DIPG-62. Reducing the levels of genomic 5-hydroxymethylcytosine by inhibiting the TET pathway induces apoptosis and decreases proliferation in Diffuse Intrinsic Pontine Glioma (DIPG)

The H3K27M mutation, observed in 70%-90% of DIPG, causes global hypomethylation. The Ten-Eleven-Translocation enzymes, TETs, which convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) regulate the methylation patterns in the genome. Previously, we observed an increase in the 5hmC levels and altered TET activity in DIPG. We hypothesize that targeting the TETs in DIPG could restore the epigenetic balance. In this study, we target the TET pathway with cell-permeable 2-hydroxyglutarate (2HG) and Bobcat339, a cytosine-based TET inhibitor. We see a dose-dependent decrease in 5hmC (46-96% reduction) and increase in 5mC (~5fold increase) in multiple DIPG cell lines treated with 2HG. We see a corresponding increase in apoptosis and reduction in proliferation (~8fold increase in cleaved-Caspase3 signal (P=0.0001) and ~55% reduction in BrdU-incorporation measured by IF (P=0.0036) in JHH DIPG16A with similar results in JHH DIPG1, HSJD007, and SUDIPG6). We see a 54-90% reduction in 5hmC and ~3fold increase in 5mC with Bobcat 339 treatment in DIPG cell lines JHH DIPG1, JHH DIPG16A, HSJD007, and SUDIPG6. We also see a corresponding increase in apoptosis as measured by cleaved PARP western blot and decrease in proliferation as measured by BrdU incorporation (approximately 32-53% decrease in proliferation). Combined treatment with 2HG and Bobcat339 lead to greater decreases in 5hmC and increases in 5mC compared to single treatment, with synergistic suppression of cell growth as measured by SynergyFinder in HSJD007, JHH DIPG1, and JHH DIPG16A. In vivo studies targeting DIPG orthotopic xenografts are currently underway. These results suggest that TETs contribute to the hypomethylated state observed in DIPG. Thus, inhibiting the TETs can be explored as a potential therapeutic approach to treat DIPG.