Developmental partitioning of SYK and ZAP70 prevents autoimmunity and cancer

Even though SYK and ZAP70 kinases share high sequence homology and serve analogous functions, their expression in B and T cells is strictly segregated throughout evolution. Here, we identified aberrant ZAP70 expression as a common feature in a broad range of B cell malignancies. We validated SYK as the kinase that sets the thresholds for negative selection of autoreactive and premalignant clones. When aberrantly expressed in B cells, ZAP70 competes with SYK at the BCR signalosome and redirects SYK from negative selection to tonic PI3K signaling, thereby promoting B cell survival. In genetic mouse models for B-ALL and B-CLL, conditional expression of Zap70 accelerated disease onset, while genetic deletion impaired malignant transformation. Inducible activation of Zap70 during B cell development compromised negative selection of autoreactive B cells, resulting in pervasive autoantibody production. Strict segregation of the two kinases is critical for normal B cell selection and represents a central safeguard against the development of autoimmune disease and B cell malignancies.

IFITM3 functions as a PIP3 scaffold to amplify PI3K signalling in B cells

Interferon-induced transmembrane protein 3 (IFITM3) has previously been identified as an endosomal protein that blocks viral infection^1-3. Here we studied clinical cohorts of patients with B cell leukaemia and lymphoma, and identified IFITM3 as a strong predictor of poor outcome. In normal resting B cells, IFITM3 was minimally expressed and mainly localized in endosomes. However, engagement of the B cell receptor (BCR) induced both expression of IFITM3 and phosphorylation of this protein at Tyr20, which resulted in the accumulation of IFITM3 at the cell surface. In B cell leukaemia, oncogenic kinases phosphorylate IFITM3 at Tyr20, which causes constitutive localization of this protein at the plasma membrane. In a mouse model, Ifitm3^-/- naive B cells developed in normal numbers; however, the formation of germinal centres and the production of antigen-specific antibodies were compromised. Oncogenes that induce the development of leukaemia and lymphoma did not transform Ifitm3^-/- B cells. Conversely, the phosphomimetic IFITM3(Y20E) mutant induced oncogenic PI3K signalling and initiated the transformation of premalignant B cells. Mechanistic experiments revealed that IFITM3 functions as a PIP3 scaffold and central amplifier of PI3K signalling. The amplification of PI3K signals depends on IFITM3 using two lysine residues (Lys83 and Lys104) in its conserved intracellular loop as a scaffold for the accumulation of PIP3. In Ifitm3^-/- B cells, lipid rafts were depleted of PIP3, which resulted in the defective expression of over 60 lipid-raft-associated surface receptors, and impaired BCR signalling and cellular adhesion. We conclude that the phosphorylation of IFITM3 that occurs after B cells encounter antigen induces a dynamic switch from antiviral effector functions in endosomes to a PI3K amplification loop at the cell surface. IFITM3-dependent amplification of PI3K signalling, which in part acts downstream of the BCR, is critical for the rapid expansion of B cells with high affinity to antigen. In addition, multiple oncogenes depend on IFITM3 to assemble PIP3-dependent signalling complexes and amplify PI3K signalling for malignant transformation.

Histone H3 trimethylation at lysine 36 guides m6A RNA modification co-transcriptionally

DNA and histone modifications exhibit noticeable impacts on gene expression¹. Being the most prevalent internal modification in mRNA, N⁶-Methyladenosine (m⁶A) mRNA modification emerges as an important post-transcriptional mechanism of gene regulation^2-4 and plays critical roles in various normal and pathological bioprocesses^5-12. However, how m⁶A is precisely and dynamically deposited in the transcriptome remains elusive. Here we report that H3K36me3 histone modification, a marker for transcription elongation, globally guides m⁶A modification. We found that m⁶A modifications enrich in the vicinity of H3K36me3 peaks, and are reduced globally when cellular H3K36me3 is depleted. Mechanistically, H3K36me3 is recognized and bound directly by METTL14, a critical component of the m⁶A methyltransferase complex (MTC), which in turn facilitates the binding of the m⁶A MTC to adjacent RNA polymerase II, and thereby delivering the m⁶A MTC to actively transcribed nascent RNAs to deposit m⁶A co-transcriptionally. In mouse embryonic stem cells, phenocopying Mettl14 silencing, H3K36me3 depletion also induces m⁶A reduction transcriptome-wide and in pluripotency transcripts, resulting in increased cell stemness. Collectively, our studies reveal the critical roles of H3K36me3 and METTL14 in determining precise and dynamic m⁶A deposition in mRNA, and uncover another layer of gene expression regulation involving crosstalk between histone modification and RNA methylation.

Abstract 4515: Lgr5 mediates positive B-cell selection and is critical for initiation and survival of B-cell malignancies

Introduction: In B-cell leukemia and lymphoma, leukemia initiating cells occur at a high frequency (Rehe 2013), are phenotypically diverse (Aoki 2015) and can arise from any stage of B-cell development (Le Viseur 2008). Unlike stem cells, where self-renewal is regulated by a developmental hierarchy, in the B-cell lineage, positive selection events, i.e. induced by antigen-receptor (BCR) signaling dictates their ability to self-renew. Leucine-rich repeat containing G-protein coupled receptor 5 (Lgr5) is a Wnt target gene and through binding to its ligand R-spondin, Lgr5 modulates Wnt signaling strength. Lgr5 is widely used as stem cell marker in multiple epithelial tissues, however the role of Lgr5 in hematopoietic cells was not explored. Results: Upon successful completion of immunoglobulin V(D)J gene recombination and first encounter of antigen represent key events in the life of a B-cell that promote survival and positive selection. Here, we found that both events result in upregulation of Lgr5 expression in B cell precursors in the bone marrow and germinal center B cells. Likewise, engagement of BCR signaling on B-cell lymphomas and oncogenic BCR-signaling mimics in leukemia strongly increased LGR5 expression, which was sensitive to inhibition of SYK and BTK kinases in the BCR pathway.In patients with B-cell leukemia, higher than median mRNA levels of LGR5 at the time of diagnosis were associated with poor clinical outcome and higher likelihood of drug-resistance and relapse. Inducible ablation of Lgr5 during earliest stages of B-cell development resulted in a >100-fold reduction of absolute B-cell numbers. Studies in epithelial cells suggest a role of Lgr5 as potentiator of WNT-signaling. However, deletion of Lgr5 in B cells caused cell death in parallel with massive accumulation of nuclear β-catenin and increased expression of β -catenin target genes. Deletion of Lgr5 abolished colony forming capacity and reduced the ability of leukemia cells to initiate fatal disease in transplant recipients. Likewise, inducible activation of a gain-of-function mutant of β-catenin resulted in rapid cell death in normal and malignant B cells. Conclusion: Lgr5-expression and positive B-cell selection is induced by BCR-engagement by antigen or oncogenic mimics of BCR signaling in B-cell malignancies (e.g. transforming oncogenes that engage the BCR pathway). Unlike in epithelial cells, LGR5 expression in B cells restricts the levels of nuclear β-catenin and enables B-cell survival and transformation through negative regulation of Wnt-signaling. Targeting Lgr5 using a novel Lgr5-ADC seems promising to deplete B-cell leukemia- and lymphoma-initiating cells.

Citation Format: Kadriye Nehir Cosgun, Anna Hecht, Xin Yang, Maurizio Mangolini,, Ali Aghajanirefah, Gang Xiao, Teresa Sadras, Zhengshan Chen, Lars Klemm, Huimin Geng, Chao Hong, Qingxiao Song, Deye Zeng, Hassan Jumaa, Defu Zeng, Hans Clevers, Markus Muschen. Lgr5 mediates positive B-cell selection and is critical for initiation and survival of B-cell malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4515.

B-Cell-Specific Diversion of Glucose Carbon Utilization Reveals a Unique Vulnerability in B Cell Malignancies

B cell activation during normal immune responses and oncogenic transformation impose increased metabolic demands on B cells and their ability to retain redox homeostasis. While the serine/threonine-protein phosphatase 2A (PP2A) was identified as a tumor suppressor in multiple types of cancer, our genetic studies revealed an essential role of PP2A in B cell tumors. Thereby, PP2A redirects glucose carbon utilization from glycolysis to the pentose phosphate pathway (PPP) to salvage oxidative stress. This unique vulnerability reflects constitutively low PPP activity in B cells and transcriptional repression of G6PD and other key PPP enzymes by the B cell transcription factors PAX5 and IKZF1. Reflecting B-cell-specific transcriptional PPP-repression, glucose carbon utilization in B cells is heavily skewed in favor of glycolysis resulting in lack of PPP-dependent antioxidant protection. These findings reveal a gatekeeper function of the PPP in a broad range of B cell malignancies that can be efficiently targeted by small molecule inhibition of PP2A and G6PD.

Abstract 2075: Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia

Introduction: Unlike other cell types, B cells are selected for an intermediate level of signaling strength. Critical survival and proliferation signals emanate from the (pre-) B cell receptor (BCR): Both attenuation below minimum (e.g. non-functional pre-BCR) and hyperactivation above maximum (e.g. autoreactive pre-BCR) thresholds of signaling strength trigger negative selection and cell death. The oncogenic BCR-ABL1 tyrosine kinase mimics active pre-BCR signaling in Ph+ acute lymphoblastic leukemia (ALL) which defines the ALL subgroup with the worst clinical outcome. Current therapy approaches are largely focused on the development of more potent tyrosine kinase inhibitors (TKI) to suppress oncogenic signaling. However resistance to TKI is developed invariably. Here, we test the hypothesis that targeting hyperactivation above a maximum threshold will selectively kill Ph+ ALL cells, similar to removal of self-reactive B cells.

Results: The Ph+ ALL cells don not express ITAM (immunoreceptor tyrosine-based activation motif) receptor Igα or Igβ on the cell surface, indicating defects for a functional pre-BCR. Reconstitution of ITAM receptor was sufficient to induce cell death through increasing pre-BCR signaling strength indicated by phosphorylation of SYK, SRC, BTK and PLCγ2. TKI-treatment, while designed to kill leukemia cells, seemingly paradoxically rescued Ph+ ALL cells in this experimental setting. Surprisingly, patient-derived Ph+ ALL cells express the ITIM (immunoreceptor tyrosine-based inhibitory motif) receptors PECAM1, CD300A and LAIR1 at high levels compared to normal pre-B cells. Importantly, high expression levels of ITIM-receptors are predictive of poor outcome in two clinical trials, including both pediatric and adult ALL patients. Genetic studies revealed that Pecam1, Cd300a and Lair1 were critical to calibrate pre-BCR signaling strength through recruitment of the inhibitory phosphatases Ptpn6 (Shp1) and Inpp5d (Ship1). Genetic deletion of Lair1, Ptpn6 or Inpp5d in BCR-ABL1 ALL caused cell death in vitro and in vivo through hyperactivation of pre-BCR signaling. Testing various components of proximal pre-BCR signaling, we found that an incremental increase of SYK tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive SYK was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Using chimeric PECAM1, CD300A and LAIR1 receptor decoys and a novel small molecule inhibitor of INPP5D, we demonstrated that pharmacological hyperactivation of pre-BCR signaling and engagement of negative B cell selection represents a promising new strategy to overcome drug-resistance in human Ph+ ALL.

Conclusion: These results indicated that inhibitory receptors and downstream phosphatases are critical regulators of pre-BCR signaling strength in Ph+ ALL, and identified targeting hyperactivation of pre-BCR signaling as a potential novel class of therapeutic strategy.

Note: This abstract was not presented at the meeting.

Citation Format: Zhengshan Chen, Seyedmehdi Shojaee, Maike Buchner, Huimin Geng, Jae Woong Lee, Lars Klemm, Eugene Park, Ying Xim Tan, Anne Satterthwaite, Elisabeth Paietta, Stephen P. Hunger, Mignon L. Loh, Jae U. Jung, John E. Coligan, Silvia Bolland, Tak W. Mak, Andre Limnander, Hassan Jumaa, Michael Reth, Arthur Weiss, Clifford A. Lowell, Markus Müschen. Signaling thresholds and negative B cell selection in acute lymphoblastic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2075. doi:10.1158/1538-7445.AM2015-2075

Erk Negative Feedback Control Enables Pre-B Cell Transformation and Represents a Therapeutic Target in Acute Lymphoblastic Leukemia

Studying mechanisms of malignant transformation of human pre-B cells, we found that acute activation of oncogenes induced immediate cell death in the vast majority of cells. Few surviving pre-B cell clones had acquired permissiveness to oncogenic signaling by strong activation of negative feedback regulation of Erk signaling. Studying negative feedback regulation of Erk in genetic experiments at three different levels, we found that Spry2, Dusp6, and Etv5 were essential for oncogenic transformation in mouse models for pre-B acute lymphoblastic leukemia (ALL). Interestingly, a small molecule inhibitor of DUSP6 selectively induced cell death in patient-derived pre-B ALL cells and overcame conventional mechanisms of drug-resistance.

Mechanisms of clonal evolution in childhood acute lymphoblastic leukemia

Childhood acute lymphoblastic leukemia (ALL) can often be traced to a pre-leukemic clone carrying a prenatal genetic lesion. Postnatally acquired mutations then drive clonal evolution toward overt leukemia. The enzymes RAG1-RAG2 and AID, which diversify immunoglobulin-encoding genes, are strictly segregated in developing cells during B lymphopoiesis and peripheral mature B cells, respectively. Here we identified small pre-BII cells as a natural subset with increased genetic vulnerability owing to concurrent activation of these enzymes. Consistent with epidemiological findings on childhood ALL etiology, susceptibility to genetic lesions during B lymphopoiesis at the transition from the large pre-BII cell stage to the small pre-BII cell stage was exacerbated by abnormal cytokine signaling and repetitive inflammatory stimuli. We demonstrated that AID and RAG1-RAG2 drove leukemic clonal evolution with repeated exposure to inflammatory stimuli, paralleling chronic infections in childhood.

Signalling thresholds and negative B-cell selection in acute lymphoblastic leukaemia

B cells are selected for an intermediate level of B-cell antigen receptor (BCR) signalling strength: attenuation below minimum (for example, non-functional BCR) or hyperactivation above maximum (for example, self-reactive BCR) thresholds of signalling strength causes negative selection. In ∼25% of cases, acute lymphoblastic leukaemia (ALL) cells carry the oncogenic BCR-ABL1 tyrosine kinase (Philadelphia chromosome positive), which mimics constitutively active pre-BCR signalling. Current therapeutic approaches are largely focused on the development of more potent tyrosine kinase inhibitors to suppress oncogenic signalling below a minimum threshold for survival. We tested the hypothesis that targeted hyperactivation--above a maximum threshold--will engage a deletional checkpoint for removal of self-reactive B cells and selectively kill ALL cells. Here we find, by testing various components of proximal pre-BCR signalling in mouse BCR-ABL1 cells, that an incremental increase of Syk tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive Syk was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in ALL cells. Unlike normal pre-B cells, patient-derived ALL cells express the inhibitory receptors PECAM1, CD300A and LAIR1 at high levels. Genetic studies revealed that Pecam1, Cd300a and Lair1 are critical to calibrate oncogenic signalling strength through recruitment of the inhibitory phosphatases Ptpn6 (ref. 7) and Inpp5d (ref. 8). Using a novel small-molecule inhibitor of INPP5D (also known as SHIP1), we demonstrated that pharmacological hyperactivation of SYK and engagement of negative B-cell selection represents a promising new strategy to overcome drug resistance in human ALL.

Identification of FOXM1 as a therapeutic target in B-cell lineage acute lymphoblastic leukaemia

Despite recent advances in the cure rate of acute lymphoblastic leukaemia (ALL), the prognosis for patients with relapsed ALL remains poor. Here we identify FOXM1 as a candidate responsible for an aggressive clinical course. We show that FOXM1 levels peak at the pre-B-cell receptor checkpoint but are dispensable for normal B-cell development. Compared with normal B-cell populations, FOXM1 levels are 2- to 60-fold higher in ALL cells and are predictive of poor outcome in ALL patients. FOXM1 is negatively regulated by FOXO3A, supports cell survival, drug resistance, colony formation and proliferation in vitro, and promotes leukemogenesis in vivo. Two complementary approaches of pharmacological FOXM1 inhibition-(i) FOXM1 transcriptional inactivation using the thiazole antibiotic thiostrepton and (ii) an FOXM1 inhibiting ARF-derived peptide-recapitulate the findings of genetic FOXM1 deletion. Taken together, our data identify FOXM1 as a novel therapeutic target, and demonstrate feasibility of FOXM1 inhibition in ALL.

Small-molecule inhibition of CBP/catenin interactions eliminates drug-resistant clones in acute lymphoblastic leukemia

Drug resistance in acute lymphoblastic leukemia (ALL) remains a major problem warranting new treatment strategies. Wnt/catenin signaling is critical for the self-renewal of normal hematopoietic progenitor cells. Deregulated Wnt signaling is evident in chronic and acute myeloid leukemia; however, little is known about ALL. Differential interaction of catenin with either the Kat3 coactivator CREBBP (CREB-binding protein (CBP)) or the highly homologous EP300 (p300) is critical to determine divergent cellular responses and provides a rationale for the regulation of both proliferation and differentiation by the Wnt signaling pathway. Usage of the coactivator CBP by catenin leads to transcriptional activation of cassettes of genes that are involved in maintenance of progenitor cell self-renewal. However, the use of the coactivator p300 leads to activation of genes involved in the initiation of differentiation. ICG-001 is a novel small-molecule modulator of Wnt/catenin signaling, which specifically binds to the N-terminus of CBP and not p300, within amino acids 1-110, thereby disrupting the interaction between CBP and catenin. Here, we report that selective disruption of the CBP/β- and γ-catenin interactions using ICG-001 leads to differentiation of pre-B ALL cells and loss of self-renewal capacity. Survivin, an inhibitor-of-apoptosis protein, was also downregulated in primary ALL after treatment with ICG-001. Using chromatin immunoprecipitation assay, we demonstrate occupancy of the survivin promoter by CBP that is decreased by ICG-001 in primary ALL. CBP mutations have been recently identified in a significant percentage of ALL patients, however, almost all of the identified mutations reported occur C-terminal to the binding site for ICG-001. Importantly, ICG-001, regardless of CBP mutational status and chromosomal aberration, leads to eradication of drug-resistant primary leukemia in combination with conventional therapy in vitro and significantly prolongs the survival of NOD/SCID mice engrafted with primary ALL. Therefore, specifically inhibiting CBP/catenin transcription represents a novel approach to overcome relapse in ALL.

AID downregulation is a novel function of the DNMT inhibitor 5-aza-deoxycytidine

Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutation (SHM) and class switch recombination (CSR) in immunoglobulin genes. However, AID can also cause mutations in host genes and contribute to cancer progression and drug resistance. In this study, molecular docking showed the interaction of free 5-aza-CdR and Zebularine (Zeb) with AID. However, only 5-aza-CdR-incorporated ssDNA bound to the active site of AID and inhibited AID expression through proteasomal degradation. 5-aza-CdR demonstrated cytotoxicity against AID-positive and -negative hematopoietic cancer cells. In contrast, Zeb exhibited a cytotoxic effect only in AID-negative cells due to its inability to inhibit AID expression. This differential effect might be due to the DNMT1 stabilization induced by AID, thus restricting the ability of Zeb to deplete DNMT1 and induce tumor suppressor genes (TSGs), such as p21, in AID-positive cells. Moreover, the in vivo anticancer effect of 5-aza-CdR but not Zeb in AID-positive hematopoietic cancer cells was demonstrated. The study not only displays the association of AID and DNMT1 and identifies a novel biological function of AID, but also provides novel information regarding the use of DNMT inhibitors to treat AID-positive hematopoietic cancers.

Vitamin D protects acute lymphoblastic leukemia cells from dexamethasone

Vitamin D deficiency has been linked with increased cancer risk, and vitamin D has been shown to be cytotoxic to some cancer cells in vitro. In the present study we evaluated whether vitamin D would have antiproliferative or cytotoxic effects on human pre-B acute lymphoblastic leukemia cells. Contrary to our hypotheses, calcitriol, the active form of vitamin D, had no effect on leukemia cell proliferation. Calcitriol actually had a modest effect to impair dexamethasone cytotoxicity and induction of apoptosis. Further studies are needed to evaluate the effects of vitamin D on leukemia cells in vivo.

Abstract 4684: Vitamin D may impair glucocorticoid cytotoxicity in acute lymphoblastic leukemia cells

Vitamin D deficiency has been associated with increased incidence and mortality in several types of cancer. Vitamin D has been shown to be cytotoxic to cancer cells in vitro, and may act synergistically with chemotherapy to kill cancer cells. Although vitamin D deficiency is common in children with cancer, the clinical implications of this remain unclear. Since acute lymphoblastic leukemia (ALL) is the most common type of cancer in children and relapsed leukemia remains a major problem, the present study was designed to test our hypothesis that active Vitamin D (Calcitriol) would induce apoptosis in ALL cells and potentiate the effect of chemotherapy on ALL cells.

We cultured several human ALL cell lines and primary human ALL cells with and without Calcitriol, and quantified survival using trypan blue exclusion. Contrary to our hypothesis, Calcitriol did not negatively affect the viability or impair proliferation in the human ALL cell lines RS4; 11, SD1, BV173, or RCH-ACV, or in primary human ALL cells US7 or TXL 2. We then tested whether Calcitriol would synergize with dexamethasone, a glucocorticoid used almost universally in the treatment of childhood ALL. Again, contrary to our hypothesis, we found that rather than augment dexamethasone-induced cytotoxicity, supraphysiological doses of Calcitriol protected RS4;11 cells from the cytotoxic effect of 72 hour exposure to 100 nM dexamethasone (dexamethasone alone: 10±5% of initially plated cells viable; dexamethasone plus 300 nM Calcitriol: 27±14% of initially plated cells viable, n=4, p=0.04). More physiological doses of Calcitriol (10nM) showed similar protection (12.5±5% vs 31.5±19% of initially plated cells were viable at 72 hrs, n=4, p=0.05). Preliminary flow cytometry using Annexin/7AAD suggests that Calcitriol decreases dexamethasone-induced apoptosis in RS4;11 cells (dexamethasone alone: 94% of cells in apoptosis; dexamethasone plus 100 nM Calcitriol: 75% of cells in apoptosis, n=1)

In summary, the active form of vitamin D appears to protect ALL cells from dexamethasone. This finding could be important, since glucocorticoid sensitivity of ALL cells is highly predictive of outcome and likelihood of relapse. Our findings could have major health implications given the high prevalence of Vitamin D deficiency in children and adolescents worldwide.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4684. doi:10.1158/1538-7445.AM2011-4684

Diet-induced obesity accelerates acute lymphoblastic leukemia progression in two murine models

Obesity is associated with an increased incidence of many cancers, including leukemia, although it is unknown whether leukemia incidence is increased directly by obesity or rather by associated genetic, lifestyle, health, or socioeconomic factors. We developed animal models of obesity and leukemia to test whether obesity could directly accelerate acute lymphoblastic leukemia (ALL) using BCR/ABL transgenic and AKR/J mice weaned onto a high-fat diet. Mice were observed until development of progressive ALL. Although obese and control BCR/ABL mice had similar median survival, older obese mice had accelerated ALL onset, implying a time-dependent effect of obesity on ALL. Obese AKR mice developed ALL significantly earlier than controls. The effect of obesity was not explained by WBC count, thymus/spleen weight, or ALL phenotype. However, obese AKR mice had higher leptin, insulin, and interleukin-6 levels than controls, and these obesity-related hormones all have potential roles in leukemia pathogenesis. In conclusion, obesity directly accelerates presentation of ALL, likely by increasing the risk of an early event in leukemogenesis. This is the first study to show that obesity can directly accelerate the progression of ALL. Thus, the observed associations between obesity and leukemia incidence are likely to be directly related to biological effects of obesity.

BCL6 is critical for the development of a diverse primary B cell repertoire

BCL6 protects germinal center (GC) B cells against DNA damage-induced apoptosis during somatic hypermutation and class-switch recombination. Although expression of BCL6 was not found in early IL-7-dependent B cell precursors, we report that IL-7Ralpha-Stat5 signaling negatively regulates BCL6. Upon productive VH-DJH gene rearrangement and expression of a mu heavy chain, however, activation of pre-B cell receptor signaling strongly induces BCL6 expression, whereas IL-7Ralpha-Stat5 signaling is attenuated. At the transition from IL-7-dependent to -independent stages of B cell development, BCL6 is activated, reaches expression levels resembling those in GC B cells, and protects pre-B cells from DNA damage-induced apoptosis during immunoglobulin (Ig) light chain gene recombination. In the absence of BCL6, DNA breaks during Ig light chain gene rearrangement lead to excessive up-regulation of Arf and p53. As a consequence, the pool of new bone marrow immature B cells is markedly reduced in size and clonal diversity. We conclude that negative regulation of Arf by BCL6 is required for pre-B cell self-renewal and the formation of a diverse polyclonal B cell repertoire.

The B cell mutator AID promotes B lymphoid blast crisis and drug resistance in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is induced by BCR-ABL1 and can be effectively treated for many years with Imatinib until leukemia cells acquire drug resistance through BCR-ABL1 mutations and progress into fatal B lymphoid blast crisis (LBC). Despite its clinical significance, the mechanism of progression into LBC is unknown. Here, we show that LBC but not CML cells express the B cell-specific mutator enzyme AID. We demonstrate that AID expression in CML cells promotes overall genetic instability by hypermutation of tumor suppressor and DNA repair genes. Importantly, our data uncover a causative role of AID activity in the acquisition of BCR-ABL1 mutations leading to Imatinib resistance, thus providing a rationale for the rapid development of drug resistance and blast crisis progression.

Pre-B cell receptor-mediated cell cycle arrest in Philadelphia chromosome-positive acute lymphoblastic leukemia requires IKAROS function

B cell lineage acute lymphoblastic leukemia (ALL) arises in virtually all cases from B cell precursors that are arrested at pre–B cell receptor–dependent stages. The Philadelphia chromosome–positive (Ph⁺) subtype of ALL accounts for 25–30% of cases of adult ALL, has the most unfavorable clinical outcome among all ALL subtypes and is defined by the oncogenic BCR-ABL1 kinase and deletions of the IKAROS gene in >80% of cases. Here, we demonstrate that the pre–B cell receptor functions as a tumor suppressor upstream of IKAROS through induction of cell cycle arrest in Ph⁺ ALL cells. Pre–B cell receptor–mediated cell cycle arrest in Ph⁺ ALL cells critically depends on IKAROS function, and is reversed by coexpression of the dominant-negative IKAROS splice variant IK6. IKAROS also promotes tumor suppression through cooperation with downstream molecules of the pre–B cell receptor signaling pathway, even if expression of the pre–B cell receptor itself is compromised. In this case, IKAROS redirects oncogenic BCR-ABL1 tyrosine kinase signaling from SRC kinase-activation to SLP65, which functions as a critical tumor suppressor downstream of the pre–B cell receptor. These findings provide a rationale for the surprisingly high frequency of IKAROS deletions in Ph⁺ ALL and identify IKAROS-mediated cell cycle exit as the endpoint of an emerging pathway of pre–B cell receptor–mediated tumor suppression.