SummaryThe maintenance of H3K9 and DNA methylation at imprinting control regions (ICRs) during early embryogenesis is key to the regulation of imprinted genes. Here, we reveal that ZFP57, its cofactor KAP1, and associated effectors bind selectively to the H3K9me3-bearing, DNA-methylated allele of ICRs in ES cells. KAP1 deletion induces a loss of heterochromatin marks at ICRs, whereas deleting ZFP57 or DNMTs leads to ICR DNA demethylation. Accordingly, we find that ZFP57 and KAP1 associated with DNMTs and hemimethylated DNA-binding NP95. Finally, we identify the methylated TGCCGC hexanucleotide as the motif that is recognized by ZFP57 in all ICRs and in several tens of additional loci, several of which are at least ZFP57-dependently methylated in ES cells. These results significantly advance our understanding of imprinting and suggest a general mechanism for the protection of specific loci against the wave of DNA demethylation that affects the mammalian genome during early embryogenesis.

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BACKGROUND In the southern region of the United States, such as in Louisiana and Texas, there are autochthonous cases of leprosy among native-born Americans with no history of foreign exposure. In the same region, as well as in Mexico, wild armadillos are infected with Mycobacterium leprae. METHODS Whole-genome resequencing of M. leprae from one wild armadillo and three U.S. patients with leprosy revealed that the infective strains were essentially identical. Comparative genomic analysis of these strains and M. leprae strains from Asia and Brazil identified 51 single-nucleotide polymorphisms and an 11-bp insertion–deletion. We genotyped these polymorphic sites, in combination with 10 variable-number tandem repeats, in M. leprae strains obtained from 33 wild armadillos from five southern states, 50 U.S. outpatients seen at a clinic in Louisiana, and 64 Venezuelan patients, as well as in four foreign reference strains. RESULTS The M. leprae genotype of patients with foreign exposure generally reflected their country of origin or travel history. However, a unique M. leprae genotype (3I-2-v1) was found in 28 of the 33 wild armadillos and 25 of the 39 U.S. patients who resided in areas where exposure to armadillo-borne M. leprae was possible. This genotype has not been reported elsewhere in the world. CONCLUSIONS Wild armadillos and many patients with leprosy in the southern United States are infected with the same strain of M. leprae. Armadillos are a large natural reservoir for M. leprae, and leprosy may be a zoonosis in the region. (Funded by the National Institute of Allergy and Infectious Diseases and others.)

Mobile elements are important evolutionary forces that challenge genomic integrity. Long interspersed element-1 (L1, also known as LINE-1) is the only autonomous transposon still active in the human genome. It displays an unusual pattern of evolution, with, at any given time, a single active L1 lineage amplifying to thousands of copies before getting replaced by a new lineage, likely under pressure of host restriction factors, which act notably by silencing L1 expression during early embryogenesis. Here, we demonstrate that in human embryonic stem (hES) cells, KAP1 (KRAB [Krü ppel-associated box domain]-associated protein 1), the master cofactor of KRAB-containing zinc finger proteins (KRAB-ZFPs) previously implicated in the restriction of endogenous retroviruses, represses a discrete subset of L1 lineages predicted to have entered the ancestral genome between 26.8 million and 7.6 million years ago. In mice, we documented a similar chronologically conditioned pattern, albeit with a much contracted time scale. We could further identify an L1-binding KRAB-ZFP, suggesting that this rapidly evolving protein family is more globally responsible for L1 recognition. KAP1 knockdown in hES cells induced the expression of KAP1-bound L1 elements, but their younger, human-specific counterparts (L1Hs) were unaffected. Instead, they were stimulated by depleting DNA methyltransferases, consistent with recent evidence demonstrating that the PIWI-piRNA (PIWI-interacting RNA) pathway regulates L1Hs in hES cells. Altogether, these data indicate that the early embryonic control of L1 is an evolutionarily dynamic process and support a model in which newly emerged lineages are first suppressed by DNA methylation-inducing small RNA-based mechanisms before KAP1-recruiting protein repressors are selected.

Reverse transcription-derived sequences account for at least half of the human genome. Although these retroelements are formidable motors of evolution, they can occasionally cause disease, and accordingly are inactivated during early embryogenesis through epigenetic mechanisms. In the mouse, at least for endogenous retroviruses, important mediators of this process are the tetrapod-specific KRAB-containing zinc finger proteins (KRAB-ZFPs) and their cofactor TRIM28. The present study demonstrates that KRAB/TRIM28-mediated regulation is responsible for controlling a very broad range of human-specific endogenous retroelements (EREs) in human embryonic stem (ES) cells and that it exerts, as a consequence, a marked effect on the transcriptional dynamics of these cells. It further reveals reciprocal dependence between TRIM28 recruitment at specific families of EREs and DNA methylation. It finally points to the importance of persistent TRIM28-mediated control of ERE transcriptional impact beyond their presumed inactivation by DNA methylation.

TRIM28 is critical for the silencing of endogenous retroviruses (ERVs) in embryonic stem (ES) cells. Here, we reveal that an essential impact of this process is the protection of cellular gene expression in early embryos from perturbation by cis-acting activators contained within these retroelements. In TRIM28-depleted ES cells, repressive chromatin marks at ERVs are replaced by histone modifications typical of active enhancers, stimulating transcription of nearby cellular genes, notably those harboring bivalent promoters. Correspondingly, ERV-derived sequences can repress or enhance expression from an adjacent promoter in transgenic embryos depending on their TRIM28 sensitivity in ES cells. TRIM28-mediated control of ERVs is therefore crucial not just to prevent retrotransposition, but more broadly to safeguard the transcriptional dynamics of early embryos.

Human cytomegalovirus (HCMV) is a highly prevalent pathogen that induces life-long infections notably through the establishment of latency in hematopoietic stem cells (HSC). Bouts of reactivation are normally controlled by the immune system, but can be fatal in immuno-compromised individuals such as organ transplant recipients. Here, we reveal that HCMV latency in human CD34+ HSC reflects the recruitment on the viral genome of KAP1, a master co-repressor, together with HP1 and the SETDB1 histone methyltransferase, which results in transcriptional silencing. During lytic infection, KAP1 is still associated with the viral genome, but its heterochromatin-inducing activity is suppressed by mTOR-mediated phosphorylation. Correspondingly, HCMV can be forced out of latency by KAP1 knockdown or pharmacological induction of KAP1 phosphorylation, and this process can be potentiated by activating NFkB with TNF-α. These results suggest new approaches both to curtail CMV infection and to purge the virus from organ transplants.DOI: http://dx.doi.org/10.7554/eLife.06068.001