3D genome mapping identifies subgroup-specific chromosome conformations and tumor-dependency genes in ependymoma.

Ependymoma is a tumor of the brain or spinal cord. The two most common and aggressive molecular groups of ependymoma are the supratentorial ZFTA-fusion associated and the posterior fossa ependymoma group A. In both groups, tumors occur mainly in young children and frequently recur after treatment. Although molecular mechanisms underlying these diseases have recently been uncovered, they remain difficult to target and innovative therapeutic approaches are urgently needed. Here, we use genome-wide chromosome conformation capture (Hi-C), complemented with CTCF and H3K27ac ChIP-seq, as well as gene expression and DNA methylation analysis in primary and relapsed ependymoma tumors, to identify chromosomal conformations and regulatory mechanisms associated with aberrant gene expression. In particular, we observe the formation of new topologically associating domains ('neo-TADs') caused by structural variants, group-specific 3D chromatin loops, and the replacement of CTCF insulators by DNA hyper-methylation. Through inhibition experiments, we validate that genes implicated by these 3D genome conformations are essential for the survival of patient-derived ependymoma models in a group-specific manner. Thus, this study extends our ability to reveal tumor-dependency genes by 3D genome conformations even in tumors that lack targetable genetic alterations.

Glioblastoma stem cells reprogram chromatin in vivo to generate selective therapeutic dependencies on DPY30 and phosphodiesterases.

Glioblastomas are universally fatal cancers and contain self-renewing glioblastoma stem cells (GSCs) that initiate tumors. Traditional anticancer drug discovery based on in vitro cultures tends to identify targets with poor therapeutic indices and fails to accurately model the effects of the tumor microenvironment. Here, leveraging in vivo genetic screening, we identified the histone H3 lysine 4 trimethylation (H3K4me3) regulator DPY30 (Dpy-30 histone methyltransferase complex regulatory subunit) as an in vivo­specific glioblastoma dependency. On the basis of the hypothesis that in vivo epigenetic regulation may define critical GSC dependencies, we interrogated active chromatin landscapes of GSCs derived from intracranial patient-derived xenografts (PDXs) and cell culture through H3K4me3 chromatin immunoprecipitation and transcriptome analyses. Intracranial-specific genes marked by H3K4me3 included FOS, NFκB, and phosphodiesterase (PDE) family members. In intracranial PDX tumors, DPY30 regulated angiogenesis and hypoxia pathways in an H3K4me3-dependent manner but was dispensable in vitro in cultured GSCs. PDE4B was a key downstream effector of DPY30, and the PDE4 inhibitor rolipram preferentially targeted DPY30-expressing cells and impaired PDX tumor growth in mice without affecting tumor cells cultured in vitro. Collectively, the MLL/SET1 (mixed lineage leukemia/SET domain-containing 1, histone lysine methyltransferase) complex member DPY30 selectively regulates H3K4me3 modification on genes critical to support angiogenesis and tumor growth in vivo, suggesting the DPY30-PDE4B axis as a specific therapeutic target in glioblastoma.

Leveraging Allele-Specific Expression for Therapeutic Response Gene Discovery in Glioblastoma.

Glioblastoma is the most prevalent primary malignant brain tumor in adults and is characterized by poor prognosis and universal tumor recurrence. Effective glioblastoma treatments are lacking, in part due to somatic mutations and epigenetic reprogramming that alter gene expression and confer drug resistance. To investigate recurrently dysregulated genes in glioblastoma, we interrogated allele-specific expression (ASE), the difference in expression between two alleles of a gene, in glioblastoma stem cells (GSC) derived from 43 patients. A total of 118 genes were found with recurrent ASE preferentially in GSCs compared with normal tissues. These genes were enriched for apoptotic regulators, including schlafen family member 11 (SLFN11). Loss of SLFN11 gene expression was associated with aberrant promoter methylation and conferred resistance to chemotherapy and PARP inhibition. Conversely, low SLFN11 expression rendered GSCs susceptible to the oncolytic flavivirus Zika. This discovery effort based upon ASE revealed novel points of vulnerability in GSCs, suggesting a potential alternative treatment strategy for chemotherapy-resistant glioblastoma. SIGNIFICANCE: Assessing allele-specific expression reveals genes with recurrent cis-regulatory changes that are enriched in glioblastoma stem cells, including SLFN11, which modulates chemotherapy resistance and susceptibility to the oncolytic Zika virus.

Author Correction: Merkel cell polyomavirus activates LSD1-mediated blockade of non-canonical BAF to regulate transformation and tumorigenesis.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Merkel cell polyomavirus activates LSD1-mediated blockade of non-canonical BAF to regulate transformation and tumorigenesis.

Merkel cell carcinoma (MCC)-a neuroendocrine cancer of the skin-is caused by the integration of Merkel cell polyomavirus and persistent expression of large T antigen and small T antigen. We report that small T antigen in complex with MYCL and the EP400 complex activates the expression of LSD1 (KDM1A), RCOR2 and INSM1 to repress gene expression by the lineage transcription factor ATOH1. LSD1 inhibition reduces the growth of MCC in vitro and in vivo. Through a forward-genetics CRISPR-Cas9 screen, we identified an antagonistic relationship between LSD1 and the non-canonical BAF (ncBAF) chromatin remodelling complex. Changes in gene expression and chromatin accessibility caused by LSD1 inhibition were partially rescued by BRD9 inhibition, revealing that LSD1 and ncBAF antagonistically regulate an overlapping set of genes. Our work provides mechanistic insight into the dependence of MCC on LSD1 and a tumour suppressor role for ncBAF in cancer.

Epigenetic Priming of Human Pluripotent Stem Cell-Derived Cardiac Progenitor Cells Accelerates Cardiomyocyte Maturation.

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) exhibit a fetal phenotype that limits in vitro and therapeutic applications. Strategies to promote cardiomyocyte maturation have focused interventions on differentiated hPSC-CMs, but this study tests priming of early cardiac progenitor cells (CPCs) with polyinosinic-polycytidylic acid (pIC) to accelerate cardiomyocyte maturation. CPCs were differentiated from hPSCs using a monolayer differentiation protocol with defined small molecule Wnt temporal modulation, and pIC was added during the formation of early CPCs. pIC priming did not alter the expression of cell surface markers for CPCs (>80% KDR+/PDGFRα+), expression of common cardiac transcription factors, or final purity of differentiated hPSC-CMs (∼90%). However, CPC differentiation in basal medium revealed that pIC priming resulted in hPSC-CMs with enhanced maturity manifested by increased cell size, greater contractility, faster electrical upstrokes, increased oxidative metabolism, and more mature sarcomeric structure and composition. To investigate the mechanisms of CPC priming, RNAseq revealed that cardiac progenitor-stage pIC modulated early Notch signaling and cardiomyogenic transcriptional programs. Chromatin immunoprecipitation of CPCs showed that pIC treatment increased deposition of the H3K9ac activating epigenetic mark at core promoters of cardiac myofilament genes and the Notch ligand, JAG1. Inhibition of Notch signaling blocked the effects of pIC on differentiation and cardiomyocyte maturation. Furthermore, primed CPCs showed more robust formation of hPSC-CMs grafts when transplanted to the NSGW mouse kidney capsule. Overall, epigenetic modulation of CPCs with pIC accelerates cardiomyocyte maturation enabling basic research applications and potential therapeutic uses. Stem Cells 2019;37:910-923.

Dual inhibition of MDM2 and MDM4 in virus-positive Merkel cell carcinoma enhances the p53 response.

Merkel cell polyomavirus (MCV) contributes to approximately 80% of all Merkel cell carcinomas (MCCs), a highly aggressive neuroendocrine carcinoma of the skin. MCV-positive MCC expresses small T antigen (ST) and a truncated form of large T antigen (LT) and usually contains wild-type p53 (TP53) and RB (RB1). In contrast, virus-negative MCC contains inactivating mutations in TP53 and RB1. While the MCV-truncated LT can bind and inhibit RB, it does not bind p53. We report here that MCV LT binds to RB, leading to increased levels of ARF, an inhibitor of MDM2, and activation of p53. However, coexpression of ST reduced p53 activation. MCV ST recruits the MYC homologue MYCL (L-Myc) to the EP400 chromatin remodeler complex and transactivates specific target genes. We observed that depletion of EP400 in MCV-positive MCC cell lines led to increased p53 target gene expression. We suspected that the MCV ST-MYCL-EP400 complex could functionally inactivate p53, but the underlying mechanism was not known. Integrated ChIP and RNA-sequencing analysis following EP400 depletion identified MDM2 as well as CK1α, an activator of MDM4, as target genes of the ST-MYCL-EP400 complex. In addition, MCV-positive MCC cells expressed high levels of MDM4. Combining MDM2 inhibitors with lenalidomide targeting CK1α or an MDM4 inhibitor caused synergistic activation of p53, leading to an apoptotic response in MCV-positive MCC cells and MCC-derived xenografts in mice. These results support dual targeting of MDM2 and MDM4 in virus-positive MCC and other p53 wild-type tumors.

Merkel cell polyomavirus recruits MYCL to the EP400 complex to promote oncogenesis.

Merkel cell carcinoma (MCC) frequently contains integrated copies of Merkel cell polyomavirus DNA that express a truncated form of Large T antigen (LT) and an intact Small T antigen (ST). While LT binds RB and inactivates its tumor suppressor function, it is less clear how ST contributes to MCC tumorigenesis. Here we show that ST binds specifically to the MYC homolog MYCL (L-MYC) and recruits it to the 15-component EP400 histone acetyltransferase and chromatin remodeling complex. We performed a large-scale immunoprecipitation for ST and identified co-precipitating proteins by mass spectrometry. In addition to protein phosphatase 2A (PP2A) subunits, we identified MYCL and its heterodimeric partner MAX plus the EP400 complex. Immunoprecipitation for MAX and EP400 complex components confirmed their association with ST. We determined that the ST-MYCL-EP400 complex binds together to specific gene promoters and activates their expression by integrating chromatin immunoprecipitation with sequencing (ChIP-seq) and RNA-seq. MYCL and EP400 were required for maintenance of cell viability and cooperated with ST to promote gene expression in MCC cell lines. A genome-wide CRISPR-Cas9 screen confirmed the requirement for MYCL and EP400 in MCPyV-positive MCC cell lines. We demonstrate that ST can activate gene expression in a EP400 and MYCL dependent manner and this activity contributes to cellular transformation and generation of induced pluripotent stem cells.

Merkel Cell Polyomavirus Small T Antigen Promotes Pro-Glycolytic Metabolic Perturbations Required for Transformation.

Merkel cell polyomavirus (MCPyV) is an etiological agent of Merkel cell carcinoma (MCC), a highly aggressive skin cancer. The MCPyV small tumor antigen (ST) is required for maintenance of MCC and can transform normal cells. To gain insight into cellular perturbations induced by MCPyV ST, we performed transcriptome analysis of normal human fibroblasts with inducible expression of ST. MCPyV ST dynamically alters the cellular transcriptome with increased levels of glycolytic genes, including the monocarboxylate lactate transporter SLC16A1 (MCT1). Extracellular flux analysis revealed increased lactate export reflecting elevated aerobic glycolysis in ST expressing cells. Inhibition of MCT1 activity suppressed the growth of MCC cell lines and impaired MCPyV-dependent transformation of IMR90 cells. Both NF-κB and MYC have been shown to regulate MCT1 expression. While MYC was required for MCT1 induction, MCPyV-induced MCT1 levels decreased following knockdown of the NF-κB subunit RelA, supporting a synergistic activity between MCPyV and MYC in regulating MCT1 levels. Several MCC lines had high levels of MYCL and MYCN but not MYC. Increased levels of MYCL was more effective than MYC or MYCN in increasing extracellular acidification in MCC cells. Our results demonstrate the effects of MCPyV ST on the cellular transcriptome and reveal that transformation is dependent, at least in part, on elevated aerobic glycolysis.

Functional comparison of herpes simplex virus 1 (HSV-1) and HSV-2 ICP27 homologs reveals a role for ICP27 in virion release.

UNLABELLED: Numerous studies have focused on the regulatory functions of ICP27, an immediate-early (IE) protein of herpes simplex virus 1 (HSV-1). However, its homolog in HSV-2, termed ICP27t2, has been little studied. Here, we used two different approaches to functionally compare ICP27t2 and ICP27. In transfection-based assays, ICP27t2 closely resembled ICP27 in its capacity to enhance HSV-1 late gene expression, suppress the splicing of a viral intron, and complement the growth of an HSV-1 ICP27 null mutant. To study ICP27t2 in the context of viral infection, we engineered K2F1, an HSV-1 mutant that encodes ICP27t2 in place of ICP27. In Vero cells, K2F1 replicated with wild-type (WT) kinetics and yields, expressed delayed-early and late proteins normally, and was fully capable of activating several cellular signal transduction pathways that are ICP27 dependent. Thus, we conclude that ICP27t2 and ICP27 are functionally very similar and that ICP27t2 can mediate all ICP27 activities that are required for HSV-1 replication in cell culture. Surprisingly, however, we found that K2F1 forms plaques that are morphologically different from those of WT HSV-1. Investigation of this trait demonstrated that it results from the decreased release of progeny virions into the culture medium. This appears to be due to a reduction in the detachment of K2F1 progeny from the extracellular surface of the infected cell. We identified two HSV-1 ICP27 amino-terminal deletion mutants with a similar release defect. Together, these results demonstrate that ICP27 plays a heretofore-unappreciated role in modulating the efficiency of progeny virion release. IMPORTANCE: ICP27 is an essential, multifunctional regulatory protein that has a number of critical roles in the HSV-1 life cycle. Although ICP27 homologs are encoded by all known members of the Herpesviridae, previous work with several of these homologs has shown that they cannot substitute for ICP27 in the context of HSV-1-infected cells. Here, we identify ICP27t2 as the first homolog that can efficiently replace ICP27 in HSV-1 infection. Unexpectedly, our results also reveal that the sequence of the ICP27 gene can affect the release of HSV-1 progeny virions from the infected cell. Thus, our comparative study has revealed a novel function for ICP27 in the regulation of virus release.

Tumor suppressor p16INK4A is necessary for survival of cervical carcinoma cell lines.

The tumor suppressor p16(INK4A) inhibits formation of enzymatically active complexes of cyclin-dependent kinases 4 and 6 (CDK4/6) with D-type cyclins. Oncogenic stress induces p16(INK4A) expression, which in turn triggers cellular senescence through activation of the retinoblastoma tumor suppressor. Subversion of oncogene-induced senescence is a key step during cancer development, and many tumors have lost p16(INK4A) activity by mutation or epigenetic silencing. Human papillomavirus (HPV)-associated tumors express high levels of p16(INK4A) in response to E7 oncoprotein expression. Induction of p16(INK4A) expression is not a consequence of retinoblastoma tumor suppressor inactivation but is triggered by a cellular senescence response and is mediated by epigenetic derepression through the H3K27-specific demethylase (KDM)6B. HPV E7 expression causes an acute dependence on KDM6B expression for cell survival. The p16(INK4A) tumor suppressor is a critical KDM6B downstream transcriptional target and its expression is critical for cell survival. This oncogenic p16(INK4A) activity depends on inhibition of CDK4/CDK6, suggesting that in cervical cancer cells where retinoblastoma tumor suppressor is inactivated, CDK4/CDK6 activity needs to be inhibited in order for cells to survive. Finally, we note that HPV E7 expression creates a unique cellular vulnerability to small-molecule KDM6A/B inhibitors.

Role of immediate early protein ICP27 in the differential sensitivity of herpes simplex viruses 1 and 2 to leptomycin B.

Leptomycin B (LMB) is a highly specific inhibitor of CRM1, a cellular karyopherin-ß that transports nuclear export signal-containing proteins from the nucleus to the cytoplasm. Previous work has shown that LMB blocks herpes simplex virus 1 (HSV-1) replication in Vero cells and that certain mutations in viral immediate early protein ICP27 can confer LMB resistance. However, little is known of the molecular mechanisms involved. Here we report that HSV-2, a close relative of HSV-1, is naturally resistant to LMB. To see whether the ICP27 gene determines this phenotypic difference, we generated an HSV-1 mutant that expresses the HSV-2 ICP27 instead of the HSV-1 protein. This recombinant was fully sensitive to LMB, indicating that one or more other viral genes must be important in determining HSV-2's LMB-resistant phenotype. In additional work, we report several findings that shed light on how HSV-1 ICP27 mutations can confer LMB resistance. First, we show that LMB treatment of HSV-1-infected cells leads to suppression of late viral protein synthesis and a block to progeny virion release. Second, we identify a novel type of ICP27 mutation that can confer LMB resistance, that being the addition of a 100-residue amino-terminal affinity purification tag. Third, by studying infections where both LMB-sensitive and LMB-resistant forms of ICP27 are present, we show that HSV-1's sensitivity to LMB is dominant to its resistance. Together, our results suggest a model in which the N-terminal portion of ICP27 mediates a nonessential activity that interferes with HSV-1 replication when CRM1 is inactive. We suggest that LMB resistance mutations weaken or abrogate this activity.