CBIO-14. EXPLORING THE OTX2 REGULATORY NETWORK: TARGETING THE “GROW AND GO” ARMS OF THE MOST AGGRESSIVE MEDULLOBLASTOMAS

Abstract

Medulloblastoma (MB) is a highly heterogeneous primary malignant pediatric brain cancer. Despite improved 5-year survival rates, MB is frequently accompanied by metastatic dissemination and poor long-term prognosis. The most aggressive tumors are refractory to conventional chemotherapy and radiation. Our goal is to identify new signaling pathways that regulate the treatment-resistant MB cellular phenotypes, namely the stem cells and metastatic or highly motile cells. To this end, we have discovered that the OTX2 gene, which is amplified or overexpressed in the majority of aggressive MBs, is a central regulator of both stem cell function or self-renewal and cell migration in these tumors. However, the molecular mechanisms by which OTX2 regulates these functions are still unknown. Here, we employed complementary bioinformatics approaches to characterize the OTX2 regulatory network and have discovered a novel relationship between OTX2 and neurodevelopmental genes associated with axon guidance signaling in Group 3 and Group 4 MB stem/progenitor cells. Following OTX2 knockdown, transcripts associated with semaphorin, netrin, ephrin and slit signaling were mostly upregulated. ChIP-sequencing identified putative OTX2 DNA binding sites and revealed statistically significant associations between OTX2 and these axon guidance pathway genes, with the presence of 1 or more OTX2 binding peaks within the transcription start site. Further, Group 3 and 4 MB patient samples were evaluated for correlations between expression of axon guidance pathway genes, OTX2 and survival. Semaphorin signaling was the most overrepresented pathway across all datasets with expression of all pathway genes being upregulated following OTX2 KD. Moreover, one semaphorin gene was identified as a novel prognostic biomarker. We anticipate that delineation of the OTX2 network will identify molecular regulators of both self-renewal and motility, and that these genes will serve as better targets for development of novel therapeutics to effectively eradicate both the “grow and go” arms of the most aggressive MB.