Molecular mechanisms of leukemogenesis mediated by MLL fusion proteins

Epigenetic tête-à-tête: the bilateral relationship between chromatin modifications and DNA methylation

The epigenome, which comprises chromatin, associated proteins, and the pattern of covalent modification of DNA by methylation, sets up and maintains gene expression programs. It was originally believed that DNA methylation was the dominant reaction in determining the chromatin structure. However, emerging data suggest that chromatin can affect DNA methylation in both directions, triggering either de novo DNA methylation or demethylation. These events are particularly important for the understanding of cellular transformation, which requires a coordinated change in gene expression profiles. While genetic alterations can explain some of the changes, the important role of epigenetic reprogramming is becoming more and more evident. Cancer cells exhibit a paradoxical coexistence of global loss of DNA methylation with regional hypermethylation.

Cdx4 and menin co-regulate Hoxa9 expression in hematopoietic cells

Background

Transcription factor Cdx4 and transcriptional coregulator menin are essential for Hoxa9 expression and normal hematopoiesis. However, the precise mechanism underlying Hoxa9 regulation is not clear.

Methods and Findings

Here, we show that the expression level of Hoxa9 is correlated with the location of increased trimethylated histone 3 lysine 4 (H3K4M3). The active and repressive histone modifications co-exist along the Hoxa9 regulatory region. We further demonstrate that both Cdx4 and menin bind to the same regulatory region at the Hoxa9 locus in vivo, and co-activate the reporter gene driven by the Hoxa9 cis-elements that contain Cdx4 binding sites. Ablation of menin abrogates Cdx4 access to the chromatin target and significantly reduces both active and repressive histone H3 modifications in the Hoxa9 locus.

Conclusion

These results suggest a functional link among Cdx4, menin and histone modifications in Hoxa9 regulation in hematopoietic cells.

Proteomics-based strategy to identify biomarkers and pharmacological targets in leukemias with t(4;11) translocations

Translocations and other aberrations involving the MLL (mixed lineage leukemia) gene result in aggressive forms of leukemias. Heterogeneity in partner genes, in chromosomal breakpoints, in MLL itself, and in the different partner genes results in heterogeneous fusion transcripts that can be alternatively spliced, which complicates deciphering a unifying mechanism of leukemogenesis. However, recent microarray studies completed with clinical leukemia specimens have uncovered several distinct mRNA signatures within MLL leukemia that differ from other types of leukemia. A global proteomics strategy using MV4-11 and RS4:11 cells in culture was employed to investigate possible protein signatures common to different MLL leukemias and to identify disease biomarkers and protein targets for pharmacological intervention. Initial proteomics screening experiments with two-dimensional differential in-gel electrophoresis revealed heat shock protein 90 alpha (HSP90alpha) as a potential target for pharmacological inhibition and nucleoside diphosphate kinase (nm23) as a biomarker for measuring treatment efficacy. Using a modified stable isotope labeling of amino acids in cell culture (SILAC) approach, coupled with two-dimensional liquid chromatography tandem mass spectrometry (2D-LC-MS/MS), changes in abundance for over 500 proteins were measured. In addition, decreased expression of the novel biomarker nm23 was observed during HSP90 inhibition with 17-allylamino-17-demethoxygeldanamycin (17-AAG) in the MV4-11 cell line. The present study validates the use of a global proteomics strategy to uncover novel biomarkers and pharmacological targets for leukemias with MLL translocations. Additionally, several proteins were found to be expressed in concordance with microarray studies of mRNA expression in specimens from patients showing the value in comparing mRNA transcript and proteomic profiles. This work represents one of the most comprehensive proteomics screens of MLL leukemias that have been conducted to date.

The MLL partial tandem duplication in acute myeloid leukaemia

Mixed lineage leukaemia gene-partial tandem duplications (MLL-PTD) characterise acute myeloid leukaemia (AML) with trisomy 11 and AML with a normal karyotype. MLL-PTD confer a worse prognosis with shortened overall and event free survival in childhood and adult AML. In spite of these clinical observations, the leukaemogenic mechanism has, so far, not been determined. This review summarises clinical studies on MLL-PTD positive AML and recent experimental findings on the putative leukaemogenic role of MLL-PTD.

Resistance of infant leukemia with MLL rearrangement to tumor necrosis factor-related apoptosis-inducing ligand: a possible mechanism for poor sensitivity to antitumor immunity

Malignant cells generally acquire some immune escape mechanisms for clonal expansion. Immune escape mechanisms also contribute to the failure of graft-versus-leukemia (GVL) effect after allogeneic hematopoietic stem cell transplantation (allo-SCT). Infant leukemias with mixed-lineage leukemia (MLL) rearrangement have a remarkably short latency, and GVL effect after allo-SCT has not been clearly evidenced in these leukemias. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)- and FasL-mediated cytotoxic pathways play important roles in cytotoxic T-lymphocyte- and natural killer cell-mediated antitumor immunity and optimal GVL activity. We investigated the in vitro sensitivity of MLL-rearranged acute lymphoblastic leukemia (ALL) and acute myeloblastic leukemia (AML) cells to TRAIL- and FasL-mediated cytotoxicity. Most of cell lines and primary leukemia cells were highly resistant to TRAIL primarily owing to low cell-surface expression of death receptors in ALL and simultaneous expression of decoy receptors in AML. Nearly half of cell lines and majority of primary leukemia cells showed low sensitivity to FasL. These results suggest that resistance to death-inducing ligands, particularly to TRAIL, could be one of the mechanisms for a rapid clonal expansion and a poor sensitivity to the GVL effect in infant leukemias with MLL rearrangement.

Leukemia stem cells

Cancer is a multifaceted disease in which cell proliferation is no longer under normal growth control. Accumulating data have suggested the existence of cancer stem cells, a minor population of tumor cells that possess the stem cell property of self-renewal and that are responsible for the initiation and maintenance of cancer. The knowledge of cancer stem cell biology is most advanced in research on the hematopoietic cancer, leukemia. With the identification of leukemia stem cells (LSCs) capable of repopulating nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, this body of research has led to conclusive proof for cancer stem cells. This review focuses on the biological characterization of LSCs for each type of leukemia, which has provided key insights into leukemogenic pathology and LSC-targeted therapies.

Molecular cytogenetic findings of acute leukemia included in the Brazilian Collaborative Study Group of Infant acute leukemia

BACKGROUND

Chromosome abnormalities often occur prenatally in childhood leukemia, characterizing an early event in leukemogenesis. The majority of the abnormalities occurring in infants involve the MLL gene on chromosome band 11q23. We describe the molecular cytogenetic findings of 207 infant acute leukemia (IAL) cases included in the Brazilian Collaborative Study Group of Infant acute leukemia.

PROCEDURE

The diagnosis of Acute Lymphoblastic leukemia (ALL) or acute myeloblastic leukemia (AML) was made according to morphology and immunophenotyping classification, followed by conventional karyotyping. Samples were then screened using RT-PCR for the presence of specific chromosome translocations. FISH assay for MLL rearrangements was performed only in cases with negative or inconclusive cytogenetic or PCR results.

RESULTS

The characteristics of children with IAL were as follows: 115 boys and 92 girls, age range 0-23 months, mean age 12 months, 145 ALL, and 62 AML. A statistically significant association was observed between pro-B ALL cases and MLL+ve (P=0.0001) cases and the age group 0-3 months with MLL+ve (P=0.008) cases. Two rare cases of pro-T ALL with MLL+ve were found. Other than MLL rearrangements, various other molecular aberrations were detected including TEL/AML1+ve (n=9), E2A/PBX1+ve (n=4), PML/RARA+ve (n=4), and AML1/ETO+ve (n=2). Cytogenetic analysis revealed hyperdiploidy (n=6), del(7) in two cases and del(11)(q23) in seven cases.

CONCLUSIONS

Our results show that not only MLL rearrangements, but also other molecular abnormalities occur before birth and may contribute to leukemogenesis.

Histone deacetylase inhibitors (HDI) cause DNA damage in leukemia cells: a mechanism for leukemia-specific HDI-dependent apoptosis?

Histone deacetylase inhibitors (HDI) increase gene expression through induction of histone acetylation. However, it remains unclear whether increases in specific gene expression events determine the apoptotic response following HDI administration. Herein, we show that a variety of HDI trigger in hematopoietic cells not only widespread histone acetylation and DNA damage responses but also actual DNA damage, which is significantly increased in leukemic cells compared with normal cells. Thus, increase in H2AX and ataxia telangiectasia mutated (ATM) phosphorylation, early markers of DNA damage, occurs rapidly following HDI administration. Activation of the DNA damage and repair response following HDI treatment is further emphasized by localizing DNA repair proteins to regions of DNA damage. These events are followed by subsequent apoptosis of neoplastic cells but not normal cells. Our data indicate that induction of apoptosis by HDI may result predominantly through accumulation of excessive DNA damage in leukemia cells, leading to activation of apoptosis.

Proteolysis of MLL family proteins is essential for taspase1-orchestrated cell cycle progression

Taspase1 was identified as the threonine endopeptidase that cleaves mixed-lineage leukemia (MLL) for proper Hox gene expression in vitro. To investigate its functions in vivo, we generated Taspase1(-/-) mice. Taspase1 deficiency results in noncleavage (nc) of MLL and MLL2 and homeotic transformations. Remarkably, our in vivo studies uncover an unexpected role of Taspase1 in the cell cycle. Taspase1(-/-) animals are smaller in size. Taspase1(-/-) mouse embryonic fibroblasts (MEFs) exhibit impaired proliferation, and acute deletion of Taspase1 leads to a marked reduction of thymocytes. Taspase1 deficiency incurs down-regulation of Cyclin Es, As, and Bs and up-regulation of p16(Ink4a) . We show that MLL and MLL2 directly target E2Fs for Cyclin expression. The uncleaved precursor MLL displays a reduced histone H3 methyl transferase activity in vitro. Accordingly, chromatin immunoprecipitation assays demonstrate a markedly decreased histone H3 K4 trimethylation at Cyclin E1 and E2 genes in Taspase1(-/-) cells. Furthermore, MLL(nc/nc;2nc/nc) MEFs are also impaired in proliferation. Our data are consistent with a model in which precursor MLLs, activated by Taspase1, target to Cyclins through E2Fs to methylate histone H3 at K4, leading to activation. Lastly, Taspase1(-/-) cells are resistant to oncogenic transformation, and Taspase1 is overexpressed in many cancer cell lines. Thus, Taspase1 may serve as a target for cancer therapeutics.

Identification and characterization of leukemia stem cells in murine MLL-AF9 acute myeloid leukemia

Using a mouse model of human acute myeloid leukemia (AML) induced by the MLL-AF9 oncogene, we demonstrate that colony-forming cells (CFCs) in the bone marrow and spleen of leukemic mice are also leukemia stem cells (LSCs). These self-renewing cells (1) are frequent, accounting for 25%-30% of myeloid lineage cells at late-stage disease; (2) generate a phenotypic, morphologic, and functional leukemia cell hierarchy; (3) express mature myeloid lineage-specific antigens; and (4) exhibit altered microenvironmental interactions by comparison with the oncogene-immortalized CFCs that initiated the disease. Therefore, the LSCs responsible for sustaining, expanding, and regenerating MLL-AF9 AML are downstream myeloid lineage cells, which have acquired an aberrant Hox-associated self-renewal program as well as other biologic features of hematopoietic stem cells.

Aberrant transcriptional regulation of the MLL fusion partner EEN by AML1-ETO and its implication in leukemogenesis

The EEN (extra eleven nineteen) gene, located on chromosome 19p13, was cloned as a fusion with MLL from a patient with acute myeloid leukemia (AML) with translocation t(11;19)(q23;p13). In this study, we characterized the genomic structure of the EEN gene, including its 5′ regulatory region and transcription start site (TSS). We found that Sp1 could bind to the guanine-cytosine (GC)–stretch of the EEN promoter and was critical for the normal EEN expression, whereas the leukemia-associated fusion protein AML1-ETO could aberrantly transactivate the EEN gene through an AML1 binding site. Of note, overexpressed EEN showed oncogenic properties, such as transforming potential in NIH3T3 cells, stimulating cell proliferation, and increasing the activity of transcriptional factor AP-1. Retroviral transduction of EEN increased self-renewal and proliferation of murine hematopoietic progenitor cells. Moreover, Kasumi-1 and HL60-cell growth was inhibited with down-regulation of EEN by RNAi. These findings demonstrate that EEN might be a common target in 2 major types of AML associated with MLL or AML1 translocations, and overexpression of EEN may play an essential role in leukemogenesis.

Leukaemic transformation by CALM-AF10 involves upregulation of Hoxa5 by hDOT1L

Chromosomal translocation is a common cause of leukaemia and the most common chromosome translocations found in leukaemia patients involve the mixed lineage leukaemia (MLL) gene. AF10 is one of more than 30 MLL fusion partners in leukaemia. We have recently demonstrated that the H3K79 methyltransferase hDOT1L contributes to MLL-AF10-mediated leukaemogenesis through its interaction with AF10 (ref. 5). In addition to MLL, AF10 has also been reported to fuse to CALM (clathrin-assembly protein-like lymphoid-myeloid) in patients with T-cell acute lymphoblastic leukaemia (T-ALL) and acute myeloid leukaemia (AML). Here, we analysed the molecular mechanism of leukaemogenesis by CALM-AF10. We demonstrate that CALM-AF10 fusion is both necessary and sufficient for leukaemic transformation. Additionally, we provide evidence that hDOT1L has an important role in the transformation process. hDOT1L contributes to CALM-AF10-mediated leukaemic transformation by preventing nuclear export of CALM-AF10 and by upregulating the Hoxa5 gene through H3K79 methylation. Thus, our study establishes CALM-AF10 fusion as a cause of leukaemia and reveals that mistargeting of hDOT1L and upregulation of Hoxa5 through H3K79 methylation is the underlying mechanism behind leukaemia caused by CALM-AF10 fusion.

The Mll-Een knockin fusion gene enhances proliferation of myeloid progenitors derived from mouse embryonic stem cells and causes myeloid leukaemia in chimeric mice

Rearrangement of the mixed lineage leukaemia (MLL) gene with extra eleven nineteen (EEN) was previously identified in an infant with acute myeloid leukaemia. Using homologous recombination, we have created a mouse equivalent of the human MLL-EEN allele and showed that when Mll(Een/+) embryonic stem (ES) cells were induced to differentiate in vitro into haemopoietic cells, there was increased proliferation of myeloid progenitors with self-renewal property. We also generated Mll(Een/+) chimeric mice, which developed leukaemia displaying enlarged livers, spleens, thymuses and lymph nodes owing to infiltration of Mll(Een/+)-expressing leukemic cells. Immunophenotyping of cells from enlarged organs and bone marrow (BM) of the Mll(Een/+) chimeras revealed an accumulation of Mac-1+/Gr-1- immature myeloid cells and a reduction in normal B- and T-cell populations. We observed differential regulation of Hox genes between myeloid cells derived from Mll(Een/+) ES cells and mouse BM leukemic cells which suggested different waves of Hox expression may be activated by MLL fusion proteins for initiation (in ES cells) and maintenance (in leukemic cells) of the disease. We believe studies of MLL fusion proteins in ES cells combined with in vivo animal models offer new approaches to the dissection of molecular events in multistep pathogenesis of leukaemia.

Topoisomerase II and the etiology of chromosomal translocations

Acute leukemias with balanced chromosomal translocations, protean morphologic and immunophenotypic presentations but generally shorter latency and absence of myelodysplasia are recognized as a complication of anti-cancer drugs that behave as topoisomerase II poisons. Translocations affecting the breakpoint cluster region of the MLL gene at chromosome band 11q23 are the most common molecular genetic aberrations in leukemias associated with the topoisomerase II poisons. These agents perturb the cleavage-religation equilibrium of topoisomerase II and increase cleavage complexes. One model suggests that this damages the DNA directly and leads to chromosomal breakage, which may result in untoward DNA recombination in the form of translocations. This review will summarize the evidence for topoisomerase II involvement in the genesis of translocations and extension of the model to acute leukemia in infants characterized by similar MLL translocations.

Dot1a-AF9 complex mediates histone H3 Lys-79 hypermethylation and repression of ENaCalpha in an aldosterone-sensitive manner

Aldosterone is a major regulator of epithelial Na(+) absorption and acts in large part through induction of the epithelial Na(+) channel (ENaC) gene in the renal collecting duct. We previously identified Dot1a as an aldosterone early repressed gene and a repressor of ENaCalpha transcription through mediating histone H3 Lys-79 methylation associated with the ENaCalpha promoter. Here, we report a novel aldosterone-signaling network involving AF9, Dot1a, and ENaCalpha. AF9 and Dot1a interact in vitro and in vivo as evidenced in multiple assays and colocalize in the nuclei of mIMCD3 renal collecting duct cells. Overexpression of AF9 results in hypermethylation of histone H3 Lys-79 at the endogenous ENaCalpha promoter at most, but not all subregions examined, repression of endogenous ENaCalpha mRNA expression and acts synergistically with Dot1a to inhibit ENaCalpha promoter-luciferase constructs. In contrast, RNA interference-mediated knockdown of AF9 causes the opposite effects. Chromatin immunoprecipitation assays reveal that overexpressed FLAG-AF9, endogenous AF9, and Dot1a are each associated with the ENaCalpha promoter. Aldosterone negatively regulates AF9 expression at both mRNA and protein levels. Thus, Dot1a-AF9 modulates histone H3 Lys-79 methylation at the ENaCalpha promoter and represses ENaCalpha transcription in an aldosterone-sensitive manner. This mechanism appears to be more broadly applicable to other aldosterone-regulated genes because overexpression of AF9 alone or in combination with Dot1a inhibited mRNA levels of three other known aldosterone-inducible genes in mIMCD3 cells.

Identification and characterization of Smyd2: a split SET/MYND domain-containing histone H3 lysine 36-specific methyltransferase that interacts with the Sin3 histone deacetylase complex

Background

Disrupting the balance of histone lysine methylation alters the expression of genes involved in tumorigenesis including proto-oncogenes and cell cycle regulators. Methylation of lysine residues is commonly catalyzed by a family of proteins that contain the SET domain. Here, we report the identification and characterization of the SET domain-containing protein, Smyd2.

Results

Smyd2 mRNA is most highly expressed in heart and brain tissue, as demonstrated by northern analysis and in situ hybridization. Over-expressed Smyd2 localizes to the cytoplasm and the nucleus in 293T cells. Although accumulating evidence suggests that methylation of histone 3, lysine 36 (H3K36) is associated with actively transcribed genes, we show that the SET domain of Smyd2 mediates H3K36 dimethylation and that Smyd2 represses transcription from an SV40-luciferase reporter. Smyd2 associates specifically with the Sin3A histone deacetylase complex, which was recently linked to H3K36 methylation within the coding regions of active genes in yeast. Finally, we report that exogenous expression of Smyd2 suppresses cell proliferation.

Conclusion

We propose that Sin3A-mediated deacetylation within the coding regions of active genes is directly linked to the histone methyltransferase activity of Smyd2. Moreover, Smyd2 appears to restrain cell proliferation, likely through direct modulation of chromatin structure.

Identification of the MLL2 complex as a coactivator for estrogen receptor alpha

A novel estrogen receptor (ER)alpha coactivator complex, the MLL2 complex, which consists of MLL2, ASH2, RBQ3, and WDR5, was identified. ERalpha directly binds to the MLL2 complex through two LXXLL motifs in a region of MLL2 near the C terminus in a ligand-dependent manner. Disrupting the interaction between ERalpha and the MLL2 complex with small interfering RNAs specific against MLL2 or an MLL2 fragment representing the interacting region with ERalpha significantly inhibited the ERalpha transcription activity. The MLL2 complex was recruited on promoters of ERalpha target genes along with ERalpha upon estrogen stimulation. Inhibition of MLL2 expression decreased the estrogen-induced expression of ERalpha target genes cathepsin D and to a lesser extent pS2. In addition, MCF-7 cell growth was also inhibited by the depletion of MLL2. These results demonstrate that the ERalpha signaling pathway is critically dependent on its direct interaction with the MLL2 complex and suggest a central role for the MLL2 complex in the growth of ERalpha-positive cancer cells.

A murine Mll-AF4 knock-in model results in lymphoid and myeloid deregulation and hematologic malignancy

The 2 most frequent human MLL hematopoietic malignancies involve either AF4 or AF9 as fusion partners; each has distinct biology but the role of the fusion partner is not clear. We produced Mll-AF4 knock-in (KI) mice by homologous recombination in embryonic stem cells and compared them with Mll-AF9 KI mice. Young Mll-AF4 mice had lymphoid and myeloid deregulation manifest by increased lymphoid and myeloid cells in hematopoietic organs. In vitro, bone marrow cells from young mice formed unique mixed pro-B lymphoid (B220(+)CD19(+)CD43(+)sIgM(-), PAX5(+), TdT(+), IgH rearranged)/myeloid (CD11b/Mac1(+), c-fms(+), lysozyme(+)) colonies when grown in IL-7- and Flt3 ligand-containing media. Mixed lymphoid/myeloid hyperplasia and hematologic malignancies (most frequently B-cell lymphomas) developed in Mll-AF4 mice after prolonged latency; long latency to malignancy indicates that Mll-AF4-induced lymphoid/myeloid deregulation alone is insufficient to produce malignancy. In contrast, young Mll-AF9 mice had predominately myeloid deregulation in vivo and in vitro and developed myeloid malignancies. The early onset of distinct mixed lymphoid/myeloid lineage deregulation in Mll-AF4 mice shows evidence for both "instructive" and "noninstructive" roles for AF4 and AF9 as partners in MLL fusion genes. The molecular basis for "instruction" and secondary cooperating mutations can now be studied in our Mll-AF4 model.

The novel CALM interactor CATS influences the subcellular localization of the leukemogenic fusion protein CALM/AF10

The Clathrin Assembly Lymphoid Myeloid leukemia gene (CALM or PICALM) was first identified as the fusion partner of AF10 in the t(10;11)(p13;q14) translocation, which is observed in acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL) and malignant lymphoma. The CALM/AF10 fusion protein plays a crucial role in t(10;11)(p13;q14) associated leukemogenesis. Using the N-terminal half of CALM as a bait in a yeast two-hybrid screen, a novel protein named CATS (CALM interacting protein expressed in thymus and spleen) was identified. Multiple tissue Northern blot analysis showed predominant expression of CATS in thymus, spleen and colon. CATS codes for two protein isoforms of 238 and 248 amino acids (aa). The interaction between CALM and CATS could be confirmed using pull down assays, co-immunoprecipitation and colocalization experiments. The CATS interaction domain of CALM was mapped to aa 221-335 of CALM. This domain is contained in the CALM/AF10 fusion protein. CATS localizes to the nucleus and shows a preference for nucleoli. Expression of CATS was able to markedly increase the nuclear localization of CALM and of the leukemogenic fusion protein CALM/AF10. The possible implications of these findings for CALM/AF10-mediated leukemogenesis are discussed.

Infection of bovine cells by the protozoan parasite Theileria annulata modulates expression of the ISGylation system

The apicomplexan parasite, Theileria annulata, dedifferentiates and induces continuous division of infected bovine myeloid cells. Re-expression of differentiation markers and a loss of proliferation occur upon treatment with buparvaquone, implying that parasite factors actively maintain the altered status of the infected cell. The factors that induce this unique transformation event have not been identified. However, parasite polypeptides (TashAT family) that are located in the infected leucocyte nucleus have been postulated to function as modulators of host cell phenotype. In this study differential RNA display and proteomic analysis were used to identify altered mRNA and polypeptide expression profiles in a bovine macrophage cell line (BoMac) transfected with TashAT2. One of the genes identified by differential display was found to encode an ubiquitin-like protease (bUBP43) belonging to the UBP43 family. The bUBP43 gene and the gene encoding its ubiquitin-like substrate, bISG15, were expressed at a low level in T. annulata-infected cells. However, infected cells were refractory to induction of elevated bISG15 expression by lipopolysaccharide or type 1 interferons while TashAT2-transfected cells showed no induction when treated with camptothecin. Modulation of the ISGylation system may be of relevance to the establishment of the transformed infected host cell, as ISGylation is associated with resistance to intracellular infection by pathogens, stimulation of the immune response and terminal differentiation of leukaemic cells.

Disruption of Sept6, a fusion partner gene of MLL, does not affect ontogeny, leukemogenesis induced by MLL-SEPT6, or phenotype induced by the loss of Sept4

Septins are evolutionarily conserved GTP-binding proteins that can heteropolymerize into filaments. Recent studies have revealed that septins are involved in not only diverse normal cellular processes but also the pathogenesis of various diseases, including cancer. SEPT6 is ubiquitously expressed in tissues and one of the fusion partner genes of MLL in the 11q23 translocations implicated in acute leukemia. However, the roles of this septin in vivo remain elusive. We have developed Sept6-deficient mice that exhibited neither gross abnormalities, changes in cytokinesis, nor spontaneous malignancy. Sept6 deficiency did not cause any quantitative changes in any of the septins evaluated in this study, nor did it cause any additional changes in the Sept4-deficient mice. Even the depletion of Sept11, a close homolog of Sept6, did not affect the Sept6-null cells in vitro, thus implying a high degree of redundancy in the septin system. Furthermore, a loss of Sept6 did not alter the phenotype of myeloproliferative disease induced by MLL-SEPT6, thus suggesting that Sept6 does not function as a tumor suppressor. To our knowledge, this is the first report demonstrating that a disruption of the translocation partner gene of MLL in 11q23 translocation does not contribute to leukemogenesis by the MLL fusion gene.

Mutations of genes in the receptor tyrosine kinase (RTK)/RAS-BRAF signal transduction pathway in therapy-related myelodysplasia and acute myeloid leukemia

Mutations of the FLT3, c-KIT, c-FMS, KRAS, NRAS, BRAF and CEBPA genes in the receptor tyrosine kinase (RTK)/RAS-BRAF signal-transduction pathway are frequent in acute myeloid leukemia (AML). We examined 140 patients with therapy-related myelodysplasia or AML (t-MDS/t-AML) for point mutations of these seven genes. In all, 11 FLT3, two c-KIT, seven KRAS, eight NRAS and three BRAF mutations were identified in 29 patients (21%). All but one patient with a FLT3 mutation presented with t-AML (P=0.0002). Furthermore, FLT3 mutations were significantly associated with previous radiotherapy without chemotherapy (P=0.03), and with a normal karyotype (P=0.004), but inversely associated with previous therapy with alkylating agents (P=0.003) and with -7/7q- (P=0.001). RAS mutations were associated with AML1 point mutations (P=0.046) and with progression from t-MDS to t-AML (P=0.008). Noteworthy, all three patients with BRAF mutations presented as t-AML of M5 subtype with t(9;11)(p22;q23) and MLL-rearrangement (P=0.01). In t-AML RAS/BRAF mutations were significantly associated with a very short survival (P=0.017). Half of the patients with a mutation in the RTK/RAS-BRAF signal-transduction pathway (denoted 'class-I' mutations) simultaneously disclosed mutation of a hematopoietic transcription factor (denoted 'class-II' mutations) (P=0.046) suggesting their cooperation in leukemogenesis.

HoxGenes: From Leukemia to Hematopoietic Stem Cell Expansion

Hox genes are clearly implicated in leukemia; however, neither the specificity of the leukemogenic potential among Hox genes of different paralog groups nor the role of the homeodomain is clear. We tested the leukemogenic potential of various NUP98-Hox fusion genes alone and with MEIS1. All genes tested had a significant overlapping effect in bone marrow cells in vitro. However, not all formed strong leukemogenic NUP98 fusion genes; but together with overexpression of MEIS1, all induced myeloid leukemia. This phenomenon was also seen with NUP98 fusions containing only the homeodomain of the corresponding Hox protein. We then exploited the strong transforming potential of NUP98-HOXD13 and NUP98-HOXA10 to establish preleukemic myeloid lines composed of early myeloid progenitors with extensive in vitro self-renewal capacity, short-term myeloid repopulating activity, and low propensity for spontaneous leukemic conversion. We also showed that MEIS1 can efficiently induce their conversion to leukemic stem cells, thus providing a novel model for the study of leukemic progression. In contrast to the leukemogenic effect of most of the Hox genes tested, HOXB4 has the ability to increase the self-renewal of hematopoietic stem cells without disrupting normal differentiation. On the basis of the discovery that the leukemogenic gene HOXA9 can also expand hematopoietic stem cells, we compared the ability of NUP98-Hox fusions to that of HOXB4 to trigger HSC expansion in vitro. Our preliminary results indicate that the expanding potential of HOXB4 is retained and even augmented by fusion to NUP98. Moreover, even greater expansion may be possible using Abd-B-like Hox fusions genes.

Identification of a chromosomal breakpoint and detection of a novel form of an MLL-AF17 fusion transcript in acute monocytic leukemia with t(11;17)(q23;q21)

More than 40 genes have been reported as translocation partners of the mixed lineage leukemia gene (MLL) in hematologic malignancies. AF17 was identified earlier than most other MLL translocation partners. On the other hand, there is only 1 report of an MLL-AF17 fusion transcript in acute myeloid leukemia (AML). Here we describe a 40-year-old man with a diagnosis of AML involving t(11;17)(q23;q21). We identified a chromosomal breakpoint for t(11;17)(q23;q21) at MLL intron 6 and AF17 intron 8. Although the previously reported form of the MLL-AF17 fusion transcript was not detected by reverse transcriptase-polymerase chain reaction (PCR) analysis, a novel form of an MLL-AF17 fusion transcript joining MLL exon 6 to AF17 exon 9 was detected by complementary DNA panhandle PCR. The fact that 2 forms of MLL-AF17 retain the leucine zipper domain of AF17 suggests that the dimerization domain of AF17 is critical for leukemogenesis by the MLL-AF17 fusion gene.

Three-way translocation involvesMLL,MLLT3, and a novel cell cycle control gene,FLJ10374, in the pathogenesis of acute myeloid leukemia with t(9;11;19)(p22;q23;p13.3)

The MLL gene, at 11q23, undergoes chromosomal translocation with a large number of partner genes in both acute lymphoblastic and acute myeloid leukemia (AML). We report a novel t(9;11;19)(p22;q23;p13.3) disrupting MLL in an infant AML patient. The 5' end of MLL fused to chromosome 9 sequences on the der(11), whereas the 3' end was translocated to chromosome 19. We developed long-distance inverse-polymerase chain reaction assays to investigate the localization of the breakpoints on der(11) and der(19). We found that intron 5 of MLL was fused to intron 5 of MLLT3 at the der(11) genomic breakpoint, resulting in a novel in-frame MLL exon 5-MLLT3 exon 6 fusion transcript. On the der(19), a novel gene annotated as FLJ10374 was disrupted by the breakpoint. Using reverse transcription-polymerase chain reaction analysis, we showed that FLJ10374 is ubiquitously expressed in human cells. Transfection of the FLJ10374 protein in different cell lines revealed that it localized exclusively to the nucleus. In serum-starved NIH-3T3 cells, the expression of FLJ10374 decreased the rate of the G1-to-S transition of the cell cycle, whereas the suppression of FLJ10374 through short interfering RNA increased cell proliferation. These results indicate that FLJ10374 negatively regulates cell cycle progression and proliferation. Thus, a single chromosomal rearrangement resulting in formation of the MLL-MLLT3 fusion gene and haplo-insufficiency of FLJ10374 may have cooperated to promote leukemogenesis in AML with t(9;11;19).

MLL chimeric protein activation renders cells vulnerable to chromosomal damage: An explanation for the very short latency of infant leukemia

MLL fusion genes are a predominant feature of acute leukemias in infants and in secondary acute myeloid leukemia (AML) associated with prior chemotherapy with topo-II poisons. The former is considered to possibly arise in utero via transplacental chemical exposure. A striking feature of these leukemias is their malignancy and remarkably brief latencies implying the rapid acquisition of any necessary additional mutations. We have suggested that these coupled features might be explained if MLL fusion gene encoded proteins rendered cells more vulnerable to further DNA damage and mutation in the presence of chronic exposure to the agent(s) that induced the MLL fusion itself. We have tested this idea by exploiting a hormone regulated MLL-ENL (MLLT1) activation system and show that MLL-ENL function in normal murine progenitor cells substantially increases the incidence of chromosomal abnormalities in proliferating cells that survive exposure to etoposide VP-16. This phenotype is associated with an altered pattern of cell cycle arrest and/or apoptosis.

Human but not yeast CHD1 binds directly and selectively to histone H3 methylated at lysine 4 via its tandem chromodomains

Defining the protein factors that directly recognize post-translational, covalent histone modifications is essential toward understanding the impact of these chromatin "marks" on gene regulation. In the current study, we identify human CHD1, an ATP-dependent chromatin remodeling protein, as a factor that directly and selectively recognizes histone H3 methylated on lysine 4. In vitro binding studies identified that CHD1 recognizes di- and trimethyl H3K4 with a dissociation constant (Kd) of approximately 5 microm, whereas monomethyl H3K4 binds CHD1 with a 3-fold lower affinity. Surprisingly, human CHD1 binds to methylated H3K4 in a manner that requires both of its tandem chromodomains. In vitro analyses demonstrate that unlike human CHD1, yeast Chd1 does not bind methylated H3K4. Our findings indicate that yeast and human CHD1 have diverged in their ability to discriminate covalently modified histones and link histone modification-recognition and non-covalent chromatin remodeling activities within a single human protein.

Continuous MLL-ENL expression is necessary to establish a "Hox Code" and maintain immortalization of hematopoietic progenitor cells

The t[(11;19)(p22;q23)] translocation, which gives rise to the MLL-ENL fusion protein, is commonly found in infant acute leukemias of both the myeloid and lymphoid lineage. To investigate the molecular mechanism of immortalization by MLL-ENL we established a Tet-regulatable system of MLL-ENL expression in primary hematopoietic progenitor cells. Immortalized myeloid cell lines were generated, which are dependent on continued MLL-ENL expression for their survival and proliferation. These cells either terminally differentiate or die when MLL-ENL expression is turned off with doxycycline. The expression profile of all 39 murine Hox genes was analyzed in these cells by real-time quantitative PCR. This analysis showed that loss of MLL-ENL was accompanied by a reduction in the expression of multiple Hoxa genes. By comparing these changes with Hox gene expression in cells induced to differentiate with granulocyte colony-stimulating factor, we show for the first time that reduced Hox gene expression is specific to loss of MLL-ENL and is not a consequence of differentiation. Our data also suggest that the Hox cofactor Meis-2 can substitute for Meis-1 function. Thus, MLL-ENL is required to initiate and maintain immortalization of myeloid progenitors and may contribute to leukemogenesis by aberrantly sustaining the expression of a "Hox code" consisting of Hoxa4 to Hoxa11.

Mechanisms of therapy-related carcinogenesis

Therapy-related cancers, defined as second primary cancers that arise as a consequence of chemotherapy and/or radiotherapy, are unusual in that they have a well-defined aetiology. Knowledge of the specific nature of the initiating exposure and exactly when it occurred has made it easier to identify crucial genetic events and to model these in vitro and in vivo. As such, the study of therapy-related cancers has led to the elucidation of discrete mechanisms of carcinogenesis, including DNA double-strand-break-induced gene translocation and genomic instability conferred by loss of DNA repair. Unsurprisingly, some of these mechanisms seem to operate in the development of sporadic cancers.

MLL: how complex does it get?

The mixed lineage leukemia (MLL) gene encodes a very large nuclear protein homologous to Drosophila trithorax (trx). MLL is required for the proper maintenance of HOX gene expression during development and hematopoiesis. The exact regulatory mechanism of HOX gene expression by MLL is poorly understood, but it is believed that MLL functions at the level of chromatin organization. MLL was identified as a common target of chromosomal translocations associated with human acute leukemias. About 50 different MLL fusion partners have been isolated to date, and while similarities exist between groups of partners, there exists no unifying property shared by all the partners. MLL gene rearrangements are found in leukemias with both lymphoid and myeloid phenotypes and are often associated with infant and secondary leukemias. The immature phenotype of the leukemic blasts suggests an important role for MLL in the early stages of hematopoietic development. Mll homozygous mutant mice are embryonic lethal and exhibit deficiencies in yolk sac hematopoiesis. Recently, two different MLL-containing protein complexes have been isolated. These and other gain- and loss-of-function experiments have provided insight into normal MLL function and altered functions of MLL fusion proteins. This article reviews the progress made toward understanding the function of the wild-type MLL protein. While many advances in understanding this multifaceted protein have been made since its discovery, many challenging questions remain to be answered.

Chromatin modifier enzymes, the histone code and cancer

In all organisms, cell proliferation is orchestrated by coordinated patterns of gene expression. Transcription results from the activity of the RNA polymerase machinery and depends on the ability of transcription activators and repressors to access chromatin at specific promoters. During the last decades, increasing evidence supports aberrant transcription regulation as contributing to the development of human cancers. In fact, transcription regulatory proteins are often identified in oncogenic chromosomal rearrangements and are overexpressed in a variety of malignancies. Most transcription regulators are large proteins, containing multiple structural and functional domains some with enzymatic activity. These activities modify the structure of the chromatin, occluding certain DNA regions and exposing others for interaction with the transcription machinery. Thus, chromatin modifiers represent an additional level of transcription regulation. In this review we focus on several families of transcription activators and repressors that catalyse histone post-translational modifications (acetylation, methylation, phosphorylation, ubiquitination and SUMOylation); and how these enzymatic activities might alter the correct cell proliferation program, leading to cancer.

Molecular genetics of acute lymphoblastic leukemia

From its beginnings two decades ago with the analysis of chromosomal translocation breakpoints, research into the molecular pathogenesis of acute lymphoblastic leukemia (ALL) has now progressed to the large-scale resequencing of candidate oncogenes and tumor suppressor genes in the genomes of ALL cases blocked at various developmental stages within the B- and T-cell lineages. In this review, we summarize the findings of these investigations and highlight how this information is being integrated into multistep mutagenesis cascades that impact specific signal transduction pathways and synergistically lead to leukemic transformation. Because of these advances, fueled by improved technology for mutational analysis and the development of small-molecule drugs and monoclonal antibodies, the future is bright for a new generation of targeted therapies. Best illustrated by the successful introduction of imatinib mesylate, these new treatments will interfere with disordered molecular pathways specific for the leukemic cells, and thus should exhibit much less toxicity and fewer long-term adverse effects than currently available therapeutic modalities.

Epigenome programming by Polycomb and Trithorax proteins

Polycomb group (PcG) and Trithorax group (TrxG) proteins work, respectively, to maintain repressed or active transcription states of developmentally regulated genes through cell division. Data accumulated in the recent years have increased our understanding of the mechanisms by which PcG and TrxG proteins regulate gene expression. The discovery that histone methylation can serve as a specific mark for PcG and TrxG complexes has provided new insight into the mechanistic function of this cell-memory system.

MLL associates specifically with a subset of transcriptionally active target genes

MLL (mixed-lineage leukemia) is a histone H3 Lys-4 specific methyltransferase that is a positive regulator of Hox expression. MLL rearrangements and amplification are common in acute lymphoid and myeloid leukemias and myelodysplastic disorders and are associated with abnormal up-regulation of Hox gene expression. Although MLL is expressed throughout hematopoiesis, Hox gene expression is sharply down-regulated during differentiation, suggesting that either the activity of MLL or its association with target promoters must be regulated. Here we show that MLL associates with actively transcribed genes but does not remain bound after transcriptional down-regulation. Surprisingly, MLL is associated not only with promoter regions but also is distributed across the entire coding regions of genes. MLL interacts with RNA polymerase II (pol II) and colocalizes with RNA pol II at a subset of actively transcribed target in vivo. Loss of function Mll results in defects in RNA pol II distribution. Together the results suggest that an intimate association between MLL and RNA pol II occurs at MLL target genes in vivo that is required for normal initiation and/or transcriptional elongation.

AF4p12, a human homologue to the furry gene of Drosophila, as a novel MLL fusion partner

More than 35 different partner genes with the mixed lineage leukemia (MLL) gene have been cloned from leukemia cells with translocations involving chromosome 11 band q23. In this study, we report on a novel fusion partner of the MLL gene, AF4p12, which we have identified as the human homologue to the furry gene of Drosophila. AF4p12, highly conserved in evolution, encodes a large protein of 3,105 amino acids. The expression of AF4p12 has been preferentially detected in colon, placenta, and brain tissues and in tumor cells of lymphoid origin. We show that the t(4;11)(p12;q23) translocation results in the creation of a chimeric RNA encoding a putative fusion protein containing 1,362 amino acids from the NH2-terminal part of MLL and 712 amino acids from the COOH-terminal part of AF4p12. FLT3 and HOXA9 genes are overexpressed in this leukemia. We found that the COOH-terminal part of AF4p12 fused to MLL contains a leucine zipper motif and exhibits transcriptional activation properties when fused to Gal4 DNA-binding domains in transient transfection assays. The AF4p12 fragment fused to MLL may contribute to the oncogenic activation of MLL, possibly through specific recruitment of the transcriptional machinery.

Cryptic MLL-AF10 fusion caused by insertion of duplicated 5′ part of MLL into 10p12 in acute leukemia: a case report

Chromosomal translocations involving the mixed lineage leukemia gene (MLL) located at 11q23 belong to common chromosomal abnormalities in both acute lymphoblastic (ALL) and acute myeloid leukemias (AML). It has been suggested that the mechanism of MLL leukemogenesis might be a result of a gain-of-function effect of the MLL fusion gene and simultaneous loss of function of one of the MLL alleles (haploinsufficiency). One of the recurrent translocations in AML-M5 involves chromosomal locus 10p12 and results in the MLL-AF10 fusion gene. Several mechanisms leading to MLL-AF10 fusion have been reported, and they have involved rearrangement of the 11q23 region. We present a detailed structural analysis of an AML case with an extra copy of the 5' part of MLL region and its insertion into the short arm of chromosome 10, resulting in an MLL-AF10 fusion without rearrangement of the MLL alleles on both chromosomes 11. Our observation supports a role for a simple MLL gain-of-function in leukemogenesis.

Crystal Structure of Human Taspase1, a Crucial Protease Regulating the Function of MLL

Taspase1 catalyzes the proteolytic processing of the mixed lineage leukemia (MLL) nuclear protein, which is required for maintaining Hox gene expression patterns. Chromosomal translocations of the MLL gene are associated with leukemia in infants. Taspase1, a threonine aspartase, is a member of the type 2 asparaginase family, but is the only protease in this family. We report here the crystal structures of human activated Taspase1 and its proenzyme, as well as the characterization of the effects of mutations in the active site region using a newly developed fluorogenic assay. The structure of Taspase1 has significant differences from other asparaginases, especially near the active site. Mutation of the catalytic nucleophile, Thr234, abolishes autocatalytic processing in cis but does not completely block proteolysis in trans. The structure unexpectedly showed the binding of a chloride ion in the active site, and our kinetic studies confirm that chlorides ions are inhibitors of this enzyme at physiologically relevant concentrations.

Alteration of host cell phenotype by Theileria annulata and Theileria parva: mining for manipulators in the parasite genomes

The apicomplexan parasites Theileria annulata and Theileria parva cause severe lymphoproliferative disorders in cattle. Disease pathogenesis is linked to the ability of the parasite to transform the infected host cell (leukocyte) and induce uncontrolled proliferation. It is known that transformation involves parasite dependent perturbation of leukocyte signal transduction pathways that regulate apoptosis, division and gene expression, and there is evidence for the translocation of Theileria DNA binding proteins to the host cell nucleus. However, the parasite factors responsible for the inhibition of host cell apoptosis, or induction of host cell proliferation are unknown. The recent derivation of the complete genome sequence for both T. annulata and T. parva has provided a wealth of information that can be searched to identify molecules with the potential to subvert host cell regulatory pathways. This review summarizes current knowledge of the mechanisms used by Theileria parasites to transform the host cell, and highlights recent work that has mined the Theileria genomes to identify candidate manipulators of host cell phenotype.

Infertility with defective spermiogenesis in mice lacking AF5q31, the target of chromosomal translocation in human infant leukemia

AF5q31 (also called MCEF) was identified by its involvement in chromosomal translocation with the gene MLL (mixed lineage leukemia), which is associated with infant acute lymphoblastic leukemia. Several potential roles have been proposed for AF5q31 and other family genes, but the specific requirements of AF5q31 during development remain unclear. Here, we show that AF5q31 is essential for spermatogenesis. Although most AF5q31-deficient mice died in utero and neonatally with impaired embryonic development and shrunken alveoli, respectively, 13% of AF5q31-deficient mice thrived as wild-type mice did. However, the male mice were sterile with azoospermia. Histological examinations revealed the arrest of germ cell development at the stage of spermiogenesis, and virtually no spermatozoa were seen in the epididymis. AF5q31 was found to be preferentially expressed in Sertoli cells. Furthermore, mutant mice displayed severely impaired expression of protamine 1, protamine 2, and transition protein 2, which are indispensable to compact the haploid genome within the sperm head, and an increase of apoptotic cells in seminiferous tubules. Thus, AF5q31 seems to function as a transcriptional regulator in testicular somatic cells and is essential for male germ cell differentiation and survival. These results may have clinical implications in the understanding of human male infertility.

mTOR regulates cell survival after etoposide treatment in primary AML cells

Acute myeloid leukemia cells have constitutive activation of phosphatidylinositol 3(PI3) kinase and require PI3 kinase activation for survival; however, the function of the PI3 kinase pathway in the survival of leukemic cells is poorly defined. We have studied the role of one PI3 kinase substrate, mTOR (mammalian target of rapamycin), in primary leukemic cells. In initial experiments, we have defined a novel growth medium that improves survival of acute myeloid leukemia (AML) blasts in long-term suspension culture and the survival of leukemic stem cells in short-term cultures. Inhibition of mTOR using rapamycin leads to a modest decrease in cell survival after 2 days of incubation with more significant decrease in survival after 7 days of culture. However, when rapamycin is added to etoposide in 2-day cultures, there is a dramatic increase in the cytotoxicity of etoposide against AML blasts. Furthermore, etoposide consistently decreased the engraftment of AML cells in nonobese diabetic/severe combined immunodeficient (NOD/SCID) animals, and this effect was enhanced by coincubation with rapamycin, demonstrating that mTOR regulates survival of AML stem cells after etoposide treatment. These results suggest that rapamycin in combination with etoposide-based chemotherapy may be efficacious in the treatment of AML.

Direct physical and functional interaction of the NuA4 complex components Yaf9p and Swc4p

Saccharomyces cerevisiae Yaf9p and the mammalian leukemia-associated protein ENL share a high degree of similarity. To investigate the biological function of Yaf9p, this protein was used to search for interacting proteins in a two-hybrid system. Here, we demonstrate that Yaf9p binds directly to Swc4p, the yeast homolog of the mammalian DNA-methyltransferase-associated protein 1. Yaf9p and Swc4p associate through C-terminal domains, and both proteins coprecipitate in vitro in pull-down experiments and in vivo by immunoprecipitation. In living cells, Swc4p is present in a megadalton protein complex that shows a fractionation behavior in gel filtration similar to that of Esa1p, the histone acetyltransferase of the NuA4 complex. Recruitment of Yaf9p to DNA leads to promoter-specific transcriptional activation that can be inhibited by dominant negative Swc4p lacking the Yaf9p binding domain. Interference with Swc4p function also increases sensitivity to the microtubule toxin benomyl, a trait that corresponds to the known phenotype of a yaf9(-) knockout strain. In summary, the results suggest that Yaf9p and Swc4p form a protein pair that has a role in chromatin modification with possible implications also for the function of their mammalian counterparts.

Mll fusions generated by Cre-loxP-mediated de novo translocations can induce lineage reassignment in tumorigenesis

Chromosomal translocations are primary events in tumorigenesis. Those involving the mixed lineage leukaemia (MLL) gene are found in various guises and it is unclear whether MLL fusions can affect haematopoietic differentiation. We have used a model in which chromosomal translocations are generated in mice de novo by Cre-loxP-mediated recombination (translocator mice) to compare the functionally relevant haematopoietic cell contexts for Mll fusions, namely pluripotent stem cells, semicommitted progenitors or committed cells. Translocations between Mll and Enl or Af9 cause myeloid neoplasias, initiating in pluripotent stem cells or multipotent myeloid progenitors. However, while Mll-Enl translocations can also cause leukaemia from T-cell progenitors, no tumours arose with Mll-Af9 translocations in the T-cell compartment. Furthermore, Mll-Enl translocations in T-cell progenitors can cause lineage reassignment into myeloid tumours. Therefore, a permissive cellular environment is required for oncogenicity of Mll-associated translocations and Mll fusions can influence haematopoietic lineage commitment.

Histone modifying and chromatin remodelling enzymes in cancer and dysplastic syndromes

Inactivation of tumour suppressor genes is central to the development of cancer. Although this inactivation was once considered to be secondary to intragenic mutations, it is now clear that silencing of these genes often occurs by epigenetic means. Hypermethylation of CpG islands associated with the tumour suppressor genes was the first manifestation of this phenomenon to be described. It is apparent, however, that this is one of a host of chromatin modifications which characterize gene silencing. Although we know little about what determines which loci are affected, our understanding of the nature of the epigenetic marks and how they are established has blossomed. There is no compelling evidence that cancer ever develops by purely epigenetic means, but it is apparent that perturbations in the apparatus which establish the epigenome may contribute to the development of cancer. This review will focus on the role of two classes of chromatin remodelling enzymes, those that alter histones by the addition or removal of acetyl and methyl groups and those of the SWI/SNF family of proteins that change the topology of the nucleosome and its DNA strand via the hydrolysis of ATP, and we shall examine the consequence of mutations in, or mis-expression of, these factors. In some cases, mutations in these factors appear to play a direct role in cancer development. However, their general role as important intermediaries involved in regulating gene expression makes them attractive therapeutic targets. In exciting developments, it has been shown that inhibition of these factors leads to the reversal of tumour suppressor gene silencing and the inhibition of cancer cell growth.

Roles of a trithorax group gene, MLL, in hematopoiesis

The mixed-lineage leukemia (MLL) gene is a trithorax group (trxG) gene that was originally identified at chromosomal translocations in patients developing acute leukemia. Although Polycomb group (PcG) genes, which counteract trxG genes, were found to play essential roles in hematopoiesis, little has been understood about the roles of trxG genes in hematopoiesis except for MLL. MLL has been found fused with 1 of more than 30 different partner genes to yield a diverse collection of MLL fusion oncoproteins that lead to the aberrant expression of HOX genes. Recent studies have revealed that MLL assembles, as do some trxG proteins, into a chromatin-modifying transcriptional regulatory supercomplex to regulate epigenetic pathways, including the methylation of histone H3 lysine 4, which is conferred by the Su (var)3-9, enhancer of zeste, and tritho-rax (SET) domain. Other studies also indicated that MLL plays a nonredundant and essential role in definitive hematopoiesis and induces the proliferation and differentiation of hematopoietic progenitors by maintaining appropriate up-regulation of HOX genes. Further progress in the field will provide novel insights into trxG- and PcG-mediated hematopoiesis and help us understand the epigenetic process by which developing stem cells coordinate proliferation and differentiation.

Targeting MLL-AF4 with short interfering RNAs inhibits clonogenicity and engraftment of t(4;11)-positive human leukemic cells

The chromosomal translocation t(4;11) marks infant acute lymphoblastic leukemia associated with a particularly dismal prognosis. The leukemogenic role of the corresponding fusion gene MLL-AF4 is not well understood. We show that transient inhibition of MLL-AF4 expression with small interfering RNAs impairs the proliferation and clonogenicity of the t(4; 11)-positive human leukemic cell lines SEM and RS4;11. Reduction of mixed-lineage leukemia (MLL)-ALL-1 fused gene from chromosome 4 (AF4) levels induces apoptosis associated with caspase-3 activation and diminished BCL-X(L) expression. Suppression of MLL-AF4 is paralleled by a decreased expression of the homeotic genes HOXA7, HOXA9, and MEIS1. MLL-AF4 depletion inhibits expression of the stem-cell marker CD133, indicating hematopoietic differentiation. Transfection of leukemic cells with MLL-AF4 siRNAs reduces leukemia-associated morbidity and mortality in SCID mice that received a xenotransplant, suggesting that MLL-AF4 depletion negatively affects leukemia-initiating cells. Our findings demonstrate that MLL-AF4 is important for leukemic clonogenicity and engraftment of this highly aggressive leukemia. Targeted inhibition of MLL-AF4 fusion gene expression may lead to an effective and highly specific treatment of this therapy-resistant leukemia.

Molecular Pathogenesis of MLL-Associated Leukemias

Chromosome translocations disrupting the MLL gene are associated with various hematologic malignancies but are particularly common in infant and secondary therapy-related acute leukemias. The normal MLL-encoded protein is an essential component of a supercomplex with chromatin-modulating activity conferred by histone acetylase and methyltransferase activities, and the protein plays a key role in the developmental regulation of gene expression, including Hox gene expression. In leukemia, this function is subverted by breakage, recombination, and the formation of chimeric fusion with one of many alternative partners. Such MLL translocations result in the replacement of the C-terminal functional domains of MLL with those of a fusion partner, yielding a newly formed MLL chimeric protein with an altered function that endows hematopoietic progenitors with self-renewing and leukemogenic activity. This potent impact of the MLL chimera can be attributed to one of 2 kinds of activity of the fusion partner: direct transcriptional transactivation or dimerization/oligomerization. Key unresolved issues currently being addressed include the set of target genes for MLL fusions, the stem cell of origin for the leukemias, the role of additional secondary mutations, and the origins or etiology of the MLL gene fusions themselves. Further elaboration of the biology of MLL gene-associated leukemia should lead to novel and specific therapeutic strategies.

New insights into MLL gene rearranged acute leukemias using gene expression profiling: shared pathways, lineage commitment, and partner genes

Rearrangements of the MLL gene occur in both acute lymphoblastic and acute myeloid leukemias (ALL, AML). This study addressed the global gene expression pattern of these two leukemia subtypes with respect to common deregulated pathways and lineage-associated differences. We analyzed 73 t(11q23)/MLL leukemias in comparison to 290 other acute leukemias and demonstrate that 11q23 leukemias combined are characterized by a common specific gene expression signature. Additionally, in unsupervised and supervised data analysis algorithms, ALL and AML cases with t(11q23) segregate according to the lineage they are derived from, that is, myeloid or lymphoid, respectively. This segregation can be explained by a highly differing transcriptional program. Through the use of novel biological network analyses, essential regulators of early B cell development, PAX5 and EBF, were shown to be associated with a clear B-lineage commitment in lymphoblastic t(11q23)/MLL leukemias. Also, the influence of the different MLL translocation partners on the transcriptional program was directly assessed. Interestingly, gene expression profiling did not reveal a clear distinct pattern associated with one of the analyzed partner genes. Taken together, the identified molecular expression pattern of MLL fusion gene samples and biological networks revealed new insights into the aberrant transcriptional program in 11q23/MLL leukemias.

The Ews-ERG fusion protein can initiate neoplasia from lineage-committed haematopoietic cells

The EWS-ERG fusion protein is found in human sarcomas with the chromosomal translocation t(21;22)(q22;q12), where the translocation is considered to be an initiating event in sarcoma formation within uncommitted mesenchymal cells, probably long-lived progenitors capable of self renewal. The fusion protein may not therefore have an oncogenic capability beyond these progenitors. To assess whether EWS-ERG can be a tumour initiator in cells other than mesenchymal cells, we have analysed Ews-ERG fusion protein function in a cellular environment not typical of that found in human cancers, namely, committed lymphoid cells. We have used Ews-ERG invertor mice having an inverted ERG cDNA cassette flanked by loxP sites knocked in the Ews intron 8, crossed with mice expressing Cre recombinase under the control of the Rag1 gene to give conditional, lymphoid-specific expression of the fusion protein. Clonal T cell neoplasias arose in these mice. This conditional Ews gene fusion model of tumourigenesis shows that Ews-ERG can cause haematopoietic tumours and the precursor cells are committed cells. Thus, Ews-ERG can function in cells that do not have to be pluripotent progenitors or mesenchymal cells.

Using a mouse model, these authors study the potential of a known cancer-related gene to cause tumors in cells committed to different lineages.

Identification of various MLL gene aberrations that lead to MLL gene mutation in patients with acute lymphoblastic leukemia (ALL) and infants with acute leukemia

Studies were done to investigate MLL gene aberrations using Conventional Cytogenetics, Southern blotting as well as FISH using a panel of probes on 218 cases which included 206 cases of pediatric/young adult ALL and 12 cases of infantile acute leukemias from Tata Memorial Hospital, India. The incidence of MLL gene rearrangements in acute lymphoblastic leukemia (ALL) was 9.4% which included infants as well as pediatric/young adults. In infantile group which included ALL as well as AML cases, MLL gene rearrangement was very common (75% frequency). Application of metaphase-FISH helped unravel MLL rearrangements not only as a result of translocations but also inversions, insertions, partial deletion, duplications, partial duplication-->self-fusion. Besides age, MLL gene rearrangements showed significant association with hyperleukocytosis, peripheral blood blast percentage and early Pre-B phenotype. Clinical outcome of patients with MLL gene rearrangements revealed unfavorable prognosis.