenger coamplifications are frequent in cancer, this principle has the potential to expand target discovery in oncology. This article is featured in Selected Articles from This Issue, p. 384.

Coordinated inheritance of extrachromosomal DNA species in human cancer cells.

The chromosomal theory of inheritance has dominated human genetics, including cancer genetics. Genes on the same chromosome segregate together while genes on different chromosomes assort independently, providing a fundamental tenet of Mendelian inheritance. Extrachromosomal DNA (ecDNA) is a frequent event in cancer that drives oncogene amplification, dysregulated gene expression and intratumoral heterogeneity, including through random segregation during cell division. Distinct ecDNA sequences, herein termed ecDNA species, can co-exist to facilitate intermolecular cooperation in cancer cells. However, how multiple ecDNA species within a tumor cell are assorted and maintained across somatic cell generations to drive cancer cell evolution is not known. Here we show that cooperative ecDNA species can be coordinately inherited through mitotic co-segregation. Imaging and single-cell analyses show that multiple ecDNAs encoding distinct oncogenes co-occur and are correlated in copy number in human cancer cells. EcDNA species are coordinately segregated asymmetrically during mitosis, resulting in daughter cells with simultaneous copy number gains in multiple ecDNA species prior to any selection. Computational modeling reveals the quantitative principles of ecDNA co-segregation and co-selection, predicting their observed distributions in cancer cells. Finally, we show that coordinated inheritance of ecDNAs enables co-amplification of specialized ecDNAs containing only enhancer elements and guides therapeutic strategies to jointly deplete cooperating ecDNA oncogenes. Coordinated inheritance of ecDNAs confers stability to oncogene cooperation and novel gene regulatory circuits, allowing winning combinations of epigenetic states to be transmitted across cell generations.

Germline Mutations Including the Rare Pathogenic Variant c.3206delC in the ATM Gene Cause Ataxia Teleangiectasia-Associated Primary Central Nervous System Lymphoma.

We here report the case of a 2-year-old patient with a primary central nervous system lymphoma of B-cell origin. Due to their past medical history of repeated respiratory tract infections and the marked chemotherapy-associated toxicity and infectious comorbidity, we suspected that the patient also suffered from an inherited immune deficiency disorder. Despite the lack of classical pathognomonic symptoms for ataxia teleangiectasia and missing evidence for a cancer predisposition syndrome in the family, genetic testing identified biallelic germline mutations, including the rare pathogenic variant c.3206delC (p.Pro1069Leufs*2), in the ataxia telangiectasia-mutated (ATM) gene. The case highlights the importance of searching for immune deficiency disorders associated with primary central nervous system lymphoma before treatment initiation and the urgent need to develop novel treatment strategies for cancer patients with underlying immunodeficiency syndromes.

H3K9me3-mediated epigenetic regulation of senescence in mice predicts outcome of lymphoma patients.

Lesion-based targeting strategies underlie cancer precision medicine. However, biological principles - such as cellular senescence - remain difficult to implement in molecularly informed treatment decisions. Functional analyses in syngeneic mouse models and cross-species validation in patient datasets might uncover clinically relevant genetics of biological response programs. Here, we show that chemotherapy-exposed primary Eµ-myc transgenic lymphomas - with and without defined genetic lesions - recapitulate molecular signatures of patients with diffuse large B-cell lymphoma (DLBCL). Importantly, we interrogate the murine lymphoma capacity to senesce and its epigenetic control via the histone H3 lysine 9 (H3K9)-methyltransferase Suv(ar)39h1 and H3K9me3-active demethylases by loss- and gain-of-function genetics, and an unbiased clinical trial-like approach. A mouse-derived senescence-indicating gene signature, termed "SUVARness", as well as high-level H3K9me3 lymphoma expression, predict favorable DLBCL patient outcome. Our data support the use of functional genetics in transgenic mouse models to incorporate basic biology knowledge into cancer precision medicine in the clinic.

CD20-Targeting Immunotherapy Promotes Cellular Senescence in B-Cell Lymphoma.

The CD20-targeting monoclonal antibody rituximab is an established component of immunochemotherapeutic regimens against B-cell lymphomas, where its coadministration with conventional anticancer agents has significantly improved long-term outcome. However, the cellular mechanisms by which rituximab exerts its antilymphoma activity are only partially understood. We show here that rituximab induces typical features of cellular senescence, a long-term growth arrest of viable cells with distinct biologic properties, in established B-cell lymphoma cell lines as well as primary transformed B cells. In addition, rituximab-based immunotherapy sensitized lymphoma cells to senescence induction by the chemotherapeutic compound adriamycin (a.k.a. doxorubicin), and, to a lesser extent, by the antimicrotubule agent vincristine. Anti-CD20 treatment further enhanced secretion of senescence-associated cytokines, and augmented the DNA damage response signaling cascade triggered by adriamycin. As the underlying prosenescence mechanism, we found intracellular reactive oxygen species (ROS) levels to be elevated in response to rituximab, and, in turn, the ROS scavenger N-acetylcysteine to largely abrogate rituximab-mediated senescence. Our results, further supported by gene set enrichment analyses in a clinical data set of chronic lymphocytic leukemia patient samples exposed to a rituximab-containing treatment regimen, provide important mechanistic insights into the biologic complexity of anti-CD20-evoked tumor responses, and unveil cellular senescence as a hitherto unrecognized effector principle of the antibody component in lymphoma immunochemotherapy. Mol Cancer Ther; 15(5); 1074-81. ©2016 AACR.

Tumor stroma-derived TGF-beta limits myc-driven lymphomagenesis via Suv39h1-dependent senescence.

Activated RAS/BRAF oncogenes induce cellular senescence as a tumor-suppressive barrier in early cancer development, at least in part, via an oncogene-evoked DNA damage response (DDR). In contrast, Myc activation-although producing a DDR as well-is known to primarily elicit an apoptotic countermeasure. Using the Emu-myc transgenic mouse lymphoma model, we show here in vivo that apoptotic lymphoma cells activate macrophages to secrete transforming growth factor beta (TGF-beta) as a critical non-cell-autonomous inducer of cellular senescence. Accordingly, neutralization of TGF-beta action, like genetic inactivation of the senescence-related histone methyltransferase Suv39h1, significantly accelerates Myc-driven tumor development via cancellation of cellular senescence. These findings, recapitulated in human aggressive B cell lymphomas, demonstrate that tumor-prompted stroma-derived signals may limit tumorigenesis by feedback senescence induction.