Different promoter affinities account for specificity in MYC-dependent gene regulation

Enhanced expression of the MYC transcription factor is observed in the majority of tumors. Two seemingly conflicting models have been proposed for its function: one proposes that MYC enhances expression of all genes, while the other model suggests gene-specific regulation. Here, we have explored the hypothesis that specific gene expression profiles arise since promoters differ in affinity for MYC and high-affinity promoters are fully occupied by physiological levels of MYC. We determined cellular MYC levels and used RNA- and ChIP-sequencing to correlate promoter occupancy with gene expression at different concentrations of MYC. Mathematical modeling showed that binding affinities for interactions of MYC with DNA and with core promoter-bound factors, such as WDR5, are sufficient to explain promoter occupancies observed in vivo. Importantly, promoter affinity stratifies different biological processes that are regulated by MYC, explaining why tumor-specific MYC levels induce specific gene expression programs and alter defined biological properties of cells

OmoMYC blunts promoter invasion by oncogenic MYC to inhibit gene expression characteristic of MYC-dependent tumors.

MYC genes have both essential roles during normal development and exert oncogenic functions during tumorigenesis. Expression of a dominant-negative allele of MYC, termed OmoMYC, can induce rapid tumor regression in mouse models with little toxicity for normal tissues. How OmoMYC discriminates between physiological and oncogenic functions of MYC is unclear. We have solved the crystal structure of OmoMYC and show that it forms a stable homodimer and as such recognizes DNA in the same manner as the MYC/MAX heterodimer. OmoMYC attenuates both MYC-dependent activation and repression by competing with MYC/MAX for binding to chromatin, effectively lowering MYC/MAX occupancy at its cognate binding sites. OmoMYC causes the largest decreases in promoter occupancy and changes in expression on genes that are invaded by oncogenic MYC levels. A signature of OmoMYC-regulated genes defines subgroups with high MYC levels in multiple tumor entities and identifies novel targets for the eradication of MYC-driven tumors

Cre recombinase expression cooperates with homozygous FLT3 internal tandem duplication knockin mouse model to induce acute myeloid leukemia

Murine models offer a valuable tool to recapitulate genetically defined subtypes of AML, and to assess the potential of compound mutations and clonal evolution during disease progression. This is of particular importance for difficult to treat leukemias such as FLT3 internal tandem duplication (ITD) positive AML. While conditional gene targeting by Cre recombinase is a powerful technology that has revolutionized biomedical research, consequences of Cre expression such as lack of fidelity, toxicity or off-target effects need to be taken into consideration. We report on a transgenic murine model of FLT3-ITD induced disease, where Cre recombinase expression alone, and in the absence of a conditional allele, gives rise to an aggressive leukemia phenotype. Here, expression of various Cre recombinases leads to polyclonal expansion of FLT3(ITD/ITD) progenitor cells, induction of a differentiation block and activation of Myc-dependent gene expression programs. Our report is intended to alert the scientific community of potential risks associated with using this specific mouse model and of unexpected effects of Cre expression when investigating cooperative oncogenic mutations in murine models of cancer

Pharmacological reactivation of MYC-dependent apoptosis induces susceptibility to anti-PD-1 immunotherapy

Elevated MYC expression sensitizes tumor cells to apoptosis but the therapeutic potential of this mechanism remains unclear. We find, in a model of MYC-driven breast cancer, that pharmacological activation of AMPK strongly synergizes with BCL-2/BCL-X-L inhibitors to activate apoptosis. We demonstrate the translational potential of an AMPK and BCL-2/BCL-X-L co-targeting strategy in ex vivo and in vivo models of MYC-high breast cancer. Metformin combined with navitoclax or venetoclax efficiently inhibited tumor growth, conferred survival benefits and induced tumor infiltration by immune cells. However, withdrawal of the drugs allowed tumor re-growth with presentation of PD-1+/CD8+ T cell infiltrates, suggesting immune escape. A two-step treatment regimen, beginning with neoadjuvant metformin+venetoclax to induce apoptosis and followed by adjuvant metformin+venetoclax+anti-PD-1 treatment to overcome immune escape, led to durable antitumor responses even after drug withdrawal. We demonstrate that pharmacological reactivation of MYC-dependent apoptosis is a powerful antitumor strategy involving both tumor cell depletion and immunosurveillance

The Action Mechanism of the Myc Inhibitor Termed Omomyc May Give Clues on How to Target Myc for Cancer Therapy

Recent evidence points to Myc – a multifaceted bHLHZip transcription factor deregulated in the majority of human cancers – as a priority target for therapy. How to target Myc is less clear, given its involvement in a variety of key functions in healthy cells. Here we report on the action mechanism of the Myc interfering molecule termed Omomyc, which demonstrated astounding therapeutic efficacy in transgenic mouse cancer models in vivo. Omomyc action is different from the one that can be obtained by gene knockout or RNA interference, approaches designed to block all functions of a gene product. This molecule – instead – appears to cause an edge-specific perturbation that destroys some protein interactions of the Myc node and keeps others intact, with the result of reshaping the Myc transcriptome. Omomyc selectively targets Myc protein interactions: it binds c- and N-Myc, Max and Miz-1, but does not bind Mad or select HLH proteins. Specifically, it prevents Myc binding to promoter E-boxes and transactivation of target genes while retaining Miz-1 dependent binding to promoters and transrepression. This is accompanied by broad epigenetic changes such as decreased acetylation and increased methylation at H3 lysine 9. In the presence of Omomyc, the Myc interactome is channeled to repression and its activity appears to switch from a pro-oncogenic to a tumor suppressive one. Given the extraordinary therapeutic impact of Omomyc in animal models, these data suggest that successfully targeting Myc for cancer therapy might require a similar twofold action, in order to prevent Myc/Max binding to E-boxes and, at the same time, keep repressing genes that would be repressed by Myc

Rationalising the role of Keratin 9 as a biomarker for Alzheimer’s disease

Keratin 9 was recently identified as an important component of a biomarker panel which demonstrated a high diagnostic accuracy (87%) for Alzheimer’s disease (AD). Understanding how a protein which is predominantly expressed in palmoplantar epidermis is implicated in AD may shed new light on the mechanisms underlying the disease. Here we use immunoassays to examine blood plasma expression patterns of Keratin 9 and its relationship to other AD-associated proteins. We correlate this with the use of an in silico analysis tool VisANT to elucidate possible pathways through which the involvement of Keratin 9 may take place. We identify possible links with Dickkopf-1, a negative regulator of the wnt pathway, and propose that the abnormal expression of Keratin 9 in AD blood and cerebrospinal fluid may be a result of blood brain barrier dysregulation and disruption of the ubiquitin proteasome system. Our findings suggest that dysregulated Keratin 9 expression is a consequence of AD pathology but, as it interacts with a broad range of proteins, it may have other, as yet uncharacterized, downstream effects which could contribute to AD onset and progression

The role of MIZ-1 in MYC-dependent tumorigenesis

A hallmark of MYC-transformed cells is their aberrant response to antimitogenic signals. Key examples include the inability of MYC-transformed cells to arrest proliferation in response to antimitogenic signals such as TGF-β or DNA damage and their inability to differentiate into adipocytes in response to hormonal stimuli. Given the plethora of antimitogenic signals to which a tumor cell is exposed, it is likely that the ability to confer resistance to these signals is central to the transforming properties of MYC in vivo. At the same time, the inability of MYC-transformed cells to halt cell-cycle progression on stress may establish a dependence on mutations that impair or disable apoptosis. We propose that the interaction of MYC with the zinc finger protein MIZ-1 mediates resistance to antimitogenic signals. In contrast to other interactions of MYC, there is currently little evidence that MIZ-1 associates with MYC in normal, unperturbed cells. The functional interaction of both proteins becomes apparent at oncogenic expression levels of MYC and association with MIZ-1 mediates both oncogenic functions of MYC as well as tumor-suppressive responses to oncogenic levels of MYC

Deletion of Gas2l3 in mice leads to specific defects in cardiomyocyte cytokinesis during development

GAS2L3 is a recently identified cytoskeleton-associated protein that interacts with actin filaments and tubulin. The in vivo function of GAS2L3 in mammals remains unknown. Here, we show that mice deficient in GAS2L3 die shortly after birth because of heart failure. Mammalian cardiomyocytes lose the ability to proliferate shortly after birth, and further increase in cardiac mass is achieved by hypertrophy. The proliferation arrest of cardiomyocytes is accompanied by binucleation through incomplete cytokinesis. We observed that GAS2L3 deficiency leads to inhibition of cardiomyocyte proliferation and to cardiomyocyte hypertrophy during embryonic development. Cardiomyocyte-specific deletion of GAS2L3 confirmed that the phenotype results from the loss of GAS2L3 in cardiomyocytes. Cardiomyocytes from Gas2l3-deficient mice exhibit increased expression of a p53-transcriptional program including the cell cycle inhibitor p21. Furthermore, loss of GAS2L3 results in premature binucleation of cardiomyocytes accompanied by unresolved midbody structures. Together these results suggest that GAS2L3 plays a specific role in cardiomyocyte cytokinesis and proliferation during heart development

A MYC-driven change in mitochondrial dynamics limits YAP/TAZ function in mammary epithelial cells and breast cancer

In several developmental lineages, an increase in MYC expression drives the transition from quiescent stem cells to transit-amplifying cells. We show that MYC activates a stereotypic transcriptional program of genes involved in cell growth in mammary epithelial cells. This change in gene expression indirectly inhibits the YAP/TAZ co-activators, which maintain the clonogenic potential of these cells. We identify a phospholipase of the mitochondrial outer membrane, PLD6, as the mediator of MYC activity. MYC-dependent growth strains cellular energy resources and stimulates AMP-activated kinase (AMPK). PLD6 alters mitochondrial fusion and fission dynamics downstream of MYC. This change activates AMPK, which in turn inhibits YAP/TAZ. Mouse models and human pathological data show that MYC enhances AMPK and suppresses YAP/TAZ activity in mammary tumors