A Cluster of Evolutionarily Recent KRAB Zinc Finger Proteins Protects Cancer Cells from Replicative Stress-Induced Inflammation.

Heterochromatin loss and genetic instability enhance cancer progression by favoring clonal diversity, yet uncontrolled replicative stress leads to mitotic catastrophe and inflammatory responses that promote immune rejection. KRAB domain-containing zinc finger proteins (KZFP) contribute to heterochromatin maintenance at transposable elements (TE). Here, we identified an association of upregulation of a cluster of primate-specific KZFPs with poor prognosis, increased copy-number alterations, and changes in the tumor microenvironment in diffuse large B-cell lymphoma (DLBCL). Depleting two of these KZFPs targeting evolutionarily recent TEs, ZNF587 and ZNF417, impaired the proliferation of cells derived from DLBCL and several other tumor types. ZNF587 and ZNF417 depletion led to heterochromatin redistribution, replicative stress, and cGAS-STING-mediated induction of an interferon/inflammatory response, which enhanced susceptibility to macrophage-mediated phagocytosis and increased surface expression of HLA-I, together with presentation of a neoimmunopeptidome. Thus, cancer cells can exploit KZFPs to dampen TE-originating surveillance mechanisms, which likely facilitates clonal expansion, diversification, and immune evasion. SIGNIFICANCE: Upregulation of a cluster of primate-specific KRAB zinc finger proteins in cancer cells prevents replicative stress and inflammation by regulating heterochromatin maintenance, which could facilitate the development of improved biomarkers and treatments.

Genetic features and genomic targets of human KRAB-zinc finger proteins.

Krüppel-associated box (KRAB) domain-containing zinc finger proteins (KZFPs) are one of the largest groups of transcription factors encoded by tetrapods, with 378 members in human alone. KZFP genes are often grouped in clusters reflecting amplification by gene and segment duplication since the gene family first emerged more than 400 million years ago. Previous work has revealed that many KZFPs recognize transposable element (TE)-embedded sequences as genomic targets, and that KZFPs facilitate the co-option of the regulatory potential of TEs for the benefit of the host. Here, we present a comprehensive survey of the genetic features and genomic targets of human KZFPs, notably completing past analyses by adding data on close to a hundred family members. General principles emerge from our study of the TE-KZFP regulatory system, which point to multipronged evolutionary mechanisms underlaid by highly complex and combinatorial modes of action with strong influences on human speciation.

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-activated POU5F1B promotes colorectal cancer growth and metastasis.

The treatment of colorectal cancer (CRC) is an unmet medical need in absence of early diagnosis. Here, upon characterizing cancer-specific transposable element-driven transpochimeric gene transcripts (TcGTs) produced by this tumor in the SYSCOL cohort, we find that expression of the hominid-restricted retrogene POU5F1B through aberrant activation of a primate-specific endogenous retroviral promoter is a strong negative prognostic biomarker. Correlating this observation, we demonstrate that POU5F1B fosters the proliferation and metastatic potential of CRC cells. We further determine that POU5F1B, in spite of its phylogenetic relationship with the POU5F1/OCT4 transcription factor, is a membrane-enriched protein that associates with protein kinases and known targets or interactors as well as with cytoskeleton-related molecules, and induces intracellular signaling events and the release of trans-acting factors involved in cell growth and cell adhesion. As POU5F1B is an apparently non-essential gene only lowly expressed in normal tissues, and as POU5F1B-containing TcGTs are detected in other tumors besides CRC, our data provide interesting leads for the development of cancer therapies.

DUX is a non-essential synchronizer of zygotic genome activation.

Some of the earliest transcripts produced in fertilized human and mouse oocytes code for DUX, a double homeodomain protein that promotes embryonic genome activation (EGA). Deleting Dux by genome editing at the one- to two-cell stage in the mouse impairs EGA and blastocyst maturation. Here, we demonstrate that mice carrying homozygous Dux deletions display markedly reduced expression of DUX target genes and defects in both pre- and post-implantation development, with, notably, a disruption of the pace of the first few cell divisions and significant rates of late embryonic mortality. However, some Dux-/- embryos give rise to viable pups, indicating that DUX is important but not strictly essential for embryogenesis.

Hominoid-Specific Transposable Elements and KZFPs Facilitate Human Embryonic Genome Activation and Control Transcription in Naive Human ESCs.

Expansion of transposable elements (TEs) coincides with evolutionary shifts in gene expression. TEs frequently harbor binding sites for transcriptional regulators, thus enabling coordinated genome-wide activation of species- and context-specific gene expression programs, but such regulation must be balanced against their genotoxic potential. Here, we show that Krüppel-associated box (KRAB)-containing zinc finger proteins (KZFPs) control the timely and pleiotropic activation of TE-derived transcriptional cis regulators during early embryogenesis. Evolutionarily recent SVA, HERVK, and HERVH TE subgroups contribute significantly to chromatin opening during human embryonic genome activation and are KLF-stimulated enhancers in naive human embryonic stem cells (hESCs). KZFPs of corresponding evolutionary ages are simultaneously induced and repress the transcriptional activity of these TEs. Finally, the same KZFP-controlled TE-based enhancers later serve as developmental and tissue-specific enhancers. Thus, by controlling the transcriptional impact of TEs during embryogenesis, KZFPs facilitate their genome-wide incorporation into transcriptional networks, thereby contributing to human genome regulation.

ZNF445 is a primary regulator of genomic imprinting.

Genomic imprinting is an epigenetic process regulated by germline-derived DNA methylation, causing parental origin-specific monoallelic gene expression. Zinc finger protein 57 (ZFP57) is critical for maintenance of this epigenetic memory during post-fertilization reprogramming, yet incomplete penetrance of ZFP57 mutations in humans and mice suggests additional effectors. We reveal that ZNF445/ZFP445, which we trace to the origins of imprinting, binds imprinting control regions (ICRs) in mice and humans. In mice, ZFP445 and ZFP57 act together, maintaining all but one ICR in vivo, whereas earlier embryonic expression of ZNF445 and its intolerance to loss-of-function mutations indicate greater importance in the maintenance of human imprints.

KAP1 facilitates reinstatement of heterochromatin after DNA replication.

During cell division, maintenance of chromatin features from the parental genome requires their proper establishment on its newly synthetized copy. The loss of epigenetic marks within heterochromatin, typically enriched in repetitive elements, endangers genome stability and permits chromosomal rearrangements via recombination. However, how histone modifications associated with heterochromatin are maintained across mitosis remains poorly understood. KAP1 is known to act as a scaffold for a repressor complex that mediates local heterochromatin formation, and was previously demonstrated to play an important role during DNA repair. Accordingly, we investigated a putative role for this protein in the replication of heterochromatic regions. We first found that KAP1 associates with several DNA replication factors including PCNA, MCM3 and MCM6. We then observed that these interactions are promoted by KAP1 phosphorylation on serine 473 during S phase. Finally, we could demonstrate that KAP1 forms a complex with PCNA and the histone-lysine methyltransferase Suv39h1 to reinstate heterochromatin after DNA replication.

Individual retrotransposon integrants are differentially controlled by KZFP/KAP1-dependent histone methylation, DNA methylation and TET-mediated hydroxymethylation in naïve embryonic stem cells.

BACKGROUND: The KZFP/KAP1 (KRAB zinc finger proteins/KRAB-associated protein 1) system plays a central role in repressing transposable elements (TEs) and maintaining parent-of-origin DNA methylation at imprinting control regions (ICRs) during the wave of genome-wide reprogramming that precedes implantation. In naïve murine embryonic stem cells (mESCs), the genome is maintained highly hypomethylated by a combination of TET-mediated active demethylation and lack of de novo methylation, yet KAP1 is tethered by sequence-specific KZFPs to ICRs and TEs where it recruits histone and DNA methyltransferases to impose heterochromatin formation and DNA methylation. RESULTS: Here, upon removing either KAP1 or the cognate KZFP, we observed rapid TET2-dependent accumulation of 5hmC at both ICRs and TEs. In the absence of the KZFP/KAP1 complex, ICRs lost heterochromatic histone marks and underwent both active and passive DNA demethylation. For KAP1-bound TEs, 5mC hydroxylation correlated with transcriptional reactivation. Using RNA-seq, we further compared the expression profiles of TEs upon Kap1 removal in wild-type, Dnmt and Tet triple knockout mESCs. While we found that KAP1 represents the main effector of TEs repression in all three settings, we could additionally identify specific groups of TEs further controlled by DNA methylation. Furthermore, we observed that in the absence of TET proteins, activation upon Kap1 depletion was blunted for some TE integrants and increased for others. CONCLUSIONS: Our results indicate that the KZFP/KAP1 complex maintains heterochromatin and DNA methylation at ICRs and TEs in naïve embryonic stem cells partly by protecting these loci from TET-mediated demethylation. Our study further unveils an unsuspected level of complexity in the transcriptional control of the endovirome by demonstrating often integrant-specific differential influences of histone-based heterochromatin modifications, DNA methylation and 5mC oxidation in regulating TEs expression.

Polyphenic trait promotes liver cancer in a model of epigenetic instability in mice.

Hepatocellular carcinoma (HCC) represents the fifth-most common form of cancer worldwide and carries a high mortality rate attributed to lack of effective treatment. Males are 8 times more likely to develop HCC than females, an effect largely driven by sex hormones, albeit through still poorly understood mechanisms. We previously identified TRIM28 (tripartite protein 28), a scaffold protein capable of recruiting a number of chromatin modifiers, as a crucial mediator of sexual dimorphism in the liver. Trim28hep-/- mice display sex-specific transcriptional deregulation of a wide range of bile and steroid metabolism genes and development of liver adenomas in males. We now demonstrate that obesity and aging precipitate alterations of TRIM28-dependent transcriptional dynamics, leading to a metabolic infection state responsible for highly penetrant male-restricted hepatic carcinogenesis. Molecular analyses implicate aberrant androgen receptor stimulation, biliary acid disturbances, and altered responses to gut microbiota in the pathogenesis of Trim28hep-/- -associated HCC. Correspondingly, androgen deprivation markedly attenuates the frequency and severity of tumors, and raising animals under axenic conditions completely abrogates their abnormal phenotype, even upon high-fat diet challenge. CONCLUSION: This work underpins how discrete polyphenic traits in epigenetically metastable conditions can contribute to a cancer-prone state and more broadly provides new evidence linking hormonal imbalances, metabolic disturbances, gut microbiota, and cancer. (Hepatology 2017;66:235-251).

Transposable Elements and Their KRAB-ZFP Controllers Regulate Gene Expression in Adult Tissues.

KRAB-containing zinc finger proteins (KRAB-ZFPs) are early embryonic controllers of transposable elements (TEs), which they repress with their cofactor KAP1 through histone and DNA methylation, a process thought to result in irreversible silencing. Using a target-centered functional screen, we matched murine TEs with their cognate KRAB-ZFP. We found the paralogs ZFP932 and Gm15446 to bind overlapping but distinguishable subsets of ERVK (endogenous retrovirus K), repress these elements in embryonic stem cells, and regulate secondarily the expression of neighboring genes. Most importantly, we uncovered that these KRAB-ZFPs and KAP1 control TEs in adult tissues, in cell culture and in vivo, where they partner up to modulate cellular genes. Therefore, TEs and KRAB-ZFPs establish transcriptional networks that likely regulate not only development but also many physiological events. Given the high degree of species specificity of TEs and KRAB-ZFPs, these results have important implications for understanding the biology of higher vertebrates, including humans.

A KRAB/KAP1-miRNA cascade regulates erythropoiesis through stage-specific control of mitophagy.

During hematopoiesis, lineage- and stage-specific transcription factors work in concert with chromatin modifiers to direct the differentiation of all blood cells. We explored the role of KRAB-containing zinc finger proteins (KRAB-ZFPs) and their cofactor KAP1 in this process. In mice, hematopoietic-restricted deletion of Kap1 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 (miRNAs) targeting mitophagy transcripts, itself secondary to a lack of repression by stage-specific KRAB-ZFPs. The KRAB/KAP1-miRNA regulatory cascade is evolutionarily conserved, as it also controls mitophagy during human erythropoiesis. Thus, a multilayered transcription regulatory system is present, in which protein- and RNA-based repressors are superimposed in combinatorial fashion to govern the timely triggering of an important differentiation event.

De novo DNA methylation of endogenous retroviruses is shaped by KRAB-ZFPs/KAP1 and ESET.

Endogenous 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 ERV-containing loci.

The KRAB-ZFP/KAP1 system contributes to the early embryonic establishment of site-specific DNA methylation patterns maintained during development.

De novo DNA methylation is an essential aspect of the epigenetic reprogramming that takes place during early development, yet factors responsible for its instatement at particular genomic loci are poorly defined. Here, we demonstrate that the KRAB-ZFP-mediated recruitment of KAP1 to DNA in embryonic stem cells (ESCs) induces cytosine methylation. This process is preceded by H3K9 trimethylation, and genome-wide analyses reveal that it spreads over short distances from KAP1-binding sites so as to involve nearby CpG islands. In sharp contrast, in differentiated cells, KRAB/KAP1-induced heterochromatin formation does not lead to DNA methylation. Correspondingly, the methylation status of CpG islands in the adult mouse liver correlates with their proximity to KAP1-binding sites in ESCs, not in hepatocytes. Therefore, KRAB-ZFPs and their cofactor KAP1 are in part responsible for the establishment during early embryogenesis of site-specific DNA methylation patterns that are maintained through development.

KAP1 regulates gene networks controlling T-cell development and responsiveness.

Chromatin remodeling at specific genomic loci controls lymphoid differentiation. Here, we investigated the role played in this process by Kruppel-associated box (KRAB)-associated protein 1 (KAP1), the universal cofactor of KRAB-zinc finger proteins (ZFPs), a tetrapod-restricted family of transcriptional repressors. T-cell-specific Kap1-deleted mice displayed a significant expansion of immature thymocytes, imbalances in CD4(+)/CD8(+) cell ratios, and altered responses to TCR and TGFß stimulation when compared to littermate KAP1 control mice. Transcriptome and chromatin studies revealed that KAP1 binds T-cell-specific cis-acting regulatory elements marked by the H3K9me3 repressive mark and enriched in Ikaros/NuRD complexes. Also, KAP1 directly controls the expression of several genes involved in TCR and cytokine signaling. Among these, regulation of FoxO1 seems to play a major role in this system. Likely responsible for tethering KAP1 to at least part of its genomic targets, a small number of KRAB-ZFPs are selectively expressed in T-lymphoid cells. These results reveal the so far unsuspected yet important role of KAP1-mediated epigenetic regulation in T-lymphocyte differentiation and activation.

Liver-specific ablation of Krüppel-associated box-associated protein 1 in mice leads to male-predominant hepatosteatosis and development of liver adenoma.

UNLABELLED: The liver is characterized by sexually dimorphic gene expression translating into sex-specific differences in lipid, drug, steroid hormone, and xenobiotic metabolism, with distinct responses of males and females to environmental challenges. Here, we investigated the role of the Krüppel-associated box (KRAB)-associated protein 1 (KAP1) epigenetic regulator in this process. Liver-specific KAP1 knockout (KO) led to strikingly sexually dimorphic phenotypic disturbances, including male-predominant steatosis and hepatic tumors with up-regulation of protein kinase B and extracellular signal-related kinases 1/2 mitogen-activated protein kinase signaling. This correlated with the sex-specific transcriptional dysregulation of a wide range of metabolic genes, notably those involved in retinol and sex hormone processing as well as in detoxification. Furthermore, chromatin immunoprecipitation followed by deep sequencing indicated that a number of dysregulated genes are direct targets of the KRAB/KAP1 repression system. Those genes include sexually dimorphic cytochrome P 450 Cyp2d9, glutathione S-transferase π, Cyp2a, Cyp2b, and Cyp3a gene clusters. Additionally, we identified a male-restricted KAP1-binding site in the fat-specific protein 27 gene, correlating with its male-predominant up-regulation upon Kap1 deletion, suggesting that the latter might be an important trigger in the development of male-specific hepatosteatosis and secondary tumorigenesis. CONCLUSION: This work reveals KRAB/KAP1-mediated transcriptional regulation as a central event in metabolic control hormones, drugs, and xenobiotics in the liver and further links disturbances in these processes with hepatic carcinogenesis.

The Krüppel-associated box repressor domain can induce reversible heterochromatization of a mouse locus in vivo.

The study of chromatin and its regulators is key to understanding and manipulating transcription. We previously exploited the Krüppel-associated box (KRAB) transcriptional repressor domain, present in hundreds of vertebrate-specific zinc finger proteins, to assess the effect of its binding to gene bodies. These experiments revealed that the ectopic and doxycycline (dox)-controlled tet repressor KRAB fusion protein (tTRKRAB) can induce reversible and long-range silencing of cellular promoters. Here, we extend this system to in vivo applications and use tTRKRAB to achieve externally controllable repression of an endogenous mouse locus. We employed lentiviral-mediated transgenesis with promoterless TetO-containing gene traps to engineer a mouse line where the endogenous kinesin family member 2A (Kif2A) promoter drives a YFP reporter gene. When these mice were crossed to animals expressing the TetO-binding tTRKRAB repressor, this regulator was recruited to the Kif2A locus, and YFP expression was reduced. This effect was reversed when dox was given to embryos or adult mice, demonstrating that the cellular Kif2A promoter was only silenced upon repressor binding. Molecular analyses confirmed that tTRKRAB induced transcriptional repression through the spread of H3K9me3-containing heterochromatin, without DNA methylation of the trapped Kif2A promoter. Therefore, we demonstrate that targeting of tTRKRAB to a gene body in vivo results in reversible transcriptional repression through the spreading of facultative heterochromatin. This finding not only sheds light on KRAB-mediated transcriptional processes, but also suggests approaches for the externally controllable and reversible modulation of chromatin and transcription in vivo.

KAP1 regulates gene networks controlling mouse B-lymphoid cell differentiation and function.

Chromatin remodeling is fundamental for B-cell differentiation. In the present study, we explored the role of KAP1, the cofactor of KRAB-ZFP transcriptional repressors, in this process. B-lymphoid-specific Kap1-KO mice displayed reduced numbers of mature B cells, lower steady-state levels of Abs, and accelerated rates of decay of neutralizing Abs after viral immunization. Transcriptome analyses of Kap1-deleted B splenocytes revealed an up-regulation of PTEN, the enzymatic counteractor of PIK3 signaling, and of genes encoding DNA-damage response factors, cell-cycle regulators, and chemokine receptors. ChIP/seq studies established that KAP1 bound at or close to several of these genes and controlled chromatin status at their promoters. Genome wide, KAP1 binding sites lacked active B cell-specific enhancers and were enriched in repressive histone marks, further supporting a role for this molecule in gene silencing in vivo. Likely responsible for tethering KAP1 to at least some of these targets, a discrete subset of KRAB-ZFPs is enriched in B lymphocytes. Our results therefore reveal the role of KRAB/KAP1-mediated epigenetic regulation in B-cell development and homeostasis.

In embryonic stem cells, ZFP57/KAP1 recognize a methylated hexanucleotide to affect chromatin and DNA methylation of imprinting control regions.

The 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.