Nicole M. Sodir

Modelling Myc inhibition as a cancer therapy

Myc is a pleiotropic basic helix–loop–helix leucine zipper transcription factor that coordinates expression of the diverse intracellular and extracellular programs that together are necessary for growth and expansion of somatic cells. In principle, this makes inhibition of Myc an attractive pharmacological approach for treating diverse types of cancer. However, enthusiasm has been muted by lack of direct evidence that Myc inhibition would be therapeutically efficacious, concerns that it would induce serious side effects by inhibiting proliferation of normal tissues, and practical difficulties in designing Myc inhibitory drugs. We have modelled genetically both the therapeutic impact and the side effects of systemic Myc inhibition in a preclinical mouse model of Ras-induced lung adenocarcinoma by reversible, systemic expression of a dominant-interfering Myc mutant. We show that Myc inhibition triggers rapid regression of incipient and established lung tumours, defining an unexpected role for endogenous Myc function in the maintenance of Ras-dependent tumours in vivo. Systemic Myc inhibition also exerts profound effects on normal regenerating tissues. However, these effects are well tolerated over extended periods and rapidly and completely reversible. Our data demonstrate the feasibility of targeting Myc, a common downstream conduit for many oncogenic signals, as an effective, efficient and tumour-specific cancer therapy.

Inhibition of Myc family proteins eradicates KRas-driven lung cancer in mice.

The principal reason for failure of targeted cancer therapies is the emergence of resistant clones that regenerate the tumor. Therapeutic efficacy therefore depends on not only how effectively a drug inhibits its target, but also the innate or adaptive functional redundancy of that target and its attendant pathway. In this regard, the Myc transcription factors are intriguing therapeutic targets because they serve the unique and irreplaceable role of coordinating expression of the many diverse genes that, together, are required for somatic cell proliferation. Furthermore, Myc expression is deregulated in most-perhaps all-cancers, underscoring its irreplaceable role in proliferation. We previously showed in a preclinical mouse model of non-small-cell lung cancer that systemic Myc inhibition using the dominant-negative Myc mutant Omomyc exerts a dramatic therapeutic impact, triggering rapid regression of tumors with only mild and fully reversible side effects. Using protracted episodic expression of Omomyc, we now demonstrate that metronomic Myc inhibition not only contains Ras-driven lung tumors indefinitely, but also leads to their progressive eradication. Hence, Myc does indeed serve a unique and nondegenerate role in lung tumor maintenance that cannot be complemented by any adaptive mechanism, even in the most aggressive p53-deficient tumors. These data endorse Myc as a compelling cancer drug target.

Endogenous Myc maintains the tumor microenvironment

The ubiquitous deregulation of Myc in human cancers makes it an intriguing therapeutic target, a notion supported by recent studies in Ras-driven lung tumors showing that inhibiting endogenous Myc triggers ubiquitous tumor regression. However, neither the therapeutic mechanism nor the applicability of Myc inhibition to other tumor types driven by other oncogenic mechanisms is established. Here, we show that inhibition of endogenous Myc also triggers ubiquitous regression of tumors in a simian virus 40 (SV40)-driven pancreatic islet tumor model. Such regression is presaged by collapse of the tumor microenvironment and involution of tumor vasculature. Hence, in addition to its diverse intracellular roles, endogenous Myc serves an essential and nonredundant role in coupling diverse intracellular oncogenic pathways to the tumor microenvironment, further bolstering its credentials as a pharmacological target.

A component of the mir-17-92 polycistronic oncomir promotes oncogene-dependent apoptosis

mir-17-92, a potent polycistronic oncomir, encodes six mature miRNAs with complex modes of interactions. In the Eμ-myc Burkitt’s lymphoma model, mir-17-92 exhibits potent oncogenic activity by repressing c-Myc-induced apoptosis, primarily through its miR-19 components. Surprisingly, mir-17-92 also encodes the miR-92 component that negatively regulates its oncogenic cooperation with c-Myc. This miR-92 effect is, at least in part, mediated by its direct repression of Fbw7, which promotes the proteosomal degradation of c-Myc. Thus, overexpressing miR-92 leads to aberrant c-Myc increase, imposing a strong coupling between excessive proliferation and p53-dependent apoptosis. Interestingly, miR-92 antagonizes the oncogenic miR-19 miRNAs; and such functional interaction coordinates proliferation and apoptosis during c-Myc-induced oncogenesis. This miR-19:miR-92 antagonism is disrupted in B-lymphoma cells that favor a greater increase of miR-19 over miR-92. Altogether, we suggest a new paradigm whereby the unique gene structure of a polycistronic oncomir confers an intricate balance between oncogene and tumor suppressor crosstalk. DOI: http://dx.doi.org/10.7554/eLife.00822.001

Myc inhibition is effective against glioma and reveals a role for Myc in proficient mitosis

Gliomas are the most common primary tumours affecting the adult central nervous system and respond poorly to standard therapy. Myc is causally implicated in most human tumours and the majority of glioblastomas have elevated Myc levels. Using the Myc dominant negative Omomyc, we previously showed that Myc inhibition is a promising strategy for cancer therapy. Here, we preclinically validate Myc inhibition as a therapeutic strategy in mouse and human glioma, using a mouse model of spontaneous multifocal invasive astrocytoma and its derived neuroprogenitors, human glioblastoma cell lines, and patient-derived tumours both in vitro and in orthotopic xenografts. Across all these experimental models we find that Myc inhibition reduces proliferation, increases apoptosis and remarkably, elicits the formation of multinucleated cells that then arrest or die by mitotic catastrophe, revealing a new role for Myc in the proficient division of glioma cells.

Ibrutinib Exerts Potent Antifibrotic and Antitumor Activities in Mouse Models of Pancreatic Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a dense stromal fibroinflammatory reaction that is a major obstacle to effective therapy. The desmoplastic stroma comprises many inflammatory cells, in particular mast cells as key components of the PDAC microenvironment, and such infiltration correlates with poor patient outcome. Indeed, it has been hypothesized that stromal ablation is critical to improve clinical response in patients with PDAC. Ibrutinib is a clinically approved Brutons tyrosine kinase inhibitor that inhibits mast cells and tumor progression in a mouse model of β-cell tumorigenesis. Here, we show that ibrutinib is highly effective at limiting the growth of PDAC in both transgenic mouse and patient-derived xenograft models of the disease. In these various experimental settings, ibrutinib effectively diminished fibrosis, extended survival, and improved the response to clinical standard-of-care therapy. Our results offer a preclinical rationale to immediately evaluate the clinical efficacy of ibrutinib in patients with PDAC.

Myc Cooperates with Ras by Programming Inflammation and Immune Suppression

Summary The two oncogenes KRas and Myc cooperate to drive tumorigenesis, but the mechanism underlying this remains unclear. In a mouse lung model of KRasG12D-driven adenomas, we find that co-activation of Myc drives the immediate transition to highly proliferative and invasive adenocarcinomas marked by highly inflammatory, angiogenic, and immune-suppressed stroma. We identify epithelial-derived signaling molecules CCL9 and IL-23 as the principal instructing signals for stromal reprogramming. CCL9 mediates recruitment of macrophages, angiogenesis, and PD-L1-dependent expulsion of T and B cells. IL-23 orchestrates exclusion of adaptive T and B cells and innate immune NK cells. Co-blockade of both CCL9 and IL-23 abrogates Myc-induced tumor progression. Subsequent deactivation of Myc in established adenocarcinomas triggers immediate reversal of all stromal changes and tumor regression, which are independent of CD4+CD8+ T cells but substantially dependent on returning NK cells. We show that Myc extensively programs an immune suppressive stroma that is obligatory for tumor progression.

Re-engineering the Pancreas Tumor Microenvironment: A "Regenerative Program" Hacked

The “hallmarks” of pancreatic ductal adenocarcinoma (PDAC) include proliferative, invasive, and metastatic tumor cells and an associated dense desmoplasia comprised of fibroblasts, pancreatic stellate cells, extracellular matrix, and immune cells. The oncogenically activated pancreatic epithelium and its associated stroma are obligatorily interdependent, with the resulting inflammatory and immunosuppressive microenvironment contributing greatly to the evolution and maintenance of PDAC. The peculiar pancreas-specific tumor phenotype is a consequence of oncogenes hacking the resident pancreas regenerative program, a tissue-specific repair mechanism regulated by discrete super enhancer networks. Defined as genomic regions containing clusters of multiple enhancers, super enhancers play pivotal roles in cell/tissue specification, identity, and maintenance. Hence, interfering with such super enhancer–driven repair networks should exert a disproportionately disruptive effect on tumor versus normal pancreatic tissue. Novel drugs that directly or indirectly inhibit processes regulating epigenetic status and integrity, including those driven by histone deacetylases, histone methyltransferase and hydroxylases, DNA methyltransferases, various metabolic enzymes, and bromodomain and extraterminal motif proteins, have shown the feasibility of disrupting super enhancer–dependent transcription in treating multiple tumor types, including PDAC. The idea that pancreatic adenocarcinomas rely on embedded super enhancer transcriptional mechanisms suggests a vulnerability that can be potentially targeted as novel therapies for this intractable disease. Clin Cancer Res; 23(7); 1647–55. ©2017 AACR. See all articles in this CCR Focus section, “Pancreatic Cancer: Challenge and Inspiration.”

PO-096 Defining the role of Myc in the pancreatic cancer super enhancer network

Introduction Cell behaviour and identity are in great part determined by transcriptional switches present in the genome that are referred to as super-enhancers (SEs). The transcription factor Myc coordinates several physiological processes required for cell growth, proliferation and spread and its expression is deregulated in the majority of cancers. Our recent studies have shown that Myc deregulation in a KRasG12D-driven mouse model drives immediate progression to aggressive and inflammatory pancreatic ductal adenocarcinoma (PDAC) accompanied by a profound desmoplastic response. Subsequent Myc de-activation triggers immediate regression of the tumour microenvironment and death of tumour cells. The complex phenotypic changes that accompany the Myc-driven transition to PDAC suggest that Myc plays a key role in driving the pancreatic cancer SE network. However, it remains unclear whether Myc establishes the PDAC SE or, instead, acts as the switch that activates a pre-configured SE network. To distinguish between these possibilities, we have addressed whether the phenotypic and gene expression changes that Myc engages in pancreatic epithelium are accompanied by modifications in the underlying tumour cell epigenome. Material and methods Primary tumour cell lines were isolated from our mouse model p48-cre;LSL-KrasG12D/+;Rosa-LSL-MycERTAM, where Myc can be switch on and off at will solely in the pancreatic epithelium. ATAC-seq and ChIP-seq was used to identify Myc-dependent changes in the accessible chromatin and signature epigenetic markers, respectively. Results and discussions We observe widespread Myc-dependent changes in gene expression and pro-tumorigenic signalling molecules, including PD-L1 (Programmed death-ligand 1), indicating rapid engagement of the SE network by Myc. Despite these changes in gene expression, we observe no significant accompanying qualitative changes in chromatin accessibility or the H3K27ac landscape. For instance, Myc binds the PD-L1 promoter and regulates its expression in a cell autonomous manner in the pancreatic tumour cell line in the absence of any changes in chromatin accessibility at the PD-L1 locus. Conclusion Our data indicate that Myc does not configure the SE landscape but, instead, serves as a master switch to engage it. Therefore, abrogating Myc’s engagement of the SE network may provide a novel therapeutic strategy for the treatment of pancreatic cancer.

Abstract 535: Deregulated Myc is an immunosuppressive switch

Early tumor evolution through sustained oncogene activity selectively bypasses the engagement of cell-intrinsic tumor-suppressor signaling and cell-extrinsic microenvironmental restrictions. How the immune system may be involved is virtually unknown. We describe here the contributions of a conditional and reversible low-level expression of Myc in a mouse model of KrasG12D-driven non-small cell lung cancer. Deregulated Myc activity results in highly expansive tumors that appear embedded in inflamed regions and leads to a rapid reduction of mouse survival. Myc activation imposes an immediate switch to an immunosuppressive and pro-angiogenic microenvironment, facilitated through IL23- and CCL9-associated recruitment of PD-L1 loaded macrophages and local exclusion of T-lymphocytes. Reversibly, blocking the activity of Myc-driven IL23 and CCL9 expression or withdrawal of deregulated Myc activity in tumors established by oncogene cooperation results in tumor cell death and regression, associated with a collapse of the established microenvironmental changes and re-engagement of cytotoxic T-cells. During oncogene cooperation with Ras, deregulated Myc directs a sufficient and necessary switch to a microenvironment that shields tumor growth and expansion from immune suppression. Citation Format: Roderik M. Kortlever, Nicole M. Sodir, Catherine H. Wilson, Deborah L. Burkhart, Lamorna Swigart, Trevor D. Littlewood, Gerard I. Evan. Deregulated Myc is an immunosuppressive switch. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 535.