Abstract P415: Etv2 Interacts With Vezf1 And Co-regulate Endothelial Gene Expression

Background: Ets transcription factors function as important developmental regulators and are known to be modulators of cell fate. Previously, in conjunction with co-factors, we have described Ets variant 2 (Etv2) as an essential regulator of the hematopoietic and endothelial lineages. But the mechanism and the Etv2 interacting partners involved in achieving this critical function remains poorly understood. Results: Using a yeast two-hybrid assay, we identified Vascular Endothelial Zinc Finger 1 (Vezf1) as an interacting factor with Etv2. Vezf1 is a conserved C2H2 zinc finger transcription factor known to regulate the formation, proliferation, and migration of endothelial cells. We verified Vezf1 as a binding partner of Etv2 through co-immunoprecipitation and GST-pull down studies. Bioinformatics analysis of ChIP-seq and Etv2-expressing single cell RNA sequencing was conducted to identify candidate genes containing both Etv2 and Vezf1 binding motifs in their regulatory regions. Histone deacetylase 7 (Hdac7) and angiomotin like protein 2 (Amotl2) were identified as candidate genes and have previously been shown to function as regulators of endothelial development. RT-qPCR analysis showed upregulation of Hdac7 and Amotl2 in response to doxycycline inducible Etv2 and Vezf1; whereas significant reduction of expression of these two genes was observed in the Etv2 and Vezf1 knockout cells. Chromatin immunoprecipitation (ChIP) and electrophoresis mobility shift assays (EMSA) confirmed Etv2-Vezf1 adjacent binding sites in the promoters of Hdac7 and Amotl2. Histone Acetyl transferase (HAT) assays was performed to investigate Etv2-Vezf1 on global histone acetylation conditions in doxycycline inducible embryoid bodies. Vezf1 overexpression results in a significant reduction of acetylated histones. Additionally, we have undertaken ATAC-Seq (Assay for Transposase-Accessible Chromatin using sequencing) and Vezf1 ChIP-Seq studies to profile the epigenetic modifications and genome-wide occupancy of Vezf1 in the presence or absence of Etv2 in endothelial progenitor cells. Conclusion: In summary, this study identifies Vezf1 as a novel binding partner of Etv2 and we further demonstrate their combined role in the regulation of downstream target genes, Amotl2 and Hdac7, during endothelial development.

Genome-wide analysis of 944 133 individuals provides insights into the etiology of haemorrhoidal disease

ObjectiveHaemorrhoidal disease (HEM) affects a large and silently suffering fraction of the population but its aetiology, including suspected genetic predisposition, is poorly understood. We report the first genome-wide association study (GWAS) meta-analysis to identify genetic risk factors for HEM to date.DesignWe conducted a GWAS meta-analysis of 218 920 patients with HEM and 725 213 controls of European ancestry. Using GWAS summary statistics, we performed multiple genetic correlation analyses between HEM and other traits as well as calculated HEM polygenic risk scores (PRS) and evaluated their translational potential in independent datasets. Using functional annotation of GWAS results, we identified HEM candidate genes, which differential expression and coexpression in HEM tissues were evaluated employing RNA-seq analyses. The localisation of expressed proteins at selected loci was investigated by immunohistochemistry.ResultsWe demonstrate modest heritability and genetic correlation of HEM with several other diseases from the GI, neuroaffective and cardiovascular domains. HEM PRS validated in 180 435 individuals from independent datasets allowed the identification of those at risk and correlated with younger age of onset and recurrent surgery. We identified 102 independent HEM risk loci harbouring genes whose expression is enriched in blood vessels and GI tissues, and in pathways associated with smooth muscles, epithelial and endothelial development and morphogenesis. Network transcriptomic analyses highlighted HEM gene coexpression modules that are relevant to the development and integrity of the musculoskeletal and epidermal systems, and the organisation of the extracellular matrix.ConclusionHEM has a genetic component that predisposes to smooth muscle, epithelial and connective tissue dysfunction.

Genome-wide analysis of 944,133 individuals provides insights into the etiology of hemorrhoidal disease

Hemorrhoidal disease (HEM) affects a large fraction of the population but its etiology including suspected genetic predisposition is poorly understood. We conducted a GWAS meta-analysis of 218,920 HEM patients and 725,213 controls of European ancestry, demonstrating modest heritability and genetic correlation with several other diseases from the gastrointestinal, neuroaffective and cardiovascular domains. HEM polygenic risk scores validated in 180,435 individuals from independent datasets allowed the identification of those at risk and correlated with younger age of onset and recurrent surgery. We identified 102 independent HEM risk loci harboring genes whose expression is enriched in blood vessels and gastrointestinal tissues, and in pathways associated with smooth muscles, epithelial and endothelial development and morphogenesis. Network transcriptomic analyses of affected tissue from HEM patients highlighted HEM gene co-expression modules that are relevant to the development and integrity of the musculoskeletal and epidermal systems, and the organization of the extracellular matrix. We conclude HEM has a genetic component that predisposes to smooth muscle, epithelial and connective tissue dysfunction.

A novel algorithm for the collective integration of single cell RNA-seq during embryogenesis

Single cell RNA-seq (scRNA-seq) over specified time periods has been widely used to dissect the cell populations during mammalian embryogenesis. Integrating such scRNA-seq data from different developmental stages and from different laboratories is critical to comprehensively define and understand the molecular dynamics and systematically reconstruct the lineage trajectories. Here, we describe a novel algorithm to integrate heterogenous temporal scRNA-seq datasets and to preserve the global developmental trajectories. We applied this algorithm and approach to integrate 3,387 single cells from seven heterogenous temporal scRNA-seq datasets, and reconstructed the cell atlas of early mouse cardiovascular development from E6.5 to E9.5. Using this integrated atlas, we identified an Etv2 downstream target, Ebf1, as an important transcription factor for mouse endothelial development.

Vegfaexpression is activated through positive and negative transcriptional regulatory networks controlled by the ETS factor Etv6in vivo

VEGFA signaling is crucial for physiological and pathological angiogenesis and hematopoiesis. Although many context-dependent signaling pathways downstream of VEGFA have been uncovered, vegfa transcriptional regulation in vivo remains unclear. Here we show that the ETS transcription factor, Etv6, positively regulates vegfa expression during Xenopus blood stem cell development through multiple transcriptional inputs. In agreement with its established repressive functions, Etv6 directly inhibits the expression of the vegfa repressor, foxo3. Surprisingly, it also directly activates the expression of the vegfa activator, klf4. Finally, it indirectly binds to the vegfa promoter where it co-localizes with Klf4. Klf4 deficiency downregulates vegfa expression and significantly decreases Etv6 binding to the vegfa promoter, indicating that Klf4 recruits Etv6 to the vegfa promoter. Thus, our work uncovers a dual function for Etv6, as both a transcriptional repressor and activator, in controlling a major signaling pathway involved in blood and endothelial development in vivo. Given the established relationships between development and cancer, this elaborate gene regulatory network may inform new strategies for the treatment of VEGFA-dependent tumorigenesis.

Discovery of novel determinants of endothelial lineage using chimeric heterokaryons

We wish to identify determinants of endothelial lineage. Murine embryonic stem cells (mESC) were fused with human endothelial cells in stable, non-dividing, heterokaryons. Using RNA-seq, it is possible to discriminate between human and mouse transcripts in these chimeric heterokaryons. We observed a temporal pattern of gene expression in the ESCs of the heterokaryons that recapitulated ontogeny, with early mesodermal factors being expressed before mature endothelial genes. A set of transcriptional factors not known to be involved in endothelial development was upregulated, one of which was POU class 3 homeobox 2 (Pou3f2). We confirmed its importance in differentiation to endothelial lineage via loss- and gain-of-function (LOF and GOF). Its role in vascular development was validated in zebrafish embryos using morpholino oligonucleotides. These studies provide a systematic and mechanistic approach for identifying key regulators in directed differentiation of pluripotent stem cells to somatic cell lineages.

VEGF and endothelium-derived retinoic acid regulate lung vascular and alveolar development

Prevention or treatment of lung diseases caused by the failure to form, or destruction of, existing alveoli, as observed in infants with bronchopulmonary dysplasia and adults with emphysema, requires understanding of the molecular mechanisms of alveolar development. In addition to its critical role in gas exchange, the pulmonary circulation also contributes to alveolar morphogenesis and maintenance by the production of paracrine factors, termed “angiocrines,” that impact the development of surrounding tissue. To identify lung angiocrines that contribute to alveolar formation, we disrupted pulmonary vascular development by conditional inactivation of the Vegf-A gene during alveologenesis. This resulted in decreased pulmonary capillary and alveolar development and altered lung elastin and retinoic acid (RA) expression. We determined that RA is produced by pulmonary endothelial cells and regulates pulmonary angiogenesis and elastin synthesis by induction of VEGF-A and fibroblast growth factor (FGF)-18, respectively. Inhibition of RA synthesis in newborn mice decreased FGF-18 and elastin expression and impaired alveolarization. Treatment with RA and vitamin A partially reversed the impaired vascular and alveolar development induced by VEGF inhibition. Thus we identified RA as a lung angiocrine that regulates alveolarization through autocrine regulation of endothelial development and paracrine regulation of elastin synthesis via induction of FGF-18 in mesenchymal cells.

Deciphering Molecular Control of VEGFR2 Regulation in Hematopoietic Progenitors: GATA1-Mediated Repression of VEGFR2 Promotes Optimum Erythropoiesis

VEGFR2 (also known as Flk1) is expressed in hemetopoietic precursors and is essential for both hematopoietic and vascular development. Interestingly, development of differentiated hematopoietic cell from hematopoietic stem cells (HSCs) is associated with VEGFR2 repression, whereas VEGFR2 expression is maintained throughout endothelial development. This differential regulation of VEGFR2 has been implicated as a key step to successfully branch out hematopoietic vs. endothelial development. However, molecular mechanisms that regulate transcriptionally active vs. repressive Vegfr2 chromatin domains in hematopoietic stem/progenitor cells (HSPCs) vs. differentiated hematopoietic cells are incompletely understood. Here, we report that transcription factor GATA1, a master-regulator of erythroid differentiation, is essential to repress VEGFR2 expression in erythroid progenitors. Genetic complementation analysis demonstrated that VEGFR2 expression in maintained in GATA1-null erythroid progenitors and rescue of GATA1-function induces VEGFR2 repression. Mechanistic studies in primary hematopoietic progenitors from mouse fetal liver and differentiating mouse embryonic stem cells (ESCs) identified a repressor element at the (-)88 kb region of the Vegfr2 locus from which GATA1 represses Vegfr2 transcription in erythroid progenitors. Furthermore, CRISPR/Cas9-mediated deletion of the Vegfr2(-)88 kb region results in reduced erythroid differentiation during fetal liver hematopoiesis. These results indicate that GATA1-mediated repression of VEGFR2 could be a determinant of optimum erythropoiesis. Disclosures No relevant conflicts of interest to declare.