The role of the MLL gene in infant leukemia

Germline Variants and Characteristic Features of Hereditary Hematological Malignancy Syndrome

Due to the proliferation of genetic testing, pathogenic germline variants predisposing to hereditary hematological malignancy syndrome (HHMS) have been identified in an increasing number of genes. Consequently, the field of HHMS is gaining recognition among clinicians and scientists worldwide. Patients with germline genetic abnormalities often have poor outcomes and are candidates for allogeneic hematopoietic stem cell transplantation (HSCT). However, HSCT using blood from a related donor should be carefully considered because of the risk that the patient may inherit a pathogenic variant. At present, we now face the challenge of incorporating these advances into clinical practice for patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) and optimizing the management and surveillance of patients and asymptomatic carriers, with the limitation that evidence-based guidelines are often inadequate. The 2016 revision of the WHO classification added a new section on myeloid malignant neoplasms, including MDS and AML with germline predisposition. The main syndromes can be classified into three groups. Those without pre-existing disease or organ dysfunction; DDX41, TP53, CEBPA, those with pre-existing platelet disorders; ANKRD26, ETV6, RUNX1, and those with other organ dysfunctions; SAMD9/SAMD9L, GATA2, and inherited bone marrow failure syndromes. In this review, we will outline the role of the genes involved in HHMS in order to clarify our understanding of HHMS.

Loss of KMT2C reprograms the epigenomic landscape in hPSCs resulting in NODAL overexpression and a failure of hemogenic endothelium specification

Germline or somatic variation in the family of KMT2 lysine methyltransferases have been associated with a variety of congenital disorders and cancers. Notably, KMT2A-fusions are prevalent in 70% of infant leukaemias but fail to phenocopy short latency leukaemogenesis in mammalian models, suggesting additional factors are necessary for transformation. Given the lack of additional somatic mutation, the role of epigenetic regulation in cell specification, and our prior results of germline KMT2C variation in infant leukaemia patients, we hypothesized that germline dysfunction of KMT2C altered haematopoietic specification. In isogenic KMT2C KO hPSCs, we found genome-wide differences in histone modifications at active and poised enhancers, leading to gene expression profiles akin to mesendoderm rather than mesoderm highlighted by a significant increase in NODAL expression and WNT inhibition, ultimately resulting in a lack of in vitro hemogenic endothelium specification. These unbiased multi-omic results provide new evidence for germline mechanisms increasing risk of early leukaemogenesis.

Only Hematopoietic Stem and Progenitor Cells from Cord Blood Are Susceptible to Malignant Transformation by MLL-AF4 Translocations

Mixed lineage leukemia (MLL) (KMT2A) rearrangements (KMT2Ar) play a crucial role in leukemogenesis. Dependent on age, major differences exist regarding disease frequency, main fusion partners and prognosis. In infants, up to 80% of acute lymphoid leukemia (ALL) bear a MLL translocation and half of them are t(4;11), resulting in a poor prognosis. In contrast, in adults only 10% of acute myeloid leukemia (AML) bear t(9;11) with an intermediate prognosis. The reasons for these differences are poorly understood. Recently, we established an efficient CRISPR/Cas9-based KMT2Ar model in hematopoietic stem and progenitor cells (HSPCs) derived from human cord blood (huCB) and faithfully mimicked the underlying biology of the disease. Here, we applied this model to HSPCs from adult bone marrow (huBM) to investigate the impact of the cell of origin and fusion partner on disease development. Both genome-edited infant and adult KMT2Ar cells showed monoclonal outgrowth with an immature morphology, myelomonocytic phenotype and elevated KMT2Ar target gene expression comparable to patient cells. Strikingly, all KMT2Ar cells presented with indefinite growth potential except for MLL-AF4 huBM cells ceasing proliferation after 80 days. We uncovered FFAR2, an epigenetic tumor suppressor, as potentially responsible for the inability of MLL-AF4 to immortalize adult cells under myeloid conditions.

MLL leukemia induction by genome editing of human CD34+ hematopoietic cells

Chromosomal rearrangements involving the mixed-lineage leukemia (MLL) gene occur in primary and treatment-related leukemias and confer a poor prognosis. Studies based primarily on mouse models have substantially advanced our understanding of MLL leukemia pathogenesis, but often use supraphysiological oncogene expression with uncertain implications for human leukemia. Genome editing using site-specific nucleases provides a powerful new technology for gene modification to potentially model human disease, however, this approach has not been used to re-create acute leukemia in human cells of origin comparable to disease observed in patients. We applied transcription activator-like effector nuclease-mediated genome editing to generate endogenous MLL-AF9 and MLL-ENL oncogenes through insertional mutagenesis in primary human hematopoietic stem and progenitor cells (HSPCs) derived from human umbilical cord blood. Engineered HSPCs displayed altered in vitro growth potentials and induced acute leukemias following transplantation in immunocompromised mice at a mean latency of 16 weeks. The leukemias displayed phenotypic and morphologic similarities with patient leukemia blasts including a subset with mixed phenotype, a distinctive feature seen in clinical disease. The leukemic blasts expressed an MLL-associated transcriptional program with elevated levels of crucial MLL target genes, displayed heightened sensitivity to DOT1L inhibition, and demonstrated increased oncogenic potential ex vivo and in secondary transplant assays. Thus, genome editing to create endogenous MLL oncogenes in primary human HSPCs faithfully models acute MLL-rearranged leukemia and provides an experimental platform for prospective studies of leukemia initiation and stem cell biology in a genetic subtype of poor prognosis leukemia.

Topoisomerase II and leukemia

Type II topoisomerases are essential enzymes that modulate DNA under- and overwinding, knotting, and tangling. Beyond their critical physiological functions, these enzymes are the targets for some of the most widely prescribed anticancer drugs (topoisomerase II poisons) in clinical use. Topoisomerase II poisons kill cells by increasing levels of covalent enzyme-cleaved DNA complexes that are normal reaction intermediates. Drugs such as etoposide, doxorubicin, and mitoxantrone are frontline therapies for a variety of solid tumors and hematological malignancies. Unfortunately, their use also is associated with the development of specific leukemias. Regimens that include etoposide or doxorubicin are linked to the occurrence of acute myeloid leukemias that feature rearrangements at chromosomal band 11q23. Similar rearrangements are seen in infant leukemias and are associated with gestational diets that are high in naturally occurring topoisomerase II-active compounds. Finally, regimens that include mitoxantrone and epirubicin are linked to acute promyelocytic leukemias that feature t(15;17) rearrangements. The first part of this article will focus on type II topoisomerases and describe the mechanism of enzyme and drug action. The second part will discuss how topoisomerase II poisons trigger chromosomal breaks that lead to leukemia and potential approaches for dissociating the actions of drugs from their leukemogenic potential.

Targeting recruitment of disruptor of telomeric silencing 1-like (DOT1L): characterizing the interactions between DOT1L and mixed lineage leukemia (MLL) fusion proteins

The MLL fusion proteins, AF9 and ENL, activate target genes in part via recruitment of the histone methyltransferase DOT1L (disruptor of telomeric silencing 1-like). Here we report biochemical, biophysical, and functional characterization of the interaction between DOT1L and MLL fusion proteins, AF9/ENL. The AF9/ENL-binding site in human DOT1L was mapped, and the interaction site was identified to a 10-amino acid region (DOT1L865-874). This region is highly conserved in DOT1L from a variety of species. Alanine scanning mutagenesis analysis shows that four conserved hydrophobic residues from the identified binding motif are essential for the interactions with AF9/ENL. Binding studies demonstrate that the entire intact C-terminal domain of AF9/ENL is required for optimal interaction with DOT1L. Functional studies show that the mapped AF9/ENL interacting site is essential for immortalization by MLL-AF9, indicating that DOT1L interaction with MLL-AF9 and its recruitment are required for transformation by MLL-AF9. These results strongly suggest that disruption of interaction between DOT1L and AF9/ENL is a promising therapeutic strategy with potentially fewer adverse effects than enzymatic inhibition of DOT1L for MLL fusion protein-associated leukemia.

Flt3 does not play a critical role in murine myeloid leukemias induced by MLL fusion genes

Leukemias harboring MLL translocations are frequent in children and adults, and respond poorly to therapies. The receptor tyrosine kinase FLT3 is highly expressed in these leukemias. In vitro studies have shown that pediatric MLL-rearranged ALL cells are sensitive to FLT3 inhibitors and clinical trials are ongoing to measure their therapeutic efficacy. We sought to determine the contribution of Flt3 in the pathogenesis of MLL-rearranged leukemias using a myeloid leukemia mouse model. Bone marrow from Flt3 null mice transduced with MLL-ENL or MLL-CBP was transplanted into host mice and Flt3^−/− leukemias were compared to their Flt3 wild type counterparts. Flt3 deficiency did not delay disease onset and had minimal impact on leukemia characteristics. To determine the anti-leukemic effect of FLT3 inhibition we studied the sensitivity of MLL-ENL leukemia cells to the FLT3 inhibitor PKC412 ex vivo. As previously reported for human MLL-rearranged leukemias, murine MLL-ENL leukemia cells with higher Flt3 levels were more sensitive to the cytotoxicity of PKC412. Interestingly, Flt3 deficient leukemia samples also displayed some sensitivity to PKC412. Our findings demonstrate that myeloid leukemias induced by MLL-rearranged genes are not dependent upon Flt3 signaling. They also highlight the discrepancy between the sensitivity of cells to Flt3 inhibition in vitro and the lack of contribution of Flt3 to the pathogenesis of MLL-rearranged leukemias in vivo.

A soluble form of CTLA-4 is present in paediatric patients with acute lymphoblastic leukaemia and correlates with CD1d+ expression

CTLA-4 is a key factor in regulating and maintaining self tolerance, providing a negative signal to the T cell and thus limiting immune responses. Several polymorphisms within the CTLA-4 gene have been associated with an increased risk of developing autoimmune diseases and, very recently, with susceptibility to human cancer. Acute lymphoblastic leukemia is a clonal disorder of lymphoid progenitors representing the most frequent malignancy of childhood. Here, we show the presence at significantly elevated levels of a circulating soluble form of CTLA-4 in 70% of B-ALL pediatric patients with active disease, the positive correlation between the percentage of leukemic B lymphocytes and the amount of serum sCTLA-4, and the expression of sCTLA-4 transcript by B cells in patients. Finally, a correlation between CD1d expression (a negative prognostic marker) and the sCTLA-4 in B-ALL patients was observed. This suggests a possible role of this soluble molecule as a marker of progression or severity of the neoplastic disease.

Molecular basis of the mixed lineage leukemia-menin interaction: implications for targeting mixed lineage leukemias

Chromosomal translocations targeting the mixed lineage leukemia (MLL) gene result in MLL fusion proteins that are found in aggressive human acute leukemias. Disruption of MLL by such translocations leads to overexpression of Hox genes, resulting in a blockage of hematopoietic differentiation that ultimately leads to leukemia. Menin, which directly binds MLL, has been identified as an essential oncogenic co-factor required for the leukemogenic activity of MLL fusion proteins. Here, we characterize the molecular basis of the MLL-menin interaction. Using (13)C-detected NMR experiments, we have mapped the residues within the intrinsically unstructured fragment of MLL that are required for binding to menin. Interestingly, we found that MLL interacts with menin with a nanomolar affinity (K(d) ∼ 10 nM) through two motifs, MBM1 and MBM2 (menin binding motifs 1 and 2). These motifs are located within the N-terminal 43-amino acid fragment of MLL, and the MBM1 represents a high affinity binding motif. Using alanine scanning mutagenesis of MBM1, we found that the hydrophobic residues Phe(9), Pro(10), and Pro(13) are most critical for binding. Furthermore, based on exchange-transferred nuclear Overhauser effect measurements, we established that MBM1 binds to menin in an extended conformation. In a series of competition experiments we showed that a peptide corresponding to MBM1 efficiently dissociates the menin-MLL complex. Altogether, our work establishes the molecular basis of the menin interaction with MLL and MLL fusion proteins and provides the necessary foundation for development of small molecule inhibitors targeting this interaction in leukemias with MLL translocations.

AF17 competes with AF9 for binding to Dot1a to up-regulate transcription of epithelial Na+ channel alpha

We previously reported that Dot1a.AF9 complex represses transcription of the epithelial Na(+) channel subunit alpha (alpha-ENaC) gene in mouse inner medullary collecting duct mIMCD3 cells and mouse kidney. Aldosterone relieves this repression by down-regulating the complex through various mechanisms. Whether these mechanisms are sufficient and conserved in human cells or can be applied to other aldosterone-regulated genes remains largely unknown. Here we demonstrate that human embryonic kidney 293T cells express the three ENaC subunits and all of the ENaC transcriptional regulators examined. These cells respond to aldosterone and display benzamil-sensitive Na(+) currents, as measured by whole-cell patch clamping. We also show that AF17 and AF9 competitively bind to the same domain of Dot1a in multiple assays and have antagonistic effects on expression of an alpha-ENaC promoter-luciferase construct. Overexpression of Dot1a or AF9 decreased mRNA expression of the ENaC subunits and their transcriptional regulators and reduced benzamil-sensitive Na(+) currents. AF17 overexpression caused the opposite effects, accompanied by redirection of Dot1a from the nucleus to the cytoplasm and reduction in histone H3 K79 methylation. The nuclear export inhibitor leptomycin B blocked the effect of AF17 overexpression on H3 K79 hypomethylation. RNAi-mediated knockdown of AF17 yielded nuclear enrichment of Dot1a and histone H3 K79 hypermethylation. As with AF9, AF17 displays nuclear and cytoplasmic co-localization with Sgk1. Therefore, AF17 competes with AF9 to bind Dot1a, decreases Dot1a nuclear expression by possibly facilitating its nuclear export, and relieves Dot1a.AF9-mediated repression of alpha-ENaC and other target genes.

Chromosomal rearrangements leading to MLL gene fusions: clinical and biological aspects

Rearrangements of the MLL gene located at 11q23 are common chromosomal abnormalities associated with acute leukemia, especially infant and therapy-related leukemias. A variety of chimeric oncoproteins resulting from these rearrangements has been described; all of these include the NH(2)-terminal region of MLL implicated in protein-protein interactions and transcriptional repression. Although the molecular basis for the oncogenic activity of MLL chimeric proteins is incompletely understood, it seems to be derived, at least in part, through activation of clustered homeobox (HOX) genes. Here, we survey MLL gene rearrangements that are associated with acute leukemia and discuss molecular pathways leading to these rearrangements.

Online immobilized metal affinity chromatography/mass spectrometric analysis of changes elicited by cCMP in the murine brain phosphoproteome

An automated online immobilized metal affinity chromatography/high-performance liquid chromatography mass spectrometric (IMAC-HPLC/MS/MS) method was developed to study cytidine 3',5'-cyclic monophosphate (cCMP)-specific protein phosphorylation, analogous to a previously successful offline IMAC method using microvolume IMAC pipette tips. The optimized method identified murine brain phosphoproteins selectively modified by challenge with cCMP, using manual interpretation of the results to confirm both phosphorylation and selectivity of response to cCMP. A number of proteins identified by this strategy have potential roles in hyperproliferation, a previously reported response to elevated levels of cCMP.

Differential expression of Ikaros isoforms in monozygotic twins with MLL-rearranged precursor-B acute lymphoblastic leukemia

Infant leukemia associated with rearrangement of the MLL gene typically presents with high-risk clinical features. Relapse is common despite aggressive therapy and perturbations in signaling pathways may contribute to disease resistance. We evaluated twin 4-month-old monozygotic baby boys who presented with MLL-rearranged precursor-B acute lymphoblastic leukemia. Two different MLL/AF4 variants were found in both the twins, the first involving MLL intron 8 and AF4 intron 3 and the second stemming from translocations of MLL exon 10 and AF4 exon 4. We detected expression of the DNA-binding Ikaros isoforms, Ik1, Ikx+, Ik2 and the dominant-negative Ik4 Ikaros isoform in both patients. However, the dominant-negative Ik8 isoform was detected in only 1 boy, suggesting a common genetic ontogeny that was modulated by leukemic evolution. Further exploration of Ikaros expression in the background of MLL rearrangements may provide new insights into disease pathogenesis and could offer targets for novel chemotherapeutic agents.

Acute myeloid leukemia induced by MLL-ENL is cured by oncogene ablation despite acquisition of complex genetic abnormalities

Chromosomal translocations involving 11q23 are frequent in infant acute leukemia and give rise to the formation of MLL fusion genes. The mechanism of leukemic transformation by these fusions has been the subject of numerous investigations. However, the dependence of acute leukemia on MLL fusion activity in vivo and the efficacy of targeting this activity to eliminate disease have not been established. We have developed a model for conditional expression of MLL-ENL in hematopoietic progenitor cells, in which expression of the fusion oncogene is turned off by doxycycline. Conditionally immortalized myeloblast cells derived from these progenitors were found to induce leukemia in vivo. Leukemic cells isolated from primary recipient mice were shown to have acquired additional genetic abnormalities and, when transplanted into secondary recipients, induced leukemia with shortened latencies. However, the leukemic cells remained dependent on MLL-ENL expression in vitro and in vivo, and its ablation resulted in regression of established leukemias. This study demonstrates that even genetically complex leukemias can be reversed on inactivation of the initiating MLL fusion and has important implications for the design of novel leukemia therapies.

Insights from clinical studies into the role of the MLL gene in infant and childhood leukemia

Translocations involving the Mixed Lineage Leukemia (MLL) gene at 11q23 are found in both acute lymphoblastic leukemia (ALL) and acute myeloblastic leukemia (AML), but have different prognostic implications depending on the phenotype of the leukemia in de novo pediatric cases. The majority of MLL gene rearrangements are associated with infant ALL, and their presence predicts a poor prognosis which worsens with earlier age of presentation. Rearrangements of the MLL gene are found in most cases of infant AML and regardless of age confer an intermediate risk. The treatment of MLL-rearranged ALL in children involves increased intensification of chemotherapy, and infants with ALL are treated with an intensive regimen of ALL- and AML-like chemotherapy, with the proportion of MLL-rearranged cases being responsible for the poor outcome in this age group. The use of DNA microarray analysis to distinguish a particular gene signature for MLL-rearranged leukemias is shedding light on the molecular mechanisms and potential therapeutic targets of these leukemias. It may also prove to have a useful role in both diagnosis and prognosis. This review considers recent advances in our understanding of the role of MLL gene rearrangements in pediatric clinical practice.

HYBRIDdb: a database of hybrid genes in the human genome

Background

Hybrid genes are candidate risk factors for human tumors by inducing mutation, translocation, inversion, or rearrangement of genes. The occurrence of hybrid genes may also have given rise to new transcripts during hominid evolution.

Description

HYBRIDdb is a database of hybrid genes in humans. This system encompasses the bioinformatics analysis of mRNA, EST, cDNA, and genomic DNA sequences in the INDC databases, and can be used to identify hybrid genes. We searched for hybrid genes among the 28,171 genes listed in the NCBI database, and analyzed their structural patterns in the human genome. The 2,344 gene pairs were detected as hybrid forms of transcriptional products. We classified the hybrid genes into two groups: chromosomal-mediated translocation fusion transcripts and transcription-mediated fusion transcripts.

Conclusion

The HYBRIDdb database will provide genome scientists with insight into potential roles for hybrid genes in human evolution and disease.

Gene expression profiling reveals intrinsic differences between T-cell acute lymphoblastic leukemia and T-cell lymphoblastic lymphoma

BACKGROUND

T-cell acute lymphoblastic leukemia (T-ALL) and T-cell lymphoblastic lymphoma (T-LL) and are often thought to represent a spectrum of a single disease. The malignant cells in T-ALL and T-LL are morphologically indistinguishable, and they share the expression of common cell surface antigens and cytogenetic characteristics. However, despite these similarities, differences in the clinical behavior of T-ALL and T-LL are observed.

PROCEDURE

We analyzed the gene expression profiles of T-ALL and T-LL samples obtained from Children's Oncology Group (COG) tumor banks using DNA arrays. Immunohistochemistry was also performed to validate the expression of selected targets.

RESULTS

Unsupervised hierarchical clustering of all samples showed complete segregation of T-ALL and T-LL into distinct clusters. Next, we identified the top 201 genes that best differentiated T-ALL from T-LL using significance analysis of microarrays (SAM), a supervised statistical approach. Genes representing several functional groups were differentially expressed in T-LL and T-ALL. Prediction analysis of microarrays (PAM) identified a subset of genes, which accurately classified all 19 T-ALL and T-LL samples with an overall misclassification error rate of 0. Immunohistochemical validation of protein expression of selected genes identified by microarray analysis confirmed overexpression of MLL-1 in T-LL tumor cells compared to T-ALL and CD47 in T-ALL tumors cells when compared to T-LL.

CONCLUSIONS

Despite significant similarities between the malignant T-cell precursors, clear differences in the gene expression profiles were observed between T-ALL and T-LL implying underlying differences in the biology of the two entities.

Temporary blast reduction after immunoglobulin administration for congenital cytomegalovirus infection masking infant leukemia with cryptic MLL rearrangement

Differentiation between reactive bone marrow suppression due to viral infection and early stages of leukemia can be difficult particularly in young infants. We report on a 2-month-old girl presenting with pancytopenia and positive markers for congenital cytomegalovirus (CMV) infection. Definitive diagnosis of coexisting pro-B cell infant leukemia with cryptic MLL rearrangement was delayed by the transient regeneration of normal hematopoiesis and reduction of abnormal blastoid cells in the bone marrow following immunoglobulin administration. Molecular diagnosis could only be established using interphase fluorescence in situ hybridization (FISH) analysis which may be considered as a valuable additional diagnostic tool in similar cases.

MLL–SEPT6 fusion transcript with a novel sequence in an infant with acute myeloid leukemia

The MLL gene at 11q23 is a site of frequent rearrangement in acute leukemia with multiple fusion partners. A relatively uncommon rearrangement, associated with infant AML-M4, fuses the MLL and SEPT6 genes. SEPT6, located at Xq24, is a member of a family of mammalian septins involved in diverse functions such as cytokinesis, cell polarity, and oncogenesis. We describe the case of an infant with acute myelogenous leukemia who showed cytogenetic evidence of rearrangement between 11q23 and Xq24 regions. Fluorescence in situ hybridization analysis suggested a possible break in the MLL gene, and molecular analysis using reverse transcriptase-polymerase chain reaction followed by sequencing confirmed the expression of an MLL-SEPT6 fusion transcript with a novel sequence. The findings emphasize the importance of combined cytogenetic and molecular analyses in the workup of acute leukemia, especially in those leukemias that occur infrequently.

Chromosomal translocations involving the MLL gene: molecular mechanisms

A wide array of recurrent, non-random chromosomal translocations are associated with hematologic malignancies; experimental models have clearly demonstrated that many of these translocations are causal events during malignant transformation. Translocations involving the MLL gene are among the most common of these non-random translocations. Leukemias with MLL translocations have been the topic of intense interest because of the unusual, biphenotypic immunophenotype of these leukemias, because of the unique clinical presentation of some MLL translocations (infant leukemia and therapy-related leukemia), and because of the large number of different chromosomal loci that partner with MLL in these translocations. This review is focused on the potential mechanisms that lead to MLL translocations, and will discuss aberrant VDJ recombination, Alu-mediated recombination, non-homologous end joining, as well as the effect of DNA topoisomerase II poisons and chromatin structure.

Individualized therapy for childhood acute lymphoblastic leukemia

In the field of oncology, a growing emphasis is now being placed on individualizing treatment in a way that maximizes chance for cure while minimizing unwanted side effects. In childhood acute lymphoblastic leukemia (ALL), several well-established clinical and biologic prognostic variables have traditionally been used to risk stratify therapy for individual patients. While this approach has been very successful, many relapses still occur unpredictably in patients characterized as having favorable features of their disease at diagnosis. Furthermore, it is likely that other children are overtreated. Therefore, current initiatives in childhood leukemia have focused on identifying new prognostic markers to refine treatment decision-making. Recent advances, which include the sequencing of the human genome, and technical developments in high-throughput genomics and proteomics, have facilitated these efforts. This review will chart the evolution of individualized therapy for ALL, the most common malignancy of children.

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.

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.

CD1d expression on B-precursor acute lymphoblastic leukemia subsets with poor prognosis

Acute lymphoblastic leukemia (ALL) is the most frequent malignancy of childhood. Although therapeutical advances have been achieved, some ALL subgroups still fare poorly. CD1d is a monomorphic molecule that provides a suitable target for immunotherapy in view of the characterization of a glycolipid, alpha-galactosylceramide (alpha-GalCer), capable of being presented to CD1d-restricted T cells with cytotoxic potential. We investigated CD1d expression in 80 pediatric B-cell precursor (BCP) ALL cases defined according to immunophenotype, cytogenetic features and age at onset. CD1d was detected on ALL cells in 15% of the patients. CD1d+ ALLs were significantly associated with infant leukemia, pro-B phenotype and mixed-lineage leukemia (MLL)/AF4 gene rearrangement. Accordingly, overall survival of patients with CD1d+ ALL was significantly shorter. CD1d+ leukemic blasts were able to present alpha-GalCer via CD1d to cytotoxic CD1d-restricted T cells, which induced apoptosis of ALL cells that was inhibited by mAb to CD1d. CD1d+ blasts loaded with alpha-GalCer elicited cytokine secretion by CD1d-restricted T cells. Analysis of bone marrow (BM) cells derived from normal donors revealed that CD19+/CD1d+ cells were mostly mature B lymphocytes. However, a minority of BCPs expressed CD1d. Thus, expression of CD1d in ALL cases heralds an adverse prognosis but may provide a therapeutic tool.

Overexpression of transcripts originating from the MMSET locus characterizes all t(4;14)(p16;q32)-positive multiple myeloma patients

Multiple myeloma (MM) is a B-lineage malignancy characterized by diverse genetic subtypes and clinical outcomes. The recurrent immunoglobulin heavy chain (IgH) switch translocation, t(4;14)(p16;q32), is associated with poor outcome, though the mechanism is unclear. Quantitative reverse-transcription-polymerase chain reaction (RT-PCR) for proposed target genes on a panel of myeloma cell lines and purified plasma cells showed that only transcripts originating from the WHSC1/MMSET/NSD2 gene are uniformly dysregulated in all t(4;14)POS patients. The different transcripts detected, multiple myeloma SET domain containing protein (MMSET I), MMSET II, Exon 4a/MMSET III, and response element II binding protein (RE-IIBP), are produced by alternative splicing and alternative transcription initiation events. Translation of the various transcripts, including those from major breakpoint region 4-2 (MB4-2) and MB4-3 breakpoint variants, was confirmed by transient transfection and immunoblotting. Green fluorescent protein (GFP)-tagged MMSET I and II, corresponding to proteins expressed in MB4-1 patients, localized to the nucleus but not nucleoli, whereas the MB4-2 and MB4-3 proteins concentrate in nucleoli. Cloning and localization of the Exon 4a/MMSET III splice variant, which contains the protein segment lost in the MB4-2 variant, identified a novel protein domain that prevents nucleolar localization. Kinetic studies using photobleaching suggest that the breakpoint variants are functionally distinct from wild-type proteins. In contrast, RE-IIBP is universally dysregulated and also potentially functional in all t(4;14)POS patients irrespective of fibroblast growth factor receptor 3 (FGFR3) expression or breakpoint type.