Genomic Instability and the Oncohistone H3K27M Drive Gliomagenesis in a Murine Model
Authors:   ,   University of Tennessee Health Science Center, Pribyl Lee 
Published: April 2021

Maintaining genome stability is crucial for human health and it is of particular importance in neural cells during early brain development. Genome maintenance occurs at two broad stages; surveillance during DNA replication and DNA damage repair in differentiating and mature cells. Neural cells are particularly sensitive to DNA strand breaks and defective DNA damage responses can result in detrimental effects on the nervous system, including cancer. Multiple DNA repair pathways play critical roles in preventing DNA damage accumulation in stem and neural progenitor cells. The mechanisms that protect progenitor genomes also suppress DNA mutations that can result in cancer. A primary objective of this dissertation is to understand the relative contributions of key DNA repair factors that prevent tumorigenesis during cortical development. We have compared the differential effects of inhibition of homologous recombination (HR), via BRCA2-inactivation and non-homologous end-joining (NHEJ), via LIG4-inactivation towards tumorigenesis by directing their deletion specifically to early cortical progenitors using an Emx1-cre recombinase driver. We find that coincident loss of either of these repair pathways with p53 inhibition result in distinct high-grade glioma (HGG) formation resulting from elevated genome instability by DNA damage accumulation during embryogenesis. Furthermore, the presence of the oncohistone H3K27M mutation, commonly found in pediatric HGGs, enhances genome instability and accelerates cortical gliomagenesis with p53 inactivation and defective HR or NHEJ. Additionally, the H3K27M resultant gliomas showed distinctive differences in increased brain tumor penetrance and diffusion. Through RNA-sequencing and whole exome sequencing we identify upregulation of genes normally controlled by bivalent gene promoter post-translational modifications, which result in transcriptional alterations in genes important for both neural development and tumorigenesis. Mechanistically, this is done by targeting specific populations of cortical cells that are more susceptible to DNA damage and transformations that may cause additional critical mutations during a limited timeframe of early cortical development which eventually result in HGGs. We provide evidence supporting that BRCA2 functions to provide DSBR and genome stability to the early-born proliferating cortical progenitor cell population, while LIG4 provides the same function but to a lesser extent to progenitor cells and more so to post-mitotic neurons. Since, epigenetic regulation is tightly connected with neural development and differentiation, we propose the specific genes that H3K27M effects may differ depending on the time period and particular cell state from which the HGG initiates. We believe this contributes to reduced heterogeneity in glioma expression signatures with H3K27M in addition to either HR- or NHEJ-deficiency. Ultimately this work highlights the power of inducible genetically engineered mouse models as an approach to better understand the complexities of providing a connection between genome instability and gliomagenesis.