Dormant SOX9-Positive Cells Facilitate MYC-Driven Recurrence of Medulloblastoma

Relapse is the leading cause of death in patients with medulloblastoma, the most common malignant pediatric brain tumor. A better understanding of the mechanisms underlying recurrence could lead to more effective therapies for targeting tumor relapses. Here, we observed that SOX9, a transcription factor and stem cell/glial fate marker, is limited to rare, quiescent cells in high-risk medulloblastoma with MYC amplification. In paired primary-recurrent patient samples, SOX9-positive cells accumulated in medulloblastoma relapses. SOX9 expression anti-correlated with MYC expression in murine and human medulloblastoma cells. However, SOX9-positive cells were plastic and could give rise to a MYC high state. To follow relapse at the single-cell level, an inducible dual Tet model of medulloblastoma was developed, in which MYC expression was redirected in vivo from treatment-sensitive bulk cells to dormant SOX9-positive cells using doxycycline treatment. SOX9 was essential for relapse initiation and depended on suppression of MYC activity to promote therapy resistance, epithelial-mesenchymal transition, and immune escape. p53 and DNA repair pathways were downregulated in recurrent tumors, whereas MGMT was upregulated. Recurrent tumor cells were found to be sensitive to treatment with an MGMT inhibitor and doxorubicin. These findings suggest that recurrence-specific targeting coupled with DNA repair inhibition comprises a potential therapeutic strategy in patients affected by medulloblastoma relapse.

SIGNIFICANCE

SOX9 facilitates therapy escape and recurrence in medulloblastoma via temporal inhibition of MYC/MYCN genes, revealing a strategy to specifically target SOX9-positive cells to prevent tumor relapse.

Subgroup-Enriched Pathways and Kinase Signatures in Medulloblastoma Patient-Derived Xenografts

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. MB is classified into four primary molecular subgroups: wingless (WNT), sonic hedgehog (SHH), Group 3 (G3), and Group 4 (G4), and further genomic and proteomic subtypes have been reported. Subgroup heterogeneity and few actionable mutations have hindered the development of targeted therapies, especially for G3 MB, which has a particularly poor prognosis. To identify novel therapeutic targets for MB, we performed mass spectrometry-based deep expression proteomics and phosphoproteomics in 20 orthotopic patient-derived xenograft (PDX) models of MB comprising SHH, G3, and G4 subgroups. We found that the proteomic profiles of MB PDX tumors are closely aligned with those of primary human MB tumors illustrating the utility of PDX models. SHH PDXs were enriched for NFκB and p38 MAPK signaling, while G3 PDXs were characterized by MYC activity. Additionally, we found a significant association between actinomycin D sensitivity and increased abundance of MYC and MYC target genes. Our results highlight several candidate pathways that may serve as targets for new MB therapies. Mass spectrometry data are available via ProteomeXchange with identifier PXD035070.

Integrated genome and tissue engineering enables screening of cancer vulnerabilities in physiologically relevant perfusable ex vivo cultures

Genetic screens are powerful tools for both resolving biological function and identifying potential therapeutic targets, but require physiologically accurate systems to glean biologically useful information. Here, we enable genetic screens in physiologically relevant ex vivo cancer tissue models by integrating CRISPR-Cas-based genome engineering and biofabrication technologies. We first present a novel method for generating perfusable tissue constructs, and validate its functionality by using it to generate three-dimensional perfusable dense cultures of cancer cell lines and sustain otherwise ex vivo unculturable patient-derived xenografts. Using this system we enable large-scale CRISPR screens in perfused tissue cultures, as well as emulate a novel point-of-care diagnostics scenario of a clinically actionable CRISPR knockout (CRISPRko) screen of genes with FDA-approved drug treatments in ex vivo PDX cell cultures. Our results reveal differences across in vitro and in vivo cancer model systems, and highlight the utility of programmable tissue engineered models for screening therapeutically relevant cancer vulnerabilities.

Functional Precision Medicine Identifies New Therapeutic Candidates for Medulloblastoma

Medulloblastoma is among the most common malignant brain tumors in children. Recent studies have identified at least four subgroups of the disease that differ in terms of molecular characteristics and patient outcomes. Despite this heterogeneity, most patients with medulloblastoma receive similar therapies, including surgery, radiation, and intensive chemotherapy. Although these treatments prolong survival, many patients still die from the disease and survivors suffer severe long-term side effects from therapy. We hypothesize that each patient with medulloblastoma is sensitive to different therapies and that tailoring therapy based on the molecular and cellular characteristics of patients' tumors will improve outcomes. To test this, we assembled a panel of orthotopic patient-derived xenografts (PDX) and subjected them to DNA sequencing, gene expression profiling, and high-throughput drug screening. Analysis of DNA sequencing revealed that most medulloblastomas do not have actionable mutations that point to effective therapies. In contrast, gene expression and drug response data provided valuable information about potential therapies for every tumor. For example, drug screening demonstrated that actinomycin D, which is used for treatment of sarcoma but rarely for medulloblastoma, was active against PDXs representing Group 3 medulloblastoma, the most aggressive form of the disease. Functional analysis of tumor cells was successfully used in a clinical setting to identify more treatment options than sequencing alone. These studies suggest that it should be possible to move away from a one-size-fits-all approach and begin to treat each patient with therapies that are effective against their specific tumor. SIGNIFICANCE: These findings show that high-throughput drug screening identifies therapies for medulloblastoma that cannot be predicted by genomic or transcriptomic analysis.

Abstract IA11: Overcoming immune evasion in pediatric brain tumors

Many immunotherapies act by enhancing T-cell killing of tumor cells. Cytotoxic T cells recognize antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. Our studies suggest that medulloblastomas and high-grade gliomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. Treatment with tumor necrosis factor or lymphotoxin beta receptor agonist rescues expression of Erap1, Tap1, and MHC-I on p53 mutant tumor cells. In vivo, TNF treatment prolongs survival and markedly augments the efficacy of the immune checkpoint inhibitor anti-PD-1. These studies identify p53 as a key regulator of immune evasion in vivo and suggest that TNF could be used to enhance sensitivity of p53-mutant tumors to immunotherapy.

Citation Format: Alexandra Garancher, Hiromichi Suzuki, Svasti Haricharan, Meher B. Masihi, Jessica M. Rusert, Paula S. Norris, Florent Carrette, Megan M. Romero, Sorana A. Morrissy, Patryk Skowron, Florence M.G. Cavalli, Hamza Farooq, Vijay Ramaswamy, Alaide Morcavallo, Jacob J. Henderson, James M. Olson, Yoon-Jae Cho, Xiao-Nan Li, Louis Chesler, Marco A. Marra, Oren J. Becher, Linda M. Bradley, Carl F. Ware, Michael D. Taylor, Robert J. Wechsler-Reya. Overcoming immune evasion in pediatric brain tumors [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr IA11.

Tumor necrosis factor overcomes immune evasion in p53-mutant medulloblastoma

Many immunotherapies act by enhancing the ability of cytotoxic T cells to kill tumor cells. Killing depends on T cell recognition of antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. In this study, we showed that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. In vitro, tumor necrosis factor (TNF) or lymphotoxin-β receptor agonist can rescue expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, low doses of TNF prolong survival and synergize with immune checkpoint inhibitors to promote tumor rejection. These studies identified p53 as a key regulator of immune evasion and suggest that TNF could be used to enhance sensitivity of tumors to immunotherapy.

TNF superfamily cytokines overcome immune evasion in medulloblastoma

Many immunotherapies act by enhancing T cell killing of tumor cells. CD8+ cytotoxic T cells recognize antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. Here we show that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. Treatment with tumor necrosis factor (TNF) or lymphotoxin beta receptor agonist (LTβRag) rescues expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, TNF treatment prolongs survival and markedly augments the efficacy of the immune checkpoint inhibitor anti-PD-1. These studies identify p53 as a key regulator of immune evasion in vivo, and suggest that TNF could be used to enhance sensitivity of p53-mutant tumors to immunotherapy.

Developmental phosphoproteomics identifies the kinase CK2 as a driver of Hedgehog signaling and a therapeutic target in medulloblastoma

A major limitation of targeted cancer therapy is the rapid emergence of drug resistance, which often arises through mutations at or downstream of the drug target or through intrinsic resistance of subpopulations of tumor cells. Medulloblastoma (MB), the most common pediatric brain tumor, is no exception, and MBs that are driven by sonic hedgehog (SHH) signaling are particularly aggressive and drug-resistant. To find new drug targets and therapeutics for MB that may be less susceptible to common resistance mechanisms, we used a developmental phosphoproteomics approach in murine granule neuron precursors (GNPs), the developmental cell of origin of MB. The protein kinase CK2 emerged as a driver of hundreds of phosphorylation events during the proliferative, MB-like stage of GNP growth, including the phosphorylation of three of the eight proteins commonly amplified in MB. CK2 was critical to the stabilization and activity of the transcription factor GLI2, a late downstream effector in SHH signaling. CK2 inhibitors decreased the viability of primary SHH-type MB patient cells in culture and blocked the growth of murine MB tumors that were resistant to currently available Hh inhibitors, thereby extending the survival of tumor-bearing mice. Because of structural interactions, one CK2 inhibitor (CX-4945) inhibited both wild-type and mutant CK2, indicating that this drug may avoid at least one common mode of acquired resistance. These findings suggest that CK2 inhibitors may be effective for treating patients with MB and show how phosphoproteomics may be used to gain insight into developmental biology and pathology.

Abstract 1935: Molecular characterization of orthotopic patient-derived xenograft models of pediatric brain tumors

[Introduction: Solid tumors of the nervous system are the most common childhood cancers after leukemias. Even though we might be able to cure more and more patients, survivors still severely suffer long-term from the intensive treatments. Therefore, new treatment strategies are urgently needed. Orthotopic patient-derived xenograft (PDX) models are an excellent platform for biomarker and preclinical drug development. However, the rarity of pediatric brain tumors and the multitude of different sub entities hinder the generation of large collection of PDX models of specific entities within single institutions. In order to generate an overview about existing PDX models in the community, we started collecting established PDX models from various centers all over the world and performed extensive molecular characterization to precisely determine the distinct molecular subgroup and constellation of genetic alterations for each PDX model, and thus identify its targetable oncogenic drivers.

Material and Methods: PDX models were established and maintained by dissociating tumor material into a single cell suspension and then orthotopically injecting it into the brain of immunodeficient animals. All PDX models and matching primary tumors (if available) have been analyzed by whole-exome and low-coverage whole-genome sequencing, as well as DNA methylation and gene expression profiling at the German Cancer Research Center (DKFZ).

Results and Discussion: Thus far, we have collected and characterized 70 established PDX models from 6 ATRTs, 8 ependymomas, 16 high-grade gliomas, 38 medulloblastomas, and 2 CNS-PNETs. PDX models always retain their molecular subtype and in the vast majority of cases also the mutations and copy number alterations when compared to their primary tumors. Only in rare cases do we observe additional aberrations, which most likely represent outgrowths of subclones from the primary tumor. Analysis of our entire cohort identified an overrepresentation of the most aggressive tumor subtypes, but also subtypes which have not been available for preclinical testing before due to lack of genetically engineered mouse models or suitable cell lines, such as Group 4 medulloblastoma. Based on our current analysis, the PDX models within the community are not yet covering the entire heterogeneity within the patient population. As a follow up, we aim to make these models and data accessible in a user-friendly manner so that the community can use them for preclinical research.

Conclusion: PDX models of pediatric brain tumors are very rare. Our molecular characterization allows researchers all over the world to find the right models for their specific scientific question. Therefore, this work will provide an unprecedented resource to study tumor biology and pave the way for improving treatment strategies for children with malignant brain tumors.

Citation Format: Sebastian Brabetz, Susanne N. Gröbner, Huriye Seker-Cin, Florian Selt, Till Milde, David T. Jones, Madison T. Wise, Jessica M. Rusert, Kyle Pedro, Andy Strand, Olaf Witt, Sarah E. Leary, Xiao-Nan Li, Robert J. Wechsler-Reya, James M. Olson, Stefan M. Pfister, Marcel Kool. Molecular characterization of orthotopic patient-derived xenograft models of pediatric brain tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1935. doi:10.1158/1538-7445.AM2017-1935

Abstract B37: Chemi-genomic analysis of patient-derived xenografts to identify personalized therapies for medulloblastoma

Medulloblastoma (MB) is the most common malignant brain tumor in children. Even with an intensive regimen of surgery, radiation and chemotherapy, one-third of patients still die from their disease. Moreover, survivors suffer devastating side effects including cognitive deficits, endocrine disorders and an increased incidence of secondary cancers later in life. Thus, more effective and less toxic therapies are desperately needed. Recent genomic analyses have identified 4 major subgroups of MB—WNT, SHH, Group 3 and Group 4—that differ in terms of mutations, gene expression profiles and patient outcomes. Despite this heterogeneity, all MB patients currently receive the same therapy. To identify novel therapies for each subgroup of MB, we have assembled a diverse panel of patient-derived xenograft (PDX) lines. These lines, established by orthotopic transplantation of tumor cells obtained from surgery, recapitulate the properties of patients' tumors more accurately than cultured cell lines. We are using these PDX lines to screen small molecule libraries and identify compounds that can inhibit tumor growth and survival. To date we have completed screening of 18 lines, including 10 representing Group 3 MB, the most aggressive and lethal form of the disease. Among the ~7800 compounds tested, we have found 20 that are effective against the majority of Group 3 PDX lines. Ongoing studies are focused on validating the activity of these compounds against additional Group 3 lines and moving the most promising ones forward into in vivo efficacy studies. Similar approaches will be pursued for each of the other subgroups of MB. Drug response data will also be compared with genomic and epigenomic data (whole exome and low coverage whole genome DNA sequencing, DNA methylation analysis, and gene expression profiling) to identify biomarkers of drug responsiveness and key pathways that may be exploited for therapy. Based on these studies, we hope to move away from a one-size-fits-all approach, and begin to treat each patient with therapies that are likely to be effective against their tumor.

Citation Format: Jessica M. Rusert, Alexandra Garancher, Yoko T. Udaka, Sebastian Brabetz, Lourdes A. Esparza, Huriye Seker-Cin, Lin Qi, Mari Kogiso, Simone Schubert, Till Milde, Yoon-Jae Cho, Xiao-Nan Li, James M. Olson, John R. Crawford, Michael L. Levy, Marcel Kool, Stefan M. Pfister, Robert J. Wechsler-Reya. Chemi-genomic analysis of patient-derived xenografts to identify personalized therapies for medulloblastoma. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr B37.

Abstract A07: Molecular characterization of patient-derived xenograft models of pediatric brain tumors

Recent genomic studies have revealed multiple molecular subtypes of pediatric brain cancers that are not only biologically but also clinically distinct. In order to develop novel treatment strategies for these often fatal diseases we need more preclinical models like orthotopic patient-derived xenograft (PDX) models that correctly reflect the many different tumor types. Prior to drug selection and testing, extensive molecular characterizations are needed to precisely assign a distinct molecular subgroup to each PDX model and to learn about its targetable oncogenic drivers.

In an international effort we aim to characterize a large repertoire of PDX models reflecting the many different molecular subtypes of pediatric brain cancer. Thus far, we have collected and characterized 64 established PDX models from 6 atypical teratoid rhabdoid tumors (AT/RT), 7 ependymomas (EPN), 16 high-grade gliomas (HGG), 32 medulloblastomas (MB), and 3 primitive neuroectodermal tumors (PNET). All PDX models and their matching primary tumors (if available) are analyzed by whole-exome and low-coverage whole-genome sequencing, as well as DNA methylation and gene expression profiling.

The DNA methylation and gene expression data showed that PDX models always cluster together with their respective brain tumor reference samples and in most cases very close to their matching primary tumor. Tumor subtype-specific oncogenic lesions could be detected by both sequencing technologies. By comparing PDX models to their primary tumor, we showed that they retain the molecular subtype, mutations and copy number alterations. Only in rare cases we observed additional aberrations in PDX models such as chromothriptic events in one MB (Group 3) or chromosome 1q gain in one EPN (posterior fossa subtype A). However, these additional aberrations are typical for this tumor subtype and were therefore most likely already present in the primary lesion at a subclonal level.

Analysis of our entire cohort identified a overrepresentation of the most aggressive tumor subtypes, but also subtypes which have not been available for preclinical testing before due to lack of genetically engineered mouse models or suitable cell lines, such as Group 4 MBs. Our molecular characterizations of PDX models provide an unprecedented resource to study tumor biology and pave the way for improving treatment strategies of malignant pediatric brain tumors.

Citation Format: Sebastian Brabetz, Huriye Seker-Cin, Susanne N. Gröbner, Norman L. Mack, Volker Hovestadt, David T. W. Jones, Till Milde, Madison T. Wise, Jessica M. Rusert, Kyle Pedro, Karina Bloom, Xiao-Nan Li, Robert J. Wechsler-Reya, James M. Olson, Stefan M. Pfister, Marcel Kool. Molecular characterization of patient-derived xenograft models of pediatric brain tumors. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr A07.

Proteomic profiling of high risk medulloblastoma reveals functional biology

Genomic characterization of medulloblastoma has improved molecular risk classification but struggles to define functional biological processes, particularly for the most aggressive subgroups. We present here a novel proteomic approach to this problem using a reference library of stable isotope labeled medulloblastoma-specific proteins as a spike-in standard for accurate quantification of the tumor proteome. Utilizing high-resolution mass spectrometry, we quantified the tumor proteome of group 3 medulloblastoma cells and demonstrate that high-risk MYC amplified tumors can be segregated based on protein expression patterns. We cross-validated the differentially expressed protein candidates using an independent transcriptomic data set and further confirmed them in a separate cohort of medulloblastoma tissue samples to identify the most robust proteogenomic differences. Interestingly, highly expressed proteins associated with MYC-amplified tumors were significantly related to glycolytic metabolic pathways via alternative splicing of pyruvate kinase (PKM) by heterogeneous ribonucleoproteins (HNRNPs). Furthermore, when maintained under hypoxic conditions, these MYC-amplified tumors demonstrated increased viability compared to non-amplified tumors within the same subgroup. Taken together, these findings highlight the power of proteomics as an integrative platform to help prioritize genetic and molecular drivers of cancer biology and behavior.

SnapShot: Medulloblastoma

Medulloblastoma (MB) is the most common malignant brain tumor in children, where one-third of patients succumb to their disease. This SnapShot describes the classification of MB subgroups, historically by histopathology and currently based on genomic information. Genomics-based classification has identified four major subgroups and provides greater opportunity for developing targeted therapies more successful than current conventional therapy.