Investigation of the Initiation, Progression and Treatment of Pediatric Gliomas

Abdallah, Aalaa  Sanad

doi:https://doi.org/10.21985/n2-t0e9-z262

Work

Investigation of the Initiation, Progression and Treatment of Pediatric Gliomas

Public

Download PDF

Download All Files (.zip)

Pediatric high-grade gliomas (pHGGs) are aggressive pediatric CNS tumors and an important subset are characterized by mutations in H3F3A, the gene that encodes Histone H3.3 (H3.3). Substitution of Glycine at position 34 of H3.3 with either Arginine or Valine (H3.3G34R/V), was recently described and characterized in a large cohort of pHGG samples as occurring in 5-20% of pHGGs. Attempts to study the mechanism of H3.3G34R have proven difficult due to the lack of knowledge regarding the cell-of-origin and the requirement for co-occurring mutations for model development. We sought to develop a biologically relevant animal model of pHGG to probe the downstream effects of the H3.3G34R mutation in the context of vital co-occurring mutations. We developed a genetically engineered mouse model (GEMM) that incorporates PDGF-A activation, TP53 loss and the H3.3G34R mutation both in the presence and loss of Alpha thalassemia/mental retardation syndrome X-linked (ATRX), which is commonly mutated in H3.3G34 mutant pHGGs. We demonstrated that ATRX loss significantly increases tumor latency in the absence of H3.3G34R and inhibits ependymal differentiation in the presence of H3.3G34R. Transcriptomic analysis revealed that ATRX loss in the context of H3.3G34R upregulates Hoxa cluster genes. We also found that the H3.3G34R overexpression leads to enrichment of neuronal markers but only in the context of ATRX loss. This study proposes a mechanism in which ATRX loss is the major contributor to many key transcriptomic changes in H3.3G34R pHGGs. The Notch signaling pathway is a well-characterized and critical regulator of embryonic development as well as various adult processes. Notch signaling is also extremely active in glioma stem cells (GSCs), the precursor cells of glioma. Our next objective was to extract information on the effects of H3.3G34R expression and/or ATRX loss on Notch signaling in pHGG and the associated underlying mechanisms. To achieve this goal, we extracted information on Notch ligands and associated genes from the RNA-Sequencing data we previously generated. In GEMMs of pHGG, we observed that ATRX and H3.3G34R expression promote Notch pathway activation independently of each other and that Notch pathway activation correlated with our previously reported changes in Dll3 regulation. We then utilized patient-derived orthotopic xenograft (PDOX) models of pHGG to confirm association of Notch pathway activation and both ATRX and H3.3G34R expression. Overall, we found that while ATRX loss and H3.3G34R expression both independently activate Notch, ATRX loss is the main contributor to NOTCH pathway activation in tumor samples. The protein growth arrest-specific protein 6 (GAS6) is a high-affinity ligand for AXL and serves as an activator of AXL signaling. Important downstream pathways of AXL/GAS6 signaling include PI3K/AKT/mTOR, JAK/STAT, NFkB, and RAS/RAF/MEK/ERK, which influence tumor cell survival, apoptosis, therapeutic resistance and angiogenesis. Aberrant expression of AXL is associated with poorer overall survival and cancer progression. AXL and GAS6 are highly expressed in a large subset of adult glioblastoma where high expression of AXL and/or AXL/GAS6 was associated with significantly shorter tumor progression and overall survival. AVB-500 is a fusion protein which binds to GAS6, ablating the AXL/GAS6 signaling pathway. We assessed the therapeutic efficacy and acute downstream molecular mechanisms of AVB-500 in PDOX mouse models of pediatric glioblastoma. Overall, AVB-500 had did extend median survival times however the difference did not reach statistical significance. We also observed that 24 hours after treatment with AVB-500, markers of angiogenesis were downregulated. AVB-500 represents a promising therapeutic target for pediatric glioblastoma though further optimization of dosage and timing are required.

Creator