Polycomb Repressive Complex 2 Loss Research Articles

EPCO-04. H3F3A K27M MUTATION DRIVES A REPRESSIVE TRANSCRIPTOME VIA REDUCED SERINE 31 PHOSPHORYLATION BY MODULATING CHROMATIN ACCESSIBILITY, INDEPENDENT OF H3K27ME3 IN DIFFUSE MIDLINE GLIOMA

Abstract Heterozygous H3.3 K27M mutations are central to the pathogenesis of diffuse midline gliomas (DMG), causing a global reduction in H3K27 trimethylation (H3K27me3) and subsequent epigenetic dysregulation. H3.3 contains a unique Serine 31 (S31) phosphorylation site crucial for mitotic checkpoint regulation. In DMG cells, reduced mitotic S31 phosphorylation correlates with chromosomal instability and aneuploidy. Using a mouse model, we demonstrate that both H3.3K27M and H3.3S31A mutations promote high-grade glioma development, whereas wild-type H3.3 does not. Importantly, S31A, which maintains normal K27me, suggests that S31 phosphorylation loss alone is oncogenic. The H3.3K27M mutation disrupts epigenetic regulation by inhibiting the Polycomb Repressive Complex 2 (PRC2), leading to decreased H3K27me3 and unintended gene induction. Concurrently, K27M exerts a compensatory repressive effect on the transcriptome, mediated by reduced S31 phosphorylation, independent of PRC2 activity. Using CRISPR-engineered isogenic DMG cells and analyzing the transcriptome and chromatin accessibility with RNA-seq and ATAC-seq, we found that the K27M mutation produces a balanced gene expression profile, while PRC2 loss primarily leads to gene induction. Notably, cells with S31A mutations also maintain this balanced profile, linking the PRC2-independent effect of K27M to reduced S31 phosphorylation. Conversely, cells with wild-type K27 but S31A mutation show a repressive genomic state, evidenced by an up/down gene ratio of 0.75, owing to functional PRC2. We further show that the genes uniquely dysregulated by K27M or S31A, independent of PRC2, overlap significantly, with dysregulated genes displaying altered chromatin accessibility. This suggests that the effect of K27M and S31A mutations on gene regulation is primarily due to changes in chromatin state. This complex interplay points to targeting K27M-associated pathways, particularly S31 phosphorylation, as a promising therapeutic avenue in DMG, a malignancy with fewer effective treatments and dismal prognosis.

Abstract 7548: Role of bromodomain extra terminal inhibitor (BETi) in combination with radiotherapy for the treatment of NF1-associated malignant peripheral nerve sheath tumor

Abstract Introduction: Neurofibromatosis Type 1 (NF-1) associated malignant peripheral nerve sheath tumors (MPNST) harbor mutations in the polycomb repressive complex 2 (PRC2) in 70% of tumors. PRC2 loss results in epigenetic changes, which increase histone acetylation and make the DNA vulnerable to binding by bromodomain proteins. This interaction could be targeted to uncouple histone acetylation from transcription using bromodomain extra terminal inhibitors (BETi). As radiation therapy (RT) is a cornerstone of MPNST treatment, we analyzed the effects of BETi with and without RT. Methods: Mice were implanted with patient-derived xenografts (PDX) of PRC2-wildtype model developed in the Torres laboratory, MPNST007. Mice were segregated into vehicle and I-BET 762 treatment groups and treated with either vehicle or drug for one week before the RT was initiated. Half of the mice in each treatment group (n = 10) received radiation (2 Gy per fraction for 5 days for a total of 10 Gy). The study endpoints were (1) tumor growth rates, (2) tumor size and weight at study termination, (3) immunohistochemical (IHC) staining for biomarkers of proliferation (Ki67), apoptosis (cleaved caspase 3, CC3), and DNA damage (γH2AX), and (4) RNA sequencing of PDXs from each experimental arm. Results: Significant tumor regression was observed in PRC2-mutant PDX tumors treated with I-BET 762 based on tumor growth kinetics over 45 days of treatment (p < 0.001). Tumor volume was significantly reduced by I-BET 762 compared to vehicle (p < 0.01). Combining I-BET 762 and RT resulted in significant tumor volume reduction (p <0.001). RNA sequencing and intersection of differentially expressed genes related to “DNA damage repair” and those repressed by I-BET 762 was performed for the MPNST007 PDXs. The expression of 259 genes related to DNA damage repair decreased after I-BET 762 treatment, including XRCC1, ERCC4, RAD50, RAD51D, and LIG4 (data not shown). IHC analysis prompted by the RNA sequencing results of the I-BET 762-treated PDXs revealed higher levels of γH2AX. An increase in necrotic and fibrotic tissue with increased CC3 in the BETi plus radiation group was observed. Conclusions: Genetic mutations in NF-1-associated MPNST tumors result in susceptibility to BETi treatment. In a PDX model of PRC2 wild-type tumors, BETi treatment resulted in tumor regression and sensitized MPNST xenografts to RT. Combining BETi and RT enhanced tumor death and increased expression of genes associated with DNA damage and apoptosis. Citation Format: Alcia A. Gingrich, Sharon Landers, Angela Bhalla, Cristian Rodriguez-Aguayo, Rossana Lazcano, Suresh Satpati, Alines Lebron-Torres, Emily Z. Keung, Christopher P. Scally, Christina L. Roland, Kelly K. Hunt, Heather Lyu, Heather Lillemoe, Ashleigh Guadagnolo, Alexander Lazar, John Slopis, Ian McCutcheon, John A. Livingston, Ahsan Farooqi, Kunal Rai, Keila E. Torres. Role of bromodomain extra terminal inhibitor (BETi) in combination with radiotherapy for the treatment of NF1-associated malignant peripheral nerve sheath tumor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7548.

Abstract 3563: SWI/SNF-associated DPF1 is a unique transcriptional regulator of malignant peripheral nerve sheath tumors

Abstract Background: Malignant peripheral nerve sheath tumors (MPNST) are rare soft tissue sarcomas that are typically therapy resistant and associated with a poor prognosis. Genetic aberrations of the polycomb repressive complex 2 (PRC2) occur in up to 75% of MPNST cases. PRC2 functions antagonistically to SWI/SNF protein complexes, regulating expression of target loci in a dynamic relationship that is critical to normal cellular development and maintenance of cell identity. The consequences of PRC2 loss on chromatin state maintained by SWI/SNF in MPNST is unexplored. Therefore, this study used functional genomics to elucidate the role of SWI/SNF in MPNST. Methods: To define the transcriptional regulatory role of SWI/SNF complexes in MPNST, SWI/SNF components were targeted via a CRISPR knock-out (KO) screen combined with a single cell RNA sequencing (RNAseq) readout. Genes of interest highlighted by this screen were further investigated using bulk CRISPR RNAseq. Phenotypic effects of the expression of these genes were studied using loss-of-function assays coupled with colony formation assays. SWI/SNF complex heterogeneity was characterized using glycerol gradient sedimentation, co-immunoprecipitation (co-IP), and mass spectrometry experiments. Results: The investigation of SWI/SNF transcriptional regulation in MPNST highlighted the Double PHD Finger family proteins (DPF1,2,3) as regulating distinct downstream targets. Notably, DPF1 had a unique transcriptional profile compared to other SWI/SNF components. Bulk CRISPR KO RNAseq confirmed these findings and highlighted a specific set of DPF1 target genes including many long non-coding RNAs. The phenotypic role of DPF1 in MPNST was investigated using in vitro siRNA and CRISPR experiments, where DPF1 KO reduced proliferation and viability of MPNST cells in both 2D and 3D cell culture assays. Further, DPF1 was found contribute to anchorage independent cell growth of MPNST cells using soft agar assays. Glycerol gradient sedimentation assays demonstrated that DPF1 co-migrated with core components of a SWI/SNF complex known as canonical BAF (cBAF) and not the alternate GBAF and PBAF SWI/SNF complexes. These findings were further validated using co-immunoprecipitation assays where core the SWI/SNF ATPase, SMARCA4, pulled down DPF1, while components unique to the GBAF and PBAF complexes did not. An ATPase directed proteolysis targeting chimers (PROTAC) was used to therapeutically target cBAF in MPNST cells, reducing their growth and viability. Combination treatment with standard of care chemotherapy synergistically increased the efficacy of this drug. Conclusions: DPF1 was identified as a unique transcriptional regulator of MPNST cells and acts as a member of the cBAF SWI/SNF complex to play important phenotypic roles in this cancer type. Citation Format: Bega Murray, Xiyuan Zhang, Shahroze Abbas, Haiyan Lei, Hilda Jafarah, Jack F. Shern. SWI/SNF-associated DPF1 is a unique transcriptional regulator of malignant peripheral nerve sheath tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3563.

Abstract A024: PRC2 loss drives MPNST metastasis and matrix remodeling

Abstract Epigenetic reprogramming alters gene transcription in response to different cellular cues and can occur at both the DNA level as well as histone post-translational modifications. Polycomb repressive complex 2 (PRC2) is a common epigenetic modifier that is responsible for adding methyl groups to the twenty-seventh lysine of histone 3. This modification is associated with decreased gene transcription and recruits complexes that further compact chromatin. PRC2 mutations have been implicated in the tumorigenesis of many cancer types; however, the role that PRC2 plays in tumorigenesis is complex and not well understood. PRC2 loss-of-function mutations have been identified in malignant peripheral nerve sheath tumors (MPNST), an aggressive sarcoma. PRC2 mutations are found in the EED and SUZ12 subunits in 70-92% of tumors. There are currently no effective therapies for MPNSTs resulting in a poor prognosis for individuals with inoperable tumors. Using a CRISPR/Cas9 system to delete EED or SUZ12, we developed in vitro and in vivo models to further study the biology of PRC2 loss in MPNSTs. Our data suggests that loss of PRC2 subunits increases the expression of extra-cellular matrix (ECM) remodeling genes, promotes cell migration in vitro, and increases pulmonary metastasis in vivo. We also observed that loss of either EED or SUZ12 increases collagen handling and a clustering metastatic phenotype. Additionally, loss of H3K27me3 was correlated with increased ECM and ECM remodeling genes as well as metastasis and overall survival in patient RNA-seq and tumor microarray data, respectively. Further characterization of the underlying mechanisms of PRC2-dependent metastasis will allow us to better understand the role of PRC2 in MPNST biology and test novel therapies for these patients. Citation Format: Qierra R. Brockman, Amanda Scherer, Gavin R. McGivney, Wade R. Gutierrez, Andrew Voigt, Alexandra Isaacson, Emily A. Laverty, Grace Roughton, Vickie Knepper-Adrian, Benjamin Darbro, Munir R. Tanas, Christopher Stipp, Rebecca D. Dodd. PRC2 loss drives MPNST metastasis and matrix remodeling. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr A024.

Single-cell sequencing reveals activation of core transcription factors in PRC2-deficient malignant peripheral nerve sheath tumor.

SUMMARYLoss-of-function mutations in the polycomb repressive complex 2 (PRC2) occur frequently in malignant peripheral nerve sheath tumor, an aggressive sarcoma that arises from NF1-deficient Schwann cells. To define the oncogenic mechanisms underlying PRC2 loss, we use engineered cells that dynamically reassemble a competent PRC2 coupled with single-cell sequencing from clinical samples. We discover a two-pronged oncogenic process: first, PRC2 loss leads to remodeling of the bivalent chromatin and enhancer landscape, causing the upregulation of developmentally regulated transcription factors that enforce a transcriptional circuit serving as the cell’s core vulnerability. Second, PRC2 loss reduces type I interferon signaling and antigen presentation as downstream consequences of hyperactivated Ras and its cross talk with STAT/IRF transcription factors. Mapping of the transcriptional program of these PRC2-deficient tumor cells onto a constructed developmental trajectory of normal Schwann cells reveals that changes induced by PRC2 loss enforce a cellular profile characteristic of a primitive mesenchymal neural crest stem cell.

Abstract 701: Loss of PRC2 enforces a mesenchymal neural crest stem cell phenotype in NF1-deficient cancer through activation of core transcription factors

Abstract Objective: Recurrent mutations in the polycomb repressive complex 2 (PRC2) occur in ~80% of malignant peripheral nerve sheath tumors (MPNST), which originate from NF1-deficient Schwann cells. In MPNST, PRC2 loss results in a diverse set of transcriptomic and phenotypic consequences, including gain of acetylated H3K27 (H3K27ac) accompanying the loss of trimethylated H3K27 (H3K27me3), hyperactivated Ras signaling, and tumor escape from immune surveillance. Mechanistic understanding of how PRC2 loss results in these diverse consequences remains unresolved. Methods: To delineate the oncogenic mechanisms mediated by PRC2 loss, we engineered MPNST cell lines to dynamically reassemble a functional PRC2 and evaluated the transcriptomic and epigenetic consequences of PRC2 restoration. We further extended these finding using single cell RNA sequencing (scRNAseq) from human MPNST. Results: Through integrative analysis of RNAseq and H3K27me3 ChIP-seq in PRC2-deficient MPNST cells, we identified 6134 H3K27me3 peaks gained when a functional PRC2 was restored. Of the 876 significantly altered genes, 699 genes were downregulated, and 177 genes were upregulated when a restored PRC2 caused genome-wide redistribution of H3K27me3 peaks. Detailed mechanistic dissection of these PRC2-regulated genes revealed a two-pronged oncogenic process mediated by PRC2 loss and they cooperatively contribute to the tumorigenesis of MPNST. First, PRC2 loss leads to the upregulation of a transcriptional circuit that remodels the enhancer landscape of MPNST cells and establishes an enhancer-driven transcription factor, FOXC1, as a master regulator and thus a core vulnerability of the cell. Second, PRC2 loss reduces type I interferon (IFN) signaling and antigen presentation as a downstream consequence of the upregulated Ras signaling. These data were further integrated with scRNAseq of human MPNST. Importantly, we discovered that the PRC2-deficient tumor cells have a corrupted transcriptional program characteristic of a mesenchymal precursor cell found during the trajectory of normal Schwann cell development. Interestingly, malignant cells expressing high levels of IFN and antigen presentation genes were missing in the metastatic MPNST, potentially allowing the tumor cell to escape the immune surveillance. The transcriptional circuit established by PRC2-regulated FOXC1 and its downstream targets sustains the malignant program in both primary and metastatic MPNST. Conclusion: In summary, we discovered the activation of a lineage specific oncogenic transcription program in PRC2-deficient MPNST, which is characteristic of a neural crest-derived mesenchymal stem cell. Our findings provide mechanistic understanding of the inherent metastatic potential, chemotherapy and radiotherapy resistance, and immune escape that are clinically characteristic of these tumors. Citation Format: Xiyuan Zhang, Hannah E. Lou, Vishaka Gopalan, Zhihui Liu, Hilda Jafarah, Haiyan Lei, Paige Jones, Carly M. Sayers, Marielle E. Yohe, Prashant Chittiboina, Brigitte C. Widemann, Carol J. Thiele, Michael C. Kelly, Sridhar Hannenhalli, Jack F. Shern. Loss of PRC2 enforces a mesenchymal neural crest stem cell phenotype in NF1-deficient cancer through activation of core transcription factors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 701.

Abstract LB060: Single-cell CRISPR screen of malignant peripheral nerve sheath tumors (MPNST)

Abstract Background: Malignant peripheral nerve sheath tumors (MPNST) are aggressive sarcomas of the peripheral nervous system. Loss of function of the polycomb repressive complex 2 (PRC2) have been identified in up to 80% of MPNST cases, via recurrent genetic mutations in the core components, EED and/or SUZ12. However, the transcriptional mis-regulation incurred by the loss of transcriptional repression in MPNST, through loss of H3K27me3, has yet to be fully deciphered. We hypothesize that PRC2 loss leads to transcriptional regulatory imbalance between key epigenetic regulating complexes in this malignancy, which may contribute to the oncogenesis of this disease. Therefore, this project aims to elucidate the role of the SWI/SNF chromatin remodeling complexes in MPNST, epigenetic regulators known to act antagonistically to PRC2. Method: This project uses a CRISPR knock out screen combined with a single cell RNA sequencing readout to target 44 known components of the SWI/SNF complexes. This technology enables large scale phenotypic assays, which we leverage to decipher the complicated epigenetic regulatory network within these lesions. Resulting genes of interest were further studied via loss-of-function assays coupled with colony formation assay and chromatin immunoprecipitation DNA sequencing (ChIP-seq) experiments. Results: This CRISPR screen highlighted core SWI/SNF components, SMARCA4 and SMARCE1, as genes of interest in MPNST. Knockdown of these genes in MPNST cell lines reduced proliferation, indicating a role for these genes in MPNST growth and viability. Further, investigations via soft agar assay found both SMARCA4 and SMARCE1 knock out to decrease the tumorigenic potential of MPNST cells. The potential dependency of SMARCA4 function in MPNST cells on PRC2 mutation was investigated using a doxycycline-inducible PRC2-restoration MPNST cell line model. ChIP-seq results suggest that loss of PRC2 does not substantially affect the binding of SMARCA4-containing SWI/SNF complexes across the genome. DPF1, a member of the neuronal-specific SWI/SNF chromatin remodeling complex, was further highlighted as a unique regulator of MPNST transcription via the CRISPR single-cell screen. CRISPR knockdown of this gene in MPNST cell lines has been found to decrease proliferation, while soft agar assay experiments highlighted that DPF1 loss reduces cell tumorigenicity. The unique dependency of this gene observed in MPNST but not in any other cancer subtypes highlights the potential of DPF1 to be a novel and unique therapeutic target in MPNST. Conclusion: Thus far, our studies have shown that members of the SWI/SNF complex play key roles in the MPNST cell proliferation and tumorigenicity. The function of these complexes in MPNST, however, may be independent of the transcriptional imbalance caused by PRC2 mutation. Citation Format: Béga Murray, Xiyuan Zhang, Shahroze Abbas, Hilda Jafarah, Jack F. Shern. Single-cell CRISPR screen of malignant peripheral nerve sheath tumors (MPNST) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB060.

CRLF2 overexpression defines an immature-like subgroup which is rescued through restoration of the PRC2 function in T-cell precursor acute lymphoblastic leukemia.

CRLF2 overexpression has been described as a biomarker of poor prognosis in T-cell acute lymphoblastic leukemia (T-ALL). In the present study, we aimed to unravel the genomic profile underlying CRLF2 overexpression (CRLF2-high) by analyzing RNA-seq, WES, and SNP-array data from 264 T-ALL patients and five cell lines deposited on the TARGET initiative, Cancer Cell Line Encyclopedia and Gene Expression Omnibus. These data allowed us to delineate the genomic landscape of CRLF2-high in T-ALL, which was associated with PTEN, JAK3, PHF6, EZH2, and RUNX1 mutations. We also observed an enrichment of CRLF2-high in early T-precursor (ETP)-ALL (23.08% vs. 4.02%, P=7.579e-06 ) and a very similar gene upregulation profile between these two entities. The inhibition of BET (iBET) proteins is a strategy previously demonstrated to reverse the gene upregulation pattern of ETP cells through restoration of polycomb repressive complex 2 (PRC2) activity. While CRLF2 expression was rescued by using this strategy in LOUCY (untreated vs. iBET P =0.0095, DMSO vs. iBET P=0.0286), a classical ETP-ALL cell line, PRC2 loss was not sufficient to promote CRLF2 upregulation in JURKAT, a more mature T-ALL cell line. Considering the role of IKZF1 in CRLF2 regulation and in recruitment of PCR2, we evaluated IKZF1 status according to CRLF2-expression subgroups. We identified that IKZF1 transcripts with intron retention were upregulated in the CRLF2-high subgroup. Here, we delineated the gene expression profile of CRLF2-high T-ALL samples and unraveled the crucial role of PRC2 in CRLF2 regulation in ETP-ALL.

Loss of Polycomb Repressive Complex 2 Function Alters Digestive Organ Homeostasis and Neuronal Differentiation in Zebrafish.

Polycomb repressive complex 2 (PRC2) mediates histone H3K27me3 methylation and the stable transcriptional repression of a number of gene expression programs involved in the control of cellular identity during development and differentiation. Here, we report on the generation and on the characterization of a zebrafish line harboring a null allele of eed, a gene coding for an essential component of the PRC2. Homozygous eed-deficient mutants present a normal body plan development but display strong defects at the level of the digestive organs, such as reduced size of the pancreas, hepatic steatosis, and a loss of the intestinal structures, to die finally at around 10–12 days post fertilization. In addition, we found that PRC2 loss of function impairs neuronal differentiation in very specific and discrete areas of the brain and increases larval activity in locomotor assays. Our work highlights that zebrafish is a suited model to study human pathologies associated with PRC2 loss of function and H3K27me3 decrease.

Genomic Analysis and Biochemical Investigation Reveal Mechanisms of Schwann Cell Transformation and Treatment Resistance in Malignant Peripheral Nerve Sheath Tumors (MPNSTs)

<h3>Purpose/Objective(s)</h3> In contrast to benign Schwann cell derived tumors such as neurofibromas and schwannomas, malignant peripheral nerve sheath tumors (MPNSTs) metastasize, and have poor outcomes. MPNSTs are associated with loss of <b>NF1</b>, a tumor suppressor that inhibits Ras signaling, and loss of Polycomb Repressive Complex 2 (PRC2) which resulting in epigenetic dysregulation. Translationally, MPNSTs are resistant to Ras/MEK inhibition with selumetinib and radiation therapy yet the mechanisms underlying how PRC2 loss cooperates with Ras misactivation to drive transformation and treatment resistance remains unclear. Here, we performed DNA methylation profiling and RNA-sequencing of Schwann cell tumors and cell lines as well as investigated <b>in vitro</b> responses to radiotherapy and selumetinib. <h3>Materials/Methods</h3> We identified 119 Schwann cell tumor resection specimens with sufficient tissue for DNA methylation profiling (66 schwannoma, 13 neurofibroma, 40 MPNSTs) and RNA-sequencing (23 schwannoma, 9 neurofibroma, 9 MPNSTs). To study cell signaling and treatment responses, we utilized primary human cell lines generated from <b>NF1</b> intact peripheral nerve, <b>NF1</b> mutant neurofibromas, and MPNSTs (n = 6). We assessed Ras signal transduction by immunoblot/immunoprecipitation for downstream effectors and performed cell viability assays following selumetinib or radiotherapy. <h3>Results</h3> Principal component analysis (PCA) of DNA methylation and RNA-sequencing data demonstrated a gradient of epigenetic signatures across Schwann cell tumors, with schwannomas comprising a distinct cluster from neurofibromas and MPNST. Moreover, differential DNA methylation and RNA-sequencing analyses both revealed enrichment for PRC2 target genes and repression of Schwann cell differentiation genes in MPNSTs. RNA-sequencing of MPNST versus neurofibroma cell lines demonstrated a PRC2 mediated signature enriched for cell differentiation genes and negative feedback regulators of Ras signaling, suggesting PRC2 dysregulation mediates de-differentiation and increased Ras pathway output in MPNSTs. In contrast to neurofibroma cells, MPNST cells were deficient in PRC2 and <b>NF1</b> while exhibiting increased basal active Ras-GTP levels insensitive to serum starvation. PRC2 deficient MPNST cell lines were more resistant to selumetinib and radiation therapy when compared to <b>NF1</b> deficient neurofibroma cells. <h3>Conclusion</h3> PRC2 loss causes epigenetic dysregulation that may mediate Schwann cell de-differentiation programs and altered Ras signaling in MPNSTs compared to neurofibromas. Translationally, these epigenetic changes may contribute to increased resistance to targeted therapy and radiation. The mechanisms underlying transformation and treatment resistance upon PRC2 loss are under further investigation.

EMBR-15. PRC2 COMPLEX ENFORCES NEURONAL LINEAGE AND SUPPRESSES TUMOR GROWTH IN SHH MEDULLOBLASTOMA

Hyperactivation of Sonic Hedgehog (SHH) signaling pathway drives tumor progression in the largest medulloblastoma subgroup. During cerebellar development, promoters of SHH target genes show inhibitory trimethylation of histone H3 at lysine 27 (H3K27me3), mediated by the Polycomb Repressive Complex 2 (PRC2). Here, we explored the regulation of cerebellar growth and medulloblastoma tumorigenesis by PRC2 complex components EED and EZH2. For developmental studies, we conditionally deleted Eed or Ezh2 in the Atoh1 lineage that gives rise to the cerebellar granule neuron progenitors (CGNP) that are cells of origin for SHH medulloblastomas. For tumor studies, we bred the conditional Eed- or Ezh2-deleted mouse lines with mice genetically engineered to develop SHH medulloblastoma. Our developmental studies showed that Eed was absolutely required for cerebellar growth. Eed-deleted CGNPs underwent aberrant, myocyte-like differentiation and spontaneous apoptosis, resulting in cerebellar hypoplasia. In contrast, Ezh2 deletion produced no developmental phenotype, despite blocking all H3K27me3 in CGNPs. Our tumor studies showed that Eed-deleted medulloblastomas similarly showed aberrant, myocyte differentiation, but unlike CGNPs, did not undergo widespread apoptosis. Eed-deleted medulloblastomas progressed more rapidly than control tumors, indicating that the inappropriate, muscle-like differentiation did not slow tumor growth. Ezh2-deleted medulloblastomas similarly progressed more rapidly than controls. Our data show that the PRC2 complex acts to enforce neuronal lineage commitment in both development and tumorigenesis and to restrain tumor growth in SHH medulloblastoma. Myocyte differentiation in Eed-deleted tumors suggests that PRC2 loss of function may contribute to the medullomyoblastomas that have been observed in patients. The differences in developmental phenotype show that EZH2 and EED functions are non-identical and can be dissociated, while similar increase in tumor progression show tumor suppressive functions for both EED and EZH2.

Loss of PRC2 Enforces a Mesenchymal Neural Crest Stem Cell Phenotype in NF1-Deficient Cancer Through Activation of Core Transcription Factors

Genetic alterations that result in the dysregulation of polycomb repressive complex 2 (PRC2) are common features of multiple cancer types. Loss-of-function mutations in the PRC2 genes, SUZ12 or EED, occur at high frequency in malignant peripheral nerve sheath tumors (MPNST), a clinically aggressive sarcoma that arises from NF1-deficient Schwann cells. To define the oncogenic mechanisms mediated by PRC2 loss, we used engineered MPNST model systems that dynamically reassemble a competent PRC2 coupled with single cell sequencing from clinical samples to evaluate the transcriptomic and epigenetic consequences of PRC2 loss. We discovered a two-pronged oncogenic process; first, PRC2 loss leads to remodeling of bivalent chromatin and the enhancer landscape of the tumor cell. These epigenetic changes result in the upregulation of developmentally regulated transcription factors, including FOXC1, which enforce a transcriptional circuit that is a core vulnerability of the cell. Second, PRC2 loss reduces type I interferon signaling and antigen presentation as a downstream consequence of hyperactivated Ras signaling and its crosstalk with STAT and IRF transcription factors. Mapping of the transcriptional program of these PRC2-deficient tumor cells onto a constructed developmental trajectory of the developing neural crest stem cell revealed that changes induced by PRC2 loss enforced a cellular profile characteristic of a primitive mesenchymal neural crest stem cell.

Abstract 3898: Loss of polycomb repressive complex 2 activates HOXB8 and remodels super enhancers in malignant peripheral nerve sheath tumors

Abstract Background: Malignant peripheral nerve sheath tumors (MPNST) are aggressive and highly metastatic soft tissue sarcomas that lack effective treatment. Loss of the functional polycomb repressive complex 2 (PRC2) through genetic alterations in its core components EED and/or SUZ12 is prevalent in this devastating disease. Consequently, the product of PRC2, trimethylation of histone H3 lysine 27 (H3K27me3), a transcriptional repression marker, was found absent in MPNST samples. However, the epigenetic consequences of PRC2 loss remain unclear. Methods: Using an inducible expression vector, we restored a functional PRC2 in three MPNST cell lines that are PRC2 deficient (PRC2-null). We then profiled the accompanied changes in histone modifications by chromatin immunoprecipitation with massively parallel DNA sequencing (ChIP-seq) and transcriptional changes by RNA sequencing (RNAseq). To test the clinical relevance of our findings, we performed single-cell RNAseq (scRNAseq) on one PRC2-null patient MPNST sample. Finally, to target the epigenetic abnormity caused by PRC2 loss, we tested the efficacy of a histone deacetylase inhibitor (HDACi) in treating MPNST cells. Results: We confirmed the tumor suppressor role of a functional PRC2 in MPNSTs by showing that PRC2 restoration caused growth inhibition in PRC2-null MPNST cells but not in those with a wildtype PRC2. Using ChIP-seq, we identified 6135 H3K27me3 peaks, which are associated with 2285 genes, that were commonly gained in all three PRC2-null MPNST cell lines with PRC2 restoration. Results of RNAseq show that more than 50% of these genes were kept silenced even without H3K27me3 and about 10% of the highly expressed genes were re-repressed by PRC2 (fold change &amp;lt; 0.7). Interestingly, among the 145 genes that are transcriptionally regulated by polycomb, 91 were found to be co-occupied by H3K4me3 at their promoters. Since co-existence of H3K27me3 and H3K4me3 at the promoter define a bivalent gene, we further investigated the epigenetic changes associated with PRC2 restoration there. Analysis of ChIP-seq results revealed that, accompanied with gains of H3K27me3, there was loss of acetylated H3K27, a marker of super enhancer (SE), at the bivalent sites. Particularly, we identified a group of oncogenic transcription factors (TFs), including homeobox B8 (HOXB8), that were activated due to the loss of PRC2 and their activation was driven by SEs. Furthermore, the analysis of scRNAseq of our patient MPNST sample confirmed that these essential SE-driven TFs can be used to separate malignant cells from the tumor microenvironment. Finally, in vitro studies show that HDACi was highly effective in treating MPNST cells by de-activating these SE-driven TFs. Conclusion: Herein, we report a novel mechanism underlying the polycomb-regulated transcriptional repression. In MPNSTs, the loss of PRC2 caused activation of a group of oncogenic transcription factors through remodeling the SE landscape, which resulted their vulnerabilities to epigenetic treatments by HDACi. Citation Format: Xiyuan Zhang, Hannah E. Lou, Haiyan Lei, Zhihui Liu, Marielle Yohe, Carol Thiele, Brigitte Widemann, Jack F. Shern. Loss of polycomb repressive complex 2 activates HOXB8 and remodels super enhancers in malignant peripheral nerve sheath tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3898.

The Role of Polycomb Repressive Complex in Malignant Peripheral Nerve Sheath Tumor.

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive soft tissue sarcomas that can arise most frequently in patients with neurofibromatosis type 1 (NF1). Despite an increasing understanding of the molecular mechanisms that underlie these tumors, there remains limited therapeutic options for this aggressive disease. One potentially critical finding is that a significant proportion of MPNSTs exhibit recurrent mutations in the genes EED or SUZ12, which are key components of the polycomb repressive complex 2 (PRC2). Tumors harboring these genetic lesions lose the marker of transcriptional repression, trimethylation of lysine residue 27 on histone H3 (H3K27me3) and have dysregulated oncogenic signaling. Given the recurrence of PRC2 alterations, intensive research efforts are now underway with a focus on detailing the epigenetic and transcriptomic consequences of PRC2 loss as well as development of novel therapeutic strategies for targeting these lesions. In this review article, we will summarize the recent findings of PRC2 in MPNST tumorigenesis, including highlighting the functions of PRC2 in normal Schwann cell development and nerve injury repair, as well as provide commentary on the potential therapeutic vulnerabilities of a PRC2 deficient tumor cell.

Histone H3K27 dimethyl loss is highly specific for malignant peripheral nerve sheath tumor and distinguishes true PRC2 loss from isolated H3K27 trimethyl loss

Malignant peripheral nerve sheath tumors contain loss of histone H3K27 trimethylation (H3K27me3) due to driver mutations affecting the polycomb repressive complex 2 (PRC2). Consequently, loss of H3K27me3 staining has served as a diagnostic marker for this tumor type. However, recent reports demonstrate H3K27me3 loss in numerous other tumors, including some in the differential diagnosis of malignant peripheral nerve sheath tumor. Since these tumors lose H3K27me3 through mechanisms distinct from PRC2 loss, we set out to determine whether loss of dimethylation of H3K27, which is also catalyzed by PRC2, might be a more specific marker of PRC2 loss and malignant peripheral nerve sheath tumor. Using mass spectrometry, we identify a near complete loss of H3K27me2 in malignant peripheral nerve sheath tumors and cell lines. Immunohistochemical analysis of 72 malignant peripheral nerve sheath tumors, seven K27M-mutant gliomas, 43 ependymomas, and 10 Merkel cell carcinomas demonstrates that while H3K27me3 loss is common across these tumor types, H3K27me2 loss is limited to malignant peripheral nerve sheath tumors and is highly concordant with H3K27me3 loss (33/34 cases). Thus, increased specificity does not come at the cost of greatly reduced sensitivity. To further compare H3K27me2 and H3K27me3 immunohistochemistry, we investigated 42 melanomas and 54 synovial sarcomas, histologic mimics of malignant peripheral nerve sheath tumor with varying degrees of H3K27me3 loss in prior reports. While global H3K27me3 loss was not seen in these tumors, weak and limited H3K27me3 staining was common. By contrast, H3K27me2 staining was more clearly retained in all cases, making it a superior binary classifier. This was confirmed by digital image analysis of stained slides. Our findings indicate that H3K27me2 loss is highly specific for PRC2 loss and that PRC2 loss is a rarer phenomenon than H3K27me3 loss. Consequently, H3K27me2 loss is a superior diagnostic marker for malignant peripheral nerve sheath tumor.

Epigenomic Reordering Induced by Polycomb Loss Drives Oncogenesis but Leads to Therapeutic Vulnerabilities in Malignant Peripheral Nerve Sheath Tumors.

Malignant peripheral nerve sheath tumor (MPNST) is an aggressive sarcoma with recurrent loss-of-function alterations in polycomb-repressive complex 2 (PRC2), a histone-modifying complex involved in transcriptional silencing. To understand the role of PRC2 loss in pathogenesis and identify therapeutic targets, we conducted parallel global epigenomic and proteomic analysis of archival formalin-fixed, paraffin-embedded (FFPE) human MPNST with and without PRC2 loss (MPNSTLOSS vs. MPNSTRET). Loss of PRC2 resulted in increased histone posttranslational modifications (PTM) associated with active transcription, most notably H3K27Ac and H3K36me2, whereas repressive H3K27 di- and trimethylation (H3K27me2/3) marks were globally lost without a compensatory gain in other repressive PTMs. Instead, DNA methylation globally increased in MPNSTLOSS. Epigenomic changes were associated with upregulation of proteins in growth pathways and reduction in IFN signaling and antigen presentation, suggesting a role for epigenomic changes in tumor progression and immune evasion, respectively. These changes also resulted in therapeutic vulnerabilities. Knockdown of NSD2, the methyltransferase responsible for H3K36me2, restored MHC expression and induced interferon pathway expression in a manner similar to PRC2 restoration. MPNSTLOSS were also highly sensitive to DNA methyltransferase and histone deacetylase (HDAC) inhibitors. Overall, these data suggest that global loss of PRC2-mediated repression renders MPNST differentially dependent on DNA methylation to maintain transcriptional integrity and makes them susceptible to therapeutics that promote aberrant transcription initiation. SIGNIFICANCE: Global profiling of histone PTMs and protein expression in archival human MPNST illustrates how PRC2 loss promotes oncogenesis but renders tumors vulnerable to pharmacologic modulation of transcription.See related commentary by Natarajan and Venneti, p. 3172.

PRC2 Inactivation Induces Resistance to Chemotherapy-Induced Apoptosis By Upregulating the TRAP1 Mitochondrial Chaperone in T-ALL

The tendency of mitochondria to undergo or resist BCL2-controlled apoptosis (so-called mitochondrial priming) is a powerful predictor of response to cytotoxic chemotherapy. Fully exploiting this finding will require unraveling the molecular genetics underlying phenotypic variability in mitochondrial priming.We analyzed pre-treatment T-ALL clinical specimens from a cohort of 47 patients (enriched for treatment failure, but with sufficient controls) treated on the COG AALL0434 or DFCI 05001 clinical trials using BH3 profiling analysis to assess mitochondrial apoptotic priming. We found that there was a strong association between resistance to mitochondrial apoptosis and a poor response to induction chemotherapy (P = 0.008). Furthermore, mitochondrial apoptosis resistance predicted significantly inferior event-free survival (65% vs. 91% at 5 years; P = 0.0376).To define the molecular determinants of this mitochondrial apoptosis resistance, we performed targeted exon sequencing and array CGH copy number analysis. This revealed that loss-of-function mutations in the polycomb repressive complex 2 (PRC2) core subunits (EZH2, EED or SUZ12) were associated with mitochondrial apoptosis resistance (P = 0.007) in clinical specimens. PRC2 is a chromatin modifying complex best known for its role in transcriptional repression, which functions as a tumor suppressor in T-ALL, but whether PRC2 regulates mitochondrial apoptosis is unknown.Using shRNA knockdown in human T-ALL cells, we found that depletion of PRC2 subunits in T-ALL cells induced mitochondrial apoptosis resistance, as assessed by BH3 profiling analysis (P < 0.001). PRC2 inactivation also induced resistance to chemotherapy-induced apoptosis (P < 0.0001), and increased T-ALL fitness following treatment with the antileukemic drug vincristine (P = 0.0001). Apoptosis resistance upon inactivation of EZH2 (a PRC2 catalytic subunit) was reversed by transduction of wild-type EZH2, but not by an EZH2 point mutant with impaired methyltransferase activity, indicating that this effect is mediated by the enzymatic activity of PRC2. In normal mouse thymocytes, heterozygous deletion of the PRC2 subunits Ezh2 or Eed was sufficient to induce apoptosis resistance in non-transformed double-negative T-cell progenitors (P < 0.010), indicating that apoptosis resistance can arise prior to oncogenic transformation.The best-known regulators of mitochondrial apoptosis are BCL2-family genes, but RNA-seq analysis of shRNA knockdown of the PRC2 subunits in a T-ALL cell line revealed that PRC2 did not regulate expression of any of the known BCL2 family members. Instead, PRC2 loss led to upregulation of TRAP1, a mitochondrially localized chaperone of the HSP90 family. TRAP1 upregulation was necessary for induction of apoptosis resistance following PRC2 inactivation, because shRNA knockdown of TRAP1 in the human CCRF-CEM cell line completely blocked induction of apoptosis resistance following PRC2 inactivation (P < 0.0001). Moreover, pharmacologic TRAP1 inhibition synergized with the antileukemic drugs dexamethasone and doxorubicin (combination index = 0.37 and 0.42, respectively).To define how PRC2 regulates TRAP1, we performed ChIP-seq analysis, which revealed that TRAP1 regulation by PRC2 is indirect. Combined ChIP-seq and RNA-seq analysis revealed a number of direct targets of PRC2, all of which were tested for their ability to upregulate TRAP1 and induce apoptosis resistance. This showed that the LIM domain transcription factor CRIP2 is a direct target of PRC2 that is necessary and sufficient for regulation of TRAP1, and for induction of apoptosis resistance downstream of PRC2 inactivation. To confirm the relevance of our findings, we used the EZH2 inhibitor GSK126 to inhibit enzymatic activity of PRC2, which revealed that EZH2 normally represses CRIP2 and TRAP1 expression in primary patient-derived xenografts. Finally, we found that increased TRAP1 expression correlates with treatment failure in T-ALL clinical specimens (P = 0.028).Taken together, our findings support a model in which loss of PRC2 induces transcriptional upregulation of its direct target CRIP2, which subsequently activates expression of TRAP1, leading to resistance to chemotherapy-induced mitochondrial apoptosis. DisclosuresAries:Pfizer: Employment. Teachey:Amgen: Consultancy; La Roche: Consultancy. Letai:AstraZeneca: Consultancy, Other: Lab research report; AbbVie: Consultancy, Other: Lab research report; Flash Therapeutics: Equity Ownership; Novartis: Consultancy, Other: Lab research report; Vivid Biosciences: Equity Ownership.

PRC2 loss induces chemoresistance by repressing apoptosis in T cell acute lymphoblastic leukemia.

The tendency of mitochondria to undergo or resist BCL2-controlled apoptosis (so-called mitochondrial priming) is a powerful predictor of response to cytotoxic chemotherapy. Fully exploiting this finding will require unraveling the molecular genetics underlying phenotypic variability in mitochondrial priming. Here, we report that mitochondrial apoptosis resistance in T cell acute lymphoblastic leukemia (T-ALL) is mediated by inactivation of polycomb repressive complex 2 (PRC2). In T-ALL clinical specimens, loss-of-function mutations of PRC2 core components (EZH2, EED, or SUZ12) were associated with mitochondrial apoptosis resistance. In T-ALL cells, PRC2 depletion induced resistance to apoptosis induction by multiple chemotherapeutics with distinct mechanisms of action. PRC2 loss induced apoptosis resistance via transcriptional up-regulation of the LIM domain transcription factor CRIP2 and downstream up-regulation of the mitochondrial chaperone TRAP1 These findings demonstrate the importance of mitochondrial apoptotic priming as a prognostic factor in T-ALL and implicate mitochondrial chaperone function as a molecular determinant of chemotherapy response.

A Clinicopathologic Study of Head and Neck Malignant Peripheral Nerve Sheath Tumors.

Head and neck high grade malignant peripheral nerve sheath tumors (HN-MPNSTs) are rare highly aggressive soft tissue sarcomas that show overlapping morphologic and immunophenotypic features with melanoma and other high grade sarcomas, resulting in diagnostic challenges, particularly in sporadic settings. Recent discoveries have implicated loss of function mutations in the polycomb repressive complex 2 (PRC2) components, including EED or SUZ12 genes, as one of the leading pathogenetic mechanisms in high grade MPNST. MPNSTs with PRC2 loss are associated with complete loss of trimethylation at lysine 27 of histone H3 (H3K27me3), which emerged as a reliable immunohistochemical marker in the diagnosis of sporadic and radiation induced MPNST. As the diagnosis of MPNST in the HN is particularly challenging to distinguish from melanoma and other sarcoma types, we carried out a clinicopathologic analysis on HN-MPNST patients managed at our institution over a 20-year period (1997-2016), using the latest diagnostic criteria including H3K27me3 staining and other molecular investigations. The overall survival of HN-MPNST was compared with other HN soft tissue sarcomas. The diagnosis of HN-MPNST was confirmed in 13 patients (seven males and six females), with a mean age of 31 years; with 3 (23%) patients being of pediatric age. The most common site was the neck soft tissue (77%). Two-thirds of patients (n = 9) had stigmata of NF1, three had prior radiotherapy and only one developed a de novo MPNST. All except one tumor (86%) tested showed loss of H3K27me3 expression, including all non-NF1 patients. The 2 and 5-year DSS rates were 50 and 30%. The 2-year DFS rate was 21%. Adverse predictors on DSS included adult age (p = 0.011), prior-history of RT (p = 0.003) and recurrence (p = 0.003). Compared to other molecularly confirmed subsets of HN sarcomas (Ewing and Ewing-like sarcoma, rhabdomyosarcoma and synovial sarcoma), HN-MPNST had the worst overall survival (p < 0.0001). We conclude that HN-MPNSTs are highly aggressive sarcomas associated with an unfavorable outcome and the utility of H3K27me3 IHC stains in the evaluation of MPNST is a reliable ancillary diagnostic adjunct.

Reduced H3K27me3 expression in Merkel cell polyoma virus-positive tumors

Merkel cell carcinoma is a primary cutaneous neuroendocrine carcinoma, which once metastatic is difficult to treat. Recent mutation analyses of Merkel cell carcinoma revealed a low number of mutations in Merkel cell polyomavirus-associated tumors, and a high number of mutations in virus-negative combined squamous cell and neuroendocrine carcinomas of chronically sun-damaged skin. We speculated that the paucity of mutations in virus-positive Merkel cell carcinoma may reflect a pathomechanism that depends on derangements of chromatin without alterations in the DNA sequence (epigenetic dysregulation). One central epigenetic regulator is the Polycomb repressive complex 2 (PRC2), which silences genomic regions by trimethylating (me3) lysine (K) 27 of histone H3, and thereby establishes the histone mark H3K27me3. Recent experimental research data demonstrated that PRC2 loss in mice skin results in the formation of Merkel cells. Prompted by these findings, we explored a possible contribution of PRC2 loss in human Merkel cell carcinoma. We examined the immunohistochemical expression of H3K27me3 in 35 Merkel cell carcinomas with pure histological features (22 primary and 13 metastatic lesions) and in 5 combined squamous and neuroendocrine carcinomas of the skin. We found a strong reduction of H3K27me3 staining in tumors with pure histologic features and virus-positive Merkel cell carcinomas. Combined neuroendocrine carcinomas had no or only minimal loss of H3K27me3 labeling. Our findings suggest that a PRC2-mediated epigenetic deregulation may play a role in the pathogenesis of virus-positive Merkel cell carcinomas and in tumors with pure histologic features.