STEM-01. Pial stromal cells adopt cancer associated fibroblast-like phenotype during pediatric brain tumor leptomeningeal colonization

Despite the profound implications leptomeningeal metastasis has on the survival of children diagnosed with brain tumors, the molecular and cellular mechanisms governing tumor cell colonization of the leptomeninges remain incompletely defined. While tumor intrinsic properties drive metastatic competence, the non-cell autonomous ways in which tumor cells interact with and co-opt stromal cells at metastatic sites are increasingly being recognized as critical, and therapeutically vulnerable, aspects of the metastatic cascade. Accordingly, we have developed an in vitro leptomeningeal organotypic model to elucidate how pediatric brain tumor cells interact with and instruct leptomeningeal stromal cells to create a receptive niche during colonization. Thus far, we have found that a wide range of tumor cell lines (medulloblastoma (SHH, Group 3, Group 4), atypical teratoid rhabdoid tumor (SHH and MYC), and posterior fossa ependymoma (Group A)) can be successfully co-cultured with primary human leptomeningeal fibroblasts in a nutrient poor, cerebrospinal fluid (CSF)-like environment. Remarkably, three distinct growth and morphological patterns have emerged, which roughly correlate with metastatic behavior observed in children. For instance, SHH medulloblastoma cells (UW426) adhere to the leptomeningeal layer as single cells but do not proliferate while Group 3 (D283, D425, MED411, and MED2112), Group 4 (CHLA-01-MED), and ATRT (BT-12) cells demonstrate nodular or laminar-like growth patterns. Laminar spreading tumor cells (Ex: D425, MED411, BT-12) uniquely affect leptomeningeal cells, disrupting cell-cell adhesions, and leading to upregulation of extracellular matrix components, contractile proteins, and inflammatory cytokines reminiscent of cancer associated fibroblasts in other tumors. Current efforts are utilizing both in vitro and in vivo approaches to define how subsets of tumor cells activate leptomeningeal cells and discern the functions of tumor-induced, leptomeningeal-derived factors on driving tumor growth. Uncovering shared mechanisms utilized by distinct tumor types could yield new therapeutic interventions for children diagnosed with highly metastatic brain tumors.