Panagiotis Ntziachristos

Genetic inactivation of the polycomb repressive complex 2 in T cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an immature hematopoietic malignancy driven mainly by oncogenic activation of NOTCH1 signaling1. In this study we report the presence of loss-of-function mutations and deletions of EZH2 and SUZ12 genes, encoding critical components of the Polycomb Repressive Complex 2 (PRC2) complex2,3, in 25% of T-ALLs. To further study the role of the PRC2 complex in T-ALL, we used NOTCH1-induced animal models of the disease, as well as human T-ALL samples, and combined locus-specific and global analysis of NOTCH1-driven epigenetic changes. These studies demonstrated that activation of NOTCH1 specifically induces loss of the repressive mark lysine-27 tri-methylation of histone 3 (H3K27me3)4 by antagonizing the activity of the Polycomb Repressive Complex 2 (PRC2) complex. These studies demonstrate a tumor suppressor role for the PRC2 complex in human leukemia and suggest a hitherto unrecognized dynamic interplay between oncogenic NOTCH1 and PRC2 function for the regulation of gene expression and cell transformation.

Contrasting roles of histone 3 lysine 27 demethylases in acute lymphoblastic leukaemia.

T-cell acute lymphoblastic leukaemia (T-ALL) is a haematological malignancy with a dismal overall prognosis, including a relapse rate of up to 25%, mainly because of the lack of non-cytotoxic targeted therapy options. Drugs that target the function of key epigenetic factors have been approved in the context of haematopoietic disorders, and mutations that affect chromatin modulators in a variety of leukaemias have recently been identified; however, ‘epigenetic’ drugs are not currently used for T-ALL treatment. Recently, we described that the polycomb repressive complex 2 (PRC2) has a tumour-suppressor role in T-ALL. Here we delineated the role of the histone 3 lysine 27 (H3K27) demethylases JMJD3 and UTX in T-ALL. We show that JMJD3 is essential for the initiation and maintenance of T-ALL, as it controls important oncogenic gene targets by modulating H3K27 methylation. By contrast, we found that UTX functions as a tumour suppressor and is frequently genetically inactivated in T-ALL. Moreover, we demonstrated that the small molecule inhibitor GSKJ4 (ref. 5) affects T-ALL growth, by targeting JMJD3 activity. These findings show that two proteins with a similar enzymatic function can have opposing roles in the context of the same disease, paving the way for treating haematopoietic malignancies with a new category of epigenetic inhibitors.

From Fly Wings to Targeted Cancer Therapies: A Centennial for Notch Signaling

Since Notch phenotypes in Drosophila melanogaster were first identified 100 years ago, Notch signaling has been extensively characterized as a regulator of cell-fate decisions in a variety of organisms and tissues. However, in the past 20 years, accumulating evidence has linked alterations in the Notch pathway to tumorigenesis. In this review, we discuss the protumorigenic and tumor-suppressive functions of Notch signaling, and dissect the molecular mechanisms that underlie these functions in hematopoietic cancers and solid tumors. Finally, we link these mechanisms and observations to possible therapeutic strategies targeting the Notch pathway in human cancers.

Notch pathway activation targets AML-initiating cell homeostasis and differentiation

Notch behaves as a tumor suppressor in AML, and Notch activation induces cell cycle arrest, differentiation, and apoptosis of AML-initiating cells.

Control of embryonic stem cell identity by BRD4-dependent transcriptional elongation of super-enhancer-associated pluripotency genes.

SUMMARY Transcription factors and chromatin-remodeling complexes are key determinants of embryonic stem cell (ESC) identity. Here, we demonstrate that BRD4, a member of the bromodomain and extraterminal domain (BET) family of epigenetic readers, regulates the self-renewal ability and pluripotency of ESCs. BRD4 inhibition resulted in induction of epithelial-tomesenchymal transition (EMT) markers and commitment to the neuroectodermal lineage while reducing the ESC multidifferentiation capacity in teratoma as-says. BRD4 maintains transcription of core stem cell genes such as OCT4 and PRDM14 by occupying their super-enhancers (SEs), large clusters of regulatory elements, and recruiting to them Mediator and CDK9, the catalytic subunit of the positive transcription elongation factor b (P-TEFb), to allow Pol-II-dependent productive elongation. Our study describes a mechanism of regulation of ESC identity that could be applied to improve the efficiency of ESC differentiation.

Deregulation of DUX4 and ERG in acute lymphoblastic leukemia

Chromosomal rearrangements deregulating hematopoietic transcription factors are common in acute lymphoblastic leukemia (ALL). Here we show that deregulation of the homeobox transcription factor gene DUX4 and the ETS transcription factor gene ERG is a hallmark of a subtype of B-progenitor ALL that comprises up to 7% of B-ALL. DUX4 rearrangement and overexpression was present in all cases and was accompanied by transcriptional deregulation of ERG, expression of a novel ERG isoform, ERGalt, and frequent ERG deletion. ERGalt uses a non-canonical first exon whose transcription was initiated by DUX4 binding. ERGalt retains the DNA-binding and transactivation domains of ERG, but it inhibits wild-type ERG transcriptional activity and is transforming. These results illustrate a unique paradigm of transcription factor deregulation in leukemia in which DUX4 deregulation results in loss of function of ERG, either by deletion or induced expression of an isoform that is a dominant-negative inhibitor of wild-type ERG function.

Histone demethylases in physiology and cancer: a tale of two enzymes, JMJD3 and UTX.

Gene regulation is fine-tuned by a dynamic balance between transcriptionally activating and repressive modifications of histone tails. It has been well-established that lysine and arginine methylation can be reversed by two groups of evolutionarily conserved enzymes known as histone demethylases, which have been shown to play critical roles in development, differentiation and diseases like cancer. Recent work has demonstrated demethylase-independent functions of these proteins, highlighting the complex mechanisms by which these proteins exert their effects on gene expression. Here, we discuss the roles of lysine 27 demethylases, JMJD3 and UTX, in cancer and potential therapeutic avenues targeting these enzymes. Despite a high degree of sequence similarity in the catalytic domain between JMJD3 and UTX, numerous studies revealed surprisingly contrasting roles in cellular reprogramming and cancer, particularly leukemia. Understanding the demethylase-dependent and demethylase-independent functions of the enzymes affecting histone methylation, their post-translational modifications and participation in different complexes, as well as in vivo modeling of the mutations affecting those enzymes in cancer, can shed light on their unique physiological roles. This information cumulated in the future will aid in the development of improved inhibitors to treat cancers affected by demethylase mutations and aberrant gene activation.

The RAG2 C-terminus and ATM protect genome integrity by controlling antigen receptor gene cleavage

Tight control of antigen-receptor gene rearrangement is required to preserve genome integrity and prevent the occurrence of leukemia and lymphoma. Nonetheless, mistakes can happen, leading to the generation of aberrant rearrangements, such as Tcra/d-Igh inter-locus translocations that are a hallmark of ATM deficiency. Current evidence indicates that these translocations arise from the persistence of unrepaired breaks converging at different stages of thymocyte differentiation. Here we show that a defect in feedback control of RAG2 activity gives rise to bi-locus breaks and damage on Tcra/d and Igh in the same T cell at the same developmental stage, which provides a direct mechanism for generating these inter-locus rearrangements. Both the RAG2 C-terminus and ATM prevent bi-locus RAG-mediated cleavage through modulation of 3D conformation (higher order loops) and nuclear organization of the two loci. This limits the number of potential substrates for translocation and provides an important mechanism for protecting genome stability.

Cardiac Myocyte KLF5 Regulates Ppara Expression and Cardiac Function

RATIONALE Fatty acid oxidation is transcriptionally regulated by peroxisome proliferator-activated receptor (PPAR)α and under normal conditions accounts for 70% of cardiac ATP content. Reduced Ppara expression during sepsis and heart failure leads to reduced fatty acid oxidation and myocardial energy deficiency. Many of the transcriptional regulators of Ppara are unknown. OBJECTIVE To determine the role of Krüppel-like factor 5 (KLF5) in transcriptional regulation of Ppara. METHODS AND RESULTS We discovered that KLF5 activates Ppara gene expression via direct promoter binding. This is blocked in hearts of septic mice by c-Jun, which binds an overlapping site on the Ppara promoter and reduces transcription. We generated cardiac myocyte-specific Klf5 knockout mice that showed reduced expression of cardiac Ppara and its downstream fatty acid metabolism-related targets. These changes were associated with reduced cardiac fatty acid oxidation, ATP levels, increased triglyceride accumulation, and cardiac dysfunction. Diabetic mice showed parallel changes in cardiac Klf5 and Ppara expression levels. CONCLUSIONS Cardiac myocyte KLF5 is a transcriptional regulator of Ppara and cardiac energetics.

A new player SETs in myeloid malignancy

Recent studies have identified recurrent mutations in SETBP1, the gene that encodes SET-binding protein 1, in several types of myeloid malignancies, including chronic myeloid and acute myeloid leukemias. The identified mutations frequently target the SKI-homologous domain, although the exact pathogenic mechanisms remain unknown.