Targeting EZHIP as a therapeutic strategy for infantile ependymomas
Posterior fossa group A (PFA) ependymomas are an especially clinically challenging type of brain tumor that occurs in young children. In order to develop new treatments to combat these tumors, we first need to understand the molecular pathways that drive the cells to grow. Recent work from our group and others found that nearly all PFA ependymoma tumors have abnormally high levels of a protein that we named EZHIP. Normally, the EZHIP gene is only turned on in cells that make up part of the testes and ovaries. Remarkably, EZHIP is somehow turned on inappropriately in the brain cells that give rise to PFA ependymoma tumors.
Molecularly, PFA ependymomas are highly similar to another brain tumor called diffuse intrinsic pontine gliomas (DIPG). Previously, we made important insights into the mechanism of the histone H3 K27M mutation found in DIPG and other midline brain tumors. Specifically, we found that the K27M mutant histone causes misregulation of genes in tumor cells by blocking the activity of an important gene regulatory factor called PRC2. In a recent study, we demonstrated that part of EZHIP is a molecular mimic of the K27M mutant histone: both proteins bind to and inhibit PRC2.
Previous work found that the K27M mutant histone is important for both the initiation and maintenance of tumor cells. Importantly, these findings suggest that targeting the K27M mutation may be a therapeutically viable strategy for DIPG. Because of the biochemical similarities between EZHIP and K27M, we now propose to explore the role of EZHIP in promoting PFA tumorigenesis.
Our previous work suggests that the EZHIP and the K27M mutant histone help drive specific types of brain tumors by targeting the PRC2 gene regulatory factor. Because brainstem tumors (DIPG) require the K27M histone for maintenance of tumor growth, we propose to determine if EZHIP is important for PFA ependymoma tumor cell proliferation. We hypothesize that EZHIP blocks PRC2 activity to promote PFA cell proliferation. To explore this possibility, we will employ different molecular strategies to test if EZHIP is necessary for PFA cells to proliferate. In addition to measuring PFA tumor cell proliferation in the presence and absence of EZHIP, we will also measure changes in chromosome structure and gene expression using molecular assays. Together these experiments will help us determine 1) if EZHIP is a critical pro-tumor factor in PFA ependymomas, and 2) which cellular pathways are misregulated by EZHIP in PFA tumors to promote proliferation.
Additionally, we will explore how EZHIP activity is regulated in tumor cells. Unlike many pro-tumor genes, the EZHIP gene is only active in very few cells and tissues in humans. We don’t understand why the gene is only activated in some cells, and we think that understanding the pathways that regulate EZHIP expression may be important for developing therapeutic strategies. Our preliminary studies point to an important epigenetic chemical modification to DNA called methylation. We will use a combination of both cell-based and biochemical experiments to comprehensively understand EZHIP gene regulation. This information may lay the groundwork for future preclinical studies with drugs that modulate EZHIP chemical modifications for therapeutic use.