A conditional model of MLL-AF4 B-cell tumourigenesis using invertor technology

Functional relevance of circRNA aberrant expression in pediatric acute leukemia with KMT2A::AFF1 fusion

ABSTRACT

Circular RNAs (circRNAs) are emerging molecular players in leukemogenesis and promising therapeutic targets. In KMT2A::AFF1 (MLL::AF4)-rearranged leukemia, an aggressive disease compared with other pediatric B-cell precursor (BCP) acute lymphoblastic leukemia (ALL), data about circRNAs are limited. Here, we disclose the circRNA landscape of infant patients with KMT2A::AFF1 translocated BCP-ALL showing dysregulated, mostly ectopically expressed, circRNAs in leukemia cells. Most of these circRNAs, apart from circHIPK3 and circZNF609, previously associated with oncogenic behavior in ALL, are still uncharacterized. An in vitro loss-of-function screening identified an oncogenic role of circFKBP5, circKLHL2, circNR3C1, and circPAN3 in KMT2A::AFF1 ALL, whose silencing affected cell proliferation and apoptosis. Further study in an extended cohort disclosed a significantly correlated expression of these oncogenic circRNAs and their putative involvement in common regulatory networks. Moreover, it showed that circAFF1 upregulation occurs in a subset of cases with HOXA KMT2A::AFF1 ALL. Collectively, functional analyses and patient data reveal oncogenic circRNA upregulation as a relevant mechanism that sustains the malignant cell phenotype in KMT2A::AFF1 ALL.

RNA-binding proteins of KHDRBS and IGF2BP families control the oncogenic activity of MLL-AF4

Chromosomal translocation generates the MLL-AF4 fusion gene, which causes acute leukemia of multiple lineages. MLL-AF4 is a strong oncogenic driver that induces leukemia without additional mutations and is the most common cause of pediatric leukemia. However, establishment of a murine disease model via retroviral transduction has been difficult owning to a lack of understanding of its regulatory mechanisms. Here, we show that MLL-AF4 protein is post-transcriptionally regulated by RNA-binding proteins, including those of KHDRBS and IGF2BP families. MLL-AF4 translation is inhibited by ribosomal stalling, which occurs at regulatory sites containing AU-rich sequences recognized by KHDRBSs. Synonymous mutations disrupting the association of KHDRBSs result in proper translation of MLL-AF4 and leukemic transformation. Consequently, the synonymous MLL-AF4 mutant induces leukemia in vivo. Our results reveal that post-transcriptional regulation critically controls the oncogenic activity of MLL-AF4; these findings might be valuable in developing novel therapies via modulation of the activity of RNA-binding proteins.

Animal models of acute lymphoblastic leukemia: Recapitulating the human disease to evaluate drug efficacy and discover therapeutic targets

Acute lymphoblastic leukemia (ALL) is a malignant hematologic tumor with highly aggressive characteristics, which is prone to relapse, has a poor prognosis and few clinically effective drugs. It is meaningful to gain a better understanding of its pathogenesis in order to discover and evaluate potential therapeutic drugs and new treatment targets. The goal of developing novel targeted drugs and treatment methods is to increase complete remission, reduce toxicity and morbidity, and that is also the most important prerequisite for modern leukemia treatment. However, the process of new drugs from research and development to clinical application is long and difficult. Many promising drugs were rejected by the USFoodandDrugAdministration(FDA) due to serious adverse drug reactions (ADR) in clinical phase I trials. Animal models provide us with an excellent tool to understand the complex pathological mechanisms of human diseases, to evaluate the potential of new targeted drugs and therapeutic approaches to treat ALL in vivo and, more importantly, to assess the potential ADR they may have on healthy organs. In this article we review ALL animal models' progression, their roles in revealing the pathogenesis of ALL and drug development. Additionally, we mainly focus on the mouse models, especially xenotransplantation and transgenic models that more closely reproduce the human phenotype. In conclusion, we summarize the advantages and limitations of each model, thereby facilitating further understanding the etiology of ALL, and eventually contributing to the effective management of the disease.

The RNA-binding protein IGF2BP3 is critical for MLL-AF4-mediated leukemogenesis

Despite recent advances in therapeutic approaches, patients with MLL-rearranged leukemia still have poor outcomes. Here, we find that the RNA-binding protein IGF2BP3, which is overexpressed in MLL-translocated leukemia, strongly amplifies MLL-Af4-mediated leukemogenesis. Deletion of Igf2bp3 significantly increases the survival of mice with MLL-Af4-driven leukemia and greatly attenuates disease, with a minimal impact on baseline hematopoiesis. At the cellular level, MLL-Af4 leukemia-initiating cells require Igf2bp3 for their function in leukemogenesis. At the molecular level, IGF2BP3 regulates a complex posttranscriptional operon governing leukemia cell survival and proliferation. IGF2BP3-targeted mRNA transcripts include important MLL-Af4-induced genes, such as those in the Hoxa locus, and the Ras signaling pathway. Targeting of transcripts by IGF2BP3 regulates both steady-state mRNA levels and, unexpectedly, pre-mRNA splicing. Together, our findings show that IGF2BP3 represents an attractive therapeutic target in this disease, providing important insights into mechanisms of posttranscriptional regulation in leukemia.

miR-130b and miR-128a are essential lineage-specific codrivers of t(4;11) MLL-AF4 acute leukemia

t(4;11) MLL-AF4 acute leukemia is one of the most aggressive malignancies in the infant and pediatric population, yet we have little information on the molecular mechanisms responsible for disease progression. This impairs the development of therapeutic regimens that can address the aggressive phenotype and lineage plasticity of MLL-AF4-driven leukemogenesis. This study highlights novel mechanisms of disease development by focusing on 2 microRNAs (miRNAs) upregulated in leukemic blasts from primary patient samples: miR-130b and miR-128a. We show that miR-130b and miR-128a are downstream targets of MLL-AF4 and can individually drive the transition from a pre-leukemic stage to an acute leukemia in an entirely murine Mll-AF4 in vivo model. They are also required to maintain the disease phenotype. Interestingly, miR-130b overexpression led to a mixed/B-cell precursor (BCP)/myeloid leukemia, propagated by the lymphoid-primed multipotent progenitor (LMPP) population, whereas miR-128a overexpression resulted in a pro-B acute lymphoblastic leukemia (ALL), maintained by a highly expanded Il7r+c-Kit+ blast population. Molecular and phenotypic changes induced by these two miRNAs fully recapitulate the human disease, including central nervous system infiltration and activation of an MLL-AF4 expression signature. Furthermore, we identified 2 downstream targets of these miRNAs, NR2F6 and SGMS1, which in extensive validation studies are confirmed as novel tumor suppressors of MLL-AF4+ leukemia. Our integrative approach thus provides a platform for the identification of essential co-drivers of MLL-rearranged leukemias, in which the preleukemia to leukemia transition and lineage plasticity can be dissected and new therapeutic approaches can be tested.

Defining the fetal origin of MLL-AF4 infant leukemia highlights specific fatty acid requirements

Infant MLL-AF4-driven acute lymphoblastic leukemia (ALL) is a devastating disease with dismal prognosis. A lack of understanding of the unique biology of this disease, particularly its prenatal origin, has hindered improvement of survival. We perform multiple RNA sequencing experiments on fetal, neonatal, and adult hematopoietic stem and progenitor cells from human and mouse. This allows definition of a conserved fetal transcriptional signature characterized by a prominent proliferative and oncogenic nature that persists in infant ALL blasts. From this signature, we identify a number of genes in functional validation studies that are critical for survival of MLL-AF4+ ALL cells. Of particular interest are PLK1 because of the readily available inhibitor and ELOVL1, which highlights altered fatty acid metabolism as a feature of infant ALL. We identify which aspects of the disease are residues of its fetal origin and potential disease vulnerabilities.

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.

The reciprocal world of MLL fusions: A personal view

Over the last 15 years the Diagnostic Center of Acute Leukemia (DCAL) at the Frankfurt University has diagnosed and elucidated the Mixed Lineage Leukemia (MLL) recombinome with >100 MLL fusion partners. When analyzing all these different events, balanced chromosomal translocations were found to comprise the majority of these cases (~70%), while other types of genetic rearrangements (3-way-translocations, spliced fusions, 11q inversions, interstitial deletions or insertion of chromosomal fragments into other chromosomes) account for about 30%. In nearly all those complex cases, functional fusion proteins can be produced by transcription, splicing and translation. With a few exceptions (10 out of 102 fusion genes which were per se out-of-frame), all these genetic rearrangements produced a direct MLL fusion gene, and in 94% of cases an additional reciprocal fusion gene. So far, 114 patients (out of 2454 = ~5%) have been diagnosed only with the reciprocal fusion allele, displaying no MLL-X allele. The fact that so many MLL rearrangements bear at least two fusion alleles, but also our findings that several direct MLL fusions were either out-of-frame fusions or missing, raises the question about the function and importance of reciprocal MLL fusions. Recent findings also demonstrate the presence of reciprocal MLL fusions in sarcoma patients. Here, we want to discuss the role of reciprocal MLL fusion proteins for leukemogenesis and beyond.

KMT2A rearranged acute lymphoblastic leukaemia: Unravelling the genomic complexity and heterogeneity of this high-risk disease

KMT2A rearranged (KMT2Ar) acute lymphoblastic leukaemia (ALL) is a high-risk genomic subtype, with long-term survival rates of less than 60% across all age groups. These cases present a complex clinical challenge, with a high incidence in infants, high-risk clinical features and propensity for aggressive relapse. KMT2A rearrangements are highly pathogenic leukaemic drivers, reflected by the high incidence of KMT2Ar ALL in infants, who carry few leukaemia-associated cooperative mutations. However, transgenic murine models of KMT2Ar ALL typically exhibit long latency and mature or mixed phenotype, and fail to recapitulate the aggressive disease observed clinically. Next-generation sequencing has revealed that KMT2Ar ALL also occurs in adolescents and adults, and potentially cooperative genomic lesions such as PI3K-RAS pathway variants are present in KMT2Ar patients of all ages. This review addresses the aetiology of KMT2Ar ALL, with a focus on the cell of origin and mutational landscape, and how genomic profiling of KMT2Ar ALL patients in the era of next-generation sequencing demonstrates that KMT2Ar ALL is a complex heterogenous disease. Ultimately, understanding the underlying biology of KMT2Ar ALL will be important in improving long-term outcomes for these high-risk patients.

Inhibition of MEK and ATR is effective in a B-cell acute lymphoblastic leukemia model driven by Mll-Af4 and activated Ras

Infant B-cell acute lymphoblastic leukemias (B-ALLs) that harbor MLL-AF4 rearrangements are associated with a poor prognosis. One important obstacle to progress for this patient population is the lack of immunocompetent models that faithfully recapitulate the short latency and aggressiveness of this disease. Recent whole-genome sequencing of MLL-AF4 B-ALL samples revealed a high frequency of activating RAS mutations; however, single-agent targeting of downstream effectors of the RAS pathway in these mutated MLL-r B-ALLs has demonstrated limited and nondurable antileukemic effects. Here, we demonstrate that the expression of activating mutant N-Ras ^G12D cooperates with Mll-Af4 to generate a highly aggressive serially transplantable B-ALL in mice. We used our novel mouse model to test the sensitivity of Mll-Af4/N-Ras ^G12D leukemia to small molecule inhibitors and found potent and synergistic preclinical efficacy of dual targeting of the Mek and Atr pathways in mouse- and patient-derived xenografts with both mutations in vivo, suggesting this combination as an attractive therapeutic opportunity that might be used to treat patients with these mutations. Our studies indicate that this mouse model of Mll-Af4/N-Ras B-ALL is a powerful tool to explore the molecular and genetic pathogenesis of this disease subtype, as well as a preclinical discovery platform for novel therapeutic strategies.

A Critical Review of Animal Models Used in Acute Myeloid Leukemia Pathophysiology

Acute myeloid leukemia (AML) is one of the most frequent, complex, and heterogeneous hematological malignancies. AML prognosis largely depends on acquired cytogenetic, epigenetic, and molecular abnormalities. Despite the improvement in understanding the biology of AML, survival rates remain quite low. Animal models offer a valuable tool to recapitulate different AML subtypes, and to assess the potential role of novel and known mutations in disease progression. This review provides a comprehensive and critical overview of select available AML animal models. These include the non-mammalian Zebrafish and Drosophila models as well as the mammalian rodent systems, comprising rats and mice. The suitability of each animal model, its contribution to the advancement of knowledge in AML pathophysiology and treatment, as well as its advantages and limitations are discussed. Despite some limitations, animal models represent a powerful approach to assess toxicity, and permit the design of new therapeutic strategies.

The RS4;11 cell line as a model for leukaemia with t(4;11)(q21;q23): Revised characterisation of cytogenetic features

BACKGROUND

Haematological malignancies harbouring rearrangements of the KMT2A gene represent a unique subtype of leukaemia, with biphenotypic clinical manifestations, a rapid and aggressive onset, and a generally poor prognosis. Chromosomal translocations involving KMT2A often cause the formation of oncogenic fusion genes, such as the most common translocation t(4;11)(q21;q23) producing the KMT2A-AFF1 chimera.

AIM

The aim of this study was to confirm and review the cytogenetic and molecular features of the KMT2A-rearranged RS4;11 cell line and put those in context with other reports of cell lines also harbouring a t(4;11) rearrangement.

METHODS AND RESULTS

The main chromosomal rearrangements t(4;11)(q21;q23) and i(7q), described when the cell line was first established, were confirmed by fluorescence in situ hybridisation (FISH) and 24-colour karyotyping by M-FISH. Additional cytogenetic abnormalities were investigated by further FISH experiments, including the presence of trisomy 18 as a clonal abnormality and the discovery of one chromosome 8 being an i(8q), which indicates a duplication of the oncogene MYC. A homozygous deletion of 9p21 containing the tumour-suppressor genes CDKN2A and CDKN2B was also revealed by FISH. The production of the fusion transcript KMT2A-AFF1 arising from the der(11)t(4;11) was confirmed by RT-PCR, but sequencing of the amplified fragment revealed the presence of multiple isoforms. Two transcript variants, resulting from alternative splicing, were identified differing in one glutamine residue in the translated protein.

CONCLUSION

As karyotype evolution is a common issue in cell lines, we highlight the need to monitor cell lines in order to re-confirm their characteristics over time. We also reviewed the literature to provide a comparison of key features of several cell lines harbouring a t(4;11). This would guide scientists in selecting the most suitable research model for this particular type of KMT2A-leukaemia.

Fetal liver Mll-AF4+ hematopoietic stem and progenitor cells respond directly to poly(I:C), but not to a single maternal immune activation

T(4;11) MLL-AF4 acute leukemia is one of the most aggressive malignancies in infant and pediatric populations. Epidemiological and functional studies have highlighted the influence of an overstimulation of the immune system on leukemia development. This study aimed at assessing if the cell-of-origin of t(4;11) MLL-AF4 acute leukemia is sensitive to a viral or bacterial mimic and if maternal immune activation can lead to a full-blown leukemia. To answer this, we used the Mll-AF4 pre-leukemia mouse model that initiates the expression of Mll-AF4 in the first definitive hematopoietic cells formed during embryonic development. We observed an increase in proliferation upon hematopoietic differentiation of fetal liver Mll-AF4+ Lineage-Sca1+ckit+ (LSK) cells exposed to the immune stimulants, poly(I:C) or LPS/lipopolysaccharide. This was accompanied by increased expression of a subset of MLL-AF4 signature genes and members of the Toll-like receptor signaling pathways in fetal liver Mll-AF4+ LSK exposed to poly(I:C), suggesting that the cell-of-origin responds to inflammatory stimuli. Maternal immune activation using a single dose of poly(I:C) did not lead to the development of leukemia in Mll-AF4+ and control offspring. Instead, aging MLL-AF4+ mice showed an increased proportion of T-lymphoid cells in the spleen, lost their B-lymphoid bias, and had decreased frequencies of hematopoietic stem and multipotent progenitor cells. Overall, this study suggests that the fetal liver Mll-AF4+ LSK cells are sensitive to direct exposure to inflammatory stimuli, especially poly(I:C); however, maternal immune activation induced by a single exposure to poly(I:C) is not sufficient to initiate MLL-AF4 leukemogenesis.

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.

Enhanced hemato-endothelial specification during human embryonic differentiation through developmental cooperation between AF4-MLL and MLL-AF4 fusions

The t(4;11)(q21;q23) translocation is associated with high-risk infant pro-B-cell acute lymphoblastic leukemia and arises prenatally during embryonic/fetal hematopoiesis. The developmental/pathogenic contribution of the t(4;11)-resulting MLL-AF4 (MA4) and AF4-MLL (A4M) fusions remains unclear; MA4 is always expressed in patients with t(4;11)+ B-cell acute lymphoblastic leukemia, but the reciprocal fusion A4M is expressed in only half of the patients. Because prenatal leukemogenesis manifests as impaired early hematopoietic differentiation, we took advantage of well-established human embryonic stem cell-based hematopoietic differentiation models to study whether the A4M fusion cooperates with MA4 during early human hematopoietic development. Co-expression of A4M and MA4 strongly promoted the emergence of hemato-endothelial precursors, both endothelial- and hemogenic-primed. Double fusion-expressing hemato-endothelial precursors specified into significantly higher numbers of both hematopoietic and endothelial-committed cells, irrespective of the differentiation protocol used and without hijacking survival/proliferation. Functional analysis of differentially expressed genes and differentially enriched H3K79me3 genomic regions by RNA-sequencing and H3K79me3 chromatin immunoprecipitation-sequencing, respectively, confirmed a hematopoietic/endothelial cell differentiation signature in double fusion-expressing hemato-endothelial precursors. Importantly, chromatin immunoprecipitation-sequencing analysis revealed a significant enrichment of H3K79 methylated regions specifically associated with HOX-A cluster genes in double fusion-expressing differentiating hematopoietic cells. Overall, these results establish a functional and molecular cooperation between MA4 and A4M fusions during human hematopoietic development.

Pediatric leukemia: Moving toward more accurate models

Leukemia is a complex genetic disease caused by errors in differentiation, growth, and apoptosis of hematopoietic cells in either lymphoid or myeloid lineages. Large-scale genomic characterization of thousands of leukemia patients has produced a tremendous amount of data that have enabled a better understanding of the differences between adult and pediatric patients. For instance, although phenotypically similar, pediatric and adult myeloid leukemia patients differ in their mutational profiles, typically involving either chromosomal translocations or recurrent single-base-pair mutations, respectively. To elucidate the molecular mechanisms underlying the biology of this cancer, continual efforts have been made to develop more contextually and biologically relevant experimental models. Leukemic cell lines, for example, provide an inexpensive and tractable model but often fail to recapitulate critical aspects of tumor biology. Likewise, murine leukemia models of leukemia have been highly informative but also do not entirely reproduce the human disease. More recent advances in the development of patient-derived xenografts (PDXs) or human models of leukemias are poised to provide a more comprehensive, and biologically relevant, approach to directly assess the impact of the in vivo environment on human samples. In this review, the advantages and limitations of the various current models used to functionally define the genetic requirements of leukemogenesis are discussed.

Rewiring the Epigenetic Networks in MLL-Rearranged Leukemias: Epigenetic Dysregulation and Pharmacological Interventions

Leukemias driven by chromosomal translocation of the mixed-lineage leukemia gene (MLL or KMT2A) are highly prevalent in pediatric oncology. The poor survival rate and lack of an effective targeted therapy for patients with MLL-rearranged (MLL-r) leukemias emphasize an urgent need for improved knowledge and novel therapeutic approaches for these malignancies. The resulting chimeric products of MLL gene rearrangements, i.e., MLL-fusion proteins (MLL-FPs), are capable of transforming hematopoietic stem/progenitor cells (HSPCs) into leukemic blasts. The ability of MLL-FPs to reprogram HSPCs toward leukemia requires the involvement of multiple chromatin effectors, including the histone 3 lysine 79 methyltransferase DOT1L, the chromatin epigenetic reader BRD4, and the super elongation complex. These epigenetic regulators constitute a complicated network that dictates maintenance of the leukemia program, and therefore represent an important cluster of therapeutic opportunities. In this review, we will discuss the role of MLL and its fusion partners in normal HSPCs and hematopoiesis, including the links between chromatin effectors, epigenetic landscapes, and leukemia development, and summarize current approaches to therapeutic targeting of MLL-r leukemias.

Molecular processes involved in B cell acute lymphoblastic leukaemia

B cell leukaemia is one of the most frequent malignancies in the paediatric population, but also affects a significant proportion of adults in developed countries. The majority of infant and paediatric cases initiate the process of leukaemogenesis during foetal development (in utero) through the formation of a chromosomal translocation or the acquisition/deletion of genetic material (hyperdiploidy or hypodiploidy, respectively). This first genetic insult is the major determinant for the prognosis and therapeutic outcome of patients. B cell leukaemia in adults displays similar molecular features as its paediatric counterpart. However, since this disease is highly represented in the infant and paediatric population, this review will focus on this demographic group and summarise the biological, clinical and epidemiological knowledge on B cell acute lymphoblastic leukaemia of four well characterised subtypes: t(4;11) MLL-AF4, t(12;21) ETV6-RUNX1, t(1;19) E2A-PBX1 and t(9;22) BCR-ABL1.

The AF4-MLL fusion transiently augments multilineage hematopoietic engraftment but is not sufficient to initiate leukemia in cord blood CD34+ cells

The translocation t(4;11)(q21;q23) is the hallmark genetic abnormality associated with infant pro-B acute lymphoblastic leukemia (B-ALL) and has the highest frequency of rearrangement in Mixed-lineage leukemia (MLL) leukemias. Unlike other MLL translocations, MLL-AF4-induced proB-ALL is exceptionally difficult to model in mice/humans. Previous work has investigated the relevance of the reciprocal translocation fusion protein AF4-MLL for t(4;11) leukemia, finding that AF4-MLL is capable of inducing proB-ALL without requirement for MLL-AF4 when expressed in murine hematopoietic stem/progenitor cells (HSPCs). Therefore, AF4-MLL might represent a key genetic lesion contributing to t(4;11)-driven leukemogenesis. Here, we aimed to establish a humanized mouse model by using AF4-MLL to analyze its transformation potential in human cord blood-derived CD34⁺ HSPCs. We show that AF4-MLL-expressing human CD34⁺ HSPCs provide enhanced long-term hematopoietic reconstitution in primary immunodeficient recipients but are not endowed with subsequent self-renewal ability upon serial transplantation. Importantly, expression of AF4-MLL in primary neonatal CD34⁺ HSPCs failed to render any phenotypic or hematological sign of disease, and was therefore not sufficient to initiate leukemia within a 36-week follow-up. Species-specific (epi)-genetic intrinsic determinants may underlie the different outcome observed when AF4-MLL is expressed in murine or human HSPCs.

Dysregulation of haematopoietic stem cell regulatory programs in acute myeloid leukaemia

Haematopoietic stem cells (HSC) are situated at the apex of the haematopoietic differentiation hierarchy, ensuring the life-long supply of mature haematopoietic cells and forming a reservoir to replenish the haematopoietic system in case of emergency such as acute blood loss. To maintain a balanced production of all mature lineages and at the same time secure a stem cell reservoir, intricate regulatory programs have evolved to control multi-lineage differentiation and self-renewal in haematopoietic stem and progenitor cells (HSPCs). Leukaemogenic mutations commonly disrupt these regulatory programs causing a block in differentiation with simultaneous enhancement of proliferation. Here, we briefly summarize key aspects of HSPC regulatory programs, and then focus on their disruption by leukaemogenic fusion genes containing the mixed lineage leukaemia (MLL) gene. Using MLL as an example, we explore important questions of wider significance that are still under debate, including the importance of cell of origin, to what extent leukaemia oncogenes impose specific regulatory programs and the relevance of leukaemia stem cells for disease development and prognosis. Finally, we suggest that disruption of stem cell regulatory programs is likely to play an important role in many other pathologies including ageing-associated regenerative failure.

Chemical exposure and infant leukaemia: development of an adverse outcome pathway (AOP) for aetiology and risk assessment research

Infant leukaemia (<1 year old) is a rare disease of an in utero origin at an early phase of foetal development. Rearrangements of the mixed-lineage leukaemia (MLL) gene producing abnormal fusion proteins are the most frequent genetic/molecular findings in infant B cell-acute lymphoblastic leukaemia. In small epidemiological studies, mother/foetus exposures to some chemicals including pesticides have been associated with infant leukaemia; however, the strength of evidence and power of these studies are weak at best. Experimental in vitro or in vivo models do not sufficiently recapitulate the human disease and regulatory toxicology studies are unlikely to capture this kind of hazard. Here, we develop an adverse outcome pathway (AOP) based substantially on an analogous disease-secondary acute leukaemia caused by the topoisomerase II (topo II) poison etoposide-and on cellular and animal models. The hallmark of the AOP is the formation of MLL gene rearrangements via topo II poisoning, leading to fusion genes and ultimately acute leukaemia by global (epi)genetic dysregulation. The AOP condenses molecular, pathological, regulatory and clinical knowledge in a pragmatic, transparent and weight of evidence-based framework. This facilitates the interpretation and integration of epidemiological studies in the process of risk assessment by defining the biologically plausible causative mechanism(s). The AOP identified important gaps in the knowledge relevant to aetiology and risk assessment, including the specific embryonic target cell during the short and spatially restricted period of susceptibility, and the role of (epi)genetic features modifying the initiation and progression of the disease. Furthermore, the suggested AOP informs on a potential Integrated Approach to Testing and Assessment to address the risk caused by environmental chemicals in the future.

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.

One Step Forward in the Challenging Arena of MLL-AF4 Leukemia

MLL-AF4 leukemia is the predominant infant acute leukemia and has a poor prognosis. No current experimental models accurately reflect the human disease. Lin et al., in this issue of Cancer Cell, describe their model that recapitulates multiple key aspects of this aggressive disease, facilitating future mechanistic and preclinical studies.

Instructive Role of MLL-Fusion Proteins Revealed by a Model of t(4;11) Pro-B Acute Lymphoblastic Leukemia

The t(4;11)(q21;q23) fuses mixed-lineage leukemia (MLL) to AF4, the most common MLL-fusion partner. Here we show that MLL fused to murine Af4, highly conserved with human AF4, produces high-titer retrovirus permitting efficient transduction of human CD34⁺ cells, thereby generating a model of t(4;11) pro-B acute lymphoblastic leukemia (ALL) that fully recapitulates the immunophenotypic and molecular aspects of the disease. MLL-Af4 induces a B ALL distinct from MLL-AF9 through differential genomic target binding of the fusion proteins leading to specific gene expression patterns. MLL-Af4 cells can assume a myeloid state under environmental pressure but retain lymphoid-lineage potential. Such incongruity was also observed in t(4;11) patients in whom leukemia evaded CD19-directed therapy by undergoing myeloid-lineage switch. Our model provides a valuable tool to unravel the pathogenesis of MLL-AF4 leukemogenesis.

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.

Mll-AF4 Confers Enhanced Self-Renewal and Lymphoid Potential during a Restricted Window in Development

MLL-AF4+ infant B cell acute lymphoblastic leukemia is characterized by an early onset and dismal survival. It initiates before birth, and very little is known about the early stages of the disease’s development. Using a conditional Mll-AF4-expressing mouse model in which fusion expression is targeted to the earliest definitive hematopoietic cells generated in the mouse embryo, we demonstrate that Mll-AF4 imparts enhanced B lymphoid potential and increases repopulation and self-renewal capacity during a putative pre-leukemic state. This occurs between embryonic days 12 and 14 and manifests itself most strongly in the lymphoid-primed multipotent progenitor population, thus pointing to a window of opportunity and a potential cell of origin. However, this state alone is insufficient to generate disease, with the mice succumbing to B cell lymphomas only after a long latency. Future analysis of the molecular details of this pre-leukemic state will shed light on additional events required for progression to acute leukemia.

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Mll-AF4 confers enhanced B cell potential and causes an expansion of pro-B cells

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Mll-AF4 increases self-renewal potential

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Mll-AF4 exerts its effects in a restricted developmental window

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The LMPP is a potential cell of origin for Mll-AF4-associated disease

Activated KRAS Cooperates with MLL-AF4 to Promote Extramedullary Engraftment and Migration of Cord Blood CD34+ HSPC But Is Insufficient to Initiate Leukemia

The MLL-AF4 (MA4) fusion gene is the genetic hallmark of an aggressive infant pro-B-acute lymphoblastic leukemia (B-ALL). Our understanding of MA4-mediated transformation is very limited. Whole-genome sequencing studies revealed a silent mutational landscape, which contradicts the aggressive clinical outcome of this hematologic malignancy. Only RAS mutations were recurrently detected in patients and found to be associated with poorer outcome. The absence of MA4-driven B-ALL models further questions whether MA4 acts as a single oncogenic driver or requires cooperating mutations to manifest a malignant phenotype. We explored whether KRAS activation cooperates with MA4 to initiate leukemia in cord blood-derived CD34(+) hematopoietic stem/progenitor cells (HSPC). Clonogenic and differentiation/proliferation assays demonstrated that KRAS activation does not cooperate with MA4 to immortalize CD34(+) HSPCs. Intrabone marrow transplantation into immunodeficient mice further showed that MA4 and KRAS(G12V) alone or in combination enhanced hematopoietic repopulation without impairing myeloid-lymphoid differentiation, and that mutated KRAS did not cooperate with MA4 to initiate leukemia. However, KRAS activation enhanced extramedullary hematopoiesis of MA4-expressing cell lines and CD34(+) HSPCs that was associated with leukocytosis and central nervous system infiltration, both hallmarks of infant t(4;11)(+) B-ALL. Transcriptional profiling of MA4-expressing patients supported a cell migration gene signature underlying the mutant KRAS-mediated phenotype. Collectively, our findings demonstrate that KRAS affects the homeostasis of MA4-expressing HSPCs, suggesting that KRAS activation in MA4(+) B-ALL is important for tumor maintenance rather than initiation. Cancer Res; 76(8); 2478-89. ©2016 AACR.

Revisiting the biology of infant t(4;11)/MLL-AF4+ B-cell acute lymphoblastic leukemia

Infant B-cell acute lymphoblastic leukemia (B-ALL) accounts for 10% of childhood ALL. The genetic hallmark of most infant B-ALL is chromosomal rearrangements of the mixed-lineage leukemia (MLL) gene. Despite improvement in the clinical management and survival (∼85-90%) of childhood B-ALL, the outcome of infants with MLL-rearranged (MLL-r) B-ALL remains dismal, with overall survival <35%. Among MLL-r infant B-ALL, t(4;11)+ patients harboring the fusion MLL-AF4 (MA4) display a particularly poor prognosis and a pro-B/mixed phenotype. Studies in monozygotic twins and archived blood spots have provided compelling evidence of a single cell of prenatal origin as the target for MA4 fusion, explaining the brief leukemia latency. Despite its aggressiveness and short latency, current progress on its etiology, pathogenesis, and cellular origin is limited as evidenced by the lack of mouse/human models recapitulating the disease phenotype/latency. We propose this is because infant cancer is from an etiologic and pathogenesis standpoint distinct from adult cancer and should be seen as a developmental disease. This is supported by whole-genome sequencing studies suggesting that opposite to the view of cancer as a "multiple-and-sequential-hit" model, t(4;11) alone might be sufficient to spawn leukemia. The stable genome of these patients suggests that, in infant developmental cancer, one "big-hit" might be sufficient for overt disease and supports a key contribution of epigenetics and a prenatal cell of origin during a critical developmental window of stem cell vulnerability in the leukemia pathogenesis. Here, we revisit the biology of t(4;11)+ infant B-ALL with an emphasis on its origin, genetics, and disease models.

Genetically engineered mouse models of human B-cell precursor leukemias

B-cell precursor acute lymphoblastic leukemias (pB-ALLs) are the most frequent type of malignancies of the childhood, and also affect an important proportion of adult patients. In spite of their apparent homogeneity, pB-ALL comprises a group of diseases very different both clinically and pathologically, and with very diverse outcomes as a consequence of their biology, and underlying molecular alterations. Their understanding (as a prerequisite for their cure) will require a sustained multidisciplinary effort from professionals coming from many different fields. Among all the available tools for pB-ALL research, the use of animal models stands, as of today, as the most powerful approach, not only for the understanding of the origin and evolution of the disease, but also for the development of new therapies. In this review we go over the most relevant (historically, technically or biologically) genetically engineered mouse models (GEMMs) of human pB-ALLs that have been generated over the last 20 years. Our final aim is to outline the most relevant guidelines that should be followed to generate an “ideal” animal model that could become a standard for the study of human pB-ALL leukemia, and which could be shared among research groups and drug development companies in order to unify criteria for studies like drug testing, analysis of the influence of environmental risk factors, or studying the role of both low-penetrance mutations and cancer susceptibility alterations.

Targeting DOT1L and HOX gene expression in MLL-rearranged leukemia and beyond

Leukemias harboring mixed-lineage leukemia gene (MLL1) abnormalities are associated with poor clinical outcomes, and new therapeutic approaches are desperately needed. Rearrangement of the MLL1 gene generates chimeric proteins that fuse the NH3 terminus of MLL1 to the COOH terminus of its translocation partners. These MLL1 fusion oncoproteins drive the expression of homeobox genes such as HOXA cluster genes and myeloid ecotropic viral integration site 1 homolog (MEIS1), which are known to induce leukemic transformation of hematopoietic progenitors. Genomewide histone methylation studies have revealed that the abnormal expression of MLL1 fusion target genes is associated with high levels of H3K79 methylation at these gene loci. The only known enzyme that catalyzes methylation of H3K79 is disruptor of telomeric-silencing 1-like (DOT1L). Loss-of-function mouse models, as well as small molecular inhibitors of DOT1L, illustrate that leukemias driven by MLL1 translocations are dependent on DOT1L enzymatic activity for proliferation and for the maintenance of HOXA gene expression. Furthermore, DOT1L also appears to be important for HOXA gene expression in other settings including leukemias with select genetic abnormalities. These discoveries have established a foundation for disease-specific therapies that target chromatin modifications in highly malignant leukemias harboring specific genetic abnormalities. This review focuses on the molecular mechanisms underlying MLL1 translocation-driven leukemogenesis and the latest progress on DOT1L-targeted epigenetic therapies for MLL1-rearranged and other leukemias.

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.

Two decades of leukemia oncoprotein epistasis: the MLL1 paradigm for epigenetic deregulation in leukemia

MLL1, located on human chromosome 11, is disrupted in distinct recurrent chromosomal translocations in several leukemia subsets. Studying the MLL1 gene and its oncogenic variants has provided a paradigm for understanding cancer initiation and maintenance through aberrant epigenetic gene regulation. Here we review the historical development of model systems to recapitulate oncogenic MLL1-rearrangement (MLL-r) alleles encoding mixed-lineage leukemia fusion proteins (MLL-FPs) or internal gene rearrangement products. These largely mouse and human cell/xenograft systems have been generated and used to understand how MLL-r alleles affect diverse pathways to result in a highly penetrant, drug-resistant leukemia. The particular features of the animal models influenced the conclusions of mechanisms of transformation. We discuss significant downstream enablers, inhibitors, effectors, and collaborators of MLL-r leukemia, including molecules that directly interact with MLL-FPs and endogenous mixed-lineage leukemia protein, direct target genes of MLL-FPs, and other pathways that have proven to be influential in supporting or suppressing the leukemogenic activity of MLL-FPs. The use of animal models has been complemented with patient sample, genome-wide analyses to delineate the important genomic and epigenomic changes that occur in distinct subsets of MLL-r leukemia. Collectively, these studies have resulted in rapid progress toward developing new strategies for targeting MLL-r leukemia and general cell-biological principles that may broadly inform targeting aberrant epigenetic regulators in other cancers.

Murine models of acute leukemia: important tools in current pediatric leukemia research

Leukemia remains the most common diagnosis in pediatric oncology and, despite dramatic progress in upfront therapy, is also the most common cause of cancer-related death in children. Much of the initial improvement in outcomes for acute lymphoblastic leukemia (ALL) was due to identification of cytotoxic agents that are active against leukemia followed by the recognition that combination of these cytotoxic agents and prolonged therapy are essential for cure. Recent data demonstrating lack of progress in patients for whom standard chemotherapy fails suggests that the ability to improve outcome for these children will not be dramatically impacted through more intensive or newer cytotoxic agents. Thus, much of the recent research focus has been in the area of improving our understanding of the genetics and the biology of leukemia. Although in vitro studies remain critical, given the complexity of a living system and the increasing recognition of the contribution of leukemia extrinsic factors such as the bone marrow microenvironment, in vivo models have provided important insights. The murine systems that are used can be broadly categorized into syngeneic models in which a murine leukemia can be studied in immunologically intact hosts and xenograft models where human leukemias are studied in highly immunocompromised murine hosts. Both of these systems have limitations such that neither can be used exclusively to study all aspects of leukemia biology and therapeutics for humans. This review will describe the various ALL model systems that have been developed as well as discuss the advantages and disadvantages inherent to these systems that make each particularly suitable for specific types of studies.

Animal models of leukemia: any closer to the real thing?

Animal models have been invaluable in the efforts to better understand and ultimately treat patients suffering from leukemia. While important insights have been gleaned from these models, limitations must be acknowledged. In this review, we will highlight the various animal models of leukemia and describe their contributions to the improved understanding and treatment of these cancers.

Ligand-independent FLT3 activation does not cooperate with MLL-AF4 to immortalize/transform cord blood CD34+ cells

MLL-AF4 fusion is hallmark in high-risk infant pro-B-acute lymphoblastic leukemia (pro-B-ALL). Our limited understanding of MLL-AF4-mediated transformation reflects the absence of human models reproducing this leukemia. Hematopoietic stem/progenitor cells (HSPCs) constitute likely targets for transformation. We previously reported that MLL-AF4 enhanced hematopoietic engraftment and clonogenic potential in cord blood (CB)-derived CD34+ HSPCs but was not sufficient for leukemogenesis, suggesting that additional oncogenic lesions are required for MLL-AF4-mediated transformation. MLL-AF4+ pro-B-ALL display enormous levels of FLT3, and occasionally FLT3-activating mutations, thus representing a candidate cooperating event in MLL-AF4+ pro-B-ALL. We have explored whether FLT3.TKD (tyrosine kinase domain) mutation or increased expression of FLT3.WT (wild type) cooperates with MLL-AF4 to immortalize/transform CB-CD34+ HSPCs. In vivo, FLT3.TKD/FLT3.WT alone, or in combination with MLL-AF4, enhances hematopoietic repopulating function of CB-CD34+ HSPCs without impairing migration or hematopoietic differentiation. None of the animals transplanted with MLL-AF4+FLT3.TKD/WT-CD34+ HSPCs showed any sign of disease after 16 weeks. In vitro, enforced expression of FLT3.TKD/FLT3.WT conveys a transient overexpansion of MLL-AF4-expressing CD34+ HSPCs associated to higher proportion of cycling cells coupled to lower apoptotic levels, but does not augment clonogenic potential nor confer stable replating. Together, FLT3 activation does not suffice to immortalize/transform MLL-AF4-expressing CB-CD34+ HSPCs, suggesting the need of alternative (epi)-genetic cooperating oncogenic lesions.

The silent mutational landscape of infant MLL-AF4 pro-B acute lymphoblastic leukemia

Over 90% of infants (< 1-year-old) diagnosed with leukemia have pro-B acute lymphoblastic leukemia (ALL) containing the MLL-AF4 fusion. When compared with other forms of paediatric ALL affecting later B-cell differentiation, MLL-AF4 pro-B is associated with a dismal prognosis with a typical 5-year disease-free survival of <20%. MLL-AF4 may be sufficient on its own for leukemogenesis or the gene-fusion product may alternatively predispose transformed cells to global genetic instability, enhancing the acquisition of additional key mutations. To gain insight into the genomic landscape of infant MLL-AF4 pro-B ALL we performed whole genome sequencing of diagnostic leukemic blasts and matched germline samples from three MLL-AF4 pro-B ALL infants. Our analysis revealed few somatic changes (copy number abnormalities, loss of heterozygosity, or single nucleotide variants), demonstrating that only a very small number of mutations are necessary to generate infant MLL-leukemia.

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.

Frequencies and prognostic impact of RAS mutations in MLL-rearranged acute lymphoblastic leukemia in infants

Acute lymphoblastic leukemia in infants represents an aggressive malignancy associated with a high incidence (approx. 80%) of translocations involving the Mixed Lineage Leukemia (MLL) gene. Attempts to mimic Mixed Lineage Leukemia fusion driven leukemogenesis in mice raised the question whether these fusion proteins require secondary hits. RAS mutations are suggested as candidates. Earlier results on the incidence of RAS mutations in Mixed Lineage Leukemia-rearranged acute lymphoblastic leukemia are inconclusive. Therefore, we studied frequencies and relation with clinical parameters of RAS mutations in a large cohort of infant acute lymphoblastic leukemia patients. Using conventional sequencing analysis, we screened neuroblastoma RAS viral (v-ras) oncogene homolog gene (NRAS), v-Ki-ras Kirsten rat sarcoma viral oncogene homolog gene (KRAS), and v-raf murine sarcoma viral oncogene homolog B1 gene (BRAF) for mutations in a large cohort (n=109) of infant acute lymphoblastic leukemia patients and studied the mutations in relation to several clinical parameters, and in relation to Homeobox gene A9 expression and the presence of ALL1 fused gene 4-Mixed Lineage Leukemia (AF4-MLL). Mutations were detected in approximately 14% of all cases, with a higher frequency of approximately 24% in t(4;11)-positive patients (P=0.04). Furthermore, we identified RAS mutations as an independent predictor (P=0.019) for poor outcome in Mixed Lineage Leukemia-rearranged infant acute lymphoblastic leukemia, with a hazard ratio of 3.194 (95% confidence interval (CI):1.211-8.429). Also, RAS-mutated infants have higher white blood cell counts at diagnosis (P=0.013), and are more resistant to glucocorticoids in vitro (P<0.05). Finally, we demonstrate that RAS mutations, and not the lack of Homeobox gene A9 expression nor the expression of AF4-MLL are associated with poor outcome in t(4;11)-rearranged infants. We conclude that the presence of RAS mutations in Mixed Lineage Leukemia-rearranged infant acute lymphoblastic leukemia is an independent predictor for a poor outcome. Therefore, future risk-stratification based on abnormal RAS-pathway activation and RAS-pathway inhibition could be beneficial in RAS-mutated infant acute lymphoblastic leukemia patients.

RUNX1 is a key target in t(4;11) leukemias that contributes to gene activation through an AF4-MLL complex interaction

The Mixed Lineage Leukemia (MLL) protein is an important epigenetic regulator required for the maintenance of gene activation during development. MLL chromosomal translocations produce novel fusion proteins that cause aggressive leukemias in humans. Individual MLL fusion proteins have distinct leukemic phenotypes even when expressed in the same cell type, but how this distinction is delineated on a molecular level is poorly understood. Here, we highlight a unique molecular mechanism whereby the RUNX1 gene is directly activated by MLL-AF4 and the RUNX1 protein interacts with the product of the reciprocal AF4-MLL translocation. These results support a mechanism of transformation whereby two oncogenic fusion proteins cooperate by activating a target gene and then modulating the function of its downstream product.

CBL mutations do not frequently occur in paediatric acute myeloid leukaemia

RAS-pathway mutations, causing a proliferative advantage, occur in acute myeloid leukaemia (AML) and MLL-rearranged leukaemia. Recently, mutations in the Casitas B lineage lymphoma (CBL) gene were reported to be involved in RAS-pathway activation in various myeloid malignancies, but their role in paediatric AML is still unknown. We performed mutation analysis of 283 newly diagnosed and 33 relapsed paediatric AML cases. Only two mutant cases (0·7%) were identified in the newly diagnosed paediatric AML samples, of which one was MLL-rearranged. Both mutant cases showed CBL mRNA expression in the range of the non-mutated cases. Phosphorylated extracellular signal-regulated kinase (pERK) was not correlated with CBL protein expression (n = 11). In conclusion, we report a very low CBL mutation frequency in paediatric AML, which, together with the lack of difference in protein and mRNA expression, illustrates the limited role of CBL in paediatric AML.

Molecular and Epigenetic Mechanisms of MLL in Human Leukemogenesis

Epigenetics is often defined as the study of heritable changes in gene expression or chromosome stability that don’t alter the underlying DNA sequence. Epigenetic changes are established through multiple mechanisms that include DNA methylation, non-coding RNAs and the covalent modification of specific residues on histone proteins. It is becoming clear not only that aberrant epigenetic changes are common in many human diseases such as leukemia, but that these changes by their very nature are malleable, and thus are amenable to treatment. Epigenetic based therapies have so far focused on the use of histone deacetylase (HDAC) inhibitors and DNA methyltransferase inhibitors, which tend to have more general and widespread effects on gene regulation in the cell. However, if a unique molecular pathway can be identified, diseases caused by epigenetic mechanisms are excellent candidates for the development of more targeted therapies that focus on specific gene targets, individual binding domains, or specific enzymatic activities. Designing effective targeted therapies depends on a clear understanding of the role of epigenetic mutations during disease progression. The Mixed Lineage Leukemia (MLL) protein is an example of a developmentally important protein that controls the epigenetic activation of gene targets in part by methylating histone 3 on lysine 4. MLL is required for normal development, but is also mutated in a subset of aggressive human leukemias and thus provides a useful model for studying the link between epigenetic cell memory and human disease. The most common MLL mutations are chromosome translocations that fuse the MLL gene in frame with partner genes creating novel fusion proteins. In this review, we summarize recent work that argues MLL fusion proteins could function through a single molecular pathway, but we also highlight important data that suggests instead that multiple independent mechanisms underlie MLL mediated leukemogenesis.

Advances in the molecular pathobiology of B-lymphoblastic leukemia

B-lymphoblastic leukemia/lymphoma, also known as B-acute lymphoblastic leukemia, is derived from B-cell progenitors. B-acute lymphoblastic leukemia occurs predominantly in children, but can occur at any age. Risk-adapted intensive chemotherapy is effective in treating most children with B-acute lymphoblastic leukemia, but this approach is less successful in adults. Recent developments in genome-wide genetic analysis in B-acute lymphoblastic leukemia have provided insights into disease pathogenesis and prognosis. B-acute lymphoblastic leukemia cases usually carry a primary genetic event, often a chromosome translocation, and a constellation of secondary genetic alterations that are acquired and selected dynamically in a nonlinear fashion. These genetic changes commonly affect cellular mechanisms that control B-cell differentiation and proliferation. The cooperative interaction between inactivation of hematopoietic transcription factors involved in differentiation (class II mutation) and activating mutations involved in cell proliferation (class I mutation) is reminiscent of the pathogenic model of acute myeloid leukemia. The resulting improved molecular understanding of B-acute lymphoblastic leukemia is helping to refine disease risk stratification and discover new therapeutic approaches for patients with refractory disease. In this review, we first summarize the clinicopathologic and immunophenotypic features of B-acute lymphoblastic leukemia and introduce current understanding of B-cell development and B-acute lymphoblastic leukemia leukemogenesis. We then focus on recent advances in genetic analysis and gene expression profiling of B-acute lymphoblastic leukemia and discuss the implications of these findings for disease evolution, risk prediction, and possible novel therapeutic approaches.

The earliest thymic T cell progenitors sustain B cell and myeloid lineage potential

The stepwise commitment from hematopoietic stem cells in the bone marrow (BM) to T lymphocyte-restricted progenitors in the thymus represents a paradigm for understanding the requirement for distinct extrinsic cues during different stages of lineage restriction from multipotent to lineage restricted progenitors. However, the commitment stage at which progenitors migrate from the BM to the thymus remains unclear. Here we provide functional and molecular evidence at the single cell level that the earliest progenitors in the neonatal thymus possessed combined granulocyte-monocyte, T and B lymphocyte, but not megakaryocyte-erythroid lineage potential. These potentials were identical to those of thymus-seeding progenitors in the BM, which were closely related at the molecular level. These findings establish the distinct lineage-restriction stage at which the T lineage commitment transits from the BM to the remote thymus.

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.

Pathogenetic, Clinical, and Prognostic Features of Adult t(4;11)(q21;q23)/MLL-AF4 Positive B-Cell Acute Lymphoblastic Leukemia

Translocation t(4;11)(q21;q23) leading to formation of MLL-AF4 fusion gene is found in about 10% of newly diagnosed B-cell acute lymphoblastic leukemia (ALL) in adult patients. Patients expressing this chromosomal aberration present typical biological, immunophenotypic, and clinical features. This form of leukemia is universally recognized as high-risk leukemia and treatment intensification with allogeneic hematopoietic stem cell transplantation (HSCT) in first complete remission (CR) could be a valid option to improve prognosis, but data obtained from the literature are controversial. In this review, we briefly describe pathogenetic, clinical, and prognostic characteristics of adult t(4;11)(q21;q23)/MLL-AF4 positive ALL and provide a review of the clinical outcome reported by the most important cooperative groups worldwide.

Methylation-mediated repression of microRNA-143 enhances MLL-AF4 oncogene expression

Fusion proteins containing the amino terminus of mixed lineage leukemia (MLL) are common in acute lymphoblastic leukemia (ALL) due to translocations. The MLL-AF4 fusion protein is generated by the translocation t(4;11)(q21;q23), and t(4;11)-positive ALL patients (MLL-AF4 ALL), have a notoriously poorer prognosis compared with patients with other MLL-associated leukemias. The detailed role of this fusion protein in leukemogenesis is not well understood. MicroRNAs (miRNAs) targeting the AF4 3' untranslated regions may modulate MLL-AF4 fusion protein levels, raising the question of whether regulation of these miRNAs are involved in the progression of MLL-AF4 ALL. In this study, we show that miR-143 was identified as a regulator of MLL-AF4 expression in MLL-AF4 ALL samples. Restoration of miR-143 in MLL-AF4-positive RS4;11 and MV4-11 cells induced apoptosis, negatively contributing to leukemia cell growth by reducing MLL-AF4 fusion protein levels. Furthermore, miR-143 was epigenetically repressed by promoter hypermethylation in MLL-AF4-positive primary blasts and cell lines, but not in normal bone marrow cells and MLL-AF4-negative primary blasts, which was directly associated with expression of the MLL-AF4 oncogene. This is the first study to show that miR-143 functions as a tumor suppressor in MLL-AF4 B-cell ALL. These data reveal the therapeutic promise of upregulating miR-143 expression for MLL-AF4 B-cell ALL.

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.