Xiao Jian Sun

The Leukemogenicity of AML1-ETO Is Dependent on Site-Specific Lysine Acetylation

A protein that drives the growth of leukemia does so only when it carries a specific posttranslational modification. The chromosomal translocations found in acute myelogenous leukemia (AML) generate oncogenic fusion transcription factors with aberrant transcriptional regulatory properties. Although therapeutic targeting of most leukemia fusion proteins remains elusive, the posttranslational modifications that control their function could be targetable. We found that AML1-ETO, the fusion protein generated by the t(8;21) translocation, is acetylated by the transcriptional coactivator p300 in leukemia cells isolated from t(8;21) AML patients, and that this acetylation is essential for its self-renewal–promoting effects in human cord blood CD34+ cells and its leukemogenicity in mouse models. Inhibition of p300 abrogates the acetylation of AML1-ETO and impairs its ability to promote leukemic transformation. Thus, lysine acetyltransferases represent a potential therapeutic target in AML.

Eriocalyxin B induces apoptosis of t(8;21) leukemia cells through NF-|[kappa]|B and MAPK signaling pathways and triggers degradation of AML1-ETO oncoprotein in a caspase-3-dependent manner

Diterpenoids isolated from Labiatae family herbs have strong antitumor activities with low toxicity. In this study, Eriocalyxin B (EriB), a diterpenoid extracted from Isodon eriocalyx, was tested on human leukemia/lymphoma cells and murine leukemia models. Acute myeloid leukemia cell line Kasumi-1 was most sensitive to EriB. Significant apoptosis was observed, concomitant with Bcl-2/Bcl-XL downregulation, mitochondrial instability and caspase-3 activation. AML1-ETO oncoprotein was degraded in parallel to caspase-3 activation. EriB-mediated apoptosis was associated with NF-κB inactivation by preventing NF-κB nuclear translocation and inducing IκBα cleavage, and disturbance of MAPK pathway by downregulating ERK1/2 phosphorylation and activating AP-1. Without affecting normal hematopoietic progenitor cells proliferation, EriB was effective on primary t(8;21) leukemia blasts and caused AML1-ETO degradation. In murine t(8;21) leukemia models, EriB remarkably prolonged the survival time or decreased the xenograft tumor size. Together, EriB might be a potential treatment for t(8;21) leukemia by targeting AML1-ETO oncoprotein and activating apoptosis pathways.

Genome-wide survey and developmental expression mapping of zebrafish SET domain-containing genes.

SET domain-containing proteins represent an evolutionarily conserved family of epigenetic regulators, which are responsible for most histone lysine methylation. Since some of these genes have been revealed to be essential for embryonic development, we propose that the zebrafish, a vertebrate model organism possessing many advantages for developmental studies, can be utilized to study the biological functions of these genes and the related epigenetic mechanisms during early development. To this end, we have performed a genome-wide survey of zebrafish SET domain genes. 58 genes total have been identified. Although gene duplication events give rise to several lineage-specific paralogs, clear reciprocal orthologous relationship reveals high conservation between zebrafish and human SET domain genes. These data were further subject to an evolutionary analysis ranging from yeast to human, leading to the identification of putative clusters of orthologous groups (COGs) of this gene family. By means of whole-mount mRNA in situ hybridization strategy, we have also carried out a developmental expression mapping of these genes. A group of maternal SET domain genes, which are implicated in the programming of histone modification states in early development, have been identified and predicted to be responsible for all known sites of SET domain-mediated histone methylation. Furthermore, some genes show specific expression patterns in certain tissues at certain stages, suggesting the involvement of epigenetic mechanisms in the development of these systems. These results provide a global view of zebrafish SET domain histone methyltransferases in evolutionary and developmental dimensions and pave the way for using zebrafish to systematically study the roles of these genes during development.

Histone H3 lysine 36 methyltransferase Hypb/Setd2 is required for embryonic vascular remodeling

HYPB is a human histone H3 lysine 36 (H3K36)–specific methyltransferase and acts as the ortholog of yeast Set2. This study explored the physiological function of mammalian HYPB using knockout mice. Homozygous disruption of Hypb impaired H3K36 trimethylation but not mono- or dimethylation, and resulted in embryonic lethality at E10.5-E11.5. Severe vascular defects were observed in the Hypb −/− embryo, yolk sac, and placenta. The abnormally dilated capillaries in mutant embryos and yolk sacs could not be remodeled into large blood vessels or intricate networks, and the aberrantly rounded mesodermal cells exhibited weakened interaction with endothelial cells. The embryonic vessels failed to invade the labyrinthine layer of placenta, which impaired the embryonic–maternal vascular connection. These defects could not be rescued by wild-type tetraploid blastocysts, excluding the possibility that they were caused by the extraembryonic tissues. Consistent with these phenotypes, gene expression profiling in wild-type and Hypb −/− yolk sacs revealed that the Hypb disruption altered the expression of some genes involved in vascular remodeling. At the cellular level, Hypb −/− embryonic stem cell–derived embryonic bodies, as well as in vitro–cultured human endothelial cells with siRNA-mediated suppression of HYPB, showed obvious defects in cell migration and invasion during vessel formation, suggesting an intrinsic role of Hypb in vascular development. Taken together, these results indicate that Hypb is required for embryonic vascular remodeling and provide a tool to study the function of H3K36 methylation in vasculogenesis/angiogenesis.

A stable transcription factor complex nucleated by oligomeric AML1-ETO controls leukaemogenesis

Transcription factors are frequently altered in leukaemia through chromosomal translocation, mutation or aberrant expression. AML1–ETO, a fusion protein generated by the t(8;21) translocation in acute myeloid leukaemia, is a transcription factor implicated in both gene repression and activation. AML1–ETO oligomerization, mediated by the NHR2 domain, is critical for leukaemogenesis, making it important to identify co-regulatory factors that ‘read’ the NHR2 oligomerization and contribute to leukaemogenesis. Here we show that, in human leukaemic cells, AML1–ETO resides in and functions through a stable AML1–ETO-containing transcription factor complex (AETFC) that contains several haematopoietic transcription (co)factors. These AETFC components stabilize the complex through multivalent interactions, provide multiple DNA-binding domains for diverse target genes, co-localize genome wide, cooperatively regulate gene expression, and contribute to leukaemogenesis. Within the AETFC complex, AML1–ETO oligomerization is required for a specific interaction between the oligomerized NHR2 domain and a novel NHR2-binding (N2B) motif in E proteins. Crystallographic analysis of the NHR2–N2B complex reveals a unique interaction pattern in which an N2B peptide makes direct contact with side chains of two NHR2 domains as a dimer, providing a novel model of how dimeric/oligomeric transcription factors create a new protein-binding interface through dimerization/oligomerization. Intriguingly, disruption of this interaction by point mutations abrogates AML1–ETO-induced haematopoietic stem/progenitor cell self-renewal and leukaemogenesis. These results reveal new mechanisms of action of AML1–ETO, and provide a potential therapeutic target in t(8;21)-positive acute myeloid leukaemia.

Setdb2 restricts dorsal organizer territory and regulates left–right asymmetry through suppressing fgf8 activity

Dorsal organizer formation is one of the most critical steps in early embryonic development. Several genes and signaling pathways that positively regulate the dorsal organizer development have been identified; however, little is known about the factor(s) that negatively regulates the organizer formation. Here, we show that Setdb2, a SET domain-containing protein possessing potential histone H3K9 methyltransferase activity, restricts dorsal organizer development and regulates left–right asymmetry by suppressing fibroblast growth factor 8 (fgf8) expression. Knockdown of Setdb2 results in a massive expansion of dorsal organizer markers floating head (flh), goosecoid (gsc), and chordin (chd), as well as a significant increase of fgf8, but not fgf4 mRNAs. Consequently, disrupted midline patterning and resultant randomization of left–right asymmetry are observed in Setdb2-deficient embryos. These characteristic changes induced by Setdb2 deficiency can be nearly corrected by either overexpression of a dominant-negative fgf receptor or knockdown of fgf8, suggesting an essential role for Setdb2–Fgf8 signaling in restricting dorsal organizer territory and regulating left–right asymmetry. These results provide unique evidence that a SET domain-containing protein potentially involved in the epigenetic control negatively regulates dorsal organizer formation during early embryonic development.

H3K36 Histone Methyltransferase Setd2 Is Required for Murine Embryonic Stem Cell Differentiation toward Endoderm

Setd2 is known as a histone-H3K36-specific methyltransferase. However, its role in physiological function remains unclear. In this study, we show that Setd2 mainly regulates differentiation of murine embryonic stem cells (mESCs) toward primitive endoderm. Furthermore, we show that downregulated endoderm-related genes in Setd2(-/-) mESCs are associated with an aberrantly low level of Erk activity and that enforced expression of Fgfr3 can rescue the defective Erk pathway in Setd2(-/-) mESCs. Interestingly, the transcriptional initiation of Fgfr3 is directly regulated through histone H3K36me3 modification in its distal promoter region by Setd2. These results indicate that Setd2 controls the primitive endoderm differentiation of mESCs by regulating the Fgfr3-Erk signaling.

The SANT Domain of p400 ATPase Represses Acetyltransferase Activity and Coactivator Function of TIP60 in Basal p21 Gene Expression

ABSTRACT The TIP60 histone acetyltransferase plays diverse roles in DNA damage responses, DNA double-strand break repair, and transcriptional regulation. TIP60 resides within a multisubunit complex that has been shown to be targeted by transcription factors and to be involved in histone acetylation and transcriptional activation. p400, an SWI2/SNF2-related ATPase that serves as an ATP-dependent chromatin remodeling enzyme, exists as an integral subunit of a TIP60 complex but also resides within a distinct complex that presumably lacks TIP60 and appears to be involved in the transcriptional repression of basal p53 target gene expression. Here, we describe a TIP60-containing p400 complex population in which the acetyltransferase activity of TIP60 is repressed by interactions with p400. We further show that an SWI3-ADA2-N-CoR-TFIIIB (SANT) domain of p400 binds directly to the histone acetyltransferase (HAT) domain of TIP60 and blocks both its enzymatic activity and its coactivator function in regulating basal p21 gene expression. Our results thus suggest that p400 represses basal p21 gene expression through dual mechanisms that include the direct inhibition of TIP60 enzymatic activity described here and the previously described ATP-dependent positioning of H2A.Z at the promoter.

The role of histone acetyltransferases in normal and malignant hematopoiesis

Histone, and non-histone, protein acetylation plays an important role in a variety of cellular events, including the normal and abnormal development of blood cells, by changing the epigenetic status of chromatin and regulating non-histone protein function. Histone acetyltransferases (HATs), which are the enzymes responsible for histone and non-histone protein acetylation, contain p300/CBP, MYST, and GNAT family members. HATs are not only protein modifiers and epigenetic factors but also critical regulators of cell development and carcinogenesis. Here, we will review the function of HATs such as p300/CBP, Tip60, MOZ/MORF, and GCN5/PCAF in normal hematopoiesis and the pathogenesis of hematological malignancies. The inhibitors that have been developed to target HATs will also be reviewed here. Understanding the roles of HATs in normal/malignant hematopoiesis will provide the potential therapeutic targets for the hematological malignancies.

Regulation of AKT signaling by Id1 controls t(8;21) leukemia initiation and progression

Transcriptional regulators are recurrently altered through translocations, deletions, or aberrant expression in acute myeloid leukemia (AML). Although critically important in leukemogenesis, the underlying pathogenetic mechanisms they trigger remain largely unknown. Here, we identified that Id1 (inhibitor of DNA binding 1) plays a pivotal role in acute myeloid leukemogenesis. Using genetically modified mice, we found that loss of Id1 inhibited t(8;21) leukemia initiation and progression in vivo by abrogating protein kinase B (AKT)1 activation, and that Id1 interacted with AKT1 through its C terminus. An Id1 inhibitor impaired the in vitro growth of AML cells and, when combined with an AKT inhibitor, triggered even greater apoptosis and growth inhibition, whereas normal hematopoietic stem/progenitor cells were largely spared. We then performed in vivo experiments and found that the Id1 inhibitor significantly prolonged the survival of t(8;21)(+) leukemic mice, whereas overexpression of activated AKT1 promoted leukemogenesis. Thus, our results establish Id1/Akt1 signaling as a potential therapeutic target in t(8;21) leukemia.