Epigenomic mapping reveals distinct B cell acute lymphoblastic leukemia chromatin architectures and regulators

H1-0 is a specific mediator of the repressive ETV6::RUNX1 transcriptional landscape in preleukemia and B cell acute lymphoblastic leukemia

ETV6::RUNX1, the most common oncogenic fusion in pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL), induces a clinically silent preleukemic state that can persist in carriers for over a decade and may progress to overt leukemia upon acquisition of secondary lesions. The mechanisms contributing to quiescence of ETV6::RUNX1+ preleukemic cells still remain elusive. In this study, we identify linker histone H1-0 as a critical mediator of the ETV6::RUNX1+ preleukemic state by employing human -induced pluripotent stem cell (hiPSC) models engineered by using CRISPR/Cas9 gene editing. Global gene expression analysis revealed upregulation of H1-0 in ETV6::RUNX1+ hiPSCs that was preserved upon hematopoietic differentiation. Moreover, whole transcriptome data of 1,727 leukemia patient samples showed significantly elevated H1-0 levels in ETV6::RUNX1+ BCP-ALL compared to other leukemia entities. Using dual-luciferase promoter assays, we show that ETV6::RUNX1 induces H1-0 promoter activity. We further demonstrate that depletion of H1-0 specifically inhibits ETV6::RUNX1 signature genes, including RAG1 and EPOR. Single-cell sequencing showed that H1-0 is highly expressed in quiescent hematopoietic cells. Importantly, H1-0 protein levels correspond to susceptibility of BCP-ALL cells towards histone deacetylase inhibitors (HDACis) and combinatorial treatment using the H1-0-inducing HDACi Quisinostat showed promising synergism with established chemotherapeutic drugs. Taken together, our data identify H1-0 as a key regulator of the ETV6::RUNX1+ transcriptome and indicate that the addition of Quisinostat may be beneficial to target non-responsive or relapsing ETV6::RUNX1+ BCP-ALL.

Discovery of Cis-Regulatory Mechanisms via Non-Coding Mutations in Acute Lymphoblastic Leukemia

The non-coding genome, constituting 98% of human DNA, remains largely unexplored, yet holds potential for identifying new biomarkers and therapeutic targets in acute lymphoblastic leukemia (ALL). In this study, we conducted a systematic analysis of recurrent somatic non-coding single nucleotide variants (SNVs) in pediatric B-cell precursor (BCP) ALL. We leveraged whole genome sequencing (WGS) data from 345 pediatric BCP ALL cases, representing all major genetic subtypes and identified 346 mutational hotspots that harbored somatic SNVs in at least three cases. Through the integration of paired RNA sequencing along with published ChIP-seq and ATAC-seq data, we found 128 non-coding hotspots associated with differentially expressed genes nearby, which were enriched for cis-regulatory elements, demonstrating the effectiveness of multi-omics integration in distinguishing pathogenic mutations from passengers. We identified one mutational hotspot that was associated with increased expression of the leukemia-associated gene NRAS in three primary ALLs. Micro-C analysis in the leukemia cell line demonstrated interactions between the hotspot region and NRAS regulatory elements. Dual luciferase assays indicated that the mutations disrupted regulatory interactions and CRISPR-mediated deletion of the region significantly upregulated NRAS, confirming the hypothesized regulatory link. Altogether, we provide new insights into the functional roles of non-coding mutations in leukemia.

Comparative Profiling of Regulatory Modules as a Tool for Identifying the Transcription Factor Network Linked to Leukemogenesis

The dynamic gene expression program of hematopoiesis is controlled by a complex network of regulatory modules consisting of transcription factors, chromatin modifiers, and genomic organizers. Genetic abnormalities or changes in the levels of these factors can disrupt normal development and often lead to malignant transformation into leukemic cells. Open chromatin regions are hallmarks of regulatory elements that can be profiled by their susceptibility to DNase I and Tn5 transposase. Genome-wide comparative profiling of open chromatin regions of normal and malignant cells can identify differentially induced regulatory elements and their associated regulatory modules in disease development. We provide an optimized bioinformatics pipeline for the processing of assay for transposase-accessible chromatin sequencing (ATAC-seq) and comparative profiling of open chromatin regions. The identified differentially induced open chromatin regions are used to investigate the changes in molecular networks that drive disease development through integrative analysis with other multi-OMICS data. Here, we demonstrate the robust application of this methodology to compare murine B-cell acute lymphoblastic leukemia cells with wild-type control, which can be applied to any two biological conditions. This integrative computational methodology can also be used for comparative profiling of genome-wide functional element screening methods such as DNaseI hypersensitive sites seq (DNase-seq) and chromatin immunoprecipitation seq (ChIP-seq).

Activating mutations remodel the chromatin accessibility landscape to drive distinct regulatory networks in KMT2A-rearranged acute leukemia

Activating FLT3 and RAS mutations commonly occur in leukemia with KMT2A-gene rearrangements (KMT2A-r). However, how these mutations cooperate with the KMT2A-r to remodel the epigenetic landscape is unknown. Using a retroviral acute myeloid leukemia (AML) mouse model driven by KMT2A::MLLT3, we show that FLT3 ITD , FLT3 N676K , and NRAS G12D remodeled the chromatin accessibility landscape and associated transcriptional networks. Although the activating mutations shared a common core of chromatin changes, each mutation exhibits unique profiles with most opened peaks associating with enhancers in intronic or intergenic regions. Specifically, FLT3 N676K and NRAS G12D rewired similar chromatin and transcriptional networks, distinct from those mediated by FLT3 ITD . Motif analysis uncovered a role for the AP-1 family of transcription factors in KMT2A::MLLT3 leukemia with FLT3 N676K and NRAS G12D , whereas Runx1 and Stat5a/Stat5b were active in the presence of FLT3 ITD . Furthermore, transcriptional programs linked to immune cell regulation were activated in KMT2A-r AML expressing NRAS G12D or FLT3 N676K , and the expression of NKG2D-ligands on KMT2A-r cells rendered them sensitive to CAR T cell-mediated killing. Human KMT2A-r AML cells could be pharmacologically sensitized to NKG2D-CAR T cells by treatment with the histone deacetylase inhibitor LBH589 (panobinostat) which caused upregulation of NKG2D-ligand levels. Co-treatment with LBH589 and NKG2D-CAR T cells enabled robust AML cell killing, and the strongest effect was observed for cells expressing NRAS G12D . Finally, the results were validated and extended to acute leukemia in infancy. Combined, activating mutations induced mutation-specific changes in the epigenetic landscape, leading to changes in transcriptional programs orchestrated by specific transcription factor networks.

Investigation of inherited noncoding genetic variation impacting the pharmacogenomics of childhood acute lymphoblastic leukemia treatment

Defining genetic factors impacting chemotherapy failure can help to better predict response and identify drug resistance mechanisms. However, there is limited understanding of the contribution of inherited noncoding genetic variation on inter-individual differences in chemotherapy response in childhood acute lymphoblastic leukemia (ALL). Here we map inherited noncoding variants associated with treatment outcome and/or chemotherapeutic drug resistance to ALL cis-regulatory elements and investigate their gene regulatory potential and target gene connectivity using massively parallel reporter assays and three-dimensional chromatin looping assays, respectively. We identify 54 variants with transcriptional effects and high-confidence gene connectivity. Additionally, functional interrogation of the top variant, rs1247117, reveals changes in chromatin accessibility, PU.1 binding affinity and gene expression, and deletion of the genomic interval containing rs1247117 sensitizes cells to vincristine. Together, these data demonstrate that noncoding regulatory variants associated with diverse pharmacological traits harbor significant effects on allele-specific transcriptional activity and impact sensitivity to antileukemic agents.

A noncoding regulatory variant in IKZF1 increases acute lymphoblastic leukemia risk in Hispanic/Latino children

Hispanic/Latino children have the highest risk of acute lymphoblastic leukemia (ALL) in the US compared to other racial/ethnic groups, yet the basis of this remains incompletely understood. Through genetic fine-mapping analyses, we identified a new independent childhood ALL risk signal near IKZF1 in self-reported Hispanic/Latino individuals, but not in non-Hispanic White individuals, with an effect size of ∼1.44 (95% confidence interval = 1.33-1.55) and a risk allele frequency of ∼18% in Hispanic/Latino populations and <0.5% in European populations. This risk allele was positively associated with Indigenous American ancestry, showed evidence of selection in human history, and was associated with reduced IKZF1 expression. We identified a putative causal variant in a downstream enhancer that is most active in pro-B cells and interacts with the IKZF1 promoter. This variant disrupts IKZF1 autoregulation at this enhancer and results in reduced enhancer activity in B cell progenitors. Our study reveals a genetic basis for the increased ALL risk in Hispanic/Latino children.

The Diverse Roles of ETV6 Alterations in B-Lymphoblastic Leukemia and Other Hematopoietic Cancers

Genetic alterations of the repressive ETS family transcription factor gene ETV6 are recurrent in several categories of hematopoietic malignancy, including subsets of B-cell and T-cell acute lymphoblastic leukemias (B-ALL and T-ALL), myeloid neoplasms, and mature B-cell lymphomas. ETV6 is essential for adult hematopoietic stem cells (HSCs), contributes to specific functions of some mature immune cells, and plays a key role in thrombopoiesis as demonstrated by familial ETV6 mutations associated with thrombocytopenia and predisposition to hematopoietic cancers, particularly B-ALL. ETV6 appears to have a tumor suppressor role in several hematopoietic lineages, as demonstrated by recurrent somatic loss-of-function (LoF) and putative dominant-negative alterations in leukemias and lymphomas. ETV6 rearrangements contribute to recurrent fusion oncogenes such as the B-ALL-associated transcription factor (TF) fusions ETV6::RUNX1 and PAX5::ETV6, rare drivers such as ETV6::NCOA6, and a spectrum of tyrosine kinase gene fusions encoding hyperactive signaling proteins that self-associate via the ETV6 N-terminal pointed domain. Another subset of recurrent rearrangements involving the ETV6 gene locus appear to function primarily to drive overexpression of the partner gene. This review surveys what is known about the biochemical and genome regulatory properties of ETV6 as well as our current understanding of how alterations in these functions contribute to hematopoietic and nonhematopoietic cancers.