Polycomb group ring finger protein 6 suppresses Myc-induced lymphomagenesis.

Max is an obligate dimerization partner for the Myc transcription factors and for several repressors, such as Mnt, Mxd1-4, and Mga, collectively thought to antagonize Myc function in transcription and oncogenesis. Mga, in particular, is part of the variant Polycomb group repressive complex PRC1.6. Here, we show that ablation of the distinct PRC1.6 subunit Pcgf6-but not Mga-accelerates Myc-induced lymphomagenesis in Eµ-myc transgenic mice. Unexpectedly, however, Pcgf6 loss shows no significant impact on transcriptional profiles, in neither pre-tumoral B-cells, nor lymphomas. Altogether, these data unravel an unforeseen, Mga- and PRC1.6-independent tumor suppressor activity of Pcgf6.

Targeting mitochondrial respiration and the BCL2 family in high-grade MYC-associated B-cell lymphoma.

Multiple molecular features, such as activation of specific oncogenes (e.g., MYC, BCL2) or a variety of gene expression signatures, have been associated with disease course in diffuse large B-cell lymphoma (DLBCL), although their relationships and implications for targeted therapy remain to be fully unraveled. We report that MYC activity is closely correlated with-and most likely a driver of-gene signatures related to oxidative phosphorylation (OxPhos) in DLBCL, pointing to OxPhos enzymes, in particular mitochondrial electron transport chain (ETC) complexes, as possible therapeutic targets in high-grade MYC-associated lymphomas. In our experiments, indeed, MYC sensitized B cells to the ETC complex I inhibitor IACS-010759. Mechanistically, IACS-010759 triggered the integrated stress response (ISR) pathway, driven by the transcription factors ATF4 and CHOP, which engaged the intrinsic apoptosis pathway and lowered the apoptotic threshold in MYC-overexpressing cells. In line with these findings, the BCL2-inhibitory compound venetoclax synergized with IACS-010759 against double-hit lymphoma (DHL), a high-grade malignancy with concurrent activation of MYC and BCL2. In BCL2-negative lymphoma cells, instead, killing by IACS-010759 was potentiated by the Mcl-1 inhibitor S63845. Thus, combining an OxPhos inhibitor with select BH3-mimetic drugs provides a novel therapeutic principle against aggressive, MYC-associated DLBCL variants.

Integrated requirement of non-specific and sequence-specific DNA binding in Myc-driven transcription.

Eukaryotic transcription factors recognize specific DNA sequence motifs, but are also endowed with generic, non-specific DNA-binding activity. How these binding modes are integrated to determine select transcriptional outputs remains unresolved. We addressed this question by site-directed mutagenesis of the Myc transcription factor. Impairment of non-specific DNA backbone contacts caused pervasive loss of genome interactions and gene regulation, associated with increased intra-nuclear mobility of the Myc protein in murine cells. In contrast, a mutant lacking base-specific contacts retained DNA-binding and mobility profiles comparable to those of the wild-type protein, but failed to recognize its consensus binding motif (E-box) and could not activate Myc-target genes. Incidentally, this mutant gained weak affinity for an alternative motif, driving aberrant activation of different genes. Altogether, our data show that non-specific DNA binding is required to engage onto genomic regulatory regions; sequence recognition in turn contributes to transcriptional activation, acting at distinct levels: stabilization and positioning of Myc onto DNA, and-unexpectedly-promotion of its transcriptional activity. Hence, seemingly pervasive genome interaction profiles, as detected by ChIP-seq, actually encompass diverse DNA-binding modalities, driving defined, sequence-dependent transcriptional responses.

Cooperation Between MYC and ß-Catenin in Liver Tumorigenesis Requires Yap/Taz.

BACKGROUND AND AIMS: Activation of MYC and catenin beta-1 (CTNNB1, encoding ß-catenin) can co-occur in liver cancer, but how these oncogenes cooperate in tumorigenesis remains unclear. APPROACH AND RESULTS: We generated a mouse model allowing conditional activation of MYC and WNT/ß-catenin signaling (through either ß-catenin activation or loss of APC - adenomatous polyposis coli) upon expression of CRE recombinase in the liver and monitored their effects on hepatocyte proliferation, apoptosis, gene expression profiles, and tumorigenesis. Activation of WNT/ß-catenin signaling strongly accelerated MYC-driven carcinogenesis in the liver. Both pathways also cooperated in promoting cellular transformation in vitro, demonstrating their cell-autonomous action. Short-term induction of MYC and ß-catenin in hepatocytes, followed by RNA-sequencing profiling, allowed the identification of a "Myc/ß-catenin signature," composed of a discrete set of Myc-activated genes whose expression increased in the presence of active ß-catenin. Notably, this signature enriched for targets of Yes-associated protein (Yap) and transcriptional coactivator with PDZ-binding motif (Taz), two transcriptional coactivators known to be activated by WNT/ß-catenin signaling and to cooperate with MYC in mitogenic activation and liver transformation. Consistent with these regulatory connections, Yap/Taz accumulated upon Myc/ß-catenin activation and were required not only for the ensuing proliferative response, but also for tumor cell growth and survival. Finally, the Myc/ß-catenin signature was enriched in a subset of human hepatocellular carcinomas characterized by comparatively poor prognosis. CONCLUSIONS: Myc and ß-catenin show a strong cooperative action in liver carcinogenesis, with Yap and Taz serving as mediators of this effect. These findings warrant efforts toward therapeutic targeting of Yap/Taz in aggressive liver tumors marked by elevated Myc/ß-catenin activity.

An early Myc-dependent transcriptional program orchestrates cell growth during B-cell activation.

Upon activation, lymphocytes exit quiescence and undergo substantial increases in cell size, accompanied by activation of energy-producing and anabolic pathways, widespread chromatin decompaction, and elevated transcriptional activity. These changes depend upon prior induction of the Myc transcription factor, but how Myc controls them remains unclear. We addressed this issue by profiling the response to LPS stimulation in wild-type and c-myc-deleted primary mouse B-cells. Myc is rapidly induced, becomes detectable on virtually all active promoters and enhancers, but has no direct impact on global transcriptional activity. Instead, Myc contributes to the swift up- and down-regulation of several hundred genes, including many known regulators of the aforementioned cellular processes. Myc-activated promoters are enriched for E-box consensus motifs, bind Myc at the highest levels, and show enhanced RNA Polymerase II recruitment, the opposite being true at down-regulated loci. Remarkably, the Myc-dependent signature identified in activated B-cells is also enriched in Myc-driven B-cell lymphomas: hence, besides modulation of new cancer-specific programs, the oncogenic action of Myc may largely rely on sustained deregulation of its normal physiological targets.

Therapeutic synergy between tigecycline and venetoclax in a preclinical model of MYC/BCL2 double-hit B cell lymphoma.

High-grade B cell lymphomas with concurrent activation of the MYC and BCL2 oncogenes, also known as double-hit lymphomas (DHL), show dismal prognosis with current therapies. MYC activation sensitizes cells to inhibition of mitochondrial translation by the antibiotic tigecycline, and treatment with this compound provides a therapeutic window in a mouse model of MYC-driven lymphoma. We now addressed the utility of this antibiotic for treatment of DHL. BCL2 activation in mouse Eµ-myc lymphomas antagonized tigecycline-induced cell death, which was specifically restored by combined treatment with the BCL2 inhibitor venetoclax. In line with these findings, tigecycline and two related antibiotics, tetracycline and doxycycline, synergized with venetoclax in killing human MYC/BCL2 DHL cells. Treatment of mice engrafted with either DHL cell lines or a patient-derived xenograft revealed strong antitumoral effects of the tigecycline/venetoclax combination, including long-term tumor eradication with one of the cell lines. This drug combination also had the potential to cooperate with rituximab, a component of current front-line regimens. Venetoclax and tigecycline are currently in the clinic with distinct indications: Our preclinical results warrant the repurposing of these drugs for combinatorial treatment of DHL.

Transcriptional integration of mitogenic and mechanical signals by Myc and YAP.

Mammalian cells must integrate environmental cues to determine coherent physiological responses. The transcription factors Myc and YAP-TEAD act downstream from mitogenic signals, with the latter responding also to mechanical cues. Here, we show that these factors coordinately regulate genes required for cell proliferation. Activation of Myc led to extensive association with its genomic targets, most of which were prebound by TEAD. At these loci, recruitment of YAP was Myc-dependent and led to full transcriptional activation. This cooperation was critical for cell cycle entry, organ growth, and tumorigenesis. Thus, Myc and YAP-TEAD integrate mitogenic and mechanical cues at the transcriptional level to provide multifactorial control of cell proliferation.

Integrative analysis of RNA polymerase II and transcriptional dynamics upon MYC activation.

Overexpression of the MYC transcription factor causes its widespread interaction with regulatory elements in the genome but leads to the up- and down-regulation of discrete sets of genes. The molecular determinants of these selective transcriptional responses remain elusive. Here, we present an integrated time-course analysis of transcription and mRNA dynamics following MYC activation in proliferating mouse fibroblasts, based on chromatin immunoprecipitation, metabolic labeling of newly synthesized RNA, extensive sequencing, and mathematical modeling. Transcriptional activation correlated with the highest increases in MYC binding at promoters. Repression followed a reciprocal scenario, with the lowest gains in MYC binding. Altogether, the relative abundance (henceforth, "share") of MYC at promoters was the strongest predictor of transcriptional responses in diverse cell types, predominating over MYC's association with the corepressor ZBTB17 (also known as MIZ1). MYC activation elicited immediate loading of RNA polymerase II (RNAPII) at activated promoters, followed by increases in pause-release, while repressed promoters showed opposite effects. Gains and losses in RNAPII loading were proportional to the changes in the MYC share, suggesting that repression by MYC may be partly indirect, owing to competition for limiting amounts of RNAPII. Secondary to the changes in RNAPII loading, the dynamics of elongation and pre-mRNA processing were also rapidly altered at MYC regulated genes, leading to the transient accumulation of partially or aberrantly processed mRNAs. Altogether, our results shed light on how overexpressed MYC alters the various phases of the RNAPII cycle and the resulting transcriptional response.

Mutual epithelium-macrophage dependency in liver carcinogenesis mediated by ST18.

The ST18 gene has been proposed to act either as a tumor suppressor or as an oncogene in different human cancers, but direct evidence for its role in tumorigenesis has been lacking thus far. Here, we demonstrate that ST18 is critical for tumor progression and maintenance in a mouse model of liver cancer, based on oncogenic transformation and adoptive transfer of primary precursor cells (hepatoblasts). ST18 messenger RNA (mRNA) and protein were detectable neither in normal liver nor in cultured hepatoblasts, but were readily expressed after subcutaneous engraftment and tumor growth. ST18 expression in liver cells was induced by inflammatory cues, including acute or chronic inflammation in vivo, as well as coculture with macrophages in vitro. Knocking down the ST18 mRNA in transplanted hepatoblasts delayed tumor progression. Induction of ST18 knockdown in pre-established tumors caused rapid tumor involution associated with pervasive morphological changes, proliferative arrest, and apoptosis in tumor cells, as well as depletion of tumor-associated macrophages, vascular ectasia, and hemorrhage. Reciprocally, systemic depletion of macrophages in recipient animals had very similar phenotypic consequences, impairing either tumor development or maintenance, and suppressing ST18 expression in hepatoblasts. Finally, RNA sequencing of ST18-depleted tumors before involution revealed down-regulation of inflammatory response genes, pointing to the suppression of nuclear factor kappa B-dependent transcription. CONCLUSION: ST18 expression in epithelial cells is induced by tumor-associated macrophages, contributing to the reciprocal feed-forward loop between both cell types in liver tumorigenesis. Our findings warrant the exploration of means to interfere with ST18-dependent epithelium-macrophage interactions in a therapeutic setting. (Hepatology 2017;65:1708-1719).

Smyd2 is a Myc-regulated gene critical for MLL-AF9 induced leukemogenesis.

The Smyd2 protein (Set- and Mynd domain containing protein 2) is a methyl-transferase that can modify both histones and cytoplasmic proteins. Smyd2 is over-expressed in several cancer types and was shown to be limiting for tumor development in the pancreas. However, genetic evidence for a role of Smyd2 in other cancers or in mouse development was missing to date. Using germ line-deleted mouse strains, we now show that Smyd2 and the related protein Smyd3 are dispensable for normal development. Ablation of Smyd2 did not affect hematopoiesis, but retarded the development of leukemia promoted by MLL-AF9, a fusion oncogene associated with acute myeloid leukemia (AML) in humans. Smyd2-deleted leukemic cells showed a competitive disadvantage relative to wild-type cells, either in vitro or in vivo. The Smyd2 gene was directly activated by the oncogenic transcription factor Myc in either MLL9-AF9-induced leukemias, Myc-induced lymphomas, or fibroblasts. However, unlike leukemias, the development of lymphomas was not dependent upon Smyd2. Our data indicate that Smyd2 has a critical role downstream of Myc in AML.

The mitochondrial translation machinery as a therapeutic target in Myc-driven lymphomas.

The oncogenic transcription factor Myc is required for the progression and maintenance of diverse tumors. This has led to the concept that Myc itself, Myc-activated gene products, or associated biological processes might constitute prime targets for cancer therapy. Here, we present an in vivo reverse-genetic screen targeting a set of 241 Myc-activated mRNAs in mouse B-cell lymphomas, unraveling a critical role for the mitochondrial ribosomal protein (MRP) Ptcd3 in tumor maintenance. Other MRP-coding genes were also up regulated in Myc-induced lymphoma, pointing to a coordinate activation of the mitochondrial translation machinery. Inhibition of mitochondrial translation with the antibiotic Tigecycline was synthetic-lethal with Myc activation, impaired respiratory activity and tumor cell survival in vitro, and significantly extended lifespan in lymphoma-bearing mice. We have thus identified a novel Myc-induced metabolic dependency that can be targeted by common antibiotics, opening new therapeutic perspectives in Myc-overexpressing tumors.

Identification of MYC-Dependent Transcriptional Programs in Oncogene-Addicted Liver Tumors.

Tumors driven by activation of the transcription factor MYC generally show oncogene addiction. However, the gene expression programs that depend upon sustained MYC activity remain unknown. In this study, we employed a mouse model of liver carcinoma driven by a reversible tet-MYC transgene, combined with chromatin immunoprecipitation and gene expression profiling to identify MYC-dependent regulatory events. As previously reported, MYC-expressing mice exhibited hepatoblastoma- and hepatocellular carcinoma-like tumors, which regressed when MYC expression was suppressed. We further show that cellular transformation, and thus initiation of liver tumorigenesis, were impaired in mice harboring a MYC mutant unable to associate with the corepressor protein MIZ1 (ZBTB17). Notably, switching off the oncogene in advanced carcinomas revealed that MYC was required for the continuous activation and repression of distinct sets of genes, constituting no more than half of all genes deregulated during tumor progression and an even smaller subset of all MYC-bound genes. Altogether, our data provide the first detailed analysis of a MYC-dependent transcriptional program in a fully developed carcinoma and offer a guide to identifying the critical effectors contributing to MYC-driven tumor maintenance. Cancer Res; 76(12); 3463-72. ©2016 AACR.

Pin1 is required for sustained B cell proliferation upon oncogenic activation of Myc.

The c-myc proto-oncogene is activated by translocation in Burkitt's lymphoma and substitutions in codon 58 stabilize the Myc protein or augment its oncogenic potential. In wild-type Myc, phosphorylation of Ser 62 and Thr 58 provides a landing pad for the peptidyl prolyl-isomerase Pin1, which in turn promotes Ser 62 dephosphorylation and Myc degradation. However, the role of Pin1 in Myc-induced lymphomagenesis remains unknown. We show here that genetic ablation of Pin1 reduces lymphomagenesis in Eµ-myc transgenic mice. In both Pin1-deficient B-cells and MEFs, the proliferative response to oncogenic Myc was selectively impaired, with no alterations in Myc-induced apoptosis or mitogen-induced cell cycle entry. This proliferative defect wasn't attributable to alterations in either Ser 62 phosphorylation or Myc-regulated transcription, but instead relied on the activity of the ARF-p53 pathway. Pin1 silencing in lymphomas retarded disease progression in mice, making Pin1 an attractive therapeutic target in Myc-driven tumors.

Selective transcriptional regulation by Myc in cellular growth control and lymphomagenesis.

The c-myc proto-oncogene product, Myc, is a transcription factor that binds thousands of genomic loci. Recent work suggested that rather than up- and downregulating selected groups of genes, Myc targets all active promoters and enhancers in the genome (a phenomenon termed 'invasion') and acts as a general amplifier of transcription. However, the available data did not readily discriminate between direct and indirect effects of Myc on RNA biogenesis. We addressed this issue with genome-wide chromatin immunoprecipitation and RNA expression profiles during B-cell lymphomagenesis in mice, in cultured B cells and fibroblasts. Consistent with long-standing observations, we detected general increases in total RNA or messenger RNA copies per cell (hereby termed 'amplification') when comparing actively proliferating cells with control quiescent cells: this was true whether cells were stimulated by mitogens (requiring endogenous Myc for a proliferative response) or by deregulated, oncogenic Myc activity. RNA amplification and promoter/enhancer invasion by Myc were separable phenomena that could occur without one another. Moreover, whether or not associated with RNA amplification, Myc drove the differential expression of distinct subsets of target genes. Hence, although having the potential to interact with all active or poised regulatory elements in the genome, Myc does not directly act as a global transcriptional amplifier. Instead, our results indicate that Myc activates and represses transcription of discrete gene sets, leading to changes in cellular state that can in turn feed back on global RNA production and turnover.

SUMOylation of Myc-family proteins.

Myc-family proteins are key controllers of the metabolic and proliferative status of the cell, and are subjected to a complex network of regulatory events that guarantee their efficient and fast modulation by extracellular stimuli. Hence, unbalances in regulatory mechanisms leading to altered Myc levels or activities are often reported in cancer cells. Here we show that c- and N-Myc are conjugated to SUMO proteins at conserved lysines in their C-terminal domain. No obvious effects of SUMOylation were detected on bulk N-Myc stability or activities, including the regulation of transcription, proliferation or apoptosis. N-Myc SUMOylation could be induced by cellular stresses, such as heat shock and proteasome inhibition, and in all instances concerned a small fraction of the N-Myc protein. We surmise that, as shown for other substrates, SUMOylation may be part of a quality-control mechanism acting on misfolded Myc proteins.

A non-redundant function of cyclin E1 in hematopoietic stem cells.

A precise balance between quiescence and proliferation is crucial for the lifelong function of hematopoietic stem cells (HSCs). Cyclins E1 and E2 regulate exit from quiescence in fibroblasts, but their role in HSCs remains unknown. Here, we report a non-redundant role for cyclin E1 in mouse HSCs. A long-term culture-initiating cell (LTC-IC) assay indicated that the loss of cyclin E1, but not E2, compromised the colony-forming activity of primitive hematopoietic progenitors. Ccne1(-/-) mice showed normal hematopoiesis in vivo under homeostatic conditions but a severe impairment following myeloablative stress induced by 5-fluorouracil (5-FU). Under these conditions, Ccne1(-/-) HSCs were less efficient in entering the cell cycle, resulting in decreased hematopoiesis and reduced survival of mutant mice upon weekly 5-FU treatment. The role of cyclin E1 in homeostatic conditions became apparent in aged mice, where HSC quiescence was increased in Ccne1(-/-) animals. On the other hand, loss of cyclin E1 provided HSCs with a competitive advantage in bone marrow serial transplantation assays, suggesting that a partial impairment of cell cycle entry may exert a protective role by preventing premature depletion of the HSC compartment. Our data support a role for cyclin E1 in controlling the exit from quiescence in HSCs. This activity, depending on the physiological context, can either jeopardize or protect the maintenance of hematopoiesis.

The methyltransferase Set7/9 (Setd7) is dispensable for the p53-mediated DNA damage response in vivo.

p53 is the central regulator of cell fate following genotoxic stress and oncogene activation. Its activity is controlled by several posttranslational modifications. Originally defined as a critical layer of p53 regulation in human cell lines, p53 lysine methylation by Set7/9 (also called Setd7) was proposed to fulfill a similar function in vivo in the mouse, promoting p53 acetylation, stabilization, and activation upon DNA damage (Kurash et al., 2008). We tested the physiological relevance of this circuit in an independent Set7/9 knockout mouse strain. Deletion of Set7/9 had no effect on p53-dependent cell-cycle arrest or apoptosis following sublethal or lethal DNA damage induced by radiation or genotoxic agents. Set7/9 was also dispensable for p53 acetylation following irradiation. c-myc oncogene-induced apoptosis was also independent of Set7/9, and analysis of p53 target genes showed that Set7/9 is not required for the p53-dependent gene expression program. Our data indicate that Set7/9 is dispensable for p53 function in the mouse.

Myc, Cdk2 and cellular senescence: Old players, new game.

The aberrant activation of oncogenic pathways promotes tumor progression, but concomitantly elicits compensatory tumor-suppressive responses, such as apoptosis or senescence. For example, Ras induces senescence, while Myc generally triggers apoptosis. Myc is in fact viewed as an anti-senescence oncogene, as it is a potent inducer of cell proliferation and immortalization, bypasses growth-inhibitory signals, and cooperates with Ras in cellular transformation. Recent reports prompt re-evaluation of Myc-induced senescence and of its role in tumor progression and therapy. We have shown that the cyclin-dependent kinase Cdk2, although redundant for cell cycle progression, has a unique role in suppressing a Myc-induced senescence program: Myc activation elicited expression of p16(INK4a) and p21(Cip1), and caused senescence in cells lacking Cdk2, but not in Cdk2-proficient cells. We show here that suppression of Myc-induced senescence by Cdk2 does not occur through phosphorylation of its purported substrate residue in Myc (Ser 62). Additional cellular activities have been identified that suppress Myc-induced senescence, including the Wrn helicase, Telomerase and Miz1. These senescencesuppressing activities were critical for tumor progression, as deficiency in either Cdk2, telomerase or Miz1 reduced the onset of Myc-induced lymphoma in transgenic mice. Other gene products like p53, SUV39H1 or TGFß promoted senescence, which together with apoptosis contributed to tumor suppression. Paradoxically, Myc directly counteracted the very same senescence program that it potentially elicited, since it positively regulated Wrn, Telomerase and Cdk2 activity. Furthermore, Cdk2 inhibition re-activated the latent senescence program in Myc expressing cells. Hence, while these molecules are instrumental to the oncogenic action of Myc, they may simultaneously constitute its Achille's heel for therapeutic development.

Chromatin association and regulation of rDNA transcription by the Ras-family protein RasL11a.

RasL11a and RasL11b are Ras super-family proteins of unknown function. Here, we show that RasL11a is a chromatin-associated modulator of pre-ribosomal RNA (pre-rRNA) synthesis. RasL11a was found in the nucleolus of interphase mouse fibroblasts, where it co-localized with the RNA polymerase I-specific transcription factor UBF. Similar to UBF, RasL11a also marked the active subset of rDNA repeats (also called nucleolar organizers, or NORs) on mitotic chromosomes. In cells, RasL11a existed in stable complexes with UBF and, as shown by chromatin immunoprecipitation, distributed along the rDNA transcription unit. Upon treatment of cells with actinomycin D, RasL11a and UBF persisted on the transcription unit beyond the release of RNA polymerase I, and remained co-localized in peri-nucleolar cap structures. Ectopic expression of RasL11a enhanced pre-rRNA levels in cells, whereas RasL11a knockdown had the opposite effect. In transient transfection experiments, RasL11a enhanced the transcriptional activity of an RNA polymerase I-specific reporter controlled by the rDNA enhancer/promoter region. We speculate that RasL11a acts in concert with UBF to facilitate initiation and/or elongation by RNA polymerase I in response to specific upstream stimuli.

Cdk2 suppresses cellular senescence induced by the c-myc oncogene.

Activated oncogenes induce compensatory tumour-suppressive responses, such as cellular senescence or apoptosis, but the signals determining the main outcome remain to be fully understood. Here, we uncover a role for Cdk2 (cyclin-dependent kinase 2) in suppressing Myc-induced senescence. Short-term activation of Myc promoted cell-cycle progression in either wild-type or Cdk2 knockout mouse embryo fibroblasts (MEFs). In the knockout MEFs, however, the initial hyper-proliferative response was followed by cellular senescence. Loss of Cdk2 also caused sensitization to Myc-induced senescence in pancreatic beta-cells or splenic B-cells in vivo, correlating with delayed lymphoma onset in the latter. Cdk2-/- MEFs also senesced upon ectopic Wnt signalling or, without an oncogene, upon oxygen-induced culture shock. Myc also causes senescence in cells lacking the DNA repair protein Wrn. However, unlike loss of Wrn, loss of Cdk2 did not enhance Myc-induced replication stress, implying that these proteins suppress senescence through different routes. In MEFs, Myc-induced senescence was genetically dependent on the ARF-p53-p21Cip1 and p16INK4a-pRb pathways, p21Cip1 and p16INK4a being selectively induced in Cdk2-/- cells. Thus, although redundant for cell-cycle progression and development, Cdk2 has a unique role in suppressing oncogene- and/or stress-induced senescence. Pharmacological inhibition of Cdk2 induced Myc-dependent senescence in various cell types, including a p53-null human cancer cell line. Our data warrant re-assessment of Cdk2 as a therapeutic target in Myc- or Wnt-driven tumours.