Abstract A012: EZH2 as a therapeutic target for glioma
Authors: Faruck Lukmanul Hakkim , Day Charles , Bhattarai Suraj , Langfald Alyssa , Hinchcliffe Edward , Robinson James 
Published: March 2024

Abstract
Diffuse midline glioma (DMG), a highly aggressive pediatric brain cancer, and glioblastoma multiforme (GBM), an intractable disease, pose significant clinical challenges. DMG is characterized by a dishearteningly short median survival period of 15 months, and GBM by a low five-year survival rate of merely 6.9 percent. A critical player in these malignancies is Enhancer of zeste homologue 2 (EZH2), a subunit of the polycomb repressive complex 2 (PRC2) that catalyzes histone 3 Lysine 27 tri-methylation (H3K27Me3). This modification suppresses gene expression, orchestrating cell fate. Dysregulated EZH2, linked to both solid and hematological malignancies, including brain cancers, heralds a poor prognosis. DMG presents a heterozygous H3K27M mutation that cripples EZH2 functionality, causing epigenetic dysregulation via H3K27Me3 loss. However, intriguingly, DMG retains residual H3K27Me3 on certain genes, propelling gliomagenesis. EZH2 overexpression in GBM and persisting H3K27Me3 in DMG underscore the malignancy. Therefore, EZH2 has emerged as a potential therapeutic target for gliomas. Despite this promise, targeted inhibitors often fail in clinical trials, thwarted by off-target effects and consequential toxicity. EZH2 inhibitors GSK126 and GSK343 were halted in trials due to their harmful side effects. Meanwhile, EPZ-6438, a more selective EZH2 inhibitor, received FDA approval for treating follicular lymphoma and advanced epithelioid sarcoma. However, recent studies found survival prolongation but subsequent recurrence and resistance in a pediatric brain tumor PDX model with high EZH2 expression treated with EPZ-6438. Recurred tumor cells exhibited minimal EZH2 expression and reduced H3K27Me3 levels, suggesting EPZ-6438 resistance. To delve into these findings, we utilized CRISPR/Cas9 technology to establish EZH2 knockout DMG and GBM cells, and subsequently examined the target specificity and anti-cancer potential of GSK126, GSK343, and EPZ-6438. Our investigations unveiled that both GSK126 and GSK343 induced cell death in EZH2 knockout DMG and GBM cells and non-transformed human cells, indicating detrimental off-target effects. In contrast, EPZ-6438 did not trigger cell death in EZH2 knockout cells but only moderately inhibited the growth of DMG and GBM cells, but also failed to fully suppress H3K27Me3. By establishing EZH2 knockout models, we have provided a platform for understanding the nuanced interactions of these drugs with glioma cells, with potential implications for refining therapeutic strategies for glioma. Our study opens a window into the influence of EZH2 inhibitor and H3K27Me3 loss on glioma cells devoid of EZH2 expression. These findings offer vital insights for discerning which glioma subsets could be effectively treated with EZH2 inhibitors and the genes and pathways regulated by H3K27Me3 enriching our therapeutic strategies against these formidable cancers.
Citation Format: Lukmanul Hakkim Faruck, Charles Day, Suraj Bhattarai, Alyssa Langfald, Edward Hinchcliffe, James Robinson. EZH2 as a therapeutic target for glioma [abstract]. In: Proceedings of the AACR Special Conference on Brain Cancer; 2023 Oct 19-22; Minneapolis, Minnesota. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_1):Abstract nr A012.