Myc and Max proteins possess distinct transcriptional activities

NMR-identification of the interaction between BRCA1 and the intrinsically disordered monomer of the Myc-associated factor X

The breast cancer susceptibility 1 (BRCA1) protein plays a pivotal role in modulating the transcriptional activity of the vital intrinsically disordered transcription factor MYC. In this regard, mutations of BRCA1 and interruption of its regulatory activity are related to hereditary breast and ovarian cancer (HBOC). Interestingly, so far, MYC's main dimerization partner MAX (MYC-associated factor X) has not been found to bind BRCA1 despite a high sequence similarity between both oncoproteins. Herein, we show that a potential reason for this discrepancy is the heterogeneous conformational space of MAX, which encloses a well-documented folded coiled-coil homodimer as well as a less common intrinsically disordered monomer state-contrary to MYC, which exists mostly as intrinsically disordered protein in the absence of any binding partner. We show that when the intrinsically disordered state of MAX is artificially overpopulated, the binding of MAX to BRCA1 can readily be observed. We characterize this interaction by nuclear magnetic resonance (NMR) spectroscopy chemical shift and relaxation measurements, complemented with ITC and SAXS data. Our results suggest that BRCA1 directly binds the MAX monomer to form a disordered complex. Though probed herein under biomimetic in-vitro conditions, this finding can potentially stimulate new perspectives on the regulatory network around BRCA1 and its involvement in MYC:MAX regulation.

Isoform-Directed Control of c-Myc Functions: Understanding the Balance from Proliferation to Growth Arrest

The transcription factor c-Myc, a key regulator of cellular processes, has long been associated with roles in cell proliferation and apoptosis. This review analyses the multiple functions of c-Myc by examining the different c-Myc isoforms in detail. The impact of different c-Myc isoforms, in particular p64 and p67, on fundamental biological processes remains controversial. It is necessary to investigate the different isoforms in the context of proto-oncogenesis. The current knowledge base suggests that neoplastic lesions may possess the means for self-destruction via increased c-Myc activity. This review presents the most relevant information on the c-Myc locus and focuses on a number of isoforms, including p64 and p67. This compilation provides a basis for the development of therapeutic approaches that target the potent growth arresting and pro-apoptotic functions of c-Myc. This information can then be used to develop targeted interventions against specific isoforms with the aim of shifting the oncogenic effects of c-Myc from pro-proliferative to pro-apoptotic. The research summarised in this review can deepen our understanding of how c-Myc activity contributes to different cellular responses, which will be crucial in developing effective therapeutic strategies; for example, isoform-specific approaches may allow for precise modulation of c-Myc function.

Difluoromethylornithine (DFMO) and Neuroblastoma: A Review

Neuroblastoma is a type of cancer that affects the sympathetic nervous system and is the most common extracranial solid tumor in children. Difluoromethylornithine (DFMO) is a drug that has shown promise as a treatment option for high-risk neuroblastoma. This review aims to provide an overview of the current research on the use of DFMO in neuroblastoma treatment. The review includes a discussion of the mechanisms of action of DFMO, as well as its potential for use in combination with other treatments such as chemotherapy and immunotherapy. The review also examines the current clinical trials involving DFMO in high-risk neuroblastoma patients and provides insights into the challenges and future directions for the use of DFMO in neuroblastoma treatment. Overall, the review highlights the potential of DFMO as a promising therapy for neuroblastoma and highlights the need for further research to fully understand its potential benefits and limitations.

MYC and therapy resistance in cancer: risks and opportunities

The MYC transcription factor, encoded by the c-MYC proto-oncogene, is activated by growth-promoting signals, and is a key regulator of biosynthetic and metabolic pathways driving cell growth and proliferation. These same processes are deregulated in MYC-driven tumors, where they become critical for cancer cell proliferation and survival. As other oncogenic insults, overexpressed MYC induces a series of cellular stresses (metabolic, oxidative, replicative, etc.) collectively known as oncogenic stress, which impact not only on tumor progression, but also on the response to therapy, with profound, multifaceted consequences on clinical outcome. On one hand, recent evidence uncovered a widespread role for MYC in therapy resistance in multiple cancer types, with either standard chemotherapeutic or targeted regimens. Reciprocally, oncogenic MYC imparts a series of molecular and metabolic dependencies to cells, thus giving rise to cancer-specific vulnerabilities that may be exploited to obtain synthetic-lethal interactions with novel anticancer drugs. Here we will review the current knowledge on the links between MYC and therapeutic responses, and will discuss possible strategies to overcome resistance through new, targeted interventions.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

Regulation of Nucleolar Activity by MYC

The nucleolus harbors the machinery necessary to produce new ribosomes which are critical for protein synthesis. Nucleolar size, shape, and density are highly dynamic and can be adjusted to accommodate ribosome biogenesis according to the needs for protein synthesis. In cancer, cells undergo continuous proliferation; therefore, nucleolar activity is elevated due to their high demand for protein synthesis. The transcription factor and universal oncogene MYC promotes nucleolar activity by enhancing the transcription of ribosomal DNA (rDNA) and ribosomal proteins. This review summarizes the importance of nucleolar activity in mammalian cells, MYC’s role in nucleolar regulation in cancer, and discusses how a better understanding (and the potential inhibition) of aberrant nucleolar activity in cancer cells could lead to novel therapeutics.

MXD3 as an Immunological and Prognostic Factor From Pancancer Analysis

MAX dimerization protein 3 (MXD3), a transcriptional regulator of the MXD3 superfamily, is a part of the MYC–MAX–MXD network. However, its role in tumors has been reported in several cancers, such as B-cell acute lymphoblastic leukemia, medulloblastoma, neuroblastoma, and glioblastoma. Based on TCGA and GEO data, our first pancancer study of MXD3 confirmed the high expression of MXD3 in cancer tissues. Our results revealed that patients suffering from cancers with higher MXD3 expression had poor OS, DSS, DFI, and PFI. We further explored the methylation status of the MXD3 gene body and gene promoter in cancer. Patients with a higher MXD3 gene body have better OS, while the prognosis of patients with a high MXD3 promoter is more complex. We also verified the differential expression of three clinical phenotypes of MXD3: age, sex, and tumor stage, in a variety of tumors, suggesting a correlation between MXD3 and clinical characteristics. We explored the negative relationship between MXD3 and TMB and MSI in most types of cancer, indicating the poor prognosis of patients with high MXD3 expression. We further investigated the relationship between MXD3 and immune infiltrating cells and identified the relationship between MXD3 and immune genes, immunosuppressive genes, and antigen-presenting genes. All of the above findings established a solid relationship between MXD3 and the immune environment and immune cells. These results demonstrated that MXD3 might also be a potential immune factor. We also found a higher expression of MXD3 and promoter according to the increasing glioma WHO grade or histologic types. Glioma patients with high MXD3 or MXD3 promoter expression had poor survival. Finally, we used IHC to verify the higher expression of MXD3 in glioma samples compared to normal samples. Our study shows that MXD3, as a poor prognostic factor, plays a significant role in many cancers, especially glioma. Although more clinical evidence for MXD3 as a clinical therapeutic target and an immunotherapy site is needed, MXD3 can play an important guiding role in multiple clinical treatments, including immunotherapy and demethylation therapy.

Obscure Involvement of MYC in Neurodegenerative Diseases and Neuronal Repair

MYC is well known as a potent oncogene involved in regulating cell cycle and metabolism. Augmented MYC expression leads to cell cycle dysregulation, intense cell proliferation, and carcinogenesis. Surprisingly, its increased expression in neurons does not induce their proliferation, but leads to neuronal cell death and consequent development of a neurodegenerative phenotype. Interestingly, while cancer and neurodegenerative diseases such as Alzheimer's disease are placed at the opposite sides of cell division spectrum, both start with cell cycle dysregulation and stimulation of proliferation. It seems that MYC action directed toward neuron cell proliferation and neural tissue repair collides with evolutional loss of regenerative capacity of CNS neurons in order to strengthen synaptic structure, to protect our cognitive abilities and therefore character. Accordingly, there are abundant mechanisms that block its expression and action specifically in the brain. Moreover, while MYC expression in brain neurons during neurodegenerative processes is related to their death, there are obvious evidences that MYC action after physical injury is beneficial in case of peripheral nerve recovery. MYC might be a useful tool to repair brain cells upon development of neurodegenerative disease or CNS trauma, including stroke and traumatic brain and spinal cord injury, as even imperfect axonal growth and regeneration strategies will likely be of profound benefit. Understanding complex control of MYC action in the brain might have important therapeutic significance, but also it may contribute to the comprehension of development of neurodegenerative diseases.

The Keap1-Nrf2 System: A Mediator between Oxidative Stress and Aging

Oxidative stress, a term that describes the imbalance between oxidants and antioxidants, leads to the disruption of redox signals and causes molecular damage. Increased oxidative stress from diverse sources has been implicated in most senescence-related diseases and in aging itself. The Kelch-like ECH-associated protein 1- (Keap1-) nuclear factor-erythroid 2-related factor 2 (Nrf2) system can be used to monitor oxidative stress; Keap1-Nrf2 is closely associated with aging and controls the transcription of multiple antioxidant enzymes. Simultaneously, Keap1-Nrf2 signaling is also modulated by a more complex regulatory network, including phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt), protein kinase C, and mitogen-activated protein kinase. This review presents more information on aging-related molecular mechanisms involving Keap1-Nrf2. Furthermore, we highlight several major signals involved in Nrf2 unbinding from Keap1, including cysteine modification of Keap1 and phosphorylation of Nrf2, PI3K/Akt/glycogen synthase kinase 3β, sequestosome 1, Bach1, and c-Myc. Additionally, we discuss the direct interaction between Keap1-Nrf2 and the mammalian target of rapamycin pathway. In summary, we focus on recent progress in research on the Keap1-Nrf2 system involving oxidative stress and aging, providing an empirical basis for the development of antiaging drugs.

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.

Unraveling the Big Sleep: Molecular Aspects of Stem Cell Dormancy and Hibernation

Tissue-resident stem cells may enter a dormant state, also known as quiescence, which allows them to withstand metabolic stress and unfavorable conditions. Similarly, hibernating mammals can also enter a state of dormancy used to evade hostile circumstances, such as food shortage and low ambient temperatures. In hibernation, the dormant state of the individual and its cells is commonly known as torpor, and is characterized by metabolic suppression in individual cells. Given that both conditions represent cell survival strategies, we here compare the molecular aspects of cellular quiescence, particularly of well-studied hematopoietic stem cells, and torpor at the cellular level. Critical processes of dormancy are reviewed, including the suppression of the cell cycle, changes in metabolic characteristics, and cellular mechanisms of dealing with damage. Key factors shared by hematopoietic stem cell quiescence and torpor include a reversible activation of factors inhibiting the cell cycle, a shift in metabolism from glucose to fatty acid oxidation, downregulation of mitochondrial activity, key changes in hypoxia-inducible factor one alpha (HIF-1α), mTOR, reversible protein phosphorylation and autophagy, and increased radiation resistance. This similarity is remarkable in view of the difference in cell populations, as stem cell quiescence regards proliferating cells, while torpor mainly involves terminally differentiated cells. A future perspective is provided how to advance our understanding of the crucial pathways that allow stem cells and hibernating animals to engage in their 'great slumbers.'

The Molecular 'Myc-anisms' Behind Myc-Driven Tumorigenesis and the Relevant Myc-Directed Therapeutics

MYC, a well-studied proto-oncogene that is overexpressed in >20% of tumors across all cancers, is classically known as “undruggable” due to its crucial roles in cell processes and its lack of a drug binding pocket. Four decades of research and creativity led to the discovery of a myriad of indirect (and now some direct!) therapeutic strategies targeting Myc. This review explores the various mechanisms in which Myc promotes cancer and highlights five key therapeutic approaches to disrupt Myc, including transcription, Myc-Max dimerization, protein stability, cell cycle regulation, and metabolism, in order to develop more specific Myc-directed therapies.

MYC gene amplification by fluorescence in situ hybridization and MYC protein expression by immunohistochemistry in the diagnosis of cutaneous angiosarcoma: Systematic review and appropriate use criteria

BACKGROUND

Secondary angiosarcoma (AS) most commonly follows breast cancer and includes postirradiation AS (PRAS) and lymphedema-associated AS. The frequent amplification of MYC (8q24.21) in secondary AS and the rising incidence of PRAS and atypical vascular lesions (AVLs) have prompted interest in the diagnostic and prognostic utility of MYC in AS.

METHODS

Retrospective series with ≥2 cases of cutaneous AS and describing the use of fluorescence in situ hybridization (FISH) for MYC amplification or immunohistochemistry (IHC) for MYC overexpression were included.

RESULTS

Sixteen studies met inclusion criteria. Overall, 93% of cases evaluated by FISH and IHC were concordant. The sensitivity of FISH in primary AS was only 6.8%, and protein overexpression occurred without amplification in sun-damaged skin. FISH and IHC were over 78% sensitive in secondary AS but negative in over 98% of AVLs. MYC amplification and FLT4 coamplification were associated with shorter overall survival in secondary AS.

CONCLUSION

FISH for MYC amplification and IHC for MYC overexpression are useful in distinguishing PRAS from AVLs and may also have prognostic value in secondary AS. In contrast, these methods have little diagnostic or prognostic value in primary AS and should not be used to distinguish primary AS from benign vascular neoplasms.

The new role for an old guy: MYC as an immunoplayer

As an oncogene, myelocytomatosis oncogene (MYC) is implicated in the concept of "oncogene addiction," where switching off the oncogene leads to the cell cycle arrest and cell differentiation. However, recent data suggest that MYC also controls the establishment of the tumour microenvironment and that "oncogene addiction" actually has a strong immune background. Evaluation of the MYC role in the immunoediting process led to the speculation that cancer just uses and distorts the physiological mechanism by which MYC normally prevents rapidly proliferating cells from the elicitation of an autoimmune response. Concordantly, elevated levels of MYC and induction of immunosuppressive molecules are observed during the processes of growth and development, tissue repair, placenta development, and so forth, implying that MYC may be involved in saving regular physiologically proliferating cells from the immune system attack. Even more, a growing body of evidence suggests MYC involvement in the shaping of the adaptive immune response, immunological memory development, and establishment of immunotolerance. This paper offers an overview of MYC actions in the context of modulation of the immune response in pathological and physiological conditions. The determination of such a new role for a well-known oncogene opens new perspectives in biomedicine, and consequently, in the treatment of various pathological conditions.

Inchworm stepping of Myc-Max heterodimer protein diffusion along DNA

Oncogenic protein Myc serves as a transcription factor to control cell metabolisms. Myc dimerizes via leucine zipper with its associated partner protein Max to form a heterodimer structure, which then binds target DNA sequences to regulate gene transcription. The regulation depends on Myc-Max binding to DNA and searching for target sequences via diffusional motions along DNA. Here, we conduct structure-based molecular dynamics (MD) simulations to investigate the diffusion dynamics of the Myc-Max heterodimer along DNA. We found that the heterodimer protein slides on the DNA in a rotation-uncoupled manner in coarse-grained simulations, as its two helical DNA binding basic regions (BRs) alternate between open and closed conformations via inchworm stepping motions. In such motions, the two BRs of the heterodimer step across the DNA strand one by one, with step sizes reaching about half of a DNA helical pitch length. Atomic MD simulations of the Myc-Max heterodimer in complex with DNA have also been conducted. Hydrogen bond interactions are revealed between the two BRs and two complementary DNA strands, respectively. In the non-specific DNA binding, the BR from Myc shows an onset of stepping on one association DNA strand and starts detaching from the other strand. Overall, our simulation studies suggest that the inchworm stepping motions of the Myc-Max heterodimer can be achieved during the protein diffusion along DNA.

MYC-Associated Factor MAX is a Regulator of the Circadian Clock

The circadian transcriptional network is based on a competition between transcriptional activator and repressor complexes regulating the rhythmic expression of clock-controlled genes. We show here that the MYC-associated factor X, MAX, plays a repressive role in this network and operates through a MYC-independent binding to E-box-containing regulatory regions within the promoters of circadian BMAL1 targets. We further show that this "clock" function of MAX is required for maintaining a proper circadian rhythm and that MAX and BMAL1 contribute to two temporally alternating transcriptional complexes on clock-regulated promoters. We also identified MAX network transcriptional repressor, MNT, as a fundamental partner of MAX-mediated circadian regulation. Collectively, our data indicate that MAX regulates clock gene expression and contributes to keeping the balance between positive and negative elements of the molecular clock machinery.

MYC in Germinal Center-derived lymphomas: Mechanisms and therapeutic opportunities

The rearrangement of immunoglobulin loci during the germinal center reaction is associated with an increased risk of chromosomal translocations that activate oncogenes such as MYC, BCL2 or BCL6, thus contributing to the development of B-cell lymphomas. MYC and BCL2 activation are initiating events in Burkitt's (BL) and Follicular Lymphoma (FL), respectively, but can occur at later stages in other subtypes such as Diffuse Large-B Cell Lymphoma (DLBCL). MYC can also be activated during the progression of FL to the transformed stage. Thus, either DLBCL or FL can give rise to aggressive double-hit lymphomas (DHL) with concurrent activation of MYC and BCL2. Research over the last three decades has improved our understanding of the functions of these oncogenes and the basis for their cooperative action in lymphomagenesis. MYC, in particular, is a transcription factor that contributes to cell activation, growth and proliferation, while concomitantly sensitizing cells to apoptosis, the latter being blocked by BCL2. Here, we review our current knowledge about the role of MYC in germinal center B-cells and lymphomas, discuss MYC-induced dependencies that can sensitize cancer cells to select pharmacological inhibitors, and illustrate their therapeutic potential in aggressive lymphomas-and in particular in DHL, in combination with BCL2 inhibitors.

Max deletion destabilizes MYC protein and abrogates Eµ-Myc lymphomagenesis

Although MAX is regarded as an obligate dimerization partner for MYC, its function in normal development and neoplasia is poorly defined. We show that B-cell-specific deletion of Max has a modest effect on B-cell development but completely abrogates Eµ-Myc-driven lymphomagenesis. While Max loss affects only a few hundred genes in normal B cells, it leads to the global down-regulation of Myc-activated genes in premalignant Eµ-Myc cells. We show that the balance between MYC-MAX and MNT-MAX interactions in B cells shifts in premalignant B cells toward a MYC-driven transcriptional program. Moreover, we found that MAX loss leads to a significant reduction in MYC protein levels and down-regulation of direct transcriptional targets, including regulators of MYC stability. This phenomenon is also observed in multiple cell lines treated with MYC-MAX dimerization inhibitors. Our work uncovers a layer of Myc autoregulation critical for lymphomagenesis yet partly dispensable for normal development.

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.

Tracing MYC Expression for Small Molecule Discovery

Our inability to effectively "drug" targets such as MYC for therapeutic purposes requires the development of new approaches. We report on the implementation of a phenotype-based assay for monitoring MYC expression in multiple myeloma cells. The open reading frame (ORF) encoding an unstable variant of GFP was engineered immediately downstream of the MYC ORF using CRISPR/Cas9, resulting in co-expression of both proteins from the endogenous MYC locus. Using fluorescence readout as a surrogate for MYC expression, we implemented a pilot screen in which ∼10,000 compounds were prosecuted. Among known MYC expression inhibitors, we identified cardiac glycosides and cytoskeletal disruptors to be quite potent. We demonstrate the power of CRISPR/Cas9 engineering in establishing phenotype-based assays to identify gene expression modulators.

Alternative Splicing of MXD3 and Its Regulation of MXD3 Levels in Glioblastoma

The transcription factor MXD3 is an atypical member of the MYC/MAX/MXD transcriptional network and has been previously shown to be an important regulator of cell proliferation. MXD3 has been shown to be overexpressed and to be required for medulloblastoma and acute lymphoblastic leukemia cell proliferation. In this study we leveraged datasets from The Cancer Genome Atlas to examine MXD3 across several cancers. We find that MXD3 transcripts are significantly overexpressed in ~72% of the available datasets. The gene itself is not frequently altered, while the promoter appears to be hypomethylated. We examine the possibility that aberrant regulation of the MXD3 message is the cause of abnormal MXD3 expression across cancers. Specifically, we looked at MXD3 alternative splicing in glioblastoma multiforme (GBM) and find notable functional differences between the splice variants. The 3′UTR confers differential message stability. Furthermore, the different coding sequences lead to different protein stabilities and localizations. Altogether, these data extend our knowledge of MXD3 in the context of human cancers while characterizing a previously unstudied splice variant of MXD3.

Therapeutic Inhibition of Myc in Cancer. Structural Bases and Computer-Aided Drug Discovery Approaches

Myc (avian myelocytomatosis viral oncogene homolog) represents one of the most sought after drug targets in cancer. Myc transcription factor is an essential regulator of cell growth, but in most cancers it is overexpressed and associated with treatment-resistance and lethal outcomes. Over 40 years of research and drug development efforts did not yield a clinically useful Myc inhibitor. Drugging the "undruggable" is problematic, as Myc inactivation may negatively impact its physiological functions. Moreover, Myc is a disordered protein that lacks effective binding pockets on its surface. It is well established that the Myc function is dependent on dimerization with its obligate partner, Max (Myc associated factor X), which together form a functional DNA-binding domain to activate genomic targets. Herein, we provide an overview of the knowledge accumulated to date on Myc regulation and function, its critical role in cancer, and summarize various strategies that are employed to tackle Myc-driven malignant transformation. We focus on important structure-function relationships of Myc with its interactome, elaborating structural determinants of Myc-Max dimer formation and DNA recognition exploited for therapeutic inhibition. Chronological development of small-molecule Myc-Max prototype inhibitors and corresponding binding sites are comprehensively reviewed and particular emphasis is placed on modern computational drug design methods. On the outlook, technological advancements may soon provide the so long-awaited Myc-Max clinical candidate.

Local genomic features predict the distinct and overlapping binding patterns of the bHLH-Zip family oncoproteins MITF and MYC-MAX

MITF and MYC are well-known oncoproteins and members of the basic helix-loop-helix leucine zipper (bHLH-Zip) family of transcription factors (TFs) recognizing hexamer E-box motifs. MITF and MYC not only share the core binding motif, but are also the two most highly expressed bHLH-Zip transcription factors in melanocytes, raising the possibility that they may compete for the same binding sites in select oncogenic targets. Mechanisms determining the distinct and potentially overlapping binding modes of these critical oncoproteins remain uncharacterized. We introduce computational predictive models using local sequence features, including a boosted convolutional decision tree framework, to distinguish MITF versus MYC-MAX binding sites with up to 80% accuracy genomewide. Select E-box locations that can be bound by both MITF and MYC-MAX form a separate class of MITF binding sites characterized by differential sequence content in the flanking region, diminished interaction with SOX10, higher evolutionary conservation, and less tissue-specific chromatin organization.

Cross-Cohort Analysis Identifies a TEAD4-MYCN Positive Feedback Loop as the Core Regulatory Element of High-Risk Neuroblastoma

High-risk neuroblastomas show a paucity of recurrent somatic mutations at diagnosis. As a result, the molecular basis for this aggressive phenotype remains elusive. Recent progress in regulatory network analysis helped us elucidate disease-driving mechanisms downstream of genomic alterations, including recurrent chromosomal alterations. Our analysis identified three molecular subtypes of high-risk neuroblastomas, consistent with chromosomal alterations, and identified subtype-specific master regulator proteins that were conserved across independent cohorts. A 10-protein transcriptional module-centered around a TEAD4-MYCN positive feedback loop-emerged as the regulatory driver of the high-risk subtype associated with MYCN amplification. Silencing of either gene collapsed MYCN-amplified (MYCNAmp) neuroblastoma transcriptional hallmarks and abrogated viability in vitro and in vivo Consistently, TEAD4 emerged as a robust prognostic marker of poor survival, with activity independent of the canonical Hippo pathway transcriptional coactivators YAP and TAZ. These results suggest novel therapeutic strategies for the large subset of MYCN-deregulated neuroblastomas.Significance: Despite progress in understanding of neuroblastoma genetics, little progress has been made toward personalized treatment. Here, we present a framework to determine the downstream effectors of the genetic alterations sustaining neuroblastoma subtypes, which can be easily extended to other tumor types. We show the critical effect of disrupting a 10-protein module centered around a YAP/TAZ-independent TEAD4-MYCN positive feedback loop in MYCNAmp neuroblastomas, nominating TEAD4 as a novel candidate for therapeutic intervention. Cancer Discov; 8(5); 582-99. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 517.

Sequence-specific DNA binding by MYC/MAX to low-affinity non-E-box motifs

The MYC oncoprotein regulates transcription of a large fraction of the genome as an obligatory heterodimer with the transcription factor MAX. The MYC:MAX heterodimer and MAX:MAX homodimer (hereafter MYC/MAX) bind Enhancer box (E-box) DNA elements (CANNTG) and have the greatest affinity for the canonical MYC E-box (CME) CACGTG. However, MYC:MAX also recognizes E-box variants and was reported to bind DNA in a “non-specific” fashion in vitro and in vivo. Here, in order to identify potential additional non-canonical binding sites for MYC/MAX, we employed high throughput in vitro protein-binding microarrays, along with electrophoretic mobility-shift assays and bioinformatic analyses of MYC-bound genomic loci in vivo. We identified all hexameric motifs preferentially bound by MYC/MAX in vitro, which include the low-affinity non-E-box sequence AACGTT, and found that the vast majority (87%) of MYC-bound genomic sites in a human B cell line contain at least one of the top 21 motifs bound by MYC:MAX in vitro. We further show that high MYC/MAX concentrations are needed for specific binding to the low-affinity sequence AACGTT in vitro and that elevated MYC levels in vivo more markedly increase the occupancy of AACGTT sites relative to CME sites, especially at distal intergenic and intragenic loci. Hence, MYC binds diverse DNA motifs with a broad range of affinities in a sequence-specific and dose-dependent manner, suggesting that MYC overexpression has more selective effects on the tumor transcriptome than previously thought.

MYC-Master Regulator of the Cancer Epigenome and Transcriptome

Overexpression of MYC is a hallmark of many human cancers. The MYC oncogene has long been thought to execute its neoplastic functions by acting as a classic transcription factor, deregulating the expression of a large number of specific target genes. However, MYC’s influence on many of these target genes is rather modest and there is little overlap between MYC regulated genes in different cell types, leaving many mechanistic questions unanswered. Recent advances in the field challenge the dogma further, revealing a role for MYC that extends beyond the traditional concept of a sequence-specific transcription factor. In this article, we review MYC’s function as a regulator of the cancer epigenome and transcriptome. We outline our current understanding of how MYC regulates chromatin structure in both a site-specific and genome-wide fashion, and highlight the implications for therapeutic strategies for cancers with high MYC expression.

c-MYC-Making Liver Sick: Role of c-MYC in Hepatic Cell Function, Homeostasis and Disease

Over 35 years ago, c-MYC, a highly pleiotropic transcription factor that regulates hepatic cell function, was identified. In recent years, a considerable increment in the number of publications has significantly shifted the way that the c-MYC function is perceived. Overexpression of c-MYC alters a wide range of roles including cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion and differentiation. The purpose of this review is to broaden the understanding of the general functions of c-MYC, to focus on c-MYC-driven pathogenesis in the liver, explain its mode of action under basal conditions and during disease, and discuss efforts to target c-MYC as a plausible therapy for liver disease.

Biophysical characterization of the b-HLH-LZ of ΔMax, an alternatively spliced isoform of Max found in tumor cells: Towards the validation of a tumor suppressor role for the Max homodimers

It is classically recognized that the physiological and oncogenic functions of Myc proteins depend on specific DNA binding enabled by the dimerization of its C-terminal basic-region-Helix-Loop-Helix-Leucine Zipper (b-HLH-LZ) domain with that of Max. However, a new paradigm is emerging, where the binding of the c-Myc/Max heterodimer to non-specific sequences in enhancers and promoters drives the transcription of genes involved in diverse oncogenic programs. Importantly, Max can form a stable homodimer even in the presence of c-Myc and bind DNA (specific and non-specific) with comparable affinity to the c-Myc/Max heterodimer. Intriguingly, alterations in the Max gene by germline and somatic mutations or changes in the gene product by alternative splicing (e.g. ΔMax) were recently associated with pheochromocytoma and glioblastoma, respectively. This has led to the proposition that Max is, by itself, a tumor suppressor. However, the actual mechanism through which it exerts such an activity remains to be elucidated. Here, we show that contrary to the WT motif, the b-HLH-LZ of ΔMax does not homodimerize in the absence of DNA. In addition, although ΔMax can still bind the E-box sequence as a homodimer, it cannot bind non-specific DNA in that form, while it can heterodimerize with c-Myc and bind E-box and non-specific DNA as a heterodimer with high affinity. Taken together, our results suggest that the WT Max homodimer is important for attenuating the binding of c-Myc to specific and non-specific DNA, whereas ΔMax is unable to do so. Conversely, the splicing of Max into ΔMax could provoke an increase in overall chromatin bound c-Myc. According to the new emerging paradigm, the splicing event and the stark reduction in homodimer stability and DNA binding should promote tumorigenesis impairing the tumor suppressor activity of the WT homodimer of Max.

Transcription Factor WRKY62 Plays a Role in Pathogen Defense and Hypoxia-Responsive Gene Expression in Rice

WRKY62 is a transcriptional repressor regulated downstream of WRKY45, a central transcription factor of the salicylic acid signaling pathway in rice. Previously, WRKY62 was reported to regulate defense negatively. However, our expressional analysis using WRKY62-knockdown rice indicated that WRKY62 positively regulates defense genes, including diterpenoid phytoalexin biosynthetic genes and their transcriptional regulator DPF. Blast and leaf blight resistance tests also showed that WRKY62 is a positive defense regulator. Yeast two-hybrid, co-immunoprecipitation and gel-shift assays showed that WRKY45 and WRKY62 can form a heterodimer, as well as homodimers, that bind to W-boxes in the DPF promoter. In transient assays in rice sheaths, the simultaneous introduction of WRKY45 and WRKY62 as effectors resulted in a strong activation of the DPF promoter:hrLUC reporter gene, whereas the activity declined with excessive WRKY62. Thus, the WRKY45-WRKY62 heterodimer acts as a strong activator, while the WRKY62 homodimer acts as a repressor. While benzothiadiazole induced equivalent numbers of WRKY45 and WRKY62 transcripts, consistent with heterodimer formation and DPF activation, submergence and nitrogen replacement induced only WRKY62 transcripts, consistent with WRKY62 homodimer formation and DPF repression. Moreover, WRKY62 positively regulated hypoxia genes, implying a role forWRKY62 in the modulation of the 'trade-off' between defense and hypoxia responses.

The Relationship of N-myc Amplification and Apoptosis in Neuroblastoma

About 30% of neuroblastomas exhibit N-myc amplification. Neuroblastomas with Nmyc amplification tend to have a stroma-poor undifferentiated histopathologic phenotype and a high mitosis-karyorrhexis index (MKI). Karyorrhectic or pyknotic cells in neuroblastomas are closely related to apoptosis. Using fluorescence in situ hybridization (FISH) technique on formal in-fixed paraffin-embedded tissue, we conducted a retrospective study on 42 cases of neuroblastomas to investigate the relationship between N-myc amplification and apoptosis. The identification of apoptotic cells was based on morphology and terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate (UTP)-biotin nick end labeling (TUNEL) method. Eleven (26%) of 42 tumors demonstrated N-myc amplification. After exclusion of nine tumors from patients who had prior chemotherapy, 3 3 tumors were available for thorough investigation. Based on the morphology of apoptotic cells, seven of the eight neuroblastomas with N-myc amplification had high apoptotic cell counts (more than 200 per 5,000 tumor cells), whereas only three of the 25 tumors without N-myc amplification revealed high apoptotic cells. Our results suggest that N-myc amplification can be readily detected in routinely processed tissue sections by FISH technique. Its presence has prognostic value and tends to be associated with a high number of apoptotic cells in neuroblastomas.

Gas6/Axl is the sensor of arginine-auxotrophic response in targeted chemotherapy with arginine-depleting agents

Many human malignancies lack de novo biosynthesis of arginine (Arg) as the key enzyme argininosuccinate synthetase 1 (ASS1) is silenced. These tumors acquire ectopic Arg for survival, and depleting this source by Arg-depleting recombinant enzyme ADI-PEG20 results in cell death. Mechanisms underlying Arg auxotrophy in these tumors and how they respond to Arg-auxotrophic stress are poorly understood. Here, we report that an immediate-early event of Arg-auxotrophic response involves reactive oxygen species-mediated secretion of Gas6, which interacts with its receptor Axl and activates the downstream Ras/PI3K/Akt growth signal leading to accumulation of c-Myc by protein stabilization. Arg-auxotrophic challenge also transcriptionally upregulates c-Myc expression, which provides a feedback mechanism to enhance Axl expression. c-Myc is a positive regulator of ASS1, but elevated ASS1 provides a feedback mechanism to suppress c-Myc and Axl. Our results revealed multiple inter-regulatory pathways in Arg-auxotrophic response, consisting of Axl, c-Myc and ASS1, which regulate Arg homeostasis and ADI-PEG20 sensitivity. These pathways provide potential targets for improving the efficacy of treating Arg-auxotrophic tumors using Arg-deprivation strategies.

Auto-regulation of the Sohlh1 gene by the SOHLH2/SOHLH1/SP1 complex: implications for early spermatogenesis and oogenesis

Tissue-specific basic helix-loop-helix (bHLH) transcription factor proteins often play essential roles in cellular differentiation. The bHLH proteins SOHLH2 and SOHLH1 are expressed specifically in spermatogonia and oocytes and are required for early spermatogonial and oocyte differentiation. We previously reported that knocking out Sohlh2 causes defects in spermatogenesis and oogenesis similar to those in Sohlh1-null mice, and that Sohlh1 is downregulated in the gonads of Sohlh2-null mice. We also demonstrated that SOHLH2 and SOHLH1 can form a heterodimer. These observations led us to hypothesize that the SOHLH2/SOHLH1 heterodimer regulates the Sohlh1 promoter. Here, we show that SOHLH2 and SOHLH1 synergistically upregulate the Sohlh1 gene through E-boxes upstream of the Sohlh1 promoter. Interestingly, we identified an SP1-binding sequence, called a GC-box, adjacent to these E-boxes, and found that SOHLH1 could bind to SP1. Furthermore, chromatin-immunoprecipitation analysis using testes from mice on postnatal day 8 showed that SOHLH1 and SP1 bind to the Sohlh1 promoter region in vivo. Our findings suggest that an SOHLH2/SOHLH1/SP1 ternary complex autonomously and cooperatively regulates Sohlh1 gene transcription through juxtaposed E- and GC-boxes during early spermatogenesis and oogenesis.

Functional interactions among members of the MAX and MLX transcriptional network during oncogenesis

The transcription factor MYC and its related family members MYCN and MYCL have been implicated in the etiology of a wide spectrum of human cancers. Compared to other oncoproteins, such as RAS or SRC, MYC is unique because its protein coding region is rarely mutated. Instead, MYC's oncogenic properties are unleashed by regulatory mutations leading to unconstrained high levels of expression. Under both normal and pathological conditions MYC regulates multiple aspects of cellular physiology including proliferation, differentiation, apoptosis, growth and metabolism by controlling the expression of thousands of genes. How a single transcription factor exerts such broad effects remains a fascinating puzzle. Notably, MYC is part of a network of bHLHLZ proteins centered on the MYC heterodimeric partner MAX and its counterpart, the MAX-like protein MLX. This network includes MXD1-4, MNT, MGA, MONDOA and MONDOB proteins. With some exceptions, MXD proteins have been functionally linked to cell cycle arrest and differentiation, while MONDO proteins control cellular metabolism. Although the temporal expression patterns of many of these proteins can differ markedly they are frequently expressed simultaneously in the same cellular context, and potentially bind to the same, or similar DNA consensus sequence. Here we review the activities and interactions among these proteins and propose that the broad spectrum of phenotypes elicited by MYC deregulation is intimately connected to the functions and regulation of the other network members. Furthermore, we provide a meta-analysis of TCGA data suggesting that the coordinate regulation of the network is important in MYC driven tumorigenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

The activities of MYC, MNT and the MAX-interactome in lymphocyte proliferation and oncogenesis

The MYC family of proteins plays essential roles in embryonic development and in oncogenesis. Efforts over the past 30 years to define the transcriptional activities of MYC and how MYC functions to promote proliferation have produced evolving models of MYC function. One picture that has emerged of MYC and its partner protein MAX is of a transcription factor complex with a seemingly unique ability to stimulate the transcription of genes that are epigenetically poised for transcription and to amplify the transcription of actively transcribed genes. During lymphocyte activation, MYC is upregulated and stimulates a pro-proliferative program in part through the upregulation of a wide variety of metabolic effector genes that facilitate cell growth and cell cycle progression. MYC upregulation simultaneously sensitizes cells to apoptosis and activated lymphocytes and lymphoma cells have pro-survival attributes that allow MYC-driven proliferation to prevail. For example, the MAX-interacting protein MNT is upregulated in activated lymphocytes and was found to protect lymphocytes from MYC-dependent apoptosis. Here we review the activities of MYC, MNT and other MAX interacting proteins in the setting of T and B cell activation and oncogenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Genome recognition by MYC

MYC dimerizes with MAX to bind DNA, with a preference for the E-box consensus CACGTG and several variant motifs. In cells, MYC binds DNA preferentially within transcriptionally active promoter regions. Although several thousand promoters are bound under physiological (low MYC) conditions, these represent only a fraction of all accessible, active promoters. MYC overexpression-as commonly observed in cancer cells-leads to invasion of virtually all active promoters, as well as of distal enhancer elements. We summarize here what is currently known about the mechanisms that may guide this process. We propose that binding site recognition is determined by low-affinity protein-protein interactions between MYC/MAX dimers and components of the basal transcriptional machinery, other chromatin-associated protein complexes, and/or DNA-bound transcription factors. DNA binding occurs subsequently, without an obligate requirement for sequence recognition. Local DNA scanning then leads to preferential stabilization of the MYC/MAX dimer on high-affinity DNA elements. This model is consistent with the invasion of all active promoters that occurs at elevated MYC levels, but posits that important differences in affinity persist between physiological target sites and the newly invaded elements, which may not all be bound in a productive regulatory mode. The implications of this model for transcriptional control by MYC in normal and cancer cells are discussed in the light of the latest literature.

Sequence specificity incompletely defines the genome-wide occupancy of Myc

BACKGROUND

The Myc-Max heterodimer is a transcription factor that regulates expression of a large number of genes. Genome occupancy of Myc-Max is thought to be driven by Enhancer box (E-box) DNA elements, CACGTG or variants, to which the heterodimer binds in vitro.

RESULTS

By analyzing ChIP-Seq datasets, we demonstrate that the positions occupied by Myc-Max across the human genome correlate with the RNA polymerase II, Pol II, transcription machinery significantly better than with E-boxes. Metagene analyses show that in promoter regions, Myc is uniformly positioned about 100 bp upstream of essentially all promoter proximal paused polymerases with Max about 15 bp upstream of Myc. We re-evaluate the DNA binding properties of full length Myc-Max proteins. Electrophoretic mobility shift assay results demonstrate Myc-Max heterodimers display significant sequence preference, but have high affinity for any DNA. Quantification of the relative affinities of Myc-Max for all possible 8-mers using universal protein-binding microarray assays shows that sequences surrounding core 6-mers significantly affect binding. Compared to the in vitro sequence preferences,Myc-Max genomic occupancy measured by ChIP-Seq is largely, although not completely, independent of sequence specificity.

CONCLUSIONS

We quantified the affinity of Myc-Max to all possible 8-mers and compared this with the sites of Myc binding across the human genome. Our results indicate that the genomic occupancy of Myc cannot be explained by its intrinsic DNA specificity and suggest that the transcription machinery and associated promoter accessibility play a predominant role in Myc recruitment.

An overview of MYC and its interactome

This review is intended to provide a broad outline of the biological and molecular functions of MYC as well as of the larger protein network within which MYC operates. We present a view of MYC as a sensor that integrates multiple cellular signals to mediate a broad transcriptional response controlling many aspects of cell behavior. We also describe the larger transcriptional network linked to MYC with emphasis on the MXD family of MYC antagonists. Last, we discuss evidence that the network has evolved for millions of years, dating back to the emergence of animals.

Joint linkage and association analysis with exome sequence data implicates SLC25A40 in hypertriglyceridemia

Hypertriglyceridemia (HTG) is a heritable risk factor for cardiovascular disease. Investigating the genetics of HTG may identify new drug targets. There are ~35 known single-nucleotide variants (SNVs) that explain only ~10% of variation in triglyceride (TG) level. Because of the genetic heterogeneity of HTG, a family study design is optimal for identification of rare genetic variants with large effect size because the same mutation can be observed in many relatives and cosegregation with TG can be tested. We considered HTG in a five-generation family of European American descent (n = 121), ascertained for familial combined hyperlipidemia. By using Bayesian Markov chain Monte Carlo joint oligogenic linkage and association analysis, we detected linkage to chromosomes 7 and 17. Whole-exome sequence data revealed shared, highly conserved, private missense SNVs in both SLC25A40 on chr7 and PLD2 on chr17. Jointly, these SNVs explained 49% of the genetic variance in TG; however, only the SLC25A40 SNV was significantly associated with TG (p = 0.0001). This SNV, c.374A>G, causes a highly disruptive p.Tyr125Cys substitution just outside the second helical transmembrane region of the SLC25A40 inner mitochondrial membrane transport protein. Whole-gene testing in subjects from the Exome Sequencing Project confirmed the association between TG and SLC25A40 rare, highly conserved, coding variants (p = 0.03). These results suggest a previously undescribed pathway for HTG and illustrate the power of large pedigrees in the search for rare, causal variants.

Chromatin dynamics at the hTERT promoter during transcriptional activation and repression by c-Myc and Mnt in Xenopus leavis oocytes

The transcription factors c-Myc and Mnt regulate gene expression through dimerization with Max and binding to E-boxes in target genes. While c-Myc activates gene expression via recruitment of histone modifying complexes, Mnt acts as a transcriptional repressor. Here, we used the Xenopus leavis oocyte system to address the effect of c-Myc and Mnt on transcription and chromatin remodeling over the E-box region in the human telomerase reverse transcriptase (hTERT) promoter. As expected we found elevated and decreased levels of hTERT transcription upon exogenously expressed c-Myc/Max and Mnt/Max, respectively. In addition, we confirmed binding of these heterodimers to both E-boxes already enriched with H3K9ac and H4K16ac. These chromatin marks were further enhanced upon c-Myc/Max binding followed by increased DNA accessibility in the E-box region. In contrast, Mnt/Max inhibited Myc-induced transcription and mediated repression through complete chromatin condensation and deacetylation of H3K9 and H4K16 across the E-box region. Importantly, Mnt was able to counteract c-Myc mediated activation even when expressed at low levels, suggesting Mnt to act as a strong repressor by closing the chromatin structure. Collectively our data demonstrate that the balance between c-Myc and Mnt activity determines the transcriptional outcome of the hTERT promoter by modulation of the chromatin architecture.

Molecular Characteristics in MRI-Classified Group 1 Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is a clinically and pathologically heterogeneous brain tumor. Previous studies of transcriptional profiling have revealed biologically relevant GBM subtypes associated with specific mutations and dysregulated pathways. Here, we applied a modified proteome to uncover abnormal protein expression profile in a MRI-classified group I GBM (GBM1), which has a spatial relationship with one of the adult neural stem cell niches, subventricular zone (SVZ). Most importantly, we identified molecular characteristics in this type of GBM that include up-regulation of metabolic enzymes, ribosomal proteins, and heat shock proteins. As GBM1 often recurs at great distances from the initial lesion, the rewiring of metabolism, and ribosomal biogenesis may facilitate cancer cells’ growth and survival during tumor progression. The intimate contact between GBM1 and the SVZ raises the possibility that tumor cells in GBM1 may be most related to SVZ cells. In support of this notion, we found that markers representing SVZ cells are highly expressed in GBM1. Emerged findings from our study provide a specific protein expression profile in GBM1 and offer better prediction or therapeutic implication for this multifocal GBM.

Peripheral neuroblastic tumors with genotype-phenotype discordance: a report from the Children's Oncology Group and the International Neuroblastoma Pathology Committee

BACKGROUND

Of 4,706 peripheral neuroblastic tumors (pNTs) registered on the Children's Cancer Group and Children's Oncology Group Neuroblastoma Study between 1989 and 2010, 51 cases (1.1%) had genotype-phenotype discordance characterized by MYCN amplification (indicating poor prognosis) and Favorable Histology (indicating better prognosis).

PROCEDURE

To distinguish prognostic subgroups in the genotype-phenotype discordant pNTs, two subgroups, "conventional" and "bull's eye," were identified based on the nuclear morphology. The "conventional" tumors (35 cases) included: Neuroblastoma, poorly differentiated subtype (NB-PD, 26 cases) with "salt-and-pepper" nuclei; neuroblastoma, differentiating subtype (4 cases); ganglioneuroblastoma, intermixed (3 cases); and ganglioneuroma, maturing subtype (2 cases). The "bull's eye" tumors included NB-PD with prominent nucleoli (16 cases). Clinicopathologic characteristics of these two subgroups were analyzed. N-myc protein expression was tested immunohistochemically on available tumors.

RESULTS

No significant difference was found between these two subgroups in the distribution of prognostic factors such as age at diagnosis, clinical stage, histopathology category/subtype, mitosis-karyorrhexis index, ploidy, 1p LOH, and unbalanced 11q LOH. However, prognosis of the patients with "conventional" tumors (5-year EFS 85.7 ± 12.2%; OS 89.3 ± 10.3%) was significantly better than those with "bull's eye" tumors (EFS 31.3 ± 13.0%; OS 42.9 ± 16.2%; P = 0.0010 and 0.0008, respectively). Immunohistochemically all (11/11) tested "conventional" tumors were negative, and 10/11 tested "bull's eye" tumors were positive for N-myc protein expression.

CONCLUSIONS

Based on the presence or absence of prominent nucleoli (the putative site of RNA synthesis/accumulation leading to N-myc protein expression), two prognostic subgroups, "conventional" with a better prognosis and "bull's eye" with a poor prognosis, were distinguished among the genotype-phenotype discordant pNTs.

Analysis of ZNF350/ZBRK1 promoter variants and breast cancer susceptibility in non-BRCA1/2 French Canadian breast cancer families

ZNF350/ZBRK1 is a transcription factor, which associates with BRCA1 to co-repress GADD45A to regulate DNA damage repair, and the expression of ZNF350 is altered in different human carcinomas. In a previous study, we identified ZNF350 genomic variants potentially involved in breast cancer susceptibility in high-risk non-BRCA1/2 breast cancer individuals, which pointed toward a potential association for variants in the 5'-UTR and promoter regions. Therefore, direct sequencing was undertaken and identified 12 promoter variants, whereas haplotype analyses put in evidence four common haplotypes with a frequency>2%. However, based on their frequency observed in breast cancer and unrelated healthy individuals, these are not statistically associated with breast cancer risk. Luciferase promoter assays in two breast cancer cell lines identified two haplotypes (H11 and H12) stimulating significantly the expression of ZNF350 transcript compared with the common haplotype H8. The high expression of the H11 allele was associated with the variant c.-874A. Using MatInspector and Transcription Element Search softwares, in silico analyses predicted that the variant c.-874A created a binding site for the factors c-Myc and myogenin. This study represents the first characterization step of the ZNF350 promoter. Additional studies in larger cohorts and other populations will be needed to further evaluate whether common and/or rare ZNF350 promoter variants and haplotypes could be associated with a modest risk of breast cancer.

Pharmacophore identification of c-Myc inhibitor 10074-G5

A structure-activity relationship (SAR) study of the c-Myc (Myc) inhibitor 10074-G5 (N-([1,1'-biphenyl]-2-yl)-7-nitrobenzo[c][1,2,5]oxadiazol-4-amine, 1) - which targets a hydrophobic domain of the Myc oncoprotein that is flanked by arginine residues - was executed in order to determine its pharmacophore. Whilst the 7-nitrobenzofurazan was found to be critical for inhibitory activity, the ortho-biphenyl could be replaced with a para-carboxyphenyl group to furnish the new inhibitor JY-3-094 (3q). Around five times as potent as the lead with an IC(50) of 33 μM for disruption of the Myc-Max heterodimer, JY-3-094 demonstrated excellent selectivity over Max-Max homodimers, with no apparent effect at 100 μM. Importantly, the carboxylic acid of JY-3-094 improves the physicochemical properties of the lead compound, which will facilitate the incorporation of additional hydrophobicity that might enhance Myc inhibitory activity further still.

MYC on the path to cancer

The MYC oncogene contributes to the genesis of many human cancers. Recent insights into its expression and function have led to therapeutic opportunities. MYC's activation by bromodomain proteins could be inhibited by drug-like molecules, resulting in tumor inhibition in vivo. Tumor growth can also be curbed by pharmacologically uncoupling bioenergetic pathways involving glucose or glutamine metabolism from Myc-induced cellular biomass accumulation. Other approaches to halt Myc on the path to cancer involve targeting Myc-Max dimerization or Myc-induced microRNA expression. Here the richness of our understanding of MYC is reviewed, highlighting new biological insights and opportunities for cancer therapies.

The miR-17-92 cluster and its target THBS1 are differentially expressed in angiosarcomas dependent on MYC amplification

Angiosarcomas (ASs) represent a heterogeneous group of malignant vascular tumors that may occur spontaneously as primary tumors or secondarily after radiation therapy or in the context of chronic lymphedema. Most secondary ASs have been associated with MYC oncogene amplification, whereas the role of MYC abnormalities in primary AS is not well defined. Twenty-two primary and secondary ASs were analyzed by array-comparative genomic hybridization (aCGH) and by deep sequencing of small RNA libraries. By aCGH and subsequently confirmed by fluorescence in situ hybridization, MYC amplification was identified in three out of six primary tumors and in 8 out of 12 secondary AS. We have also found MAML1 as a new potential oncogene in MYC-amplified AS. Significant upregulation of the miR-17-92 cluster was observed in MYC-amplified AS compared to AS lacking MYC amplification and the control group (other vascular tumors, nonvascular sarcomas). Moreover, MYC-amplified ASs were associated with a significantly lower expression of thrombospondin-1 (THBS1) than AS without MYC amplification or controls. Altogether, our study implicates MYC amplification not only in the pathogenesis of secondary AS but also in a subset of primary AS. Thus, MYC amplification may play a crucial role in the angiogenic phenotype of AS through upregulation of the miR-17-92 cluster, which subsequently downregulates THBS1, a potent endogenous inhibitor of angiogenesis. © 2012 Wiley Periodicals, Inc.

MAD1 and its life as a MYC antagonist: an update

The MYC/MAX/MAD network is of central importance for controlling cell physiology. The network is compiled of transcriptional regulators that form different heterodimers, which can either activate or repress the expression of target genes. Thus these proteins function as a molecular switch to control gene expression. MAD1, a member of this network, acts as a transcriptional repressor. It interacts with MAX to form the OFF position of the switch, antagonizing MYC/MAX complexes that define the ON position. MAD1 regulates cell proliferation and apoptosis through a number of target genes. In addition recent evidence indicates that the expression and activity of MAD1 are regulated at multiple levels. Here the recent developments are summarized, in comparison to MYC, of our understanding how the expression of the MAD1 gene and protein are controlled and what the functional consequences and downstream effectors of MAD1 are, which relay its activity as a transcriptional regulator.

Miz-1 is required to coordinate the expression of TCRbeta and p53 effector genes at the pre-TCR "beta-selection" checkpoint

Miz-1 is a Broad-complex, Tramtrack and Bric-à-brac/pox virus zinc finger domain (BTB/POZ)-containing protein expressed in lymphoid precursors that can activate or repress transcription. We report in this article that mice expressing a nonfunctional Miz-1 protein lacking the BTB/POZ domain (Miz-1(ΔPOZ)) have a severe differentiation block at the pre-T cell "β-selection" checkpoint, evident by a drastic reduction of CD4(-)CD8(-) double-negative-3 (DN3) and DN4 cell numbers. T cell-specific genes including Rag-1, Rag-2, CD3ε, pTα, and TCRβ are expressed in Miz-1-deficient cells and V(D)J recombination is intact, but few DN3/DN4 cells express a surface pre-TCR. Miz-1-deficient DN3 cells are highly apoptotic and do not divide, which is consistent with enhanced expression of p53 target genes such as Cdkn1a, PUMA, and Noxa. However, neither coexpression of the antiapoptotic protein Bcl2 nor the deletion of p21(CIP1) nor the combination of both relieved Miz-1-deficient DN3/DN4 cells from their differentiation block. Only the coexpression of rearranged TCRαβ and Bcl2 fully rescued Miz-1-deficient DN3/DN4 cell numbers and enabled them to differentiate into DN4TCRβ(+) and double-positive cells. We propose that Miz-1 is a critical factor for the β-selection checkpoint and is required for both the regulation of p53 target genes and proper expression of the pre-TCR to support the proliferative burst of DN3 cells during T cell development.

Premetazoan ancestry of the Myc-Max network

The origin of metazoans required the evolution of mechanisms for maintaining differentiated cell types within a multicellular individual, in part through spatially differentiated patterns of gene transcription. The unicellular ancestor of metazoans was presumably capable of regulating gene expression temporally in response to changing environmental conditions, and spatial cell differentiation in metazoans may represent a co-option of preexisting regulatory mechanisms. Myc is a critical regulator of cell growth, proliferation, and death that is found in all metazoans but absent in other multicellular lineages, including fungi and plants. Homologs of Myc and its binding partner, Max, exist in two of the closest living relatives of animals, the choanoflagellate Monosiga brevicollis (Mb) and Capsaspora owczarzaki, a unicellular opisthokont that is closely related to metazoans and choanoflagellates. We find that Myc and Max from M. brevicollis heterodimerize and bind to both canonical and noncanonical E-boxes, the DNA-binding sites through which metazoan Myc proteins act. Moreover, in M. brevicollis, MbMyc protein can be detected in nuclear and flagellar regions. Like metazoan Max proteins, MbMax can form homodimers that bind to E-boxes. However, cross-species dimerization between Mb and human Myc and Max proteins was not observed, suggesting that the binding interface has diverged. Our results reveal that the Myc/Max network arose before the divergence of the choanoflagellate and metazoan lineages. Furthermore, core features of metazoan Myc function, including heterodimerization with Max, binding to E-box sequences in DNA, and localization to the nucleus, predate the origin of metazoans.