EPIG-03EPIGENETIC SUPPRESSION OF EGFR SIGNALING IN G-CIMP + GLIOBLASTOMAS

Abstract

While it is understood that glioblastomas with the Glioma CpG island methylator phenotype (G-CIMP) phenotype harbor extensive methylation in the CpG islands of a large number of genetic loci, how these methylations patterns translate into the modulation intrinsic signaling cascades remain poorly understood. Using independent, published mRNA signatures associated with EGFR activation, we demonstrate that G-CIMP+ tumors harbor decreased EGFR signaling in three independent datasets, including the REMBRANDT dataset (n = 288), The Cancer Genome Atlas (TCGA; n = 406), and the Chinese Glioma Genome Atlas (CGGA; n = 155). Supporting these findings, analysis of an independent collection of 25 fresh-frozen glioblastomas confirmed lowered pERK levels in G-CIMP+ specimens (p < 0.001), supporting suppressed EGFR signaling in G-CIMP+ glioblastomas. To better understand the mechanism underlying this suppression, we analyzed the TCGA glioblastomas database for the expression level of genes required for EGFR signaling and found that G-CIMP+ glioblastomas harbored lowered mRNA levels for EGFR and H-Ras. In independent glioblastoma models, induction of G-CIMP+ state by exogenous expression of a mutated isocitrate dehydrogenase 1, IDH1-R132H, or by chronic 2-hydroxyglurate treatment suppressed EGFR and H-Ras protein expression as well as pERK accumulation. These suppressions were associated with increased deposition of the repressive histone markers, H3K9me3 and H3K27me3, in the EGFR and H-Ras promoter regions. The IDH1-R132H expression-induced pERK suppression can be reversed by exogenous expression of H-RasG12V but not be exogenous expression of EGFRvIII, suggesting epigenetic suppression H-Ras as the rate limiting step of EGFR signaling in G-CIMP+ glioblastomas. Further supporting suppressed EGFR signaling in G-CIMP+ glioblastomas, we observed increased resistance to the EGFR inhibitor, Gefitnib, in an engineered G-CIMP+ Ink4a-Arf-/- EGFRvIII glioblastoma line relative to its isogenic G-CIMP- counterpart. These results suggest that mitogenic signaling cascades in glioblastomas are epigenetically regulated and a role for IDH1 mutation serves as a predictive biomarker for EGFR inhibitor sensitivity in glioblastoma patients.