Diffuse intrinsic pontine glioma (DIPG) is a rare cancer constitutes the largest number of deaths caused by brain tumours in people under the age of 19. The average age of DIPG diagnosis is between 6 and 8 years old and most of those children die within the first year of diagnosis. While many other paediatric tumours have had significant progress, DIPG survival rates have stagnated over the past four decades for this very aggressive cancer. More than 60–70% of DIPG tumors have lysine27 to methionine mutations (K27M). H3K27M results in the substitution of lysine with methionine in the isoforms H3.1 and H3.3. The Histone H3K27M mutations reprogramme epigenome with a global reduction of H3K27 methylation reduced DNA methylation. H3K27M mutation accelerates tumorigenesis, especially for brainstem HGG and high grade DIPG. PRC2 plays an important role in silencing gene transcription. PRC2 consists of 4 core subunits (EZH1/2, EED, SUZ12, and RBBP4/7) and associated to some non-core subunits. The signature activity of PRC2 is to methylate histone H3 of lysin27 which is catalysed by EZH1/2 via s-adenosyl methionine.
EZH1 and EZH2 of PRC2 catalyse methylation of H3K27 and are potential target for cancer therapy. Therefore, EZH1 and EZH2 inhibitors can be exciting new area for cancer therapy.
We assessed the impact of small molecule compounds on the inhibition of histone methyltransferase activity of EZH1 and EZH2. First, we investigated the effects of GSK126 (a highly selective EZH2 inhibitor) and UNC1999 (a dual EZH1 and EZH2 inhibitor) that both are an S-Adenosyl methionine (SAM) competitive inhibitor of EZH1/EZH2 on cell viability in DIPG cell lines. DIPG cell lines, H3.3K27M (DIPG17 and DIPG25) and H3.1K27M mutant (DIPG33, DIPG36), were treated with increasing concentrations of GSK126 and UNC1999 for 3 and 6 days. All the H3K27M expressing cell lines showed reduced proliferation in response to EZH2-inhibitor treatment. GSK126 significantly decreased viable cell numbers in a dose-dependent manner as compared to similar concentrations of diluted DMSO. However, UNC1999 exhibited a superior ability to suppress cell proliferation as compared to GSK126 in all the H3K27M mutant cell lines tested. To establish the sensitivity of cells to the drugs, The IC50 of GSK126 and UNC1999 were determined for all cell lines. UNC1999 showed a lower IC50 value than GSK126 over 6-days. In parallel, cell confluence was scanned by Incucyte-live cell analysis every 12 h for 6 days. The Incucyte analysis determined that UNC1999 reduced cell confluence in a dose and time-dependent manner better than GSK126. Inhibition of both EZH1/2 resulted in higher antitumor activity than only inhibiting EZH2. Collectively, our research data suggested that targeting both EZH1/2 could provide new therapeutic options for the treatment of PRC2-dependent cancers, like DIPG.