SWI/SNF Blockade Disrupts PU.1-Directed Enhancer Programs in Normal Hematopoietic Cells and Acute Myeloid Leukemia

Pharmacologic Blockade of a Pioneer Transcription Factor

Cancers frequently co-opt lineage-specific transcription factors (TF) utilized in normal development to sustain proliferation. However, the effects of these TFs on tumor development depend considerably on where in the genome they bind. A new article by Taylor and colleagues expands on previously developed diamidine compounds that obstruct the DNA binding sites of the pioneer TF PU.1 (SPI1) in acute myeloid leukemia. Immobilization and sequencing of genomic DNA targeted by these compounds revealed that these inhibitors alter the genomic binding patterns of PU.1. The authors report that their strategy constrains the genomic binding preferences of PU.1, leading to redistribution of PU.1 to promoters and other gene-proximal regions with elevated guanine/cytosine content. In this study, we discuss recent developments for targeting PU.1 in hematologic malignancies. We also explore the shared functional roles of PU.1 and SWI/SNF ATP-dependent chromatin remodeling complexes, which not only work together to sustain the enhancer landscape needed for tumor cell proliferation but also play key roles in nontumor settings.

Smarcd1 subunit of SWI/SNF chromatin-remodeling complexes collaborates with E2a to promote murine lymphoid specification

Lymphocyte development from murine hematopoietic stem cells (HSCs) entails a loss of self-renewal capacity and a progressive restriction of developmental potential. Previous research from our laboratory suggests that specialized assemblies of ATP-dependent SWI/SNF chromatin-remodeling complexes play lineage-specific roles during murine hematopoiesis. Here, we demonstrate that the Smarcd1 subunit is essential for specification of lymphoid cell fate from multipotent progenitors. Acute deletion of Smarcd1 in murine adult hematopoiesis leads to lymphopenia, characterized by a near-complete absence of early lymphoid progenitors and mature B and T cells, while the myeloid and erythroid lineages remain unaffected. Mechanistically, we demonstrate that Smarcd1 is essential for the coordinated activation of a lymphoid gene signature in murine multipotent progenitors. This is achieved by interacting with the E2a transcription factor at proximal promoters and by regulating the activity of distal enhancers. Globally, these findings identify Smarcd1 as an essential chromatin remodeler that governs lymphoid cell fate.

Notch induces transcription by stimulating release of paused RNA polymerase II

Notch proteins undergo ligand-induced proteolysis to release a nuclear effector that influences a wide range of cellular processes by regulating transcription. Despite years of study, however, how Notch induces the transcription of its target genes remains unclear. Here, we comprehensively examine the response to human Notch1 across a time course of activation using high-resolution genomic assays of chromatin accessibility and nascent RNA production. Our data reveal that Notch induces target gene transcription primarily by releasing paused RNA polymerase II (RNAPII). Moreover, in contrast to prevailing models suggesting that Notch acts by promoting chromatin accessibility, we found that open chromatin was established at Notch-responsive regulatory elements prior to Notch signal induction through SWI/SNF-mediated remodeling. Together, these studies show that the nuclear response to Notch signaling is dictated by the pre-existing chromatin state and RNAPII distribution at the time of signal activation.

Opening and changing: mammalian SWI/SNF complexes in organ development and carcinogenesis

The switch/sucrose non-fermentable (SWI/SNF) subfamily are evolutionarily conserved, ATP-dependent chromatin-remodelling complexes that alter nucleosome position and regulate a spectrum of nuclear processes, including gene expression, DNA replication, DNA damage repair, genome stability and tumour suppression. These complexes, through their ATP-dependent chromatin remodelling, contribute to the dynamic regulation of genetic information and the maintenance of cellular processes essential for normal cellular function and overall genomic integrity. Mutations in SWI/SNF subunits are detected in 25% of human malignancies, indicating that efficient functioning of this complex is required to prevent tumourigenesis in diverse tissues. During development, SWI/SNF subunits help establish and maintain gene expression patterns essential for proper cellular identity and function, including maintenance of lineage-specific enhancers. Moreover, specific molecular signatures associated with SWI/SNF mutations, including disruption of SWI/SNF activity at enhancers, evasion of G0 cell cycle arrest, induction of cellular plasticity through pro-oncogene activation and Polycomb group (PcG) complex antagonism, are linked to the initiation and progression of carcinogenesis. Here, we review the molecular insights into the aetiology of human malignancies driven by disruption of the SWI/SNF complex and correlate these mechanisms to their developmental functions. Finally, we discuss the therapeutic potential of targeting SWI/SNF subunits in cancer.

Electrochemical impedance spectroscopy unmasks high-risk atherosclerotic features in human coronary artery disease

Coronary plaque rupture remains the prominent mechanism of myocardial infarction. Accurate identification of rupture-prone plaque may improve clinical management. This study assessed the discriminatory performance of electrochemical impedance spectroscopy (EIS) in human cardiac explants to detect high-risk atherosclerotic features that portend rupture risk. In this single-center, prospective study, n = 26 cardiac explants were collected for EIS interrogation of the three major coronary arteries. Vessels in which advancement of the EIS catheter without iatrogenic plaque disruption was rendered impossible were not assessed. N = 61 vessels underwent EIS measurement and histological analyses. Plaques were dichotomized according to previously established high rupture-risk parameter thresholds. Diagnostic performance was determined via receiver operating characteristic areas-under-the-curve (AUC). Necrotic cores were identified in n = 19 vessels (median area 1.53 mm2) with a median fibrous cap thickness of 62 μm. Impedance was significantly greater in plaques with necrotic core area ≥1.75 mm2 versus <1.75 mm2 (19.8 ± 4.4 kΩ vs. 7.2 ± 1.0 kΩ, p = .019), fibrous cap thickness ≤65 μm versus >65 μm (19.1 ± 3.5 kΩ vs. 6.5 ± 0.9 kΩ, p = .004), and ≥20 macrophages per 0.3 mm-diameter high-power field (HPF) versus <20 macrophages per HPF (19.8 ± 4.1 kΩ vs. 10.2 ± 0.9 kΩ, p = .002). Impedance identified necrotic core area ≥1.75 mm2, fibrous cap thickness ≤65 μm, and ≥20 macrophages per HPF with AUCs of 0.889 (95% CI: 0.716-1.000) (p = .013), 0.852 (0.646-1.000) (p = .025), and 0.835 (0.577-1.000) (p = .028), respectively. Further, phase delay discriminated severe stenosis (≥70%) with an AUC of 0.767 (0.573-0.962) (p = .035). EIS discriminates high-risk atherosclerotic features that portend plaque rupture in human coronary artery disease and may serve as a complementary modality for angiography-guided atherosclerosis evaluation.

Chromatin remodellers as therapeutic targets

Large-scale cancer genome sequencing studies have revealed that chromatin regulators are frequently mutated in cancer. In particular, more than 20% of cancers harbour mutations in genes that encode subunits of SWI/SNF (BAF) chromatin remodelling complexes. Additional links of SWI/SNF complexes to disease have emerged with the findings that some oncogenes drive transformation by co-opting SWI/SNF function and that germline mutations in select SWI/SNF subunits are the basis of several neurodevelopmental disorders. Other chromatin remodellers, including members of the ISWI, CHD and INO80/SWR complexes, have also been linked to cancer and developmental disorders. Consequently, therapeutic manipulation of SWI/SNF and other remodelling complexes has become of great interest, and drugs that target SWI/SNF subunits have entered clinical trials. Genome-wide perturbation screens in cancer cell lines with SWI/SNF mutations have identified additional synthetic lethal targets and led to further compounds in clinical trials, including one that has progressed to FDA approval. Here, we review the progress in understanding the structure and function of SWI/SNF and other chromatin remodelling complexes, mechanisms by which SWI/SNF mutations cause cancer and neurological diseases, vulnerabilities that arise because of these mutations and efforts to target SWI/SNF complexes and synthetic lethal targets for therapeutic benefit.

Baf155 controls hematopoietic differentiation and regeneration through chromatin priming

Chromatin priming promotes cell-type-specific gene expression, lineage differentiation, and development. The mechanism of chromatin priming has not been fully understood. Here, we report that mouse hematopoietic stem and progenitor cells (HSPCs) lacking the Baf155 subunit of the BAF (BRG1/BRM-associated factor) chromatin remodeling complex produce a significantly reduced number of mature blood cells, leading to a failure of hematopoietic regeneration upon transplantation and 5-fluorouracil (5-FU) injury. Baf155-deficient HSPCs generate particularly fewer neutrophils, B cells, and CD8+ T cells at homeostasis, supporting a more immune-suppressive tumor microenvironment and enhanced tumor growth. Single-nucleus multiomics analysis reveals that Baf155-deficient HSPCs fail to establish accessible chromatin in selected regions that are enriched for putative enhancers and binding motifs of hematopoietic lineage transcription factors. Our study provides a fundamental mechanistic understanding of the role of Baf155 in hematopoietic lineage chromatin priming and the functional consequences of Baf155 deficiency in regeneration and tumor immunity.

Transcription factor dependencies identify BAF-dependent cancers

In Cancer Cell, Bolomsky et al., Duplaquet et al., and He et al. identify cancers that are dependent on the BAF chromatin remodeling complex, specifically IRF4-driven multiple myeloma and POU2F3-subtype small cell lung cancer, highlighting potential therapeutic applications for BAF complex inhibitors/degraders.

Targeting the mSWI/SNF complex in POU2F-POU2AF transcription factor-driven malignancies

The POU2F3-POU2AF2/3 transcription factor complex is the master regulator of the tuft cell lineage and tuft cell-like small cell lung cancer (SCLC). Here, we identify a specific dependence of the POU2F3 molecular subtype of SCLC (SCLC-P) on the activity of the mammalian switch/sucrose non-fermentable (mSWI/SNF) chromatin remodeling complex. Treatment of SCLC-P cells with a proteolysis targeting chimera (PROTAC) degrader of mSWI/SNF ATPases evicts POU2F3 and its coactivators from chromatin and attenuates downstream signaling. B cell malignancies which are dependent on the POU2F1/2 cofactor, POU2AF1, are also sensitive to mSWI/SNF ATPase degraders, with treatment leading to chromatin eviction of POU2AF1 and IRF4 and decreased IRF4 signaling in multiple myeloma cells. An orally bioavailable mSWI/SNF ATPase degrader significantly inhibits tumor growth in preclinical models of SCLC-P and multiple myeloma without signs of toxicity. This study suggests that POU2F-POU2AF-driven malignancies have an intrinsic dependence on the mSWI/SNF complex, representing a therapeutic vulnerability.

ARID1A suppresses R-loop-mediated STING-type I interferon pathway activation of anti-tumor immunity

Clinical trials have identified ARID1A mutations as enriched among patients who respond favorably to immune checkpoint blockade (ICB) in several solid tumor types independent of microsatellite instability. We show that ARID1A loss in murine models is sufficient to induce anti-tumor immune phenotypes observed in ARID1A mutant human cancers, including increased CD8+ T cell infiltration and cytolytic activity. ARID1A-deficient cancers upregulated an interferon (IFN) gene expression signature, the ARID1A-IFN signature, associated with increased R-loops and cytosolic single-stranded DNA (ssDNA). Overexpression of the R-loop resolving enzyme, RNASEH2B, or cytosolic DNase, TREX1, in ARID1A-deficient cells prevented cytosolic ssDNA accumulation and ARID1A-IFN gene upregulation. Further, the ARID1A-IFN signature and anti-tumor immunity were driven by STING-dependent type I IFN signaling, which was required for improved responsiveness of ARID1A mutant tumors to ICB treatment. These findings define a molecular mechanism underlying anti-tumor immunity in ARID1A mutant cancers.

CUX1 regulates human hematopoietic stem cell chromatin accessibility via the BAF complex

CUX1 is a homeodomain-containing transcription factor that is essential for the development and differentiation of multiple tissues. CUX1 is recurrently mutated or deleted in cancer, particularly in myeloid malignancies. However, the mechanism by which CUX1 regulates gene expression and differentiation remains poorly understood, creating a barrier to understanding the tumor-suppressive functions of CUX1. Here, we demonstrate that CUX1 directs the BAF chromatin remodeling complex to DNA to increase chromatin accessibility in hematopoietic cells. CUX1 preferentially regulates lineage-specific enhancers, and CUX1 target genes are predictive of cell fate in vivo. These data indicate that CUX1 regulates hematopoietic lineage commitment and homeostasis via pioneer factor activity, and CUX1 deficiency disrupts these processes in stem and progenitor cells, facilitating transformation.

Targeting the mSWI/SNF Complex in POU2F-POU2AF Transcription Factor-Driven Malignancies

The POU2F3-POU2AF2/3 (OCA-T1/2) transcription factor complex is the master regulator of the tuft cell lineage and tuft cell-like small cell lung cancer (SCLC). Here, we found that the POU2F3 molecular subtype of SCLC (SCLC-P) exhibits an exquisite dependence on the activity of the mammalian switch/sucrose non-fermentable (mSWI/SNF) chromatin remodeling complex. SCLC-P cell lines were sensitive to nanomolar levels of a mSWI/SNF ATPase proteolysis targeting chimera (PROTAC) degrader when compared to other molecular subtypes of SCLC. POU2F3 and its cofactors were found to interact with components of the mSWI/SNF complex. The POU2F3 transcription factor complex was evicted from chromatin upon mSWI/SNF ATPase degradation, leading to attenuation of downstream oncogenic signaling in SCLC-P cells. A novel, orally bioavailable mSWI/SNF ATPase PROTAC degrader, AU-24118, demonstrated preferential efficacy in the SCLC-P relative to the SCLC-A subtype and significantly decreased tumor growth in preclinical models. AU-24118 did not alter normal tuft cell numbers in lung or colon, nor did it exhibit toxicity in mice. B cell malignancies which displayed a dependency on the POU2F1/2 cofactor, POU2AF1 (OCA-B), were also remarkably sensitive to mSWI/SNF ATPase degradation. Mechanistically, mSWI/SNF ATPase degrader treatment in multiple myeloma cells compacted chromatin, dislodged POU2AF1 and IRF4, and decreased IRF4 signaling. In a POU2AF1-dependent, disseminated murine model of multiple myeloma, AU-24118 enhanced survival compared to pomalidomide, an approved treatment for multiple myeloma. Taken together, our studies suggest that POU2F-POU2AF-driven malignancies have an intrinsic dependence on the mSWI/SNF complex, representing a therapeutic vulnerability.

BRG1/BRM inhibitor targets AML stem cells and exerts superior preclinical efficacy combined with BET or menin inhibitor

ABSTRACT

BRG1 (SMARCA4) and BRM (SMARCA2) are the mutually exclusive core ATPases of the chromatin remodeling BAF (BRG1/BRM-associated factor) complexes. They enable transcription factors/cofactors to access enhancers/promoter and modulate gene expressions responsible for cell growth and differentiation of acute myeloid leukemia (AML) stem/progenitor cells. In AML with MLL1 rearrangement (MLL1r) or mutant NPM1 (mtNPM1), although menin inhibitor (MI) treatment induces clinical remissions, most patients either fail to respond or relapse, some harboring menin mutations. FHD-286 is an orally bioavailable, selective inhibitor of BRG1/BRM under clinical development in AML. Present studies show that FHD-286 induces differentiation and lethality in AML cells with MLL1r or mtNPM1, concomitantly causing perturbed chromatin accessibility and repression of c-Myc, PU.1, and CDK4/6. Cotreatment with FHD-286 and decitabine, BET inhibitor (BETi) or MI, or venetoclax synergistically induced in vitro lethality in AML cells with MLL1r or mtNPM1. In models of xenografts derived from patients with AML with MLL1r or mtNPM1, FHD-286 treatment reduced AML burden, improved survival, and attenuated AML-initiating potential of stem-progenitor cells. Compared with each drug, cotreatment with FHD-286 and BETi, MI, decitabine, or venetoclax significantly reduced AML burden and improved survival, without inducing significant toxicity. These findings highlight the FHD-286-based combinations as a promising therapy for AML with MLL1r or mtNPM1.

S, Abbineni C, Vaishampayan U, Lyssiotis CA, Parolia A, Xiao L, Chinnaiyan AM. Development of an orally bioavailable mSWI/SNF ATPase degrader and acquired mechanisms of resistance in prostate cancer

Mammalian switch/sucrose nonfermentable (mSWI/SNF) ATPase degraders have been shown to be effective in enhancer-driven cancers by functioning to impede oncogenic transcription factor chromatin accessibility. Here, we developed AU-24118, an orally bioavailable proteolysis-targeting chimera (PROTAC) degrader of mSWI/SNF ATPases (SMARCA2 and SMARCA4) and PBRM1. AU-24118 demonstrated tumor regression in a model of castration-resistant prostate cancer (CRPC) which was further enhanced with combination enzalutamide treatment, a standard of care androgen receptor (AR) antagonist used in CRPC patients. Importantly, AU-24118 exhibited favorable pharmacokinetic profiles in preclinical analyses in mice and rats, and further toxicity testing in mice showed a favorable safety profile. As acquired resistance is common with targeted cancer therapeutics, experiments were designed to explore potential mechanisms of resistance that may arise with long-term mSWI/SNF ATPase PROTAC treatment. Prostate cancer cell lines exposed to long-term treatment with high doses of a mSWI/SNF ATPase degrader developed SMARCA4 bromodomain mutations and ABCB1 (ATP binding cassette subfamily B member 1) overexpression as acquired mechanisms of resistance. Intriguingly, while SMARCA4 mutations provided specific resistance to mSWI/SNF degraders, ABCB1 overexpression provided broader resistance to other potent PROTAC degraders targeting bromodomain-containing protein 4 and AR. The ABCB1 inhibitor, zosuquidar, reversed resistance to all three PROTAC degraders tested. Combined, these findings position mSWI/SNF degraders for clinical translation for patients with enhancer-driven cancers and define strategies to overcome resistance mechanisms that may arise.

ARID1A orchestrates SWI/SNF-mediated sequential binding of transcription factors with ARID1A loss driving pre-memory B cell fate and lymphomagenesis

ARID1A, a subunit of the canonical BAF nucleosome remodeling complex, is commonly mutated in lymphomas. We show that ARID1A orchestrates B cell fate during the germinal center (GC) response, facilitating cooperative and sequential binding of PU.1 and NF-kB at crucial genes for cytokine and CD40 signaling. The absence of ARID1A tilts GC cell fate toward immature IgM+CD80-PD-L2- memory B cells, known for their potential to re-enter new GCs. When combined with BCL2 oncogene, ARID1A haploinsufficiency hastens the progression of aggressive follicular lymphomas (FLs) in mice. Patients with FL with ARID1A-inactivating mutations preferentially display an immature memory B cell-like state with increased transformation risk to aggressive disease. These observations offer mechanistic understanding into the emergence of both indolent and aggressive ARID1A-mutant lymphomas through the formation of immature memory-like clonal precursors. Lastly, we demonstrate that ARID1A mutation induces synthetic lethality to SMARCA2/4 inhibition, paving the way for potential precision therapy for high-risk patients.

S, Abbineni C, Lyssiotis CA, Parolia A, Xiao L, Chinnaiyan AM. Development of an orally bioavailable mSWI/SNF ATPase degrader and acquired mechanisms of resistance in prostate cancer

Mammalian switch/sucrose non-fermentable (mSWI/SNF) ATPase degraders have been shown to be effective in enhancer-driven cancers by functioning to impede oncogenic transcription factor chromatin accessibility. Here, we developed AU-24118, a first-in-class, orally bioavailable proteolysis targeting chimera (PROTAC) degrader of mSWI/SNF ATPases (SMARCA2 and SMARCA4) and PBRM1. AU-24118 demonstrated tumor regression in a model of castration-resistant prostate cancer (CRPC) which was further enhanced with combination enzalutamide treatment, a standard of care androgen receptor (AR) antagonist used in CRPC patients. Importantly, AU-24118 exhibited favorable pharmacokinetic profiles in preclinical analyses in mice and rats, and further toxicity testing in mice showed a favorable safety profile. As acquired resistance is common with targeted cancer therapeutics, experiments were designed to explore potential mechanisms of resistance that may arise with long-term mSWI/SNF ATPase PROTAC treatment. Prostate cancer cell lines exposed to long-term treatment with high doses of a mSWI/SNF ATPase degrader developed SMARCA4 bromodomain mutations and ABCB1 overexpression as acquired mechanisms of resistance. Intriguingly, while SMARCA4 mutations provided specific resistance to mSWI/SNF degraders, ABCB1 overexpression provided broader resistance to other potent PROTAC degraders targeting bromodomain-containing protein 4 (BRD4) and AR. The ABCB1 inhibitor, zosuquidar, reversed resistance to all three PROTAC degraders tested. Combined, these findings position mSWI/SNF degraders for clinical translation for patients with enhancer-driven cancers and define strategies to overcome resistance mechanisms that may arise.

Canonical BAF complex regulates the oncogenic program in human T-cell acute lymphoblastic leukemia

ABSTRACT

Acute leukemia cells require bone marrow microenvironments, known as niches, which provide leukemic cells with niche factors that are essential for leukemic cell survival and/or proliferation. However, it remains unclear how the dynamics of the leukemic cell-niche interaction are regulated. Using a genome-wide CRISPR screen, we discovered that canonical BRG1/BRM-associated factor (cBAF), a variant of the switch/sucrose nonfermenting chromatin remodeling complex, regulates the migratory response of human T-cell acute lymphoblastic leukemia (T-ALL) cells to a niche factor CXCL12. Mechanistically, cBAF maintains chromatin accessibility and allows RUNX1 to bind to CXCR4 enhancer regions. cBAF inhibition evicts RUNX1 from the genome, resulting in CXCR4 downregulation and impaired migration activity. In addition, cBAF maintains chromatin accessibility preferentially at RUNX1 binding sites, ensuring RUNX1 binding at these sites, and is required for expression of RUNX1-regulated genes, such as CDK6; therefore, cBAF inhibition negatively impacts cell proliferation and profoundly induces apoptosis. This anticancer effect was also confirmed using T-ALL xenograft models, suggesting cBAF as a promising therapeutic target. Thus, we provide novel evidence that cBAF regulates the RUNX1-driven leukemic program and governs migration activity toward CXCL12 and cell-autonomous growth in human T-ALL.

The IL6/JAK/STAT3 signaling axis is a therapeutic vulnerability in SMARCB1-deficient bladder cancer

SMARCB1 loss has long been observed in many solid tumors. However, there is a need to elucidate targetable pathways driving growth and metastasis in SMARCB1-deficient tumors. Here, we demonstrate that SMARCB1 deficiency, defined as genomic SMARCB1 copy number loss associated with reduced mRNA, drives disease progression in patients with bladder cancer by engaging STAT3. SMARCB1 loss increases the chromatin accessibility of the STAT3 locus in vitro. Orthotopically implanted SMARCB1 knockout (KO) cell lines exhibit increased tumor growth and metastasis. SMARCB1-deficient tumors show an increased IL6/JAK/STAT3 signaling axis in in vivo models and patients. Furthermore, a pSTAT3 selective inhibitor, TTI-101, reduces tumor growth in SMARCB1 KO orthotopic cell line-derived xenografts and a SMARCB1-deficient patient derived xenograft model. We have identified a gene signature generated from SMARCB1 KO tumors that predicts SMARCB1 deficiency in patients. Overall, these findings support the clinical evaluation of STAT3 inhibitors for the treatment of SMARCB1-deficient bladder cancer.

Pioneer factors: roles and their regulation in development

Pioneer factors are a subclass of transcription factors that can bind and initiate opening of silent chromatin regions. Pioneer factors subsequently regulate lineage-specific genes and enhancers and, thus, activate the zygotic genome after fertilization, guide cell fate transitions during development, and promote various forms of human cancers. As such, pioneer factors are useful in directed cell reprogramming. In this review, we define the structural and functional characteristics of pioneer factors, how they bind and initiate opening of closed chromatin regions, and the consequences for chromatin dynamics and gene expression during cell differentiation. We also discuss emerging mechanisms that modulate pioneer factors during development.

The ncBAF Complex Regulates Transcription in AML Through H3K27ac Sensing by BRD9

The non-canonical BAF complex (ncBAF) subunit BRD9 is essential for acute myeloid leukemia (AML) cell viability but has an unclear role in leukemogenesis. Because BRD9 is required for ncBAF complex assembly through its DUF3512 domain, precise bromodomain inhibition is necessary to parse the role of BRD9 as a transcriptional regulator from that of a scaffolding protein. To understand the role of BRD9 bromodomain function in regulating AML, we selected a panel of five AML cell lines with distinct driver mutations, disease classifications, and genomic aberrations and subjected these cells to short-term BRD9 bromodomain inhibition. We examined the bromodomain-dependent growth of these cell lines, identifying a dependency in AML cell lines but not HEK293T cells. To define a mechanism through which BRD9 maintains AML cell survival, we examined nascent transcription, chromatin accessibility, and ncBAF complex binding genome-wide after bromodomain inhibition. We identified extensive regulation of transcription by BRD9 bromodomain activity, including repression of myeloid maturation factors and tumor suppressor genes, while standard AML chemotherapy targets were repressed by inhibition of the BRD9 bromodomain. BRD9 bromodomain activity maintained accessible chromatin at both gene promoters and gene-distal putative enhancer regions, in a manner that qualitatively correlated with enrichment of BRD9 binding. Furthermore, we identified reduced chromatin accessibility at GATA, ETS, and AP-1 motifs and increased chromatin accessibility at SNAIL-, HIC-, and TP53-recognized motifs after BRD9 inhibition. These data suggest a role for BRD9 in regulating AML cell differentiation through modulation of accessibility at hematopoietic transcription factor binding sites.

SIGNIFICANCE

The bromodomain-containing protein BRD9 is essential for AML cell viability, but it is unclear whether this requirement is due to the protein's role as an epigenetic reader. We inhibited this activity and identified altered gene-distal chromatin regulation and transcription consistent with a more mature myeloid cell state.

Reactivation of the G1 enhancer landscape underlies core circuitry addiction to SWI/SNF

Several cancer core regulatory circuitries (CRCs) depend on the sustained generation of DNA accessibility by SWI/SNF chromatin remodelers. However, the window when SWI/SNF is acutely essential in these settings has not been identified. Here we used neuroblastoma (NB) cells to model and dissect the relationship between cell-cycle progression and SWI/SNF ATPase activity. We find that SWI/SNF inactivation impairs coordinated occupancy of non-pioneer CRC members at enhancers within 1 hour, rapidly breaking their autoregulation. By precisely timing inhibitor treatment following synchronization, we show that SWI/SNF is dispensable for survival in S and G2/M, but becomes acutely essential only during G1 phase. We furthermore developed a new approach to analyze the oscillating patterns of genome-wide DNA accessibility across the cell cycle, which revealed that SWI/SNF-dependent CRC binding sites are enriched at enhancers with peak accessibility during G1 phase, where they activate genes involved in cell-cycle progression. SWI/SNF inhibition strongly impairs G1-S transition and potentiates the ability of retinoids used clinically to induce cell-cycle exit. Similar cell-cycle effects in diverse SWI/SNF-addicted settings highlight G1-S transition as a common cause of SWI/SNF dependency. Our results illustrate that deeper knowledge of the temporal patterns of enhancer-related dependencies may aid the rational targeting of addicted cancers.

An alternative NURF complex sustains acute myeloid leukemia by regulating the accessibility of insulator regions

Efficient treatment of acute myeloid leukemia (AML) patients remains a challenge despite recent therapeutic advances. Here, using a CRISPRi screen targeting chromatin factors, we identified the nucleosome-remodeling factor (NURF) subunit BPTF as an essential regulator of AML cell survival. We demonstrate that BPTF forms an alternative NURF chromatin remodeling complex with SMARCA5 and BAP18, which regulates the accessibility of a large set of insulator regions in leukemic cells. This ensures efficient CTCF binding and boundary formation between topologically associated domains that is essential for maintaining the leukemic transcriptional programs. We also demonstrate that the well-studied PHD2-BROMO chromatin reader domains of BPTF, while contributing to complex recruitment to chromatin, are dispensable for leukemic cell growth. Taken together, our results uncover how the alternative NURF complex contributes to leukemia and provide a rationale for its targeting in AML.

Global identification of SWI/SNF targets reveals compensation by EP400

Mammalian SWI/SNF chromatin remodeling complexes move and evict nucleosomes at gene promoters and enhancers to modulate DNA access. Although SWI/SNF subunits are commonly mutated in disease, therapeutic options are limited by our inability to predict SWI/SNF gene targets and conflicting studies on functional significance. Here, we leverage a fast-acting inhibitor of SWI/SNF remodeling to elucidate direct targets and effects of SWI/SNF. Blocking SWI/SNF activity causes a rapid and global loss of chromatin accessibility and transcription. Whereas repression persists at most enhancers, we uncover a compensatory role for the EP400/TIP60 remodeler, which reestablishes accessibility at most promoters during prolonged loss of SWI/SNF. Indeed, we observe synthetic lethality between EP400 and SWI/SNF in cancer cell lines and human cancer patient data. Our data define a set of molecular genomic features that accurately predict gene sensitivity to SWI/SNF inhibition in diverse cancer cell lines, thereby improving the therapeutic potential of SWI/SNF inhibitors.

Chromatin Profiling of CBFA2T3-GLIS2 AMLs Identifies Key Transcription Factor Dependencies and BRG1 Inhibition as a Novel Therapeutic Strategy

Oncogenic fusions involving transcription factors are present in the majority of pediatric leukemias; however, the context-specific mechanisms they employ to drive cancer remain poorly understood. CBFA2T3-GLIS2 (C/G) fusions occur in treatment-refractory acute myeloid leukemias and are restricted to young children. To understand how the C/G fusion drives oncogenesis we applied CUT&RUN chromatin profiling to an umbilical cord blood/endothelial cell (EC) co-culture model of C/G AML that recapitulates the biology of this malignancy. We find C/G fusion binding is mediated by its zinc finger domains. Integration of fusion binding sites in C/G- transduced cells with Polycomb Repressive Complex 2 (PRC2) sites in control cord blood cells identifies MYCN, ZFPM1, ZBTB16 and LMO2 as direct C/G targets. Transcriptomic analysis of a large pediatric AML cohort shows that these genes are upregulated in C/G patient samples. Single cell RNA-sequencing of umbilical cord blood identifies a population of megakaryocyte precursors that already express many of these genes despite lacking the fusion. By integrating CUT&RUN data with CRISPR dependency screens we identify BRG1/SMARCA4 as a vulnerability in C/G AML. BRG1 profiling in C/G patient-derived cell lines shows that the CBFA2T3 locus is a binding site, and treatment with clinically-available BRG1 inhibitors reduces fusion levels and downstream C/G targets including N-MYC, resulting in C/G leukemia cell death and extending survival in a murine xenograft model.

Transcription factor genetics and biology in predisposition to bone marrow failure and hematological malignancy

Transcription factors (TFs) play a critical role as key mediators of a multitude of developmental pathways, with highly regulated and tightly organized networks crucial for determining both the timing and pattern of tissue development. TFs can act as master regulators of both primitive and definitive hematopoiesis, tightly controlling the behavior of hematopoietic stem and progenitor cells (HSPCs). These networks control the functional regulation of HSPCs including self-renewal, proliferation, and differentiation dynamics, which are essential to normal hematopoiesis. Defining the key players and dynamics of these hematopoietic transcriptional networks is essential to understanding both normal hematopoiesis and how genetic aberrations in TFs and their networks can predispose to hematopoietic disease including bone marrow failure (BMF) and hematological malignancy (HM). Despite their multifaceted and complex involvement in hematological development, advances in genetic screening along with elegant multi-omics and model system studies are shedding light on how hematopoietic TFs interact and network to achieve normal cell fates and their role in disease etiology. This review focuses on TFs which predispose to BMF and HM, identifies potential novel candidate predisposing TF genes, and examines putative biological mechanisms leading to these phenotypes. A better understanding of the genetics and molecular biology of hematopoietic TFs, as well as identifying novel genes and genetic variants predisposing to BMF and HM, will accelerate the development of preventative strategies, improve clinical management and counseling, and help define targeted treatments for these diseases.