An insertional mutagenesis screen identifies genes that cooperate with Mll-AF9 in a murine leukemogenesis model

RNA-seq analysis reveals candidate genes associated with proliferation, invasion, and migration in BCL11A knockdown B-NHL cell lines

B-cell lymphoma/leukemia 11A (BCL11A) is highly expressed in B-cell non-Hodgkin lymphoma (B-NHL), blocks cell differentiation, and inhibits cell apoptosis. However, little is known about BCL11A in the proliferation, invasion, and migration of B-NHL cells. Here, we found increased expression of BCL11A in B-NHL patients and cell lines. Knockdown of BCL11A suppressed the proliferation, invasion, and migration of B-NHL cells in vitro and reduced tumor growth in vivo. RNA sequencing (RNA-seq) and KEGG pathway analysis demonstrated that BCL11A-targeted genes were significantly enriched in the PI3K/AKT signaling pathway, focal adhesion, and extracellular matrix (ECM)-receptor interaction (including COL4A1, COL4A2, FN1, SPP1), and SPP1 was the most significantly downregulated gene. qRT‒PCR, western blotting, and immunohistochemistry revealed that silencing BCL11A reduced the expression level of SPP1 in Raji cells. Our study suggested that high level of BCL11A may promote B-NHL proliferation, invasion, and migration, and the BCL11A-SPP1 regulatory axis may play an important role in Burkitt's lymphoma.

BCL11A promotes myeloid leukemogenesis by repressing PU.1 target genes

BCL11A promotes myeloid leukemogenesis via the repression of PU.1 target genes.

Inhibition of corepressors abrogates the BCL11A function, inducing growth suppression and inhibition of engraftment in AML.

The transcriptional repressor BCL11A is involved in hematological malignancies, B-cell development, and fetal-to-adult hemoglobin switching. However, the molecular mechanism by which it promotes the development of myeloid leukemia remains largely unknown. We find that Bcl11a cooperates with the pseudokinase Trib1 in the development of acute myeloid leukemia (AML). Bcl11a promotes the proliferation and engraftment of Trib1-expressing AML cells in vitro and in vivo. Chromatin immunoprecipitation sequencing analysis showed that, upon DNA binding, Bcl11a is significantly associated with PU.1, an inducer of myeloid differentiation, and that Bcl11a represses several PU.1 target genes, such as Asb2, Clec5a, and Fcgr3. Asb2, as a Bcl11a target gene that modulates cytoskeleton and cell-cell interaction, plays a key role in Bcl11a-induced malignant progression. The repression of PU.1 target genes by Bcl11a is achieved by sequence-specific DNA-binding activity and recruitment of corepressors by Bcl11a. Suppression of the corepressor components HDAC and LSD1 reverses the repressive activity. Moreover, treatment of AML cells with the HDAC inhibitor pracinostat and the LSD1 inhibitor GSK2879552 resulted in growth inhibition in vitro and in vivo. High BCL11A expression is associated with worse prognosis in humans with AML. Blocking of BCL11A expression upregulates the expression of PU.1 target genes and inhibits the growth of HL-60 cells and their engraftment to the bone marrow, suggesting that BCL11A is involved in human myeloid malignancies via the suppression of PU.1 transcriptional activity.

Genome-Wide Association Study of Susceptibility Loci for TCF3-PBX1 Acute Lymphoblastic Leukemia in Children

Acute lymphoblastic leukemia (ALL) is the most common cancer in children. TCF3-PBX1 fusion defines a common molecular subtype of ALL with unique clinical features, but the molecular basis of its inherited susceptibility is unknown. In a genome-wide association study of 1494 ALL cases and 2057 non-ALL controls, we identified a germline risk locus located in an intergenic region between BCL11A and PAPOLG: rs2665658, P = 1.88 × 10-8 for TCF3-PBX1 ALL vs non-ALL, and P = 1.70 × 10-8 for TCF3-PBX1 ALL vs other-ALL. The lead variant was validated in a replication cohort, and conditional analyses pointed to a single causal variant with subtype-specific effect. The risk variant is located in a regulatory DNA element uniquely activated in ALL cells with the TCF3-PBX1 fusion and may distally modulate the transcription of the adjacent gene REL. Our results expand the understanding of subtype-specific ALL susceptibility and highlight plausible interplay between germline variants and somatic genomic abnormalities in ALL pathogenesis.

In vivo functional screening for systems-level integrative cancer genomics

With the genetic portraits of all major human malignancies now available, we next face the challenge of characterizing the function of mutated genes, their downstream targets, interactions and molecular networks. Moreover, poorly understood at the functional level are also non-mutated but dysregulated genomes, epigenomes or transcriptomes. Breakthroughs in manipulative mouse genetics offer new opportunities to probe the interplay of molecules, cells and systemic signals underlying disease pathogenesis in higher organisms. Herein, we review functional screening strategies in mice using genetic perturbation and chemical mutagenesis. We outline the spectrum of genetic tools that exist, such as transposons, CRISPR and RNAi and describe discoveries emerging from their use. Genome-wide or targeted screens are being used to uncover genomic and regulatory landscapes in oncogenesis, metastasis or drug resistance. Versatile screening systems support experimentation in diverse genetic and spatio-temporal settings to integrate molecular, cellular or environmental context-dependencies. We also review the combination of in vivo screening and barcoding strategies to study genetic interactions and quantitative cancer dynamics during tumour evolution. These scalable functional genomics approaches are transforming our ability to interrogate complex biological systems.

BCL11A: a potential diagnostic biomarker and therapeutic target in human diseases

Transcription factor B-cell lymphoma/leukemia 11A (BCL11A) gene encodes a zinc-finger protein that is predominantly expressed in brain and hematopoietic tissue. BCL11A functions mainly as a transcriptional repressor that is crucial in brain, hematopoietic system development, as well as fetal-to-adult hemoglobin switching. The expression of this gene is regulated by microRNAs, transcription factors and genetic variations. A number of studies have recently shown that BCL11A is involved in β-hemoglobinopathies, hematological malignancies, malignant solid tumors, 2p15-p16.1 microdeletion syndrome, and Type II diabetes. It has been suggested that BCL11A may be a potential prognostic biomarker and therapeutic target for some diseases. In this review, we summarize the current research state of BCL11A, including its biochemistry, expression, regulation, function, and its possible clinical application in human diseases.

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.

Characterization of functional transposable element enhancers in acute myeloid leukemia

Transposable elements (TEs) have been shown to have important gene regulatory functions and their alteration could lead to disease phenotypes. Acute myeloid leukemia (AML) develops as a consequence of a series of genetic changes in hematopoietic precursor cells, including mutations in epigenetic factors. Here, we set out to study the gene regulatory role of TEs in AML. We first explored the epigenetic landscape of TEs in AML patients using ATAC-seq data. We show that a large number of TEs in general, and more specifically mammalian-wide interspersed repeats (MIRs), are more enriched in AML cells than in normal blood cells. We obtained a similar finding when analyzing histone modification data in AML patients. Gene Ontology enrichment analysis showed that genes near MIRs in open chromatin regions are involved in leukemogenesis. To functionally validate their regulatory role, we selected 19 MIR regions in AML cells, and tested them for enhancer activity in an AML cell line (Kasumi-1) and a chronic myeloid leukemia (CML) cell line (K562); the results revealed several MIRs to be functional enhancers. Taken together, our results suggest that TEs are potentially involved in myeloid leukemogenesis and highlight these sequences as potential candidates harboring AML-associated variation.

Meningioma 1 is indispensable for mixed lineage leukemia-rearranged acute myeloid leukemia

Mixed lineage leukemia (MLL/KMT2A) rearrangements (MLL-r) are one of the most frequent chromosomal aberrations in acute myeloid leukemia. We evaluated the function of Meningioma 1 (MN1), a co-factor of HOXA9 and MEIS1, in human and murine MLL-rearranged leukemia by CRISPR-Cas9 mediated deletion of MN1. MN1 was required for in vivo leukemogenicity of MLL positive murine and human leukemia cells. Loss of MN1 inhibited cell cycle and proliferation, promoted apoptosis and induced differentiation of MLL-rearranged cells. Expression analysis and chromatin immunoprecipitation with sequencing from previously reported data sets demonstrated that MN1 primarily maintains active transcription of HOXA9 and HOXA10, which are critical downstream genes of MLL, and their target genes like BCL2, MCL1 and Survivin. Treatment of MLL-rearranged primary leukemia cells with anti-MN1 siRNA significantly reduced their clonogenic potential in contrast to normal CD34⁺ hematopoietic progenitor cells, suggesting a therapeutic window for MN1 targeting. In summary, our findings demonstrate that MN1 plays an essential role in MLL fusion leukemias and serve as a therapeutic target in MLL-rearranged acute myeloid leukemia.

Ex Vivo Transposon-Mediated Genetic Screens for Cancer Gene Discovery

Transposon mutagenesis has emerged as a powerful methodology for functionally annotating cancer genomes. Although in vivo transposon-mediated forward genetic screens have proven to be valuable for cancer gene identification, they are also time consuming and resource intensive. To facilitate the rapid and cost-effective identification of genes that regulate tumor-promoting pathways, we developed a complementary ex vivo transposon mutagenesis approach wherein human or mouse cells growing in culture are mutagenized and screened for the acquisition of specific phenotypes in vitro or in vivo, such as growth factor independence or tumor-forming ability. This approach allows discovery of both gain- and loss-of-function mutations in the same screen. Transposon insertions sites are recovered by high-throughput sequencing. We recently applied this system to comprehensively identify and validate genes that promote growth factor independence and transformation of murine Ba/F3 cells. Here we describe a method for performing ex vivo Sleeping Beauty-mediated mutagenesis screens in these cells, which may be adapted for the acquisition of many different phenotypes in distinct cell types.

Identification of the Gene Expression Rules That Define the Subtypes in Glioma

As a common brain cancer derived from glial cells, gliomas have three subtypes: glioblastoma, diffuse astrocytoma, and anaplastic astrocytoma. The subtypes have distinctive clinical features but are closely related to each other. A glioblastoma can be derived from the early stage of diffuse astrocytoma, which can be transformed into anaplastic astrocytoma. Due to the complexity of these dynamic processes, single-cell gene expression profiles are extremely helpful to understand what defines these subtypes. We analyzed the single-cell gene expression profiles of 5057 cells of anaplastic astrocytoma tissues, 261 cells of diffuse astrocytoma tissues, and 1023 cells of glioblastoma tissues with advanced machine learning methods. In detail, a powerful feature selection method, Monte Carlo feature selection (MCFS) method, was adopted to analyze the gene expression profiles of cells, resulting in a feature list. Then, the incremental feature selection (IFS) method was applied to the obtained feature list, with the help of support vector machine (SVM), to extract key features (genes) and construct an optimal SVM classifier. Several key biomarker genes, such as IGFBP2, IGF2BP3, PRDX1, NOV, NEFL, HOXA10, GNG12, SPRY4, and BCL11A, were identified. In addition, the underlying rules of classifying the three subtypes were produced by Johnson reducer algorithm. We found that in diffuse astrocytoma, PRDX1 is highly expressed, and in glioblastoma, the expression level of PRDX1 is low. These rules revealed the difference among the three subtypes, and how they are formed and transformed. These genes are not only biomarkers for glioma subtypes, but also drug targets that may switch the clinical features or even reverse the tumor progression.

Childhood Leukemia and Primary Prevention

Leukemia is the most common pediatric cancer, affecting 3800 children per year in the United States. Its annual incidence has increased over the last decades, especially among Latinos. Although most children diagnosed with leukemia are now cured, many suffer long-term complications, and primary prevention efforts are urgently needed. The early onset of leukemia-usually before 5 years of age-and the presence at birth of "pre-leukemic" genetic signatures indicate that pre- and postnatal events are critical to the development of the disease. In contrast to most pediatric cancers, there is a growing body of literature-in the United States and internationally-that has implicated several environmental, infectious, and dietary risk factors in the etiology of childhood leukemia, mainly for acute lymphoblastic leukemia, the most common subtype. For example, exposures to pesticides, tobacco smoke, solvents, and traffic emissions have consistently demonstrated positive associations with the risk of developing childhood leukemia. In contrast, intake of vitamins and folate supplementation during the preconception period or pregnancy, breastfeeding, and exposure to routine childhood infections have been shown to reduce the risk of childhood leukemia. Some children may be especially vulnerable to these risk factors, as demonstrated by a disproportionate burden of childhood leukemia in the Latino population of California. The evidence supporting the associations between childhood leukemia and its risk factors-including pooled analyses from around the world and systematic reviews-is strong; however, the dissemination of this knowledge to clinicians has been limited. To protect children's health, it is prudent to initiate programs designed to alter exposure to well-established leukemia risk factors rather than to suspend judgment until no uncertainty remains. Primary prevention programs for childhood leukemia would also result in the significant co-benefits of reductions in other adverse health outcomes that are common in children, such as detriments to neurocognitive development.

Comprehensive Ex Vivo Transposon Mutagenesis Identifies Genes That Promote Growth Factor Independence and Leukemogenesis

Aberrant signaling through cytokine receptors and their downstream signaling pathways is a major oncogenic mechanism underlying hematopoietic malignancies. To better understand how these pathways become pathologically activated and to potentially identify new drivers of hematopoietic cancers, we developed a high-throughput functional screening approach using ex vivo mutagenesis with the Sleeping Beauty transposon. We analyzed over 1,100 transposon-mutagenized pools of Ba/F3 cells, an IL3-dependent pro-B-cell line, which acquired cytokine independence and tumor-forming ability. Recurrent transposon insertions could be mapped to genes in the JAK/STAT and MAPK pathways, confirming the ability of this strategy to identify known oncogenic components of cytokine signaling pathways. In addition, recurrent insertions were identified in a large set of genes that have been found to be mutated in leukemia or associated with survival, but were not previously linked to the JAK/STAT or MAPK pathways nor shown to functionally contribute to leukemogenesis. Forced expression of these novel genes resulted in IL3-independent growth in vitro and tumorigenesis in vivo, validating this mutagenesis-based approach for identifying new genes that promote cytokine signaling and leukemogenesis. Therefore, our findings provide a broadly applicable approach for classifying functionally relevant genes in diverse malignancies and offer new insights into the impact of cytokine signaling on leukemia development.

BCL11A expression in acute myeloid leukemia

BACKGROUND

BCL11A encodes a C2H2 type zinc-finger protein. During normal haematopoietic cell differentiation BCL11A expression is down-regulated. Data in mice suggest up-regulation of BCL11A is involved in the pathogenesis of myeloid leukaemias. BCL11A expression in persons with acute myeloid leukaemia (AML) is not systematically studied.

OBJECTIVE

Interrogate associations between BCL11A expression at diagnosis and clinical and laboratory valuables and outcomes in newly-diagnosed persons with AML.

METHODS

We determined BCL11A mRNA levels in bone marrow and blood mononuclear cells in 292 consecutive newly-diagnosed subjects with AML by reverse transcript and real-time polymerase chain reaction. Data were compared to mRNA levels in bone marrow cells of normals.

RESULTS

Subjects with BCL11A transcript levels at diagnosis exceeding the median value of 2.434 (±3.423 SD; 25th-75th inter-quartile range, 1.33-4.29) had higher WBC levels, a greater proportion of bone marrow myeloblasts, were more likely to be FAB M0 subtype, less likely to be FAB M3 subtype, more likely to be in the intermediate cytogenetic risk cohort, less likely to have a complex karyotype and more likely to have DNMT3A(R882) and FLT3-ITD mutations than subjects with transcript levels below the median value. In 89 subjects receiving conventional induction chemotherapy the complete remission rate was 54% (95% confidence interval [CI]; 33, 75%) in the lower BCL11A cohort and 65% (45, 85%; P=0.26) in the higher BCL11A cohort. 3 year survival was 33% (2, 65%) in the lower BCL11A cohort and 15% (0, 39%; P=0.35) in the high BCL11A cohort.

CONCLUSION

BCL11A transcript levels at diagnosis was significantly associated with several clinical and laboratory variables. There were also non-significant associations with complete remission rate and survival. These data suggest a possible role for BCL11A expression in AML biology.

Pontin is a critical regulator for AML1-ETO-induced leukemia

The oncogenic fusion protein AML1-ETO, also known as RUNX1-RUNX1T1 is generated by the t(8;21)(q22;q22) translocation, one of the most frequent chromosomal rearrangements in acute myeloid leukemia (AML). Identifying the genes that cooperate with or are required for the oncogenic activity of this chimeric transcription factor remains a major challenge. Our previous studies showed that Drosophila provides a genuine model to study how AML1-ETO promotes leukemia. Here, using an in vivo RNA interference screen for suppressors of AML1-ETO activity, we identified pontin/RUVBL1 as a gene required for AML1-ETO-induced lethality and blood cell proliferation in Drosophila. We further show that PONTIN inhibition strongly impaired the growth of human t(8;21)(+) or AML1-ETO-expressing leukemic blood cells. Interestingly, AML1-ETO promoted the transcription of PONTIN. Moreover, transcriptome analysis in Kasumi-1 cells revealed a strong correlation between PONTIN and AML1-ETO gene signatures and demonstrated that PONTIN chiefly regulated the expression of genes implicated in cell cycle progression. Concordantly, PONTIN depletion inhibited leukemic self-renewal and caused cell cycle arrest. All together our data suggest that the upregulation of PONTIN by AML1-ETO participate in the oncogenic growth of t(8;21) cells.

Constitutive IRF8 expression inhibits AML by activation of repressed immune response signaling

Myeloid differentiation is blocked in acute myeloid leukemia (AML), but the molecular mechanisms are not well characterized. Meningioma 1 (MN1) is overexpressed in AML patients and confers resistance to all-trans retinoic acid-induced differentiation. To understand the role of MN1 as a transcriptional regulator in myeloid differentiation, we fused transcriptional activation (VP16) or repression (M33) domains with MN1 and characterized these cells in vivo. Transcriptional activation of MN1 target genes induced myeloproliferative disease with long latency and differentiation potential to mature neutrophils. A large proportion of differentially expressed genes between leukemic MN1 and differentiation-permissive MN1VP16 cells belonged to the immune response pathway like interferon-response factor (Irf) 8 and Ccl9. As MN1 is a cofactor of MEIS1 and retinoic acid receptor alpha (RARA), we compared chromatin occupancy between these genes. Immune response genes that were upregulated in MN1VP16 cells were co-targeted by MN1 and MEIS1, but not RARA, suggesting that myeloid differentiation is blocked through transcriptional repression of shared target genes of MN1 and MEIS1. Constitutive expression of Irf8 or its target gene Ccl9 identified these genes as potent inhibitors of murine and human leukemias in vivo. Our data show that MN1 prevents activation of the immune response pathway, and suggest restoration of IRF8 signaling as therapeutic target in AML.

Evaluating risks of insertional mutagenesis by DNA transposons in gene therapy

Investigational therapy can be successfully undertaken using viral- and nonviral-mediated ex vivo gene transfer. Indeed, recent clinical trials have established the potential for genetically modified T cells to improve and restore health. Recently, the Sleeping Beauty (SB) transposon/transposase system has been applied in clinical trials to stably insert a chimeric antigen receptor (CAR) to redirect T-cell specificity. We discuss the context in which the SB system can be harnessed for gene therapy and describe the human application of SB-modified CAR(+) T cells. We have focused on theoretical issues relating to insertional mutagenesis in the context of human genomes that are naturally subjected to remobilization of transposons and the experimental evidence over the last decade of employing SB transposons for defining genes that induce cancer. These findings are put into the context of the use of SB transposons in the treatment of human disease.

In focus: MLL-rearranged leukemia

The molecular mechanisms underlying oncogenesis in leukemias associated with rearrangement of the Mixed Lineage Leukemia (MLL) gene have received a considerable amount of attention over the last two decades. In this review we will focus on recent studies, published over the past year, that reveal new insights into the multi-protein complexes formed by MLL and MLL fusion proteins, the role of epigenetic deregulation in MLL fusion function, downstream transcriptional target genes, the importance of the leukemia cell of origin, the role played by microRNAs, cooperating mutations and the implications that recent research has for the therapy of MLL-rearranged leukemia.