EPZ-005687

The most aggressive subtype of medulloblastoma (MB), Group 3, is characterized by MYC amplifications. However, targeting MYC has proven unsuccessful, and there remains a lack of therapeutic targets for treating MB. Studies have shown that the B7 homolog 3 (B7‑H3) promotes cell proliferation and tumor cell invasion in a variety of cancers. Similarly, it was recently revealed that B7‑H3 promotes angiogenesis in Group 3 MB and likely facilitates MB metastasis through exosome biogenesis. While therapies targeting B7‑H3 remain in the early stages of development, targeting upstream regulators of B7‑H3 expression may be more effective for halting MB progression. Notably, MYC and the enhancer of zeste homolog 2 (EZH2) are known to regulate B7‑H3 expression, and a previous study by the authors suggested that B7‑H3 amplifications present in MB are likely the result of EZH2‑MYC mediated activities. In the present study, it was reported that overexpression of EZH2 is associated with lower overall survival in Group 3 MB patients. It was also revealed that inhibition of EZH2 significantly reduces B7‑H3 and MYC transcript levels and upregulates miR‑29a, indicating that EZH2 post‑transcriptionally regulates B7‑H3 expression in Group 3 MB cells. Pharmacological inhibition of EZH2 using EPZ005687 attenuated MB cell viability and reduced the expression of B7‑H3. Similarly, pharmacological inhibition and knockdown of EZH2 led to the downregulation of MYC, B7‑H3, and H3K27me3. Further, EZH2 silencing induced apoptosis and reduced colony‑forming ability in MB cells, whereas EZH2 inhibition in MYC‑amplified C17.2 neural stem cells induced G2/M phase arrest while downregulating B7‑H3 expression. Collectively, the current study positions EZH2 as a viable target for the future development of MB treatments and that targeting EZH2 in combination with B7‑H3 immunotherapy may be an effective treatment for halting MB progression.

Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase and a catalytic subunit of the polycomb repressive complex 2 (PRC2) that catalyzes the mono-, di-, and tri-methylation of histone H3 at Lys 27 (H3K27me3) to facilitate chromatin-remodeling and gene-silencing functions. Previous reports showed a significant association of EZH2 aberrations in pediatric cancers, such as soft tissue sarcomas and glioblastoma. Recent reports in human subjects and animal models have also suggested a central role of EZH2 in the induction and progression of acute myeloid leukemia. In this study, we aimed to investigate the molecular status of EZH in cell lines derived from distinct pediatric leukemia to assess the efficacy of targeting EZH2 to suppress cancer cell survival and proliferation. Our results showed that EZH2 protein is overexpressed in the pediatric monocytic cell-line THP-1, but not in other leukemia-derived cell lines MV4;11 and SEM. Screening a panel of methyltransferase inhibitors revealed that three inhibitors; GSK126, UNC1999 and EPZ-5687 are the most potent inhibitors that suppressed EZH2 activity selectively on lysine 27 which resulted in increased apoptosis and inhibition of AKT and ERK protein phosphorylation in THP-1 cells. Our data demonstrated a significant increase in apoptosis in cells treated with drug combination (EZH2i and selinexor) compared to EZH2i inhibitors alone. Taken together, our data provide initial evidence that targeting EZH2 is a promising therapeutic strategy for the treatment of subtypes of pediatric AML. Also, combining EZH2 inhibitors with selinexor may increase the treatment efficacy in these patients.