Therapy-related myeloid neoplasms resembling juvenile myelomonocytic leukemia: a case series and review of the literature

Therapy-related myeloid neoplasms (t-MN) are a distinct subgroup of myeloid malignancies with a poor prognosis that include cases of therapy-related myelodysplastic syndrome (t-MDS), therapy-related myeloproliferative neoplasms (t-MPN) and therapy-related acute myeloid leukemia (t-AML). Here, we report a series of patients with clinical features consistent with juvenile myelomonocytic leukemia (JMML), an overlap syndrome of MDS and myeloproliferative neoplasms that developed after treatment for another malignancy.

Non-canonical H3K79me2-dependent pathways promote the survival of MLL-rearranged leukemia

MLL-rearranged leukemia depends on H3K79 methylation. Depletion of this transcriptionally activating mark by DOT1L deletion or high concentrations of the inhibitor pinometostat downregulates HOXA9 and MEIS1, and consequently reduces leukemia survival. Yet, some MLL-rearranged leukemias are inexplicably susceptible to low-dose pinometostat, far below concentrations that downregulate this canonical proliferation pathway. In this context, we define alternative proliferation pathways that more directly derive from H3K79me2 loss. By ICeChIP-seq, H3K79me2 is markedly depleted at pinometostat-downregulated and MLL-fusion targets, with paradoxical increases of H3K4me3 and loss of H3K27me3. Although downregulation of polycomb components accounts for some of the proliferation defect, transcriptional downregulation of FLT3 is the major pathway. Loss-of-FLT3-function recapitulates the cytotoxicity and gene expression consequences of low-dose pinometostat, whereas overexpression of constitutively active STAT5A, a target of FLT3-ITD-signaling, largely rescues these defects. This pathway also depends on MLL1, indicating combinations of DOT1L, MLL1 and FLT3 inhibitors should be explored for treating FLT3-mutant leukemia.

Tet1 is not required for myeloid leukemogenesis by MLL-ENL in novel mouse models

The Ten Eleven Translocation 1 (TET1) gene encodes an epigenetic modifying molecule that is involved in demethylation of 5-methylcytosine. In hematological malignancies, loss-of-function mutations of TET2, which is one of the TET family genes including TET1, are frequently found, while the mutations of TET1 are not. However, clinical studies have revealed that TET1 is highly expressed in some cases of the hematological malignancies including acute myeloid leukemia. Indeed, studies by mouse models using conventional Tet1 knockout mice demonstrated that Tet1 is involved in myeloid leukemogenesis by Mixed Lineage Leukemia (MLL) fusion gene or TET2 mutant. Meanwhile, the other study showed that Tet1 is highly expressed in hematopoietic stem cells (HSCs), and that deletion of Tet1 in HSCs enhances potential self-renewal capacity, which is potentially associated with myeloid leukemogenesis. To examine the role of Tet1 in myeloid leukemogenesis more precisely, we generated novel conditional Tet1-knockout mice, which were used to generate the compound mutant mice by crossing with the inducible MLL-ENL transgenic mice that we developed previously. The leukemic immortalization in vitro was not critically affected by conditional ablation of Tet1 in HSCs with the induced expression of MLL-ENL or in hematopoietic progenitor cells retrovirally transduced with MLL-ENL. In addition, the leukemic phenotypes caused by the induced expression of MLL-ENL in vivo was not also critically affected in the compound mutant mouse model by conditional ablation of Tet1, although we found that the expression of Evi1, which is one of critical target genes of MLL fusion gene, in tumor cells was remarkably low under Tet1-ablated condition. These results revealed that Tet1 was dispensable for the myeloid leukemogenesis by MLL-ENL, suggesting that the therapeutic application of Tet1 inhibition may need careful assessment.

The molecular functions of common and atypical MLL fusion protein complexes

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

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

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

Modulation of FLT3 through decitabine-activated C/EBPa-PU.1 signal pathway in FLT3-ITD positive cells

FMS-like tyrosine kinase 3 (FLT3)-mutant acute myeloid leukemia (AML) which occurs in approximately 30% of all AML patients still has a poor prognosis. This study aimed to examine the effect of decitabine (DAC) on FLT3-ITD positive AML. In our study, we found that expression of FLT3 and its downstream targets was decreased in FLT3-ITD mutant cell lines treated with DAC. DAC treatment could increase the percentage of apoptotic cells and CD11b positive cells tested by flow cytometry and upregulate the expression of cleaved caspase3, cleaved PARP, C/EBPa and PU.1 detected by western blot. To explore the effect of increased expression of PU.1 on FLT3 protein, we transiently transfected MOLM13 and MV4-11 cells with siRNA against PU.1 and a siRNA control. In both FLT3-ITD positive cells, the effect of DAC on downregulation of FLT3 was diminished in PU.1-konckdown MOLM13 and MV4-11 cells and there was a decrease of CD11b expression after PU.1 knockdown. Furthermore, the percentage of apoptotic cells was also decreased in PU.1-konckdown cells compared with siRNA control-expressing cells with the same dose of DAC. These findings indicated that DAC upregulated PU.1 to induce downregulation of FLT3 to trigger apoptosis. DAC was also found efficacious in mouse xenograft models of FLT3-ITD AML in our study. These findings may provide a novel theoretical basis for treatment of FLT3-ITD positive AML patients.

The Impact of PI3-kinase/RAS Pathway Cooperating Mutations in the Evolution of KMT2A-rearranged Leukemia

Leukemia is an evolutionary disease and evolves by the accrual of mutations within a clone. Those mutations that are systematically found in all the patients affected by a certain leukemia are called "drivers" as they are necessary to drive the development of leukemia. Those ones that accumulate over time but are different from patient to patient and, therefore, are not essential for leukemia development are called "passengers." The first studies highlighting a potential cooperating role of phosphatidylinositol 3-kinase (PI3K)/RAS pathway mutations in the phenotype of KMT2A-rearranged leukemia was published 20 years ago. The recent development in more sensitive sequencing technologies has contributed to clarify the contribution of these mutations to the evolution of KMT2A-rearranged leukemia and suggested that these mutations might confer clonal fitness and enhance the evolvability of KMT2A-leukemic cells. This is of particular interest since this pathway can be targeted offering potential novel therapeutic strategies to KMT2A-leukemic patients. This review summarizes the recent progress on our understanding of the role of PI3K/RAS pathway mutations in initiation, maintenance, and relapse of KMT2A-rearranged leukemia.

Eya2 is critical for the E2A‑HLF‑mediated immortalization of mouse hematopoietic stem/progenitor cells

The immunoglobulin enhancer‑binding factor/hepatic leukemia factor (E2A‑HLF) oncogenic fusion gene, generated by t(17;19)(q22;p13) translocation in childhood B‑cell acute lymphoblastic leukemia with a very poor prognosis, encodes a chimeric transcription factor in which the transactivation domains of E2A are fused to the DNA‑binding and dimerization domain of HLF. E2A‑HLF has been demonstrated to have an anti‑apoptotic effect. However, the molecular mechanism underlying E2A‑HLF‑mediated leukemogenesis remains unclear. The present study identified EYA transcriptional coactivator and phosphatase 2 (Eya2), the forced expression of which is known to immortalize mouse hematopoietic stem/progenitor cells (HSPCs), as a direct target molecule downstream of E2A‑HLF. E2A‑HLF‑immortalized mouse HSPCs expressed Eya2 at a high level in the aberrant self‑renewal program. Chromatin immunoprecipitation‑quantitative polymerase chain reaction and a reporter assay revealed that E2A‑HLF enhanced the Eya2 expression by binding to the promoter region containing the E2A‑HLF‑binding consensus sequence. Eya2 knockdown in E2A‑HLF‑immortalized cells resulted in reduced colony‑forming efficiency. These results suggest a critical role of Eya2 in E2A‑HLF‑mediated leukemogenesis.

A case of KMT2A-SEPT9 fusion-associated acute megakaryoblastic leukemia

Acute megakaryoblastic leukemia (AMKL) constitutes ∼5%-15% of cases of non-Down syndrome AML in children, and in the majority of cases, chimeric oncogenes resulting from recurrent gene rearrangements are identified. Based on these rearrangements, several molecular subsets have been characterized providing important prognostic information. One such subset includes a group of patients with translocations involving the KMT2A gene, which has been associated with various fusion partners in patients with AMKL. Here we report the molecular findings of a 2-yr-old girl with AMKL and t(11;17)(q23;25) found to have a KMT2A-SEPT9 fusion identified through targeted RNA sequencing. A KMT2A-SEPT9 fusion in this subset of patients has not previously been reported.

De novo activating mutations drive clonal evolution and enhance clonal fitness in KMT2A-rearranged leukemia

Activating signaling mutations are common in acute leukemia with KMT2A (previously MLL) rearrangements (KMT2A-R). These mutations are often subclonal and their biological impact remains unclear. Using a retroviral acute myeloid mouse leukemia model, we demonstrate that FLT3^ITD, FLT3^N676K, and NRAS^G12D accelerate KMT2A-MLLT3 leukemia onset. Further, also subclonal FLT3^N676K mutations accelerate disease, possibly by providing stimulatory factors. Herein, we show that one such factor, MIF, promotes survival of mouse KMT2A-MLLT3 leukemia initiating cells. We identify acquired de novo mutations in Braf, Cbl, Kras, and Ptpn11 in KMT2A-MLLT3 leukemia cells that favored clonal expansion. During clonal evolution, we observe serial genetic changes at the Kras^G12D locus, consistent with a strong selective advantage of additional Kras^G12D. KMT2A-MLLT3 leukemias with signaling mutations enforce Myc and Myb transcriptional modules. Our results provide new insight into the biology of KMT2A-R leukemia with subclonal signaling mutations and highlight the importance of activated signaling as a contributing driver.

In acute leukemia with KMT2A rearrangements (KMT2A-R), activating signaling mutations are common. Here, the authors use a retroviral acute myeloid mouse leukemia model to show that subclonal de novo activating mutations drive clonal evolution in acute leukemia with KMT2A-R and enhance clonal fitness.

WDR5 high expression and its effect on tumorigenesis in leukemia

WD repeat domain 5 (WDR5) plays an important role in various biological functions through the epigenetic regulation of gene transcription. However, the oncogenic effect of WDR5 in leukemia remains largely unknown. Here, we found WDR5 expression is increased in leukemia patients. High expression of WDR5 is associated with high risk leukemia; Patients with WDR5 and MLL1 high expression have poor complete remission rate. We further identified the global genomic binding of WDR5 in leukemic cells and found the genomic co-localization of WDR5 binding with H3K4me3 enrichment. Moreover, WDR5 knockdown by shRNA suppresses cell proliferation, induces apoptosis, inhibits the expression of WDR5 targets, and blocks the H3K4me3 enrichment on the promoter of its targets. We also observed the positive correlation of WDR5 expression with these targets in the cohort study of leukemia patients. Our data reveal that WDR5 may have oncogenic effect and WDR5-mediated H3K4 methylation plays an important role in leukemogenesis.

Evolution of AF6-RAS association and its implications in mixed-lineage leukemia

Elucidation of activation mechanisms governing protein fusions is essential for therapeutic development. MLL undergoes rearrangement with numerous partners, including a recurrent translocation fusing the epigenetic regulator to a cytoplasmic RAS effector, AF6/afadin. We show here that AF6 employs a non-canonical, evolutionarily conserved α-helix to bind RAS, unique to AF6 and the classical RASSF effectors. Further, all patients with MLL-AF6 translocations express fusion proteins missing only this helix from AF6, resulting in exposure of hydrophobic residues that induce dimerization. We provide evidence that oligomerization is the dominant mechanism driving oncogenesis from rare MLL translocation partners and employ our mechanistic understanding of MLL-AF6 to examine how dimers induce leukemia. Proteomic data resolve association of dimerized MLL with gene expression modulators, and inhibiting dimerization disrupts formation of these complexes while completely abrogating leukemogenesis in mice. Oncogenic gene translocations are thus selected under pressure from protein structure/function, underscoring the complex nature of chromosomal rearrangements.

Several rearrangements of the MLL gene are associated with acute leukemia, including the fusion of MLL with a RAS effector protein, AF6. Here the authors show that the truncated AF6 can induce AF6-MLL dimerization and drive its oncogenic activity.

Recurrent SPI1 (PU.1) fusions in high-risk pediatric T cell acute lymphoblastic leukemia

The outcome of treatment-refractory and/or relapsed pediatric T cell acute lymphoblastic leukemia (T-ALL) is extremely poor, and the genetic basis for this is not well understood. Here we report comprehensive profiling of 121 cases of pediatric T-ALL using transcriptome and/or targeted capture sequencing, through which we identified new recurrent gene fusions involving SPI1 (STMN1-SPI1 and TCF7-SPI1). Cases positive for fusions involving SPI1 (encoding PU.1), accounting for 3.9% (7/181) of the examined pediatric T-ALL cases, showed a double-negative (DN; CD4^-CD8^-) or CD8⁺ single-positive (SP) phenotype and had uniformly poor overall survival. These cases represent a subset of pediatric T-ALL distinguishable from the known T-ALL subsets in terms of expression of genes involved in T cell precommitment, establishment of T cell identity, and post-β-selection maturation and with respect to mutational profile. PU.1 fusion proteins retained transcriptional activity and, when constitutively expressed in mouse stem/progenitor cells, induced cell proliferation and resulted in a maturation block. Our findings highlight a unique role of SPI1 fusions in high-risk pediatric T-ALL.

Eya2, a Target Activated by Plzf, Is Critical for PLZF-RARA-Induced Leukemogenesis

PLZF is a transcription factor that confers aberrant self-renewal in leukemogenesis, and the PLZF-RARA fusion gene causes acute promyelocytic leukemia (APL) through differentiation block. However, the molecular mechanisms of aberrant self-renewal underlying PLZF-mediated leukemogenesis are poorly understood. To investigate these mechanisms, comprehensive expression profiling of mouse hematopoietic stem/progenitor cells transduced with Plzf was performed, which revealed the involvement of a key transcriptional coactivator, Eya2, a target molecule shared by Plzf and PLZF-RARA, in the aberrant self-renewal. Indeed, PLZF-RARA as well as Plzf rendered those cells immortalized through upregulation of Eya2. Eya2 also led to immortalization without differentiation block, while depletion of Eya2 suppressed clonogenicity in cells immortalized by PLZF-RARA without influence on differentiation and apoptosis. Interestingly, cancer outlier profile analysis of human samples of acute myeloid leukemia (AML) in The Cancer Genome Atlas (TCGA) revealed a subtype of AML that strongly expressed EYA2 In addition, gene set enrichment analysis of human AML samples, including TCGA data, showed that this subtype of AML was more closely associated with the properties of leukemic stem cells in its gene expression signature than other AMLs. Therefore, EYA2 may be a target for molecular therapy in this subtype of AML, including PLZF-RARA APL.

Mouse models of MLL leukemia: recapitulating the human disease

Chromosome translocations involving the mixed lineage leukemia (MLL) gene fuse it in frame with multiple partner genes creating novel fusion proteins (MLL-FPs) that cause aggressive acute leukemias in humans. Animal models of human disease are important for the exploration of underlying disease mechanisms as well as for testing novel therapeutic approaches. Patients carrying MLL-FPs have very few cooperating mutations, making MLL-FP driven leukemias ideal for animal modeling. The fact that the MLL-FP is the main driver mutation has allowed for a wide range of different experimental model systems designed to explore different aspects of MLL-FP leukemogenesis. In addition, MLL-FP driven acute myeloid leukemia (AML) in mice is often used as a general model for AML. This review provides an overview of different MLL-FP mouse model systems and discusses how well they have recapitulated aspects of the human disease as well as highlights the biological insights each model has provided into MLL-FP leukemogenesis. Many promising new drugs fail in the early stages of clinical trials. Lessons learned from past and present MLL-FP models may serve as a paradigm for designing more flexible and dynamic preclinical models for these as well as other acute leukemias.

Deciphering KRAS and NRAS mutated clone dynamics in MLL-AF4 paediatric leukaemia by ultra deep sequencing analysis

To induce and sustain the leukaemogenic process, MLL-AF4+ leukaemia seems to require very few genetic alterations in addition to the fusion gene itself. Studies of infant and paediatric patients with MLL-AF4+ B cell precursor acute lymphoblastic leukaemia (BCP-ALL) have reported mutations in KRAS and NRAS with incidences ranging from 25 to 50%. Whereas previous studies employed Sanger sequencing, here we used next generation amplicon deep sequencing for in depth evaluation of RAS mutations in 36 paediatric patients at diagnosis of MLL-AF4+ leukaemia. RAS mutations including those in small sub-clones were detected in 63.9% of patients. Furthermore, the mutational analysis of 17 paired samples at diagnosis and relapse revealed complex RAS clone dynamics and showed that the mutated clones present at relapse were almost all originated from clones that were already detectable at diagnosis and survived to the initial therapy. Finally, we showed that mutated patients were indeed characterized by a RAS related signature at both transcriptional and protein levels and that the targeting of the RAS pathway could be of beneficial for treatment of MLL-AF4+ BCP-ALL clones carrying somatic RAS mutations.

Novel working hypothesis for pathogenesis of hematological malignancies: combination of mutations-induced cellular phenotypes determines the disease (cMIP-DD)

Recent progress in high-speed sequencing technology has revealed that tumors harbor novel mutations in a variety of genes including those for molecules involved in epigenetics and splicing, some of which were not categorized to previously thought malignancy-related genes. However, despite thorough identification of mutations in solid tumors and hematological malignancies, how these mutations induce cell transformation still remains elusive. In addition, each tumor usually contains multiple mutations or sometimes consists of multiple clones, which makes functional analysis difficult. Fifteen years ago, it was proposed that combination of two types of mutations induce acute leukemia; Class I mutations induce cell growth or inhibit apoptosis while class II mutations block differentiation, co-operating in inducing acute leukemia. This notion has been proven using a variety of mouse models, however most of recently found mutations are not typical class I/II mutations. Although some novel mutations have been found to functionally work as class I or II mutation in leukemogenesis, the classical class I/II theory seems to be too simple to explain the whole story. We here overview the molecular basis of hematological malignancies based on clinical and experimental results, and propose a new working hypothesis for leukemogenesis.

Proteasome inhibitors induce FLT3-ITD degradation through autophagy in AML cells

Internal tandem duplication of the Fms-like tyrosine kinase-3 receptor (FLT3) internal tandem duplication (ITD) is found in 30% of acute myeloid leukemia (AML) and is associated with a poor outcome. In addition to tyrosine kinase inhibitors, therapeutic strategies that modulate the expression of FLT3-ITD are also promising. We show that AML samples bearing FLT3-ITD mutations are more sensitive to proteasome inhibitors than wild-type samples and this sensitivity is strongly correlated with a higher FLT3-ITD allelic burden. Using pharmacologic inhibitors of autophagy, specific downregulation of key autophagy proteins including Vps34, autophagy gene (Atg)5, Atg12, Atg13, biochemical, and microscopy studies, we demonstrated that proteasome inhibitors induced cytotoxic autophagy in AML cells. FLT3-ITD molecules were detectable within autophagosomes after bortezomib treatment indicating that autophagy induction was responsible for the early degradation of FLT3-ITD, which preceded the inhibition of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), PI3K/AKT, and STAT5 pathways, and subsequent activation of cell death. Moreover, proteasome inhibitors overcome resistance to quizartinib induced by mutations in the kinase domain of FLT3, suggesting that these compounds may prevent the emergence of mutant clones arising from tyrosine kinase inhibitor treatments. In xenograft mice models, bortezomib stimulated the conversion of LC3-I to LC3-II, indicating induction of autophagy in vivo, downregulated FLT3-ITD protein expression and improved overall survival. Therefore, selecting patients according to FLT3-ITD mutations could be a new way to detect a significant clinical activity of proteasome inhibitors in AML patients.

Molecular mechanisms of MLL-associated leukemia

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

Mutated Ptpn11 alters leukemic stem cell frequency and reduces the sensitivity of acute myeloid leukemia cells to Mcl1 inhibition

PTPN11 encodes the Shp2 non-receptor protein-tyrosine phosphatase implicated in several signaling pathways. Activating mutations in Shp2 are commonly associated with juvenile myelomonocytic leukemia (JMML) but are not as well defined in other neoplasms. Here we report that Shp2 mutations occur in human acute myeloid leukemia (AML) at a rate of 6.6% (6/91) in the ECOG E1900 dataset. We examined the role of mutated Shp2 in leukemias harboring MLL translocations which co-occur in human AML. The hyperactive Shp2E76K mutant, commonly observed in leukemia patients, significantly accelerated MLL-AF9 mediated leukemogenesis in vivo. Shp2E76K increased leukemic stem cell frequency and affords MLL-AF9 leukemic cells IL3 cytokine hypersensitivity. As Shp2 is reported to regulate anti-apoptotic genes, we investigated Bcl2, Bcl-xL and Mcl1 expression in MLL-AF9 leukemic cells with and without Shp2E76K. While the Bcl2 family of genes was upregulated in Shp2E76K cells, Mcl1 showed the highest upregulation in MLL-AF9 cells in response to Shp2E76K. Indeed, expression of Mcl1 in MLL-AF9 cells phenocopies expression of Shp2E76K suggesting Shp2 mutations cooperate through activation of anti-apoptotic genes. Finally, we show Shp2E76K mutations reduce sensitivity of AML cells to small molecule mediated Mcl1 inhibition suggesting reduced efficacy of drugs targeting MCL1 in patients with hyperactive Shp2.

Septin functions in organ system physiology and pathology

Human septins comprise a family of 13 genes that encode for >30 protein isoforms with ubiquitous and tissue-specific expressions. Septins are GTP-binding proteins that assemble into higher-order oligomers and filamentous polymers, which associate with cell membranes and the cytoskeleton. In the last decade, much progress has been made in understanding the biochemical properties and cell biological functions of septins. In parallel, a growing number of studies show that septins play important roles for the development and physiology of specific tissues and organs. Here, we review the expression and function of septins in the cardiovascular, immune, nervous, urinary, digestive, respiratory, endocrine, reproductive, and integumentary organ systems. Furthermore, we discuss how the tissue-specific functions of septins relate to the pathology of human diseases that arise from aberrations in septin expression.

A novel cell-cycle-indicator, mVenus-p27K-, identifies quiescent cells and visualizes G0-G1 transition

The quiescent (G0) phase of the cell cycle is the reversible phase from which the cells exit from the cell cycle. Due to the difficulty of defining the G0 phase, quiescent cells have not been well characterized. In this study, a fusion protein consisting of mVenus and a defective mutant of CDK inhibitor, p27 (p27K⁻) was shown to be able to identify and isolate a population of quiescent cells and to effectively visualize the G0 to G1 transition. By comparing the expression profiles of the G0 and G1 cells defined by mVenus-p27K⁻, we have identified molecular features of quiescent cells. Quiescence is also an important feature of many types of stem cells, and mVenus-p27K⁻-transgenic mice enabled the detection of the quiescent cells with muscle stem cell markers in muscle in vivo. The mVenus-p27K⁻ probe could be useful in investigating stem cells as well as quiescent cells.

Sequential gene targeting to make chimeric tumor models with de novo chromosomal abnormalities

The discovery of chromosomal translocations in leukemia/lymphoma and sarcomas presaged a widespread discovery in epithelial tumors. With the advent of new-generation whole-genome sequencing, many consistent chromosomal abnormalities have been described together with putative driver and passenger mutations. The multiple genetic changes required in mouse models to assess the interrelationship of abnormalities and other mutations are severe limitations. Here, we show that sequential gene targeting of embryonic stem cells can be used to yield progenitor cells to generate chimeric offspring carrying all the genetic changes needed for cell-specific cancer. Illustrating the technology, we show that MLL-ENL fusion is sufficient for lethal leukocytosis and proof of genome integrity comes from germline transmission of the sequentially targeted alleles. This accelerated technology leads to a reduction in mouse numbers (contributing significantly to the 3Rs), allows fluorescence tagging of cancer-initiating cells, and provides a flexible platform for interrogating the interaction of chromosomal abnormalities with mutations.

The molecular basis of myeloid malignancies

Myeloid malignancies consist of acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and myeloproliferative neoplasm (MPN). The latter two diseases have preleukemic features and frequently evolve to AML. As with solid tumors, multiple mutations are required for leukemogenesis. A decade ago, these gene alterations were subdivided into two categories: class I mutations stimulating cell growth or inhibiting apoptosis; and class II mutations that hamper differentiation of hematopoietic cells. In mouse models, class I mutations such as the Bcr-Abl fusion kinase induce MPN by themselves and some class II mutations such as Runx1 mutations induce MDS. Combinations of class I and class II mutations induce AML in a variety of mouse models. Thus, it was postulated that hematopoietic cells whose differentiation is blocked by class II mutations would autonomously proliferate with class I mutations leading to the development of leukemia. Recent progress in high-speed sequencing has enabled efficient identification of novel mutations in a variety of molecules including epigenetic factors, splicing factors, signaling molecules and proteins in the cohesin complex; most of these are not categorized as either class I or class II mutations. The functional consequences of these mutations are now being extensively investigated. In this article, we will review the molecular basis of hematological malignancies, focusing on mouse models and the interfaces between these models and clinical findings, and revisit the classical class I/II hypothesis.

Plzf drives MLL-fusion-mediated leukemogenesis specifically in long-term hematopoietic stem cells

Oncogenic transformation requires unlimited self-renewal. Currently, it remains unclear whether a normal capacity for self-renewal is required for acquiring an aberrant self-renewal capacity. Our results in a new conditional transgenic mouse showed that a mixed lineage leukemia (MLL) fusion oncogene, MLL-ENL, at an endogenous-like expression level led to leukemic transformation selectively in a restricted subpopulation of hematopoietic stem cells (HSCs) through upregulation of promyelocytic leukemia zinc finger (Plzf). Interestingly, forced expression of Plzf itself immortalized HSCs and myeloid progenitors in vitro without upregulation of Hoxa9/Meis1, which are well-known targets of MLL fusion proteins, whereas its mutant lacking the BTB/POZ domain did not. In contrast, depletion of Plzf suppressed the MLL-fusion-induced leukemic transformation of HSCs in vitro and in vivo. Gene expression analyses of human clinical samples showed that a subtype of PLZF-high MLL-rearranged myeloid leukemia cells was closely associated with the gene expression signature of HSCs. These findings suggested that MLL fusion protein enhances the self-renewal potential of normal HSCs to develop leukemia, in part through a Plzf-driven self-renewal program.

Mechanisms of mixed-lineage leukemia

Advances in our understanding of the genetic determinants of leukemia have translated to better treatment options and improved survival of patients with acute myeloid and acute lymphoid leukemia. However, some leukemias, such as those bearing 11q23 (MLL) translocations, result in aggressive diseases with a relatively poor prognosis, despite improved treatments such as allogeneic hematopoietic stem cell transplantation. This article will briefly review the functions and regulation of wild-type MLL during normal hematopoiesis, while focusing on recent advances in our understanding of the molecular mechanisms governing MLL leukemias. The transcriptional targets, cooperating signaling pathways and molecular machinery involved in MLL-associated leukemias will be discussed, as well as how these may be harnessed for more personalized treatment of this disease.

FLT3 activation cooperates with MLL-AF4 fusion protein to abrogate the hematopoietic specification of human ESCs

Mixed-lineage leukemia (MLL)-AF4 fusion arises prenatally in high-risk infant acute pro-B-lymphoblastic leukemia (pro-B-ALL). In human embryonic stem cells (hESCs), MLL-AF4 skewed hematoendothelial specification but was insufficient for transformation, suggesting that additional oncogenic insults seem required for MLL-AF4-mediated transformation. MLL-AF4+ pro-B-ALL expresses enormous levels of FLT3, occasionally because of activating mutations, thus representing a candidate cooperating event in MLL-AF4+ pro-B-ALL. Here, we explored the developmental impact of FLT3 activation alone, or together with MLL-AF4, in the hematopoietic fate of hESCs. FLT3 activation does not affect specification of hemogenic precursors but significantly enhances the formation of CD45(+) blood cells, and CD45(+)CD34(+) blood progenitors with clonogenic potential. However, overexpression of FLT3 mutations or wild-type FLT3 (FLT3-WT) completely abrogates hematopoietic differentiation from MLL-AF4-expressing hESCs, indicating that FLT3 activation cooperates with MLL-AF4 to inhibit human embryonic hematopoiesis. Cell cycle/apoptosis analyses suggest that FLT3 activation directly affects hESC specification rather than proliferation or survival of hESC-emerging hematopoietic derivatives. Transcriptional profiling of hESC-derived CD45(+) cells supports the FLT3-mediated inhibition of hematopoiesis in MLL-AF4-expressing hESCs, which is associated with large transcriptional changes and downregulation of genes involved in hematopoietic system development and function. Importantly, FLT3 activation does not cooperate with MLL-AF4 to immortalize/transform hESC-derived hematopoietic cells, suggesting the need of alternative (epi)-genetic cooperating hits.

STAT5 is crucial to maintain leukemic stem cells in acute myelogenous leukemias induced by MOZ-TIF2

MOZ-TIF2 is a leukemogenic fusion oncoprotein that confers self-renewal capability to hematopoietic progenitor cells and induces acute myelogenous leukemia (AML) with long latency in bone marrow transplantation assays. Here, we report that FLT3-ITD transforms hematopoietic cells in cooperation with MOZ-TIF2 in vitro and in vivo. Coexpression of FLT3-ITD confers growth factor independent survival/proliferation, shortens disease latency, and results in an increase in the number of leukemic stem cells (LSC). We show that STAT5, a major effector of aberrant FLT3-ITD signal transduction, is both necessary and sufficient for this cooperative effect. In addition, STAT5 signaling is essential for MOZ-TIF2-induced leukemic transformation itself. Lack of STAT5 in fetal liver cells caused rapid differentiation and loss of replating capacity of MOZ-TIF2-transduced cells enriched for LSCs. Furthermore, mice serially transplanted with Stat5(-/-) MOZ-TIF2 leukemic cells develop AML with longer disease latency and finally incomplete penetrance when compared with mice transplanted with Stat5(+/+) MOZ-TIF2 leukemic cells. These data suggest that STAT5AB is required for the self-renewal of LSCs and represents a combined signaling node of FLT3-ITD and MOZ-TIF2 driven leukemogenesis. Therefore, targeting aberrantly activated STAT5 or rewired downstream signaling pathways may be a promising therapeutic option.

FLT3 in lineage specification and plasticity

FLT3 is a receptor tyrosine kinase that is expressed in CD34⁺ hematopoietic stem/progenitor cells (HSPCs) and is important for both normal myeloid and lymphoid differentiation. FLT3 expression in Pax5 negative lymphoid precursors coincides with a window of multilineage differentiation potential in mice and humans. Recent work has shown that FLT3 activating mutations can collaborate with a Nup98-HoxD13 mutation to induce an aggressive acute leukemia. The leukemic initiating population in this model displayed properties of both lymphoid and myeloid precursors, making it a useful tool to study the role of FLT3 in lineage plasticity. Through a variety of assays, the leukemic initiating population was shown to be restricted to myeloid differentiation, suggesting that the B-lineage properties in these cells are due to the priming of lymphoid transcription programs in multipotent progenitors rather than a true capacity for B-cell maturation. The development of an undifferentiated myeloid leukemia in this model, also has implications for the role of FLT3 in the inhibition of myeloid differentiation. Here we discuss the insights gained from this model.

MicroRNA-125b-1 accelerates a C-terminal mutant of C/EBPα (C/EBPα-C(m))-induced myeloid leukemia

MicroRNA-125b-1 (miR-125b-1) is a target of the chromosomal translocations t(11;14)(q24;q32) and t(2;11)(p21;q23), which are found in human B-lymphoid and myeloid malignancies, respectively. These translocations result in overexpression of mature miR-125b, consisting of 22 nucleotides. To analyze the role of miR-125b-1 in leukemogenesis, we created a bone marrow transplantation model using a retrovirus vector containing GFP expression elements. Sole transduction of miR-125b-1 into bone marrow cells resulted in expansion of hematopoietic cells expressing GFP. Compared with cells lacking GFP expression, we observed that GFP(+)/CD11b(+) or GFP(+)/Gr(-)1(+) cells were increased in the bone marrow and spleen. Although previous studies reported sole induction of miR-125b-induced leukemia, we did not find leukemic transformation in our model. Transduction of miR-125b-1 did accelerate myeloid tumors induced by a C-terminal mutant of CAAT-enhancer binding protein (C/EBPα-C(m)), a class II-like mutation. As miR-125b has been shown to hasten the development of leukemia in a BCR/ABL-transduced animal model, our present results support the conclusion that overexpression of miR-125b cooperates with other genetic alterations in the pathogenesis of myeloid malignancies.

Knock-in of a FLT3/ITD mutation cooperates with a NUP98-HOXD13 fusion to generate acute myeloid leukemia in a mouse model

Constitutive activation of FLT3 by internal tandem duplication (ITD) is one of the most common molecular alterations in acute myeloid leukemia (AML). FLT3/ITD mutations have also been observed in myelodysplastic syndrome patients both before and during progression to AML. Previous work has shown that insertion of an FLT3/ITD mutation into the murine Flt3 gene induces a myeloproliferative neoplasm, but not progression to acute leukemia, suggesting that additional cooperating events are required. We therefore combined the FLT3/ITD mutation with a model of myelodysplastic syndrome involving transgenic expression of the Nup98-HoxD13 (NHD13) fusion gene. Mice expressing both the FLT3/ITD and NHD13 transgene developed AML with 100% penetrance and short latency. These leukemias were driven by mutant FLT3 expression and were susceptible to treatment with FLT3 tyrosine kinase inhibitors. We also observed a spontaneous loss of the wild-type Flt3 allele in these AMLs, further modeling the loss of the heterozygosity phenomenon that is seen in human AML with FLT3-activating mutations. Because resistance to FLT3 inhibitors remains an important clinical issue, this model may help identify new molecular targets in collaborative signaling pathways.

A human ESC model for MLL-AF4 leukemic fusion gene reveals an impaired early hematopoietic-endothelial specification

The MLL-AF4 fusion gene is a hallmark genomic aberration in high-risk acute lymphoblastic leukemia in infants. Although it is well established that MLL-AF4 arises prenatally during human development, its effects on hematopoietic development in utero remain unexplored. We have created a human-specific cellular system to study early hemato-endothelial development in MLL-AF4-expressing human embryonic stem cells (hESCs). Functional studies, clonal analysis and gene expression profiling reveal that expression of MLL-AF4 in hESCs has a phenotypic, functional and gene expression impact. MLL-AF4 acts as a global transcriptional activator and a positive regulator of homeobox gene expression in hESCs. Functionally, MLL-AF4 enhances the specification of hemogenic precursors from hESCs but strongly impairs further hematopoietic commitment in favor of an endothelial cell fate. MLL-AF4 hESCs are transcriptionally primed to differentiate towards hemogenic precursors prone to endothelial maturation, as reflected by the marked upregulation of master genes associated to vascular-endothelial functions and early hematopoiesis. Furthermore, we report that MLL-AF4 expression is not sufficient to transform hESC-derived hematopoietic cells. This work illustrates how hESCs may provide unique insights into human development and further our understanding of how leukemic fusion genes, known to arise prenatally, regulate human embryonic hematopoietic specification.

Mutations in the transmembrane and juxtamembrane domains enhance IL27R transforming activity

Cytokines and their receptors regulate haemopoiesis by controlling cellular growth, survival and differentiation. Thus it is not surprising that mutations of cytokine receptors contribute to the formation of haemopoietic disorders, including cancer. We recently identified transforming properties of IL27R, the ligand-binding component of the receptor for interleukin-27. Although wild-type IL27R exhibits transforming properties in haemopoietic cells, in the present study we set out to determine if the transforming activity of IL27R could be enhanced by mutation. We identified three mutations of IL27R that enhance its transforming activity. One of these mutations is a phenylalanine to cysteine mutation at residue 523 (F523C) in the transmembrane domain of the receptor. The two other mutations identified involve deletions of amino acids in the cytoplasmic juxtamembrane region of the receptor. Expression of each of these mutant IL27R proteins led to rapid cytokine-independent transformation in haemopoietic cells. Moreover, the rate of transformation induced by these mutants was significantly greater than that induced by wild-type IL27R. Expression of these IL27R mutants also induced enhanced activation of JAK (Janus kinase)/STAT (signal transducer and activator of transcription) signalling compared with wild-type. An activating deletion mutation of IL27R enhanced homodimerization of the receptor by a mechanism that may involve disulfide bonding. These transforming IL27R mutants displayed equal or greater transforming activity than bona fide haemopoietic oncogenes such as BCR-ABL (breakpoint cluster region-Abelson murine leukaemia viral oncogene homologue) and JAK2-V617F. Since IL27R is expressed on haemopoietic stem cells, lymphoid cells and myeloid cells, including acute myeloid leukaemia blast cells, mutation of this receptor has the potential to contribute to a variety of haemopoietic neoplasms.

An integrated approach to dissecting oncogene addiction implicates a Myb-coordinated self-renewal program as essential for leukemia maintenance

Although human cancers have complex genotypes and are genomically unstable, they often remain dependent on the continued presence of single-driver mutations-a phenomenon dubbed "oncogene addiction." Such dependencies have been demonstrated in mouse models, where conditional expression systems have revealed that oncogenes able to initiate cancer are often required for tumor maintenance and progression, thus validating the pathways they control as therapeutic targets. Here, we implement an integrative approach that combines genetically defined mouse models, transcriptional profiling, and a novel inducible RNAi platform to characterize cellular programs that underlie addiction to MLL-AF9-a fusion oncoprotein involved in aggressive forms of acute myeloid leukemia (AML). We show that MLL-AF9 contributes to leukemia maintenance by enforcing a Myb-coordinated program of aberrant self-renewal involving genes linked to leukemia stem cell potential and poor prognosis in human AML. Accordingly, partial and transient Myb suppression precisely phenocopies MLL-AF9 withdrawal and eradicates aggressive AML in vivo without preventing normal myelopoiesis, indicating that strategies to inhibit Myb-dependent aberrant self-renewal programs hold promise as effective and cancer-specific therapeutics. Together, our results identify Myb as a critical mediator of oncogene addiction in AML, delineate relevant Myb target genes that are amenable to pharmacologic inhibition, and establish a general approach for dissecting oncogene addiction in vivo.

Loss of AML1/Runx1 accelerates the development of MLL-ENL leukemia through down-regulation of p19ARF

Dysfunction of AML1/Runx1, a transcription factor, plays a crucial role in the development of many types of leukemia. Additional events are often required for AML1 dysfunction to induce full-blown leukemia; however, a mechanistic basis of their cooperation is still elusive. Here, we investigated the effect of AML1 deficiency on the development of MLL-ENL leukemia in mice. Aml1 excised bone marrow cells lead to MLL-ENL leukemia with shorter duration than Aml1 intact cells in vivo. Although the number of MLL-ENL leukemia-initiating cells is not affected by loss of AML1, the proliferation of leukemic cells is enhanced in Aml1-excised MLL-ENL leukemic mice. We found that the enhanced proliferation is the result of repression of p19(ARF) that is directly regulated by AML1 in MLL-ENL leukemic cells. We also found that down-regulation of p19(ARF) induces the accelerated onset of MLL-ENL leukemia, suggesting that p19(ARF) is a major target of AML1 in MLL-ENL leukemia. These results provide a new insight into a role for AML1 in the progression of leukemia.

MLL-SEPTIN gene fusions in hematological malignancies

The mixed lineage leukemia (MLL) locus is involved in more than 60 different rearrangements with a remarkably diverse group of fusion partners in approximately 10% of human leukemias. MLL rearrangements include chromosomal translocations, gene internal duplications, chromosome 11q deletions or inversions and MLL gene insertions into other chromosomes, or vice versa. MLL fusion partners can be classified into four distinct categories: nuclear proteins, cytoplasmatic proteins, histone acetyltransferases and septins. Five different septin genes (SEPT2, SEPT5, SEPT6, SEPT9, and SEPT11) have been identified as MLL fusion partners, giving rise to chimeric fusion proteins in which the N terminus of MLL is fused, in frame, to almost the entire open reading frame of the septin partner gene. The rearranged alleles result from heterogeneous breaks in distinct introns of both MLL and its septin fusion partner, originating distinct gene fusion variants. MLL-SEPTIN rearrangements have been repeatedly identified in de novo and therapy related myeloid neoplasia in both children and adults, and some clinicopathogenetic associations are being uncovered. The fundamental roles of septins in cytokinesis, membrane remodeling and compartmentalization can provide some clues on how abnormalities in the septin cytoskeleton and MLL deregulation could be involved in the pathogenesis of hematological malignancies.

Enforced expression of MLL-AF4 fusion in cord blood CD34+ cells enhances the hematopoietic repopulating cell function and clonogenic potential but is not sufficient to initiate leukemia

Infant acute lymphoblastic leukemia harboring the fusion mixed-lineage leukemia (MLL)-AF4 is associated with a dismal prognosis and very brief latency. Our limited understanding of transformation by MLL-AF4 is reflected in murine models, which do not accurately recapitulate the human disease. Human models for MLL-AF4 disease do not exist. Hematopoietic stem or progenitor cells (HSPCs) represent probable targets for transformation. Here, we explored in vitro and in vivo the impact of the enforced expression of MLL-AF4 in human cord blood-derived CD34(+) HSPCs. Intrabone marrow transplantation into NOD/SCID-IL2Rγ(-/-) mice revealed an enhanced multilineage hematopoietic engraftment, efficiency, and homing to other hematopoietic sites on enforced expression of MLL-AF4. Lentiviral transduction of MLL-AF4 into CD34(+) HSPCs increased the in vitro clonogenic potential of CD34(+) progenitors and promoted their proliferation. Consequently, cell cycle and apoptosis analyses suggest that MLL-AF4 conveys a selective proliferation coupled to a survival advantage, which correlates with changes in the expression of genes involved in apoptosis, sensing DNA damage and DNA repair. However, MLL-AF4 expression was insufficient to initiate leukemogenesis on its own, indicating that either additional hits (or reciprocal AF4-MLL product) may be required to initiate ALL or that cord blood-derived CD34(+) HSPCs are not the appropriate cellular target for MLL-AF4-mediated ALL.

Two types of C/EBPα mutations play distinct but collaborative roles in leukemogenesis: lessons from clinical data and BMT models

Two types of mutations of a transcription factor CCAAT-enhancer binding protein α (C/EBPα) are found in leukemic cells of 5%-14% of acute myeloid leukemia (AML) patients: N-terminal mutations expressing dominant negative p30 and C-terminal mutations in the basic leucine zipper domain. Our results showed that a mutation of C/EBPα in one allele was observed in AML after myelodysplastic syndrome, while the 2 alleles are mutated in de novo AML. Unlike an N-terminal frame-shift mutant (C/EBPα-Nm)–transduced cells, a C-terminal mutant (C/EBPα-Cm)–transduced cells alone induced AML with leukopenia in mice 4-12 months after bone marrow transplantation. Coexpression of both mutants induced AML with marked leukocytosis with shorter latencies. Interestingly, C/EBPα-Cm collaborated with an Flt3-activating mutant Flt3-ITD in inducing AML. Moreover, C/EBPα-Cm strongly blocked myeloid differentiation of 32Dcl3 cells, suggesting its class II mutation-like role in leukemogenesis. Although C/EBPα-Cm failed to inhibit transcriptional activity of wild-type C/EBPα, it suppressed the synergistic effect between C/EBPα and PU.1. On the other hand, C/EBPα-Nm inhibited C/EBPα activation in the absence of PU.1, despite low expression levels of p30 protein generated by C/EBPα-Nm. Thus, 2 types of C/EBPα mutations are implicated in leukemo-genesis, involving different and cooperating molecular mechanisms.

Insights into the cellular origin and etiology of the infant pro-B acute lymphoblastic leukemia with MLL-AF4 rearrangement

Infant acute lymphoblastic leukemia (ALL) involving mixed-lineage leukemia (MLL) fusions has attracted a huge interest in basic and clinical research because of its prenatal origin, mixed-lineage phenotype, dismal prognosis and extremely short latency. Over 90% of infant ALLs are pro-B ALL harboring the leukemic fusion MLL-AF4. Despite the fact that major achievements have provided a better understanding about the etiology of infant MLL-AF4+ ALL over the last two decades, key questions remain unanswered. Epidemiological and genetic studies suggest that the in utero origin of MLL rearrangements in infant leukemia may be the result of prenatal exposure to genotoxic compounds. In fact, chronic exposure of human embryonic stem cells (hESCs) to etoposide induces MLL rearrangements and makes hESC more prone to acquire subsequent chromosomal abnormalities than postnatal CD34(+) cells, linking embryonic exposure to topoisomerase II inhibitors to genomic instability and MLL rearrangements. Unfortunately, very little is known about the nature of the target cell for transformation. Neuron-glial antigen 2 expression was initially claimed to be specifically associated with MLL rearrangements and was recently shown to be readily expressed in CD34+CD38+, but not CD34+CD38- cells suggesting that progenitors rather than stem cells may be the target cell for transformation. Importantly, the recent findings showing that MLL-AF4 rearrangement is present and expressed in mesenchymal stem cells from infant patients with MLLAF4+ ALL challenged our current view of the etiology and cellular origin of this leukemia. It becomes therefore crucial to determine where the leukemia relapses come from and how the tumor-stroma relationship is defined at the molecular level. Finally, MLL-AF4 leukemogenesis has been particularly difficult to model and bona fide MLL-AF4 disease models do not exist so far. It is likely that the current disease models are missing some essential ingredients of leukemogenesis in the human embryo/fetus. We thus propose modeling MLL-AF4+ infant pro-B ALL using prenatal hESCs.

A translocation in acute lymphoblastic leukemia that cytogenetically mimics the recurrent MLL-AFF1 translocation and fuses SEPT11 to MLL

A 55-year-old man sought care for aggressive acute lymphoblastic leukemia (ALL), which developed 8 years after he had received chemotherapeutic treatment for nephrotic syndrome. The sole cytogenetic abnormality observed in bone marrow-derived metaphases was a t(4;11)(q21;q23), which is a frequently occurring translocation in ALL. However, subsequent reverse transcriptase-polymerase chain reaction for the expected mixed lineage leukemia [trithorax homolog, Drosophila] (MLL)-AFF1 fusion transcript was negative. Further fluorescence in situ hybridization (FISH) analysis narrowed the 4q21 breakpoint down to a 250-kb region proximal of AFF1. This comprised four genes, of which septin11 (SEPT11) was further analyzed. Reverse transcriptase-polymerase chain reaction revealed expression of a chimeric MLL-SEPT11 transcript, thus identifying what is to our knowledge a hitherto undescribed translocation in ALL. Sequence analysis of cDNA showed in-frame fusion of MLL exon 11 to SEPT11 exon 2. This MLL-SEPT11 fusion is cytogenetically indistinguishable from the recurrent t(4;11)(q21;q23). Thus, it is crucial to characterize cytogenetic aberrations in leukemia by molecular methods, even in cases where a known recurrent translocation is presumed. This report expands the spectrum of ALL-related translocations and hypothesizes on the mechanism leading to the MLL-SEPT11 fusion. Five septins have been identified thus far as MLL fusion partners in leukemia. Their putative oncogenic role may be related to forced MLL dimerization by the septin coiled coil and GTP-binding domains, which could convert MLL to an oncogene.

Ubiquitin conjugase UBCH8 targets active FMS-like tyrosine kinase 3 for proteasomal degradation

The class III receptor tyrosine kinase FMS-like tyrosine kinase 3 (FLT3) regulates normal hematopoiesis and immunological functions. Nonetheless, constitutively active mutant FLT3 (FLT3-ITD) causally contributes to transformation and is associated with poor prognosis of acute myeloid leukemia (AML) patients. Histone deacetylase inhibitors (HDACi) can counteract deregulated gene expression profiles and decrease oncoprotein stability, which renders them candidate drugs for AML treatment. However, these drugs have pleiotropic effects and it is often unclear how they correct oncogenic transcriptomes and proteomes. We report here that treatment of AML cells with the HDACi LBH589 induces the ubiquitin-conjugating enzyme UBCH8 and degradation of FLT3-ITD. Gain- and loss-of-function approaches show that UBCH8 and the ubiquitin-ligase SIAH1 physically interact with and target FLT3-ITD for proteasomal degradation. These ubiquitinylating enzymes though have a significantly lesser effect on wild-type FLT3. Furthermore, physiological and pharmacological stimulation of FLT3 phosphorylation, inhibition of FLT3-ITD autophosphorylation and analysis of kinase-inactive FLT3-ITD revealed that tyrosine phosphorylation determines degradation of FLT3 and FLT3-ITD by the proteasome. These results provide novel insights into antileukemic activities of HDACi and position UBCH8, which have been implicated primarily in processes in the nucleus, as a previously unrecognized important modulator of FLT3-ITD stability and leukemic cell survival.

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

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

Molecular pathology of mixed-lineage leukemia

Genomic rearrangements of the locus 11q23 are frequently observed in aggressive acute leukemias with poor prognosis. These chromosomal aberrations fuse the mixed-lineage leukemia (MLL) gene to one of more than 50 partners. The resulting mixed-lineage leukemia fusions often code for chimeric transcriptional activators, which are able to transform normal hematopoietic cells through the deregulation of leukemogenic target genes. This review provides a concise overview about the known functions encoded in MLL and the respective fusion partners. Additionally, the roles of some target genes, as well as co-factors of mixed-lineage leukemia fusion proteins, are described with an emphasis on recent advances potentially uncovering novel therapeutic targets.

DNA copy-number abnormalities do not occur in infant ALL with t(4;11)/MLL-AF4

The pathogenesis of infant acute lymphoblastic leukemia (ALL) is still not well defined. Short latency to leukemia and very high concordance rate for ALL in Mixed-Lineage Leukemia (MLL)-positive infant twins suggest that the MLL rearrangement itself could be sufficient for overt leukemia. Attempts to generate a suitable mouse model for MLL-AF4-positive ALL did not thoroughly resolve the issue of whether cooperating mutations are required to reduce latency and to generate overt leukemia in vivo. In this study, we applied single-nucleotide polymorphism array technology to perform genomic profiling of 28 infant ALL cases carrying t(4;11) to detect MLL-cooperating aberrations hidden to conventional techniques and to gain new insights into infant ALL pathogenesis. In contrast to pediatric, adolescent and adult ALL cases, the MLL rearrangement in infant ALL is associated with an exceptionally low frequency of copy-number abnormalities, thus confirming the unique nature of this disease. By contrast, additional genetic aberrations are acquired at disease relapse. Small-segmental uniparental disomy traits were frequently detected, mostly constitutional, and widely distributed throughout the genome. It can be argued that the MLL rearrangement as a first hit, rather than inducing the acquisition of additional genetic lesions, has a major role to drive and hasten the onset of leukemia.