Histone H3 lysine 4 methylation (H3K4Me) is most often associated with chromatin activation, and removing H3K4 methyl groups has been shown to be coincident with gene repression. H3K4Me demethylase KDM1a/LSD1 is a therapeutic target for multiple diseases, including for the potential treatment of b-globinopathies (sickle cell disease and b-thalassemia) since it is a component of g-globin repressor complexes, and LSD1 inactivation leads to robust induction of the fetal globin genes. The effects of LSD1 inhibition in definitive erythropoiesis are not well characterized, so we examined the consequences of conditional inactivation of Lsd1 in adult red blood cells using a new Gata1creERT2 BAC transgene. Erythroid-specific loss of Lsd1 activity in mice led to a block in erythroid progenitor differentiation and to the expansion of GMP-like cells, converting hematopoietic differentiation potential from an erythroid to a myeloid fate. The analogous phenotype was also observed in human hematopoietic stem and progenitor cells (HSPC), coincident with induction of myeloid transcription factors (e.g. PU.1 and CEBPa). Finally, blocking the activity of transcription factors PU.1 or RUNX1 at the same time as LSD1 inhibition rescued myeloid lineage conversion to an erythroid phenotype. These data show that LSD1 promotes erythropoiesis by repressing myeloid cell fate in adult erythroid progenitors, and that inhibition of the myeloid differentiation pathway reverses the lineage switch induced by LSD1 inactivation.

Key Points Notch1 cofactor Zmiz1 induces a subset of Notch target genes and drives pre–T-cell proliferation during normal and stress thymopoiesis. Disrupting the Zmiz1-Notch1 protein-protein interaction impairs Myc induction, pre–T-cell expansion, and leukemic proliferation.

Allelic exclusion is a vital mechanism for the generation of monospecificity to foreign Ags in B and T lymphocytes. In this study, we developed a high-throughput barcoded method to simultaneously analyze the VDJ recombination status of both mouse TCR-b alleles in hundreds of single cells using next-generation sequencing.

Tissue factor (TF) is an evolutionarily conserved protein necessary for initiation of hemostasis. Zebrafish have two copies of the tissue factor gene (f3a and f3b) as the result of an ancestral teleost fish duplication event (so called ohnologs). In vivo physiologic studies of TF function have been difficult given early lethality of TF knockout in the mouse. We used genome editing to produce knockouts of both f3a and f3b in zebrafish. Since ohnologs arose through sub- or neofunctionalization, they can unmask unknown functions of non-teleost genes and could reveal whether mammalian TF has developmental functions distinct from coagulation. Here we show that a single copy of either f3a or f3b is necessary and sufficient for normal lifespan. Complete loss of TF results in lethal hemorrhage by 2–4 months despite normal embryonic and vascular development. Larval vascular endothelial injury reveals predominant roles for TFa in venous circulation and TFb in arterial circulation. Finally, we demonstrate that loss of TF predisposes to a stress-induced cardiac tamponade independent of its role in fibrin formation. Overall, our data suggest partial subfunctionalization of TFa and TFb. This multigenic zebrafish model has the potential to facilitate study of the role of TF in different vascular beds.