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Mutations in the gene Additional Sex-Combs Like 1 (ASXL1) are recurrent in myeloid malignancies as well as the pre-malignant condition clonal hematopoiesis, where they are universally associated with poor prognosis. However, the role of ASXL1 in myeloid lineage maturation is incompletely described. To define the role of ASXL1 in myelopoiesis, we employed single cell RNA sequencing and a murine model of hematopoietic-specific Asxl1 deletion. In granulocyte progenitors, Asxl1 deletion leads to hyperactivation of MYC and a quantitative decrease in neutrophil production. This loss of granulocyte production was not accompanied by significant changes in the landscape of covalent histone modifications. However, Asxl1 deletion results in a decrease in RNAPII promoter-proximal pausing in granulocyte progenitors, indicative of a global increase in productive transcription. These results suggest that ASXL1 inhibits productive transcription in granulocyte progenitors, identifying a new role for this epigenetic regulator in myeloid development.

Normalized Read Pileup and Differential Enrichment Clusters of MOLM13 H3K27Ac CUT&Tag (2 Hours After Drug Treatment)

STAT5 and MYC play a key role in the response to FLT3/LSD1 inhibition.

RNA PolII Pause Index Analysis from MOLM13 RBP1 CUT&Tag (6 Hours After Drug Treatment)

GO Analysis from Regions of Differential MOLM13 H3K27Ac CUT&Tag (6 Hours After Drug Treatment) Pileup at Enhancers and Promoters

Supplementary Materials and Methods

Log2 Normalized Counts of Differentially Expressed Genes from Primary AML Blast Bulk RNA-seq (24 Hours After Drug Treatment)

Stat5 over-expression diminishes the efficacy of combined FLT3/LSD1 inhibition.

Single cell ATAC-seq cell population dynamics in primary patient samples follow-ing treatment with dual FLT3/LSD1 inhibition.

Combined FLT3/LSD1 inhibition does not substantially alter myeloid differentiation.

Differential Phosphoprotein Network Enrichment in MOLM13 Cells (24 Hours After Drug Treatment)

Efficacy of dual FLT3/LSD1 inhibition in MOLM13 cells.

Normalized Read Pileup and Differential Enrichment Clusters of MOLM13 H3K27Ac CUT&Tag (6 Hours After Drug Treatment) at Enhancers and Promoters

Epigenetic impact of dual FLT3/LSD1 inhibition.

SPI1 knockdown does not disrupt dual FLT3/LSD1 inhibition synergy.

Dual FLT3/LSD1 inhibition suppresses MYC expression and activity.

Mutations in Fms-like tyrosine kinase 3 (FLT3) are common drivers in acute myeloid leukemia (AML) yet FLT3 inhibitors only provide modest clinical benefit. Prior work has shown that inhibitors of lysine-specific demethylase 1 (LSD1) enhance kinase inhibitor activity in AML. Here we show that combined LSD1 and FLT3 inhibition induces synergistic cell death in FLT3-mutant AML. Multi-omic profiling revealed that the drug combination disrupts STAT5, LSD1, and GFI1 binding at the MYC blood superenhancer, suppressing superenhancer accessibility as well as MYC expression and activity. The drug combination simultaneously results in the accumulation of repressive H3K9me1 methylation, an LSD1 substrate, at MYC target genes. We validated these findings in 72 primary AML samples with the nearly every sample demonstrating synergistic responses to the drug combination. Collectively, these studies reveal how epigenetic therapies augment the activity of kinase inhibitors in FLT3-ITD (internal tandem duplication) AML.Implications:This work establishes the synergistic efficacy of combined FLT3 and LSD1 inhibition in FLT3-ITD AML by disrupting STAT5 and GFI1 binding at the MYC blood-specific superenhancer complex.

Transcription Factor Enrichment Analysis of Differentially Downregulated Genes by The Drug Combination Previously Identified as Depleting Genes in a Genome-Wide CRISPR Dropout Screen

Transcription Factor Enrichment Analysis of MOLM13 Bulk RNA-Seq (24 Hours After Drug Treatment)

Transcription Factor Enrichment Analysis of Differentially Downregulated Genes by The Drug Combination Previously Identified as Depleting Genes in a Genome-Wide CRISPR Dropout Screen

Transcription factors are critical regulators of cellular gene expression programs. Disruption of normal transcription factor regulation is associated with a broad range of diseases. In order to understand the mechanisms that underly disease pathogenesis, it is critical to detect aberrant transcription factor activity. We have developed Priori, a computational method to predict transcription factor activity from RNA sequencing data. Priori has several key advantages over existing methods. Priori utilizes literature-supported regulatory relationship information to identify known transcription factor target genes. Using these transcriptional relationships, Priori uses linear models to determine the impact and direction of transcription factor regulation on the expression of its target genes. In our work, we evaluated the ability of Priori and 16 other methods to detect aberrant activity from 124 single-gene perturbation experiments. We show that Priori identifies perturbed transcription factors with greater sensitivity and specificity than other methods. Furthermore, our work demonstrates that Priori can be used to discover significant determinants of survival in breast cancer as well as identify mediators of drug response in leukemia from primary patient samples.

Extracellular vesicles (EVs) have been shown as key mediators of extracellular small RNA transport. However, carriers of cell-free messenger RNA (cf-mRNA) in human biofluid and their association with cancer remain poorly understood. Here, we performed a transcriptomic analysis of size-fractionated plasma from lung cancer, liver cancer, multiple myeloma, and healthy donors. Morphology and size distribution analysis showed the successful separation of medium and small EVs and non-vesicular carriers. We developed a strategy to purify and sequence ultra-low amounts of cf-mRNA from vesicular and non-vesicular subpopulations with the implementation of RNA spike-ins to control for technical variability and to normalize for intrinsic drastic differences in the amount of cf-mRNA carried in each plasma fraction. We found that the majority of cf-mRNA was enriched and protected in EVs with remarkable stability in RNase-rich environments. We observed specific enrichment patterns of cancer-associated cf-mRNA in each vesicular and non-vesicular subpopulation. The EV-enriched differentiating genes were associated with specific biological pathways, such as immune systems, liver function, and toxic substance regulation in lung cancer, liver cancer, and multiple myeloma, respectively. Our results suggest that dissecting the complexity of EVs subpopulations illuminates their biological significance and offers a promising liquid biopsy approach.

The characterization of cancer genomes has provided insight into somatically altered genes across tumors, transformed our understanding of cancer biology, and enabled tailoring of therapeutic strategies. However, the function of most cancer alleles remains mysterious, and many cancer features transcend their genomes. Consequently, tumor genomic characterization does not influence therapy for most patients. Approaches to understand the function and circuitry of cancer genes provide complementary approaches to elucidate both oncogene and non-oncogene dependencies. Emerging work indicates that the diversity of therapeutic targets engendered by non-oncogene dependencies is much larger than the list of recurrently mutated genes. Here we describe a framework for this expanded list of cancer targets, providing novel opportunities for clinical translation.

Mutations in Fms-like tyrosine kinase 3 (FLT3) are common drivers in acute myeloid leukemia (AML) yet FLT3 inhibitors only provide modest clinical benefit. Prior work has shown that inhibitors of lysine-specific demethylase 1 (LSD1) enhance kinase inhibitor activity in AML. Here we show that combined LSD1 and FLT3 inhibition induces synergistic cell death in FLT3-mutant AML. Multi-omic profiling revealed that the drug combination disrupts STAT5, LSD1, and GFI1 binding at the MYC blood superenhancer, suppressing superenhancer accessibility as well as MYC expression and activity. The drug combination simultaneously results in the accumulation of repressive H3K9me1 methylation, an LSD1 substrate, at MYC target genes. We validated these findings in 72 primary AML samples with the nearly every sample demonstrating synergistic responses to the drug combination. Collectively, these studies reveal how epigenetic therapies augment the activity of kinase inhibitors in FLT3-ITD (internal tandem duplication) AML. Implications: This work establishes the synergistic efficacy of combined FLT3 and LSD1 inhibition in FLT3-ITD AML by disrupting STAT5 and GFI1 binding at the MYC blood-specific superenhancer complex.

Responses to kinase-inhibitor therapy in AML are frequently short-lived due to the rapid development of resistance, limiting the clinical efficacy. Combination therapy may improve initial therapeutic responses by targeting pathways used by leukemia cells to escape monotherapy. Here we report that combined inhibition of KIT and lysine-specific demethylase 1 (LSD1) produces synergistic cell death in KIT-mutant AML cell lines and primary patient samples. This drug combination evicts both MYC and PU.1 from chromatin driving cell cycle exit. Using a live cell biosensor for AKT activity, we identify early adaptive changes in kinase signaling following KIT inhibition that are reversed with the addition of LSD1 inhibitor via modulation of the GSK3a/b axis. Multi-omic analyses, including scRNA-seq, ATAC-seq and CUT&Tag, confirm these mechanisms in primary KIT-mutant AML. Collectively, this work provides rational for a clinical trial to assess the efficacy of KIT and LSD1 inhibition in patients with KIT-mutant AML.

Activating mutations in the KIT tyrosine receptor kinase confer an adverse prognosis for patients with acute myeloid leukemia (AML). Outside of bone marrow transplantation, treatment options are limited. Here we demonstrate combined KIT and LSD1 inhibition produces synergistic cell death against KIT mutant AML cells. This combination suppresses MYC expression to drive cell cycle exit and apoptosis. This decreased MYC expression results from a loss of PU.1 binding at downstream MYC enhancers. The drug combination also inactivates PI3K/AKT/GSK3a/b signaling to decrease MYC protein abundance. KIT-mutant AML cells rapidly adapt to KIT inhibitor monotherapy by restoring PI3K/AKT activity, but cannot when treated with combined KIT and LSD1 inhibitor. In addition, we validate MYC suppression as a mechanism of synergy in KIT-mutant AML patient samples. Collectively, this work provides rational for a clinical trial to assess the efficacy of KIT and LSD1 inhibition in patients with KIT-mutant AML.Statement of significanceEffective treatment options for AML are limited. We describe the synergistic response to combined KIT and LSD1 inhibition in KIT-mutant AML and identify key biomarkers of drug response. The specificity and efficacy of this combination in cell lines and patient samples provides rationale for investigation in early phase clinical trials.

SummaryMechanisms of therapeutic resistance manifest in metastatic cancers as tumor cell intrinsic alterations and extrinsic microenvironmental influences that can change during treatment. To support the development of methods for the identification of these mechanisms in individual patients, we present here an Omic and Multidimensional Spatial (OMS) Atlas generated from four serial biopsies of a metastatic breast cancer patient during 3.5 years of therapy. This resource links detailed, longitudinal clinical metadata including treatment times and doses, anatomic imaging, and blood-based response measurements to exploratory analytics including comprehensive DNA, RNA, and protein profiles, images of multiplexed immunostaining, and 2- and 3-dimensional scanning electron micrographs. These data reveal aspects of therapy-associated heterogeneity and evolution of the cancer’s genome, signaling pathways, immune microenvironment, cellular composition and organization, and ultrastructure. We present illustrative examples showing how integrative analyses of these data provide insights into potential mechanisms of response and resistance, and suggest novel therapeutic vulnerabilities.

Mutations in the gene Additional Sex-Combs Like 1 (ASXL1) are recurrent in myeloid malignancies as well as the pre-malignant condition clonal hematopoiesis, where they are universally associated with poor prognosis. An epigenetic regulator, ASXL1 canonically directs the deposition of H3K27me3 via the polycomb repressive complex 2. However, its precise role in myeloid lineage maturation is incompletely described. We utilized single cell RNA sequencing (scRNA-seq) on a murine model of hematopoietic-specific ASXL1 deletion and identified a specific role for ASXL1 in terminal granulocyte maturation. Terminal maturation is accompanied by down regulation of Myc expression and cell cycle exit. ASXL1 deletion leads to hyperactivation of Myc in granulocyte precursors and a quantitative decrease in neutrophil production. This failure of normal developmentallyassociated Myc suppression is not accompanied by significant changes in the landscape of covalent histone modifications including H3K27me3. Examining the genome-wide localization of ASXL1 in myeloid progenitors revealed strong co-localization with RNA Polymerase II (RNAPII) at the promoters and spread across the gene bodies of transcriptionally active genes. ASXL1 deletion results in a decrease in RNAPII promoter-proximal pausing in granulocyte progenitors, indicative of a global increase in productive transcription, consistent with the known role of ASXL1 as a mediator of RNAPII pause release. These results suggest that ASXL1 inhibits productive transcription in granulocyte progenitors, identifying a new role for this epigenetic regulator and highlighting a novel potential oncogenic mechanism for ASXL1 mutations in myeloid malignancies.

Mechanisms of therapeutic resistance and vulnerability evolve in metastatic cancers as tumor cells and extrinsic microenvironmental influences change during treatment. To support the development of methods for identifying these mechanisms in individual people, here we present an omic and multidimensional spatial (OMS) atlas generated from four serial biopsies of an individual with metastatic breast cancer during 3.5 years of therapy. This resource links detailed, longitudinal clinical metadata that includes treatment times and doses, anatomic imaging, and blood-based response measurements to clinical and exploratory analyses, which includes comprehensive DNA, RNA, and protein profiles; images of multiplexed immunostaining; and 2- and 3-dimensional scanning electron micrographs. These data report aspects of heterogeneity and evolution of the cancer genome, signaling pathways, immune microenvironment, cellular composition and organization, and ultrastructure. We present illustrative examples of how integrative analyses of these data reveal potential mechanisms of response and resistance and suggest novel therapeutic vulnerabilities.

Myotonic dystrophy (dystrophia myotonica, DM) is a multi-systemic disease caused by expanded CTG or CCTG microsatellite repeats. Characterized by symptoms in muscle, heart and central nervous system, among others, it is one of the most variable diseases known. A major pathogenic event in DM is the sequestration of muscleblind-like proteins by CUG or CCUG repeat-containing RNAs transcribed from expanded repeats, and differences in the extent of MBNL sequestration dependent on repeat length and expression level may account for some portion of the variability. However, many other cellular pathways are reported to be perturbed in DM, and the severity of specific disease symptoms varies among individuals. To help understand this variability and facilitate research into DM, we generated 120 RNASeq transcriptomes from skeletal and heart muscle derived from healthy and DM1 biopsies and autopsies. A limited number of DM2 and Duchenne muscular dystrophy samples were also sequenced. We analyzed splicing and gene expression, identified tissue-specific changes in RNA processing and uncovered transcriptome changes strongly correlating with muscle strength. We created a web resource at http://DMseq.org that hosts raw and processed transcriptome data and provides a lightweight, responsive interface that enables browsing of processed data across the genome.

SETBP1 (SET Binding Protein 1) mutations are associated with exceptionally poor prognosis in myeloid neoplasms. Despite this, SETBP1's role in oncogenesis remains incompletely understood. In this study, we find that SETBP1 leads to a marked upregulation of the Myc oncogene and associated pro-stem and progenitor programs through epigenetic dysregulation. We further identify an epigenetic modulatory drug that normalizes SETBP1-driven Myc overexpression and synergizes with disease-relevant therapy. SETBP1 has documented roles in both the regulation of tumor suppressor pathways and modulation of transcription. To understand how SETBP1 modulates leukemia biology and leverage this mechanistic insight to develop novel therapeutic strategies, we turned to a genetically well-defined model system, chronic neutrophilic leukemia (CNL). SETBP1 is mutated in approximately half of all cases of CNL, a myeloproliferative neoplasm characterized by the presence of signaling-activating mutations in Colony Stimulating Factor 3 Receptor (CSF3R). By expressing SETBP1 and CSF3R mutations in mouse hematopoietic progenitors, we have generated models of CNL that can be leveraged for mechanistic studies and drug development. In a hematopoietic colony forming unit (CFU) assay, murine hematopoietic progenitors co-expressing mutant SETBP1 and mutant CSF3R have a high proliferation phenotype compared to those with mutant CSF3R alone. When cells co-expressing mutant SETBP1 and CSF3R are transplanted into lethally irradiated mice, they develop a rapidly lethal disease relative to the control mice. This is associated with an expansion of the granulocyte lineage, quantified by complete blood count, flow cytometry and histology. We find that SETBP1 is essential for the induction of a pro-proliferative transcriptional program. One of the most prominent SETBP1-associated signatures is that of Myc target genes. Myc itself is one of the top differentially expressed genes driven by SETBP1. Congruent with its increased expression, we also find higher Myc transcriptional activity in cells overexpressing SETBP1. To better understand the epigenetic regulation of Myc and progenitor pathways by SETBP1, we employed a low input profiling methods called CUT&Tag to assess the activation of regulatory elements. We began by profiling two epigenetic marks associated with active enhancers-H3K4me1 and H3K27Ac. We also assessed activation of the Myc promoter by measuring H3K27Ac and H3K4me3. Together this data helps us to understand the epigenetic underpinnings for dysregulation of Myc-driven programs by SETBP1. Therapeutic strategies that normalize aberrant Myc activity may be effective against SETBP1-driven disease. We found that SETBP1 and CSF3R transformed hematopoietic progenitors are highly sensitive to inhibitors of the epigenetic regulator lysine specific demethylase 1 (LSD1). LSD1 inhibition restores Myc expression to physiological levels and represses Myc promotor activity in vitro. Furthermore, LSD1 inhibitors are highly synergistic with JAK inhibitors, which block signaling downstream of CSF3R. Together these data establish the importance of Myc activation in SETBP1-driven malignancies and identify a therapeutic approach to normalize aberrant Myc activity. In future, we will use the insight gained in this genetically well defined disease to inform studies on the mechanistic basis and therapeutic vulnerabilities of other SETBP1-mutant myeloid malignancies. Disclosures Maxson: Gilead Sciences: Research Funding; Ionis Pharmaceuticals: Other: Joint oversight committee for a collaboration between OHSU and Ionis Pharmaceuticals.

Responses to kinase inhibitors in AML are short-lived with the inevitable development of resistance. Combination therapy with agents from distinct functional classes is one strategy to overcome this resistance. Using AML cell lines and primary patient samples harboring oncogenic cKIT mutations we demonstrate that inhibition of the epigenetic regulator Lysine-Specific Demethylase 1 (LSD1) markedly augments the cytotoxic effect of the KIT inhibitor avapritinib. Transcriptomic and epigenetic profiling revealed synergistic suppression of Myc activity accompanied by a global loss of acetylation and decreased expression of key leukemia cell cycle genes. This epigenetic reorganization results from disruption of key transcription factor networks responsible for maintaining leukemia cell self-renewal and survival. These findings demonstrate that combined KIT and LSD1 inhibition is an exciting new therapeutic avenue for KIT mutant AML. Previously, we have shown targeting the leukemic epigenome as a possible mechanism to augment the efficacy of kinase inhibitors. Thus, we hypothesize that targeting both the cKIT and CBF mutations with combination therapy will result in a deeper molecular response. Kasumi-1 cells harbor mutant cKIT, making an ideal model to study this combination. We observed a synergistic response to a clinically available KIT inhibitor, avapritinib, and two different LSD1 inhibitors, ORY-1001 and GSK-LSD1. Colony formation of healthy CD34+ cells and cell growth of cKIT WT cells were not significantly altered by the combination, thus showing the specificity of this combination by targeting mutant KIT. Bulk RNA-seq and master regulator analysis revealed greater suppression of Myc target genes with dual inhibition of KIT and LSD1. Using Cleavage Under Targets and Tagmentation (CUT&Tag), we observed synergistic loss of H3K27Ac at the promoters of genes necessary for cell proliferation. Additionally, we used Cleavage Under Targets and Release Using Nuclease (CUT&RUN) to interrogate Myc localization, and found a greater loss of Myc binding with dual inhibition, overlapping with the loss of acetylation at these loci. This study demonstrates the synergistic efficacy of combination therapy and identifies key biomarkers for drug activity, namely loss of histone acetylation and Myc binding. Collectively, combined KIT and LSD1 inhibition may be an effective therapeutic approach for cKIT mutant AML. Citation Format: Brittany M. Smith, Daniel J. Coleman, Lauren Maloney, Cody Coblentz, Zachary Schonrock, Joseph Estabrook, Theresa Lusardi, Brian J. Druker, Julia E. Maxson, Theodore P. Braun. Combined inhibition of cKIT and Lysine-Specific Demethylase 1 as a therapeutic strategy in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1133.

The unstable (CTG·CAG)n trinucleotide repeat in the myotonic dystrophy type 1 (DM1) locus is bidirectionally transcribed from genes with terminal overlap. By transcription in the sense direction, the DMPK gene produces various alternatively spliced mRNAs with a (CUG)n repeat in their 3′ UTR. Expression in opposite orientation reportedly yields (CAG)n-repeat containing RNA, but both structure and biologic significance of this antisense gene (DM1-AS) are largely unknown. Via a combinatorial approach of computational and experimental analyses of RNA from unaffected individuals and DM1 patients we discovered that DM1-AS spans >6 kb, contains alternative transcription start sites and uses alternative polyadenylation sites up- and downstream of the (CAG)n repeat. Moreover, its primary transcripts undergo alternative splicing, whereby the (CAG)n segment is removed as part of an intron. Thus, in patients a mixture of DM1-AS RNAs with and without expanded (CAG)n repeat are produced. DM1-AS expression appears upregulated in patients, but transcript abundance remains very low in all tissues analyzed. Our data suggest that DM1-AS transcripts belong to the class of long non-coding RNAs. These and other biologically relevant implications for how (CAG)n-expanded transcripts may contribute to DM1 pathology can now be explored experimentally.