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Glucocorticoids (GCs) are a mainstay of contemporary, multidrug chemotherapy in the treatment of childhood acute lymphoblastic leukemia (ALL), and resistance to GCs remains a major clinical concern. Resistance to GCs is predictive of ALL relapse and poor clinical outcome, and therefore represents a major hurdle limiting further improvements in survival rates. While advances have been made in identifying genes implicated in GC resistance, there remains an insufficient understanding of the impact of cis-regulatory disruptions in resistance. To address this, we mapped the gene regulatory response to GCs in two ALL cell lines using functional genomics and high-throughput reporter assays and identified thousands of GC-responsive changes to chromatin state, including the formation of over 250 GC-responsive super-enhancers and a depletion of AP-1 bound cis-regulatory elements implicated in cell proliferation and anti-apoptotic processes. By integrating our GC response maps with genetic and epigenetic datasets in primary ALL cells from patients, we further uncovered cis-regulatory disruptions at GC-responsive genes that impact GC resistance in childhood ALL. Overall, these data indicate that GCs initiate pervasive effects on the leukemia epigenome, and that alterations to the GC gene regulatory network contribute to GC resistance.

Understanding the genomic and epigenetic mechanisms of drug resistance in pediatric acute lymphoblastic leukemia (ALL) is critical for further improvements in treatment outcomes. The role of transcriptomic response in conferring resistance to l-asparaginase (LASP) is poorly understood beyond asparagine synthetase (ASNS). We defined reproducible LASP response genes in LASP-resistant and LASP-sensitive ALL cell lines as well as primary leukemia samples from newly diagnosed patients. Defining target genes of the amino acid stress response-related transcription factor activating transcription factor 4 (ATF4) in ALL cell lines using chromatin immunoprecipitation sequencing (ChIP-seq) revealed 45% of genes that changed expression after LASP treatment were direct targets of the ATF4 transcription factor, and 34% of these genes harbored LASP-responsive ATF4 promoter binding events. SLC7A11 was found to be a response gene in cell lines and patient samples as well as a direct target of ATF4. SLC7A11 was also one of only 2.4% of LASP response genes with basal level gene expression that also correlated with LASP ex vivo resistance in primary leukemia cells. Experiments using chemical inhibition of SLC7A11 with sulfasalazine, gene overexpression, and partial gene knockout recapitulated LASP resistance or sensitivity in ALL cell lines. These findings show the importance of assessing changes in gene expression following treatment with an antileukemic agent for its association with drug resistance and highlight that many response genes may not differ in their basal expression in drug-resistant leukemia cells.

SUMMARYB-cell lineage acute lymphoblastic leukemia (B-ALL) is comprised of diverse molecular subtypes and while transcriptional and DNA methylation profiling of B-ALL subtypes has been extensively examined, the accompanying chromatin landscape is not well characterized for many subtypes. We therefore mapped chromatin accessibility using ATAC-seq for 10 B-ALL molecular subtypes in primary ALL cells from 154 patients. Comparisons with B-cell progenitors identified candidate B-ALL cell-of-origin and AP-1-associatedcis-regulatory rewiring in B-ALL.Cis-regulatory rewiring promoted B-ALL-specific gene regulatory networks impacting oncogenic signaling pathways that perturb normal B-cell development. We also identified that over 20% of B-ALL accessible chromatin sites exhibit strong subtype enrichment, with transcription factor (TF) footprint profiling identifying candidate TFs that maintain subtype-specific chromatin architectures. Over 9000 inherited genetic variants were further uncovered that contribute to variability in chromatin accessibility among individual patient samples. Overall, our data suggest that distinct chromatin architectures are driven by diverse TFs and inherited genetic variants which promote unique gene regulatory networks that contribute to transcriptional differences among B-ALL subtypes.HIGHLIGHTSPro-B progenitor cells as the most common cell-of-origin for B-ALLAP-1 TF-associatedcis-regulatory rewiring in B-ALLSubtype-specific accessible chromatin signatures representing 20% of all B-ALL sitesRole for distinct TFs in promoting subtype-specific chromatin architecturesThousands of inherited genetic variants identified impacting chromatin state

Although acute lymphoblastic leukemia (ALL) is the most common childhood cancer, there is limited understanding of the contribution of inherited genetic variation on inter-individual differences in chemotherapy response. Defining genetic factors impacting therapy failure can help better predict response and identify drug resistance mechanisms. We therefore mapped inherited noncoding variants associated with chemotherapeutic drug resistance and/or treatment outcome to ALLcis-regulatory elements and investigated their gene regulatory potential and genomic connectivity using massively parallel reporter assays and promoter capture Hi-C, respectively. We identified 53 variants with reproducible allele-specific effects on transcription and high-confidence gene targets. Subsequent functional interrogation of the top variant (rs1247117) determined that it disrupted a PU.1 consensus motif and PU.1 binding affinity. Importantly, deletion of the genomic interval containing rs1247117 sensitized ALL cells to vincristine. Together, these data demonstrate that noncoding regulatory variation associated with diverse pharmacological traits harbor significant effects on allele-specific transcriptional activity and impact sensitivity to chemotherapeutic agents in ALL.

Glucocorticoids (GCs; i.e., steroids) are important chemotherapeutic agents in the treatment of B-cell precursor acute lymphoblastic leukemia (B-ALL) andde novoGC resistance predicts relapse and poor clinical outcome in patients. Glucocorticoids induce B-ALL cell apoptosis through activation of glucocorticoid receptor (GR), a ligand-induced nuclear receptor transcription factor (TF). We previously identified disruptions to glucocorticoid receptor (GR)-boundcis-regulatory elements controllingTLE1expression in GC-resistant primary B-ALL cells from patients.TLE1is a GC-response gene up-regulated by steroids and functions as a canonical Wnt signaling repressor. To better understand the mechanistic relationship between GC signaling and canonical Wnt signaling, we performed diverse functional analyses that identified extensive crosstalk and mutual antagonism between these two signaling pathways in B-ALL. We determined that crosstalk and antagonism was driven by the binding of GR and the canonical Wnt signaling TFs LEF1 and TCF7L2 to overlapping sets ofcis-regulatory elements associated with genes impacting cell death and cell proliferation, and was further accompanied by overlapping and opposing transcriptional programs. Our data additionally suggest thatcis-regulatory disruptions atTLE1are linked to GC resistance through a dampening of the GC response and GC-mediated apoptosis via enhanced canonical Wnt signaling. As a result of the extensive genomic and gene regulatory connectivity between these two signaling pathways, our data supports the importance of canonical Wnt signaling in mediating GC resistance in B-ALL.

Acute lymphoblastic leukemia (ALL) is a hematopoietic malignancy comprised of molecular subtypes largely characterized by aneuploidy or recurring chromosomal rearrangements. Despite extensive information on the ALL transcriptome and methylome, there is limited understanding of the ALL chromatin landscape. We therefore mapped accessible chromatin in 24 primary ALL cell biospecimens comprising three common molecular subtypes (DUX4/ERG, ETV6-RUNX1 and hyperdiploid) from patients treated at St. Jude Children's Research Hospital. Our findings highlight extensive chromatin reprogramming in ALL, including the identification ALL subtype-specific chromatin landscapes that are additionally modulated by genetic variation. Chromatin accessibility differences between ALL and normal B-cells implicate the activation of B-cell repressed chromatin domains and detail the disruption of normal B-cell development in ALL. Among ALL subtypes, we uncovered roles for basic helix-loop-helix, homeodomain and activator protein 1 transcription factors in promoting subtype-specific chromatin accessibility and distinct gene regulatory networks. In addition to chromatin subtype-specificity, we further identified over 3500 DNA sequence variants that alter the ALL chromatin landscape and contribute to inter-individual variability in chromatin accessibility. Collectively, our data suggest that subtype-specific chromatin landscapes and gene regulatory networks impact ALL biology and contribute to transcriptomic differences among ALL subtypes.

Acute lymphoblastic leukemia (ALL) is the most prevalent childhood cancer and despite improved survival rates, relapsed ALL is still among the most common causes of cancer death in children. Although changes in the expression of specific genes have been linked to chemotherapeutic resistance, relatively little is understood of the pharmacogenomic impact of the noncoding, cis-regulatory landscape governing gene regulation. Glucocorticoids (GCs; i.e. steroids) are a mainstay of contemporary, multi-drug chemotherapy in ALL, and GC resistance is predictive of both relapse and poor clinical outcome in ALL. Because GCs function through activation of glucocorticoid receptor (GR), a nuclear receptor transcription factor that interacts directly with cis-regulatory elements, unveiling the glucocorticoid gene regulatory network (GC-GRN) in leukemia cells is crucial to understanding not only the biological mechanism of apoptosis, but also illuminating gene regulatory mechanisms contributing to GC resistance. To test the hypothesis that alterations to the GC-GRN are important contributors to steroid resistance in ALL, we comprehensively mapped cellular responses to GCs in human ALL cell lines using >100 independent functional genomic datasets. This comprehensive approach uncovered thousands of genes and cis-regulatory elements that were responsive to GCs, and further identified >38,000 high-confidence glucocorticoid response elements (GREs) in the ALL genome. A closer examination of these data revealed GR binding profiles that were consistent with the long-range flexible billboard model of gene regulation. By further integrating our results with genetic and epigenetic data in primary ALL cells from patients enrolled on St. Jude clinical trials, we identified 45 DNA sequence variants associated with ex vivo GC resistance that map to GREs and functionally validated an associated variant within the TLE1 gene locus. We also uncovered 1929 accessible chromatin sites (FDR10,000 sgRNAs targeting >1000 GR binding events at putative GC-resistance accessible chromatin sites identified a subset of GR binding sites implicated in GC resistance. Overall, these data indicate that GCs initiate pervasive, genome-wide effects on the leukemia epigenome and transcriptome, and that genetic and epigenetic alterations to GREs are mechanisms contributing to GC resistance in childhood ALL. Disclosures Pui: Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Novartis: Other: Data Monitoring Committee. Evans: Princess Máxima Center for Pediatric Oncology, Scientific Advisory Board, Chair: Membership on an entity's Board of Directors or advisory committees; BioSkryb, Inc.: Membership on an entity's Board of Directors or advisory committees; St. Jude Children's Research Hospital, Emeritus Member (began Jan 2021): Ended employment in the past 24 months.

Understanding the genomic and epigenetic mechanisms of drug resistance in pediatric acute lymphoblastic leukemia (ALL) is critical for further improvements in treatment outcome. The role of transcriptomic response in conferring resistance to l-asparaginase (LASP) is poorly understood, beyond asparagine synthetase (ASNS). We defined reproducible LASP response genes in LASP resistant and sensitive ALL cell lines (n = 7) as well as primary leukemia samples from newly diagnosed patients. We identified 2219 response genes (absolute log 2FC > 1.5, FDR p-value Disclosures Stock: Pfizer: Consultancy, Honoraria, Research Funding; amgen: Honoraria; agios: Honoraria; jazz: Honoraria; kura: Honoraria; kite: Honoraria; morphosys: Honoraria; servier: Honoraria; syndax: Consultancy, Honoraria; Pluristeem: Consultancy, Honoraria. Mullighan: Amgen: Current equity holder in publicly-traded company; Illumina: Membership on an entity's Board of Directors or advisory committees; AbbVie: Research Funding; Pfizer: Research Funding. Pui: Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Novartis: Other: Data Monitoring Committee. Evans: Princess Máxima Center for Pediatric Oncology, Scientific Advisory Board, Chair: Membership on an entity's Board of Directors or advisory committees; BioSkryb, Inc.: Membership on an entity's Board of Directors or advisory committees; St. Jude Children's Research Hospital, Emeritus Member (began Jan 2021): Ended employment in the past 24 months.

Acute lymphoblastic leukemia (ALL) is the most common malignancy in pediatric patients spanning both B- and T-cell lineages. ALL also occurs less commonly in adults but with cure rates of only around 30%. Past work has characterized ALL into molecular subtypes spanning a range of aberrant chromosomal rearrangements and oncogene chimeric fusions driving malignancy. While transcriptional profiling of these subtypes has been extensively examined, the accompanying chromatin accessibility landscape and corresponding gene regulatory repertoire is not well characterized for many subtypes. To better profile the ALL epigenomic and gene regulatory repertoire we examined chromatin accessibility of 12 distinct molecular subtypes (BCR-ABL1, ETV6-RUNX1, Hyperdiploid, Hypodiploid, KMT2A rearranged, Ph-Like, PAX5, DUX4/ERG, TCF3-PBX1, T-ALL, Early T Precursor and B-other) across 189 primary patient samples of pediatric ALL (n = 106) and adult ALL (n = 83) origin using ATAC-seq. To our knowledge, this represents the largest collection of chromatin accessibility data in primary ALL samples spanning multiple molecular subtypes to date. Collectively, we identified over 600,000 accessible chromatin sites in the ALL genome with over 50,000 regions of differentially accessible chromatin encompassing both common and subtype-specific modalities. Further, transcription factor (TF) footprint profiling of ATAC-seq yielded tens of thousands of candidate TF binding events and identified key TF drivers within distinct molecular subtypes. We additionally performed H3K27ac ChIP-seq in a subset of 12 primary ALL patient samples, with integration of these histone data for select patient samples allowing inferences about candidate super-enhancer drivers of ALL molecular subtypes. Overall, these analyses and data offer a window into the gene regulatory and epigenetic landscape of ALL, and further highlight the complexity and heterogeneity of accessible chromatin landscapes among distinct molecular subtypes of ALL. Disclosures Stock: Pfizer: Consultancy, Honoraria, Research Funding; amgen: Honoraria; agios: Honoraria; jazz: Honoraria; kura: Honoraria; kite: Honoraria; morphosys: Honoraria; servier: Honoraria; syndax: Consultancy, Honoraria; Pluristeem: Consultancy, Honoraria. Pui: Adaptive Biotechnologies: Membership on an entity's Board of Directors or advisory committees; Novartis: Other: Data Monitoring Committee. Evans: Princess Máxima Center for Pediatric Oncology, Scientific Advisory Board, Chair: Membership on an entity's Board of Directors or advisory committees; St. Jude Children's Research Hospital, Emeritus Member (began Jan 2021): Ended employment in the past 24 months; BioSkryb, Inc.: Membership on an entity's Board of Directors or advisory committees.