EMBR-33. YAP1 FUNCTION IN SHH MEDULLOBLASTOMA PROGRESSION AND IMMUNE EVASION

Our incomplete understanding of the key players in Medulloblastoma (MB) development and progression, and their roles in modulating highly Immune desert-like microenvironment in MBs present major hurdles in successfully applying existing therapies and developing new therapies for MBs. Here, we demonstrate that Yap1 acts as a critical modulator of SHH MB (fSmoM2; GFAPcre (SG) and Ptch;p53 SHH-MB mouse models) progression and immune evasion. Yap1 genetic deletion in SG mice significantly extends survival and normalizes brain development by increasing neuronal differentiation. Both bulk and single-cell RNA sequencing analyses show that Yap1 deleted tumors contain cells with more differentiated molecular signatures similar to late CGNPs and differentiating neurons, and less stem-like cells, compared to SG tumors. Additionally, integrated analyses of ChiPseq, RNAseq, and scRNAq data suggest that Yap1 directly binds to the Super enhancer region containing Sox2 and promotes Sox2 expression in SHH MB cells. We postulate that Yap1 expression is maintained or re-activated in SHH MB cells to generate long-term self-renewing tumor cells. Consistently, Yap1-deleted SHH MB or Verteporfin (a small molecule inhibitor of Yap1) -treated Ptch;p53 MB cells lose self-renewal ability in vitro. Furthermore, we hypothesize that a molecular mechanism underlying this stemness promoting function is mediated through Sox2 expression. Intriguingly, Yap1 deletion in SG MBs is accompanied by a significant change in the immune microenvironment, when compared to age-matched SG MBs. There is a significant increase in the number of bone marrow-derived immune cells (including cytotoxic T-cells, neutrophils, and macrophages). RNAseq analysis of rescued tumors shows marked enrichment of interferon-gamma response genes and pro-inflammatory cytokines. This study highlights Yap1 as a crucial mediator of MB progression and a molecular regulator of inflammatory immune cell infiltration into SHH MBs. Consequently, our work paves the way for improving immunotherapy treatments in brain malignancies.