MLL-AFX requires the transcriptional effector domains of AFX to transform myeloid progenitors and transdominantly interfere with forkhead protein function

Unraveling MLL1-fusion leukemia: Epigenetic revelations from an iPS cell point mutation

Our understanding of acute leukemia pathology is heavily dependent on 11q23 chromosomal translocations involving the mixed lineage leukemia-1 (MLL1) gene, a key player in histone H3 lysine 4 (H3K4) methylation. These translocations result in MLL1-fusion (MLL1F) proteins that are thought to drive leukemogenesis. However, the mechanism behind increased H3K4 trimethylation in MLL1F-leukemic stem cells (MLL1F-LSCs), following loss of the catalytic SET domain of MLL1 (known for H3K4 monomethylation and dimethylation) remains unclear. In our investigation, we introduced a homozygous loss-of-function point mutation in MLL1 within human-induced pluripotent stem cells. This mutation mimics the histone methylation, gene expression, and epithelial-mesenchymal transition phenotypes of MLL1F-LSCs-without requiring a translocation or functional WT MLL1. The mutation caused a genome-wide redistribution of the H3K4 trimethyl mark and upregulated LSC-maintenance genes like HoxA9-A13, Meis1, and the HOTTIP long noncoding RNA. Epithelial-mesenchymal transition markers such as ZEB1, SNAI2, and HIC-5 were also increased leading to enhanced cellular migration and invasiveness. These observations underscore the essential role of MLL1's enzymatic activity in restraining the cascade of epigenetic changes associated with the gene-activating H3K4 trimethylation mark, which we show may be catalyzed by mislocalized SETd1a H3K4 trimethyltransferase in the absence of MLL1's enzymatic activity. Challenging existing models, our findings imply that MLL1F-induced leukemias arise from a dominant-negative impact on MLL1's histone methyltransferase activity. We propose targeting SETd1a in precision medicine as a new therapeutic approach for MLL1-associated leukemias.

Protein lysine acetyltransferase CBP/p300: A promising target for small molecules in cancer treatment

CBP and p300 are homologous proteins exhibiting remarkable structural and functional similarity. Both proteins function as acetyltransferase and coactivator, underscoring their significant roles in cellular processes. The function of histone acetyltransferases is to facilitate the release of DNA from nucleosomes and act as transcriptional co-activators to promote gene transcription. Transcription factors recruit CBP/p300 by co-condensation and induce transcriptional bursting. Disruption of CBP or p300 functions is associated with different diseases, especially cancer, which can result from either loss of function or gain of function. CBP and p300 are multidomain proteins containing HAT (histone acetyltransferase) and BRD (bromodomain) domains, which perform acetyltransferase activity and maintenance of HAT signaling, respectively. Inhibitors targeting HAT and BRD have been explored for decades, and some BRD inhibitors have been evaluated in clinical trials for treating hematologic malignancies or advanced solid tumors. Here, we review the development and application of CBP/p300 inhibitors. Several inhibitors have been evaluated in vivo, exhibiting notable potency but limited selectivity. Exploring these inhibitors emphasizes the promise of targeting CBP and p300 with small molecules in cancer therapy.

Role of the MOZ/MLL-mediated transcriptional activation system for self-renewal in normal hematopoiesis and leukemogenesis

Homeostasis in the blood system is maintained by the balance between self-renewing stem cells and nonstem cells. To promote self-renewal, transcriptional regulators maintain epigenetic information during multiple rounds of cell division. Mutations in such transcriptional regulators cause aberrant self-renewal, leading to leukemia. MOZ, a histone acetyltransferase, and MLL, a histone methyltransferase, are transcriptional regulators that promote the self-renewal of hematopoietic stem cells. Gene rearrangements of MOZ and MLL generate chimeric genes encoding fusion proteins that function as constitutively active forms. These MOZ and MLL fusion proteins constitutively activate transcription of their target genes and cause aberrant self-renewal in committed hematopoietic progenitors, which normally do not self-renew. Recent progress in the field suggests that MOZ and MLL are part of a transcriptional activation system that activates the transcription of genes with nonmethylated CpG-rich promoters. The nonmethylated state of CpGs is normally maintained during cell divisions from the mother cell to the daughter cells. Thus, the MOZ/MLL-mediated transcriptional activation system replicates the expression profile of mother cells in daughter cells by activating the transcription of genes previously transcribed in the mother cell. This review summarizes the functions of the components of the MOZ/MLL-mediated transcriptional activation system and their roles in the promotion of self-renewal.

Leukemogenesis via aberrant self-renewal by the MLL/AEP-mediated transcriptional activation system

Homeostasis of the hematopoietic system is achieved in a hierarchy, with hematopoietic stem cells at the pinnacle. Because only hematopoietic stem cells (HSCs) can self-renew, the size of the hematopoietic system is strictly controlled. In hematopoietic reconstitution experiments, 1 HSC can reconstitute the entire hematopoietic system, whereas 50 multipotent progenitors cannot. This indicates that only HSCs self-renew, whereas non-HSC hematopoietic progenitors are programmed to differentiate or senesce. Oncogenic mutations of the mixed lineage leukemia gene (MLL) overcome this "programmed differentiation" by conferring the self-renewing ability to non-HSC hematopoietic progenitors. In leukemia, mutated MLL proteins constitutively activate a broad range of previously transcribed CpG-rich promoters by an MLL-mediated transcriptional activation system. This system promotes self-renewal by replicating an expression profile similar to that of the mother cell in its daughter cells. In this transcriptional activation system, MLL binds to unmethylated CpG-rich promoters and recruits RNA polymerase II. MLL recruits p300/CBP through its transcriptional activation domain, which acetylates histone H3 at lysines 9, 18, and 27. The AF4 family/ENL family/P-TEFb complex (AEP) binds to acetylated H3K9/18/27 to activate transcription. Gene rearrangements of MLL with AEP- or CBP/p300-complex components generate constitutively active transcriptional machinery of this transcriptional activation system, which causes aberrant self-renewal of leukemia stem cells. Inhibitors of the components of this system effectively decrease their leukemogenic potential.

Molecular Dynamics Simulations of Human FOXO3 Reveal Intrinsically Disordered Regions Spread Spatially by Intramolecular Electrostatic Repulsion

The human transcription factor FOXO3 (a member of the 'forkhead' family of transcription factors) controls a variety of cellular functions that make it a highly relevant target for intervention in anti-cancer and anti-aging therapies. FOXO3 is a mostly intrinsically disordered protein (IDP). Absence of knowledge of its structural properties outside the DNA-binding domain constitutes a considerable obstacle to a better understanding of structure/function relationships. Here, I present extensive molecular dynamics (MD) simulation data based on implicit solvation models of the entire FOXO3/DNA complex, and accelerated MD simulations under explicit solvent conditions of a central region of particular structural interest (FOXO3^120-530). A new graphical tool for studying and visualizing the structural diversity of IDPs, the Local Compaction Plot (LCP), is introduced. The simulations confirm the highly disordered nature of FOXO3 and distinguish various degrees of folding propensity. Unexpectedly, two 'linker' regions immediately adjacent to the DNA-binding domain are present in a highly extended conformation. This extended conformation is not due to their amino acid composition, but rather is caused by electrostatic repulsion of the domains connected by the linkers. FOXO3 is thus an IDP present in an unusually extended conformation to facilitate interaction with molecular interaction partners.

The molecular functions of common and atypical MLL fusion protein complexes

Mixed-lineage leukemia (MLL) fuses with a variety of partners to produce a functionally altered MLL complex that is not expressed in normal cells, which transforms normal hematopoietic progenitors into leukemia cells. Because more than 80 fusion partners have been identified to date, the molecular functions of MLL fusion protein complexes appear diverse. However, over the past decade, the common functions utilized for leukemic transformation have begun to be elucidated. It appears that most (if not all) MLL fusion protein complexes utilize the AF4/ENL/P-TEFb and DOT1L complexes to some extent. Based on an understanding of the underlying molecular mechanisms, several molecular targeting drugs are being developed, opening paths to novel therapies. Here, we review the recent progress made in identifying the molecular functions of various MLL fusions and categorize the numerous fusion partners into several functionally-distinct groups to help discern commonalities and differences among various MLL fusion protein complexes.

Learning from mouse models of MLL fusion gene-driven acute leukemia

5-10% of human acute leukemias carry chromosomal translocations involving the mixed lineage leukemia (MLL) gene that result in the expression of chimeric protein fusing MLL to >80 different partners of which AF4, ENL and AF9 are the most prevalent. In contrast to many other leukemia-associated mutations, several MLL-fusions are powerful oncogenes that transform hematopoietic stem cells but also more committed progenitor cells. Here, I review different approaches that were used to express MLL fusions in the murine hematopoietic system which often, but not always, resulted in highly penetrant and transplantable leukemias that closely phenocopied the human disease. Due to its simple and reliable nature, reconstitution of irradiated mice with bone marrow cells retrovirally expressing the MLL-AF9 fusion became the most frequently in vivo model to study the biology of acute myeloid leukemia (AML). I review some of the most influential studies that used this model to dissect critical protein interactions, the impact of epigenetic regulators, microRNAs and microenvironment-dependent signals for MLL fusion-driven leukemia. In addition, I highlight studies that used this model for shRNA- or genome editing-based screens for cellular vulnerabilities that allowed to identify novel therapeutic targets of which some entered clinical trials. Finally, I discuss some inherent characteristics of the widely used mouse model based on retroviral expression of the MLL-AF9 fusion that can limit general conclusions for the biology of AML. This article is part of a Special Issue entitled: The MLL family of proteins in normal development and disease edited by Thomas A Milne.

The FOXO's Advantages of Being a Family: Considerations on Function and Evolution

The nematode Caenorhabditis elegans possesses a unique (with various isoforms) FOXO transcription factor DAF-16, which is notorious for its role in aging and its regulation by the insulin-PI3K-AKT pathway. In humans, five genes (including a protein-coding pseudogene) encode for FOXO transcription factors that are targeted by the PI3K-AKT axis, such as in C. elegans. This common regulation and highly conserved DNA-binding domain are the pillars of this family. In this review, I will discuss the possible meaning of possessing a group of very similar proteins and how it can generate additional functionality to more complex organisms. I frame this discussion in relation to the much larger super family of Forkhead proteins to which they belong. FOXO members are very often co-expressed in the same cell type. The overlap of function and expression creates a certain redundancy that might be a safeguard against the accidental loss of FOXO function, which could otherwise lead to disease, particularly, cancer. This is one of the points that will be examined in this "family affair" report.

FOXO3a from the Nucleus to the Mitochondria: A Round Trip in Cellular Stress Response

Cellular stress response is a universal mechanism that ensures the survival or negative selection of cells in challenging conditions. The transcription factor Forkhead box protein O3 (FOXO3a) is a core regulator of cellular homeostasis, stress response, and longevity since it can modulate a variety of stress responses upon nutrient shortage, oxidative stress, hypoxia, heat shock, and DNA damage. FOXO3a activity is regulated by post-translational modifications that drive its shuttling between different cellular compartments, thereby determining its inactivation (cytoplasm) or activation (nucleus and mitochondria). Depending on the stress stimulus and subcellular context, activated FOXO3a can induce specific sets of nuclear genes, including cell cycle inhibitors, pro-apoptotic genes, reactive oxygen species (ROS) scavengers, autophagy effectors, gluconeogenic enzymes, and others. On the other hand, upon glucose restriction, 5′-AMP-activated protein kinase (AMPK) and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) -dependent FOXO3a mitochondrial translocation allows the transcription of oxidative phosphorylation (OXPHOS) genes, restoring cellular ATP levels, while in cancer cells, mitochondrial FOXO3a mediates survival upon genotoxic stress induced by chemotherapy. Interestingly, these target genes and their related pathways are diverse and sometimes antagonistic, suggesting that FOXO3a is an adaptable player in the dynamic homeostasis of normal and stressed cells. In this review, we describe the multiple roles of FOXO3a in cellular stress response, with a focus on both its nuclear and mitochondrial functions.

Forkhead Domains of FOXO Transcription Factors Differ in both Overall Conformation and Dynamics

FOXO transcription factors regulate cellular homeostasis, longevity and response to stress. FOXO1 (also known as FKHR) is a key regulator of hepatic glucose production and lipid metabolism, and its specific inhibition may have beneficial effects on diabetic hyperglycemia by reducing hepatic glucose production. Moreover, all FOXO proteins are considered potential drug targets for drug resistance prevention in cancer therapy. However, the development of specific FOXO inhibitors requires a detailed understanding of structural differences between individual FOXO DNA-binding domains. The high-resolution structure of the DNA-binding domain of FOXO1 reported in this study and its comparison with structures of other FOXO proteins revealed differences in both their conformation and flexibility. These differences are encoded by variations in protein sequences and account for the distinct functions of FOXO proteins. In particular, the positions of the helices H1, H2 and H3, whose interface form the hydrophobic core of the Forkhead domain, and the interactions between hydrophobic residues located on the interface between the N-terminal segment, the H2-H3 loop, and the recognition helix H3 differ among apo FOXO1, FOXO3 and FOXO4 proteins. Therefore, the availability of apo structures of DNA-binding domains of all three major FOXO proteins will support the development of FOXO-type-specific inhibitors.

Forkhead box O4 transcription factor in human neoplasms: Cannot afford to lose the novel suppressor

Forkhead box O4 (FOXO4), a member of FOXO family, has been highlighted as an essential transcriptional regulator in many diverse carcinomas. Accumulated studies have demonstrated that FOXO4 is downregulated and associated with tumorigenesis, invasiveness, and metastasis of most human cancer. FOXO4 alteration is also closely linked to the prognosis of various types of cancer. The aim of this review is to comprehensively present the clinical and pathological significance of FOXO4 in human cancer. Additionally, the potential clinical applications of future FOXO4 research are discussed. Collectively, the information reviewed here should increase the potential of FOXO4 as a therapeutic target for cancer.

Transcription factors FOXO in the regulation of homeostatic hematopoiesis

PURPOSE OF REVIEW

Work in the past decade has revealed key functions of the evolutionary conserved transcription factors Forkhead box O (FOXO) in the maintenance of homeostatic hematopoiesis. Here the diverse array of FOXO functions in normal and diseased hematopoietic stem and progenitor cells is reviewed and the main findings in the past decade are highlighted. Future work should reveal FOXO-regulated networks whose alterations contribute to hematological disorders.

RECENT FINDINGS

Recent studies have identified unanticipated FOXO functions in hematopoiesis including in hematopoietic stem and progenitor cells (HSPC), erythroid cells, and immune cells. These findings suggest FOXO3 is critical for the regulation of mitochondrial and metabolic processes in hematopoietic stem cells, the balanced lineage determination, the T and B homeostasis, and terminal erythroblast maturation and red blood cell production. In aggregate these findings highlight the context-dependent function of FOXO in hematopoietic cells. Recent findings also question the nature of FOXO's contribution to heme malignancies as well as the mechanisms underlying FOXO's regulation in HSPC.

SUMMARY

FOXO are safeguards of homeostatic hematopoiesis. FOXO networks and their regulators and coactivators in HSPC are greatly complex and less well described. Identifications and characterizations of these FOXO networks in disease are likely to uncover disease-promoting mechanisms.

FOXO in Neural Cells and Diseases of the Nervous System

The evolutionarily conserved FOXO family of transcription factors has emerged as a significant arbiter of neural cell fate and function in mammals. From the neural stem cell (NSC) state through mature neurons under both physiological and pathological conditions, they have been found to modulate neural cell survival, stress responses, lineage commitment, and neuronal signaling. Lineage-specific FOXO knockout mice have provided an invaluable tool for the dissection of FOXO biology in the nervous system. Within the NSC compartments of the brain, FOXOs are required for the maintenance of NSC quiescence and for the clearance of reactive oxygen species. Within mature neurons, FOXO transcriptional activity is essential for the prevention of age-dependent axonal degeneration. Acutely, FOXO3 has been found to cause axonal degeneration upon withdrawal of neurotrophic factors. In more active neural signaling, FOXO6 promotes increased dendritic spine density of hippocampal neurons and is required for the consolidation of memories. In addition to the central nervous system (CNS), FOXOs also influence the functionality of the peripheral nervous system (PNS). FOXO1 knockout within the PNS results in a reduction of sympathetic tone and decreased levels of brain-derived norepinephrine and lower energy expenditure. FOXO3 knockout mice have impaired hearing which may be due to defects in synapse localization within the ear. Given the scope of FOXO activities in both the CNS and PNS, it will be of interest to study FOXOs within the context of neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, and amyotrophic lateral sclerosis. From within the nervous system, FOXOs may also regulate important parameters such as whole-body metabolism, motor function, and catecholamine production, making FOXOs key players in physiologic homeostasis.

FOXOs Maintaining the Equilibrium for Better or for Worse

A paradigm shift is emerging within the FOXO field and accumulating evidence indicates that we need to reappreciate the role of FOXOs, at least in cancer development. Here, we discuss the possibility that FOXOs are both tumor suppressors as well as promoters of tumor progression. This is mostly dependent on the biological context. Critical to this dichotomous role is the notion that FOXOs are central in preserving cellular homeostasis in redox control, genomic stability, and protein turnover. From this perspective, a paradoxical role in both suppressing and enhancing tumor progression can be reconciled. As many small molecules targeting the PI3K pathway are developed by big pharmaceutical companies and/or are in clinical trial, we will discuss what the consequences may be for the context-dependent role of FOXOs in tumor development in treatment options based on active PI3K signaling in tumors.

Mechanisms controlling the anti-neoplastic functions of FoxO proteins

The Forkhead box O (FoxO) proteins comprise a family of evolutionarily conserved transcription factors that predominantly function as tumor suppressors. These proteins assume diverse roles in the cellular anti-neoplastic response, including regulation of apoptosis and autophagy, cancer metabolism, cell-cycle arrest, oxidative stress and the DNA damage response. More recently, FoxO proteins have been implicated in cancer immunity and cancer stem-cell (CSC) homeostasis. Interestingly, in some sporadic sub-populations, FoxO protein function may also be manipulated by factors such as β-catenin whereby they instead can facilitate cancer progression via maintenance of CSC properties or promoting drug resistance or metastasis and invasion. This review highlights the essential biological functions of FoxOs and explores the areas that may be exploited in FoxO protein signaling pathways in the development of novel cancer therapeutic agents.

Structural characterization of interactions between transactivation domain 1 of the p65 subunit of NF-κB and transcription regulatory factors

p65 is a member of the NF-κB family of transcriptional regulatory proteins that functions as the activating component of the p65-p50 heterodimer. Through its acidic transactivation domain (TAD), p65 has the capacity to form interactions with several different transcriptional regulatory proteins, including TFIIB, TFIIH, CREB-binding protein (CBP)/p300 and TAFII31. Like other acidic TADs, the p65 TAD contains two subdomains (p65TA1 and p65TA2) that interact with different regulatory factors depending on the target gene. Despite its role in controlling numerous NF-κB target genes, there are no high-resolution structures of p65TA1 bound to a target transcriptional regulatory factor. In this work, we characterize the interaction of p65TA1 with two factors, the Tfb1/p62 subunit of TFIIH and the KIX domain of CBP. In these complexes, p65TA1 transitions into a helical conformation that includes its characteristic ΦXXΦΦ motif (Φ = hydrophobic amino acid). Structural and functional studies demonstrate that the two binding interfaces are primarily stabilized by three hydrophobic amino acids within the ΦXXΦΦ motif and these residues are also crucial to its ability to activate transcription. Taken together, the results provide an atomic level description of how p65TA1 is able to bind different transcriptional regulatory factors needed to activate NF-κB target genes.

miR-150 exerts antileukemia activity in vitro and in vivo through regulating genes in multiple pathways

MicroRNAs, a class of small noncoding RNAs, have been implicated to regulate gene expression in virtually all important biological processes. Although accumulating evidence demonstrates that miR-150, an important regulator in hematopoiesis, is deregulated in various types of hematopoietic malignancies, the precise mechanisms of miR-150 action are largely unknown. In this study, we found that miR-150 is downregulated in samples from patients with acute lymphoblastic leukemia, acute myeloid leukemia, and chronic myeloid leukemia, and normalized after patients achieved complete remission. Restoration of miR-150 markedly inhibited growth and induced apoptosis of leukemia cells, and reduced tumorigenicity in a xenograft leukemia murine model. Microarray analysis identified multiple novel targets of miR-150, which were validated by quantitative real-time PCR and luciferase reporter assay. Gene ontology and pathway analysis illustrated potential roles of these targets in small-molecule metabolism, transcriptional regulation, RNA metabolism, proteoglycan synthesis in cancer, mTOR signaling pathway, or Wnt signaling pathway. Interestingly, knockdown one of four miR-150 targets (EIF4B, FOXO4B, PRKCA, and TET3) showed an antileukemia activity similar to that of miR-150 restoration. Collectively, our study demonstrates that miR-150 functions as a tumor suppressor through multiple mechanisms in human leukemia and provides a rationale for utilizing miR-150 as a novel therapeutic agent for leukemia treatment.

FOXO transcription factors in cancer development and therapy

The forkhead box O (FOXO) transcription factors are considered as tumor suppressors that limit cell proliferation and induce apoptosis. FOXO gene alterations have been described in a limited number of human cancers, such as rhabdomyosarcoma, leukemia and lymphoma. In addition, FOXO proteins are inactivated by major oncogenic signals such as the phosphatidylinositol-3 kinase pathway and MAP kinases. Their expression is also repressed by micro-RNAs in multiple cancer types. FOXOs are mediators of the tumor response to various therapies. However, paradoxical roles of FOXOs in cancer progression were recently described. FOXOs contribute to the maintenance of leukemia-initiating cells in acute and chronic myeloid leukemia. These factors may also promote invasion and metastasis of subsets of colon and breast cancers. Resistance to treatment was also ascribed to FOXO activation in multiple cases, including targeted therapies. In this review, we discuss the complex role of FOXOs in cancer development and response to therapy.

The epigenetic regulators CBP and p300 facilitate leukemogenesis and represent therapeutic targets in acute myeloid leukemia

Growing evidence links abnormal epigenetic control to the development of hematological malignancies. Accordingly, inhibition of epigenetic regulators is emerging as a promising therapeutic strategy. The acetylation status of lysine residues in histone tails is one of a number of epigenetic post-translational modifications that alter DNA-templated processes, such as transcription, to facilitate malignant transformation. Although histone deacetylases are already being clinically targeted, the role of histone lysine acetyltransferases (KAT) in malignancy is less well characterized. We chose to study this question in the context of acute myeloid leukemia (AML), where, using in vitro and in vivo genetic ablation and knockdown experiments in murine models, we demonstrate a role for the epigenetic regulators CBP and p300 in the induction and maintenance of AML. Furthermore, using selective small molecule inhibitors of their lysine acetyltransferase activity, we validate CBP/p300 as therapeutic targets in vitro across a wide range of human AML subtypes. We proceed to show that growth retardation occurs through the induction of transcriptional changes that induce apoptosis and cell-cycle arrest in leukemia cells and finally demonstrate the efficacy of the KAT inhibitors in decreasing clonogenic growth of primary AML patient samples. Taken together, these data suggest that CBP/p300 are promising therapeutic targets across multiple subtypes in AML.

Redox regulation of FoxO transcription factors

Transcription factors of the forkhead box, class O (FoxO) family are important regulators of the cellular stress response and promote the cellular antioxidant defense. On one hand, FoxOs stimulate the transcription of genes coding for antioxidant proteins located in different subcellular compartments, such as in mitochondria (i.e. superoxide dismutase-2, peroxiredoxins 3 and 5) and peroxisomes (catalase), as well as for antioxidant proteins found extracellularly in plasma (e.g., selenoprotein P and ceruloplasmin). On the other hand, reactive oxygen species (ROS) as well as other stressful stimuli that elicit the formation of ROS, may modulate FoxO activity at multiple levels, including posttranslational modifications of FoxOs (such as phosphorylation and acetylation), interaction with coregulators, alterations in FoxO subcellular localization, protein synthesis and stability. Moreover, transcriptional and posttranscriptional control of the expression of genes coding for FoxOs is sensitive to ROS. Here, we review these aspects of FoxO biology focusing on redox regulation of FoxO signaling, and with emphasis on the interplay between ROS and FoxOs under various physiological and pathophysiological conditions. Of particular interest are the dual role played by FoxOs in cancer development and their key role in whole body nutrient homeostasis, modulating metabolic adaptations and/or disturbances in response to low vs. high nutrient intake. Examples discussed here include calorie restriction and starvation as well as adipogenesis, obesity and type 2 diabetes.

Forkhead followed by disordered tail: The intrinsically disordered regions of FOXO3a

Forkhead box Class O is one of 19 subfamilies of the Forkhead box family, comprising 4 human transcription factors: FOXO1, FOXO3a, FOXO4, and FOXO6, which are involved in many crucial cellular processes. FOXO3a is a tumor suppressor involved in multiple physiological and pathological processes, and plays essential roles in metabolism, cell cycle arrest, DNA repair, and apoptosis. In its role as a transcription factor, the FOXO3a binds a consensus Forkhead response element DNA sequence, and recruits transcriptional coactivators to activate gene transcription. FOXO3a has additional functions, such as regulating p53-mediated apoptosis and activating kinase ATM. With the exception of the structured DNA-binding forkhead domain, most of the FOXO3a sequence comprises intrinsically disordered regions (IDRs), including 3 regions (CR1-3) that are conserved within the FOXO subfamily. Numerous studies have demonstrated that these IDRs directly mediate many of the diverse functions of FOXO3a. These regions contain post-translational modification and protein-protein interaction sites that integrate upstream signals to maintain homeostasis. Thus, the FOXO3a IDRs are emerging as key mediators of diverse regulatory processes, and represent an important target for the future development of therapeutics for FOXO3a-related diseases.

Critical role of retinoid/rexinoid signaling in mediating transformation and therapeutic response of NUP98-RARG leukemia

While the nucleoporin 98-retinoic acid receptor gamma (NUP98-RARG) is the first RARG fusion protein found in acute leukemia, its roles and the molecular basis in oncogenic transformation are currently unknown. Here, we showed that homodimeric NUP98-RARG not only acquired unique nuclear localization pattern and ability of recruiting both RXRA and wild-type NUP98, but also exhibited similar transcriptional properties as RARA fusions found in acute promyelocytic leukemia (APL). Using murine bone marrow retroviral transduction/transformation assay, we further demonstrated that NUP98-RARG fusion protein had gained transformation ability of primary hematopoietic stem/progenitor cells, which was critically dependent on the C-terminal GLFG domain of NUP98 and the DNA binding domain (DBD) of RARG. In contrast to other NUP98 fusions, cells transformed by the NUP98-RARG fusion were extremely sensitive to all-trans retinoic acid (ATRA) treatment. Interestingly, while pan-RXR agonists, SR11237 and LGD1069 could specifically inhibit NUP98-RARG transformed cells, mutation of the RXR interaction domain in NUP98-RARG had little effect on its transformation, revealing that therapeutic functions of rexinoid can be independent of the direct biochemical interaction between RXR and the fusion. Together, these results indicate that deregulation of the retinoid/rexinoid signaling pathway has a major role and may represent a potential therapeutic target for NUP98-RARG-mediated transformation.

The PAX3-FOXO1 fusion protein present in rhabdomyosarcoma interferes with normal FOXO activity and the TGF-β pathway

PAX3-FOXO1 (PAX3-FKHR) is the fusion protein produced by the genomic translocation that characterizes the alveolar subtype of Rhabdomyosarcoma, a pediatric sarcoma with myogenic phenotype. PAX3-FOXO1 is an aberrant but functional transcription factor. It retains PAX3-DNA-binding activity and functionally overlaps PAX3 function while also disturbing it, in particular its role in myogenic differentiation. We herein show that PAX3-FOXO1 interferes with normal FOXO function. PAX3-FOXO1 affects FOXO-family member trans-activation capability and the FOXO-dependent TGF-β response. PAX3-FOXO1 may contribute to tumor formation by inhibiting the tumor suppressor activities which are characteristic of both FOXO family members and TGF-β pathways. The recognition of this mechanism raises new questions about how FOXO family members function.

Molecular mechanisms of MLL-associated leukemia

Gene rearrangements of the mixed lineage leukemia (MLL) gene cause aggressive leukemia. The fusion of MLL and its partner genes generates various MLL fusion genes, and their gene products trigger aberrant self-renewal of hematopoietic progenitors leading to leukemia. Since the identification of the MLL gene two decades ago, a substantial amount of information has been obtained regarding the mechanisms by which MLL mutations cause leukemia. Wild-type MLL maintains the expression of Homeobox (HOX) genes during development. MLL activates the expression of posterior HOX-A genes in the hematopoietic lineage to stimulate the expansion of immature progenitors. MLL fusion proteins constitutively activate the HOX genes, causing aberrant self-renewal. The modes of transcriptional activation vary depending on the fusion partners and can be categorized into at least four groups. Here I review the recent progress in research related to the molecular mechanisms of MLL fusion-dependent leukemogenesis.

Transcriptional/epigenetic regulator CBP/p300 in tumorigenesis: structural and functional versatility in target recognition

In eukaryotic cells, gene transcription is regulated by sequence-specific DNA-binding transcription factors that recognize promoter and enhancer elements near the transcriptional start site. Some coactivators promote transcription by connecting transcription factors to the basal transcriptional machinery. The highly conserved coactivators CREB-binding protein (CBP) and its paralog, E1A-binding protein (p300), each have four separate transactivation domains (TADs) that interact with the TADs of a number of DNA-binding transcription activators as well as general transcription factors (GTFs), thus mediating recruitment of basal transcription machinery to the promoter. Most promoters comprise multiple activator-binding sites, and many activators contain tandem TADs, thus multivalent interactions may stabilize CBP/p300 at the promoter, and intrinsically disordered regions in CBP/p300 and many activators may confer adaptability to these multivalent complexes. CBP/p300 contains a catalytic histone acetyltransferase (HAT) domain, which remodels chromatin to 'relax' its superstructure and enables transcription of proximal genes. The HAT activity of CBP/p300 also acetylates some transcription factors (e.g., p53), hence modulating the function of key transcriptional regulators. Through these numerous interactions, CBP/p300 has been implicated in complex physiological and pathological processes, and, in response to different signals, can drive cells towards proliferation or apoptosis. Dysregulation of the transcriptional and epigenetic functions of CBP/p300 is associated with leukemia and other types of cancer, thus it has been recognized as a potential anti-cancer drug target. In this review, we focus on recent exciting findings in the structural mechanisms of CBP/p300 involving multivalent and dynamic interactions with binding partners, which may pave new avenues for anti-cancer drug development.

miR-421 induces cell proliferation and apoptosis resistance in human nasopharyngeal carcinoma via downregulation of FOXO4

microRNAs have been demonstrated to play important roles in cancer development and progression. Hence, identifying functional microRNAs and better understanding of the underlying molecular mechanisms would provide new clues for the development of targeted cancer therapies. Herein, we reported that a microRNA, miR-421 played an oncogenic role in nasopharyngeal carcinoma. Upregulation of miR-421 induced, whereas inhibition of miR-421 repressed cell proliferation and apoptosis resistance. Furthermore, we found that upregulation of miR-421 inhibited forkhead box protein O4 (FOXO4) signaling pathway following downregulation of p21, p27, Bim and FASL expression by directly targeting FOXO4 3'UTR. Additionally, we demonstrated that FOXO4 expression is critical for miR-421-induced cell growth and apoptosis resistance. Taken together, our findings not only suggest that miR-421 promotes nasopharyngeal carcinoma cell proliferation and anti-apoptosis, but also uncover a novel regulatory mechanism for inactivation of FOXO4 in nasopharyngeal carcinoma.

Molecular and Epigenetic Mechanisms of MLL in Human Leukemogenesis

Epigenetics is often defined as the study of heritable changes in gene expression or chromosome stability that don’t alter the underlying DNA sequence. Epigenetic changes are established through multiple mechanisms that include DNA methylation, non-coding RNAs and the covalent modification of specific residues on histone proteins. It is becoming clear not only that aberrant epigenetic changes are common in many human diseases such as leukemia, but that these changes by their very nature are malleable, and thus are amenable to treatment. Epigenetic based therapies have so far focused on the use of histone deacetylase (HDAC) inhibitors and DNA methyltransferase inhibitors, which tend to have more general and widespread effects on gene regulation in the cell. However, if a unique molecular pathway can be identified, diseases caused by epigenetic mechanisms are excellent candidates for the development of more targeted therapies that focus on specific gene targets, individual binding domains, or specific enzymatic activities. Designing effective targeted therapies depends on a clear understanding of the role of epigenetic mutations during disease progression. The Mixed Lineage Leukemia (MLL) protein is an example of a developmentally important protein that controls the epigenetic activation of gene targets in part by methylating histone 3 on lysine 4. MLL is required for normal development, but is also mutated in a subset of aggressive human leukemias and thus provides a useful model for studying the link between epigenetic cell memory and human disease. The most common MLL mutations are chromosome translocations that fuse the MLL gene in frame with partner genes creating novel fusion proteins. In this review, we summarize recent work that argues MLL fusion proteins could function through a single molecular pathway, but we also highlight important data that suggests instead that multiple independent mechanisms underlie MLL mediated leukemogenesis.

PRMT1 interacts with AML1-ETO to promote its transcriptional activation and progenitor cell proliferative potential

Fusion protein AML1-ETO, resulting from t(8;21) translocation, is highly related to leukemia development. It has been reported that full-length AML1-ETO blocks AML1 function and requires additional mutagenic events to promote leukemia. We have previously shown that the expression of AE9a, a splice isoform of AML1-ETO, can rapidly cause leukemia in mice. To understand how AML1-ETO is involved in leukemia development, we took advantage of our AE9a leukemia model and sought to identify its interacting proteins from primary leukemic cells. Here, we report the discovery of a novel AE9a binding partner PRMT1 (protein arginine methyltransferase 1). PRMT1 not only interacts with but also weakly methylates arginine 142 of AE9a. Knockdown of PRMT1 affects expression of a specific group of AE9a-activated genes. We also show that AE9a recruits PRMT1 to promoters of AE9a-activated genes, resulting in enrichment of H4 arginine 3 methylation, H3 Lys9/14 acetylation, and transcription activation. More importantly, knockdown of PRMT1 suppresses the self-renewal capability of AE9a, suggesting a potential role of PRMT1 in regulating leukemia development.

Structures of KIX domain of CBP in complex with two FOXO3a transactivation domains reveal promiscuity and plasticity in coactivator recruitment

Forkhead box class O 3a (FOXO3a) is a transcription factor and tumor suppressor linked to longevity that determines cell fate through activating transcription of cell differentiation, survival, and apoptotic genes. Recruitment of the coactivator CBP/p300 is a crucial step in transcription, and we revealed that in addition to conserved region 3 (CR3) of FOXO3a, the C-terminal segment of CR2 (CR2C) binds CBP/p300 and contributes to transcriptional activity. CR2C and CR3 of FOXO3a interact with the KIX domain of CBP/p300 at both "MLL" and "c-Myb" binding sites simultaneously. A FOXO3a CR2C-CR3 peptide in complex with KIX exists in equilibrium between two equally populated conformational states, one of which has CR2C bound to the MLL site and CR3 bound to the c-Myb site, whereas in the other, CR2C and CR3 bind the c-Myb and MLL sites, respectively. This promiscuous interaction between FOXO3a and CBP/p300 is further supported by additional binding sites on CBP/p300, namely, the TAZ1 and TAZ2 domains. In functional studies, our structure-guided mutagenesis showed that both CR2C and CR3 are involved in the activation of certain endogenous FOXO3a target genes. Further, phosphorylation of S626, a known AMP-dependent protein kinase target in CR3, increased affinity for CBP/p300 and the phosphomimetic mutation enhanced transactivation of luciferase. These findings underscore the significance of promiscuous multivalent interactions and posttranslational modification in the recruitment of transcriptional coactivators, which may allow transcription factors to adapt to various gene-specific genomic and chromatin structures and respond to cell signals.

Oncogenic activation of FOXR1 by 11q23 intrachromosomal deletion-fusions in neuroblastoma

Neuroblastoma tumors frequently show loss of heterozygosity of chromosome 11q with a shortest region of overlap in the 11q23 region. These deletions are thought to cause inactivation of tumor suppressor genes leading to haploinsufficiency. Alternatively, micro-deletions could lead to gene fusion products that are tumor driving. To identify such events we analyzed a series of neuroblastomas by comparative genomic hybridization and single-nucleotide polymorphism arrays and integrated these data with Affymetrix mRNA profiling data with the bioinformatic tool R2 (http://r2.amc.nl). We identified three neuroblastoma samples with small interstitial deletions at 11q23, upstream of the forkhead-box R1 transcription factor (FOXR1). Genes at the proximal side of the deletion were fused to FOXR1, resulting in fusion transcripts of MLL-FOXR1 and PAFAH1B2-FOXR1. FOXR1 expression has only been detected in early embryogenesis. Affymetrix microarray analysis showed high FOXR1 mRNA expression exclusively in the neuroblastomas with micro-deletions and rare cases of other tumor types, including osteosarcoma cell line HOS. RNAi silencing of FOXR1 strongly inhibited proliferation of HOS cells and triggered apoptosis. Expression profiling of these cells and reporter assays suggested that FOXR1 is a negative regulator of fork-head box factor-mediated transcription. The neural crest stem cell line JoMa1 proliferates in culture conditional to activity of a MYC-ER transgene. Over-expression of the wild-type FOXR1 could functionally replace MYC and drive proliferation of JoMa1. We conclude that FOXR1 is recurrently activated in neuroblastoma by intrachromosomal deletion/fusion events, resulting in overexpression of fusion transcripts. Forkhead-box transcription factors have not been previously implicated in neuroblastoma pathogenesis. Furthermore, this is the first identification of intrachromosomal fusion genes in neuroblastoma.

Akt, FoxO and regulation of apoptosis

Forkhead box O (FoxO) transcription factors are downstream targets of the serine/threonine protein kinase B (PKB)/Akt. The Akt kinase regulates processes of cellular proliferation and survival. Phosphorylation of FoxOs by Akt inhibits transcriptional functions of FoxOs and contributes to cell survival, growth and proliferation. Emerging evidence suggests involvement of FoxOs in diverse intracellular signaling pathways with critical roles in a number of physiological as well as pathological conditions including cancer. The FoxO signaling is regulated by their interactions with other intracellular proteins as well as their post-translational modifications such as phosphorylation. FoxOs promote cell growth inhibitory and/or apoptosis signaling by either inducing expression of multiple pro-apoptotic members of the Bcl2-family of mitochondria-targeting proteins, stimulating expression of death receptor ligands such as Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), or enhancing levels of various cyclin-dependent kinase inhibitors (CDKIs). Coupled with their ability to cross-talk with p53, FoxOs represent an important class of tumor suppressors in a variety of cancers. This review summarizes our current understanding of mechanisms by which Akt and FoxOs regulate cell growth and survival that in turn offers opportunities for development of novel strategies to combat cancer. This article is part of a Special Issue entitled: P13K-AKT-FOxO axis in cancer and aging.

Histone H3 lysine 79 methyltransferase Dot1 is required for immortalization by MLL oncogenes

Chimeric oncoproteins resulting from fusion of MLL to a wide variety of partnering proteins cause biologically distinctive and clinically aggressive acute leukemias. However, the mechanism of MLL-mediated leukemic transformation is not fully understood. Dot1, the only known histone H3 lysine 79 (H3K79) methyltransferase, has been shown to interact with multiple MLL fusion partners including AF9, ENL, AF10, and AF17. In this study, we utilize a conditional Dot1l deletion model to investigate the role of Dot1 in hematopoietic progenitor cell immortalization by MLL fusion proteins. Western blot and mass spectrometry show that Dot1-deficient cells are depleted of the global H3K79 methylation mark. We find that loss of Dot1 activity attenuates cell viability and colony formation potential of cells immortalized by MLL oncoproteins but not by the leukemic oncoprotein E2a-Pbx1. Although this effect is most pronounced for MLL-AF9, we find that Dot1 contributes to the viability of cells immortalized by other MLL oncoproteins that are not known to directly recruit Dot1. Cells immortalized by MLL fusions also show increased apoptosis, suggesting the involvement of Dot1 in survival pathways. In summary, our data point to a pivotal requirement for Dot1 in MLL fusion protein-mediated leukemogenesis and implicate Dot1 as a potential therapeutic target.

MLL fusions: pathways to leukemia

Human leukemias with chromosomal band 11q23 aberrations that disrupt the MLL/HRX/ALL-1 gene portend poor prognosis. MLL associated leukemias account for the majority of infant leukemia, approximately 10% of adult de novo leukemia and approximately 33% of therapy related acute leukemia with a balanced chromosome translocation. The 500 kD MLL precursor is processed by Taspase1 to generate mature MLL(N320/C180), which orchestrates many aspects of biology such as embryogenesis, cell cycle, cell fate and stem cell maintenance. Leukemogenic MLL translocations fuse the common MLL N-terminus (approximately 1,400 aa) in frame with more than 60 translocation partner genes (TPGs). Recent studies on MLL and MLL leukemia have greatly advanced our knowledge concerning the normal function of MLL and its deregulation in leukemogenesis. Here, we summarize the critical biological and pathological activities of MLL and MLL fusions, and discuss available models and potential therapeutic targets of MLL associated leukemias.

Self-association mediated by the Ras association 1 domain of AF6 activates the oncogenic potential of MLL-AF6

MLL is a common target for chromosomal translocations associated with acute leukemia resulting in its fusion with a large variety of nuclear or cytoplasmic proteins that may activate its oncogenic properties by distinct but poorly understood mechanisms. The MLL-AF6 fusion gene represents the most common leukemogenic fusion of mixed lineage leukemia (MLL) to a cytoplasmic partner protein. Here, we identified a highly conserved Ras association (RA1) domain at the amino-terminus of AF6 as the minimal region sufficient for MLL-AF6 mediated myeloid progenitor immortalization in vitro and short latency leukemogenesis in vivo. Moreover, the ability of RA1 to activate MLL oncogenesis is conserved with its Drosophila ortholog, Canoe. Although the AF6 RA1 domain has previously been defined as an interaction surface for guanosine triphosphate-bound Ras, single amino acid substitutions known to abolish the AF6-Ras interaction did not abrogate MLL-AF6-mediated oncogenesis. Furthermore, fusion of MLL to heterologous RA domains of c-Raf1 or RalGDS, or direct fusion of MLL to constitutively active K-RAS, H-RAS, or RAP1 was not sufficient for oncogenic activation of MLL. Rather, the AF6 RA1 domain efficiently mediated self-association, suggesting that constitutive MLL self-association is a more common pathogenic mechanism for MLL oncogenesis than indicated by previous studies of rare MLL fusion partners.

MLL/AF10(OM-LZ)-immortalized cells expressed cytokines and induced host cell proliferation in a mouse bone marrow transplantation model

Several mouse models studying the MLL fusion-induced leukemic transformation showed that a myeloproliferation stage precedes leukemia or occurred as the only phenotype of hematological disorder in mice. We established 6 MLL/AF10(OM-LZ)-immortalized cell lines by retrovirally transducing the fusion gene into bone marrow cells from B6 or congenic GFP-B6 mice. Immunophenotypic and cytological analyses revealed that the immortalized cell lines could be divided into 2 types. Type I had a high percentage of cells expressing monocytic lineage marker CD115 in the medium containing IL3 and could terminally differentiate into granulocytes and monocytes in response to granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor (M-CSF) treatments, respectively. On the other hand, type II had a low percentage of cells expressing CD115. The type II cell lines could not differentiate into granulocytes by G-CSF treatment and died rapidly in response to M-CSF treatment. Transplantation of both types I and II cells induced lethal myeloproliferative disease (MPD)-like myeloid leukemia in most of the sublethally irradiated B6 mice. Flow cytometric analysis of GFP and lineage markers of the peripheral blood cells from MPD mice revealed that the monocytes and granulocytes were generated not only from the donor cells but also from the host cells. RT-PCR analysis revealed that the MLL/AF10(OM-LZ)-immortalized cells expressed mRNAs encoding colony-stimulating factors (CSFs) of M-CSF and GM-CSF and inflammatory cytokines of IL-1alpha, IL-1beta and TNF-alpha. Our results showed that the MLL/AF10(OM-LZ)-immortalized cells could induce host cell proliferation in the transplanted mice, probably through stimulation by CSFs or cytokines produced by the donor cells.

A higher-order complex containing AF4 and ENL family proteins with P-TEFb facilitates oncogenic and physiologic MLL-dependent transcription

AF4 and ENL family proteins are frequently fused with MLL, and they comprise a higher order complex (designated AEP) containing the P-TEFb transcription elongation factor. Here, we show that AEP is normally recruited to MLL-target chromatin to facilitate transcription. In contrast, MLL oncoproteins fused with AEP components constitutively form MLL/AEP hybrid complexes to cause sustained target gene expression, which leads to transformation of hematopoietic progenitors. Furthermore, MLL-AF6, an MLL fusion with a cytoplasmic protein, does not form such hybrid complexes, but nevertheless constitutively recruits AEP to target chromatin via unknown alternative mechanisms. Thus, AEP recruitment is an integral part of both physiological and pathological MLL-dependent transcriptional pathways. Bypass of its normal recruitment mechanisms is the strategy most frequently used by MLL oncoproteins.

Synergistic interplay between promoter recognition and CBP/p300 coactivator recruitment by FOXO3a

FOXO3a is a transcription factor belonging to the forkhead box O-Class (FOXO) subfamily, and it regulates metabolism, cell-cycle arrest, cell differentiation, and apoptosis through activating or suppressing gene transcription. FOXO3a contains a well-folded DNA-binding forkhead (FH) domain, but a large portion of the remaining protein sequence (75% of the total) is predicted to comprise intrinsically disordered regions (IDRs). Within the IDRs, there are three conserved regions (CR1-CR3), and it has been shown that CR3 (residues D610-N650) is a transactivation domain that recruits the coactivator histone acetyltransferase (HAT) CBP/p300, through binding to its KIX domain. In a previous study, we determined the solution structure of the FH domain and identified an intramolecular interaction between FH and CR3 domains of FOXO3a. Here we illustrate that the KIX domain of CBP interacts with the central core region (L620-A635) of CR3, which also internally interacts with the FH domain. In this heterotypic interplay, FH prevents CR3 from binding to KIX; however, upon binding to the Forkhead response element (FRE) DNA, the FH domain releases the CR3 domain, allowing it to interact with KIX. While previous studies have shown that the transactivation domains of c-Myb and MLL bind to distinct sites on KIX, our results indicate that FOXO3a CR3 has an ability to bind to both of these sites. These results suggest a model of FOXO3a-dependent coactivator recruitment in which the dynamic interplay between KIX and FH domains for binding to CR3 plays a key regulatory role in gene transcription activation.

The FOX transcription factors regulate vascular pathology, diabetes and Tregs

A small number of upstream master genes in "higher hierarchy" controls the expression of a large number of downstream genes and integrates the signaling pathways underlying the pathogenesis of cardiovascular diseases with or without autoimmune inflammatory mechanisms. In this brief review, we organize our analysis of recent progress in characterization of forkhead (FOX) transcription factor family members in vascular pathology, diabetes and regulatory T cells into the following sections: (1) Overview of the FOX transcription factor superfamily; (2) Vascular pathology of mice deficient in FOX transcription factors; (3) Roles of FOX transcription factors in endothelial cell pathology; (4) Roles of FOX transcription factors in vascular smooth muscle cells; (5) Roles of FOX transcription factors in the pathogenesis of diabetes; and (6) Immune system phenotypes of mice deficient in FOX transcription factors. Advances in these areas suggest that the FOX transcription factor family plays important roles in vascular development and in the pathogenesis of autoimmune inflammatory cardiovascular diseases.

Retroviral/Lentiviral transduction and transformation assay

Non-random chromosomal translocations can be found in about half of acute leukaemia patients and mostly lead to either over-expression of proto-oncogenes or creation of novel fusion genes. To assess the oncogenic potential and characterize the underlying mechanisms mediated by these candidate oncoproteins, a retroviral transduction/transformation assay (RTTA) has been successfully employed to study the biological impacts of a number of proto-oncoproteins and novel fusion proteins in primary hematopoietic cells both in vitro and in vivo. To further widen the application of the RTTA, a lentiviral transduction/transformation assay (LTTA) has also been developed to target the most quiescent hematopoietic stem cells (HSCs). This chapter will cover both the RTTA and LTTA for studying candidate oncogenes involved in human leukaemia.

Epigenetic mechanisms in AML - a target for therapy

Epigenetics refers to a stable, mitotically perpetuated regulatory mechanism of gene expression without an alteration of the coding sequence. Epigenetic mechanism include DNA methylation and histone tail modifications. Epigenetic regulation is part of physiologic development and becomes abnormal in neoplasia, where silencing of critical genes by DNA methylation or histone deacetylation can contribute to leukemogenesis as an alternative to deletion or loss-of-function mutation. In acute myelogenous leukemia (AML), aberrant DNA methylation can be observed in multiple functionally relevant genes such as p15, p 73, E-cadherin, ID 4, RARbeta2. Abnormal activities of histone tail-modifying enzymes have also been seen in AML, frequently as a direct result of chromosomal translocations. It is now clear that these epigenetic changes play a significant role in development and progression of AML, and thus constitute important targets of therapy. The aim of targeting epigenetic effector protein or "epigenetic therapy" is to reverse epigenetic silencing and reactive various genes to induce a therapeutic effect such as differentiation, growth arrest, or apoptosis. Recent clinical studies have shown the relative safety and efficacy of such epigenetic therapies.

The forkhead transcription factors play important roles in vascular pathology and immunology

Transcription factor families are a small number of upstream master genes in "higher hierarchy" that control the expression of a large number of downstream genes. These transcription factors have been found to integrate the signaling pathways underlying the pathogenesis of cardiovascular diseases with or without autoimmune inflammatory mechanisms. In this chapter, we organize our analysis of recent progress in characterization of forkhead (Fox) transcription factor family members in vascular pathology and immune regulation into the following sections: (1) Introduction of the FOX transcription factor superfamily; (2) FOX transcription factors and endotheial cell pathology; (3) FOX transcription factors and vascular smooth muscle cells; and (4) FOX transcription factors, inflammation and immune system. Advances in these areas suggest that the FOX transcription factor family is important in regulating vascular development and the pathogenesis of autoimmune inflammatory cardiovascular diseases.

Identification and analysis of evolutionary selection pressures acting at the molecular level in five forkhead subfamilies

Background

Members of the forkhead gene family act as transcription regulators in biological processes including development and metabolism. The evolution of forkhead genes has not been widely examined and selection pressures at the molecular level influencing subfamily evolution and differentiation have not been explored. Here, in silico methods were used to examine selection pressures acting on the coding sequence of five multi-species FOX protein subfamily clusters; FoxA, FoxD, FoxI, FoxO and FoxP.

Results

Application of site models, which estimate overall selection pressures on individual codons throughout the phylogeny, showed that the amino acid changes observed were either neutral or under negative selection. Branch-site models, which allow estimated selection pressures along specified lineages to vary as compared to the remaining phylogeny, identified positive selection along branches leading to the FoxA3 and Protostomia clades in the FoxA cluster and the branch leading to the FoxO3 clade in the FoxO cluster. Residues that may differentiate paralogs were identified in the FoxA and FoxO clusters and residues that differentiate orthologs were identified in the FoxA cluster. Neutral amino acid changes were identified in the forkhead domain of the FoxA, FoxD and FoxP clusters while positive selection was identified in the forkhead domain of the Protostomia lineage of the FoxA cluster. A series of residues under strong negative selection adjacent to the N- and C-termini of the forkhead domain were identified in all clusters analyzed suggesting a new method for refinement of domain boundaries. Extrapolation of domains among cluster members in conjunction with selection pressure information allowed prediction of residue function in the FoxA, FoxO and FoxP clusters and exclusion of known domain function in residues of the FoxA and FoxI clusters.

Conclusion

Consideration of selection pressures observed in conjunction with known functional information allowed prediction of residue function and refinement of domain boundaries. Identification of residues that differentiate orthologs and paralogs provided insight into the development and functional consequences of paralogs and forkhead subfamily composition differences among species. Overall we found that after gene duplication of forkhead family members, rapid differentiation and subsequent fixation of amino acid changes through negative selection has occurred.

Glycogen synthase kinase 3 in MLL leukaemia maintenance and targeted therapy

Glycogen synthase kinase 3 (GSK3) is a multifunctional serine/threonine kinase that participates in numerous signalling pathways involved in diverse physiological processes. Several of these pathways are implicated in disease pathogenesis, which has prompted efforts to develop GSK3-specific inhibitors for therapeutic applications. However, before now, there has been no strong rationale for targeting GSK3 in malignancies. Here we report pharmacological, physiological and genetic studies that demonstrate an oncogenic requirement for GSK3 in the maintenance of a specific subtype of poor prognosis human leukaemia, genetically defined by mutations of the MLL proto-oncogene. In contrast to its previously characterized roles in suppression of neoplasia-associated signalling pathways, GSK3 paradoxically supports MLL leukaemia cell proliferation and transformation by a mechanism that ultimately involves destabilization of the cyclin-dependent kinase inhibitor p27(Kip1). Inhibition of GSK3 in a preclinical murine model of MLL leukaemia provides promising evidence of efficacy and earmarks GSK3 as a candidate cancer drug target.

Unravelling the tumor-suppressive functions of FOXO proteins

Members of the forkhead box O (FOXO) family of transcription factors have been postulated to be tumor suppressors because of their established roles in cell-cycle arrest, apoptosis, DNA-damage repair and scavenging of reactive oxygen species. Recently, several animal model studies have shown that the FOXO proteins are indeed tumor suppressors. Furthermore, FOXO proteins have recently been implicated in the negative regulation of signaling by the hypoxia-inducible factor 1 during vascular development, raising the possibility that the FOXO proteins suppress not only tumor formation but also tumor angiogenesis and, possibly, metastasis. Here, we discuss recent advances in the understanding of the roles of FOXO family members in tumor suppression.

Review: genetic models of acute myeloid leukaemia

The use of genetically engineered mice (GEM) have been critical in understanding disease states such as cancer, and none more so than acute myelogenous leukaemia (AML), a disease characterized by over 100 distinct chromosomal translocations. A substantial proportion of cases exhibiting recurrent reciprocal translocations at diagnosis, such as t(8;21) or t(15;17) have been exhaustively studied and are currently employed in clinical diagnosis. However, a definitive conclusion regarding the leukaemogenic potential of defined transgenes for this disease remains elusive. While it is increasingly apparent that a number of cooperating mutations are necessary to develop a leukaemic phenotype, the number of models reflecting these synergisms remains few. Furthermore, little emphasis has been paid to the effect of chromosomal translocations other than recurrent genetic abnormalities, with no models reflecting the multiple abnormalities observed in high-risk cases of AML accounting for 8-10% of adult AML. Here we review the differing technologies employed in generation of GEM of AML. We discuss the relevance of GEM AML from embryonic stem cell-mediated (for example retinoic acid receptor-alpha fusions and AML1/ETO) models; through to the valuable retroviral-mediated gene transfer models. The latter have been used to great effect in defining the transforming properties of chromosomal translocation products such as MLL (found in 5-6% of all AML cases) and NUP98 (denoting poor prognosis in therapy-related disease) and particularly when co-transduced with bad prognostic factors such as Flt3 mutations. Finally, we comment on the emergence of newer transduction technologies, which can regulate the level of expression to defined cell lineages in both primary murine and human xenografts, and discuss how combining multiple genetic modalities, more relevant models of this complex disease are being generated.