Despite rapid progress in characterizing transcription factor-driven reprogramming of somatic cells to an induced pluripotent stem cell (iPSC) state, many mechanistic questions still remain. To gain insight into the earliest events in the reprogramming process, we systematically analyzed the transcriptional and epigenetic changes that occur during early factor induction after discrete numbers of divisions. We observed rapid, genome-wide changes in the euchromatic histone modification, H3K4me2, at more than a thousand loci including large subsets of pluripotency-related or developmentally regulated gene promoters and enhancers. In contrast, patterns of the repressive H3K27me3 modification remained largely unchanged except for focused depletion specifically at positions where H3K4 methylation is gained. These chromatin regulatory events precede transcriptional changes within the corresponding loci. Our data provide evidence for an early, organized, and population-wide epigenetic response to ectopic reprogramming factors that clarify the temporal order through which somatic identity is reset during reprogramming.

In embryonic stem (ES) cells, bivalent chromatin domains with overlapping repressive (H3 lysine 27 tri-methylation) and activating (H3 lysine 4 tri-methylation) histone modifications mark the promoters of more than 2,000 genes. To gain insight into the structure and function of bivalent domains, we mapped key histone modifications and subunits of Polycomb-repressive complexes 1 and 2 (PRC1 and PRC2) genomewide in human and mouse ES cells by chromatin immunoprecipitation, followed by ultra high-throughput sequencing. We find that bivalent domains can be segregated into two classes -- the first occupied by both PRC2 and PRC1 (PRC1-positive) and the second specifically bound by PRC2 (PRC2-only). PRC1-positive bivalent domains appear functionally distinct as they more efficiently retain lysine 27 tri-methylation upon differentiation, show stringent conservation of chromatin state, and associate with an overwhelming number of developmental regulator gene promoters. We also used computational genomics to search for sequence determinants of Polycomb binding. This analysis revealed that the genomewide locations of PRC2 and PRC1 can be largely predicted from the locations, sizes, and underlying motif contents of CpG islands. We propose that large CpG islands depleted of activating motifs confer epigenetic memory by recruiting the full repertoire of Polycomb complexes in pluripotent cells.

Mutations of subunits of the SWI/SNF chromatin remodeling complexes occur commonly in cancers of different lineages, including advanced thyroid cancers. Here we show that thyroid-specific loss of Arid1a, Arid2, or Smarcb1 in mouse BRAFV600E-mutant tumors promotes disease progression and decreased survival, associated with lesion-specific effects on chromatin accessibility and differentiation. As compared with normal thyrocytes, BRAFV600E-mutant mouse papillary thyroid cancers have decreased lineage transcription factor expression and accessibility to their target DNA binding sites, leading to impairment of thyroid-differentiated gene expression and radioiodine incorporation, which is rescued by MAPK inhibition. Loss of individual SWI/SNF subunits in BRAF tumors leads to a repressive chromatin state that cannot be reversed by MAPK pathway blockade, rendering them insensitive to its redifferentiation effects. Our results show that SWI/SNF complexes are central to the maintenance of differentiated function in thyroid cancers, and their loss confers radioiodine refractoriness and resistance to MAPK inhibitor-based redifferentiation therapies. SIGNIFICANCE: Reprogramming cancer differentiation confers therapeutic benefit in various disease contexts. Oncogenic BRAF silences genes required for radioiodine responsiveness in thyroid cancer. Mutations in SWI/SNF genes result in loss of chromatin accessibility at thyroid lineage specification genes in BRAF-mutant thyroid tumors, rendering them insensitive to the redifferentiation effects of MAPK blockade.This article is highlighted in the In This Issue feature, p. 995.

The myeloproliferative neoplasms (MPN) frequently progress to blast phase disease, an aggressive form of acute myeloid leukemia. To identify genes that suppress disease progression, we performed a focused CRISPR/Cas9 screen and discovered that depletion of LKB1/Stk11 led to enhanced in vitro self-renewal of murine MPN cells. Deletion of Stk11 in a mouse MPN model caused rapid lethality with enhanced fibrosis, osteosclerosis, and an accumulation of immature cells in the bone marrow, as well as enhanced engraftment of primary human MPN cells in vivo. LKB1 loss was associated with increased mitochondrial reactive oxygen species and stabilization of HIF1α, and downregulation of LKB1 and increased levels of HIF1α were observed in human blast phase MPN specimens. Of note, we observed strong concordance of pathways that were enriched in murine MPN cells with LKB1 loss with those enriched in blast phase MPN patient specimens, supporting the conclusion that STK11 is a tumor suppressor in the MPNs. SIGNIFICANCE: Progression of the myeloproliferative neoplasms to acute myeloid leukemia occurs in a substantial number of cases, but the genetic basis has been unclear. We discovered that loss of LKB1/STK11 leads to stabilization of HIF1a and promotes disease progression. This observation provides a potential therapeutic avenue for targeting progression.This article is highlighted in the In This Issue feature, p. 1307.

T cell receptor (TCR) signal strength is a key determinant of T cell responses. We developed a cancer mouse model in which tumor-specific CD8 T cells (TST cells) encounter tumor antigens with varying TCR signal strength. High-signal-strength interactions caused TST cells to up-regulate inhibitory receptors (IRs), lose effector function, and establish a dysfunction-associated molecular program. TST cells undergoing low-signal-strength interactions also up-regulated IRs, including PD1, but retained a cell-intrinsic functional state. Surprisingly, neither high- nor low-signal-strength interactions led to tumor control in vivo, revealing two distinct mechanisms by which PD1hi TST cells permit tumor escape; high signal strength drives dysfunction, while low signal strength results in functional inertness, where the signal strength is too low to mediate effective cancer cell killing by functional TST cells. CRISPR-Cas9-mediated fine-tuning of signal strength to an intermediate range improved anti-tumor activity in vivo. Our study defines the role of TCR signal strength in TST cell function, with important implications for T cell-based cancer immunotherapies.

Chromosomal instability (CIN) and epigenetic alterations are characteristics of advanced and metastatic cancers1-4, but whether they are mechanistically linked is unknown. Here we show that missegregation of mitotic chromosomes, their sequestration in micronuclei5,6 and subsequent rupture of the micronuclear envelope7 profoundly disrupt normal histone post-translational modifications (PTMs), a phenomenon conserved across humans and mice, as well as in cancer and non-transformed cells. Some of the changes in histone PTMs occur because of the rupture of the micronuclear envelope, whereas others are inherited from mitotic abnormalities before the micronucleus is formed. Using orthogonal approaches, we demonstrate that micronuclei exhibit extensive differences in chromatin accessibility, with a strong positional bias between promoters and distal or intergenic regions, in line with observed redistributions of histone PTMs. Inducing CIN causes widespread epigenetic dysregulation, and chromosomes that transit in micronuclei experience heritable abnormalities in their accessibility long after they have been reincorporated into the primary nucleus. Thus, as well as altering genomic copy number, CIN promotes epigenetic reprogramming and heterogeneity in cancer.

Mutations in genes encoding components of chromatin modifying and remodeling complexes are among the most frequently observed somatic events in human cancers. For example, missense and nonsense mutations targeting the mixed lineage leukemia family member 3 (MLL3, encoded by KMT2C) histone methyltransferase occur in a range of solid tumors, and heterozygous deletions encompassing KMT2C occur in a subset of aggressive leukemias. Although MLL3 loss can promote tumorigenesis in mice, the molecular targets and biological processes by which MLL3 suppresses tumorigenesis remain poorly characterized. Here, we combined genetic, epigenomic, and animal modeling approaches to demonstrate that one of the mechanisms by which MLL3 links chromatin remodeling to tumor suppression is by co-activating the Cdkn2a tumor suppressor locus. Disruption of Kmt2c cooperates with Myc overexpression in the development of murine hepatocellular carcinoma (HCC), in which MLL3 binding to the Cdkn2a locus is blunted, resulting in reduced H3K4 methylation and low expression levels of the locus-encoded tumor suppressors p16/Ink4a and p19/Arf. Conversely, elevated KMT2C expression increases its binding to the CDKN2A locus and co-activates gene transcription. Endogenous Kmt2c restoration reverses these chromatin and transcriptional effects and triggers Ink4a/Arf-dependent apoptosis. Underscoring the human relevance of this epistasis, we found that genomic alterations in KMT2C and CDKN2A were associated with similar transcriptional profiles in human HCC samples. These results collectively point to a new mechanism for disrupting CDKN2A activity during cancer development and, in doing so, link MLL3 to an established tumor suppressor network.

DNA amplifications in cancer do not only harbor oncogenes. We sought to determine whether passenger coamplifications could create collateral therapeutic vulnerabilities. Through an analysis of >3,000 cancer genomes followed by the interrogation of CRISPR-Cas9 loss-of-function screens across >700 cancer cell lines, we determined that passenger coamplifications are accompanied by distinct dependency profiles. In a proof-of-principle study, we demonstrate that the coamplification of the bona fide passenger gene DEAD-Box Helicase 1 (DDX1) creates an increased dependency on the mTOR pathway. Interaction proteomics identified tricarboxylic acid (TCA) cycle components as previously unrecognized DDX1 interaction partners. Live-cell metabolomics highlighted that this interaction could impair TCA activity, which in turn resulted in enhanced mTORC1 activity. Consequently, genetic and pharmacologic disruption of mTORC1 resulted in pronounced cell death in vitro and in vivo. Thus, structurally linked coamplification of a passenger gene and an oncogene can result in collateral vulnerabilities.

MYCN amplification drives one in six cases of neuroblastoma. The supernumerary gene copies are commonly found on highly rearranged, extrachromosomal circular DNA (ecDNA). The exact amplicon structure has not been described thus far and the functional relevance of its rearrangements is unknown. Here, we analyze the MYCN amplicon structure using short-read and Nanopore sequencing and its chromatin landscape using ChIP-seq, ATAC-seq and Hi-C. This reveals two distinct classes of amplicons which explain the regulatory requirements for MYCN overexpression. The first class always co-amplifies a proximal enhancer driven by the noradrenergic core regulatory circuit (CRC). The second class of MYCN amplicons is characterized by high structural complexity, lacks key local enhancers, and instead contains distal chromosomal fragments harboring CRC-driven enhancers. Thus, ectopic enhancer hijacking can compensate for the loss of local gene regulatory elements and explains a large component of the structural diversity observed in MYCN amplification.

Oncogenes only transform cells under certain cellular contexts, a phenomenon called oncogenic competence. Using a combination of a human pluripotent stem cell–derived cancer model along with zebrafish transgenesis, we demonstrate that the transforming ability of BRAFV600E along with additional mutations depends on the intrinsic transcriptional program present in the cell of origin. In both systems, melanocytes are less responsive to mutations, whereas both neural crest and melanoblast populations are readily transformed. Profiling reveals that progenitors have higher expression of chromatin-modifying enzymes such as ATAD2, a melanoma competence factor that forms a complex with SOX10 and allows for expression of downstream oncogenic and neural crest programs. These data suggest that oncogenic competence is mediated by regulation of developmental chromatin factors, which then allow for proper response to those oncogenes.

The pace of human brain development is highly protracted compared with most other species1-7. The maturation of cortical neurons is particularly slow, taking months to years to develop adult functions3-5. Remarkably, such protracted timing is retained in cortical neurons derived from human pluripotent stem cells (hPSCs) during in vitro differentiation or upon transplantation into the mouse brain4,8,9. Those findings suggest the presence of a cell-intrinsic clock setting the pace of neuronal maturation, although the molecular nature of this clock remains unknown. Here we identify an epigenetic developmental programme that sets the timing of human neuronal maturation. First, we developed a hPSC-based approach to synchronize the birth of cortical neurons in vitro which enabled us to define an atlas of morphological, functional and molecular maturation. We observed a slow unfolding of maturation programmes, limited by the retention of specific epigenetic factors. Loss of function of several of those factors in cortical neurons enables precocious maturation. Transient inhibition of EZH2, EHMT1 and EHMT2 or DOT1L, at progenitor stage primes newly born neurons to rapidly acquire mature properties upon differentiation. Thus our findings reveal that the rate at which human neurons mature is set well before neurogenesis through the establishment of an epigenetic barrier in progenitor cells. Mechanistically, this barrier holds transcriptional maturation programmes in a poised state that is gradually released to ensure the prolonged timeline of human cortical neuron maturation.

Polycomb proteins are epigenetic regulators that localize to developmental loci in the early embryo where they mediate lineage-specific gene repression. In Drosophila, these repressors are recruited to sequence elements by DNA binding proteins associated with Polycomb repressive complex 2 (PRC2). However, the sequences that recruit PRC2 in mammalian cells have remained obscure. To address this, we integrated a series of engineered bacterial artificial chromosomes into embryonic stem (ES) cells and examined their chromatin. We found that a 44 kb region corresponding to the Zfpm2 locus initiates de novo recruitment of PRC2. We then pinpointed a CpG island within this locus as both necessary and sufficient for PRC2 recruitment. Based on this causal demonstration and prior genomic analyses, we hypothesized that large GC-rich elements depleted of activating transcription factor motifs mediate PRC2 recruitment in mammals. We validated this model in two ways. First, we showed that a constitutively active CpG island is able to recruit PRC2 after excision of a cluster of activating motifs. Second, we showed that two 1 kb sequence intervals from the Escherichia coli genome with GC-contents comparable to a mammalian CpG island are both capable of recruiting PRC2 when integrated into the ES cell genome. Our findings demonstrate a causal role for GC-rich sequences in PRC2 recruitment and implicate a specific subset of CpG islands depleted of activating motifs as instrumental for the initial localization of this key regulator in mammalian genomes.

Immune checkpoint blockade (ICB) has demonstrated clinical success in "inflamed" tumors with substantial T cell infiltrates, but tumors with an immune-desert tumor microenvironment (TME) fail to benefit. The tumor cell-intrinsic molecular mechanisms of the immune-desert phenotype remain poorly understood. Here, we demonstrated that inactivation of the polycomb-repressive complex 2 (PRC2) core components embryonic ectoderm development (EED) or suppressor of zeste 12 homolog (SUZ12), a prevalent genetic event in malignant peripheral nerve sheath tumors (MPNSTs) and sporadically in other cancers, drove a context-dependent immune-desert TME. PRC2 inactivation reprogramed the chromatin landscape that led to a cell-autonomous shift from primed baseline signaling-dependent cellular responses (e.g., IFN-γ signaling) to PRC2-regulated developmental and cellular differentiation transcriptional programs. Further, PRC2 inactivation led to diminished tumor immune infiltrates through reduced chemokine production and impaired antigen presentation and T cell priming, resulting in primary resistance to ICB. Intratumoral delivery of inactivated modified vaccinia virus Ankara (MVA) enhanced tumor immune infiltrates and sensitized PRC2-loss tumors to ICB. Our results identify molecular mechanisms of PRC2 inactivation-mediated, context-dependent epigenetic reprogramming that underline the immune-desert phenotype in cancer. Our studies also point to intratumoral delivery of immunogenic viruses as an initial therapeutic strategy to modulate the immune-desert TME and capitalize on the clinical benefit of ICB.

The nucleoporin 98 gene (NUP98) is fused to a variety of partner genes in multiple hematopoietic malignancies. Here, we demonstrate that NUP98 fusion proteins, including NUP98-HOXA9 (NHA9), NUP98-HOXD13 (NHD13), NUP98-NSD1, NUP98-PHF23, and NUP98-TOP1 physically interact with mixed lineage leukemia 1 (MLL1) and the non-specific lethal (NSL) histone-modifying complexes. Chromatin immunoprecipitation sequencing illustrates that NHA9 and MLL1 co-localize on chromatin and are found associated with Hox gene promoter regions. Furthermore, MLL1 is required for the proliferation of NHA9 cells in vitro and in vivo. Inactivation of MLL1 leads to decreased expression of genes bound by NHA9 and MLL1 and reverses a gene expression signature found in NUP98-rearranged human leukemias. Our data reveal a molecular dependency on MLL1 function in NUP98-fusion-driven leukemogenesis.

UDP-glucose pyrophosphorylase 2 (UGP2), the enzyme that synthesizes uridine diphosphate (UDP)-glucose, rests at the convergence of multiple metabolic pathways, however, the role of UGP2 in tumor maintenance and cancer metabolism remains unclear. Here, we identify an important role for UGP2 in the maintenance of pancreatic ductal adenocarcinoma (PDAC) growth in both in vitro and in vivo tumor models. We found that transcription of UGP2 is directly regulated by the Yes-associated protein 1 (YAP)-TEA domain transcription factor (TEAD) complex, identifying UGP2 as a bona fide YAP target gene. Loss of UGP2 leads to decreased intracellular glycogen levels and defects in N-glycosylation targets that are important for the survival of PDACs, including the epidermal growth factor receptor (EGFR). These critical roles of UGP2 in cancer maintenance, metabolism, and protein glycosylation may offer insights into therapeutic options for otherwise intractable PDACs.

Prostate cancer exhibits a lineage-specific dependence on androgen signaling. Castration resistance involves reactivation of androgen signaling or activation of alternative lineage programs to bypass androgen requirement. We describe an aberrant gastrointestinal-lineage transcriptome expressed in ∼5% of primary prostate cancer that is characterized by abbreviated response to androgen-deprivation therapy and in ∼30% of castration-resistant prostate cancer. This program is governed by a transcriptional circuit consisting of HNF4G and HNF1A. Cistrome and chromatin analyses revealed that HNF4G is a pioneer factor that generates and maintains enhancer landscape at gastrointestinal-lineage genes, independent of androgen-receptor signaling. In HNF4G/HNF1A-double-negative prostate cancer, exogenous expression of HNF4G at physiologic levels recapitulates the gastrointestinal transcriptome, chromatin landscape, and leads to relative castration resistance.

Rearrangements of MLL (encoding lysine-specific methyltransferase 2A and officially known as KMT2A; herein referred to as MLL to denote the gene associated with mixed-lineage leukemia) generate MLL fusion proteins that bind DNA and drive leukemogenic gene expression. This gene expression program is dependent on the disruptor of telomeric silencing 1-like histone 3 lysine 79 (H3K79) methyltransferase DOT1L, and small-molecule DOT1L inhibitors show promise as therapeutics for these leukemias. However, the mechanisms underlying this dependency are unclear. We conducted a genome-scale RNAi screen and found that the histone deacetylase SIRT1 is required for the establishment of a heterochromatin-like state around MLL fusion target genes after DOT1L inhibition. DOT1L inhibits chromatin localization of a repressive complex composed of SIRT1 and the H3K9 methyltransferase SUV39H1, thereby maintaining an open chromatin state with elevated H3K9 acetylation and minimal H3K9 methylation at MLL fusion target genes. Furthermore, the combination of SIRT1 activators and DOT1L inhibitors shows enhanced antiproliferative activity against MLL-rearranged leukemia cells. These results indicate that the dynamic interplay between chromatin regulators controlling the activation and repression of gene expression could provide novel opportunities for combination therapy.

The histone variant H2A.Z has been implicated in nucleosome exchange, transcriptional activation and Polycomb repression. However, the relationships among these seemingly disparate functions remain obscure.

In acute myeloid leukemia (AML), chemotherapy resistance remains prevalent and poorly understood. Using functional proteomics of patient AML specimens, we identified MEF2C S222 phosphorylation as a specific marker of primary chemoresistance. We found that Mef2cS222A/S222A knock-in mutant mice engineered to block MEF2C phosphorylation exhibited normal hematopoiesis, but were resistant to leukemogenesis induced by MLL-AF9 MEF2C phosphorylation was required for leukemia stem cell maintenance and induced by MARK kinases in cells. Treatment with the selective MARK/SIK inhibitor MRT199665 caused apoptosis and conferred chemosensitivity in MEF2C-activated human AML cell lines and primary patient specimens, but not those lacking MEF2C phosphorylation. These findings identify kinase-dependent dysregulation of transcription factor control as a determinant of therapy response in AML, with immediate potential for improved diagnosis and therapy for this disease.Significance: Functional proteomics identifies phosphorylation of MEF2C in the majority of primary chemotherapy-resistant AML. Kinase-dependent dysregulation of this transcription factor confers susceptibility to MARK/SIK kinase inhibition in preclinical models, substantiating its clinical investigation for improved diagnosis and therapy of AML. Cancer Discov; 8(4); 478-97. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 371.

Oncogenes can initiate tumors only in certain cellular contexts, which is referred to as oncogenic competence. In melanoma, whether cells in the microenvironment can endow such competence remains unclear. Using a combination of zebrafish transgenesis coupled with human tissues, we demonstrate that GABAergic signaling between keratinocytes and melanocytes promotes melanoma initiation by BRAFV600E. GABA is synthesized in melanoma cells, which then acts on GABA-A receptors in keratinocytes. Electron microscopy demonstrates specialized cell-cell junctions between keratinocytes and melanoma cells, and multielectrode array analysis shows that GABA acts to inhibit electrical activity in melanoma/keratinocyte cocultures. Genetic and pharmacologic perturbation of GABA synthesis abrogates melanoma initiation in vivo. These data suggest that GABAergic signaling across the skin microenvironment regulates the ability of oncogenes to initiate melanoma.