Abstract 3831: Ash2L and p300 mediate histone H3 modifications at EGFR during its developmental silencing and re-expression in gliomas

Gliomas are the most frequent and aggressive primary brain tumors, and are universally fatal once they progress to glioblastomas. Glioma cells commonly share abnormalities in pathways that normally control proliferation, migration and differentiation of glial progenitors; one such example is signaling through the epidermal growth factor receptor (EGFR). We show that EGFR is highly expressed in neural progenitors during fetal brain development, and that it becomes silenced during adulthood in most glia, except for those in the subventricular zone (SVZ) where adult neural progenitors are thought to reside. Interestingly, most low- and high-grade diffuse gliomas show strong re-expression of EGFR. The mechanism of this pathological overexpression is not clearly understood and cannot be accounted for by genomic abnormalities alone, since less than half of primary glioblastomas and almost no low-grade gliomas contain EGFR-activating gene amplification or mutations. We hypothesized that EGFR silencing during normal glial differentiation and its aberrant re-expression in gliomas is at least partly mediated via local epigenetic mechanisms at its promoter. To test this hypothesis, we have performed targeted epigenetic analyses at the EGFR promoter, measuring both DNA methylation levels via bisulfite sequencing and histone H3 methylation (me) and acetylation (ac) levels via chromatin immunoprecipitation (ChIP) using primary human samples. Surprisingly, we find that DNA methylation patterns at EGFR are conserved regardless of the EGFR expression status in non-neoplastic glia and in gliomas. In contrast, ChIP analysis reveals enrichment of the activating modifications H3K27ac and H3K4me3 during fetal brain development when EGFR is highly expressed, and their loss in adult white matter where EGFR is silenced. In the SVZ, where some glial cells continue to express EGFR at lower levels, these modifications are moderately enriched. The repressing modification H3K27me3, in contrast, is low during fetal development but becomes highly enriched in adult white matter. Interestingly, we also observe robust enrichment of H3K27ac and H3K4me3 in adult glioma specimens. Furthermore, we find significant binding of the histone methyltransferase complex Ash2L at EGFR only in fetal and glioma samples that show high levels of H3K4me3, and of the histone acetyltransferase p300 only in samples showing enrichment of H3K27ac, implicating these two histone modifying enzymes in the dynamic epigenetic regulation of EGFR expression during glial differentiation and glioma formation. Our studies use endogenous human material and point to an important, region-specific role for chromatin remodeling in EGFR expression during gliomagenesis, which will undoubtedly open a broad range of new potential therapeutic tumor targets. Citation Format: Parsa Erfani, Jessica Tome Garcia, Peter Canoll, Nadejda Tsankova. Ash2L and p300 mediate histone H3 modifications at EGFR during its developmental silencing and re-expression in gliomas. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3831. doi:10.1158/1538-7445.AM2015-3831