Targeting MYC Expression through G-Quadruplexes

Solution structure of the major G-quadruplex formed in the human VEGF promoter in K+: insights into loop interactions of the parallel G-quadruplexes

Vascular endothelial growth factor (VEGF) proximal promoter region contains a poly G/C-rich element that is essential for basal and inducible VEGF expression. The guanine-rich strand on this tract has been shown to form the DNA G-quadruplex structure, whose stabilization by small molecules can suppress VEGF expression. We report here the nuclear magnetic resonance structure of the major intramolecular G-quadruplex formed in this region in K(+) solution using the 22mer VEGF promoter sequence with G-to-T mutations of two loop residues. Our results have unambiguously demonstrated that the major G-quadruplex formed in the VEGF promoter in K(+) solution is a parallel-stranded structure with a 1:4:1 loop-size arrangement. A unique capping structure was shown to form in this 1:4:1 G-quadruplex. Parallel-stranded G-quadruplexes are commonly found in the human promoter sequences. The nuclear magnetic resonance structure of the major VEGF G-quadruplex shows that the 4-nt middle loop plays a central role for the specific capping structures and in stabilizing the most favored folding pattern. It is thus suggested that each parallel G-quadruplex likely adopts unique capping and loop structures by the specific middle loops and flanking segments, which together determine the overall structure and specific recognition sites of small molecules or proteins.

LAY SUMMARY

The human VEGF is a key regulator of angiogenesis and plays an important role in tumor survival, growth and metastasis. VEGF overexpression is frequently found in a wide range of human tumors; the VEGF pathway has become an attractive target for cancer therapeutics. DNA G-quadruplexes have been shown to form in the proximal promoter region of VEGF and are amenable to small molecule drug targeting for VEGF suppression. The detailed molecular structure of the major VEGF promoter G-quadruplex reported here will provide an important basis for structure-based rational development of small molecule drugs targeting the VEGF G-quadruplex for gene suppression.

Synthesis, G-quadruplex stabilisation, docking studies, and effect on cancer cells of indolo[3,2-b]quinolines with one, two, or three basic side chains

G-quadruplex (G4) DNA structures in telomeres and oncogenic promoter regions are potential targets for cancer therapy, and G4 ligands have been shown to modulate telomerase activity and oncogene transcription. Herein we report the synthesis and G4 thermal stabilisation effects, determined by FRET melting assays, of 20 indolo[3,2-b]quinolines mono-, di-, and trisubstituted with basic side chains. Molecular modelling studies were also performed in an attempt to rationalise the ligands' binding poses with G4. Overall, the results suggest that ligand binding and G4 DNA thermal stabilisation increase with an N5-methyl or a 7-carboxylate group and propylamine side chains, whereas selectivity between G4 and duplex DNA appears to be modulated by the number and relative position of basic side chains. From all the indoloquinoline derivatives studied, the novel trisubstituted compounds 3 d and 4 d, bearing a 7-(aminoalkyl)carboxylate side chain, stand out as the most promising compounds; they show high G4 thermal stabilisation (ΔTm values between 17 and 8 °C) with an inter-G4 ΔTm trend of Hsp90A>KRas21R≈F21T>c-Kit2, 10-fold selectivity for G4 over duplex DNA, and 100-fold selectivity for the HCT116 cancer cell line (IC50 and IC90: <10 μM) over primary rat hepatocytes. Compounds 3 d and 4 d also decreased protein expression levels of Hsp90 and KRas in HCT116 cancer cells.

Molecular Pathways: Targeting MYC-induced metabolic reprogramming and oncogenic stress in cancer

MYC is a multifunctional transcription factor that is deregulated in many human cancers. MYC impacts a collaborative genetic program that orchestrates cell proliferation, metabolism, and stress responses. Although the progression of MYC-amplified tumors shows robust dependence on MYC activity, directly targeting MYC as a therapeutic method has proven to be technically difficult. Therefore, alternative approaches are currently under development with a focus on interference with MYC-mediated downstream effects. To fuel rapid cell growth, MYC reprograms cancer cell metabolism in a way that is substantially different from normal cells. The MYC-induced metabolic signature is characterized by enhanced glucose and glutamine uptake, increased lactate production, and altered amino acid metabolism. Targeting MYC-reprogrammed cancer cell metabolism is considered to be promising based on multiple preclinical studies. In addition, the increased biosynthetic demand of MYC-driven tumors coupled with limited nutrient access within tumor microenvironments create multiple levels of oncogenic stress, which can also be used as tumor-specific targets for pharmacologic intervention. Presumably, the best therapeutic strategy for treating MYC-amplified tumors is combined targeting of multiple MYC-mediated pathways, especially those involved in regulating cell proliferation, metabolism, and oncogenic stress.

Mechanistic studies for the role of cellular nucleic-acid-binding protein (CNBP) in regulation of c-myc transcription

BACKGROUND

Guanine-rich sequence of c-myc nuclease hypersensitive element (NHE) III1 is known to fold in G-quadruplex and subsequently serves as a transcriptional silencer. Cellular nucleic-acid-binding protein (CNBP), a highly conserved zinc-finger protein with multiple biological functions, could bind to c-myc NHE III1 region, specifically to the single strand G-rich sequence.

METHODS

In the present study, a variety of methods, including cloning, expression and purification of protein, EMSA, CD, FRET, Ch-IP, RNA interference, luciferase reporter assay, SPR, co-immunoprecipitation, and co-transfection, were applied to investigate the mechanism for the role of CNBP in regulating c-myc transcription.

RESULTS

We found that human CNBP specifically bound to the G-rich sequence of c-myc NHE III1 region both in vitro and in cellulo, and subsequently promoted the formation of G-quadruplex. CNBP could induce a transient decrease followed by an increase in c-myc transcription in vivo. The interaction of CNBP with NM23-H2 was responsible for the increase of c-myc transcription.

CONCLUSIONS

Based on above experimental results, a new mechanism, involving G-quadruplex related CNBP/NM23-H2 interaction, for the regulation of c-myc transcription was proposed.

GENERAL SIGNIFICANCE

These findings indicated that the regulation of c-myc transcription through NHE III1 region might be governed by mechanisms involving complex protein-protein interactions, and suggested a new possibility of CNBP as a potential anti-cancer target based on CNBP's biological function in c-myc transcription.

c-Myc and cancer metabolism

The processes of cellular growth regulation and cellular metabolism are closely interrelated. The c-Myc oncogene is a "master regulator" which controls many aspects of both of these processes. The metabolic changes which occur in transformed cells, many of which are driven by c-Myc overexpression, are necessary to support the increased need for nucleic acids, proteins, and lipids necessary for rapid cellular proliferation. At the same time, c-Myc overexpression results in coordinated changes in level of expression of gene families which result in increased cellular proliferation. This interesting duality of c-Myc effects places it in the mainstream of transformational changes and gives it a very important role in regulating the "transformed phenotype." The effects induced by c-Myc can occur either as a "primary oncogene" which is activated by amplification or translocation or as a downstream effect of other activated oncogenes. In either case, it appears that c-Myc plays a central role in sustaining the changes which occur with transformation. Although efforts to use c-Myc as a therapeutic target have been quite frustrating, it appears that this may change in the next few years.

In vivo NCL targeting affects breast cancer aggressiveness through miRNA regulation

Numerous studies have described the altered expression and the causal role of microRNAs (miRNAs) in human cancer. However, to date, efforts to modulate miRNA levels for therapeutic purposes have been challenging to implement. Here we find that nucleolin (NCL), a major nucleolar protein, posttranscriptionally regulates the expression of a specific subset of miRNAs, including miR-21, miR-221, miR-222, and miR-103, that are causally involved in breast cancer initiation, progression, and drug resistance. We also show that NCL is commonly overexpressed in human breast tumors and that its expression correlates with that of NCL-dependent miRNAs. Finally, inhibition of NCL using guanosine-rich aptamers reduces the levels of NCL-dependent miRNAs and their target genes, thus reducing breast cancer cell aggressiveness both in vitro and in vivo. These findings illuminate a path to novel therapeutic approaches based on NCL-targeting aptamers for the modulation of miRNA expression in the treatment of breast cancer.

Transcriptional stalling in B-lymphocytes: a mechanism for antibody diversification and maintenance of genomic integrity

B cells utilize three DNA alteration strategies-V(D)J recombination, somatic hypermutation (SHM) and class switch recombination (CSR)-to somatically mutate their genome, thereby expressing a plethora of antibodies tailor-made against the innumerable antigens they encounter while in circulation. Of these three events, the single-strand DNA cytidine deaminase, Activation Induced cytidine Deaminase (AID), is responsible for SHM and CSR. Recent advances, discussed in this review article, point toward various components of RNA polymerase II "stalling" machinery as regulators of AID activity during antibody diversification and maintenance of B cell genome integrity.

Possible regulatory roles of promoter g-quadruplexes in cardiac function-related genes - human TnIc as a model

G-quadruplexes (G4s) are four-stranded DNA secondary structures, which are involved in a diverse range of biological processes. Although the anti-cancer potential of G4s in oncogene promoters has been thoroughly investigated, the functions of promoter G4s in non-cancer-related genes are not well understood. We have explored the possible regulatory roles of promoter G4s in cardiac function-related genes using both computational and a wide range of experimental approaches. According to our bioinformatics results, it was found that potential G4-forming sequences are particularly enriched in the transcription regulatory regions (TRRs) of cardiac function-related genes. Subsequently, the promoter of human cardiac troponin I (TnIc) was chosen as a model, and G4s found in this region were subjected to biophysical characterisations. The chromosome 19 specific minisatellite G4 sequence (MNSG4) and near transcription start site (TSS) G4 sequence (−80 G4) adopt anti-parallel and parallel structures respectively in 100 mM KCl, with stabilities comparable to those of oncogene G4s. It was also found that TnIc G4s act cooperatively as enhancers in gene expression regulation in HEK293 cells, when stabilised by a synthetic G4-binding ligand. This study provides the first evidence of the biological significance of promoter G4s in cardiac function-related genes. The feasibility of using a single ligand to target multiple G4s in a particular gene has also been discussed.

Not all G-quadruplexes are created equally: an investigation of the structural polymorphism of the c-Myc G-quadruplex-forming sequence and its interaction with the porphyrin TMPyP4

G-quadruplexes, DNA tertiary structures highly localized to functionally important sites within the human genome, have emerged as important new drug targets. The putative G-quadruplex-forming sequence (Pu27) in the NHE-III(1) promoter region of the c-Myc gene is of particular interest as stabilization of this G-quadruplex with TMPyP4 has been shown to repress c-Myc transcription. In this study, we examine the Pu27 G-quadruplex-forming sequence and its interaction with TMPyP4. We report that the Pu27 sequence exists as a heterogeneous mixture of monomeric and higher-order G-quadruplex species in vitro and that this mixture can be partially resolved by size exclusion chromatography (SEC) separation. Within this ensemble of configurations, the equilibrium can be altered by modifying the buffer composition, annealing procedure, and dialysis protocol thereby affecting the distribution of G-quadruplex species formed. TMPyP4 was found to bind preferentially to higher-order G-quadruplex species suggesting the possibility of stabilization of the junctions of the c-Myc G-quadruplex multimers by porphyrin end-stacking. We also examined four modified c-Myc sequences that have been previously reported and found a narrower distribution of G-quadruplex configurations compared to the parent Pu27 sequence. We could not definitively conclude whether these G-quadruplex structures were selected from the original ensemble or if they are new G-quadruplex structures. Since these sequences differ considerably from the wild-type promoter sequence, it is unclear whether their structures have any actual biological relevance. Additional studies are needed to examine how the polymorphic nature of G-quadruplexes affects the interpretation of in vitro data for c-Myc and other G-quadruplexes. The findings reported here demonstrate that experimental conditions contribute significantly to G-quadruplex formation and should be carefully considered, controlled, and reported in detail.

A cyclometallated platinum complex as a selective optical switch for quadruplex DNA

Hits the spot: A cyclometalled platinum(II) complex with a substituted phenanthroline ligand is reported. The complex has high in vitro affinity for quadruplex DNA and upon binding its emission is switched on. The complex can be easily delivered to the cell by using a metallo-cage as a carrier (see illustration). By means of confocal microscopy, it is shown that the complex is released inside the cell, penetrates the nucleus and localises in the nucleoli.

Non-B DB v2.0: a database of predicted non-B DNA-forming motifs and its associated tools

The non-B DB, available at http://nonb.abcc.ncifcrf.gov, catalogs predicted non-B DNA-forming sequence motifs, including Z-DNA, G-quadruplex, A-phased repeats, inverted repeats, mirror repeats, direct repeats and their corresponding subsets: cruciforms, triplexes and slipped structures, in several genomes. Version 2.0 of the database revises and re-implements the motif discovery algorithms to better align with accepted definitions and thresholds for motifs, expands the non-B DNA-forming motifs coverage by including short tandem repeats and adds key visualization tools to compare motif locations relative to other genomic annotations. Non-B DB v2.0 extends the ability for comparative genomics by including re-annotation of the five organisms reported in non-B DB v1.0, human, chimpanzee, dog, macaque and mouse, and adds seven additional organisms: orangutan, rat, cow, pig, horse, platypus and Arabidopsis thaliana. Additionally, the non-B DB v2.0 provides an overall improved graphical user interface and faster query performance.

Promoter G-quadruplex sequences are targets for base oxidation and strand cleavage during hypoxia-induced transcription

The G-quadruplex, a non-B DNA motif that forms in certain G-rich sequences, is often located near transcription start sites in growth regulatory genes. Multiple lines of evidence show that reactive oxygen species generated as second messengers during physiologic signaling target specific DNA sequences for oxidative base modifications. Because guanine repeats are uniquely sensitive to oxidative damage, and G4 sequences are known "hot spots" for genetic mutation and DNA translocation, we hypothesized that G4 sequences are targeted for oxidative base modifications in hypoxic signaling. Approximately 25% of hypoxia-regulated genes in pulmonary artery endothelial cells harbored G4 sequences within their promoters. Chromatin immunoprecipitation showed that common base oxidation product 8-oxoguanine was selectively introduced into G4s, in promoters of hypoxia up-, down-, and nonregulated genes. Additionally, base excision DNA repair (BER) enzymes were recruited, and transient strand breaks formed in these sequences. Transcription factor Sp1, constitutively bound to G4 sequences in normoxia, was evicted as 8-oxoguanine accumulated during hypoxic exposure. Blocking hypoxia-induced oxidant production prevented both base modifications and decreased Sp1 binding. These findings suggest that oxidant stress in hypoxia causes oxidative base modifications, recruitment of BER enzymes, and transient strand breaks in G4 promoter sequences potentially altering G4 integrity and function.

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.

Role of DLC1 tumor suppressor gene and MYC oncogene in pathogenesis of human hepatocellular carcinoma: potential prospects for combined targeted therapeutics (review)

Hepatocellular carcinoma (HCC) is the third leading cause of cancer death, and its incidence is increasing worldwide in an alarming manner. The development of curative therapy for advanced and metastatic HCC is a high clinical priority. The HCC genome is complex and heterogeneous; therefore, the identification of recurrent genomic and related gene alterations is critical for developing clinical applications for diagnosis, prognosis and targeted therapy of the disease. This article focuses on recent research progress and our contribution in identifying and deciphering the role of defined genetic alterations in the pathogenesis of HCC. A significant number of genes that promote or suppress HCC cell growth have been identified at the sites of genomic reorganization. Notwithstanding the accumulation of multiple genetic alterations, highly recurrent changes on a single chromosome can alter the expression of oncogenes and tumor suppressor genes (TSGs) whose deregulation may be sufficient to drive the progression of normal hepatocytes to malignancy. A distinct and highly recurrent pattern of genomic imbalances in HCC includes the loss of DNA copy number (associated with loss of heterozygosity) of TSG-containing chromosome 8p and gain of DNA copy number or regional amplification of protooncogenes on chromosome 8q. Even though 8p is relatively small, it carries an unusually large number of TSGs, while, on the other side, several oncogenes are dispersed along 8q. Compelling evidence demonstrates that DLC1, a potent TSG on 8p, and MYC oncogene on 8q play a critical role in the pathogenesis of human HCC. Direct evidence for their role in the genesis of HCC has been obtained in a mosaic mouse model. Knockdown of DLC1 helps MYC in the induction of hepatoblast transformation in vitro, and in the development of HCC in vivo. Therapeutic interventions, which would simultaneously target signaling pathways governing both DLC1 and MYC functions in hepatocarcinogenesis, could result in progress in the treatment of liver cancer.

Gaining insights into the small molecule targeting of the G-quadruplex in the c-MYC promoter using NMR and an allele-specific transcriptional assay

G-quadruplexes (four-stranded DNA secondary structures) are showing promise as new targets for anticancer therapies. Specifically, G-quadruplexes in the proximal promoter region of regulatory genes have the potential to act as silencer elements and thereby turn off transcription. Thus, compounds that are capable of binding to and stabilizing G-quadruplexes would be of great benefit. In this chapter we describe two recent studies from our labs. In the first case, we use NMR to elucidate the structure of a 2:1 complex between a small molecule and the G-quadruplex in the c-MYC promoter. In the second case, we use an allele-specific transcription assay to demonstrate that the effect of a G-quadruplex-interactive compound is mediated directly through the G-quadruplex. Finally, we use this information to propose models for the interaction of various small molecules with the c-MYC G-quadruplex.

Solution structure of a 2:1 quindoline-c-MYC G-quadruplex: insights into G-quadruplex-interactive small molecule drug design

Unimolecular parallel-stranded G-quadruplex structures are found to be prevalent in gene promoters. The nuclease hypersensitivity element III(1) (NHE III(1)) of the c-MYC promoter can form transcriptionally active and silenced forms, and the formation of DNA G-quadruplex structures has been shown to be critical for c-MYC transcriptional silencing. The solution structure of a 2:1 quindoline-G-quadruplex complex has been solved and shows unexpected features, including the drug-induced reorientation of the flanking sequences to form a new binding pocket. While both 3' and 5' complexes show overall similar features, there are identifiable differences that emphasize the importance of both stacking and electronic interactions. For the first time, we describe the importance of the shape of the ligand as well as the two flanking bases in determining drug binding specificity. These structures provide important insights for the structure-based rational design of drugs that bind to unimolecular parallel G-quadruplexes commonly found in promoter elements.

Demonstration that drug-targeted down-regulation of MYC in non-Hodgkins lymphoma is directly mediated through the promoter G-quadruplex

Most transcription of the MYC proto-oncogene initiates in the near upstream promoter, within which lies the nuclease hypersensitive element (NHE) III(1) region containing the CT-element. This dynamic stretch of DNA can form at least three different topologies: single-stranded DNA, double-stranded DNA, or higher order secondary structures that silence transcription. In the current report, we identify the ellipticine analog GQC-05 (NSC338258) as a high affinity, potent, and selective stabilizer of the MYC G-quadruplex (G4). In cells, GQC-05 induced cytotoxicity with corresponding decreased MYC mRNA and altered protein binding to the NHE III(1) region, in agreement with a G4 stabilizing compound. We further describe a unique feature of the Burkitt's lymphoma cell line CA46 that allowed us to clearly demonstrate the mechanism and location of action of GQC-05 within this region of DNA and through the G4. Most importantly, these data present, as far as we are aware, the most direct evidence of intracellular G4-mediated control of a particular promoter.

Anticancer targets in the glycolytic metabolism of tumors: a comprehensive review

CANCER IS A METABOLIC DISEASE AND THE SOLUTION OF TWO METABOLIC EQUATIONS: to produce energy with limited resources and to fulfill the biosynthetic needs of proliferating cells. Both equations are solved when glycolysis is uncoupled from oxidative phosphorylation in the tricarboxylic acid cycle, a process known as the glycolytic switch. This review addresses in a comprehensive manner the main molecular events accounting for high-rate glycolysis in cancer. It starts from modulation of the Pasteur Effect allowing short-term adaptation to hypoxia, highlights the key role exerted by the hypoxia-inducible transcription factor HIF-1 in long-term adaptation to hypoxia, and summarizes the current knowledge concerning the necessary involvement of aerobic glycolysis (the Warburg effect) in cancer cell proliferation. Based on the many observations positioning glycolysis as a central player in malignancy, the most advanced anticancer treatments targeting tumor glycolysis are briefly reviewed.

Evaluation of fluorescent analogs of deoxycytidine for monitoring DNA transitions from duplex to functional structures

Topological variants of single-strand DNA (ssDNA) structures, referred to as “functional DNA,” have been detected in regulatory regions of many genes and are thought to affect gene expression. Two fluorescent analogs of deoxycytidine, Pyrrolo-dC (PdC) and 1,3-diaza-2-oxophenoxazine (tC^°), can be incorporated into DNA. Here, we describe spectroscopic studies of both analogs to determine fluorescent properties that report on structural transitions from double-strand DNA (dsDNA) to ssDNA, a common pathway in the transition to functional DNA structures. We obtained fluorescence-detected circular dichroism (FDCD) spectra, steady-state fluorescence spectra, and fluorescence lifetimes of the fluorophores in DNA. Our results show that PdC is advantageous in fluorescence lifetime studies because of a distinct ~2 ns change between paired and unpaired bases. However, tC^° is a better probe for FDCD experiments that report on the helical structure of DNA surrounding the fluorophore. Both fluorophores provide complementary data to measure DNA structural transitions.

Targeting G-quadruplexes in gene promoters: a novel anticancer strategy?

G-quadruplexes are four-stranded DNA structures that are over-represented in gene promoter regions and are viewed as emerging therapeutic targets in oncology, as transcriptional repression of oncogenes through stabilization of these structures could be a novel anticancer strategy. Many gene promoter G-quadruplexes have physicochemical properties and structural characteristics that might make them druggable, and their structural diversity suggests that a high degree of selectivity might be possible. Here, we describe the evidence for G-quadruplexes in gene promoters and discuss their potential as therapeutic targets, as well as progress in the development of strategies to harness this potential through intervention with small-molecule ligands.

Learning about the functions of NME/NM23: lessons from knockout mice to silencing strategies

The human NME gene family (also known as NM23) comprises ten genes that are involved in diverse physiological and pathological processes including proliferation, differentiation, development, ciliary functions, and metastasis. For the moment, only the NME1, NME2, and NME7 genes have been inactivated in transgenic knockout mice, as well as a double NME1-NME2 gene knockout. Mice lacking NME1 or NME2 grow to adulthood without health problems, although NME1 (-/-) mice have modest growth retardation. Double knockout NME1 (-/-)-NME2 (-/-) mice, by contrast, are highly hypotrophic and die at birth from profound anemia due to impaired erythroblast development. Evidence for a metastasis suppressor function of NME1 in vivo comes from crossing NME1 (-/-) mice with mice prone to develop hepatocellular carcinoma; the double transgenic mice present a higher incidence of lung metastases. Silencing of NME1 by siRNA interference has confirmed this function by conferring a "metastatic phenotype" on non-invasive human epithelial cancer cell lines. This function is specific to NME1 and is not observed when the NME2 is silenced. The data indicate that NME1 loss is causally involved at the early stages of the metastatic cascade. NME2 (-/-) mice and NME2 silencing experiments reveal a specific role of NME2 in activation of heterotrimeric G proteins and of KCa3.1 channel in T cells, pointing to a role of NME2 as a histidine phosphotransferase. Regarding NME7, consistent with its expression in axonemal structures, NME7 (-/-) mice present lesions similar to primary ciliary dyskinesia. This review summarizes the recent data obtained by knockout and silencing of NME/NM23 genes that provide mechanistic insights into their respective roles in physiology and pathology.

The evolving world of protein-G-quadruplex recognition: a medicinal chemist's perspective

The physiological and pharmacological role of nucleic acids structures folded into the non canonical G-quadruplex conformation have recently emerged. Their activities are targeted at vital cellular processes including telomere maintenance, regulation of transcription and processing of the pre-messenger or telomeric RNA. In addition, severe conditions like cancer, fragile X syndrome, Bloom syndrome, Werner syndrome and Fanconi anemia J are related to genomic defects that involve G-quadruplex forming sequences. In this connection G-quadruplex recognition and processing by nucleic acid directed proteins and enzymes represents a key event to activate or deactivate physiological or pathological pathways. In this review we examine protein-G-quadruplex recognition in physiologically significant conditions and discuss how to possibly exploit the interactions' selectivity for targeted therapeutic intervention.

Enigmatic MYC Conducts an Unfolding Systems Biology Symphony

The enigmatic MYC oncogene, which participates broadly in cancers, revealed itself recently as the maestro of an unfolding symphony of cell growth, proliferation, death, and metabolism. The study of MYC is arguably most challenging to its students but at the same time exhilarating when MYC reveals its deeply held secrets. It is the excitement of our richer understanding of MYC that is captured in each review of this special issue of Genes & Cancer. Collectively, our deeper understanding of MYC reveals that it is a symphony conductor, controlling a large orchestra of target genes. Although MYC controls many orchestra sections, which are necessary but not sufficient for Myc function, ribosome biogenesis stands out to reveal Myc's primordial function particularly in fruit flies. Because ribosome biogenesis and the associated translational machinery are bioenergetically demanding, Myc's other target genes involved in energy metabolism must be coupled with energy demand to ensure that cells can replicate their genome and produce daughter cells. Normal cells have feedback loops that diminish MYC expression when nutrients are scarce. On the other hand, when deregulated Myc transforms cells, their constitutive bioenergetic demand can trigger cell death when energy is unavailable. This special issue captures the unfolding symphony of MYC-mediated tumorigenesis through reviews that span from a timeline of MYC research, fundamental understanding of how the MYC gene itself is regulated, the study of Myc in model organisms, Myc function, and target genes to translational research in search of new therapeutic modalities for the treatment of cancer.