RNA-binding protein RBM5 plays an essential role in acute myeloid leukemia by activating the oncogenic protein HOXA9

BACKGROUND

The oncogenic protein HOXA9 plays a critical role in leukemia transformation and maintenance, and its aberrant expression is a hallmark of most aggressive acute leukemia. Although inhibiting the upstream regulators of HOXA9 has been proven as a significant therapeutic intervention, the comprehensive regulation network controlling HOXA9 expression in leukemia has not been systematically investigated.

RESULTS

Here, we perform genome-wide CRISPR/Cas9 screening in the HOXA9-driven reporter acute leukemia cells. We identify a poorly characterized RNA-binding protein, RBM5, as the top candidate gene required to maintain leukemia cell fitness. RBM5 is highly overexpressed in acute myeloid leukemia (AML) patients compared to healthy individuals. RBM5 loss triggered by CRISPR knockout and shRNA knockdown significantly impairs leukemia maintenance in vitro and in vivo. Through domain CRISPR screening, we reveal that RBM5 functions through a noncanonical transcriptional regulation circuitry rather than RNA splicing, such an effect depending on DNA-binding domains. By integrative analysis and functional assays, we identify HOXA9 as the downstream target of RBM5. Ectopic expression of HOXA9 rescues impaired leukemia cell proliferation upon RBM5 loss. Importantly, acute protein degradation of RBM5 through auxin-inducible degron system immediately reduces HOXA9 transcription.

CONCLUSIONS

We identify RBM5 as a new upstream regulator of HOXA9 and reveal its essential role in controlling the survival of AML. These functional and molecular mechanisms further support RBM5 as a promising therapeutic target for myeloid leukemia treatment.

Systematic characterization of the HOXA9 downstream targets in MLL-r leukemia by noncoding CRISPR screens

Accumulating evidence indicates that HOXA9 dysregulation is necessary and sufficient for leukemic transformation and maintenance. However, it remains largely unknown how HOXA9, as a homeobox transcriptional factor, binds to noncoding regulatory sequences and controls the downstream genes. Here, we conduct dropout CRISPR screens against 229 HOXA9-bound peaks identified by ChIP-seq. Integrative data analysis identifies reproducible noncoding hits, including those located in the distal enhancer of FLT3 and intron of CDK6. The Cas9-editing and dCas9-KRAB silencing of the HOXA9-bound sites significantly reduce corresponding gene transcription and impair cell proliferation in vitro, and in vivo by transplantation into NSG female mice. In addition, RNA-seq, Q-PCR analysis, chromatin accessibility change, and chromatin conformation evaluation uncover the noncoding regulation mechanism of HOXA9 and its functional downstream genes. In summary, our work improves our understanding of how HOXA9-associated transcription programs reconstruct the regulatory network specifying MLL-r dependency.

Interrogating bromodomain inhibitor resistance in KMT2A-rearranged leukemia through combinatorial CRISPR screens

Bromo- and extra-terminal domain inhibitors (BETi) have exhibited therapeutic activities in many cancers. However, the mechanisms controlling BETi response and resistance are not well understood. We conducted genome-wide loss-of-function CRISPR screens using BETi-treated KMT2A-rearranged (KMT2A-r) cell lines. We revealed that Speckle-type POZ protein (SPOP) gene (Speckle Type BTB/POZ Protein) deficiency caused significant BETi resistance, which was further validated in cell lines and xenograft models. Proteomics analysis and a kinase-vulnerability CRISPR screen indicated that cells treated with BETi are sensitive to GSK3 perturbation. Pharmaceutical inhibition of GSK3 reversed the BETi-resistance phenotype. Based on this observation, a combination therapy regimen inhibiting both BET and GSK3 was developed to impede KMT2A-r leukemia progression in patient-derived xenografts in vivo. Our results revealed molecular mechanisms underlying BETi resistance and a promising combination treatment regimen of ABBV-744 and CHIR-98014 by utilizing unique ex vivo and in vivo KMT2A-r PDX models.

Auxin-inducible degron 2 system deciphers functions of CTCF domains in transcriptional regulation

BACKGROUND

CTCF is a well-established chromatin architectural protein that also plays various roles in transcriptional regulation. While CTCF biology has been extensively studied, how the domains of CTCF function to regulate transcription remains unknown. Additionally, the original auxin-inducible degron 1 (AID1) system has limitations in investigating the function of CTCF.

RESULTS

We employ an improved auxin-inducible degron technology, AID2, to facilitate the study of acute depletion of CTCF while overcoming the limitations of the previous AID system. As previously observed through the AID1 system and steady-state RNA analysis, the new AID2 system combined with SLAM-seq confirms that CTCF depletion leads to modest nascent and steady-state transcript changes. A CTCF domain sgRNA library screening identifies the zinc finger (ZF) domain as the region within CTCF with the most functional relevance, including ZFs 1 and 10. Removal of ZFs 1 and 10 reveals genomic regions that independently require these ZFs for DNA binding and transcriptional regulation. Notably, loci regulated by either ZF1 or ZF10 exhibit unique CTCF binding motifs specific to each ZF.

CONCLUSIONS

By extensively comparing the AID1 and AID2 systems for CTCF degradation in SEM cells, we confirm that AID2 degradation is superior for achieving miniAID-tagged protein degradation without the limitations of the AID1 system. The model we create that combines AID2 depletion of CTCF with exogenous overexpression of CTCF mutants allows us to demonstrate how peripheral ZFs intricately orchestrate transcriptional regulation in a cellular context for the first time.

Acute depletion of CTCF rewires genome-wide chromatin accessibility

BACKGROUND

The transcription factor CTCF appears indispensable in defining topologically associated domain boundaries and maintaining chromatin loop structures within these domains, supported by numerous functional studies. However, acute depletion of CTCF globally reduces chromatin interactions but does not significantly alter transcription.

RESULTS

Here, we systematically integrate multi-omics data including ATAC-seq, RNA-seq, WGBS, Hi-C, Cut&Run, and CRISPR-Cas9 survival dropout screens, and time-solved deep proteomic and phosphoproteomic analyses in cells carrying auxin-induced degron at endogenous CTCF locus. Acute CTCF protein degradation markedly rewires genome-wide chromatin accessibility. Increased accessible chromatin regions are frequently located adjacent to CTCF-binding sites at promoter regions and insulator sites associated with enhanced transcription of nearby genes. In addition, we use CTCF-associated multi-omics data to establish a combinatorial data analysis pipeline to discover CTCF co-regulatory partners. We successfully identify 40 candidates, including multiple established partners. Interestingly, many CTCF co-regulators that have alterations of their respective downstream gene expression do not show changes of their own expression levels across the multi-omics measurements upon acute CTCF loss, highlighting the strength of our system to discover hidden co-regulatory partners associated with CTCF-mediated transcription.

CONCLUSIONS

This study highlights that CTCF loss rewires genome-wide chromatin accessibility, which plays a critical role in transcriptional regulation.

Functional interrogation of HOXA9 regulome in MLLr leukemia via reporter-based CRISPR/Cas9 screen

Aberrant HOXA9 expression is a hallmark of most aggressive acute leukemias, notably those with KMT2A (MLL) gene rearrangements. HOXA9 overexpression not only predicts poor diagnosis and outcome but also plays a critical role in leukemia transformation and maintenance. However, our current understanding of HOXA9 regulation in leukemia is limited, hindering development of therapeutic strategies. Here, we generated the HOXA9-mCherry knock-in reporter cell lines to dissect HOXA9 regulation. By utilizing the reporter and CRISPR/Cas9 screens, we identified transcription factors controlling HOXA9 expression, including a novel regulator, USF2, whose depletion significantly down-regulated HOXA9 expression and impaired MLLr leukemia cell proliferation. Ectopic expression of Hoxa9 rescued impaired leukemia cell proliferation upon USF2 loss. Cut and Run analysis revealed the direct occupancy of USF2 at HOXA9 promoter in MLLr leukemia cells. Collectively, the HOXA9 reporter facilitated the functional interrogation of the HOXA9 regulome and has advanced our understanding of the molecular regulation network in HOXA9-driven leukemia.

Discovery of regulatory noncoding variants in individual cancer genomes by using cis-X

We developed cis-X, a computational method for discovering regulatory noncoding variants in cancer by integrating whole-genome and transcriptome sequencing data from a single cancer sample. cis-X first finds aberrantly cis-activated genes that exhibit allele-specific expression accompanied by an elevated outlier expression. It then searches for causal noncoding variants that may introduce aberrant transcription factor binding motifs or enhancer hijacking by structural variations. Analysis of 13 T-lineage acute lymphoblastic leukemias identified a recurrent intronic variant predicted to cis-activate the TAL1 oncogene, a finding validated in vivo by chromatin immunoprecipitation sequencing of a patient-derived xenograft. Candidate oncogenes include the prolactin receptor PRLR activated by a focal deletion that removes a CTCF-insulated neighborhood boundary. cis-X may be applied to pediatric and adult solid tumors that are aneuploid and heterogeneous. In contrast to existing approaches, which require large sample cohorts, cis-X enables the discovery of regulatory noncoding variants in individual cancer genomes.

A cis-element within the ARF locus mediates repression of p16INK4A expression via long-range chromatin interactions

Loss of function of CDKN2A/B, also known as INK4/ARF [encoding p16INK4A, p15INK4B, and p14ARF (mouse p19Arf)], confers susceptibility to cancers, whereas its up-regulation during organismal aging provokes cellular senescence and tissue degenerative disorders. To better understand the transcriptional regulation of p16INK4A, a CRISPR screen targeting open, noncoding chromatin regions adjacent to p16INK4A was performed in a human p16INK4A-P2A-mCherry reporter cell line. We identified a repressive element located in the 3' region adjacent to the ARF promoter that controls p16INK4A expression via long-distance chromatin interactions. Coinfection of lentiviral dCas9-KRAB with selected single-guide RNAs against the repressive element abrogated the ARF/p16INK4A chromatin contacts, thus reactivating p16INK4A expression. Genetic CRISPR screening identified candidate transcription factors inhibiting p16INK4A regulation, including ZNF217, which was confirmed to bind the ARF/p16INK4A interaction loop. In summary, direct physical interactions between p16INK4A and ARF genes provide mechanistic insights into their cross-regulation.

Acute depletion of CTCF directly affects MYC regulation through loss of enhancer-promoter looping

Numerous pieces of evidence support the complex, 3D spatial organization of the genome dictates gene expression. CTCF is essential to define topologically associated domain boundaries and to facilitate the formation of insulated chromatin loop structures. To understand CTCF's direct role in global transcriptional regulation, we integrated the miniAID-mClover3 cassette to the endogenous CTCF locus in a human pediatric B-ALL cell line, SEM, and an immortal erythroid precursor cell line, HUDEP-2, to allow for acute depletion of CTCF protein by the auxin-inducible degron system. In SEM cells, CTCF loss notably disrupted intra-TAD loops and TAD integrity in concurrence with a reduction in CTCF-binding affinity, while showing no perturbation to nuclear compartment integrity. Strikingly, the overall effect of CTCF's loss on transcription was minimal. Whole transcriptome analysis showed hundreds of genes differentially expressed in CTCF-depleted cells, among which MYC and a number of MYC target genes were specifically downregulated. Mechanically, acute depletion of CTCF disrupted the direct interaction between the MYC promoter and its distal enhancer cluster residing ∼1.8 Mb downstream. Notably, MYC expression was not profoundly affected upon CTCF loss in HUDEP-2 cells suggesting that CTCF could play a B-ALL cell line specific role in maintaining MYC expression.

CHK2 kinase promotes pre-mRNA splicing via phosphorylating CDK11(p110)

Checkpoint kinase 2 (CHK2) kinase is a key mediator in many cellular responses to genotoxic stresses, including ionizing radiation (IR) and topoisomerase inhibitors. Upon IR, CHK2 is activated by ataxia telangiectasia mutated kinase and regulates the S-phase and G1-S checkpoints, apoptosis and DNA repair by phosphorylating downstream target proteins, such as p53 and Brca1. In addition, CHK2 is thought to be a multi-organ cancer susceptibility gene. In this study, we used a tandem affinity purification strategy to identify proteins that interact with CHK2 kinase. Cyclin-dependent kinase 11 (CDK11)(p110) kinase, implicated in pre-mRNA splicing and transcription, was identified as a CHK2-interacting protein. CHK2 kinase phosphorylated CDK11(p110) on serine 737 in vitro. Unexpectedly, CHK2 kinase constitutively phosphorylated CDK11(p110) in a DNA damage-independent manner. At a molecular level, CDK11(p110) phosphorylation was required for homodimerization without affecting its kinase activity. Overexpression of CHK2 promoted pre-mRNA splicing. Conversely, CHK2 depletion decreased endogenous splicing activity. Mutation of the phosphorylation site in CDK11(p110) to alanine abrogated its splicing-activating activity. These results provide the first evidence that CHK2 kinase promotes pre-mRNA splicing via phosphorylating CDK11(p110).

Mammalian ChlR1 has a role in heterochromatin organization

The ChlR1 DNA helicase, encoded by DDX11 gene, which is responsible for Warsaw breakage syndrome (WABS), has a role in sister-chromatid cohesion. In this study, we show that human ChlR1 deficient cells exhibit abnormal heterochromatin organization. While constitutive heterochromatin is discretely localized at perinuclear and perinucleolar regions in control HeLa cells, ChlR1-depleted cells showed dispersed localization of constitutive heterochromatin accompanied by disrupted centromere clustering. Cells isolated from Ddx11(-/-) embryos also exhibited diffuse localization of centromeres and heterochromatin foci. Similar abnormalities were found in HeLa cells depleted of combinations of HP1α and HP1β. Immunofluorescence and chromatin immunoprecipitation showed a decreased level of HP1α at pericentric regions in ChlR1-depleted cells. Trimethyl-histone H3 at lysine 9 (H3K9-me3) was also modestly decreased at pericentric sequences. The abnormality in pericentric heterochromatin was further supported by decreased DNA methylation within major satellite repeats of Ddx11(-/-) embryos. Furthermore, micrococcal nuclease (MNase) assay revealed a decreased chromatin density at the telomeres. These data suggest that in addition to a role in sister-chromatid cohesion, ChlR1 is also involved in the proper formation of heterochromatin, which in turn contributes to global nuclear organization and pleiotropic effects.

The RNA binding motif protein 15B (RBM15B/OTT3) is a functional competitor of serine-arginine (SR) proteins and antagonizes the positive effect of the CDK11p110-cyclin L2α complex on splicing

Here, we report the identification of the RNA binding motif protein RBM15B/OTT3 as a new CDK11(p110) binding partner that alters the effects of CDK11 on splicing. RBM15B was initially identified as a binding partner of the Epstein-Barr virus mRNA export factor and, more recently, as a cofactor of the nuclear export receptor NXF1. In this study, we found that RBM15B co-elutes with CDK11(p110), cyclin L2α, and serine-arginine (SR) proteins, including SF2/ASF, in a large nuclear complex of ∼1-MDa molecular mass following size exclusion chromatography. Using co-immunoprecipitation experiments and in vitro pulldown assays, we mapped two distinct domains of RBM15B that are essential for its direct interaction with the N-terminal extension of CDK11(p110), cyclin L2α, and SR proteins such as 9G8 and SF2/ASF. Finally, we established that RBM15B is a functional competitor of the SR proteins SF2/ASF and 9G8, inhibits formation of the functional spliceosomal E complex, and antagonizes the positive effect of the CDK11(p110)-cyclin L2α complex on splicing both in vitro and in vivo.

Synthetic mRNA splicing modulator compounds with in vivo antitumor activity

We report our progress on the development of new synthetic anticancer lead compounds that modulate the splicing of mRNA. We also report the synthesis and evaluation of new biologically active ester and carbamate analogues. Further, we describe initial animal studies demonstrating the antitumor efficacy of compound 5 in vivo. Additionally, we report the enantioselective and diastereospecific synthesis of a new 1,3-dioxane series of active analogues. We confirm that compound 5 inhibits the splicing of mRNA in cell-free nuclear extracts and in a cell-based dual-reporter mRNA splicing assay. In summary, we have developed totally synthetic novel spliceosome modulators as therapeutic lead compounds for a number of highly aggressive cancers. Future efforts will be directed toward the more complete optimization of these compounds as potential human therapeutics.

Perturbation of HP1 localization and chromatin binding ability causes defects in sister-chromatid cohesion

Sister-chromatid cohesion, the machinery used in eukaryote organisms to prevent aneuploidy, tethers sister chromatids together after their replication in S phase until mitosis. Previous studies in fission yeast, Drosophila and mammals have demonstrated the requirement for the heterochromatin formation pathway for proper centromeric cohesion. However, the exact role of heterochromatin protein 1 (HP1) in sister-chromatid cohesion in mammals is still unknown. In this study, we disrupted endogenous HP1 expression in HeLa cells using a dominant-negative mutant of HP1beta and wild-type or mutant forms of HP1alpha. We then examined their effects on chromosome alignment, segregation and cohesion. Enforced expression of these constructs leads to frequent chromosome misalignment and missegregation. Mitotic chromosomes from these cells also exhibit a loosened primary constriction and separated sister chromatids. We further demonstrate that alignment of the cohesin proteins around kinetochores was also aberrant and that cohesin complexes bound less tightly in these cells. Unexpectedly, we observed a "wavy" chromosome morphology resembling that seen upon depletion of condensin proteins in cells with over-expression of HP1alpha, but not in cells expressing the HP1beta mutant. These results indicate that proper HP1 status is required for sister-chromatid cohesion in mammalian cells, and suggest that HP1alpha might be required for chromosome condensation.