Neural stem cells (NSCs) are controlled by diffusible factors. The transcription factor Sox2 is expressed by NSCs and Sox2 mutations in humans cause defects in the brain and, in particular, in the hippocampus. We deleted Sox2 in the mouse embryonic brain. At birth, the mice showed minor brain defects; shortly afterwards, however, NSCs and neurogenesis were completely lost in the hippocampus, leading to dentate gyrus hypoplasia. Deletion of Sox2 in adult mice also caused hippocampal neurogenesis loss. The hippocampal developmental defect resembles that caused by late sonic hedgehog (Shh) loss. In mutant mice, Shh and Wnt3a were absent from the hippocampal primordium. A SHH pharmacological agonist partially rescued the hippocampal defect. Chromatin immunoprecipitation identified Shh as a Sox2 target. Sox2-deleted NSCs did not express Shh in vitro and were rapidly lost. Their replication was partially rescued by the addition of SHH and was almost fully rescued by conditioned medium from normal cells. Thus, NSCs control their status, at least partly, through Sox2-dependent autocrine mechanisms.

The transcription factor Sox2 is active in neural stem cells, and Sox2 'knockdown' mice show defects in neural stem/progenitor cells in the hippocampus and eye, and possibly some neurons. In humans, heterozygous Sox2 deficiency is associated with eye abnormalities, hippocampal malformation and epilepsy. To better understand the role of Sox2, we performed in vitro differentiation studies on neural stem cells cultured from embryonic and adult brains of 'knockdown' mutants. Sox2 expression is high in undifferentiated cells, and declines with differentiation, but remains visible in at least some of the mature neurons. In mutant cells, neuronal, but not astroglial, differentiation was profoundly affected. ␤-Tubulin-positive cells were abundant, but most failed to progress to more mature neurons, and showed morphological abnormalities. Overexpression of Sox2 in neural cells at early, but not late, stages of differentiation, rescued the neuronal maturation defect. In addition, it suppressed GFAP expression in glial cells. Our results show an in vitro requirement for Sox2 in early differentiating neuronal lineage cells, for maturation and for suppression of alternative lineage markers. Finally, we examined newly generated neurons from Sox2 'knockdown' newborn and adult mice. GABAergic neurons were greatly diminished in number in newborn mouse cortex and in the adult olfactory bulb, and some showed abnormal morphology and migration properties. GABA deficiency represents a plausible explanation for the epilepsy observed in some of the knockdown mice, as well as in SOX2-deficient individuals.

In mouse and human neural progenitor and glioblastoma "stem-like" cells, we identified key targets of the Polycomb-group protein BMI1 by combining ChIP-seq with in vivo RNAi screening. We discovered that Bmi1 is important in the cellular response to the transforming growth factor-β/bone morphogenetic protein (TGF-β/BMP) and endoplasmic reticulum (ER) stress pathways, in part converging on the Atf3 transcriptional repressor. We show that Atf3 is a tumor-suppressor gene inactivated in human glioblastoma multiforme together with Cbx7 and a few other candidates. Acting downstream of the ER stress and BMP pathways, ATF3 binds to cell-type-specific accessible chromatin preloaded with AP1 and participates in the inhibition of critical oncogenic networks. Our data support the feasibility of combining ChIP-seq and RNAi screens in solid tumors and highlight multiple p16(INK4a)/p19(ARF)-independent functions for Bmi1 in development and cancer.

Polycomb group (PcG) proteins form transcriptional repressor complexes with well-established functions during cell-fate determination. Yet, the mechanisms underlying their regulation remain poorly understood. Here, we extend the role of Polycomb complexes in the temporal control of neural progenitor cell (NPC) commitment by demonstrating that the PcG protein Ezh2 is necessary to prevent the premature onset of gliogenesis. In addition, we identify the chromodomain helicase DNA-binding protein 4 (Chd4) as a critical interaction partner of Ezh2 required specifically for PcG-mediated suppression of the key astrogenic marker gene GFAP. Accordingly, in vivo depletion of Chd4 in the developing neocortex promotes astrogenesis. Collectively, these results demonstrate that PcG proteins operate in a highly dynamic, developmental stage-dependent fashion during neural differentiation and suggest that target gene-specific mechanisms regulate Polycomb function during sequential cell-fate decisions

In vivo deletion of USP3, a deubiquitinating enzyme involved in DNA damage repair, increases the incidence of spontaneous cancer and impairs the proliferation and repopulation ability of HSCs.

DOT 1L methylates histone H3K79 and is aberrantly regulated in MLL ‐rearranged leukemia. Inhibitors have been developed to target DOT 1L activity in leukemia, but cellular mechanisms that regulate DOT 1L are still poorly understood. We have identified the histone deacetylase Rpd3 as a negative regulator of budding yeast Dot1. At its target genes, the transcriptional repressor Rpd3 restricts H3K79 methylation, explaining the absence of H3K79me3 at a subset of genes in the yeast genome. Similar to the crosstalk in yeast, inactivation of the murine Rpd3 homolog HDAC 1 in thymocytes led to an increase in H3K79 methylation. Thymic lymphomas that arise upon genetic deletion of Hdac1 retained the increased H3K79 methylation and were sensitive to reduced DOT 1L dosage. Furthermore, cell lines derived from Hdac1 Δ/Δ thymic lymphomas were sensitive to a DOT 1L inhibitor, which induced apoptosis. In summary, we identified an evolutionarily conserved crosstalk between HDAC 1 and DOT 1L with impact in murine thymic lymphoma development.

DNA damage tolerance (DDT) enables bypassing of DNA lesions during replication, thereby preventing fork stalling, replication stress, and secondary DNA damage related to fork stalling. Three modes of DDT have been documented: translesion synthesis (TLS), template switching (TS), and repriming. TLS and TS depend on sitespecific PCNA K164 monoubiquitination and polyubiquitination, respectively. To investigate the role of DDT in maintaining hematopoietic stem cells (HSCs) and progenitors, we used Pcna K164R/K164R mice as a unique DDT-defective mouse model. Analysis of the composition of HSCs and HSC-derived multipotent progenitors (MPPs) revealed a significantly reduced number of HSCs, likely owing to increased differentiation of HSCs toward myeloid/erythroidassociated MPP2s. This skewing came at the expense of the number of lymphoid-primed MPP4s, which appeared to be compensated for by increased MPP4 proliferation. Furthermore, defective DDT decreased the numbers of MPP-derived common lymphoid progenitor (CLP), common myeloid progenitor (CMP), megakaryocyte-erythroid progenitor (MEP), and granulocyte-macrophage progenitor (GMP) cells, accompanied by increased cell cycle arrest in CMPs. The HSC and MPP phenotypes are reminiscent of premature aging and stressed hematopoiesis, and indeed progressed with age and were exacerbated on cisplatin exposure. Bone marrow transplantations revealed a strong cell intrinsic defect of DDT-deficient HSCs in reconstituting lethally irradiated mice and a strong competitive disadvantage when cotransplanted with wild-type HSCs. These findings indicate a critical role of DDT in maintaining HSCs and progenitor cells, and in preventing premature aging.proliferating cell nuclear antigen K164 ubiquitination | DNA damage tolerance | hematopoietic stem and progenitor cells | translesion synthesis | template switching

Class I histone deacetylases are critical regulators of gene transcription by erasing lysine acetylation. Targeting histone deacetylases using relative non-specific small molecule inhibitors is of major interest in the treatment of cancer, neurological disorders and acquired immune deficiency syndrome. Harnessing the therapeutic potential of histone deacetylase inhibitors requires full knowledge of individual histone deacetylases in vivo. As hematologic malignancies show increased sensitivity towards histone deacetylase inhibitors we targeted deletion of class I Hdac1 and Hdac2 to hematopoietic cell lineages. Here, we show that Hdac1 and Hdac2 together control hematopoietic stem cell homeostasis, in a cell-autonomous fashion. Simultaneous loss of Hdac1 and Hdac2 resulted in loss of hematopoietic stem cells and consequently bone marrow failure. Bone-marrow-specific deletion of Sin3a, a major Hdac1/2 co-repressor, phenocopied loss of Hdac1 and Hdac2 indicating that Sin3a-associated HDAC1/2-activity is essential for hematopoietic stem cell homeostasis. Although Hdac1 and Hdac2 show compensatory and overlapping functions in hematopoiesis, mice expressing mono-allelic Hdac1 or Hdac2 revealed that Hdac1 and Hdac2 contribute differently to the development of specific hematopoietic lineages.