The Polycomb Group Protein L3MBTL1 Represses a SMAD5-Mediated Hematopoietic Transcriptional Program in Human Pluripotent Stem Cells

SummaryEpigenetic regulation of key transcriptional programs is a critical mechanism that controls hematopoietic development, and, thus, aberrant expression patterns or mutations in epigenetic regulators occur frequently in hematologic malignancies. We demonstrate that the Polycomb protein L3MBTL1, which is monoallelically deleted in 20q- myeloid malignancies, represses the ability of stem cells to drive hematopoietic-specific transcriptional programs by regulating the expression of SMAD5 and impairing its recruitment to target regulatory regions. Indeed, knockdown of L3MBTL1 promotes the development of hematopoiesis and impairs neural cell fate in human pluripotent stem cells. We also found a role for L3MBTL1 in regulating SMAD5 target gene expression in mature hematopoietic cell populations, thereby affecting erythroid differentiation. Taken together, we have identified epigenetic priming of hematopoietic-specific transcriptional networks, which may assist in the development of therapeutic approaches for patients with anemia

The AIM2 inflammasome exacerbates atherosclerosis in clonal haematopoiesis

Clonal haematopoiesis, which is highly prevalent in older individuals, arises from somatic mutations that endow a proliferative advantage to haematopoietic cells. Clonal haematopoiesis increases the risk of myocardial infarction and stroke independently of traditional risk factors(1). Among the common genetic variants that give rise to clonal haematopoiesis, the JAK2(V617F) (JAK2(VF)) mutation, which increases JAK-STAT signalling, occurs at a younger age and imparts the strongest risk of premature coronary heart disease(1,2). Here we show increased proliferation of macrophages and prominent formation of necrotic cores in atherosclerotic lesions in mice that express Jak2(VF) selectively in macrophages, and in chimeric mice that model clonal haematopoiesis. Deletion of the essential inflammasome components caspase 1 and 11, or of the pyroptosis executioner gasdermin D, reversed these adverse changes. Jak2(VF) lesions showed increased expression of AIM2, oxidative DNA damage and DNA replication stress, and Aim2 deficiency reduced atherosclerosis. Single-cell RNA sequencing analysis of Jak2(VF) lesions revealed a landscape that was enriched for inflammatory myeloid cells, which were suppressed by deletion of Gsdmd. Inhibition of the inflammasome product interleukin-1 beta reduced macrophage proliferation and necrotic formation while increasing the thickness of fibrous caps, indicating that it stabilized plaques. Our findings suggest that increased proliferation and glycolytic metabolism in Jak2(VF) macrophages lead to DNA replication stress and activation of the AIM2 inflammasome, thereby aggravating atherosclerosis. Precise application of therapies that target interleukin-1 beta or specific inflammasomes according to clonal haematopoiesis status could substantially reduce cardiovascular risk

A method to sequence and quantify DNA integration for monitoring outcome in gene therapy

Human genetic diseases have been successfully corrected by integration of functional copies of the defective genes into human cells, but in some cases integration of therapeutic vectors has activated proto-oncogenes and contributed to leukemia. For this reason, extensive efforts have focused on analyzing integration site populations from patient samples, but the most commonly used methods for recovering newly integrated DNA suffer from severe recovery biases. Here, we show that a new method based on phage Mu transposition in vitro allows convenient and consistent recovery of integration site sequences in a form that can be analyzed directly using DNA barcoding and pyrosequencing. The method also allows simple estimation of the relative abundance of gene-modified cells from human gene therapy subjects, which has previously been lacking but is crucial for detecting expansion of cell clones that may be a prelude to adverse events

LiPS-A3S, a human genomic site for robust expression of inserted transgenes

Transgenesis of human pluripotent stem cells (hPSCs) can enable and empower a variety of studies in stem cell research, including lineage tracing and functional genetics studies. While in recent years much progress has been made in the development of tools for gene targeting, little attention has been given to the identification of sites in the human genome where transgenes can be inserted and reliably expressed. In order to find human genomic sites capable of supporting long-term and high-level transgene expression in hPSCs, we performed a lentiviral screen in human induced pluripotent stem cells (iPSCs). We isolated 40 iPSC clones each harboring a single vector copy and characterized the level of transgene expression afforded by each unique integration site. We selected one clone, LiPS-A3 with an integration site in chromosome 15 maintaining robust expression without silencing and demonstrate that different transgenes can be inserted therein rapidly and efficiently through recombinase-mediated cassette exchange (RMCE). The LiPS-A3 line can greatly facilitate the insertion of reporter and other genes in hPSCs. Targeting transgenes in the LiPS-A3S genomic locus can find broad applications in stem cell research and possibly cell and gene therapy