Molecular markers and potential therapeutic targets in non-WNT/non-SHH (group 3 and group 4) medulloblastomas

Childhood medulloblastomas (MB) are heterogeneous and are divided into four molecular subgroups. The provisional non-wingless-activated (WNT)/non-sonic hedgehog-activated (SHH) category combining group 3 and group 4 represents over two thirds of all MBs, coupled with the highest rates of metastases and least understood pathology. The molecular era expanded our knowledge about molecular aberrations involved in MB tumorigenesis, and here, we review processes leading to non-WNT/non-SHH MB formations.The heterogeneous group 3 and group 4 MBs frequently harbor rare individual genetic alterations, yet the emerging profiles suggest that infrequent events converge on common, potentially targetable signaling pathways. A mutual theme is the altered epigenetic regulation, and in vitro approaches targeting epigenetic machinery are promising. Growing evidence indicates the presence of an intermediate, mixed signature group along group 3 and group 4, and future clarifications are imperative for concordant classification, as misidentifying patient samples has serious implications for therapy and clinical trials.To subdue the high MB mortality, we need to discern mechanisms of disease spread and recurrence. Current preclinical models do not represent the full scale of group 3 and group 4 heterogeneity: all of existing group 3 cell lines are MYC-amplified and most mouse models resemble MYC-activated MBs. Clinical samples provide a wealth of information about the genetic divergence between primary tumors and metastatic clones, but recurrent MBs are rarely resected. Molecularly stratified treatment options are limited, and targeted therapies are still in preclinical development. Attacking these aggressive tumors at multiple frontiers will be needed to improve stagnant survival rates

MBRS-12. A TRANSPOSON MUTAGENESIS SCREEN IDENTIFIES Rreb1 AS A DRIVER FOR GROUP 3 MEDULLOBLASTOMA

Abstract Medulloblastoma (MB) is the most common malignant childhood brain tumor. MB can be divided into four major subgroups – WNT, Sonic hedgehog (SHH), Group 3 (G3), and Group 4 (G4) – that exhibit distinct genetic alterations, gene expression profiles, and clinical outcomes. Patients with G3-MB have the worst prognosis, and a deeper understanding of this disease is critical for development of new therapies. Most G3-MBs express high levels of the MYC oncogene, suggesting that MYC plays an important role in tumorigenesis. To identify genes that cooperate with MYC to promote formation of G3-MB, we performed an in vivo mutagenesis screen using mice expressing the Sleeping Beauty (SB) transposon. Cerebellar stem cells from transposon/transposase-expressing mice were infected with viruses encoding Myc, and transplanted into the cerebellum of adult hosts. The resulting tumors were sequenced to identify transposon-targeted genes, and these genes were functionally analyzed to determine whether they could cooperate with Myc to drive G3-MB. These studies identified the transcription factor Ras-responsive element binding protein 1 (Rreb1) as a potent Myc-cooperating gene. Tumors driven by Myc and Rreb1 resemble G3-MB at a histological and molecular level. Moreover, RREB1 is overexpressed in human G3-MB, and knockdown of RREB1 impairs growth of G3-MB cell lines and patient-derived xenografts. Ongoing studies are aimed at identifying the mechanisms by which Rreb1 contributes to tumor growth. Our studies demonstrate an important role for RREB1 in G3-MB, and provide a new model that can be used to identify therapeutic targets and develop more effective therapies for medulloblastoma.</jats:p

TMOD-35. CAN RARE SOX9-POSITIVE CELLS INCITE MYC-DRIVEN MEDULLOBLASTOMA RECURRENCE?

Tumor recurrence is the main cause of death among children with medulloblastoma, the most frequent type of malignant pediatric brain tumors. The medulloblastoma subgroup Group 3 has the poorest survival of all four subgroups, and is associated with a high rate of tumor recurrence in children. Mechanisms behind medulloblastoma recurrence are not yet well understood. We found that the transcription factor SOX9 marks quiescent brain tumor stem cells and is suppressed by MYC overexpression in aggressive Group 3 tumors. By using our inducible Tet-OFF transgenic (GTML) mouse model for malignant MYCN-driven Group 3 tumors and human Group 3 MYC-driven patient-derived xenograft (PDX) models we identified rare SOX9-positive, slow-cycling brain tumor cells that are more resistant to standard chemotherapy. Dox treatment normally cures GTML transgenic animals that developed aggressive medulloblastoma by turning MYCN off. However, when crossing the Tet-OFF GTML model with a Tet-ON rtTA-Sox9 model we can redirect MYCN expression to the control of the Sox9 promoter - ultimately driving brain tumor recurrence from rare SOX9-positive cells with 100% penetrance. These recurrent tumors were actively disseminating from the hindbrain into the forebrain. Expression profiling shows that recurring tumors have increased levels of SOX9, are more inflammatory and have elevated levels of MGMT methyltransferase, compared to the primary tumors. Overexpressing SOX9 into Group 3 MB cells directly inhibited MYC, and decreased cell proliferation while promoting metastasis. Paired primary and recurrent human Group 3 and Group 4 tumor biopsies also showed significantly higher levels of SOX9 at recurrence. PDX models of Group 3 tumors further showed increased levels of SOX9 positivity in metastatic compartments. Our data unveils complex mechanisms by which dormant medulloblastoma cells fail to respond to standard therapy and generate tumor relapses

TMOD-31. RARE SOX9+ CELLS BEHIND MYC-DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death among children with medulloblastoma, the most frequent type of malignant pediatric brain tumor. The mechanisms behind medulloblastoma recurrence are not fully understood. We found that the transcription factor SOX9 marks quiescent brain tumor stem cells and is suppressed by MYC overexpression in aggressive Group 3 tumors. By using an inducible Tet-OFF transgenic (GTML) mouse model for malignant MYCN-driven Group 3 tumors and human Group 3 MYC-driven PDX models we identified rare SOX9+, slow-cycling brain tumor cells that are more resistant to standard chemotherapy. Dox treatment normally cures GTML transgenic animals that developed aggressive medulloblastoma by turning MYCN off. However, when crossing the Tet-OFF GTML model with a Tet-ON rtTA-Sox9 model we can redirect MYCN expression to the SOX9 promoter ultimately driving brain tumor recurrence from rare SOX9+ cells with 100% penetrance. These recurrent tumors were actively disseminating from the hindbrain to the spinal cord and into the forebrain. Expression profiling comparing primary to recurrent tumors shows that recurring tumors maintain their molecular subgroup but have severely defective DNA repair system and present an increased inflammatory immune response. By overexpressing SOX9 into human Group 3 MB cells MYC was directly inhibited and decreased cell proliferation while promoting migration and metastasis. Paired primary and recurrent human Group 3 and Group 4 tumor biopsies further showed significantly higher levels of SOX9 at recurrence. Finally, PDX models of Group 3 tumors showed increased levels of SOX9 positivity in metastatic compartments. To summarize, our data clarify important and complex mechanisms by which dormant medulloblastoma cells fail to respond to standard therapy and generate relapses

TMOD-25. LATENT SOX9-POSITIVE CELLS BEHIND MYC-DRIVEN MEDULLOBLASTOMA RELAPSE

Abstract Tumor recurrence developing from therapy resistance, immune escape and metastasis is the leading cause of death in medulloblastoma, the most frequent malignant pediatric brain tumor. Amplification of MYC genes is the most common genetic alteration in Group 3 and Group 4 subgroups that constitute two thirds of medulloblastoma. SOX9 is a transcription factor present in stem cells in the normal brain but is limited to rare, quiescent cells in medulloblastoma patients with MYC gene amplifications. By studying paired primary-recurrent patient samples and patient-derived xenografts we here identified significant accumulation of SOX9-positive cells in Group 3 and Group 4 relapses. To follow relapse at the single cell level we developed an inducible dual Tet model of MYC-driven MB, where MYC was re-directed from the treatment-sensitive bulk cells to resistant, dormant SOX9-positive cells by doxycycline. In this model, distant recurrent tumors and spinal metastases developed. SOX9 promoted immune escape, DNA repair suppression and was essential for recurrence. Tumor cell dormancy was non-hierarchical, migratory and depended on MYC suppression by SOX9 to promote relapse. By using computational modeling and treatment we also showed how doxorubicin and MGMT inhibitors were specifically targeting recurrent cells that could be of potential use in future treatments for patients affected by these fatal relapses

Lsd1 as a therapeutic target in Gfi1-activated medulloblastoma.

Drugs that modify the epigenome are powerful tools for treating cancer, but these drugs often have pleiotropic effects, and identifying patients who will benefit from them remains a major clinical challenge. Here we show that medulloblastomas driven by the transcription factor Gfi1 are exquisitely dependent on the enzyme lysine demethylase 1 (Kdm1a/Lsd1). We demonstrate that Lsd1 physically associates with Gfi1, and that these proteins cooperate to inhibit genes involved in neuronal commitment and differentiation. We also show that Lsd1 is essential for Gfi1-mediated transformation: Gfi1 proteins that cannot recruit Lsd1 are unable to drive tumorigenesis, and genetic ablation of Lsd1 markedly impairs tumor growth in vivo. Finally, pharmacological inhibitors of Lsd1 potently inhibit growth of Gfi1-driven tumors. These studies provide important insight into the mechanisms by which Gfi1 contributes to tumorigenesis, and identify Lsd1 inhibitors as promising therapeutic agents for Gfi1-driven medulloblastoma