Abstract 384: Epigenetic derangements in histone and cytosine methylation are unique to diffuse intrinsic pontine glioma (DIPG)

Abstract

Abstract Diffuse intrinsic pontine glioma (DIPG) is a universally fatal tumor that comprises approximately 10% of pediatric brain tumors. Eighty percent of DIPG patients have a mutation in the H3F3A gene resulting in a lysine to methionine conversion at position 27 of Histone 3.3 (H3K27M). This mutation inhibits methylation at this site by enhancer of zeste homologue 2 (EZH2) methyltransferase and leads to global chromatin changes. DIPG is also associated with global DNA hypomethylation. Recent evidence suggests that 5-methylcytosine (5mC) can be converted to cytosine in a multistep enzymatic process with 5-hydroxymethylcytosine (5hmC) being the initial intermediate. 5hmC itself represents an important epigenetic mark and together with 5mC and histone methylation plays a key role in neural development. Due to the global hypomethylation noted in DIPGs, we hypothesized that 5mC and 5hmC would be low. We expected H3K27 trimethylation (H3K27me3) to be low and associated with the H3F3A mutation common in DIPGs. Tissue microarrays created from post mortem tumor tissue from 24 pediatric DIPG cases were compared to adult and pediatric glioblastoma (GBM) tissue arrays. Immunohistochemical (IHC) assay using antibodies for H3K27me3, 5hmC, and 5mC was used to identify epigenetic differences between the brain tumor groups. H-scores (H) (0-200) were obtained by multiplying intensity of stain (0: no stain, 1: weak stain, 2: strong stain) by percentage (0-100) of cells showing the staining. We found H3K27me3 immunoreactivity was significantly lower for DIPG compared to pediatric GBM (p<0.01), adult GBM (p<0.0001), and normal brain (p<0.0001) tissue. Surprisingly, 5hmC immunoreactivity was significantly higher for DIPG compared to pediatric GBM (p<0.0001) and adult GBM (p<0.0001). Lastly, 5mC was significantly lower in DIPGs compared to pediatric GBM (p<0.001), adult GBM (p<0.01), and normal brain (p<0.01). This data indicates that DIPGs are different from their GBM counterparts and have a unique epigenetic landscape with derangements in both histone and cytosine methylation. This may shift the normal neural development of hindbrain progenitors into a pathological state and promote tumorigenicity. The presence of high-levels of 5hmC in the context of global hypomethylation may be a key indicator of epigenetic derangement in DIPG and may provide an opportunity for therapeutic intervention. Citation Format: Sama Ahsan, Eric Raabe, Michael C. Haffner, Javad Nazarian, Katherine Warren, Leo Ballester, Martha Quezado, Charles Eberhart, Fausto J. Rodriguez. Epigenetic derangements in histone and cytosine methylation are unique to diffuse intrinsic pontine glioma (DIPG). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 384. doi:10.1158/1538-7445.AM2014-384