More than forty per cent of the mammalian genome is derived from retroelements, of which about one-quarter are endogenous retroviruses (ERVs). Some are still active, notably in mice the highly polymorphic early transposon (ETn)/MusD and intracisternal A-type particles (IAP). ERVs are transcriptionally silenced during early embryogenesis by histone and DNA methylation (and reviewed in ref. 7), although the initiators of this process, which is essential to protect genome integrity, remain largely unknown. KAP1 (KRAB-associated protein 1, also known as tripartite motif-containing protein 28, TRIM28) represses genes by recruiting the histone methyltransferase SETDB1, heterochromatin protein 1 (HP1) and the NuRD histone deacetylase complex, but few of its physiological targets are known. Two lines of evidence suggest that KAP1-mediated repression could contribute to the control of ERVs: first, KAP1 can trigger permanent gene silencing during early embryogenesis, and second, a KAP1 complex silences the retrovirus murine leukaemia virus in embryonic cells. Consistent with this hypothesis, here we show that KAP1 deletion leads to a marked upregulation of a range of ERVs, in particular IAP elements, in mouse embryonic stem (ES) cells and in early embryos. We further demonstrate that KAP1 acts synergistically with DNA methylation to silence IAP elements, and that it is enriched at the 5' untranslated region (5'UTR) of IAP genomes, where KAP1 deletion leads to the loss of histone 3 lysine 9 trimethylation (H3K9me3), a hallmark of KAP1-mediated repression. Correspondingly, IAP 5'UTR sequences can impose in cis KAP1-dependent repression on a heterologous promoter in ES cells. Our results establish that KAP1 controls endogenous retroelements during early embryonic development.

In metazoan embryos, transcription is mostly silent for a few cell divisions, until release of a first major wave of embryonic transcripts by so-called zygotic genome activation (ZGA) 1. Maternally provided ZGA-triggering factors have been identified in Drosophila melanogaster and Danio rerio 2,3, but their mammalian homologues are still undefined. Here, we reveal that the DUX family of transcription factors 4,5 is essential to this process in human and mouse. First, human DUX4 and murine Dux are both expressed prior to ZGA in their respective species. Second, both orthologues bind the promoters and activate the transcription of ZGA genes. Third, Dux knockout in mouse embryonic stem cells (mESCs) prevents their cycling through a 2-cell-like state. Finally, zygotic depletion of Dux leads to impaired early embryonic development and defective ZGA. We conclude that DUX proteins are key inducers of zygotic genome activation in placental mammals.

New neurons are continuously generated from neural stem cells with astrocyte properties, which reside in close proximity to the ventricle in the postnatal and adult brain. In this study we found that microRNA-124 (miR-124) dictates postnatal neurogenesis in the mouse subventricular zone. Using a transgenic reporter mouse we show that miR-124 expression is initiated in the rapid amplifying progenitors and remains expressed in the resulting neurons. When we stably inhibited miR-124 in vivo, neurogenesis was blocked, leading to the appearance of ectopic cells with astrocyte characteristics in the olfactory bulb. Conversely, when we overexpressed miR-124, neural stem cells were not maintained in the subventricular zone and neurogenesis was lost. In summary, our results demonstrate that miR-124 is a neuronal fate determinant in the subventricular zone.

SUMMARYEndogenous retroviruses (ERVs) undergo de novo DNA methylation during the first few days of mammalian embryogenesis, although the factors that control the targeting of this process are largely unknown. We asked whether KAP1 (KRAB-associated protein 1) is involved in this mechanism because of its previously defined role in maintaining the silencing of ERVs through the histone methyltransferase ESET and histone H3 lysine 9 trimethylation. Here, we demonstrate that introduced ERV sequences are sufficient to direct rapid de novo methylation of a flanked promoter in embryonic stem (ES) cells. This mechanism requires the presence of an ERV sequence-recognizing KRAB zinc-finger protein (ZFP) and both KAP1 and ESET. Furthermore, this process can also take place on a strong cellular promoter and leads to methylation signatures that are subsequently maintained in vivo throughout embryogenesis. Finally, we show that methylation of ERVs residing in the genome is affected by knockout of KAP1 in early embryos. KRAB-ZFPs, KAP1 and ESET are thus likely to be responsible for the early embryonic instatement of stable epigenetic marks at ERVcontaining loci. KEY WORDS:De novo DNA methylation, Endogenous retroviral silencing, KRAB-associated protein 1 (KAP1), TRIM28, TIF1, KRAB zincfinger protein (KRAB-ZFP), ESET (SETDB1)De novo DNA methylation of endogenous retroviruses is shaped by KRAB-ZFPs/KAP1 and ESET

During hematopoiesis, lineage-and stage-specific transcription factors work in concert with chromatin modifiers to direct the differentiation of all blood cells. Here, we explored the role of KRAB-containing zinc finger proteins (KRAB-ZFPs) and their cofactor KAP1 in this process. Hematopoietic-restricted deletion of Kap1 in the mouse resulted in severe hypoproliferative anemia. Kap1-deleted erythroblasts failed to induce mitophagy-associated genes and retained mitochondria. This was due to persistent expression of microRNAs targeting mitophagy transcripts, itself secondary to a lack of repression by stage-specific KRAB-ZFPs. The KRAB/ KAP1-miRNA regulatory cascade is evolutionary conserved, as it also controls mitophagy during human erythropoiesis. Thus, a multilayered transcription regulatory system is present, where protein-and RNA-based repressors are super-imposed in combinatorial fashion to govern the timely triggering of an important differentiation event.Through the process of erythropoiesis, about one hundred billion new red cells are generated every day in the human adult bone marrow. This process is initiated by the differentiation of hematopoietic stem cells (HSC) into the earliest erythroid progenitor, which was identified ex vivo as a slowly growing burst-forming unit-erythroid (BFU-E). This erythroid progenitor morphs into the rapidly dividing CFU-E (colony-forming unit-erythroid), the proliferation of which is stimulated by the hypoxia-induced hormone erythropoietin. Further differentiation occurs through a highly sophisticated program orchestrated by lineage-and stage-specific combinations of protein-and RNA-based transcription regulators (1-3). It culminates in the elimination of intracellular organelles including mitochondria and the nucleus to yield the fully mature erythrocyte, containing on the order of 250 million molecules of hemoglobin as almost sole cargo. Much is still to be learned about the molecular mechanisms of these events, not only to understand the cause of red cell disorders, but also to aid the in vitro manufacturing of the large supplies of oxygen-carrying cells for transfusion.Higher vertebrate genomes encode hundreds of KRAB-ZFPs that can bind DNA in a sequence-specific fashion through a C-terminal array of C2H2 zinc fingers and recruit the ‡ Corresponding author. didier.trono@epfl.ch.