Background
Pediatric high-grade glioma is the leading cause of cancer death in children [
1]. Diffuse intrinsic pontine glioma (DIPG) is a high-grade glioma that arises in the brainstem, with an overall survival of only 11 months [
2]. DIPG cannot be removed surgically, and there is no known effective treatment. Standard management is with radiotherapy, which temporarily relieves symptoms but only confers a survival benefit of three months [
3]. Since DIPGs ultimately exhibit treatment resistance and disease progression, better understanding of tumor biology is desperately needed to identify effective therapeutic therapy [
4].
Recent studies suggest that changes in epigenetic regulation of gene transcription play a key role in pediatric gliomagenesis and response to treatment [
5‐
7]. Dynamic post-translational modifications (PTMs) on the histone N-terminal tail, rendered by histone modifying enzymes, alter chromatin structure to facilitate open reading (euchromatin) or silencing (heterochromatin) throughout the genome [
5,
8‐
10]. Specific patterns of histone PTMs represent an epigenetic signature, or histone code, reflecting a relative transcriptional state. In up to 80% of DIPGs, heterozygous somatic mutations in genes encoding histone H3 isoforms H3.1 and H3.3 (
HIST1H3B and
H3F3A, respectively) substitute methionine for lysine at amino acid residue 27 on the H3 N-terminal tail (H3K27M) [
11]. This H3K27M mutant protein alters histone H3 PTMs in DIPG, impacting chromatin structure and hence gene expression [
5,
7,
12].
For example, histone H3K27 di- and tri-methylation (H3K27me2/3), transcriptionally silencing marks, are reduced in H3K27M DIPG, de-repressing transcription [
13‐
15]. H3K27me2/3 loss is accompanied by increased H3K27ac and the formation of heterotypic H3K27M/H3K27ac nucleosomes that colocalize with bromodomain proteins 2 and 4 (BRD2/4) and RNA polymerase II, suggesting a potential role of BRD proteins in DIPG pathogenesis [
7,
16]. We demonstrated pre-clinical efficacy of bromodomain inhibition (BRDi) with JQ1, a bromodomain and extra terminal domain (BET) inhibitor, resulting in reduction of H3K27ac and restoration of H3K27me3 [
7]. These findings suggest that changes in epigenetic regulation of gene expression in DIPG may be reflected by histone codes, and detection of these signatures could shed light on mechanisms of tumorigenesis. Here, we employ targeted mass spectrometry via multiple reaction monitoring (MRM) to characterize 93 distinct PTM states in H3.3K27M mutant DIPG specimens, and investigate the effects of radiation and bromodomain inhibition on tumor epigenetic signatures.
Materials and methods
Tissue specimens
Pediatric DIPG tumor tissue (
n = 9, H3.3K27M mutant) was collected postmortem (IRB #Pro00001339). H3.3K27 wild-type (WT) adult glioma tumor tissue specimens (
n = 2), and one non-tumor frontal lobe tissue specimen were collected during the course of treatment (IRB# STU00095863). All diagnoses were made radiographically by a neuroradiologist, then confirmed by neuropathological evaluation upon tissue collection. Patient identifiers were removed and a single sequential numerical identifier assigned to each specimen (Table
1). Tissues were stored at − 80 °C prior to proteomic analysis (IRB #STU00202063).
Table 1
Summary of specimens analyzed with histone tail mass spectrometry
1 | DIPG tissue | H3.3K27M | F | 5 |
2 | DIPG tissue | H3.3K27M | M | 8 |
3 | DIPG tissue | H3.3K27M | M | 7 |
4 | DIPG tissue | H3.3K27M | M | 19.6 |
5 | DIPG tissue | H3.3K27M | F | 7 |
6 | DIPG tissue | H3.3K27M | M | 7.6 |
7 | DIPG tissue | H3.3K27M | M | 6 |
8 | DIPG tissue | H3.3K27M | M | 6 |
9 | DIPG tissue | H3.3K27M | M | 6.25 |
10 | GBM tissue | H3.3G34R | N/A | 45 |
11 | GBM tissue | Wild type | N/A | 50 |
12 | Focal cortical dysplasia, frontal lobe | Wild type | N/A | 62 |
SF7761 | DIPG cell line | H3.3K27M | N/A | N/A |
SF8628 | DIPG cell line | H3.3K27M | N/A | N/A |
DIPG 007 | DIPG cell line | H3.3K27M | N/A | N/A |
KNS 42 | GBM cell line | H3.3G34V | N/A | N/A |
SF9427 | Pediatric GBM cell line | Wild type | N/A | N/A |
U87 | Adult GBM cell line | Wild type | N/A | N/A |
NHA | Normal human astrocytes | Wild type | N/A | N/A |
Cell lines
Patient-derived pediatric DIPG cell lines SF8628, SF7761, DIPG007 (H3.3K27M mutant) and high-grade glioma cells KNS42 (H3.3G34V mutant) and SF9427 (H3.3G34 wild-type) were provided by Dr. Rintaro Hashizume (Northwestern University) as previously described [
17‐
19]. H3 wild-type adult glioma cells (U87) and normal human astrocytes (NHA) were obtained from American Type Culture Collection and ScienCell (#1800), respectively. Cells were treated with 300 nM JQ1 (Selleckchem S7110) or DMSO (Sigma D2650) for 24 and 48 h. Cell irradiation (9 Gy) was performed with the RadSOURCE-2000 X-Ray Irradiator, and cells analyzed at 24 and 48 h. Cell pellets were generated by harvesting with trypsin and washing twice with PBS, then stored at − 80 °C prior to proteomic analysis.
Western blotting
Histones were extracted from cell pellets using Histone Extraction Kit (ab113476, Abcam) according to the manufacturer’s protocol. 0.5 μg Histones were loaded and separated by SDS-PAGE using a 12% Mini-PROTEAN® TGX™ Precast Protein Gels and transferred by using Trans-Blot Turbo Mini 0.2 μm PVDF Transfer Packs at 2.5A, 25 V for 3 min in a Trans-Blot Turbo™ Transfer Systems (Bio-Rad). The membrane was blocked prior to the addition of the primary antibody with 5% milk in Tris buffered saline with 0.1% of Tween 20. The membrane was incubated overnight with primary antibody in TBS buffer with 0.1% Tween and 5% milk. The dilution of primary antibody was as follows: Rabbit monoclonal anti-Histone H3 (mutated K27M) antibody (ab190631) at a dilution of 1:1000, Rabbit mAb Acetyl-Histone H3 (Lys27) (D5E4) XP ((#8173, Cell Signaling Technology) at a dilution of 1:1000, Mouse mAb Histone H3 (1B1B2) (#14269, Cell Signaling Technology) at a dilution of 1:1000. The membrane was washed 3 times with TBS/0.1% Tween and incubated with an anti-rabbit or anti-mouse IgG conjugated to horse radish peroxidase (7074 and 7076, Cell Signaling Technology) at a 1:2000 dilution in TBS/0.1% Tween and 5% milk. Pierce ECL Plus Western Blotting Substrate (CN: 32132, Thermo Scientific) was used according to the manufacturer’s protocol to visualize proteins and quantify band intensity. Experiments were performed in triplicate for statistical analysis. The intensity of protein bands were analyzed by ImageJ, with the relative protein value expressed via the ratio of specific protein/H3.
Gene expression profiling
Total cell RNA was extracted for gene expression profiling based on manufacturer protocol (Qiagen). All samples were adjusted to a concentration of 20 ng/μl RNA prior to submission for gene expression analysis using the NanoString nCounter Plexset Analysis System DNA damage repair 96 gene codeset (NanoString Technologies, Seattle, WA). Gene expression values were determined and converted to fold change in expression level for each treatment condition compared to DMSO controls in order to determine statistically significant differences in gene expression levels between groups (NanoString). Functional pathways and gene ontology analysis was then performed on fold change expression values (Inginuity Pathways Analysis, Qiagen).
Histone preparation for targeted mass spectrometry
Nuclei were isolated in buffer (15 mM Tris-HCl pH 7.5, 60 mM KCl, 15 mM NaCl, 5 mM MgCl
2, 1 mM CaCl
2, 250 mM sucrose; 1 mM DTT, 1:100 HALT protease inhibitor (Thermo Scientific)), and 10 mM sodium butyrate added immediately prior to use containing 0.3% NP-40. Nuclei were centrifuged at 600 x
g for five minutes at 4 °C and washed twice with nuclear isolation buffer. Histones were extracted by via five volumes of H
2SO
4 and incubated at room temperature one hour. Debris was removed by centrifugation at 4000 x
g for five minutes. Trichloroacetic acid was added to the supernatant at a final concentration of 20% (v/v) and incubated overnight at 4 °C. Precipitated histones were pelleted at 10,000 x
g for five minutes, washed once with ice-cold acetone with 0.1% HCl and twice with acetone at 15,000 x
g for five minutes. Residual acetone was dried in a fume hood, and samples stored at − 20 °C until further processing. Histones were derivatized with a single round of propionylation, digested with 1 μg trypsin (Promega, Madison, WI), and derivatized again with a single round of propionylation according to Garcia et al. [
20].
Targeted LC-MS/MS of histone peptides
Histone peptides were resuspended in Loading Buffer (0.1% TFA in water) and analyzed on a Thermo TSQ Quantiva (Thermo Scientific) in-line with a Dionex nano-LC (Sunnyvale, CA). Peptides were loaded onto a trapping column (3 cm × 150 μm, packed with ProntoSIL C18-AQ, 3 μm, 200 Å resin (New Objective, Woburn, MA)) for 10 min at 2.5 μL/min with 100% Loading Buffer. Elution from the trapping column and separation on a PicoChip analytical capillary column (10 cm × 75 μm packed with ProntoSIL C18-AQ, 3 μm, 200 Å resin (New Objective)) was achieved by decreasing the percentage of Solvent A (0.1% formic acid in water) and increasing the percentage of Solvent B (0.1% formic acid in 95% acetonitrile) from 1 to 35% at a flow rate of 0.30 μL/min over 45 min. The peptides were introduced into the triple quadrupole mass spectrometer by electrospray from an emitter with a 10 μm tip. The instrument settings were as follows: collision gas pressure of 1.5 mTorr; Q1 peak width of 0.7 (FWHM); cycle time of 3 s; skimmer offset of 10 V; electrospray voltage of 2.5 kV. All injections were performed in technical triplicate. Targeted methods were performed as previously published [
21]. A list of monitored peptide transitions is provided (Supplementary material
1).
Quantitation of histone modification states
Raw data were imported into Skyline with Savitzky-Golay smoothing for analysis. Peak area assignments were manually confirmed, and total peak areas used to determine the percent relative abundance of histone modifications. The percent relative peptide abundance was determined by dividing the area under the curve for a given peptide of interest by the sum of the areas under the curve for all peptides with the same sequence, then multiplying by 100. Heatmaps were generated with R using gplots package, and show modifications that were above the limit of detection in all samples.
Statistical analysis
Differences in peptide abundance were analyzed using independent-sample t test for between-group comparisons, or one-way ANOVA and Tukey’s post hoc test for multi-group analyses using SPSS (IBM, Version 25), and expressed as mean ± standard deviation (SD). Pearson Correlation was performed to analyze the relationship between H3.3K27M mutation and detected histone codes. Multivariable analysis was performed using linear regression with Stepwise entry (p = 0.05) and removal (p = 0.10). All p values were two-sided, with p < 0.05 considered statistically significant.
Discussion
DIPG is an incurable pediatric brain tumor, with a high rate of somatic missense mutations in genes encoding histone H3.1 and H3.3 proteins. Here, we present comprehensive characterization of histone N-terminal tail modification states in pediatric glioma specimens, including H3.3K27M mutant DIPG tissue, using novel, high-throughput proteomic analysis, revealing the effects of radiotherapy and BRDi on the tumor epigenetic landscape. The greatest differences in PTMs between H3K27M mutant and WT specimens were observed at H3K27, H3K36, and H4K16ac, while H3K36me2 was the highest co-occurring modification on the H3.3K27M peptide, and H4K16 was the most acetylated residue. These results suggest that H3K36me2 and H4K16ac may serve as epigenetic signatures in DIPG, and that targeting these modification states may represent a potential therapeutic strategy.
Multiple groups have identified distinct DIPG molecular subtypes, with H3.3K27M mutant tumors associated with shorter survival [
22‐
30]. We found that H3.3K27M significantly affects the histone PTM landscape, with cell and tissue profiles clustering by mutation status. Further, a decrease in global H3K27me2/3 has been observed in H3.3K27M tumors, and our results corroborate these data, [
5,
7,
8,
10,
12,
13,
31]. Of note, a new subgroup of DIPG lacking H3K27M mutation but still exhibiting genomic H3K27me3 loss was recently reported [
32]: we did not observe this phenomenon in our cohort (Supplementary material
4). Our H3K27M mutant specimens also exhibit distinct modification states at H3K36 and H4K16. Previous work showed that PRC2 loss coincides with increased H3K36me2, and that an increase in H3K36 methylation impacts methylation at H3K27 [
33‐
35]. Our results also confirm an increase in H3K36me2 in DIPG specimens on the H3.3K27M mutant peptide, as previously detected via mass spectrometry of H3.3K27M transgenic PDGF-driven murine glioblastoma [
13]. Further, we detected high H4K16ac abundance in DIPG cells and tissue specimens. A recently published report characterizing high-throughput screening with nucleosome substrate methods demonstrates small-molecule inhibitors of the human histone lysine methyltransferase NSD2 [
36], the writer of mono- and di-methylation of H3K36 [
37]; another report identified DC_M01_7 as a novel inhibitor of MOF, the H4K16ac-specific acetyltransferase [
38,
39]. Therefore, H3K36me2 and H4K16ac may represent potential therapeutic targets in DIPG, and further investigation of this strategy is warranted.
Using our method, we also characterized H3K27 and K36 modification states
in cis, confirming me2/3 as the predominant modifications, and acetylation as the least abundant, on the H3.3K27MK36 peptide. We observed positive correlation between H3.3K27M and H3.1K27K36me2, and negative correlation between H3.3K27M and H3.1K27K36me3. Taken together, these results confirm that H3.3K27M affects PTMs on both mutant and WT H3.1 and H3.3 N-terminal tails. Importantly, our study is the first, to our best knowledge, to quantify the relative amount of mutant H3.3K27M peptide in DIPG tumor specimens. It is reported that relative abundance of mutant epidermal growth factor receptor (EGFR) can predict benefit from EGFR-tyrosine kinase inhibitors for advanced non-small-cell lung cancer [
40]. Longitudinal quantification of the H3.3K27M peptide in patient-derived specimens might similarly serve as a predictive biomarker for targeted therapy, and hence is worthy of consideration when designing future clinical trials.
Radiation therapy (RT) is standard treatment for DIPG, providing transient symptom relief but no change in disease outcome [
41]. Unfortunately, the mechanism of radiation resistance is unknown, and radio-sensitizers provide no benefit [
42]. Patients with the H3.3K27M mutation also respond more poorly to RT [
24]. Our study reveals RT can induce cell-specific and peptide-specific changes in histone acetylation, underscoring the heterogeneity of DIPG and warranting consideration when using histone deacetylase / acetyltransferase inhibitors in combination with RT. Importantly, H3K9me3 abundance increased in all four cell lines after RT, which kills rapidly proliferating cancer cells by inducing DNA damage beyond cellular capacity for repair, including DNA double-strand breaks (DSBs) [
43‐
45]. DSB repair is linked to rapid changes in epigenetic modifications, including increased H3K9 methylation, to recruit DNA repair proteins. Importantly, H3K9 methyltransferase G9a inhibition (BIX-01294) increases radio-sensitivity in glioma [
46]. Indeed, on functional pathways analysis of cell gene expression profiles, we found DSB repair pathways implicated in RT treated cell lines relative to controls. These data suggest that combination therapy RT with G9a inhibition, aimed at reducing H3K9me3 levels and hence DNA DSB repair, may also be a useful therapeutic strategy in DIPG.
We found distinct DIPG histone codes after treatment with JQ1, a BET/ bromodomain inhibitor. BET domain proteins are epigenetic readers that bind acetylated histones via their bromodomain region to regulate gene transcription [
46‐
48]. BET inhibitors dislodge acetylated histone readers from chromatin, leading to oncogene repression [
49‐
51]. JQ1 is the best known BET inhibitor, and induces a dose-dependent reduction in DIPG cell viability and genomic H3K27ac in vitro and in vivo [
7,
52,
53]. Our results showed increased mono- and/or di-methylation of multiple residues with JQ1 treatment, consistent with previous reports that JQ1 globally reduces acetylation and increases methylation [
7]. Since mono-methylation of H3K27, H3K79, and H4K20 are all linked to gene activation, whereas tri-methylation of H3K27 and H3K79 are linked to repression [
54], these findings may provide additional insight into the mechanism by which BRDi alters tumorigenic gene expression in DIPG. H4K20me, which is functional in DNA repair, represents a binding site for the 53BP1 protein; H3K9me3 and H4K20me3 represent epigenetic markers important to the function of 53BP1 in non-homologous end joining (NHEJ) repair [
55]. In line with these known functions, pathways analysis of differential gene expression profiles implicated nucleotide excision repair and DNA mismatch repair via NHEJ as top activated pathways in JQ1 treated cells, compared to controls. As such, additional studies of the mechanism by which JQ1 may augment response to RT are necessary. Lastly, we also observed that reduction of histone acetylation in DIPG was greatest at 24 h, which may be important to consider when determining JQ1 dosing regimens.
Because fresh DIPG tissue is very rare, we chose to analyze archival post-mortem tissue specimens for this study, raising the possibility of treatment-induced effects. However, the patterns of peptide abundance observed in tumor tissues were similar to treatment-naïve cell lines. Analysis of treatment fresh tumor tissue specimens with this approach will be crucial to teasing out tumor biology from the effects of therapy. Optimization of our technical approach to minimize the amount of starting tumor tissue necessary for this purpose is currently underway.
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