The eukaryotic translation initiation factor eIF4E reprograms alternative splicing.

Aberrant splicing is typically attributed to splice-factor (SF) mutation and contributes to malignancies including acute myeloid leukemia (AML). Here, we discovered a mutation-independent means to extensively reprogram alternative splicing (AS). We showed that the dysregulated expression of eukaryotic translation initiation factor eIF4E elevated selective splice-factor production, thereby impacting multiple spliceosome complexes, including factors mutated in AML such as SF3B1 and U2AF1. These changes generated a splicing landscape that predominantly supported altered splice-site selection for ~800 transcripts in cell lines and ~4,600 transcripts in specimens from high-eIF4E AML patients otherwise harboring no known SF mutations. Nuclear RNA immunoprecipitations, export assays, polysome analyses, and mutational studies together revealed that eIF4E primarily increased SF production via its nuclear RNA export activity. By contrast, eIF4E dysregulation did not induce known SF mutations or alter spliceosome number. eIF4E interacted with the spliceosome and some pre-mRNAs, suggesting its direct involvement in specific splicing events. eIF4E induced simultaneous effects on numerous SF proteins, resulting in a much larger range of splicing alterations than in the case of mutation or dysregulation of individual SFs and providing a novel paradigm for splicing control and dysregulation.

Immune Checkpoint Inhibitor Therapy and Associations with Clonal Hematopoiesis.

Cancer cohorts are now known to be associated with increased rates of clonal hematopoiesis (CH). We sort to characterize the hematopoietic compartment of patients with melanoma and non-small cell lung cancer (NSCLC) given our recent population level analysis reporting evolving rates of secondary leukemias. The advent of immune checkpoint blockade (ICB) has dramatically changed our understanding of cancer biology and has altered the standards of care for patients. However, the impact of ICB on hematopoietic myeloid clonal expansion remains to be determined. We studied if exposure to ICB therapy affects hematopoietic clonal architecture and if their evolution contributed to altered hematopoiesis. Blood samples from patients with melanoma and NSCLC (n = 142) demonstrated a high prevalence of CH. Serial samples (or post ICB exposure samples; n = 25) were evaluated in melanoma and NSCLC patients. Error-corrected sequencing of a targeted panel of genes recurrently mutated in CH was performed on peripheral blood genomic DNA. In serial sample analysis, we observed that mutations in DNMT3A and TET2 increased in size with longer ICB exposures in the melanoma cohort. We also noted that patients with larger size DNMT3A mutations with further post ICB clone size expansion had longer durations of ICB exposure. All serial samples in this cohort showed a statistically significant change in VAF from baseline. In the serial sample analysis of NSCLC patients, we observed similar epigenetic expansion, although not statistically significant. Our study generates a hypothesis for two important questions: (a) Can DNMT3A or TET2 CH serve as predictors of a response to ICB therapy and serve as a novel biomarker of response to ICB therapy? (b) As ICB-exposed patients continue to live longer, the myeloid clonal expansion may portend an increased risk for subsequent myeloid malignancy development. Until now, the selective pressure of ICB/T-cell activating therapies on hematopoietic stem cells were less known and we report preliminary evidence of clonal expansion in epigenetic modifier genes (also referred to as inflammatory CH genes).

BCL6 maintains survival and self-renewal of primary human acute myeloid leukemia cells.

B-cell lymphoma 6 (BCL6) is a transcription repressor and proto-oncogene that plays a crucial role in the innate and adaptive immune system and lymphoid neoplasms. However, its role in myeloid malignancies remains unclear. Here, we explored the role of BCL6 in acute myeloid leukemia (AML). BCL6 was expressed at variable and often high levels in AML cell lines and primary AML samples. AMLs with higher levels of BCL6 were generally sensitive to treatment with BCL6 inhibitors, with the exception of those with monocytic differentiation. Gene expression profiling of AML cells treated with a BCL6 inhibitor revealed induction of BCL6-repressed target genes and transcriptional programs linked to DNA damage checkpoints and downregulation of stem cell genes. Ex vivo treatment of primary AML cells with BCL6 inhibitors induced apoptosis and decreased colony-forming capacity, which correlated with the levels of BCL6 expression. Importantly, inhibition or knockdown of BCL6 in primary AML cells resulted in a significant reduction of leukemia-initiating capacity in mice, suggesting ablation of leukemia repopulating cell functionality. In contrast, BCL6 knockout or inhibition did not suppress the function of normal hematopoietic stem cells. Treatment with cytarabine further induced BCL6 expression, and the levels of BCL6 induction were correlated with resistance to cytarabine. Treatment of AML patient-derived xenografts with BCL6 inhibitor plus cytarabine suggested enhanced antileukemia activity with this combination. Hence, pharmacologic inhibition of BCL6 might provide a novel therapeutic strategy for ablation of leukemia-repopulating cells and increased responsiveness to chemotherapy.

2021 Update on MRD in acute myeloid leukemia: a consensus document from the European LeukemiaNet MRD Working Party.

Measurable residual disease (MRD) is an important biomarker in acute myeloid leukemia (AML) that is used for prognostic, predictive, monitoring, and efficacy-response assessments. The European LeukemiaNet (ELN) MRD Working Party evaluated standardization and harmonization of MRD in an ongoing manner and has updated the 2018 ELN MRD recommendations based on significant developments in the field. New and revised recommendations were established during in-person and online meetings, and a 2-stage Delphi poll was conducted to optimize consensus. All recommendations are graded by levels of evidence and agreement. Major changes include technical specifications for next-generation sequencing-based MRD testing and integrative assessments of MRD irrespective of technology. Other topics include use of MRD as a prognostic and surrogate end point for drug testing; selection of the technique, material, and appropriate time points for MRD assessment; and clinical implications of MRD assessment. In addition to technical recommendations for flow- and molecular-MRD analysis, we provide MRD thresholds and define MRD response, and detail how MRD results should be reported and combined if several techniques are used. MRD assessment in AML is complex and clinically relevant, and standardized approaches to application, interpretation, technical conduct, and reporting are of critical importance.

Small-molecule inhibitors targeting Polycomb repressive complex 1 RING domain.

Polycomb repressive complex 1 (PRC1) is an essential chromatin-modifying complex that monoubiquitinates histone H2A and is involved in maintaining the repressed chromatin state. Emerging evidence suggests PRC1 activity in various cancers, rationalizing the need for small-molecule inhibitors with well-defined mechanisms of action. Here, we describe the development of compounds that directly bind to RING1B-BMI1, the heterodimeric complex constituting the E3 ligase activity of PRC1. These compounds block the association of RING1B-BMI1 with chromatin and inhibit H2A ubiquitination. Structural studies demonstrate that these inhibitors bind to RING1B by inducing the formation of a hydrophobic pocket in the RING domain. Our PRC1 inhibitor, RB-3, decreases the global level of H2A ubiquitination and induces differentiation in leukemia cell lines and primary acute myeloid leukemia (AML) samples. In summary, we demonstrate that targeting the PRC1 RING domain with small molecules is feasible, and RB-3 represents a valuable chemical tool to study PRC1 biology.

Single-cell RNA sequencing reveals a signature of sexual commitment in malaria parasites.

Pathogens have to balance transmission with persistence. For Plasmodium falciparum, the most widespread and virulent malaria parasite, persistence within its human host requires continuous asexual replication within red blood cells, while its mosquito-borne transmission depends on intra-erythrocytic differentiation into non-replicating sexual stages called gametocytes. Commitment to either fate is determined during the preceding cell cycle that begins with invasion by a single, asexually committed merozoite and ends, 48 hours later, with a schizont releasing newly formed merozoites, all committed to either continued asexual replication or differentiation into gametocytes. Sexual commitment requires the transcriptional activation of ap2-g (PF3D7_1222600), the master regulator of sexual development, from an epigenetically silenced state during asexual replication. AP2-G expression during this 'commitment cycle' prepares gene expression in nascent merozoites to initiate sexual development through a hitherto unknown mechanism. To maintain a persistent infection, the expression of ap2-g is limited to a sub-population of parasites (1-30%, depending on genetic background and growth conditions). As sexually committed schizonts comprise only a sub-population and are morphologically indistinguishable from their asexually committed counterparts, defining their characteristic gene expression has been difficult using traditional, bulk transcriptome profiling. Here we use highly parallel, single-cell RNA sequencing of malaria cultures undergoing sexual commitment to determine the transcriptional changes induced by AP2-G within this sub-population. By analysing more than 18,000 single parasite transcriptomes from a conditional AP2-G knockdown line and NF54 wild-type parasites at multiple stages of development, we show that sexually committed, AP2-G+ mature schizonts specifically upregulate additional regulators of gene expression, including other AP2 transcription factors, histone-modifying enzymes, and regulators of nucleosome positioning. These epigenetic regulators may act to facilitate the expression and/or repression of genes that are necessary for the initiation of gametocyte development in the subsequent cell cycle.

The epichaperome is an integrated chaperome network that facilitates tumour survival.

Transient, multi-protein complexes are important facilitators of cellular functions. This includes the chaperome, an abundant protein family comprising chaperones, co-chaperones, adaptors, and folding enzymes-dynamic complexes of which regulate cellular homeostasis together with the protein degradation machinery. Numerous studies have addressed the role of chaperome members in isolation, yet little is known about their relationships regarding how they interact and function together in malignancy. As function is probably highly dependent on endogenous conditions found in native tumours, chaperomes have resisted investigation, mainly due to the limitations of methods needed to disrupt or engineer the cellular environment to facilitate analysis. Such limitations have led to a bottleneck in our understanding of chaperome-related disease biology and in the development of chaperome-targeted cancer treatment. Here we examined the chaperome complexes in a large set of tumour specimens. The methods used maintained the endogenous native state of tumours and we exploited this to investigate the molecular characteristics and composition of the chaperome in cancer, the molecular factors that drive chaperome networks to crosstalk in tumours, the distinguishing factors of the chaperome in tumours sensitive to pharmacologic inhibition, and the characteristics of tumours that may benefit from chaperome therapy. We find that under conditions of stress, such as malignant transformation fuelled by MYC, the chaperome becomes biochemically 'rewired' to form a network of stable, survival-facilitating, high-molecular-weight complexes. The chaperones heat shock protein 90 (HSP90) and heat shock cognate protein 70 (HSC70) are nucleating sites for these physically and functionally integrated complexes. The results indicate that these tightly integrated chaperome units, here termed the epichaperome, can function as a network to enhance cellular survival, irrespective of tissue of origin or genetic background. The epichaperome, present in over half of all cancers tested, has implications for diagnostics and also provides potential vulnerability as a target for drug intervention.

Analysis of Plant-Plant Interactions Reveals the Presence of Potent Antileukemic Compounds.

A method to identify anticancer compounds in plants was proposed based on the hypothesis that these compounds are primarily present in plants to provide them with an ecological advantage over neighboring plants and other competitors. According to this view, identifying plants that contain compounds that inhibit or interfere with the development of other plant species may facilitate the discovery of novel anticancer agents. The method was developed and tested using Magnolia grandiflora, Gynoxys verrucosa, Picradeniopsis oppositifolia, and Hedyosmum racemosum, which are plant species known to possess compounds with cytotoxic activities. Plant extracts were screened for growth inhibitory activity, and then a thin-layer chromatography bioautography assay was conducted. This located the major antileukemic compounds 1, 2, 4, and 5 in the extracts. Once the active compounds were located, they were extracted and purified, and their structures were determined. The growth inhibitory activity of the purified compounds showed a significant correlation with their antileukemic activity. The proposed approach is rapid, inexpensive, and can easily be implemented in areas of the world with high biodiversity but with less access to advanced facilities and biological assays.

Chaperome heterogeneity and its implications for cancer study and treatment.

The chaperome is the collection of proteins in the cell that carry out molecular chaperoning functions. Changes in the interaction strength between chaperome proteins lead to an assembly that is functionally and structurally distinct from each constituent member. In this review, we discuss the epichaperome, the cellular network that forms when the chaperome components of distinct chaperome machineries come together as stable, functionally integrated, multimeric complexes. In tumors, maintenance of the epichaperome network is vital for tumor survival, rendering them vulnerable to therapeutic interventions that target critical epichaperome network components. We discuss how the epichaperome empowers an approach for precision medicine cancer trials where a new target, biomarker, and relevant drug candidates can be correlated and integrated. We introduce chemical biology methods to investigate the heterogeneity of the chaperome in a given cellular context. Lastly, we discuss how ligand-protein binding kinetics are more appropriate than equilibrium binding parameters to characterize and unravel chaperome targeting in cancer and to gauge the selectivity of ligands for specific tumor-associated chaperome pools.

Chaperome Networks - Redundancy and Implications for Cancer Treatment.

The chaperome is a large family of proteins composed of chaperones, co-chaperones and a multitude of other factors. Elegant studies in yeast and other organisms have paved the road to how we currently understand the complex organization of this large family into protein networks. The goal of this chapter is to provide an overview of chaperome networks in cancer cells, with a focus on two cellular states defined by chaperome network organization. One state characterized by chaperome networks working in isolation and with little overlap, contains global chaperome networks resembling those of normal, non-transformed, cells. We propose that in this state, redundancy in chaperome networks results in a tumor type unamenable for single-agent chaperome therapy. The second state comprises chaperome networks interconnected in response to cellular stress, such as MYC hyperactivation. This is a state where no redundant pathways can be deployed, and is a state of vulnerability, amenable for chaperome therapy. We conclude by proposing a change in how we discover and implement chaperome inhibitor strategies, and suggest an approach to chaperome therapy where the properties of chaperome networks, rather than genetics or client proteins, are used in chaperome inhibitor implementation.

Synthesis, Crystallography, and Anti-Leukemic Activity of the Amino Adducts of Dehydroleucodine.

Dehydroleucodine is a bioactive sesquiterpene lactone. Herein, four dehydroleucodine amino derivatives were synthesized using the amines proline, piperidine, morpholine, and tyramine, and spectroscopic methods and single-crystal X-ray diffraction unambiguously established their structures. The cytotoxic activity of these compounds was evaluated against eight acute myeloid leukemia cell lines, and their toxicity to peripheral blood mononuclear cells was also determined. The proline adduct was the most active compound, it showed anti-leukemic activity, upregulated heme oxygenase 1 (HMOX1) and the primary stress-inducible isoform of the heath shock 70 kDa protein 1 (HSPA1A), and downregulated NFkB1 transcription, it was also found to be about 270 times more water soluble than dehydroleucodine.

A novel tetrazole analogue of resveratrol is a potent anticancer agent.

A series of novel tetrazole analogues of resveratrol were synthesized and evaluated for their anti-leukemic activity against an extensive panel of human cancer cell lines and against the MV4-11 AML cell line. These molecules were designed as drug-like derivatives of the resveratrol analogue DMU-212 and its cyano derivatives. Four compounds 8g, 8h, 10a and 10b exhibited LD50 values of 4.60 µM, 0.02 µM, 1.46 µM, and 1.08 µM, respectively, against MV4-11 leukemia cells. The most potent compounds, 8h and 10b, were also found to be active against an extensive panel of human hematological and solid tumor cell lines; compound 8h was the most potent compound with GI50 values <10 nM against more than 90% of the human cancer cell lines in the 60-cell panel. Analogues 8g, 8h, 10a and 10b were also tested for their ability to inhibit the polymerization of tubulin, and compound 8h was found to be the most potent analogue. Molecular modeling studies demonstrated that 8h binds to the colchicine binding site on tubulin. Thus, compound 8h is considered to be a lead druglike molecule from this tetrazole series of compounds.

Parthenolide and DMAPT induce cell death in primitive CML cells through reactive oxygen species.

Tyrosine kinase inhibitors (TKI) have become a first-line treatment for chronic myeloid leuakemia (CML). TKIs efficiently target bulk CML cells; however, they are unable to eliminate the leukaemic stem cell (LSC) population that causes resistance and relapse in CML patients. In this study, we assessed the effects of parthenolide (PTL) and dimethyl amino parthenolide (DMAPT), two potent inhibitors of LSCs in acute myeloid leukaemia (AML), on CML bulk and CML primitive (CD34+ lin- ) cells. We found that both agents induced cell death in CML, while having little effect on the equivalent normal hematopoietic cells. PTL and DMAPT caused an increase in reactive oxygen species (ROS) levels and inhibited NF-κB activation. PTL and DMAPT inhibited cell proliferation and induced cell cycle arrest in G0 and G2 phases. Furthermore, we found cell cycle inhibition to correlate with down-regulation of cyclin D1 and cyclin A. In summary, our study shows that PTL and DMAPT have a strong inhibitory effect on CML cells. Given that cell cycle arrest was not dependent on ROS induction, we speculate that this effect could be a direct consequence of NF-κB inhibition and if this mechanism was to be evaded, PTL and DMAPT induced cell death would be potentiated.

Phase I trial of plerixafor combined with decitabine in newly diagnosed older patients with acute myeloid leukemia.

Acute myeloid leukemia carries a dismal prognosis in older patients. The objective of this study was to investigate the safety and efficacy of decitabine combined with the CXCR4 antagonist plerixafor in newly diagnosed older patients with acute myeloid leukemia and to evaluate the effects of plerixafor on leukemia stem cells. Patients were treated with monthly cycles of decitabine 20 mg/m2 days 1-10 and escalating doses of plerixafor (320-810 mcg/kg) days 1-5. Sixty-nine patients were treated, with an overall response rate of 43%. Adverse karyotype did not predict response (P=0.31). Prior hypomethylating agent treatment was the strongest independent predictor of adverse overall survival (hazard ratio 3.1; 95%CI: 1.3-7.3; P=0.008) and response (14% in previously treated patients, 46% in treatment naïve; P=0.002). As expected, the most common toxicities were myelosuppression and infection. Plerixafor induced mobilization of leukemia stem and progenitor cells, but did not cause clinically significant hyperleukocytosis. Reduction in leukemia stem cells appeared to correlate with duration of response. Plerixafor can be safely added to decitabine in poor-prognosis, elderly acute myeloid leukemia patients. The maximum tolerated dose of the combination was 810 mcg/kg. While mobilization of leukemia stem cells was observed in some patients, the clinical benefit of adding plerixafor was uncertain. This trial was registered at clinicaltrials.gov identifier: 01352650.

Minimal Residual Disease in Acute Myeloid Leukemia.

Monitoring measurable (minimal) residual disease (MRD) in acute myeloid leukemia (AML) has greatly increased our ability to assess chemosensisitivity to treatment as well as the duration of treatment responses. There is strong evidence to support its prognostic value for long-term outcomes at different time points and across assays and targets. It's role as a surrogate endpoint to define risk-adapted strategies is still under evaluation. In this chapter, we will discuss the definition of MRD in AML, the potential contribution of leukemia stem cells (LSCs) to MRD and we will review all the current approaches to assess residual disease including the 2018 European Leukemia Network (ELN) working group recommendations for MRD standardization in AML. In addition, a summary of MRD studies associated to prognosis will be described.

Dehydroleucodine, a Sesquiterpene Lactone from Gynoxys verrucosa, Demonstrates Cytotoxic Activity against Human Leukemia Cells.

The sesquiterpene lactones dehydroleucodine (1) and leucodine (2) were isolated from Gynoxys verrucosa, a species used in traditional medicine in southern Ecuador. The activity of these compounds was determined against eight acute myeloid leukemia (AML) cell lines and compared with their activity against normal peripheral blood mononuclear cells. Compound 1 showed cytotoxic activity against the tested cell lines, with LD50 values between 5.0 and 18.9 µM. Compound 2 was inactive against all of the tested cell lines, demonstrating that the exocyclic methylene in the lactone ring is required for cytotoxic activity. Importantly, compound 1 induced less toxicity to normal blood cells than to AML cell lines and was active against human AML cell samples from five patients, with an average LD50 of 9.4 µM. Mechanistic assays suggest that compound 1 has a similar mechanism of action to parthenolide (3). Although these compounds have significant structural differences, their lipophilic surface signatures show striking similarities.

Single-cell mass cytometry reveals intracellular survival/proliferative signaling in FLT3-ITD-mutated AML stem/progenitor cells.

Understanding the unique phenotypes and complex signaling pathways of leukemia stem cells (LSCs) will provide insights and druggable targets that can be used to eradicate acute myeloid leukemia (AML). Current work on AML LSCs is limited by the number of parameters that conventional flow cytometry (FCM) can analyze because of cell autofluorescence and fluorescent dye spectral overlap. Single-cell mass cytometry (CyTOF) substitutes rare earth elements for fluorophores to label antibodies, which allows measurements of up to 120 parameters in single cells without correction for spectral overlap. The aim of this study was the evaluation of intracellular signaling in antigen-defined stem/progenitor cell subsets in primary AML. CyTOF and conventional FCM yielded comparable results on LSC phenotypes defined by CD45, CD34, CD38, CD123, and CD99. Intracellular phosphoprotein responses to ex vivo cell signaling inhibitors and cytokine stimulation were assessed in myeloid leukemia cell lines and one primary AML sample. CyTOF and conventional FCM results were confirmed by western blotting. In the primary AML sample, we investigated the cell responses to ex vivo stimulation with stem cell factor and BEZ235-induced inhibition of PI3K and identified activation patterns in multiple PI3K downstream signaling pathways including p-4EBP1, p-AKT, and p-S6, particularly in CD34(+) subsets. We evaluated multiple signaling pathways in antigen-defined subpopulations in primary AML cells with FLT3-ITD mutations. The data demonstrated the heterogeneity of cell phenotype distribution and distinct patterns of signaling activation across AML samples and between AML and normal samples. The mTOR targets p-4EBP1 and p-S6 were exclusively found in FLT3-ITD stem/progenitor cells, but not in their normal counterparts, suggesting both as novel targets in FLT3 mutated AML. Our data suggest that CyTOF can identify functional signaling pathways in antigen-defined subpopulations in primary AML, which may provide a rationale for designing therapeutics targeting LSC-enriched cell populations.

Concise review: Leukemia stem cells in personalized medicine.

Despite increased comprehension of acute myelogenous leukemia (AML) pathogenesis, current treatment strategies have done little to improve upon standard induction chemotherapy to induce long-term remissions. Since the identification of the leukemic stem cell, efforts have been placed on identifying therapeutically actionable pathways that distinguish this increasingly important cellular compartment. With the advent of increased genome sequencing efforts and phenotypic characterization, opportunities for personalized treatment strategies are rapidly emerging. In this review, we highlight recent advances in the understanding of leukemic stem cell biology and their potential for translation into clinically relevant therapeutics. NF-kappa B activation, Bcl-2 expression, oxidative and metabolic state, and epigenetic modifications all bear their own clinical implications. With advancements in genetic, epigenetic, and metabolic profiling, personalized strategies may be feasible in the near future to improve outcomes for AML patients.

Understanding the interaction between leukaemia stem cells and their microenvironment to improve therapeutic approaches.

Although chemotherapeutic regimens can eliminate blasts in leukaemia patients, such therapies are associated with toxicity and often fail to eliminate all malignant cells resulting in disease relapse. Disease relapse has been attributed to the persistence of leukaemia cells in the bone marrow (BM) with the capacity to recapitulate disease; these cells are often referred to as leukaemia stem cells (LSCs). Although LSCs have distinct characteristics in terms of pathobiology and immunophenotype, they are still regulated by their interactions with the surrounding microenvironment. Thus, understanding the interaction between LSCs and their microenvironment is critical to identify effective therapies. To this end, there are numerous efforts to develop models to study such interactions. In this review, we will focus on the reciprocal interactions between LSCs and their milieu in the BM. Furthermore, we will highlight relevant therapies targeting these interactions and discuss some of the promising in vitro models designed to mimic such relationship. LINKED ARTICLES: This article is part of a themed issue on Cancer Microenvironment and Pharmacological Interventions. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.2/issuetoc.

Glucocorticoids paradoxically promote steroid resistance in B cell acute lymphoblastic leukemia through CXCR4/PLC signaling.

Glucocorticoid (GC) resistance in childhood relapsed B-cell acute lymphoblastic leukemia (B-ALL) represents an important challenge. Despite decades of clinical use, the mechanisms underlying resistance remain poorly understood. Here, we report that in B-ALL, GC paradoxically induce their own resistance by activating a phospholipase C (PLC)-mediated cell survival pathway through the chemokine receptor, CXCR4. We identify PLC as aberrantly activated in GC-resistant B-ALL and its inhibition is able to induce cell death by compromising several transcriptional programs. Mechanistically, dexamethasone (Dex) provokes CXCR4 signaling, resulting in the activation of PLC-dependent Ca2+ and protein kinase C signaling pathways, which curtail anticancer activity. Treatment with a CXCR4 antagonist or a PLC inhibitor improves survival of Dex-treated NSG mice in vivo. CXCR4/PLC axis inhibition significantly reverses Dex resistance in B-ALL cell lines (in vitro and in vivo) and cells from Dex resistant ALL patients. Our study identifies how activation of the PLC signalosome in B-ALL by Dex limits the upfront efficacy of this chemotherapeutic agent.