Genes Preserving Stem Cell State in Medulloblastoma Contribute to Therapy Evasion and Relapse

Abstract

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Out of the four molecular subgroups (WNT, SHH, Group 3 and Group 4), Group 3 patients face the highest incidence of leptomeningeal spread and overall patient survival of less than 50%. Current clinical trials for recurrent MB patients based on genomic profiles of primary, treatment-naïve tumors, provide limited clinical benefit since recurrent metastatic MBs are highly genetically divergent from their primary tumors. The paucity of patient matched primary and recurrent MB samples has contributed to the lack of molecular targets specific to medulloblastoma recurrence, limiting relapsing MB patients to palliation. Our previous in silico analyses revealed enriched expression of many stem cell self-renewal regulatory genes in Group 3 MB.

In this work, I have set out to investigate whether by identifying genes contributing to self-renewal of Group 3 MB cells, we can characterize a population of cells responsible for therapy evasion and subsequent tumor relapse. Initially, we have adapted the existing COG (Children’s Oncology Group) protocol for children with newly diagnosed high-risk MB for treatment of immuno-deficient mice intracranially xenografted with human MB cells. Cell populations recovered separately from brains and spines mice during the course of tumor development and therapy were comprehensively profiled for gene expression analysis, stem cell and molecular features to generate a global, comparative profile of MB cells through therapy. Additionally, we have investigated therapeutic potential of small molecules targeting BMI1, a known self-renewal regulating gene. In the setting of recurrent Group 3 MB, pharmacological inhibition of BMI1, led to a remarkable decrease in cell proliferation and self-renewal in vitro as well as reduction of local and spinal metastatic disease in vivo. Finally, by combining the established therapy-adapted patient-derived xenograft mouse model and BMI1 inhibitor, PTC-596, we have demonstrated an additive effect of two modalities and provided the pre-clinical data for the upcoming Phase I trial.

Biological investigations into the drivers of MB recurrence will lead to development of new therapeutic options for children who are frequently limited to palliation. Clinically relevant mouse models of MB recurrence can serve as platforms for pre-clinical testing and validation of new treatments aimed to provide therapeutic intervention rather than palliation.