The identification and functional characterization of chemical modifications on an mRNA molecule, in particular N6-methyladenosine (m6A) modification, significantly broadened our understanding of RNA function and regulation. While interactions between RNA modifications and other RNA features have been proposed, direct evidence showing correlation is limited. Here, using Oxford Nanopore long-read direct RNA sequencing (dRNA-seq), we simultaneously interrogate the transcriptome and epitranscriptome of a human leukemia cell line to investigate the correlation between m6A modifications, mRNA abundance, mRNA stability, polyadenylation (poly(A)) tail length, and alternative splicing. High-quality dRNA-seq is important for unbiased and large-scale correlative analyses. Global assessments indicated a negative association between poly(A) tail length and mRNA abundance while uncovering pathway-specific responses upon depletion of the m6A-forming enzyme METTL3. Overall, our study presented a rich dRNA-seq data resource that has been validated and can be further exploited to inquire into the complexity of RNA modifications and potential interplays between RNA regulatory elements.

In the rapidly evolving landscape of medical research, the emergence of RNA-based therapeutics is paradigm shifting. It is mainly driven by the molecular adaptability and capacity to provide precision in targeting. The coronavirus disease 2019 pandemic crisis underscored the effectiveness of the mRNA therapeutic development platform and brought it to the forefront of RNA-based interventions. These RNA-based therapeutic approaches can reshape gene expression, manipulate cellular functions, and correct the aberrant molecular processes underlying various diseases. The new technologies hold the potential to engineer and deliver tailored therapeutic agents to tackle genetic disorders, cancers, and infectious diseases in a highly personalized and precisely tuned manner. The review discusses the most recent advancements in the field of mRNA therapeutics for cancer treatment, with a focus on the features of the most utilized RNA-based therapeutic interventions, current pre-clinical and clinical developments, and the remaining challenges in delivery strategies, effectiveness, and safety considerations.

Protein synthesis is frequently deregulated during tumorigenesis. However, the precise contexts of selective translational control and the regulators of such mechanisms in cancer is poorly understood. Here, we uncovered CNOT3, a subunit of the CCR4-NOT complex, as an essential modulator of translation in myeloid leukemia. Elevated CNOT3 expression correlates with unfavorable outcomes in patients with acute myeloid leukemia (AML). CNOT3 depletion induces differentiation and apoptosis and delayed leukemogenesis. Transcriptomic and proteomic profiling uncovers c-MYC as a critical downstream target which is translationally regulated by CNOT3. Global analysis of mRNA features demonstrates that CNOT3 selectively influences expression of target genes in a codon usage dependent manner. Furthermore, CNOT3 associates with the protein network largely consisting of ribosomal proteins and translation elongation factors in leukemia cells. Overall, our work elicits the direct requirement for translation efficiency in tumorigenesis and propose targeting the post-transcriptional circuitry via CNOT3 as a therapeutic vulnerability in AML.

RNA deadenylation, the process of shortening of the 3' poly(A) tail of an RNA molecule, is one of the key steps of post-transcriptional regulation of gene expression in eukaryotic cells. PAN2/3 and CCR4-NOT (CNOT) are the two dominant RNA deadenylation complexes, which play central roles in mediating mRNA decay and translation. While degradation is the final fate of virtually all RNAs in their life cycles, selection of RNA targets as well as control of the rate and timing of RNA decay, in coordination with other molecular pathways, including translation, can be modulated in certain contexts. Such regulation influences cell growth, proliferation, and differentiation at the cellular level; and contributes to establish polarity and regulate signaling at the tissue level. Dysregulation of deadenylation processes have also been implicated in human diseases ranging from cardiac diseases and neurodevelopmental disorders to cancers. In this review, we will discuss mechanisms of gene expression control mediated by the RNA deadenylation complexes and highlight relevant evidence supporting the emerging roles of RNA deadenylation and its regulatory proteins during development and in diseases. A systemic understanding of these mechanisms will be a critical foundation for development of effective strategies to therapeutically target them.

Multiple myeloma (MM) is an incurable malignancy of plasma cells. To identify targets for MM immuno-therapy, we develop an integrated pipeline based on mass spectrometry analysis of seven MM cell lines and RNA sequencing (RNA-seq) from 900+ patients. Starting from 4,000+ candidates, we identify the most highly expressed cell surface proteins. We annotate candidate protein expression in many healthy tissues and validate the expression of promising targets in 30+ patient samples with relapsed/refractory MM, as well as in primary healthy hematopoietic stem cells and T cells by flow cytometry. Six candidates (ILT3, SEMA4A, CCR1, LRRC8D, FCRL3, IL12RB1) and B cell maturation antigen (BCMA) present the most favor-able profile in malignant and healthy cells. We develop a bispecific T cell engager targeting ILT3 that shows potent killing effects in vitro and decreased tumor burden and prolonged mice survival in vivo, suggesting therapeutic relevance. Our study uncovers MM-associated antigens that hold great promise for immune -based therapies of MM.

Tissue homeostasis is maintained after stress by engaging and activating the hematopoietic stem and progenitor compartments in the blood. Hematopoietic stem cells (HSCs) are essential for long-term repopulation after secondary transplantation. Here, using a conditional knockout mouse model, we revealed that the RNA-binding protein SYNCRIP is required for maintenance of blood homeostasis especially after regenerative stress due to defects in HSCs and progenitors. Mechanistically, we find that SYNCRIP loss results in a failure to maintain proteome homeostasis that is essential for HSC maintenance. SYNCRIP depletion results in increased protein synthesis, a dysregulated epichaperome, an accumulation of misfolded proteins and induces endoplasmic reticulum stress. Additionally, we find that SYNCRIP is required for translation of CDC42 RHO-GTPase, and loss of SYNCRIP results in defects in polarity, asymmetric segregation, and dilution of unfolded proteins. Forced expression of CDC42 recovers polarity and in vitro replating activities of HSCs. Taken together, we uncovered a post-transcriptional regulatory program that safeguards HSC self-renewal capacity and blood homeostasis.

Regulation of gene expression at the RNA level is an important regulatory mechanism in cancer. However, posttranscriptional molecular pathways underlying tumorigenesis remain largely unexplored. In this study, we uncovered a functional axis consisting of microRNA (miR)-148a-3p, RNA helicase DDX6, and its downstream target thioredoxininteracting protein (TXNIP) in acute myeloid leukemia (AML). Using a DROSHA-knockout cell system to evaluate miR-mediated gene expression control, we comprehensively profiled putative transcripts regulated by miR-148a-3p and identified DDX6 as a direct target of miR148a-3p in AML cells. DDX6 depletion induced cell cycle arrest, apoptosis, and differentiation, although delaying leukemia development in vivo. Genome-wide assessment of DDX6-binding transcripts and gene expression profiling of DDX6-depleted cells revealed TXNIP, a tumor suppressor, as the functional downstream target of DDX6. Overall, our study identified DDX6 as a posttranscriptional regulator that is required for AML survival. We proposed the regulatory link between miR-148a-3p and DDX6 as a potential therapeutic target in leukemia.

RNA modifications play an important role in various cancers including blood cancers by controlling gene expression programs critical for survival, proliferation and differentiation of cancer cells. While hundreds of RNA modifications have been identified, many have not been functionally characterized. With development of enabling technologies to identify and map RNA modifications, tremendous advancement has been made in our understanding of the biological functions of these molecular markers in diverse cellular contexts. In the last 5 years, N6-methyladenosine (m6A), the most prevalent internal mRNA modification, has been extensively implicated in many facets of leukemogenesis. Other types of RNA modifications are also involved in the regulation of cell fate decisions and tumorigenesis. Here, we summarize existing knowledge and recent discoveries regarding the role of RNA modifications in leukemia. We choose to highlight cutting-edge techniques to characterize and profile RNA modifications while discussing critical functions of key modifiers and regulatory mechanisms in the pathogenesis of hematological malignancies and touch on therapeutic strategies targeting RNA modifications. These important advancements in the field will continue to foster a strong foundation for the development of innovative treatments for hematological malignancies.

Post-transcriptional RNA modifications determine RNA fate by influencing numerous processes such as translation, decay and localization. One of the most abundant RNA modifications is N6-methyladenoside (m6A), which has been shown to be important in healthy as well as malignant hematopoiesis. Several proteins representing key players in m6A RNA biology, such as m6A writers, erasers and readers, were recently reported to be essential for hematopoietic stem cell (HSC) function. In leukemia, expression of m6A regulators has been shown to be increased, opening up potential opportunities for therapeutic exploitation by targeting them in blood malignancies. These recent discoveries were the focus of the Fall 2021 International Society for Experimental Hematology New Investigators webinar. We review here the latest findings in the field of mRNA modifications in normal and malignant hematopoiesis and how this might open up novel therapeutic options. (c) 2022 ISEH - Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.

How processes that affect post-transcriptional and translational regulation impact leukemia development is not well studied. The CCR4-NOT (CNOT) complex is one of two major multi-subunit polyA deadenylation complexes which mediate the shortening of the poly(A) tails and is recruited to m6A transcripts to mediate mRNA degradation. We surveyed several genome-wide CRISPR screens and observed several subunits of the CNOT complex ranked as genes essential for survival of AML cells. The highest ranked CNOT3 adaptor subunit demonstrated elevated expression in AML patients and associated with a poor prognosis in independent AML patient cohorts. Depletion of CNOT3 in human AML cells resulted in inhibition of cell growth, increased myeloid differentiation, cell cycle arrest and delayed leukemia development in vivo. Meanwhile, CNOT3 loss in normal cord-blood CD34+ cells promoted differentiation without inducing cell death, suggesting a potential therapeutic window to target CNOT3 in leukemia. Using a CRISPR saturation mutagenesis screen, we identified NOT box in the C-term domain of CNOT3 to be most critical for cell viability, indicating a functional dependence of CNOT3 on interaction with the deadenylation complex. Transcriptomic and proteomic profiling upon CNOT3 depletion revealed activation of p53 pathways and decrease of MYC- target genes. We further observed a marked decrease in c-MYC protein without alternation in total abundancy and half-life of c-MYC mRNA upon CNOT3 ablation. Mass spectrometry analysis of CNOT3 immunoprecipitation captured proteins involved in translation machinery. Moreover, depletion of CNOT3 significantly reduced the abundancy of c-MYC mRNA in the actively translating polysome fraction. Overall, we uncovered a critical role of the CNOT deadenylation complex in myeloid leukemia and translational control of critical oncogenic targets including c-MYC. How processes that affect post-transcriptional and translational regulation impact leukemia development is not well studied. The CCR4-NOT (CNOT) complex is one of two major multi-subunit polyA deadenylation complexes which mediate the shortening of the poly(A) tails and is recruited to m6A transcripts to mediate mRNA degradation. We surveyed several genome-wide CRISPR screens and observed several subunits of the CNOT complex ranked as genes essential for survival of AML cells. The highest ranked CNOT3 adaptor subunit demonstrated elevated expression in AML patients and associated with a poor prognosis in independent AML patient cohorts. Depletion of CNOT3 in human AML cells resulted in inhibition of cell growth, increased myeloid differentiation, cell cycle arrest and delayed leukemia development in vivo. Meanwhile, CNOT3 loss in normal cord-blood CD34+ cells promoted differentiation without inducing cell death, suggesting a potential therapeutic window to target CNOT3 in leukemia. Using a CRISPR saturation mutagenesis screen, we identified NOT box in the C-term domain of CNOT3 to be most critical for cell viability, indicating a functional dependence of CNOT3 on interaction with the deadenylation complex. Transcriptomic and proteomic profiling upon CNOT3 depletion revealed activation of p53 pathways and decrease of MYC- target genes. We further observed a marked decrease in c-MYC protein without alternation in total abundancy and half-life of c-MYC mRNA upon CNOT3 ablation. Mass spectrometry analysis of CNOT3 immunoprecipitation captured proteins involved in translation machinery. Moreover, depletion of CNOT3 significantly reduced the abundancy of c-MYC mRNA in the actively translating polysome fraction. Overall, we uncovered a critical role of the CNOT deadenylation complex in myeloid leukemia and translational control of critical oncogenic targets including c-MYC.

RNA binding proteins (RBPs) have been increasingly recognized as an important class of regulators of normal and malignant hematopoiesis. However, the exact function and underpinning mechanisms of the RBPs that govern hematopoietic stem cells (HSCs) remains poorly characterized. We had previously identified SYNCRIP as a critical RBP that controls leukemia stem cell program in myeloid leukemia. Here, using the novel murine genetic conditional knockout (cKO) model, we delineated the role of SYNCRIP in regulating the low-output HSC. We developed a Syncrip cKO allele and crossed Syncripf/f mice to the interferon (IFN) -a-inducible Mx-1-Cre mice to create Syncripf/f Mx-1-Cre+. We consistently obtained near complete depletion of SYNCRIP 3 weeks after two consecutive Poly(I:C) injections. We observed that SYNCRIP is dispensable for static hematopoiesis and Syncrip KO animals showed equivalent number and frequencies of stem and progenitor cells (Lin-Sca+cKit+ (LSK)- LT-HSC (CD48-CD150+); MPP1 (CD48-CD150-); MPP2 (CD48+CD150+); MPP4 (CD48-CD150-)). However, KO SyncripD/D deficient cells were outcompeted by WT Syncripf/f cells in the transplantation setting (bone marrow (BM) chimerism WT (n=9) 38% ± 7.8% vs. KO (n=9) 2.7% ± 0.8%, p<0.001 at 16 weeks post-transplant) and completely lost their ability to repopulate in secondary recipient animals (WT (n=5) 58% ± 7.4% vs. KO (n=5) 7.2 %± 2.9%, p<0.001 at 16 weeks post-transplant). These data strongly indicate that SYNCRIP is critical for maintenance of long-term self-renewal of HSCs. To decipher the effect of Syncrip deletion on the transcriptomic changes in different cell types upon Syncrip loss, we performed single cell RNA sequencing analysis (scRNA-seq) of sorted LK cells (Lin-cKit+ cells) from KO SyncripD/D (n=3) vs. WT Syncrip f/f (n=3) mice. While there is no significant change in frequencies of stem and progenitor compartments, we found defective trajectory from the HSC that is closely identified as low-output HSC based on previously performed barcoding studies. We observed a strong activation of cellular response to stress and unfolded proteins, in particular the HSF1-dependent pathways upon Syncrip depletion specifically within the HSC population. To further investigate the impacts of SYNCRIP loss in the HSC unfolded protein stress response, we evaluated unfolded proteins in cells using tetraphenylethene maleimide (TMI)-based flow cytometry. The abundance of accessible thiols in unfolded proteins, which is bound by TMI serves as a surrogate measurement for the state of the unfolded proteome. We consistently observed almost 2.5-fold increase in TMI signals specifically in LT-HSC, but not ST-HSCs or MPPs upon SYNCRIP deletion indicating that SYNCRIP is required to maintain high protein quality in HSCs. Similar results were obtained with the epichaperome probe PU-FITC, which consists of HSP90 inhibitor PU-H71 conjugated to FITC. PU-H71 selectively binds to the altered epichaperome, which reflects an accumulation of chaperon networks in an aberrant cellular stress condition. Altogether, these data further confirmed that SYNCRIP depletion tips off the proteostatic balance. To understand the molecular mechanisms underpinning the functional requirement of SYNCRIP in HSPCs, we identified 534 direct mRNA targets of SYNCRIP using hyper-TRIBE method. We performed transcriptomic and proteomic analysis of sorted LT- HSCs and LSKs respectively upon SYNCRIP deletion. We integrated these datasets and found a strong enrichment of SYNCRIP targets in control of cytoskeleton and RHO GTPase related pathways. Using immunofluorescence imaging, we confirmed that SYNCRIP deletion in HSCs resulted in 2-fold reduction in RHO GTPase CDC42 expression coupled with reduced tubulin and a loss of cellular polarity (percentage of tubulin polarized cells 56% WT vs. 40% KO). We also observed that Syncrip deficient HSCs demonstrated reduced expression of lysosomal-associated membrane protein 1 (LAMP-1) and less asymmetric distribution of LAMP1 marked lysosomes during cell division (LAMP1 asymmetric division 25% WT vs. 19% KO). Overexpression of CDC42 restored cell polarity and partly rescued ability of KO SyncripD/D to serially replate. Overall, SYNCRIP is required for maintenance of protein homeostasis and cell polarity of the reserve HSCs. Our study uncovers a new regulatory axis that controls stem cell stress responses to preserve HSC self-renewal.

RNA-binding proteins (RBPs) are key arbiters of post-transcriptional regulation and are found to be dysregulated in hematological malignancies. Here we identify the RBP RNA-binding motif protein, X-linked (RBMX; also known as hnRNPG), and its retrogene RBMXL1 to be required for murine and human myeloid leukemogenesis. RBMX and RBMXL1 were overexpressed in individuals with acute myeloid leukemia (AML) compared to healthy individuals, and RBMX/RBMXL1 loss delayed leukemia development. RBMX/RBMXL1 loss lead to global changes in chromatin accessibility as well as chromosomal breaks and gaps. We found that RBMX and RBMXL1 directly bind to mRNAs, affect transcription of multiple loci, including CBX5 (also known as heterochromatin protein 1 alpha (HP1-α)), and control the nascent transcription of the CBX5 locus. Forced CBX5 expression rescued the RBMX/RBMXL1 depletion effects on cell growth and apoptosis. Overall, we determined that RBMX and RBMXL1 control leukemia cell survival by regulating chromatin state through the downstream target CBX5. These findings identify a mechanism for RBPs directly promoting transcription and suggest RBMX and RBMXL1, as well as CBX5, as potential therapeutic targets in myeloid malignancies. Kharas and colleagues identify the RNA-binding proteins RBMX and RBMXL1 as AML tumor promoters that alter chromatin compaction and hence cell survival via transcriptional regulation of the heterochromatin protein encoded by CBX5.

The cell-context dependency for RNA binding proteins (RBPs) mediated control of stem cell fate remains to be defined. Here we adapt the HyperTRIBE method using an RBP fused to a Drosophila RNA editing enzyme (ADAR) to globally map the mRNA targets of the RBP MSI2 in mammalian adult normal and malignant stem cells. We reveal a unique MUSASHI-2 (MSI2) mRNA binding network in hematopoietic stem cells that changes during transition to multipotent progenitors. Additionally, we discover a significant increase in RNA binding activity of MSI2 in leukemic stem cells compared with normal hematopoietic stem and progenitor cells, resulting in selective regulation of MSI2's oncogenic targets. This provides a basis for MSI2 increased dependency in leukemia cells compared to normal cells. Moreover, our study provides a way to measure RBP function in rare cells and suggests that RBPs can achieve differential binding activity during cell state transition independent of gene expression.

Stem cells balance cellular fates through asymmetric and symmetric divisions in order to self-renew or to generate downstream progenitors. Symmetric commitment divisions in stem cells are required for rapid regeneration during tissue damage and stress. The control of symmetric commitment remains poorly defined. Using single-cell RNA sequencing (scRNA-seq) in combination with transcriptomic profiling of HSPCs (hematopoietic stem and progenitor cells) from control and m6A methyltransferase Mettl3 conditional knockout mice, we found that m6A-deficient hematopoietic stem cells (HSCs) fail to symmetrically differentiate. Dividing HSCs are expanded and are blocked in an intermediate state that molecularly and functionally resembles multipotent progenitors. Mechanistically, RNA methylation controls Myc mRNA abundance in differentiating HSCs. We identified MYC as a marker for HSC asymmetric and symmetric commitment. Overall, our results indicate that RNA methylation controls symmetric commitment and cell identity of HSCs and may provide a general mechanism for how stem cells regulate differentiation fate choice.

Hematopoietic development and differentiation are highly regulated processes, and recent studies focusing on m(6)A mRNA methylation have uncovered how this mark controls cell fate in both normal and malignant hematopoietic states. In this review, we focus on how writers, readers, and erasers of RNA methylation can mediate distinct phenotypes on mRNAs and on cells. Targeting the RNA methylation program has emerged as a potential novel therapeutic strategy, and we explore the role for these regulators in both normal and dysregulated cell contexts. Significance: RNA methylation is required for cancer cell survival in solid tumors and in acute myeloid leukemia, and targeting this pathway has been proposed as a new therapeutic strategy in cancer. However, understanding the role for RNA methylation in both normal and malignant states is essential for understanding the potential consequences for therapeutic intervention.

One of the biggest challenges in treating acute myeloid leukemia (AML) is relapse of aggressive disease after treatment. In this issue of Cancer Cell, Boyd et al. characterize a molecularly distinct population of chemotherapy-induced transient leukemic regenerating cells (LRCs), which can be exploited to prevent AML recurrence.

One of the biggest challenges in treating acute myeloid leukemia (AML) is relapse of aggressive disease after treatment. In this issue of Cancer Cell, Boyd et al. characterize a molecularly distinct population of chemotherapy-induced transient leukemic regenerating cells (LRCs), which can be exploited to prevent AML recurrence.

N-6-methyladenosine (m6A) is one of the most abundant posttranscriptional modifications in eukaryotic mRNAs and long noncoding RNAs. We previously found a critical role for m6A in promoting human myeloid leukemia (Vu et al. Nature Medicine 2017). Targeting the RNA methylation program in leukemias has been suggested as a potential novel therapeutic strategy. However, it is unknown whether the m6A modification controls normal adult hematopoiesis and hematopoietic stem cells (HSC) function.

Acute myeloid leukemia (AML) is characterized by a block in the development of myeloid cells, often due to dysregulation of genes involved in key processes including self-renewal, proliferation, and differentiation. Somatic mutations and aberrant expression of RNA binding proteins (RBPs) have recently been found to be important in hematological malignancies. For example, our group and others have recently determined that increased expression of MUSASHI-2 and SYNCRIP drives aggressive leukemia. To discover novel RBP regulators of leukemia, we performed an in vivo pooled shRNA screen of 127 MSI2 direct protein interactors and associated genes (Vu et al. Nat Gen. 2017). In this screen, shRNAs specific to the RBP RBMX (RNA binding motif protein, X-linked) were selectively depleted in murine MLL-AF9 driven leukemia. RBMX has been implicated in regulating alternative splicing, chromatin cohesion, and DNA-damage response, but its function in hematopoiesis and leukemia is not known. We confirmed that depletion of RBMX with shRNAs in murine MLL-AF9 leukemia cells resulted in reduced myeloid colony formation, increased apoptosis, and increased differentiation as determined by flow analysis of myeloid cell surface markers Gr-1 and Mac-1 (mean of 61-65% shRNA versus mean of 12.95% control). Furthermore, RBMX is highly expressed among human myeloid leukemia cell lines (n=10/11) and primary AML patient samples (n=2/4). Depletion of RBMX with shRNAs led to a dramatic decrease in cell proliferation and 3-fold induction of apoptosis in several human myeloid leukemia cell lines (MOLM-13, THP-1, K562, and KCL-22). Additionally, RBMX depletion in AML cells induced myeloid differentiation and significantly delayed leukemogenesis cells in vivo (median survival of 51.5 days in control versus median ‘not reached’ in shRNA1 and shRNA2). To determine if there is a differential requirement of RBMX in survival of leukemia cells compared to normal hematopoietic stem and progenitor cells (HSPCs), we depleted RBMX with shRNAs in normal murine bone marrow c-Kit+ cells and found no significant changes in colony formation. Depleting RBMX with shRNAs in human cord blood derived CD34+ HSPCs resulted in reduced colony formation but no increase in apoptosis. Thus, these data suggest that there is a differential requirement for RBMX in myeloid leukemia cells compared to normal cells. To uncover the mechanism of RBMX function, we performed RNA-sequencing of human AML cells (MOLM-13) depleted for RBMX. Gene set enrichment analysis demonstrated a loss of cell cycle and DNA repair associated programs in RBMX depleted cells. Complex chromosomal karyotyping analysis of these cells revealed increased metaphases with breaks and gaps (mean of 30.67% shRNA versus mean of 13.33% control) and irregular chromatin compaction (mean of 47.67%shRNA versus mean of 20% control), while cell cycle analysis showed significantly increased S-phase arrest as determined by flow analysis of Hoechst stained cells (mean of 37-40% shRNA versus of 24.18% control). Reanalysis of RBMX transcriptome-wide binding sites in 293T cells revealed that RBMX directly binds to heterochromatin protein HP1α transcripts (Liu et al. Nucleic Acids Res. 2017). HP1α, also called CBX5, is a key heterochromatin protein that binds to histone H3-K9 tri-methylation marks to promote heterochromatin formation, which is critical in chromatin condensation and chromosome segregation. HP1α has also been determined to be required for MLL leukemia stem cell maintenance. We demonstrated that RBMX depletion resulted in a significant decrease of HP1α mRNA expression without affecting its mRNA stability in AML cells. We confirmed that RBMX depletion reduced the protein abundance of HP1α. Moreover, overexpression of HP1α rescued the effect of RBMX depletion on cell growth and apoptosis. Our study finds that RBMX binds to HP1α mRNA and regulates the transcriptional activity of the HP1α locus, which then maintains proper chromatin compaction in leukemia cells. Overall, we determine that RBMX function is critical for myeloid leukemia survival and has potential as a novel therapeutic target in AML.