Non-canonical functions of UHRF1 maintain DNA methylation homeostasis in cancer cells.

DNA methylation is an essential epigenetic chromatin modification, and its maintenance in mammals requires the protein UHRF1. It is yet unclear if UHRF1 functions solely by stimulating DNA methylation maintenance by DNMT1, or if it has important additional functions. Using degron alleles, we show that UHRF1 depletion causes a much greater loss of DNA methylation than DNMT1 depletion. This is not caused by passive demethylation as UHRF1-depleted cells proliferate more slowly than DNMT1-depleted cells. Instead, bioinformatics, proteomics and genetics experiments establish that UHRF1, besides activating DNMT1, interacts with DNMT3A and DNMT3B and promotes their activity. In addition, we show that UHRF1 antagonizes active DNA demethylation by TET2. Therefore, UHRF1 has non-canonical roles that contribute importantly to DNA methylation homeostasis; these findings have practical implications for epigenetics in health and disease.

A novel bioinformatic approach reveals cooperation between Cancer/Testis genes in basal-like breast tumors.

Breast cancer is the most prevalent type of cancer in women worldwide. Within breast tumors, the basal-like subtype has the worst prognosis, prompting the need for new tools to understand, detect, and treat these tumors. Certain germline-restricted genes show aberrant expression in tumors and are known as Cancer/Testis genes; their misexpression has diagnostic and therapeutic applications. Here we designed a new bioinformatic approach to examine Cancer/Testis gene misexpression in breast tumors. We identify several new markers in Luminal and HER-2 positive tumors, some of which predict response to chemotherapy. We then use machine learning to identify the two Cancer/Testis genes most associated with basal-like breast tumors: HORMAD1 and CT83. We show that these genes are expressed by tumor cells and not by the microenvironment, and that they are not expressed by normal breast progenitors; in other words, their activation occurs de novo. We find these genes are epigenetically repressed by DNA methylation, and that their activation upon DNA demethylation is irreversible, providing a memory of past epigenetic disturbances. Simultaneous expression of both genes in breast cells in vitro has a synergistic effect that increases stemness and activates a transcriptional profile also observed in double-positive tumors. Therefore, we reveal a functional cooperation between Cancer/Testis genes in basal breast tumors; these findings have consequences for the understanding, diagnosis, and therapy of the breast tumors with the worst outcomes.

Locus-level L1 DNA methylation profiling reveals the epigenetic and transcriptional interplay between L1s and their integration sites.

Long interspersed element 1 (L1) retrotransposons are implicated in human disease and evolution. Their global activity is repressed by DNA methylation, but deciphering the regulation of individual copies has been challenging. Here, we combine short- and long-read sequencing to unveil L1 methylation heterogeneity across cell types, families, and individual loci and elucidate key principles involved. We find that the youngest primate L1 families are specifically hypomethylated in pluripotent stem cells and the placenta but not in most tumors. Locally, intronic L1 methylation is intimately associated with gene transcription. Conversely, the L1 methylation state can propagate to the proximal region up to 300 bp. This phenomenon is accompanied by the binding of specific transcription factors, which drive the expression of L1 and chimeric transcripts. Finally, L1 hypomethylation alone is typically insufficient to trigger L1 expression due to redundant silencing pathways. Our results illuminate the epigenetic and transcriptional interplay between retrotransposons and their host genome.

E4F1 and ZNF148 are transcriptional activators of the -57A > C and wild-type TERT promoter.

Point mutations within the TERT promoter are the most recurrent somatic noncoding mutations identified across different cancer types, including glioblastoma, melanoma, hepatocellular carcinoma, and bladder cancer. They are most abundant at -146C > T and -124C > T, and rarer at -57A > C, with the latter originally described as a familial case, but subsequently shown also to occur somatically. All three mutations create de novo E26-specific (ETS) binding sites and result in activation of the TERT gene, allowing cancer cells to achieve replicative immortality. Here, we used a systematic proteomics screen to identify transcription factors preferentially binding to the -146C > T, -124C > T, and -57A > C mutations. Although we confirmed binding of multiple ETS factors to the mutant -146C > T and -124C > T sequences, we identified E4F1 as a -57A > C-specific binder and ZNF148 as a TERT wild-type (WT) promoter binder that showed reduced interaction with the -124C > T allele. Both proteins are activating transcription factors that bind specifically to the -57A > C and WT (at position 124) TERT promoter sequence in corresponding cell lines, and up-regulate TERT transcription and telomerase activity. Our work describes new regulators of TERT gene expression with possible roles in cancer.

A genetic screen identifies BEND3 as a regulator of bivalent gene expression and global DNA methylation.

Epigenetic mechanisms are essential to establish and safeguard cellular identities in mammals. They dynamically regulate the expression of genes, transposable elements and higher-order chromatin structures. Consequently, these chromatin marks are indispensable for mammalian development and alterations often lead to disease, such as cancer. Bivalent promoters are especially important during differentiation and development. Here we used a genetic screen to identify new regulators of a bivalent repressed gene. We identify BEND3 as a regulator of hundreds of bivalent promoters, some of which it represses, and some of which it activates. We show that BEND3 is recruited to a CpG-containg consensus site that is present in multiple copies in many bivalent promoters. Besides having direct effect on the promoters it binds, the loss of BEND3 leads to genome-wide gains of DNA methylation, which are especially marked at regions normally protected by the TET enzymes. DNA hydroxymethylation is reduced in Bend3 mutant cells, possibly as consequence of altered gene expression leading to diminished alpha-ketoglutarate production, thus lowering TET activity. Our results clarify the direct and indirect roles of an important chromatin regulator, BEND3, and, more broadly, they shed light on the regulation of bivalent promoters.

A genome-wide screen reveals new regulators of the 2-cell-like cell state.

In mammals, only the zygote and blastomeres of the early embryo are totipotent. This totipotency is mirrored in vitro by mouse '2-cell-like cells' (2CLCs), which appear at low frequency in cultures of embryonic stem cells (ESCs). Because totipotency is not completely understood, we carried out a genome-wide CRISPR knockout screen in mouse ESCs, searching for mutants that reactivate the expression of Dazl, a gene expressed in 2CLCs. Here we report the identification of four mutants that reactivate Dazl and a broader 2-cell-like signature: the E3 ubiquitin ligase adaptor SPOP, the Zinc-Finger transcription factor ZBTB14, MCM3AP, a component of the RNA processing complex TREX-2, and the lysine demethylase KDM5C. All four factors function upstream of DPPA2 and DUX, but not via p53. In addition, SPOP binds DPPA2, and KDM5C interacts with ncPRC1.6 and inhibits 2CLC gene expression in a catalytic-independent manner. These results extend our knowledge of totipotency, a key phase of organismal life.

Unchanged PCNA and DNMT1 dynamics during replication in DNA ligase I-deficient cells but abnormal chromatin levels of non-replicative histone H1.

DNA ligase I (LigI), the predominant enzyme that joins Okazaki fragments, interacts with PCNA and Pol δ. LigI also interacts with UHRF1, linking Okazaki fragment joining with DNA maintenance methylation. Okazaki fragments can also be joined by a relatively poorly characterized DNA ligase IIIα (LigIIIα)-dependent backup pathway. Here we examined the effect of LigI-deficiency on proteins at the replication fork. Notably, LigI-deficiency did not alter the kinetics of association of the PCNA clamp, the leading strand polymerase Pol ε, DNA maintenance methylation proteins and core histones with newly synthesized DNA. While the absence of major changes in replication and methylation proteins is consistent with the similar proliferation rate and DNA methylation levels of the LIG1 null cells compared with the parental cells, the increased levels of LigIIIα/XRCC1 and Pol δ at the replication fork and in bulk chromatin indicate that there are subtle replication defects in the absence of LigI. Interestingly, the non-replicative histone H1 variant, H1.0, is enriched in the chromatin of LigI-deficient mouse CH12F3 and human 46BR.1G1 cells. This alteration was not corrected by expression of wild type LigI, suggesting that it is a relatively stable epigenetic change that may contribute to the immunodeficiencies linked with inherited LigI-deficiency syndrome.

DNA methylation atlas and machinery in the developing and regenerating annelid Platynereis dumerilii.

BACKGROUND: Methylation of cytosines in DNA (5mC methylation) is a major epigenetic modification that modulates gene expression and constitutes the basis for mechanisms regulating multiple aspects of embryonic development and cell reprogramming in vertebrates. In mammals, 5mC methylation of promoter regions is linked to transcriptional repression. Transcription regulation by 5mC methylation notably involves the nucleosome remodeling and deacetylase complex (NuRD complex) which bridges DNA methylation and histone modifications. However, less is known about regulatory mechanisms involving 5mC methylation and their function in non-vertebrate animals. In this paper, we study 5mC methylation in the marine annelid worm Platynereis dumerilii, an emerging evolutionary and developmental biology model capable of regenerating the posterior part of its body post-amputation. RESULTS: Using in silico and experimental approaches, we show that P. dumerilii displays a high level of DNA methylation comparable to that of mammalian somatic cells. 5mC methylation in P. dumerilii is dynamic along the life cycle of the animal and markedly decreases at the transition between larval to post-larval stages. We identify a full repertoire of mainly single-copy genes encoding the machinery associated with 5mC methylation or members of the NuRD complex in P. dumerilii and show that this repertoire is close to the one inferred for the last common ancestor of bilaterians. These genes are dynamically expressed during P. dumerilii development and regeneration. Treatment with the DNA hypomethylating agent Decitabine impairs P. dumerilii larval development and regeneration and has long-term effects on post-regenerative growth. CONCLUSIONS: Our data reveal high levels of 5mC methylation in the annelid P. dumerilii, highlighting that this feature is not specific to vertebrates in the bilaterian clade. Analysis of DNA methylation levels and machinery gene expression during development and regeneration, as well as the use of a chemical inhibitor of DNA methylation, suggest an involvement of 5mC methylation in P. dumerilii development and regeneration. We also present data indicating that P. dumerilii constitutes a promising model to study biological roles and mechanisms of DNA methylation in non-vertebrate bilaterians and to provide new knowledge about evolution of the functions of this key epigenetic modification in bilaterian animals.

Lysine Methylation Regulators Moonlighting outside the Epigenome.

Landmark discoveries made nearly two decades ago identified known transcriptional regulators as histone lysine methyltransferases. Since then, the field of lysine methylation signaling has been dominated by studies of how this small chemical posttranslational modification regulates gene expression and other chromatin-based processes. However, recent advances in mass-spectrometry-based proteomics have revealed that histones are just a subset of the thousands of eukaryotic proteins marked by lysine methylation. As the writers, erasers, and readers of histone lysine methylation are emerging as a promising therapeutic target class for cancer and other diseases, a key challenge for the field is to define the full spectrum of activities for these proteins. Here we summarize recent discoveries implicating non-histone lysine methylation as a major regulator of diverse cellular processes. We further discuss recent technological innovations that are enabling the expanded study of lysine methylation signaling. Collectively, these findings are shaping our understanding of the fundamental mechanisms of non-histone protein regulation through this dynamic and multi-functional posttranslational modification.

Genetic screens reveal mechanisms for the transcriptional regulation of tissue-specific genes in normal cells and tumors.

The proper tissue-specific regulation of gene expression is essential for development and homeostasis in metazoans. However, the illegitimate expression of normally tissue-restricted genes-like testis- or placenta-specific genes-is frequently observed in tumors; this promotes transformation, but also allows immunotherapy. Two important questions are: how is the expression of these genes controlled in healthy cells? And how is this altered in cancer? To address these questions, we used an unbiased approach to test the ability of 350 distinct genetic or epigenetic perturbations to induce the illegitimate expression of over 40 tissue-restricted genes in primary human cells. We find that almost all of these genes are remarkably resistant to reactivation by a single alteration in signaling pathways or chromatin regulation. However, a few genes differ and are more readily activated; one is the placenta-expressed gene ADAM12, which promotes invasion. Using cellular systems, an animal model, and bioinformatics, we find that a non-canonical but druggable TGF-ß/KAT2A/TAK1 axis controls ADAM12 induction in normal and cancer cells. More broadly, our data show that illegitimate gene expression in cancer is an heterogeneous phenomenon, with a few genes activatable by simple events, and most genes likely requiring a combination of events to become reactivated.

Structure of the UHRF1 Tandem Tudor Domain Bound to a Methylated Non-histone Protein, LIG1, Reveals Rules for Binding and Regulation.

The protein UHRF1 is crucial for DNA methylation maintenance. The tandem Tudor domain (TTD) of UHRF1 binds histone H3K9me2/3 with micromolar affinity, as well as unmethylated linker regions within UHRF1 itself, causing auto-inhibition. Recently, we showed that a methylated histone-like region of DNA ligase 1 (LIG1K126me2/me3) binds the UHRF1 TTD with nanomolar affinity, permitting UHRF1 recruitment to chromatin. Here we report the crystal structure of the UHRF1 TTD bound to a LIG1K126me3 peptide. The data explain the basis for the high TTD-binding affinity of LIG1K126me3 and reveal that the interaction may be regulated by phosphorylation. Binding of LIG1K126me3 switches the overall structure of UHRF1 from a closed to a flexible conformation, suggesting that auto-inhibition is relieved. Our results provide structural insight into how UHRF1 performs its key function in epigenetic maintenance.

Depletion of ZBTB38 potentiates the effects of DNA demethylating agents in cancer cells via CDKN1C mRNA up-regulation.

DNA methyltransferase inhibitor (DNMTi) treatments have been used for patients with myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), and have shown promising beneficial effects in some other types of cancers. Here, we demonstrate that the transcriptional repressor ZBTB38 is a critical regulator of the cellular response to DNMTi. Treatments with 5-azacytidine, or its derivatives decitabine and zebularine, lead to down-regulation of ZBTB38 protein expression in cancer cells, in parallel with cellular damage. The depletion of ZBTB38 by RNA interference enhances the toxicity of DNMTi in cell lines from leukemia and from various solid tumor types. Further we observed that inactivation of ZBTB38 causes the up-regulation of CDKN1C mRNA, a previously described indirect target of DNMTi. We show that CDKN1C is a key actor of DNMTi toxicity in cells lacking ZBTB38. Finally, in patients with MDS a high level of CDKN1C mRNA expression before treatment correlates with a better clinical response to a drug regimen combining 5-azacytidine and histone deacetylase inhibitors. Collectively, our results suggest that the ZBTB38 protein is a target of DNMTi and that its depletion potentiates the toxicity of DNMT inhibitors in cancer cells, providing new opportunities to enhance the response to DNMT inhibitor therapies in patients with MDS and other cancers.

Comparative methylome analysis of ICF patients identifies heterochromatin loci that require ZBTB24, CDCA7 and HELLS for their methylated state.

Alterations of DNA methylation landscapes and machinery are a hallmark of many human diseases. A prominent case is the ICF syndrome, a rare autosomal recessive immunological/neurological disorder diagnosed by the loss of DNA methylation at (peri)centromeric repeats and its associated chromosomal instability. It is caused by mutations in the de novo DNA methyltransferase DNMT3B in about half of the patients (ICF1). In the remainder, the striking identification of mutations in factors devoid of DNA methyltransferase activity, ZBTB24 (ICF2), CDCA7 (ICF3) or HELLS (ICF4), raised key questions about common or distinguishing DNA methylation alterations downstream of these mutations and hence, about the functional link between the four factors. Here, we established the first comparative methylation profiling in ICF patients with all four genotypes and we provide evidence that, despite unifying hypomethylation of pericentromeric repeats and a few common loci, methylation profiling clearly distinguished ICF1 from ICF2, 3 and 4 patients. Using available genomic and epigenomic annotations to characterize regions prone to loss of DNA methylation downstream of ICF mutations, we found that ZBTB24, CDCA7 and HELLS mutations affect CpG-poor regions with heterochromatin features. Among these, we identified clusters of coding and non-coding genes mostly expressed in a monoallelic manner and implicated in neuronal development, consistent with the clinical spectrum of these patients' subgroups. Hence, beyond providing blood-based biomarkers of dysfunction of ICF factors, our comparative study unveiled new players to consider at certain heterochromatin regions of the human genome.

Methylation of DNA Ligase 1 by G9a/GLP Recruits UHRF1 to Replicating DNA and Regulates DNA Methylation.

DNA methylation is an essential epigenetic mark in mammals that has to be re-established after each round of DNA replication. The protein UHRF1 is essential for this process; it has been proposed that the protein targets newly replicated DNA by cooperatively binding hemi-methylated DNA and H3K9me2/3, but this model leaves a number of questions unanswered. Here, we present evidence for a direct recruitment of UHRF1 by the replication machinery via DNA ligase 1 (LIG1). A histone H3K9-like mimic within LIG1 is methylated by G9a and GLP and, compared with H3K9me2/3, more avidly binds UHRF1. Interaction with methylated LIG1 promotes the recruitment of UHRF1 to DNA replication sites and is required for DNA methylation maintenance. These results further elucidate the function of UHRF1, identify a non-histone target of G9a and GLP, and provide an example of a histone mimic that coordinates DNA replication and DNA methylation maintenance.

MBD5 and MBD6 interact with the human PR-DUB complex through their methyl-CpG-binding domain.

MBD5 and MBD6 are two members of the methyl-CpG-binding domain (MBD) family of proteins that are poorly characterized. Studies performed thus far have failed to show binding of the MBD5 and MBD6 MBD to methylated DNA. Here, we show that both MBD5 and MBD6 interact with the mammalian PR-DUB Polycomb protein complex in a mutually exclusive manner. Strikingly, the MBD of MBD5 and MBD6 is both necessary and sufficient to mediate this interaction. Chromatin immunoprecipitation analyses reveal that MBD6 and FOXK2/PR-DUB share a subset of genomic target genes, suggesting a functional interaction in vivo. Finally, we show that MBD6, but not MBD5, is recruited to sites of DNA damage in a PR-DUB independent manner. Our study thus implies a shared function for MBD5 and MBD6 through an interaction with PR-DUB, as well as an MBD6-specific recruitment to sites of DNA damage.

Maternal diets trigger sex-specific divergent trajectories of gene expression and epigenetic systems in mouse placenta.

Males and females responses to gestational overnutrition set the stage for subsequent sex-specific differences in adult onset non communicable diseases. Placenta, as a widely recognized programming agent, contibutes to the underlying processes. According to our previous findings, a high-fat diet during gestation triggers sex-specific epigenetic alterations within CpG and throughout the genome, together with the deregulation of clusters of imprinted genes. We further investigated the impact of diet and sex on placental histology, transcriptomic and epigenetic signatures in mice. Both basal gene expression and response to maternal high-fat diet were sexually dimorphic in whole placentas. Numerous genes showed sexually dimorphic expression, but only 11 genes regardless of the diet. In line with the key role of genes belonging to the sex chromosomes, 3 of these genes were Y-specific and 3 were X-specific. Amongst all the genes that were differentially expressed under a high-fat diet, only 16 genes were consistently affected in both males and females. The differences were not only quantitative but remarkably qualitative. The biological functions and networks of genes dysregulated differed markedly between the sexes. Seven genes of the epigenetic machinery were dysregulated, due to effects of diet, sex or both, including the Y- and X-linked histone demethylase paralogues Kdm5c and Kdm5d, which could mark differently male and female epigenomes. The DNA methyltransferase cofactor Dnmt3l gene expression was affected, reminiscent of our previous observation of changes in global DNA methylation. Overall, this striking sexual dimorphism of programming trajectories impose a considerable revision of the current dietary interventions protocols.