ZFTA-RELA Dictates Oncogenic Transcriptional Programs to Drive Aggressive Supratentorial Ependymoma

More than 60% of supratentorial ependymomas harbor a ZFTA-RELA (ZRfus) gene fusion (formerly C11orf95-RELA). To study the biology of ZRfus, we developed an autochthonous mouse tumor model using in utero electroporation (IUE) of the embryonic mouse brain. Integrative epigenomic and transcriptomic mapping was performed on IUE-driven ZRfus tumors by CUT&RUN, chromatin immunoprecipitation sequencing, assay for transposase-accessible chromatin sequencing, and RNA sequencing and compared with human ZRfus-driven ependymoma. In addition to direct canonical NFκB pathway activation, ZRfus dictates a neoplastic transcriptional program and binds to thousands of unique sites across the genome that are enriched with PLAGL family transcription factor (TF) motifs. ZRfus activates gene expression programs through recruitment of transcriptional coactivators (Brd4, Ep300, Cbp, Pol2) that are amenable to pharmacologic inhibition. Downstream ZRfus target genes converge on developmental programs marked by PLAGL TF proteins, and activate neoplastic programs enriched in Mapk, focal adhesion, and gene imprinting networks. SIGNIFICANCE: Ependymomas are aggressive brain tumors. Although drivers of supratentorial ependymoma (ZFTA- and YAP1-associated gene fusions) have been discovered, their functions remain unclear. Our study investigates the biology of ZFTA-RELA-driven ependymoma, specifically mechanisms of transcriptional deregulation and direct downstream gene networks that may be leveraged for potential therapeutic testing.This article is highlighted in the In This Issue feature, p. 2113.

Cross-Species Genomics Reveals Oncogenic Dependencies in ZFTA/C11orf95 Fusion-Positive Supratentorial Ependymomas

Molecular groups of supratentorial ependymomas comprise tumors with ZFTA-RELA or YAP1-involving fusions and fusion-negative subependymoma. However, occasionally supratentorial ependymomas cannot be readily assigned to any of these groups due to lack of detection of a typical fusion and/or ambiguous DNA methylation-based classification. An unbiased approach with a cohort of unprecedented size revealed distinct methylation clusters composed of tumors with ependymal but also various other histologic features containing alternative translocations that shared ZFTA as a partner gene. Somatic overexpression of ZFTA-associated fusion genes in the developing cerebral cortex is capable of inducing tumor formation in vivo, and cross-species comparative analyses identified GLI2 as a key downstream regulator of tumorigenesis in all tumors. Targeting GLI2 with arsenic trioxide caused extended survival of tumor-bearing animals, indicating a potential therapeutic vulnerability in ZFTA fusion-positive tumors. SIGNIFICANCE: ZFTA-RELA fusions are a hallmark feature of supratentorial ependymoma. We find that ZFTA acts as a partner for alternative transcriptional activators in oncogenic fusions of supratentorial tumors with various histologic characteristics. Establishing representative mouse models, we identify potential therapeutic targets shared by ZFTA fusion-positive tumors, such as GLI2.This article is highlighted in the In This Issue feature, p. 2113.

ZFTA Translocations Constitute Ependymoma Chromatin Remodeling and Transcription Factors

ZFTA (C11orf95)-a gene of unknown function-partners with a variety of transcriptional coactivators in translocations that drive supratentorial ependymoma, a frequently lethal brain tumor. Understanding the function of ZFTA is key to developing therapies that inhibit these fusion proteins. Here, using a combination of transcriptomics, chromatin immunoprecipitation sequencing, and proteomics, we interrogated a series of deletion-mutant genes to identify a tripartite transformation mechanism of ZFTA-containing fusions, including: spontaneous nuclear translocation, extensive chromatin binding, and SWI/SNF, SAGA, and NuA4/Tip60 HAT chromatin modifier complex recruitment. Thereby, ZFTA tethers fusion proteins across the genome, modifying chromatin to an active state and enabling its partner transcriptional coactivators to promote promiscuous expression of a transforming transcriptome. Using mouse models, we validate further those elements of ZFTA-fusion proteins that are critical for transformation-including ZFTA zinc fingers and partner gene transactivation domains-thereby unmasking vulnerabilities for therapeutic targeting. SIGNIFICANCE: Ependymomas are hard-to-treat brain tumors driven by translocations between ZFTA and a variety of transcriptional coactivators. We dissect the transforming mechanism of these fusion proteins and identify protein domains indispensable for tumorigenesis, thereby providing insights into the molecular basis of ependymoma tumorigenesis and vulnerabilities for therapeutic targeting.This article is highlighted in the In This Issue feature, p. 2113.

EPEN-01. C11ORF95-RELA DICTATES ONCOGENIC TRANSCRIPTIONAL PROGRAMS TO DRIVE AGGRESSIVE SUPRATENTORIAL EPENDYMOMA

Over 60% of supratentorial (ST) ependymomas harbor a gene fusion between C11orf95, an uncharacterized gene, and RELA (also known as p65), a main component of the NF-ĸB family of transcription factors. While its sufficiency to drive tumor has been established, the mechanism of tumorigenesis remains elusive. To tackle this question, we developed a natively forming mouse tumor model using in utero electroporation (IUE) of the embryonic mouse brain and performed integrative epigenomic and transcriptomic mapping. Our findings indicate that in addition to direct canonical NF-ĸB pathway activation, C11orf95-RELA (CRfus) dictates a neoplastic transcriptional program and binds to unique sites across the genome enriched with Plagl family transcription factor motifs. CRfus modulates the transcriptional landscape by recruiting transcription co-activators (Brd4, EP300, Cbp, Pol2) which are amenable to pharmacologic inhibition. Downstream CRfus target genes converge on developmental programs marked by Plagl family of transcription factors and activate neoplastic programs enriched in Mapk, focal adhesion, and gene imprinting networks, many of which contain previously unreported therapeutic leads in C11orf95-RELA ependymoma.

EPEN-02. FUNCTION AND DEPENDENCY OF NF-KB ACTIVITY IN C11ORF95-RELA FUSION EPENDYMOMA

Introduction

Ependymoma is an aggressive type of pediatric brain tumor resistant to chemotherapy, with treatment to date limited to surgical resection and radiation. Thus, identification and validation of molecular targets that can translate into clinical trials in ependymoma is desperately needed to improve patient outcomes. Over 70% of supratentorial ependymoma are driven by an oncogenic fusion between C11orf95 and Rela (denoted CRFUS). CRFUS expression initiates ependymoma development in mice by potentially acting as an oncogenic transcription factor and disrupting gene expression programs. We hypothesized that specific CRFUS interacting proteins are required for tumor formation and could represent lead therapeutic targets.

Methods

To study CRFUS ependymoma, a natively-forming tumor model of CRFUS generated by in utero electroporation of the developing mouse brain was utilized. Tumor cells were isolated and then subjected to nuclear Rapid Immunoprecipitation and Mass Spectrometry Analysis of Endogenous Proteins (RIME) of HA-tagged CRFUS protein. Immunoprecipitation and Western Blot (IP-WB) were utilized to probe for leading protein interactions.

Results

We identified NF-kB proteins consistent with canonical Rela mediated transcription (NFKB1 and NFKB2) as well as novel protein interactomes that converged on RNA splicing and translational regulation. In addition, we identified a large series of novel chromatin-binding proteins as candidates potentially required for CRFUS mediated tumorigenesis.

Conclusions

Further study is ongoing to validate key CRFUS protein interaction dependency on tumor development. ChIP-Seq (chromatin immunoprecipitation with massively parallel DNA sequencing) and CUT&RUN (cleavage under target and release using nuclease) assays have been employed to further analyze the functional role of canonical Rela pathway members. By interrogating these mechanisms, novel therapeutic targets and pathways may be identified in parallel with dissecting the molecular basis of CRFUS driven ependymoma.

EMBR-09. EXAMINING THE ROLE OF THE DEVELOPMENTALLY ENCODED TRANSCRIPTION FACTOR, LHX9, IN GROUP 3 MEDULLOBLASTOMA

Medulloblastoma (MB) is the most common malignant brain tumor of childhood. Despite major advances in our understanding of the biology of MB, novel treatments remain urgently needed. Using a chemical-genomics driven drug repositioning strategy, we identified the cardiac glycoside family of compounds as potential treatments for Group 3 MB. We subsequently demonstrated that single-agent treatment with digoxin prolongs survival in a patient-derived xenograft model (PDOX) of Group 3 MB to a degree comparable to radiation therapy, a mainstay in the treatment of MB. Finally, we examined the mechanism of digoxin-mediated cell killing using RNA-seq. This work identified LHX9, a member of the LIM homeobox family of transcription factors, as the gene most significantly down-regulated following treatment (Huang and Injac et al, Sci Trans Medicine, 2018). Homologs of LHX9 play key roles in cerebellar development via spatially and temporally restricted expression and LHX9 has been proposed as a core transcription factor (TF) in the regulatory circuitry of Group 3 tumors. Loss of function of other core TFs has been shown to impact MB growth. The role of LHX9 in MB, however, has not been previously experimentally evaluated. We now report that knockdown of LHX9 in MB-derived cell lines results in marked growth inhibition. RNA-seq analysis of LHX9-depleted cells showed changes which included alterations in extracellular matrix-receptor interactions and TGFb signaling. These findings raise the possibility that loss of LHX9 plays a major role in digoxin-mediated cell killing and that LHX9 represents a key dependency required for the growth of Group 3 MB. Clinical targeting of core TFs would represent a novel approach to targeting this devastating disease.

EPEN-30. C11ORF95-RELA FUSION PROTEIN ENGAGES NOVEL GENOMIC LOCI TO DRIVE MURINE EPENDYMOMA GROWTH

RATIONALE

Over 70% of supratentorial (ST) ependymoma are characterized by an oncogenic fusion between C11ORF95 and RELA. C11ORF95-RELA fusion is frequently the sole genetic driver detected in ST ependymoma, thus ranking this genomic event as a lead target for therapeutic investigation. RELA is a transcription factor (TF) central to mediating NF-kB pathway activation in processes such as inflammation, cellular metabolism, and chemotaxis. HYPOTHESIS: We posited that C11ORF95-RELA acts as an oncogenic TF that aberrantly shapes the tumor epigenome to drive aberrant transcription. Approach: To this end we developed an in utero electroporation (IUE) mouse model of ependymoma to express C11ORF95-RELA during embryonic development. Our IUE approach allowed us to develop C11ORF95-RELA driven tumor models and cell lines. We comprehensively characterized the epigenome and transcriptome of C11ORF95-RELA fusion driven mouse cells by H3K27ac ChIP-seq, ATAC-seq, and RNA-seq.

RESULTS

This data revealed that: 1) C11ORF95-RELA directly engages ‘open’ chromatin and is enriched at regions with known RELA TF binding sites as well as novel genomic loci/motifs, 2) C11ORF95-RELA preferentially binds to both H3K27ac (active) enhancers and promoters, and 3) Bound C11ORF95-RELA promoter loci are associated with increased transcription of genes shared with human ependymoma.

CONCLUSION

Our findings shed light on the transcriptional mechanisms of C11ORF95-RELA, and reveal downstream targets that may represent cancer dependency genes and molecular targets.