A DNA Contact Map for the Mouse Runx1 Gene Identifies Novel Haematopoietic Enhancers

Dynamic Runx1 chromatin boundaries affect gene expression in hematopoietic development

The transcription factor RUNX1 is a critical regulator of developmental hematopoiesis and is frequently disrupted in leukemia. Runx1 is a large, complex gene that is expressed from two alternative promoters under the spatiotemporal control of multiple hematopoietic enhancers. To dissect the dynamic regulation of Runx1 in hematopoietic development, we analyzed its three-dimensional chromatin conformation in mouse embryonic stem cell (ESC) differentiation cultures. Runx1 resides in a 1.1 Mb topologically associating domain (TAD) demarcated by convergent CTCF motifs. As ESCs differentiate to mesoderm, chromatin accessibility, Runx1 enhancer-promoter (E-P) interactions, and CTCF-CTCF interactions increase in the TAD, along with initiation of Runx1 expression from the P2 promoter. Differentiation to hematopoietic progenitor cells is associated with the formation of tissue-specific sub-TADs over Runx1, a shift in E-P interactions, P1 promoter demethylation, and robust expression from both Runx1 promoters. Deletion of promoter-proximal CTCF sites at the sub-TAD boundaries has no obvious effects on E-P interactions but leads to partial loss of domain structure, mildly affects gene expression, and delays hematopoietic development. Together, our analysis of gene regulation at a large multi-promoter developmental gene reveals that dynamic sub-TAD chromatin boundaries play a role in establishing TAD structure and coordinated gene expression.

Circular Chromosome Conformation Capture Sequencing (4C-Seq ) in Primary Adherent Cells

The three-dimensional structure of the genome is highly organized and is an important aspect of gene regulation. Chromatin interactions can be identified using chromosome conformation capture-based techniques, which rely on proximity ligation. Of these techniques, circular chromosome conformation capture sequencing (4C-seq) is used to identify all chromatin interactions occurring with a single chromosomal location (one versus all). Here we describe a 4C-seq protocol that has been optimized for primary adherent cells, for which the first digestion step is inefficient using standard 4C-seq protocols. It can, however, also be applied to other cell or tissue types. This protocol utilizes a standard DNA library preparation method using a commercial kit, and includes a description of the data processing steps.

SON inhibits megakaryocytic differentiation via repressing RUNX1 and the megakaryocytic gene expression program in acute megakaryoblastic leukemia

A high incidence of acute megakaryoblastic leukemia (AMKL) in Down syndrome patients implies that chromosome 21 genes have a pivotal role in AMKL development, but the functional contribution of individual genes remains elusive. Here, we report that SON, a chromosome 21-encoded DNA- and RNA-binding protein, inhibits megakaryocytic differentiation by suppressing RUNX1 and the megakaryocytic gene expression program. As megakaryocytic progenitors differentiate, SON expression is drastically reduced, with mature megakaryocytes having the lowest levels. In contrast, AMKL cells express an aberrantly high level of SON, and knockdown of SON induced the onset of megakaryocytic differentiation in AMKL cell lines. Genome-wide transcriptome analyses revealed that SON knockdown turns on the expression of pro-megakaryocytic genes while reducing erythroid gene expression. Mechanistically, SON represses RUNX1 expression by directly binding to the proximal promoter and two enhancer regions, the known +23 kb enhancer and the novel +139 kb enhancer, at the RUNX1 locus to suppress H3K4 methylation. In addition, SON represses the expression of the AP-1 complex subunits JUN, JUNB, and FOSB which are required for late megakaryocytic gene expression. Our findings define SON as a negative regulator of RUNX1 and megakaryocytic differentiation, implicating SON overexpression in impaired differentiation during AMKL development.

Transcriptional Regulation of RUNX1: An Informatics Analysis

The RUNX1/AML1 gene encodes a developmental transcription factor that is an important regulator of haematopoiesis in vertebrates. Genetic disruptions to the RUNX1 gene are frequently associated with acute myeloid leukaemia. Gene regulatory elements (REs), such as enhancers located in non-coding DNA, are likely to be important for Runx1 transcription. Non-coding elements that modulate Runx1 expression have been investigated over several decades, but how and when these REs function remains poorly understood. Here we used bioinformatic methods and functional data to characterise the regulatory landscape of vertebrate Runx1. We identified REs that are conserved between human and mouse, many of which produce enhancer RNAs in diverse tissues. Genome-wide association studies detected single nucleotide polymorphisms in REs, some of which correlate with gene expression quantitative trait loci in tissues in which the RE is active. Our analyses also suggest that REs can be variant in haematological malignancies. In summary, our analysis identifies features of the RUNX1 regulatory landscape that are likely to be important for the regulation of this gene in normal and malignant haematopoiesis.

Developmental trajectory of prehematopoietic stem cell formation from endothelium

Hematopoietic stem and progenitor cells (HSPCs) in the bone marrow are derived from a small population of hemogenic endothelial (HE) cells located in the major arteries of the mammalian embryo. HE cells undergo an endothelial to hematopoietic cell transition, giving rise to HSPCs that accumulate in intra-arterial clusters (IAC) before colonizing the fetal liver. To examine the cell and molecular transitions between endothelial (E), HE, and IAC cells, and the heterogeneity of HSPCs within IACs, we profiled ∼40 000 cells from the caudal arteries (dorsal aorta, umbilical, vitelline) of 9.5 days post coitus (dpc) to 11.5 dpc mouse embryos by single-cell RNA sequencing and single-cell assay for transposase-accessible chromatin sequencing. We identified a continuous developmental trajectory from E to HE to IAC cells, with identifiable intermediate stages. The intermediate stage most proximal to HE, which we term pre-HE, is characterized by increased accessibility of chromatin enriched for SOX, FOX, GATA, and SMAD motifs. A developmental bottleneck separates pre-HE from HE, with RUNX1 dosage regulating the efficiency of the pre-HE to HE transition. A distal candidate Runx1 enhancer exhibits high chromatin accessibility specifically in pre-HE cells at the bottleneck, but loses accessibility thereafter. Distinct developmental trajectories within IAC cells result in 2 populations of CD45+ HSPCs; an initial wave of lymphomyeloid-biased progenitors, followed by precursors of hematopoietic stem cells (pre-HSCs). This multiomics single-cell atlas significantly expands our understanding of pre-HSC ontogeny.

RUNX1-targeted therapy for AML expressing somatic or germline mutation in RUNX1

RUNX1 transcription factor regulates normal and malignant hematopoiesis. Somatic or germline mutant RUNX1 (mtRUNX1) is associated with poorer outcome in acute myeloid leukemia (AML). Knockdown or inhibition of RUNX1 induced more apoptosis of AML expressing mtRUNX1 versus wild-type RUNX1 and improved survival of mice engrafted with mtRUNX1-expressing AML. CRISPR/Cas9-mediated editing-out of RUNX1 enhancer (eR1) within its intragenic super-enhancer, or BET protein BRD4 depletion by short hairpin RNA, repressed RUNX1, inhibited cell growth, and induced cell lethality in AML cells expressing mtRUNX1. Moreover, treatment with BET protein inhibitor or degrader (BET-proteolysis targeting chimera) repressed RUNX1 and its targets, inducing apoptosis and improving survival of mice engrafted with AML expressing mtRUNX1. Library of Integrated Network-based Cellular Signatures 1000-connectivity mapping data sets queried with messenger RNA signature of RUNX1 knockdown identified novel expression-mimickers (EMs), which repressed RUNX1 and exerted in vitro and in vivo efficacy against AML cells expressing mtRUNX1. In addition, the EMs cinobufagin, anisomycin, and narciclasine induced more lethality in hematopoietic progenitor cells (HPCs) expressing germline mtRUNX1 from patients with AML compared with HPCs from patients with familial platelet disorder (FPD), or normal untransformed HPCs. These findings highlight novel therapeutic agents for AML expressing somatic or germline mtRUNX1.

Packaging development: how chromatin controls transcription in zebrafish embryogenesis

How developmental gene expression is activated, co-ordinated and maintained is one of the biggest questions in developmental biology. While transcription factors lead the way in directing developmental gene expression, their accessibility to the correct repertoire of genes can depend on other factors such as DNA methylation, the presence of particular histone variants and post-translational modifications of histones. Collectively, factors that modify DNA or affect its packaging and accessibility contribute to a chromatin landscape that helps to control the timely expression of developmental genes. Zebrafish, perhaps better known for their strength as a model of embryology and organogenesis during development, are coming to the fore as a powerful model for interpreting the role played by chromatin in gene expression. Several recent advances have shown that zebrafish exhibit both similarities and differences to other models (and humans) in the way that they employ chromatin mechanisms of gene regulation. Here, I review how chromatin influences developmental transcriptional programmes during early zebrafish development, patterning and organogenesis. Lastly, I briefly highlight the importance of zebrafish chromatin research towards the understanding of human disease and transgenerational inheritance.