MDB-37. Dissecting the role of the leptomeningeal microenvironment in promoting chemotherapy resistance in Group 3 medulloblastoma

Relapse following standard of care therapy portends a dismal prognosis for children diagnosed with brain tumors such as medulloblastoma. This is particularly true for distant relapse that occurs within leptomeninges of the brain or spinal cord. The proclivity for metastatic , as opposed to local relapse, especially for Group 3 tumors, suggests that metastatically competent may be inherently more resistant to chemotherapy. Alternatively, it is possible that factors within the leptomeningeal environment contribute to therapy resistance, though this has not been rigorously explored due to the intrinsic difficulty of studying the leptomeninges. Using an in vitro organotypic co-culture model, we identified that adhering to the subpial matrix promotes persistence following chemotherapy through the acquisition of a transient polyploid cell state in multiple Group 3 medulloblastoma cell lines. Intriguingly, using an acellular extracellular matrix mimetic with adjustable stiffness, we implicate the increased biophysical tissue stiffness medulloblastoma cells encounter upon adhering to pial cells as a key contributor to persistence after chemotherapy. DNA damage response proteins such as CHK1/2 have been nominated as potential therapeutic strategies for Group 3 medulloblastoma. Strikingly, we find that increasing matrix stiffness promotes activation of proteins such as CHK2 in a chemotherapy-independent fashion. CHK2 is known to promote cell cycle arrest to allow for DNA repair. Additionally, cell cycle arrest can lead to endocyling that promotes the generation of polyploid cells. Accordingly, we are currently using our models to test the hypothesis that adhesion-induced CHK2 activation primes cells for chemotherapy persistent, polyploid cell states. Collectively, our data highlight that identifying non-cell autonomous mechanisms of chemotherapy persistence provides novel therapeutic vulnerabilities to eradicate tumor cells from the leptomeninges.