The KRAB zinc finger protein ZFP809 is required to initiate epigenetic silencing of endogenous retroviruses

Mechanistic regulation of HERV activation in tumors and implications for translational research in oncology

Transcription of distinct loci of human endogenous retroviruses (HERVs) and in some cases, translation of these transcripts have been consistently observed in many types of cancer. It is still debated whether HERV activation serves as a trigger for carcinogenesis or rather occurs as a consequence of epigenetic alterations and other molecular sequelae that characterize cellular transformation. Here we review the known molecular and epigenetic mechanisms of HERV activation in cancer cells as well as its potential contribution to carcinogenesis. Further, we describe the use of HERV expression in cancer diagnostic and characterize the potential of HERV-derived antigens to serve as novel targets for cancer immunotherapy. We believe this review, which summarizes both what is known as well as unknown in this rapidly developing field, will boost interest in research on the therapeutic potential of targeting HERV elements in tumors and the impact of HERV activation in oncogenesis.

Diversity and functional specialization of H3K9-specific histone methyltransferases

Histone modifications play a critical role in the control over activities of the eukaryotic genome; among these chemical alterations, the methylation of lysine K9 in histone H3 (H3K9) is one of the most extensively studied. The number of enzymes capable of methylating H3K9 varies greatly across different organisms: in fission yeast, only one such methyltransferase is present, whereas in mammals, 10 are known. If there are several such enzymes, each of them must have some specific function, and they can interact with one another. Thus arises a complex system of interchangeability, "division of labor," and contacts with each other and with diverse proteins. Histone methyltransferases specialize in the number of methyl groups that they attach and have different intracellular localizations as well as different distributions on chromosomes. Each also shows distinct binding to different types of sequences and has a specific set of nonhistone substrates.

HERVs: Expression Control Mechanisms and Interactions in Diseases and Human Immunodeficiency Virus Infection

Human endogenous retroviruses (HERVs) are the result of retroviral infections acquired millions of years ago; nowadays, they compose around 8% of human DNA. Multiple mechanisms have been employed for endogenous retroviral deactivation, rendering replication and retrotransposition defective, while some of them have been co-opted to serve host evolutionary advantages. A pleiad of mechanisms retains the delicate balance of HERV expression in modern humans. Thus, epigenetic modifications, such as DNA and histone methylation, acetylation, deamination, chromatin remodeling, and even post-transcriptional control are recruited. In this review, we aim to summarize the main HERV silencing pathways, revisit paradigms of human disease with a HERV component, and emphasize the human immunodeficiency virus (HIV) and HERV interactions during HIV infection.

Regulation of endogenous retroviruses in murine embryonic stem cells and early embryos

Endogenous retroviruses (ERVs) are important components of transposable elements that constitute ∼40% of the mouse genome. ERVs exhibit dynamic expression patterns during early embryonic development and are engaged in numerous biological processes. Therefore, ERV expression must be closely monitored in cells. Most studies have focused on the regulation of ERV expression in mouse embryonic stem cells (ESCs) and during early embryonic development. This review touches on the classification, expression, and functions of ERVs in mouse ESCs and early embryos and mainly discusses ERV modulation strategies from the perspectives of transcription, epigenetic modification, nucleosome/chromatin assembly, and post-transcriptional control.

The infection-tolerant white-footed deermouse tempers interferon responses to endotoxin in comparison to the mouse and rat

The white-footed deermouse Peromyscus leucopus, a long-lived rodent, is a key reservoir in North America for agents of several zoonoses, including Lyme disease, babesiosis, anaplasmosis, and a viral encephalitis. While persistently infected, this deermouse is without apparent disability or diminished fitness. For a model for inflammation elicited by various pathogens, the endotoxin lipopolysaccharide (LPS) was used to compare genome-wide transcription in blood by P. leucopus, Mus musculus, and Rattus norvegicus and adjusted for white cell concentrations. Deermice were distinguished from the mice and rats by LPS response profiles consistent with non-classical monocytes and alternatively-activated macrophages. LPS-treated P. leucopus, in contrast to mice and rats, also displayed little transcription of interferon-gamma and lower magnitude fold-changes in type 1 interferon-stimulated genes. These characteristics of P. leucopus were also noted in a Borrelia hermsii infection model. The phenomenon was associated with comparatively reduced transcription of endogenous retrovirus sequences and cytoplasmic pattern recognition receptors in the deermice. The results reveal a mechanism for infection tolerance in this species and perhaps other animal reservoirs for agents of human disease.

Metastable epialleles in humans

First identified in isogenic mice, metastable epialleles (MEs) are loci where the extent of DNA methylation (DNAm) is variable between individuals but correlates across tissues derived from different germ layers within a given individual. This property, termed systemic interindividual variation (SIV), is attributed to stochastic methylation establishment before germ layer differentiation. Evidence suggests that some putative human MEs are sensitive to environmental exposures in early development. In this review we introduce key concepts pertaining to human MEs, describe methods used to identify MEs in humans, and review their genomic features. We also highlight studies linking DNAm at putative human MEs to early environmental exposures and postnatal (including disease) phenotypes.

TNRC18 engages H3K9me3 to mediate silencing of endogenous retrotransposons

Trimethylation of histone H3 lysine 9 (H3K9me3) is crucial for the regulation of gene repression and heterochromatin formation, cell-fate determination and organismal development1. H3K9me3 also provides an essential mechanism for silencing transposable elements1-4. However, previous studies have shown that canonical H3K9me3 readers (for example, HP1 (refs. 5-9) and MPP8 (refs. 10-12)) have limited roles in silencing endogenous retroviruses (ERVs), one of the main transposable element classes in the mammalian genome13. Here we report that trinucleotide-repeat-containing 18 (TNRC18), a poorly understood chromatin regulator, recognizes H3K9me3 to mediate the silencing of ERV class I (ERV1) elements such as LTR12 (ref. 14). Biochemical, biophysical and structural studies identified the carboxy-terminal bromo-adjacent homology (BAH) domain of TNRC18 (TNRC18(BAH)) as an H3K9me3-specific reader. Moreover, the amino-terminal segment of TNRC18 is a platform for the direct recruitment of co-repressors such as HDAC-Sin3-NCoR complexes, thus enforcing optimal repression of the H3K9me3-demarcated ERVs. Point mutagenesis that disrupts the TNRC18(BAH)-mediated H3K9me3 engagement caused neonatal death in mice and, in multiple mammalian cell models, led to derepressed expression of ERVs, which affected the landscape of cis-regulatory elements and, therefore, gene-expression programmes. Collectively, we describe a new H3K9me3-sensing and regulatory pathway that operates to epigenetically silence evolutionarily young ERVs and exert substantial effects on host genome integrity, transcriptomic regulation, immunity and development.

Unraveling the role of ZNF506 as a human PBS-pro-targeting protein for ERVP repression

Krüppel-associated box zinc finger proteins (KZFPs) function as a defense mechanism to maintain the genome stability of higher vertebrates by regulating the transcriptional activities of transposable elements (TEs). While previous studies have characterized ZFP809 as responsible for binding and repressing ERVs containing a proline tRNA primer-binding site (PBS-Pro) in mice, comparable KZFPs have not been identified in humans yet. Here, we identified ZNF506 as a PBS-Pro-binding protein in humans, which functions as a transcriptional repressor of PBS-Pro-utilizing retroviruses by recruiting heterochromatic modifications. Although they have similar functions, the low protein similarities between ZNF506, ZFP809 and KZFPs of other species suggest their independent evolution against the invasion of PBS-Pro-utilizing retroviruses into their respective ancestor genomes after species divergence. We also explored the link between ZNF506 and leukemia. Our findings suggest that ZNF506 is a unique human KZFP that can bind to PBS-Pro, highlighting the diverse evolution of KZFPs in defending against retroviral invasions. Additionally, our study provides insights into the potential role of ZNF506 in leukemia, contributing to the expanding knowledge of KZFPs' crucial function in disease and genome stability.

The white-footed deermouse, an infection-tolerant reservoir for several zoonotic agents, tempers interferon responses to endotoxin in comparison to the mouse and rat

The white-footed deermouse Peromyscus leucopus, a long-lived rodent, is a key reservoir for agents of several zoonoses, including Lyme disease. While persistently infected, this deermouse is without apparent disability or diminished fitness. For a model for inflammation elicited by various pathogens, the endotoxin lipopolysaccharide (LPS) was used to compare genome-wide transcription in blood by P. leucopus, Mus musculus and Rattus norvegicus and adjusted for white cell concentrations. Deermice were distinguished from the mice and rats by LPS response profiles consistent with non-classical monocytes and alternatively-activated macrophages. LPS-treated P. leucopus, in contrast to mice and rats, also displayed little transcription of interferon-gamma and lower magnitude fold-changes in type 1 interferon-stimulated genes. This was associated with comparatively reduced transcription of endogenous retrovirus sequences and cytoplasmic pattern recognition receptors in the deermice. The results reveal a mechanism for infection tolerance in this species and perhaps other animal reservoirs for agents of human disease.

The Regulation and Immune Signature of Retrotransposons in Cancer

Advances in sequencing technologies and the bioinformatic analysis of big data facilitate the study of jumping genes' activity in the human genome in cancer from a broad perspective. Retrotransposons, which move from one genomic site to another by a copy-and-paste mechanism, are regulated by various molecular pathways that may be disrupted during tumorigenesis. Active retrotransposons can stimulate type I IFN responses. Although accumulated evidence suggests that retrotransposons can induce inflammation, the research investigating the exact mechanism of triggering these responses is ongoing. Understanding these mechanisms could improve the therapeutic management of cancer through the use of retrotransposon-induced inflammation as a tool to instigate immune responses to tumors.

POGZ suppresses 2C transcriptional program and retrotransposable elements

The POGZ gene has been found frequently mutated in neurodevelopmental disorders (NDDs) such as autism spectrum disorder (ASD) and intellectual disability (ID). We have recently shown that POGZ maintains mouse embryonic stem cells (ESCs). However, the exact mechanisms remain unclear. Here, we show that POGZ plays an important role in the maintenance of ESCs by silencing Dux and endogenous retroviruses (ERVs). POGZ maintains a silent chromatin state at Dux and ERVs by associating with and recruiting TRIM28 and SETDB1, and its loss leads to decreased levels of H3K9me3/H4K20me3, resulting in up-regulation of 2C transcripts and ESC transition to a 2C-like state. POGZ suppresses different classes of ERVs through direct (IAPEy, the intracisternal A-type particle elements) and indirect regulation (MERVL). Activation of POGZ-bound ERVs is associated with up-regulation of nearby neural disease genes such as Serpina3m. Our findings provide important insights into understanding the disease mechanism caused by POGZ dysfunction.

Transposable elements drive the evolution of metazoan zinc finger genes

Cys2-His2 zinc finger genes (ZNFs) form the largest family of transcription factors in metazoans. ZNF evolution is highly dynamic and characterized by the rapid expansion and contraction of numerous subfamilies across the animal phylogeny. The forces and mechanisms underlying rapid ZNF evolution remain poorly understood, but there is growing evidence that, in tetrapods, the targeting and repression of lineage-specific transposable elements (TEs) plays a critical role in the evolution of the Krüppel-associated box ZNF (KZNF) subfamily. Currently, it is unknown whether this function and coevolutionary relationship is unique to KZNFs or is a broader feature of metazoan ZNFs. Here, we present evidence that genomic conflict with TEs has been a central driver of the diversification of ZNFs in animals. Sampling from 3221 genome assemblies, we show that the copy number of retroelements correlates with that of ZNFs across at least 750 million years of metazoan evolution. Using computational predictions, we show that ZNFs preferentially bind TEs in diverse animal species. We further investigate the largest ZNF subfamily found in cyprinid fish, which is characterized by a conserved sequence we dubbed the fish N-terminal zinc finger-associated (FiNZ) domain. Zebrafish possess approximately 700 FiNZ-ZNFs, many of which are evolving adaptively under positive selection. Like mammalian KZNFs, most zebrafish FiNZ-ZNFs are expressed at the onset of zygotic genome activation, and blocking their translation using morpholinos during early embryogenesis results in derepression of transcriptionally active TEs. Together, these data suggest that ZNF diversification has been intimately connected to TE expansion throughout animal evolution.

Transposable Elements: Epigenetic Silencing Mechanisms or Modulating Tools for Vertebrate Adaptations? Two Sides of the Same Coin

Transposable elements constitute one of the main components of eukaryotic genomes. In vertebrates, they differ in content, typology, and family diversity and played a crucial role in the evolution of this taxon. However, due to their transposition ability, TEs can be responsible for genome instability, and thus silencing mechanisms were evolved to allow the coexistence between TEs and eukaryotic host-coding genes. Several papers are highlighting in TEs the presence of regulatory elements involved in regulating nearby genes in a tissue-specific fashion. This suggests that TEs are not sequences merely to silence; rather, they can be domesticated for the regulation of host-coding gene expression, permitting species adaptation and resilience as well as ensuring human health. This review presents the main silencing mechanisms acting in vertebrates and the importance of exploiting these mechanisms for TE control to rewire gene expression networks, challenging the general view of TEs as threatening elements.

Interplay between RNA interference and transposable elements in mammals

RNA interference (RNAi) plays pleiotropic roles in animal cells, from the post-transcriptional control of gene expression via the production of micro-RNAs, to the inhibition of RNA virus infection. We discuss here the role of RNAi in regulating the expression of self RNAs, and particularly transposable elements (TEs), which are genomic sequences capable of influencing gene expression and disrupting genome architecture. Dicer proteins act as the entry point of the RNAi pathway by detecting and degrading RNA of TE origin, ultimately leading to TE silencing. RNAi similarly targets cellular RNAs such as repeats transcribed from centrosomes. Dicer proteins are thus nucleic acid sensors that recognize self RNA in the form of double-stranded RNA, and trigger a silencing RNA interference response.

Co-option of endogenous retroviruses through genetic escape from TRIM28 repression

Endogenous retroviruses (ERVs) have rewired host gene networks. To explore the origins of co-option, we employed an active murine ERV, IAPEz, and an embryonic stem cell (ESC) to neural progenitor cell (NPC) differentiation model. Transcriptional silencing via TRIM28 maps to a 190 bp sequence encoding the intracisternal A-type particle (IAP) signal peptide, which confers retrotransposition activity. A subset of "escapee" IAPs (∼15%) exhibits significant genetic divergence from this sequence. Canonical repressed IAPs succumb to a previously undocumented demarcation by H3K9me3 and H3K27me3 in NPCs. Escapee IAPs, in contrast, evade repression in both cell types, resulting in their transcriptional derepression, particularly in NPCs. We validate the enhancer function of a 47 bp sequence within the U3 region of the long terminal repeat (LTR) and show that escapee IAPs convey an activating effect on nearby neural genes. In sum, co-opted ERVs stem from genetic escapees that have lost vital sequences required for both TRIM28 restriction and autonomous retrotransposition.

RNA-mediated heterochromatin formation at repetitive elements in mammals

The failure to repress transcription of repetitive genomic elements can lead to catastrophic genome instability and is associated with various human diseases. As such, multiple parallel mechanisms cooperate to ensure repression and heterochromatinization of these elements, especially during germline development and early embryogenesis. A vital question in the field is how specificity in establishing heterochromatin at repetitive elements is achieved. Apart from trans-acting protein factors, recent evidence points to a role of different RNA species in targeting repressive histone marks and DNA methylation to these sites in mammals. Here, we review recent discoveries on this topic and predominantly focus on the role of RNA methylation, piRNAs, and other localized satellite RNAs.

Endogenous Retrovirus Elements Are Co-Expressed with IFN Stimulation Genes in the JAK-STAT Pathway

Background: Endogenous retrovirus (ERV) elements can act as proximal regulatory elements in promoting interferon (IFN) responses. Previous relevant studies have mainly focused on IFN-stimulated genes (ISGs). However, the role of ERV elements as cis-regulatory motifs in regulating genes of the JAK-STAT pathway remains poorly understood. In our study, we analyzed the changes in ERV elements and genes under both IFN stimulation and blockade of the signaling pathway. Methods: The effects of interferon on cells under normal conditions and knockout of the receptor were compared based on the THP1_IFNAR1_KO and THP1_IFNAR2_mutant cell lines. The correlation between differentially expressed ERVs (DHERVs) and differentially expressed genes (DEGs) as DEHERV-G pairs was explored with construction of gene regulatory networks related to ERV and induced by proinflammatory cytokines. Results: A total of 430 DEHERV loci and 190 DEGs were identified in 842 DEHERV-G pairs that are common to the three groups. More than 87% of DEHERV-G pairs demonstrated a consistent expression pattern. ISGs such as AIM2, IFIT1, IFIT2, IFIT3, STAT1, and IRF were activated via the JAK-STAT pathway in response to interferon stimulation. Thus, STAT1, STAT2, and IRF1 appear to play core roles in regulatory networks and are closely associated with ERVs. Conclusions: The RNA expression of ISGs and ERV elements is correlated, indicating that ERV elements are closely linked to host innate immune responses.

The sound of silence: Transgene silencing in mammalian cell engineering

To elucidate principles operating in native biological systems and to develop novel biotechnologies, synthetic biology aims to build and integrate synthetic gene circuits within native transcriptional networks. The utility of synthetic gene circuits for cell engineering relies on the ability to control the expression of all constituent transgene components. Transgene silencing, defined as the loss of expression over time, persists as an obstacle for engineering primary cells and stem cells with transgenic cargos. In this review, we highlight the challenge that transgene silencing poses to the robust engineering of mammalian cells, outline potential molecular mechanisms of silencing, and present approaches for preventing transgene silencing. We conclude with a perspective identifying future research directions for improving the performance of synthetic gene circuits.

Primate-specific transposable elements shape transcriptional networks during human development

The human genome contains more than 4.5 million inserts derived from transposable elements (TEs), the result of recurrent waves of invasion and internal propagation throughout evolution. For new TE copies to be inherited, they must become integrated in the genome of the germline or pre-implantation embryo, which requires that their source TE be expressed at these stages. Accordingly, many TEs harbor DNA binding sites for the pluripotency factors OCT4, NANOG, SOX2, and KLFs and are transiently expressed during embryonic genome activation. Here, we describe how many primate-restricted TEs have additional binding sites for lineage-specific transcription factors driving their expression during human gastrulation and later steps of fetal development. These TE integrants serve as lineage-specific enhancers fostering the transcription, amongst other targets, of KRAB-zinc finger proteins (KZFPs) of comparable evolutionary age, which in turn corral the activity of TE-embedded regulatory sequences in a similarly lineage-restricted fashion. Thus, TEs and their KZFP controllers play broad roles in shaping transcriptional networks during early human development.

Transposon control as a checkpoint for tissue regeneration

Tissue regeneration requires precise temporal control of cellular processes such as inflammatory signaling, chromatin remodeling and proliferation. The combination of these processes forms a unique microenvironment permissive to the expression, and potential mobilization of, transposable elements (TEs). Here, we develop the hypothesis that TE activation creates a barrier to tissue repair that must be overcome to achieve successful regeneration. We discuss how uncontrolled TE activity may impede tissue restoration and review mechanisms by which TE activity may be controlled during regeneration. We posit that the diversification and co-evolution of TEs and host control mechanisms may contribute to the wide variation in regenerative competency across tissues and species.

Epigenetic mechanisms of Strip2 in differentiation of pluripotent stem cells

Significant evidence points to Strip2 being a key regulator of the differentiation processes of pluripotent embryonic stem cells. However, Strip2 mediated epigenetic regulation of embryonic differentiation and development is quite unknown. Here, we identified several interaction partners of Strip2, importantly the co-repressor molecular protein complex nucleosome remodeling deacetylase/Tripartite motif-containing 28/Histone deacetylases/Histone-lysine N-methyltransferase SETDB1 (NuRD/TRIM28/HDACs/SETDB1) histone methyltransferase, which is primarily involved in regulation of the pluripotency of embryonic stem cells and its differentiation. The complex is normally activated by binding of Krueppel-associated box zinc-finger proteins (KRAB-ZFPs) to specific DNA motifs, causing methylation of H3 to Lysin-9 residues (H3K9). Our data showed that Strip2 binds to a DNA motif (20 base pairs), like the KRAB-ZFPs. We establish that Strip2 is an epigenetic regulator of pluripotency and differentiation by modulating DNA KRAB-ZFPs as well as the NuRD/TRIM28/HDACs/SETDB1 histone methyltransferase complex.

A satellite DNA array barcodes chromosome 7 and regulates totipotency via ZFP819

Mammalian genomes are a battleground for genetic conflict between repetitive elements and KRAB-zinc finger proteins (KZFPs). We asked whether KZFPs can regulate cell fate by using ZFP819, which targets a satellite DNA array, ZP3AR. ZP3AR coats megabase regions of chromosome 7 encompassing genes encoding ZSCAN4, a master transcription factor of totipotency. Depleting ZFP819 in mouse embryonic stem cells (mESCs) causes them to transition to a 2-cell (2C)-like state, whereby the ZP3AR array switches from a poised to an active enhancer state. This is accompanied by a global erosion of heterochromatin roadblocks, which we link to decreased SETDB1 stability. These events result in transcription of active LINE-1 elements and impaired differentiation. In summary, ZFP819 and TRIM28 partner up to close chromatin across Zscan4, to promote exit from totipotency. We propose that satellite DNAs may control developmental fate transitions by barcoding and switching off master transcription factor genes.

The regulation of totipotency transcription: Perspective from in vitro and in vivo totipotency

Totipotency represents the highest developmental potency. By definition, totipotent stem cells are capable of giving rise to all embryonic and extraembryonic cell types. In mammalian embryos, totipotency occurs around the zygotic genome activation period, which is around the 2-cell stage in mouse embryo or the 4-to 8-cell stage in human embryo. Currently, with the development of in vitro totipotent-like models and the advances in small-scale genomic methods, an in-depth mechanistic understanding of the totipotency state and regulation was enabled. In this review, we explored and summarized the current views about totipotency from various angles, including genetic and epigenetic aspects. This will hopefully formulate a panoramic view of totipotency from the available research works until now. It can also help delineate the scaffold and formulate new hypotheses on totipotency for future research works.

Widespread expression of the ancient HERV-K (HML-2) provirus group in normal human tissues

Human endogenous retrovirus (HERV) transcripts are known to be highly expressed in cancers, yet their activity in nondiseased tissue is largely unknown. Using the GTEx RNA-seq dataset from normal tissue sampled at autopsy, we characterized individual expression of the recent HERV-K (HML-2) provirus group across 13,000 different samples of 54 different tissues from 948 individuals. HML-2 transcripts could be identified in every tissue sampled and were elevated in the cerebellum, pituitary, testis, and thyroid. A total of 37 different individual proviruses were expressed in 1 or more tissues, representing all 3 LTR5 subgroups. Nine proviruses were identified as having long terminal repeat (LTR)-driven transcription, 7 of which belonged to the most recent LTR5HS subgroup. Proviruses of different subgroups displayed a bias in tissue expression, which may be associated with differences in transcription factor binding sites in their LTRs. Provirus expression was greater in evolutionarily older proviruses with an earliest shared ancestor of gorilla or older. HML-2 expression was significantly affected by biological sex in 1 tissue, while age and timing of death (Hardy score) had little effect. Proviruses containing intact gag, pro, and env open reading frames (ORFs) were expressed in the dataset, with almost every tissue measured potentially expressing at least 1 intact ORF (gag).

Human endogenous retrovirus (HERV) transcripts are known to be highly expressed in cancers, but what is their activity in normal tissue? This study uses unique patterns of HERV-K RNA expression in the large GEx dataset from non-diseased tissue sites to provide new insights into both the coevolution of HERV-K with our primate ancestors and their current role in human biology.

Transcriptional Regulation of Endogenous Retroviruses and Their Misregulation in Human Diseases

Endogenous retroviruses (ERVs), deriving from exogenous retroviral infections of germ line cells occurred millions of years ago, represent ~8% of human genome. Most ERVs are highly inactivated because of the accumulation of mutations, insertions, deletions, and/or truncations. However, it is becoming increasingly apparent that ERVs influence host biology through genetic and epigenetic mechanisms under particular physiological and pathological conditions, which provide both beneficial and deleterious effects for the host. For instance, certain ERVs expression is essential for human embryonic development. Whereas abnormal activation of ERVs was found to be involved in numbers of human diseases, such as cancer and neurodegenerative diseases. Therefore, understanding the mechanisms of regulation of ERVs would provide insights into the role of ERVs in health and diseases. Here, we provide an overview of mechanisms of transcriptional regulation of ERVs and their dysregulation in human diseases.

Hijacking of transcriptional condensates by endogenous retroviruses

Most endogenous retroviruses (ERVs) in mammals are incapable of retrotransposition; therefore, why ERV derepression is associated with lethality during early development has been a mystery. Here, we report that rapid and selective degradation of the heterochromatin adapter protein TRIM28 triggers dissociation of transcriptional condensates from loci encoding super-enhancer (SE)-driven pluripotency genes and their association with transcribed ERV loci in murine embryonic stem cells. Knockdown of ERV RNAs or forced expression of SE-enriched transcription factors rescued condensate localization at SEs in TRIM28-degraded cells. In a biochemical reconstitution system, ERV RNA facilitated partitioning of RNA polymerase II and the Mediator coactivator into phase-separated droplets. In TRIM28 knockout mouse embryos, single-cell RNA-seq analysis revealed specific depletion of pluripotent lineages. We propose that coding and noncoding nascent RNAs, including those produced by retrotransposons, may facilitate ‘hijacking’ of transcriptional condensates in various developmental and disease contexts.

TRIM28 depletion in embryonic stem cells disconnects transcriptional condensates from super-enhancers, which is rescued by knockdown of endogenous retroviruses.

The Intricate Evolutionary Balance between Transposable Elements and Their Host: Who Will Kick at Goal and Convert the Next Try?

Transposable elements (TEs) are mobile DNA sequences that can jump from one genomic locus to another and that have colonized the genomes of all living organisms. TE mobilization and accumulation are an important source of genomic innovations that greatly contribute to the host species evolution. To ensure their maintenance and amplification, TE transposition must occur in the germ cell genome. As TE transposition is also a major threat to genome integrity, the outcome of TE mobility in germ cell genomes could be highly dangerous because such mutations are inheritable. Thus, organisms have developed specialized strategies to protect the genome integrity from TE transposition, particularly in germ cells. Such effective TE silencing, together with ongoing mutations and negative selection, should result in the complete elimination of functional TEs from genomes. However, TEs have developed efficient strategies for their maintenance and spreading in populations, particularly by using horizontal transfer to invade the genome of novel species. Here, we discuss how TEs manage to bypass the host's silencing machineries to propagate in its genome and how hosts engage in a fightback against TE invasion and propagation. This shows how TEs and their hosts have been evolving together to achieve a fine balance between transposition and repression.

Transcriptional states of retroelement-inserted regions and specific KRAB zinc finger protein association are correlated with DNA methylation of retroelements in human male germ cells

DNA methylation, repressive histone modifications, and PIWI-interacting RNAs are essential for controlling retroelement silencing in mammalian germ lines. Dysregulation of retroelement silencing is associated with male sterility. Although retroelement silencing mechanisms have been extensively studied in mouse germ cells, little progress has been made in humans. Here, we show that the Krüppel-associated box domain zinc finger proteins are associated with DNA methylation of retroelements in human primordial germ cells. Further, we show that the hominoid-specific retroelement SINE-VNTR-Alus (SVA) is subjected to transcription-directed de novo DNA methylation during human spermatogenesis. The degree of de novo DNA methylation in SVAs varies among human individuals, which confers significant inter-individual epigenetic variation in sperm. Collectively, our results highlight potential molecular mechanisms for the regulation of retroelements in human male germ cells.

Species-specific KRAB-ZFPs function as repressors of retroviruses by targeting PBS regions

Hosts often target the relatively conserved regions in rapidly mutating retroviruses to inhibit their replication. One of these regions is called a primer binding site (PBS), which has to be complementary to the host tRNA to initiate reverse transcription. By analyzing endogenous retroviral elements, we found that host cells use this sequence as a target in efforts to block the expression of viral elements. A specific type of zinc finger protein targets the PBS in a host genome, which not only inhibits the transcription of endogenous viruses but also inhibits the replication of exogenous retroviruses with the same PBS. Thus, our study sheds light on a strategy for searching for host restriction factors targeting retroviruses.

Eukaryotic genomes harbor sequences derived from the chromosomal integration of ancient viruses, such as endogenous retroviruses (ERVs), which comprise 8% of the human genome. Like exogenous retroviruses, ERVs retain many common functional elements, including the corresponding DNA sequences of transfer RNA (tRNA) primer binding sites (PBSs), which are utilized for reverse transcription initiation by exogenous retroviruses. Here, through a medium-scale analysis of PBS loci positioned within ERVs, coupled with chromatin immunoprecipitation sequencing (ChIP-seq) of Kruppel-associated box zinc finger proteins (KRAB-ZFPs), we identified multiple ZFPs that specifically bind to different PBS loci. Among these, we focused on PBS-Lys, which is utilized by HIV-1, and identified its specific binding proteins to be mouse ZFP961 and human ZNF417/ZNF587. We found that these proteins not only repress ERV transcription but also inhibit retrovirus integration and transcription. Disruption of these ZFPs rendered cells more susceptible to HIV-1 infection. Thus, our research provides a methodology for identifying potential host factors that target retroviruses by ERVs.

Stress Decreases Host Viral Resistance and Increases Covid Susceptibility in Embryonic Stem Cells

Stress-induced changes in viral receptor and susceptibility gene expression were measured in embryonic stem cells (ESC) and differentiated progeny. Rex1 promoter-Red Fluorescence Protein reporter ESC were tested by RNAseq after 72hr exposures to control stress hyperosmotic sorbitol under stemness culture (NS) to quantify stress-forced differentiation (SFD) transcriptomic programs. Control ESC cultured with stemness factor removal produced normal differentiation (ND). Bulk RNAseq transcriptomic analysis showed significant upregulation of two genes involved in Covid-19 cell uptake, Vimentin (VIM) and Transmembrane Serine Protease 2 (TMPRSS2). SFD increased the hepatitis A virus receptor (Havcr1) and the transplacental Herpes simplex 1 (HSV1) virus receptor (Pvrl1) compared with ESC undergoing ND. Several other coronavirus receptors, Glutamyl Aminopeptidase (ENPEP) and Dipeptidyl Peptidase 4 (DPP4) were upregulated significantly in SFD>ND. Although stressed ESC are more susceptible to infection due to increased expression of viral receptors and decreased resistance, the necessary Covid-19 receptor, angiotensin converting enzyme (ACE)2, was not expressed in our experiments. TMPRSS2, ENPEP, and DPP4 mediate Coronavirus uptake, but are also markers of extra-embryonic endoderm (XEN), which arise from ESC undergoing ND or SFD. Mouse and human ESCs differentiated to XEN increase TMPRSS2 and other Covid-19 uptake-mediating gene expression, but only some lines express ACE2. Covid-19 susceptibility appears to be genotype-specific and not ubiquitous. Of the 30 gene ontology (GO) groups for viral susceptibility, 15 underwent significant stress-forced changes. Of these, 4 GO groups mediated negative viral regulation and most genes in these increase in ND and decrease with SFD, thus suggesting that stress increases ESC viral susceptibility. Taken together, the data suggest that a control hyperosmotic stress can increase Covid-19 susceptibility and decrease viral host resistance in mouse ESC. However, this limited pilot study should be followed with studies in human ESC, tests of environmental, hormonal, and pharmaceutical stressors and direct tests for infection of stressed, cultured ESC and embryos by Covid-19.

Harnessing natural variation to identify cis regulators of sex-biased gene expression in a multi-strain mouse liver model

Sex differences in gene expression are widespread in the liver, where many autosomal factors act in tandem with growth hormone signaling to regulate individual variability of sex differences in liver metabolism and disease. Here, we compare hepatic transcriptomic and epigenetic profiles of mouse strains C57BL/6J and CAST/EiJ, representing two subspecies separated by 0.5-1 million years of evolution, to elucidate the actions of genetic factors regulating liver sex differences. We identify 144 protein coding genes and 78 lncRNAs showing strain-conserved sex bias; many have gene ontologies relevant to liver function, are more highly liver-specific and show greater sex bias, and are more proximally regulated than genes whose sex bias is strain-dependent. The strain-conserved genes include key growth hormone-dependent transcriptional regulators of liver sex bias; however, three other transcription factors, Trim24, Tox, and Zfp809, lose their sex-biased expression in CAST/EiJ mouse liver. To elucidate the observed strain specificities in expression, we characterized the strain-dependence of sex-biased chromatin opening and enhancer marks at cis regulatory elements (CREs) within expression quantitative trait loci (eQTL) regulating liver sex-biased genes. Strikingly, 208 of 286 eQTLs with strain-specific, sex-differential effects on expression were associated with a complete gain, loss, or reversal of the sex differences in expression between strains. Moreover, 166 of the 286 eQTLs were linked to the strain-dependent gain or loss of localized sex-biased CREs. Remarkably, a subset of these CREs apparently lacked strain-specific genetic variants yet showed coordinated, strain-dependent sex-biased epigenetic regulation. Thus, we directly link hundreds of strain-specific genetic variants to the high variability in CRE activity and expression of sex-biased genes and uncover underlying genetically-determined epigenetic states controlling liver sex bias in genetically diverse mouse populations.

Transposable Element Dynamics and Regulation during Zygotic Genome Activation in Mammalian Embryos and Embryonic Stem Cell Model Systems

Transposable elements (TEs) are mobile genetic sequences capable of duplicating and reintegrating at new regions within the genome. A growing body of evidence has demonstrated that these elements play important roles in host genome evolution, despite being traditionally viewed as parasitic elements. To prevent ectopic activation of TE transposition and transcription, they are epigenetically silenced in most somatic tissues. Intriguingly, a specific class of TEs-retrotransposons-is transiently expressed at discrete phases during mammalian development and has been linked to the establishment of totipotency during zygotic genome activation (ZGA). While mechanisms controlling TE regulation in somatic tissues have been extensively studied, the significance underlying the unique transcriptional reactivation of retrotransposons during ZGA is only beginning to be uncovered. In this review, we summarize the expression dynamics of key retrotransposons during ZGA, focusing on findings from in vivo totipotent embryos and in vitro totipotent-like embryonic stem cells (ESCs). We then dissect the functions of retrotransposons and discuss how their transcriptional activities are finetuned during early stages of mammalian development.

Targeting Epigenetic Mechanisms to Treat Alcohol Use Disorders (AUD)

BACKGROUND

The impact of abusive alcohol consumption on human health is remarkable. According to the World Health Organization (WHO), approximately 3.3 million people die annually because of harmful alcohol consumption (the figure represents around 5.9% of global deaths). Alcohol Use Disorder (AUD) is a chronic disease where individuals exhibit compulsive alcohol drinking and present negative emotional states when they do not drink. In the most severe manifestations of AUD, the individuals lose control over intake despite a decided will to stop drinking. Given the multiple faces and the specific forms of this disease, the term AUD often appears in the plural (AUDs). Since only a few approved pharmacological treatments are available to treat AUD and they do not apply to all individuals or AUD forms, the search for compounds that may help to eliminate the burden of the disease and complement other therapeutical approaches is necessary.

METHODS

This work reviews recent research focused on the involvement of epigenetic mechanisms in the pathophysiology of AUD. Excessive drinking leads to chronic and compulsive consumption that eventually damages the organism. The central nervous system is a key target and is the focus of this study. The search for the genetic and epigenetic mechanisms behind the intricated dysregulation induced by ethanol will aid researchers in establishing new therapy approaches.

CONCLUSION

Recent findings in the field of epigenetics are essential and offer new windows for observation and research. The study of small molecules that inhibit key epienzymes involved in nucleosome architecture dynamics is necessary in order to prove their action and specificity in the laboratory and to test their effectivity and safety in clinical trials with selected patients bearing defined alterations caused by ethanol.

A cis-acting structural variation at the ZNF558 locus controls a gene regulatory network in human brain development

The human forebrain has expanded in size and complexity compared to chimpanzees despite limited changes in protein-coding genes, suggesting that gene expression regulation is an important driver of brain evolution. Here, we identify a KRAB-ZFP transcription factor, ZNF558, that is expressed in human but not chimpanzee forebrain neural progenitor cells. ZNF558 evolved as a suppressor of LINE-1 transposons but has been co-opted to regulate a single target, the mitophagy gene SPATA18. ZNF558 plays a role in mitochondrial homeostasis, and loss-of-function experiments in cerebral organoids suggests that ZNF558 influences developmental timing during early human brain development. Expression of ZNF558 is controlled by the size of a variable number tandem repeat that is longer in chimpanzees compared to humans, and variable in the human population. Thus, this work provides mechanistic insight into how a cis-acting structural variation establishes a regulatory network that affects human brain evolution.

LentiRILES, a miRNA-ON sensor system for monitoring the functionality of miRNA in cancer biology and therapy

A major unresolved challenge in miRNA biology is the capacity to monitor the spatiotemporal activity of miRNAs expressed in animal disease models. We recently reported that the miRNA-ON monitoring system called RILES (RNAi-inducible expression Luciferase system) implanted in lentivirus expression system (LentiRILES) offers unique opportunity to decipher the kinetics of miRNA activity in vitro, in relation with their intracellular trafficking in glioblastoma cells. In this study, we describe in detail the method for the production of LentiRILES stable cell lines and employed it in several applications in the field of miRNA biology and therapy. We show that LentiRILES is a robust, highly specific and sensitive miRNA sensor system that can be used in vitro as a single-cell miRNA monitoring method, cell-based screening platform for miRNA therapeutics and as a tool to analyse the structure-function relationship of the miRNA duplex. Furthermore, we report the kinetics of miRNA activity upon the intracranial delivery of miRNA mimics in an orthotopic animal model of glioblastoma. This information is exploited to evaluate the tumour suppressive function of miRNA-200c as locoregional therapeutic modality to treat glioblastoma. Our data provide evidence that LentiRILES is a robust system, well suited to resolve the activity of endogenous and exogenously expressed miRNAs from basic research to gene and cell therapy.

Structure, Activity and Function of the SETDB1 Protein Methyltransferase

The SET Domain Bifurcated Histone Lysine Methyltransferase 1 (SETDB1) is a prominent member of the Suppressor of Variegation 3–9 (SUV39)-related protein lysine methyltransferases (PKMTs), comprising three isoforms that differ in length and domain composition. SETDB1 is widely expressed in human tissues, methylating Histone 3 lysine 9 (H3K9) residues, promoting chromatin compaction and exerting negative regulation on gene expression. SETDB1 has a central role in normal physiology and nervous system development, having been implicated in the regulation of cell cycle progression, inactivation of the X chromosome, immune cells function, expression of retroelements and formation of promyelocytic leukemia (PML) nuclear bodies (NB). SETDB1 has been frequently deregulated in carcinogenesis, being implicated in the pathogenesis of gliomas, melanomas, as well as in lung, breast, gastrointestinal and ovarian tumors, where it mainly exerts an oncogenic role. Aberrant activity of SETDB1 has also been implicated in several neuropsychiatric, cardiovascular and gastrointestinal diseases, including schizophrenia, Huntington’s disease, congenital heart defects and inflammatory bowel disease. Herein, we provide an update on the unique structural and biochemical features of SETDB1 that contribute to its regulation, as well as its molecular and cellular impact in normal physiology and disease with potential therapeutic options.

Functions and Regulation of Endogenous Retrovirus Elements during Zygotic Genome Activation: Implications for Improving Somatic Cell Nuclear Transfer Efficiency

Endogenous retroviruses (ERVs), previously viewed as deleterious relics of ancestral retrovirus infections, are silenced in the vast majority of cells to minimize the risk of retrotransposition. Counterintuitively, bursts of ERV transcription usually occur during maternal-to-zygotic transition (MZT) in preimplantation embryos; this is regarded as a major landmark event in the zygotic genome activation (ZGA) process, indicating that ERVs play an active part in ZGA. Evolutionarily, the interaction between ERVs and hosts is mutually beneficial. The endogenization of retrovirus sequences rewires the gene regulatory network during ZGA, and ERV repression may lower germline fitness. Unfortunately, owing to various limitations of somatic cell nuclear transfer (SCNT) technology, both developmental arrest and ZGA abnormalities occur in a high percentage of cloned embryos, accompanied by ERV silencing, which may be caused by the activation failure of upstream ERV inducers. In this review, we discuss the functions and regulation of ERVs during the ZGA process and the feasibility of temporal control over ERVs in cloned embryos via exogenous double homeobox (DUX). We hypothesize that further accurate characterization of the ERV-rewired gene regulatory network during ZGA may provide a novel perspective on the development of preimplantation embryos.

Endogenous retroviruses in the origins and treatment of cancer

Endogenous retroviruses (ERVs) are emerging as promising therapeutic targets in cancer. As remnants of ancient retroviral infections, ERV-derived regulatory elements coordinate expression from gene networks, including those underpinning embryogenesis and immune cell function. ERV activation can promote an interferon response, a phenomenon termed viral mimicry. Although ERV expression is associated with cancer, and provisionally with autoimmune and neurodegenerative diseases, ERV-mediated inflammation is being explored as a way to sensitize tumors to immunotherapy. Here we review ERV co-option in development and innate immunity, the aberrant contribution of ERVs to tumorigenesis, and the wider biomedical potential of therapies directed at ERVs.

Targeting Epigenetics to Cure HIV-1: Lessons From (and for) Cancer Treatment

The Human Immunodeficiency Virus type 1 (HIV-1) integrates in the host genome as a provirus resulting in a long-lived reservoir of infected CD4 cells. As a provirus, HIV-1 has several aspects in common with an oncogene. Both the HIV-1 provirus and oncogenes only cause disease when expressed. A successful cure of both cancer and HIV-1 includes elimination of all cells with potential to regenerate the disease. For over two decades, epigenetic drugs developed against cancer have been used in the HIV-1 field to modulate the state of the proviral chromatin. Cells with an intact HIV-1 provirus exist in three states of infection: productive, inducible latent, and non-inducible latent. Here focus is on HIV-1, transcription control and chromatin structure; how the inducible proviruses are maintained in a chromatin structure that allows reactivation of transcription; and how transcription switches between different stages to allow for an abundance of different transcripts from a single promoter. Recently it was shown that a functional cure of HIV can be achieved by encapsulating all intact HIV-1 proviruses in heterochromatin, giving hope that epigenetic interventions may be used to end the HIV-1 epidemic.

An immune epigenetic insight to COVID-19 infection

Severe acute respiratory syndrome coronavirus-2 is a positive-sense RNA virus, a causal agent of ongoing COVID-19 pandemic. ACE2R methylation across three CpG sites (cg04013915, cg08559914, cg03536816) determines the host cell's entry. It regulates ACE2 expression by controlling the SIRT1 and KDM5B activity. Further, it regulates Type I and III IFN response by modulating H3K27me3 and H3K4me3 histone mark. SARS-CoV-2 protein with bromodomain and protein E mimics bromodomain histones and evades from host immune response. The 2'-O MTases mimics the host's cap1 structure and plays a vital role in immune evasion through Hsp90-mediated epigenetic process to hijack the infected cells. Although the current review highlighted the critical epigenetic events associated with SARS-CoV-2 immune evasion, the detailed mechanism is yet to be elucidated.

The Regulation and Functions of Endogenous Retrovirus in Embryo Development and Stem Cell Differentiation

Endogenous retroviruses (ERVs) are repetitive sequences in the genome, belonging to the retrotransposon family. During the course of life, ERVs are associated with multiple aspects of chromatin and transcriptional regulation in development and pathological conditions. In mammalian embryos, ERVs are extensively activated in early embryo development, but with a highly restricted spatial-temporal pattern; and they are drastically silenced during differentiation with exceptions in extraembryonic tissue and germlines. The dynamic activation pattern of ERVs raises questions about how ERVs are regulated in the life cycle and whether they are functionally important to cell fate decision during early embryo and somatic cell development. Therefore, in this review, we focus on the pieces of evidence demonstrating regulations and functions of ERVs during stem cell differentiation, which suggests that ERV activation is not a passive result of cell fate transition but the active epigenetic and transcriptional regulation during mammalian development and stem cell differentiation.

Genomic properties of variably methylated retrotransposons in mouse

BACKGROUND

Transposable elements (TEs) are enriched in cytosine methylation, preventing their mobility within the genome. We previously identified a genome-wide repertoire of candidate intracisternal A particle (IAP) TEs in mice that exhibit inter-individual variability in this methylation (VM-IAPs) with implications for genome function.

RESULTS

Here we validate these metastable epialleles and discover a novel class that exhibit tissue specificity (tsVM-IAPs) in addition to those with uniform methylation in all tissues (constitutive- or cVM-IAPs); both types have the potential to regulate genes in cis. Screening for variable methylation at other TEs shows that this phenomenon is largely limited to IAPs, which are amongst the youngest and most active endogenous retroviruses. We identify sequences enriched within cVM-IAPs, but determine that these are not sufficient to confer epigenetic variability. CTCF is enriched at VM-IAPs with binding inversely correlated with DNA methylation. We uncover dynamic physical interactions between cVM-IAPs with low methylation ranges and other genomic loci, suggesting that VM-IAPs have the potential for long-range regulation.

CONCLUSION

Our findings indicate that a recently evolved interplay between genetic sequence, CTCF binding, and DNA methylation at young TEs can result in inter-individual variability in transcriptional outcomes with implications for phenotypic variation.

Conserved paradoxical relationships among the evolutionary, structural and expressional features of KRAB zinc-finger proteins reveal their special functional characteristics

Background

One striking feature of the large KRAB domain-containing zinc finger protein (KZFP) family is its rapid evolution, leading to hundreds of member genes with various origination time in a certain mammalian genome. However, a comprehensive genome-wide and across-taxa analysis of the structural and expressional features of KZFPs with different origination time is lacking. This type of analysis will provide valuable clues about the functional characteristics of this special family.

Results

In this study, we found several conserved paradoxical phenomena about this issue. 1) Ordinary young domains/proteins tend to be disordered, but most of KRAB domains are completely structured in 64 representative species across the superclass of Sarcopterygii and most of KZFPs are also highly structured, indicating their rigid and unique structural and functional characteristics; as exceptions, old-zinc-finger-containing KZFPs have relatively disordered KRAB domains and linker regions, contributing to diverse interacting partners and functions. 2) In general, young or highly structured proteins tend to be spatiotemporal specific and have low abundance. However, by integrated analysis of 29 RNA-seq datasets, including 725 samples across early embryonic development, embryonic stem cell differentiation, embryonic and adult organs, tissues in 7 mammals, we found that KZFPs tend to express ubiquitously with medium abundance regardless of evolutionary age and structural disorder degree, indicating the wide functional requirements of KZFPs in various states. 3) Clustering and correlation analysis reveal that there are differential expression patterns across different spatiotemporal states, suggesting the specific-high-expression KZFPs may play important roles in the corresponding states. In particular, part of young-zinc-finger-containing KZFPs are highly expressed in early embryonic development and ESCs differentiation into endoderm or mesoderm. Co-expression analysis revealed that young-zinc-finger-containing KZFPs are significantly enriched in five co-expression modules. Among them, one module, including 13 young-zinc-finger-containing KZFPs, showed an ‘early-high and late-low’ expression pattern. Further functional analysis revealed that they may function in early embryonic development and ESC differentiation via participating in cell cycle related processes.

Conclusions

This study shows the conserved and special structural, expressional features of KZFPs, providing new clues about their functional characteristics and potential causes of their rapid evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00346-w.

The fibronectin type-III (FNIII) domain of ATF7IP contributes to efficient transcriptional silencing mediated by the SETDB1 complex

BACKGROUND

The histone methyltransferase SETDB1 (also known as ESET) represses genes and various types of transposable elements, such as endogenous retroviruses (ERVs) and integrated exogenous retroviruses, through a deposition of trimethylation on lysine 9 of histone H3 (H3K9me3) in mouse embryonic stem cells (mESCs). ATF7IP (also known as MCAF1 or AM), a binding partner of SETDB1, regulates the nuclear localization and enzymatic activities of SETDB1 and plays a crucial role in SETDB1-mediated transcriptional silencing. In this study, we further dissected the ATF7IP function with its truncated mutants in Atf7ip knockout (KO) mESCs.

RESULTS

We demonstrated that the SETDB1-interaction region within ATF7IP is essential for ATF7IP-dependent SETDB1 nuclear localization and silencing of both ERVs and integrated retroviral transgenes, whereas its C-terminal fibronectin type-III (FNIII) domain is dispensable for both these functions; rather, it has a role in efficient silencing mediated by the SETDB1 complex. Proteomic analysis identified a number of FNIII domain-interacting proteins, some of which have a consensus binding motif. We showed that one of the FNIII domain-binding proteins, ZMYM2, was involved in the efficient silencing of a transgene by ATF7IP. RNA-seq analysis of Atf7ip KO and WT or the FNIII domain mutant of ATF7IP-rescued Atf7ip KO mESCs showed that the FNIII domain mutant re-silenced most de-repressed SETDB1/ATF7IP-targeted ERVs compared to the WT. However, the silencing activity of the FNIII domain mutant was weaker than that of the ATF7IP WT, and some of the de-repressed germ cell-related genes in Atf7ip KO mESCs were not silenced by the FNIII domain mutant. Such germ cell-related genes are targeted and silenced by the MAX/MGA complex, and MGA was also identified as another potential binding molecule of the ATF7IP FNIII domain in the proteomic analysis. This suggests that the FNIII domain of ATF7IP acts as a binding hub of ATF7IP-interacting molecules possessing a specific interacting motif we named FAM and contributes to one layer of the SETDB1/ATF7IP complex-mediated silencing mechanisms.

CONCLUSIONS

Our findings contributed to further understanding the function of ATF7IP in the SETDB1 complex, revealed the role of the FNIII domain of ATF7IP in transcriptional silencing, and suggested a potential underlying molecular mechanism for it.

Host Gene Regulation by Transposable Elements: The New, the Old and the Ugly

The human genome has been under selective pressure to evolve in response to emerging pathogens and other environmental challenges. Genome evolution includes the acquisition of new genes or new isoforms of genes and changes to gene expression patterns. One source of genome innovation is from transposable elements (TEs), which carry their own promoters, enhancers and open reading frames and can act as ‘controlling elements’ for our own genes. TEs include LINE-1 elements, which can retrotranspose intracellularly and endogenous retroviruses (ERVs) that represent remnants of past retroviral germline infections. Although once pathogens, ERVs also represent an enticing source of incoming genetic material that the host can then repurpose. ERVs and other TEs have coevolved with host genes for millions of years, which has allowed them to become embedded within essential gene expression programmes. Intriguingly, these host genes are often subject to the same epigenetic control mechanisms that evolved to combat the TEs that now regulate them. Here, we illustrate the breadth of host gene regulation through TEs by focusing on examples of young (The New), ancient (The Old), and disease-causing (The Ugly) TE integrants.

Genome-wide CRISPR knockout screen identifies ZNF304 as a silencer of HIV transcription that promotes viral latency

Despite the widespread use of anti-retroviral therapy, human immunodeficiency virus (HIV) still persists in an infected cell reservoir that harbors transcriptionally silent yet replication-competent proviruses. While significant progress has been made in understanding how the HIV reservoir is established, transcription repression mechanisms that are enforced on the integrated viral promoter have not been fully revealed. In this study, we performed a whole-genome CRISPR knockout screen in HIV infected T cells to identify host genes that potentially promote HIV latency. Of several top candidates, the KRAB-containing zinc finger protein, ZNF304, was identified as the top hit. ZNF304 silences HIV gene transcription through associating with TRIM28 and recruiting to the viral promoter heterochromatin-inducing methyltransferases, including the polycomb repression complex (PRC) and SETB1. Depletion of ZNF304 expression reduced levels of H3K9me3, H3K27me3 and H2AK119ub repressive histone marks on the HIV promoter as well as SETB1 and TRIM28, ultimately enhancing HIV gene transcription. Significantly, ZNF304 also promoted HIV latency, as its depletion delayed the entry of HIV infected cells into latency. In primary CD4⁺ cells, ectopic expression of ZNF304 silenced viral transcription. We conclude that by associating with TRIM28 and recruiting host transcriptional repressive complexes, SETB1 and PRC, to the HIV promoter, ZNF304 silences HIV gene transcription and promotes viral latency.

Antiretroviral therapy has significantly decreased the morbidity and mortality associated with HIV infection. However, a complete cure remains out of reach, as HIV persists in a cell reservoir that is highly stable in the face of therapy. While developing novel therapeutic strategies to eliminate the reservoir is a well-recognized goal, knowledge of the molecular events that establish HIV latency is still not complete. To obtain insights into the silencing mechanisms of HIV gene transcription and the establishment of viral latency, a genome-wide CRISPR screen was employed to identify host factors that control viral latency. We identified zinc-finger protein 304 (ZNF304) and showed that through association with TRIM28, it recruits the histone methyltransferases SETB1 and PRC to deposit repressive marks on chromatin of the HIV promoter, thereby facilitating the silencing of viral gene transcription. Moreover, we found that depletion of ZNF304 expression activated HIV gene expression, while ZNF304 overexpression repressed viral gene transcription both in a T cell line and in primary CD4⁺ cells. Finally, our study showed that ZNF304 is also involved in modulating HIV latency, as its depletion delayed entry of the virus into a latency state. Our results offer an additional mechanistic explanation for how host histone repression complexes are tethered to the HIV promoter to promote chromatin compaction, thereby defining a potentially new target for perturbing the establishment of the viral reservoir.

Silencing and Transcriptional Regulation of Endogenous Retroviruses: An Overview

Almost half of the human genome is made up of transposable elements (TEs), and about 8% consists of endogenous retroviruses (ERVs). ERVs are remnants of ancient exogenous retrovirus infections of the germ line. Most TEs are inactive and not detrimental to the host. They are tightly regulated to ensure genomic stability of the host and avoid deregulation of nearby gene loci. Histone-based posttranslational modifications such as H3K9 trimethylation are one of the main silencing mechanisms. Trim28 is one of the identified master regulators of silencing, which recruits most prominently the H3K9 methyltransferase Setdb1, among other factors. Sumoylation and ATP-dependent chromatin remodeling factors seem to contribute to proper localization of Trim28 to ERV sequences and promote Trim28 interaction with Setdb1. Additionally, DNA methylation as well as RNA-mediated targeting of TEs such as piRNA-based silencing play important roles in ERV regulation. Despite the involvement of ERV overexpression in several cancer types, autoimmune diseases, and viral pathologies, ERVs are now also appreciated for their potential positive role in evolution. ERVs can provide new regulatory gene elements or novel binding sites for transcription factors, and ERV gene products can even be repurposed for the benefit of the host.

Analysis of KAP1 expression patterns and human endogenous retrovirus Env proteins in ovarian cancer

BACKGROUND

Human endogenous retroviruses (HERVs) constitute around 8% of the human genome and have important roles in human health and disease, including cancers. Previous studies showed that HERV envelope (Env) proteins are highly expressed in cancer tissues and co-related with cancer progression. KAP1 has been reported to play a key role in regulating retrotransposons, including HERV-K, through epigenetic silencing.

OBJECTIVE

The relationship between KAP-1 and HERV Envs expressions was analyzed only in tumor cell lines and has not yet been studied in cancer tissues. In this study, we analyzed the expression patterns and relationship between KAP1 and HERV Env proteins in ovarian cancer tissues.

METHOD

The expression patterns of KAP-1 and HERV Env proteins, including HERV-K and HERV-R, were analyzed in ovarian cancer tissue microarrays that contained 80 surgical specimens, including normal ovary and malignant ovarian cancers.

RESULTS

The expression of HERV-R Env and KAP1 proteins is significantly higher in ovarian cancer compared with normal ovary tissues. However, the expression of HERV-K Env did not change significantly in cancer tissues. The expression patterns of HERV-K Env and HERV-R Env significantly increased in early stages of cancer and KAP1 expression was higher in certain stage and types of cancers. However, the expression of HERV-K Env, HERV-R Env, and KAP1 did not change in different age groups. The correlation between the expression of KAP1 and HERV-Env, including HERV-K and HERV-R, was not significantly correlated.

CONCLUSIONS

The results of this study showed that there was no significant correlation between the expression of KAP1 and HERV Env proteins in ovarian cancer tissues, unlike studies with cell lines in vitro. These results suggest that the actual expression of HERV Env proteins in ovarian cancer tissues may be regulated through various complex factors as well as KAP1.

HOXA-AS2 promotes type I endometrial carcinoma via miRNA-302c-3p-mediated regulation of ZFX

Background

HOXA cluster antisense RNA2 (HOXA-AS2), a long-chain non-coding RNA, plays an important role in the behavior of various malignant tumors. The roles of HOXA-AS2 in endometrial cancer remain unclear.

Methods

We test expression levels of HOXA-AS2, miRNA-302c-3p, the transcription factor zinc finger X-chromosomal protein (ZFX), and the chitinase-like protein YKL-40 in endometrial carcinoma by qRT-PCR and western blotting. Luciferase reporter and qRT-PCR assays were conducted to identify potential binding sites of HOXA-AS2 to miRNA-302c-3p. Cell cycle, migration and invasion ability of endometrial cancer cells were investigated using flow-cytometric analysis, CCK-8 and transwell assays, respectively.

Results

HOXA-AS2 levels were significantly increased in endometrial cancer specimens compared to normal endometrial specimens. Upregulated HOXA-AS2 promoted invasion and proliferation of type I endometrial cancer cells. HOXA-AS2 silenced miRNA-302c-3p by binding to it. MiRNA-302c-3p negatively regulates ZFX and YKL-40. Thus HOXA-AS2 promotes the development of type I endometrial cancer via miRNA-302c-3p-mediated regulation of ZFX.

Conclusions

These findings suggest that HOXA-AS2 can act as a new therapeutic target for type I endometrial cancer.