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MLL1 promotes IL-7 responsiveness and survival during B cell differentiation

ABSTRACT: B-lymphocyte differentiation is an exquisitely regulated homeostatic process resulting in continuous production of appropriately selected B cells. Relatively small changes in gene expression can result in deregulation of this process, leading acute lymphocytic leukemia, immune deficiency or autoimmunity. Translocation of Mll1 (Kmt2a) often results in a pro-B cell acute lymphocytic leukemia (B-ALL), but little is known about its role in normal B cell differentiation. Using a Rag1-cre knock-in to selectively delete Mll1 in developing lymphocytes, we show that B-cell, but not T-cell homeostasis depends on MLL1. Mll1-/- B progenitors fail to differentiate efficiently through the pro- to pre-B cell transition, resulting in a persistent reduction in B cell populations. Cells inefficiently transit the pre-B cell receptor (pre-BCR) checkpoint, despite normal to higher levels of pre-BCR components and rearranged IgH expression fails to rescue this differentiation block. Instead of IgH rearrangement defects, we find that Mll1-/- pre-B cells exhibit attenuated RAS/MAPK signaling downstream of the pre-BCR, resulting in reduced survival in physiologic levels of IL-7. Genome-wide expression data illustrate that MLL1 is connected to B-cell differentiation and IL-7-dependent survival through a complex transcriptional network. Overall, our data demonstrate that wild type MLL1 is a regulator of pre-BCR signaling and B-cell differentiation, and further suggest that targeting its function in B-ALL may be more broadly effective than previously anticipated.

ORGANISM(S): Mus musculus

PROVIDER: GSE108093 | GEO | 2018/06/30

REPOSITORIES: GEO

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Project description:Integrative epigenomic analysis identifies biomarkers and therapeutic targets in adult B-acute lymphoblastic leukemia. We performed DNA methylation (HELP array) and gene expression profiling in 215 samples of adult B-lineage acute lymphoblastic leukemia (ALL) and 12 normal preB samples. Adult B-lineage acute lymphoblastic leukemia (B-ALL) is an aggressive disease with <40% long-term survival. Genetic alterations such as BCR/ABL, E2A/PBX1 and MLL rearrangement (tMLL) define distinct B-ALL subtypes, which are associated with poor clinical outcome. It has been shown that these B-ALL subtypes have distinct expression profiles. However, the role of the epigenome in shaping these expression profiles and how the aberrant epigenetic gene regulation contributes to the biological and clinical features of those ALL subtypes is largely unknown. To address this question, we performed genome-wide DNA methylation and gene expression profiling on a large cohort of 215 well-characterized adult B-ALL specimens from the ECOG E2993 phase III clinical trial and a cohort of normal precursor B (preB) cells from 12 healthy bone marrows. The integrative analysis of these profiles led to the identification of key gene networks deregulated at the epigenetic and transcriptional levels within each subtype. In BCR/ABL, we identified a network centered on IL2RA(CD25), which is itself hypomethylated and overexpressed in most BCR/ABL B-ALL and confers poor clinical outcomes. In the tMLL subtype, we uncovered aberrant epigenetic and transcriptional activities that include hypomethylation and upregulation of FLT3 and BCL6. After showing that MLL/AF4 fusion protein binds to these genes as well as other hypomethylated and overexpressed genes in tMLL ALL cells, we showed that a specific BCL6 inhibitor, RI-BPI, kills tumor cells in both tMLL ALL cell lines and patient samples. BCL6 inhibition may therefore represent a novel therapeutic strategy for B-ALL patients with MLL translocations. RUNX1 is a key target gene in MLL-AF4 leukemias and contributes to gene activation by interacting with the AF4-MLL complex. The Mixed Lineage Leukemia 1 protein (MLL1) is an important epigenetic protein that is required for the maintenance of gene activation during development, but is also mutated in a subset of aggressive human leukemias. The most common leukemogenic MLL1 mutations are chromosome translocations that fuse MLL1 in-frame to produce novel fusion proteins. Different MLL1 fusion proteins cause unique leukemias even when they are expressed in the same cell type, suggesting that they function through unique molecular mechanisms. We used ChIP-seq in MLL-AF4 patient cell lines to identify target genes that are involved in leukemogenesis. ChIP-seq using MLLN, AF4, H3K4me3 and H3K79me2 antibodies in RS4;11 cells.

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MLL1 promotes IL-7 responsiveness and survival during B cell differentiation

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