Taimur Hassan

Background: Glioblastoma multiform (GBM) is the most malignant brain tumor and has a median overall survivability (OS) of less than one year since diagnosis.¹ Single-nucleotide polymorphisms (SNPs) and amplification of the epidermal growth factor receptor (EGFR) gene contribute to more than 50% of the cases where EGFR-related signaling pathways are over-stimulated.¹ Prior EGFR-targeted therapies have been largely unsuccessful.¹ EGFR-amplification plays a crucial role in changing the 3D structure of chromatin, exposing oncogenes to transcription factors, or suppressing tumor repressor genes likely sequestering them in sections of condensed chromatin.¹ Errors in genes regulating the three processes of initiation of tumor growth, evasion of senescence, and enabling of immortal growth can all lead to gliomagenesis.² Blocking the signaling pathway through EGFR can provide the ultimate basis of a therapy as it provides the initial signal to all three processes.

Objective: In this narrative review, we explored the primary mechanisms of tumorigenesis and a potential novel therapy for EGFR-dependent GBM.

Search Methods: An online search in the PubMed database was conducted from 2017 to 2023 using the following keywords: “chromatin”, “EGFR”, “glioblastoma”.

Results: Studies indicate that EGFR regulates the expression of GLUT3 that is dependent on the super-enhancer function of SOX9 in GBM through the EGFR-SOX9-GLUT3 axis.³ Increased GLUT3 causes increased facilitation of glucose transport across the plasma membrane.³ EGFR intron 1 has open chromatin regions containing super enhancers, such as CE1 and CE2, and deletion of these regions decrease EGFR transcript and protein formation.⁴ AP-1 transcription factors also play a role in intron 1-mediated EGFR expression.⁴ Loss of different BET proteins, modulators of CE1 and CE2, through knockdowns was also found to have a strong downregulation of EGFR expression.⁴ Targeted therapies towards open chromatin regions in EGFR intron 1 could be utilized where therapies aim to tighten the chromatin structure making it refractory to factors that need to gain access to the DNA template.¹ Another study presented TEAD1/4 motifs as potential targets in the transcriptionally accessible regions (open chromatin) and their crucial role in GBM cell migration using AQP4 and CDH11.⁵ Histone deacetylase (HDAC) inhibitors are another therapeutic option as they lead to increase in open chromatin but subsequent reduction in effectiveness of human GBM cells.⁶ Combined inhibition of HDACs and EGFR through synergism was more effective than inhibiting either target alone in reducing cell division.⁶ Inhibition through these inhibitors causes compensatory increases in mRNA expression of the signal transducer STAT3 through BRD4 activation of a cascade that leads to high JAK expression.⁶ This causes increased transcription of cell growth factors and anti-apoptotic molecules, such as BCL2 and CCND1, eventually maintaining tumor cell growth and survival.⁷

Conclusion: While HDAC inhibitors cause some opening of chromatin, they have vital benefits in cell-cycle arrest. The negative consequences can be averted using an HDAC-STAT3-EGFR triple inhibitor therapy, leading to a complete blockade of EGFR-dependent GBM tumorigenesis. EGFR inhibition would make possible deletion of CE1 and CE2 enhancers in intron 1 and TEAD1/4 motifs redundant as upstream EGFR signals on the membranes are blocked.

Works Cited:

1. Yang Q, Jiang N, Zou H, et al. Alterations in 3D chromatin organization contribute to tumorigenesis of EGFR-amplified glioblastoma. Comput Struct Biotechnol J. 2022;20:1967-1978. Published 2022 Apr 8. doi:10.1016/j.csbj.2022.04.007

1. Xu H, Zong H, Ma C, et al. Epidermal growth factor receptor in glioblastoma. Oncol Lett. 2017;14(1):512-516. doi:10.3892/ol.2017.6221
2. Chen S, Yang L, Li Z, et al. EGFR/EGFRvIII partly regulates the tumourigenesis of glioblastoma through the SOX9-GLUT3 axis. Am J Transl Res. 2021;13(6):6055-6065. Published 2021 Jun 15.
3. Jameson NM, Ma J, Benitez J, et al. Intron 1-Mediated Regulation of EGFR Expression in EGFR-Dependent Malignancies Is Mediated by AP-1 and BET Proteins. Mol Cancer Res. 2019;17(11):2208-2220. doi:10.1158/1541-7786.MCR-19-0747
4. Tome-Garcia J, Erfani P, Nudelman G, et al. Analysis of chromatin accessibility uncovers TEAD1 as a regulator of migration in human glioblastoma. Nat Commun. 2018;9(1):4020. Published 2018 Oct 1. doi:10.1038/s41467-018-06258-2
5. Buendia Duque M, Pinheiro KV, Thomaz A, et al. Combined Inhibition of HDAC and EGFR Reduces Viability and Proliferation and Enhances STAT3 mRNA Expression in Glioblastoma Cells. J Mol Neurosci. 2019;68(1):49-57. doi:10.1007/s12031-019-01280-5
6. Ren Y, Li S, Zhu R, et al. Discovery of STAT3 and Histone Deacetylase (HDAC) Dual-Pathway Inhibitors for the Treatment of Solid Cancer. J Med Chem. 2021;64(11):7468-7482. doi:10.1021/acs.jmedchem.1c00136