Medulloblastoma Therapy Research Articles

Oncolytic virus-driven immune remodeling revealed in mouse medulloblastomas at single cell resolution

Oncolytic viruses, modified for tumor-restricted infection, are a promising cancer immunotherapeutic, yet much remains to be understood about factors driving their activity and outcome in the tumor microenvironment. Here, we report that oncolytic herpes simplex virus C134, previously found to exert Tcell-dependent efficacy in mouse models of glioblastoma, exerts Tcell-independent efficacy in mouse models of medulloblastoma, indicating this oncolytic virus uses different mechanisms in different tumors. We investigated C134's behavior in mouse medulloblastomas, using single cell RNA sequencing to map C134-induced gene expression changes across cell types, timepoints, and medulloblastoma subgroup models at whole-transcriptome resolution. Our work details substantial oncolytic virus-induced transcriptional remodeling of medulloblastoma-infiltrating immune cells, 10 subpopulations of monocytes and macrophages collectively demonstrating M1-like responses to C134, and suggests C134 be investigated as a potential new therapy for medulloblastoma.

A qualitative exploration of parental caregivers' experience caring for children who have survived medulloblastoma.

Approximately 70% of children diagnosed with a medulloblastoma will become long-term survivors. Medulloblastoma therapy frequently causes long-term morbidities in survivors, which places a considerable burden on parental caregivers. We aimed to explore the experience of parental caregivers caring for medulloblastoma survivors. We conducted a qualitative study using grounded theory thematic analysis. We used semi-structured parental caregiver interviews to explore family experiences, social circumstances, and family-reported impact within families of children who had survived medulloblastoma. Parental caregivers were recruited from specialized survivor clinics at two large quaternary centers in Toronto, Canada. Sixteen of 22 eligible families participated, and 20 parental caregiver interviews were completed. Survivors were a median age of 6years (range: 1-9years) at diagnosis, and were 9.5years (range: 5-12years) from treatment at the time of the interview. Three major themes and associated subthemes emerged: (i) parental caregivers described significant long-term challenges associated with their child's survivorship. Subthemes included medical treatment sequelae, school issues and behavioral concerns, and surveillance and access to care. (ii) Parental caregivers recognized the impact that their child's quality of life (QOL) had on both their personal and family QOL. Subthemes included parental QOL, parental mental health and coping, spousal relationships, and effects on the family unit as a whole. (iii) Parental caregivers reported experiencing conflicting emotions related to their child's survivorship status and long-term effects. Subthemes included feeling happiness with concurrent worry, fear, and stress, as well as concerns about the future. Parental caregivers of medulloblastoma survivors experience long-term challenges, with personal and family impacts. Further work is needed to improve care models and support systems for families with a child who has survived medulloblastoma.

Circ_103128 is associated with the tumorigenesis of medulloblastoma.

Medulloblastoma (MB) is a primary malignant tumor of the brain. They are categorized as WHO grade IV neoplasms, and mostly occur in children. The traditional therapy for MB is surgery, followed by radiation and chemotherapy, but the clinical outcome is still poor and has a high possibility of recurrence. The mechanism underlying the development of MB should be further investigated to develop novel therapeutic strategies. Research has demonstrated that circRNAs contribute to tumorigenesis, but the functional mechanism of circRNAs in MB has not been fully explored and remains vague. The differentially expressed circRNAs between MB and normal cerebellar tissues were analyzed based on the microarray expression profiles to characterize the potential mechanism of circRNAs in MB. The results revealed that circRNA_103128 was highly expressed in MB, and cellular and animal experiments were performed to verify its tumorigenic effect in MB. Furthermore, a bioinformatics analysis and literature review previous literature were performed, confirming miR-129-5p as a target gene downstream of circRNA_103128. In addition, SOX4 was predicted to be a downstream target protein of miR-129-5p. Subsequently, miR-129-5p expression was inhibited, which revealed the regulatory mechanism of circRNA_103128. The latter promotes MB cell growth, migration, and invasion by the sponge effect of miR-129-5p, thereby affecting the expression of SOX4. This study is the first to systematically demonstrate that circRNA_103128 may play an important regulatory role in MB through a sponge effect with miR-129 -5p, which affects SOX4 expression and regulates tumorigenesis and tumor cell development in MB.

Transmorphic phage-guided systemic delivery of TNFα gene for the treatment of human pediatric medulloblastoma.

Medulloblastoma is the most common childhood brain tumor with an unfavorable prognosis and limited options of harmful treatments that are associated with devastating long-term side effects. Therefore, the development of safe, noninvasive, and effective therapeutic approaches is required to save the quality of life of young medulloblastoma survivors. We postulated that therapeutic targeting is a solution. Thus, we used a recently designed tumor-targeted bacteriophage (phage)-derived particle, named transmorphic phage/AAV, TPA, to deliver a transgene expressing the tumor necrosis factor-alpha (TNFα) for targeted systemic therapy of medulloblastoma. This vector was engineered to display the double-cyclic RGD4C ligand to selectively target tumors after intravenous administration. Furthermore, the lack of native phage tropism in mammalian cells warrants safe and selective systemic delivery to the tumor microenvironment. In vitro RGD4C.TPA.TNFα treatment of human medulloblastoma cells generated efficient and selective TNFα expression, subsequently triggering cell death. Combination with the chemotherapeutic drug cisplatin used clinically against medulloblastoma resulted in augmented effect through the enhancement of TNFα gene expression. Systemic administration of RGD4C.TPA.TNFα to mice-bearing subcutaneous medulloblastoma xenografts resulted in selective tumor homing of these particles and consequently, targeted tumor expression of TNFα, apoptosis, and destruction of the tumor vasculature. Thus, our RGD4C.TPA.TNFα particle provides selective and efficient systemic delivery of TNFα to medulloblastoma, yielding a potential TNFα anti-medulloblastoma therapy while sparing healthy tissues from the systemic toxicity of this cytokine.

Optimizing reirradiation for relapsed medulloblastoma: identifying the ideal patient and tumor profiles.

First-line therapies for medulloblastoma(MBL) are obtaining higher survival-rates while decreasing late-effects, but treatment at relapse is not standardized. We report here the experience with MBL re-irradiation(re-RT), its timing and outcome in different clinical settings and tumor groups. Patient's staging/treatment at diagnosis, histotypes/molecular subgroups, relapse site/s, re-treatments outcome are reported. 25 patients were included, with a median age of 11.4 years; 8 had metastases. According to 2016-2021 WHO-classification, 14 had SHH subgroup tumors(six TP53 mutated,one + MYC,one + NMYC amplification), 11 non-WNT/non-SHH (two with MYC/MYCN amplification).Thirteen had received HART-CSI, 11 standard-CSI, one HFRT; all post-radiation chemotherapy(CT), 16 also pre-RT. Median time to relapse (local-LR in nine, distant-DR in 14, LR + DR in two) was 26 months. Fourteen patients were re-operated, in five cases excising single DR-sites, thereafter three received CT, two after re-RT; out of 11 patients not re-operated, four had re-RT as first treatment and seven after CT. Re-RT was administered at median 32 months after first RT: focally in 20 cases, craniospinal-CSI in five. Median post-relapse-PFS/after re-RT was 16.7/8.2 months, while overall survival-OS was 35.1/23.9 months, respectively. Metastatic status both at diagnosis/relapse negatively affected outcome and re-surgery was prognostically favorable. PD after re-RT was however significantly more frequent in SHH (with a suggestive association with TP53 mutation, p = 0.050). We did not observe any influence of biological subgroups on PFS from recurrence while SHH showed apparently worse OS compared to non-WNT/non-SHH group. Re-surgery + reRT can prolong survival; a substantial fraction of patients with worse outcome belongs to the SHH-subgroup.

MDB-37. CDK9 IS A DRUGGABLE MEDIATOR SUSTAINING MYC-DRIVEN TRANSCRIPTIONAL CIRCUITRY IN MEDULLOBLASTOMA

Abstract Though long recognized as a master regulator of cell proliferation across a wide range of cancers, Myc has proven elusive to direct therapeutic targeting. The CDK9-containing PTEFb, complexed with either BRD4 or SEC, facilitates Myc-driven transcriptional programs and is necessary for sustaining expression of Myc itself. Advances in development of clinical-grade CDK9 inhibitors creates an opportunity to examine this as a rational therapy for Myc-driven medulloblastoma (Myc-MB). In order to define the enhancer-promoter circuitry driving Myc expression in medulloblastoma, we performed chromosome conformation capture (Hi-C) from primary patient Myc-MB samples. Using a combination of CUT&RUN and RNA-seq, we find that treatment with the CDK9 inhibitor zotiraciclib depletes Myc binding and transcriptional activity at enhancer elements within the Myc topologically associated domain, largely abrogating transcriptional output from the Myc promoter. This leads to a decrease in Myc binding genome-wide with a concordant downregulation of hallmark Myc-driven transcriptional programs. Myc-MB cell lines are sensitive to CDK9 inhibition at low nanomolar concentrations, with a loss of fitness as measured by decreased proliferative or clonogenic potential and a marked induction of apoptosis. Clinically relevant CDK9 inhibitors show variable efficacy in vivo, but CNS-penetrant agents achieved a significant prolongation in xenograft survival. Together, these data demonstrate that CDK9 catalytic activity represents a druggable vulnerability underpinning Myc-driven transcriptional programs. The development of CNS-penetrant CDK9 inhibitors may open new avenues for rational therapy in these high-risk medulloblastomas.

MDB-38. COMPUTATIONAL DRUG SENSITIVITY PREDICTS MEDULLOBLASTOMA SUBGROUP-SPECIFIC THERAPEUTICS

Abstract Medulloblastoma is the most common malignant pediatric brain tumor. Tumors are typically characterized as Group 3, Group 4, SHH, or WNT. Current standard-of-care includes surgery, radiation, and chemotherapy; however, treatment response and prognosis vary widely between subgroups. Additionally, surviving children frequently suffer from lifelong neurocognitive deficits. Therefore, novel therapeutic options are urgently needed. Despite extensive characterization of medulloblastoma tumors into four molecular subgroups, few subgroup specific therapies have advanced to the clinic. To address this, we have developed a novel platform called DrugSeq, which predicts drug sensitivities in patients and can stratify tumors by subgroup. DrugSeq also identifies key pharmacotranscriptomic differences between primary and recurrent tumors. We first calculated disease signatures for each patient by normalizing gene expression to low grade glioma samples from the posterior fossa. Using previously developed transcriptional consensus signatures (TCSs) that represent the affect that a drug has on the gene expression across a panel of cancer cell lines, we then calculated the discordance between each patient disease signature and drug TCS. Finally, using an ANOVA analysis we identified drugs which are predicted to differentially target each medulloblastoma subgroup. Among our top predicted anti-cancer compounds we found several kinase inhibitors, bromodomain inhibitors, and several psychiatric drugs with known brain penetrance. Additionally, we show distinct differences in drug sensitivity predictions between newly diagnosed and recurrent tumors, such as sensitivity to BET inhibition for recurrent Group 3 and Group 4 tumors despite no predicted sensitivity in the newly diagnosed tumors. Future studies with larger datasets may be able to further subdivide patients by age or molecular features within subgroups. Collectively, we show that DrugSeq may identify novel therapies and facilitate patient stratification in clinical trials, leading to more successful targeted medulloblastoma therapies.

Survival and prognostic factors in pediatric patients with medulloblastoma treated at a national pediatric hospital in Peru: a retrospective cohort.

The objectives of this study were to determine the overall survival (OS) and event-free survival (EFS) rates of patients with medulloblastoma treated in a national pediatric hospital in Peru, as well as to identify demographic, clinical, imaging, postoperative, and histopathological characteristics and prognostic factors associated with OS and EFS. The authors conducted a retrospective study analyzing information from the medical records of children with a diagnosis of medulloblastoma who underwent surgical treatment at the Instituto Nacional de Salud del Niño-San Borja, a public hospital in Lima, Peru, from 2015 to 2020. Clinical-epidemiological variables, degree of disease extension, risk stratification, extent of resection, postoperative complications, status of oncological treatment received, histological subtype, and neurological sequelae were taken into account. The Kaplan-Meier method and Cox regression analysis were used to estimate OS, EFS, and prognostic factors. Of the 57 children evaluated with complete medical records, only 22 children (38.6%) underwent complete oncological treatment. OS was 37% (95% CI 0.25-0.55) at 48 months. EFS was 44% (95% CI 0.31-0.61) at 23 months. High-risk stratification-meaning patients with ≥ 1.5 cm2 of residual postoperative tumor, those younger than 3 years, those with disseminated disease (HR 9.69, 95% CI 1.40-67.0, p = 0.02), and those who underwent subtotal resection (HR 3.78, 95% CI 1.09-13.2, p = 0.04)-was negatively associated with OS. Failure to receive complete oncological treatment was negatively associated with OS (HR 20.0, 95% CI 4.84-82.6, p < 0.001) and EFS (HR 7.82, 95% CI 2.47-24.7, p < 0.001). OS and EFS of patients with medulloblastoma in the author's milieu are below those reported in developed countries. Incomplete treatment and treatment abandonment in the authors' cohort were also high compared with high-income country statistics. Failure to complete oncological treatment was the most important factor associated with poor prognosis, both in terms of OS and EFS. High-risk patients and subtotal resection were negatively associated with OS. Interventions are needed to promote the completion of adjuvant oncological therapy for medulloblastoma in the disadvantaged Peruvian population.

Combination of PARP and WEE1 inhibitors invitro: Potential for use in the treatment of SHH medulloblastoma.

Medulloblastoma (MB), grouped as either WNT‑activated, Sonic hedgehog (SHH)‑activated, or non-WNT/non-SHH group 3, accounts for almost 20% of all childhood brain cancers. In spite of current intensive treatments, not all patients are cured and survivors suffer from severe side‑effects. The present study therefore examined the effects of the poly‑ADP‑ribose polymerase (PARP) and WEE1‑like protein kinase (WEE1) inhibitors, BMN673 and MK‑1775, respectively, alone or in combination on four MB cell lines. More specifically, the MB cell lines, DAOY, UW228‑3, MED8A and D425, were tested for their sensitivity to BMN673 and MK‑1775 alone or in combination, using cell viability, cell confluency and cytotoxicity assays. The effects on the cell cycle phases were also examined using FACS analysis. Monotherapy with BMN673 and MK‑1775 exerted dose‑dependent inhibitory effects on the viability of almost all MB cell lines. Notably, when BMN673 and MK‑1775 were used in combination, synergistic effects were noted in the SHH group cell lines (DAOY and UW228‑3), but not in the already WEE1‑sensitive group 3 (MED8A and D425) lines. Moreover, the combination treatment decreased the percentage of cells in the G1 phase and induced the novel distribution of both DAOY and UW228‑3 cells in the S and G2/M phases, with the UW228‑3 cells exhibiting a greater delay. To conclude, MK‑1775 was efficient in all and BMN673 in most cell lines, and their combined use exerted synergistic effects on the SHH, but not the group 3 cell lines. These data suggest that MK‑1775 alone may be of interest for all MB cell lines, and that the combination of PARP/WEE1 inhibitors may provide possible therapeutic opportunities for the therapy of SHH MBs. Their use warrants further investigations in the future.

Drug Resistance in Medulloblastoma Is Driven by YB-1, ABCB1 and a Seven-Gene Drug Signature

Therapy resistance represents an unmet challenge in the treatment of medulloblastoma. Accordingly, the identification of targets that mark drug-resistant cell populations, or drive the proliferation of resistant cells, may improve treatment strategies. To address this, we undertook a targeted approach focused on the multi-functional transcription factor YB-1. Genetic knockdown of YB-1 in Group 3 medulloblastoma cell lines diminished cell invasion in 3D in vitro assays and increased sensitivity to standard-of-care chemotherapeutic vincristine and anti-cancer agents panobinostat and JQ1. For vincristine, this occurred in part by YB-1-mediated transcriptional regulation of multi-drug resistance gene ABCB1, as determined by chromatin immunoprecipitation. Whole transcriptome sequencing of YB-1 knockdown cells identified a role for YB-1 in the regulation of tumourigenic processes, including lipid metabolism, cell death and survival and MYC and mTOR pathways. Stable cisplatin- and vincristine-tolerant Group 3 and SHH cell lines were generated to identify additional mechanisms driving resistance to standard-of-care medulloblastoma therapy. Next-generation sequencing revealed a vastly different transcriptomic landscape following chronic drug exposure, including a drug-tolerant seven-gene expression signature, common to all sequenced drug-tolerant cell lines, representing therapeutically targetable genes implicated in the acquisition of drug tolerance. Our findings provide significant insight into mechanisms and genes underlying therapy resistance in medulloblastoma.

Characterization of an RNA binding protein interactome reveals a context-specific post-transcriptional landscape of MYC-amplified medulloblastoma

Pediatric medulloblastoma (MB) is the most common solid malignant brain neoplasm, with Group 3 (G3) MB representing the most aggressive subgroup. MYC amplification is an independent poor prognostic factor in G3 MB, however, therapeutic targeting of the MYC pathway remains limited and alternative therapies for G3 MB are urgently needed. Here we show that the RNA-binding protein, Musashi-1 (MSI1) is an essential mediator of G3 MB in both MYC-overexpressing mouse models and patient-derived xenografts. MSI1 inhibition abrogates tumor initiation and significantly prolongs survival in both models. We identify binding targets of MSI1 in normal neural and G3 MB stem cells and then cross referenced these data with unbiased large-scale screens at the transcriptomic, translatomic and proteomic levels to systematically dissect its functional role. Comparative integrative multi-omic analyses of these large datasets reveal cancer-selective MSI1-bound targets sharing multiple MYC associated pathways, providing a valuable resource for context-specific therapeutic targeting of G3 MB.

TMET-01. DISRUPTION OF THE SERINE-PRODUCING PATHWAYS SLOWS PROGRESSION OF THE SONIC HEDGEHOG-DRIVEN MEDULLOBLASTOMA

Abstract We have found that medulloblastoma metabolism is specialized to promote growth in the CNS and can be targeted for anti-tumor therapy. Prior studies show that serine is scarce in the CNS and that metastatic tumors require serine-producing enzymes to grow in the brain. We analyzed whether medulloblastoma, a primary brain tumor, showed similar serine dependency. We found that Sonic Hedgehog signaling, which induces proliferation in cerebellar granule neuron progenitors in normal development and medulloblastoma formation when hyperactivated, also induces PHGDH and SHMT1, which each catalyze key steps in different serine-production pathways. To define the functional role of PHGDH and SHMT1 in medulloblastomas, we bred mice genetically engineered to develop SHH medulloblastomas with mice carrying deletions of each of these genes. We found that mice with Phgdh-deleted medulloblastomas showed slower progression and longer survival times, compared to mice with Phgdh-intact medulloblastomas. Medulloblastomas with combined deletion of Phgdh and Shmt1 show even slower tumor progression. Stable-isotope flux analysis suggests that different serine-producing pathways compensate for the loss of individual serine-producing enzymes, providing a rationale for the greater anti-tumor effect of disrupting multiple serine-producing enzymes simultaneously. Our data show that serine-producing enzymes can be targeted as a novel approach to medulloblastoma therapy, and underscore the importance of targeting multiple serine-production pathways in complex metabolic interventions.

Pathological implications of metabolic reprogramming and its therapeutic potential in medulloblastoma.

Tumor-specific alterations in metabolism have been recognized to sustain the production of ATP and macromolecules needed for cell growth, division and survival in many cancer types. However, metabolic heterogeneity poses a challenge for the establishment of effective anticancer therapies that exploit metabolic vulnerabilities. Medulloblastoma (MB) is one of the most heterogeneous malignant pediatric brain tumors, divided into four molecular subgroups (Wingless, Sonic Hedgehog, Group 3 and Group 4). Recent progresses in genomics, single-cell sequencing, and novel tumor models have updated the classification and stratification of MB, highlighting the complex intratumoral cellular diversity of this cancer. In this review, we emphasize the mechanisms through which MB cells rewire their metabolism and energy production networks to support and empower rapid growth, survival under stressful conditions, invasion, metastasis, and resistance to therapy. Additionally, we discuss the potential clinical benefits of currently available drugs that could target energy metabolism to suppress MB progression and increase the efficacy of the current MB therapies.

28P SHH pathway-targeted therapy in a relapsed medulloblastoma

Recurrent medulloblastoma is a deleterious disease with a 5-year survival of less than 5% and no recognized standard treatment. In recent years, we have learned about the molecular characterization of the different types of medulloblastoma with four subgroups also appearing different subgroups, each with its oncogenic pathway activation. SHH activation is the hallmark of medulloblastoma group 2 and several molecules have been developed to block this signaling mechanism, including SMO inhibitors, that are approved for the treatment of basal cell carcinoma.

P04.01.B High impact of ITGB1 on Pi3K/AKT expression in medulloblastoma

Abstract Medulloblastoma, an embryonal tumor of the cerebellum, is one of the most frequent malignant brain tumors. Despite the increasing use of genetic variation in treatment stratification, high-risk patients characterized by light meningeal spread, TP53 mutations, or MYC amplification still have poor survival. Phosphatidylinositol-3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling is one of the most important intracellular pathways, which can be considered as a master regulator for cancer. In tissue samples obtained from medulloblastoma patients, the significant upregulation of PI3K/AKT was associated with a lifting expression level of integrin β1(ITGB1). To understand the underlying mechanism, we investigated the effect of ITGB1 on the PI3K/AKT pathway in medulloblastoma cell lines. Transfection of this ITGB1 reduced proliferation and invasion of several medulloblastoma cell lines and inhibit epithelial-mesenchymal transition. In addition, knocking down ITGB1 expression can significantly inhibit the activation of PI3K/AKT signaling pathway. In conclusion, ITGB1 may selectively activation the pathophysiological effect of aberrant PI3K/AKT expression and serve as a targeted approach for medulloblastoma therapy.

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.

STAT3 in medulloblastoma: a key transcriptional regulator and potential therapeutic target.

Medulloblastoma is the most common malignant brain tumor of childhood accounting for about 60% of all pediatric embryonal tumors. Despite improvements in the overall survival rate, this tumor still lacks an efficient, reliable, and less toxic therapeutic approach. Characterization of the molecular mechanisms involved in medulloblastoma initiation and progression is a crucial step for the development of effective therapies. Signal transducer and activator of transcription 3 is a convergence point for several signaling cascades that are implicated in medulloblastoma tumorigenesis. Accumulated evidence has revealed the pivotal role of signal transducer and activator of transcription 3 in medulloblastoma pathogenesis such as proliferation, survival, angiogenesis, and immunosuppression as well as maintenance, drug resistance, and recurrence. In this review, we focus on the role of signal transducer and activator of transcription 3 in medulloblastoma tumorigenesis and discuss the recent advances of signal transducer and activator of transcription 3 inhibition as a promising developed strategy for medulloblastoma therapy.

Abstract 5369: LOXL1-AS1 contributes to medulloblastoma metastasis: A potential target for mesenchymal stem cell exosome-delivered gene therapy

Abstract Introduction: Medulloblastoma (MB) is the most common malignant tumor in children. MB-related treatment failures and deaths mostly result from metastasis. One of the key genetic drivers in MB is MYCN, whose aberrant expressions are associated with tumor aggressiveness and poor prognosis. Long non-coding (lnc)RNAs have been implied as potential targets for cancer treatment. More insights into the role of lncRNAs in MYCN-mediated MB progression would have both prognostic and therapeutic significance in metastatic MB. Cell-derived vesicles such as exosome can encapsulate siRNAs to improve their bioavailability and targeted delivery to brain tumors. Mesenchymal stem cell (MSC) is a readily available and modifiable source to produce functional exosomes. Objective: To identify the downstream lncRNAs of MYCN that contributes to MB metastasis, and to develop a novel gene therapy for MB by MSC exosome-delivered siRNAs to target these prometastatic lncRNAs. Methods: An RNA sequencing analysis was used to identify LOXL1-AS1 as a MYCN targeting gene in MYCN-overexpressing MB cells. The association of LOXL1-AS1 and MYCN was validated in MB cell lines by RT-qPCR and Western blot assay, and in tumor tissues by in-situ hybridization and immunohistochemistry. The anti-tumor effect of LOXL1-AS1 knockdown in MYCN-overexpressing MB cells was assessed in vitro by migration and sphere formation assays, and in an orthotopic xenograft mouse model by intracranial injection. To generate therapeutic exosomes, MSCs were transfected with exogenous LOXL1-AS1 siRNAs and then subjected to exosome characterization of size distribution and marker expression. The effect of exosome treatment on MB cells was evaluated using RT-qPCR and wound healing assay. Results: Elevated LOXL1-AS1 was found in MYCN-overexpressed MB cells compared to parental cells, and this association was confirmed in clinical tissue samples. LOXL1-AS1 expression was correlated with poor prognosis in SHH-α and SHH-γ MB subtypes of patients. Knockdown of LOXL1-AS1 significantly reduced cell migration and cancer stemness properties of MB cells in vitro, and suppressed orthotopic tumor growth, metastasis, and enhanced survival in xenografted mice. Exogenous siRNAs were successfully packaged into MSC exosomes by cell transfection. Treatment of MB cells with LOXL1-AS1 siRNA-loaded exosomes effectively reduced LOXL1-AS1 expression and inhibited cell migration compared to those treated with unmodified exosomes. Conclusion: Our study has proven the contributive role of LOXL1-AS1 in promoting cancer progression and metastasis of MYCN-associated MB, and provided in vitro evidence to support the feasibility of LOXL1-AS1 gene therapy through siRNA-loaded MSC exosomes. In vivo biodistribution and therapeutic efficacy assessment of MSC exosome-delivered LOXL1-AS1 siRNAs are being investigated in animal models. Citation Format: Anh Duy Do, Chia-Ling Hsieh, Tai-Tong Wong, Shian-Ying Sung. LOXL1-AS1 contributes to medulloblastoma metastasis: A potential target for mesenchymal stem cell exosome-delivered gene therapy [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 5369.

MEDB-16. Persistent radiological lesions at the end of primary therapy in childhood medulloblastoma: residual lesion or active residual tumor?

BACKGROUND: Magnetic resonance imaging (MRI) of patients with medulloblastoma (MB) often shows persistent residual findings after primary treatment. Criteria for characterizing these lesions and consensus on further therapeutic approaches are not established. MATERIAL AND METHODS: Eighty-four patients ≥4 years with centrally reviewed residual lesions on MRI at the end of primary therapy with initial surgery between 2000 and 2018 were identified. Data were extracted from the German HIT-MED database. RESULTS: Median age at initial diagnosis was 9.3 (4.0-20.8) years. 91.7% were histologically classified as CMB, 7.1% as LC/AMB and 1.2% as DMB. The majority (65.5%) of the evaluated cohort was assigned to molecular subgroup 4, 24.1% to group 3, 6.8% to WNT, 3.4% to SHH. Median follow-up for survivors was 5.96 (1.41-16.67) years. Univariate analysis revealed that patients showing an overall partial response (PR) to primary therapy have a significantly lower risk of progression of residual lesions compared to patients with stable disease (SD) (5-year PFS [PR]: 62.5±7,0; 5-year PFS [SD]: 35.9±12.8; 5-year OS [PR]: 85.6±5.1; 5-year OS [SD]: 54.1±13.7; p=0.02 [PFS], p=0.04 [OS]). Additionally, patients with multiple residual lesions (M+ and R+) were at higher risk of progression (5-year PFS [R+ only]: 72.4±12.0, 5-year PFS [R+/M+]: 22.9±17.9; p=0.02 [PFS]). Further procedures after the end of primary therapy (additional resections, chemotherapy, radiotherapy) did not impact on PFS and OS. These results were confirmed by multivariate Cox regression. For molecular or histological type no significant effect was found, presumably due to small cohort. CONCLUSION: PFS in patients with residual lesions at the end of primary treatment depends on the overall response to primary therapy. Additional procedures do not seem to be superior compared to watch-and-wait strategies. Decisions regarding further therapies should be scrutinized on a case-by-case basis. Further identification of biomarkers is warranted.

MEDB-28. CDK9 is a druggable mediator sustaining Myc-driven circuitry in medulloblastoma

BACKGROUND: Though long recognized as a master regulator of cell proliferation across a wide range of cancers, Myc has proven elusive to direct therapeutic targeting. The CDK9-containing PTEFb, complexed with either BRD4 or SEC, facilitates Myc-driven transcriptional programs and is necessary for sustaining expression of Myc itself. Advances in development of clinical-grade CDK9 inhibitors creates an opportunity to examine this as a rational therapy for Myc-driven medulloblastoma. METHODS: We used both RNAi depletion and a panel of pharmacologic agents to characterize the mechanistic and functional consequences of CDK9 inhibition in Myc-driven medulloblastoma. We used a combination of clonogenic assays and live cell imaging to assess the cytotoxic effects of CDK9 activity loss. We then performed a combination of CUT&RUN and RNA-seq to evaluate alterations to Myc binding and downstream Myc-driven transcriptional programs. Finally, we employed orthotopic xenograft models of medulloblastoma to assess CNS penetration, tolerability, and anti-tumor efficacy of lead CDK9i candidate compounds. RESULTS: Genetic or pharmacologic inhibition of CDK9 leads to a loss of Myc expression and downregulation of hallmark Myc-driven transcriptional programs. This corresponds to a loss of cell fitness, as measured by decreased proliferation and clonogenic potential. Clinically relevant CDK9 inhibitors show variable efficacy in vivo, but the CNS-penetrant zotiraciclib achieved a significant prolongation in xenograft survival. CONCLUSION: CDK9 catalytic activity represents a druggable vulnerability underpinning Myc-driven transcriptional programs. The development of CNS-penetrant CDK9 inhibitors may open new avenues for rational therapy in these high-risk medulloblastomas.